{"gene":"XBP1","run_date":"2026-04-28T23:00:23","timeline":{"discoveries":[{"year":2001,"finding":"XBP1 mRNA is a substrate of IRE1 endoribonuclease: upon ER stress, IRE1 performs unconventional cytoplasmic splicing of XBP1 pre-mRNA, producing a frameshift that generates a highly active transcription factor (spliced XBP1); unspliced XBP1 mRNA is also transcriptionally induced by ATF6. Only the spliced form efficiently activates UPR target genes.","method":"Molecular cloning, mRNA splicing assays, reporter gene assays, identification of ATF6 as transcriptional inducer of XBP1","journal":"Cell","confidence":"High","confidence_rationale":"Tier 1 — foundational mechanistic study, >3000 citations, multiple orthogonal methods","pmids":["11779464"],"is_preprint":false},{"year":2003,"finding":"IRE1-mediated splicing of XBP1 mRNA is required for plasma cell differentiation and immunoglobulin production; signals from B cell differentiation (IL-4 controls XBP1 transcription; immunoglobulin synthesis controls post-transcriptional XBP1 processing); spliced XBP1 also controls IL-6 production essential for plasma cell survival.","method":"XBP1-/- mouse B cells reconstituted with spliced XBP1; genetic rescue experiments; cytokine measurement","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined cellular phenotype, genetic rescue, replicated across studies","pmids":["12612580"],"is_preprint":false},{"year":2005,"finding":"Unspliced XBP1 (XBP1u) protein is constitutively synthesized and rapidly degraded by proteasomal mechanisms (both ubiquitin-dependent and ubiquitin-independent); the C-terminus of XBP1u mediates its cytoplasmic trafficking and rapid degradation; XBP1u degradation prevents uncontrolled UPR activation.","method":"Protein stability assays, proteasome inhibitor treatment, mutagenesis of XBP1u C-terminus, reconstitution of XBP1-/- MEFs with XBP1u stability mutants","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — multiple orthogonal methods including mutagenesis and functional reconstitution","pmids":["16332684"],"is_preprint":false},{"year":2007,"finding":"Genome-wide mapping of XBP1 transcriptional targets in skeletal muscle and secretory cells identified a core group of ER function genes regulated in all cell types, plus tissue-specific targets; XBP1 regulates distinct target sets through different sequence motifs; XBP1 directly targets Mist1 (a regulator of differentiation), linking XBP1 loss-of-function to developmental defects.","method":"Genome-wide ChIP, microarray expression profiling, reporter assays","journal":"Molecular cell","confidence":"High","confidence_rationale":"Tier 2 — genome-wide approach with multiple orthogonal methods, highly cited","pmids":["17612490"],"is_preprint":false},{"year":2009,"finding":"The IRE1α-XBP1 pathway is required for adipogenesis: C/EBPβ directly binds the Xbp1 promoter to induce its expression; spliced XBP1 then binds the Cebpa promoter to activate C/EBPα expression; only spliced XBP1 rescues adipogenic defects in XBP1-deficient cells.","method":"XBP1-deficient MEFs and XBP1/IRE1α knockdown 3T3-L1 cells; promoter ChIP; rescue with spliced vs. unspliced XBP1","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — genetic KO with defined phenotype, ChIP, isoform-specific rescue","pmids":["19490910"],"is_preprint":false},{"year":2009,"finding":"Spliced XBP1 (XBP1s) augments the CDP-choline pathway for phosphatidylcholine biosynthesis and drives ER membrane biogenesis; ATF6α can also drive ER expansion and PtdCho biosynthesis independently of XBP1s, through partially distinct mechanisms.","method":"Overexpression of constitutively active ATF6α or XBP1s; phospholipid biosynthesis assays; ER morphology analysis; XBP1-deficient cells","journal":"Journal of cell science","confidence":"High","confidence_rationale":"Tier 2 — biochemical lipid assays, genetic controls, multiple cell systems","pmids":["19420237"],"is_preprint":false},{"year":2012,"finding":"miR-30c-2* (miR-30c-2-3p) is induced by the PERK pathway of the UPR and directly suppresses XBP1 expression post-transcriptionally, thereby limiting the magnitude of XBP1-mediated adaptive UPR gene expression and influencing cell fate.","method":"miRNA overexpression/inhibition, reporter assays, PERK pathway genetic manipulation, UPR target gene expression analysis","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 — direct regulatory link between PERK-induced miRNA and XBP1, single lab with multiple methods","pmids":["22431749"],"is_preprint":false},{"year":2014,"finding":"XBP1 drives triple-negative breast cancer tumorigenicity by forming a transcriptional complex with HIF1α that recruits RNA polymerase II to regulate HIF1α target genes; genome-wide mapping defined this XBP1-HIF1α regulatory network.","method":"ChIP-seq, Co-IP, RNA pol II recruitment assays, XBP1 depletion in TNBC models, patient cohort analysis","journal":"Nature","confidence":"High","confidence_rationale":"Tier 1-2 — reconstituted complex, genome-wide ChIP-seq, functional KD, high citations","pmids":["24670641"],"is_preprint":false},{"year":2015,"finding":"XBP1 activation in tumor-associated dendritic cells (driven by lipid peroxidation byproducts) induces a triglyceride biosynthetic program leading to abnormal lipid accumulation and inhibition of DC immunostimulatory capacity; DC-specific XBP1 deletion restores anti-tumor T cell responses.","method":"DC-specific XBP1 deletion mouse model, nanoparticle-mediated XBP1 silencing, lipid accumulation assays, T cell functional assays","journal":"Cell","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific KO with defined immunological phenotype, multiple orthogonal methods","pmids":["26073941"],"is_preprint":false},{"year":2015,"finding":"Nuclear translocation of spliced XBP1 (sXBP1) is impaired in diabetic nephropathy podocytes; sXBP1 lies downstream of insulin receptor/PI3K p85α/p85β signaling; interaction of sXBP1 with p85α and p85β is required for its nuclear translocation and podocyte homeostasis; disruption promotes ATF6/CHOP-mediated maladaptive ER stress.","method":"Podocyte-specific XBP1 KO mice, insulin receptor/p85 KO models, Co-IP of sXBP1 with p85 subunits, nuclear fractionation","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — multiple genetic KO models, Co-IP, functional nuclear localization assays","pmids":["25754093"],"is_preprint":false},{"year":2015,"finding":"XBP1 is selectively required for eosinophil differentiation: eosinophil progenitors selectively activate IRE1α and splice Xbp1 mRNA without inducing parallel ER stress pathways; without XBP1, nascent eosinophils show massive defects in post-translational maturation of granule proteins.","method":"XBP1-deficient mouse models, lineage-specific analysis, granule protein maturation assays, IRE1α activation assays","journal":"Nature immunology","confidence":"High","confidence_rationale":"Tier 2 — clean KO with defined differentiation phenotype and molecular mechanism","pmids":["26147683"],"is_preprint":false},{"year":2015,"finding":"SEC63 exists in a complex with polycystin-1 (PC1) and with IRE1α; SEC63 deficiency selectively activates IRE1α-XBP1; XBP1 is required for GPS cleavage and maturation of PC1; enforced XBP1s expression enhances PC1 GPS cleavage and ameliorates cystic disease.","method":"Murine genetic models with combined SEC63/XBP1 inactivation, Co-IP of SEC63 with PC1, XBP1s overexpression, in vivo cystic phenotype rescue","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — reciprocal Co-IP, genetic epistasis, in vivo rescue","pmids":["25844898"],"is_preprint":false},{"year":2015,"finding":"IRE1α/XBP1 signaling regulates osteoclastogenesis: XBP1 directly binds the NFATc1 promoter and induces its transcription; IRE1α activation during osteoclastogenesis is partially dependent on Ca2+ oscillations mediated by IP3 receptors ITPR2 and ITPR3; conditional IRE1α ablation in bone marrow increases bone mass due to defective osteoclastic bone resorption.","method":"ChIP for XBP1 binding at NFATc1 promoter; conditional IRE1α KO mice; pharmacological/genetic inhibition of ITPR2/3; osteoclast differentiation assays","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — ChIP, genetic KO with defined phenotype, upstream signaling identified","pmids":["26193638"],"is_preprint":false},{"year":2016,"finding":"The PERK-ATF4 pathway enhances IRE1α-XBP1 signaling by inducing IRE1α expression under ER stress, increasing the splicing ratio of XBP1 mRNA and prolonging the duration of pathway activation.","method":"IRE1α expression measurement under PERK/ATF4 pathway activation, XBP1 splicing ratio assays, genetic manipulation of PERK pathway","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 — functional cross-pathway regulation with splicing assays, single lab","pmids":["27052593"],"is_preprint":false},{"year":2017,"finding":"XBP1 binds the IL-6 promoter and activates its expression; IRE1α RNase activity is required for IL-6 induction; activated IRE1α/XBP1 pathway promotes melanoma cell proliferation via autocrine/paracrine IL-6/STAT3 signaling.","method":"ChIP for XBP1 at IL-6 promoter, IRE1α RNase inhibition, ectopic XBP1s expression, JAK/STAT3 pathway analysis","journal":"Journal of translational medicine","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP validation, functional IRE1α inhibition, single lab","pmids":["28222747"],"is_preprint":false},{"year":2017,"finding":"LOXL2 accumulates in the ER and interacts with HSPA5 (BiP), leading to IRE1-XBP1 pathway activation; XBP1 then directly transcriptionally activates EMT transcription factors SNAI1, SNAI2, ZEB2, and TCF3; IRE1 inhibition blocks LOXL2-dependent EMT.","method":"Co-IP of LOXL2 with HSPA5, ChIP for XBP1 at EMT-TF promoters, IRE1 inhibitor experiments, LOXL2 overexpression","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP, ChIP, pharmacological inhibition, single lab","pmids":["28332555"],"is_preprint":false},{"year":2017,"finding":"Unspliced XBP1 (XBP1-u) stabilizes MDM2 protein by binding to it (via its unique C-terminal degradation domain) and inhibiting MDM2 self-ubiquitination, thereby enhancing p53 ubiquitination and degradation, downregulating p21, and promoting cell cycle progression and tumorigenesis.","method":"shRNA library screen, Co-IP of XBP1-u with MDM2, ubiquitination assays, domain mapping, cell cycle analysis","journal":"Science advances","confidence":"High","confidence_rationale":"Tier 2 — Co-IP, ubiquitination assay, domain mutagenesis, isoform specificity established","pmids":["29057323"],"is_preprint":false},{"year":2018,"finding":"IRE1α-XBP1 activation in ovarian cancer-infiltrating T cells suppresses mitochondrial respiration and IFNγ production by limiting glutamine carrier abundance, restricting glutamine influx needed for mitochondrial respiration under glucose-depleted conditions; XBP1-deficient T cells show transcriptional reprogramming and improved anti-tumor effector capacity.","method":"XBP1-deficient T cells, metabolic flux assays, glutamine transporter expression analysis, mitochondrial respiration assays, patient specimen analysis, mouse OvCa model","journal":"Nature","confidence":"High","confidence_rationale":"Tier 2 — T cell-specific KO, mechanistic metabolomics, in vivo mouse model, human specimens","pmids":["30305738"],"is_preprint":false},{"year":2018,"finding":"MYC directly controls IRE1 transcription by binding to its promoter and enhancer; MYC also forms a transcriptional complex with spliced XBP1 and enhances its transcriptional activity; XBP1 is a synthetic lethal partner of MYC.","method":"ChIP for MYC at IRE1 promoter/enhancer, Co-IP of MYC with XBP1, synthetic lethality screen, patient-derived xenograft models","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — ChIP, Co-IP, in vivo PDX models, multiple orthogonal methods","pmids":["29480818"],"is_preprint":false},{"year":2018,"finding":"IRE1α-XBP1 pathway in pancreatic β cells regulates oxidative proinsulin folding by inducing expression of five protein disulfide isomerases (PDI, PDIR, P5, ERp44, ERp46); IRE1α CKO mice exhibit diabetic phenotype with decreased insulin secretion and impaired proinsulin folding; reconstitution of the IRE1α-XBP1 pathway restores PDI expression, proinsulin content, and insulin secretion.","method":"β cell-specific Ire1α conditional KO mice, Cre-loxP insulinoma cell lines, PDI expression assays, proinsulin folding assays, IRE1α-XBP1 reconstitution","journal":"The Journal of cell biology","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific KO with defined molecular targets and functional rescue","pmids":["29507125"],"is_preprint":false},{"year":2018,"finding":"The