Localization and expression of BCAT during pregnancy and lactation in the rat mammary gland

Adipocytes Are the Only Site of Glutamine Synthetase Expression Within the Lactating Mouse Mammary Gland

Background

Glutamine in milk is believed to play an important role in neonatal intestinal maturation and immune function. For lactating mothers, glutamine utilization is increased to meet the demands of the enlarged intestine and milk production. However, the source of such glutamine during lactation has not been studied.

Objectives

We aimed to assess the effects of lactation on the expression of glutamine synthetase (GS) in the mammary gland and other tissues of lactating mice.

Methods

Mouse tissues were sampled at 4 time points: 8-wk-old (virgin, control), post-delivery day 5 (PD5, early lactation), PD15 (peak lactation), and involution (4 days after weaning at PD21). We examined the gene expression and protein concentrations of GS and the first 2 enzymes of branched-chain amino acid catabolism: branched-chain aminotransferase 2 (BCAT2) and branched-chain ketoacid dehydrogenase subunit E1α (BCKDHA).

Results

The messenger RNA (mRNA) expression and protein concentrations of GS in mammary glands were significantly lower at PD5 and PD15 compared with the control but were restored at involution. Within the mammary gland, GS protein was only detected in adipocytes with no evidence of presence in mammary epithelial cells. Compared with the control, mRNA and protein concentrations of BCAT2 and BCKDHA in mammary glands significantly decreased during lactation and involution. No changes in GS protein concentrations during lactation were found in the liver, skeletal muscle, and lung. In non-mammary adipose tissue, GS protein abundance was higher during lactation compared with the virgin.

Conclusions

This work shows that, within the mouse mammary gland, GS is only expressed in adipocytes and that the relative GS abundance in mammary gland sections is lower during lactation. This suggests that mammary adipocytes may be a site of glutamine synthesis in the lactating mouse. Identifying the sources of glutamine production during lactation is important for optimizing milk glutamine concentration to enhance neonatal and maternal health.

Amino Acid Catabolism: An Overlooked Area of Metabolism

Amino acids have been extensively studied in nutrition, mainly as key elements for maintaining optimal protein synthesis in the body as well as precursors of various nitrogen-containing compounds. However, it is now known that amino acid catabolism is an important element for the metabolic control of different biological processes, although it is still a developing field to have a deeper understanding of its biological implications. The mechanisms involved in the regulation of amino acid catabolism now include the contribution of the gut microbiota to amino acid oxidation and metabolite generation in the intestine, the molecular mechanisms of transcriptional control, and the participation of specific miRNAs involved in the regulation of amino acid degrading enzymes. In addition, molecules derived from amino acid catabolism play a role in metabolism as they are used in the epigenetic regulation of many genes. Thus, this review aims to examine the mechanisms of amino acid catabolism and to support the idea that this process is associated with the immune response, abnormalities during obesity, in particular insulin resistance, and the regulation of thermogenesis.

Immunohistochemical and ultrastructural evidence for telocytes in the different physiological stages of the female rat mammary gland

INTRODUCTION

Telocytes (TCs) are recently described to integrate a variety of different cells.

AIM OF THE WORK

The aim was to investigate the presence of TCs in the rat mammary gland at its different physiological stages.

MATERIAL AND METHODS

Twenty four adult female albino rats were classified into 4 groups: resting, mid-pregnancy, lactating, and involution groups. Inguinal mammary glands were processed for immunohistochemical and transmission electron microscopic (TEM) examination.

RESULTS

TCs were immune-positive for c-kit and CD34 and showed significant differences in the different studied groups indicating variable roles at the different stages. TEM results characterized TCs by its shape and the long slender and moniliform telopodes linking the cells into stromal networks. The extracellular exosomes, homo-cellular synapsis and hetero-cellular synapsis were observed.

CONCLUSION

Our study provides evidence for the presence of TCs in all stages of the gland; not only in the resting stage as proved by other studies, but with immune-labeling differences suggesting different structural and physiological roles of TCs according to the stage requirements. These functions might via controlling the proliferation during pregnancy and lactation and the involution of the gland after weaning. Thus, more future functional studies of TCs will be important to help understanding the mechanism by which TCs contribute to tissue homeostasis concerning the role of the stromal/epithelial interactions in mammary gland biology and pathology including breast cancer which would be revolutionary for future therapeutic applications.

