Identification of a human nuclear receptor defines a new signaling pathway for CYP 3 A induction

Abstract

Nuclear receptors regulate metabolic pathways in response to changes in the environment by appropriate alterations in gene expression of key metabolic enzymes. Here, a computational search approach based on iteratively built hidden Markov models of nuclear receptors was used to identify a human nuclear receptor, termed hPAR, that is expressed in liver and intestines. hPAR was found to be efficiently activated by pregnanes and by clinically used drugs including rifampicin, an antibiotic known to selectively induce human but not murine CYP3A expression. The CYP3A drugmetabolizing enzymes are expressed in gut and liver in response to environmental chemicals and clinically used drugs. Interestingly, hPAR is not activated by pregnenolone 16␣-carbonitrile, which is a potent inducer of murine CYP3A genes and an activator of the mouse receptor PXR.1. Furthermore, hPAR was found to bind to and trans-activate through a conserved regulatory sequence present in human but not murine CYP3A genes. These results provide evidence that hPAR and PXR.1 may represent orthologous genes from different species that have evolved to regulate overlapping target genes in response to pharmacologically distinct CYP3A activators, and have potential implications for the in vitro identification of drug interactions important to humans.The recent progress made in determining the full genomic sequences of model organisms as well as the rapid accumulation of sequence data from the human genome has opened up new possibilities to determine the functional organization of genomes by computational approaches (1). Multiple alignments of members of different protein families followed by homology searching are powerful methods to infer gene function from sequence data as well as to identify novel genes within a given gene family. Hidden Markov models (HMMs) are a general statistical modeling technique that can be used as formal, fully probabilistic forms of sequence profiles (2, 3), describing the consensus of a set of sequences. This approach toward the identification and functional characterization of novel genes is particularly amenable to evolutionary conserved gene families consisting of a large number of orthologs and paralogs. The nuclear receptors constitute one such large gene family that is structurally and functionally conserved and represented within different metazoan phylae from cnidarians to vertebrates (4). These receptors are conditionally regulated transcription factors that exert their effects by interacting with small lipophilic ligands followed by sequence-specific binding of the receptor to DNA sequences called hormone response elements (HREs). Binding of the receptor to DNA results in changes in gene expression of specific target genes (5).Steroid hormones were the first group of small, lipophilic molecules identified as nuclear receptor ligands. Today, the number of substances known to regulate the activity of this group of receptors is represented by a large and chemically diverse group of molecules including retinoids, vi...

Retinoids induce cytochrome P450 3A4 through RXR/VDR-mediated pathway

“…It has been well characterized that CYP3A4 gene is regulated by PXR [5][6][7][8], CAR [4], and VDR [9,10]. Induction of Cyp3a11 (homologous of human CYP3A4) mRNA by retinoids in hepatocytes which are deficient in both PXR and CAR would strongly suggest the role of VDR in regulation of Cyp3a11.…”

“…It also participates in the metabolism of xenobiotics and bio-activation of environmental pro-carcinogens. Several members of the nuclear receptor superfamily including CAR [4], PXR [5][6][7][8], VDR [9,10], and the glucocorticoid receptor (GR) [11] have been shown to be responsible for endobiotic-and xenobiotic-mediated CYP3A induction. Human PXR (hPXR) or steroids and xenobiotic receptor (SXR) [12,13], CAR [4], and VDR [9,10] control CYP3A4 expression by targeting the same cis-acting elements located in the regulatory region of target genes.…”

Intestinal cell-specific vitamin D receptor (VDR)-mediated transcriptional regulation of CYP3A4 gene

“…CYP3A4 induction by xenobiotics and hormones is mediated by the pregnane X receptor (PXR, NR1I2) [5][6][7], constitutive androstane receptor (CAR, NR1I3) [8,9], vitamin D receptor (VDR, NR1I1) [10,11], glucocorticoid receptor- (GR, NR3C1) [12], hepatocyte nuclear factor-4 (HNF4, NR2A1) [13] and retinoid X receptor- (RXR, NR2B1) in the liver [3]. In addition, basal (constitutive) CYP3A4 expression in the absence of an inducing agent has been found to correlate with expression of CAR, PXR and HNF4 in the liver [12,14].…”

Nuclear receptor mediated induction of cytochrome P450 3A4 by anticancer drugs: a key role for the pregnane X receptor

“…CYP3A4 expression is transcriptionally regulated by members of the NR1I nuclear receptor subfamily of ligandactivated transcription factors, which constitutes the human pregnane X receptor (hPXR; NR1I2) [3,5,6], the vitamin D 3 receptor (VDR; NR1I1) [7] and the constitutive androstane receptor (CAR; NRI3) [8,9]. Only a few agonists are known for CAR and VDR, while PXR is activated by a wide variety of structurally unrelated compounds that include rifampicin, phenobarbital and hyperforin, but also anticancer drugs like paclitaxel [10] and tamoxifen [11].…”