The p53 tumor suppressor protein is phosphorylated by a number of DNA damage-inducible kinases, such as ATM, and is a key effector of the DNA damage response by promoting cell cycle arrest or apoptosis. Deregulation of the Rb-E2F1 pathway also results in the activation of p53 and promotion of apoptosis and this contributes to the suppression of tumor development. Here we describe a novel connection between E2F1 and the ATM DNA damage response pathway. In primary human fibroblasts lacking functional ATM, the ability of E2F1 to induce the phosphorylation of p53 and apoptosis is impaired. In contrast, ATM status has no effect on transcriptional activation of target genes or the stimulation of DNA synthesis by E2F1. Cells containing mutant Nijmegen breakage syndrome protein (NBS1), a component of the Mre11-Rad50 DNA repair complex, also have attenuated p53 phosphorylation and apoptosis in response to E2F1 expression. Moreover, the ATM pathway effector kinase Chk2 is phosphorylated at threonine 68 and thereby activated as a result of E2F1 expression in an ATM and NBS1 dependent manner. Despite extensive involvement of a classic DNA damage response pathway in E2F1 mediated apoptosis, our observations suggest that E2F1 does not activate the ATM signaling pathway by inducing DNA damage. Delayed histone H2AX phosphorylation and the absence of ATM autophosphorylation at serine 1981 in response to E2F1 expression suggest that E2F1 stimulates ATM through a unique mechanism that is distinct from agents that cause DNA double-strand breaks. These findings identify new roles for several DNA damage response factors by demonstrating that they also participate in the oncogenic stress signaling pathway between E2F1 and p53.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]