Expression of the facilitated glucose transporters (GLUT1 and GLUT3) by a choriocarcinoma cell line (JAr) and cytotrophoblast cells in culture

Fetomaternal Expression of Glucose Transporters (GLUTs)-Biochemical, Cellular and Clinical Aspects

Several types of specialized glucose transporters (GLUTs) provide constant glucose transport from the maternal circulation to the developing fetus through the placental barrier from the early stages of pregnancy. GLUT1 is a prominent protein isoform that regulates placental glucose transfer via glucose-facilitated diffusion. The GLUT1 membrane protein density and permeability of the syncytial basal membrane (BM) are the main factors limiting the rate of glucose diffusion in the fetomaternal compartment in physiological conditions. Besides GLUT1, the GLUT3 and GLUT4 isoforms are widely expressed across the human placenta. Numerous medical conditions and molecules, such as hormones, adipokines, and xenobiotics, alter the GLUT's mRNA and protein expression. Diabetes upregulates the BM GLUT's density and promotes fetomaternal glucose transport, leading to excessive fetal growth. However, most studies have found no between-group differences in GLUTs' placental expression in macrosomic and normal control pregnancies. The fetomaternal GLUTs expression may also be influenced by several other conditions, such as chronic hypoxia, preeclampsia, and intrahepatic cholestasis of pregnancy.

Endometrial Glucose Transporters in Health and Disease

Pregnancy loss is a frequent occurrence during the peri-implantation period, when there is high glucose demand for embryonic development and endometrial decidualization. Glucose is among the most essential uterine fluid components required for those processes. Numerous studies associate abnormal glucose metabolism in the endometrium with a higher risk of adverse pregnancy outcomes. The endometrium is incapable of synthesizing glucose, which thus must be delivered into the uterine lumen by glucose transporters (GLUTs) and/or the sodium-dependent glucose transporter 1 (SGLT1). Among the 26 glucose transporters (14 GLUTs and 12 SGLTs) described, 10 (9 GLUTs and SGLT1) are expressed in rodents and 8 (7 GLUTs and SGLT1) in the human uterus. This review summarizes present knowledge on the most studied glucose transporters in the uterine endometrium (GLUT1, GLUT3, GLUT4, and GLUT8), whose data regarding function and regulation are still lacking. We present the recently discovered SGLT1 in the mouse and human endometrium, responsible for controlling glycogen accumulation essential for embryo implantation. Moreover, we describe the epigenetic regulation of endometrial GLUTs, as well as signaling pathways included in uterine GLUT’s expression. Further investigation of the GLUTs function in different endometrial cells is of high importance, as numerous glucose transporters are associated with infertility, polycystic ovary syndrome, and gestational diabetes.

Human placental glucose transport in fetoplacental growth and metabolism

While efficient glucose transport is essential for all cells, in the case of the human placenta, glucose transport requirements are two-fold; provision of glucose for the growing fetus in addition to the supply of glucose required the changing metabolic needs of the placenta itself. The rapidly evolving environment of placental cells over gestation has significant consequences for the development of glucose transport systems. The two-fold transport requirement of the placenta means also that changes in expression will have effects not only for the placenta but also for fetal growth and metabolism. This review will examine the localization, function and evolution of placental glucose transport systems as they are altered with fetal development and the transport and metabolic changes observed in pregnancy pathologies.

Metabolomic Analysis Reveals Vitamin D-induced Decrease in Polyol Pathway and Subtle Modulation of Glycolysis in HEK293T Cells

We combined ¹H NMR metabolomics with functional and molecular biochemical assays to describe the metabolic changes elicited by vitamin D in HEK293T, an embryonic proliferative cell line adapted to high-glucose concentrations. Activation of the polyol pathway, was the most important consequence of cell exposure to high glucose concentration, resembling cells exposed to hyperglycemia. Vitamin D induced alterations in HEK293T cells metabolism, including a decrease in sorbitol, glycine, glutamate, guanine. Vitamin D modulated glycolysis by increasing phosphoglycerate mutase and decreasing enolase activities, changing carbon fate without changing glucose consumption, lactate export and Krebs cycle. The decrease in sorbitol intracellular concentration seems to be related to vitamin D regulated redox homeostasis and protection against oxidative stress, and helped maintaining the high proliferative phenotype, supported by the decrease in glycine and guanine and orotate concentration and increase in choline and phosphocholine concentration. The decrease in orotate and guanine indicated an increased biosynthesis of purine and pyrimidines. Vitamin D elicited metabolic alteration without changing cellular proliferation and mitochondrial respiration, but reclaiming reductive power. Our study may contribute to the understanding of the metabolic mechanism of vitamin D upon exposure to hyperglycemia, suggesting a role of protection against oxidative stress.

