[Facilitative glucose transporters: expression, distribution and the relationship to diseases]

Facilitative glucose transporters (GLUT) are proteins that mediate glucose transmembrane transport in the form of facilitated diffusion, which play an important role in regulating cell energy metabolism. There are many breakthroughs in researches of facilitative GLUT in recent years. It has been known that there are 14 subtypes of facilitative GLUT with obvious tissue specificity in distribution and physiological function. In the present review, the tissue and cellular distribution, subcellular localization, expression regulation, physiological function and the relationship to diseases of facilitative GLUT subtypes were summarized, in order to further understand their physiological and pathophysiological significances.

[Regulation mechanism of HIFs, PPARs and AMPK in hypoxic training-induced reduction of body weight]

Hypoxic exposure activates hypoxia inducible factors (HIFs) to up-regulate the expression of its target genes. These genes encode glucose metabolism related proteins, such as glucose transporters (GLUTs) and glycolysis related enzymes, including lactate dehydrogenase A (LDHA) and aldolase A (ALDA). Therefore, HIFs participate in oxygenolysis of glucose and play an important role in mediating hypoxia response and weight loss. Exercise training influences fatty acid metabolism, insulin sensitivity and body energy balance through activating peroxisome proliferator-activated receptors (PPARs), which plays an active role in losing weight. In addition, hypoxic exposure or exercise training can activate energy sensor 5'-AMP activated protein kinase (AMPK) in cells and promote oxidation of glucose and fatty acid and weight loss. It has been shown that hypoxic training exerts a better effects on controlling weight, compared with either hypoxic exposure or exercise training alone. This paper reviewed synergistic interactions among HIFs, PPARs and AMPK under hypoxic training and proposed possible mechanisms of hypoxic training-induced weight loss via AMPK-HIFs axis or AMPK-PPARs axis, thus providing theoretical guidance for application of hypoxic training in weight control.

DKB114, A Mixture of Chrysanthemum Indicum Linne Flower and Cinnamomum Cassia (L.) J. Presl Bark Extracts, Improves Hyperuricemia through Inhibition of Xanthine Oxidase Activity and Increasing Urine Excretion

Chrysanthemum indicum Linne flower (CF) and Cinnamomum cassia (L.) J. Presl bark (CB) extracts have been used as the main ingredients in several prescriptions to treat the hyperuricemia and gout in traditional medicine. In the present study, we investigated the antihyperuricemic effects of DKB114, a CF, and CB mixture, and the underlying mechanisms in vitro and in vivo. DKB114 markedly reduced serum uric acid levels in normal rats and rats with PO-induced hyperuricemia, while increasing renal uric acid excretion. Furthermore, it inhibited the activity of xanthine oxidase (XOD) in vitro and in the liver in addition to reducing hepatic uric acid production. DKB114 decreased cellular uric acid uptake in oocytes and HEK293 cells expressing human urate transporter (hURAT)1 and decreased the protein expression levels of urate transporters, URAT1, and glucose transporter, GLUT9, associated with the reabsorption of uric acid in the kidney. DKB114 exerts antihyperuricemic effects and uricosuric effects, which are accompanied, partially, by a reduction in the production of uric acid and promotion of uric acid excretion via the inhibition of XOD activity and reabsorption of uric acid. Therefore, it may have potential as a treatment for hyperuricemia and gout.

[Recent advances in urate metabolism]

In the last fifteen years, genomics and other -omics sciences have revolutionized our understanding of biological processes at the molecular level. An illustrative example is urate metabolism. Before the publication of the complete human genome, in 2003 it was believed that a single enzyme (urate oxidase) was responsible for uricolysis that is the conversion of urate into the more soluble allantoin. Now we know with great detail that this process requires the consecutive action of three enzymes that have been lost by gene inactivation in our hominoid ancestor. Similarly, a single urate transporter (URAT1) was known at that time. Now we have evidence that urate homeostasis depends on a complex set of transporters located on the epithelial cells of the kidney and the intestine. In this review article, we give an account of the recent discoveries on urate metabolism and how these discoveries can be applied to the development of novel drugs to treat hyperuricemia, tumor lysis syndrome and the Lesch-Nyhan disease.

