Multimodal function of the sweet taste receptor expressed in pancreatic β-cells: generation of diverse patterns of intracellular signals by sweet agonists

Taste receptor T1R3 regulates testosterone synthesis via the cAMP-PKA-SP1 pathway in testicular Leydig cells

Taste receptor type 1 subunit 3 (T1R3) is a G protein-coupled receptor encoded by the TAS1R3 gene that can be specifically activated by certain sweeteners or umami agents for sweet/umami recognition. T1R3 is a potential target for regulating male reproduction. However, studies on the impact of non-nutritive sweeteners on reproduction are limited. In the present study, we evaluated the impact of the non-nutritive sweeteners (saccharin sodium, sucralose and acesulfame-K) on testosterone synthesis in testicular Leydig cells of Xiang pigs by comparing the relative abundance of mRNA transcripts and protein expression of T1R3, steroidogenic related factors, and intracellular cyclic adenosine monophosphate (cAMP), protein kinase A (PKA), as well as testosterone levels using Western blotting, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) and enzyme-linked immunosorbent assay (ELISA). To clarify the specific mechanism, a dual luciferase assay was used to uncover the relationship between the transcription factors and steroidogenic enzyme. The acute intratesticular injection of a typical non-nutritive sweeteners was conducted to verify this impact in mouse. The results showed that saccharin sodium not only enhanced T1R3 expression in Leydig cells of Xiang pigs, but also caused significant increases in testosterone, cAMP, PKA, phosphorylation of specificity protein 1 (p-SP1), total protein of specificity protein 1 (SP1), steroidogenic acute regulatory protein (StAR), and 3β-hydroxysteroid dehydrogenase type 1 (3β-HSD1) (P < 0.05). Similarly, treatment of Leydig cells with sucralose and acesulfame-K also increased testosterone level, protein expression of T1R3, 17-α-hydroxylase/17, 20-lyase (CYP17A1), and 3β-HSD1 (P < 0.05). Treatment with SQ22536 (an adenylate cyclas inhibitor) or H89 (a PKA inhibitor) significantly reduced saccharin sodium-induced protein levels of p-SP1, StAR, CYP17A1, and 3β-HSD1 (P < 0.05). In addition, a dual luciferase assay further demonstrated that SP1 significantly increased the promoter activity of CYP17A1 (P < 0.05). When mouse testes were injected with saccharin sodium, T1R3, p-SP1, CYP17A1, and 3β-HSD1 were upregulated, leading to a significant testicular increase in testosterone and cAMP levels (P < 0.05). These results suggest a mechanism by which the taste receptor T1R3 regulates testosterone production, and this mechanism may be linked to the cAMP-PKA pathway. Understanding the interrelationship between T1R3 and the cAMP-PKA-SP1 pathway contributes to clarify the regulatory mechanisms of male reproduction.

Three New Triterpene Glycosides from the Roots of Deutzia x Hybrida "Strawberry Fields" (Hydrangeaceae)

Three new triterpene glycosides were isolated from Deutzia x hybrida "Strawberry Fields" cultivar via aqueous-ethanolic extraction of the roots, including one derivative of sumaresinolic acid and two of echinocystic acid: 3-O-β-D-glucuronopyranosylsumaresinolic acid 28-O-β-D-xylopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl ester, 3-O-β-D-glucuronopyranosylechinocystic acid 28-O-β-D-xylopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl ester, and 3-O-α-L-arabinopyranosyl-(1→3)-β-D-glucuronopyranosylechinocystic acid 28-O-β-D-xylopyranosyl-(1→4)-α-L-rhamnopyranosyl-(1→2)-α-L-arabinopyranosyl ester. As none of the isolated saponins were previously documented in the literature, their structural elucidation required extensive 1D and homo- and heteronuclear 2D NMR spectroscopy, as well as mass spectrometry analysis. All three glycosides were tested for their stimulatory activity of the sweet taste receptor TAS1R2/TAS1R3. It is the first chemical and biological investigation of Deutzia x hybrida "Strawberry Fields" as well as the first report of sumaresinolic acid glycosides in Deutzia genus.

Interkingdom Detection of Bacterial Quorum-Sensing Molecules by Mammalian Taste Receptors

Bitter and sweet taste G protein-coupled receptors (known as T2Rs and T1Rs, respectively) were originally identified in type II taste cells on the tongue, where they signal perception of bitter and sweet tastes, respectively. Over the past ~15 years, taste receptors have been identified in cells all over the body, demonstrating a more general chemosensory role beyond taste. Bitter and sweet taste receptors regulate gut epithelial function, pancreatic β cell secretion, thyroid hormone secretion, adipocyte function, and many other processes. Emerging data from a variety of tissues suggest that taste receptors are also used by mammalian cells to "eavesdrop" on bacterial communications. These receptors are activated by several quorum-sensing molecules, including acyl-homoserine lactones and quinolones from Gram-negative bacteria such as Pseudomonas aeruginosa, competence stimulating peptides from Streptococcus mutans, and D-amino acids from Staphylococcus aureus. Taste receptors are an arm of immune surveillance similar to Toll-like receptors and other pattern recognition receptors. Because they are activated by quorum-sensing molecules, taste receptors report information about microbial population density based on the chemical composition of the extracellular environment. This review summarizes current knowledge of bacterial activation of taste receptors and identifies important questions remaining in this field.

Sweet Taste Receptor T1R3 Expressed in Leydig Cells Is Closely Related to Homeostasis of the Steroid Hormone Metabolism Profile

Taste receptor type 1 subunit 3 (T1R3) is initially expressed in mammal tongue for recognition and response of sweet/umami tastants and is critical to nutrient absorption, even endocrine. In this study, down-regulation of related steroidogenic enzymes such as StAR, 3β-HSD, CYP17A1, and 17β-HSD with the decrease of T1R3 expression was found in Leydig cells treated by a T1R3 inhibitor (lactisole). The abundances of progesterone, 17a-hydroxyprogesterone, androstenedione, testosterone, and deoxycorticosterone were down-regulated by 2.3, 3.5, 1.4, 1.6, and 2.2 times, respectively, after T1R3 inhibition. In addition, opposite results were found in saccharin sodium treatment. T1R3 activation contributed to intracellular cyclic adenosine monophosphate (cAMP) accumulation (14.41 ± 0.58 vs 20.21 ± 0.65) and increased testosterone (20.31 ± 3.49 vs 50.01 ± 7.44) and steroidogenic metabolite levels. Coadministration of human chorionic gonadotropin and saccharin sodium resulted in elevating the testosterone and cAMP levels and enhancing the expression levels of steroidogenic-related factors. Similarly, intratesticular injection of lactisole and saccharin sodium further confirmed that T1R3 inhibition/activation affected the expression of related steroidogenic enzymes and the testosterone levels in mice. The above findings suggest that T1R3 plays a role in testicular steroidogenesis.

The elusive cephalic phase insulin response: triggers, mechanisms, and functions

The cephalic phase insulin response (CPIR) is classically defined as a head receptor-induced early release of insulin during eating that precedes a postabsorptive rise in blood glucose. Here we discuss, first, the various stimuli that elicit the CPIR and the sensory signaling pathways (sensory limb) involved; second, the efferent pathways that control the various endocrine events associated with eating (motor limb); and third, what is known about the central integrative processes linking the sensory and motor limbs. Fourth, in doing so, we identify open questions and problems with respect to the CPIR in general. Specifically, we consider test conditions that allow, or may not allow, the stimulus to reach the potentially relevant taste receptors and to trigger a CPIR. The possible significance of sweetness and palatability as crucial stimulus features and whether conditioning plays a role in the CPIR are also discussed. Moreover, we ponder the utility of the strict classical CPIR definition based on what is known about the effects of vagal motor neuron activation and thereby acetylcholine on the β-cells, together with the difficulties of the accurate assessment of insulin release. Finally, we weigh the evidence of the physiological and clinical relevance of the cephalic contribution to the release of insulin that occurs during and after a meal. These points are critical for the interpretation of the existing data, and they support a sharper focus on the role of head receptors in the overall insulin response to eating rather than relying solely on the classical CPIR definition.

