{"gene":"NR1H4","run_date":"2026-04-29T11:37:57","timeline":{"discoveries":[{"year":1995,"finding":"NR1H4 (then called FXR) was identified as an orphan nuclear receptor that forms a heterodimeric complex with retinoid X receptor (RXR) and is activated by farnesol and related metabolites, establishing it as a ligand-modulated transcription factor.","method":"Receptor isolation, ligand screening, heterodimerization assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — original discovery paper with functional activation assays and heterodimerization demonstrated","pmids":["7774010"],"is_preprint":false},{"year":1997,"finding":"RIP14 (NR1H4) can be activated by all-trans-retinoic acid and the synthetic retinoid TTNPB; it binds DNA as a heterodimer with RXR, and mutagenesis of the ligand-dependent activation function showed TTNPB activates the RIP14 component while 9-cis-RA and LG1069 activate the RXR component.","method":"Reporter gene activation assays, site-directed mutagenesis of AF-2 domain, DNA binding assays","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis combined with functional activation assays","pmids":["9223286"],"is_preprint":false},{"year":1999,"finding":"Bile acids (chenodeoxycholic acid, lithocholic acid, deoxycholic acid) are physiological ligands for FXR/NR1H4; ligand-bound FXR repressed CYP7A1 transcription (rate-limiting enzyme in bile acid synthesis) and activated intestinal bile acid-binding protein (IBABP) expression, establishing FXR as a bile acid sensor that transcriptionally regulates bile acid biosynthesis and enterohepatic transport.","method":"Radioligand binding assays, reporter gene assays, Northern blot analysis of CYP7A1 and IBABP","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — direct ligand identification with functional transcriptional readouts, independently replicated in same year by multiple labs","pmids":["10334992","10334993","10360171"],"is_preprint":false},{"year":1999,"finding":"Physiological concentrations of free and conjugated chenodeoxycholic acid, lithocholic acid, and deoxycholic acid activate FXR and promote coactivator (SRC-1 peptide) recruitment, demonstrating a nuclear bile acid signaling pathway.","method":"Transactivation assays, coactivator interaction assays (SRC-1 peptide recruitment)","journal":"Science","confidence":"High","confidence_rationale":"Tier 1 — coactivator recruitment assay plus transactivation, replicated across labs","pmids":["10334993"],"is_preprint":false},{"year":2000,"finding":"FXR induces expression of SHP-1 (small heterodimer partner), which then represses CYP7A1 by inhibiting the activity of LRH-1 (liver receptor homolog-1); this FXR→SHP-1→LRH-1 regulatory cascade provides the molecular basis for bile acid feedback suppression of CYP7A1.","method":"Nonsteroidal FXR ligand treatment, reporter assays, mRNA expression analysis, epistasis with SHP-1 and LRH-1","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — epistatic cascade defined with multiple components and replicated in two simultaneous papers","pmids":["11030332","11030331"],"is_preprint":false},{"year":2000,"finding":"Targeted disruption of FXR/BAR in mice resulted in elevated serum bile acids, cholesterol, and triglycerides; increased hepatic cholesterol and triglycerides; reduced bile acid pools; and loss of bile acid-mediated regulation of CYP7A1 and IBABP, establishing FXR's in vivo role as an intracellular bile acid sensor critical for bile acid and lipid homeostasis.","method":"FXR knockout mice, serum lipid measurements, hepatic lipid measurements, gene expression analysis","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — clean knockout with multiple defined phenotypic and molecular readouts","pmids":["11030617"],"is_preprint":false},{"year":2000,"finding":"FXR (as an RXR heterodimer) activates ABC1 expression and represses CYP7A1; rexinoid treatment phenocopies these effects, establishing FXR-RXR as a key regulator of reverse cholesterol transport and bile acid synthesis downstream of bile acid sensing.","method":"Receptor-selective agonist treatment in mice, gene expression analysis, ABC1 and CYP7A1 reporter assays","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — in vivo pharmacological and genetic approach with receptor-selective compounds","pmids":["10968783"],"is_preprint":false},{"year":2002,"finding":"Guggulsterone is a highly efficacious antagonist of FXR; its cholesterol-lowering activity depends on FXR, as guggulsterone failed to decrease hepatic cholesterol in FXR-null mice fed a high-cholesterol diet.","method":"FXR reporter assays, FXR knockout mice, high-cholesterol diet model","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — antagonist function validated in FXR knockout, confirming on-target mechanism","pmids":["11988537"],"is_preprint":false},{"year":2003,"finding":"FXR directly regulates expression of FGF19 (FGF-15 in mice), a secreted growth factor; FGF19 signals through FGFR4 to strongly suppress CYP7A1 expression in hepatocytes via a c-Jun N-terminal kinase (JNK)-dependent pathway, defining a novel endocrine feedback loop for repression of bile acid biosynthesis.","method":"FXR agonist treatment, FGF19 expression in primary hepatocytes, FGFR4 signaling, JNK pathway inhibitor studies, CYP7A1 reporter assays","journal":"Genes & development","confidence":"High","confidence_rationale":"Tier 2 — mechanistic cascade with multiple pathway components and signaling assays","pmids":["12815072"],"is_preprint":false},{"year":2003,"finding":"FXR activates the human PPARα gene promoter via a defined FXR response element (αFXRE) in the PPARα promoter, as shown by gel shift analysis (FXR binding to αFXRE), cotransfection assays, and mutation analysis; this cross-talk is species-specific (not present in mice).","method":"Gel shift (EMSA), cotransfection with FXR/RXR, mutation analysis, reporter assays in HepG2 cells and primary hepatocytes","journal":"Molecular endocrinology","confidence":"High","confidence_rationale":"Tier 1 — EMSA direct binding plus mutational analysis and functional reporter assays","pmids":["12554753"],"is_preprint":false},{"year":2005,"finding":"FXR-activating ligands downregulate rabbit ASBT (apical sodium-dependent bile acid transporter) expression through the regulatory cascade FXR→SHP→FTF (alpha-fetoprotein transcription factor); a functional FTF binding site at -1166/-1158 of the rabbit ASBT promoter is required, and only FXR-activating ligands (not non-activating bile acids) repress ASBT.","method":"In vivo rabbit feeding experiments, promoter cloning and deletion analysis, Caco-2 cell transfection, mRNA quantification","journal":"American journal of physiology. Gastrointestinal and liver physiology","confidence":"High","confidence_rationale":"Tier 2 — cascade defined with promoter deletion and in vivo validation","pmids":["15591588"],"is_preprint":false},{"year":2006,"finding":"FXR activation by GW4064 or constitutively active FXR adenovirus significantly lowered blood glucose in diabetic db/db mice by repressing hepatic gluconeogenic genes and increasing hepatic glycogen synthesis via enhanced insulin sensitivity; FXR-null mice showed glucose intolerance and insulin insensitivity, with blunted insulin signaling in skeletal muscle and liver. The FXR-SHP cascade mediates glucose metabolism.","method":"FXR agonist (GW4064) treatment, adenoviral FXR overexpression, FXR knockout mice, hyperinsulinemic euglycemic clamp, gene expression analysis","journal":"Proceedings of the National Academy of Sciences / Journal of Clinical Investigation","confidence":"High","confidence_rationale":"Tier 2 — gain- and loss-of-function with mechanistic pathway placement, replicated across two independent labs","pmids":["16410358","16557297"],"is_preprint":false},{"year":2006,"finding":"Androsterone (a testosterone metabolite) directly binds the FXR ligand-binding domain (LBD) as shown by NMR spectroscopy, recruits SRC-1 coactivator peptide, and activates FXR; site-directed mutagenesis of hFXR-LBD identified Asn354 and Ser345 as critical for differential species sensitivity to CDCA and androsterone, respectively. Crystal structure-guided mutations (M265I, R331H) further dissected the binding mode.","method":"NMR spectroscopy of purified hFXR-LBD, site-directed mutagenesis, Gal4-LBD reporter assays, in vivo androsterone treatment of castrated mice","journal":"Endocrinology","confidence":"High","confidence_rationale":"Tier 1 — direct binding by NMR plus mutagenesis and crystal structure interpretation","pmids":["16675527"],"is_preprint":false},{"year":2006,"finding":"FXR-null mice have reduced expression of the major hepatic canalicular bile acid transport protein (BSEP); nuclear receptor-dependent bile acid signaling via FXR is required for normal liver regeneration after partial hepatectomy, as decreased bile acid levels inhibit liver regrowth and FXR absence prevents normal regeneration.","method":"FXR knockout mice, partial hepatectomy model, bile acid manipulation, gene expression analysis","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — clean knockout with defined regeneration phenotype and bile acid manipulation","pmids":["16614213"],"is_preprint":false},{"year":2007,"finding":"FXRα (NR1H4) acquired ligand specificity for bile salts late in vertebrate evolution; deletion of an extra LBD sequence in a FXRβ homolog from skate (which normally responds weakly to bile salts) conferred responsiveness to GW4064 and ursodeoxycholic acid, demonstrating that the bile acid-binding specificity of FXRα is conferred by the structure of its LBD.","method":"Phylogenetic analysis, luciferase reporter assays, LBD deletion mutagenesis in heterologous cells","journal":"American journal of physiology. Regulatory, integrative and comparative physiology","confidence":"Medium","confidence_rationale":"Tier 2 — LBD deletion mutagenesis plus reporter assays in single study","pmids":["17567710"],"is_preprint":false},{"year":2008,"finding":"FXR directly induces GLUT4 transcription through a defined FXR response element (FXRE) in the GLUT4 promoter; EMSA and RXR knockdown showed FXR binds the GLUT4-FXRE as a monomer (RXR-independent); FXR activation does not interfere with insulin-induced GLUT4 translocation to the plasma membrane.","method":"5'-deletion and site-mutation analysis of GLUT4 promoter, EMSA, RXR siRNA knockdown, in vivo CDCA treatment","journal":"Cellular physiology and biochemistry","confidence":"High","confidence_rationale":"Tier 1 — EMSA direct binding plus promoter mutagenesis and in vivo validation","pmids":["18769028"],"is_preprint":false},{"year":2009,"finding":"FXR activation by CDCA promotes endothelial cell motility and tube formation via FAK phosphorylation at Y397 (but not Y576/577 or Y925), which activates paxillin; FAK phosphorylation at Y397 is required for CDCA-induced MMP-9 upregulation; both FXR and MMP-9 siRNA blocked CDCA-induced motility, establishing a FXR→FAK(pY397)→paxillin→MMP-9 pathway for EC motility.","method":"Time-lapse video microscopy, siRNA knockdown of FXR/MMP-9/FAK, phosphodeficient FAK mutant, phospho-specific immunoblotting","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"High","confidence_rationale":"Tier 2 — multiple siRNAs plus phosphodeficient mutant defining mechanistic pathway","pmids":["19150878"],"is_preprint":false},{"year":2009,"finding":"FXR is expressed by cells of innate immunity (macrophages, dendritic cells); FXR activation stabilizes the nuclear corepressor NCoR on the NF-κB responsive element of the IL-1β promoter, thereby transrepressing NF-κB-dependent inflammatory genes (TNF-α, IL-1β, IL-6, COX-1, COX-2, iNOS); FXR-null mice show exacerbated colitis.","method":"FXR-null mice colitis models (TNBS, DSS), LPS-activated macrophage treatment, ChIP (NCoR on IL-1β promoter), gene expression analysis","journal":"Journal of immunology","confidence":"High","confidence_rationale":"Tier 2 — ChIP demonstrating NCoR stabilization plus knockout phenotype in two colitis models","pmids":["19864602"],"is_preprint":false},{"year":2011,"finding":"FXR activation by INT-747 (obeticholic acid) in vivo protects against DSS- and TNBS-induced colitis by downregulating pro-inflammatory cytokines and preserving epithelial barrier function; FXR-null mice show exacerbated colitis, establishing FXR as a regulator of intestinal inflammation and barrier integrity.","method":"FXR-null mice, DSS/TNBS colitis models, INT-747 treatment, epithelial permeability assay, cytokine ELISA, in vitro Caco-2/HT29 cells","journal":"Gut","confidence":"High","confidence_rationale":"Tier 2 — knockout plus pharmacological rescue with multiple mechanistic readouts","pmids":["21242261"],"is_preprint":false},{"year":2014,"finding":"FXR suppresses hepatic autophagy in the fed state by binding to promoters of autophagy genes (including Atg7, Ulk1, Tfeb) and disrupting the functional CREB-CRTC2 coactivator complex; FXR and CREB compete for shared sites in autophagic gene promoters with opposite transcriptional outputs. FXR-knockout mice show partial defects in fed-state suppression of hepatic autophagy.","method":"ChIP-seq from mouse liver, pharmacological FXR activation/knockout, co-immunoprecipitation of CREB-CRTC2 complex disruption, autophagy flux assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1 — ChIP-seq, reconstitution of complex disruption, and knockout mice — replicated in two simultaneous Nature papers","pmids":["25383523","25383539"],"is_preprint":false},{"year":2014,"finding":"FXR is a molecular target for the metabolic benefits of vertical sleeve gastrectomy (VSG); VSG increases circulating bile acids, and in FXR-knockout mice, the ability of VSG to reduce body weight and improve glucose tolerance is substantially reduced, establishing FXR as a key mediator of VSG metabolic effects.","method":"FXR knockout mice with VSG surgery, gut microbiota analysis, circulating bile acid measurement, glucose tolerance tests","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — genetic loss-of-function with defined surgical phenotype","pmids":["24670636"],"is_preprint":false},{"year":2014,"finding":"FXR directly regulates renal AQP2 (aquaporin 2) expression by binding to a FXR response element in the AQP2 gene promoter; FXR agonist treatment increased urine osmolality and upregulated AQP2, while FXR-knockout mice showed impaired urine concentrating ability and reduced AQP2 expression.","method":"FXR knockout mice, FXR agonist treatment, luciferase reporter assay with AQP2 promoter FXRE, primary inner medullary collecting duct cell culture","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 — promoter FXRE characterization plus knockout phenotype","pmids":["24464484"],"is_preprint":false},{"year":2015,"finding":"FXR directly binds an IR9 DNA motif within the SOCS3 promoter (by EMSA and ChIP) and enhances SOCS3 transcription; FXR-induced SOCS3 upregulates p21 and represses STAT3 phosphorylation, mediating anti-tumor effects in HCC; SOCS3 siRNA knockdown abolished FXR's