Evolution of promiscuous nuclear hormone receptors: LXR, FXR, VDR, PXR, and CAR

Abstract

Nuclear hormone receptors (NHRs) are transcription factors that work in concert with co-activators and co-repressors to regulate gene expression. Some examples of ligands for NHRs include endogenous compounds such as bile acids, retinoids, steroid hormones, thyroid hormone, and vitamin D. This review describes the evolution of liver X receptors α and β (NR1H3 and 1H2, respectively), farnesoid X receptor (NR1H4), vitamin D receptor (NR1I1), pregnane X receptor (NR1I2), and constitutive androstane receptor (NR1I3). These NHRs participate in complex, overlapping transcriptional regulation networks involving cholesterol homeostasis and energy metabolism. Some of these receptors, particularly PXR and CAR, are promiscuous with respect to the structurally wide range of ligands that act as agonists. A combination of functional and computational analyses has shed light on the evolutionary changes of NR1H and NR1I receptors across vertebrates, and how these receptors may have diverged from ancestral receptors that first appeared in invertebrates.

Fig. 1

Endogenous and synthetic ligands for NR1H and NR1I receptors. A. Most of the known endogenous ligands for LXR, FXR, VDR, PXR, and CAR are products formed from cholesterol, which can be converted to oxysterols, steroid hormones, bile salts, and vitamin D. B. Endogenous ligands for NR1H and NR1I receptors include: 5α-bile alcohols (planar structure, ‘ancestral’ bile salts; FXRs, PXRs), 5β-bile acids (bent structure, evolutionarily ‘recent’ bile salts; FXRs, VDRs, PXRs), calcitriol (VDRs), 5β-pregnan-3,20-dione (PXRs), 5α-androstan-3α-ol (PXRs, CARs), farnesol (FXRs), and 3-aminoethylbenzoate (frog PXRs). The bile alcohol shown is 5α-myxinol disulfate (3β,7α,16α,27-tetrahydroxy-5α-cholestan-3,27-disulfate) from the hagfish. The bile acid shown is taurochenodeoxycholic acid, a common bile acid found in teleost fish, birds, and mammals. C. Synthetic ligands for NR1H and NR1I receptors include: GW4064 (mammalian and zebrafish FXRs), fexaramine (mammalian FXRs), T-0901317 (LXRs, FXRs, PXRs), GW3965 (LXRs), and TCPOBOP (PXRs, CARs).

Fig. 2

Pharmacology of liver X and farnesoid X receptors across species. The tables list the receptors found in the corresponding animal(s) organized according to the standard phylogenetic tree on the left. One liver X receptor (LXR) gene has been detected in non-mammalian species (including the invertebrate Ciona intestinalis) while two LXR genes (termed LXRα and LXRβ) are found in mammals. Most animals have a single farnesoid X receptor (FXR) gene except for a few mammalian species that have an additional functional FXRβ gene. The agonists for LXRs from mammals, amphibians, and teleost fish are very similar, including oxysterols and the synthetic agonists GW3965 and T-0901317. The LXR from Ciona differs in pharmacology from vertebrate LXRs in not being activated by GW3965 and T-0901317. The pharmacology of avian, reptile, and sea lamprey LXRs have not been reported. Vertebrate FXRs studied so far share the common feature of being activated by species-specific primary (1°) bile salts. Outside mammals, the synthetic agonists fexaramine and GW4064 are generally inactive except for GW4064 as an agonist for the zebrafish FXR. The Ciona FXR is activated by sulfated steroids (steroid SO₄) and AM-580 but not by bile salts. The synthetic agonists marked by an asterisk (*) have submicromolar potency at the receptor indicated.

Fig. 3

Pharmacology of vitamin D, pregnane X, and constitutive androstane receptors across species. The tables list the receptors found in the corresponding animal(s) organized as in Fig. 2. We follow the convention of referring to non-mammalian PXR/CAR-like receptors as PXRs, although it is debatable whether PXR or CAR is the ancestral receptor. Vertebrate vitamin D receptors (VDRs) are all activated by vitamin D derivatives. Mammalian VDRs are also activated by secondary (2°) bile acids. The vertebrate PXRs studied so far, with the exception of frog PXRs, are activated by bile salts, steroid hormones, and xenobiotics, although with substantial cross-species differences in ligand specificity. The frog PXRs are selectively activated by a class of benzoate ligands that may be unique to amphibians. Only one putative ortholog to vertebrate NR1I receptors has been cloned and characterized from the invertebrate Ciona intestinalis. This receptor has markedly different pharmacology from vertebrate VDRs, PXRs, and CARs. There are several major evolutionary changes in NR1I receptors indicated on the phylogeny: *, duplication of a single receptor gene to separate VDR and PXR genes; **, divergence of function and ligand specificity for frog PXRs; and ***, duplication of single PXR/CAR gene to separate PXR and CAR genes. The “Ligand affinity” columns classifies the ligands into whether they have EC₅₀ values for activation of the receptor of 10 µM or higher (low affinity), 1–10 µM (medium affinity), or less than 1 µM (high affinity). Xenobiotics at PXRs have a range of affinities, including a small number such as hyperforin than have affinities in the nanomolar range.