2,3,7,8-Tetrachlorodibenzo-p-dioxin causes increases in expression of c-erb-A and levels of protein-tyrosine kinases in selected tissues of responsive mouse strains

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

TCDD (dioxin) induces a rapid inflammatory response from 3T3-L1 adipocytes as judged by prominent induction of the mRNA expression of prostaglandin-endperoxide synthase 2 (Cox-2) along with other inflammation markers within 1 h. This action of TCDD is clearly antagonized by cell pretreatment with AACOCF3 (an inhibitor of cPLA2), nifedipine (a Ca(2+) channel blocker), or 3'-methyl-4'-nitroflavone (MNF), an antagonist of the Ah receptor (AhR), suggesting the possible involvement of the nongenomic pathway of action of TCDD as shown previously in MCF10A cells [Dong, B., and Matsumura, F. (2008) Mol. Pharmacol. 74 (1), 255-263]. This early inflammatory action of TCDD is clearly different from that mediated by its classical action pathway in that the former is mediated by protein kinases such as PKC, PKA, and tyrosine kinases, but not by ARNT. Furthermore, the former is not blocked by two "DRE-decoy" treatments. Such an inflammatory effect of TCDD on 3T3-L1 adipocyes persists at least for 5 days, when the affected adipocytes exhibit significant reduction in their adipocyte characteristics. To assess the cause for the long-lasting influence of this nongenomic action of TCDD, we tested the effects of AACOCF3, exogenous arachidonic acid (AA), and H89 (an inhibitor of PKA) on the 5 day action of TCDD. These agents clearly antagonized all the long-term actions of TCDD except that on CYP1A1 induction, indicating that the influence of the nongenomic action of TCDD lasts a long time in this cell material. One of the major factors mediating its long-lasting effects has been identified to be PKA.

Dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin) enhances triggered afterdepolarizations in rat ventricular myocytes

The effects of 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) on action potential and afterdepolarizations were studied in rat ventricular myocytes using nystatin-perforated whole-cell patch-clamp technique. TCDD treatment, in the concentration range of 1 to 100 nM, significantly prolonged action potential duration measured at 90% of repolarization (APD90). The triggered delayedafterdepolarizations (DADs) were observed in 6 out of 8 cells after exposure of TCDD (10 nM). In the presence of isoproterenol (ISO, 10 nM) or Bay K 8644 (1 microM), TCDD (10 nM) markedly augmented the amplitude and frequency of the arrhythmogenic DADs and triggered sustained spontaneous firings in ventricular myocytes. Voltage-clamp data indicated that TCDD (10 nM) exposure significantly enhanced the transient inward current (Iti). The triggered earlyafterdepolarizations (EADs) were evoked only in cells simultaneously exposed to TCDD (10 nM) and ISO (or Bay K 8644). Further study indicated that TCDD treatment increased L-type Ca2+ current. These results indicate that activation of TCDD signaling pathway can prolong action potential duration and cause abnormal triggered afterdepolarizations. These effects may lead to clinically relevant ventricular arrhythmia especially when susceptible individuals are under elevated sympathetic stress or suffering from other myocardiopathies coincided with Ca2+-overload.

CYP1 and AhR expression in 7,12-dimethylbenz[a]anthracene-induced mammary carcinoma of rats prenatally exposed to 3,3′,4,4′,5-pentachlorobiphenyl

We previously reported the finding that prenatal exposure to a relatively low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) acted as an enhancing agent for 17-beta-estradiol (E2)-dependent 7,12-dimethylbenz[a]anthracene (DMBA)-induced rat mammary carcinoma, while a high dose decreased it. E2 is a known risk factor for mammary carcinoma, and CYP1A1 and 1B1 (CYP1) are the major enzymes catalyzing 2- and 4-hydroxylation of E2, respectively. We investigated the induction of CYP1 and aryl hydrocarbon receptor (AhR) in DMBA-induced mammary carcinoma using female Sprague-Dawley rats whose dams had been treated (i.g.) with 2.5 ng, 250 ng, 7.5 microg of PCB126/kg or the vehicle on days 13-19 post-conception. Immunohistochemical analysis revealed that the mammary carcinoma of the 250 ng group showed a significantly higher number of nuclei expressing estrogen receptor alpha (ER) and proliferating cell nuclear antigen (PCNA) compared to those of the other groups. Quantitative real-time RT-PCR analysis revealed that the 7.5 microg group showed a significantly higher level of CYP1A1 mRNA, and that the 250 ng group showed significantly higher levels of CYP1B1 mRNA. The level of AhR mRNA was significantly higher in both the 7.5 microg and 250 ng groups. Western blotting analysis was consistent with mRNA changes. It has been revealed that CYP1B1 catalyzes a step in the formation of 4-hydroxylated E2 metabolites, which show quite high mammary carcinogenicity. This study indicates that the enhancement of DMBA-induced mammary carcinogenicity in a relatively low PCB126 dose group might partially involve the higher expression of CYP1B1 and AhR in these carcinomas.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced impairment of glucose-stimulated insulin secretion in isolated rat pancreatic islets

We have explored the effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) administration on the secretory function of isolated rat pancreatic islets. Twenty-four hours after TCDD administration (1 microg/kg b.w., i.p.), rats showed no significant differences in plasma glucose, insulin, triglycerides and leptin levels whereas plasma-free fatty acids were significantly increased with respect to untreated controls. In isolated islets, DNA and protein content were unchanged, whereas insulin content was significantly decreased in TCDD-treated rats. Incubation with different concentrations of glucose demonstrated a significant impairment of glucose-stimulated insulin secretion in islets isolated from TCDD-treated rats, whereas insulin release was better preserved upon alpha-ketoisocaproate stimulation. A significant reduction of [3H]-2-deoxy-glucose uptake was observed in pancreatic tissue of TCDD-treated rats, whereas no significant reduction in GLUT-2 protein levels was detectable by immunoblotting in islets from TCDD-treated rats. We concluded that low-dose TCDD could rapidly induce significant alterations of the pancreatic endocrine function in the rat.

Aryl hydrocarbon receptor, cell cycle regulation, toxicity, and tumorigenesis

Most effects of exposure to halogenated and polycyclic aromatic hydrocarbons are mediated by the aryl hydrocarbon receptor (AHR). It has long been recognized that the AHR is a ligand-activated transcription factor that plays a central role in the induction of drug-metabolizing enzymes and hence in xenobiotic detoxification. Of late, it has become evident that outside this well-characterized role, the AHR also functions as a modulator of cellular signaling pathways. In this Prospect, we discuss the involvement of the AHR in pathways critical to cell cycle regulation, mitogen-activated protein kinase cascades, immediate-early gene induction, and the functions of the RB protein. Ultimately, the toxicity of AHR xenobiotic ligands may be intrinsically connected with the perturbation of these pathways and depend on the many critical signaling pathways and effectors with which the AHR itself interacts.

Dioxin increases C/EBPbeta transcription by activating cAMP/protein kinase A

The environmental pollutant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD = dioxin) has been shown to increase the expression of C/EBPbeta. The modulated expression of C/EBPbeta has been suggested to be associated with toxic responses of TCDD such as wasting syndrome, diabetes, and inhibition of adipocyte differentiation. This study focused on the regulatory mechanism of TCDD-mediated transcriptional activation of C/EBPbeta. Elevated C/EBPbeta mRNA and protein levels in mouse embryonic fibroblasts (C3H10T(1/2)) and in mouse hepatoma cells (Hepa1c1c7) were correlated with increased binding affinity of the C/EBPbeta protein. Transfection studies with different deletion constructs of the CCAAT/enhancer-binding protein promoter indicated that a small region located 60-120 bp upstream of the start site of transcription is required for activation of the C/EBPbeta gene by TCDD in both cell lines tested. Further analysis using mutation constructs of the C/EBPbeta promoter demonstrated that activation of the C/EBPbeta promoter is mediated through incomplete cAMP-response element-binding protein (CREB) sites located close to the TATA box of the C/EBPbeta gene. The protein kinase A (PKA) inhibitor H89 completely blocks the TCDD-dependent effect on C/EBPbeta promoter activity, indicating that TCDD activates CREB binding via a cAMP/PKA pathway, which is supported by the increased cAMP level and PKA activity observed after TCDD treatment. Gel shift analyses demonstrated that CREB itself binds to the putative CREB motif that mediates the TCDD-dependent effect on C/EBPbeta gene transcription. Cotransfection experiments with CREB and PKA expression plasmids further supported our conclusions that the TCDD-dependent effect on C/EBPbeta transcription is mediated via PKA-dependent CREB activation.

