Role of the Raf/mitogen-activated protein kinase pathway in p21(ras) desensitization

Desensitization of p21(ras) after stimulation of cells by growth factors and phorbol 12-myristate 13-acetate (PMA) correlates with hyperphosphorylation of the guanine nucleotide exchange factor Son-of-sevenless (Sos) and its dissociation from the adaptor protein Grb2 (Cherniack, A., Klarlund, J. K., Conway, B. R., and Czech, M. P. (1995) J. Biol. Chem. 270, 1485-1488). To test the role of the Raf/mitogen-activated protein (MAP) kinase pathway, we utilized cells expressing a chimera composed of the catalytic domain of p74Raf-1 and the hormone binding domain of the estradiol receptor (Delta Raf-1:ER), Estradiol markedly stimulated Delta Raf-1:ER and the downstream MEK and MAP kinases in these cells as well as Sos phosphorylation, However, the dissociation of Grb2 from Sos observed in response to PMA was not apparent upon Delta Raf-1:ER activation, Furthermore, stimulation of Delta Raf-1:ER did not impair GTP loading of p21(ras) in response to platelet derived growth factor or epidermal growth factor, We conclude that activation of the Raf/MAP kinase pathway alone in these cells is insufficient to cause disassembly of Sos from Grb2 or to interrupt the ability of Sos to catalyze activation of p21(ras).