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. 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. The aryl hydrocarbon receptor nuclear translocator (ARNT) 1The abbreviations and trivial names used are: ARNT, aryl hydrocarbon receptor nuclear translocator; Arntfxneo, hypomorphic Arnt allele; AHR, aryl hydrocarbon receptor; ALT, alanine aminotransferase; bHLH, basic helix-loop-helix; dioxin, 2,3,7,8-tetrachlorodibenzo-p-dioxin; DV, ductus venosus; EROD, ethoxyresorufin O-deethylase; HIF, hypoxia-inducible factor; Neo, neomycin resistance cassette; PAH, polycyclic aromatic hydrocarbon; PAS, Period-ARNT-Singleminded. and the Ah receptor (AHR) are founding members of the PAS superfamily of transcriptional regulators (1Gu Y.Z. Hogenesch J. Bradfield C. Annu. Rev. Pharmacol. Toxicol. 2000; 40: 519-561Crossref PubMed Scopus (862) Google Scholar). These proteins were originally identified as the result of their involvement in the regulation of an adaptive metabolic response to certain xenobiotics, such as polycyclic aromatic hydrocarbons (PAHs) (2Schmidt J.V. Bradfield C.A. Annu. Rev. Cell Dev. Biol. 1996; 12: 55-89Crossref PubMed Scopus (813) Google Scholar, 3Whitlock Jr., J.P. Chichester C.H. Bedgood R.M. Okino S.T. Ko H.P. Ma Q. Dong L. Li H. Clarke-Katzenberg R. Drug Metab. Rev. 1997; 29: 1107-1127Crossref PubMed Scopus (65) Google Scholar). In this pathway, PAH molecules bind to the AHR, which then translocates to the nucleus and heterodimerizes with its transcriptional partner, ARNT. The AHR·ARNT complex then interacts with specific response elements to up-regulate a battery of xenobiotic metabolizing enzymes that include the cytochrome p450 enzymes, Cyp1a1, Cyp1a2, Cyp1b1 as well as the phase II enzymes Gst-a1 and Ugt1–06 (2Schmidt J.V. Bradfield C.A. Annu. Rev. Cell Dev. Biol. 1996; 12: 55-89Crossref PubMed Scopus (813) Google Scholar, 4Hankinson O. Annu. Rev. Pharmacol. Toxicol. 1995; 35: 307-340Crossref PubMed Scopus (1438) Google Scholar). Given that each of the up-regulated enzymes metabolize PAHs, we refer to this process as the “adaptive pathway” of AHR. In addition to its role in the adaptive metabolism of xenobiotics, the AHR also mediates two other biological pathways that we refer to as “toxic” and “developmental.” In the toxic pathway, exposure to potent agonists such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (dioxin) results in a response that includes end points such as hepatotoxicity, thymic involution, epithelial hyperplasia, and cleft palate (5Poland A. Knutson J.C. Annu. Rev. Pharmacol. Toxicol. 1982; 22: 517-554Crossref PubMed Scopus (2342) Google Scholar). A compelling body of genetic and pharmacological evidence has demonstrated that the AHR mediates most, if not all, of these toxic responses (5Poland A. Knutson J.C. Annu. Rev. Pharmacol. Toxicol. 1982; 22: 517-554Crossref PubMed Scopus (2342) Google Scholar, 6Bradfield C.A. Kende A.S. Poland A. Mol. Pharmacol. 1988; 34: 229-237PubMed Google Scholar, 7Poland A. Glover E. Mol. Pharmacol. 1980; 17: 86-94PubMed Google Scholar). With regard to the developmental pathway, a highly reproducible phenotype arising in Ahr null mouse models is a reduced liver size. We have proposed that the smaller liver size is the result of a persistent fetal vascular shunt known as the ductus venosus (DV), and furthermore, that the failure of the DV to close at parturition decreases the portal blood supply to the liver (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). As a consequence, nutrient supply is limited, and the liver size is decreased. We view AHR signal transduction as encompassing three physiological processes: 1) the adaptive pathway ensuring metabolism of PAHs, 2) the toxic pathway mediating the deleterious effects of dioxin exposure, and 3) the developmental pathway guiding the resolution of fetal liver vascular architecture. We hypothesize that the fundamental mechanisms behind these three distinct biological processes are dependent upon AHR·ARNT heterodimerization, interaction with cognate responsive elements, and up-regulation of transcriptional targets. The ability to produce three distinct biological events from one signal