Hostname: page-component-586b7cd67f-t8hqh Total loading time: 0 Render date: 2024-11-25T18:12:00.712Z Has data issue: false hasContentIssue false
  • English
  • Français

Amino acid transportation, sensing and signal transduction in the mammary gland: key molecular signalling pathways in the regulation of milk synthesis

Published online by Cambridge University Press:  10 March 2020

Zhihui Wu ,
Jinghui Heng ,
Min Tian ,
Hanqing Song ,
Fang Chen ,
Wutai Guan  and
Shihai Zhang Open the ORCID record for Shihai Zhang [Opens in a new window]
Zhihui Wu
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China
Jinghui Heng
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China
Min Tian
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China
Hanqing Song
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China
Fang Chen
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China College of Animal Science and National Engineering Research Center for Breeding Swine Industry, South China Agricultural University, Guangzhou510642, China
Wutai Guan*
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China College of Animal Science and National Engineering Research Center for Breeding Swine Industry, South China Agricultural University, Guangzhou510642, China
Shihai Zhang*
Affiliation:
Guangdong Province Key Laboratory of Animal Nutrition Control, College of Animal Science, South China Agricultural University, Guangzhou, 510642, China College of Animal Science and National Engineering Research Center for Breeding Swine Industry, South China Agricultural University, Guangzhou510642, China Department of Molecular Biology, University of Texas Southwestern Medical Center, Dallas, TX, USA
*
*Corresponding authors: Wutai Guan, email [email protected]; Shihai Zhang, email [email protected]
*Corresponding authors: Wutai Guan, email [email protected]; Shihai Zhang, email [email protected]
Rights & Permissions [Opens in a new window]

Abstract

The mammary gland, a unique exocrine organ, is responsible for milk synthesis in mammals. Neonatal growth and health are predominantly determined by quality and quantity of milk production. Amino acids are crucial maternal nutrients that are the building blocks for milk protein and are potential energy sources for neonates. Recent advances made regarding the mammary gland further demonstrate that some functional amino acids also regulate milk protein and fat synthesis through distinct intracellular and extracellular pathways. In the present study, we discuss recent advances in the role of amino acids (especially branched-chain amino acids, methionine, arginine and lysine) in the regulation of milk synthesis. The present review also addresses the crucial questions of how amino acids are transported, sensed and transduced in the mammary gland.

Keywords

Mammary glandAmino acidsMilk proteinMilk fatSignalling pathways
Type
Review Article
Information
Nutrition Research Reviews , Volume 33 , Issue 2 , December 2020 , pp. 287 - 297
Copyright
© The Author(s) 2020

Introduction

Milk is primarily composed of milk protein, fat, lactose, vitamins and minerals, which are important nutrient sources for neonates. Maternal nutrients are considered as building blocks for milk synthesis. In humans, breast-feeding decreases the risk of neonatal acute illnesses, diarrhoea and overweight/obesity (breast-feeding for more than 6 months)(Reference Pattison, Kraschnewski and Lehman1). However, in pigs, sufficient and quality colostrum (first 24 h) supplies are important for neonates to gain higher weaning weight and have better growth performance later in life(Reference Theil, Lauridsen and Quesnel2). Calves fed adequate colostrum during the first week of their life exhibit significantly enhanced metabolic and immunological status(Reference Rauprich, Hammon and Blum3). Thus, understanding nutritional strategies to regulate milk synthesis and its underlying mechanism is important for human beings and other mammals.

Amino acids are not only basic components of proteins, but also act as functional regulators in a variety of biological processes. The functions of amino acids have been extensively studied in the gut, liver, muscle and adipose tissue, especially in the field of protein(Reference Wu4) and fat metabolism(Reference Zhang, Zeng and Ren5). In the mammary gland, amino acid uptake from blood is almost equal to milk output based on a nitrogen basis in dairy cows(Reference Omphalius, Lapierre and Guinard-Flament6–Reference Haque, Guinard-Flament and Lamberton8), goats(Reference Safayi and Nielsen9), sows(Reference Trottier, Shipley and Easter10) and ewes(Reference Davis, Bickerstaffe and Hart11). However, the destinies of different amino acids in mammary cells are different(Reference Omphalius, Lapierre and Guinard-Flament6–Reference Haque, Guinard-Flament and Lamberton8,Reference Trottier, Shipley and Easter10–Reference Mepham and Linzell12) . The mammary amino acid uptake:output ratios could be larger (for example, valine, isoleucine, leucine and arginine), equal (for example, methionine and histidine) or less than 1 (for example, asparagine and proline) (mammary amino acid uptake:output ratios in cows, goats, sows and ewes are shown in Table 1)(Reference Omphalius, Lapierre and Guinard-Flament6–Reference Haque, Guinard-Flament and Lamberton8,Reference Trottier, Shipley and Easter10–Reference Mepham and Linzell12) . Those amino acids with uptake:output ratios greater than 1 can be metabolised to CO2, urea, polyamine or simply other non-essential amino acids(Reference Mepham13). In addition, various amino acids (especially branched-amino acids, methionine and arginine) are involved in the regulation of milk synthesis. A variety of signalling molecules have been proposed to cooperate with amino acids to regulate biological functions in the mammary gland. Signalling pathways regulating mammary epithelial cell proliferation and differentiation have been well characterised previously(Reference Hennighausen and Robinson14). However, the underlying mechanisms and signalling pathways by which amino acids regulate milk and fat synthesis were largely unknown until recently. The aim of the present review is to describe how amino acids are transported, sensed and transduced in the mammary gland, as well as their functions in the regulation of milk synthesis.

Table 1. Mammary amino acid uptake:output ratios in different mammals

Branched-chain amino acids

Transportation and metabolism of branched-chain amino acids in the mammary gland

The plasma membrane transport system L is the most critical amino acid transporter system for branched-chain amino acids (BCAA) in mammary cells(Reference Jackson, Bryson and Wang15). Transporters from the L system have been well characterised to regulate cell growth and proliferation by directly transporting branched or aromatic amino acids into the cytoplasm in a range of cell lines(Reference Luo, Coon and Su16–Reference Kurayama, Ito and Nishibori18). These transporters are heterodimeric proteins, which comprise of a catalytic subunit (l-type amino acid transporter 1 (LAT1), encoded by SLC7A5, or l-type amino acid transporter 2 (LAT2), encoded by SLC7A6) and a glycoprotein 4F2 heavy chain (4F2hc). Both LAT1 and LAT2 are highly expressed in the mammary tissues(Reference Jackson, Bryson and Wang15), but which subtype is predominately expressed in the mammary gland seems to be different among species(Reference Matsumoto, Nakamura and Nakamura19–Reference Chen, Zhang and Deng21). In rats, the gene expression of LAT1 in the mammary gland is greater than that of LAT2 during the lactation period(Reference Matsumoto, Nakamura and Nakamura19). The gene expression of LAT2, but not LAT1, is significantly increased with the progression of lactation in sows(Reference Chen, Zhang and Deng21). In bovine mammary glands, more research has been focused on the effects of LAT1(Reference Lin, Duan and Lv20,Reference Duan, Lin and Lv22) . Depletion of LAT1 in the bovine mammary gland dephosphorylates and inhibits the activity of mTORC1 (mammalian target of rapamycin complex 1), thereby blunting cell viability and β-casein synthesis(Reference Lin, Duan and Lv20). Additionally, the inhibition of mTORC1 can be rescued by re-expressing LAT1(Reference Lin, Duan and Lv20). However, whether LAT2 also plays an important role in the bovine mammary gland has not been determined and remains to be studied.

BCAA catabolism in mammary tissue is similar to that in other tissues (such as muscle, liver and intestine)(Reference Li, Knabe and Kim23). Leucine, isoleucine and valine share a number of BCAA catabolic enzymes, such as branched-chain aminotransferase (BCAT) and branched-chain α-keto acid dehydrogenase (BCKD)(Reference Wohlt, Clark and Derrig24). In the porcine mammary gland, the protein levels of BCAT are higher than those in the small intestine, skeletal muscle and liver(Reference Li, Knabe and Kim23), which indicates that BCAA are actively metabolised in the mammary gland. Mammary BCAA catabolism primarily produces glutamine and aspartate(Reference Li, Knabe and Kim23). Notably, as BCAA share the same catabolic enzymes, excessive supplementation of either BCAA might affect the metabolism of other BCAA.

