2,3,7,8-Tetrachlorodibenzo-p-dioxin-induced porphyria in genetically inbred mice: partial antagonism and mechanistic studies

The aryl hydrocarbon receptor ligand omeprazole inhibits breast cancer cell invasion and metastasis

BACKGROUND

Patients with ER-negative breast tumors are among the most difficult to treat and exhibit low survival rates due, in part, to metastasis from the breast to various distal sites. Aryl hydrocarbon receptor (AHR) ligands show promise as antimetastatic drugs for estrogen receptor (ER)-negative breast cancer.

METHODS

Triple negative MDA-MB-231 breast cancer cells were treated with eight AHR-active pharmaceuticals including 4-hydroxtamoxifen, flutamide leflunomide, mexiletine, nimodipine, omeprazole, sulindac and tranilast, and the effects of these compounds on cell proliferation (MTT assay) and cell migration (Boyden chamber assay) were examined. The role of the AHR in mediating inhibition of MDA-MB-231 cell invasion was investigated by RNA interference (RNAi) and knockdown of AHR or cotreatment with AHR agonists. Lung metastasis of MDA-MB-231 cells was evaluated in mice administered cells by tail vein injection and prometastatic gene expression was examined by immunohistochemistry.

RESULTS

We showed that only the proton pump inhibitor omeprazole decreased MDA-MB-231 breast cancer cell invasion in vitro. Omeprazole also significantly decreased MDA-MB-231 cancer cell metastasis to the lung in a mouse model (tail vein injection), and in vitro studies showed that omeprazole decreased expression of at least two prometastatic genes, namely matrix metalloproteinase-9 (MMP-9) and C-X-C chemokine receptor 4 (CXCR4). Results of RNA interference studies confirmed that omeprazole-mediated downregulation of CXCR4 (but not MMP-9) was AHR-dependent. Chromatin immunoprecipitation assays demonstrated that omeprazole recruited the AHR to regions in the CXCR4 promoter that contain dioxin response elements (DREs) and this was accompanied by the loss of pol II on the promoter and decreased expression of CXCR4.

CONCLUSIONS

AHR-active pharmaceuticals such as omeprazole that decrease breast cancer cell invasion and metastasis may have important clinical applications for late stage breast cancer chemotherapy.

Role of the aryl hydrocarbon receptor in carcinogenesis and potential as a drug target

The aryl hydrocarbon receptor (AHR) is highly expressed in multiple organs and tissues, and there is increasing evidence that the AHR plays an important role in cellular homeostasis and disease. The AHR is expressed in multiple tumor types, in cancer cell lines, and in tumors from animal models, and the function of the AHR has been determined by RNA interference, overexpression, and inhibition studies. With few exceptions, knockdown of the AHR resulted in decreased proliferation and/or invasion and migration of cancer cell lines, and in vivo studies in mice overexpressing the constitutively active AHR exhibited enhanced stomach and liver cancers, suggesting a pro-oncogenic role for the AHR. In contrast, loss of the AHR in transgenic mice that spontaneously develop colonic tumors and in carcinogen-induced liver tumors resulted in increased carcinogenesis, suggesting that the receptor may exhibit antitumorigenic activity prior to tumor formation. AHR ligands also either enhanced or inhibited tumorigenesis, and these effects were highly tumor specific, demonstrating that selective AHR modulators that exhibit agonist or antagonist activities represent an important new class of anticancer agents that can be directed against multiple tumors.

Comparisons of differential gene expression elicited by TCDD, PCB126, βNF, or ICZ in mouse hepatoma Hepa1c1c7 cells and C57BL/6 mouse liver

The aryl hydrocarbon receptor (AhR) is a promiscuous receptor activated by structurally diverse synthetic and natural compounds. AhR activation may lead to ligand-specific changes in gene expression despite similarities in mode of action. Therefore, differential gene expression elicited by four structurally diverse, high affinity AhR ligands (2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD; 10nM, 30 μg/kg), 3,3',4,4',5-pentachlorobiphenyl (PCB126; 100nM, 300μg/kg), β-naphthoflavone (βNF; 10 μM, 90 mg/kg), and indolo[3,2-b]carbazole (ICZ; 1μM)) in mouse Hepa1c1c7 hepatoma cells and C57BL/6 mouse liver samples were compared. A total of 288, 183, 119, and 131 Hepa1c1c7 genes were differentially expressed (|fold-change|≥ 1.5, P1(t)≥ 0.9999) by TCDD, βNF, PCB126, and ICZ, respectively. Only ∼35% were differentially expressed by all 4 ligands in Hepa1c1c7 cells. In vivo, 661, 479, and 265 hepatic genes were differentially expressed following treatment with TCDD, βNF, and PCB126, respectively. Similar to Hepa1c1c7 cells, ≤ 34% of gene expression changes were common across all ligands. Principal components analysis identified time-dependent gene expression divergence. Comparisons of ligand-elicited expression between Hepa1c1c7 cells and mouse liver identified only 11 common gene expression changes across all ligands. Although metabolism may explain some ligand-specific gene expression changes, PCB126, βNF, and ICZ also elicited divergent expression compared to TCDD, suggestive of selective AhR modulation.

