Direct Small Molecule BAX Inhibition as a Novel Therapeutic Strategy to Protect the Heart Against Cancer Therapies
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Although advancements in chemotherapeutics have led to a dramatic increase in the number of cancer survivors, many of these patients experience unintended cardiotoxic effects as a direct result of their cancer treatments. Both traditional and targeted cancer therapies cause severe side effects such as cardiomyopathy, which can progress to lethal heart failure. Therefore, cardioprotective therapies that mitigate chemotherapy-induced cardiomyopathy while not compromising the anti-cancer effects would greatly benefit cancer patients.;The most commonly used traditional cancer chemotherapeutic agent is doxorubicin, widely used both in solid tumors and leukemia. The precise mechanism of doxorubicininduced cardiotoxicity is not well understood, but a major contributing factor is cardiomyocyte death, which is mediated by two major processes: apoptosis and necrosis. Apoptosis is a highly conserved and actively mediated cell suicide program important for organismal development and homeostasis. While necrosis was once thought to be accidental and unregulated, a growing body of research has demonstrated that a significant proportion of necrotic cell death is actively mediated through highly regulated mechanisms. The active nature of these forms of cell death suggests the possibility that doxorubicin-induced cardiotoxicity may be prevented through inhibition of cardiomyocyte death.;Since both apoptosis and necrosis play critical roles in the pathogenesis of doxorubicininduced cardiotoxicity, we have sought a molecular target that promotes both. Recently, it was demonstrated that the protein BAX, long recognized as a central activator of apoptosis, also plays a critical role in cardiomyocyte necrosis in vivo. Hence, we hypothesized that BAX may play a crucial role in mediating doxorubicin-induced cardiomyopathy and its antagonism may provide a means to simultaneously inhibit both apoptosis and necrosis, and thereby limit doxorubicin-induced cardiac toxicity. Indeed, deletion of BAX in mice protected against doxorubicin-induced apoptotic and necrotic cardiac cell death and cardiomyopathy. Accordingly, we identified small molecule BAX activation inhibitors (BAIs). NMR studies demonstrated that the lead compound, BAI1, previously identified in a screen for inhibitors of tBID-induced cytochrome c release from isolated mitochondria, directly binds BAX and inhibits its conformational activation. BAI1 and several analogs potently block apoptotic and necrotic cell death triggered by a variety of death stimuli. Inhibition of cell death by BAI1 is specific as it requires the presence of BAX. Mechanistically, BAI1 inhibits BAX mitochondrial translocation and membrane insertion and blocks BAX-mediated permeabilization of the outer mitochondrial membrane.;Importantly, BAI1 decreases doxorubicin-induced death in primary rodent and human induced pluripotent stem cell-derived cardiomyocytes. In animal studies, BAI1 prevents doxorubicin-induced cardiomyopathy in zebrafish. Testing in mice demonstrates that BAI compounds limit both acute and chronic doxorubicin-induced cardiomyopathy, cardiac apoptosis and necrosis. Moreover, the anti-human epidermal growth factor receptor 2 antibody, trastuzumab, was employed in combination with doxorubicin to model a common breast cancer therapy regimen. BAI1 also successfully protects against cardiac dysfunction in this model. Interestingly, BAI1 does not interfere with doxorubicin's ability to kill human breast cancer and acute myeloid leukemia cell lines. Furthermore, BAI1 does not inhibit doxorubicin-mediated reduction of tumor burden in breast cancer and acute myeloid leukemia mouse models. In the latter case, BAI1 even exhibits cardioprotection in the same leukemic mice. We delineated that one mechanism that allows the heart to be protected by BAX inhibition without interfering with chemotherapeutic efficacy is the high BAX levels and cell death-priming in cancer cells.;In conclusion, the results from these studies reveal the protein BAX as an important mediator of chemotherapy-induced cardiomyopathy, cardiac apoptosis and necrosis, and provide BAX inhibitor compounds as novel drug prototypes for this disease. This study further reinforces the essential role that programmed cell death plays in cardiotoxicities and provides evidence that cell death pathways can be pharmacologically manipulated to mitigate disease progression.
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