Establishment of a confluent monolayer model with human primary trophoblast cells: novel insights into placental glucose transport

STUDY HYPOTHESIS

Using optimized conditions, primary trophoblast cells isolated from human term placenta can develop a confluent monolayer in vitro, which morphologically and functionally resembles the microvilli structure found in vivo.

STUDY FINDING

We report the successful establishment of a confluent human primary trophoblast monolayer using pre-coated polycarbonate inserts, where the integrity and functionality was validated by cell morphology, biophysical features, cellular marker expression and secretion, and asymmetric glucose transport.

WHAT IS KNOWN ALREADY

Human trophoblast cells form the initial barrier between maternal and fetal blood to regulate materno-fetal exchange processes. Although the method for isolating pure human cytotrophoblast cells was developed almost 30 years ago, a functional in vitro model with primary trophoblasts forming a confluent monolayer is still lacking.

STUDY DESIGN, SAMPLES/MATERIALS, METHODS

Human term cytotrophoblasts were isolated by enzymatic digestion and density gradient separation. The purity of the primary cells was evaluated by flow cytometry using the trophoblast-specific marker cytokeratin 7, and vimentin as an indicator for potentially contaminating cells. We screened different coating matrices for high cell viability to optimize the growth conditions for primary trophoblasts on polycarbonate inserts. During culture, cell confluency and polarity were monitored daily by determining transepithelial electrical resistance (TEER) and permeability properties of florescent dyes. The time course of syncytia-related gene expression and hCG secretion during syncytialization were assessed by quantitative RT-PCR and enzyme-linked immunosorbent assay, respectively. The morphology of cultured trophoblasts after 5 days was determined by light microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Membrane makers were visualized using confocal microscopy. Additionally, glucose transport studies were performed on the polarized trophoblasts in the same system.

MAIN RESULTS AND THE ROLE OF CHANCE

During 5-day culture, the highly pure trophoblasts were cultured on inserts coated with reconstituted basement membrane matrix . They exhibited a confluent polarized monolayer, with a modest TEER and a size-dependent apparent permeability coefficient (Papp) to fluorescently labeled compounds (MW ∼400-70 000 Da). The syncytialization progress was characterized by gradually increasing mRNA levels of fusogen genes and elevating hCG secretion. SEM analyses confirmed a confluent trophoblast layer with numerous microvilli, and TEM revealed a monolayer with tight junctions. Immunocytochemistry on the confluent trophoblasts showed positivity for the cell-cell adhesion molecule E-cadherin, the tight junction protein 1 (ZO-1) and the membrane proteins ATP-binding cassette transporter A1 (ABCA1) and glucose transporter 1 (GLUT1). Applying this model to study the bidirectional transport of a non-metabolizable glucose derivative indicated a carrier-mediated placental glucose transport mechanism with asymmetric kinetics.

LIMITATIONS, REASONS FOR CAUTION

The current study is only focused on primary trophoblast cells isolated from healthy placentas delivered at term. It remains to be evaluated whether this system can be extended to pathological trophoblasts isolated from diverse gestational diseases.

WIDER IMPLICATIONS OF THE FINDINGS

These findings confirmed the physiological properties of the newly developed human trophoblast barrier, which can be applied to study the exchange of endobiotics and xenobiotics between the maternal and fetal compartment, as well as intracellular metabolism, paracellular contributions and regulatory mechanisms influencing the vectorial transport of molecules.

LARGE-SCALE DATA

Not applicable.

