The Na+-independent D-glucose transporter in the enterocyte basolateral membrane: orientation and cytochalasin B binding characteristics

Glucose absorption regulation and mechanism of the compounds in Lilium lancifolium Thunb on Caco-2 cells

In this paper, the Caco-2 cell was used to study the glucose absorption regulation and mechanism of kaempferol, caffeic acid and quercetin-3-O-β-D-galactoside in Lilium lancifolium Thunb in vitro. Glucose oxidase-peroxidase (GOD-POD) method was used to measure glucose consumption in supernatant. The fluorescent D-glucose analog (2-NBDG) was used as a tracer probe to study the changes in the fluorescence intensity of 2-NBDG uptake by Caco-2 cells with an inverted fluorescence microscope. Western blotting and quantitative real-time PCR were used to detect the protein expression and mRNA transcription of SGLT1 and GLUT2. The results showed that caffeic acid and quercetin-3-O-β-D-galactoside could significantly promote the absorption of glucose by normal Caco-2 cells compared with the control group (P < 0.001). Both caffeic acid and quercetin-3-O-β-D-galactoside could significantly promote the uptake of glucose tracer 2-NBDG on Caco-2 cells. Caffeic acid and quercetin-3-O-β-D-galactoside could significantly promote SGLT1 and GLUT2 protein expression levels and mRNA transcription (P < 0.001, P < 0.01, P < 0.05). The mechanism might be related to the promotion of SGLT1 and GLUT2 protein expression levels and mRNA transcription.

Does apical membrane GLUT2 have a role in intestinal glucose uptake?

It has been proposed that the non-saturable component of intestinal glucose absorption, apparent following prolonged exposure to high intraluminal glucose concentrations, is mediated via the low affinity glucose and fructose transporter, GLUT2, upregulated within the small intestinal apical border. The evidence that the non-saturable transport component is mediated via an apical membrane sugar transporter is that it is inhibited by phloretin, after exposure to phloridzin. Since the other apical membrane sugar transporter, GLUT5, is insensitive to inhibition by either cytochalasin B, or phloretin, GLUT2 was deduced to be the low affinity sugar transport route. As in its uninhibited state, polarized intestinal glucose absorption depends both on coupled entry of glucose and sodium across the brush border membrane and on the enterocyte cytosolic glucose concentration exceeding that in both luminal and submucosal interstitial fluids, upregulation of GLUT2 within the intestinal brush border will usually stimulate downhill glucose reflux to the intestinal lumen from the enterocytes; thereby reducing, rather than enhancing net glucose absorption across the luminal surface. These states are simulated with a computer model generating solutions to the differential equations for glucose, Na and water flows between luminal, cell, interstitial and capillary compartments. The model demonstrates that uphill glucose transport via SGLT1 into enterocytes, when short-circuited by any passive glucose carrier in the apical membrane, such as GLUT2, will reduce transcellular glucose absorption and thereby lead to increased paracellular flow. The model also illustrates that apical GLUT2 may usefully act as an osmoregulator to prevent excessive enterocyte volume change with altered luminal glucose concentrations.

Transepithelial glucose transport and Na+/K+ homeostasis in enterocytes: an integrative model

The uptake of glucose and the nutrient coupled transcellular sodium traffic across epithelial cells in the small intestine has been an ongoing topic in physiological research for over half a century. Driving the uptake of nutrients like glucose, enterocytes must have regulatory mechanisms that respond to the considerable changes in the inflow of sodium during absorption. The Na-K-ATPase membrane protein plays a major role in this regulation. We propose the hypothesis that the amount of active Na-K-ATPase in enterocytes is directly regulated by the concentration of intracellular Na(+) and that this regulation together with a regulation of basolateral K permeability by intracellular ATP gives the enterocyte the ability to maintain ionic Na(+)/K(+) homeostasis. To explore these regulatory mechanisms, we present a mathematical model of the sodium coupled uptake of glucose in epithelial enterocytes. Our model integrates knowledge about individual transporter proteins including apical SGLT1, basolateral Na-K-ATPase, and GLUT2, together with diffusion and membrane potentials. The intracellular concentrations of glucose, sodium, potassium, and chloride are modeled by nonlinear differential equations, and molecular flows are calculated based on experimental kinetic data from the literature, including substrate saturation, product inhibition, and modulation by membrane potential. Simulation results of the model without the addition of regulatory mechanisms fit well with published short-term observations, including cell depolarization and increased concentration of intracellular glucose and sodium during increased concentration of luminal glucose/sodium. Adding regulatory mechanisms for regulation of Na-K-ATPase and K permeability to the model show that our hypothesis predicts observed long-term ionic homeostasis.

