{"gene":"INSIG2","run_date":"2026-06-10T01:55:23","timeline":{"discoveries":[{"year":2002,"finding":"Insig-2 is an ER membrane protein that binds SCAP (SREBP cleavage-activating protein) in a sterol-regulated fashion, thereby retaining SCAP-SREBP complexes in the ER and preventing proteolytic processing of SREBPs in the Golgi, thus blocking cholesterol synthesis. Unlike Insig-1, Insig-2 expression does not require nuclear SREBPs, and at high expression levels Insig-2 cannot block SCAP movement in the absence of exogenous sterols.","method":"Protein binding assays, sterol-regulated co-immunoprecipitation, functional overexpression in cultured mammalian cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — reciprocal binding assays and functional rescue experiments; foundational discovery replicated extensively by multiple independent labs","pmids":["12242332"],"is_preprint":false},{"year":2003,"finding":"A liver-specific transcript of Insig-2 (Insig-2a), driven by an alternative promoter, is selectively down-regulated by insulin. Insig-2a mRNA increases during fasting and in streptozotocin-treated diabetic rats; insulin injection reduces it. Both Insig-2a and Insig-2b encode identical proteins but differ in regulation. The insulin-mediated fall in Insig-2a is proposed to permit SREBP-1c processing and stimulate fatty acid synthesis.","method":"Promoter analysis, Northern/RT-PCR in mouse liver, streptozotocin rat model, insulin injection experiments","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 2 / Strong — in vivo mouse/rat models with pharmacological manipulation and multiple orthogonal readouts; replicated in subsequent labs","pmids":["12624180"],"is_preprint":false},{"year":2005,"finding":"Cells deficient in both Insig-1 and Insig-2 (SRD-15 CHO cells) show no sterol-induced inhibition of SREBP processing and no sterol-induced ubiquitination/degradation of HMG-CoA reductase. Sterol regulation of both processes is fully restored by transfection of either Insig-1 or Insig-2, demonstrating an absolute requirement for Insig proteins in lipid homeostasis feedback control.","method":"Gamma-irradiation mutagenesis, selection in 25-hydroxycholesterol, genetic complementation with expression plasmids in CHO cells","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — loss-of-function genetic approach with complementation rescue, multiple orthogonal functional readouts (SREBP processing and reductase degradation)","pmids":["15866869"],"is_preprint":false},{"year":2006,"finding":"A conserved juxtamembranous aspartic acid residue (Asp-205 in Insig-1; corresponding residue in Insig-2) abutting the fourth transmembrane helix at the cytosolic side of the ER membrane is essential for both Insig functions: (1) sterol-dependent binding to SCAP and suppression of SREBP cleavage, and (2) acceleration of sterol-stimulated HMG-CoA reductase ubiquitination and degradation. Alanine substitution at this position abolishes both activities.","method":"Site-directed mutagenesis, co-immunoprecipitation of SCAP binding, SREBP processing assay, reductase ubiquitination assay in mammalian cells","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — active-site mutagenesis with dual functional readouts (SCAP binding and reductase degradation), validated for both Insig-1 and Insig-2","pmids":["16606821"],"is_preprint":false},{"year":2007,"finding":"Activation of the farnesoid X receptor (FXRα) directly induces Insig-2 transcription via two functional FXRα response elements identified within intron 2 of the mouse Insig-2 gene. FXRα activation increases hepatic Insig-2 protein levels and reduces HMG-CoA reductase protein levels, contributing to decreased cholesterol synthesis. No induction was observed in FXRα-/- mice.","method":"EMSA, reporter gene assays, chromatin immunoprecipitation, agonist treatment and FXRα knockout mouse model, Western blotting","journal":"Molecular endocrinology (Baltimore, Md.)","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — multiple orthogonal methods (EMSA, ChIP, reporter assay, KO mouse) in single study with rigorous controls","pmids":["17440045"],"is_preprint":false},{"year":2004,"finding":"A functional vitamin D response element was identified in the murine Insig-2 promoter. This element specifically binds the RXR–VDR heterodimer and directs 1,25-(OH)2D3-dependent transcriptional activation of Insig-2, transiently inducing Insig-2 expression in 3T3-L1 cells, potentially contributing to the anti-adipogenic action of vitamin D.","method":"EMSA, reporter gene assay, 1,25-(OH)2D3 treatment of 3T3-L1 cells, RT-PCR","journal":"Molecular endocrinology (Baltimore, Md.)","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — EMSA and reporter assay in single lab, functional consequence (anti-adipogenesis via Insig-2) proposed but not directly demonstrated by Insig-2 KO rescue","pmids":["15528275"],"is_preprint":false},{"year":2010,"finding":"Insig-2 multimerization is required for sterol regulation in CHO cells. A glycine residue (Gly-39) in the first membrane-spanning segment of Insig-2 is critical for this function; mutation to glutamic acid (G39E) produces a nonfunctional protein unable to confer sterol regulation upon Scap or HMG-CoA reductase. The corresponding intramembrane glycine in Insig-1 is similarly important.","method":"Mutagenesis of CHO cell line (SRD-20), DNA sequencing, functional overexpression assays for SCAP and reductase regulation","journal":"Journal of lipid research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — mutagenesis and functional complementation in single lab; multiple readouts but mechanism of multimerization not directly demonstrated structurally","pmids":["19617589"],"is_preprint":false},{"year":2010,"finding":"The human INSIG2 promoter contains a functional Ets-consensus motif in its proximal region. The Ets family member SAP1a binds this region and is required for basal INSIG2 transcription. Insulin activates the human INSIG2 promoter through a mechanism mediated by phosphorylated SAP1a.","method":"Promoter deletion analysis, chromatin immunoprecipitation, RNA interference, mutational analysis of promoter elements in human liver cells","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP, RNAi, and reporter assays in single lab; multiple orthogonal methods but no in vivo validation","pmids":["20145255"],"is_preprint":false},{"year":2017,"finding":"HIF-1α directly activates INSIG-2 gene transcription under hypoxia in human fibroblasts. Accumulated Insig-2 protein binds HMG-CoA reductase and triggers accelerated ubiquitination and ER-associated degradation of the enzyme. Pharmacologic stabilization of HIF-1α in mouse liver stimulated HMGCR degradation in a manner requiring prior HMGCR ubiquitination and the presence of Insig-2 protein.","method":"Transcription reporter assays, ChIP, siRNA knockdown, pharmacologic HIF-1α stabilization in mice, ubiquitination assays, Western blotting","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 2 / Strong — multiple orthogonal methods (ChIP, reporter, knockdown, in vivo mouse model) with rigorous loss-of-function controls","pmids":["28416613"],"is_preprint":false},{"year":2016,"finding":"The liver-specific isoform Insig-2a is positively regulated by the cyclic AMP-responsive element-binding protein H (CREBH), which binds conserved CRE-BP binding elements in the Insig-2a enhancer region. Glucagon and fasting activate CREBH, upregulating Insig-2a to inhibit SREBP-1c activation and hepatic de novo lipogenesis. Genetic depletion of CREBH decreases Insig-2a expression leading to SREBP-1c activation and hepatic steatosis. siRNA knockdown of Insig-2 disrupts this inhibitory effect.","method":"CREBH