Characterization of the aryl hydrocarbon receptor repressor gene and association of its Pro185Ala polymorphism with micropenis

BACKGROUND: Genetic background of a fetus contributes to the abnormal development after teratogen exposure. In rodents, in utero exposure to dioxins affects male external genital development. The effects of dioxins are mediated via the aryl hydrocarbon receptor (AHR) and its binding protein, aryl hydrocarbon receptor nuclear translocator (ARNT). In mice, aryl hydrocarbon receptor repressor (AHRR), which binds to ARNT in competition with AHR, plays a critical negative regulatory role in AHR signaling. We attempt to characterize the human AHRR gene and investigate the relationship between AHRR polymorphisms and the incidence of micropenis, a phenotype of undermasculinization. METHODS: We identified and characterized the human homolog of mouse AHRR, taking advantage of the publicly available draft version of the human genome sequence. After detecting an AHRR protein polymorphism by the direct sequencing of pooled human genomic DNA, we evaluated the association between the polymorphism and the presence or absence of micropenis (< -2.5 SD) in patients with micropenis and control subjects. RESULTS: The deduced sequence for human AHRR (715 residues) and the mouse AHRR protein exhibited 81% sequence homology to each other. The Pro185Ala polymorphism was identified between the PAS-A region and the highly conserved arginine/cysteine-rich RCFRCRL/VRC region. Forty-six percent (27/59) of patients with micropenis and 27% (22/80) of the controls were homozygous for 185Pro; this difference in frequencies was significant (P = 0.03). CONCLUSIONS: Homozygosity for the 185Pro allele of AHRR may increase the susceptibility of a fetus to the undermasculinizing effects of dioxin exposure in utero, presumably through the diminished inhibition of AHR-mediated signaling.     

Conditional disruption of the aryl hydrocarbon receptor nuclear translocator (Arnt) gene leads to loss of target gene induction by the aryl hydrocarbon receptor and hypoxia-inducible factor 1alpha

To determine the function of the aryl hydrocarbon receptor nuclear translocator (ARNT), a conditional gene knockout mouse was made using the Cre-loxP system. Exon 6, encoding the conserved basic-helix-loop-helix domain of the protein, was flanked by loxP sites and introduced into the Arnt gene by standard gene disruption techniques using embryonic stem cells. Mice homozygous for the floxed allele were viable and had no readily observable phenotype. The Mx1-Cre transgene, in which Cre is under control of the interferon-gamma promoter, was introduced into the Arnt-floxed mouse line. Treatment with polyinosinic-polycytidylic acid to induce expression of Cre resulted in complete disruption of the Arnt gene and loss of ARNT messenger RNA (mRNA) expression in liver. To determine the role of ARNT in gene control in the intact animal mouse liver, expression of target genes under control of an ARNT dimerization partner, the aryl hydrocarbon receptor (AHR), was monitored. Induction of CYP1A1, CYP1A2, and UGT1*06 mRNAs by the AHR ligand 2,3,7,8-tetrachlorodibenzo-p-dioxin was absent in livers of Arnt-floxed/Mx1-Cre mice treated with polyinosinic-polycytidylic. These data demonstrate that ARNT is required for AHR function in the intact animal. Partial deletion of the Arnt allele was found in kidney, heart, intestine, and lung. Despite more than 80% loss of the ARNT expression in lung, maximal induction of CYP1A1 was found, indicating that the expression level of ARNT is not limiting to AHR signaling. Cobalt chloride induction of the glucose transporter-1 and heme oxygenase-1 mRNAs was also markedly abrogated in mice lacking ARNT expression, suggesting an inhibition of HIF-1alpha activity. These studies establish a critical role for ARNT in AHR and HIF-1alpha signal transduction in the intact mouse.     

Identification of Cys255 in HIF‐1α as a novel site for development of covalent inhibitors of HIF‐1α/ARNT PasB domain protein–protein interaction

The heterodimer HIF‐1α (hypoxia inducible factor)/HIF‐β (also known as ARNT‐aryl hydrocarbon nuclear translocator) is a key mediator of cellular response to hypoxia. The interaction between these monomer units can be modified by the action of small molecules in the binding interface between their C‐terminal heterodimerization (PasB) domains. Taking advantage of the presence of several cysteine residues located in the allosteric cavity of HIF‐1α PasB domain, we applied a cysteine‐based reactomics “hotspot identification” strategy to locate regions of HIF‐1α PasB domain critical for its interaction with ARNT. COMPOUND 5 was identified using a mass spectrometry‐based primary screening strategy and was shown to react specifically with Cys255 of the HIF‐1α PasB domain. Biophysical characterization of the interaction between PasB domains of HIF‐1α and ARNT revealed that covalent binding of COMPOUND 5 to Cys255 reduced binding affinity between HIF‐1α and ARNT PasB domains approximately 10‐fold. Detailed NMR structural analysis of HIF‐1α‐PasB‐COMPOUND 5 conjugate showed significant local conformation changes in the HIF‐1α associated with key residues involved in the HIF‐1α/ARNT PasB domain interaction as revealed by the crystal structure of the HIF‐1α/ARNT PasB heterodimer. Our screening strategy could be applied to other targets to identify pockets surrounding reactive cysteines suitable for development of small molecule modulators of protein function.