Aryl hydrocarbon receptor activation by cAMP vs. dioxin: divergent signaling pathways

Lack of ligand-selective binding of the aryl hydrocarbon receptor to putative DNA binding sites regulating expression of Bax and paraoxonase 1 genes

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor that mediates the biological and toxicological effects of structurally diverse chemicals through its ability to bind specific DNA recognition sites (dioxin responsive elements (DREs)), and activate transcription of adjacent genes. While the DRE has a highly conserved consensus sequence, it has been suggested that the nucleotide specificity of AhR DNA binding may be ligand-dependent. The upstream regulatory regions of the murine Bax and human paraoxonase 1 (PON1) genes reportedly contain unique DRE-like sequences that respond to AhRs activated by some ligands but not others. Given the significant implications of this observation to understanding the diversity in AhR responses and that of other ligand-dependent nuclear receptors, a combination of DNA binding, nuclear translocation and gene expression analysis was used to investigate the molecular mechanisms underlying these ligand-selective responses. Although known AhR agonists stimulated AhR nuclear translocation, DRE binding and gene expression, the ligand-selective DRE-like DNA elements identified in the Bax and PON1 upstream regulatory regions failed to bind ligand-activated AhR or confer AhR-responsiveness upon a reporter gene. These results argue against the reported ligand-selectivity of AhR DNA binding and suggest DNA binding by ligand activated AhR involves DRE-containing DNA.

Zinc finger nuclease-mediated knockout of AHR or ARNT in human breast cancer cells abolishes basal and ligand-dependent regulation of CYP1B1 and differentially affects estrogen receptor α transactivation

In this study, we used zinc finger nuclease-mediated knockout of the aryl hydrocarbon receptor (AHR) or AHR nuclear translocator (ARNT) in MCF7 and AHR knockout in MDA-MB-231 human breast cancer cells to investigate cross talk among AHR, ARNT, and estrogen receptor α (ERα). Knockout of AHR or ARNT prevented the 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-dependent induction of all AHR target genes examined. Knockout of AHR or ARNT also significantly reduced basal cytochrome P4501B1 (CYP1B1) expression levels, which were restored with overexpression of either protein but not with a DNA binding-deficient AHR mutant. Basal and TCDD-, 17β-estradiol (E2)-, or TCDD + E2-dependent recruitment of AHR, ARNT, ERα, NCoA3, and RNA polymerase II to CYP1B1 as well as CYP1B1 mRNA levels were abolished in MCF7-AHR((ko)) and MDA-MB-231 AHR(ko) cells. However, reduced but significant E2-dependent recruitment of ERα, NCoA3, and RNA polymerase II to CYP1B1 and weak increases in CYP1B1 mRNA levels were observed in MCF7 ARNT((ko)) cells. Interestingly, E2-dependent increases in trefoil factor 1, but not growth regulation by estrogen in breast cancer 1 (GREB1) mRNA levels, were dependent on ARNT expression. Moreover, the TCDD-dependent increases in the proteolytic degradation of ERα were prevented by the loss of AHR or ARNT. Our data show that AHR and ARNT play critical roles in the basal, TCDD, and E2-induced regulation of CYP1B1 but also reveal distinct roles for both proteins in ERα transactivation.

Somatostatin analogues increase AIP expression in somatotropinomas, irrespective of Gsp mutations

Germline aryl hydrocarbon receptor interacting protein (AIP) gene mutations confer a predisposition to pituitary adenoma (PA), predominantly GH-secreting (GH-PA). As recent data suggest a role for AIP in the pathogenesis of sporadic GH-PA and their response to somatostatin analogues (SSA), the expression of AIP and its partner, aryl hydrocarbon receptor (AHR), was determined by semiquantitative immunohistochemistry scoring in 62 sporadic GH-PA (37 treated with SSA preoperatively). The influence of Gsp status was studied in a subset of tumours (n=39, 14 Gsp(+)) and six GH-PA were available for primary cultures. AIP and AHR were detected in most cases, with a positive correlation between AIP and cytoplasmic AHR (P=0.012). Low AIP expression was significantly more frequent in untreated vs SSA-treated tumours (44.0 vs 20.5%, P=0.016). AHR expression or localisation did not differ between the two groups. Similarly, in vitro octreotide induced a median twofold increase in AIP expression (range 1.2-13.9, P=0.027) in GH-PA. In SSA-treated tumours, the AIP score was significantly higher in the presence of preoperative IGF1 decrease or tumour shrinkage (P=0.008 and P=0.014 respectively). In untreated tumours, low AIP expression was significantly associated with invasiveness (P=0.028) and suprasellar extension (P=0.019). The only effect of Gsp status was a significantly lower nuclear AHR score in Gsp(+) vs Gsp(-) tumours (P=0.025), irrespective of SSA. In conclusion, AIP is involved in the aggressiveness of sporadic GH-PA, regardless of Gsp status, and AIP up-regulation in SSA-treated tumours is associated with a better preoperative response, with no clear role for AHR.

Aip regulates cAMP signalling and GH secretion in GH3 cells

Mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene have been linked to predisposition to pituitary adenomas. However, the mechanism by which this occurs remains unknown. AIP interacts with a number of interesting proteins, including members of the cAMP signalling pathway that has been shown to be consistently altered in pituitary tumours. The functional role of Aip was investigated using both over-expression and knock down of Aip in GH3 cells. cAMP signalling and its downstream effectors, including GH secretion, were then investigated. cAMP signalling was analysed using cAMP assays, cAMP-response element-promoter luciferase reporter assays, real-time PCR and finally secreted GH quantification. Over-expression of wild-type (WT)-Aip reduced forskolin-induced cAMP signalling at the total cAMP level, luciferase reporter activity and target gene expression, when compared with empty vector and the non-functional R304X mutant. Additionally, GH secretion was reduced in WT-Aip over-expressing GH3 cells treated with forskolin. Knock down of endogenous Aip resulted in increased cAMP signalling but a decrease in GH secretion was also noted. Inhibition of phosphodiesterase activity using general and selective inhibitors did not completely ablate the effect of Aip on forskolin-augmented cAMP signalling. A mechanism by which Aip acts as a tumour suppressor, by maintaining a low cAMP signalling and concentration, is suggested. Mutations of Aip render the protein incapable of such activity. This effect appears not to be mediated by the AIP-PDE interaction, suggesting the involvement of other interacting partners in mediating this outcome.

