TCDD activates Mdm2 and attenuates the p53 response to DNA damaging agents

Effect of oxidative stress induced by 2,3,7,8- tetrachlorodibenzo-p-dioxin on DNA damage

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic persistent organic pollutant (POP) that can induce DNA damage within cells. Although oxidative stress is one of the primary mechanisms causing DNA damage, its role in the process of TCDD-induced DNA damage remains unclear. In this study, the TCDD-induced production of reactive oxygen species (ROS) and the occurrence of DNA damage at the AP site were monitored simultaneously. Further investigation revealed that TCDD impaired the activities of superoxide dismutase (SOD) and catalase (CAT), compromising the cellular antioxidant defense system. Consequently, this led to an increase in the production of O2.- and NO, thus inducing DNA damage at the AP site under oxidative stress. Our findings were further substantiated by the upregulation of key genes in the base excision repair (BER) pathway and the absence of DNA AP site damage after inhibiting O2.- and NO. In addition, transcriptome sequencing revealed that TCDD induces DNA damage by upregulating genes associated with oxidative stress in the mitogen-activated protein kinase (MAPK), cyclic adenosine monophosphate (cAMP), and breast cancer pathways. This study provides important insights into the toxicity mechanisms of TCDD.

Approaches for the setting of occupational exposure limits (OELs) for carcinogens

This article addresses issues of importance for occupational exposure limits (OELs) and chemical carcinogens with a focus on non-threshold carcinogens. It comprises scientific as well as regulatory issues. It is an overview, not a comprehensive review. A central topic is mechanistic research and insights, and its implications for cancer risk assessment. Alongside scientific advancements, the approaches of hazard identification and qualitative and quantitative risk assessment have developed over the years. The key steps in a quantitative risk assessment are outlined, with special attention given to the dose-response assessment and the derivation of an OEL using risk calculations or default assessment factors. The work procedures of several bodies performing cancer hazard identifications and quantitative risk assessments, as well as regulatory procedures to derive OELs for non-threshold carcinogens, are presented. Non-threshold carcinogens for which the European Union (EU) introduced binding OELs in 2017-2019 serve as illustrations together with some currently used strategies in the EU and elsewhere. Available knowledge supports the derivation of health-based OELs (Hb-OELs) for non-threshold carcinogens, and the use of a risk-based approach with low-dose linear extrapolation (linear non-threshold, LNT) as the default for non-threshold carcinogens. However, there is a need to develop methods that allow recent years' advances in cancer research to be used for improving risk estimates. It is recommended that defined risk levels (terminology and numerical values) are harmonised, and that both collective and individual risks are considered and clearly communicated. Socioeconomic aspects should be dealt with transparently and separated from the scientific health risk assessment.

Phospholipase A and acyltransferase 4/retinoic acid receptor responder 3 at the intersection of tumor suppression and pathogen restriction

Phospholipase A and acyltransferase (PLAAT) 4 is a class II tumor suppressor with phospholipid metabolizing abilities. It was characterized in late 2000s, and has since been referred to as 'tazarotene-induced gene 3' (TIG3) or 'retinoic acid receptor responder 3' (RARRES3) as a key downstream effector of retinoic acid signaling. Two decades of research have revealed the complexity of its function and regulatory roles in suppressing tumorigenesis. However, more recent findings have also identified PLAAT4 as a key anti-microbial effector enzyme acting downstream of interferon regulatory factor 1 (IRF1) and interferons (IFNs), favoring protection from virus and parasite infections. Unveiling the molecular mechanisms underlying its action may thus open new therapeutic avenues for the treatment of both cancer and infectious diseases. Herein, we aim to summarize a brief history of PLAAT4 discovery, its transcriptional regulation, and the potential mechanisms in tumor prevention and anti-pathogen defense, and discuss potential future directions of PLAAT4 research toward the development of therapeutic approaches targeting this enzyme with pleiotropic functions.

Inorganic arsenic induces MDM2, p53, and their phosphorylation and affects the MDM2/p53 complex in vitro

Arsenic, as a human carcinogen, has posed a certain threat to environmental health globally. However, the underlying mechanism of the arsenic carcinogenic effect remains largely undetermined. The up-regulation of MDM2 seems to play a crucial part in tumors in especial carcinomas of the diffuse type. The interaction of MDM2 and p53 is closely relevant to the pathogenesis of tumors. In this study, we aimed to investigate the effect on MDM2, p53, and their phosphorylation after As(III). In the epidemiological study, we investigated that MDM2 expression was up-regulation and was positively linked to methylated metabolites (monomethylarsonic acid (MMA) and dimethylarsinic acid (DMA)) after As(III)-exposure. In vitro studies employing A549 and 16HBE cells confirmed the epidemiological data. Studies on MDM2 phosphorylation sites consisting of Ser166, Ser260, and Ser394 in response to arsenic exposure, which have not been studied presently, indicated that As(III) could induce the expression of MDM2 phosphorylation. Moreover, we studied the alterations of p53 and its N-terminus phosphorylation sites of Ser9, Ser15, and Ser33, which demonstrated that p53 and its phosphorylation were highly expressed after As(III) exposure. Subsequently, Co-immunoprecipitation assays validated our hypothesis that the bonding of MDM2 and p53 was altered by arsenic exposure. What's more, outcomes coming from different cell types of A549, 16HBE, and 60 T-16HBE revealed that MDM2 and its phosphorylation expression existed a significant difference. The study provides evidence that As(III) and its methylated metabolites modulate the expression of MDM2, p53, and their phosphorylation and then affect the interaction between MDM2 and p53.

Aberrant Upregulation of Indoleamine 2,3-Dioxygenase 1 Promotes Proliferation and Metastasis of Hepatocellular Carcinoma Cells via Coordinated Activation of AhR and β-Catenin Signaling