STUDY FUNDING AND COMPETING INTERESTS

This study was supported by the Swiss National Center of Competence in Research, NCCR TransCure, University of Bern, Switzerland, and the Swiss National Science Foundation (grant no. 310030_149958, C.A.). All authors declare that their participation in the study did not involve factual or potential conflicts of interests.

Placental glucose transporter 3 (GLUT3) is up-regulated in human pregnancies complicated by late-onset intrauterine growth restriction

INTRODUCTION

Transport of glucose from maternal blood across the placental trophoblastic tissue barrier is critical to sustain fetal growth. The mechanism by which GLUTs are regulated in trophoblasts in response to ischemic hypoxia encountered with intrauterine growth restriction (IUGR) has not been suitably investigated.

OBJECTIVE

To investigate placental expression of GLUT1, GLUT3 and GLUT4 and possible mechanisms of GLUT regulation in idiopathic IUGR.

METHODS

We analyzed clinical, biochemical and histological data from placentas collected from women affected by idiopathic full-term IUGR (n = 10) and gestational age-matched healthy controls (n = 10).

RESULTS

We found increased GLUT3 protein expression in the trophoblast (cytotrophoblast greater than syncytiotrophoblast) on the maternal aspect of the placenta in IUGR compared to normal placenta, but no differences in GLUT1 or GLUT4 were found. No differential methylation of the GLUT3 promoter between normal and IUGR placentas was observed. Increased GLUT3 expression was associated with an increased nuclear concentration of HIF-1α, suggesting hypoxia may play a role in the up-regulation of GLUT3.

DISCUSSION

Further studies are needed to elucidate whether increased GLUT3 expression in IUGR is a marker for defective villous maturation or an adaptive response of the trophoblast in response to chronic hypoxia.

CONCLUSIONS

Patients with IUGR have increased trophoblast expression of GLUT3, as found under the low-oxygen conditions of the first trimester.

Glucose transporter 3 (GLUT3) protein expression in human placenta across gestation

Conflicting information regarding expression of GLUT3 protein in the human placenta has been reported and the localization and pattern of expression of GLUT3 protein across gestation has not been clearly defined. The objective of this study was characterization of syncytial GLUT3 protein expression across gestation. We hypothesized that GLUT3 protein is present in the syncytial microvillous membrane and that its expression decreases over gestation. GLUT3 protein was measured in samples from a range of gestational ages (first to third trimester), with human brain and human bowel used as a positive and negative control respectively. As an additional measure of specificity, we transfected BeWo choriocarcinoma cells, a trophoblast cell line expressing GLUT3, with siRNA directed against GLUT3 and analyzed expression by Western blotting. GLUT3 was detected in the syncytiotrophoblast at all gestational ages by immunohistochemistry. Using Western blotting GLUT3 was detected as an integral membrane protein at a molecular weight of ∼50 kDa in microvillous membranes from all trimesters but not in syncytial basal membranes. The identity of the primary antibody target was confirmed by demonstrating that expression of the immunoblotting signal in GLUT3 siRNA-treated BeWo was decreased to 18 ± 6% (mean ± SEM) of that seen in cells transfected with a non-targeting siRNA. GLUT3 expression in microvillous membranes detected by Western blot decreased through the trimesters such that expression in the second trimester (wks 14-26) was 48 ± 7% of that in the first trimester and by the third trimester (wks 31-40) only 34 ± 10% of first trimester expression. In addition, glucose uptake into BeWo cells treated with GLUT3 siRNA was reduced to 60% of that measured in cells treated with the non-targeting siRNA. This suggests that GLUT3-mediated uptake comprises approximately 50% of glucose uptake into BeWo cells. These results confirm the hypothesis that GLUT3 is present in the syncytial microvillous membrane early in gestation and decreases thereafter, supporting the idea that GLUT3 is of greater importance for glucose uptake early in gestation.

Do glucose transporters have other roles in addition to placental glucose transport during early pregnancy?

