Regulation of the human Na(+)-glucose cotransporter gene, SGLT1, by HNF-1 and Sp1

Influence of postruminal casein infusion and exogenous glucagon-like peptide 2 administration on the jejunal mucosal transcriptome in cattle

We previously demonstrated that postruminal casein infusion and exogenous glucagon-like peptide 2 (GLP-2) administration independently stimulated growth and carbohydrase activity of the pancreas and jejunal mucosa in cattle. The objective of the current study was to profile the jejunal mucosal transcriptome of cattle using next-generation RNA sequencing in response to postruminal casein infusion and exogenous GLP-2. Twenty-four Holstein steers [250 ± 23.1 kg body weight (BW)] received a continuous abomasal infusion of 3.94 g raw corn starch/kg of BW combined with either 0 or 1.30 g casein/kg of BW for 7 d. Steers received subcutaneous injections at 0800 and 2000 h to provide either 0 or 100 μg GLP-2/kg of BW per day. At the end of the 7-d treatment period, steers were slaughtered for collection of the jejunal mucosa. Total RNA was extracted from jejunal mucosal tissue, strand-specific cDNA libraries were prepared, and RNA sequencing was conducted to generate 150-bp paired-end reads at a depth of 40 M reads per sample. Differentially expressed genes (DEG), KEGG pathway enrichment, and gene ontology enrichment were determined based on the FDR-corrected P-value (padj). Exogenous GLP-2 administration upregulated (padj < 0.05) 667 genes and downregulated 1,101 genes of the jejunal mucosa. Sphingolipid metabolism, bile secretion, adherens junction, and galactose metabolism were among the top KEGG pathways enriched with upregulated DEG (padj < 0.05) in response to exogenous GLP-2 administration. The top gene ontologies enriched with upregulated DEG (padj < 0.05) in response to exogenous GLP-2 administration included nutrient metabolic processes, brush border and bicellular tight junction assembly, and enzyme and transporter activities. Exogenous GLP-2 administration increased or tended to increase (padj < 0.10) brush border carbohydrase (MGAM, LCT, TREH), hexose transporter (SLC5A1, SLC2A2), and associated transcription factor (HNF1, GATA4, KAT2B) mRNA expression of the jejunal mucosa. Gene ontologies and KEGG pathways that were downregulated (padj < 0.05) in response to exogenous GLP-2 were related to genetic information processing. Postruminal casein infusion downregulated (padj < 0.05) 7 jejunal mucosal genes that collectively did not result in enriched KEGG pathways or gene ontologies. This study highlights some of the transcriptional mechanisms associated with increased growth, starch assimilation capacity, and barrier function of the jejunal mucosa in response to exogenous GLP-2 administration.

Marginal Zinc Deficiency Promotes Pancreatic Islet Enlargement While Zinc Supplementation Improves the Pancreatic Insulin Response in Zucker Diabetic Fatty Rats

Zinc deficiency has been associated with the worsening of diabetes while zinc supplementation has been proposed to ameliorate diabetes. This study examined the effects of marginal zinc deficiency (MZD) and zinc supplementation (ZS) on obesity, glycemic control, pancreatic islets, hepatic steatosis and renal function of Zucker diabetic fatty (ZDF) rats. Male ZDF rats were fed an MZD, zinc control (ZC) or ZS diet (4, 30 and 300 mg Zn/kg diet, respectively), and lean Zucker rats were fed a ZC diet for 8 weeks. MZD and ZS did not alter body weight or whole-body composition in ZDF rats. MZD ZDF rats had reduced zinc concentrations in the femur and pancreas, a greater number of enlarged pancreatic islets and a diminished response to an oral glucose load based on a 1.8-fold greater incremental area-under-the-curve (AUC) for glucose compared to ZC ZDF. ZS ZDF rats had elevated serum, femur and pancreatic zinc concentrations, unchanged pancreatic parameters and a 50% reduction in the AUC for insulin compared to ZC ZDF rats, suggesting greater insulin sensitivity. Dietary zinc intake did not alter hepatic steatosis, creatinine clearance, or levels of proteins that contribute to insulin signaling, inflammation or zinc transport in epididymal fat. Potential adverse effects of ZS were suggested by reduced hepatic copper concentrations and elevated serum urea compared to ZC ZDF rats. In summary, ZS improved the pancreatic insulin response but not the glucose handling. In contrast, reduced zinc status in ZDF rats led to impaired glucose tolerance and a compensatory increase in the number and size of pancreatic islets which could lead to β-cell exhaustion.

A Rare Olive Compound Oleacein Improves Lipid and Glucose Metabolism, and Inflammatory Functions: A Comprehensive Whole-Genome Transcriptomics Analysis in Adipocytes Differentiated from Healthy and Diabetic Adipose Stem Cells

Oleacein (OLE), a rare natural compound found in unfiltered extra virgin olive oil, has been shown to have anti-inflammatory and anti-obesity properties. However, little is known regarding the mechanisms by which OLE influences metabolic processes linked to disease targets, particularly in the context of lipid metabolism. In the present study, we conducted whole-genome DNA microarray analyses in adipocytes differentiated from human adipose-derived stem cells (hASCs) and diabetic hASCs (d-hASCs) to examine the effects of OLE on modulating metabolic pathways. We found that OLE significantly inhibited lipid formation in adipocytes differentiated from both sources. In addition, microarray analysis demonstrated that OLE treatment could significantly downregulate lipid-metabolism-related genes and modulate glucose metabolism in both adipocyte groups. Transcription factor enrichment and protein-protein interaction (PPI) analyses identified potential regulatory gene targets. We also found that OLE treatment enhanced the anti-inflammatory properties in adipocytes. Our study findings suggest that OLE exhibits potential benefits in improving lipid and glucose metabolism, thus holding promise for its application in the management of metabolic disorders.

The Lack of Bmal1, a Core Clock Gene, in the Intestine Decreases Glucose Absorption in Mice

The circadian clock network is an evolutionarily conserved system that regulates systemic metabolism, such as glucose homeostasis. Intestinal tissue is a pivotal organ for the regulation of glucose metabolism, mainly via glucose absorption into the circulation; however, the significance of the intestinal circadian clock network for glucose metabolism remains largely unclear. We herein utilized a mouse model in which Bmal1, a core clock gene, was deleted in an intestine-specific manner (Bmal1Int-/- mice) and demonstrated a rhythmic expression of Sglt1 with its peak at zeitgeber time (ZT) 10.7 ± 2.8 in control mice, whereas this was lost in Bmal1Int-/- mice. Mechanistically, chromatin immunoprecipitation analysis revealed rhythmic binding of CLOCK to the E-box elements in the Sglt1 gene in control mice; however, this was absent in Bmal1Int-/- mice. Accordingly, SGLT1 protein levels were decreased during the dark phase in Bmal1Int-/- mice and this was associated with impaired glucose absorption, leading to a decline in hepatic glycogen levels at ZT4, which was restored by ingestion of high-sucrose water. Additionally, when mice were starved from ZT0, greater expression of the lipolysis-related gene Pnpla2 was observed in adipose tissue of Bmal1Int-/- mice, and this was not noted when glycogen storage was restored by high-sucrose water prior to fasting, suggesting that higher Pnpla2 expression in Bmal1Int-/- mice was likely caused by lower glycogen storage. These results indicate that disruption of the intestinal circadian clock system impairs glucose absorption in the intestine and affects systemic glucose homeostasis.

Glycaemia and body weight are regulated by sodium-glucose cotransporter 1 (SGLT1) expression via O-GlcNAcylation in the intestine

Objective

Methods

Results

Conclusion

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Intestine-specific OGT depletion results in weight loss and hypoglycaemia.

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It reduces SGLT1 expression, resulting in glucose absorption from the gut.

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OGT knockdown may contribute to diminish glucose-induced incretin secretion.

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OGT may regulate SGLT1 expression via the cAMP/CREB-dependent pathway.

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O-GlcNAcylation regulates SGLT1 expression in the intestine and the kidney.