Potential signalling pathway of branched-chain amino acids in the mammary gland

Leucine

In the mammary gland, leucine regulates various biological processes, such as cell proliferation and milk synthesis (αs-casein, β-casein and κ-casein) (as shown in Table 2). Numerous studies have demonstrated that mTOR functions as a critical regulator of these processes. Until recently, it was not clear of how leucine regulates mTOR until recently (Fig. 1). Before scientists started studying the effect of leucine on mTOR signalling in mammary cells, most pioneering studies were conducted in human embryonic kidney 293 (HEK 293) cells, which is a classical cell line for research investigating the cellular signalling pathway. In HEK 293 cells, it has been demonstrated that leucine regulates the mTOR signalling pathway primarily through mTOR complex 1 (mTORC1) which consists of regulatory-associated protein of mTOR (Raptor), mammalian lethal with SEC13 protein 8 (mLST8), 40 kDa proline-rich protein kinase B (Akt) substrate (PRAS40) and DEP domain-containing mTOR-interacting protein (DEPTOR). After being absorbed into the cytosol, leucine beings to regulate mTORC1 activation by first dephosphorylating Sestrin2(Reference Saxton, Knockenhauer and Wolfson25,Reference Kimball, Gordon and Moyer26) . Dephosphorylated Sestrin2 induces the dissociation of GTPase-activating protein activity toward Rags 2 (GATOR2), which further inhibits the function of GTPase-activating protein activity toward Rags 1 (GATOR1, a negative regulator of RagA/B)(Reference Wolfson, Chantranupong and Saxton27). Finally, activated RagA/B promotes the translocation of mTORC1 to lysosomes for further activation(Reference Wolfson, Chantranupong and Saxton27).

Table 2. Effects of branched-chain amino acids (BCAA) on mammary gland function and its potential signalling pathways

mTORC1, mammalian target of rapamycin complex 1; eIF, eukaryotic initiation factor; mTOR, mammalian target of rapamycin; S6K1, S6 kinase 1; Akt, protein kinase B; LAT1, l-type amino acid transporter 1; CSN1S1, 2, 3, casein αs1, 2, 3; JAK2, Janus kinase 2; STAT5, signal transducers and activators of transcription 5; AMPK, AMP-activated protein kinase; SREBP-1, sterol regulatory element-binding protein 1; ERK, extracellular signal-regulated kinase.

Fig. 1. Branched-chain amino acid (BCAA) and mammalian target of rapamycin complex 1 (mTORC1) signalling networks in the mammary gland. Note: l-type amino acid transporter 1/4F2 heavy chain (LAT1/4F2hc) and l-type amino acid transporter 2 (LAT2)/4F2hc derived from transporter system L are highly expressed and play a dominant role in BCAA transportation in the mammary gland. All three BCAA activate mTORC1 pathways in mammary glands. Leucine induces dephosphorylation of Sestrin2 and further promotes mTORC1 activation through GTPase-activating protein activity toward Rags (GATOR) 2, GATOR1 and RagA/B. In addition, GCG12, SH3-domain binding protein 4 (SH3BP4) and leucyl-tRNA synthetase (LeuRS) are crucial regulators in leucine-related mTORC1 activation. Extracellular valine activates G-protein-coupled receptors (GPRC) T1R1/T1R3, increases phospholipase Cβ (PLCβ) activity and further enhances an influx of intracellular Ca2+. Increased Ca2+ regulates the mTORC1 signalling pathway through extracellular signal-regulated kinase 1/2–tuberous sclerosis complex 1/2–Rheb (ERK1/2–TSC1/2–Rheb) signalling. Intracellular isoleucine activates mTORC1 through an unknown mechanism. In the mammary gland, activated mTORC1 not only increases milk protein synthesis but also milk fat synthesis through lipin 1 (Lpn1)–sterol regulatory element-binding protein 1c (SREBP-1c) pathways. ER, endoplasmic reticulum. Please refer to the main text for details.

Recent advances also strongly suggest that the leucine-regulated mTOR signalling pathway is also conserved in the mammary gland. In bovine mammary glands, overexpression of Sestrin2 depresses mTORC1 activity and synthesis of casein, indicating that Sestrin2 plays a major physical role in mammary gland cells(Reference Luo, Zheng and Zhao28). Danio rerio SH3-domain binding protein 4 (SH3BP4) has been previously reported to abrogate mTORC1 activation by hydrolysing GTP to GDP of RagB in HEK 293 cells(Reference Kim, Stone and Hwang29). In the mammary gland, SH3BP4 is also proposed to play a vital role between Sestrin 2 and Rag GTPase(Reference Luo, Zheng and Zhao28). Interestingly, not only leucine but also other essential amino acids and non-essential amino acids can also regulate mTORC1 through Sestrin2 in the mammary gland(Reference Luo, Zhao and Zhang30), which is inconsistent with observations in HEK 293 cell lines and warrants further investigation.

In the mammary gland, the other potential leucine-mediated mTORC1 signalling pathway is through guanine nucleotide-binding protein subunit γ-12 (GNG12) and leucyl-tRNA synthetase (LeuRS). GNG12 regulates mTORC1 via interaction with Regulator(Reference Luo, Zhao and Dai31), which affects the translocation of mTORC1 to lysosomal membranes(Reference Sancak, Bar-Peled and Zoncu32). LeuRS acts as a vital intracellular leucine sensor that can directly bind to Rag GTPase and activate mTORC1(Reference Han, Jeong and Park33). In mouse mammary cells, LeuRS activates the mTOR signalling pathway and increases cell proliferation(Reference McGuckin, Manjarin and Peterson34). In bovine mammary cells, GNG12 enhances cell growth and milk protein synthesis by activating the mTORC1 signalling pathway(Reference Luo, Zhao and Dai31).

Valine and isoleucine

In addition to l-leucine, l-isoleucine and l-valine are also proposed to regulate milk synthesis. Dietary supplementation of l-isoleucine and l-valine during the whole lactation period enhances milk synthesis in sows and supports increased weaning weight of their litters(Reference Richert, Goodband and Tokach35,Reference Richert, Tokach and Goodband36) . Similarly, l-isoleucine and l-valine deficiency during the mid-lactation period inhibits milk synthesis in dairy cows(Reference Haque, Rulquin and Lemosquet37). Furthermore, recent advances indicate that both l-isoleucine and l-valine can activate the mTOR signalling pathway and have the potential to enhance milk protein synthesis(Reference Appuhamy, Knoebel and Nayananjalie38–Reference Che, Xu and Gao40). Additionally, l-valine enhances fatty acid synthesis through activation of the mTOR/sterol regulatory element-binding protein 1 (SREBP-1) pathway(Reference Che, Xu and Gao40). SREBP-1 is a transcription factor and is proposed to be an important regulator of mammary gland fat synthesis in sheep(Reference Carcangiu, Mura and Daga41), cows(Reference Ma and Corl42) and mice(Reference Rudolph, McManaman and Phang43). Lipin 1, a phosphatidic acid phosphatase, is a critical link between mTOR and SREBP-1(Reference Peterson, Sengupta and Harris44). When mTORC1 is activated, it phosphorylates lipin 1, which releases SREBP-1 and activates SREBP-1-regulated lipogenetic gene expression(Reference Peterson, Sengupta and Harris44). As l-valine regulates lipogenesis through activation of mTORC1, this finding strongly suggests that l-leucine and l-isoleucine might also participate in lipogenesis progression. The isotope tracing experiment showed that the carbon from l-leucine can incorporate into milk fat on goat mammary explants, which provides direct evidence that l-leucine is involved in milk fat production(Reference Roets, Massart-Leën and Peeters45).