FGT-1 is a mammalian GLUT2-like facilitative glucose transporter in Caenorhabditis elegans whose malfunction induces fat accumulation in intestinal cells

Caenorhabditis elegans (C. elegans) is an attractive animal model for biological and biomedical research because it permits relatively easy genetic dissection of cellular pathways, including insulin/IGF-like signaling (IIS), that are conserved in mammalian cells. To explore C. elegans as a model system to study the regulation of the facilitative glucose transporter (GLUT), we have characterized the GLUT gene homologues in C. elegans: fgt-1, R09B5.11, C35A11.4, F53H8.3, F48E3.2, F13B12.2, Y61A9LA.1, K08F9.1 and Y37A1A.3. The exogenous expression of these gene products in Xenopus oocytes showed transport activity to unmetabolized glucose analogue 2-deoxy-D-glucose only in FGT-1. The FGT-1-mediated transport activity was inhibited by the specific GLUT inhibitor phloretin and exhibited a Michaelis constant (K_m) of 2.8 mM. Mannose, galactose, and fructose were able to inhibit FGT-1-mediated 2-deoxy-D-glucose uptake (P < 0.01), indicating that FGT-1 is also able to transport these hexose sugars. A GFP fusion protein of FGT-1 was observed only on the basolateral membrane of digestive tract epithelia in C. elegans, but not in other tissues. FGT-1::eGFP expression was observed from early embryonic stages. The knockdown or mutation of fgt-1 resulted in increased fat staining in both wild-type and daf-2 (mammalian insulin receptor homologue) mutant animals. Other common phenotypes of IIS mutant animals, including dauer formation and brood size reduction, were not affected by fgt-1 knockdown in wild-type or daf-2 mutants. Our results indicated that in C. elegans, FGT-1 is mainly a mammalian GLUT2-like intestinal glucose transporter and is involved in lipid metabolism.

Effect of the artificial sweetener, acesulfame potassium, a sweet taste receptor agonist, on glucose uptake in small intestinal cell lines

INTRODUCTION

Sweet taste receptors may enhance glucose absorption.

AIM

This study aimed to explore the cell biology of sweet taste receptors on glucose uptake.

HYPOTHESIS

Artificial sweeteners increase glucose uptake via activating sweet taste receptors in the enterocyte to translocate GLUT2 to the apical membrane through the PLC βII pathway.

METHODS

Caco-2, RIE-1, and IEC-6 cells, starved from glucose for 1 h were pre-incubated with 10 mM acesulfame potassium (AceK). Glucose uptake was measured by incubating cells for 1 to 10 min with 0.5-50 mM glucose with or without U-73122, chelerythrine, and cytochalasin B.

RESULTS

In Caco-2 and RIE-1 cells, 10 mM AceK increased glucose uptake by 20-30 % at glucose >25 mM, but not in lesser glucose concentrations (<10 mM), nor at 1 min or 10 min incubations. U-73122 (PLC βII inhibitor) inhibited uptake at glucose >25 mM and for 5 min incubation; chelerythrine and cytochalasin B had similar effects. No effect occurred in IEC-6 cells. Activation of sweet taste receptors had no effect on glucose uptake in low (<25 mM) glucose concentrations but increased uptake at greater concentrations (>25 mM).

CONCLUSIONS

Role of artificial sweeteners on glucose uptake appears to act in part by effects on the enterocyte itself.

Translocation of transfected GLUT2 to the apical membrane in rat intestinal IEC-6 cells

AIM

In this study, we transfected the full length cDNA of glucose transporter 2 (GLUT2) into IEC-6 cells (which lack GLUT2 expression) to investigate GLUT2 translocation in enterocytes. The purpose of this study was to investigate cellular mechanisms of GLUT2 translocation and its signaling pathway.

METHODS

Rat GLUT2 cDNA was transfected into IEC-6 cells. Glucose uptake was measured by incubating cell monolayers with glucose (0.5-50 mM), containing (14)C-D-glucose and (3)H-L-glucose, to measure stereospecific, carrier-mediated and passive uptake. We imaged GLUT2 immunoreactivity by confocal fluorescence microscopy. We evaluated the GLUT2 inhibitor (1 mM phloretin), SGLT1 inhibitor (0.5 mM phlorizin), disrupting microtubular integrity (2 μM nocodazole and 0.5 μM cytochalasin B), protein kinase C (PKC) inhibitors (50 nM calphostin C and 10 μM chelerythrine), and PKC activator (50 nM phorbol 12-myristate 13-acetate: PMA).

