The brain specific Ras exchange factor CDC25 Mm: modulation of its activity through Gi-protein-mediated signals.

CDC25Mm is a mouse guanine nucleotide exchange factor specific for Ras, exclusively expressed in the brain. We used a reporter gene containing a Ras-responsive fos-promoter in order to gain information on the role played by this exchange factor in signal transduction. Transient expression of CDC25Mm in CHO cells activates Ras. Moreover serum, but not insulin, can upregulate the response mediated by CDC25Mm and this modulation requires that the CDC25Mm maintains its N-terminal region. NIH3T3 fibroblasts, stably overexpressing this exchange factor, show a partially transformed phenotype, suggesting that the Ras-dependent pathway is constitutively active. In these cells serum and lysophosphatidic acid (LPA) stimulate Ras activity above the basal level while PDGF does not. Both serum and LPA-induced Ras activations in CDC25Mm overexpressing cells can be completely inhibited by pertussis toxin. Moreover, these responses are strongly reduced by coexpression of a truncated version of CDC25Mm lacking the C-terminal catalytic portion. This construct behaves in a dominant negative manner suggesting that it may compete with CDC25Mm by sequestering in an unproductive way signalling components activated by these factors. The data presented indicate that CDC25Mm does not participate in connecting tyrosine kinase receptors with Ras, while it could mediate Ras activation induced by pertussis toxin sensitive Gi-coupled receptors.