Over-expression of facilitative glucose transporter genes in human cancer

Multiple-drug-resistance phenomenon in the yeast Saccharomyces cerevisiae: involvement of two hexose transporters

In the yeast Saccharomyces cerevisiae, multidrug resistance to unrelated chemicals can result from overexpression of ATP-binding cassette (ABC) transporters such as Pdr5p, Snq2p, and Yor1p. Expression of these genes is under the control of two homologous zinc finger-containing transcription regulators, Pdr1p and Pdr3p. Here, we describe the isolation, by an in vivo screen, of two new Pdr1p-Pdr3p target genes: HXT11 and HXT9. HXT11 and HXT9, encoding nearly identical proteins, have a high degree of identity to monosaccharide transporters of the major facilitator superfamily (MFS). In this study, we show that the HXT11 product, which allows glucose uptake in a glucose permease mutant (rag1) strain of Kluyveromyces lactis, is also involved in the pleiotropic drug resistance process. Loss of HXT11 and/or HXT9 confers cycloheximide, sulfomethuron methyl, and 4-NQO (4-nitroquinoline-N-oxide) resistance. Conversely, HXT11 overexpression increases sensitivity to these drugs in the wild-type strain, an effect which is more pronounced in a strain having both PDR1 and PDR3 deleted. These data show that the two putative hexose transporters Hxt11p and Hxt9p are transcriptionally regulated by the transcription factors Pdr1p and Pdr3p, which are known to regulate the production of ABC transporters required for drug resistance in yeast. We thus demonstrate the existence of genetic interactions between genes coding for two classes of transporters (ABC and MFS) to control the multidrug resistance process.

Effects of antitumor agents on 3H-2-deoxyglucose uptake in tumor cells and their relationship with the main targets of the antitumor agents

To investigate the effects of antitumor drugs on 3H-2-deoxyglucose (DG) uptake in tumor cells, we performed DG uptake studies of the short-term treatment of four kinds of antitumor drugs in a cell culture system. The antitumor drugs adriamycin (ADM) and cisplatin (cDDP), which affect on DNA synthesis, did not greatly affect DG uptake, but DG uptake was lowered by antitumor drugs, actinomycin D (AcD) and cycloheximide (CHX), which target the gene expression system. To investigate the mechanism of DG uptake changes, we also tested the effects of some glucose metabolic inhibitors on DG uptake. An inhibitor of glycolytic flow (iodoacetate) lowered DG uptake whereas mitochondrial inhibition increased DG uptake. These results on the inhibition of glucose metabolism indicated that there were two types of factors affecting DG uptake directly; one affects glycolysis and the other affects oxidative phosphorylation. The two antitumor drugs with effects on gene expression were thought to act by the former. The effects of the drug treatments for tumors on DG uptake could be divided into three groups; glycolysis inhibition, mitochondrial inhibition and no relation to glucose metabolism. With the further observations of FDG uptake changes based on this prediction, the biochemical relationship between treatment effects and FDG uptake changes will be clarified.

GLUT1 glucose transporter expression in benign and malignant thyroid nodules

Malignant cells exhibit increased rates of glycolysis and glucose uptake, the latter of which is mediated by glucose transport proteins. Because several types of cancer have been shown to express high levels of the GLUT1 glucose transporter isoform, we hypothesized that expression of GLUT1 might distinguish malignant from benign thyroid tissue. Archival thyroid tissue obtained at surgery was immunostained for GLUT1 protein. There were 38 benign cases (24 follicular adenoma, 1 Hürthle cell adenoma, 8 nodular goiter, 3 Hashimoto's thyroiditis, 2 Graves' disease) and 28 cases of thyroid cancer (17 papillary and its follicular variant, 6 follicular, 1 Hurthle cell, 2 anaplastic, 2 medullary). Normal thyroid tissue adjacent to nodules showed no thyrocyte staining in any case. No GLUT1 staining was seen in thyrocytes in benign nodular tissue, except for a single case of Hashimoto's thyroiditis in which a few Hurthle cells showed weak staining. Among the thyroid cancers, 13 of 28 (46%) showed tumor cell GLUT1 staining in at least some areas. This included 9 of 17 cases of papillary carcinoma and its follicular variant, 2 of 6 cases of follicular carcinoma and 2 of 2 cases of anaplastic carcinoma. Tumor cell GLUT1 staining was seen in two patterns: circumferential plasma membrane staining focally within the tumor, or asymmetric staining of the basilar aspect of tumor cells adjacent to stroma in some cases of papillary carcinoma. We conclude that GLUT1 expression is frequently detectable by immunostaining in thyroid cancer, but not in benign nodules or normal thyroid. GLUT1 expression may be a clinically useful molecular marker for thyroid cancer.

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

The expression of GLUT1 and GLUT3 mRNA and protein in human placental trophoblast-derived cells was investigated. A dividing choriocarcinoma derived cell line (JAr) was compared to differentiating cytotrophoblast cells, isolated from human term placenta, following 18 (mononucleate) and 66 h (multinucleate) in culture. JAr cells treated with 8-bromoadenosine, which inhibits growth and induces differentiation, were also studied. GLUT1 mRNA and protein expression were similar in the four groups of cells. However, GLUT3 mRNA expression was significantly higher (six- to sevenfold) in both control and 8-bromoadenosine-treated JAr cells compared to cytotrophoblast cells and was also significantly higher in untreated versus treated JAr cells. Western blotting showed that GLUT3 protein was undetectable in either cytotrophoblast cell groups, but was abundant in both groups of JAr cells. GLUT3 protein in JAr cells treated with 8-bromoadenosine was also significantly lower than in untreated JAr cells, in agreement with the mRNA data. We conclude that GLUT1 expression is unaffected by either growth or differentiation of trophoblast cells whereas GLUT3 expression is associated with dividing cells. We propose that in the placenta, GLUT3 may be involved in maintaining metabolic requirements of dividing trophoblast cells, rather than having a direct role in transport of glucose to the fetus.

Glucose-transporter-type-I-gene amplification correlates with sialyl-Lewis-X synthesis and proliferation in lung cancer

Increased glucose transport is a common characteristic of most tumors. To examine the role of elevated glucose uptake in lung cancer, we performed PCR amplification of 2 facilitative glucose transporter genes (GLUT1 and GLUT3) and immunohistochemical staining for GLUT1, proliferating cell nuclear antigen (PCNA), and sialyl Lewis x (sLe(x)) on tumor specimens from 327 patients with lung cancer who underwent surgical resection from 1980 to 1993. To evaluate the relationship between GLUT, alpha-2,3-sialyltransferase (ST), and alpha-1,3-fucosyltransferase (Fuc-T) genes, PCR amplification of the ST3N and Fuc-TVII also was performed. Amplification of GLUT1 was significantly greater than that of GLUT3. GLUT1 and GLUT3 amplification correlated with PCNA staining (p < 0.01). In addition, GLUT1 amplification correlated with the grading of sLe(x) staining as well as with the grading of GLUT1 staining (p < .03, p < 0.01). GLUT1 was co-amplified with ST3N and Fuc-TVII genes, which are involved in the synthesis of sLe(x) (p < 0.01). The survival of patients whose tumors showed GLUT1 amplification was significantly shorter than that of patients whose tumors did not (p < 0.01). In a multivariate analysis of survival, GLUT1 remained a statistically significant prognostic factor. Our results suggest that GLUT1 amplification may participate in sLe(x) synthesis as well as in proliferation, and may be of prognostic value in lung cancer.

