Dietary flavonoid naringenin induces regulatory T cells via an aryl hydrocarbon receptor mediated pathway

New Trends in Aryl Hydrocarbon Receptor Biology

Traditionally considered as a critical intermediate in the toxic and carcinogenic response to dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin, TCDD), the Aryl hydrocarbon/Dioxin receptor (AhR) has proven to be also an important regulator of cell physiology and organ homeostasis. AhR has become an interesting and actual area of research mainly boosted by a significant number of recent studies analyzing its contribution to the proper functioning of the immune, hepatic, cardiovascular, vascular and reproductive systems. At the cellular level, AhR establishes functional interactions with signaling pathways governing cell proliferation and cell cycle, cell morphology, cell adhesion and cell migration. Two exciting new aspects in AhR biology deal with its implication in the control of cell differentiation and its more than likely involvement in cell pluripotency and stemness. In fact, it is possible that AhR could help modulate the balance between differentiation and pluripotency in normal and transformed tumor cells. At the molecular level, AhR regulates an increasingly large array of physiologically relevant genes either by traditional transcription-dependent mechanisms or by unforeseen processes involving genomic insulators, chromatin dynamics and the transcription of mobile genetic elements. AhR is also closely related to epigenetics, not only from the point of view of target gene expression but also with respect to its own regulation by promoter methylation. It is reasonable to consider that deregulation of these many functions could have a causative role, or at least contribute to, human disease. Consequently, several laboratories have proposed that AhR could be a valuable tool as diagnostic marker and/or therapeutic target in human pathologies. An additional point of interest is the possibility of regulating AhR activity by endogenous non-toxic low weight molecules agonist or antagonist molecules that could be present or included in the diet. In this review, we will address these molecular and functional features of AhR biology within physiological and pathological contexts.

Naringenin Inhibits Superoxide Anion-Induced Inflammatory Pain: Role of Oxidative Stress, Cytokines, Nrf-2 and the NO-cGMP-PKG-KATP Channel Signaling Pathway

In the present study, the effect and mechanism of action of the flavonoid naringenin were evaluated in superoxide anion donor (KO₂)-induced inflammatory pain in mice. Naringenin reduced KO₂-induced overt-pain like behavior, mechanical hyperalgesia, and thermal hyperalgesia. The analgesic effect of naringenin depended on the activation of the NO−cGMP−PKG−ATP-sensitive potassium channel (K_ATP) signaling pathway. Naringenin also reduced KO₂-induced neutrophil recruitment (myeloperoxidase activity), tissue oxidative stress, and cytokine production. Furthermore, naringenin downregulated KO₂-induced mRNA expression of gp91phox, cyclooxygenase (COX)-2, and preproendothelin-1. Besides, naringenin upregulated KO₂-reduced nuclear factor (erythroid-derived 2)-like 2 (Nrf2) mRNA expression coupled with enhanced heme oxygenase (HO-1) mRNA expression. In conclusion, the present study demonstrates that the use of naringenin represents a potential therapeutic approach reducing superoxide anion-driven inflammatory pain. The antinociceptive, anti-inflammatory and antioxidant effects are mediated via activation of the NO−cGMP−PKG−K_ATP channel signaling involving the induction of Nrf2/HO-1 pathway.

Cancer prevention and therapy through the modulation of the tumor microenvironment

Cancer arises in the context of an in vivo tumor microenvironment. This microenvironment is both a cause and consequence of tumorigenesis. Tumor and host cells co-evolve dynamically through indirect and direct cellular interactions, eliciting multiscale effects on many biological programs, including cellular proliferation, growth, and metabolism, as well as angiogenesis and hypoxia and innate and adaptive immunity. Here we highlight specific biological processes that could be exploited as targets for the prevention and therapy of cancer. Specifically, we describe how inhibition of targets such as cholesterol synthesis and metabolites, reactive oxygen species and hypoxia, macrophage activation and conversion, indoleamine 2,3-dioxygenase regulation of dendritic cells, vascular endothelial growth factor regulation of angiogenesis, fibrosis inhibition, endoglin, and Janus kinase signaling emerge as examples of important potential nexuses in the regulation of tumorigenesis and the tumor microenvironment that can be targeted. We have also identified therapeutic agents as approaches, in particular natural products such as berberine, resveratrol, onionin A, epigallocatechin gallate, genistein, curcumin, naringenin, desoxyrhapontigenin, piperine, and zerumbone, that may warrant further investigation to target the tumor microenvironment for the treatment and/or prevention of cancer.

