{"gene":"FAP","run_date":"2026-04-28T17:46:03","timeline":{"discoveries":[{"year":1994,"finding":"FAP (fibroblast activation protein alpha) was identified as a type II integral membrane protein with a large extracellular domain, sharing 48% amino acid sequence identity with CD26/DPPIV and conserving three catalytic serine protease domains. Immunochemical analysis of COS-1 cells co-expressing FAP and CD26 revealed that the two molecules form heteromeric cell surface complexes, identifying CD26 as the previously described FAP-associated protein FAP-beta.","method":"Expression cloning in COS-1 cells, immunochemical co-expression analysis, sequence analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — original molecular cloning with functional domain characterization and direct co-expression experiment showing heteromeric complex formation","pmids":["7911242"],"is_preprint":false},{"year":1997,"finding":"Seprase (the 170-kDa melanoma membrane-bound gelatinase) was identified as identical to FAP-alpha; it is a homodimer of N-glycosylated 97-kDa subunits. Proteolytic (gelatinase) activity requires the dimeric form and is abolished upon dissociation into 97-kDa subunits. The active site serine was confirmed by affinity labeling with [³H]diisopropyl fluorophosphate, and activity was blocked by serine-protease inhibitors, establishing FAP/seprase as a serine integral membrane protease.","method":"Protein purification, reverse transcription-PCR cloning, COS-7 cell transfection, [³H]DFP affinity labeling, serine protease inhibitor assays, dissociation experiments","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — biochemical reconstitution of activity, active-site labeling, and mutagenesis-equivalent dissociation experiments showing dimer requirement","pmids":["9065413","9247085"],"is_preprint":false},{"year":1999,"finding":"FAP exhibits both dipeptidyl peptidase IV activity and a collagenolytic/gelatinolytic endopeptidase activity capable of degrading gelatin and type I collagen. Mutation of the putative catalytic serine residue to alanine abolishes both enzymatic activities. FAP enzyme activity was detected in human cancerous tissues but not in matched normal tissues using an immunocapture assay, demonstrating it is active as a cell surface-bound collagenase in tumor stroma.","method":"Recombinant protein expression, active-site serine-to-alanine mutagenesis, in vitro collagen/gelatin degradation assays, immunocapture enzyme activity assay on tissue extracts","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — dual enzymatic activity demonstrated with purified recombinant protein and confirmed by catalytic serine mutagenesis abolishing both activities","pmids":["10593948"],"is_preprint":false},{"year":1999,"finding":"Type I collagen substratum induces the association of α3β1 integrin with seprase/FAP as a complex at invadopodia of aggressive tumor cells. In the absence of collagen, α3β1 integrin and seprase exist as non-associating membrane proteins, establishing that integrin serves as a collagen-dependent docking protein for FAP at sites of cell invasion.","method":"Co-immunoprecipitation, immunofluorescence localization at invadopodia, collagen-dependent association assay","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal co-IP with collagen-dependence control, but single study","pmids":["10455171"],"is_preprint":false},{"year":2002,"finding":"FAP/seprase and DPPIV form a functional protease complex at invadopodia of migratory fibroblasts that is required for cell invasion and migration on collagenous matrix. This complex elicits both gelatin-binding and gelatinase activities localized at invadopodia; serine protease inhibitors block the gelatinase activity and gelatin degradation, and antibodies to the gelatin-binding domain of DPPIV reduce degradation without affecting adhesion.","method":"Co-immunoprecipitation of seprase-DPPIV complex, gelatinase activity assays, serine protease inhibitor blocking, antibody neutralization, wound-closure migration assays","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — biochemical complex isolation, functional activity assays, multiple inhibitory approaches confirming mechanism, replicated in endothelial cells","pmids":["12023964","16651416"],"is_preprint":false},{"year":2005,"finding":"The crystal structure of FAP-alpha apoenzyme was solved at high resolution, revealing a DPPIV-like fold with an alpha/beta-hydrolase domain and an eight-bladed beta-propeller domain. A critical difference from DPPIV is Ala657 in FAP (vs. Asp663 in DPPIV) within the active site, which reduces acidity in the substrate-binding pocket and explains FAP's lower affinity for N-terminal amines and its endopeptidase activity. The FAP/A657D mutant showed ~60-fold increased catalytic efficiency for dipeptide substrates and ~350-fold reduced efficiency for endopeptidase substrates, confirming Ala657 as the molecular determinant of substrate specificity.","method":"X-ray crystallography (apoenzyme structure), kinetic analysis of wild-type and A657D mutant FAP, comparison with DPPIV crystal structure","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — crystal structure plus mutagenesis with quantitative kinetic validation, strong mechanistic insight","pmids":["15809306"],"is_preprint":false},{"year":2005,"finding":"Circulating antiplasmin-cleaving enzyme (APCE) is a soluble form of FAP. APCE and recombinant FAP are homodimers with identical pH optima, extinction coefficients, tryptic peptide sequences, and antibody cross-reactivity. Both cleave α2-antiplasmin at Pro3-Leu4 and Pro12-Asn13 bonds, with ~16-fold higher kcat/Km for the Pro12-Asn13 site, identifying Met-α2-antiplasmin as a physiological substrate of FAP and establishing a role for soluble FAP in fibrinolysis.","method":"Comparative biochemical characterization of APCE and recombinant FAP, kinetic analysis of cleavage site preferences, tryptic peptide sequencing, antibody cross-reactivity","journal":"Blood","confidence":"High","confidence_rationale":"Tier 1 — direct comparative biochemistry with purified proteins, kinetic quantification of substrate cleavage, identification of physiological substrate","pmids":["16223769"],"is_preprint":false},{"year":2005,"finding":"FAP overexpression in LX-2 human hepatic stellate cells increased cell adhesion and migration on extracellular matrix proteins (collagen-I, fibronectin, Matrigel) and enhanced invasion across transwells, and also enhanced staurosporine-induced apoptosis. Importantly, the enzymatic activity of FAP was not required for these functions (non-enzymatic role). FAP overexpression increased MMP-2 and CD44 expression and reduced integrin-β1 expression.","method":"GFP-FAP fusion protein overexpression in LX-2 and HEK293T cells, adhesion assays, transwell migration/invasion assays, Western blot for downstream targets, enzyme-activity-dead controls","journal":"Hepatology","confidence":"Medium","confidence_rationale":"Tier 2 — overexpression with enzyme-dead controls distinguishing enzymatic from non-enzymatic functions, multiple functional readouts, single study","pmids":["16175601"],"is_preprint":false},{"year":2011,"finding":"FAP enzymatically remodels extracellular matrix by modulating fibronectin and collagen fiber organization and protein levels in a 3D matrix system. FAP-dependent architectural alterations of the ECM promote pancreatic cancer cell invasion along parallel fiber orientations (enhanced directionality and velocity), and this phenotype is reversed by inhibition of FAP enzymatic activity during matrix production. The FAP+ matrix-induced tumor invasion phenotype is β1-integrin/FAK mediated.","method":"Tetracycline-inducible FAP overexpression, 3D in vivo-like matrix system, fiber orientation analysis, time-lapse invasion assays, FAP enzymatic inhibitor treatment, Western blot for β1-integrin/FAK","journal":"BMC cancer","confidence":"Medium","confidence_rationale":"Tier 2 — inducible system with enzyme inhibitor controls linking FAP activity to ECM remodeling and invasion phenotype, pathway placement via β1-integrin/FAK","pmids":["21668992"],"is_preprint":false},{"year":2015,"finding":"FAP directly participates in collagen catabolism in concert with MMPs. In two mouse models of pulmonary fibrosis (bleomycin and thoracic irradiation), FAP-deficient mice showed increased mortality and lung fibrosis, with accumulation of intermediate-sized collagen fragments consistent with FAP mediating proteolytic processing of MMP-derived collagen cleavage products. FAP-mediated collagen processing increased collagen internalization (via Endo180 receptor) without altering receptor expression; pharmacological FAP inhibition decreased collagen internalization; restoration of FAP expression in FAP-deficient mouse lungs normalized collagen content.","method":"FAP-knockout mouse models, two independent fibrosis models, lung ECM analysis, in vitro collagen processing assays, pharmacological FAP inhibition, viral rescue (FAP re-expression in KO lungs), hydroxyproline quantification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1–2 — genetic KO with two independent disease models plus pharmacological inhibition and rescue experiment, multiple orthogonal methods","pmids":["26663085"],"is_preprint":false},{"year":2015,"finding":"FAP triggers induction of a cancer-associated fibroblast subset with an inflammatory phenotype via persistent activation of STAT3 through a uPAR-dependent FAK-Src-JAK2 signaling pathway. Enforcing FAP expression in normal fibroblasts was sufficient to activate STAT3 and upregulate CCL2, which promoted recruitment of myeloid-derived suppressor cells (MDSCs) via CCR2. FAP+-CAF-mediated tumor promotion and MDSC recruitment was abrogated in Ccr2-deficient mice.","method":"FAP overexpression in normal fibroblasts, signaling pathway inhibitors (FAK, Src, JAK2), STAT3 reporter assays, murine liver tumor model, Ccr2-deficient mice, CCL2 ELISA","journal":"Cancer research","confidence":"Medium","confidence_rationale":"Tier 2 — genetic and pharmacological dissection of signaling pathway with in vivo validation in KO mice, single laboratory","pmids":["27216177"],"is_preprint":false},{"year":2016,"finding":"FAP cleaves and inactivates fibroblast growth factor 21 (FGF21). A selective FAP chemical inhibitor, FAP immunodepletion, or genetic Fap deletion stabilized recombinant human FGF21 in serum. Administration of a selective FAP inhibitor acutely increased circulating intact FGF21 levels in cynomolgus monkeys, establishing FGF21 as a physiological substrate of FAP.","method":"In vitro cleavage assays with purified FAP, selective FAP inhibitor treatment, immunodepletion of FAP from serum, Fap-knockout mice, in vivo FAP inhibitor dosing in monkeys with intact FGF21 quantification","journal":"The Journal of biological chemistry","confidence":"High","confidence_rationale":"Tier 1 — multiple orthogonal approaches (inhibitor, immunodepletion, genetic KO) plus in vivo primate confirmation identifying FGF21 as FAP substrate","pmids":["26797127"],"is_preprint":false},{"year":2016,"finding":"Pharmacological FAP inhibition (using talabostat) in diet-induced obese mice led to decreased body weight, reduced food intake, improved glucose tolerance and insulin sensitivity, and elevated plasma FGF21 levels. FAP inhibition showed no metabolic effect in FGF21-knockout obese animals, and in vitro FAP was shown to degrade human FGF21 at both termini in the absence of inhibitor, confirming FGF21 as the critical substrate mediating FAP's metabolic role.","method":"In vitro FGF21 degradation assay with purified FAP and inhibitor, diet-induced obese mouse model, FGF21-knockout mice, talabostat pharmacological intervention, metabolic phenotyping","journal":"Molecular metabolism","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro enzyme assay corroborated by FGF21-KO epistasis experiment establishing FGF21 as the obligate downstream mediator","pmids":["27689014"],"is_preprint":false},{"year":2016,"finding":"Specific interfacial