Cytochrome c induces caspase-dependent apoptosis in intact hematopoietic cells and overrides apoptosis suppression mediated by bcl-2, growth factor signaling, MAP-kinase-kinase, and malignant change.

It has been shown that cytochrome c is released from mitochondria during apoptosis, activates pro-caspase CPP32 (caspase III), and induces DNA fragmentation in mixtures of cytosolic extracts and isolated nuclei. To establish whether cytochrome c can primarily induce apoptosis in intact cells, we used direct electroporation of cytochrome c into murine interleukin-3 (IL-3)-dependent cells. Electroporation of micromolar external concentrations of cytochrome c rapidly induced apoptosis (2 to 4 hours) that was concentration-dependent, did not affect mitochondrial transmembrane potential, and was independent of cell growth. Only certain isoforms of cytochrome c were apoptogenic; yeast cytochrome c and other redox proteins were inactive. Cytochrome c-induced apoptosis was dependent on heme attachment to the apo-enzyme and was completely abolished by caspase inhibitors. Nonapoptogenic isoforms of cytochrome c did not compete for apoptogenic cytochrome c. Although apoptosis induced by IL-3 withdrawal was inhibited by bcl-2 overexpression and expression of an activated MAP-kinase-kinase (MAP-KK), cytochrome c induced apoptosis in the presence of IL-3 signaling, bcl-2 over-expression, expression of activated MAP-KK, and the combined antiapoptotic action of all three. Cytochrome c also induced apoptosis in the leukemic cell line WEHI 3b. However, human HL60 and CEM cells were resistant to cytochrome c-induced apoptosis. HL60 cells did not electroporate, but CEM cells were efficiently electroporated. Our studies with IL-3-dependent cells confirm that the apoptogenic attributes of cytochrome c are identical in intact cells to those in cell extracts. We conclude that cytochrome c can be a prime initiator of apoptosis in intact growing cells and acts downstream of bcl-2 and mitochondria, but that other cells are resistant to its apoptogenic activity. The system described offers a novel, simple approach for investigating regulation of apoptosis by cytochrome c and provides a model linking growth factor signaling to metabolism, survival, and apoptosis control.