Comparison of aspartame- and sugar-sweetened soft drinks on postprandial metabolism

Aim: We compared the impact of artificially- and sugar-sweetened beverages co-ingested with a mixed meal on postprandial fat and carbohydrate oxidation, blood glucose, and plasma insulin and triglyceride concentrations. Methods: Eight college-aged, healthy males completed three randomly assigned trials, which consisted of a mixed macronutrient meal test with 20oz of Diet-Coke (AS), Coca-Cola (NS), or water (CON). One week separated each trial and each participant served as his own control. Resting energy expenditure (REE) via indirect calorimetry, blood pressure, and blood samples were obtained immediately before, 5, 10, 30, 60, 120, and 180 min after meal and beverage ingestion. A two-way (treatment × time) repeated-measures ANOVA was conducted to assess REE, fat and carbohydrate oxidation rates, blood glucose, and plasma insulin and triglyceride concentrations. Results: There was a significant main effect of treatment on total fat oxidation (P = 0.006), fat oxidation was significantly higher after AS (P = 0.006) and CON (P = 0.001) compared to following NS. There was a significant main effect of treatment on total carbohydrate oxidation (P = 0.005), carbohydrate oxidation was significantly lower after AS (P = 0.014) and CON (P = 0.001) compared to following NS. Plasma insulin concentration AUC was significantly lower after AS (P = 0.019) and trended lower in CON (P = 0.054) compared to following NS. Conclusion: Ingestion of a mixed meal with an artificially-sweetened beverage does not impact postprandial metabolism, whereas a sugar-sweetened beverage suppresses fat oxidation and increases carbohydrate oxidation compared to artificially-sweetened beverage and water.

The dietary sweetener sucralose is a negative modulator of T cell-mediated responses

Artificial sweeteners are used as calorie-free sugar substitutes in many food products and their consumption has increased substantially over the past years1. Although generally regarded as safe, some concerns have been raised about the long-term safety of the consumption of certain sweeteners2-5. In this study, we show that the intake of high doses of sucralose in mice results in immunomodulatory effects by limiting T cell proliferation and T cell differentiation. Mechanistically, sucralose affects the membrane order of T cells, accompanied by a reduced efficiency of T cell receptor signalling and intracellular calcium mobilization. Mice given sucralose show decreased CD8+ T cell antigen-specific responses in subcutaneous cancer models and bacterial infection models, and reduced T cell function in models of T cell-mediated autoimmunity. Overall, these findings suggest that a high intake of sucralose can dampen T cell-mediated responses, an effect that could be used in therapy to mitigate T cell-dependent autoimmune disorders.

Gene expression analyses of TAS1R taste receptors relevant to the treatment of cardiometabolic disease

The sweet taste receptor (STR) is a G protein-coupled receptor (GPCR) responsible for mediating cellular responses to sweet stimuli. Early evidence suggests that elements of the STR signaling system are present beyond the tongue in metabolically active tissues, where it may act as an extraoral glucose sensor. This study aimed to delineate expression of the STR in extraoral tissues using publicly available RNA-sequencing repositories. Gene expression data was mined for all genes implicated in the structure and function of the STR, and control genes including highly expressed metabolic genes in relevant tissues, other GPCRs and effector G proteins with physiological roles in metabolism, and other GPCRs with expression exclusively outside the metabolic tissues. Since the physiological role of the STR in extraoral tissues is likely related to glucose sensing, expression was then examined in diseases related to glucose-sensing impairment such as type 2 diabetes. An aggregate co-expression network was then generated to precisely determine co-expression patterns among the STR genes in these tissues. We found that STR gene expression was negligible in human pancreatic and adipose tissues, and low in intestinal tissue. Genes encoding the STR did not show significant co-expression or connectivity with other functional genes in these tissues. In addition, STR expression was higher in mouse pancreatic and adipose tissues, and equivalent to human in intestinal tissue. Our results suggest that STR expression in mice is not representative of expression in humans, and the receptor is unlikely to be a promising extraoral target in human cardiometabolic disease.

Activation of a Sweet Taste Receptor by Oleanane-Type Glycosides from Wisteria sinensis

The phytochemical study of Wisteria sinensis (Sims) DC. (Fabaceae), commonly known as the Chinese Wisteria, led to the isolation of seven oleanane-type glycosides from an aqueous-ethanolic extract of the roots. Among the seven isolated saponins, two have never been reported before: 3-O-α-L-rhamnopyranosyl-(1→2)-β-D-glucopyranosyl-(1→2)-β-D-glucuronopyranosyl-22-O-acetylolean-12-ene-3β,16β,22β,30-tetrol, and 3-O-β-D-xylopyranosyl-(1→2)-β-D-glucuronopyranosylwistariasapogenol A. Based on the close structures between the saponins from W. sinensis, and the glycyrrhizin from licorice, the stimulation of the sweet taste receptor TAS1R2/TAS1R3 by these glycosides was evaluated.

Is the Use of Artificial Sweeteners Beneficial for Patients with Diabetes Mellitus? The Advantages and Disadvantages of Artificial Sweeteners

Artificial sweeteners have been developed as substitutes for sugar. Sucralose, acesulfame K (ACE K), aspartame, and saccharin are artificial sweeteners. Previously, artificial sweeteners were thought to be effective in treating obesity and diabetes. Human meta-analyses have reported that artificial sweeteners have no effect on body weight or glycemic control. However, recent studies have shown that artificial sweeteners affect glucose absorption in the intestinal tract as well as insulin and incretin secretion in humans and animals. Moreover, artificial sweeteners alter the composition of the microbiota and worsen the glycemic control owing to changes in the gut microbiota. The early intake of ACE K was also shown to suppress the taste response to sugar. Furthermore, a large cohort study showed that high artificial sweetener intake was associated with all-cause mortality, cardiovascular risk, coronary artery disease risk, cerebrovascular risk, and cancer risk. The role of artificial sweeteners in the treatment of diabetes and obesity should be reconsidered, and the replacement of sugar with artificial sweeteners in patients will require the long-term tracking of not only intake but also changes in blood glucose and weight as well as future guidance based on gut bacteria data. To utilize the beneficial properties of artificial sweeteners in treatment, further studies are needed.

Sweet Taste Is Complex: Signaling Cascades and Circuits Involved in Sweet Sensation

Sweetness is the preferred taste of humans and many animals, likely because sugars are a primary source of energy. In many mammals, sweet compounds are sensed in the tongue by the gustatory organ, the taste buds. Here, a group of taste bud cells expresses a canonical sweet taste receptor, whose activation induces Ca²⁺ rise, cell depolarization and ATP release to communicate with afferent gustatory nerves. The discovery of the sweet taste receptor, 20 years ago, was a milestone in the understanding of sweet signal transduction and is described here from a historical perspective. Our review briefly summarizes the major findings of the canonical sweet taste pathway, and then focuses on molecular details, about the related downstream signaling, that are still elusive or have been neglected. In this context, we discuss evidence supporting the existence of an alternative pathway, independent of the sweet taste receptor, to sense sugars and its proposed role in glucose homeostasis. Further, given that sweet taste receptor expression has been reported in many other organs, the physiological role of these extraoral receptors is addressed. Finally, and along these lines, we expand on the multiple direct and indirect effects of sugars on the brain. In summary, the review tries to stimulate a comprehensive understanding of how sweet compounds signal to the brain upon taste bud cells activation, and how this gustatory process is integrated with gastro-intestinal sugar sensing to create a hedonic and metabolic representation of sugars, which finally drives our behavior. Understanding of this is indeed a crucial step in developing new strategies to prevent obesity and associated diseases.

Comprehensive structure-activity-relationship studies of sensory active compounds in licorice (Glycyrrhiza glabra)

Licorice saponins, the main constituents of Glycyrrhiza glabra L. roots, are highly appreciated by the consumer for their pleasant sweet and long lasting licorice taste. The objective of the present study was to understand the molecular features that contribute to bitter, sweet and licorice sensation of licorice roots, and whether individual compounds elicit more than one of these sensations. Therefore, a sensomics approach was conducted, followed by purification of the compounds with highest sensory impact, and by synthesis as well as full characterization via HRESIMS, ESIMS/MS and 1D/2D-NMR experiments. This led to the discovery and structure determination of 28 sweet, bitter and licorice tasting key phytochemicals, including two unknown compounds. A combination of sensorial, cell-based and computational analysis revealed distinct structural features, such as spatial arrangement of functional groups in the triterpenoid E-ring, driving to different taste sensations and sweet receptor hTAS1R2/R3 stimulation.

