The inhibition of cell proliferation at a critical cell density is a characteristic feature of normal cells that is lost in most tumor cells. Several lines of evidence suggest that regulation of protein kinase C (PKC) plays an important role in density-dependent growth inhibition. This process appears to involve the generation of membrane-associated growth inhibitory molecules. Many tumor cells appear to have lost the ability to respond to growth inhibitory factors, although they retain such factors on their plasma membranes. The studies in this proposal are designed to investigate the role of the ether-linked monoglyceride, alkylglycerol, as a novel mediator of density dependent cell proliferation. Alkylglycerol accumulates in contact inhibited tissue culture cells in a density dependent manner, and this accumulation appears to coincide with the down regulation of PKC. Treatment of cultured cells with synthetic alkylglycerol potently inhibits PKC activation and DNA synthesis. The proposed research will address the following specific aims: 1) to define the mechanism of cell growth inhibition by alkylglycerol. The applicant will examine the effects of alkylglycerol on the cell cycle and on induction of DNA synthesis and PKC translocation by specific mitogens; 2) to examine the production and release of alkylglycerol from ras-transformed cells. Alkylglycerol levels are highly elevated in fibroblasts transformed by oncogenic H-ras. These studies will characterize the production and release of alkylglycerol from ras-transformed cells and characterize its growth inhibitory properties; and 3) to determine the effects of alkylglycerol on PKC and cell proliferation in H-ras 3T3 fibroblasts. Experiments will be performed to determine if the elevated levels of alkylglycerol in ras-transformed cells are responsible for down-regulation of PKC. The results of these studies will provide important information on a novel mechanism of growth control in mammalian cells.
