This study aims to determine the effects of D-methionine (D-Met) isomer and the methionine precursor 2-hydroxy-4-methylthiobutanoic acid i (HMBi) supplementation on milk protein synthesis on immortalized bovine mammary epithelial cell (MAC-T). MAC-T cells were seeded using 10-cm dishes and cultured in Dulbecco’s modified Eagle’s medium/F12 (DMEM/F12) basic medium. The basic medium of DMEM/F12 was replaced with the lactogenic DMEM/F12 differentiation medium when 90% of MAC-T cells reached confluency. The best dosage at 0.6 mM of D-Met and HMBi and incubation time at 72 h were used uniformly for all treatments. Each treatment was replicated six times wherein treatments were randomly assigned in a 6-well plate. Cell, medium, and total protein were determined using a bicinchoninic acid protein assay kit. Genes, proteomics and metabolomics analyses were also done to determine the mechanism of the milk protein synthesis pathway. Data were analyzed by two-way analysis of variance (ANOVA) with supplement type and plate as fixed effects. The least significant difference test was used to evaluate the differences among treatments. The HMBi treatment group had the highest beta-casein and S6 kinase beta-1 (S6K1) mRNA gene expression levels. HMBi and D-Met treatments have higher gene expressions compared to the control group. In terms of medium protein content, HMBi had a higher medium protein quantity than the control although not significantly different from the D-Met group. HMBi supplementation stimulated the production of eukaryotic translation initiation factor 3 subunit protein essential for protein translation initiation resulting in higher medium protein synthesis in the HMBi group than in the control group. The protein pathway analysis results showed that the D-Met group stimulated fructose–galactose metabolism, glycolysis pathway, phosphoinositide 3 kinase, and pyruvate metabolism. The HMBi group stimulated the pentose phosphate and glycolysis pathways. Metabolite analysis revealed that the D-Met treatment group increased seven metabolites and decreased uridine monophosphate (UMP) production. HMBi supplementation increased the production of three metabolites and decreased UMP and N-acetyl-L-glutamate production. Taken together, D-Met and HMBi supplementation are effective in stimulating milk protein synthesis in MAC-T cells by genes, proteins, and metabolites stimulation linked to milk protein synthesis.

With deep knowledge and numerous researches about DNA in addition to recent advances in molecular biology and omics analysis methodology, manipulation of DNA transcription becomes possible that may eventually affect protein expression. Possible posttranscriptional modifications such as phosphorylation of proteins could also greatly affect protein expression. There are vast evidences showing that synthesis of milk protein is strongly regulated by several factors such as insulin, amino acids and their specific amino acid transporters as well as various energy sources through transcriptional and posttranscriptional pathways with insulin-mTOR pathway serving as the central point. Available latest data suggest the possibility of nutrigenomic interventions to manipulate milk protein synthesis by supplying amino acids and different energy sources. This could greatly contribute to major progress in understanding milk protein synthesis. With improvement of 'omics' tools, utilization of these tools in our future studies would help us fully understand the mechanism behind. This could lead to possible control and manipulation of milk protein synthesis in mammary gland.

The objective of this study was to determine the effects of supplementing L-isoleucine (L-Ile) on milk protein synthesis, using an immortalized bovine mammary epithelial (MAC-T) cell line. In this case, the cells were treated with 0, 0.3, 0.6, 0.9, 1.2 and 1.5 mM of supplemental Isoleucine (Ile), and the most efficient time for protein synthesis for each amino acid was determined by measuring the cell, medium and total protein at 0, 24, 48, 72 and 96 h. Confirmatory tests showed that 48h incubation time and 0.6 mM dosage of L-Ile are considered as the optimal time and dosage. The mechanism of milk protein synthesis was elucidated through proteomics analysis to clarify the metabolic pathway. When the L-Ile was supplemented, extracellular protein (medium protein) reached a peak at 48h, whereas in the case of the intracellular cell protein, it was shown to have reached to its peak at 24h in all L-Ile dosage treatments. In total, it is noted that there were 63 upregulated and 52 downregulated proteins. The results of the protein pathway analysis showed that the L-Ile group stimulated insulin/IGF pathway-mitogen activated protein kinase kinase/MAP kinase cascade, insulin/IGF pathway-protein kinase B signaling cascade, p53 pathway, de novo purine biosynthesis, Wnt signaling pathway, glycolysis, pentose phosphate pathway, and ATP synthesis which are pathways involved and related to protein and energy metabolism. Together, these results demonstrate that L-Ile supplementation was effective in stimulating β-casein synthesis by stimulating genes and pathways which are significantly related to protein and energy metabolism.

This study was conducted to investigate the effect of supplementing different types of methionine (L and D-type) and its precursor (HMBi) on milk protein synthesis using immortalized bovine mammary epithelial cell line (MAC-T Cell); D-methionine (D-Met), L-methionine (L-met) and 2-hydroxy-4-methylthiobutanoic acid I (HMBi), an isopropyl ester of the hydroxy analogue precursor of methionine. The underlying mechanism of milk protein synthesis by adding D- and L-type amino acid as well as HMBi was elucidated through omics analysis to verify the metabolism pathway. Results showed that HMBi group showed the highest beta casein mRNA expression levels compared to D- and L-Met groups and highest medium protein although not different with the L-Met treatment. The observed upregulated (>2 protein expression vs. control) and downregulated (<0.5 protein expression vs. control) proteins in L-Met, D-Met and HMBi treated groups were: 39, 77; 46, 68; and 40, 78, respectively. Interestingly, based on protein pathway analysis, L-Met treated group stimulated the ATP synthesis, PI3 kinase and pyruvate metabolism. On the other hand, the D-Met group stimulated fructose-galactose metabolism, glycolysis pathway, PI3 kinase and pyruvate metabolism. And lastly HMBi-treated group stimulated pentose phosphate pathway and glycolysis pathway. Metabolite analysis revealed that L-Met treated group resulted in the increase in 11 metabolites. On the other hand, D-Met treated group showed increase in 7 metabolites and decreased of uridine monophosphate (UMP). HMBi supplementation caused increases of 3 metabolites and decreased of UMP and N-acetyl-L-glutamate Addition of different isoforms of Met stimulated the production of intermediate metabolites for energy production. Addition of L-Met stimulated the production of energy metabolites such as pyruvate, malate, and fumarate, well-known as intermediates of Krebs cycle. On the other hand, HMBi supplementation resulted in increases of energy metabolite glucose 1-phosphate and 6-phosphogluconate. Results showed that HMBi-treated group exhibited highest expression of β-casein mRNA expression by stimulating proteins and metabolites as well as protein and metabolic pathways involved in protein and energy synthesis. As a result, HMBi-treated group resulted in highest protein concentration but not significantly different with L-Met. Both the D- and L-isoforms has considerably the same medium protein concentration and β-casein mRNA expression higher than the control. So, D- and L-Met isoforms can be used alternatively without any significant change in protein synthesis efficiency in bovine mammary epithelial cells.