The Ah receptor (AHR) is a ligand-activated transcription factor that mediates a pleiotropic response to environmental contaminants such as benzo [a]pyrene and 2, 3,7,8-tetrachlorodibenzo-p-dioxin. In an effort to gain insight into the physiological role of the AHR and to develop models useful in risk assessment, gene targeting was used to inactivate the murineAhr gene by homologous recombination. Ahr-l mice are viable and fertile but show a spectrum of hepatic defects that indicate a role for the AHR in normal liver growth and development. TheAhr-1-phenotype is most severe between 0-3 weeks ofage and involves slowed early growth and hepatic defects, including reduced liver weight, transient microvesicular fatty metamorphosis, prolonged extramedullary hematopoiesis, and portal hypercellularity with thickening and fibrosis.The Ah receptor (AHR) is a ligand-activated transcription factor that regulates a biphasic pleiotropic response to a variety of structurally related environmental contaminants (1, 2). Upon binding polycyclic aromatic hydrocarbons (PAHs), such as benzo [a]pyrene, the AHR increases the expression of xenobiotic metabolizing enzymes, including the cytochrome P450IA1, P450IA2 and P450IB1-dependent monooxygenases, the glutathione S-transferase Ya subunit, quinone oxidoreductase, and UDP-glucuronosyltransferase (3-8). In response to more potent halogenated aromatic agonists like 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), the AHR induces xenobiotic metabolism and also mediates a spectrum of toxic responses, including thymic involution, teratogenesis, tumor promotion, wasting, and epithelial hyperplasia and metaplasia (9-11).The AHR is a member of a family of transcription factors containing basic/helix-loop-helix and PAS homology domains (bHLH-PAS) (1). In response to agonist binding within the PAS domain, the cytosolic AHR undergoes a conformational change, translocates to the nucleus, dissociates from the 90-kDa heat shock protein, and dimerizes with a second bHLH-PAS protein known as the Ah receptor nuclear translocator (ARNT) (12-16). This heterodimer interacts with dioxin-responsive enhancer elements upstream of target genes and activates their transcription. Despite our mechanistic understanding of the role of the AHR in regulating xenobiotic metabolism, we still have little understanding of how the AHR mediates the toxic responses of halogenated agonists and why such responses are produced only by high affinity, poorly metabolized ligands such as TCDD.To examine the importance of the AHR in normal vertebrate biology, we have used gene targeting to create mice with a mutation at theAhr locus. We anticipated that such a mutant could provide insights into additional physiological roles of the AHR and would represent a powerful model to understand the toxicology of halogenated aromatic pollutants like TCDD.

The aryl hydrocarbon (Ah) receptor has occupied the attention of toxicologists for over two decades. Interest arose from the early observation that this soluble protein played key roles in the adaptive metabolic response to polycyclic aromatic hydrocarbons and in the toxic mechanism of halogenated dioxins and dibenzofurans. More recent investigations have provided a fairly clear picture of the primary adaptive signaling pathway, from agonist binding to the transcriptional activation of genes involved in the metabolism of xenobiotics. Structure-activity studies have provided an understanding of the pharmacology of this receptor; recombinant DNA approaches have identified the enhancer sequences through which this factor regulates gene expression; and functional analysis of cloned cDNAs has allowed the characterization of the major signaling components in this pathway. Our objective is to review the Ah receptor's role in regulation of xenobiotic metabolism and use this model as a framework for understanding the less well-characterized mechanism of dioxin toxicity. In addition, it is hoped that this information can serve as a model for future efforts to understand an emerging superfamily of related signaling pathways that control biological responses to an array of environmental stimuli.

The arylhydrocarbon-receptor nuclear translocator (ARNT) is a member of the basic-helix-loop-helix-PAS family of heterodimeric transcription factors which includes the arylhydrocarbon receptor (AHR), hypoxia-inducible factor-1alpha (HIF-1alpha) and the Drosophila single-minded protein (Sim). ARNT forms heterodimeric complexes with the arylhydrocarbon receptor, HIF-1alpha, Sim and the PAS protein Per. In response to environmental pollutants, AHR-ARNT heterodimers regulate genes involved in the metabolism of xenobiotics, whereas ARNT-HIF-1alpha heterodimers probably regulate those involved in the response to oxygen deprivation. By generating a targeted disruption of the Arnt locus in the mouse, we show here that Arnt-/- embryonic stem cells fail to activate genes that normally respond to low oxygen tension. Arnt-/- ES cells also failed to respond to a decrease in glucose concentration, indicating that ARNT is crucial in the response to hypoxia and to hypoglycaemia. Arnt-/- embryos were not viable past embryonic day 10.5 and showed defective angiogenesis of the yolk sac and branchial arches, stunted development and embryo wasting. The defect in blood vessel formation in Arnt-/- yolk sacs is similar to the angiogenic abnormalities reported for mice deficient in vascular endothelial growth factor or tissue factor. On the basis of these findings, we propose a model in which increasing tissue mass during organogenesis leads to the formation of hypoxic/nutrient-deprived cells, the subsequent activation of ARNT, and a concomitant increase in the expression of genes (including that encoding vascular endothelial growth factor) that promote vascularization of the developing yolk sac and solid tissues.

Delta-like 1homolog (Dlk1) is an imprinted gene encoding a transmembrane protein whose increased expression has been associated with muscle hypertrophy in animal models. However, the mechanisms by which Dlk1 regulates skeletal muscle plasticity remain unknown. Here we combine conditional gene knockout and over-expression analyses to investigate the role of Dlk1 in mouse muscle development, regeneration and myogenic stem cells (satellite cells). Genetic ablation of Dlk1 in the myogenic lineage resulted in reduced body weight and skeletal muscle mass due to reductions in myofiber numbers and myosin heavy chain IIB gene expression. In addition, muscle-specific Dlk1 ablation led to postnatal growth retardation and impaired muscle regeneration, associated with augmented myogenic inhibitory signaling mediated by NF-κB and inflammatory cytokines. To examine the role of Dlk1 in satellite cells, we analyzed the proliferation, self-renewal and differentiation of satellite cells cultured on their native host myofibers. We showed that ablation of Dlk1 inhibits the expression of the myogenic regulatory transcription factor MyoD, and facilitated the self-renewal of activated satellite cells. Conversely, Dlk1 over-expression inhibited the proliferation and enhanced differentiation of cultured myoblasts. As Dlk1 is expressed at low levels in satellite cells but its expression rapidly increases upon myogenic differentiation in vitro and in regenerating muscles in vivo, our results suggest a model in which Dlk1 expressed by nascent or regenerating myofibers non-cell autonomously promotes the differentiation of their neighbor satellite cells and therefore leads to muscle hypertrophy.