Glucose transporters in the small intestine in health and disease

Absorption of monosaccharides is mainly mediated by Na+-D-glucose cotransporter SGLT1 and the facititative transporters GLUT2 and GLUT5. SGLT1 and GLUT2 are relevant for absorption of D-glucose and D-galactose while GLUT5 is relevant for D-fructose absorption. SGLT1 and GLUT5 are constantly localized in the brush border membrane (BBM) of enterocytes, whereas GLUT2 is localized in the basolateral membrane (BLM) or the BBM plus BLM at low and high luminal D-glucose concentrations, respectively. At high luminal D-glucose, the abundance SGLT1 in the BBM is increased. Hence, D-glucose absorption at low luminal glucose is mediated via SGLT1 in the BBM and GLUT2 in the BLM whereas high-capacity D-glucose absorption at high luminal glucose is mediated by SGLT1 plus GLUT2 in the BBM and GLUT2 in the BLM. The review describes functions and regulations of SGLT1, GLUT2, and GLUT5 in the small intestine including diurnal variations and carbohydrate-dependent regulations. Also, the roles of SGLT1 and GLUT2 for secretion of enterohormones are discussed. Furthermore, diseases are described that are caused by malfunctions of small intestinal monosaccharide transporters, such as glucose-galactose malabsorption, Fanconi syndrome, and fructose intolerance. Moreover, it is reported how diabetes, small intestinal inflammation, parental nutrition, bariatric surgery, and metformin treatment affect expression of monosaccharide transporters in the small intestine. Finally, food components that decrease D-glucose absorption and drugs in development that inhibit or downregulate SGLT1 in the small intestine are compiled. Models for regulations and combined functions of glucose transporters, and for interplay between D-fructose transport and metabolism, are discussed.

Cardiac sodium-dependent glucose cotransporter 1 is a novel mediator of ischaemia/reperfusion injury

AIMS

We previously reported that sodium-dependent glucose cotransporter 1 (SGLT1) is highly expressed in cardiomyocytes and is further up-regulated in ischaemia. This study aimed to determine the mechanisms by which SGLT1 contributes to ischaemia/reperfusion (I/R) injury.

METHODS AND RESULTS

Mice with cardiomyocyte-specific knockdown of SGLT1 (TGSGLT1-DOWN) and wild-type controls were studied. In vivo, the left anterior descending coronary artery was ligated for 30 min and reperfused for 48 h. Ex vivo, isolated perfused hearts were exposed to 20 min no-flow and up to 2 h reperfusion. In vitro, HL-1 cells and isolated adult murine ventricular cardiomyocytes were exposed to 1 h hypoxia and 24 h reoxygenation (H/R). We found that TGSGLT1-DOWN hearts were protected from I/R injury in vivo and ex vivo, with decreased infarct size, necrosis, dysfunction, and oxidative stress. 5'-AMP-activated protein kinase (AMPK) activation increased SGLT1 expression, which was abolished by extracellular signal-related kinase (ERK) inhibition. Co-immunoprecipitation studies showed that ERK, but not AMPK, interacts directly with SGLT1. AMPK activation increased binding of the hepatocyte nuclear factor 1 and specificity protein 1 transcription factors to the SGLT1 gene, and HuR to SGLT1 mRNA. In cells, up-regulation of SGLT1 during H/R was abrogated by AMPK inhibition. Co-immunoprecipitation studies showed that SGLT1 interacts with epidermal growth factor receptor (EGFR), and EGFR interacts with protein kinase C (PKC). SGLT1 overexpression activated PKC and NADPH oxidase 2 (Nox2), which was attenuated by PKC inhibition, EGFR inhibition, and/or disruption of the interaction between EGFR and SGLT1.

CONCLUSION

During ischaemia, AMPK up-regulates SGLT1 through ERK, and SGLT1 interacts with EGFR, which in turn increases PKC and Nox2 activity and oxidative stress. SGLT1 may represent a novel therapeutic target for mitigating I/R injury.

Null mutations of NEUROG3 are associated with delayed-onset diabetes mellitus

Biallelic mutations of the gene encoding the transcription factor NEUROG3 are associated with a rare disorder that presents in neonates as generalized malabsorption - due to a complete absence of enteroendocrine cells - followed, in early childhood or beyond, by insulin-dependent diabetes mellitus (IDDM). The commonly delayed onset of IDDM suggests a differential requirement for NEUROG3 in endocrine cell generation in the human pancreas versus the intestine. However, previously identified human mutations were hypomorphic and, hence, may have had residual function in pancreas. We report 2 patients with biallelic functionally null variants of the NEUROG3 gene who nonetheless did not present with IDDM during infancy but instead developed permanent IDDM during middle childhood ages. The variants showed no evidence of function in traditional promoter-based assays of NEUROG3 function and also failed to exhibit function in a variety of potentially novel in vitro and in vivo molecular assays designed to discern residual NEUROG3 function. These findings imply that, unlike in mice, pancreatic endocrine cell generation in humans is not entirely dependent on NEUROG3 expression and, hence, suggest the presence of unidentified redundant in vivo pathways in human pancreas capable of yielding β cell mass sufficient to maintain euglycemia until early childhood.

BMAL1 controls glucose uptake through paired-homeodomain transcription factor 4 in differentiated Caco-2 cells

The transcription factor aryl hydrocarbon receptor nuclear translocator-like protein-1 (BMAL1) is an essential regulator of the circadian clock, which controls the 24-h cycle of physiological processes such as nutrient absorption. To examine the role of BMAL1 in small intestinal glucose absorption, we used differentiated human colon adenocarcinoma cells (Caco-2 cells). Here, we show that BMAL1 regulates glucose uptake in differentiated Caco-2 cells and that this process is dependent on the glucose transporter sodium-glucose cotransporter 1 (SGLT1). Mechanistic studies show that BMAL1 regulates glucose uptake by controlling the transcription of SGLT1 involving paired-homeodomain transcription factor 4 (PAX4), a transcriptional repressor. This is supported by the observation that clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated endonuclease Cas9 (Cas9) knockdown of PAX4 increases SGLT1 and glucose uptake. Chromatin immunoprecipitation (ChIP) and ChIP-quantitative PCR assays show that the knockdown or overexpression of BMAL1 decreases or increases the binding of PAX4 to the hepatocyte nuclear factor 1-α binding site of the SGLT1 promoter, respectively. These findings identify BMAL1 as a critical mediator of small intestine carbohydrate absorption and SGLT1.

Sodium-glucose transporter as a novel therapeutic target in disease

Glucose is the primary energy fuel of life. A glucose transporter, the sodium-glucose transporter (SGLT), is receiving attention as a novel therapeutic target in disease. This review summarizes the physiological role of SGLT in cerebral ischemia, cancer, cardiac disease, and intestinal ischemia, which has encouraged analysis of SGLT function. In cerebral ischemia and cardiomyopathy, SGLT-1 is involved in worsening of the injury. In addition, SGLT-1 promotes the development of cancer. On the other hand, SGLT-1 has a protective effect against cardiac and intestinal ischemia. Interestingly, SGLT-1 expression levels are increased in some diseased tissue, such as in cerebral ischemia and cancer. This suggests that SGLT-1 may have an important role in many diseases. This review discusses the potential of SGLT as a target for novel therapeutic agents.

Gut Mechanisms Linking Intestinal Sweet Sensing to Glycemic Control

Sensing nutrients within the gastrointestinal tract engages the enteroendocrine cell system to signal within the mucosa, to intrinsic and extrinsic nerve pathways, and the circulation. This signaling provides powerful feedback from the intestine to slow the rate of gastric emptying, limit postprandial glycemic excursions, and induce satiation. This review focuses on the intestinal sensing of sweet stimuli (including low-calorie sweeteners), which engage similar G-protein-coupled receptors (GPCRs) to the sweet taste receptors (STRs) of the tongue. It explores the enteroendocrine cell signals deployed upon STR activation that act within and outside the gastrointestinal tract, with a focus on the role of this distinctive pathway in regulating glucose transport function via absorptive enterocytes, and the associated impact on postprandial glycemic responses in animals and humans. The emerging role of diet, including low-calorie sweeteners, in modulating the composition of the gut microbiome and how this may impact glycemic responses of the host, is also discussed, as is recent evidence of a causal role of diet-induced dysbiosis in influencing the gut-brain axis to alter gastric emptying and insulin release. Full knowledge of intestinal STR signaling in humans, and its capacity to engage host and/or microbiome mechanisms that modify glycemic control, holds the potential for improved prevention and management of type 2 diabetes.

Duodenal Sodium/Glucose Cotransporter 1 Expression Under Fasting Conditions Is Associated With Postload Hyperglycemia

CONTEXT

Type 2 diabetes (T2DM) is associated with a higher intestinal expression of the glucose transporters sodium/glucose cotransporter 1 (SGLT-1) and glucose transporter 2 (GLUT-2). It is currently unsettled whether prediabetes conditions characterized by postprandial hyperglycemia, such as impaired glucose tolerance (IGT) and normal glucose tolerance (NGT) with 1-hour postload glucose ≥155 mg/dL (8.6 mmol/L) (NGT-1h-high) are associated with increased expression of these glucose carriers in the intestine.

OBJECTIVE

We evaluated whether duodenal abundance of SGLT-1 and GLUT-2 is augmented in subjects with IGT and NGT-1h-high, in comparison with subjects with NGT and 1-hour postload glucose ˂155 mg/dL (NGT-1h-low).

DESIGN

Cross-sectional.

