Role Of Extra-Oral Bitter Taste Receptor Agonists in Cancer Treatment
The Role of Bitter Taste Receptors in Cancer: A Systematic Review
Background: Since it is known that bitter taste receptors (TAS2Rs) are expressed and functionally active in various extra-oral cells, their genetic variability and functional response initiated by their activation have become of broader interest, including in the context of cancer. Methods: A systematic research was performed in PubMed and Google Scholar to identify relevant publications concerning the role of TAS2Rs in cancer. Results: While the findings on variations of TAS2R genotypes and phenotypes and their association to the risk of developing cancer are still inconclusive, gene expression analyses revealed that TAS2Rs are expressed and some of them are predominately downregulated in cancerous compared to non-cancerous cell lines and tissue samples. Additionally, receptor-specific, agonist-mediated activation induced various anti-cancer effects, such as decreased cell proliferation, migration, and invasion, as well as increased apoptosis. Furthermore, the overexpression of TAS2Rs resulted in a decreased tumour incidence in an in vivo study and TAS2R activation could even enhance the therapeutic effect of chemotherapeutics in vitro. Finally, higher expression levels of TAS2Rs in primary cancerous cells and tissues were associated with an improved prognosis in humans. Conclusion: Since current evidence demonstrates a functional role of TAS2Rs in carcinogenesis, further studies should exploit their potential as (co-)targets of chemotherapeutics.
An update on extra-oral bitter taste receptors
Bitter taste-sensing type 2 receptors (TAS2Rs or T2Rs), belonging to the subgroup of family A G-protein coupled receptors (GPCRs), are of crucial importance in the perception of bitterness. Although in the first instance, TAS2Rs were considered to be exclusively distributed in the apical microvilli of taste bud cells, numerous studies have detected these sensory receptor proteins in several extra-oral tissues, such as in pancreatic or ovarian tissues, as well as in their corresponding malignancies. Critical points of extra-oral TAS2Rs biology, such as their structure, roles, signaling transduction pathways, extensive mutational polymorphism, and molecular evolution, have been currently broadly studied. The TAS2R cascade, for instance, has been recently considered to be a pivotal modulator of a number of (patho)physiological processes, including adipogenesis or carcinogenesis. The latest advances in taste receptor biology further raise the possibility of utilizing TAS2Rs as a therapeutic target or as an informative index to predict treatment responses in various disorders. Thus, the focus of this review is to provide an update on the expression and molecular basis of TAS2Rs functions in distinct extra-oral tissues in health and disease. We shall also discuss the therapeutic potential of novel TAS2Rs targets, which are appealing due to their ligand selectivity, expression pattern, or pharmacological profiles.
The distribution of hTAS2R transcripts among human non-cancer breast epithelial cell line (MCF-10A line), as well as in cells derived from estrogen-dependent (MCF-7 line) and estrogen-independent metastatic (MDA-MB-231 line) breast adenocarcinoma, has been investigated, highlighting a strong heterogeneity in hTAS2R expression (Table 4) [238–https://doi.org/10.1007/s11010-019-03679-5
." href="https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-021-03067-y#ref-CR240" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 240">240]. In particular, the expression of hTAS2R4 at both mRNA and protein levels, and rapid mobilisation of intracellular calcium as a result of its activation, were significantly lower in MDA-MB-231 and MCF-7 cells than in MCF-10A (P < 0.001). This suggested the existence of invasion-metastasis mechanism(s) in which evasion of tumour suppressor genes is modulated by the expression of key genes, such as hTAS2R4 in breast cancer . In contrast, it can be assumed that TAS2Rs determine the body's physiological defence reaction to metastatic cancer cells, which is supported by the observations of Singh et al. [https://doi.org/10.1007/s11010-019-03679-5
." href="https://translational-medicine.biomedcentral.com/articles/10.1186/s12967-021-03067-y#ref-CR240" data-track="click" data-track-action="reference anchor" data-track-label="link" data-test="citation-ref" aria-label="Reference 240">240]. The authors showed that the activation of TAS2R4 and 14 induced apoptosis, and inhibited cell proliferation and chemotactic migration due to down-regulation of matrix metalloproteinase (MMP)-9 secretion in metastatic breast adenocarcinoma MDA-MB-231.
Ovarian and prostate cancer
Researchers have demonstrated heterologous expression of the receptors in tumours compared to in normal tissue (fallopian tube and uterine tissue; BPH-1 benign prostatic hyperplasia line) in biopsies, along with expression in human ovarian (OVCAR4, OVCAR8, SKOV3 and IGROV1 lines), endometrial (HEC-1a line), and prostate cancer cells (PC3, LNCAP, and DU145 lines) (Table 4) . Low expression of hTAS2R14 and 38 in almost all ovarian cancer lines was accompanied by overexpression of hTAS2R4 and -10, similar to that observed in breast adenocarcinoma cells. In contrast, prostate cancer cells were mostly characterized by reduced expression of the dedicated receptors, except for that of hTAS2R38 in the PC3 line . In addition, 25 μM noscapine (NOS), a bitter isoquinoline alkaloid, induced tumour cell apoptosis in a hTAS2R14-dependent manner, and cell viability after 72 h decreased proportionally with increase in the concentration of the used phytochemical (0–100 μM). Although NOS possesses anti-microtubulin properties similar to paclitaxel, it inhibits the growth of paclitaxel-resistant cancer cells , which indicates a difference in the mechanism of action of these two compounds. Therefore, hTAS2R14 appears to be the new target of NOS, which may constitute a new and promising anti-cancer strategy for taxane-resistant cases .
