A glucose sensor hiding in a family of transporters

Heteroaryl-O-glucosides as novel sodium glucose co-transporter 2 inhibitors. Part 1

A series of benzo-fused heteroaryl-O-glucosides was synthesized and evaluated in SGLT1 and 2 cell-based functional assays. Indole-O-glucoside 10a and benzimidazole-O-glucoside 18 exhibited potent in vitro SGLT2 inhibitory activity.

Inhibition of intestinal and renal Na+-glucose cotransporter by naringenin

Reduction in glucose uptake constitutes a possible means of controlling diabetic hyperglycemia. Using purified intestinal brush border membrane vesicles and everted intestinal sleeves, we have demonstrated that naringenin, a flavonoid present in citrus fruits and juices, significantly inhibited glucose uptake in the intestine. In addition, naringenin also elicited inhibitory actions towards glucose uptake in renal brush border membrane vesicles. Naringin, a glycoside of naringenin, was totally inactive in these aspects. Naringenin exhibited moderate inhibitory action on glucose uptake in rabbit intestinal brush border membrane vesicles, and showed strong inhibitory action in rat everted intestinal sleeves. The IC(50) values were 205.9 and 2.4 micromol/l, respectively. Lineweaver-Burk analysis demonstrated that naringenin inhibited glucose uptake in rat everted intestinal sleeves in a competitive manner with a K(i) value of 1.1 micromol/l. Glucose uptake activities in both the intestinal and renal brush border membrane vesicles of diabetic rats were significantly higher than in normal rats. Naringenin (500 microM) reduced glucose uptake by more than 60% in both the intestinal and renal brush border membrane vesicles of diabetic rats to a level similar to that of the normal rats. The IC(50) values of naringenin in the renal brush border membrane vesicles of normal and diabetic rats were 323.9 and 166.1 micromol/l, respectively. These results suggest that inhibition of intestinal glucose uptake and renal glucose reabsorption explains, in part at least, the in vivo antihyperglycemic action of naringenin and its derivatives. The possible application of these natural compounds in controlling hyperglycemia warrants further investigations.

Sodium-dependent glucose transporter reduces peroxynitrite and cell injury caused by cisplatin in renal tubular epithelial cells

Cisplatin causes nephropathy accompanied by two types of cell death, necrosis and apoptosis, according to its dosage. The mechanisms of necrosis are still unclear. In this study, we examined how high doses of cisplatin induce cell injury and whether a high affinity sodium-dependent glucose transporter (SGLT1) has a cytoprotective function in renal epithelial LLC-PK(1) cells. Cisplatin decreased in transepithelial electrical resistance (TER) and increased in the number of necrotic dead cells in a time dependent manner. Phloridzin, a potent SGLT1 inhibitor, enhanced both TER decrease and increase of necrotic dead cells caused by cisplatin. Cisplatin increased in the intracellular nitric oxide, superoxide anion and peroxynitrite productions. Phloridzin enhanced the peroxynitrite production caused by cisplatin. The intracellular diffusion of ZO-1 and TER decrease caused by cisplatin were inhibited by N-nitro-l-arginine methyl ester, a nitric oxide synthase inhibitor. Protein kinase C was not involved in the cisplatin-induced injury. 5,10,15,20-tetrakis-(4-sulfonatophenyl)-porphyrinato iron (III) and reduced glutathione, peroxynitrite scavengers, inhibited the cisplatin-induced ZO-1 diffusion, TER decrease, and increase of necrotic dead cells. These results suggest that peroxynitrite is a key mediator in the nephrotoxicity caused by high doses of cisplatin. SGLT1 endogenously carries out the cytoprotective function by the reduction of peroxynitrite production.