anti-HCC effects.","method":"EMSA, ChIP assay, luciferase reporter assay, SOCS3 siRNA, HCC xenograft mouse model","journal":"Oncotarget","confidence":"High","confidence_rationale":"Tier 1 — direct DNA binding by EMSA and ChIP plus functional epistasis with siRNA and in vivo model","pmids":["26416445"],"is_preprint":false},{"year":2015,"finding":"Intestinal FXR inhibition (by Gly-MCA) improves obesity-related metabolic dysfunction; mechanistically, reduced intestinal FXR signaling decreases biosynthesis of intestinal ceramides, which directly compromise beige fat thermogenic function; intestine-specific FXR knockout mice were unresponsive to Gly-MCA benefits.","method":"Intestine-specific FXR knockout mice, Gly-MCA treatment, ceramide measurement, beige fat thermogenesis assays, FXR agonist blockade experiments","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific knockout plus ceramide supplementation rescue experiment","pmids":["26670557"],"is_preprint":false},{"year":2015,"finding":"FXR activation by GW4064 induces SREBP-2 mRNA and miR-33 expression via a defined FXR response element in intron 10 of the Srebp-2 gene; however, FXR simultaneously induces INSIG-2A, which prevents processing of pSREBP-2 to nuclear SREBP-2, uncoupling miR-33 induction from SREBP-2 target gene regulation.","method":"ChIP-seq (FXR response element in Srebp-2 intron 10), FXR agonist treatment in mice, Scap-knockout mice, INSIG-2A expression analysis, miR-33 target gene analysis","journal":"Arteriosclerosis, thrombosis, and vascular biology","confidence":"High","confidence_rationale":"Tier 2 — ChIP-seq plus genetic validation in Scap-null mice","pmids":["25593129"],"is_preprint":false},{"year":2016,"finding":"FXR activation suppresses renal fibrosis by inhibiting the transcriptional activity of the Smad3 gene promoter (luciferase reporter assay); FXR-mediated repression of fibrosis is alleviated by ectopic Smad3 overexpression, and FXR agonist protected against fibrosis in UUO mice while downregulating Smad3.","method":"Luciferase reporter assay (Smad3 promoter), Smad3 overexpression, FXR antagonist, FXR agonist in UUO mouse model","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — promoter assay and epistasis (Smad3 rescue) in single lab","pmids":["27853248"],"is_preprint":false},{"year":2017,"finding":"A CK2-RNF4 interplay coordinates non-canonical FXR SUMOylation and degradation: CK2 phosphorylates Ser-327 of FXR, enabling SUMO2 conjugation at Lys-325 (a pSuM motif) by Ubc9/PIAS1; Lys-325 SUMOylation promotes efficient ligand activation and transcriptional coactivation. Constitutive SUMOylation directs FXR ubiquitination by E3 ligase RNF4 and proteasomal degradation; RNF4 is required for maximal FXR-dependent gene regulation.","method":"Site-directed mutagenesis (K325R, S327A/E), Co-IP of SUMO2/ubiquitin, PIAS1/Ubc9 interaction, RNF4 knockdown, ChIP, CK2 inhibition","journal":"Journal of molecular cell biology","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis of modification sites combined with Co-IP and functional transcriptional readouts","pmids":["28201649"],"is_preprint":false},{"year":2019,"finding":"FXR restricts abnormal Lgr5+ intestinal cancer stem cell growth; bile acids antagonizing intestinal FXR (T-βMCA, DCA) induce proliferation and DNA damage in Lgr5+ cells, while selective intestinal FXR activation curtails CRC progression; this implicates FXR in coordinating intestinal self-renewal with bile acid levels.","method":"Intestinal organoids, Lgr5+ cell sorting, FXR agonist/antagonist treatment, APC-mutant/high-fat-diet mouse model, DNA damage assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — genetic and pharmacological manipulation in organoid and in vivo models with defined cellular phenotype","pmids":["30794774"],"is_preprint":false},{"year":2019,"finding":"FXR activation inhibits Src kinase (Tyr416 phosphorylation), leading to increased Ser127 phosphorylation and cytosolic retention of YAP (via Hippo kinase complex stabilization), thereby protecting against renal fibrosis; FXR-knockout mice show increased fibrosis markers and nuclear YAP.","method":"FXR agonist (GW4064) in HK2 cells and UUO mice, FXR-KO mice, Src inhibitor (PP2), phospho-specific immunoblotting, Co-IP of Hippo kinases","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 — pharmacological and genetic FXR manipulation with mechanistic Src-YAP pathway definition","pmids":["31298930"],"is_preprint":false},{"year":2020,"finding":"Enhanced FXR SUMOylation in activated hepatic stellate cells (HSCs) limits their response to FXR agonists in liver fibrosis; SUMOylation inhibitors rescue FXR signaling and synergize with OCA against HSC activation. FXR upregulates Perilipin-1 to stabilize lipid droplets and prevent HSC activation.","method":"SUMOylation assays in HSCs, SUMOylation inhibitors, OCA treatment, CCl4/BDL/NASH mouse fibrosis models, Perilipin-1 ChIP and siRNA knockdown","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — SUMOylation mechanism defined with inhibitors and functional rescue in multiple fibrosis models","pmids":["31932588"],"is_preprint":false},{"year":2020,"finding":"FXR functions as a T cell-intrinsic sensor of nutrient availability; T cell-specific FXR deletion prevented starvation-induced loss of lymphocytes and increased effector T cell fitness under low-glucose conditions by increasing glutamine and fatty acid contribution to respiration and enhancing cell survival.","method":"T cell-specific FXR knockout mice, infection models, metabolic flux analysis (glutamine and fatty acid use), glucose rescue experiments","journal":"Proceedings of the National Academy of Sciences","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific knockout with metabolic flux readouts","pmids":["33318189"],"is_preprint":false},{"year":2020,"finding":"FXR and GPBAR1 (TGR5) exert antagonistic effects on autophagy: FXR represses autophagy-related genes (Atgs) in the fed state, while GPBAR1 promotes autophagy via cAMP-CREB pathway; mechanistically, GPBAR1 agonism promotes CREB recruitment to the LC3 promoter CRE and reverses FXR-mediated repression of autophagy.","method":"Gpbar1-null mice, FXR-null mice, FXR antagonist, GPBAR1 agonist (BAR501), ChIP (CREB on LC3 promoter), autophagy flux assays in liver and WAT","journal":"FASEB journal","confidence":"High","confidence_rationale":"Tier 2 — dual genetic knockouts and ChIP demonstrating opposing mechanisms at shared promoters","pmids":["33368684"],"is_preprint":false},{"year":2020,"finding":"PPARγ directly binds the PPAR-responsive element in the FXR gene promoter in a PPARγ agonist-dependent manner (ChIP assay), driving FXR expression during adipogenesis; FXR activation then induces SCD (stearoyl-CoA desaturase) by binding a FXRE in the SCD gene promoter, promoting lipogenesis in adipocytes.","method":"ChIP assay (PPARγ binding to FXR promoter), FXRE identification in SCD promoter, reporter assay, troglitazone/CDCA/guggulsterone treatment in 3T3-L1 cells","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP plus functional readouts, single lab study","pmids":["32446390"],"is_preprint":false},{"year":2021,"finding":"Hepatic FXR protects against NAFLD primarily through two distinct intestinal and hepatic mechanisms: hepatic FXR controls lipogenic genes (repressing Scd1, Dgat2, Lpin1 independently of SHP and SREBP1c), while intestinal FXR controls lipid absorption via regulation of bile acids; tissue-specific FXR knockout mice dissected these two pathways.","method":"Tissue-specific FXR knockout mice (hepatic vs intestinal), comprehensive lipidomics, bile acid replenishment experiments, GSK2324 FXR agonist treatment","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific knockouts with comprehensive lipidomics providing mechanistic dissection","pmids":["34270928"],"is_preprint":false},{"year":2022,"finding":"FXR is a direct transcriptional regulator of ACE2 in gastrointestinal and respiratory tissues; FXR inhibition by UDCA or z-guggulsterone downregulates ACE2 expression in human lung, cholangiocyte, and intestinal organoids, in mouse and hamster tissues, and in ex situ perfused human lungs, reducing susceptibility to SARS-CoV-2 infection.","method":"FXR agonist/antagonist treatment in human organoids, ex vivo perfused human organs, mouse and hamster in vivo models, ACE2 reporter assays, SARS-CoV-2 infection assays","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal model systems (organoids, ex vivo organs, in vivo animals) with functional infection readout","pmids":["36470304"],"is_preprint":false},{"year":2022,"finding":"SIRT6 deacetylates FXR at the protein level, thereby elevating FXR transcriptional activity and protecting against APAP-induced hepatotoxicity; FXR ablation abolished SIRT6 overexpression-mediated hepatoprotection, and pharmacological FXR activation rescued SIRT6-knockout mice from APAP injury.","method":"Hepatocyte-specific SIRT6 and FXR knockout mice, SIRT6 overexpression and pharmacological activation, deacetylation assays, APAP hepatotoxicity model","journal":"Cellular and molecular gastroenterology and hepatology","confidence":"High","confidence_rationale":"Tier 2 — dual genetic knockouts with mechanistic deacetylation and functional hepatoprotection readouts","pmids":["35526796"],"is_preprint":false},{"year":2022,"finding":"Under liver injury conditions, FXR acetylation at K217 (near the nuclear localization signal) blocks recognition by importin KPNA3, preventing nuclear import; simultaneously, reduced phosphorylation at T442 (within nuclear export signal) promotes recognition by exportin CRM1, facilitating FXR nuclear export and cytosolic retention; cytosolic FXR is then degraded by the E3 ligase CHIP. SIRT1 activators reduce FXR acetylation, prevent cytosolic degradation, and synergize with FXR agonists against liver injury.","method":"Acetylation site mutagenesis (K217, T442), importin/exportin binding assays (KPNA3, CRM1), CHIP E3 ligase Co-IP, SIRT1 activator treatment, acute/chronic liver injury mouse models","journal":"Acta pharmaceutica Sinica. B","confidence":"High","confidence_rationale":"Tier 1 — mutagenesis of modification sites combined with importin/exportin binding assays and in vivo models","pmids":["36873184"],"is_preprint":false},{"year":2020,"finding":"BRD4 is a co-factor required for FXR-mediated regulation of bile acid homeostasis; liver-specific BRD4 downregulation disrupted bile acid homeostasis, and FXR-mediated regulation of SHP and CYP7A1 was BRD4-dependent. In cholestasis, OCA-activated FXR increased binding of FXR and co-repressor SMRT at inflammatory genes, decreased NF-κB binding, and repressed inflammatory genes in a BRD4-dependent manner.","method":"Liver-specific BRD4 knockdown, cholestatic mouse models, ChIP for FXR/SMRT/NF-κB at inflammatory gene promoters, JQ1 (BRD4 inhibitor) and OCA treatment","journal":"JCI insight","confidence":"High","confidence_rationale":"Tier 2 — ChIP demonstrating co-factor requirement plus liver-specific knockdown with gene expression readouts","pmids":["33290278"],"is_preprint":false}],"current_model":"NR1H4 (FXR) is a bile acid-activated nuclear receptor that heterodimerizes with RXR and directly binds bile acid response elements to transcriptionally regulate bile acid synthesis (via FXR→SHP→LRH-1→CYP7A1 cascade and FXR→FGF15/19→FGFR4→JNK cascade), bile acid transport (BSEP, ASBT), lipid and glucose metabolism, autophagy (competing with CREB-CRTC2 at autophagy gene promoters), intestinal barrier integrity, and innate immunity (NCoR-stabilization on NF-κB promoters); its activity is post-translationally regulated by SUMOylation (CK2/RNF4-mediated at K325/S327), acetylation (SIRT6/SIRT1-controlled at K217 governing importin KPNA3 recognition and nuclear import), phosphorylation (at T442 controlling CRM1-mediated export), and BRD4-dependent coactivation, with cytosolic FXR being targeted for proteasomal degradation by the E3 ligase CHIP."},"narrative":{"teleology":[{"year":1995,"claim":"Identification of NR1H4 as a ligand-modulated orphan nuclear receptor that heterodimerizes with RXR established it as a transcription factor responsive to isoprenoid metabolites, setting the stage for ligand deorphanization.","evidence":"Receptor cloning, farnesol activation assays, and RXR heterodimerization assays in mammalian cells","pmids":["7774010"],"confidence":"High","gaps":["Physiological ligand unidentified","No in vivo target genes known","Tissue-specific roles undefined"]},{"year":1999,"claim":"Identification of bile acids as the endogenous FXR ligands and demonstration that FXR represses CYP7A1 while inducing IBABP transformed the receptor from an orphan into the central bile acid sensor governing enterohepatic circulation.","evidence":"Radioligand binding, reporter assays, and Northern blot for CYP7A1/IBABP across three independent labs","pmids":["10334992","10334993","10360171"],"confidence":"High","gaps":["Molecular mechanism of CYP7A1 repression unknown","In vivo confirmation in knockout animals pending"]},{"year":2000,"claim":"The FXR→SHP→LRH-1 cascade and FXR-null mouse phenotype jointly defined the molecular pathway for bile acid feedback repression of CYP7A1 and established FXR as essential for bile acid and lipid homeostasis in vivo.","evidence":"Epistatic reporter/expression analysis of SHP-LRH-1 cascade; FXR knockout mice with elevated serum bile acids, cholesterol, and triglycerides","pmids":["11030332","11030331","11030617"],"confidence":"High","gaps":["SHP-independent repression mechanisms unexplored","FXR roles beyond liver not yet characterized"]},{"year":2003,"claim":"Discovery that FXR induces intestinal FGF19, which signals through FGFR4–JNK to repress hepatic CYP7A1, revealed a second, endocrine axis for bile acid feedback that operates across the gut–liver axis.","evidence":"FXR agonist induction of FGF19 in hepatocytes, JNK inhibitor studies, CYP7A1 reporter assays","pmids":["12815072"],"confidence":"High","gaps":["Relative contributions of SHP vs FGF19 axes in humans unclear","FGF19 regulation of non-bile-acid targets not defined"]},{"year":2006,"claim":"FXR was shown to regulate glucose metabolism through SHP-dependent repression of gluconeogenic genes and enhanced insulin sensitivity, and to be required for liver regeneration after partial hepatectomy, broadening its role beyond bile acid homeostasis.","evidence":"GW4064 treatment and adenoviral FXR overexpression in db/db mice; hyperinsulinemic clamp in FXR-null mice; partial hepatectomy in FXR-null mice","pmids":["16410358","16557297","16614213"],"confidence":"High","gaps":["Direct FXR target genes for gluconeogenesis incompletely mapped","Whether regeneration defect is cell-autonomous or bile acid-mediated unclear"]},{"year":2009,"claim":"FXR was established as an anti-inflammatory transcription factor in innate immune cells, acting through NCoR stabilization on NF-κB-dependent promoters, and as a regulator of endothelial cell motility via FAK–paxillin–MMP-9 