Resveratrol inhibits dioxin-induced expression of human CYP1A1 and CYP1B1 by inhibiting recruitment of the aryl hydrocarbon receptor complex and RNA polymerase II to the regulatory regions of the corresponding genes

Mechanisms and therapeutic prospects of polyphenols as modulators of the aryl hydrocarbon receptor

Polyphenolic AhR modulators displayed concentration-, XRE-, gene-, species- and cell-specific agonistic/antagonistic activity.

Chemopreventive effect of natural dietary compounds on xenobiotic-induced toxicity

Contaminants (or pollutants) that affect human health have become an important issue, spawning a myriad of studies on how to prevent harmful contaminant-induced effects. Recently, a variety of biological functions of natural dietary compounds derived from consumed foods and plants have been demonstrated in a number of studies. Natural dietary compounds exhibited several beneficial effects for the prevention of disease and the inhibition of chemically-induced carcinogenesis. Contaminant-induced toxicity and carcinogenesis are mostly attributed to the mutagenic activity of reactive metabolites and the disruption of normal biological functions. Therefore, the metabolic regulation of hazardous chemicals is key to reducing contaminant-induced adverse health effects. Moreover, promoting contaminant excretion from the body through Phase I and II metabolizing enzymes is also a useful strategy for reducing contaminant-induced toxicity. This review focuses on summarizing the natural dietary compounds derived from common dietary foods and plants and their possible mechanisms of action in the prevention/suppression of contaminant-induced toxicity.

Role of Aryl Hydrocarbon Receptor in Circadian Clock Disruption and Metabolic Dysfunction

The prevalence of metabolic syndrome, a clustering of three or more risk factors that include abdominal obesity, increased blood pressure, and high levels of glucose, triglycerides, and high-density lipoproteins, has reached dangerous and costly levels worldwide. Increases in morbidity and mortality result from a combination of factors that promote altered glucose metabolism, insulin resistance, and metabolic dysfunction. Although diet and exercise are commonly touted as important determinants in the development of metabolic dysfunction, other environmental factors, including circadian clock disruption and activation of the aryl hydrocarbon receptor (AhR) by dietary or other environmental sources, must also be considered. AhR binds a range of ligands, which prompts protein-protein interactions with other Per-Arnt-Sim (PAS)-domain-containing proteins and subsequent transcriptional activity. This review focuses on the reciprocal crosstalk between the activated AhR and the molecular circadian clock. AhR exhibits a rhythmic expression and time-dependent sensitivity to activation by AhR agonists. Conversely, AhR activation influences the amplitude and phase of expression of circadian clock genes, hormones, and the behavioral responses of the clock system to changes in environmental illumination. Both the clock and AhR status and activation play significant and underappreciated roles in metabolic homeostasis. This review highlights the state of knowledge regarding how AhR may act together with the circadian clock to influence energy metabolism. Understanding the variety of AhR-dependent mechanisms, including its interactions with the circadian timing system that promote metabolic dysfunction, reveals new targets of interest for maintenance of healthy metabolism.

[Effects of Xenobiotics on Drug Pharmacokinetics and Safety]

The use of nanotechnology has increased over the past 10 years, and various nanomaterials with a wide range of applications have been developed. Carbon nanotubes (CNTs), which are cylindrical molecules consisting of hexagonally arranged carbon atoms, are nanomaterials with high utility. Recently, applications of single-walled CNT (SWCNT) in the medical field for drug-delivery and as gene-delivery agents have been proposed. Due to its structural characteristics and physicochemical properties, the inhalation of SWCNT could be considered as one route for targeted drug delivery into the lungs. Therefore, it is necessary to investigate the effects of SWCNT on the physiological state and response of the cells upon delivery into the lung. We clarified the different response of two carcinoma cell lines to SWCNT exposure, and determined these differences may be due to different cell functions. Furthermore, SWCNT exposure resulted in a global downregulation of stress-responsive genes in normal human bronchial epithelial cells, thereby indicating that the factors involved in the stress responses were not activated by SWCNT. We then tried to ascertain the possible effect of SWCNT on the fate of drugs delivered with SWCNT. Exposure to SWCNT down-regulated the mRNA expression and enzymatic activity of CYP1A1 and CYP1B1 by preventing the binding of activated aryl hydrocarbon receptors to the enhancer region of these genes. This review provides basic information for the prediction of human responses to SWCNT exposure by inhalation, and in its use as a drug delivery carrier.

Chronic cardiovascular disease-associated gene network analysis in human umbilical vein endothelial cells exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin

The association of dioxin exposure with increased morbidity or mortality of chronic cardiovascular diseases (CVDs) has been established by many epidemiological studies. However, the precise global gene expression alterations caused by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in the cardiovascular system need to be further elucidated. In this study, we profiled the gene expression of human umbilical vein endothelial cells (HUVECs) exposed to different concentrations of TCDD by high-throughput sequencing. Expression of 1,838 genes was changed significantly after TCDD stimulation. The FunDO analysis suggested that some CVDs were highly associated with TCDD treatment, including atherosclerosis, thromboangiitis obliterans, pulmonary arterial hypertension (PAH), and hypertension. KEGG pathway analysis showed that many genes in the signaling pathways of vascular smooth muscle contraction and apoptosis were altered distinctly. In addition, we revealed evidence regarding the gene network changes of chronic CVDs including atherosclerosis, thrombosis, myocardial infarction (MI), hypertension, and PAH in TCDD-exposed HUVECs. We found that gene expression of β1-adrenoceptors (ADRB1), β2-adrenoceptors (ADRB2), endothelin-converting enzyme 1 (ECE1), and endothelin-1 gene (EDN1) that are involved in the blood pressure regulation pathway decreased apparently under TCDD treatment. Moreover, the transcripts of interleukin 1 beta (IL-1β) and tumor necrosis factor α (TNFα), which are related to atherosclerosis, were up-regulated by TCDD stimulation. In addition, the transcripts of Homo sapiens collagen, type IV, alpha 1 (COL4A1), and isoforms that trigger the MI pathway were up-regulated after TCDD exposure. Finally, we found enhanced platelet-derived growth factor (PDGF) and signal transducer and activator of transcription 5 (Stat5) expression with TCDD treatment in endothelial cells, which are involved in PAH induced by vascular injury.