PATIENTS

A total of 54 individuals underwent an upper gastrointestinal endoscopy.

MAIN OUTCOME MEASURES

Duodenal SGLT-1 and GLUT-2 protein and messenger RNA levels were assessed by Western blot and reverse transcription polymerase chain reaction, respectively.

RESULTS

Of the 54 subjects examined, 18 had NGT-1h-low, 12 had NGT-1h-high, 12 had IGT, and 12 had T2DM. Duodenal SGLT-1 protein and messenger RNA levels were significantly higher in individuals with NGT-1h-high, IGT, or T2DM in comparison with NGT-1h-low subjects. GLUT-2 abundance was higher in individuals with T2DM in comparison with NGT-1h-low subjects; no substantial increase in GLUT-2 expression was observed in NGT-1h-high or IGT individuals. Univariate correlations showed that duodenal SGLT-1 abundance was positively correlated with 1-hour postload plasma glucose levels (r = 0.44; P = 0.003) but not with fasting or 2-hour postload glucose levels.

CONCLUSIONS

Duodenal SGLT-1 expression is increased in individuals with 1-hour postload hyperglycemia or IGT, as well as in subjects with T2DM, and it positively correlates with early postload glucose excursion.

High Glucose-Induced Hypomethylation Promotes Binding of Sp-1 to Myo-Inositol Oxygenase: Implication in the Pathobiology of Diabetic Tubulopathy

The catabolic enzyme myo-inositol oxygenase (MIOX) is expressed in proximal tubules and up-regulated in the diabetic state. Previously, we reported its transcriptional and translation regulation by high glucose (HG), osmolytes, and fatty acids. However, its epigenetic regulation is unknown. Bisulfite sequencing revealed that both human and mouse MIOX promoters, enriched with CpG sites, are hypomethylated and unmethylated under HG ambience and hyperglycemic states associated with increased MIOX expression. Eletrophoretic mobility shift assays revealed increased binding of unmethylated oligos with nucleoproteins of cells maintained under HG. In addition, a strong binding of specificity protein (Sp)-1 transcription factor with MIOX promoter was observed under HG, especially with unmethylated Sp-1 oligo. Specificity of binding was established by supershift assays and treatment with the Sp-1 inhibitor mithramycin. Promoter analysis revealed an increase in luciferase activity under HG, which was reduced after mutation of the Sp-1-binding site. Sp1 siRNA treatment reduced mRNA and protein expression of Sp-1 and MIOX and generation of reactive oxygen species derived from NADPH oxidase (NOX)-4 and mitochondrial sources. In addition, there was reduced expression of hypoxia-inducible factor-1α relevant in the pathogenesis of diabetic nephropathy. Sp1 siRNA treatment reduced fibronectin expression, an extracellular matrix protein that is increased in diabetic nephropathy and tubulopathy. HG-induced MIOX expression was also reduced with the treatment of apelin-13, which deacetylates histones. Overall, these findings highlight the epigenetic regulation of MIOX in the pathogenesis of diabetic tubulopathy.

Sodium glucose cotransporter SGLT1 as a therapeutic target in diabetes mellitus

INTRODUCTION

Glycemic control is important in diabetes mellitus to minimize the progression of the disease and the risk of potentially devastating complications. Inhibition of the sodium-glucose cotransporter SGLT2 induces glucosuria and has been established as a new anti-hyperglycemic strategy. SGLT1 plays a distinct and complementing role to SGLT2 in glucose homeostasis and, therefore, SGLT1 inhibition may also have therapeutic potential.

AREAS COVERED

This review focuses on the physiology of SGLT1 in the small intestine and kidney and its pathophysiological role in diabetes. The therapeutic potential of SGLT1 inhibition, alone as well as in combination with SGLT2 inhibition, for anti-hyperglycemic therapy are discussed. Additionally, this review considers the effects on other SGLT1-expressing organs like the heart.

EXPERT OPINION

SGLT1 inhibition improves glucose homeostasis by reducing dietary glucose absorption in the intestine and by increasing the release of gastrointestinal incretins like glucagon-like peptide-1. SGLT1 inhibition has a small glucosuric effect in the normal kidney and this effect is increased in diabetes and during inhibition of SGLT2, which deliver more glucose to SGLT1 in late proximal tubule. In short-term studies, inhibition of SGLT1 and combined SGLT1/SGLT2 inhibition appeared to be safe. More data is needed on long-term safety and cardiovascular consequences of SGLT1 inhibition.

An Essential Role of cAMP Response Element Binding Protein in Ginsenoside Rg1-Mediated Inhibition of Na+/Glucose Cotransporter 1 Gene Expression

The Na(+)/glucose cotransporter 1 (SGLT1) is responsible for glucose uptake in intestinal epithelial cells. It has been shown that the intestinal SGLT1 level is significantly increased in diabetic individuals and positively correlated with the pathogenesis of diabetes. The development of targeted therapeutics that can reduce the intestinal SGLT1 expression level is, therefore, important. In this study, we showed that ginsenoside Rg1 effectively decreased intestinal glucose uptake through inhibition of SGLT1 gene expression in vivo and in vitro. Transient transfection analysis of the SGLT1 promoter revealed an essential cAMP response element (CRE) that confers the Rg1-mediated inhibition of SGLT1 gene expression. Chromatin immunoprecipitation assay and targeted CRE-binding protein (CREB) silencing demonstrated that Rg1 reduced the promoter binding of CREB and CREB binding protein associated with an inactivated chromatin status. In addition, further studies showed that the epidermal growth factor receptor (EGFR) signaling pathway also plays an essential role in the inhibitory effect of Rg1; taken together, our study demonstrates the involvement of the EGFR-CREB signaling pathway in the Rg1-mediated downregulation of SGLT1 expression, which offers a potential strategy in the development of antihyperglycemic and antidiabetic treatments.

Activation of cerebral sodium-glucose transporter type 1 function mediated by post-ischemic hyperglycemia exacerbates the development of cerebral ischemia

The regulation of post-ischemic hyperglycemia plays an important role in suppressing neuronal damage in therapeutic strategies for cerebral ischemia. We previously reported that the cerebral sodium-glucose transporter (SGLT) was involved in the post-ischemic hyperglycemia-induced exacerbation of cerebral ischemic neuronal damage. Cortical SGLT-1, one of the cerebral SGLT isoforms, is dramatically increased by focal cerebral ischemia. In this study, we focused on the involvement of cerebral SGLT-1 in the development of cerebral ischemic neuronal damage. It was previously reported that activation of 5'-adenosine monophosphate-activated protein kinase (AMPK) increases SGLT-1 expression. Moreover, ischemic stress-induced activation of AMPK exacerbates cerebral ischemic neuronal damage. Therefore, we directly confirmed the relationship between cerebral SGLT-1 and cerebral AMPK activation using in vitro primary culture of mouse cortical neurons. An in vivo mouse model of focal cerebral ischemia was generated using a middle cerebral artery occlusion (MCAO). The development of infarct volume and behavioral abnormalities on day 3 after MCAO were ameliorated in cerebral SGLT-1 knock down mice. Cortical and striatal SGLT-1 expression levels were significantly increased at 12h after MCAO. Immunofluorescence revealed that SGLT-1 and the neuronal nuclear antigen (NeuN) were co-localized in the cortex and striatum of MCAO mice. In the in vitro study, primary cortical neurons were cultured for five days before each treatment with reagents. Concomitant treatment with hydrogen peroxide and glucose induced the elevation of SGLT-1 and phosphorylated AMPK/AMPK ratio, and this elevation was suppressed by compound C, an AMPK inhibitor in primary cortical neurons. Moreover, compound C suppressed neuronal cell death induced by concomitant hydrogen peroxide/glucose treatment in primary cortical neurons. Therefore, we concluded that enhanced cerebral SGLT-1 function mediated by post-ischemic hyperglycemia exacerbates the development of cerebral ischemic neuronal damage. One of the mechanisms of cerebral SGLT-1 up-regulation may be involved in the AMPK activation after cerebral ischemia.

An essential role of cAMP response element-binding protein in epidermal growth factor-mediated induction of sodium/glucose cotransporter 1 gene expression and intestinal glucose uptake

The sodium/glucose cotransporter 1 (SGLT1) is responsible for glucose uptake in intestinal epithelial cells. Its expression is decreased in individuals with intestinal inflammatory disorders and is correlated with the pathogenesis of disease. The aim of this study was to understand the regulatory mechanism of the SGLT1 gene. Using the trinitrobenzene sulfonic acid-induced mouse models of intestinal inflammation, we observed decreased SGLT1 expression in the inflamed intestine was positively correlated with the mucosal level of epidermal growth factor (EGF) and activated CREB. Overexpression of EGF demonstrated that the effect of EGF on intestinal glucose uptake was primarily due to the increased level of SGLT1. We identified an essential cAMP binding element (CRE) confers EGF inducibility in the human SGLT1 gene promoter. ChIP assay further demonstrated the increased binding of CREB and CBP to the SGLT1 gene promoter in EGF-treated cells. In addition, the EGFR- and PI3K-dependent CREB phosphorylations are involved in the EGF-mediated SGLT1 expression. This is the first report to demonstrate that CREB is involved in EGF-mediated transcription regulation of SGLT1 gene in the normal and inflamed intestine, which can provide potential therapeutic applications for intestinal inflammatory disorders.

