Developmental changes in the feeding-induced activation of the insulin-signaling pathway in neonatal pigs

Abstract

In neonatal animals, feeding stimulates skeletal muscle protein synthesis, a response that declines with development. Both the magnitude of the feeding response and its developmental decline can be reproduced by insulin infusion, suggesting that an altered responsiveness to insulin is a primary determinant of the developmental decline in the stimulation of protein synthesis by feeding. In this study, 7- and 26-day-old pigs were either fasted overnight or fed porcine milk after an overnight fast. We examined the abundance and degree of tyrosine phosphorylation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and IRS-2 in skeletal muscle and, for comparison, liver. We also evaluated the association of IRS-1 and IRS-2 with phosphatidylinositol 3-kinase (PI 3-kinase). The abundance of IR protein in muscle was twofold higher at 7 than at 26 days, but IRS-1 and IRS-2 abundances were similar in muscle of 7- and 26-day-old pigs. The feeding-induced phosphorylations were greater at 7 than at 26 days of age for IR (28- vs. 13-fold), IRS-1 (14- vs. 8-fold), and IRS-2 (21- vs. 12-fold) in muscle. The associations of IRS-1 and IRS-2 with PI 3-kinase were also increased by refeeding to a greater extent at 7 than at 26 days (9- vs. 5-fold and 6- vs. 4-fold, respectively). In liver, the abundance of IR, IRS-1, and IRS-2 was similar at 7 and 26 days of age. Feeding increased the activation of IR, IRS-1, IRS-2, and PI 3-kinase in liver only twofold, and these responses were unaffected by age. Thus our findings demonstrate that the feeding-induced activation of IR, IRS-1, IRS-2, and PI 3-kinase in skeletal muscle decreases with development. Further study is needed to ascertain whether the developmental decline in the feeding-induced activation of early insulin-signaling components contributes to the developmental decline in translation initiation in skeletal muscle.

Protein-tyrosine-phosphatase 1B activation is regulated developmentally in muscle of neonatal pigs

“…The PTP1B association with IR in muscle, but not liver, was lower in 7-compared with 26-day-old pigs (* P Ͻ 0.05). high insulin sensitivity of protein synthesis in skeletal muscle of 7-compared with 26-day-old pigs (14,37,46). Importantly, no developmental change in PTP1B activation in liver was detected, which is consistent with the previously reported lack of developmental change in insulin sensitivity in the liver (14,37).…”

“…Recent studies from our laboratory have shown an enhanced activation of the insulin-signaling pathway leading to translation initiation in skeletal muscle of the neonatal pig after food consumption (16,26,37). This feeding-induced activation of the IR, IRS-I, PI 3-kinase, and protein kinase B decreases with development in parallel with the developmental decline in the feeding-induced activation of translation initiation factors that regulate the binding of mRNA to 43S ribosomal complex.…”

“…Pigs were killed after the 18-h fast or 30 min after the second gavage feeding, and samples of longissimus dorsi muscle and liver were rinsed in ice-cold saline and rapidly frozen. The protocol, which we have previously described (16,26,37), was approved by the Animal Care and Use Committee of Baylor College of Medicine and was conducted in accordance with the National Research Council's Guide for the Care and Use of Laboratory Animals.…”

“…neonate; insulin signaling; insulin sensitivity; protein synthesis THE ENHANCED ACTIVATION of the insulin-signaling pathway leading to translation initiation in skeletal muscle of the neonate after food consumption has an important role in the enhanced responsiveness of muscle protein synthesis to feeding in the neonate (13,16,26,27,37). We have shown that the feeding-induced activation of the insulin receptor (IR), insulin receptor substrate-1 (IRS-1), and phosphatidylinositol 3-kinase (PI 3-kinase), as well as the abundance of the IR, is enhanced in skeletal muscle of the neonatal pig and decreases markedly with development.…”

“…Furthermore, we also studied the mechanisms involved in PTP1B activation by measuring the association of PTP1B with the IR, the tyrosine phosphorylation of PTP1B, and the association of PTP1B with Grb2 adaptor protein. For comparison, similar measurements were also made in liver, an organ in which insulin does not stimulate protein synthesis (14,15) and which shows no developmental decline in insulin signaling (37).…”