Resveratrol for breast cancer prevention and therapy: Preclinical evidence and molecular mechanisms

Globally, breast cancer is the most frequently diagnosed cancer among women. The major unresolved problems with metastatic breast cancer is recurrence after receiving objective response to chemotherapy, drug-induced side effects of first line chemotherapy and delayed response to second line of treatment. Unfortunately, very few options are available as third line treatment. It is clear that under such circumstances there is an urgent need for new and effective drugs. Phytochemicals are among the most promising chemopreventive treatment options for the management of cancer. Resveratrol (3,5,4'-trihydroxy-trans-stilbene), a non-flavonoid polyphenol present in several dietary sources, including grapes, berries, soy beans, pomegranate and peanuts, has been shown to possess a wide range of health benefits through its effect on a plethora of molecular targets.The present review encompasses the role of resveratrol and its natural/synthetic analogue in the light of their efficacy against tumor cell proliferation, metastasis, epigenetic alterations and for induction of apoptosis as well as sensitization toward chemotherapeutic drugs in various in vitro and in vivo models of breast cancer. The roles of resveratrol as a phytoestrogen, an aromatase inhibitor and in stem cell therapy as well as adjuvent treatment are also discussed. This review explores the full potential of resveratrol in breast cancer prevention and treatment with current limitations, challenges and future directions of research.

Critical review of resveratrol in xenobiotic-induced hepatotoxicity

Use of natural products is increasingly popular. In fact, many patients with liver diseases self-medicate with herbal supplements. Resveratrol (RSV), in particular, is a common natural product that can reduce injury in experimental models of liver disease. Xenobiotic hepatotoxicity is a particularly important area-of-need for therapeutics. Drug-induced liver injury, for example, is the most common cause of acute liver failure (ALF) and ALF-induced deaths in many countries. Importantly, RSV protects against hepatotoxicity in animal models in vivo caused by several drugs and chemicals and may be an effective intervention. Although many mechanisms have been proposed to explain the protection, not all are consistent with other data. Furthermore, RSV suffers from other issues, including limited bioavailability due to extensive hepatic metabolism. The purpose of this article is to summarize recent findings on the protective effects of RSV in xenobiotic-induced liver injury and other forms of liver injury and to provide a critical review of the underlying mechanisms. New mechanisms that are more consistent with data emerging from the toxicology field are suggested. Efforts to move RSV into clinical use are also considered. Overall, RSV is a promising candidate for therapeutic use, but additional studies are needed to better understand its effects.

Differential Effects of Glycyrrhiza Species on Genotoxic Estrogen Metabolism: Licochalcone A Downregulates P450 1B1, whereas Isoliquiritigenin Stimulates It

Estrogen chemical carcinogenesis involves 4-hydroxylation of estrone/estradiol (E1/E2) by P450 1B1, generating catechol and quinone genotoxic metabolites that cause DNA mutations and initiate/promote breast cancer. Inflammation enhances this effect by upregulating P450 1B1. The present study tested the three authenticated medicinal species of licorice [Glycyrrhiza glabra (GG), G. uralensis (GU), and G. inflata (GI)] used by women as dietary supplements for their anti-inflammatory activities and their ability to modulate estrogen metabolism. The pure compounds, liquiritigenin (LigF), its chalcone isomer isoliquiritigenin (LigC), and the GI-specific licochalcone A (LicA) were also tested. The licorice extracts and compounds were evaluated for anti-inflammatory activity by measuring inhibition of iNOS activity in macrophage cells: GI ≫ GG > GU and LigC ≅ LicA ≫ LigF. The Michael acceptor chalcone, LicA, is likely responsible for the anti-inflammatory activity of GI. A sensitive LC-MS/MS assay was employed to quantify estrogen metabolism by measuring 2-MeOE1 as nontoxic and 4-MeOE1 as genotoxic biomarkers in the nontumorigenic human mammary epithelial cell line, MCF-10A. GG, GU, and LigC increased 4-MeOE1, whereas GI and LicA inhibited 2- and 4-MeOE1 levels. GG, GU (5 μg/mL), and LigC (1 μM) also enhanced P450 1B1 expression and activities, which was further increased by inflammatory cytokines (TNF-α and IFN-γ). LicA (1, 10 μM) decreased cytokine- and TCDD-induced P450 1B1 gene expression and TCDD-induced xenobiotic response element luciferase reporter (IC50 = 12.3 μM), suggesting an antagonistic effect on the aryl hydrocarbon receptor, which regulates P450 1B1. Similarly, GI (5 μg/mL) reduced cytokine- and TCDD-induced P450 1B1 gene expression. Collectively, these data suggest that, of the three licorice species that are used in botanical supplements, GI represents the most promising chemopreventive licorice extract for women's health. Additionally, the differential effects of the Glycyrrhiza species on estrogen metabolism emphasize the importance of standardization of botanical supplements to species-specific bioactive compounds.

Cytochrome P450 1 family and cancers

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcriptional factor that dimerizes with aryl hydrocarbon receptor nuclear translocator (ARNT). This complex binds to xenobiotics response element (XREs), and then starts the expressions of downstream genes including cytochrome P450 (CYP) 1 family members: CYP1A1, CYP1A2 and CYP1B1. Role of CYP1 family is involved in the metabolism of endogenous hormones, xenobiotics and drug. The expression of CYP1 family is regulated by estradiol (E2) or xenobiotics in diverse cancers. In breast cancers expressing estrogen receptors (ERs), level of CYP1B1 is increased by E2 and reversed by an estrogen receptor antagonist, ICI 182,780 or 4-hydrotamoxifen, which indicates that the expression of CYP1 family in downstream region of AhR is regulated by an activation of ERα. In metabolic pathways, E2 is converted into 4-hydroxyestradiol by CYP1B1, which can be converted into mainly estradiol-3,4-quinone, a potential carcinogen, by peroxidase. Increased expression of CYP1 family indicates the possibility of carcinogenesis by exposure of xenobiotics in endometrial and ovarian cancers. Apart from roles of CYP1 family in relation with ER pathway, CYP1 family is over-expressed in ER independent cancers. CYP1A1 exhibits hydroxylase activity in oxidation of arachidonic acid, which has been transformed to 12(R)-hydrxyeicosatetraenoic (HETEs), a potent activator of AhR activity. On the basis of results, phytoestrogens and dexamethasone are provided as cancer therapy regulating the expression of CYP1 family. Thus, this review focuses on the role(s) of CYP1 family in ER-dependent or ER-independent cancers and the potential for cancer therapy to target CYP1 family in these cancers.