Aryl hydrocarbon receptor control of adaptive immunity

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that belongs to the family of basic helix-loop-helix transcription factors. Although the AhR was initially recognized as the receptor mediating the pathologic effects of dioxins and other pollutants, the activation of AhR by endogenous and environmental factors has important physiologic effects, including the regulation of the immune response. Thus, the AhR provides a molecular pathway through which environmental factors modulate the immune response in health and disease. In this review, we discuss the role of AhR in the regulation of the immune response, the source and chemical nature of AhR ligands, factors controlling production and degradation of AhR ligands, and the potential to target the AhR for therapeutic immunomodulation.

Aryl hydrocarbon receptor-mediated induction of the microRNA-132/212 cluster promotes interleukin-17-producing T-helper cell differentiation

Aryl hydrocarbon receptor (AHR) plays critical roles in various autoimmune diseases such as multiple sclerosis by controlling interleukin-17 (IL-17)-producing T-helper (TH17) and regulatory T cells. Although various transcription factors and cytokines have been identified as key participants in TH17 generation, the role of microRNAs in this process is poorly understood. In this study, we found that expression of the microRNA (miR)-132/212 cluster is up-regulated by AHR activation under TH17-inducing, but not regulatory T-inducing conditions. Deficiency of the miR-132/212 cluster prevented the enhancement of TH17 differentiation by AHR activation. We also identified B-cell lymphoma 6, a negative regulator of TH17 differentiation, as a potential target of the miR-212. Finally, we investigated the roles of the miR-132/212 cluster in experimental autoimmune encephalomyelitis, a murine model of multiple sclerosis. Mice deficient in the miR-132/212 cluster exhibited significantly higher resistance to the development of experimental autoimmune encephalomyelitis and lower frequencies of both TH1 and TH17 cells in draining lymph nodes. Our findings reveal a unique mechanism of AHR-dependent TH17 differentiation that depends on the miR-132/212 cluster.

Use of natural AhR ligands as potential therapeutic modalities against inflammatory disorders

The aim of this review is to discuss research involving ligands for the aryl hydrocarbon receptor (AhR) and their role in immunomodulation. While activation of the AhR is well known for its ability to regulate the biochemical and toxic effects of environmental chemicals, more recently an exciting discovery has been made indicating that AhR ligation can also regulate T-cell differentiation, specifically through activation of Foxp3(+) regulatory T cells (Tregs) and downregulation of the proinflammatory Th17 cells. Such findings have opened new avenues of research on the possibility of targeting the AhR to treat inflammatory and autoimmune diseases. Specifically, this review will discuss the current research involving natural and dietary AhR ligands. In addition, evidence indicating the potential use of these ligands in regulating inflammation in various diseases will be highlighted. The importance of the AhR in immunological processes can be illustrated by expression of this receptor on a majority of immune cell types. In addition, AhR signaling pathways have been reported to influence a number of genes responsible for mediating inflammation and other immune responses. As interest in the AhR and its ligands increases, it seems prudent to consolidate current research on the contributions of these ligands to immune regulation during the course of inflammatory diseases.

Familial isolated pituitary adenomas (FIPA) and the pituitary adenoma predisposition due to mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene

Pituitary adenomas are one of the most frequent intracranial tumors and occur with a prevalence of approximately 1:1000 in the developed world. Pituitary adenomas have a serious disease burden, and their management involves neurosurgery, biological therapies, and radiotherapy. Early diagnosis of pituitary tumors while they are smaller may help increase cure rates. Few genetic predictors of pituitary adenoma development exist. Recent years have seen two separate, complimentary advances in inherited pituitary tumor research. The clinical condition of familial isolated pituitary adenomas (FIPA) has been described, which encompasses the familial occurrence of isolated pituitary adenomas outside of the setting of syndromic conditions like multiple endocrine neoplasia type 1 and Carney complex. FIPA families comprise approximately 2% of pituitary adenomas and represent a clinical entity with homogeneous or heterogeneous pituitary adenoma types occurring within the same kindred. The aryl hydrocarbon receptor interacting protein (AIP) gene has been identified as causing a pituitary adenoma predisposition of variable penetrance that accounts for 20% of FIPA families. Germline AIP mutations have been shown to associate with the occurrence of large pituitary adenomas that occur at a young age, predominantly in children/adolescents and young adults. AIP mutations are usually associated with somatotropinomas, but prolactinomas, nonfunctioning pituitary adenomas, Cushing disease, and other infrequent clinical adenoma types can also occur. Gigantism is a particular feature of AIP mutations and occurs in more than one third of affected somatotropinoma patients. Study of pituitary adenoma patients with AIP mutations has demonstrated that these cases raise clinical challenges to successful treatment. Extensive research on the biology of AIP and new advances in mouse Aip knockout models demonstrate multiple pathways by which AIP may contribute to tumorigenesis. This review assesses the current clinical and therapeutic characteristics of more than 200 FIPA families and addresses research findings among AIP mutation-bearing patients in different populations with pituitary adenomas.

The circadian clock circuitry and the AHR signaling pathway in physiology and pathology

Life forms populating the Earth must face environmental challenges to assure individual and species survival. The strategies predisposed to maintain organismal homeostasis and grant selective advantage rely on anticipatory phenomena facing periodic modifications, and compensatory phenomena facing unpredictable changes. Biological processes bringing about these responses are respectively driven by the circadian timing system, a complex of biological oscillators entrained to the environmental light/dark cycle, and by regulatory and metabolic networks that precisely direct the body's adjustments to variations of external conditions and internal milieu. A critical role in organismal homeostatic functions is played by the aryl hydrocarbon receptor (AHR) complex, which senses environmental and endogenous compounds, influences metabolic responses controlling phase I/II gene expression, and modulates vital phenomena such as development, inflammation and adaptive immunity. A physiological cross-talk between circadian and AHR signaling pathways has been evidenced. The alteration of AHR signaling pathway deriving from genetic damage with polymorphisms or mutations, or produced by exogenous or endogenous AHR activation, and chronodisruption caused by mismatch between the body's internal clock and geophysical time/social schedules, are capable of triggering pathological mechanisms involved in metabolic, immune-related and neoplastic diseases. On the other hand, the molecular components of the circadian clock circuitry and AHR signaling pathway may represent useful tools for preventive interventions and valuable targets of therapeutic approaches.

