[Abstract unavailable

ATM and PARP-1 are two of the most important players in the cell's response to DNA damage. PARP-1 and ATM recognize and bound to both single and double strand DNA breaks in response to different triggers. Here we report that ATM and PARP-1 form a molecular complex in vivo in undamaged cells and this association increases after γ-irradiation. ATM is also modified by PARP-1 during DNA damage. We have also evaluated the impact of PARP-1 absence or inhibition on ATM-kinase activity and have found that while PARP-1 deficient cells display a defective ATM-kinase activity and reduced γ-H2AX foci formation in response to γ-irradiation, PARP inhibition on itself is able to activate ATM-kinase. PARP inhibition induced γ H2AX foci accumulation, in an ATM-dependent manner. Inhibition of PARP also induces DNA double strand breaks which were dependent on the presence of ATM. As consequence ATM deficient cells display an increased sensitivity to PARP inhibition. In summary our results show that while PARP-1 is needed in the response of ATM to gamma irradiation, the inhibition of PARP induces DNA double strand breaks (which are resolved in and ATM-dependent pathway) and activates ATM kinase.

To ensure the accurate transmission of genetic information in dividing cells, specific biochemical pathways maintain integrity. Fundamental to these pathways is the recognition, by specific proteins, of genomic lesions, which signal the presence of DNA damage to other nuclear and cytoplasmic factors. DNA strand breaks, generated either directly by genotoxic agents (oxygen radicals, ionizing radiations, or monofunctional alkylating agents) or indirectly after enzymatic incision of a DNA-base lesion, trigger the synthesis of poly(ADP-ribose) by the enzyme poly(ADP-ribose) polymerase (PARP [E.C.2.4.2.30]). PARP is a nuclear zinc-finger DNA-binding protein that detects DNA strand breaks. At a breakage site, PARP catalyzes the transfer of the ADP-ribose moiety, from the respiratory coenzyme NAD ϩ to a limited number of protein acceptors. These PARP substrates may influence chromatin architecture, as with histones H1, H2B, and lamin B, or they may act in DNA metabolism, as with DNA-replication factors and PARP itself (reviewed by de Murcia and Menissier-de Murcia 1994; Oei et al. 1997). Because of the high negative charge on ADP-ribose polymers (fig. 1A), poly(ADP-ribosylated) proteins lose their affinity for DNA and hence, in many cases, their biological activities. PARP and other modified proteins may be restored to their native state after poly(ADP-ribose) glycohydrolase. Therefore, poly(ADP-ribosylation) is an immediate posttranslational modification of nuclear DNA-binding proteins, induced by DNA damaging agents. The physiological role of PARP has been much debated during this past decade, but molecular and genetic