Ratio of lysine to methionine alters expression of genes involved in milk protein transcription and translation and mTOR phosphorylation in bovine mammary cells

L-leucine promotes the synthesis of milk protein and milk fat in bovine mammary epithelial cells through the AKT/mTOR signaling pathway under hypoxic conditions

Hypoxia stress has been demonstrated to impede animal embryonic development, spermatogenesis, and lactation, leading to decreased animal production performance. However, the impact of hypoxia-induced activation of hypoxia inducible factor-1 (HIF-1) signaling on milk protein and fat synthesis remains unclear. L-leucine, a branched-chain amino acid, is known to modulate milk protein and fat synthesis. Therefore, our study aimed to evaluate the effect of L-leucine on milk protein and fat synthesis under hypoxic conditions and shed light on the molecular mechanism using an in vitro model. The results indicated that hypoxia treatment significantly decreased the synthesis of α-casein and β-casein, as well as inhibited factors related to milk fat synthesis in bovine mammary epithelial cells (MAC-T). Additionally, hypoxia stress suppressed the activities of the mammalian target of rapamycin (mTOR) and protein kinase B (AKT). Interfering with HIF-1α significantly reversed the expression of AKT, mTOR and factors related to milk synthesis. Importantly, supplementation with L-leucine activated AKT/mTOR signaling, thereby enhancing milk protein and fat synthesis in MAC-T cells to some extent. In conclusion, these findings suggest that HIF-1 signaling plays an important role in milk synthesis and that L-leucine may stimulate the synthesis of milk protein and fat by activating the AKT/mTOR signaling pathway under hypoxic conditions, making it a potential additive for promoting milk synthesis inhibited by hypoxia.

Rumen-protected methionine supplementation improves lactation performance and alleviates inflammation during a subclinical mastitis challenge in lactating dairy cows

This study aimed to evaluate the effects of rumen-protected Met on lactation performance, inflammation and immune response, and liver glutathione of lactating dairy cows during a subclinical mastitis challenge (SMC). Thirty-two Holstein cows (145 ± 51 DIM) were enrolled in a randomized complete block design. At -21 d relative to the SMC, cows were assigned to dietary treatments, and data were collected before and during the SMC. Cows were blocked according to parity, DIM, and milk yield and received a basal diet (17.4% CP; Lys 7.01% MP and Met 2.14% MP) plus 100 g/d of ground corn (CON; n = 16) or a basal diet plus 100 g/d of ground corn and rumen-protected Met (SM, Smartamine M at 0.09% of dietary DM; n = 16), fed as a top-dress. At 0 d, the mammary gland's rear right quarter was infused with 100,000 cfu of Streptococcus uberis (O140J). Milk yield was recorded twice daily from 0 until 3 d relative to SMC. Milk samples were collected during each milking from 0 to 3 d relative to SMC, blood samples were collected at 0, 6, 12, 24, 48, and 72 h relative to SMC. The mTOR pathway activation was assessed in immune cells in blood and milk samples by measuring quantity and phosphorylation status of mTOR-related proteins, including AKT, S6RP, and 4EBP1. For the ratio of phosphorylated to total AKT, S6RP, and 4EBP1, blood samples were collected at 0, 12, and 24 h, and milk samples at 24 h relative to SMC. Liver biopsies were performed at -10 d and 24 h relative to SMC for measurement of glutathione. Linear mixed models with repeated measures were used to analyze the results. There was a trend for greater milk yield per milking (+ 0.8 kg) and per day (+1.7 kg) after SMC in SM cows compared with CON. The DMI was not affected by dietary treatments. Reactive oxygen metabolites (ROM) were lower in SM cows than in CON. Milk somatic cell linear score was not affected by dietary treatments, and a score >4 at 24 h confirmed subclinical mastitis. The SM cows had greater milk fat percentage at 24 and 36 h post SMC, resulting in overall greater milk fat. Milk protein tended to be greater in SM cows than in CON. We observed greater liver glutathione in SM cows than in CON. Among inflammation biomarkers, ceruloplasmin was lower for SM cows compared with CON. In milk, greater pAKT:AKT and pS6RP:S6RP ratios were observed in immune cell populations from SM cows compared with CON. Blood neutrophils had a greater p4EBP1:4EBP1 ratio in SM cows compared with CON. Overall, our results show that Met supplementation during an SMC positively affected milk performance, lowered the risk of oxidative stress, and attenuated inflammation partially by increasing liver glutathione and immune cells' protein synthesis via mTOR signaling.

Effects of lysine and threonine on milk yield, amino acid metabolism, and fecal microbiota of Yili lactating mares

The nutritional benefits of mare milk are attracting increasing consumer interest. Limited availability due to low yield poses a challenge for widespread adoption. Although lysine and threonine are often used to enhance protein synthesis and muscle mass in horses, their impact on mare milk yield and nutrient composition remains underexplored. This study investigated the effects of lysine and threonine supplementation on 24 healthy Yili mares, mares at day 30 of lactation, over a 120-day period. The mares were divided into control and three experimental groups (six mares each) under pure grazing conditions. The control group received no amino acid supplementation, while experimental groups received varying daily doses of lysine and threonine: Group I (40 g lysine + 20 g threonine), Group II (60 g lysine + 40 g threonine), and Group III (80 g lysine + 60 g threonine). Supplementation in Group II notably increased milk yield, while Groups I and II showed higher milk fat percentages, and all experimental groups exhibited improved milk protein percentages. Additionally, blood levels of total protein, albumin, triglycerides, and glucose were reduced. Detailed analyses from Group II at peak lactation (day 60) included targeted metabolomics and microbial sequencing of milk, blood, and fecal samples. Amino acid metabolomics assessed amino acid content in mare milk and serum, while 16S rRNA gene sequencing evaluated rectal microbial composition. The results indicated that lysine and threonine supplementation significantly increased levels of threonine and creatine in the blood, and lysine, threonine, glutamine, and alanine in mare milk. Microbial analysis revealed a higher prevalence of certain bacterial families and genera, including Prevotellaceae, p_251_o5, and Rikenellaceae at the family level, and unclassified_p_251_o5, Prevotellaceae_UCG_001, and Rikenellaceae_RC9_gut_group at the genus level. Multi-omics analysis showed positive correlations between specific fecal genera and amino acids in mare milk. For instance, Prevotellaceae_UCG_003, unclassified Bacteroidetes_BS11_gut_group, and Corynebacterium were positively correlated with lysine, while unclassified Prevotellaceae was positively correlated with alanine and threonine, and Unclassified_Bacteroidales_BS11_gut_group was positively correlated with glutamine. In summary, lysine and threonine supplementation in grazing lactating mares enhanced milk production and improved milk protein and fat quality. It is recommended that herders, veterinarians, and technicians consider amino acid content in the diet of lactating mares. The optimal supplementation levels under grazing conditions for Yili horses were determined to be 60 g lysine and 40 g threonine per day. Future research should explore the molecular mechanisms by which these amino acids influence milk protein and lipid synthesis in mare mammary epithelial cells.

Varying the ratio of Lys: Met through enhancing methionine supplementation improved milk secretion ability through regulating the mRNA expression in bovine mammary epithelial cells under heat stress

Introduction

The ratio of lysine (Lys) to methionine (Met) with 3.0: 1 is confirmed as the "ideal" profile for milk protein synthesis, but whether this ratio is suitable for milk protein synthesis under HS needs to be further studied.

Methods

To evaluate the molecular mechanism by which HS and Lys to Met ratios affect mammary cell functional capacity, an immortalized bovine mammary epithelial cell line (MAC-T) is incubated with 5 doses of Met while maintaining a constant concentration of Lys. The MAC-T cells was treated for 6 h as follow: Lys: Met 3.0: 1 (control 37°C and IPAA 42°C) or treatments under HS (42°C) with different ratios of Lys: Met at 2.0: 1 (LM20), 2.5: 1 (LM25), 3.5: 1 (LM35) and 4.0: 1 (LM40). RNA sequencing was used to assess transcriptome-wide alterations in mRNA abundance.

Results

The significant difference between control and other groups was observed base on PCA analysis. A total of 2048 differentially expressed genes (DEGs) were identified in the IPAA group relative to the control group. Similarly, 226, 306, 148, 157 DEGs were detected in the LM20, LM25, LM35 and LM40 groups, respectively, relative to the IPAA group. The relative mRNA abundance of HSPA1A was upregulated and anti-apoptotic genes (BCL2L1 and BCL2) was down-regulated in the IPAA group, compared to the control group (p < 0.05). Compared with the IPAA group, the relative mRNA abundance of anti-apoptotic genes and casein genes (CSN1S2 and CSN2) was up-regulated in the LM25 group (p < 0.05). The DEGs between LM25 and IPAA groups were associated with the negative regulation of transcription RNA polymerase II promoter in response to stress (GO: 0051085, DEGs of BAG3, DNAJB1, HSPA1A) as well as the mTOR signaling pathway (ko04150, DEGs of ATP6V1C2, WNT11, WNT3A, and WNT9A). Several DEGs involved in amino acids metabolism (AFMID, HYKK, NOS3, RIMKLB) and glycolysis/gluconeogenesis (AFMID and MGAT5B) were up-regulated while DEGs involved in lipolysis and beta-oxidation catabolic processes (ALOX12 and ALOX12B) were down-regulated.

Conclusion

These results suggested that increasing Met supply (Lys: Met at 2.5: 1) may help mammary gland cells resist HS-induced cell damage, while possibly maintaining lactation capacity through regulation of gene expression.

