Familial isolated pituitary adenoma is independent of Ahr genotype in a novel mouse model of disease.

Human familial isolated pituitary adenoma (FIPA) has been linked to germline heterozygous mutations in the gene encoding the aryl hydrocarbon receptor-interacting protein (AIP, also known as ARA9, XAP2, FKBP16, or FKBP37). To investigate the hypothesis that AIP is a pituitary adenoma tumor suppressor via its role in aryl hydrocarbon receptor (AHR) signaling, we have compared the pituitary phenotype of our global null Aip (AipΔC) mouse model with that of a conditional null Aip model (Aipfx/fx) carrying the same deletion, as well as pituitary phenotypes of Ahr global null and Arnt conditional null animals. We demonstrate that germline AipΔC heterozygosity results in a high incidence of pituitary tumors in both sexes, primarily somatotropinomas, at 16 months of age. Biallelic deletion of Aip in Pit-1 cells (Aipfx/fx:rGHRHRcre) increased pituitary tumor incidence and also accelerated tumor progression, supporting a loss-of-function/loss-of-heterozygosity model of tumorigenesis. Tumor development exhibited sexual dimorphism in wildtype and Aipfx/fx:rGHRHRcre animals. Despite the role of AHR as a tumor suppressor in other cancers, the observation that animals lacking AHR in all tissues, or ARNT in Pit-1 cells, do not develop somatotropinomas argues against the hypothesis that pituitary tumorigenesis in AIP-associated FIPA is related to decreased activities of either the Ahr or Arnt gene products.

Generation of an Allelic Series at the Ahr Locus Using an Edited Recombinant Approach.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor and a member of the PER-ARNT-SIM (PAS) superfamily of environmental sensors. The AHR is involved in a series of biological processes including adaptive metabolism of xenobiotics, toxicity of certain environmental pollutants, vascular development, fertility, and immune function. Mouse models, including the Ahr null and Ahr conditional null (Ahrfx) mice, are widely used for the study of AHR-mediated biology and toxicity. The Ahr conditional null mouse harbors the low-affinity Ahrd allele that exhibits approximately a 10-fold lower binding affinity for certain xenobiotic AHR ligands than the widely used C57BL/6 mouse that harbors the higher affinity Ahrb1 allele. Here, we report a novel mouse model that introduces a V375A polymorphism that converts the low-affinity allele into a high-affinity allele, offering a more sensitive conditional model. In the generation of this novel conditional allele, two additional mutants arose, including a 3-bp deletion in the PAS-B domain (AhrNG367R) and an early termination codon in the PAS-B domain (AhrTer383). The AhrNG367R allele presents as a phenocopy of the null and the AhrTer383 allele presents as an antimorph when assessing for the ductus venosus and liver lobe weight endpoints. These new models represent a series of tools that will be useful in further characterizing AHR biology.

Retinal pathology in the PPCD1 mouse.

Retinal phenotypes of the PPCD1 mouse, a mouse model of posterior polymorphous corneal dystrophy, have been characterized. PPCD1 mice on the DBA/2J background (D2.Ppcd1) have previously been reported to develop an enlarged anterior chamber due to epithelialization and proliferation of the corneal endothelium and subsequent blockage of the iridocorneal angle. Results presented here show that D2.Ppcd1 mice develop increased intraocular pressure (IOP), with measurements at three months of age revealing significant increases in IOP. Significant retinal ganglion cell layer cell loss is observed at five months of age. D2.Ppcd1 animals also exhibit marked degeneration of the outer nuclear layer in association with hyperplasia of the retinal pigment epithelium. Evidence of retinal detachment is present as early as three weeks of age. By 3.5 months of age, focal areas of outer nuclear layer loss are observed. Although the GpnmbR150X mutation leads to increased IOP and glaucoma in DBA/2J mice, development of anterior segment and retinal defects in D2.Ppcd1 animals does not depend upon presence of the GpnmbR150X mutation.