SIN3A, generally regarded as a transcriptional repressor, is required for induction of gene transcription by the aryl hydrocarbon receptor

CYP1A1 bioactivates several procarcinogens and detoxifies several xenobiotic compounds. Transcription of CYP1A1 is highly induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) via the aryl hydrocarbon receptor. We recently described an RNAi high throughput screening performed in the Hepa-1 mouse hepatoma cell line, which revealed that SIN3A is necessary for the induction of CYP1A1-dependent ethoxyresorufin-o-deethylase (EROD) enzymatic activity by TCDD. In the current studies, we sought to provide insight into the role of SIN3A in this process, particularly because studies on SIN3A have usually focused on its repressive activity on transcription. We report that ectopic expression of human SIN3A in Hepa-1 cells enhanced EROD induction by TCDD and efficiently rescued TCDD induction of EROD activity in cells treated with an siRNA to mouse SIN3A, thus validating a role for SIN3A in CYP1A1 induction. We demonstrate that SIN3A is required for TCDD induction of the CYP1A1 protein in Hepa-1 cells but not for expression of the aryl hydrocarbon receptor protein. In addition, siRNAs for SIN3A decreased TCDD-mediated induction of CYP1A1 mRNA and EROD activity in human hepatoma cell line Hep3B. We establish that TCDD treatment of Hepa-1 cells rapidly increases the degree of SIN3A binding to both the proximal promoter and enhancer of the Cyp1a1 gene and demonstrate that increased binding to the promoter also occurs in human Hep3B, HepG2, and MCF-7 cells. These studies establish that SIN3A physically interacts with the CYP1A1 gene and extends the transcriptional role of SIN3A to a gene that is very rapidly and dramatically induced.

Smilax China root extract detoxifies nicotine by reducing reactive oxygen species and inducing CYP2A6

Resveratrol has a beneficial effect of lowering reactive oxygen species (ROS) and reduces cellular oxidative stress. We hypothesized that ethanol extract of Smilax china root (EESC) rich in resveratrol (RES) and oxyresveratrol (OXY) could reduce ROS caused by nicotine and promoting nicotine turnover by induction of CYP2A6. The amount of cotinine converted from nicotine was quantified by the direct barbiturate assay method. Expression of CYP2A6 was unregulated by RES, OXY, or EESC, respectively. Pretreatment of RES (50, 100, and 250 μM), OXY (50, 100, and 250 μM), and RES+OXY (50 and 100 μM) inhibited cytotoxicity and ROS production caused by nicotine in a dose-dependent manner. EESC pretreatment (1.8 mg/mL) increased cell viability by 1.5-fold higher than the control (nicotine only), and lowered cellular ROS levels. A significant amount of the conversion of nicotine to cotinine was observed in EESC pretreatment by CYP2A6 induction in HepG2 cells. These results suggested that hepatic induction of CYP2A6 and ROS reduction by EESC activate nicotine metabolism and reduce cellular oxidative stress.

PRACTICAL APPLICATION

Nicotine exposure due to smoking is very concerning because it is the major factor for lung diseases and cardiovascular disorders. It is necessary to examine natural ingredients that can detoxify from nicotine to cotinine as well as neutralize free radicals induced from nicotine. Results from the current study suggest potential applications of Smilax china root for detoxification of nicotine in the food industry.

Resveratrol protects against polychlorinated biphenyl-mediated impairment of glucose homeostasis in adipocytes

Resveratrol (RSV) is a plant polyphenol that exhibits several favorable effects on glucose homeostasis in adipocytes. Recent studies from our laboratory demonstrated that coplanar polychlorinated biphenyls (PCBs) that are ligands of the aryl hydrocarbon receptor impair glucose homeostasis in mice. PCB-induced impairment of glucose homeostasis was associated with augmented expression of inflammatory cytokines in adipose tissue, a site for accumulation of lipophilic PCBs. This study determined if RSV protects against PCB-77 induced impairment of glucose disposal in vitro and in vivo and if these beneficial effects are associated with enhanced nuclear factor erythoid 2-related factor 2 (Nrf2) signaling in adipose tissue. PCB-77 increased oxidative stress and abolished insulin stimulated 2-deoxy-d-glucose uptake in 3 T3-L1 adipocytes. These effects were restored by RSV, which resulted in a concentration-dependent increase in NAD(P)H:quinone oxidoreductase 1 (NQO1), the downstream target of Nrf2 signaling. We quantified glucose and insulin tolerance and components of Nrf2 and insulin signaling cascades in adipose tissue of male C57BL/6 mice administered vehicle or PCB-77 (50 mg/kg) and fed a diet with or without resVida (0.1%, or 160 mg/kg per day). PCB-77 impaired glucose and insulin tolerance, and these effects were reversed by RSV. PCB-77 induced reductions in insulin signaling in adipose tissue were also abolished by RSV, which increased NQO1 expression. These results demonstrate that coplanar PCB-induced impairment of glucose homeostasis in mice can be prevented by RSV, potentially through stimulation of Nrf2 signaling and enhanced insulin stimulated glucose disposal in adipose tissue.

Genetic polymorphisms in the CYP1A1 and CYP1B1 genes and susceptibility to bladder cancer: a meta-analysis

The current meta-analysis of case-control studies was conducted to evaluated the relationships of genetic polymorphisms in the CYP1A1 and CYP1B1 genes with the susceptibility to bladder cancer, aiming at determine whether these polymorphisms may contribute to the pathogenesis of bladder cancer. Related articles were determined via searching the following electronic databases without any language restrictions: PubMed, CISCOM, CINAHL, Web of Science, Google Scholar, EBSCO, Cochrane Library, and CBM databases for relevant articles published before November 1st, 2013. STATA 12.0 software was also selected to deal with statistical data. The relationships were evaluated using the pooled odds ratios (ORs) and their 95% confidence intervals (CI). Eleven case-control studies with a total of 2,609 bladder cancer patients and 2,634 healthy subjects met the inclusion criteria. The results of our meta-analysis demonstrated that CYP1A1 genetic polymorphisms were associated with increased risks of bladder cancer (allele model: RR = 1.18, 95% CI 1.07-1.30, P = 0.001; dominant model: RR = 1.15, 95% CI 1.05-1.27, P = 0.003; respectively), especially among 11599G>C, 2455A>G, 3810T>C, and 113T>C polymorphisms. A subgroup analysis by ethnicity was conducted to investigate its effect on susceptibility to bladder cancer. The subgroup analysis results revealed positive significant correlations between CYP1A1 genetic polymorphisms and bladder cancer risk among Asians (allele model: RR = 1.26, 95% CI 1.10-1.44, P = 0.001; dominant model: RR = 1.22, 95% CI 1.08-1.38, P = 0.001), but not among Caucasians (all P < 0.05). Nevertheless, we observed no significant correlations between CYP1B1 genetic polymorphisms and bladder cancer risk (all P > 0.05). Our meta-analysis indicates that CYP1A1 genetic polymorphisms may be involved in the pathogenesis of bladder cancer, especially among 11599G>C, 2455A>G, 3810T>C, and 113T>C polymorphisms. However, CYP1B1 genetic polymorphisms may not be important determinants of bladder cancer susceptibility.