signaling.","evidence":"ChIP for NCoR on IL-1β promoter in macrophages; FXR-null colitis models; siRNA/phosphodeficient FAK mutant in endothelial cells","pmids":["19864602","19150878"],"confidence":"High","gaps":["FXR target gene specificity in immune cells beyond NF-κB unclear","Physiological relevance of endothelial FXR signaling in vivo not confirmed"]},{"year":2014,"claim":"Genome-wide ChIP-seq revealed that FXR suppresses hepatic autophagy in the fed state by directly occupying autophagy gene promoters and disrupting CREB–CRTC2 coactivator complexes, linking nutrient sensing to autophagic flux.","evidence":"ChIP-seq in mouse liver, CREB–CRTC2 co-IP disruption, autophagy flux assays in FXR-null mice","pmids":["25383523","25383539"],"confidence":"High","gaps":["Whether FXR directly represses autophagy in non-hepatic tissues not established","Structural basis for CREB–FXR competition unknown"]},{"year":2017,"claim":"Definition of the CK2→pS327→SUMO2-K325→RNF4 ubiquitin–proteasome axis revealed that SUMOylation is a dual-purpose post-translational switch that both licenses FXR transcriptional activity and programs its degradation.","evidence":"Site-directed mutagenesis (K325R, S327A/E), Co-IP of SUMO2/ubiquitin, RNF4 knockdown, ChIP","pmids":["28201649"],"confidence":"High","gaps":["In vivo quantitative contribution of SUMOylation to FXR turnover unknown","Whether other E3 ligases cooperate with RNF4 not tested"]},{"year":2022,"claim":"Mapping of acetylation at K217 (controlling KPNA3-dependent nuclear import) and phosphorylation at T442 (controlling CRM1-dependent nuclear export), together with identification of CHIP as the cytosolic E3 ligase for FXR, defined a complete nucleocytoplasmic trafficking code that is disrupted in liver injury.","evidence":"Acetylation/phosphorylation site mutagenesis, importin/exportin binding assays, CHIP Co-IP, SIRT1 activator rescue in liver injury models","pmids":["36873184"],"confidence":"High","gaps":["Kinase responsible for T442 phosphorylation not identified","Whether the same trafficking code operates in intestinal FXR is unknown"]},{"year":2022,"claim":"FXR was identified as a direct transcriptional regulator of ACE2 in gastrointestinal and respiratory epithelia, with FXR inhibition by UDCA reducing SARS-CoV-2 susceptibility, revealing an unexpected connection between bile acid signaling and viral entry.","evidence":"FXR agonist/antagonist treatment in human organoids, ex vivo perfused human lungs, mouse and hamster in vivo models, SARS-CoV-2 infection assays","pmids":["36470304"],"confidence":"High","gaps":["Long-term consequences of FXR-mediated ACE2 suppression on cardiometabolic function unknown","Whether FXR regulates other viral receptors unexplored"]},{"year":null,"claim":"Key unresolved questions include the structural basis for FXR's competition with CREB at autophagy promoters, the kinase(s) governing T442 phosphorylation, the relative tissue-specific contributions of hepatic versus intestinal FXR to metabolic disease, and the full scope of FXR's immune-regulatory transcriptome.","evidence":"","pmids":[],"confidence":"High","gaps":["No co-crystal structure of FXR with CREB–CRTC2 exists","No systematic comparison of hepatic vs intestinal FXR cistrome in humans","T442 kinase identity unknown"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,1,2,4,9,15,19,22,34]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[1,9,15,22]},{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[2,3,12]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[17,37]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,1,9,15,19,22,36]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[36]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,2,4,9,15,19,22,34]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[2,5,6,8,11,23,24,33]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[8,16,28]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[17,30]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[19,31]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[5,10,13]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[24]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[26,36]}],"complexes":["FXR-RXR heterodimer"],"partners":["RXRA","NR0B2","BRD4","NCOR1","CHIP","KPNA3","XPO1","RNF4"],"other_free_text":[]},"mechanistic_narrative":"NR1H4 (FXR) is a bile acid-activated nuclear receptor that heterodimerizes with RXR to bind bile acid response elements and transcriptionally coordinate bile acid synthesis, transport, lipid and glucose metabolism, autophagy, intestinal barrier integrity, and innate immune responses [PMID:10334992, PMID:11030617, PMID:25383523, PMID:19864602]. FXR represses bile acid biosynthesis through two major cascades: an intracellular FXR→SHP→LRH-1 pathway that silences CYP7A1, and an endocrine FXR→FGF19→FGFR4→JNK axis that suppresses hepatic CYP7A1 from the intestine [PMID:11030332, PMID:12815072]. FXR transrepresses NF-κB-dependent inflammatory genes by stabilizing the corepressor NCoR on target promoters, and in the fed state it suppresses hepatic autophagy by competing with CREB–CRTC2 at autophagy gene promoters [PMID:19864602, PMID:25383523]. FXR transcriptional activity is post-translationally tuned by CK2-dependent SUMOylation at K325/S327 (coupled to RNF4-mediated proteasomal turnover), by SIRT1/SIRT6-controlled deacetylation at K217 governing importin KPNA3-mediated nuclear import, by phosphorylation at T442 controlling CRM1-dependent nuclear export, and by BRD4-dependent coactivation [PMID:28201649, PMID:36873184, PMID:35526796, PMID:33290278]."},"prefetch_data":{"uniprot":{"accession":"Q96RI1","full_name":"Bile acid receptor","aliases":["Farnesoid X-activated receptor","Farnesol receptor HRR-1","Nuclear receptor subfamily 1 group H member 4","Retinoid X receptor-interacting protein 14","RXR-interacting protein 14"],"length_aa":486,"mass_kda":55.9,"function":"Ligand-activated transcription factor. Receptor for bile acids (BAs) such as chenodeoxycholic acid (CDCA), lithocholic acid, deoxycholic acid (DCA) and allocholic acid (ACA). Plays a essential role in BA homeostasis through the regulation of genes involved in BA synthesis, conjugation and enterohepatic circulation. Also regulates lipid and glucose homeostasis and is involved innate immune response (PubMed:10334992, PubMed:10334993, PubMed:21383957, PubMed:22820415). The FXR-RXR heterodimer binds predominantly to farnesoid X receptor response elements (FXREs) containing two inverted repeats of the consensus sequence 5'-AGGTCA-3' in which the monomers are spaced by 1 nucleotide (IR-1) but also to tandem repeat DR1 sites with lower affinity, and can be activated by either FXR or RXR-specific ligands. It is proposed that monomeric nuclear receptors such as NR5A2/LRH-1 bound to coregulatory nuclear responsive element (NRE) halfsites located in close proximity to FXREs modulate transcriptional activity (By similarity). In the liver activates transcription of the corepressor NR0B2 thereby indirectly inhibiting CYP7A1 and CYP8B1 (involved in BA synthesis) implicating at least in part histone demethylase KDM1A resulting in epigenomic repression, and SLC10A1/NTCP (involved in hepatic uptake of conjugated BAs). Activates transcription of the repressor MAFG (involved in regulation of BA synthesis) (By similarity). Activates transcription of SLC27A5/BACS and BAAT (involved in BA conjugation), ABCB11/BSEP (involved in bile salt export) by directly recruiting histone methyltransferase CARM1, and ABCC2/MRP2 (involved in secretion of conjugated BAs) and ABCB4 (involved in secretion of phosphatidylcholine in the small intestine) (PubMed:12754200, PubMed:15471871, PubMed:17895379). Activates transcription of SLC27A5/BACS and BAAT (involved in BA conjugation), ABCB11/BSEP (involved in bile salt export) by directly recruiting histone methyltransferase CARM1, and ABCC2/MRP2 (involved in secretion of conjugated BAs) and ABCB4 (involved in secretion of phosphatidylcholine in the small intestine) (PubMed:10514450, PubMed:15239098, PubMed:16269519). In the intestine activates FGF19 expression and secretion leading to hepatic CYP7A1 repression (PubMed:12815072, PubMed:19085950). The function also involves the coordinated induction of hepatic KLB/beta-klotho expression (By similarity). Regulates transcription of liver UGT2B4 and SULT2A1 involved in BA detoxification; binding to the UGT2B4 promoter seems to imply a monomeric transactivation independent of RXRA (PubMed:12806625, PubMed:16946559). Modulates lipid homeostasis by activating liver NR0B2/SHP-mediated repression of SREBF1 (involved in de novo lipogenesis), expression of PLTP (involved in HDL formation), SCARB1 (involved in HDL hepatic uptake), APOE, APOC1, APOC4, PPARA (involved in beta-oxidation of fatty acids), VLDLR and SDC1 (involved in the hepatic uptake of LDL and IDL remnants), and inhibiting expression of MTTP (involved in VLDL assembly (PubMed:12554753, PubMed:12660231, PubMed:15337761). Increases expression of APOC2 (promoting lipoprotein lipase activity implicated in triglyceride clearance) (PubMed:11579204). Transrepresses APOA1 involving a monomeric competition with NR2A1 for binding to a DR1 element (PubMed:11927623, PubMed:21804189). Also reduces triglyceride clearance by inhibiting expression of ANGPTL3 and APOC3 (both involved in inhibition of lipoprotein lipase) (PubMed:12891557). Involved in glucose homeostasis by modulating hepatic gluconeogenesis through activation of NR0B2/SHP-mediated repression of respective genes. Modulates glycogen synthesis (inducing phosphorylation of glycogen synthase kinase-3) (By similarity). Modulates glucose-stimulated insulin secretion and is involved in insulin resistance (PubMed:20447400). Involved in intestinal innate immunity. Plays a role in protecting the distal small intestine against bacterial overgrowth and preservation of the epithelial barrier (By similarity). Down-regulates inflammatory cytokine expression in several types of immune cells including macrophages and mononuclear cells (PubMed:21242261). Mediates trans-repression of TLR4-induced cytokine expression; the function seems to require its sumoylation and prevents N-CoR nuclear receptor corepressor clearance from target genes such as IL1B and NOS2 (PubMed:19864602). Involved in the TLR9-mediated protective mechanism in intestinal inflammation. Plays an anti-inflammatory role in liver inflammation; proposed to inhibit pro-inflammatory (but not antiapoptotic) NF-kappa-B signaling) (By similarity) Promotes transcriptional activation of target genes NR0B2/SHP (inducible by unconjugated CDCA), SLC51B/OSTB (inducible by unconjugated CDCA and DCA) and FABP6/IBAP; low activity for ABCB11/BSEP (inducible by unconjugated CDCA, DCA and ACA); not inducible by taurine- and glycine-amidated CDCA Promotes transcriptional activation of target genes ABCB11/BSEP (inducible by unconjugated CDCA, DCA and ACA), NR0B2/SHP (inducible by unconjugated CDCA DCA and ACA), SLC51B/OSTB (inducible by unconjugated CDCA and DCA) and FABP6/IBAP; not inducible by taurine- and glycine-amidated CDCA Promotes transcriptional activation of target genes NR0B2/SHP (inducible by unconjugated CDCA), SLC51B/OSTB (inducible by unconjugated CDCA and DCA) and IBAP; low activity for ABCB11/BSEP (inducible by unconjugated CDCA, DCA and ACA); not inducible by taurine- and glycine-amidated CDCA Promotes transcriptional activation of target genes ABCB11/BSEP (inducible by unconjugated CDCA, ACA and DCA), NR0B2/SHP (inducible by unconjugated CDCA, ACA and DCA), SLC51B/OSTB (inducible by unconjugated CDCA and DCA) and FABP6/IBAP; most efficient isoform compared to isoforms 1 to 3; not inducible by taurine- and glycine-amidated CDCA","subcellular_location":"Nucleus","url":"https://www.uniprot.org/uniprotkb/Q96RI1/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/NR1H4","classification":"Not Classified","n_dependent_lines":8,"n_total_lines":1208,"dependency_fraction":0.006622516556291391},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/NR1H4","total_profiled":1310},"omim":[{"mim_id":"617386","title":"NUCLEAR RECEPTOR SUBFAMILY 1, GROUP H, MEMBER 5, PSEUDOGENE; NR1H5P","url":"https://www.omim.org/entry/617386"},{"mim_id":"617049","title":"CHOLESTASIS, PROGRESSIVE FAMILIAL INTRAHEPATIC, 5; PFIC5","url":"https://www.omim.org/entry/617049"},{"mim_id":"613291","title":"BILE ACID MALABSORPTION, PRIMARY, 1; PBAM1","url":"https://www.omim.org/entry/613291"},{"mim_id":"605552","title":"ABDOMINAL OBESITY-METABOLIC SYNDROME 1; AOMS1","url":"https://www.omim.org/entry/605552"},{"mim_id":"605495","title":"SOLUTE CARRIER ORGANIC ANION TRANSPORTER FAMILY, MEMBER 1B3; SLCO1B3","url":"https://www.omim.org/entry/605495"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"Supported","locations":[{"location":"Nucleoplasm","reliability":"Supported"},{"location":"Nuclear speckles","reliability":"Supported"},{"location":"Primary cilium tip","reliability":"Additional"},{"location":"Cytosol","reliability":"Additional"}],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in some","driving_tissues":[{"tissue":"intestine","ntpm":36.8},{"tissue":"liver","ntpm":112.7}],"url":"https://www.proteinatlas.org/search/NR1H4"},"hgnc":{"alias_symbol":["FXR","RIP14","HRR1","HRR-1"],"prev_symbol":[]},"alphafold":{"accession":"Q96RI1","domains":[{"cath_id":"3.30.50.10","chopping":"147-212","consensus_level":"high","plddt":88.3227,"start":147,"end":212},{"cath_id":"1.10.565.10","chopping":"262-484","consensus_level":"high","plddt":88.3828,"start":262,"end":484}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96RI1","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q96RI1-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q96RI1-F1-predicted_aligned_error_v6.png","plddt_mean":68.81},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=NR1H4","jax_strain_url":"https://www.jax.org/strain/search?query=NR1H4"},"sequence":{"accession":"Q96RI1","fasta_url":"https://rest.uniprot.org/uniprotkb/Q96RI1.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q96RI1/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q96RI1"}},"corpus_meta":[{"pmid":"30397356","id":"PMC_30397356","title":"Gut microbiota and intestinal FXR mediate the clinical benefits of metformin.","date":"2018","source":"Nature medicine","url":"https://pubmed.ncbi.nlm.nih.gov/30397356","citation_count":791,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25920087","id":"PMC_25920087","title":"New paradigms in the treatment of hepatic cholestasis: from UDCA to FXR, PXR and beyond.","date":"2015","source":"Journal of hepatology","url":"https://pubmed.ncbi.nlm.nih.gov/25920087","citation_count":407,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"30794774","id":"PMC_30794774","title":"FXR Regulates Intestinal Cancer Stem Cell Proliferation.","date":"2019","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/30794774","citation_count":369,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"26579439","id":"PMC_26579439","title":"Bile acid nuclear receptor FXR and digestive system diseases.","date":"2015","source":"Acta pharmaceutica Sinica. 