Cyclic nucleotide regulation of Na+/glucose cotransporter (SGLT1) mRNA stability. Interaction of a nucleocytoplasmic protein with a regulatory domain in the 3'-untranslated region critical for stabilization

Adrenomedullin Enhances Mouse Gustatory Nerve Responses to Sugars via T1R-Independent Sweet Taste Pathway

On the tongue, the T1R-independent pathway (comprising glucose transporters, including sodium-glucose cotransporter (SGLT1) and the K_ATP channel) detects only sugars, whereas the T1R-dependent (T1R2/T1R3) pathway can broadly sense various sweeteners. Cephalic-phase insulin release, a rapid release of insulin induced by sensory signals in the head after food-related stimuli, reportedly depends on the T1R-independent pathway, and the competitive sweet taste modulators leptin and endocannabinoids may function on these two different sweet taste pathways independently, suggesting independent roles of two oral sugar-detecting pathways in food intake. Here, we examined the effect of adrenomedullin (ADM), a multifunctional regulatory peptide, on sugar sensing in mice since it affects the expression of SGLT1 in rat enterocytes. We found that ADM receptor components were expressed in T1R3-positive taste cells. Analyses of chorda tympani (CT) nerve responses revealed that ADM enhanced responses to sugars but not to artificial sweeteners and other tastants. Moreover, ADM increased the apical uptake of a fluorescent D-glucose derivative into taste cells and SGLT1 mRNA expression in taste buds. These results suggest that the T1R-independent sweet taste pathway in mouse taste cells is a peripheral target of ADM, and the specific enhancement of gustatory nerve responses to sugars by ADM may contribute to caloric sensing and food intake.

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.

Glucagon-Like Peptide-2 and the Enteric Nervous System Are Components of Cell-Cell Communication Pathway Regulating Intestinal Na+/Glucose Co-transport

The Na⁺/glucose cotransporter 1, SGLT1 is the major route for transport of dietary glucose from the lumen of the intestine into absorptive enterocytes. Sensing of dietary sugars and artificial sweeteners by the sweet taste receptor, T1R2-T1R3, expressed in the enteroendocrine L-cell regulates SGLT1 expression in neighboring absorptive enterocytes. However, the mechanism by which sugar sensing by the enteroendocrine cell is communicated to the absorptive enterocytes is not known. Here, we show that glucagon-like peptide-2 (GLP-2) secreted from the enteroendocrine cell in response to luminal sugars regulates SGLT1 mRNA and protein expression in absorptive enterocytes, via the enteric neurons. Glucose and artificial sweeteners induced secretion of GLP-2 from mouse small intestine, which was inhibited by the sweet-taste receptor inhibitor, gurmarin. In wild type mice there was an increase in sugar-induced SGLT1 mRNA and protein abundance that was not observed in GLP-2 receptor knockout mice. GLP-2 receptor is expressed in enteric neurons, and not in absorptive enterocytes ruling out a paracrine effect of GLP-2. Electric field stimulation of the intestine resulted in upregulation of SGLT1 expression that was abolished by the nerve blocking agent tetrodotoxin. We conclude that GLP-2 and the enteric nervous system are components of the enteroendocrine-absorptive enterocyte communication pathway regulating intestinal glucose transport.

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.

Involvement of the Niacin Receptor GPR109a in the LocalControl of Glucose Uptake in Small Intestine of Type 2Diabetic Mice

Niacin is a popular nutritional supplement known to reduce the risk of cardiovascular diseases by enhancing high-density lipoprotein levels. Despite such health benefits, niacin impairs fasting blood glucose. In type 2 diabetes (T2DM), an increase in jejunal glucose transport has been well documented; however, this is intriguingly decreased during niacin deficient state. In this regard, the role of the niacin receptor GPR109a in T2DM jejunal glucose transport remains unknown. Therefore, the effects of diabetes and high-glucose conditions on GPR109a expression were studied using jejunal enterocytes of 10-week-old m+/db and db/db mice, as well as Caco-2 cells cultured in 5.6 or 25.2 mM glucose concentrations. Expression of the target genes and proteins were quantified using real-time polymerase chain reaction (RT-PCR) and Western blotting. Glucose uptake in Caco-2 cells and everted mouse jejunum was measured using liquid scintillation counting. 10-week T2DM increased mRNA and protein expression levels of GPR109a in jejunum by 195.0% and 75.9%, respectively, as compared with the respective m+/db control; high-glucose concentrations increased mRNA and protein expression of GPR109a in Caco-2 cells by 130.2% and 69.0%, respectively, which was also confirmed by immunohistochemistry. In conclusion, the enhanced GPR109a expression in jejunal enterocytes of T2DM mice and high-glucose treated Caco-2 cells suggests that GPR109a is involved in elevating intestinal glucose transport observed in diabetes.

