Liver tumour promoting activity of 3,4,5,3',4'-pentachlorobiphenyl and its interaction with 2,3,7,8-tetrachlorodibenzo-p-dioxin

Hair analysis to document a clinical case of TCDD over-exposure

Dioxins and related compounds (furans) are persistent environmental contaminants that cause adverse biological effects. Their influence on humans is still unclear, except for accidental high-dose exposure. Chronic exposure to these compounds seems to be involved in cancer, endocrine disruption and neurobehavioral effects. For several years, a large concern about the potential health risks of dioxins is emerging in Europe and United States. The case of a 50-year-old man victim of an acute over-exposure to tetrachloro dibenzo-p-dioxin (TCDD or Seveso dioxin) is reported here with particular focus on hair investigations. The developed method involved the decontamination of the hair strand using picograde level methylene chloride, the homogenization of hair segments with scissors and their extraction in presence of 13C12-marked dioxin congeners under reflux of toluene using a Soxhlet, 8h at 130 degrees C. After reduction of the toluene fraction to 1 ml and addition of purification marker (37Cl4-2,3,7,8-TCDD), dioxins purification was achieved using three successive columns: silica, alumina/sodium sulfate and carbon/Celite columns. Finally, the toluene eluent was evaporated and the extract injected in the analytical system. After chromatographic separation, detection was achieved in single ion monitoring mode using a high resolution mass spectrometer operating in electron impact ionization mode (40 eV, minimal resolution of 10,000). The analysis of the first hair segment (0-6 cm) revealed the presence of 2,3,7,8-TCDD at 65 fg/mg when the distal one remained negative (LOQ=0.3 fg/mg). All other congeners (n=16) were in the range of those determined in the general population (0.62 and 0.89 fg/mg in the two hair segments, respectively). The extremely low dioxin levels generally found in hair specimens (low fg/mg range) lead us to analyze them using the very sensitive and specific gas chromatography-high resolution mass spectrometry apparatus. From the best of our knowledge, this is the first case of over-exposure to Seveso dioxin through hair analysis reported in the literature.

The aryl hydrocarbon receptor agonist 3,3',4,4',5-pentachlorobiphenyl induces distinct patterns of gene expression between hepatoma and glioma cells: chromatin remodeling as a mechanism for selective effects

Genome-wide oligonucleotide DNA microarrays and real time RT-PCR were used to assess differential gene expression in rat glioma and hepatoma cell lines after exposure to the aryl hydrocarbon receptor (AhR) agonist 3,3',4,4',5-pentachlorobiphenyl (penta-CB). Under maximal inducing concentrations for cytochrome P450 1A1 (CYP1A1) in H4IIE rat hepatoma cells, both H4IIE and C6 rat glioma cells were exposed to sub-micromolar concentrations of penta-CB for 24h. Differential gene expression for approximately 28,000 gene probes were computationally analyzed and compared. As expected, penta-CB potently activated CYP1A1/2 transcription in liver-derived H4IIE hepatoma cells yet did not do so in brain-derived C6 glioma cells. Additionally, we show that penta-CB causes: (1) distinct patterns of gene expression between tumor cells derived from liver or brain; (2) robust transcriptional activation of select C6 glioma gene ontologies; (3) over-expression of H4IIE hepatoma genes associated with tumor progression in liver; (4) greater than 100-fold over-expression of C6 glioma genes associated with protein processing and programmed cell death and/or metastasis; (5) tissue-selective histone deacetylase inhibition in C6 glioma, but not H4IIE hepatoma cells as signaled by galectin-1 over-expression.

The 2005 World Health Organization reevaluation of human and Mammalian toxic equivalency factors for dioxins and dioxin-like compounds

In June 2005, a World Health Organization (WHO)-International Programme on Chemical Safety expert meeting was held in Geneva during which the toxic equivalency factors (TEFs) for dioxin-like compounds, including some polychlorinated biphenyls (PCBs), were reevaluated. For this reevaluation process, the refined TEF database recently published by Haws et al. (2006, Toxicol. Sci. 89, 4-30) was used as a starting point. Decisions about a TEF value were made based on a combination of unweighted relative effect potency (REP) distributions from this database, expert judgment, and point estimates. Previous TEFs were assigned in increments of 0.01, 0.05, 0.1, etc., but for this reevaluation, it was decided to use half order of magnitude increments on a logarithmic scale of 0.03, 0.1, 0.3, etc. Changes were decided by the expert panel for 2,3,4,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.3), 1,2,3,7,8-pentachlorodibenzofuran (PeCDF) (TEF = 0.03), octachlorodibenzo-p-dioxin and octachlorodibenzofuran (TEFs = 0.0003), 3,4,4',5-tetrachlorbiphenyl (PCB 81) (TEF = 0.0003), 3,3',4,4',5,5'-hexachlorobiphenyl (PCB 169) (TEF = 0.03), and a single TEF value (0.00003) for all relevant mono-ortho-substituted PCBs. Additivity, an important prerequisite of the TEF concept was again confirmed by results from recent in vivo mixture studies. Some experimental evidence shows that non-dioxin-like aryl hydrocarbon receptor agonists/antagonists are able to impact the overall toxic potency of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and related compounds, and this needs to be investigated further. Certain individual and groups of compounds were identified for possible future inclusion in the TEF concept, including 3,4,4'-TCB (PCB 37), polybrominated dibenzo-p-dioxins and dibenzofurans, mixed polyhalogenated dibenzo-p-dioxins and dibenzofurans, polyhalogenated naphthalenes, and polybrominated biphenyls. Concern was expressed about direct application of the TEF/total toxic equivalency (TEQ) approach to abiotic matrices, such as soil, sediment, etc., for direct application in human risk assessment. This is problematic as the present TEF scheme and TEQ methodology are primarily intended for estimating exposure and risks via oral ingestion (e.g., by dietary intake). A number of future approaches to determine alternative or additional TEFs were also identified. These included the use of a probabilistic methodology to determine TEFs that better describe the associated levels of uncertainty and "systemic" TEFs for blood and adipose tissue and TEQ for body burden.