binding to Insig-2a promoter/enhancer, siRNA knockdown of CREBH and Insig-2, glucagon treatment, streptozotocin mouse model, lipid accumulation assays","journal":"Scientific reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function (siRNA, CREBH KO) with promoter binding analysis in single lab; multiple methods but no ChIP confirmation of CREBH-Insig-2a binding reported","pmids":["27582413"],"is_preprint":false},{"year":2020,"finding":"Insig-2 is ubiquitylated on Cys215 by the E3 ubiquitin ligase gp78, leading to its degradation in hepatocytes and undifferentiated C2C12 myoblasts. During myoblast differentiation into myotubes, elevated reactive oxygen species (ROS) oxidize Cys215, preventing ubiquitylation and stabilizing Insig-2. Stabilized Insig-2 downregulates lipogenesis via inhibition of the SREBP pathway in myotubes. The YECK tetrapeptide containing Cys215 is highly conserved in amniotes.","method":"gp78-deficient mice (tissue fractionation), ubiquitylation assays, ROS manipulation, site-directed mutagenesis of Cys215, lipogenesis assays, evolutionary sequence analysis","journal":"Nature communications","confidence":"High","confidence_rationale":"Tier 1–2 / Strong — in vitro ubiquitylation assays, mutagenesis, gp78 KO mice, and ROS manipulation with multiple orthogonal readouts in single rigorous study","pmids":["31953408"],"is_preprint":false},{"year":2021,"finding":"Cryo-electron microscopy of the human Scap–Insig-2 complex in the presence of 25-hydroxycholesterol (25HC) at 3.7 Å average resolution revealed that a 25HC molecule is sandwiched between the S4–S6 segments of Scap's sterol-sensing domain and TMs 3 and 4 of Insig-2 in the luminal leaflet of the membrane. Unwinding of the middle of the Scap-S4 segment is critical for 25HC binding and Insig-2 association.","method":"Cryo-electron microscopy structural determination","journal":"Science (New York, N.Y.)","confidence":"High","confidence_rationale":"Tier 1 / Strong — cryo-EM structure at near-atomic resolution directly revealing the molecular interface and sterol-binding site","pmids":["33446483"],"is_preprint":false},{"year":2021,"finding":"CD36, activated by insulin, forms a complex with Insig-2 that disrupts the SCAP–Insig-2 interaction, thereby allowing SREBP1 to translocate from ER to Golgi for processing and activating hepatic de novo lipogenesis. Treatment with 25-hydroxycholesterol or betulin (which enhance SCAP–Insig interaction) reversed the effect of CD36 on SREBP1 cleavage.","method":"Co-immunoprecipitation, proximity ligation assay, CD36 liver-specific knockout mice, CD36 overexpression in HepG2 cells, pharmacological rescue with 25HC/betulin","journal":"Molecular metabolism","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal co-IP and proximity ligation plus in vivo KO mouse model in single lab; mechanistic interpretation relies on displacement of SCAP–Insig-2 complex","pmids":["34974159"],"is_preprint":false},{"year":2021,"finding":"INSIG-1 and INSIG-2 mediate oxysterol-dependent activation of the PERK–eIF2α–ATF4 axis. Oxysterols with high affinity for Insig (27-HC and 25-HC) markedly induce ATF4 protein upregulation, and this is attenuated in INSIG1/2-deficient CHO cells and rescued by either INSIG1 or INSIG2. Binding of 25HC to INSIG is critical for ATF4 induction via PERK–eIF2α activation, promoting cell death through Chop, Chac1, and Trb3.","method":"INSIG1/2 double-deficient CHO cells, INSIG1/INSIG2 single knockout in Huh7 cells, genetic rescue, oxysterol treatment, Western blotting for PERK-eIF2α-ATF4 pathway","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — loss-of-function with genetic rescue in two cell systems; single lab but multiple cell-line validation","pmids":["34298014"],"is_preprint":false},{"year":2024,"finding":"Eicosapentaenoic acid (EPA), a polyunsaturated fatty acid, inhibits fatty acid synthesis specifically through Insig-2 by activating adenylate cyclase, which induces PKA to phosphorylate serine-106 in Insig-2. Phospho-Insig-2 selectively blocks proteolytic processing of SREBP-1 (not SREBP-2), thereby specifically blocking fatty acid synthesis without affecting cholesterol synthesis. Insig-1 lacks serine-106 and is not phosphorylated at this site. EPA inhibition was reduced by S106A mutation of Insig-2, by PKA inhibitor KT5720, and was absent in fibroblasts lacking Insig-2.","method":"In vitro PKA phosphorylation assay, site-directed mutagenesis (S106A), PKA inhibitor treatment, Insig-2-deficient human fibroblasts, SREBP-1 and SREBP-2 processing assays in human fibroblasts and rat hepatocytes","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 / Strong — in vitro phosphorylation reconstitution, mutagenesis, pharmacological inhibition, and Insig-2-null cell rescue; multiple orthogonal approaches in single rigorous study","pmids":["39145929"],"is_preprint":false},{"year":2017,"finding":"miR-96 directly targets the 3' UTR of INSIG2 (but not INSIG1) and reduces Insig-2 protein levels, resulting in increased nuclear SREBP-1 and SREBP-2 forms and upregulation of their target gene mRNAs. This was demonstrated in INSIG1 knockout human fibroblasts to isolate the Insig-2-specific effect.","method":"3' UTR reporter assays, miR-96 overexpression in INSIG1 KO human fibroblasts, Western blotting for SREBP nuclear forms, RT-PCR for target genes","journal":"Animal cells and systems","confidence":"Medium","confidence_rationale":"Tier 3 / Moderate — 3' UTR reporter and functional overexpression; single lab, limited orthogonal validation","pmids":["30460077"],"is_preprint":false},{"year":2008,"finding":"Overexpression of Insig-2 in colon cancer cells localizes to the mitochondria/heavy membrane fraction and associates with conformationally changed Bax protein, suppressing Bax expression and activation upon chemotherapeutic drug treatment, thereby inhibiting apoptosis. Insig-2 overexpression also increased cell proliferation, invasion, and anchorage-independent growth.","method":"Insig-2 overexpression in colon cancer cells, subcellular fractionation, co-immunoprecipitation with Bax, apoptosis assays, proliferation and invasion assays","journal":"International journal of cancer","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, single co-IP with Bax; localization to mitochondria unexpected for a canonical ER protein, not independently replicated","pmids":["18464289"],"is_preprint":false},{"year":2008,"finding":"Overexpression of Insig-2 in 3T3-L1 cells (via pEGFP-C3-insig2) results in expression of the fusion protein in the endoplasm and down-regulation of adiponectin mRNA and AP2 mRNA transcription, suggesting Insig-2 influences fat metabolism gene expression in preadipocytes.","method":"Eukaryotic expression plasmid construction, lipofectamine transfection, RT-PCR for downstream gene expression, fluorescence microscopy","journal":"Xi bao yu fen zi mian yi xue za zhi = Chinese journal of cellular and molecular immunology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single overexpression experiment, single lab, limited mechanistic follow-up","pmids":["17428382"],"is_preprint":false},{"year":2018,"finding":"Hepatic mTOR–AKT2–Insig-2a signaling contributes to the improvement of hepatic steatosis after Roux-en-Y Gastric Bypass (RYGB). Adenovirus-mediated S6K1 overexpression suppressed AKT2 while stimulating Insig-2a expression, and inhibited SREBP-1c and downstream lipogenic genes, reducing lipid deposition in hepatocytes. RYGB enhanced mTOR activity and Insig-2a while decreasing AKT2 and lipogenic pathways.","method":"Diet-induced obese mouse model, RYGB surgery, rapamycin treatment, Ad-S6K1 adenovirus infusion, Western blotting, primary hepatocyte oleic acid