Inhibition of 7,12-dimethylbenz[a]anthracene-induced rat mammary tumor growth by aryl hydrocarbon receptor agonists

Role of Aryl Hydrocarbon Receptor and Oxidative Stress in the Regioselective Toxicities of Hydroxychrysenes in Embryonic Japanese Medaka (Oryzias latipes)

Oxygenated polycyclic aromatic hydrocarbons (oxy-PAHs) are environmental contaminants that can be created through oxidation of parent PAHs. Previous studies have found that 2-hydroxychrysene (2-OHCHR) caused anemia in embryonic Japanese medaka whereas 6-hydroxychrysene (6-OHCHR) did not, an example of regioselective toxicity. Anemia was prevented by cytochrome P450 (CYP) inhibition, which reduced the formation of the potential oxidatively active metabolite, 1,2-catechol, from 2-OHCHR. 2-OHCHR has also been found to be a four-fold more potent aryl hydrocarbon receptor (AhR) agonist compared with 6-OHCHR. These findings led us to hypothesize that AhR activation and/or oxidative stress play an important role in 2-OHCHR toxicity. Although treatments with the AhR agonists polychlorinated biphenyl (PCB)126 and 2-methoxychrysene (2-MeOCHR) did not cause significant toxicity, pretreatments with the AhR antagonist, CH-223191, reduced anemia by 97.2 ± 0.84% and mortality by 96.6 ± 0.69%. Aryl hydrocarbon receptor inhibition by the antagonist was confirmed by significant reductions (91.0 ± 9.94%) in induced ethoxyresorufin-O-deethylase activity. Thiobarbituric acid reactive substances concentrations were 32.9 ± 3.56% higher (p < 0.05) in 2-OHCHR treatments at 100 hours postfertilization compared with controls. Staining 2-OHCHR-treated embryos with the reactive oxygen species (ROS) scavenger 2',7'-dichlorofluorescin diacetate revealed 32.6 ± 2.69% of 2-OHCHR-treated embryos exhibiting high concentrations of ROS in caudal tissues, which is a site for embryonic hematopoiesis in medaka. Pretreatment with antioxidants, N-acetylcysteine (NAC) or vitamin E (Vit E) significantly reduced 2-OHCHR-induced anemia (NAC: 80.7 ± 1.12% and Vit E: 99.1 ± 0.43%) and mortality (NAC: 67.1 ± 1.69% and Vit E: 98.9 ± 0.66%). These results indicate that AhR may mediate 2-OHCHR toxicity through canonical signaling by up-regulating CYP1, enhancing the formation of reactive metabolites of 2-OHCHR that generate ROS within caudal hematopoietic tissues, potentially disrupting hematopoiesis, leading to anemia and subsequent mortality. Environ Toxicol Chem 2023;42:698-706. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

The Role of the Aryl Hydrocarbon Receptor (AhR) and Its Ligands in Breast Cancer

Breast cancer is a complex disease which is defined by numerous cellular and molecular markers that can be used to develop more targeted and successful therapies. The aryl hydrocarbon receptor (AhR) is overexpressed in many breast tumor sub-types, including estrogen receptor -positive (ER+) tumors; however, the prognostic value of the AhR for breast cancer patient survival is not consistent between studies. Moreover, the functional role of the AhR in various breast cancer cell lines is also variable and exhibits both tumor promoter- and tumor suppressor- like activity and the AhR is expressed in both ER-positive and ER-negative cells/tumors. There is strong evidence demonstrating inhibitory AhR-Rα crosstalk where various AhR ligands induce ER degradation. It has also been reported that different structural classes of AhR ligands, including halogenated aromatics, polynuclear aromatics, synthetic drugs and other pharmaceuticals, health promoting phytochemical-derived natural products and endogenous AhR-active compounds inhibit one or more of breast cancer cell proliferation, survival, migration/invasion, and metastasis. AhR-dependent mechanisms for the inhibition of breast cancer by AhR agonists are variable and include the downregulation of multiple genes/gene products such as CXCR4, MMPs, CXCL12, SOX4 and the modulation of microRNA levels. Some AhR ligands, such as aminoflavone, have been investigated in clinical trials for their anticancer activity against breast cancer. In contrast, several publications have reported that AhR agonists and antagonists enhance and inhibit mammary carcinogenesis, respectively, and differences between the anticancer activities of AhR agonists in breast cancer may be due in part to cell context and ligand structure. However, there are reports showing that the same AhR ligand in the same breast cancer cell line gives opposite results. These differences need to be resolved in order to further develop and take advantage of promising agents that inhibit mammary carcinogenesis by targeting the AhR.

Emodin inhibits U87 glioblastoma cells migration by activating aryl hydrocarbon receptor (AhR) signaling pathway

Aryl hydrocarbon receptor (AhR) is a ligand-activated receptor to mediates the biological reactions of many environmental and natural compounds, which is highly expressed in glioblastoma. Although it has been reported that AhR agonist emodin can suppress some kinds of tumors, its inhibitory effect on glioblastoma migration and its relationship with AhR remain unclear. Based on the complexity of tumor pathogenesis and the tissue specificity of AhR, we hope can further understand the effect of emodin on glioblastoma and explore its mechanism. We found that the inhibitory effect of emodin on the migration of U87 glioblastoma cells increased with time, and the cell migration ability was inhibited by about 25% after 36 h exposure. In this process, emodin promoted the expression of the tumor suppressor IL24 by activating the AhR signaling pathway. Reducing the expression of AhR or IL24 by interfering RNA could block or relieve the inhibitory effect of emodin on the U87 cells migration, which indicates the inhibition of emodin on the migration of glioblastoma is mediated by the AhR-IL24 axis. Our data proved the AhR-IL24 signal axis is an important pathway for emodin to inhibit the migration of glioblastoma, and the AhR signaling pathway can be used as a key target to research the regulation effect and its mechanism of compounds on glioblastoma migration.

Gut bacterial metabolites modulate endoplasmic reticulum stress

Background

The endoplasmic reticulum (ER) is a membranous organelle that maintains proteostasis and cellular homeostasis, controlling the fine balance between health and disease. Dysregulation of the ER stress response has been implicated in intestinal inflammation associated with inflammatory bowel disease (IBD), a chronic condition characterized by changes to the mucosa and alteration of the gut microbiota. While the microbiota and microbially derived metabolites have also been implicated in ER stress, examples of this connection remain limited to a few observations from pathogenic bacteria. Furthermore, the mechanisms underlying the effects of bacterial metabolites on ER stress signaling have not been well established.

Results

Utilizing an XBP1s-GFP knock-in reporter colorectal epithelial cell line, we screened 399 microbiome-related metabolites for ER stress pathway modulation. We find both ER stress response inducers (acylated dipeptide aldehydes and bisindole methane derivatives) and suppressors (soraphen A) and characterize their activities on ER stress gene transcription and translation. We further demonstrate that these molecules modulate the ER stress pathway through protease inhibition or lipid metabolism interference.

Conclusions

Our study identified novel links between classes of gut microbe-derived metabolites and the ER stress response, suggesting the potential for these metabolites to contribute to gut ER homeostasis and providing insight into the molecular mechanisms by which gut microbes impact intestinal epithelial cell homeostasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-021-02496-8.

Regulation of Aryl Hydrocarbon Receptor Signaling Pathway and Dioxin Toxicity by Novel Agonists and Antagonists

Dioxins, mostly through activation of aryl hydrocarbon receptor (AhR), are potent toxic substances widely distributed in the environment, while moderated suppression of AhR also exhibits anti-tumor effects. Therefore, the proper modulation of AhR activity may counteract AhR-mediated toxicities and certain diseases. In this investigation, we identified several novel AhR moderate agonists and antagonists using chemical biology approaches. The mechanisms and mode of interactions with AhR by these hits were also revealed using both experimental and computational studies. The newly identified AhR moderate agonists and antagonists were predicted to bind to AhR and modulate AhR signaling. The structure-activity relationships of moderate agonists and antagonists and their unique binding features with AhR have created a solid framework for further optimization of the next generation of AhR modulators.

