Essential role of the AH receptor in the dysfunction of heme metabolism induced by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Abstract

The dysfunction of hepatic heme synthesis by 2,3,7,8-tetrachlordibenzo- p-dioxin (TCDD) in mice, enhanced by iron, leads to accumulation of uroporphyrins I and III (uroporphyria) and resembles the human disorder porphyria cutanea tarda (PCT) precipitated by alcohol and estrogenic drugs. Although consequences of TCDD are considered entirely dependent on the aryl hydrocarbon receptor (AHR), this is not proven for uroporphyria. Administration of TCDD (75 microg/kg) caused uroporphyria in susceptible C57BL/6J mice with high-affinity AHR after 5 weeks (>600-fold increase in hepatic uroporphyrins). Transcriptomics showed significant modified gene expressions for intermediary, heme, and iron metabolism as well as for oxidative stress and cell injury. Resistant low-affinity AHR DBA/2 mice (no increase in porphyrins) showed far fewer changes. At this dose of TCDD, persistent up-regulation of some traditional AH battery genes occurred in both strains. Essentiality of AHR was demonstrated with C57BL/6 Ahr knockout mice. Elevation of hepatic uroporphyrins was 964-fold in Ahr (+/+) mice, lower in Ahr (+/-) (60-fold), but undetectable with Ahr (-/-) . Consistent with an oxidative mechanism, iron overload enhanced porphyria as well as general liver injury in Ahr (+/+) and Ahr (+/-) mice but had no interactive effect in Ahr (-/-) . In contrast, when iron-treated mice received, instead of TCDD, the heme precursor 5-aminolevulinic acid (ALA), causing uroporphyia in Ahr (+/+) mice (242-fold rise in uroporphyrins), elevation of uroporphyrins I and III (42-fold) also occurred in Ahr (-/-) mice and was seemingly associated with AHR-independent expression of Cyp1a2. The findings prove that AHR is a key factor in porphyria induced in mice by TCDD. However, in other models of human PCT, participation of AHR may not be an essential requirement.