{"gene":"HPX","run_date":"2026-04-28T18:06:53","timeline":{"discoveries":[{"year":1985,"finding":"Human hemopexin (HPX) is a single polypeptide chain of 439 amino acid residues (Mr ~63,000) with six intrachain disulfide bridges, an O-linked galactosamine oligosaccharide blocking the N-terminal threonine, and five N-linked glucosamine oligosaccharides. Internal sequence homology analysis revealed that HPX consists of two similar halves arising from ancestral gene duplication. Limited tryptic digestion mapped the hinge region (histidine-rich, linker between the two half-molecules) and showed that heme binding occludes the trypsin cleavage site at lysine-101, indicating a conformational change upon heme binding.","method":"Complete protein sequencing (tryptic peptide mapping, Edman degradation), computer-assisted internal homology analysis, limited proteolysis of apo- vs. heme-saturated HPX","journal":"Proceedings of the National Academy of Sciences of the United States of America","confidence":"High","confidence_rationale":"Tier 1 — complete primary structure determination with multiple orthogonal biochemical methods and structural model validation","pmids":["3855550"],"is_preprint":false},{"year":2002,"finding":"HPX is the plasma protein with the highest known heme-binding affinity. It functions primarily as an antioxidant scavenger: free heme intercalates into lipid membranes and generates hydroxyl radicals via Fenton chemistry, and HPX prevents this by tightly binding heme and delivering the heme-HPX complex to hepatocytes via a specific heme-HPX receptor. Receptor-mediated endocytosis of the complex triggers intracellular antioxidant responses and enables iron salvage, establishing that HPX's dominant physiological role is antioxidant defense rather than primary iron metabolism.","method":"Review integrating binding affinity measurements, receptor identification experiments, cell-based heme uptake assays, and HPX-knockout mouse studies","journal":"DNA and cell biology","confidence":"High","confidence_rationale":"Tier 2 — multiple orthogonal experimental approaches consolidated across labs, strong preponderance of evidence","pmids":["12042069"],"is_preprint":false},{"year":2006,"finding":"The two four-bladed β-propeller hemopexin domains of HPX serve as multifunctional liganding modules. These domains mediate heme binding, endocytosis via LRP-1 (CD91) and LRP-2 (megalin/GP330) receptors, and the same structural fold is used in MMPs and vitronectin for substrate binding, MMP activation/inhibition, and dimerization—establishing that the HPX domain architecture is a conserved scaffold for diverse ligand-receptor interactions.","method":"Domain structural analysis, receptor-binding assays (LRP-1/LRP-2), domain-swapping experiments across HPX-domain-containing proteins","journal":"Journal of leukocyte biology","confidence":"Medium","confidence_rationale":"Tier 2 — integrative structural and binding data, but review/synthesis paper drawing on multiple prior studies","pmids":["17185359"],"is_preprint":false},{"year":2009,"finding":"Hemopexin (HPX/Hx) physically associates with apoA-1-containing HDL particles in both atherogenic mouse models and coronary heart disease patients. This association is dependent on the presence of haptoglobin (Hp), as HDL from Hp−/− mice under atherogenic conditions does not accumulate hemoglobin and remains anti-inflammatory. The Hb×Hp×Hx complex on HDL correlates with pro-inflammatory properties, and treatment with an apoA-1 mimetic peptide dissociates these complexes and restores HDL anti-inflammatory function.","method":"Co-immunoprecipitation/pulldown, HDL isolation and protein association analysis in mouse models (apoE−/−, Hp−/−) and human CHD patients, apoA-1 mimetic peptide treatment experiments","journal":"The Journal of biological chemistry","confidence":"Medium","confidence_rationale":"Tier 2 — reciprocal genetic (Hp−/− mouse) and pharmacological dissection with functional readout, single lab","pmids":["19433579"],"is_preprint":false},{"year":2025,"finding":"HPX suppresses hepatocellular carcinoma (HCC) cell proliferation, migration, invasion, and lenvatinib resistance. Mechanistically, phthalate exposure activates the MAPK signaling pathway, and HPX overexpression inhibits this MAPK activation. This HPX-MAPK axis was confirmed by Western blot in cell lines and validated in vivo in mouse models, identifying HPX as a negative regulator of MAPK signaling in HCC.","method":"HPX overexpression functional assays (proliferation, migration, invasion, drug resistance), Western blot for MAPK pathway components, in vivo mouse xenograft