RNA ligation step of XBP1 mRNA splicing is performed by RtcB ligase; CNP (2',3'-cyclic nucleotide phosphodiesterase) suppresses RtcB-mediated XBP1 splicing by hydrolyzing the 2',3'-cyclic phosphate on cleaved exon termini; RtcA (RNA 3'-terminal cyclase) facilitates splicing by converting 2'-phosphate back to 2',3'-cyclic phosphate; mammalian Trpt1 does not contribute to XBP1 splicing.","method":"In vitro XBP1 splicing assays, CNP/RtcA/Trpt1 genetic manipulation in mouse and human cell lines","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — in vitro reconstitution splicing assays with defined enzymatic components","pmids":["30355738"],"is_preprint":false},{"year":2019,"finding":"Functional XBP1 directly transactivates PTGS2 (Cox-2) and PTGES (mPGES-1) genes to enable optimal prostaglandin E2 biosynthesis in myeloid cells; IRE1α-deficient or XBP1-deficient myeloid cells show compromised inducible PGE2 biosynthesis; mice lacking IRE1α-XBP1 in leukocytes show reduced pain behaviors in PGE2-dependent pain models.","method":"Myeloid-specific IRE1α/XBP1 KO mice, ChIP/reporter assays for XBP1 binding at PTGS2/PTGES promoters, PGE2 ELISA, in vivo pain behavioral assays","journal":"Science","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific KO, ChIP-based direct target validation, in vivo functional assays","pmids":["31320508"],"is_preprint":false},{"year":2019,"finding":"TLR/MyD88 signaling in skeletal muscle activates XBP1 (downstream of IRE1α) to promote muscle wasting during cancer cachexia; muscle-specific XBP1 deletion ameliorates muscle wasting; overexpression of active XBP1 causes myotube atrophy.","method":"Muscle-specific XBP1 KO mice, LLC tumor-bearing cachexia model, adenoviral XBP1 overexpression in myotubes, siRNA knockdown","journal":"Molecular and cellular biology","confidence":"High","confidence_rationale":"Tier 2 — tissue-specific KO with defined phenotype, gain-of-function validation","pmids":["31138662"],"is_preprint":false},{"year":2020,"finding":"Insulin-activated AKT directly phosphorylates IRE1 at S724, promoting XBP1u mRNA splicing to generate XBP1s in the liver; XBP1s stimulates lipogenic gene expression; conversely, XBP1u (predominant in fasting) increases PKA-stimulated gluconeogenic gene (G6pc, Pck1) expression and glucose production; the two isoforms thus regulate opposing metabolic programs in fed vs. fasted states.","method":"AKT kinase assay for IRE1 S724 phosphorylation, liver-specific XBP1 KO/reexpression, XBP1u/s overexpression in primary hepatocytes and mice, gluconeogenic gene expression assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro kinase assay, genetic rescue, isoform-specific functional dissection","pmids":["35863429"],"is_preprint":false},{"year":2020,"finding":"XBP1s binds to the hepatic 12-hour cistrome to directly regulate 12-hour oscillatory transcription of rate-limiting metabolic genes; liver-specific XBP1 ablation disrupts the hepatic 12-hour clock and promotes spontaneous NAFLD; XBP1 loss reduces membrane fluidity and impairs fatty acid monounsaturated and phospholipid remodeling pathways.","method":"Liver-specific XBP1 KO mice, ChIP-seq mapping of XBP1s binding sites, 12-hour rhythmicity analysis, phospholipid and membrane fluidity assays","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 2 — ChIP-seq, liver-specific KO, biochemical lipid assays, multiple methods","pmids":["33277471"],"is_preprint":false},{"year":2021,"finding":"Unspliced XBP1 (XBP1u) directly binds β-catenin via amino acids 205-230 in its C-terminal degradation domain, promoting β-catenin ubiquitin-proteasomal degradation (independently of GSK-3β), thereby inhibiting β-catenin/TCF-mediated Runx2 and Msx2 transcription and suppressing vascular calcification.","method":"Interactome analysis, Co-IP, domain mapping (aa 205-230), ubiquitin-proteasomal degradation assays, smooth muscle cell-specific XBP1 KO mice, β-catenin knockdown rescue","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 2 — Co-IP with domain mapping, in vivo KO, rescue experiment, ubiquitination assays","pmids":["34870453"],"is_preprint":false},{"year":2021,"finding":"IRE1α-XBP1 signaling in myofibers promotes skeletal muscle regeneration through a cell non-autonomous mechanism by augmenting satellite cell proliferation via Notch signaling and canonical NF-κB pathway.","method":"Myofiber-specific IRE1α/XBP1 KO mice, ex vivo myofiber satellite cell cultures, Notch and NF-κB pathway analysis, mdx muscular dystrophy model","journal":"eLife","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific KO, ex vivo cultures, defined signaling pathway, disease model","pmids":["34812145"],"is_preprint":false},{"year":2021,"finding":"XBP1 directly represses PUMA (a pro-apoptotic gene) and IRE1 mRNA expression during the UPR, representing a novel feedback repressive role that promotes cell survival and limits apoptosis.","method":"XBP1s-inducible human cell line, XBP1 silencing during pharmacological ER stress, next-generation RNA sequencing, bioinformatic analysis of XBP1-binding motifs, PUMA protein/cell death assays","journal":"Cellular and molecular life sciences","confidence":"Medium","confidence_rationale":"Tier 2 — inducible system, genome-wide sequencing, functional cell death assays, single lab","pmids":["34636989"],"is_preprint":false},{"year":2021,"finding":"XBP1 regulates macrophage BNIP3 transcription by directly binding to the Bnip3 promoter (shown by ChIP), inhibiting BNIP3-mediated mitophagy, thereby promoting mitochondrial DNA cytosolic leakage and cGAS/STING/NLRP3 activation, contributing to liver fibrosis.","method":"ChIP assay for XBP1 binding at Bnip3 promoter, myeloid-specific XBP1 KO mice, RNA sequencing, STING/NLRP3 pathway analysis, mitophagy assays","journal":"JHEP reports","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP, myeloid-specific KO, defined molecular pathway, single lab","pmids":["36185574"],"is_preprint":false},{"year":2022,"finding":"Unspliced XBP1 (XBP1-u) colocalizes with SREBP2 and inhibits its ubiquitination/proteasomal degradation, stabilizing SREBP2 to activate HMGCR transcription and enhance cholesterol biosynthesis in hepatocellular carcinoma.","method":"Co-IP/colocalization of XBP1-u with SREBP2, ubiquitination assays, HMGCR promoter activity, XBP1-u KD/OE in HCC cells","journal":"Cellular and molecular life sciences","confidence":"Medium","confidence_rationale":"Tier 2-3 — Co-IP, ubiquitination assay, functional cell assays, single lab","pmids":["35933495"],"is_preprint":false},{"year":2022,"finding":"Spliced XBP1 in macrophages directly binds the NLRP3 promoter and enhances its activity (shown by luciferase reporter assay), promoting NLRP3 inflammasome activation and contributing to renal ischemia/reperfusion injury.","method":"Luciferase reporter assay for XBP1 binding at NLRP3 promoter, XBP1 siRNA knockdown in TCMK-1 cells, in vivo renal IRI model","journal":"Cell death discovery","confidence":"Medium","confidence_rationale":"Tier 3 — luciferase reporter assay, in vivo model, but single lab and limited mechanistic validation","pmids":["36801911"],"is_preprint":false},{"year":2022,"finding":"XBP1 in cancer cells promotes cholesterol synthesis and secretion via small extracellular vesicles; cholesterol is internalized by MDSCs through macropinocytosis, activating MDSCs and causing immunosuppression; XBP1 depletion reduces MDSC abundance and triggers anti-tumor responses.","method":"XBP1 genetic/pharmacological depletion in cancer cells, extracellular vesicle isolation, cholesterol content measurement, MDSC functional assays, in vivo tumor models","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 2 — genetic and pharmacological KO, defined secretion/uptake mechanism, in vivo model","pmids":["36351432"],"is_preprint":false},{"year":2022,"finding":"IRE1 phosphorylates Pumilio (RNA-binding protein) in a kinase-dependent manner during ER stress; phosphorylated Pumilio then binds Xbp1-spliced mRNA and protects it from RIDD (regulated IRE1-dependent decay), selectively stabilizing the spliced XBP1 product.","method":"Drosophila genetics, RNA pull-down of Pumilio with Xbp1 mRNAs, in vitro kinase assay (hIRE1 phosphorylating Pumilio), RIDD assay, Pumilio phospho-mutant analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1-2 — in vitro kinase assay, genetic Drosophila experiments, RNA binding and mRNA stability assays","pmids":["35332141"],"is_preprint":false},{"year":2022,"finding":"XBP1 macrophage signaling inhibits hepatic IRI: macrophage Foxo1 colocalizes with XBP1 and activates target gene Zc3h15; XBP1 functions as a transcriptional coactivator of Foxo1 in regulating NOD1-driven liver inflammation and calcineurin/TRPM7-induced cell death.","method":"Myeloid-specific RIPK3/Foxo1 KO mice, ChIP-seq (Foxo1 colocalized with XBP1), adoptive macrophage transfer, hepatocyte/macrophage co-culture","journal":"JHEP reports","confidence":"Medium","confidence_rationale":"Tier 2 — ChIP-seq, multiple genetic KO, in vivo adoptive transfer, single lab","pmids":["37841640"],"is_preprint":false},{"year":2023,"finding":"Spliced XBP1 overexpression in the aged hippocampus restores synaptic function and reduces cell senescence; IRE1 genetic disruption accelerates age-related cognitive decline; XBP1 regulates expression of synaptic factors and proteostasis pathways in the brain.","method":"IRE1 KO mouse cognitive assays, XBP1 active form overexpression in aged mice, hippocampal proteomic profiling","journal":"The EMBO journal","confidence":"Medium","confidence_rationale":"Tier 2 — genetic KO and gain-of-function mouse models, proteomic profiling, defined behavioral phenotype","pmids":["36314651"],"is_preprint":false},{"year":2023,"finding":"In T cells in multiple myeloma, XBP1 directly binds the SLC38A2 (glutamine transporter) promoter and inhibits its expression, resulting in decreased glutamine uptake and immune dysfunction of cytotoxic T cells.","method":"Single-cell RNA sequencing, ChIP/promoter binding assay for XBP1 at SLC38A2 promoter, SLC38A2 silencing, glutamine uptake assays","journal":"Cancer letters","confidence":"Medium","confidence_rationale":"Tier 2-3 — promoter binding assay, functional glutamine assay, consistent with prior XBP1-glutamine transporter biology","pmids":["37054944"],"is_preprint":false},{"year":2023,"finding":"Overexpression of XBP1 increases Hrd1 (E3 ubiquitin ligase) expression; Hrd1 then interacts with and ubiquitinates Nrf2, promoting its degradation and increasing cell susceptibility to ferroptosis in diabetic nephropathy context; shown by Co-IP and ubiquitylation assays.","method":"Co-IP of Hrd1 with Nrf2, ubiquitylation assay, XBP1 overexpression, ferroptosis assays in HK-2 cells and DN mouse model","journal":"Biomedicine & pharmacotherapy","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP, ubiquitylation assay, single lab with defined molecular chain","pmids":["37224754"],"is_preprint":false},{"year":2024,"finding":"IRE1α/XBP1 signaling in ILC3s is selectively activated by IL-23 through mitochondrial ROS (mtROS); this activation is required for cytokine (IL-22) production by ILC3s; Ire1α deletion in ILC3s (Ire1αΔRorc mice) renders mice highly susceptible to infections and colitis.","method":"ILC3-specific Ire1α KO mice, IL-23 stimulation assays, mtROS manipulation, IL-22 cytokine measurement, human IBD specimens, IRE1 inhibitor studies","journal":"The Journal of clinical investigation","confidence":"High","confidence_rationale":"Tier 2 — cell-type-specific KO, defined upstream signal (IL-23/mtROS), human specimen validation","pmids":["38722686"],"is_preprint":false},{"year":2016,"finding":"PIN1 prolyl isomerase interacts with spliced XBP1 (XBP1s) in a phosphorylation-dependent manner and promotes XBP1s stability; PIN1 depletion reduces XBP1s expression and inhibits cell proliferation; XBP1s activates NF-κB, AP1, and Myc oncogenic pathways and also represses PIN1 transcription via p53 induction as a negative-feedback mechanism.","method":"Co-IP of PIN1 with XBP1s, phosphorylation-dependent interaction assays, PIN1 KD/KO, XBP1s stability measurement, oncogenic pathway reporter assays","journal":"The Biochemical journal","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP with functional follow-up, single lab","pmids":["27334111"],"is_preprint":false},{"year":2018,"finding":"Spliced XBP1 upregulates eNOS (NOS3) mRNA through 3'UTR-mediated stabilization and increased translation; XBP1s, Akt, and eNOS form a protein complex (shown by Co-IP), resulting in Akt and eNOS nuclear relocation; miR-24 participates in XBP1s-induced eNOS upregulation; XBP1 splicing