DNA damage causes cell cycle arrest in G(1), S, or G(2) to prevent replication on damaged
DNA or to prevent aberrant mitosis. The G(1) arrest requires the p53
tumor suppressor, yet the
topoisomerase I inhibitor SN38 induces p53 after the G(1) checkpoint such that the cells only arrest in S or G(2). 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 G(2) arrest.
UCN-01 abrogated S and G(2) arrest 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 G(2) 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.