Individual functions of the histone acetyl transferases CBP and p300 in regulating the inflammatory response of synovial fibroblasts.

Chromatin remodeling, and a persistent histone 3 lysine 27 acetylation (H3K27ac) in particular, are associated with a sustained inflammatory response of synovial fibroblasts (SF) in rheumatoid arthritis (RA). Here we investigated individual functions of the writers of H3K27ac marks, the homologues histone acetyl transferases (HAT) CBP and p300, in controlling the constitutive and inflammatory gene expression in RA SF. We applied a silencing strategy, followed by RNA-sequencing and pathway analysis, complemented with the treatment of SF with inhibitors targeting the HAT (C646) or bromo domains (I-CBP) of CBP and p300. We showed that CBP and p300 undertook overlapping and, in particular at gene levels, distinct regulatory functions in SF. p300 is the major HAT for H3K27ac in SF and regulated more diverse pathways than CBP. Whereas both factors regulated genes associated with extracellular matrix remodeling, adhesion and proliferation, p300 specifically controlled developmental genes associated with limb development. Silencing of CBP specifically down regulated the TNF-induced expression of interferon-signature genes. In contrast, silencing of p300 resulted in anti- and pro-inflammatory effects. Integration of data sets derived from RNA-sequencing and chromatin immunoprecipitation sequencing for H3K27ac revealed that changes in gene expression after CBP or p300 silencing could be only partially explained by changes in levels of H3K27ac. Inhibition of CBP/p300 using HAT and bromo domain inhibitors strongly mirrored effects obtained by silencing of p300, including anti- and pro-inflammatory effects, indicating that such inhibitors are not sufficient to be used as anti-inflammatory drugs.