Elevated levels of diacylglycerol and decreased phorbol ester sensitivity in ras-transformed fibroblasts

Abstract

Diacylglycerol (DG) plays a central role in phospholipid metabolism and is an endogenous activator of protein kinase C (refs 1–3). We have suggested^4,5 that constitutive activation of this kinase is one mechanism by which oncogenes transform cells. The ras-encoded proteins are similar to regulatory G-proteins⁶ and are candidates for the unknown G-protein that modulates phosphatidylinositol (PI) turnover^1,7,8. Differences in polyphos-phoinositide metabolism have been reported for ras-transformed cells⁹. But because these experiments were performed on confluent cultures of established cell lines, the differences are difficult to attribute to ras transformation. Here we show that exponentially growing NIH 3T3 fibroblasts recently transformed by Ha-ras or Ki-ras possess elevated DG concentrations without significant alterations in the levels of other polyphosphoinositide metabolites. The basal phosphorylation of protein kinase C substrate of relative molecular mass (M_r) 80,000 (80K) is significantly increased in all the ras-transformed cell lines. Surprisingly, however, further phosphorylation of this protein on addition of phorbol ester was greatly reduced. Ha-ras cells also show less binding of phorbol ester than control cells, suggesting that elevation of DG causes partial down-regulation in addition to activation of protein kinase C.