residues in the FAP transmembrane (TM) domain (G10, S14, A18, forming a small-X3-small motif) are required for FAP homodimerization. Mutations G10L, S14L, and A18L reduced FAP TM-CYTO dimerization as measured by the AraTM bacterial assay, while off-interface residues showed no change. The G10L mutation significantly decreased FAP endopeptidase activity by more than 25% and reduced cell-surface versus intracellular FAP expression, linking TM-mediated dimerization to both protease activity and proper trafficking.","method":"AraTM bacterial dimerization assay, site-directed mutagenesis of TM interface residues, endopeptidase activity assays, cell-surface vs. intracellular expression quantification","journal":"Biochimica et biophysica acta","confidence":"Medium","confidence_rationale":"Tier 2 — systematic mutagenesis of predicted interface residues with functional readouts for activity and trafficking, single study","pmids":["27155568"],"is_preprint":false},{"year":2015,"finding":"FAP expression in activated cardiac fibroblasts after myocardial infarction is induced by TGF-β1 via the canonical SMAD2/SMAD3 pathway (blocked by TGFbR1 inhibitor SB431542, MAPK inhibitor U0126, and siRNA targeting SMAD2/SMAD3). FAP depletion in fibroblasts reduced migratory capacity (Boyden chamber assay) without affecting proliferation, and FAP exhibited gelatinase activity by gelatin zymography, indicating roles in cardiac wound healing and remodeling.","method":"TGF-β1 stimulation of human cardiac fibroblasts, TGFbR1 inhibitor, MAPK inhibitor, SMAD2/3 siRNA knockdown, modified Boyden-chamber migration assay, BrdU proliferation assay, gelatin zymography","journal":"Journal of molecular and cellular cardiology","confidence":"Medium","confidence_rationale":"Tier 2 — pathway dissection with multiple inhibitors and siRNA converging on SMAD2/3, functional migration and gelatinase readouts, single study","pmids":["26319660"],"is_preprint":false},{"year":2023,"finding":"BNP (brain natriuretic peptide) is a novel physiological substrate of FAP that mediates post-ischemic angiogenesis. FAP degrades BNP to inhibit vascular endothelial cell migration and tube formation. Genetic or pharmacological inhibition of FAP in mice increased angiogenesis in the peri-infarct zone after myocardial infarction and improved cardiac function. The cardioprotective effect of FAP inhibition was abolished in mice deficient in Nppb (encoding pre-proBNP) or Npr1 (encoding the BNP receptor), establishing BNP as the obligate mediator.","method":"In vitro BNP cleavage assay, endothelial cell migration and tube formation assays, FAP genetic KO and pharmacological inhibition in mouse MI model, Nppb-KO and Npr1-KO epistasis, echocardiography, histological analysis, RNA-sequencing","journal":"Circulation research","confidence":"High","confidence_rationale":"Tier 1–2 — in vitro substrate cleavage assay corroborated by double-KO epistasis experiments establishing BNP as the obligate downstream mediator, multiple orthogonal methods","pmids":["36756875"],"is_preprint":false},{"year":2014,"finding":"FAP silencing in oral squamous cell carcinoma (OSCC) cells inhibited growth and metastasis in vitro and in vivo; mechanistically, FAP knockdown inactivated PTEN/PI3K/AKT and Ras-ERK signaling. The inhibitory effects of FAP knockdown on proliferation and metastasis were rescued by PTEN silencing, placing FAP upstream of PTEN/PI3K/AKT in OSCC.","method":"FAP siRNA knockdown, in vitro proliferation/migration/invasion assays, xenograft mouse models, Western blot for PI3K/AKT and Ras-ERK pathway components, PTEN rescue experiment","journal":"Cell death & disease","confidence":"Medium","confidence_rationale":"Tier 2 — epistasis rescue experiment placing FAP upstream of PTEN/PI3K/AKT, in vivo validation, single laboratory","pmids":["24722280"],"is_preprint":false},{"year":2020,"finding":"FAP overexpression in oral squamous cell carcinoma induces epithelial-mesenchymal transition (EMT) by down-regulating DPP9 (dipeptidyl peptidase 9). FAP was identified as interacting intracellularly with DPP9 by immunoprecipitation-mass spectrometry. DPP9 overexpression reversed the FAP-induced proliferation, migration, invasion, and EMT, establishing a FAP→DPP9 suppression axis. This mechanism is non-enzymatic (FAP promotes EMT by reducing DPP9, not via its protease activity).","method":"Immunoprecipitation-mass spectrometry (IP-MS) to identify FAP interactors, FAP overexpression, DPP9 knockdown/overexpression, in vitro and in vivo functional assays","journal":"OncoTargets and therapy","confidence":"Medium","confidence_rationale":"Tier 2–3 — IP-MS identifies intracellular DPP9 as FAP binding partner with functional rescue experiment, single study","pmids":["32273729"],"is_preprint":false},{"year":2023,"finding":"FAP in adipose tissue macrophages (ATM) specifically mediates CCL8 chemokine expression, contributing to recruitment of proinflammatory monocyte-derived macrophages in obese adipose tissue. Macrophage-specific FAP deficiency protected mice from diet-induced obesity and improved insulin resistance; CCL8 overexpression restored metabolic phenotypes in FAP-deficient mice, establishing CCL8 as the downstream effector. FAP-deficient ATMs also showed decreased monoamine oxidase expression, leading to elevated norepinephrine and increased lipolysis.","method":"Macrophage-specific FAP knockout mice, high-fat diet model, CCL8 overexpression rescue, flow cytometry, norepinephrine measurement, monoamine oxidase expression analysis","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"Medium","confidence_rationale":"Tier 2 — cell-type-specific KO with CCL8 rescue epistasis establishing pathway, multiple metabolic readouts, single laboratory","pmids":["38100414"],"is_preprint":false},{"year":2023,"finding":"FAP promotes metastasis and chemoresistance in mucinous colorectal adenocarcinoma by interacting with MPRIP (myosin phosphatase Rho-interacting protein) and regulating the Rho/Hippo/YAP signaling pathway, leading to TAM recruitment and M2 macrophage polarization. MPRIP was identified as a direct FAP-interacting protein.","method":"Co-immunoprecipitation identifying MPRIP as FAP interactor, FAP overexpression/knockdown, in vitro and in vivo functional assays, Rho/Hippo/YAP pathway analysis, macrophage polarization assays","journal":"iScience","confidence":"Low","confidence_rationale":"Tier 3 — single Co-IP identifying MPRIP interaction, pathway placement based on overexpression/knockdown, single study with limited mechanistic depth","pmids":["37213233"],"is_preprint":false},{"year":2025,"finding":"FAP+ cancer-associated fibroblasts in breast cancer secrete high levels of fibronectin 1 (FN1), which engages integrin α5β1 on macrophages to activate FAK-AKT-STAT3 signaling, driving M2-like macrophage polarization. Pharmacological disruption of FN1-integrin α5β1 signaling with Cilengitide reprogrammed the tumor immune landscape and suppressed tumor growth in mouse models.","method":"scRNA-seq, co-culture experiments, FAP+ CAF conditioned medium, integrin α5β1 blocking antibody and Cilengitide treatment, Western blot for FAK-AKT-STAT3 pathway, in vivo mouse tumor models","journal":"Oncogene","confidence":"Medium","confidence_rationale":"Tier 2 — mechanistic pathway (FN1→integrin α5β1→FAK-AKT-STAT3) established by pharmacological blockade with in vivo validation, single study","pmids":["40263422"],"is_preprint":false}],"current_model":"FAP (fibroblast activation protein alpha/seprase) is a type II transmembrane serine protease that functions as an obligate homodimer (dimerization requiring a specific TM small-X3-small motif) with dual enzymatic activities — dipeptidyl peptidase IV-like exopeptidase and post-prolyl endopeptidase (collagenolytic) activity — both requiring the catalytic serine (S624); its endopeptidase specificity over DPPIV is conferred by Ala657 in the active site; it cleaves physiological substrates including type I collagen/gelatin, α2-antiplasmin, FGF21 (inactivating it), and BNP (inhibiting angiogenesis after MI); it forms a functional protease complex with DPPIV/CD26 at invadopodia to mediate ECM degradation and cell migration; it remodels stromal ECM to promote tumor invasion via β1-integrin/FAK signaling; and it activates STAT3 via uPAR-FAK-Src-JAK2 signaling in fibroblasts to drive immunosuppressive CCL2/MDSC recruitment, while also promoting M2 macrophage polarization through FN1-integrin α5β1-FAK-AKT-STAT3 and regulating metabolic inflammation through CCL8 in adipose tissue macrophages."},"narrative":{"teleology":[{"year":1994,"claim":"Molecular cloning established FAP as a type II membrane serine protease with homology to DPPIV and revealed that FAP and CD26/DPPIV form heteromeric complexes on the cell surface, providing the first molecular identity for the fibroblast activation protein.","evidence":"Expression cloning in COS-1 cells with immunochemical co-expression analysis","pmids":["7911242"],"confidence":"High","gaps":["Enzymatic activity not yet characterized","Physiological substrates unknown","In vivo function not addressed"]},{"year":1997,"claim":"Demonstration that seprase and FAP are identical proteins and that gelatinase activity requires the homodimeric form resolved a key structural prerequisite for catalytic function.","evidence":"Purification, RT-PCR cloning, [³H]DFP active-site labeling, and dissociation experiments in COS-7 cells","pmids":["9065413","9247085"],"confidence":"High","gaps":["Specific cleavage sites not mapped","Dimerization interface not structurally defined","Endopeptidase versus exopeptidase activity not distinguished"]},{"year":1999,"claim":"Mutagenesis of the catalytic serine showed that a single active site governs both DPPIV-like exopeptidase and collagenolytic endopeptidase activities, and collagen-dependent recruitment of FAP to invadopodia via α3β1 integrin provided a spatial mechanism for ECM degradation at the invasion front.","evidence":"S→A mutagenesis with in vitro collagen/gelatin degradation assays; co-IP showing collagen-dependent α3β1 integrin–FAP complex at invadopodia","pmids":["10593948","10455171"],"confidence":"High","gaps":["Structural basis for dual activity unknown","In vivo collagenolytic role not tested","No physiological substrates beyond gelatin/collagen identified"]},{"year":2002,"claim":"Isolation of a functional FAP–DPPIV protease complex at invadopodia demonstrated that the two related proteases cooperate in gelatin degradation and cell migration, establishing a composite proteolytic unit for ECM invasion.","evidence":"Co-IP of FAP–DPPIV complex, gelatinase activity assays with serine protease inhibitors and anti-gelatin-binding antibodies, wound-closure migration assays","pmids":["12023964","16651416"],"confidence":"High","gaps":["Relative contribution of each protease to invasion unclear","Stoichiometry and regulation of complex assembly unresolved"]},{"year":2005,"claim":"The crystal structure of FAP revealed that a single residue difference (Ala657 vs. Asp663 in DPPIV) governs endopeptidase specificity, and identification of α2-antiplasmin as a physiological substrate linked soluble FAP to fibrinolysis regulation.","evidence":"X-ray crystallography with A657D kinetic analysis; comparative biochemistry of APCE/FAP showing cleavage of α2-antiplasmin at defined sites","pmids":["15809306","16223769"],"confidence":"High","gaps":["Physiological relevance of α2-antiplasmin cleavage in vivo not confirmed in genetic models","Structural basis for collagen recognition not resolved"]},{"year":2005,"claim":"Overexpression studies in hepatic stellate cells demonstrated that FAP promotes adhesion, migration, and invasion through non-enzymatic mechanisms independent of its protease activity, expanding FAP's functional repertoire beyond catalysis.","evidence":"GFP-FAP overexpression with enzyme-dead controls in LX-2 cells; adhesion, migration, and invasion assays; MMP-2/CD44/integrin-β1 expression changes","pmids":["16175601"],"confidence":"Medium","gaps":["Mechanism of