Gestational exposure of Ahr and Arnt hypomorphs to dioxin rescues vascular development

Abstract

The aryl hydrocarbon receptor (AHR) is commonly known for its role in the adaptive metabolism of xenobiotics and in the toxic events that follow exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). Previously, we have demonstrated that the AHR and its heterodimeric partner, the AHR nuclear translocator (ARNT), play a role in the developmental closure of a hepatic vascular shunt known as the ductus venosus (DV). To investigate the mechanism of DV closure, we generated hypomorphic alleles of the Ahr and Arnt loci. Using these models, we then asked whether this vascular defect could be rescued by receptor activation during late development. By manipulating gestational exposure, the patent DV in AHR or ARNT hypomorphs could be efficiently closed by dioxin exposure as early as embryonic day 12.5 and as late as embryonic day 18.5. These findings define the temporal regulation of receptor activation during normal ontogeny and provide evidence to support the idea that receptor activation and AHR-ARNT heterodimerization are essential for normal vascular development. Taken in the broader context, these data demonstrate that similar AHR signaling steps govern all major aspects of AHR biology.

Aryl hydrocarbon receptor-dependent liver development and hepatotoxicity are mediated by different cell types

“…The conditional Ahr allele (Ahr fx ) contains exon 2 of Ahr flanked by loxP sites (''floxed'') for later excision by a cell-specific Cre recombinase (Cre). The Ahr fx mice were generated from the Ahr fxneo allele (8). Mice homozygous for the Ahr fxneo allele were crossed to a transgenic line carrying Cre under the control of the Ella promoter (Cre Ella , strain name: FVB͞N-Tg(EIIa-cre)C5379Lmgd͞J) (The Jackson Laboratory) to generate partial recombinants and obtain the conditional Ahr fx allele lacking the neomycin gene (11,12).…”

“…Mice homozygous for the Ahr fxneo allele were crossed to a transgenic line carrying Cre under the control of the Ella promoter (Cre Ella , strain name: FVB͞N-Tg(EIIa-cre)C5379Lmgd͞J) (The Jackson Laboratory) to generate partial recombinants and obtain the conditional Ahr fx allele lacking the neomycin gene (11,12). Discrimination between the WT and floxed allele was accomplished by PCR amplification of the region surrounding the loxP site located 3Ј to exon 2, as described (8). Excision of the neomycin gene was detected by Southern blot analysis of BglIIdigested genomic DNA by using a 500-bp probe specific to exon 2 of Ahr.…”

“…Using this model, we have provided evidence to suggest that the intracellular details of AHR signal transduction are similar for the adaptive, toxic, and developmental pathways. Through the use of recombinant Ahr and Arnt alleles, we have shown that AHR activation, AHR translocation to the nucleus, AHR dimerization with the aryl hydrocarbon receptor nuclear translocator (ARNT), and AHR-ARNT binding to dioxin responsive elements within the genome are required for adaptive metabolism, dioxin toxicity, and closure of the DV within the developing liver (3,4,7,8,10).The question of how the AHR is able to produce multiple biological events from a similar signal transduction mechanism remains unclear. We hypothesize that receptor signaling in distinct cell types is responsible for these various aspects of AHR biology.…”

“…In response to halogenateddibenzo-p-dioxins, AHR activation results in the induction of xenobiotic metabolism plus a variety of toxic responses that include hepatocellular damage, thymic involution, epithelial hyperplasia, teratogenesis, and cancer (3)(4)(5)(6). Finally, in response to an unknown developmental cue, the AHR influences normal vascular development, most notably the closure of a fetal vascular structure known as the ductus venosus (DV) (3,(7)(8)(9).…”

The Aryl hydrocarbon receptor is activated by modified low-density lipoprotein

“…These include a patent ductus venosus (DV), persistent hyaloid arteries in the eye, decreased hepatic perfusion, a confused vascularization in the corneal limbus, and cardiac hypertrophy (4-6). Experiments using Ahr and Arnt hypomorphic mice have demonstrated that, similar to its role in xenobiotic metabolism, the developmental role of AHR requires activation of the receptor and formation of an AHR/ARNT dimer complex (7,8). Additionally, experiments using tissue-specific AHR null mice have demonstrated that AHR expression within endothelial and/or hematopoietic cells is necessary for closure of the DV (9).…”