Aryl hydrocarbon receptor-microRNA-212/132 axis in human breast cancer suppresses metastasis by targeting SOX4

BACKGROUND

MicroRNAs (miRNAs) are a class of short non-coding RNAs that pave a new avenue for understanding immune responses and cancer progression. Although the miRNAs are involved in breast cancer development, their axis with the transcription factors that show therapeutic potential in breast cancer is largely unknown. Previous studies showed anti-metastatic roles of agonist-activated aryl hydrocarbon receptor (Ahr) in various breast cancer cell lines. Recently, we demonstrated that agonist-activated Ahr induced a highly conserved miRNA cluster, named miR-212/132, in murine cellular immune compartment. Therefore, current study was performed to examine if this miRNA cluster mediates the anti-metastatic properties of Ahr agonists.

METHODS

The expression of miR-212/132 cluster and coding genes were examined by real-time PCR, and the protein levels were detected by western blot. The 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 3,3'-diindolylmethane (DIM) were used to activate Ahr in MDA-MB-231 and T47D breast cancer cells. Chromatin immunoprecipitation (ChIP) assay was used to identify the binding site(s) for Ahr on miR-212/132 promoter. For prediction of potentially target gene of the miRNA cluster, bioinformatics analysis was carried out, and to test targeting, luciferase activity was quantified. Besides, biological effects of Ahr-miR-212/132 axis were examined in vitro by cell migration, expansion and invasion, and examined in vivo by orthotopic model of spontaneous metastasis.

RESULTS

The miR-212/132 cluster was transcriptionally activated in MDA-MB-231 and T47D cells by TCDD and DIM, and this activation was regulated by Ahr. A reciprocal correlation was identified between Ahr agonists-induced miR-212/132 and the pro-metastatic SRY-related HMG-box4 (SOX4), and a new specific binding sites for miR-212/132 were identified on the untranslated region (3'UTR) of SOX4. Interestingly, miR-212/132 over-expression showed direct anti-migration, anti-expansion and anti-invasion properties, and an inhibition of the miRNA cluster mitigated the anti-invasive properties of TCDD and DIM. Further in vivo studies demonstrated that the Ahr-miR-212/132-SOX4 module was induced by Ahr activation.

CONCLUSION

Taken together, the findings provide the first evidences of the synergistic anti-metastatic properties of miR-212/132 cluster through suppression of SOX4. Also, current study suggest a new miRNA-based mechanism elucidating the anti-metastatic properties of Ahr agonists, suggesting possibility of using miR-212/132 to control metastasis in breast cancer patients.

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.

Ligand promiscuity of aryl hydrocarbon receptor agonists and antagonists revealed by site-directed mutagenesis

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that can be activated by structurally diverse chemicals. To examine the mechanisms responsible for the promiscuity in AhR ligand binding, we determined the effects of mutations within the AhR ligand-binding domain (LBD) on the activity of diverse AhR ligands. Site-directed mutagenesis identified Ile319 of the mouse AhR and, to a lesser extent, Phe318 as residues involved in ligand-selective modulation of AhR transformation using a panel of 12 AhR ligands. These ligands could be categorized into four distinct structurally related groups based on their ability to activate AhR mutants at position 319 in vitro. The mutation I319K was selectively activated by FICZ and not by other examined ligands in vitro and in cell culture. F318L and F318A mutations resulted in the conversion of AhR agonists β-naphthoflavone and 3-methylcholanthrene, respectively, into partial agonists/antagonists. Hsp90 binding to the AhR was decreased with several mutations and was inversely correlated with AhR ligand-binding promiscuity. Together, these data define overlapping amino acid residues within the AhR LBD involved in the selectivity of ligand binding, the agonist or antagonist mode of ligand binding, and hsp90 binding and provide insights into the ligand diversity of AhR activators.

Role of the aryl hydrocarbon receptor in carcinogenesis and potential as a drug target

The aryl hydrocarbon receptor (AHR) is highly expressed in multiple organs and tissues, and there is increasing evidence that the AHR plays an important role in cellular homeostasis and disease. The AHR is expressed in multiple tumor types, in cancer cell lines, and in tumors from animal models, and the function of the AHR has been determined by RNA interference, overexpression, and inhibition studies. With few exceptions, knockdown of the AHR resulted in decreased proliferation and/or invasion and migration of cancer cell lines, and in vivo studies in mice overexpressing the constitutively active AHR exhibited enhanced stomach and liver cancers, suggesting a pro-oncogenic role for the AHR. In contrast, loss of the AHR in transgenic mice that spontaneously develop colonic tumors and in carcinogen-induced liver tumors resulted in increased carcinogenesis, suggesting that the receptor may exhibit antitumorigenic activity prior to tumor formation. AHR ligands also either enhanced or inhibited tumorigenesis, and these effects were highly tumor specific, demonstrating that selective AHR modulators that exhibit agonist or antagonist activities represent an important new class of anticancer agents that can be directed against multiple tumors.

Activation of the aryl hydrocarbon receptor by TCDD inhibits mammary tumor metastasis in a syngeneic mouse model of breast cancer

Treatment with aryl hydrocarbon receptor (AhR) agonists can slow or reverse the growth of primary mammary tumors in rodents, which has fostered interest in developing selective AhR modulators for treatment of breast cancer. However, the major goal of breast cancer therapy is to inhibit metastasis, the primary cause of mortality in women with this disease. Studies conducted using breast cancer cell lines have demonstrated that AhR agonists suppress proliferation, invasiveness, and colony formation in vitro; however, further exploration using in vivo models of metastasis is warranted. To test the effect of AhR activation on metastasis, 4T1.2 mammary tumor cells were injected into the mammary gland fat pad of syngeneic Balb/c mice treated with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Primary tumor growth was monitored for 4 weeks, at which time metastasis was determined. TCDD treatment suppressed metastasis by approximately 50%, as measured both in the lung and in mammary glands at sites distant from the primary tumor. Primary tumor growth was not suppressed by TCDD exposure nor was proliferation of 4T1.2 cells affected by TCDD treatment in vitro. Taken together, these results suggest that the protective effect of AhR activation was selective for the metastatic process and not simply the result of a direct decrease in tumor cell proliferation or survival at the primary site. These observations in immunologically intact animals warrant further investigation into the mechanism of the protective effects of AhR activation and support the promise for use of AhR modulators to treat breast cancer.

Eicosapentaenoic acid protects against 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced hepatic toxicity in cultured rat hepatocytes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a persistent and ubiquitous environmental contaminant. The health impact of TCDD exposure is of great concern to the general public. Recent reports have implied that eicosapentaenoic acid (EPA) might be a potential chemopreventive agent and influence hepatotoxicity. The aim of the current study was to explore the effectiveness of EPA in alleviating the toxicity of TCDD on primary cultured rat hepatocytes. EPA (5, 10 and 20 μM) was added to cultures alone or simultaneously with TCDD (5 and 10 μM). Rat hepatocytes were treated with TCDD and EPA for 48 h, and then cytotoxicity was detected by [3-(4,5-dimethyl-thiazol-2-yl) 2,5-diphenyltetrazolium bromide] (MTT) assay and lactate dehydrogenase (LDH) release, while total antioxidant capacity (TAC) and total oxidative stress (TOS) levels were determined to evaluate the oxidative injury. The DNA damage was also analyzed by liver micronucleus assay (LMN) and 8-oxo-2-deoxyguanosine (8-OH-dG). The results of MTT and LDH assays showed that TCDD but not EPA decreased cell viability. TCDD also increased TOS level and significantly decreased TAC level in rat hepatocytes in a clear dose dependent manner. On the basis of increasing doses, the dioxin caused significant increases of micronucleated hepatocytes (MNHEPs) and 8-OH-dG as compared to control culture. Whereas, in cultures treated with EPA alone, TOS level did not change and the level of TAC significantly increased. The presence of EPA with TCDD minimized the toxic effects of the dioxin on primary hepatocytes cultures. Noteworthy, EPA has a protective effect against TCDD-mediated DNA damages.