non-enzymatic signaling not defined","Overexpression system may not reflect physiological levels","No identification of the non-enzymatic binding partner"]},{"year":2011,"claim":"FAP's enzymatic remodeling of 3D extracellular matrix was shown to organize collagen/fibronectin fibers into parallel orientations that promote directional tumor cell invasion via β1-integrin/FAK signaling, linking FAP catalytic activity to a mechanistic ECM-guided invasion program.","evidence":"Tetracycline-inducible FAP expression, 3D matrix fiber orientation analysis, FAP enzymatic inhibitor rescue, β1-integrin/FAK Western blot","pmids":["21668992"],"confidence":"Medium","gaps":["Specific ECM cleavage products driving fiber reorganization unidentified","In vivo validation of fiber-orientation phenotype lacking"]},{"year":2015,"claim":"Genetic and pharmacological studies in fibrosis models revealed that FAP processes MMP-generated collagen fragments to promote Endo180-mediated collagen internalization, establishing an in vivo collagen catabolic function; concurrently, TGF-β1/SMAD2/3 was identified as the upstream inducer of FAP expression in cardiac fibroblasts.","evidence":"FAP-KO mice in bleomycin/irradiation fibrosis models with viral rescue; TGF-β1 stimulation with SMAD2/3 siRNA and pharmacological inhibitors in cardiac fibroblasts","pmids":["26663085","26319660"],"confidence":"High","gaps":["Precise collagen fragment cleavage sites in vivo unmapped","Contribution of soluble vs. membrane FAP in fibrosis not distinguished"]},{"year":2016,"claim":"FGF21 was established as a key physiological substrate through convergent evidence from FAP inhibitor treatment, immunodepletion, Fap-KO mice, and primate dosing, with FGF21-KO epistasis confirming it as the obligate mediator of FAP's metabolic effects; separately, TM domain mutagenesis defined the small-X3-small dimerization motif required for protease activity and surface trafficking.","evidence":"In vitro cleavage, selective FAP inhibitor in cynomolgus monkeys, FGF21-KO epistasis in obese mice; AraTM bacterial dimerization assay with TM point mutations","pmids":["26797127","27689014","27155568"],"confidence":"High","gaps":["Full spectrum of FAP-regulated metabolic pathways beyond FGF21 unclear","Structural model of the TM dimer interface at atomic resolution lacking"]},{"year":2016,"claim":"FAP was placed upstream of STAT3 activation via a uPAR–FAK–Src–JAK2 signaling cascade in cancer-associated fibroblasts, driving CCL2-dependent MDSC recruitment—demonstrating a non-enzymatic immunomodulatory function validated by Ccr2-KO epistasis in vivo.","evidence":"FAP overexpression in fibroblasts with FAK/Src/JAK2 inhibitors, STAT3 reporter, murine liver tumor model, Ccr2-KO mice","pmids":["27216177"],"confidence":"Medium","gaps":["Whether protease activity contributes to STAT3 activation not formally excluded","Generalizability beyond hepatocellular carcinoma model not established"]},{"year":2023,"claim":"BNP was identified as a novel FAP substrate mediating post-ischemic angiogenesis, with FAP inhibition improving cardiac function after MI in a BNP- and NPR1-dependent manner, establishing a cardiovascular axis for FAP protease activity; in parallel, macrophage-specific FAP deletion revealed a CCL8-dependent role in adipose tissue inflammation and metabolic regulation.","evidence":"FAP-KO and pharmacological inhibition in mouse MI model with Nppb-KO and Npr1-KO epistasis; macrophage-specific FAP-KO on HFD with CCL8 overexpression rescue","pmids":["36756875","38100414"],"confidence":"High","gaps":["BNP cleavage site(s) by FAP not precisely mapped","Relative contribution of membrane vs. soluble FAP to BNP inactivation in vivo unknown","CCL8 transcriptional regulation by FAP mechanism uncharacterized"]},{"year":2025,"claim":"FAP+ CAF-derived fibronectin 1 was shown to drive M2 macrophage polarization via integrin α5β1–FAK–AKT–STAT3 signaling, revealing a paracrine mechanism by which FAP shapes the immunosuppressive tumor microenvironment.","evidence":"scRNA-seq, co-culture with conditioned medium, Cilengitide and integrin-blocking antibody treatment, mouse tumor models","pmids":["40263422"],"confidence":"Medium","gaps":["Whether FAP enzymatic activity is required for FN1 upregulation not tested","Single tumor type (breast cancer) limits generalizability"]},{"year":null,"claim":"Major open questions include the full substrate repertoire of FAP in vivo, the structural basis for collagen and BNP recognition, the molecular mechanism by which FAP exerts non-enzymatic signaling functions, and whether membrane-bound and soluble FAP have distinct physiological roles.","evidence":"","pmids":[],"confidence":"High","gaps":["No co-crystal structure with any physiological substrate","Membrane vs. soluble FAP contributions not genetically separated in vivo","Non-enzymatic signaling mechanism molecularly undefined"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0140096","term_label":"catalytic activity, acting on a protein","supporting_discovery_ids":[1,2,5,6,9,11,12,15]},{"term_id":"GO:0016787","term_label":"hydrolase activity","supporting_discovery_ids":[1,2,5,6,9]}],"localization":[{"term_id":"GO:0005886","term_label":"plasma membrane","supporting_discovery_ids":[0,1,3,4,13]},{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[6]},{"term_id":"GO:0031012","term_label":"extracellular matrix","supporting_discovery_ids":[8,9]}],"pathway":[{"term_id":"R-HSA-1474244","term_label":"Extracellular matrix organization","supporting_discovery_ids":[2,8,9]},{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[10,16,20]},{"term_id":"R-HSA-168256","term_label":"Immune System","supporting_discovery_ids":[10,18,20]},{"term_id":"R-HSA-1430728","term_label":"Metabolism","supporting_discovery_ids":[11,12]},{"term_id":"R-HSA-392499","term_label":"Metabolism of proteins","supporting_discovery_ids":[5,6,11,15]}],"complexes":["FAP homodimer","FAP-DPPIV/CD26 heteromeric complex"],"partners":["DPP4","ITGB1","ITGA3","DPP9","MPRIP","FN1","UPAR"],"other_free_text":[]},"mechanistic_narrative":"FAP is a type II transmembrane serine protease that functions as an obligate homodimer with dual enzymatic activities—dipeptidyl peptidase IV-like exopeptidase and post-prolyl endopeptidase—both dependent on a single catalytic serine (S624), with endopeptidase specificity over DPPIV conferred by Ala657 in the active-site pocket [PMID:15809306, PMID:10593948]. FAP cleaves physiological substrates including type I collagen (cooperating with MMPs to facilitate collagen catabolism and Endo180-mediated internalization), α2-antiplasmin (regulating fibrinolysis), FGF21 (controlling metabolic homeostasis), and BNP (modulating post-ischemic angiogenesis), with obligate substrate dependence confirmed by epistasis experiments in FGF21-KO and Nppb-KO mice [PMID:26663085, PMID:16223769, PMID:26797127, PMID:36756875]. Beyond its protease function, FAP exerts non-enzymatic signaling roles: it activates STAT3 via a uPAR–FAK–Src–JAK2 cascade in cancer-associated fibroblasts to drive CCL2-dependent MDSC recruitment, promotes M2 macrophage polarization through FN1–integrin α5β1–FAK–AKT–STAT3 signaling, and mediates CCL8-dependent proinflammatory macrophage recruitment in adipose tissue [PMID:27216177, PMID:40263422, PMID:38100414]. FAP forms a functional complex with DPPIV/CD26 at invadopodia and associates with α3β1 integrin in a collagen-dependent manner to mediate ECM degradation and tumor cell invasion [PMID:12023964, PMID:10455171]."},"prefetch_data":{"uniprot":{"accession":"Q12884","full_name":"Prolyl endopeptidase FAP","aliases":["170 kDa melanoma membrane-bound gelatinase","Dipeptidyl peptidase FAP","Fibroblast activation protein alpha","FAPalpha","Gelatine degradation protease FAP","Integral membrane serine protease","Post-proline cleaving enzyme","Serine integral membrane protease","SIMP","Surface-expressed protease","Seprase"],"length_aa":760,"mass_kda":87.7,"function":"Cell surface glycoprotein serine protease that participates in extracellular matrix degradation and involved in many cellular processes including tissue remodeling, fibrosis, wound healing, inflammation and tumor growth. Both plasma membrane and soluble forms exhibit post-proline cleaving endopeptidase activity, with a marked preference for Ala/Ser-Gly-Pro-Ser/Asn/Ala consensus sequences, on substrate such as alpha-2-antiplasmin SERPINF2 and SPRY2 (PubMed:14751930, PubMed:16223769, PubMed:16410248, PubMed:16480718, PubMed:17381073, PubMed:18095711, PubMed:21288888, PubMed:24371721). Degrade also gelatin, heat-denatured type I collagen, but not native collagen type I and IV, vitronectin, tenascin, laminin, fibronectin, fibrin or casein (PubMed:10347120, PubMed:10455171, PubMed:12376466, PubMed:16223769, PubMed:16651416, PubMed:18095711, PubMed:2172980, PubMed:7923219, PubMed:9065413). Also has dipeptidyl peptidase activity, exhibiting the ability to hydrolyze the prolyl bond two residues from the N-terminus of synthetic dipeptide substrates provided that the penultimate residue is proline, with a preference for Ala-Pro, Ile-Pro, Gly-Pro, Arg-Pro and Pro-Pro (PubMed:10347120, PubMed:10593948, PubMed:16175601, PubMed:16223769, PubMed:16410248, PubMed:16651416, PubMed:17381073, PubMed:21314817, PubMed:24371721, PubMed:24717288). Natural neuropeptide hormones for dipeptidyl peptidase are the neuropeptide Y (NPY), peptide YY (PYY), substance P (TAC1) and brain natriuretic peptide 32 (NPPB) (PubMed:21314817). The plasma membrane form, in association with either DPP4, PLAUR or integrins, is involved in the pericellular proteolysis of the extracellular matrix (ECM), and hence promotes cell adhesion, migration and invasion through the ECM. Plays a role in tissue remodeling during development and wound healing. Participates in the cell invasiveness towards the ECM in malignant melanoma cancers. Enhances tumor growth progression by increasing angiogenesis, collagen fiber degradation and apoptosis and by reducing antitumor response of the immune system. Promotes glioma cell invasion through the brain parenchyma by degrading the proteoglycan brevican. Acts as a tumor suppressor in melanocytic cells through regulation of cell proliferation and survival in a serine protease activity-independent manner","subcellular_location":"Cytoplasm","url":"https://www.uniprot.org/uniprotkb/Q12884/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/FAP","classification":"Not Classified","n_dependent_lines":0,"n_total_lines":1208,"dependency_fraction":0.0},"opencell":{"profiled":false,"resolved_as":"","ensg_id":"","cell_line_id":"","localizations":[],"interactors":[],"url":"https://opencell.sf.czbiohub.org/search/FAP","total_profiled":1310},"omim":[{"mim_id":"620657","title":"AMYLOIDOSIS, HEREDITARY SYSTEMIC 3; AMYLD3","url":"https://www.omim.org/entry/620657"},{"mim_id":"613659","title":"GASTRIC CANCER","url":"https://www.omim.org/entry/613659"},{"mim_id":"611767","title":"MICRO RNA 126; MIR126","url":"https://www.omim.org/entry/611767"},{"mim_id":"611731","title":"APC REGULATOR OF WNT SIGNALING PATHWAY; APC","url":"https://www.omim.org/entry/611731"},{"mim_id":"610155","title":"TYPE 1 DIABETES MELLITUS 19; T1D19","url":"https://www.omim.org/entry/610155"}],"hpa":{"profiled":true,"resolved_as":"","reliability":"","locations":[],"tissue_specificity":"Tissue enhanced","tissue_distribution":"Detected in many","driving_tissues":[{"tissue":"endometrium 1","ntpm":29.4}],"url":"https://www.proteinatlas.org/search/FAP"},"hgnc":{"alias_symbol":["DPPIV"],"prev_symbol":[]},"alphafold":{"accession":"Q12884","domains":[{"cath_id":"2.140.10.30","chopping":"114-268","consensus_level":"medium","plddt":97.6383,"start":114,"end":268},{"cath_id":"2.140.10.30","chopping":"280-438","consensus_level":"medium","plddt":97.3748,"start":280,"end":438},{"cath_id":"-","chopping":"444-500","consensus_level":"medium","plddt":98.0389,"start":444,"end":500},{"cath_id":"3.40.50.1820","chopping":"501-755","consensus_level":"medium","plddt":98.2021,"start":501,"end":755}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/Q12884","model_url":"https://alphafold.ebi.ac.uk/files/AF-Q12884-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-Q12884-F1-predicted_aligned_error_v6.png","plddt_mean":95.62},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=FAP","jax_strain_url":"https://www.jax.org/strain/search?query=FAP"},"sequence":{"accession":"Q12884","fasta_url":"https://rest.uniprot.org/uniprotkb/Q12884.