Functional evolution of the vitamin D and pregnane X receptors

Erica J Reschly,Lee R Hagey,Junhai Ou,Nathan Bahary,Matthew D Krasowski,Raman Venkataramanan,Sripal R Mada,Afonso Celso Dias Bainy,Jaco Joaquim Mattos

doi:10.1186/1471-2148-7-222

Abstract

BackgroundThe vitamin D receptor (VDR) and pregnane X receptor (PXR) are nuclear hormone receptors of the NR1I subfamily that show contrasting patterns of cross-species variation. VDR and PXR are thought to have arisen from duplication of an ancestral gene, evident now as a single gene in the genome of the chordate invertebrate Ciona intestinalis (sea squirt). VDR genes have been detected in a wide range of vertebrates including jawless fish. To date, PXR genes have not been found in cartilaginous fish. In this study, the ligand selectivities of VDRs were compared in detail across a range of vertebrate species and compared with those of the Ciona VDR/PXR. In addition, several assays were used to search for evidence of PXR-mediated hepatic effects in three model non-mammalian species: sea lamprey (Petromyzon marinus), zebrafish (Danio rerio), and African clawed frog (Xenopus laevis).ResultsHuman, mouse, frog, zebrafish, and lamprey VDRs were found to have similar ligand selectivities for vitamin D derivatives. In contrast, using cultured primary hepatocytes, only zebrafish showed evidence of PXR-mediated induction of enzyme expression, with increases in testosterone 6β-hydroxylation activity (a measure of cytochrome P450 3A activity in other species) and flurbiprofen 4-hydroxylation activity (measure of cytochrome P450 2C activity) following exposure to known PXR activators. A separate assay in vivo using zebrafish demonstrated increased hepatic transcription of another PXR target, multidrug resistance gene (ABCB5), following injection of the major zebrafish bile salt, 5α-cyprinol 27-sulfate. The PXR target function, testosterone hydroxylation, was detected in frog and sea lamprey primary hepatocytes, but was not inducible in these two species by a wide range of PXR activators in other animals. Analysis of the sea lamprey draft genome also did not show evidence of a PXR gene.ConclusionOur results show tight conservation of ligand selectivity of VDRs across vertebrate species from Agnatha to mammals. Using a functional approach, we demonstrate classic PXR-mediated effects in zebrafish, but not in sea lamprey or African clawed frog liver cells. Using a genomic approach, we failed to find evidence of a PXR gene in lamprey, suggesting that VDR may be the original NR1I gene.

Highlights

The vitamin D receptor (VDR) and pregnane X receptor (PXR) are nuclear hormone receptors of the NR1I subfamily that show contrasting patterns of cross-species variation
All five vertebrate receptors were activated by 1α,25-(OH)2vitamin D3, 1α-hydroxyvitamin D2, 1αhydroxyvitamin D3, 25-hydroxyvitamin D3, and 24(R),25-(OH)2-vitamin D3 (Figure 2A–E; Table 1). xlVDR has lower potency for the five vitamin D derivatives studied, similar to the initial report published on X. laevis VDR [10]
There were few major differences between the five receptors with regard to vitamin D derivatives. This is consistent with the high degree of sequence conservation across vertebrate VDRs at positions shown to interact with ligands in x-ray crystallographic structures of human [3234], rat [35,36], and zebrafish VDRs [37,38] (Additional file 2)

Summary

Introduction

The vitamin D receptor (VDR) and pregnane X receptor (PXR) are nuclear hormone receptors of the NR1I subfamily that show contrasting patterns of cross-species variation. VDR genes have been detected in a wide range of vertebrates including jawless fish. The ligand selectivities of VDRs were compared in detail across a range of vertebrate species and compared with those of the Ciona VDR/PXR. The vitamin D receptor (VDR, NR1I1) and pregnane X receptor (PXR, NR1I2) are members of the nuclear hormone receptor (NR) superfamily of ligand-activated transcription factors. VDRs bind 1α,25-(OH)2-vitamin D3 (calcitriol) with high affinity and mediate classic calcitriol effects such as regulation of calcium and phosphate homeostasis (see Figure 1 for chemical structure of calcitriol).

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Discussion

Conclusion