Significance of the nongenomic, inflammatory pathway in mediating the toxic action of TCDD to induce rapid and long-term cellular responses in 3T3-L1 adipocytes

A New Insight into the Potential Role of Tryptophan-Derived AhR Ligands in Skin Physiological and Pathological Processes

The aryl hydrocarbon receptor (AhR) plays a crucial role in environmental responses and xenobiotic metabolism, as it controls the transcription profiles of several genes in a ligand-specific and cell-type-specific manner. Various barrier tissues, including skin, display the expression of AhR. Recent studies revealed multiple roles of AhR in skin physiology and disease, including melanogenesis, inflammation and cancer. Tryptophan metabolites are distinguished among the groups of natural and synthetic AhR ligands, and these include kynurenine, kynurenic acid and 6-formylindolo[3,2-b]carbazole (FICZ). Tryptophan derivatives can affect and regulate a variety of signaling pathways. Thus, the interest in how these substances influence physiological and pathological processes in the skin is expanding rapidly. The widespread presence of these substances and potential continuous exposure of the skin to their biological effects indicate the important role of AhR and its ligands in the prevention, pathogenesis and progression of skin diseases. In this review, we summarize the current knowledge of AhR in skin physiology. Moreover, we discuss the role of AhR in skin pathological processes, including inflammatory skin diseases, pigmentation disorders and cancer. Finally, the impact of FICZ, kynurenic acid, and kynurenine on physiological and pathological processes in the skin is considered. However, the mechanisms of how AhR regulates skin function require further investigation.

A delayed proinflammatory response of human preadipocytes to PCB126 is dependent on the aryl hydrocarbon receptor

Inflammation in adipose tissue is recognized as a causative factor in the development of type II diabetes. Adipocyte hypertrophy as well as bacterial and environmental factors have been implicated in causing inflammation in mature adipocytes. Exposure to persistent organic pollutants such as polychlorinated biphenyls (PCBs) has been associated with the development of type II diabetes. We show here that PCB126, a dioxin-like PCB, activates a robust proinflammatory state in fat cell precursors (preadipocytes). The response was found to be dependent on aryl hydrocarbon receptor (AhR) activation, although induction of the response was delayed compared to upregulation of CYP1A1, a classic AhR-responsive gene. Treatment of preadipocytes with a nuclear factor kappa-light-chain-enhancer of activated B cell (NF-κB) inhibitor partially attenuated the PCB126-induced inflammatory response and partly, but not completely, ameliorated disruption of adipogenesis caused by PCB126. Our results indicate a role for PCB126 in mediating an inflammatory response through AhR in preadipocytes that interferes with adipogenesis.

Aryl Hydrocarbon Receptor Activity of Tryptophan Metabolites in Young Adult Mouse Colonocytes

The tryptophan microbiota metabolites indole-3-acetate, indole-3-aldehyde, indole, and tryptamine are aryl hydrocarbon receptor (AhR) ligands, and in this study we investigated their AhR agonist and antagonist activities in nontransformed young adult mouse colonocyte (YAMC) cells. Using Cyp1a1 mRNA as an Ah-responsive end point, we observed that the tryptophan metabolites were weak AhR agonists and partial antagonists in YAMC cells, and the pattern of activity was different from that previously observed in CaCo2 colon cancer cells. However, expansion of the end points to other Ah-responsive genes including the Cyp1b1, the AhR repressor (Ahrr), and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-inducible poly(ADP-ribose) polymerase (TiParp) revealed a highly complex pattern of AhR agonist/antagonist activities that were both ligand- and gene-dependent. For example, the magnitude of induction of Cyp1b1 mRNA was similar for TCDD, tryptamine, and indole-3-acetate, whereas lower induction was observed for indole and indole-3-aldehyde was inactive. These results suggest that the tryptophan metabolites identified in microbiota are selective AhR modulators.

TCDD‑induced chick cardiotoxicity is abolished by a selective cyclooxygenase‑2 (COX‑2) inhibitor NS398

Halogenated aromatic hydrocarbons, including 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), are known to cause severe heart defects in avian species. However, the mechanism of TCDD-induced chick cardiovascular toxicity is unclear. In this study, we investigated cyclooxygenase-2 (COX-2) as a possible mechanism of TCDD-induced cardiotoxicity. Fertile chicken eggs were injected with TCDD and a COX-2 selective inhibitor, NS398, and we investigated chick heart failure on day 10. We found that the chick heart to body weight ratio and atrial natriuretic factor mRNA expression were increased, but this increase was abolished with treatment of NS398. In addition, the morphological abnormality of an enlarged ventricle resulting from TCDD exposure was also abolished with co-treatment of TCDD and NS398. Our results suggested that TCDD-induced chick heart defects are mediated via the nongenomic pathway and that they do not require the genomic pathway.