A dynamic role for the Ah receptor in cell signaling? Insights from a diverse group of Ah receptor interacting proteins

The aryl hydrocarbon (Ah) receptor (AhR) is a member of the basic helix-loop-helix PER-ARNT-SIM (PAS) transcription factor family. Consistent with the notion that PAS proteins are biological sensors, AhR binding to Ah toxicants induces or represses transcription of a wide range of genes and results in a cascade of toxic responses. However, an endogenous role for AhR in development and homeostasis is supported by (1) the discovery of low affinity, endogenous ligands; (2) studies demonstrating a role for the receptor in development of liver and vascular systems, that were established using mice lacking AhR expression; and (3) the presence of functional dioxin-responsive elements in promoter regions of genes involved in cellular growth and differentiation. A large body of recent literature has implicated AhR in multiple signal transduction pathways. AhR is known to interact with signaling pathways that are mediated by estrogen receptor and other hormone receptors, hypoxia, nuclear factor kappaB, and retinoblastoma protein. In addition, AhR complexes may affect cellular signaling through interactions with various other regulatory and signaling proteins, including PAS heterodimerization partners (ARNT), chaperone and immunophilin-like proteins (e.g. HSP90, XAP2/ARA9/AIP, p23), protein kinases and phosphatases (e.g. tyrosine kinases, casein kinase 2, protein kinase C), and coactivators (e.g. SRC-1, RIP 140, CBP/p300). Here we summarize the types of molecular cross talk that have been identified between AhR and cell signaling pathways.

Role of the aryl hydrocarbon receptor in cell cycle regulation

Traditionally, the aryl hydrocarbon receptor (AHR) is considered to be a ligand-activated receptor and transcription factor responsible for the induction of drug-metabolizing enzymes. Its role in the combinatorial matrix of cell functions was neatly established long before the first report of an AHR cDNA sequence was published. Only recently, other functions of this protein have begun to be recognized. This review addresses novel findings relating to AHR functions that have resulted from experimental approaches markedly outside traditional receptor analyses. Here we examine the aspects of AHR biology relevant to its role in cell cycle regulation, from the activation of mitogen-activated protein kinases to the cross-talk between AHR and the RAS pathway and the functional significance of the interaction between AHR and the retinoblastoma protein. We have attempted to provide the reader with a balanced interpretation of the evidence, highlighting areas of consensus as well as areas still being contested.

Polychlorinated biphenyls, polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans as endocrine disrupters--what we have learned from Yusho disease

Polychlorinated biphenyls (PCBs), polychlorinated dibenzo-p-dioxins, and polychlorinated dibenzofurans (PCDFs) are persistent environmental pollutants. In some areas wildlife reproduction has been affected by these compounds, which are recognized as endocrine disrupters. In 1968 in northern Kyushu in Japan about 2000 people were poisoned by PCBs and PCDFs (pyrolysis products of PCBs) which contaminated rice oil. Their condition was named "Yusho" disease. A similar poisoning by PCBs in Taiwan was named "Yu-Cheng" disease. The major symptoms of Yusho disease were dermal and ocular lesions, but some of the symptoms, such as irregular menstrual cycles and altered immune responses, were notable with respect to the endocrine disrupting activities of PCBs and related compounds. Several important observations relevant to the mechanisms of Yusho have been made from animal studies. For example, a coplanar PCB congener was shown to cause atrophy of the thymus and PCB administration was thought to alter androgen metabolism. The most tragic aspect of Yusho and Yu-Cheng diseases was the exposure of children to PCBs. In the case of Yu-Cheng, children exposed to PCBs in utero and lactationally were reported to have poor cognitive development. Intellectual impairment was also observed in children born to women who had eaten fish contaminated with PCBs in the United States. From animal studies, alterations in thyroid hormone status, modulation of protein kinase C, and changes in dopamine levels, etc. were proposed as the possible mechanisms for the adverse effects of PCBs on brain development. Whereas coplanar PCB and related congeners, e.g., 2,3,7,8-tetrachlorodibenzo-p-dioxin, induce gene expression via a ligand-dependent transactivating factor, the arylhydrocarbon receptor, alternative pathways for gene expression, e.g., c-Src and cross talk with the MAP kinase pathway, are also reviewed with respect to understanding the toxic mechanisms of these compounds. Finally, the "precautionary principle" is discussed for prevention of the health hazards caused by exposure to endocrine disrupters.

Expression of the aryl hydrocarbon receptor/transcription factor (AhR) and AhR-regulated CYP1 gene transcripts in a rat model of mammary tumorigenesis

Exposure to ubiquitous environmental chemicals, such as polycyclic aromatic hydrocarbons (PAH), may contribute to human breast cancer. In animals, PAH induce tumors in part by activating the aryl hydrocarbon receptor (AhR)/transcription factor. Historically, investigations into AhR-regulated carcinogenesis have focused on AhR-dependent transcriptional regulation of cytochrome P450 (CYP) enzymes which oxidize PAH to mutagenic intermediates. However, recent studies suggest that the AhR directly regulates cell growth. Given the postulated role of the AhR in carcinogenesis, we predicted that: (1) tissue predisposed to PAH tumorigenesis would express the AhR and (2) aberrant AhR and/or AhR-regulated gene expression would accompany malignant transformation. To test these hypotheses, AhR and CYP1 protein and/or mRNA levels were evaluated in rat mammary tumors induced with 7, 12-dimethylbenz[a]anthracene (DMBA), a prototypic PAH and AhR ligand. Results indicate modest AhR expression in normal mammary myoepithelial and ductal epithelial cells. In contrast, high AhR levels were detected in DMBA-induced tumors. Nuclear AhR localization in tumors suggested constitutive AhR activation. In situ hybridization and quantitative RT-PCR assays indicated high AhR mRNA levels in neoplastic epithelial cells. While both AhR-regulated CYP1A1 and CYP1B1 mRNAs were induced in breast tissue within 6 h of DMBA gavage, only CYP1B1 mRNA remained elevated in tumors. These results: (1) help explain targeting of breast tissue by carcinogenic PAH, (2) imply that AhR and CYP1B1 hyper-expression represent molecular biomarkers for, at least, PAH-induced mammary cell transformation, and (3) suggest mechanisms through which the AhR may contribute to carcinogenesis well after exogenous AhR ligands have been eliminated.

Expression of dioxin-related transactivating factors and target genes in human eutopic endometrial and endometriotic tissues

OBJECTIVE

Although an association between dioxin exposure and endometriosis has been proposed, the effects of this environmental toxin on human endometriosis are not known. To understand the potential underlying molecular mechanisms we studied the expressions of cytochrome P-450 genes (CYP1A1, CYP1A2, and CYP1B1 ), which are induced by dioxin, and the expressions of cytosolic receptor for dioxin, aryl hydrocarbon receptor, and its nuclear translocator, aryl hydrocarbon receptor nuclear translocator protein, in endometriotic and eutopic endometrial tissues.

STUDY DESIGN

Levels of transcripts of CYP1A1, CYP1A2, CYP1B1, aryl hydrocarbon receptor, and aryl hydrocarbon receptor nuclear translocator protein were determined by a quantitative reverse transcriptase-polymerase chain reaction and Southern blot assay in total ribonucleic acid samples from endometriotic and eutopic endometrial tissues. Eutopic endometrial tissue samples (n = 33) and endometriotic tissue samples (n = 10) were obtained at the time of uterine curettage and laparoscopy from disease-free women and from patients with endometriosis. Portions of these eutopic endometrial and endometriotic tissues were obtained simultaneously from the same patients (n = 8 pairs of samples). Levels of transcripts of CYP1A1, CYP1A2, CYP1B1, aryl hydrocarbon receptor, and aryl hydrocarbon receptor nuclear translocator protein were determined in endometrial and endometriotic tissues during follicular and luteal phases of the cycle and in cultured endometriotic stromal cells treated with forskolin, phorbol diacetate, medroxyprogesterone acetate,and serum.