Cyprodinil as an activator of aryl hydrocarbon receptor

Cyprodinil is a pyrimidinamine fungicide, used worldwide by agriculture. It is used to protect fruit plants and vegetables from a wide range of pathogens. Benzo[a]pyrene (BaP) and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are toxic environmental pollutants and are prototypes of aryl hydrocarbon receptor (AHR) ligands. Although the structure of cyprodinil distinctly differs from those of BaP and TCDD, our results show that cyprodinil induced nuclear translocation of the AHR, and induced the transcriptional activity of aryl hydrocarbon response element (AHRE). Cyprodinil induced the expression of cytochrome P450 (CYP) 1A1, a well-known AHR-targeted gene, in ovarian granulosa cells, HO23, and hepatoma cells, Hepa-1c1c7. Its induction did not appear in AHR signal-deficient cells, and was blocked by the AHR antagonist, CH-223191. Cyprodinil decreased AHR expression in HO23 cells, resulting in CYP1A1 expression decreasing after it peaked at 9h of treatment in HO23 cells. Dexamethasone is a synthetic agonist of glucocorticoids. Cyprodinil enhanced dexamethasone-induced gene expression, and conversely, its induction of CYP1A1 expression was decreased by dexamethasone in HO23 cells, indicating its induction of crosstalk between the AHR and glucocorticoid receptor and its role as a potential endocrine disrupter. In addition to BaP, TCDD, and an AHR agonist, β-NF, cyprodinil also phosphorylated extracellular signal-regulated kinase (ERK) in HO23 and Hepa-1c1c7 cells, indicating its deregulation of ERK activity. In summary, our results demonstrate that cyprodinil, similar to BaP, acts as an AHR activator, a potential endocrine disrupter, and an ERK disrupter.

The AhR twist: ligand-dependent AhR signaling and pharmaco-toxicological implications

The aryl hydrocarbon receptor (AhR) is a transcription factor which is activated by diverse compounds and regulates the expression of xenobiotic metabolism genes. Recent studies have unraveled unsuspected physiological roles and novel alternative ligand-specific pathways for this receptor. In this review, we discuss these novel aspects and focus on the different responses elicited by the diverse endogenous and/or exogenous AhR ligands. In addition to challenging the relevance of the 'agonist/antagonist' classification of ligands, we introduce the concept of AhR plasticity as a primordial factor in the generation of these pathways. Finally, we suggest several promising perspectives for the pharmacological modulation of these responses.

Serum from methimazole-treated patients induces activation of aryl hydrocarbon receptor, a transcription factor that binds to dioxin-response elements

BACKGROUND

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by xenobiotic substances such as dioxin. After activation, it binds to dioxin response elements of DNA, thereby inducing transcription of a variety of xenobiotic metabolizing enzymes. To investigate whether AhR-activating substances accumulate in patients with endocrine disorders, we tested serum samples for AhR-stimulating activity.

METHODS

Serum AhR-stimulating activity was evaluated by exposing the HepG2 cells transiently transfected with an AhR-responsive reporter plasmid to serum samples. On the basis of preliminary findings that implicated methimazole (MMI), wild-type and AhR-null mice were treated with MMI, and their plasma AhR-stimulating activities and thyroxine levels were quantified.

RESULTS

In 28 randomly chosen patients, 7 out of 10 Graves' disease patients exhibited increased serum AhR-stimulating activity. The increased activity did not correlate with thyroid hormone status. However, we hypothesized that it might be caused by MMI. Subsequent analyses revealed that in 25 of 26 MMI-treated Graves' patients, serum samples collected after the MMI treatment had significantly higher AhR-stimulating activity compared to samples obtained when the same patients were not on MMI. By contrast, serum AhR-stimulating activity was unchanged in samples from the seven patients on propylthiouracil (PTU) compared to serum taken before the PTU treatment. In vitro experiments demonstrated that an MMI metabolite 3-methyl-2-thiohydantoin, but not MMI, activated AhR. MMI increased plasma AhR-stimulating activities and reduced plasma thyroxine concentrations, in both wild-type and AhR-deficient mice.

CONCLUSIONS

Graves' patients taking MMI have increased serum AhR-stimulating activity, which is unrelated to thyroid hormone status, but correlates with MMI treatment. The AhR activation is likely caused by 3-methyl-2-thiohydantoin. Further studies are required to determine the potency of 3-methyl-2-thiohydantoin as an AhR activator and the significance of the differences between MMI and PTU observed in this study.

The aryl hydrocarbon receptor modulates acute and late mast cell responses

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor whose activity is modulated by xenobiotics as well as physiological ligands. These compounds may modulate inflammatory responses and contribute to the rising prevalence of allergic diseases observed in industrialized countries. Mast cells (MCs), located within tissues at the boundary of the external environment, represent a potential target of AhR ligands. In this study, we report that murine and human MCs constitutively express AhR, and its activation by the high-affinity ligand 6-formylindolo[3,2-b]carbazole (FICZ) determines a boost in degranulation. On the contrary, repeated exposure to FICZ inhibits MC degranulation. Accordingly, histamine release, in an in vivo passive systemic anaphylactic model, is exacerbated by a single dose and is attenuated by repetitive stimulation of AhR. FICZ-exposed MCs produce reactive oxygen species and IL-6 in response to cAMP-dependent signals. Moreover, AhR-activated MCs produce IL-17, a critical player in chronic inflammation and autoimmunity, suggesting a novel pathway for MC activation in the pathogenesis of these diseases. Indeed, histological analysis of patients with chronic obstructive pulmonary disease revealed an enrichment in AhR/IL-6 and AhR/IL-17 double-positive MCs within bronchial lamina propria. Thus, tissue-resident MCs could translate external chemical challenges through AhR by modulating allergic responses and contributing to the generation of inflammation-related diseases.

