Background/Purpose: Huntingtin-interacting protein 1 (Hip1) is a new arthritis severity gene recently identified in the Pristane and Collagen-induced arthritis (PIA, CIA) quantitative trait locus Cia25/Pia42 on rat chromosome 12. This study aimed to characterize the mechanism of action of Hip1 in arthritis.

Methods: Synovial tissues and fibroblast-like synoviocytes (FLS) were obtained from DA and DA.ACI(Cia25/Pia42) subcongenic R6 (R6) rats (day 21 after the onset of PIA). FLS from DA and the DA.ACI(Cia25/Pia42) subcongenic R6 (R6) were treated with PDGF, EGF, 10% FBS or 0.1% FBS and studied for a) in vitro invasion in a two-chamber assay through Matrigel, b) proliferation, c) gene expression (qPCR) analysis. DA and rheumatoid arthritis (RA) FLS were also studied following siRNA knockdown of Hip1. Tissues from Hip1 knockout mice were used for qPCR.

Results:  Synovial tissues from DA rats expressed higher levels of Hip1 compared with DA.ACI(Cia25/Pia42) (1.78-fold). FLS from DA were five-fold more invasive than FLS from protected DA.ACI(Cia25/Pia42)-R6 subcongenics. Hip1 siRNA knockdown reduced the invasiveness of DA FLS and RA FLS by 60% and 50%, respectively demonstrating a critical role for Hip1 in cell invasion. Hip1 is known to interact with receptor tyrosine kinases (RTKs). Therefore, we examined 84 PDGF pathway genes in FLS and detected 16 up and 12 down in DA, compared with R6 subcongenics, including PDGFR, IGFR and EGFR. siRNA knock down of Hip1 in DA FLS reduced the levels of these RTKs to nearly zero, with similar reductions in expression levels detected in tissues from Hip1 knock-out mice. We examined cell invasion in response to PDGFb and detected a more pronounced increase on mean DA FLS invasiveness (12-fold) compared with R6 FLS (7-fold). In the presence of low serum conditions (0.1% FBS) DA FLS also had increased growth rate compared with R6 subcongenic FLS, a characteristic previously reported in Hip1 transfected cells.

Conclusion:  We describe new evidence suggesting increased expression and activity of Hip1 in arthritis-derived FLS. Our results suggest that Hip1 regulates different aspect of FLS behavior favoring survival and proliferation, as well as invasivess in FLS from arthritic rats and from patients with RA. Hip1 also regulates the expression levels and activity of RTKs and their response to ligands such as PDGFb. These new discoveries provide new understanding about event regulating FLS behavior in arthritis and have the potential to generate a new prognostic biomarkers and a new intra-cellular target for therapy.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/huntingtin-interactin-protein-1-hip1-regulates-receptor-tyrosine-kinases-mediated-activity-and-cell-invasiness-in-fibroblast-like-synoviocytes/