Introductory Chapter: Milk Protein Synthesis, Progress, and Projections

1. Introduction

Milk protein is one of the most important nutrients to the neonate drives their nutritional and immunological properties. It contains a variety of essential amino acids required by the body to maintain a variety of potential biological functions. Milk proteins fall into the category in which they are synthesized by the cells of the mammary gland. Milk protein is synthesized by precursors from the diet and the availability of amino acids (AA) passes from the blood to the lumen of the mammary alveoli. It is necessary to consider the basis for the two routes of the passage of specific proteins across the mammary epithelial cell [1, 2]. The first is the paracellular route that proteins pass from the extracellular fluid (ECF) to the lumen of the alveoli between adjacent secretory cells. The second is the transcellular route, which is proteins across the apical secretory cell membrane. The sequence of a series of steps viz. (i) the passage of amino acids from the blood capillary to the extracellular fluid (ECF), (ii) passage from the ECF to the secretory cell, (iii) the intracellular synthesis of milk specific proteins, (iv) intracellular translocation of the protein to the apical membrane of the mammary cell, (v) the protein across the apical membrane into the alveolar lumen. Therefore, the process necessitates the factors controlling each step, which are involved in a process of controlled polymerization within the mammary cell. The current knowledge of the mechanisms involved in these separate steps.

The knowledge in the area of milk protein continues to be one of rapid progress in the subject of lactation. Several papers review the finding involving milk protein in detail in this area. Since, the process of milk protein synthesis requires the accumulation of information from the testing of hypotheses in a wide range of species. New research approaches have provided a further understanding of the control mechanism of milk protein synthesis at both cellular and molecular levels. The process of the mammary gland and milk protein secretion impinge directly on many diverse topics such as sociology, medicine involving breast and food allergens feeding, agriculture involving the dairy and pig industry, and economic, etc.

2. Milk protein and area of research

Since the synthesis of milk protein during lactation is a complex biological activity. The mechanisms of milk protein synthesis are concerned with the mechanisms of other major milk constituents with lactose and milk fat which are widespread and diverse. Multifactorial have involved these lines of approach for the research area in both cellular or molecular studies of milk protein synthesis still being investigated by scientists. The mammary gland is the target organ of a complex system to the major pathway that a supply of substrates for the synthesis of milk components and the initiation and maintenance of all of the mechanisms of lactation in the secretory cells. The pathways for protein, lactose, and fat synthesis in the mammary cell require energy in the form of ATP used for the common relationship of the system. Many of the cellular subcomponents play a direct role in the synthesis of certain milk constituents and may be involved with more than one. The relationship of the systems to the major pathway in the mammary cell involving a complex interrelationship has been proposed [3].

Milk proteins are classified into caseins and non-casein (whey) proteins. Caseins also are milk-specific proteins. There are four basic types of molecules: the αs₁, αs₂, β, and κ in bovine milk which each type comprises many molecular species [4]. Κ-casein is a glycoprotein that stabilizes the micelle against precipitation by calcium ions. Non-casein (whey) proteins comprise both milk-specific and serum proteins which in some species (e.g. rodents) contain few specific proteins while carnivores contain many. Although the basic mechanism of milk protein synthesis is identical in all species. Variations in the activity of which would be expressed in different milk protein compositions. The milk-specific protein like serum protein occurs in milk, especially immunoglobulins being high in colostrum in many species (e.g. cows, sheep, and pigs) in which the structure of the placental membranes prevents their transfer from the maternal to the fetal plasma. The question then arises, whether this protein in milk comes from blood plasma or by the synthesis in plasma cells situated in the mammary gland via either transcellular route or paracellular route. The milk-specific non-casein proteins are α-lactalbumin and β-lactoglobulin. The role of α-lactalbumin is known to involve in lactose biosynthesis and increase milk yield. This example indicates that complex mechanisms of metabolic pathways occur in the mammary gland. The elucidation of these and many other metabolic differences among species in the synthesis of milk is an area with an important related application for some that should receive continued and increasing attention.

An increase in the synthesis of protein, lactose, and fat in milk is a complex mechanism that improves the efficiency of milk production. The achievement is proposed by combining genetic improvements with good management including improving the nutritional availability of the basic compounds used by the mammary gland to produce milk. The advanced study of molecular biology and discovery of DNA that manipulation of DNA transcription is a powerful means to affect phenotypic outcomes. In addition to molecular study transcription, many posttranscriptional regulations can greatly affect phenotype, among which phosphorylation of proteins constitutes a primary one.

