1-O-OCTADECYL-2-O-METHYLGLYCEROPHOSPHOCHOLINE INHIBITS PROTEIN-KINASEC-DEPENDENT PHOSPHORYLATION OF ENDOGENOUS PROTEINS IN MCF-7 CELLS

Studies with leukaemic cells, based primarily on in vitro assays, have suggested that antitumour ether lipids have only a moderate effect on protein kinase C (PKC) activity, and, furthermore, inhibition of PKC is unlikely to be involved in the mechanism of inhibition of cell prol iferation by these compounds. To determine if this is also the case fo r epithelial cancer cells, we examined the effect of I-0-octadecyl-2-0 -methylglycerophosphocholine (ET18-OCH3) on PKC-induced phosphorylatio n of endogenous proteins in MCF-7 cells under incubation conditions wh ere the drug inhibited cell proliferation. As expected, stimulation of quiescent P-32-labelled MCF-7 cells with 1 mu M PMA resulted in the p hosphorylation of a number of proteins. The PMA-induced phosphorylatio n of the proteins was abolished by preincubation of the cells with Ro 31-8220 (5 mu M) for 20 min, or 10 mu g/ml ET18-OCH3 for 3 h before st imulation with PMA. Thus under incubation conditions where ET18-OCH3 i nhibited the proliferation of MCF-7 cells, the ether lipid potently in hibited the activity of PKC in intact cells. This inhibition was unlik ely to be due to the effect of the compound on PKC translocation since there was little effect of ET18-OCH3 on the translocation of the alph a, gamma and epsilon species of PKC. These results suggest that a role for the inhibition of PKC activity by ET18-OCH3 in the mechanism of i nhibition of cell proliferation by ET18-OCH3 cannot yet be discounted in epithelial cancer cells. In addition, we also observed that ET18-OC H3 enhanced the phosphorylation of selected proteins under basal unsti mulated conditions. Although some of these proteins were also observed to be phosphorylated in response to PMA stimulation, the phosphorylat ion induced by ET18-OCH3, was not inhibited by Ro 31-8220, indicating that this was not mediated by PKC.