RESULTS

In GLUT2-IEC cells, the K(m) (54.5 mM) increased compared with non-transfected IEC-6 cells (7.8 mM); phloretin (GLUT2 inhibitor) inhibited glucose uptake to that of non-transfected IEC-6 cells (P < 0.05). Nocodazole and cytochalasin B (microtubule disrupters) inhibited uptake by 43-58% only at glucose concentrations ≥25 and 50 mM and the 10-min incubations. Calphostin C (PKC inhibitor) reproduced the inhibition of nocodazole; PMA (a PKC activator) enhanced glucose uptake by 69%. Exposure to glucose increased the GFP signal at the apical membrane of GLUT-1EC cells.

CONCLUSION

IEC-6 cells lacking GLUT2 translocate GLUT2 apically when transfected to express GLUT2. Translocation of GLUT2 occurs through glucose stimulation via a PKC-dependent signaling pathway and requires integrity of the microtubular skeletal structure.

Mechanisms of glucose uptake in intestinal cell lines: role of GLUT2

BACKGROUND

GLUT2 is translocated to the apical membrane of enterocytes exposed to glucose concentrations >∼50 mM. Mechanisms of GLUT2-mediated glucose uptake in cell culture models of enterocytes have not been studied.

AIM

To explore mechanism(s) of glucose uptake in 3 enterocyte-like cell lines.

METHODS

Glucose uptake was measured in Caco-2, RIE-1, and IEC-6 cell lines using varying concentrations of glucose (0.5-50 mM). Effects of phlorizin (SGLT1 inhibitor), phloretin (GLUT2 inhibitor), nocodazole and cytochalasin B (disrupters of cytoskeleton), calphostin C and chelerythrine (PKC inhibitors), and phorbol 12-myristate 13-acetate (PKC activator) were evaluated.

RESULTS

Phlorizin inhibited glucose uptake in all 3 cell lines. Phloretin inhibited glucose uptake in Caco-2 and RIE-1 cells. Starving cells decreased glucose uptake in Caco-2 and RIE-1 cells. Glucose uptake was saturated at >10 mM glucose in all 3 cell lines when exposed briefly (<1 min) to glucose. After exposure for >5 min in Caco-2 and RIE-1 cells, glucose uptake did not saturate and K(m) and V(max) increased. This increase in glucose uptake was inhibited by phloretin, nocodazole, cytochalasin B, calphostin C, and chelerythrine. PMA enhanced glucose uptake by 20%. Inhibitors and PMA had little or no effect in the IEC-6 cells.

CONCLUSION

Constitutive expression of GLUT2 in the apical membrane along with additional translocation of cytoplasmic GLUT2 to the apical membrane via an intact cytoskeleton and activated PKC appears responsible for enhanced carrier-mediated glucose uptake at greater glucose concentrations (>20 mM) in Caco-2 and RIE-1 cells. IEC-6 cells do not appear to express functional GLUT2.

Intestinal sugar transport

Carbohydrates are an important component of the diet. The carbohydrates that we ingest range from simple monosaccharides (glucose, fructose and galactose) to disaccharides (lactose, sucrose) to complex polysaccharides. Most carbohydrates are digested by salivary and pancreatic amylases, and are further broken down into monosaccharides by enzymes in the brush border membrane (BBM) of enterocytes. For example, lactase-phloridzin hydrolase and sucrase-isomaltase are two disaccharidases involved in the hydrolysis of nutritionally important disaccharides. Once monosaccharides are presented to the BBM, mature enterocytes expressing nutrient transporters transport the sugars into the enterocytes. This paper reviews the early studies that contributed to the development of a working model of intestinal sugar transport, and details the recent advances made in understanding the process by which sugars are absorbed in the intestine.

Inhibition of intestinal glucose uptake by aporphines and secoaporphines

The effects of aporphines and secoaporphines on glucose uptake by isolated intestinal brush-border membrane vesicles (BBMV) or basolateral membrane vesicles (BLMV) and glucose absorption during in situ intestinal perfusion were studied. Of the tested compounds, N-allylsecoboldine was the most potent glucose uptake inhibitor, with IC50 values of 159 microM and 121 microM, respectively, for uptake by BBMV and BLMV. While thaliporphine competitively inhibited glucose uptake by both membrane preparations, inhibition by N-allylsecoboldine was competitive using BBMV and noncompetitive using BLMV. In addition, N-allylsecoboldine significantly reduced both glucose absorption during in situ intestinal perfusion and blood glucose levels in the oral glucose tolerance test. The results demonstrate that levels of both aporphines and secoaporphines achievable by oral administration have an inhibitory effect on intestinal glucose uptake and suggest that the hypoglycemic effects of these compounds merit attention.