Recently, mTORC1 stimulation was observed before amino acid absorption into the cytostome(Reference Nelson, Chandrashekar and Hoon46–Reference Liu, Wang and Li48). T1R1/T1R3, a G-protein-coupled receptor (GPCR) in the cell membrane, is an important player in this process(Reference Nelson, Chandrashekar and Hoon46). It has been demonstrated that amino acids regulate milk protein synthesis through T1R1/T1R3 in the mouse mammary gland(Reference Wang, Liu and Wu47,Reference Liu, Wang and Li48) . Activation of the G-protein-coupled receptors (GPRC) T1R1/T1R3 increases phospholipase Cβ (PLCβ) activity and further enhances an influx of intracellular Ca2+(Reference Wauson, Zaganjor and Lee49,Reference Wauson, Zaganjor and Cobb50) . Extracellular signal-regulated kinase (ERK) 1 and 2 (ERK1/2), which are increased with Ca2+ concentration, regulate the activation of mTORC1 directly via the phosphorylation of Raptor (regulatory-associated protein of mTOR)(Reference Carriere, Romeo and Acosta-Jaquez51). The other possible pathway by which ERK1/2 activates mTORC1 signalling is through the inactivation of tuberous sclerosis complex 2 (TSC2), which is an inhibitor of mTORC1(Reference Rolfe, McLeod and Pratt52). As T1R1/T1R3 can be widely activated by l-amino acids(Reference Nelson, Chandrashekar and Hoon46), it is supposed that all of the BCAA (leucine, valine and isoleucine) could activate mTORC1 through this signalling pathway in the mammary gland. However, in bovine mammary glands, valine, not isoleucine and leucine, regulates the mTOR signalling pathway through the membrane GPRC receptor T1R1/T1R3(Reference Zhou, Zhou and Peng53), which might be due to insufficient supplementation with isoleucine and leucine. Future experiments are warranted to verify these results in the mammary cells of other species.

Methionine

Transportation system of methionine in the mammary gland

In the mammary gland, three amino acid transporter systems are involved in methionine transportation, namely systems A, ASC and L(Reference Verma and Kansal54). System A consists of Na-coupled neutral amino acid transporter 1 (SNAT1; detected in pigs) and Na-coupled neutral amino acid transporter 2 (SNAT2; detected in rats and cows)(Reference Shennan and Boyd55). System ASC primarily contains Na-dependent alanine cotransport 1 (ASCT1) (detected in humans, mice, cows and pigs) and Na-dependent alanine cotransport 2 (ASCT2) (detected in rats and cows)(Reference Shennan and Boyd55). System L is composed of two heteromeric Na+-independent transporters LAT1/4F2hc (detected in humans, rats, mice, cows) and LAT2/4F2hc (detected in rats and cows)(Reference Chillaron, Roca and Valencia56). In bovine mammary glands, SNAT2 inhibition strongly prevents the activation of mTORC1 caused by decreased methionine transportation(Reference Qi, Meng and Jin57). The functional evaluation of the importance of specific methionine transporters is still insufficient and warrants further research to demonstrate which transporter system may play a dominant role in the mammary gland.

Potential signalling pathway of methionine in the mammary gland

The effects of methionine on milk synthesis in the mammary gland have been demonstrated for many years (Table 3). It has been shown through meta-analyses that methionine is one of the first two limiting amino acids in dairy cows(Reference Schwab, Satter and Clay58,Reference Rulquin, Pisulewski and Vérité59) and is an important limiting amino acid in lactating sows(Reference Dourmad, Etienne and Valancogne60,61) . Intriguingly, the effect of methionine on milk fat synthesis is not linked to the use of methionine carbon in fatty acid synthesis since its ratio of mammary uptake to milk output is always at 1 in dairy cows(Reference Lapierre, Lobley and Doepel62). Advanced research in HEK 293 cells has demonstrated that when methionine is deficient, the cellular methyl donor S-adenosylmethionine (SAM) level will be reduced, which further increases the association of SAMTOR (SAM sensor) with GATOR2 and inhibits mTORC1 signalling(Reference Gu, Orozco and Saxton63). However, to the best of our knowledge, whether SAMTOR also acts as a conserved SAM sensor in the mammary gland has not been determined and merits further research.

Table 3. Effects of methionine on mammary gland function and its potential signalling pathways

NFE2L2, nuclear factor erythroid 2-like 2; mTOR, mammalian target of rapamycin; S6K1, S6 kinase 1; 4E-BP1, eukaryotic initiation factor 4E-binding protein 1; FABP5, fatty acid-binding protein 5; SREBP-1c, sterol regulatory element-binding protein 1c; SNAT2, Na-coupled neutral amino acid transporter 2; PI3K, inositol 1,4,5-trisphosphate 3-kinase; mTORC1, mammalian target of rapamycin complex 1; Akt, protein kinase B.

Two other potential methionine-regulated mTORC1 signalling pathways have been verified in the mammary gland (Fig. 2). One possible approach is the inositol 1,4,5-trisphosphate 3-kinase (PI3K)/Akt signalling pathway. Activated PI3K/Akt/mTORC1 significantly stimulates milk protein synthesis(Reference Qi, Meng and Jin57,Reference Ma, Batistel and Xu64) . Furthermore, methionine also plays a crucial role in milk fat synthesis. Fatty acid-binding protein 5 (FABP5) is a crucial regulator that activates SREBP-1c for milk fatty synthesis(Reference Li, Yu and Zhou65), which can be partly activated by PI3K(Reference Lv, Wang and Zhang66,Reference Li, Li and Wang67) . All of this information indicates that methionine may regulate milk lipid synthesis through the PI3K/Akt/FABP5/SREBP-1c signalling pathway. The other novel and crucial signalling pathway is the T1R1/T1R3 signalling pathway. Similar to isoleucine and valine, methionine also increases the influx of intracellular Ca2+ and regulates the effects of mTORC1 through T1R1/T1R3 in the mammary gland(Reference Zhou, Zhou and Peng53).

Fig. 2. Methionine and mammalian target of rapamycin complex 1 (mTORC1) signalling networks in the mammary gland. Note: sodium-coupled neutral amino acid transporter 1 (SNAT1) and SNAT2 originate from transporter system A and are crucial methionine transporters in the mammary gland. Intracellular methionine increases cellular S-adenosylmethionine (SAM) levels, which decreases the association of SAMTOR (SAM sensor) with GTPase-activating protein activity toward Rags 2 (GATOR2) and inhibits the mTORC1 signalling pathway. In addition, intracellular methionine regulates mTORC1 through the inositol 1,4,5-trisphosphate 3-kinase/protein kinase B/Rheb (PI3K/Akt/Rheb) signalling pathway. Extracellular methionine activates the G-protein-coupled receptors (GPCR) T1R1/T1R3, increases phospholipase Cβ (PLCβ) activity and further enhances the influx of intracellular Ca2+. Increased Ca2+ regulates the mTORC1 signalling pathway through extracellular signal-regulated kinase 1/2–tuberous sclerosis complex 1/2–Rheb (ERK1/2–TSC1/2–Rheb) signalling. Activated mTORC1 increases milk protein synthesis and regulates milk fat synthesis through sterol regulatory element-binding protein 1 (SREBP-1) and fatty acid-binding protein 5 (FABP5). ER, endoplasmic reticulum. Please refer to the main text for details.

PI3K/Akt was previously demonstrated to be regulated by hormones, but not amino acids, in cell models. One possible cause of this discrepancy is that an unknown link between methionine and PI3K exists in the mammary gland. As IGF-1/PI3K/Akt/mTOR is the canonical pathway in cells(Reference Latres, Amini and Amini68,Reference Stitt, Drujan and Clarke69) , the other possible cause is that dietary methionine deficiency might indirectly inhibit PI3K activation via decreased IGF-1 secretion(Reference Miller, Buehner and Chang70–Reference Stubbs, Wheelhouse and Lomax72).