triggered by wounding promotes endothelial cell migration.","method":"Co-IP of XBP1s/Akt/eNOS complex, immunofluorescence nuclear colocalization, XBP1s overexpression/IRE1α knockdown, 3'UTR reporter assay, wound healing/migration assays","journal":"Experimental cell research","confidence":"Medium","confidence_rationale":"Tier 3 — Co-IP, 3'UTR reporter, functional migration assays, single lab","pmids":["29352987"],"is_preprint":false},{"year":2009,"finding":"IRE1α controls cyclin A1 expression through XBP1: IRE1α activity correlates positively with cell proliferation; XBP1 siRNA knockdown slows proliferation; genome-wide expression analysis identified cyclin A1 as the only differentially expressed cell cycle gene downstream of IRE1α/XBP1.","method":"Molecular/chemical genetic IRE1α control, siRNA XBP1 knockdown, genome-wide mRNA expression, cyclin A1/cyclin A protein level assays","journal":"Cell stress & chaperones","confidence":"Medium","confidence_rationale":"Tier 2-3 — genome-wide screen, genetic manipulation, single lab","pmids":["20013084"],"is_preprint":false},{"year":2015,"finding":"Spliced XBP1 promotes macrophage autophagy; transient XBP1s overexpression induces autophagy and proliferation, whereas sustained overexpression leads to apoptosis; XBP1s interacts with Beclin-1 (Co-IP), and XBP1 downregulation ablates rapamycin-induced autophagosome formation.","method":"Adenoviral XBP1s overexpression, siRNA XBP1 knockdown, Co-IP of XBP1s with Beclin-1, autophagy assays","journal":"Biochemical and biophysical research communications","confidence":"Low","confidence_rationale":"Tier 3 — single Co-IP, single lab, limited mechanistic follow-up","pmids":["26026678"],"is_preprint":false},{"year":2018,"finding":"XBP1s promotes colorectal cancer cell proliferation by directly binding to the TAp73 promoter and suppressing its transcriptional activity; TAp73 overexpression cancels XBP1s-induced proliferation and colony formation.","method":"ChIP/promoter reporter for XBP1s at TAp73 promoter, XBP1s overexpression/knockdown, TAp73 rescue experiments","journal":"Biochemical and biophysical research communications","confidence":"Medium","confidence_rationale":"Tier 3 — ChIP/reporter, functional rescue, single lab","pmids":["30473215"],"is_preprint":false}],"current_model":"XBP1 is a bZIP transcription factor whose activity is primarily controlled by IRE1-mediated unconventional cytoplasmic splicing of XBP1 mRNA (facilitated by RtcB ligase, regulated by CNP/RtcA) during ER stress; the spliced isoform (XBP1s) translocates to the nucleus to activate genes governing ER biogenesis, protein folding, lipid synthesis, and cellular differentiation (plasma cells, eosinophils, adipocytes, osteoclasts), while forming transcriptional complexes with partners such as HIF1α and Foxo1; the unspliced isoform (XBP1u) has distinct functions including stabilizing MDM2 to suppress p53, promoting β-catenin degradation, and activating gluconeogenesis; the pathway is regulated upstream by ATF6 (which transcriptionally induces XBP1), by AKT phosphorylation of IRE1 at S724, by PERK-ATF4 enhancement of IRE1 levels, and post-transcriptionally by miR-30c-2*, while XBP1s stability is enhanced by PIN1 and spliced XBP1 mRNA is selectively protected from RIDD by IRE1-phosphorylated Pumilio."},"narrative":{"teleology":[{"year":2001,"claim":"The discovery that IRE1 performs unconventional cytoplasmic splicing of XBP1 mRNA—and that ATF6 transcriptionally induces XBP1—established the two-step activation mechanism that makes spliced XBP1 the principal transcriptional effector of the mammalian UPR.","evidence":"Molecular cloning, mRNA splicing assays, and reporter gene assays in mammalian cells","pmids":["11779464"],"confidence":"High","gaps":["RNA ligation enzyme performing the second step of splicing was unknown","nuclear translocation mechanism of XBP1s was undefined","genome-wide transcriptional targets not yet mapped"]},{"year":2003,"claim":"Demonstrating that IRE1-mediated XBP1 splicing is required for plasma cell differentiation and immunoglobulin secretion established XBP1 as a lineage-determining factor for secretory cells, not merely a stress-response effector.","evidence":"XBP1−/− mouse B cells reconstituted with spliced XBP1; genetic rescue; cytokine measurement","pmids":["12612580"],"confidence":"High","gaps":["Whether XBP1 controls other hematopoietic lineages was unknown","direct transcriptional targets in plasma cells not identified"]},{"year":2005,"claim":"Showing that XBP1u protein is constitutively made and rapidly degraded by the proteasome revealed a built-in dampening mechanism that prevents inappropriate UPR activation in unstressed cells.","evidence":"Protein stability assays, proteasome inhibitors, C-terminal mutagenesis, reconstitution of XBP1−/− MEFs","pmids":["16332684"],"confidence":"High","gaps":["Identity of the E3 ligase(s) targeting XBP1u was unknown","whether XBP1u has its own signaling functions was not addressed"]},{"year":2007,"claim":"Genome-wide ChIP mapping of XBP1 binding sites revealed a core set of ER-function target genes shared across tissues plus tissue-specific targets including the differentiation regulator Mist1, establishing XBP1 as a context-dependent transcriptional organizer.","evidence":"ChIP-chip, microarray expression profiling, reporter assays in skeletal muscle and secretory cells","pmids":["17612490"],"confidence":"High","gaps":["Tissue-specific cofactors enabling differential target selection were unidentified","chromatin-level mechanisms of target selectivity unknown"]},{"year":2009,"claim":"Identification of XBP1s as a direct activator of C/EBPα in adipogenesis, and of phosphatidylcholine biosynthesis for ER membrane expansion, extended XBP1's functional scope to lipid metabolism and developmental differentiation programs beyond stress response.","evidence":"XBP1-deficient MEFs and 3T3-L1 cells with isoform-specific rescue; promoter ChIP; phospholipid biosynthesis assays","pmids":["19490910","19420237"],"confidence":"High","gaps":["Whether XBP1 directly transactivates phospholipid biosynthetic genes or acts indirectly was unclear","relationship between ER expansion and differentiation timing not resolved"]},{"year":2014,"claim":"Discovery that XBP1 forms a transcriptional complex with HIF1α to co-regulate hypoxia-responsive genes in triple-negative breast cancer revealed a non-canonical partnership through which XBP1 drives tumorigenicity independently of canonical ER stress.","evidence":"ChIP-seq, Co-IP, RNA Pol II recruitment assays, XBP1 depletion in TNBC models","pmids":["24670641"],"confidence":"High","gaps":["Structural basis of the XBP1–HIF1α interaction undefined","whether this complex operates in non-cancer hypoxic contexts was untested"]},{"year":2015,"claim":"A series of cell-type-specific knockouts collectively established that XBP1 is required for eosinophil granule protein maturation, osteoclast differentiation via NFATc1 transactivation, podocyte homeostasis via PI3K p85-dependent nuclear import, and polycystin-1 maturation—each representing a distinct tissue-autonomous function.","evidence":"Eosinophil-specific, podocyte-specific, and osteoclast-lineage XBP1/IRE1α KO mice; ChIP at NFATc1 promoter; Co-IP of XBP1s with p85α/p85β; SEC63-XBP1 genetic epistasis and PC1 maturation rescue","pmids":["26147683","26193638","25754093","25844898"],"confidence":"High","gaps":["Whether p85-mediated nuclear import is universal or podocyte-specific was unknown","mechanism by which XBP1 controls GPS cleavage of PC1 not fully resolved"]},{"year":2015,"claim":"DC-specific XBP1 deletion showed that lipid-peroxidation-driven XBP1 activation in tumor-associated dendritic cells causes pathological triglyceride accumulation and immune suppression, establishing XBP1 as a cell-intrinsic checkpoint limiting anti-tumor immunity.","evidence":"DC-specific XBP1 KO mice, nanoparticle-mediated XBP1 silencing, lipid accumulation and T cell functional assays","pmids":["26073941"],"confidence":"High","gaps":["Direct lipogenic transcriptional targets of XBP1 in DCs not fully catalogued","applicability beyond ovarian cancer models untested"]},{"year":2016,"claim":"Identification of PIN1 as a phosphorylation-dependent stabilizer of XBP1s revealed a post-translational layer of regulation and a negative-feedback loop wherein XBP1s represses PIN1 transcription via p53.","evidence":"Co-IP of PIN1 with XBP1s, phosphorylation-dependent interaction assays, PIN1 KD/KO, XBP1s stability assays","pmids":["27334111"],"confidence":"Medium","gaps":["Specific phosphorylation site(s) on XBP1s recognized by PIN1 not mapped","single-lab finding without independent confirmation"]},{"year":2017,"claim":"Demonstrating that XBP1u stabilizes MDM2 by blocking its self-ubiquitination—thereby promoting p53 degradation and cell-cycle progression—revealed the first major oncogenic function attributable specifically to the unspliced isoform.","evidence":"shRNA screen, Co-IP of XBP1u with MDM2, domain mapping, ubiquitination assays, cell cycle analysis","pmids":["29057323"],"confidence":"High","gaps":["In vivo tumorigenesis models testing XBP1u-MDM2 axis not provided","stoichiometry and structural basis of the interaction unknown"]},{"year":2018,"claim":"Three studies collectively defined upstream and downstream wiring: MYC directly induces IRE1 and co-activates transcription with XBP1s; XBP1s induces five PDIs to control oxidative proinsulin folding in β cells; and RtcB was identified as the RNA ligase completing XBP1 splicing, with CNP and RtcA acting as opposing regulators of the ligation substrate.","evidence":"ChIP for MYC at IRE1 locus and Co-IP with XBP1; β-cell-specific IRE1α CKO and PDI reconstitution; in vitro XBP1 splicing reconstitution with RtcB/CNP/RtcA manipulation","pmids":["29480818","29507125","30355738"],"confidence":"High","gaps":["Relative contributions of CNP vs. RtcA in vivo not quantified","whether MYC-XBP1 co-regulation operates in normal (non-cancer) physiology unclear"]},{"year":2018,"claim":"Showing that XBP1 activation in tumor-infiltrating T cells suppresses mitochondrial respiration by limiting glutamine transporter abundance established a metabolic checkpoint through which ER stress impairs anti-tumor T cell function.","evidence":"XBP1-deficient T cells, metabolic flux and mitochondrial respiration assays, glutamine transporter expression, mouse ovarian cancer model and human specimens","pmids":["30305738"],"confidence":"High","gaps":["Identity of the specific XBP1-repressed glutamine transporter gene was not pinpointed until later work","whether this mechanism operates in other tumor types was untested"]},{"year":2020,"claim":"Discovery that AKT phosphorylates IRE1 at S724 to promote XBP1 splicing in the fed state, while XBP1u activates gluconeogenic genes during fasting, established the two XBP1 isoforms as metabolic switches controlling opposing hepatic metabolic programs.","evidence":"In vitro kinase assay, liver-specific XBP1 KO/reexpression, isoform-specific overexpression in primary hepatocytes and mice","pmids":["35863429"],"confidence":"High","gaps":["Whether other kinases besides AKT phosphorylate IRE1 S724 in vivo unclear","XBP1u gluconeogenic mechanism (direct vs. indirect) not fully resolved"]},{"year":2020,"claim":"ChIP-seq in liver revealed that XBP1s directly binds a 12-hour oscillatory cistrome controlling rate-limiting metabolic genes, and that liver-specific XBP1 loss disrupts ultradian rhythms, membrane fluidity, and lipid remodeling, promoting spontaneous NAFLD.","evidence":"Liver-specific XBP1 KO mice, ChIP-seq, 12-hour rhythmicity analysis, phospholipid and membrane fluidity assays","pmids":["33277471"],"confidence":"High","gaps":["What entrains XBP1s to a 12-hour cycle rather than circadian rhythm is unknown","relationship to circadian clock machinery not defined"]},{"year":2021,"claim":"Identification of XBP1u as a direct binding partner of β-catenin that promotes its ubiquitin-proteasomal degradation independently of GSK-3β expanded the non-transcriptional repertoire of the unspliced isoform to Wnt pathway regulation and vascular calcification suppression.","evidence":"Co-IP, domain mapping (aa 205–230), ubiquitination assays, smooth-muscle-cell-specific XBP1 KO mice, β-catenin knockdown rescue","pmids":["34870453"],"confidence":"High","gaps":["E3 ligase mediating XBP1u-promoted β-catenin ubiquitination not identified","whether this function is conserved outside vascular smooth muscle unknown"]},{"year":2022,"claim":"Demonstrating that IRE1 phosphorylates Pumilio, which then selectively protects spliced XBP1 mRNA from IRE1's own RIDD