Abstract B28: Determining the apoptotic mechanisms of bromodomain and extraterminal (BET) inhibitors

Abstract

Abstract Inhibitors of the bromodomain and extraterminal domain family (BETi), represent a novel class of targeted therapy to treat aggressive B-cell lymphomas. Specifically, BETi are shown to inhibit proliferation and induce apoptosis in lymphoma cells arising from Myc transgenic mice. However, the underlying apoptotic mechanisms are still not well understood. Dissecting apoptotic mechanisms required by BETi could provide strong rationale on how to best utilize BETi to treat patients with aggressive B-cell lymphomas. It was the primary purpose of this study to delineate the molecular mechanisms underlying BETi induced apoptosis. Secondly, by determining critical apoptotic proteins/pathways, investigate the potential of BETi in novel combination therapy strategies. For this, we conducted in vitro studies utilizing primary and compound mutant lymphomas derived from the Eµ-myc murine model of B-cell lymphoma. The BETi IBET762 was used to treat lymphomas, and over all cell death, apoptosis, mitochondria damage, cell cycle distribution were assessed by flow cytometry. Protein expression of Myc, bromodomain 3 and 4 (BRD 3 and BRD4), Bcl-2 and activated caspase 3 were assessed by western blot. IBET762 induces apoptosis in Eµ-myc lymphomas in a time and dose dependent manner at low nanomolar concentrations. Apoptosis was dependent on the intrinsic apoptotic pathway, but independent of p53 or p19arf. Loss of apoptosome function delayed the kinetics of BETi mediated cell death. Analysis of BRD3, BRD4, Bcl-2 and Myc protein expression is ongoing. Because apoptosis induction by BETi was blocked by the overexpression of Bcl-2, we tested the efficacy of the small molecule Bcl-2 inhibitor, ABT-263. Combining low dose ABT-263 with IBET762 resulted in induction of apoptosis in Eµ-myc/Bcl-2 lymphomas. These data indicate mitochondrial damage to be critical for BETi mediated apoptosis. Finally, our current studies will provide vision and strong rationale on how best to apply BET inhibitors to the clinic to provide maximum benefit to patients with aggressive B-cell lymphomas. Citation Format: Elena Lasorsa, Leigh Ellis. Determining the apoptotic mechanisms of bromodomain and extraterminal (BET) inhibitors. [abstract]. In: Proceedings of the AACR Special Conference on Myc: From Biology to Therapy; Jan 7-10, 2015; La Jolla, CA. Philadelphia (PA): AACR; Mol Cancer Res 2015;13(10 Suppl):Abstract nr B28.