RESULTS

Transcripts of dioxin receptor, its nuclear translocator, and two dioxin-induced target genes (CYP1A2 and CYP1B1) were demonstrated during follicular and luteal phases of the cycle in both eutopic endometrial tissues and tissues affected by pelvic endometriosis, with no readily detectable differences between these tissues. On the other hand, levels of transcripts of another dioxin-induced gene, CYP1A1, were found to be strikingly higher in endometriotic tissues than in the eutopic endometrium. Mean levels in endometriotic tissues were 8.7 times those found in eutopic endometrium. Various hormonal treatments of endometriotic stromal cells did not significantly alter these levels.

CONCLUSION

We demonstrated for the first time the expression of dioxin-related transcription factors aryl hydrocarbon receptor and aryl hydrocarbon receptor nuclear translocator protein and target genes CYP1A1, CYP1A2, and CYP1B1 in endometriotic tissues and stromal cells. Strikingly elevated CYP1A1 transcripts in endometriosis may give rise to significantly increased P-4501A1 enzyme activity and thus promote the development and growth of endometriosis by either activating procarcinogens or inducing the formation of catechol estrogens or both. In fact, the proposed link between dioxin exposure and endometriosis may be explained in part by the up-regulation of the CYP1A1 gene expression in endometriotic tissues.

Effects of environmental synthetic chemicals on thyroid function

Synthetic chemicals are released into the environment by design (pesticides) or as a result of industrial activity. It is well known that natural environmental chemicals can cause goiter or thyroid imbalance. However, the effects of synthetic chemicals on thyroid function have received little attention, and there is much controversy over their potential clinical impact, because few studies have been conducted in humans. This article reviews the literature on possible thyroid disruption in wildlife, humans, and experimental animals and focuses on the most studied chemicals: the pesticides DDT, amitrole, and the thiocarbamate family, including ethylenethiourea, and the industrial chemicals polyhalogenated hydrocarbons, phenol derivatives, and phthalates. Wildlife observations in polluted areas clearly demonstrate a significant incidence of goiter and/or thyroid imbalance in several species. Experimental evidence in rodents, fish, and primates confirms the potentiality for thyroid disruption of several chemicals and illustrates the mechanisms involved. In adult humans, however, exposure to background levels of chemicals does not seem to have a significant negative effect on thyroid function, while exposure at higher levels, occupational or accidental, may produce mild thyroid changes. The impact of transgenerational, background exposure in utero on fetal neurodevelopment and later childhood cognitive function is now under scrutiny. There are several studies linking a lack of optimal neurological function in infants and children with high background levels of exposure to polychlorinated biphenyls (PCBs), dioxins, and/or co-contaminants, but it is unclear if the effects are caused by thyroid disruption in utero or direct neurotoxicity.

Alterations in the growth factor signal transduction pathways and modulators of the cell cycle in endocervical cells from macaques exposed to TCDD

After more than a year had elapsed since a single oral exposure to 2 and 4 microgram 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)/kg, there was an apparent dose-related increased incidence of significant endocervical squamous metaplasia in a group of cynomolgus macaques (Scott et al., 1998). In the present experiments we investigated the mechanisms by which chemicals like TCDD could induce epithelial cell transdifferentiation in the primate endocervix. One focus of investigation was epidermal growth factor receptor (EGFR) and the key cytosolic signaling kinases, c-Src and protein tyrosine kinase (PTK), whose responses to TCDD are well characterized. A second focus was the distal kinase Erk2 that transduces the cytosolic signal into a nuclear signal, and which in combination with nuclear casein kinase II (CKII), can lead to activation of p53. Finally, we studied three key target proteins of activated p53 (wafl/p21, Cdc2 p34, and Cdk4), whose modulation could produce cell cycle effects. The studies were carried out using primary cell cultures prepared from endocervical epithelium recovered at necropsy from TCDD-treated (2 and 4 microgram TCDD/kg) and untreated macaques. There was a significant decrease in EGFR binding activity in cells from TCDD-treated animals as compared to controls. A marked increase in the protein amount of H-Ras and a significant increase in the activity of c-Src kinase, PTK, and Erk2 were found in cells from TCDD-treated animals. A significant decrease in the activity of CKII and in the protein amount of p53, wafl/p21, and Cdc2 p34 was found. On the other hand, a substantial increase in the protein amount of Cdk4 and DNA binding activity of AP-1 was found in cells from TCDD-treated animals. In vitro experiments using primary cultures of endocervical cells from untreated macaques revealed that these cells have AhR, and that c-Src protein is functionally attached to the AhR and is specifically activated upon ligand binding as judged by the following criteria. (1) A structure-activity relationship study with TCDD and three dioxin congeners revealed a rank order for their potency in activation of AhR-associated c-Src kinase from cervical cells which was identical to that of previously determined toxicity indices. (2) TCDD-induced, AhR-associated c-Src kinase activity was abolished when an AhR immunoprecipitate from cervical cells was preincubated with alpha-naphthoflavone (AhR blocker) or geldanamycin (Src kinase inhibitor) prior to the addition of TCDD. (3) The analysis of the AhR complex showed three proteins of molecular weights of 100 (AhR), 90, and 60 kDa. (4) The same protein with molecular weight 60 kDa was found when the immunoprecipitate with anti AhR-antibody was analyzed by SDS-PAGE, then transferred into nitrocellulose membrane followed by immunobloting the membrane with anti c-Src-antibody. Our data suggest that TCDD induced pathology in endocervical cells through changes in growth factor receptor signaling, other cytosolic signaling proteins, tumor suppressor proteins, and cell cycle proteins.

The Ah receptor is not involved in 2,3,7,8-tetrachlorodibenzo- p-dioxin-mediated apoptosis in human leukemic T cell lines

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a common environmental pollutant causing public concern. Its toxic effects include disruption of the immune, endocrine, and reproductive systems, impairment of fetal development, carcinogenicity, and lethality in rodents. Here, we report that TCDD induces apoptosis in two cultured human leukemic lymphoblastic T cell lines. This cell death was found not to be dependent on an aryl hydrocarbon receptor and to be inhibited by the inhibitor of tyrosine kinases and caspases. Apoptosis-linked c-Jun N-terminal kinase is rapidly activated in these cells by the treatment with TCDD. A dominant-negative mutant of c-Jun N-terminal kinase prevented cell death in the treatment with TCDD. Furthermore, TCDD decreases the Bcl-2 protein level in these cell lines. These findings will help in the understanding of the molecular mechanism underlying TCDD-mediated immunotoxicity.

Macrophage Activation by Polycyclic Aromatic Hydrocarbons: Evidence for the Involvement of Stress-Activated Protein Kinases, Activator Protein-1, and Antioxidant Response Elements

Polycyclic aromatic hydrocarbons (PAH) contained in fossil fuel combustion particles enhance the allergic response to common environmental Ags. A key question is: what are molecular pathways in the immune system by which PAH and conversion products drive allergic inflammation? Circumstantial evidence suggests that macrophages are involved in PAH-induced responses. We demonstrate that a representative PAH, β-napthoflavone (BNF), and a representative quinone metabolite, tert-butylhydroxyquinone (tBHQ), induce Jun kinase and p38 mitogen-activated protein kinase activities in parallel with the generation of activator protein-1 (AP-1) mobility shift complexes in THP-1 and RAW264.7 macrophage cell lines. Activation of mitogen-activated protein kinases was dependent on generation of oxidative stress, and could be inhibited by N-acetylcysteine. Another genetic response pathway linked to PAH is the antioxidant response element (ARE), which regulates expression of detoxifying enzymes. BNF and tBHQ activated a human ARE (hARE) reporter gene in RAW264.7 cells. Interestingly, bacterial lipopolysaccharide also induced hARE/chloramphenicol acetyltransferase activity. While the hARE core, GTGACTCAGC, contains a consensus AP-1 sequence (underlined), AP-1 was not required for hARE activation. This suggests that PAH and their conversion products operate via ARE-specific transcription factors in the immune system. BNF and tBHQ did, however, induce AP-1 binding to the hARE, while constitutively active Jun kinase interfered in hARE/chloramphenicol acetyltransferase activation. This suggests that AP-1 proteins negatively regulate the hARE. These data establish important activation pathways for PAH in the immune system and provide us with targets to modulate the effect of environmental pollutants on allergic inflammation.

