The Role of NBS1 in the Insulin-Like Growth Factor-1 Signaling

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.