Hexokinase 1 and 2 mediates glucose utilization to regulate the synthesis of kappa casein via ribosome protein subunit 6 kinase 1 in bovine mammary epithelial cells

Glucose plays a vital part in milk protein synthesis through the mTOR signaling pathway in bovine mammary epithelial cells (BMEC). The objectives of this study were to determine how glucose affects hexokinase (HK) activity in BMEC and investigate the regulatory effect of HK in kappa casein (CSN3) synthesis via the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway in BMEC. For this, HK1 and HK2 were knocked out in BMEC using the CRISPR/Cas9 system. The gene and protein expression, glucose uptake, and cell proliferation were measured. We found that glucose uptake, cell proliferation, CSN3 gene expression levels, and expression of HK1 and HK2 increased with increasing glucose concentrations. Notably, glucose uptake was significantly reduced in HK2 knockout (HK2KO) BMEC treated with 17.5 mM glucose. Moreover, under the same glucose treatment conditions, the proliferative ability and abundance of CSN3 were significantly diminished in both HK1 knockout (HK1KO) and HK2KO BMEC compared with that in wild-type BEMC. We further observed that the phosphorylation levels of ribosome protein subunit 6 kinase 1 (S6K1) were reduced in HK1KO and HK2KO BMEC following treatment with 17.5 mM glucose. As expected, the levels of glucose-6-phosphate and the mRNA expression levels of glycolysis-related genes were decreased in both HK1KO and HK2KO BMEC following glucose treatment. These results indicated that the knockout of HK1 and HK2 inhibited cell proliferation and CSN3 expression in BMEC under glucose treatment, which may be associated with the inactivation of the S6K1 and inhibition of glycolysis.

Effect of dietary supplementation with rumen-protected amino acids, lysine and methionine, on the performance of Comisana ewes and on the growth of their lambs

The purpose of the study was to evaluate the effect of supplementing the diets of Comisana ewes with rumen protected methionine and lysine (RP-ML) for a duration of 60 days on quantitative-qualitative production of milk, antioxidant parameters of milk and blood, biochemical parameters of blood, and lambs' growth performance. Two groups of 15 ewes with equal average body weight were considered for the trial. The control group was administered a standard diet (Control, C) and the experimental group the standard diet supplemented with 1.5% lysine and 1.0% methionine (Treated, T). Results showed that RP-ML supplementation positively influenced milk yield, fat and protein content, and casein (p < 0.01). Furthermore, lambs of the T group, compared with those of the C group, showed a 15% higher growth rate during the suckling period of 42 days. Finally, the total antioxidant capacity of milk, measured by ferric reducing antioxidant power and 2,2'-azino-bis(3-ethylbenzotiazolin-6-sulfonic acid assay, was significantly lower in T groups (p < 0.05). In conclusion, the results showed that the integration of RP-ML in ewes could be a valid strategy in order to improve their performances but further investigations are necessary to define the right concentration to be administered to the animals.

Methionine Promotes Milk Protein Synthesis via the PI3K-mTOR Signaling Pathway in Human Mammary Epithelial Cells

Breast milk is widely considered to be the most natural, safe, and complete food for infants. However, current breastfeeding rates fall short of the recommendations established by the World Health Organization. Despite this, there are few studies that have focused on the promotion of human lactation through nutrient supplementation. Therefore, the aim of this study was to investigate the effect of methionine on milk synthesis in human mammary epithelial cells (MCF-10A cells) and to explore the underlying mechanisms. To achieve this, MCF-10A cells were cultured with varying concentrations of methionine, ranging from 0 to 1.2 mM. Our results indicated that 0.6 mM of methionine significantly promoted the synthesis of milk protein. An RNA-seq analysis revealed that methionine acted through the PI3K pathway. This finding was validated through real-time quantitative polymerase chain reaction (RT-qPCR) and Western blotting. In addition, PI3K inhibition assays confirmed that methionine upregulated the expression of both mTOR and p-mTOR through activation of PI3K. Taken together, these findings suggest that methionine positively regulates milk protein synthesis in MCF-10A cells through the PI3K-mTOR signaling pathway.

Enhancing Metabolism and Milk Production Performance in Periparturient Dairy Cattle through Rumen-Protected Methionine and Choline Supplementation

For dairy cattle to perform well throughout and following lactations, precise dietary control during the periparturient phase is crucial. The primary issues experienced by periparturient dairy cows include issues like decreased dry matter intake (DMI), a negative energy balance, higher levels of non-esterified fatty acids (NEFA), and the ensuing inferior milk output. Dairy cattle have always been fed a diet high in crude protein (CP) to produce the most milk possible. Despite the vital function that dairy cows play in the conversion of dietary CP into milk, a sizeable percentage of nitrogen is inevitably expelled, which raises serious environmental concerns. To reduce nitrogen emissions and their production, lactating dairy cows must receive less CP supplementation. Supplementing dairy cattle with rumen-protected methionine (RPM) and choline (RPC) has proven to be a successful method for improving their ability to use nitrogen, regulate their metabolism, and produce milk. The detrimental effects of low dietary protein consumption on the milk yield, protein yield, and dry matter intake may be mitigated by these nutritional treatments. In metabolic activities like the synthesis of sulfur-containing amino acids and methylation reactions, RPM and RPC are crucial players. Methionine, a limiting amino acid, affects the production of milk protein and the success of lactation in general. According to the existing data in the literature, methionine supplementation has a favorable impact on the pathways that produce milk. Similarly, choline is essential for DNA methylation, cell membrane stability, and lipid metabolism. Furthermore, RPC supplementation during the transition phase improves dry matter intake, postpartum milk yield, and fat-corrected milk (FCM) production. This review provides comprehensive insights into the roles of RPM and RPC in optimizing nitrogen utilization, metabolism, and enhancing milk production performance in periparturient dairy cattle, offering valuable strategies for sustainable dairy farming practices.

Genetics, environmental stress, and amino acid supplementation affect lactational performance via mTOR signaling pathway in bovine mammary epithelial cells

Mammary glands are known for their ability to convert nutrients present in the blood into milk contents. In cows, milk synthesis and the proliferation of cow mammary epithelial cells (CMECs) are regulated by various factors, including nutrients such as amino acids and glucose, hormones, and environmental stress. Amino acids, in particular, play a crucial role in regulating cell proliferation and casein synthesis in mammalian epithelial cells, apart from being building blocks for protein synthesis. Studies have shown that environmental factors, particularly heat stress, can negatively impact milk production performance in dairy cattle. The mammalian target of rapamycin complex 1 (mTORC1) pathway is considered the primary signaling pathway involved in regulating cell proliferation and milk protein and fat synthesis in cow mammary epithelial cells in response to amino acids and heat stress. Given the significant role played by the mTORC signaling pathway in milk synthesis and cell proliferation, this article briefly discusses the main regulatory genes, the impact of amino acids and heat stress on milk production performance, and the regulation of mTORC signaling pathway in cow mammary epithelial cells.

The functional and regulatory entities underlying free and peptide-bound amino acid transporters in the bovine mammary gland

Free and peptide-bound AA act as building blocks and key regulators of milk protein. To improve milk protein production, mammary epithelial cells of lactating mammals require extensive AA movement across the plasma membrane via multiple transport systems. Recent studies on bovine mammary cells/tissues have expanded the number of AA transporter systems identified and the knowledge on their contribution to AA utilization for milk protein synthesis and the regulatory machinery. However, in lactating cows, the exact intracellular location of mammary AA transporters and the extent of mammary net AA utilization for milk protein production remain unclear. This review highlights the existing knowledge on various characteristics, such as substrate specificity, kinetics, their effects on AA uptake and utilization, and regulatory mechanism, of recently examined bovine mammary free and peptide-bound AA transporters.

Overview of the effect of rumen-protected limiting amino acids (methionine and lysine) and choline on the immunity, antioxidative, and inflammatory status of periparturient ruminants

Overproduction of reactive oxygen species (ROS) is a well-known phenomenon experienced by ruminants, especially during the transition from late gestation to successful lactation. This overproduction of ROS may lead to oxidative stress (OS), which compromises the immune and anti-inflammatory systems of animals, thus predisposing them to health issues. Besides, during the periparturient period, metabolic stress is developed due to a negative energy balance, which is followed by excessive fat mobilization and poor production performance. Excessive lipolysis causes immune suppression, abnormal regulation of inflammation, and enhanced oxidative stress. Indeed, OS plays a key role in regulating the metabolic activity of various organs and the productivity of farm animals. For example, rapid fetal growth and the production of large amounts of colostrum and milk, as well as an increase in both maternal and fetal metabolism, result in increased ROS production and an increased need for micronutrients, including antioxidants, during the last trimester of pregnancy and at the start of lactation. Oxidative stress is generally neutralized by the natural antioxidant system in the body. However, in some special phases, such as the periparturient period, the animal's natural antioxidant system is unable to cope with the situation. The effect of rumen-protected limiting amino acids and choline on the regulation of immunity, antioxidative, and anti-inflammatory status and milk production performance, has been widely studied in ruminants. Thus, in the current review, we gathered and interpreted the data on this topic, especially during the perinatal and lactational stages.