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gastrectomy.","date":"2014","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/24670636","citation_count":760,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16557297","id":"PMC_16557297","title":"Farnesoid X receptor is essential for normal glucose homeostasis.","date":"2006","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/16557297","citation_count":726,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"21242261","id":"PMC_21242261","title":"Farnesoid X receptor activation inhibits inflammation and preserves the intestinal barrier in inflammatory bowel disease.","date":"2011","source":"Gut","url":"https://pubmed.ncbi.nlm.nih.gov/21242261","citation_count":710,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"33961781","id":"PMC_33961781","title":"Dual proteome-scale networks reveal cell-specific remodeling of the human 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1950)","url":"https://pubmed.ncbi.nlm.nih.gov/19864602","citation_count":530,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16614213","id":"PMC_16614213","title":"Nuclear receptor-dependent bile acid signaling is required for normal liver regeneration.","date":"2006","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/16614213","citation_count":520,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16460270","id":"PMC_16460270","title":"LXRS and FXR: the yin and yang of cholesterol and fat metabolism.","date":"2006","source":"Annual review of physiology","url":"https://pubmed.ncbi.nlm.nih.gov/16460270","citation_count":490,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26670557","id":"PMC_26670557","title":"Intestine-selective farnesoid X receptor inhibition improves obesity-related metabolic dysfunction.","date":"2015","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/26670557","citation_count":460,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"25383539","id":"PMC_25383539","title":"Nutrient-sensing nuclear receptors coordinate autophagy.","date":"2014","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/25383539","citation_count":431,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"26871637","id":"PMC_26871637","title":"Widespread Expansion of Protein Interaction Capabilities by Alternative Splicing.","date":"2016","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/26871637","citation_count":423,"is_preprint":false,"source_track":"gene2pubmed"},{"pmid":"16344560","id":"PMC_16344560","title":"Diversification of transcriptional modulation: large-scale identification and characterization of putative alternative promoters of human genes.","date":"2005","source":"Genome 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\"confidence_rationale\": \"Tier 1 — functional mutagenesis combined with transactivation assays in a foundational paper\",\n      \"pmids\": [\"9223286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"FXR was identified as a bile acid receptor and biological sensor: the primary bile acid chenodeoxycholic acid (CDCA) directly binds and activates FXR, which then mediates feedback suppression of CYP7A1 (the rate-limiting enzyme in bile acid synthesis) and activates intestinal bile acid binding protein (IBABP) involved in enterohepatic bile acid circulation.\",\n      \"method\": \"Ligand-binding assays, reporter gene assays, target gene expression studies\",\n      \"journal\": \"Trends in cardiovascular medicine\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — de-orphanization replicated across multiple labs with consistent mechanistic framework\",\n      \"pmids\": [\"11150726\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Androsterone (a metabolic product of testosterone) directly binds to the human FXR ligand-binding domain (LBD) and recruits the coactivator SRC-1, activating FXR. NMR spectroscopy confirmed direct androsterone-FXR LBD binding. Site-directed mutagenesis identified Asn354 and Ser345 as critical for differential species sensitivity to CDCA and androsterone, respectively, and crystal structure data revealed the steroid nucleus of bile acids binds in reversed orientation compared to other nuclear receptors.\",\n      \"method\": \"NMR spectroscopy (direct binding), site-directed mutagenesis, Gal4-LBD reporter assays, TR-FRET coactivator recruitment assay, in vivo SHP induction in castrated mice\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding by NMR, mutagenesis, and crystal structure interpretation in single study\",\n      \"pmids\": [\"16675527\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Evolutionary analysis of FXR from marine skate (FXRbeta) showed that FXRalpha (NR1H4) acquired ligand specificity for bile acids late in vertebrate evolution; deletion of an extra sequence in the skate FXR LBD and replacement with the corresponding human FXRalpha sequence conferred responsiveness to bile acid ligands, demonstrating that this LBD region determines bile acid ligand specificity.\",\n      \"method\": \"Phylogenetic analysis, chimeric/mutant LBD luciferase reporter assays, transactivation assays\",\n      \"journal\": \"American journal of physiology. Regulatory, integrative and comparative physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1/2 — mutagenesis with functional validation, single lab\",\n      \"pmids\": [\"17567710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FXR-activating bile acid ligands downregulate rabbit ASBT (apical sodium-dependent bile acid transporter) expression via a FXR→SHP→FTF (alpha-fetoprotein transcription factor) cascade: activated FXR induces SHP, which inhibits FTF-dependent transactivation of the ASBT promoter through a defined FTF binding element.\",\n      \"method\": \"In vivo bile acid feeding, promoter cloning, deletion/mutation analysis, transfection in Caco-2 cells, SHP overexpression\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — promoter mutagenesis + in vivo validation + epistasis cascade, replicated in vivo and in vitro\",\n      \"pmids\": [\"15591588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FXR directly induces GLUT4 transcription through an FXR response element (FXRE) in the GLUT4 promoter; EMSA showed FXR binds the GLUT4-FXRE as a monomer (RXR is not required); progressive 5'-deletion and site-mutation analysis confirmed the FXRE requirement; FXR does not interfere with insulin-induced GLUT4 membrane translocation.\",\n      \"method\": \"Reporter gene assay with 5'-deletion analysis and site-directed mutagenesis, EMSA, RXR knockdown, in vivo CDCA treatment in mice, Western blot\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — promoter mutagenesis + EMSA + in vivo confirmation with multiple orthogonal methods\",\n      \"pmids\": [\"18769028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FXR promotes endothelial cell motility and tube formation through a FXR→MMP-9→FAK signaling cascade: CDCA-activated FXR transcriptionally induces MMP-9, which activates FAK phosphorylation at Y397 (but not Y576/577 or Y925), enabling downstream paxillin activation and focal adhesion assembly required for cell migration; phosphodeficient FAK Y397 mutant blocked CDCA-induced MMP-9 upregulation and cell motility.\",\n      \"method\": \"Time-lapse video microscopy, siRNA knockdown of FXR and MMP-9, FAK phospho-specific mutants, tube formation assay\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal methods including siRNA, phospho-mutants, and functional migration assays\",\n      \"pmids\": [\"19150878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FXR is a direct transcriptional regulator of renal aquaporin 2 (AQP2): a putative FXR response element in the AQP2 gene promoter is bound and activated by FXR, increasing AQP2 transcription in collecting duct cells; FXR knockout mice show markedly reduced AQP2 expression and impaired urine concentrating ability (polyuria phenotype); FXR agonist treatment increased AQP2 expression and urine osmolality in vivo.\",\n      \"method\": \"FXR knockout mouse phenotyping (urine volume/osmolality), FXR agonist treatment in vivo, promoter analysis with FXRE identification, in vitro primary collecting duct cell experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — KO phenotype + FXRE promoter identification + in vitro and in vivo orthogonal validation\",\n      \"pmids\": [\"24464484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FXR induces SOCS3 expression by directly binding to an IR9 DNA motif within the SOCS3 promoter (demonstrated by EMSA and ChIP assay), thereby inhibiting STAT3 phosphorylation and suppressing hepatocellular carcinoma cell growth and G1 cell cycle arrest; SOCS3 knockdown abolished the anti-tumor effects of FXR activation.\",\n      \"method\": \"EMSA, ChIP assay, reporter gene assay, siRNA knockdown, cell cycle analysis, xenograft mouse model\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — direct DNA binding by EMSA and ChIP plus functional epistasis via SOCS3 siRNA, confirmed in vivo\",\n      \"pmids\": [\"26416445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"FXR is SUMOylated in activated hepatic stellate cells (HSCs), and this SUMOylation limits FXR signaling activity and its anti-fibrotic effects; SUMOylation inhibitors rescue FXR signaling and enhance FXR agonist (OCA) efficacy against HSC activation. FXR directly transcriptionally upregulates Perilipin-1 to stabilize lipid droplets and prevent HSC activation.\",\n      \"method\": \"SUMOylation assays, co-treatment with SUMOylation inhibitors + FXR agonists in multiple fibrosis mouse models (CCl4, BDL, NASH), target gene (Perilipin-1) identification\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanism in multiple orthogonal disease models with genetic/pharmacological rescue\",\n      \"pmids\": [\"31932588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A CK2-RNF4 interplay coordinates FXR activity and degradation: CK2 phosphorylates Ser-327 of FXR, enabling Ubc9/PIAS1-mediated SUMO2 conjugation at the adjacent Lys-325 (a non-canonical pSuM motif); this SUMOylation is required for efficient ligand activation and transcriptional coactivation; constitutive pSuM activation directs FXR to proteasomal degradation via the SUMO-dependent E3 ubiquitin ligase RNF4.\",\n      \"method\": \"Site-directed mutagenesis of pSuM motif, co-immunoprecipitation, SUMOylation assays, ubiquitination assays, reporter assays, phospho-mimic mutants, RNF4 functional studies\",\n      \"journal\": \"Journal of molecular cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of specific residues combined with biochemical reconstitution of the SUMOylation-ubiquitination cascade\",\n      \"pmids\": [\"28201649\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FXR restricts colorectal cancer progression by limiting Lgr5+ cancer stem cell proliferation; bile acids that antagonize intestinal FXR (T-βMCA and DCA) induce proliferation and DNA damage in Lgr5+ cells, while selective intestinal FXR activation curtails this growth, mechanistically linking FXR activity to coordination of intestinal self-renewal with bile acid levels.\",\n      \"method\": \"Intestinal organoids, genetic mouse models (APC mutation + HFD), FXR agonist/antagonist treatment, bile acid profiling, Lgr5+ cell sorting and functional assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological manipulations in multiple models with defined cellular phenotype\",\n      \"pmids\": [\"30794774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FXR activation inhibits the nonreceptor tyrosine kinase Src (Tyr416 phosphorylation), which in turn increases Ser127 phosphorylation and cytosolic retention of YAP, thereby activating Hippo core kinases and protecting against renal fibrosis; FXR knockout mice showed increased YAP nuclear localization and fibrosis markers.\",\n      \"method\": \"FXR agonist treatment in HK2 cells and UUO mouse model, FXR knockout mice, Src kinase inhibitor (PP2), phospho-specific antibodies, Co-IP of Hippo kinase components\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO phenotype + pharmacological rescue + defined phosphorylation mechanism, single lab\",\n      \"pmids\": [\"31298930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FXR is a direct transcriptional regulator of ACE2 in gastrointestinal and respiratory tissues; FXR inhibition (by UDCA or z-guggulsterone) downregulates ACE2 expression in human lung, cholangiocyte, and intestinal organoids, in vivo in mice and hamsters, and in ex situ perfused human livers and lungs, reducing susceptibility to SARS-CoV-2 infection.\",\n      \"method\": \"Human organoids (lung, cholangiocyte, intestinal), in vivo mouse and hamster models, ex situ perfused human organs, SARS-CoV-2 infection assays in vitro and in vivo, FXR reporter assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — direct FXR-ACE2 transcriptional link validated across multiple orthogonal human and animal systems\",\n      \"pmids\": [\"36470304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FXR acts as a T cell-intrinsic sensor of nutrient availability: during feeding reduction or starvation, FXR deletion in T cells prevents starvation-induced lymphocyte loss and increases effector T cell fitness under low-glucose conditions by increasing glutamine and fatty acid utilization toward respiration and enhancing survival.\",\n      \"method\": \"T cell-specific FXR knockout mice, infection models with imposed fasting, metabolic flux analysis (glutamine/fatty acid respiration), glucose rescue experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with defined metabolic phenotype, single lab\",\n      \"pmids\": [\"33318189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FXR activation inhibits the transcriptional activity of the Smad3 gene promoter (demonstrated by luciferase reporter assay), suppressing Smad3 expression and thereby attenuating TGFβ-Smad signaling-driven renal fibrosis; ectopic Smad3 expression rescued fibrosis despite FXR activation, establishing a FXR/Smad3 epistatic pathway.\",\n      \"method\": \"Luciferase reporter assay (Smad3 promoter), Smad3 overexpression rescue, FXR agonist and antagonist treatment, UUO mouse model\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter assay + epistasis rescue + in vivo validation, single lab\",\n      \"pmids\": [\"27853248\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FXR represses autophagy in the fed state by reversing CREB activity on autophagy-related genes (Atgs); FXR gene ablation partially abrogates feeding-mediated repression of autophagy in liver; FXR activation represses Atgs even in the fasted state, while GPBAR1 agonism promotes autophagy through cAMP-CREB and can reverse FXR-mediated autophagy repression.