Association of a Chromosomal Rearrangement Event with Mouse Posterior Polymorphous Corneal Dystrophy and Alterations in Csrp2bp, Dzank1, and Ovol2 Gene Expression.

We have previously described a mouse model of human posterior polymorphous corneal dystrophy (PPCD) and localized the causative mutation to a 6.2 Mbp region of chromosome 2, termed Ppcd1. We now show that the gene rearrangement linked to mouse Ppcd1 is a 3.9 Mbp chromosomal inversion flanked by 81 Kbp and 542 bp deletions. This recombination event leads to deletion of Csrp2bp Exons 8 through 11, Dzank1 Exons 20 and 21, and the pseudogene Znf133. In addition, we identified translocation of novel downstream sequences to positions adjacent to Csrp2bp Exon 7 and Dzank1 Exon 20. Twelve novel fusion transcripts involving Csrp2bp or Dzank1 linked to downstream sequences have been identified. Eight are expressed at detectable levels in PPCD1 but not wildtype eyes. Upregulation of two Csrp2bp fusion transcripts, as well as upregulation of the adjacent gene, Ovol2, was observed. Absence of the PPCD1 phenotype in animals haploinsufficient for Csrp2bp or both Csrp2bp and Dzank1 rules out haploinsufficiency of these genes as a cause of mouse PPCD1. Complementation experiments confirm that PPCD1 embryonic lethality is due to disruption of Csrp2bp expression. The ocular expression pattern of Csrp2bp is consistent with a role for this protein in corneal development and pathogenesis of PPCD1.

Loss of BMAL1 in ovarian steroidogenic cells results in implantation failure in female mice.

The circadian clock plays a significant role in many aspects of female reproductive biology, including estrous cycling, ovulation, embryonic implantation, onset of puberty, and parturition. In an effort to link cell-specific circadian clocks to their specific roles in female reproduction, we used the promoter that controls expression of Steroidogenic Factor-1 (SF1) to drive Cre-recombinase-mediated deletion of the brain muscle arnt-like 1 (Bmal1) gene, known to encode an essential component of the circadian clock (SF1-Bmal1(-/-)). The resultant SF1-Bmal1(-/-) females display embryonic implantation failure, which is rescued by progesterone supplementation, or bilateral or unilateral transplantation of wild-type ovaries into SF1-Bmal1(-/-) dams. The observation that the central clock, and many other peripheral clocks, are fully functional in this model allows the assignment of the implantation phenotype to the clock in ovarian steroidogenic cells and distinguishes it from more general circadian related systemic pathology (e.g., early onset arthropathy, premature aging, ovulation, late onset of puberty, and abnormal estrous cycle). Our ovarian transcriptome analysis reveals that deletion of ovarian Bmal1 disrupts expression of transcripts associated with the circadian machinery and also genes critical for regulation of progesterone production, such as steroidogenic acute regulatory factor (Star). Overall, these data provide a powerful model to probe the interlocking and synergistic network of the circadian clock and reproductive systems.

Liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxin is dependent on the aryl hydrocarbon receptor and TNF/IL-1 receptors.

We set out to better understand the signal transduction pathways that mediate liver tumor promotion by 2,3,7,8-tetrachlorodibenzo-p-dioxn ("dioxin"). To this end, we first employed congenic mice homozygous for either the Ahr(b1) or Ahr(d) alleles (encoding an aryl hydrocarbon receptor (AHR) with high or low binding affinity for dioxin, respectively) and demonstrated that hepatocellular tumor promotion in response to dioxin segregated with the Ahr locus. Once we had genetic evidence for the importance of AHR signaling, we then asked if tumor promotion by dioxin was influenced by "interleukin-1 (IL-1)-like" inflammatory cytokines. The importance of this question arose from our earlier observation that aspects of the acute hepatocellular toxicity of dioxin are dependent upon IL1-like cytokine signaling. To address this issue, we employed a triple knock-out (TKO) mouse model with null alleles at the loci encoding the three relevant receptors for tumor necrosis factors α and ß and IL-1α and IL-1ß (i.e., null alleles at the Tnfrsf1a, Tnfrsf1b, and Il-1r1 loci). The observation that TKO mice were resistant to the tumor promoting effects of dioxin in liver suggests that inflammatory cytokines play an important step in dioxin mediated liver tumor promotion in the mouse. Collectively, these data support the idea that the mechanism of dioxin acute hepatotoxicity and its activity as a promoter in a mouse two stage liver cancer model may be similar, i.e., tumor promotion by dioxin, like acute hepatotoxicity, are mediated by the linked action of two receptor systems, the AHR and the receptors for the "IL-1-like" cytokines.

