Department of Biochemistry, Boston University School of Medicine, Boston, MA, USA.
Oncogene. 2010 Apr 1;29(13):1952-62. doi: 10.1038/onc.2009.480. Epub 2010 Jan 11.
Knockout of heat shock protein Hsp72 was shown to promote chromosomal instability and increase radiation sensitivity of mouse fibroblasts. Here, we report that downregulation of Hsp72 in human tumor cells leads to suppression of a specific branch of the DNA damage response (DDR) that facilitates DNA repair following genotoxic insults, that is, reduced accumulation of the phosphorylated form of histone H2AX (gammaH2AX). This inhibition was due to decreased expression of H2AX as well as higher rate of gammaH2AX dephosphorylation. Formation of gammaH2AX and MDC1 radiation-induced foci was impaired in Hsp72-depleted cells, which in turn enhanced DNA damage, resulting in sensitization of cells to gamma-radiation and doxorubicin. These effects of Hsp72 knockdown were dependent on activation of the p53/p21-signaling pathway. Overall, permanent activation of the p53/p21 signaling in Hsp72-depleted cells specifically impaired the gammaH2AX pathway of the DDR, enhanced DNA damage following genotoxic insults, and led to further stimulation of the p53/p21 pathway, thus creating a positive feedback loop. The resulting strong induction of p21 precipitated senescence following exposure to DNA-damaging agents, thus accounting for higher sensitivity of cells to genotoxic stresses.
敲除热休克蛋白 Hsp72 被证明可促进染色体不稳定性并增加小鼠成纤维细胞对辐射的敏感性。在这里，我们报告说，人肿瘤细胞中 Hsp72 的下调导致促进 DNA 修复的特定 DNA 损伤反应 (DDR) 分支受到抑制，即，磷酸化组蛋白 H2AX（γH2AX）的积累减少。这种抑制是由于 H2AX 的表达降低以及 γH2AX 去磷酸化的速度更快所致。在 Hsp72 耗尽的细胞中，γH2AX 和 MDC1 辐射诱导焦点的形成受损，这反过来又增强了 DNA 损伤，导致细胞对 γ 辐射和阿霉素的敏感性增加。Hsp72 敲低的这些影响取决于 p53/p21 信号通路的激活。总的来说，在 Hsp72 耗尽的细胞中 p53/p21 信号的永久激活特异性地破坏了 DDR 的 γH2AX 途径，增强了遗传毒性应激后的 DNA 损伤，并进一步刺激了 p53/p21 途径，从而形成了正反馈回路。p21 的强烈诱导导致 DNA 损伤剂暴露后衰老，从而导致细胞对遗传毒性应激的敏感性增加。
