Rapid activation of c-Src kinase by dioxin is mediated by the Cdc37-HSP90 complex as part of Ah receptor signaling in MCF10A cells

Multi-omics reveals bufadienolide Q-markers of Bufonis Venenum based on antitumor activity and cardiovascular toxicity in zebrafish

BACKGROUND

Bufonis Venenum (BV) is a traditional animal-based Chinese medicine with therapeutic effects against cancer. However, its clinical use is significantly restricted due to associated cardiovascular risks. BV's value in China's market is typically assessed based on "content priority," focusing on indicator components. However, these components of BV possess both antitumor activity and toxicity, and the correlation between the antitumor activity and toxicity of BV has not yet been elucidated.

PURPOSE

This study employs an integrated multi-omics approach to identify bufadienolide Q-markers and explore the correlation between BV's antitumor activity and toxicity. The aim is to establish a more comprehensive method for BV's quality.

METHODS

Normal zebrafish and HepG2 xenograft zebrafish were chosen as activity and toxicity evaluation models. Ultra-high performance liquid chromatography (UHPLC) coupled with a linear ion trap orbitrap (LTQ-Orbitrap) mass spectrometry was used to quantify eight batches of BV and key "toxic and effective" components were screened out. Transcriptomic and metabolomic analyses were performed to elucidate the regulatory mechanisms underlying the antitumor activity and cardiovascular toxicity of the key components in BV.

RESULTS

Eight key "toxic and effective" compounds were identified: resibufogenin, cinobufagin, arenobufagin, bufotalin, bufalin, gamabufotalin, desacetylcinobufagin, and telocinobufagin. The findings showed that bufalin and cinobufagin interfered with calcium homeostasis through CaV and CaSR, induced cardiotoxicity, and upregulated CASP9 to activate myocardial cell apoptosis. However, desacetylcinobufagin exhibited greater potential in terms of anti-tumor effects. Combining the results of untargeted and targeted metabolomics revealed that desacetylcinobufagin could have a callback effect on differential lipids and correct abnormal energy and amino acid metabolism caused by cancer, similar to cinobufagin and bufalin. Microscale thermophoresis (MST) ligand binding measurements also showed that the binding of desacetylcinobufagin to GPX4 has a more potent ability to induce ferroptosis in tumor cells compared to cinobufagin.

CONCLUSION

An innovative evaluation method based on the zebrafish was developed to investigate the relationship between the toxicity and efficacy of BV. This study identified toxicity and activity Q-markers and explored the mechanism between the two effects of BV. The research data could offer valuable insights into the efficacy of BV. Additionally, desacetylcinobufagin, an active ingredient with low toxicity, was found to enhance the quality of BV.

Mechanisms of breast cancer progression induced by environment-polluting aryl hydrocarbon receptor agonists

Breast cancer is the most common invasive malignancy among women worldwide and constitutes a complex and heterogeneous disease. Interest has recently grown in the role of the aryl hydrocarbon receptor (AhR) in breast cancer and the contribution of environment-polluting AhR agonists. Here, we present a literature review addressing AhR ligands, including pesticides hexachlorobenzene and chlorpyrifos, polycyclic aromatic hydrocarbons, polychlorinated dibenzo-p-dioxins and dibenzofurans, polychlorinated biphenyls, parabens, and phthalates. The objectives of this review are a) to summarize recent original experimental, preclinical, and clinical studies on the biological mechanisms of AhR agonists which interfere with the regulation of breast endocrine functions, and b) to examine the biological effects of AhR ligands and their impact on breast cancer development and progression. We discuss biological mechanisms of action in cell viability, cell cycle, proliferation, epigenetic changes, epithelial to mesenchymal transition, and cell migration and invasion. In addition, we examine the effects of AhR ligands on angiogenic processes, metastasis, chemoresistance, and stem cell renewal. We conclude that exposure to AhR agonists stimulates pathways that promote breast cancer development and may contribute to tumor progression. Given the massive use of industrial and agricultural chemicals, ongoing evaluation of their effects in laboratory assays and preclinical studies in breast cancer at environmentally relevant doses is deemed essential. Likewise, awareness should be raised in the population regarding the most harmful toxicants to eradicate or minimize their use.

Kynurenine promotes neonatal heart regeneration by stimulating cardiomyocyte proliferation and cardiac angiogenesis

Indoleamine 2,3 dioxygenase-1 (IDO1) catalyzes tryptophan-kynurenine metabolism in many inflammatory and cancer diseases. Of note, acute inflammation that occurs immediately after heart injury is essential for neonatal cardiomyocyte proliferation and heart regeneration. However, the IDO1-catalyzed tryptophan metabolism during heart regeneration is largely unexplored. Here, we find that apical neonatal mouse heart resection surgery led to rapid and consistent increases in cardiac IDO1 expression and kynurenine accumulation. Cardiac deletion of Ido1 gene or chemical inhibition of IDO1 impairs heart regeneration. Mechanistically, elevated kynurenine triggers cardiomyocyte proliferation by activating the cytoplasmic aryl hydrocarbon receptor-SRC-YAP/ERK pathway. In addition, cardiomyocyte-derived kynurenine transports to endothelial cells and stimulates cardiac angiogenesis by promoting aryl hydrocarbon receptor nuclear translocation and enhancing vascular endothelial growth factor A expression. Notably, Ahr deletion prevents indoleamine 2,3 dioxygenase -kynurenine-associated heart regeneration. In summary, increasing indoleamine 2,3 dioxygenase-derived kynurenine level promotes cardiac regeneration by functioning as an endogenous regulator of cardiomyocyte proliferation and cardiac angiogenesis.

