Effects of tripeptides and lactogenic hormones on oligopeptide transporter 2 in bovine mammary gland

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.

Enhancing Milk Production by Nutrient Supplements: Strategies and Regulatory Pathways

The enhancement of milk production is essential for dairy animals, and nutrient supplements can enhance milk production. This work summarizes the influence of nutrient supplements-including amino acids, peptides, lipids, carbohydrates, and other chemicals (such as phenolic compounds, prolactin, estrogen and growth factors)-on milk production. We also attempt to provide possible illuminating insights into the subsequent effects of nutrient supplements on milk synthesis. This work may help understand the strategy and the regulatory pathway of milk production promotion. Specifically, we summarize the roles and related pathways of nutrients in promoting milk protein and fat synthesis. We hope this review will help people understand the relationship between nutritional supplementation and milk production.

Effects of L-arginine and arginine-arginine dipeptide on amino acids uptake and αS1-casein synthesis in bovine mammary epithelial cells

Arginine (Arg), as an important functional amino acids (AA), is essential for milk protein synthesis in lactating ruminants. Arg shares transporters with cationic and neutral AA in mammary epithelial cells. Therefore, competitive inhibition might exist among these AA in uptake by mammary epithelial cells. In this study, cultured bovine mammary epithelial cells (BMEC) were used as the model to investigate whether the availability of L-Arg (0.7, 1.4, 2.8, 5.6, and 11.2 mM) affects the uptake of other AA and if this related to αS1-casein synthesis, and whether Arginine-Arginine (Arg-Arg) substituting part of free L-Arg can alleviate competitive inhibition among Arg and other AA, so as to promote αS1-casein synthesis. Our results showed that 2.8 mM L-Arg generated the greatest positive effects on αS1-casein synthesis and the activation of mammalian target of rapamycin (mTOR) signaling pathway (P < 0.01). With L-Arg supply increasing from 0.7 to 11.2 mM, the net-uptake of other AA (except Glu and Ala) decreased linearly and quadratically (Plinear < 0.01; Pquadratic < 0.01). Compared with 2.8 mM, the net-uptake of essential amino acids (EAA) and total amino acids (TAA) were lower at 11.2 mM L-Arg group, while greater at 1.4 mM L-Arg group (P < 0.01). Arg-Arg dipeptide replacing 10% free L-Arg increased αS1-casein synthesis (P < 0.05), net-uptake of EAA and TAA, as well as phosphorylation level of mTOR and p70 ribosomal protein S6 kinase (P70S6K) and mRNA expression of oligopeptide transporter 2 (PepT2; P < 0.01). These observations suggested that the increased αS1-casein synthesis by 10% Arg-Arg dipeptide might be related to the increase of AA availability and the activation of mTOR signaling pathway in BMEC.

Biology of Peptide Transporter 2 in Mammals: New Insights into Its Function, Structure and Regulation

Peptide transporter 2 (PepT2) in mammals plays essential roles in the reabsorption and conservation of peptide-bound amino acids in the kidney and in maintaining neuropeptide homeostasis in the brain. It is also of significant medical and pharmacological significance in the absorption and disposing of peptide-like drugs, including angiotensin-converting enzyme inhibitors, β-lactam antibiotics and antiviral prodrugs. Understanding the structure, function and regulation of PepT2 is of emerging interest in nutrition, medical and pharmacological research. In this review, we provide a comprehensive overview of the structure, substrate preferences and localization of PepT2 in mammals. As PepT2 is expressed in various organs, its function in the liver, kidney, brain, heart, lung and mammary gland has also been addressed. Finally, the regulatory factors that affect the expression and function of PepT2, such as transcriptional activation and posttranslational modification, are also discussed.

Lactation Activity and Mechanism of Milk-Protein Synthesis by Peptides from Oyster Hydrolysates

