ITIH5 is a secreted inter-alpha-inhibitor heavy-chain family glycoprotein expressed prominently in placenta and adipose tissue and also by fibroblasts. It contains VIT and von Willebrand factor A domains but no Kunitz protease- inhibitor domain. In human fibroblasts, heavy chain 5 associates with bikunin and cell-surface hyaluronan in a TSG-6-dependent pericellular matrix that supports TGF-beta1-induced myofibroblast differentiation. ITIH5 also acts as an adipose-derived extracellular signal: mammalian genetic and recombinant- protein studies indicate that it restrains adipocyte differentiation, while direct treatment of human endothelium reduces leukocyte adhesion. ITIH5 expression is frequently lost through promoter methylation in several cancers, and experimental re-expression can suppress tumor-cell proliferation.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004867 serine-type endopeptidase inhibitor activity | IEA GO_REF:0000002 | REMOVE | Summary: InterPro electronically propagates serine-type endopeptidase inhibitor activity from the inter-alpha-inhibitor family. Reason: Direct chain biochemistry assigns inhibitory activity to the 30-kDa bikunin light chain and explicitly identifies the associated heavy chains as noninhibitory. ITIH5 is a heavy-chain protein without a Kunitz inhibitor domain, so this is component-level family-complex overpropagation. Supporting Evidence: PMID:2476436 Inter-alpha-trypsin inhibitor contains noninhibitory heavy chains of 65,000 and 70,000 Da file:human/ITIH5/ITIH5-deep-research-manual.md ITIH5 encodes a heavy-chain family protein and has no Kunitz inhibitor domain. |
| GO:0005576 extracellular region | IEA GO_REF:0000044 | ACCEPT | Summary: UniProt subcellular-location mapping places ITIH5 in the extracellular region. Reason: The reviewed canonical precursor has a cleaved signal peptide, and direct human-fibroblast experiments place endogenous HC5 in a cell-surface hyaluronan matrix. Two independent human ECM proteomic studies provide additional localization evidence. Supporting Evidence: PMID:27143355 HC5 was released by hyaluronidase treatment, confirming its association with cell surface HA. file:human/ITIH5/ITIH5-uniprot.txt FT SIGNAL 1..16 |
| GO:0030212 hyaluronan metabolic process | IEA GO_REF:0000002 | ACCEPT | Summary: InterPro transfers hyaluronan metabolic process from the inter-alpha- inhibitor heavy-chain family. Reason: Independent direct human-fibroblast evidence validates the family-based inference. TSG-6 and bikunin are required for cell-surface HC5, HC5 is released by hyaluronidase, and blocking heavy-chain transfer or depleting HC5 disrupts the HA-dependent matrix phenotype. Supporting Evidence: PMID:27143355 The current study describes a novel mechanism linking the TSG-6 transfer of the newly described HC5 to the HA-dependent control of cell phenotype. |
| GO:0031012 extracellular matrix | HDA PMID:28675934 Characterization of the Extracellular Matrix of Normal and D... | ACCEPT | Summary: ITIH5 was recovered in a mass-spectrometric analysis of ECM-enriched normal and diseased human tissues. Reason: The HDA is consistent with canonical secretion and with direct recovery of endogenous HC5 from a hyaluronan-rich cell-surface matrix. It supports extracellular-matrix localization without implying a disease-specific role. Supporting Evidence: PMID:28675934 analyze ECM peptides by mass spectrometry |
| GO:0031012 extracellular matrix | HDA PMID:25037231 Extracellular matrix signatures of human primary metastatic ... | ACCEPT | Summary: ITIH5 was recovered in ECM-enriched proteomes from human colorectal tumors, liver metastases, and matched tissues. Reason: The HDA supports extracellular-matrix localization and is independently coherent with ITIH5 secretion and the HC5-hyaluronan matrix mechanism. It does not by itself establish a colorectal-cancer function. Supporting Evidence: PMID:25037231 We have used enrichment of extracellular matrix (ECM) from human patient samples and proteomics |
| GO:0005540 hyaluronic acid binding | IDA PMID:27143355 Tumor Necrosis Factor-stimulated Gene 6 (TSG-6)-mediated Int... | NEW | Summary: Endogenous HC5 associates with the hyaluronan-rich surface matrix of human lung fibroblasts and is released by hyaluronidase. Reason: Hyaluronidase-sensitive retention supplies direct biochemical localization evidence for association with hyaluronan, while TSG-6/bikunin perturbations establish the expected heavy-chain matrix pathway. Supporting Evidence: PMID:27143355 HC5 was released by hyaluronidase treatment, confirming its association with cell surface HA. |
| GO:0009986 cell surface | IDA PMID:27143355 Tumor Necrosis Factor-stimulated Gene 6 (TSG-6)-mediated Int... | NEW | Summary: Endogenous HC5 was detected at the human fibroblast cell surface. Reason: TSG-6 or bikunin knockdown prevents HC5 surface expression, and hyaluronidase releases the surface pool, demonstrating an extracellular pericellular-matrix localization. Supporting Evidence: PMID:27143355 its cell surface expression was prevented by siRNA inhibition of TSG-6 or bikunin. |
| GO:1904762 positive regulation of myofibroblast differentiation | IMP PMID:27143355 Tumor Necrosis Factor-stimulated Gene 6 (TSG-6)-mediated Int... | NEW | Summary: HC5 knockdown impairs TGF-beta1-induced differentiation of human lung fibroblasts to myofibroblasts. Reason: The targeted loss-of-function phenotype directly establishes that endogenous HC5 positively supports this differentiation program. Supporting Evidence: PMID:27143355 HC5 knockdown by siRNA confirmed its role in myofibroblast differentiation. |