Apical GLUT2: a major pathway of intestinal sugar absorption

Understanding the mechanisms that determine postprandial fluctuations in blood glucose concentration is central for effective glycemic control in the management of diabetes. Intestinal sugar absorption is one such mechanism, and studies on its increase in experimental diabetes led us to propose a new model of sugar absorption. In the apical GLUT2 model, the glucose transported by the Na(+)/glucose cotransporter SGLT1 promotes insertion of GLUT2 into the apical membrane within minutes, so that the mechanism operates during assimilation of a meal containing high-glycemic index carbohydrate to provide a facilitated component of absorption up to three times greater than by SGLT1. Here we review the evidence for the apical GLUT2 model and describe how apical GLUT2 is a target for multiple short-term nutrient-sensing mechanisms by dietary sugars, local and endocrine hormones, cellular energy status, stress, and diabetes. These mechanisms suggest that apical GLUT2 is a potential therapeutic target for novel dietary or pharmacological approaches to control intestinal sugar delivery and thereby improve glycemic control.

How did the neurotransmitter cross the bilayer? A closer view

Plasma membrane neurotransmitter transporters for monoamines, GABA, glycine and excitatory amino acids are homologous to two sizable families of bacterial amino acid transporters. Recently, a high resolution structure was determined for a thermophilic glutamate transporter. Also, a bacterial tryptophan transporter related to the family of biogenic amine neurotransmitter transporters was functionally expressed. Structural insights from these and other bacterial transporters will help to rationalize the mechanisms for the increasingly complex functions that have been described for mammalian transporters, in addition to their modes of regulation. We touch on recent insights into the functions of neurotransmitter transporters in their physiological contexts.

Reorganization of ZO-1 by sodium-dependent glucose transporter activation after heat stress in LLC-PK1 cells

Heat stress (HS) induces activation of high-affinity sodium-dependent glucose transporter (SGLT1) in porcine renal LLC-PK(1) cells. In this study, we investigated the roles of SGLT1 activation in reorganization of zonula occludens-1 (ZO-1), a cytosolic tight junction (TJ) protein, after HS. HS (42 degrees C, 3 h) caused decrease in transepithelial electrical resistance (TER). Subsequent incubation at 37 degrees C for 12 h increased TER above pre-HS level. The treatment of phloridzin, a potent SGLT1 inhibitor, or the replacement of glucose with a nonmetabolizable glucose analog blocked the recovery of TER and increased the transepithelial flux of FITC-dextran (4,000 Da). Immunofluorescent staining of ZO-1 showed that HS diffused ZO-1 from cell contact to cytosolic sites. Furthermore, the fraction of ZO-1 was distributed from the Triton X-100 insoluble to the Triton X-100 soluble pool. After incubation at 37 degrees C for 12 h, cell contact and ZO-1 extractability with Triton X-100 returned to pre-HS conditions, but the recovery was completely prevented by phloridzin. Tyrosine kinases activity was increased by HS that was inhibited by phloridzin. Genistein and CGP77675, tyrosine kinases inhibitors, blocked the recovery of TER and increased the transepithelial flux of FITC-dextran. Furthermore, these inhibitors prevented the recovery of cell contact and ZO-1 extractability with Triton X-100 as same as phloridzin. These findings suggested that the activation of SGLT1 reorganized ZO-1 mediated by elevation of tyrosine kinases activity after heat injury.

The Broadly Selective Human Na+/Nucleoside Cotransporter(hCNT3) Exhibits Novel Cation-coupled Nucleoside TransportCharacteristics

