Functional molecular switches of mammalian G protein-coupled bitter-taste receptors.

Bitter taste receptors (TAS2Rs) are a poorly understood subgroup of G protein-coupled receptors (GPCRs). The experimental structure of these receptors has yet to be determined, and key-residues controlling their function remain mostly unknown. We designed an integrative approach to improve comparative modeling of TAS2Rs. Using current knowledge on class A GPCRs and existing experimental data in the literature as constraints, we pinpointed conserved motifs to entirely re-align the amino-acid sequences of TAS2Rs. We constructed accurate homology models of human TAS2Rs. As a test case, we examined the accuracy of the TAS2R16 model with site-directed mutagenesis and in vitro functional assays. This combination of in silico and in vitro results clarifies sequence-function relationships and proposes functional molecular switches that encode agonist sensing and downstream signaling mechanisms within mammalian TAS2Rs sequences.

Interactions between Chemesthesis and Taste: Role of TRPA1 and TRPV1.

In addition to the sense of taste and olfaction, chemesthesis, the sensation of irritation, pungency, cooling, warmth, or burning elicited by spices and herbs, plays a central role in food consumption. Many plant-derived molecules demonstrate their chemesthetic properties via the opening of transient receptor potential ankyrin 1 (TRPA1) and transient receptor potential vanilloid 1 (TRPV1) channels. TRPA1 and TRPV1 are structurally related thermosensitive cation channels and are often co-expressed in sensory nerve endings. TRPA1 and TRPV1 can also indirectly influence some, but not all, primary taste qualities via the release of substance P and calcitonin gene-related peptide (CGRP) from trigeminal neurons and their subsequent effects on CGRP receptor expressed in Type III taste receptor cells. Here, we will review the effect of some chemesthetic agonists of TRPA1 and TRPV1 and their influence on bitter, sour, and salt taste qualities.

Suppression of hTAS2R16 Signaling by Umami Substances.

Interaction between umami and bitter taste has long been observed in human sensory studies and in neural responses in animal models, however, the molecular mechanism for their action has not been delineated. Humans detect diverse bitter compounds using 25-30 members of the type 2 taste receptor (TAS2R) family of G protein-coupled receptor. In this study, we investigated the putative mechanism of antagonism by umami substances using HEK293T cells expressing hTAS2R16 and two known probenecid-insensitive mutant receptors, hTAS2R16 N96T and P44T. In wild type receptor, Glu-Glu, inosine monophosphate (IMP), and l-theanine behave as partial insurmountable antagonists, and monosodium glutamate (MSG) acts as a surmountable antagonist in comparison with probenecid as a full insurmountable antagonist. The synergism with IMP of umami substances still stands in the suppression of hTAS2R16 signaling. In mutagenesis analysis, we found that Glu-Glu, MSG, and l-theanine share at least one critical binding site on N96 and P44 with probenecid. These results provide the first evidence for a direct binding of umami substances to the hTAS2R16 through the probenecid binding pocket on the receptor, resulting in the suppression of bitterness.

Kokumi Taste Active Peptides Modulate Salt and Umami Taste.

Kokumi taste substances exemplified by γ-glutamyl peptides and Maillard Peptides modulate salt and umami tastes. However, the underlying mechanism for their action has not been delineated. Here, we investigated the effects of a kokumi taste active and inactive peptide fraction (500-10,000 Da) isolated from mature (FIIm) and immature (FIIim) Ganjang, a typical Korean soy sauce, on salt and umami taste responses in humans and rodents. Only FIIm (0.1-1.0%) produced a biphasic effect in rat chorda tympani (CT) taste nerve responses to lingual stimulation with 100 mM NaCl + 5 µM benzamil, a specific epithelial Na+ channel blocker. Both elevated temperature (42 °C) and FIIm produced synergistic effects on the NaCl + benzamil CT response. At 0.5% FIIm produced the maximum increase in rat CT response to NaCl + benzamil, and enhanced salt taste intensity in human subjects. At 2.5% FIIm enhanced rat CT response to glutamate that was equivalent to the enhancement observed with 1 mM IMP. In human subjects, 0.3% FIIm produced enhancement of umami taste. These results suggest that FIIm modulates amiloride-insensitive salt taste and umami taste at different concentration ranges in rats and humans.