The regulation of gastric ghrelin secretion

Ghrelin is a gastric hormone with multiple physiological functions, including the stimulation of food intake and adiposity. It is well established that circulating ghrelin levels are closely associated with feeding patterns, rising strongly before a meal and lowering upon food intake. However, the mechanisms underlying the modulation of ghrelin secretion are not fully understood. The purpose of this review is to discuss current knowledge on the circadian oscillation of circulating ghrelin levels, the neural mechanisms stimulating fasting ghrelin levels and peripheral mechanisms modulating postprandial ghrelin levels. Furthermore, the therapeutic potential of targeting the ghrelin pathway is discussed in the context of the treatment of various metabolic disorders, including obesity, type 2 diabetes, diabetic gastroparesis and Prader-Willi syndrome. Moreover, eating disorders including anorexia nervosa, bulimia nervosa and binge-eating disorder are also discussed.

Sucralose and Cardiometabolic Health: Current Understanding from Receptors to Clinical Investigations

The excess consumption of added sugar is consistently found to be associated with weight gain, and a higher risk of type 2 diabetes mellitus, coronary heart disease, and stroke. In an effort to reduce the risk of cardiometabolic disease, sugar is frequently replaced by low- and null-calorie sweeteners (LCSs). Alarmingly, though, emerging evidence indicates that the consumption of LCSs is associated with an increase in cardiovascular mortality risk that is amplified in those who are overweight or obese. Sucralose, a null-caloric high-intensity sweetener, is the most commonly used LCS worldwide, which is regularly consumed by healthy individuals and patients with metabolic disease. To explore a potential causal role for sucralose in increased cardiovascular risk, this present review summarizes the preclinical and clinical data from current research detailing the effects of sucralose on systems controlling food intake, glucose homeostasis, and gut microbiota.

Nutrient-sensing components of the mouse stomach and the gastric ghrelin cell

BACKGROUND

The ability of the gut to detect nutrients is critical to the regulation of gut hormone secretion, food intake, and postprandial blood glucose control. Ingested nutrients are detected by specific gut chemosensors. However, knowledge of these chemosensors has primarily been derived from the intestine, while available information on gastric chemosensors is limited. This study aimed to investigate the nutrient-sensing repertoire of the mouse stomach with particular emphasis on ghrelin cells.

METHODS

Quantitative RT-PCR was used to determine mRNA levels of nutrient chemosensors (protein: G protein-coupled receptor 93 [GPR93], calcium-sensing receptor [CaSR], metabotropic glutamate receptor type 4 [mGluR4]; fatty acids: CD36, FFAR2&4; sweet/umami taste: T1R3), taste transduction components (TRPM5, GNAT2&3), and ghrelin and ghrelin-processing enzymes (PC1/3, ghrelin O-acyltransferase [GOAT]) in the gastric corpus and antrum of adult male C57BL/6 mice. Immunohistochemistry was performed to assess protein expression of chemosensors (GPR93, T1R3, CD36, and FFAR4) and their co-localization with ghrelin.

KEY RESULTS

Most nutrient chemosensors had higher mRNA levels in the antrum compared to the corpus, except for CD36, GNAT2, ghrelin, and GOAT. Similar regional distribution was observed at the protein level. At least 60% of ghrelin-positive cells expressed T1R3 and FFAR4, and over 80% expressed GPR93 and CD36.

CONCLUSIONS AND INFERENCES

The cellular mechanisms for the detection of nutrients are expressed in a region-specific manner in the mouse stomach and gastric ghrelin cells. These gastric nutrient chemosensors may play a role modulating gastrointestinal responses, such as the inhibition of ghrelin secretion following food intake.

An alternative pathway for sweet sensation: possible mechanisms and physiological relevance

Sweet substances are detected by taste-bud cells upon binding to the sweet-taste receptor, a T1R2/T1R3 heterodimeric G protein-coupled receptor. In addition, experiments with mouse models lacking the sweet-taste receptor or its downstream signaling components led to the proposal of a parallel "alternative pathway" that may serve as metabolic sensor and energy regulator. Indeed, these mice showed residual nerve responses and behavioral attraction to sugars and oligosaccharides but not to artificial sweeteners. In analogy to pancreatic β cells, such alternative mechanism, to sense glucose in sweet-sensitive taste cells, might involve glucose transporters and K_ATP channels. Their activation may induce depolarization-dependent Ca²⁺ signals and release of GLP-1, which binds to its receptors on intragemmal nerve fibers. Via unknown neuronal and/or endocrine mechanisms, this pathway may contribute to both, behavioral attraction and/or induction of cephalic-phase insulin release upon oral sweet stimulation. Here, we critically review the evidence for a parallel sweet-sensitive pathway, involved signaling mechanisms, neural processing, interactions with endocrine hormonal mechanisms, and its sensitivity to different stimuli. Finally, we propose its physiological role in detecting the energy content of food and preparing for digestion.

Sucralose can improve glucose tolerance and upregulate expression of sweet taste receptors and glucose transporters in an obese rat model

OBJECTIVES

Non-nutritive sweeteners (NNS) are widely used as replacements for table sugar in beverages and dessert. However, the metabolic effects of NNS remain controversial. This study aimed to investigate the effects of various sucralose loads on glucose metabolism and expression of sweet taste receptors (STR) and glucose transporters in a high-fat diet (HFD) rats.

METHODS

Four-week-old male Sprague Dawley rats were fed a HFD for 8 weeks, then randomly divided into eight groups (6 in each group). All were gavaged with either saline, sucralose (0.54 mM or 0.78 mM), or sucrose (324 mM) with/without gurmarin, a sweet taste inhibitor, for 4 weeks, followed by an intragastric glucose tolerance test (IGGTT) with blood glucose, and plasma insulin, GLP-1 and glucose-dependent insulinotropic polypeptide (GIP) measurements. In the following week, the rats were sacrificed and the small intestine was removed for measurement of sweet taste receptor and glucose transporter expression by quantitative Reverse Transcription-Polymerase Chain Reaction.

RESULTS

In HFD rats, blood glucose levels were decreased at 30, 60, and 120 min during the IGGTT after 4 weeks supplementation with 0.78 mM sucralose. TIR3 expression was increased in the duodenum and TIR2 was increased in the ileum after 324 mM sucrose supplementation. T1R3 expression was increased after 0.54 mM and 0.78 mM sucralose in the ileum, but there was no change in the expression of TIRs in the duodenum after sucralose treatments. SGLT-1 expression was increased after both 0.78 mM sucralose and 324 mM sucrose in the ileum, and only increased in the duodenum after 324 mM sucrose supplementation.

CONCLUSIONS

The effects of sucralose on glucose metabolism in HFD rats are dose-dependent and related to enhanced expression of sweet taste receptors and glucose transporters. Further studies are needed to clarify the molecular mechanisms involved.

Characterization of the taste receptor-related G-protein, α-gustducin, in pancreatic β-cells

AIMS/INTRODUCTION

Taste receptors, T1rs and T2rs, and the taste-selective G-protein, α-gustducin, are expressed outside the taste-sensing system, such as enteroendocrine L cells. Here, we examined whether α-gustducin also affects nutrition sensing and insulin secretion by pancreatic β-cells.

MATERIALS AND METHODS

The expression of α-gustducin and taste receptors was evaluated in β-cell lines, and in rat and mouse islets either by quantitative polymerase chain reaction or fluorescence immunostaining. The effects of α-gustducin knockdown on insulin secretion and on cyclic adenosine monophosphate and intracellular Ca2+ levels in rat INS-1 cells were estimated. Sucralose (taste receptor agonist)-induced insulin secretion was investigated in INS-1 cells with α-gustducin suppression and in islets from mouse disease models.

RESULTS

The expression of Tas1r3 and α-gustducin was confirmed in β-cell lines and pancreatic islets. Basal levels of cyclic adenosine monophosphate, intracellular calcium and insulin secretion were significantly enhanced with α-gustducin knockdown in INS-1 cells. The expression of α-gustducin was decreased in high-fat diet-fed mice and in diabetic db/db mice. Sucralose-induced insulin secretion was not attenuated in INS-1 cells with α-gustducin knockdown or in mouse islets with decreased expression of α-gustducin.

CONCLUSIONS

α-Gustducin is involved in the regulation of cyclic adenosine monophosphate, intracellular calcium levels and insulin secretion in pancreatic β-cells in a manner independent of taste receptor signaling. α-Gustducin might play a novel role in β-cell physiology and the development of type 2 diabetes.

