Interaction between Iron Metabolism and 2,3,7,8-Tetrachlorodibenzo-p-dioxin in Mice with Variants of the Ahr Gene: A Hepatic Oxidative Mechanism
- Andrew G. Smith1,
- Bruce Clothier1,
- Susan Robinson1,
- Matthew J. Scullion1,
- Philip Carthew1,
- Richard Edwards1,
- Jinli Luo1,
- Chang Kee Lim1 and
- Michel Toledano1,2
- 1Medical Research Council Toxicology Unit, University of Leicester, Leicester LE1 9HN, UK (A.G.S, B.C, M.S, P.C, R.E, J.L, C.K.L, S.R.), and 2Environmental and Occupational Health Sciences Institute, New Jersey Medical School and Rutgers University, Piscataway, New Jersey 08855-1179 (M.T.)
Abstract
The binding of 2,3,7,8 tetrachlorodibenzo-p-dioxin (TCDD) with the aryl hydrocarbon (AH) receptor and subsequent changes in gene expression have been studied intensively, but the mechanisms by which these lead to toxicity are unclear. We investigated the influence of iron, previously implicated in TCDD-induced hepatic porphyria, in mice with alleles of Ahr that encode receptors with varied affinity for TCDD. The administration of iron toAhrb-1 C57BL/6J (AH-responsive) mice before a single dose of TCDD (75 μg/kg) markedly potentiated not only the hepatic porphyria but also general hepatocellular damage and elevation of plasma hepatic enzymes. The formation of hydroxylated and peroxylated derivatives of uroporphyrins formed from uroporphyrinogen and the induction of a μ-glutathione transferase (GST) were consistent with the operation of an oxidative mechanism. In a comparison of C57BL/6J mice with Ahrb-2BALB/c (AH-responsive) and Ahrd SWR and DBA/2 (AH-nonresponsive) mice, iron overcame the weak hepatic porphyria and toxicity responses in BALB/c and SWR strains but not in DBA/2. CYP1A isoforms are strongly implicated in the mechanism of porphyria, but activities were lowered by 20–30% with iron treatment, and a comparison of levels between strains did not fully account for the resistance of DBA/2 mice. Studies with the use of gel shift assays and cytosolic aconitase of the capacity of the iron regulatory protein controlling the translation of some iron metabolism proteins showed a significant difference between C57BL/6J and DBA/2 mice after the administration of TCDD. We conclude that iron potentiates both the hepatic porphyria and toxicity of TCDD in susceptible mice in an oxidative process with disturbance of iron regulatory protein capacity. Iron even overcomes the AH-nonresponsiveAhrd allele in the SWR strain but not in DBA/2 mice, which remain resistant.
Footnotes
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Send reprint requests to: Dr. A. G. Smith, MRC Toxicology Unit, Hodgkin Building, University of Leicester, Lancaster Road, P.O. Box 138, Leicester LE1 9HN, United Kingdom. E-mail:ags5{at}le.ac.uk
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↵1 Current affiliation: Department de Biologie Cellulaire et Moleculaire, CEA/Saclay, F-91191, Gif-sur-Yvette, France.
- Abbreviations:
- TCDD
- 2,3,7,8-tetrachlorodibenzo-p-dioxin
- GST
- μ-glutathione transferase
- AH
- aryl hydrocarbon
- AHR
- aryl hydrocarbon receptor
- ALT
- alanine aminotransferase
- IRP
- iron regulatory protein
- IRE
- iron responsive element
- HPLC
- high performance liquid chromatography
- UROD
- uroporphyrinogen decarboxylase
- HCB
- hexachlorobenzene
- PCNA
- proliferative cell nuclear antigen
- PCT
- porphyria cutanea tarda
- PCB
- polychlorinated biphenyl
- EROD
- ethoxyresorufin dealkylation
- MROD
- methoxyresorufin dealkylation
- BROD
- benzyloxyresorufin dealkylation
- HEPES
- 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid
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- Received May 19, 1997.
- Accepted September 12, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
