ABSTRACT
PARP-/– Fibroblasts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69 4.3 Function of PARP in Endotoxin-Induced Septic Shock . . . . . . . . . . . . . . . .73 4.4 Human PARP Protects PARP Knockout Mice from High Doses
of γ Radiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75 4.5 Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77 Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Poly(ADP-ribosyl)ation is an important post-translational modification of nuclear proteins induced by DNA damage. Poly(ADP-ribose) polymerase (PARP, or PARP1, EC 2.4.2.30) is a constitutive chromatin-associated protein representing the major cellular poly(ADP-ribosyl)ating activity.1 The enzyme is found in most eukaryotes with the exception of yeast. In response to DNA damage induced by alkylating agents, ionizing radiation, or free radicals, PARP binds rapidly to DNA strand breaks
and undergoes automodification by forming long branched ADP-ribose polymers using NAD+ as a substrate.2 The polymer synthesis seems to occur quantitatively according to the number of DNA strand breaks.1 Therefore, overactivation of PARP induced by massive DNA damage can cause the depletion of intracellular NAD+. The negative charge of ADP-ribose polymers causes auto-modified PARP to be subsequently dissociated from DNA ends, allowing the DNA repair process to take place.2 Poly(ADP-ribose) polymers are short-lived in vivo because they are rapidly degraded by poly(ADP-ribose) glycohydrolase (PARG).3 Studies using specific inhibitors of PARP and a dominant-negative mutant of the enzyme, as well as cell lines devoid of PARP activity, have indicated that PARP and poly(ADP-ribosyl)ation play a multifunctional role in a range of cellular processes such as cell proliferation and replication, stress response, cell toxicity and apoptosis, DNA repair and recombination, as well as the maintenance of chromosomal stability 1,4,5
Together with chemical inhibitor studies, the loss-of-function and overexpression approaches have been useful to study the biological function of PARP in vivo6-13 These studies have shown that constitutive overexpression of a dominant-negative PARP mutant (DNA-binding domain) renders cells’ hypersensitive to alkylating agents and chromosomal instability7,10 and interferes with tumor formation because of increased apoptosis.14 Perhaps the most powerful approach was the generation of PARP knockout mice by gene targeting,15-17 which proved to be a valuable tool in determining biological functions of PARP.
