Expression of GLUT12 in the fetal membranes of the human placenta

AMPK regulates homeostasis of invasion and viability in trophoblasts by redirecting glucose metabolism: Implications for pre-eclampsia

Pre-eclampsia (PE) is deemed an ischemia-induced metabolic disorder of the placenta due to defective invasion of trophoblasts during placentation; thus, the driving role of metabolism in PE pathogenesis is largely ignored. Since trophoblasts undergo substantial glycolysis, this study aimed to investigate its function and regulatory mechanism by AMPK in PE development. Metabolomics analysis of PE placentas was performed by gas chromatography-mass spectrometry (GC-MS). Trophoblast-specific AMPKα1-deficient mouse placentas were generated to assess morphology. A mouse PE model was established by Reduced Uterine Perfusion Pressure, and placental AMPK was modulated by nanoparticle-delivered A769662. Trophoblast glucose uptake was measured by 2-NBDG and 2-deoxy-d-[³ H] glucose uptake assays. Cellular metabolism was investigated by the Seahorse assay and GC-MS.PE complicated trophoblasts are associated with AMPK hyperactivation due not to energy deficiency. Thereafter, AMPK activation during placentation exacerbated PE manifestations but alleviated cell death in the placenta. AMPK activation in trophoblasts contributed to GLUT3 translocation and subsequent glucose metabolism, which were redirected into gluconeogenesis, resulting in deposition of glycogen and accumulation of phosphoenolpyruvate; the latter enhanced viability but compromised trophoblast invasion. However, ablation of AMPK in the mouse placenta resulted in decreased glycogen deposition and structural malformation. These data reveal a novel homeostasis between invasiveness and viability in trophoblasts, which is mechanistically relevant for switching between the 'go' and 'grow' cellular programs.

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.

Role of Placental Glucose Transporters in Determining Fetal Growth

Maternal nutrient availability and its transport through the placenta are crucial for fetal development. Nutrients are transported to the fetus via specific transporters present on the microvillous (MVM) and basal membrane (BM) of the placenta. Glucose is the most abundant nutrient transferred to the fetus and plays a key role in the fetal growth and development. The transfer of glucose across the human placenta is directly proportional to maternal glucose concentrations, and is mediated by glucose transporter family proteins (GLUTs). Maternal glucose concentration influences expression and activity of GLUTs in the MVM (glucose uptake) and BM (glucose delivery). Alteration in the number and function of these transporters may affect the growth and body composition of the fetus. The thin-fat phenotype of the Indian baby (low ponderal index, high adiposity) is proposed as a harbinger of future metabolic risk. We propose that placental function mediated through nutrient transporters contributes to the phenotype of the baby, specifically that glucose transporters will influence neonatal fat. This review discusses the role of various glucose transporters in the placenta in determining fetal growth and body composition, in light of the above hypothesis.

Steroids Regulate SLC2A1 and SLC2A3 to Deliver Glucose Into Trophectoderm for Metabolism via Glycolysis

The conceptuses (embryo/fetus and placental membranes) of pigs require energy to support elongation and implantation, and amounts of glucose and fructose increase in the uterine lumen during the peri-implantation period. Conceptuses from day 16 of pregnancy were incubated with either 14C-glucose or 14C-fructose and amounts of radiolabeled CO2 released from the conceptuses measured to determine rates of oxidation of glucose and fructose. Glucose and fructose both transport into conceptuses, and glucose is preferentially metabolized in the presence of fructose, whereas fructose is actively metabolized in the absence of glucose and to a lesser extent in the presence of glucose. Endometrial and placental expression of glucose transporters SLC2A1, SLC2A2, SCL2A3, and SLC2A4 were determined. SLC2A1 messenger RNA (mRNA) and protein, and SLC2A4 mRNA were abundant in the uterine luminal epithelium of pregnant compared to cycling gilts, and increased in response to progesterone and conceptus-secreted estrogen. SLC2A2 mRNA was expressed weakly by conceptus trophectoderm on day 15 of pregnancy, whereas SLC2A3 mRNA was abundant in trophectoderm/chorion throughout pregnancy. Therefore, glucose can be transported into the uterine lumen by SLC2A1, and then into conceptuses by SLC2A3. On day 60 of gestation, the cell-specific expression of these transporters was more complex, suggesting that glucose and fructose transporters are precisely regulated in a spatial-temporal pattern along the uterine-placental interface of pigs to maximize hexose sugar transport to the pig conceptus/placenta.