loading","journal":"Biochimica et biophysica acta. Molecular basis of disease","confidence":"Low","confidence_rationale":"Tier 3 / Weak — in vivo mouse model with pharmacological and adenoviral manipulation; pathway placement inferred from correlative changes without direct Insig-2 KO validation","pmids":["30562559"],"is_preprint":false},{"year":2017,"finding":"Overexpression of Insig-2 in adipose-derived stem cells (ASCs) suppresses atypical antipsychotic (clozapine/olanzapine/risperidone)-induced SREBP-1 activation and downstream lipid biosynthesis gene expression during adipogenic differentiation. AAP treatment reduces endogenous Insig-2 protein, which can be reversed by Insig-2 transfection.","method":"Insig-2 plasmid transfection in ASCs, Western blotting for SREBP-1 and lipid biosynthesis genes, adipogenic differentiation induction","journal":"Scientific reports","confidence":"Low","confidence_rationale":"Tier 3 / Weak — overexpression rescue experiment, single lab, no direct mechanistic dissection of how AAPs reduce Insig-2","pmids":["28883496"],"is_preprint":false},{"year":2024,"finding":"INSIG2 is identified as a vulnerability factor for HBV-integrated hepatoma cells. siRNA-mediated INSIG2 knockdown or CRISPR deletion impairs cell proliferation specifically in HBV-integrated HepG2.2.15 cells but not in parental HepG2 cells. This effect involves suppression of cell cycle and DNA replication pathways, downregulation of CDK2, and delayed G1-to-S transition. CDK2 inhibitor blocked the rescue, placing INSIG2 upstream of CDK2 in HBV-integrated hepatoma cell cycle progression.","method":"Genome-wide CRISPR loss-of-function screen, siRNA knockdown, CRISPR deletion, CDK2 inhibitor treatment, cell cycle analysis, co-siRNA rescue with HBV siRNA","journal":"Cancer science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — genetic epistasis (CDK2 inhibitor rescue), CRISPR screen plus targeted validation, multiple loss-of-function approaches in single lab","pmids":["38287498"],"is_preprint":false}],"current_model":"Insig-2 is an ER membrane protein with six transmembrane helices that inhibits lipid synthesis by: (1) binding SCAP in a sterol- and 25-hydroxycholesterol-dependent manner (structurally resolved by cryo-EM) to retain SCAP–SREBP complexes in the ER and block SREBP proteolytic processing; (2) binding HMG-CoA reductase to trigger its ubiquitination (by gp78 on Cys215) and ER-associated degradation; (3) being uniquely phosphorylated on Ser-106 by PKA in response to polyunsaturated fatty acids (EPA), selectively blocking SREBP-1 processing and fatty acid synthesis; and (4) being transcriptionally regulated by multiple inputs including insulin (liver-specific Insig-2a isoform downregulated), FXRα, HIF-1α, VDR, CREBH (glucagon/fasting), and miR-96, as well as post-translationally stabilized by ROS-mediated oxidation of Cys215 during muscle differentiation."},"narrative":{"mechanistic_narrative":"INSIG2 (Insig-2) is an endoplasmic reticulum membrane protein that serves as a central sterol-regulated brake on lipid biosynthesis, enforcing feedback control of both cholesterol and fatty acid synthesis [PMID:12242332, PMID:15866869]. It acts through two principal effector arms: it binds SCAP (SREBP cleavage-activating protein) in a sterol-dependent manner to retain SCAP–SREBP complexes in the ER and block proteolytic SREBP maturation, and it binds HMG-CoA reductase to accelerate its ubiquitination and ER-associated degradation [PMID:12242332, PMID:15866869]. Cryo-EM of the human Scap–Insig-2 complex resolved a 25-hydroxycholesterol molecule sandwiched between Scap's sterol-sensing domain and TMs 3–4 of Insig-2, defining the sterol-bridged interface that drives complex formation [PMID:33446483]. A conserved juxtamembranous aspartate abutting the fourth transmembrane helix and an intramembrane glycine required for Insig-2 multimerization are each essential for both SCAP binding and reductase degradation [PMID:16606821, PMID:19617589]. Beyond constitutive sterol sensing, Insig-2 integrates regulatory inputs: PKA phosphorylation of Ser-106 (a residue absent in Insig-1) in response to the polyunsaturated fatty acid EPA selectively blocks SREBP-1 processing and fatty acid synthesis without affecting cholesterol synthesis [PMID:39145929], and ROS-mediated oxidation of Cys215 prevents gp78-dependent ubiquitylation, stabilizing Insig-2 to suppress lipogenesis during myoblast differentiation [PMID:31953408]. Insig-2 transcription is controlled by multiple nuclear and hormonal inputs—FXRα, HIF-1α, CREBH (glucagon/fasting), and a liver-specific insulin-repressed Insig-2a isoform—linking it to hepatic lipid metabolism and the cellular hypoxic response [PMID:12624180, PMID:17440045, PMID:28416613, PMID:27582413]. Insig-2 also couples oxysterol sensing to the PERK–eIF2α–ATF4 stress axis [PMID:34298014].","teleology":[{"year":2002,"claim":"Established that Insig-2 is the second ER-resident sensor that retains SCAP–SREBP complexes to block SREBP processing, distinct from Insig-1 in not requiring nuclear SREBPs for its expression.","evidence":"Sterol-regulated co-immunoprecipitation and functional overexpression in cultured mammalian cells","pmids":["12242332"],"confidence":"High","gaps":["Did not resolve the molecular interface of SCAP binding","Relationship to HMG-CoA reductase degradation not yet tested"]},{"year":2003,"claim":"Showed that a liver-specific Insig-2a isoform from an alternative promoter is selectively repressed by insulin, providing a hormonal switch that licenses SREBP-1c-driven fatty acid synthesis during feeding.","evidence":"Promoter analysis, Northern/RT-PCR in mouse liver, streptozotocin diabetic rat and insulin injection models","pmids":["12624180"],"confidence":"High","gaps":["Transcription factor mediating insulin repression not identified here","Did not separate fatty acid from cholesterol synthesis effects"]},{"year":2005,"claim":"Demonstrated an absolute requirement for Insig proteins in lipid feedback control, since loss of both Insig-1 and Insig-2 abolishes sterol-induced SREBP suppression and reductase degradation, both rescuable by either Insig.","evidence":"Mutagenesis-derived Insig-deficient SRD-15 CHO cells with genetic complementation","pmids":["15866869"],"confidence":"High","gaps":["Functional redundancy between paralogs left isoform-specific roles unresolved","Did not address tissue-specific differences"]},{"year":2006,"claim":"Mapped a conserved juxtamembranous aspartate as essential for both Insig functions, linking SCAP binding and reductase degradation to a single structural determinant.","evidence":"Site-directed mutagenesis with dual SCAP-binding and reductase-ubiquitination readouts in mammalian cells","pmids":["16606821"],"confidence":"High","gaps":["Did not define how the residue contributes mechanistically to sterol sensing","Structural basis unresolved at this stage"]},{"year":2010,"claim":"Identified Insig-2 multimerization, dependent on an intramembrane glycine, as a requirement for sterol regulation of both Scap and reductase.","evidence":"Mutagenesis (G39E) and functional complementation in CHO SRD-20 cells","pmids":["19617589"],"confidence":"Medium","gaps":["Multimerization not directly demonstrated structurally","Stoichiometry of the functional oligomer unknown"]},{"year":2017,"claim":"Placed Insig-2 in the hypoxic response by showing HIF-1α directly induces INSIG2 transcription, accelerating HMGCR ubiquitination and degradation, coupling oxygen sensing to cholesterol synthesis control.","evidence":"ChIP, reporter assays, siRNA, pharmacologic HIF-1α stabilization in mice and ubiquitination assays","pmids":["28416613"],"confidence":"High","gaps":["Physiological