Measurement of the intracellular ph in human stomach cells: a novel approach to evaluate the gastric acid secretory potential of coffee beverages

Identification of mozambioside roasting products and their bitter taste receptor activation

Arabica coffee contains the bitter-tasting diterpene glycoside mozambioside, which degrades during coffee roasting, leading to yet unknown structurally related degradation products with possibly similar bitter-receptor-activating properties. The study aimed at the generation, isolation, and structure elucidation of individual pyrolysis products of mozambioside and characterization of bitter receptor activation by in vitro analysis in HEK 293T-Gα16gust44 cells. The new compounds 17-O-β-d-glucosyl-11-hydroxycafestol-2-on, 11-O-β-d-glucosyl-16-desoxycafestol-2-on, 11-O-β-d-glucosyl-(S)-16-desoxy-17-oxocafestol-2-on, 11-O-β-d-glucosyl-15,16-dehydrocafestol-2-on, and 11-O-β-d-glucosyl-(R)-16-desoxy-17-oxocafestol-2-on were isolated from pyrolyzed mozambioside by HPLC and identified by NMR and UHPLC-ToF-MS. Roasting products 11-O-β-d-glucosyl-(S)-16-desoxy-17-oxocafestol-2-on, 11-O-β-d-glucosyl-15,16-dehydrocafestol-2-on, and 11-O-β-d-glucosyl-(R)-16-desoxy-17-oxocafestol-2-on had lower bitter receptor activation thresholds compared to mozambioside. Molecular docking simulations revealed the binding modes of the compounds 11-O-β-d-glucosyl-15,16-dehydrocafestol-2-on and 11-O-β-d-glucosyl-(R)-16-desoxy-17-oxocafestol-2-on and their aglycone 11-hydroxycafestol-2-on in the two cognate receptors TAS2R43 and TAS2R46. The newly discovered roasting products 17-O-β-d-glucosyl-11-hydroxycafestol-2-on, 11-O-β-d-glucosyl-(S)-16-desoxy-17-oxocafestol-2-on, 11-O-β-d-glucosyl-15,16-dehydrocafestol-2-on, and 11-O-β-d-glucosyl-(R)-16-desoxy-17-oxocafestol-2-on were detected in authentic roast coffee brew by UHPLC-ToF-MS and could contribute to coffee's bitter taste impression.

Extracellular Vesicles and Particles Modulate Proton Secretion in a Model of Human Parietal Cells

The secretion of extracellular vesicles and particles (EVPs) is an important mechanism of cellular communication. In this work, we demonstrate a functional role of EVPs in mechanisms regulating gastric acid secretion. HGT-1 cells were used as a model system to assess proton secretion. First, in order to prove EVP secretion by HGT-1 cells, EVPs were isolated by size exclusion chromatography and characterized by nanoparticle tracking analysis, Western blot, and cryo transmission electron microscopy. For examination of the potential role of EVPs in proton secretion, HGT-1 cells were treated with pharmacological EV-inhibitors, resulting in a reduction of histamine-induced proton secretion. To demonstrate the functional role of EVPs in the mechanism of proton secretion, EVP-conditioned supernatant was collected after stimulation of HGT-1 cells with histamine, fractionated, and subjected to an activity screening. The results revealed constituents of the HGT-1-derived secretome with an MW of >100 kDa (including EVPs) to modulate proton secretion, while smaller constituents had no effect. Finally, a dose-dependent modulatory effect on proton secretion of HGT-1 cells was demonstrated by isolated HGT-1-derived EVPs. Hence, this study presents first results on the potential function of EVPs as a previously undiscovered mechanism of regulation of gastric acid secretion by parietal cells.

