Aryl Hydrocarbon Receptor Splice Variants in the Dioxin-Resistant Rat: Tissue Expression and Transactivational Activity

Abstract

The AHR locus encodes the aryl hydrocarbon receptor (AHR), a transcriptional regulator of multiple drug-metabolizing enzymes and mediator of toxicity of dioxin-like chemicals. The Han/ Wistar (Kuopio) rat strain (H/W) is remarkably resistant to lethal effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) because of a point mutation in the exon/intron 10 boundary in AHR genomic structure that leads to use of 3 alternative cryptic splice sites, potentially creating 3 alternative transcripts and 2 protein products. The deletion variant (DV), which lacks 43 amino acids in the transactivation domain, has the highest intrinsic transactivation activity in vitro; amino acids 766 to 783 suppress transactivation function. However, DV expression levels in H/W rats in vivo are low in liver, lung, thymus, kidney, and testis; insertion variant mRNAs (IVs) are the dominant mRNA forms in H/W rats in which wild-type AHR mRNA is undetectable. In dioxin-sensitive rat strains and lines that are homozygous for wild-type AHR alleles, wild-type AHR mRNA is the most abundant transcript but some IV transcripts are detectable. TCDD treatment in vivo increases transcript levels for both the DV and IVs in H/W rats and increases wild-type transcript levels in dioxin-sensitive rats but does not alter which transcript forms are expressed. In silico modeling indicates that the DV mRNA has lost considerable secondary structure, whereas at the protein level, the transactivation domain of the IV in the dioxin-resistant H/W rat has greater ␣-helical content and a more hydrophobic terminus than wild-type AHR, which may produce a protein conformation that is less amenable to interaction with other regulatory proteins.

Possible involvement of arylhydrocarbon receptor variants in TCDD-induced thymic atrophy and XRE-dependent transcriptional activity in Wistar Hannover GALAS rats

“…4). However, AhRdv was found to have the highest activity mediated by the synthesized XRE among the four AhR variants protein encoded by AhR hw/hw could regulate the XREdependent pathway differently from the proteins encoded by the other AhR variants, which is consistent with the recent data of Moffat and coworkers (Moffat et al, 2007), who showed, using the transactivation domain of AhR variants and the GAL4-DNA binding domain, that -ity in the absence of exogenous ligands such as TCDD.…”

Aryl Hydrocarbon Receptor-Dependent Induction of Flavin-Containing Monooxygenase mRNAs in Mouse Liver

“…B. Okey, manuscript in preparation). There are dmd.aspetjournals.org substantial differences in AH receptor structure between C57BL/6J mice and wild-type rat AHR (Moffat et al, 2007b;Okey, 2007). To determine whether these differences underlie the lack of FMO induction in rats we measured FMO2 and FMO3 mRNA levels in TCDDtreated transgenic mice that express wild-type rat AHR in a C57BL/6J background.…”

When evolution is the solution to pollution: Key principles, and lessons from rapid repeated adaptation of killifish (Fundulus heteroclitus) populations

“…Among HAHs, ligand affinity is a good predictor of toxicity; compounds that bind with greatest affinity are the most potent at altering gene expression and causing toxic effects (Safe, 1994). Differences in AHR properties, including affinity for ligand binding, explain differences in sensitivity to DLCs in several vertebrate systems, including strains of rodents (Moffat, Roblin, Harper, Okey, & Pohjanvirta, 2007; Poland, Palen, & Glover, 1994), species of birds (Farmahin et al., 2013; Karchner, Franks, Kennedy, & Hahn, 2006), and populations of some fish (Atlantic tomcod) (Wirgin et al., 2011). (At the extreme end of this spectrum are invertebrates, which have AHR proteins that lack the ability to bind DLCs [Powell‐Coffman, Bradfield, & Wood, 1998; Butler, Kelley, Powell, Hahn, & Van Beneden, 2001] and also are largely insensitive to DLC toxicity [Hahn, 1998a]. )…”