\",\n      \"method\": \"FXR KO mice, GPBAR1 KO mice, genetic and pharmacological approaches, ChIP (CREB recruitment to LC3 promoter), fed/fasted physiological states\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — KO models with ChIP validation, single lab with multiple orthogonal tools\",\n      \"pmids\": [\"33368684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FXR acetylation governs nucleocytoplasmic shuttling: acetylation at K217 (near the nuclear localization signal) blocks recognition by importin KPNA3, preventing nuclear import; concomitantly, reduced phosphorylation at T442 (within the nuclear export signal) promotes recognition by exportin CRM1, facilitating nuclear export. Cytosolic FXR is then degraded by the E3 ligase CHIP. SIRT1 deacetylates FXR, preventing this cytosolic accumulation and degradation.\",\n      \"method\": \"Site-directed mutagenesis (K217, T442), Co-IP with importin KPNA3 and exportin CRM1, CHIP ubiquitin ligase interaction assays, SIRT1 activator treatment in acute/chronic liver injury models\",\n      \"journal\": \"Acta pharmaceutica Sinica. B\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of specific PTM sites combined with interactor Co-IP and functional rescue in disease models\",\n      \"pmids\": [\"36873184\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SIRT6 deacetylates FXR and elevates its transcriptional activity; in the context of acetaminophen-induced liver injury, SIRT6 knockout impairs FXR-mediated hepatoprotection, while pharmacological FXR activation rescues SIRT6 KO hepatotoxicity, establishing SIRT6 as an upstream regulator of FXR activity through deacetylation.\",\n      \"method\": \"Hepatocyte-specific SIRT6 KO mice, FXR KO mice, SIRT6 overexpression/pharmacological activation, FXR agonist rescue in SIRT6 KO mice, RNA sequencing\",\n      \"journal\": \"Cellular and molecular gastroenterology and hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic epistasis with KO rescue experiments, single lab\",\n      \"pmids\": [\"35526796\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"BRD4 is a cofactor required for FXR-mediated regulation of bile acid homeostasis genes including SHP and CYP7A1; liver-specific BRD4 downregulation disrupts BA homeostasis; OCA-activated FXR increases FXR and SMRT corepressor binding at inflammatory genes while decreasing NF-κB binding in a BRD4-dependent manner, demonstrating BRD4 is required for the anti-inflammatory actions of FXR.\",\n      \"method\": \"Liver-specific BRD4 knockdown, cholestasis mouse models, ChIP assays (FXR, SMRT, NF-κB binding at target gene promoters), OCA and JQ1 pharmacological treatments\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP validation + genetic knockdown + pharmacological epistasis, single lab\",\n      \"pmids\": [\"33290278\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FXR activation by GW4064 increases hepatic Srebp-2 mRNA, precursor SREBP-2 protein, and miR-33 levels via an FXR response element within intron 10 of the Srebp-2 gene (identified by ChIP-seq); however, FXR simultaneously induces INSIG-2A, which prevents processing of precursor SREBP-2 to its nuclear active form, thereby uncoupling miR-33 induction from SREBP-2 target gene activation.\",\n      \"method\": \"ChIP-seq (FXR response element in Srebp-2 intron 10), in vivo FXR agonist treatment in mice, Scap KO mice (epistasis), Western blot for SREBP-2 processing forms, miR-33 target gene expression\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1/2 — ChIP-seq identification of FXRE + genetic epistasis with Scap KO + in vivo validation with multiple orthogonal methods\",\n      \"pmids\": [\"25593129\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FXR is O-GlcNAcylated in response to glucose and directly interacts with ChREBP, acting as a repressor on the carbohydrate response element (ChoRE) of glycolytic genes in liver; O-GlcNAcylation modifies the mutual affinity and transcriptional activity of the FXR-ChREBP complex.\",\n      \"method\": \"O-GlcNAcylation assays, co-immunoprecipitation (FXR-ChREBP interaction), reporter assays on glycolytic gene ChoRE elements\",\n      \"journal\": \"Frontiers in endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP plus reporter assay; review article synthesizing primary data from referenced studies\",\n      \"pmids\": [\"25628602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"PPARγ directly binds to a PPAR-responsive element (PPRE) in the FXR gene promoter in a PPARγ agonist-dependent manner (demonstrated by ChIP assay), driving FXR expression during adipogenesis; activated FXR then binds to an FXRE in the stearoyl-CoA desaturase (SCD) gene promoter to promote lipogenesis in adipocytes.\",\n      \"method\": \"ChIP assay (PPARγ binding to FXR promoter PPRE), promoter FXRE identification, lipid accumulation assays in 3T3-L1 cells, FXR agonist/antagonist treatments\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP validation of direct binding + functional FXRE in SCD promoter, single lab\",\n      \"pmids\": [\"32446390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"The FXR-FGF15/19 endocrine axis operates as follows: bile acids activate FXR in ileal enterocytes to induce FGF15 (mouse)/FGF19 (human) expression; FGF15/19 travels via portal circulation to act on hepatic FGFR4-β-klotho receptor complex, repressing bile acid synthesis (CYP7A1) and gluconeogenesis while stimulating glycogen and protein synthesis and gallbladder filling.\",\n      \"method\": \"Mouse genetic models, tissue-specific FXR activation studies, FGF15/19 administration, FGFR4-β-klotho receptor pharmacology (multiple labs)\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — replicated across multiple labs with genetic and pharmacological tools; foundational enterohepatic signaling axis\",\n      \"pmids\": [\"35682726\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NR1H4/FXR is a ligand-activated nuclear receptor that heterodimerizes with RXRα to bind FXR response elements and regulate transcription; primary bile acids (especially CDCA) and androsterone serve as direct ligands, and activated FXR controls bile acid homeostasis (via SHP-FTF cascade suppressing CYP7A1 and ASBT, and inducing BSEP), drives the ileal FXR-FGF15/19 endocrine axis to repress hepatic bile acid synthesis, directly regulates metabolic genes including GLUT4, AQP2, SOCS3, SREBP-2/miR-33, SCD, and ACE2, and is itself regulated by multiple post-translational modifications including phosphorylation-dependent SUMOylation at K325 (by CK2/Ubc9/PIAS1 enabling RNF4-mediated ubiquitination and degradation), acetylation at K217 (blocking KPNA3-mediated nuclear import and promoting CRM1-dependent export and CHIP-mediated cytosolic degradation reversed by SIRT1/SIRT6), and SUMOylation in activated hepatic stellate cells that limits its anti-fibrotic activity.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1995,\n      \"finding\": \"NR1H4 (then called FXR) was identified as an orphan nuclear receptor that forms a heterodimeric complex with retinoid X receptor (RXR) and is activated by farnesol and related metabolites, establishing it as a ligand-modulated transcription factor.\",\n      \"method\": \"Receptor isolation, ligand screening, heterodimerization assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original discovery paper with functional activation assays and heterodimerization demonstrated\",\n      \"pmids\": [\"7774010\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"RIP14 (NR1H4) can be activated by all-trans-retinoic acid and the synthetic retinoid TTNPB; it binds DNA as a heterodimer with RXR, and mutagenesis of the ligand-dependent activation function showed TTNPB activates the RIP14 component while 9-cis-RA and LG1069 activate the RXR component.\",\n      \"method\": \"Reporter gene activation assays, site-directed mutagenesis of AF-2 domain, DNA binding assays\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis combined with functional activation assays\",\n      \"pmids\": [\"9223286\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Bile acids (chenodeoxycholic acid, lithocholic acid, deoxycholic acid) are physiological ligands for FXR/NR1H4; ligand-bound FXR repressed CYP7A1 transcription (rate-limiting enzyme in bile acid synthesis) and activated intestinal bile acid-binding protein (IBABP) expression, establishing FXR as a bile acid sensor that transcriptionally regulates bile acid biosynthesis and enterohepatic transport.\",\n      \"method\": \"Radioligand binding assays, reporter gene assays, Northern blot analysis of CYP7A1 and IBABP\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct ligand identification with functional transcriptional readouts, independently replicated in same year by multiple labs\",\n      \"pmids\": [\"10334992\", \"10334993\", \"10360171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Physiological concentrations of free and conjugated chenodeoxycholic acid, lithocholic acid, and deoxycholic acid activate FXR and promote coactivator (SRC-1 peptide) recruitment, demonstrating a nuclear bile acid signaling pathway.\",\n      \"method\": \"Transactivation assays, coactivator interaction assays (SRC-1 peptide recruitment)\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — coactivator recruitment assay plus transactivation, replicated across labs\",\n      \"pmids\": [\"10334993\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"FXR induces expression of SHP-1 (small heterodimer partner), which then represses CYP7A1 by inhibiting the activity of LRH-1 (liver receptor homolog-1); this FXR→SHP-1→LRH-1 regulatory cascade provides the molecular basis for bile acid feedback suppression of CYP7A1.\",\n      \"method\": \"Nonsteroidal FXR ligand treatment, reporter assays, mRNA expression analysis, epistasis with SHP-1 and LRH-1\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistatic cascade defined with multiple components and replicated in two simultaneous papers\",\n      \"pmids\": [\"11030332\", \"11030331\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"Targeted disruption of FXR/BAR in mice resulted in elevated serum bile acids, cholesterol, and triglycerides; increased hepatic cholesterol and triglycerides; reduced bile acid pools; and loss of bile acid-mediated regulation of CYP7A1 and IBABP, establishing FXR's in vivo role as an intracellular bile acid sensor critical for bile acid and lipid homeostasis.\",\n      \"method\": \"FXR knockout mice, serum lipid measurements, hepatic lipid measurements, gene expression analysis\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with multiple defined phenotypic and molecular readouts\",\n      \"pmids\": [\"11030617\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2000,\n      \"finding\": \"FXR (as an RXR heterodimer) activates ABC1 expression and represses CYP7A1; rexinoid treatment phenocopies these effects, establishing FXR-RXR as a key regulator of reverse cholesterol transport and bile acid synthesis downstream of bile acid sensing.\",\n      \"method\": \"Receptor-selective agonist treatment in mice, gene expression analysis, ABC1 and CYP7A1 reporter assays\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — in vivo pharmacological and genetic approach with receptor-selective compounds\",\n      \"pmids\": [\"10968783\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"Guggulsterone is a highly efficacious antagonist of FXR; its cholesterol-lowering activity depends on FXR, as guggulsterone failed to decrease hepatic cholesterol in FXR-null mice fed a high-cholesterol diet.\",\n      \"method\": \"FXR reporter assays, FXR knockout mice, high-cholesterol diet model\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — antagonist function validated in FXR knockout, confirming on-target mechanism\",\n      \"pmids\": [\"11988537\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"FXR directly regulates expression of FGF19 (FGF-15 in mice), a secreted growth factor; FGF19 signals through FGFR4 to strongly suppress CYP7A1 expression in hepatocytes via a c-Jun N-terminal kinase (JNK)-dependent pathway, defining a novel endocrine feedback loop for repression of bile acid biosynthesis.\",\n      \"method\": \"FXR agonist treatment, FGF19 expression in primary hepatocytes, FGFR4 signaling, JNK pathway inhibitor studies, CYP7A1 reporter assays\",\n      \"journal\": \"Genes & development\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic cascade with multiple pathway components and signaling assays\",\n      \"pmids\": [\"12815072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"FXR activates the human PPARα gene promoter via a defined FXR response element (αFXRE) in the PPARα promoter, as shown by gel shift analysis (FXR binding to αFXRE), cotransfection assays, and mutation analysis; this cross-talk is species-specific (not present in mice).\",\n      \"method\": \"Gel shift (EMSA), cotransfection with FXR/RXR, mutation analysis, reporter assays in HepG2 cells and primary hepatocytes\",\n      \"journal\": \"Molecular endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — EMSA direct binding plus mutational analysis and functional reporter assays\",\n      \"pmids\": [\"12554753\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FXR-activating ligands downregulate rabbit ASBT (apical sodium-dependent bile acid transporter) expression through the regulatory cascade FXR→SHP→FTF (alpha-fetoprotein transcription factor); a functional FTF binding site at -1166/-1158 of the rabbit ASBT promoter is required, and only FXR-activating ligands (not non-activating bile acids) repress ASBT.\",\n      \"method\": \"In vivo rabbit feeding experiments, promoter cloning and deletion analysis, Caco-2 cell transfection, mRNA quantification\",\n      \"journal\": \"American journal of physiology. Gastrointestinal and liver physiology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cascade defined with promoter deletion and in vivo validation\",\n      \"pmids\": [\"15591588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"FXR activation by GW4064 or constitutively active FXR adenovirus significantly lowered blood glucose in diabetic db/db mice by repressing hepatic gluconeogenic genes and increasing hepatic glycogen synthesis via enhanced insulin sensitivity; FXR-null mice showed glucose intolerance and insulin insensitivity, with blunted insulin signaling in skeletal muscle and liver. The FXR-SHP cascade mediates glucose metabolism.\",\n      \"method\": \"FXR agonist (GW4064) treatment, adenoviral FXR overexpression, FXR knockout mice, hyperinsulinemic euglycemic clamp, gene expression analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences / Journal of Clinical Investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — gain- and loss-of-function with mechanistic pathway placement, replicated across two independent labs\",\n      \"pmids\": [\"16410358\", \"16557297\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"Androsterone (a testosterone metabolite) directly binds the FXR ligand-binding domain (LBD) as shown by NMR spectroscopy, recruits SRC-1 coactivator peptide, and activates FXR; site-directed mutagenesis of hFXR-LBD identified Asn354 and Ser345 as critical for differential species sensitivity to CDCA and androsterone, respectively. Crystal structure-guided mutations (M265I, R331H) further dissected the binding mode.