Lithium Chloride Promotes Milk Protein and Fat Synthesis in Bovine Mammary Epithelial Cells via HIF-1α and β-Catenin Signaling Pathways

Lithium is one of the trace elements with many physiological properties, such as being anti-cancer, anti-viral, and anti-inflammatory. However, little is known about its effect on milk synthesis during lactation. Therefore, we selected different concentrations (5 mM, 10 mM, and 20 mM) of lithium chloride (LiCl) and assessed the effect of LiCl on bovine mammary epithelial (MAC-T) cells that underwent 4 days of differentiation induction. Moreover, we analyzed the effect of LiCl on the expression of genes related to milk fat and milk protein synthesis. Herein, LiCl (5-20 mM) significantly increased the expression of β-casein, promoted mRNA expression and phosphorylated protein expression of the signal transduction molecule and activator of transcription 5β (STAT5-β), and inhibited mRNA and protein expression of suppressor of cytokine signaling 2 (SOCS2). In contrast, 5 and 10 mM LiCl significantly inhibited expression of SOCS3. LiCl at concentration of 5-20 mM enhanced phosphorylation level of mTOR protein; at 10 mM and 20 mM, LiCl significantly promoted expression and phosphorylation of downstream ribosomal protein S6 kinase beta-1 (S6K1) protein. Considering milk fat synthesis, mRNA expression of acetyl CoA carboxylase (ACC) and lipoprotein lipase (LPL) genes was considerably increased in the presence of LiCl (5-20 mM). Additionally, increased protein expression levels of stearoyl-CoA desaturase (SCD), peroxisome proliferator-activated receptor-γ (PPARγ), and sterol regulatory element-binding protein 1 (SREBP1) were observed at all LiCl concentrations tested. Subsequently, LiCl (5-20 mM) significantly promoted protein expression and phosphorylation of β-catenin, while 10 mM and 20 mM of LiCl significantly promoted protein expression of hypoxia-inducible factor-1α (HIF-1α). Collectively, it has been shown that 10 mM LiCl can effectively activate HIF-1α, β-catenin, and β-catenin downstream signaling pathways. Conversely, at 10 mM, LiCl inhibited SOCS2 and SOCS3 protein expression through JAK2/STAT5, mTOR, and SREBP1 signaling pathways, improving synthesis of milk protein and fat. Therefore, LiCl can be used as a potential nutrient to regulate milk synthesis in dairy cows.

The Importance of Dietary Antioxidants on Oxidative Stress, Meat and Milk Production, and Their Preservative Aspects in Farm Animals: Antioxidant Action, Animal Health, and Product Quality-Invited Review

The biological effects of oxidative stress and associated free radicals on farm animal performance, productivity, and product quality may be managed via dietary interventions-specifically, the provision of feeds, supplements, and forages rich in antioxidants. To optimize this approach, it is important first to understand the development of free radicals and their contributions to oxidative stress in tissue systems of farm animals or the human body. The interactions between prooxidants and antioxidants will impact redox homeostasis and, therefore, the well-being of farm animals. The impact of free radical formation on the oxidation of lipids, proteins, DNA, and biologically important macromolecules will likewise impact animal performance, meat and milk quality, nutritional value, and longevity. Dietary antioxidants, endogenous antioxidants, and metal-binding proteins contribute to the 'antioxidant defenses' that control free radical formation within the biological systems. Different bioactive compounds of varying antioxidant potential and bio-accessibility may be sourced from tailored feeding systems. Informed and successful provision of dietary antioxidants can help alleviate oxidative stress. However, knowledge pertaining to farm animals, their unique biological systems, and the applications of novel feeds, specialized forages, bioactive compounds, etc., must be established. This review summarized current research to direct future studies towards more effective controls for free radical formation/oxidative stress in farm animals so that productivity and quality of meat and milk can be optimized.

mTORC1 Mediates the Processes of Lysine Regulating Satellite Cells Proliferation, Apoptosis, and Autophagy

Lysine (Lys) is essential for skeletal muscle growth and protein synthesis in mammals. However, the regulatory network underlying Lys-regulated skeletal muscle development is unknown. To determine whether any cross-talk occurs among mammalian targets of rapamycin complex 1 (mTORC1) and Lys in the regulation of muscle satellite cells (SCs) proliferation, we applied the treatment rapamycin (a mTORC1 inhibitor) and MHY1485 (a mTORC1 activator) on Lys-added or -deficient SCs. The results show Lys deprivation significantly decreases SCs viability, protein synthesis, and cell cycling, increases autophagy and apoptosis, and inhibits the mTORC1 signaling pathway. Restoration of Lys content significantly attenuates this effect. mTORC1 signaling pathway activation during Lys deprivation or mTORC1 signaling pathway inhibition during Lys addition attenuates the effect of Lys deprivation or addition on SCs viability, protein synthesis, cell cycling, autophagy, and apoptosis. In conclusion, Lys could improve SCs proliferation, and inhibit SCs apoptosis and autophagy, via the mTORC1 signaling pathway.

Enhancement of BCAT2-Mediated Valine Catabolism Stimulates β-Casein Synthesis via the AMPK-mTOR Signaling Axis in Bovine Mammary Epithelial Cells

Valine, a kind of branched-chain amino acid, plays a regulatory role beyond that of a building block in milk protein synthesis. However, the underlying molecular mechanism through which valine stimulates β-casein synthesis has not been clarified. Therefore, our study aimed to evaluate the effect of valine on β-casein synthesis and shed light into the molecular mechanism using an in vitro model. Results showed that valine supplementation significantly increased β-casein synthesis in bovine mammary epithelial cells (BMECs). Meanwhile, the supplementation of valine resulted in high levels of branched-chain aminotransferase transaminase 2 (BCAT2), TCA-cycle intermediate metabolites, and ATP, AMP-activated protein kinase (AMPK) inhibition, and mammalian target of rapamycin (mTOR) activation. Furthermore, the inhibition of BCAT2 decreased the β-casein synthesis and downregulated the AMPK-mTOR pathway, with similar results observed for AMPK activation. Together, the present data indicate that valine promotes the synthesis of β-casein by affecting the AMPK-mTOR signaling axis and that BCAT2-mediated valine catabolism is the key target.

Depletion of serum-derived exosomes aggravates heat stress-induced damage of bovine mammary epithelial cells

BACKGROUND

Exosomes are involved in intercellular communication, affecting many physiological and pathological process. The present study evaluated the effects of serum exosomes on the function of bovine mammary epithelial cells (BMECs) and milk synthesis under heat stress.

METHODS AND RESULTS

We cultured the BMECs in fetal bovine serum (FBS) or exosome-free FBS medium and examined, their viability using CCK-8 kit. The results showed that culturing the cells in an exosome-free medium decreased viability and increased the levels of reactive oxygen species. The BMECs cultured in the exosome-free medium had reduced mitochondrial membrane potential, decreased manganese superoxide dismutase activity, and disrupted mitochondrial dynamics. They exhibited apoptosis due to upregulated Drp1, Fis1, Bax and HSP70. Lastly, we observed downregulation of milk fat and lactoprotein-related genes: mTOR, PPARγ, p-mTOR and ADD1 and SREBP1, ELF5, and CSN2, respectively, after culturing the cells in an exosome-free medium. These negative effects of the exosome-free medium on the BMECs could be further reinforced under heat stress.

CONCLUSION

Our results demonstrated that exosomes from serum are critical for maintaining the normal function of BMECs.

Rumen-protected methionine during heat stress alters mTOR, insulin signaling, and 1-carbon metabolism protein abundance in liver, and whole-blood transsulfuration pathway genes in Holstein cows

We investigated effects of rumen-protected Met (RPM) during a heat stress (HS) challenge on (1) hepatic abundance of mTOR, insulin, and antioxidant signaling proteins, (2) enzymes in 1-carbon metabolism, and (3) innate immunity. Holstein cows (n = 32; mean ± standard deviation, 184 ± 59 d in milk) were randomly assigned to 1 of 2 environmental groups, and 1 of 2 diets [total mixed ration (TMR) with RPM (Smartamine M; 0.105% dry matter as top-dress) or TMR without (CON); n = 16/diet] in a split-plot crossover design. There were 2 periods with 2 phases. During phase 1 (9 d), all cows were in thermoneutral conditions (TN; temperature-humidity index = 60 ± 3) and fed ad libitum. During phase 2 (9 d), half the cows (n = 8/diet) were exposed to HS using electric heat blankets. The other half (n = 8/diet) remained in TN, but was pair-fed to HS counterparts. After a 14-d washout and 7-d adaptation period, the study was repeated (period 2) and environmental treatments were inverted relative to phase 2, but dietary treatments were the same. Blood was collected on d 6 of each phase 2 to measure immune function and isolate whole-blood RNA. Liver biopsies were performed at the end of each period for cystathione β-synthase (CBS) and methionine adenosyltransferase activity, glutathione concentration, and protein abundance. Data were analyzed using PROC MIXED in SAS. Abundance of CUL3, inhibitor of antioxidant responses, tended to be downregulated by HS suggesting increased oxidative stress. Heat-shock protein 70 abundance was upregulated by HS. Phosphorylated mTOR abundance was greater overall with RPM, suggesting an increase in pathway activity. An environment × diet (E × D) effect was observed for protein kinase B (AKT), whereas there was a tendency for an interaction for phosphorylated AKT. Abundance of AKT was upregulated in CON cows during HS versus TN, this was not observed in RPM cows. For phosphorylated AKT, tissue from HS cows fed CON had greater abundance compared with all other treatments. The same effect was observed for EIF2A (translation initiation) and SLC2A4 (insulin-induced glucose uptake). An E × D effect was observed for INSR due to upregulation in CON cows during HS versus TN cows fed CON or RPM. There was an E × D effect for CBS, with lower activity in RPM versus CON cows during HS. The CON cows tended to have greater CBS during HS versus TN. An E × D effect was observed for methionine adenosyltransferase, with lower activity in RPM versus CON during HS. Although activity increased in CON during HS versus TN, RPM cows tended to have greater activity during TN. Neutrophil and monocyte oxidative burst and monocyte phagocytosis decreased with HS. An (E × D) effect was observed for whole-blood mRNA abundance of CBS, SOD1 and CSAD; RPM led to upregulation during TN versus HS. Regardless of diet, CDO1, CTH, and SOD1 decreased with HS. Although HS increased hepatic HSP70 and seemed to alter antioxidant signaling, feeding RPM may help cows maintain homeostasis in mTOR, insulin signaling, and 1-carbon metabolism. Feeding RPM also may help maintain whole-blood antioxidant response during HS, which is an important aspect of innate immune function.