Exactly the same but different: promiscuity and diversity in the molecular mechanisms of action of the aryl hydrocarbon (dioxin) receptor

The Ah receptor (AhR) is a ligand-dependent transcription factor that mediates a wide range of biological and toxicological effects that result from exposure to a structurally diverse variety of synthetic and naturally occurring chemicals. Although the overall mechanism of action of the AhR has been extensively studied and involves a classical nuclear receptor mechanism of action (i.e., ligand-dependent nuclear localization, protein heterodimerization, binding of liganded receptor as a protein complex to its specific DNA recognition sequence and activation of gene expression), details of the exact molecular events that result in most AhR-dependent biochemical, physiological, and toxicological effects are generally lacking. Ongoing research efforts continue to describe an ever-expanding list of ligand-, species-, and tissue-specific spectrum of AhR-dependent biological and toxicological effects that seemingly add even more complexity to the mechanism. However, at the same time, these studies are also identifying and characterizing new pathways and molecular mechanisms by which the AhR exerts its actions and plays key modulatory roles in both endogenous developmental and physiological pathways and response to exogenous chemicals. Here we provide an overview of the classical and nonclassical mechanisms that can contribute to the differential sensitivity and diversity in responses observed in humans and other species following ligand-dependent activation of the AhR signal transduction pathway.

Aryl hydrocarbon receptor activation during pregnancy, and in adult nulliparous mice, delays the subsequent development of DMBA-induced mammary tumors

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), the prototypic ligand for the aryl hydrocarbon receptor (AhR), promotes tumor formation in some model systems. However, with regard to breast cancer, epidemiological and animal studies are inconclusive as to whether exposure increases tumor incidence or may instead be protective. We have previously reported that mice exposed to TCDD during pregnancy have impaired differentiation of mammary tissue, including decreased branching and poor development of lobuloalveolar structures. Because normal pregnancy-induced mammary differentiation may protect against subsequent neoplastic transformation, we hypothesized that TCDD-treated mice would be more susceptible to chemical carcinogenesis after parturition. To test this, mice were treated with TCDD or vehicle during pregnancy. Four weeks later, 7,12-dimethylbenz[a]anthracene (DMBA) was administered to induce mammary tumor formation. Contrary to our hypothesis, TCDD-exposed parous mice showed a 4-week delay in tumor formation relative to controls, and they had a lower tumor incidence throughout the 27-week time course. The same results were obtained in nulliparous mice given TCDD and DMBA on the same schedule. We next addressed whether the delayed tumor incidence was a reflection of decreased tumor initiation, by testing the formation of DMBA-DNA adducts and preneoplastic lesions, induction of cytochrome P450s, and cell proliferation. None of these markers of tumor initiation differed between vehicle- and TCDD-treated animals. The expression of CXCL12 and CXCR4 was also measured to address their possible role in tumorigenesis. Taken together, our results suggest that AhR activation by TCDD slows the promotion of preneoplastic lesions to overt mammary tumors.

The aryl hydrocarbon receptor as a target for estrogen receptor-negative breast cancer chemotherapy

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and the relatively non-toxic selective aryl hydrocarbon receptor (AhR) modulator 6-methyl-1,3,8-trichlorodibenzo-furan (MCDF) induced CYP1A1-dependent ethoxyresorufin O-deethylase activity and inhibited proliferation of seven estrogen receptor (ER) negative breast cancer cell lines. MCDF, TCDD and structurally related 2,3,7,8-tetrachlorodibenzofuran, 1,2,3,7,8-pentachlorodibenzo-p-dioxin, 2,3,4,7,8-pentachlorodibenzofuran, and 3,3',4,4',5-pentachlorobiphenyl induced CYP1A1 and inhibited proliferation of BT-474 and MDA-MB-468 cells. In BT474 and MDA-MB-468 cells transfected with a small inhibitory RNA for the AhR, the antiproliferative activity of the chlorinated aromatic compounds was reversed, whereas for MCDF, only partial reversal was observed, suggesting that this compound acts through both AhR-dependent and AhR-independent pathways in these two cell lines. MCDF also inhibited tumor growth in athymic nude mice in which MDA-MB-468 cells were injected directly into the mammary fat pad. These results suggest that the AhR is a potential drug target for treatment of ER-negative breast cancer.

Potential therapeutic significance of increased expression of aryl hydrocarbon receptor in human gastric cancer

AIM: To determine the functional significance of aryl hydrocarbon receptor (AhR) in gastric carcinogenesis, and to explore the possible role of AhR in gastric cancer (GC) treatment.

METHODS: RT-PCR, real-time PCR, and Western blotting were performed to detect AhR expression in 39 GC tissues and five GC cell lines. AhR protein was detected by immunohistochemistry (IHC) in 190 samples: 30 chronic superficial gastritis (CSG), 30 chronic atrophic gastritis (CAG), 30 intestinal metaplasia (IM), 30 atypical hyperplasia (AH), and 70 GC. The AhR agonist tetrachlorodibenzo-para-dioxin (TCDD) was used to treat AGS cells. MTT assay and flow cytometric analysis were performed to measure the viability, cell cycle and apoptosis of AGS cells.

RESULTS: AhR expression was significantly increased in GC tissues and GC cell lines. IHC results indicated that the levels of AhR expression gradually increased, with the lowest levels in CSG, followed by CAG, IM, AH and GC. AhR expression and nuclear translocation were significantly higher in GC than in precancerous tissues. TCDD inhibited proliferation of AGS cells via induction of growth arrest at the G1-S phase.

CONCLUSION: AhR plays an important role in gastric carcinogenesis. AhR may be a potential therapeutic target for GC treatment.

The selective aryl hydrocarbon receptor modulator 6-methyl-1,3,8-trichlorodibenzofuran inhibits prostate tumor metastasis in TRAMP mice

The aryl hydrocarbon receptor (AhR) is a basic-helix-loop-helix transcription factor that binds halogenated aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and endogenous compounds. We previously reported that AhR null (Ahr(-/-)) transgenic adenocarcinoma of the mouse prostate (TRAMP) mice on a C57BL/6J background develop prostate tumors with much greater frequency than AhR wild-type (Ahr(+/+)) TRAMP mice, suggesting that the AhR has tumor suppressor properties. Because AhR signaling pathway inactivation increased susceptibility to prostate tumorigenesis, we tested the hypothesis that a selective AhR modulator (SAhRM), 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), can protect against prostate tumorigenesis. TRAMP mice on the standard C57BL/6JxFVB genetic background were fed 0, 10, or 40mg 6-MCDF/kg diet beginning at 8 weeks of age. Tumor incidence, pelvic lymph node metastasis, and serum vascular endothelial growth factor (VEGF) concentrations were determined at 140 days of age. Prostate tumor incidence and size were not significantly reduced in mice fed 6-MCDF. However, the frequency of pelvic lymph node metastasis was reduced fivefold in mice fed the 40mg 6-MCDF/kg diet. Serum VEGF concentrations were also reduced by 6-MCDF treatment, particularly in mice without prostate tumors, and 6-MCDF was shown to act directly on cultured prostates to inhibit VEGF secretion. Together, these results suggest that 6-MCDF inhibits metastasis, in part, by inhibiting prostatic VEGF production prior to tumor formation. This is the first report that 6-MCDF can confer protection against prostate cancer in vivo.