Mechanisms of gender-specific TCDD-induced toxicity in guinea pig adipose tissue

After treatment with TCDD, the activities of cytosolic AhR-associated c-Src kinase, microsomal protein kinase C (nPKC epsilon), microsomal c-Src kinase, nuclear p44/42 MAPK, c-Jun N terminus kinase, and the amount of microsomal pan-Ras protein were different in males and females. TCDD did not decrease body or adipose tissue weights in transgenic src-deficient male mice as compared to their wild-type littermates, and the activity of AhR-associated c-Src kinase was not increased by TCDD in src-deficient male mice. Similar results were obtained when TCDD was given to male guinea pigs treated with the Src-kinase inhibitor, geldanamycin. Treatment with estradiol protected male guinea pigs from TCDD-induced wasting. TCDD induced similar changes in protein tyrosine kinase activity in adipose tissues of castrated male and intact female guinea pigs. The gender-specific mechanisms of TCDD-induced toxicity appear to involve c-Src kinase, nPKC epsilon, and pan-Ras, as well as overlap in the cytosolic signal transduction pathways of TCDD and sex steroids.

Sustained increase in intracellular free calcium and activation of cyclooxygenase-2 expression in mouse hepatoma cells treated with dioxin

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a non-genotoxic environmental pollutant that causes multiple adverse effects in experimental animals and in humans. We show here that TCDD treatment of mouse hepatoma cells causes a rapid mobilization of intracellular calcium both in wild type Hepa-1 cells and in its c2 variant, a cell line that has highly reduced levels of functional aromatic hydrocarbon (Ah) receptor (AHR). In wild type cells, but not in the c2 variant, TCDD treatment leads to a sustained elevation of cytosolic free calcium. TCDD also induces elevated levels of cyclooxygenase-2 (COX-2) mRNA in wild type and in c37, a CYP1A1-deficient cell line, but not in c2 cells. Induction of Cox-2 is in fact dependent on the presence of a functional Ah receptor, since it can be blocked by antisense oligonucleotides to Ah receptor mRNA. Most likely as a consequence of Cox-2 induction, we find a significant increase in the level of 12-hydroxyheptadecatrienoic acid (12-HHT) secreted from TCDD-treated Hepa-1 cells. In addition, we observe elevated levels of 6-keto prostaglandin F1alpha in c2 cells and high levels of secreted prostaglandin F2alpha in c2, c37 and c4, the variant cell line lacking aromatic hydrocarbon nuclear translocator protein. These data suggest that Cox-2 activation by TCDD leads to the release of prostaglandins, eicosanoids and other mediators which may have an important role in the biological and toxic effects of TCDD.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) causes an Ah receptor-dependent and ARNT-independent increase in membrane levels and activity of p60(Src)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is known to affect various cellular activities including growth factor signal transduction, hormone responses, and cell differentiation. The purpose of this study was to examine more closely the very early effects of TCDD on protein tyrosine kinase activity, specifically p60(Src). We found that TCDD causes rapid changes in the plasma-microsomal membrane levels and activity of p60(Src) in Hepa 1c1c7, Hepa c4 cells, and SR3Y1 cells, a p60(v-Src) overexpressing cell line. Such cellular changes occur within 30 minutes after 10 nM TCDD treatment, as measured by Western blot analysis. TCDD's ability to increase p60(Src) levels was found to be: (1) dose-dependent, with an estimated EC(50) between 10(-10) and 10(-11) M TCDD; (2) Ah receptor (AhR)-dependent, since TCDD's effect was blocked by co-administration with 1 μM α-naphthoflavone, an AhR antagonist; and interestingly (3) ARNT-independent, since TCDD's effect was observed in Hepa c4 cells, an ARNT(-) mutant cell line. Since ARNT is a heterodimerization partner of the AhR required for binding of the ligand-activated AhR to dioxin-responsive elements on DNA in the nucleus to transactivate genes controlled by the AhR, an alternative mechanism for TCDD's action is discussed which does not require ARNT. Along with increased membrane levels of p60(Src), we observed a corresponding increase in the activity of a 60 kDa protein tyrosine kinase using two different kinase detection assays. This effect of TCDD was also found to be AhR-dependent, ARNT-independent, and independent of de novo protein synthesis since cycloheximide was unable to completely abolish TCDD's effect. The present findings provide a potentially important mechanism by which TCDD can alter cell growth and differentiation.

Environmental toxins and endometriosis

The effect of natural, environmental, or manufactured substances on endocrine organs is a current topic of discussion. Recently, some toxicants have been suggested to promote endometriosis; therefore, attention has turned to the role of environmental factors as a stimulant for endometriosis growth and maintenance. This article reviews the influence of various toxicants on endometriosis.

Phenobarbital induction of CYP1A1 gene expression in a primary culture of rainbow trout hepatocytes

In mammals, phenobarbital (PB) is an in vivo inducer of the cytochrome P4502B (CYP2B) family, whereas in teleosts PB induction of cytochrome P450 is unclear. We show that teleost cytochrome P4502K1 (CYP2K1) protein levels and 7-pentoxyresorufin-O-deethylase activity were not induced by exposure of primary cultures of rainbow trout hepatocytes to PB. Instead, cytochrome P4501A1 (CYP1A1) gene expression was strongly induced by PB, based upon observations of marked increases in CYP1A1 mRNA, CYP1A1 protein, and 7-ethoxyresorufin-O-deethylase activity. In accordance with these data we provide a temporal study employing antibodies for the aromatic hydrocarbon (Ah) receptor that showed an increase in Ah receptor in nuclear extracts prepared from cells exposed to PB. Employment of the electrophoretic mobility shift assay (EMSA) showed PB to cause activation or "transformation" of the Ah receptor in nuclear extracts. Studies employing actinomycin D and cycloheximide indicated that PB induction of CYP1A1 was regulated at both the transcriptional and post-transcriptional levels. Nuclear run-off experiments confirm that PB causes an increase in CYP1A1 transcription. Inhibition of protein synthesis led to the superinduction of CYP1A1 mRNA, suggesting the regulation of teleost CYP1A1 may involve a labile repressor protein. These findings suggest that PB induction of the CYP1A1 gene involves the Ah receptor and is via transcriptional activation.

Phosphorylation of c-Jun stimulated in primary cultured rat liver parenchymal cells by a coplanar polychlorinated biphenyl

Phosphorylation of c-Jun was stimulated in primary cultured rat liver parenchymal cells by treatment with a coplanar polychlorinated biphenyl congener, 3,3'4,4'5-pentachlorobiphenyl (PenCB), as well as by epidermal growth factor, but was not stimulated by the non-coplanar form. However, the amount of c-Jun mRNA did not increase with PenCB treatment. PenCB may activate a signal-transducing pathway consisting of protein kinases.

Developmental effects of dioxins

The potent developmental toxicity of dioxin in multiple species has been known for a number of years. However, recent studies have indicated that dioxin also induces functional developmental defects, many of which are delayed. Subtle structural deficits, not detectable at birth, have also been described in multiple species and in both sexes. Certain defects have been reported not only in animals but also in children prenatally exposed to complex mixtures containing dioxinlike compounds. None of the effects can be attributed to modulation of any one endocrine system. For example, dioxin does not bind to the estrogen receptor, but it can cause effects that are both estrogenic and antiestrogenic. However, viewing dioxin and related compounds as endocrine disruptors that may alter multiple pathways sheds some light on the complexities of this potent class of growth dysregulators.