Indazole-Cl inhibits hypoxia-induced cyclooxygenase-2 expression in vascular smooth muscle cells.

Atherosclerosis is the most common root cause of arterial disease, such as coronary artery disease and carotid artery disease. Hypoxia is associated with the formation of macrophages and increased inflammation and is known to be present in lesions of atherosclerotic. Vascular smooth muscle cells (VSMCs) are one of the major components of blood vessels, and hypoxic conditions affect VSMC inflammation, proliferation and migration, which contribute to vascular stenosis and play a major role in the atherosclerotic process. Estrogen receptor (ER)-ß is thought to play an important role in preventing the inflammatory response in VSMCs. In this report, we studied the anti-inflammatory effect of indazole (In)-Cl, an ERß-specific agonist, under conditions of hypoxia. Expression of cyclooxygenase-2 reduced by hypoxia was inhibited by In-Cl treatment in VSMCs, and this effect was antagonized by an anti-estrogen compound. Additionally, the production of reactive oxygen species induced under conditions of hypoxia was reduced by treatment with In-Cl. Increased cell migration and invasion by hypoxia were also dramatically decreased following treatment with In-Cl. The increase in cell proliferation following treatment with platelet-derived growth factor was attenuated by In-Cl in VSMCs. RNA sequencing analysis was performed to identify changes in inflammation-related genes following In-Cl treatment in the hypoxic state. Our results suggest that ERß is a potential therapeutic target for the suppression of hypoxia-induced inflammation in VSMCs.

Ginsenoside Rf inhibits cyclooxygenase-2 induction via peroxisome proliferator-activated receptor gamma in A549 cells.

BACKGROUND: Ginsenoside Rf is a ginseng saponin found only in Panax ginseng that affects lipid metabolism. It also has neuroprotective and antiinflammatory properties. We previously showed that Korean Red Ginseng (KRG) inhibited the expression of cyclooxygenase-2 (COX-2) by hypoxia via peroxisome proliferator-activated receptor gamma (PPARγ). The aim of the current study was to evaluate the possibility of ginsenoside Rf as an active ingredient of KRG in the inhibition of hypoxia-induced COX-2 via PPARγ. METHODS: The effects of ginsenoside Rf on the upregulation of COX-2 by hypoxia and its antimigration effects were evaluated in A549 cells. Docking of ginsenoside Rf was performed with the PPARγ structure using Surflex-Dock in Sybyl-X 2.1.1. RESULTS: PPARγ protein levels and peroxisome proliferator response element promoter activities were promoted by ginsenoside Rf. Inhibition of COX-2 expression by ginsenoside Rf was blocked by the PPARγ-specific inhibitor, T0070907. The PPARγ inhibitor also blocked the ability of ginsenoside Rf to suppress cell migration under hypoxia. The docking simulation results indicate that ginsenoside Rf binds to the active site of PPARγ. CONCLUSIONS: Our results demonstrate that ginsenoside Rf inhibits hypoxia induced-COX-2 expression and cellular migration, which are dependent on PPARγ activation. These results suggest that ginsenoside Rf has an antiinflammatory effect under hypoxic conditions. Moreover, docking analysis of ginsenoside Rf into the active site of PPARγ suggests that the compound binds to PPARγ in a position similar to that of known agonists.

Ginseng on Nuclear Hormone Receptors.

