Functional and mechanistic aspects of Ras participation in signaling pathways initiated by receptor tyrosine kinase were studied in eukaryotic models of Ras-mediated cell proliferation and differentiation. It was shown that Ras proteins are obligatory intermediates in the process of insulin-induced differentiation of 3T3 L1 cells into adipocytes. It was also demonstrated in this differentiation system that Ras proteins are necessary and sufficient for insulin-induced activation of cytosolic kinases including Raf-1, MAP and RSK. However, in contrast to other reports in proliferating cells, Raf1 kinase activation is completely dissociated from activation of the MAP kinases by insulin in these differentiating cells. These results suggest that Ras proteins may elicit different signal networks activating downstream kinases depending on whether they are in a differentiating or proliferating cellular context. Analysis of the expression patterns of the Ras guanine nucleotide exchangers (GEF) hGRF and hSOS1 revealed the existence of multiple isoforms that are expressed differentially in various human fetal and adult tissues or in different tumor cell lines analyzed. It was also demonstrated that at least some of the isoforms are significantly different functionally in vivo and in vitro. Alternatively spliced, functionally different GEF isoforms with differential tissue expression and distribution may play important regulatory roles in the fine control of ras activation in different tissues or at different developmental stages.
