Activation of Cell Death Mediated by Apoptosis-Inducing Factor Due to the Absence of Poly(ADP-ribose) Glycohydrolase

We previously demonstrated that the absence of poly(ADP-ribose) glycohydrolase (PARG) led to increased cell death following DNA-damaging treatments. Here, we investigated cell death pathways following UV treatment. Decreased amounts of PARG-null embryonic trophoblast stem (TS) cells were observed following doses of 10-100J/m(2) as compared to wild-type cells, in wild-type cells, caspase-cleaved poly(ADPribose) polymerase-1 (PARP-I) and activated caspase-3 were detected 12-24 h after UV treatment. Surprisingly, both were detected at decreased levels only after 24 h in PARG-null TS cells, indicating a decreased level and delayed presence of caspase-mediated events. Further, a time- and dose-dependent accumulation of poly(ADP-ribose) (PAR) levels after UV was observed in PARG-null TS cells and not in wild-type cells. Determination of the levels of nicotinamide adenine dinucleotide (NAD(+)), the substrate for PAR synthesis and a coenzyme in cellular redox reactions, demonstrated a UV dose-dependent decrease in the level of NAD(+) in wild-type cells, while NAD(+) levels in PARG-null TS cells remained at higher. levels. This indicates no depletion of NAD+ in PARG-null TS cells following increased levels of PAR Lastly, cell death mediated by apoptosis-inducing factor (AlF) was analyzed because of its dependence on increased PAR levels. The results demonstrate nuclear AlF translocation only in PARG-null TS cells, which demonstrates the presence of AIF-mediated cell death. Herein, we provide compelling evidence that the absence of PARC leads to decreased caspase-3 activity and the specific activation of AIF-mediated cell death. Therefore, the absence of PARC may provide a strategy for specifically inducing an alternative apoptotic pathway.