A gut microbial metabolite of ginsenosides, compound K, induces intestinal glucose absorption and Na(+) /glucose cotransporter 1 gene expression through activation of cAMP response element binding protein

SCOPE

The Na(+) /glucose cotransporter 1 (SGLT1) plays a crucial role in glucose uptake in intestinal epithelial cells (IECs), which has been shown essential in ameliorating intestinal inflammation. Ginseng has historically been used to treat inflammatory disorders. Understanding the regulatory mechanism of ginseng-mediated induction of SGLT1 gene expression in human intestinal cells is therefore important.

METHODS AND RESULTS

We demonstrate that ginsenoside compound K (CK) enhances SGLT1-mediated glucose uptake in mice and human intestinal Caco-2 cells. Transient transfection analysis using SGLT1 promoter-luciferase reporters demonstrated that the presence of an essential cAMP response element (CRE) is required for CK-mediated induction of SGLT1 gene expression. The ChIP assays indicated that increased CRE-binding protein (CREB) and CREB-binding protein (CBP) binding to the SGLT1 promoter in CK-treated cells is associated with an activated chromatin state. Our result showed that the increased CREB phosphorylation is directly correlated with SGLT1 expression in IECs. Further studies indicated that the epidermal growth factor receptor (EGFR) signaling pathway is involved in the CK-mediated effect.

CONCLUSION

These findings provide a novel mechanism for the CK-mediated upregulation of SGLT1 expression through EGFR-CREB signaling activation, which could contribute to reducing gut inflammation.

SPAK-sensitive regulation of glucose transporter SGLT1

The WNK-dependent STE20/SPS1-related proline/alanine-rich kinase SPAK is a powerful regulator of ion transport. The study explored whether SPAK similarly regulates nutrient transporters, such as the Na(+)-coupled glucose transporter SGLT1 (SLC5A1). To this end, SGLT1 was expressed in Xenopus oocytes with or without additional expression of wild-type SPAK, constitutively active (T233E)SPAK, WNK-insensitive (T233A)SPAK or catalytically inactive (D212A)SPAK, and electrogenic glucose transport determined by dual-electrode voltage-clamp experiments. Moreover, Ussing chamber was employed to determine the electrogenic glucose transport in intestine from wild-type mice (spak(wt/wt)) and from gene-targeted mice carrying WNK-insensitive SPAK (spak(tg/tg)). In SGLT1-expressing oocytes, but not in water-injected oocytes, the glucose-dependent current (I(g)) was significantly decreased following coexpression of wild-type SPAK and (T233E)SPAK, but not by coexpression of (T233A)SPAK or (D212A)SPAK. Kinetic analysis revealed that SPAK decreased maximal I(g) without significantly modifying the glucose concentration required for halfmaximal I(g) (K(m)). According to the chemiluminescence experiments, wild-type SPAK but not (D212A)SPAK decreased SGLT1 protein abundance in the cell membrane. Inhibition of SGLT1 insertion by brefeldin A (5 μM) resulted in a decline of I(g), which was similar in the absence and presence of SPAK, suggesting that SPAK did not accelerate the retrieval of SGLT1 protein from the cell membrane but rather down-regulated carrier insertion into the cell membrane. Intestinal electrogenic glucose transport was significantly lower in spak(wt/wt) than in spak(tg/tg) mice. In conclusion, SPAK is a powerful negative regulator of SGLT1 protein abundance in the cell membrane and thus of electrogenic glucose transport.

Circadian regulation of epithelial functions in the intestine

Many physiological functions exhibit a diurnal rhythmicity that is influenced by biological clocks and feeding rhythms. In this review, we discuss the growing evidence showing the important role of circadian rhythms in regulating intestinal mucosa. First, we introduce the molecular timing system and the interrelationship between the master biological clock in the suprachiasmatic nuclei of the brain and the peripheral intestinal clock and provide evidence that the intestinal clock is entrained with the external environment. Second, we review the circadian rhythmicity of enterocyte proliferation and the largely unknown regulatory mechanisms behind these rhythms. Finally, we focus on the circadian clock control of food processing that functions by regulating the expression of digestive enzymes and intestinal nutrient and salt transporters. The concepts to be discussed highlight the ability of the intestinal epithelium to utilize self-sustained clock signals together with signals associated with changes in the cellular environment and to use endogenous temporal control of the gastrointestinal functions to meet varying physiological and pathophysiological demands. The fact that internal de-synchronizations within the body, such as those that occur in shift workers or with changes in food intake behaviour, are often associated with malfunctions of the gastrointestinal tract indicates that more information about the connections between the circadian clock and intestinal mucosa/transporting enterocytes could provide clues for future therapies.

Relevance of sodium/glucose cotransporter-1 (SGLT1) to diabetes mellitus and obesity in dogs

Glucose transport across the enterocyte brush border membrane by sodium/glucose cotransporter-1 (SGLT1, coded by Slc5a1) is the rate-limiting step for intestinal glucose transport. The relevance of SGLT1 expression in predisposition to diabetes mellitus and to obesity was investigated in dogs. Cultured Caco-2/TC7 cells were shown to express SGLT1 in vitro. A 2-kbp fragment of the Slc5a1 5' flanking region was cloned from canine genomic DNA, ligated into reporter gene plasmids, and shown to drive reporter gene expression in these cells above control (P < 0.001). To determine the effect of the 3 known SNPs in this region on promoter function, new promoter/reporter constructs (all permutations of these 3 SNPs) were created by site-directed mutagenesis. No significant differences in promoter function were seen, suggesting that these SNPs do not have a significant effect on the constitutive transcription of SGLT1 mRNA in dogs. A search for novel SNPs in this region in dogs was made in 2 breeds predisposed to diabetes mellitus (Samoyed and cairn terrier), 2 breeds that rarely develop diabetes (boxer and German shepherd), and 2 breeds predisposed to obesity (Labrador retriever and cocker spaniel). The Slc5a1 5' flanking region was amplified from 10 healthy individuals of each of these breeds by high-fidelity PCR with the use of breed-labeled primers and sequenced by pyrosequencing. The sequence of the Slc5a1 5' flanking region in all individuals of all breeds tested was identical. On this evidence, variations in Slc5a1 promoter sequence between dogs do not influence the pathogenesis of diabetes mellitus or obesity in these breeds.

Loss of claudins 2 and 15 from mice causes defects in paracellular Na+ flow and nutrient transport in gut and leads to death from malnutrition

BACKGROUND & AIMS

The intestinal symport system moves nutrients across membranes via transporters, and is required for absorption of major nutrients such as glucose, amino acids, and bile acids (which are required for fat absorption). Most of these transporters are regulated by Na(+), but the standard diet does not provide sufficient levels of this ion to the intestinal lumen to support this system. Claudins form paracellular barriers between epithelial cells, and claudin-2 and -15 regulate paracellular ion flow in the intestine. We investigated how cell adherence, tight junction barriers, and claudins regulate the supply of Na(+) to the intestinal lumen in mice.

METHODS

We created Cldn2(-/-)Cldn15(-/-) (double-knockout) mice and analyzed intestinal tissues by reverse-transcription polymerase chain reaction, immunoblot, immunofluorescence, electron microscopy, and H&E analyses. We also measured paracellular Na(+) flow, luminal Na(+) concentration, and absorption of glucose, amino acids, and fats, which were administered orally to the mice.

RESULTS

Paracellular flow of Na(+) from the intestinal submucosa to the lumen, and therefore the concentration of Na(+) in the lumen, was greatly reduced in intestines of Cldn2(-/-)Cldn15(-/-) mice. Absorption of glucose, amino acids, and fats also decreased in the mice, which died by postnatal day 25 from malnutrition.

CONCLUSIONS

The paracellular flow of Na(+) from the intestinal submucosa is regulated by tight junctions that contain claudin-2 and -15. This system is required for the absorption of glucose, amino acids, and fats; disruption of this system in mice leads to infant death as a result of malabsorption.

