Effects of glucose on lactose synthesis in mammary epithelial cells from dairy cow
© The Author(s). 2016
Received: 23 July 2015
Accepted: 17 May 2016
Published: 26 May 2016
Lactose, as the primary osmotic component in milk, is the major determinant of milk volume. Glucose is the primary precursor of lactose. However, the effect of glucose on lactose synthesis in dairy cow mammary glands and the mechanism governing this process are poorly understood.
Here we showed that glucose has the ability to induce lactose synthesis in dairy cow mammary epithelial cells, as well as increase cell viability and proliferation. A concentration of 12 mM glucose was the optimum concentration to induce cell growth and lactose synthesis in cultured dairy cow mammary epithelial cells. In vitro, 12 mM glucose enhanced lactose content, along with the expression of genes involved in glucose transportation and the lactose biosynthesis pathway, including GLUT1, SLC35A2, SLC35B1, HK2, β4GalT-I, and AKT1. In addition, we found that AKT1 knockdown inhibited cell growth and lactose synthesis as well as expression of GLUT1, SLC35A2, SLC35B1, HK2, and β4GalT-I.
Glucose induces cell growth and lactose synthesis in dairy cow mammary epithelial cells. Protein kinase B alpha acts as a regulator of metabolism in dairy cow mammary gland to mediate the effects of glucose on lactose synthesis.
The basic function of the lactating mammary gland is to produce milk, providing nutrients for growth and development of the offspring. The main constituents of bovine milk are lactose, proteins, and lipids . Among these, lactose is the major carbohydrate of milk. It plays a primary role in milk production because it represents the main osmotic constituent in milk and draws water into the mammary epithelial cells .
Lactose is synthesized from free glucose and uridine diphosphate (UDP)-galactose. In lactating ruminants, the mammary gland can consume up to 85 % of circulating glucose . Uridine diphosphate-galactose is also derived from glucose. Analysis of lactose synthesis in rodent and bovine models of secretory activation has demonstrated increases in a number of enzyme activities and carbohydrate metabolites that together form the fundamental framework of the lactose synthesis pathway [4, 5]. The lactose synthesis reaction is catalyzed by lactose synthase (UDP-galactose : D-glucose l-galactosyl transferase, EC 22.214.171.124), a complex of β-1, 4-galactosyltransferase (β4GalT-I) and the essential cofactor α-lactalbumin (α-LA), in the Golgi compartment [6, 7]. The expression of α-LA is very low during pregnancy and increases significantly with parturition . Beginning in late pregnancy, the β4GalT-I level in the mammary gland is estimated to increase about 50-fold in preparation for lactose biosynthesis . As the substrate of lactose synthase, the glucose concentration in the lactating mammary epithelial cells is higher than usual . Generally, animal nutrition has a substantial effect on milk composition . Glucose is a dietary factor that affects milk synthesis , but whether glucose supplementation can affect lactose synthesis in dairy cow mammary gland is not well understood.
Lactose synthesis and secretion by the mammary gland involve the expression of a large number of genes . However, few studies have focused on the effects of glucose supplementation on the expression of genes involved in lactose biosynthesis. The protein kinase B (PKB, also known as AKT) is a critical downstream effector in multiple signal transduction pathways and regulates cellular proliferation, survival, and metabolism . Among the members of the AKT family, only AKT1 is upregulated in the mammary gland during pregnancy and lactation . Knockout studies have demonstrated that AKT1 has roles in the functional differentiation of the secretory epithelium and in metabolic pathways that regulate milk synthesis . Loss of AKT1 results in failure of the coordinated metabolic responses required for the establishment of lactation at parturition, including increased glucose uptake and lipid synthesis, which in turn results in decreased milk production . The lactation defect observed in AKT1 –/– mice is thus due to a metabolic defect that results from a failure to upregulate glucose transporter 1 (GLUT1) and other AKT1-specific target genes . It is unknown whether glucose supplementation modulation of lactose synthesis could be related to AKT1 expression in dairy cow mammary gland.
