p53-mediated downregulation of Chk1 abrogates the DNA damage-induced G2M checkpoint in K562 cells, resulting in increased apoptosis.

BCR-ABL confers apoptotic resistance to a range of genotoxic agents, and this protection is mediated in part by prolonging the G2 checkpoint. The p53 tumour suppressor protein regulates the transcription of regulatory genes involved in cell cycle arrest and apoptosis. To investigate the effect of p53 on the BCR-ABL-mediated G2M checkpoint response, we transiently transfected the BCR-ABL-positive, p53-negative cell line K562 with wild-type human p53. The p53-transfected cells showed a decreased ability to arrest in G2 and an increase in apoptosis in response to etoposide treatment, relative to the control mock-transfected cells. p53-transfected and control cells were treated with etoposide and trapped at mitosis with nocodazole. The mitotic index of p53-transfected cells was higher than that of the control cells, which suggests that p53 abrogates the G2 checkpoint response to etoposide treatment in K562 cells. We found that the expression of the cell cycle checkpoint protein Chk1 was reduced in the etoposide-treated p53-transfected cells by 24 h, and this correlated with a reduction in the extent of etoposide-induced phosphorylation of CDK1 at tyrosine 15 (Y15). We conclude, therefore, that p53 overrides the strong G2 checkpoint response to etoposide in K562 cells, by directly or indirectly downregulating Chk1 expression, which, in turn, contributes to the proapoptotic effect of p53.