Background: Previous studies have shown that the antitumor ether lipid, 1-O-Octadecyl-2-O-methyl-rac-glycerophosphocholine (ET-18-OCH3), inhibits the activation of the MAPK pathway in EGF- and serum-stimulated MCF-7 cells. The activation of the MAPK pathway subsequent to growth factor stimulation requires the recruitment of Raf-1 from the cytosol to the membrane. ET-18-OCH3 decreased the level of membrane-associated Raf-1 relative to untreated control cells. Since ET-18-OCH3 did not inhibit the activities of the kinases in the cascade, the reduced Raf-1 levels appeared to be the cause for the reduction in the magnitude and duration of MAPK activity. In this study we have investigated whether the reduced Raf-1 levels arise from a perturbation of the interaction of Raf-1 with activated Ras, which is the event that mediates the membrane recruitment.
Materials and methods: The interaction of Raf-1 with Ras was examined by investigating the association of cytosolic Raf-1, from ET-18-OCH3-treated and untreated cells with purified GST-Ras-GTP-gamma-S bound to agarose beads. The level of associating Raf was determined by Western blot analysis. The effect of naturally occurring phospholipids on the Raf-1-Ras interaction was also examined to assess the specificity of the results.
Results: In cells preincubated with ET-18-OCH3, the interaction of GST-Ras-GTP-gamma-S with cytosolic Raf was reduced. The addition of ET-18-OCH3 to the cytosolic fraction isolated from untreated cells also reduced the binding of Raf to activated GST-Ras-GTP-gamma-S. Cytosolic Raf-1 from cells incubated with natural lysophospholipids was similar to that of controls.
Conclusion: Our findings suggest that ET-18-OCH3 associates specifically with Raf-1 in the cytosol and interferes in the interaction of Raf-1 with activated Ras, thereby reducing the levels that are translocated to the membrane for activation. Thus Raf-1 appears to be a molecular target of ET-18-OCH3.