Effect of dioxins on regulation of tyrosine hydroxylase gene expression by aryl hydrocarbon receptor: a neurotoxicology study

Sex-specific Effects of Endocrine-disrupting Chemicals on Brain Monoamines and Cognitive Behavior

The period of brain sexual differentiation is characterized by the development of hormone-sensitive neural circuits that govern the subsequent presentation of sexually dimorphic behavior in adulthood. Perturbations of hormones by endocrine-disrupting chemicals (EDCs) during this developmental period interfere with an organism's endocrine function and can disrupt the normative organization of male- or female-typical neural circuitry. This is well characterized for reproductive and social behaviors and their underlying circuitry in the hypothalamus and other limbic regions of the brain; however, cognitive behaviors are also sexually dimorphic, with their underlying neural circuitry potentially vulnerable to EDC exposure during critical periods of brain development. This review provides recent evidence for sex-specific changes to the brain's monoaminergic systems (dopamine, serotonin, norepinephrine) after developmental EDC exposure and relates these outcomes to sex differences in cognition such as affective, attentional, and learning/memory behaviors.

The role of tyrosine hydroxylase-dopamine pathway in Parkinson's disease pathogenesis

BACKGROUND

Parkinson's disease (PD) is characterized by selective and progressive dopamine (DA) neuron loss in the substantia nigra and other brain regions, with the presence of Lewy body formation. Most PD cases are sporadic, whereas monogenic forms of PD have been linked to multiple genes, including Leucine kinase repeat 2 (LRRK2) and PTEN-induced kinase 1 (PINK1), two protein kinase genes involved in multiple signaling pathways. There is increasing evidence to suggest that endogenous DA and DA-dependent neurodegeneration have a pathophysiologic role in sporadic and familial PD.

METHODS

We generated patient-derived dopaminergic neurons and human midbrain-like organoids (hMLOs), transgenic (TG) mouse and Drosophila models, expressing both mutant and wild-type (WT) LRRK2 and PINK1. Using these models, we examined the effect of LRRK2 and PINK1 on tyrosine hydroxylase (TH)-DA pathway.

RESULTS

We demonstrated that PD-linked LRRK2 mutations were able to modulate TH-DA pathway, resulting in up-regulation of DA early in the disease which subsequently led to neurodegeneration. The LRRK2-induced DA toxicity and degeneration were abrogated by wild-type (WT) PINK1 (but not PINK1 mutations), and early treatment with a clinical-grade drug, α-methyl-L-tyrosine (α-MT), a TH inhibitor, was able to reverse the pathologies in human neurons and TG Drosophila models. We also identified opposing effects between LRRK2 and PINK1 on TH expression, suggesting that functional balance between these two genes may regulate the TH-DA pathway.

CONCLUSIONS

Our findings highlight the vital role of the TH-DA pathway in PD pathogenesis. LRRK2 and PINK1 have opposing effects on the TH-DA pathway, and its balance affects DA neuron survival. LRRK2 or PINK1 mutations can disrupt this balance, promoting DA neuron demise. Our findings provide support for potential clinical trials using TH-DA pathway inhibitors in early or prodromic PD.

Pharmacological blockage of the AHR-CYP1A1 axis: a call for in vivo evidence

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that can be activated by structurally diverse compounds arising from the environment and the microbiota and host metabolism. Expanding evidence has been shown that the modulation of the canonical pathway of AHR occurs during several chronic diseases and that its abrogation might be of clinical interest for metabolic and inflammatory pathological processes. However, most of the evidence on the pharmacological abrogation of the AHR-CYP1A1 axis has been reported in vitro, and therefore, guidance for in vivo studies is needed. In this review, we cover the state-of-the-art of the pharmacodynamic and pharmacokinetic properties of AHR antagonists and CYP1A1 inhibitors in different in vivo rodent (mouse or rat) models of disease. This review will serve as a road map for those researchers embracing this emerging therapeutic area targeting the AHR. Moreover, it is a timely opportunity as the first AHR antagonists have recently entered the clinical stage of drug development.