The mechanism of dioxin toxicity: relationship to risk assessment

Risk characterization involves hazard identification, determination of dose-response relationships, and exposure assessment. Improvement of the risk assessment process requires inclusion of the best available science. Recent findings in the area of dioxin toxicity have led to a major effort to reassess its risk. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), commonly referred to as "dioxin," is the most toxic member of a class of related chemicals including the polyhalogenated dibenzo-p-dioxins, dibenzofurans, biphenyls, naphthalenes, azo- and azoxy-benzenes, whose toxicities can be expressed as fractional equivalencies of TCDD. These chemicals exert their effects through interaction with a specific intracellular protein, the Ah receptor. While binding to the receptor is necessary, it is not sufficient to bring about a chain of events leading to various responses including enzyme induction, immunotoxicity, reproductive and endocrine effects, developmental toxicity, chloracne, tumor promotion, etc. Some of these responses appear to be linear at low doses. Immunotoxicity and effects on the reproductive system appear to be among the most sensitive responses. The Ah receptor functions as a transcriptional enhancer, interacting with a number of other regulatory proteins (heat shock proteins, kinases, translocases, DNA binding species). Interaction with specific base sequences in the DNA appear to be modulated by the presence of other growth factors, hormones and their receptors as well as other regulatory proteins. Thus, dioxin appears to function as a hormone, initiating a cascade of events that is dependent upon the environment of each cell and tissue. While Ah receptor variants exist, all vertebrates examined have demonstrated such a protein with similar numbers of receptors and binding affinity for TCDD. Most species respond similarly to dioxin and related compounds. While a given species may be an outlier for a given response, it will behave like other animals for other responses. For both in vivo and in vitro end points where animal and human data exist, such as enzyme induction, chloracne, immunotoxicity, developmental toxicity, and cancer, the sensitivity of humans appears similar to that of experimental animals. Current levels of environmental exposure to this class of chemicals may be resulting in subtle responses in populations at special risk such as subsistence fisherman and the developing infant, as well as in the general population. Increased understanding of the mechanism of dioxin's effects as well as elucidation of exposure-dose relationships is leading to the development of a biologically based dose-response model in the ongoing process of incorporating the best science into the risk assessment of TCDD and related compounds.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced reduction of adenosine deaminase activity in vivo and in vitro

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent immunosuppressant in several animal species. The purpose of this study was to determine if TCDD affected the activity of adenosine deaminase (ADA), a purine metabolizing enzyme that is vital to the proper functioning of the immune system. The effect of TCDD on ADA activity was studied in various tissues of male Balb/c mice (a TCDD-responsive strain) and DBA/2 mice (a less-responsive strain). Of the tissues examined after administration of TCDD in vivo (115 micrograms/kg, i.p.), ADA activity was found to be significantly reduced in thymic and splenic tissues of Balb/c mice at 24 hours postadministration. The enzyme activity in these affected tissues remained consistently low through 10 days postadministration. Such an effect of TCDD was both dose and time related in the thymic tissue of Balb/c mice. In contrast, no appreciable alterations in ADA activity were evident in any of the tissues of DBA/2 mice at any of the sampling intervals, indicating that such an effect of TCDD is likely to be mediated through the Ah receptor. This in vivo effect of TCDD on thymic ADA activity was also reproducible in situ where isolated whole thymuses were directly incubated with 10 nM TCDD. In this model, TCDD's effects on ADA activity were antagonized by known protein kinase or phosphorylation inhibitors such as quercetin, genistein, tyrphostin, and neomycin. These results indicate that the effect of TCDD on ADA activity in the thymus may be related to its property to elevate protein kinase activities in this tissue. ADA activity was also reduced in 3T3 cells that were treated with 10 nM TCDD in a low (1%) serum media. In contrast, 25 ng/mL epidermal growth factor (EGF) under such conditions consistently stimulated ADA activity. Interestingly, EGF at a similar concentration failed to elicit a stimulatory effect on ADA activity when cells were pretreated with TCDD. The property of TCDD to lower ADA activity under in vivo, in situ, as well as in vitro conditions appears to be largely related to its action to modulate protein phosphorylation activities.

Significance of TCDD-induced changes in protein phosphorylation in the adipocyte of male guinea pigs

The chemical TCDD (2,3,7,8-tetrachlorodibenzo-p-dioxin) at very low concentrations has been found to cause a rapid rise in protein phosphorylation activities in the extranuclear fraction (i.e., cytosol and cellular membranes) of the adipose tissue from male guinea pigs. The effect occurred both in vivo and when TCDD was added to adipose in tissue culture (in situ). To study the cause for such a TCDD-induced biochemical change, isolated adipose tissues were treated with TCDD in conjunction with various diagnostic agents known to affect protein synthesis (actinomycin D and cycloheximide) and protein phosphorylation activities (genistein). The stimulatory effect of TCDD on protein phosphorylation was not totally abolished by actinomycin D; however, cycloheximide and genistein partially suppressed the effect of TCDD. Such an action of TCDD was accompanied by a quick rise in ras GTP binding activity as well as a rise in phosphorylation of nuclear c-myc protein product within 10 minutes after TCDD addition, indicating that TCDD is likely to activate the signal transduction pathway for growth factors. In view of the absence of an inhibitory action of actinomycin D and the short time required for TCDD to induce these changes, we have formulated a working hypothesis that stimulation of protein phosphorylation activities by TCDD may not be mediated by a transcriptional process. To test this possibility, an extranuclear fraction was obtained from the homogenate of adipose tissue, and the effect of TCDD was studied in vitro cell- and nucleus-free conditions. It was found that TCDD was capable of activating protein phosphorylation activity under cell free conditions even in the absence of nuclei. Such an action of TCDD is dependent on the action of the Ah receptor.

Biphasic modulation of protein kinase C (PKC) activity by polychlorinated dibenzo-p-dioxins (PCDDs) in serum-deprived rat aortic smooth muscle cells

Previous studies in this laboratory have shown that benzo(a)pyrene (BaP) modulates protein kinase C (PKC)-mediated phosphorylation of aortic smooth muscle cell (SMC) proteins. This observation is consistent with the ability of other aromatic hydrocarbons (AHs), such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), to modulate kinase activities in cells of hepatic, testicular, and thymic origin. Because all these chemicals share the ability to bind the aryl hydrocarbon receptor (AhR), the present studies were conducted to determine if changes in PKC activity by AHs conform with established structure-activity relationships. Experiments were conducted to examine the effects of TCDD, 2,3,7,8-tetrachlorodibenzofuran (TCDF), and 2,8-dichlorodibenzodioxin (DCDD) on the phosphorylation of exogenous histone type-III under basal and PKC-activating conditions. These congeners exhibit both high (TCDD and TCDF) and low (DCDD) AhR agonist activities. Measurements of kinase activity were conducted in the cytosolic and particulate fractions of growth-arrested (i.e., serum-deprived) cultured rat aortic SMCs incubated with 10 nM TCDD, TCDF, and DCDD for 0.5, 12, or 24 hours. No changes in basal kinase activity were induced by these chemicals at any of the times tested. Significant decreases in cytosolic and particulate PKC activity relative to controls were observed upon exposure of SMCs for 0.5 hours to 10 nM TCDD, TCDF, and DCDD. In contrast, SMCs exposed to TCDD and TCDF for 12 hours exhibited a significant increase in PKC activity in both cytosolic and particulate fractions. The PKC activity in cells exposed to DCDD for 12 hours was not altered.(ABSTRACT TRUNCATED AT 250 WORDS)

3,3',4,4'-Tetrachlorobiphenyl inhibits proliferation of immature thymocytes in fetal thymus organ culture