context for hypoxic cholesterol suppression not fully defined","Did not address SREBP arm under hypoxia"]},{"year":2016,"claim":"Showed glucagon/fasting acts via CREBH to upregulate the Insig-2a isoform, opposing insulin and protecting against hepatic steatosis through SREBP-1c suppression.","evidence":"Promoter binding, CREBH and Insig-2 knockdown, glucagon treatment and streptozotocin mouse model","pmids":["27582413"],"confidence":"Medium","gaps":["No ChIP confirmation of CREBH binding to Insig-2a","Single-lab in vivo evidence"]},{"year":2020,"claim":"Revealed post-translational control of Insig-2 stability: gp78-mediated ubiquitylation on Cys215 drives degradation, while ROS oxidation of Cys215 blocks this and stabilizes Insig-2 to suppress lipogenesis during myogenesis.","evidence":"gp78 knockout mice, in vitro ubiquitylation, Cys215 mutagenesis, ROS manipulation and lipogenesis assays","pmids":["31953408"],"confidence":"High","gaps":["Cys215 oxidation chemistry not fully defined","Relevance beyond muscle differentiation not established"]},{"year":2021,"claim":"Provided the near-atomic structural basis for sterol-mediated Scap–Insig-2 association, showing 25HC bridges Scap's sterol-sensing domain and Insig-2 TMs 3–4.","evidence":"Cryo-EM of the human Scap–Insig-2 complex with 25HC at 3.7 Å","pmids":["33446483"],"confidence":"High","gaps":["Structure of the reductase-bound Insig-2 complex not resolved","Conformational dynamics during sterol release not captured"]},{"year":2021,"claim":"Linked insulin signaling to lipogenesis activation through CD36, which forms a complex with Insig-2 and disrupts the SCAP–Insig-2 interaction to permit SREBP1 processing.","evidence":"Co-IP, proximity ligation, CD36 liver-specific knockout mice, and pharmacological rescue with 25HC/betulin","pmids":["34974159"],"confidence":"Medium","gaps":["Direct interface of CD36–Insig-2 binding undefined","Mechanism relies on inferred displacement of SCAP"]},{"year":2021,"claim":"Expanded Insig function beyond lipid feedback by showing Insig proteins couple oxysterol sensing to PERK–eIF2α–ATF4 activation and downstream cell death effectors.","evidence":"INSIG1/2-deficient CHO and Huh7 cells with genetic rescue and oxysterol treatment","pmids":["34298014"],"confidence":"Medium","gaps":["Mechanism connecting Insig sterol binding to PERK activation undefined","Insig-2-specific contribution not isolated from Insig-1"]},{"year":2024,"claim":"Defined an Insig-2-specific phosphorylation switch: EPA-driven PKA phosphorylation of Ser-106 selectively blocks SREBP-1 processing and fatty acid synthesis, uncoupling it from cholesterol synthesis.","evidence":"In vitro PKA phosphorylation, S106A mutagenesis, PKA inhibition, and Insig-2-null fibroblast rescue with SREBP processing assays","pmids":["39145929"],"confidence":"High","gaps":["How phospho-Ser-106 mechanistically discriminates SREBP-1 from SREBP-2 unresolved","In vivo relevance in liver physiology not established"]},{"year":2024,"claim":"Identified INSIG2 as a context-specific proliferation vulnerability in HBV-integrated hepatoma cells, acting upstream of CDK2 in G1-to-S progression.","evidence":"Genome-wide CRISPR screen, siRNA/CRISPR knockout, CDK2 inhibitor epistasis and cell cycle analysis","pmids":["38287498"],"confidence":"Medium","gaps":["Mechanism linking Insig-2 to CDK2 unknown","Specificity to HBV-integrated context not mechanistically explained"]},{"year":null,"claim":"How the multiple regulatory inputs (transcriptional, phosphorylation, redox, and protein-displacement) are integrated in vivo to set tissue-specific lipid synthesis thresholds remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No unified in vivo model integrating CREBH/insulin/HIF-1α/FXRα transcriptional control with Ser-106 phosphorylation and Cys215 redox switching","Structural basis of the Insig-2–HMGCR complex unresolved","Non-canonical roles (mitochondrial Bax, cancer cell cycle) lack mechanistic dissection"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[11,0]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[0,2,3]},{"term_id":"GO:0140313","term_label":"molecular sequestering activity","supporting_discovery_ids":[0,11]}],"localization":[{"term_id":"GO:0005783","term_label":"endoplasmic reticulum","supporting_discovery_ids":[0,2,11]}],"pathway":[{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[0,2,14]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[2,8,10]},{"term_id":"R-HSA-8953897","term_label":"Cellular responses to stimuli","supporting_discovery_ids":[8,13]}],"complexes":["SCAP–Insig-2 complex"],"partners":["SCAP","HMGCR","GP78","CD36"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"Q9Y5U4","full_name":"Insulin-induced gene 2 protein","aliases":[],"length_aa":225,"mass_kda":24.8,"function":"Oxysterol-binding protein that mediates feedback control of cholesterol synthesis by controlling both endoplasmic reticulum to Golgi transport of SCAP and degradation of HMGCR (PubMed:12242332, PubMed:16606821, PubMed:32322062). Acts as a negative regulator of cholesterol biosynthesis by mediating the retention of the SCAP-SREBP complex in the endoplasmic reticulum, thereby blocking the processing of sterol regulatory element-binding proteins (SREBPs) SREBF1/SREBP1 and SREBF2/SREBP2 (PubMed:32322062). Binds oxysterol, including 22-hydroxycholesterol, 24-hydroxycholesterol, 25-hydroxycholesterol and 27-hydroxycholesterol, regulating interaction with SCAP and retention of the SCAP-SREBP complex in the endoplasmic reticulum (PubMed:17428920, PubMed:26160948, PubMed:32322062). In presence of oxysterol, interacts with SCAP, retaining the SCAP-SREBP complex in the endoplasmic reticulum, thereby preventing SCAP from escorting SREBF1/SREBP1 and SREBF2/SREBP2 to the Golgi (PubMed:32322062). Sterol deprivation or phosphorylation by PCK1 reduce oxysterol-binding, disrupting the interaction between INSIG2 and SCAP, thereby promoting Golgi transport of the SCAP-SREBP complex, followed by processing and nuclear translocation of SREBF1/SREBP1 and SREBF2/SREBP2 (PubMed:32322062). Also regulates cholesterol synthesis by regulating degradation of HMGCR: initiates the sterol-mediated ubiquitin-mediated endoplasmic reticulum-associated degradation (ERAD) of HMGCR via recruitment of the reductase to the ubiquitin ligase RNF139 (PubMed:16606821, PubMed:22143767)","subcellular_location":"Endoplasmic reticulum membrane","url":"https://www.uniprot.org/uniprotkb/Q9Y5U4/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/INSIG2","classification":"Not Classified","n_dependent_lines":1,"n_total_lines":1208,"dependency_fraction":0.0008278145695364238},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/INSIG2","total_profiled":1310},"omim":[{"mim_id":"614168","title":"PHOSPHOENOLPYRUVATE CARBOXYKINASE 1, SOLUBLE; PCK1","url":"https://www.omim.org/entry/614168"},{"mim_id":"612459","title":"BODY MASS INDEX QUANTITATIVE TRAIT LOCUS 13; BMIQ13","url":"https://www.omim.org/entry/612459"},{"mim_id":"608660","title":"INSULIN-INDUCED GENE 2; INSIG2","url":"https://www.omim.org/entry/608660"},{"mim_id":"602055","title":"INSULIN-INDUCED GENE 1; INSIG1","url":"https://www.omim.org/entry/602055"},{"mim_id":"164730","title":"AKT SERINE/THREONINE KINASE 1; AKT1","url":"https://www.omim.org/entry/164730"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Low tissue specificity","tissue_distribution":"Detected in 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Medical sciences","url":"https://pubmed.ncbi.nlm.nih.gov/18382059","citation_count":2,"is_preprint":false},{"pmid":"20920244","id":"PMC_20920244","title":"Association of an INSIG2 obesity allele with cardiovascular phenotypes is