In this study we report that protein kinase C j j (PKC j j), one of the atypical isoforms of the PKC family located predominantly in cytosol, is redistributed by C2-ceramide treatment in isolated hepatocytes. PKC j j increased in membrane and nuclear fractions after 30 min of treatment with C2-ceramide in a dose-and time-dependent manner. The action of C2-ceramide was inhibited by wortmannin and LY 294002, indicating that C2-ceramide-induced PKC j j increase in both nucleus and membrane fractions is mediated by phosphatidylinositol 3-kinase (PI3-kinase) activation. In addition, a significant translocation of PI3-kinase to the nucleus was observed after C2-ceramide treatment. ß
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Protein tyrosine phosphorylation, modulated by the rate of both protein tyrosine kinase and protein tyrosine phosphatase activities, is critical for cellular signal transduction cascades. We report that endothelin-l stimulation of rabbit platelets resulted in a dose-and time-dependent tyrosine phosphorylation of four groups of proteins in the molecular mass ranges of 50,60,70-100 and 100-200 kDa and that one of these corresponds to focal adhesion kinase. This effect is also related to the approximately 60% decrease in protein tyrosine phosphatase activity. Moreover, this inhibited activity was less sensitive to orthovanadate. In the presence of forskolin that increases the cAMP level a dose-dependent inhibition of the endothelin-stimulated tyrosine phosphorylation of different protein substrates and a correlation with an increase in the protein tyrosine phosphatase activity (1l.6-fold compared to control) have been found. Further studies by immunoblotting of immunoprecipitated soluble fraction with anti-protein tyrosine phosphatase-l C from endothelin-stimulated platelets have demonstrated that the tyrosine phosphorylation of platelet protein tyrosine phosphatase-l C is correlated with the decrease in its phosphatase activity. As a consequence, modulation and regulation by endothelin-l in rabbit platelets can be proposed through a cAMP-dependent pathway and a tyrosine phosphorylation process that may affect some relevant proteins such as focal adhesion kinase.
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