Aryl hydrocarbon receptor (AHR)-active pharmaceuticals are selective AHR modulators in MDA-MB-468 and BT474 breast cancer cells

Leflunomide, flutamide, nimodipine, mexiletine, sulindac, tranilast, 4-hydroxytamoxifen, and omeprazole are pharmaceuticals previously characterized as aryl hydrocarbon receptor (AHR) agonists in various cell lines and animal models. In this study, the eight AHR-active pharmaceuticals were investigated in highly aggressive aryl hydrocarbon (Ah)-responsive BT474 and MDA-MB-468 breast cancer cell lines, and their effects on AHR protein, CYP1A1 (protein and mRNA), CYP1B1 (mRNA), and cell migration were determined. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) was used as a positive control. The AHR agonist activities of the pharmaceuticals depended on structure, response, and cell context. Most compounds induced one or more AHR-mediated responses in BT474 cells, whereas in Ah-responsive MDA-MB-468 cells effects of the AHR-active pharmaceuticals were highly variable. 4-Hydroxytamoxifen, mexiletine, and tranilast did not induce CYP1A1 in MDA-MB-468 cells; moreover, in combination with TCDD, mexiletine was a potent AHR antagonist, tranilast was a partial antagonist, and 4-hydroxytamoxifen also exhibited some AHR antagonist activity. Omeprazole and, to a lesser extent, sulindac and leflunomide were full and partial AHR agonists, respectively, in both breast cancer cell lines. These data indicate that the AHR-active pharmaceuticals are selective AHR modulators, and applications of these drugs for targeting the AHR must be confirmed by studies using the most relevant cell context.

Aryl hydrocarbon receptor agonists induce microRNA-335 expression and inhibit lung metastasis of estrogen receptor negative breast cancer cells

The aryl hydrocarbon receptor (AHR) was initially identified as a receptor that bound 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related environmental toxicants; however, there is increasing evidence that the AHR is an important new drug target for treating multiple diseases including breast cancer. Treatment of estrogen receptor (ER)-negative MDA-MB-231 and BT474 breast cancer cells with TCDD or the selective AHR modulator 6-methyl-1,3,-trichlorodibenzofuran (MCDF) inhibited breast cancer cell invasion in a Boyden chamber assay. These results were similar to those previously reported for the antimetastic microRNA-335 (miR-335). Both TCDD and MCDF induced miR-335 in MDA-MB-231 and BT474 cells and this was accompanied by downregulation of SOX4, a miR-335-regulated (inhibited) gene. The effects of TCDD and MCDF on miR-335 and SOX4 expression and interactions of miR-335 with the 3'-UTR target sequence in the SOX4 gene were all inhibited in cells transfected with an oligonucleotide (iAHR) that knocks down the AHR, thus confirming AHR-miR-335 interactions. MCDF (40 mg/kg/d) also inhibited lung metastasis of MDA-MB-231 cells in a tail vein injection model, showing that the AHR is a potential new target for treating patients with ER-negative breast cancer, a disease where treatment options and their effectiveness are limited.

The aryl hydrocarbon receptor as a target for estrogen receptor-negative breast cancer chemotherapy

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and the relatively non-toxic selective aryl hydrocarbon receptor (AhR) modulator 6-methyl-1,3,8-trichlorodibenzo-furan (MCDF) induced CYP1A1-dependent ethoxyresorufin O-deethylase activity and inhibited proliferation of seven estrogen receptor (ER) negative breast cancer cell lines. MCDF, TCDD and structurally related 2,3,7,8-tetrachlorodibenzofuran, 1,2,3,7,8-pentachlorodibenzo-p-dioxin, 2,3,4,7,8-pentachlorodibenzofuran, and 3,3',4,4',5-pentachlorobiphenyl induced CYP1A1 and inhibited proliferation of BT-474 and MDA-MB-468 cells. In BT474 and MDA-MB-468 cells transfected with a small inhibitory RNA for the AhR, the antiproliferative activity of the chlorinated aromatic compounds was reversed, whereas for MCDF, only partial reversal was observed, suggesting that this compound acts through both AhR-dependent and AhR-independent pathways in these two cell lines. MCDF also inhibited tumor growth in athymic nude mice in which MDA-MB-468 cells were injected directly into the mammary fat pad. These results suggest that the AhR is a potential drug target for treatment of ER-negative breast cancer.

Development of selective aryl hydrocarbon receptor modulators for treatment of breast cancer

The aryl hydrocarbon receptor (AhR) is a basic helix-loop-helix DNA-binding protein that forms a transcriptionally-active heterodimer with the AhR nuclear translocator (Arnt) protein. The nuclear AhR complex is a ligand-induced transcription factor and the environmental toxicant 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is a high affinity ligand for the AhR. TCDD induces a diverse spectrum of tissue-, sex- and species-specific biochemical and toxic responses in Ah-responsive cells/tissues including the inhibition of 17beta-oestradiol (E2)-induced gene expression in the rodent uterus and mammary and in human breast cancer cell lines. TCDD also inhibits spontaneous and carcinogen-induced mammary tumour formation and growth in rodent models. Research in this laboratory has utilised the AhR as a target for developing anticancer drugs for treatment of breast cancer and two different structural classes of selective AhR modulators (SAhRMs) have been developed. Alternate-substituted (1,3,6,8- and 2,4,6,8-) alkyl polychlorinated dibenzofurans (PCDFs) and substituted diindolylmethanes (DIMs) bind the AhR and induce a pattern of AhR-oestrogen receptor (ER) inhibitory cross-talk similar to that observed for TCDD including inhibition of mammary tumour growth at doses < 1.0 mg/kg/day. In contrast, effective doses of these compounds do not induce hepatic CYP1A1-dependent activity or other AhR-mediated toxic responses induced by TCDD. These results indicate that SAhRMs may be an important new class of drugs for clinical treatment of breast cancer via AhR-ER inhibitory cross-talk.