Bitter Sensing TAS2R50 Mediates the trans-Resveratrol-Induced Anti-inflammatory Effect on Interleukin 6 Release in HGF-1 Cells in Culture

Recent data have shown anti-inflammatory effects for trans-resveratrol (RSV) and rosmarinic acid (RA) in various immune-competent cell models through reduction of lipopolysaccharide (LPS)-induced interleukin 6 (IL-6) release. Because both compounds have been reported to taste bitter, we hypothesized an involvement of human bitter taste sensing receptors (TAS2Rs) on IL-6 release in LPS-treated human gingival fibroblasts (HGF-1). First, the bitter taste intensity of RSV and RA was compared in a sensory trial with 10 untrained panelists, of whom 90% rated a 50 ppm of RSV in water solution more bitter than 50 ppm of RA. A mean 19 ± 6% reduction of the RSV-induced bitter taste intensity was achieved by co-administration of 50 ppm of the bitter-masking, TAS2R43 antagonist homoeriodictyol (HED). Mechanistic experiments in a stably CRISPR-Cas9-edited TAS2R43ko gastric cell model revealed involvement of TAS2R43 in the HED-evoked effect on RSV-induced proton secretion, whereas the cellular response to RSV did not depend upon TAS2R43. Next, the IL-6 modulatory effect of 100 μM RSV was studied in LPS-treated immune-competent HGF-1 cells. After 6 h of treatment, RSV reduced the LPS-induced IL-6 gene expression and protein release by -46.2 ± 12.7 and -73.9 ± 2.99%, respectively. This RSV-evoked effect was abolished by co-administration of HED. Because real-time quantitative polymerase chain reaction analyses revealed a regulation of TAS2R50 in RSV with or without HED-treated HGF-1 cells, an siRNA knockdown approach of TAS2R50 was applied to verify TAS2R50 involvement in the RSV-induced reduction of the LPS-evoked IL-6 release in HGT-1 cells.

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.

Reduction of βN-alkanoyl-5-hydroxytryptamides and diterpenes by yeast supplementation to green coffee during wet processing

Coffee is one of the most consumed non-alcoholic beverages in the world. It is well known that some compounds present in coffee beans have important biological activities. In this study, evidence was turned to ^βN-alkanoyl-5-hydroxytryptamides (C-5HTs) and to the furokaurane diterpenes cafestol and kahweol, associated with gastric irritation and increasing of blood cholesterol, respectively. Fermentation in coffee post-harvest wet process was induced by three Saccharomyces cerevisiae yeasts (for bakery, white and sparkling wines) as starter cultures. Variations in mass, time, temperature and pH (56 experiments under fractional factorial and mixture experimental designs) were tested. Substantial reductions for C-5HTs (up to 38% reduction for C20-5HT and 26% for C22-5HT) as well as for diterpenes (54% for cafestol and 53% for kahweol) were obtained after treating green coffee beans with 0.6 g of a 1:1:1 mixture the three yeasts for 12 h at 15 °C and pH 4. Caffeine and 5-CQA content, monitored in the green coffee beans, did not change. Therefore, the use of starter cultures during coffee post-harvest wet process has influence on the amount of some important compounds related to health and improves the sensory quality of the beverage.

Impact of free Nε-carboxymethyllysine, its precursor glyoxal and AGE-modified BSA on serotonin release from human parietal cells in culture

Advanced glycation end products (AGEs) are frequently encountered in a western diet, in addition to their formation in vivo. N-Epsilon-carboxymethyllysine (CML), one of the chemically diverse compounds formed in the reaction between reducing carbohydrates and amines, is often used as a marker of advanced glycation, and has been shown to stimulate serotonin release from cells representing the central (SH-SY5Y cells) and the peripheral (Caco-2 cells) serotonin system in vitro. Here, we investigated the effect of glyoxal, free CML, and protein-linked AGE-BSA on serotonin release from human gastric tumour cells, which originate from an adenocarcinoma of the stomach and have recently been shown to be capable of serotonin synthesis and release. Microarray experiments showed both CML and glyoxal to alter genes associated with serotonin receptors. Furthermore, treatment with glyoxal resulted in a small change in RAGE expression while CML did not alter its expression. On a functional level, treatment with 500 μM CML increased extracellular serotonin content by 341 ± 241%, while treatment with 1 mg mL-1 AGE-BSA led to a reduction by 49 ± 11% compared to non-treated cells. The CML-induced serotonin release was reduced by the HTR3 antagonist granisetron. Incubation with the RAGE antagonist FPS-ZM1 abolished the effect of AGE-BSA on serotonin release, while no impact on CML-induced serotonin release was observed. Furthermore, treatment with 5 mM CML stimulated proton secretion as a functional outcome measure, assessed using a pH sensitive dye. Taken together, these results indicate a likely HTR3-mediated, RAGE-independent effect of free CML on serotonin release and a RAGE-dependent mechanism for the protein linked AGE-BSA.