In this study, we hypothesized glucose supplementation could affect lactose synthesis in lactating mammary gland of dairy cow. Additionally, glucose induced lactose synthesis is related to AKT1 expression in lactating cow mammary epithelial cells. To meet these objectives, we evaluated the effects of different concentrations of glucose on mammary epithelial cell survival, proliferation, and lactose synthesis. The expression of genes known to be involved in glucose transportation and lactose synthesis was examined by quantitative real-time PCR (qPCR) and western blot when cells were cultured with DMEM containing 12 mM glucose. To evaluate if glucose modulates lactose synthesis via AKT1 activation, siRNA-mediated knockdown of AKT1 in cultured mammary epithelia cells was performed.
Effect of glucose on lactose synthesis in dairy cow mammary epithelial cells
To detect the effects of glucose on mammary epithelial cell survival, proliferation, and lactose biosynthesis, mammary epithelial cells were cultured in DMEM with or without glucose for 24 h. Cell viability (Fig. 1c) and proliferation (Fig. 1d) were both markedly increased in mammary epithelial cells cultured with high glucose compared with those in glucose-free medium (P < 0.05). We also measured the lactose secreted from mammary epithelial cells into the medium. The amount of lactose in medium collected from cells cultured with high glucose for 24 h was significantly higher than that from glucose free-cultured cells (P < 0.01, Fig. 1e). These data suggested that glucose induced mammary epithelial cell growth and lactose biosynthesis.
Effect of glucose on expression of genes involved in lactose synthesis in dairy cow mammary epithelial cells
On the basis of the mRNA expression data, we analyzed protein expression of GLUT1, SLC35A2, SLC35B1, HK2, β4GalT-I, AKT1, and pAKT1 in mammary epithelial cells by western blot. Compared with the control, the levels of GLUT1, SLC35A2, and SLC35B1 were significantly increased (P < 0.05) in mammary epithelial cells cultured in DMEM with 12 mM glucose for 24 h. Consistent with this, HK2 and β4GalT-I levels were also increased (P < 0.05) (Fig. 3c, d). Protein kinase B alpha and its active form, pAKT1, were also upregulated in mammary epithelial cells cultured in DMEM with 12 mM glucose compared with the control (P < 0.05; Fig. 3e, f). Overall, these results strongly suggested that glucose induced lactose synthesis in mammary epithelial cells by upregulating GLUT1, SLC35A2, SLC35B1, HK2, β4GalT-I, and AKT1.
Relationship between AKT1 and glucose modulation of lactose synthesis in dairy cow mammary gland
We then investigated the effect of AKT1 knockdown on lactose synthesis in mammary epithelial cells and measured lactose production under both basal (without glucose) and stimulated (glucose concentrations ranging from 4 to 20 mM) conditions. Compared with the control, AKT1 knockdown decreased lactose synthesis in both basal and glucose-stimulated conditions. The effect at 0 mM (basal) was not significant, and the inhibition effect was significant at 8 mM and 12 mM glucose (Fig. 4d). We further examined the effect of AKT1 knockdown on cell viability and proliferation when cells were cultured in medium with different concentrations of glucose (from 0 to 20 mM). Compared with the control, cell viability and proliferation were both significantly decreased when AKT1 was knocked down (Fig. 4e, f). However, cell viability and proliferation were increased by treatment with 12 mM glucose compared to other glucose concentrations when AKT1 was knocked down.
Lactose synthesis is related to milk volume. Glucose is the primary precursor of lactose. Once taken up by the lactating mammary epithelial cell, glucose is either used in the synthesis of lactose or processed by glycolysis to provide energy . Compared with non-lactating dairy cows, lactating dairy cows have an approximately 4-fold higher requirement for glucose . Because lactating dairy cows need more glucose than non-lactating dairy cows, one might reasonably expect that glucose availability could affect lactose synthesis in lactating dairy cow mammary gland. In this study, we observed that glucose increased lactose synthesis in cultured dairy cow mammary epithelial cells, as well as cell viability and proliferation. A concentration of 12 mM glucose was the optimum concentration to induce lactose synthesis in cultured dairy cow mammary epithelial cells within 24 h. Protein kinase B alpha was activated by glucose in lactose synthesis process. Glucose transporter 1, SLC35A2, SLC35B1, HK2, and β4GalT-I expression were also increased as a result of AKT1 signaling in dairy cow mammary gland.