models, molecular docking","journal":"Ecotoxicology and environmental safety","confidence":"Medium","confidence_rationale":"Tier 2 — functional overexpression with defined pathway readout and in vivo validation, single lab","pmids":["40858030"],"is_preprint":false},{"year":2026,"finding":"In an experimental ICI-related myocarditis model, HPX acts upstream of the Nrf2/HO-1 antioxidant pathway. Quantitative proteomics and Western blot confirmed that crocins (saffron-derived compounds) ameliorate myocardial damage by targeting the Hpx/Nrf2/HO-1 axis. Molecular docking showed crocin II binds directly to Hpx, placing HPX as an upstream regulator of Nrf2 and HO-1 in cardiac oxidative stress responses.","method":"Quantitative proteomics, Western blot, molecular docking, subcutaneous xenotransplant tumor model in nude mice with ICI-related myocarditis phenotyping","journal":"International journal of molecular sciences","confidence":"Low","confidence_rationale":"Tier 3 — pathway placement supported by proteomics and docking but limited direct mechanistic validation of HPX-Nrf2 interaction; single lab","pmids":["41596558"],"is_preprint":false}],"current_model":"Human HPX (hemopexin) is a two-domain, hinge-linked plasma glycoprotein that binds free heme with the highest known affinity among plasma proteins; it delivers the heme-HPX complex to hepatocytes via LRP-1/LRP-2-mediated endocytosis, thereby preventing heme-driven Fenton-type oxidative damage and iron loss, and also associates with HDL particles where it modulates inflammatory properties; additionally, HPX negatively regulates MAPK signaling in hepatocellular carcinoma and acts upstream of the Nrf2/HO-1 antioxidant pathway in cardiac tissue."},"narrative":{"teleology":[{"year":1985,"claim":"Determination of the complete primary structure of HPX established its two-domain architecture arising from gene duplication and demonstrated that heme binding induces a conformational change at the inter-domain hinge, providing the first structural framework for understanding heme–HPX interaction.","evidence":"Complete protein sequencing (Edman degradation, tryptic peptide mapping) and limited proteolysis of apo- vs. heme-saturated HPX","pmids":["3855550"],"confidence":"High","gaps":["Atomic-resolution crystal structure of the heme–HPX complex was not yet available","Identity of the hepatocyte receptor for heme–HPX complex was unknown","Stoichiometry and thermodynamics of heme binding were not precisely defined in this study"]},{"year":2002,"claim":"Consolidation of binding, uptake, and knockout studies established that HPX possesses the highest heme-binding affinity among plasma proteins and functions primarily as an antioxidant scavenger—not merely an iron transport protein—by delivering heme to hepatocytes via receptor-mediated endocytosis.","evidence":"Integration of binding affinity measurements, cell-based heme uptake assays, and HPX-knockout mouse phenotyping","pmids":["12042069"],"confidence":"High","gaps":["Molecular identity of the hepatocyte receptor had been proposed but receptor-specific knockdown was not yet shown","Intracellular signaling events triggered upon heme–HPX endocytosis were incompletely mapped"]},{"year":2006,"claim":"Identification of LRP-1 (CD91) and LRP-2 (megalin) as the endocytic receptors for heme–HPX resolved the receptor question and revealed that the four-bladed β-propeller HPX domain is a conserved scaffold used across diverse proteins for ligand-receptor interactions.","evidence":"Domain structural analysis and receptor-binding assays with LRP-1/LRP-2","pmids":["17185359"],"confidence":"Medium","gaps":["Relative contributions of LRP-1 vs. LRP-2 to hepatocyte heme uptake in vivo were not dissected","Whether HPX is recycled or degraded after receptor-mediated endocytosis remained debated"]},{"year":2009,"claim":"Discovery of HPX's physical association with HDL particles in a haptoglobin-dependent manner expanded the functional repertoire of HPX beyond heme scavenging, linking it to HDL inflammatory remodeling in atherosclerosis.","evidence":"Co-immunoprecipitation and HDL protein analysis in apoE−/− and Hp−/− mouse models and human coronary heart disease patients; apoA-1 mimetic peptide dissociation experiments","pmids":["19433579"],"confidence":"Medium","gaps":["Direct binding interface between HPX and apoA-1 or other HDL components was not mapped","Whether HPX modifies HDL function independently of hemoglobin/haptoglobin cargo was unclear","Causal contribution of HPX–HDL association to atherogenesis was not tested in HPX-knockout models"]},{"year":2025,"claim":"Functional studies in hepatocellular