A role for the aryl hydrocarbon receptor in mammary gland tumorigenesis

The aryl hydrocarbon receptor (AhR) is an evolutionarily conserved transcription factor bound and activated by ubiquitous environmental pollutants. Historically, the AhR has been studied for its transcriptional regulation of genes encoding cytochrome P450 enzymes, which metabolize many of these chemicals into mutagenic and toxic intermediates. However, recent studies demonstrate that the AhR plays an important role in the biology of several cell types in the absence of environmental chemicals. Here, this paradigm shift is discussed in the context of a putative role for the AhR in mammary gland tumorigenesis. Data demonstrating high levels of constitutively active AhR in mammary tumors are summarized. Particular focus is placed on the likelihood that the AhR contributes to ongoing mammary tumor cell growth and on the possibility that the AhR inhibits apoptosis while promoting transition to an invasive, metastatic phenotype. A working model is proposed that may help explain the sometimes contradictory outcomes observed after AhR manipulation and that serves as a blueprint for the design of therapeutics which target the AhR in breast cancer. The theme that malignant cells reveal the functions for which the AhR has been evolutionarily conserved is presented throughout this discussion.

Effect of biochanin A on the aryl hydrocarbon receptor and cytochrome P450 1A1 in MCF-7 human breast carcinoma cells

Phytoestrogen biochanin A is an isoflavone derivative isolated from red clover Trifolium pratense with anticarcinogenic properties. This study examined the action of biochanin A with the carcinogen activation pathway that is mediated by the aryl hydrocarbon receptor (AhR) in MCF-7 breast carcinoma cells. Treating the cells with biochanin A alone caused the accumulation of CYP1A1 mRNA and an increase in CYP1A1-specific 7-ethoxyresorufin O-deethylase (EROD) activity in a dose dependent manner. A concomitant treatment with 7,12-dimethylbenz[a]anthracene (DMBA) and biochanin A markedly reduced the DMBA-inducible EROD activity and CYP1A1 mRNA level. In addition, the biochanin A treatment alone activated the DNA-binding capacity of the AhR for the dioxin-response element (DRE) of CYP1A1, as measured by the electrophoretic-mobility shift assay (EMSA). EMSA revealed that biochanin A reduced the level of the DMBA-inducible AhR-DRE binding complex. Furthermore, biochanin A competed with the prototypical AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), for binding to the AhR in an isolated rat cytosol. The biochanin A competitively inhibited the metabolic activation of DMBA, as measured by the formation of the DMBA-DNA adducts. These results suggest that biochanin A may thus be a natural ligand to bind on AhR. Therefore, biochanin A may be due to act an antagonist/agonist of the AhR pathway.

Role of the aryl hydrocarbon receptor in drug metabolism

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that regulates the transcription of certain key enzymes involved in the metabolism of xenobiotic substances including some drugs. The AhR can be activated by a wide range of classes of compounds (e.g. polycyclic aromatic hydrocarbons, benzimidazoles and flavonoids), and interacts with a number of other proteins, including nuclear hormone receptors such as the oestrogen and androgen receptors. Activation of the AhR antagonises the oestrogen receptor and can lead to modulation of its transcriptional activity; thus, activating the AhR may serve as a target for breast cancer therapy. Disruption of normal signalling by drug interactions with the AhR or downstream components of this pathway could result in adverse effects, such as the bioactivation of procarcinogens or the disruption of normal homeostasis. The cytochrome P450s CYP1A1, -1B1, -1A2 and -2S1 are regulated by the AhR, and they are all involved in the metabolism of endogenous substrates as well as xenobiotics. Polymorphisms in the AhR, or polymorphisms in enzymes regulated by the AhR, may cause variations in response to certain drugs in different individuals; this needs to be taken into consideration when administering drugs that interact with this pathway.

Functional interaction of nuclear receptor coactivator 4 with aryl hydrocarbon receptor

Aryl hydrocarbon receptor (AhR) transcriptional activity is enhanced by interaction with p160 coactivators. We demonstrate here that NcoA4, a nuclear receptor coactivator, interacts with and amplifies AhR action. NcoA4-AhR and NcoA4-ARNT interactions were demonstrated by immunoprecipitation in T47D breast cancer and COS cells and was independent of ligand. Overexpression of NcoA4 enhanced AhR transcriptional activity 3.2-fold in the presence of dioxin, whereas overexpression of a splice variant, NcoA4beta, as well as a variant lacking the C-terminal region enhanced AhR transcriptional activity by only 1.6-fold. Enhanced AhR signaling by NcoA4 was independent of the LXXLL and FXXLF motifs or of the activation domain. NcoA4 protein localized to cytoplasm in the absence of dioxin and in both the cytoplasm and nucleus following dioxin treatment. NcoA4-facilitation of AhR activity was abolished by overexpression of androgen receptor, suggesting a potential competition of AhR and androgen receptor for NcoA4. These findings thus demonstrate a functional interaction between NcoA4 and AhR that may alter AhR activity to affect disease development and progression.

Growth of a human mammary tumor cell line is blocked by galangin, a naturally occurring bioflavonoid, and is accompanied by down-regulation of cyclins D3, E, and A

Introduction

This study was designed to determine if and how a non-toxic, naturally occurring bioflavonoid, galangin, affects proliferation of human mammary tumor cells. Our previous studies demonstrated that, in other cell types, galangin is a potent inhibitor of the aryl hydrocarbon receptor (AhR), an environmental carcinogen-responsive transcription factor implicated in mammary tumor initiation and growth control. Because some current breast cancer therapeutics are ineffective in estrogen receptor (ER) negative tumors and since the AhR may be involved in breast cancer proliferation, the effects of galangin on the proliferation of an ER^-, AhR^high line, Hs578T, were studied.

Methods

AhR expression and function in the presence or absence of galangin, a second AhR inhibitor, α-naphthoflavone (α-NF), an AhR agonist, indole-3-carbinol, and a transfected AhR repressor-encoding plasmid (FhAhRR) were studied in Hs578T cells by western blotting for nuclear (for instance, constitutively activated) AhR and by transfection of an AhR-driven reporter construct, pGudLuc. The effects of these agents on cell proliferation were studied by ³H-thymidine incorporation and by flow cytometry. The effects on cyclins implicated in mammary tumorigenesis were evaluated by western blotting.

Results

Hs578T cells were shown to express high levels of constitutively active AhR. Constitutive and environmental chemical-induced AhR activity was profoundly suppressed by galangin as was cell proliferation. However, the failure of α-NF or FhAhRR transfection to block proliferation indicated that galangin-mediated AhR inhibition was either insufficient or unrelated to its ability to significantly block cell proliferation at therapeutically relevant doses (IC₅₀ = 11 μM). Galangin inhibited transition of cells from the G₀/G₁ to the S phases of cell growth, likely through the nearly total elimination of cyclin D3. Expression of cyclins A and E was also suppressed.

Conclusion

Galangin is a strong inhibitor of Hs578T cell proliferation that likely mediates this effect through a relatively unique mechanism, suppression of cyclin D3, and not through the AhR. The results suggest that this non-toxic bioflavonoid may be useful as a chemotherapeutic, particularly in combination with agents that target other components of the tumor cell cycle and in situations where estrogen receptor-specific therapeutics are ineffective.

The aryl hydrocarbon receptor constitutively represses c-myc transcription in human mammary tumor cells

The aryl hydrocarbon receptor (AhR) is an environmental carcinogen-activated transcription factor associated with tumorigenesis. High levels of apparently active AhR characterize a variety of tumors, even in the absence of environmental ligands. Despite this association between transformation and AhR upregulation, little is known of the transcriptional consequences of constitutive AhR activation. Here, the effects of constitutively active and environmental ligand-induced AhR on c-myc, an oncogene whose promoter contains six AhR-binding sites (AhREs (aryl hydrocarbon response elements)), were investigated. A reporter containing the human c-myc promoter, with its six AhREs and two NF-kappaB-binding sites, was constructed. This vector, and variants with deletions in the NF-kappaB and/or AhR-binding sites, was transfected into a human breast cancer cell line, Hs578T, which expresses high levels of apparently active, nuclear AhR. Results indicate that: (1) the AhR constitutively binds the c-myc promoter; (2) there is a low but significant baseline level of c-myc promoter activity, which is not regulated by NF-kappaB and is not affected by an environmental AhR ligand; (3) deletion of any one of the AhREs has no effect on constitutive reporter activity, while deletion of all six increases reporter activity approximately fivefold; (4) a similar increase in reporter activity occurs when constitutively active AhR is suppressed by transfection with an AhR repressor plasmid (AhRR); (5) AhRR transfection significantly increases background levels of endogenous c-myc mRNA and c-Myc protein. These results suggest that the AhR influences the expression of c-Myc, a protein critical to malignant transformation.