The aryl hydrocarbon receptor-interacting protein (AIP) is required for dioxin-induced hepatotoxicity but not for the induction of the Cyp1a1 and Cyp1a2 genes.

The aryl hydrocarbon receptor (AHR) plays an essential role in the toxic response to environmental pollutants such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin), in the adaptive up-regulation of xenobiotic metabolizing enzymes, and in hepatic vascular development. In our model of AHR signaling, the receptor is found in a cytosolic complex with a number of molecular chaperones, including Hsp90, p23, and the aryl hydrocarbon receptor-interacting protein (AIP), also known as ARA9 and XAP2. To understand the role of AIP in adaptive and toxic aspects of AHR signaling, we generated a conditional mouse model where the Aip locus can be deleted in hepatocytes. Using this model, we demonstrate two important roles for the AIP protein in AHR biology. (i) The expression of AIP in hepatocytes is essential to maintain high levels of functional cytosolic AHR protein in the mammalian liver. (ii) Expression of the AIP protein is essential for dioxin-induced hepatotoxicity. Interestingly, classical AHR-driven genes show differential dependence on AIP expression. The Cyp1b1 and Ahrr genes require AIP expression for normal up-regulation by dioxin, whereas Cyp1a1 and Cyp1a2 do not. This differential dependence on AIP provides evidence that the mammalian genome contains more than one class of AHR-responsive genes and suggests that a search for AIP-dependent, AHR-responsive genes may guide us to the targets of the dioxin-induced hepatotoxicity.

Abnormal liver development and resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity in mice carrying a mutation in the DNA-binding domain of the aryl hydrocarbon receptor.

The aryl hydrocarbon receptor (AHR) is known for its role in the adaptive and toxic responses to a large number of environmental contaminants, as well as its role in hepatovascular development. The classical AHR pathway involves ligand binding, nuclear translocation, heterodimerization with the AHR nuclear translocator (ARNT), and binding of the heterodimer to dioxin response elements (DREs), thereby modulating the transcription of an array of genes. The AHR has also been implicated in signaling events independent of nuclear localization and DNA binding, and it has been suggested that such pathways may play important roles in the toxicity of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Here, we report the generation of a mouse model that expresses an AHR protein capable of ligand binding, interactions with chaperone proteins, functional heterodimerization with ARNT, and nuclear translocation, but is unable to bind DREs. Using this model, we provide evidence that DNA binding is required AHR-mediated liver development, as Ahr(dbd/dbd) mice exhibit a patent ductus venosus, similar to what is seen in Ahr(-/-) mice. Furthermore, Ahr(dbd/dbd) mice are resistant to TCDD-induced toxicity for all endpoints tested. These data suggest that DNA binding is necessary for AHR-mediated developmental and toxic signaling.

Deletion of the aryl hydrocarbon receptor-associated protein 9 leads to cardiac malformation and embryonic lethality.

The aryl hydrocarbon receptor-associated protein 9, ARA9 (also known as XAP2 or AIP1), is a chaperone that is found in complexes with certain xenobiotic receptors, such as the aryl hydrocarbon receptor (AHR) and the peroxisome proliferator-activated receptor alpha (PPARalpha). In an effort to better understand the physiological role of ARA9 outside of its role in xenobiotic signal transduction, we generated a null allele at the Ara9 locus in mice. Mice with a homozygous deletion of this gene die at various time points throughout embryonic development. Embryonic lethality is accompanied by decreased blood flow to head and limbs, as well as a range of heart deformations, including double outlet right ventricle, ventricular-septal defects, and pericardial edema. The early cardiovascular defects observed in Ara9-null mice suggest an essential role for the ARA9 protein in cardiac development. The observation that the developmental aberrations in Ara9-null mice are distinct from those observed for disrupted alleles at Ahr or Pparalpha indicates that the role of ARA9 in cardiac development is independent of its interactions with its known xenobiotic receptor partners.

