Document Detail

Carcinogenic risks of dioxin: mechanistic considerations.
MedLine Citation:
PMID:  16054739     Owner:  NLM     Status:  MEDLINE    
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.
Michael Schwarz; Klaus E Appel
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Publication Detail:
Type:  Journal Article; Review    
Journal Detail:
Title:  Regulatory toxicology and pharmacology : RTP     Volume:  43     ISSN:  0273-2300     ISO Abbreviation:  Regul. Toxicol. Pharmacol.     Publication Date:  2005 Oct 
Date Detail:
Created Date:  2005-09-12     Completed Date:  2006-07-12     Revised Date:  -    
Medline Journal Info:
Nlm Unique ID:  8214983     Medline TA:  Regul Toxicol Pharmacol     Country:  United States    
Other Details:
Languages:  eng     Pagination:  19-34     Citation Subset:  IM    
Institute of Pharmacology and Toxicology, Department of Toxicology, University of Tübingen, Wilhelmstr. 56, 72074 Tübingen, Germany.
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MeSH Terms
Apoptosis / drug effects
Carcinogenicity Tests
Carcinogens, Environmental / toxicity*
Cytochrome P-450 CYP1A1 / biosynthesis*
Cytochrome P-450 CYP1A2 / biosynthesis*
Dioxins / toxicity*
Dose-Response Relationship, Drug
Drug Synergism
Environmental Exposure*
Enzyme Induction
Gene Expression Regulation / drug effects
Liver / drug effects*,  enzymology,  metabolism
Liver Neoplasms / chemically induced*
No-Observed-Adverse-Effect Level
Receptors, Aryl Hydrocarbon / drug effects*,  genetics,  metabolism
Risk Assessment
Transcription Factors / genetics,  metabolism
Reg. No./Substance:
0/Carcinogens, Environmental; 0/Dioxins; 0/Receptors, Aryl Hydrocarbon; 0/Transcription Factors; EC P-450 CYP1A1; EC P-450 CYP1A2

From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine

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