The Effects of Amino Acid Nutritional Deficiency on the Expression of Protein Metabolism-Related Genes in the Mammary Gland and Muscle Tissues of Lactating Mice

The mammary gland tissue exhibits a series of responses that are different from those of muscle and other peripheral tissues under amino acid deficiency. So, this present study was designed to investigate the effects of amino acid nutritional deficiency on the expression of protein metabolism-related genes in the mammary gland and muscle tissues of lactating mice. A total of 60 postpartum, lactating Kunming white mice were selected and randomly divided into 5 groups; each group contained 12 mice. Group A was the control group. The mice in group A were maintained on a normal diet after the initiation of lactation. Group B (starved) was given normal saline via intragastric administration. Group C (energy) was given glucose solution via intragastric administration. Groups D and E received a sodium caseinate solution via intragastric administration, which provided 0.5 g protein/d and 1.5 g protein/d, respectively. The results showed the following. 1) When the mice were exposed to nutritional stress caused by dietary amino acid deficiency, the β-casein mRNA expression level was increased in the mammary gland tissue. The increase in β-casein expression was the most significant in the energy-supplemented group, followed by the starved group (P < 0.01). In addition, the expression level of myosin heavy chain 2A (MHC2a) mRNA was markedly reduced in the muscle tissue (P < 0.05). 2) Different dietary treatments significantly affected the mRNA expression of ubiquitin system enzymes such as PolyUbC, E214k and C2 (P < 0.01). The expression levels of the enzymes were upregulated. 3) The blood level of insulin-like growth factor 1 (IGF-1) was significantly decreased in the treatment groups (P < 0.01). 4) The phosphorylation level of the p70S6 kinase (p70S6K) was markedly enhanced in the mammary gland tissues collected from the treatment groups (P < 0.05). 5) The phosphorylation level of p70S6K was elevated in the muscle tissues collected from the treatment groups. However, the magnitude of the increase was far smaller compared to that in the mammary gland tissues.