Aryl hydrocarbon receptor-mediated inhibition of LNCaP prostate cancer cell growth and hormone-induced transactivation

LNCaP prostate cancer cells express the aryl hydrocarbon receptor (AhR), and treatment with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) induces CYP1A1 protein and an Ah-responsive reporter gene. Similar results were obtained with the selective AhR modulator 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF); however, TCDD but not 6-MCDF induced degradation of the AhR protein. TCDD and 6-MCDF inhibited growth of LNCaP cells, and inhibitory AhR-androgen receptor (AR) crosstalk was investigated in cells transfected with constructs containing the androgen-responsive probasin promoter (-288 to +28) (pPB) or three copies of the -244 to -96 region of this promoter (pARR(3)). Ten nanomolar dihydrotestosterone (DHT) and 17 beta-estradiol (E2) induced transactivation in LNCaP cells transfected with pPB or pARR(3); however, inhibitory AhR-AR crosstalk was observed only with the latter construct. 6-MCDF and TCDD did not inhibit DHT- or E2-induced transactivation in ZR-75 human breast cancer cells, indicating that these interactions were promoter and cell context-dependent. Both E2 and DHT stabilized AR protein in LNCaP cells; however, cotreatment with TCDD or 6-MCDF decreased AR protein levels. These results indicate that inhibitory AhR-AR crosstalk in prostate cancer cells is complex and for some responses, AR protein stability may play a role.

Experimental hepatic uroporphyria induced by the diphenyl-ether herbicide fomesafen in male DBA/2 mice

Hepatic uroporphyria can be readily induced by a variety of treatments in mice of the C57BL strains, whereas DBA/2 mice are almost completely resistant. However, feeding of the protoporphyrinogen oxidase-inhibiting herbicide fomesafen (0.25% in the diet for 18 weeks) induced hepatic uroporphyria in male DBA/2N mice (liver porphyrin content up to 150 nmol/g, control animals 1 nmol/g), whereas fomesafen-treated male C57BL/6N mice displayed only a slight elevation of liver porphyrins (approximately 5 nmol/g). The profile of accumulated hepatic porphyrins in fomesafen-treated DBA/2N mice resembled the well-characterised uroporphyria induced by polyhalogenated aromatic hydrocarbons, while histological examination confirmed the presence of uroporphyria-specific cytoplasmic inclusions in the hepatocytes. Uroporphyrinogen decarboxylase activity decreased to about 30% of control values in fomesafen-treated DBA/2N mice; microsomal methoxyresorufin O-dealkylase activity was slightly reduced. The amount of CYP1A1 and CYP1A2 mRNA, as determined by real-time PCR, was not significantly changed; mRNA encoding the housekeeping 5-aminolevulinic acid synthase was elevated 10-fold. Total liver iron was slightly increased. A similar uroporphyria was induced by the herbicide formulation Blazer, containing a structurally related herbicide acifluorfen, when fed to DBA/2N mice at a dose corresponding to 0.25% of acifluorfen in the diet. Since DBA/2 mice are almost completely resistant to all well-characterised porphyrogenic chemicals, the results suggest the possible existence of a yet unknown mechanism of uroporphyria induction, to which the DBA/2 mouse strain is more sensitive than the C57BL strain.

Contribution of planar (0-1 ortho) and nonplanar (2-4 ortho) fractions of Aroclor 1260 to the induction of altered hepatic foci in female Sprague-Dawley rats

The hepatic tumor promoting activity of the planar 0-1 ortho ( approximately 9.7% w/w) and the nonplanar 2-4 ortho ( approximately 90.3% w/w) fraction of the commercial PCB mixture Aroclor 1260 was studied using a medium-term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. Fractionation was carried out on an activated charcoal column. The composition of the effluent from the column was tested by GC-ECD. The absence of planar compounds in the 2-4 ortho fraction was confirmed by GC-MS analysis. The dioxin-like toxic potency of the fractions was determined with the DR-CALUX assay. The animal experiment was started with the initiation procedure (diethylnitrosamine injection, 30 mg/kg body wt ip, 24 h after (2)/(3) hepatectomy), followed 6 weeks later by the promotion treatment, which consisted of a weekly subcutaneous injection during 20 weeks. Exposure groups (n = 10) received the following treatments (dose/kg body wt/week): Aroclor 1260 (10 mg), 0-1 ortho fraction (0.97 mg), 2-4 ortho fraction (1, 3, or 9 mg), a reconstituted 0-4 ortho fraction (9.97 mg), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153; 1 or 9 mg), 2,3,7,8-TCDD (1 microg; positive control) or corn oil (1 ml; vehicle control). One group did not receive a promotion treatment. All exposure groups exhibited a significantly increased volume fraction of the liver occupied by hepatic foci positive for the placental form of glutathione-S-transferase-p compared to the corn oil control, except for the groups treated with 0-1 ortho fraction and 1 mg PCB 153/kg body wt/week. Approximately 80% of the total tumor promoting capacity of the reconstituted 0-4 ortho fraction could be explained by the 2-4 ortho PCB fraction while the 0-1 ortho fraction had only a negligible contribution. These results suggest that the majority of the tumor promotion potential of PCB mixtures resides in the non-dioxin-like fraction, which is not taken into account in the toxic equivalency factor (TEF) approach for risk assessment of PCBs. This may result in an underestimation of the tumor promotion potential of environmental PCB mixtures.

