TPA stimulates S6 phosphorylation but not protein synthesis in Ehrlich cells.

Increased phosphorylation of ribosomal protein S6 has been extensively correlated with an increased rate of protein synthesis. We report here that under two separate conditions in Ehrlich cells an increase in the level of S6 phosphorylation does not result in any increase in the rate of protein synthesis. 1) In glutamine-deprived cells TPA stimulates S6 phosphorylation but has no effect on the rate of protein synthesis, 2) In cells deprived of serum growth factors, addition of serum stimulates both S6 phosphorylation and protein synthesis while TPA stimulates only S6 phosphorylation. These results show that increased phosphorylation of S6 is not sufficient to cause increased rates of protein synthesis, and suggest that additional factors may play a more direct role.