Astringent Gallic Acid in Red Wine Regulates Mechanisms of Gastric Acid Secretion via Activation of Bitter Taste Sensing Receptor TAS2R4.

Red wine is rich in phenolic compounds, which chiefly determine its characteristic taste. One of its major phenolic acid constituents for which an astringency, yet no clear contribution to bitter taste has been reported, is gallic acid (GA). In previous studies, we have demonstrated bitter-tasting constituents to regulate cellular proton secretion (PS) as a key mechanism of gastric acid secretion via activation of bitter taste sensing receptors (TAS2Rs). Here, we hypothesized a contributing role of GA to the red wine-stimulated effect on PS in human gastric tumor cells (HGT-1 cells). Sensory analyses revealed that 10 µM GA as the lowest concentration tested more bitter than tap water, with increasing bitter ratings up to 1000 µM. In HGT-1 cells, the concentration of 10 µM GA evoked the most pronounced effect on PS secretion, either when added to cells as in-water solution or when spiked to a red wine matrix. GA-spiking of Zweigelt and Blaufränkisch red wine samples up to a concentration of 10 µM resulted in an equally stimulated PS, whereas the non-GA-spiked wine samples demonstrated contrary effects on PS, indicating a functional role of GA on PS. Involvement of TAS2R4 in the GA-induced PS was verified by means of an HGT-1 homozygote CRISPR-Cas9 TAS2R4 knockout approach. Moreover, gene expression analyses revealed GA to increase TAS2R4. These results demonstrate a functional role of TAS2R4 in GA-evoked PS as a key mechanism of gastric acid secretion aiding digestion. Moreover, our data provide mechanistic insights, which will help to produce stomach-friendly red wines.

Gastric Serotonin Biosynthesis and Its Functional Role in L-Arginine-Induced Gastric Proton Secretion.

Among mammals, serotonin is predominantly found in the gastrointestinal tract, where it has been shown to participate in pathway-regulating satiation. For the stomach, vascular serotonin release induced by gastric distension is thought to chiefly contribute to satiation after food intake. However, little information is available on the capability of gastric cells to synthesize, release and respond to serotonin by functional changes of mechanisms regulating gastric acid secretion. We investigated whether human gastric cells are capable of serotonin synthesis and release. First, HGT-1 cells, derived from a human adenocarcinoma of the stomach, and human stomach specimens were immunostained positive for serotonin. In HGT-1 cells, incubation with the tryptophan hydroxylase inhibitor p-chlorophenylalanine reduced the mean serotonin-induced fluorescence signal intensity by 27%. Serotonin release of 147 ± 18%, compared to control HGT-1 cells (set to 100%) was demonstrated after treatment with 30 mM of the satiating amino acid L-Arg. Granisetron, a 5-HT3 receptor antagonist, reduced this L-Arg-induced serotonin release, as well as L-Arg-induced proton secretion. Similarly to the in vitro experiment, human antrum samples released serotonin upon incubation with 10 mM L-Arg. Overall, our data suggest that human parietal cells in culture, as well as from the gastric antrum, synthesize serotonin and release it after treatment with L-Arg via an HTR3-related mechanism. Moreover, we suggest not only gastric distension but also gastric acid secretion to result in peripheral serotonin release.

Identification of Bitter-Taste Intensity and Molecular Weight as Amino Acid Determinants for the Stimulating Mechanisms of Gastric Acid Secretion in Human Parietal Cells in Culture.

Secretion of gastric acid, aimed at preventing bacterial growth and aiding the digestion of foods in the stomach, is chiefly stimulated by dietary intake of protein and amino acids (AAs). However, AAs' key structural determinants responsible for their effects on mechanisms regulating gastric acid secretion (GAS) have not been identified yet. In this study, AAs have been tested in the parietal cell model HGT-1 on GAS and on mRNA expression of genes regulating GAS. AAs' taste intensities from 0 (not bitter at all) to 10 (very bitter) were assessed in a sensory study, in which ARG (l: 6.42 ± 0.41; d: 4.62 ± 0.59) and ILE (l: 4.21 ± 0.43; d: 2.28 ± 0.33) were identified as bitter-tasting candidates in both isomeric forms. Pearson correlation showed that GAS in HGT-1 cells is directly associated with the bitter taste quality ( r: -0.654) in combination with the molecular weight of l-AA ( r: -0.685).

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.