Abstract
Key mechanisms of apopotic cytochrome c (Cytc) efflux are not fully understood. We focused on the roles for ATM and PNA-PKcs signaling, Bax/Bak, and caspase-2 in p53-dependent apoptosis in IR-irradiated murine wild-typeST4 and Scid STH1a pre-T cells (gift of I. Radford) expressing p53. We found that ST4 and STH1a cells underwent the similar highly radiosensitive apoptosis, with a slightly enhanced apoptosis due to defective NHEJ of IR-induced DSBs in the DNA-PKcs mutant STH1a cells. The stabilized p53 protein by ATM but not DNA-PKcs in cells in response to DSBs upregulated BH3-only PUMA and Noxa, which in turn activated Bax to result in Bax oligomerization. Furthermore, we identified the induction of novel Bax (Bak)/VDAC1 hybrid dimer channels in stressed mitochondria. Independently, damage-activated caspase-2 also participated in apoptotic Cytc release, suppressible by the caspase-2 and pan-caspase inhibitors. Thus, the ATM-p53 signal relay plays a pivotal role for downstream mitochondrial apoptosis through PUMA/Noxa-mediated Bax/Bak activation and p53/PIDDosome-mediated caspase-2 activation, both of which mark the highly radiosensitive apoptotic phenotype in both wild-type ST4 and Scid STH1a pre-T cells.
