β_IIprotein kinase C is required for the G₂/M phase transition of cell cycle

Entry into mitosis requires the coordinated action of multiple mitotic protein kinases. In this report, we investigate the involvement of protein kinase C in the control of mitosis in human cells. Treatment of synchronized HL60 cells with the highly selective protein kinase C (PKC) inhibitor chelerythrine chloride leads to profound cell cycle arrest in G₂ phase. The cellular effects of chelerythrine are not due to either direct or indirect inhibition of the known mitotic regulator p34^cdc2/cyclin B kinase. Rather, several lines of evidence demonstrate that chelerythrine-mediated G² phase arrest results from selective inhibition and degradation of β_II protein kinase C. First, chelerythrine causes dose-dependent inhibition of β_II PKC in vitro with an IC₅₀ identical to that for G₂ phase blockade in whole cells. Second, chelerythrine specifically inhibits β_II PKC-mediated lamin B phosphorylation and mitotic nuclear lamina disassembly. Third, chelerythrine leads to selective loss of β_II PKC during G₂ phase in synchronized cells. Fourth, chelerythrine mediates activation-dependent degradation of PKC, indicating that β_II PKC is selectively activated during G₂ phase of cell cycle. Taken together, these data demonstrate that β_II PKC activation at G₂ phase is required for mitotic nuclear lamina disassembly and entry into mitosis and that β_II PKC-mediated phosphorylation of nuclear lamin B is important in these events.