The Expression of GLUT8, GLUT9a, and GLUT9b in the Mouse Testis and Sperm

The objective of this study is to confirm the precise glucose transporter (GLUT) 8 localization and determine the expression of GLUT9a and GLUT9b by Western blot and confocal and immunoelectron microscopy in the mouse testis and sperm. GLUT8, GLUT9a, and GLUT9b proteins are expressed in the most intraseminiferous tubula cells and Leydig cells. GLUT8 localizes in the midpiece and principal piece as well as in the acrosomal region of the sperm. Immunoelectron microscopic analysis shows that GLUT8 is strongly detectable at the acrosome and neck region of the sperm. In the midpiece, GLUT8 localizes at the outer dense fibers (odf) as well as at the circumference of the spiral mitochondria. In the principal piece, GLUT8 localizes at the odf. GLUT9a strictly localizes in the midpiece, but GLUT9b localizes in the acrosome, midpiece, and principal piece of the sperm. These results suggest that glucose uptake via GLUT8, GLUT9a, and GLUT9b likely affects normal spermatogenesis, steroidogenesis, and sperm function in the mouse.

Facilitative glucose transporters: Implications for cancer detection, prognosis and treatment

It is long recognized that cancer cells display increased glucose uptake and metabolism. In a rate-limiting step for glucose metabolism, the glucose transporter (GLUT) proteins facilitate glucose uptake across the plasma membrane. Fourteen members of the GLUT protein family have been identified in humans. This review describes the major characteristics of each member of the GLUT family and highlights evidence of abnormal expression in tumors and cancer cells. The regulation of GLUTs by key proliferation and pro-survival pathways including the phosphatidylinositol 3-kinase (PI3K)-Akt, hypoxia-inducible factor-1 (HIF-1), Ras, c-Myc and p53 pathways is discussed. The clinical utility of GLUT expression in cancer has been recognized and evidence regarding the use of GLUTs as prognostic or predictive biomarkers is presented. GLUTs represent attractive targets for cancer therapy and this review summarizes recent studies in which GLUT1, GLUT3, GLUT5 and others are inhibited to decrease cancer growth.

Cancer metabolism as a therapeutic target

Cancer is now recognized to be a disease arising from both genetic and metabolic abnormalities. In the mid-1900s, Otto Warburg described the phenomenon of elevated glucose consumption and aerobic glycolysis, and the dependence of cancer cells on this phenomenon for proliferation and growth. The Warburg effect has formed the basis of such diagnostic and prognostic imaging modalities as positron emission tomography (PET); however, we have not yet capitalized on this phenomenon for therapy. Several mechanisms have now been shown to contribute to the Warburg effect.Ongoing studies are attempting to understand the reasons that tumor cells engage in aerobic glycolysis in lieu of oxidative phosphorylation, and the advantages that accrue to them as a result. In this review, we discuss known benefits to tumor cells from this metabolic switch, and we highlight key enzymes that play a role in aerobic glycolysis. We also describe novel therapeutic options targeting glucose metabolism and the importance of continuing to understand the metabolic plasticity of cancer.

The expression of particular glucose transporters and insulin resistance indicators in the risk groups of type 2 diabetes--a two-year follow-up

INTRODUCTION

The proper expression of particular glucotransporter (GLUT) isoforms determines a sufficient supply of glucose to tissues. The impairment of cellular glucose transport observed in insulin resistance leads to glucose metabolism disturbances. The aim of this study was the estimation of insulin resistance indicators and the quantitative expression of GLUT-1, GLUT-3 and GLUT-4 on peripheral blood lymphocytes in prediabetic subjects and persons with a positive family history of type 2 diabetes during 24 months of observation.

