Phorbol ester, serum, and rous sarcoma virus transforming gene product induce similar phosphorylations of ribosomal protein S6.

The addition of phorbol 12-myristate 13-acetate (PMA), a potent tumor promoter, to serum-starved quiescent chicken embryo fibroblasts (CEF) or C127 murine cells resulted in increased phosphorylation of 40S ribosomal protein S6. The effect of PMA on S6 phosphorylation in quiescent CEF was half-maximal at approximately equal to 100 nM and was readily observed at 16 nM. In addition, S6 phosphorylation was increased in serum-starved CEF incubated with the diacylglycerol derivative, 1-oleoyl-2-acetylglycerol. S6 phosphorylation in PMA-stimulated, serum-stimulated, and serum-starved Rous sarcoma virus-transformed CEF was analyzed by phospho amino acid analysis, two-dimensional polyacrylamide gel electrophoresis, limited proteolysis with V8 protease, and two-dimensional thin-layer electrophoresis of chymotryptic digests. Comparison of S6 phosphorylation by these methods suggests that phosphorylation of S6 stimulated by PMA, serum, or oncogenic transformation with Rous sarcoma virus occurs through common pathways. This is further supported by the observation that the simultaneous addition of PMA and serum to CEF or of either PMA or serum to Rous sarcoma virus-transformed CEF did not significantly further increase the incorporation of phosphate into S6.