Hepatocellular carcinoma (HCC) is the fourth most common cause of cancer-related death worldwide. Chronic liver inflammation due to hepatitis virus infection and other major effectors is a major risk factor of HCC. Indoleamine 2,3-dioxygenase 1 (IDO1), a heme enzyme highly expressed upon stimulation with proinflammatory cytokines such as interferon-γ (IFN-γ), is activated to modulate the tumor microenvironment and potentially crucial in the development of certain cancer types. Earlier studies have majorly reported an immunomodulatory function of IDO1. However, the specific role of IDO1 in cancer cells, particularly HCC, remains to be clarified. Analysis of The Cancer Genome Atlas Liver Hepatocellular Carcinoma (TCGA LIHC) dataset in the current study revealed a significant correlation between IDO1 expression and HCC. We further established inducible IDO1-expressing cell models by coupling lentivirus-mediated knockdown and IFN-γ induction of IDO1 in normal and HCC cells. In functional assays, proliferation and motility-related functions of HCC cells were compromised upon suppression of IDO1, which may partially be rescued by its enzymatic product, kynurenine (KYN), while normal hepatocytes were not affected. Aryl hydrocarbon receptor (AhR), a reported endogenous KYN receptor, is suggested to participate in tumorigenesis. In mechanistic studies, IDO1 activation promoted both AhR and β-catenin activity and nuclear translocation. Immunofluorescence staining and co-immunoprecipitation assays further disclosed interactions between AhR and β-catenin. In addition, we identified a Src-PTEN-PI3K/Akt-GSK-3β axis involved in β-catenin stabilization and activation following IDO1-mediated AhR activation. IDO1-induced AhR and β-catenin modulated the expression of proliferation- and EMT-related genes to facilitate growth and metastasis of HCC cells. Our collective findings provide a mechanistic basis for the design of more efficacious IDO1-targeted therapy for HCC.

TCDD promotes liver fibrosis through disordering systemic and hepatic iron homeostasis

2, 3, 7, 8-Tetrachlorodibenzo-p-dioxin (TCDD) is a toxic environmental pollutant which can cause severe health problems, such as fibrosis. However, the toxic effects and related mechanism of TCDD on the liver remain largely unknown. In this study, we established a liver fibrosis mouse model upon exposure of TCDD, as evidenced by increased collagen I, tumor growth factor β1 (TGFβ1), α-smooth muscle actin (α-SMA), and Masson staining. Meanwhile, there was also a significant increase of inflammatory factors and TUNEL-positive hepatocytes in liver, indicating that liver inflammation and hepatic cell apoptosis occurred. In addition, increased serum and liver iron were concomitant with liver injury induced by TCDD. We further investigated the mechanism underlying TCDD-induced hepatocyte apoptosis through apoptosis polymerase chain reaction array, and found that a crucial apoptosis-related gene, cell death-inducing DFF45-like effector b (Cideb), was significantly increased in primary hepatocytes from TCDD-exposed mice, and accompanied by liver iron deposition in hepcidin knockout mice. Therefore, Cideb depletion could effectively attenuated TCDD or iron induced cell death related genes expression. In conclusion, our results showed that iron-induced Cideb expression played a critical role in promoting TCDD-induced hepatocyte apoptosis and liver fibrosis, which provide a novel mechanism for understanding TCDD-induced liver injury.

Multigenerational and Transgenerational Effects of Dioxins

Dioxins are ubiquitous and persistent environmental contaminants whose background levels are still reason for concern. There is mounting evidence from both epidemiological and experimental studies that paternal exposure to the most potent congener of dioxins, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), can lower the male/female ratio of offspring. Moreover, in laboratory rodents and zebrafish, TCDD exposure of parent animals has been reported to result in reduced reproductive performance along with other adverse effects in subsequent generations, foremost through the paternal but also via the maternal germline. These impacts have been accompanied by epigenetic alterations in placenta and/or sperm cells, including changes in methylation patterns of imprinted genes. Here, we review recent key studies in this field with an attempt to provide an up-to-date picture of the present state of knowledge to the reader. These studies provide biological plausibility for the potential of dioxin exposure at a critical time-window to induce epigenetic alterations across multiple generations and the significance of aryl hydrocarbon receptor (AHR) in mediating these effects. Currently available data do not allow to accurately estimate the human health implications of these findings, although epidemiological evidence on lowered male/female ratio suggests that this effect may take place at realistic human exposure levels.

Effects of PCB-126 on aryl hydrocarbon receptor, ubiquitin and p53 expression levels in Sparus aurata

The aim of the present study is to determine if Ahr ligands as PCB-126, a dioxin-like, might contribute to inhibition of the tumor suppressor p53 by promoting its degradation through proteasome-ubiquitin system (UPS). The findings show, in the presence of PCB-126, a significant increase of p53 immunoreactivity in fish compared to the control. Subsequently, there is a decrease of p53 immunoreactivity at 24h which is maintained even at 72h. At the same time there is a slight decrease of ubiquitin immunoreactivity to 12h compared to the control and a marked decrease to 24 and 72h. The induction of ubiquitin expression is resulted very marked in the control and preserved at 12h. It's very important to underline as in our study we demonstrate a marked decrease of ubiquitin and p53 immunoreactivity at 24h and 72h. AHR activation, by ligands as PCB-126, increases p53 ubiquitation inhibiting its expression, in addition it decreases the free ubiquitin promoting disruption of Ub homeostasis; this is the first report that establishes a relationship between AhR, increases p53 ubiquitation, and reduction of free ubiquitin. Our result emphasize the need to deeply the role of this receptor in UPS regulation as potential therapeutic target for cancer treatment.

Long lasting MDM2/Translocator protein modulator: a new strategy for irreversible apoptosis of human glioblastoma cells

The development of multi-target drugs and irreversible modulators of deregulated signalling proteins is the major challenge for improving glioblastoma multiforme (GBM) treatment. Reversible single-target drugs are not sufficient to sustain a therapeutic effect over time and may favour the activation of alternative signalling pathways and the onset of resistance phenomena. Thus, a multi-target compound that has a long-lasting mechanism of action might have a greater and longer life span of anti-proliferative activity. Recently, a dual-target indol-3ylglyoxyldipeptide derivative, designed to bind to the Translocator Protein (TSPO) and reactivate p53 function via dissociation from its physiological inhibitor, murine double minute 2 (MDM2), has been developed as a potent GBM pro-apoptotic agent. In this study, this derivative was chemically modified to irreversibly bind MDM2 and TSPO. The new compound elicited a TSPO-mediated mitochondrial membrane dissipation and restored p53 activity, triggering a long-lasting apoptosis of GBM cells. These effects were sustained over time, involved a stable activation of extracellular signal regulated kinases and were specifically observed in cancer cells, in which these protein kinases are deregulated. Dual-targeting and irreversible binding properties combined in the same molecule may represent a useful strategy to overcome the time-limited effects elicited by classical chemotherapies.