carcinoma revealed that HPX negatively regulates MAPK signaling, suppressing tumor cell proliferation, migration, and drug resistance, extending HPX's biology into cancer-relevant pathways.","evidence":"HPX overexpression in HCC cell lines with Western blot for MAPK pathway components and in vivo mouse xenograft validation","pmids":["40858030"],"confidence":"Medium","gaps":["Direct molecular target through which HPX inhibits MAPK activation was not identified","Whether this anti-tumor effect is heme-binding-dependent or independent was not tested","Findings have not been replicated in independent cohorts or labs"]},{"year":null,"claim":"Key unresolved questions include the precise mechanism by which HPX engages the Nrf2/HO-1 antioxidant axis, whether HPX's roles in MAPK suppression and HDL remodeling are mechanistically linked to its heme-binding function, and the structural basis for receptor selectivity between LRP-1 and LRP-2.","evidence":"","pmids":[],"confidence":"Low","gaps":["HPX–Nrf2 pathway placement relies on proteomics and docking without direct interaction validation","No structure of full-length HPX in complex with LRP-1 or LRP-2 exists","Heme-binding-dependent vs. -independent functions of HPX have not been systematically dissected"]}],"mechanism_profile":{"molecular_activity":[{"term_id":"GO:0008289","term_label":"lipid binding","supporting_discovery_ids":[0,1,2]},{"term_id":"GO:0016209","term_label":"antioxidant activity","supporting_discovery_ids":[1,2]},{"term_id":"GO:0140104","term_label":"molecular carrier activity","supporting_discovery_ids":[1,2]},{"term_id":"GO:0098772","term_label":"molecular function regulator activity","supporting_discovery_ids":[4]}],"localization":[{"term_id":"GO:0005576","term_label":"extracellular region","supporting_discovery_ids":[0,1,3]}],"pathway":[{"term_id":"R-HSA-162582","term_label":"Signal Transduction","supporting_discovery_ids":[4]},{"term_id":"R-HSA-382551","term_label":"Transport of small molecules","supporting_discovery_ids":[1,2]}],"complexes":[],"partners":["LRP1","LRP2","APOA1","HP"],"other_free_text":[]},"mechanistic_narrative":"Hemopexin (HPX) is a plasma glycoprotein that functions as the primary extracellular scavenger of free heme, preventing heme-driven Fenton-chemistry oxidative damage and enabling hepatocyte iron salvage. HPX is a single-chain, two-domain β-propeller protein (439 residues, six disulfide bridges) whose heme binding induces a conformational change at the inter-domain hinge, and the heme–HPX complex is internalized by hepatocytes via LRP-1/LRP-2 receptor-mediated endocytosis [PMID:3855550, PMID:12042069, PMID:17185359]. HPX also physically associates with apoA-1-containing HDL particles in a haptoglobin-dependent manner, and this association modulates HDL inflammatory properties in atherosclerotic settings [PMID:19433579]. Overexpression of HPX suppresses MAPK signaling and hepatocellular carcinoma cell proliferation in vitro and in vivo, indicating an additional role as a negative regulator of mitogenic signaling [PMID:40858030]."},"prefetch_data":{"uniprot":{"accession":"P02790","full_name":"Hemopexin","aliases":["Beta-1B-glycoprotein"],"length_aa":462,"mass_kda":51.7,"function":"Binds heme and transports it to the liver for breakdown and iron recovery, after which the free hemopexin returns to the circulation","subcellular_location":"Secreted","url":"https://www.uniprot.org/uniprotkb/P02790/entry"},"depmap":{"release":"DepMap","has_data":true,"is_common_essential":false,"resolved_as":"","url":"https://depmap.org/portal/gene/HPX","classification":"Not 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many","driving_tissues":[{"tissue":"liver","ntpm":4942.7}],"url":"https://www.proteinatlas.org/search/HPX"},"hgnc":{"alias_symbol":[],"prev_symbol":[]},"alphafold":{"accession":"P02790","domains":[{"cath_id":"2.110.10.10","chopping":"51-227","consensus_level":"high","plddt":95.2041,"start":51,"end":227},{"cath_id":"2.110.10.10","chopping":"253-459","consensus_level":"high","plddt":94.4078,"start":253,"end":459}],"viewer_url":"https://alphafold.ebi.ac.uk/entry/P02790","model_url":"https://alphafold.ebi.ac.uk/files/AF-P02790-F1-model_v6.cif","pae_url":"https://alphafold.ebi.ac.uk/files/AF-P02790-F1-predicted_aligned_error_v6.png","plddt_mean":87.56},"mouse_models":{"mgi_url":"https://www.informatics.jax.org/marker/summary?nomen=HPX","jax_strain_url":"https://www.jax.org/strain/search?query=HPX"},"sequence":{"accession":"P02790","fasta_url":"https://rest.uniprot.org/uniprotkb/P02790.fasta","uniprot_url":"https://www.uniprot.org/uniprotkb/P02790/entry","alphafold_viewer_url":"https://alphafold.ebi.ac.uk/entry/P02790"}},"corpus_meta":[{"pmid":"15967999","id":"PMC_15967999","title":"Substantial 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homeostasis, and shows functional divergence from its teleost paralogs WAP65-1 and WAP65-2 after gene duplication, with HPX showing a faster rate of evolution compared to both WAP65 paralogs.