Heat shock-induced apoptosis
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Apoptosis is a conserved program of cell death that promotes organism homeostasis in all stages of life. Two main pathways activate caspases, which are cysteinyl-aspartate proteases that execute apoptosis. The extrinsic pathway is initiated by cell surface death receptors, while the intrinsic pathway is initiated by intracellular signals that cause permeabilization of the outer mitochondrial membrane (MOMP). The Bcl-2 protein family regulates MOMP, which causes the release of several pro-apoptotic proteins (such as cytochrome c, Smac) into the cytosol. Bcl-2 proteins share homology in up to four "BH" domains and are subdivided into three subgroups. Pro-apoptotic Bax and Bak catalyze pore formation in the mitochondria, while anti-apoptotic members (Bcl-2, Mcl-1) inhibit MOMP. The third subgroup, termed BH3-only, promotes MOMP by either antagonizing Bcl-2 proteins or by directly activating Bax/Bak, and initiate apoptosis in response to various stressors, including heat shock (HS). Hyperthermia or acute HS reportedly induces apoptosis through caspase-2-mediated cleavage of BID, engaging the intrinsic pathway. However, additional evidence suggests that this pathway could represent an amplification loop. Thus we hypothesized that during HS, another BH3-only protein such as BIM, that does not require cleavage, could engage MOMP. Herein, we report that BIM mediates an alternative HS-induced apoptosis pathway. Cells lacking BIM are resistant to HS and exhibit better short and long-term survival than either Bid[superscript -/-] or Bax[superscript -/-]Bak[superscript -/-]. Moreover, caspase-2 induces apoptosis in Bim[superscript -/-] but not Bid[superscript -/-] cells, implying that caspase-2 kills exclusively through BID. Interestingly, Bim[superscript -/-] and Bax[superscript -/-]Bak[superscript -/-] cells are entirely resistant to MOMP, but the Bax[superscript -/-]Bak[superscript -/-] cells still undergo caspase-3 activation and remain partially sensitive to HS, indicating that BIM triggers caspase-3 activation upstream of mitochondria. Thus, BIM plays an important role in HS-induced apoptosis. Hyperthermia has clinical applications for the treatment of solid tumors. Unfortunately, a practical limitation is the development of thermotolerance, which confers resistance not only to subsequent HS but also to radiotherapy and chemotherapy. Therefore, a better understanding of the molecular mechanisms involved both in heat-induced apoptosis and thermotolerance could lead to new therapeutic interventions. Here we also show evidence for a putative role for the stress kinase JNK signaling pathway in the regulation of thermotolerance.