Immunohistochemical expression of glucose transporter-1 in human penile proliferative lesions

Glucose transporters (GLUTs) are a family of membrane proteins responsible for the transport of glucose across cellular membranes. In terms of their mRNA levels, they have been reported to be expressed in some human tumours. However, the immunohistochemical localization of GLUTs in human urogenital lesions has rarely been studied. This study was performed to evaluate the expression of GLUT1 in penile proliferative lesions (18 cases of penile carcinoma and 13 cases of condyloma acuminatum). Using an isoform-specific anti-GLUT1 antibody, formalin-fixed paraffin-embedded sections were stained by the avidin-biotin complex method. In all cases of penile carcinoma, GLUT1 staining was diffusely recognized on the cell membrane of the carcinoma cells in the mainly infiltrating areas. However, the inner areas of the tumour were more weakly and focally stained. The intensity of staining for the penile carcinoma (staining score = 2.8 +/- 0.6) was stronger than that for condyloma acuminatum and that for adjacent non-proliferative areas. All cases of condyloma acuminatum showed a diffuse staining on the cell membrane in the basal and intermediate layers (staining score = 2.4 +/- 0.5). Non-proliferative (histologically normal) glans areas adjacent to the above lesions expressed the weakest GLUT1 staining only in the stratum basale (staining score = 1.8 +/- 0.5). These three areas showed significantly different staining scores from each other (p < 0.01). In conclusion, GLUT1 is expressed dominantly in penile proliferative lesions, especially in infiltrating areas of penile carcinoma.

Synthesis of O-methylsulfonyl derivatives of D-glucose as potential alkylating agents for targeted drug delivery to the brain. Evaluation of their interaction with the human erythrocyte GLUT1 hexose transporter

In order to obtain hydrophilic analogues of 1,4-dimethylsulfonyloxybutane (busulfan) with enhanced selectivity and improved brain penetration, we have synthesized 6-O-methylsulfonyl-D-glucose, 3-O-methylsulfonyl-D-glucose, 3,6-di-O-methylsulfonyl-D-glucose, 4-O-methylsulfonyl-D-glucose, and 4,6-di-O-methylsulfonyl-D-glucose, and we have studied their interactions with the human erythrocyte GLUT1 hexose transport system. Mesylation of OH-4 and OH-6 of glucose resulted in a slightly diminished affinity for the GLUT1 glucose transporter, whereas mesylation of OH-3 led to complete loss of affinity.

Nuclear medicine techniques provide unique physiologic characterization of suspected and known soft tissue and bone sarcomas

Primary bone and soft tissue tumors have a range of physiologic, biochemical and genetic characteristics which differentiate them from benign tumors and normal tissues. These "fingerprints" are amenable to noninvasive detection and quantification using nuclear medicine techniques. Functional characterization using radiotracers imaged using a gamma camera or positron emission tomography (PET) can provide unique but complementary information to that provided by anatomically-based imaging modalities such as plain radiography, computed tomography (CT) and magnetic resonance imaging (MRI). This is particularly important following therapy when residual mass lesions may cause a dilemma regarding the need for further treatment. Lack of understanding of the role of functional imaging has impeded more rational use of nuclear medicine in this field. This review addresses the role of nuclear medicine techniques in the primary evaluation, staging, and therapeutic monitoring of soft tissue and bone sarcomas. The potential of delivering targeted radiotherapy to these tumors with radiopharmaceuticals which exploit the unique pathophysiology of each individual malignant lesion is also addressed.

Synthesis of glucose-chlorambucil derivatives and their recognition by the human GLUT1 glucose transporter

A limitation of the use of chemotherapeutic agents against intracerebral tumors lies on their poor uptake into the central nervous system. An approach to enhance brain delivery is to design agents that are transported into the brain by one of the saturable nutrient carriers of the blood-brain barrier, the highly efficient brain and erythrocyte glucose transporter isoform GLUT1. Since the GLUT1 hexose transporter of the blood-brain barrier is also present on erythrocytes, new compounds designed to be transported by the GLUT1 transporter were studied on human erythrocytes, which represent unique, easily accessible human GLUT1 expressing cells. In this paper we describe the synthesis of four glucose-chlorambucil derivatives, namely methyl 6-O-4[bis(2-chloroethyl)amino]benzenebut anoyl-beta-D-glucopyranosi de (3), 6-O-4-[bis(2-chloroethyl)amino]benzenebu tanoyl-D-glucopyranose (6), methyl 6-[4-[bis(2-chloroethyl)amino]benzenebut anoylamido]-6-deoxy-beta-D-glucopyranoside (9) and 6-[4-[bis(2-chloroethyl)amino]benzenebut anoyl amido]-6-deoxy-D-glucopyranose (10), and the study of their interactions with the GLUT1 transporter of the human erythrocytes. All four compounds were able to inhibit [14C]glucose uptake in a concentration-dependent manner. One of them, compound 6, exhibited an approximately 160-fold higher inhibition of [14C]glucose uptake by the GLUT1 transporter than glucose itself. Compound 6 was also able to inhibit [3H]cytochalasin B binding to erythrocytes with approximately 1000-fold higher efficacy than does glucose. The inhibition of glucose uptake was entirely reversible, indicating that it was not due to alkylation of a nucleophilic group of the hexose transporter. The above results suggested specific interactions of compound 6 with the hexose transporter protein. Uptake studies of [14C]compound 6 indicated, in addition, some non-specific interactions with intact and open erythrocyte membranes: only a small amount of the bound [14C]compound 6 can be displaced by cytochalasin B. Collectively, these findings led us to conclude that the interactions of compound 6 with GLUT1 are presumably that of a non-transported inhibitor.

Oncological applications of positron emission tomography with fluorine-18 fluorodeoxyglucose

Positron emission tomography (PET) is now primarily used in oncological indication owing to the successful application of fluorine-18 fluorodeoxyglucose (FDG) in an increasing number of clinical indications at different stages of diagnosis, and for staging and follow-up. This review first considers the biological characteristics of FDG and then discusses methodological considerations regarding its use. Clinical indications are considered, and the results achieved in respect of various organs and tumour types are reviewed in depth. The review concludes with a brief consideration of the ways in which clinical PET might be improved.