activity, revealed a built-in selectivity mechanism ensuring the productive splicing product survives the destructive arm of IRE1 signaling.","evidence":"Drosophila genetics, RNA pull-down, in vitro kinase assay (hIRE1 phosphorylating Pumilio), RIDD and mRNA stability assays, Pumilio phospho-mutant analysis","pmids":["35332141"],"confidence":"High","gaps":["Conservation of Pumilio-mediated protection in mammalian systems not demonstrated in vivo","specific Pumilio phosphorylation sites not fully characterized"]},{"year":2022,"claim":"Two studies showed XBP1 controls cholesterol metabolism: XBP1u stabilizes SREBP2 to drive HMGCR-dependent cholesterol biosynthesis in HCC, while XBP1 in cancer cells promotes cholesterol secretion via extracellular vesicles that activate immunosuppressive MDSCs.","evidence":"Co-IP of XBP1u with SREBP2, ubiquitination assays, HMGCR reporter; XBP1 genetic/pharmacological depletion, EV cholesterol assays, MDSC functional assays, in vivo tumor models","pmids":["35933495","36351432"],"confidence":"Medium","gaps":["Whether XBP1u-SREBP2 stabilization is direct or involves bridging factors unknown","mechanism linking XBP1 to vesicular cholesterol loading not resolved"]},{"year":2024,"claim":"Demonstrating that IL-23 activates IRE1α/XBP1 in ILC3s through mitochondrial ROS, and that this is required for IL-22 production and mucosal immunity, extended XBP1's immune functions to innate lymphoid cells and mucosal defense.","evidence":"ILC3-specific IRE1α KO mice, IL-23 stimulation, mtROS manipulation, IL-22 measurement, human IBD specimens, IRE1 inhibitor studies","pmids":["38722686"],"confidence":"High","gaps":["Direct transcriptional targets of XBP1 in ILC3s not mapped","how mtROS activates IRE1α mechanistically not defined"]},{"year":null,"claim":"Despite extensive characterization of tissue-specific functions, the structural basis of XBP1's interactions with diverse transcriptional partners (HIF1α, Foxo1, MYC), the E3 ligases targeting XBP1u and its binding partners (β-catenin, MDM2), and the in vivo relevance of Pumilio-mediated spliced mRNA protection in mammals remain unresolved.","evidence":"","pmids":[],"confidence":"Low","gaps":["No crystal or cryo-EM structure of XBP1 or its complexes","E3 ligases for XBP1u-mediated substrate degradation unidentified","mammalian in vivo validation of Pumilio-XBP1 mRNA protection lacking"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140110","term_label":"transcription regulator activity","supporting_discovery_ids":[0,3,4,7,12,14,15,21,24,28,30,35,42]},{"term_id":"GO:0003677","term_label":"DNA binding","supporting_discovery_ids":[3,4,7,12,14,21,24,28,35]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[16,25,29]}],"localization":[{"term_id":"GO:0005634","term_label":"nucleus","supporting_discovery_ids":[0,3,7,9,12,24]},{"term_id":"GO:0005829","term_label":"cytosol","supporting_discovery_ids":[2,16,25]}],"pathway":[{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[0,3,4,7,12,14,21,24]},{"term_id":"R-HSA-8953854","term_label":"Metabolism of RNA","supporting_discovery_ids":[0,20,32]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[0,2,5,19,27]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[1,8,10,17,37]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[5,23,24,29,31]},{"term_id":"R-HSA-1266738","term_label":"Developmental Biology","supporting_discovery_ids":[1,4,10,12]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,16,19,25,36]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[9,23,37]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[7,8,17,31]}],"complexes":[],"partners":["HIF1A","FOXO1","MYC","PIN1","PIK3R1","PIK3R2","MDM2","CTNNB1"],"other_free_text":[]},"mechanistic_narrative":"XBP1 is a bZIP transcription factor that functions as the principal effector of the IRE1α branch of the unfolded protein response, with broad roles in ER homeostasis, lipid metabolism, immune cell differentiation, and metabolic regulation. Upon ER stress, IRE1α cleaves XBP1 mRNA in the cytoplasm and RtcB ligase re-ligates the exons, generating a frameshifted spliced isoform (XBP1s) whose nuclear translocation—facilitated by PI3K p85 subunits—activates transcription of ER chaperones, protein disulfide isomerases, phospholipid biosynthetic enzymes, and cell-type-specific targets including NFATc1 in osteoclasts, C/EBPα in adipocytes, and IL-6 in plasma cells [PMID:11779464, PMID:30355738, PMID:25754093, PMID:19490910, PMID:26193638, PMID:12612580, PMID:29507125]. XBP1s also forms transcriptional complexes with HIF1α, Foxo1, and MYC to co-regulate context-dependent gene programs in cancer cells, immune cells, and hepatocytes, while governing hepatic 12-hour ultradian transcriptional rhythms that maintain membrane lipid homeostasis [PMID:24670641, PMID:37841640, PMID:29480818, PMID:33277471]. The constitutively produced unspliced isoform (XBP1u) exerts distinct non-transcriptional functions: it stabilizes MDM2 to promote p53 degradation, targets β-catenin for ubiquitin-proteasomal degradation, stabilizes SREBP2 to enhance cholesterol synthesis, and activates gluconeogenic gene expression during fasting [PMID:29057323, PMID:34870453, PMID:35933495, PMID:35863429]."},"prefetch_data":{"uniprot":{"accession":"P17861","full_name":"X-box-binding protein 1","aliases":["Tax-responsive element-binding protein 5","TREB-5"],"length_aa":261,"mass_kda":28.7,"function":"Functions as a transcription factor during endoplasmic reticulum (ER) stress by regulating the unfolded protein response (UPR). Required for cardiac myogenesis and hepatogenesis during embryonic development, and the development of secretory tissues such as exocrine pancreas and salivary gland (By similarity). Involved in terminal differentiation of B lymphocytes to plasma cells and production of immunoglobulins (PubMed:11460154). Modulates the cellular response to ER stress in a PIK3R-dependent manner (PubMed:20348923). Binds to the cis-acting X box present in the promoter regions of major histocompatibility complex class II genes (PubMed:8349596). Involved in VEGF-induced endothelial cell (EC) proliferation and retinal blood vessel formation during embryonic development but also for angiogenesis in adult tissues under ischemic conditions. Also functions as a major regulator of the UPR in obesity-induced insulin resistance and type 2 diabetes for the management of obesity and diabetes prevention (By similarity) Plays a role in the unconventional cytoplasmic splicing processing of its own mRNA triggered by the endoplasmic reticulum (ER) transmembrane endoribonuclease ERN1: upon ER stress, the emerging XBP1 polypeptide chain, as part of a mRNA-ribosome-nascent chain (R-RNC) complex, cotranslationally recruits its own unprocessed mRNA through transient docking to the ER membrane and translational pausing, therefore facilitating efficient IRE1-mediated XBP1 mRNA isoform 2 production (PubMed:19394296, PubMed:21233347). In endothelial cells (EC), associated with KDR, promotes IRE1-mediated XBP1 mRNA isoform 2 productions in a vascular endothelial growth factor (VEGF)-dependent manner, leading to EC proliferation and angiogenesis (PubMed:23529610). Functions as a negative feed-back regulator of the potent transcription factor XBP1 isoform 2 protein levels through proteasome-mediated degradation, thus preventing the constitutive activation of the ER stress response signaling pathway (PubMed:16461360, PubMed:25239945). Inhibits the transactivation activity of XBP1 isoform 2 in myeloma cells (By similarity). Acts as a weak transcriptional factor (PubMed:8657566). Together with HDAC3, contributes to the activation of NFE2L2-mediated HMOX1 transcription factor gene expression in a PI(3)K/mTORC2/Akt-dependent signaling pathway leading to EC survival under disturbed flow/oxidative stress (PubMed:25190803). Binds to the ER stress response element (ERSE) upon ER stress (PubMed:11779464). Binds to the consensus 5'-GATGACGTG[TG]N(3)[AT]T-3' sequence related to cAMP responsive element (CRE)-like sequences (PubMed:8657566). Binds the Tax-responsive element (TRE) present in the long terminal repeat (LTR) of T-cell leukemia virus type 1 (HTLV-I) and to the TPA response elements (TRE) (PubMed:1903538, PubMed:2196176, PubMed:2321018, PubMed:8657566). Associates preferentially to the HDAC3 gene promoter region in a static flow-dependent manner (PubMed:25190803). Binds to the CDH5/VE-cadherin gene promoter region (PubMed:19416856) Functions as a stress-inducible potent transcriptional activator during endoplasmic reticulum (ER) stress by inducing unfolded protein response (UPR) target genes via binding to the UPR element (UPRE). Up-regulates target genes encoding ER chaperones and ER-associated degradation (ERAD) components to enhance the capacity of productive folding and degradation mechanism, respectively, in order to maintain the homeostasis of the ER under ER stress (PubMed:11779464, PubMed:25239945). Plays a role in the production of immunoglobulins and interleukin-6 in the presence of stimuli required for plasma cell differentiation (By similarity). Induces phospholipid biosynthesis and ER expansion (PubMed:15466483). Contributes to the VEGF-induced endothelial cell (EC) growth and proliferation in a Akt/GSK-dependent and/or -independent signaling pathway, respectively, leading to beta-catenin nuclear translocation and E2F2 gene expression (PubMed:23529610). Promotes umbilical vein EC apoptosis and atherosclerotisis development in a caspase-dependent signaling pathway, and contributes to VEGF-induced EC proliferation and angiogenesis in adult tissues under ischemic conditions (PubMed:19416856, PubMed:23529610). Involved in the regulation of endostatin-induced autophagy in EC through BECN1 transcriptional activation (PubMed:23184933). Plays a role as an oncogene by promoting tumor progression: stimulates zinc finger protein SNAI1 transcription to induce epithelial-to-mesenchymal (EMT) transition, cell migration and invasion of breast cancer cells (PubMed:25280941). Involved in adipocyte differentiation by regulating lipogenic gene expression during lactation. Plays a role in the survival of both dopaminergic neurons of the substantia nigra pars compacta (SNpc), by maintaining protein homeostasis and of myeloma cells. Increases insulin sensitivity in the liver as a response to a high carbohydrate diet, resulting in improved glucose tolerance. Also improves glucose homeostasis in an ER stress- and/or insulin-independent manner through both binding and proteasome-induced degradation of the transcription factor FOXO1, hence resulting in suppression of gluconeogenic genes expression and in a reduction of blood glucose levels. Controls the induction of de novo fatty acid synthesis in hepatocytes by regulating the expression of a subset of lipogenic genes in an ER stress- and UPR-independent manner (By similarity). Associates preferentially to the HDAC3 gene promoter region in a disturbed flow-dependent manner (PubMed:25190803). Binds to the BECN1 gene promoter region (PubMed:23184933). Binds to the CDH5/VE-cadherin gene promoter region (PubMed:19416856). Binds to the ER stress response element (ERSE) upon ER stress (PubMed:11779464). Binds to the 5'-CCACG-3' motif in the PPARG promoter (By similarity). Binds to the HEPN1 gene promoter region and activates transcription (PubMed:25448679)","subcellular_location":"Cytoplasm; Nucleus","url":"https://www.uniprot.org/uniprotkb/P17861/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/XBP1","classification":"Not Classified","n_dependent_lines":6,"n_total_lines":1208,"dependency_fraction":0.004966887417218543},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/XBP1","total_profiled":1310},"omim":[{"mim_id":"621097","title":"IMMUNODEFICIENCY 131; IMD131","url":"https://www.omim.org/entry/621097"},{"mim_id":"621029","title":"RING FINGER PROTEIN 183; RNF183","url":"https://www.omim.org/entry/621029"},{"mim_id":"620714","title":"DEAFNESS, AUTOSOMAL RECESSIVE 122; DFNB122","url":"https://www.omim.org/entry/620714"},{"mim_id":"620670","title":"IMMUNODEFICIENCY, COMMON VARIABLE, 15; CVID15","url":"https://www.omim.org/entry/620670"},{"mim_id":"620534","title":"NEUTROPENIA, SEVERE CONGENITAL, 10, AUTOSOMAL RECESSIVE; SCN10","url":"https://www.omim.org/entry/620534"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in all","driving_tissues":[{"tissue":"pancreas","ntpm":1102.3},{"tissue":"salivary gland","ntpm":792.7}],"url":"https://www.proteinatlas.org/search/XBP1"},"hgnc":{"alias_symbol":[],"prev_symbol":["XBP2"]},"alphafold":{"accession":"P17861","domains":[],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P17861","model_url":"https://alphafold.ebi.ac.uk/files/AF-P17861-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P17861-F1-predicted_aligned_error_v6.png","plddt_mean":68.31},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=XBP1","jax_strain_url":"https://www.jax.org/strain/search?query=XBP1"},"sequence":{"accession":"P17861","fasta_url":"https://rest.uniprot.org/uniprotkb/P17861.