Trans-dominant inhibition of poly(ADP-ribosyl)ation leads to decreased recovery from ionizing radiation-induced cell killing.

Poly(ADP-ribosyl)ation is a post-translational modification of nuclear proteins catalysed by poly(ADP-ribose)polymerase (PARP). PARP is strongly activated by DNA strand breaks and is thought to be involved in DNA repair, and various chemical agents that inhibit poly(ADP-ribosyl)ation have radiosensitizing properties. An alternative, highly specific (trans-dominant) inhibition of PARP function has been made possible with a molecular genetic approach where CO60 hamster cells were transfected with the PARP DNA-binding domain (DBD) under the control of a Dexamethasone (Dex)-inducible promoter. Stable transfectants were incubated with or without Dex and the impact of poly(ADP-ribosyl)ation on cellular radiation response (clonogenic survival) was measured following irradiation at high or low dose-rate or when potentially lethal damage (PLD) recovery was allowed. For acute exposures the radiosensitizing effect of PARP inhibition could be confirmed and a large enhancement ratio (calculated from the respective mean inactivation doses) of 2.2 was found for plateau phase cells. Both cellular recovery phenomena (dose-rate sparing and PLD) were decreased in the presence of Dex, and particularly PLD-recovery was nearly completely abolished due to the inhibition of poly(ADP-ribosyl)ation. The latter finding strongly suggests an involvement of PARP in the repair of DNA double-strand breakage.