Oxazaphosphorine bioactivation and detoxification The role of xenobiotic receptors

Oxazaphosphorines, with the most representative members including cyclophosphamide, ifosfamide, and trofosfamide, constitute a class of alkylating agents that have a broad spectrum of anticancer activity against many malignant ailments including both solid tumors such as breast cancer and hematological malignancies such as leukemia and lymphoma. Most oxazaphosphorines are prodrugs that require hepatic cytochrome P450 enzymes to generate active alkylating moieties before manifesting their chemotherapeutic effects. Meanwhile, oxazaphosphorines can also be transformed into non-therapeutic byproducts by various drug-metabolizing enzymes. Clinically, oxazaphosphorines are often administered in combination with other chemotherapeutics in adjuvant treatments. As such, the therapeutic efficacy, off-target toxicity, and unintentional drug-drug interactions of oxazaphosphorines have been long-lasting clinical concerns and heightened focuses of scientific literatures. Recent evidence suggests that xenobiotic receptors may play important roles in regulating the metabolism and clearance of oxazaphosphorines. Drugs as modulators of xenobiotic receptors can affect the therapeutic efficacy, cytotoxicity, and pharmacokinetics of coadministered oxazaphosphorines, providing a new molecular mechanism of drug-drug interactions. Here, we review current advances regarding the influence of xenobiotic receptors, particularly, the constitutive androstane receptor, the pregnane X receptor and the aryl hydrocarbon receptor, on the bioactivation and detoxification of oxazaphosphorines, with a focus on cyclophosphamide and ifosfamide.

Clinical and genetic aspects of familial isolated pituitary adenomas

Pituitary adenomas represent a group of functionally diverse neoplasms with relatively high prevalence in the general population. Most occur sporadically, but inherited genetic predisposing factors are increasingly recognized. Familial isolated pituitary adenoma is a recently defined clinical entity, and is characterized by hereditary presentation of pituitary adenomas in the absence of clinical and genetic features of syndromic disease such as multiple endocrine neoplasia type 1 and Carney complex. Familial isolated pituitary adenoma is inherited in an autosomal dominant manner and accounted for approximately 2-3% of pituitary tumors in some series. Germline mutations in the aryl-hydrocarbon interacting protein gene are identified in around 25% of familial isolated pituitary adenoma kindreds. Pituitary adenomas with mutations of the aryl-hydrocarbon interacting protein gene are predominantly somatotropinomas and prolactinomas, but non-functioning adenomas, Cushing disease, and thyrotropinoma may also occur. These tumors may present as macroadenomas in young patients and are often relatively difficult to control. Furthermore, recent evidence indicates that aryl-hydrocarbon interacting protein gene mutations occur in >10% of patients with sporadic macroadenomas that occur before 30 years of age, and in >20% of children with macroadenomas. Genetic screening for aryl-hydrocarbon interacting protein gene mutations is warranted in selected high-risk patients who may benefit from early recognition and follow-up.

Expression profiling of CYP1B1 in oral squamous cell carcinoma: counterintuitive downregulation in tumors

Oral Squamous Cell Carcinoma (OSCC) has a very flagitious treatment regime. A prodrug approach is thought to aid in targeting chemotherapy. CYP1B1, a member of cytochrome P450 family, has been implicated in chemical carcinogenesis. There exists a general accordance that this protein is overexpressed in a variety of cancers, making it an ideal candidate for a prodrug therapy. The activation of the prodrug facilitated by CYP1B1 would enable the targeting of chemotherapy to tumor tissues in which CYP1B1 is specifically overexpressed as a result reducing the non-specific side effects that the current chemotherapy elicits. This study was aimed at validating the use of CYP1B1 as a target for the prodrug therapy in OSCC. The expression profile of CYP1B1 was analysed in a panel of 51 OSCC tumors, their corresponding normal tissues, an epithelial dysplasia lesion and its matched normal tissue by qRT-PCR, Western blotting and Immunohistochemistry. CYP1B1 was found to be downregulated in 77.78% (28/36) tumor tissues in comparison to their corresponding normal tissues as well as in the epithelial dysplasia lesion compared to its matched normal tissue at the transcriptional level, and in 92.86% (26/28) of tumor tissues at the protein level. This report therefore clearly demonstrates the downregulation of CYP1B1 at the transcriptional and translational levels in tumor tissues in comparison to their corresponding normal tissues. These observations indicate that caution should be observed as this therapy may not be applicable universally to all cancers and also suggest the possibility of a prophylactic therapy for oral cancer.

The role of endogenous aryl hydrocarbon receptor signaling in cardiovascular physiology

The aryl hydrocarbon receptor (AHR) is an orphan nuclear receptor with a primary function of mediating xenobiotic metabolism through transcriptional activation of Phase I and Phase II drug-metabolizing enzymes. Although no high-affinity physiological activators of AHR have been discovered, the endogenous signaling of the AHR pathway is believed to play an important role in the development and function of the cardiovascular system, based on the observations on ahr gene-deficient mice. The AHR knockout mice develop cardiac hypertrophy, abnormal vascular structure in multiple organs and altered blood pressure depending on their host environment. In this review, the endogenous role of AHR in cardiovascular physiology, including heart function, vascular development and blood pressure regulation has been summarized and discussed.

Aryl hydrocarbon receptor in association with RelA modulates IL-6 expression in non-smoking lung cancer

Aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that is activated by cigarette smoke. Previously, we demonstrated that AhR is overexpressed in lung adenocarcinomas (ADs). In this study we observed that AhR expression is significantly correlated with nuclear RelA (a nuclear factor-κB (NFκB) subunit) and cytosolic interleukin-6 (IL-6) in 200 non-small cell lung cancer patients, especially among never smokers. Overexpression of AhR increased IL-6 expression in H1355 cells and immortalized human bronchial epithelial cells BEAS-2B. As NFκB inhibitor and knockdown RelA expression greatly reduced constitutive AhR-induced IL-6 expression, we hypothesized that AhR expression, in the absence of exogenous ligand, is able to modulate NFκB activity and subsequently upregulate IL-6 expression, thus promoting the development of lung AD. Specifically, AhR overexpression significantly increased NFκB activity, whereas interference with AhR expression significantly reduced NFκB activity and IL-6 expression in H1355 cells. We demonstrated that AhR associates with RelA in the cytosol and nucleus of human lung cells. Furthermore, AhR overexpression enhanced nuclear localization of AhR and RelA, and increased the association of AhR-RelA with the NFκB response element of the IL-6 promoter. However, p50 was not involved. Our results indicate that AhR, without exposure to a ligand, associates with RelA, which then positively modulates NFκB activity and then upregulates IL-6 expression in human lung cells. Thus we have identified a new mechanism for lung tumorigenesis in non-smokers.