The p53 inhibitor, pifithrin-α, disrupts microtubule organization, arrests growth, and induces polyploidy in the rainbow trout gill cell line, RTgill-W1

Pifithrin-α (PFT-α) blocks p53-dependent transcription and is an example of the many drugs being developed to target the p53 pathway in humans that could be released into the environment with potential impacts on aquatic animals if they were to become successful pharmaceuticals. In order to understand how p53 drugs might act on fish, the effects of PFT-α on rainbow trout gill epithelial cell line, RTgill-W1, were studied. PFT-α was not cytotoxic to RTgill-W1 in cultures with or without fetal bovine serum (FBS), but at 5.25μg/ml, PFT-α completely arrested proliferation. When FBS was present, PFT-α increased the number of polyploid cells over 12days. Those results suggest that like in mammals, p53 appears to regulate ploidy in fish. However, several effects were seen that have not been observed with mammalian cells. PFT-α caused a transient rise in the mitotic index and a disruption in cytoskeletal microtubules. These results suggest that in fish cells PFT-α affects microtubules either directly through an off-target action on tubulin or indirectly through an on-target action on p53-regulated transcription.

The adverse outcome pathway for rodent liver tumor promotion by sustained activation of the aryl hydrocarbon receptor

An Adverse Outcome Pathway (AOP) represents the existing knowledge of a biological pathway leading from initial molecular interactions of a toxicant and progressing through a series of key events (KEs), culminating with an apical adverse outcome (AO) that has to be of regulatory relevance. An AOP based on the mode of action (MOA) of rodent liver tumor promotion by dioxin-like compounds (DLCs) has been developed and the weight of evidence (WoE) of key event relationships (KERs) evaluated using evolved Bradford Hill considerations. Dioxins and DLCs are potent aryl hydrocarbon receptor (AHR) ligands that cause a range of species-specific adverse outcomes. The occurrence of KEs is necessary for inducing downstream biological responses and KEs may occur at the molecular, cellular, tissue and organ levels. The common convention is that an AOP begins with the toxicant interaction with a biological response element; for this AOP, this initial event is binding of a DLC ligand to the AHR. Data from mechanistic studies, lifetime bioassays and approximately thirty initiation-promotion studies have established dioxin and DLCs as rat liver tumor promoters. Such studies clearly show that sustained AHR activation, weeks or months in duration, is necessary to induce rodent liver tumor promotion--hence, sustained AHR activation is deemed the molecular initiating event (MIE). After this MIE, subsequent KEs are 1) changes in cellular growth homeostasis likely associated with expression changes in a number of genes and observed as development of hepatic foci and decreases in apoptosis within foci; 2) extensive liver toxicity observed as the constellation of effects called toxic hepatopathy; 3) cellular proliferation and hyperplasia in several hepatic cell types. This progression of KEs culminates in the AO, the development of hepatocellular adenomas and carcinomas and cholangiolar carcinomas. A rich data set provides both qualitative and quantitative knowledge of the progression of this AOP through KEs and the KERs. Thus, the WoE for this AOP is judged to be strong. Species-specific effects of dioxins and DLCs are well known--humans are less responsive than rodents and rodent species differ in sensitivity between strains. Consequently, application of this AOP to evaluate potential human health risks must take these differences into account.

The aryl hydrocarbon receptor (AhR) mediates resistance to apoptosis induced in breast cancer cells

The aryl hydrocarbon receptor (AhR) is well known as a ligand binding transcription factor regulating various biological effects. Previously we have shown that long-term exposure to estrogen in breast cancer cells caused not only down regulation of estrogen receptor (ER) but also overexpression of AhR. The AhR interacts with several cell signaling pathways associated with induction of tyrosine kinases, cytokines and growth factors which may support the survival roles of AhR escaping from apoptosis elicited by a variety of apoptosis inducing agents in breast cancer. In this study, we studied the anti-apoptotic role of AhR in different breast cancer cells when apoptosis was induced by exposure to UV light and chemotherapeutic agents. Activation of AhR by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in AhR overexpressing breast cancer cells effectively suppressed the apoptotic response induced by UV-irradiation, doxorubicin, lapatinib and paclitaxel. The anti-apoptotic response of TCDD was uniformly antagonized by the treatment with 3'methoxy-4'nitroflavone (MNF), a specific antagonist of AhR. TCDD's survival action of apoptosis was accompanied with the induction of well-known inflammatory genes, such as cyclooxygenase-2 (COX-2) and NF-κB subunit RelB. Moreover, TCDD increased the activity of the immunosuppressive enzyme indoleamine 2, 3-dioxygenase (IDO), which metabolizes tryptophan to kynurenine (Kyn) and mediates tumor immunity. Kyn also acts as an AhR ligand like TCDD, and kyn induced an anti-apoptotic response in breast cancer cells. Accordingly, our present study suggests that AhR plays a pivotal role in the development of breast cancer via the suppression of apoptosis, and provides an idea that the use of AhR antagonists with chemotherapeutic agents may effectively synergize the elimination of breast cancer cells.

TCDD promotes lung tumors via attenuation of apoptosis through activation of the Akt and ERK1/2 signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a multiple-site, multiple-species carcinogen that induces cancer in multiple organs. The molecular mechanisms underlying TCDD-induced lung tumorigenesis remain unclear. In the present study, a two-stage lung tumorigenesis model was established by administrating a single low dose of 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) combined with TCDD to female A/J mice. The results indicated that TCDD combined with low-dose NNK has a significant tumor-promoting effect compared with TCDD or low-dose NNK alone. Resistance to apoptosis is a hallmark of cancer and is thought to be one of the tumor-promoting mechanisms regulated by TCDD. We performed an additional series of experiments in the normal human bronchial epithelial cell line Beas2B cells, in which TCDD was combined with the apoptosis inducer staurosporine. Our in vitro results confirmed that TCDD could rescue cells from apoptosis induced by staurosporine. The inhibition of apoptosis is likely mediated by the activation of the Akt and ERK1/2 pathways, as determined by the effectiveness of pathway-specific inhibitors in abrogating the anti-apoptotic activity of TCDD. In conclusion, we demonstrated that TCDD promoted NNK-induced lung tumorigenesis and revealed that TCDD inhibits staurosporine-induced apoptosis, at least in part, through the Akt and ERK1/2 signaling pathways.

2,3,7,8-Tetrachlorodibenzo-p-dioxin induces apoptosis by disruption of intracellular calcium homeostasis in human neuronal cell line SHSY5Y

The persistent xenobiotic agent 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces neurotoxic effects that alters neurodevelopment and behavior both during development and adulthood. There are many ongoing efforts to determine the molecular mechanisms of TCDD-mediated neurotoxicity, the signaling pathways involved and its molecular targets in neurons. In this work, we have used SHSY5Y human neuroblastoma cells to characterize the TCDD-induced toxicity. TCDD produces a loss of viability linked to an increased caspase-3 activity, PARP-1 fragmentation, DNA laddering, nuclear fragmentation and hypodiploid (apoptotic) DNA content, in a similar way than staurosporine, a prototypical molecule of apoptosis induction. In addition, TCDD produces a decrease of mitochondrial membrane potential and an increase of intracellular calcium concentration (P < 0.05). Finally, based on the high lipophilic properties of the dioxin, we test the TCDD effect on the membrane integrity using sarcoplasmic reticulum vesicles as a model. TCDD produces calcium efflux through the membrane and an anisotropy decrease (P < 0.05) that reflects an increase in membrane fluidity. Altogether these results support the hypothesis that TCDD toxicity in SHSY5Y neuroblastoma cells provokes the disruption of calcium homeostasis, probably affecting membrane structural integrity, leading to an apoptotic process.