Complementary transcriptomic and proteomic analyses reveal regulatory mechanisms of milk protein production in dairy cows consuming different forages

Forage plays a critical role in the milk production of dairy cows; however, the mechanisms regulating bovine milk synthesis in dairy cows fed high forage rations with different basal forage types are not well-understood. In the study, rice straw (RS, low-quality) and alfalfa hay (AH, high-quality) diets were fed to lactating cows to explore how forage quality affected the molecular mechanisms regulating milk production using RNA-seq transcriptomic method with iTRAQ proteomic technique. A total of 554 transcripts (423 increased and 131 decreased) and 517 proteins (231 up-regulated and 286 down-regulated) were differentially expressed in the mammary glands of the two groups. The correlation analysis demonstrated seven proteins (six up-regulated and one down-regulated) had consistent mRNA expression. Functional analysis of the differentially expressed transcripts/proteins suggested that enhanced capacity for energy and fatty acid metabolism, increased protein degradation, reduced protein synthesis, decreased amino acid metabolism and depressed cell growth were related to RS consumption. The results indicated cows consuming RS diets may have had depressed milk protein synthesis because these animals had decreased capacity for protein synthesis, enhanced proteolysis, inefficient energy generation and reduced cell growth. Additional work evaluating RS- and AH-based rations may help better isolate molecular adaptations to low nutrient availability during lactation.

The response of newly born mice to odors of murine colostrum and milk: unconditionally attractive, conditionally discriminated

It is a general rule that milk conveys chemosensory cues that are attractive to mammalian neonates. This study investigated whether compositional fluctuations in milk along lactation induce variations in newborn mouse pups' (Mus musculus, strain BALB/c) attraction to milk odor. Pups differing in suckling experience were exposed to the odor of milk sampled from females varying in lactational stage. Immediately after birth, suckling-inexperienced (P0) and suckling-experienced (P0suck ) pups were assayed in a series of paired-choice tests contrasting murine milk [of lactation days 0, 3, 15 (abridged L0, L3, L15, respectively)] and a blank (water) to evaluate olfactory detection and attraction of milk odor. Preference tests further paired these milk two-by-two to assess their relative attraction. Results showed first that P0 and P0suck pups detect and positively orient to any milk odor. When L0 is presented against L15 milk, P0 pups orient for a similar duration towards these odor stimuli, whereas P0suck pups spend more time toward the odor of L0 than of L15 milk. Finally, P0suck pups orient similarly to odors of L0 milk collected before/after the first suckling episode (L0 and L0suck , respectively), but the odor of L0 milk was more attractive than that of L3 milk. Thus, mouse pups' positive orientation toward the odors of murine colostrum (assumed to correspond to L0/L0suck milk) and later-lactation milk appears unconditional of previous suckling experience, whereas their ability to discriminate or display preference between milk differing in lactation stage appears conditional on postnatal exposure effects.

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.

Temporal changes in concentrations of branched-chain amino acids in plasma on healthy mares and foals from birth to 24 weeks of age

The concentrations of branched-chain amino acids (BCAA; valine, leucine, isoleucine) were determined in plasma of 7 healthy thoroughbred mares and their foals from birth (0 week) to 24 weeks of age, using automated high-performance liquid chromatography. In foals, the concentrations of plasma valine were significantly high (p<0.05) at 16, 20 and 24 weeks. The concentrations of plasma leucine were significantly high (p<0.05) at 1 and 3 weeks. The concentrations of plasma isoleucine were significantly high (p<0.05) from 1 to 24 weeks. In mares, the concentrations of plasma valine were significantly high (p<0.05) at 16 and 24 weeks. The concentrations of plasma leucine and isoleucine were significantly high (p<0.05) at 16 weeks. It was clear that the concentrations of plasma BCAA in foals and mares were at different levels at various times after birth. Since mares and foals were kept in health during this study, we could get the base data of the concentrations of BCAA in plasma of healthy foals and mares from birth to 24 weeks.

Lactating porcine mammary tissue catabolizes branched-chain amino acids for glutamine and aspartate synthesis

The uptake of branched-chain amino acids (BCAA) from plasma by lactating porcine mammary gland substantially exceeds their output in milk, whereas glutamine output is 125% greater than its uptake from plasma. In this study, we tested the hypothesis that BCAA are catabolized for glutamine synthesis in mammary tissue. Mammary tissue slices from sows on d 28 of lactation were incubated at 37 degrees C for 1 h in Krebs buffer containing 0.5 or 2 mmol/L l-[1-(14)C]- or l-[U-(14)C]-labeled leucine, isoleucine, or valine. Rates of BCAA transport and degradation in mammary tissue were high, with approximately 60% of transaminated BCAA undergoing oxidative decarboxylation and the remainder being released as branched-chain alpha-ketoacids (BCKA). Most ( approximately 70%) of the decarboxylated BCAA were oxidized to CO(2). Rates of net BCAA transamination were similar to rates of glutamate, glutamine, aspartate, asparagine, and alanine synthesis. Consistent with the metabolic data, mammary tissue expressed BCAA aminotransferase (BCAT), BCKA decarboxylase, glutamine synthetase (GS), glutamate-oxaloacetate aminotransferase, glutamate-pyruvate aminotransferase, and asparagine synthetase, but no phosphate-activated glutaminase, activity. Western blot analysis indicated relatively high levels of mitochondrial and cytosolic isoforms of BCAT, as well as BCKA dehydrogenase and GS proteins in mammary tissue. Our results demonstrate that glutamine and aspartate (abundant amino acids in milk protein) were the major nitrogenous products of BCAA catabolism in lactating porcine mammary tissue and provide a biochemical basis to explain an enrichment of glutamine and aspartate in sow milk.