Critical Role of TAK1-Dependent Nuclear Factor-κB Signaling in 2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced Astrocyte Activation and Subsequent Neuronal Death

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been recently shown to elicit inflammatory response in a number of cell-types. However, whether TCDD could provoke inflammation in astrocytes, the most abundant glial cells in central nervous system (CNS), remains virtually unknown. In the present study, we showed that TCDD exposure could induce evident astrocyte activation both in vivo and in vitro. Further, we found that TGF-β-activated kinase 1 (TAK1), a critical regulator of NF-κB signaling, was rapidly phosphorylated in the process of TCDD-induced reactive astroglia. Exposure to TCDD led to rapid TAK1 and NF-κB p65 phosphorylation, as well as IKBα degradation. Moreover, blockage of TAK1 using siRNA oligos or TAK1 inhibitor 5Z-7-oxozeaenol significantly attenuated TCDD-induced astrocyte activation as well as the release of TNF-α. Finally, we showed that the conditioned medium of TCDD-treated astrocytes promoted the apoptosis of PC12 neuronal cells, which could be blocked with the pre-treatment of TAK1 inhibitor. Taken together, these findings suggested that TCDD could promote the inflammatory activation of astrocytes through modulating TAK1-NF-κB cascade, implicating that reactive astrocytes might contribute to TCDD-induced adverse effects on CNS system.

Omeprazole Inhibits Pancreatic Cancer Cell Invasion through a Nongenomic Aryl Hydrocarbon Receptor Pathway

Omeprazole and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) are aryl hydrocarbon receptor (AhR) agonists that inhibit the invasion of breast cancer cells through inhibition of CXCR4 transcription. Treatment of highly invasive Panc1 pancreatic cancer cells with TCDD, omeprazole, and seven other AhR-active pharmaceuticals showed that only omeprazole and tranilast, but not TCDD, inhibited invasion in a Boyden chamber assay. Similar results were observed in MiaPaCa2 cells, another quasimensenchymal pancreatic ductal adenocarcinoma (QM-PDA) pancreatic cancer cell line, whereas invasion was not observed with BxPC3 or L3.6pL cells, which are classified as classical (less invasive) pancreatic cancer cells. It was also observed in QM-PDA cells that TCDD, omeprazole, and tranilast did not induce CYP1A1 or CXCR4 and that treatment with these compounds did not result in nuclear uptake of AhR. In contrast, treatment of BxPC3 and L3.6pL cells with these AhR ligands resulted in induction of CYP1A1 (by TCDD) and nuclear uptake of AhR, which was similar to that observed for Ah-responsive MDA-MB-468 breast and HepG2 liver cancer cell lines. Results of AhR and AhR nuclear translocator (Arnt) knockdown experiments in Panc1 and MiaPaCa2 cells demonstrated that omeprazole- and tranilast-mediated inhibition of invasion was AhR-dependent but Arnt-independent. These results demonstrate that in the most highly invasive subtype of pancreatic cancer cells (QM-PDA) the selective AhR modulators omeprazole and tranilast inhibit invasion through a nongenomic AhR pathway.

Histopathological, ultrastructural, and immunohistochemical assessment of hippocampus structures of rats exposed to TCDD and high doses of tocopherol and acetylsalicylic acid

The effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on central nervous system consists of changing expression of estrogen receptors, whereas the result of chronic inflammatory reaction caused by dioxin is occurrence of destructive changes in various organs connected with disturbed metabolism of connective tissue and damage of cells. The aim of the study was to determine the effect of dioxins on function, ultrastructure, and cytological and histological structure of hippocampus, particularly on expression of estrogen receptors in central nervous system as well as to define protective influence of tocopherol (TCP) and acetylsalicylic acid (ASA) on the decrease in activity of proinflammatory effects in central nervous system. It was shown that TCDD contributes to destructive and inflammatory changes along with demyelization of myelin sheaths and atrophy of estrogen receptors in hippocampus. Dioxin contributes to atrophy of estrogen receptors in hippocampus, in which also destructive and inflammatory changes were found along with demyelination of myelin sheaths. Histopathological and ultrastructural image of hippocampus areas in rats, in which both TCP and ASA were used, is characterized by poorly expressed degenerative changes and smaller inflammatory reactivity. Using both TCP and ASA has a protective effect on functions of central nervous system.

