Identification of the xenosensors regulating human 5-aminolevulinate synthase

Abstract

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 upregulated, 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.

Effects of Hepatic Drug-metabolizing Enzyme Induction on Clinical Pathology Parameters in Animals and Man

“…Phase II enzymes additionally conjugate several nonsteroidal and some steroidal hormones; the UGT1A family is particularly responsible for conjugation of thyroid and estrogenic hormones (Kato et al 2008;Mackenzie et al 2000;Takahashi et al 2008), and the UGT2B family has a significant role in glucuronidation of human androgens (Barbier and Bélanger 2003). Drug-metabolizing enzyme induction has also been suspected of altering other endogenous human products, including circulating insulin, glucose, lipids, bilirubin, and vitamins D, K, B12, and folate (Benedetti et al 2005;Lahtela 1986;Moreau et al 2008;Podvinec et al 2004;Putignano et al 1998;Roth et al 2008;Xie et al 2003;You 2004). In this section, only clinical pathology linking DME induction to human thyroid and reproductive hormone levels will be discussed owing to limited comparable information on these other effects with laboratory animals.…”

Aquisition, Mobilization and Utilization of Cellular Iron and Heme: Endless Findings and Growing Evidence of Tight Regulation

“…The level of intracellular uncommitted heme is extremely low since increased levels of free heme are toxic, so heme biosynthesis is tightly regulated at the step of ALAS1 by multiple mechanisms such as (1) preventing the transfer of ALAS1 precursor to the mitochondria, (2) decreasing the stability of ALAS1 mRNA, and (3) repressing the transcription of ALAS1 mRNA in mammals (Munakata et al 2004). On the other hand, the drug-responsive elements that mediate the direct activation of the transcription were identified in the promoter region of the murine, chicken and human ALAS1 genes (Fraser et al 2002;Podvinec et al 2004). The binding of nuclear receptors such as constitutive androstane receptor and pregnane X receptor to the drug-responsive elements led to activation of the transcription, the mechanisms being similar to those for the transcriptional activation of druginducible cytochrome P-450s (Podvinec et al 2004).…”

“…On the other hand, the drug-responsive elements that mediate the direct activation of the transcription were identified in the promoter region of the murine, chicken and human ALAS1 genes (Fraser et al 2002;Podvinec et al 2004). The binding of nuclear receptors such as constitutive androstane receptor and pregnane X receptor to the drug-responsive elements led to activation of the transcription, the mechanisms being similar to those for the transcriptional activation of druginducible cytochrome P-450s (Podvinec et al 2004).…”

An alternatively‐spliced exon in the 5′‐UTR of human ALAS1 mRNA inhibits translation and renders it resistant to haem‐mediated decay

“…The ALAS1 gene is expressed in both erythroid and non-erythroid cells, although its expression varies between tissues [2]. Highest levels are found in the liver where ALAS1 transcription can be induced directly by compounds such as phenobarbital to co-ordinate an increased demand for haem with increased apo-cytochrome synthesis [3,4]. Whilst haem has been reported to regulate transcription of ALAS1 in some cells [2], but not others [5], it primarily controls its own synthesis post-transcriptionally: through inhibition of the import of pre-ALAS1 to the mitochondrial matrix [6,7] and regulation of ALAS1 mRNA stability [5,[8][9][10].…”