\",\n      \"method\": \"Evolutionary/phylogenetic analysis combined with 3D structural modeling of WAP65 and HPX proteins\",\n      \"journal\": \"The Journal of heredity\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 4 — computational/evolutionary analysis only, no direct biochemical experiment on HPX mechanism\",\n      \"pmids\": [\"24344252\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"HPX suppresses HCC cell proliferation, migration, invasion, and lenvatinib resistance; mechanistically, HPX inhibits activation of the MAPK signaling pathway that is triggered by phthalate exposure, as confirmed by Western blot and in vivo mouse models.\",\n      \"method\": \"Functional assays (proliferation, migration, invasion), overexpression experiments, Western blot for MAPK pathway components, in vivo mouse xenograft models\",\n      \"journal\": \"Ecotoxicology and environmental safety\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2–3 — multiple functional assays with pathway validation by Western blot and in vivo, single lab\",\n      \"pmids\": [\"40858030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"HPX acts upstream of the Nrf2/HO-1 pathway in the context of ICI-related myocarditis; crocins target this Hpx/Nrf2/HO-1 axis to ameliorate myocardial damage, oxidative stress, and inflammation, with molecular docking showing direct binding of crocin II to Hpx.\",\n      \"method\": \"Quantitative proteomics, Western blot, molecular docking, animal model of ICI-related myocarditis\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — single lab, pathway placement inferred from proteomics and docking without direct reconstitution or mutagenesis of HPX\",\n      \"pmids\": [\"41596558\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human hemopexin (HPX) is a plasma protein that binds free heme to maintain iron homeostasis; emerging evidence places HPX as an inhibitor of MAPK signaling in hepatocellular carcinoma and as an upstream regulator of the Nrf2/HO-1 antioxidant pathway, but detailed mechanistic characterization of its catalytic or binding mechanism in mammalian systems remains limited in the available literature.\"\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\nI'll classify each paper first, then extract discoveries only from KEEP papers.\n\n**Classification:**\n\n**KEEP (canonical human HPX - hemopexin):**\n- [10] HPX-MAPK pathway in HCC — hemopexin protein, functional assays ✓\n- [11] Hpx/Nrf2/HO-1 pathway in myocarditis — hemopexin ✓\n- [23] Hemopexin domains as multifunctional liganding modules — hemopexin domains ✓\n- [24] Complete amino acid sequence of human hemopexin ✓\n- [25] Hemopexin associates with apoA-1/HDL particles ✓\n- [10 gene2pubmed] Hemopexin structure, function, regulation review ✓\n\n**EXCLUDE (alias collision / off-target):**\n- [1] Hpx- mutants of *E. coli* — bacterial knockout for catalase/peroxidase genes, not human HPX ✗\n- [2] Hypotransferrinemic (hpx) mouse — \"hpx\" here = hypotransferrinemia mutation in transferrin gene, not HPX gene ✗\n- [3] ApoA1, HPX, POTEE in breast cancer — expression/proteomics only, no mechanism ✗\n- [4] hpx genetic system in *Klebsiella pneumoniae* — bacterial hypoxanthine catabolism, symbol collision ✗\n- [5] Pig plasma HPX phenotyping — protein polymorphism/electrophoresis, no mechanism ✗\n- [6] Rat Hpx gene expression in vaccine toxicity — expression correlation only ✗\n- [7] WAP65/HPX evolutionary divergence — evolutionary analysis, no direct mechanism experiment ✗\n- [8] HPX-2 in *C. elegans* — nematode heme peroxidase, symbol/function collision (different protein class) ✗\n- [9] Urinary HPX in SLE — biomarker/correlation study ✗\n- [12] Preprint DEN-induced HCC proteomics — HPX mentioned as differentially expressed, no mechanism ✗\n- gene2pubmed [1] MGC cDNA sequences — no mechanism ✗\n- gene2pubmed [2,3,4,5] BioPlex interactome networks — mass AP-MS, HPX not specifically studied ✗\n- gene2pubmed [6] Human plasma proteome list — no mechanism ✗\n- gene2pubmed [7] GO phylogenetic propagation — no mechanism ✗\n- gene2pubmed [8] Urinary exosome proteomics — detection only ✗\n- gene2pubmed [9] Plasma N-glycoproteome — detection only ✗\n- gene2pubmed [11] HCC transcriptome — expression only ✗\n- gene2pubmed [12] Serum interactome — detection only ✗\n- gene2pubmed [13] BioID/AP-MS chromatin — not about HPX ✗\n- gene2pubmed [14] CFTR interactome — not about HPX ✗\n- gene2pubmed [15] Human liver protein interaction network — no HPX-specific mechanism ✗\n- gene2pubmed [16] ECM proteomics — detection only ✗\n- gene2pubmed [17] Salivary glycoproteins — detection only ✗\n- gene2pubmed [18] CVD lipid GWAS — no mechanism ✗\n- gene2pubmed [19] N-glycoprotein screening — detection only ✗\n- gene2pubmed [20] Prostatic secretion exosomes — detection only ✗\n- gene2pubmed [21] Podocyte exosome proteomics — detection only ✗\n- gene2pubmed [22] Bile proteomics — detection only ✗\n- gene2pubmed [26] CSF proteomics — detection only ✗\n- gene2pubmed [27] CSF markers in AD — biomarker/correlation only ✗\n- gene2pubmed [28] CD44 binding through HPX-like domain of MT1-MMP — this is the HPX domain of MMP14, not the HPX gene itself ✗\n- gene2pubmed [29] Plasma microvesicle proteomics — detection only ✗\n- gene2pubmed [30] Chromatin demethylase complex — not about HPX ✗\n\n```json\n{\n  \"discoveries\": [\n    {\n      \"year\": 1985,\n      \"finding\": \"Human hemopexin (HPX) is a single polypeptide chain of 439 amino acid residues (Mr ~63,000) with six intrachain disulfide bridges, an O-linked galactosamine oligosaccharide blocking the N-terminal threonine, and five N-linked glucosamine oligosaccharides. Internal sequence homology analysis revealed that HPX consists of two similar halves arising from ancestral gene duplication. Limited tryptic digestion mapped the hinge region (histidine-rich, linker between the two half-molecules) and showed that heme binding occludes the trypsin cleavage site at lysine-101, indicating a conformational change upon heme binding.\",\n      \"method\": \"Complete protein sequencing (tryptic peptide mapping, Edman degradation), computer-assisted internal homology analysis, limited proteolysis of apo- vs. heme-saturated HPX\",\n      \"journal\": \"Proceedings of the National Academy of Sciences of the United States of America\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 1 — complete primary structure determination with multiple orthogonal biochemical methods and structural model validation\",\n      \"pmids\": [\"3855550\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2002,\n      \"finding\": \"HPX is the plasma protein with the highest known heme-binding affinity. It functions primarily as an antioxidant scavenger: free heme intercalates into lipid membranes and generates hydroxyl radicals via Fenton chemistry, and HPX prevents this by tightly binding heme and delivering the heme-HPX complex to hepatocytes via a specific heme-HPX receptor. Receptor-mediated endocytosis of the complex triggers intracellular antioxidant responses and enables iron salvage, establishing that HPX's dominant physiological role is antioxidant defense rather than primary iron metabolism.\",\n      \"method\": \"Review integrating binding affinity measurements, receptor identification experiments, cell-based heme uptake assays, and HPX-knockout mouse studies\",\n      \"journal\": \"DNA and cell biology\",\n      \"confidence\": \"High\",\n      \"confidence_rationale\": \"Tier 2 — multiple orthogonal experimental approaches consolidated across labs, strong preponderance of evidence\",\n      \"pmids\": [\"12042069\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2006,\n      \"finding\": \"The two four-bladed β-propeller hemopexin domains of HPX serve as multifunctional liganding modules. These domains mediate heme binding, endocytosis via LRP-1 (CD91) and LRP-2 (megalin/GP330) receptors, and the same structural fold is used in MMPs and vitronectin for substrate binding, MMP activation/inhibition, and dimerization—establishing that the HPX domain architecture is a conserved scaffold for diverse ligand-receptor interactions.\",\n      \"method\": \"Domain structural analysis, receptor-binding assays (LRP-1/LRP-2), domain-swapping experiments across HPX-domain-containing proteins\",\n      \"journal\": \"Journal of leukocyte biology\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — integrative structural and binding data, but review/synthesis paper drawing on multiple prior studies\",\n      \"pmids\": [\"17185359\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2009,\n      \"finding\": \"Hemopexin (HPX/Hx) physically associates with apoA-1-containing HDL particles in both atherogenic mouse models and coronary heart disease patients. This association is dependent on the presence of haptoglobin (Hp), as HDL from Hp−/− mice under atherogenic conditions does not accumulate hemoglobin and remains anti-inflammatory. The Hb×Hp×Hx complex on HDL correlates with pro-inflammatory properties, and treatment with an apoA-1 mimetic peptide dissociates these complexes and restores HDL anti-inflammatory function.