Promotion of regulatory T cell induction by immunomodulatory herbal medicine licorice and its two constituents

Regulatory T cells (Treg) play a critical role to control immune responses and to prevent autoimmunity, thus selective increase of Treg cells in vivo has broad therapeutic implications for autoimmune and inflammatory diseases. Licorice is a well-known herbal medicine used worldwide for over thousands of years, and accumulating evidence has shown its immunomodulatory potential. However, it is not clear whether licorice could regulate the induction and function of Treg cells. Here we found licorice extract could promote Treg cell induction, and then we used a rational approach to isolate its functional fractions and constituents. The results showed that two constituents, isoliquiritigenin and naringenin, promoted Treg cell induction both in vitro and in vivo. The effective fractions and two constituents of licorice also enhanced immune suppression of Treg cells, and they further reduced severity of DSS-induced colitis in mice. This study suggested that promotion of regulatory T cell induction could be an underlying mechanism of the historically and widely used herbal medicine licorice, providing its two effective molecules against autoimmune and inflammatory diseases.

Food, immunity, and the microbiome

There is increasing evidence that ingested diet-borne components are involved in the pathogenesis of disorders such as inflammatory bowel diseases, atherosclerosis, and type 2 diabetes. Nutrients can have short- and long-term effects in shaping the composition of the microbiota. Western diets (enriched in fat, phosphatidylcholine, and L-carnitine) promote inflammation and atherosclerosis through specific fatty acids and degradation products such as trimethylamine N-oxide. Other dietary factors such as carbazoles or tryptophan-enriched proteins have anti-inflammatory properties-partly via activation of aryl hydrocarbon receptors. The microbiota and its metabolic machinery produce a myriad of metabolites that serve as important messengers between the diet, microbiota, and host. Short-chain fatty acids affect immune responses and epithelial integrity via G-protein-coupled receptors and epigenetic mechanisms. By increasing our understanding of interactions between diet, immunity, and the microbiota, we might develop food-based approaches to prevent or treat many diseases. There now is scientific evidence to support the adage "we are what we eat," and this process begins in early life.

Impact of dietary components on NK and Treg cell function for cancer prevention

An important characteristic of cancer is that the disease can overcome the surveillance of the immune system. A possible explanation for this resistance arises from the ability of tumor cells to block the tumoricidal activity of host immune cells such as natural killer (NK) cells by inducing the localized accumulation of regulatory T (Treg) cells. Evidence exists that components in commonly consumed foods including vitamins A, D, and E, water-soluble constituents of mushrooms, polyphenolics in fruits and vegetables, and n-3 fatty acids in fish oil can modulate NK cell activities, Treg cell properties, and the interactions between those two cell types. Thus, it is extremely important for cancer prevention to understand the involvement of dietary components with the early stage dynamics of interactions among these immune cells. This review addresses the potential significance of diet in supporting the function of NK cells, Treg cells, and the balance between those two cell types, which ultimately results in decreased cancer risk.

Aryl hydrocarbon receptor ligands in cancer: friend and foe

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that is best known for mediating the toxicity and tumour-promoting properties of the carcinogen 2,3,7,8-tetrachlorodibenzo-p-dioxin, commonly referred to as ‘dioxin’. AHR influences the major stages of tumorigenesis — initiation, promotion, progression and metastasis — and physiologically relevant AHR ligands are often formed during disease states or during heightened innate and adaptive immune responses. Interestingly, ligand specificity and affinity vary between rodents and humans. Studies of aggressive tumours and tumour cell lines show increased levels of AHR and constitutive localization of this receptor in the nucleus. This suggests that the AHR is chronically activated in tumours, thus facilitating tumour progression. This Review discusses the role of AHR in tumorigenesis and the potential for therapeutic modulation of its activity in tumours.

