GLUT12 expression and regulation in murine small intestine and human Caco-2 cells.

GLUT12 was cloned from the mammary cancer cell line MCF-7, but its physiological role still needs to be elucidated. To gain more knowledge of GLUT12 function in the intestine, we investigated GLUT12 subcellular localization in the small intestine and its regulation by sugars, hormones, and intracellular mediators in Caco-2 cells and mice. Immunohistochemical methods were used to determine GLUT12 subcellular localization in human and murine small intestine. Brush border membrane vesicles were isolated for western blot analyses. Functional studies were performed in Caco-2 cells by measuring α-methyl-d-glucose (αMG) uptake in the absence of sodium. GLUT12 is located in the apical cytoplasm, below the brush border membrane, and in the perinuclear region of murine and human enterocytes. In Caco-2 cells, GLUT12 translocation to the apical membrane and α-methyl- d-glucose uptake by the transporter are stimulated by protons, glucose, insulin, tumor necrosis factor-α (TNF-α), protein kinase C, and AMP-activated protein kinase. In contrast, hypoxia decreases GLUT12 expression in the apical membrane. Upregulation of TNF-α and hypoxia-inducible factor-1α ( HIF-1α) genes is found in the jejunal mucosa of diet-induced obese mice. In these animals, GLUT12 expression in the brush border membrane is slightly decreased compared with lean animals. Moreover, an intraperitoneal injection of insulin does not induce GLUT12 translocation to the membrane, as it occurs in lean animals. GLUT12 rapid translocation to the enterocytes' apical membrane in response to glucose and insulin could be related to GLUT12 participation in sugar absorption during postprandial periods. In obesity, in which insulin sensitivity is reduced, the contribution of GLUT12 to sugar absorption is affected.

Basolateral presence of the proinflammatory cytokine tumor necrosis factor -α and secretions from adipocytes and macrophages reduce intestinal sugar transport.

We have previously demonstrated in Caco-2 cells that tumor necrosis factor-α (TNF-α) inhibits sugar uptake, acting from the apical membrane, by decreasing the expression of the Na+ -glucose cotransporter SGLT1 in the brush border membrane. The goal was to investigate the hypothesis that TNF-α from abdominal adipose tissue (adipocytes and macrophages) would decrease sugar and amino acid transport acting from the basolateral membrane of the enterocytes. TNF-α placed in the basal compartment of Caco-2 cells decreased α-methyl- d-glucose (αMG) and glutamine uptake. The apical medium derived from these Caco-2 cells apically placed in another set of cells, also reduced sugar and glutamine transport. Reverse-transcription polymerase chain reaction analysis demonstrated upregulation of TNF-α, IL-1ß, and MCP1 expression in Caco-2 cells exposed to basal TNF-α. Similarly, αMG uptake was inhibited after Caco-2 cells were incubated, in the basal compartment, with medium from visceral human mesenchymal stem cells-derived adipocytes of overweight individuals. The apical medium collected from those Caco-2 cells, and placed in the upper side of other set of cells, also decreased sugar uptake. Basal presence of medium derived from lipopolysaccharide-activated macrophages and nonactivated macrophages decreased αMG uptake as well. Diet-induced obese mice showed an increase in the visceral adipose tissue surrounding the intestine. In this physiological condition, there was a reduction on αMG uptake in jejunal everted rings. Altogether, these results suggest that basolateral TNF-α, which can be produced by adipocytes and macrophages during obesity, would be able to activate TNF-α and other proinflammatory proteins expression in the small intestine and diminish intestinal sugar and amino acids transport.

EPA blocks TNF-α-induced inhibition of sugar uptake in Caco-2 cells via GPR120 and AMPK.