Aryl hydrocarbon receptor as a new therapeutic target for cancer and immune disorders

The aryl hydrocarbon receptor (AhR) was discovered more than three decades ago, and initially was characterized as a transcription factor with a role in xenobiotic metabolism. However, based on subsequent observations that AhR remains active under physiological conditions, exhibits constitutive expression during development, and has a high degree of conservation among species, it was hypothesized that AhR is responsible for functions in addition to its role in detoxification. Correspondingly, recent studies have elucidated novel physiological roles for this ligand-dependent transcription factor that link it to several pathways associated with disease development. In this review, studies are presented that support a role for AhR in cell proliferation, apoptosis, and immune homeostasis, thereby highlighting the therapeutic potential of this receptor for cancer and immune disorders.

miR-125b transcriptionally increased by Nrf2 inhibits AhR repressor, which protects kidney from cisplatin-induced injury

MicroRNAs (miRNAs) have a role in the cellular defense mechanism. Nuclear factor erythroid-2-related factor 2 (Nrf2) increases antioxidant enzyme capacity. However, miRNA transcriptionally controlled by Nrf2 had been uncharacterized. Here we report that miR-125b is transactivated by Nrf2 and inhibits aryl hydrocarbon receptor (AhR) repressor (AhRR). Bioinformatic approaches enabled us to extract six candidate miRNAs. Of them, only miR-125b was increased in the kidney of mice treated with oltipraz. Nrf2 overexpression enhanced primary, precursor and mature miR-125b levels. Functional assays revealed MIR125B1 is a bona fide target gene of Nrf2. Oltipraz treatment protected the kidney from cisplatin toxicity with increase of miR-125b. Consistently, Nrf2 knockout abrogated an adaptive increase of miR-125b elicited by cisplatin, augmenting kidney injury. An integrative network of miRNA and messenger RNA changes enabled us to predict miR-125b as an inhibitor of AhRR for the control of AhR activity and cell survival. In our molecular study, miR-125b inhibited AhRR and thereby activated AhR, leading to the induction of mdm2. Consistently, p53 activation by cisplatin was diminished by either miR-125b or oltipraz treatment. The results of experiments using miR-125b mimic or small interfering RNA of AhRR verified the role of miR-125b in AhRR regulation for kidney protection. In conclusion, miR-125b is transcriptionally activated by Nrf2 and serves as an inhibitor of AhRR, which contributes to protecting kidney from acute injury.

Recent advances in 2D and 3D in vitro systems using primary hepatocytes, alternative hepatocyte sources and non-parenchymal liver cells and their use in investigating mechanisms of hepatotoxicity, cell signaling and ADME

This review encompasses the most important advances in liver functions and hepatotoxicity and analyzes which mechanisms can be studied in vitro. In a complex architecture of nested, zonated lobules, the liver consists of approximately 80 % hepatocytes and 20 % non-parenchymal cells, the latter being involved in a secondary phase that may dramatically aggravate the initial damage. Hepatotoxicity, as well as hepatic metabolism, is controlled by a set of nuclear receptors (including PXR, CAR, HNF-4α, FXR, LXR, SHP, VDR and PPAR) and signaling pathways. When isolating liver cells, some pathways are activated, e.g., the RAS/MEK/ERK pathway, whereas others are silenced (e.g. HNF-4α), resulting in up- and downregulation of hundreds of genes. An understanding of these changes is crucial for a correct interpretation of in vitro data. The possibilities and limitations of the most useful liver in vitro systems are summarized, including three-dimensional culture techniques, co-cultures with non-parenchymal cells, hepatospheres, precision cut liver slices and the isolated perfused liver. Also discussed is how closely hepatoma, stem cell and iPS cell-derived hepatocyte-like-cells resemble real hepatocytes. Finally, a summary is given of the state of the art of liver in vitro and mathematical modeling systems that are currently used in the pharmaceutical industry with an emphasis on drug metabolism, prediction of clearance, drug interaction, transporter studies and hepatotoxicity. One key message is that despite our enthusiasm for in vitro systems, we must never lose sight of the in vivo situation. Although hepatocytes have been isolated for decades, the hunt for relevant alternative systems has only just begun.

Subchronic oral exposure to benzo(a)pyrene leads to distinct transcriptomic changes in the lungs that are related to carcinogenesis

We have previously shown that acute oral exposure to the environmental carcinogen benzo(a)pyrene (BaP) elicits comparable levels of DNA adducts, but distinct transcriptomic changes, in mouse lungs and livers, the two main BaP bioactivating organs. Oral BaP exposure is predominantly associated with lung cancer and not hepatic cancer in some animal models, suggesting that gene expression differences may provide insight into the drivers of tissue-specific carcinogenesis. In the present study, we examine pulmonary DNA adduct formation, lacZ mutant frequency, and mRNA profiles in adult male MutaMouse following subchronic (28 day) oral exposure to BaP (0, 25, 50, and 75 mg/kg/day) and sacrificed 3 days postexposure. The results are compared with those obtained from livers of the same mice (previously published). Although there was a 1.8- to 3.3-fold increase in the levels of DNA adducts in lung compared with liver, the lacZ transgene mutant frequency was similar in both tissues. At the transcriptomic level, a transition from activation of the DNA damage response p53 pathway at the low dose to the induction of genes involved in angiogenesis, evasion of apoptosis and growth signals at the high doses was evident only in the lungs. These results suggest that tissue DNA adducts and mutant frequency are sensitive markers of target tissue exposure and mode of action, whereas early changes in gene expression may provide a better indication of the likelihood of carcinogenic transformation in selected tissues. Moreover, the study provides new information on the underlying mechanisms that contribute to tissue-specific responses to BaP.

Regulation of the activity and expression of aryl hydrocarbon receptor by ethanol in mouse hepatic stellate cells

BACKGROUND

During the course of alcohol-induced liver damage, hepatic stellate cells are transformed into proliferative, fibrogenic, and contractile myofibroblasts. Aryl hydrocarbon receptor (AhR) is a transcription factor that controls the expression of genes involved in the metabolism of xenobiotics, inflammation, cell proliferation, and death.