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/Q12884/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/Q12884"}},"corpus_meta":[{"pmid":"1651563","id":"PMC_1651563","title":"Mutations of chromosome 5q21 genes in FAP and colorectal cancer patients.","date":"1991","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/1651563","citation_count":1669,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"35365629","id":"PMC_35365629","title":"Single-cell and spatial analysis reveal interaction of FAP+ fibroblasts and SPP1+ macrophages in colorectal cancer.","date":"2022","source":"Nature communications","url":"https://pubmed.ncbi.nlm.nih.gov/35365629","citation_count":675,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"7536343","id":"PMC_7536343","title":"FAP-1: a protein tyrosine phosphatase that associates with Fas.","date":"1995","source":"Science (New York, N.Y.)","url":"https://pubmed.ncbi.nlm.nih.gov/7536343","citation_count":668,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"27216177","id":"PMC_27216177","title":"FAP Promotes Immunosuppression by Cancer-Associated Fibroblasts in the Tumor Microenvironment via STAT3-CCL2 Signaling.","date":"2016","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/27216177","citation_count":603,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"25979873","id":"PMC_25979873","title":"Tumor-Promoting Desmoplasia Is Disrupted by Depleting FAP-Expressing Stromal Cells.","date":"2015","source":"Cancer research","url":"https://pubmed.ncbi.nlm.nih.gov/25979873","citation_count":413,"is_preprint":false,"source_track":"pubmed_title"},{"pmid":"24470260","id":"PMC_24470260","title":"Understanding fibroblast activation protein (FAP): substrates, activities, expression and targeting for cancer therapy.","date":"2014","source":"Proteomics. 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apoptosis), and FAP-1 overexpression inhibits IGF-I survival signaling by reducing PI3K activity (~80%) and Akt activation (~55%) via decreased IRS-1 tyrosine phosphorylation.\",\n      \"method\": \"Antisense knockdown, TUNEL assay, PI3K activity assay, Akt phosphorylation immunoblot\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — loss-of-function with defined signaling pathway readout, multiple biochemical measurements\",\n      \"pmids\": [\"12354757\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"FAP-1 is expressed in human thyrocytes and protects them from Fas-mediated programmed cell death; a synthetic SLV tripeptide competing for Fas–FAP-1 interaction significantly increases thyrocyte death, implicating FAP-1 as a functional inhibitor of Fas-induced apoptosis in thyroid cells.\",\n      \"method\": \"RNase protection assay, immunohistochemistry, flow cytometry, competitive peptide inhibition, cell death assay\",\n      \"journal\": \"Endocrinology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — competitive peptide approach with functional readout, single lab\",\n      \"pmids\": [\"10537175\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2013,\n      \"finding\": \"Autophagy selectively degrades FAP-1 (Fap-1) in Type I cells, thereby promoting Fas-mediated apoptosis; in cells with high basal autophagy, Fap-1 levels are reduced and Fas apoptosis is enhanced, while autophagy inhibition stabilizes Fap-1 and protects these cells from Fas-induced death.\",\n      \"method\": \"Flow cytometry cell sorting by autophagy level, immunoblot for Fap-1 after autophagy modulation, functional apoptosis assays\",\n      \"journal\": \"Nature cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell sorting combined with functional apoptosis assays and pharmacological/genetic autophagy modulation; Moderate evidence strength\",\n      \"pmids\": [\"24316673\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"Fibroblast activation protein (FAP, fibroblast activation protein α) functions as an atypical serine protease with both dipeptidyl peptidase and endopeptidase activities, cleaving substrates at post-proline bonds, and acts as a homodimer; its enzymatic activities have been characterized in vitro with defined substrates.\",\n      \"method\": \"In vitro enzymatic assays, inhibitor structure-activity relationship studies\",\n      \"journal\": \"Proteomics. Clinical applications\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical characterization, replicated across multiple labs\",\n      \"pmids\": [\"24470260\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FAP (fibroblast activation protein α) expressed on cancer-associated fibroblasts activates fibroblastic STAT3 through a uPAR-dependent FAK-Src-JAK2 signaling pathway, leading to CCL2 upregulation; FAP+ CAFs promote MDSC recruitment via CCL2-CCR2 signaling to suppress anti-tumor immunity.\",\n      \"method\": \"FAP overexpression in normal fibroblasts, signaling pathway inhibitors, Ccr2-knockout mice, tumor growth assays, IHC co-localization\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — epistasis via KO mice, pharmacological inhibition, and gain-of-function with defined pathway, multiple readouts\",\n      \"pmids\": [\"27216177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAP (fibroblast activation protein) participates directly in collagen catabolism by processing MMP-derived intermediate collagen cleavage fragments, promoting collagen internalization via Endo180; FAP-deficient mice accumulate intermediate collagen fragments in the lung and show increased fibrosis and mortality after bleomycin or irradiation injury, which is rescued by restoring FAP expression.\",\n      \"method\": \"FAP-knockout mice, lung fibrosis models (bleomycin, irradiation), ECM analysis, collagen internalization assays, FAP inhibitor pharmacology, AAV-mediated FAP rescue\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro collagen processing assay plus in vivo genetic rescue with defined biochemical and histological readouts\",\n      \"pmids\": [\"26663085\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"The FAP transmembrane domain mediates homodimerization via a small-X3-small (GxxxA) motif; mutations G10L, S14L, and A18L in the TM domain reduce homodimerization measured by AraTM assay, and G10L significantly decreases FAP endopeptidase activity (>25% reduction) and reduces cell-surface versus intracellular FAP expression, linking TM dimerization to both trafficking and proteolytic activity.\",\n      \"method\": \"AraTM bacterial dimerization assay, site-directed mutagenesis, endopeptidase activity assay, cell-surface vs. intracellular expression analysis\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro reconstitution of dimerization with mutagenesis plus functional enzymatic and trafficking readouts\",\n      \"pmids\": [\"27155568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Pharmacological inhibition of FAP with talabostat in diet-induced obese mice elevates total and intact plasma FGF21; FAP degrades FGF21 in vitro, and the metabolic benefits of talabostat (weight loss, reduced adiposity, improved glucose tolerance) are absent in FGF21 knockout mice, establishing FGF21 as a physiological substrate of FAP.\",\n      \"method\": \"In vitro FAP-FGF21 cleavage assay, FAP inhibitor pharmacology, FGF21-KO and lean mouse models, metabolic phenotyping\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — in vitro substrate assay plus genetic rescue (FGF21-KO) confirming substrate dependency in vivo\",\n      \"pmids\": [\"27689014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2017,\n      \"finding\": \"A homogeneous fluorescence assay using FGF21 as a natural substrate specifically detects FAP endopeptidase activity in serum, distinguishing it from related enzymes DPPIV and PREP; structural modeling indicates the mechanistic basis for FAP substrate specificity, and the assay detects elevated FAP activity in liver cirrhosis patients, validated using Fap-deficient mice.\",\n      \"method\": \"In vitro enzymatic assay with FAP-specific substrate (FGF21), Fap-KO mouse validation, structural modeling\",\n      \"journal\": \"Scientific reports\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — in vitro biochemical assay with genetic KO validation and structural rationale\",\n      \"pmids\": [\"28970566\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAP degrades BNP (brain natriuretic peptide) to inhibit vascular endothelial cell migration and tube formation; genetic or pharmacological FAP inhibition after myocardial infarction stabilizes BNP, promotes angiogenesis in the peri-infarct zone, limits scar expansion, and improves cardiac function. Cardioprotective effects are absent in Nppb- or Npr1-deficient mice, confirming BNP as a physiological FAP substrate.\",\n      \"method\": \"FAP-KO mice, pharmacological FAP inhibition, Nppb-KO and Npr1-KO epistasis, in vitro endothelial cell migration/tube formation assays, echocardiography, histology\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1-2 — substrate identification with multiple genetic KO epistasis models and in vitro functional validation\",\n      \"pmids\": [\"36756875\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAP (fibroblast activation protein α) on cancer-associated stromal cells is required for maintenance of the provisional tumor stroma; depletion of FAP+ cells by CAR T cell adoptive transfer reduces extracellular matrix proteins, glycosaminoglycans, and tumor vascular density, restraining desmoplastic tumor growth in an immune-independent fashion.\",\n      \"method\": \"FAP-targeted CAR T cell adoptive transfer, xenograft and syngeneic tumor models, ECM protein analysis, vascular density quantification\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple tumor models with defined ECM and vascular readouts, immune-independence established using immunodeficient xenograft model\",\n      \"pmids\": [\"25979873\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FAP overexpression in oral squamous cell carcinoma promotes EMT via a non-enzymatic intracellular interaction with DPP9; FAP binds DPP9 intracellularly (identified by IP-MS), downregulates DPP9 expression, and DPP9 overexpression reverses FAP-induced proliferation, migration, invasion and EMT.\",\n      \"method\": \"Immunoprecipitation-mass spectrometry (IP-MS), overexpression and knockdown, in vitro and in vivo tumor assays\",\n      \"journal\": \"OncoTargets and therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 3 — single IP-MS identification with functional rescue, single lab\",\n      \"pmids\": [\"32273729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2018,\n      \"finding\": \"In breast tumors, FAP+PDPN+ cancer-associated fibroblasts suppress T cell proliferation in a nitric oxide-dependent manner, while FAP+PDPN- pericytes do not; these two FAP-expressing populations differ in transcriptome (CAFs enriched in TGFβ signaling/fibrosis genes), spatial localization (CAFs at tumor edge, pericytes around vessels), and immunosuppressive function.\",\n      \"method\": \"Flow cytometry sorting, co-culture T cell proliferation assay with nitric oxide inhibitors, spatial immunofluorescence, transcriptomics\",\n      \"journal\": \"Cancer immunology research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — functional co-culture assay with mechanism (nitric oxide), spatial validation, transcriptomic characterization\",\n      \"pmids\": [\"30266714\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAP expression in adipose tissue macrophages (ATMs) specifically mediates CCL8 expression, promoting recruitment of proinflammatory monocyte-derived macrophages to obese adipose tissue; macrophage-specific FAP deficiency also enhances energy expenditure associated with decreased monoamine oxidase expression and increased norepinephrine in white adipose tissue.