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Vol. 53, Issue 1, 52-61, January 1998
Medical 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
Environmental and Occupational Health Sciences Institute, New
Jersey Medical School and Rutgers University, Piscataway, New Jersey
08855-1179 (M.T.)
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 to
Ahrb-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-2
BALB/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-nonresponsive
Ahrd allele in the SWR strain but not in
DBA/2 mice, which remain resistant.
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