Julia A. Yaglom

Summary: Activation of the Her2 (ErbB2) oncogene is implicated in the development of breast, ovary and other cancers. Here, we show that expression of NeuT, a mutant‐activated rodent isoform of Her2, in immortalized breast epithelial cells, while promoting senescence‐associated morphological changes, up‐regulation of senescence‐associated β‐galactosidase activity, and accumulation of the cyclin‐dependent kinase inhibitor p21, failed to trigger the major senescence end‐point, i.e. permanent growth arrest. Similar senescence‐associated phenotype with incomplete growth arrest, which we dubbed senescence with incomplete growth arrest (SWING), could also be triggered by the expression of the Ras oncogene. SWING phenotype was stable, and persisted in tumor xenografts established from NeuT‐transduced cells. Furthermore, a significant population of cells in SWING state was found in tumors in the MMTV/NeuT transgenic mouse model. SWING cells showed downregulation of histone H2AX, critical for repair of double‐stranded DNA breaks, and impaired activation of Chk1 kinase. Overall, SWING cells were characterized by increased DNA instability and hypersensitivity to genotoxic stresses. We propose that the SWING state could be a stage in the process of cancer development.

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 (γH2AX). This inhibition was due to decreased expression of H2AX as well as higher rate of γH2AX dephosphorylation. Formation of γH2AX and MDC1 radiation-induced foci was impaired in Hsp72-depleted cells, which in turn enhanced DNA damage, resulting in sensitization of cells to γ-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 γH2AX 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.