Mitochondrial cytochrome c release in apoptosis occurs upstream of DEVD-specific caspase activation and independently of mitochondrial transmembrane depolarization.

Mitochondrial cytochrome c, which functions as an electron carrier in the respiratory chain, translocates to the cytosol in cells undergoing apoptosis, where it participates in the activation of DEVD-specific caspases. The apoptosis inhibitors Bcl-2 or Bcl-xL prevent the efflux of cytochrome c from mitochondria. The mechanism responsible for the release of cytochrome c from mitochondria during apoptosis is unknown. Here, we report that cytochrome c release from mitochondria is an early event in the apoptotic process induced by UVB irradiation or staurosporine treatment in CEM or HeLa cells, preceding or at the time of DEVD-specific caspase activation and substrate cleavage. A reduction in mitochondrial transmembrane potential (Deltapsim) occurred considerably later than cytochrome c translocation and caspase activation, and was not necessary for DNA fragmentation. Although zVAD-fmk substantially blocked caspase activity, a reduction in Deltapsim and cell death, it failed to prevent the passage of cytochrome c from mitochondria to the cytosol. Thus the translocation of cytochrome c from mitochondria to cytosol does not require a mitochondrial transmembrane depolarization.