The environmental pollutant 3,3',4,4'-tetrachlorobiphenyl (TCB) leads to thymic atrophy and immunosuppression, the former possibly causing the latter. TCB binds to the cytosolic aryl-hydrocarbon receptor (AhR) and transforms it into a DNA-binding state. The development of fetal thymocytes is severely affected by TCB and other AhR-binding xenobiotics, leading to a skewed pattern of thymocyte maturation stages. Murine thymocyte proliferation after exposure to TCB was studied in fetal thymus organ culture (FTOC). C57BL/6 fetus thymic lobes from day 15 of gestation were explanted and grown for 2, 4, 6, and 8 days in organ culture in the presence or absence of 3.3 microM TCB. Subsets of thymocytes were defined by CD4 and CD8 surface markers, and their cell cycle was analysed by DNA staining with 7-amino-actinomycin D (7-AAD). Exposure of fetal thymi in vitro to 3.3 microM TCB significantly reduced the total number of thymocytes, and fewer thymocytes were in S/G2M phase. The inhibition of cell proliferation induced by TCB treatment affected mainly the CD4-CD8- (double-negative, DN) and CD4-CD8+ (single-positive, SP) subsets, and these inhibition appeared mainly in more immature thymocytes, i.e. DNCD3- and CD8+CD3- subpopulations, whereas no effect of TCB on CD4+CD8+ (double-positive, DP) cell proliferative activity was observed. Analysis of the relation of cell proliferation and development of subsets in differentiating fetal thymocytes suggests that TCB enhanced thymocyte differentiation into mature CD8+ cells.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD)-induced changes in glucose transporting activity in guinea pigs, mice, and rats in vivo and in vitro

The toxic action of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on glucose uptake was studied on different species, sexes, and strains of animals by using a nonmetabolizable glucose analog, 3-0-methyl D [1-3H] glucose (3H-Me-glc). We have found a drastic reduction in glucose uptake in different organs from male guinea pigs in vivo as well as in vitro. Female guinea pigs were less responsive to this effect of TCDD compared to males. Highly TCDD-responsive mice strains responded in the same way as the male guinea pigs, while less TCDD-responsive mice strains did not. A reduction in glucose uptake was found in explant adipose tissue cultures (in vitro) of male mice (highly TCDD-responsive) after a 4 hour incubation with 10(-8) M TCDD. In less responsive mice strains, stimulation was observed using the same model (in vitro). In the case of male rats, as well as male guinea pigs, glucose uptake was highly reduced by 10(-8) M TCDD using an in vitro system of explant adipose tissue. The Ah-receptor blocker, 4,7-phenanthroline, abolished the effect of TCDD on inhibition of glucose uptake in adipose tissue of male guinea pigs both in vivo and in vitro. Transcription and translation inhibitors (actinomycin D and cycloheximide, respectively) were tested separately or in combination with TCDD using the guinea pig adipose tissue culture model. It was found that actinomycin D could block the effect of TCDD on glucose uptake throughout the experiment time (360 min) while cycloheximide blocked TCDD action for about 60 minutes. Protein kinase inhibitors (e.g., genistein, neomycin) did not change the effect of TCDD on glucose uptake in male guinea pig adipose tissue. Based on these observations, we conclude that TCDD inhibits Me-glc uptake in various organs in guinea pigs, mice, and rats, and this response is mediated by the Ah-receptor.

Polychlorinated biphenyls modulate protooncogene expression in Chang liver cells

n Chang liver cells we studied the influence of polychlorinated biphenyls (PCB) on the expression of different protooncogenes. In cells incubated with medium supplemented with PCB we observed an early effect after 3 h on c-erbA and c-erbB RNA level. This reduction of RNA was due to a delayed transcriptional activation of the genes. The PCB congener 3,3',4,4',5-pentachlorobiphenyl (5-CB) in a 1,000-fold lower concentration influenced protooncogene expression in the same manner. In contrast c-raf RNA level increased transiently after 24 h. The fact that persistent chemicals like PCB interfere with protooncogene expression is particularly interesting in view of their tumor promoting activity.

Expression of glutathione S-transferase P-form in primary cultured rat liver parenchymal cells by coplanar polychlorinated biphenyl congeners is suppressed by protein kinase inhibitors and dexamethasone

Glutathione S-transferase P-form (GST-P, EC 2.5.1.18) mRNA was expressed by epidermal growth factor as well as by 3,4,5,3',4'-penta-chlorinated biphenyl (PenCB) in primary cultured rat liver parenchymal cells. The expression of GST-P was suppressed by inhibitors of protein kinase C and dexamethasone, an antagonist of AP-1 transcription factor activity, whereas expression of cytochrome P450IA2 by PenCB was not affected by these reagents. The AP-1 related transcription factor may be essential for the expression of GST-P by PenCB as also may be a protein kinase C type enzyme.

Neutrophil function after exposure to polychlorinated biphenyls in vitro

Polychlorinated biphenyls (PCBs) are known to be immunotoxic, yet the effects on neutrophil (PMN) function are not well characterized. We incubated PMNs isolated from rat peritoneum with a mixture of PCB congeners, Aroclor 1242, in the absence or presence of either phorbol myristate acetate (PMA) to stimulate generation of superoxide anion (O2-) or N-formyl-methionyl-leucyl-phenylalanine (fMLP) to induce degranulation (measured as release of beta-glucuronidase). Aroclor 1242 alone stimulated O2- production at a concentration of 10 micrograms/ml. Significant cytotoxicity was not observed under these conditions. This concentration of Aroclor 1242 also increased O2- generation in PMNs activated with 20 ng PMA/ml. In the presence of a concentration of PMA (2 ng/ml) that by itself did not stimulate production of O2-, 1 microgram Aroclor 1242/ml caused significant generation of O2-, indicating synergy between Aroclor 1242 and PMA. Aroclor 1242 caused release of beta-glucuronidase from quiescent PMNs; however, in PMNs stimulated with fMLP to undergo degranulation, Aroclor 1242 inhibited release of beta-glucuronidase. The effects of two PCB congeners, one that binds to the Ah receptor (3,3', 4,4'-tetrachlorobiphenyl) and one that has little affinity for this receptor (2,2', 4,4'-tetrachlorobiphenyl) were examined. 3,3', 4,4'-Tetrachlorobiphenyl had no effect on PMN function in vitro, whereas 2,2', 4,4'-tetrachlorobiphenyl had effects similar to those observed with Aroclor 1242. These results indicate that PCBs affect PMN function in vitro in a complex manner, stimulating or inhibiting function under different conditions. These effects are apparently not mediated through the Ah receptor.

Role of the Ah receptor and the dioxin-inducible [Ah] gene battery in toxicity, cancer, and signal transduction

1. On the basis of our current knowledge about the evolution of drug-metabolizing enzymes, it appears to be extremely likely that these enzymes play a critical role in maintaining steady-state levels of the ligands involved in ligand-modulated transcription of genes effecting growth, differentiation, homeostasis, and neuroendocrine functions. 2. The original observations about genetic differences in CYP1A1 (cytochrome P1-450) induction by TCDD or benzo[a]pyrene in the mouse have led to an appreciation for a similar polymorphism in the human and the recent cloning of the murine Ah receptor (Ahr) and human Ah receptor nuclear translocator (ARNT) genes. It is most likely that the correlation between genetic differences in human or murine CYP1A1 inducibility by polycyclic hydrocarbons or TCDD and increased risk of cancer will be explained by differences in the AHR gene, leading to enhanced tumor promotion (rather than in the CYP1A1 structural gene). Perhaps the same will be found for birth defects, immunotoxicity, and other forms of toxic damage caused by these environmental chemicals. 3. In a manner similar to that of the phorbol ester tumor promoter, TCDD induces intracellular Ca2+ changes, accumulation of FOS and JUN mRNAs, and large increases in AP-1 transcription factor activity. Interestingly, these early effects of TCDD, and also of benzo[a]pyrene, appear not to require the Ah receptor. 4. Many genes are induced by TCDD, and many others are induced by electrophilic metabolites such as quinones and H2O2; using several mouse experimental systems, we have defined a subset of six of these genes as constituting the [Ah] battery by the sole criterion that a functional CYP1A1 or CYP1A2 enzyme is able to repress the expression of genes that are members of this gene battery.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) alters pancreatic membrane tyrosine phosphorylation following acute treatment

To understand the basic mechanisms of TCDD's action to cause hypoinsulinemia in several experimental animals, we have studied TCDD-induced changes in various protein kinase activities in membrane preparations of guinea pig pancreas. For this purpose, young male guinea pigs were treated through a single intraperitoneal injection with 1 or 3 micrograms/kg of TCDD in vivo, and, after given time periods, pancreas samples were obtained and membranes were isolated through homogenization and centrifugation procedures. Several sets of incubation conditions were selected for protein kinase activity assay, each favoring a specific type of protein kinase. It was found that overall protein phosphorylation activities were higher in the preparation from TCDD-treated animals as compared to those found in pair-fed controls and that this trend was more pronounced when the assay medium contained Mn2+ in place of Mg2+ and EGTA. These are the conditions that are known to favor protein tyrosine kinases. Other types of protein kinases from the treated animals did not show any significant differences from the pair-fed control animals, though that of protein kinase C in the treated preparation showed a modest increase. To establish that the type of protein kinases stimulated by TCDD are protein tyrosine kinases, we have carried out phosphoamino acid analyses, KOH digestion, and western blot analyses using an antibody to phosphotyrosine. All the results were consistent in supporting the idea that TCDD causes a rise in protein-tyrosine kinases in pancreas at early stages of poisoning.

