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Lysine and BCAA deficiencies decrease abundances of S6K and eIF2Bå in the mammary glands of lactating dairy cows
There have been recent investigations into the potential role of mRNA translational control in the regulation of synthesis of milk protein in the lactating bovine mammary gland. An infusion subtraction protocol was used to explore effects on abundance and activity state of regulators of mRNA translation in response to specific essential amino acid (EAA) deficiencies and imbalances. Five lactating cows on a diet of 11.7% protein were infused abomasally for 5 d with saline, 563 g/d of a complete EAA mix (equivalent to EAA in 1kg casein), or EAA without lysine (Lys), leucine (Leu) or the branched-chain amino acids (BCAA; isoleucine (Ile), Leu, and valine (Val)) in a 5 × 5 Latin square design. Data was analyzed using the MIXED procedure in SAS where period and treatment were considered classification effects and cow considered a random effect. The translation factors examined in mammary biopsies collected on d 5 of each period were ribosomal protein S6 kinase (S6K) to indicate mTOR (mammalian target of rapamycin) activity, eukaryotic initiation factor 2 alpha (eIF2α) to indicate uncharged tRNA signalling, and eukaryotic initiation factor 2B epsilon (eIF2Bε) to indicate insulin effects. Milk protein yield increased in response to EAA compared to saline (P = 0.001), while Lys, Leu, and BCAA deficiencies depressed milk protein yield compared to EAA (P < 0.03). Infusion of the complete EAA mix did not affect mTOR activity (P = 0.65) but subtraction of BCAA from the mix decreased phosphorylated S6K abundance (P = 0.05) and subtraction of Leu tended to decrease phosphorylated S6K (P = 0.10). Similarly, abundance of total eIF2Bε was not affected by infusion of EAA (P = 0.39) but decreased when BCAA (P = 0.04) or Leu (P = 0.06) were subtracted. There was a correlation of 0.58 between abundances of phosphorylated S6K and total eIF2Bε. Lys subtraction had no effect on mammary mTOR/eIF2Bε signalling, but the abundance of total S6K tended to be lower during Lys deficiency compared to saline (P = 0.06) and EAA (P = 0.09). Phosphorylation state of eIF2α was not increased by any of the imbalances or deficiencies. It was concluded that Lys deficiency may impair milk protein yield through a decline in translational activation capacity, indicated by S6K abundance. BCAA deficiencies may impair milk protein yields through deactivation of mTOR-mediated up-regulation of eIF2Bε abundance.
Keywords: milk protein synthesis; mRNA translation regulation, essential amino acid