Etoposide-mediated deregulation of the G2M checkpoint in myeloid leukaemic cell lines results in loss of cell survival

The K562 leukaemic cell line expresses an inherent survival signal due to the antiapoptotic properties of Bcr-abl, which is, in part, mediated by prolonging the G2M checkpoint and allowing DNA repair mechanisms to operate post genotoxic insult. Arrest of the cell cycle is mediated by retaining an inactivating state of phosphorylation of cyclin-dependent kinase 1 (Cdk1) on tyrosine 15. Our data confirmed that cell survival in K562 was promoted by cell cycle arrest at G2M in response to the genotoxin etoposide. There was no predicted cell cycle arrest in Bcr-abl-positive derivative cell lines of K562 that did not survive the same genotoxic insult but, paradoxically, Cdk1 tyrosine phosphorylation was enhanced to a higher extent compared with the parental cell line where arrest of the cell cycle was observed. To ascertain that this was not an anomaly of the derivative lines, HL60 cells were treated with concentrations of etoposide that induced arrest of the cell cycle or apoptosis. Only HL60 cells that subsequently underwent apoptosis elicited the same effect of increased Cdk1 tyrosine phosphorylation. It is proposed that the augmented tyrosine phosphorylation status of Cdk1 is associated with the abolition of cell survival, in addition to the previously reported induction of cell cycle arrest in myeloid cell lines.