β_II Protein 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. Treatment of synchronized HL60 cells with the highly selective PKC inhibitor chelerythrine chloride leads to profound cell cycle arrest in G2 phase. The cellular effects of chelerythrine are not due either direct or indirect inhibition of the known mitotic regulator, 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 βu 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 phosphorvlation of nuclear lamin B is important in these events.