\",\n      \"method\": \"Co-immunoprecipitation/pulldown, HDL isolation and protein association analysis in mouse models (apoE−/−, Hp−/−) and human CHD patients, apoA-1 mimetic peptide treatment experiments\",\n      \"journal\": \"The Journal of biological chemistry\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — reciprocal genetic (Hp−/− mouse) and pharmacological dissection with functional readout, single lab\",\n      \"pmids\": [\"19433579\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2025,\n      \"finding\": \"HPX suppresses hepatocellular carcinoma (HCC) cell proliferation, migration, invasion, and lenvatinib resistance. Mechanistically, phthalate exposure activates the MAPK signaling pathway, and HPX overexpression inhibits this MAPK activation. This HPX-MAPK axis was confirmed by Western blot in cell lines and validated in vivo in mouse models, identifying HPX as a negative regulator of MAPK signaling in HCC.\",\n      \"method\": \"HPX overexpression functional assays (proliferation, migration, invasion, drug resistance), Western blot for MAPK pathway components, in vivo mouse xenograft models, molecular docking\",\n      \"journal\": \"Ecotoxicology and environmental safety\",\n      \"confidence\": \"Medium\",\n      \"confidence_rationale\": \"Tier 2 — functional overexpression with defined pathway readout and in vivo validation, single lab\",\n      \"pmids\": [\"40858030\"],\n      \"is_preprint\": false\n    },\n    {\n      \"year\": 2026,\n      \"finding\": \"In an experimental ICI-related myocarditis model, HPX acts upstream of the Nrf2/HO-1 antioxidant pathway. Quantitative proteomics and Western blot confirmed that crocins (saffron-derived compounds) ameliorate myocardial damage by targeting the Hpx/Nrf2/HO-1 axis. Molecular docking showed crocin II binds directly to Hpx, placing HPX as an upstream regulator of Nrf2 and HO-1 in cardiac oxidative stress responses.\",\n      \"method\": \"Quantitative proteomics, Western blot, molecular docking, subcutaneous xenotransplant tumor model in nude mice with ICI-related myocarditis phenotyping\",\n      \"journal\": \"International journal of molecular sciences\",\n      \"confidence\": \"Low\",\n      \"confidence_rationale\": \"Tier 3 — pathway placement supported by proteomics and docking but limited direct mechanistic validation of HPX-Nrf2 interaction; single lab\",\n      \"pmids\": [\"41596558\"],\n      \"is_preprint\": false\n    }\n  ],\n  \"current_model\": \"Human HPX (hemopexin) is a two-domain, hinge-linked plasma glycoprotein that binds free heme with the highest known affinity among plasma proteins; it delivers the heme-HPX complex to hepatocytes via LRP-1/LRP-2-mediated endocytosis, thereby preventing heme-driven Fenton-type oxidative damage and iron loss, and also associates with HDL particles where it modulates inflammatory properties; additionally, HPX negatively regulates MAPK signaling in hepatocellular carcinoma and acts upstream of the Nrf2/HO-1 antioxidant pathway in cardiac tissue.\"\n}\n```","stage2_raw":"```json\n{\n  \"mechanistic_narrative\": \"Hemopexin (HPX) is a plasma glycoprotein that binds free heme in the bloodstream and participates in iron homeostasis [PMID:24344252]. HPX suppresses hepatocellular carcinoma cell proliferation, migration, invasion, and drug resistance by inhibiting MAPK signaling pathway activation [PMID:40858030]. HPX also functions upstream of the Nrf2/HO-1 antioxidant pathway, where it modulates oxidative stress and inflammation in the context of immune checkpoint inhibitor–related myocarditis [PMID:41596558].\",\n  \"teleology\": [\n    {\n      \"year\": 2013,\n      \"claim\": \"Phylogenetic analysis established that HPX is a heme-binding plasma protein involved in iron homeostasis, placing it functionally within the broader hemopexin/WAP65 family after gene duplication.\",\n      \"evidence\": \"Evolutionary/phylogenetic analysis with 3D structural modeling of HPX and teleost WAP65 paralogs\",\n      \"pmids\": [\"24344252\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Computational analysis only; no direct biochemical measurement of heme-binding kinetics or stoichiometry\",\n        \"Functional divergence inferred from evolutionary rates, not from mutagenesis or domain-swap experiments\",\n        \"No mammalian in vivo data on HPX iron homeostasis function provided\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Functional studies revealed that HPX acts as a tumor suppressor in hepatocellular carcinoma by inhibiting MAPK pathway activation, directly linking its activity to cell proliferation, migration, invasion, and drug resistance.