An Orphan Nuclear Receptor Activated by Pregnanes Defines a Novel Steroid Signaling Pathway

Abstract

Steroid hormones exert profound effects on differentiation, development, and homeostasis in higher eukaryotes through interactions with nuclear receptors. We describe a novel orphan nuclear receptor, termed the pregnane X receptor (PXR), that is activated by naturally occurring steroids such as pregnenolone and progesterone, and synthetic glucocorticoids and antiglucocorticoids. PXR exists as two isoforms, PXR.1 and PXR.2, that are differentially activated by steroids. Notably, PXR.1 is efficaciously activated by pregnenolone 16alpha-carbonitrile, a glucocorticoid receptor antagonist that induces the expression of the CYP3A family of steroid hydroxylases and modulates sterol and bile acid biosynthesis in vivo. Our results provide evidence for the existence of a novel steroid hormone signaling pathway with potential implications in the regulation of steroid hormone and sterol homeostasis.

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

“…We have recently reported that retinoids activate the RXRα/hPXR-mediated pathway and induce endogenous CYP3A4 activity in Huh7 human hepatoma cells [14]. Given that hPXR and VDR share 64% amino acid identity in their ligand binding domain [7], and the networks of hPXR, VDR, and CAR control CYP3A4 gene expression, we anticipated that retinoids may also regulate CYP3A4 gene expression through the RXR/VDR signaling pathway. Using transient transfection assays, real-time PCR for quantification of mRNA levels, enzyme activity assays, gene knockout models, and ChIP assays, we have unequivocally demonstrated that retinoid-activated RXRs/VDR heterodimers and RXRα homodimers, are responsible for the induction of CYP3A4.…”

“…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.…”

“…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. One of the shared motifs is pER6, an everted repeat separated by 6 nucleotides, in the proximal region of the CYP3A4 promoter [7]. The second is dXREM, a distal xenobiotic-responsive enhancer module [4].…”

The apical conjugate efflux pump ABCC2 (MRP2)

“…In addition, a hormone response element in the rat Abcc2 promoter (ER-8) was identified, which is bound by heterodimers of the retinoid X receptor α (RXRα, NR2B1 [129]) with the ligand-activated transcription factors farnesoid X receptor (FXR, NR1H4), pregnane X receptor (PXR, NR1I2), or constitutive androstane receptor (CAR, NR1I3) [73]. These nuclear receptors are, for example, activated by bile acids via FXR [132] by various xenobiotics such as the antibiotic rifampicin, the synthetic glucocorticoid dexamethasone, and pregnenolone 16α-carbonitrile via PXR [8, 41,85], or by phenobarbital via CAR [161]. Knowledge of the presence of the hormone response element ER-8 in the rat Abcc2 promoter [73] may now explain studies which describe the induction of Abcc2 mRNA and Abcc2 protein in primary cultures of rat hepatocytes by a number of xenobiotics, including the carcinogen 2-acetylaminofluorene, the anticancer drug cisplatin, the antifungal agent clotrimazole, the antibiotic cycloheximide, dexamethasone, the chemopreventive agents oltipraz and sulforaphane, phenobarbital, and pregnenolone 16α-carbonitrile [23, 73,75,98,137].…”