Identification of the xenosensors regulating human 5-aminolevulinate synthase.
Heme is an essential component of numerous hemoproteins with functions including oxygen transport, energy metabolism, and drug biotransformation. In nonerythropoietic cells, 5-aminolevulinate synthase (ALAS1) is the rate-limiting enzyme in heme biosynthesis. Upon exposure to drugs that induce cytochromes P450 and other drug-metabolizing enzymes, ALAS1 is transcriptionally up-regulated, increasing the rate of heme biosynthesis to provide heme for cytochrome P450 hemoproteins. We used a combined in silico-in vitro approach to identify sequences in the ALAS1 gene that mediate direct transcriptional response to xenobiotic challenge. We have characterized two enhancer elements, located 20 and 16 kb upstream of the transcriptional start site. Both elements respond to prototypic inducer drugs and interact with the human pregnane X receptor NR1I2 and the human constitutive androstane receptor NR1I3. Our results suggest that the fundamental mechanism of drug induction is the same for cytochromes P450 and ALAS1. Transcriptional activation of the ALAS1 gene is the first step in the coordinated up-regulation of apoprotein and heme synthesis in response to exogenous and endogenous signals controlling heme levels. Understanding the direct effects of drugs on heme synthesis is of clinical interest, particularly in patients with hepatic porphyrias.
Podvinec
Michael
M
Division of Pharmacology and Neurobiology, Biozentrum, University of Basel, Klingelbergstrasse 50-70, CH-4056 Basel, Switzerland.
Handschin
Christoph
C
Looser
Renate
R
Meyer
Urs A
UA
D024506:5' Flanking Region / Q000235:genetics; D000624:5-Aminolevulinate Synthetase / Q000235:genetics / Q000378:metabolism*; D001483:Base Sequence; D002460:Cell Line; D000090702:Constitutive Androstane Receptor; D004742:Enhancer Elements, Genetic / Q000187:drug effects / Q000235:genetics; D004790:Enzyme Induction / Q000187:drug effects; D006801:Humans; D008969:Molecular Sequence Data; D000077297:Pregnane X Receptor; D011485:Protein Binding; D018160:Receptors, Cytoplasmic and Nuclear / Q000819:agonists / Q000378:metabolism; D011987:Receptors, Steroid / Q000819:agonists / Q000378:metabolism; D014157:Transcription Factors / Q000819:agonists / Q000378:metabolism; D015533:Transcriptional Activation / Q000187:drug effects; D015262:Xenobiotics / Q000494:pharmacology*
D000090702:Constitutive Androstane Receptor; C000625852:NR1I2 protein, human; C000716327:NR1I3 protein, human; D000077297:Pregnane X Receptor; D018160:Receptors, Cytoplasmic and Nuclear; D011987:Receptors, Steroid; D014157:Transcription Factors; D015262:Xenobiotics; D000624:5-Aminolevulinate Synthetase