The Role of AhR in the Hallmarks of Brain Aging: Friend and Foe

In recent years, aryl hydrocarbon receptor (AhR), a ligand-activated transcription factor, has been considered to be involved in aging phenotypes across several species. This receptor is a highly conserved biosensor that is activated by numerous exogenous and endogenous molecules, including microbiota metabolites, to mediate several physiological and toxicological functions. Brain aging hallmarks, which include glial cell activation and inflammation, increased oxidative stress, mitochondrial dysfunction, and cellular senescence, increase the vulnerability of humans to various neurodegenerative diseases. Interestingly, many studies have implicated AhR signaling pathways in the aging process and longevity across several species. This review provides an overview of the impact of AhR pathways on various aging hallmarks in the brain and the implications for AhR signaling as a mechanism in regulating aging-related diseases of the brain. We also explore how the nature of AhR ligands determines the outcomes of several signaling pathways in brain aging processes.

DNA methylome perturbations: an epigenetic basis for the emergingly heritable neurodevelopmental abnormalities associated with maternal smoking and maternal nicotine exposure†

Maternal smoking during pregnancy is associated with an ensemble of neurodevelopmental consequences in children and therefore constitutes a pressing public health concern. Adding to this burden, contemporary epidemiological and especially animal model research suggests that grandmaternal smoking is similarly associated with neurodevelopmental abnormalities in grandchildren, indicative of intergenerational transmission of the neurodevelopmental impacts of maternal smoking. Probing the mechanistic bases of neurodevelopmental anomalies in the children of maternal smokers and the intergenerational transmission thereof, emerging research intimates that epigenetic changes, namely DNA methylome perturbations, are key factors. Altogether, these findings warrant future research to fully elucidate the etiology of neurodevelopmental impairments in the children and grandchildren of maternal smokers and underscore the clear potential thereof to benefit public health by informing the development and implementation of preventative measures, prophylactics, and treatments. To this end, the present review aims to encapsulate the burgeoning evidence linking maternal smoking to intergenerational epigenetic inheritance of neurodevelopmental abnormalities, to identify the strengths and weaknesses thereof, and to highlight areas of emphasis for future human and animal model research therein.

Tyroxine Hydroxylase-Positive Neuronal Cell Population is Increased by Temporal Dioxin Exposure at Early Stage of Differentiation from Human Embryonic Stem Cells

Background: The neurological effects of short-term dioxin exposure during the fetal period is an important health risk in humans. Here, we investigated the effects of dioxin on neural differentiation using human embryonic stem cells (hESCs) to evaluate human susceptibility to dioxin. Methods: Using an enzymatic bulk passage, neural differentiation from human ESCs was carried out. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was added to various stages of culture. The expression levels of the neuronal markers microtubule-associated protein 2 (MAP2) and thyroxine hydroxylase (TH) were measured by RT-qPCR and image analysis of immunostaining. Results: Although early-stage neuronal cells are quite resistant to TCDD, the numbers of neural rosettes and increases in mRNA expression levels and the number of cells positive for MAP2 and TH were significant by temporal exposure at embryoid body stage (Day9-exposure group). In contrast, the TCDD exposures against ESCs (Day0-exposure group) and differentiated neural cells (Day35-exposure group) were not affected at all. The increment was similarly observed by continuous exposure of TCDD from Day9 through Day60. Conclusions: These results indicated that dioxin exposure during the early stage of differentiation from hESCs increases the contents of neuronal cells, especially TH-positive neuronal cells. Regulations of aryl hydrocarbon receptor (AHR) signaling in an early stage of embryogenesis should be investigated extensively to understand the mechanism underlying the increase in neuronal cell populations and to apply the knowledge to regenerative medicine.

