Nichols K, Doelman J, Kim J J M, Carson M, Metcalf J A, Cant J P
Department of Animal Biosciences, University of Guelph, ON, N1G 2W1 Canada.
Trouw Nutrition Agresearch Canada, Guelph, ON, N1G 4T2 Canada.
J Dairy Sci. 2017 Jul;100(7):5909-5921. doi: 10.3168/jds.2016-12387. Epub 2017 May 3.
The phosphorylation of mammalian target of rapamycin complex 1 (mTORC1) components and integrated stress response networks in the mammary glands of lactating cows have not accounted for the stimulation of milk protein yield by chronic supplementation with AA or glucose. Faster milk protein synthesis could be a consequence of increased milk protein mRNA per cell, the number of ribosomes per cell, the secretory capacity of cells, or the mammary cell number. To investigate these 4 possibilities using a translational and transcriptional approach, we performed protein and gene expression analyses of mammary and longissimus dorsi tissue collected from lactating dairy cows after 5 d of abomasal infusion with saline or 844 or 1,126 g/d of an essential AA (EAA) mixture, with and without 1,000 g/d glucose. Infusion with EAA increased milk protein yield but did not affect the phosphorylation of mTORC1-related proteins in the mammary gland. In skeletal muscle, phosphorylation of 4EBP1 (eIF4E-binding protein 1) increased in response to both EAA and glucose, and phosphorylated S6K1 (70-kDa ribosomal protein S6 kinase) increased with glucose. In response to EAA, mammary mRNA expression of the marker genes for milk proteins, ribosome biogenesis, and cell proliferation were not upregulated. Instead, reciprocal regulation of 2 arms of the unfolded protein response occurred. Infusion of EAA for 5 d activated XBP1 (X-box binding protein 1) mRNA, encoding a transcription factor for endoplasmic reticulum biogenesis, and it decreased the mRNA expression of genes encoding pro-apoptotic protein CHOP (C/EBP homologous protein) and downstream GADD34 (growth arrest and DNA damage-inducible 34). These findings implicate non-stress-related, adaptive capabilities of the unfolded protein response in the long-term nutritional regulation of milk protein yield in lactating dairy cows.
在泌乳奶牛的乳腺中，雷帕霉素靶蛋白复合物1（mTORC1）组分的磷酸化以及整合应激反应网络，并未解释长期补充氨基酸（AA）或葡萄糖对乳蛋白产量的刺激作用。更快的乳蛋白合成可能是由于每个细胞中乳蛋白mRNA增加、每个细胞中核糖体数量增加、细胞的分泌能力增强或乳腺细胞数量增加所致。为了使用翻译和转录方法研究这4种可能性，我们对从泌乳奶牛采集的乳腺和背最长肌组织进行了蛋白质和基因表达分析，这些奶牛经真胃灌注生理盐水或844或1126 g/d的必需氨基酸（EAA）混合物5天，同时添加或不添加1000 g/d葡萄糖。灌注EAA可提高乳蛋白产量，但不影响乳腺中mTORC1相关蛋白的磷酸化。在骨骼肌中，4EBP1（真核翻译起始因子4E结合蛋白1）的磷酸化对EAA和葡萄糖均有反应而增加，磷酸化的S6K1（70 kDa核糖体蛋白S6激酶）随葡萄糖增加。对EAA的反应中，乳蛋白、核糖体生物合成和细胞增殖的标记基因的乳腺mRNA表达未上调。相反，未折叠蛋白反应的两个分支发生了相互调节。灌注EAA 5天激活了XBP1（X盒结合蛋白1）mRNA，其编码内质网生物合成的转录因子，并降低了编码促凋亡蛋白CHOP（C/EBP同源蛋白）和下游GADD34（生长停滞和DNA损伤诱导蛋白34）的基因的mRNA表达。这些发现表明，未折叠蛋白反应的非应激相关适应性能力参与了泌乳奶牛乳蛋白产量的长期营养调控。