Relationships between CYP1A1 genetic polymorphisms and bladder cancer risk: a meta-analysis

This meta-analysis aims at evaluating the relationships between CYP1A1 genetic polymorphisms and bladder cancer risk. The PubMed, CISCOM, CINAHL, Web of Science, Google Scholar, EBSCO, Cochrane Library, and CBM databases were searched from inception through November 1st, 2013 without language restrictions. Meta-analysis was conducted with the use of the STATA 12.0 software. The relationships were evaluated by calculating the pooled odds ratios (ORs) and their 95% confidence intervals (CIs). Eight case-control studies with a total of 2120 bladder cancer patients and 2061 healthy subjects met the inclusion criteria. Ten common polymorphisms in the CYP1A1 gene were assessed. The results of our meta-analysis suggested that CYP1A1 genetic polymorphisms might be strongly correlated with an increased risk of bladder cancer (allele model: OR=1.23, 95%CI=1.08-1.39, p=0.001; dominant model: OR=1.25, 95%CI=1.07-1.46, p=0.005; respectively), especially for 11599G>C, 2455A>G, 3810T>C, and 113T>C polymorphisms. A subgroup analysis was done to investigate the effect of ethnicity on an individual's risk of bladder cancer. Our results revealed positive significant correlations between CYP1A1 genetic polymorphisms and an increased risk of bladder cancer among Asians (allele model: OR=1.33, 95%CI=1.08-1.65, p=0.009; dominant model: OR=1.37, 95%CI=1.02-1.85, p=0.034; respectively), but not among Caucasians (all p<0.05). Our findings provide convincing evidence that CYP1A1 genetic polymorphisms may contribute to susceptibility to bladder cancer, especially for 11599G>C, 2455A>G, 3810T>C, and 113T>C polymorphisms among Asians.

Resveratrol potentiates effects of simvastatin on inhibition of rat ovarian theca-interstitial cells steroidogenesis

BACKGROUND

Polycystic ovary syndrome (PCOS) is characterized by ovarian enlargement, hyperplastic theca compartment and increased androgen production due to, at least in part, excessive expression of several key genes involved in steroidogenesis. Previously, our group has demonstrated that simvastatin, competitive inhibitor of 3-hydroxy-3-methyl-glutaryl-CoA reductase (HMG-CoA reductase), a rate-limiting step of the mevalonate pathway, reduces rat-theca interstitial cell steroidogenesis by inhibiting Cyp17a1 gene expression, the key enzyme of the androgen biosynthesis pathway. Recently, we demonstrated that resveratrol, a bioflavonoid abundant in red grapes, decreases rat theca-interstitial cell steroidogenesis and this suppressive effect is mediated through mechanisms independent of the mevalonate pathway. The present study evaluated the effect of combining simvastatin and resveratrol treatments on rat theca-interstitial cell steroidogenesis.

METHODS

Rat theca-interstitial cells isolated from 30 day-old female rats were cultured for up to 48 h with or without simvastatin (1 μM) and/or resveratrol (3-10 μM). Steroidogenic enzymes gene expression was evaluated by quantitative real time PCR and steroid levels were measured by liquid chromatography-mass spectrometry. Comparisons between groups were performed using ANOVA and Tukey test.

RESULTS

Resveratrol potentiated inhibitory effects of simvastatin on androstenedione and androsterone production in theca-interstitial cells. This suppressive effect correlated with profound inhibition in Cyp17a1 mRNA expression in the presence of a combination of resveratrol and simvastatin.

CONCLUSIONS

The present findings indicate that resveratrol potentiates the simvastatin-induced inhibitory effect on theca-interstitial cell androgen production, raising the possibility of development of novel treatments of PCOS.

Resveratrol Downregulates Cyp2e1 and Attenuates Chemically Induced Hepatocarcinogenesis in SD Rats

Cyp2e1 plays an important role in chemically induced hepatocarcinogenesis. Resveratrol (REV) is known to prevent diethylnitrosamine (DEN)-induced hepatocarcinogenesis, but its effects on this process induced by DEN and 2-acetylaminofluorene (2-AAF) and the role of Cyp2e1 remain unclear. In this study, glutathione S-transferase placental form (GST-P)-positive foci were used as a marker of hepatocarcinogenesis. REV or diallyl disulfide (DADS, an inhibitor of Cyp2e1) significantly reduced both the area and number of GST-P-positive foci induced by DEN and 2-AAF. Treatment with REV or DADS also markedly decreased the expression of Cyp2e1 in the rat liver. By immunohistochemical staining of serial liver sections, we found that the expression of Cyp2e1 in GST-P-positive foci showed three distinct patterns: decreased in GST-P foci, increased in GST-P foci when compared with surrounding liver tissue and mixed type. The number of GST-P foci with increased Cyp2e1 expression was greater than the number of GST-P foci with decreased Cyp2e1. Protein levels of GST-P and Cyp2e1 were also higher in foci compared with surrounding liver tissue. REV or DADS significantly reduced the expression of GST-P and Cyp2e1 in both foci and surrounding liver tissue. Taken together, these results suggested that REV has a significant inhibitory effect on chemically induced hepatocarcinogenesis, which may be attributed to downregulation of Cyp2e1.