Oyster meat has a tender texture and delicate flavor, and the oyster is an aquatic shellfish with high nutritional and economic values. As they are rich in protein, oysters serve as a good source for the preparation of bioactive peptides. However, research on the lactation effect and mechanism of the synthesis of polypeptides from oyster hydrolysates is yet to be observed. This study aimed to analyze the lactation activity of the fraction UEC4-1 and explore its mechanism. The results show that, in an in vivo experiment, UEC4-1 could significantly increase the concentration of PRL in the serum and mammary tissue and the concentration of PRLR in the mammary tissue in rats with postpartum hypogalactia. UEC4-1 promoted the development of mammary tissue structure, resulting in active lactation. UEC4-1 promoted the proliferation of MCF-10A in a dose-dependent manner and could significantly upregulate the gene expression levels of PRL, PRLR, CSN1S1, CSN2, CSN3 and CCND1. UEC4-1 could also significantly increase the expression of mTOR, AKT1, RPS6KB1 and STAT5A in MCF-10A and improve its phosphorylation level. These results show that UEC4-1 had the ability to upregulate the proliferation and PRL synthesis of MCF-10A and promote lactation. The ability of UEC4-1 to regulate the milk-protein synthesis signaling pathway is the mechanism behind this. Oysters had a remarkable effect on lactating mothers’ sweating irritability after childbirth and may serve as an everyday diet to promote lactation. Postpartum dysgalactia is a common problem for lactating women. The study of the oyster’s lactation-active peptide can provide dietary nutrition guidance for postpartum lactating mothers, and it has the potential to be used for the development of drugs for the treatment of postpartum hypogalactia or oligogalactia.

Effect of Corticosteroids on Peptide Transporter 2 Function and Induction of Innate Immune Response by Bacterial Peptides in Alveolar Epithelial Cells

In the lung alveolar region, the innate immune system serves as an important host defense system. We recently reported that peptide transporter 2 (PEPT2) has an essential role in the uptake of bacterial peptides and induction of innate immune response in alveolar epithelial cells. In this study, we aimed to clarify the effects of corticosteroids on PEPT2 function and PEPT2-dependent innate immune response. NCI-H441 (H441) cells were used as an in vitro model of human alveolar type II epithelial cells, and the effects of dexamethasone (DEX) and budesonide (BUD) on the transport function of PEPT2 and the innate immune response induced by bacterial peptides were examined. PEPT2 function, estimated by measuring β-alanyl-Nε-(7-amino-4-methyl-2-oxo-2H-1-benzopyran-3-acetyl)-L-lysine (β-Ala-Lys-AMCA) uptake in H441 cells, was suppressed by treatment with DEX and BUD in a concentration- and time-dependent manner. The suppression of PEPT2 function was partially recovered by a glucocorticoid receptor antagonist. The expression of PEPT2 and nucleotide-binding oligomerization domain 1 (NOD1) mRNAs was suppressed by treatment with DEX and BUD, while PEPT2 protein level was not changed by these treatment conditions. Additionally, the increased mRNA expression of interleukin (IL)-8 and the increased secretion of IL-8 into the culture medium induced by bacterial peptides were also suppressed by treatment with these corticosteroids. Taken together, these results clearly suggest that corticosteroids suppress PEPT2 function and bacterial peptide-induced innate immune response in alveolar epithelial cells. Therefore, PEPT2- and NOD1-dependent innate immune response induced by bacterial peptides in the lung alveolar region may be suppressed during the inhaled corticosteroid therapy.

Regulation of Milk Protein Synthesis by Free and Peptide-Bound Amino Acids in Dairy Cows

Milk protein (MP) synthesis in the mammary gland of dairy cows is a complex biological process. As the substrates for protein synthesis, amino acids (AAs) are the most important nutrients for milk synthesis. Free AAs (FAAs) are the main precursors of MP synthesis, and their supplies are supplemented by peptide-bound AAs (PBAAs) in the blood. Utilization of AAs in the mammary gland of dairy cows has attracted the great interest of researchers because of the goal of increasing MP yield. Supplying sufficient and balanced AAs is critical to improve MP concentration and yield in dairy cows. Great progress has been made in understanding limiting AAs and their requirements for MP synthesis in dairy cows. This review focuses on the effects of FAA and PBAA supply on MP synthesis and their underlying mechanisms. Advances in our knowledge in the field can help us to develop more accurate models to predict dietary protein requirements for dairy cows MP synthesis, which will ultimately improve the nitrogen utilization efficiency and lactation performance of dairy cows.

Cloning, tissue distribution and functional characterization of the donkey (Equus asinus) oligopeptide transporter 2

Oligopeptide transporter 2 (PepT2) is an important transporter of oligopeptides. In the present study, we describe the molecular cloning, tissue distribution and functional characterization of a donkey (Equus asinus) PepT2. The cloned cDNA sequence was 2202 bp at full length, encoding a 733 amino acid peptide with a molecular weight of 81.9 kDa and a theoretical pI of 8.92. Bioinformatics analysis showed that the deduced peptide sequence possessed all the characteristic features of PepT2. The expression of PepT2 in the kidney and lung was significantly higher than that observed in the ileum, duodenum, jejunum, spleen, liver, heart and stomach. Functional characterization by heterologous expression in Chinese hamster ovary cells showed that the uptake of β-Ala-Lys-N-7-amino-4-methylcoumarin-3-acetic acid (β-Ala-Lys-AMCA) by donkey PepT2-Chinese hamster ovary cells was dependent on time, pH and substrate concentration, with a low Km value of 91.51 ± 14.14 μM and a maximum velocity of 41.37 ± 2.193 pmol/min/mg protein. In the present study, for the first time, the expression and functional characteristics of donkey PepT2 were evaluated, the results of which provide new insights and a better understanding of its crucial role in oligopeptide transport in donkeys.