Rat C6 glioma cell growth is related to glucose transport and metabolism

In order to establish whether growth of glioma cells is associated with glucose transport and metabolism, we investigated expression of the glucose transporter and hexokinase, as well as glucose transport and glucose phosphorylation in rat C6 glioma cells growing at different rates. Rat C6 glioma cells were subcloned to produce four different cell lines (CL1, CL2, CL3 and CL4) differing in growth, differentiation and morphology: CL1 cells were slow-growing with an astrocytic appearance whereas CL4 cells grew rapidly and were small and spindle-shaped. Immunocytochemical analysis using glial fibrillary acidic protein and galactocerebroside antibodies revealed that CL1 and CL4 cells differentiate to astrocytes and oligodendrocytes respectively. Both of these cell lines expressed GLUT1 mRNA predominantly, whereas little GLUT3 mRNA was evident by Northern-blot analysis. The GLUT1 mRNA level was much higher in CL4 than in CL1 cells, and the uptake of 2-deoxy-D-glucose and 3-O-methyl-D-glucose by CL4 cells was markedly higher than that by CL1 cells, indicating a correlation between the growth rate, glucose transporter (GLUT1) level and glucose-transport rate of C6 glioma cells. We then studied glucose metabolism by CL1 and CL4 cells by measuring their hexokinase activities and intracellular concentrations of glucose and ATP. The mitochondrial hexokinase activity of CL4 cells was about three times higher than that of CL1 cells, whereas the cytosolic hexokinase activity of CL4 cells was only about half that of CL1 cells. As the total amount of cellular hexokinase protein in CL4 cells was only slightly higher (about 20%) than that in CL1 cells, the hexokinase protein of CL4 cells was considered to have moved from the cytosol to the mitochondrial membranes. Consistent with the increased mitochondrial hexokinase activity of CL4 cells, the intracellular glucose concentration was undetectable, and the ATP concentration was higher than that of CL1 cells, suggesting that glucose transport is the rate-limiting factor for overall glucose metabolism is rapidly growing C6 cells. Therefore the present data demonstrate that glioma cell growth is related to glucose transport, which is closely associated with glucose metabolism.

Regional glucose metabolism and histopathology of gliomas. A study based on positron emission tomography-guided stereotactic biopsy

BACKGROUND

Positron emission tomography (PET) with 18F-2-fluoro-2-deoxy-D-glucose (FDG) is widely applied to the study of gliomas. The histology of most gliomas is regionally heterogeneous. The relationship between histologic features and glucose metabolism evaluated by PET with FDG may therefore vary within the limits of the tumor. PET with FDG integrated in the planning of stereotactic brain biopsy allows precise comparison between local FDG uptake and histology. Using this approach, the authors investigated whether glucose metabolism of gliomas is related to anaplasia, and whether PET with FDG detects metabolic heterogeneity that parallels histologic heterogeneity of gliomas.

METHODS

A total of 161 biopsy samples collected from 20 PET-guided procedures performed in patients with gliomas (8 low grade astrocytomas, 8 anaplastic astrocytomas, 1 anaplastic oligoastrocytoma, and 3 glioblastomas) were analyzed for the presence or absence of 8 histologic features. Stereotactic coordinates were used to calculate the metabolic rate of glucose (MRGlu) in the region of each biopsy sample. Gray and white matter MRGlu were used to define four metabolic grades that were compared with local histology.

RESULTS

The difference in MRGlu expressed as micromoles per 100 g per minute was highly significant between anaplastic and nonanaplastic samples; the median +/- quartile deviation was 23 +/- 16 in anaplastic samples and 18 +/- 5 in nonanaplastic samples (P < 0.005). Even more significant differences were found when MRGlu was normalized to the cortex or to the white matter. Metabolic grades were different in anaplastic and nonanaplastic samples (P < 0.0001). Approximately 75% of samples metabolically graded 3 or 4 demonstrated signs of anaplasia, compared with 10% of samples graded 0 or 1.

CONCLUSIONS

FDG uptake in gliomas is anatomically heterogeneous and is regionally related to the presence of anaplasia.

Prognostic value positron emission tomography with [18F]fluoro-2-deoxy-D-glucose in the low-grade glioma

OBJECTIVE

The natural history of the supratentorial low-grade glioma (LGG) of the adult is variable, and its malignant transformation is hardly predictable. Because positron emission tomography with [18F]fluoro-2-deoxy-D-glucose (FDG) has prognostic value in high-grade gliomas, this study was designed to search for a possible relationship between glucose metabolism and risk of malignant evolution in LGGs.

METHODS

Positron emission tomography with FDG was performed in 28 patients with LGGs (22 at the time of diagnosis and 6 after the diagnosis). A metabolic grading system based on the visual inspection of the positron emission tomographic images was used.

RESULTS

In 19 patients, no area of FDG uptake higher than in the white matter was detected (metabolic Grade 1). All of those patients were alive at the end of the follow-up period. Only one of the patients presented a histological modification 7 months after the diagnosis. Nine patients presented areas of increased FDG uptake (metabolic Grade 2 or 3). Those areas were found in the tumor area in eight patients and in an area of radionecrosis in one. Of the nine patients with FDG "hot spots," six died, two had recurrence but were alive at the end of the follow-up period, and the patient with radionecrosis had no signs of recurrence.

CONCLUSIONS

The presence of areas of increased FDG uptake in a histologically proven LGG predicts, in most cases, a deleterious evolution. This metabolic feature, detectable with a noninvasive procedure, may provide a clue to cellular changes, announcing malignant transformation in a tumor that retains the histological features of an LGG. Protocols with aggressive therapeutic strategies in this situation should be considered for evaluation.

The interaction among glucose transport, hexokinase, and glucose-6-phosphatase with respect to 3H-2-deoxyglucose retention in murine tumor models

The development of new diagnostic/therapeutic modalities for cancer requires a specific understanding of how tumors differ from normal tissues. Though the key components involved in the selective accumulation of 2-deoxy-D-glucose (2-DG) analogs in tumors are known, the relative importance of each is controversial. For this reason glucose transport protein (GLUT) density, hexokinase/glucose-6-phosphatase (GP) activity, and 2-DG biodistribution were measured together in four tumor models and normal murine tissues. Direct binding studies with 3H-cytochalasin B showed that GLUT density was elevated 20-fold in LX-1 tumors. Immunohistochemically in all tumors, the expression of GLUT-1 was highest in the necrotic/ perinecrotic foci and similar in cells not adjacent to necrotic foci. As the retention of 3H-2-DG was similar in all tumors, these data suggest that the GLUT-1 in perinecrotic tumor cells were not rate limiting for 3H-2-DG uptake. Kidney, liver, and lung had high GP activity and rapid clearance of 3H-2-DG. Sodium orthovanadate (5 mumol), a GP inhibitor, increased the concentration of 3H-2-DG in these tissues, suggesting that GP is a rate-limiting enzyme for 3H-2-DG clearance. All tumor homogenates had low GP activity, and hexokinase activity was not elevated compared to normal tissues. Thus, in the tumors studied, the selective accumulation of 3H-2-DG consistently occurred in the absence of significant GP activity without the marked overexpression of hexokinase or GLUT.

