Caspases and mitochondria in c-Myc-induced apoptosis: identification of ATM as a new target of caspases

The mechanism(s) of c-Myc transcription factor-induced apoptosis is still o bscure. The activation of c-Myc has been found to lead into the processing/ activation of caspases (caspase-3), but the significance of this for the ce ll demise is debatable, Here we report that several targets of caspases (PK C delta, MDM2, PARP, replication factor C, 70 kDa UlsnRNP, fodrin and lamin s) are cleaved during c-Myc-induced apoptosis in Rat-1 MycER(TM) cells, ind icating an important role for caspases in the apoptotic process. We further found that the ATM (ataxia telangiectasia mutated) - protein is a novel ke y substrate of caspases, In in vitro assays, purified recombinant ATM prote in was found to be cleaved by the effector caspases 3 and 7, The functional significance of the ATM cleavage is supported by the finding that ectopic expression of ATM protected in part against apoptosis, We also show that c- Myc-induced apoptosis involves loss of mitochondrial transmembrane potentia l, release of cytochrome c from mitochondria into the cytosol and subsequen t processing of caspase-9, The cleavage of caspase-9 is, hone, er, minimal and a much later event than the processing/activation of caspase-3, suggest ing that it is not the apical caspase, Evidence is provided that there is, nevertheless, an upstream caspase(s) regulating the functions of caspase-3 and mitochondria, Additionally, it mas found that p53 becomes upregulated, together,vith its transcriptional targets MDM2 and p21, upon c-Myc inductio n, but this occurs also at a later time than the activation of caspase-3.