THE G(2) BLOCK INDUCED BY DNA-DAMAGE - A CAFFEINE-RESISTANT COMPONENTINDEPENDENT OF CDC25C, MPM-2 PHOSPHORYLATION, AND H1 KINASE-ACTIVITY

Treatment of cells with agents that cause DNA damage often results in a delay in G(2). There is convincing evidence showing that inhibition of p34(cdc2) kinase activation is involved in the DNA damage-induced G (2) delay. In this study, we have demonstrated the existence of an add itional pathway, independent of the p34(cdc2) kinase activation pathwa y, that leads to a G(2) arrest in etoposide-treated cells. Both the X- ray-induced and the etoposide-induced G(2) arrest were associated with inhibition of the p34(cdc2) H1 kinase activation pathway as judged by p34(cdc2) H1 kinase activity and phosphorylation of cdc25C. Caffeine treatment restored these activities after either of the treatments. Ho wever, the etoposide-treated cells did not resume cycling, revealing t he presence of an alternative pathway Leading to a G(2) arrest. To exp lore the possibility that this additional pathway involved phosphoryla tion of the MPM-2 epitope that is shared by a large family of mitotic phosphoproteins, we monitored the phosphorylation status of the MPM-2 epitope after DNA damage and after treatment with caffeine. Phosphoryl ation of the MPM-2 epitope was depressed in both X-ray and etoposide-t reated cells, and the depression was reversed by caffeine in both case s. The results indicate that the pathway affecting MPM-2 epitope phosp horylation is involved in the G(2) delay caused by DNA damage. However , it is not part of the caffeine-insensitive pathway leading to a G(2) block seen in etoposide-treated cells.