Effects of short-term fasting on the Akt-mediated pathway involved in protein metabolism in chicken skeletal muscle

Regulation of Autophagy in Chick Skeletal Muscle: Effect of mTOR Inhibition

Autophagy in the skeletal muscle increases under catabolic conditions resulting in muscle atrophy. This study investigated the effect of inhibition of mechanistic target of rapamycin (mTOR) on autophagy in chick skeletal muscle. We examined the effects of Torin1, an mTOR inhibitor, on autophagy. Chick myotubes were incubated with Torin1 (100 nM) for 3 h. It was observed that Torin1 inhibited the phosphorylation of AKT (Ser473), p70 ribosomal S6 kinase 1 (S6K1, Thr389), S6 ribosomal protein (Ser235/236), and eukaryotic translation initiation factor 4E-binding protein 1 (4E-BP1, Thr37/46), which are used for measurement of mTOR activity. Torin1 significantly (P< 0.01) increased the LC3-II/LC3-I ratio, an index for autophagosome formation, while it did not influence the expression of autophagy-related genes (LC3B, GABARAPL1, and ATG12). In addition, Torin1 increased atrogin-1/MAFbx (a muscle-specific ubiquitin ligase) mRNA expression. Fasting for 24 h inhibited the phosphorylation of AKT (Ser473), S6K1 (Ther389), S6 ribosomal protein (Ser235/236), and 4E-BP1 (Thr37/46) in chick skeletal muscle and significantly (P<0.01) increased the LC3-II/LC3-I ratio. Fasting also increased GABARAPL1 and atrogin-1/MAFbx mRNA expression but not LC3B or ATG12 mRNA expression. These results indicate that mTOR signaling regulates autophagy and the ubiquitin-proteasome proteolytic pathway in chick skeletal muscle.

Differential regulation of protein synthesis by skeletal muscle type in chickens

In mammalian skeletal muscles, protein synthesis rates vary according to fiber types. We herein demonstrated differences in the regulatory mechanism underlying the protein synthesis in the pectoralis major (a glycolytic twitch muscle), adductor superficialis (an oxidative twitch muscle), and adductor profound (a tonic muscle) muscles of 14-day-old chickens. Under ad libitum feeding conditions, protein synthesis is significantly higher in the adductor superficialis muscle than in the pectoralis major muscle, suggesting that protein synthesis is upregulated in oxidative muscles in chickens, similar to that in mammals. In the pectoralis major muscle, fasting significantly inhibited the Akt/S6 pathway and protein synthesis with a corresponding decrease in plasma insulin concentration. Conversely, the insulin like growth factor-1 (IGF-1) mRNA levels significantly increased. These findings suggest that the insulin/Akt/S6 pathway plays an important role in the regulation of protein synthesis in the pectoralis major muscle. Interestingly, protein synthesis in the adductor superficialis muscle appears to be regulated in an Akt-independent manner, because fasting significantly decreased S6 phosphorylation and protein synthesis without affecting Akt phosphorylation. In the adductor profound muscle, IGF-1 expression, phosphorylation of Akt and S6, and protein synthesis were decreased by fasting, suggesting that insulin and/or skeletal IGF-1 appear contribute to protein synthesis via the Akt/S6 pathway. These findings revealed the differential regulation of protein synthesis depending on skeletal muscle types in chickens.

Role of Insulin-like Growth Factor-1 in the Central Regulation of Feeding Behavior in Chicks

Insulin-like growth factor-1 (IGF-1) is a key regulator of muscle development and metabolism in chickens. Recently, we have demonstrated that intracerebroventricular administration of IGF-1 significantly decreased food intake in broiler chicks. However, the molecular mechanisms underlying the IGF-1-induced anorexia and the anorexigenic effect of IGF-1 in different strains of commercial chicks have not been investigated. Neuropeptide Y (NPY, a hypothalamic orexigenic neuropeptide), agouti-related protein (AgRP, a hypothalamic orexigenic neuropeptide), and proopiomelanocortin (POMC, the precursor of hypothalamic anorexigenic neuropeptides) play important roles in the regulation of food intake in both mammals and chickens. Evidence shows that several cell signaling pathways in the hypothalamus are involved in regulating the feeding behavior of mammals. In the present study, we first investigated the effects of IGF-1 on the expression of appetite-regulating neuropeptides and phosphorylation of signaling molecules in the hypothalamus of broiler chicks. Intracerebroventricular administration of IGF-1 significantly increased the mRNA levels of POMC, whereas the mRNA levels of NPY and AgRP were not significantly altered. IGF-1 also significantly induced the phosphorylation of v-Akt murine thymoma viral oncogene homolog 1 (AKT) in the hypothalamus of chicks, but did not influence the phosphorylation of forkhead box O1, S6 protein, AMP-activated protein kinase, and extracellular signal-regulated kinase 1/2. We also compared the effect of IGF-1 on food intake in broiler chicks (a hyperphagic strain of chickens) and layer chicks. Results demonstrated that the threshold of IGF-1-induced anorexia in broiler chicks was higher than that in layer chicks. Our observations suggest that hypothalamic POMC and AKT may be involved in the IGF-1-induced anorexigenic pathway and that high threshold of IGF-1-induced anorexia in broiler chicks might be one of the causes of hyperphagia in broiler chicks. Overall, it appears that IGF-1 plays important roles in the central regulation of feeding behavior in chicks.

