Histone deacetylase is a target of valproic acid-mediated cellular differentiation

Valproic acid (VPA), a well-established therapy for seizures and bipolar disorder, has recently been shown to inhibit histone deacetylases (HDACs). Similar to more widely studied HDAC inhibitors, VPA can cause growth arrest and induce differentiation of transformed cells in culture. Whether this effect of VPA is through inhibition of HDACs or modulation of another target of VPA has not been tested. We have used a series of VPA analogs to establish a pharmacological profile for HDAC inhibition. We find that VPA and its analogs inhibit multiple HDACs from class I and class II (but not HDAC6 or HDAC10) with a characteristic order of potency in vitro. These analogs also induce hyperacetylation of core histones H3 and H4 in intact cells with an order of potency that parallels in vitro inhibition. VPA and VPA analogs induce differentiation in hematopoietic cell lines in a p21-dependent manner, and the order of potency for induction of differentiation parallels the potencies for inhibition in vitro, as well as for acetylation of histones associated with the p21 promoter, supporting the argument that differentiation caused by VPA is mediated through inhibition of HDACs. These findings provide additional evidence that VPA, a well-tolerated, orally administered drug with extensive clinical experience, may serve as an effective chemotherapeutic agent through targeting of HDACs.