The concentrative nucleoside transporter (CNT) protein family in humans is represented by three members, hCNT1, hCNT2, and hCNT3. hCNT3, a Na+/nucleoside symporter, transports a broad range of physiological purine and pyrimidine nucleosides as well as anticancer and antiviral nucleoside drugs, and belongs to a different CNT subfamily than hCNT1/2. H+-dependent Escherichia coli NupC and Candida albicans CaCNT are also CNT family members. The present study utilized heterologous expression in Xenopus oocytes to investigate the specificity, mechanism, energetics, and structural basis of hCNT3 cation coupling. hCNT3 exhibited uniquely broad cation interactions with Na+, H+, and Li+ not shared by Na+-coupled hCNT1/2 or H+-coupled NupC/CaCNT. Na+ and H+ activated hCNT3 through mechanisms to increase nucleoside apparent binding affinity. Direct and indirect methods demonstrated cation/nucleoside coupling stoichiometries of 2:1 in the presence of Na+ and both Na+ plus H+, but only 1:1 in the presence of H+ alone, suggesting that hCNT3 possesses two Na+-binding sites, only one of which is shared by H+. The H+-coupled hCNT3 did not transport guanosine or 3'-azido-3'-deoxythymidine and 2',3'-dideoxycytidine, demonstrating that Na+- and H+-bound versions of hCNT3 have significantly different conformations of the nucleoside binding pocket and/or translocation channel. Chimeric studies between hCNT1 and hCNT3 located hCNT3-specific cation interactions to the C-terminal half of hCNT3, setting the stage for site-directed mutagenesis experiments to identify the residues involved.

Adverse effect of cadmium on binding of transcription factor Sp1 to the GC-rich regions of the mouse sodium-glucose cotransporter 1, SGLT1, promoter

Exposure of the kidney to cadmium can cause glucosuria. Effect of cadmium on sodium-glucose cotransporter 1, (SGLT1) mRNA molecules in cultured mouse kidney cortical cells was determined by quantitative competitive RT-PCR. SGLT1 mRNA molecules decreased from 58 x 10(4) microg(-1) total RNA in untreated cells to 29 x 10(4) microg(-1) total RNA in cells exposed to 5 microM cadmium. Increasing cadmium to 7.5 and 10 microM, reduced mRNA molecules to 21 x 10(4) and 12 x 10(4) microg(-1) total RNA, respectively. The half-life of SGLT1 mRNA in control and in cells exposed to 7.5 microM cadmium were almost the same and calculated to be 9.1 h (S.E.+/-2.7) for the former and 8.5 h (S.E.+/-2.2) for the latter. We also analyzed mouse SGLT1 promoter sequences and identified two conserved Sp1 binding sites. The Sp1 binding sequences were used as probes in electrophoretic mobility shift assay (EMSA) with nuclear proteins from cultured cells. Intensity of complexes of the 5' and the 3' Sp1 probes with nuclear Sp1 from cells treated with 7.5 microM cadmium were 84% (S.E.+/-4) and 61% (S.E.+/-14) of controls, respectively. Cadmium had no effect on expression of Sp1 mRNA or protein level. Cadmium-induced inhibition of glucose uptake in kidney may be the result of transcriptional down-regulation of SGLT1 mediated through modification of Sp1 binding to its promoter.

SLC5A9/SGLT4, a new Na+-dependent glucose transporter, is an essential transporter for mannose, 1,5-anhydro-D-glucitol, and fructose

We isolated a cDNA clone of SLC5A9/SGLT4 from human small intestinal full-length cDNA libraries, and functionally characterized it in vitro. The messenger RNA encoding SGLT4 was mainly expressed in the small intestine and kidney, among the human tissues tested. COS-7 cells transiently expressing SGLT4 exhibited Na(+)-dependent alpha-methyl-D-glucopyranoside (AMG) transport activity with an apparent K(m) of 2.6 mM, suggesting that SGLT4 is a low affinity-type transporter. The rank order of naturally occurring sugar analogs for the inhibition of AMG transport was: D-mannose (Man) >> D-glucose (Glc) > D-fructose (Fru) = 1,5-anhydro-D-glucitol (1,5AG) > D-galactose (Gal). Recognition of Man as a substrate was confirmed by direct uptake of Man into the cell. COS-7 cells expressing a putative murine SGLT4 ortholog showed similar Na(+)-dependent AMG transport activity and a similar deduced substrate specificity. These results suggest that SGLT4 would have unique physiological functions (i.e., absorption and/or reabsorption of Man, 1,5AG, and Fru, in addition to Glc).