Hormonal environment in the induction of breast cancer in castrated rats using dimethylbenzanthracene: influence of the presence or absence of ovarian activity and of treatment with estradiol, tibolone, and raloxifene

OBJECTIVE

The influence of hormone therapy on the induction or the promotion of breast cancer has yet to be determined. Recent studies establish a cause-effect relation between hormones and cancer, although epidemiological data and studies of tumor behavior give rise to doubts. The aim of the study was to observe and evaluate the influence of different hormonal environments on the induction of breast cancer in a well-established experimental model.

DESIGN

In this experimental animal study, breast cancer was induced by using a single intragastric dose of 20 mg of dimethylbenzanthracene in prepubertal Sprague-Dawley rats randomized into five groups: group 1 (control); group 2 (castrated prepubertal animals); and groups 3, 4, and 5 (castration of prepubertal animals followed by hormonal treatment starting at puberty [11 weeks] with tibolone, raloxifene, and estradiol, respectively). Follicle-stimulating hormone and estradiol levels were measured at 6, 11, 16, and 31 weeks.

RESULTS

Absence of ovarian activity was observed in groups 2, 3, 4, and 5, as well as the expected variations in hormone levels in all groups. Breast cancers were obtained in 100% of the animals in the control group, with an average of four (two to seven) tumors per animal in this group. Only one cancer appeared in groups 2, 3, and 4, and none appeared in group 5.

CONCLUSIONS

In this experimental model and using the hormone treatments chosen, neither the treatments nor the absence of ovarian activity induced breast cancer.

Development of selective aryl hydrocarbon receptor modulators for treatment of breast cancer

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix DNA-binding protein that forms a transcriptionally-active heterodimer with the AhR nuclear translocator (Arnt) protein. The nuclear AhR complex is a ligand-induced transcription factor and the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a high affinity ligand for the AhR. TCDD induces a diverse spectrum of tissue-, sex- and species-specific biochemical and toxic responses in Ah-responsive cells/tissues including the inhibition of 17beta-oestradiol (E2)-induced gene expression in the rodent uterus and mammary and in human breast cancer cell lines. TCDD also inhibits spontaneous and carcinogen-induced mammary tumour formation and growth in rodent models. Research in this laboratory has utilised the AhR as a target for developing anticancer drugs for treatment of breast cancer and two different structural classes of selective AhR modulators (SAhRMs) have been developed. Alternate-substituted (1,3,6,8- and 2,4,6,8-) alkyl polychlorinated dibenzofurans (PCDFs) and substituted diindolylmethanes (DIMs) bind the AhR and induce a pattern of AhR-oestrogen receptor (ER) inhibitory cross-talk similar to that observed for TCDD including inhibition of mammary tumour growth at doses < 1.0 mg/kg/day. In contrast, effective doses of these compounds do not induce hepatic CYP1A1-dependent activity or other AhR-mediated toxic responses induced by TCDD. These results indicate that SAhRMs may be an important new class of drugs for clinical treatment of breast cancer via AhR-ER inhibitory cross-talk.

Activation of Nur77 by selected 1,1-Bis(3'-indolyl)-1-(p-substituted phenyl)methanes induces apoptosis through nuclear pathways

Nur77 is an orphan receptor and a member of the nerve growth factor-I-B subfamily of the nuclear receptor family of transcription factors. Based on the results of transactivation assays in pancreatic and other cancer cell lines, we have now identified for the first time Nur77 agonists typified by 1,1-bis(3-indolyl)-1-(p-anisyl)methane that activate GAL4-Nur77 chimeras expressing wild-type and the ligand binding domain (E/F) of Nur77. In Panc-28 pancreatic cancer cells, Nur77 agonists activate the nuclear receptor, and downstream responses include decreased cell survival and induction of cell death pathways, including tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) and poly(ADP-ribose) polymerase (PARP) cleavage. Moreover, the transactivation and apoptotic responses are also induced in other pancreatic, prostate, and breast cancer cells that express Nur77. In Panc-28 cells, small inhibitory RNA for Nur77 reverses ligand-dependent transactivation and induction of TRAIL and PARP cleavage. Nur77 agonists also inhibit tumor growth in vivo in athymic mice bearing Panc-28 cell xenografts. These results identify compounds that activate Nur77 through the ligand binding domain and show that ligand-dependent activation of Nur77 through nuclear pathways in cancer cells induces cell death and these compounds are a novel class of anticancer agents.

Lack of effects of postnatal exposure to a mixture of aryl hydrocarbon-receptor agonists on the development of methylnitrosourea-induced mammary tumors in sprague-dawley rats

There are concerns that early life exposure to organochlorines, including aryl hydrocarbon receptor (AhR) agonists, may lead to long-term effects and increase the risk of developing breast cancer. Our objective was to test if postnatal exposure to a mixture of 2,3,7,8-tetrachlorodibenzodioxin (TCDD)-like chemicals would modulate the development of methylnitrosourea (MNU)-induced mammary tumors. Females received by gavage a mixture containing 3 non-ortho-polychlorinated biphenyls (PCBs), 6 polychlorinated dibenzodioxins (PCDDs), and 7 polychlorinated dibenzofurans (PCDFs), at 1, 5, 10, 15, and 20d of age. The doses were equivalent to 0, 1, 10, 100, or 1000 times the amount ingested through breast milk by a human infant during its first 24 d of life. Subgroups of 1000 x reated rats and controls were sacrificed at 21 d of age for assessment of mammary-gland development, cell death, and proliferation. Mammary-tumor development was assessed in MNU (30 mg/kg body weight ip at 50 days of age)-induced rats pre-exposed to the mixture (MNU-0, MNU-1, MNU-10, MNU-100, MNU-1000). Rats were sacrificed when their mammary tumors reached 1 cm in diameter, or when the rats reached > or = 32 wk of age. Mammary-gland whole mounts were analyzed with all palpable and microscopic lesions (n = 1563) histologically classified and grouped as benign, intraductal proliferations, or malignant. There were no marked effects on age at onset of puberty (vaginal opening) and estrous cyclicity. Despite a significant decrease in proliferating cell nuclear antigen (PCNA)-positive mammary cells in 1000 x treated 21-d-old rats, there were no long-term dose-response effects on mammary-gland morphology and tumor development. In conclusion, postnatal exposure to the mixture of AhR agonists had no significant effects on the development of MNU-initiated mammary tumors.

The Ah receptor inhibits estrogen-induced estrogen receptor β in breast cancer cells

We have studied the effect of the aryl hydrocarbon receptor ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on estrogen receptor (ER) beta gene expression in the human breast cancer cell line, T47D. TCDD inhibited 17beta-estradiol (E2)-induced up-regulation of both ER beta wild type and ER beta cx mRNA. Cycloheximide pre-treatment had no inhibitory effect, and the estimated half-life of ER beta mRNA of about 33 min was not changed by any hormone administration. Chromatin immunoprecipitation experiments showed recruitment of ER alpha to the ER beta promoter. Gel mobility shift experiments revealed an E2-induced protein binding to a half site estrogen response element in the ER beta promoter, and TCDD reduced that binding. These results show that ER alpha regulates the expression of its own heterodimerization partner, ER beta, in T47D cells. TCDD, an anti-estrogenic compound, inhibits ER alpha-mediated induction of ER beta mRNA. These findings add to our understanding of cross talk between dioxin and estrogen signaling in human cells.