In mammals, glucose transport across the basolateral membrane of epithelial cells is carried out by glucose transporters . In the current research, we examined the expression of GLUT1, GLUT4, GLUT8, and GLUT12, which has been previously reported in mammary gland [19–21]. Glucose induced significant increases in GLUT1 mRNA and protein levels in dairy cow mammary epithelial cells but did not affect GLUT4, GLUT8, and GLUT12 mRNA, suggesting that GLUT1 is one of the glucose transporters induced by glucose in dairy cow mammary gland for lactose synthesis. Glucose transporter 1 is abundantly expressed in bovine mammary gland and increases during pregnancy and lactation . Our result is in agreement with a report that GLUT1 is the main extracellular and intercellular (Golgi membrane) glucose transporter in mammary epithelial cells . In mouse and dairy goat mammary epithelial cells, GLUT1 is mainly distributed along the plasma membrane and Golgi apparatus [28, 29]. The presence of GLUT1 on the Golgi membrane appears to be specific to mammary epithelial cells, as most cells do not have this glucose transporter on the Golgi membrane .
In rat and mouse mammary glands, GLUT4 expression markedly decreases during pregnancy and practically disappears during lactation [30, 31]. This is because GLUT4 is mainly located in the adipose compartment, not in the plasma membrane . In bovine mammary gland, the expression of GLUT8 increases 10-fold from prepartum to postpartum, and fairly strong GLUT8 staining is observed at both the apical and basolateral membrane of lactating bovine mammary epithelial cells . But our results showed that GLUT8 expression in cells cultured with 12 mM glucose did not changed compared to the control (without glucose), which is consistent with the report that GLUT8 mRNA is not affected by various concentration of glucose in cultured bovine mammary epithelial cells . Although GLUT12 is detected in the apical plasma membrane of epithelial cells during lactation in rats, forced weaning results in decreased cytoplasmic GLUT1 staining intensity but no change in GLUT12 staining . Our data, along with previous evidence, demonstrate that GLUT1 is the main glucose transporter in dairy cow mammary epithelial cells for lactose synthesis, and its levels are regulated by glucose.
The synthesis of lactose takes place in the Golgi compartment of mammary epithelial cells. Lactose synthesis paralleled the induction of expression of proteins involved in UDP-galactose synthesis and its transport across the Golgi membrane. The initial step in the synthesis of UDP-galactose is phosphorylation of glucose by HK. Bovine mammary epithelial cells grown in higher glucose concentrations have higher HK activity . In rodents, HK1 is expressed in the mammary gland throughout pregnancy and lactation, whereas HK2 is expressed only following parturition . In the current study, we detected HK2 mRNA and protein expression in cultured dairy cow mammary epithelial cells, and this expression was upregulated when cells were treated with 12 mM glucose for 24 h. These expression data are positively correlated with lactose synthesis, indicating that HK2 expression was affected by glucose during lactation. Glucose has the ability to induce HK2 expression in mammary epithelial cells, but various concentrations of glucose do not elevate HK2 mRNA expression levels . Uridine diphosphate-galactose is actively transported into the Golgi lumen by SLC35A2 and SLC35B1. We found that the levels of SLC35A2 and SLC35B1 were markedly increased in mammary epithelial cells treated with 12 mM glucose for 24 h. Similarly, the expression of SLC35A2 and SLC35B1 strongly correlated with lactose concentration.