Development of cardiac hypertrophy by sunitinib in vivo and in vitro rat cardiomyocytes is influenced by the aryl hydrocarbon receptor signaling pathway

Sunitinib (SUN) is a new tyrosine kinase inhibitor that possesses both anti-angiogenic and anti-tumor activities. Although SUN has improved survival rate in cancer patients, cardiotoxicity has been reported as a significant side effect. Several studies suggested a role for the aryl hydrocarbon receptor (AhR) and its regulated genes such as cytochrome P4501A1 (CYP1A1) in the pathogenesis of heart failure and cardiac hypertrophy. To test the hypothesis that SUN induces cardiac hypertrophy through the modulation of AhR, Wistar albino rats were treated for 15 and 30 days with increasing doses of SUN (25, 50, and 100 mg/kg), whereas at the in vitro level, rat cardiomyocyte H9c2 cells were incubated with SUN (1, 2.5, and 5 μM). Thereafter, cardiac hypertrophy parameters were determined at the biochemical, histopathology, and gene expression levels. SUN treatment causes increase in cardiac enzymes, changes in histopathology, and induction in several hypertrophic markers. This was associated with proportional increase in the CYP1A1 gene in a concentration- and time-dependent manner. The direct involvement of AhR in the SUN-induced cardiac hypertrophy in H9c2 cells was supported by the ability of resveratrol, an AhR antagonist, to block the SUN-induced hypertrophy and the ability of SB203580, a novel AhR agonist, to potentiate SUN-induced hypertrophic genes. This is the first demonstration that SUN induces hypertrophic genes in vivo and in vitro rat cardiomyocyte through AhR/CYP1A1-mediated mechanism.

Modulation of carcinogen-metabolizing cytochromes P450 by phytochemicals in humans

INTRODUCTION

Cytochrome P450 (CYP) families 1 - 3, besides oxidizing environmental and dietary chemicals, leading to their elimination, catalyze the bioactivation of exogenous as well as endogenous carcinogens. Phytochemicals, particularly those which are active food components, were shown to be able to affect specific CYP expression and/or activity in animal models and in human in vitro systems. Human intervention studies involving healthy volunteers were also performed. This review describes human CYP modulation by naturally occurring phytochemicals which can not only affect carcinogen metabolism in humans, but also change the drug response.

AREAS COVERED

The authors present an overview of carcinogens metabolizing human CYP modulation in different model systems as well as studies on human dietary intervention. Furthermore, the authors provide examples of the phytochemicals that affect CYP expression and activity.

EXPERT OPINION

CYP, which are involved in carcinogen activation, can metabolize a range of substrates and inducing CYP by one substrate may also increase the metabolism of another. The ultimate proof of the efficacy of CYP modulation strategy for chemoprevention may be provided by clinical trials involving risk populations, which are difficult to perform. The new human-like models are highly desired for the study of modulation of carcinogen-metabolizing CYP.

Obesity is mediated by differential aryl hydrocarbon receptor signaling in mice fed a Western diet

BACKGROUND

Obesity is a growing worldwide problem with genetic and environmental causes, and it is an underlying basis for many diseases. Studies have shown that the toxicant-activated aryl hydrocarbon receptor (AHR) may disrupt fat metabolism and contribute to obesity. The AHR is a nuclear receptor/transcription factor that is best known for responding to environmental toxicant exposures to induce a battery of xenobiotic-metabolizing genes.

OBJECTIVES

The intent of the work reported here was to test more directly the role of the AHR in obesity and fat metabolism in lieu of exogenous toxicants.

METHODS

We used two congenic mouse models that differ at the Ahr gene and encode AHRs with a 10-fold difference in signaling activity. The two mouse strains were fed either a low-fat (regular) diet or a high-fat (Western) diet.

RESULTS

The Western diet differentially affected body size, body fat:body mass ratios, liver size and liver metabolism, and liver mRNA and miRNA profiles. The regular diet had no significant differential effects.

CONCLUSIONS

The results suggest that the AHR plays a large and broad role in obesity and associated complications, and importantly, may provide a simple and effective therapeutic strategy to combat obesity, heart disease, and other obesity-associated illnesses.

Resveratrol enhances solar UV-induced responses in normal human epidermal keratinocytes

Resveratrol (RV) differentially affects UV-induced death/pro-survival pathways in normal and tumor cells. On these grounds, RV-containing topical products have been developed to prevent UV-associated tumorigenesis/damage to human skin. In this study, we evaluated mechanisms of combined effects of RV and low-dose solar simulated UVA+UVB or 6-formylindo[3,2-b]carbazole (FICZ), a product of tryptophan photo-oxidation known to mediate UV effects, on the inflammatory, metabolic and proliferative responses of cultured normal human epidermal keratinocytes (HEK). Applied alone, RV, UV and FICZ induced time- and dose-dependent activation of aryl hydrocarbon receptor (AhR) pathway followed by over-expression of Cyp1A1 (metabolic response), UV and RV induced IL-8 expression (inflammatory response), while RV enhanced also HEK proliferation revealed by MTT assay and (3)H-thymidine incorporation. In the combined treatment, RV synergized with both UV and FICZ, leading to further activation of AhR machine, Cyp1A1 transcription and IL-8 expression, the latter partly AhR-dependent as assessed by AhR silencing. RV enhanced UV-induced NFkappaB activation and nuclear translocation of epidermal growth factor receptor. By contrast, proliferative effect of RV was abolished in the presence of UV, whereas synergic anti-proliferative action of RV+UV was observed in the Nrf2-silenced HEK. Our data suggest cooperative effects of RV-specific and UV-/FICZ-activated transcription factors leading to deregulated inflammatory, metabolic and proliferative responses of HEK.

The anticancer effects of resveratrol: modulation of transcription factors

Resveratrol (3, 4', 5-trihydroxystilbene), a naturally occurring phytoalexin readily available in the diet, is reported to possess both chemopreventive and chemotherapeutic activities in several cancers. However, despite the identification of numerous molecular targets, the underlying mechanisms involved in the anticancer activities of resveratrol are not completely understood. Resveratrol is postulated to function as a potential signaling pathway modulator and, as such, is demonstrated to affect a multitude of signal transduction pathways associated with tumorigenesis and/or carcinogenesis; it is likely that this collective activity, rather than just a single effect, may play an important role in the anticancer properties of resveratrol. Since transcription factors control the expression of many genes, the elucidation of molecular targets of resveratrol involved in transcriptional regulation is necessary to better understand how this dietary phytochemical affects chemopreventive and chemotherapeutic processes. As a result, investigators have increasingly searched for and examined possible targets of resveratrol. In this review, we summarize the current knowledge on molecular targets, specifically transcription factors, that contribute to the observed anticancer effects of resveratrol related to 1) inhibition of carcinogenic activation and induction of carcinogen detoxification, 2) induction of growth arrest and apoptosis, and 3) suppression of proinflammatory signaling pathways related to cancer progression.