Human placenta regulates the transport of maternal molecules to the fetus. It is known that glucose transport occurs via glucose transporters (GLUTs) in the feto-placental unit. Data on the expression of GLUTs during implantation are very scarce. Moreover, the question of how the decidual leukocytes obtain the energy for their activation during implantation mechanism is still under investigation. We studied the distributions of GLUT1, GLUT3, and GLUT4 in tissue sections of first trimester pregnancies the human maternal-fetal interface. GLUT1 was present in apical microvilli of the syncytiotrophoblast, in cytotrophoblast, and in vascular patterns of the villous core, whereas GLUT3 was localized in cytotrophoblasts of placental villi and in some fetal endothelial cells. Moreover, the proliferating cells of the proximal cell columns were also immunopositive for GLUT1 and GLUT3. We did not observe any positive immunoreactivity for GLUT4 in placental and decidual tissues. Essentially, GLUT3 and also to some extent GLUT1 was present in maternal leukocytes and platelets. In conclusion, our results suggest that the glucose taken up via GLUT1 and GLUT3 from the maternal circulation might not only be needed for placental functions but also for successful implantation by trophoblast invasion, proliferation and also by having a role to support energy for maternal leukocytes.

Growth and function of the normal human placenta

The placenta is the highly specialised organ of pregnancy that supports the normal growth and development of the fetus. Growth and function of the placenta are precisely regulated and coordinated to ensure the exchange of nutrients and waste products between the maternal and fetal circulatory systems operates at maximal efficiency. The main functional units of the placenta are the chorionic villi within which fetal blood is separated by only three or four cell layers (placental membrane) from maternal blood in the surrounding intervillous space. After implantation, trophoblast cells proliferate and differentiate along two pathways described as villous and extravillous. Non-migratory, villous cytotrophoblast cells fuse to form the multinucleated syncytiotrophoblast, which forms the outer epithelial layer of the chorionic villi. It is at the terminal branches of the chorionic villi that the majority of fetal/maternal exchange occurs. Extravillous trophoblast cells migrate into the decidua and remodel uterine arteries. This facilitates blood flow to the placenta via dilated, compliant vessels, unresponsive to maternal vasomotor control. The placenta acts to provide oxygen and nutrients to the fetus, whilst removing carbon dioxide and other waste products. It metabolises a number of substances and can release metabolic products into maternal and/or fetal circulations. The placenta can help to protect the fetus against certain xenobiotic molecules, infections and maternal diseases. In addition, it releases hormones into both the maternal and fetal circulations to affect pregnancy, metabolism, fetal growth, parturition and other functions. Many placental functional changes occur that accommodate the increasing metabolic demands of the developing fetus throughout gestation.

Multiple hexose transporters of Schizosaccharomyces pombe

We have identified a family of six hexose transporter genes (Ght1 to Ght6) in the fission yeast Schizosaccharomyces pombe. Sequence homology to Saccharomyces cerevisiae and mammalian hexose transporters (Hxtp and GLUTp, respectively) and secondary-structure predictions of 12 transmembrane domains for each of the Ght proteins place them into the sugar porter subfamily within the major facilitator superfamily. Interestingly, among this sugar porter family, the emerging S. pombe hexose transporter family clusters are separate from monosaccharide transporters of other yeasts (S. cerevisiae, Kluyveromyces lactis, and Candida albicans) and of humans, suggesting that these proteins form a distinct structural family of hexose transporters. Expression of the Ght1, Ght2, Ght5, and Ght6 genes in the S. cerevisiae mutant RE700A may functionally complement its D-glucose uptake-deficient phenotype. Northern blot analysis and reverse transcription-PCR showed that among all Ght's of S. pombe, Ght5 is the most prominently expressed hexose transporter. Ght1p, Ght2p, and Ght5p displayed significantly higher specificities for D-glucose than for D-fructose. Analysis of the previously described S. pombe D-glucose transport-deficient mutant YGS-5 revealed that this strain is defective in the Ght1, Ght5, and Ght6 genes. Based on an analysis of three S. pombe strains bearing single or double mutations in Ght3 and Ght4, we conclude that the Ght3p function is required for D-gluconate transport in S. pombe. The function of Ght4p remains to be clarified. Ght6p exhibited a slightly higher affinity to D-fructose than to D-glucose, and among the Ght's it is the transporter with the highest specificity for D-fructose.