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.

A novel quercetin analogue from a medicinal plant promotes peak bone mass achievement and bone healing after injury and exerts an anabolic effect on osteoporotic bone: the role of aryl hydrocarbon receptor as a mediator of osteogenic action

We recently reported that extracts made from the stem bark of Ulmus wallichiana promoted peak bone mass achievement in growing rats and preserved trabecular bone mass and cortical bone strength in ovariectomized (OVX) rats. Further, 6-C-β-D-glucopyranosyl-(2S,3S)-(+)-3',4',5,7-tetrahydroxyflavanol (GTDF), a novel flavonol-C-glucoside isolated from the extracts, had a nonestrogenic bone-sparing effect on OVX rats. Here we studied the effects of GTDF on osteoblast function and its mode of action and in vivo osteogenic effect. GTDF stimulated osteoblast proliferation, survival, and differentiation but had no effect on osteoclastic or adipocytic differentiation. In cultured osteoblasts, GTDF transactivated the aryl hydrocarbon receptor (AhR). Activation of AhR mediated the stimulatory effect of GTDF on osteoblast proliferation and differentiation. Furthermore, GTDF stimulated cAMP production, which mediated osteogenic gene expression. GTDF treatments given to 1- to 2-day-old rats or adult rats increased the mRNA levels of AhR target genes in calvaria or bone marrow stromal cells. In growing female rats, GTDF promoted parameters of peak bone accrual in the appendicular skeleton, including increased longitudinal growth, bone mineral density, bone-formation rate (BFR), cortical deposition, and bone strength. GTDF promoted the process of providing newly generated bone to fill drill holes in the femurs of both estrogen-sufficient and -deficient rats. In osteopenic OVX rats, GTDF increased BFR and significantly restored trabecular bone compared with the ovaries-intact group. Together our data suggest that GTDF stimulates osteoblast growth and differentiation via the AhR and promotes modeling-directed bone accrual, accelerates bone healing after injury, and exerts anabolic effects on osteopenic rats likely by a direct stimulatory effect on osteoprogenitors. Based on these preclinical data, clinical evaluation of GTDF as a potential bone anabolic agent is warranted.

Stress-related methylation of the catechol-O-methyltransferase Val 158 allele predicts human prefrontal cognition and activity

DNA methylation at CpG dinucleotides is associated with gene silencing, stress, and memory. The catechol-O-methyltransferase (COMT) Val(158) allele in rs4680 is associated with differential enzyme activity, stress responsivity, and prefrontal activity during working memory (WM), and it creates a CpG dinucleotide. We report that methylation of the Val(158) allele measured from peripheral blood mononuclear cells (PBMCs) of Val/Val humans is associated negatively with lifetime stress and positively with WM performance; it interacts with stress to modulate prefrontal activity during WM, such that greater stress and lower methylation are related to reduced cortical efficiency; and it is inversely related to mRNA expression and protein levels, potentially explaining the in vivo effects. Finally, methylation of COMT in prefrontal cortex and that in PBMCs of rats are correlated. The relationship of methylation of the COMT Val(158) allele with stress, gene expression, WM performance, and related brain activity suggests that stress-related methylation is associated with silencing of the gene, which partially compensates the physiological role of the high-activity Val allele in prefrontal cognition and activity. Moreover, these results demonstrate how stress-related DNA methylation of specific functional alleles impacts directly on human brain physiology beyond sequence variation.

AIP and its interacting partners

Germline mutations in the aryl hydrocarbon receptor-interacting protein gene (AIP) predispose to young-onset pituitary tumours, most often to GH- or prolactin-secreting adenomas, and most of these patients belong to familial isolated pituitary adenoma families. The molecular pathway initiated by the loss-of-function AIP mutations leading to pituitary tumour formation is unknown. AIP, a co-chaperone of heat-shock protein 90 and various nuclear receptors, belongs to the family of tetratricopeptide repeat (TPR)-containing proteins. It has three antiparallel α-helix motifs (TPR domains) that mediate the interaction of AIP with most of its partners. In this review, we summarise the known interactions of AIP described so far. The identification of AIP partners and the understanding of how AIP interacts with these proteins might help to explain the specific phenotype of the families with heterozygous AIP mutations, to gain deeper insight into the pathological process of pituitary tumour formation and to identify novel drug targets.

Aryl hydrocarbon receptor activation in lactotropes and gonadotropes interferes with estradiol-dependent and -independent preprolactin, glycoprotein alpha and luteinizing hormone beta gene expression

Arylhydrocarbon receptor (Ahr) activation by 2,3,7,8-tetrachlordibenzo-p-dioxin (TCDD) interferes with female reproductive functions, but there is little information on the specific targets of TCDD in the hypothalamic-pituitary-gonadal (HPG) axis. In these studies, we found that TCDD upregulated known AhR target genes, cytochrome p450 1a1 (Cyp1a1), Cyp1a2 and Cyp1b1 in the rat pituitary gland. Moreover, 75% of pituitary lactotropes and 45% of gonadotropes contained Ahr mRNA, and most Ahr-containing cells were estrogen receptor 1 (Esr1)-positive. TCDD abrogated estradiol (E(2))-induced prolactin (Prl) expression in vivo and in vitro; conversely, E(2) blocked TCDD upregulation of luteinizing hormone beta (Lhb) and glycoprotein hormone alpha polypeptide (Cga) expression. TCDD had no effect on levels of Ahr mRNA, but upregulated Esr1 mRNA. E(2) independently repressed Ahr and Esr1 expression and blocked TCDD upregulation of Esr1. Thus, complex interactions between Ahr and Esr alter Prl and luteinizing hormone (LH) synthesis by direct actions in lactotropes and gonadotropes. These findings provide important insights into how TCDD disrupts female reproductive functions.