Single nucleotide polymorphisms in the human corticosteroid-binding globulin promoter alter transcriptional activity

Corticosteroid-binding globulin (CBG) is a high-affinity plasma protein that transports glucocorticoids and progesterone. Others and we have reported non-synonymous single nucleotide polymorphisms (SNPs) that influence CBG production or steroid-binding activity. However, no promoter polymorphisms affecting the transcription of human CBG gene (Cbg) have been reported. In the present study we investigated function implications of six promoter SNPs, including -26 C/G, -54 C/T, -144 G/C, -161 A/G, -205 C/A, and -443/-444 AG/-, five of which are located within the first 205 base pairs of 5'-flanking region and close to the highly conserved footprinted elements, TATA-box, or CCAAT-box. Luciferase reporter assays demonstrated that basal activity of the promoter carrying -54 T or -161 G was significantly enhanced. The first three polymorphisms, -26 C/G, -54 C/T, and -144 G/C located close to the putative hepatic nuclear factor (HNF) 1 binding elements, altered the transactivation effect of HNF1β. We also found a negative promoter response to dexamethasone-activated glucocorticoid receptor (GR) α, although none of the SNPs affected its transrepression function. Our results suggest that human Cbg -26 C/G, -54 C/T, -144 G/C, and -161 A/G promoter polymorphisms alter transcriptional activity, and further studies are awaited to explore their association with physiological and pathological conditions.

PER1 modulates SGLT1 transcription in vitro independent of E-box status

BACKGROUND AND AIMS

The intestine demonstrates profound circadian rhythmicity in glucose absorption in rodents, mediated entirely by rhythmicity in the transcription, translation, and function of the sodium glucose co-transporter SGLT1 (Slc5a1). Clock genes are rhythmic in the intestine and have been implicated in the regulation of rhythmicity of other intestinal genes; however, their role in the regulation of SGLT1 is unknown. We investigated the effects of one clock gene, PER1, on SGLT1 transcription in vitro.

METHODS

Caco-2 cells were stably transfected with knockdown vectors for PER1 and mRNA expression of clock genes and SGLT1 determined using quantitative polymerase chain reaction (qPCR). Chinese hamster ovary (CHO) cells were transiently cotransfected with combinations of the PER1 expression vectors and the wild-type SGLT1-luciferase promoter construct or the promoter with mutated E-box sequences.

RESULTS

Knockdown of PER1 increased native SGLT1 expression in Caco-2 enterocytes, while promoter studies confirmed that the inhibitory activity of PER1 on SGLT1 occurs via the proximal 1 kb of the SGLT1 promoter. E-box sites exerted a suppressive effect on the SGLT1 promoter; however, mutation of E-boxes had little effect on the inhibitory activity of PER1 on the SGLT1 promoter suggesting that the actions of PER1 on SGLT1 are independent of E-boxes.

CONCLUSIONS

Our findings suggest that PER1 exerts an indirect suppressive effect on SGLT1, possibly acting via other clock-controlled genes binding to non-E-box sites on the SGLT1 promoter. Understanding the regulation of rhythmicity of SGLT1 may lead to new treatments for the modulation of SGLT1 expression in conditions such as malabsorption, diabetes, and obesity.

Constitutive activation of the MEK/ERK pathway inhibits intestinal epithelial cell differentiation

The Ras/Raf/MEK/ERK cascade regulates intestinal epithelial cell proliferation. Indeed, while barely detectable in differentiated cells of the villi, ERK1/2-activated forms are detected in the nucleus of undifferentiated human intestinal crypt cells. In addition, we and others have reported that ERKs are selectively inactivated during enterocyte differentiation. However, whether inactivation of the ERK pathway is necessary for inhibition of both proliferation and induction of differentiation of intestinal epithelial cells is unknown. Human Caco-2/15 cells, undifferentiated crypt IEC-6 cells, and differentiating Cdx3-expressing IEC-6 cells were infected with retroviruses encoding either a hemagglutinin (HA)-tagged MEK1 wild type (wtMEK) or a constitutively active S218D/S222D MEK1 mutant (caMEK). Protein and gene expression was assessed by Western blotting, semiquantitative RT-PCR, and real-time PCR. Morphology was analyzed by transmission electron microscopy. We found that 1) IEC-6/Cdx3 cells formed multicellular layers after confluence and differentiated after 30 days in culture, as assessed by increased polarization, microvilli formation, expression of differentiation markers, and ERK1/2 inhibition; 2) while activated MEK prevented neither the inhibition of ERK1/2 activities nor the differentiation process in postconfluent Caco-2/15 cells, caMEK expression prevented ERK inhibition in postconfluent IEC-6/Cdx3 cells, thus leading to maintenance of elevated ERK1/2 activities; 3) caMEK-expressing IEC-6/Cdx3 cells exhibited altered multicellular structure organization, poorly defined tight junctions, reduced number of microvilli on the apical surface, and decreased expression of the hepatocyte nuclear factor 1α transcription factor and differentiation markers, namely apolipoprotein A-4, fatty acid-binding protein, calbindin-3, mucin 2, alkaline phosphatase, and sucrase-isomaltase; and 4) increased Cdx3 phosphorylation on serine-60 (S60) in IEC-6/Cdx3 cells expressing caMEK led to decreased Cdx2 transactivation potential. These results indicate that inactivation of the ERK pathway is required to ensure the full Cdx2/3 transcriptional activity necessary for intestinal epithelial cell terminal differentiation.

Biology of human sodium glucose transporters

There are two classes of glucose transporters involved in glucose homeostasis in the body, the facilitated transporters or uniporters (GLUTs) and the active transporters or symporters (SGLTs). The energy for active glucose transport is provided by the sodium gradient across the cell membrane, the Na(+) glucose cotransport hypothesis first proposed in 1960 by Crane. Since the cloning of SGLT1 in 1987, there have been advances in the genetics, molecular biology, biochemistry, biophysics, and structure of SGLTs. There are 12 members of the human SGLT (SLC5) gene family, including cotransporters for sugars, anions, vitamins, and short-chain fatty acids. Here we give a personal review of these advances. The SGLTs belong to a structural class of membrane proteins from unrelated gene families of antiporters and Na(+) and H(+) symporters. This class shares a common atomic architecture and a common transport mechanism. SGLTs also function as water and urea channels, glucose sensors, and coupled-water and urea transporters. We also discuss the physiology and pathophysiology of SGLTs, e.g., glucose galactose malabsorption and familial renal glycosuria, and briefly report on targeting of SGLTs for new therapies for diabetes.

Diet-induced epigenetic regulation in vivo of the intestinal fructose transporter Glut5 during development of rat small intestine

Metabolic complications arising from excessive fructose consumption are increasing dramatically even in young children, but little is known about ontogenetic mechanisms regulating Glut5 [glucose transporter 5; encoded by the Slc2a5 (solute carrier family 2 member 5) gene]. Glut5 expression is low postnatally and does not increase, unless luminal fructose and systemic glucocorticoids are present, until ≥ 14 days of age, suggesting substrate-inducible age- and hormone-sensitive regulation. In the present study, we perfused intestines of 10- and 20-day-old rats with either fructose or glucose then analysed the binding of Pol II (RNA polymerase II) and GR (glucocorticoid receptor), as well as acetylation of histones H3 and H4 by chromatin immunoprecipitation. Abundance of Glut5 mRNA increased only with fructose perfusion and age, a pattern that matched that of Pol II binding and histone H3 acetylation to the Glut5 promoter. Although many regions of the Glut5 promoter respond to developmental signals, fewer regions perceive dietary signals. Age- but not fructose-dependent expression of Sglt1 [sodium-dependent glucose co-transporter 1 encoded by the Slc5a1(solute carrier family 5 member 1) gene] also correlated with Pol II binding and histone H3 acetylation. In contrast, G6Pase (glucose-6-phosphatase; encoded by the G6pc gene) expression, which decreases with age and increases with fructose, is associated only with age-dependent changes in histone H4 acetylation. Induction of Glut5 during ontogenetic development appears to be specifically mediated by GR translocation to the nucleus and subsequent binding to the Glut5 promoter, whereas the glucocorticoid-independent regulation of Sglt1 by age was not associated with any GR binding to the Sglt1 promoter.