Interaction of the aryl hydrocarbon receptor ligand 6-methyl-1,3,8-trichlorodibenzofuran with estrogen receptor alpha

The polycyclic aromatic hydrocarbon 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) is related to the industrial byproduct dioxin and is a weak agonist and partial antagonist at the aryl hydrocarbon receptor (AhR). Tamoxifen is used for the treatment and prevention of breast cancer and interferes with the interaction of estrogen with estrogen receptor alpha (ER). The combination of MCDF and tamoxifen lowered the effective dose of both drugs required to inhibit 7,12-dimethylbenz(a)anthracene-induced mammary tumor growth in rats and protected against the estrogenic effects of tamoxifen on the uterus in rats (A. McDougal et al., Cancer Res 2001;61:3902-7), pointing to the potential use of MCDF in breast cancer treatment. Potential AhR-ER cross-talk is evidenced by the antiestrogenic activity of MCDF and the degradative effect of MCDF on ER protein levels. Our studies confirmed that MCDF degraded the ER. MCDF displayed antiestrogenic activity at higher concentrations in MCF-7 human breast cancer cells, but MCDF alone (10(-6) M) stimulated the growth of MCF-7 cells. MCDF also activated an estrogen response element (ERE)-luciferase reporter and increased mRNA levels of the estrogen-responsive gene transforming growth factor (TGF)-alpha. The estrogenic effects of MCDF are ER dependent because they were blocked by the pure antiestrogen ICI 182,780. MCDF induced ER-coactivator interaction in glutathione S-transferase pull-down assays and the formation of an ER.ERE complex in gel mobility shift assays, further indicating that the estrogenic actions of MCDF are mediated by the ER. In addition, knockdown of the AhR with small interfering RNA did not affect MCDF-induced ERE-luciferase activity. Overall, these data support the conclusion that MCDF is a partial agonist at the ER. This study provides the first evidence for the direct interaction of the ER with MCDF and challenges the view that MCDF is simply an AhR-specific ligand.

Aryl hydrocarbon receptor-mediated inhibition of LNCaP prostate cancer cell growth and hormone-induced transactivation

LNCaP prostate cancer cells express the aryl hydrocarbon receptor (AhR), and treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 protein and an Ah-responsive reporter gene. Similar results were obtained with the selective AhR modulator 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF); however, TCDD but not 6-MCDF induced degradation of the AhR protein. TCDD and 6-MCDF inhibited growth of LNCaP cells, and inhibitory AhR-androgen receptor (AR) crosstalk was investigated in cells transfected with constructs containing the androgen-responsive probasin promoter (-288 to +28) (pPB) or three copies of the -244 to -96 region of this promoter (pARR(3)). Ten nanomolar dihydrotestosterone (DHT) and 17 beta-estradiol (E2) induced transactivation in LNCaP cells transfected with pPB or pARR(3); however, inhibitory AhR-AR crosstalk was observed only with the latter construct. 6-MCDF and TCDD did not inhibit DHT- or E2-induced transactivation in ZR-75 human breast cancer cells, indicating that these interactions were promoter and cell context-dependent. Both E2 and DHT stabilized AR protein in LNCaP cells; however, cotreatment with TCDD or 6-MCDF decreased AR protein levels. These results indicate that inhibitory AhR-AR crosstalk in prostate cancer cells is complex and for some responses, AR protein stability may play a role.

Increased arylhydrocarbon receptor expression offers a potential therapeutic target for pancreatic cancer

The arylhydrocarbon receptor (AhR) was initially identified as a member of the adaptive metabolic and toxic response pathway to polycyclic aromatic hydrocarbons and to halogenated dibenzo-p-dioxins and dibenzofurans. In the present study, we sought to determine the functional significance of the AhR pathway in pancreatic carcinogenesis. AhR expression was analysed by Northern blotting. The exact site of AhR expression was analysed by in situ hybridization and immunohistochemistry. The effects of TCDD and four selective AhR agonists on pancreatic cancer cell lines were investigated by growth assays, apoptosis assays, and induction of the cyclin-dependent kinase inhibitor p21. There was strong AhR mRNA expression in 14 out of 15 pancreatic cancer samples, weak expression in chronic pancreatitis tissues, and faint expression in all normal pancreata. In pancreatic cancer tissues, AhR mRNA and protein expression were localized in the cytoplasm of pancreatic cancer cells. TCDD and the four AhR agonists inhibited pancreatic cancer cell growth in a dose-dependent manner, and decreased anchorage-independent cell growth. DAPI staining did not reveal nuclear fragmentation and CYP1A1 and was not induced by TCDD and AhR agonists. In contrast, TCDD and AhR agonists induced the expression of the cyclin-dependent kinase inhibitor p21. In conclusion, the relatively non-toxic AhR agonists caused growth inhibition in pancreatic cancer cells with high AhR expression levels via cell cycle arrest. In addition, almost all human pancreatic cancer tissues expressed this receptor at high levels, suggesting that these or related compounds may play a role in the therapy of pancreatic cancer in the future.

Endocrine disruptors: a new scientific role for clinical pharmacologists? Impact on human health, wildlife, and the environment

It is important for the clinical pharmacologist to understand the potential human health implications of exposure to environmental chemicals that may act as hormonally active agents. It is necessary to have an understanding of how pharmaceutical and personal care products and other chemicals affect the ecosystem of planet Earth and to understand how they may negatively contribute to human disease. Clinical pharmacologists must understand the various definitions of endocrine disruptors and be able to "decipher" these terms for their patients. Understanding the need for the EPA endocrine disruptor screening program and possessing knowledge of the screening assays used to assess endocrine activity potential are two essential components relevant to the topic of endocrine disruptors. Clinical pharmacologists have an opportunity to play an important role in resolving the question of what role endocrine disruptors play in initiating human disease since some scientists argue that the present evidence is not compelling. Clinical pharmacologists can also play an important role in the evaluation of the risk assessment and use of risk management and risk communication tools required to address public health concerns related to actions of endocrine disruptors. It is important that clinical pharmacologists work with veterinary clinical pharmacologists, toxicologists, industrial chemists, regulators, the scientific community, the general public, and environmental groups to understand the impact of endocrine disruptors on human health, wildlife, and the environment with an ultimate goal to minimize and/or alleviate the unwanted, detrimental effects of the endocrine disruptors.

Molecular biology of the Ah receptor and its role in carcinogenesis

The aryl hydrocarbon receptor (AhR) is a ligand-activated nuclear transcription factor that mediates responses to toxic halogenated aromatic toxins such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic hydrocarbons, combustion products, and numerous phytochemicals such as flavonoids and indole-3-carbinol (I3C). The nuclear AhR complex is a heterodimer containing the AhR and AhR nuclear translocator (Arnt) proteins, and the molecular mechanism of AhR action is associated with binding of the heterodimer to dioxin responsive elements (DREs) in regulatory regions of Ah-responsive genes. TCDD, a 'xenodioxin', is a multi-site carcinogen in several species and possibly in humans, whereas natural AhR ligands including I3C and flavonoids tend to protect against cancer. Both TCDD and phytochemicals inhibit estrogen-induced breast and endometrial cancer, and the molecular mechanisms of this common response will be described.

Methyl-substituted diindolylmethanes as inhibitors of estrogen-induced growth of T47D cells and mammary tumors in rats

Diindolylmethane (DIM) is formed by acid catalyzed dimerization of the phytochemical indole-3-carbinol, and both compounds inhibit formation and/or growth of mammary tumors in rodents. In this study, we have investigated the aryl hydrocarbon receptor (AhR) agonist activity and inhibitory AhR-estrogen receptor crosstalk induced by the following methyl-substituted DIMs: 1,1'-dimethyl-, 2,2'-dimethyl-, 5,5'-dimethyl-, 6,6'-dimethyl-, and 7,7'-dimethylDIM and 1,1',2,2'-tetramethylDIM. The six compounds bound to the rat cytosolic AhR in a transformation assay but, at concentrations < or = 10 microM, exhibited minimal to non-detectable AhR agonist or antagonist activities associated with CYP1A1 induction. In contrast, the methyl-substituted DIMs inhibited estrogen-induced T47D human breast cancer cell growth and the four most active compounds (1,1'-, 2,2'-, 5,5'-dimethylDIM and 1,1',2,2'-tetramethylDIM) inhibited one or more estrogen-induced responses in the 21-day-old female B6C3F1 mice at a dose of 100 mg/kg/day (X3). Induction of hepatic CYP1A1-dependent activity was not observed at this high dose. The antitumorigenic activity of these compounds was examined in 7,12-dimethylbenz[a]anthracene-induced rat mammary tumor model in which the DIM analogs were orally administered (by gavage in corn oil) at a dose of 1 mg/kg/day (X10). 1,1'-DimethylDIM, 5,5'-dimethylDIM and 1,1',2,2'-tetramethylDIM significantly inhibited mammary tumor growth, and this was not accompanied by changes in organ/body weights or histopathology. These studies demonstrate that methyl-substituted DIMs are selective AhR modulators (SAhRMs) with potential for clinical treatment of breast cancer.