Role and therapeutic value of dendritic cells in central nervous system autoimmunity

Dendritic cells (DCs) are professional antigen-presenting cells that control the generation of adaptive immunity. Consequently, DCs have a central role in the induction of protective immunity to pathogens and also in the pathogenic immune response responsible for the development and progression of autoimmune disorders. Thus the study of the molecular pathways that control DC development and function is likely to result in new strategies for the therapeutic manipulation of the immune response. In this review, we discuss the role and therapeutic value of DCs in autoimmune diseases, with a special focus on multiple sclerosis.

6-shogaol, a major compound in ginger, induces aryl hydrocarbon receptor-mediated transcriptional activity and gene expression

Xenobiotics are usually detoxified by drug-metabolizing enzymes and excreted from the body. The expression of many of drug-metabolizing enzymes is regulated by the aryl hydrocarbon receptor (AHR). Some substances in vegetables have the potential to be AHR ligands. To search for vegetable components that exhibit AHR-mediated transcriptional activity, we assessed the activity of vegetable extracts and identified the active compounds using the previously established stable AHR-responsive HepG2 cell line. Among the hot water extracts of vegetables, the highest activity was found in ginger. The ethyl acetate fraction of the ginger hot water extract remarkably induced AHR-mediated transcriptional activity, and the major active compound was found to be 6-shogaol. Subsequently, the mRNA levels of AHR-targeting drug-metabolizing enzymes (CYP1A1, UGT1A1, and ABCG 2) and the protein level of CYP1A1 in HepG2 cells were shown to be increased by 6-shogaol. This is the first report that 6-shogaol can regulate the expression of detoxification enzymes by AHR activation.

Characterization of natural aryl hydrocarbon receptor agonists from cassia seed and rosemary

Many recent studies have suggested that activation of the aryl hydrocarbon receptor (AhR) reduces immune responses, thus suppressing allergies and autoimmune diseases. In our continuing study on natural AhR agonists in foods, we examined the influence of 37 health food materials on the AhR using a reporter gene assay, and found that aqueous ethanol extracts of cassia seed and rosemary had particularly high AhR activity. To characterize the AhR-activating substances in these samples, the chemical constituents of the respective extracts were identified. From an active ethyl acetate fraction of the cassia seed extract, eight aromatic compounds were isolated. Among these compounds, aurantio-obtusin, an anthraquinone, elicited marked AhR activation. Chromatographic separation of an active ethyl acetate fraction of the rosemary extract gave nine compounds. Among these compounds, cirsimaritin induced AhR activity at 10-10² μM, and nepitrin and homoplantagenin, which are flavone glucosides, showed marked AhR activation at 10-10³ μM.

Modulation of intestinal functions by dietary substances: an effective approach to health promotion

Food contains a variety of substances than can modulate transport,barrier, detoxification, and immune functions of the intestines. Functional foods with those substances will be beneficial in promoting gut health, and eventually prevent lifestyle-related diseases.

Establishment of a stable aryl hydrocarbon receptor-responsive HepG2 cell line

The aryl hydrocarbon receptor (AHR) is a ligand-dependent transcription factor. It heterodimerizes with aryl hydrocarbon nuclear translocator, binds to the xenobiotic-responsive element (XRE), and enhances the transcription of genes encoding xenobiotic metabolizing enzymes. AHR also plays important roles in the inhibition of intestinal carcinogenesis and the modulation of gut immunity. It is very important to screen for AHR-activating compounds because those are expected to produce the AHR-mediated physiological functions. Until now, AHR-mediated transcriptional activity represented by the transcriptional activity of CYP1A1 in luciferase assay has been applied as a screening procedure for AHR-activating compounds. However, the AHR-mediated transcriptional activity did not necessarily correspond with the CYP1A1 transcriptional activity. To evaluate AHR-mediated transcriptional activity more specifically, and to screen for AHR-activating compounds, we establish a stable AHR-responsive HepG2 cell line by co-transfection of an AHR expression vector and an AHR-responsive vector (pGL3-XRE) containing a luciferase gene and three tandemly arranged XRE elements into a human hepatoma derived cell line, HepG2. The induction of luciferase activity in the stable AHR-responsive HepG2 cell line by typical AHR activators occurred in time- and concentration-dependent manners. By assessing the AHR target genes CYP1A1, UGT1A1, and ABCG2, an AHR activator-mediated induction was observed at mRNA level. Furthermore, the AHR activator-mediated induction of luciferase activity was positively correlated with the mRNA levels of CYP1A1, UGT1A1, and ABCG2. These findings verified the usefulness of the established stable AHR-responsive HepG2 cell line for the screening of AHR-activating compounds.