Genistein is a natural inhibitor of hexose and dehydroascorbic acid transport through the glucose transporter, GLUT1

Genistein is a dietary-derived plant product that inhibits the activity of protein-tyrosine kinases. We show here that it is a potent inhibitor of the mammalian facilitative hexose transporter GLUT1. In human HL-60 cells, which express GLUT1, genistein inhibited the transport of dehydroascorbic acid, deoxyglucose, and methylglucose in a dose-dependent manner. Transport was not affected by daidzein, an inactive genistein analog that does not inhibit protein-tyrosine kinase activity, or by the general protein kinase inhibitor staurosporine. Genistein inhibited the uptake of deoxyglucose and dehydroascorbic acid in Chinese hamster ovary (CHO) cells overexpressing GLUT1 in a similar dose-dependent manner. Genistein also inhibited the uptake of deoxyglucose in human erythrocytes indicating that its effect on glucose transporter function is cell-independent. The inhibitory action of genistein on transport was instantaneous, with no additional effect observed in cells preincubated with it for various periods of time. Genistein did not alter the uptake of leucine by HL-60 cells, indicating that its inhibitory effect was specific for the glucose transporters. The inhibitory effect of genistein was of the competitive type, with a Ki of approximately 12 microM for inhibition of the transport of both methylglucose and deoxyglucose. Binding studies showed that genistein inhibited glucose-displaceable binding of cytochalasin B to GLUT1 in erythrocyte ghosts in a competitive manner, with a Ki of 7 microM. These data indicate that genistein inhibits the transport of dehydroascorbic acid and hexoses by directly interacting with the hexose transporter GLUT1 and interfering with its transport activity, rather than as a consequence of its known ability to inhibit protein-tyrosine kinases. These observations indicate that some of the many effects of genistein on cellular physiology may be related to its ability to disrupt the normal cellular flux of substrates through GLUT1, a hexose transporter universally expressed in cells, and is responsible for the basal uptake of glucose.

Expression of the fructose transporter GLUT5 in human breast cancer

The primary metabolic characteristic of malignant cells is an increased uptake of glucose and its anaerobic metabolism. We studied the expression and function of the glucose transporters in human breast cancer cell lines and analyzed their expression in normal and neoplastic primary human breast tissue. Hexose uptake assays and immunoblotting experiments revealed that the breast carcinoma cell lines MCF-7 and MDA-468 express the glucose transporters GLUT1 and GLUT2, isoforms expressed in both normal and neoplastic breast tissue. We also found that the breast cancer cell lines transport fructose and express the fructose transporter GLUT5. Immunolocalization studies revealed that GLUT5 is highly expressed in vivo in human breast cancer but is absent in normal human breast tissue. These findings indicate that human breast cancer cells have a specialized capacity to transport fructose, a metabolic substrate believed to be used by few human tissues. Identification of a high-affinity fructose transporter on human breast cancer cells opens opportunities to develop novel strategies for early diagnosis and treatment of breast cancer.

Isoenzyme-specific regulation of genes involved in energy metabolism by hypoxia: similarities with the regulation of erythropoietin

Recent studies have indicated that regulatory mechanisms underlying the oxygen-dependent expression of the haematopoietic growth factor erythropoietin are widely operative in non-erythropoietin-producing cells and are involved in the regulation of other genes. An important characteristic of this system is that the inducible response to hypoxia is mimicked by exposure to particular transition metals such as cobaltous ions, and by iron chelation. We have investigated the extent of operation of this system in the regulation of a range of genes concerned with energy metabolism. The effects of hypoxia (1% oxygen), cobaltous ions and desferrioxamine on gene expression in tissue-culture cells was studied using RNase protection assays. Hypoxia induced the expression of glucose transporters in an isoform-specific manner; GLUT-1 and GLUT-3 were induced by hypoxia, whereas expression of GLUT-2 was decreased. Isoenzyme-specific regulation by hypoxia was also observed for genes encoding phosphofructokinase, aldolase and lactate dehydrogenase. For all of these genes, responses to cobaltous ions and desferrioxamine correlated in both direction and magnitude with the response to hypoxia. In contrast, a reduction in mitochondrial transcripts was observed in hypoxia, but these changes were not mimicked by either cobaltous ions or desferrioxamine. These findings indicate that similarities with erythropoietin regulation extend to the oxygen-dependent regulation of genes encoding glucose transporters and glycolytic enzymes but not to the regulation of mitochondrial transcripts, and they show that in glucose metabolism regulation by this system is isoenzyme- or isoform-specific.

A tumor-associated glycosylation change in the glucose transporter GLUT1 controlled by tumor suppressor function in human cell hybrids

Studies of human cell hybrids have provided evidence that the tumorigenicity of a cervical carcinoma (HeLa) is under the control of a putative tumor suppressor on chromosome 11. Using these human cell hybrids, we found a tumor-associated glycosylation change in the glucose transporter GLUT1, which is an N-linked glycoprotein at the plasma membrane. The non-tumorigenic HeLa × fibroblast cell hybrid CGL1 and the normal diploid fibroblast WI38 expressed the 50–55 kDa GLUT1, whereas in a tumorigenic segregant hybrid, CGL4, as well as in parental HeLa cells, GLUT1 glycosylation was altered and its molecular mass was about 70 kDa. However, the altered GLUT1 glycosylation was not observed in SV40-transformed WI38 cells, suggesting a correlation between this glycosylation change and a putative tumor suppressor function. Further investigations using glycosidases, glycosylation inhibitors and lectin-affinity chromatography demonstrated that the tumor-associated glycosylation change in GLUT1 was mainly due to the increase in N-acetyl-lactosamine repeats in the N-linked oligosaccharides. In accordance with the altered glycosylation, affinity for 2-deoxyglucose in the tumorigenic CGL4 cells increased 2-fold, but there was little change in the Vmax. These results suggest there may be a functional role for the modulation by glycosylation of GLUT1 in the tumorigenic behavior of CGL4 and HeLa cells.