Enhanced absorption of 3-O-methyl glucose following gastrointestinal injury induced by repeated oral administration of 5-FU in mice

The absorption of nutrients is mainly mediated by specific carriers and generally retarded following gastrointestinal injury. The aim of this study was to assess the effect of repeated oral administration of 5-fluorouracil (5-FU) on the intestinal absorption of glucose by using 3-O-methyl-D-glucose (3-OMG), a glucose analogue that is not metabolized, as a probe. Repeated administration of 5-FU (60 mg/kg/day for 3 days) readily induced intestinal mucosal injury assessed by visual observation and loss of intestinal wet weight. At the same time, the carrier-dependent absorption clearance of 3-OMG was increased 1.8-fold, while the carrier-independent absorption assessed by L-glucose transport was not affected. Phloretin, a glucose transporter 2 (GLUT2) inhibitor, completely abolished the absorption of 3-OMG in both control and 5-FU-treated mice, indicating the specific effect on the carrier-dependent process. Protein and mRNA expressions of GLUT2 were significantly higher in 5-FU-treated mice compared to the control mice. Sodium (Na(+)) glucose co-transporter 1 (SGLT1) expressions were also moderately elevated in 5-FU-treated mice. Concomitantly, the uptake of D-glucose into both isolated brush border and basolateral membrane vesicles was significantly increased. These results indicate that repeated oral administration of 5-FU did not hamper, but unexpectedly induced, SGLT1 and GLUT2 expression to enhance glucose absorption.

Efflux properties of basolateral peptide transporter in human intestinal cell line Caco-2

Small peptides and some pharmacologically active compounds are absorbed from the small intestine by the apical H(+)-coupled peptide transporter 1 (PEPT1) and the basolateral peptide transporter. Here we investigated the efflux properties of the basolateral peptide transporter in Caco-2 cells using two strategies, efflux measurements and a kinetic analysis of transepithelial transport of glycylsarcosine (Gly-Sar). [(14)C]Gly-Sar efflux through the basolateral membrane was not affected significantly by the external pH. Both approaches revealed that the basolateral peptide transporter was saturable in the efflux direction, and that the affinity was lower than that in the influx direction. For two peptide-like drugs, there was no difference in substrate recognition by the basolateral peptide transporter between the two sides of the membrane. Using the kinetic parameters of PEPT1 and the basolateral peptide transporter, a computational model of Gly-Sar transport in Caco-2 cells was constructed. The simulation fitted the experimental data well. Our findings suggested that substrate affinity of the basolateral peptide transporter is apparently asymmetric, but pH-dependence and substrate specificity are symmetric for the two directions of transport. The behaviour of Gly-Sar in Caco-2 cells could be predicted by a mathematical model describing the peptide transporters.

The diffusive component of intestinal glucose absorption is mediated by the glucose-induced recruitment of GLUT2 to the brush-border membrane

We have investigated the mechanism responsible for the diffusive component of intestinal glucose absorption, the major route by which glucose is absorbed. In perfused rat jejunum in vivo, absorption was strongly inhibited by phloretin, an inhibitor of GLUT2. The GLUT2 level at the brush-border membrane increased some 2-fold when the luminal glucose concentration was changed from 0 to 100 mM. The phloretin-sensitive or diffusive component of absorption appeared superficially linear and consistent with simple diffusion, but was in fact carrier-mediated and co-operative (n=1.6, [G(1/2)]=56 mM; where [G(1/2)] is the glucose concentration at half V(max)) because of the glucose-induced activation and recruitment of GLUT2 to the brush-border membrane. Diffusive transport by paracellular flow was negligible. The phloretin-insensitive, SGLT1-mediated, component of glucose absorption showed simple saturation kinetics with [G(1/2)]=27 mM: the activation of protein kinase C (PKC) betaII, the isoenzyme of PKC that most probably controls GLUT2 trafficking [Helliwell, Richardson, Affleck and Kellett (2000) Biochem. J. 350, 149-154], also showed simple saturation kinetics, with [G(1/2)]=21 mM. We conclude that the principal route for glucose absorption is by GLUT2-mediated facilitated diffusion across the brush-border membrane, which is up to 3-fold greater than that by SGLT1; the magnitude of the diffusive component at any given glucose concentration correlates with the SGLT1-dependent activation of PKC betaII. The implications of these findings for the assimilation of sugars immediately after a meal are discussed.