Arginine and lysine

Transportation system of arginine and lysine in the mammary gland

Arginine and lysine are both cationic amino acids and have the same amino acid transporter systems in the mammary gland(Reference Broer73). Four cationic amino acid transporter (CAT) systems have been identified in the mammary gland as follows: (1) y+ system: CAT-1 (detected in humans, cows, pigs and rats) and CAT-2 (detected in pigs); (2) y+L system: y+LAT1 (detected in pigs and cows) and y+LAT2 (detected in pigs); (3) b0,+ system: b0,+AT (detected in pigs); and (4) B0,+ system: ATB0,+ (detected in pigs, humans, rats)(Reference Laspiur, Burton and Weber74–Reference Shennan, McNeillie and Jamieson77). Among all transporters, CAT-1 seems to play a central role in arginine uptake in the mammary gland. In mammary MCF-7 cells, when 50 % cellular CAT-1 was knocked down, arginine uptake was inhibited by 35–40 %(Reference Abdelmagid, Rickard and McDonald78,Reference Too, Abdelmagid, Patel, Preedy and Rajendram79) . Furthermore, blocking ATB0,+ also inhibits arginine uptake in the MCF-7 cell line(Reference Karunakaran, Ramachandran and Coothankandaswamy80). Recently, ATB0,+ has also been considered as the most crucial lysine transporter in the mammary gland. In bovine mammary gland epithelial cells, lysine activates the mTOR signalling pathway, which is inhibited by blockade of the lysine transporter ATB0,+(Reference Lin, Li and Zou81). In the sow mammary gland, the transportation of lysine is partly inhibited by excessive arginine supplementation(Reference Hurley, Wang and Bryson82). This evidence indicates that arginine and lysine have the same critical transporter system (CAT-1 and ATB0,+) in the mammary gland.

Potential signalling pathway of arginine in the mammary gland

Positive effects of arginine on placental growth and fetal survival and growth have been demonstrated in pigs, rats, mice and sheep(Reference Wu, Bazer and Satterfield83), whereas its functions in the mammary gland were not determined until recently (Table. 4). In the mammary gland, arginine catabolism produces proline, ornithine, urea, glutamate, glutamine, CO2 and polyamines (putrescine, spermidine and spermine)(Reference O’Quinn, Knabe and Wu84). In sows, supplementation with 0·5 or 1·0 % l-Arg-HCl activates the milk synthesis and increases the litter weight of sucking piglets(Reference Cui, Guo and Gao85). Similar to other amino acids, arginine also regulates milk protein synthesis through mTORC1(Reference Ma, Hu and Bannai86,Reference Wang, Xu and Wang87) . The central regulator linking arginine to the mTORC1 signalling pathway is cellular arginine sensor for mTORC1 (CASTOR1)(Reference Chantranupong, Scaria and Saxton88,Reference Saxton, Chantranupong and Knockenhauer89) . Sufficient arginine dissociates GATOR2 from CASTOR1 and further activates the mTOR signalling pathway(Reference Chantranupong, Scaria and Saxton88,Reference Saxton, Chantranupong and Knockenhauer89) (Fig. 3). The other crucial function of arginine in the mammary gland is primarily achieved through its metabolite NO(Reference Kim and Wu90). Briefly, NO increases the mammary blood vessel density and diameter, which might enhance the transportation of nutrients to the mammary gland and support milk synthesis(Reference Holanda, Marcolla and Guimarães91).

Table 4. Effects of arginine on mammary gland function and its potential signalling pathways

mTOR, mammalian target of rapamycin; GCN2, general control non-derepressible 2; eIF, eukaryotic initiation factor.

Fig. 3. Lysine and arginine regulate the mammalian target of rapamycin complex 1 (mTORC1) signalling network in the mammary gland. Note: cationic amino acid transporter-1 (CAT-1) and ATB0,+ are critical cationic amino acid transporters for arginine and lysine transportation in the mammary gland. The intracellular arginine regulator mTORC1 acts through the cellular arginine sensor for mTORC1–GTPase-activating protein activity toward Rags 2–GTPase-activating protein activity toward Rags 1–RagA/B (CASTOR1–GATOR2–GATOR1–RagA/B) signalling pathway, whereas extracellular lysine regulates mTORC1 through the G-protein-coupled receptor (GPCR) GPCR6A. As a Gαi/Gαq receptor, GPCR6A can activate milk protein synthesis through the GPRC6A–inositol 1,4,5-trisphosphate 3-kinase–protein kinase B–tuberous sclerosis complex 1/2–Rheb (GPRC6A–PI3K–Akt–TSC1/2–Rheb) and GPRC6A–extracellular signal-regulated kinase 1/2 (ERK1/2)–TSC1/2–Rheb pathways. Dashed lines represent potential signalling pathways that have not been verified in the mammary gland. FABP5, fatty acid-binding protein 5; SREBP-1, sterol regulatory element-binding protein 1. Please refer to the main text for details.

Potential signalling pathway of lysine in the mammary gland

Similar to methionine, lysine is one of the first two limiting amino acids both in dairy cows(Reference Rulquin, Pisulewski and Vérité59,92) and sows(Reference Dourmad, Etienne and Valancogne60,61) . In the bovine mammary gland, the use of lysine is dose dependent as it can be the first limiting amino acid. In case of low supply, lysine is mainly utilised in milk protein synthesis with the ratio of lysine uptake to lysine output close to 1(Reference Haque, Guinard-Flament and Lamberton8). However, when sufficient amount of lysine is provided through the diet, it can also be used to either synthesise non-essential amino acids(Reference Lapierre, Lobley and Doepel62,Reference Lapierre, Doepel and Milne93) or be oxidised into CO2 as BCAA(Reference Lin, Li and Zou81). When lysine is deficient, milk protein synthesis is inhibited(Reference Doelman, Kim and Carson94), with a decrease in mTORC1 activity in dairy cows(Reference Dong, Zhou and Saremi95). However, the function of lysine in the mammary gland has largely not been determined. Recent advances have found that lysine increases milk fat synthesis through the GPRC6A/PI3K/FABP5 signalling pathway(Reference Li, Li and Wang67). GPRC6A is a G protein-coupled receptor that is specific for cationic amino acid sensing(Reference Clemmensen, Smajilovic and Wellendorph96). As a Gαi/Gαq receptor, GPRC6A has the potential to regulate cellular cAMP levels and activate the MAPK signalling pathway(Reference Husted, Trauelsen and Rudenko97). Thus, both GPRC6A/PI3K/Akt/mTOR and GPRC6A/ERK/mTOR can be the potential signalling pathways for lysine to regulate milk protein and fat synthesis in the mammary gland.

Conclusion

Amino acids play crucial roles in the synthesis of milk protein and fat in the mammary gland. The dominant amino acid transposers (BCAA, methionine, lysine and arginine) of the mammary gland are summarised in the present review. In addition, our review has focused on a number of canonical and novel signalling molecules involved in amino acid signalling pathway in the mammary gland. Remarkably, mTORC1 acts as the central node of the amino acid-regulated signalling pathway and can be activated intracellularly and extracellularly (through a G-protein-coupled receptor (GPCR)). Currently, the amino acid signalling pathway in the mammary gland still warrants further investigation. Achieving a better understanding of the amino acid signalling pathway might help us to optimise the amino acid profiles in maternal diets for human beings and other mammals in the future.

Acknowledgements

The present review was supported by the National Natural Science Foundation of the Peopleʼs Republic of China (no. 31802067 and 31872364) and the Natural Science Foundation of Guangdong Province (no. 2018A030310201).

S. Z. initiated the idea, the scope, and the outline of this review paper. Z. W., J. H., M. T., H. S. and F. C. studied and analysed all of the publications cited in this paper and were involved in manuscript preparation. W. G. conducted the final editing and proofreading. All authors read and approved the final manuscript.

The authors declare that they have no competing interests.