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.

Characterization of Bitter Compounds via Modulation of Proton Secretion in Human Gastric Parietal Cells in Culture

Humans perceive bitterness via around 25 different bitter receptors. Therefore, the identification of antagonists remains a complex challenge. We previously demonstrated several bitter-tasting compounds such as caffeine to induce acid secretion in the stomach and in a human gastric tumor cell line (HGT-1). Here, the results of a fluorescent-based in vitro assay using HGT-1 cells and a human sensory panel testing nine selected potential bitter modulators, with or without the bitter compounds caffeine or theobromine, were compared. Of the bitter-modulating compounds tested, eriodictyol, matairesinol, enterolacton, lariciresinol, and homoeriodictyol reduced the effect of caffeine on proton secretion by -163 ± 14.0, -152 ± 12.4, -74 ± 16.4, -58 ± 7.2, and -44.6 ± 16.5%, respectively, and reduced the bitter intensity of caffeine in the human sensory panel. In contrast, naringenin and 5,7-dihydroxy-4(4-hydroxyphenyl)chroman-2-one neither reduced the caffeine-induced proton secretion in HGT-1 cells nor showed an effect on bitter intensity perceived by the sensory panel. Results for theobromine were not as pronounced as those for caffeine, but followed a similar trend. The results demonstrate that the HGT-1 in vitro assay is a useful tool to identify potential bitter-masking compounds. Nevertheless, a sensory human panel is necessary to quantify the bitter-masking potency.

Identification of Catechin, Syringic Acid, and Procyanidin B2 in Wine as Stimulants of Gastric Acid Secretion

Organic acids of wine, in addition to ethanol, have been identified as stimulants of gastric acid secretion. This study characterized the influence of other wine compounds, particularly phenolic compounds, on proton secretion. Forty wine parameters were determined in four red wines and six white wines, including the contents of organic acids and phenolic compounds. The secretory activity of the wines was determined in a gastric cell culture model (HGT-1 cells) by means of a pH-sensitive fluorescent dye. Red wines stimulated proton secretion more than white wines. Lactic acid and the phenolic compounds syringic acid, catechin, and procyanidin B2 stimulated proton secretion and correlated with the pro-secretory effect of the wines. Addition of the phenolic compounds to the least active white wine sample enhanced its proton secretory effect by 65 ± 21% (p < 0.05). These results indicate that not only malic and lactic acid but also bitter and astringent tasting phenolic compounds in wine contribute to its stimulatory effect on gastric acid secretion.

Modulation of 3',5'-cyclic AMP homeostasis in human platelets by coffee and individual coffee constituents

3',5'-Cyclic AMP (cAMP) is one of the most important second messengers in mammalian cells, mediating a multitude of diverse cellular signalling responses. Its homeostasis is primarily regulated by adenylate cyclases and phosphodiesterases (PDE), the activities of which are partially dependent on the downstream events of adenosine receptor signalling. The present study was conducted to determine whether coffee constituents other than caffeine can influence the homeostasis of intracellular cAMP in vitro and in vivo by evaluating the effects of selected constituents present in coffee, coffee brews and coffee extracts on platelet PDE activity. In addition, to evaluate the potential effects of these constituents on platelet cAMP concentrations and PDE activity in humans, a 7-week pilot intervention study with eight subjects was conducted. The subjects consumed a regular commercial coffee and a low-caffeine coffee at a rate of 750 ml/d for 2 weeks each. The in vivo results revealed a highly significant inhibition of PDE activity (P< 0·001) after coffee intervention that was not directly dependent on the caffeine content of coffee. Although our in vitro and in vivo findings suggest that caffeine plays some role in the modulation of platelet cAMP status, other natural and roasting-associated compounds such as pyrazines and other currently unidentified species also appear to contribute significantly. In conclusion, moderate consumption of coffee can modulate platelet PDE activity and cAMP concentrations in humans, which may contribute to the putative beneficial health effects of coffee. Further detailed mechanistic investigations will be required to substantiate these beneficial effects and to elucidate the underlying mechanisms.