MATERIAL AND METHODS

The study included 25 prediabetic subjects (according to WHO criteria) and 24 normoglycaemic individuals with a positive family history of type 2 diabetes. Twenty three healthy subjects with no family history of type 2 diabetes, matched with BMI, served as a control group. All participants were recommended to perform physical activity for at least 140 minutes per week and to maintain a low calorie diet. The peripheral blood lymphocytes demonstrating expression of GLUT-1, GLUT-3 and GLUT-4 were labelled with the use of indirect immunofluorescence. The expression of GLUT isoforms was investigated by flow cytometry. Cells were stained by using anti-human GLUT antibodies and FITC-conjugated immunoglobulin. Flow cytometry was performed using a FACS Calibur (Becton-Dickinson). Additionally, we determined: fasting plasma glucose (FPG), insulin and C peptide concentrations, HOMA-IR, BMI and WHR. All the tests were performed at baseline, and after 12 and 24 months.

RESULTS

At baseline, prediabetics and subjects with a positive family history of type 2 diabetes were characterised by a much higher expression of GLUT-4 compared to control subjects. Twenty four months of lifestyle modification resulted in significant lowering of the expression of GLUT-4 on the surface of PBL in both studied groups, with no differences in the expression of GLUT-1 or GLUT-3. Both prediabetic subjects and individuals with a positive family history of type 2 diabetes revealed no significant differences in determined insulin resistance markers after 24 months of the observation compared to the baseline values.

CONCLUSIONS

The estimation of typical GLUT isoforms present on the peripheral blood lymphocytes, as well as the evaluation of insulin resistance indicators, are obviously insufficient for monitoring the metabolic disorders progression in the risk groups of type 2 diabetes. The decrease in GLUT-4 lymphocyte expression may reflect a positive influence of lifestyle modification on a tissue redistribution of this crucial insulin-dependent glucotransporter. The determination of GLUT-4 on the surface of peripheral blood lymphocytes can be a useful tool for the evaluation of the efficacy of therapeutic actions in subjects at high risk of type 2 diabetes.

Quantitative analysis of glucose transporter mRNAs in endometrial stromal cells reveals critical role of GLUT1 in uterine receptivity

Recurrent miscarriages affect about 1-2% of couples trying to conceive; however, mechanisms leading to this complication are largely unknown. Most studies focus on the early embryo, but proper development and implantation of the blastocyst are also dependent on optimal endometrial progression into a receptive state. One of the key steps in the uterine preparation for embryo receptivity, known as decidualization, is the differentiation of endometrial stromal cells (ESCs) into decidual cells. During this transition, the ESCs undergo a drastic change in glucose metabolism. The efficiency of glucose uptake is determined by a family of facilitative glucose transporters (GLUTs), and many have been identified in the stroma. The primary focus of this work was to quantify the absolute amount of GLUT mRNAs in this cell type before and after decidualization. We used primary ESCs isolated from murine and human uteri. We developed and validated cDNA-based calibration curves for each GLUT and used these primers to arrive at absolute mRNA copy numbers. Here, we report all the GLUT mRNAs that are present in the ESCs and their abundance under both conditions, control and decidualized. GLUT1 mRNA is the most abundant and critical transporter in ESCs of both species, because knocking down this GLUT with sort hairpin RNA leads to dramatically reduced decidualization. These findings suggest that GLUT1 mRNA expression is essential for decidualization and we are the first to determine a possible mechanism to explain how maternal conditions of abnormal glucose utilization may impair implantation at the level of the ESCs.

