{"gene":"FN1","run_date":"2026-06-09T23:54:44","timeline":{"discoveries":[{"year":1977,"finding":"LETS protein (FN1) is located predominantly at the cell-substrate interface and in cell-cell contact regions in subconfluent cultures, and forms a fibrillar network in dense cultures; transformed cells show greatly reduced surface LETS protein. Perturbation experiments showed that actin microfilament bundles and LETS protein respond coordinately to certain perturbants, supporting a functional link between FN1 and cell adhesion/cytoskeleton.","method":"Immunofluorescence, cytoskeletal perturbation agents (cytochalasin, colchicine), cell morphology analysis","journal":"The Journal of cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization experiments with functional perturbants, single lab, multiple conditions tested","pmids":["925079"],"is_preprint":false},{"year":1977,"finding":"LETS protein (FN1) is synthesized and secreted by normal cells into the medium at higher rates than transformed cells; exogenous addition of purified LETS protein to transformed cells restores cell attachment, spreading, alignment, and actin cable formation, establishing a direct functional role for FN1 in cell adhesion and cytoskeletal organization.","method":"Immunoprecipitation, biosynthesis assays, addition of purified LETS protein to transformed cells, morphological readout","journal":"Journal of supramolecular structure","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — purified protein addition with defined cellular phenotype, single lab, multiple orthogonal readouts","pmids":["616487"],"is_preprint":false},{"year":1978,"finding":"FN1 (LETS protein) colocalizes with actin microfilament bundles in a fibrillar pattern during and after cell spreading, with 80–100% correspondence when fibrillar patterns develop, suggesting a transmembrane relationship between microfilament bundles and FN1 that contributes to formation of attachment plaques.","method":"Double-label immunofluorescence for fibronectin and actin/intermediate filaments during cell spreading time course","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct co-localization experiment with temporal resolution, single lab, two-marker approach","pmids":["365353"],"is_preprint":false},{"year":1978,"finding":"Addition of purified LETS protein (FN1) to normal or transformed cells increases cell migration as measured by phagokinetic track formation on gold-coated coverslips and on plastic; added FN1 attaches in a fibrillar network with greater binding to normal than transformed cells.","method":"Phagokinetic track assay on gold particle-coated coverslips, cell migration on plastic, immunofluorescence of added protein","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — purified protein addition with quantitative migration readout, single lab, two assay types","pmids":["667950"],"is_preprint":false},{"year":1977,"finding":"During myoblast fusion (myogenesis), the fibrillar surface distribution of LETS protein (FN1) disappears from myotubes and total LETS protein is quantitatively reduced, as measured by immunofluorescence and radioimmunoassay, establishing a regulated downregulation of FN1 during muscle differentiation.","method":"Indirect immunofluorescence, radioimmunoassay","journal":"Cell","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — two orthogonal quantitative methods, single lab, defined developmental model","pmids":["321128"],"is_preprint":false},{"year":1978,"finding":"LETS protein (FN1) is expressed on endothelial cells, choroid epithelial cells, fibroblasts, and leptomeningeal cells in the nervous system, but not on glial or neuronal cells; subcellular localization shows FN1 at the luminal surface of endothelial cells and at the basal end (not apical) of choroid epithelial cells, demonstrating cell-type-specific and polarized subcellular distribution.","method":"Immunofluorescence in mammalian and avian nervous tissue sections across developmental stages","journal":"Brain research","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct localization across multiple species and developmental stages, single lab","pmids":["21348357"],"is_preprint":false},{"year":1978,"finding":"In somatic cell hybrids, high levels of LETS protein (FN1) and extensive microfilament bundles correlate with normal growth control, while reduced or absent LETS protein correlates with transformed growth characteristics, supporting a role for FN1 in normal growth regulation linked to cytoskeletal organization.","method":"Lactoperoxidase-catalyzed radioiodination for LETS protein quantification, indirect immunofluorescence for actin/myosin, hybrid cell growth control assays","journal":"Journal of cell science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple hybrid lines with quantitative readouts, single lab, genetic epistasis approach","pmids":["363730"],"is_preprint":false},{"year":2016,"finding":"In phosphaturic mesenchymal tumors (PMTs), recurrent FN1-FGFR1 fusion genes (present in ~42% of cases) and a novel FN1-FGF1 fusion gene (present in ~6%) were identified; FN1-FGF1 fusion protein is predicted to be secreted and to serve as a ligand activating FGFR1 via an autocrine loop, implicating FN1 domain-driven FGF signaling in PMT pathogenesis.","method":"RNA sequencing, Sanger sequencing validation, FISH, western blot, immunohistochemistry","journal":"Modern pathology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal validation methods (RNA-seq, Sanger, FISH, WB), single large series","pmids":["27443518"],"is_preprint":false},{"year":2016,"finding":"In calcifying aponeurotic fibroma (CAF), recurrent FN1-EGF fusion genes were identified; FN1 exons are fused to EGF exon 17 or 19, with strong FN1 promoter activity driving inappropriate expression of the biologically active EGF portion, detected immunohistochemically; this suggests an autocrine/paracrine EGF signaling mechanism driven by the FN1 promoter.","method":"Chromosome banding, FISH, RNA sequencing, RT-PCR, immunohistochemistry","journal":"The Journal of pathology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multiple orthogonal methods, validated in index and additional cases, single lab","pmids":["26691015"],"is_preprint":false},{"year":2019,"finding":"FN1-ACVR2A rearrangements are recurrent in synovial chondromatosis (57%) and chondrosarcoma secondary to synovial chondromatosis (75%), as demonstrated by FISH and RNA sequencing; FN1 is also rearranged in soft tissue chondromas with FN1-FGFR2 fusions, establishing FN1 gene rearrangement as the defining molecular event in these cartilaginous tumors.","method":"FISH, RNA sequencing, RT-PCR","journal":"Modern pathology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — reciprocal validation by FISH and RNA-seq, multi-case series, single lab","pmids":["31273315"],"is_preprint":false},{"year":2018,"finding":"FN1 protein binds ITGA5 (integrin alpha-5) in colorectal cancer cells, as demonstrated by co-immunoprecipitation; ITGA5 overexpression reverses the suppression of proliferation, migration, invasion, and apoptosis caused by FN1 knockdown, establishing FN1-ITGA5 interaction as functionally required for FN1-mediated colorectal cancer tumorigenesis.","method":"siRNA knockdown, western blot, Co-IP for FN1-ITGA5 interaction, proliferation/migration/invasion assays, apoptosis assay","journal":"Journal of cellular biochemistry","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP plus rescue experiment, single lab, multiple functional readouts","pmids":["29274284"],"is_preprint":false},{"year":2020,"finding":"FN1 is degraded via the autophagy-lysosome pathway in a p62/SQSTM1-dependent manner: rapamycin and EBSS promote autophagy and increase FN1 degradation, autophagy inhibitors reduce FN1 degradation, and immunoprecipitation assays show p62/SQSTM1 physically interacts with FN1 as an autophagy adapter; p62 mutation abolishes this interaction.","method":"Pharmacological autophagy modulation, lysosomal/proteasomal inhibitors (MG132, bafilomycin A1, chloroquine), immunoprecipitation, p62 mutant cell lines","journal":"International journal of oral science","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — Co-IP with mutant validation, multiple pharmacological orthogonal approaches, single lab","pmids":["33318468"],"is_preprint":false},{"year":2023,"finding":"Exercise induces skeletal muscle secretion of FN1 into circulation; muscle-secreted FN1 activates hepatic autophagy via the hepatic receptor α5β1 integrin and downstream IKKα/β-JNK1-BECN1 signaling pathway, mediating systemic insulin sensitization; this was demonstrated by proteomic identification of FN1 as an exercise-induced circulating factor, and by muscle-specific FN1 knockout experiments.","method":"Proteomics of exercise mouse plasma, muscle-specific knockout, hepatic autophagy assays, integrin receptor blocking, insulin sensitization assays","journal":"Cell metabolism","confidence":"High","confidence_rationale":"Tier 1-2 / Strong — multiple orthogonal methods (proteomics, genetic KO, pathway dissection), single rigorous study with in vivo validation","pmids":["36812915"],"is_preprint":false},{"year":2019,"finding":"IRE1α regulates colon cancer cell metastasis by controlling FN1 expression: IRE1α activates XBP1s, which binds the FN1 promoter to initiate FN1 transcription; FN1 in turn activates Src/FAK phosphorylation and downstream GTPases (RhoA, Rac1, CDC42); exogenous FN1 addition rescues Src/FAK phosphorylation and migration inhibited by IRE1α knockdown.","method":"IRE1α siRNA knockdown, XBP1s ChIP on FN1 promoter, exogenous FN1 rescue, Src/FAK phosphorylation assays, GTPase activation assays, migration/invasion assays","journal":"The international journal of biochemistry & cell biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP plus rescue experiment plus phosphorylation readouts, single lab, multiple orthogonal methods","pmids":["31326465"],"is_preprint":false},{"year":2016,"finding":"HMGA2 directly binds the FN1 promoter and transcriptionally activates FN1 expression in colorectal cancer, as demonstrated by chromatin immunoprecipitation-PCR and luciferase assays; HMGA2-driven FN1 upregulation contributes to enhanced migration and invasion in vitro and metastasis in vivo.","method":"Chromatin immunoprecipitation (ChIP)-PCR, luciferase reporter assay, ectopic HMGA2 expression/silencing, in vivo metastasis model","journal":"Carcinogenesis","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct promoter binding confirmed by ChIP and luciferase, in vivo validation, single lab","pmids":["26964871"],"is_preprint":false},{"year":2022,"finding":"HOXA13 directly binds the FN1 promoter region to enhance FN1 transcription, activating the FAK/Src signaling axis; co-immunoprecipitation confirmed that FN1 interacts with ITGA5 and ITGB1 (integrin α5β1) in gastric cancer cells; rescue experiments showed FN1 is required for HOXA13-mediated promotion of gastric cancer metastasis.","method":"ChIP, dual luciferase assay, Co-IP (FN1-ITGA5/ITGB1), FAK/Src phosphorylation assays, rescue experiments, in vivo model","journal":"Experimental hematology & oncology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — ChIP + Co-IP + rescue, multiple orthogonal methods, single lab","pmids":["35197128"],"is_preprint":false},{"year":2024,"finding":"A rare FN1 variant (rs140926439) is protective against APOEε4-mediated Alzheimer's disease risk (OR=0.29) and delays age at onset; in zebrafish, loss-of-function mutations in fn1b (FN1 ortholog) reduced gliosis, enhanced gliovascular remodeling, and potentiated microglial response, suggesting that pathological FN1 accumulation at the blood-brain barrier impairs toxic protein clearance.","method":"Whole-genome sequencing, genetic association analysis, zebrafish loss-of-function models, immunofluorescence for gliosis and microglial markers","journal":"Acta neuropathologica","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — in vivo genetic KO in zebrafish with defined cellular phenotypes, large human cohort validation, single study","pmids":["38598053"],"is_preprint":false},{"year":2021,"finding":"In calcified chondroid mesenchymal neoplasms, FN1 fuses with multiple receptor tyrosine kinase genes (FGFR2, FGFR1, MERTK, NTRK1, TEK); breakpoints in FN1 range from exons 11–48 retaining signal peptide, FN1, FN2, and/or FN3 domains, while partner genes retain transmembrane and tyrosine kinase domains, suggesting FN1 acts as a membrane-targeting/secretion signal driving constitutive RTK activation.","method":"Targeted RNA-seq (115-gene panel), Sanger sequencing for breakpoint validation, FISH","journal":"Modern pathology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — precise breakpoint mapping by RNA-seq and sequencing in 12-case series, single lab","pmids":["33727696"],"is_preprint":false},{"year":2023,"finding":"A novel pathogenic FN1 mutation (c.3415G>A) causes glomerular fibronectin-specific deposition in a gain-of-function manner; the variant increases fibronectin binding to integrin (maintaining podocyte adhesion) but decreases fibronectin's capacity to bind COL4A3/4 (collagen IV), providing a mechanism for both the glomerulopathy and associated thin basement membrane nephropathy.","method":"Genetic sequencing, in vitro binding assays (FN1 variant vs. integrin and collagen IV), patient tissue analysis","journal":"Pathology","confidence":"Medium","confidence_rationale":"Tier 1-2 / Moderate — functional binding assays with specific variant and defined substrates, single lab","pmids":["36774238"],"is_preprint":false},{"year":2021,"finding":"FN1 promotes glioblastoma cell proliferation by inducing PTPRM promoter methylation, which reduces PTPRM expression and leads to increased STAT3 phosphorylation; FN1 overexpression decreases PTPRM protein, and demethylating agent 5-aza reverses FN1-induced STAT3 phosphorylation and cell viability, placing FN1 upstream of PTPRM methylation and STAT3 activation.","method":"Lentiviral overexpression/knockdown, methylation-specific PCR, 5-aza demethylation treatment, STAT3 inhibitor (stattic), western blot, cell viability assay","journal":"Pharmaceutical biology","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — pharmacological and genetic manipulation with multiple mechanistic readouts, single lab","pmids":["34225581"],"is_preprint":false},{"year":2023,"finding":"FN1 promotes breast cancer progression by activating the YAP1/Hippo pathway (via reduced YAP1 phosphorylation) and upregulating SLC1A3-mediated aspartate uptake; silencing FN1 enhances YAP1 phosphorylation, reduces aspartate uptake, and inhibits proliferation, invasion and migration; combined FN1 inhibition with YAP1 or SLC1A3 inhibitors synergistically suppresses tumor growth.","method":"FN1 siRNA knockdown, RNA-seq, LC-MS metabolomics, western blot, in vivo tumor models, YAP1/SLC1A3 inhibitor combination studies","journal":"Breast cancer research and treatment","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — multi-omics plus in vivo validation, single lab, multiple orthogonal methods","pmids":["37458908"],"is_preprint":false},{"year":2022,"finding":"FN1 activates the FAK/Src signaling axis and downstream NF-κB pathway in thyroid carcinoma; overexpression of FN1 in MDA-T85 cells promoted growth, migration and invasion with increased p-IκB-α, p-IKK-β, and NF-κB p65 expression, while FN1 knockdown in MDA-T41 cells had the opposite effect.","method":"FN1 overexpression/knockdown, western blot for NF-κB pathway components, CCK-8, EDU, scratch, transwell assays","journal":"Protein and peptide letters","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, correlative pathway activation without direct rescue or epistasis confirmation","pmids":["36278453"],"is_preprint":false},{"year":2024,"finding":"DPSCs-secreted FN1 promotes endothelial cell proliferation, migration, and tube formation via ITGA5 (integrin α5) and downstream PI3K/AKT signaling during dental pulp development; this was validated by FN1 