The first record of ginseng use dates back over two millennia, and ginseng is now popular in more than 35 countries. Ginsenosides are the pharmacological constituents responsible for the beneficial effects of ginseng. There is increasing evidence that ginseng and its bioactive ingredients are involved in the regulation of nuclear receptors, molecules that act in response to the specific binding of hormones, which link to a diverse array of signaling pathways, such as the ERK and PI3K/Akt pathways. Knowledge of the mechanism of how ginseng mediates these complexes is essential for the development of multi-target phytomedicine as possible therapy for different diseases. Here, we discuss the literature on the effects of ginseng and its constituents on estrogen, glucocorticoid, peroxisome proliferator-activated, and androgen nuclear hormone receptors, as well as how ginseng and its constituents exert their biological function in the treatment of cancer, obesity, and cardiovascular and neurological disorders. The accumulated results definitely show that the nuclear receptors are cellular targets of ginsenosides, but more rigorous data are required to establish and provide a scientific basis to confirm the suggested efficacy of ginseng or products with ginsenosides.

4,5-Di-O-Caffeoylquinic Acid from Ligularia fischeri Suppresses Inflammatory Responses Through TRPV1 Activation.

Ligularia fischeri (Ledeb.) Turcz., a perennial plant native to northeastern Asia, has long been used as folk remedies for the alleviation of inflammatory symptoms. We investigated whether the extract of L. fischeri (LFEx) and caffeoylquinic acid (CQA) derivatives, the pharmacologically active ingredients identified from L. fischeri, regulate inflammation via a transient receptor potential vanilloid 1 (TRPV1)-mediated pathway. Changes in intracellular Ca2+ levels to the LFEx and trans-5-O-CQA, 3,4-di-O-CQA, 3,5-di-O-CQA, and 4,5-di-O-CQA were monitored in TRPV1-expressing human embryonic kidney cell HEK 293T. LFEx and 4,5-di-O-CQA (EC50  = 69.34 ± 1.12 µM) activated TRPV1, and these activations were significantly inhibited by ruthenium red, a general blocker of TRP channels, and capsazepine, a specific antagonist of TRPV1. 4,5-Di-O-CQA has been determined having antiinflammatory effect under hypoxic conditions by detecting the expression of cyclooxygenase-2 (COX-2), a representative inflammatory marker, and cellular migration in human pulmonary epithelial A549 cells. 4,5-Di-O-CQA suppressed COX-2 expression and cell migration, and this inhibition was countered by co-treatment with capsazepine. This study provides evidence that L. fischeri is selective to inflammatory responses via a TRPV1-mediated pathway, and 4,5-di-O-CQA might play a key role to create these effects. Copyright © 2017 John Wiley & Sons, Ltd.

Identification of a key umami-active fraction in modernized Korean soy sauce and the impact thereof on bitter-masking.

Food protein hydrolysates created by natural fermentation have been used for centuries as food flavorings. The aim of this study was to define the key umami-active fraction of modernized Korean soy sauce (mJGN) and the impact thereof on bitter-masking of human sensory and bitter-taste receptor-expressing cells. We found strong correlations between taste profiles of mJGN and a contained fraction (F05). The latter contained compounds of less than 500Da, and elicits a distinct umami taste. Both free amino acids and Glu-enriched oligopeptides are suggested to be crucial in terms of the effects of F05 on taste. F05 not only reduced human-perceived bitterness, but also effectively suppressed the intracellular Ca2+ response induced by caffeine in the hTAS2R43 and hTAS2R46 human bitter-taste receptor-expressing cells. This suggests that F05, a key umami-active fraction of mJGN, contains components that at least partially modulate human bitter-taste receptor action, improving food flavor.

Cyclic-AMP regulates postnatal development of neural and behavioral responses to NaCl in rats.