References

Pattison, KL, Kraschnewski, JL, Lehman, E, et al. (2019) Breastfeeding initiation and duration and child health outcomes in the first baby study. Prev Med 118, 1–6.CrossRefGoogle ScholarPubMed
Theil, PK, Lauridsen, C & Quesnel, H (2014) Neonatal piglet survival: impact of sow nutrition around parturition on fetal glycogen deposition and production and composition of colostrum and transient milk. Animal 8, 1021–1030.CrossRefGoogle ScholarPubMed
Rauprich, A, Hammon, H & Blum, J (2000) Influence of feeding different amounts of first colostrum on metabolic, endocrine, and health status and on growth performance in neonatal calves. J Anim Sci 78, 896–908.CrossRefGoogle ScholarPubMed
Wu, G (2009) Amino acids: metabolism, functions, and nutrition. Amino Acids 37, 1–17.CrossRefGoogle ScholarPubMed
Zhang, S, Zeng, X, Ren, M, et al. (2017) Novel metabolic and physiological functions of branched chain amino acids: a review. J Anim Sci Biotechnol 8, 10.CrossRefGoogle ScholarPubMed
Omphalius, C, Lapierre, H, Guinard-Flament, J, et al. (2019) Amino acid efficiencies of utilization vary by different mechanisms in response to energy and protein supplies in dairy cows: study at mammary-gland and whole-body levels. J Dairy Sci 102, 9883–9901.CrossRefGoogle ScholarPubMed
Raggio, G, Lemosquet, S, Lobley, G, et al. (2006) Effect of casein and propionate supply on mammary protein metabolism in lactating dairy cows. J Dairy Sci 89, 4340–4351.CrossRefGoogle ScholarPubMed
Haque, M, Guinard-Flament, J, Lamberton, P, et al. (2015) Changes in mammary metabolism in response to the provision of an ideal amino acid profile at 2 levels of metabolizable protein supply in dairy cows: consequences on efficiency. J Dairy Sci 98, 3951–3968.CrossRefGoogle ScholarPubMed
Safayi, S & Nielsen, MO (2013) Intravenous supplementation of acetate, glucose or essential amino acids to an energy and protein deficient diet in lactating dairy goats: effects on milk production and mammary nutrient extraction. Small Ruminant Res 112, 162–173.CrossRefGoogle Scholar
Trottier, N, Shipley, C & Easter, R (1997) Plasma amino acid uptake by the mammary gland of the lactating sow. J Anim Sci 75, 1266–1278.CrossRefGoogle ScholarPubMed
Davis, S, Bickerstaffe, R & Hart, D (1978) Amino acid uptake by the mammary gland of the lactating ewe. Aust J Biol Sci 31, 123–132.CrossRefGoogle ScholarPubMed
Mepham, T & Linzell, J (1966) A quantitative assessment of the contribution of individual plasma amino acids to the synthesis of milk proteins by the goat mammary gland. Biochem J 101, 76–83.CrossRefGoogle ScholarPubMed
Mepham, T (1982) Amino acid utilization by lactating mammary gland. J Dairy Sci 65, 287–298.CrossRefGoogle ScholarPubMed
Hennighausen, L & Robinson, GW (2001) Signaling pathways in mammary gland development. Dev Cell 1, 467–475.CrossRefGoogle ScholarPubMed
Jackson, S, Bryson, J, Wang, H, et al. (2000) Cellular uptake of valine by lactating porcine mammary tissue. J Anim Sci 78, 2927–2932.CrossRefGoogle ScholarPubMed
Luo, X, Coon, JS, Su, E, et al. (2010) LAT1 regulates growth of uterine leiomyoma smooth muscle cells. Reprod Sci 17, 791–797.Google Scholar
Fan, X, Ross, DD, Arakawa, H, et al. (2010) Impact of system l amino acid transporter 1 (LAT1) on proliferation of human ovarian cancer cells: a possible target for combination therapy with anti-proliferative aminopeptidase inhibitors. Biochem Pharmacol 80, 811–818.CrossRefGoogle ScholarPubMed
Kurayama, R, Ito, N, Nishibori, Y, et al. (2011) Role of amino acid transporter LAT2 in the activation of mTORC1 pathway and the pathogenesis of crescentic glomerulonephritis. Lab Invest 91, 992–1006.CrossRefGoogle ScholarPubMed
Matsumoto, T, Nakamura, E, Nakamura, H, et al. (2013) The production of free glutamate in milk requires the leucine transporter LAT1. Am J Physiol Cell Physiol 305, C623–C631.CrossRefGoogle ScholarPubMed
Lin, Y, Duan, X, Lv, H, et al. (2018) The effects of l-type amino acid transporter 1 on milk protein synthesis in mammary glands of dairy cows. J Dairy Sci 101, 1687–1696.CrossRefGoogle ScholarPubMed
Chen, F, Zhang, S, Deng, Z, et al. (2018) Regulation of amino acid transporters in the mammary gland from late pregnancy to peak lactation in the sow. J Anim Sci Biotechnol 9, 35.CrossRefGoogle ScholarPubMed
Duan, X, Lin, Y, Lv, H, et al. (2017) Methionine induces LAT1 expression in dairy cow mammary gland by activating the mTORC1 signaling pathway. DNA Cell Biol 36, 1126–1133.CrossRefGoogle ScholarPubMed
Li, P, Knabe, DA, Kim, SW, et al. (2006) Lactating porcine mammary tissue catabolizes branched-chain amino acids for glutamine and aspartate synthesis. J Nutr 139, 1502–1509.CrossRefGoogle Scholar
Wohlt, J, Clark, J, Derrig, R, et al. (1977) Valine, leucine, and isoleucine metabolism by lactating bovine mammary tissue. J Dairy Sci 60, 1875–1882.CrossRefGoogle ScholarPubMed
Saxton, RA, Knockenhauer, KE, Wolfson, RL, et al. (2016) Structural basis for leucine sensing by the Sestrin2–mTORC1 pathway. Science 351, 53–58.CrossRefGoogle ScholarPubMed
Kimball, SR, Gordon, BS, Moyer, JE, et al. (2016) Leucine induced dephosphorylation of Sestrin2 promotes mTORC1 activation. Cell Signal 28, 896–906.CrossRefGoogle ScholarPubMed
Wolfson, RL, Chantranupong, L, Saxton, RA, et al. (2016) Sestrin2 is a leucine sensor for the mTORC1 pathway. Science 351, 43–48.CrossRefGoogle ScholarPubMed
Luo, C, Zheng, N, Zhao, S, et al. (2019) Sestrin2 negatively regulates casein synthesis through the SH3BP4–mTORC1 pathway in response to AA depletion or supplementation in cow mammary epithelial cells. J Agric Food Chem 67, 4849–4859.CrossRefGoogle ScholarPubMed
Kim, Y-M, Stone, M, Hwang, TH, et al. (2012) SH3BP4 is a negative regulator of amino acid–Rag GTPase–mTORC1 signaling. Mol Cell 46, 833–846.CrossRefGoogle ScholarPubMed
Luo, C, Zhao, S, Zhang, M, et al. (2018) SESN2 negatively regulates cell proliferation and casein synthesis by inhibition the amino acid-mediated mTORC1 pathway in cow mammary epithelial cells. Sci Rep 8, 3912.CrossRefGoogle ScholarPubMed
Luo, C, Zhao, S, Dai, W, et al. (2018) Proteomic analyses reveal GNG12 regulates cell growth and casein synthesis by activating the Leu-mediated mTORC1 signaling pathway. Biochim Biophys Acta Proteins Proteom 1866, 1092–1101.CrossRefGoogle ScholarPubMed
Sancak, Y, Bar-Peled, L, Zoncu, R, et al. (2010) Ragulator–Rag complex targets mTORC1 to the lysosomal surface and is necessary for its activation by amino acids. Cell 141, 290–303.CrossRefGoogle ScholarPubMed
Han, JM, Jeong, SJ, Park, MC, et al. (2012) Leucyl-tRNA synthetase is an intracellular leucine sensor for the mTORC1-signaling pathway. Cell 149, 410–424.CrossRefGoogle ScholarPubMed
McGuckin, M, Manjarin, R, Peterson, D (2016) Leucine supplementation increases mouse mammary cell proliferation in vitro. J Anim Sci 94, 98–98.CrossRefGoogle Scholar
Richert, B, Goodband, R, Tokach, M, et al. (1997) Increasing valine, isoleucine, and total branched-chain amino acids for lactating sows. J Anim Sci 75, 2117–2128.CrossRefGoogle ScholarPubMed
Richert, B, Tokach, M, Goodband, R, et al. (1996) Valine requirement of the high-producing lactating sow. J Anim Sci 74, 1307–1313.CrossRefGoogle ScholarPubMed
Haque, M, Rulquin, H & Lemosquet, S (2013) Milk protein responses in dairy cows to changes in postruminal supplies of arginine, isoleucine, and valine. J Dairy Sci 96, 420–430.CrossRefGoogle ScholarPubMed
Appuhamy, JRN, Knoebel, NA, Nayananjalie, WD, et al. (2012) Isoleucine and leucine independently regulate mTOR signaling and protein synthesis in MAC-T cells and bovine mammary tissue slices. J Nutr 142, 484–491.CrossRefGoogle ScholarPubMed
