Decreased cyclin B1 expression contributes to G(2) delay in human brain tumor cells after treatment with camptothecin

DNA damage produces delayed mitosis (G(2)/M delay) in proliferating cells, and shortening the delay sensitizes human malignant glioma and medulloblast oma cells to cytotoxic chemotherapy. Although activation of the cyclin-depe ndent kinase CDC2 mediates G(2)/M transition in all tumor cells studied to date, regulation of CDC2 varies between tumor types. Persistent hyperphosph orylation of kinase and reduced cyclin expression have been implicated as m ediators of treatment-induced G(2) delay in different tumor models. To eval uate regulation of G(2)/M transition in human brain tumors, we studied the expression and/or activity of CDC2 kinase and cyclins A and B1 in U-251 MG and DAOY medulloblastoma cells after their treatment with camptothecin (CPT ). Synchronized cells were treated during S phase, then harvested at predet ermined intervals for evaluation of cell cycle kinetics, kinase activity mR NA, and protein expression. CPT produced G(2) delay associated with decreas ed CDC2 kinase activity and cyclin B1 expression. Kinase activity was assoc iated with CDC2 bound to cyclin B1, not cyclin A, in both cell lines. Cycli n A mRNA and protein expression were reduced after CPT treatment; however, decreased protein expression was short lived and moderate in the glioma and primitive neuroectodermal tumor/medulloblastoma cells, respectively. We co nclude that G(2) delay is a common response of brain tumor cells to chemoth erapy with topoisomerase I inhibitors and that a mechanism of this delay ma y he reduced expression of cyclin B1.