Plasma essential amino acid concentration and profile are associated with performance of lactating dairy cows as revealed through meta-analysis and hierarchical clustering

Our aim was to explore whether changes in plasma essential AA (EAA) concentration ([EAA]_p) or profile (defined here as the molar proportion of individual [EAA]_p relative to the total [EAA]_p) may serve as an indicator of the EAA status of a cow. We undertook a meta-analysis with the objectives to determine if different plasma EAA profiles exist among cows and to explore the association of [EAA]_p or the profile of EAA with lactating cow performance and measures of N utilization. We hypothesized the existence of differences in [EAA]_p and different plasma EAA profile for cows with greater milk output, feed efficiency, and greater N use efficiency (NUE; milk true protein-N:N intake) compared with cows with lower milk output, feed efficiency, and lower NUE. The data set included 22 feeding trials and 96 dietary treatments. First, a mixed-effect model analysis was used to predict [EAA]_p in response to the categorical fixed effect of EAA, continuous fixed effect of National Research Council model-predicted metabolizable protein (MP) supply, continuous fixed effect of body weight, the fixed effect of EAA and MP supply interaction, the fixed effect of EAA and body weight interaction, and the random effect of study. Then, residuals of the model were standardized based on Z-score and clustered using the hierarchical method (Euclidean distance and Ward's minimum variance method) resulting in 2 clusters. Finally, a fixed-effect model was used to evaluate the significance with which clusters were associated with [EAA]_p, cow performance, feed efficiency, and NUE. The total concentration of [EAA]_p was lower (784 vs. 983 µM) and the concentration of each EAA was on average 22 µM lower for cows in cluster 1 compared with cluster 2 with the smallest and greatest difference found for Met (4 µM) and Val (59 µM), respectively. The percentage difference in [EAA]_p was the smallest for Thr (-5.3%) and the greatest for Leu (-37.1%). There was no difference between clusters for Arg, His, and Met molar proportions; however, cows in cluster 1 had a lower molar proportion of Leu and a tendency for lower molar proportion of Val compared with cows in cluster 2. Additionally, cows in cluster 1 had greater molar proportions of Ile, Lys, and Thr and a tendency for greater molar proportion of Phe compared with cows in cluster 2. The fixed-effect model analysis indicated that cows in cluster 1 had higher milk energy output (+3.2 Mcal/d), true protein yield (+87 g/d) and fat yield (+236 g/d), feed efficiency (milk Mcal:dry matter intake; +8% unit), and a tendency for greater MP efficiency (Milk true protein/MP supply; +2.3% unit) than cows in cluster 2. These results suggested greater use of EAA by the mammary gland (as reflected by greater milk protein synthesis) and lower hepatic catabolism of AA (as reflected by a tendency to greater MP efficiency) in cows of cluster 1 compared with cluster 2. Our findings should be evaluated further, including whether the relative molar proportions of plasma EAA might serve as a holistic indicator of the EAA status of cows as related to their productivity, feed efficiency and N utilization.

Effects of Encapsulated Methionine on Skeletal Muscle Growth and Development and Subsequent Feedlot Performance and Carcass Characteristics in Beef Steers

Two studies were conducted to evaluate the effect of encapsulated methionine on live performance, carcass characteristics, and skeletal muscle development in feedlot steers. In Experiment 1, 128 crossbred steers (body weight [BW] = 341 ± 36.7 kg) were used in a randomized complete block design and supplemented with 0, 4, 8, or 12 g/(head day [d]) of ruminally protected methionine (0MET, 4MET, 8MET, and 12MET, respectively) for 111 d or 139 d. In Exp. 2, 20 steers (BW = 457 ± 58 kg) were stratified by BW and randomly assigned to either the 0MET or 8MET treatment; longissimus muscle (LM) biopsies were collected on d 0, 14, 28, 42, and 56, and analyzed for mRNA and protein expression. Additionally, immunohistochemical analysis was performed to measure fiber type area and distribution as well as the density of muscle nuclei and satellite cells (Myf5, Pax7, and Myf5/Pax7). In Experiment 1, no significant differences were observed for live performance (p ≥ 0.09). There was, however, a linear relationship between LM area and methionine supplementation (p = 0.04), with a 9% increase in the area when steers were supplemented with 12MET compared to 0MET. In Exp. 2, There were no treatment × day interactions (p ≥ 0.10) for expression of mRNA or protein abundance. Although mRNA expression and protein abundance of all genes were influenced by day (p ≤ 0.04), methionine supplementation did not have a significant effect (p ≥ 0.08). There was a significant treatment × day interaction for distribution of MHC-I fibers (p = 0.03), where 8MET supplemented cattle had a greater proportion of MHC-I fibers after 56 d of supplementation than did 0MET steers. Cross-sectional area was increased over time regardless of fiber type (p < 0.01) but was unaffected by treatment (p ≥ 0.36). While nuclei density was not impacted by treatment (p = 0.55), the density of myonuclei increased nearly 55% in 8MET supplemented cattle (p = 0.05). The density of Myf5 positive satellite cells tended to decrease with methionine supplementation (p = 0.10), while the density of Pax7 expressing cells tended to increase (p = 0.09). These results indicate that encapsulated methionine supplementation may influence markers of skeletal muscle growth, and potential improvements in the LM area may exist.

Effects of rumen-protected methionine supplementation on the performance of high production dairy cows in the tropics

Increasing methionine availability in dairy cow diets during the first third of lactation may enhance their performance and health. The aim of this study was to determine the effect of supplementing rumen-protected methionine (Smartamine® M, SM) in a lactation diet with protein and energy levels calculated according to the literature. Seventy-six multiparous Holstein cows (39.1 ± 6.8 kg of milk/d and 65 ± 28 DIM) were assigned to 1 of 2 dietary treatments (38/treatment) according to a randomized complete block design with a 2-wk (covariate) and 10-wk experimental period. Treatments were a basal diet (CON; 3.77 Lys:1Met); and CON + 23 g SM (2.97 Lys:1 Met). Individual milk samples were taken every 2 weeks to determine milk composition. Blood was collected from 24 cows on d+30 d to measure plasma AA levels. Body weight and body condition score (BCS) were measured at the beginning and the end of the experiment. The SM diet promoted higher milk yield (41.7 vs. 40.1 kg/d; P = 0.03). Energy-corrected milk yield (41.0 vs. 38.0 kg/d), milk protein yield (1.30 vs. 1.18 kg/d), milk protein (3.14% vs. 2.97%) and casein (2.39% vs. 2.28%) were also different (P < 0.01) as well as milk fat yield (1.42 vs. 1.29 kg/d; P = 0.02). A trend (P = 0.06) for higher milk fat % (3.41% vs. 3.21%) was observed. Both diets resulted in similar body weight, but CON-fed cows tended (P = 0.08) to have higher BCS. Higher plasma methionine levels were determined with SM compared with CON (29.6 vs. 18.4 μM; P < 0.01), but lysine and histidine were not different. Dietary supplementation of RPM improved productive performance by increasing milk yield and milk components yields, suggesting better dietary AA utilization when Met levels are adjusted in Lys-adequate lactation diets.

LPS-induced SOCS3 antagonizes the JAK2-STAT5 pathway and inhibits β-casein synthesis in bovine mammary epithelial cells

Bovine mammary epithelial cells (BMECs) are essential for lactation in the dairy cow mammary gland, and are often used as a cellular model to study changes in inflammatory responses and lactation functions with exogenous stimuli. Prolactin (PRL) promotes milk protein synthesis by continuously activating the Janus kinase 2 and signal transducer and activator of transcription 5 (JAK2-STAT5) pathway. Lipopolysaccharides (LPS) activates inflammatory responses in cells and inhibits casein synthesis, but the exact mechanism is still unclear. Suppressor of cytokine signaling-3 (SOCS3) is a negative regulator of the JAK-STATs signaling pathway, and regulates a variety of inflammatory responses by inhibiting STAT3. Previous studies also suggested that SOCS3 plays a role in the development and involution of bovine mammary glands. The purpose of this study was to investigate whether LPS activated SOCS3, and whether SOCS3 resisted the regulation of casein synthesis by PRL in a JAK2-STAT5-dependent manner. We treated in vitro BMECs with 125 ng/mL PRL, 10 μg/mL LPS, SOCS3 siRNA (silencing), a SOCS3-GFP adenovirus overexpression vector, or combinations, to determine β-casein expression. We demonstrated that PRL up-regulated phospho-JAK2, phsopho-STAT5 and β-casein expression, whereas LPS caused the opposite effects, and activated SOCS3. SOCS3 overexpression interrupted the JAK2-STAT5 pathway in BMECs. With SOCS3 was silenced, LPS could not activate the JAK2-STAT5 pathway, and no inhibition of β-casein expression was observed. In conclusion, we showed that LPS activated SOCS3 in BMECs, antagonized the JAK2-STAT5 pathway via SOCS3 regulation, and ultimately reduced β-casein expression in these cells.

