Glucose-6-phosphate hydrolysis by lactating rat mammary gland

Glucose Metabolism and Dynamics of Facilitative Glucose Transporters (GLUTs) under the Influence of Heat Stress in Dairy Cattle

Heat stress is one of the main threats to dairy cow production; in order to resist heat stress, the animal exhibits a variety of physiological and hormonal responses driven by complex molecular mechanisms. Heat-stressed cows have high insulin activity, decreased non-esterified fatty acids, and increased glucose disposal. Glucose, as one of the important biochemical components of the energetic metabolism, is affected at multiple levels by the reciprocal changes in hormonal secretion and adipose metabolism under the influence of heat stress in dairy cattle. Therefore, alterations in glucose metabolism have negative consequences for the animal’s health, production, and reproduction under heat stress. Lactose is a major sugar of milk which is affected by the reshuffle of the whole-body energetic metabolism during heat stress, contributing towards milk production losses. Glucose homeostasis is maintained in the body by one of the glucose transporters’ family called facilitative glucose transporters (GLUTs encoded by SLC2A genes). Besides the glucose level, the GLUTs expression level is also significantly changed under the influence of heat stress. This review aims to describe the effect of heat stress on systemic glucose metabolism, facilitative glucose transporters, and its consequences on health and milk production.

GLUT1 and lactose synthetase are critical genes for lactose synthesis in lactating sows

Background

Lactose synthesis rate is an important factor in milk production and quality in mammals. Understanding the lactose synthesis mechanism is crucial for the improvement of milk quantity and quality. However, research on the temporal gene changes regarding lactose synthesis during the whole lactation is still limited. The objective of this study was to determine gene expression profiles related to lactose synthesis in sows during lactation, and further identify the critical steps or key factors in the lactose synthesis pathway.

Methods

To determine the temporal change of factors related to lactose synthesis in sows, milk from eight multiparous Yorkshire sows (parity 3 to 6) was collected at 0 h, 2 h, 6 h, 12 h, 24 h, day 2, 3, 4, 7, 14, and 21 after birth of the first piglet. Lactose content, prolactin and progesterone concentration, and gene or protein expression related to lactose synthesis were measured.

Results

The lactose yield increased gradually from D2 to D21 and reached a maximum at D14 (3-fold from D2) during lactation (P < 0.05). A similar trend was observed in IGF-1 and insulin concentrations in milk, both of which were greatest at D3 with a subsequent decrease during middle to late lactation. Conversely, milk prolactin and progesterone concentrations moderately decreased with the progression of lactation. The mRNA or protein expressions related to glucose transportation (GLUT1), glucose-galactose interconversion (HK1 and UGP2), UDP-galactose transportation (SLC35A2), and lactose synthetase (LALBA and B4GALT1) in the lactose synthesis pathway were significantly upregulated during early to middle lactation and plateaued by late lactation (P < 0.05).

Conclusions

These novel findings suggest that the increased lactose synthesis in lactation was related to the coordinated upregulation of genes or enzymes in the lactose synthesis pathway, and glucose transportation (GLUT1) and lactose synthetase (LALBA and B4GALT1) might be the critical steps in the lactose synthesis pathway of sows during lactation.

Buffering agent-induced lactose content increases via growth hormone-mediated activation of gluconeogenesis in lactating goats

Dairy goats are often fed a high-concentrate (HC) diet to meet their lactation demands; however, long-term concentrate feeding is unhealthy and leads to milk yield and lactose content decreases. Therefore, we tested whether a buffering agent is able to increase the output of glucose in the liver and influence lactose synthesis. Eight lactating goats were randomly assigned to two groups: one group received a HC diet (Concentrate : Forage = 6:4, HG) and the other group received the same diet with a buffering agent added (0.2 % NaHCO(3), 0.1 % MgO, BG) over a 19-week experimental period. The total volatile fatty acids and lipopolysaccharide (LPS) declined in the rumen, which led the rumen pH to become stabile in the BG goats. The milk yield and lactose content increased. The alanine aminotransferase, aspartate transaminase, alkaline phosphatase, pro-inflammatory cytokines, LPS and lactate contents in the plasma significantly decreased, whereas the prolactin and growth hormone levels increased. The hepatic vein glucose content increased. In addition, pyruvate carboxylase (PC), phosphoenolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (G6PC) expression in the liver was significantly up-regulated. In the mammary glands, the levels of glucose transporter type 1, 8, 12 as well as of sodium-glucose cotransporter 1 increased. Cumulative buffering agent treatment increased the blood concentrations of glucose via gluconeogenesis and promoted its synthesis in the liver. This treatment may contribute to the increase of the milk yield and lactose synthesis of lactating goats.