Changes in messenger RNA abundance of amino acid transporters in rat mammary gland during pregnancy, lactation, and weaning

During lactation, the mammary gland increases the needs for nutrients to fulfill the milk production requirements. Among these nutrients, amino acids play an important role for the synthesis of milk proteins. Amino acids are supplied to the mammary gland through amino acid transporters, although some are synthesized in situ. The purpose of this study was to establish the pattern of changes in messenger RNA abundance of the amino acid transporters ASC, system L, EAAC1, GLAST, CAT-1, and Tau in the mammary gland of the rat during different stages of pregnancy and lactation. Rats were fed during pregnancy and lactation a 20% casein diet. Food intake increased significantly during the lactation period. Amino acid transporter ASC expression increased during the first days of pregnancy about 2-fold, and it was increased in a lesser extent again during the peak of lactation. The expression of system L (LAT-1) and CAT-1 transporters was increased only during the lactation period. On the other hand, the expression of the transporters for anionic amino acids EAAC1 and GLAST was low during both stages. Finally, taurine transporter expression decreased during pregnancy; and it was significantly lower during lactation. These results showed that amino acid transporters were not expressed similarly in the mammary gland during pregnancy and lactation, indicating that the expression of these transporters did not respond only to the metabolic needs of the gland but depended on the dietary protein supply and possibly the specific hormonal changes that occur during pregnancy and lactation.

Characterization and regulation of the gene expression of amino acid transport system A (SNAT2) in rat mammary gland

Amino acid transport via system A plays an important role during lactation, promoting the uptake of small neutral amino acids, mainly alanine and glutamine. However, the regulation of gene expression of system A [sodium-coupled neutral amino acid transporter (SNAT)2] in mammary gland has not been studied. The aim of the present work was to understand the possible mechanisms of regulation of SNAT2 in the rat mammary gland. Incubation of gland explants in amino acid-free medium induced the expression of SNAT2, and this response was repressed by the presence of small neutral amino acids or by actinomycin D but not by large neutral or cationic amino acids. The half-life of SNAT2 mRNA was 67 min, indicating a rapid turnover. In addition, SNAT2 expression in the mammary gland was induced by forskolin and PMA, inducers of PKA and PKC signaling pathways, respectively. Inhibitors of PKA and PKC pathways partially prevented the upregulation of SNAT2 mRNA during adaptive regulation. Interestingly, SNAT2 mRNA was induced during pregnancy and to a lesser extent at peak lactation. beta-Estradiol stimulated the expression of SNAT2 in mammary gland explants; this stimulation was prevented by the estrogen receptor inhibitor ICI-182780. Our findings clearly demonstrated that the SNAT2 gene is regulated by multiple pathways, indicating that the expression of this amino acid transport system is tightly controlled due to its importance for the mammary gland during pregnancy and lactation to prepare the gland for the transport of amino acids during lactation.

Synthesis of long-chain polyunsaturated fatty acids in lactating mammary gland: role of Δ5 and Δ6 desaturases, SREBP-1, PPARα, and PGC-1

The purpose of this work was to study whether rat lactating mammary gland can synthesize PUFAs through the expression of Delta5 and Delta6 desaturases (Delta5D and Delta6D), whether these enzymes are regulated by the transcription factors sterol-regulatory element binding protein 1 (SREBP-1) and peroxisome proliferator-activated receptor alpha (PPARalpha) and the coactivator peroxisome proliferator-activated receptor gamma coactivator 1beta (PGC-1beta), and whether these desaturases are regulated by the lipid concentration in the diet. The results showed that on day 12 of lactation, approximately 35% of the linoleic acid in the diet, which is the precursor of PUFAs, is transferred to the mammary gland. There was expression of Delta5D and Delta6D in mammary gland, and it was regulated by the corn oil content in the diet. The higher the corn oil content in the diet, the lower the expression of both desaturases. Induction of Delta5D and Delta6D was associated positively with similar changes in SREBP-1 and PGC-1beta. Expression of PPARalpha was barely detected and was not affected by the corn oil content in the diet, whereas PGC-1beta expression increased as the corn oil in the diet increased. These results indicate that the lactating mammary gland has the capacity to synthesize PUFAs and can be regulated by the lipid content in the diet.

