Background and objectives Chemical inhibitors of histone deacetylases (HDACis) possess anti-inflammatory properties in both in vitro and in vivo models of rheumatoid arthritis (RA). Currently available HDACis show little selectivity for class I (HDAC 1–3, 8) and class II (HDAC 4–7, 9, 10) HDACs. However, preliminary evidence suggests that targeting specific class I HDACs could result in amelioration of disease in models of RA. The present study aimed to better characterise potential differential contributions of class I and class II HDAC family members to the inflammatory activation of RA fibroblast-like synoviocytes (FLS).

Materials and methods The effects of pan-specific and selective HDAC1/2, HDAC3/6, HDAC6, and HDAC8 inhibitors on RA FLS global protein and histone lysine acetylation was analysed by Western blot, while effects on gene expression were analysed by qPCR. FLS were transfected with either control siRNA or with specific HDAC3 siRNA or IFNAR1 siRNA and gene expression was analysed by qPCR. Analysis of transcription factor expression and activity was performed using Western blotting and ELISA-based DNA-binding assays.

Results Impairment of class I HDAC3 and class IIb HDAC6 activity by the use of a selective HDAC3/6 inhibitor (HDAC3/6i) reduced the expression of IL-beta-induced inflammatory genes suppressed by pan-HDACi in FLS. In contrast, selective inhibition of class I HDAC1/2 and HDAC8 showed poor effects on gene modulation. Silencing of HDAC3 reproduced the effects of HDAC3/6i on gene regulation (p = 0.0156), contrary to HDAC6 specific inhibition. Screening of candidate transcription factors enriched in these gene targets, as well as IFNAR1 silencing, revealed that HDAC3/6i selectively abrogates STAT1 Tyr701 phosphorylation and STAT1 activity, but does not affect STAT1 acetylation. Impairment of STAT1 activity was a consequence of HDAC3-mediated suppression of IFNβ production.

Conclusions Our results demonstrate that impairment of class I HDAC3, either through pharmacological inhibition or gene silencing, reproduces the effects of pan-HDACi in suppressing inflammatory gene expression in RA FLS. A subset of these genes are dependent upon HDAC3 regulation of IFNβ production and subsequent STAT1 phosphorylation. Taken together our data demonstrate an essential role of HDAC3 in the modulation of the inflammatory gene expression program in RA FLS, and suggests that specific epigenetic intervention may be of therapeutic benefit in the treatment of inflammatory diseases such as RA.