Ahr and Cyp1a2 genotypes both affect susceptibility to motor deficits following gestational and lactational exposure to polychlorinated biphenyls

Polychlorinated biphenyls (PCBs) are persistent organic pollutants known to cause adverse health effects and linked to neurological deficits in both human and animal studies. Children born to exposed mothers are at highest risk of learning and memory and motor deficits. We developed a mouse model that mimics human variation in the aryl hydrocarbon receptor and cytochrome P450 1A2 (CYP1A2) to determine if genetic variation increases susceptibility to developmental PCB exposure. In our previous studies, we found that high-affinity Ahr^bCyp1a2(-/-) and poor-affinity Ahr^dCyp1a2(-/-) knockout mice were most susceptible to learning and memory deficits following developmental PCB exposure compared with Ahr^bCyp1a2(+/+) wild type mice (C57BL/6J strain). Our follow-up studies focused on motor deficits, because human studies have identified PCBs as a potential risk factor for Parkinson's disease. Dams were treated with an environmentally relevant PCB mixture at gestational day 10 and postnatal day 5. We used a motor battery that included tests of nigrostriatal function as well as cerebellar function, because PCBs deplete thyroid hormone, which is essential to normal cerebellar development. There was a significant effect of PCB treatment in the rotarod test with impaired performance in all three genotypes, but decreased motor learning as well in the two Cyp1a2(-/-) knockout lines. Interestingly, we found a main effect of genotype with corn oil-treated control Cyp1a2(-/-) mice performing significantly worse than Cyp1a2(+/+) wild type mice. In contrast, we found that PCB-treated high-affinity Ahr^b mice were most susceptible to disruption of nigrostriatal function with the greatest deficits in Ahr^bCyp1a2(-/-) mice. We conclude that differences in AHR affinity combined with the absence of CYP1A2 protein affect susceptibility to motor deficits following developmental PCB exposure.

Dioxins as potential risk factors for autism spectrum disorder

Autism spectrum disorder (ASD) has emerged as a major public health concern due to its fast-growing prevalence in recent decades. Environmental factors are thought to contribute substantially to the variance in ASD. Interest in environmental toxins as causes of ASD has arisen due to the high sensitivity of the developing human brain to toxic chemicals, particularly to dioxin and certain dioxin-like compounds (dioxins). As a group of typical persistent organic pollutants, dioxins have been found to exert adverse effects on human brain development. In this paper, we review the evidence for association of exposure to dioxins with neurodevelopmental abnormalities related to ASD based on both human epidemiological and animal studies. It has been documented that exposure to dioxins during critical developmental periods increased risk for ASD. This notion has been demonstrated in different populations exposed to high or background level of dioxins. Furthermore, the effects and mechanisms of action of dioxins relevant to the pathophysiology and pathogenesis of ASD are summarized, describing potential underlying mechanisms linking dioxin exposure with ASD onset. Further studies focusing on effects of prenatal/perinatal exposure to individual dioxin congeners or to mixtures of dioxins on ASD-associated behavioral and neurobiological consequences in animal models, and on the mechanisms of actions of dioxins, are needed in order to better understand how dioxin exposure might contribute to increased risk for ASD.

Sleep disorders among Yusho patients highly intoxicated with dioxin-related compounds: A 140-case series

PURPOSE

Patients with Yusho, a condition caused by exposure to dioxins and dioxin-like compounds, have diverse mental and physical complaints. However, the relationship between dioxins and sleep disorders has not yet been examined. This cross-sectional study was designed to investigate problems associated with sleep among patients with Yusho.

METHODS

A total of 140 participants (52.9% men, average age: 67.1 ± 12.2 years) were examined using questionnaires and medical interviews by an expert on sleep medicine. Demographic and clinical characteristics, including blood concentrations of 2,3,4,7,8-pentachlorodibenzofuran (PeCDF), which is the major cause of Yusho, were obtained from the results of recent surveys conducted by the Yusho Study Group.