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.

Could GLUT12 be a Potential Therapeutic Target in Cancer Treatment? A Preliminary Report

BACKGROUND

Recent studies proposed GLUT12 to be a major glucose transporter involved in the glycolytic metabolism of cancer cells.

METHODS

GLUT12 expression was determined by immunohistochemistry in a selection of cancer cell lines and a tumour spheroid model.

RESULTS

GLUT12 expression was high in A549 and RH-36; low in HT29; and absent in NB-EB cancer cell lines. GLUT12 expression was located in the necrotic centre of HT29 spheroids, which is characterised by anaerobic metabolism.

CONCLUSION

The data supports the involvement of GLUT12 in the glycolytic metabolism of cancer cells and therefore, its potential as a novel therapeutic target for cancer treatment.

Hepatic expression and cellular distribution of the glucose transporter family

Glucose and other carbohydrates are transported into cells using members of a family of integral membrane glucose transporter (GLUT) molecules. To date 14 members of this family, also called the solute carrier 2A proteins have been identified which are divided on the basis of transport characteristics and sequence similarities into several families (Classes 1 to 3). The expression of these different receptor subtypes varies between different species, tissues and cellular subtypes and each has differential sensitivities to stimuli such as insulin. The liver is a contributor to metabolic carbohydrate homeostasis and is a major site for synthesis, storage and redistribution of carbohydrates. Situations in which the balance of glucose homeostasis is upset such as diabetes or the metabolic syndrome can lead metabolic disturbances that drive chronic organ damage and failure, confirming the importance of understanding the molecular regulation of hepatic glucose homeostasis. There is a considerable literature describing the expression and function of receptors that regulate glucose uptake and release by hepatocytes, the most import cells in glucose regulation and glycogen storage. However there is less appreciation of the roles of GLUTs expressed by non parenchymal cell types within the liver, all of which require carbohydrate to function. A better understanding of the detailed cellular distribution of GLUTs in human liver tissue may shed light on mechanisms underlying disease pathogenesis. This review summarises the available literature on hepatocellular expression of GLUTs in health and disease and highlights areas where further investigation is required.

Differential regulation of glucose transporters mediated by CRH receptor type 1 and type 2 in human placental trophoblasts

Glucose transport across the placenta is mediated by glucose transporters (GLUT), which is critical for normal development and survival of the fetus. Regulatory mechanisms of GLUT in placenta have not been elucidated. Placental CRH has been implicated to play a key role in the control of fetal growth and development. We hypothesized that CRH, produced locally in placenta, could act to modulate GLUT in placenta. To investigate this, we obtained human placentas from uncomplicated term pregnancies and isolated and cultured trophoblast cells. GLUT1 and GLUT3 expressions in placenta were determined, and effects of CRH on GLUT1 and GLUT3 were examined. GLUT1 and GLUT3 were identified in placental villous syncytiotrophoblasts and the endothelium of vessels. Treatment of cultured placental trophoblasts with CRH resulted in an increase in GLUT1 expression while a decrease in GLUT3 expression in a dose-dependent manner. Cells treated with either CRH antibody or nonselective CRH receptor (CRH-R) antagonist astressin showed a decrease in GLUT1 and an increase in GLUT3 expression. CRH-R1 antagonist antalarmin decreased GLUT1 expression while increased GLUT3 expression. CRH-R2 antagonist astressin2b increased the expression of both GLUT1 and GLUT3. Knockdown of CRH-R1 decreased GLUT1 expression while increased GLUT3 expression. CRH-R2 knockdown caused an increase in both GLUT1 and GLUT3 expression. Our data suggest that, in placenta, CRH produced locally regulates GLUT1 and GLUT3 expression, CRHR1 and CRHR2-mediated differential regulation of GLUT1 and GLUT3 expression. Placental CRH may regulate the growth of fetus and placenta by modulating the expression of GLUT in placenta during pregnancy.

