¹MRC Toxicology Unit, Hodgkin Building, University of Leicester, Leicester, UK

Correspondence: Professor GM Cohen, MRC Toxicology Unit, Hodgkin Building, University of Leicester, PO Box 138, Lancaster Road, Leicester LE1 9HN, UK. E-mail: gmc2@le.ac.uk

Received 19 January 2007; Revised 2 April 2007; Accepted 18 April 2007; Published online 24 May 2007.

Abstract

Several histone deacetylase inhibitors (HDACi), which have recently entered early clinical trials, exert their anticancer activity in part through the induction of apoptosis although the precise mechanism of this induction is not known. Induction of apoptosis by structurally diverse HDACi in primary cells from patients with chronic lymphocytic leukemia (CLL) and different leukemic cell lines was mediated by the Bcl-2 regulated intrinsic pathway and demonstrated a requirement for de novo protein synthesis. A marked time-dependent induction of the pro-apoptotic BH3-only proteins, Bim, Noxa and Bmf was observed, which preceded the induction of apoptosis. A key role for both Bim and Noxa was proposed in HDACi-mediated apoptosis based on our findings that siRNA for Bim and Noxa but not Bmf largely prevented the HDACi-induced loss in mitochondrial membrane potential, caspase processing and phosphatidylserine externalization. Noxa, induced by HDACi, in CLL cells and tumor cell lines, bound extensively to Mcl-1, a major anti-apoptotic Bcl-2 family member present in CLL cells. Our data strongly suggests that HDACi induce apoptosis primarily through inactivation of anti-apoptotic Bcl-2 family members by increases in Bim and Noxa and highlights these increases as a potential clinical target for CLL/lymphoma therapy.