Unique adaptations in neonatal hepatic transcriptome, nutrient signaling, and one-carbon metabolism in response to feeding ethyl cellulose rumen-protected methionine during late-gestation in Holstein cows

BACKGROUND

Methionine (Met) supply during late-pregnancy enhances fetal development in utero and leads to greater rates of growth during the neonatal period. Due to its central role in coordinating nutrient and one-carbon metabolism along with immune responses of the newborn, the liver could be a key target of the programming effects induced by dietary methyl donors such as Met. To address this hypothesis, liver biopsies from 4-day old calves (n = 6/group) born to Holstein cows fed a control or the control plus ethyl-cellulose rumen-protected Met for the last 28 days prepartum were used for DNA methylation, transcriptome, metabolome, proteome, and one-carbon metabolism enzyme activities.

RESULTS

Although greater withers and hip height at birth in Met calves indicated better development in utero, there were no differences in plasma systemic physiological indicators. RNA-seq along with bioinformatics and transcription factor regulator analyses revealed broad alterations in 'Glucose metabolism', 'Lipid metabolism, 'Glutathione', and 'Immune System' metabolism due to enhanced maternal Met supply. Greater insulin sensitivity assessed via proteomics, and efficiency of transsulfuration pathway activity suggested beneficial effects on nutrient metabolism and metabolic-related stress. Maternal Met supply contributed to greater phosphatidylcholine synthesis in calf liver, with a role in very low density lipoprotein secretion as a mechanism to balance metabolic fates of fatty acids arising from the diet or adipose-depot lipolysis. Despite a lack of effect on hepatic amino acid (AA) transport, a reduction in metabolism of essential AA within the liver indicated an AA 'sparing effect' induced by maternal Met.

CONCLUSIONS

Despite greater global DNA methylation, maternal Met supply resulted in distinct alterations of hepatic transcriptome, proteome, and metabolome profiles after birth. Data underscored an effect on maintenance of calf hepatic Met homeostasis, glutathione, phosphatidylcholine and taurine synthesis along with greater efficiency of nutrient metabolism and immune responses. Transcription regulators such as FOXO1, PPARG, E2F1, and CREB1 appeared central in the coordination of effects induced by maternal Met. Overall, maternal Met supply induced better immunometabolic status of the newborn liver, conferring the calf a physiologic advantage during a period of metabolic stress and suboptimal immunocompetence.

Multifaceted role of one-carbon metabolism on immunometabolic control and growth during pregnancy, lactation and the neonatal period in dairy cattle

Dairy cattle undergo dramatic metabolic, endocrine, physiologic and immune changes during the peripartal period largely due to combined increases in energy requirements for fetal growth and development, milk production, and decreased dry matter intake. The negative nutrient balance that develops results in body fat mobilization, subsequently leading to triacylglycerol (TAG) accumulation in the liver along with reductions in liver function, immune dysfunction and a state of inflammation and oxidative stress. Mobilization of muscle and gluconeogenesis are also enhanced, while intake of vitamins and minerals is decreased, contributing to metabolic and immune dysfunction and oxidative stress. Enhancing post-ruminal supply of methyl donors is one approach that may improve immunometabolism and production synergistically in peripartal cows. At the cellular level, methyl donors (e.g. methionine, choline, betaine and folic acid) interact through one-carbon metabolism to modulate metabolism, immune responses and epigenetic events. By modulating those pathways, methyl donors may help increase the export of very low-density lipoproteins to reduce liver TAG and contribute to antioxidant synthesis to alleviate oxidative stress. Thus, altering one-carbon metabolism through methyl donor supplementation is a viable option to modulate immunometabolism during the peripartal period. This review explores available data on the regulation of one-carbon metabolism pathways in dairy cows in the context of enzyme regulation, cellular sensors and signaling mechanisms that might respond to increased dietary supply of specific methyl donors. Effects of methyl donors beyond the one-carbon metabolism pathways, including production performance, immune cell function, mechanistic target or rapamycin signaling, and fatty acid oxidation will also be highlighted. Furthermore, the effects of body condition and feeding system (total mixed ration vs. pasture) on one-carbon metabolism pathways are explored. Potential effects of methyl donor supply during the pepartum period on dairy calf growth and development also are discussed. Lastly, practical nutritional recommendations related to methyl donor metabolism during the peripartal period are presented. Nutritional management during the peripartal period is a fertile area of research, hence, underscoring the importance for developing a systems understanding of the potential immunometabolic role that dietary methyl donors play during this period to promote health and performance.

Supplementing conjugated and non-conjugated L-methionine and acetate alters expression patterns of CSN2, proteins and metabolites related to protein synthesis in bovine mammary cells

The experiments reported in this research paper aimed to determine the effect of supplementing different forms of L-methionine (L-Met) and acetate on protein synthesis in immortalized bovine mammary epithelial cell line (MAC-T cells). Treatments were Control, L-Met, conjugated L-Met and acetate (CMA), and non-conjugated L-Met and Acetate (NMA). Protein synthesis mechanism was determined by omics method. NMA group had the highest protein content in the media and CSN2 mRNA expression levels (P < 0.05). The number of upregulated and downregulated proteins observed were 39 and 77 in L-Met group, 62 and 80 in CMA group and 50 and 81 in NMA group from 448 proteins, respectively (P < 0.05). L-Met, NMA and CMA treatments stimulated pathways related to protein and energy metabolism (P < 0.05). Metabolomic analysis also revealed that L-Met, CMA and NMA treatments resulted in increases of several metabolites (P < 0.05). In conclusion, NMA treatment increased protein concentration and expression level of CSN2 mRNA in MAC-T cells compared to control as well as L-Met and CMA treatments through increased expression of milk protein synthesis-related genes and production of the proteins and metabolites involved in energy and protein synthesis pathways.

Quantitative determination of histone methylation via fluorescence resonance energy transfer (FRET) technology in immortalized bovine mammary alveolar epithelial cells supplemented with methionine

Methionine (Met) is an essential precursor of S-adenosylmethionine (SAM), which is the primary methyl donor required for biological processes such as DNA and histone methylation, which alter gene expression. In dairy cows, dietary Met has been observed to exert transcriptional alterations with beneficial effects on milk biosynthesis; however, the extent of these effects via SAM remains unknown. Therefore, we evaluated the effect of Met supply on histone methylation in lysine residues K9 and K27 in the histone tail H3 via a fluorescence resonance energy transfer (FRET) system in immortalized bovine mammary alveolar epithelial cells (MACT) incubated varying concentration of Met. The histone methylation data was complemented with global DNA methylation, cellular protein synthesis, and RT-qPCR analysis of genes related to Met cycle, DNA and histone methylation, AA transporters, and protein synthesis. The histone methylation data was performed on MACT cells seeded at 30,000 cells/well in 96-well plates 24 h prior to transfection. The transfections of FRET gene reporter plasmids H3K9 and H3K27 was performed with 0.3 μL/well of Lipofectamine® 3000 and 50 ng of plasmid DNA per well. At 24 h post-transfection, cells were treated with 0, 125, 250, and 500 μM of Met, and quantification of histone methylation was performed at 0, 12, and 24 h post-treatment as well as cell viability at 24 h using CellProfiler software. An inverted microscope for live imagining (EVOS^® FL Auto) equipped with a motorized scanning stage, and an environment-controlled chamber at 37˚C and 5.0% of CO₂ was used to take 4 pictures/well at 4x magnification. A more defined response on histone methylation was observed in H3K9 than H3K27 to Met supply, where maximal histone methylation in H3K9 was observed with 125 μM of Met. This greater histone methylation in H3K9 at 125 μM was accompanied by greater cellular protein concentration. The linear increase in Met supply causes a linear decrease in global DNA methylation, while linearly upregulating genes related to the Met cycle (i.e., MAT1A, PEMT, SAHH, and MTR). The histone methylation data suggest that, to some extent, methyl-donors such as Met may affect the methylation sites, H3K9 and H3K27, and consequently causing a different epigenetic alteration. In the context of the dairy cow, further refinement to this FRET assay to study histone methylation could lead to establishing novel potential mechanisms of how dietary methyl donors may control the structural conformation of the bovine genome and, by extension, gene expression.

ASCT2 Is Involved in SARS-Mediated β-Casein Synthesis of Bovine Mammary Epithelial Cells with Methionine Supply

The methionine (Met) uptake into mammary cells depends upon the corresponding amino acid (AA) transporters, which play a regulatory role in the mammary protein production beyond transport. Our previous studies have identified that seryl-tRNA synthetase (SARS) could be a novel mediator to regulate essential AA-stimulated casein synthesis in primary bovine mammary epithelial cells (BMECs). However, the regulatory mechanisms of Met in milk protein production in dairy cows remain further clarified. Here, we aimed to investigate the effects of Met on milk protein synthesis in BMECs and explore the underlying mechanism. The effects of Met on the AA transporter, casein synthesis, and the related signaling pathway were evaluated in the BMECs treated with 0.6 mM Met for 6 h combined with or without the inhibition of AA transporter (ASCT2, a neutral AA transporter) activity by the corresponding inhibitor (GPNA). Besides, the effects of SARS on the cells were mainly evaluated in the BMECs treated with 0.6 mM Met for 6 h together with or without SARS knockdown by RNAi interference. The gene expression of AA transporters and pathway-related genes were analyzed by the real-time quantitative polymerase chain reaction method, and the protein expression of related proteins were determined by the western blot assay. Results showed that 0.6 mM Met remarkably enhanced cell growth and β-casein synthesis compared to the supply of other Met concentrations. Among 13 amino acid transporters, 0.6 mM Met highly increased ASCT2 expression. This Met-stimulated ASCT2 expression and the enhanced mammary intracellular Met uptake were both decreased by the addition of 500 μM GPNA, an inhibitor of ASCT2. In the presence of 0.6 mM Met, the inhibition of ASCT2 activity (by GPNA) and SARS expression (by RNAi) both reduced β-casein synthesis. Additionally, 0.6 mM Met increased the gene expression of mTOR, S6K1, 4EBP1, and Akt; in contrast, the inhibition of ASCT2 by GPNA lowered the gene expression of these four genes. Collectively, this work suggests that ASCT2 is involved in the SARS-mediated Met stimulation of β-casein synthesis through enhancing mammary Met uptake and activating the mTOR signaling pathway in BMECs.