Glucose transporters in the human placenta

The availability of antibodies and cDNA probes specific for the various members of the facilitated-diffusion glucose transporter (GLUT) family has enabled researchers to obtain a much clearer picture of the mechanisms for placental uptake and transplacental transport of glucose. This review examines studies of human placental glucose transport with the aim of providing a model which describes the transporter isoforms present in the placenta, their cellular localization and functional significance. The GLUT1 glucose transporter, present on both the microvillous and basal membranes of the syncytial barrier, is the primary isoform involved in the transplacental movement of glucose. Although GLUT3 mRNA is widely distributed, GLUT3 protein is localized to the arterial component of the vascular endothelium, where it may play a role in enhancing transplacental glucose transport. This data is in contrast to the situation in other mammalian species, such as the mouse, rat and sheep, where GLUT3 protein is not only present in those epithelial cells which carry out transplacental transport but becomes an increasingly prominent isoform as gestation progresses. The asymmetric distribution of GLUT1 in the human syncytiotrophoblast (microvillous>basal) means that basal GLUT1 acts as the rate limiting step in transplacental transfer. Changes in basal GLUT1 therefore have the potential to cause alterations in transplacental transport of glucose. Although there appear to be no changes in syncytial GLUT1 expression in intrauterine growth retardation, in diabetic pregnancies increases in basal GLUT1 expression and activity have been observed, with significant consequences for the maternal-fetal flux of glucose. Little is known of glucose transporter regulation in the placenta save for the effects of hyper- and hypoglycemia. GLUT1 expression and activity appear to be inversely related to extracellular glucose concentration, however within the physiological range, GLUT1 expression is relatively refractory to glucose concentration. Information is still needed on gestational development, on the expression and activity in well-defined conditions of intrauterine growth retardation, on the mechanisms and consequences of the changes observed in diabetic pregnancy and on the role of external agents other than glucose in regulating placental glucose transport.

Characterization of glucose transport and glucose transporters in the human choriocarcinoma cell line, BeWo

In this study we have characterized 2-deoxyglucose (2DG) transport and hexose transporter expression in the human choriocarcinoma cell line, BeWo. 2DG uptake in BeWo cells displayed saturable kinetics (V(max), 29+/-1.5 nmol/min/mg protein;K(m), 1.5+/-0.02 m m) and was significantly inhibited in the presence of 2-deoxyglucose, mannose and 3- O -methyl glucose (all at a competing concentration of 30 m m) by up to 97 per cent, but not by galactose or fructose. Glucose uptake was not Na(+)-dependent, but was inhibited by cytochalasin B (by approx 85 per cent) indicating that hexose uptake was mediated via a facilitative glucose transport mechanism. Northern and immunoblot analyses revealed that BeWo cells expressed GLUT1 and GLUT5, but not GLUT2 or GLUT3. On immunoblots, GLUT1 migrated as a broad protein band on SDS-gels (average M(r)of 55 kDa) and treatment with N -glycanase resulted in a significant shift in its electrophoretic mobility; the core protein migrating as a 40 kDa band indicating that the carrier was heavily glycosylated. GLUT5 was detected as a discrete 60 kDa band and like GLUT1, the observed immunoreactive signal was lost when using antiserum that had been pre-adsorbed with the antigenic peptide. Our findings indicate that BeWo cells express a facilitative glucose transport system with characteristics broadly similar to those reported in isolated human placental membrane vesicles and that they are likely to serve as a useful experimental system for studying the regulation of placental glucose transport and transporter expression.

Nutrient transport across the placenta

The placenta forms a selective barrier that functions to transport nutrients that are of critical use to the fetus. Nutrient transport across the placenta is regulated by many different active transporters found on the surface of both maternal and fetal facing membranes of the placenta. The presence of these transporters in the placenta has been implicated in the facilitation of nutrient diffusion and proper fetal growth. In this review, recent developments concerning nutrient transporters that regulate glucose, amino acid, fatty acid, and nucleoside transplacental movement are discussed.

