Background: Acute gouty arthritis is triggered by endogenously formed monosodium urate (MSU) crystals, which cause caspase-1-dependent activation of interleukin(IL)-1β. However, for fullblown inflammation and cytokine production an additional stimulus is required. Toll-like receptor (TLR) ligands such as LPS or saturated free fatty acids (FFA) can provide such a signal and stimulate the initial transcription of pro-IL-1β. The combination of TLR ligands and MSU crystals produce synergistic inflammatory cytokine responses. Recently, we have shown that the short-chain fatty acid butyrate inhibits FFA+MSU-induced cytokine production via inhibition of class I histone deacetylases (HDACs). However, butyrate has been reported to have HDAC-independent effects as well. For this reason we envisage that inhibition of one or several HDAC isoforms with highly specific synthetic compounds may be a new therapeutic option for the treatment of acute gouty arthritis.
Methods: A monocyteenriched cell population was isolated from blood of healthy donors by Ficoll and Percoll density gradient and stimulated for 24 hours with MSU and palmitic acid (C16.0) in the presence or absence of a broad range of epigenetic modulators. This library included HDAC and sirtuin inhibitors as well as methylation modulators and retinoic acid pathway inhibitors and agonists. Cytokine responses were measured with specific ELISAs.
Results: The most interesting finding in the epigenetic compound screening was that combined specific HDAC1 and HDAC2 inhibitors added at 1 uM decreased C16.0+MSU-induced IL-1β production. Interestingly, IL-1Ra production stayed the same or was even increased.
Discussion: Our earlier findings pinpoint the cytokine-suppressive effects of butyrate on C16.0+MSU stimulation to class I HDAC inhibition (HDAC1, -2, -3, and -8). Screening of the epigenetic compound library revealed that specific combined HDAC1 and HDAC2 inhibition decreased IL-1β, but not IL-1Ra. Possibly, broad HDAC inhibitors such as Panobinostat and butyrate induce multiple effects by robust class I HDAC inhibition, whereas combined specific HDAC1 and -2 inhibition reveals a decrease in IL-1β but does not affect IL-1Ra production. This specific effect could potentially be of high therapeutic value in acute gouty arthritis.