transduction mechanism may lie in either the transcriptional up-regulation of the same battery of genes with some temporal and/or spatial specificity or the up-regulation of gene sets unique to each biological event. Although ARNT has a well defined role in adaptive metabolism, its role in mediating the developmental signaling and dioxin-induced toxicity of AHR has not been fully addressed. Although it may be assumed that all features of AHR signal transduction are dependent upon ARNT heterodimerization, a formal demonstration of this hypothesis is lacking. In this regard, it is important to note that at least three putative homologs of ARNT have been identified in the human genome, ARNT2, MOP3, and MOP9, (9Hirose K. Morita M. Ema M. Mimura J. Hamada H. Fujii H. Saijo Y. Gotoh O. Sogawa K. Fujii-Kuriyama Y. Mol. Cell. Biol. 1996; 16: 1706-1713Crossref PubMed Scopus (223) Google Scholar, 10Drutel G. Kathmann M. Heron A. Schwartz J.C. Arrang J.M. Biochem. Biophys. Res. Commun. 1996; 225: 333-339Crossref PubMed Scopus (75) Google Scholar, 11Hogenesch J.B. Chan W.K. Jackiw V.H. Brown R.C. Gu Y.Z. Pray-Grant M. Perdew G.H. Bradfield C.A. J. Biol. Chem. 1997; 272: 8581-8593Abstract Full Text Full Text PDF PubMed Scopus (399) Google Scholar, 12Hogenesch J. Gu Y.-Z. Moran S. Shimomura K Radcliffe L. Takahashi J. Bradfield C. J. Neurosci. 2000; 20: 1-5Crossref PubMed Google Scholar). Furthermore, the ARNT2 protein has been shown to form transcriptionally active partnerships with the AHR in transient transfection experiments (9Hirose K. Morita M. Ema M. Mimura J. Hamada H. Fujii H. Saijo Y. Gotoh O. Sogawa K. Fujii-Kuriyama Y. Mol. Cell. Biol. 1996; 16: 1706-1713Crossref PubMed Scopus (223) Google Scholar). In an effort to examine the possibility that the toxic and developmental aspects of AHR biology are dependent upon ARNT heterodimerization, we used gene targeting to generate a hypomorphic or low-expressing Arnt allele, designated Arntfxneo. This novel allele allowed us to bypass the known embryonic lethality of the Arnt null mutation, and therefore, to assess the effect of limiting ARNT on AHR-dependent toxicity and development (13Maltepe E. Schmidt J.V. Baunoch D. Bradfield C.A. Simon M.C. Nature. 1997; 386: 403-407Crossref PubMed Scopus (636) Google Scholar, 14Kozak K.R. Abbott B. Hankinson O. Dev. Biol. 1997; 191: 297-305Crossref PubMed Scopus (264) Google Scholar). Through this examination, we provide evidence to support the idea that heterodimerization is an essential feature of adaptive, toxic, and developmental AHR signaling. Generation of Arntfxneo Mice—A bacteriophage (P1) clone containing the murine Arnt locus was obtained from Genome Systems (St. Louis, MO) (13Maltepe E. Schmidt J.V. Baunoch D. Bradfield C.A. Simon M.C. Nature. 1997; 386: 403-407Crossref PubMed Scopus (636) Google Scholar). The strategy for creation of the targeting construct (PL910) involved insertion of a neomycin resistance cassette (Neo), flanked by loxp recombination sequences, into the EcoRI site 3′ of exon 6 (Fig. 1B). A third loxp site was inserted into the intron 5′ of exon 6, replacing the internal BglII site. Our homologous recombination protocols using embryonic have been Moran Glover E. G.P. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). were for homologous recombination by of using a 5′ to the end of the targeting construct were into and were to to of the to were to for at least was by on from using the and the These a from the allele and a from the allele to the insertion of the loxp site. The was for in a containing of at and of each were in a on with and to the and by the of ARNT was in and by of from the and Moran Glover E. G.P. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J.V. Bradfield C.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). In were by with of dioxin in or with were with an of of body blood was obtained by for of alanine liver perfusion to DV were and Liver were in and in for and and ethoxyresorufin were on and liver Moran Glover E. G.P. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, J.V. Bradfield C.A. J. Biol. Chem. Full Text PDF PubMed Google Scholar). In could be an of was and was used to with a of the DV was by perfusion of the liver with (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). were the portal and the was to the liver with of was into the of in the liver perfusion a patent To this was to the of into the portal To liver vascular were obtained (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). Generation of Arntfxneo that a of the Arnt locus results in developmental lethality at embryonic we the idea that the of a hypomorphic allele provide ARNT to bypass the developmental and produce that were in the of ARNT for signal transduction pathways (13Maltepe E. Schmidt J.V. Baunoch D. Bradfield C.A. Simon M.C. Nature. 1997; 386: 403-407Crossref PubMed Scopus (636) Google Scholar, 14Kozak K.R. Abbott B. Hankinson O. Dev. Biol. 1997; 191: 297-305Crossref PubMed Scopus (264) Google Scholar). targeting events in demonstrated that insertion of the gene to the exon the basic in the of a hypomorphic allele Moran Glover E. G.P. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). we inserted a cassette flanked by loxp to exon 6, which the of Arnt (Fig. 1B). A third loxp site was inserted of exon 6 for creation of the A of the Arnt allele, which we as Arntfxneo, is shown in shown is a of the Arnt gene as well as the construct used for embryonic homologous recombination in embryonic and and were by (Fig. were used to generate and these of the Arntfxneo A was developed to on of the loxp of the exon the (Fig. The Arntfxneo in The of this on ARNT protein was by of protein from from and for the Arntfxneo allele This that ARNT was in all and (Fig. A and We that from in mouse liver was reduced to of on with liver protein obtained from The from in other was to be and This that the Arntfxneo allele is hypomorphic for ARNT protein To examine the ARNT and the of Arnt we also protein from and mouse (Fig. a hypomorphic allele is with a allele is with the As results in a not of protein This is with the idea that protein is a of from all and is an that locus is not of Arntfxneo to the embryonic for the Arnt null we the of hypomorphic Arntfxneo allele to as the on fetal for Arntfxneo allele were with the were and The of to the embryonic lethality arising from the The that a of that of Arnt from the hypomorphic allele is the to bypass the developmental and were The of ARNT also demonstrated that the hypomorphic Arntfxneo allele to have a effect on the ability of the to a to produce to The that are to the is evidence that the embryonic lethality in hypomorphic is in Although the failure in to the of not the of the the is that hypomorphic be by of the with ARNT hypomorphs from these are and the hypomorphs are ARNT for of the studies from have demonstrated that the Ahr null are a vascular phenotype the fetal DV to Because AHR signaling is important in vascular resolution and ARNT is the partner of the AHR, we ARNT hypomorphs a vascular phenotype to that in the mice. of the liver vascular the distinct vascular of and mice. of in and portal of the liver vascular and (Fig. the vascular of the was we that the the liver the portal and the DV into the (Fig. it the liver (Fig. As a we also the vascular structure of the Ahr null As observed also the DV in Ahr null thus that the ARNT hypomorphs and the Ahr null have an identical liver vascular phenotype (Fig. (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). Liver perfusion studies using a to of the vascular also the DV phenotype and that the DV in of the mice. Moreover, this phenotype is of of the not The Arntfxneo the AHR an effort to the of the hypomorphic Arntfxneo allele on the adaptive and toxic signaling pathways of AHR, we the response of and to a of of dioxin liver were and the of the adaptive metabolic response was is a of and enzymes, which are adaptive responses to dioxin exposure. from all in the of the AHR In response to dioxin, from and are that the adaptive AHR signal transduction pathway is in ARNT hypomorphs at this (Fig. with the idea that liver dioxin exposure is at least the result of of the liver in and was to the same with their (Fig. Liver were also in the with dioxin with their Although the of the DV with a smaller the in liver in ARNT hypomorphs is of the of the DV (Fig. (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). The Arntfxneo to an effort to the role of ARNT in the toxic pathway of AHR we the ARNT to examine two classical end points of dioxin toxicity. exposure of and to of dioxin for were and for and of the classical end points of dioxin toxicity is thymic (5Poland A. Knutson J.C. Annu. Rev. Pharmacol. Toxicol. 