The aryl hydrocarbon receptor (AHR) mediates the toxic effects of halogenated aromatic hydrocarbons (HAHs), including 2,3,7,8-tetrachlorodibenzo-p-dioxin, 2,3,4,7,8-pentachlorodibenzofuran and 2,3,7,8-tetrachlorodibenzofuran. Y322 is believed to play a role in binding-independent activation of AHR by atypical inducers, such as omeprazole. I examined AHR-mediated regulation of and coactivator recruitment to CYP1A1, CYP1B1, HES1 and TiPARP in T-47D and HuH7 cells. All compounds induced expression of each gene in both cell lines, with some temporal differences between the HAHs and omeprazole. Chromatin immunoprecipitation assays demonstrated activator-, cell line- and gene-selectivity in AHR coactivator recruitment. Omeprazole induced AHR degradation which was prevented by MG-132 pre-treatment. Y322 was found to be important for maximal AHR activation by 2,3,7,8-TCDD and 2,3,4,7,8-PeCDF, but required for 2,3,7,8-TCDF and Omp in an AHR-deficient MCF-7 cells. My findings provide further evidence for cell-, gene- and ligand-dependent differences in AHR-mediated gene expression and coactivator recruitment, and a role for Y322 in AHR activation.

Developmental exposure to aryl hydrocarbon receptor (AhR) agonists in fish causes severe defects in the cardiovascular system. However, the effects of acute AhR agonist exposure on the adult fish cardiovascular system as well as the genes mediating developmental AhR-induced deformities remain unclear. In this thesis, two studies were carried out to address these issues. Before experiments could begin, methods for quantitative real-time reverse transcriptase polymerase chain reaction (rtrt-PCR) as well as larval exposure and rearing were developed, validated, and optimized.<p> Following method development, a series of experiments was performed on adult zebrafish (<i>Danio rerio</i>) to assess how expression of cytochrome P450 (CYP) and cyclooxygenase (COX) enzyme mRNA in hepatic and vascular tissues is altered after intraperitoneal injection of AhR agonists benzo(a)pyrene (BaP) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone and in combination with the purported AhR antagonists resveratrol (Res) or alpha-naphthoflavone (ANF). Both TCDD and BaP induced similar patterns of gene expression in arteries, although with different efficacies, and had slightly different effects in hepatic tissues. Resveratrol was generally without effect in all treatment groups and tissues with the exception of reducing TCDD-induced CYP1C2 in vascular tissues. In contrast, ANF antagonized TCDD- and BaP-induced changes, as well as reduced baseline gene expression in liver. However, in arteries, ANF alone acted as an agonist to increase expression of several of the genes investigated.<p> The second series of experiments involved zebrafish eggs aqueously exposed to BaP or TCDD alone and in combination with Res or ANF. Whole larvae CYP and COX isoform mRNA expression was quantified at 5 and 10 days post-fertilization (dpf), then correlated with developmental phenotype. Both TCDD and BaP caused concentration-dependent AhR-associated deformities with a significant increase in mortalities by 10 dpf and increased CYP1A mRNA expression, while TCDD alone decreased CYP1C2 expression. BaP/ANF co-exposure exhibited the highest rate of deformities and mortalities at both 5 and 10 dpf, caused marked alterations in cardiac and vascular morphology at 10 dpf, and increased CYP1A expression. Furthermore, ANF exhibited additive agonistic effects on gene expression with both BaP and TCDD. Correlation analyses revealed that gene expression at 5 dpf, but not 10 dpf, was strongly linked to abnormal cardiac and vascular phenotypes at 10 dpf with several genes related to cardiac development and one primary gene linked to vascular development.