Identification of organic acids in wine that stimulate mechanisms of gastric acid secretion

Wine may cause stomach irritation due to its stimulatory effect on gastric acid secretion, although the mechanisms by which wine or components thereof activate pathways of gastric acid secretion are poorly understood. Gastric pH was measured with a noninvasive intragastric probe, demonstrating that administration of 125 mL of white or red wine to healthy volunteers stimulated gastric acid secretion more potently than the administration of equivalent amounts of ethanol. Between both beverages, red wine showed a clear trend for being more active in stimulating gastric acid secretion than white wine (p = 0.054). Quantification of the intracellular proton concentration in human gastric tumor cells (HGT-1), a well-established indicator of proton secretion and, in turn, stomach acid formation in vivo, confirmed the stronger effect of red wine as compared to white wine. RT-qPCR experiments on cells exposed to red wine also revealed a more pronounced effect than white wine on the fold change expression of genes associated with gastric acid secretion. Of the quantitatively abundant organic acids in wine, malic acid and succinic acid most actively stimulated proton secretion in vitro. However, addition of ethanol to individual organic acids attenuated the secretory effect of tartaric acid, but not that of the other organic acids. It was concluded that malic acid for white wine and succinic acid for red wine are key organic acids that contribute to gastric acid stimulation.

Identification of beer bitter acids regulating mechanisms of gastric acid secretion

Beer, one of the most consumed beverages worldwide, has been shown to stimulate gastric acid secretion. Although organic acids, formed by fermentation of glucose, are known to be stimulants of gastric acid secretion, very little is known about the effects of different types of beer or the active constituents thereof. In the present study, we compared the effects of different beers on mechanisms of gastric acid secretion. To investigate compound-specific effects on mechanisms of gastric acid secretion, organic acids and bitter compounds were quantified by HPLC-DAD and UPLC-MS/MS and tested in human gastric cancer cells (HGT-1) by means of a pH-sensitive fluorescent dye which determines the intracellular pH as an indicator of proton secretion. The expression of relevant genes, coding the H(+)/K(+)-ATPase, ATP4A, the histamine receptor, HRH2, the acetylcholine receptor, CHRM3, and the somatostatin receptor, SSTR2, was determined by qPCR. Ethanol and the organic acids succinic acid, malic acid, and citric acid were demonstrated to contribute to some extent to the effect of beer. The bitter acids comprising α-, β-, and iso-α-acids were identified as potential key components promoting gastric acid secretion and up-regulation of CHRM3 gene expression by a maximum factor of 2.01 compared to that of untreated control cells with a correlation to their respective bitterness.

Multi-parametric approach to identify coffee components that regulate mechanisms of gastric acid secretion

SCOPE

Chlorogenic acid (CA), caffeine (CAFF), pyrogallol (PYR), catechol (CAT), (β)N-alkanoyl-hydroxytryptamides (C5HT) and N-methylpyridinium (N-MP) were evaluated for their influence on mechanisms of gastric acid secretion as single compounds and in biomimetic mixtures.