\",\n      \"evidence\": \"Overexpression and functional assays (proliferation, migration, invasion), Western blot for MAPK components, in vivo mouse xenograft models\",\n      \"pmids\": [\"40858030\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Mechanism by which HPX inhibits MAPK signaling is not defined (direct binding vs. indirect heme-scavenging effect)\",\n        \"Single-laboratory finding; independent replication pending\",\n        \"Whether MAPK inhibition depends on HPX heme-binding activity is untested\"\n      ]\n    },\n    {\n      \"year\": 2026,\n      \"claim\": \"Proteomic and molecular docking studies positioned HPX upstream of the Nrf2/HO-1 antioxidant axis in immune checkpoint inhibitor–related myocarditis, extending its role beyond heme scavenging to oxidative stress regulation in cardiac tissue.\",\n      \"evidence\": \"Quantitative proteomics, Western blot, molecular docking of crocin II to Hpx, animal model of ICI-related myocarditis\",\n      \"pmids\": [\"41596558\"],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"Pathway placement inferred from proteomics and docking without direct reconstitution or mutagenesis of HPX\",\n        \"Whether HPX directly activates Nrf2 or acts indirectly through heme delivery/sequestration is unknown\",\n        \"Crocin–HPX binding demonstrated only by docking, not by biophysical assay\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"The direct molecular mechanism by which HPX modulates intracellular signaling pathways (MAPK, Nrf2/HO-1) — whether through heme scavenging, receptor-mediated endocytosis, or direct protein–protein interactions — remains undefined.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"No structure–function studies with HPX domain mutants exist in the timeline\",\n        \"The relationship between HPX heme-binding activity and its signaling functions has not been dissected\",\n        \"No receptor-level mechanism for HPX signal transduction has been characterized\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 1]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"GO:0162582\", \"supporting_discovery_ids\": [1]}\n    ],\n    \"complexes\": [],\n    \"partners\": [],\n    \"other_free_text\": []\n  }\n}\n```\n\n--- ROUND 2 (R2/R3, augmented corpus) ---\n\n```json\n{\n  \"mechanistic_narrative\": \"Hemopexin (HPX) is a plasma glycoprotein that functions as the primary extracellular scavenger of free heme, preventing heme-driven Fenton-chemistry oxidative damage and enabling hepatocyte iron salvage. HPX is a single-chain, two-domain β-propeller protein (439 residues, six disulfide bridges) whose heme binding induces a conformational change at the inter-domain hinge, and the heme–HPX complex is internalized by hepatocytes via LRP-1/LRP-2 receptor-mediated endocytosis [PMID:3855550, PMID:12042069, PMID:17185359]. HPX also physically associates with apoA-1-containing HDL particles in a haptoglobin-dependent manner, and this association modulates HDL inflammatory properties in atherosclerotic settings [PMID:19433579]. Overexpression of HPX suppresses MAPK signaling and hepatocellular carcinoma cell proliferation in vitro and in vivo, indicating an additional role as a negative regulator of mitogenic signaling [PMID:40858030].\",\n  \"teleology\": [\n    {\n      \"year\": 1985,\n      \"claim\": \"Determination of the complete primary structure of HPX established its two-domain architecture arising from gene duplication and demonstrated that heme binding induces a conformational change at the inter-domain hinge, providing the first structural framework for understanding heme–HPX interaction.\",\n      \"evidence\": \"Complete protein sequencing (Edman degradation, tryptic peptide mapping) and limited proteolysis of apo- vs. heme-saturated HPX\",\n      \"pmids\": [\"3855550\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Atomic-resolution crystal structure of the heme–HPX complex was not yet available\",\n        \"Identity of the hepatocyte receptor for heme–HPX complex was unknown\",\n        \"Stoichiometry and thermodynamics of heme binding were not precisely defined in this study\"\n      ]\n    },\n    {\n      \"year\": 2002,\n      \"claim\": \"Consolidation of binding, uptake, and knockout studies established that HPX possesses the highest heme-binding affinity among plasma proteins and functions primarily as an antioxidant scavenger—not merely an iron transport protein—by delivering heme to hepatocytes via receptor-mediated endocytosis.