CIS‐diamminedichloroplatinum(II) inhibits p34cdc2 protein kinase in human lung‐cancer cells

cis‐Diamminedichloroplatinum(II) (CDDP) induced G2‐phase arrest in PC‐9 human cancer cells. To elucidate how CDDP acts on cell‐cycle regulation, we analyzed the effect of CDDP on cell‐cycle regulators such as p34cdc2 protein kinase. p34cdc2 protein kinase activity was maximum in G2 phase and decreased after G2/M transition in synchronized PC‐9 human lung cancer cells. Evidence for a phosphorylated p34cdc2 protein kinase complexed with cyclin B was obtained from cells in G2 phase and the p34cdc2 protein kinase appeared to be dephosphorylated at M phase. After exposure to CDDP in G, phase, PC‐9 cells were arrested in G2 phase. The activation of p34cdc2 protein kinase was inhibited by CDDP. Cyclin A and wee‐1 kinase were not affected by the exposure to CDDP. Cyclin B was degraded in M phase in PC‐9 cells. Exposure to CDDP did not affect the degradation of cyclin B. Our data suggest that the effect of CDDP on cell‐cycle phase might be regulated by the dephosphorylation of p34cdc2 protein kinase. To determine whether the p34cdc2 protein kinase is a primary target for CDDP, we examined the direct effect of CDDP on tyrosine dephosphorylation of p34cdc2 protein kinase in cellular extracts. Cell lysates from synchronized PC‐9 in G2 phase were immunoprecipitated with p 13‐Sepharose beads. In vitro dephosphorylation of phosphotyrosine of p34cdc2 protein kinase was observed after exposure to okadaic acid in a concentration‐dependent manner. The dephosphorylation of p34cdc2 protein kinase by okadaic acid was inhibited by CDDP. We hypothesize that inhibition of p34cdc2 dephorphorylation by CDDP is important for its growth‐inhibiting properties.