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.

E-cadherin gene 3'-UTR C/T polymorphism in Turkish patients with nephrolithiasis

Nephrolithiasis is a complex disease and many gene polymorphisms have been associated with stone formation. In this study we aimed to investigate another possible relationship between E-cadherin gene (CHD1) 3'-UTR C/T polymorphism and calcium oxalate nephrolithiasis in the Turkish population. Study population was composed of 143 patients with nephrolithiasis and 158 control subjects. CHD1 3'-UTR C/T polymorphism was analysed using polymerase chain reaction (PCR)-restriction fragment length polymorphism (RFLP) technique. Genotype distribution of the investigated polymorphism was not deviated from Hardy-Weinberg equilibrium (HWE) in patients and control subjects (P > 0.05). C allele frequency was 85.7 and 85.1% in patients and controls, respectively (P = 0.836). Genotype distributions of the CHD1 3'-UTR C/T polymorphism among patients were also not significantly different from those among control subjects (P = 0.636). Our results showed that there is no association between the CHD1 gene 3'-UTR C/T polymorphism and nephrolithiasis in our population.

Association of E-cadherin gene 3'-UTR C/T polymorphism with primary open angle glaucoma

PURPOSE

E-cadherin (E-CDH) is one of the most important cell surface glycoproteins involved in cell morphogenesis. In primary open angle glaucoma (POAG), the extracellular matrixes of trabecular meshwork and lamina cribrosa in the optic nerve head are out of balance. We suspected that E-CDH by way of metalloproteinases is closely related to POAG. We therefore investigated the relationship between CDH-1 gene 3' untranslated region (3'-UTR) polymorphism and POAG patients in order to support this hypothesis.

PATIENTS AND METHODS

We enrolled 60 POAG patients and 103 healthy volunteers from the Department of Ophthalmology at the China Medical University Hospital, Taichung, Taiwan, ROC. None of the control subjects had a history of eye disease and all underwent the same examination as the POAG patients. PCR-based analysis of the restriction fragment length polymorphism was used to test the CDH-1 gene 3'-UTR polymorphism. All statistical analyses were performed by the chi(2) test.

RESULT

There was a significant difference in the distribution of the CDH-1 gene 3'-UTR C/T polymorphism between POAG patients and the normal controls (p <0.000). The odds ratio of the 'C' allele was al so significantly different between both groups (odds ratio = 5.510, 95% confidence interval = 3.171-9.574).

CONCLUSION

CDH-1 is closely related to metalloproteinase and plays an important but not well-understood role in the onset and progression of POAG. The exact role of CDH-1 in POAG could be resolved by the posttranslated products of the gene and the protein-protein interaction of the gene products in the future.

Foxl1 null mice have abnormal intestinal epithelia, postnatal growth retardation, and defective intestinal glucose uptake

Mice lacking the mesenchymal winged helix transcription factor Foxl1 exhibit markedly abnormal small intestinal epithelia and postnatal growth retardation. We investigated whether defects in intestinal nutrient uptake and specific transport processes exist in mice homozygous for a Foxl1 null allele (Foxl1-/-). Foxl1-/- mice and controls on a defined genetic background were weighed regularly and killed at 2, 4, and 12 wk of age. Intestinal uptake studies, quantitative real-time PCR, RNase protection assays, and Western blot analyses were performed. Foxl1-/- mice have dysmorphic small intestinal epithelia and postnatal growth retardation. Foxl1-/- mice demonstrate decreased small intestinal uptake of D-glucose in all age groups studied. Intestinal uptake of D-fructose and two amino acids, L-proline and L-leucine, is not altered. Consistent with these findings, Foxl1-/- mice show decreased levels of the intestinal D-glucose transporter SGLT1. Expression of sucrase-isomaltase, lactase, GLUT2, and Na+-K+ ATPase are not changed. Foxl1-/- mice demonstrate markedly abnormal intestinal epithelia, postnatal growth retardation, and decreased intestinal uptake of D-glucose. The specific effect of Foxl1 on intestinal d-glucose uptake is due to decreased production of SGLT1 protein in the small intestine. Thus we identified, for the first time, a link between a mesenchymal factor, Foxl1, and the regulation of a specific epithelial transport process.

