Involvement of CPP32/Caspase-3 in c-Myc-induced apoptosis.

c-Myc is a transcriptional activator implicated in the control of cell proliferation, differentiation and transformation, but is also involved in the regulation of programmed cell death, apoptosis. Despite intensive research, the molecular mechanisms by which c-Myc triggers and executes cell death remain still elusive. Here, we made use of Rat 1A MycER cells expressing a conditionally active c-Myc protein and tested first the hypothesis that ornithine decarboxylase (ODC), which is a transcriptional target of c-Myc, were a mediator of c-Myc-induced apoptosis. However, our results show that the activity of ODC is not required for the c-Myc-mediated apoptosis to occur in these cells. We also found that the expression of p53, p21waf1/cip1, Bcl-2, Bax, Bcl-xL, Bad and cyclins D1, E, A and B did not show any significant changes following c-Myc induction. But, our studies revealed that the c-Myc induced apoptosis is associated with a specific cleavage of poly(ADPribose) polymerase (PARP), suggesting that a cysteine protease of the ICE/CED-3 family is involved. Moreover, we found that the cysteine protease CPP32/Caspase-3, which is known to cleave PARP, is processed from its inactive form to an active protease composed of 17 and 12 kDa subunits; whilst Ich-1/Caspase-2 belonging to another subset of this protease family was not processed/ activated following c-Myc activation. The activation of CPP32 and apoptotic cell death were inhibited by addition of Z-VAD-fmk, a universal inhibitor of ICE-like proteases. Further, a selective inhibitor of CPP32-like proteases (Z-DEVD-fmk) partly inhibited apoptosis. These results provide evidence that the ICE/CED3-family proteases, CPP32 and likely others, play a critical role in the execution of a nuclear proto-oncogene, c-Myc-induced apoptosis.