Regulation of Akt Signaling in Skeletal Muscle Is Altered by Prematurity in a Neonatal Piglet Model

Objectives Preterm birth is a risk factor for growth faltering. We recently showed in a neonatal piglet model of prematurity that premature birth decreases weight gain and skeletal muscle protein synthesis. This is associated with reduced insulin-induced activation of signaling components downstream of Akt but with no change in the activation of the IR/IRS-1/PI3K axis upstream of Akt. The aim of this study was to identify key regulatory molecules involved in Akt activation that are responsible for the differential response of skeletal muscle to insulin in preterm compared to term pigs. Methods Piglets were delivered by cesarean section 11 d (preterm/PT) or 2 d (term/T) before term birth and a jugular vein catheter was placed for delivery of total parenteral nutrition. On day 3, after 4 h fasting, piglets were fasted one additional h or fed an elemental diet (31.5 kcal/kg, 1.3 g/kg carbohydrate, 2.7 g/kg amino acids, and 1.6 g/kg lipid), similar in composition to sow milk, via oral gavage. Longissimus dorsi muscle was collected following euthanasia (fasted state or 60 min after feeding). Positive and negative regulators of Akt activity were determined by immunoprecipitation and immunoblotting assays. Results Akt1 and Akt2 phosphorylation were lower in PT than in T pigs (P < 0.05), whereas Akt3 phosphorylation was not affected by prematurity. The phosphorylation of Akt activators PDK1 and mTORC2, but not FAK, was lower in PT than in T pigs (P < 0.05). The abundance of Ubl4A, a positive regulator of Akt, was lower in PT than in T pigs (P < 0.05). The abundance Akt inhibitors PHLPP and SHIP2, but not PTEN and IP6K1, was significantly higher in PT than in T pigs (P < 0.05). Activation of the phosphatase PP2A was lower in PT than in T pigs (P < 0.05), but its activation was not affected by feeding. However, PP2A activation was inhibited by feeding in T pigs (P < 0.05). Conclusions These results showed that following preterm birth, the postprandial activation of positive regulators of Akt is reduced whereas the activation of negative regulators of Akt is enhanced. Our findings suggest that premature birth impairs the activation of Akt that is essential for channeling dietary nutrients for anabolism and likely contributes to the extrauterine growth faltering of prematurity. Funding Sources NIH and USDA.