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 1 alpha

To determine the function of the aryl hydrocarbon receptor nuclear transloc ator (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 Amt gene by sta ndard gene disruption techniques using embryonic stem cells. Mice homozygou s for the flexed allele were viable and had no readily observable phenotype . The Mx1-Cre transgene, in which Cre is under control of the interferon-ga mma promoter, was introduced into the Arnt-floxed mouse line. Treatment wit h polyinosinic-polycytidylic acid to induce expression of Cre resulted in c omplete disruption of the Amt gene and loss of ARNT messenger RNA (mRNA) ex pression in liver. To determine the role of ARNT in gene control in the int act 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-te trachlorodibenzo-p-dioxin was absent in livers of Arnt-floxed/Mx1-Cre mice treated with polyinosinic-polycytidylic. These data demonstrate that ARNT i s required for AHR function in the intact animal. Partial deletion of the A mt allele was found in kidney, heart, intestine, and lung. Despite more tha n 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 expressi on, suggesting an inhibition of HIF-1 alpha activity. These studies establi sh a critical role for ARNT in AHR and HIF-1 alpha signal transduction in t he intact mouse.