Carcinogenic risks of dioxin: Mechanistic considerations

Dioxins and dioxin-like chemicals demonstrate high affinity binding to the aryl hydrocarbon receptor (AhR), a ligand activated transcription factor, which mediates most, if not all, of the toxic responses of these agents. Since dioxins are not directly genotoxic their carcinogenic effect is likely the result of their tumor promoting activity produced by activation of the AhR. For the purpose of risk assessment extrapolation from effects in the observable high dose range to background dietary exposure is necessary. In the present review, we discuss various aspects of low-dose-response of receptor-mediated processes in general, including threshold phenomena with regard to tumor promotion during multi-stage carcinogenesis. In this connection the reversibility of tumor promotion plays an important role but this may not be valid for dioxins due to their long half life. The relevance of cytochrome P 4501 A-induction as biomarker for prediction of carcinogenic effects of dioxins at low doses is considered. Dioxins may act in concert with endogenous ligands of the AhR, an effect which becomes particularly relevant at low toxicant concentrations. At present, however, the nature and role of these postulated ligands are unknown. Furthermore, it is unclear whether dioxins produce synergistic tumor promotional effects with non-dioxin-like chemicals to which humans are also exposed. Dioxins and, e.g., non-dioxin-like PCBs act through different receptors and there is, albeit yet limited, experimental evidence from experimental studies to suggest that they may act on different target cell populations within the same target organ. From the available data the existence of a (physiological) threshold of effects cannot be proven and may not even exist. For regulatory purposes the application of a so called "practical threshold" for the carcinogenic effect of dioxins is proposed. Further mechanistic studies should be conducted to get insight into the dose-response characteristics of relevant events of dioxin-like and non-dioxin-like agents and into the consequences of potential interactions between both group of compounds during carcinogenesis.

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.

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.

Interactive effects of three structurally different polychlorinated biphenyls in a rat liver tumor promotion bioassay

Interactive effects between the non-ortho-substituted 3,3', 4,4', 5-pentachlorobiphenyl (PCB126), the mono-ortho-substituted 2,3,3',4, 4'-pentachlorobiphenyl (PCB105), and the di-ortho-substituted 2,2',4, 4',5,5'-hexachlorobiphenyl (PCB153) were studied in an initiation/promotion bioassay. Female Sprague-Dawley rats were injected with 30 mg/kg ip of N-nitrosodiethylamine 24 h after partial hepatectomy. Five weeks later, weekly sc administrations of the three PCBs in 15 systematically selected dose combinations started. After 20 weeks of administration, the animals were killed and the livers were analyzed for areas expressing placental glutathione-S-transferase as a marker of preneoplastic foci. In addition, concentration of liver and kidney retinoids and plasma retinol was analyzed, as well as body and organ weights, plasma transaminases, and induction of hepatic cytochrome P450 1A1/2 (CYP1A1/2) and CYP2B1/2 activities. Data were analyzed with a multivariate method. At the doses applied in this study, weak antagonism was observed between PCB126 and PCB153 for effects on volume fraction of foci, number of foci/cm3, concentration of plasma retinol and liver retinoids, relative liver weight, and induction of CYP2B1/2 activity. Weak antagonism was also observed between PCB126 and PCB105 for effects on volume fraction of foci, number of foci/cm3, and plasma retinol concentration. No interactions other than pure additivity were observed between PCB105 and PCB153. Synergism was not observed within the dose ranges investigated in this study. Knowledge of interactive effects is important for risk assessment of environmental mixtures of dioxin-like compounds. Antagonism between congeners generally results in risk assessments that overestimate human risk. The significance to human risk assessment of the relatively weak antagonism observed in this study is however unclear, considering many other uncertainties involved in the toxic equivalency factor (TEF) concept. A change of the TEF concept for risk assessments of dioxin-like substances is not motivated based on the results of this study.