The aim of the present work was to investigate in Caco-2 cells whether eicosapentaenoic acid (EPA), an omega-3 polyunsaturated fatty acid, could block the inhibitory effect of tumor necrosis factor-α (TNF-α) on sugar transport, and identify the intracellular signaling pathways involved. After pre-incubation of the Caco-2 cells with TNF-α and EPA for 1 hr, EPA prevented the inhibitory effect of the cytokine on α-methyl-d-glucose (αMG) uptake (15 min) and on SGLT1 expression at the brush border membrane, measured by Western blot. The ERK1/2 inhibitor PD98059 and the AMPK activator AICAR also prevented the inhibitory effect of TNF-α on both αMG uptake and SGLT1 expression. Interestingly, the AMPK inhibitor, Compound C, abolished the ability of EPA to prevent TNF-α-induced reduction of sugar uptake and transporter expression. The GPR120 antagonist, AH7614, also blocked the preventive effect of EPA on TNF-α-induced decrease of αMG uptake and AMPK phosphorylation. In summary, TNF-α inhibits αMG uptake by decreasing SGLT1 expression in the brush border membrane through the activation of ERK1/2 pathway. EPA prevents the inhibitory effect of TNF-α through the involvement of GPR120 and AMPK activation.

Functional role of glucose metabolism, osmotic stress, and sodium-glucose cotransporter isoform-mediated transport on Na+/H+ exchanger isoform 3 activity in the renal proximal tubule.

Na(+)-glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na(+)-H(+) exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLT-mediated glucose uptake regulates NHE3-mediated NaHCO3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLT-mediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.

TNFα regulates sugar transporters in the human intestinal epithelial cell line Caco-2.

PURPOSE: During intestinal inflammation TNFα levels are increased and as a consequence malabsorption of nutrients may occur. We have previously demonstrated that TNFα inhibits galactose, fructose and leucine intestinal absorption in animal models. In continuation with our work, the purpose of the present study was to investigate in the human intestinal epithelial cell line Caco-2, the effect of TNFα on sugar transport and to identify the intracellular mechanisms involved. METHODS: Caco-2 cells were grown on culture plates and pre-incubated during different periods with various TNFα concentrations before measuring the apical uptake of galactose, α-methyl-glucoside (MG) or fructose for 15 min. To elucidate the signaling pathway implicated, cells were pre-incubated for 30min with the PKA inhibitor H-89 or the PKC inhibitor chelerythrine, before measuring the sugar uptake. The expression in the apical membrane of the transporters implicated in the sugars uptake process (SGLT1 and GLUT5) was determined by Western blot. RESULTS: TNFα inhibited 0.1mM MG uptake after pre-incubation of the cells for 6-48h with the cytokine and in the absence of cytokine pre-incubation. In contrast, 5mM fructose uptake was stimulated by TNFα only after long pre-incubation times (24 and 48 h). These effects were mediated by the binding of the cytokine to its specific receptor TNFR1, present in the apical membrane of the Caco-2 cells. Analysis of the expression of the MG and fructose transporters at the brush border membrane of the cells, after 24h pre-incubation with the cytokine, revealed decrease on the amount of SGLT1 and increase on the amount of GLUT5 proteins. Short-term inhibition of MG transport by TNFα was not modified by H-89 but was blocked by chelerythrine. CONCLUSIONS: SGLT1 and GLUT5 expression in the plasma membrane is regulated by TNFα in the human epithelial cell line Caco-2 cells, leading to alteration on sugars transport, suggesting that TNFα could be considered as a physiological local regulator of nutrients absorption in response to an intestinal inflammatory status.

LX4211 increases serum glucagon-like peptide 1 and peptide YY levels by reducing sodium/glucose cotransporter 1 (SGLT1)-mediated absorption of intestinal glucose.

LX4211 [(2S,3R,4R,5S,6R)-2-(4-chloro-3-(4-ethoxybenzyl)phenyl)-6-(methylthio)tetrahydro-2H-pyran-3,4,5-triol], a dual sodium/glucose cotransporter 1 (SGLT1) and SGLT2 inhibitor, is thought to decrease both renal glucose reabsorption by inhibiting SGLT2 and intestinal glucose absorption by inhibiting SGLT1. In clinical trials in patients with type 2 diabetes mellitus (T2DM), LX4211 treatment improved glycemic control while increasing circulating levels of glucagon-like peptide 1 (GLP-1) and peptide YY (PYY). To better understand how LX4211 increases GLP-1 and PYY levels, we challenged SGLT1 knockout (-/-) mice, SGLT2-/- mice, and LX4211-treated mice with oral glucose. LX4211-treated mice and SGLT1-/- mice had increased levels of plasma GLP-1, plasma PYY, and intestinal glucose during the 6 hours after a glucose-containing meal, as reflected by area under the curve (AUC) values, whereas SGLT2-/- mice showed no response. LX4211-treated mice and SGLT1-/- mice also had increased GLP-1 AUC values, decreased glucose-dependent insulinotropic polypeptide (GIP) AUC values, and decreased blood glucose excursions during the 6 hours after a challenge with oral glucose alone. However, GLP-1 and GIP levels were not increased in LX4211-treated mice and were decreased in SGLT1-/- mice, 5 minutes after oral glucose, consistent with studies linking decreased intestinal SGLT1 activity with reduced GLP-1 and GIP levels 5 minutes after oral glucose. These data suggest that LX4211 reduces intestinal glucose absorption by inhibiting SGLT1, resulting in net increases in GLP-1 and PYY release and decreases in GIP release and blood glucose excursions. The ability to inhibit both intestinal SGLT1 and renal SGLT2 provides LX4211 with a novel dual mechanism of action for improving glycemic control in patients with T2DM.