RESULTS

Moderate to severe symptoms of insomnia were present in 51.8% of the patients. The median Pittsburgh Sleep Quality Index global score (PSQI GS) was 8 (interquartile range: 5-11). The prevalence of restless legs syndrome/Willis-Ekbom disease (RLS/WED) was 30.7%; 24.3% of patients had severe RLS/WED (distressing symptoms with a frequency ≥ 1day per week). A higher blood concentration of 2,3,4,7,8-PeCDF (≥72.27 pg/g lipid) and severe RLS/WED were associated with higher odds of a PSQI GS ≥8, after adjusting for covariates (odds ratio [95% confidence interval]: 4.84 [1.10-21.25] and 4.15 [1.53-11.28], respectively).

CONCLUSIONS

Symptoms of insomnia were frequent, and the prevalence of RLS/WED was high in patients with Yusho. In addition to the presence of RLS/WED, a higher blood concentration of 2,3,4,7,8-PeCDF was associated with lower subjective sleep quality.

The Aryl Hydrocarbon Receptor and the Nervous System

The aryl hydrocarbon receptor (or AhR) is a cytoplasmic receptor of pollutants. It translocates into the nucleus upon binding to its ligands, and forms a heterodimer with ARNT (AhR nuclear translocator). The heterodimer is a transcription factor, which regulates the transcription of xenobiotic metabolizing enzymes. Expressed in many cells in vertebrates, it is mostly present in neuronal cell types in invertebrates, where it regulates dendritic morphology or feeding behavior. Surprisingly, few investigations have been conducted to unravel the function of the AhR in the central or peripheral nervous systems of vertebrates. In this review, we will present how the AhR regulates neural functions in both invertebrates and vertebrates as deduced mainly from the effects of xenobiotics. We will introduce some of the molecular mechanisms triggered by the well-known AhR ligand, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), which impact on neuronal proliferation, differentiation, and survival. Finally, we will point out the common features found in mice that are exposed to pollutants, and in AhR knockout mice.

Dipentylammonium Binds to the Sigma-1 Receptor and Protects Against Glutamate Toxicity, Attenuates Dopamine Toxicity and Potentiates Neurite Outgrowth in Various Cultured Cell Lines

Alzheimer's disease is a neurodegenerative disease that affects 44 million people worldwide, costing the world $605 billion to care for those affected not taking into account the physical and psychological costs for those who care for Alzheimer's patients. Dipentylammonium is a simple amine, which is structurally similar to a number of other identified sigma-1 receptor ligands with high affinities such as (2R-trans)-2butyl-5-heptylpyrrolidine, stearylamine and dodecylamine. This study investigates whether dipentylammonium is able to provide neuroprotective effects similar to those of sigma-1 receptor agonists such as PRE-084. Here we identify dipentylammonium as a sigma-1 receptor ligand with nanomolar affinity. We have found that micromolar concentrations of dipentylammonium protect from glutamate toxicity and prevent NFκB activation in HT-22 cells. Micromolar concentrations of dipentylammonium also protect stably expressing amyloid precursor protein Swedish mutant (APP/Swe) Neuro2A cells from toxicity induced by 150 μM dopamine, suggesting that dipentylammonium may be useful for the treatment of Parkinsonian symptoms in Alzheimer's patients which are often associated with a more rapid deterioration of cognitive and physical ability. Finally, we found that low micromolar concentrations of dipentylammonium could out preform known sigma-1 receptor agonist PRE-084 in potentiating neurite outgrowth in Neuro2A cells, further suggesting that dipentylammonium has a potential use in the treatment of neurodegenerative diseases and could be acting through the sigma-1 receptor.