Mammary alveolar cell as in vitro evaluation system for casein gene expression involved in glucose level

Objective

Glucose is an essential fuel in the energy metabolism and synthesis pathways of all mammalian cells. In lactating animals, glucose is the major precursor for lactose and is a substrate for the synthesis of milk proteins and fat in mammary secretory (alveolar) epithelial cells. However, clear utilization of glucose in mammary cells during lactogenesis is still unknown, due to the lack of in vitro analyzing models. Therefore, the objective of this study was to test the reliability of the mammary alveolar (MAC-T) cell as an in vitro study model for glucose metabolism and lactating system.

Methods

Undifferentiated MAC-T cells were cultured in three types of Dulbecco’s modified Eagle’s medium with varying levels of glucose (no-glucose: 0 g/L, low-glucose: 1 g/L, and high-glucose: 4.5 g/L) for 8 d, after which differentiation to casein secretion was induced. Cell proliferation and expression levels of apoptotic genes, Insulin like growth factor-1 (IGF1) receptor, oxytocin receptor, αS1, αS2, and β casein genes were analyzed at 1, 2, 4, and 8 d after differentiation.

Results

The proliferation of MAC-T cells with high-glucose treatment was seen to be significantly higher. Expression of apoptotic genes was not affected in any group. However, expression levels of the mammary development related gene (IGF1 receptor) and lactation related gene (oxytocin receptor) were significantly higher in the low-glucose group. Expressions of αS1-casein, αS2-casein, and β-casein were also higher in the low-glucose treated group as compared to that in the no-glucose and high-glucose groups.

Conclusion

The results demonstrated that although a high-glucose environment increases cell proliferation in MAC-T cells, a low-glucose treatment to MAC-T cells induces higher expression of casein genes. Our results suggest that the MAC-T cells may be used as an in vitro model to analyze mammary cell development and lactation connected with precise biological effects.

Biology of glucose transport in the mammary gland

Glucose is the major precursor of lactose, which is synthesized in Golgi vesicles of mammary secretory alveolar epithelial cells during lactation. Glucose is taken up by mammary epithelial cells through a passive, facilitative process, which is driven by the downward glucose concentration gradient across the plasma membrane. This process is mediated by facilitative glucose transporters (GLUTs), of which there are 14 known isoforms. Mammary glands mainly express GLUT1 and GLUT8, and GLUT1 is the predominant isoform with a Km of ~10 mM and transport activity for mannose and galactose in addition to glucose. Mammary glucose transport activity increases dramatically from the virgin state to the lactation state, with a concomitant increase in GLUT expression. The increased GLUT expression during lactogenesis is not stimulated by the accepted lactogenic hormones. New evidence indicates that a possible low oxygen tension resulting from increased metabolic rate and oxygen consumption may play a major role in stimulating glucose uptake and GLUT1 expression in mammary epithelial cells during lactogenesis. In addition to its primary presence on the plasma membrane, GLUT1 is also expressed on the Golgi membrane of mammary epithelial cells and is likely involved in facilitating the uptake of glucose and galactose to the site of lactose synthesis. Because lactose synthesis dictates milk volume, regulation of GLUT expression and trafficking represents potentially fruitful areas for further research in dairy production. In addition, this research will have pathological implications for the treatment of breast cancer because glucose uptake and GLUT expression are up-regulated in breast cancer cells to accommodate the increased glucose need.