In the Golgi compartment, the biosynthesis of lactose involves the combination of free glucose and UDP-galactose, which is catalyzed by the lactose synthase system. This enzyme complex is composed of β4GalT-I and α-LA . β-1, 4-galactosyltransferase is a constitutively expressed, trans-Golgi resident type II membrane-bound glycoprotein . In mammals, β4GalT-I is required for the tissue-specific production of lactose, which takes place exclusively in the lactating mammary gland. α-lactalbumin is a noncatalytic mammalian protein expressed de novo exclusively in the mammary gland during lactation [35, 36]. The physiological function of α-LA is to lower the glucose K m of β4GalT-I so that it may be used maximally for the synthesis of lactose and it changes the specificity of β4GalT-I from N-acetylglucosamine to glucose . Mice with a null mutation of the α-LA gene fail to lactate . In this study, we found that β4GalT-I levels were upregulated in the presence of additional glucose, but the expression of α-LA in dairy cow mammary epithelial cells was not affected by glucose. This result is in agreement with the previous report that elevated level of glucose increases β4GalT-I expression, but not α-LA in bovine mammary epithelial cells . Our data, along with previous evidence, indicate that glucose supplementation may stimulate lactose synthesis partly by altering the expression of β4GalT-I, but not α-LA.
In this study, we also demonstrate that glucose modulation of lactose synthesis is likely related to AKT1 in lactating mammary glands of dairy cows. Protein kinase B alpha serves as a potent survival factor for secretory epithelial cells . Protein kinase B alpha is upregulated in the mammary gland during pregnancy and lactation, and knockout studies have demonstrated that this particular serine-threonine protein kinase plays a role in the functional differentiation of the secretory epithelium and in metabolic pathways that regulate milk synthesis . In our experiments, AKT1 knockdown decreased both basal and glucose-stimulated lactose synthesis, as well as cell viability and proliferation, indicating that AKT1 is required for lactose synthesis in response to glucose.
The protein kinase B family stimulates transport and metabolism of glucose . In hematopoietic cell line, AKT1 activation increases the localization of GLUT1 to the cell surface and maintenance of HK function . In mammary gland, previous studies have shown that mice lacking AKT1 exhibit a pronounced metabolic defect during late pregnancy and lactation that results from a failure to upregulate GLUT1 [11, 13]. Our data showed that knockdown of AKT1 in dairy cow mammary epithelial cells decreased both the mRNA and protein expression of GLUT1 in response to glucose. These data, along with previous evidence, demonstrate that AKT1 is required for upregulation of glucose uptake in dairy cow mammary epithelial cells. Consistent with a role in upregulation of GLUT1, AKT1 knockdown also resulted in deceased expression of SLC35A2 and SLC35B1, as well as HK2 and β4GalT-I, in dairy cow mammary epithelial cells.
Glucose supplementation has the ability to induce lactose synthesis in in-vitro bovine mammary epithelial cells. Glucose modulation of lactose synthesis is related to AKT1 activation in lactating mammary glands of dairy cows. Additionally, GLUT1, SLC35A2, SLC35B1, HK2, and β4GalT-I expression were increased as a result of AKT1 signaling in dairy cow mammary gland.
Mammary epithelial cell cultures
Primary mammary epithelial cells were prepared from mid-lactating mammary tissues of dairy cows as described previously . Briefly, lactating cows were slaughtered and several pieces of mammary parenchyma tissue were aseptically removed from the midregion of the mammary glands. Mammary epithelial cells were isolated from mammary tissues by collagenase digestion and cultured in DMEM/F12 (12400–024, Life Technologies, Carlsbad, CA, USA) containing 10 % (v/v) fetal bovine serum (16000–044, Life Technologies, Carlsbad, CA, USA), 100 U/mL penicillin, 100 μg/mL streptomycin, and lactating hormones (5 μg/mL insulin, 1 μg/mL prolactin, and 1 μg/mL hydrocortisone; Sigma-Aldrich, St Louis, MO, USA) on cell culture dishes coated with 0.5 % rat tail collagen. Cells were maintained at 37 °C in an atmosphere containing 5 % CO2. When cells grew to 80 % confluency, the primary mammary epithelial cells were trypsinized with 0.25 % trypsin plus 0.02 % EDTA and passaged. The pure mammary epithelial cells were isolated after 3 to 4 passages.