\",\n      \"method\": \"Macrophage-specific FAP knockout mice, high-fat diet model, CCL8 overexpression rescue, flow cytometry, NE measurement\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with defined mechanistic rescue (CCL8 overexpression) and metabolic readouts\",\n      \"pmids\": [\"38100414\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FAP+ CAFs secrete fibronectin 1 (FN1), which engages integrin α5β1 on macrophages to activate FAK-AKT-STAT3 signaling, driving M2-like macrophage polarization; pharmacological disruption of FN1-integrin α5β1 with Cilengitide reverses immunosuppression and suppresses tumor growth in vivo.\",\n      \"method\": \"Single-cell RNA-seq, co-culture assays, signaling pathway analysis, Cilengitide pharmacological inhibition, in vivo mouse tumor models\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — defined signaling pathway with pharmacological rescue, in vivo validation, single lab\",\n      \"pmids\": [\"40263422\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2024,\n      \"finding\": \"In periodontitis, senescent gingival fibroblasts upregulate FAP through mTOR pathway activation; recombinant FAP increases pro-inflammatory cytokine secretion and osteoclast differentiation in macrophages, and FAP inhibition reduces macrophage inflammation, collagen degradation, and bone resorption in experimental periodontitis. Rapamycin restores FAP/OLN balance.\",\n      \"method\": \"Single-cell RNA-seq, recombinant FAP treatment of macrophages, FAP inhibitor in vivo, rapamycin treatment, bone resorption histology\",\n      \"journal\": \"Advanced science\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2-3 — in vitro recombinant FAP functional assay plus in vivo inhibitor study, single lab\",\n      \"pmids\": [\"39716898\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FAP (fibroblast activation protein α) is a type II transmembrane serine protease that homodimerizes via a TM small-X3-small motif to acquire both dipeptidyl peptidase and endopeptidase activities; it cleaves post-proline substrates including collagen fragments, FGF21, and BNP to regulate extracellular matrix remodeling, metabolic hormone levels, and cardiac repair, while on cancer-associated fibroblasts it activates STAT3 via uPAR-FAK-Src-JAK2 and drives immunosuppression through CCL2-mediated MDSC recruitment and FN1-integrin α5β1-mediated macrophage M2 polarization; additionally, the homologous phosphatase FAP-1 (PTPN13) binds the C-terminal 15 amino acids of Fas to retain Fas intracellularly and inhibit apoptosis, and is itself subject to selective autophagic degradation that controls cell fate decisions.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper and extract discoveries only from KEEP papers.\n\n**Classification notes:**\n- Papers about FAP (Fibroblast Activation Protein α, the serine protease / seprase) → KEEP\n- Papers about FAP-1/PTPL1 (Fas-Associated Phosphatase-1, a tyrosine phosphatase) → EXCLUDE (alias collision — different protein)\n- Papers about familial adenomatous polyposis (FAP syndrome, APC gene) → EXCLUDE (alias collision)\n- Papers about TTR-FAP (transthyretin familial amyloid polyneuropathy) → EXCLUDE (alias collision)\n- Papers about DPPIV/CD26 as the primary subject (not FAP) → EXCLUDE (different protein, though related family)\n- Papers about fluorogen-activating protein (bacterial FAP) → EXCLUDE (symbol collision)\n- Papers about FAP+ fibroblasts in TME with mechanistic findings about the FAP protein → KEEP\n- Papers about FAP-1 (tyrosine phosphatase): PMID 7536343, 24316673, 12354757, 12724420, 10398166, 10537175 → EXCLUDE\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1994,\n      \"finding\": \"FAP (fibroblast activation protein alpha) was identified as a type II integral membrane protein with a large extracellular domain, sharing 48% amino acid sequence identity with CD26/DPPIV and conserving three catalytic serine protease domains. Immunochemical analysis of COS-1 cells co-expressing FAP and CD26 revealed that the two molecules form heteromeric cell surface complexes, identifying CD26 as the previously described FAP-associated protein FAP-beta.\",\n      \"method\": \"Expression cloning in COS-1 cells, immunochemical co-expression analysis, sequence analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — original molecular cloning with functional domain characterization and direct co-expression experiment showing heteromeric complex formation\",\n      \"pmids\": [\"7911242\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1997,\n      \"finding\": \"Seprase (the 170-kDa melanoma membrane-bound gelatinase) was identified as identical to FAP-alpha; it is a homodimer of N-glycosylated 97-kDa subunits. Proteolytic (gelatinase) activity requires the dimeric form and is abolished upon dissociation into 97-kDa subunits. The active site serine was confirmed by affinity labeling with [³H]diisopropyl fluorophosphate, and activity was blocked by serine-protease inhibitors, establishing FAP/seprase as a serine integral membrane protease.\",\n      \"method\": \"Protein purification, reverse transcription-PCR cloning, COS-7 cell transfection, [³H]DFP affinity labeling, serine protease inhibitor assays, dissociation experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — biochemical reconstitution of activity, active-site labeling, and mutagenesis-equivalent dissociation experiments showing dimer requirement\",\n      \"pmids\": [\"9065413\", \"9247085\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"FAP exhibits both dipeptidyl peptidase IV activity and a collagenolytic/gelatinolytic endopeptidase activity capable of degrading gelatin and type I collagen. Mutation of the putative catalytic serine residue to alanine abolishes both enzymatic activities. FAP enzyme activity was detected in human cancerous tissues but not in matched normal tissues using an immunocapture assay, demonstrating it is active as a cell surface-bound collagenase in tumor stroma.\",\n      \"method\": \"Recombinant protein expression, active-site serine-to-alanine mutagenesis, in vitro collagen/gelatin degradation assays, immunocapture enzyme activity assay on tissue extracts\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — dual enzymatic activity demonstrated with purified recombinant protein and confirmed by catalytic serine mutagenesis abolishing both activities\",\n      \"pmids\": [\"10593948\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 1999,\n      \"finding\": \"Type I collagen substratum induces the association of α3β1 integrin with seprase/FAP as a complex at invadopodia of aggressive tumor cells. In the absence of collagen, α3β1 integrin and seprase exist as non-associating membrane proteins, establishing that integrin serves as a collagen-dependent docking protein for FAP at sites of cell invasion.\",\n      \"method\": \"Co-immunoprecipitation, immunofluorescence localization at invadopodia, collagen-dependent association assay\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal co-IP with collagen-dependence control, but single study\",\n      \"pmids\": [\"10455171\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"FAP/seprase and DPPIV form a functional protease complex at invadopodia of migratory fibroblasts that is required for cell invasion and migration on collagenous matrix. This complex elicits both gelatin-binding and gelatinase activities localized at invadopodia; serine protease inhibitors block the gelatinase activity and gelatin degradation, and antibodies to the gelatin-binding domain of DPPIV reduce degradation without affecting adhesion.\",\n      \"method\": \"Co-immunoprecipitation of seprase-DPPIV complex, gelatinase activity assays, serine protease inhibitor blocking, antibody neutralization, wound-closure migration assays\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — biochemical complex isolation, functional activity assays, multiple inhibitory approaches confirming mechanism, replicated in endothelial cells\",\n      \"pmids\": [\"12023964\", \"16651416\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"The crystal structure of FAP-alpha apoenzyme was solved at high resolution, revealing a DPPIV-like fold with an alpha/beta-hydrolase domain and an eight-bladed beta-propeller domain. A critical difference from DPPIV is Ala657 in FAP (vs. Asp663 in DPPIV) within the active site, which reduces acidity in the substrate-binding pocket and explains FAP's lower affinity for N-terminal amines and its endopeptidase activity. The FAP/A657D mutant showed ~60-fold increased catalytic efficiency for dipeptide substrates and ~350-fold reduced efficiency for endopeptidase substrates, confirming Ala657 as the molecular determinant of substrate specificity.\",\n      \"method\": \"X-ray crystallography (apoenzyme structure), kinetic analysis of wild-type and A657D mutant FAP, comparison with DPPIV crystal structure\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — crystal structure plus mutagenesis with quantitative kinetic validation, strong mechanistic insight\",\n      \"pmids\": [\"15809306\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"Circulating antiplasmin-cleaving enzyme (APCE) is a soluble form of FAP. APCE and recombinant FAP are homodimers with identical pH optima, extinction coefficients, tryptic peptide sequences, and antibody cross-reactivity. Both cleave α2-antiplasmin at Pro3-Leu4 and Pro12-Asn13 bonds, with ~16-fold higher kcat/Km for the Pro12-Asn13 site, identifying Met-α2-antiplasmin as a physiological substrate of FAP and establishing a role for soluble FAP in fibrinolysis.\",\n      \"method\": \"Comparative biochemical characterization of APCE and recombinant FAP, kinetic analysis of cleavage site preferences, tryptic peptide sequencing, antibody cross-reactivity\",\n      \"journal\": \"Blood\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — direct comparative biochemistry with purified proteins, kinetic quantification of substrate cleavage, identification of physiological substrate\",\n      \"pmids\": [\"16223769\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2005,\n      \"finding\": \"FAP overexpression in LX-2 human hepatic stellate cells increased cell adhesion and migration on extracellular matrix proteins (collagen-I, fibronectin, Matrigel) and enhanced invasion across transwells, and also enhanced staurosporine-induced apoptosis. Importantly, the enzymatic activity of FAP was not required for these functions (non-enzymatic role). FAP overexpression increased MMP-2 and CD44 expression and reduced integrin-β1 expression.\",\n      \"method\": \"GFP-FAP fusion protein overexpression in LX-2 and HEK293T cells, adhesion assays, transwell migration/invasion assays, Western blot for downstream targets, enzyme-activity-dead controls\",\n      \"journal\": \"Hepatology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — overexpression with enzyme-dead controls distinguishing enzymatic from non-enzymatic functions, multiple functional readouts, single study\",\n      \"pmids\": [\"16175601\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2011,\n      \"finding\": \"FAP enzymatically remodels extracellular matrix by modulating fibronectin and collagen fiber organization and protein levels in a 3D matrix system. FAP-dependent architectural alterations of the ECM promote pancreatic cancer cell invasion along parallel fiber orientations (enhanced directionality and velocity), and this phenotype is reversed by inhibition of FAP enzymatic activity during matrix production. The FAP+ matrix-induced tumor invasion phenotype is β1-integrin/FAK mediated.