Malignant neoplasms of the GI tract often develop asymptomatically, which results in diagnosis at advanced stages of the disease. In addition, anti-cancer therapy is often ineffective and is associated with various side effects. This "status quo" indicates the need to conduct extensive scientific research that can reveal new and safer approaches for increasing the effectiveness of oncotherapy. The importance of TAS2Rs in pancreatic ductal adenocarcinoma (PDAC) pathogenesis was recognized relatively recently, when Gaida et al. observed cytoplasmic expression of functional TAS2R38 in SU8686, T3M4, and MiaPaCa-2 lines, and in tumour tissues derived from 88 patients. Activation of the above receptor with a bona fide ligand, PTU, or a bacterial quorum detection molecule, AHL-12, led to phosphorylation of p38 mitogen-activated protein (p38MAPK) and ERK1/2 kinases. This was followed by overexpression of the transcription factor—nuclear factor of activated Tcells 1, and the multidrug-resistance protein ABCB1, a transmembrane transporter participating in shuttling a plethora of drugs, such as chemotherapeutics or antibiotics . Another extensive study has systematically investigated the functional role of TAS2R10 expressed on the surface and in the cytoplasm of PDAC tissue and in tumour-derived cell lines (AsPC-1, BxPC-3, Capan-1, COLO-357, MiaPaCa-2, SU.86.86, PANC-1, and T3M4) in the context of chemoresistance (Table (Table4)4) . This study demonstrated that caffeine, the natural ligand of this receptor, sensitises cancer cells to two standard chemotherapeutics, gemcitabine and 5-fluorouracil. Knocking down of T2R10 in the BxPC-3 cell line reduced the caffeine-induced effect. Possibly, caffeine triggered Akt phosphorylation and subsequently down-regulated ABCG2 via T2R10, another multi-drug resistance protein that participates in rendering cells resistant to various chemotherapeutics.
The association between TAS2R gene variants and malignant GI tumour susceptibility is currently gaining attention. A study by Choi et al. in the Korean population showed that that the heterozygous TAS2R38 diplotype (PAV/AVI) significantly increased gastric cancer risk (OR = 1.513; 95% CI = 1.148–1.994) . Similarly, the presence of the AVI/AVI combination of TAS2R38 was associated with enhanced risk of developing colorectal cancer (CRC) in two different Caucasian populations compared to those with two PAV haplotypes (OR = 1.15; 95% CI = 0.80–1.66 for the Czech population and OR = 1.52; 95% CI = 1.05–2.21 for the German population) .
Barontini et al. did not observe any strong influence of the selected SNPs of TAS2R16 on CRC susceptibility. This is possibly because SNPs in this gene are modulated by lifestyle factor or dietary habit, which were not considered in the cited analysis. However, after stratification by histology (colon vs. rectum), rs1525489 was found to be associated with increased risk of rectal cancer (Ptrend = 0.0071) . Despite the abundance of results, the role of TAS2R polymorphic variants in carcinogenesis is not clear. On one hand, distinct phenotypes, such as AVI/AVI (non-tester) of TAS2R38, reflect differential receptor function and the inability to detect bitter compounds, which may also be a marker for impaired function of the receptors in GI; non-taster individuals eliminate potentially harmful/carcinogenic xenobiotics from the gut slowly and consequently are at high risk of developing GI malignancies . On the other hand, another study has revealed that the AVI/AVI diplotype is not simply a functional marker for the impaired TAS2R38 variant protein, as expression of the homozygous AVI transcript was detected, and subjects with this genotype reacted to other bitter tastes as strongly as individuals with the homozygous PAV variant . This observation may indicate that the structural perturbation of the AVI variant protein may enhance the sensing of other unknown bitter-tasting, potentially carcinogenic molecules, which might initiate appropriate anti-cancer mechanisms . This protective effect of the AVI/AVI diplotype was evident in Choi et al.’s study, in which the AVI haplotype tended to reduce risk of CRC in the Korean population (OR = 0.74; 95% CI = 0.56–0.99) . This may also explain the significantly high diversity observed in TAS2R genes . However, these are only hypotheses, which have to be confirmed using scientific research.
