Unscheduled Activation of Cydin B1/Cdc2 Kinase in Human Promyelocytic Leukemia Cell Line HL6O Cells Undergoing Apoptosis Induced by DNA Damage

We have studied changes in cydlinAand Bi-dependent kinases during apoptosis induced In human promyelocytic leukemia (HL6O)cells treated with the topoisomerase I Inhibitor camptothecin. We found that cydlin BIJCdC2 kinase activity transiently increases within 30 mm after camp tothecin treatmentS This increase is followed by a rapid Inactivation of the cydin B1/CdC2 kinase that Is associated with Cdc2 tyrosine phosphoryl ation without any change in Cdc2 or cydlin Bi protein levels. The DNA polymerase inhibitor aphidicolin abrogates camptothecin-induced changes In cyclin B1/CdC2 kinase activity, indicating that DNA replica don-induced DNA damage is essential for both Cdc2 alterations and apoptosis activation. Apoptosis and the initial cyclin B1/CdC2 kinase activation were amplified using synchronized S-phase cells, and cydlin A/cdk2 kinase did not change under these conditions. The same transient activation and subsequent inactivation of cydlin B1JCdC2 kinase were observed after DNA damage by etoposide or bis-(2-chloroethyl)methyl amine hydrochloride. These observations suggest that DNA damage pro motes the transient and unscheduled stimulation of cyclin BIJCdC2 kinase activity in 11L60 cells prior to apoptosis.