Multiple enhancer units mediate drug induction of CYP2H1 by xenobiotic-sensing orphan nuclear receptor chicken xenobiotic receptor.

Binding of nuclear receptors to drug-responsive enhancer units mediates transcriptional activation of cytochromes P-450 (P-450) by drugs and xenobiotics. In previous studies, a 264-base-pair (bp) phenobarbital-responsive enhancer unit (PBRU) located at -1671 to -1408 upstream of the chicken CYP2H1 transcriptional start-site increased gene expression when activated by the chicken xenobiotic-sensing orphan nuclear receptor CXR. In extension of these studies, we now have functionally analyzed a second distal drug-responsive element and delimited a 643- and a 240-bp PBRU located between 5 and 6 kilobases upstream of the transcriptional start site of CYP2H1. Both PBRUs were activated by CXR after treatment with different drugs. A nuclear receptor binding site, a direct repeat-4 (DR-4) hexamer repeat, was identified on the 240-bp PBRU. Site-directed mutagenesis of this DR-4 abolished activity in reporter gene assays in the chicken hepatoma cells leghorn male hepatoma as well as transactivation of the 240-bp PBRU by CXR in CV-1 cells. CXR bound to this PBRU in electromobility shift assays and the complex remained unaffected by unlabeled 240-bp PBRU with a mutated DR-4. In cross-species experiments, both the human xenobiotic-sensing nuclear receptors pregnane X receptor and constitutive androstane receptor bound to this element, suggesting sequence conservation between chicken and mammalian PBRUs and between the DNA binding domains of these receptors. Of two orphan nuclear receptors involved in cholesterol and bile acid homeostasis, only chicken liver X receptor (LXR) but not chicken farnesoid X receptor bound to the 240-bp PBRU. These results suggest that CYP2H1 induction is explained by the combined effect of multiple distal enhancer elements interacting with multiple transcription factors, including CXR and LXR.

Purification, cloning, and characterization of a second arylalkylamine N-acetyltransferase from Drosophila melanogaster.

In insects, amine acetylation by the enzyme arylalkylamine N-acetyltransferase (AANAT) is involved in melatonin formation, sclerotization, and neurotransmitter inactivation. This wide spectrum of activities suggests that several AANAT enzymes are present. We recently purified a protein fraction with AANAT activity from Drosophila melanogaster and cloned the corresponding gene, aaNAT1. Following the same strategy, we now report the purification of an additional AANAT from D. melanogaster, AANAT2, and the cloning of the corresponding cDNA. The isolated protein differs from AANAT1a and AANAT1b in its molecular weight and isoelectric point. The AANAT2 shares about 30% identity with the products of the aaNAT1 gene. The enzyme does not follow one-site Michaelis-Menten kinetics when assayed with various concentrations of the arylalkylamine tryptamine and a constant concentration (0.5 mM) of the cofactor acetyl coenzyme A. The data can be interpreted in terms of an enzyme with two kinetic regimes (K(m1) = 7.2 microM, K(m2) = 0.6 mM, and v(max2) = 2.7 v(max1)) that are governed by binding of the substrate to a regulatory site (K(r) = 6.2 mM). These findings demonstrate the presence of a second expressed gene encoding an AANAT in D. melanogaster. Northern blot analysis revealed no diurnal variation of aaNAT2 transcription, similar to the results obtained for aaNAT1a and aaNAT1b.

Molecular and biochemical characterization of the aaNAT1 (Dat) locus in Drosophila melanogaster: differential expression of two gene products.

In insects, arylalkylamine N-acetyltransferases (AANATs) have been implicated in several physiological processes, including sclerotization, inactivation of certain neurotransmitters, and, similar to the function in vertebrates, catalysis of the rate-limiting step in melatonin biosynthesis. Here, we report an extensive biochemical and functional analysis of the products of the aaNAT1 gene of Drosophila melanogaster. The aaNAT1 gene generates two transcripts through alternative first-exon usage. These transcripts are under tissue-specific and developmental control and encode proteins which differ in their N-terminus with respect to their starting methionine. The more abundant isoform, AANATlb, is first expressed during late embryogenesis in the brain, the ventral nerve cord, and the midgut; in adults, AANATlb is still detectable in the brain and midgut. The less abundant isoform, AANATla, appears only during late pupal stages and in adults is found predominantly in the brain. We demonstrate that the mutation Dat(lo) represents a hypomorphic allele of aaNAT1b, in which an insertion of two transposable elements, MDG412 and blastopia, has occurred within the first intron of the gene. Using a deficiency which removes the aaNAT1 gene, we provide evidence that aaNAT1 is not essential for the process of sclerotization. Furthermore, neither of the two enzyme isoforms shows circadian regulation of RNA or protein levels. The differing levels of abundance and distinct developmental control of AANAT1a and AANAT1b suggest different in vivo functions for these two enzymes.

Cloning of an arylalkylamine N-acetyltransferase (aaNAT1) from Drosophila melanogaster expressed in the nervous system and the gut.

In insects, neurotransmitter catabolism, melatonin precursor formation, and sclerotization involve arylalkylamine N-acetyltransferase (aaNAT, EC 2.3.1.87) activity. It is not known if one or multiple aaNAT enzymes are responsible for these activities. We recently have purified an aaNAT from Drosophila melanogaster. Here, we report the cloning of the corresponding aaNAT cDNA (aaNAT1) that upon COS cell expression acetylates dopamine, tryptamine, and the immediate melatonin precursor serotonin. aaNAT1 represents a novel gene family unrelated to known acetyl-transferases, except in two weakly conserved amino acid motifs. In situ hybridization studies of aaNAT1 mRNA in embryos reveal hybridization signals in the brain, the ventral cord, the gut, and probably in oenocytes, indicating a broad tissue distribution of aaNAT1 transcripts. Moreover, in day/ night studies we demonstrate a diurnal rhythm of melatonin concentration without a clear-cut change in aaNAT1 mRNA levels. The data suggest that tissue-specific regulation of aaNAT1 may be associated with different enzymatic functions and do not exclude the possibility of additional aaNAT genes.