Ha Linan, Ceryak Susan, Patierno Steven R
Department of Pharmacology, Program in Molecular and Cellular Oncology, The George Washington University Medical Center, Washington, D. C. 20037, USA.
J Biol Chem. 2003 May 16;278(20):17885-94. doi: 10.1074/jbc.M210560200. Epub 2003 Mar 10.
The ataxia telangiectasia mutated (ATM) protein plays a central role in early stages of DNA double strand break (DSB) detection and controls cellular responses to this damage. Although hypersensitive to ionizing radiation-induced clonogenic lethality, ataxia telangiectasia cells are paradoxically deficient in their ability to undergo ionizing radiation-induced apoptosis. This contradiction illustrates the complexity of the central role of ATM in DNA damage response and the need for further understanding. Certain hexavalent chromium (Cr(VI)) compounds are implicated as occupational respiratory carcinogens at doses that are both genotoxic and cytotoxic. Cr(VI) induces a broad spectrum of DNA damage, but Cr(VI)-induced DSBs have not been reported. Here, we examined the role of ATM in the cellular response to Cr(VI) and found that Cr(VI) activates ATM. We also show that physiological targets of ATM, p53 Ser-15 and Chk2 Thr-68, were phosphorylated by Cr(VI) exposure in an ATM-dependent fashion. We found that ATM-/- cells were markedly resistant to Cr(VI)-induced apoptosis but considerably more sensitive to Cr(VI)-induced clonogenic lethality than wild type cells, indicating that resistance to Cr(VI)-induced apoptosis did not confer a selective survival advantage. However, analysis of long term growth arrest revealed a striking difference: ATM-/- cells were markedly less able to recover from Cr(VI)-induced growth arrest. This indicates that terminal growth arrest is the fate of these apoptosis-resistant cells. In summary, ATM is involved in cellular response to a complex genotoxin that may not directly induce DSBs. Our data suggest that ATM is a major signal initiator for genotoxin-induced apoptosis but, paradoxically, also contributes to maintenance of cell survival by facilitating recovery/escape from terminal growth arrest. The results also strongly suggest that terminal growth arrest is not merely an extended or even irreversible form of checkpoint arrest, but instead an independent and unique cell fate pathway.
共济失调毛细血管扩张症突变（ATM）蛋白在DNA双链断裂（DSB）检测的早期阶段发挥核心作用，并控制细胞对这种损伤的反应。尽管共济失调毛细血管扩张症细胞对电离辐射诱导的克隆形成致死高度敏感，但矛盾的是，它们在经历电离辐射诱导的凋亡方面能力不足。这种矛盾说明了ATM在DNA损伤反应中的核心作用的复杂性以及进一步理解的必要性。某些六价铬（Cr(VI)）化合物在具有基因毒性和细胞毒性的剂量下被认为是职业性呼吸道致癌物。Cr(VI)会诱导广泛的DNA损伤，但尚未有Cr(VI)诱导DSB的报道。在此，我们研究了ATM在细胞对Cr(VI)反应中的作用，发现Cr(VI)激活了ATM。我们还表明，ATM的生理靶点p53丝氨酸15和Chk2苏氨酸68在暴露于Cr(VI)时以ATM依赖的方式被磷酸化。我们发现ATM基因敲除细胞对Cr(VI)诱导的凋亡具有显著抗性，但比野生型细胞对Cr(VI)诱导的克隆形成致死更敏感，这表明对Cr(VI)诱导凋亡的抗性并未赋予选择性生存优势。然而，对长期生长停滞的分析揭示了一个显著差异：ATM基因敲除细胞从Cr(VI)诱导的生长停滞中恢复的能力明显较弱。这表明终末生长停滞是这些抗凋亡细胞的命运。总之，ATM参与细胞对一种可能不直接诱导DSB的复杂基因毒素的反应。我们的数据表明，ATM是基因毒素诱导凋亡的主要信号启动子，但矛盾的是，它也通过促进从终末生长停滞中恢复/逃脱来维持细胞存活。结果还强烈表明，终末生长停滞不仅仅是检查点停滞的延长甚至不可逆形式，而是一种独立且独特的细胞命运途径。