Elevation of the Blood Glucose Level is Involved in an Increase in Expression of Sweet Taste Receptors in Taste Buds of Rat Circumvallate Papillae

The gustation system for sweeteners is well-known to be regulated by nutritional and metabolic conditions, but there is no or little information on the underlying mechanism. Here, we examined whether elevation of the blood glucose level was involved in alteration of the expression of sweet taste receptors in circumvallate papillae (CP) and sweet taste sensitivity in male Sprague-Dawley rats. Rats under 4 h-fed conditions following 18 h-fasting exhibited elevated blood glucose levels and decreased pancreatic T1R3 expression, compared to rats after 18 h-fasting treatment, and they exhibited increased protein expression of sweet taste receptors T1R2 and T1R3 in CP. Under streptozotocin (STZ)-induced diabetes mellites (DM) conditions, the protein expression levels of T1R2 and T1R3 in CP were higher than those under control conditions, and these DM rats exhibited increased lick ratios in a low sucrose concentration range in a brief access test with a mixture of sucrose and quinine hydrochloride (QHCl). These findings indicate that the elevation of blood glucose level is a regulator for an increase in sweet taste receptor protein expression in rat CP, and such alteration in STZ-induced DM rats is involved in enhancement of their sweet taste sensitivity.

Non-nutritive Sweeteners and Glycaemic Control

PURPOSE OF REVIEW

The consumption of foods and beverages containing non-nutritive sweeteners (NNS) has increased worldwide over the last three decades. Consumers' choice of NNS rather than sugar or other nutritive sweeteners may be attributable to their potential to reduce weight gain.

RECENT FINDINGS

It is not clear what the effects of NNS consumption are on glycaemic control and the incidence of type 2 diabetes. This review aims to examine this question in epidemiological, human intervention and animal studies. It is not clear that NNS consumption has an effect on the incidence of type 2 diabetes or on glycaemic control even though there is some evidence for the modification of the microbiome and for interaction with sweet taste receptors in the oral cavity and the intestines' modification of secretion of glucagon-like peptide-1 (GLP-1), peptide YY (PYY), ghrelin and glucose-dependent insulinotropic polypeptide (GIP), which may affect glycaemia following consumption of NNS. In conclusion, long-term studies of NNS consumption are required to draw a firm conclusion about the role of NNS consumption on glycaemic control.

Low Calorie Sweeteners Differ in Their Physiological Effects in Humans

Low calorie sweeteners (LCS) are prevalent in the food supply for their primary functional property of providing sweetness with little or no energy. Though tested for safety individually, there has been extremely limited work on the efficacy of each LCS. It is commonly assumed all LCS act similarly in their behavioral and physiological effects. However, each LCS has its own chemical structure that influences its metabolism, making each LCS unique in its potential effects on body weight, energy intake, and appetite. LCS may have different behavioral and physiological effects mediated at the sweet taste receptor, in brain activation, with gut hormones, at the microbiota and on appetitive responses. Further elucidation of the unique effects of the different commercially available LCS may hold important implications for recommendations about their use for different health outcomes.

Acute exposure to 3‑deoxyglucosone at high glucose levels impairs insulin secretion from β‑cells by downregulating the sweet taste receptor signaling pathway

Sweet taste receptors (STRs) expressed on β‑cells stimulate insulin secretion in response to an increase in the circulating level of glucose, maintaining glucose homeostasis. 3‑Deoxyglucosone (3DG), a highly reactive α‑dicarbonyl compound, has been previously described as an independent factor associate with the development of prediabetes. In our previous study, pathological plasma levels of 3DG were induced in normal rats with a single intravenous injection of 50 mg/kg 3DG, and an acute rise in circulating 3DG induced glucose intolerance by impairing the function of pancreatic β‑cells. The present study aimed to investigate whether the deleterious effects of pathological plasma levels of 3DG on β‑cell function and insulin secretion were associated with STRs. INS‑1 cells, an in vitro model to study rat β‑cells, were treated with various concentrations of 3DG (1.85, 30.84 and 61.68 mM) or lactisole (5 mM). Pancreatic islets were collected from rats 2 h after a single intravenous injection of 50 mg/kg 3DG + 0.5 g/kg glucose. The insulin concentration was measured by ELISA. The protein expression levels of components of the STR signaling pathways were determined by western blot analysis. Treatment with 3DG and 25.5 mM glucose for 1 h significantly reduced insulin secretion by INS‑1 cells, which was consistent with the phenotype observed in INS‑1 cells treated with the STR inhibitor lactisole. Accordingly, islets isolated from rats treated with 3DG exhibited a significant reduction in insulin secretion following treatment with 25.5 mM glucose. Furthermore, acute exposure of INS‑1 cells to 3DG following treatment with 25.5 mM glucose for 1 h significantly reduced the protein expression level of the STR subunit taste 1 receptor member 3 and its downstream factors, transient receptor potential cation channel subfamily M member 5 and glucose transporter 2. Notably, islet tissues collected from rats treated with 3DG exhibited a similar downregulation of these factors. The present results suggested that acute exposure to pathologically relevant levels of 3DG in presence of high physiological levels of glucose decreased insulin secretion from β‑cells by, at least in part, downregulating the STR signaling pathway.

A randomized controlled trial contrasting the effects of 4 low-calorie sweeteners and sucrose on body weight in adults with overweight or obesity

BACKGROUND

Low-calorie sweeteners (LCSs) provide sweetness with little or no energy. However, each LCS's unique chemical structure has potential to elicit different sensory, physiological, and behavioral responses that affect body weight.

OBJECTIVE

The purpose of this trial was to compare the effects of consumption of 4 LCSs and sucrose on body weight, ingestive behaviors, and glucose tolerance over a 12-wk intervention in adults (18-60 y old) with overweight or obesity (body mass index 25-40 kg/m2).

METHODS

In a parallel-arm design, 154 participants were randomly assigned to consume 1.25-1.75 L of beverage sweetened with sucrose (n = 39), aspartame (n = 30), saccharin (n = 29), sucralose (n = 28), or rebaudioside A (rebA) (n = 28) daily for 12 wk. The beverages contained 400-560 kcal/d (sucrose treatments) or <5 kcal/d (LCS treatments). Anthropometric indexes, energy intake, energy expenditure, appetite, and glucose tolerance were measured at baseline. Body weight was measured every 2 wk with energy intake, expenditure, and appetite assessed every 4 wk. Twenty-four-hour urine collections were completed every 4 wk to determine study compliance via para-aminobenzoic acid excretion.

RESULTS

Of the participants enrolled in the trial, 123 completed the 12-wk intervention. Sucrose and saccharin consumption led to increased body weight across the 12-wk intervention (Δweight = +1.85 ± 0.36 kg and +1.18 ± 0.36 kg, respectively; P ≤ 0.02) and did not differ from each other. There was no significant change in body weight with consumption of the other LCS treatments compared with baseline, but change in body weight for sucralose was negative and significantly lower compared with all other LCSs at week 12 (weight difference ≥ 1.37 ± 0.52 kg, P ≤ 0.008). Energy intake decreased with sucralose consumption (P = 0.02) and ingestive frequency was lower for sucralose than for saccharin (P = 0.045). Glucose tolerance was not significantly affected by any of the sweetener treatments.

CONCLUSIONS

Sucrose and saccharin consumption significantly increase body weight compared with aspartame, rebA, and sucralose, whereas weight change was directionally negative and lower for sucralose compared with saccharin, aspartame, and rebA consumption. LCSs should be categorized as distinct entities because of their differing effects on body weight. This trial was registered at clinicaltrials.gov as NCT02928653.

Green Fluorescent Protein-Based Glucose Indicators Report Glucose Dynamics in Living Cells

Glucose is the most important energy source for living animals. Here, we developed a series of single fluorescent protein (FP)-based glucose indicators, named as "Green Glifons", to understand the hierarchal and mutual relationships between molecules involved in energy metabolism. Three indicators showed a different EC₅₀ for glucose (50, 600, and 4000 μM), producing a ∼7-fold change in fluorescence intensity in response to glucose. The indicators could visualize glucose dynamics in the cytoplasm, plasma membrane, nucleus and mitochondria of living HeLa cells and in vivo, in the pharyngeal muscle of C. elegans and could measure murine blood glucose levels. Finally, the indicators were applicable to dual-color imaging, revealing the dynamic interplay between glucose and Ca²⁺ in mouse pancreatic MIN6 m9 β cells. We propose that these indicators will facilitate and contribute to in vivo and multicolor imaging of energy metabolism.