Characterization of the avian GLUT1 glucose transporter: differential regulation of GLUT1 and GLUT3 in chicken embryo fibroblasts

Vertebrate cells that are transformed by oncogenes such as v-src or are stimulated by mitogens have increased rates of glucose uptake. In rodent cells, the mechanisms whereby glucose transport is up-regulated are well understood. Stimulation of glucose transport involves an elevation in mRNA encoding the GLUT1 glucose transporter that is controlled at the levels of both transcription and mRNA stability. Cloning and sequencing of chicken GLUT1 cDNA showed that it shares 95% amino acid sequence similarity to mammalian GLUT1s. Nevertheless, unlike mammalian GLUT1 mRNA, it was not induced by v-src, serum addition, or treatment with the tumor promoter 12-O-tetradecanoylphorbol 13-acetate in chicken embryo fibroblasts. Rather, the induction of glucose transport in chicken embryo fibroblasts by v-src, serum, and 12-O-tetradecanoylphorbol 13-acetate was associated with induction of GLUT3 mRNA level and GLUT3 transcription. Rat fibroblasts were also found to express both GLUT1 and GLUT3 isoforms, but v-src induced GLUT1 and not GLUT3. This suggests that animal cells require both a basal and an upregulatable glucose transporter and that these functions have been subsumed by different GLUT isoforms in avian and mammalian cells.

The estrogen connection: the etiological relationship between diabetes, cancer, rheumatoid arthritis and psychiatric disorders

For some considerable time, there has been a growing awareness that defective essential fatty acid metabolism plays a causal role in the pathogenesis of both schizophrenia and non-insulin-dependent diabetes mellitus (NIDDM) but the influence of defective essential fatty acid metabolism in the pathogenesis of rheumatoid arthritis and cancer is less well appreciated. An EFA deficiency, or defective EFA metabolism, negatively influences prostaglandin synthesis and glucose regulation and transport. Moreover, defective EFA metabolism negatively influences estrogen availability which contributes to the observed gender bias some of these illnesses manifest. While fluctuations of estrogen are known to contribute to the pathogenesis of these conditions, so also do fluctuations of IGF-II and there is some suggestion that IGF-II and insulin may well be inversely regulated. In addition, insulin-dependent diabetes mellitus (IDDM), rheumatoid arthritis, and schizophrenia are thought to be autoimmune disorders, while cancer is associated with immune system failure. Consequently, this paper aims to examine the pathophysiological similarities and differences between mental illness, diabetes, rheumatoid arthritis and cancer in respect of which the causal relationship that obtains between essential fatty acids, estrogen, IGF-II, glucose regulation and autoimmunity will be addressed.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Gene expression of GLUT3 and GLUT1 glucose transporters in human brain tumors

GLUT3 glucose transporter gene expression is confined to neurons, while GLUT1 gene expression is limited to endothelial cells in normal brain. Thus far, neither of the GLUT genes has been shown to be consistently expressed in glial cells in adult brain in vivo under normal conditions. However, GLUT gene expression may be aberrant in human brain glial tumors. The present investigation shows that the GLUT1 and GLUT3 transcripts are differentially expressed in a series of 20 human brain tumors. The GLUT1/actin mRNA ratio increased in parallel to the astrocytoma grade, compared to a control human brain cortex, although no change in this ratio was seen in 5 meningiomas. Immunoreactive GLUT1 protein was not detectable in human brain tumors, including high-grade gliomas. Both 4.2 or 2.7 kb GLUT3/actin mRNA ratios showed a linear correlation with the glioma grade (P < 0.025), and the GLUT3-immunoreactive protein was also expressed in high grade gliomas. These studies provide evidence for induction of GLUT1 and GLUT3 gene expression in malignant glial cells, and the mRNA levels correlate with the biologic aggressiveness of the tumor. The detection of immunoreactive GLUT3, but not GLUT1, in the high grade gliomas suggest the GLUT3 isoform may be the predominant glucose transporter in highly malignant glial cells of human brain.

Effect of glucose on pHin and [Ca2+]in in NIH-3T3 cells transfected with the yeast P-type H(+)-ATPase

NIH-3T3 cells transfected with yeast H(+)-ATPases (RN1a cells) are tumorigenic (Perona and Serrano, 1988, Nature, 334:438). We have previously shown that RN1a cells maintain a chronically high intracellular pH (pHin) under physiological conditions. We have also shown that RN1a cells are serum-independent for growth, maintain a higher intracellular Ca2+ ([Ca2+]in), and glycolyze more rapidly than their non-transformed counterparts (Gillies et al., Proc. Natl. Acad. Sci., 1990, 87:7414; Gillies et al., Cell. Physiol. Biochem., 1992, 2:159). The present study was aimed to understand the interrelationships between glycolysis, pHin, and [Ca2+]in in RN1a cells and their non-transformed counterparts, NIH-3T3 cells. Our data show that the higher rate of glycolysis observed in RN1a cells is due to the presence of low affinity glucose transporters. Consequently, the higher rate of glycolysis is exacerbated at high glucose concentration in RN1a cells. Moreover, the maximal velocity (Vmax) for glucose utilization is up to sixfold higher in RN1a cells than in the NIH-3T3 cells, suggesting that the number of glucose transporters is higher in RN1a than NIH-3T3 cells. Glucose addition to NIH-3T3 cells results in modest decreases in both pHin and [Ca2+]in. In contrast, RN1a cells respond to glucose with a large decrease in pHin, followed by a large decrease in [Ca2+]in. The decrease in [Ca2+]in observed upon glucose addition is likely due to activation of Ca(2+)-ATPase by glycolysis, since the Ca2+ decrease is abolished by the Ca2+ ATPase inhibitors thapsigargin and cyclopiazonic acid. Glucose addition to ATP-depleted cells results in a decrease in [Ca2+]in, suggesting that ATP furnished by glycolysis is utilized by this pump.

FDG uptake, tumor proliferation and expression of glycolysis associated genes in animal tumor models

To determine the influence of tumor cell proliferation and changes in the genetic program in malignant cells on the fluorodeoxyglucose (FDG) uptake we performed PET studies in several animal tumors: spontaneous mammary fibroadenoma, chemically-induced mammary adenocarcinoma and Dunning prostate adenocarcinoma. The expression of the glucose transporter (GLUT1) and of hexokinase (Hk) was measured using 32P-labeled cDNA probes and densitometry. Furthermore the proliferative activity was determined with one-dimensional flow cytometry. The FDG uptake and the proliferation parameters were not correlated. The normalized amounts of GLUT and Hk mRNA were lower in spontaneous fibroadenomas and prostate tumors than in chemically induced mammary. The FDG uptake was correlated to GLUT1 expression with r = 0.83 and to Hk expression with r = 0.77. Multiple regression analysis revealed a relation of FDG uptake to GLUT1 and HK with r = 0.87. Our results show that the FDG uptake in our study was related not to differences in proliferation, but rather to differences in the transcription of glycolysis associated genes.