Developmental exposure to aryl hydrocarbon receptor (AhR) agonists in fish causes severe defects in the cardiovascular system. However, the effects of acute AhR agonist exposure on the adult fish cardiovascular system as well as the genes mediating developmental AhR-induced deformities remain unclear. In this thesis, two studies were carried out to address these issues. Before experiments could begin, methods for quantitative real-time reverse transcriptase polymerase chain reaction (rtrt-PCR) as well as larval exposure and rearing were developed, validated, and optimized.<p> Following method development, a series of experiments was performed on adult zebrafish (<i>Danio rerio</i>) to assess how expression of cytochrome P450 (CYP) and cyclooxygenase (COX) enzyme mRNA in hepatic and vascular tissues is altered after intraperitoneal injection of AhR agonists benzo(a)pyrene (BaP) or 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone and in combination with the purported AhR antagonists resveratrol (Res) or alpha-naphthoflavone (ANF). Both TCDD and BaP induced similar patterns of gene expression in arteries, although with different efficacies, and had slightly different effects in hepatic tissues. Resveratrol was generally without effect in all treatment groups and tissues with the exception of reducing TCDD-induced CYP1C2 in vascular tissues. In contrast, ANF antagonized TCDD- and BaP-induced changes, as well as reduced baseline gene expression in liver. However, in arteries, ANF alone acted as an agonist to increase expression of several of the genes investigated.<p> The second series of experiments involved zebrafish eggs aqueously exposed to BaP or TCDD alone and in combination with Res or ANF. Whole larvae CYP and COX isoform mRNA expression was quantified at 5 and 10 days post-fertilization (dpf), then correlated with developmental phenotype. Both TCDD and BaP caused concentration-dependent AhR-associated deformities with a significant increase in mortalities by 10 dpf and increased CYP1A mRNA expression, while TCDD alone decreased CYP1C2 expression. BaP/ANF co-exposure exhibited the highest rate of deformities and mortalities at both 5 and 10 dpf, caused marked alterations in cardiac and vascular morphology at 10 dpf, and increased CYP1A expression. Furthermore, ANF exhibited additive agonistic effects on gene expression with both BaP and TCDD. Correlation analyses revealed that gene expression at 5 dpf, but not 10 dpf, was strongly linked to abnormal cardiac and vascular phenotypes at 10 dpf with several genes related to cardiac development and one primary gene linked to vascular development.

<p>Estrogens and the estrogen receptor (ER) have been implicated in the etiology of breast cancer and osteoporosis. However, the mechanisms by which this receptor-ligand complex manifest their regulatory activities in these processes is not completely understood. The development and subsequent definition of the molecular mechanism of action of selective ER modulators (SERMs), compounds with differential relative agonist/antagonist activity, has uncovered an unanticipated complexity in this signaling pathway. Furthermore, these analyses indicat that it is likely that in addition to the classical steroidal estrogens, which exhibit agonist properties, endogenous compounds exist that interact with ER and function as physiological SERMs. Recently, 27-hydroxycholesterol (27HC) was identified as an endogenous ER ligand with tissue-specific estrogenic/anti-estrogenic activities. Indeed, we determined that 27HC exhibited the three basic properties of a SERM: 1) it bound competitively with estradiol (E2) to both genetic subtypes of ER, ER&#945; and ER&#946;; 2) it induced a unique conformation of ER that is likely related to its biological activity; and 3) it displayed tissue-specific ER modulatory activity in the cardiovascular system, breast, and bone. In particular, we undertook a series of in vivo studies to show that a pathological elevation of 27HC was associated with decreased bone quantity, an effect that was partially rescued by E2 supplementation. The ability of 27HC to decrease bone density in the absence of endogenous estrogens suggests that the circulating level of 27HC may be of critical importance in determining osteoporosis risk in post-menopausal women. Interestingly, cholesterol-lowering statins have been shown to improve bone density; thus, given the stoichiometric relationship between circulating cholesterol and 27HC, our data provide a possible explanation for the observed bone sparing actions of this class of drugs. </p><p>In general, it is considered that SERM activity can be explained by the ability to induce differential alterations in ER structure and the impact that this has on the recruitment of functionally distinct cofactors. The results of our studies reveal a much more complex picture and suggest that some SERM pharmacology can be ascribed to actions in pathways that do not include ER. Specifically, we have determined that the SERM 4-hydroxy-tamoxifen (4OHT) can bind to and activate the aryl hydrocarbon receptor (AHR). Given that AHR controls the expression of E2-metabolizing enzymes, our finding that 4OHT regulates AHR in the context of breast cancer could have important pharmacological and pathological implications. Interestingly, our preliminary in vitro data indicate that the ability of 4OHT to inhibit osteoclast (OC) differentiation, and thus aid in preserving bone density in post-menopausal women, is primarily dependent on expression of AHR, not ER. Conversely, the inhibitory activity of raloxifene (RAL), another SERM, on OC differentiation was absolutely dependent on ER. Thus, the activity of 4OHT in bone is likely to be a composite response requiring its actions on both ER and AHR. </p><p>Many new aspects of the estrogen and ER signaling pathways have been uncovered as we learn more about ligands that modulate ER by altering its conformation and thus its ability to engage in protein-protein interactions. Collectively, our findings demonstrate that the intersection between cholesterol metabolism, ER signaling, and the AHR pathway will have important consequences in regulating cellular function, and may be involved in the development or progression of multiple disease states.</p> / Dissertation