| GO:0008285 negative regulation of cell population proliferation | IMP PMID:17653090 The extracellular matrix protein ITIH5 is a novel prognostic... | NEW | Summary: Forced full-length ITIH5 expression reduces proliferation of MDA-MB-231 breast-cancer cells. Reason: The experimental perturbation supports the term in a tumor-cell context. It is retained as non-core biology because ectopic expression in one cancer line does not establish a general physiological anti-proliferative role. Supporting Evidence: PMID:17653090 ITIH5-expressing clones showed a 40% reduced proliferation rate compared to mock-transfected cells. |
| GO:0045599 negative regulation of fat cell differentiation | ISS PMID:39198923 Genetic deletion of ITIH5 leads to increased development of ... | NEW | Summary: Mouse Itih5 deletion increases adipogenic differentiation of adipose stem cells, and recombinant ITIH5 reverses the phenotype. Reason: Knockout and protein-rescue experiments support a conserved mammalian anti-adipogenic role. ISS is used because the direct genetic cells and animals were mouse, even though the reviewed product is human ITIH5. Supporting Evidence: PMID:39198923 ITIH5-/- cells exhibited increased proliferation and adipogenic differentiation PMID:39198923 the application of the rITIH5 protein reversed the observed knockout effects in ASCs. |
| GO:1904995 negative regulation of leukocyte adhesion to vascular endothelial cell | IDA PMID:42025695 Systems genetics reveals ITIH5 as a key mediator of adipocyt... | NEW | Summary: Recombinant ITIH5 treatment reduces leukocyte adhesion to human aortic endothelial monolayers. Reason: The direct human endothelial-cell adhesion assay precisely supports this process term and avoids overextending the broader in vivo mouse immune phenotypes. Supporting Evidence: PMID:42025695 ITIH5 protein alone blunted this adhesion of leukocytes |
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Download this section (compressed HTML)Q: Which receptor or matrix partner transduces ITIH5 signals in adipose stem cells and vascular endothelium?
Q: How is the TSG-6/bikunin/HC5 hyaluronan pathway related to the adipose- endothelial signaling activity?
Q: Which ITIH5 isoforms are secreted in fibroblasts, adipocytes, placenta, and tumors?
Q: Is reduced tumor-cell proliferation a physiological ITIH5 function or a context-specific consequence of forced expression?
Experiment: Perform cell-surface ligand-receptor capture with recombinant ITIH5 in human adipose stem cells and aortic endothelial cells, then test candidate-receptor knockouts for loss of ITIH5-dependent differentiation and leukocyte-adhesion phenotypes with receptor rescue.
Hypothesis: ITIH5 binds a cell-surface receptor that is required for its anti-adipogenic and anti-adhesive endothelial effects.
Type: Ligand-receptor discovery and genetic epistasis
Experiment: Compare wild-type ITIH5 with cleavage/transfer-deficient and VWA-domain mutants in TSG-6/bikunin-dependent fibroblast matrices, adipose-stem-cell differentiation, and endothelial leukocyte-adhesion assays.
Hypothesis: Covalent heavy-chain matrix transfer is required for fibroblast differentiation but dispensable for adipose-endothelial signaling.
Type: Separation-of-function mutagenesis and cellular rescue
Experiment: Express each tagged isoform at matched levels, quantify secretion and subcellular localization, and perform isoform-specific rescue after endogenous ITIH5 depletion in fibroblasts and adipose stem cells.
Hypothesis: N-terminally truncated ITIH5 isoforms remain intracellular and have activities distinct from secreted isoform 1.
Type: Isoform-resolved localization and functional complementation
What is not known β curated, literature-grounded statements of the open unknowns (the inverse of core functions).
Gap: The molecular receptor or binding partner that transduces ITIH5 signaling in adipose stromal and endothelial cells is unknown.
OPEN BIOLOGY MF_DARK
What is known: Recombinant protein and genetic perturbations establish cellular effects, but no receptor, direct signaling complex, or separation-of-function allele has been identified.
Significance: Identifying the receptor is necessary to distinguish an autonomous signaling function from an indirect consequence of extracellular-matrix remodeling.
What would resolve it: Use affinity purification and cell-surface ligand-receptor screening with recombinant ITIH5, followed by receptor knockout and rescue in adipose stem cells and human endothelium.
Provenance (the field's own admissions):
Gap: Isoform-specific secretion and function are unresolved, particularly for isoforms 3 and 4 that lack the canonical N-terminal signal peptide.
OPEN BIOLOGY CC_DARK
What is known: Sequence records define four isoforms, but the primary functional studies do not identify which endogenous protein isoforms were detected.
Significance: Isoform resolution is required to know whether all ITIH5 products enter the extracellular matrix or whether truncated forms act intracellularly.
What would resolve it: Develop isoform-specific transcript and peptide assays, then couple secretion measurements with isoform-resolved rescue experiments.
Provenance (the field's own admissions):
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