TCDD and omeprazole prime platelets through the aryl hydrocarbon receptor (AhR) non-genomic pathway

The role of the aryl hydrocarbon receptor (AhR) in hemostasis has recently gained increased attention. Here, we demonstrate, by qRT-PCR and western blot, that human platelets express both AhR mRNA and AhR protein. AhR protein levels increase in a dose dependent manner when incubated with either 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) or omeprazole. Treatment of platelets with puromycin blocks increased AhR protein synthesis in the presence of AhR activators. Additionally, treatment of platelets with either activator results in phosphorylation of p38MAPK and cPLA2, two key signaling molecules in platelet activation pathways. Using the AhR competitive inhibitors alpha naphthoflavone and CH-223191, we show that phosphorylation of p38MAPK is AhR dependent. Further, inhibition of p38MAPK blocks downstream cPLA2 phosphorylation induced by TCDD or omeprazole. Treatment with AhR activators results in platelet priming, as demonstrated by increased platelet aggregation, which is inhibited by AhR antagonists. Our data support a model of the platelet AhR non-genomic pathway in which treatment with AhR activators results in increased expression of the AhR, phosphorylation of p38MAPK and cPLA2, leading to platelet priming in response to agonist.

Hypothesis: Activation of rapid signaling by environmental estrogens and epigenetic reprogramming in breast cancer

Environmental and lifestyle factors are considered significant components of the increasing breast cancer risk in the last 50 years. Specifically, exposure to environmental endocrine disrupting compounds is correlated with cancer susceptibility in a variety of tissues. In both human and rodent models, the exposure to ubiquitous environmental estrogens during early life has been shown to disrupt normal mammary development and cause permanent adverse effects. Recent studies indicate that environmental estrogens not only have the ability to disrupt estrogen receptor (ER) signaling, but can also reprogram the epigenome by altering DNA and histone methylation through rapid, nongenomic ER actions. We have observed xenoestrogen-mediated activation of several nongenomic signaling pathways and have identified a target for epigenetic reprogramming in MCF-7 breast cancer cells. These observations, in addition to data from the literature, support the hypothesis that activation of rapid signaling by environmental estrogens can lead to epigenetic reprogramming and contribute to the progression of breast cancer.

Benzyl butyl phthalate induces migration, invasion, and angiogenesis of Huh7 hepatocellular carcinoma cells through nongenomic AhR/G-protein signaling

Background

The widespread use of phthalates as plasticizers has raised public health concerns regarding their adverse effects, including an association with cancer. Although animal investigations have suggested an association between phthalate exposure and hepatocellular carcinoma, the mechanisms are unknown.

Methods

The hepatocellular carcinoma cell line Huh7 was treated with benzyl butyl phthalate (BBP), and then analyzed by total internal reflection fluorescence microscopy, confocal microscopy and double immunogold transmission electron microscopy. Following BBP treatment, mRNA levels were measured by RT-PCR, protein levels were measured using western blot, and vascular endothelial growth factor levels were measured by an enzyme-linked immunosorbent assay. Cell migration and invasion assays were evaluated by transwell, and angiogenesis were performed by a tube formation assay. Nude mice were used to investigate metastasis and angiogenesis in vivo.

Results

BBP affected hepatocellular carcinoma progression through the aryl hydrocarbon receptor (AhR) and that benzyl butyl phthalate (BBP) stimulated AhR at the cell surface, which then interacted with G proteins and triggered a downstream signaling cascade. BBP activated AhR through a nongenomic action involving G-protein signaling rather than the classical genomic AhR action. BBP treatment promoted cell migration and invasion in vitro and metastasis in vivo via the AhR/G_β/PI3K/Akt/NF-κB pathway. In addition, BBP induced both in vitro and in vivo angiogenesis through the AhR/ERK/VEGF pathway.

Conclusions

These findings suggest a novel nongenomic AhR mechanism involving G-protein signaling induced by phthalates, which contributes to tumor progression of hepatocellular carcinoma.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2407-14-556) contains supplementary material, which is available to authorized users.

Association of dioxin and other persistent organic pollutants (POPs) with diabetes: epidemiological evidence and new mechanisms of beta cell dysfunction

The worldwide explosion of the rates of diabetes and other metabolic diseases in the last few decades cannot be fully explained only by changes in the prevalence of classical lifestyle-related risk factors, such as physical inactivity and poor diet. For this reason, it has been recently proposed that other "nontraditional" risk factors could contribute to the diabetes epidemics. In particular, an increasing number of reports indicate that chronic exposure to and accumulation of a low concentration of environmental pollutants (especially the so-called persistent organic pollutants (POPs)) within the body might be associated with diabetogenesis. In this review, the epidemiological evidence suggesting a relationship between dioxin and other POPs exposure and diabetes incidence will be summarized, and some recent developments on the possible underlying mechanisms, with particular reference to dioxin, will be presented and discussed.