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P17861/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P17861"}},"corpus_meta":[{"pmid":"11779464","id":"PMC_11779464","title":"XBP1 mRNA is induced by ATF6 and spliced by IRE1 in response to ER stress to produce a highly active transcription factor.","date":"2001","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/11779464","citation_count":3312,"is_preprint":false},{"pmid":"26073941","id":"PMC_26073941","title":"ER Stress Sensor XBP1 Controls Anti-tumor Immunity by Disrupting Dendritic Cell Homeostasis.","date":"2015","source":"Cell","url":"https://pubmed.ncbi.nlm.nih.gov/26073941","citation_count":744,"is_preprint":false},{"pmid":"12612580","id":"PMC_12612580","title":"Plasma cell differentiation and the unfolded protein response intersect at the transcription factor XBP-1.","date":"2003","source":"Nature immunology","url":"https://pubmed.ncbi.nlm.nih.gov/12612580","citation_count":716,"is_preprint":false},{"pmid":"24670641","id":"PMC_24670641","title":"XBP1 promotes triple-negative breast cancer by controlling the HIF1α pathway.","date":"2014","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/24670641","citation_count":697,"is_preprint":false},{"pmid":"17612490","id":"PMC_17612490","title":"XBP1 controls diverse cell type- and condition-specific transcriptional regulatory networks.","date":"2007","source":"Molecular cell","url":"https://pubmed.ncbi.nlm.nih.gov/17612490","citation_count":686,"is_preprint":false},{"pmid":"15342372","id":"PMC_15342372","title":"XBP1 is essential for survival under hypoxic conditions and is required for tumor growth.","date":"2004","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/15342372","citation_count":457,"is_preprint":false},{"pmid":"30305738","id":"PMC_30305738","title":"IRE1α-XBP1 controls T cell function in ovarian cancer by regulating mitochondrial activity.","date":"2018","source":"Nature","url":"https://pubmed.ncbi.nlm.nih.gov/30305738","citation_count":345,"is_preprint":false},{"pmid":"19490910","id":"PMC_19490910","title":"The IRE1alpha-XBP1 pathway of the unfolded protein response is required for adipogenesis.","date":"2009","source":"Cell metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/19490910","citation_count":240,"is_preprint":false},{"pmid":"19420237","id":"PMC_19420237","title":"ATF6alpha induces XBP1-independent expansion of the endoplasmic reticulum.","date":"2009","source":"Journal of cell science","url":"https://pubmed.ncbi.nlm.nih.gov/19420237","citation_count":234,"is_preprint":false},{"pmid":"14960590","id":"PMC_14960590","title":"Hepatitis C virus suppresses the IRE1-XBP1 pathway of the unfolded protein response.","date":"2004","source":"The Journal of biological chemistry","url":"https://pubmed.ncbi.nlm.nih.gov/14960590","citation_count":193,"is_preprint":false},{"pmid":"29480818","id":"PMC_29480818","title":"Pharmacological targeting of MYC-regulated IRE1/XBP1 pathway suppresses MYC-driven breast cancer.","date":"2018","source":"The Journal of clinical investigation","url":"https://pubmed.ncbi.nlm.nih.gov/29480818","citation_count":190,"is_preprint":false},{"pmid":"25754093","id":"PMC_25754093","title":"Defective podocyte insulin signalling through p85-XBP1 promotes ATF6-dependent maladaptive ER-stress response in diabetic nephropathy.","date":"2015","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/25754093","citation_count":155,"is_preprint":false},{"pmid":"26147683","id":"PMC_26147683","title":"The transcription factor XBP1 is selectively required for eosinophil differentiation.","date":"2015","source":"Nature immunology","url":"https://pubmed.ncbi.nlm.nih.gov/26147683","citation_count":151,"is_preprint":false},{"pmid":"25986851","id":"PMC_25986851","title":"Targeting the IRE1α-XBP1 branch of the unfolded protein response in human diseases.","date":"2015","source":"Seminars in cancer biology","url":"https://pubmed.ncbi.nlm.nih.gov/25986851","citation_count":148,"is_preprint":false},{"pmid":"36351432","id":"PMC_36351432","title":"Cancer cell-intrinsic XBP1 drives immunosuppressive reprogramming of intratumoral myeloid cells by promoting cholesterol production.","date":"2022","source":"Cell metabolism","url":"https://pubmed.ncbi.nlm.nih.gov/36351432","citation_count":139,"is_preprint":false},{"pmid":"35292349","id":"PMC_35292349","title":"Role of XBP1 in regulating the progression of non-alcoholic steatohepatitis.","date":"2022","source":"Journal of hepatology","url":"https://pubmed.ncbi.nlm.nih.gov/35292349","citation_count":114,"is_preprint":false},{"pmid":"31320508","id":"PMC_31320508","title":"IRE1α-XBP1 signaling in leukocytes controls prostaglandin biosynthesis and pain.","date":"2019","source":"Science (New York, 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Only the spliced form efficiently activates UPR target genes.\",\n      \"method\": \"Molecular cloning, mRNA splicing assays, reporter gene assays, identification of ATF6 as transcriptional inducer of XBP1\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — foundational mechanistic study, >3000 citations, multiple orthogonal methods\",\n      \"pmids\": [\"11779464\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"IRE1-mediated splicing of XBP1 mRNA is required for plasma cell differentiation and immunoglobulin production; signals from B cell differentiation (IL-4 controls XBP1 transcription; immunoglobulin synthesis controls post-transcriptional XBP1 processing); spliced XBP1 also controls IL-6 production essential for plasma cell survival.\",\n      \"method\": \"XBP1-/- mouse B cells reconstituted with spliced XBP1; genetic rescue experiments; cytokine measurement\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined cellular phenotype, genetic rescue, replicated across studies\",\n      \"pmids\": [\"12612580\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Unspliced XBP1 (XBP1u) protein is constitutively synthesized and rapidly degraded by proteasomal mechanisms (both ubiquitin-dependent and ubiquitin-independent); the C-terminus of XBP1u mediates its cytoplasmic trafficking and rapid degradation; XBP1u degradation prevents uncontrolled UPR activation.\",\n      \"method\": \"Protein stability assays, proteasome inhibitor treatment, mutagenesis of XBP1u C-terminus, reconstitution of XBP1-/- MEFs with XBP1u stability mutants\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — multiple orthogonal methods including mutagenesis and functional reconstitution\",\n      \"pmids\": [\"16332684\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Genome-wide mapping of XBP1 transcriptional targets in skeletal muscle and secretory cells identified a core group of ER function genes regulated in all cell types, plus tissue-specific targets; XBP1 regulates distinct target sets through different sequence motifs; XBP1 directly targets Mist1 (a regulator of differentiation), linking XBP1 loss-of-function to developmental defects.\",\n      \"method\": \"Genome-wide ChIP, microarray expression profiling, reporter assays\",\n      \"journal\": \"Molecular cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genome-wide approach with multiple orthogonal methods, highly cited\",\n      \"pmids\": [\"17612490\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"The IRE1α-XBP1 pathway is required for adipogenesis: C/EBPβ directly binds the Xbp1 promoter to induce its expression; spliced XBP1 then binds the Cebpa promoter to activate C/EBPα expression; only spliced XBP1 rescues adipogenic defects in XBP1-deficient cells.\",\n      \"method\": \"XBP1-deficient MEFs and XBP1/IRE1α knockdown 3T3-L1 cells; promoter ChIP; rescue with spliced vs. unspliced XBP1\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO with defined phenotype, ChIP, isoform-specific rescue\",\n      \"pmids\": [\"19490910\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Spliced XBP1 (XBP1s) augments the CDP-choline pathway for phosphatidylcholine biosynthesis and drives ER membrane biogenesis; ATF6α can also drive ER expansion and PtdCho biosynthesis independently of XBP1s, through partially distinct mechanisms.\",\n      \"method\": \"Overexpression of constitutively active ATF6α or XBP1s; phospholipid biosynthesis assays; ER morphology analysis; XBP1-deficient cells\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — biochemical lipid assays, genetic controls, multiple cell systems\",\n      \"pmids\": [\"19420237\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2012,\n      \"finding\": \"miR-30c-2* (miR-30c-2-3p) is induced by the PERK pathway of the UPR and directly suppresses XBP1 expression post-transcriptionally, thereby limiting the magnitude of XBP1-mediated adaptive UPR gene expression and influencing cell fate.\",\n      \"method\": \"miRNA overexpression/inhibition, reporter assays, PERK pathway genetic manipulation, UPR target gene expression analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — direct regulatory link between PERK-induced miRNA and XBP1, single lab with multiple methods\",\n      \"pmids\": [\"22431749\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"XBP1 drives triple-negative breast cancer tumorigenicity by forming a transcriptional complex with HIF1α that recruits RNA polymerase II to regulate HIF1α target genes; genome-wide mapping defined this XBP1-HIF1α regulatory network.\",\n      \"method\": \"ChIP-seq, Co-IP, RNA pol II recruitment assays, XBP1 depletion in TNBC models, patient cohort analysis\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — reconstituted complex, genome-wide ChIP-seq, functional KD, high citations\",\n      \"pmids\": [\"24670641\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"XBP1 activation in tumor-associated dendritic cells (driven by lipid peroxidation byproducts) induces a triglyceride biosynthetic program leading to abnormal lipid accumulation and inhibition of DC immunostimulatory capacity; DC-specific XBP1 deletion restores anti-tumor T cell responses.\",\n      \"method\": \"DC-specific XBP1 deletion mouse model, nanoparticle-mediated XBP1 silencing, lipid accumulation assays, T cell functional assays\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with defined immunological phenotype, multiple orthogonal methods\",\n      \"pmids\": [\"26073941\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Nuclear translocation of spliced XBP1 (sXBP1) is impaired in diabetic nephropathy podocytes; sXBP1 lies downstream of insulin receptor/PI3K p85α/p85β signaling; interaction of sXBP1 with p85α and p85β is required for its nuclear translocation and podocyte homeostasis; disruption promotes ATF6/CHOP-mediated maladaptive ER stress.\",\n      \"method\": \"Podocyte-specific XBP1 KO mice, insulin receptor/p85 KO models, Co-IP of sXBP1 with p85 subunits, nuclear fractionation\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple genetic KO models, Co-IP, functional nuclear localization assays\",\n      \"pmids\": [\"25754093\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"XBP1 is selectively required for eosinophil differentiation: eosinophil progenitors selectively activate IRE1α and splice Xbp1 mRNA without inducing parallel ER stress pathways; without XBP1, nascent eosinophils show massive defects in post-translational maturation of granule proteins.\",\n      \"method\": \"XBP1-deficient mouse models, lineage-specific analysis, granule protein maturation assays, IRE1α activation assays\",\n      \"journal\": \"Nature immunology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — clean KO with defined differentiation phenotype and molecular mechanism\",\n      \"pmids\": [\"26147683\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"SEC63 exists in a complex with polycystin-1 (PC1) and with IRE1α; SEC63 deficiency selectively activates IRE1α-XBP1; XBP1 is required for GPS cleavage and maturation of PC1; enforced XBP1s expression enhances PC1 GPS cleavage and ameliorates cystic disease.