Abstract
Exposure to ultraviolet (UV) light causes several types of DNA damage in cells. Unrepaired lesions encountered by the DNA replication machinery during S-phase cause stalling of the replication fork, which may lead to cell death unless DNA synthesis resumes. Recruitment of RAD18 to stalled replication forks facilitates monoubiquitination of PCNA during S-phase, promoting translesion synthesis at the UV irradiation-induced DNA damaged sites.
Recently, it was demonstrated that RAD18-null HCT116 cells are sensitive to both X-ray irradiation and camptothecin, due to the defective repair of single-strand DNA breaks that arise during S- phase (Shiomi et al, 2007). These findings suggest that RAD18 is involved in the cellular response to DSBs via PCNA- and Polh-independent mechanisms. Therefore, in the present study, we investigated the regulation and function of RAD18 in response to DSBs. In cells exposed to DSB-inducing agents, many proteins involved in the DNA damage-response pathway, including g-H2AX, BRCA1, 53BP1, NBS1 and phosphorylated ATM accumulate in nuclear foci. These foci have been designated IR-induced nuclear foci (IRIFs). g-H2AX, which is immediately phosphorylated at its C-terminus in response to DSBs, plays a crucial role in regulating IRIF formation. IRIF formation is thought to be vital for cellular DNA repair and the activation of DNA-damage checkpoints. The function of RAD18 in IR-irradiated cells and the potential targets of its E3 ligase activity are unknown. Therefore, in the present study we sought to identify the role of RAD18 in the DSB response, and to determine the relationship between IRIFs and RAD18 foci.