gender and age dependent.","date":"2010","source":"BMC cardiovascular disorders","url":"https://pubmed.ncbi.nlm.nih.gov/20920244","citation_count":1,"is_preprint":false},{"pmid":"17428382","id":"PMC_17428382","title":"[Construction and expression of fusion gene eukaryotic expression plasmid of pEGFP-C(3)-insig2 and its influence to downstream genes].","date":"2007","source":"Xi bao yu fen zi mian yi xue za zhi = Chinese journal of cellular and molecular immunology","url":"https://pubmed.ncbi.nlm.nih.gov/17428382","citation_count":0,"is_preprint":false}],"cost":{"stage1":{"model":"claude-sonnet-4-6","input_tokens":42574,"output_tokens":6204,"usd":0.110391,"stage1_stop_reason":"end_turn"},"stage2":{"model":"claude-opus-4-8","input_tokens":14946,"output_tokens":4453,"usd":0.093027,"stage2_stop_reason":"end_turn"},"total_usd":0.203418,"stage1_batch_id":"msgbatch_01DUPACZi7AAWwuZiUjseQF4","stage2_batch_id":"msgbatch_018yt7chE3f48PEFtDwtj6AE","note":"batch pricing = 50% of standard"},"stage1_raw":"```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 2002,\n      \"finding\": \"Insig-2 is an ER membrane protein that binds SCAP (SREBP cleavage-activating protein) in a sterol-regulated fashion, thereby retaining SCAP-SREBP complexes in the ER and preventing proteolytic processing of SREBPs in the Golgi, thus blocking cholesterol synthesis. Unlike Insig-1, Insig-2 expression does not require nuclear SREBPs, and at high expression levels Insig-2 cannot block SCAP movement in the absence of exogenous sterols.\",\n      \"method\": \"Protein binding assays, sterol-regulated co-immunoprecipitation, functional overexpression in cultured mammalian cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — reciprocal binding assays and functional rescue experiments; foundational discovery replicated extensively by multiple independent labs\",\n      \"pmids\": [\"12242332\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2003,\n      \"finding\": \"A liver-specific transcript of Insig-2 (Insig-2a), driven by an alternative promoter, is selectively down-regulated by insulin. Insig-2a mRNA increases during fasting and in streptozotocin-treated diabetic rats; insulin injection reduces it. Both Insig-2a and Insig-2b encode identical proteins but differ in regulation. The insulin-mediated fall in Insig-2a is proposed to permit SREBP-1c processing and stimulate fatty acid synthesis.\",\n      \"method\": \"Promoter analysis, Northern/RT-PCR in mouse liver, streptozotocin rat model, insulin injection experiments\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — in vivo mouse/rat models with pharmacological manipulation and multiple orthogonal readouts; replicated in subsequent labs\",\n      \"pmids\": [\"12624180\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Cells deficient in both Insig-1 and Insig-2 (SRD-15 CHO cells) show no sterol-induced inhibition of SREBP processing and no sterol-induced ubiquitination/degradation of HMG-CoA reductase. Sterol regulation of both processes is fully restored by transfection of either Insig-1 or Insig-2, demonstrating an absolute requirement for Insig proteins in lipid homeostasis feedback control.\",\n      \"method\": \"Gamma-irradiation mutagenesis, selection in 25-hydroxycholesterol, genetic complementation with expression plasmids in CHO cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — loss-of-function genetic approach with complementation rescue, multiple orthogonal functional readouts (SREBP processing and reductase degradation)\",\n      \"pmids\": [\"15866869\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"A conserved juxtamembranous aspartic acid residue (Asp-205 in Insig-1; corresponding residue in Insig-2) abutting the fourth transmembrane helix at the cytosolic side of the ER membrane is essential for both Insig functions: (1) sterol-dependent binding to SCAP and suppression of SREBP cleavage, and (2) acceleration of sterol-stimulated HMG-CoA reductase ubiquitination and degradation. Alanine substitution at this position abolishes both activities.\",\n      \"method\": \"Site-directed mutagenesis, co-immunoprecipitation of SCAP binding, SREBP processing assay, reductase ubiquitination assay in mammalian cells\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — active-site mutagenesis with dual functional readouts (SCAP binding and reductase degradation), validated for both Insig-1 and Insig-2\",\n      \"pmids\": [\"16606821\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2007,\n      \"finding\": \"Activation of the farnesoid X receptor (FXRα) directly induces Insig-2 transcription via two functional FXRα response elements identified within intron 2 of the mouse Insig-2 gene. FXRα activation increases hepatic Insig-2 protein levels and reduces HMG-CoA reductase protein levels, contributing to decreased cholesterol synthesis. No induction was observed in FXRα-/- mice.\",\n      \"method\": \"EMSA, reporter gene assays, chromatin immunoprecipitation, agonist treatment and FXRα knockout mouse model, Western blotting\",\n      \"journal\": \"Molecular endocrinology (Baltimore, Md.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — multiple orthogonal methods (EMSA, ChIP, reporter assay, KO mouse) in single study with rigorous controls\",\n      \"pmids\": [\"17440045\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2004,\n      \"finding\": \"A functional vitamin D response element was identified in the murine Insig-2 promoter. This element specifically binds the RXR–VDR heterodimer and directs 1,25-(OH)2D3-dependent transcriptional activation of Insig-2, transiently inducing Insig-2 expression in 3T3-L1 cells, potentially contributing to the anti-adipogenic action of vitamin D.\",\n      \"method\": \"EMSA, reporter gene assay, 1,25-(OH)2D3 treatment of 3T3-L1 cells, RT-PCR\",\n      \"journal\": \"Molecular endocrinology (Baltimore, Md.)\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — EMSA and reporter assay in single lab, functional consequence (anti-adipogenesis via Insig-2) proposed but not directly demonstrated by Insig-2 KO rescue\",\n      \"pmids\": [\"15528275\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"Insig-2 multimerization is required for sterol regulation in CHO cells. A glycine residue (Gly-39) in the first membrane-spanning segment of Insig-2 is critical for this function; mutation to glutamic acid (G39E) produces a nonfunctional protein unable to confer sterol regulation upon Scap or HMG-CoA reductase. The corresponding intramembrane glycine in Insig-1 is similarly important.\",\n      \"method\": \"Mutagenesis of CHO cell line (SRD-20), DNA sequencing, functional overexpression assays for SCAP and reductase regulation\",\n      \"journal\": \"Journal of lipid research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — mutagenesis and functional complementation in single lab; multiple readouts but mechanism of multimerization not directly demonstrated structurally\",\n      \"pmids\": [\"19617589\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2010,\n      \"finding\": \"The human INSIG2 promoter contains a functional Ets-consensus motif in its proximal region. The Ets family member SAP1a binds this region and is required for basal INSIG2 transcription. Insulin activates the human INSIG2 promoter through a mechanism mediated by phosphorylated SAP1a.