Stable expression of activated Ki-Ras does not constitutively activate themitogen-activated protein kinase pathway but attenuates epidermal growth factor receptor activation in human astrocytoma cells

Mutation in the Ras oncogene is one of the most commonly reported genetic a berrations in human cancer. Activated Pas mutants are thought to play a maj or role in promoting the growth and malignancy of tumor cells. Ras protein plays a central role in transmitting mitogenic signals from cell surface-to -nucleus by activating signaling pathways in response to receptor activatio n. Pas protein by recruiting c-Raf-l kinase to the plasma membrane activate s the mitogen-activated protein (MAP) kinase pathway. Expression of activat ed Ras mutants in rodent fibroblast has been reported to constitutively act ivate the MAP kinase pathway, suggesting that constitutive activation of th is pathway contributes to Ras influence on proliferation and transformation . In this study, we investigated whether stable expression of an activated Ki-Ras oncogenic mutant (G12V) in human astrocytoma cells leads to constitu tive activation of the MAP kinase pathway and how this may influence cellul ar proliferation and signaling by epidermal growth factor (EGF) receptor. W e discovered that Ki-Ras stable expression does not lead to constitutive ac tivation of the MAP kinase pathway, rather expression of Ki-Ras plays a rol e in attenuating the activation of this pathway in response to EGF stimulat ion. Furthermore, we provide evidence that stable Ki-Ras expression attenua tes the ability of EGF receptor to activate the MAP kinase pathway by inter fering with the receptor ability to autophosphorylate at tyrosine residues and not by down regulating receptor expression.