S Shimizu

Although apoptosis and necrosis are morphologically distinct manifestations of cell death, apoptosis and some necroses share common features in the death signaling pathway involving functional steps of death-driving interleukin 1beta-converting enzyme family proteases and anti-cell death protein Bcl-2. One evident physiological difference in cells undergoing apoptosis versus necrosis is in intracellular levels of ATP. In this study, we specifically addressed the question of whether apoptosis depends on intracellular ATP levels, since longer incubation under ATP-depleting conditions results in necrotic cell death. Incubation of cells in glucose-free medium with an inhibitor of mitochondrial F0F1-ATPases reduces intracellular ATP levels and completely blocks Fas/Apo-1-stimulated apoptosis. ATP supplied through glycolysis or oxidative phosphorylation restores the apoptotic cell death pathway. ATP depletion also leads to a block in Fas-induced activation of CPP32/Yama(-like) proteases, and when ATP is depleted after the activation of the proteases, subsequent apoptosis is significantly blocked. Thus, ATP-dependent steps exist both upstream and downstream of CPP32/Yama(-like) protease activation in apoptotic signal transduction. Treatment with the calcium ionophore induces apoptosis under ATP-supplying conditions but induces necrotic cell death under ATP-depleting conditions, indicating that ATP levels are a determinant of manifestation of cell death.

Apoptosis, a mechanism involving programmed cell death, is important for normal development and maintenance of tissue homeostasis of multicellular organisms. Apoptotic cells are defined by their fragmented nuclei with condensed chromatin, fragmented or condensed cytoplasm and formation of apoptotic bodies. The apoptotic signal transducing pathways activated by a variety of stimuli, including depletion of growth factors, heat shock, cytokines, DNA damaging reagents and crosslinking of Fas receptor, finally converge into the phylogenically conserved apoptotic main machinery, consisting of death-driving ICE-family proteases and anti-cell death protein Bcl-2. Recently, we noted that necrotic cell death induced by chemical hypoxia shares at least some part of the apoptotic main machinery. Using this system, we have shown that Bcl-2 prevents the loss of the mitochondrial membrane potential observed in both apoptotic and necrotic cell death. We also showed that the ICE protease cascade operates in apoptosis and that Bcl-2 functions upstream of the ICE prolease cascade. Here, we review the signal transducing pathway of the apoptotic main machinery.