Aryl hydrocarbon receptor ligand effects in RBL2H3 cells

The aryl hydrocarbon receptor (AHR) mediates toxic effects of dioxin and xenobiotic metabolism. AHR has an emerging role in the immune system, but its physiological ligands and functional role in immunocytes remain poorly understood. Mast cells are immunocytes that are central to inflammatory responses and release a spectrum of pro-inflammatory mediators including histamine, mast cell proteases, and pro-inflammatory cytokines such as IL-6 upon stimulation. The aim was to investigate the AHR in model mast cells and examine how both putative and known AHR ligands, e.g., kynurenine, kynurenic acid (KA), Resveratrol, indolmycin, and violacein, affect mast cell activation and signaling. These ligands were tested on calcium signaling, degranulation, and gene expression. The data show that AHR is present in three model mast cell lines, and that various known and putative AHR ligands regulate gene expression of Cyp1a1, a gene down-stream of AHR. Furthermore, it was found that calcium influxes and mast cell secretory responses were enhanced or suppressed after chronic treatment with AHR agonists or antagonists, and that AHR ligands modified RBL2H3 cell degranulation. AHR ligands can chronically change cytokine gene expression in activated mast cells, as exemplified by IL-6. The antagonist Resveratrol repressed expression of induced IL-6 gene expression. Although KA and kynurenine are both AHR agonists, these ligands behaved differently in regards to degranulation and IL-6 expression, indicating that they may function outside of AHR pathways. These data suggest considerable complexity in RBL2H3 responses to AHR ligands, with implications for understanding of both dioxin pathology and the immunological effects of endogenous AHR ligands.

Sensitivity to dioxin decreases as zebrafish mature

The embryos of teleost fish are exquisitely sensitive to the toxic effects of exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). However, several lines of evidence suggest that adults are less sensitive to TCDD. To better understand and characterize this difference between early life stage and adults, we exposed zebrafish (Danio rerio) to graded TCDD concentrations at different ages. The LD(50) for embryos exposed at 1 day post-fertilization (dpf) was more than an order of magnitude lower than it was for juveniles exposed at 30 dpf. The latency between exposure and response also increased with age. Embryo toxicity was characterized by marked cardiovascular collapse and heart malformation, whereas juveniles exposed at 30 dpf had no detectable cardiovascular toxicity. In juveniles, the effects of TCDD exposure included stunted growth, altered pigmentation, and skeletal malformations. Furthermore, the transcriptional profile produced in hearts exposed to TCDD as embryos had very little overlap with the transcriptional changes induced by TCDD at 30 dpf. The early cardiotoxic response was associated with fish exposed prior to metamorphosis from the larval to the adult body plan at approximately 14 dpf. Our results show conclusively that the developmental stage at the time of exposure controls the toxic response to TCDD.

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

The aryl hydrocarbon receptor (AhR), a transcription factor mediating xenobiotic detoxification, plays a considerable role in regulatory T cell (Treg) induction. Tregs regulate the immune system, thus suppressing allergies and autoimmune diseases. This study aims to identify new types of antiallergic dietary factors, with focus on the flavonoids with potential AhR agonistic activity. Among 25 dietary flavonoid samples tested using a reporter assay, 8 showed marked induction of AhR-dependent transcriptional activity. The subsequent T cell proliferation suppression assay identified naringenin as the only sample capable of stimulating Treg induction; notably, this induction was eliminated by cotreatment with AhR antagonists. Indeed, naringenin induced CD4(+)Foxp3(+) Tregs, irrespective of the presence of the transforming growth factor-β (TGF-β), indicating that the conventional TGF-β-dependent signaling pathway might not be involved.

Changes in expression of drug-metabolizing enzymes by single-walled carbon nanotubes in human respiratory tract cells

Single-walled carbon nanotubes (SWCNTs) have attracted attention for biomedical and biotechnological applications, such as drug delivery. However, there are concerns about the safety of SWCNTs for use in humans. To investigate the potential use of SWCNTs for targeted drug delivery to the lung, we examined their effect on drug-metabolizing enzymes in primary normal human bronchial epithelial (NHBE) cells from two donors and the lung carcinoma A549 cell line. Exposure of NHBE and A549 cells to SWCNTs dysregulated some of the important drug-metabolizing enzymes expressed in the human respiratory organs. Exposure of NHBE cells to SWCNTs for 24 h had a pronounced effect on expression of CYP1A1 and CYP1B1 mRNAs, which were reduced to less than 1% of control cells. These effects were also observed in A549 cells. Exposure of A549, HepG2 hepatic carcinoma cells, and MCF-7 breast carcinoma cells to tetrachlorodibenzo-p-dioxin induced the expression and enzymatic activity of CYP1A1 and CYP1B1, which were also suppressed by SWCNTs, suggesting that SWCNTs down-regulated both basal and induced CYP1A1 and CYP1B1 activities. Chromatin immunoprecipitation assays revealed that the down-regulatory effect of SWCNTs may be due to inhibition of activated aryl hydrocarbon receptor binding to the enhancer regions of the CYP1A1 and CYP1B1 genes. Down-regulation of CYP1A1 and CYP1B1 genes by SWCNTs may affect the defense mechanisms by reducing procarcinogen bioactivation in the human lung.

BRCA-1 promoter hypermethylation and silencing induced by the aromatic hydrocarbon receptor-ligand TCDD are prevented by resveratrol in MCF-7 cells

Epigenetic mechanisms may contribute to reduced expression of the tumor suppressor gene BRCA-1 in sporadic breast cancers. Through environmental exposure and diet, humans are exposed to xenobiotics and food compounds that bind the aromatic hydrocarbon receptor (AhR). AhR-ligands include the dioxin-like and tumor promoter 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD). The activated AhR regulates transcription through binding to xenobiotic response elements (XREs=GCGTG) and interactions with transcription cofactors. Previously, we reported on the presence of several XREs in the proximal BRCA-1 promoter and that the expression of endogenous AhR was required for silencing of BRCA-1 expression by TCDD. Here, we document that in estrogen receptor-α-positive and BRCA-1 wild-type MCF-7 breast cancer cells, the treatment with TCDD attenuated 17β-estradiol-dependent stimulation of BRCA-1 protein and induced hypermethylation of a CpG island spanning the BRCA-1 transcriptional start site of exon-1a. Additionally, we found that TCDD enhanced the association of the AhR; DNA methyl transferase (DNMT)1, DNMT3a and DNMT3b; methyl binding protein (MBD)2; and trimethylated H3K9 (H3K9me3) with the BRCA-1 promoter. Conversely, the phytoalexin resveratrol, selected as a prototype dietary AhR antagonist, antagonized at physiologically relevant doses (1 μmol/L) the TCDD-induced repression of BRCA-1 protein, BRCA-1 promoter methylation and the recruitment of the AhR, MBD2, H3K9me3 and DNMTs (1, 3a and 3b). Taken together, these observations provide mechanistic evidence for AhR agonists in the establishment of BRCA-1 promoter hypermethylation and the basis for the development of prevention strategies based on AhR antagonists.

