Exogenous essential amino acids stimulate an adaptive unfolded protein response in the mammary glands of lactating cows

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.