Aquaporins in spermatozoa and testicular germ cells: identification and potential role

Mammalian spermatozoa have relatively high water permeability and swell readily, as in the hypo-osmotic swelling test used in the andrology clinic. Physiologically, spermatozoa experience changes in the osmolality of the surrounding fluids in both the male and the female tracts on their journey from the testis to the ovum. Sperm volume regulation in response to such osmotic challenges is important to maintain a stable cell size for the normal shape and function of the sperm tail. Alongside ion channels for the fluxes of osmolytes, water channels would be crucial for sperm volume regulation. In contrast to the deep knowledge and numerous studies on somatic cell aquaporins (AQPs), the understanding of sperm AQPs is limited. Among the 13 AQPs, convincing evidence for their presence in spermatozoa has been confined to AQP7, AQP8 and AQP11. Overall, current findings indicate a major role of AQP8 in water influx and efflux for sperm volume regulation, which is required for natural fertilization. The preliminary data suggestive of a role for AQP7 in sperm glycerol metabolism needs further substantiation. The association of AQP11 with the residual cytoplasm of elongated spermatids and the distal tail of spermatozoa supports the hypothesis of more than just a role in conferring water permeability and also in the turnover and recycling of surplus cellular components made redundant during spermiogenesis and spermiation. This would be crucial for the maintenance of a germinal epithelium functioning efficiently in the production of spermatozoa.

Glucose-transporter-mediated positive inotropic effects in human myocardium of diabetic and nondiabetic patients

Insulin causes inotropic effects via Ca(2+)-dependent and Ca(2+)-independent pathways. The latter one is potentially glucose dependent. We examined inotropic responses and signal transduction of insulin in human atrial myocardium of diabetic and nondiabetic patients to test for the role of glucose transporters. Experiments were performed in isolated atrial myocardium of 88 patients undergoing cardiac surgery and 28 ventricular muscle samples of explanted hearts. Influence of insulin (0.02 micromol/L) on isometric twitch force was examined with and without blocking glucose transporter (GLUT) 4 translocation (latrunculin), sodium-coupled glucose transporter (SGLT) 1 (phlorizin, T-1095A), or PI3-kinase (wortmannin). Experiments were performed in Tyrode solution containing glucose or pyruvate as energetic substrate. Messenger RNA expression of glucose transporters (GLUT1, GLUT4, SGLT1, SGLT2) was analyzed in atrial and ventricular myocardium of both diabetic and nondiabetic patients. Developed force increases after insulin (to 117.8% +/- 2.4% and 115.8% +/- 1.9%) in trabeculae from patients with and without diabetes. Inotropic effect was reduced after displacing glucose with pyruvate as well as after PI3-kinase inhibition (to 103% +/- 2%) or inhibition of glucose transporters GLUT4 (to 105% +/- 2%) and SGLT1 (phlorizin to 106% +/- 2%, T-1095A to 105% +/- 2%), without differences between the 2 groups. In glucose-free pyruvate-containing solution, only inhibition of PI3-kinase but not blocking glucose transporters resulted in further inhibitory effects. Messenger RNA expression did not show significant differences between patients with or without diabetes. Insulin exerts positive inotropic effects in human atrial myocardium. These effects are mediated via a PI3-kinase-sensitive and a glucose-transport-sensitive pathway. Differences in functional effects or messenger RNA expression of glucose transporters were not detectable between patients with and without diabetes.

Facilitative glucose transporter 9, a unique hexose and urate transporter

GLUT9 is a novel, facilitative glucose transporter isoform that exists as two alternative splice variants encoding two proteins that differ in their NH(2)-terminal sequence (GLUT9a and GLUT9b). Both forms of GLUT9 protein and mRNA are expressed in the epithelia of various tissues; however, the two splice variants are expressed differentially within polarized cells, with GLUT9a localized predominantly on the basolateral surfaces and GLUT9b expressed on apical surfaces. Protein expression of GLUT9 drops under conditions of starvation but increases with addition of glucose and under hyperglycemic conditions. The substrate specificity of GLUT9 is unique since, in addition to transporting hexose sugars, it also is a high-capacity uric acid transporter. Several recent large-scale human genetic studies show a correlation between SNPs mapped to GLUT9 and the serum uric acid levels in several different cohorts. The relationship between GLUT9 and uric acid is highly clinically significant. Elevated uric acid levels have been associated with metabolic syndrome, obesity, diabetes, hypertension, and chronic renal failure. Although some believe uric acid is elevated as a result of these diseases, there is now evidence that uric acid may play a role in the pathogenesis of these diseases. It is also known that GLUT9 is expressed in articular cartilage and is a uric acid transporter, and thus it is possible that GLUT9 plays a role in gout, a disease of uric acid deposition in the joints. In addition, some studies have suggested that intake of fructose plays an important role in causing elevated serum uric acid levels, especially in diabetes and obesity. It is possible that GLUT9, which seems to be both a fructose and a uric acid transporter, plays an important role in these conditions associated with hyperuricemia.