Mechanisms of inhibitory aryl hydrocarbon receptor-estrogen receptor crosstalk in human breast cancer cells

The aryl hydrocarbon receptor (AhR) is a ligand-activated transcription factor that forms a functional heterodimeric complex with the AhR nuclear translocator (Arnt) protein. The environmental toxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), is a high affinity ligand for the AhR and has been extensively used to investigate AhR-mediated biochemical and toxic responses. TCDD modulates several endocrine pathways including inhibition of 17beta-estradiol-induced responses in the immature and ovariectomized rodent uterus and mammary gland and in human breast cancer cell lines. TCDD inhibits formation and growth of mammary tumors in carcinogen-induced rodent models and relatively nontoxic selective AhR modulators (SAhRMs) are being developed for treatment of breast cancer. The mechanisms of inhibitory AhR-estrogen receptor (ER) crosstalk have been investigated in MCF-7 breast cancer cells by analysis of promoter regions of genes induced by E2 and inhibited by TCDD. AhR-mediated inhibition of E2-induced cathepsin D, pS2, c-fos, and heat shock protein 27 gene expression involves direct interaction of the AhR complex with inhibitory pentanucleotide (GCGTG) dioxin responsive elements (iDREs) resulting in disruption of interactions between proteins binding DNA elements required for ER action and the basal transcription machinery. Mechanisms of inhibitory AhR-ER crosstalk indicate that functional iDREs are required for inhibition of some genes; however, results indicate that other interaction pathways are important including AhR-mediated proteasome-dependent degradation of the ER.

Induction of altered hepatic foci by a mixture of dioxin-like compounds with and without 2,2',4,4',5,5'-hexachlorobiphenyl in female Sprague-Dawley rats

The hepatic tumor-promoting activity of a mixture of polyhalogenated aromatic hydrocarbons (PHAHs) was studied in a medium term two-stage initiation/promotion bioassay in female Sprague-Dawley rats. The PHAH mixture contained 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), 1, 2,3,7,8-pentachlorodibenzo-p-dioxin (PeCDD), 2,3,4,7, 8-pentachlorodibenzofuran (PeCDF), 3,3',4,4',5-pentachlorobiphenyl (PCB 126), 2,3',4,4',5-pentachlorobiphenyl (PCB 118), 2,3,3',4,4', 5-hexachlorobiphenyl (PCB 156), 2,2',4,4',5,5'-hexachlorobiphenyl (PCB 153) and covered >90% of the total toxic equivalents (TEQ) present in Baltic herring. To determine possible interactive effects of di-ortho-substituted PCBs, the PHAH mixture was tested with (PHAH+) and without (PHAH-) PCB 153. Rats were initiated by a diethylnitrosamine injection (30 mg/kg body wt i.p.) 24 h after a partial 23 hepatectomy. Six weeks after initiation, the PHAH mixtures were administered once a week by subcutaneous injections for 20 weeks. Treatment with the PHAH mixtures caused liver enlargement and an increased activity of the hepatic cytochrome P4501A1/2 and P4502B1/2. All PHAH exposure groups exhibited an increased occurrence of hepatic foci positive for the placental form of glutathione-S-transferase. In the PHAH-group dosed 1 microgram TEQ/kg body wt/week, the volume fraction of the liver occupied by foci was significantly lower compared to the TEQ equivalent dosed TCDD group (3.8 vs 8.7%). The volume fraction was significantly increased in the groups treated with 0.5, 1, or 2 micrograms TEQ/kg body wt/week of the PHAH+ mixture (4.5, 5.2, and 6.6%, respectively) compared to the corn oil group (2.0%), but to a lower extent than expected on basis of the TEQ doses. Overall, the TEQ-based administered dose overestimated the observed tumor-promoting effects of this PHAH mixture. The applicability of the toxic equivalency factor concept, the role of differences in toxicokinetic properties and interactive effects of PCB 153 on hepatic deposition of the dioxin-like congeners are discussed.

Toxicokinetics of an environmentally relevant mixture of dioxin-like PHAHs with or without a non-dioxin-like PCB in a semi-chronic exposure study in female Sprague Dawley rats

Female Sprague Dawley rats were treated subcutaneously for 20 weeks with an environmentally relevant mixture of dioxin-like PHAHs with (PHAH+) or without (PHAH-) 2,2',4,4',5,5'-hexachlorobiphenyl. The hepatic retention (% of given dose) of the various PHAH congeners differed considerably and in the following order: 2,3,4,7,8-pentachlorodibenzofuran (30.5-43.1%), 3,3',4,4',5-pentachlorobiphenyl (12.8-17.6%), 1,2,3,7,8-pentachlorodibenzo-p-dioxin (6.9-10.8%), 2,3,7,8-tetrachlorodibenzo-p-dioxin (3.2-4.5%), 2,3,3',4,4',5-hexachlorobiphenyl (1.0-1.7%), 2,2',4,4',5,5'-hexachlorobiphenyl (0.5-0.8%) and 2,3',4,4',5-pentachlorobiphenyl (0.2-0.4%). A decrease of the hepatic retention of 2,3,7,8-TCDD, 1,2,3,7,8-PeCDD and 2,3,4,7,8-PeCDF was found at increasing doses of the PHAH+ mixture. 2,2',4,4',5,5'-Hexachlorobiphenyl increased the hepatic retention (1.3-2 times) of all congeners in the PHAH+ group, compared to the TEQ equivalent dosed PHAH- group. No interactions were observed on the ethoxyresorufin-O-deethylase activity.