METHODS

Immortal mouse hepatic stellate cells (MHSCs) were isolated from transgenic mice that expressed a thermolabile SV40 tumor antigen. Quantitative real-time reverse transcription polymerase chain reaction assays, Western blot analysis, promoter activity assays, and chromatin immunoprecipitation analyses were performed for studying the effect of ethanol (EtOH) on AhR expression and transcriptional activity.

RESULTS

Treatment of MHSCs with 50 to 200 mM EtOH for 6 hours induced AhR nuclear translocation, enhanced the promoter activity of cytochrome P450 (CYP) 1A1, increased the amount of AhR bound to the promoter of CYP1A1 and 1B1, and up-regulated the mRNA expression of these AhR target genes in a dose-dependent manner. In contrast, EtOH exposure down-regulated AhR mRNA and protein expression. Similarly, benzo(a)pyrene (BaP) at 10 nM reduced AhR and increased CYP1A1 and 1B1 mRNAs. Pretreatment of MHSCs with 50 mM EtOH for 7 days diminished the capacity of MHSCs to express CYP1A1 and 1B1 induced by a 200 mM EtOH challenge, or by 10 nM BaP. However, the up-regulatory effect of EtOH on solute carrier family 16, member 6 (SLC16a6) was unaffected by EtOH pretreatment. Similar to EtOH, dimethyl sulfoxide (DMSO) at concentrations of 50 to 100 mM down-regulated AhR and up-regulated CYP1A1 mRNA expression in a dose-dependent manner.

CONCLUSIONS

These data, for the first time, demonstrate that EtOH activates MHSC AhR and down-regulates its expression. Chronic EtOH pretreatment lowers the availability of AhR, and specifically diminishes the inducibility of CYP genes. The effect on AhR appears to not be an EtOH-specific response, as DMSO alone (and possibly other organic solvents) was also able to activate AhR.

Multivariate toxicity profiles and QSAR modeling of non-dioxin-like PCBs--an investigation of in vitro screening data from ultra-pure congeners

The non-dioxin-like PCBs (NDL-PCBs) found in food and human samples have a complex spectrum of adverse effects, but lack a detailed risk assessment. The toxicity profiles of 21 carefully selected PCBs (19 NDL-PCBs) were identified by in vitro screening in 17 different assays on specific endpoints related to neurotoxicity, endocrine disruption and tumor promotion. To ensure that the test results were not affected by polychlorinated dioxins, dibenzofurans or DL-PCB contaminants, the NDL-PCB congeners were thoroughly purified before testing. Principal component analysis (PCA) was used to derive general toxicity profiles from the in vitro screening data. The toxicity profiles indicated different structure-activity relationships (SAR) and distinct mechanisms of action. The analysis also indicated that the NDL-PCBs could be divided into two groups. The first group included generally smaller, ortho-substituted congeners, comprising PCB 28, 47, 51, 52, 53, 95, 100, 101, 104 and 136, with PCB 95, 101 and 136 as generally being most active. The second group comprising PCB 19, 74, 118, 122, 128, 138, 153, 170, 180 and 190 had lower biological activity in many of the assays, except for three endocrine-related assays. The most abundant congeners, PCB 138, 153, 170, 180 and 190, cluster in the second group, and thereby show similar SAR. Two quantitative structure-activity relationship (QSAR) models could be developed that added information to the SAR and could aid in risk assessments of NDL-PCBs. The QSAR models predicted a number of congeners as active and among these e.g., PCB 18, 25, 45 and 49 have been found in food or human samples.

The aryl hydrocarbon receptor interacts with ATP5α1, a subunit of the ATP synthase complex, and modulates mitochondrial function

Dioxins, including 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD), produce a wide range of toxic effects in mammals. Most, if not all, of these toxic effects are regulated by the aryl hydrocarbon receptor (AHR). The AHR is a ligand activated transcription factor that has been shown to interact with numerous proteins capable of influencing the receptor's function. The ability of secondary proteins to alter AHR-mediated transcriptional events, a necessary step for toxicity, led us to determine whether additional interacting proteins could be identified. To this end, we have employed tandem affinity purification (TAP) of the AHR in Hepa1c1c7 cells. TAP of the AHR, followed by mass spectrometry (MS) identified ATP5α1, a subunit of the ATP synthase complex, as a strong AHR interactor in the absence of ligand. The interaction was lost upon exposure to TCDD. The association was confirmed by co-immunoprecipitation in multiple cell lines. In addition, cell fractionation experiments showed that a fraction of the AHR is found in the mitochondria. To ascribe a potential functional role to the AHR:ATP5α1 interaction, TCDD was shown to induce a hyperpolarization of the mitochondrial membrane in an AHR-dependent and transcription-independent manner. These results suggest that a fraction of the total cellular AHR pool is localized to the mitochondria and contributes to the organelle's homeostasis.

Dioxin toxicity, aryl hydrocarbon receptor signaling, and apoptosis-persistent pollutants affect programmed cell death

Exogenous ligands of the aryl hydrocarbon receptor (AhR) such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related substances are highly toxic pollutants ubiquitously present in the environment. They cause a variety of toxic effects to different organs and tissues. Among other effects, TCDD exposure to laboratory animals leads to thymus atrophy and immunosuppression on the one hand, and to tumor formation on the other. Apoptosis appears to be involved in both these toxic effects: AhR activation by TCDD was discussed to induce apoptosis of immune cells, leading to the depletion of thymocytes and ultimately immunosuppression. This mechanism could help to explain the highly immunotoxic actions of TCDD but it is nevertheless under debate whether this is the mode of action for immunosuppression by this class of chemical substances. In other cell types, especially liver cells, TCDD inhibits apoptosis induced by genotoxic treatment. In initiation-promotion studies, TCDD was shown to be a potent liver tumor promoter. Among other theories it was hypothesized that TCDD acts as a tumor promoter by preventing initiated cells from undergoing apoptosis. The exact mechanisms of apoptosis inhibition by TCDD are not fully understood, but both in vivo and in vitro studies consistently showed an involvement of the tumor suppressor p53 in this effect. Various strings of evidence have been established linking apoptosis to the detrimental effects of exogenous activation of the AhR. Within this article, studies elucidating the effects of TCDD and related substances on apoptosis signaling, be it inducing or repressing, is to be reviewed.

