The epidermal growth factor receptor modulates DNA double-strand break repair by regulating non-homologous end-joining

In mammalian cells repair of radiation-induced DNA damage appears to be also controlled by the epidermal growth factor receptor (EGFR) with a special impact on DNA double-strand break (DSB) repair. Aim of this study was to demonstrate this interaction between EGFR signalling and DNA DSB repair and to identify the underlying downstream pathways. We especially wanted to know in how far non-homologous end-joining (NHEJ) as the most important DSB repair pathway is involved in this interaction. Overall DSB repair was determined by counting γH2AX foci remaining 24 after irradiation, while NHEJ activity was monitored by using a specially designed repair construct stably integrated into the genome. The overall DSB repair capacity was clearly enhanced when EGFR was activated by its natural ligand EGF and, vice versa, was reduced when EGFR was blocked either by the specific antibody Cetuximab or the tyrosine kinase inhibitor erlotinib, whereby reduction was clearly stronger for erlotinib. There was also a difference in the pathways affected. While erlotinib lead to a block of both, MAPK as well as AKT signalling, Cetuximab only affected MAPK. As demonstrated by specific inhibitors (PD98059, AKTIII) EGFR interacts with DSB repair mostly via MAPK pathway. Also for NHEJ activity, there was a substantial increase, when EGFR was activated by EGF as determined for two different reporter cell lines (A549.EJ and H1299.EJ) and, vice versa, a reduction was seen when EGFR signalling was blocked by Cetuximab or erlotinib. There was, however, no difference for the two inhibitors used. This regulation of NHEJ by EGFR was only blocked when ERK was affected by siRNA but not when AKT was knocked down. These data indicate that EGFR modulates DSB repair by regulating NHEJ via MAPK signalling.