Inhibition of 7,12-dimethylbenz[a]anthracene-induced rat mammary tumor growth by aryl hydrocarbon receptor agonists

The antitumorigenic activities of 6-methyl-1,3,8-trichlorodibenzofuran (6-MCDF), 8-methyl-1,3,6-trichlorodibenzofuran (8-MCDF) and 6-cyclohexyl-1,3,8-trichlorodibenzofuran (6-CHDF) were investigated in the 7,12-dimethylbenz[a]anthracene (DMBA) rat mammary tumor model. At doses of 5, 10 or 25 mg/kg/week, both 6-MCDF and 8-MCDF significantly inhibited mammary tumor growth and at the 5 mg/kg/week dose >50% growth inhibition was observed. In contrast, 6-CHDF was inactive at the 5 mg/kg/week dose and the structure-antitumorigenicity relationships (6-MCDF/8-MCDF > 6-CHDF) correlated with structure-antiestrogenicity (rat uterus) studies and the relative binding affinities of these compounds for the aryl hydrocarbon receptor (AhR). The antitumorigenic activity of 6-MCDF or 8-MCDF in the mammary was not accompanied by any significant changes in liver/body weight ratios, liver morphology or induction of hepatic CYP1A1-dependent activity which is one of the most sensitive indicators of exposure to AhR agonists. RT-PCR and Western blot analysis of mammary tumor mRNA and protein extracts, respectively, confirmed the presence of AhR suggesting that AhR-mediated signaling pathways are functional in rat mammary tumors. These results define a relatively non-toxic group of AhR agonists which exhibit potent antitumorigenic activity in the DMBA-induced rat mammary tumor model (<1 mg/kg/day), and therefore represent a new class of indirect-acting antiestrogens which have potential for clinical treatment of mammary cancer.

Modulation of gene expression and endocrine response pathways by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related compounds

The aryl hydrocarbon (Ah) receptor binds several different structural classes of chemicals, including halogenated aromatics, typified by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), polynuclear aromatic and heteropolynuclear aromatic hydrocarbons. TCDD induces expression of several genes including CYP1A1, and molecular biology studies show that the Ah receptor acts as a nuclear ligand-induced transcription factor that interacts with xenobiotic or dioxin responsive elements located in 5'-flanking regions of responsive genes. TCDD also elicits diverse toxic effects, modulates endocrine pathways and inhibits a broad spectrum of estrogen (17 beta-estradiol)-induced responses in rodents and human breast cancer cell lines. Molecular biology studies show that TCDD inhibited 17 beta-estradiol-induced cathepsin D gene expression by targeted interaction of the nuclear Ah receptor with imperfect dioxin responsive elements strategically located within the estrogen receptor-Sp1 enhancer sequence of this gene.

Porphyrin studies in TCDD-exposed workers

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) has been shown to inhibit uroporphyrinogen decarboxylase activity resulting in chronic hepatic porphyria. From a cross-sectional study of 170 workers in chemical industry 68 showed elevated coproporphyrin levels, interpreted as secondary coproporphyrinuria. Three persons suffered from chronic hepatic porphyria in subclinical stages. None of the workers showed an overt porphyria cutanea tarda. A low-grade zinc protoporphyrinemia was observed in three persons. Forty-three of the 170 workers were evaluable for investigating the effect of TCDD on porphyrin levels. No significant correlation was found between TCDD concentration in adipose tissue and the level of uroporphyrin and coproporphyrin. The influence of a chloracne history is described.

Inhibition of estrogen-induced progesterone receptor in MCF-7 human breast cancer cells by aryl hydrocarbon (Ah) receptor agonists

17 beta-Estradiol (E2) induces progesterone receptor (PR) binding, immunoreactive protein, nuclear PR formation and PR mRNA levels in MCF-7 human breast cancer cells. Gel mobility shift analysis of nuclear extracts from E2-treated cells also exhibited a higher intensity retarded band associated with formation of a PR complex with a consensus [32P]progesterone/glucocorticoid responsive element. In contrast, 1 nM 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) alone did not alter or decrease these same responses in MCF-7 cells; however, in cells co-treated with 1 nM TCDD plus 1 nM E2, TCDD significantly inhibited all the E2-induced responses. Scatchard analysis of PR binding demonstrated that TCDD decreased the number of E2-induced PR cellular binding sites but not the binding affinity of the PR for a radiolabeled promegestrone. In parallel studies, 3-methylcholanthrene, a prototypical polynuclear aromatic hydrocarbon, also inhibited E2-induced PR binding and immunoreactive protein. For a series of halogenated aromatics including 2,3,7,8- and 1,2,7,8-tetrachlorodibenzofuran, 1,3,7,8-TCDD and 6-methyl-1,3,8-trichlorodibenzofuran, their rank order potency for inhibiting E2-induced PR binding paralleled their rank order binding to the aryl hydrocarbon (Ah) receptor. These results support a role for the Ah receptor in mediating the antiestrogenic activity of polynuclear and halogenated aromatic hydrocarbons and illustrate cross-talk between the Ah and estrogen receptor signal transduction pathways.