Cancer is a leading cause of human mortality worldwide, and is expected to soon become the overall leading cause of death in the United States. Some cancers are hormone-related, including the sex-specific cancers of the breast (predominantly in women) and prostate (in men). In both cases, early stage tumors are responsive to inhibitory endocrine-based therapies. However, both cancers progress to hormone-nonresponsive states and this is in part due to altered properties of the primary nuclear hormone receptor signaling pathway (estrogen receptor [ER] in breast; androgen receptor [AR] in prostate). Other nuclear receptors are thus being investigated as therapeutic targets due to their crosstalk with hormone receptor pathways and these include the aryl hydrocarbon receptor (AhR), peroxisome proliferator activated receptor γ(PPARγ, retinoic acid receptor and retinoid X receptor (RAR/RXR), and vitamin D receptor (VDR). Previous studies have demonstrated that the AhR mediates chemoprotective, antiestrogenic, and tumoristatic effects in experimental models, and relatively non-toxic selective aryl hydrocarbon receptor modulators (SAhRMs) have been developed. Studies in this dissertation have investigated the therapeutic properties of a new class of compounds related to the SAhRM 3,3‘-diindolylmethane (DIM) in models of breast cancer. Additionally, the potential therapeutic role of the AhR in human prostate cancer cells has been investigated. Several ring- and methylene-substituted DIMs exhibited antiestrogenic and tumoristatic activities in breast cancer cells and in carcinogen-induced rat mammary tumors. At least some of the methylene-substituted DIMs act through PPARγ. The AhR is expressed in LNCaP and iv 22Rv1 prostate cancer cells and AhR agonists inhibit cell growth and AR-induced transactivation through pathways independent of androgen receptor downregulation.

Thesis (Ph. D.)--University of Washington, 2003. / Vita. Includes bibliographical references (leaves 135-141).