2,3,7,8-Tetrachlorodibenzo-p-dioxin promotes astrocyte activation and the secretion of tumor necrosis factor-α via PKC/SSeCKS-dependent mechanisms

2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a ubiquitous environmental pollutant that could induce significant toxic effects in the human nervous system. However, the underlying molecular mechanism has not been entirely elucidated. Reactive astrogliosis has implicated in various neurological diseases via the production of a variety of pro-inflammatory mediators. Herein, we investigated the potential role of TCDD in facilitating astrocyte activation and the underlying molecular mechanisms. We showed that TCDD induced rapid astrocyte activation following TCDD exposure, which was accompanied by significantly elevated expression of Src-Suppressed-C Kinase Substrate (SSeCKS), a protein involved in protein kinase C (PKC)-mediated Nuclear Factor kappa B signaling, suggesting a possible involvement of PKC-induced SSeCKS activation in TCDD-triggered reactive astroglia. In keeping with the finding, we found that the level of phosphorylated Nuclear Factor kappa B p65 was remarkably increased after TCDD treatment. Furthermore, interference of SSeCKS attenuated TCDD-induced inducible nitric oxide synthase, glial fibrillary acidic protein, phospho-p65 expression, and tumor necrosis factor-α secretion in astrocytes. In addition, pre-treatment with PKC inhibitor also attenuated TCDD-induced astrocyte activation, as well as SSeCKS expression. Interestingly, we found that TCDD treatment could lead to SSeCKS perinuclear localization, which could be abolished after treatment with PKC inhibitor. Finally, we showed that inhibition of PKC activity or SSeCKS expression would impair TCDD-triggered tumor necrosis factor-α secretion. Our results suggested that TCDD exposure could lead to astrocyte activation through PKC/SSeCKS-dependent mechanisms, highlighting that astrocytes might be important target of TCDD-induced neurotoxicity. 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) elicits neurotoxic effects. Here, we show TCDD induces pro-inflammatory responses in astrocytes. TCDD initiates an increase of [Ca2+]i, followed by the activation of PKC, which then induces the activation of Src-suppressed C-kinase substrate (SSeCKS). SSeCKS promotes NF-κB activation and the secretion of TNF-α and nitric oxide in astrocytes.

The aryl hydrocarbon receptor-activating effect of uremic toxins from tryptophan metabolism: a new concept to understand cardiovascular complications of chronic kidney disease

Patients with chronic kidney disease (CKD) have a higher risk of cardiovascular diseases and suffer from accelerated atherosclerosis. CKD patients are permanently exposed to uremic toxins, making them good candidates as pathogenic agents. We focus here on uremic toxins from tryptophan metabolism because of their potential involvement in cardiovascular toxicity: indolic uremic toxins (indoxyl sulfate, indole-3 acetic acid, and indoxyl-β-d-glucuronide) and uremic toxins from the kynurenine pathway (kynurenine, kynurenic acid, anthranilic acid, 3-hydroxykynurenine, 3-hydroxyanthranilic acid, and quinolinic acid). Uremic toxins derived from tryptophan are endogenous ligands of the transcription factor aryl hydrocarbon receptor (AhR). AhR, also known as the dioxin receptor, interacts with various regulatory and signaling proteins, including protein kinases and phosphatases, and Nuclear Factor-Kappa-B. AhR activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin and some polychlorinated biphenyls is associated with an increase in cardiovascular disease in humans and in mice. In addition, this AhR activation mediates cardiotoxicity, vascular inflammation, and a procoagulant and prooxidant phenotype of vascular cells. Uremic toxins derived from tryptophan have prooxidant, proinflammatory, procoagulant, and pro-apoptotic effects on cells involved in the cardiovascular system, and some of them are related with cardiovascular complications in CKD. We discuss here how the cardiovascular effects of these uremic toxins could be mediated by AhR activation, in a “dioxin-like” effect.

The aryl receptor inhibitor epigallocatechin-3-gallate protects INS-1E beta-cell line against acute dioxin toxicity

The aim of this research was to investigate the mechanism(s) underlying the acute toxicity of dioxin in pancreatic beta cells and to evaluate the protective effects of epigallocatechin-3-gallate (EGCG), the most abundant of the green tea's catechins and a powerful inhibitor of the aryl hydrocarbon receptor (AhR). Using the insulin-secreting INS-1E cell line we have explored the effect of 1h exposure to different concentrations of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), alone or in the presence of EGCG, on: (a) cell survival; (b) cellular ultrastructure; (c) intracellular calcium levels; (d) mitochondrial membrane potential; (e) glucose-stimulated insulin secretion and (f) activation of MAP kinases. Our results demonstrate that TCDD is highly toxic for INS-1E cells, suggesting that pancreatic beta cells should be considered a relevant and sensitive target for dioxin acute toxicity. EGCG significantly protects INS-1E cells against TCDD-induced toxicity in terms of both cell survival and preservation of cellular ultrastructure. The mechanism of this protective effect seems to be related to: (a) the ability of EGCG to preserve the mitochondrial function and thus to prevent the TCDD-induced inhibition of glucose-stimulated insulin secretion and (b) the ability of EGCG to inhibit the TCDD-induced activation of selected kinases, such as e.g. ERK 1/2 and JNK. Our results clearly show that EGCG is able to protect pancreatic beta cells against dioxin acute toxicity and indicate the mitochondrion as the most likely target for this beneficial effect.