Branched-chain amino acids: enzyme and substrate regulation

The three branched-chain amino acids (BCAAs) are the most hydrophobic of the amino acids and play crucial roles in determining the structures of globular proteins as well as the interaction of the transmembrane domains of membranous proteins with phospholipid bilayers. However, the three BCAAs do not behave identically. In terms of protein secondary structure, valine and isoleucine exhibit a definite preference for the beta-structure, whereas leucine has a higher preference for the alpha-helix. Although mutation of one BCAA to another is commonly regarded as conservative, there are well-documented examples of such substitutions that have a significant effect on protein function. The occurrence of BCAA in nature is, therefore, attributable to their primary role in protein structure, not to their secondary metabolic roles. These functions are important for almost all proteins; therefore, BCAA commonly account for approximately 20-25% of most dietary proteins. Dietary BCAA largely escape first-pass splanchnic metabolism. The first steps in their catabolism are common to all three, involving the BCAA aminotransferase (BCAT) and branched-chain alpha-keto acid dehydrogenase (BCKD). Their further metabolism employs distinct pathways to different end-products (glucose and/or ketone bodies). However, the fact that the flux-generating step for the catabolism of the three BCAAs occurs at one of the common steps indicates that the production of these downstream products are not individually regulated and, hence, may not play important individual roles. The catabolism of the BCAAs is highly regulated by both allosteric and covalent mechanisms. BCKD is inhibited by phosphorylation and activated by dephosphorylation. Allosteric inhibition of the kinase by the branched-chain keto acids (BCKA) (particularly by alpha-ketoisocaproate) serves both as a mechanism for promoting the catabolism of excess quantities of these amino acids as well as for conserving low concentrations of these dietary essential amino acids. Cytosolic and mitochondrial isoenzymes of BCAT have been identified. They are thought to play an important role in brain neurotransmitter metabolism.

Mitochondrial branched chain aminotransferase gene expression in AS-30D hepatoma rat cells and during liver regeneration after partial hepatectomy in rat

Branched chain aminotransferase (BCAT) is the first enzyme in the catabolism of branched chain amino acids (BCAA). Unlike other amino acid degrading enzymes present in liver, BCAT is only expressed in extrahepatic tissues, and is not regulated by dietary protein, glucagon or glucocorticoids. However, the mitochondrial (m) isoform of BCAT is highly expressed in the fetal liver and rapidly decays after birth. The purpose of the present work was to establish if liver cells under conditions of rapid cell proliferation such as in hepatoma AS30D cells or during liver regeneration after partial hepatectomy were associated with an increase in the activity and expression of BCATm. BCAT activity in mitochondria of AS30D cells was 18.6 mU/mg protein. Western, Northern blot, and immunohistochemical analysis revealed that AS30D hepatoma cells expressed only BCATm. The apparent Km of BCATm in isolated AS30D cells mitochondria for leucine, isoleucine and valine was 1.0+/-0.02, 1.3+/-0.1 and 2.1+/-0.1 mM, respectively. The regenerated liver showed BCAT activity from day 3 to day 6, and the maximal BCAT activity (7.0 mU/mg protein) was on day 5. By day 14 after partial hepatectomy BCAT activity and expression was almost undetectable. Interestingly, there was a relationship between BCAT activity and the Mr. of the immunoreactive band of BCATm. The presence of a 41 kDa band was associated with BCAT activity, whereas the 43 kDa band with undetectable activity. The results of this study indicate that BCATm activity is required in liver cells under conditions of rapid cell proliferation.

Ontogeny and subcellular localization of rat liver mitochondrial branched chain amino-acid aminotransferase

Branched chain amino-acid aminotransferase (BCAT) activity is present in fetal liver but the developmental pattern of mitochondrial BCAT (BCATm) expression in rat liver has not been studied. The aim of this study was to determine the activity, protein and mRNA concentration of BCATm in fetal and postnatal rat liver, and to localize this enzyme at the cellular and subcellular levels at both developmental stages. Maximal BCAT activity and BCATm mRNA expression occurred at 17 days' gestation in fetal rat liver and then declined significantly immediately after birth. This pattern was observed only in liver; rat heart showed a different developmental pattern. Fetal liver showed intense immunostaining to BCATm in the nuclei and mitochondria of hepatic cells and blood cell precursors; in contrast, adult liver showed mild immunoreactivity located only in the mitochondria of hepatocytes. BCAT activity in isolated fetal liver nuclei was 0.64 mU x mg(-1) protein whereas it was undetectable in adult liver nuclei. By Western blot analysis the BCATm antibody recognized a 41-kDa protein in fetal liver nuclei, and proteins of 41 and 43 kDa in fetal liver supernatant. In adult rat liver supernatant, the BCATm antibody recognized only a 43-kDa protein; however, neither protein was detected in adult rat liver nuclei. The appearance of the 41-kDa protein was associated with the presence of the highly active form of BCATm. These results suggest the existence of active and inactive forms of BCAT in rat liver.