Abstract
Valproic acid (VPA) is a well-tolerated anticonvulsant drug that has been recently recognized as powerful histone deacetylase (HDCA) inhibitor. VPA induces hyperacetylation of histone H3 and H4 and inhibits both class I and II HDCACs. Recently it has been shown that VPA exerts in vitro and in vivo anti-tumor activity against solid cancers and its in vitro anti-Multiple Myeloma (MM) activity has been previously reported. However, the molecular mechanisms are still unclear. Here we have investigated molecular changes induced by VPA as well as its in vivo activity in murine models of MM. We first studied the in vitro activity of VPA against IL-6 independent as well as IL-6 dependent MM cells. A time- and dose-dependent decrease in proliferation and survival of MM cell lines was observed (IC50 in the range of 1–3 mM). Gene expression profile following treatment with VPA at 2 and 5 mM showed down-regulation of genes involved in cell cycle regulation, DNA replication and transcription as well as up-regulation of genes implicated in apoptosis and chemokine pathways. The signaling pathway analysis performed by Ingenuity Systems Software identified the cell growth, cell cycle, cell death as well as DNA replication and repair as the most important networks modulated by VPA treatment. We next evaluated the in vivo activity of VPA using two xenograft models of human MM. A cohort of SCID mice bearing subcutaneous MM1s or OPM1 were treated i.p. daily with VPA (200 mg/kg, and 300 mg/kg, n=5 mice, respectively), or vehicle alone (n=5 mice) for 16 consecutive days. Tumors were measured every 2 days, and survival was calculated using the Kaplan Mayer method. Following VPA treatment, we found a significant (p=0.006) inhibition of tumor growth in mice bearing subcutaneous MM-1s cells treated with VPA at 200 mg/kg compared to control group, which translated into a significant (p= 0.002) survival advantage in the VPA treated animals. Similar results were obtained in animals bearing subcutaneous OPM1 cells. Flow cytometry analysis performed on retrieved tumor tissues from animals showed reduction of G2-M and S phase in tumor specimens following VPA treatment, versus untreated tumors, strongly suggesting in vivo effects of VPA on cell cycle regulation. Taken together, our data demonstrate the in vitro and in vivo anti-tumor activity of VPA, delineate potential molecular targets triggered by this agent and provide a preclinical rationale for its clinical evaluation, both as a single agent or in combination, to improve patient outcome in MM.
Structure of the histone mRNA hairpin required for cell cycle regulation of histone gene expression