Liu, G, Hanigan, M, Lin, X, et al. (2017) Methionine, leucine, isoleucine, or threonine effects on mammary cell signaling and pup growth in lactating mice. J Dairy Sci 100, 4038–4050.CrossRefGoogle ScholarPubMed
Che, L, Xu, M, Gao, K, et al. (2019) Valine increases milk fat synthesis in mammary gland of gilts through stimulating AKT/MTOR/SREBP1 pathway. Biol Reprod 101, 126–137.CrossRefGoogle Scholar
Carcangiu, V, Mura, MC, Daga, C, et al. (2013) Association between SREBP-1 gene expression in mammary gland and milk fat yield in Sarda breed sheep. Meta Gene 1, 43–49.CrossRefGoogle ScholarPubMed
Ma, L & Corl, B (2012) Transcriptional regulation of lipid synthesis in bovine mammary epithelial cells by sterol regulatory element binding protein-1. J Dairy Sci 95, 3743–3755.CrossRefGoogle ScholarPubMed
Rudolph, MC, McManaman, JL, Phang, T, et al. (2007) Metabolic regulation in the lactating mammary gland: a lipid synthesizing machine. Physiol Genomics 28, 323–336.CrossRefGoogle ScholarPubMed
Peterson, TR, Sengupta, SS, Harris, TE, et al. (2011) mTOR complex 1 regulates lipin 1 localization to control the SREBP pathway. Cell 146, 408–420.CrossRefGoogle ScholarPubMed
Roets, E, Massart-Leën, A-M, Peeters, G, et al. (1983) Metabolism of leucine by the isolated perfused goat udder. J Dairy Res 50, 413–424.CrossRefGoogle ScholarPubMed
Nelson, G, Chandrashekar, J, Hoon, MA, et al. (2002) An amino-acid taste receptor. Nature 416, 199–202.CrossRefGoogle Scholar
Wang, Y, Liu, J, Wu, H, et al. (2017) Amino acids regulate mTOR pathway and milk protein synthesis in a mouse mammary epithelial cell line is partly mediated by T1R1/T1R3. Eur J Nutr 56, 2467–2474.CrossRefGoogle Scholar
Liu, J, Wang, Y, Li, D, et al. (2017) Milk protein synthesis is regulated by T1R1/T1R3, a G protein-coupled taste receptor, through the mTOR pathway in the mouse mammary gland. Mol Nutr Food Res 61, 1601017.CrossRefGoogle Scholar
Wauson, EM, Zaganjor, E, Lee, A-Y, et al. (2012) The G protein-coupled taste receptor T1R1/T1R3 regulates mTORC1 and autophagy. Mol Cell 47, 851–862.CrossRefGoogle Scholar
Wauson, EM, Zaganjor, E & Cobb, MH (2013) Amino acid regulation of autophagy through the GPCR TAS1R1-TAS1R3. Autophagy 9, 418–419.CrossRefGoogle ScholarPubMed
Carriere, A, Romeo, Y, Acosta-Jaquez, HA, et al. (2011) ERK1/2 phosphorylate Raptor to promote Ras-dependent activation of mTOR complex 1 (mTORC1). J Biol Chem 286, 567–577.CrossRefGoogle Scholar
Rolfe, M, McLeod, LE, Pratt, PF, et al. (2005) Activation of protein synthesis in cardiomyocytes by the hypertrophic agent phenylephrine requires the activation of ERK and involves phosphorylation of tuberous sclerosis complex 2 (TSC2). Biochem J 388, 973–984.CrossRefGoogle Scholar
Zhou, Y, Zhou, Z, Peng, J, et al. (2018) Methionine and valine activate the mammalian target of rapamycin complex 1 pathway through heterodimeric amino acid taste receptor (TAS1R1/TAS1R3) and intracellular Ca2+ in bovine mammary epithelial cells. J Dairy Sci 101, 11354–11363.CrossRefGoogle ScholarPubMed
Verma, N & Kansal, VK (1993) Characterisation of the routes of methionine transport in mouse mammary glands. Indian J Med Res 98, 297–304.Google ScholarPubMed
Shennan, D & Boyd, C (2014) The functional and molecular entities underlying amino acid and peptide transport by the mammary gland under different physiological and pathological conditions. J Mammary Gland Biol Neoplasia 19, 19–33.CrossRefGoogle ScholarPubMed
Chillaron, J, Roca, R, Valencia, A, et al. (2001) Heteromeric amino acid transporters: biochemistry, genetics, and physiology. Am J Physiol Renal Physiol 281, F995–F1018.CrossRefGoogle Scholar
Qi, H, Meng, C, Jin, X, et al. (2018) Methionine promotes milk protein and fat synthesis and cell proliferation via the SNAT2–PI3K signaling pathway in bovine mammary epithelial cells. J Agric Food Chem 66, 11027–11033.CrossRefGoogle ScholarPubMed
Schwab, CG, Satter, L & Clay, A (1976) Response of lactating dairy cows to abomasal infusion of amino acids. J Dairy Sci 59, 1254–1270.CrossRefGoogle ScholarPubMed
Rulquin, H, Pisulewski, P, Vérité, R, et al. (1993) Milk production and composition as a function of postruminal lysine and methionine supply: a nutrient-response approach. Livest Prod Sci 37, 69–90.CrossRefGoogle Scholar
Dourmad, J-Y, Etienne, M, Valancogne, A, et al. (2008) InraPorc: a model and decision support tool for the nutrition of sows. Anim Feed Sci Technol 143, 372–386.CrossRefGoogle Scholar
National Research Council (1998) Nutrient Requirements of Swine, 10th ed. Washington, DC: National Academies Press.Google Scholar
Lapierre, H, Lobley, GE, Doepel, L, et al. (2012) Triennial Lactation Symposium: Mammary metabolism of amino acids in dairy cows. J Anim Sci 90, 1708–1721.CrossRefGoogle ScholarPubMed
Gu, X, Orozco, JM, Saxton, RA, et al. (2017) SAMTOR is an S-adenosylmethionine sensor for the mTORC1 pathway. Science 358, 813–818.CrossRefGoogle ScholarPubMed
Ma, Y, Batistel, F, Xu, T, et al. (2019) Phosphorylation of AKT serine/threonine kinase and abundance of milk protein synthesis gene networks in mammary tissue in response to supply of methionine in periparturient Holstein cows. J Dairy Sci 102, 4264–4274.CrossRefGoogle ScholarPubMed
Li, P, Yu, M, Zhou, C, et al. (2019) FABP5 is a critical regulator of methionine- and estrogen-induced SREBP-1c gene expression in bovine mammary epithelial cells. J Cell Physiol 234, 537–549.CrossRefGoogle Scholar
Lv, Q, Wang, G, Zhang, Y, et al. (2019) FABP5 regulates the proliferation of clear cell renal cell carcinoma cells via the PI3K/AKT signaling pathway. Int J Oncol 54, 1221–1232.Google ScholarPubMed
Li, X, Li, P, Wang, L, et al. (2019) Lysine enhances the stimulation of fatty acids on milk fat synthesis via the GPRC6A–PI3K–FABP5 signaling in bovine mammary epithelial cells. J Agric Food Chem 67, 7005–7015.CrossRefGoogle ScholarPubMed
Latres, E, Amini, AR, Amini, AA, et al. (2005) Insulin-like growth factor-1 (IGF-1) inversely regulates atrophy-induced genes via the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin (PI3K/Akt/mTOR) pathway. J Biol Chem 280, 2737–2744.CrossRefGoogle ScholarPubMed
Stitt, TN, Drujan, D, Clarke, BA, et al. (2004) The IGF-1/PI3K/Akt pathway prevents expression of muscle atrophy-induced ubiquitin ligases by inhibiting FOXO transcription factors. Mol Cell 14, 395–403.CrossRefGoogle ScholarPubMed
Miller, RA, Buehner, G, Chang, Y, et al. (2005) Methionine-deficient diet extends mouse lifespan, slows immune and lens aging, alters glucose, T4, IGF-I and insulin levels, and increases hepatocyte MIF levels and stress resistance. Aging Cell 4, 119–125.CrossRefGoogle ScholarPubMed
Carew, L, McMurtry, J & Alster, F (2003) Effects of methionine deficiencies on plasma levels of thyroid hormones, insulin-like growth factors-I and-II, liver and body weights, and feed intake in growing chickens. Poult Sci 82, 1932–1938.CrossRefGoogle ScholarPubMed
Stubbs, A, Wheelhouse, N, Lomax, M, et al. (2002) Nutrient–hormone interaction in the ovine liver: methionine supply selectively modulates growth hormone-induced IGF-I gene expression. J Endocrinol 174, 335–341.CrossRefGoogle ScholarPubMed
Broer, S (2008) Amino acid transport across mammalian intestinal and renal epithelia. Physiol Rev 88, 249–286.CrossRefGoogle ScholarPubMed
Laspiur, JP, Burton, J, Weber, P, et al. (2004) Amino acid transporters in porcine mammary gland during lactation. J Dairy Sci 87, 3235–3237.CrossRefGoogle ScholarPubMed
Manjarin, R, Steibel, J, Zamora, V, et al. (2011) Transcript abundance of amino acid transporters, β-casein, and α-lactalbumin in mammary tissue of periparturient, lactating, and postweaned sows. J Dairy Sci 94, 3467–3476.CrossRefGoogle ScholarPubMed
