Introduction
The bitterness of medicinal products used for functional dyspepsia and irritable bowel syndrome containing herbal extracts is well known, although the underlying mechanisms are poorly understood. In previous works of our group, bitter tasting phenolic plant compounds were shown to elicit (1) an anti-inflammatory activity in immune competent cells, and (2) stimulate proton secretion in parietal cells as key mechanism of gastric acid secretion.1-3 Both effects were mediated through bitter taste receptors (TAS2Rs), and were demonstrated to correlate with the compound’s bitter taste in their effect size.1-4
Aims & Methods
In this work, a commercially available liquid herb formulation used to aid digestive discomfort was tested in a concentration range of 5 – 500 µg/ml for its TAS2R-mediated effects on the mechanisms of proton secretion by means of a pH-sensitive fluorescent dye in the well-established human parietal HGT-1 cell model.4 This native cell line expresses all 25 TAS2Rs and all the elements necessary for signal transduction of functional gastric acid secretion. Individual plant extracts of the more effective formulation, namely extracts from Artemisia absinthium, Cichorium, Erythraea centaurium, Gentiana lutea, Imperatoria ostruthium, Juniperus communis, Millefolium, Salvia officinalis, and Taraxacum, were studied for their effects on proton secretion and TAS2R gene regulation by means of RT-qPCR. Moreover, total phenolic content, expressed as gallic acid equivalents (GAE), were analyzed by using the colorimetric Folin-Ciocalteu method.
Results
Treatment of the HGT-1 cells with the ethanolic plant extracts (cytotoxic effects were excluded; cell viability ≥ 95%) revealed a concentration-dependent (1 – 300 µg/mL herbal extract) stimulation of proton secretion by seven of nine extracts, of which those from Imperatoria ostruthium and Juniperus communis demonstrated the strongest effects. Gene expression experiments resulted a regulation of 16 out of 25 TAS2Rs after exposure of the HGT-1 cells to the herbal product. Without exception, these regulations were reproduced by treatment of the cells with the individual plant extracts. For example, exposure of HGT-1 cells to the liquid formulation led to a regulation of TAS2R4 and TAS2R5, which are known to be activated by polyphenolic compounds.2,5 These regulations were also found for the ethanolic extracts from Juniperus communis and Imperatoria ostruthium (TAS2R4 and TAS2R5) and Gentiana lutea (TAS2R4). Quantitation of the total polyphenol content showed a high polyphenol concentration (> 1500 µg/mL GAE) for four out of the nine extracts, including that from Imperatoria ostruthium (2063.7 ± 31.6 µg/mL GAE) and Juniperus communis (2859.5 ± 13.6 µg/mL GAE), which also had a strong effect on proton secretion. In contrast, extracts with lower polyphenol concentrations (< 1500 µg/mL GAE) demonstrated a less pronounced effect on or no effects on proton secretion.
Conclusion
In summary, exposure of parietal HGT-1 cells to herbal plant extracts with a high polyphenol content lead to a stronger stimulation of proton secretion compared to extracts with low polyphenol contents (R2 = 0.77; p < 0.0001). For the most active extracts from Imperatoria ostruthium and Juniperus communis, a gene regulation of TAS2R4 and TAS2R5 was demonstrated. Next experimental steps will include (1) a functional involvement of TAS2R4 and TAS2R5 by means of knock-out (CRISPR-Cas9) or knock-down (by means of specific siRNA) approaches, and (2) identification of key active phenolic compounds by mass-spectrometry.
References
1 Tiroch, J. et al. & Somoza, V. (2023). Human Gingival Fibroblasts as a Novel Cell Model Describing the Association between Bitter Taste Thresholds and Interleukin-6 Release. Journal of Agricultural and Food Chemistry (Vol. 71, Issue 13, pp. 5314–5325). https://doi.org/10.1021/acs.jafc.2c06979
2 Sterneder, S. et al. & Somoza, V. (2021). Astringent Gallic Acid in Red Wine Regulates Mechanisms of Gastric Acid Secretion via Activation of Bitter Taste Sensing Receptor TAS2R4. Journal of Agricultural and Food Chemistry (Vol. 69, Issue 36, pp. 10550–10561). https://doi.org/10.1021/acs.jafc.1c03061
3 Tiroch, J. et al. & Somoza, V. (2021). Bitter Sensing TAS2R50 Mediates the trans-Resveratrol-Induced Anti-inflammatory Effect on Interleukin 6 Release in HGF-1 Cells in Culture. Journal of Agricultural and Food Chemistry (Vol. 69, Issue 45, pp. 13339–13349). https://doi.org/10.1021/acs.jafc.0c07058
4 Liszt, K. I. et al. & Somoza, V. (2017). Caffeine induces gastric acid secretion via bitter taste signaling in gastric parietal cells. Proceedings of the National Academy of Sciences (Vol. 114, Issue 30). Proceedings of the National Academy of Sciences. https://doi.org/10.1073/pnas.1703728114
5 Soares, S. et al. & De Freitas, V. (2013). Different Phenolic Compounds Activate Distinct Human Bitter Taste Receptors. Journal of Agricultural and Food Chemistry (Vol. 61, Issue 7, pp. 1525–1533). https://doi.org/10.1021/jf304198k
Disclosure
M-R. Piqué-Borràs and G. Künstle are employees of Weleda AG, Arlesheim, Basel-Country, Switzerland.