recombinant protein treatment of HUVECs and in vivo mouse experiments.","method":"Single-cell sequencing, recombinant FN1 protein treatment, ITGA5 blocking, PI3K/AKT pathway inhibition, tube formation assay, in vivo mouse model, western blot","journal":"Stem cell reviews and reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — recombinant protein plus receptor blocking plus in vivo validation, single lab","pmids":["38418738"],"is_preprint":false},{"year":2025,"finding":"CAF-derived FN1 activates the integrin-PI3K/AKT signaling pathway in pancreatic cancer cells to promote metastasis; combined inhibition of PI3K/AKT and integrins synergistically suppressed tumor invasion in vitro; high FN1 expression correlated with M2 macrophage/Treg immunosuppressive microenvironment.","method":"Transcriptomic/single-cell sequencing analysis, in vitro co-culture, Transwell invasion assay, PI3K/AKT inhibitor, integrin blocking, western blot","journal":"Frontiers in oncology","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, pathway activation assays without direct FN1 rescue in co-culture model","pmids":["40678072"],"is_preprint":false},{"year":2022,"finding":"GATA6 transcription factor directly binds the FN1 promoter and activates FN1 transcription in oral squamous cell carcinoma; GATA6 knockdown-mediated suppression of proliferation, colony formation, invasion, and migration is rescued by FN1 overexpression.","method":"Dual-luciferase reporter assay, chromatin immunoprecipitation (ChIP), GATA6 knockdown/FN1 overexpression rescue experiments","journal":"Molecular medicine reports","confidence":"Medium","confidence_rationale":"Tier 2 / Moderate — direct promoter binding confirmed by ChIP and luciferase plus rescue, single lab","pmids":["35088888"],"is_preprint":false},{"year":2022,"finding":"SOX2 transcriptionally upregulates FN1 expression in Schwann cell-like cells (iSCs), promoting proliferation and migration (fibronectin fibrillogenesis); exosomes secreted by iSCs also increase Schwann cell viability and migration; the SOX2/FN1 axis was demonstrated by RNA-seq and functional experiments in sciatic nerve repair.","method":"SC RNA-seq, SOX2 overexpression/knockdown, scratch wound assay, EdU proliferation assay, sciatic nerve transection animal model","journal":"International journal of molecular medicine","confidence":"Low","confidence_rationale":"Tier 3 / Weak — single lab, SOX2-FN1 link by RNA-seq correlation and overexpression, no direct promoter binding assay reported","pmids":["35475578"],"is_preprint":false}],"current_model":"FN1 (fibronectin 1) is a secreted extracellular matrix glycoprotein that acts at the cell surface to promote adhesion, spreading, and migration via transmembrane linkage to actin microfilament bundles; it signals intracellularly through integrin receptors (particularly α5β1/ITGA5-ITGB1), activating FAK/Src, PI3K/AKT, and downstream GTPase/NF-κB/YAP1 pathways; its transcription is directly regulated by factors including HMGA2, HOXA13, and GATA6 binding to its promoter, and it is degraded via the p62/SQSTM1-autophagy-lysosome pathway; muscle-secreted FN1 also functions as a circulating exercise-induced factor that activates hepatic autophagy and insulin sensitization through α5β1 integrin and IKKα/β-JNK1-BECN1 signaling; oncogenic FN1 gene fusions (e.g., FN1-FGFR1, FN1-EGF, FN1-ACVR2A) place receptor tyrosine kinase or growth factor domains under FN1 promoter/secretion control, driving constitutive signaling in multiple mesenchymal tumor types."},"narrative":{"mechanistic_narrative":"FN1 encodes a secreted extracellular matrix glycoprotein that organizes cell adhesion, spreading, and migration by linking the cell surface to actin microfilament bundles, with surface FN1 forming fibrillar networks whose loss accompanies cellular transformation and loss of normal growth control [PMID:925079, PMID:616487, PMID:363730]. FN1 colocalizes with actin bundles during cell spreading, implying a transmembrane connection that nucleates attachment plaques, and exogenous purified FN1 restores attachment, spreading, alignment, and migration to transformed cells [PMID:365353, PMID:667950]. Its distribution is cell-type-specific and polarized—present at endothelial luminal surfaces and basal choroid epithelium—and is developmentally downregulated during myoblast fusion [PMID:321128, PMID:21348357]. FN1 signals into cells chiefly through the α5β1 integrin (ITGA5/ITGB1), an interaction shown by co-immunoprecipitation and required for FN1-driven proliferation, migration, and invasion, which activates FAK/Src and downstream RhoA/Rac1/CDC42 GTPases as well as PI3K/AKT, NF-κB, STAT3 (via PTPRM promoter methylation), and YAP1/Hippo with SLC1A3-mediated aspartate uptake in multiple tumor contexts [PMID:29274284, PMID:31326465, PMID:35197128, PMID:34225581, PMID:37458908]. FN1 transcription is directly driven by HMGA2, HOXA13, and GATA6 binding to its promoter and by the IRE1α–XBP1s axis, while FN1 protein is turned over through p62/SQSTM1-dependent autophagy-lysosome degradation [PMID:33318468, PMID:31326465, PMID:26964871, PMID:35197128, PMID:35088888]. Beyond its matrix role, exercise-induced muscle-secreted FN1 acts as a circulating factor that engages hepatic α5β1 integrin and IKKα/β–JNK1–BECN1 signaling to activate hepatic autophagy and systemic insulin sensitization [PMID:36812915]. Recurrent oncogenic FN1 gene fusions place receptor tyrosine kinase or growth-factor domains (FGFR1, FGF1, EGF, ACVR2A, FGFR2, MERTK, NTRK1, TEK) under FN1 promoter and secretion/membrane-targeting control, driving constitutive signaling across phosphaturic mesenchymal tumors, calcifying aponeurotic fibroma, synovial chondromatosis, and calcified chondroid mesenchymal neoplasms [PMID:27443518, PMID:26691015, PMID:31273315, PMID:33727696]. A gain-of-function FN1 variant that raises integrin binding while reducing collagen IV (COL4A3/4) binding causes glomerular fibronectin deposition, and a rare FN1 variant modifies APOEε4-mediated Alzheimer's disease risk through altered blood-brain barrier clearance [PMID:38598053, PMID:36774238].","teleology":[{"year":1978,"claim":"Established that FN1 (LETS protein) is a cell-surface/matrix glycoprotein physically and functionally coupled to the actin cytoskeleton, answering whether an adhesion molecule could transduce structure across the membrane.","evidence":"Immunofluorescence localization, cytoskeletal perturbants, and double-label co-localization with actin during cell spreading in cultured cells","pmids":["925079","365353"],"confidence":"Medium","gaps":["No molecular identification of the transmembrane linker","Correlative co-localization does not prove direct receptor engagement"]},{"year":1978,"claim":"Demonstrated FN1 is a sufficient functional effector of adhesion and motility, since purified protein added back to transformed cells restored attachment, spreading, and migration—linking reduced surface FN1 to the transformed phenotype and growth control.","evidence":"Addition of purified LETS protein to normal/transformed cells, phagokinetic track migration assays, radioiodination quantification in somatic cell hybrids","pmids":["616487","667950","363730"],"confidence":"Medium","gaps":["No receptor or signaling pathway identified at this stage","Growth-control correlation in hybrids is genetic association, not direct mechanism"]},{"year":1978,"claim":"Showed FN1 expression is cell-type-restricted, polarized, and developmentally regulated, indicating its deposition is spatially and temporally controlled rather than constitutive.","evidence":"Immunofluorescence across nervous tissue cell types and species; immunofluorescence/radioimmunoassay during myoblast fusion","pmids":["21348357","321128"],"confidence":"Medium","gaps":["Transcriptional regulators driving polarization unknown","Functional consequence of downregulation during myogenesis not tested"]},{"year":2018,"claim":"Identified the integrin α5β1 receptor (ITGA5/ITGB1) as the direct functional partner