The metabolic and biochemical impact of glucose 6-sulfonate (sulfoquinovose), a dietary sugar, on carbohydrate metabolism.

Increased activity of the main carbohydrate pathways (glycolysis, pentose phosphate, and hexosamine biosynthetic pathways) is one of the hallmarks of metabolic diseases such as cancer. Sulfoquinovosyl diacylglycerol is a sulfoglycolipid found in the human diet that possesses anticancer activity that is absent when its carbohydrate moiety (glucose 6-sulfonate or sulfoquinovose) is removed. This work used bacterial systems to further understand the metabolism of this sugar through three main carbohydrate processing pathways and how this could influence its biological activity. Using (13)C NMR spectroscopy and enzyme assays, we showed that glucose 6-sulfonate cannot enter the pentose phosphate pathway, hence decreasing pentose and nucleotide biosyntheses. In glycolysis, glucose 6-sulfonate only provides one pyruvate per monosaccharide molecule, decreasing the flux of this pathway by half when compared to glucose 6-phosphate. Glucose 6-sulfonate can enter the hexosamine biosynthetic pathway by producing glucosamine 6-sulfonate, which is a reported antibacterial agent that competitively inhibits hexosamine production. All these interactions with carbohydrate routes might help explain the observed anticancer activity that glucose 6-sulfonate has in vitro. This adds to our knowledge of how vegetables rich in glucose 6-sulfonate can also act as metabolic inhibitors of pathways that are increased in metabolic diseases.

D-glucose acts via sodium/glucose cotransporter 1 to increase NHE3 in mouse jejunal brush border by a Na+/H+ exchange regulatory factor 2-dependent process.

BACKGROUND & AIMS: Oral rehydration solutions reduce diarrhea-associated mortality. Stimulated sodium absorption by these solutions is mediated by the Na(+)/H(+) hydrogen exchanger NHE3 and is increased by Na(+)-glucose co-transport in vitro, but the mechanisms of this up-regulated process are only partially understood. METHODS: Intracellular pH was measured in jejunal enterocytes of wild-type mice and mice with disrupted Na+/H+ exchange regulatory co-factor 2 (NHERF2-/- mice) by multiphoton microscopy. Diarrhea was induced by cholera toxin. Caco-2BBe cells that express NHE3 and the sodium/glucose cotransporter 1 (SGLT1) were studied by fluorometry, before and after siRNA-mediated knockdown of NHERF1 or NHERF2. NHE3 distribution was assessed by cell-surface biotinylation and confocal microscopy. Brush-border mobility was determined by fluorescence recovery after photobleaching and confocal microscopy. RESULTS: The nonmetabolized SGLT1 substrate α-methyl-D-Glu (α-MD-G) activated jejunal NHE3; this process required Akt and NHERF2. α-MD-G normalized NHE3 activity after cholera toxin-induced diarrhea. α-MD-G-stimulated jejunal NHE3 activity was defective in NHERF2-/- mice and cells with NHERF2 knockdown, but occurred normally with NHERF1 knockdown; was associated with increased NHE3 surface expression in Caco-2 cells, which also was NHERF2-dependent; was associated with dissociation of NHE3 from NHERF2 and an increase in the NHE3 mobile fraction from the brush border; and was accompanied by a NHERF2 ezrin-radixin-moesin-binding domain-dependent increase in co-precipitation of ezrin with NHE3. CONCLUSIONS: SGLT1-mediated Na-glucose co-transport stimulates NHE3 activity in vivo by an Akt- and NHERF2-dependent signaling pathway. It is associated with increased brush-border NHE3 and association between ezrin and NHE3. Activation of NHE3 corrects cholera toxin-induced defects in Na absorption and might contribute to the efficacy of oral rehydration solutions.