Effect of development and hypoxic-ischemia upon rabbit brain glucose transporter expression

We have cloned and sequenced a full length rabbit GLUT 1 and partial rabbit GLUT 3 cDNAs. The derived rabbit GLUT 3 peptide revealed 84% homology to the mouse, 82% to the rat, human, dog, and sheep, and 69% to the chicken GLUT 3 peptides. Using Northern blot analysis, we investigated the tissue and brain cellular distribution of GLUT 1 and GLUT 3 expression. In addition, we examined the effect of development and hypoxic-ischemia upon brain GLUT 1 and GLUT 3 mRNA levels. While GLUT 1 mRNA was observed in most tissues, GLUT 3 was expressed predominantly in the brain, placenta, stomach, and lung with minor amounts in the heart, kidney and skeletal muscle. In the brain, both GLUT 1 and GLUT 3 were noted in neuron- and glial-enriched cultures. Both GLUT 1 and GLUT 3 mRNA levels demonstrated a similar developmental progression (p<0.05) secondary to post-transcriptional mechanisms. Further, while hypoxic-ischemia did not significantly affect brain GLUT 1 mRNA and protein, it altered GLUT 3 mRNA levels in a region-specific manner, with a three-fold increase in the cerebral cortex, a two-fold increase in the hippocampus, and a 50% increase in the caudate nucleus (p<0.05). We conclude, that the rabbit GLUT 3 peptide sequence exhibits 82-84% homology to that of other species in the coding region with a 62-89% sequence identity in the 3'-untranslated region. The tissue-specific expression of rabbit GLUT 3 mimics that of the human closely. Postnatal development and hypoxic-ischemia with reperfusion injury cause an increase in brain GLUT 3 expression, as a response to synaptogenesis and substrate deprivation, respectively.

Hyperglycemia regulates the glucose-transport system of clonal choriocarcinoma cells in vitro. A potential molecular mechanism contributing to the adjunct effect of glucose in tumor therapy

Glucose is taken up by tumor cells via sodium-independent facilitated diffusion along a concentration gradient. To examine the regulation of this process by substrate concentration, we investigated the effect of hyperglycemia on the glucose-transport system of choriocarcinoma-derived JAR and JEG-3 cells by culturing them for 24, 48 and 96 hr in medium containing either 5.5 (normoglycemia) or 25 (hyperglycemia) mM D-glucose, respectively. Immunocytochemically, choriocarcinoma cells expressed the high-affinity glucose transporter isoforms GLUT1 and GLUT3. Based on initial uptake measurements using 3-O-[14C]methyl-D-glucose, kinetic parameters were calculated as Km = 15 mM and Vmax = 95 fmol/sec per cell for JAR and Km = 9 mM and Vmax = 64 fmol/sec per cell for JEG-3 cells. In JAR cells cultured under hyperglycemic conditions, uptake rates were significantly increased at 15, 20 and 25 mM exogenous D-glucose concentrations as compared with normoglycemic conditions. This effect was due to an increase in Vmax, whereas Km remained unchanged. Using Northern blotting, GLUT1 mRNA levels were higher but GLUT3 transcripts were reduced upon hyperglycemia. Western blotting revealed elevated GLUT1 and GLUT3 expression under hyperglycemic conditions. Hyperglycemia did not significantly influence the glucose-transport system of JEG-3 cells. We conclude that sustained hyperglycemia stimulates the glucose-transport system of JAR, but not of JEG-3, choriocarcinoma cells in vitro due to changes in GLUT1 and GLUT3 expression levels. We speculate that this mechanism may contribute to the beneficial effects of induced hyperglycemia as an adjuvant in tumor therapy.

Glycaemic regulation of glucose transporter expression and activity in the human placenta

To determine whether the expression and activity of glucose transporters in human trophoblast are regulated by glucose, syncytiotrophoblast cells, choriocarcinoma cells, and villous fragments were incubated with a range of glucose concentrations (0-20 mM, 24 h). Expression of GLUT1 and GLUT3 glucose transporters was measured by immunoblotting, while glucose transporter activity was determined by [3H]2-deoxyglucose uptake in the cultured cells. GLUT1 expression in syncytial cells was enhanced following incubation in absence of glucose, reduced by incubation in 20 mM glucose but was not altered by incubation at 1 or 12 mM glucose. Transporter activity was inversely related to extracellular glucose over the entire range of concentrations tested (0-20 mM). Incubation of villous fragments in 20 mM glucose produced a limited suppression of GLUT1 expression, but no effects were noted following incubation at 0 or 1 mM glucose. Neither GLUT1 expression in JAr and JEG-3 choriocarcinoma cells nor transport activity in JEG-3 cells was affected by extracellular glucose concentration. Unlike syncytial cells, JAr, JEG-3 and BeWo all expressed GLUT3 protein in addition to GLUT1. These results show that while syncytiotrophoblast GLUT1 expression is altered at the extremes of extracellular glucose concentration, it is refractory to glucose alone at lower concentrations. By contrast, an inverse relationship exists between glucose transporter activity and extracellular glucose. This suggests that there are post-translational regulatory mechanisms which may respond to changes in extracellular glucose concentration.