Protein tyrosine phosphorylation as an indicator of 2,3,7,8-tetrachloro-p-dioxin exposure in vivo and in vitro

A dose-dependent increase in tyrosine phosphorylation of five hepatic intracellular proteins with approximate molecular weights of 17, 21, 27, 29, and 34 kDa was seen 24 h after administration of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) to C57BL/6J female mice. The ED50 values for tyrosylphosphorylation of these five proteins, respectively, were 0.26, 0.21, 0.26, 0.31, and 0.38 micrograms TCDD/Kg. TCDD induction of 7-ethoxyresorufin O-deethylase activity (EROD) was characterized by an ED50 of 2.5 micrograms/Kg. An eighteen h exposure of a human lymphoblastoma cell line (X3) to TCDD increased tyrosylphosphorylation status of ten proteins with approximate molecular weights of 16, 17, 24, 26, 27, 32, 33, 34, 35, and 36 kDa in a dose-dependent manner. The EC50 values for these TCDD-dependent tyrosylphosphorylation ranged from 0.01 to 0.07 nM TCDD. EROD induction by TCDD in X3 cells exhibited an EC50 of 0.14 nM. These data indicate that TCDD alters intracellular protein tyrosine phosphorylation and these changes are more sensitive biological indicators of TCDD exposure than induction of EROD.

The interactions of 2,3,7,8-tetrachlorodibenzo-p-dioxin and 6-methyl-1,3,8-trichlorodibenzofuran in chick embryo hepatocytes

Treatment of chick embryo hepatocytes in ovo and in culture with 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) resulted in a dose-dependent induction of microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities. Significant induction was observed in the hepatocytes at TCDD doses as low as 10(-11) mol/egg (in ovo) and 10(-10)m (in culture). In contrast, 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) was a relatively weak inducer of these activities and only 10-20% of the induction responses observed for TCDD were elicited by MCDF at doses of 10(-6) mol/egg or 10(-7)m in culture. Co-treatment of the chick embryo hepatocytes with TCDD (10(-10) mol/egg in ovo; 10(-10)m in culture) and different concentrations of MCDF (10(-6) to 10(-8) mol/egg in ovo and 10(-7) and 10(-8)m in culture) resulted in minimal inhibition of TCDD-induced enzyme activities in ovo and a 37 to 50% inhibition in culture. The partial antagonist activity of MCDF in the chick embryo hepatocytes in culture paralleled the interactive effects previously reported in rodent liver and transformed rodent cell lines. TCDD (10(-7) to 10(-10)m) also caused an accumulation of hepta- and octacarboxyporphyrins in chick embryo hepatocytes (10(-7) to 10(-9)m); however, MCDF (10(-6) and 10(-5)m) elicited similar responses and MCDF did not significantly decrease the TCDD-induced porphyrogenic response in these cells. These results suggest that chick embryo hepatocytes in culture will serve as a useful model for investigating TCDD-induced gene transcription and the effects and mechanism of action of antagonists.

6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) as an antiestrogen in human and rodent cancer cell lines: evidence for the role of the Ah receptor

6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) is a relatively nontoxic analog of 2,3,7,8-tetrachlorodibenzo-p-dioxin. Treatment of aryl hydrocarbon (Ah)-responsive MCF-7 human breast cancer cells with 100 nM MCDF resulted in the inhibition of 17 beta-estradiol-induced proliferation and the secretion of the 34-, 52-, and 160-kDa proteins. After treatment of the cells with 17 beta-[3H]estradiol, 100 nM of MCDF caused a decrease in the accumulation of the radiolabeled nuclear estrogen receptor (ER) complex in these cells. In parallel experiments, the antiestrogenic effects of MCDF were also determined in Ah-responsive wild-type Hepa 1c1c7 cells and Ah-nonresponsive class 1 and class 2 mutant cells. Treatment of the wild-type cells with 17 beta-[3H]estradiol and 100 nM MCDF caused a decrease in the accumulation of radiolabeled nuclear ER complex in these cells whereas no significant effects were observed in the mutant cells as determined by velocity sedimentation analysis. Comparable results were obtained using ER antibodies to measure the decrease in immunoreactive nuclear ER. In addition, both actinomycin D and cycloheximide inhibited the MCDF-mediated decrease of nuclear ER levels in the Hepa 1c1c7 wild-type cells. Although 100 nM MCDF did not induce cytochrome P-450-dependent monooxygenases in the MCF-7 or Hepa 1c1c7 cell lines, incubation of nuclear extracts from the MCF-7 cells treated with 100 nM MCDF with a synthetic consensus dioxin responsive element (an oligonucleotide duplex of 26 bases) gave a retarded band in a gel-retardation assay. The data suggest that the antiestrogenic effects of MCDF does not require the induction of the CYP1A1 gene expression but may involve the induction of other genes.

Polychlorinated biphenyls in the environment

This review surveys the problems arising from the release of PCBs into the environment from the point of view of the analytical chemist. These problems are very complex and interdependent and so it is essential to recognize their mutual links rather than to separate one problem from another (sources of contamination, fate in the environment, toxic properties and particular capabilities, limitations and purposes of analytical methods). Prominent attention should be paid in the future to congener-specific analyses of "toxic" congeners using high-resolution gas chromatography and to toxicity-assessing biological methods.