Calcitriol mediates the activity of SGLT1 through an extranuclear initiated mechanism that involves intracellular signaling pathways.

The present study explored whether calcitriol plays a role in the regulation of sodium-dependent glucose transporter protein 1 (SGLT1) activity. For this purpose, alpha-methyl glucoside (AMG) uptake in stable transfected Chinese hamster ovary (CHO-G6D3) cells expressing rabbit SGLT1 (rbSGLT1) was used. The involvement of second messengers, intracellular signaling pathways, and pro-inflammatory cytokines were examined using specific inhibitors before incubation with calcitriol for 15 min. The present study demonstrated the involvement of second messengers produced by phospholipase A(2), phospholipase C, calmodulin, diacylglycerol kinase, and phosphoinositide 3 kinase on calcitriol-regulated AMG uptake. Pretreatment with inhibitors of the mitogen-activated protein kinase (MAPK) signaling pathway increased calcitriol-induced AMG uptake. In contrast, inhibition of the phosphoinositide 3-kinase PI3K/Akt/mTOR signaling pathway decreased the effect of calcitriol on AMG uptake. These findings suggest that calcitriol regulates rbSGLT1 activity through a rapid, extranuclear initiated mechanism of action stimulated by MAPK and inhibited by PI3K/Akt/mTOR. Another important finding was the effect of pro-inflammatory cytokines on calcitriol-induced AMG uptake. Interleukin-6 increased while tumor necrosis factor-alpha decreased calcitriol-induced AMG uptake. In conclusion, the present study demonstrates the involvement of calcitriol in the regulation of rbSGLT1 activity. This is due to the activation of intracellular signaling pathways triggered by second messenger molecules and cytokines after a short time (15 min) exposure to calcitriol.

Activation of sodium-glucose cotransporter 1 ameliorates hyperglycemia by mediating incretin secretion in mice.

Glucose ingestion stimulates the secretion of the incretin hormones, glucose-dependent insulinotropic peptide (GIP) and glucagon-like peptide-1 (GLP-1). Despite the critical role of incretins in glucose homeostasis, the mechanism of glucose-induced incretin secretion has not been established. We investigated the underlying mechanism of glucose-induced incretin secretion in vivo in mice. Injection of glucose at 1 g/kg in the upper intestine significantly increased plasma GIP and GLP-1 levels, whereas injection of glucose in the colon did not increase GIP or GLP-1 levels. This finding indicates that the glucose sensor for glucose-induced incretin secretion is in the upper intestine. Coadministration of a sodium-glucose cotransporter-1 (SGLT1) inhibitor, phloridzin, with glucose in the upper intestine blocked glucose absorption and glucose-induced incretin secretion. alpha-methyl-d-glucopyranoside (MDG), an SGLT1 substrate that is a nonmetabolizable sugar, significantly increased plasma GIP and GLP-1 levels, whereas phloridzin blocked these increases, indicating that concomitant transport of sodium ions and glucose (substrate) via SGLT1 itself triggers incretin secretion without the need for subsequent glucose metabolism. Interestingly, oral administration of MDG significantly increased plasma GIP, GLP-1, and insulin levels and reduced blood glucose levels during an intraperitoneal glucose tolerance test. Furthermore, chronic MDG treatment in drinking water (3%) for 13 days reduced blood glucose levels after a 2-h fast and in an oral glucose tolerance test in diabetic db/db mice. Our findings indicate that SGLT1 serves as the intestinal glucose sensor for glucose-induced incretin secretion and that a noncalorigenic SGLT1 substrate ameliorates hyperglycemia by stimulating incretin secretion.

Influence of luminal monosaccharides on secretion of glutathione conjugates from rat small intestine in vitro.