Antioxidant action and potential antidiabetic properties of an isoflavonoid-containing soyabean phytochemical extract (SPE)

The potential role of oestrogenic agents, antioxidants and intestinal glucose-uptake inhibitors in the treatment of diabetes is briefly reviewed. Reports in the literature suggest that oestrogen replacement therapy may favourably modulate glucose homeostasis. A soya phytochemical extract (SPE) containing the isoflavone phytoestrogens genistein and daidzein (mostly in their glycone forms as genistin and daidzin) was investigated as an antioxidant and modulator of intestinal glucose-transport. In the present study, SPE was found to protect against glucose-induced oxidation of human low density lipoproteins (LDL) in vitro. Equol (a gut bacterial metabolite of daidzein) was a more effective antioxidant than daidzein or genistein in this system and was of similar antioxidant potency to the dietary flavonols quercetin and kaempferol and to the endogenous antioxidant 17beta-oestradiol. SPE was found to be an inhibitor of glucose uptake into rabbit intestinal brush border membrane vesicles in vitro, though of weaker potency than the classical sodium dependent glucose transporter (SGLT) inhibitor, phlorizin. Thus SPE displays a range of properties which may be of benefit in diabetes, namely as an oestrogenic agent, an inhibitor of intestinal glucose-uptake and a preventive agent for glucose-induced lipid peroxidation.

Transport of citrate across the brush border and basolateral membrane of rat small intestine

It was the aim of the present study to investigate the transport of tricarboxylates (citrate, tricarballylate) across the basolateral membrane (BLM) of the small intestine. Experiments were performed using BLM vesicles isolated from the jejunum of rats. For comparison, some experiments with brush border membrane (BBM) vesicles were also performed. Finally, transfer of citrate and tricarballylate across the intestinal wall was investigated using sacs of everted small intestine. Uptake of citrate by BBM vesicles occurs by a Na+ gradient-driven transport mechanism specific for tri- and dicarboxylates. The partially protonated forms of citrate seem to be much better transported than the completely dissociated form, since lowering the extravesicular pH from 7.8 to 5.6 resulted in a marked stimulation of Na(+)-dependent citrate uptake. In contrast to citrate uptake across the BBM, uptake of citrate across the BLM was neither influenced by Na+ nor by pH changes. Neither structurally related tri- and dicarboxylates (tricarballylate, succinate) nor other organic and inorganic anions (e.g. lactate, p-aminohippurate, sulfate, chloride, bicarbonate) significantly influenced citrate uptake by BLM vesicles under cis-conditions. Uptake of citrate as a function of the extravesicular substrate concentration was linear over a concentration range from 0.1 to 10 mmol/l. Thus, citrate uptake under these conditions seems to be Na(+)-independent and not to be mediated by a carrier. However, preloading the BLMV with citrate clearly trans-stimulated the uptake of citrate and tricarballylate, respectively. Furthermore, citrate significantly inhibited tricarballylate uptake into BLMV preloaded with citrate. These results indicate uptake of tricarboxylates across the BLM by an exchange mechanism. Using sacs of everted small intestine, no transfer of intact citrate against a concentration gradient occurred, but some evidence for metabolization of citrate within the intestinal wall was obtained. In contrast, the non-metabolizable tricarboxylate tricarballylate was significantly accumulated in the serosal compartment of everted intestinal sacs.

Hypertonic upregulation of amino acid transport system A in vascular smooth muscle cells

The A10 line of vascular smooth muscle cells has Na+ dependent transport systems for alanine, proline, and Pi, whereas uptake of leucine, myo-inositol and D-glucose is Na+ independent. When A10 cells were incubated for 4 h in medium made hypertonic by addition of sucrose, there was a marked increase in Na(+)-dependent transport of alanine and proline but no change in Na(+)-dependent Pi uptake or Na(+)-independent uptake of leucine and inositol. Intracellular alanine content was increased 61% by the hypertonic treatment. Other nonpenetrating solutes, such as cellobiose and mannitol, reproduced the effect of sucrose, but urea, a penetrating solute, did not. Studies with 2-(methylamino)-isobutyric acid revealed that the upregulation by hypertonicity involved only system A. Increases in alanine and proline uptake also occurred after incubating the cells in isotonic medium containing 0.1 mM ouabain, suggesting that an increase in intracellular Na+ may be part of the intracellular signal for upregulation of system A. Hypertonic upregulation of Na(+)-dependent alanine transport occurred also in primary cultures of vascular smooth muscle cells. The response was blocked by actinomycin D and cycloheximide, indicating that gene transcription and protein synthesis play important roles in the mechanism leading to increased alanine uptake. We conclude that vascular smooth muscle cells, during prolonged hypertonic stress, activate system A and accumulate specific neutral amino acids which may act as organic osmolytes to help maintain normal cell volume.