\",\n      \"method\": \"Tetracycline-inducible FAP overexpression, 3D in vivo-like matrix system, fiber orientation analysis, time-lapse invasion assays, FAP enzymatic inhibitor treatment, Western blot for β1-integrin/FAK\",\n      \"journal\": \"BMC cancer\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — inducible system with enzyme inhibitor controls linking FAP activity to ECM remodeling and invasion phenotype, pathway placement via β1-integrin/FAK\",\n      \"pmids\": [\"21668992\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAP directly participates in collagen catabolism in concert with MMPs. In two mouse models of pulmonary fibrosis (bleomycin and thoracic irradiation), FAP-deficient mice showed increased mortality and lung fibrosis, with accumulation of intermediate-sized collagen fragments consistent with FAP mediating proteolytic processing of MMP-derived collagen cleavage products. FAP-mediated collagen processing increased collagen internalization (via Endo180 receptor) without altering receptor expression; pharmacological FAP inhibition decreased collagen internalization; restoration of FAP expression in FAP-deficient mouse lungs normalized collagen content.\",\n      \"method\": \"FAP-knockout mouse models, two independent fibrosis models, lung ECM analysis, in vitro collagen processing assays, pharmacological FAP inhibition, viral rescue (FAP re-expression in KO lungs), hydroxyproline quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — genetic KO with two independent disease models plus pharmacological inhibition and rescue experiment, multiple orthogonal methods\",\n      \"pmids\": [\"26663085\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAP triggers induction of a cancer-associated fibroblast subset with an inflammatory phenotype via persistent activation of STAT3 through a uPAR-dependent FAK-Src-JAK2 signaling pathway. Enforcing FAP expression in normal fibroblasts was sufficient to activate STAT3 and upregulate CCL2, which promoted recruitment of myeloid-derived suppressor cells (MDSCs) via CCR2. FAP+-CAF-mediated tumor promotion and MDSC recruitment was abrogated in Ccr2-deficient mice.\",\n      \"method\": \"FAP overexpression in normal fibroblasts, signaling pathway inhibitors (FAK, Src, JAK2), STAT3 reporter assays, murine liver tumor model, Ccr2-deficient mice, CCL2 ELISA\",\n      \"journal\": \"Cancer research\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — genetic and pharmacological dissection of signaling pathway with in vivo validation in KO mice, single laboratory\",\n      \"pmids\": [\"27216177\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"FAP cleaves and inactivates fibroblast growth factor 21 (FGF21). A selective FAP chemical inhibitor, FAP immunodepletion, or genetic Fap deletion stabilized recombinant human FGF21 in serum. Administration of a selective FAP inhibitor acutely increased circulating intact FGF21 levels in cynomolgus monkeys, establishing FGF21 as a physiological substrate of FAP.\",\n      \"method\": \"In vitro cleavage assays with purified FAP, selective FAP inhibitor treatment, immunodepletion of FAP from serum, Fap-knockout mice, in vivo FAP inhibitor dosing in monkeys with intact FGF21 quantification\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — multiple orthogonal approaches (inhibitor, immunodepletion, genetic KO) plus in vivo primate confirmation identifying FGF21 as FAP substrate\",\n      \"pmids\": [\"26797127\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Pharmacological FAP inhibition (using talabostat) in diet-induced obese mice led to decreased body weight, reduced food intake, improved glucose tolerance and insulin sensitivity, and elevated plasma FGF21 levels. FAP inhibition showed no metabolic effect in FGF21-knockout obese animals, and in vitro FAP was shown to degrade human FGF21 at both termini in the absence of inhibitor, confirming FGF21 as the critical substrate mediating FAP's metabolic role.\",\n      \"method\": \"In vitro FGF21 degradation assay with purified FAP and inhibitor, diet-induced obese mouse model, FGF21-knockout mice, talabostat pharmacological intervention, metabolic phenotyping\",\n      \"journal\": \"Molecular metabolism\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro enzyme assay corroborated by FGF21-KO epistasis experiment establishing FGF21 as the obligate downstream mediator\",\n      \"pmids\": [\"27689014\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2016,\n      \"finding\": \"Specific interfacial residues in the FAP transmembrane (TM) domain (G10, S14, A18, forming a small-X3-small motif) are required for FAP homodimerization. Mutations G10L, S14L, and A18L reduced FAP TM-CYTO dimerization as measured by the AraTM bacterial assay, while off-interface residues showed no change. The G10L mutation significantly decreased FAP endopeptidase activity by more than 25% and reduced cell-surface versus intracellular FAP expression, linking TM-mediated dimerization to both protease activity and proper trafficking.\",\n      \"method\": \"AraTM bacterial dimerization assay, site-directed mutagenesis of TM interface residues, endopeptidase activity assays, cell-surface vs. intracellular expression quantification\",\n      \"journal\": \"Biochimica et biophysica acta\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — systematic mutagenesis of predicted interface residues with functional readouts for activity and trafficking, single study\",\n      \"pmids\": [\"27155568\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2015,\n      \"finding\": \"FAP expression in activated cardiac fibroblasts after myocardial infarction is induced by TGF-β1 via the canonical SMAD2/SMAD3 pathway (blocked by TGFbR1 inhibitor SB431542, MAPK inhibitor U0126, and siRNA targeting SMAD2/SMAD3). FAP depletion in fibroblasts reduced migratory capacity (Boyden chamber assay) without affecting proliferation, and FAP exhibited gelatinase activity by gelatin zymography, indicating roles in cardiac wound healing and remodeling.\",\n      \"method\": \"TGF-β1 stimulation of human cardiac fibroblasts, TGFbR1 inhibitor, MAPK inhibitor, SMAD2/3 siRNA knockdown, modified Boyden-chamber migration assay, BrdU proliferation assay, gelatin zymography\",\n      \"journal\": \"Journal of molecular and cellular cardiology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — pathway dissection with multiple inhibitors and siRNA converging on SMAD2/3, functional migration and gelatinase readouts, single study\",\n      \"pmids\": [\"26319660\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"BNP (brain natriuretic peptide) is a novel physiological substrate of FAP that mediates post-ischemic angiogenesis. FAP degrades BNP to inhibit vascular endothelial cell migration and tube formation. Genetic or pharmacological inhibition of FAP in mice increased angiogenesis in the peri-infarct zone after myocardial infarction and improved cardiac function. The cardioprotective effect of FAP inhibition was abolished in mice deficient in Nppb (encoding pre-proBNP) or Npr1 (encoding the BNP receptor), establishing BNP as the obligate mediator.\",\n      \"method\": \"In vitro BNP cleavage assay, endothelial cell migration and tube formation assays, FAP genetic KO and pharmacological inhibition in mouse MI model, Nppb-KO and Npr1-KO epistasis, echocardiography, histological analysis, RNA-sequencing\",\n      \"journal\": \"Circulation research\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1–2 — in vitro substrate cleavage assay corroborated by double-KO epistasis experiments establishing BNP as the obligate downstream mediator, multiple orthogonal methods\",\n      \"pmids\": [\"36756875\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2014,\n      \"finding\": \"FAP silencing in oral squamous cell carcinoma (OSCC) cells inhibited growth and metastasis in vitro and in vivo; mechanistically, FAP knockdown inactivated PTEN/PI3K/AKT and Ras-ERK signaling. The inhibitory effects of FAP knockdown on proliferation and metastasis were rescued by PTEN silencing, placing FAP upstream of PTEN/PI3K/AKT in OSCC.\",\n      \"method\": \"FAP siRNA knockdown, in vitro proliferation/migration/invasion assays, xenograft mouse models, Western blot for PI3K/AKT and Ras-ERK pathway components, PTEN rescue experiment\",\n      \"journal\": \"Cell death & disease\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — epistasis rescue experiment placing FAP upstream of PTEN/PI3K/AKT, in vivo validation, single laboratory\",\n      \"pmids\": [\"24722280\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2020,\n      \"finding\": \"FAP overexpression in oral squamous cell carcinoma induces epithelial-mesenchymal transition (EMT) by down-regulating DPP9 (dipeptidyl peptidase 9). FAP was identified as interacting intracellularly with DPP9 by immunoprecipitation-mass spectrometry. DPP9 overexpression reversed the FAP-induced proliferation, migration, invasion, and EMT, establishing a FAP→DPP9 suppression axis. This mechanism is non-enzymatic (FAP promotes EMT by reducing DPP9, not via its protease activity).\",\n      \"method\": \"Immunoprecipitation-mass spectrometry (IP-MS) to identify FAP interactors, FAP overexpression, DPP9 knockdown/overexpression, in vitro and in vivo functional assays\",\n      \"journal\": \"OncoTargets and therapy\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — IP-MS identifies intracellular DPP9 as FAP binding partner with functional rescue experiment, single study\",\n      \"pmids\": [\"32273729\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAP in adipose tissue macrophages (ATM) specifically mediates CCL8 chemokine expression, contributing to recruitment of proinflammatory monocyte-derived macrophages in obese adipose tissue. Macrophage-specific FAP deficiency protected mice from diet-induced obesity and improved insulin resistance; CCL8 overexpression restored metabolic phenotypes in FAP-deficient mice, establishing CCL8 as the downstream effector. FAP-deficient ATMs also showed decreased monoamine oxidase expression, leading to elevated norepinephrine and increased lipolysis.\",\n      \"method\": \"Macrophage-specific FAP knockout mice, high-fat diet model, CCL8 overexpression rescue, flow cytometry, norepinephrine measurement, monoamine oxidase expression analysis\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — cell-type-specific KO with CCL8 rescue epistasis establishing pathway, multiple metabolic readouts, single laboratory\",\n      \"pmids\": [\"38100414\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2023,\n      \"finding\": \"FAP promotes metastasis and chemoresistance in mucinous colorectal adenocarcinoma by interacting with MPRIP (myosin phosphatase Rho-interacting protein) and regulating the Rho/Hippo/YAP signaling pathway, leading to TAM recruitment and M2 macrophage polarization. MPRIP was identified as a direct FAP-interacting protein.\",\n      \"method\": \"Co-immunoprecipitation identifying MPRIP as FAP interactor, FAP overexpression/knockdown, in vitro and in vivo functional assays, Rho/Hippo/YAP pathway analysis, macrophage polarization assays\",\n      \"journal\": \"iScience\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single Co-IP identifying MPRIP interaction, pathway placement based on overexpression/knockdown, single study with limited mechanistic depth\",\n      \"pmids\": [\"37213233\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"FAP+ cancer-associated fibroblasts in breast cancer secrete high levels of fibronectin 1 (FN1), which engages integrin α5β1 on macrophages to activate FAK-AKT-STAT3 signaling, driving M2-like macrophage polarization. Pharmacological disruption of FN1-integrin α5β1 signaling with Cilengitide reprogrammed the tumor immune landscape and suppressed tumor growth in mouse models.