Intestinal efflux transporters can significantly reduce the absorption of the drug after peroral application. In this work we studied secretion of glutathione conjugates triggered by glucose at the luminal side of the intestine. Glucose stimulated secretion of DNPSG, NEMSG and CDNB. We used some different monosaccharides and determined that glucose, galactose and alpha-methylglucopyranoside trigger the secretion, while mannitol and fructose do not. We concluded that interaction with SGLT transporter is the key process necessary for this triggering. To determine which of possible glutathione conjugate transporters (MRP2, MRP4, BCRP or RLIP76) is responsible for the secretion of glutathione conjugates, we used benzbromarone, a MRP inhibitor, and sulfanitran and furosemide, two allosteric MRP2 activators. Benzbromarone inhibited glucose stimulated DNPSG secretion, while allosteric activators additionally increased the secretion. We concluded that MRP2 transporter is related to glucose stimulated DNPSG secretion. Regarding the work of Kubitz et al. we tested the effect of changed medium osmolarity on DNPSG transport and determined that hypoosmolar conditions trigger secretion of DNPSG. These findings suggest that intestinal MRP2 activity has no basal level, but can be stimulated by hypoosmolarity and SGLT transport.

Dissociation between sensing and metabolism of glucose in sugar sensing neurones.

Some of the neurones controlling sleep, appetite and hormone release act as specialized detectors of ambient glucose. Their sugar sensing is conventionally thought to involve glucokinase-dependent metabolism of glucose to ATP, which then alters membrane excitability by modulating ATP-dependent channels or transporters, such as ATP-inhibited K(+) channels (K(ATP)). However, recent studies also provide examples of both glucose-excited (GE) and glucose-inhibited (GI) neurones that sense glucose independently of such metabolic pathways. Two-thirds of hypothalamic GE neurones in primary cultures are also excited by the non-metabolizable glucose analogue alpha-methylglucopyranoside (alpha-MDG), which acts as a substrate for electrogenic (depolarizing) sodium-glucose cotransporter (SGLT). The excitatory responses to both glucose and alpha-MDG are abolished by arresting SGLT activity by sodium removal or the SGLT inhibitor phloridzin. Direct depolarization and excitation by glucose-triggered SGLT activity may ensure that GE neurones continue to sense glucose in 'high-energy' states, when K(ATP) channels are closed. A major class of hypothalamic GI neurones, the orexin/hypocretin cells, also appear to use a non-metabolic sensing strategy. In these cells, glucose-induced hyperpolarization and inhibition are unaffected by glucokinase inhibitors such as alloxan, D-glucosamine, and N-acetyl-D-glucosamine, and mimicked by the non-metabolizable glucose analogue 2-deoxyglucose, but not by stimulating intracellular ATP production with lactate. The dissociation between sensing and metabolism of sugar may allow the brain to predict and prevent adverse changes in extracellular glucose levels with minimal impact on the flow of intracellular fuel.

Protein kinase-A affects sorting and conformation of the sodium-dependent glucose co-transporter SGLT1.

In Chinese hamster ovary cells expressing rabbit sodium-dependent glucose transporter (rbSGLT1) protein kinase A (PKA) activators (forskolin and 8-Br-cAMP) stimulated alpha-methyl D-glucopyranoside uptake. Kinetic analysis revealed an increase in both V(max) and affinity of the transport. Immunohistochemistry and biotinylation experiments showed that this stimulation was accompanied by an increased amount of SGLT1 localized into the plasma membrane, which explains the higher V(max) of the transport. Cytochalasin D only partly attenuated the effect of forskolin as did deletion of the PKA phosphorylation site of SGLT1 in transient transfection studies. Experiments using an anti-phosphopeptide antibody revealed that forskolin also increased the extent of phosphorylation of SGLT1 in the membrane fraction. These results suggested that regulation of SGLT1 mediated glucose transport involves an additional direct effect on SGLT1 by phosphorylation. To evaluate this assumption further, phosphorylation studies of recombinant human SGLT1 (hSGLT1) in vitro were performed. In the presence of the catalytic subunit PKA and [(32)P] ATP 1.05 mol of phosphate were incorporated/mol of hSGLT1. Additionally, phosphorylated hSGLT1 demonstrated a reduction in tryptophan fluorescence intensity and a higher quenching by the hydrophilic Trp quencher acrylamide, particularly in the presence of D-glucose. These results indicate that PKA-mediated phosphorylation of SGLT1 changes the conformation of the empty carrier and the glucose carrier complex, probably causing the increase in transport affinity. Thus, PKA-mediated phosphorylation of the transporter represents a further mechanism in the regulation of SGLT1-mediated glucose transport in epithelial cells, in addition to a change in surface membrane expression.