Calvert, D & Shennan, D (1996) Evidence for an interaction between cationic and neutral amino acids at the blood-facing aspect of the lactating rat mammary epithelium. J Dairy Res 63, 25–33.CrossRefGoogle ScholarPubMed
Shennan, D, McNeillie, S, Jamieson, E, et al. (1994) Lysine transport in lactating rat mammary tissue: evidence for an interaction between cationic and neutral amino acids. Acta Physiol Scand 151, 461–466.CrossRefGoogle ScholarPubMed
Abdelmagid, SA, Rickard, JA, McDonald, WJ, et al. (2011) CAT-1-mediated arginine uptake and regulation of nitric oxide synthases for the survival of human breast cancer cell lines. J Cell Biochem 112, 1084–1092.CrossRefGoogle ScholarPubMed
Too, CK & Abdelmagid, SA (2017) l-Arginine uptake and its role in the survival of breast cancer cells. In l-Arginine in Clinical Nutrition, pp. 253–268 [Patel, VB, Preedy, VR and Rajendram, R, editors]. Cham: Springer.CrossRefGoogle Scholar
Karunakaran, S, Ramachandran, S, Coothankandaswamy, V, et al. (2011) SLC6A14 (ATB0,+) protein, a highly concentrative and broad specific amino acid transporter, is a novel and effective drug target for treatment of estrogen receptor-positive breast cancer. J Biol Chem 286, 31830–31838.CrossRefGoogle Scholar
Lin, X, Li, S, Zou, Y, et al. (2018) Lysine stimulates protein synthesis by promoting the expression of ATB0,+ and activating the mTOR pathway in bovine mammary epithelial cells. J Nutr 148, 1426–1433.CrossRefGoogle ScholarPubMed
Hurley, W, Wang, H, Bryson, J, et al. (2000) Lysine uptake by mammary gland tissue from lactating sows. J Anim Sci 78, 391–395.CrossRefGoogle ScholarPubMed
Wu, G, Bazer, FW, Satterfield, MC, et al. (2013) Impacts of arginine nutrition on embryonic and fetal development in mammals. Amino Acids 45, 241–256.CrossRefGoogle ScholarPubMed
O’Quinn, P, Knabe, D & Wu, G (2002) Arginine catabolism in lactating porcine mammary tissue. J Anim Sci 80, 467–474.CrossRefGoogle ScholarPubMed
Cui, Z, Guo, C-Y, Gao, K-G, et al. (2017) Dietary arginine supplementation in multiparous sows during lactation improves the weight gain of suckling piglets. J Integr Agr 16, 648–655.Google Scholar
Ma, Q, Hu, S, Bannai, M, et al. (2018) l-Arginine regulates protein turnover in porcine mammary epithelial cells to enhance milk protein synthesis. Amino Acids 50, 621–628.CrossRefGoogle ScholarPubMed
Wang, M, Xu, B, Wang, H, et al. (2014) Effects of arginine concentration on the in vitro expression of casein and mTOR pathway related genes in mammary epithelial cells from dairy cattle. PLOS ONE 9, e95985.CrossRefGoogle ScholarPubMed
Chantranupong, L, Scaria, SM, Saxton, RA, et al. (2016) The CASTOR proteins are arginine sensors for the mTORC1 pathway. Cell 165, 153–164.CrossRefGoogle ScholarPubMed
Saxton, RA, Chantranupong, L, Knockenhauer, KE, et al. (2016) Mechanism of arginine sensing by CASTOR1 upstream of mTORC1. Nature 536, 229–233.CrossRefGoogle ScholarPubMed
Kim, SW & Wu, G (2009) Regulatory role for amino acids in mammary gland growth and milk synthesis. Amino Acids 37, 89–95.CrossRefGoogle ScholarPubMed
Holanda, D, Marcolla, C, Guimarães, S, et al. (2019) Dietary l-arginine supplementation increased mammary gland vascularity of lactating sows. Animal 13, 790–798.CrossRefGoogle ScholarPubMed
National Research Council (2001) Nutrient Requirements of Dairy Cattle, 7th revised ed. Washington, DC: The National Academies Press.Google Scholar
Lapierre, H, Doepel, L, Milne, E, et al. (2009) Responses in mammary and splanchnic metabolism to altered lysine supply in dairy cows. Animal 3, 360–371.CrossRefGoogle ScholarPubMed
Doelman, J, Kim, JJ, Carson, M, et al. (2015) Branched-chain amino acid and lysine deficiencies exert different effects on mammary translational regulation. J Dairy Sci 98, 7846–7855.CrossRefGoogle ScholarPubMed
Dong, X, Zhou, Z, Saremi, B, et al. (2018) 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. J Dairy Sci 101, 1708–1718.CrossRefGoogle ScholarPubMed
Clemmensen, C, Smajilovic, S, Wellendorph, P, et al. (2014) The GPCR, class C, group 6, subtype A (GPRC6A) receptor: from cloning to physiological function. Br J Pharmacol 171, 1129–1141.CrossRefGoogle Scholar
Husted, AS, Trauelsen, M, Rudenko, O, et al. (2017) GPCR-mediated signaling of metabolites. Cell Metab 25, 777–796.CrossRefGoogle ScholarPubMed
Gao, H-N, Hu, H, Zheng, N, et al. (2015) Leucine and histidine independently regulate milk protein synthesis in bovine mammary epithelial cells via mTOR signaling pathway. J Zhejiang Univ Sci B 16, 560–572.CrossRefGoogle ScholarPubMed
Zhao, Y, Yan, S, Chen, L, et al. (2019) Effect of interaction between leucine and acetate on the milk protein synthesis in bovine mammary epithelial cells. Anim Sci J 90, 81–89.CrossRefGoogle ScholarPubMed
Tian, W, Wu, T, Zhao, R, et al. (2017) Responses of milk production of dairy cows to jugular infusions of a mixture of essential amino acids with or without exclusion leucine or arginine. Anim Nutr 3, 271–275.CrossRefGoogle ScholarPubMed
Zhang, J, He, W, Yi, D, et al. (2019) Regulation of protein synthesis in porcine mammary epithelial cells by l-valine. Amino Acids 51, 717–726.CrossRefGoogle ScholarPubMed
Han, L, Batistel, F, Ma, Y, et al. (2018) Methionine supply alters mammary gland antioxidant gene networks via phosphorylation of nuclear factor erythroid 2-like 2 (NFE2L2) protein in dairy cows during the periparturient period. J Dairy Sci 101, 8505–8512.CrossRefGoogle ScholarPubMed
Lu, L, Gao, X, Li, Q, et al. (2012) Comparative phosphoproteomics analysis of the effects of l-methionine on dairy cow mammary epithelial cells. Can J Anim Sci 92, 433–442.CrossRefGoogle Scholar
Zhang, Y, Wang, P, Lin, S, et al. (2018) mTORC1 signaling-associated protein synthesis in porcine mammary glands was regulated by the local available methionine depending on methionine sources. Amino Acids 50, 105–115.CrossRefGoogle ScholarPubMed
Rosa, F & Osorio, J (2018) In vitro histone manipulation of bovine mammary epithelial cells through methionine supplementation. Dairy Science Publication Database, 1977. https://openprairie.sdstate.edu/dairy_pubdb/1977 (accessed March 2020).Google Scholar
Salama, A, Duque, M, Wang, L, et al. (2019) 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. J Dairy Sci 102, 2469–2480.CrossRefGoogle ScholarPubMed
Ding, L, Shen, Y, Wang, Y, et al. (2019) Jugular arginine supplementation increases lactation performance and nitrogen utilization efficiency in lactating dairy cows. J Anim Sci Biotechnol 10, 3.CrossRefGoogle ScholarPubMed
Zhao, F, Wu, T, Wang, H, et al. (2018) Jugular arginine infusion relieves lipopolysaccharide-triggered inflammatory stress and improves immunity status of lactating dairy cows. J Dairy Sci 101, 5961–5970.CrossRefGoogle ScholarPubMed
Wu, T, Wang, C, Ding, L, et al. (2016) Arginine relieves the inflammatory response and enhances the casein expression in bovine mammary epithelial cells induced by lipopolysaccharide. Mediators Inflamm 2016, 9618795.CrossRefGoogle ScholarPubMed
Xia, X, Che, Y, Gao, Y, et al. (2016) Arginine supplementation recovered the IFN-γ-mediated decrease in milk protein and fat synthesis by inhibiting the GCN2/eIF2α pathway, which induces autophagy in primary bovine mammary epithelial cells. Mol Cells 39, 410–417.Google ScholarPubMed
Xia, X, Gao, Y, Zhang, J, et al. (2016) Autophagy mediated by arginine depletion activation of the nutrient sensor GCN2 contributes to interferon-γ-induced malignant transformation of primary bovine mammary epithelial cells. Cell Death Discov 2, 15065.CrossRefGoogle ScholarPubMed
Chen, L, Li, Z, Wang, M, et al. (2013) Preliminary report of arginine on synthesis and gene expression of casein in bovine mammary epithelial cell. Int Res J Agric Sci Soil Sci 3, 17–23.Google Scholar
Figure 0