Mechanism of benzo[a]pyrene-induced Cyp1a-1 gene expression in mouse Hepa 1c1c7 cells: role of the nuclear 6 s and 4 s proteins

Treatment of wild-type (wt) aryl hydrocarbon (Ah)-responsive mouse Hepa 1c1c7 cells with benzo[a]pyrene (B[a]P) caused a concentration-dependent induction of ethoxyresorufin O-deethylase (EROD) activity. In contrast, B[a]P was inactive as an inducer in Ah nonresponsive class 1 and class 2 mutant cell lines. In parallel experiments, the nuclear fractions from wt cells treated with 10(-7) M [3H]B[a]P contained both the 4 s carcinogen binding protein and the 6 s (Ah receptor) complexes, whereas only the 4 s complex was present in the nuclear fraction of the class 2 mutant cells. The results obtained from cotreatment of wt Hepa 1c1c7 cells with 10(-6) or 10(-7) M B[a]P and 5 x 10(-7) or 10(-7) M 6-methyl-1,3,8-trichlorodibenzofuran (MCDF) showed that MCDF inhibited the induction of EROD activity and Cyp1a-1 mRNA levels by B[a]P. Moreover, using 10(-7) M [3H]B[a]P and unlabeled MCDF, it was shown that MCDF not only inhibited the induction response but also caused a concentration-dependent decrease in levels of the nuclear 6 s complex but not the 4 s complex. In contrast, in situ competition studies with unlabeled 10(-6) M benzo[ghi]-perylene (B[ghi]P) resulted in the elimination of the nuclear [3H]B[a]P 4 s complex (but not the 6 s complex); however, the EROD activity and Cyp1a-1 mRNA levels in cells treated with 10(-7) M B[a]P in the presence or absence of 10(-6) M B[ghi]P were not significantly different. These results indicate that the 4 s binding protein is not required for the induction of Cyp1a-1 gene expression in Hepa 1c1c7 cells and suggest that B[a]P and 2,3,7,8-tetrachlorodibenzo-p-dioxin induce Cyp1a-1 gene expression via a common mechanism which involves binding to the Ah receptor.

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) and related compounds as antioestrogens: characterization and mechanism of action

In the female Sprague-Dawley rat uterus 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds exhibited a broad spectrum of antioestrogenic responses. For example 2,3,7,8-TCDD inhibited the 17 beta-oestradiol-induced uterine wet weight increase, peroxidase activity, oestrogen and progesterone receptor levels, epidermal growth factor (EGF) receptor binding, and EGF receptor and c-fos protooncogene mRNA levels. The aryl hydrocarbon (Ah) receptor was identified in the rat uterus and the antioestrogenic activities of TCDD and related compounds were structure-dependent. In parallel studies, the effects of TCDD as an antioestrogen in MCF-7 human breast cancer cells was also investigated. TCDD inhibited the 17 beta-oestradiol-induced proliferation of these cells and the secretion of the 34-, 52- and 160-kDa proteins. Treatment of MCF-7 cells with 1 nM [3H]-17 beta-oestradiol resulted in a rapid accumulation of nuclear oestrogen receptor (ER) complexes. Pretreatment of the cells with TCDD caused a rapid decrease in nuclear ER binding activity and immunoreactive protein; moreover, the structure-dependent potencies of TCDD and related compounds as antioestrogens were similar to their Ah receptor binding affinities. TCDD also caused a decrease in nuclear ER levels in wild-type Ah-responsive Hepa 1c1c7 cells but was inactive in Ah non-responsive mutant Hepa 1c1c7 cells. Moreover, in the wild-type cells, both actinomycin D and cycloheximide blocked the effects of TCDD. 6-Methyl-1,3,8-trichlorodibenzofuran (MCDF) has previously been characterized as a TCDD antagonist in rodents and in transformed rodent cell lines. However, like TCDD, MCDF also exhibited a broad spectrum of antioestrogenic activities in both the female Sprague-Dawley rat uterus and MCF-7 cells. MCDF is relatively non-toxic compared to TCDD and is being investigated as a compound which may be clinically useful for the treatment of mammary cancer.

Liver high performance liquid chromatographic porphyrin profiles in experimental porphyria induced by peroxidizing herbicides

The effect of peroxidizing herbicides on the liver porphyrin content of experimental animals was examined. Mice treated with the herbicide oxadiazon accumulated uroporphyrin and protoporphyrin in the liver. Formesafen-treated mice accumulated uroporphyrin and heptacarboxylic porphyrin.

Structure-dependent induction of aryl hydrocarbon hydroxylase activity in C57BL/6 mice by 2,3,7,8-tetrachlorodibenzo-p-dioxin and related congeners: mechanistic studies

The time- and dose-dependent induction of murine hepatic microsomal aryl hydrocarbon hydroxylase (AHH) and ethoxyresorufin O-deethylase (EROD) activities by five polychlorinated dibenzo-p-dioxin and dibenzofuran congeners showed that the order of induction potency was 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) greater than 2,3,7,8-tetrachlorodibenzofuran (TCDF) greater than 1,2,3,7,8-pentachlorodibenzo-p-dioxin (PCDD) greater than 1,2,3,7,8-pentachlorodibenzofuran (PCDF) greater than 2,3,7-trichlorodibenzo-p-dioxin (TrCDD). These structure-induction relationships were comparable to the structure-toxicity and competitive structure-receptor binding relationships previously reported for these compounds. However, using the corresponding radiolabeled congeners, the direct binding Kd values for dissociation of the cytosolic receptor-ligand complexes were 9.52, 7.96, 1.27, 3.10, and 8.31 nM for the 2,3,7,8-TCDD, 2,3,7,8-TCDF, 2,3,7-TrCDD, 1,2,3,7,8-PCDD, and 1,2,3,7,8-PCDF congeners and these data were clearly not structure dependent (i.e., similar to the structure-activity relationships). Some of the molecular properties for several radioligand-receptor complexes were similar; for example, the sedimentation coefficients for the cytosolic and nuclear receptor complexes varied from 8.8-10.4 S and 5.98-7.0 S, respectively, and the nuclear receptor complexes for all the radioligands eluted from a DNA-Sepharose column at salt concentrations of 0.27-0.29 M. Treatment of the mice with a maximum inducing dose of 2,3,7,8-[3H]TCDD resulted in a time-dependent formation of the nuclear receptor complex which was maximized between 16-24 hr and subsequently decreased up to 72 hr after initial exposure. In parallel studies, the nuclear receptor complex levels were determined 16 hr after treatment of the mice with different doses (2.25, 4.5, and 45 micrograms/kg) of all five radioligands. The results showed that at submaximal induction of the monooxygenase enzyme activities there was a linear correlation between the induced AHH or EROD activities (after 32 hr) and the corresponding nuclear receptor complex levels. It was also apparent from the data that the relative levels of nuclear receptor complex were structure dependent and this suggests that the transformation or activation of cytosolic receptor complexes may be a ligand structure-dependent process which correlates with the observed structure-activity relationships for 2,3,7,8-TCDD and related compounds.

