Background Epigenetic mechanisms, including reversible acetylation and methylation of histones, play a pivotal role in regulating cellular inflammatory responses. Aberrant activity and expression of histone deacetylases (HDACs) contributes to pathology in inflammatory lung diseases, and recent reports have suggested that HDAC activity is deregulated in rheumatoid arthritis (RA), and associates with synovial expression of inflammatory cytokines.
Objectives To analyze potential associations of synovial protein acetylation and HDAC expression with RA patient disease activity and synovial expression of inflammatory mediators, and to characterize the effects of inflammatory stimulation on HDAC expression in RA fibroblast-like synoviocytes (FLS).
Methods The study was performed on synovial tissue sections of 12 RA and 12 osteoarthritis (OA) patients undergoing joint replacement surgery, and 20 RA patients undergoing arthroscopy. Synovial protein acetylation and cellular composition of the synovial tissue was assessed by immunohistochemistry, and quantified by digital image analysis. mRNA expression of HDAC family members, TNFα, IL-6 and MMP-1 in the synovial tissue, and changes in mRNA levels of HDACs expressed in FLS were measured by quantitative PCR.
Results Similar levels of acetylated lysine and acetylated H3 were detected in RA and osteoarthritis (OA) synovial tissue, while a modest reduction in H4 acetylation was observed in RA compared to OA (P=0.04). Total protein acetylation levels in the synovial tissue of 20 RA patients failed to correlate with clinical parameters and synovial T cell or FLS numbers. On the other hand, a strong positive association of acetylated lysine, acetylated H3, acetylated H4 and dimethylated H3 levels with synovial macrophage numbers was observed (R≥0.539, P<0.02). Synovial expression of TNFα and MMP-1 positively correlated with mRNA levels of HDAC1 (TNFα: R=0.651, P=0.003; MMP-1: R=0.502, P=0.029) and other Class I HDACs. A significant negative correlation was observed between HDAC5 expression and both RA patient disease activity (CRP: R= -0.664, P=0.007; ESR: R= -0.557, P=0.013; DAS28: R= -0.567, P=0.011) and synovial IL-6 expression (R= -0.477, P=0.039). mRNA expression of HDAC5 in RA FLS, but not other HDACs, was rapidly suppressed following IL-1β stimulation.
Conclusions This study provides several novel findings regarding the relationship between HDAC-mediated epigenetic regulatory mechanisms and inflammation in RA. First, our data suggests that differences in HDAC expression or activity that might be present in RA synovial tissue are not sufficient to result in global changes in total protein or histone acetylation associated with disease activity. Second, cellular protein hyperacetylation represents a cell type-specific phenomenon characteristic for synovial macrophages in RA, not directly related to diagnosis or disease activity. Finally, we confirm previous studies suggesting that high synovial expression of Class I HDACs is associated with local expression of inflammatory mediators, but identify cytokine-mediated reduction of HDAC5 as a potential marker of systemic inflammatory processes.
Disclosure of Interest None Declared
Statistics from Altmetric.com
If you wish to reuse any or all of this article please use the link below which will take you to the Copyright Clearance Center’s RightsLink service. You will be able to get a quick price and instant permission to reuse the content in many different ways.