Critical role of cellular ras proteins in proliferative signal transduction.
In the experiments described above, a neutralizing anti-ras antibody was utilized to study the role of ras protein in normal cell proliferation. Initially, it was demonstrated that the antibody was specific for ras protein, and that ras activity was efficiently inhibited. With the neutralizing antibody, it was first shown that ras activity is required for the proliferation of all normal cell types tested. ras activity was required just prior to initiation of S phase. The transforming activity of several retroviral oncogenes was also blocked following anti-ras injection. This included the tyrosine kinase, plasma-membrane-associated proteins, and an oncogene derived from a growth factor. On the other hand, cytoplasmic oncogenes with serine kinase activity were not dependent on ras activity for expression of the transformed phenotype. These observations form the basis of our model for proliferative signal transduction. We propose that the action of either growth factors, their receptor molecules, or related oncogenes initiate an intracellular signal received by ras proteins and then transferred by ras to cytoplasmic serine kinase oncogenes. This signal transduction system directly regulates cellular proliferation. Although further evidence in support of this model is needed, it appears from our studies that the mechanism of signaling between tyrosine kinases and ras proteins might be at the level of phospholipid metabolism. This observation is based on the fact that the mitogenic lipid molecules tested were remarkably dependent on ras activity, even more so than the growth factors or related oncogenes tested. Finally, our work suggests a fundamental distinction between normal and tumor cells. All the normal cell types tested were efficiently inhibited in proliferation by the injected antibody. Tumor cells, on the other hand, were never completely inhibited by the antibody and often were not inhibited at all. The presence of an activated ras oncogene within the tumor assured at least a partial role for ras activity in the proliferation of the mature tumor line. The significance of the observed distinction between normal and tumor cells is not known. The fact that this distinction involves a protein with an apparently critical role in normal proliferation suggests that the observation might be important.