Inhibition of p34cdc2Kinase Activation, p34cdc2Tyrosine Dephosphorylation, and Mitotic Progression in Chinese Hamster Ovary Cells Exposed to Etoposide1

II.M"''kinase, an enzyme essential for mitosis in mammalian cells, may play a role in etoposide-induced G2 phase arrest of Chinese hamster ovary cells. In this study, etoposide is shown to cause inhibition of a specific p3-4"''-'kinase activation pathway, that of tyrosine dephosphorylation. Exposure of asynchronous!}' dividing cells to etoposide caused a simultaneous rapid decline of both mitotic index and p.V4"''-'kinase activity, suggesting that the kinase was not activated and that the arrest point was in late »... phase. Using synchronized cells, p34cdc2kinase exhibited maximal activity at the Gz/M transition. Activation of the kinase and the onset of mitosis were accompanied by increased electrophoretic mobility and tyrosine dephosphorylation of the p34"'' ' protein. A 1-h exposure to etoposide during early <•.. phase inhibited p34cdc2 kinase activation, its shift in electrophoretic mobility, and its tyrosine dephosphorylation, all of which correlated with a delay in mitotic pro gression. The interaction between the p.M"'"' and cyclin B proteins appeared unaffected under etoposide exposure conditions which resulted in greater than 70% inhibition of p.M"''' kinase activity and almost complete cessation of transition into mitosis. These data suggest that mammalian cells express a DNA damage-responsive mechanism which controls mitotic progression at the level of p34rfc2 tyrosine