This study shows that the membrane-permeable stereospecific
1-oleoyl-2-acetyl-sn-glycerol (OAG), which is the analog of the natural
1,2-diacylglycerol (DAG), can stimulate the growth of
ascites tumor cells. OAG can fully replace high serum concentrations in the culture medium and stimulates
DNA synthesis in a dose-dependent manner. Investigation of the
protein kinase C (PKC) isolated from a Triton extract of a 100,000g membrane pellet revealed that OAG can directly activate this
enzyme. Concomitantly the phosphorylation of several cytosolic
proteins with the molecular weights of 26, 33, 49, 55, 64, and 90 kDa is observed which is also found in serum-stimulated cells. Since DAG as a second messenger molecule originates from the hydrolysis of
phosphoinositides we have investigated the metabolism of these
lipids after labeling the cells with [3H]
inositol. In detail, we have measured the amount of radioactive
inositol trisphosphate (IP3) and the
phosphodiesterase hydrolyzing phosphatidylinositol-4,5-bisphosphate (PIP2). The decreased radioactivity level of IP3 in OAG-stimulated cells as compared to non-growing cells (1-2% serum) indicates a feedback regulation of PIP2 hydrolysis which is substantiated by a profound reduction of PIP2-specific
phospholipase C activity. The reduced IP3 formation has apparently no inhibitory effect on the cytoplasmic free Ca2+ concentration of OAG-stimulated cells, suggesting that the Ca2+ release is not directly correlated to the amount of IP3, which is also demonstrated for the non-growing cells. These data indicate that OAG apparently has a duel effect on the
inositol phospholipid-mediated signal transfer system.(ABSTRACT TRUNCATED AT 250 WORDS)