Glucose as a hormone: receptor-mediated glucose sensing in the yeast Saccharomyces cerevisiae

Because glucose is the principal carbon and energy source for most cells, most organisms have evolved numerous and sophisticated mechanisms for sensing glucose and responding to it appropriately. This is especially apparent in the yeast Saccharomyces cerevisiae, where these regulatory mechanisms determine the distinctive fermentative metabolism of yeast, a lifestyle it shares with many kinds of tumour cells. Because energy generation by fermentation of glucose is inefficient, yeast cells must vigorously metabolize glucose. They do this, in part, by carefully regulating the first, rate-limiting step of glucose utilization: its transport. Yeast cells have learned how to sense the amount of glucose that is available and respond by expressing the most appropriate of its 17 glucose transporters. They do this through a signal transduction pathway that begins at the cell surface with the Snf3 and Rgt2 glucose sensors and ends in the nucleus with the Rgt1 transcription factor that regulates expression of genes encoding glucose transporters. We explain this glucose signal transduction pathway, and describe how it fits into a highly interconnected regulatory network of glucose sensing pathways that probably evolved to ensure rapid and sensitive response of the cell to changing levels of glucose.

Mice without the regulator gene Rsc1A1 exhibit increased Na+-D-glucose cotransport in small intestine and develop obesity

The product of the intronless single copy gene RSC1A1, named RS1, is an intracellular 617-amino-acid protein that is involved in the regulation of the Na(+)-d-glucose cotransporter SGLT1. We generated and characterized RS1 knockout (RS1(-/-) mice. In the small intestines of RS1(-/-) mice, the SGLT1 protein was up-regulated sevenfold compared to that of wild-type mice but was not changed in the kidneys. The up-regulation of SGLT1 was posttranscriptional. Small intestinal d-glucose uptake measured in jointly perfused small bowel and liver was increased twofold compared to that of the wild-type, with increased peak concentrations of d-glucose in the portal vein. At birth, the weights of RS1(-/-) and wild-type mice were similar. At the age of 3 months, male RS1(-/-) mice had 5% higher weights and 15% higher food intakes, whereas their energy expenditures and serum leptin concentrations were similar to those of wild-type mice. At the age of 5 months, male and female RS1(-/-) mice were obese, with 30% increased body weight, 80% increased total fat, and 30% increased serum cholesterol. At this age, serum leptin was increased, whereas food intake was the same as for wild-type mice. The data suggest that the removal of RS1 leads to leptin-independent up-regulation of food intake, which causes obesity.

Surprising versatility of Na+-glucose cotransporters: SLC5

SLC5 is an ancient gene family with 11 members in the human genome. These membrane proteins have diverse, multiple functions ranging from actively transporting solutes, ions, and water, to channeling water and urea, to sensing glucose in cholinergic neurons. Metabolic disorders have been identified that are associated with congenital mutations in two of the human genes.

Analysis of Transport Activity of Arabidopsis Sugar Alcohol Permease Homolog AtPLT5

The transporters responsible for sugar uptake into non-photosynthetic sink tissues in plants, such as roots and flowers, have not been fully identified and analyzed. Plants encode around 100 putative sugar transporters within the major facilitator superfamily, yet only a few have been studied. Here we report the analysis of a sugar alcohol permease homolog (AtPLT5, At3g18830) from Arabidopsis. A wide range of sugars including hexoses, pentoses, tetroses, a sugar acid, and sugar alcohols but not disaccharides induced inward currents in oocytes expressing AtPLT5. AtPLT5 expression also resulted in 14C-labeled substrate uptake in oocytes, indicating that AtPLT5 encodes an ion-coupled uptake transporter. K(0.5) values for glucose and sorbitol were highly dependent on external pH. Expression of AtPLT5 was found primarily in sink tissues: in the elongation zone of roots, in the inflorescence stem, and several floral structures, especially in the floral abscission zone. Expression was induced by mechanical wounding and insect feeding. Analysis of transport properties and expression in Arabidopsis indicate that AtPLT5 functions to transport a wide range of sugars into specific sink tissues in the plant.

