Prematurity Blunts Feeding‐Induced Stimulation of Translation Initiation Signaling and Protein Synthesis in Muscle of Neonatal Piglets

Abstract

BackgroundPreterm infants commonly experience extrauterine growth faltering, with many weighing less than the 10th percentile for age at hospital discharge. Growth restriction during the neonatal period may contribute to both short‐ and long‐term morbidities.ObjectiveTo determine how preterm birth alters the protein anabolic response of muscle to feeding in a neonatal piglet model.MethodsPiglets were delivered at term (112 d) or preterm (103 d) gestation by Cesarean section and immediately fitted with a jugular catheter for delivery of total parenteral nutrition (240 mL/kg/d). Following a 4 hr fast at 3 d of age, pigs were fasted an additional hour or fed an enteral elemental meal (4 groups: PF, preterm fasted; PP, preterm postprandial; TF, term fasted; TP, term postprandial). Flooding dose of L[4‐3H]phenylalanine was injected at 30 min relative to the meal for measurement of protein synthesis, and tissues collected at 60 min.ResultsBirth weight of preterm (n = 48, 4 litters) pigs was lower (P < 0.01) than that of term (n = 42, 4 litters) pigs. Body weight gain was also lower (g/kg/d; P = 0.01) in preterm than term pigs despite equivalent nutrient delivery. Preterm pigs had greater kidney (g/kg; P = 0.03) and liver (P < 0.01) masses, but lower (P = 0.04) longissimus dorsi (LD) mass than term pigs. Gestational age at birth (GAB) did not affect gastrocnemius, soleus, or heart masses. GAB did not affect plasma glucose or insulin of fasted pigs, but among fed pigs both plasma insulin and glucose rose more rapidly, and reached a higher peak, in term than preterm pigs. Plasma insulin‐like growth factor‐1 (IGF‐1) was lower (P = 0.045) in preterm than term pigs at 60 min. Feeding increased LD fractional protein synthesis rates (Ks) in both GAB groups, but the response was lower in preterm than term pigs (PF = TF < PP < TP; P < 0.01). LD protein kinase B phosphorylation followed a similar pattern (PF = TF < PP < TP; P < 0.01), indicating GAB‐dependent insulin signaling pathway activation. Formation of active eukaryotic initiation factor (eIF) eIF4E·eIF4G complex, and phosphorylation of S6 kinase (S6K1) and eIF4E‐binding protein 1 (4EBP1) in the LD also reflected this trend (all proteins PF = TF < PP < TP; P < 0.01), indicating enhanced translation initiation in term vs. preterm fed pigs. Microtubule‐associated protein light chain 3 II (LC3II) to total LC3 ratio in LD was decreased (P < 0.01) by feeding regardless of GAB, indicating reduced autophagy‐lysosome system activation. There were no differences between groups in LD muscle RING‐finger protein‐1 (MuRF1) or atrogin‐1 abundance, indices of ubiquitin‐proteasome system activity. Similarly, no differences were observed between groups in phosphorylation of eukaryotic elongation factor 2 (eEF2) or eukaryotic initiation factor 2α (eIF2α), or in abundance of late endosomal/lysosomal adaptor, MAPK and MTOR activator 2 (LAMTOR2), Ras‐related GTP‐binding protein A (RagA), RagC, or DEP domain containing MTOR‐interacting protein (Deptor).ConclusionsPreterm piglets exhibit decreased body weight gain and feeding‐induced muscle protein synthesis compared to term piglets. These differences are due to blunted insulin response and reduced translation initiation signaling.Support or Funding InformationNIH HD085573, USDA CRIS 6250‐51000‐055, NIH HD072891, USDA NIFA 2013‐67015‐20438.