Placental CpG methylation of HPA-axis genes is associated with cognitive impairment at age 10 among children born extremely preterm

A major component of the neuroendocrine system is the hypothalamus-pituitary adrenal (HPA) axis. HPA axis genes are also known to play a role in placental physiology. Thus, disruptions in the signaling of HPA axis-associated genes may adversely impact the placenta as well as fetal development, with adverse consequences for health and development of the child. In support of this, recent studies have shown that placental epigenetic methylation of HPA axis genes has an impact on infant behavior. In this study, we evaluated CpG methylation of 14 placental HPA axis-associated genes from a subcohort (n=228) of the Extremely Low Gestational Age Newborns (ELGAN) cohort in relation to cognitive function in mid-childhood (e.g. 10 yrs). Multivariable logistic regression revealed that placental CpG methylation of 10 HPA-axis associated genes were significantly associated with cognition at age 10. Specifically, placental CpG methylation levels of the glucocorticoid receptor gene, Nuclear Receptor Subfamily Group 3 C Member 1 (NR3C1 ) and Brain-derived Neurotropic Factor (BDNF ) were significantly associated with increased odds in developing moderate/severe adverse cognitive impairment at age 10. Methyl-CpG Binding Protein 2 (MECP2) was the major transcriptional regulator of the ten identified HPA genes. The data suggest that placental CpG methylation is associated with cognitive outcomes in mid-childhood.

Neurocognitive and physical functioning in the Seveso Women's Health Study

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is neurotoxic in animals but few studies have investigated its effects on the human brain. Related dioxin-like compounds have been linked to poorer cognitive and motor function in older adults, with effects more pronounced in women, perhaps due to the loss of neuro-protective estrogen in menopause. On 10 July 1976, a chemical explosion in Seveso, Italy, resulted in one of the highest known residential exposures to TCDD. In 1996, we initiated the Seveso Women's Health Study, a retrospective cohort study of the health of the women who were newborn to 40 years old in 1976. Here, we investigate whether TCDD exposure is associated with physical functioning and working memory more than 20 years later. Individual TCDD concentration (ppt) was measured in archived serum collected soon after the explosion. In 1996 and 2008, we measured physical functioning (n=154) and working memory (n=459), respectively. We examined associations between serum TCDD and motor and cognitive outcomes with multivariate linear regression and semi-parametric estimators. A 10-fold increase in serum TCDD was not associated with walking speed (adjusted β=0.0006ft/s, 95% Confidence Interval (CI): -0.13, 0.13), upper body mobility (adjusted β=-0.06, 95% CI: -0.36, 0.23), or manual dexterity (adjusted β=0.34, 95% CI: -0.65, 1.33). We observed an inverted U-shaped association in grip strength, with poorer strength in the lowest and highest TCDD exposure levels. There was no association between TCDD and the Wechsler digit and spatial span tests. Neither menopause status at assessment nor developmental timing of exposure modified associations between TCDD and working memory. Our findings, in one of the only studies of TCDD's effects on neuropsychological and physical functioning in women, do not indicate an adverse effect on these domains, with the exception of a U-shaped relationship with grip strength. Given the limited assessment and relative youth of the women at this follow-up, future work examining additional neuropsychological outcomes is warranted.

Conserved Upstream Regulatory Regions in Mammalian Tyrosine Hydroxylase

Tyrosine hydroxylase (Th) encodes the rate-limiting enzyme in catecholamine biosynthesis, and the regulation of its transcription is critical for the specification and maintenance of catecholaminergic neuron phenotypes. For many genes, regulatory genomic DNA sequences that are upstream of the proximal promoter control expression levels as well as region-specific expression patterns. The regulatory architecture of the genomic DNA upstream of the Th proximal promoter, however, is poorly understood. In this study, we examined the 11 kb upstream nucleotide sequence of Th from nine mammalian species and identified five highly conserved regions. Using cultured human cells and mouse olfactory bulb tissue, chromatin immunoprecipitation (ChIP) assays show that these conserved regions recruit transcription factors that are established regulators of Th transcription (such as NURR1, PITX3, FOXA2, MEIS2, and PAX6). This analysis also identified a conserved binding site for CTCF, and functional studies in cultured human cells and ChIP assays with mouse tissue show that CTCF is a novel regulator of Th transcription in the forebrain. Together, the findings in this study provide key insights into the upstream regulatory genomic architecture and regulatory mechanisms controlling mammalian Th gene transcription.