Non-genomic action of TCDD to induce inflammatory responses in HepG2 human hepatoma cells and in liver of C57BL/6J mice

To assess the significance of the non-genomic signaling of TCDD (=dioxin) on liver of C57BL/6 mice and HepG2 human hepatoma cells, we first determined the group of markers that are susceptible to inhibition by parthenolide, a compound known to specifically suppress NF-κB-mediated inflammation. Of those, the most consistent marker turned out to be SOCS3 (a suppressor of cytokine signaling) known to respond to inflammation. An early diagnostic test on the action of TCDD on HepG2 cells in vitro within 3-6 h indicated that Cox-2 and SOCS3 are mainly induced via a non-genomic route, whereas PAI-2 appears to be induced through the classical action route. More detailed diagnostic tests at later stages of action of TCDD in HepG2 cells revealed that induction of IL-1β, BAFF, and iNOS are largely mediated by the protein kinase-dependent non-genomic route. An in vivo study on the 7 day action of TCDD on liver of AhR(NLS) mice showed that several early markers (e.g., Cox-2, MCP-1 and SOCS3) are induced, but not late markers such as IL-1β. Together, these results show that the non-genomic pathway contributes significantly to the early stress response reactions to TCDD that includes inflammation in hepatoma cells as well as in the liver.

Characterization of aryl hydrocarbon receptor interacting protein (AIP) mutations in familial isolated pituitary adenoma families

Familial isolated pituitary adenoma (FIPA) is an autosomal dominant condition with variable genetic background and incomplete penetrance. Germline mutations of the aryl hydrocarbon receptor interacting protein (AIP) gene have been reported in 15–40% of FIPA patients. Limited data are available on the functional consequences of the mutations or regarding the regulation of the AIP gene. We describe a large cohort of FIPA families and characterize missense and silent mutations using minigene constructs, luciferase and β-galactosidase assays, as well as in silico predictions. Patients with AIP mutations had a lower mean age at diagnosis (23.6±11.2 years) than AIP mutation-negative patients (40.4±14.5 years). A promoter mutation showed reduced in vitro activity corresponding to lower mRNA expression in patient samples. Stimulation of the protein kinase A-pathway positively regulates the AIP promoter. Silent mutations led to abnormal splicing resulting in truncated protein or reduced AIP expression. A two-hybrid assay of protein–protein interaction of all missense variants showed variable disruption of AIP-phosphodiesterase-4A5 binding. In summary, exonic, promoter, splice-site, and large deletion mutations in AIP are implicated in 31% of families in our FIPA cohort. Functional characterization of AIP changes is important to identify the functional impact of gene sequence variants. Hum Mutat 31:1–11, 2010. © 2010 Wiley-Liss, Inc.

Aryl hydrocarbon receptor nuclear translocator in hepatocytes is required for aryl hydrocarbon receptor-mediated adaptive and toxic responses in liver

The aryl hydrocarbon receptor (AHR) plays a central role in the toxic responses to halogenated dibenzo-p-dioxins ("dioxins"), in the metabolic adaptation to polycyclic aromatic hydrocarbons, and in the development of the mature vascular system. A number of lines of evidence support the idea that the regulation of adaptive metabolism requires an AHR partnership with the aryl hydrocarbon receptor nuclear translocator (ARNT). Yet, for AHR-dependent vascular development and dioxin toxicity, the role of ARNT is less certain. In fact, numerous models have been proposed over the years to suggest that the AHR signals in important ways via ARNT-independent events. In an effort to clarify the role of ARNT in AHR-mediated dioxin hepatotoxicity, we generated a conditional Arnt mouse model. Such a model was essential because global inactivation of Arnt results in embryonic lethality presumably due to this protein's role as a heterodimeric partner for the hypoxia-inducible factors (HIFs). Using a hepatocyte-specific Arnt deletion, we were able to demonstrate that hepatocyte ARNT is required for major aspects of AHR-mediated dioxin toxicity in the liver. Results from this conditional Arnt allele are also consistent with a model where hepatocyte ARNT is unrelated to AHR-mediated hepatovascular development. In sum, these data suggest that AHR-ARNT dimers within the hepatocyte direct the toxic and adaptive and developmental functions associated with the AHR and that developmental vascular events arise due to signaling in a distinct cell type expressing this dimeric pair.

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.

PRKAR1A and the evolution of pituitary tumors

Carney complex (CNC) is an inherited tumor predisposition associated with pituitary tumors, including GH-producing pituitary adenomas and rare reports of prolactinomas. This disease is caused by mutations in PRKAR1A, which encodes the type 1A regulatory subunit of the cAMP-dependent protein kinase, PKA. Loss of PRKAR1A causes enhanced PKA signaling, which leads to pituitary tumorigenesis. Mutations in the gene have not been detected in sporadic pituitary tumors, but there is some data to suggest that non-genomic mechanisms may cause loss of protein expression. Unlike CNC patients, mice heterozygous for Prkar1a mutations do not develop pituitary tumors, although complete knockout of the gene in the Pit1 lineage of the pituitary produces GH-secreting pituitary adenomas. These data indicate that complete loss of Prkar1a/PRKAR1A is able to cause pituitary tumors in mice and men. The pattern of tumors is likely related to the signaling pathways employed in specific pituitary cell types.

Role of the aryl hydrocarbon receptor-interacting protein in familial isolated pituitary adenoma

Pituitary adenomas are typically sporadic benign tumors. However, approximately 5% of cases have been found to be familial in origin. Of these, approximately 40% occur in the absence of multiple endocrine neoplasia type 1 or Carney complex and have been termed 'familial isolated pituitary adenoma' (FIPA). Recently, germline mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene have been described in 15-20% of these families, identifying an autosomal dominant condition with incomplete penetrance termed 'pituitary adenoma predisposition'. Pituitary adenoma predisposition cohorts show a marked disposition to develop large, aggressive somatotroph, somatolactotroph or lactotroph adenomas, typically presenting at a young age. AIP mutation families have a distinct clinical phenotype compared with AIP mutation-negative FIPA families. Current evidence suggests that AIP is a tumor-suppressor gene. AIP has been demonstrated to interact with a number of cellular proteins, including several nuclear receptors, heat-shock protein 90 and survivin, although the mechanism of the tumor-suppressor effect is unknown. This article summarizes available data regarding the role of AIP in pituitary tumorigenesis and the clinical features of FIPA.