mediating FN1's pro-tumorigenic effects, answering how secreted FN1 signals into cells.","evidence":"Co-immunoprecipitation and ITGA5 rescue of FN1 knockdown phenotypes in colorectal cancer cells; Co-IP of FN1-ITGA5/ITGB1 in gastric cancer","pmids":["29274284","35197128"],"confidence":"Medium","gaps":["Binding interface and stoichiometry not defined","Single-cell-line Co-IP without structural validation"]},{"year":2019,"claim":"Connected FN1 to defined intracellular signaling cascades, showing it activates Src/FAK and downstream Rho-family GTPases to drive migration, with exogenous FN1 rescuing pathway activity.","evidence":"IRE1α/XBP1s ChIP on FN1 promoter, phosphorylation and GTPase activation assays, exogenous FN1 rescue in colon cancer","pmids":["31326465"],"confidence":"Medium","gaps":["Receptor proximal to FAK/Src activation not directly demonstrated here","Single cancer context"]},{"year":2022,"claim":"Defined the transcriptional control of FN1, showing HMGA2, HOXA13, and GATA6 directly bind its promoter to drive FN1-dependent invasion and metastasis, while IRE1α-XBP1s provides a stress-responsive input.","evidence":"ChIP-PCR, dual-luciferase reporter assays, and FN1-overexpression rescue across colorectal, gastric, and oral squamous carcinoma models with in vivo validation","pmids":["26964871","35197128","35088888"],"confidence":"Medium","gaps":["Combinatorial regulation among these factors unresolved","Promoter elements bound not finely mapped"]},{"year":2021,"claim":"Expanded the downstream signaling repertoire of FN1 to STAT3 (via PTPRM promoter methylation), NF-κB, PI3K/AKT, and YAP1/Hippo-coupled metabolic uptake, establishing FN1 as a node feeding multiple oncogenic pathways.","evidence":"Genetic/pharmacological manipulation with methylation-specific PCR, demethylation, pathway inhibitor combinations, multi-omics, and in vivo models in glioblastoma, breast, thyroid, and pancreatic cancer","pmids":["34225581","37458908","36278453","40678072"],"confidence":"Medium","gaps":["Pathway selectivity across tumor types unexplained","Thyroid and pancreatic studies are correlative without direct FN1 rescue"]},{"year":2020,"claim":"Identified how FN1 protein levels are controlled post-translationally, showing p62/SQSTM1 acts as an autophagy adapter targeting FN1 to lysosomal degradation.","evidence":"Pharmacological autophagy/lysosome modulation, immunoprecipitation, and p62 mutant cell lines","pmids":["33318468"],"confidence":"Medium","gaps":["Whether intracellular or secreted FN1 pool is degraded not fully resolved","Single Co-IP system"]},{"year":2023,"claim":"Revealed an endocrine role for FN1 beyond the matrix: exercise-induced muscle-secreted FN1 signals to liver via α5β1 integrin to activate hepatic autophagy and systemic insulin sensitization.","evidence":"Plasma proteomics, muscle-specific FN1 knockout, hepatic autophagy assays, integrin blockade, and insulin sensitization assays in mice","pmids":["36812915"],"confidence":"High","gaps":["Human relevance of the muscle-liver FN1 axis not established","Distinction between this circulating FN1 and matrix FN1 forms unclear"]},{"year":2021,"claim":"Established recurrent FN1 gene fusions as defining oncogenic events, where the FN1 promoter and secretion/membrane-targeting domains drive constitutive RTK or growth-factor signaling across distinct mesenchymal tumors.","evidence":"RNA sequencing, FISH, Sanger breakpoint mapping, IHC, and Western blot across PMT, CAF, synovial chondromatosis, and calcified chondroid mesenchymal neoplasms","pmids":["27443518","26691015","31273315","33727696"],"confidence":"Medium","gaps":["Functional reconstitution of fusion signaling largely predicted not directly assayed","Therapeutic targetability not tested in these series"]},{"year":2024,"claim":"Linked FN1 sequence variation directly to human disease through gain-of-function binding changes, providing mechanism for glomerular fibronectin deposition and a modifier role in Alzheimer's disease.","evidence":"Genetic sequencing with in vitro integrin/collagen IV binding assays for the c.3415G>A glomerulopathy variant; whole-genome association plus zebrafish fn1b loss-of-function for the APOEε4-protective variant","pmids":["36774238","38598053"],"confidence":"Medium","gaps":["Causality of the Alzheimer's variant rests on ortholog modeling","Tissue-specific consequences of altered binding not fully characterized"]},{"year":null,"claim":"How the distinct FN1 functional pools—matrix-bound adhesive fibronectin, autophagy-degraded intracellular FN1, and circulating endocrine FN1—are differentially produced, modified, and targeted to specific receptors and tissues remains unresolved.","evidence":"","pmids":[],"confidence":"Medium","gaps":["No structural model of FN1-integrin engagement in the timeline","Mechanism distinguishing endocrine vs matrix FN1 function unknown","Whether fusion-driven signaling requires the same domains as native FN1 secretion untested"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0098631","term_label":"cell adhesion mediator activity","supporting_discovery_ids":[0,1,2,3,10,15]},{"term_id":"GO:0048018","term_label":"receptor ligand activity","supporting_discovery_ids":[12,22]},{"term_id":"GO:0060089","term_label":"molecular transducer activity","supporting_discovery_ids":[10,13,15,22]},{"term_id":"GO:0005198","term_label":"structural molecule activity","supporting_discovery_ids":[0,2]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[1,12]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[0,2]},{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,5]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[10,13,15,20,22]},{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[0,1,2]},{"term_id":"R-HSA-9612973","term_label":"Autophagy","supporting_discovery_ids":[11,12]},{"term_id":"R-HSA-1643685","term_label":"Disease","supporting_discovery_ids":[7,8,9,17,18]},{"term_id":"R-HSA-74160","term_label":"Gene expression (Transcription)","supporting_discovery_ids":[13,14,15,24]}],"complexes":[],"partners":["ITGA5","ITGB1","SQSTM1","COL4A3"],"other_free_text":[]}},"prefetch_data":{"uniprot":{"accession":"P02751","full_name":"Fibronectin","aliases":["Cold-insoluble globulin","CIG"],"length_aa":2477,"mass_kda":272.3,"function":"Fibronectins bind cell surfaces and various compounds including collagen, fibrin, heparin, DNA, and actin (PubMed:3024962, PubMed:3593230, PubMed:3900070, PubMed:7989369). Fibronectins are involved in cell adhesion, cell motility, opsonization, wound healing, and maintenance of cell shape (PubMed:3024962, PubMed:3593230, PubMed:3900070, PubMed:7989369). Involved in osteoblast compaction through the fibronectin fibrillogenesis cell-mediated matrix assembly process, essential for osteoblast mineralization (By similarity). Participates in the regulation of type I collagen deposition by osteoblasts (By similarity). Acts as a ligand for the LILRB4 receptor, inhibiting FCGR1A/CD64-mediated monocyte activation (PubMed:34089617) Binds fibronectin and induces fibril formation. This fibronectin polymer, named superfibronectin, exhibits enhanced adhesive properties. Both anastellin and superfibronectin inhibit tumor growth, angiogenesis and metastasis. Anastellin activates p38 MAPK and inhibits lysophospholipid signaling Secreted by contracting muscle, induces liver autophagy, a degradative pathway for nutrient mobilization and damage removal, and systemic insulin sensitization via hepatic ITGA5:ITGB1 integrin receptor signaling","subcellular_location":"Secreted, extracellular space, extracellular matrix; Secreted","url":"https://www.uniprot.org/uniprotkb/P02751/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FN1","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/FN1","total_profiled":1310},"omim":[{"mim_id":"620638","title":"POTASSIUM CHANNEL TETRAMERIZATION DOMAIN-CONTAINING PROTEIN 4; KCTD4","url":"https://www.omim.org/entry/620638"},{"mim_id":"619811","title":"UHRF1-BINDING PROTEIN 1-LIKE; 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Perturbation experiments showed that actin microfilament bundles and LETS protein respond coordinately to certain perturbants, supporting a functional link between FN1 and cell adhesion/cytoskeleton.