Harmaline and harmalol inhibit the carcinogen-activating enzyme CYP1A1 via transcriptional and posttranslational mechanisms

Dioxins are known to cause several human cancers through activation of the aryl hydrocarbon receptor (AhR). Harmaline and harmalol are dihydro-β-carboline compounds present in several medicinal plants such as Peganum harmala. We have previously demonstrated the ability of P. harmala extract to inhibit TCDD-mediated induction of Cyp1a1 in murine hepatoma Hepa 1c1c7 cells. Therefore, the aim of this study is to examine the effect of harmaline and its main metabolite, harmalol, on dioxin-mediated induction of CYP1A1 in human hepatoma HepG2 cells. Our results showed that harmaline and harmalol at concentrations of (0.5-12.5μM) significantly inhibited the dioxin-induced CYP1A1 at mRNA, protein and activity levels in a concentration-dependent manner. The role of AhR was determined by the inhibition of the TCDD-mediated induction of AhR-dependent luciferase activity and the AhR/ARNT/XRE formation by both harmaline and harmalol. In addition, harmaline significantly displaced [(3)H]TCDD in the competitive ligand binding assay. At posttranslational level, both harmaline and harmalol decreased the protein stability of CYP1A1, suggesting that posttranslational modifications are involved. Moreover, the posttranslational modifications of harmaline and harmalol involve ubiquitin-proteasomal pathway and direct inhibitory effects of both compounds on CYP1A1 enzyme. These data suggest that harmaline and harmalol are promising agents for preventing dioxin-mediated effects.

Plant polyphenols differentially modulate inflammatory responses of human keratinocytes by interfering with activation of transcription factors NFκB and AhR and EGFR-ERK pathway

Molecular mechanisms underlying modulation of inflammatory responses in primary human keratinocytes by plant polyphenols (PPs), namely the glycosylated phenylpropanoid verbascoside, the stilbenoid resveratrol and its glycoside polydatin, and the flavonoid quercetin and its glycoside rutin were evaluated. As non-lethal stimuli, the prototypic ligand for epidermal growth factor receptor (EGFR) transforming growth factor alpha (TGFalpha), the combination of tumor necrosis factor (TNFalpha) and interferon (IFNgamma) (T/I), UVA+UVB irradiation, and bacterial lipopolysaccharide (LPS) were used. We demonstrated differential modulation of inflammatory responses in keratinocytes at signal transduction, gene transcription, and protein synthesis levels as a function of PP chemical structure, the pro-inflammatory trigger used, and PP interaction with intracellular detoxifying systems. The PPs remarkably inhibited constitutive, LPS- and T/I-induced but not TGFalpha-induced ERK phosphorylation. They also suppressed NFkappaB activation by LPS and T/I. Verbascoside and quercetin invariably impaired EGFR phosphorylation and UV-associated aryl hydrocarbon receptor (AhR)-mediated signaling, while rutin, polydatin and resveratrol did not affect EGFR phosphorylation and further activated AhR machinery in UV-exposed keratinocytes. In general, PPs down-regulated gene expression of pro-inflammatory cytokines/enzymes, except significant up-regulation of IL-8 observed under stimulation with TGFalpha. Both spontaneous and T/I-induced release of IL-8 and IP-10 was suppressed, although 50μM resveratrol and polydatin up-regulated IL-8. At this concentration, resveratrol activated both gene expression and de novo synthesis of IL-8 and AhR-mediated mechanisms were involved. We conclude that PPs differentially modulate the inflammatory response of human keratinocytes through distinct signal transduction pathways, including AhR and EGFR.

Comparative gene responses to collected ambient particles in vitro: endothelial responses

Epidemiologic studies associate exposure to ambient particulate matter (APM) with increased cardiovascular mortality. Since both pulmonary inflammation and systemic circulation of ultrafine particles are hypothesized as initiating cardiovascular effects, we examined responses of potential target cells in vitro. Human aortic endothelial cells (HAEC) were exposed to 10 μg/ml fine and ultrafine APM collected in an urban setting in summer 2006 or winter 2007 in the San Joaquin Valley, California. RNA isolated after 3 h was analyzed with high-density oligonucleotide arrays. Summer APM treatment affected genes involved in xenobiotic and oxidoreductase activity, transcription factors, and inflammatory responses in HAEC, while winter APM had a robust xenobiotic but lesser inflammatory response. Real-time polymerase chain reaction analysis confirmed that particulate matter (PM)-treated HAEC increased mRNA levels of xenobiotic response enzymes CYP1A1, ALDH1A3, and TIPARP and cellular stress response transcription factor ATF3. Inflammatory response genes included E-selectin, PTGS2, CXCL-2 (MIP-2α), and CCL-2 (MCP-1). Multiplex protein assays showed secretion of IL-6 and MCP-1 by HAEC. Since induction of CYP1A1 is mediated through the ligand-activated aryl hydrocarbon receptor (AhR), we demonstrated APM induced AhR nuclear translocation by immunofluorescence and Western blotting and activation of the AhR response element using a luciferase reporter construct. Inhibitor studies suggest differential influences of polycyclic aromatic hydrocarbon signaling, ROS-mediated responses and endotoxin alter stress and proinflammatory endothelial cell responses. Our findings demonstrate gene responses correlated with current concepts that systemic inflammation drives cardiovascular effects of particulate air pollution. We also demonstrate a unique pattern of gene responses related to xenobiotic metabolism in PM-exposed HAEC.