TCDD induces UbcH7 expression and synphilin-1 protein degradation in the mouse ventral midbrain

UbcH7 is an ubiquitin-conjugating enzyme that interacts with parkin, an E3 ligase. The UbcH7-parkin complex promotes the ubiquitination and degradation of several proteins via the 26S proteasome. Cellular accumulation of the UbcH7-parkin targets alpha-synuclein and synphilin-1 has been associated with Parkinson disease. In mouse liver, 2,3,7,8-tetrachlorodibenzo-p-dioxin, an aryl hydrocarbon receptor ligand, induces UbcH7 expression. Therefore, the aim of the present study was to determine whether 2,3,7,8-tetrachlorodibenzo-p-dioxin induces Ubch7 mRNA and UbcH7 protein expression in the mouse brain, to characterize the molecular mechanism, and the effect on synphilin-1 half-life. We found that 2,3,7,8-tetrachlorodibenzo-p-dioxin promotes the aryl hydrocarbon receptor binding to Ubch7 gene promoter as well as its transactivation, resulting in an induction of UbcH7 levels in the olfactory bulb, ventral midbrain, hippocampus, striatum, cerebral cortex, brain stem, and medulla oblongata. In parallel, 2,3,7,8-tetrachlorodibenzo-p-dioxin promoted synphilin-1 degradation in an aryl hydrocarbon receptor-dependent way.

Methamphetamine induces Shati/Nat8L expression in the mouse nucleus accumbens via CREB- and dopamine D1 receptor-dependent mechanism

Shati/Nat8L significantly increased in the nucleus accumbens (NAc) of mice after repeated methamphetamine (METH) treatment. We reported that Shati/Nat8L overexpression in mouse NAc attenuated METH-induced hyperlocomotion, locomotor sensitization, and conditioned place preference. We recently found that Shati/Nat8L overexpression in NAc regulates the dopaminergic neuronal system via the activation of group II mGluRs by elevated N-acetylaspartylglutamate following N-acetylaspartate increase due to the overexpression. These findings suggest that Shati/Nat8L suppresses METH-induced responses. However, the mechanism by which METH increases the Shati/Nat8L mRNA expression in NAc is unclear. To investigate the regulatory mechanism of Shati/Nat8L mRNA expression, we performed a mouse Shati/Nat8L luciferase assay using PC12 cells. Next, we investigated the response of METH to Shati/Nat8L expression and CREB activity using mouse brain slices of NAc, METH administration to mice, and western blotting for CREB activity of specific dopamine receptor signals in vivo and ex vivo. We found that METH activates CREB binding to the Shati/Nat8L promoter to induce the Shati/Nat8L mRNA expression. Furthermore, the dopamine D1 receptor antagonist SCH23390, but not the dopamine D2 receptor antagonist sulpiride, inhibited the upregulation of Shati/Nat8L and CREB activities in the mouse NAc slices. Thus, the administration of the dopamine D1 receptor agonist SKF38393 increased the Shati/Nat8L mRNA expression in mouse NAc. These results showed that the Shati/Nat8L mRNA was increased by METH-induced CREB pathway via dopamine D1 receptor signaling in mouse NAc. These findings may contribute to development of a clinical tool for METH addiction.