During postnatal development rats demonstrate an age-dependent increase in NaCl chorda tympani (CT) responses and the number of functional apical amiloride-sensitive epithelial Na+ channels (ENaCs) in salt sensing fungiform (FF) taste receptor cells (TRCs). Currently, the intracellular signals that regulate the postnatal development of salt taste have not been identified. We investigated the effect of cAMP, a downstream signal for arginine vasopressin (AVP) action, on the postnatal development of NaCl responses in 19-23 day old rats. ENaC-dependent NaCl CT responses were monitored after lingual application of 8-chlorophenylthio-cAMP (8-CPT-cAMP) under open-circuit conditions and under ±60 mV lingual voltage clamp. Behavioral responses were tested using 2 bottle/24h NaCl preference tests. The effect of [deamino-Cys1, D-Arg8]-vasopressin (dDAVP, a specific V2R agonist) was investigated on ENaC subunit trafficking in rat FF TRCs and on cAMP generation in cultured adult human FF taste cells (HBO cells). Our results show that in 19-23 day old rats, the ENaC-dependent maximum NaCl CT response was a saturating sigmoidal function of 8-CPT-cAMP concentration. 8-CPT-cAMP increased the voltage-sensitivity of the NaCl CT response and the apical Na+ response conductance. Intravenous injections of dDAVP increased ENaC expression and γ-ENaC trafficking from cytosolic compartment to the apical compartment in rat FF TRCs. In HBO cells dDAVP increased intracellular cAMP and cAMP increased trafficking of γ- and δ-ENaC from cytosolic compartment to the apical compartment 10 min post-cAMP treatment. Control 19-23 day old rats were indifferent to NaCl, but showed clear preference for appetitive NaCl concentrations after 8-CPT-cAMP treatment. Relative to adult rats, 14 day old rats demonstrated significantly less V2R antibody binding in circumvallate TRCs. We conclude that an age-dependent increase in V2R expression produces an AVP-induced incremental increase in cAMP that modulates the postnatal increase in TRC ENaC and the neural and behavioral responses to NaCl.

Methyl syringate, a TRPA1 agonist represses hypoxia-induced cyclooxygenase-2 in lung cancer cells.

BACKGROUND: We have previously found that methyl syringate is a specific and selective agonist of the human transient receptor potential channel ankyrin 1 (TRPA1) and suppresses food intake and gastric emptying in imprinting control region mice. Because TRPA1 has been implicated in inflammatory responses, and inflammation and tumorigenesis are stimulated by the cyclooxygenase-2 (COX-2)/prostaglandin E2 pathway in hypoxic cancer cells. PURPOSE: This study examined the effects of methyl syringate on hypoxia-induced COX-2 in human distal lung epithelial A549 cells. STUDY DESIGN: The effect of the methyl syringate on suppression of hypoxia-induced COX-2 in A549 cells were determined by Western blot and/or quantitative real-time polymerase chain reaction. The anti-invasive effect of methyl syringate was evaluated on A549 cells using matrigel invasion assay. RESULTS: Methyl syringate suppressed hypoxia-induced COX-2 protein and mRNA expression and promoter activity and reduced hypoxia-induced cell migration and invasion and secretion of vascular endothelial growth factor. These effects were antagonized by a TRPA1 antagonist, implying their mediation by the TRPA1 pathway. CONCLUSION: Together, these results indicate that methyl syringate inhibits the hypoxic induction of COX-2 expression and cell invasion through TRPA1 activation. These findings suggest that methyl syringate could be effective to suppress hypoxia-induced inflammation and indicate an additional functional effect of methyl syringate.

Limonin, a Component of Dictamni Radicis Cortex, Inhibits Eugenol-Induced Calcium and cAMP Levels and PKA/CREB Signaling Pathway in Non-Neuronal 3T3-L1 Cells.