Extracellular acidosis stimulates breast cancer cell motility through aryl hydrocarbon receptor and c-Src kinase activation

A reduction in extracellular pH (pHe) is a characteristic of most malignant tumors. The aryl hydrocarbon receptor (AhR) is a transcription factor localized in a cytosolic complex with c-Src, which allows it to trigger non-genomic effects through c-Src. Considering that the slightly acidic tumor microenvironment promotes breast cancer progression in a similar way to the AhR/c-Src axis, our aim was to evaluate whether this pathway could be activated by low pHe. We examined the effect of pHe 6.5 on AhR/c-Src axis using two breast cancer cell lines (MDA-MB-231 and LM3) and mammary epithelial cells (NMuMG) and found that acidosis increased c-Src phosphorylation only in tumor cells. Moreover, the presence of AhR inhibitors prevented c-Src activation. Low pHe reduced intracellular pH (pHi), while amiloride treatment, which is known to reduce pHi, induced c-Src phosphorylation through AhR. Analyses were conducted on cell migration and metalloproteases (MMP)-2 and -9 activities, with results showing an acidosis-induced increase in MDA-MB-231 and LM3 cell migration and MMP-9 activity, but no changes in NMuMG cells. Moreover, all these effects were blocked by AhR and c-Src inhibitors. In conclusion, acidosis stimulates the AhR/c-Src axis only in breast cancer cells, increasing cell migration and MMP-9 activity. Although the AhR activation mechanism still remains elusive, a reduction in pHi may be thought to be involved. These findings suggest a critical role for the AhR/c-Src axis in breast tumor progression stimulated by an acidic microenvironment. This article is protected by copyright. All rights reserved.

Endocrine disruptor hexachlorobenzene induces cell migration and invasion, and enhances aromatase expression levels in human endometrial stromal cells

Endometriosis is the presence and growth of endometrial tissue outside of the uterus. Previous studies have suggested that endocrine disrupting chemicals such as organochlorine pesticides could be a risk factor for endometriosis. Hexachlorobenzene (HCB) is a weak ligand of the aryl hydrocarbon receptor (AhR) and promotes metalloproteinase and cyclooxygenase-2 (COX-2) expression, as well as, c-Src activation in human endometrial stromal cells (T-HESC) and in rat endometriosis model. Our aim was to evaluate the effect of HCB exposure on oestrogen receptor (ER) ɑ and β, progesterone receptor (PR) and aromatase expression, as well as, on cell migration and invasion in T-HESC and primary cultures of endometrial stromal cells from eutopic endometria of control subjects (ESC). Results show that HCB increases ERɑ and aromatase protein levels and reduces PR content in both T-HESC and ESC. However, the pesticide only increases ERβ expression in ESC, without changes in T-HESC. Moreover, cell migration and invasion are promoted by pesticide exposure involving the AhR, c-Src, COX-2 and ER pathways in T-HESC. HCB also triggers ERɑ activation via phosphorylation in Y537 through AhR/c-Src pathway. Our results provide experimental evidence that HCB induces alterations associated with endometriosis, suggesting that these mechanisms could contribute to pesticide exposure-induced endometriosis development.

Selective beta-cell toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin on isolated pancreatic islets

An association between exposure to environmental pollutants and diabetes risk has been repeatedly shown by epidemiological studies. However, the biological basis of this association still need to be clarified. In this research we explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) exposure on isolated pancreatic islets. After 1, 6 and 24 h exposure of isolated islets to different concentrations (1-50 nM) of TCDD we assayed: i) cell survival; ii) ultrastructure; iii) glucose-stimulated insulin secretion (GSIS); iv) expression of selected genes. A significant, dose-related increase of both necrosis and apoptosis was observed isolated rat islets after 24 h exposure to TCDD. The electron microscopic analysis revealed, at the same time point, the presence of several ultrastructural alterations (mitochondrial swelling, increased mitophagy, dilation of the endoplasmic reticulum) that, very interestingly, were exclusively observed in beta cells and not in other endocrine cells. Similar results were obtained in isolated human islets. GSIS was rapidly (1 h) and persistently (6 and 24 h) decreased by TCDD exposure even at the smallest concentration (1 nM). TCDD exposure significantly affected gene expression in isolated islets: Glut2, Gck, Bcl-xL, MafA, Pdx1 FoxO1 and IRE1 gene expression was significantly decreased, whereas Puma, DP5, iNOS and Chop gene expression was significantly increased after 6 h exposure to TCDD. In conclusion, our results clearly indicated that pancreatic beta cells represent not only a sensitive but also a specific target of the toxic action of dioxin.

Epigenetic Regulations of AhR in the Aspect of Immunomodulation

Environmental factors contribute to autoimmune disease manifestation, and as regarded today, AhR has become an important factor in studies of immunomodulation. Besides immunological aspects, AhR also plays a role in pharmacological, toxicological and many other physiological processes such as adaptive metabolism. In recent years, epigenetic mechanisms have provided new insight into gene regulation and reveal a new contribution to autoimmune disease pathogenesis. DNA methylation, histone modifications, chromatin alterations, microRNA and consequently non-genetic changes in phenotypes connect with environmental factors. Increasing data reveals AhR cross-roads with the most significant in immunology pathways. Although study on epigenetic modulations in autoimmune diseases is still not well understood, therefore future research will help us understand their pathophysiology and help to find new therapeutic strategies. Present literature review sheds the light on the common ground between remodeling chromatin compounds and autoimmune antibodies used in diagnostics. In the proposed review we summarize recent findings that describe epigenetic factors which regulate AhR activity and impact diverse immunological responses and pathological changes.

