A Death Receptor-associated Anti-apoptotic Protein, BRE, Inhibits Mitochondrial Apoptotic Pathway

Abstract

BRE, brain and reproductive organ-expressed protein, was found previously to bind the intracellular juxtamembrane domain of a ubiquitous death receptor, tumor necrosis factor receptor 1 (TNF-R1), and to downregulate TNF-␣-induced activation of NF-B. Here we show that BRE also binds to another death receptor, Fas, and upon overexpression conferred resistance to apoptosis induced by TNF-␣, anti-Fas agonist antibody, cycloheximide, and a variety of stress-related stimuli. However, down-regulation of the endogenous BRE by small interfering RNA increased apoptosis to TNF-␣, but nottoetoposide,indicatingthatthephysiologicalantiapoptotic role of this protein is specific to death receptormediated apoptosis. We further demonstrate that BRE mediates antiapoptosis by inhibiting the mitochondrial apoptotic machinery but without translocation to the mitochondria or nucleus or down-regulation of the cellular level of truncated Bid. Dissociation of BRE rapidly from TNF-R1, but not from Fas, upon receptor ligation suggests that this protein interacts with the death inducing signaling complex during apoptotic induction. Increased association of BREwith phosphorylated, sumoylated, and ubiquitinated proteins after death receptor stimulation was also detected. We conclude that in contrast to the truncated Bid that integrates mitochondrial apoptosis to death receptor-triggered apoptotic cascade, BRE inhibits the integration. We propose that BRE inhibits, by ubiquitination-like activity, components in or proximal to the death-inducing signaling complexes that are necessary for activation of the mitochondria.

BRE is an antiapoptotic protein in vivo and overexpressed in human hepatocellular carcinoma

“…Thus, mice with homozygous knockout of Bid, or transgenic overexpression of either BCL-2 or BCL-X L in the liver, showed significantly attenuated Fasmediated hepatitis (Lacronique et al, 1996;Rodriguez et al, 1996;de la Coste et al, 1999a;Yin et al, 1999). The present demonstration of better survival of the liver-specific BRE transgenic mice compared with the non-transgenic littermates in the model is consistent with our previous findings in cell lines showing inhibition of t-BID-induced mitochondrial release of proapoptotic proteins by overexpressed BRE (Li et al, 2004). However, we have also noted that our transgenic mice were not as well protected from the Fas-induced lethal liver failure as the BCL-2 transgenic and Bid-knockout mice.…”

“…The following experiments were performed to establish the antigenic specificity of the two monoclonal antibodies. As shown in Figure 1a, western blotting by Mab489-7 and Mab246 of GSB2 (Li et al, 2004), a stable Jurkat transfectant expressing a V5 and His-tagged BRE (GS-BRE), yielded two major protein bands, which correspond to the endogenous BRE and the transfected GS-BRE (lanes 2 and 4). These antibodies were also able to stain the 44-kDa recombinant BRE expressed in insect cell line Sf9, transduced by the recombinant baculovirus (lanes 1 and 3).…”

“…Thus, BRE is a death receptor-specific antiapoptotic protein, which acts like a functional converse of truncated BID (t-BID) by blocking the crosstalk between the extrinsic and intrinsic apoptotic pathways. Contrary to the depletion of BRE, overexpression of the protein by transfection can affect the intrinsic apoptosis by downregulating the apoptotic response to a variety of stress-related and genotoxic stimuli (Li et al, 2004). This asymmetric sensitivity of intrinsic apoptosis to BRE levels implies that the protein is normally sequestered from the mitochondrial apoptotic system, unless it is overexpressed or the death receptors are activated.…”

“…Subcellular fractionation has confirmed that BRE is not located in the mitochondria; it was found in the cytoplasmic fraction as expected, and also in the nucleus. Intriguingly, unlike t-BID, BRE is not translocated to the mitochondria during apoptotic induction (Korsmeyer et al, 2000;Li et al, 2004). The protein is also not a member of the BCL-2 family, well known to play key roles in regulating mitochondrial apoptosis (Green and Reed, 1998;Zamzami et al, 1998;Gross et al, 1999;Cheng et al, 2001).…”

High BRE expression in pediatric MLL-rearranged AML is associated with favorable outcome

“…Intriguingly, BRE transcription has been shown to be downregulated after DNA damage and retinoic acid treatment. 38 Li et al 39 showed that BRE is a death receptorassociated antiapoptotic protein, inhibiting the mitochondrial apoptotic pathway. We could not find evidence that overexpression of BRE influenced apoptosis in an Previous studies demonstrated that overexpression of BRE enhances tumor growth, rather than initiating tumor formation in vivo.…”