It has been suggested that ras proteins participate in transduction of cellular proliferative signals; however, their specific role is still undefined, and we and others have shown that they are not regulatory elements of any of the two main signaling pathways in mammalian cells, i.e., the adenylate cyclase- and the phosphoinositide pathways. Recently, we have also shown that malignant transformation mediated by cytoplasmic and membrane-associated oncogenes including ras involves common biochemical pathways that result in pleiotropic alterations of phospholipid signaling pathways (i.e., constitutive activation of PLA2 and altered coupling [uncoupling or supercoupling] between external signals and intracellular PLC.) These changes are not due to altered function of receptors, G proteins, or effector enzymes but, rather, involve faulty interactions between otherwise normal components of the different signaling pathways. Microinjected ras oncogene proteins can induce meiotic maturation of Xenopus oocytes. We have recently found that microinjected ras proteins; however, in the absence of protein synthesis, ras proteins still induce meiotic maturation, and mos antisense oligonucleotides do not block ras- induced maturation. In addition, ras-induced maturation in the presence of cycloheximide is specifically associated with phosphorylation of specific cellular substrates. Our results in mammalian and amphibian cells suggest the occurrence of a common biochemical pathway for transduction of mitogenic and proliferative signals where the biochemical activities of membrane-associated proto- oncogene products (src-like or receptor-like tyrosine kinases) precedes those of the ras gene products and these, in turn, are followed by the participation of cytoplasmic serine or threonine kinases such as mos or raf. These studies suggest also that this common pathway involves a cascade of phosphorylation events where ras proteins exert, directly or indirectly, a key regulatory protein.