Regulatory Mechanisms of Intracellular Distribution of Na+-dependent Glucose Transporter and the Role in Recovery from Cellular Injury

Exposure of cells or organs to sublethal stress induces the expression of some heat-shock proteins (Hsp), including Hsp70. In porcine renal LLC-PK(1) cells, heat stress (HS) elevates Hsp70 expression and Na(+)-dependent glucose transport. We examined whether Na(+)-dependent glucose transporter (SGLT1) interacts with Hsp70 to elevate SGLT1 activity and whether SGLT1 activation is involved in the recovery from HS injury. HS (42 degrees C for 3 h) elevated SGLT1 activity and expression of SGLT1 in the apical membrane fraction. HS increased the maximal transport rate (V(max)), but did not affect the apparent affinity constant (K(m)) for glucose. The HS-induced SGLT1 activation was inhibited by anti-transforming growth factor (TGF)-beta1 antibody. Furthermore, transfection of anti-Hsp70 antibody into the cells inhibited SGLT1 activation. These results suggest that HS induces TGF-beta1 secretion, and then Hsp70 forms a complex with SGLT1 and increases the distribution of SGLT1 in the apical membrane. Next, we examined the effect of HS on plasma membrane integrity. Accumulation of calcein, a membrane-impermeable fluorescent dye, was decreased by HS and then returned to basal level. This recovery was inhibited by phloridzin, a selective SGLT inhibitor, and nonmetabolizable glucose analogues. Anti-TGF-beta1 antibody also inhibited the recovery of calcein accumulation. Taken together, the present results show that HS elevates SGLT1 activity mediated via the TGF-beta1 signaling pathway and that the increase in glucose uptake is necessary to repair plasma membrane injury.

Neuronal glucosensing: what do we know after 50 years?

Glucosensing neurons are specialized cells that use glucose as a signaling molecule to alter their action potential frequency in response to variations in ambient glucose levels. Glucokinase (GK) appears to be the primary regulator of most neuronal glucosensing, but other regulators almost certainly exist. Glucose-excited neurons increase their activity when glucose levels rise, and most use GK and an ATP-sensitive K(+) channel as the ultimate effector of glucose-induced signaling. Glucose-inhibited (GI) neurons increase their activity at low glucose levels. Although many use GK, it is unclear what the final pathway of GI neuronal glucosensing is. Glucosensing neurons are located in brain sites and respond to and integrate a variety of hormonal, metabolic, transmitter, and peptide signals involved in the regulation of energy homeostasis and other biological functions. Although it is still uncertain whether daily fluctuations in blood glucose play a specific regulatory role in these physiological functions, it is clear that large decreases in glucose availability stimulate food intake and counterregulatory responses that restore glucose levels to sustain cerebral function. Finally, glucosensing is altered in obesity and after recurrent bouts of hypoglycemia, and this altered sensing may contribute to the adverse outcomes of these conditions. Thus, although much is known, much remains to be learned about the physiological function of brain glucosensing neurons.

AtSUC8 and AtSUC9 encode functional sucrose transporters, but the closely related AtSUC6 and AtSUC7 genes encode aberrant proteins in different Arabidopsis ecotypes

Three members of the Arabidopsis sucrose transporter gene family, AtSUC6-AtSUC8 (At5g43610; At1g66570; At2g14670), share a high degree of sequence homology in their coding regions and even in their introns and in their 5'- and 3'-flanking regions. A fourth sucrose transporter gene, AtSUC9 (At5g06170), which is on the same branch of the AtSUC-phylogenetic tree, shows only slightly less sequence homology. Here we present data demonstrating that two genes from this subgroup, AtSUC6 and AtSUC7, encode aberrant proteins and seem to represent sucrose transporter pseudogenes, whereas AtSUC8 and AtSUC9 encode functional sucrose transporters. These results are based on analyses of splice patterns and polymorphic sites between these genes in different Arabidopsis ecotypes, as well as on functional analyses by cDNA expression in baker's yeast. For one of these genes, AtSUC7 (At1g66570), different, ecotype-specific splice patterns were observed in Wassilewskija (Ws), C24, Columbia wild type (Col-0) and Landsberg erecta (Ler). No incorrect splicing and no sequence polymorphism were detected in the cDNAs of AtSUC8 and AtSUC9, which encode functional sucrose transporters and are expressed in floral tissue. Finally, promoter-reporter gene plants and T-DNA insertion lines were analyzed for AtSUC8 and AtSUC9.