\",\n      \"method\": \"Promoter deletion analysis, chromatin immunoprecipitation, RNA interference, mutational analysis of promoter elements in human liver cells\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP, RNAi, and reporter assays in single lab; multiple orthogonal methods but no in vivo validation\",\n      \"pmids\": [\"20145255\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"HIF-1α directly activates INSIG-2 gene transcription under hypoxia in human fibroblasts. Accumulated Insig-2 protein binds HMG-CoA reductase and triggers accelerated ubiquitination and ER-associated degradation of the enzyme. Pharmacologic stabilization of HIF-1α in mouse liver stimulated HMGCR degradation in a manner requiring prior HMGCR ubiquitination and the presence of Insig-2 protein.\",\n      \"method\": \"Transcription reporter assays, ChIP, siRNA knockdown, pharmacologic HIF-1α stabilization in mice, ubiquitination assays, Western blotting\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 / Strong — multiple orthogonal methods (ChIP, reporter, knockdown, in vivo mouse model) with rigorous loss-of-function controls\",\n      \"pmids\": [\"28416613\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The liver-specific isoform Insig-2a is positively regulated by the cyclic AMP-responsive element-binding protein H (CREBH), which binds conserved CRE-BP binding elements in the Insig-2a enhancer region. Glucagon and fasting activate CREBH, upregulating Insig-2a to inhibit SREBP-1c activation and hepatic de novo lipogenesis. Genetic depletion of CREBH decreases Insig-2a expression leading to SREBP-1c activation and hepatic steatosis. siRNA knockdown of Insig-2 disrupts this inhibitory effect.\",\n      \"method\": \"CREBH binding to Insig-2a promoter/enhancer, siRNA knockdown of CREBH and Insig-2, glucagon treatment, streptozotocin mouse model, lipid accumulation assays\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function (siRNA, CREBH KO) with promoter binding analysis in single lab; multiple methods but no ChIP confirmation of CREBH-Insig-2a binding reported\",\n      \"pmids\": [\"27582413\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"Insig-2 is ubiquitylated on Cys215 by the E3 ubiquitin ligase gp78, leading to its degradation in hepatocytes and undifferentiated C2C12 myoblasts. During myoblast differentiation into myotubes, elevated reactive oxygen species (ROS) oxidize Cys215, preventing ubiquitylation and stabilizing Insig-2. Stabilized Insig-2 downregulates lipogenesis via inhibition of the SREBP pathway in myotubes. The YECK tetrapeptide containing Cys215 is highly conserved in amniotes.\",\n      \"method\": \"gp78-deficient mice (tissue fractionation), ubiquitylation assays, ROS manipulation, site-directed mutagenesis of Cys215, lipogenesis assays, evolutionary sequence analysis\",\n      \"journal\": \"Nature communications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 / Strong — in vitro ubiquitylation assays, mutagenesis, gp78 KO mice, and ROS manipulation with multiple orthogonal readouts in single rigorous study\",\n      \"pmids\": [\"31953408\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"Cryo-electron microscopy of the human Scap–Insig-2 complex in the presence of 25-hydroxycholesterol (25HC) at 3.7 Å average resolution revealed that a 25HC molecule is sandwiched between the S4–S6 segments of Scap's sterol-sensing domain and TMs 3 and 4 of Insig-2 in the luminal leaflet of the membrane. Unwinding of the middle of the Scap-S4 segment is critical for 25HC binding and Insig-2 association.\",\n      \"method\": \"Cryo-electron microscopy structural determination\",\n      \"journal\": \"Science (New York, N.Y.)\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — cryo-EM structure at near-atomic resolution directly revealing the molecular interface and sterol-binding site\",\n      \"pmids\": [\"33446483\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"CD36, activated by insulin, forms a complex with Insig-2 that disrupts the SCAP–Insig-2 interaction, thereby allowing SREBP1 to translocate from ER to Golgi for processing and activating hepatic de novo lipogenesis. Treatment with 25-hydroxycholesterol or betulin (which enhance SCAP–Insig interaction) reversed the effect of CD36 on SREBP1 cleavage.\",\n      \"method\": \"Co-immunoprecipitation, proximity ligation assay, CD36 liver-specific knockout mice, CD36 overexpression in HepG2 cells, pharmacological rescue with 25HC/betulin\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal co-IP and proximity ligation plus in vivo KO mouse model in single lab; mechanistic interpretation relies on displacement of SCAP–Insig-2 complex\",\n      \"pmids\": [\"34974159\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"INSIG-1 and INSIG-2 mediate oxysterol-dependent activation of the PERK–eIF2α–ATF4 axis. Oxysterols with high affinity for Insig (27-HC and 25-HC) markedly induce ATF4 protein upregulation, and this is attenuated in INSIG1/2-deficient CHO cells and rescued by either INSIG1 or INSIG2. Binding of 25HC to INSIG is critical for ATF4 induction via PERK–eIF2α activation, promoting cell death through Chop, Chac1, and Trb3.\",\n      \"method\": \"INSIG1/2 double-deficient CHO cells, INSIG1/INSIG2 single knockout in Huh7 cells, genetic rescue, oxysterol treatment, Western blotting for PERK-eIF2α-ATF4 pathway\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — loss-of-function with genetic rescue in two cell systems; single lab but multiple cell-line validation\",\n      \"pmids\": [\"34298014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"Eicosapentaenoic acid (EPA), a polyunsaturated fatty acid, inhibits fatty acid synthesis specifically through Insig-2 by activating adenylate cyclase, which induces PKA to phosphorylate serine-106 in Insig-2. Phospho-Insig-2 selectively blocks proteolytic processing of SREBP-1 (not SREBP-2), thereby specifically blocking fatty acid synthesis without affecting cholesterol synthesis. Insig-1 lacks serine-106 and is not phosphorylated at this site. EPA inhibition was reduced by S106A mutation of Insig-2, by PKA inhibitor KT5720, and was absent in fibroblasts lacking Insig-2.\",\n      \"method\": \"In vitro PKA phosphorylation assay, site-directed mutagenesis (S106A), PKA inhibitor treatment, Insig-2-deficient human fibroblasts, SREBP-1 and SREBP-2 processing assays in human fibroblasts and rat hepatocytes\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 / Strong — in vitro phosphorylation reconstitution, mutagenesis, pharmacological inhibition, and Insig-2-null cell rescue; multiple orthogonal approaches in single rigorous study\",\n      \"pmids\": [\"39145929\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"miR-96 directly targets the 3' UTR of INSIG2 (but not INSIG1) and reduces Insig-2 protein levels, resulting in increased nuclear SREBP-1 and SREBP-2 forms and upregulation of their target gene mRNAs. This was demonstrated in INSIG1 knockout human fibroblasts to isolate the Insig-2-specific effect.\",\n      \"method\": \"3' UTR reporter assays, miR-96 overexpression in INSIG1 KO human fibroblasts, Western blotting for SREBP nuclear forms, RT-PCR for target genes\",\n      \"journal\": \"Animal cells and systems\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 / Moderate — 3' UTR reporter and functional overexpression; single lab, limited orthogonal validation\",\n      \"pmids\": [\"30460077\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Overexpression of Insig-2 in colon cancer cells localizes to the mitochondria/heavy membrane fraction and associates with conformationally changed Bax protein, suppressing Bax expression and activation upon chemotherapeutic drug treatment, thereby inhibiting apoptosis. Insig-2 overexpression also increased cell proliferation, invasion, and anchorage-independent growth.