GLUT8, the enigmatic intracellular hexose transporter

GLUT8 is a class III sugar transporter predominantly expressed in testis and brain. In contrast to the class I and class II transporters, hydrophobicity plots predict a short extracellular loop between transmembrane domain (TM)1 and TM2 and a long extracellular loop between TM9 and TM10 that contains the only N-glycosylation site. In vitro translated GLUT8 migrates as a 35-kDa protein that is glycosylated in the presence of microsomal membranes. In heterologous expression systems, glucose transport activity (Km of 2 mM) was inhibited by fructose and galactose. The transporter carries an NH2-terminal endosomal/lysosomal targeting motif ([DE]XXXL[LI]). Accordingly, constitutive GLUT8 has been found to be associated with endosomes and lysosomes but also with membranes of the endoplasmic reticulum. A similar distribution was detected after overexpression of wild-type or tagged GLUT8 in different cell systems. In these cells, none of the conventional signals tested induced a translocation of GLUT8 to the plasma membrane. Therefore, GLUT8 appears to catalyze transport of sugars or sugar derivatives through intracellular membranes. Slc2a8 knockout mice were viable, developed normally, and showed mild alterations in brain (increased proliferation of neuronal cells in dentate gyrus of the hippocampus, hyperactivity), heart (impaired transmission of electrical wave through the atrium), and sperm cells (reduced number of motile sperm cells associated with reduced mitochondrial membrane potential and ATP levels in sperm). The links between molecular function, cellular localization and phenotype of the knockout mouse is unclear and remains to be determined.

A highly conserved hydrophobic motif in the exofacial vestibule of fructose transporting SLC2A proteins acts as a critical determinant of their substrate selectivity

The substrate specificity of the facilitated hexose transporter, GLUT, family, (gene SLC2A) is highly varied. Some appear to be able to translocate both glucose and fructose, while the ability to handle 2-deoxyglucose and galactose does not necessarily correlate with the other two hexoses. It has become generally accepted that a central substrate binding/translocation site determines which hexoses can be transported. However, a recent study showed that a single point mutation of a hydrophobic residue in GLUTs 2, 5 & 7 removed their ability to transport fructose without affecting the kinetics of glucose permeation. This residue is in the 7th transmembrane helix, facing the aqueous pore and lies close to the opening of the exofacial vestibule. This study expands these observations to include the other class II GLUTs (9 & 11) and shows that a three amino acid motif (NXI/NXV) appears to be critical in determining if fructose can access the translocation mechanism. GLUT11 can also transport fructose, but it has the motif DSV at the same position, which appears to function in the same manner as NXI and when all three residues are replaced with NAV fructose transport lost. These results are discussed in relation to possible roles for hydrophobic residues lining the aqueous pore at the opening of the exofacial vestibule. Finally, the possibility that the translocation binding site may not be the sole determinant of substrate specificity for these proteins is examined.