Expression of the aryl hydrocarbon receptor/transcription factor (AhR) and AhR-regulated CYP1 gene transcripts in a rat model of mammary tumorigenesis

Exposure to ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAH), may contribute to human breast cancer. In animals, PAH induce tumors in part by activating the aryl hydrocarbon receptor (AhR)/transcription factor. Historically, investigations into AhR-regulated carcinogenesis have focused on AhR-dependent transcriptional regulation of cytochrome P450 (CYP) enzymes which oxidize PAH to mutagenic intermediates. However, recent studies suggest that the AhR directly regulates cell growth. Given the postulated role of the AhR in carcinogenesis, we predicted that: (1) tissue predisposed to PAH tumorigenesis would express the AhR and (2) aberrant AhR and/or AhR-regulated gene expression would accompany malignant transformation. To test these hypotheses, AhR and CYP1 protein and/or mRNA levels were evaluated in rat mammary tumors induced with 7, 12-dimethylbenz[a]anthracene (DMBA), a prototypic PAH and AhR ligand. Results indicate modest AhR expression in normal mammary myoepithelial and ductal epithelial cells. In contrast, high AhR levels were detected in DMBA-induced tumors. Nuclear AhR localization in tumors suggested constitutive AhR activation. In situ hybridization and quantitative RT-PCR assays indicated high AhR mRNA levels in neoplastic epithelial cells. While both AhR-regulated CYP1A1 and CYP1B1 mRNAs were induced in breast tissue within 6 h of DMBA gavage, only CYP1B1 mRNA remained elevated in tumors. These results: (1) help explain targeting of breast tissue by carcinogenic PAH, (2) imply that AhR and CYP1B1 hyper-expression represent molecular biomarkers for, at least, PAH-induced mammary cell transformation, and (3) suggest mechanisms through which the AhR may contribute to carcinogenesis well after exogenous AhR ligands have been eliminated.

Mechanisms of inhibitory aryl hydrocarbon receptor-estrogen receptor crosstalk in human breast cancer cells

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that forms a functional heterodimeric complex with the AhR nuclear translocator (Arnt) protein. The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a high affinity ligand for the AhR and has been extensively used to investigate AhR-mediated biochemical and toxic responses. TCDD modulates several endocrine pathways including inhibition of 17beta-estradiol-induced responses in the immature and ovariectomized rodent uterus and mammary gland and in human breast cancer cell lines. TCDD inhibits formation and growth of mammary tumors in carcinogen-induced rodent models and relatively nontoxic selective AhR modulators (SAhRMs) are being developed for treatment of breast cancer. The mechanisms of inhibitory AhR-estrogen receptor (ER) crosstalk have been investigated in MCF-7 breast cancer cells by analysis of promoter regions of genes induced by E2 and inhibited by TCDD. AhR-mediated inhibition of E2-induced cathepsin D, pS2, c-fos, and heat shock protein 27 gene expression involves direct interaction of the AhR complex with inhibitory pentanucleotide (GCGTG) dioxin responsive elements (iDREs) resulting in disruption of interactions between proteins binding DNA elements required for ER action and the basal transcription machinery. Mechanisms of inhibitory AhR-ER crosstalk indicate that functional iDREs are required for inhibition of some genes; however, results indicate that other interaction pathways are important including AhR-mediated proteasome-dependent degradation of the ER.

Inhibition of carcinogen-induced rat mammary tumor growth and other estrogen-dependent responses by symmetrical dihalo-substituted analogs of diindolylmethane

90%) by the haloDIMs at concentrations of 5 or 10 microM, and only 4, 4'-dichloroDIM alone increased cell proliferation. With the exception of 5,5'-difluoroDIM, the remaining compounds also inhibited E2-induced growth of MCF-7 human breast cancer cells. DihaloDIMs (100 mg/kg/dayx3) were not estrogenic in the immature female B6C3F1 mouse uterus; however, in animals co-treated with E2 (0.02 microg/mouse), 5,5'-dichloro- and 6,6'-dichloroDIM inhibited uterine progesterone receptor (PR) binding and uterine peroxidase activity, whereas 5,5'-dichloro- and 5,5'-dichloro-2,2'-dimethylDIM inhibited only the latter response. The antitumorigenic activities of the dihaloDIMs were determined by their inhibition of carcinogen-induced mammary tumor growth in female Sprague-Dawley rats. 4,4'-Dichloro-, 5,5'-dibromo- and 6,6'-dichloroDIM, significantly inhibited mammary tumor growth at doses of 1 mg/kg every second day, and no significant changes in organ weights or liver and kidney histopathology were observed. These three compounds were more active than DIM in the same in vivo assay.

Estrogen and aryl hydrocarbon receptor expression and crosstalk in human Ishikawa endometrial cancer cells

Ishikawa endometrial cancer cells express the estrogen receptor (ER), and this study investigates aryl hydrocarbon receptor (AhR) expression and inhibitory AhR-ER crosstalk in this cell line. Treatment of Ishikawa cells with the AhR agonist [3H]2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) gave a radiolabeled nuclear complex that sedimented at 6.0 S in sucrose density gradients, and Western blot analysis confirmed that Ishikawa cells expressed human AhR and AhR nuclear translocator (Arnt) proteins. Treatment of Ishikawa cells with 10 nM TCDD induced a 9.7-fold increase in CYP1A1-dependent ethoxyresorufin O-deethylase (EROD) activity and a 10.5-fold increase in chloramphenicol acetyltransferase (CAT) activity in cells transfected with pRNH11c containing an Ah-responsive human CYP1A1 gene promoter insert (-1142 to +2434). Inhibitory AhR-ER crosstalk was investigated in Ishikawa cells using E2-induced cell proliferation and transcriptional activation assays in cells transfected with E2-responsive constructs containing promoter inserts from the progesterone receptor and vitellogenin A2 genes. AhR agonists including TCDD, benzo[a]pyrene (BaP) and 6-methyl-1,3,8-trichlorodibenzofuran, inhibited 32-47% of the E2-induced responses. In contrast, neither estrogen nor progesterone inhibited EROD activity induced by TCDD in Ishikawa cells, whereas inhibitory ER-AhR crosstalk was observed in ECC-1 endometrial cells suggesting that these interactions were cell context-dependent.

