Histone deacetylase HDAC8 associates with smooth muscle alpha-actin and is essential for smooth muscle cell contractility

Although originally characterized as nuclear enzymes controlling the stability of nucleosomes, histone deacetylases (HDACs) may also exert their activity within the cytosol. Recently, we have demonstrated that HDAC8, a class I HDAC, is a novel, prominently cytosolic marker of smooth muscle differentiation. As HDAC8 displays a striking stress fiber-like pattern of distribution and is coexpressed in vivo with smooth muscle alpha-actin (alpha-SMA) and smooth muscle myosin heavy chain, we have explored the possible participation of this HDAC in smooth muscle cytoskeleton regulation. Cell fractionation assays performed with primary human smooth muscle cells (HSMCs) showed that HDAC8, in contrast to HDAC1 and HDAC3, was enriched in cytoskeleton-bound protein fractions and insoluble cell pellets, suggesting an association of HDAC8 with the cystoskeleton. Coimmunoprecipitation experiments using HSMCs, NIH-3T3 cells, and human prostate tissue lysates further demonstrated that HDAC8 associates with alpha-SMA but not with beta-actin. HDAC8 silencing through RNA interference strongly reduced the capacity of HSMCs to contract collagen lattices. Mock transfections had no effect on HSMC contractily, and transfections with small interfering RNAs (siRNAs) specific for HDAC6, a cytosolic HDAC that functions as an alpha-tubulin deacetylase, resulted in a weak contraction inhibition. Although mock- and HDAC6 siRNA-transfected HSMCs showed no noticeable morphological changes, HDAC8 siRNA-transfected HSMCs displayed a size reduction with diminished cell spreading after replating. Altogether, our findings indicate that HDAC8 associates with the smooth muscle actin cytoskeleton and may regulate the contractile capacity of smooth muscle cells.