Glucose uptake and growth of glucose-limited chemostat cultures of Aspergillus niger and a disruptant lacking MstA, a high-affinity glucose transporter

This is a study of high-affinity glucose uptake in Aspergillus niger and the effect of disruption of a high-affinity monosaccharide-transporter gene, mstA. The substrate saturation constant (K(s)) of a reference strain was about 15 microM in glucose-limited chemostat culture. Disruption of mstA resulted in a two- to fivefold reduction in affinity for glucose and led to expression of a low-affinity glucose transport gene, mstC, at high dilution rate. The effect of mstA disruption was more subtle at low and intermediate dilution rates, pointing to some degree of functional redundancy in the high-affinity uptake system of A. niger. The mstA disruptant and a reference strain were cultivated in glucose-limited chemostat cultures at low, intermediate and high dilution rate (D=0.07 h(-1), 0.14 h(-1) and 0.20 h(-1)). Mycelium harvested from steady-state cultures was subjected to glucose uptake assays, and analysed for expression of mstA and two other transporter genes, mstC and mstF. The capacity for glucose uptake (v(max)) of both strains was significantly reduced at low dilution rate. The glucose uptake assays revealed complex uptake kinetics. This impeded accurate determination of maximum specific uptake rates (v(max)) and apparent affinity constants ( ) at intermediate and high dilution rate. Two high-affinity glucose transporter genes, mstA and mstF, were expressed at all three dilution rates in chemostat cultures, in contrast to batch culture, where only mstC was expressed. Expression patterns of the three transporter genes suggested differential regulation and functionality of their products.

Hypoxic regulation of glucose transport, anaerobic metabolism and angiogenesis in cancer: novel pathways and targets for anticancer therapeutics

Cancer cells require a steady source of metabolic energy in order to continue their uncontrolled growth and proliferation. Accelerated glycolysis is one of the biochemical characteristics of cancer cells. Recent work indicates that glucose transport and metabolism are essential for the posttreatment survival of tumor cells, leading to poor prognosis. Glycolytic breakdown of glucose is preceded by the transport of glucose across the cell membrane, a rate-limiting process mediated by facilitative glucose transporter proteins belonging to the facilitative glucose transporter/solute carrier GLUT/SLC2A family. Tumors frequently show overexpression of GLUTs, especially the hypoxia-responsive GLUT1 and GLUT3 proteins. There are also studies that have reported associations between GLUT expression and proliferative indices, whilst others suggest that GLUT expression may be of prognostic significance. In this article we revisit Warburg's original hypothesis and review the recent clinical and basic research on the expression of GLUT family members in human cancers and in cell lines derived from human tumors. We also explore the links between hypoxia-induced genes, glucose transporters and angiogenic factors. Hypoxic tumors are significantly more malignant, metastatic, radio- and chemoresistant and have a poor prognosis. With the discovery the oxygen-sensitive transcription factor hypoxia-inducible factor (HIF-1) has come a new understanding of the molecular link between hypoxia and deregulated glucose metabolism. HIF-1 induces a number of genes integral to angiogenesis, e.g. vascular endothelial growth factor (VEGF), a process intimately involved with metastatic spread. This knowledge may enhance existing chemotherapeutic strategies so that treatment can be more rationally applied and personalized for cancer patients.

Identification and functional expression of the Arabidopsis thaliana vacuolar glucose transporter 1 and its role in seed germination and flowering

Sugar compartmentation into vacuoles of higher plants is a very important physiological process, providing extra space for transient and long-term sugar storage and contributing to the osmoregulation of cell turgor and shape. Despite the long-standing knowledge of this subcellular sugar partitioning, the proteins responsible for these transport steps have remained unknown. We have identified a gene family in Arabidopsis consisting of three members homologous to known sugar transporters. One member of this family, Arabidopsis thaliana vacuolar glucose transporter 1 (AtVGT1), was localized to the vacuolar membrane. Moreover, we provide evidence for transport activity of a tonoplast sugar transporter based on its functional expression in bakers' yeast and uptake studies in isolated yeast vacuoles. Analyses of Atvgt1 mutant lines indicate an important function of this vacuolar glucose transporter during developmental processes like seed germination and flowering.