Transcriptional activation of c-fos protooncogene by 17beta-estradiol: mechanism of aryl hydrocarbon receptor-mediated inhibition

17Beta-estradiol (E2) induced c-fos protooncogene mRNA levels in MCF-7 human breast cancer cells, and maximal induction was observed within 1 h after treatment. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) inhibited the E2-induced response within 2 h. The molecular mechanism of this response was further investigated using pFC2-CAT, a construct containing a -1400 to +41 sequence from the human c-fos protooncogene linked to a bacterial chloramphenicol acetyltransferase (CAT) reporter gene. In MCF-7 cells transiently transfected with pFC2-CAT, 10 nM E2 induced an 8.5-fold increase of CAT activity, and cotreatment with 10 nM TCDD decreased this response by more than 45%. Alpha-Naphthoflavone, an aryl hydrocarbon receptor (AhR) antagonist, blocked the inhibitory effects of TCDD; moreover, the inhibitory response was not observed in variant Ah-nonresponsive MCF-7 cells, suggesting that the AhR complex was required for estrogen receptor cross-talk. The E2-responsive sequence (-1220 to -1155) in the c-fos gene promoter contains two putative core pentanucleotide dioxin-responsive elements (DREs) at -1206 to -1202 and -1163 to -1159. In transient transfection assays using wild-type and core DRE mutant constructs, the downstream core DRE (at -1163 to -1159) was identified as a functional inhibitory DRE. The results of photo-induced cross-linking, gel mobility shift, and in vitro DNA footprinting assays showed that the AhR complex interacted with the core DRE that also overlapped the E2-responsive GC-rich site (-1168 to -1161), suggesting that the mechanism for AhR-mediated inhibitory effects may be due to quenching or masking at the Sp1-binding site.

Estrogen and aryl hydrocarbon responsiveness of ECC-1 endometrial cancer cells

ECC-1 endometrial cancer cells express estrogen receptor alpha (ER(alpha)), and 17beta-estradiol (E2) induces cell proliferation, cathepsin D mRNA levels, and reporter gene activity in cells transiently transfected with constructs derived from the human cathepsin D and creatine kinase B (pCD and pCKB, respectively) gene promoters. The comparative antiestrogenic activity of aryl hydrocarbon receptor (AhR) agonists and ER(alpha) antagonists were also determined in these endometrial cancer cells. A functional AhR was expressed in ECC-1 cells and AhR agonists including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) inhibited E2-induced cell proliferation and transactivation. This was comparable to inhibitory AhR-ER crosstalk in breast cancer cell lines. The pure ER antagonist ICI 182,780 also exhibited antiestrogenic activity in ECC-1 cells; however, the results obtained for 4'-hydroxytamoxifen were response-specific. 4'-Hydroxytamoxifen alone did not induce ECC-1 cell proliferation but completely inhibited E2-induced cell proliferation. 4'-Hydroxytamoxifen primarily exhibited ER antagonist activities in transactivation assays and this contrasted to the predominant ER agonist responses observed in other endometrial cancer cell lines. The unique cellular context of ECC-1 cells was confirmed using pCKB and constructs expressing wild-type ER or ER variants expressing activation function 1 (AF1) or AF2 (ER-AF1 and ER-AF2, respectively). 4'-Hydroxytamoxifen did not induce reporter gene activity in cells cotransfected with pCKB and ER-AF1 or ER-AF2; however, in cotreatment studies (4'-hydroxytamoxifen plus E2), 4'-hydroxytamoxifen inhibited E2-induced transcriptional activation by ER-AF1 or ER-AF2. Thus, the primarily antiestrogenic activity observed for 4'-hydroxytamoxifen in ECC-1 cells may be related to the inability to activate gene expression through AF1-dependent pathways.

3,3'4,4'-Tetrachlorobiphenyl exhibits antiestrogenic and antitumorigenic activity in the rodent uterus and mammary cells and in human breast cancer cells

3,3',4,4'-Tetrachlorobiphenyl (tetraCB) binds to the aryl hydrocarbon receptor (AhR), and several reports have demonstrated that AhR agonists exhibit antiestrogenic and antitumorigenic activities in human breast cancer cells, the rodent uterus and breast. In contrast, a recent study showed that 3,3',4,4'-tetraCB bound the estrogen receptor (ER) and exhibited ER agonist activities, and we therefore have reinvestigated the estrogenic and antiestrogenic activities of 3,3',4,4'-tetraCB. Our results showed that 3,3',4,4'tetraCB and a structurally related analog, 3,3',4,4',5-pentaCB, did not bind the mouse uterine or human ER, did not induce proliferation of MCF-7 or T47D human breast cancer cells or induce reporter gene activity in cells transfected with E2-responsive constructs derived from the creatine kinase B (pCKB) or cathepsin D (pCD) gene promoters. Moreover, 3,3',4,4'-tetraCB and 3,3',4,4',5-pentaCB did not induce an increase in uterine wet weight, peroxidase activity or progesterone receptor binding in the 21-25-day-old female B6C3F1 mouse uterus. In contrast, both compounds inhibited 17beta-estradiol (E2)-induced cell proliferation and transactivation in MCF-7/T47D cells and uterine responses in B6C3F1 mice; surprisingly inhibition of E2-induced reporter gene activity was not observed in T47D cells transfected with pCKB, and this was observed as a cell-specific response with other AhR agonists. Additionally, 3,3',4,4'-tetraCB significantly inhibited mammary tumor growth in female Sprague-Dawley rats initiated with 7,12-dimethylbenzanthracene. Our results indicate that 3,3',4,4'-tetraCB does not exhibit ER agonist activity but exhibits a broad spectrum of antiestrogenic responses consistent with ligand-mediated AhR-ER crosstalk.

Ah receptor agonists as endocrine disruptors: antiestrogenic activity and mechanisms

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds induce a broad spectrum of biochemical and toxic responses and disrupt multiple endocrine pathways. Research in this laboratory has focused on characterizing aryl hydrocarbon receptor (AhR)-mediated antiestrogenicity in the rodent uterus and mammary and in human breast cancer cells. TCDD inhibits multiple estrogen (E2)-induced responses in these tissues including development or growth of human mammary and endometrial cancer cells, carcinogen-induced mammary cancer in rats, and mammary cancer in mice bearing breast cancer cell xenografts. The mechanisms of AhR-mediated antiestrogenicity are complex; however, studies on the molecular biology of cross-talk between the AhR and estrogen-receptor (ER) signaling pathways have been initiated using several E2-regulated genes as models. The results indicate that the nuclear AhR complex targets specific genomic core inhibitory dioxin responsive elements (iDREs) in promoter regions of some E2-responsive target genes to inhibit hormone-induced transactivation. The pS2, cathepsin and c-fos genes have functional iDREs, whereas the iDRE in the progesterone receptor gene promoter was not functional. Research has also focused on development of AhR-based antiestrogens which inhibit mammary tumor development and growth but do not exhibit prototypical AhR-induced toxic responses.

Aryl hydrocarbon receptor-mediated antiestrogenicity in MCF-7 cells: modulation of hormone-induced cell cycle enzymes

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) inhibits 17beta-estradiol (E2) mammary tumor growth in rodents and in MCF-7 human breast cancer cells; however, the cell cycle genes/proteins which are inhibited have not been determined. Initial studies showed that treatment of MCF-7 cells with 10 nM E2 significantly increased cyclin D1 (protein and mRNA), cdk2- and cdk4-dependent kinase activities, and hyperphosphorylation of retinoblastoma (RB) protein. In contrast to results of recent studies (M. D. Planas-Silva and R. A. Weinberg, 1997, Mol. Cell. Biol. 17, 4059-4069), E2 induced dissociation of both cdk2 and cdk4 proteins from the p21 protein complex and significantly increased cdk7-dependent kinase activity. Treatment of MCF-7 cells with E2 also induced cdc25A phosphatase protein, which was accompanied by increased cdk2 and cdk4 proteins containing unphosphorylated tyrosine residues. Although TCDD alone has minimal effects on cell cycle proteins/enzymes, several E2-induced responses were significantly inhibited in MCF-7 cells cotreated with E2 plus TCDD. For example, TCDD significantly inhibited E2-induced hyperphosphorylation of RB, cyclin D1 protein, and cdk2-, cdk4-, and cdk7-dependent kinase activities. Inhibition of E2-induced cdk4-dependent kinase activity by TCDD may be related to the parallel decrease of E2-induced cyclin D1 protein, and inhibition of induced cdk2- and cdk4-dependent kinase activities may be due to significantly increased p21 levels in cells cotreated with TCDD plus E2. These results demonstrate that the antiestrogenic activity of TCDD is due to downregulation of several E2-induced cell cycle proteins/activities and this illustrates the complex cross talk between the aryl hydrocarbon and the E2 receptor signaling pathways.