Does NLRP3 Inflammasome and Aryl Hydrocarbon Receptor Play an Interlinked Role in Bowel Inflammation and Colitis-Associated Colorectal Cancer?

Inflammation is a hallmark in many forms of cancer; with colitis-associated colorectal cancer (CAC) being a progressive intestinal inflammation due to inflammatory bowel disease (IBD). While this is an exemplification of the negatives of inflammation, it is just as crucial to have some degree of the inflammatory process to maintain a healthy immune system. A pivotal component in the maintenance of such intestinal homeostasis is the innate immunity component, inflammasomes. Inflammasomes are large, cytosolic protein complexes formed following stimulation of microbial and stress signals that lead to the expression of pro-inflammatory cytokines. The NOD-, LRR- and pyrin domain-containing protein 3 (NLRP3) inflammasome has been extensively studied in part due to its strong association with colitis and CAC. The aryl hydrocarbon receptor (AhR) has recently been acknowledged for its connection to the immune system aside from its role as an environmental sensor. AhR has been described to play a role in the inhibition of the NLRP3 inflammasome activation pathway. This review will summarise the signalling pathways of both the NLRP3 inflammasome and AhR; as well as new-found links between these two signalling pathways in intestinal immunity and some potential therapeutic agents that have been found to take advantage of this link in the treatment of colitis and CAC.

AhR ligands reactivate LINE-1 retrotransposon in triple-negative breast cancer cells MDA-MB-231 and non-tumorigenic mammary epithelial cells NMuMG

Breast cancer is the most common cancer type in females worldwide. Environmental exposure to pesticides affecting hormonal homeostasis does not necessarily induce DNA mutations but may influence gene expression by disturbances in epigenetic regulation. Expression of long interspersed nuclear element-1 (LINE-1) has been associated with tumorigenesis in several cancers. In nearly all somatic cells, LINE-1 is silenced by DNA methylation in the 5́'UTR and reactivated during disease initiation and/or progression. Strong ligands of aryl hydrocarbon receptor (AhR) activate LINE-1 through the transforming growth factor-β1 (TGF-β1)/Smad pathway. Hexachlorobenzene (HCB) and chlorpyrifos (CPF), both weak AhR ligands, promote cell proliferation and migration in breast cancer cells, as well as tumor growth in rat models. In this context, our aim was to examine the effect of these pesticides on LINE-1 expression and ORF1p localization in the triple-negative breast cancer cell line MDA-MB-231 and the non-tumorigenic epithelial breast cell line NMuMG, and to evaluate the role of TGF-β1 and AhR pathways. Results show that 0.5 μM CPF and 0.005 μM HCB increased LINE-1 mRNA expression through Smad and AhR signaling in MDA-MB-231. In addition, the methylation of the first sites in 5́'UTR of LINE-1 was reduced by pesticide exposure, although the farther sites remained unaffected. Pesticides modulated ORF1p localization in MDA-MB-231: 0.005 μM HCB and 50 μM CPF increased nuclear translocation, while both induced cytoplasmic retention at 0.5 and 5 μM. Moreover, both stimulated double-strand breaks, enhancing H2AX phosphorylation, coincidentally with ORF1p nuclear localization. In NMuMG similar results were observed, since they heighten LINE-1 mRNA levels. CPF effect was through AhR and TGF-β1 signaling, whereas HCB action depends only of AhR. In addition, both pesticides increase ORF1p expression and nuclear localization. Our results provide experimental evidence that HCB and CPF exposure modify LINE-1 methylation levels and induce LINE-1 reactivation, suggesting that epigenetic mechanisms could contribute to pesticide-induced breast cancer progression.

Impact of endocrine disruptor hexachlorobenzene on the mammary gland and breast cancer: The story thus far

Breast cancer incidence is increasing globally and exposure to endocrine disruptors has gained importance as a potential risk factor. Hexachlorobenzene (HCB) was once used as a fungicide and, despite being banned, considerable amounts are still released into the environment. HCB acts as an endocrine disruptor in thyroid, uterus and mammary gland and was classified as possibly carcinogenic to human. This review provides a thorough analysis of results obtained in the last 15 years of research and evaluates data from assays in mammary gland and breast cancer in diverse animal models. We discuss the effects of environmentally relevant HCB concentrations on the normal mammary gland and different stages of carcinogenesis, and attempt to elucidate its mechanisms of action at molecular level. HCB weakly binds to the aryl hydrocarbon receptor (AhR), activating both membrane (c-Src) and nuclear pathways. Through c-Src stimulation, AhR signaling interacts with other membrane receptors including estrogen receptor-α, insulin-like growth factor-1 receptor, epidermal growth factor receptor and transforming growth factor beta 1 receptors. In this way, several pathways involved in mammary morphogenesis and breast cancer development are modified, inducing tumor progression. HCB thus stimulates epithelial cell proliferation, preneoplastic lesions and alterations in mammary gland development as well as neoplastic cell migration and invasion, metastasis and angiogenesis in breast cancer. In conclusion, our findings support the hypothesis that the presence and bioaccumulation of HCB in high-fat tissues and during highly sensitive time windows such as pregnancy, childhood and adolescence make exposure a risk factor for breast tumor development.

