Methionine, leucine, isoleucine, or threonine effects on mammary cell signaling and pup growth in lactating mice

Two studies were undertaken to assess the effects of individual essential AA supplementation of a protein-deficient diet on lactational performance in mice using litter growth rates as a response variable. The first study was designed to establish a dietary protein response curve, and the second to determine the effects of Leu, Ile, Met, and Thr supplementation of a protein-deficient diet on lactational performance. In both studies, dams were fed test diets from parturition through d 17 of lactation, when the studies ended. Mammary tissue was collected on d 17 from mice on the second experiment and analyzed for mammalian target of rapamycin (mTOR) pathway signaling. Supplementation with Ile, Leu, or Met independently increased litter weight gain by 11, 9, and 10%, respectively, as compared with the protein-deficient diet. These responses were supported by independent phosphorylation responses for mTOR and eIF4E binding protein 1 (4eBP1). Supplementation of Ile, Leu, and Met increased phosphorylation of mTOR by 55, 34, and 47%, respectively, as compared with the protein-deficient diet. Phosphorylation of 4eBP1 increased in response to Ile and Met supplementation by 60 and 40%, respectively. Supplementation of Ile and Met increased phosphorylation of Akt/protein kinase B (Akt) by 41 and 59%, respectively. This work demonstrated that milk production responds nonlinearly to protein supply, and milk production and the mTOR pathway responded independently to supplementation of individual AA. The former demonstrates that a linear breakpoint model is an inappropriate description of the responses, and the latter demonstrates that no single factor limits AA for lactation. Incorporation of a multiple-limiting AA concept and nonlinear responses into milk protein response models will help improve milk yield predictions and allow derivation of diets that will increase postabsorptive N efficiency and reduce N excretion by lactating animals.