A role for taste receptors in (neuro)endocrinology?

The sense of taste is positioned at the forefront when it comes to the interaction of our body with foodborne chemicals. However, the role of our taste system, and in particular its associated taste receptors, is not limited to driving food preferences leading to ingestion or rejection before other organs take over responsibility for nutrient digestion, absorption and metabolic regulation. Taste sensory elements do much more. On the one hand, extra-oral taste receptors from the brain to the gut continue to sense nutrients and noxious substances after ingestion and, on the other hand, the nutritional state feeds back on the taste system. This intricate regulatory network is orchestrated by endocrine factors that are secreted in response to taste receptor signalling and, in turn regulate the taste receptor cells themselves. The present review summarises current knowledge on the endocrine regulation of the taste perceptual system and the release of hunger/satiety regulating factors by gastrointestinal taste receptors. Furthermore, the regulation of blood glucose levels via the activation of pancreatic sweet taste receptors and subsequent insulin secretion, as well as the influence of bitter compounds on thyroid hormone release, is addressed. Finally, the central effects of tastants are discussed briefly.

Taste receptor function

This chapter summarizes the available data about taste receptor functions and their role in perception of food with emphasis on the human system. In addition we illuminate the widespread presence of these receptors throughout the body and discuss some of their extraoral functions. Finally, we describe clinical aspects where taste receptor signaling could be relevant.

3-Deoxyglucosone Induces Glucagon-Like Peptide-1 Secretion from STC-1 Cells via Upregulating Sweet Taste Receptor Expression under Basal Conditions

3-Deoxyglucosone (3DG) is derived from D-glucose during food processing and storage and under physiological conditions. We reported that glucagon-like peptide-1 (GLP-1) secretion in response to an oral glucose load in vivo and high-glucose stimulation in vitro was decreased by acute 3DG administration. In this study, we determined the acute effect of 3DG on GLP-1 secretion under basal conditions and investigated the possible mechanisms. Normal fasting rats were given a single acute intragastric administration of 50 mg/kg 3DG. Plasma basal GLP-1 levels and duodenum 3DG content and sweet taste receptor expression were measured. STC-1 cells were acutely exposed to 3DG (80, 300, and 1000 ng/ml) for 1 h under basal conditions (5.6 mM glucose), and GLP-1 secretion, intracellular concentrations of cyclic adenosine monophosphate (cAMP) and Ca²⁺, and molecular expression of STR signaling pathway were measured. Under the fasted state, plasma GLP-1 levels, duodenum 3DG content, and duodenum STR expression were elevated in 3DG-treated rats. GLP-1 secretion was increased in 3DG-treated cells under either 5.6 mM glucose or glucose-free conditions. 3DG-induced acute GLP-1 secretion from STC-1 cells under 5.6 mM glucose was inhibited in the presence of the STR inhibitor lactisole, which was consistent with the observation under glucose-free conditions. Moreover, acute exposure to 3DG increased the protein expression of TAS1R2 and TAS1R3 under either 5.6 mM glucose or glucose-free conditions, with affecting other components of STR signaling pathway, including the upregulation of transient receptor potential channel type M5 TRPM5 and the increment of intracellular Ca²⁺ concentration. In summary, the glucose-free condition was used to first demonstrate the involvement of STR in 3DG-induced acute GLP-1 secretion. These results first showed that acute 3DG administration induces basal GLP-1 secretion in part through upregulation of STR expression.

Electrophysiology of the pancreatic islet β-cell sweet taste receptor TIR3

Over recent years, the presence of the sweet taste receptor TIR3 in rodent and human insulin-producing pancreatic islet β-cells was documented. The activation of this receptor by sweet-tasting sucralose mimics several biochemical and functional effects of D-glucose in the β-cells. The present study extends this analogy to the bioelectrical response of β-cells. In this respect, sucralose was inefficient in the absence of D-glucose, but induced on occasion electrical activity in mouse β-cells exposed to low non-stimulatory concentrations of the hexose and potentiated, in a concentration-related manner, the response to stimulatory concentrations of D-glucose. These data indicate that sucralose, acting as an agonist of the TIR3 receptor, exerts an excitatory effect upon pancreatic β-cell bioelectrical activity.

Oral Post-Oral Actions of Low-Calorie Sweeteners: A Tale of Contradictions and Controversies

OBJECTIVE

Many scientists and laypeople alike have concerns about low-calorie sweeteners (LCSs). These concerns stem from both a dissatisfaction with the taste of LCSs and reports that they cause metabolic disruptions (e.g., weight gain, glucose intolerance).

METHODS

This article provides a critical review of the literature on LCSs from the standpoint of their taste, gastrointestinal, and metabolic effects; biological fate in the body; and impact on ingestion and glucose homeostasis.

RESULTS AND CONCLUSIONS

Mammals can readily discriminate between LCSs and sugars because both types of sweetener activate distinct oral and post-oral sensory pathways. LCSs differ in their ability to access post-oral tissues, but few studies have incorporated this observation into their design. It is difficult to extrapolate results between mice, rats, and humans because of interspecies differences in the taste and post-oral actions of LCSs and the fact that investigators often use different response measures in rodents and humans. There is confounding in the experimental design of some of the most widely cited studies of LCS-induced metabolic disruptions. The uncritical acceptance of these studies has generated considerable controversy. More work is needed to obtain a clearer understanding of the metabolic effects of LCSs.

Noncaloric Sweeteners Induce Peripheral Serotonin Secretion via the T1R3-Dependent Pathway in Human Gastric Parietal Tumor Cells (HGT-1)

The role of sweet taste in energy intake and satiety regulation is still controversial. Noncaloric artificial sweeteners (NCSs) are thought to help reduce energy intake, although little is known about their impact on the satiating neurotransmitter serotonin (5-HT). In the gastrointestinal (GI) tract, 5-HT regulates gastric acid secretion and gastric motility, both part of the complex network of mechanisms regulating food intake and satiety. This study demonstrated a stimulating impact compared to controls (100%) on 5-HT release in human gastric tumor cells (HGT-1) by the NCSs cyclamate (50 mM, 157% ± 6.3%), acesulfame potassium (Ace K, 50 mM, 197% ± 8.6%), saccharin (50 mM, 147% ± 6.7%), sucralose (50 mM, 194% ± 11%), and neohesperidin dihydrochalcone (NHDC, 1 mM, 201% ± 13%). Although these effects were not associated with the sweet taste intensity of the NCSs tested, involvement of the sweet receptor subunit T1R3 in the NCS-evoked response was demonstrated by mRNA expression of TAS1R3, co-incubation experiments using the T1R3 receptor antagonist lactisole, and a TAS1R3 siRNA knockdown approach. Analysis of the downstream signaling revealed activation of the cAMP/ERK/Ca²⁺ cascade. Co-treatment experiments with 10 mM glucose enhanced the 5-HT release induced by cyclamate, Ace K, saccharin, and sucralose, thereby supporting the enhancing effect of glucose on a NCS-mediated response. Overall, the results obtained identify NCSs as potent inducers of 5-HT release via T1R3 in human gastric parietal cells in culture and warrant in vivo studies to demonstrate their efficacy.