Hexachlorobenzene modulates the crosstalk between the aryl hydrocarbon receptor and transforming growth factor-β1 signaling, enhancing human breast cancer cell migration and invasion

Given the number of women affected by breast cancer, considerable interest has been raised in understanding the relationships between environmental chemicals and disease onset. Hexachlorobenzene (HCB) is a dioxin-like compound that is widely distributed in the environment and is a weak ligand of the aryl hydrocarbon receptor (AhR). We previously demonstrated that HCB acts as an endocrine disruptor capable of stimulating cell proliferation, migration, invasion, and metastasis in different breast cancer models. In addition, increasing evidence indicates that transforming growth factor-β1 (TGF-β1) can contribute to tumor maintenance and progression. In this context, this work investigated the effect of HCB (0.005, 0.05, 0.5, and 5μM) on TGF-β1 signaling and AhR/TGF-β1 crosstalk in the human breast cancer cell line MDA-MB-231 and analyzed whether TGF-β1 pathways are involved in HCB-induced cell migration and invasion. RT-qPCR results indicated that HCB reduces AhR mRNA expression through TGF-β1 signaling but enhances TGF-β1 mRNA levels involving AhR signaling. Western blot analysis demonstrated that HCB could increase TGF-β1 protein levels and activation, as well as Smad3, JNK, and p38 phosphorylation. In addition, low and high doses of HCB were determined to exert differential effects on AhR protein levels, localization, and activation, with a high dose (5μM) inducing AhR nuclear translocation and AhR-dependent CYP1A1 expression. These findings also revealed that c-Src and AhR are involved in HCB-mediated activation of Smad3. HCB enhances cell migration (scratch motility assay) and invasion (Transwell assay) through the Smad, JNK, and p38 pathways, while ERK1/2 is only involved in HCB-induced cell migration. These results demonstrate that HCB modulates the crosstalk between AhR and TGF-β1 and consequently exacerbates a pro-migratory phenotype in MDA-MB-231 cells, which contributes to a high degree of malignancy. Taken together, our findings help to characterize the molecular mechanism underlying the effects of HCB on breast cancer progression.

Discovery and identification of Cdc37-derived peptides targeting the Hsp90–Cdc37 protein–protein interaction

In order to explore the key residues of the Hsp90–Cdc37 binding interface for further design of peptide inhibitors, a combined strategy of molecular dynamics simulation and MM-PBSA analysis was performed.

Activation of the aryl hydrocarbon receptor by the widely used Src family kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(dimethylethyl)pyrazolo[3,4-d]pyrimidine (PP2)

Small molecular weight protein kinase inhibitors are frequently used tools to unravel the complex network of cellular signal transduction under certain physiological and pathophysiological conditions. 4-amino-5-(4-chlorophenyl)-7-(dimethylethyl)pyrazolo[3,4-d]pyrimidine (PP2) is a widely used compound to block the activity of Src family kinases, the major group of non-receptor tyrosine kinases, which trigger multiple cellular signaling pathways. Here, we show that PP2 induces cytochrome P450 1A1 mRNA expression and enzyme activity in a dose-dependent manner in human HepG2 hepatoma cells and NCTC 2544 keratinocytes. By means of reporter gene assays, RNA interference, electrophoretic mobility shift assay, and competitive ligand-binding assay, we further demonstrate that PP2 is a ligand for the aryl hydrocarbon receptor (AHR), an intracellular chemosensor that regulates xenobiotic metabolism, environmental stress responses, and immune functions. Upon ligand-dependent activation, the AHR translocates into the nucleus and dimerizes with the AHR nuclear translocator (ARNT) to modulate the expression of its target genes. In addition, AHR activation is frequently accompanied by an activation of the tyrosine kinase c-Src, resulting in stimulation of cell-surface receptors and downstream signal transduction. As PP2 activates the AHR/ARNT pathway by simultaneously blocking c-Src-mediated alternative signaling routes, this compound may be a suitable tool to study the contribution of the different AHR-dependent signaling pathways to biological processes and adverse outcomes. On the other hand, the unexpected property of PP2 to stimulate AHR/ARNT signaling should be carefully taken into account in future investigations in order to avoid a false interpretation of experimental results and molecular interrelations.

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.

Uremic toxin indoxyl 3-sulfate regulates the differentiation of Th2 but not of Th1 cells to lessen allergic asthma

Immune system dysfunctions including the increased Th1/Th2 ratio are common in chronic kidney disease (CKD) patients, and a wide variety of skin diseases including Th1-mediated uremic pruritis are associated with CKD. Although there are more than 90 uremic toxins reported, it is yet to be known which uremic solute is associated with the unbalanced Th1/Th2 ratio and how it works. Indoxyl 3-sulfate (I3S), one of uremic toxins and a potent aryl hydrocarbon receptor (AhR) ligand, accumulates in blood and tissues, increasing up to 81.04 μM in CKD patients, compared with 1.03 μM in healthy subjects. I3S activates NF-κB and AhR. Thus, we investigated roles of I3S in the differentiation of Th1 and Th2 cells. I3S inhibited Th2 differentiation but showed little or no effect on Th1 differentiation. I3S suppressed Th2-mediated ovalbumin-induced allergic asthma in mice and decreased the frequency of IL-4 producing CD4 T cells in the lungs. I3S inhibited phosphorylation of STAT5 and STAT6, transcription factors associated with Th2 differentiation. Effects of I3S on Th2 differentiation were suppressed by α-naphtoflavone, an AhR antagonist, indicating that I3S regulates Th2 differentiation AhR-dependently.