Limonin, one of the major components in dictamni radicis cortex (DRC), has been shown to play various biological roles in cancer, inflammation, and obesity in many different cell types and tissues. Recently, the odorant-induced signal transduction pathway (OST) has gained attention not only because of its function in the perception of smell but also because of its numerous physiological functions in non-neuronal cells. However, little is known about the effects of limonin and DRC on the OST pathway in non-neuronal cells. We investigated odorant-stimulated increases in Ca(2+) and cAMP, major second messengers in the OST pathway, in non-neuronal 3T3-L1 cells pretreated with limonin and ethanol extracts of DRC. Limonin and the extracts significantly decreased eugenol-induced Ca(2+) and cAMP levels and upregulated phosphorylation of CREB and PKA. Our results demonstrated that limonin and DRC extract inhibit the OST pathway in non-neuronal cells by modulating Ca(2+) and cAMP levels and phosphorylation of CREB.

Five hTRPA1 Agonists Found in Indigenous Korean Mint, Agastache rugosa.

Transient receptor potential ankyrin1 (TRPA1) and transient receptor potential vanilloid 1 (TRPV1) are members of the TRP superfamily of structurally related, nonselective cation channels and mediators of several signaling pathways. Previously, we identified methyl syringate as an hTRPA1 agonist with efficacy against gastric emptying. The aim of this study was to find hTRPA1 and/or hTRPV1 activators in Agastache rugosa (Fisch. et Meyer) O. Kuntze (A.rugosa), commonly known as Korean mint to improve hTRPA1-related phenomena. An extract of the stem and leaves of A.rugosa (Labiatae) selectively activated hTRPA1 and hTRPV1. We next investigated the effects of commercially available compounds found in A.rugosa (acacetin, 4-allylanisole, p-anisaldehyde, apigenin 7-glucoside, L-carveol, ß-caryophyllene, trans-p-methoxycinnamaldehyde, methyl eugenol, pachypodol, and rosmarinic acid) on cultured hTRPA1- and hTRPV1-expressing cells. Of the ten compounds, L-carveol, trans-p-methoxycinnamaldehyde, methyl eugenol, 4-allylanisole, and p-anisaldehyde selectively activated hTRPA1, with EC50 values of 189.1±26.8, 29.8±14.9, 160.2±21.9, 1535±315.7, and 546.5±73.0 µM, respectively. The activities of these compounds were effectively inhibited by the hTRPA1 antagonists, ruthenium red and HC-030031. Although the five active compounds showed weaker calcium responses than allyl isothiocyanate (EC50=7.2±1.4 µM), our results suggest that these compounds from the stem and leaves of A.rugosa are specific and selective agonists of hTRPA1.

Regulatory Effects of Ca2+ and H+ on the Rat Chorda Tympani Response to NaCl and KCl.

Modulatory effects of pHi and [Ca(2+)]i on taste receptor cell (TRC) epithelial sodium channel (ENaC) were investigated by monitoring chorda tympani (CT) responses to NaCl and KCl at various lingual voltages, before and after lingual application of ionomycin and with 0-10mM CaCl2 in the stimulus and rinse solutions adjusted to pHo 2.0-9.7. 0.1 and 0.5M KCl responses varied continuously with voltage and were fitted to an apical ion channel kinetic model using the same parameters. ENaC-dependent NaCl CT response was fitted to the same channel model but with parameters characteristic of ENaC. A graded increase in TRC [Ca(2+)]i decreased the ENaC-dependent NaCl CT response, and inhibited and ultimately eliminated its pH sensitivity. CT responses to KCl were pHi- and [Ca(2+)]i-independent. Between ±60 mV applied lingual potential, the data were well described by a linear approximation to the nonlinear channel equation and yielded 2 parameters, the open-circuit response and the negative of the slope of the line in the CT response versus voltage plot, designated the response conductance. The ENaC-dependent NaCl CT response conductance was a linear function of the open-circuit response for all pHi-[Ca(2+)]i combinations examined. Analysis of these data shows that pHi and [Ca(2+)]i regulate TRC ENaC exclusively through modulation of the maximum CT response.

Modulation of sweet taste by umami compounds via sweet taste receptor subunit hT1R2.

