Ras Proteins as Potential Activators of Protein Kinase C Function
The structural and functional relationships of the products of the ras genes with known G proteins have suggested that ras-p21 s may be important mediators of signal transduction pathways involved in cellular proliferation and differentiation. One of the best characterized pathways constitutes the activation of the Ca++sensitive, phospholipid dependent protein kinase C (PKC), following receptor mediated hydrolysis of inositol phospholipids by phospholipase C (PLC). We have investigated the putative involvement of the ras-p21 proteins in the modulation of PLC, responsible for the generation of 1,2-diacylglycerol (DAG) and inositol phosphates (IPs), as well as the consequent activation of PKC by the generated DAG. Utilizing Xenopus laevis oocytes we have detected a rapid increase in the generation of DAG after microinjection of the transforming H-ras p21 protein. Elevated basal levels of DAG were also observed in NIH-3T3 cells transformed by a variety of mutated ras proteins without increased levels of the corresponding IPs. These results suggest a novel source for the generated DAG other than the hydrolysis of phosphatidyl inositol phosphates. We have also observed that microinjection of mutated ras proteins into cells previously devoided of endogenous PKC by chronic treatment with phorbol esters, lacked its otherwise potent mitogenic activity. The mitogenic function of ras-p21 was regenerated under identical conditions by co-microinjection with PKC . These results suggest the requirement of functional PKC for the mitogenic activity of ras. Finally, our previous findings that microinjection of ras-p21 into 3T3 cells induces a rapid increase in the intracellular pH of 0.15–0.2 pH units (Hagag et al. 1987) demonstrate the ras-p21 can induce the activation of PKC under normal conditions. Taken together, all these results imply that ras-p21 proteins may function as regulators of PKC activity.
KeywordsPhorbol Ester Mitogenic Activity Xenopus Laevis Oocyte Phospholipid Dependent Protein Kinase Methyl Thymidine
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