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.

Khellin and visnagin differentially modulate AHR signaling and downstream CYP1A activity in human liver cells

Khellin and visnagin are two furanochromones that can be frequently found in ethnomedical formulations in Asia and the Middle East. Both compounds possess anti-inflammatory and analgesic properties, therefore modern medicine uses these compounds or structurally related derivatives for treatment of vitiligo, bronchial asthma and renal colics. Despite their frequent usage, the potential toxic properties of visnagin and khellin are not well characterized up-to-now. Many natural compounds modulate the expression and activity of cytochrome P450 1A1 (CYP1A1), which is well-known to bioactivate pro-carcinogens. The expression of this enzyme is controlled by the aryl hydrocarbon receptor (AHR), a ligand-activated transcription factor and regulator of drug metabolism. Here, we investigated the influence of both furanochromones on AHR signaling in human HepG2 hepatocarcinoma cells and primary human hepatocytes. Both compounds transactivated xenobiotic response element (XRE)-driven reporter gene activity in a dose-dependent manner and induced CYP1A1 transcription in HepG2 cells and primary hepatocytes. The latter was abolished in presence of a specific AHR antagonist. CYP1A enzyme activity assays done in HepG2 cells and primary hepatocytes revealed an inhibition of enzyme activity by both furanochromones, which may become relevant regarding the metabolism of xenobiotics and co-administered therapeutic drugs. The observed induction of several other members of the AHR gene battery, whose gene products are involved in regulation of cell growth, differentiation and migration, indicates that a further toxicological characterization of visnagin and khelllin is urgently required in order to minimize potential drug-drug interactions and other toxic side-effects that may occur during therapeutic usage of these furanochromones.

The emerging roles of AhR in physiology and immunity

The aryl hydrocarbon receptor (AhR) is traditionally defined as a transcriptional regulator involved in adaptive xenobiotic response, however, emerging evidence supports physiological functions of AhR in normal cell development and immune response. The role of AhR in immunomodulation is multi-dimensional. On the one hand, activation of AhR by TCDD and other ligands leads to profound immunosuppression, potentially via skewed Th1/Th2 cell balance toward Th1 dominance, and boosted Treg cell differentiation. On the other hand, activation of AhR can also induce Th17 cell polarization and increase the severity of autoimmune disease. In addition to T lymphocytes, the AhR also appears to play a vital role in B cell maturation, and regulates the activity of macrophages, dendritic cells and neutrophils following lipopolysaccharide challenge or influenza virus infection. In these scenarios, activation of AhR is associated with decreased host response and reduced survival. Furthermore, gene knock out studies suggest that AhR is indispensable for the postnatal maintenance of intestinal intraepithelial lymphocytes and skin-resident dendritic epidermal gamma delta T cells, providing a potential link between AhR and gut immunity and wound healing. It is well accepted that the magnitude and the type of immune response is dependent on the local cytokine milieu and the AhR appears to be one of the key factors involved in the fine turning of this cytokine balance.

Indoxyl 3-sulfate stimulates Th17 differentiation enhancing phosphorylation of c-Src and STAT3 to worsen experimental autoimmune encephalomyelitis

Although AhR activation regulates CD4T cell differentiation, how it works has yet to be elucidated. In the present study, using in vitro Th17 differentiation model, we examined effects of AhR activation by indoxyl 3-sulfate (I3S), a uremic toxin, on Th17 differentiation and investigated underlying mechanisms. I3S increased expression of RORγt, the master transcription factor for Th17 differentiation, and stimulated Th17 differentiation, in a comparative manner as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a prototypical AhR ligand. Activation of STAT3, which is phosphorylated by the IL-6 signaling pathways and thus is necessary for Th17 differentiation, was strongly stimulated by I3S and TCDD. Phosphorylation of c-Src, which was shown to be activated by AhR ligands, was also increased by I3S and TCDD, and blocking of c-Src activity by 4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine (PP2) inhibited phosphorylation of both c-Src and STAT3, raising a possibility that stimulatory activities of I3S and TCDD on Th17 differentiation could be exerted via increased phosphorylation of c-Src, which in turn stimulates STAT3 activation. Finally, we found that I3S worsened experimental autoimmune encephalomyelitis (EAE), which is primarily mediated by Th17 cells, enhancing the frequency of IL-17-producing cells in draining lymph nodes.

Action of hexachlorobenzene on tumor growth and metastasis in different experimental models

Hexachlorobenzene (HCB) is a widespread organochlorine pesticide, considered a possible human carcinogen. It is a dioxin-like compound and a weak ligand of the aryl hydrocarbon receptor (AhR). We have found that HCB activates c-Src/HER1/STAT5b and HER1/ERK1/2 signaling pathways and cell migration, in an AhR-dependent manner in MDA-MB-231 breast cancer cells. The aim of this study was to investigate in vitro the effect of HCB (0.005, 0.05, 0.5, 5μM) on cell invasion and metalloproteases (MMPs) 2 and 9 activation in MDA-MB-231 cells. Furthermore, we examined in vivo the effect of HCB (0.3, 3, 30mg/kg b.w.) on tumor growth, MMP2 and MMP9 expression, and metastasis using MDA-MB-231 xenografts and two syngeneic mouse breast cancer models (spontaneous metastasis using C4-HI and lung experimental metastasis using LM3). Our results show that HCB (5μM) enhances MMP2 expression, as well as cell invasion, through AhR, c-Src/HER1 pathway and MMPs. Moreover, HCB increases MMP9 expression, secretion and activity through a HER1 and AhR-dependent mechanism, in MDA-MB-231 cells. HCB (0.3 and 3mg/kg b.w.) enhances subcutaneous tumor growth in MDA-MB-231 and C4-HI in vivo models. In vivo, using MDA-MB-231 model, the pesticide (0.3, 3 and 30mg/kg b.w.) activated c-Src, HER1, STAT5b, and ERK1/2 signaling pathways and increased MMP2 and MMP9 protein levels. Furthermore, we observed that HCB stimulated lung metastasis regardless the tumor hormone-receptor status. Our findings suggest that HCB may be a risk factor for human breast cancer progression.