1982; 22: 517-554Crossref PubMed Scopus (2342) Google Scholar, Annu. Rev. Pharmacol. Toxicol. PubMed Scopus Google Scholar). in and was reduced by in response to dioxin exposure (Fig. In thymic in hypomorphs was that observed in the and mice. The dioxin toxicity in the of demonstrated that the hypomorphic allele is to this end The resistance of the hypomorphs to dioxin-induced thymic with the of a the of thymic in Arnt was in thymic S. H. S. K. S. A. A. J. Y. Y. J. PubMed Scopus Google Scholar). To assess the of the Arntfxneo allele on dioxin-induced hepatotoxicity, we two As of hepatotoxicity, we the of in to of with dioxin displayed that were in (Fig. This in was reduced in two hypomorphic a resistance to dioxin-induced In we observed a a response as a of the of hypomorphic these from or are not from As of hepatotoxicity, a examination was This also a in the of in ARNT with their with dioxin displayed and all of the liver as well as with of (Fig. In liver from with dioxin displayed and in the (Fig. is difficult to assess the role of ARNT in models the null allele at this locus results in embryonic lethality at embryonic (13Maltepe E. Schmidt J.V. Baunoch D. Bradfield C.A. Simon M.C. Nature. 1997; 386: 403-407Crossref PubMed Scopus (636) Google Scholar, 14Kozak K.R. Abbott B. Hankinson O. Dev. Biol. 1997; 191: 297-305Crossref PubMed Scopus (264) Google Scholar). This developmental is to be to the role of ARNT as the transcriptional partner to the Proc. Natl. Acad. Sci. U. S. A. 1995; PubMed Scopus Google Scholar, E. M. Dev. 12: PubMed Scopus Google Scholar, H. S.L. Dev. 1997; PubMed Scopus Google Scholar, Y.Z. Moran Hogenesch J.B. L. Bradfield C.A. Google Scholar). on are two to the essential of ARNT in The is to generate a and the is to a hypomorphic allele that has reduced ARNT upon in the we a strategy that the to such models from a targeting in embryonic The of Arnt be in In this we the of the ARNT in the role of this protein in AHR-mediated toxicity and The of ARNT in AHR-mediated is evidence to demonstrate that ARNT is involved in the classical end points of dioxin toxicity. a role for ARNT is an important a of models have been proposed that not the ARNT models include the signaling of AHR cellular factors such as the and A. C. J. Biol. Chem. 2000; Full Text Full Text PDF PubMed Scopus Google Scholar, J. Biol. Chem. Full Text Full Text PDF PubMed Scopus (223) Google Scholar, A. Mol. Pharmacol. 1997; PubMed Scopus Google Scholar, Y. S. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, D. L. S. R. D. G. 2000; PubMed Scopus Google Scholar, K. Biochem. Biophys. 2000; PubMed Scopus Google Scholar, Jr., J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). we to the role that ARNT plays in AHR-mediated dioxin toxicity. In this regard, we that if the toxic pathway the adaptive pathway, then toxic events be in ARNT hypomorphic we are a toxicity is the result of transcriptional we are also a of toxicity. In that it is proposed that toxicity may result from the of ARNT upon dioxin-induced of the AHR (1Gu Y.Z. Hogenesch J. Bradfield C. Annu. Rev. Pharmacol. Toxicol. 2000; 40: 519-561Crossref PubMed Scopus (862) Google Scholar). As a of this other pathways could be W.K. G. Gu Y.Z. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar, K. J. R. L. M. L. Mol. Cell. Biol. 1996; 16: PubMed Scopus Google Scholar, R.S. Mol. Pharmacol. PubMed Scopus Google Scholar). is at then ARNT hypomorphic demonstrate an response to we are the idea that ARNT is involved in the AHR pathway and not its ARNT2, MOP3, or (9Hirose K. Morita M. Ema M. Mimura J. Hamada H. Fujii H. Saijo Y. Gotoh O. Sogawa K. Fujii-Kuriyama Y. Mol. Cell. Biol. 1996; 16: 1706-1713Crossref PubMed Scopus (223) Google Scholar, 10Drutel G. Kathmann M. Heron A. Schwartz J.C. Arrang J.M. Biochem. Biophys. Res. Commun. 