Contribution of aquaporin 9 and multidrug resistance-associated protein 2 to differential sensitivity to arsenite between primary cultured chorion and amnion cells prepared from human fetal membranes

Arsenic trioxide (arsenite, As(III)) has shown a remarkable clinical efficacy, whereas its side effects are still a serious concern. Therefore, it is critical to understand the effects of As(III) on human-derived normal cells for revealing the mechanisms underlying these side effects. We examined the effects of As(III) on primary cultured chorion (C) and amnion (A) cells prepared from human fetal membranes. A significant dose-dependent As(III)-mediated cytotoxicity was observed in the C-cells accompanied with an increase of lactate dehydrogenase (LDH) release. Higher concentrations of As(III) were required for the A-cells to show cytotoxicity and LDH release, suggesting that the C-cells were more sensitive to As(III) than the A-cells. The expression levels of aquaporin 9 (AQP9) were approximately 2 times higher in the C-cells than those in the A-cells. Both intracellular arsenic accumulation and its cytotoxicity in the C-cells were significantly abrogated by sorbitol, a competitive AQP9 inhibitor, in a dose-dependent manner. The protein expression levels of multidrug resistance-associated protein (MRP) 2 were downregulated by As(III) in the C-cells, but not in the A-cells. No significant differences in the expression levels of MRP1 were observed between C- and A-cells. The protein expression of P-glycoprotein (P-gp) was hardly detected in both cells, although a detectable amount of its mRNA was observed. Cyclosporine A, a broad-spectrum inhibitor for ABC transporters, and MK571, a MRP inhibitor, but not PGP-4008, a P-gp specific inhibitor, potently sensitized both cells to As(III)-mediated cytotoxicity. These results suggest that AQP9 and MRP2 are involved in controlling arsenic accumulation in these normal cells, which then contribute to differential sensitivity to As(III) cytotoxicity between these cells.

Asymmetric syncytial expression of GLUT9 splice variants in human term placenta and alterations in diabetic pregnancies

Glucose transport from the maternal to fetal side of the placenta is critical for fetal growth and development due to the absence of fetal gluconeogenesis. Human GLUT9, existing as 2 isoforms, is a novel member of the transporter family. This study investigated the localization and relative expression levels of these isoforms in the human term placenta from both control and diabetic patients. Placenta samples were collected from normal pregnancies and those complicated by maternal diabetes (White classifications A1, A2, and B). Antibodies specific for the different isoforms were used to detect expression. Both forms of the protein are expressed in syncytiotrophoblast cells. Subcellular fractionation revealed an asymmetrical expression pattern with GLUT9a on basal membranes, whereas GLUT9b localizes to microvillus membranes. Expression of both isoforms is significantly increased in placental tissue from diabetic pregnancies. Altered expression of GLUT9 in the placenta may play a role in the fetal pathophysiology associated with diabetes-complicated pregnancies.

Potential role of sugar transporters in cancer and their relationship with anticancer therapy

Sugars, primarily glucose and fructose, are the main energy source of cells. Because of their hydrophilic nature, cells use a number of transporter proteins to introduce sugars through their plasma membrane. Cancer cells are well known to display an enhanced sugar uptake and consumption. In fact, sugar transporters are deregulated in cancer cells so they incorporate higher amounts of sugar than normal cells. In this paper, we compile the most significant data available about biochemical and biological properties of sugar transporters in normal tissues and we review the available information about sugar carrier expression in different types of cancer. Moreover, we describe the possible pharmacological interactions between drugs currently used in anticancer therapy and the expression or function of facilitative sugar transporters. Finally, we also go into the insights about the future design of drugs targeted against sugar utilization in cancer cells.

Molecular and metabolic retinoid pathways in human amniotic membranes

Vitamin A (retinol) and its active derivatives (the retinoids) are essential for the growth and development of the mammalian fetus and placenta. The amniotic membranes are extra-embryonic structures that are indispensable for normal gestation in mammals. Although placental involvement of retinoids is clearly established, little is known about the roles of retinoids for the associated amniotic membranes. The aim of this study was to define the metabolic and molecular pathways of retinoic signaling in human fetal membranes. The expression of retinoid receptors (RARalpha, beta and RXRalpha, beta) was established at transcript and protein levels. Enzymes involved in retinoic acid generation were also detected. The enzymatic generation of functional retinoids was confirmed using specific inhibitors of retinol metabolism. Finally, the functionality of retinoid pathways was demonstrated by inducing established retinoid target gene expression. Our results clearly demonstrated that the molecular and metabolic actors of retinoic signaling pathways are functional in human fetal membranes.