Zinc potentiates an uncoupled anion conductance associated with the dopamine transporter

Binding of Zn(2+) to an endogenous binding site in the dopamine transporter (DAT) leads to inhibition of dopamine (DA) uptake and enhancement of carrier-mediated substrate efflux. To elucidate the molecular mechanism for this dual effect, we expressed the DAT and selected mutants in Xenopus laevis oocytes and applied the two-electrode voltage clamp technique together with substrate flux studies employing radiolabeled tracers. Under voltage clamp conditions we found that Zn(2+) (10 mum) enhanced the current induced by both DA and amphetamine. This was not accompanied by a change in the uptake rate but by a marked increase in the charge/DA flux coupling ratio as assessed from concomitant measurements of [(3)H]DA uptake and currents in voltage-clamped oocytes. These data suggest that Zn(2+) facilitates an uncoupled ion conductance mediated by DAT. Whereas this required substrate in the wild type (WT), we observed that Zn(2+) by itself activated such a conductance in a previously described mutant (Y335A). This signifies that the conductance is not strictly dependent on an active transport process. Ion substitution experiments in Y335A, as well as in WT, indicated that the uncoupled conductance activated by Zn(2+) was mainly carried by Cl(-). Experiments in oocytes under non-voltage-clamped conditions revealed furthermore that Zn(2+) could enhance the depolarizing effect of substrates in oocytes expressing WT. The data suggest that by potentiating an uncoupled Cl(-) conductance, Zn(2+) is capable of modulating the membrane potential of cells expressing DAT and as a result cause simultaneous inhibition of uptake and enhancement of efflux.

Gut-derived signaling molecules and vagal afferents in the control of glucose and energy homeostasis

PURPOSE OF REVIEW

The control of glucose and energy homeostasis, including feeding behaviour, is tightly regulated by gut-derived peptidic and nonpeptidic endocrine mediators, autonomic nervous signals, as well as nutrients such as glucose. We will review recent findings on the role of the gastrointestinal tract innervation and of portal vein glucose sensors; we will review selected data on the action of gastrointestinally released hormones.

RECENT FINDINGS

The involvement of mechanosensory vagal afferents in postprandial meal termination has been clarified using mouse models with selective impairments of genes required for development of mechanosensory fibres. These activate central glucogen-like peptide-1/glucogen-like peptide-2 containing ascending pathways linking the visceroceptive brainstem neurons to hypothalamic nuclei. Mucosal terminals comprise the chemosensory vagal afferents responsive to postprandially released gastrointestinal hormones. The mechanism by which the hepatoportal glucose sensor stimulates glucose utilization by muscles was demonstrated, using genetically modified mice, to be insulin-independent but to require GLUT4 and AMP-kinase. This sensor is a key site of glucogen-like peptide-1 action and plays a critical role in triggering first phase insulin secretion. PeptideYY and ghrelin target intracerebral receptors as they are bidirectionally transported across the blood brain barrier. The anorectic functions of peripherally released peptideYY may however be mediated both via vagal afferents and intracerebral Y2 receptors in the brainstem and arcuate nucleus.

SUMMARY

These recent findings demonstrate that the use of improved anatomical and physiological techniques and animal models with targeted gene modifications lead to an improved understanding of the complex role of gastrointestinal signals in the control of energy homeostasis.