Post-column mobile phase adjustment: a strategy to eliminate the contradiction between liquid chromatography and mass spectrometry in the determination of flavonoids in rat plasma

Flavonoids are a group of important naturally occurring polyphenolic compounds with a wide range of biological effects. In this study, a sensitive liquid chromatography tandem mass spectrometry method was developed to simultaneously determine multiple active flavonoids, including quercetin (Que), kaempferol (Kae), apigenin (Api), isorhamnetin (Iso), luteolin (Lut), and naringenin (Nar), in rat plasma. To achieve a satisfied peak shape and LC separation, formic acid with the concentration between 0.05 and 0.2%, or in some case 5%, was generally used to acidify the LC mobile phase in reported studies. Here we found that even 0.05% formic acid could lead to strong mass signal suppression, and the absence of formic acid could reverse the signal suppression but cause serious peak tailing. There is an irreconcilable contradiction between liquid chromatography (LC) and mass spectrometry (MS). In order to simultaneously satisfy LC and MS, LC mobile phase with 0.00075% formic acid and post column mobile phase adjustment with 0.0677% ammonium solution in isopropanol were applied. Compared with the conventional method with mobile phase containing 0.05% formic acid, the mass signal response of Que, Kae, Api, Iso, Lut, Nar, and Oka increased 26.2, 18.6, 13.6, 23.5, 17.5, 15.6 and 15.4 fold, respectively. In addition, the post column mobile phase addition exhibited the better peak shape for the reduction of analytes longitudinal diffusion. The method has been fully validated according to FDA guidelines within the linear range between 0.328 ng mL⁻¹ and 168 ng mL⁻¹, and successfully applied to a pilot pharmacokinetic study of rats after administering 5.43 g kg⁻¹ Pollen of Brassica campestris.

Toward understanding the role of aryl hydrocarbon receptor in the immune system: current progress and future trends

The immune system is regulated by distinct signaling pathways that control the development and function of the immune cells. Accumulating evidence suggest that ligation of aryl hydrocarbon receptor (Ahr), an environmentally responsive transcription factor, results in multiple cross talks that are capable of modulating these pathways and their downstream responsive genes. Most of the immune cells respond to such modulation, and many inflammatory response-related genes contain multiple xenobiotic-responsive elements (XREs) boxes upstream. Active research efforts have investigated the physiological role of Ahr in inflammation and autoimmunity using different animal models. Recently formed paradigm has shown that activation of Ahr by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or 3,3′-diindolylmethane (DIM) prompts the differentiation of CD4⁺Foxp3⁺ regulatory T cells (Tregs) and inhibits T helper (Th)-17 suggesting that Ahr is an innovative therapeutic strategy for autoimmune inflammation. These promising findings generate a basis for future clinical practices in humans. This review addresses the current knowledge on the role of Ahr in different immune cell compartments, with a particular focus on inflammation and autoimmunity.

Regulation of central nervous system autoimmunity by the aryl hydrocarbon receptor

The ligand-activated transcription factor aryl hydrocarbon receptor controls the activity of several components of the immune system, many of which play an important role in neuroinflammation. This review discusses the role of AhR in T cells and dendritic cells, its relevance for the control of autoimmunity in the central nervous system, and its potential as a therapeutic target for immune-mediated disorders.