Single-step genome-wide association studies (GWAS) and post-GWAS analyses to identify genomic regions and candidate genes for milk yield in Brazilian Girolando cattle

Milk production is economically important to the Brazilian agribusiness, and the majority of the country's milk production derives from Girolando (Gir × Holstein) cows. This study aimed to identify quantitative trait loci (QTL) and candidate genes associated with 305-d milk yield (305MY) in Girolando cattle. In addition, we investigated the SNP-specific variances for Holstein and Gir breeds of origin within the sequence of candidate genes. A single-step genomic BLUP procedure was used to identify QTL associated with 305MY, and the most likely candidate genes were identified through follow-up analyses. Genomic breeding values specific for Holstein and Gir were estimated in the Girolando animals using a model that uses breed-specific partial relationship matrices, which were converted to breed of origin SNP effects. Differences between breed of origin were evaluated by comparing estimated SNP variances between breeds. From 10 genome regions explaining most additive genetic variance for 305MY in Girolando cattle, 7 candidate genes were identified on chromosomes 1, 4, 6, and 26. Within the sequence of these 7 candidate genes, Gir breed of origin SNP alleles showed the highest genetic variance. These results indicated QTL regions that could be further explored in genomic selection panels and which may also help in understanding the gene mechanisms involved in milk production in the Girolando breed.

Transporters in the Mammary Gland-Contribution to Presence of Nutrients and Drugs into Milk

A large number of nutrients and bioactive ingredients found in milk play an important role in the nourishment of breast-fed infants and dairy consumers. Some of these ingredients include physiologically relevant compounds such as vitamins, peptides, neuroactive compounds and hormones. Conversely, milk may contain substances-drugs, pesticides, carcinogens, environmental pollutants-which have undesirable effects on health. The transfer of these compounds into milk is unavoidably linked to the function of transport proteins. Expression of transporters belonging to the ATP-binding cassette (ABC-) and Solute Carrier (SLC-) superfamilies varies with the lactation stages of the mammary gland. In particular, Organic Anion Transporting Polypeptides 1A2 (OATP1A2) and 2B1 (OATP2B1), Organic Cation Transporter 1 (OCT1), Novel Organic Cation Transporter 1 (OCTN1), Concentrative Nucleoside Transporters 1, 2 and 3 (CNT1, CNT2 and CNT3), Peptide Transporter 2 (PEPT2), Sodium-dependent Vitamin C Transporter 2 (SVCT2), Multidrug Resistance-associated Protein 5 (ABCC5) and Breast Cancer Resistance Protein (ABCG2) are highly induced during lactation. This review will focus on these transporters overexpressed during lactation and their role in the transfer of products into the milk, including both beneficial and harmful compounds. Furthermore, additional factors, such as regulation, polymorphisms or drug-drug interactions will be described.

Functional Characterization of Peptide Transporters in Bovine Mammary Epithelial Cells

The objective of this study was to characterize the expression profile, transport kinetics, and regulation of peptide transporters in bovine mammary epithelial cells (BMECs). Quantitative reverse-transcription real-time PCR, Western blotting, and immunofluorescence staining were used to investigate the expression of peptide transporters in bovine mammary tissues. The effects of time, pH, concentration, and specific inhibitors on β-alanyl-l-lysyl- Nε-7-amino-4-methyl-coumarin-3-acetic acid (β-Ala-Lys-AMCA) uptake in BMECs were also studied. The results showed that the peptide transporters PepT2 and PhT1 are both expressed in bovine mammary glands. The optimal pH for the uptake of β-Ala-Lys-AMCA in BMECs was 6.5. The transport-kinetics study suggested that the uptake of β-Ala-Lys-AMCA in BMECs is saturable over the tested concentration, with a K_m value of 82 ± 18 μM and a V_max of 124 ± 11 pmol/min per milligram of protein. Other dipeptides, including Gly-Sar, Met-Gly, and Met-Met, competitively inhibited β-Ala-Lys-AMCA uptake in BMECs. However, histidine had no effect on β-Ala-Lys-AMCA uptake. Furthermore, knocking down PepT2 could significantly reduce β-Ala-Lys-AMCA uptake, but PhT1 interference had no effect on peptide uptake in BMECs. The inhibition of PI3K and Akt decreased the uptake of β-Ala-Lys-AMCA. The above results revealed functional characteristics of peptide transporters and demonstrated that PepT2 may play a major role in β-Ala-Lys-AMCA uptake in BMECs. Moreover, the PI3K-Akt signaling pathway may regulate the uptake of β-Ala-Lys-AMCA in BMECs.