3. Role of amino acid supply for milk protein synthesis

It is known that the rate of substrates supplying to the mammary gland is determined by the concentration of substrates in the plasma and mammary blood flow. Thus, the sustainable milk protein synthesis in the mammary gland is dependent upon its blood supply to provide amino acids at appropriate rates. The rate of milk protein production depends on the function of a number of secretory cells and their metabolic activity. However, the mechanisms can be divided into three main levels of regulation, which are the arterial flow of amino acids in the mammary gland, the amino acids extraction by the mammary gland via amino acids transport systems, and the metabolic and secretion activities of the mammary epithelial cell. A dynamic exchange of amino acids between extracellular and intracellular compartments is dependent on changes driven by environmental and physiological stimuli. There is evidence that the transport of amino acids to the lactating cell is a limiting step for milk protein synthesis which depends on the concentration in plasma and extraction rates by the mammary gland. Some amino acids, for example, methionine and lysine are identified as the most limiting amino acids for milk protein synthesis in dairy cows [5] as well as in humans and other species [6]. Methionine uptake by the bovine mammary gland occurs via the sodium-independent neutral and basic amino acids transporter system [7]. Amino acids transporters may also act as sensors of intracellular and extracellular amino acids abundances regulating directly or indirectly protein synthesis. The implications and the extent of the amino acids sensing by amino acids transporters in milk protein synthesis are currently unknown but may be a phenomenon worth investigating.

4. Regulation of protein synthesis by hormone supply and the energy utilization

It has been realized that during late pregnancy, lactogenesis occurs concurrently with mammary development, and many hormones are needed for maximal stimulation of lactogenesis. Many factors have been reported to be capable of influencing lactation persistency particularly in ruminants. The hormonal control of substrates uptake of the mammary gland and milk protein yield is complex. Milk protein synthesis is highly regulated by amino acids levels, amino acid transporters, and insulin via transcriptional and post-transcriptional routes, with the insulin-mTOR pathway playing a central role [8, 9].

The mammary gland is known to be hypersensitive to insulin during lactation and at the onset of pregnancy, with an increased sensitivity until the end of pregnancy, primarily due to an augmented kinase activity of the insulin receptor [10]. One of the primary functions of insulin in the mammary gland is the control of milk protein synthesis by inducing translation via activation of mTOR pathway and activation of signal transducer and activator of transcription (STAT5) through phosphorylation [9].

During lactation, the mammary gland becomes a metabolically active and the highest energetically demanding tissue. In ruminants, increases in energy utilization for milk protein synthesis are due to an increase in mRNA translation in the mammary gland [9]. In addition, the transformation of dietary nitrogen into milk proteins [11], with a high rate of protein turnover [12]. In the goat, the daily tissue protein synthesis can account for as much as 88% of the total protein synthesized, of which 50% of ATP generated by the lactating mammary glands is used for the synthesis of extra-mammary tissue (nonmilk) protein [13]. Insulin can also affect milk protein synthesis by increasing the uptake of amino acids, particularly the branched-chain and the essential amino acids [9, 14]. An increase in intracellular glucose can enhance protein synthesis by increasing ATP production with subsequent inhibition of 5′ AMP-activated protein kinase activity, one of the main negative regulators of mTOR [15].

The availability of amino acids is not only essential for milk protein synthesis, but they can also activate the translational machinery through mTOR pathway [9] and affect the expression of milk protein genes through nutrient/gene interactions [16]. Methionine is well known for its essential role in the initiation of mRNA translation, increasing protein expression and phosphorylation of STAT5a and mTOR and protein expression of casein [17]. In addition to methionine, other amino acids, such as tryptophan, arginine, and isoleucine, have positive effects on milk protein synthesis via phosphorylation of mTOR pathway-related proteins in bovine mammary epithelial cells [18].

The role of energy and amino acids, as well as amino acids transporters in controlling milk protein synthesis through posttranscriptional regulation by an insulin-mTOR signaling pathway, has been previously proposed as a detailed model in the regulation of milk protein synthesis [19]. A review on the translational regulation of milk protein synthesis, including a historical overview of the progress toward understanding post-transcriptional regulation, has been reported [20]. Among transcription factors important for the regulation of casein genes, signal transducer and activator of transcription 5 (STAT5), is the most important due to its role in controlling the expression of various caseins, genes, and lactalbumin [9]. Although insulin plays an important role in the activation of STAT5, other hormones, such as prolactin and glucocorticoids, can also regulate the activity of STAT5 [21]. Interestingly, the level of regulation of STAT5 through Jak/Stat signaling by hormones may vary across species, which has been reviewed previously [9].

5. Conclusion

Milk protein is one of the most important nutrients in milk. Research on the general mechanisms involved in the synthesis of milk protein provides evidence that the synthesis of milk protein is complex in the mammary gland and is regulated by multiple factors and the protein components in milk are specific to the mammary tissue. During lactogenesis, several hormones are needed for maximal stimulation of milk production. The role of insulin has been shown to participate in the regulation of milk protein synthesis with a pivotal role in perturbation with the mTOR and STAT5 signaling pathways. However, the control of milk protein synthesis is still needed to be completely understood, to open new insight into research not previously considered. For example, the elucidation of the mechanism of milk protein synthesis and other metabolic differences among species. High environmental temperatures are known to affect milk secretion at various levels of mechanisms both directly and indirectly in lactating animals in the tropic. Many technologies are required using to elucidate the complex mechanism of milk protein synthesis. The molecular study using omics technologies is a technic that has been introduced to provide the possibilities to further investigate related complex mechanisms of milk protein synthesis. These novel research approaches may contribute to revealing the mechanism of milk protein synthesis and the novel biomarkers in milk affected by some factors. However, the elucidation of these and many other metabolic differences among species in the synthesis of milk protein is an area with an important related application for some that should receive continued and increasing attention.