Influence of CLOCK on cytotoxicity induced by diethylnitrosamine in mouse primary hepatocytes

The Clock gene is a core clock factor that plays an essential role in generating circadian rhythms. In the present study, it was investigated whether the Clock gene affects the response to diethylnitrosamine (DEN)-induced cytotoxicity using mouse primary hepatocytes. DEN-induced cytotoxicity, after 24h exposure, was caused by apoptosis in hepatocytes isolated from wild-type mouse. On the other hand, Clock mutant mouse (Clk/Clk) hepatocytes showed resistance to apoptosis. Because apoptosis is an important pathway for suppressing carcinogenesis after genomic DNA damage, the mechanisms that underlie resistance to DEN-induced apoptosis were examined in Clk/Clk mouse hepatocytes. The mRNA levels of metabolic enzymes bioactivating DEN and apoptosis-inducing factors before DEN exposure were lower in Clk/Clk cells than in wild-type cells. The accumulation of p53 and Ser15 phosphorylated p53 after 8h DEN exposure was seen in wild-type cells but not in Clk/Clk cells. Caspase-3/7 activity was elevated during 24h DEN exposure in wild-type cells but not in Clk/Clk cells. In addition, resistance to DEN-induced apoptosis in Clk/Clk cells affected the cell viability. These studies suggested that the lower expression levels of metabolic enzymes bioactivating DEN and apoptosis inducing factors affected the resistance to DEN-induced apoptosis in Clk/Clk cells, and the Clock gene plays an important role in cytotoxicity induced by DEN.

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.

Estrogen-like effects of cadmium in vivo do not appear to be mediated via the classical estrogen receptor transcriptional pathway

BACKGROUND

Cadmium (Cd), a ubiquitous food contaminant, has been proposed to be an endocrine disruptor by inducing estrogenic responses in vivo. Several in vitro studies suggested that these effects are mediated via estrogen receptors (ERs).

OBJECTIVE

We performed this study to clarify whether Cd-induced effects in vivo are mediated via classical ER signaling through estrogen responsive element (ERE)-regulated genes or if other signaling pathways are involved.

METHODS

We investigated the estrogenic effects of cadmium chloride (CdCl2) exposure in vivo by applying the Organisation for Economic Co-operation and Development (OECD) rodent uterotrophic bioassay to transgenic ERE-luciferase reporter mice. Immature female mice were injected subcutaneously with CdCl2 (5, 50, or 500 µg/kg body weight) or with 17α-ethinylestradiol (EE2) on 3 consecutive days. We examined uterine weight and histology, vaginal opening, body and organ weights, Cd tissue retention, activation of mitogen-activated protein kinase (MAPK) pathways, and ERE-dependent luciferase expression.

RESULTS

CdCl2 increased the height of the uterine luminal epithelium in a dose-dependent manner without increasing the uterine wet weight, altering the timing of vaginal opening, or affecting the luciferase activity in reproductive or nonreproductive organs. However, we observed changes in the phosphorylation of mouse double minute 2 oncoprotein (Mdm2) and extracellular signal-regulated kinase (Erk1/2) in the liver after CdCl2 exposure. As we expected, EE2 advanced vaginal opening and increased uterine epithelial height, uterine wet weight, and luciferase activity in various tissues.

CONCLUSION

Our data suggest that Cd exposure induces a limited spectrum of estrogenic responses in vivo and that, in certain targets, effects of Cd might not be mediated via classical ER signaling through ERE-regulated genes.

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.

Ube2l3 gene expression is modulated by activation of the aryl hydrocarbon receptor: implications for p53 ubiquitination

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a halogenated aromatic hydrocarbon and environmental contaminant, results in several deleterious effects, including fetal malformation and cancer. These effects are mediated by the aryl hydrocarbon receptor (AhR), a ligand-activated receptor that regulates the expression of genes encoding xenobiotic-metabolizing enzymes. Several reports suggest that AhR function is beyond the adaptive chemical response. In the present study, we analyzed and compared gene expression profiles of C57BL/6N wild-type (WT) and Ahr-null mice. DNA microarray and quantitative RT-PCR analyses revealed changes in the expression of genes involved in the ubiquitin-proteasome system (UPS). UPS has an important role in cellular homeostasis control and dysfunction of this pathway has been implicated in the development of several human pathologies. Protein ubiquitination is a multi-step enzymatic process that regulates the stability, function, and/or localization of the modified proteins. This system is highly regulated post-translationally by covalent modifications. However, little information regarding the transcriptional regulation of the genes encoding ubiquitin (Ub) proteins is available. Therefore, we investigated the role of the AhR in modulation of the UPS and regulation of Ube2l3 transcription, an E2 ubiquitin-conjugating enzyme, as well as the effects on p53 degradation. Our results indicate that AhR inactivation decreases on liver proteasome activity, probably due to a down-regulation on the expression of several proteasome subunits. On the other hand, AhR activation increases Ube2l3 mRNA and protein levels by controlling Ube2l3 gene expression, resulting in increased p53 ubiquitination and degradation. In agreement with this, induction of apoptosis was attenuated by the AhR activation.

2,3,7,8-tetrachlorodibenzo-p-dioxin counteracts the p53 response to a genotoxicant by upregulating expression of the metastasis marker agr2 in the hepatocarcinoma cell line HepG2

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is an environmental pollutant that binds the aryl hydrocarbon receptor (AhR), a transcription factor that triggers various biological responses. In this study, we show that TCDD treatment counteracts the p53 activation (phosphorylation and acetylation) elicited by a genotoxic compound, etoposide, in the human hepatocarcinoma cell line HepG2 and we delineated the mechanisms of this interaction. Using small interfering RNA knockdown experiments, we found that the newly described metastasis marker, anterior gradient-2 (AGR2), is involved in this effect. Both AGR2 messenger RNA (mRNA) and protein levels were increased (sixfold and fourfold, respectively) by TCDD treatment, and this effect was mediated by the AhR receptor. The half-life of AGR2 mRNA was unchanged by TCDD treatment. Analysis of the promoter of the AGR2 gene revealed three putative xenobiotic-responsive elements (XREs) in the proximal 3.5-kb promoter. Transient transfection of HepG2 cells by the Gaussia luciferase reporter gene driven by various deleted and mutated fragments of the promoter indicated that only the most proximal XRE was active. Binding of the AhR to the endogenous AGR2 promoter was also triggered by TCDD treatment. These results suggest that AhR ligands such as TCDD might contribute to tumor progression by inhibiting p53 regulation (phosphorylation and acetylation) triggered by genotoxicants via the increased expression of the metastasis marker AGR2.