Table 1. Mammary amino acid uptake:output ratios in different mammals

Figure 1

Table 2. Effects of branched-chain amino acids (BCAA) on mammary gland function and its potential signalling pathways

Figure 2

Fig. 1. Branched-chain amino acid (BCAA) and mammalian target of rapamycin complex 1 (mTORC1) signalling networks in the mammary gland. Note: l-type amino acid transporter 1/4F2 heavy chain (LAT1/4F2hc) and l-type amino acid transporter 2 (LAT2)/4F2hc derived from transporter system L are highly expressed and play a dominant role in BCAA transportation in the mammary gland. All three BCAA activate mTORC1 pathways in mammary glands. Leucine induces dephosphorylation of Sestrin2 and further promotes mTORC1 activation through GTPase-activating protein activity toward Rags (GATOR) 2, GATOR1 and RagA/B. In addition, GCG12, SH3-domain binding protein 4 (SH3BP4) and leucyl-tRNA synthetase (LeuRS) are crucial regulators in leucine-related mTORC1 activation. Extracellular valine activates G-protein-coupled receptors (GPRC) T1R1/T1R3, increases phospholipase Cβ (PLCβ) activity and further enhances an influx of intracellular Ca2+. Increased Ca2+ regulates the mTORC1 signalling pathway through extracellular signal-regulated kinase 1/2–tuberous sclerosis complex 1/2–Rheb (ERK1/2–TSC1/2–Rheb) signalling. Intracellular isoleucine activates mTORC1 through an unknown mechanism. In the mammary gland, activated mTORC1 not only increases milk protein synthesis but also milk fat synthesis through lipin 1 (Lpn1)–sterol regulatory element-binding protein 1c (SREBP-1c) pathways. ER, endoplasmic reticulum. Please refer to the main text for details.

Figure 3

Table 3. Effects of methionine on mammary gland function and its potential signalling pathways

Figure 4

Fig. 2. Methionine and mammalian target of rapamycin complex 1 (mTORC1) signalling networks in the mammary gland. Note: sodium-coupled neutral amino acid transporter 1 (SNAT1) and SNAT2 originate from transporter system A and are crucial methionine transporters in the mammary gland. Intracellular methionine increases cellular S-adenosylmethionine (SAM) levels, which decreases the association of SAMTOR (SAM sensor) with GTPase-activating protein activity toward Rags 2 (GATOR2) and inhibits the mTORC1 signalling pathway. In addition, intracellular methionine regulates mTORC1 through the inositol 1,4,5-trisphosphate 3-kinase/protein kinase B/Rheb (PI3K/Akt/Rheb) signalling pathway. Extracellular methionine activates the G-protein-coupled receptors (GPCR) T1R1/T1R3, increases phospholipase Cβ (PLCβ) activity and further enhances the influx of intracellular Ca2+. Increased Ca2+ regulates the mTORC1 signalling pathway through extracellular signal-regulated kinase 1/2–tuberous sclerosis complex 1/2–Rheb (ERK1/2–TSC1/2–Rheb) signalling. Activated mTORC1 increases milk protein synthesis and regulates milk fat synthesis through sterol regulatory element-binding protein 1 (SREBP-1) and fatty acid-binding protein 5 (FABP5). ER, endoplasmic reticulum. Please refer to the main text for details.

Figure 5

Table 4. Effects of arginine on mammary gland function and its potential signalling pathways

Figure 6

Fig. 3. Lysine and arginine regulate the mammalian target of rapamycin complex 1 (mTORC1) signalling network in the mammary gland. Note: cationic amino acid transporter-1 (CAT-1) and ATB0,+ are critical cationic amino acid transporters for arginine and lysine transportation in the mammary gland. The intracellular arginine regulator mTORC1 acts through the cellular arginine sensor for mTORC1–GTPase-activating protein activity toward Rags 2–GTPase-activating protein activity toward Rags 1–RagA/B (CASTOR1–GATOR2–GATOR1–RagA/B) signalling pathway, whereas extracellular lysine regulates mTORC1 through the G-protein-coupled receptor (GPCR) GPCR6A. As a Gαi/Gαq receptor, GPCR6A can activate milk protein synthesis through the GPRC6A–inositol 1,4,5-trisphosphate 3-kinase–protein kinase B–tuberous sclerosis complex 1/2–Rheb (GPRC6A–PI3K–Akt–TSC1/2–Rheb) and GPRC6A–extracellular signal-regulated kinase 1/2 (ERK1/2)–TSC1/2–Rheb pathways. Dashed lines represent potential signalling pathways that have not been verified in the mammary gland. FABP5, fatty acid-binding protein 5; SREBP-1, sterol regulatory element-binding protein 1. Please refer to the main text for details.