Brain antioxidant capacity in rat models of betacytotoxic-induced experimental sporadic Alzheimer's disease and diabetes mellitus

It is believed that oxidative stress plays a central role in the pathogenesis of metabolic diseases like diabetes mellitus (DM) and its complications (like peripheral neuropathy) as well as in neurodegenerative disorders like sporadic Alzheimer's disease (sAD). Representative experimental models of these diseases are streptozotocin (STZ)-induced diabetic rats and STZ-intracerebroventricularly (STZ-icv) treated rats, in which antioxidant capacity against peroxyl (ORAC(-ROO)*) and hydroxyl (ORAC(-OH)*) free radical was measured in three different brain regions (hippocampus, cerebellum, and brain stem) by means of oxygen radical absorbance capacity (ORAC) assay. In the brain of both STZ-induced diabetic and STZ-icv treated rats decreased antioxidant capacity has been found demonstrating regionally specific distribution. In the diabetic rats these abnormalities were not associated with the development of peripheral diabetic neuropathy. Also, these abnormalities were not prevented by the icv pretreatment of glucose transport inhibitor 5-thio-D-glucose in the STZ-icv treated rats, suggesting different mechanism for STZ-induced central effects from those at the periphery. Similarities in the oxidative stress alterations in the brain of STZ-icv rats and humans with sAD could be useful in the search for new drugs in the treatment of sAD that have antioxidant activity.

Transmembrane Segment 12 of the Glut1 Glucose Transporter Is an Outer Helix and Is Not Directly Involved in the Transport Mechanism

A model has been proposed for the exofacial configuration of the Glut1 glucose transporter in which eight transmembrane domains form an inner helical bundle stabilized by four outer helices. The role of transmembrane segment 12, predicted to be an outer helix in this hypothetical model, was examined by cysteine-scanning mutagenesis and the substituted cysteine accessibility method using the membrane-impermeant, sulfhydryl-specific reagent, p-chloromercuribenzenesulfonate (pCMBS). A previously characterized functional cysteine-less Glut1 molecule was used to produce 21 Glut1 point mutants by changing each residue along helix 12 to a cysteine residue. These mutants were then expressed in Xenopus oocytes, and their protein levels, functional activities, and sensitivities to pCMBS were determined. Strikingly, in contrast to all nine other predicted Glut1 transmembrane helices that have been previously examined by this method, none of the 21 helix 12 single-cysteine mutants exhibited significant inhibition of specific transport activity. Also unlike most other Glut1 transmembrane domains in which solvent-accessible residues lie along a single face of the helix, mutations in five consecutive residues predicted to lie close to the exofacial face of the membrane resulted in sensitivity to pCMBS-induced transport inhibition. These results suggest that helix 12 plays a passive stabilizing role in the structure of Glut1 and is not directly involved in the transport mechanism. Additionally, the pCMBS data indicate that the predicted exoplasmic end of helix 12 is completely exposed to the external solvent when the transporter is in its exofacial configuration.

Characterization of the human SLC2A11 (GLUT11) gene: alternative promoter usage, function, expression, and subcellular distribution of three isoforms, and lack of mouse orthologue

GLUT11 (SLC2A11) is a class II sugar transport facilitator which exhibits highest similarity with the fructose transporter GLUT5 (about 42%). Here we demonstrate that separate exons 1 (exon 1A, exon 1B, and exon 1C) of the SLC2A11 gene generate mRNAs of three GLUT11 variants (GLUT11-A, GLUT11-B, and GLUT11-C) that differ in the amino acid sequence of their N-termini. All three 5'-flanking regions of exon 1A, exon 1B and exon 1C exhibited promoter activity when expressed as luciferase fusion constructs in COS-7 cells. 5'-RACE-PCR, quantitative real-time PCR, and Northern blot analysis performed with specific probes for exon 1A, 1B and 1C demonstrated that GLUT11-A is expressed in heart, skeletal muscle, and kidney, GLUT11-B in kidney, adipose tissue, and placenta, and GLUT11-C in adipose tissue, heart, skeletal muscle, and pancreas. Surprisingly, mice and rats lack the SLC2A11 gene. When expressed in Xenopus oocytes, all three GLUT11 isoforms transport glucose and fructose but not galactose. There was no apparent difference in the subcellular distribution of the three isoforms expressed in COS-7 cells. Our data indicate that different promoters and splicing of the human SLC2A11 gene generate three GLUT11 isoforms which are expressed in a tissue specific manner but do not appear to differ in their functional characteristics.

