Abstract

The Nbs1 protein (nibrin, p95) is a member of the DNA repair/checkpoint complex Mre11/Rad50/Nbs1 (MRN), which plays a critical role in the cellular responses to DNA damage, cell cycle checkpoints, and telomere and genome stability. Many transgenic models in mice and clinical symptoms of NBS patients have clearly shown that Nbs1 exerts pleiotropic actions in growth and development of mammals. However, the molecular role of Nbs1 in mitogenic signaling pathways which could explain the growth retardation, developmental defects and impaired proliferation capacity of NBS patient cells has not been demonstrated, so far. This study shows that after repression of endogenous Nbs1 levels using short interference RNA, hTERT-immortalized RPE cells exhibit decreased proliferation ability and poor response to IGF-1 stimulation. After release from G1 arrest, NBS1 siRNA-transfected cells display disturbances in periodical oscillations of cyclin E and A, and delayed cell cycle progression. Remarkably, lower phosphorylation levels of c-Raf, and diminished activity of ERK1/2 in response to IGF-1 suggest a link between NBS1, IGF-1 signaling, and Ras/Raf/MEK/ERK cascade. The functional relevance of NBS1 in mitogenic signaling and initiation of cell cycle progression are demonstrated in NBS1 siRNA-transfected cells where IGF-1 has a limited capacity to induce expressions of FOS and CCND1. The impact of NBS1 on the IGF-1 signaling cascade is finally identified by the reduction of IGF1R, SOS1 and SOS2 expression in NBS1 siRNA-transfected cells. The disturbed IGF-1 signaling, a consequence of diminished expression of the key components of the cascade, results in a failure of IGF-1 to rescue NBS1 siRNA-transfected cells from gamma radiation-induced cell death. In conclusion, this study provides the first evidence that, by modulating the IGF-1 signaling cascade, NBS1 has a functional role in the promotion of cell cycle progression, cell proliferation, and cellular radio-resistance in addition to its well known function for proper DNA double strand break signaling.