Abstract
Parthanatos is a form of programed cell death, which occurs in a PARP1 (PARP) 1-dependent, caspase-independent manner. PARP1 binds DNA-strand breaks via its DNA-binding domain, located near the N-terminus, and then forms poly(ADP-ribose) (PAR) through its catalytic domain, located near the C-terminus; PAR may be attached to the PARP1 automodification domain and nuclear acceptor proteins. Thereafter, some PAR polymers are translocated to the cytoplasm, where they serve as signaling molecules to provide nuclear information to the cytoplasm and mitochondria. After translocation to the cytoplasm, PAR polymers bind to the PAR-binding motif of apoptosis-inducing factor (AIF) and hexokinase 1. PAR promotes the cleavage and release of AIF from mitochondrial membranes. AIF then translocates to the nucleus where it induces DNA fragmentation. In addition, PAR binding to hexokinase 1 inhibits glycolysis, resulting in ATP depletion. A key step in the parthanatos pathway is PAR translocation from the nucleus to the cytoplasm, but its mechanism is unknown.
In apoptosis, PARP1 is cleaved by caspases 3 and 7 at its caspase-cleavage site into 24-kDa and 89-kDa fragments. The 24-kDa PARP1 fragment contains the DNA-binding motif and the nuclear localization signal, while the 89-kDa PARP1 fragment contains the automodification and catalytic domains.
In the current study, we found that parthanatos occurs downstream of caspase-dependent reactions following exposure of Hela cells to staurosporine and actinomycin D, conventional apoptosis inducers. Caspase activation induced PARP1 fragmentation, generating 89-kDa and 24-kDa PARP1 fragments. This fragmentation may occur after poly(ADP-ribosyl)ation of PARP1. Activation of caspases then generates 89-kDa PARP1 fragments with attached PAR polymers containing the automodification and catalytic domains, which translocated to the cytoplasm, while 24-kDa fragments were associated with DNA lesions. In the cytoplasm, AIF binding to PAR and its translocation required the association with PAR polymers attached to the 89-kDa PARP1 fragment. Thus, the 89-kDa PARP1 fragment serves as a carrier of PAR from the nucleus to the cytoplasm to induce parthanatos in caspase-dependent apoptosis.
