T1R3 and gustducin in gut sense sugars to regulate expression of Na+-glucose cotransporter 1

Abstract

Dietary sugars are transported from the intestinal lumen into absorptive enterocytes by the sodium-dependent glucose transporter isoform 1 (SGLT1). Regulation of this protein is important for the provision of glucose to the body and avoidance of intestinal malabsorption. Although expression of SGLT1 is regulated by luminal monosaccharides, the luminal glucose sensor mediating this process was unknown. Here, we show that the sweet taste receptor subunit T1R3 and the taste G protein gustducin, expressed in enteroendocrine cells, underlie intestinal sugar sensing and regulation of SGLT1 mRNA and protein. Dietary sugar and artificial sweeteners increased SGLT1 mRNA and protein expression, and glucose absorptive capacity in wild-type mice, but not in knockout mice lacking T1R3 or alpha-gustducin. Artificial sweeteners, acting on sweet taste receptors expressed on enteroendocrine GLUTag cells, stimulated secretion of gut hormones implicated in SGLT1 up-regulation. Gut-expressed taste signaling elements involved in regulating SGLT1 expression could provide novel therapeutic targets for modulating the gut's capacity to absorb sugars, with implications for the prevention and/or treatment of malabsorption syndromes and diet-related disorders including diabetes and obesity.

Fig. 1.

Increased SGLT1 expression in response to dietary carbohydrate in wild-type, but not in Gα_gust or T1R3 knockout, mice. Wild-type (WT), Gα_gust, and T1R3 knockout mice were given low (L) or high (H) carbohydrate diets for two weeks. (A) Steady-state levels of SGLT1 mRNA determined by QPCR were normalized to β-actin mRNA. (B) SGLT1 protein from brush-border membrane vesicles (BBMV) isolated from mid small intestine was detected in Western blots (Left). Densitometric analysis (Right) of Western blots normalized SGLT1 protein expression to that of β-actin. (C) SGLT1-mediated glucose uptake was measured by Na⁺-dependent d-[U¹⁴C] glucose uptake into BBMV. Mean uptake rates are presented as arbitrary units relative to rates measured in BBMV of wild-type mice maintained on the low-carbohydrate diet (defined as 100). All values are expressed relative to SGLT1 in wild-type mice on low-carbohydrate diets as means ± SD. Data were generated in triplicate, with n = 4 animals in each group. Statistically significant results determined by Student's unpaired two-tailed t test are indicated by *, P < 0.05; **, P < 0.005.

Fig. 2.

Increased SGLT1 expression in response to dietary supplementation with artificial sweeteners in wild-type, but not in Gα_gust or T1R3 knockout, mice. Wild-type (WT), Gα_gust, and T1R3 knockout mice were given a low-carbohydrate diet without (L), or with 2 mM sucralose (L+suc) for 2 weeks. (A–C) Steady-state levels of SGLT1 mRNA (A), SGLT1 protein (B), and SGLT1-mediated glucose transport rates (C), were measured (see Fig. 1). (D) Steady-state levels of SGLT1 mRNA were measured in wild-type (WT) mice that had been maintained for 2 weeks on a high-carbohydrate diet (H) or low-carbohydrate diet without (L) or with the indicated artificial sweeteners aspartame (L+asp), acesulfame K (L+ace-K), or saccharin (L+sac). All data are expressed relative to SGLT1 in wild-type mice on low-carbohydrate diets as means ± SD. Data were generated in triplicate, with n = 3 animals in each group. Statistically significant results determined by Student's unpaired two-tailed t test are indicated by *, P < 0.05; **, P < 0.005; or ***, P < 0.001.

Fig. 3.

Detection and localization of T1R receptors and Gα_gust along the crypt–villus axis of small intestine. (A) In situ hybridization with antisense riboprobes to T1R2, T1R3, and Gα_gust. Complementary sense probes to all targets did not hybridize to any transcripts within the tissue sections (see SI Fig. 6). (B and C) Immunofluorescent detection of the T1R3 taste receptor subunit (red) and Gα_gust (green) in serial wax sections of mouse duodenum. (D) Immunofluorescent detection of the T1R3 (red) and T1R2 (green) taste receptor subunits in a single wax section of human duodenum. (E–G) Immunofluorescent detection of the T1R2 and T1R3 taste receptor subunits (green), and Gα_gust (red) in a single wax section of human duodenum. (H) Chromogenic detection of SGLT1 in wax sections of mouse small intestine. (I) Chromogenic detection of Gα_gust and chromogranin in serial wax sections of mouse proximal intestine. The boxed cell expresses both Gα_gust and chromogranin.

Fig. 4.

Sucralose stimulation of endogenously expressed sweet taste receptors in GLUTag cells leads to GLP-1 and GIP release. (A) GLP-1 release into the culture medium of GLUTag cells was monitored after treatment of cells with buffer alone or buffer containing sucralose (50 mM final concentration). Addition of sucralose led to increased GLP-1 release from GLUTag cells (P = 0.00098). Preincubation (15 min) of the GLUTag cells with gurmarin (3 μg/ml) blocked most of the sucralose-dependent increase in released GLP-1 (P = 0.00574 vs. sucralose alone). n = 4 samples per group; the experiment was carried out in triplicate; a representative experiment is shown, and levels are expressed ± SEM. (B) GIP release into the culture medium of GLUTag cells was monitored after treatment of cells with buffer alone or buffer containing sucralose (50 mM final concentration). The addition of sucralose led to a large increase over baseline in GIP release from GLUTag cells (P = 0.047). Preincubation (15 min) of the GLUTag cells with the sweet taste receptor inhibitor gurmarin (3 μg/ml) blocked most of the sucralose-dependent increase in released GIP (P = 0.00260 vs. sucralose alone). n = 2–4 samples per group; the experiment was carried out in duplicate; a representative experiment is shown, and levels are expressed ± SEM.