Mutations in the facilitative glucose transporter GLUT10 alter angiogenesis and cause arterial tortuosity syndrome

Arterial tortuosity syndrome (ATS) is an autosomal recessive disorder characterized by tortuosity, elongation, stenosis and aneurysm formation in the major arteries owing to disruption of elastic fibers in the medial layer of the arterial wall. Previously, we used homozygosity mapping to map a candidate locus in a 4.1-Mb region on chromosome 20q13.1 (ref. 2). Here, we narrowed the candidate region to 1.2 Mb containing seven genes. Mutations in one of these genes, SLC2A10, encoding the facilitative glucose transporter GLUT10, were identified in six ATS families. GLUT10 deficiency is associated with upregulation of the TGFbeta pathway in the arterial wall, a finding also observed in Loeys-Dietz syndrome, in which aortic aneurysms associate with arterial tortuosity. The identification of a glucose transporter gene responsible for altered arterial morphogenesis is notable in light of the previously suggested link between GLUT10 and type 2 diabetes. Our data could provide new insight on the mechanisms causing microangiopathic changes associated with diabetes and suggest that therapeutic compounds intervening with TGFbeta signaling represent a new treatment strategy.

Comparison of metabolic pathways between cancer cells and stromal cells in colorectal carcinomas: a metabolic survival role for tumor-associated stroma

Understanding tumor metabolism is important for the development of anticancer therapies. Immunohistochemical evaluation of colorectal adenocarcinomas showed that cancer cells share common enzyme/transporter activities suggestive of an anaerobic metabolism [high lactate dehydrogenase 5 (LDH5)/hypoxia-inducible factor alphas (HIFalphas)] with high ability for glucose absorption and lactate extrusion [high glucose transporter 1 (GLUT1)/monocarboxylate transporter (MCT1)]. The tumor-associated fibroblasts expressed proteins involved in lactate absorption (high MCT1/MCT2), lactate oxidation (high LDH1 and low HIFalphas/LDH5), and reduced glucose absorption (low GLUT1). The expression profile of the tumor-associated endothelium indicated aerobic metabolism (high LDH1 and low HIFalphas/LDH5), high glucose absorption (high GLUT1), and resistance to lactate intake (lack of MCT1). It is suggested that the newly formed stroma and vasculature express complementary metabolic pathways, buffering and recycling products of anaerobic metabolism to sustain cancer cell survival. Tumors survive and grow because they are capable of organizing the regional fibroblasts and endothelial cells into a harmoniously collaborating metabolic domain.

Effect of PAO (phenylarsine oxide) on the inhibitory effect of insulin and IGF-1 on insulin release from INS-1 cells

Phenylarsine oxide (PAO) which complexes vicinal thiol groups is a valuable pharmacological tool to investigate the interaction of peptides such as insulin with their receptors and the signal transduction from the receptor to the cell interior. This tool was now used to elucidate the inhibitory effects of insulin and IGF-1 on insulin secretion via their receptors. Insulin and IGF-1 inhibited insulin release from INS-1 cells, an insulin secreting cell line. PAO was able to reverse this inhibitory effect of both hormones. Dimercaptopropanol (DMP), which is well known to antagonize PAO effects, inhibited the abolishment of PAO effect on the inhibitory effect of insulin and IGF-1 regarding insulin release. Membrane bound GLUT2 in INS-1 cells was increased by either insulin and IGF-1 which is counteracted by PAO. Thus the inhibitory effect of insulin and IGF-1 on insulin release is operative and can be disturbed by a thiol interacting compound such as PAO. This may happen at the receptor level or at the sub-receptor level.