Regulation of the cell cycle machinery by oncogenic ras.

The ras proto-oncogene has been implicated during the formation of tumors in vivo as well as the transformation of cell lines in culture. Conditional expression of an activated ras mutant in Balb/c-3T3 fibroblasts failed to stimulate S phase entry in the absence of plasma-derived progression factors, but did shorten the G1 interval from 12 to 6 h and abrogate the normal proliferative requirement for platelet-derived growth factor. Ras-dependent alteration of the 3T3 cell cycle was accompanied by a dramatic increase in the expression of the G1 regulatory protein, cyclin D1, while expression of cyclin E and cyclin A proteins were only weakly induced. Cyclin/cdk complexes assembled in response to ectopic ras expression in the absence of growth factor stimulation bound the cdk inhibitory factor, Kip1, and were inactive. However, plasma-stimulated regulatory pathways functioned co-operatively with the oncogenic ras molecule to decrease Kip1 levels, induce the kinase activities associated with cyclins D, E and A, and trigger the initiation of DNA replication. Our results suggest that a ras-activated signal transduction pathway may link environmental mitogenic stimuli to the cell cycle machinery via modulation of G1 cyclin expression.