\",\n      \"method\": \"Immunofluorescence, cytoskeletal perturbation agents (cytochalasin, colchicine), cell morphology analysis\",\n      \"journal\": \"The Journal of cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization experiments with functional perturbants, single lab, multiple conditions tested\",\n      \"pmids\": [\"925079\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1977,\n      \"finding\": \"LETS protein (FN1) is synthesized and secreted by normal cells into the medium at higher rates than transformed cells; exogenous addition of purified LETS protein to transformed cells restores cell attachment, spreading, alignment, and actin cable formation, establishing a direct functional role for FN1 in cell adhesion and cytoskeletal organization.\",\n      \"method\": \"Immunoprecipitation, biosynthesis assays, addition of purified LETS protein to transformed cells, morphological readout\",\n      \"journal\": \"Journal of supramolecular structure\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — purified protein addition with defined cellular phenotype, single lab, multiple orthogonal readouts\",\n      \"pmids\": [\"616487\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1978,\n      \"finding\": \"FN1 (LETS protein) colocalizes with actin microfilament bundles in a fibrillar pattern during and after cell spreading, with 80–100% correspondence when fibrillar patterns develop, suggesting a transmembrane relationship between microfilament bundles and FN1 that contributes to formation of attachment plaques.\",\n      \"method\": \"Double-label immunofluorescence for fibronectin and actin/intermediate filaments during cell spreading time course\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct co-localization experiment with temporal resolution, single lab, two-marker approach\",\n      \"pmids\": [\"365353\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1978,\n      \"finding\": \"Addition of purified LETS protein (FN1) to normal or transformed cells increases cell migration as measured by phagokinetic track formation on gold-coated coverslips and on plastic; added FN1 attaches in a fibrillar network with greater binding to normal than transformed cells.\",\n      \"method\": \"Phagokinetic track assay on gold particle-coated coverslips, cell migration on plastic, immunofluorescence of added protein\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — purified protein addition with quantitative migration readout, single lab, two assay types\",\n      \"pmids\": [\"667950\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1977,\n      \"finding\": \"During myoblast fusion (myogenesis), the fibrillar surface distribution of LETS protein (FN1) disappears from myotubes and total LETS protein is quantitatively reduced, as measured by immunofluorescence and radioimmunoassay, establishing a regulated downregulation of FN1 during muscle differentiation.\",\n      \"method\": \"Indirect immunofluorescence, radioimmunoassay\",\n      \"journal\": \"Cell\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — two orthogonal quantitative methods, single lab, defined developmental model\",\n      \"pmids\": [\"321128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1978,\n      \"finding\": \"LETS protein (FN1) is expressed on endothelial cells, choroid epithelial cells, fibroblasts, and leptomeningeal cells in the nervous system, but not on glial or neuronal cells; subcellular localization shows FN1 at the luminal surface of endothelial cells and at the basal end (not apical) of choroid epithelial cells, demonstrating cell-type-specific and polarized subcellular distribution.\",\n      \"method\": \"Immunofluorescence in mammalian and avian nervous tissue sections across developmental stages\",\n      \"journal\": \"Brain research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct localization across multiple species and developmental stages, single lab\",\n      \"pmids\": [\"21348357\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1978,\n      \"finding\": \"In somatic cell hybrids, high levels of LETS protein (FN1) and extensive microfilament bundles correlate with normal growth control, while reduced or absent LETS protein correlates with transformed growth characteristics, supporting a role for FN1 in normal growth regulation linked to cytoskeletal organization.\",\n      \"method\": \"Lactoperoxidase-catalyzed radioiodination for LETS protein quantification, indirect immunofluorescence for actin/myosin, hybrid cell growth control assays\",\n      \"journal\": \"Journal of cell science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple hybrid lines with quantitative readouts, single lab, genetic epistasis approach\",\n      \"pmids\": [\"363730\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In phosphaturic mesenchymal tumors (PMTs), recurrent FN1-FGFR1 fusion genes (present in ~42% of cases) and a novel FN1-FGF1 fusion gene (present in ~6%) were identified; FN1-FGF1 fusion protein is predicted to be secreted and to serve as a ligand activating FGFR1 via an autocrine loop, implicating FN1 domain-driven FGF signaling in PMT pathogenesis.\",\n      \"method\": \"RNA sequencing, Sanger sequencing validation, FISH, western blot, immunohistochemistry\",\n      \"journal\": \"Modern pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal validation methods (RNA-seq, Sanger, FISH, WB), single large series\",\n      \"pmids\": [\"27443518\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"In calcifying aponeurotic fibroma (CAF), recurrent FN1-EGF fusion genes were identified; FN1 exons are fused to EGF exon 17 or 19, with strong FN1 promoter activity driving inappropriate expression of the biologically active EGF portion, detected immunohistochemically; this suggests an autocrine/paracrine EGF signaling mechanism driven by the FN1 promoter.\",\n      \"method\": \"Chromosome banding, FISH, RNA sequencing, RT-PCR, immunohistochemistry\",\n      \"journal\": \"The Journal of pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multiple orthogonal methods, validated in index and additional cases, single lab\",\n      \"pmids\": [\"26691015\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"FN1-ACVR2A rearrangements are recurrent in synovial chondromatosis (57%) and chondrosarcoma secondary to synovial chondromatosis (75%), as demonstrated by FISH and RNA sequencing; FN1 is also rearranged in soft tissue chondromas with FN1-FGFR2 fusions, establishing FN1 gene rearrangement as the defining molecular event in these cartilaginous tumors.\",\n      \"method\": \"FISH, RNA sequencing, RT-PCR\",\n      \"journal\": \"Modern pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — reciprocal validation by FISH and RNA-seq, multi-case series, single lab\",\n      \"pmids\": [\"31273315\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"FN1 protein binds ITGA5 (integrin alpha-5) in colorectal cancer cells, as demonstrated by co-immunoprecipitation; ITGA5 overexpression reverses the suppression of proliferation, migration, invasion, and apoptosis caused by FN1 knockdown, establishing FN1-ITGA5 interaction as functionally required for FN1-mediated colorectal cancer tumorigenesis.\",\n      \"method\": \"siRNA knockdown, western blot, Co-IP for FN1-ITGA5 interaction, proliferation/migration/invasion assays, apoptosis assay\",\n      \"journal\": \"Journal of cellular biochemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP plus rescue experiment, single lab, multiple functional readouts\",\n      \"pmids\": [\"29274284\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FN1 is degraded via the autophagy-lysosome pathway in a p62/SQSTM1-dependent manner: rapamycin and EBSS promote autophagy and increase FN1 degradation, autophagy inhibitors reduce FN1 degradation, and immunoprecipitation assays show p62/SQSTM1 physically interacts with FN1 as an autophagy adapter; p62 mutation abolishes this interaction.