IGF-I and insulin regulate eIF4F formation by different mechanisms in muscle and liver in the ovine fetus

Abstract

The mechanisms by which insulin-like growth factor I (IGF-I) and insulin regulate eukaryotic initiation factor (eIF)4F formation were examined in the ovine fetus. Insulin infusion increased phosphorylation of eIF4E-binding protein (4E-BP1) in muscle and liver. IGF-I infusion did not alter 4E-BP1 phosphorylation in liver. In muscle, IGF-I increased 4E-BP1 phosphorylation by 27%; the percentage in the gamma-form in the IGF-I group was significantly lower than that in the insulin group. In liver, only IGF-I increased eIF4G. Both IGF-I and insulin increased eIF4E. eIF4G binding in muscle, but only insulin decreased the amount of 4E-BP1 associated with eIF4E. In liver, only IGF-I increased eIF4E. eIF4G binding. Insulin increased the phosphorylation of p70 S6 kinase (p70(S6k)) in both muscle and liver and protein kinase B (PKB/Akt) in muscle, two indicative signal proteins in the phosphatidylinositol (PI) 3-kinase pathway. IGF-I increased PKB/Akt phosphorylation in muscle but had no effect on p70(S6k) phosphorylation in muscle or liver. We conclude that insulin and IGF-I modulate eIF4F formation; however, the two hormones have different regulatory mechanisms. Insulin increases phosphorylation of 4E-BP1 and eIF4E. eIF4G binding in muscle, whereas IGF-I regulates eIF4F formation by increasing total eIF4G. Insulin, but not IGF-I, decreased 4E-BP1 content associated with eIF4E. Insulin regulates translation initiation via the PI 3-kinase-p70(S6k) pathway, whereas IGF-I does so mainly via mechanisms independent of the PI 3-kinase-p70(S6k) pathway.

Changes in 4E-BP1 and p70S6K Phosphorylation in Skeletal Muscle of the Ovine Fetus after Prolonged Maternal Fasting: Effects of Insulin and IGF-I

“…The percentage of highly phosphorylated 4E-BP1 was 28% (Fig. 1), whereas this figure was 40% in a similar fed study (13), suggesting that decreased fetal protein synthesis associated with maternal fasting is in part due to a decrease in the efficiency of translation initiation.…”

“…Highly phosphorylated p70S6K migrates more slowly than less phosphorylated protein (13). Using a p70S6K pan-antibody, we found that both insulin and IGF-I slowed down p70S6K migration rate on SDS-PAGE, suggesting that the two peptides increase p70S6K phosphorylation (Fig.…”

“…Insulin stimulates neonatal muscle protein synthesis via mechanisms that are associated with increased 4E-BP1 phosphorylation and enhanced assembly of the eIF4E⅐eIF4G complex (12). In the fetus, we have previously demonstrated that insulin increases skeletal muscle protein synthesis in part by increasing the formation of the eIF4E⅐eIF4G complex (13).…”

“…Insulin also acts on translation initiation by enhancing the activity of p70S6K, a serine/threonine kinase that phosphorylates the ribosomal protein 6 (12)(13)(14). Increased p70S6K phosphorylation correlates with accelerated rates of translation of mRNAs containing a 5'-terminal oligopyrimidines (5'TOP).…”

“…This present investigation was conducted to examine the ability of insulin and IGF-I to modulate translation initiation, by effecting changes in 4E-BP1 phosphorylation and p70S6K phosphorylation in skeletal muscle of the ovine fetus after 5 d of maternal fasting; the results are compared with those of the fed study we reported previously (13). The results indicate that the decreased insulin-induced protein synthesis in skeletal muscle in the fasted fetus is due, in part, to impaired insulin sensitivity to 4E-BP1 phosphorylation/eIF4F formation/ GSK3␤ phosphorylation; however, not all insulin-induced signaling pathways are impaired by maternal fasting.…”