The lipid fraction of human milk initiates adipocyte differentiation in 3T3-L1 cells

BACKGROUND

The prevalence of childhood obesity has increased worldwide over the past decade. Despite evidence that human milk lowers the risk of childhood obesity, the mechanism is not fully understood.

AIMS

We investigated the direct effect of human milk on differentiation of 3T3-L1 preadipocytes.

STUDY DESIGN AND SUBJECTS

3T3-L1 preadipocytes were treated with donated human milk only or the combination of the standard hormone mixture; insulin, dexamethasone (DEX), and 3-isobututyl-1-methylxanthine (IBMX). Furthermore, the induction of preadipocyte differentiation by extracted lipids from human milk was tested in comparison to the cells treated with lipid extracts from infant formula. Adipocyte differentiation, specific genes as well as formation of lipid droplets were examined.

RESULTS

We clearly show that lipids present in human milk initiate 3T3-L1 preadipocyte differentiation. In contrast, this effect was not observed in response to lipids present in infant formula. The initiation of preadipocyte differentiation by human milk was enhanced by adding the adipogenic hormone, DEX or insulin. The expression of late adipocyte markers in Day 7 adipocytes that have been induced into differentiation with human milk lipid extracts was comparable to those in control cells initiated by a standard adipogenic hormone cocktail.

CONCLUSIONS

These results demonstrate that human milk contains bioactive lipids that can initiate preadipocyte differentiation in the absence of the standard adipogenic compounds via a unique pathway.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) induces microglial nitric oxide production and subsequent rat primary cortical neuron apoptosis through p38/JNK MAPK pathway

It has been widely accepted that microglia, which are the innate immune cells in the brain, upon activation can cause neuronal damage. In the present study, we investigated the role of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in regulating microglial nitric oxide production and its role in causing neuronal damage. The study revealed that TCDD stimulates the expression of inducible nitric oxide synthase (iNOS) as well as the production of nitric oxide (NO) in a dose- and time-dependent manner. Further, a rapid activation of p38 and JNK MAPKs was found in HAPI microglia following TCDD treatment. Blockage of p38 and JNK kinases with their specific inhibitors, SB202190 and SP600125, significantly reduced TCDD-induced iNOS expression and NO production. In addition, it was demonstrated through treating rat primary cortical neurons with media conditioned with TCDD treated microglia that microglial iNOS activation mediates neuronal apoptosis. Lastly, it was also found that p38 and JNK MAPK inhibitors could attenuate the apoptosis of rat cortical neurons upon exposure to medium conditioned by TCDD-treated HAPI microglial cells. Based on these observations, we highlight that the p38/JNK MAPK pathways play an important role in TCDD-induced iNOS activation in rat HAPI microglia and in the subsequent induction of apoptosis in primary cortical neurons.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced inflammatory activation is mediated by intracellular free calcium in microglial cells

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been known to induce inflammatory signaling in a number of cell types and tissues. However, the adverse effects of TCDD on the central nervous system (CNS) have not been entirely elucidated. In this study, using reverse transcriptase PCR (RT-PCR) and ELISA, we showed that TCDD up-regulated the expression and secretion of tumor necrosis factor-alpha (TNF-α) in a time-dependent manner in cultured HAPI microglial cells. TCDD also caused a fast (within 30min as judged by the increase in its mRNA level) activation of cytosolic phospholipase A2 (cPLA2). This initial action was accompanied by up-regulation of cyclooxygenase-2 (COX-2), an important inflammation marker within 1h after TCDD treatment. These pro-inflammatory responses were inhibited by two types of Ca(2+) blockers, bis-(o-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid acetoxymethyl ester (BAPTA-AM) and nifedipine, thus, indicating that the effects are triggered by initial increase in the intracellular concentration of free Ca(2+) ([Ca(2+)]i). Further, TCDD exposure could induce phosphorylation- and ubiquitination-dependent degradation of IкBα, and the translocation of NF-κB p65 from the cytosol to the nucleus in this microglial cell line. Thus, the NF-κB signaling pathway can be activated after TCDD treatment. However, Ca(2+) blockers also obviously attenuated NF-κB activation and transnuclear transport induced by TCDD. In concert with these results, we highlighted that the secretion of pro-inflammatory cytokine and NF-κB activation induced by TCDD can be mediated by elevation of [Ca(2+)]i in HAPI microglial cells.