Molecular targets that link dioxin exposure to toxicity phenotypes

Many toxicology studies have elucidated health effects associated with exposure to various chemicals, but few have identified the molecular targets that cause specific endpoints of toxicity. Our understanding of the toxicity of dioxins, a group of chemicals capable of causing toxicity at environmentally relevant levels of exposure, is no exception. Dioxins are unique compared to most chemicals that we are exposed to in the environment because they activate a high affinity receptor, aryl hydrocarbon receptor (AhR), that was identified more than three decades ago. In recent years, several lines of experimental evidence have provided clues for opening the "black box" that contains the molecular mechanisms of dioxin action. These clues have emerged by toxicologists beginning to identify the molecular targets that link AhR signaling to tissue-specific toxicity phenotypes. Endpoints of dioxin toxicity for which downstream molecular targets have begun to be elucidated are observed in developmental or tissue regeneration processes, and include impaired prostate development and hydronephrosis in mouse fetuses and pups, reduced midbrain blood flow and jaw malformation in zebrafish embryos, and impaired fin regeneration in larval and adult zebrafish. Significant progress in identifying molecular targets for dioxin-induced hepatotoxicity in adult mice also has occurred. Misregulation of AhR downstream pathways, such as conversion of arachidonic acid to prostanoids via cyclooxygenase-2, and altered Wnt/β-catenin signaling downregulating Sox9, and signaling by receptors for inflammatory cytokines have been implicated in tissue-specific endpoints of dioxin toxicity. These findings may not only begin to clarify the molecular targets of dioxin action but shed light on new molecular events associated with development and disease.

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.

Impacts of environmental toxicants on male reproductive dysfunction

Male infertility caused by exposure to environmental toxicants such as cadmium, mercury, bisphenol A (BPA) and dioxin is a global problem, particularly in industrialized countries. Studies in the testis and other organs have illustrated the importance of environmental toxicant-induced oxidative stress in mediating disruption to cell junctions. This, in turn, is regulated by the activation of PI3K/c-Src/FAK and MAPK signaling pathways, with the involvement of polarity proteins. This leads to reproductive dysfunction such as reduced sperm count and reduced quality of semen. In this review, we discuss how these findings can improve understanding of the modes of action of environmental toxicants in testicular dysfunction. Thus, specific inhibitors and/or antagonists against signaling molecules in these pathways may be able to 'reverse' and/or 'block' the disruptive effects of toxicant-induced damage. Additional studies comparing high-level acute exposure versus low-level chronic exposure to environmental toxicants are also needed to fully elucidate the underlying molecular mechanism(s) by which these toxicants disrupt male reproductive function.

Activation of c-Src/HER1/STAT5b and HER1/ERK1/2 signaling pathways and cell migration by hexachlorobenzene in MDA-MB-231 human breast cancer cell line

Hexachlorobenzene (HCB) is a widespread environmental pollutant. It is a dioxin-like compound and a weak ligand of the aryl hydrocarbon receptor (AhR) protein. HCB is a tumor cocarcinogen in rat mammary gland and an inducer of cell proliferation and c-Src kinase activity in MCF-7 breast cancer cells. This study was carried out to investigate HCB action on c-Src and the human epidermal growth factor receptor (HER1) activities and their downstream signaling pathways, Akt, extracellular-signal-regulated kinase (ERK1/2), and signal transducers and activators of transcription (STAT) 5b, as well as on cell migration in a human breast cancer cell line, MDA-MB-231. We also investigated whether the AhR is involved in HCB-induced effects. We have demonstrated that HCB (0.05μM) produces an early increase of Y416-c-Src, Y845-HER1, Y699-STAT5b, and ERK1/2 phosphorylation. Moreover, our results have shown that the pesticide (15 min) activates these pathways in a dose-dependent manner (0.005, 0.05, 0.5, and 5μM). In contrast, HCB does not alter T308-Akt activation. Pretreatment with a specific inhibitor for c-Src (4-amino-5-(4-chlorophenyl)-7-(t-butyl) pyrazolo[3,4-d]pyrimidine [PP2]) prevents Y845-HER1 and Y699-STAT5b phosphorylation. AG1478, a specific HER1 inhibitor, abrogates HCB-induced STAT5b and ERK1/2 activation, whereas 4,7-orthophenanthroline and α-naphthoflavone, two AhR antagonists, prevent HCB-induced STAT5b and ERK1/2 phosphorylation. HCB enhances cell migration evaluated by scratch motility and transwell assays. Pretreatment with PP2, AG1478, and 4,7-orthophenanthroline suppresses HCB-induced cell migration. These results demonstrate that HCB stimulates c-Src/HER1/STAT5b and HER1/ERK1/2 signaling pathways in MDA-MB-231. c-Src, HER1, and AhR are involved in HCB-induced increase in cell migration. The present study makes a significant contribution to the molecular mechanism of action of HCB in mammary carcinogenesis.