The aryl hydrocarbon receptor (AhR) in the regulation of cell-cell contact and tumor growth

The aryl hydrocarbon receptor (AhR) is a ligand-dependent transcription factor, which is activated by a large group of environmental pollutants including polycyclic aromatic hydrocarbons, dioxins and planar polychlorinated biphenyls. Ligand binding leads to dimerization of the AhR with aryl hydrocarbon receptor nuclear translocator and transcriptional activation of several xenobiotic phase I and phase II metabolizing enzymes, such as cytochrome P4501A1 and glutathione-S-transferase, respectively. Since phase I enzymes convert inert carcinogens to active genotoxins, the AhR plays a key role in tumor initiation. Besides this classical route, the AhR mediates tumor promotion and recent evidence suggests that the AhR also plays a role in tumor progression. To date, no mechanistic link could be established between the canonical pathway involving xenobiotic metabolism and AhR-dependent tumor promotion and progression. A hallmark of tumor promotion is unbalanced proliferation, whereas tumor progression is characterized by dedifferentiation, increased motility and metastasis of tumor cells. Tumor progression and presumably also tumor promotion are triggered by loss of cell-cell contact. Cell-cell contact is known to be a critical regulator of proliferation, differentiation and cell motility in vitro and in vivo. Increasing evidence suggests that activation of the AhR may lead to deregulation of cell-cell contact, thereby inducing unbalanced proliferation, dedifferentiation and enhanced motility. In line with this is the finding of increased AhR expression and malignancy in some animal and human cancers. Here, we summarize our current knowledge on non-canonical AhR-driven pathways being involved in deregulation of cell-cell contact and discuss the data with respect to tumor initiation, promotion and progression.

Clinical, genetic and molecular characterization of patients with familial isolated pituitary adenomas (FIPA)

Familial pituitary adenomas can occur in MEN1 and Carney complex, as well as in the recently characterized familial isolated pituitary adenoma (FIPA) syndrome. FIPA is an autosomal dominant disease with incomplete penetrance, characterized by early-onset disease, often aggressive tumor growth and a predominance of somatotroph and lactotroph adenomas. In 20% of FIPA families, heterozygous mutations have been described in the aryl hydrocarbon receptor interacting (AIP) gene, whereas in other families the causative gene(s) are unknown. It has been suggested that AIP is a tumor suppressor gene and although experimental data support this hypothesis, the exact molecular mechanism by which its disruption leads to tumorigenesis is unclear. Here we discuss the clinical, genetic and molecular features of patients with FIPA.

Potential health-modulating effects of isoflavones and metabolites via activation of PPAR and AhR

Isoflavones have multiple actions on cell functions. The most prominent one is the activation of estrogen receptors. Other functions are often overlooked, but are equally important and explain the beneficial health effects of isoflavones. Isoflavones are potent dual PPARα/γ agonists and exert anti-inflammatory activity, which may contribute to the prevention of metabolic syndrome, atherosclerosis and various other inflammatory diseases. Some isoflavones are potent aryl hydrocarbon receptor (AhR) agonists and induce cell cycle arrest, chemoprevention and modulate xenobiotic metabolism. This review discusses effects mediated by the activation of AhR and PPARs and casts a light on the concerted action of isoflavones.

The aryl hydrocarbon receptor: regulation of hematopoiesis and involvement in the progression of blood diseases

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix protein that belongs to the superfamily of environment-sensing PAS (Per-ARNT-Sim) proteins. A large number of ligands have been described to bind AhR and promote its nuclear translocation. In the nucleus, the AhR and its dimerization partner the AhR nuclear translocator (ARNT) form a DNA-binding complex that acts as a transcriptional regulator. Animal and human data suggest that, beyond its mediating responses to xenobiotic and/or unknown endogenous ligands, the AhR has a role, although as yet undefined, in the regulation of cell cycle and inflammation. The AhR also appears to regulate the hematopoietic and immune systems during development and adult life in a cell-specific manner. While accidental exposure to xenobiotic AhR ligands has been associated with leukemia in humans, the specific mechanisms of AhR involvement are still not completely understood. However, recent data are consistent with a functional role of the AhR in the maintenance of hematopoietic stem and/or progenitor cells (HSCs/HPCs). Studies highlighting AhR regulation of HSCs/HPCs provide a rational framework to understand their biology, a role of the AhR in hematopoietic diseases, and a means to develop interventions for these diseases.

The mammalian aryl hydrocarbon (Ah) receptor: from mediator of dioxin toxicity toward physiological functions in skin and liver

The mammalian Ah receptor (AhR) is a ligand-activated transcription factor with multiple functions in adaptive metabolism, development and dioxin toxicity in a variety of organs and cell systems. Phenotypes observed following sustained activation by dioxin or in AhR-null mice suggest organ-dependent physiological functions. These functions are probably deregulated following exposure to dioxin. We focus on skin and liver to facilitate discussion of mechanisms linking phenotypes and AhR-modulated genotypes. After a brief summary of currently discussed AhR ligand candidates, two groups of direct AhR target genes/proteins and associated functions are highlighted: (i) xenobiotic-metabolizing enzymes which are also involved in homeostasis of endogenous ligands and (ii) proteins controlling cell proliferation/apoptosis, differentiation and inflammation. Homeostatic feedback loops might not only include CYP1A1 but also Phase II enzymes such as UGT1A1 which controls the antioxidant AhR ligand bilirubin. The AhR is involved in extensive crosstalk with other transcription factors and multiple signaling pathways. Efforts elucidating the pathway toward identification of physiological functions of the AhR remain challenging and promising.

Molecular mechanisms of the physiological functions of the aryl hydrocarbon (dioxin) receptor, a multifunctional regulator that senses and responds to environmental stimuli

The aryl hydrocarbon receptor (AhR) was originally identified as a ligand-activated transcription factor that is involved in the induction of xenobiotic-metabolizing Cytochrome P4501A1 (CYP1A1). For several decades, AhR has been studied in relation to toxicology and pharmacology. With recent discoveries on novel AhR functions, AhR research has expanded into multiple aspects of physiology, such as reproduction, innate immunity and tumor suppression. In this review, we summarize and discuss recent progress in mechanistic and functional studies on AhR with particular emphasis on physiological processes.

