Although Erk kinase has been recently reported to function in the DNA damage response, the mechanism governing this process is unknown. We report here that hydroxyurea (HU) activates Erk via MEK1, a process that is sensitized by a constitutively active MEK1 (MEK1Q56P) and attenuated by a dominant-negative MEK1 (MEK1K97M). While ectopic MEK1Q56P sensitized HU-induced S-phase arrest, inhibition of Erk activation via U0126, PD98059, and MEK1K97M attenuated the arrest, and thereby enhanced cells to HU-induced toxicity. Taken together, we demonstrate an important contribution of Erk to the activation of the S-phase DNA damage checkpoint. This can be attributed to Erk's regulatory role in modulating ATR function. Inhibition of Erk activation with U0126/PD98059 and MEK1K97M substantially reduced HU-induced ATR nuclear foci, leading to a dramatic reduction of cH2AX and its nuclear foci. Reduction of MEK1 function by a small interference RNA (siRNA) MEK1 and ectopic MEK1K97M significantly decreased HU-induced cH2AX. Conversely, ectopic MEK1Q56P enhanced cH2AX foci. Furthermore, immunofluorescent and cell fractioning experiments revealed cytosolic and nuclear localization of ATR. HU treatment caused the redistribution of ATR from the cytosol to the nucleus, a process that is inhibited by U0126. Collectively, we show that Erk kinase modulates HU-initiated DNA damage response by regulating ATR function.

While p14(ARF) suppression of tumorigenesis in a p53-dependent manner is well studied, the mechanism by which p14(ARF) inhibits tumorigenesis independently of p53 remains elusive. A variety of factors have been reported to play a role in this latter process. We report here that p14(ARF) displays different effects on the anchorage-dependent and -independent growth of p53-null/Mdm2 wild type cells. p14(ARF) blocks both the anchorage-dependent and-independent (soft agar) proliferation of 293T and p53(-/-) HCT116, but not p53-null H1299 lung carcinoma cells. While p14(ARF) had no effect on the anchorage-dependent proliferation of p53(-/-) MEFs and Ras12V-transformed p53(-/-) MEFs, it inhibited the growth of Ras12V-transformed p53(-/-) MEFs in soft agar. Furthermore, ectopic expression of p14(ARF) did not lead to degradation of the E2F1 protein and did not result in the reduction of E2F1 activity detected by two E2F1 responsible promoters, Apaf1 and p14(ARF) promoter, in 293T, p53(-/-) HCT116, and H1299 cells. This is consistent with our observations that p14(ARF) did not result in G1 arrest, but induced apoptosis via Bax up-regulation. Taken together, our data demonstrate that the response of p53-null cells to ARF is cell type dependent and involves factors other than Mdm2 and E2F1.

Bcl-2 can both promote and attenuate tumorigenesis. Although the former function is relatively well characterized, the mechanism of the latter remains elusive. We report here that enforced Bcl-2 expression in MCF7 cells stabilizes p53, induces phosphorylation of p53 serine 15 (p53pSer15) and inhibits MCF7 cell growth. Consistent with p53 Ser15 being a target of ataxia telangiectasia mutated protein(ATM)/ATR (ATM-and rad3-related) in the DNA damage response, Bcl-2 activates ATM by inducing ATM Ser1981 phosphorylation, which is accompanied with the phosphorylaton of two additional ATM substrates, Chk2 Thr68 and H2AX Ser139. Downregulation of ATM using a specific small interference RNA fragment (ATMRNAi) abolished Bcl-2-induced p53pSer15 and Bcl-2-mediated growth inhibition of MCF7 cells. Ectopic expression of a dominant-negative p53 mutant, p53175H, partially rescued this growth inhibition. Taken together, these observations demonstrate the contribution of ATM-p53 function to Bcl-2-mediated inhibition of MCF7 cell growth, indicating an ATM-mediated surveillance system for regulating Bcl-2 overexpression. Consistent with this concept, we found that MCF7 cells express Bcl-2 heterogeneously with 34.5% of cells being Bcl-2 negative. In general, Bcl-2-positive MCF7 cells proliferate slower than those of Bcl-2 negative. Thus, we provide evidence suggesting that activation of ATM suppresses Bcl-2-induced tumorigenesis, and that attenuation of ATM function may be an important event in breast cancer progression.

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