A Modified Tripeptide Motif of RS1 (RSC1A1) Down-Regulates Exocytotic Pathways of Human Na+-d-glucose Cotransporters SGLT1, SGLT2, and Glucose Sensor SGLT3 in the Presence of Glucose.

A domain of protein RS1 (RSC1A1) called RS1-Reg down-regulates the plasma membrane abundance of Na+-d-glucose cotransporter SGLT1 by blocking the exocytotic pathway at the trans-Golgi. This effect is blunted by intracellular glucose but prevails when serine in a QSP (Gln-Ser-Pro) motif is replaced by glutamate [RS1-Reg(S20E)]. RS1-Reg binds to ornithine decarboxylase (ODC) and inhibits ODC in a glucose-dependent manner. Because the ODC inhibitor difluoromethylornithine (DFMO) acts like RS1-Reg(S20E), and DFMO and RS1-Reg(S20E) are not cumulative, we raised the hypothesis that RS1-Reg(S20E) down-regulates the exocytotic pathway of SGLT1 at the trans-Golgi by inhibiting ODC. We investigated whether QEP down-regulates human SGLT1 (hSGLT1) like hRS1-Reg(S20E) and whether human Na+-d-glucose cotransporter hSGLT2 and the human glucose sensor hSGLT3 are also addressed. We expressed hSGLT1, hSGLT1 linked to yellow fluorescent protein (hSGLT1-YFP), hSGLT2-YFP and hSGLT3-YFP in oocytes of Xenopus laevis, injected hRS1-Reg(S20E), QEP, DFMO, and/or α-methyl-d-glucopyranoside (AMG), and measured AMG uptake, glucose-induced currents, and plasma membrane-associated fluorescence after 1 hour. We also performed in vitro AMG uptake measurements into small intestinal mucosa of mice and human. The data indicate that QEP down-regulates the exocytotic pathway of SGLT1 similar to hRS1-Reg(S20E). Our results suggests that both peptides also down-regulate hSGLT2 and hSGLT3 via the same pathway. Thirty minutes after application of 5 mM QEP in the presence of 5 mM d-glucose, hSGLT1-mediated AMG uptake into small intestinal mucosa was decreased by 40% to 50%. Thus oral application of QEP in a formulation that optimizes uptake into enterocytes but prevents entry into the blood is proposed as novel antidiabetic therapy.

Duodenal-jejunal bypass improves glycemia and decreases SGLT1-mediated glucose absorption in rats with streptozotocin-induced type 2 diabetes.

OBJECTIVE: To elucidate whether duodenal-jejunal-bypass (DJB), which improves blood glucose control, changes activity of Na-D-glucose cotransporter SGLT1 in small intestine. BACKGROUND: DJB has been shown to improve oral glucose tolerance in normal rats and a genetic diabetic rat model. Because intestinal D-glucose absorption is mediated by SGLT1 localized in the brush border membrane of small intestinal enterocytes, it is unclear whether function of SGLT1 is altered by DJB and whether this contributes to the improvement of glycemic control. METHODS: A high-fat diet and low-dose streptozotocin administration were used to induce a type 2 diabetes in male Lewis rats. The diabetic animals underwent DJB or sham surgery. An oral glucose tolerance test (OGTT) was used to evaluate glucose control 3 weeks after surgery. SGLT1-mediated glucose transport was assessed using everted rings of different small intestinal segments. SGLT1 mRNA expression was determined by quantitative reverse transcription polymerase chain reaction (RT-PCR). RESULTS: DJB improved the OGTT significantly (P < 0.001) compared with sham-operated rats while body weight was not different among the surgical groups. DJB induced a 50% reduction of SGLT1-mediated glucose uptake into enterocytes of duodenum and jejunum (P < 0.001). The concentration of D-glucose in the blood following glucose gavage increased more slowly after DJB versus sham. CONCLUSIONS: The data indicate that DJB surgery decreases glucose absorption in the small intestine by downregulation of SGLT1-mediated glucose uptake. We suggest that the downregulation of SGLT1 contributes to the body-weight independent improvement of diabetes type 2.

Ileal Interposition in Rats with Experimental Type 2 Like Diabetes Improves Glycemic Control Independently of Glucose Absorption.

Bariatric operations in obese patients with type 2 diabetes often improve diabetes before weight loss is observed. In patients mainly Roux-en-Y-gastric bypass with partial stomach resection is performed. Duodenojejunal bypass (DJB) and ileal interposition (IIP) are employed in animal experiments. Due to increased glucose exposition of L-cells located in distal ileum, all bariatric surgery procedures lead to higher secretion of antidiabetic glucagon like peptide-1 (GLP-1) after glucose gavage. After DJB also downregulation of Na(+)-d-glucose cotransporter SGLT1 was observed. This suggested a direct contribution of decreased glucose absorption to the antidiabetic effect of bariatric surgery. To investigate whether glucose absorption is also decreased after IIP, we induced diabetes with decreased glucose tolerance and insulin sensitivity in male rats and investigated effects of IIP on diabetes and SGLT1. After IIP, we observed weight-independent improvement of glucose tolerance, increased insulin sensitivity, and increased plasma GLP-1 after glucose gavage. The interposed ileum was increased in diameter and showed increased length of villi, hyperplasia of the epithelial layer, and increased number of L-cells. The amount of SGLT1-mediated glucose uptake in interposed ileum was increased 2-fold reaching the same level as in jejunum. Thus, improvement of glycemic control by bariatric surgery does not require decreased glucose absorption.