Histone deacetylase 8 suppresses osteogenic differentiation of bone marrow stromal cells by inhibiting histone H3K9 acetylation and RUNX2 activity

Bone marrow stromal cells (BMSCs) are multipotent progenitor cells with capacities to differentiate into the various cell types and hold great promise in regenerative medicine. The regulatory roles of histone deacetylases (HDACs) in osteoblast differentiation process have been increasingly recognized; however, little is known about the precise roles of HDAC8 in the osteogenic differentiation of BMSCs. Herein we aimed to investigate the roles of HDAC8 in the osteogenic differentiation of rat BMSCs by pharmacological and genetic manipulations in vitro. During osteogenic differentiation of BMSCs, pharmacological inhibition of HDAC8 by HDAC inhibitor valproic acid (VPA) promoted the level of histone H3 lysine 9 acetylation (H3K9Ac) and significantly enhanced the expression of osteogenesis-related genes Runx2, Osterix, osteocalcin (OCN), osteopontin (OPN) and alkaline phosphatase (ALP). Similarly, knockdown of HDAC8 using short interfering RNA triggered H3K9Ac and enhanced osteogenic differentiation of BMSCs, largely phenocopied the effects of VPA-mediated HDAC8 depletion. However, enforced expression of HDAC8 significantly reduced the level of H3K9Ac and inhibited osteogenic differentiation of BMSCs, which can be attenuated by VPA addition. Mechanistically, HDAC8 suppressed osteogenesis-related genes expression by removing the acetylation of histone H3K9, thus leading to transcriptional inhibition during osteogenic differentiation of BMSCs. Importantly, we found that HDAC8 physically associated with Runx2 to repress its transcriptional activity and this association decreased when BMSCs underwent osteogenic differentiation. Taken together, these results indicate that epigenetic regulation of Runx2 by HDAC8-mediated histone H3K9 acetylation is required for the proper osteogenic differentiation of BMSCs.