Identification of more than 200 glucose-responsive Arabidopsis genes none of which responds to 3-O-methylglucose or 6-deoxyglucose

The response of some plant genes to glucose analogues 3-O-methylglucose (3OMG) or 6-deoxyglucose (6DOG) has been cited as evidence for metabolism-independent glucose signalling. To analyse such signalling using a genetic approach, we sought to identify Arabidopsis glucose-responsive genes which also respond to 3OMG and 6DOG in seedlings. Microarray analysis of gene expression in glucose-treated seedlings and RT-PCR analysis of glucose-treated leaf sections identified more than 200 glucose-responsive genes, but none responded to 3OMG or 6DOG. These data together with other published data on individual genes fail to identify any Arabidopsis sugar-responsive genes which also respond to 3OMG or 6DOG.

Ezrin regulates NHE3 translocation and activation after Na+-glucose cotransport

Initiation of Na(+)-glucose cotransport in intestinal epithelial cells leads to activation of the apical Na(+)-H(+) exchanger NHE3 and subsequent increases in cytoplasmic pH (pH(i)). This process requires activation of p38 mitogen-activated protein (MAP) kinase, but additional signaling intermediates have not been identified. One candidate is the cytoskeletal linker protein ezrin, which interacts with NHE3 via specific regulatory proteins. The data show that initiation of Na(+)-glucose cotransport resulted in rapid increases in both apical membrane-associated NHE3 and cytoskeletal-associated ezrin and occurred in parallel with ezrin phosphorylation at threonine 567. Phosphorylation at this site is known to activate ezrin and increase its association with actin. Consistent with a central role for ezrin activation in this NHE3 regulation, an N-terminal dominant negative ezrin construct inhibited both NHE3 recruitment and pH(i) increases after Na(+)-glucose cotransport. Ezrin phosphorylation occurred in parallel with p38 MAP kinase activation, and the latter proceeded normally in cells expressing dominant negative ezrin. In contrast, inhibition of p38 MAP kinase prevented increases in ezrin phosphorylation after initiation of Na(+)-glucose cotransport. Thus, ezrin phosphorylation after Na(+)-glucose cotransport requires p38 MAP kinase activity, but p38 MAP kinase activation does not require ezrin function. These data describe a specific role for ezrin in the coordinate regulation of Na(+)-glucose cotransport and Na(+)-H(+) exchange. Intact ezrin function is necessary for NHE3 recruitment to the apical membrane and NHE3-dependent pH(i) increases triggered by Na(+)-glucose cotransport. The data also define a pathway of p38 MAP kinase-dependent ezrin activation.

The human tumour suppressor gene SLC5A8 expresses a Na+-monocarboxylate cotransporter

The orphan cotransport protein expressed by the SLC5A8 gene has been shown to play a role in controlling the growth of colon cancers, and the silencing of this gene is a common and early event in human colon neoplasia. We expressed this protein in Xenopus laevis oocytes and have found that it transports small monocarboxylic acids. The electrogenic activity of the cotransporter, which we have named SMCT (sodium monocarboxylate transporter), was dependent on external Na(+) and was compatible with a 3 : 1 stoichiometry between Na(+) and monocarboxylates. A portion of the SMCT-mediated current was also Cl(-) dependent, but Cl(-) was not cotransported. SMCT transports a variety of monocarboxylates (similar to unrelated monocarboxylate transport proteins) and most transported monocarboxylates demonstrated K(m) values near 100 microm, apart from acetate and d-lactate, for which the protein showed less affinity. SMCT was strongly inhibited by 1 mm probenecid or ibuprofen. In the absence of external substrate, a Na(+)-independent leak current was also observed to pass through SMCT. SMCT activity was strongly inhibited after prolonged exposure to high external concentrations of monocarboxylates. The transport of monocarboxylates in anionic form was confirmed by the observation of a concomitant alkalinization of the cytosol. SMCT, being expressed in colon and kidney, represents a novel means by which Na(+), short-chain fatty acids and other monocarboxylates are transported in these tissues. The significance of a Na(+)-monocarboxylate transporter to colon cancer presumably stems from the transport of butyrate, which is well known for having anti-proliferative and apoptosis-inducing activity in colon epithelial cells.