\",\n      \"method\": \"scRNA-seq, co-culture experiments, FAP+ CAF conditioned medium, integrin α5β1 blocking antibody and Cilengitide treatment, Western blot for FAK-AKT-STAT3 pathway, in vivo mouse tumor models\",\n      \"journal\": \"Oncogene\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — mechanistic pathway (FN1→integrin α5β1→FAK-AKT-STAT3) established by pharmacological blockade with in vivo validation, single study\",\n      \"pmids\": [\"40263422\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"FAP (fibroblast activation protein alpha/seprase) is a type II transmembrane serine protease that functions as an obligate homodimer (dimerization requiring a specific TM small-X3-small motif) with dual enzymatic activities — dipeptidyl peptidase IV-like exopeptidase and post-prolyl endopeptidase (collagenolytic) activity — both requiring the catalytic serine (S624); its endopeptidase specificity over DPPIV is conferred by Ala657 in the active site; it cleaves physiological substrates including type I collagen/gelatin, α2-antiplasmin, FGF21 (inactivating it), and BNP (inhibiting angiogenesis after MI); it forms a functional protease complex with DPPIV/CD26 at invadopodia to mediate ECM degradation and cell migration; it remodels stromal ECM to promote tumor invasion via β1-integrin/FAK signaling; and it activates STAT3 via uPAR-FAK-Src-JAK2 signaling in fibroblasts to drive immunosuppressive CCL2/MDSC recruitment, while also promoting M2 macrophage polarization through FN1-integrin α5β1-FAK-AKT-STAT3 and regulating metabolic inflammation through CCL8 in adipose tissue macrophages.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"FAP (fibroblast activation protein α) is a type II transmembrane serine protease that homodimerizes through a GxxxA transmembrane motif to acquire dipeptidyl peptidase and endopeptidase activities cleaving post-proline bonds in substrates including collagen fragments, FGF21, and BNP, thereby regulating extracellular matrix turnover, metabolic hormone levels, and cardiac repair [PMID:24470260, PMID:27155568, PMID:26663085, PMID:27689014, PMID:36756875]. On cancer-associated fibroblasts, FAP activates STAT3 via uPAR–FAK–Src–JAK2 signaling to upregulate CCL2 and recruit myeloid-derived suppressor cells, and FAP+ CAF-derived fibronectin 1 engages integrin α5β1 on macrophages to drive M2 polarization, collectively establishing an immunosuppressive tumor microenvironment [PMID:27216177, PMID:40263422]. The homologous phosphatase FAP-1 (PTPN13) binds the C-terminal 15 amino acids of Fas to retain Fas intracellularly and inhibit Fas-mediated apoptosis, a function regulated by selective autophagic degradation of FAP-1 that shifts cells toward apoptotic sensitivity [PMID:7536343, PMID:12724420, PMID:24316673]. In adipose tissue macrophages, FAP drives CCL8-dependent proinflammatory monocyte recruitment and suppresses norepinephrine-mediated energy expenditure, linking FAP to obesity-associated inflammation [PMID:38100414].\",\n  \"teleology\": [\n    {\n      \"year\": 1995,\n      \"claim\": \"The discovery that FAP-1 (PTPN13) physically binds the Fas C-terminus and inhibits Fas-induced apoptosis established the first link between a tyrosine phosphatase and death receptor signaling.\",\n      \"evidence\": \"Protein interaction screen, gene transfer overexpression, and apoptosis assay in T cells\",\n      \"pmids\": [\"7536343\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Mechanism by which phosphatase activity opposes Fas signaling was not defined\", \"In vivo relevance in immune regulation not tested\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Demonstration that FAP-1 protects human thyrocytes from Fas-mediated death using a competitive SLV peptide confirmed the physiological relevance of the FAP-1–Fas interaction beyond T cells.\",\n      \"evidence\": \"Competitive peptide inhibition of Fas–FAP-1 binding, cell death assay in primary thyrocytes\",\n      \"pmids\": [\"10537175\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Peptide competition is indirect; in vivo confirmation in thyroid tissue not provided\", \"Downstream phosphatase substrates not identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Showing that FAP-1 phosphatase activity suppresses IRS-1/PI3K/Akt signaling revealed FAP-1 as a broader signaling regulator beyond Fas, connecting it to growth factor survival pathways.\",\n      \"evidence\": \"Antisense knockdown, TUNEL assay, PI3K activity and Akt phosphorylation measurements in MCF7 cells\",\n      \"pmids\": [\"12354757\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Direct dephosphorylation target on IRS-1 not mapped\", \"Relationship between anti-apoptotic Fas retention and pro-apoptotic IGF pathway suppression in the same cell not resolved\"]\n    },\n    {\n      \"year\": 2003,\n      \"claim\": \"The finding that FAP-1 retains Fas in an intracellular cytoskeletal compartment, reducing surface Fas, provided the trafficking mechanism underlying FAP-1's anti-apoptotic function.\",\n      \"evidence\": \"siRNA knockdown, dominant-negative expression, confocal localization, flow cytometry, Fas mutagenesis\",\n      \"pmids\": [\"12724420\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Identity of the cytoskeletal retention compartment not fully characterized\", \"Whether FAP-1 catalytic activity or scaffolding mediates Fas retention was unclear\"]\n    },\n    {\n      \"year\": 2013,\n      \"claim\": \"Discovery that selective autophagy degrades FAP-1 to sensitize Type I cells to Fas-mediated apoptosis established autophagy as a decision-layer controlling FAP-1 abundance and cell fate.\",\n      \"evidence\": \"Flow cytometry sorting by autophagy level, immunoblot for Fap-1 after autophagy modulation, functional apoptosis assays\",\n      \"pmids\": [\"24316673\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Autophagy receptor mediating selective FAP-1 degradation not identified\", \"Whether this mechanism operates in vivo during immune surveillance not shown\"]\n    },\n    {\n      \"year\": 2014,\n      \"claim\": \"Biochemical characterization of FAP α as a homodimeric serine protease with both dipeptidyl peptidase and endopeptidase activities cleaving post-proline bonds defined its dual catalytic nature.\",\n      \"evidence\": \"In vitro enzymatic assays, inhibitor structure-activity relationship studies\",\n      \"pmids\": [\"24470260\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for endopeptidase versus dipeptidyl peptidase selectivity not fully resolved\", \"Physiological substrates not yet identified at this point\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Two studies established FAP's extracellular functions: FAP processes MMP-derived collagen fragments to promote collagen clearance via Endo180, and FAP+ stromal cells maintain provisional tumor stroma including ECM and vasculature.\",\n      \"evidence\": \"FAP-KO mice with bleomycin/irradiation fibrosis models plus AAV rescue; FAP-targeted CAR T cells in xenograft/syngeneic tumors with ECM quantification\",\n      \"pmids\": [\"26663085\", \"25979873\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FAP's collagen-processing and tumor-stromal roles involve the same enzymatic activity not distinguished\", \"Relative contribution of dipeptidyl peptidase vs. endopeptidase activity in vivo not delineated\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"Three advances converged: transmembrane GxxxA-mediated homodimerization was shown to be required for endopeptidase activity and surface trafficking; FGF21 was validated as a physiological substrate linking FAP to metabolic regulation; and FAP on CAFs was shown to activate STAT3–CCL2–MDSC immunosuppressive signaling.\",\n      \"evidence\": \"AraTM dimerization assay with mutagenesis; FAP inhibitor in obese mice with FGF21-KO epistasis; FAP overexpression in fibroblasts with signaling inhibitors and Ccr2-KO mice\",\n      \"pmids\": [\"27155568\", \"27689014\", \"27216177\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether dimerization is regulated in vivo (e.g., by lipid composition) unknown\", \"Whether STAT3 activation requires FAP enzymatic activity or scaffolding not resolved\"]\n    },\n    {\n      \"year\": 2018,\n      \"claim\": \"Distinguishing FAP+PDPN+ CAFs from FAP+PDPN− pericytes revealed that immunosuppressive T cell inhibition via nitric oxide is specific to the CAF subset, refining the cellular context of FAP-mediated immunosuppression.\",\n      \"evidence\": \"Flow sorting, co-culture T cell proliferation assays with NO inhibitors, spatial immunofluorescence, transcriptomics in breast tumors\",\n      \"pmids\": [\"30266714\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FAP enzymatic activity is required for NO-dependent T cell suppression not tested\", \"Heterogeneity of FAP+ CAFs across tumor types not explored\"]\n    },\n    {\n      \"year\": 2020,\n      \"claim\": \"FAP was found to interact intracellularly with DPP9 in a non-enzymatic manner to promote EMT in oral squamous cell carcinoma, revealing an intracellular signaling role independent of its protease activity.\",\n      \"evidence\": \"IP-MS identification, overexpression/knockdown rescue in OSCC cells and xenografts\",\n      \"pmids\": [\"32273729\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Single IP-MS identification without reciprocal validation\", \"Structural basis for FAP–DPP9 intracellular interaction not defined\", \"Generalizability beyond OSCC not established\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"Identification of BNP as a FAP substrate established that FAP limits post-MI cardiac repair by degrading a pro-angiogenic natriuretic peptide, and separately, macrophage-specific FAP was shown to drive adipose inflammation via CCL8 and suppress catecholamine-mediated thermogenesis.\",\n      \"evidence\": \"FAP-KO mice with Nppb-KO and Npr1-KO epistasis after MI; macrophage-specific FAP-KO mice on HFD with CCL8 rescue\",\n      \"pmids\": [\"36756875\", \"38100414\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Whether FAP cleaves additional natriuretic peptides (ANP, CNP) not assessed\", \"Whether macrophage FAP enzymatic activity or expression per se drives CCL8 not distinguished\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"FAP+ CAFs were shown to polarize macrophages to an M2 phenotype through FN1–integrin α5β1–FAK–AKT–STAT3 signaling, with pharmacological reversal by Cilengitide, providing a defined paracrine immunosuppressive mechanism.\",\n      \"evidence\": \"scRNA-seq, co-culture signaling analysis, Cilengitide treatment in vivo\",\n      \"pmids\": [\"40263422\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether FN1 secretion depends on FAP enzymatic activity not resolved\", \"Single-lab finding awaiting independent replication\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include whether FAP's immunosuppressive functions require its protease activity or scaffolding, the structural basis for substrate selectivity among post-proline peptides, and the identity of the autophagy receptor targeting FAP-1 for selective degradation.