Central administration of insulin and refeeding lead to the phosphorylation of AKT, but not FOXO1, in the hypothalamus of broiler chicks

Several studies in rodents and layer chickens have demonstrated that insulin upregulates hypothalamic AKT-mediated signaling and expression of proopiomelanocortin (POMC, the precursor of alpha-melanocyte stimulating hormone, an anorexigenic peptide) and suppresses appetite in these animals. However, a previous study has also reported that insulin fails to suppress food intake in broiler chicks. In the present study, no significant differences were observed in hypothalamic AKT and forkhead box O1 (FOXO1) phosphorylation levels between broiler and layer chicks. The phosphorylation rate of AKT, but not that of FOXO1, increased in the hypothalami of broilers refed for 1 h after a 24-h fast, with a corresponding increase in plasma insulin concentration. Intracerebroventricular (ICV) administration of 50 pmol insulin, which could decrease food intake in broiler chicks, significantly increased the AKT phosphorylation rate, whereas no significant change was observed in FOXO1 phosphorylation or POMC expression after ICV insulin administration. These findings suggest that hypothalamic AKT responds to insulin in broiler chicks, but FOXO1-mediated regulation of POMC expression is not induced by insulin, which may be one of the causes of excessive food intake in broiler chickens.

Myostatin Increases Smad2 Phosphorylation and Atrogin-1 Expression in Chick Embryonic Myotubes

Skeletal muscle mass is an important trait in poultry meat production. In mammals, myostatin, a negative regulator of skeletal muscle growth, activates Smad transcription factors and induces the expression of atrogin-1 by regulating the Akt/FOXO pathway. Although the amino acid sequence of chicken myostatin is known to be completely identical to its mammalian counterpart, previous studies in chicken skeletal muscles have implied that the physiological roles of chicken myostatin are different from those of mammals. Furthermore, it remains to be elucidated whether myostatin affects cellular signaling factors and atrogin-1 expression. In this study, using chick embryonic myotubes, we found that myostatin significantly increased the phosphorylation rate of Smad2 and mRNA levels of atrogin-1. No significant change was observed in the phosphorylation of Akt and FOXO1. These in vitro results suggest that the molecular mechanisms underlying myostatin-induced expression of atrogin-1 might be different between chickens and mammals.

Feed Restriction Reveals Distinct Serum Metabolome Profiles in Chickens Divergent in Feed Efficiency Traits

Restrictive feeding influences systemic metabolism of nutrients; however, this impact has not been evaluated in chickens of diverging feed efficiency. This study investigated the effect of ad libitum versus restrictive feeding (85% of ad libitum) on the serum metabolome and white blood cell composition in chickens of diverging residual feed intake (RFI; metric for feed efficiency). Blood samples were collected between days 33 and 37 post-hatch. While serum glucose was similar, serum uric acid and cholesterol were indicative of the nutritional status and chicken’s RFI, respectively. Feed restriction and RFI rank caused distinct serum metabolome profiles, whereby restrictive feeding also increased the blood lymphocyte proportion. Most importantly, 10 amino acids were associated with RFI rank in birds, whereas restrictive feeding affected almost all detected lysophosphatidylcholines, with 3 being higher and 6 being lower in restrictively compared to ad libitum fed chickens. As indicated by relevance networking, isoleucine, lysine, valine, histidine, and ornithine were the most discriminant for high RFI, whereas 3 biogenic amines (carnosine, putrescine, and spermidine) and 3 diacyl-glycerophospholipids (38:4, 38:5, and 40:5) positively correlated with feed intake and body weight gain, respectively. Only for taurine, feed intake mostly explained the RFI-associated variation, whereas for most metabolites, other host physiological factors played a greater role for the RFI-associated differences, and was potentially related to insulin-signaling, phospholipase A2, and arachidonic acid metabolism. Alterations in the hepatic synthesis of long-chain fatty acids and the need for precursors for gluconeogenesis due to varying energy demand may explain the marked differences in serum metabolite profiles in ad libitum and restrictively fed birds.

Regulation of Autophagy in Chick Myotubes: Effects of Insulin, Insulin-Like Growth Factor-I, and Amino Acids

Autophagy, an intracellular bulk protein degradation system in skeletal muscle, is increased under catabolic conditions resulting in muscle atrophy. This study aimed to investigate the effects of insulin, insulin-like growth factor (IGF)-I, and amino acids on autophagy (LC3-II content and expression of autophagy-related genes) in chick myotubes. Chick myotubes were incubated with insulin (1 µg/ml), IGF-I (100 ng/ml), and amino acids for 3 h. The LC3-II content, an index of autophagosome formation, and mRNA expression of LC3B and GABARAPL1 were significantly decreased by insulin. The LC3-II content, but not mRNA expression of autophagy-related genes, was also significantly decreased by IGF-I. The LC3-II content and LC3B mRNA level were also significantly decreased by amino acids. The mRNA expression of atrogin-1/MAFbx, a muscle-specific ubiquitin ligase, was also significantly decreased by insulin, IGF-I, and amino acids in chick myotubes. These results indicated that insulin, IGF-I, and amino acids regulate autophagy as well as the ubiquitin-proteasome proteolytic pathway in chick myotubes.