\",\n      \"method\": \"Murine genetic models with combined SEC63/XBP1 inactivation, Co-IP of SEC63 with PC1, XBP1s overexpression, in vivo cystic phenotype rescue\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal Co-IP, genetic epistasis, in vivo rescue\",\n      \"pmids\": [\"25844898\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"IRE1α/XBP1 signaling regulates osteoclastogenesis: XBP1 directly binds the NFATc1 promoter and induces its transcription; IRE1α activation during osteoclastogenesis is partially dependent on Ca2+ oscillations mediated by IP3 receptors ITPR2 and ITPR3; conditional IRE1α ablation in bone marrow increases bone mass due to defective osteoclastic bone resorption.\",\n      \"method\": \"ChIP for XBP1 binding at NFATc1 promoter; conditional IRE1α KO mice; pharmacological/genetic inhibition of ITPR2/3; osteoclast differentiation assays\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP, genetic KO with defined phenotype, upstream signaling identified\",\n      \"pmids\": [\"26193638\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The PERK-ATF4 pathway enhances IRE1α-XBP1 signaling by inducing IRE1α expression under ER stress, increasing the splicing ratio of XBP1 mRNA and prolonging the duration of pathway activation.\",\n      \"method\": \"IRE1α expression measurement under PERK/ATF4 pathway activation, XBP1 splicing ratio assays, genetic manipulation of PERK pathway\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional cross-pathway regulation with splicing assays, single lab\",\n      \"pmids\": [\"27052593\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"XBP1 binds the IL-6 promoter and activates its expression; IRE1α RNase activity is required for IL-6 induction; activated IRE1α/XBP1 pathway promotes melanoma cell proliferation via autocrine/paracrine IL-6/STAT3 signaling.\",\n      \"method\": \"ChIP for XBP1 at IL-6 promoter, IRE1α RNase inhibition, ectopic XBP1s expression, JAK/STAT3 pathway analysis\",\n      \"journal\": \"Journal of translational medicine\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP validation, functional IRE1α inhibition, single lab\",\n      \"pmids\": [\"28222747\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"LOXL2 accumulates in the ER and interacts with HSPA5 (BiP), leading to IRE1-XBP1 pathway activation; XBP1 then directly transcriptionally activates EMT transcription factors SNAI1, SNAI2, ZEB2, and TCF3; IRE1 inhibition blocks LOXL2-dependent EMT.\",\n      \"method\": \"Co-IP of LOXL2 with HSPA5, ChIP for XBP1 at EMT-TF promoters, IRE1 inhibitor experiments, LOXL2 overexpression\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP, ChIP, pharmacological inhibition, single lab\",\n      \"pmids\": [\"28332555\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Unspliced XBP1 (XBP1-u) stabilizes MDM2 protein by binding to it (via its unique C-terminal degradation domain) and inhibiting MDM2 self-ubiquitination, thereby enhancing p53 ubiquitination and degradation, downregulating p21, and promoting cell cycle progression and tumorigenesis.\",\n      \"method\": \"shRNA library screen, Co-IP of XBP1-u with MDM2, ubiquitination assays, domain mapping, cell cycle analysis\",\n      \"journal\": \"Science advances\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP, ubiquitination assay, domain mutagenesis, isoform specificity established\",\n      \"pmids\": [\"29057323\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"IRE1α-XBP1 activation in ovarian cancer-infiltrating T cells suppresses mitochondrial respiration and IFNγ production by limiting glutamine carrier abundance, restricting glutamine influx needed for mitochondrial respiration under glucose-depleted conditions; XBP1-deficient T cells show transcriptional reprogramming and improved anti-tumor effector capacity.\",\n      \"method\": \"XBP1-deficient T cells, metabolic flux assays, glutamine transporter expression analysis, mitochondrial respiration assays, patient specimen analysis, mouse OvCa model\",\n      \"journal\": \"Nature\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — T cell-specific KO, mechanistic metabolomics, in vivo mouse model, human specimens\",\n      \"pmids\": [\"30305738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"MYC directly controls IRE1 transcription by binding to its promoter and enhancer; MYC also forms a transcriptional complex with spliced XBP1 and enhances its transcriptional activity; XBP1 is a synthetic lethal partner of MYC.\",\n      \"method\": \"ChIP for MYC at IRE1 promoter/enhancer, Co-IP of MYC with XBP1, synthetic lethality screen, patient-derived xenograft models\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP, Co-IP, in vivo PDX models, multiple orthogonal methods\",\n      \"pmids\": [\"29480818\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"IRE1α-XBP1 pathway in pancreatic β cells regulates oxidative proinsulin folding by inducing expression of five protein disulfide isomerases (PDI, PDIR, P5, ERp44, ERp46); IRE1α CKO mice exhibit diabetic phenotype with decreased insulin secretion and impaired proinsulin folding; reconstitution of the IRE1α-XBP1 pathway restores PDI expression, proinsulin content, and insulin secretion.\",\n      \"method\": \"β cell-specific Ire1α conditional KO mice, Cre-loxP insulinoma cell lines, PDI expression assays, proinsulin folding assays, IRE1α-XBP1 reconstitution\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with defined molecular targets and functional rescue\",\n      \"pmids\": [\"29507125\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"The RNA ligation step of XBP1 mRNA splicing is performed by RtcB ligase; CNP (2',3'-cyclic nucleotide phosphodiesterase) suppresses RtcB-mediated XBP1 splicing by hydrolyzing the 2',3'-cyclic phosphate on cleaved exon termini; RtcA (RNA 3'-terminal cyclase) facilitates splicing by converting 2'-phosphate back to 2',3'-cyclic phosphate; mammalian Trpt1 does not contribute to XBP1 splicing.\",\n      \"method\": \"In vitro XBP1 splicing assays, CNP/RtcA/Trpt1 genetic manipulation in mouse and human cell lines\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution splicing assays with defined enzymatic components\",\n      \"pmids\": [\"30355738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"Functional XBP1 directly transactivates PTGS2 (Cox-2) and PTGES (mPGES-1) genes to enable optimal prostaglandin E2 biosynthesis in myeloid cells; IRE1α-deficient or XBP1-deficient myeloid cells show compromised inducible PGE2 biosynthesis; mice lacking IRE1α-XBP1 in leukocytes show reduced pain behaviors in PGE2-dependent pain models.\",\n      \"method\": \"Myeloid-specific IRE1α/XBP1 KO mice, ChIP/reporter assays for XBP1 binding at PTGS2/PTGES promoters, PGE2 ELISA, in vivo pain behavioral assays\",\n      \"journal\": \"Science\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO, ChIP-based direct target validation, in vivo functional assays\",\n      \"pmids\": [\"31320508\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"TLR/MyD88 signaling in skeletal muscle activates XBP1 (downstream of IRE1α) to promote muscle wasting during cancer cachexia; muscle-specific XBP1 deletion ameliorates muscle wasting; overexpression of active XBP1 causes myotube atrophy.\",\n      \"method\": \"Muscle-specific XBP1 KO mice, LLC tumor-bearing cachexia model, adenoviral XBP1 overexpression in myotubes, siRNA knockdown\",\n      \"journal\": \"Molecular and cellular biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — tissue-specific KO with defined phenotype, gain-of-function validation\",\n      \"pmids\": [\"31138662\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Insulin-activated AKT directly phosphorylates IRE1 at S724, promoting XBP1u mRNA splicing to generate XBP1s in the liver; XBP1s stimulates lipogenic gene expression; conversely, XBP1u (predominant in fasting) increases PKA-stimulated gluconeogenic gene (G6pc, Pck1) expression and glucose production; the two isoforms thus regulate opposing metabolic programs in fed vs. fasted states.\",\n      \"method\": \"AKT kinase assay for IRE1 S724 phosphorylation, liver-specific XBP1 KO/reexpression, XBP1u/s overexpression in primary hepatocytes and mice, gluconeogenic gene expression assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro kinase assay, genetic rescue, isoform-specific functional dissection\",\n      \"pmids\": [\"35863429\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"XBP1s binds to the hepatic 12-hour cistrome to directly regulate 12-hour oscillatory transcription of rate-limiting metabolic genes; liver-specific XBP1 ablation disrupts the hepatic 12-hour clock and promotes spontaneous NAFLD; XBP1 loss reduces membrane fluidity and impairs fatty acid monounsaturated and phospholipid remodeling pathways.\",\n      \"method\": \"Liver-specific XBP1 KO mice, ChIP-seq mapping of XBP1s binding sites, 12-hour rhythmicity analysis, phospholipid and membrane fluidity assays\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-seq, liver-specific KO, biochemical lipid assays, multiple methods\",\n      \"pmids\": [\"33277471\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Unspliced XBP1 (XBP1u) directly binds β-catenin via amino acids 205-230 in its C-terminal degradation domain, promoting β-catenin ubiquitin-proteasomal degradation (independently of GSK-3β), thereby inhibiting β-catenin/TCF-mediated Runx2 and Msx2 transcription and suppressing vascular calcification.\",\n      \"method\": \"Interactome analysis, Co-IP, domain mapping (aa 205-230), ubiquitin-proteasomal degradation assays, smooth muscle cell-specific XBP1 KO mice, β-catenin knockdown rescue\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — Co-IP with domain mapping, in vivo KO, rescue experiment, ubiquitination assays\",\n      \"pmids\": [\"34870453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"IRE1α-XBP1 signaling in myofibers promotes skeletal muscle regeneration through a cell non-autonomous mechanism by augmenting satellite cell proliferation via Notch signaling and canonical NF-κB pathway.\",\n      \"method\": \"Myofiber-specific IRE1α/XBP1 KO mice, ex vivo myofiber satellite cell cultures, Notch and NF-κB pathway analysis, mdx muscular dystrophy model\",\n      \"journal\": \"eLife\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO, ex vivo cultures, defined signaling pathway, disease model\",\n      \"pmids\": [\"34812145\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"XBP1 directly represses PUMA (a pro-apoptotic gene) and IRE1 mRNA expression during the UPR, representing a novel feedback repressive role that promotes cell survival and limits apoptosis.\",\n      \"method\": \"XBP1s-inducible human cell line, XBP1 silencing during pharmacological ER stress, next-generation RNA sequencing, bioinformatic analysis of XBP1-binding motifs, PUMA protein/cell death assays\",\n      \"journal\": \"Cellular and molecular life sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — inducible system, genome-wide sequencing, functional cell death assays, single lab\",\n      \"pmids\": [\"34636989\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"XBP1 regulates macrophage BNIP3 transcription by directly binding to the Bnip3 promoter (shown by ChIP), inhibiting BNIP3-mediated mitophagy, thereby promoting mitochondrial DNA cytosolic leakage and cGAS/STING/NLRP3 activation, contributing to liver fibrosis.\",\n      \"method\": \"ChIP assay for XBP1 binding at Bnip3 promoter, myeloid-specific XBP1 KO mice, RNA sequencing, STING/NLRP3 pathway analysis, mitophagy assays\",\n      \"journal\": \"JHEP reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP, myeloid-specific KO, defined molecular pathway, single lab\",\n      \"pmids\": [\"36185574\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Unspliced XBP1 (XBP1-u) colocalizes with SREBP2 and inhibits its ubiquitination/proteasomal degradation, stabilizing SREBP2 to activate HMGCR transcription and enhance cholesterol biosynthesis in hepatocellular carcinoma.\",\n      \"method\": \"Co-IP/colocalization of XBP1-u with SREBP2, ubiquitination assays, HMGCR promoter activity, XBP1-u KD/OE in HCC cells\",\n      \"journal\": \"Cellular and molecular life sciences\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — Co-IP, ubiquitination assay, functional cell assays, single lab\",\n      \"pmids\": [\"35933495\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"Spliced XBP1 in macrophages directly binds the NLRP3 promoter and enhances its activity (shown by luciferase reporter assay), promoting NLRP3 inflammasome activation and contributing to renal ischemia/reperfusion injury.