Regulation of fibroblast-like synoviocyte function by cadherin 6 in rheumatoid arthritis

Cadherins (CDH), such as CDH11, are glycoprotein adhesion molecules contributing to cell-cell interactions in health and disease. CDH11 has demonstrated important functions in rheumatoid arthritis (RA) fibroblast-like synoviocytes (FLS). In transcriptome expression studies, we observed that Cadherin 6 (CDH6) expression was higher in RA compared to osteoarthritis (OA). CDH6 is associated with cancer progression, but little information is known on the role of CDH6 in RA. The present study investigates CDH6 expression, regulation, function in FLS, and distribution in RA synovia.

Synovial tissue and FLS were obtained from RA or OA patients undergoing joint replacement. CDH6 epigenetic marks and expression in RA and OA FLS were evaluated using public databases. CDH6 expression was determined by RT-PCR, Western blot, and immunostaining. RA and OA FLS were stimulated with cytokines and growth factors, and CDH6 mRNA expression was determined. CDH6 was silenced using siRNA, and the effect on migration, cell growth, apoptosis, autophagy, cell cycle, and signaling was studied.

In our analysis of cadherin family expression, CDH6 expression was higher in RA than OA FLS. This was associated with differential chromatin accessibility and histone marks in the CDH6 promoter of RA FLS. H3K27ac was identified as an important regulator of CDH6 expression in RA FLS based on experiments using histone deacetylase inhibitors. TGFß, but not IL-1β, TNF, IL-17A, IFNγ, IL-6, or PDGF, increased CDH6 expression of cultured RA FLS. CDH6 knockdown significantly decreased RA FLS migration and cell growth. The latter was associated with increased apoptosis in CDH6 deficient FLS. Immunofluorescence showed CDH6 protein distribution in the membrane, perinuclear, and nuclear regions of cultured FLS. In RA synovial tissue, CDH6 expression was noted in FLS and macrophages within the lining and sublining regions.

CDH6 expression is elevated in RA FLS due to epigenetic and local conditions of synovitis promoting migration, survival and cell growth, which are characteristic features of aggressive RA FLS. The intracellular distribution suggests additional functions beyond adhesion and homotypic aggregation, such as signaling and gene regulation. These data suggest CDH6 contributes to RA pathogenesis by influencing pathologic FLS behavior and could be a therapeutic target.