Transmembrane IV of the high-affinity sodium-glucose cotransporter participates in sugar binding.

Investigation of the structure/function relationships of the sodium-glucose transporter (SGLT1) is crucial to understanding the cotransporter mechanism. In the present study, we used cysteine-scanning mutagenesis and chemical modification by methanethiosulfonate (MTS) derivatives to test whether predicted transmembrane IV participates in sugar binding. Five charged and polar residues (K139, Q142, T156, K157, and D161) and two glucose/galactose malabsorption missense mutations (I147 and S159) were replaced with cysteine. Mutants I147C, T156C, and K157C exhibited sufficient expression to be studied in detail using the two-electrode voltage-clamp method in Xenopus laevis oocytes and COS-7 cells. I147C was similar in function to wild-type and was not studied further. Mutation of lysine-157 to cysteine (K157C) causes loss of phloridzin and alpha-methyl-D-glucopyranoside (alphaMG) binding. These functions are restored by chemical modification with positively charged (2-aminoethyl) methanethiosulfonate hydrobromide (MTSEA). Mutation of threonine-156 to cysteine (T156C) reduces the affinity of alphaMG and phloridzin for T156C by approximately 5-fold and approximately 20-fold, respectively. In addition, phloridzin protects cysteine-156 in T156C from alkylation by MTSEA. Therefore, the presence of a positive charge or a polar residue at 157 and 156, respectively, affects sugar binding and sugar-induced Na(+) currents.

Biotransformation and pharmacokinetics of 4-(3,4-dihydroxybenzoyloxymethyl)phenyl-O-beta-D-glucopyranoside, an antioxidant isolated from Origanum vulgare.

4-(3,4-Dihydroxybenzoyloxymethyl)phenyl- O-beta-D-glucopyranoside (OV-16) is a polyphenolic glycoside isolated from oregano (Origanum vulgare L.), which is a popular Chinese herb and a common spice in Western diet. To understand the biotransformation and pharmacokinetics of OV-16, rats were orally administered OV-16 and oregano decoction. Blood samples were withdrawn at specific time points. The presence of OV-16 and its metabolites protocatechuic acid (PCA) and p-hydroxybenzyl alcohol (HBA) in serum were determined by HPLC method, whereas their conjugated metabolites were assayed indirectly through hydrolysis with beta-glucuronidase and sulfatase. Our results showed that when OV-16 was orally administered, free forms of OV-16, PCA, and HBA were not present in blood and the major metabolites were the glucuronides/sulfates of PCA and HBA sulfate. The serum metabolites of OV-16 exhibited free radical scavenging activity. When oregano decoction was given, the glucuronides and sulfates of PCA were the major metabolites in blood.

Absence of Na+/sugar cotransport activity in Barrett's metaplasia.

AIM: To evaluate the presence of Na+-dependent, active, sugar transport in Barrett's epithelia as an intestinal biomarker, based on the well-documented, morphological intestinal phenotype of Barrett's esophagus (BE). METHODS: We examined uptake of the nonmeta-bolizable glucose analogue, alpha-methyl-D-glucoside (AMG), a substrate for the entire sodium glucose cotransporter (SGLT) family of transport proteins. During upper endoscopy, patients with BE or with uncomplicated gastroesophageal reflux disease (GERD) allowed for duodenal, gastric fundic, and esophageal mucosal biopsies to be taken. Biopsies were incubated in bicarbonate-buffered saline (KRB) containing 0.1 mmol/L 14C-AMG for 60 min at 20 centigrade. Characterized by abundant SGLT, duodenum served as a positive control while gastric fundus and normal esophagus, known to lack SGLT, served as negative controls. RESULTS: Duodenal biopsies accumulated 249.84+/-35.49 (SEM) picomoles AMG/microg DNA (n=12), gastric fundus biopsies 36.20+/-6.62 (n=12), normal esophagus 12.10+/-0.59 (n=3) and Barrett's metaplasia 29.79+/-5.77 (n=8). There was a statistical difference (P<0.01) between biopsies from duodenum and each other biopsy site but there was no statistically significant difference between normal esophagus and BE biopsies. 0.5 mmol/L phlorizin (PZ) inhibited AMG uptake into duodenal mucosa by over 89%, but had no significant effect on AMG uptake into gastric fundus, normal esophagus, or Barrett's tissue. In the absence of Na+ (all Na+ salts replaced by Li+ salts), AMG uptake in duodenum was decreased by over 90%, while uptake into gastric, esophageal or Barrett's tissue was statistically unaffected. CONCLUSION: Despite the intestinal enterocyte phenotype of BE, Na+-dependent, sugar transport activity is not present in these cells.

