Alterations of G-protein coupling function in phosphoinositide signaling pathways of cells transformed by ras and other membrane-associated and cytoplasmic oncogenes.

We showed previously that transformation by cytoplasmic and membrane-associated oncogenes including ras results in uncoupling between surface stimulation by platelet-derived growth factor, bombesin, and serum and activation of intracellular phospholipase C (PLC); this uncoupling does not involve alterations at the receptor or effector enzyme levels (T. Alonso, R. O. Morgan, J. C. Marvizon, H. Zarbl, and E. Santos, Proc. Natl. Acad. Sci. USA 85:4271-4275, 1988). In this study, we stimulated normal and oncogene-transformed NIH 3T3 cells with fluoroaluminate (AIF4-), thus directly activating PLC-associated G protein(s) and bypassing the receptor step. A1F4(-)-elicited PLC responses were significantly impaired in transformed cells when compared with those in their normal counterparts, suggesting that the uncoupling of PLC is the result, at least in part, of functional impairment at the G-protein level. Transformation by ras oncogenes has also been reported to result in enhanced PLC response to bradykinin resulting from increased receptor numbers (G. Parries, R. Hoebel, and E. Racker, Proc. Natl. Acad. Sci. USA 84:2648-2652, 1987; J. Downward, J. de Gunzburg, R. Riehl, and R. Weinberg, Proc. Natl. Acad. Sci. USA 85:5774-5778, 1988). We demonstrate here that transformation by other membrane-associated and cytoplasmic oncogenes also results in increased responsiveness to bradykinin ("supercoupling") and enhanced receptor numbers. However, there is no direct correlation between the number of receptors and the enhancement in responsiveness, suggesting that other factors besides receptor number are also involved in the enhanced responses. We propose that a common effect of transformation by cytoplasmic and membrane-associated oncogenes is functional alteration of coupling G proteins and that a similar modification of different kinds of G proteins may account for the pleiotropic alterations of signal transduction (uncoupling and supercoupling) observed.