Sustained hyperglycemia in vitro down-regulates the GLUT1 glucose transport system of cultured human term placental trophoblast: a mechanism to protect fetal development?

The trophoblast of human placenta is directly exposed to the maternal circulation. It forms the main barrier to maternal-fetal glucose transport. The present study investigated the effect of sustained hyperglycemia in vitro on the glucose transport system of these cells. Trophoblasts isolated from term placentas and immunopurified were cultured for 24, 48, and 96 h in DMEM containing either 5.5 (normoglycemia) or 25 mmol/l D-glucose (hyperglycemia), respectively. Initial uptake of glucose was measured using 3-O-[14C]methyl-D-glucose. Kinetic parameters were calculated as K(M) = 73 mmol/l and Vmax = 29 fmol s(-1) per trophoblast cell. Uptake rates of cells cultured under hyperglycemic conditions did not differ at exogenous D-glucose concentrations in the physiological range (1, 5.5, 10, and 15 mmol/l), but were significantly decreased by 25% (P<0.05) at diabetes-like concentrations (20 and 25 mmol/l) as compared to normoglycemic conditions. This effect was due to a decrease in Vmax (-50%), whereas K(M) remained virtually unaffected. GLUT1 mRNA levels were lower by 50% (P<0.05; Northern blotting) and GLUT1 protein was reduced by 16% (P<0.05; Western blotting) in trophoblast cells cultured under hyperglycemic vs. normoglycemic conditions. We conclude that prolonged hyperglycemia in vitro reduces trophoblast glucose uptake at substrate concentrations corresponding to blood levels of poorly controlled diabetic gravidas. This effect is due to diminished GLUT1 mRNA and protein expression in the trophoblast.

Transport mechanisms for vitamin C in the JAR human placental choriocarcinoma cell line

We investigated the transport pathways available for the uptake of vitamin C in the human placental choriocarcinoma cell line, JAR. These cells were found to possess the capacity to accumulate the vitamin when presented either in the oxidized form (dehydroascorbic acid) or in the reduced form (ascorbate). Dithiothreitol and 5,5'-dithiobis(2-nitrobenzoic acid) were used to maintain vitamin C as ascorbate and dehydroascorbic acid, respectively. The uptake of these two forms of vitamin C in JAR cells was found to occur by different mechanisms. The uptake of the dehydroascorbic acid was Na(+)-independent and was mediated by facilitative glucose transporters as evidenced from the inhibition of the uptake process by glucose. On the other hand, the uptake of ascorbate was Na(+)-dependent and was not sensitive to inhibition by glucose. Substitution of Na+ with other monovalent cations abolished the uptake of ascorbate completely. The uptake process was, however, not influenced by anions. Kinetic analysis indicated the presence of a single saturable transport system for ascorbate with a Michaelis-Menten constant of 22 +/- 1 microM. The dependence of the uptake rare of ascorbate on Na+ concentration exhibited sigmoidal kinetics, suggesting interaction of more than one Na+ ion with the transporter. The Hill coefficient for the Na+ interaction was 2, indicating that the Na(+)-dependent ascorbate transport is electrogenic. The Na(+)-dependent stimulation of ascorbate uptake was primarily due to an increase in the affinity of the transporter for ascorbate in the presence of Na+. It is concluded that the JAR placental trophoblast cell line expresses two different transport systems for vitamin C: one for the reduced form of the vitamin ascorbate; and the other for the oxidized form of the vitamin dehydroascorbic acid.