Abrogation of the S Phase DNA Damage Checkpoint Results in S Phase Progression or Premature Mitosis Depending on the Concentration of 7-Hydroxystaurosporine and the Kinetics of Cdc25C Activation

DNA damage causes cell cycle arrest in G1, S, or G2 to prevent replication on damaged DNA or to prevent aberrant mitosis. The G1 arrest requires the p53 tumor suppressor, yet the topoisomerase I inhibitor SN38 induces p53 after the G1 checkpoint such that the cells only arrest in S or G2. Hence, SN38 facilitates comparison of p53 wild-type and mutant cells with regard to the efficacy of drugs such as 7-hydroxystaurosporine (UCN-01) that abrogate S and G2 arrest. UCN-01 abrogated S and G2arrest in the p53 mutant breast tumor cell line MDA-MB-231 but not in the p53 wild-type breast line, MCF10a. This resistance to UCN-01 in the p53 wild-type cells correlated with suppression of cyclins A and B. In the p53 mutant cells, low concentrations of UCN-01 caused S phase cells to progress to G2 before undergoing mitosis and death, whereas high concentrations caused rapid premature mitosis and death of S phase cells. UCN-01 inhibits Chk1/2, which should activate the mitosis-inducing phosphatase Cdc25C, yet this phosphatase remained inactive during S phase progression induced by low concentrations of UCN-01, probably because Cdc25C is also inhibited by the constitutive kinase, C-TAK1. High concentrations of UCN-01 caused rapid activation of Cdc25C, which is attributed to inhibition of C-TAK1, as well as Chk1/2. Hence, UCN-01 has multiple effects depending on concentration and cell phenotype that must be considered when investigating mechanisms of checkpoint regulation.