Chronic ethanol exposure increases the binding of HuR to the TNFalpha 3'-untranslated region in macrophages

Tumor necrosis factor alpha (TNFalpha) expression is a key mediator of ethanol-induced liver disease. Increased lipopolysaccharide (LPS)-stimulated TNFalpha expression in macrophages after chronic ethanol feeding is associated with a stabilization of TNFalpha mRNA (Kishore, R., McMullen, M. R., and Nagy, L. E. (2001) J. Biol. Chem. 276, 41930-41937). Here we show that the 3'-UTR of murine TNFalpha mRNA was sufficient to mediate increased LPS-stimulated expression of a luciferase reporter in RAW 264.7 macrophages after chronic ethanol exposure. Further, we show that HuR, a nuclear/cytoplasmic shuttling protein, which binds to TNFalpha mRNA, is required for increased expression of TNFalpha after chronic ethanol. In Kupffer cells, HuR was primarily localized to the nucleus and then translocated to the cytosol in response to LPS in both pair- and ethanol-fed rats. After chronic ethanol feeding, HuR quantity in the cytosol was greater, both at baseline and in response to LPS, compared with pair-fed controls. Using RNA gel shift assays, we found that LPS treatment increased HuR binding to the 65-nucleotide A + U-rich element of the TNFalpha 3'-UTR by 2-fold over baseline in Kupffer cells from pair-fed rats. After chronic ethanol feeding, HuR binding to the TNFalpha A + U-rich element was increased by more than 5-fold at baseline and in response to LPS, compared with pair-fed controls. Down-regulation of HuR expression by RNA interference prevented the chronic ethanol-induced increase in expression of luciferase reporters containing the TNFalpha 3'-UTR. Taken together, these data demonstrate that increased binding of HuR to the TNFalpha 3'-UTR contributes to increased LPS-stimulated TNFalpha expression in macrophages after chronic ethanol exposure.

Estradiol stabilizes estrogen receptor messenger ribonucleic acid in sheep endometrium via discrete sequence elements in its 3'-untranslated region

The preovulatory surge of estrogen up-regulates estrogen receptor-alpha (ER) gene expression in the uterus during the estrous/menstrual cycles of female mammals. Previously, we demonstrated that the 5-fold increase in ER mRNA levels in endometrium of ovariectomized ewes treated with a physiological dose of estradiol (E2) is entirely due to an increase in ER mRNA stability. Our current work confirms that the E2 effect is specific to ER mRNA. The sequence of ER mRNA, cloned from sheep endometrium, shows a high degree of conservation with those of other species, even in the 5'- and the very long 3'-untranslated regions. In a cell-free assay, ER mRNA demonstrates greater stability with endometrial extracts from E2-treated ewes compared with those from untreated ovariectomized ewes. The E2-enhanced stability of ER mRNA was ablated by prior treatment of the extracts with proteinase K, 70 C heat, and oxidizing and alkylating reagents, indicating that a protein is responsible for stabilization of the message. The 3'-untranslated region of ER mRNA contains discrete sequences required for E2-enhanced stability, four of which were identified by extensive deletion mutant analyses. Transfer of two of the four minimal E2-modulated stability sequences conferred E2-enhanced stability to a heterologous RNA. These minimal E2-modulated stability sequences contain a common 10-base, uridine-rich sequence that is predicted to reside in a loop structure. Throughout our studies, estrogen stabilization of ER mRNA in sheep endometrium resembled that of vitellogenin mRNA in frog liver, indicating conservation of this ancient mechanism for enhancing gene expression in response to estrogen.

Nutritional control of mRNA stability is mediated by a conserved AU-rich element that binds the cytoplasmic shuttling protein HuR

The cationic amino acid transporter, Cat-1, is a high affinity transporter of the essential amino acids, arginine and lysine. Expression of the cat-1 gene increases during nutritional stress as part of the adaptive response to starvation. Amino acid limitation induces coordinate increases in stability and translation of the cat-1 mRNA, at a time when global protein synthesis decreases. It is shown here that increased cat-1 mRNA stability requires an 11 nucleotide AU-rich element within the distal 217 bases of the 3'-untranslated region. When this 217-nucleotide nutrient sensor AU-rich element (NS-ARE) is present in a chimeric mRNA it confers mRNA stabilization during amino acid starvation. HuR is a member of the ELAV family of RNA-binding proteins that has been implicated in regulating the stability of ARE-containing mRNAs. We show here that the cytoplasmic concentration of HuR increases during amino acid starvation, at a time when total cellular HuR levels decrease. In addition, RNA gel shift experiments in vitro demonstrated that HuR binds to the NS-ARE and binding was dependent on the 11 residue AU-rich element. Moreover, HuR binding to the NS-ARE in extracts from amino acid-starved cells increased in parallel with the accumulation of cytoplasmic HuR. It is proposed that an adaptive response of cells to nutritional stress results in increased mRNA stability mediated by HuR binding to the NS-ARE.