FRET analysis of protein tyrosine kinase c-Src activation mediated via aryl hydrocarbon receptor

BACKGROUND

Activation of the protein tyrosine kinase c-Src (c-Src kinase) induced by the exposure to the environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) has been shown in various cell types. Most previous works used Western blot analysis to detect the phosphorylation on the Tyr416 residue, which activates c-Src kinase.

METHODS

Here we compared the results of c-Src tyrosine phosphorylation via aryl hydrocarbon receptor (AhR)-dependent mechanisms from Western blot analysis with fluorescent resonance energy transfer (FRET) assay detecting c-Src activation after treatment with TCDD to activate AhR in two different human cell types.

RESULTS

Western blot analyses show time-dependent phosphorylation of c-Src by TCDD in HepG2 and MCF-10A cells. Data from FRET assay visualized and quantified the activation of c-Src kinase induced by TCDD in living cells of both cell types. The FRET efficiency decreased by 20%, 5 min after TCDD treatment and continued decreasing until the end of the experiment, 25 min after TCDD treatment. PP2, a c-Src specific inhibitor, suppressed both TCDD- and epidermal growth factor- (EGF) induced c-Src activation. In contrast, the AhR antagonist 3'-methoxy-4'nitroflavone (MNF) blocked only TCDD- but not EGF-induced activation of c-Src.

CONCLUSIONS

The current study shows that the early activation of c-Src via EGF and AhR signaling pathways can be visualized in living cells using the FRET assay which is in line with Western blot analysis.

GENERAL SIGNIFICANCE

The FRET assay provides a useful tool to visualize and quantify c-Src kinase activation via AhR in living cells.

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

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

The inhibitory mechanisms of the tyrosine kinase inhibitors herbimycin a, genistein, and tyrphostin B48 with regard to the function of the aryl hydrocarbon receptor in Caco-2 cells

The aryl hydrocarbon receptor (AhR) is a transcription factor that is activated by dioxin and related xenobiotics. Although the activation of AhR is inhibited by tyrosine kinase inhibitors, the molecular mechanism has not been clarified. In the current study, the inhibitory mechanisms of several inhibitors of tyrosine kinase, herbimycin A, genistein, and tyrphostin B48, on AhR activation was analyzed in human Caco-2 cells. All the inhibitors suppressed the transcriptional activation of AhR induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Herbimycin A induced down-regulation of the AhR protein by inhibiting its molecular chaperone heat shock protein 90 (HSP90). In contrast, genistein and tyrphostin B48 inhibited the nuclear localization of AhR induced by TCDD, although the amount of AhR protein was not altered. The inhibitory effects of genistein and tyrphostin B48 on endogenous tyrosine kinase activity were evaluated by detection of alterations in the tyrosine phosphorylation states of cellular proteins.

Hexachlorobenzene induces cell proliferation and IGF-I signaling pathway in an estrogen receptor alpha-dependent manner in MCF-7 breast cancer cell line

Hexachlorobenzene (HCB) is an organochlorine pesticide widely distributed in the biosphere. ERalpha regulates the expression of genes involved in growth and development, and plays an important role in breast cancer. The present study focuses attention on the effect of HCB (0.005, 0.05, 0.5, 5 microM) on cell proliferation in estrogen receptor alpha (ERalpha)(+) MCF-7, and ERalpha(-) MDA-MB-231 breast cancer cell lines. We also studied the insulin growth factor-I (IGF-I) signaling pathway in MCF-7 cells. HCB (0.005 and 0.05 microM) stimulated cell proliferation in MCF-7, but not in MDA-MB-231 cells. The pesticide increased apoptosis in MCF-7, at HCB (0.5 and 5 microM). At these doses, HCB induced the expression of the aryl hydrocarbon receptor (AhR)-regulated gene cytochrome P4501A1. MCF-7 cells exposed to HCB (0.005 and 0.05 microM) overexpressed IGF-IR and insulin receptor (IR). IRS-1-phosphotyrosine content was increased in a dose dependent manner. ICI 182,780 prevented HCB-induced cell proliferation and IGF-I signaling in MCF-7 cells incubated in phenol-red free media. In addition, HCB (0.005 microM) increased c-Src activation, Tyr537-ERalpha phosphorylation and ERalpha down-regulation. Taken together, our data indicate that HCB stimulation of cell proliferation and IGF-I signaling is ERalpha dependent in MCF-7 cells.

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.

A comparison of Hsp90alpha and Hsp90beta interactions with cochaperones and substrates

Hsp90 chaperone complexes function in assembly, folding, and activation of numerous substrates. The 2 vertebrate homologues encoded by the genes hsp90a and hsp90b are differentially expressed in embryonic and adult tissues and during stress; however, it is not known whether they possess identical functional activities in chaperone complexes. This question was addressed by examining potential differences between the Hsp90 isoforms with respect to both cochaperone and substrate interactions. Epitope-tagged proteins were expressed in mammalian cells or Xenopus oocytes and subjected to immunoprecipitation with an array of co-chaperones. Both isoforms were shown to participate equally in multichaperone complexes, and no significant differences in cochaperone distribution were observed. The substrates Raf-1, HSF1, Cdc37, and MEK1 interacted with both Hsp90alpha and Hsp90beta, and the relative patterns of these interactions were not affected by heat shock. The substrate kinases c-Src, CKIIB, A-raf, and Erk interacted with both isoforms; however, significantly more Hsp90alpha was recovered after heat shock. The data demonstrate that Hsp90alpha and Hsp90beta exhibit similar interactions with co-chaperones, but significantly different behaviors with respect to substrate interactions under stress conditions. These results reveal both functional similarities and key functional differences in the individual members of this protein family.