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Enzymatic vs. non-enzymatic contributions to immunosuppression not genetically dissected\", \"No full-length FAP structure with bound physiological substrate\", \"Selective autophagy receptor for FAP-1 unknown\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [5, 7, 8, 9, 10, 11]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [5, 7, 9, 11]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [5, 8, 12]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [7, 9, 11]},\n      {\"term_id\": \"GO:0005856\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [7, 12]},\n      {\"term_id\": \"R-HSA-5357801\", \"supporting_discovery_ids\": [0, 1, 4]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [6, 14, 15, 16]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [2, 6, 16]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [9, 15]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"FAS\",\n      \"DPP9\",\n      \"FGF21\",\n      \"NPPB\",\n      \"FN1\",\n      \"UPAR\",\n      \"IRS1\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"FAP is a type II transmembrane serine protease that functions as an obligate homodimer with dual enzymatic activities—dipeptidyl peptidase IV-like exopeptidase and post-prolyl endopeptidase—both dependent on a single catalytic serine (S624), with endopeptidase specificity over DPPIV conferred by Ala657 in the active-site pocket [PMID:15809306, PMID:10593948]. FAP cleaves physiological substrates including type I collagen (cooperating with MMPs to facilitate collagen catabolism and Endo180-mediated internalization), α2-antiplasmin (regulating fibrinolysis), FGF21 (controlling metabolic homeostasis), and BNP (modulating post-ischemic angiogenesis), with obligate substrate dependence confirmed by epistasis experiments in FGF21-KO and Nppb-KO mice [PMID:26663085, PMID:16223769, PMID:26797127, PMID:36756875]. Beyond its protease function, FAP exerts non-enzymatic signaling roles: it activates STAT3 via a uPAR–FAK–Src–JAK2 cascade in cancer-associated fibroblasts to drive CCL2-dependent MDSC recruitment, promotes M2 macrophage polarization through FN1–integrin α5β1–FAK–AKT–STAT3 signaling, and mediates CCL8-dependent proinflammatory macrophage recruitment in adipose tissue [PMID:27216177, PMID:40263422, PMID:38100414]. FAP forms a functional complex with DPPIV/CD26 at invadopodia and associates with α3β1 integrin in a collagen-dependent manner to mediate ECM degradation and tumor cell invasion [PMID:12023964, PMID:10455171].\",\n  \"teleology\": [\n    {\n      \"year\": 1994,\n      \"claim\": \"Molecular cloning established FAP as a type II membrane serine protease with homology to DPPIV and revealed that FAP and CD26/DPPIV form heteromeric complexes on the cell surface, providing the first molecular identity for the fibroblast activation protein.\",\n      \"evidence\": \"Expression cloning in COS-1 cells with immunochemical co-expression analysis\",\n      \"pmids\": [\"7911242\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Enzymatic activity not yet characterized\", \"Physiological substrates unknown\", \"In vivo function not addressed\"]\n    },\n    {\n      \"year\": 1997,\n      \"claim\": \"Demonstration that seprase and FAP are identical proteins and that gelatinase activity requires the homodimeric form resolved a key structural prerequisite for catalytic function.\",\n      \"evidence\": \"Purification, RT-PCR cloning, [³H]DFP active-site labeling, and dissociation experiments in COS-7 cells\",\n      \"pmids\": [\"9065413\", \"9247085\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Specific cleavage sites not mapped\", \"Dimerization interface not structurally defined\", \"Endopeptidase versus exopeptidase activity not distinguished\"]\n    },\n    {\n      \"year\": 1999,\n      \"claim\": \"Mutagenesis of the catalytic serine showed that a single active site governs both DPPIV-like exopeptidase and collagenolytic endopeptidase activities, and collagen-dependent recruitment of FAP to invadopodia via α3β1 integrin provided a spatial mechanism for ECM degradation at the invasion front.\",\n      \"evidence\": \"S→A mutagenesis with in vitro collagen/gelatin degradation assays; co-IP showing collagen-dependent α3β1 integrin–FAP complex at invadopodia\",\n      \"pmids\": [\"10593948\", \"10455171\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Structural basis for dual activity unknown\", \"In vivo collagenolytic role not tested\", \"No physiological substrates beyond gelatin/collagen identified\"]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Isolation of a functional FAP–DPPIV protease complex at invadopodia demonstrated that the two related proteases cooperate in gelatin degradation and cell migration, establishing a composite proteolytic unit for ECM invasion.\",\n      \"evidence\": \"Co-IP of FAP–DPPIV complex, gelatinase activity assays with serine protease inhibitors and anti-gelatin-binding antibodies, wound-closure migration assays\",\n      \"pmids\": [\"12023964\", \"16651416\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Relative contribution of each protease to invasion unclear\", \"Stoichiometry and regulation of complex assembly unresolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"The crystal structure of FAP revealed that a single residue difference (Ala657 vs. Asp663 in DPPIV) governs endopeptidase specificity, and identification of α2-antiplasmin as a physiological substrate linked soluble FAP to fibrinolysis regulation.\",\n      \"evidence\": \"X-ray crystallography with A657D kinetic analysis; comparative biochemistry of APCE/FAP showing cleavage of α2-antiplasmin at defined sites\",\n      \"pmids\": [\"15809306\", \"16223769\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Physiological relevance of α2-antiplasmin cleavage in vivo not confirmed in genetic models\", \"Structural basis for collagen recognition not resolved\"]\n    },\n    {\n      \"year\": 2005,\n      \"claim\": \"Overexpression studies in hepatic stellate cells demonstrated that FAP promotes adhesion, migration, and invasion through non-enzymatic mechanisms independent of its protease activity, expanding FAP's functional repertoire beyond catalysis.\",\n      \"evidence\": \"GFP-FAP overexpression with enzyme-dead controls in LX-2 cells; adhesion, migration, and invasion assays; MMP-2/CD44/integrin-β1 expression changes\",\n      \"pmids\": [\"16175601\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Mechanism of non-enzymatic signaling not defined\", \"Overexpression system may not reflect physiological levels\", \"No identification of the non-enzymatic binding partner\"]\n    },\n    {\n      \"year\": 2011,\n      \"claim\": \"FAP's enzymatic remodeling of 3D extracellular matrix was shown to organize collagen/fibronectin fibers into parallel orientations that promote directional tumor cell invasion via β1-integrin/FAK signaling, linking FAP catalytic activity to a mechanistic ECM-guided invasion program.\",\n      \"evidence\": \"Tetracycline-inducible FAP expression, 3D matrix fiber orientation analysis, FAP enzymatic inhibitor rescue, β1-integrin/FAK Western blot\",\n      \"pmids\": [\"21668992\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Specific ECM cleavage products driving fiber reorganization unidentified\", \"In vivo validation of fiber-orientation phenotype lacking\"]\n    },\n    {\n      \"year\": 2015,\n      \"claim\": \"Genetic and pharmacological studies in fibrosis models revealed that FAP processes MMP-generated collagen fragments to promote Endo180-mediated collagen internalization, establishing an in vivo collagen catabolic function; concurrently, TGF-β1/SMAD2/3 was identified as the upstream inducer of FAP expression in cardiac fibroblasts.\",\n      \"evidence\": \"FAP-KO mice in bleomycin/irradiation fibrosis models with viral rescue; TGF-β1 stimulation with SMAD2/3 siRNA and pharmacological inhibitors in cardiac fibroblasts\",\n      \"pmids\": [\"26663085\", \"26319660\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Precise collagen fragment cleavage sites in vivo unmapped\", \"Contribution of soluble vs. membrane FAP in fibrosis not distinguished\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"FGF21 was established as a key physiological substrate through convergent evidence from FAP inhibitor treatment, immunodepletion, Fap-KO mice, and primate dosing, with FGF21-KO epistasis confirming it as the obligate mediator of FAP's metabolic effects; separately, TM domain mutagenesis defined the small-X3-small dimerization motif required for protease activity and surface trafficking.\",\n      \"evidence\": \"In vitro cleavage, selective FAP inhibitor in cynomolgus monkeys, FGF21-KO epistasis in obese mice; AraTM bacterial dimerization assay with TM point mutations\",\n      \"pmids\": [\"26797127\", \"27689014\", \"27155568\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"Full spectrum of FAP-regulated metabolic pathways beyond FGF21 unclear\", \"Structural model of the TM dimer interface at atomic resolution lacking\"]\n    },\n    {\n      \"year\": 2016,\n      \"claim\": \"FAP was placed upstream of STAT3 activation via a uPAR–FAK–Src–JAK2 signaling cascade in cancer-associated fibroblasts, driving CCL2-dependent MDSC recruitment—demonstrating a non-enzymatic immunomodulatory function validated by Ccr2-KO epistasis in vivo.\",\n      \"evidence\": \"FAP overexpression in fibroblasts with FAK/Src/JAK2 inhibitors, STAT3 reporter, murine liver tumor model, Ccr2-KO mice\",\n      \"pmids\": [\"27216177\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether protease activity contributes to STAT3 activation not formally excluded\", \"Generalizability beyond hepatocellular carcinoma model not established\"]\n    },\n    {\n      \"year\": 2023,\n      \"claim\": \"BNP was identified as a novel FAP substrate mediating post-ischemic angiogenesis, with FAP inhibition improving cardiac function after MI in a BNP- and NPR1-dependent manner, establishing a cardiovascular axis for FAP protease activity; in parallel, macrophage-specific FAP deletion revealed a CCL8-dependent role in adipose tissue inflammation and metabolic regulation.\",\n      \"evidence\": \"FAP-KO and pharmacological inhibition in mouse MI model with Nppb-KO and Npr1-KO epistasis; macrophage-specific FAP-KO on HFD with CCL8 overexpression rescue\",\n      \"pmids\": [\"36756875\", \"38100414\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\"BNP cleavage site(s) by FAP not precisely mapped\", \"Relative contribution of membrane vs. soluble FAP to BNP inactivation in vivo unknown\", \"CCL8 transcriptional regulation by FAP mechanism uncharacterized\"]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"FAP+ CAF-derived fibronectin 1 was shown to drive M2 macrophage polarization via integrin α5β1–FAK–AKT–STAT3 signaling, revealing a paracrine mechanism by which FAP shapes the immunosuppressive tumor microenvironment.\",\n      \"evidence\": \"scRNA-seq, co-culture with conditioned medium, Cilengitide and integrin-blocking antibody treatment, mouse tumor models\",\n      \"pmids\": [\"40263422\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\"Whether FAP enzymatic activity is required for FN1 upregulation not tested\", \"Single tumor type (breast cancer) limits generalizability\"]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Major open questions include the full substrate repertoire of FAP in vivo, the structural basis for collagen and BNP recognition, the molecular mechanism by which FAP exerts non-enzymatic signaling functions, and whether membrane-bound and soluble FAP have distinct physiological roles.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"High\",\n      \"gaps\": [\"No co-crystal structure with any physiological substrate\", \"Membrane vs. soluble FAP contributions not genetically separated in vivo\", \"Non-enzymatic signaling mechanism molecularly undefined\"]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0140096\", \"supporting_discovery_ids\": [1, 2, 5, 6, 9, 11, 12, 15]},\n      {\"term_id\": \"GO:0016787\", \"supporting_discovery_ids\": [1, 2, 5, 6, 9]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005886\", \"supporting_discovery_ids\": [0, 1, 3, 4, 13]},\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [6]},\n      {\"term_id\": \"GO:0031012\", \"supporting_discovery_ids\": [8, 9]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-1474244\", \"supporting_discovery_ids\": [2, 8, 9]},\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [10, 16, 20]},\n      {\"term_id\": \"R-HSA-168256\", \"supporting_discovery_ids\": [10, 18, 20]},\n      {\"term_id\": \"R-HSA-1430728\", \"supporting_discovery_ids\": [11, 12]},\n      {\"term_id\": \"R-HSA-392499\", \"supporting_discovery_ids\": [5, 6, 11, 15]}\n    ],\n    \"complexes\": [\n      \"FAP homodimer\",\n      \"FAP-DPPIV/CD26 heteromeric complex\"\n    ],\n    \"partners\": [\n      \"DPP4\",\n      \"ITGB1\",\n      \"ITGA3\",\n      \"DPP9\",\n      \"MPRIP\",\n      \"FN1\",\n      \"UPAR\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}