Evidence for a lactate-anion exchanger in the rat jejunal basolateral membrane

BACKGROUND/AIMS

The mechanism by which lactate, absorbed from the intestinal lumen or generated within the epithelium, crosses the basolateral membrane of the enterocyte and enters the bloodstream has not previously been characterized in detail.

METHODS

L-lactate uptake into and efflux from isolated jejunal basolateral membrane vesicles was investigated at room temperature using rapid filtration techniques.

RESULTS

Furosemide sensitive uptake of L-lactate was unaffected by cis sodium or proton gradients but could be stimulated by a trans gradient of bicarbonate and chloride. Kinetic analysis showed uptake to consist of a saturable component with a Michaelis constant (Km) of 3.2 mmol/L and a maximum velocity (Vmax) of 67 pmol.mg protein-1 x s-1 and a nonsaturable alpha-4-hydroxy-cinnamic acid insensitive component. Pyruvate, butyrate, acetate, valerate, and propionate competitively inhibited lactate uptake into the vesicles. Efflux of lactate from preloaded vesicles was furosemide sensitive and accelerated by a trans bicarbonate gradient as well as by 10 mmol/L acetate, butyrate, and pyruvate.

CONCLUSIONS

It is concluded that there is a short chain-fatty acid carrier system in the intestinal basolateral membrane, which operates as an anion exchanger.

GLUT2 is the transporter for fructose across the rat intestinal basolateral membrane

BACKGROUND

The exact roles of disaccharidases and GLUT5 in the brush border membrane and GLUT2 in the basolateral membrane in the absorption of fructose across the intestine have not been fully determined. This paper describes characterization of fructose transport across the jejunal basolateral membrane using isolated membrane vesicles.

METHODS

Transport of fructose was measured using rapid filtration of vesicles. Luminal perfusion in vivo with glucose and fructose before vesicle preparation was used to assess modulation of GLUT2 activity. Western blotting measured the abundance of GLUT2 in the membrane.

RESULTS

The maximal rate of transport for fructose was 1100 pmol/mg protein/s and the Michaelis constant was 16 mmol/L. Fructose and glucose could completely inhibit the transport of each other. Perfusion of the intestinal lumen with fructose or glucose saline for 4 hours produced a fourfold increase in maximal fructose transport.

CONCLUSIONS

These data indicate that the one transport protein, GLUT2, is responsible for moving both fructose and glucose out of the enterocyte across the basolateral membrane under basal conditions. The activity of this, or a closely related carrier, is rapidly upregulated by the presence of hexoses in the intestinal lumen, explaining the potentiation of fructose absorption by luminal glucose and obviating any need to involve apical disaccharidases.

Role of liver-type glucose transporter (GLUT2) in transport across the basolateral membrane in rat jejunum

To obtain information on the regulation of glucose transport across the basolateral membrane (BLM) of intestinal epithelial cells, we measured the number of [3H]cytochalasin B binding sites and the level of liver-type glucose transporter (GLUT2) protein in the BLM in the jejunum of rats (i) with diabetes (ii) given a high-carbohydrate diet or (iii) with experimental hyperglycemia (12 h infusion of a high-glucose solution). A glucose uptake and the number of D-glucose inhibitable [3H]cytochalasin B binding sites in BLM vesicles were significantly increased in all three conditions. Western blot analysis showed that the amount of GLUT2 protein in BLM vesicles was increased in rats with diabetes and those given a high-carbohydrate diet, but not in those with experimental hyperglycemia. These results suggest that there is a mechanism for rapid regulation of glucose transport in the BLM that does not depend on change in the amount of GLUT2.

d-glucose transport systems in rat jejunal brush border membrane: Influence of ageing

Jejunal brush border membranes were isolated from rats of different ages (very young, young, adult and old); the gamma-GT specific activity and the vesicle volumes were unaffected by ageing, whilst protein content was significantly reduced in brush border from old rats. Vesicles were used to investigate the kinetics of Na-glucose cotransport under voltage-clamped and zero-trans conditions over a wide range of D-glucose concentrations (0.005-70 mM). Results provide evidence that in all the ages tested D-glucose can cross the brush border membrane both by a passive diffusional component and by two Na-dependent saturable transport systems, namely one with high-affinity and low-capacity and the other with low-affinity and high-capacity. However, in some old rats only one saturable and a very small passive component occur. The two Na-dependent transport systems were analyzed to define the stoichiometry of coupling between Na and glucose fluxes. In all the ages tested the Na:glucose ratio is higher in the high-affinity system than in the low-affinity one. Accordingly the effect of a superimposed membrane potential is more evident for the high-affinity transport mechanism. In conclusion, D-glucose transport systems seem to be unaffected by ageing from very young to adult rats; only in old animals age-related alterations can be observed.