Although the five basic taste qualities-sweet, sour, bitter, salty and umami-can be recognized by the respective gustatory system, interactions between these taste qualities are often experienced when food is consumed. Specifically, the umami taste has been investigated in terms of whether it enhances or reduces the other taste modalities. These studies, however, are based on individual perception and not on a molecular level. In this study we investigated umami-sweet taste interactions using umami compounds including monosodium glutamate (MSG), 5'-mononucleotides and glutamyl-dipeptides, glutamate-glutamate (Glu-Glu) and glutamate-aspartic acid (Glu-Asp), in human sweet taste receptor hT1R2/hT1R3-expressing cells. The sensitivity of sucrose to hT1R2/hT1R3 was significantly attenuated by MSG and umami active peptides but not by umami active nucleotides. Inhibition of sweet receptor activation by MSG and glutamyl peptides is obvious when sweet receptors are activated by sweeteners that target the extracellular domain (ECD) of T1R2, such as sucrose and acesulfame K, but not by cyclamate, which interact with the T1R3 transmembrane domain (TMD). Application of umami compounds with lactisole, inhibitory drugs that target T1R3, exerted a more severe inhibitory effect. The inhibition was also observed with F778A sweet receptor mutant, which have the defect in function of T1R3 TMD. These results suggest that umami peptides affect sweet taste receptors and this interaction prevents sweet receptor agonists from binding to the T1R2 ECD in an allosteric manner, not to the T1R3. This is the first report to define the interaction between umami and sweet taste receptors.

Expression of human olfactory receptor 10J5 in heart aorta, coronary artery, and endothelial cells and its functional role in angiogenesis.

ORs are ectopically expressed in non-chemosensory tissues including muscle, kidney, and keratinocytes; however, their physiological roles are largely unknown. We found that human olfactory receptor 10J5 (OR10J5) is expressed in the human aorta, coronary artery, and umbilical vein endothelial cells (HUVEC). Lyral induces Ca(2+) and phosphorylation of AKT in HUVEC. A knockdown study showed the inhibition of the lyral-induced Ca(2+) and the phosphorylation AKT and implied that these processes are mediated by OR10J5. In addition, lyral enhanced migration of HUVEC, which were also inhibited by RNAi in a migration assay. In addition, matrigel plug assay showed that lyral enhanced angiogenesis in vivo. Together these data demonstrate the physiological role of OR10J5 in angiogenesis and represent roles of ORs in HUVEC cells.

Piperine, a component of black pepper, decreases eugenol-induced cAMP and calcium levels in non-chemosensory 3T3-L1 cells.

This study investigated the effects of an ethanol extract of black pepper and its constituent, piperine, on odorant-induced signal transduction in non-chemosensory cells. An ethanol extract of black pepper decreased eugenol-induced cAMP and calcium levels in preadipocyte 3T3-L1 cells with no toxicity. Phosphorylation of CREB (cAMP response element-binding protein) was down-regulated by the black pepper extract. The concentration (133.8 mg/g) and retention time (5.5 min) of piperine in the ethanol extract were quantified using UPLC-MS/MS. Pretreatment with piperine decreased eugenol-induced cAMP and calcium levels in 3T3-L1 cells. Piperine also decreased the phosphorylation of CREB, which is up-regulated by eugenol. These results suggest that piperine inhibits the eugenol-induced signal transduction pathway through modulation of cAMP and calcium levels and phosphorylation of CREB in non-chemosensory cells.

Different spatial expressions of c-Fos in the nucleus of the solitary tract following taste stimulation with sodium, potassium, and ammonium ions in rats.