T2R bitter taste receptors regulate apoptosis and may be associated with survival in head and neck squamous cell carcinoma
Better management of head and neck squamous cell carcinomas (HNSCCs) requires a clearer understanding of tumor biology and disease risk. Bitter taste receptors (T2Rs) have been studied in several cancers, including thyroid, salivary, and GI, but their role in HNSCC has not been explored. We found that HNSCC patient samples and cell lines expressed functional T2Rs on both the cell and nuclear membranes. Bitter compounds, including bacterial metabolites, activated T2R‐mediated nuclear Ca2+responses leading to mitochondrial depolarization, caspase activation, and ultimately apoptosis. Buffering nuclear Ca2+ elevation blocked caspase activation. Furthermore, increased expression of T2Rs in HNSCCs from The Cancer Genome Atlas is associated with improved overall survival. This work suggests that T2Rs are potential biomarkers to predict outcomes and guide treatment selection, may be leveraged as therapeutic targets to stimulate tumor apoptosis, and may mediate tumor‐microbiome crosstalk in HNSCC.
The influence of bitter-taste receptor (TAS2R) expression in pharmacological response to Chloroquine in obese patients with COVID-19
Recently, a retrospective analysis of 3,615 patients with COVID-19 at the New York academic hospital system reported that approximately 38% of these patients were obese (Body Mass Index, BMI >30 kg/m2). Moreover, obese patients under 60 years of age had twofold susceptibility to being admitted in critical care compared to non-obese individuals (8).
Obesity-related conditions worsen the impact of COVID-19 symptoms due to complications associated with excessive body weight, metabolic dysfunction, cardiovascular risk, sleep apnea, vitamin D deficiency, dysregulation of the renin-angiotensin-aldosterone system, and sarcopenia (10). Furthermore, obese patients have altered levels of circulating cytokines. Notably, these individuals exhibit higher concentrations of TNF-alpha, MCP-1, and IL-6, which are generated by visceral and subcutaneous adipose tissue (11). This alteration in inflammatory profile appears to predict the severity and prognosis of obese patients and COVID-19 (11,12). Thus, special attention should be considered for this population.
In the absence of efficient pharmacotherapy, and due to the public health emergency, a significant number of potential drugs was proposed, such as antiviral agents, chloroquine and hydroxychloroquine, and corticosteroids (13,14).
Chloroquine (CQ) is an amine acidotropic form of quinine. In the past, this compound was the drug of choice against malaria and has been reported to be a potential broad-spectrum antiviral drug (13). The mechanism of action of CQ and its analog hydroxychloroquine (HCQ) consists in blocking viral entry into cells by interacting with viral particles that bind to human cell surface receptors (15). Both CQ and HCQ can have immunomodulatory effects through the modulation of pro-inflammatory cytokines (13).
CQ is also an active agonist of bitter-taste receptors (TAS2Rs), first identified in the bud cells of the tongue. These receptors signal information to the brain about the nutritive value or toxicity of ingested foods and beverages (16,17). Aside from their role in chemosensory cells, these receptors are expressed in an extensive range of tissues and perform varied functions (18). A recent study conducted by Grassin-Delyle et al. (19) suggested that TAS2Rs 3, 4, 5, 9, 10, 14, 30, 39, and 40 were involved in the inhibition of cytokine production. Moreover, Li et al. (12) hypothesized that TAS2R10 might help to prevent the cytokine storm - a key event for patients with more aggressive COVID-19 symptoms - because its receptor regulates natural killer cell-mediated cytotoxicity, and chemokine, T-cell receptor, and TNF signaling pathways (12). CQ binding to TAS2Rs may also result in functional changes in respiratory tract cells, such as smooth muscle, ciliated cells, and blood cell markers. This suggests that the extra-oral effects of CQ could be beneficial against pulmonary diseases (20).
Interestingly, extra-oral TAS2R expression could be reduced in obese subjects compared to non-obese individuals (18). This may result in a decreased sensitivity to bitter taste, which is often observed in obese individuals, and may affect food choices (18). Considering the role of TAS2R and its agonists on immune responses, and the fact that CQ is a TAS2R agonist, we hypothesized that COVID-19-infected obese patients could respond differently to pharmacological treatment with CQ and side effects could occur more frequently due to overdosage in this population. This hypothesis is supported by a recent study that evaluated the risk of retinopathy - one of the most important long-term side effects of HCQ treatment - related to clinical characteristics and HCQ blood levels of 537 patients (21). The authors found a significant association between higher BMI and a higher risk of HCQ toxicity, suggesting a limited HCQ dosage at 400 mg daily, irrespective of how high is the patient’s BMI (21).
Moreover, according to Karalis et al. (22), dosage regimens of CQ/HCQ used for treating COVID-19 symtoms could be linked to high toxicity risk. Thus, to avoid adverse effects due to CQ/HCQ overdosage, the authors proposed dosage regimens tailored to the patient’s characteristics, such as body weight, age, and severity of COVID-19 symptoms (22). However, more studies are required to further address the influence of individual factors in the CQ/HCQ pharmacodynamics and the optimal dosage of CQ/HCQ for treating COVID-19 symptoms (15,23).