\",\n      \"method\": \"Pharmacological autophagy modulation, lysosomal/proteasomal inhibitors (MG132, bafilomycin A1, chloroquine), immunoprecipitation, p62 mutant cell lines\",\n      \"journal\": \"International journal of oral science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — Co-IP with mutant validation, multiple pharmacological orthogonal approaches, single lab\",\n      \"pmids\": [\"33318468\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"Exercise induces skeletal muscle secretion of FN1 into circulation; muscle-secreted FN1 activates hepatic autophagy via the hepatic receptor α5β1 integrin and downstream IKKα/β-JNK1-BECN1 signaling pathway, mediating systemic insulin sensitization; this was demonstrated by proteomic identification of FN1 as an exercise-induced circulating factor, and by muscle-specific FN1 knockout experiments.\",\n      \"method\": \"Proteomics of exercise mouse plasma, muscle-specific knockout, hepatic autophagy assays, integrin receptor blocking, insulin sensitization assays\",\n      \"journal\": \"Cell metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 / Strong — multiple orthogonal methods (proteomics, genetic KO, pathway dissection), single rigorous study with in vivo validation\",\n      \"pmids\": [\"36812915\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2019,\n      \"finding\": \"IRE1α regulates colon cancer cell metastasis by controlling FN1 expression: IRE1α activates XBP1s, which binds the FN1 promoter to initiate FN1 transcription; FN1 in turn activates Src/FAK phosphorylation and downstream GTPases (RhoA, Rac1, CDC42); exogenous FN1 addition rescues Src/FAK phosphorylation and migration inhibited by IRE1α knockdown.\",\n      \"method\": \"IRE1α siRNA knockdown, XBP1s ChIP on FN1 promoter, exogenous FN1 rescue, Src/FAK phosphorylation assays, GTPase activation assays, migration/invasion assays\",\n      \"journal\": \"The international journal of biochemistry & cell biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP plus rescue experiment plus phosphorylation readouts, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"31326465\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"HMGA2 directly binds the FN1 promoter and transcriptionally activates FN1 expression in colorectal cancer, as demonstrated by chromatin immunoprecipitation-PCR and luciferase assays; HMGA2-driven FN1 upregulation contributes to enhanced migration and invasion in vitro and metastasis in vivo.\",\n      \"method\": \"Chromatin immunoprecipitation (ChIP)-PCR, luciferase reporter assay, ectopic HMGA2 expression/silencing, in vivo metastasis model\",\n      \"journal\": \"Carcinogenesis\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct promoter binding confirmed by ChIP and luciferase, in vivo validation, single lab\",\n      \"pmids\": [\"26964871\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"HOXA13 directly binds the FN1 promoter region to enhance FN1 transcription, activating the FAK/Src signaling axis; co-immunoprecipitation confirmed that FN1 interacts with ITGA5 and ITGB1 (integrin α5β1) in gastric cancer cells; rescue experiments showed FN1 is required for HOXA13-mediated promotion of gastric cancer metastasis.\",\n      \"method\": \"ChIP, dual luciferase assay, Co-IP (FN1-ITGA5/ITGB1), FAK/Src phosphorylation assays, rescue experiments, in vivo model\",\n      \"journal\": \"Experimental hematology & oncology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — ChIP + Co-IP + rescue, multiple orthogonal methods, single lab\",\n      \"pmids\": [\"35197128\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"A rare FN1 variant (rs140926439) is protective against APOEε4-mediated Alzheimer's disease risk (OR=0.29) and delays age at onset; in zebrafish, loss-of-function mutations in fn1b (FN1 ortholog) reduced gliosis, enhanced gliovascular remodeling, and potentiated microglial response, suggesting that pathological FN1 accumulation at the blood-brain barrier impairs toxic protein clearance.\",\n      \"method\": \"Whole-genome sequencing, genetic association analysis, zebrafish loss-of-function models, immunofluorescence for gliosis and microglial markers\",\n      \"journal\": \"Acta neuropathologica\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — in vivo genetic KO in zebrafish with defined cellular phenotypes, large human cohort validation, single study\",\n      \"pmids\": [\"38598053\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"In calcified chondroid mesenchymal neoplasms, FN1 fuses with multiple receptor tyrosine kinase genes (FGFR2, FGFR1, MERTK, NTRK1, TEK); breakpoints in FN1 range from exons 11–48 retaining signal peptide, FN1, FN2, and/or FN3 domains, while partner genes retain transmembrane and tyrosine kinase domains, suggesting FN1 acts as a membrane-targeting/secretion signal driving constitutive RTK activation.\",\n      \"method\": \"Targeted RNA-seq (115-gene panel), Sanger sequencing for breakpoint validation, FISH\",\n      \"journal\": \"Modern pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — precise breakpoint mapping by RNA-seq and sequencing in 12-case series, single lab\",\n      \"pmids\": [\"33727696\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"A novel pathogenic FN1 mutation (c.3415G>A) causes glomerular fibronectin-specific deposition in a gain-of-function manner; the variant increases fibronectin binding to integrin (maintaining podocyte adhesion) but decreases fibronectin's capacity to bind COL4A3/4 (collagen IV), providing a mechanism for both the glomerulopathy and associated thin basement membrane nephropathy.\",\n      \"method\": \"Genetic sequencing, in vitro binding assays (FN1 variant vs. integrin and collagen IV), patient tissue analysis\",\n      \"journal\": \"Pathology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 1-2 / Moderate — functional binding assays with specific variant and defined substrates, single lab\",\n      \"pmids\": [\"36774238\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2021,\n      \"finding\": \"FN1 promotes glioblastoma cell proliferation by inducing PTPRM promoter methylation, which reduces PTPRM expression and leads to increased STAT3 phosphorylation; FN1 overexpression decreases PTPRM protein, and demethylating agent 5-aza reverses FN1-induced STAT3 phosphorylation and cell viability, placing FN1 upstream of PTPRM methylation and STAT3 activation.\",\n      \"method\": \"Lentiviral overexpression/knockdown, methylation-specific PCR, 5-aza demethylation treatment, STAT3 inhibitor (stattic), western blot, cell viability assay\",\n      \"journal\": \"Pharmaceutical biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — pharmacological and genetic manipulation with multiple mechanistic readouts, single lab\",\n      \"pmids\": [\"34225581\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FN1 promotes breast cancer progression by activating the YAP1/Hippo pathway (via reduced YAP1 phosphorylation) and upregulating SLC1A3-mediated aspartate uptake; silencing FN1 enhances YAP1 phosphorylation, reduces aspartate uptake, and inhibits proliferation, invasion and migration; combined FN1 inhibition with YAP1 or SLC1A3 inhibitors synergistically suppresses tumor growth.\",\n      \"method\": \"FN1 siRNA knockdown, RNA-seq, LC-MS metabolomics, western blot, in vivo tumor models, YAP1/SLC1A3 inhibitor combination studies\",\n      \"journal\": \"Breast cancer research and treatment\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — multi-omics plus in vivo validation, single lab, multiple orthogonal methods\",\n      \"pmids\": [\"37458908\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"FN1 activates the FAK/Src signaling axis and downstream NF-κB pathway in thyroid carcinoma; overexpression of FN1 in MDA-T85 cells promoted growth, migration and invasion with increased p-IκB-α, p-IKK-β, and NF-κB p65 expression, while FN1 knockdown in MDA-T41 cells had the opposite effect.