To confirm that the cells we isolated were pure mammary epithelial cells, cells plated on glass coverslips were washed twice with PBS and fixed in ice-cold methanol for 10 min. Cells were blocked with 10 % normal rabbit serum (ZLI-9025, ZSGB-BIO, Beijing, China) in PBS at room temperature for 1 h and then treated with cytokeratin 18 antibody (diluted 1:200 in PBS containing 10 % normal rabbit serum; sc-31700, Santa Cruz, CA, USA) at 4 °C overnight. Following three washes with PBS, cells were incubated with fluorescein isothiocyanate (FITC)-conjugated AffiniPure rabbit anti-goat IgG (diluted 1:50 in PBS containing 10 % normal rabbit serum; ZF-0314, ZSGB-BIO, Beijing, China) for 1 h at room temperature. To stain the nuclei, cells were treated with propidium iodide (Sigma-Aldrich, St Louis, MO, USA) for 10 min at room temperature, followed by three washes with PBS, and mounted with DABCO (Sigma-Aldrich, St Louis, MO, USA). Fluorescence was assessed by confocal microscopy (TCS SP2, Leica Microsystems, Wetzlar, GmbH, Germany).
To investigate if glucose has the ability to induce cell growth and lactose synthesis, mammary epithelial cells were plated at 6 × 104 cells/well in 12-well culture plates. When cells grew to 80 % confluency, the medium was changed to DMEM with high glucose (25 mM, 11965–092, Life Technologies, Carlsbad, CA, USA) or without glucose (11966–025, Life Technologies, Carlsbad, CA, USA). After 24 h of treatment, cells in triplicate wells were collected for viability and proliferation analyses, and medium in triplicate wells was collected for lactose content analysis.
To detect the effects of different concentrations of glucose on cell growth and lactose synthesis, mammary epithelial cells were plated at 6 × 104 cells/well in 12-well culture plates. When cells grew to 80 % confluency, the medium was changed to DMEM with different concentrations of glucose (ranging from 0 to 20 mM). After 12, 24, 36, and 48 h of treatment, cells in triplicate wells were collected for viability and proliferation analyses, and medium was collected for lactose content analysis.
To explore the molecular process by which glucose induces lactose synthesis, mammary epithelial cells were plated at 2 × 105 cells/well in 6-well culture plates. When cells grew to 80 % confluency, the medium was changed to DMEM with 12 mM glucose or without glucose. After 24 h of treatment, cells in triplicate wells were collected for indicated assays.
Cell viability and proliferation analyses
Cell viability and proliferation were assayed with the CASY-TT Analyzer System (Schärfe System GmbH, Reutlingen, Germany) as described previously . Briefly, 100 μL mammary epithelial cell-suspension aliquots were transferred to a CASY cup containing 10 mL CASY ton, mixed by inverting three times, and placed in the CASY cell counter. All experiments were performed in triplicate.
Lactose content in medium was measured with the Lactose/d-Glucose (Rapid) Assay kit (K-LACGAR, Megazyme, Ireland, UK) as described . All experiments were performed in triplicate.
Primers sequences used for qPCR
Primer sequence (5′ to 3′)2
Small interfering RNA (siRNA)-mediated gene knockdown
The siRNA against AKT1 and scrambled siRNA (Scr) were chemically synthesized by GenePharma Co., Ltd, Shanghai, China. Gene knockdown was achieved by transient transfection with AKT1 siRNA or scrambled siRNA as negative control using Lipofectamine 2000 Reagent (11668–019, Life Technologies, Carlsbad, CA, USA) as described previously . Transfection efficiency was detected by fluorescence microscope (DM LB2, Leica Microsystems, Wetzlar, GmbH, Germany) after 6 h of transfection (Additional file 2). After 48 h of transfection, cells were extracted for the AKT1 mRNA and protein expression detection.
To investigate the effect of AKT1 knockdown on lactose synthesis in mammary epithelial cells, after 48 h of transfection, the medium was changed to DMEM with different concentrations of glucose (ranging from 0 to 20 mM). After 24 h of treatment, cells and medium in triplicate wells were collected for indicated assays.