Gene expression of GLUT3 glucose transporter regulated by glucose in vivo in mouse brain and in vitro in neuronal cell cultures from rat embryos

This study was designed to determine whether glucose regulates the gene expression of glucose transporter GLUT3 in neurons. We examined the regulation of GLUT3 mRNA by glucose in vivo in mouse brain and in vitro by using neuronal cultures from rat embryos. Hypoglycaemia (< 30 mg/dl), produced by 72 h of starvation, increased GLUT3 mRNA in mouse brain by 2-fold. Hybridization studies in situ demonstrated that hypoglycaemia-induced increases in GLUT3 mRNA expression were observed selectively in brain regions including the hippocampus, dentate gyrus, cerebral cortex and piriform cortex, but not the cerebellum. Primary neuronal cultures from rat embryos deprived of glucose for 48 h also showed an increase (4-fold over control) in GLUT3 mRNA, indicating that glucose can directly regulate expression of GLUT3 mRNA. In contrast with hypoglycaemia, hyperglycaemia produced by streptozotocin did not alter the expression of GLUT3 mRNA. We also confirmed previous findings that hypoglycaemia increases GLUT1 mRNA expression in brain. The increase in GLUT1 expression was probably limited to the blood-brain barrier in vivo, since GLUT1 mRNA could not be detected in neurons of the mouse cerebrum. Thus we conclude that up-regulation of neuronal GLUT3 in response to glucose starvation represents a protective mechanism against energy depletion in neurons.

Presence and differential expression of SGLT1, GLUT1, GLUT2, GLUT3 and GLUT5 hexose-transporter mRNAs in Caco-2 cell clones in relation to cell growth and glucose consumption

Seven clones from the Caco-2 cell line, three isolated from passage 29 (PD7, PD10, PF11) and four from passage 198 (TB10, TC7, TF3, TG6), all of them selected on the basis of differences in the levels of expression of sucrase-isomaltase and rates of glucose consumption, were analysed for the expression of hexose-transporter mRNAs (SGLT1, GLUT1-GLUT5) in relation to the phases of cell growth and the associated variations of the rates of glucose consumption. All clones showed a similar pattern of evolution of the rates of glucose consumption, which decreased from the exponential to the late-stationary phase, but differed, in a 1-40-fold range, in the values observed at late postconfluency. According to these values, clones could be divided into high- (PD10, PF11) and low-glucose-consuming cells (PD7, TB10, TC7, TF3 and TG6). GLUT1 and GLUT3 mRNAs were expressed in all clones and showed a similar pattern of evolution: their level decreased, from the exponential to the stationary phase, in close correlation with the decrease in rates of glucose consumption, with only high-glucose-consuming clones maintaining high levels in the stationary phase. In contrast, SGLT1, GLUT2 and GLUT5 mRNAs were only expressed, like sucrase-isomaltase mRNA, in the low-glucose-consuming clones, and their level increased from the exponential to the stationary phase, in parallel with the differentiation of the cells. GLUT4 was undetectable in all the clones. Glucose deprivation generally resulted in a discrete decrease in the levels of all transporter mRNAs in all clones, one exception being GLUT2, which in the high-glucose-consuming clones is only detectable when the cells are grown in low glucose. These clones should be ideal tools with which to study in vitro, at the single-cell level, how these transporters concur to the utilization and transport of hexoses and how their exclusive or co-ordinated expression is regulated.

Expression of glucose transporters in head-and-neck tumors

The expression of glucose transporter genes (GLUTI-4) was studied in 20 head-and-neck tumors of 18 patients. All tumors--16 of which were squamous-cell carcinomas (SCC)--expressed GLUT1 and/or GLUT3 mRNA, while detectable levels of GLUT2 or GLUT4 mRNA were not observed. The signals for GLUT1 and GLUT3 mRNAs varied markedly throughout the SCC tumor population but no clear relationship with the grade of differentiation was found. The high GLUT expression observed in some tumors was not associated with amplification or rearrangement of the corresponding genes. Immunohistochemistry of 5 SCCs showed that GLUT1 protein was located in tumor-cell membranes in relation to the level of mRNA expression. We conclude that both GLUT1 and GLUT3 are involved in basal glucose uptake of extracranial head-and-neck tumors. The increased expression of these high-affinity GLUT isoforms may be related to the growth maintenance of cancer tissue in cases of limited supply of substrate e.g., in poorly vascularized tumor areas.

Facilitative glucose transporters

Facilitative glucose transport is mediated by members of the Glut protein family that belong to a much larger superfamily of 12 transmembrane segment transporters. Six members of the Glut family have been described thus far. These proteins are expressed in a tissue- and cell-specific manner and exhibit distinct kinetic and regulatory properties that reflect their specific functional roles. Glut1 is a widely expressed isoform that provides many cells with their basal glucose requirement. It also plays a special role in transporting glucose across epithelial and endothelial barrier tissues. Glut2 is a high-Km isoform expressed in hepatocytes, pancreatic beta cells, and the basolateral membranes of intestinal and renal epithelial cells. It acts as a high-capacity transport system to allow the uninhibited (non-rate-limiting) flux of glucose into or out of these cell types. Glut3 is a low-Km isoform responsible for glucose uptake into neurons. Glut4 is expressed exclusively in the insulin-sensitive tissues, fat and muscle. It is responsible for increased glucose disposal in these tissues in the postprandial state and is important in whole-body glucose homeostasis. Glut5 is a fructose transporter that is abundant in spermatozoa and the apical membrane of intestinal cells. Glut7 is the transporter present in the endoplasmic reticulum membrane that allows the flux of free glucose out of the lumen of this organelle after the action of glucose-6-phosphatase on glucose 6-phosphate. This review summarizes recent advances concerning the structure, function, and regulation of the Glut proteins.

