Generation of diacylglycerol molecular species through the cell cycle: a role for 1-stearoyl, 2-arachidonyl glycerol in the activation of nuclear protein kinase C-βII at G2/M

Protein kinase C (PKC) is a family of 11 isoenzymes that are differentially involved in the regulation of cell proliferation. PKC-βII, a mitotic lamin kinase, has been shown previously to translocate to the nucleus at G₂/M and this was coupled to the generation of nuclear diacylglycerol. However, it is not clear how isoenzyme selective translocation and nuclear targeting is achieved during cell cycle. To investigate further the role of nuclear diacylglycerol we measured PKC isoenzyme translocation and analysed diacylglycerol species at different stages of the cell cycle in U937 cells synchronized by centrifugal elutriation. Translocation of PKC-βII to the membrane fraction, an indicator of activation, occurred at S and G₂/M, although PKC-βII was targeted to the nucleus only at G₂/M. Levels of nuclear diacylglycerol, specifically tetraunsaturated species, increased during G₂/M. By contrast, there were no obvious changes in nuclear phosphatidic acid species or mass. 1-stearoyl, 2-arachidonyl glycerol (SAG), the major polyunsaturated nuclear diacylglycerol, was able to activate classical PKC isoenzymes (PKC-α andβ), but was less effective for activation of novel isoenzymes(PKC-δ), in an in vitro PKC assay. We propose that PKC-βII nuclear translocation during G₂/M phase transition is mediated in part by generation of SAG at the nucleus.