\",\n      \"method\": \"Luciferase reporter assay for XBP1 binding at NLRP3 promoter, XBP1 siRNA knockdown in TCMK-1 cells, in vivo renal IRI model\",\n      \"journal\": \"Cell death discovery\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — luciferase reporter assay, in vivo model, but single lab and limited mechanistic validation\",\n      \"pmids\": [\"36801911\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"XBP1 in cancer cells promotes cholesterol synthesis and secretion via small extracellular vesicles; cholesterol is internalized by MDSCs through macropinocytosis, activating MDSCs and causing immunosuppression; XBP1 depletion reduces MDSC abundance and triggers anti-tumor responses.\",\n      \"method\": \"XBP1 genetic/pharmacological depletion in cancer cells, extracellular vesicle isolation, cholesterol content measurement, MDSC functional assays, in vivo tumor models\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological KO, defined secretion/uptake mechanism, in vivo model\",\n      \"pmids\": [\"36351432\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"IRE1 phosphorylates Pumilio (RNA-binding protein) in a kinase-dependent manner during ER stress; phosphorylated Pumilio then binds Xbp1-spliced mRNA and protects it from RIDD (regulated IRE1-dependent decay), selectively stabilizing the spliced XBP1 product.\",\n      \"method\": \"Drosophila genetics, RNA pull-down of Pumilio with Xbp1 mRNAs, in vitro kinase assay (hIRE1 phosphorylating Pumilio), RIDD assay, Pumilio phospho-mutant analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro kinase assay, genetic Drosophila experiments, RNA binding and mRNA stability assays\",\n      \"pmids\": [\"35332141\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"XBP1 macrophage signaling inhibits hepatic IRI: macrophage Foxo1 colocalizes with XBP1 and activates target gene Zc3h15; XBP1 functions as a transcriptional coactivator of Foxo1 in regulating NOD1-driven liver inflammation and calcineurin/TRPM7-induced cell death.\",\n      \"method\": \"Myeloid-specific RIPK3/Foxo1 KO mice, ChIP-seq (Foxo1 colocalized with XBP1), adoptive macrophage transfer, hepatocyte/macrophage co-culture\",\n      \"journal\": \"JHEP reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — ChIP-seq, multiple genetic KO, in vivo adoptive transfer, single lab\",\n      \"pmids\": [\"37841640\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Spliced XBP1 overexpression in the aged hippocampus restores synaptic function and reduces cell senescence; IRE1 genetic disruption accelerates age-related cognitive decline; XBP1 regulates expression of synaptic factors and proteostasis pathways in the brain.\",\n      \"method\": \"IRE1 KO mouse cognitive assays, XBP1 active form overexpression in aged mice, hippocampal proteomic profiling\",\n      \"journal\": \"The EMBO journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic KO and gain-of-function mouse models, proteomic profiling, defined behavioral phenotype\",\n      \"pmids\": [\"36314651\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"In T cells in multiple myeloma, XBP1 directly binds the SLC38A2 (glutamine transporter) promoter and inhibits its expression, resulting in decreased glutamine uptake and immune dysfunction of cytotoxic T cells.\",\n      \"method\": \"Single-cell RNA sequencing, ChIP/promoter binding assay for XBP1 at SLC38A2 promoter, SLC38A2 silencing, glutamine uptake assays\",\n      \"journal\": \"Cancer letters\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — promoter binding assay, functional glutamine assay, consistent with prior XBP1-glutamine transporter biology\",\n      \"pmids\": [\"37054944\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Overexpression of XBP1 increases Hrd1 (E3 ubiquitin ligase) expression; Hrd1 then interacts with and ubiquitinates Nrf2, promoting its degradation and increasing cell susceptibility to ferroptosis in diabetic nephropathy context; shown by Co-IP and ubiquitylation assays.\",\n      \"method\": \"Co-IP of Hrd1 with Nrf2, ubiquitylation assay, XBP1 overexpression, ferroptosis assays in HK-2 cells and DN mouse model\",\n      \"journal\": \"Biomedicine & pharmacotherapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP, ubiquitylation assay, single lab with defined molecular chain\",\n      \"pmids\": [\"37224754\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"IRE1α/XBP1 signaling in ILC3s is selectively activated by IL-23 through mitochondrial ROS (mtROS); this activation is required for cytokine (IL-22) production by ILC3s; Ire1α deletion in ILC3s (Ire1αΔRorc mice) renders mice highly susceptible to infections and colitis.\",\n      \"method\": \"ILC3-specific Ire1α KO mice, IL-23 stimulation assays, mtROS manipulation, IL-22 cytokine measurement, human IBD specimens, IRE1 inhibitor studies\",\n      \"journal\": \"The Journal of clinical investigation\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO, defined upstream signal (IL-23/mtROS), human specimen validation\",\n      \"pmids\": [\"38722686\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"PIN1 prolyl isomerase interacts with spliced XBP1 (XBP1s) in a phosphorylation-dependent manner and promotes XBP1s stability; PIN1 depletion reduces XBP1s expression and inhibits cell proliferation; XBP1s activates NF-κB, AP1, and Myc oncogenic pathways and also represses PIN1 transcription via p53 induction as a negative-feedback mechanism.\",\n      \"method\": \"Co-IP of PIN1 with XBP1s, phosphorylation-dependent interaction assays, PIN1 KD/KO, XBP1s stability measurement, oncogenic pathway reporter assays\",\n      \"journal\": \"The Biochemical journal\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP with functional follow-up, single lab\",\n      \"pmids\": [\"27334111\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Spliced XBP1 upregulates eNOS (NOS3) mRNA through 3'UTR-mediated stabilization and increased translation; XBP1s, Akt, and eNOS form a protein complex (shown by Co-IP), resulting in Akt and eNOS nuclear relocation; miR-24 participates in XBP1s-induced eNOS upregulation; XBP1 splicing triggered by wounding promotes endothelial cell migration.\",\n      \"method\": \"Co-IP of XBP1s/Akt/eNOS complex, immunofluorescence nuclear colocalization, XBP1s overexpression/IRE1α knockdown, 3'UTR reporter assay, wound healing/migration assays\",\n      \"journal\": \"Experimental cell research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — Co-IP, 3'UTR reporter, functional migration assays, single lab\",\n      \"pmids\": [\"29352987\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"IRE1α controls cyclin A1 expression through XBP1: IRE1α activity correlates positively with cell proliferation; XBP1 siRNA knockdown slows proliferation; genome-wide expression analysis identified cyclin A1 as the only differentially expressed cell cycle gene downstream of IRE1α/XBP1.\",\n      \"method\": \"Molecular/chemical genetic IRE1α control, siRNA XBP1 knockdown, genome-wide mRNA expression, cyclin A1/cyclin A protein level assays\",\n      \"journal\": \"Cell stress & chaperones\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — genome-wide screen, genetic manipulation, single lab\",\n      \"pmids\": [\"20013084\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"Spliced XBP1 promotes macrophage autophagy; transient XBP1s overexpression induces autophagy and proliferation, whereas sustained overexpression leads to apoptosis; XBP1s interacts with Beclin-1 (Co-IP), and XBP1 downregulation ablates rapamycin-induced autophagosome formation.\",\n      \"method\": \"Adenoviral XBP1s overexpression, siRNA XBP1 knockdown, Co-IP of XBP1s with Beclin-1, autophagy assays\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"26026678\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"XBP1s promotes colorectal cancer cell proliferation by directly binding to the TAp73 promoter and suppressing its transcriptional activity; TAp73 overexpression cancels XBP1s-induced proliferation and colony formation.\",\n      \"method\": \"ChIP/promoter reporter for XBP1s at TAp73 promoter, XBP1s overexpression/knockdown, TAp73 rescue experiments\",\n      \"journal\": \"Biochemical and biophysical research communications\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — ChIP/reporter, functional rescue, single lab\",\n      \"pmids\": [\"30473215\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"XBP1 is a bZIP transcription factor whose activity is primarily controlled by IRE1-mediated unconventional cytoplasmic splicing of XBP1 mRNA (facilitated by RtcB ligase, regulated by CNP/RtcA) during ER stress; the spliced isoform (XBP1s) translocates to the nucleus to activate genes governing ER biogenesis, protein folding, lipid synthesis, and cellular differentiation (plasma cells, eosinophils, adipocytes, osteoclasts), while forming transcriptional complexes with partners such as HIF1α and Foxo1; the unspliced isoform (XBP1u) has distinct functions including stabilizing MDM2 to suppress p53, promoting β-catenin degradation, and activating gluconeogenesis; the pathway is regulated upstream by ATF6 (which transcriptionally induces XBP1), by AKT phosphorylation of IRE1 at S724, by PERK-ATF4 enhancement of IRE1 levels, and post-transcriptionally by miR-30c-2*, while XBP1s stability is enhanced by PIN1 and spliced XBP1 mRNA is selectively protected from RIDD by IRE1-phosphorylated Pumilio.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"XBP1 is a bZIP transcription factor that functions as the principal effector of the IRE1α branch of the unfolded protein response, with broad roles in ER homeostasis, lipid metabolism, immune cell differentiation, and metabolic regulation. Upon ER stress, IRE1α cleaves XBP1 mRNA in the cytoplasm and RtcB ligase re-ligates the exons, generating a frameshifted spliced isoform (XBP1s) whose nuclear translocation—facilitated by PI3K p85 subunits—activates transcription of ER chaperones, protein disulfide isomerases, phospholipid biosynthetic enzymes, and cell-type-specific targets including NFATc1 in osteoclasts, C/EBPα in adipocytes, and IL-6 in plasma cells [PMID:11779464, PMID:30355738, PMID:25754093, PMID:19490910, PMID:26193638, PMID:12612580, PMID:29507125]. XBP1s also forms transcriptional complexes with HIF1α, Foxo1, and MYC to co-regulate context-dependent gene programs in cancer cells, immune cells, and hepatocytes, while governing hepatic 12-hour ultradian transcriptional rhythms that maintain membrane lipid homeostasis [PMID:24670641, PMID:37841640, PMID:29480818, PMID:33277471]. The constitutively produced unspliced isoform (XBP1u) exerts distinct non-transcriptional functions: it stabilizes MDM2 to promote p53 degradation, targets β-catenin for ubiquitin-proteasomal degradation, stabilizes SREBP2 to enhance cholesterol synthesis, and activates gluconeogenic gene expression during fasting [PMID:29057323, PMID:34870453, PMID:35933495, PMID:35863429].\",\n  \"teleology\": [\n    {\n      \"year\": 2001,\n      \"claim\": \"The discovery that IRE1 performs unconventional cytoplasmic splicing of XBP1 mRNA—and that ATF6 transcriptionally induces XBP1—established the two-step activation mechanism that makes spliced XBP1 the principal transcriptional effector of the mammalian UPR.\",\n      \"evidence\": \"Molecular cloning, mRNA splicing assays, and reporter gene assays in mammalian cells\",\n      \"pmids\": [\"11779464\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"RNA ligation enzyme performing the second step of splicing was unknown\", \"nuclear translocation mechanism of XBP1s was undefined\", \"genome-wide transcriptional targets not yet mapped\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Demonstrating that IRE1-mediated XBP1 splicing is required for plasma cell differentiation and immunoglobulin secretion established XBP1 as a lineage-determining factor for secretory cells, not merely a stress-response effector.\",\n      \"evidence\": \"XBP1−/− mouse B cells reconstituted with spliced XBP1; genetic rescue; cytokine measurement\",\n      \"pmids\": [\"12612580\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether XBP1 controls other hematopoietic lineages was unknown\", \"direct transcriptional targets in plasma cells not identified\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Showing that XBP1u protein is constitutively made and rapidly degraded by the proteasome revealed a built-in dampening mechanism that prevents inappropriate UPR activation in unstressed cells.