Characterization of a murine Ahr null allele: involvement of the Ah receptor in hepatic growth and development.

Abstract

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.

Loss of teratogenic response to 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD) in mice lacking the Ah (dioxin) receptor

“…Many such symptomatic discrepancies are apparent between the present report and previous reports. The other pathological effects observed in homozygous Ahr mutant mice will be discussed elsewhere in reference to previous reports from other laboratories (Fernandez-Salguero et al 1995;Schmidt et al 1996).…”

“…Although two separate studies have already been published on mice which lack the AhR gene, these groups reported considerable phenotypic differences, but with consistent observations being a reduced liver weight and a loss of inducibility of the drug-metabolizing enzymes (Fernandez-Salguero et al 1995;Schmidt et al 1996). It has recently been reported that adult Ahr -/-mice became resistant to the toxic effects of TCDD in the thymus and liver (Fernandez-Salguero et al 1996).…”

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

“…In the population genomic study, CYP1A duplications were found in three of the four tolerant populations; in the three northern tolerant populations, individuals have up to eight CYP1A copies (Reid et al., 2016). Even in the absence of exogenous chemical exposure the AHR may help to maintain basal CYP1A expression; loss of AHR signaling in mice reduces this expression (Schmidt, Su, Reddy, Simon, & Bradfield, 1996). Thus, CYP1A duplications might have been favored in tolerant fish with suppressed AHR signaling to maintain constitutive levels of CYP1A needed for physiological functions.…”

Recent advances in P450 research

“…Because the [Ah] gene battery comprises at least six and probably many more genes, the AhR controls constitutive expression of several xenobiotic-metabolizing enzymes. AhR-null mice have been constructed by several laboratories 43,51,52 and these animals exhibited immune system impairment and hepatic fibrosis confirming that AhR has a developmental and physiological role. Activation of the AhR by polycyclic aromatic hydrocarbons (PAHs) and associated compounds is associated with disruption of multiple hormone systems resulting in developmental and reproductive impairment.…”

“…Thus, mouse lines have also been constructed that lack Cyp1a2. 43,44 CYP1B1 shares many properties with CYP1A1 such as the metabolic activation of polycyclic aromatic hydrocarbons (PAHs). In addition to activating polycyclic aromatic hydrocarbons, CYP1B1 functions as a physiological regulator in that a deficiency in the enzyme has been linked to congenital glaucoma in humans.…”