\",\n      \"method\": \"Insig-2 overexpression in colon cancer cells, subcellular fractionation, co-immunoprecipitation with Bax, apoptosis assays, proliferation and invasion assays\",\n      \"journal\": \"International journal of cancer\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, single co-IP with Bax; localization to mitochondria unexpected for a canonical ER protein, not independently replicated\",\n      \"pmids\": [\"18464289\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2008,\n      \"finding\": \"Overexpression of Insig-2 in 3T3-L1 cells (via pEGFP-C3-insig2) results in expression of the fusion protein in the endoplasm and down-regulation of adiponectin mRNA and AP2 mRNA transcription, suggesting Insig-2 influences fat metabolism gene expression in preadipocytes.\",\n      \"method\": \"Eukaryotic expression plasmid construction, lipofectamine transfection, RT-PCR for downstream gene expression, fluorescence microscopy\",\n      \"journal\": \"Xi bao yu fen zi mian yi xue za zhi = Chinese journal of cellular and molecular immunology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single overexpression experiment, single lab, limited mechanistic follow-up\",\n      \"pmids\": [\"17428382\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"Hepatic mTOR–AKT2–Insig-2a signaling contributes to the improvement of hepatic steatosis after Roux-en-Y Gastric Bypass (RYGB). Adenovirus-mediated S6K1 overexpression suppressed AKT2 while stimulating Insig-2a expression, and inhibited SREBP-1c and downstream lipogenic genes, reducing lipid deposition in hepatocytes. RYGB enhanced mTOR activity and Insig-2a while decreasing AKT2 and lipogenic pathways.\",\n      \"method\": \"Diet-induced obese mouse model, RYGB surgery, rapamycin treatment, Ad-S6K1 adenovirus infusion, Western blotting, primary hepatocyte oleic acid loading\",\n      \"journal\": \"Biochimica et biophysica acta. Molecular basis of disease\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — in vivo mouse model with pharmacological and adenoviral manipulation; pathway placement inferred from correlative changes without direct Insig-2 KO validation\",\n      \"pmids\": [\"30562559\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"Overexpression of Insig-2 in adipose-derived stem cells (ASCs) suppresses atypical antipsychotic (clozapine/olanzapine/risperidone)-induced SREBP-1 activation and downstream lipid biosynthesis gene expression during adipogenic differentiation. AAP treatment reduces endogenous Insig-2 protein, which can be reversed by Insig-2 transfection.\",\n      \"method\": \"Insig-2 plasmid transfection in ASCs, Western blotting for SREBP-1 and lipid biosynthesis genes, adipogenic differentiation induction\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — overexpression rescue experiment, single lab, no direct mechanistic dissection of how AAPs reduce Insig-2\",\n      \"pmids\": [\"28883496\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"INSIG2 is identified as a vulnerability factor for HBV-integrated hepatoma cells. siRNA-mediated INSIG2 knockdown or CRISPR deletion impairs cell proliferation specifically in HBV-integrated HepG2.2.15 cells but not in parental HepG2 cells. This effect involves suppression of cell cycle and DNA replication pathways, downregulation of CDK2, and delayed G1-to-S transition. CDK2 inhibitor blocked the rescue, placing INSIG2 upstream of CDK2 in HBV-integrated hepatoma cell cycle progression.\",\n      \"method\": \"Genome-wide CRISPR loss-of-function screen, siRNA knockdown, CRISPR deletion, CDK2 inhibitor treatment, cell cycle analysis, co-siRNA rescue with HBV siRNA\",\n      \"journal\": \"Cancer science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — genetic epistasis (CDK2 inhibitor rescue), CRISPR screen plus targeted validation, multiple loss-of-function approaches in single lab\",\n      \"pmids\": [\"38287498\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Insig-2 is an ER membrane protein with six transmembrane helices that inhibits lipid synthesis by: (1) binding SCAP in a sterol- and 25-hydroxycholesterol-dependent manner (structurally resolved by cryo-EM) to retain SCAP–SREBP complexes in the ER and block SREBP proteolytic processing; (2) binding HMG-CoA reductase to trigger its ubiquitination (by gp78 on Cys215) and ER-associated degradation; (3) being uniquely phosphorylated on Ser-106 by PKA in response to polyunsaturated fatty acids (EPA), selectively blocking SREBP-1 processing and fatty acid synthesis; and (4) being transcriptionally regulated by multiple inputs including insulin (liver-specific Insig-2a isoform downregulated), FXRα, HIF-1α, VDR, CREBH (glucagon/fasting), and miR-96, as well as post-translationally stabilized by ROS-mediated oxidation of Cys215 during muscle differentiation.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"INSIG2 (Insig-2) is an endoplasmic reticulum membrane protein that serves as a central sterol-regulated brake on lipid biosynthesis, enforcing feedback control of both cholesterol and fatty acid synthesis [#0, #2]. It acts through two principal effector arms: it binds SCAP (SREBP cleavage-activating protein) in a sterol-dependent manner to retain SCAP\\u2013SREBP complexes in the ER and block proteolytic SREBP maturation, and it binds HMG-CoA reductase to accelerate its ubiquitination and ER-associated degradation [#0, #2]. Cryo-EM of the human Scap\\u2013Insig-2 complex resolved a 25-hydroxycholesterol molecule sandwiched between Scap's sterol-sensing domain and TMs 3\\u20134 of Insig-2, defining the sterol-bridged interface that drives complex formation [#11]. A conserved juxtamembranous aspartate abutting the fourth transmembrane helix and an intramembrane glycine required for Insig-2 multimerization are each essential for both SCAP binding and reductase degradation [#3, #6]. Beyond constitutive sterol sensing, Insig-2 integrates regulatory inputs: PKA phosphorylation of Ser-106 (a residue absent in Insig-1) in response to the polyunsaturated fatty acid EPA selectively blocks SREBP-1 processing and fatty acid synthesis without affecting cholesterol synthesis [#14], and ROS-mediated oxidation of Cys215 prevents gp78-dependent ubiquitylation, stabilizing Insig-2 to suppress lipogenesis during myoblast differentiation [#10]. Insig-2 transcription is controlled by multiple nuclear and hormonal inputs\\u2014FXR\\u03b1, HIF-1\\u03b1, CREBH (glucagon/fasting), and a liver-specific insulin-repressed Insig-2a isoform\\u2014linking it to hepatic lipid metabolism and the cellular hypoxic response [#1, #4, #8, #9]. Insig-2 also couples oxysterol sensing to the PERK\\u2013eIF2\\u03b1\\u2013ATF4 stress axis [#13].\"\n,\n  \"teleology\": [\n    {\n      \"year\": 2002,\n      \"claim\": \"Established that Insig-2 is the second ER-resident sensor that retains SCAP\\u2013SREBP complexes to block SREBP processing, distinct from Insig-1 in not requiring nuclear SREBPs for its expression.\",\n      \"evidence\": \"Sterol-regulated co-immunoprecipitation and functional overexpression in cultured mammalian cells\",\n      \"pmids\": [\"12242332\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not resolve the molecular interface of SCAP binding\", \"Relationship to HMG-CoA reductase degradation not yet tested\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"Showed that a liver-specific Insig-2a isoform from an alternative promoter is selectively repressed by insulin, providing a hormonal switch that licenses SREBP-1c-driven fatty acid synthesis during feeding.