Identification of bile acid precursors as endogenous ligands for the nuclear xenobiotic pregnane X receptor

Abstract

Sterol 27-hydroxylase (CYP27A1) is required for bile acid synthesis by both the classical and alternate pathways. Cyp27a1 ؊/؊ mice exhibit a dramatic increase in the activity of cytochrome P450 3A (CYP3A), which catalyzes side-chain hydroxylations of bile acid intermediates, thereby facilitating their excretion in the bile and urine. We examine the role of the nuclear xenobiotic receptor PXR (pregnane X receptor) in this process. We demonstrate that expression of Cyp3a11 and other established PXR target genes is increased in the Cyp27a1 ؊/؊ mice. WhenCyp27a1 ؊/؊ mice are fed a diet containing either cholic acid or chenodeoxycholic acid, expression of CYP7A1, which catalyzes the rate-limiting step in bile acid biosynthesis, is strongly suppressed. In parallel, the induction of Cyp3a11 observed in these mice is reversed, suggesting that bile acid intermediates serve as PXR activators. In support of this hypothesis, three potentially toxic sterols (7␣-hydroxy-4-cholesten-3-one, 5␤-cholestan-3␣,7␣,12␣-triol, and 4-cholesten-3-one), including two that are known to accumulate in Cyp27a1 ؊/؊ mice, are efficacious activators of mouse PXR. All three compounds are more potent activators of mouse PXR than of human PXR, which may explain in part why humans who lack functional CYP27A1 do not display a corresponding increase in CYP3A activity and are stricken with the disease cerebrotendinous xanthomatosis. Taken together, these results reveal the existence of a feedforward regulatory loop by which potentially toxic bile acid intermediates activate PXR and induce their own metabolism. In addition, this study demonstrates that animal models with alterations in gene expression can be used to identify endogenous ligands for orphan nuclear receptors.

Neurosteroid regulation of central nervous system development

“…Unlike most steroid hormone receptors that have dissociation constants in the nanomolar range, and activate gene expression when steroids are present at 1-100 nM, the Kd for allopregnanolone binding to PXR is in the micromolar range, and requires 10-50 μM allopregnanolone for activation of gene expression in cell transfection assays (Lamba et al, 2004). PXR is activated by a wide range of structurally dissimilar compounds, consistent with its function in drug/xenobiotic sensing (Dixit et al, 2005;Ekins and Erickson, 2002;Kliewer et al, 2002;Poso and Honkakoski, 2006;Zhu et al, 2004), bile acid metabolism (Dussault et al, 2003;Goodwin et al, 2003), owing to its large ligand binding pocket (Ekins and Schuetz, 2002;Orans et al, 2005;Schuster and Langer, 2005;Watkins et al, 2002;Watkins et al, 2001). …”

Nuclear receptors in macrophages: A link between metabolism and inflammation

“…CYP27 can be induced by retinoids and PPAR-c only in human macrophages [50]. Deletion of CYP27 does not induce atherosclerosis in mice [53][54][55] only in men [45,46]. LXR can induce its own transcription only in human cells [26,56,57].…”

Regulatory network of lipid-sensing nuclear receptors: roles for CAR, PXR, LXR, and FXR

“…lithocholic acid) have been shown to directly activate PXR at concentrations between 10-100 M [50,51]. Moreover, three bile acid precursors (7-hydroxy-4-cholesten-3-one, 5-cholestan-3,7,12-triol, and 4-cholesten-3-one) activate mouse PXR in the low -8 -micromolar range but are less potent activators of its human ortholog [52]. This species difference in ligand specificity extends to other xenobiotic ligands [53].…”