Effects of rumen-protected methionine on lactation performance and physiological variables during a heat stress challenge in lactating Holstein cows

Milk yield, content, and composition are altered by heat stress. Thirty-two multiparous, lactating Holstein cows [balanced by days in milk (mean ± standard deviation; 184 ± 59); body surface area (5.84 ± 0.34 m²)] were randomly assigned to 1 of 2 dietary treatments [total mixed ration with rumen-protected Met (RPM; Smartamine M; Adisseo Inc., Antony, France; 1.05 g of RPM/kg of dry matter intake) or total mixed ration without RPM (CON)], and within each dietary treatment group cows were randomly assigned to 1 of 2 environmental treatment groups in a split-plot crossover design. The study was divided into 2 periods with 2 phases per period. In phase 1 (9 d), all cows were in thermoneutral conditions and fed ad libitum. In phase 2 (9 d), group 1 (n = 16) was exposed to a heat stress challenge (HSC) using electric heat blankets. Group 2 (n = 16) remained in thermoneutral conditions but was pair-fed (PFTN) to HSC counterparts. After a 21-d washout period, the study was repeated (period 2) and the environmental treatments were inverted relative to treatments from phase 2 of period 1, whereas dietary treatments (RPM or CON) remained the same for each cow. Cows were milked 3× per day and samples were taken on d 1, 5, and 9 of each phase. Vaginal temperature was measured every 10 min, rectal temperature and skin temperature were measured 3× per day, and respiration rate and heart rate were recorded once per day. Cow activity was measured using an accelerometer. Paired difference values were calculated for each cow for each period based on the difference between phase 1 baseline means and phase 2 values for each variable. Cows in HSC had a greater increase in vaginal temperature and respiration rate (+0.2°C and +13.7 breaths/min, respectively) compared with cows in PFTN (0.0°C and -1.6 breaths/min, respectively). Cows in PFTN had a greater decrease in dry matter intake and milk yield (-3.9 and -2.6 kg/d, respectively) compared with cows in HSC (-3.2 and -0.9 kg/d, respectively). Cows in CON had a greater decrease in milk protein concentration for PFTN (-0.10 percentage units) and HSC (-0.06 percentage units) compared with cows in RPM for PFTN (0.00 percentage units) and HSC (-0.02 percentage units). Cows in CON for HSC had greater decrease in milk fat concentration compared with cows in RPM for HSC (-0.10 and +0.12 percentage units, respectively). In conclusion, HSC altered physiological and production parameters of cows. Additionally, RPM helped maintain milk protein and fat concentration during HSC, whereas dry matter intake, milk yield, and feed efficiencies were not affected by RPM.

Supply of methionine and arginine alters phosphorylation of mechanistic target of rapamycin (mTOR), circadian clock proteins, and α-s1-casein abundance in bovine mammary epithelial cells

Methionine (Met) and arginine (Arg) regulate casein protein abundance through alterations in activity of the mechanistic target of rapamycin complex 1 (mTORC1) signaling pathway.

Camellia (Camellia oleifera Abel.) seed oil promotes milk fat and protein synthesis-related gene expression in bovine mammary epithelial cells

Camellia (Camellia oleifera Abel.) seed oil is a commonly used edible oil of China. In ancient Chinese literature, it is mentioned to be helpful for postpartum repair and lactation in women. Research on camellia seed oil (CO) as a feed additive for dairy cattle is less. We investigated the effect of CO on the expression of milk fat and protein syntheses-related genes in differentiated bovine mammary epithelial cells (MAC-T) using soybean oil (SO) as the control. The results showed that CO increased the expression of genes related to de novo synthesis of fatty acids including sterol regulatory element-binding protein 1 (SREBP1), acetyl-CoA carboxylase 1 (ACC), fatty acid synthase (FASN), lipoprotein lipase (LPL), and stearoyl-CoA desaturase (SCD) (p < .05). Among the milk protein genes analyzed, CO increased β-casein mRNA expression (p < .05) and decreased αS1-casein mRNA expression (p < .05) in MAC-T cells. CO upregulated the pathways related to milk protein synthesis with increased mRNA levels of phosphoinositide 3-kinase (PI3K), RAC-alpha serine/threonine-protein kinase (AKT1), and mammalian target of rapamycin (mTOR) (p < .05) in MAC-T cells. Ribosomal protein S6 kinase beta-1 (S6K1) gene was upregulated, and eukaryotic initiation factor 4E (eIF4E) gene (p < .05) was downregulated with CO treatment. The mRNA expression levels of janus kinase 2 (JAK2), activator of transcription 5-β (STAT5-β), and E74-like factor 5 (ELF5) were elevated in MAC-T cells treated with CO (p < .05). Meanwhile, the protein expression levels of S6K1, STAT5-β, phosphorylated mTOR (p-mTOR), p-S6K1, and p-STAT5-β increased in MAC-T cells treated with CO (p < .05). In summary, CO promoted β-casein synthesis by regulating PI3K-mTOR-S6K1 and JAK2-STAT5 signaling pathways and influenced fatty acid synthesis by regulating SREBP1-related genes in MAC-T cells. We need to further confirm the function of CO using in vivo models.

Seryl-tRNA synthetase is involved in methionine stimulation of β-casein synthesis in bovine mammary epithelial cells

Despite the well-characterised mechanisms of amino acids (AA) regulation of milk protein synthesis in mammary glands (MG), the underlying specific AA regulatory machinery in bovine MG remains further elucidated. As methionine (Met) is one of the most important essential and limiting AA for dairy cows, it is crucial to expand how Met exerts its regulatory effects on dairy milk protein synthesis. Our previous work detected the potential regulatory role of seryl-tRNA synthetase (SARS) in essential AA (EAA)-stimulated bovine casein synthesis. Here, we investigated whether and how SARS participates in Met stimulation of casein production in bovine mammary epithelial cells (BMEC). With or without RNA interference against SARS, BMEC were treated with the medium in the absence (containing all other EAA and devoid of Met alone)/presence (containing 0·6 mm of Met in the medium devoid of Met alone) of Met. The protein abundance of β-casein and members of the mammalian target of rapamycin (mTOR) and general control nonderepressible 2 (GCN2) pathways was determined by immunoblot assay after 6 h treatment, the cell viability and cell cycle progression were determined by cell counting and propidium iodide-staining assay after 24 h treatment, and protein turnover was determined by l-[ring-3H5]phenylalanine isotope tracing assay after 48 h treatment. In the absence of Met, there was a general reduction in cell viability, total protein synthesis and β-casein production; in contrast, total protein degradation was enhanced. SARS knockdown strengthened these changes. Finally, SARS may work to promote Met-stimulated β-casein synthesis via affecting mTOR and GCN2 routes in BMEC.

Effect of all-trans retinoic acid on casein and fatty acid synthesis in MAC-T cells

Objective

Caseins and fatty acids of milk are synthesized and secreted by the epithelial cells of the mammary gland. All-trans retinoic acid (ATRA), an active metabolite of vitamin A, has been shown to promote mammary development. This study was conducted to determine the effect of ATRA on casein synthesis and fatty acid composition in MAC-T cells.

Methods

MAC-T cells were allowed to differentiate for 4 d, treated with ATRA (0, 1.0, 1.5, and 2.0 μM), and incubated for 3 d. We analyzed the fatty acid composition, the mRNA expression of casein and fatty acid synthesis-related genes, and the phosphorylation of casein synthesis-related proteins of MAC-T cells by gas chromatography, quantitative polymerase chain reaction, and western blotting, respectively.

Results

In MAC-T cells, ATRA increased the mRNA levels of α_S1-casein and β-casein, janus kinase 2 (JAK2) and E74-like factor 5 of the signal transducer and activator of transcription 5 β (STAT5-β) pathway, ribosomal protein S6 kinase beta-1 (S6K1) and eukaryotic translation initiation factor 4E binding protein 1 of the mammalian target of rapamycin (mTOR) pathway, inhibited the mRNA expression of phosphoinositide 3-kinase and eukaryotic initiation factor 4E of the mTOR pathway, and promoted the phosphorylation of STAT5-β and S6K1 proteins. Additionally, ATRA increased the de novo synthesis of fatty acids, reduced the content of long-chain fatty acids, the ratio of monounsaturated fatty acids to saturated fatty acids (SFA), the ratio of polyunsaturated fatty acids (PUFA) to SFA, and the ratio of ω-6 to ω-3 PUFA. The mRNA levels of acetyl-CoA carboxylase 1, fatty acid synthase, lipoprotein lipase, stearoyl-CoA desaturase, peroxisome proliferator-activated receptor gamma, and sterol regulatory element-binding protein 1 (SREBP1) were enhanced by ATRA.