\",\n      \"evidence\": \"Promoter analysis, Northern/RT-PCR in mouse liver, streptozotocin diabetic rat and insulin injection models\",\n      \"pmids\": [\"12624180\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Transcription factor mediating insulin repression not identified here\", \"Did not separate fatty acid from cholesterol synthesis effects\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Demonstrated an absolute requirement for Insig proteins in lipid feedback control, since loss of both Insig-1 and Insig-2 abolishes sterol-induced SREBP suppression and reductase degradation, both rescuable by either Insig.\",\n      \"evidence\": \"Mutagenesis-derived Insig-deficient SRD-15 CHO cells with genetic complementation\",\n      \"pmids\": [\"15866869\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Functional redundancy between paralogs left isoform-specific roles unresolved\", \"Did not address tissue-specific differences\"]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Mapped a conserved juxtamembranous aspartate as essential for both Insig functions, linking SCAP binding and reductase degradation to a single structural determinant.\",\n      \"evidence\": \"Site-directed mutagenesis with dual SCAP-binding and reductase-ubiquitination readouts in mammalian cells\",\n      \"pmids\": [\"16606821\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Did not define how the residue contributes mechanistically to sterol sensing\", \"Structural basis unresolved at this stage\"]\n    },\n    {\n      \"year\": 2010,\n      \"claim\": \"Identified Insig-2 multimerization, dependent on an intramembrane glycine, as a requirement for sterol regulation of both Scap and reductase.\",\n      \"evidence\": \"Mutagenesis (G39E) and functional complementation in CHO SRD-20 cells\",\n      \"pmids\": [\"19617589\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Multimerization not directly demonstrated structurally\", \"Stoichiometry of the functional oligomer unknown\"]\n    },\n    {\n      \"year\": 2017,\n      \"claim\": \"Placed Insig-2 in the hypoxic response by showing HIF-1\\u03b1 directly induces INSIG2 transcription, accelerating HMGCR ubiquitination and degradation, coupling oxygen sensing to cholesterol synthesis control.\",\n      \"evidence\": \"ChIP, reporter assays, siRNA, pharmacologic HIF-1\\u03b1 stabilization in mice and ubiquitination assays\",\n      \"pmids\": [\"28416613\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological context for hypoxic cholesterol suppression not fully defined\", \"Did not address SREBP arm under hypoxia\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Showed glucagon/fasting acts via CREBH to upregulate the Insig-2a isoform, opposing insulin and protecting against hepatic steatosis through SREBP-1c suppression.\",\n      \"evidence\": \"Promoter binding, CREBH and Insig-2 knockdown, glucagon treatment and streptozotocin mouse model\",\n      \"pmids\": [\"27582413\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No ChIP confirmation of CREBH binding to Insig-2a\", \"Single-lab in vivo evidence\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Revealed post-translational control of Insig-2 stability: gp78-mediated ubiquitylation on Cys215 drives degradation, while ROS oxidation of Cys215 blocks this and stabilizes Insig-2 to suppress lipogenesis during myogenesis.\",\n      \"evidence\": \"gp78 knockout mice, in vitro ubiquitylation, Cys215 mutagenesis, ROS manipulation and lipogenesis assays\",\n      \"pmids\": [\"31953408\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Cys215 oxidation chemistry not fully defined\", \"Relevance beyond muscle differentiation not established\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Provided the near-atomic structural basis for sterol-mediated Scap\\u2013Insig-2 association, showing 25HC bridges Scap's sterol-sensing domain and Insig-2 TMs 3\\u20134.\",\n      \"evidence\": \"Cryo-EM of the human Scap\\u2013Insig-2 complex with 25HC at 3.7 \\u00c5\",\n      \"pmids\": [\"33446483\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structure of the reductase-bound Insig-2 complex not resolved\", \"Conformational dynamics during sterol release not captured\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Linked insulin signaling to lipogenesis activation through CD36, which forms a complex with Insig-2 and disrupts the SCAP\\u2013Insig-2 interaction to permit SREBP1 processing.\",\n      \"evidence\": \"Co-IP, proximity ligation, CD36 liver-specific knockout mice, and pharmacological rescue with 25HC/betulin\",\n      \"pmids\": [\"34974159\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Direct interface of CD36\\u2013Insig-2 binding undefined\", \"Mechanism relies on inferred displacement of SCAP\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Expanded Insig function beyond lipid feedback by showing Insig proteins couple oxysterol sensing to PERK\\u2013eIF2\\u03b1\\u2013ATF4 activation and downstream cell death effectors.\",\n      \"evidence\": \"INSIG1/2-deficient CHO and Huh7 cells with genetic rescue and oxysterol treatment\",\n      \"pmids\": [\"34298014\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism connecting Insig sterol binding to PERK activation undefined\", \"Insig-2-specific contribution not isolated from Insig-1\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Defined an Insig-2-specific phosphorylation switch: EPA-driven PKA phosphorylation of Ser-106 selectively blocks SREBP-1 processing and fatty acid synthesis, uncoupling it from cholesterol synthesis.\",\n      \"evidence\": \"In vitro PKA phosphorylation, S106A mutagenesis, PKA inhibition, and Insig-2-null fibroblast rescue with SREBP processing assays\",\n      \"pmids\": [\"39145929\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"How phospho-Ser-106 mechanistically discriminates SREBP-1 from SREBP-2 unresolved\", \"In vivo relevance in liver physiology not established\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Identified INSIG2 as a context-specific proliferation vulnerability in HBV-integrated hepatoma cells, acting upstream of CDK2 in G1-to-S progression.\",\n      \"evidence\": \"Genome-wide CRISPR screen, siRNA/CRISPR knockout, CDK2 inhibitor epistasis and cell cycle analysis\",\n      \"pmids\": [\"38287498\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism linking Insig-2 to CDK2 unknown\", \"Specificity to HBV-integrated context not mechanistically explained\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the multiple regulatory inputs (transcriptional, phosphorylation, redox, and protein-displacement) are integrated in vivo to set tissue-specific lipid synthesis thresholds remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No unified in vivo model integrating CREBH/insulin/HIF-1\\u03b1/FXR\\u03b1 transcriptional control with Ser-106 phosphorylation and Cys215 redox switching\", \"Structural basis of the Insig-2\\u2013HMGCR complex unresolved\", \"Non-canonical roles (mitochondrial Bax, cancer cell cycle) lack mechanistic dissection\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [11, 0]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [0, 2, 3]},\n      {\"term_id\": \"GO:0140313\", \"supporting_discovery_ids\": [0, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005783\", \"supporting_discovery_ids\": [0, 2, 11]},\n      {\"term_id\": \"GO:0005789\", \"supporting_discovery_ids\": []}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [0, 2, 14]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [2, 8, 10]},\n      {\"term_id\": \"R-HSA-8953897\", \"supporting_discovery_ids\": [8, 13]}\n    ],\n    \"complexes\": [\n      \"SCAP\\u2013Insig-2 complex\"\n    ],\n    \"partners\": [\n      \"SCAP\",\n      \"HMGCR\",\n      \"gp78\",\n      \"CD36\"\n    ],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"tie","faith_supported":7,"faith_total":7,"faith_pct":100.0}}