No abstract

Bromodomain- and extra-terminal domain (BET) proteins are epigenetic reader proteins that regulate transcription of their target genes by binding to acetylated histone side chains. Small molecule inhibitors, such as I-BET151, have anti-inflammatory properties in fibroblast-like synoviocytes (FLS) and in animal models of arthritis. Here, we investigated whether BET inhibition can also affect the levels of histone modifications, a novel mechanism underlying BET protein inhibition. On the one hand, FLSs were treated with I-BET151 (1 µM) for 24 h in absence and presence of TNF. On the other hand, FLSs were washed with PBS after 48 h of I-BET151 treatment, and the effects were measured 5 days after I-BET151 treatment or after an additional 24 h stimulation with TNF (5 d + 24 h). Mass spectrometry analysis indicated that I-BET151 induced profound changes in histone modifications, with a global reduction in acetylation on different histone side chains 5 days after treatment. We confirmed changes on acetylated histone side chains in independent samples by Western blotting. I-BET151 treatment reduced mean TNF-induced levels of total acetylated histone 3 (acH3), H3K18ac, and H3K27ac. In line with these changes, the TNF-induced expression of BET protein target genes was suppressed 5 d after I-BET151 treatment. Our data indicate that BET inhibitors not only prevent the reading of acetylated histones but directly influence overall chromatin organization, in particular after stimulation with TNF.

Most forms of arthritis, have a distinctive topographical pattern of joint involvement. Beyond these differences among diseases, there are also differences in phenotype and response to treatment between joints of the same type of arthritis, suggesting that molecular mechanisms may differ depending on joint location. Here we show that there are joint-specific molecular and tissue changes in the synovium and in local stromal cells (synovial fibroblasts;SF). The long non-coding RNA HOTAIR, expressed only in lower extremities SF, regulates much of this site-specific gene expression in SF. Downregulation of HOTAIR after TNF stimulation regulated relevant inflammatory pathways by epigenetic and transcriptional mechanisms and modified the migratory function of SF, decreased SF-mediated osteoclastogenesis, and increased the attraction of B cells by SF. Since site-specific expression of HOTAIR was also measured in the skin, spine and gastrointestinal tract, we propose HOTAIR as important epigenetic factor that modulates site-specific phenotypes of chronic inflammation.