2,3,7,8-Tetrachlorodibenzo-P-dioxin induced alterations in protein phosphorylation in guinea pig adipose tissue

An in situ (explant tissue culture) model has been developed to study the effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), hormones, and growth factors either alone or in combination. In our model system, the effect of TCDD on protein phosphorylation was greatly affected by the presence or the absence of externally added D-glucose. In the presence of a physiologically relevant level of glucose (13.3 mM), TCDD clearly stimulated protein phosphorylation as in the case of in vivo data. However, in the absence of D-glucose, TCDD clearly inhibited protein phosphorylation. On the other hand, TCDD reduced the glucose uptake activity in isolated adipose tissue either in the presence or absence of D-glucose (13.3 mM). Therefore, the TCDD-induced reduction of glucose transport does not appear to be related directly to the simultaneous rise in protein phosphorylation. For comparison, several agents which are known to affect protein phosphorylation were tested. These hormonal agents generally affected the TCDD-untreated adipose tissues in the manner expected from their known actions, indicating that this in situ model is an adequate system to study their independent actions. The TCDD-treated adipose tissue samples showed only mild or insignificant response to these hormonal stimuli. In terms of the changes in the pattern of protein phosphorylation activities, the action of TCDD appeared to resemble that of EGF and T3. Since under in situ conditions no agents such as EGF or T3 can be expected to be present, the observed TCDD-induced changes are suggestive of the basic intracellular changes in cellular activities. The types of TCDD-induced protein kinases appear to be protein tyrosine kinases and protein kinase C.

Environmental toxicology of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans

Few environmental compounds have generated as much interest and controversy within the scientific community and in the lay public as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Their ubiquitous presence in the environment and the risk of accidental exposure has raised concern over a possible threat of PCDDs or PCDFs to human health. The most extensively studied and potent isomer is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD or dioxin). Dioxin is a multisite toxicant in laboratory rodents resulting in a number of tissue-, species-, and sex-dependent responses. Much has been learned about the mechanism of dioxin's effects, especially for the induction of cytochrome P-450 enzymes. Binding of PCDDs and PCDFs to a receptor protein, termed the dioxin or Ah receptor, is necessary for most biological and toxic responses. The most common toxic response used for evaluating the human health risk posed by PCDDs and PCDFs is the hepatocarcinogenic response observed primarily in rodents. Despite extensive research efforts, the effects of PCDDs and PCDFs on humans are not well characterized. However, available data indicate there is good agreement between known effects of dioxin in laboratory animals and those described in epidemiological studies for effects in humans. The sequence in events initiated by the Ah receptor interacting with dioxin-responsive genes and ending with altered patterns of differentiation and growth must be sought in order to understand tissue, species, sex, and interindividual variation in biological responses and the health risk posed by PCDDs and PCDFs.

Stimulatory effect of the CYP1A1 inducer 2,3,7,8-tetrachlorodibenzo-p-dioxin on the reproduction of HIV-1 in human lymphoid cell culture

1. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was an inducer of microsomal benzo[alpha]pyrene hydroxylase (AHH) and 7-ethoxyresorufin O-deethylase (EROD) in MT-4 human lymphoid cell culture. 2. The monoclonal antibody (Mab), 1-7-1 and the immunodepleted polyclonal antibody (Pab), anti-CYP1A1(-A2), inhibit AHH and EROD activities pre-induced by 10 nM TCDD in MT-4 cells. Hence, the specific monooxygenase isoform induced in the lymphoid cells by TCDD appears to be CYP1A1 the expression of which is mediated by the Ah receptor. 3. Incubation of MT-4 cells with TCDD at 10, 50 and 150 nM for 1.5 and 48 h followed by infection of the cells with human immunodeficiency virus 1 (HIV-1) was accompanied by a 3-6-fold increase in the activity of viral RNA-dependent DNA-polymerase. The most marked effect on reverse transcriptase activity occurred with 10 nM TCDD 5-9 days after HIV-1 infection. 4. In the same period there was accumulation of viral protein, determined by ELISA, with a 4-8-fold increase in production of viral protein. The above effects of TCDD have been observed even when MT-4 cells were washed 1.5 h after beginning the incubation with TCDD.

Characterization of protein phosphorylation by 2,3,7,8-tetrachlorodibenzo-p-dioxin in murine lymphocytes: indirect evidence for a role in the suppression of humoral immunity

Studies were undertaken to more thoroughly characterize 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced stimulation of kinase activity in murine lymphocytes. In female B6C3F1 mice, TCDD-induced phosphorylation of 29, 45, 52 and 63 KDa proteins was selective for B cells, with little or no enhancement observed in T cells. When B cells were purified and separated by density on a percoll gradient, phosphorylation was only observed in the band composed of activated B cells, and was not enhanced in the band composed of resting B cells. TCDD-stimulated phosphorylation was associated with both the cytosol (45 and 52 KDa species) and membrane (52 KDa species) fractions. Purified B cells from both DBA/2 (Ahdd) and C57B16 (Ahbb) mice demonstrated equivalent enhancement of phosphorylation in response to TCDD. Administration of human gamma interferon (Hu-IFNg) at concentrations from 0.5 to 500 Units/ml produced a dose-related reversal of TCDD-induced suppression of in vitro antibody responses to both the polyclonal B cell activator, LPS, and the T-dependent antigen, sRBC in whole splenocytes isolated from female B6C3F1 mice. These concentrations of Hu-IFNg did not affect the magnitude of either response in the absence of TCDD, and did not reverse dexamethasone-induced suppression of either in vitro antibody response. TCDD-induced suppression of the T-dependent response was reversed only when Hu-IFNg was added to culture within the first 18 hours after treatment with TCDD and sRBC. These studies demonstrate that Hu-IFNg can reverse TCDD-induced in vitro Ab response suppression if it is administered during the period of susceptibility to TCDD. TCDD-induced phosphorylation in isolated B cells was also antagonized following co-incubation with Hu-IFNg. The profile of TCDD-induced increases in protein phosphorylation, including the selective effect on activated B cells, the general involvement of both cytosolic and membrane proteins, the lack of segregation with the Ah-dependent processes, and the ability of Hu-IFNg to reverse both the suppression of the Ab response and the increase in phosphorylation, supports the interpretation that such phosphorylation is involved in TCDD-induced suppression of the Ab response.

Dioxin induces expression of c-fos and c-jun proto-oncogenes and a large increase in transcription factor AP-1

Among environmental pollutants, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; dioxin) is one of the most potent tumor promoters and teratogens known. The molecular mechanisms responsible for the biological activity of TCDD, however, remain largely unknown. In this report, we show that the first observable effects of TCDD in cultured murine hepatoma cells are a rapid, transient increase in Ca2+ influx and a minor but significant elevation of activated, membrane-bound protein kinase C. These changes are then followed by induction of the immediate early proto-oncogenes c-fos, jun-B, c-jun, and jun-D, and by large increases in AP-1 transcription factor activity. Induction of these changes by TCDD is delayed compared with that by phorbol esters, although the magnitude of the effects caused by both treatments is similar, and both induction processes can be blocked by staurosporine, a protein kinase C inhibitor. In cultured cells, proto-oncogene induction by TCDD appears to be independent of the presence of a functional aryl hydrocarbon (Ah) receptor or nuclear translocation protein. These results reveal early events that may lead to the elucidation of the molecular basis of TCDD-induced tumor promotion.

Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation

Transcriptional activation of the murine Cyp1a-1 (cytochrome P(1)450) gene by inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (dioxin) requires the aromatic hydrocarbon (Ah) receptor and the interaction of an inducer-receptor complex with one or more of the Ah-responsive elements (AhREs) located about 1 kb upstream from the transcriptional initiation site. We find that treatment of mouse hepatoma Hepa-1 cells with 2-aminopurine, an inhibitor of protein kinase activity, inhibits CYP1A1 mRNA induction by TCDD as well as the concomitant increase in CYP1A1 enzyme activity. Formation of DNA-protein complexes between the Ah receptor and its AhRE target is also inhibited by 2-aminopurine, as determined by gel mobility shift assays. Phosphorylation is required for the formation of Ah receptor-specific complexes, since in vitro dephosphorylation of nuclear extracts from TCDD-treated Hepa-1 cells abolishes the capacity of the Ah receptor to form specific complexes with its cognate AhRE sequences. To determine whether any one of several known protein kinases was involved in the transcriptional regulation of the Cyp1a-1 gene, we treated Hepa-1 cells with nine other protein kinase inhibitors prior to induction with TCDD; nuclear extracts from these cells were analyzed for their capacity to form specific DNA-protein complexes. Only extracts from cells treated with staurosporine, a protein kinase C inhibitor, were unable to form these complexes. In addition, staurosporine completely inhibited CYP1A1 mRNA induction by TCDD. Depletion of protein kinase C by prolonged treatment with phorbol ester led to the complete suppression of CYP1A1 mRNA induction by TCDD. We conclude that (i) phosphorylation is necessary for the formation of a transcriptional complex and for transcriptional activation of the Cyp1a-1 gene; (ii) the phosphorylation site(s) exists on at least one of the proteins constituting the transcriptional complex, possibly the Ah receptor itself; and (iii) the enzyme responsible for the phosphorylation is likely to be protein kinase C.

Dioxin-dependent activation of murine Cyp1a-1 gene transcription requires protein kinase C-dependent phosphorylation

Transcriptional activation of the murine Cyp1a-1 (cytochrome P(1)450) gene by inducers such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (dioxin) requires the aromatic hydrocarbon (Ah) receptor and the interaction of an inducer-receptor complex with one or more of the Ah-responsive elements (AhREs) located about 1 kb upstream from the transcriptional initiation site. We find that treatment of mouse hepatoma Hepa-1 cells with 2-aminopurine, an inhibitor of protein kinase activity, inhibits CYP1A1 mRNA induction by TCDD as well as the concomitant increase in CYP1A1 enzyme activity. Formation of DNA-protein complexes between the Ah receptor and its AhRE target is also inhibited by 2-aminopurine, as determined by gel mobility shift assays. Phosphorylation is required for the formation of Ah receptor-specific complexes, since in vitro dephosphorylation of nuclear extracts from TCDD-treated Hepa-1 cells abolishes the capacity of the Ah receptor to form specific complexes with its cognate AhRE sequences. To determine whether any one of several known protein kinases was involved in the transcriptional regulation of the Cyp1a-1 gene, we treated Hepa-1 cells with nine other protein kinase inhibitors prior to induction with TCDD; nuclear extracts from these cells were analyzed for their capacity to form specific DNA-protein complexes. Only extracts from cells treated with staurosporine, a protein kinase C inhibitor, were unable to form these complexes. In addition, staurosporine completely inhibited CYP1A1 mRNA induction by TCDD. Depletion of protein kinase C by prolonged treatment with phorbol ester led to the complete suppression of CYP1A1 mRNA induction by TCDD. We conclude that (i) phosphorylation is necessary for the formation of a transcriptional complex and for transcriptional activation of the Cyp1a-1 gene; (ii) the phosphorylation site(s) exists on at least one of the proteins constituting the transcriptional complex, possibly the Ah receptor itself; and (iii) the enzyme responsible for the phosphorylation is likely to be protein kinase C.

Induction of glutathione S-transferase P-form in primary cultured rat liver parenchymal cells by co-planar polychlorinated biphenyl congeners

Alterations in protein synthesis in primary cultured rat liver parenchymal cells were examined after their exposure to the potent carcinogens, polychlorinated biphenyl (PCB) congeners. Co-planar PCB congeners (3,4,5,3',4'-PCB and 3,4,5,3',4',5'-PCB) (10 nM) induced a protein, the Mr of which was 25,000 (25 k protein) under denaturing conditions. However, non-co-planar PCB congeners and several xenobiotics, which induce microsomal proteins, did not induce the 25 k protein. By using immunoblotting, the 25 k protein was identified as glutathione S-transferase P-form (GST-P, 7-7, EC 2.5.1.18).

Multifunctional receptor model for dioxin and related compound toxic action: possible thyroid hormone-responsive effector-linked site

Molecular/theoretical modeling studies have revealed that thyroid hormones and toxic chlorinated aromatic hydrocarbons of environmental significance (for which dioxin or TCDD is the prototype) have similar structural properties that could be important in molecular recognition in biochemical systems. These molecular properties include a somewhat rigid, sterically accessible and polarizable aromatic ring and size-limited, hydrophobic lateral substituents, usually contained in opposite adjoining rings of a diphenyl compound. These molecular properties define the primary binding groups thought to be important in molecular recognition of both types of structures in biochemical systems. Similar molecular reactivities are supported by the demonstration of effective specific binding of thyroid hormones and chlorinated aromatic hydrocarbons with four different proteins, enzymes, or receptor preparations that are known or suspected to be involved in the expression of thyroid hormone activity. These binding interactions represent both aromatic-aromatic (stacking) and molecular cleft-type recognition processes. A multiple protein or multifunctional receptor-ligand binding mechanism model is proposed as a way of visualizing the details and possible role of both the stacking and cleft type molecular recognition factors in the expression of biological activity. The model suggests a means by which hormone-responsive effector-linked sites (possible protein-protein-DNA complexes) can maintain highly structurally specific control of hormone action. Finally, the model also provides a theoretical basis for the design and conduct of further biological experimentation on the molecular mechanism(s) of action of toxic chlorinated aromatic hydrocarbons and thyroid hormones.

Effects of TCDD on the EGF receptor of XB mouse keratinizing epithelial cells

TCDD was found to cause a marked inhibition of 125I-epidermal growth factor (EGF) binding to its receptor on the cell surface of XB mouse keratinizing epithelial cells (XB cells) cultured in vitro. The EC50 concentration was estimated to be on the order of 3 x 10(-11) M 24 hours after TCDD administration. As early as 12 hours after the addition of 10(-9) M of TCDD, XB cells showed signs of a decline in 125I-EGF binding levels. The level of such EGF receptor downregulation reached a maximum at 24 hours, continued until day 2, but completely recovered by day 3. This was accompanied by a rise in protein kinase activities, particularly those of the protein tyrosine kinases during the initial period of 6-24 hours. To test the hypothesis that the EGF receptors of the cells, by showing TCDD-induced symptoms of downregulation, actually are being activated and triggering EGF-like signals, we examined the effects of both TCDD and exogenously added EGF on cell morphology, colony formation degree of keratinization, the pattern of activation of protein kinases and de novo protein synthesis, and EGF receptor phosphorylation. Based on the similarity of cell responses to these between TCDD- and EGF-treated cells, we concluded that TCDD, directly or indirectly, causes activation of the EGF receptor. In contrast, 12-O-tetradencanoylphorbol-13-acetate (TPA), which is known to downregulate EGF receptors by blocking their protein tyrosine kinase, produced dissimilar end results. The balance of evidence support the notion that the action of TCDD in this cell line is tightly coupled to the activation of the EGF receptor and that one of the key consequences of such a biochemical change is that it signals these cells to commit to terminal differentiation.