Toxic effects of lactational exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on development of male reproductive system: involvement of antioxidants, oxidants, and p53 protein

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent endocrine disruptor compound and induces multiple organ dysfunctions. The effect of TCDD exposure both in adults and in utero has been well established. However, little is known about the effects of TCDD acquired through mother's milk on the development of the male reproductive system. The aim of this study was to investigate the effects and mechanisms of TCDD from lactational exposure. TCDD (1 microg/kg) was administered to C57BL/6 mouse mothers for 4 days from the day of delivery. On postnatal day 30 (PND 30) and postnatal day 60 (PND 60), body weight, body length, and anogenital distance (AGD) of male offspring were measured. The weights of the testes and epididymides were also measured. Epididymides were used for sperm counts, and testes were used to measure the activity of antioxidant enzymes (SOD, CAT, GPX, GR), the parameters of oxidative stress (hydrogen peroxide, MDA), and testosterone. In addition, expression of p53 and the proapoptotic protein, Bax, were analyzed by Western blot. TCDD exposure decreased body weight, body length, and AGD in both PND 30 and PND 60 groups compared with the control group. The activity of all antioxidant enzymes at PND 60 was decreased after TCDD treatment. TCDD treatment decreased testicular testosterone levels in both the PND 30 and PND 60 groups. The expression of p53 and Bax were also upregulated by TCDD and did not return to normal levels by PND 60. These data suggest that TCDD affects development of male offspring when the mother is exposed to TCDD during lactation. In addition, oxidative stress is a major mediator of TCDD-induced adverse effects, and p53 may play an important role in this mechanism.

Inhibition of apoptosis by 2,3,7,8-tetrachlorodibenzo-p-dioxin depends on protein biosynthesis

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic persistent organic pollutant. Most of the toxic effects of TCDD are believed to be mediated by high-affinity binding to the aryl hydrocarbon receptor (AhR) and subsequent effects on gene transcription and protein expression. TCDD causes cancer in multiple tissues in different animal species and is classified as a class 1 human carcinogen. In initiation-promotion studies, TCDD was shown to be a potent liver-tumor promotor. Among other theories it has been hypothesized that TCDD promotes tumor growth by preventing initiated cells from correctly executing apoptosis. In this study, we examined the effects of TCDD on apoptosis induced by UV-C light, ochratoxin A (OTA), and cycloheximide (CHX) in primary rat hepatocytes. Both UV-C light and OTA caused caspase activation and nuclear apoptotic effects. CHX did not activate caspases but nevertheless caused DNA fragmentation and chromatin condensation. TCDD inhibited UV-C light-induced apoptosis and this effect seemed to be dependent on AhR-activation as was shown by employing an AhR antagonist. In contrast to UV-C light-induced apoptosis, TCDD failed to protect primary rat hepatocytes from OTA- or CHX-induced apoptosis. Since both of these compounds inhibit protein biosynthesis as was demonstrated by measuring the incorporation of radiolabeled leucin and protein expression of cytochrome P450 1A1, we propose that the inhibition of apoptosis by TCDD depends on protein biosynthesis. Either TCDD induces some anti-apoptotic protein in an AhR-dependent manner or inhibits pro-apoptotic proteins induced by UV irradiation.

Inhibition of UV-C light-induced apoptosis in liver cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic pollutant ubiquitously present in the environment. Most of the toxic effects of TCDD are believed to be mediated by high-affinity binding to the aryl hydrocarbon receptor (AhR) and subsequent effects on gene transcription. TCDD causes cancer in multiple tissues in different animal species and is classified as a class 1 human carcinogen. In initiation-promotion studies TCDD was shown to be a potent liver tumor promotor. Among other theories it has been hypothesized that TCDD acts as a tumor promotor by preventing initiated cells from undergoing apoptosis. We examined the effects of TCDD on ultraviolet C (UV-C) light-induced apoptosis in primary rat hepatocytes and Huh-7 human hepatoma cells. TCDD inhibits UV-C light-induced apoptosis in both cell types. This effect is seen with chromatin condensation and fragmentation and appears to be mediated by the AhR in rat hepatocytes. Apoptosis induced by UV-C light in these cells is caspase-dependent and is accompanied by alterations in apoptosis-related gene expression such as up-regulation of proapoptotic bcl-2 family genes like bak and bax, and a marked down regulation of the expression of the antiapoptotic bcl-2. TCDD treatment of irradiated hepatocytes induces the expression of some apoptosis-related genes (birc3, dad1, pycard, tnf). Upstream apoptotic events, namely caspase activation and caspase substrate cleavage are not inhibited by TCDD treatment. We hypothesize that TCDD inhibits late-stage apoptotic events that lead to internucleosomal DNA fragmentation, maintaining chromosomal integrity probably in order to sustain metabolic capacity and hepatic elimination of substrates despite of an initiation of apoptosis.

Exposure of HepG2 cells to low levels of PAH-containing extracts from contaminated soils results in unpredictable genotoxic stress responses

Contaminated soil is a serious environmental problem, constituting a risk to humans and the environment. Polycyclic aromatic hydrocarbons (PAHs) are often present at contaminated sites. However, risk levels are difficult to estimate because of the complexity of contaminants present. Here, we compare cellular effects of extracts from contaminated soils collected at six industrial settings in Sweden. Chemical analysis showed that all soils contained complex mixtures of PAHs and oxy-PAHs. Western blotting and immunocytochemistry were used to investigate DNA damage signaling in HepG2 cells exposed to extracts from these soils. The effects on phosphorylated Mdm2, p53, Erk, H2AX, 53BP1, and Chk2, cell cycle regulating proteins (cyclin D1 and p21), and cell proliferation were compared. We found that most soil extracts induced phosphorylation of Mdm2 at the 2A10 epitope at low concentrations. This is in line with previous studies suggesting that this endpoint reflects readily repaired DNA-damage. However, we found concentration- and time-dependent gammaH2AX and 53BP1 responses that were sustained for 48 hr. These endpoints may reflect the presence of different types of persistent DNA-damage. High concentrations of soil extracts decreased cyclin D1 and increased p21 response, indicating cell cycle arrest. Phosphorylation of Mdm2 at Ser166, which attenuates the p53 response and is induced by many tumor promoters, was induced in a time-dependent manner and was associated with Erk phosphorylation. Taken together, the PAH extracts elicited unpredictable signaling responses that differed between samples. More polar compounds, i.e., oxy-PAHs, also contributed to the complexity.

Activation of the aryl hydrocarbon receptor by TCDD inhibits senescence: a tumor promoting event?

