The Chk1 protein kinase and the Cdc25C regulatory pathways are targets of the anticancer agent UCN-01

A checkpoint operating in the G(2) phase of the cell cycle prevents entry i nto mitosis in the presence of DNA damage, UCN-01, a protein kinase inhibit or currently undergoing clinical trials for cancer treatment, abrogates G(2 ) checkpoint function and sensitizes p53-defective cancer cells to DNA dama ging agents. In most species, the G(2) checkpoint prevents the Cdc25 phosph atase from removing inhibitory phosphate groups from the mitosis-promoting kinase Cdc2. This is accomplished by maintaining Cdc25 in a phosphorylated form that binds 14-3-3 proteins. The checkpoint kinases, Chk1 and Cds1, are proposed to regulate the interactions between human Cdc25C and 14-3-3 prot eins by phosphorylating Cdc25C on serine 216. 14-3-3 proteins, in turn, fun ction to keep Cdc25C out of the nucleus. Here we report that UCN-01 caused loss of both serine 216 phosphorylation and 14-3-3 binding to Cdc25C in DNA -damaged cells. In addition, UCN-01 potently inhibited the ability of Chk1 to phosphorylate Cdc25C in vitro. In contrast, Cds1 was refractory to inhib ition by UCN-01 in vitro, and Cds1 was still phosphorylated in irradiated c ells treated with UCN-01. Thus, neither Cds1 nor kinases upstream of Cds1, such as ataxia telangiectasia-mutated, are targets of UCN-01 action in vivo . Taken together our results identify the Chk1 kinase and the Cdc25C pathwa y as potential targets of G(2) checkpoint abrogation by UCN-01.