\",\n      \"evidence\": \"Protein stability assays, proteasome inhibitors, C-terminal mutagenesis, reconstitution of XBP1−/− MEFs\",\n      \"pmids\": [\"16332684\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the E3 ligase(s) targeting XBP1u was unknown\", \"whether XBP1u has its own signaling functions was not addressed\"]\n    },\n    {\n      \"year\": 2007,\n      \"claim\": \"Genome-wide ChIP mapping of XBP1 binding sites revealed a core set of ER-function target genes shared across tissues plus tissue-specific targets including the differentiation regulator Mist1, establishing XBP1 as a context-dependent transcriptional organizer.\",\n      \"evidence\": \"ChIP-chip, microarray expression profiling, reporter assays in skeletal muscle and secretory cells\",\n      \"pmids\": [\"17612490\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Tissue-specific cofactors enabling differential target selection were unidentified\", \"chromatin-level mechanisms of target selectivity unknown\"]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Identification of XBP1s as a direct activator of C/EBPα in adipogenesis, and of phosphatidylcholine biosynthesis for ER membrane expansion, extended XBP1's functional scope to lipid metabolism and developmental differentiation programs beyond stress response.\",\n      \"evidence\": \"XBP1-deficient MEFs and 3T3-L1 cells with isoform-specific rescue; promoter ChIP; phospholipid biosynthesis assays\",\n      \"pmids\": [\"19490910\", \"19420237\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether XBP1 directly transactivates phospholipid biosynthetic genes or acts indirectly was unclear\", \"relationship between ER expansion and differentiation timing not resolved\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Discovery that XBP1 forms a transcriptional complex with HIF1α to co-regulate hypoxia-responsive genes in triple-negative breast cancer revealed a non-canonical partnership through which XBP1 drives tumorigenicity independently of canonical ER stress.\",\n      \"evidence\": \"ChIP-seq, Co-IP, RNA Pol II recruitment assays, XBP1 depletion in TNBC models\",\n      \"pmids\": [\"24670641\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis of the XBP1–HIF1α interaction undefined\", \"whether this complex operates in non-cancer hypoxic contexts was untested\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"A series of cell-type-specific knockouts collectively established that XBP1 is required for eosinophil granule protein maturation, osteoclast differentiation via NFATc1 transactivation, podocyte homeostasis via PI3K p85-dependent nuclear import, and polycystin-1 maturation—each representing a distinct tissue-autonomous function.\",\n      \"evidence\": \"Eosinophil-specific, podocyte-specific, and osteoclast-lineage XBP1/IRE1α KO mice; ChIP at NFATc1 promoter; Co-IP of XBP1s with p85α/p85β; SEC63-XBP1 genetic epistasis and PC1 maturation rescue\",\n      \"pmids\": [\"26147683\", \"26193638\", \"25754093\", \"25844898\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether p85-mediated nuclear import is universal or podocyte-specific was unknown\", \"mechanism by which XBP1 controls GPS cleavage of PC1 not fully resolved\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"DC-specific XBP1 deletion showed that lipid-peroxidation-driven XBP1 activation in tumor-associated dendritic cells causes pathological triglyceride accumulation and immune suppression, establishing XBP1 as a cell-intrinsic checkpoint limiting anti-tumor immunity.\",\n      \"evidence\": \"DC-specific XBP1 KO mice, nanoparticle-mediated XBP1 silencing, lipid accumulation and T cell functional assays\",\n      \"pmids\": [\"26073941\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct lipogenic transcriptional targets of XBP1 in DCs not fully catalogued\", \"applicability beyond ovarian cancer models untested\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Identification of PIN1 as a phosphorylation-dependent stabilizer of XBP1s revealed a post-translational layer of regulation and a negative-feedback loop wherein XBP1s represses PIN1 transcription via p53.\",\n      \"evidence\": \"Co-IP of PIN1 with XBP1s, phosphorylation-dependent interaction assays, PIN1 KD/KO, XBP1s stability assays\",\n      \"pmids\": [\"27334111\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific phosphorylation site(s) on XBP1s recognized by PIN1 not mapped\", \"single-lab finding without independent confirmation\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Demonstrating that XBP1u stabilizes MDM2 by blocking its self-ubiquitination—thereby promoting p53 degradation and cell-cycle progression—revealed the first major oncogenic function attributable specifically to the unspliced isoform.\",\n      \"evidence\": \"shRNA screen, Co-IP of XBP1u with MDM2, domain mapping, ubiquitination assays, cell cycle analysis\",\n      \"pmids\": [\"29057323\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"In vivo tumorigenesis models testing XBP1u-MDM2 axis not provided\", \"stoichiometry and structural basis of the interaction unknown\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Three studies collectively defined upstream and downstream wiring: MYC directly induces IRE1 and co-activates transcription with XBP1s; XBP1s induces five PDIs to control oxidative proinsulin folding in β cells; and RtcB was identified as the RNA ligase completing XBP1 splicing, with CNP and RtcA acting as opposing regulators of the ligation substrate.\",\n      \"evidence\": \"ChIP for MYC at IRE1 locus and Co-IP with XBP1; β-cell-specific IRE1α CKO and PDI reconstitution; in vitro XBP1 splicing reconstitution with RtcB/CNP/RtcA manipulation\",\n      \"pmids\": [\"29480818\", \"29507125\", \"30355738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contributions of CNP vs. RtcA in vivo not quantified\", \"whether MYC-XBP1 co-regulation operates in normal (non-cancer) physiology unclear\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Showing that XBP1 activation in tumor-infiltrating T cells suppresses mitochondrial respiration by limiting glutamine transporter abundance established a metabolic checkpoint through which ER stress impairs anti-tumor T cell function.\",\n      \"evidence\": \"XBP1-deficient T cells, metabolic flux and mitochondrial respiration assays, glutamine transporter expression, mouse ovarian cancer model and human specimens\",\n      \"pmids\": [\"30305738\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the specific XBP1-repressed glutamine transporter gene was not pinpointed until later work\", \"whether this mechanism operates in other tumor types was untested\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Discovery that AKT phosphorylates IRE1 at S724 to promote XBP1 splicing in the fed state, while XBP1u activates gluconeogenic genes during fasting, established the two XBP1 isoforms as metabolic switches controlling opposing hepatic metabolic programs.\",\n      \"evidence\": \"In vitro kinase assay, liver-specific XBP1 KO/reexpression, isoform-specific overexpression in primary hepatocytes and mice\",\n      \"pmids\": [\"35863429\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether other kinases besides AKT phosphorylate IRE1 S724 in vivo unclear\", \"XBP1u gluconeogenic mechanism (direct vs. indirect) not fully resolved\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"ChIP-seq in liver revealed that XBP1s directly binds a 12-hour oscillatory cistrome controlling rate-limiting metabolic genes, and that liver-specific XBP1 loss disrupts ultradian rhythms, membrane fluidity, and lipid remodeling, promoting spontaneous NAFLD.\",\n      \"evidence\": \"Liver-specific XBP1 KO mice, ChIP-seq, 12-hour rhythmicity analysis, phospholipid and membrane fluidity assays\",\n      \"pmids\": [\"33277471\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"What entrains XBP1s to a 12-hour cycle rather than circadian rhythm is unknown\", \"relationship to circadian clock machinery not defined\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Identification of XBP1u as a direct binding partner of β-catenin that promotes its ubiquitin-proteasomal degradation independently of GSK-3β expanded the non-transcriptional repertoire of the unspliced isoform to Wnt pathway regulation and vascular calcification suppression.\",\n      \"evidence\": \"Co-IP, domain mapping (aa 205–230), ubiquitination assays, smooth-muscle-cell-specific XBP1 KO mice, β-catenin knockdown rescue\",\n      \"pmids\": [\"34870453\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"E3 ligase mediating XBP1u-promoted β-catenin ubiquitination not identified\", \"whether this function is conserved outside vascular smooth muscle unknown\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Demonstrating that IRE1 phosphorylates Pumilio, which then selectively protects spliced XBP1 mRNA from IRE1's own RIDD activity, revealed a built-in selectivity mechanism ensuring the productive splicing product survives the destructive arm of IRE1 signaling.\",\n      \"evidence\": \"Drosophila genetics, RNA pull-down, in vitro kinase assay (hIRE1 phosphorylating Pumilio), RIDD and mRNA stability assays, Pumilio phospho-mutant analysis\",\n      \"pmids\": [\"35332141\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Conservation of Pumilio-mediated protection in mammalian systems not demonstrated in vivo\", \"specific Pumilio phosphorylation sites not fully characterized\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Two studies showed XBP1 controls cholesterol metabolism: XBP1u stabilizes SREBP2 to drive HMGCR-dependent cholesterol biosynthesis in HCC, while XBP1 in cancer cells promotes cholesterol secretion via extracellular vesicles that activate immunosuppressive MDSCs.\",\n      \"evidence\": \"Co-IP of XBP1u with SREBP2, ubiquitination assays, HMGCR reporter; XBP1 genetic/pharmacological depletion, EV cholesterol assays, MDSC functional assays, in vivo tumor models\",\n      \"pmids\": [\"35933495\", \"36351432\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether XBP1u-SREBP2 stabilization is direct or involves bridging factors unknown\", \"mechanism linking XBP1 to vesicular cholesterol loading not resolved\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Demonstrating that IL-23 activates IRE1α/XBP1 in ILC3s through mitochondrial ROS, and that this is required for IL-22 production and mucosal immunity, extended XBP1's immune functions to innate lymphoid cells and mucosal defense.\",\n      \"evidence\": \"ILC3-specific IRE1α KO mice, IL-23 stimulation, mtROS manipulation, IL-22 measurement, human IBD specimens, IRE1 inhibitor studies\",\n      \"pmids\": [\"38722686\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct transcriptional targets of XBP1 in ILC3s not mapped\", \"how mtROS activates IRE1α mechanistically not defined\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Despite extensive characterization of tissue-specific functions, the structural basis of XBP1's interactions with diverse transcriptional partners (HIF1α, Foxo1, MYC), the E3 ligases targeting XBP1u and its binding partners (β-catenin, MDM2), and the in vivo relevance of Pumilio-mediated spliced mRNA protection in mammals remain unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\"No crystal or cryo-EM structure of XBP1 or its complexes\", \"E3 ligases for XBP1u-mediated substrate degradation unidentified\", \"mammalian in vivo validation of Pumilio-XBP1 mRNA protection lacking\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140110\", \"supporting_discovery_ids\": [0, 3, 4, 7, 12, 14, 15, 21, 24, 28, 30, 35, 42]},\n      {\"term_id\": \"GO:0003677\", \"supporting_discovery_ids\": [3, 4, 7, 12, 14, 21, 24, 28, 35]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [16, 25, 29]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005634\", \"supporting_discovery_ids\": [0, 3, 7, 9, 12, 24]},\n      {\"term_id\": \"GO:0005829\", \"supporting_discovery_ids\": [2, 16, 25]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [0, 3, 4, 7, 12, 14, 21, 24]},\n      {\"term_id\": \"R-HSA-8953854\", \"supporting_discovery_ids\": [0, 20, 32]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [0, 2, 5, 19, 27]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [1, 8, 10, 17, 37]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [5, 23, 24, 29, 31]},\n      {\"term_id\": \"R-HSA-1266738\", \"supporting_discovery_ids\": [1, 4, 10, 12]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 16, 19, 25, 36]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [9, 23, 37]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [7, 8, 17, 31]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"HIF1A\",\n      \"FOXO1\",\n      \"MYC\",\n      \"PIN1\",\n      \"PIK3R1\",\n      \"PIK3R2\",\n      \"MDM2\",\n      \"CTNNB1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}