Activation of the aryl hydrocarbon receptor (AHR) by the agonist, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown to promote tumor formation in both liver and skin. In the liver, but not the skin, the AHR-mediated events that contribute to TCDD's tumor promoting activities have been studied in some detail and are thought to involve perturbation of cell fate processes. However, studies performed using cultured cells have often resulted in apparent contradictory results indicating that the impact of TCDD on cell fate processes may be cell context dependent. We and others have shown that in primary cultured keratinocytes TCDD increases post-confluent proliferation and increases late differentiation. Further, our studies performed in these cells indicate that TCDD can also inhibit culture-induced senescence. While senescence, a permanent cell cycle arrest, is emerging as an important process regulated by oncogenes and considered to be of therapeutic importance, its role with respect to TCDD/AHR mediated tumor promotion has not been fully considered. The intent of this article is to focus primarily on senescence as a cell process relevant to skin tumorigenesis and explore the idea that the inhibition of senescence by TCDD could be an important mechanism by which it may exert its tumor promoting effects in the skin.

The aryl hydrocarbon receptor at the crossroads of multiple signaling pathways

The aryl hydrocarbon receptor (AHR) has long been recognized as a ligand-activated transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was established long before the first report of an AHR cDNA sequence was published. It is only recently that other functions of this protein have begun to be recognized, and it is now clear that the AHR also functions in pathways outside of its well-characterized role in xenobiotic enzyme induction. Perturbation of these pathways by xenobiotic ligands may ultimately explain much of the toxicity of these compounds. This chapter focuses on the interactions of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, differentiation and apoptosis. Ultimately, the effect of a particular AHR ligand on the biology of the organism will depend on the milieu of critical pathways and proteins expressed in specific cells and tissues with which the AHR itself interacts.

Dioxin-induced toxicity on vascular remodeling of the placenta

Arylhydrocarbon receptor (AhR) activated by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) triggers its downstream signaling pathway to exert adverse effects on vasculature development, which can be initiated by vasculogenesis, followed by angiogenesis, or vascular remodeling, in a variety of animals including avians, piscines and mammals. The placenta, a mammalian organ rich in vasculature, consists of endothelial and trophoblast cells of fetal origin, which proliferate and differentiate under hypoxic condition in the uterine horn. Our studies demonstrated that vascular remodeling occurs prominently in the placenta of the control Holtzman rat strain during the late period of gestation, and induces changes in cell shape and elimination by apoptosis of trophoblasts. As a result, the net volumes of both maternal and fetal blood in the placenta increase to cope with the essential requirements of oxygen and nutrients in the late period of gestation. On the other hand, in utero exposure to TCDD markedly suppressed the development of sinusoids and trophoblast cells and the apoptosis of trophoblast cells with a concomitant increase in the incidence of fetal death under hypoxic condition. A crosstalk between the hypoxia-inducible factor (HIF)-mediated pathway and AhR-mediated pathway is considered to play an important role in this physiological process. No such changes were observed in the Sprague-Dawley rat strain that turned out to have an AhR conformation identical to that of the Holtzman rat strain. In this commentary, we will discuss a possible link of the TCDD toxicities with the AhR signaling pathway and gestation-related diseases.

The aryl hydrocarbon receptor cross-talks with multiple signal transduction pathways

Exposure to toxic polycyclic aromatic hydrocarbons raises a number of toxic and carcinogenic responses in experimental animals and humans mediated for the most part by the aryl hydrocarbon -- or dioxin -- receptor (AHR). The AHR is a ligand-activated transcription factor whose central role in the induction of drug-metabolizing enzymes has long been recognized. For quite some time now, it has become clear that the AHR also functions in pathways outside of its role in detoxification and that perturbation of these pathways by xenobiotic ligands may be an important part of the toxicity of these compounds. AHR activation by some of its ligands participates among others in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, cross-talk within the RB/E2F axis and mobilization of crucial calcium stores. Ultimately, the effect of a particular AHR ligand may depend as much on the adaptive interactions that it established with pathways and proteins expressed in a specific cell or tissue as on the toxic responses that it raises.

Dioxin-mediated tumor progression through activation of mitochondria-to-nucleus stress signaling

The environmental toxin 2,3,7,8-tetrachlorodibenzodioxin (TCDD) is a known human carcinogen; however, its precise mechanism of action remains unclear. Here we show that TCDD induces mitochondrial dysfunction, stress signaling, and tumor invasion by a mechanism similar to that described for mtDNA-depleted cells. Treatment of C2C12 cells with TCDD disrupted mitochondrial transmembrane potential in a time-dependent fashion and inhibited mitochondrial transcription and translation. TCDD also increased cytosolic [Ca(2+)](c) and RyR1-specific Ca(2+) release. These changes were associated with increased calcineurin (CnA) levels and activation of CnA-sensitive NF-kappaB/Rel (IkappaBbeta-dependent) factors. Cells treated with TCDD displayed resistance to apoptosis, increased expression of the tumor marker cathepsin L, and a high degree of invasiveness as tested by the Matrigel membrane invasion assay. These effects were reversed by the CnA inhibitor FK506, and CnA mRNA silencing suggesting that TCDD triggers a signaling pathway similar to mtDNA depletion. Taken together, these results reveal that TCDD may promote tumor progression in vivo by directly targeting mitochondrial transcription and induction of mitochondrial stress signaling.

Carcinogenicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin in experimental models

The contaminant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a prototype compound of a whole class of halogenated aromatic hydrocarbons termed 'dioxinlike' contaminants present in food, human tissue, mothers milk, and environmental samples. Among the various adverse effects caused by TCDD in animal experiments, its carcinogenic effects caused particular concern. In rodents, long-term TCDD treatment leads to the development of tumors of the liver, thyroid, lung, skin, oral cavity and other sites. The occurrence of liver tumors mainly observed in female rats has been used as a basis for quantitative cancer risk assessment for TCDD. TCDD does not behave like a 'complete carcinogen', i. e. no DNA binding of the parent compound or metabolites thereof could be detected. However, enhanced oxidative damage of hepatic DNA was observed, probably resulting from a dramatic induction of cytochrome P450 enzymes, which are under the regulatory, transcriptional control of the TCDD-activated aryl hydrocarbon receptor. The marked enhancement of TCDD-related oxidative liver DNA damage in rats by estrogens warrants further mechanistic investigation. Furthermore, TCDD acts as a tumor promoter, i. e. it facilitates the growth of putative preneoplastic hepatic lesions after initiation with a complete carcinogen. The mechanisms underlying this effect may be related to altered intracellular signaling involving pronounced changes in the phosphorylation pattern of proteins regulating growth and apoptosis. These effects are thought to result in an enhanced survival of preneoplastic cells, some of which can undergo further steps on the way to malignancy. In summary, a better understanding of the mechanisms of the carcinogenicity of TCDD is mandatory to provide a rational basis for a better inter-species extrapolation. The final aim of these efforts is a more reliable risk assessment for the carcinogenic potency of the class of dioxinlike contaminants in humans.