Advances in nutritional research on regulatory T-cells

Many clinical and animal studies have shown that certain dietary components exert anti-inflammatory properties that aid in the amelioration of chronic inflammatory diseases. Among the various proposed channels through which dietary components affect immune responses, regulatory T-cells (Tregs) are emerging as key targets for the dietary prevention of chronic inflammatory diseases. In this review, immunoregulation by Tregs is briefly described, followed by a summary of recent advances and possible applications of techniques for the study of Tregs. In addition, this review provides an overview of the current knowledge on Treg regulation by certain dietary components, including vitamins, omega-3 polyunsaturated fatty acids, and polyphenols. The caveats of previous studies are also discussed in order to highlight the distinctions between dietary studies and immunological approaches. Consequently, this review may help to clarify the means by which nutritional components influence Tregs.

Modulation of immune function by polyphenols: possible contribution of epigenetic factors

Several biological activities have been described for polyphenolic compounds, including a modulator effect on the immune system. The effects of these biologically active compounds on the immune system are associated to processes as differentiation and activation of immune cells. Among the mechanisms associated to immune regulation are epigenetic modifications as DNA methylation of regulatory sequences, histone modifications and posttranscriptional repression by microRNAs that influences the gene expression of key players involved in the immune response. Considering that polyphenols are able to regulate the immune function and has been also demonstrated an effect on epigenetic mechanisms, it is possible to hypothesize that there exists a mediator role of epigenetic mechanisms in the modulation of the immune response by polyphenols.

Multidrug resistance reversal and apoptosis induction in human colon cancer cells by some flavonoids present in citrus plants

Multidrug resistance (MDR) of cancer cells constitutes one of the main reasons for chemotherapy failure. The search for nontoxic modulators that reduce MDR is a task of great importance. An ability to enhance apoptosis of resistant cells would also be beneficial. In the present study, the MDR reversal and apoptosis-inducing potency of three flavonoids produced by Citrus plants, namely, naringenin (1a), aromadendrin (2), and tangeretin (3), and the methylated naringenin derivatives (1b, 1c), have been studied in sensitive (LoVo) and multidrug-resistant (LoVo/Dx) human colon adenocarcinoma cells. Cytotoxicity of methoxylated flavonoids was higher as compared to hydroxylated analogues. Only 3 turned out to inhibit P-glycoprotein, as demonstrated by a rhodamine 123 accumulation assay. It also increased doxorubicin accumulation in LoVo/Dx cells and enabled doxorubicin to enter cellular nuclei. In addition, 3 was found to be an effective MDR modulator in resistant cells by sensitizing them to doxorubicin. Tangeretin-induced caspase-3 activation and elevated surface phosphatidylserine exposure demonstrated its apoptosis-inducing activity in LoVo/Dx cells, while the other flavonoids evaluated were not active. Additionally, 3 was more toxic to resistant rather than to sensitive cancer cells. Its apoptosis-inducing activity was also higher in LoVo/Dx than in LoVo cells. It was concluded that the activity of 3 against multidrug-resistant cancer cells may be enhanced by its apoptosis-inducing activity.

The citrus flavanone naringenin suppresses CYP1B1 transactivation through antagonising xenobiotic-responsive element binding

Exposure to environmental toxicants or exogenous oestrogen increases the risk of cancer. Some toxicants such as polycyclic aromatic hydrocarbons (PAH) undergo biotransformation to become genotoxic agents. Cytochrome p450 (CYP) 1B1 is an enzyme catalysing this transformation. Consumption of fruit and vegetables is considered to be protective against carcinogenesis, and naringenin can be found abundantly in citrus fruits. In the present study, the effect of naringenin on the regulation of CYP1B1 was investigated in MCF-7 cells. Enzyme inhibition assays revealed that naringenin inhibited CYP1B1 at or above 5 μm but not CYP1A1 activity. Quantitative PCR analysis also demonstrated that 1 μm-naringenin reduced CYP1B1 mRNA expression induced by 7,12-dimethylbenz(α)anthracene (DMBA). Further study illustrated that the suppression was at the transcriptional level. Since previous studies have shown that oestrogen response element (ERE) and xenobiotic-responsive element (XRE) are functional binding sequences in the promoter region of CYP1B1, interference of DNA binding on these two elements was pursued. Employing reporter gene assays as well as the electromobility shift assay, we verified that naringenin counteracted DMBA-induced XRE binding at − 1675. These results supported the notion that fruit consumption could be a protective measure against PAH biotransformation.