Glucose transporter and hypoxia-associated gene expression in the mammary gland of transition dairy cattle

Glucose is an important energy substrate, especially needed by dairy cows postpartum to support the onset of lactation. The prioritization and regulation of glucose uptake is accomplished, in part, by changes in expression of cellular glucose transport molecules (GLUT) within the mammary gland. The objectives of this study were to (1) evaluate the expression and cell-type specific localization of GLUT and hypoxia-associated genes that may regulate GLUT expression over the transition period and through lactation in bovine mammary tissue and (2) determine functionality of GLUT on primary bovine mammary endothelial cells (BMEC). Mammary tissue biopsies were taken from cows at 15 d before calving and again at 1, 15, 30, 60, 120, and 240 d post-parturition for quantitative real-time PCR analysis of GLUT and hypoxia-associated genes. Additional mammary tissue samples were used to localize GLUT within the cells of the lobulo-alveolar system via fluorescence microscopy. Cultures of primary bovine mammary endothelial cells were used to confirm the functionality of GLUT with a fluorescent glucose analog uptake assay. Significant increases in GLUT1 gene expression were observed during early lactation, whereas both GLUT3 and GLUT4 gene expression increased during late lactation. The gene expression for 2 receptors of vascular endothelial growth factor increased significantly during early lactation and remained increased throughout lactation when compared with gene expression during the transition period. All GLUT were detected on cultured BMEC and were capable of internalizing glucose through GLUT-mediated mechanisms. These data suggest mammary vascular tissues express GLUT during lactation and BMEC express functional glucose transporters. A better understanding of glucose uptake at the endothelial level may prove to be critical to improve glucose absorption from the blood for utilization by mammary epithelial cells.

Expression and regulation of glucose transporters in the bovine mammary gland

Glucose is the primary precursor for the synthesis of lactose, which controls milk volume by maintaining the osmolarity of milk. Glucose uptake in the mammary gland plays a key role in milk production. Glucose transport across the plasma membranes of mammalian cells is carried out by 2 distinct processes: facilitative transport, mediated by a family of facilitative glucose transporters (GLUT); and sodium-dependent transport, mediated by the Na+/glucose cotransporters (SGLT). Transport kinetic studies indicate that glucose transport across the plasma membrane of the lactating bovine mammary epithelial cell has a K(m) value of 8.29 mM for 3-O-methyl-D-glucose and can be inhibited by both cytochalasin-B and phloretin, indicating a facilitative transport process. This is consistent with the observation that in the lactating bovine mammary gland, GLUT1 is the predominant glucose transporter. However, the bovine lactating mammary gland also expresses GLUT3, GLUT4, GLUT5, GLUT8, GLUT12, and sodium-dependent SGLT1 and SGLT2 at different levels. Studies of protein expression and cellular and subcellular localizations of these transporters are needed to address their physiological functions in the mammary gland. From late pregnancy to early lactation, expression of GLUT1, GLUT8, GLUT12, SGLT1, and SGLT2 mRNA increases from at least 5-fold to several hundred-fold, suggesting that these transporters may be regulated by lactogenic hormones and have roles in milk synthesis. The GLUT1 protein is detected in lactating mammary epithelial cells. Its expression level decreases from early to late lactation stages and becomes barely detectable in the nonlactating gland. Both GLUT1 mRNA and protein levels in the lactating mammary gland are not significantly affected by exogenous bovine growth hormone, and, in addition, GLUT1 mRNA does not appear to be affected by leptin.