Expression of facilitative glucose transporter isoforms in human brain tumors

The expression of facilitative glucose transporter (GLUT) isoforms in human astrocytic tumors was examined. Reverse transcriptase-polymerase chain reaction of a surgically biopsied glioblastoma was carried out using the degenerative oligonucleotide primers corresponding to the sequences of the human facilitative glucose transporter family, and polymerase chain reaction products were hybridized with human GLUT1, GLUT2, GLUT3, GLUT4, and GLUT5 cDNA probes. The results showed that a biopsied glioblastoma expressed GLUT1, GLUT3, and GLUT4 glucose transporter genes. Northern blot analysis of total RNA (10 micrograms) from a biopsied glioblastoma showed the transcripts of only GLUT1 and GLUT3, suggesting that the expression of insulin-responsive glucose transporter GLUT4 mRNA is relatively low. Immunoblot analysis of biopsied glioblastoma tissues by polyclonal antibodies against the C-terminal synthetic peptides of GLUT1, GLUT3, and GLUT4 showed a single band of each polypeptide. However, elevated expression of GLUT1 and GLUT3 glucose transporters was not observed in the glioblastoma. Astrocytic tumor tissues (n = 14) were also examined immunohistochemically. Reactive products for GLUT1 were observed in the luminal surface of capillaries in all cases, whereas tumor cells were positive for GLUT1 in only two of 14 cases. GLUT3 was positive in astrocytic tumor cells in all cases. Three of 14 cases expressed the GLUT4 protein, which was localized in the cytoplasm of tumor cells. These results suggest that the facilitative glucose transport may be altered in astrocytic tumor cells and thus display a significant change in glucose metabolism.

Neuron-specific glucose transporter (NSGT): CNS distribution of GLUT3 rat glucose transporter (RGT3) in rat central neurons

The identity of the glucose transporters (GLUT) expressed in neurons in situ has yet to be fully established. In the present study we have isolated the GLUT3 (RGT3) cDNA and produced anti RGT3 polyclonal antibody allowing us to investigate the cellular localization and tissue distributions of RGT3 mRNA and protein in the central nervous system of the rat by the methods of in situ hybridization and immunohistochemistry. Here we demonstrate the direct evidence that RGT3 is present in neurons in adult rat brain. In situ hybridization showed the expression of RGT3 mRNA mostly in the regions of hippocampus, cerebral cortex, striatum, and the granule cell layer of the cerebellum, indicating that RGT3 mRNA is predominantly expressed within neurons. Immunohistochemistry showed that RGT3 protein is widely distributed in the rat brain, and concentrated on the plasma membrane of neurons. Double labeling studies with anti-RGT3, glial fibrillary acidic protein (GFAP), and neuron specific enolase (NSE) antibodies revealed the specific expression of RGT3 protein in neurons. Thus, RGT3 is indicated to be a neuron specific glucose transporter isoform (NSGT), and suggested to play a functionally significant role in rat central neurons.

Overexpression of Glut-1 glucose transporter in human breast cancer. An immunohistochemical study

BACKGROUND

Breast cancers have higher than normal glucose metabolism, but the mechanism of glucose entry into these tumors is not well understood.

METHODS

The expression of five facilitative glucose transporters, Glut-1 (erythrocyte type), Glut-2 (liver type), Glut-3 (brain type), Glut-4 (muscle/fat type), and Glut-5 (small intestine type), was studied by immunohistochemistry of paraffin sections from 12 primary human breast cancers and 8 lymph node metastases from 2 patients. Rat tissues known to express these glucose transporters were used as controls.

RESULTS

All the primary breast cancers and the lymph node metastases were positive for Glut-1. This transporter was expressed on the cell membrane and in the cytoplasm of the tumor cells, but exhibited marked intratumoral and intertumoral variability in the proportions of positive cells and the intensity of staining. Staining of the normal mammary epithelium, if present, was much lower than observed in tumor cells from the same patient. Glut-2 was expressed in all of the tumors, but the intensity of staining was not consistently stronger than that seen in healthy breast. Clusters of Glut-4-positive granule were observed in cells in six of the tumors. None of the tumors or the healthy breast in the tissues studied expressed Glut-3 or Glut-5.

CONCLUSIONS

Higher expression of the glucose transporter Glut-1 by breast cancer cells compared with the healthy breast tissue is common. Increased glucose transporter protein expression may contribute to the increased uptake of 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) by these tumors observed by positron emission tomography (PET) imaging.

Overexpression of Glut-1 glucose transporter in human breast cancer. An immunohistochemical study

BACKGROUND

Breast cancers have higher than normal glucose metabolism, but the mechanism of glucose entry into these tumors is not well understood.

METHODS

The expression of five facilitative glucose transporters, Glut-1 (erythrocyte type), Glut-2 (liver type), Glut-3 (brain type), Glut-4 (muscle/fat type), and Glut-5 (small intestine type), was studied by immunohistochemistry of paraffin sections from 12 primary human breast cancers and 8 lymph node metastases from 2 patients. Rat tissues known to express these glucose transporters were used as controls.

RESULTS

All the primary breast cancers and the lymph node metastases were positive for Glut-1. This transporter was expressed on the cell membrane and in the cytoplasm of the tumor cells, but exhibited marked intratumoral and intertumoral variability in the proportions of positive cells and the intensity of staining. Staining of the normal mammary epithelium, if present, was much lower than observed in tumor cells from the same patient. Glut-2 was expressed in all of the tumors, but the intensity of staining was not consistently stronger than that seen in healthy breast. Clusters of Glut-4-positive granule were observed in cells in six of the tumors. None of the tumors or the healthy breast in the tissues studied expressed Glut-3 or Glut-5.

CONCLUSIONS

Higher expression of the glucose transporter Glut-1 by breast cancer cells compared with the healthy breast tissue is common. Increased glucose transporter protein expression may contribute to the increased uptake of 2-[18F]-fluoro-2-deoxy-D-glucose (FDG) by these tumors observed by positron emission tomography (PET) imaging.

Identification of insulin-like growth factor (IGF) and glucose transporter-1 and -3 mRNA in CNS tumors

Glucose metabolism is increased in CNS tumors and correlates with malignant grade. We have previously investigated the role of IGFs in regulating CNS tumor growth and metabolism. In the present study we examined total cellular RNA from human CNS tumors for the presence for glucose transporter (Glut) and IGF mRNA. Human meningiomas and gliomas were frozen in liquid nitrogen at the time of surgery and then stored at -80 degrees C. Total cellular RNA was prepared by acid-guanidinium phenol-chloroform extraction and 20 micrograms of RNA was loaded for agarose-formaldehyde gel electrophoresis and transfer. RNA integrity in 5 meningiomas and 2 gliomas was confirmed by ethidium bromide staining of 28S and 18S ribosomal RNA and hybridization with a cDNA probe for beta-actin. For analysis, membranes were hybridized to radioactively labeled human Glut-1, Glut-3, IGF-I, and IGF-II cDNA probes, and mRNA transcripts were identified by autoradiography. All 7 tumors expressed Glut-1 and Glut-3 mRNA and Glut-3 appeared to be more abundant in meningiomas. IGF-II mRNA was detected in 2 of 6 meningiomas and in both gliomas. IGFs may play an important role in the regulation of glucose metabolism in CNS tumors. IGFs and specific glucose transporters may prove useful as markers of malignancy and potential targets for future therapy.

