Host: The Japan Radiation Research Society, Chairman of the 52nd Annual Meeting, Toshiteru Okubo (Radiation Effects Research Foundation)
We showed that heat stress (44oC/30 min) or SOD-mimic nitroxide Tempo (10 mM) induces apoptosis, while their combination swiches apoptosis to nonapoptotic cell death (Zhao et al., FBRM 40: 1131, 2006); however, this switch mechamism remains elusive. Here we identify that the above co-treatment induces autophagy in U937 cells. This high-dose of Tempo inhibited heat-induced apoptosis by specifically inhibiting the processing of activated procaspases-2, -8, -9 and -3 to active caspases downstream of tBid/Bax-mediated cytochrome c release, and instead induced the caspase-independent autophagy. Caspases were inactivated by a memchanism by which highly oxidative oxo-ammonium generated during Tempo's dismutation of superoxide oxidizes active-site-CysSH of all caspases. Autophagy was induced by the permeability transition pore opening-mediated mitochondrial dysfunction after the above co-treatment. The autophagic cells underwent propidium iodide-positive necrotic death in a delayed fashion, leading to complete inhibition of proliferation. Remarkably, ruthenium red and BAPTA (inhibitors of mitochondrial calcium transport) or cyclosporin A (a cyclophilin D inhibitor) facilitated autophagic cells to die of necrosis. In contrast, 5 mM Tempop-plus-44oC/10 min or 44oC/30 min induced more potently caspase-dependent apoptosis through tBid/Bax-mediated cytochrome c release than Tempo alone. Thus, Tempo is a unique thermosensitizer to synergistically induce apoptosis and autophagic cell death.