Glucose sensing and signalling; regulation of intestinal glucose transport

Epithelial cells lining the inner surface of the intestinal epithelium are in direct contact with a lumenal environment that varies dramatically with diet. It has long been suggested that the intestinal epithelium can sense the nutrient composition of lumenal contents. It is only recently that the nature of intestinal nutrient-sensing molecules and underlying mechanisms have been elucidated. There are a number of nutrient sensors expressed on the luminal membrane of endocrine cells that are activated by various dietary nutrients. We showed that the intestinal glucose sensor, T1R2+T1R3 and the G-protein, gustducin are expressed in endocrine cells. Eliminating sweet transduction in micein vivoby deletion of either gustducin or T1R3 prevented dietary monosaccharide- and artificial sweetener-induced up-regulation of the Na+/glucose cotransporter, SGLT1 observed in wild-type mice. Transgenic mice, lacking gustducin or T1R3 had deficiencies in secretion of glucagon-like peptide 1 (GLP-1) and, glucose-dependent insulinotrophic peptide (GIP). Furthermore, they had an abnormal insulin profile and prolonged elevation of postprandial blood glucose in response to orally ingested carbohydrates. GIP and GLP-1 increase insulin secretion, while glucagon-like peptide 2 (GLP-2) modulates intestinal growth, blood flow and expression of SGLT1. The receptor for GLP-2 resides in enteric neurons and not in any surface epithelial cells, suggesting the involvement of the enteric nervous system in SGLT1 up-regulation. The accessibility of the glucose sensor and the important role that it plays in regulation of intestinal glucose absorption and glucose homeostasis makes it an attractive nutritional and therapeutic target for manipulation.

The human organic anion transporter genes OAT5 and OAT7 are transactivated by hepatocyte nuclear factor-1α (HNF-1α)

Organic anion transporters (OATs) are anion exchangers that transport small hydrophilic anions and diuretics, antibiotics, nonsteroidal anti-inflammatory drugs, antiviral nucleoside analogs, and antitumor drugs across membrane barriers of epithelia of diverse organs. Three OATs are present in human liver: OAT2, OAT5, and OAT7. Given that hepatocyte nuclear factor-1α (HNF-1α) has previously been shown to regulate the expression of several hepatocellular transporter genes, we investigated whether the liver-specific human OAT genes are also regulated by HNF-1α. Short interfering RNAs targeting HNF-1α reduced endogenous expression of OAT5 and OAT7, but not OAT2, in human liver-derived Huh7 cells. Luciferase reporter gene constructs containing the OAT5 (SLC22A10) and OAT7 (SLC22A9) promoter regions were transactivated by HNF-1α in HepG2 cells. Two putative HNF-1α binding elements in the proximal OAT5 promoter, located at nucleotides -68/-56 and -173/-160, and one element in the OAT7 promoter, located at nucleotides -14/-2 relative to the transcription start site, were shown to bind HNF-1α in electromobility shift assays, and these promoter regions also interacted with HNF-1α in chromatin immunoprecipitation assays. A correlation between HNF-1α and OAT5 (r = 0.134, P < 0.05) or OAT7 (r = 0.461, P < 0.001) mRNA expression levels in surgical liver biopsies from 75 patients further supported an important role of HNF-1α in the regulation of OAT gene expression.

SGLT1, a novel cardiac glucose transporter, mediates increased glucose uptake in PRKAG2 cardiomyopathy

Human mutations in the gene PRKAG2 encoding the gamma2 subunit of AMP-activated protein kinase (AMPK) cause a glycogen storage cardiomyopathy. Transgenic mice (TG(T400N)) with the human T400N mutation exhibit inappropriate activation of AMPK and consequent glycogen storage in the heart. Although increased glucose uptake and activation of glycogen synthesis have been documented in PRKAG2 cardiomyopathy, the mechanism of increased glucose uptake has been uncertain. Wildtype (WT), TG(T400N), and TG(alpha2DN) (carrying a dominant negative, kinase dead alpha2 catalytic subunit of AMPK) mice were studied at ages 2-8 weeks. Cardiac mRNA expression of sodium-dependent glucose transporter 1 (SGLT1), but not facilitated-diffusion glucose transporter 1 (GLUT1) or GLUT4, was increased approximately 5- to 7-fold in TG(T400N) mice relative to WT. SGLT1 protein was similarly increased at the cardiac myocyte sarcolemma in TG(T400N) mice. Phlorizin, a specific SGLT1 inhibitor, attenuated cardiac glucose uptake in TG(T400N) mice by approximately 40%, but not in WT mice. Chronic phlorizin treatment reduced cardiac glycogen content by approximately 25% in TG(T400N) mice. AICAR, an AMPK activator, increased cardiac SGLT1 mRNA expression approximately 3-fold in WT mice. Relative to TG(T400N) mice, double transgenic (TG(T400N)/TG(alpha2DN)) mice had decreased ( approximately 50%) cardiac glucose uptake and decreased (approximately 70%) cardiac SGLT1 expression. TG(T400N) hearts had increased binding activity of the transcription factors HNF-1 and Sp1 to the promoter of the gene encoding SGLT1. Our data suggest that upregulation of cardiac SGLT1 is responsible for increased cardiac glucose uptake in the TG(T400N) mouse. Increased AMPK activity leads to upregulation of SGLT1, which in turn mediates increased cardiac glucose uptake.

Zinc finger transcription factor Zn3-Sp1 reactions with Cd2+

Cadmium is a major environmental pollutant that causes kidney failure including the inability to resorb nutrients such as glucose. In a mouse kidney cell culture model, Cd(2+) inhibits Na(+)-dependent glucose uptake mediated by SGLT transporters. This defect has been traced to the down-regulation of SGLT mRNA synthesis mediated by the zinc-finger transcription factor, Zn(3)-Sp1. Incubation of Cd(2+) with Zn(2+)-Sp1 inhibited its capacity to bind to GC1, its binding site in the SGLT1 promoter. The extent of reaction was reduced as increasing concentrations of Zn(2+) are simultaneously present in the reaction mixture. The results are consistent with a Cd(2+)-Zn(2+) exchange reaction that inactivates the DNA binding function of the protein. The equilibrium constant for this reaction was calculated as 14 +/- 3 and 7 +/- 4 for the reactions measured by the binding to GC1 and an analogous SGLT2 promoter site. Sequential addition of Cd(2+) and Zn(2+) to Zn(3)-Sp1 failed to inhibit the reduction in DNA binding seen with Cd(2+) alone, indicating that substitution of Zn(2+) by Cd(2+) was followed by a second reaction that failed to respond to Zn(2+). Buffers for the DNA binding reaction (electrophoretic mobility shift assay) contain EDTA and Cd-EDTA is active in the same concentration range as Cd(2+). During the standard 15 min incubation, Cd(2+) down-regulates Zn(3)-Sp1 but is inactive against the adduct, Zn(3)-Sp1.GC1. Kinetic studies demonstrated that with 5 muM Cd(2+), Zn(3)-Sp1 was about 75% inactivated in 15 min, whereas, Zn(3)-Sp1.GC1 was slowly dissociated with 50% still remaining after 60 min. In contrast, Zn(3)-Sp1 bound to a cognate consensus site resisted any reaction over 60 min. An adduct of Zn(3)-Sp1.(polydI-dC) was just as reactive with Cd(2+) as Zn(3)-Sp1. Reexamination of the NMR structure of Zn- and Cd-finger peptides related to Sp1 fingers has revealed subtle changes in conformation of the metalbinding site and DNA-binding helix that occur when Cd(2+) is substituted by Zn(2+).

Cadmium down-regulation of kidney Sp1 binding to mouse SGLT1 and SGLT2 gene promoters: possible reaction of cadmium with the zinc finger domain of Sp1

Cadmium (Cd) exposure causes glucosuria (glucose in the urine). Previously, it was shown that Cd exposure of primary cultures of mouse kidney cells (PMKC) decreased mRNA levels of the glucose transporters, SGLT1 and SGLT2 and that Sp1 from Cd-exposed cells displayed reduced binding to the GC boxes of the mouse SGLT1 promoter in vitro. Here, we identified a GC box upstream of mouse SGLT2 gene. ChIP assays on PMKC revealed that exposure to 5 microM Cd abolished Sp1 binding to SGLT1 GC box while it decreased Sp1 binding to SGLT2 GC sequence by 30% in vivo. The in vitro DNA binding assay, EMSA, demonstrated that binding of Sp1 from Cd (7.5 microM)-treated PMKC to the SGLT2 GC probe was 86% lower than in untreated cells. Sp1 is a zinc finger protein. Compared to PMKC exposed to 5 microM Cd alone, inclusion of 5 microM Zn restored SGLT1 and 2 mRNA levels by 15% and 30%, respectively. Cd (10 microM) decreased the binding of recombinant Sp1 (rhSp1) to SGLT1 and SGLT2 GC probes to 12% and 8% of untreated controls. Cd exerted no effect on GC-bound rhSp1. Co-treatment with Cd and Zn showed that added Zn significantly restored rhSp1 binding to the SGLT1 and SGLT2. Addition of Zn post Cd treatment was not stimulatory. We conclude that Cd can replace Zn in Sp1 DNA binding domain to reduce its binding to GC sites in mouse SGLT1 and SGLT2 promoters.