Human Sweet Receptor T1R3 is Functional in Human Gastric Parietal Tumor Cells (HGT-1) and Modulates Cyclamate and Acesulfame K-Induced Mechanisms of Gastric Acid Secretion

The noncaloric sweeteners (NCSs) cyclamate (Cycl) and acesulfame K (AceK) are widely added to foods and beverages. Little is known about their impact on gastric acid secretion (GAS), which is stimulated by dietary protein and bitter-tasting compounds. Since Cycl and AceK have a bitter off taste in addition to their sweet taste, we hypothesized they modulate mechanisms of GAS in human gastric parietal cells (HGT-1). HGT-1 cells were exposed to sweet tastants (50 mM of glucose, d-threonine, Cycl, or AceK) and analyzed for their intracellular pH index (IPX), as an indicator of proton secretion by means of a pH-sensitive dye, and for mRNA levels of GAS-associated genes by RT-qPCR. Since the NCSs act via the sweet taste-sensing receptor T1R2/T1R3, mRNA expression of the corresponding genes was analyzed in addition to immunocytochemical localization of the T1R2 and T1R3 receptor proteins. Exposure of HGT-1 cells to AceK or d-threonine increased the IPX to 0.60 ± 0.05 and 0.80 ± 0.04 ( P ≤ 0.05), respectively, thereby indicating a reduced secretion of protons, whereas Cycl demonstrated the opposite effect with IPX values of -0.69 ± 0.08 ( P ≤ 0.05) compared to controls (IPX = 0). Cotreatment with the T1R3-inhibitor lactisole as well as a TAS1R3 siRNA knock-down approach reduced the impact of Cycl, AceK, and d-thr on proton release ( P ≤ 0.05), whereas cotreatment with 10 mM glucose enhanced the NCS-induced effect ( P ≤ 0.05). Overall, we demonstrated Cycl and AceK as modulators of proton secretion in HGT-1 cells and identified T1R3 as a key element in this response.

Taste receptors in the upper airway

Taste receptors were named for their originally-identified expression on the tongue and role in the sensation of taste (gustation). They are now known to be involved in many chemosensory processes outside the tongue. Expression of the receptors for bitter, sweet, and umami was recently identified in many organs, including the brain, airway, gastrointestinal tract, and reproductive systems. We do not yet know the full roles of these receptors in all of these tissues, nor do we know all of the endogenous ligands that activate them. However, taste receptors are emerging as potentially important therapeutic targets. Moreover, they may mediate some off target effects of drugs, as many medications in common clinical use are known to be bitter. The focus of this review is on recent basic and clinical data describing the expression of bitter (T2R) and sweet (T1R) receptors in the airway and their activation by secreted bacterial compounds. These receptors play important roles in innate immune nitric oxide production and antimicrobial peptide secretion, and may be useful targets for stimulating immune responses in the upper respiratory tract via topical therapies. Moreover, genetic variation in these receptors may play a role in the differential susceptibility of patients to certain types of respiratory infections as well as to differential outcomes in patients with chronic rhinosinusitis (CRS). CRS is a syndrome of chronic upper respiratory infection and inflammation and has a significant detrimental impact on patient quality of life. CRS treatment accounts for approximately 20% of adult antibiotic prescriptions and is thus a large driver of the public health crisis of antibiotic resistance. Taste receptors represent a novel class of therapeutic target to potentially stimulate endogenous immune responses and treat CRS patients without conventional antibiotics.

Sweet Taste Receptors Mediated ROS-NLRP3 Inflammasome Signaling Activation: Implications for Diabetic Nephropathy

Previous studies demonstrated that ROS-NLRP3 inflammasome signaling activation was involved in the pathogenesis of diabetic nephropathy (DN). Recent research has shown that sweet taste receptors (STRs) are important sentinels of innate immunity. Whether high glucose primes ROS-NLRP3 inflammasome signaling via STRs is unclear. In this study, diabetic mouse model was induced by streptozotocin (STZ) in vivo; mouse glomerular mesangial cells (GMCs) and human proximal tubular cells were stimulated by high glucose (10, 20, and 30 mmol/L) in vitro; STR inhibitor lactisole was used as an intervention reagent to evaluate the role and mechanism of the STRs in the pathogenesis of DN. Our results showed that the expression of STRs and associated signaling components (Gα-gustducin, PLCβ2, and TRPM5) was obviously downregulated under the condition of diabetes in vivo and in vitro. Furthermore, lactisole significantly mitigated the production of intracellular ROS and reversed the high glucose-induced decrease of Ca²⁺ and the activation of NLRP3 inflammasome signaling in vitro (p < 0.05). These combined results support the hypothesis that STRs could be involved in the activation of ROS-NLRP3 inflammasome signaling in the pathogenesis of DN, suggesting that STRs may act as new therapeutic targets of DN.

The cephalic phase insulin response to nutritive and low-calorie sweeteners in solid and beverage form

The purpose of the study was to examine the role of the cephalic phase insulin response (CPIR) following exposure to nutritive and low-calorie sweeteners in solid and beverage form in overweight and obese adults. In addition, the role of learning on the CPIR to nutritive and low-calorie sweetener exposure was tested. Sixty-four overweight and obese adults (age: 18-50years, BMI: 24-37kg/m2, body fat percentage>25% for men and >32% for women) were sham-fed (at 2-minute intervals for 14min) a randomly assigned test load comprised of a nutritive (sucrose) or low-calorie sweetener (sucralose) in beverage or solid form in phase 1 of the study. A 2-3ml blood sample was collected before and 2, 6, 10, 14, 61, 91 and 121min after oral exposure for serum insulin and glucose analysis. During phase 2, participants underwent a 2-week training period to facilitate associative learning between the sensory properties of test loads and their post-ingestive effects. In phase 3, participants were retested for their cephalic phase responses as in phase 1. Participants were classified as responders if they demonstrated a positive insulin response (rise of serum insulin above baseline i.e. Δ insulin) 2min post-stimulus in phase 1. Among responders exposed to the same sweetener in Phases 1 and 3, the proportion of participants that displayed a rise of insulin with oral exposure to sucralose was significantly greater when the stimulus was in the solid form compared to the beverage form. Sucralose and sucrose exposure elicited similarly significant increases in serum insulin 2min after exposure and significant decreases after 2min in responders in both food forms. The solid food form elicited greater CPIR over 2, 6 and 10min than the beverage form. There was no effect of learning on insulin responses after training. The results indicate the presence of a significant CPIR in a subset of individuals with overweight or obesity after oral exposure to sucralose, especially when present in solid food form. Future studies must confirm the reliability of this response.

A 12-week randomized clinical trial investigating the potential for sucralose to affect glucose homeostasis

The discovery of gut sweet taste receptors has led to speculations that non-nutritive sweeteners, including sucralose, may affect glucose control. A double-blind, parallel, randomized clinical trial, reported here and previously submitted to regulatory agencies, helps to clarify the role of sucralose in this regard. This was primarily an out-patient study, with 4-week screening, 12-week test, and 4-week follow-up phases. Normoglycemic male volunteers (47) consumed ∼333.3 mg encapsulated sucralose or placebo 3x/day at mealtimes. HbA1c, fasting glucose, insulin, and C-peptide were measured weekly. OGTTs were conducted in-clinic overnight, following overnight fasting twice during screening phase, twice during test phase, and once at follow-up. Throughout the study, glucose, insulin, C-peptide and HbA1c levels were within normal range. No statistically significant differences between sucralose and placebo groups in change from baseline for fasting glucose, insulin, C-peptide and HbA1c, no clinically meaningful differences in time to peak levels or return towards basal levels in OGTTs, and no treatment group differences in mean glucose, insulin, or C-peptide AUC change from baseline were observed. The results of other relevant clinical trials and studies of gastrointestinal sweet taste receptors are compared to these findings. The collective evidence supports that sucralose has no effect on glycemic control.

Sucralose activates an ERK1/2-ribosomal protein S6 signaling axis

The sweetener sucralose can signal through its GPCR receptor to induce insulin secretion from pancreatic β cells, but the downstream signaling pathways involved are not well‐understood. Here we measure responses to sucralose, glucagon‐like peptide 1, and amino acids in MIN6 β cells. Our data suggest a signaling axis, whereby sucralose induces calcium and cAMP, activation of ERK1/2, and site‐specific phosphorylation of ribosomal protein S6. Interestingly, sucralose acted independently of mTORC1 or ribosomal S6 kinase (RSK). These results suggest that sweeteners like sucralose can influence β‐cell responses to secretagogues like glucose through metabolic as well as GPCR‐mediated pathways. Future investigation of novel sweet taste receptor signaling pathways in β cells will have implications for diabetes and other emergent fields involving these receptors.