Conclusion

ATRA promotes the synthesis of casein by regulating JAK2/STAT5 pathway and downstream mTOR signaling pathway, and it improves the fatty acid composition of MAC-T cells by regulating SREBP1-related genes.

Effects of varying extracellular amino acid profile on intracellular free amino acid concentrations and cell signaling in primary mammary epithelial cells

Extracellular amino acid profiles affect intracellular AA concentrations and profile as well as signaling proteins that regulate protein translation rates. The objective of this study was to assess whether various extracellular AA profiles and varied ratios of Lys to Met would increase the phosphorylation of signaling proteins related to protein metabolism. Six AA profiles, reflecting Dulbecco's modified Eagle's medium (DMEM), blood meal (BM), corn gluten meal (CGM), casein (CAS), plasma of lactating cows (PLA), and a negative control (NEG) represented the first factor (F1), and the ratio of Lys to Met (unaltered or set to 3:1) was the second factor (F2). Treatments were arranged in a 6 × 2 factorial manner, resulting in 12 treatments that were replicated 4 times. The total AA masses for all treatments were set to 659 mg/L (63% of DMEM) except NEG (0 mg/L). Confluent mammary epithelial cells were exposed to treatment media for 80 min (SD = 7.4). Intracellular concentrations of 17 AA were changed according to F1. The Met and Leu percent of total intracellular AA mass, as an example, varied from 0.58 (PLA) to 6.94 (NEG, +F2) for Met and 0.05 (NEG, -F2) to 4.63 (CGM, +F2) for Leu. Overall, balancing for Lys and Met at a 3:1 ratio increased intracellular concentrations of Lys and Met by 54 and 71%, respectively. Within the mechanistic target of rapamycin (mTOR) pathway, phosphorylation of mTOR (Ser2448), ribosomal protein S6 (Ser235/236), and eukaryotic initiation factor 4E binding protein 1 (Thr37/46) (4EBP1) were increased by all 5 AA profiles compared with the NEG control. We found no differences in phosphorylation state among the 5 AA profiles, indicating lack of sensitivity to various AA profiles. This lack of sensitivity between AA profiles might also be due to assay imprecision or other experimental limitations. Only phosphorylation of 4EBP1 was increased for F2. Phosphorylation of eukaryotic initiation factor 2 α subunit (Ser51) was unaffected by either F1 or F2 factors. Regression analyses indicated that intracellular concentrations of Met, Thr, Ile, and Leu predicted phosphorylation of mTOR-related proteins with adequate precision and accuracy, suggesting that multiple EAA dictate regulation, regardless of AA ratios. Changes in extracellular AA profiles translated to modified intracellular AA profiles, and no single profile uniquely stimulated phosphorylation of the mTOR pathway-related proteins.

Meta-analysis of mammary RNA seq datasets reveals the molecular understanding of bovine lactation biology

A better understanding of the biology of lactation, both in terms of gene expression and the identification of candidate genes for the production of milk and its components, is made possible by recent advances in RNA seq technology. The purpose of this study was to understand the synthesis of milk components and the molecular pathways involved, as well as to identify candidate genes for milk production traits within whole mammary transcriptomic datasets. We performed a meta-analysis of publically available RNA seq transcriptome datasets of mammary tissue/milk somatic cells. In total, 11 562 genes were commonly identified from all RNA seq based mammary gland transcriptomes. Functional annotation of commonly expressed genes revealed the molecular processes that contribute to the synthesis of fats, proteins, and lactose in mammary secretory cells and the molecular pathways responsible for milk synthesis. In addition, we identified several candidate genes responsible for milk production traits and constructed a gene regulatory network for RNA seq data. In conclusion, this study provides a basic understanding of the lactation biology of cows at the gene expression level.

Amino Acid Metabolism in Dairy Cows and their Regulation in Milk Synthesis

Background:

Reducing dietary Crude Protein (CP) and supplementing with certain Amino Acids (AAs) has been known as a potential solution to improve Nitrogen (N) efficiency in dairy production. Thus understanding how AAs are utilized in various sites along the gut is critical.

Objective:

AA flow from the intestine to Portal-drained Viscera (PDV) and liver then to the mammary gland was elaborated in this article. Recoveries in individual AA in PDV and liver seem to share similar AA pattern with input: output ratio in mammary gland, which subdivides essential AA (EAA) into two groups, Lysine (Lys) and Branchedchain AA (BCAA) in group 1, input: output ratio > 1; Methionine (Met), Histidine (His), Phenylalanine (Phe) etc. in group 2, input: output ratio close to 1. AAs in the mammary gland are either utilized for milk protein synthesis or retained as body tissue, or catabolized. The fractional removal of AAs and the number and activity of AA transporters together contribute to the ability of AAs going through mammary cells. Mammalian Target of Rapamycin (mTOR) pathway is closely related to milk protein synthesis and provides alternatives for AA regulation of milk protein synthesis, which connects AA with lactose synthesis via α-lactalbumin (gene: LALBA) and links with milk fat synthesis via Sterol Regulatory Element-binding Transcription Protein 1 (SREBP1) and Peroxisome Proliferatoractivated Receptor (PPAR).

Conclusion:

Overall, AA flow across various tissues reveals AA metabolism and utilization in dairy cows on one hand. While the function of AA in the biosynthesis of milk protein, fat and lactose at both transcriptional and posttranscriptional level from another angle provides the possibility for us to regulate them for higher efficiency.

Enhanced supply of methionine or arginine alters mechanistic target of rapamycin signaling proteins, messenger RNA, and microRNA abundance in heat-stressed bovine mammary epithelial cells in vitro

Heat stress (HS) causes reductions in milk production, but it is unclear whether this effect is due to reduced number or functional capacity (or both) of mammary cells. Methionine supplementation improves milk protein, whereas Arg is taken up in excess by mammary cells to produce energy and nonessential AA that can be incorporated into milk protein. To evaluate molecular mechanisms by which mammary functional capacity is affected by HS and Met or Arg, mammary alveolar (MAC-T) cells were incubated at thermal-neutral (37°C) or HS (42°C) temperatures. Treatments were optimal AA profiles (control; Lys:Met = 2.9:1.0; Lys:Arg = 2.1:1.0), control plus Met (Lys:Met = 2.5:1.0), or control plus Arg (Lys:Arg = 1.0:1.0). After incubation for 6 h, cells were harvested and RNA and protein were extracted for quantitative real-time PCR and Western blotting. Protein abundance of mechanistic target of rapamycin (MTOR), eukaryotic initiation factor 2a, serine-threonine protein kinase (AKT), 4E binding protein 1 (EIF4EBP1), and phosphorylated EIF4EBP1 was lower during HS. The lower phosphorylated EIF4EBP1 with HS would diminish translation initiation and reduce protein synthesis. Both Met and Arg had no effect on MTOR proteins, but the phosphorylated EIF4EBP1 decreased by AA, especially Arg. Additionally, Met but not Arg decreased the abundance of phosphorylated eukaryotic elongation factor 2, which could be positive for protein synthesis. Although HS upregulated the heat shock protein HSPA1A, the apoptotic gene BAX, and the translation inhibitor EIF4EBP1, the mRNA abundance of PPARG, FASN, ACACA (lipogenesis), and BCL2L1 (antiapoptotic) decreased. Greater supply of Met or Arg reversed most of the effects of HS occurring at the mRNA level and upregulated the abundance of HSPA1A. In addition, compared with the control, supply of Met or Arg upregulated genes related to transcription and translation (MAPK1, MTOR, SREBF1, RPS6KB1, JAK2), insulin signaling (AKT2, IRS1), AA transport (SLC1A5, SLC7A1), and cell proliferation (MKI67). Upregulation of microRNA related to cell growth arrest and apoptosis (miR-34a, miR-92a, miR-99, and miR-184) and oxidative stress (miR-141 and miR-200a) coupled with downregulation of fat synthesis-related microRNA (miR-27ab and miR-221) were detected with HS. Results suggest that HS has a direct negative effect on synthesis of protein and fat, mediated in part by coordinated changes in mRNA, microRNA, and protein abundance of key networks. The positive responses with Met and Arg raise the possibility that supplementation with these AA during HS might have a positive effect on mammary metabolism.

