The automated deep-research attempt was made with Falcon and the configured Perplexity-lite fallback. Falcon returned HTTP 402 and the fallback returned HTTP 401, so this review uses the reviewed UniProt record, cached primary publications, and a manual inspection of the open-access article for PMID:27143355. Detailed evidence is recorded in ITIH5-deep-research-manual.md.
ITIH5 (UniProt Q86UX2) is a 942-residue inter-alpha-trypsin inhibitor heavy-chain family protein with an N-terminal signal peptide, VIT domain, and von Willebrand factor type A domain. The founding study noted that ITIH5 diverged early from the other heavy-chain subfamilies PMID:14744536. Canonical isoform 1 is predicted to enter the secretory pathway. Isoforms 3 and 4 lack the canonical N terminus and signal peptide; none of the functional studies reviewed here resolves the tested isoform.
In human lung fibroblasts, HC5 forms chondroitinase-sensitive high-molecular-weight complexes with bikunin. TSG-6 and bikunin are required for cell-surface HC5, and hyaluronidase releases HC5, directly tying it to cell-surface hyaluronan PMID:27143355. More than 95% HC5 knockdown reduced TGF-beta1-induced alpha-smooth-muscle actin by more than half, supporting a positive role in fibroblast-to-myofibroblast differentiation. This establishes a human HC5-bikunin-TSG-6-hyaluronan matrix pathway.
The family name does not make the heavy chain a serine protease inhibitor. Biochemical work on inter-alpha-inhibitor explicitly describes the heavy subunits as noninhibitory PMID:2476436. The Kunitz-containing bikunin light chain carries protease-inhibitory activity. ITIH5 lacks a Kunitz domain, so the InterPro-derived inhibitor annotation is removed as component-level overpropagation.
Mouse Itih5 deletion increased adipose mass and caused adipose stem cells to proliferate and differentiate more strongly toward adipocytes; recombinant ITIH5 reversed the cellular phenotype PMID:39198923. This supports a conserved negative effect on fat-cell differentiation, transferred to human ITIH5 with ISS rather than direct human experimental evidence.
A 2026 study found a direct effect of recombinant ITIH5 on human aortic endothelial monolayers: ITIH5 reduced leukocyte recruitment PMID:42025695. The same study connects adipocyte-derived ITIH5 to endothelial and immune behavior, suggesting an extracellular signaling role that is at least partly separable from stable heavy-chain linkage to bikunin.
ITIH5 is frequently reduced by promoter methylation in breast cancer. Forced expression in MDA-MB-231 cells reduced proliferation PMID:17653090. This supports a context-dependent negative regulation of cell population proliferation annotation, but tumor suppression is kept outside the conserved core-function summary.
| GO annotation | Decision | Rationale |
|---|---|---|
| serine-type endopeptidase inhibitor activity | REMOVE | Inhibitory activity belongs to bikunin; ITIH heavy chains are noninhibitory. |
| extracellular region | ACCEPT | Signal peptide plus direct extracellular/cell-surface matrix evidence. |
| hyaluronan metabolic process | ACCEPT | HC5 participates in TSG-6-dependent heavy-chain organization on hyaluronan. |
| extracellular matrix, PMID:25037231 | ACCEPT | Direct human matrisome proteomics. |
| extracellular matrix, PMID:28675934 | ACCEPT | Direct human tissue ECM proteomics. |