Effects of Dietary Crude Protein Levels and Cysteamine Supplementation on Protein Synthetic and Degradative Signaling in Skeletal Muscle of Finishing Pigs

Dietary protein levels and cysteamine (CS) supplementation can affect growth performance and protein metabolism of pigs. However, the influence of dietary protein intake on the growth response of CS-treated pigs is unclear, and the mechanisms involved in protein metabolism remain unknown. Hence, we investigated the interactions between dietary protein levels and CS supplementation and the effects of dietary crude protein levels and CS supplementation on protein synthetic and degradative signaling in skeletal muscle of finishing pigs. One hundred twenty barrows (65.84 ± 0.61 kg) were allocated to a 2 × 2 factorial arrangement with five replicates of six pigs each. The primary variations were dietary crude protein (CP) levels (14% or 10%) and CS supplemental levels (0 or 700 mg/kg). The low-protein (LP) diets (10% CP) were supplemented with enough essential amino acids (EAA) to meet the NRC AA requirements of pigs and maintain the balanced supply of eight EAA including lysine, methionine, threonine, tryptophan, valine, phenylalanine, isoleucine, and leucine. After 41 days, 10 pigs per treatment were slaughtered. We found that LP diets supplemented with EAA resulted in decreased concentrations of plasma somatostatin (SS) (P<0.01) and plasma urea nitrogen (PUN) (P<0.001), while dietary protein levels did not affect other traits. However, CS supplementation increased the average daily gain (P<0.001) and lean percentage (P<0.05), and decreased the feed conversion ratio (P<0.05) and back fat (P<0.05). CS supplementation also increased the concentrations of plasma insulin-like growth factor 1 (IGF-1) (P<0.001), and reduced the concentrations of leptin, SS, and PUN (P<0.001). Increased mRNA abundance of Akt1 and IGF-1 signaling (P<0.001) and decreased mRNA abundance of Forkhead Box O (FOXO) 4 (P<0.01) and muscle atrophy F-box (P<0.001) were observed in pigs receiving CS. Additionally, CS supplementation increased the protein levels for the phosphorylated mammalian target of rapamycin (mTOR), eIF-4E binding protein 1, and ribosomal protein S6 kinase 1 (P<0.001). There were no interactions between dietary protein levels and CS supplementation for all traits. In conclusion, dietary protein levels and CS supplementation influenced growth and protein metabolism through independent mechanisms in pigs. In addition, LP diets supplemented with EAA did not affect growth performance and other traits except the concentrations of SS and PUN probably through maintenance of protein synthesis and degradation signaling. Moreover, CS supplementation improved growth performance by increasing plasma IGF-1 concentrations possibly through alterations of mTOR and Akt/FOXO signaling pathways in skeletal muscle of finishing pigs.

日粮蛋白质水平与半胱胺（cysteamine, CS）添加均可影响猪的生长性能与蛋白质代谢。然而，日粮蛋白质摄入对经CS处理猪的生长响应的影响尚不明确，其参与蛋白质代谢的调控机制亦尚未阐明。因此，本研究旨在探究日粮蛋白质水平与CS添加之间的交互作用，以及日粮粗蛋白质（crude protein, CP）水平与CS添加对育肥猪骨骼肌蛋白质合成与降解信号通路的影响。本试验选取120头阉公猪（初始体重65.84 ± 0.61 kg），采用2×2因子试验设计，设置5个重复，每个重复6头猪。试验因子为日粮粗蛋白质（CP）水平（14%或10%）与CS添加水平（0或700 mg/kg）。低蛋白质（low-protein, LP）日粮（CP为10%）补充足量的必需氨基酸（essential amino acids, EAA）以满足猪的NRC氨基酸需求标准，并维持赖氨酸、蛋氨酸、苏氨酸、色氨酸、缬氨酸、苯丙氨酸、异亮氨酸及亮氨酸这8种必需氨基酸的平衡供给。试验期为41天，试验结束后每处理组屠宰10头猪。试验结果表明：补充EAA的LP日粮可降低血浆生长抑素（somatostatin, SS）与血浆尿素氮（plasma urea nitrogen, PUN）的浓度（P<0.01、P<0.001），但日粮蛋白质水平对其余测定性状无显著影响。然而，CS添加可提高平均日增重（average daily gain, ADG）与瘦肉率（P<0.001、P<0.05），并降低料重比（feed conversion ratio, FCR）与背膘厚度（P<0.05）。CS添加还可升高血浆胰岛素样生长因子1（insulin-like growth factor 1, IGF-1）的浓度（P<0.001），并降低瘦素、生长抑素与血浆尿素氮的浓度（P<0.001）。经CS处理的猪骨骼肌中，Akt1及IGF-1信号通路相关基因的mRNA丰度显著升高（P<0.001），而叉头框O（Forkhead Box O, FOXO）4与肌肉萎缩F-box（muscle atrophy F-box）的mRNA丰度则显著降低（P<0.01、P<0.001）。此外，CS添加可升高磷酸化哺乳动物雷帕霉素靶蛋白（phosphorylated mammalian target of rapamycin, mTOR）、真核翻译起始因子4E结合蛋白1与核糖体蛋白S6激酶1的蛋白水平（P<0.001）。日粮蛋白质水平与CS添加对所有测定性状均无显著交互作用。综上，日粮蛋白质水平与CS添加可通过独立的调控机制影响猪的生长性能与蛋白质代谢。此外，补充EAA的LP日粮除降低血浆生长抑素与血浆尿素氮浓度外，对其余生长性能及相关性状无显著影响，这可能与其维持蛋白质合成与降解的信号通路平衡有关。进一步而言，CS添加可通过调控育肥猪骨骼肌中的mTOR与Akt/FOXO信号通路，升高血浆IGF-1浓度，从而改善其生长性能。