\",\n      \"method\": \"FN1 overexpression/knockdown, western blot for NF-κB pathway components, CCK-8, EDU, scratch, transwell assays\",\n      \"journal\": \"Protein and peptide letters\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, correlative pathway activation without direct rescue or epistasis confirmation\",\n      \"pmids\": [\"36278453\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"DPSCs-secreted FN1 promotes endothelial cell proliferation, migration, and tube formation via ITGA5 (integrin α5) and downstream PI3K/AKT signaling during dental pulp development; this was validated by FN1 recombinant protein treatment of HUVECs and in vivo mouse experiments.\",\n      \"method\": \"Single-cell sequencing, recombinant FN1 protein treatment, ITGA5 blocking, PI3K/AKT pathway inhibition, tube formation assay, in vivo mouse model, western blot\",\n      \"journal\": \"Stem cell reviews and reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — recombinant protein plus receptor blocking plus in vivo validation, single lab\",\n      \"pmids\": [\"38418738\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"CAF-derived FN1 activates the integrin-PI3K/AKT signaling pathway in pancreatic cancer cells to promote metastasis; combined inhibition of PI3K/AKT and integrins synergistically suppressed tumor invasion in vitro; high FN1 expression correlated with M2 macrophage/Treg immunosuppressive microenvironment.\",\n      \"method\": \"Transcriptomic/single-cell sequencing analysis, in vitro co-culture, Transwell invasion assay, PI3K/AKT inhibitor, integrin blocking, western blot\",\n      \"journal\": \"Frontiers in oncology\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, pathway activation assays without direct FN1 rescue in co-culture model\",\n      \"pmids\": [\"40678072\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"GATA6 transcription factor directly binds the FN1 promoter and activates FN1 transcription in oral squamous cell carcinoma; GATA6 knockdown-mediated suppression of proliferation, colony formation, invasion, and migration is rescued by FN1 overexpression.\",\n      \"method\": \"Dual-luciferase reporter assay, chromatin immunoprecipitation (ChIP), GATA6 knockdown/FN1 overexpression rescue experiments\",\n      \"journal\": \"Molecular medicine reports\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 / Moderate — direct promoter binding confirmed by ChIP and luciferase plus rescue, single lab\",\n      \"pmids\": [\"35088888\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2022,\n      \"finding\": \"SOX2 transcriptionally upregulates FN1 expression in Schwann cell-like cells (iSCs), promoting proliferation and migration (fibronectin fibrillogenesis); exosomes secreted by iSCs also increase Schwann cell viability and migration; the SOX2/FN1 axis was demonstrated by RNA-seq and functional experiments in sciatic nerve repair.\",\n      \"method\": \"SC RNA-seq, SOX2 overexpression/knockdown, scratch wound assay, EdU proliferation assay, sciatic nerve transection animal model\",\n      \"journal\": \"International journal of molecular medicine\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 / Weak — single lab, SOX2-FN1 link by RNA-seq correlation and overexpression, no direct promoter binding assay reported\",\n      \"pmids\": [\"35475578\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FN1 (fibronectin 1) is a secreted extracellular matrix glycoprotein that acts at the cell surface to promote adhesion, spreading, and migration via transmembrane linkage to actin microfilament bundles; it signals intracellularly through integrin receptors (particularly α5β1/ITGA5-ITGB1), activating FAK/Src, PI3K/AKT, and downstream GTPase/NF-κB/YAP1 pathways; its transcription is directly regulated by factors including HMGA2, HOXA13, and GATA6 binding to its promoter, and it is degraded via the p62/SQSTM1-autophagy-lysosome pathway; muscle-secreted FN1 also functions as a circulating exercise-induced factor that activates hepatic autophagy and insulin sensitization through α5β1 integrin and IKKα/β-JNK1-BECN1 signaling; oncogenic FN1 gene fusions (e.g., FN1-FGFR1, FN1-EGF, FN1-ACVR2A) place receptor tyrosine kinase or growth factor domains under FN1 promoter/secretion control, driving constitutive signaling in multiple mesenchymal tumor types.\"\n}\n```","stage2_raw":"{\n  \"mechanistic_narrative\": \"FN1 encodes a secreted extracellular matrix glycoprotein that organizes cell adhesion, spreading, and migration by linking the cell surface to actin microfilament bundles, with surface FN1 forming fibrillar networks whose loss accompanies cellular transformation and loss of normal growth control [#0, #1, #6]. FN1 colocalizes with actin bundles during cell spreading, implying a transmembrane connection that nucleates attachment plaques, and exogenous purified FN1 restores attachment, spreading, alignment, and migration to transformed cells [#2, #3]. Its distribution is cell-type-specific and polarized\\u2014present at endothelial luminal surfaces and basal choroid epithelium\\u2014and is developmentally downregulated during myoblast fusion [#4, #5]. FN1 signals into cells chiefly through the \\u03b15\\u03b21 integrin (ITGA5/ITGB1), an interaction shown by co-immunoprecipitation and required for FN1-driven proliferation, migration, and invasion, which activates FAK/Src and downstream RhoA/Rac1/CDC42 GTPases as well as PI3K/AKT, NF-\\u03baB, STAT3 (via PTPRM promoter methylation), and YAP1/Hippo with SLC1A3-mediated aspartate uptake in multiple tumor contexts [#10, #13, #15, #19, #20]. FN1 transcription is directly driven by HMGA2, HOXA13, and GATA6 binding to its promoter and by the IRE1\\u03b1\\u2013XBP1s axis, while FN1 protein is turned over through p62/SQSTM1-dependent autophagy-lysosome degradation [#11, #13, #14, #15, #24]. Beyond its matrix role, exercise-induced muscle-secreted FN1 acts as a circulating factor that engages hepatic \\u03b15\\u03b21 integrin and IKK\\u03b1/\\u03b2\\u2013JNK1\\u2013BECN1 signaling to activate hepatic autophagy and systemic insulin sensitization [#12]. Recurrent oncogenic FN1 gene fusions place receptor tyrosine kinase or growth-factor domains (FGFR1, FGF1, EGF, ACVR2A, FGFR2, MERTK, NTRK1, TEK) under FN1 promoter and secretion/membrane-targeting control, driving constitutive signaling across phosphaturic mesenchymal tumors, calcifying aponeurotic fibroma, synovial chondromatosis, and calcified chondroid mesenchymal neoplasms [#7, #8, #9, #17]. A gain-of-function FN1 variant that raises integrin binding while reducing collagen IV (COL4A3/4) binding causes glomerular fibronectin deposition, and a rare FN1 variant modifies APOE\\u03b54-mediated Alzheimer's disease risk through altered blood-brain barrier clearance [#16, #18].\",\n  \"teleology\": [\n    {\n      \"year\": 1978,\n      \"claim\": \"Established that FN1 (LETS protein) is a cell-surface/matrix glycoprotein physically and functionally coupled to the actin cytoskeleton, answering whether an adhesion molecule could transduce structure across the membrane.\",\n      \"evidence\": \"Immunofluorescence localization, cytoskeletal perturbants, and double-label co-localization with actin during cell spreading in cultured cells\",\n      \"pmids\": [\"925079\", \"365353\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No molecular identification of the transmembrane linker\", \"Correlative co-localization does not prove direct receptor engagement\"]\n    },\n    {\n      \"year\": 1978,\n      \"claim\": \"Demonstrated FN1 is a sufficient functional effector of adhesion and motility, since purified protein added back to transformed cells restored attachment, spreading, and migration\\u2014linking reduced surface FN1 to the transformed phenotype and growth control.