Molecular genetics of the aip gene in familial pituitary tumorigenesis

Pituitary adenomas usually occur as sporadic tumors, but familial cases are now increasingly identified. As opposed to multiple endocrine neoplasia type 1 and Carney complex, in familial isolated pituitary adenoma (FIPA) syndrome no other disease is associated with the familial occurrence of pituitary adenomas. It is an autosomal dominant disease with incomplete variable penetrance. Approximately 20% of patients with FIPA harbour germline mutations in the aryl hydrocarbon receptor-interacting protein (AIP) gene located on 11q13. Patients with AIP mutations have an overwhelming predominance of somatotroph and lactotroph adenomas, which often present in childhood or young adulthood. AIP, originally identified as a molecular co-chaperone of several nuclear receptors, is thought to act as a tumor suppressor gene; overexpression of wild-type, but not mutant AIP, reduces cell proliferation while knockdown of AIP stimulates it. AIP is shown to bind various proteins, including the aryl hydrocarbon receptor, Hsp90, phosphodiesterases, survivin, RET and the glucocorticoid receptor, but currently it is not clear which interaction has the leading role in pituitary tumorigenesis. This chapter summarizes the available clinical and molecular data regarding the role of AIP in the pituitary gland.

The expression of AIP-related molecules in elucidation of cellular pathways in pituitary adenomas

Germline mutations in the aryl hydrocarbon receptor interacting protein (AIP) gene predispose to the development of pituitary adenomas. Here, we characterized AIP mutation positive (AIPmut+) and AIP mutation negative (AIPmut-) pituitary adenomas by immunohistochemistry. The expressions of the AIP-related proteins aryl hydrocarbon receptor (AHR), AHR nuclear translocator (ARNT), cyclin-dependent kinase inhibitor 1B encoding p27(Kip1), and hypoxia-inducible factor 1-alpha were examined in 14 AIPmut+ and 53 AIPmut- pituitary adenomas to detect possible expression differences. In addition, the expression of CD34, an endothelial and hematopoietic stem cell marker, was analyzed. We found ARNT to be less frequently expressed in AIPmut+ pituitary adenomas (P = 0.001), suggesting that AIP regulates the ARNT levels. AIP small interfering RNA-treated HeLa, HEK293, or Aip-null mouse embryonic fibroblast cells did not show lowered expression of ARNT. Instead, in the pituitary adenoma cell line GH3, Aip silencing caused a partial reduction of Arnt and a clear increase in cell proliferation. We also observed a trend for increased expression of nuclear AHR in AIPmut+ samples, although the difference was not statistically significant (P = 0.06). The expressions of p27(Kip1), hypoxia-inducible factor 1-alpha, or CD34 did not differ between tumor types. The present study shows that the expression of ARNT protein is significantly reduced in AIPmut+ tumors. We suggest that the down-regulation of ARNT may be connected to an imbalance in AHR/ARNT complex formation arising from aberrant cAMP signaling.

The aryl hydrocarbon receptor in immunity

Low-molecular-weight chemicals or xenobiotics might contribute to the increasing prevalence of allergies and autoimmunity. Certain chemicals can alter immune responses via their action on the cytosolic transcription factor aryl hydrocarbon receptor (AhR). AhR recognizes numerous small xenobiotic and natural molecules, such as dioxin and the tryptophan photoproduct 6-formylindolo[3,2-b]carbazole. Although AhR is best known for mediating dioxin toxicity, knockout studies have indicated that AhR also plays a role in normal physiology, including certain immune responses. In particular, Th17 cells and dendritic cells express high levels of AhR. We review here current evidence for the physiological role of AhR in the immune system, focussing in particular on T-cell biology.

Aryl hydrocarbon receptor activation and overexpression upregulated fibroblast growth factor-9 in human lung adenocarcinomas

We had previously reported that aryl hydrocarbon receptors (AhRs) are overexpressed in lung adenocarcinomas. Benzo[a]pyrene (BaP), an AhR agonist, increased FGF-9 expression in human lung adenocarcinoma cells. Similarly, several AhR agonists increased FGF-9 mRNA levels, and BaP-induced FGF-9 expression was prevented by cotreatment with AhR antagonist in human lung adenocarcinoma cells. Furthermore, AhR agonists increased transcriptional activity of FGF-9 promoter. Modulation of AhR expression via RNA interference or transient overexpression respectively reduced or increased both constitutive and BaP-induced FGF-9 expression in human lung cells. These results suggested that AhR activation and overexpression increased FGF-9 expression in lung cells. FGF-9 increased growth of lung fibroblasts but not that of lung adenocarcinoma cells. However, conditioned media collected from FGF-9-treated fibroblasts increased cell growth of lung adenocarcinoma cells. Furthermore, lung adenocarcinoma cells expressed FGF receptor 2 and cotreatment with anti-FGF receptor 2 prevented the interaction between fibroblasts and tumor cells. It is likely that FGF-9-stimulated fibroblasts secreted unknown factors, which activated FGF receptor 2 and subsequently promoted growth of lung adenocarcinoma cells. We further compared AhR and FGF-9 expression in 146 non-small cell lung cancer (NSCLC) cases by immunohistochemistry. FGF-9 expression was more common in adenocarcinomas than in squamous cell carcinomas. Furthermore, FGF-9 and AhR expression were well correlated in lung adenocarcinomas. These results suggest that AhR expression correlated positively with FGF-9 expression in lung adenocarcinomas, which might promote tumor growth by modulating communication between tumor cells and fibroblasts. Preventing AhR overexpression or disturbing FGF-9 function may reduce the development of lung adenocarcinomas. (c) 2009 UICC.

The aryl hydrocarbon receptor: a perspective on potential roles in the immune system

The aryl hydrocarbon receptor (AHR) is a protein best known for its role in mediating toxicity. Over 30 years of research has uncovered additional roles for the AHR in xenobiotic metabolism and normal vascular development. Activation of the AHR has long been known to cause immunotoxicity, including thymic involution. Recent data suggesting a role for the AHR in regulatory T-cell (Treg) and T-helper 17 (Th17) cell development have only added to the excitement about this biology. In this review, we will attempt to illustrate what is currently known about AHR biology in the hope that data from fields as diverse as evolutionary biology and pharmacology will help elucidate the mechanism by which AHR modifies immune responses. We also will discuss the complexities of AHR pharmacology and genetics that may influence future studies of AHR in the immune system.