The bromodomain protein inhibitor I-BET151 suppresses expression of inflammatory genes and matrix degrading enzymes in rheumatoid arthritis synovial fibroblasts

Objective To investigate the effects of BET bromodomain protein inhibition on inflammatory activation and functional properties of rheumatoid arthritis synovial fibroblasts (RASF). Methods The expression of the BET bromodomain proteins BRD2, BRD3 and BRD4 was analysed in synovial tissue by immunohistochemistry. RASE were stimulated with tumour necrosis factor (TNF)-alpha, interleukin (IL)-1 beta and toll-like receptor (TLR) ligands (Pam3, pIC and lipopolysaccharide (LPS)) in the presence or absence of the BET inhibitor I-BET151, or siRNA targeting BRD2, BRD3 and BRD4. RASF expression of inflammatory mediators, including MMP1, MMP3, IL-6 and IL-8, was measured by q-PCR, q-PCR array and ELISA. Cellular viability, apoptosis, proliferation and chemoattractive properties of RASF were investigated using MTT, cell apoptosis ELISA, BrdU-based proliferation and transwell migration assays. Results BRD2, BRD3 and BRD4 proteins were detected in rheumatoid arthritis (RA) synovial tissue, expressed in both RASF and macrophages. I-BET151 suppressed cytokine and TLR ligand-induced secretion of MMP1, MMP3, IL-6 and IL-8, and mRNA expression of more than 70% of genes induced by TNF-alpha and IL-1 beta. Combined silencing of BRD2, BRD3 and BRD4 significantly reduced cytokine and TLR ligand-induced expression of a subset of gene products targeted by I-BET151, including MMP1, CXCL10 and CXCL11. I-BET151 treatment of RASF reduced RASF proliferation, and the chemotactic potential for peripheral blood leucocytes of RASF conditioned medium. Conclusions Inhibition of BET family proteins suppresses the inflammatory, matrix-degrading, proliferative and chemoattractive properties of RASF and suggests a therapeutic potential in the targeting of epigenetic reader proteins in RA.