Western blot analysis
For western blot analysis, dairy cow mammary epithelial cells were washed twice with ice-cold PBS and lysed with RIPA buffer (P0013C, Beyotime Biotechnology, Shanghai, China) containing 0.5 mM PMSF (ST506, Beyotime Biotechnology, Shanghai, China),5 μg/mL aprotinin (A1153, Sigma-Aldrich, St Louis, MO, USA) and 5 μg/mL leupeptin (L2884, Sigma-Aldrich, St Louis, MO, USA ) on ice. The cell lysates were centrifuged at 17,800 × g for 10 min at 4 °C. Protein quantification was performed using the BCA Protein Assay kit (23227, Thermo Fisher Scientific Inc., Waltham, MA, USA). Equal amounts of protein (40 μg) were separated by 10 % SDS-PAGE and transferred onto nitrocellulose membranes. Membranes were blocked for 1 h with 5 % nonfat milk in Tris-buffered saline with 0.1 % Tween 20 (TBST) at room temperature and incubated overnight at 4 °C with primary antibodies specific to GLUT1 (1:200 dilution; sc-7903, Santa Cruz, CA, USA), SLC35A2 (1:200 dilution; sc-82032, Santa Cruz, CA, USA), SLC35B1 (1:200 dilution; sc-132864, Santa Cruz, CA, USA), HK2 (1:200 dilution; sc-6521, Santa Cruz, CA, USA), β4GalT-I (1:200 dilution; sc-22279, Santa Cruz, CA, USA), AKT1 (1:200 dilution; sc-1618, Santa Cruz, CA, USA), pAKT1 (1:1000 dilution, Abcam, Cambridge, MA, USA), and β-ACTIN (1:1000 dilution; 4970, Cell Signaling Technology, Beverly, MA, USA). The primary antibodies used were all diluted in 5 % BSA in TBST. The blots were then treated with horseradish peroxidase (HRP)–conjugated anti-goat IgG (1:500 dilution; ZB-2306, ZSGB-BIO, Beijing, China) or HRP-conjugated anti-rabbit IgG (1:500 dilution; ZB-2301, ZSGB-BIO, Beijing, China) for 1 h at room temperature. Proteins were visualized using the SignalFire ECL Reagent (6683, Cell Signaling Technology, Beverly, MA, USA). Three samples were assessed per time point or treatment. The protein bands were used for densitometric analysis using Image-Pro Plus 6.0 (Media Cybernetics, Inc., Warrendale, PA, USA) to obtain relative protein levels expressed as integrated density values. The integrated density, representing the mean density of pixels multiplied by the area was determined from equal sized rectangles drawn around the bands of interest minus the background to remove non-specific antibody staining. The protein levels were normalized to β-ACTIN.
Experimental data were analyzed using GraphPad Prism 6 (GraphPad Software, La Jolla, CA). Quantitative data is presented as the mean ± SEM. A two-tailed unpaired t-test was used for two condition comparison. Two-way or one-way ANOVA with Bonferroni multiple comparison was used for multiple conditions comparison. P < 0.05 was considered to be statistically significant.
UDP, uridine diphosphate; β4GalT-I, β-1, 4-galactosyltransferase, α-LA: α-lactalbumin; PKB, the protein kinase B, also known as AKT; GLUT, glucose transporter; qPCR, quantitative real-time PCR; SLC35A2, solute carrier family 35 member A2; SLC35B1, solute carrier family 35 member B1; HK2, hexokinase 2; Scr, scrambled siRNA; FITC, fluorescein isothiocyanate.
This work was supported by grants from National Basic Research Program (2011CB100804) from the Ministry of Science and Technology of China and the National Natural Science Foundation of China (31401109 to YL and 31200984 to XH).
Availability of data and material
The datasets supporting the conclusions of this article are included within the article and its additional files.
YL devised the hypothesis, designed the experiments and wrote the manuscript. XS performed the experiments. XH performed the data analysis and made the figures. BQ helped with the confocal microscopy. XG helped with experimental design. QL is also responsible for the writing, revision and finalization of the manuscript as well as for the decision to submit the manuscript for publication. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Consent for publication
All procedures involving dairy cows were performed in accordance with institutional and national guidelines and approved by the Institutional Animal Care and Use Committee of Northeast Agricultural University (China).
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
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