HuR binds a cyclic nucleotide-dependent, stabilizing domain in the 3' untranslated region of Na(+)/glucose cotransporter (SGLT1) mRNA

Differentiation-dependent expression of the Na(+)/glucose cotransporter (SGLT1) is accompanied by a large, cAMP-dependent increase in stability of its mRNA. Stabilization is mediated by protein binding to a critical uridine-rich element (URE) in its 3' untranslated region. In the present study, we demonstrate that HuR, an RNA binding protein of the embryonic lethal abnormal vision family, binds the SGLT1 URE. HuR binding was increased after elevation of intracellular cAMP levels and was dependent on protein phosphorylation. This interaction was prevented by a substitution mutation previously shown to block cAMP-dependent reporter message stabilization. These results implicate HuR as a key mediator of cAMP-dependent SGLT1 mRNA stabilization.

The plasma membrane-associated protein RS1 decreases transcription of the transporter SGLT1 in confluent LLC-PK1 cells

Previously we cloned RS1, a 67-kDa polypeptide that is associated with the intracellular side of the plasma membrane. Upon co-expression in Xenopus laevis oocytes, human RS1 decreased the concentration of the Na(+)-D-glucose co-transporter hSGLT1 in the plasma membrane (Valentin, M., Kühlkamp, T., Wagner, K., Krohne, G., Arndt, P., Baumgarten, K., Weber, W.-M., Segal, A., Veyhl, M., and Koepsell, H. (2000) Biochim. Biophys. Acta 1468, 367-380). Here, the porcine renal epithelial cell line LLC-PK1 was used to investigate whether porcine RS1 (pRS1) plays a role in transcriptional up-regulation of SGLT1 after confluence and in down-regulation of SGLT1 by high extracellular D-glucose concentrations. Western blots indicated a dramatic decrease of endogenous pRS1 protein at the plasma membrane after confluence but no significant effect of D-glucose. In confluent LLC-PK1 cells overexpressing pRS1, SGLT1 mRNA, protein, and methyl-alpha-D-glucopyranoside uptakes were drastically decreased; however, the reduction of methyl-alpha-D-glucopyranoside uptake after cultivation with 25 mm D-glucose remained. In confluent pRS1 antisense cells, the expression of SGLT1 mRNA and protein was strongly increased, whereas the reduction of SGLT1 expression during cultivation with high D-glucose was not influenced. Nuclear run-on assays showed that the transcription of SGLT1 was 10-fold increased in the pRS1 antisense cells. The data suggest that RS1 participates in transcriptional up-regulation of SGLT1 after confluence but not in down-regulation by D-glucose.

Intestinal bile acid transport: biology, physiology, and pathophysiology

Intestinal reabsorption of bile salts plays a crucial role in human health and disease. This process is primarily localized to the terminal ileum and is mediated by a 48-kd sodium-dependent bile acid cotransporter (SLC10A2 = ASBT). ASBT is also expressed in renal tubule cells, cholangiocytes, and the gallbladder. Exon skipping leads to a truncated version of ASBT, which sorts to the basolateral surface and mediates efflux of bile salts. Inherited mutation of ASBT leads to congenital diarrhea secondary to bile acid malabsorption. Partial inhibition of ASBT may be useful in the treatment of hypercholesterolemia and intrahepatic cholestasis. During normal development in the rat ileum, ASBT undergoes a biphasic pattern of expression with a prenatal onset, postnatal repression, and reinduction at the time of weaning. The bile acid responsiveness of the ASBT gene is not clear and may be dependent on both the experimental model used and the species being investigated. Future studies of the transcriptional and posttranscriptional regulation of the ASBT gene and analysis of ASBT knockout mice will provide further insight into the biology, physiology, and pathophysiology of intestinal bile acid transport.

A cis-dominant cyclic nucleotide-dependent regulatory domain in the 3'-untranslated region of Na(+)/glucose cotransporter (SGLT1) mRNA

A 122 nt uridine-rich sequence (URE) in the Na(+)/glucose cotransporter (SGLT1) mRNA 3'-untranslated region is critical for cAMP-dependent message stabilization. Its function was investigated in LLC-PK(1) cells stably expressing beta-globin reporter transcripts. Insertion of the SGLT1 URE downstream from an unrelated destabilizing sequence, the c-fos ARE, evoked cAMP-dependent message stabilization. Stabilization was blocked by a substitution mutation within the SLGT1 URE. These observations indicate that the SGLT1 URE is sufficient to transmit cAMP-dependent, cis-dominant mRNA stabilization in the presence of appropriate trans-acting factors and appears to function independently of the nature of the destabilizing domain.