Upregulation of ABCG2 by romidepsin via the aryl hydrocarbon receptor pathway

Histone deacetylase inhibitors (HDACI) are promising anticancer agents and their use in combination with conventional anticancer drugs is currently under investigation. We previously reported cell line-specific upregulation of ABCG2, a multidrug resistance transporter shown to control oral bioavailability and CNS penetration, by the HDACI romidepsin, although the precise mechanism in a particular cell line remains to be determined. The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that can be activated by numerous environmental contaminants and has been shown to be a client protein of heat shock protein 90 (Hsp90). A xenobiotic response element was defined in the ABCG2 promoter and was shown to mediate AhR signaling. Activated AhR was found to be associated with the ABCG2 promoter only in cell line models that respond to romidepsin with ABCG2 upregulation. Our data suggest that romidepsin acetylated Hsp70 and inhibited the chaperone function of Hsp90, thereby allowing the dissociation of AhR from Hsp90. The dissociation of AhR from Hsp90 may be a prerequisite for the differential upregulation of ABCG2 by romidepsin. Increasing our understanding of the mechanism(s) governing differential upregulation of ABCG2 in response to romidepsin could provide an insight into strategies needed to tackle resistance to HDACIs in cancer therapeutics.

Effects of combined treatment with resveratrol and indole-3-carbinol

Male Wistar rats received a semisynthetic diet with resveratrol (100 mg/kg), indole-3-carbinol (20 mg/kg), or a mixture of these compounds in the same doses for 1 week. Activities of ethoxyresorufin dealkylase (EROD), methoxyresorufin dealkylase (MROD), pentoxyresorufin dealkylase (PROD), and 6β-testosterone hydroxylase (6β-TH) and the content of mRNA for CYP1A1, CYP1A2, and CYP3A1 were elevated in the liver of rats receiving indole-3-carbinol. These changes were accompanied by an increase in activity of phase II xenobiotic metabolism enzymes (quinone reductase, hemoxygenase-1, glutathione transferase, and UDP glucuronosyl transferase). Resveratrol did not modify activity of these enzymes. After combined treatment with the test compounds, resveratrol suppressed the indole-3-carbinol-induced increase in activities of EROD, MROD, PROD, and 6β-TH, and expression of the corresponding genes. Combined treatment was characterized by potentiation of the antioxidant effects of these compounds.

Induction of cytochromes P450 1A1 and 1A2 by tanshinones in human HepG2 hepatoma cell line

Diterpenoid tanshinones including tanshinone IIA (TIIA), cryptotanshinone (CTS), tanshinone I (TI) and dihydrotanshinone I (DHTI) are the major bioactive components from Danshen. The major aim of our present study was to investigate the induction potential of these four main components of tanshinones (TIIA, CTS, TI, and DHTI) on the expression of CYP1A1 and CYP1A2 in HepG2 cells. Our results showed that all of these four tanshinones caused a significant time- and concentration-dependent increase in the amount of CYP1A1/2 expression in HepG2 cells. These induction effects were further characterized through transcriptional regulation: the induction of CYP1A1/2 mRNA level by tanshinones was completely blocked by the transcription inhibitor actinomycin D; the expression of CYP1A1/2 heterogeneous nuclear RNA was induced by tanshinone treatment; and CYP1A1 mRNA stability was not influenced by these tanshinones. Interestingly, tanshinones plus B[a]P produced additive/synergistic effect on CYP1A1/2 induction. In addition, the tanshinone-induced CYP1A1/2 expression was abolished by the aryl hydrocarbon receptor (AhR) antagonist resveratrol, suggesting an AhR dependent transcription mechanism. In the reporter gene assay, while TI and DHTI significantly induced AhR-dependent luciferase activity, TIIA and CTS failed to induce this activity. Collectively, the tanshinones could induce CYP1A1 and CYP1A2 expression through transcriptional activation mechanism and exert differential effects on activating AhR in HepG2 cells. Our findings suggest that rational administration of tanshinones should be considered with respect to their effect on AhR and CYP1A1/2 expression.

Inhibition of aryl hydrocarbon receptor-dependent transcription by resveratrol or kaempferol is independent of estrogen receptor α expression in human breast cancer cells

Resveratrol and kaempferol are natural chemopreventative agents that are also aryl hydrocarbon receptor (AHR) antagonists and estrogen receptor (ER) agonists. In this study we evaluated the role of ERα in resveratrol- and kaempferol-mediated inhibition of AHR-dependent transcription. Kaempferol or resveratrol inhibited dioxin-induced cytochrome P450 1A1 (CYP1A1) and CYP1B1 expression levels and recruitment of AHR, ERα and co-activators to CYP1A1 and CYP1B1. Both phytochemicals induced the expression and recruitment of ERα to gene amplified in breast cancer 1 (GREB1). RNAi-mediated knockdown of ERα in T-47D cells did not affect the inhibitory action of either phytochemical on AHR activity. Both compounds also inhibited AHR-dependent transcription in ERα-negative MDA-MB-231 and BT-549 breast cancer cells. These data show that ERα does not contribute to the AHR-inhibitory activities of resveratrol and kaempferol.

Estrogen receptor expression is required for low-dose resveratrol-mediated repression of aryl hydrocarbon receptor activity

The putative cardioprotective and chemopreventive properties of the red wine phenolic resveratrol (RES) have made it the subject of a growing body of clinical and basic research. We have begun investigations focusing on the effects of RES on the activity of the aryl hydrocarbon receptor (AHR) complex. Our evidence suggests that RES is a potent repressor of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible gene transcription in estrogen receptor (ER)-positive human breast, lung, and colon cancer cell lines. RES activates the transcription of the ER target genes to the same degree as estradiol (E(2)) in human MCF-7 breast cancer cells. Unlike E(2), which can only diminish TCDD-inducible CYP1A1 gene transcription by approximately 50%, RES can completely abrogate this response. Furthermore, 50% repression of TCDD-inducible transcription can be achieved with 100 nM RES, approximately 2.5 orders of magnitude lower than concentrations required for maximal inhibition, suggesting that multiple mechanisms are responsible for this effect. RES (100 nM) does not prevent ligand binding of a TCDD analog, nor does it prevent AHR from binding to its response element in the 5'-regulatory region of the CYP1A1 gene. Small inhibitory RNAs directed to ERα have demonstrated that RES-mediated repression of CYP1A1 depends on ERα. Whereas CYP1A1 protein levels in MCF-7 cells are refractory to the low-dose transcriptional effects of RES, a concomitant decrease in CYP1A1 protein levels is observed in Caco-2 cells. These results highlight a low-dose RES effect that could occur at nutritionally relevant exposures and are distinct from the high-dose effects often characterized.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.