The main goal of this thesis was to develop new methods and knowledge that will explain and predict species differences in sensitivity to dioxin-like compounds (DLCs) in birds. The important achievements and results obtained from the four experimental chapters of this thesis are summarized as follow: (1) an efficient luciferase reporter gene (LRG) assay was developed for use with 96-well cell culture plates; (2) the results obtained from LRG assay were shown to be highly correlated to available in ovo toxicity data; (3) amino acids at positions 324 and 380 within the aryl hydrocarbon receptor 1 ligand binding domain (AHR1 LBD) were shown to be responsible for reduced Japanese quail (Coturnix japonica) AHR1 activity to induce a dioxin-responsive reporter gene in comparison to chicken (Gallus gallus domesticus), and ring-necked pheasant (Phasianus colchicus) AHR1 in response to different DLCs; (4) AHR1 LBD sequences of 86 avian species were studied and differences at amino acid sites 256, 257, 297, 324, 337 and 380 were identified. It was discovered that only positions 324 and 380 play a role in AHR1 activity to induce a dioxin-responsive gene; (5) in COS-7 cells expressing chicken AHR1, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) are equipotent inducers of the reporter gene and bind with similar affinity to chicken AHR1, however, in the cells expressing pheasant, Japanese quail and common tern (Sterna hirundo) AHR1, PeCDF is a stronger inducer than TCDD. PeCDF also binds with higher affinity to pheasant and quail AHR1 than TCDD. The results of this thesis show that embryo lethal effect of DLCs in avian species can be predicted by use of two new non-lethal methods: (1) the LRG assay and (2) determination of the identity of the amino acids at positions 324 and 380. The findings and methods described in this thesis will be of use for environmental risk assessments of DLCs.

<p>Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous environmental contaminants formed from the incomplete combustion of fossil fuels and are found in the environment as complex mixtures. PAHs are developmentally toxic to fish, causing yolk sac edema, hemorrhaging, craniofacial malformations and cardiac defects including impaired heart looping, elongated heart, decreased blood flow, and pericardial effusion. Previous research has shown that many of the toxic effects of PAHs are mediated through the aryl hydrocarbon receptor (AHR), which upregulates phase I and II metabolic genes, but the underlying mechanisms of PAH-induced toxicity are not yet known. The primary goal of this dissertation was to better understand the molecular mechanisms by which PAH mixtures cause developmental toxicity in fish. To this end, the zebrafish (Danio rerio) was used as a developmental model. Simple mixtures consisting of a PAH that is an AHR agonist (benzo[a]pyrene or benzo[k]fluoranthene) and a PAH that is a cytochrome P450 1 (CYP1) inhibitor (fluoranthene) were used in these experiments along with the dioxin-like compound 3,3',4,4',5-pentachlorobiphenyl (PCB-126). Morpholino gene knockdown was used to examine the role of specific genes in response to PAHs, gene expression changes in response to PAH exposures were examined via QPCR, quantification of pericardial effusion was used as a metric for cardiac toxicity, and CYP1 activity was measured as an indication of AHR pathway induction. First, PAH mixtures consisting of an AHR agonist (BkF) and a CYP1 inhibitor (FL) induced cardiac toxicity that was preceded by upregulation of CYP1 and redox-responsive gene expression, and these effects were dependent upon the AHR2. Second, knockdown of glutathione s-transferase pi class 2 (GSTp2), part of phase II metabolism, exacerbated PAH-induced toxicity but did not affect PCB-126-induced toxicity. Third, knockdown of another isoform of the AHR, AHR1, exacerbated PAH- and PCB-126-induced toxicity and increased CYP1 activity but did not affect CYP expression in response to these agonists. Simultaneous knockdown of AHR1A and AHR2 did not exacerbate nor ameliorate PAH-induced toxicity but did prevent PCB-126-induced toxicity. Fourth, to examine AHR2-dependent and AHR2-independent gene induction in zebrafish hearts in response to PAHs, microarrays were used. Gene expression changes caused by PAHs were largely AHR2-dependent and consisted of genes involved in cell adhesion, oxidation-reduction, and TGF-&beta signaling processes as well as genes involved in heart structure and function. These findings help to elucidate how PAHs elicit deformities during development and highlight differences between PAHs and other AHR agonists. Additionally, these experiments have identified other genes in addition to AHR2 that are involved in mediating or responding to the toxicity of PAHs.</p> / Dissertation

Diss. (sammanfattning) Stockholm : Karolinska institutet, 2007. / Härtill 4 uppsatser.

Diss. (sammanfattning) Stockholm : Karol. inst., 2003. / Härtill 5 uppsatser.