Poly (ADP ribose) (PAR) formation catalyzed by PAR polymerase 1 in response to genotoxic stress mediates cell death due to necrosis and apoptosis. PAR glycohydrolase (PARG) has been thought to be the only enzyme responsible for hydrolysis of PAR in vivo. However, we show an alternative PAR-degradation pathway, resulting from action of ADP ribosyl-acceptor hydrolase (ARH) 3. PARG and ARH3, acting in tandem, regulate nuclear and cytoplasmic PAR degradation following hydrogen peroxide (H2O2) exposure. PAR is responsible for induction of parthanatos, a mechanism for caspase-independent cell death, triggered by apoptosis-inducing factor (AIF) release from mitochondria and its translocation to the nucleus, where it initiates DNA cleavage. PARG, by generating protein-free PAR from poly-ADP ribosylated protein, makes PAR translocation possible. A protective effect of ARH3 results from its lowering of PAR levels in the nucleus and the cytoplasm, thereby preventing release of AIF from mitochondria and its accumulation in the nucleus. Thus, PARG release of PAR attached to nuclear proteins, followed by ARH3 cleavage of PAR, is essential in regulating PAR-dependent AIF release from mitochondria and parthanatos.
Keywords: cytotoxicity; posttranslational modification.