Lipid-Lowering Pharmaceutical Clofibrate Inhibits Human Sweet Taste

T1R2-T1R3 is a heteromeric receptor that binds sugars, high potency sweeteners, and sweet taste blockers. In rodents, T1R2-T1R3 is largely responsible for transducing sweet taste perception. T1R2-T1R3 is also expressed in non-taste tissues, and a growing body of evidence suggests that it helps regulate glucose and lipid metabolism. It was previously shown that clofibric acid, a blood lipid-lowering drug, binds T1R2-T1R3 and inhibits its activity in vitro The purpose of this study was to determine whether clofibric acid inhibits sweetness perception in humans and is, therefore, a T1R2-T1R3 antagonist in vivo Fourteen participants rated the sweetness intensity of 4 sweeteners (sucrose, sucralose, Na cyclamate, acesulfame K) across a broad range of concentrations. Each sweetener was prepared in solution neat and in mixture with either clofibric acid or lactisole. Clofibric acid inhibited sweetness of every sweetener. Consistent with competitive binding, inhibition by clofibric acid was diminished with increasing sweetener concentration. This study provides in vivo evidence that the lipid-lowering drug clofibric acid inhibits sweetness perception and is, therefore, a T1R carbohydrate receptor inhibitor. Our results are consistent with previous in vitro findings. Given that T1R2-T1R3 may in part regulate glucose and lipid metabolism, future studies should investigate the metabolic effects of T1R inhibition.

Supplementation of oligofructose, but not sucralose, decreases high-fat diet induced body weight gain in mice independent of gustducin-mediated gut hormone release

SCOPE

Enteroendocrine cells sense nutrients through taste receptors similar to those on the tongue. Sweet and fatty acid taste receptors (FFAR) coupled to the gustatory G-protein, gustducin, on enteroendocrine cells play a role in gut hormone release. We studied if supplementation of artificial (sucralose) or prebiotic (oligofructose; OFS) sweeteners target gustducin-mediated signaling pathways to alter gut hormone release and reduce obesity-associated disorders.

METHODS AND RESULTS

Wild-type (WT) and α-gustducin knockout (α-gust^-/- ) mice were fed a high-fat diet and gavaged once daily (8 wk) with water or equisweet concentrations of sweeteners. OFS but not sucralose decreased body weight gain (-19 ± 3%, p < 0.01), fat pad mass (-55 ± 6%, p < 0.001), and insulin resistance (-39 ± 5%, p < 0.001) independent of α-gustducin. Neither sweetener improved glucose intolerance, while solely OFS improved the disturbed colonic permeability. OFS decreased (-65 ± 8%, p < 0.001) plasma glucagon-like peptide 1 (GLP-1) but not ghrelin and peptide YY (PYY) levels in WT mice. Cecal acetate and butyrate levels were reduced by OFS in both genotypes suggesting enhanced uptake of SCFAs that may target FFAR2 (upregulated expression) in adipose tissue.

CONCLUSION

OFS, but not sucralose, reduced body weight gain and decreased intestinal permeability, but not glucose intolerance. Effects were not mediated by altered gut hormone levels or gustducin-mediated signaling. Artificial sweeteners do not affect gut hormone levels and are metabolically inert in mice on a high-fat diet. In contrast, prebiotic oligosaccharides (OFS) prevent body weight gain but not glucose intolerance. Alterations in sweet and short-chain fatty acid receptors (FFAR) (studied in WT and α-gust^-/- mice) that regulate gut hormone levels are not mandatory for the positive effects of OFS. Enhanced uptake of SCFAs may favor interaction with FFAR2/3 on adipose tissue to induce weight loss.

Sweet Taste Receptor Serves to Activate Glucose- and Leptin-Responsive Neurons in the Hypothalamic Arcuate Nucleus and Participates in Glucose Responsiveness

The hypothalamic feeding center plays an important role in energy homeostasis. In the feeding center, whole-body energy signals including hormones and nutrients are sensed, processed, and integrated. As a result, food intake and energy expenditure are regulated. Two types of glucose-sensing neurons exist in the hypothalamic arcuate nucleus (ARC): glucose-excited neurons and glucose-inhibited neurons. While some molecules are known to be related to glucose sensing in the hypothalamus, the mechanisms underlying glucose sensing in the hypothalamus are not fully understood. The sweet taste receptor is a heterodimer of taste type 1 receptor 2 (T1R2) and taste type 1 receptor 3 (T1R3) and senses sweet tastes. T1R2 and T1R3 are distributed in multiple organs including the tongue, pancreas, adipose tissue, and hypothalamus. However, the role of sweet taste receptors in the ARC remains to be clarified. To examine the role of sweet taste receptors in the ARC, cytosolic Ca²⁺ concentration ([Ca²⁺]_i) in isolated single ARC neurons were measured using Fura-2 fluorescent imaging. An artificial sweetener, sucralose at 10⁻⁵–10⁻² M dose dependently increased [Ca²⁺]_i in 12–16% of ARC neurons. The sucralose-induced [Ca²⁺]_i increase was suppressed by a sweet taste receptor inhibitor, gurmarin. The sucralose-induced [Ca²⁺]_i increase was inhibited under an extracellular Ca²⁺-free condition and in the presence of an L-type Ca²⁺ channel blocker, nitrendipine. Sucralose-responding neurons were activated by high-concentration of glucose. This response to glucose was markedly suppressed by gurmarin. More than half of sucralose-responding neurons were activated by leptin but not ghrelin. Percentages of proopiomelanocortin (POMC) neurons among sucralose-responding neurons and sweet taste receptor expressing neurons were low, suggesting that majority of sucralose-responding neurons are non-POMC neurons. These data suggest that sweet taste receptor-mediated cellular activation mainly occurs on non-POMC leptin-responding neurons and contributes to glucose responding. Endogenous sweet molecules including glucose may regulate energy homeostasis through sweet taste receptors on glucose-and leptin-responsive neurons in the ARC.

Positive Allosteric Modulation of the Calcium-sensing Receptor by Physiological Concentrations of Glucose

The calcium-sensing receptor (CaSR) is activated by various cations, cationic compounds, and amino acids. In the present study we investigated the effect of glucose on CaSR in HEK293 cells stably expressing human CaSR (HEK-CaSR cells). When glucose concentration in the buffer was raised from 3 to 25 mm, a rapid elevation of cytoplasmic Ca²⁺ concentration ([Ca²⁺]_c) was observed. This elevation was immediate and transient and was followed by a sustained decrease in [Ca²⁺]_c The effect of glucose was detected at a concentration of 4 mm and reached its maximum at 5 mm 3-O-Methylglucose, a non-metabolizable analogue of glucose, reproduced the effect of glucose. Sucrose also induced an elevation of [Ca²⁺]_c in HEK-CaSR cells. Similarly, sucralose was nearly as effective as glucose in inducing elevation of [Ca²⁺]_c Glucose was not able to increase [Ca²⁺]_c in the absence of extracellular Ca²⁺ The effect of glucose on [Ca²⁺]_c was inhibited by NPS-2143, an allosteric inhibitor of CaSR. In addition, NPS-2143 also inhibited the [Ca²⁺]_c responses to sucralose and sucrose. Glucose as well as sucralose decreased cytoplasmic cAMP concentration in HEK-CaSR cells. The reduction of cAMP induced by glucose was blocked by pertussis toxin. Likewise, sucralose reduced [cAMP]_c Finally, glucose increased [Ca²⁺]_c in PT-r parathyroid cells and in Madin-Darby canine kidney cells, both of which express endogenous CaSR. These results indicate that glucose acts as a positive allosteric modulator of CaSR.

Sucralose, an activator of the glucose-sensing receptor, increases ATP by calcium-dependent and -independent mechanisms

Sucralose is an artificial sweetener and activates the glucose-sensing receptor expressed in pancreatic β-cells. Although sucralose does not enter β-cells nor acts as a substrate for glucokinase, it induces a marked elevation of intracellular ATP ([ATP]c). The present study was conducted to identify the signaling pathway responsible for the elevation of [ATP]c induced by sucralose. Previous studies have shown that sucralose elevates cyclic AMP (cAMP), activates phospholipase C (PLC) and stimulates Ca(2+) entry by a Na(+)-dependent mechanism in MIN6 cells. The addition of forskolin induced a marked elevation of cAMP, whereas it did not affect [ATP]c. Carbachol, an activator of PLC, did not increase [ATP]c. In addition, activation of protein kinase C by dioctanoylglycerol did not affect [ATP]c. In contrast, nifedipine, an inhibitor of the voltage-dependent Ca(2+) channel, significantly reduced [ATP]c response to sucralose. Removal of extracellular Na(+) nearly completely blocked sucralose-induced elevation of [ATP]c. Stimulation of Na(+) entry by adding a Na(+) ionophore monensin elevated [ATP]c. The monensin-induced elevation of [ATP]c was only partially inhibited by nifedipine and loading of BAPTA, both of which completely abolished elevation of [Ca(2+)]c. These results suggest that Na(+) entry is critical for the sucralose-induced elevation of [ATP]c. Both calcium-dependent and -independent mechanisms are involved in the action of sucralose.