Proteomic analyses reveal GNG12 regulates cell growth and casein synthesis by activating the Leu-mediated mTORC1 signaling pathway

In cow mammary epithelial cells (CMECs), cell growth and casein synthesis are regulated by amino acids (AAs), and lysosomes are important organelles in this regulatory process, but the mechanisms remain unclear. Herein, lysosomal membrane proteins (LMPs) in CMECs in the presence (Leu+) and absence (Leu-) of leucine were quantitatively analysed using Sequential Windowed Acquisition of All Theoretical Fragment Ion (SWATH) mass spectrometry. In identified LMPs, Guanine nucleotide-binding protein subunit gamma-12 (GNG12) was a markedly up-regulated protein in Leu+ group. CMECs were treated with Leu+ or Leu-, expression and lysosomal localization of GNG12 were decreased in response to Leu absence. Overexpressing or inhibiting GNG12 demonstrated that cell growth, casein synthesis and activation of the mammalian target of rapamycin complex 1 (mTORC1) signaling pathway were all up-regulated by GNG12. Cell growth, casein synthesis and mTORC1 signaling pathway were decreased in response to Leu absence, but these decreases were partially restored by GNG12 overexpression, and those effects were partially reversed by inhibiting GNG12. Co-immunoprecipitation analysis showed that GNG12 activates the mTORC1 pathway via interaction with Ragulator. Taken together, these results suggest that GNG12 is a positive regulator of the Leu-mediated mTORC1 signaling pathway in CMECs that promotes cell growth and casein synthesis.

d-Glucose and amino acid deficiency inhibits casein synthesis through JAK2/STAT5 and AMPK/mTOR signaling pathways in mammary epithelial cells of dairy cows

Amino acids and energy deficiency lead to lower milk protein content in dairy cows. However, the known mechanisms involved in this process do not adequately explain the variability of milk protein concentration in the mammary gland. We hypothesized that a deficiency in d-glucose (d-Glc) or AA would inhibit casein synthesis by regulating signaling pathways in mammary epithelial cells. Cow mammary epithelial cells (CMEC) were subjected to combinations of 1 of 3 concentrations of d-Glc (0, 2.50, or 17.5 mM) and 1 of 3 concentrations of AA (0, 1.03, or 7.20 mM). The effect of each mixture on cell cycle stage was assessed by flow cytometry. The expression levels of β-casein and κ-casein (encoded by CSN2 and CSN3) were measured by quantitative real-time PCR and Western blotting. Phosphorylation of Janus kinase 2 (Jak2), signal transducer and activator of transcription 5a (Stat5a), AMP-activated protein kinase (AMPK), mammalian target of rapamycin (mTOR), ribosomal protein S6 kinase 1 (S6K1), and eukaryotic factor 4E-binding protein 1 (4EBP1) were analyzed by Western blotting. The percentages of cells in the DNA postsynthetic (G2) and DNA synthesis (S) phases would decrease, with the level of d-Glc or AA declining individually, but no interaction was observed between the d-Glc and AA effects. The CSN2 and CSN3 mRNA and protein were downregulated when d-Glc or AA decreased individually from 17.5 to 2.50 mM or from 7.20 to 1.03 mM, but d-Glc deficiency had a greater effect according to the regression analysis. The phosphorylation ratio of Jak2 (Tyr^1007/1008), Stat5a (Tyr⁶⁹⁴), mTOR (Ser²⁴⁴⁸), S6K1 (Thr³⁸⁹), and 4EBP1 (Thr³⁷) was downregulated with the level of d-Glc or AA decline, whereas the phosphorylation ratio of AMPK (Thr^183/172) was upregulated. And the change of d-Glc level had a more marked effect than AA in regulating the activity of these signaling protein above according to the regression analysis. Thus, d-Glc or AA deficiency likely reduced casein transcription via inhibition of the Jak2/Stat5 pathway, and reduced translation via suppression of the mTOR pathway by activation of AMPK, but d-Glc deficiency had a more marked effect. These indicated that deficiency of AA, and especially Glc, suppressed proliferation of CMEC and casein gene and protein expression, associated with inhibition of JAK2/STAT5 and AMPK/mTOR signaling pathways.

Varying the ratio of Lys:Met while maintaining the ratios of Thr:Phe, Lys:Thr, Lys:His, and Lys:Val alters mammary cellular metabolites, mammalian target of rapamycin signaling, and gene transcription

Amino acids are not only precursors for but also signaling molecules regulating protein synthesis. Regulation of protein synthesis via AA occurs at least in part by alterations in the phosphorylation status of mammalian target of rapamycin (mTOR) pathway proteins. Although the ideal profile of Lys:Met to promote milk protein synthesis during established lactation in dairy cows has been proposed to be 3:1, aside from being the most-limiting AA for milk protein synthesis, the role of Met in other key biologic pathways such as methylation is not well characterized in the bovine. The objective of this study was to determine the influence of increasing supplemental Met, based on the ideal 3:1 ratio of Lys to Met, on intracellular metabolism related to protein synthesis and mTOR pathway phosphorylation status. MAC-T cells, an immortalized bovine mammary epithelial cell line, were incubated (n = 5 replicates/treatment) for 12 h with 3 incremental doses of Met while holding Lys concentration constant to achieve the following: Lys:Met 2.9:1 (ideal AA ratio; IPAA), Lys:Met 2.5:1 (LM2.5), and Lys:Met 2.0:1 (LM2.0). The ratios of Thr:Phe (1.05:1), Lys:Thr (1.8:1), Lys:His (2.38:1), and Lys:Val (1.23:1) were the same across the 3 treatments. Applying gas chromatography-mass spectrometry metabolomics revealed distinct clusters of differentially concentrated metabolites in response to Lys:Met. Lower Phe, branched-chain AA, and putrescine concentrations were observed with LM2.5 compared with IPAA. Apart from greater intracellular Met concentrations, further elevations in Met level (LM2.0) led to greater intracellular concentrations of nonessential AA (Pro, Glu, Gln, and Gly) compared with IPAA and greater essential AA (EAA; Met, Ile, and Leu) and nonessential AA (Pro, Gly, Ala, Gln, and Glu) compared with LM2.5. However, compared with IPAA, mRNA expression of β-casein and AA transporters (SLC7A5, SLC36A1, SLC38A2, SLC38A9, and SLC43A1) and mTOR phosphorylation were lower in response to LM2.5 and LM2.0. Overall, the results of this study provide evidence that increasing Met while Lys and the ratios of Phe, Thr, His, and Val relative to Lys were held constant could increase the concentration and utilization of intracellular EAA, in particular branched-chain AA, potentially through improving the activity of AA transporters partly controlled by mTOR signaling. Because EAA likely are metabolized by other tissues upon absorption, a question for future in vivo studies is whether formulating diets for optimal ratios of EAA in the metabolizable protein is sufficient to provide the desired levels of these AA to the mammary cells.

mTORC1 signaling-associated protein synthesis in porcine mammary glands was regulated by the local available methionine depending on methionine sources

Mechanistic target of rapamycin complex1 (mTORC1) activation and protein synthesis varied with methionine sources; however, the related mechanisms are largely unknown. Porcine mammary epithelial cells (PMEC) and mammary tissue slices (MTS) were used to test whether methionine precursors differ in providing the available methionine and thus differ in mTORC1 signaling-associated protein synthesis. PMEC with methionine deprivation for 8 h and MTS from lactating sows were cultured for 24 and 2 h, respectively, with treatment media without methionine (negative control, NC) or supplemented with 0.6 mM (for PMEC) and 0.1 mM (for MTS) of L-methionine (L-MET), D-methionine (D-MET), DL-2-hydroxy-4-(methylthio) butyric acid (HMTBA), or keto-methyl(thio)butanoic acid (KMB). The measurements included: phosphorylation of mTORC1 signaling, fractional protein synthesis rate (FSR), amino acids (AA) profile, and enzyme activities. Compared with the NC treatment, activated mTORC1 signaling as manifested by higher (P < 0.05) protein abundance of phosphorylated-S6 Kinase 1 (P-S6K1) and phosphorylated-4E-binding Protein 1 (P-4E-BP1) in PMEC and MTS, and increased protein synthesis as indicated by higher (P < 0.05) FSR in MTS occurred in L-MET and HMTBA treatments rather than in D-MET treatment. Compared with the NC treatment, methionine concentration and ratio of methionine to lysine in MTS increased (P < 0.05) in L-MET and HMTBA treatments but not in D-MET treatment, and activities of enzymes responsible for conversion of D-MET and HMTBA to keto-methionine in mammary tissues were about 10 and 50%, respectively, of that in liver. Taken together, mTORC1 signaling-associated protein synthesis in porcine mammary glands was regulated by the local available methionine depending on methionine sources.

Optimal ratios of essential amino acids stimulate β-casein synthesis via activation of the mammalian target of rapamycin signaling pathway in MAC-T cells and bovine mammary tissue explants

Amino acids are the building blocks of proteins and serve as key molecular components upstream of the signaling pathways that regulate protein synthesis. The objective of this study was to systematically investigate the effect of essential AA ratios on milk protein synthesis in vitro and to elucidate some of the underlying mechanisms. Triplicate cultures of MAC-T cells and bovine mammary tissue explants (MTE) were incubated with the optimal AA ratio (OPAA; Lys:Met, 2.9:1; Thr:Phe, 1.05:1; Lys:Thr, 1.8:1; Lys:His, 2.38:1; and Lys:Val, 1.23:1) in the presence of rapamycin (control), OPAA, a Lys:Thr ratio of 2.1:1, a Lys:Thr ratio of 1.3:1, a Lys:His ratio of 3.05:1, or a Lys:Val ratio of 1.62:1 for 12 h; the other AA concentrations were equal to OPAA. In some experiments, the cells were cultured with OPAA with or without rapamycin (100 ng/mL) or with mammalian target of rapamycin (mTOR) small interference RNA, and the MTE were exposed to OPAA with rapamycin for β-casein expression. Among the treatments, the expression of β-casein was greatest in the MTE cultured with OPAA. In MAC-T cells, the OPAA upregulated the mRNA expression of SLC1A5 and SLC7A5 but downregulated the expression of IRS1, AKT3, EEF1A1, and EEF2 compared with the control. The OPAA had no effect on the mTOR phosphorylation status but increased the phosphorylation of S6K1 and RPS6. When the MTE were treated with rapamycin in the presence of OPAA, the expression of β-casein was markedly decreased. The phosphorylation of RPS6 and 4EBP1 also was reduced in MAC-T cells. A similar negative effect on the expression of RPS6KB1 and EIF4EBP1 was detected when the cells were cultured with either rapamycin or mTOR small interference RNA. The optimal AA ratio stimulated β-casein expression partly by enhancing the transport of AA into the cells, cross-talk with insulin signaling and a subsequent enhancement of mTOR signaling, or translation elongation in both MAC-T cells and bovine MTE.