Ras transformation of cloned rat embryo fibroblasts results in increased rates of protein synthesis and phosphorylation of eukaryotic initiation factor 4E.

Abstract

Eukaryotic initiation factor 4E (eIF-4E) is a 25-kDa phosphoprotein that binds to the 7-methylguanosine cap of mRNA and acts, along with other eIF-4 polypeptides, to unwind mRNA secondary structure at the 5' terminus. Recent studies have indicated that eIF-4E acts as a protooncogene, but only in its phosphorylated state. In order to determine the role of eIF-4E in oncogenesis, we examined its regulation and expression in cloned rat embryo fibroblasts transformed with the Harvey ras (Ha-ras) oncogene. The expression of Ha-ras increased the rate of protein synthesis but did not increase the levels of eIF-4E mRNA or protein. However, a dramatic increase (7-fold) in phosphate incorporation into eIF-4E was observed. The percentage of eIF-4E in the phosphorylated state was the same in transfected and control cells, indicating that both phosphorylation and dephosphorylation of eIF-4E were increased. Phosphopeptide mapping of eIF-4E from transformed cells indicated a single site of phosphorylation at Ser-53, which is the same as that identified previously in eIF-4E from reticulocytes and HeLa cells. These results indicate that p21ras is part of the signal transduction pathway leading to phosphorylation of eIF-4E. These findings also provide a potential mechanism for cell transformation by p21ras which involves the preferential stimulation of translation of certain mRNAs.