Cation-specific epithelial receptors on the tongue have been well demonstrated. However, active regions along the nucleus of the solitary tract (NST) for cations Na(+), K(+), NH4(+) are still unclear, even though the best responses of NST neurons to taste stimuli vary depending on the cell. In the present study, the spatial distribution patterns of cation-specific active regions in the NST are investigated. The tongues of urethane-anesthetized Sprague-Dawley rats (n = 25) were stimulated with artificial saliva (control), 0.5 M NaCl, 1.0 M NaCl, 0.5 M KCl, and 0.3 M NH(4) Cl. Then, the three-dimensional positions of c-Fos-like-immunoreactive (cFLI) cells in the NST were generated. The spatial distributions of cFLI cells in the NST were compared among five taste stimulations. cFLI cells were observed throughout the NST, irrespective of the stimulus; however, the intermediate-medial central regions of the NST had higher numbers of cFLI cells than the other regions in all taste stimulations. Analysis of images revealed that the activated regions in the NST differed significantly depending on the cations. The intermediate-dorsal-central region and the caudal-ventral region were activated by a 0.5 M concentration of sodium, the rostral-ventral region and the intermediate-dorsal/ventral region were activated by a 1.0 M concentration of sodium, the intermediate-dorsal/ventral region was activated by potassium ions, and the rostral-ventral region and the intermediate-ventral central region were activated by ammonium ions. These results suggest that the responses of NST cells to cation salt ions are regulated differentially.

Umami-bitter interactions: the suppression of bitterness by umami peptides via human bitter taste receptor.

Taste-taste interactions often showed in human psychophysical studies. Considering that each tastant in foodstuffs individually stimulates its responsible gustatory systems to elicit relevant taste modalities, taste-taste interaction should be performed in taste receptor cell-based assay. While umami substances have been proposed to suppress the bitterness of various chemicals in human sensory evaluation, the bitter-umami interaction has not been explored in bitter taste receptors, TAS2Rs. We investigated umami-bitter taste interactions by presenting umami peptides with bitter substance (salicin) on Ca(2+)-flux signaling assay using hTAS2R16-expressing cells. Five representative umami peptides (Glu-Asp, Glu-Glu, Glu-Ser, Asp-Glu-Ser, and Glu-Gly-Ser) derived from soybean markedly attenuated the salicin-induced intracellular calcium influx in a time-dependent manner, respectively, while Gly-Gly, a tasteless peptide did not. The efficacies of Glu-Glu suppressing salicin-induced activation of hTAS2R16 were higher than that of probenecid, a specific antagonist of hTAS2R16. According to Ca(2+)-flux signaling assay using the mixtures of salicin and umami peptides, all five umami peptides suppressed salicin-induced intracellular calcium influx in a noncompetitive manner. These results may provide evidence that umami peptides suppress bitter taste via bitter taste receptor(s). This is the first report which defines the interaction between bitter and umami taste in taste receptor level.

Spergularia marina induces glucagon-like peptide-1 secretion in NCI-H716 cells through bile acid receptor activation.

Spergularia marina Griseb. (SM) is a halophyte that grows in mud flats. The aerial portions of SM have been eaten as vegetables and traditionally used to prevent chronic diseases in Korea. However, there has been no scientific report that demonstrates the pharmacological effects of SM. Glucagon-like peptide-1 (GLP-1) is important for the maintenance of glucose and energy homeostasis through acting as a signal in peripheral and neural systems. To discover a functional food for regulating glucose and energy homeostasis, we evaluated the effect of an aqueous ethanolic extract (AEE) of SM on GLP-1 release from enteroendocrine NCI-H716 cells. In addition, we explored the Takeda G-protein-coupled receptor 5 (TGR5) agonist activity of AEE-SM in Chinese hamster ovary (CHO)-K1 cells transiently transfected with human TGR5. As a result, treatment of NCI-H716 cells with AEE-SM increased GLP-1 secretion and intracellular Ca(2+) and cyclic AMP (cAMP) levels in a dose-dependent manner. Transfection of NCI-H716 cells with TGR5-specific small interference RNA inhibited AEE-SM-induced GLP-1 secretion and the increase in Ca(2+) and cAMP levels. Moreover, AEE-SM showed that the TGR5 agonist activity in CHO-K1 cells transiently transfected with TGR5. The results suggest that AEE-SM might be a candidate for a functional food to regulate glucose and energy homeostasis.