METHODS AND RESULTS

Compounds were tested in coffee representative concentrations. Human gastric cancer cells (HGT-1) were used to study the proton secretory activity by Ussing chamber experiments and FACS analysis. For activation of EGFr, Akt1, ERK1/2, ATF-2 and cAMP levels, we performed pathway screening assays. Time-dependent expression of related genes were determined by real-time PCR. Part of the data was used for neural network modeling to identify the most relevant compounds. N-MP increased the expression of the anti-secretory somatostatin receptor by 114%, whereas C5HT decreased its expression by 52%. N-MP down-regulated the pro-secretory CHRM3 receptor by 36% and the H⁺,K⁺-ATPase by 36%. CAFF stimulated the secretory activity in the functional assays, whereas N-MP and CA decreased proton secretion. After applying a pathway analysis, we were able to discriminate between CAFF, CA, CAT, C5HT, PYR and histamine-activating EGFr signaling and N-MP-associated ERK1/2 signaling.

CONCLUSION

By applying a multi-parametric approach, N-MP was shown to effectively down-regulate mechanisms of gastric acid secretion in human parietal gastric cells.

Gastric secretion

PURPOSE OF REVIEW

This review summarizes the past year's literature regarding the regulation of gastric exocrine and endocrine secretion at the central, peripheral, and cellular levels.

RECENT FINDINGS

Gastric acid secretion is an intricate and dynamic process that is regulated by neural (efferent and afferent), hormonal (e.g., gastrin), and paracrine (e.g., histamine, ghrelin, somatostatin) pathways as well as mechanical (e.g., distension) and chemical (e.g., protein, glutamate, coffee, and ethanol) stimuli. Secretion of hydrochloric acid by the parietal cell involves recruitment and fusion of HK-adenosine triphosphatase (HK-ATPase)-containing cytoplasmic tubulovesicles with the apical membrane with subsequent electroneutral transport of hydronium ions in exchange for potassium; the source of the latter is the potassium channel, KCNQ1. Concomitantly, chloride exits via the cystic fibrosis transmembrane regulator. Inhibition of the HK-ATPase by proton pump inhibitors leads to a compensatory hypergastrinemia which, if prolonged, results in parietal and enterochromaffin-like cell hyperplasia. The clinical consequence is rebound acid secretion which may induce dyspeptic symptoms in healthy individuals and exacerbate reflux symptoms in patients with gastroesophageal reflux disease.

SUMMARY

We continue to make progress in our understanding of the regulation of gastric acid secretion in health and disease. A better understanding of the pathways and mechanisms regulating acid secretion should lead to improved management of patients with acid-induced disorders as well as those who secrete too little acid.

Activity-guided fractionation to characterize a coffee beverage that effectively down-regulates mechanisms of gastric acid secretion as compared to regular coffee

In some individuals, the consumption of coffee beverages is related to symptoms of gastric irritation. Hot water steam-treatment of raw coffee beans is hypothesized to reduce the contents of stomach irritating compounds, and products to which this technology is applied are launched as stomach-friendly coffee. However, data on the effect of steam-treated coffee on gastric acid secretion are conflicting and it has not been proven yet as to which coffee components act as pro- or antisecretory stimulants. The work presented here aimed at the characterization of a coffee beverage that effectively down-regulates mechanisms of proton secretion in human gastric cells (HGT-1). At first, a regular coffee beverage was fractionated by using solvents of different polarity: water, ethylacetate, dichloromethane, and pentane. Functional assays on the proton secretory activity (PSA) of these solvent fractions revealed the least pronounced effect for the water fraction, for which quantitative analyses demonstrated the highest distribution of chlorogenic acid (95%), (beta)N-alkanoyl-5-hydroxytryptamides (55%), and N-methylpyridinium (N-MP, >99%) among all fractions. Following experiments demonstrated that HGT-1 cells treated with regular coffee fortified with N-MP at a concentration of about 20 mg/mL N-MP showed a significantly decreased PSA as compared to cells which were exposed to coffee beverages containing higher (32-34 mg/L) or lower (5 mg/L) N-MP concentrations. Results from cellular pathway analyses of transcription (ATF-1 and Akt1) and signaling (cAMP and EGFr) factors and kinases (ERK1/2), and experiments on the gene expression of pro (histamine-HRH2 and acetylcholine-CHRM3)- and anti (somatostatin-SSTR1)-secretory receptors and H(+),K(+)-ATPase verified this antisecretory activity of N-MP in coffee beverages.