Functional characterization of oligopeptide transporter 1 of dairy cows

Background

It is well known that peptides play a vital role in the nutrition and health of dairy cows. Bovine oligopeptide transporter 1 (bPepT1) is involved in the peptide transport process in the gastrointestinal tracts of dairy cows. However, little information is known in the characteristics of bPepT1. Therefore, the purpose of this study was to characterize bPepT1 functionally using a mammalian cell expression system. The uptake of radiolabeled dipeptide glycyl-sarcosine ([³H]-Gly-Sar) into the bPepT1-transfected Chinese hamster ovary cells was measured at various pH and substrate concentrations and with or without 15 other small peptides that contained Met or Lys.

Results

Western blot results showed that the abundance of bPepT1 protein in the jejunum and ileum are the highest in the gastrointestinal tract of dairy cows. The uptake of [³H]-Gly-Sar by bPepT1-Chinese hamster ovary cells was dependent on time, pH, and substrate concentration, with a low K_m value of 0.94 ± 0.06 mmol/L and a maximum velocity of 20.80 ± 1.74 nmol/(mg protein • 5 min). Most of the di- and tripeptides were the substrates of bPepT1, based on substrate-competitive studies. However, bPepT1 has a higher affinity to the peptides with shorter chains, greater hydrophobicity, and negative or neutral charges.

Conclusions

These results demonstrated for the first time the functional characteristics of bPepT1, and they provide a new insight and better understanding into its vital role in absorbing a wide range of peptides from the digestive tract of dairy cows.

Food Deprivation Affects the miRNome in the Lactating Goat Mammary Gland

Background

Nutrition affects milk composition thus influencing its nutritional properties. Nutrition also modifies the expression of mammary genes, whose regulation is not fully understood. MicroRNAs (miRNA) are small non coding RNA which are important post-transcriptional regulators of gene expression by targeting messenger RNAs. Our goal was to characterize miRNA whose expression is regulated by nutrition in the lactating goat mammary gland, which may provide clues to deciphering regulations of the biosynthesis and secretion of milk components.

Methodology/principal findings

Using high-throughput sequencing technology, miRNomes of the lactating mammary gland were established from lactating goats fed ad libitum or deprived of food for 48h affecting milk production and composition. High throughput miRNA sequencing revealed 30 miRNA with an expression potentially modulated by food deprivation; 16 were down-regulated and 14 were up-regulated. Diana-microT predictive tools suggested a potential role for several nutriregulated miRNA in lipid metabolism. Among the putative targets, 19 were previously identified as differently expressed genes (DEG). The functions of these 19 DEG revealed, notably, their involvement in tissue remodelling.

Conclusion/significance

In conclusion, this study offers the first evidence of nutriregulated miRNA in the ruminant mammary gland. Characterization of these 30 miRNA could contribute to a clearer understanding of gene regulation in the mammary gland in response to nutrition.

Methionyl-Methionine Promotes α-s1 Casein Synthesis in Bovine Mammary Gland Explants by Enhancing Intracellular Substrate Availability and Activating JAK2-STAT5 and mTOR-Mediated Signaling Pathways

BACKGROUND

Interest is increasing in the role of peptide-bound amino acids (AAs) in milk protein synthesis because studies have found that the uptake of some essential AAs by the mammary gland cannot meet the requirements for milk protein synthesis. Although the role of dipeptides in milk protein synthesis is clearly established, little is known about the underlying mechanisms.

OBJECTIVE

The objective of this study was to determine whether small peptides can be taken up intact by the peptide transporters in mammary tissue explants and the underlying mechanisms of the effects of methionyl-methionine (Met-Met) supplementation on milk protein synthesis.