Homocysteine induces COX-2 expression in macrophages through ROS generated by NMDA receptor-calcium signaling pathways

Homocysteine (Hcy) at elevated levels is a putative risk factor for many cardiovascular disorders including atherosclerosis. In the present study, we investigated the effect of Hcy on the expression of cyclooxygenase (COX)-2 in murine macrophages and the mechanisms involved. Hcy increased the expression of COX-2 mRNA and protein in dose- and time-dependent manners, but did not affect COX-1 expression. Hcy-induced COX-2 expression was attenuated not only by the calcium chelators, EGTA and BAPTA-AM, but also by an antioxidant, N-acetylcysteine. Calcium chelators also attenuated Hcy-induced reactive oxygen species (ROS) production in macrophages, indicating that Hcy-induced COX-2 expression might be mediated through ROS generated by calcium-dependent signaling pathways. In another series of experiments, Hcy increased the intracellular concentration of calcium in a dose-dependent manner, which was attenuated by MK-801, an N-methyl-D-aspartate (NMDA) receptor inhibitor, but not by bicuculline, a gamma-aminobutyric acid receptor inhibitor. Molecular inhibition of NMDA receptor using small interfering RNA also attenuated Hcy-induced increases in intracellular calcium. Furthermore, both ROS production and Hcy-induced COX-2 expression were also inhibited by MK-801 as well as by molecular inhibition of NMDA receptor. Taken together, these findings suggest that Hcy enhances COX-2 expression in murine macrophages by ROS generated via NMDA receptor-mediated calcium signaling pathways.

Quantitative phosphoproteomic analysis of early alterations in protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin

A comprehensive quantitative analysis of changes in protein phosphorylation preceding or accompanying transcriptional activation by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in 5L rat hepatoma cells was performed using the SILAC approach. Following exposure of the cells to DMSO or 1 nM TCDD for 0.5 to 2 h, 5648 phosphorylated peptides corresponding to 2156 phosphoproteins were identified. Eight peptides exhibited a statistically significantly altered phosphorylation because of TCDD exposure and 22 showed a regulation factor of ≥ 1.5 in one of the experiments per time point. The vast majority of the TCCD-induced phosphorylation changes had not been reported before. The transcription factor ARNT, the obligate partner for gene activation by the TCDD-bound Ah receptor, exhibited an up-regulation of its Ser77 phosphorylation, a modification known to control the differential binding of ARNT homodimers and heterodimers to different enhancers suggesting that this phosphorylation represents a novel mechanism contributing to the alteration of gene expression by TCDD. Other proteins with altered phosphorylation included, among others, various transcriptional coregulators previously unknown to participate in TCDD-induced gene activation, regulators of small GTPases of the Ras superfamily, UBX domain-containing proteins and the oncogenic protein LYRIC. The results open up new directions for research on the molecular mechanisms of dioxin action and toxicity.

Aryl hydrocarbon receptor ligand effects in RBL2H3 cells

The aryl hydrocarbon receptor (AHR) mediates toxic effects of dioxin and xenobiotic metabolism. AHR has an emerging role in the immune system, but its physiological ligands and functional role in immunocytes remain poorly understood. Mast cells are immunocytes that are central to inflammatory responses and release a spectrum of pro-inflammatory mediators including histamine, mast cell proteases, and pro-inflammatory cytokines such as IL-6 upon stimulation. The aim was to investigate the AHR in model mast cells and examine how both putative and known AHR ligands, e.g., kynurenine, kynurenic acid (KA), Resveratrol, indolmycin, and violacein, affect mast cell activation and signaling. These ligands were tested on calcium signaling, degranulation, and gene expression. The data show that AHR is present in three model mast cell lines, and that various known and putative AHR ligands regulate gene expression of Cyp1a1, a gene down-stream of AHR. Furthermore, it was found that calcium influxes and mast cell secretory responses were enhanced or suppressed after chronic treatment with AHR agonists or antagonists, and that AHR ligands modified RBL2H3 cell degranulation. AHR ligands can chronically change cytokine gene expression in activated mast cells, as exemplified by IL-6. The antagonist Resveratrol repressed expression of induced IL-6 gene expression. Although KA and kynurenine are both AHR agonists, these ligands behaved differently in regards to degranulation and IL-6 expression, indicating that they may function outside of AHR pathways. These data suggest considerable complexity in RBL2H3 responses to AHR ligands, with implications for understanding of both dioxin pathology and the immunological effects of endogenous AHR ligands.

