Abstract

Simple SummaryCells of the mammary gland obtain their necessary nutrients from the blood to produce milk components, such as casein. To achieve higher productivity, cows are excessively supplemented, thus generating a higher cost of production and affecting the environment. Therefore, this triggers the need for a reduction in the supplementation of essential amino acids without affecting the milk composition. The present in vitro study shows that, through homeostatic and homeorhetic processes, cells have the ability to maintain stable casein levels despite decreasing the percentage of essential amino acids (EAAs) supplied. These findings could contribute to the proposal of more efficient nutritional strategies at lower environmental and economic costs.Mammary epithelial cells (MECs) in culture are a useful model for elucidating mammary gland metabolism and changes that occur under different nutrient disponibility. MECs were exposed to different treatments: 100% EAA for 8 h and 24 h restriction (R); 2% EAA for 8 h and 24 h R; 2% EAA for 8 h and 24 h + 100% EAA for 8 h and 24 h restriction + re-feeding (R + RF). Western blotting and protein quantification was performed. The Kyoto Encyclopedia of Genes and Genomes (KEGG) software identified the amino acids (AAs) and signaling pathways. The chi-squared test, multiple classification analysis, and analysis of variance were used for the purification and identification of data. Intracellular casein levels were not affected. The KEGG analysis revealed that the important pathways of metabolism of AAs, which were involved in processes related to metabolism and biosynthesis of phenylalanine, tyrosine, and tryptophan (fumarate, acetyl-CoA, and tricarboxylic acid (TCA) cycle), were affected by both R and R + RF treatments, mainly through the glutamic-oxaloacetic transaminase-2 enzyme. Additionally, metabolic processes mediated by the mitochondrial malate dehydrogenase, S-adenosylmethionine synthetase, and asparagine synthase proteins positively regulated the carbohydrate pathway, pyruvate, and TCA cycles, as well as the metabolism of alanine, aspartate, and glutamate metabolism (carbohydrate and TCA cycle). We hypothesized that MECs have the capacity to utilize alternative pathways that ensure the availability of substrates for composing milk proteins.

Highlights

  • 20% of the protein offered is eliminated as feces; 15% is eliminated as ammonium from the rumen; 40% is catabolized after absorption; and only 25% is recovered in milk protein [2]

  • Intracellular casein levels were not affected by treatment with a mean of 0.086 ± 0.02 for the control group (T1) and a mean of 0.081 ± 0.002 and 0.069 ± 0.004 with 2% essential amino acids (EAAs) R

  • We propose that the increase in MDH2 protein levels in cysteine and methionine metabolism could indicate that when performing the R and R + RF of EAA, the Mammary epithelial cells (MECs) optimize the processes for the energy metabolism of molecules, such as pyruvate, and try to keep the metabolic processes stable

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Summary

Introduction

Given the current feeding conditions, where only 30% of nitrogen (N) offered to cattle via diet is metabolized [1], large losses are excreted to the environment, thereby increasing the costs of production. 20% of the protein offered is eliminated as feces; 15% is eliminated as ammonium from the rumen; 40% is catabolized after absorption; and only 25% is recovered in milk protein [2]. The above-mentioned points reflect the inefficiency of the usage of proteins in lactating cows [3]. Optimal amino acid (AA) profile of dietary protein for these types of cows is not specified by nutritional models [4]. In the field, the levels of raw protein in the diet are usually higher than those required by animals [5]

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