The goal of our research is to understand the modulation of signal transduction mechanisms during the expression of a well-defined oncogene. We had previously found that ras transformation of MDCK cells caused a selective loss of glucagon receptors and this loss can be restored by treatment of cells with PGE2. The effect of PGE2 seems to be mediated by cyclic AMP. We have now found that, subsequent to the elevation of cyclic AMP, but preceding the appearance of glucagon sensitivity, PGE2 decreased the level of inositol 1,4,5-triphosphate in the transformed MDCK cells. Both effects of PGE2 on IP3 and on the induction of hormone sensitivity were blocked by phorbol ester. We conclude that the perturbation of the IP3 pathway by cyclic AMP is likely to be causally related to the induction of glucagon response. In order to examine the causal relationship between the expression of ras gene and the various changes of signal transduction more closely, we have transfected cells with plasmid containing inducible ras oncogenes to allow more precise control of p2l production. Using transfected 3T3 cells (obtained from others) and MDCK cells, we found that cyclic AMP-mediated hormone sensitivity is unaltered even when ras is fully expressed. In contrast, EGF receptors are rapidly desensitized, due to a loss of high affinity sites, when ras is expressed. Since diacylglycerol is also elevated when p2l is induced by dexamethasone, the desensitization of EGF receptors is likely caused by an activation of protein kinase. We are now screening the transfected GH3 cell line, which possesses high density EGF receptors, for clones expressing inducible ras, so that we can study the mechanism responsible for the desensitization of EGF receptors by protein kinase C. We have found a rapid increase in transfected 3T3 cells and a rapid decrease of PGE2 production in transfected MDCK cells, upon the expression of ras gene. This is consistent with previous findings using virally transformed cells.