The aryl hydrocarbon receptor signaling pathway is modified through interactions with a Kelch protein.

The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor with important roles in metabolic adaptation, dioxin toxicology, and vascular development. To understand the details of this signal transduction pathway, we have used the yeast two-hybrid system to identify proteins that physically interact with the AHR in a ligand-dependent manner. Using this strategy, we identified a novel modifier of the AHR signaling pathway that we named Ah-receptor associated protein 3 (ARA3). Coexpression of ARA3 with an AHR chimera in yeast and mammalian cells enhances signaling in response to agonists. The human full-length cDNA previously was described as influenza virus nonstructural protein-1 binding protein (NS1BP). This protein contains four apparent domains-a "broad complex/tramtrack/bric-a-brac" (BTB) domain, a "kelch" domain, a "BTB and C-terminal kelch" (BACK) domain, and an intervening region (IVR). The carboxyl terminus of the AHR "Per-ARNT-Sim" (periodicity/AHR nuclear translocator/simple-minded) domain and the BACK/IVR domains of ARA3 mediate the AHR-ARA3 interaction. The BACK/IVR domains of ARA3 also are sufficient to modify AHR signaling in yeast and mammalian cells. In an effort to provide a preliminary model of NS1BP activity in AHR signaling, we demonstrate that NS1BP regulates the concentration of functional AHR in mammalian cells.

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

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 endothelial/hematopoietic 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.

The aryl hydrocarbon receptor is required for developmental closure of the ductus venosus in the neonatal mouse.

A developmental role for the Ahr locus has been indicated by the observation that mice harboring a null allele display a portocaval vascular shunt throughout life. To define the ontogeny and determine the identity of this shunt, we developed a visualization approach in which three-dimensional (3D) images of the developing liver vasculature are generated from serial sections. Applying this 3D visualization approach at multiple developmental times allowed us to demonstrate that the portocaval shunt observed in Ahr-null mice is the remnant of an embryonic structure and is not acquired after birth. We observed that the shunt is found in late-stage wild-type embryos but closes during the first 48 h of postnatal life. In contrast, the same structure fails to close in Ahr-null mice and remains open throughout adulthood. The ontogeny of this shunt, along with its 3D position, allowed us to conclude that this shunt is a patent developmental structure known as the ductus venosus (DV). Upon searching for a physiological cause of the patent DV, we observed that during the first 48 h, most major hepatic veins, such as the portal and umbilical veins, normally decrease in diameter but do not change in Ahr-null mice. This observation suggests that failure of the DV to close may be the consequence of increased blood pressure or a failure in vasoconstriction in the developing liver.

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

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.

A maternal Ahr null genotype sensitizes embryos to chemical teratogenesis.

The aryl hydrocarbon receptor (encoded by the Ahr locus) is a ligand-activated transcription factor that mediates the toxicology and teratology of 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin). In an effort to understand the role of the maternal compartment in dioxin teratology, we designed a breeding strategy that allowed us to compare the teratogenic response in embryos from Ahr(-/-) (null) and Ahr(+/+) (wild-type) dams. Using this strategy, we demonstrate that embryos from the Ahr(-/-) dams are 5-fold more sensitive to dioxin-induced cleft palate and hydronephrosis as compared with embryos from an Ahr(+/+) dam. Moreover, this increased teratogenic sensitivity extends beyond dioxin, because embryos from Ahr(-/-) dams exhibited a 9-fold increase in their sensitivity to the fetotoxic effects of the glucocorticoid, dexamethasone. In searching for an explanation for this increased sensitivity, we found that more dioxin and dexamethasone reached the embryos from Ahr(-/-) dams as compared with embryos from Ahr(+/+) dams. We propose that increased deposition of teratogens/fetotoxicants to the embryonic compartment is the result of porto-systemic shunting and/or blocked P4501A induction in Ahr(-/-) dams. In addition to demonstrating the importance of maternal AHR in teratogenesis, these data may have implications that reach beyond the mechanism of action of dioxin. In this regard, the Ahr(-/-) mouse may provide a system that allows pharmacological agents and toxicants to be more easily studied in a model where first pass clearance is a significant obstacle.