METHODS

Mammary tissue explants were cultured in conditional medium and then treated with different concentrations of Met-Met that replaced 0%, 5%, 10%, 15%, 20%, and 25% of free Met for another 24 h. In some experiments, explants were cultured with an optimal dose of Met-Met with or without the inhibitors of peptide transporter 2 [PepT2; diethylpyrocarbonate (DEPC), 0.1 mmol/L] and aminopeptidase N (APN; bestatin, 20 μmol/L) for 24 h.

RESULTS

The substitutions of 15% free Met with Met-Met significantly promoted α-s1 casein (αs1-CN) expression in the mammary explants (P < 0.05). The inhibition of the PepT2 by DEPC or APN by bestatin significantly decreased the Met-Met-stimulated increase of αs1-CN expression (P < 0.05). Compared with the control group (0% Met-Met), absorption of Val, Met, Leu, Phe, Lys, and His was improved, and mRNA abundance of the neutral and basic AA transporter was increased in the 15% Met-Met group (P < 0.05). In addition, the mRNA abundance of the mammalian target of rapamycin (mTOR), p70 ribosomal S6 kinase 1 gene, eukaryotic initiation factor 4E binding protein 1 gene , Janus kinase 2 (JAK2), and signal transducer and activator of transcription 5 (STAT5) was increased in the 15% Met-Met-treated group (P < 0.05).

CONCLUSION

Met-Met promoted αs1-CN synthesis in cultured bovine mammary gland explants, and this stimulation may be mediated by enhanced intracellular substrate availability and by activating JAK2-STAT5 and mTOR signaling pathways.

Establishment and characterization of an omasal epithelial cell model derived from dairy calves for the study of small peptide absorption

The objective of this study was to establish a primary culture of omasal epithelial cells (OECs) derived from dairy calves and to characterize its function in small peptide absorption. Bovine omasal tissues were obtained from newborn Chinese Holstein calves and digested with a 2.5% trypsin solution to obtain OECs. The isolated cells were later cultured in DMEM containing 10% fetal bovine serum, 5 μg/ml insulin, 10 ng/ml epidermal growth factor, 100 U/ml penicillin, 100 μg/ml streptomycin, 50 μg/ml gentamycin and 2.5 μg/ml amphotericin B. Hematoxylin and eosin staining of omasal tissue after digestion indicated that the cultured cells originated from the epithelial strata. Pure epithelial cells displayed an epithelial cell-like morphology, similar to cobblestone, with few visible fibroblasts and were cytokeratin 18-positive according to immunocytochemical analyses. The OECs were morphologically characterized with desmosomes, tight junctions and microvilli. These cells exhibited normal growth properties, as assessed using a cell growth curve, and were stably cultured for 10 passages. The OECs expressed the peptide transporter 1 (PEPT1) mRNA and absorbed intact glycylsarcosine (Gly-Sar). The uptake of Gly-Sar by OECs was pH-dependent with an optimal pH of 5.5–6.5. Furthermore, the uptake of Gly-Sar was also time-dependent, concentration-dependent and temperature-dependent. Moreover, PEPT1 was saturated with Gly-Sar at a concentration of 2.5 mM. The uptake via PEPT1 was higher compared with that via passive route at low substrate concentrations (<1.5 mM). This result suggested that PEPT1 contributed more to total small peptide absorption at low concentrations. In addition, this uptake could be competitively inhibited by methionine-glycine. Taken together, these data suggested that PEPT1 contributes to small peptide absorption in OECs. Thus, OECs may serve as a useful culture model for the study of the absorption of small peptides in bovine omasum.

The functional and molecular entities underlying amino acid and peptide transport by the mammary gland under different physiological and pathological conditions

This review describes the properties and regulation of the membrane transport proteins which supply the mammary gland with aminonitrogen to support metabolism under different physiological conditions (i.e. pregnancy, lactation and involution). Early studies focussed on characterising amino acid and peptide transport pathways with respect to substrate specificity, kinetics and hormonal regulation to allow a broad picture of the systems within the gland to be established. Recent investigations have concentrated on identifying the individual transporters at the molecular level (i.e. mRNA and protein). Many of the latter studies have identified the molecular correlates of the transport systems uncovered in the earlier functional investigations but in turn have also highlighted the need for more amino acid transport studies to be performed. The transporters function as either cotransporters and exchangers (or both) and act in a coordinated and regulated fashion to support the metabolic needs of the gland. However, it is apparent that a physiological role for a number of the transport proteins has yet to be elucidated. This article highlights the many gaps in our knowledge regarding the precise cellular location of a number of amino acid transporters within the gland. We also describe the role of amino acid transport in mammary cell volume regulation. Finally, the important role that individual mammary transport proteins may have in the growth and proliferation of mammary tumours is discussed.