Apoptosis in the absence of caspase 3

MCF-7 human breast Cancer cells do not express Caspase 3, thought by some to be a critical component of the Apoptosis cascade. Nonetheless, both mock- and bcl-2-transfected MCF-7 cells undergo Apoptosis after treatment with a variety of stimuli, including the DNA-cleaving antimitotic agent, neocarzinostatin (NCS). Transfection with Bcl-2 shifts the concentration-response curve to NCS but does not change the phenomenology of Apoptosis when it occurs. In both cases, NCS treatment results in condensation and fragmentation of MCF-7 cell nuclei and release of cytochrome c from the mitochondria to the cytosol. This Apoptosis is accompanied by decreased levels of Bcl-2 and increased levels of Bax. Using a series of Caspase inhibitors with overlapping specificities, enzyme-specific chromogenic substrates, and an antibody specific for activated Caspase 7, we have determined that Apoptosis in MCF-7 cells proceeds via sequential activation of caspases 9, 7 and 6. P21 is detected only after activation of Caspase 7, and P53 is neither expressed at baseline nor up-regulated with Apoptosis induction. This pathway bypasses the need for activated Caspase 3 in these cells.