\",\n      \"method\": \"NMR spectroscopy of purified hFXR-LBD, site-directed mutagenesis, Gal4-LBD reporter assays, in vivo androsterone treatment of castrated mice\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct binding by NMR plus mutagenesis and crystal structure interpretation\",\n      \"pmids\": [\"16675527\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"FXR-null mice have reduced expression of the major hepatic canalicular bile acid transport protein (BSEP); nuclear receptor-dependent bile acid signaling via FXR is required for normal liver regeneration after partial hepatectomy, as decreased bile acid levels inhibit liver regrowth and FXR absence prevents normal regeneration.\",\n      \"method\": \"FXR knockout mice, partial hepatectomy model, bile acid manipulation, gene expression analysis\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean knockout with defined regeneration phenotype and bile acid manipulation\",\n      \"pmids\": [\"16614213\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"FXRα (NR1H4) acquired ligand specificity for bile salts late in vertebrate evolution; deletion of an extra LBD sequence in a FXRβ homolog from skate (which normally responds weakly to bile salts) conferred responsiveness to GW4064 and ursodeoxycholic acid, demonstrating that the bile acid-binding specificity of FXRα is conferred by the structure of its LBD.\",\n      \"method\": \"Phylogenetic analysis, luciferase reporter assays, LBD deletion mutagenesis in heterologous cells\",\n      \"journal\": \"American journal of physiology. Regulatory, integrative and comparative physiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — LBD deletion mutagenesis plus reporter assays in single study\",\n      \"pmids\": [\"17567710\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"FXR directly induces GLUT4 transcription through a defined FXR response element (FXRE) in the GLUT4 promoter; EMSA and RXR knockdown showed FXR binds the GLUT4-FXRE as a monomer (RXR-independent); FXR activation does not interfere with insulin-induced GLUT4 translocation to the plasma membrane.\",\n      \"method\": \"5'-deletion and site-mutation analysis of GLUT4 promoter, EMSA, RXR siRNA knockdown, in vivo CDCA treatment\",\n      \"journal\": \"Cellular physiology and biochemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — EMSA direct binding plus promoter mutagenesis and in vivo validation\",\n      \"pmids\": [\"18769028\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FXR activation by CDCA promotes endothelial cell motility and tube formation via FAK phosphorylation at Y397 (but not Y576/577 or Y925), which activates paxillin; FAK phosphorylation at Y397 is required for CDCA-induced MMP-9 upregulation; both FXR and MMP-9 siRNA blocked CDCA-induced motility, establishing a FXR→FAK(pY397)→paxillin→MMP-9 pathway for EC motility.\",\n      \"method\": \"Time-lapse video microscopy, siRNA knockdown of FXR/MMP-9/FAK, phosphodeficient FAK mutant, phospho-specific immunoblotting\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple siRNAs plus phosphodeficient mutant defining mechanistic pathway\",\n      \"pmids\": [\"19150878\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"FXR is expressed by cells of innate immunity (macrophages, dendritic cells); FXR activation stabilizes the nuclear corepressor NCoR on the NF-κB responsive element of the IL-1β promoter, thereby transrepressing NF-κB-dependent inflammatory genes (TNF-α, IL-1β, IL-6, COX-1, COX-2, iNOS); FXR-null mice show exacerbated colitis.\",\n      \"method\": \"FXR-null mice colitis models (TNBS, DSS), LPS-activated macrophage treatment, ChIP (NCoR on IL-1β promoter), gene expression analysis\",\n      \"journal\": \"Journal of immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP demonstrating NCoR stabilization plus knockout phenotype in two colitis models\",\n      \"pmids\": [\"19864602\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"FXR activation by INT-747 (obeticholic acid) in vivo protects against DSS- and TNBS-induced colitis by downregulating pro-inflammatory cytokines and preserving epithelial barrier function; FXR-null mice show exacerbated colitis, establishing FXR as a regulator of intestinal inflammation and barrier integrity.\",\n      \"method\": \"FXR-null mice, DSS/TNBS colitis models, INT-747 treatment, epithelial permeability assay, cytokine ELISA, in vitro Caco-2/HT29 cells\",\n      \"journal\": \"Gut\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — knockout plus pharmacological rescue with multiple mechanistic readouts\",\n      \"pmids\": [\"21242261\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FXR suppresses hepatic autophagy in the fed state by binding to promoters of autophagy genes (including Atg7, Ulk1, Tfeb) and disrupting the functional CREB-CRTC2 coactivator complex; FXR and CREB compete for shared sites in autophagic gene promoters with opposite transcriptional outputs. FXR-knockout mice show partial defects in fed-state suppression of hepatic autophagy.\",\n      \"method\": \"ChIP-seq from mouse liver, pharmacological FXR activation/knockout, co-immunoprecipitation of CREB-CRTC2 complex disruption, autophagy flux assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — ChIP-seq, reconstitution of complex disruption, and knockout mice — replicated in two simultaneous Nature papers\",\n      \"pmids\": [\"25383523\", \"25383539\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FXR is a molecular target for the metabolic benefits of vertical sleeve gastrectomy (VSG); VSG increases circulating bile acids, and in FXR-knockout mice, the ability of VSG to reduce body weight and improve glucose tolerance is substantially reduced, establishing FXR as a key mediator of VSG metabolic effects.\",\n      \"method\": \"FXR knockout mice with VSG surgery, gut microbiota analysis, circulating bile acid measurement, glucose tolerance tests\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic loss-of-function with defined surgical phenotype\",\n      \"pmids\": [\"24670636\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FXR directly regulates renal AQP2 (aquaporin 2) expression by binding to a FXR response element in the AQP2 gene promoter; FXR agonist treatment increased urine osmolality and upregulated AQP2, while FXR-knockout mice showed impaired urine concentrating ability and reduced AQP2 expression.\",\n      \"method\": \"FXR knockout mice, FXR agonist treatment, luciferase reporter assay with AQP2 promoter FXRE, primary inner medullary collecting duct cell culture\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — promoter FXRE characterization plus knockout phenotype\",\n      \"pmids\": [\"24464484\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FXR directly binds an IR9 DNA motif within the SOCS3 promoter (by EMSA and ChIP) and enhances SOCS3 transcription; FXR-induced SOCS3 upregulates p21 and represses STAT3 phosphorylation, mediating anti-tumor effects in HCC; SOCS3 siRNA knockdown abolished FXR's anti-HCC effects.\",\n      \"method\": \"EMSA, ChIP assay, luciferase reporter assay, SOCS3 siRNA, HCC xenograft mouse model\",\n      \"journal\": \"Oncotarget\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct DNA binding by EMSA and ChIP plus functional epistasis with siRNA and in vivo model\",\n      \"pmids\": [\"26416445\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Intestinal FXR inhibition (by Gly-MCA) improves obesity-related metabolic dysfunction; mechanistically, reduced intestinal FXR signaling decreases biosynthesis of intestinal ceramides, which directly compromise beige fat thermogenic function; intestine-specific FXR knockout mice were unresponsive to Gly-MCA benefits.\",\n      \"method\": \"Intestine-specific FXR knockout mice, Gly-MCA treatment, ceramide measurement, beige fat thermogenesis assays, FXR agonist blockade experiments\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific knockout plus ceramide supplementation rescue experiment\",\n      \"pmids\": [\"26670557\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FXR activation by GW4064 induces SREBP-2 mRNA and miR-33 expression via a defined FXR response element in intron 10 of the Srebp-2 gene; however, FXR simultaneously induces INSIG-2A, which prevents processing of pSREBP-2 to nuclear SREBP-2, uncoupling miR-33 induction from SREBP-2 target gene regulation.\",\n      \"method\": \"ChIP-seq (FXR response element in Srebp-2 intron 10), FXR agonist treatment in mice, Scap-knockout mice, INSIG-2A expression analysis, miR-33 target gene analysis\",\n      \"journal\": \"Arteriosclerosis, thrombosis, and vascular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-seq plus genetic validation in Scap-null mice\",\n      \"pmids\": [\"25593129\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FXR activation suppresses renal fibrosis by inhibiting the transcriptional activity of the Smad3 gene promoter (luciferase reporter assay); FXR-mediated repression of fibrosis is alleviated by ectopic Smad3 overexpression, and FXR agonist protected against fibrosis in UUO mice while downregulating Smad3.\",\n      \"method\": \"Luciferase reporter assay (Smad3 promoter), Smad3 overexpression, FXR antagonist, FXR agonist in UUO mouse model\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — promoter assay and epistasis (Smad3 rescue) in single lab\",\n      \"pmids\": [\"27853248\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A CK2-RNF4 interplay coordinates non-canonical FXR SUMOylation and degradation: CK2 phosphorylates Ser-327 of FXR, enabling SUMO2 conjugation at Lys-325 (a pSuM motif) by Ubc9/PIAS1; Lys-325 SUMOylation promotes efficient ligand activation and transcriptional coactivation. Constitutive SUMOylation directs FXR ubiquitination by E3 ligase RNF4 and proteasomal degradation; RNF4 is required for maximal FXR-dependent gene regulation.\",\n      \"method\": \"Site-directed mutagenesis (K325R, S327A/E), Co-IP of SUMO2/ubiquitin, PIAS1/Ubc9 interaction, RNF4 knockdown, ChIP, CK2 inhibition\",\n      \"journal\": \"Journal of molecular cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of modification sites combined with Co-IP and functional transcriptional readouts\",\n      \"pmids\": [\"28201649\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FXR restricts abnormal Lgr5+ intestinal cancer stem cell growth; bile acids antagonizing intestinal FXR (T-βMCA, DCA) induce proliferation and DNA damage in Lgr5+ cells, while selective intestinal FXR activation curtails CRC progression; this implicates FXR in coordinating intestinal self-renewal with bile acid levels.\",\n      \"method\": \"Intestinal organoids, Lgr5+ cell sorting, FXR agonist/antagonist treatment, APC-mutant/high-fat-diet mouse model, DNA damage assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological manipulation in organoid and in vivo models with defined cellular phenotype\",\n      \"pmids\": [\"30794774\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FXR activation inhibits Src kinase (Tyr416 phosphorylation), leading to increased Ser127 phosphorylation and cytosolic retention of YAP (via Hippo kinase complex stabilization), thereby protecting against renal fibrosis; FXR-knockout mice show increased fibrosis markers and nuclear YAP.\",\n      \"method\": \"FXR agonist (GW4064) in HK2 cells and UUO mice, FXR-KO mice, Src inhibitor (PP2), phospho-specific immunoblotting, Co-IP of Hippo kinases\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — pharmacological and genetic FXR manipulation with mechanistic Src-YAP pathway definition\",\n      \"pmids\": [\"31298930\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Enhanced FXR SUMOylation in activated hepatic stellate cells (HSCs) limits their response to FXR agonists in liver fibrosis; SUMOylation inhibitors rescue FXR signaling and synergize with OCA against HSC activation. FXR upregulates Perilipin-1 to stabilize lipid droplets and prevent HSC activation.\",\n      \"method\": \"SUMOylation assays in HSCs, SUMOylation inhibitors, OCA treatment, CCl4/BDL/NASH mouse fibrosis models, Perilipin-1 ChIP and siRNA knockdown\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — SUMOylation mechanism defined with inhibitors and functional rescue in multiple fibrosis models\",\n      \"pmids\": [\"31932588\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FXR functions as a T cell-intrinsic sensor of nutrient availability; T cell-specific FXR deletion prevented starvation-induced loss of lymphocytes and increased effector T cell fitness under low-glucose conditions by increasing glutamine and fatty acid contribution to respiration and enhancing cell survival.\",\n      \"method\": \"T cell-specific FXR knockout mice, infection models, metabolic flux analysis (glutamine and fatty acid use), glucose rescue experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific knockout with metabolic flux readouts\",\n      \"pmids\": [\"33318189\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FXR and GPBAR1 (TGR5) exert antagonistic effects on autophagy: FXR represses autophagy-related genes (Atgs) in the fed state, while GPBAR1 promotes autophagy via cAMP-CREB pathway; mechanistically, GPBAR1 agonism promotes CREB recruitment to the LC3 promoter CRE and reverses FXR-mediated repression of autophagy.\",\n      \"method\": \"Gpbar1-null mice, FXR-null mice, FXR antagonist, GPBAR1 agonist (BAR501), ChIP (CREB on LC3 promoter), autophagy flux assays in liver and WAT\",\n      \"journal\": \"FASEB journal\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — dual genetic knockouts and ChIP demonstrating opposing mechanisms at shared promoters\",\n      \"pmids\": [\"33368684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"PPARγ directly binds the PPAR-responsive element in the FXR gene promoter in a PPARγ agonist-dependent manner (ChIP assay), driving FXR expression during adipogenesis; FXR activation then induces SCD (stearoyl-CoA desaturase) by binding a FXRE in the SCD gene promoter, promoting lipogenesis in adipocytes.\",\n      \"method\": \"ChIP assay (PPARγ binding to FXR promoter), FXRE identification in SCD promoter, reporter assay, troglitazone/CDCA/guggulsterone treatment in 3T3-L1 cells\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP plus functional readouts, single lab study\",\n      \"pmids\": [\"32446390\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Hepatic FXR protects against NAFLD primarily through two distinct intestinal and hepatic mechanisms: hepatic FXR controls lipogenic genes (repressing Scd1, Dgat2, Lpin1 independently of SHP and SREBP1c), while intestinal FXR controls lipid absorption via regulation of bile acids; tissue-specific FXR knockout mice dissected these two pathways.