What Does Diabetes "Taste" Like?

The T1R2 (taste type 1 receptor, member 2)/T1R3 (taste type 1 receptor, member 3) sweet taste receptor is expressed in taste buds on the tongue, where it allows the detection of energy-rich carbohydrates of food. This single receptor responds to all compounds perceived as sweet by humans, including natural sugars and natural and artificial sweeteners. Importantly, the T1R2/T1R3 sweet taste receptor is also expressed in extra-oral tissues, including the stomach, pancreas, gut, liver, and brain. Although its physiological role remains to be established in numerous organs, T1R2/T1R3 is suspected to be involved in the regulation of metabolic processes, such as sugar sensing, glucose homeostasis, and satiety hormone release. In this review, the physiological role of the sweet taste receptor in taste perception and metabolic regulation is discussed by focusing on dysfunctions leading to diabetes. Current knowledge of T1R2/T1R3 inhibitors making this receptor a promising therapeutic target for the treatment of type 2 diabetes is also summarized and discussed.

Glucose-Sensing Receptor T1R3: A New Signaling Receptor Activated by Glucose in Pancreatic β-Cells

Subunits of the sweet taste receptors T1R2 and T1R3 are expressed in pancreatic β-cells. Compared with T1R3, mRNA expression of T1R2 is considerably lower. At the protein level, expression of T1R2 is undetectable in β-cells. Accordingly, a major component of the sweet taste-sensing receptor in β-cells may be a homodimer of T1R3 rather than a heterodimer of T1R2/T1R3. Inhibition of this receptor by gurmarin or deletion of the T1R3 gene attenuates glucose-induced insulin secretion from β-cells. Hence the T1R3 homodimer functions as a glucose-sensing receptor (GSR) in pancreatic β-cells. When GSR is activated by the T1R3 agonist sucralose, elevation of intracellular ATP concentration ([ATP]i) is observed. Sucralose increases [ATP]i even in the absence of ambient glucose, indicating that sucralose increases [ATP]i not simply by activating glucokinase, a rate-limiting enzyme in the glycolytic pathway. In addition, sucralose augments elevation of [ATP]i induced by methylsuccinate, suggesting that sucralose activates mitochondrial metabolism. Nonmetabolizable 3-O-methylglucose also increases [ATP]i and knockdown of T1R3 attenuates elevation of [ATP]i induced by high concentration of glucose. Collectively, these results indicate that the T1R3 homodimer functions as a GSR; this receptor is involved in glucose-induced insulin secretion by activating glucose metabolism probably in mitochondria.

An epigenomic signature of postprandial hyperglycemia in peripheral blood leukocytes

Postprandial hyperglycemia is known to be one of the earliest signs of abnormal glucose homeostasis associated with type 2 diabetes. This study aimed to assess clinical significance of a 1-h postprandial glucose level for the development of diabetes, and identify epigenetic biomarkers of postprandial hyperglycemia. We analyzed clinical data from the oral glucose tolerance tests for healthy subjects (n=4502). The ratio (Glu60/Glu0) of 1-h glucose levels to fasting glucose levels was significantly associated with an insulin sensitive index (QUICKI, quantitative insulin sensitivity check index) (β=0.055, P=1.25E-04) as well as a risk of future pre-diabetic and diabetic conversion. Next, DNA methylation profile analyses of 24 matched pairs of the high and low Glu60/Glu0 ratio subjects showed that specific DNA methylation levels in the promoter region of an olfactory receptor gene (olfactory receptor gene family10 member A4, OR10A4) were associated with the Glu60/Glu0 ratios (β=0.337, P=0.03). Moreover, acute oral glucose challenges decreased the DNA methylation levels of OR10A4 but not the global DNA methylation in peripheral leukocytes of healthy subjects (n=7), indicating that OR10A4 is a specific epigenomic target of postprandial hyperglycemia. This work suggests possible relevance of olfactory receptor genes to an earlier molecular biomarker of peripheral hyperglycemia and diabetic conversion.

Glucose Evokes Rapid Ca2+ and Cyclic AMP Signals by Activating the Cell-Surface Glucose-Sensing Receptor in Pancreatic β-Cells

Glucose is a primary stimulator of insulin secretion in pancreatic β-cells. High concentration of glucose has been thought to exert its action solely through its metabolism. In this regard, we have recently reported that glucose also activates a cell-surface glucose-sensing receptor and facilitates its own metabolism. In the present study, we investigated whether glucose activates the glucose-sensing receptor and elicits receptor-mediated rapid actions. In MIN6 cells and isolated mouse β-cells, glucose induced triphasic changes in cytoplasmic Ca(2+) concentration ([Ca(2+)]c); glucose evoked an immediate elevation of [Ca(2+)]c, which was followed by a decrease in [Ca(2+)]c, and after a certain lag period it induced large oscillatory elevations of [Ca(2+)]c. Initial rapid peak and subsequent reduction of [Ca(2+)]c were independent of glucose metabolism and reproduced by a nonmetabolizable glucose analogue. These signals were also blocked by an inhibitor of T1R3, a subunit of the glucose-sensing receptor, and by deletion of the T1R3 gene. Besides Ca(2+), glucose also induced an immediate and sustained elevation of intracellular cAMP ([cAMP]c). The elevation of [cAMP]c was blocked by transduction of the dominant-negative Gs, and deletion of the T1R3 gene. These results indicate that glucose induces rapid changes in [Ca(2+)]c and [cAMP]c by activating the cell-surface glucose-sensing receptor. Hence, glucose generates rapid intracellular signals by activating the cell-surface receptor.

Lactisole inhibits the glucose-sensing receptor T1R3 expressed in mouse pancreatic β-cells

Glucose activates the glucose-sensing receptor T1R3 and facilitates its own metabolism in pancreatic β-cells. An inhibitor of this receptor would be helpful in elucidating the physiological function of the glucose-sensing receptor. The present study was conducted to examine whether or not lactisole can be used as an inhibitor of the glucose-sensing receptor. In MIN6 cells, in a dose-dependent manner, lactisole inhibited insulin secretion induced by sweeteners, acesulfame-K, sucralose and glycyrrhizin. The IC50 was ∼4 mmol/l. Lactisole attenuated the elevation of cytoplasmic Ca2+ concentration ([Ca2+]c) evoked by sucralose and acesulfame-K but did not affect the elevation of intracellular cAMP concentration ([cAMP]c) induced by these sweeteners. Lactisole also inhibited the action of glucose in MIN6 cells. Thus, lactisole significantly reduced elevations of intracellular [NADH] and intracellular [ATP] induced by glucose, and also inhibited glucose-induced insulin secretion. To further examine the effect of lactisole on T1R3, we prepared HEK293 cells stably expressing mouse T1R3. In these cells, sucralose elevated both [Ca2+]c and [cAMP]c. Lactisole attenuated the sucralose-induced increase in [Ca2+]c but did not affect the elevation of [cAMP]c. Finally, lactisole inhibited insulin secretion induced by a high concentration of glucose in mouse islets. These results indicate that the mouse glucose-sensing receptor was inhibited by lactisole. Lactisole may be useful in assessing the role of the glucose-sensing receptor in mouse pancreatic β-cells.

The endocrinology of taste receptors

Levels of obesity have reached epidemic proportions on a global scale, which has led to considerable increases in health problems and increased risk of several diseases, including cardiovascular and pulmonary diseases, cancer and diabetes mellitus. People with obesity consume more food than is needed to maintain an ideal body weight, despite the discrimination that accompanies being overweight and the wealth of available information that overconsumption is detrimental to health. The relationship between energy expenditure and energy intake throughout an individual's lifetime is far more complicated than previously thought. An improved comprehension of the relationships between taste, palatability, taste receptors and hedonic responses to food might lead to increased understanding of the biological underpinnings of energy acquisition, as well as why humans sometimes eat more than is needed and more than we know is healthy. This Review discusses the role of taste receptors in the tongue, gut, pancreas and brain and their hormonal involvement in taste perception, as well as the relationship between taste perception, overeating and the development of obesity.