Hexoneogenesis in the human breast during lactation

Lactose is the major osmotic agent in milk. Therefore, lactose synthesis indirectly regulates milk volume. The aim of this study was to determine the source of glucose and galactose in lactose. Six healthy lactating women were studied twice, during a 24-h fast and during ingestion of a mixed macronutrient drink (Sustacal) using [U-13C]glucose and [2-13C]glycerol. Six additional lactating women were studied on one single occasion during ingestion of glucose labeled with [1-13C]glucose. Using the ratios of [13C6] enrichments of glucose in lactose and plasma glucose and that of galactose in lactose and plasma glucose, we determined that 98 +/- 3% of glucose and 68 +/- 7% of galactose in lactose were derived from plasma glucose in the fed state, and 72 +/- 4 and 51 +/- 3%, respectively, after a 24-h fast. Virtually identical results (97 +/- 6 and 64 +/- 4%, respectively) were obtained during the glucose feeding study. On the basis of the [13C1] enrichment of glucose and galactose in lactose (derived from [2-13C]glycerol), glycerol contributes to the production of galactose but not glucose within the breast. Thus, plasma glucose is an important source of lactose, but significant amounts of glucose and galactose in lactose are generated within the breast, a process denoted hexoneogenesis. In this process, glycerol is a precursor for milk galactose but not glucose.

Localization and gene expression of glucose transporters in bovine mammary gland

Glucose uptake in the mammary gland is a rate-limiting step in milk synthesis. To study glucose transporters in the bovine mammary gland, the erythrocyte-type glucose transporter (GLUT1) and the insulin-responsive glucose transporter (GLUT4) proteins were assessed by Western blotting and immunohistochemical staining, using polyclonal antibodies against the C-terminal peptide of GLUT1 and GLUT4. Our results demonstrated that the bovine mammary gland expressed a relatively high level of GLUT1 protein, whereas GLUT4 protein was not detected in the mammary gland of either lactating or dry cows. The absence of GLUT4 may indicate that glucose transport is not regulated by insulin in the lactating and dry bovine mammary gland. The anti-GLUT1 antibody strongly stained the single layer of epithelial cells of mammary alveoli. The expression of GLUT1 mRNA was similar in the mammary gland of late lactation and non-lactating cows. However, a smaller molecular weight species (38 kDa) of GLUT1 protein was detected in the mammary gland of non-lactating cows where its abundance in crude membrane preparation was 80% higher than in lactating animals. There were no significant differences in GLUT1 mRNA in bovine mammary gland at 118 d and 181 d postpartum, however, GLUT1 protein expression tended to be greater at 118 d postpartum.

Glucose 6-phosphatase activity in pregnant and lactating mammary glands of the mouse

Glucose 6-phosphatase activity was studied in the secretory epithelial cell and other cell types composing alveoli of the mammary gland (cytochemical study) and in the whole mammary gland (biochemical study) of pregnant and lactating mice. The reaction product for the enzyme activity was seen in the endoplasmic reticulum and nuclear envelope in secretory epithelial cells from all animals examined (days 7 and 14 of pregnancy, and days 0, 3, 10, and 20 of lactation. The amounts of the reaction product appeared scarce at day 7 of pregnancy, moderate at day 14 of pregnancy and day 0 of lactation, and abundant at days 3 and 10 of lactation. The reaction product, however, became generally scarce at day 20 of lactation. Biochemical activity was relatively low at days 7 and 14 of pregnancy and days 0 and 20 of lactation, while it was high at days 3 and 10 of lactation. The increased activity is probably related to functions of secretory epithelial cells in the lactating gland.

Lactose synthesis: the possibilities of regulation

Elucidation of the details of lactose synthesis, in particular its dependence upon alpha-lactalbumin and its location within the lumen of the Golgi apparatus, now allows one to ask useful questions pertaining to its regulation. Attention is directed towards galactosyltransferase itself (EC 2.4.1.22), which appears to be rate-limiting in the uridine nucleotide cycle that supports lactose synthesis, and to those factors that may affect its activity. In laboratory animals alpha-lactalbumin appears to be the major agent of regulation during lactogenesis but is not necessarily limiting at other times, whereas the increase in amount of galactosyltransferase seems largely to account for the rising yield of lactose during lactation. Studies with pinched-off Golgi membrane vesicles, together with measurements of intracellular chemical concentrations, suggest that beta-glucose and uridine diphosphategalactose do not saturate lactose synthesis and are, therefore potentially regulatory features of this process. Further aspects of lactose synthesis that may offer points of regulation include calcium ions, generation of protons within the Golgi lumen, and the generally rate-limiting nature of the Golgi membrane.