pH in human tumour xenografts: effect of intravenous administration of glucose

pH frequency distributions of tumours grown s.c. from 30 human tumour xenograft lines in rnu/rnu rats were analysed with the use of H+ ion-sensitive semi-microelectrodes prior to and following stimulation of tumour cell glycolysis by i.v. infusion of glucose. At normoglycemia, the average pH of the tumours investigated was 6.83 (range, 6.72-7.01; n = 268). Without exception, all xenografts responded to the temporary increase in plasma glucose concentration (PGC) from 6 +/- 1 to 30 +/- 3 mM by an accumulation of acidic metabolites, as indicated by a pH reduction to an average value of 6.43 (range, 6.12-6.78; n = 292). This pH value corresponds to a ten-fold increase in H+ ion activity in tumour tissue as compared to arterial blood. Tumour pH approached minimum values at 2-4 h after the onset of glucose administration and could be maintained at acidic levels for 24 h by controlled glucose infusion. Irrespective of pH variations between tumours grown from individual xenograft lines, there was no major difference in pH response to glucose between the four main histopathological tumour entities investigated, i.e. breast, lung and gastrointestinal carcinomas, and sarcomas. In tumours from several xenograft lines, an increase in blood glucose to only 2.5-times the normal value (14 mM) was sufficient to reduce the mean pH to 6.4. Glucose-induced acidosis was tumour-specific. The pH frequency distributions in liver, kidney and skeletal muscle of tumour-bearing rnu/rnu rats were only marginally sensitive to hyperglycemia (average pH, 6.97 vs normal value of 7.14). Tumour-selective activation of pH-sensitive anti-cancer agents, e.g. alkylating drugs, acid-labile prodrugs or pH-sensitive immunoconjugates may thus be feasible in a wide variety of human cancers.

Yeast sugar transporters

Transport of sugars is a fundamental property of all eukaryotic cells. Of particular importance is the uptake of glucose, a preferred carbon and energy source. The rate of glucose utilization in yeast is often dictated by the activity and concentration of glucose transporters in the plasma membrane. Given the importance of transport as a site of control of glycolytic flux, the regulation of glucose transporters is necessarily complex. The molecular analysis of these transporters in Saccharomyces has revealed the existence of a multigene family of sugar carriers. Recent data have raised the question of the actual role of all of these proteins in sugar catabolism, as some appear to be lowly expressed, and point mutations of these genes may confer pleiotropic phenotypes, inconsistent with a simple role as catabolic transporters. The transporters themselves appear to be intimately involved in the process of sensing glucose, a model for which there is growing support.

Tumour-associated hypoglycaemia in a murine cachexia model

Animals bearing a cachexia-inducing tumour, the MAC16 adenocarcinoma, showed a progressive decrease in blood glucose levels with increasing weight loss, while animals bearing a histologically similar tumour, the MAC13 adenocarcinoma, showed no change in either body weight or blood glucose levels with growth of the tumour. The effect of the MAC16 tumour on blood glucose levels appeared to be unrelated to food intake, glucose consumption by the tumour, or to the production of increased levels of IGF-I and IGF-II mRNA by the tumour cells. The relationship between the induction of cachexia and alteration in blood glucose levels remains unknown.

Pretranslational regulation of two cardiac glucose transporters in rats exposed to hypobaric hypoxia

To investigate the mechanism by which cardiac glucose utilization increases during hypoxia and increased work load, we studied the effect of 2 and 14 days of hypobaric hypoxia on the expression of two subtypes of the facilitative D-glucose transporter, the GLUT-4 or "insulin-regulatable" isoform and the GLUT-1 isoform thought to mediate basal transport. Rats lose weight when exposed to hypobaric hypoxia, so fasting controls were used in the 2-day studies and pair-fed controls in the 14-day experiments. Hypobaric hypoxia (PO2 69 mmHg) resulted in right ventricular (RV), but not left ventricular (LV), hypertrophy. RV and LV GLUT-1 mRNA levels increased 2- to 3-fold after 2 days and 1.5- to 2-fold after 14 days of hypobaric hypoxia compared with both fasted rats and normal controls. RV GLUT-1 protein increased approximately 3-fold and LV GLUT-1 protein increased 1.5-fold after 14 days of hypobaric hypoxia vs. both pair-fed and normal controls. RV GLUT-4 mRNA decreased to 26% and RV GLUT-4 protein decreased to 54% of normal control levels as a result of 2 days of hypobaric hypoxia. RV GLUT-4 mRNA decreased to 64% of normal control levels with no change in RV GLUT-4 protein as a result of 2 days of fasting. We conclude that hypobaric hypoxia increases cardiac GLUT-1 expression at the pretranslational level in both ventricles. The greater increase in GLUT-1 protein on the right suggests an additive effect of pressure overload. GLUT-4 expression is reduced early in the development of RV hypertrophy.

Polarized distribution of glucose transporter isoforms in Caco-2 cells

We have examined the expression and cellular location of facilitated glucose transporter proteins (GLUT1, -3, and -5) in a human colonic epithelial cell line (Caco-2) by using peptide-specific antibodies. A differential cellular distribution of these transporters was observed in differentiated (greater than 14 days postconfluence) Caco-2 cells by immunofluorescence and immunoelectron microscopy. GLUT1 was localized primarily to the basolateral membrane, whereas GLUT3 was predominantly localized to the apical membrane. GLUT5, which was detected in only approximately 40% of fully differentiated Caco-2 cells, was found primarily in the apical membrane but was also present in both basolateral and intracellular membranes. A Na(+)-independent glucose transport system in the apical membrane of Caco-2 cells has been described previously [Blais, A., Bissonnette, A. & Berteloot, A. (1987) J. Membr. Biol. 99, 113-125], and we propose that GLUT3 mediates this process. The amino acid sequence identity (57%) and structural conservation between GLUT1 and GLUT3 may make these transporters an ideal model system for examining the molecular basis for polarized sorting of membrane proteins.

Neurons and microvessels express the brain glucose transporter protein GLUT3

To elucidate glucose transport mechanisms in brain and to demonstrate the cellular expression of the brain-type glucose transporter (GLUT3), antisera to a synthetic peptide corresponding to the C terminus were prepared and used as probes for this isoform of the facilitative glucose transporter family. Immunocytochemistry of frozen sections of dog and rat brain demonstrated GLUT3 antigen in pyramidal cell bodies and processes, in microvessels, and in intima pia or glia limitans. Immunoanalysis of Western blots identified a protein (Mr, 45,000) that was present in both neuron/neuropil and microvessel fractions. The presence of the GLUT3 message in brain was confirmed by Northern blot analysis and by amplifying and partially sequencing GLUT3 cDNA by PCR. These findings demonstrate a neuron glucose transporter in tissue and suggest that GLUT3 may play an important role in brain metabolism under physiological and pathophysiological conditions.