Novel shuttling domain in a regulator (RSC1A1) of transporter SGLT1 steers cell cycle-dependent nuclear location

The gene product of RSC1A1, RS1, participates in the regulation of the Na(+)-D-glucose cotransporter SGLT1. RS1 inhibits release of SGLT1 from the trans Golgi network. In subconfluent LLC-PK(1) cells, RS1 migrates into the nucleus and modulates transcription of SGLT1, whereas most confluent cells do not contain RS1 in the nuclei. We showed that confluence-dependent nuclear location of RS1 is because of different phases of the cell cycle and identified a RS1 nuclear shuttling domain (RNS) with an associated protein kinase C (PKC) phosphorylation site (RNS-PKC) that mediates cell cycle-dependent nuclear location. RNS-PKC contains a novel non-conventional nuclear localization signal interacting with importin beta1, a nuclear export signal mediating export via protein CRM1 and a Ca(2+)-dependent calmodulin binding site. PKC and calmodulin compete for binding to RNS-PKC. Mutagenesis experiments and analyses of the phosphorylation status suggest the following sequences of events. Subconfluent cells without and with synchronization to the G2/M phase contain non-phosphorylated RNS-PKC that mediates nuclear import of RS1 but not its export. During confluence or synchronization of subconfluent cells to the G2/M phase, phosphorylation of RNS-PKC mediates rapid nuclear export of RS1.

The role of the neutral amino acid transporter B0AT1 (SLC6A19) in Hartnup disorder and protein nutrition

Hartnup disorder (OMIM 234500) is an autosomal recessive disorder, which was first described in 1956 as an aminoaciduria of neutral amino acids accompanied by a variety of symptoms, such as a photo‐sensitive skin‐rash and cerebellar ataxia. The disorder is caused by mutations in the neutral amino acid transporter B⁰AT1 (SLC6A19)1. To date 21 mutations have been identified in more than twenty families. SLC6A19 requires either collectrin or angiotensin‐converting enzyme 2 for surface expression in the kidney and intestine, respectively. This ties SLC6A19 together with more complex functions such as blood‐pressure control, glomerular structure, and exocytosis. © 2009 IUBMB IUBMB Life, 61(6): 591–599, 2009

Establishment of intestinal identity and epithelial-mesenchymal signaling by Cdx2

We demonstrate that conditional ablation of the homeobox transcription factor Cdx2 from early endoderm results in the replacement of the posterior intestinal epithelium with keratinocytes, a dramatic cell fate conversion caused by ectopic activation of the foregut/esophageal differentiation program. This anterior homeotic transformation of the intestine was first apparent in the early embryonic Cdx2-deficient gut by a caudal extension of the expression domains of several key foregut endoderm regulators. While the intestinal transcriptome was severely affected, Cdx2 deficiency only transiently modified selected posterior Hox genes and the primary enteric Hox code was maintained. Further, we demonstrate that Cdx2-directed intestinal cell fate adoption plays an important role in the establishment of normal epithelial-mesenchymal interactions, as multiple signaling pathways involved in this process were severely affected. We conclude that Cdx2 controls important aspects of intestinal identity and development, and that this function is largely independent of the enteric Hox code.

Enhanced expressions of sodium-glucose cotransporters in the kidneys of diabetic Zucker rats

Diabetes-mediated changes in mRNA expressions of kidney glucose transporters SGLT1 and SGLT2 were investigated in Zucker rats. SGLTs expressions in pre-diabetic obese rats were similar to leans. SGLT1 and SGLT2 levels in diabetic obese rats were 1.6 (P<0.03) and 4.8 (P<0.002) folds higher than age-matched leans, respectively.

Role of Sp1 and HNF1 transcription factors in SGLT1 regulation during chronic intestinal inflammation

In a rabbit model of chronic intestinal inflammation, we previously demonstrated that the activity of Na-glucose cotransporter (SGLT1), SLC5A1, is inhibited. This inhibition is secondary to a decrease in the number of cotransporters, indicating that the regulation of SGLT1 during chronic inflammation is at the level of transcription. However, the regulation of SGLT1 expression and the transcription factors involved in the regulation are not yet known. In this report, we describe the cloning and characterization of rabbit SGLT1 promoter and the identification of transcription factors affected in villus cells during chronic intestinal inflammation. The promoter sequence for SGLT1 gene was identified by using the publicly available rabbit genomic sequence. Even though rabbit SGLT1 promoter did not have considerable overall homology with other mammalian SGLT1 promoters, two specificity protein 1 (Sp1) and a hepatocyte nuclear factor 1 (HNF1) binding sites were highly conserved among the species. Rabbit SGLT1 cDNA was encoded by 15 exons. Minimal promoter region determination showed that 196 nucleotides upstream of the transcription start site were sufficient for optimal promoter activity. This region encompassed two transcription factor binding sites, Sp1 and HNF1. For maximal SGLT1 promoter activity, these two transcription factor binding sites were essential, and their effect was synergistic, indicating that two separate regulatory pathways might be involved in their regulation. Using mobility shift assays, we further demonstrated that the binding of both Sp1 and HNF1 transcription factors to SGLT1 promoter regions were affected during chronic intestinal inflammation. Thus this report demonstrates that Sp1 and HNF1 transcription factors act in concert to regulate SGLT1 transcription in the chronically inflamed intestine.

Defining the transcriptional regulation of the intestinal sodium-glucose cotransporter using RNA-interference mediated gene silencing

BACKGROUND

The sodium glucose cotransporter (SGLT1) is responsible for all active intestinal glucose uptake. Hepatocyte nuclear factors 1 alpha and beta (HNF 1 alpha and HNF 1 beta) activate the SGLT1 promoter, whereas GATA-binding protein 5 (GATA-5) and caudal-type homeobox protein 2 (CDX2) regulate transcription of other intestinal genes. We investigated SGLT1 regulation by these transcription factors using promoter studies and RNA interference.

METHODS

Chinese hamster ovary (CHO) cells were transiently cotransfected with an SGLT1-luciferase promoter construct and combinations of expression vectors for HNF 1 alpha, HNF 1 beta, CDX2, and GATA-5. Caco-2 cells were stably transfected with knockdown vectors for either HNF 1 alpha or HNF 1 beta. mRNA levels of HNF 1 alpha, HNF 1 beta, and SGLT1 were determined using quantitative polymerase chain reaction (qPCR).

RESULTS

HNF 1 alpha, GATA-5, and HNF 1 beta significantly activated the SGLT1 promoter (P < .05). Cotransfection of GATA-5 with HNF 1 alpha had an additive effect, whereas HNF 1 beta and CDX2 antagonized HNF 1 alpha and GATA-5. SGLT1 expression was significantly reduced in HNF 1 alpha or HNF 1 beta knockdowns (P < .001). HNF alpha knockdown significantly reduced HNF 1 beta expression and vice versa (P < .005).

CONCLUSIONS

HNF 1 alpha and HNF 1 beta are important transcription factors for endogenous SGLT1 expression by cultured enterocytes. GATA-5 and CDX2 also regulate SGLT1 promoter activity and show cooperativity with the HNF1s. We, therefore, propose a multifactorial model for SGLT1 regulation, with interactions between HNF1, GATA-5, and CDX2 modulating intestinal glucose absorption.

Critical roles of Sp1 in gene expression of human and rat H+/organic cation antiporter MATE1

A H+/organic cation antiporter (multidrug and toxin extrusion 1: MATE1/SLC47A1) plays important roles in the tubular secretion of various clinically important cationic drugs such as cimetidine. We have recently found that the regulation of this transporter greatly affects the pharmacokinetic properties of cationic drugs in vivo. No information is available about the regulatory mechanisms for the MATE1 gene. In the present study, therefore, we examined the gene regulation of human (h) and rat (r) MATE1, focusing on basal expression. A deletion analysis suggested that the regions spanning -65/-25 and -146/-38 were essential for the basal transcriptional activity of the hMATE1 and rMATE1 promoter, respectively, and that both regions contained putative Sp1-binding sites. Functional involvement of Sp1 was confirmed by Sp1 overexpression, a mutational analysis of Sp1-binding sites, mithramycin A treatment, and an electrophoretic mobility shift assay. Furthermore, we found a single nucleotide polymorphism (SNP) in the promoter region of hMATE1 (G-32A), which belongs to a Sp1-binding site. The allelic frequency of this rSNP was 3.7%, and Sp1-binding and promoter activity were significantly decreased. This is the first study to clarify the transcriptional mechanisms of the MATE1 gene and to identify a SNP affecting the promoter activity of hMATE1.

