Bcl-2 Inhibits Nuclear Homologous Recombination by Localizing BRCA1 to the Endomembranes

Abstract

Genetic stability requires coordination of a network of pathways including DNA repair/recombination and apoptosis. In addition to its canonical anti-apoptotic role, Bcl-2 negatively impacts genome stability. In this study, we identified the breast cancer tumor suppressor BRCA1, which plays an essential role in homologous recombination (HR), as a target for Bcl-2 in the repression of HR. Indeed, ionizing radiation-induced BRCA1 foci assembly was repressed when Bcl-2 was expressed ectopically, in human SV40 fibroblasts, or spontaneously, in lymphoma t(14:18) cells and in HeLa and H460 cancer cell lines. Moreover, we showed that the transmembrane (TM) domain of Bcl-2 was required for both inhibition of BRCA1 foci assembly and the inhibition of HR induced by a double-strand break targeted into an intrachromosomal HR substrate by the meganuclease I-SceI. Fluorescence confocal microscopy, proximity ligation assay, and electron microscopy analyses as well as Western blot analysis of subcellular fractions showed that Bcl-2 and BRCA1 colocalized to mitochondria and endoplasmic reticulum in a process requiring the TM domain of Bcl-2. Targeting BRCA1 to the endomembranes depletes BRCA1 from the nucleus and, thus, accounts for the inhibition of HR. Furthermore, our findings support an apoptosis-stimulatory role for the cytosolic form of BRCA1, suggesting a new tumor suppressor function of BRCA1. Together, our results reveal a new mode of BRCA1 regulation and for HR in the maintenance of genome stability. Cancer Res; 71(10); 3590-602. Ó2011 AACR.

The secret life of Bcl-2: Apoptosis-independent inhibition of DNA repair by Bcl-2 family members

“…Moreover, in another study, the massive translocation of Bcl-2 into the nucleus after irradiation was not observed in cells overexpressing either ectopic or endogenous Bcl-2 [47].…”

Combination of AZD2281 (Olaparib) and GX15-070 (Obatoclax) results in synergistic antitumor activities in preclinical models of pancreatic cancer

“…Emerging evidence has demonstrated that Bcl-2, Bcl-xL and Mcl-1 possess non-apoptotic functions, in addition to their traditional roles in the mitochondrial apoptosis pathway, and are involved in the DNA damage response [5,19,38,44]. PARP inhibitors have demonstrated efficacy in cells with impaired HR due to BRCA1/2 mutations, as well as mutations in other HR-related genes [7,24,32].…”

“…This pan-Bcl-2 inhibitor has been reported to directly induce apoptosis in a variety of cultured cancer cell lines and primary patient samples through the mitochondrial apoptotic pathway as well as non-apoptotic cell death [16,28,33]. Recent studies have found compelling evidence to suggest that the anti-apoptotic Bcl-2 family proteins can regulate DNA double-strand break repair independent of their pro-survival functions [2,11,14,17,19,31,[36][37][38]42,44]. We hypothesize that using a pan-Bcl-2 inhibitor may sensitize pancreatic cancer cells to PARP inhibitors.…”

Homologous Recombination in Mammals

“…Some metabolic pathways can more or less directly repress HR. This is the • case in p53 (for a review, see Bertrand et al 2004 ) , in the activation of AKT1 (Plo et al 2008 ;Plo and Lopez 2009 ) or in expression of members of the Bc1-2 family (Laulier et al 2011a ) . Other cellular metabolic pathways affecting HR are likely to be discovered.…”

“…Bc1-2 was initially found to be overexpressed in B cell lymphoma with recurrent translocation t(14:18), but it is also overexpressed in numerous tumors. Overexpression of Bc1-2 leads to the relocalization of BRCA1 in endomembranes (endoplasmic reticulum, mitochondria), resulting in inhibition of HR (Laulier et al 2011a ) .…”