Stimulation of translation in 3T3-L1 cells in response to insulin and phorbol ester is directly correlated with increased phosphate labelling of initiation factor (eIF-) 4F and ribosomal protein S6.

One of the earliest responses to growth-promoting compounds in quiescent or serum-starved cells is stimulation of phosphorylation of ribosomal protein S6. Exposure of 3T3-L1 cells to insulin or phorbol ester (phorbol 12-myristate 13-acetate, PMA) also promotes phosphorylation of specific initiation factors. In this study, stimulation of phosphate labelling of S6, eIF-4F and eIF-4B in response to insulin and PMA has been examined in 3T3-L1 cells and compared with changes in protein synthesis. The rate of phosphate incorporation into eIF-4F and S6 is rapid and parallels the transient stimulation of protein synthesis observed with insulin. Whilst a similar correlation exists with PMA, the response is not as great as with insulin, but is more sustained. A role for the co-ordinate phosphorylation of initiation factors and ribosomal protein S6 in the stimulation of protein synthesis is discussed.