Ras-independent oncogenic transformation by an EGF-receptor mutant

Mutations in the ligand-binding domain of the epidermal growth factor recep tor have been identified in several types of human cancers, including malig nant gliomas, These mutations render signaling by this receptor to be const itutively ligand-independent. In fibroblasts transformed with ligand-indepe ndent epidermal growth factor receptor mutants, there is a correlation betw een the formation of a unique phosphotyrosine protein complex and oncogenic transformation. This phosphoprotein complex includes Grb2, She, Sos, tyros ine-phosphorylated form of caldesmon, and two, as yet, unidentified protein s. The presence of Grb2, She, and Sos in this complex implicates Ras in lig and-independent signaling by these oncogenic epidermal growth factor recept or mutants, We, therefore, have used retroviral co-infections of cultured p rimary fibroblasts to determine if Ras activation is required for phosphopr otein complex formation, stress fiber loss, or transformation. As predicted , expression of a dominant-negative Ras mutant (N17Ras) completely abrogate s ligand-stimulated soft agar colony growth of primary fibroblasts, In cont rast, N17Ras expression has no effect on v-ErbB mediated stress fiber disas sembly, soft agar colony growth, or phosphoprotein complex assembly. In add ition, our data suggest that ligand-dependent Ras activation may be suppres sed by oncogenic v-ErbB expression. Together these observations suggest tha t oncogenic signaling by v-ErbB does not require Ras activation, and implic ate an alternative signal transduction pathway in ligand-independent epider mal growth factor receptor oncogenic signaling.