Nuclear Receptors Constitutive Androstane Receptor and Pregnane X Receptor Activate a Drug-responsive Enhancer of the Murine 5-aminolevulinic Acid Synthase Gene

Nuclear receptors have been implicated in the transcriptional regulation of expression of a growing number of genes, including cytochromes P450 and 5-aminolevulinate synthase (ALAS1), the first and rate-limiting enzyme in the heme biosynthesis pathway. Although drugs that induce cytochromes P450 also induce ALAS1, the regulatory mechanisms governing these pathways have not been fully elucidated. We have identified a drug-responsive enhancer in the murine ALAS1 gene. This sequence mediates transcriptional activation by a wide range of compounds including typical cytochrome P450 pan-inducers phenobarbital and metyrapone, as well as specific activators of the pregnane X receptor and the constitutive androstane receptor. ALAS1 drug-responsive enhancer sequences were identified by transient transfection of reporter gene constructs in the drug-responsive leghorn male hepatoma cell line. Using the NUBIScan algorithm, DR4 nuclear receptor binding sites were identified within the elements and their roles in mediating transcriptional activation of ALAS1 were confirmed by site-directed mutagenesis. Electrophoretic mobility shift assays demonstrate clear interactions of mouse pregnane X receptor and constitutive androstane receptor on the ADRES. Transactivation assays in CV-1 cells implicate the nuclear receptors as major contributors to transcriptional activation of ALAS1. Moreover, in vivo studies in knock-out animals confirm the induction of ALAS1 is mediated at least in part by nuclear receptors. These studies are the first to explain drug induction via drug response elements for mammalian ALAS1.