Localisation of the high affinity facilitative glucose transporter protein GLUT 1 in the placenta of human, marmoset monkey (Callithrix jacchus) and rat at different developmental stages

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.

Transplacental Nutrient Transport Mechanisms of Intrauterine Growth Restriction in Rodent Models and Humans

Although the causes of intrauterine growth restriction (IUGR) have been intensively investigated, important information is still lacking about the role of the placenta as a link from adverse maternal environment to adverse pregnancy outcomes of IUGR and preterm birth. IUGR is associated with an increased risk of cardiovascular, metabolic, and neurological diseases later in life. Determination of the most important pathways that regulate transplacental transport systems is necessary for identifying marker genes as diagnostic tools and for developing drugs that target the molecular pathways. Besides oxygen, the main nutrients required for appropriate fetal development and growth are glucose, amino acids, and fatty acids. Dysfunction in transplacental transport is caused by impairments in both placental morphology and blood flow, as well as by factors such as alterations in the expression of insulin-like growth factors and changes in the mTOR signaling pathway leading to a change in nutrient transport. Animal models are important tools for systematically studying such complex events. Debate centers on whether the rodent placenta is an appropriate tool for investigating the alterations in the human placenta that result in IUGR. This review provides an overview of the alterations in expression and activity of nutrient transporters and alterations in signaling associated with IUGR and compares these findings in rodents and humans. In general, the data obtained by studies of the various types of rodent and human nutrient transporters are similar. However, direct comparison is complicated by the fact that the results of such studies are controversial even within the same species, making the interpretation of the results challenging. This difficulty could be due to the absence of guidelines of the experimental design and, especially in humans, the use of trophoblast cell culture studies instead of clinical trials. Nonetheless, developing new therapy concepts for IUGR will require the use of animal models for gathering robust data about mechanisms leading to IUGR and for testing the effectiveness and safety of the intervention among pregnant women.

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.