Diabetes and glucose transporter gene expression in rat small intestine

The expressions of Na(+)-dependent glucose transporter (SGLT1) and five facilitative glucose transporter genes (GLUT1-5) in the small intestine of streptozotocin (STZ)-induced diabetic rats were examined by RNA blotting analysis. The transcripts of SGLT1 mRNA gave bands of 4.5 Kilobases (Kb) and 2.8Kb (very faint band). The levels of SGLT1 mRNA were significantly increased in 30- and 60-day STZ rats, but not changed in acute diabetic rats (2- to 10- day STZ rats). The GLUT2 mRNA levels changed in parallel with the D-galactose transport activity, being increased about 4-fold in 5-day STZ rats. The transcripts of GLUT5 mRNA gave three bands of 5.1Kb, 2.8Kb and 2.OKb, whose levels were significantly reduced in 30- and 60-day STZ rats. These results suggest that the facilitative glucose transporter (GLUT2), in addition to the Na(+)-dependent glucose transporter (SGLT1), may play an important role in intestinal glucose transport in diabetic rats.

Differential localization of two glucose transporter isoforms in rat kidney

The localization of two glucose transporter isoforms was mapped in the rat kidney: the high-Michaelis constant (Km; 15-20 mM) low-affinity "liver" transporter and the low-Km (1-2 mM) high-affinity "erythroid/brain" transporter. Both are basolateral membrane proteins, but the liver transporter was present exclusively in the S1 part of the proximal tubule, whereas the erythroid/brain transporter was expressed at variable levels in different nephron segments. Staining intensity was low in the straight proximal tubule (S3), intermediate in the medullary thin and thick ascending limbs, and highest in connecting segments and collecting ducts. In the collecting duct, the erythroid/brain glucose transporter was expressed at the highest level in intercalated cells; less was present in principal cells. In the papilla, only intercalated cells expressed this transporter isoform. These results suggest specific involvements of each transporter isoform in transepithelial glucose reabsorption by different segments of the proximal tubule. They also indicate that while the liver glucose transporter is present in gluconeogenic cells, there is a good correlation between the level of expression of the erythroid/brain glucose transporter and the glycolytic activity of the different nephron segments.

Liver glucose transporter: a basolateral protein in hepatocytes and intestine and kidney cells

The "liver" isoform of the facilitated diffusion glucose transporter is expressed predominantly in liver, intestine, kidney, and pancreatic islet beta-cells. The apparent molecular mass of the transporter in liver, kidney, and intestine is different, as detected by Western blot analysis of membrane proteins using antipeptide antibodies. However, as assessed by Northern blot analysis and molecular cloning, the same mRNA is expressed in these tissues, indicating that there are tissue-specific posttranslational modifications of the same transporter polypeptide. As determined by immunofluorescence analysis on frozen tissue sections, the liver glucose transporter is present on the sinusoidal membrane of hepatocytes, on the basolateral membrane of fully differentiated absorptive intestine epithelial cells, and on the basolateral membrane of proximal tubule cells of the kidney nephron. This localization is consistent with the involvement of the liver glucose transporter in several key steps of glucose metabolism: glucose uptake and release by the liver and absorption or reabsorption by epithelial cells of the intestine and kidney, respectively.

Facilitative glucose transporters: an expanding family

The uptake of glucose into most eukaryotic cells is accomplished by a carrier-mediated transport system, facilitative diffusion, which transports glucose down its chemical gradient in a stereospecific manner. Recent studies have shown that facilitative transport of glucose across the plasma membrane is mediated by a family of structurally related proteins. This review summarizes the structural and functional features of the family of facilitative glucose transporters.

Localization of the pancreatic beta cell glucose transporter to specific plasma membrane domains

Immunocytochemical techniques revealed that the "liver-type" glucose transporter is present in the insulin-producing beta cells of rat pancreatic islets but not in other islet endocrine cells. Ultrastructural analysis of the transporter by the protein A-gold technique showed that it is restricted to certain domains of the plasma membrane, its density being sixfold higher in microvilli facing adjacent endocrine cells than in the flat regions of the plasma membrane. These results support a possible role for this glucose transporter in glucose sensing by beta cells and provide evidence that these cells are polarized.