Interleukin-6 stimulates alpha-MG uptake in renal proximal tubule cells: involvement of STAT3, PI3K/Akt, MAPKs, and NF-kappaB.

Recent studies have shown that interleukin 6 (IL-6) acts on the cellular proliferation-activating transduction signals during cellular regeneration. Therefore, this study examined the effect of IL-6 on the activation of Na(+)/glucose cotransporters (SGLTs) and its related signaling pathways in primary cultured renal proximal tubule cells (PTCs). IL-6 increased the level of alpha-methyl-d-[(14)C]glucopyranoside (alpha-MG) uptake in time- and dose-dependent manners. IL-6 also increased SGLT1 plus SGLT2 mRNA and protein expression level. The IL-6 receptors (IL-6Ralpha and gp 130) were expressed in PTCs. In addition, genistein and herbimycin A completely blocked the IL-6-induced increases in alpha-MG uptake and the protein expression level of SGLTs. On the other hand, IL-6 increased the level of 5-(and-6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate-sensitive cellular reactive oxygen species (ROS), and IL-6-induced increases in alpha-MG uptake and the protein expression level of SGLTs were blocked by ascorbic acid or taurine (antioxidants). IL-6 also increased the phosphorylation of signal transducer and activator of transcription-3 (STAT3), phosphoinositide-3 kinase (PI3K)/Akt, and mitogen-activated protein kinases (MAPKs) in a time-dependent manner. A pretreatment with STAT3 inhibitor LY 294002, an Akt inhibitor, or MAPK inhibitors significantly blocked the IL-6-induced increase in alpha-MG uptake. In addition, IL-6 increased the level of nuclear factor-kappaB (NF-kappaB) phosphorylation. A pretreatment with SN50 or BAY 11-7082 also blocked the IL-6-induced increase in alpha-MG uptake. In conclusion, IL-6 increases the SGLT activity through ROS, and its action in renal PTCs is associated with the STAT3, PI3K/Akt, MAPKs, and NF-kappaB signaling pathways.

Genetic requirements for growth of Escherichia coli K12 on methyl-alpha-D-glucopyranoside and the five alpha-D-glucosyl-D-fructose isomers of sucrose.

Strains of Escherichia coli K12, including MG-1655, accumulate methyl-alpha-D-glucopyranoside via the phosphoenolpyruvate-dependent glucose:phosphotransferase system (IICB(Glc)/IIA(Glc)). High concentrations of intracellular methyl-alpha-D-glucopyranoside 6-phosphate are toxic, and cell growth is prevented. However, transformation of E. coli MG-1655 with a plasmid (pAP1) encoding the gene aglB from Klebsiella pneumoniae resulted in excellent growth of the transformant MG-1655 (pAP1) on the glucose analog. AglB is an unusual NAD+/Mn2+-dependent phospho-alpha-glucosidase that promotes growth of MG-1655 (pAP1) by catalyzing the in vivo hydrolysis of methyl-alpha-D-glucopyranoside 6-phosphate to yield glucose 6-phosphate and methanol. When transformed with plasmid pAP2 encoding the K. pneumoniae genes aglB and aglA (an alpha-glucoside-specific transporter AglA (IICB(Agl))), strain MG-1655 (pAP2) metabolized a variety of other alpha-linked glucosides, including maltitol, isomaltose, and the following five isomers of sucrose: trehalulose alpha(1-->1), turanose alpha(1-->3), maltulose alpha(1-->4), leucrose alpha(1-->5), and palatinose alpha(1-->6). Remarkably, MG-1655 (pAP2) failed to metabolize sucrose alpha(1-->2). The E. coli K12 strain ZSC112L (ptsG::cat manXYZ nagE glk lac) can neither grow on glucose nor transport methyl-alpha-D-glucopyranoside. However, when transformed with pTSGH11 (encoding ptsG) or pAP2, this organism provided membranes that contained either the PtsG or AglA transporters, respectively. In vitro complementation of transporter-specific membranes with purified general phosphotransferase components showed that although PtsG and AglA recognized glucose and methyl-alpha-D-glucopyranoside, only AglA accepted other alpha-D-glucosides as substrates. Complementation experiments also revealed that IIA(Glc) was required for functional activity of both PtsG and AglA transporters. We conclude that AglA, AglB, and IIA(Glc) are necessary and sufficient for growth of E. coli K12 on methyl-alpha-D-glucoside and related alpha-D-glucopyranosides.