\",\n      \"evidence\": \"Integration of binding affinity measurements, cell-based heme uptake assays, and HPX-knockout mouse phenotyping\",\n      \"pmids\": [\"12042069\"],\n      \"confidence\": \"High\",\n      \"gaps\": [\n        \"Molecular identity of the hepatocyte receptor had been proposed but receptor-specific knockdown was not yet shown\",\n        \"Intracellular signaling events triggered upon heme–HPX endocytosis were incompletely mapped\"\n      ]\n    },\n    {\n      \"year\": 2006,\n      \"claim\": \"Identification of LRP-1 (CD91) and LRP-2 (megalin) as the endocytic receptors for heme–HPX resolved the receptor question and revealed that the four-bladed β-propeller HPX domain is a conserved scaffold used across diverse proteins for ligand-receptor interactions.\",\n      \"evidence\": \"Domain structural analysis and receptor-binding assays with LRP-1/LRP-2\",\n      \"pmids\": [\"17185359\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Relative contributions of LRP-1 vs. LRP-2 to hepatocyte heme uptake in vivo were not dissected\",\n        \"Whether HPX is recycled or degraded after receptor-mediated endocytosis remained debated\"\n      ]\n    },\n    {\n      \"year\": 2009,\n      \"claim\": \"Discovery of HPX's physical association with HDL particles in a haptoglobin-dependent manner expanded the functional repertoire of HPX beyond heme scavenging, linking it to HDL inflammatory remodeling in atherosclerosis.\",\n      \"evidence\": \"Co-immunoprecipitation and HDL protein analysis in apoE−/− and Hp−/− mouse models and human coronary heart disease patients; apoA-1 mimetic peptide dissociation experiments\",\n      \"pmids\": [\"19433579\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct binding interface between HPX and apoA-1 or other HDL components was not mapped\",\n        \"Whether HPX modifies HDL function independently of hemoglobin/haptoglobin cargo was unclear\",\n        \"Causal contribution of HPX–HDL association to atherogenesis was not tested in HPX-knockout models\"\n      ]\n    },\n    {\n      \"year\": 2025,\n      \"claim\": \"Functional studies in hepatocellular carcinoma revealed that HPX negatively regulates MAPK signaling, suppressing tumor cell proliferation, migration, and drug resistance, extending HPX's biology into cancer-relevant pathways.\",\n      \"evidence\": \"HPX overexpression in HCC cell lines with Western blot for MAPK pathway components and in vivo mouse xenograft validation\",\n      \"pmids\": [\"40858030\"],\n      \"confidence\": \"Medium\",\n      \"gaps\": [\n        \"Direct molecular target through which HPX inhibits MAPK activation was not identified\",\n        \"Whether this anti-tumor effect is heme-binding-dependent or independent was not tested\",\n        \"Findings have not been replicated in independent cohorts or labs\"\n      ]\n    },\n    {\n      \"year\": null,\n      \"claim\": \"Key unresolved questions include the precise mechanism by which HPX engages the Nrf2/HO-1 antioxidant axis, whether HPX's roles in MAPK suppression and HDL remodeling are mechanistically linked to its heme-binding function, and the structural basis for receptor selectivity between LRP-1 and LRP-2.\",\n      \"evidence\": \"\",\n      \"pmids\": [],\n      \"confidence\": \"Low\",\n      \"gaps\": [\n        \"HPX–Nrf2 pathway placement relies on proteomics and docking without direct interaction validation\",\n        \"No structure of full-length HPX in complex with LRP-1 or LRP-2 exists\",\n        \"Heme-binding-dependent vs. -independent functions of HPX have not been systematically dissected\"\n      ]\n    }\n  ],\n  \"mechanism_profile\": {\n    \"molecular_activity\": [\n      {\"term_id\": \"GO:0008289\", \"supporting_discovery_ids\": [0, 1, 2]},\n      {\"term_id\": \"GO:0016209\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0140104\", \"supporting_discovery_ids\": [1, 2]},\n      {\"term_id\": \"GO:0098772\", \"supporting_discovery_ids\": [4]}\n    ],\n    \"localization\": [\n      {\"term_id\": \"GO:0005576\", \"supporting_discovery_ids\": [0, 1, 3]}\n    ],\n    \"pathway\": [\n      {\"term_id\": \"R-HSA-162582\", \"supporting_discovery_ids\": [4]},\n      {\"term_id\": \"R-HSA-382551\", \"supporting_discovery_ids\": [1, 2]}\n    ],\n    \"complexes\": [],\n    \"partners\": [\n      \"LRP1\",\n      \"LRP2\",\n      \"APOA1\",\n      \"HP\"\n    ],\n    \"other_free_text\": []\n  }\n}\n```"}