Aryl hydrocarbon receptor response to indigoids in vitro and in vivo.

Indigo and indirubin have been reported to be present at low levels in human urine. The possibility that indigoids are physiological ligands of the aryl hydrocarbon receptor (AhR) has been suggested by initial studies in yeast, where indirubin was found to be 50 times more potent than 2,3,7,8-tetrachlorodibenzo[p]dioxin (TCDD), and indigo was found to be equipotent. To demonstrate that these indigoids are bona fide agonists in mammalian systems, we employed a number of in vitro and in vivo measures of AhR agonist potency. In a hepatoma cell reporter system, indigo yielded an EC50 of approximately 5x10(-6)M (indirubin 3' -oxime EC50 approximately 5x10(-7)M, indirubin EC50 approximately 1x10(-7)M). A comparison of these EC50 values with that of 2,3,7,8-tetrachlorodibenzofuran (TCDBF) ( approximately 3x10(-9)M) indicated that these compounds are less potent than classic halogenated-dibenzofurans or -dibenzo-p-dioxins. Competitive binding assays for AhR occupancy showed similar IC50 values for indirubin and TCDBF ( approximately 2x10(-9) and 5x10(-9)M), with the IC50 values of indigo and indirubin 3' -oxime being approximately 10-fold higher. When rats were treated with these indigoids in the range of 1.5-50mg/kg, induction of hepatic cytochrome P450 1A1 was detected. Differences in the rank-order of potency observed in vivo and in vitro could, in part, be explained by metabolism. Although their biological potencies are not as high as has been previously suggested, collectively the results show that these indole-derived pigments are agonists of AhR in vivo. The in vivo results suggest that solubility, distribution, and metabolism influence the response to the compounds.

Patent ductus venosus and dioxin resistance in mice harboring a hypomorphic Arnt allele.

The Ah receptor nuclear translocator (ARNT) is the dimeric partner of hypoxia-inducible factors and thus plays a pivotal role in cellular adaptation to low oxygen environments. ARNT is also a dimeric partner for the Ah receptor (AHR), and this complex is essential in regulating the adaptive metabolic response to polycyclic aromatic hydrocarbons. Because of the essential role of ARNT in hypoxia-driven developmental events, it has been difficult to study the physiological significance of AHR.ARNT heterodimers in vivo. To address this issue, we developed a hypomorphic Arnt allele that displayed normal development and allowed the examination of the role of ARNT in AHR biology. In this regard, the AHR is also known to mediate two additional biological processes: the toxicological response to compounds such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) and the developmental closure of a fetal vascular structure known as the ductus venosus. Although the mechanism of the adaptive pathway has been well described, the mechanism of AHR-mediated signal transduction in the toxic and developmental pathways is not well understood. Liver perfusion studies demonstrated that ARNT hypomorphs have a patent ductus venosus, identical to that observed in the Ahr null mice. Parallel dioxin toxicity studies demonstrated that the ARNT hypomorphs exhibited resistance to the end points of dioxin exposure. Moreover, we observed that toxicity could be segregated from the classical adaptive responses such as P4501A induction. Taken in sum, these experiments demonstrate that ARNT is an essential component of AHR developmental signaling and shed light on the mechanism of dioxin toxicity.

Resistance to 2,3,7,8-tetrachlorodibenzo-p-dioxin toxicity and abnormal liver development in mice carrying a mutation in the nuclear localization sequence of the aryl hydrocarbon receptor.

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.