| Serum stimulates protein synthesis in quiescent serum-deprived cells as part of an overall proliferative response. Serum-deprived Ehrlich cells were used to characterize both this effect on protein synthesis, and the factors in serum responsible for it. In cells maintained in serum-free medium for 16 hours the rate of protein synthesis was about 50% of the rate in well-fed cells. Adding 10% calf serum stimulated protein synthesis within ten minutes, through a nontranscriptional mechanism operating at the level of polypeptide chain initiation.;Eukaryotic initiation factor 2 (eIF-2) is more highly phosphorylated in serum-deprived cells than in fed cells, and the phosphorylated form of eIF-2 is known to inhibit protein synthesis by inhibiting Guanine Nucleotide Exchange Factor. eIF-2 was rapidly dephosphorylated in response to serum, returning to near basal levels after 10 minutes. The close temporal correlation between eIF-2 dephosphorylation and increased rate of protein synthesis suggests that eIF-2 plays an important role in the regulation of protein synthesis by serum.;Fresh serum-free medium slightly stimulated protein synthesis in serum-deprived cells, but did not alter their response to the subsequent addition of serum. The serum effect was due to nondialyzable serum growth factors which were sensitive to treatment with dithiothreitol and iodoacetamide. Albumin also stimulated protein synthesis in serum-deprived cells, probably due to a contaminating growth factor. Although several purification protocols were employed, albumin was never completely separated from the stimulatory activity. However, fractions were generated which contained only small quantities of albumin and were enriched for stimulatory activity.;The phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) did not stimulate protein synthesis in serum-deprived Ehrlich cells, nor did overnight treatment of serum-deprived cells with TPA alter their response to the subsequent addition of serum. TPA treatment of serum deprived cells did result in increased phosphorylation of ribosomal protein S6, which is also phosphorylated by serum refeeding. Increased phosphorylation of S6 has been correlated with increased rates of protein synthesis, although the mechanism of action is not known. These results suggest that phosphorylation of S6 is not sufficient for stimulation of protein synthesis in Ehrlich cells. |