\",\n      \"evidence\": \"Addition of purified LETS protein to normal/transformed cells, phagokinetic track migration assays, radioiodination quantification in somatic cell hybrids\",\n      \"pmids\": [\"616487\", \"667950\", \"363730\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No receptor or signaling pathway identified at this stage\", \"Growth-control correlation in hybrids is genetic association, not direct mechanism\"]\n    },\n    {\n      \"year\": 1978,\n      \"claim\": \"Showed FN1 expression is cell-type-restricted, polarized, and developmentally regulated, indicating its deposition is spatially and temporally controlled rather than constitutive.\",\n      \"evidence\": \"Immunofluorescence across nervous tissue cell types and species; immunofluorescence/radioimmunoassay during myoblast fusion\",\n      \"pmids\": [\"21348357\", \"321128\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Transcriptional regulators driving polarization unknown\", \"Functional consequence of downregulation during myogenesis not tested\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Identified the integrin \\u03b15\\u03b21 receptor (ITGA5/ITGB1) as the direct functional partner mediating FN1's pro-tumorigenic effects, answering how secreted FN1 signals into cells.\",\n      \"evidence\": \"Co-immunoprecipitation and ITGA5 rescue of FN1 knockdown phenotypes in colorectal cancer cells; Co-IP of FN1-ITGA5/ITGB1 in gastric cancer\",\n      \"pmids\": [\"29274284\", \"35197128\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Binding interface and stoichiometry not defined\", \"Single-cell-line Co-IP without structural validation\"]\n    },\n    {\n      \"year\": 2019,\n      \"claim\": \"Connected FN1 to defined intracellular signaling cascades, showing it activates Src/FAK and downstream Rho-family GTPases to drive migration, with exogenous FN1 rescuing pathway activity.\",\n      \"evidence\": \"IRE1\\u03b1/XBP1s ChIP on FN1 promoter, phosphorylation and GTPase activation assays, exogenous FN1 rescue in colon cancer\",\n      \"pmids\": [\"31326465\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Receptor proximal to FAK/Src activation not directly demonstrated here\", \"Single cancer context\"]\n    },\n    {\n      \"year\": 2022,\n      \"claim\": \"Defined the transcriptional control of FN1, showing HMGA2, HOXA13, and GATA6 directly bind its promoter to drive FN1-dependent invasion and metastasis, while IRE1\\u03b1-XBP1s provides a stress-responsive input.\",\n      \"evidence\": \"ChIP-PCR, dual-luciferase reporter assays, and FN1-overexpression rescue across colorectal, gastric, and oral squamous carcinoma models with in vivo validation\",\n      \"pmids\": [\"26964871\", \"35197128\", \"35088888\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Combinatorial regulation among these factors unresolved\", \"Promoter elements bound not finely mapped\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Expanded the downstream signaling repertoire of FN1 to STAT3 (via PTPRM promoter methylation), NF-\\u03baB, PI3K/AKT, and YAP1/Hippo-coupled metabolic uptake, establishing FN1 as a node feeding multiple oncogenic pathways.\",\n      \"evidence\": \"Genetic/pharmacological manipulation with methylation-specific PCR, demethylation, pathway inhibitor combinations, multi-omics, and in vivo models in glioblastoma, breast, thyroid, and pancreatic cancer\",\n      \"pmids\": [\"34225581\", \"37458908\", \"36278453\", \"40678072\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Pathway selectivity across tumor types unexplained\", \"Thyroid and pancreatic studies are correlative without direct FN1 rescue\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"Identified how FN1 protein levels are controlled post-translationally, showing p62/SQSTM1 acts as an autophagy adapter targeting FN1 to lysosomal degradation.\",\n      \"evidence\": \"Pharmacological autophagy/lysosome modulation, immunoprecipitation, and p62 mutant cell lines\",\n      \"pmids\": [\"33318468\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether intracellular or secreted FN1 pool is degraded not fully resolved\", \"Single Co-IP system\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Revealed an endocrine role for FN1 beyond the matrix: exercise-induced muscle-secreted FN1 signals to liver via \\u03b15\\u03b21 integrin to activate hepatic autophagy and systemic insulin sensitization.\",\n      \"evidence\": \"Plasma proteomics, muscle-specific FN1 knockout, hepatic autophagy assays, integrin blockade, and insulin sensitization assays in mice\",\n      \"pmids\": [\"36812915\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Human relevance of the muscle-liver FN1 axis not established\", \"Distinction between this circulating FN1 and matrix FN1 forms unclear\"]\n    },\n    {\n      \"year\": 2021,\n      \"claim\": \"Established recurrent FN1 gene fusions as defining oncogenic events, where the FN1 promoter and secretion/membrane-targeting domains drive constitutive RTK or growth-factor signaling across distinct mesenchymal tumors.\",\n      \"evidence\": \"RNA sequencing, FISH, Sanger breakpoint mapping, IHC, and Western blot across PMT, CAF, synovial chondromatosis, and calcified chondroid mesenchymal neoplasms\",\n      \"pmids\": [\"27443518\", \"26691015\", \"31273315\", \"33727696\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Functional reconstitution of fusion signaling largely predicted not directly assayed\", \"Therapeutic targetability not tested in these series\"]\n    },\n    {\n      \"year\": 2024,\n      \"claim\": \"Linked FN1 sequence variation directly to human disease through gain-of-function binding changes, providing mechanism for glomerular fibronectin deposition and a modifier role in Alzheimer's disease.\",\n      \"evidence\": \"Genetic sequencing with in vitro integrin/collagen IV binding assays for the c.3415G>A glomerulopathy variant; whole-genome association plus zebrafish fn1b loss-of-function for the APOE\\u03b54-protective variant\",\n      \"pmids\": [\"36774238\", \"38598053\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Causality of the Alzheimer's variant rests on ortholog modeling\", \"Tissue-specific consequences of altered binding not fully characterized\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"How the distinct FN1 functional pools\\u2014matrix-bound adhesive fibronectin, autophagy-degraded intracellular FN1, and circulating endocrine FN1\\u2014are differentially produced, modified, and targeted to specific receptors and tissues remains unresolved.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"No structural model of FN1-integrin engagement in the timeline\", \"Mechanism distinguishing endocrine vs matrix FN1 function unknown\", \"Whether fusion-driven signaling requires the same domains as native FN1 secretion untested\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098631\", \"supporting_discovery_ids\": [0, 1, 2, 3, 10, 15]},\n      {\"term_id\": \"GO:0048018\", \"supporting_discovery_ids\": [12, 22]},\n      {\"term_id\": \"GO:0060089\", \"supporting_discovery_ids\": [10, 13, 15, 22]},\n      {\"term_id\": \"GO:0005198\", \"supporting_discovery_ids\": [0, 2]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [1, 12]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [0, 2]},\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 5]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 13, 15, 20, 22]},\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"R-HSA-9612973\", \"supporting_discovery_ids\": [11, 12]},\n      {\"term_id\": \"R-HSA-1643685\", \"supporting_discovery_ids\": [7, 8, 9, 17, 18]},\n      {\"term_id\": \"R-HSA-74160\", \"supporting_discovery_ids\": [13, 14, 15, 24]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\"ITGA5\", \"ITGB1\", \"SQSTM1\", \"COL4A3\"],\n    \"other_free_text\": []\n  }\n}","audit_flag":null,"evaluation":{"pairwise":"win","faith_supported":8,"faith_total":8,"faith_pct":100.0}}