Bcl-XL and Mcl-1 upregulation by calreticulin promotes apoptosis resistance of fibroblast-like synoviocytes via activation of PI3K/Akt and STAT3 pathways in rheumatoid arthritis.

OBJECTIVES: Fibroblast-like synoviocytes (FLS) play key roles in synovium hyperplasia and pannus formation in rheumatoid arthritis (RA). The present study was undertaken to explore the mechanisms that calreticulin (CRT) promoted anti-apoptosis of RA FLS.

METHODS: The expression of CRT and anti-apoptotic proteins Bcl-XL and Mcl-1 in RA synovium were detected by immunohistochemistry. The expression of Bcl-XL and Mcl-1 in RA FLS by CRT were determined. The phosphorylation of Akt and STAT3 was detected by western blot. The effect of CRT on proliferation of RA FLS was examined by MTT assay. The ability of CRT to inhibit RA FLS apoptosis was assessed by flow cytometry.

RESULTS: Increased expressions of CRT, Bcl-XL and Mcl-1 were detected in RA synovium compared with osteoarthritis (OA). Moreover, CRT expression correlated positively with Bcl-XL and Mcl-1 in RA, respectively. In vitro, CRT induced upregulation of Bcl-XL and Mcl-1 protein levels in RA FLS, in dose/time dependent manners. Upregulated expression of Bcl-XL and Mcl-1 induced by CRT were inhibited by PI3K/Akt or STAT3 pathways inhibitors in RA FLS, respectively. The increased phosphorylation levels of Akt and STAT3 were also detected with CRT incubation, in dose/time dependent manners. Additionally, CRT rescued apoptosis of RA FLS mediated by FasL.

CONCLUSIONS: This study showed that upregulation of Bcl-XL and Mcl-1 expression in RA FLS by CRT were PI3K/Akt and STAT3 signal pathways dependent, and promoted the anti-apoptosis of RA FLS. Therefore, this may represent a therapeutic target for the treatment of RA.