Na(+)-D-glucose cotransporter in muscle capillaries increases glucose permeability

By immunohistochemistry, we demonstrated the localization of the Na(+)-D-glucose cotransporter SGLT1 in capillaries of rat heart and skeletal muscle, but not in capillaries of small intestine and submandibular gland. mRNA of SGLT1 was identified in skeletal muscle and primary cultured coronary endothelial cells. The functional relevance of SGLT1 for glucose transport across capillary walls in muscle was tested by measuring the extraction of D-glucose from the perfusate during non-recirculating perfusion of isolated rat hindlimbs. In this model, D-glucose extraction from the perfusate is increased by insulin which accelerates D-glucose uptake into myocytes by increasing the concentration of glucose transporter GLUT4 in the plasma membrane. The insulin-induced increase of D-glucose extraction from the perfusate was abolished after blocking SGLT1 with the specific inhibitor phlorizin. The data show that SGLT1 in capillaries of skeletal muscle is required for the action of insulin on D-glucose supply of myocytes.

The sodium/glucose cotransport family SLC5

The sodium/glucose cotransporter family (SLCA5) has 220 or more members in animal and bacterial cells. There are 11 human genes expressed in tissues ranging from epithelia to the central nervous system. The functions of nine have been revealed by studies using heterologous expression systems: six are tightly coupled plasma membrane Na(+)/substrate cotransporters for solutes such as glucose, myo-inositol and iodide; one is a Na(+)/Cl(-)/choline cotransporter; one is an anion transporter; and another is a glucose-activated ion channel. The exon organization of eight genes is similar in that each comprises 14-15 exons. The choline transporter (CHT) is encoded in eight exons and the Na(+)-dependent myo-inositol transporter (SMIT) in one exon. Mutations in three genes produce genetic diseases (glucose-galactose malabsorption, renal glycosuria and hypothyroidism). Members of this family are multifunctional membrane proteins in that they also behave as uniporters, urea and water channels, and urea and water cotransporters. Consequently it is a challenge to determine the role(s) of these genes in human physiology and pathology.

Intestinal absorption in health and disease--sugars

Carbohydrates are mostly digested to glucose, fructose and galactose before absorption by the small intestine. Absorption across the brush border and basolateral membranes of enterocytes is mediated by sodium-dependent and -independent membrane proteins. Glucose and galactose transport across the brush border occurs by a Na(+)/glucose (galactose) co-transporter (SGLT1), whereas passive fructose transport is mediated by a uniporter (GLUT5). The passive exit of all three sugars out of the cell across the basolateral membrane occurs through two uniporters (GLUT2 and GLUT5). Mutations in SGLT1 cause a major defect in glucose and galactose absorption (glucose-galactose Malabsorption), but mutations in GLUT2 do not appear to disrupt glucose and galactose absorption. Studies on GLUT1 null mice and Fanconi-Bickel patients suggest that there is another exit pathway for glucose and galactose that may involve exocytosis. There are no known defects of fructose absorption.

The sodium/glucose cotransport family SLC5

The sodium/glucose cotransporter family (SLCA5) has 220 or more members in animal and bacterial cells. There are 11 human genes expressed in tissues ranging from epithelia to the central nervous system. The functions of nine have been revealed by studies using heterologous expression systems: six are tightly coupled plasma membrane Na(+)/substrate cotransporters for solutes such as glucose, myo-inositol and iodide; one is a Na(+)/Cl(-)/choline cotransporter; one is an anion transporter; and another is a glucose-activated ion channel. The exon organization of eight genes is similar in that each comprises 14-15 exons. The choline transporter (CHT) is encoded in eight exons and the Na(+)-dependent myo-inositol transporter (SMIT) in one exon. Mutations in three genes produce genetic diseases (glucose-galactose malabsorption, renal glycosuria and hypothyroidism). Members of this family are multifunctional membrane proteins in that they also behave as uniporters, urea and water channels, and urea and water cotransporters. Consequently it is a challenge to determine the role(s) of these genes in human physiology and pathology.