\",\n      \"method\": \"Tissue-specific FXR knockout mice (hepatic vs intestinal), comprehensive lipidomics, bile acid replenishment experiments, GSK2324 FXR agonist treatment\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific knockouts with comprehensive lipidomics providing mechanistic dissection\",\n      \"pmids\": [\"34270928\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FXR is a direct transcriptional regulator of ACE2 in gastrointestinal and respiratory tissues; FXR inhibition by UDCA or z-guggulsterone downregulates ACE2 expression in human lung, cholangiocyte, and intestinal organoids, in mouse and hamster tissues, and in ex situ perfused human lungs, reducing susceptibility to SARS-CoV-2 infection.\",\n      \"method\": \"FXR agonist/antagonist treatment in human organoids, ex vivo perfused human organs, mouse and hamster in vivo models, ACE2 reporter assays, SARS-CoV-2 infection assays\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal model systems (organoids, ex vivo organs, in vivo animals) with functional infection readout\",\n      \"pmids\": [\"36470304\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SIRT6 deacetylates FXR at the protein level, thereby elevating FXR transcriptional activity and protecting against APAP-induced hepatotoxicity; FXR ablation abolished SIRT6 overexpression-mediated hepatoprotection, and pharmacological FXR activation rescued SIRT6-knockout mice from APAP injury.\",\n      \"method\": \"Hepatocyte-specific SIRT6 and FXR knockout mice, SIRT6 overexpression and pharmacological activation, deacetylation assays, APAP hepatotoxicity model\",\n      \"journal\": \"Cellular and molecular gastroenterology and hepatology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — dual genetic knockouts with mechanistic deacetylation and functional hepatoprotection readouts\",\n      \"pmids\": [\"35526796\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Under liver injury conditions, FXR acetylation at K217 (near the nuclear localization signal) blocks recognition by importin KPNA3, preventing nuclear import; simultaneously, reduced phosphorylation at T442 (within nuclear export signal) promotes recognition by exportin CRM1, facilitating FXR nuclear export and cytosolic retention; cytosolic FXR is then degraded by the E3 ligase CHIP. SIRT1 activators reduce FXR acetylation, prevent cytosolic degradation, and synergize with FXR agonists against liver injury.\",\n      \"method\": \"Acetylation site mutagenesis (K217, T442), importin/exportin binding assays (KPNA3, CRM1), CHIP E3 ligase Co-IP, SIRT1 activator treatment, acute/chronic liver injury mouse models\",\n      \"journal\": \"Acta pharmaceutica Sinica. B\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — mutagenesis of modification sites combined with importin/exportin binding assays and in vivo models\",\n      \"pmids\": [\"36873184\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"BRD4 is a co-factor required for FXR-mediated regulation of bile acid homeostasis; liver-specific BRD4 downregulation disrupted bile acid homeostasis, and FXR-mediated regulation of SHP and CYP7A1 was BRD4-dependent. In cholestasis, OCA-activated FXR increased binding of FXR and co-repressor SMRT at inflammatory genes, decreased NF-κB binding, and repressed inflammatory genes in a BRD4-dependent manner.\",\n      \"method\": \"Liver-specific BRD4 knockdown, cholestatic mouse models, ChIP for FXR/SMRT/NF-κB at inflammatory gene promoters, JQ1 (BRD4 inhibitor) and OCA treatment\",\n      \"journal\": \"JCI insight\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP demonstrating co-factor requirement plus liver-specific knockdown with gene expression readouts\",\n      \"pmids\": [\"33290278\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"NR1H4 (FXR) is a bile acid-activated nuclear receptor that heterodimerizes with RXR and directly binds bile acid response elements to transcriptionally regulate bile acid synthesis (via FXR→SHP→LRH-1→CYP7A1 cascade and FXR→FGF15/19→FGFR4→JNK cascade), bile acid transport (BSEP, ASBT), lipid and glucose metabolism, autophagy (competing with CREB-CRTC2 at autophagy gene promoters), intestinal barrier integrity, and innate immunity (NCoR-stabilization on NF-κB promoters); its activity is post-translationally regulated by SUMOylation (CK2/RNF4-mediated at K325/S327), acetylation (SIRT6/SIRT1-controlled at K217 governing importin KPNA3 recognition and nuclear import), phosphorylation (at T442 controlling CRM1-mediated export), and BRD4-dependent coactivation, with cytosolic FXR being targeted for proteasomal degradation by the E3 ligase CHIP.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"NR1H4 (FXR) is a bile acid–activated nuclear receptor that functions as a master transcriptional regulator of bile acid homeostasis, lipid and glucose metabolism, and tissue-protective programs across hepatic, intestinal, renal, and immune compartments. FXR heterodimerizes with RXRα or binds DNA as a monomer at FXR response elements, with chenodeoxycholic acid (CDCA) and androsterone serving as direct endogenous ligands that induce coactivator recruitment; ligand binding triggers a transcriptional cascade—including induction of SHP to repress CYP7A1 and ASBT, induction of ileal FGF15/19 to suppress hepatic bile acid synthesis via FGFR4–β-klotho, and direct activation of targets such as BSEP, GLUT4, AQP2, SOCS3, SREBP-2/miR-33, SCD, Perilipin-1, and ACE2 [PMID:11150726, PMID:15591588, PMID:35682726, PMID:18769028, PMID:24464484, PMID:26416445, PMID:36470304]. FXR activity is tuned by post-translational modifications: CK2 phosphorylation at Ser-327 enables Ubc9/PIAS1-mediated SUMO2 conjugation at Lys-325, coupling transcriptional activation to RNF4-dependent proteasomal degradation; acetylation at Lys-217 blocks KPNA3-mediated nuclear import and promotes CRM1-dependent export and CHIP-mediated cytosolic degradation, a process reversed by SIRT1 and SIRT6 deacetylation [PMID:28201649, PMID:36873184, PMID:35526796]. FXR additionally represses hepatic autophagy in the fed state by opposing CREB-driven autophagy gene transcription, limits colorectal cancer stem cell expansion, and acts as a T cell–intrinsic nutrient sensor that modulates metabolic fuel utilization during nutrient deprivation [PMID:33368684, PMID:30794774, PMID:33318189].\",\n  \"teleology\": [\n    {\n      \"year\": 1997,\n      \"claim\": \"Identification of RIP14/FXR as an orphan nuclear receptor that heterodimerizes with RXR established the foundational molecular framework—a ligand-activated transcription factor—but left the physiological ligand unknown.\",\n      \"evidence\": \"Reporter gene assays and activation-function mutagenesis in transfected cells\",\n      \"pmids\": [\"9223286\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological ligand unidentified\", \"No known target genes\", \"In vivo relevance not established\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"De-orphanization of FXR as a bile acid receptor (CDCA as direct ligand) and identification of CYP7A1 suppression and IBABP induction as target gene programs answered the central question of FXR's physiological role—bile acid homeostasis.\",\n      \"evidence\": \"Ligand-binding assays and target gene expression studies replicated across multiple laboratories\",\n      \"pmids\": [\"11150726\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of bile acid recognition not resolved\", \"Downstream signaling cascade incompletely mapped\", \"Species differences in ligand selectivity not characterized\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Delineation of the FXR→SHP→FTF cascade repressing ASBT transcription established a defined epistatic pathway through which FXR coordinates both synthesis and reabsorption arms of bile acid homeostasis.\",\n      \"evidence\": \"Promoter deletion/mutation analysis in Caco-2 cells combined with in vivo bile acid feeding\",\n      \"pmids\": [\"15591588\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether this cascade operates identically in human intestine\", \"Additional FXR-regulated transporters not surveyed\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Demonstration that androsterone directly binds the FXR LBD (by NMR) and identification of residues Asn354/Ser345 governing species-specific ligand selectivity resolved how FXR discriminates among steroid ligands and revealed a reversed steroid-binding orientation unique among nuclear receptors.\",\n      \"evidence\": \"NMR spectroscopy, mutagenesis, TR-FRET coactivator recruitment, crystal structure interpretation\",\n      \"pmids\": [\"16675527\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full co-crystal structure with androsterone not deposited\", \"Physiological relevance of androgen–FXR axis in vivo incompletely tested\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Evolutionary reconstruction using skate FXRβ chimeras showed that bile acid responsiveness was acquired late in vertebrate evolution through a specific LBD insertion, answering when and how FXR was repurposed as a bile acid sensor.\",\n      \"evidence\": \"Chimeric LBD reporter assays and phylogenetic analysis\",\n      \"pmids\": [\"17567710\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Structural mechanism of the insertion enabling bile acid binding not determined\", \"Limited to a single non-mammalian species comparison\"]\n    },\n    {\n      \"year\": 2008,\n      \"claim\": \"Identification of a functional FXRE in the GLUT4 promoter bound by FXR as a monomer (without RXR) expanded FXR's role beyond bile acid metabolism into glucose uptake regulation and revealed an alternative DNA-binding mode.\",\n      \"evidence\": \"EMSA, promoter deletion/mutagenesis, RXR knockdown, in vivo CDCA treatment in mice\",\n      \"pmids\": [\"18769028\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological significance of monomeric versus heterodimeric binding genome-wide unknown\", \"Whether FXR regulates other glucose transporters not tested\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Discovery of the FXR→MMP-9→FAK(Y397) signaling cascade driving endothelial migration and tube formation revealed a non-genomic-like signaling output linking FXR to vascular remodeling.\",\n      \"evidence\": \"siRNA, FAK phospho-mutants, time-lapse microscopy, tube formation assays in endothelial cells\",\n      \"pmids\": [\"19150878\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether MMP-9 induction is direct transcriptional or indirect not fully resolved\", \"In vivo angiogenesis phenotype not shown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Demonstration that FXR directly transactivates AQP2 and that FXR knockout mice exhibit polyuria established FXR as a regulator of renal water homeostasis, a function entirely distinct from bile acid metabolism.\",\n      \"evidence\": \"FXR KO mouse phenotyping, FXRE identification in AQP2 promoter, primary collecting duct cell experiments, in vivo agonist rescue\",\n      \"pmids\": [\"24464484\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether bile acids reach sufficient renal concentrations to activate FXR physiologically\", \"Contribution relative to vasopressin/V2R axis not quantified\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"ChIP-seq identification of an FXRE in Srebp-2 intron 10 combined with simultaneous INSIG-2A induction showed that FXR uncouples miR-33 production from SREBP-2 nuclear activation, revealing a sophisticated dual-output regulatory logic at a single locus.\",\n      \"evidence\": \"ChIP-seq, Scap KO epistasis, in vivo FXR agonist treatment, Western blot for SREBP-2 processing\",\n      \"pmids\": [\"25593129\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional consequences of isolated miR-33 induction on cholesterol efflux not fully tested\", \"Human relevance of the intronic FXRE not confirmed\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Direct FXR binding to an IR9 element in the SOCS3 promoter (by EMSA and ChIP) and epistatic dependence of FXR's anti-proliferative effect on SOCS3 established a mechanism linking FXR to JAK-STAT3 suppression and hepatocellular carcinoma growth inhibition.\",\n      \"evidence\": \"EMSA, ChIP, siRNA epistasis, xenograft mouse model\",\n      \"pmids\": [\"26416445\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether SOCS3 induction mediates anti-tumor effects in other cancer types unknown\", \"Contribution relative to other FXR anti-proliferative mechanisms not dissected\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Biochemical reconstitution of the CK2(Ser327)→Ubc9/PIAS1(SUMO2-Lys325)→RNF4(ubiquitin) cascade revealed a phosphorylation-dependent SUMOylation switch (pSuM) that couples FXR transcriptional activation to its own proteasomal turnover, answering how FXR activity is temporally self-limited.\",\n      \"evidence\": \"Site-directed mutagenesis, SUMOylation/ubiquitination assays, phospho-mimic mutants, RNF4 functional studies\",\n      \"pmids\": [\"28201649\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological kinetics of this turnover cycle in vivo not measured\", \"Whether other kinases besides CK2 phosphorylate Ser327 not tested\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Demonstration that FXR restricts Lgr5+ intestinal cancer stem cell proliferation and that bile acid antagonists of FXR promote DNA damage in these cells established FXR as a tumor-suppressive gatekeeper coordinating intestinal renewal with luminal bile acid composition.\",\n      \"evidence\": \"Intestinal organoids, APC-mutant + HFD genetic mouse models, bile acid profiling, Lgr5+ cell sorting\",\n      \"pmids\": [\"30794774\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct FXR target genes mediating stem cell quiescence not fully catalogued\", \"Whether FXR agonists prevent human colorectal cancer progression unknown\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"SUMOylation of FXR in activated hepatic stellate cells was shown to limit its anti-fibrotic signaling, and SUMOylation inhibitors rescued FXR agonist efficacy; identification of Perilipin-1 as an FXR target stabilizing lipid droplets provided a mechanistic link between FXR and HSC quiescence.\",\n      \"evidence\": \"SUMOylation assays, co-treatment with SUMOylation inhibitors in CCl4/BDL/NASH mouse models, Perilipin-1 target gene identification\",\n      \"pmids\": [\"31932588\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific SUMOylation sites on FXR in HSCs not mapped\", \"Whether Perilipin-1 induction is sufficient for HSC quiescence not tested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Genetic studies with T cell–specific FXR knockouts showed that FXR senses nutrient availability in T cells: FXR deletion prevented starvation-induced lymphocyte attrition by shifting fuel usage toward glutamine and fatty acid oxidation, establishing FXR as an immune-metabolic checkpoint.