The epinephrine increases tyrosine hydroxylase expression through upregulating thioredoxin-1 in PC12 cells

Epinephrine is a stress hormone which is sharply increased in response to acute stress and is continuously elevated during persistent stress. Thioredoxin-1 (Trx-1) is a redox regulating protein and is induced under various stresses. Our previous study has shown that epinephrine induces the expression of Trx-1. Tyrosine hydroxylase (TH) is the major rate-limiting enzyme in catecholamine biosynthesis in response to stress. However, how TH is regulated by epinephrine is still unknown. In the present study, we found that epinephrine increased the expression of TH in a dose- and time-dependent manner in PC12 cells, which was inhibited by propranolol (β-adrenergic receptor inhibitor), but not by phenoxybenzamine (α-adrenergic receptor inhibitor). The increase of TH was also inhibited by SQ22536 (adenylyl cyclase inhibitor), H-89(PKA inhibitor) and LY294002 (phosphatidylinositol 3 kinase inhibitor). More importantly, overexpression of Trx-1 significantly enhanced the expression of TH, while Trx-1 siRNA suppressed TH expression induced by epinephrine. These results suggest that Trx-1 is involved in TH expression induced by epinephrine in PC12 cells.

Neurodegeneration in Parkinson's disease: interactions of oxidative stress, tryptophan catabolites and depression with mitochondria and sirtuins

The biological underpinnings to the etiology and course of neurodegeneration in Parkinson's disease are an area of extensive research that has yet to produce an early biological marker or disease-slowing or preventative treatment. Recent conceptualizations of Parkinson's disease have integrated immuno-inflammation and oxidative and nitrosative stress occurring in depression, somatization and peripheral inflammation into the course of Parkinson's disease. We review the data showing the importance of immuno-inflammatory processes and oxidative and nitrosative stress in such classically conceived 'comorbidities', suggesting that lifetime, prodromal and concurrent depression and somatization may be intricately involved in the etiology and course of Parkinson's disease, rather than psychiatric comorbidities. This produces a longer term developmental perspective of Parkinson's disease, which incorporates tryptophan catabolites (TRYCATs), lipid peroxidation, sirtuins, cyclic adenosine monophosphate, aryl hydrocarbon receptor, and circadian genes. This integrates wider bodies of data pertaining to neuronal loss in Parkinson's disease, emphasizing how these interact with susceptibility genes to drive changes in mitochondria, blood-brain barrier permeability and intercellular signalling. We review this data here in the context of neurodegeneration in Parkinson's disease and to the future directions indicated for slowing disease progression.

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.

Tyrosine hydroxylase assay: a bioassay for aryl hydrocarbon receptor-active compounds based on tyrosine hydroxylase promoter activation

Tyrosine hydroxylase (TH) assay is a novel bioassay for screening aryl hydrocarbon receptor (AhR)-active compounds with diverse toxicities. The TH assay employs reporter genes to evaluate TH promoter activity, which is linked to dopamine synthesis and regulated by the AhR-aryl hydrocarbon receptor responsive element-III (AHRE-III)-mediated pathway. The toxic equivalency factors for higher chlorinated dioxin congeners in the TH assay (TH-TEF) exhibit the same tendency as those for the WHO-TEF, indicating that the activity of the TH assay is consistent with that of existing methods. Furthermore, other chemicals, such as lower chlorinated dioxin congeners that were minimally investigated in the past could be detected preferentially by this assay.

Theoretical targets for TCDD: a bioinformatics approach

Dioxins are a group of highly toxic molecules that exert their toxicity through the activation of the aryl hydrocarbon receptor (AhR). The most important agonist of the AhR, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a highly toxic compound. Although most of the effects related to TCDD exposure have been linked to the activation of AhR, the objective of this work was to use a bioinformatics approach to identify possible new targets for TCDD. The Target Fishing Docking (TarFisDock) Server was used to find target proteins for TCDD. This virtual screening allowed the identification of binding sites with high affinity for TCDD in diverse proteins, such as metallopeptidases 8 and 3, oxidosqualene cyclase, and myeloperoxidase. Some of these proteins are well known for their biochemical role in some pathological effects of dioxin exposure, including endometriosis, diabetes, inflammation and liver damage. These results suggest that TCDD could also be interacting with cellular targets though AhR-independent pathways.