The conversion of rapid TCCD nongenomic signals to persistent inflammatory effects via select protein kinases in MCF10A cells

Previously we found that 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces rapid inflammatory cellular responses in MCF10A mammary epithelial cells through a distinct nongenomic pathway by activating cytosolic phospholipase A2 and Src kinase within 30 min. In the current study we investigated how such an initial, seemingly transient signaling induced by TCDD is subsequently converted into more stable long-term messages. We found that TCDD causes prolonged activation of the binding activity of nuclear proteins to the oligonucleotide probes representing consensus activator protein 1 and CCAAT enhancer binding protein response element sequences, followed by later induction of some diagnostic marker including cyclooxgenase-2, matrix metalloproteinase-2, colony stimulating factor-1, and cytochrome P450 19 (or aromatase). Blocking the early steps of the nongenomic pathway inhibits this action of TCDD. It was also found that Src kinase is mainly responsible for the increase of binding activity to the activator protein 1 probe, and another kinase, protein kinase A (PKA), is accountable for most of the increase of binding activity to the CCAAT enhancer binding protein probe. The induction of those diagnostic markers is also affected by 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (a Src kinase inhibitor) or H89 (a PKA inhibitor). These results indicate that Src kinase and PKA act as the second messengers in propagating the initial nongenomic signaling of TCDD.

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.

Growth factors, cytokines and their receptors as downstream targets of arylhydrocarbon receptor (AhR) signaling pathways

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a widespread environmental pollutant, which causes a variety of severe health effects, e.g. immunosuppression, hepatotoxicity, and carcinogenesis. The main mediator of TCDD toxicity is the arylhydrocarbon receptor (AhR), which, upon activation, translocates into the nucleus and enforces gene expression. Since most of the pleiotropic effects caused by TCDD are associated with alterations in cell growth and differentiation, the analysis of the interference of the AhR with factors controlling these cellular functions seems to be a promising target regarding the prevention and treatment of chemical-provoked diseases. Cell growth and differentiation are regulated by numerous growth factors and cytokines. These multifunctional peptides promote or inhibit cell growth and regulate differentiation and other cellular processes, depending on cell-type and developmental stage. They are involved in the regulation of a broad range of physiological processes, including immune response, hematopoiesis, neurogenesis, and tissue remodeling. The complex network of growth factors and cytokines is accurately regulated and disturbances of this system are associated with adverse health effects. The molecular mechanisms by which the AhR interferes with this signaling network are multifaceted and the physiological consequences of this cross-talk are quite enigmatic. The investigation of this complex interaction is an exciting task, especially with respect to the recently described non-genomic and/or ligand-independent activities of AhR. Therefore, we summarize the current knowledge about the interaction of the AhR with three cytokine-/growth factor-related signal transducers -- the epidermal growth factor (EGF) family, tumor necrosis factor-alpha (TNF-alpha), and transforming growth factor-beta (TGF-beta) -- with regard to pathophysiological findings.

The role of caveolin-1 in PCB77-induced eNOS phosphorylation in human-derived endothelial cells

Polychlorinated biphenyls (PCBs) may contribute to the pathology of atherosclerosis by activating inflammatory responses in vascular endothelial cells. Endothelial nitric oxide synthase (eNOS) is colocalized with caveolae and is a critical regulator of vascular homeostasis. PCBs may be proatherogenic by causing dysfunctional eNOS signaling. The objective of this study was to investigate the role of caveolin-1 in PCB-induced endothelial dysfunction with a focus on mechanisms associated with eNOS signaling. Cells derived from an immortalized human vascular endothelial cell line were treated with PCB77 to study nitrotyrosine formation through eNOS signaling. Phosphorylation studies of eNOS, caveolin-1, and kinases, such as Src, phosphatidylinositol 3-kinase (PI3K), and Akt, were conducted in cells containing either functional or small-interfering RNA-silenced caveolin-1 protein. We also investigated caveolin-1-regulated mechanisms associated with PCB-induced markers of peroxynitrite formation and DNA binding of NF-kappaB. Cellular exposure to PCB77 increased eNOS phosphorylation and nitric oxide production, as well as peroxynitrite levels. A subsequent PCB-induced increase in NF-kappaB DNA binding may have implications in oxidative stress-mediated inflammatory mechanisms. The activation of eNOS by PCB77 treatment was blocked by inhibitors of the Src/PI3K/Akt pathway. PCB77 also increased phosphorylation of caveolin-1, indicating caveolae-dependent endocytosis. Caveolin-1 silencing abolished both the PCB-stimulated Akt and eNOS phosphorylation, suggesting a regulatory role of caveolae in PCB-induced eNOS signaling. These findings suggest that PCB77 induces eNOS phosphorylation in endothelial cells through a Src/PI3K/Akt-dependent mechanism, events regulated by functional caveolin-1. Our data provide evidence that caveolae may play a critical role in regulating vascular endothelial cell activation and toxicity induced by persistent environmental pollutants such as coplanar PCBs.