\",\n      \"evidence\": \"T cell–specific FXR KO mice, infection models with fasting, metabolic flux analysis\",\n      \"pmids\": [\"33318189\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether bile acids reach T cells at activating concentrations in vivo\", \"Downstream transcriptional targets in T cells not defined\", \"Single laboratory finding\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"FXR was shown to repress hepatic autophagy in the fed state by opposing CREB-mediated transcription of Atg genes, with GPBAR1 acting antagonistically; this established FXR as a nutrient-state–dependent brake on autophagy.\",\n      \"evidence\": \"FXR KO and GPBAR1 KO mice, ChIP for CREB at LC3 promoter, fed/fasted paradigms\",\n      \"pmids\": [\"33368684\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct FXR binding to Atg gene promoters not demonstrated\", \"Relative contribution versus mTOR-mediated autophagy suppression unclear\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identification of BRD4 as a required cofactor for FXR-dependent bile acid gene regulation and anti-inflammatory gene repression (via SMRT corepressor recruitment displacing NF-κB) clarified how FXR achieves gene-selective activation versus repression at chromatin.\",\n      \"evidence\": \"Liver-specific BRD4 knockdown, ChIP for FXR/SMRT/NF-κB, cholestasis models, OCA and JQ1 treatments\",\n      \"pmids\": [\"33290278\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether BRD4 interaction is direct or through Mediator/other bridging factors\", \"Genome-wide map of BRD4-dependent versus -independent FXR targets unavailable\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Mechanistic dissection of FXR acetylation at K217 showed it blocks importin KPNA3 recognition while reduced T442 phosphorylation promotes CRM1-mediated export, leading to CHIP-dependent cytosolic degradation reversible by SIRT1—providing a complete nucleocytoplasmic trafficking model for FXR activity control.\",\n      \"evidence\": \"Site-directed mutagenesis of K217 and T442, Co-IP with KPNA3/CRM1/CHIP, SIRT1 activator in liver injury models\",\n      \"pmids\": [\"36873184\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Acetyltransferase(s) responsible for K217 acetylation not identified\", \"Whether K217 acetylation state is dynamically regulated by feeding/fasting cycles unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"SIRT6 was identified as a second sirtuin deacetylase acting on FXR: hepatocyte-specific SIRT6 KO impaired FXR-dependent hepatoprotection after acetaminophen injury, and FXR agonists rescued the SIRT6 KO phenotype, placing SIRT6 upstream of FXR in liver stress responses.\",\n      \"evidence\": \"Hepatocyte-specific SIRT6 KO mice, FXR agonist rescue, RNA-seq\",\n      \"pmids\": [\"35526796\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific FXR residue(s) deacetylated by SIRT6 not mapped\", \"Whether SIRT1 and SIRT6 act on distinct or overlapping sites unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstration that FXR directly transactivates ACE2 across lung, intestinal, and cholangiocyte organoids—and that FXR inhibition reduces SARS-CoV-2 infection—revealed an unexpected link between bile acid signaling and viral entry receptor expression.\",\n      \"evidence\": \"Human organoids, in vivo mouse/hamster models, ex situ perfused human lungs and livers, SARS-CoV-2 infection assays\",\n      \"pmids\": [\"36470304\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"FXRE in ACE2 promoter not precisely mapped\", \"Long-term consequences of FXR inhibition on ACE2-dependent RAS homeostasis unknown\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"A genome-wide, cell-type–resolved map of direct FXR binding sites across its diverse tissue contexts (liver, intestine, kidney, immune cells) and integration with the multiple PTM regulatory codes (SUMOylation, acetylation, O-GlcNAcylation, phosphorylation) to explain gene-selective and context-dependent transcriptional outputs remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No integrated multi-tissue cistrome exists\", \"Rules determining monomer vs. heterodimer binding genome-wide undefined\", \"How O-GlcNAcylation, SUMOylation, and acetylation are coordinately regulated in different metabolic states is unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 1, 4, 5, 7, 8, 13, 20, 22]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [0, 5, 8, 20]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 5, 8, 10, 17]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [17]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 1, 4, 5, 7, 8, 13, 20, 22]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [1, 4, 20, 21, 23]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [6, 12, 23]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [10, 17]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [16]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [4, 5]}\n    ],\n    \"complexes\": [\n      \"FXR-RXRα heterodimer\"\n    ],\n    \"partners\": [\n      \"RXRA\",\n      \"SHP\",\n      \"BRD4\",\n      \"SIRT1\",\n      \"SIRT6\",\n      \"KPNA3\",\n      \"CHIP\",\n      \"RNF4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"NR1H4 (FXR) is a bile acid-activated nuclear receptor that heterodimerizes with RXR to bind bile acid response elements and transcriptionally coordinate bile acid synthesis, transport, lipid and glucose metabolism, autophagy, intestinal barrier integrity, and innate immune responses [PMID:10334992, PMID:11030617, PMID:25383523, PMID:19864602]. FXR represses bile acid biosynthesis through two major cascades: an intracellular FXR→SHP→LRH-1 pathway that silences CYP7A1, and an endocrine FXR→FGF19→FGFR4→JNK axis that suppresses hepatic CYP7A1 from the intestine [PMID:11030332, PMID:12815072]. FXR transrepresses NF-κB-dependent inflammatory genes by stabilizing the corepressor NCoR on target promoters, and in the fed state it suppresses hepatic autophagy by competing with CREB–CRTC2 at autophagy gene promoters [PMID:19864602, PMID:25383523]. FXR transcriptional activity is post-translationally tuned by CK2-dependent SUMOylation at K325/S327 (coupled to RNF4-mediated proteasomal turnover), by SIRT1/SIRT6-controlled deacetylation at K217 governing importin KPNA3-mediated nuclear import, by phosphorylation at T442 controlling CRM1-dependent nuclear export, and by BRD4-dependent coactivation [PMID:28201649, PMID:36873184, PMID:35526796, PMID:33290278].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"Identification of NR1H4 as a ligand-modulated orphan nuclear receptor that heterodimerizes with RXR established it as a transcription factor responsive to isoprenoid metabolites, setting the stage for ligand deorphanization.\",\n      \"evidence\": \"Receptor cloning, farnesol activation assays, and RXR heterodimerization assays in mammalian cells\",\n      \"pmids\": [\"7774010\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological ligand unidentified\", \"No in vivo target genes known\", \"Tissue-specific roles undefined\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Identification of bile acids as the endogenous FXR ligands and demonstration that FXR represses CYP7A1 while inducing IBABP transformed the receptor from an orphan into the central bile acid sensor governing enterohepatic circulation.\",\n      \"evidence\": \"Radioligand binding, reporter assays, and Northern blot for CYP7A1/IBABP across three independent labs\",\n      \"pmids\": [\"10334992\", \"10334993\", \"10360171\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Molecular mechanism of CYP7A1 repression unknown\", \"In vivo confirmation in knockout animals pending\"]\n    },\n    {\n      \"year\": 2000,\n      \"claim\": \"The FXR→SHP→LRH-1 cascade and FXR-null mouse phenotype jointly defined the molecular pathway for bile acid feedback repression of CYP7A1 and established FXR as essential for bile acid and lipid homeostasis in vivo.\",\n      \"evidence\": \"Epistatic reporter/expression analysis of SHP-LRH-1 cascade; FXR knockout mice with elevated serum bile acids, cholesterol, and triglycerides\",\n      \"pmids\": [\"11030332\", \"11030331\", \"11030617\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"SHP-independent repression mechanisms unexplored\", \"FXR roles beyond liver not yet characterized\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Discovery that FXR induces intestinal FGF19, which signals through FGFR4–JNK to repress hepatic CYP7A1, revealed a second, endocrine axis for bile acid feedback that operates across the gut–liver axis.\",\n      \"evidence\": \"FXR agonist induction of FGF19 in hepatocytes, JNK inhibitor studies, CYP7A1 reporter assays\",\n      \"pmids\": [\"12815072\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of SHP vs FGF19 axes in humans unclear\", \"FGF19 regulation of non-bile-acid targets not defined\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"FXR was shown to regulate glucose metabolism through SHP-dependent repression of gluconeogenic genes and enhanced insulin sensitivity, and to be required for liver regeneration after partial hepatectomy, broadening its role beyond bile acid homeostasis.\",\n      \"evidence\": \"GW4064 treatment and adenoviral FXR overexpression in db/db mice; hyperinsulinemic clamp in FXR-null mice; partial hepatectomy in FXR-null mice\",\n      \"pmids\": [\"16410358\", \"16557297\", \"16614213\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct FXR target genes for gluconeogenesis incompletely mapped\", \"Whether regeneration defect is cell-autonomous or bile acid-mediated unclear\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"FXR was established as an anti-inflammatory transcription factor in innate immune cells, acting through NCoR stabilization on NF-κB-dependent promoters, and as a regulator of endothelial cell motility via FAK–paxillin–MMP-9 signaling.\",\n      \"evidence\": \"ChIP for NCoR on IL-1β promoter in macrophages; FXR-null colitis models; siRNA/phosphodeficient FAK mutant in endothelial cells\",\n      \"pmids\": [\"19864602\", \"19150878\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"FXR target gene specificity in immune cells beyond NF-κB unclear\", \"Physiological relevance of endothelial FXR signaling in vivo not confirmed\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Genome-wide ChIP-seq revealed that FXR suppresses hepatic autophagy in the fed state by directly occupying autophagy gene promoters and disrupting CREB–CRTC2 coactivator complexes, linking nutrient sensing to autophagic flux.\",\n      \"evidence\": \"ChIP-seq in mouse liver, CREB–CRTC2 co-IP disruption, autophagy flux assays in FXR-null mice\",\n      \"pmids\": [\"25383523\", \"25383539\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FXR directly represses autophagy in non-hepatic tissues not established\", \"Structural basis for CREB–FXR competition unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Definition of the CK2→pS327→SUMO2-K325→RNF4 ubiquitin–proteasome axis revealed that SUMOylation is a dual-purpose post-translational switch that both licenses FXR transcriptional activity and programs its degradation.\",\n      \"evidence\": \"Site-directed mutagenesis (K325R, S327A/E), Co-IP of SUMO2/ubiquitin, RNF4 knockdown, ChIP\",\n      \"pmids\": [\"28201649\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo quantitative contribution of SUMOylation to FXR turnover unknown\", \"Whether other E3 ligases cooperate with RNF4 not tested\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Mapping of acetylation at K217 (controlling KPNA3-dependent nuclear import) and phosphorylation at T442 (controlling CRM1-dependent nuclear export), together with identification of CHIP as the cytosolic E3 ligase for FXR, defined a complete nucleocytoplasmic trafficking code that is disrupted in liver injury.\",\n      \"evidence\": \"Acetylation/phosphorylation site mutagenesis, importin/exportin binding assays, CHIP Co-IP, SIRT1 activator rescue in liver injury models\",\n      \"pmids\": [\"36873184\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Kinase responsible for T442 phosphorylation not identified\", \"Whether the same trafficking code operates in intestinal FXR is unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"FXR was identified as a direct transcriptional regulator of ACE2 in gastrointestinal and respiratory epithelia, with FXR inhibition by UDCA reducing SARS-CoV-2 susceptibility, revealing an unexpected connection between bile acid signaling and viral entry.\",\n      \"evidence\": \"FXR agonist/antagonist treatment in human organoids, ex vivo perfused human lungs, mouse and hamster in vivo models, SARS-CoV-2 infection assays\",\n      \"pmids\": [\"36470304\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Long-term consequences of FXR-mediated ACE2 suppression on cardiometabolic function unknown\", \"Whether FXR regulates other viral receptors unexplored\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the structural basis for FXR's competition with CREB at autophagy promoters, the kinase(s) governing T442 phosphorylation, the relative tissue-specific contributions of hepatic versus intestinal FXR to metabolic disease, and the full scope of FXR's immune-regulatory transcriptome.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal structure of FXR with CREB–CRTC2 exists\", \"No systematic comparison of hepatic vs intestinal FXR cistrome in humans\", \"T442 kinase identity unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 1, 2, 4, 9, 15, 19, 22, 34]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [1, 9, 15, 22]},\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [2, 3, 12]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [17, 37]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 1, 9, 15, 19, 22, 36]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [36]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": []},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 2, 4, 9, 15, 19, 22, 34]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [2, 5, 6, 8, 11, 23, 24, 33]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [8, 16, 28]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [17, 30]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [19, 31]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [5, 10, 13]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [24]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [26, 36]}\n    ],\n    \"complexes\": [\n      \"FXR-RXR heterodimer\"\n    ],\n    \"partners\": [\n      \"RXRA\",\n      \"NR0B2\",\n      \"BRD4\",\n      \"NCOR1\",\n      \"CHIP\",\n      \"KPNA3\",\n      \"XPO1\",\n      \"RNF4\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}