Determination of endotoxin by the measurement of the acetylated methyl glycoside derivative of Kdo with gas-liquid chromatography-mass spectrometry.

A gas-liquid chromatographic-mass spectrometric (GLC-MS) method was applied to the detection of 3-deoxy-d-manno-2-octulosonic acid (Kdo), a constituent of bacterial lipopolysaccharide (LPS, endotoxin). Samples containing LPS were dried, methanolyzed with 2 M HCl in methanol at 60 degrees C for 1 h and acetylated with acetic anhydride and pyridine (1:1, v/v) solution at 100 degrees C for 30 min, then the products were analyzed by GLC-MS or GLC-MSMS. Four acetylated methylglycoside methyl ester derivatives of Kdo are formed in these conditions, namely one with pyranose ring (Kdo1), two derivatives in the furanose form (Kdo2 and 3) and one derivative of anhydro Kdo (Kdo4), as results from their mass fragmentation patterns. Synthetic Kdo produced mainly Kdo4 derivative, whereas Kdo1 of pyranose ring shape was the predominating derivative formed from LPS. The ion fragment of m/z 375 was selected for the specific detection of this Kdo1 derivative, which might be applied for the endotoxin determination. That approach was used for the analysis of preparations of bacteria, bacteriophages and samples of animal sera. In order to ensure the removal of phosphate substitutions from Kdo, methanolyzed samples can be treated with alkaline phosphatase (2.6 U, pH 9.2, 37 degrees C, 15 min), what was elaborated on Vibrio LPS preparation.

ATP stimulates Na+-glucose cotransporter activity via cAMP and p38 MAPK in renal proximal tubule cells.

Extracellular ATP plays an important role in the regulation of renal function. However, the effect of ATP on the Na(+)-glucose cotransporters (SGLTs) has not been elucidated in proximal tubule cells (PTCs). Therefore, this study was performed to examine the action of ATP on SGLTs and their related signal pathways in primary cultured rabbit renal PTCs. ATP increased [(14)C]-alpha-methyl-d-glucopyranoside (alpha-MG) uptake in a time-dependent (>1 h) and dose-dependent (>10(-6) M) manner. ATP stimulated alpha-MG uptake by increasing in V(max) without affecting K(m). ATP-induced increase of alpha-MG uptake was correlated with the increase in both SGLT1 and SGLT2 protein expression levels. ATP-induced stimulation of alpha-MG uptake was blocked by suramin (nonspecific P2 receptor antagonist), RB-2 (P2Y receptor antagonist), and MRS-2179 (P2Y(1) receptor antagonist), suggesting a role for the P2Y receptor. ATP-induced stimulation of alpha-MG uptake was blocked by pertussis toxin (PTX, a G(i) protein inhibitor), SQ-22536 (an adenylate cyclase inhibitor), and PKA inhibitor amide 14-22 (PKI). ATP also increased cAMP formation, which was blocked by PTX and RB-2. However, pretreatment of adenosine deaminase did not block ATP-induced cAMP formation. In addition, ATP-induced stimulation of alpha-MG uptake was blocked by SB-203580 (p38 MAPK inhibitor), but not by PD-98059 (p44/42 MAPK inhibitor) or SP-600125 (JNK inhibitor). Indeed, ATP induced phosphorylation of p38 MAPK. In conclusion, ATP increases alpha-MG uptake via cAMP and p38 MAPK in renal PTCs.