1996; 225: 333-339Crossref PubMed Scopus (75) Google Scholar, 11Hogenesch J.B. Chan W.K. Jackiw V.H. Brown R.C. Gu Y.Z. Pray-Grant M. Perdew G.H. Bradfield C.A. J. Biol. Chem. 1997; 272: 8581-8593Abstract Full Text Full Text PDF PubMed Scopus (399) Google Scholar, 12Hogenesch J. Gu Y.-Z. Moran S. Shimomura K Radcliffe L. Takahashi J. Bradfield C. J. Neurosci. 2000; 20: 1-5Crossref PubMed Google Scholar). of the response of the ARNT to dioxin exposure us to the mechanism of the toxic pathway signaling of AHR. In response to dioxin, that the ARNT hypomorphs their metabolic a resistance to two toxic end thymic and on these we a of the role of ARNT in toxicity. The is that ARNT homologs are not a in and thymic ARNT homologs a we have the of the toxicological end points to be by the of the hypomorphic Arntfxneo The is that the AHR is dependent on ARNT for the toxic and the developmental the AHR were of ARNT, then the hypomorphic Arntfxneo allele not the of dioxin exposure. The is that the mechanism of toxicity is not at Our was that if was ARNT hypomorphs be to toxic end Our in the of toxicity in the ARNT hypomorphs with dioxin, and therefore, the for these end The of dioxin toxicity that the mechanisms by which the AHR mediates adaptive metabolic and toxic responses the ARNT These are with a and thymic are dependent on AHR·ARNT Although not in this the possibility is that toxicity is dependent upon This idea is by that with a in the of AHR are to these same toxic end points Moran Glover E. G.P. Bradfield C.A. J. Biol. Chem. Full Text Full Text PDF PubMed Scopus Google Scholar). important that be in the development of of toxicity is the that was in ARNT hypomorphs with mice. This result that the of these is of the toxic end points in this This be or by examination of the null models developed by other R.S. Biochem. Biophys. Res. Commun. 2000; PubMed Scopus Google Scholar, Li H. A. Proc. Natl. Acad. Sci. U. S. A. 1996; PubMed Scopus Google Scholar, S. U. J. J.M. Proc. Natl. Acad. Sci. U. S. A. PubMed Scopus Google Scholar). The of ARNT in AHR-mediated of the ARNT also into the mechanism by which the AHR liver development and vascular Given that ARNT is the partner to the AHR, we to the idea that the AHR·ARNT is important in upon their highly reproducible we on DV closure and regulation of liver size as developmental end to the toxicity models a models that could the mechanism by which the AHR development in an is hypothesis that the AHR·ARNT in a that is to its role in adaptive metabolism and toxicity. on this we that ARNT hypomorphs liver that in the Ahr null mice. of the of from ARNT we observed a of patent DV, as well as a reduced liver size in the and A patent DV is a in and the of a patent DV in was for the Ahr (8Lahvis G.P. Lindell S.L. Thomas R.S. McCuskey R.S. Murphy C. Glover E. Bentz M. Southard J. Bradfield C.A. Proc. Natl. Acad. Sci. U. S. A. 2000; 97: 10442-10447Crossref PubMed Scopus (317) Google Scholar). The that the ARNT the Ahr null is a of genetic these two in either partner of this result in an identical vascular This the hypothesis that AHR and ARNT are in the resolution of the DV and that ARNT is an essential component of the developmental pathway of AHR signal Given that ARNT is a partner to PAS with evidence that ARNT is involved in all aspects of AHR it is to the of ARNT for each In this regard, of the ARNT has of this protein for signaling the of reduced ARNT protein by this the embryonic lethality is and the adaptive up-regulation of is In these limiting is failure in DV and dioxin-induced thymic and is Taken in sum, these a for the ARNT protein by biological In we have evidence to demonstrate that a hypomorphic Arntfxneo allele be by insertion of the neomycin gene to exon 6 of the Arnt The that ARNT in this mouse is allowed us to the role of this protein in aspects of its biology AHR. In this have to signal this we have evidence that the toxic and the developmental pathways of AHR signal transduction on with ARNT and a role for in these This AHR·ARNT to be an essential for the toxic end points of dioxin exposure as well as the AHR-dependent closure of the We and for