RS1 (RSC1A1) regulates the exocytotic pathway of Na+-d-glucose cotransporter SGLT1

The product of gene RSC1A1, named RS1, participates in transcriptional and posttranscriptional regulation of the sodium-d-glucose cotransporter SGLT1. Using coexpression in oocytes of Xenopus laevis, posttranscriptional inhibition of human SGLT1 (hSGLT1) and some other transporters by human RS1 (hRS1) was demonstrated previously. In the present study, histidine-tagged hRS1 was expressed in oocytes or Sf9 cells and purified using nickel(II)-charged nitrilotriacetic acid-agarose. hRS1 protein was injected into oocytes expressing hSGLT1 or the human organic cation transporter hOCT2, and the effect on hSGLT1-mediated uptake of methyl-α-d-[14C]glucopyranoside ([14C]AMG) or hOCT2-mediated uptake of [14C]tetraethylammonium ([14C]TEA) was measured. Within 30 min after the injection of hRS1 protein, hSGLT1-expressed AMG uptake or hOCT2-expressed TEA uptake was inhibited by ∼50%. Inhibition of AMG uptake was decreased when a dominant negative mutant of dynamin I was coexpressed and increased after stimulation of PKC. Inhibition remained unaltered when endocytosis was inhibited by chlorpromazine, imipramine, or filipin but was prevented when exocytosis was inhibited by botulinum toxin B or when the release of vesicles from the TGN and endosomes was inhibited by brefeldin A. Inhibition of hSGLT1-mediated AMG uptake and hOCT2-mediated TEA uptake by hRS1 protein were decreased at an enhanced intracellular AMG concentration. The data suggest that hRS1 protein exhibits glucose-dependent, short-term inhibition of hSGLT1 and hOCT2 by inhibiting the release of vesicles from the trans-Golgi network.

Mutant neurogenin-3 in congenital malabsorptive diarrhea

BACKGROUND

Neurogenin-3 (NEUROG3) is expressed in endocrine progenitor cells and is required for endocrine-cell development in the pancreas and intestine. The NEUROG3 gene (NEUROG3) is therefore a candidate for the cause of a newly discovered autosomal recessive disorder characterized by generalized malabsorption and a paucity of enteroendocrine cells.

METHODS

We screened genomic DNA from three unrelated patients with sparse enteroendocrine cells for mutations of NEUROG3. We then tested the ability of the observed mutations to alter NEUROG3 function, using in vitro and in vivo assays.

RESULTS

The patients had few intestinal enteroendocrine cells positive for chromogranin A, but they had normal numbers of Paneth's, goblet, and absorptive cells. We identified two homozygous mutations in NEUROG3, both of which rendered the NEUROG3 protein unable to activate NEUROD1, a downstream target of NEUROG3, and compromised the ability of NEUROG3 to bind to an E-box element in the NEUROD1 promoter. The injection of wild-type but not mutant NEUROG3 messenger RNA into xenopus embryos induced NEUROD1 expression.

CONCLUSIONS

A newly discovered disorder characterized by malabsorptive diarrhea and a lack of intestinal enteroendocrine cells is caused by loss-of-function mutations in NEUROG3.

Organic anion and cation transporter expression and function during embryonic kidney development and in organ culture models

Organic anion and cation transporters (OATs, OCTs, and OCTNs) mediate the proximal tubular secretion of numerous clinically important compounds, including various commonly prescribed pharmaceuticals. Here, we report determination of the ontogeny of these transporters and of NaP(i)2 and SGLT1, using quantitative polymerase chain reaction (QPCR) to determine expression levels of transporter genes in rat embryonic kidneys on each day of gestation from embryonic day (ed) 13 to ed18, in cultures of induced and uninduced metanephric mesenchyme (MM), and on each day of 1 week of whole embryonic kidney (WEK) culture. We also examined ontogeny of Oat1 protein expression in rat embryonic kidney by immunohistochemistry. Finally, we used uptake of fluorescein (FL) as a novel in vitro functional assay of OAT expression in WEK and MM. Developmental induction of OAT and OCT genes does not occur uniformly: some genes are induced early (e.g., Oat1 and Oat3, potential early markers of proximal tubulogenesis), and others after kidney development is relatively advanced (e.g., Oct1, a potential marker of terminal differentiation). The ontogeny of transporter genes in WEK and MM is similar to that observed in vivo, indicating that these organ culture systems may represent convenient in vitro models to study the developmental induction of OATs, OCTs, and OCTNs. Functional transport was evidenced by accumulation of FL in the developing tubule in WEK and MM organ cultures. Our findings on the renal ontogeny of OATs and OCTs could carry implications both for the development of more rational therapeutics for premature infants, as well as for our understanding of proximal tubule differentiation.

Hepatocyte nuclear factor-1alpha is required for expression but dispensable for histone acetylation of the lactase-phlorizin hydrolase gene in vivo

Hepatocyte nuclear factor-1alpha (HNF-1alpha) is a modified homeodomain-containing transcription factor that has been implicated in the regulation of intestinal genes. To define the importance and underlying mechanism of HNF-1alpha for the regulation of intestinal gene expression in vivo, we analyzed the expression of the intestinal differentiation markers and putative HNF-1alpha targets lactase-phlorizin hydrolase (LPH) and sucrase-isomaltase (SI) in hnf1alpha null mice. We found that in adult jejunum, LPH mRNA in hnf1alpha(-/-) mice was reduced 95% compared with wild-type controls (P < 0.01, n = 4), whereas SI mRNA was virtually identical to that in wild-type mice. Furthermore, SI mRNA abundance was unchanged in the absence of HNF-1alpha along the length of the adult mouse small intestine as well as in newborn jejunum. We found that HNF-1alpha occupies the promoters of both the LPH and SI genes in vivo. However, in contrast to liver and pancreas, where HNF-1alpha regulates target genes by recruitment of histone acetyl transferase activity to the promoter, the histone acetylation state of the LPH and SI promoters was not affected by the presence or absence of HNF-1alpha. Finally, we showed that a subset of hypothesized intestinal target genes is regulated by HNF-1alpha in vivo and that this regulation occurs in a defined tissue-specific and developmental context. These data indicate that HNF-1alpha is an activator of a subset of intestinal genes and induces these genes through an alternative mechanism in which it is dispensable for chromatin remodeling.

Alterations in transporter expression in liver, kidney, and duodenum after targeted disruption of the transcription factor HNF1alpha

The transcription factor hepatocyte nuclear factor 1alpha (HNF1alpha) is involved in regulation of glucose metabolism and transport, and in the expression of several drug and bile acid metabolizing enzymes. Targeted disruption of the HNF1alpha gene results in decreased Cyp1a2, and Cyp2e1 expression, and increased Cyp4a1 and Cyp7a1 expression, suggesting these enzymes are HNF1alpha target genes. Since hepatic metabolism can be coordinately linked with drug and metabolite transport, this study aims to demonstrate whether HNF1alpha regulates expression of a variety of organic anion and cation transporters through utilization of an HNF1alpha-null mouse model. Expression of 32 transporters, including members of the Oat, Oatp, Oct, Mrp, Mdr, bile acid and sterolin families, was quantified in three different tissues: liver, kidney, and duodenum. The expression of 17 of 32 transporters was altered in liver, 21 of 32 in kidney, and 6 of 32 in duodenum of HNF1alpha-null mice. This includes many novel observations, including marked downregulation of Oats in kidney, as well as upregulation of many Mrp and Mdr family members in all three tissues. These data indicate that disruption of HNF1alpha causes a marked attenuation of several Oat and Oatp uptake transporters in liver and kidney, and increased expression of efflux transporters such as Mdrs and Mrps, thus suggesting that HNF1alpha is a central mediator in regulating hepatic, renal, and intestinal transporters.

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.

Characterization of the human peptide transporter PEPT1 promoter: Sp1 functions as a basal transcriptional regulator of human PEPT1

H+-coupled peptide transporter 1 (PEPT1, SLC15A1) localized at the brush-border membranes of intestinal epithelial cells plays an important role in the intestinal absorption of small peptides and a variety of peptidemimetic drugs. PEPT1 is regulated by various factors, including hormones, dietary conditions, some pharmaceutics, and diurnal rhythm. But there is little information about the transcriptional regulation of PEPT1. In the present study, therefore, we cloned the human (h)PEPT1 promoter region and examined its promoter activity using a human intestinal cell line, Caco-2. Deletion analysis of the hPEPT1 promoter suggested that the region spanning -172 to -35 bp was essential for basal transcriptional activity. This region lacked a TATA-box but contained some GC-rich sites that supposedly bind with the transcription factor Sp1. Mutational analysis revealed that three of these putative Sp1 sites contributed to the transcriptional activity. EMSA showed that Sp1 bound to two GC-rich sites. Furthermore, inhibition of Sp1 binding by mithramycin A treatment significantly reduced the transcriptional activity. Finally, overexpression of Sp1 increased the transcriptional activity in a dose-dependent manner. This study reports the first characterization of the hPEPT1 promoter and shows the significant role of Sp1 in the basal transcriptional regulation of hPEPT1.

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.

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.