Inter-strain heterogeneity in rat hepatic transcriptomic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

The biochemical and toxic effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) have been the subject of intense study for decades. It is now clear that essentially all TCDD-induced toxicities are mediated by DNA-protein interactions involving the Aryl Hydrocarbon Receptor (AHR). Nevertheless, it remains unknown which AHR target genes cause TCDD toxicities. Several groups, including our own, have developed rodent model systems to probe these questions. mRNA expression profiling of these model systems has revealed significant inter-species heterogeneity in rodent hepatic responses to TCDD. It has remained unclear if this variability also exists within a species, amongst rodent strains. To resolve this question, we profiled the hepatic transcriptomic response to TCDD of diverse rat strains (L-E, H/W, F344 and Wistar rats) and two lines derived from L-E×H/W crosses, at consistent age, sex, and dosing (100 μg/kg TCDD for 19 h). Using this uniquely consistent dataset, we show that the majority of TCDD-induced alterations in mRNA abundance are strain/line-specific: only 11 genes were affected by TCDD across all strains, including well-known dioxin-responsive genes such as Cyp1a1 and Nqo1. Our analysis identified two novel universally dioxin-responsive genes as well as 4 genes induced by TCDD in dioxin-sensitive rats only. These 6 genes are strong candidates to explain TCDD-related toxicities, so we validated them using 152 animals in time-course (0 to 384 h) and dose-response (0 to 3000 μg/kg) experiments. This study reveals that different rat strains exhibit dramatic transcriptional heterogeneity in their hepatic responses to TCDD and that inter-strain comparisons can help identify candidate toxicity-related genes.

Celecoxib, a selective cyclooxygenase-2 inhibitor, lowers plasma cholesterol and attenuates hepatic lipid peroxidation during carbon-tetrachloride-associated hepatotoxicity in rats

Cyclooxygenase-2 (COX-2) expression and prostaglandin production are suggested to play important, complex roles in the pathogenesis of various liver diseases. Studies on the effects of COX-2 inhibitors on the progression of liver fibrosis present controversial results, and the proposed therapeutic potential of these agents in chronic liver disease is predicated largely on their effectiveness in modulating hepatic stellate cell activation in vitro. This study investigated the modulatory effect of celecoxib, a selective COX-2 inhibitor, in CCl(4)-mediated hepatotoxicity in rats. Thirty Wistar albino rats, weighing 120-180 g, were assigned into five groups of 6 rats/group. Groups 1 and 2 received saline (10 mL/kg) and CCl(4) (80 mg/kg), respectively. Group 3 was given celecoxib (5.7 mg/kg), whereas groups 4 and 5 were pretreated with 2.9 and 5.7 mg/kg/day of celecoxib, respectively, 1 hour before CCl(4) treatment. Plasma aspartate aminotransferase, alanine aminotransferase, and alkaline phosphatase activities increased significantly by 118.5, 150.0, and 51.3%, respectively, with an accompanying decrease (P < 0.05) in total protein and albumin after CCl(4) treatment. Hepatotoxicity was associated with a significant increase in plasma cholesterol, hepatic lipid peroxidation (LPO), and severe hepatic necrosis with marked fatty and cellular (i.e., mononuclear cells) infiltration. Although celecoxib neither reduced CCl(4)-induced increases in marker enzymes of hepatotoxicity nor significantly attenuated hepatic necrosis, it, however, was effective in reducing elevated cholesterol by 16.5 and 20.8% and LPO by 12.9 and 35.5% at 2.9 and 5.7 mg/kg, respectively. Data suggest that COX-2 inhibitors may be effective in controlling hypercholesterolemia and peroxidative changes associated with liver injury.

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

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

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

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

TCDD induced pericardial edema and relative COX-2 expression in medaka (Oryzias Latipes) embryos

Exposure to dioxin and other aryl hydrocarbon receptor (AhR) ligands results in multiple, specific developmental cardiovascular phenotypes including pericardial edema and circulatory failure in small aquarium fish models. Although phenotypes are well described, mechanistic underpinnings for such toxicities remain elusive. Here we suggest that AhR activation results in stimulation of inflammation and "eicosanoid" pathways, which contribute to the observed developmental, cardiovascular phenotypes. We demonstrate that medaka embryos exposed to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0.05-1 ppb) during early development result in a dose-related increase in the prevalence of pericardial edema and that this phenotype correlates with an increase in cyclooxygenase-2 (COX-2) gene expression. Those individuals exhibiting the edema phenotype had significantly greater COX-2 mRNA than their nonedematous cohort. Selective pharmacological inhibition of COX-2, with NS-398, and genetic knock down of COX-2 with a translation initiation morpholino significantly attenuated prevalence and severity of edema phenotype. Subsequently, exposures of medaka embryos to arachidonic acid (AA) resulted in recapitulation of the pericardial edema phenotype and significantly increased COX-2 expression only in those individuals exhibiting the edema phenotype compared with their nonedematous cohort. AA exposure does not result in significant induction of cytochrome P450 1A expression, suggesting that pericardial edema can be induced independent of AhR/aryl hydrocarbon receptor nuclear translocator/dioxin response element interactions. Results from this study demonstrate that developmental exposure to TCDD results in an induction of inflammatory mediators including COX-2, which contribute to the onset, and progression of heart dysmorphogenesis in the medaka model.