The mechanism of action of alpha-naphthoflavone as an inhibitor of 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced CYP1A1 gene expression

Treatment of rat hepatoma H-4-II E cells with alpha-naphthoflavone (alpha NF) (10(-8), 10(-7), 10(-6)M) resulted in only minimum induction of ethoxyresorufin O-deethylase (EROD) activity and cytochrome P4501A1 mRNA levels only at 10(-6)M. In contrast, 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) caused maximum or near maximum induction responses at 10(-8) and 10(-9)M. In a time-course study with TCDD (10(-9)M), and TCDD plus alpha NF (cotreated), alpha NF significantly inhibited the induction of EROD activity and cytochrome P4501A1 mRNA levels by TCDD for 6-24 h after initial exposure of the cells to the chemicals. In addition, treatment of the cells with 10(-9)M TCDD in the presence or absence of 10(-8), 10(-7), and 10(-9)M alpha NF showed that the latter compound inhibited the induction effects by TCDD in a concentration-dependent manner and these inhibitory effects could be overcome, in part, by a higher concentration of TCDD (10(-8)M). Treatment of the rat hepatoma H-4-II E cells with [3H]TCDD showed that within 60 min, there was an initial rapid increase in nuclear [3H]TCDD receptor complex levels (38 fmol/mg protein) which decreased to less than 10 fmol/mg protein within 4 h and remained relatively constant for up to 24 h. However, in cells treated with [3H]TCDD (10(-9)M) plus alpha NF (10(-6)M) the levels of the nuclear [3H]TCDD receptor complex were less than 5 fmol/mg protein throughout the 24-h time course. These data, coupled with the results which indicate that the alpha NF competitively inhibits the binding of [3H]-TCDD to the cytosolic aryl hydrocarbon (Ah) receptor, suggest that alpha NF inhibits the TCDD-mediated induction of CYP1A1 gene transcription and translation by direct competition for cytosolic Ah receptor binding sites.

Oxidative stress induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is one of the most potent toxins and tumor promoters known to man. It is prototypical of many halogenated polycyclic hydrocarbons that occur as environmental contaminants. Pathologic lesions produced by these compounds are mediated by an intracellular receptor protein called the TCDD (Ah) receptor which functions as a trans-acting effector of gene expression. However, the ultimate posttranslational pathways and mechanisms involved in the expression of the toxic manifestations of TCDD have received little attention and remain unclear, yet constitute an important segment in our understanding of the overall mechanism of action of TCDD. Recent studies have demonstrated that an oxidative stress occurs in various tissues of TCDD-treated animals. Evidence indicating production of an oxidative stress by TCDD in rodents is reviewed and includes:enhanced in vitro and in vivo hepatic and extrahepatic lipid peroxidation; increased hepatic and macrophage DNA damage; increased urinary excretion of malondialdehyde; decreased hepatic membrane fluidity; increased production of superoxide anion by peritoneal macrophage; and decreased glutathione, nonprotein sulfhydryl, and NADPH contents in liver. The potential role of reactive oxygen species in tumor promotion by TCDD is discussed. Possible sources and mechanisms of production of reactive oxygen species in response to TCDD are considered in light of current information. Evidence demonstrating the involvement of iron in TCDD-induced formation of reactive oxygen species and DNA damage is reviewed. Oxidative damage may contribute to many of the toxic responses produced by TCDD and its bioisosteres, and may be common to most of the tissue-damaging effects.

Polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), and related compounds: environmental and mechanistic considerations which support the development of toxic equivalency factors (TEFs)

Halogenated aromatic compounds, typified by the polychlorinated dibenzo-p-dioxins (PCDDs), dibenzofurans (PCDFs), biphenyls (PCBs), and diphenylethers (PCDEs), are industrial compounds or byproducts which have been widely identified in the environment and in chemical-waste dumpsites. Halogenated aromatics are invariably present in diverse analytes as highly complex mixtures of isomers and congeners and this complicates the hazard and risk assessment of these compounds. Several studies have confirmed the common receptor-mediated mechanism of action of toxic halogenated aromatics and this has resulted in the development of structure-activity relationships for this class of chemicals. The most toxic halogenated aromatic is 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and based on in vivo and in vitro studies the relative toxicities of individual halogenated aromatics have been determined relative to TCDD (i.e., toxic equivalents). The derived toxic equivalents can be used for hazard and risk assessment of halogenated aromatic mixtures; moreover, for more complex mixtures containing congeners for which no standards are available (e.g., bromo/chloro mixtures), several in vitro or in vivo assays can be utilized for hazard or risk assessment.