A physiological examination of mice harboring a null allele at the aryl hydrocarbon (Ah) locus revealed that the encoded aryl hydrocarbon receptor plays a role in the resolution of fetal vascular structures during development. Although the aryl hydrocarbon receptor is more commonly studied for its role in regulating xenobiotic metabolism and dioxin toxicity, a developmental role of this protein is supported by the observation that Ah null mice display smaller livers, reduced fecundity, and decreased body weights. Upon investigating the liver phenotype, we found that the decrease in liver size is directly related to a reduction in hepatocyte size. We also found that smaller hepatocyte size is the result of massive portosystemic shunting in null animals. Colloidal carbon uptake and microsphere perfusion studies indicated that 56% of portal blood flow bypasses the liver sinusoids. Latex corrosion casts and angiography demonstrated that shunting is consistent with the existence of a patent ductus venosus in adult animals. Importantly, fetal vascular structures were also observed at other sites. Intravital microscopy demonstrated an immature sinusoidal architecture in the liver and persistent hyaloid arteries in the eyes of adult Ah null mice, whereas corrosion casting experiments described aberrations in kidney vascular patterns.T he aryl hydrocarbon receptor (AHR) is a member of the per-arnt-sim (PAS) superfamily of proteins. The AHR regulates biological responses to a variety of environmental contaminants, such as the polycyclic aromatic hydrocarbons found in cigarette smoke, the polychlorinated dioxins that contaminate industrial chemicals, and the wartime defoliant Agent Orange (1-5). These chemical ligands bind to the AHR, leading to receptor dimerization with another PAS protein known as the aryl hydrocarbon nuclear translocator (ARNT). This heterocomplex interacts with genomic enhancer elements upstream of a battery of target genes that encode xenobiotic metabolizing enzymes (1, 6, 7). The observation that the up-regulated enzymes often have metabolic activity toward AHR agonists has led to the idea that this pathway represents an adaptive metabolic response that protects an organism from exposure to certain classes of toxic environmental contaminants. Although this adaptive role has considerable experimental support, this pathway is not always protective. Exposure to high-affinity AHR agonists, like the chlorinated dioxins, can result in cancer (8), immunosuppression (9), liver damage (10), and birth defects (11). The mechanisms underlying these toxic effects are unknown but appear to be AHR mediated.Because of its role in mediating responses to environmental contaminants, the biology of the AHR has been extensively characterized from a toxicological viewpoint. However, several observations suggest an additional role for the AHR in vertebrate development. First, a phylogenetic survey indicates that the AHR arose over 450 million years ago, with functional orthologs found in species that have evolved in vario...

The aryl hydrocarbon receptor (AHR) plays a role in three areas of biology that include the adaptive metabolism of xenobiotics, the toxic responses associated with exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin), and vascular remodeling of the developing embryo. To test the hypothesis that receptor signaling in different cell types is responsible for these aspects of AHR biology, we generated a conditional Ahr allele where exon 2 is flanked by loxP sites. Through the use of Cre-lox technology, we then investigated the role of AHR signaling in hepatocytes or endothelial cells in mediating prototypical endpoints of adaptive, toxic, or developmental signaling. Using this model, we provide evidence that AHR signaling in endothelial͞hematopoietic cells is necessary for developmental closure of the ductus venosus, whereas AHR signaling in hepatocytes is necessary to generate adaptive and toxic responses of the liver in response to dioxin exposure. Taken together, these data illustrate the importance of cell-specific receptor signaling for the generation of distinct AHR-dependent physiological outcomes.Cre recombinase ͉ ductus venosus ͉ endothelial cell ͉ hepatocyte ͉ dioxin T he aryl hydrocarbon receptor (AHR) is a basic helix-loophelix͞Per-Arnt-Sim protein that plays an essential role in three areas of biology. In response to polycyclic aromatic hydrocarbons, the AHR up-regulates a battery of xenobiotic metabolizing enzymes that include the cytochromes P450, CYP1A1, CYP1A2, and CYP1B1 as well as the phase II enzymes GST-A1 and UGT1-06 (1, 2). 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 (3-6). 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, 7-9).The mouse liver is a powerful model for investigations related to AHR biology. The mouse system allows the production of recombinant loci by gene targeting, whereas the liver provides a representation of each of the three aspects of AHR signal transduction. 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 ...

The Ah receptor (AHR) mediates the metabolic adaptation to a number of planar aromatic chemicals. Essential steps in this adaptive mechanism include AHR binding of ligand in the cytosol, translocation of the receptor to the nucleus, dimerization with the Ah receptor nuclear translocator, and binding of this heterodimeric transcription factor to dioxin-responsive elements (DREs) upstream of promoters that regulate the expression of genes involved in xenobiotic metabolism. The AHR is also involved in other aspects of mammalian biology, such as the toxicity of molecules like 2,3,7,8-tetrachlorodibenzo-p-dioxin as well as regulation of normal liver development. In an effort to test whether these additional AHR-mediated processes require a nuclear event, such as DRE binding, we used homologous recombination to generate mice with a mutation in the AHR nuclear localization/DRE binding domain. These Ahr nls mice were found to be resistant to all 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced toxic responses that we examined, including hepatomegaly, thymic involution, and cleft palate formation. Moreover, aberrations in liver development observed in these mice were identical to that observed in mice harboring a null allele at the Ahr locus. Taken in sum, these data support a model where most, if not all, of AHR-regulated biology requires nuclear localization. The aryl hydrocarbon receptor (AHR)1 regulates an adaptive metabolic response to a variety of planar aromatic chemicals that are widely dispersed in the environment. Over the last 20 years, the mechanistic details of this adaptive signaling pathway have been well characterized (1-4). The AHR is a basic helix-loop-helix-PAS (bHLH-PAS) transcription factor. Upon binding agonists, the AHR translocates from the cytoplasm to the nucleus, where it forms a heterodimer with another bHLH-PAS protein known as the aryl hydrocarbon nuclear translocator (ARNT). This heterodimeric complex interacts with dioxin-responsive elements (DREs) within the genome and upregulates the transcription of a battery of xenobiotic metabolizing enzymes (XMEs). These regulated XMEs include the cytochrome P450s Cyp1a1, Cyp1b1, and Cyp1a2 and the phase II enzymes Gst-a1 and Ugt1-06 (reviewed in Refs. 2 and 3).In addition to regulating an adaptive metabolic response, the AHR also mediates toxic responses to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) and plays an important role in normal development. Early genetic and pharmacological experiments provided evidence that the AHR mediates toxic responses to TCDD and related pollutants (5). Highly reproducible toxic endpoints in rodent species include thymic involution, hepatomegaly, epithelial hyperplasia, and teratogenesis. More recently, generation of null alleles at the Ahr locus in mice revealed that the AHR also plays an important role in normal mammalian development (6 -9). Across laboratories, the most reproducible phenotype associated with the homozygous null allele is a smaller liver. We have proposed that smaller liver size is the result of the pers...

No abstract