Cytochalasin B binding to rabbit proximal tubular basolateral membranes

Cytochalasin B binds to the Na+-independent D-glucose transporter in non-renal tissues. We have shown previously that the Na+-independent D-glucose transporter of the rabbit renal proximal tubular cell is localized exclusively in the basolateral membrane. To determine whether cytochalasin B binds to this renal transporter we measured binding of [3H]cytochalasin B to proximal tubular basolateral membranes isolated from rabbit kidneys. A steady state of binding is reached by 15 minutes at 20 degrees C over a concentration range of 0.01 to 50 microM. Non-linear regression analysis of cytochalasin B binding from 0.01 to 20 microM plotted according to Scatchard reveals two classes of binding sites with Kd 5.88 x 10(-8) M, Bmax 16.1 pmol/mg protein; and Kd 5.62 x 10(-5) M, Bmax 2816 pmol/mg protein. [3H]cytochalasin B (0.1 microM) binding to basolateral membranes is a reversible process; it is displacable by excess unlabeled cytochalasin B with a time course similar to binding of [3H]cytochalasin B. Binding of [3H]cytochalasin B is inhibited by 500 mM D-glucose (21%), 2-deoxy-D-glucose (57%) and 3-O-methyl-D-glucose (64%), but not by L-glucose. [3H]cytochalasin B binding is reduced 71% by 0.1 mM phloretin, but only 26% by 0.1 mM phlorizin. Such substrate specificity and inhibitor sensitivity are similar to those previously demonstrated in non-renal tissues by others as well as in rabbit renal proximal tubular basolateral membranes by us. Our data suggest that cytochalasin B binds to the Na+-independent D-glucose transporter or a component of the transporter in the renal proximal tubular basolateral membrane.

Sugar transport in isolated rat kidney papillary collecting duct cells

D-Glucose is an important substrate of energy metabolism and osmolyte synthesis in the renal papillary collecting duct. In order to characterize the cellular entry of D-glucose in this tubular segment, collecting duct cells were isolated from rat kidney papilla and the rate of D-glucose uptake was measured indirectly by monitoring the D-glucose-dependent O2 uptake in the presence of the uncoupler CCCP. D-Glucose uptake was found to be sodium-independent and not sensitive to phlorizin even at a concentration of 10(-3) M. Uptake was, however, completely inhibited by 10(-5) M cytochalasin B and 10(-4) M phloretin. The apparent Ki for cytochalasin B was 1.5 x 10(-6) M and for phloretin 2.0 x 10(-5) M. Studies on the substrate specificity revealed that at 1 mM D-mannose is taken up and metabolized to the same extent as D-glucose. A 50-fold higher concentration of 2-deoxy-D-glucose and 2-amino-2-deoxy-D-glucose inhibited D-glucose uptake completely whereas alpha-methyl-D-glucoside, D-allose, and D-galactose were without effect. Under conditions where D-glucose utilization was maximally stimulated an apparent Km of 1.2 mM and a Vmax of 1 mmol D-glucose/g protein.hour was found for D-glucose uptake. These results indicate that the D-glucose uptake into papillary collecting duct cells is probably mediated by a transport system similar to the one found in basal-lateral membranes of polarized renal, intestinal, and liver cells as well as in nonpolarized fat cells and erythrocytes.

Cloning and functional expression in bacteria of a novel glucose transporter present in liver, intestine, kidney, and beta-pancreatic islet cells

The well-characterized erythrocyte glucose transporter is also expressed in brain, adipocytes, kidney, muscle, and certain transformed cells, but not in liver, intestine, or the islets of Langerhans. Using as probe a cDNA encoding the rat brain glucose transporter, we isolated from a rat liver cDNA library a clone encoding a protein 55% identical in sequence to the rat brain transporter, and with a superimpossible hydropathy plot. We expressed this protein in an E. coli mutant defective in glucose uptake; the protein was incorporated into the bacterial membrane and functioned as a glucose transporter. This new transporter is expressed in liver, intestine, kidney, and the islets of Langerhans; immunofluorescence analysis showed that it is present in the plasma membrane of the insulin-producing beta cells. Insulinoma cells express, inappropriately, the erythrocyte glucose transporter, and we suggest that this may be related to their inability to secrete insulin in response to elevations in glucose.