1-O-OCTADECYL-2-O-METHYL-GLYCEROPHOSPHOCHOLINE INHIBITS THE TRANSDUCTION OF GROWTH SIGNALS VIA THE MAPK CASCADE IN CULTURED MCF-7 CELLS

1-O-Octadecyl-2-O-methyl-glycerophosphocholine (ET18-OCH3) is an ether lipid with selective antiproliferative properties whose mechanism of action is still unresolved, We hypothesized that since ET18-OCH, affec ts a wide variety of cells, its mechanism of action was likely to invo lve the inhibition of a common widely used pathway for transducing gro wth signals such as the mitogen-activated protein kinase (MAPK) cascad e. To test this, we established conditions whereby quiescent MCF-7 cel ls took up ET18-OCH3 in sufficient quantities that inhibited cell prol iferation subsequent to the addition of growth medium and examined the activation of components of the MAPK cascade under these conditions. ET18-OCH3 inhibited the sustained phosphorylation of MAPK resulting in a decrease in the magnitude and duration of activation of MAPK in cel ls stimulated with serum or EGF. ET18-OCH3 had no effect on the bindin g of EGF to its receptors, their activation, or p21(ras) activation. H owever, an interference in the association of Raf-1 with membranes and a resultant decrease in Raf-l kinase activity in membranes of ET18-OC H3-treated cells was observed. ET18-OCH3 had no direct effect on MAPK or Raf-l kinase activity. A direct correlation between ET18-OCH3 accum ulation, inhibition of cell proliferation, Raf association with the me mbrane, and MAPK activation was also established, These results sugges t that inhibition of the MAPK cascade by ET18-OCH3 as a result of its effect on Raf-l activation may be an important mechanism by which ET18 -OCH3 inhibits cell proliferation.