Hypoxia and acidosis activate cardiac myocyte death through the Bcl-2 family protein BNIP3

Abstract

Coronary artery disease leads to injury and loss of myocardial tissue by deprivation of blood flow (ischemia) and is a major underlying cause of heart failure. Prolonged ischemia causes necrosis and apoptosis of cardiac myocytes and vascular cells; however, the mechanisms of ischemia-mediated cell death are poorly understood. Ischemia is associated with both hypoxia and acidosis due to increased glycolysis and lactic acid production. We recently reported that hypoxia does not induce cardiac myocyte apoptosis in the absence of acidosis. We now report that hypoxia-acidosisassociated cell death is mediated by BNIP3, a member of the Bcl-2 family of apoptosis-regulating proteins. Chronic hypoxia induced the expression and accumulation of BNIP3 mRNA and protein in cardiac myocytes, but acidosis was required to activate the death pathway. Acidosis stabilized BNIP3 protein and increased the association with mitochondria. Cell death by hypoxia-acidosis was blocked by pretreatment with antisense BNIP3 oligonucleotides. The pathway included extensive DNA fragmentation and opening of the mitochondrial permeability transition pore, but no apparent caspase activation. Overexpression of wild-type BNIP3, but not a translocation-defective mutant, activated cardiac myocyte death only when the myocytes were acidic. This pathway may figure significantly in muscle loss during myocardial ischemia.heart ͉ apoptosis ͉ ischemia ͉ mitochondria ͉ antisense

Cardiac reanimation: targeting cardiomyocyte death by BNIP3 and NIX/BNIP3L

“…The germ-line BNIP3 knockout mouse showed no detectable developmental or functional abnormalities in any organ system, including the hematopoietic system wherein increased numbers of lymphoid, myeloid and erythroid cells have been described for other BH3-only factor gene knockout models (Bouillet et al, 1999;Ranger et al, 2003;Diwan et al, 2007a). This is consistent with the concept that BNIP3 functions almost exclusively in an inducible manner, and that ischemia is critical to increase its expression and possibly to activate it post-translationally (Kubasiak et al, 2002;Kubli et al, 2008). Accordingly, systemic anti-BNIP3 therapy has the potential to prevent post-ischemic cardiac remodeling without untoward consequences in other tissues.…”

“…More importantly, they also suggest that during ischemic stress, factors other than simple upregulation are important in generating the full apoptotic response to BNIP3 in ischemic injury. Whether this is hypoxia/acidosis-mediated post-translational modification and mitochondrial translocation of BNIP3 as has been suggested (Kubasiak et al, 2002;Kubli et al, 2008), or some other as-yet unidentified factor such as nuclear translocation that is seen in ischemic brain injury (Schmidt-Kastner et al, 2004;Burton et al, 2005), is not currently known.…”

“…The growth factors epidermal growth factor and insulinlike growth factor appear to inhibit BNIP3-induced cell death in CHO and MCF7 cells (Kothari et al, 2003). Work by the Webster laboratory indicated that acidosis during hypoxia enhances BNIP3-induced mitochondrial PTP opening and cell death, suggesting either that acidosis stabilizes BNIP3 protein, or that it can provide an as-yet undefined activating signal (Kubasiak et al, 2002;Frazier et al, 2006). Ubiquitination and rapid proteasomal degradation of BNIP3 and NIX/BNIP3L was mentioned above (Chen et al, 1999;Cizeau et al, 2000).…”

Aberrant DNA methylation associated with silencing BNIP3 gene expression in haematopoietic tumours

“…It is noteworthy that because HIF-1 is involved in both survival and apoptosis of tumour cells, abrogation of the apoptotic pathway caused by silencing BNIP3 may enhance HIF-1-induced survival signals within tumours. Expression of BNIP3, which is induced by hypoxic stimuli (Guo et al, 2001), has been detected in several human cancer cell lines and cardiac myocytes subjected to hypoxia (Crow, 2002;Kubasiak et al, 2002;Regula et al, 2002). In addition, Sowter et al (2003) recently reported that BNIP3 is highly expressed in the hypoxic regions of high-grade breast cancer.…”

Alterations of intracellular pH homeostasis in apoptosis: origins and roles

“…44 Concerning BNIP3, a very recent report demonstrates that acidosis is necessary for this protein to induce apoptosis in hypoxic cardiac cells, likely via an effect on protein translation or stability. 45 Besides an effect on protein, acidification might also control the expression of the Bcl-2 family genes; indeed, the downregulation of antiapoptotic Mcl-1 expression, observed upon staurosporine treatment of MDCK cells, has been shown to be inhibited by a blocker of Cl À /HCO 3 À exchange. 46 This effect is interesting as expression of Mcl-1 in CHO cells has been reported to inhibit the acidification induced by staurosporine.…”