Severe toxicity and cyclooxygenase (COX)-2 mRNA increase by lithium in the neonatal mouse kidney

Functions of the kidney of mammals are immature during the neonatal period, and the neonatal kidney could be susceptible to chemicals, including drugs and environmental toxicants. Among these chemicals, cyclooxygenase (COX)-inducing chemicals should be given attentions as the potential kidney toxicants during the period, and we hypothesized that lithium chloride (LiCl) has such toxicity. Neonatal mice of C57BL/J strain were intraperitoneally injected with LiCl (2 mmol/kg body weight) daily until 21 days of age, and examined on 7 days and 21 days of age. Neonatal treatment of LiCl caused a significant increase in COX-2 mRNA and a decrease in mRNAs of aquaporins on day 7 of age. Osmolarity of urine from LiCl-treated neonates was significantly lower than that of control neonate. Most of the LiCl-treated neonates died during the second week of age. Histological examination revealed renal cysts on day 7 and hydronephrosis on day 21. in the surviving neonates. The present results showed that the kidney of mouse neonates is vulnerable to lithium, and suggested the possibility that COX-2 upregulation is responsible for the severe renal toxicity including hydronephrosis.

Induction of hepatic cyclooxygenase-2 by hyperhomocysteinemia via nuclear factor-κB activation

Hyperhomocysteinemia, an elevation of blood homocysteine (Hcy), is a metabolic disorder associated with dysfunction of multiple organs. Apart from endothelial dysfunction, Hcy can cause hepatic lipid accumulation and liver injury. However, the mechanism responsible for Hcy-induced liver injury is poorly understood. The aim of this study was to investigate the regulation of cyclooxygenase-2 (COX-2), a proinflammatory factor, expression in the liver during the initial phase of hyperhomocysteinemia. Sprague-Dawley rats were fed a high-methionine diet for 1 or 4 wk. Serum and liver concentrations of Hcy were significantly elevated after 1 or 4 wk of dietary treatment. COX-2 mRNA and protein levels were significantly elevated in the liver of hyperhomocysteinemic rats. The induction of COX-2 expression was more prominent in 1-wk hyperhomocysteinemic rats than that in the 4-wk group. EMSA revealed an activation of NF-κB in the same liver tissue in which COX-2 was induced. Administration of a NF-κB inhibitor to hyperhomocysteinemic rats effectively abolished hepatic COX-2 expression, inhibited the formation of inflammatory foci, and improved liver function. Further investigation revealed that oxidative stress due to increased superoxide generation was responsible for increased phosphorylation and degradation of IκBα leading to NF-κB activation in the liver. Administration of 4-hydroxy-tetramethyl-piperidine-1-oxyl, an SOD mimetic, to hyperhomocysteinemic rats not only inhibited NF-κB activation but also prevented hepatic COX-2 induction and improved liver function. These results suggest that hyperhomocysteinemia-induced COX-2 expression is mediated via NF-κB activation. Increased oxidative stress and inflammatory response may contribute to liver injury associated with hyperhomocysteinemia.

TCDD-induced cyclooxygenase-2 expression is mediated by the nongenomic pathway in mouse MMDD1 macula densa cells and kidneys

Cyclooxygenase-2 (Cox-2) plays a critical role in TCDD-induced hydronephrosis in mouse neonates. In this study we found that induction of Cox-2 by TCDD in MMDD1, a mouse macula densa cell line, is accompanied with a rapid increase in the enzymatic activity of cytosolic phospholipase A2 (cPLA2) as well as activation of protein kinases. Calcium serves as a trigger for such an action of TCDD in this cell line. These observations indicate that the basic mode of action of TCDD to induce the rapid inflammatory response in MMDD1 is remarkably similar to those mediated by the nongenomic pathway of aryl hydrocarbon receptor (AhR) found in other types of cells. Such an action of TCDD to induce Cox-2 in MMDD1 was not affected by "DRE decoy oligonucleotides" treatment or by introduction of a mutation on the DRE site of Cox-2 promoter, suggesting that this route of action of TCDD is clearly different from that mediated by the classical genomic pathway. An in vivo study with Ahr(nls) mouse model has shown that TCDD-induces Cox-2 and renin expression in the kidneys of the Ahr(nls) mice as well as Ahr(+/-) mice, but not in the Ahr(-/-) mice, indicating that this initial action of TCDD in mouse kidney does not require the translocation of AhR into the nucleus, supporting our conclusion that induction of Cox-2 by TCDD in mouse kidney is largely mediated by the nongenomic pathway of TCDD-activated AhR.