Abstract
The evolution, molecular behavior, and physiological function of nuclear receptors are of particular interest given their diverse roles in regulating essential biological processes. The vitamin D receptor (VDR) is well known for its canonical roles in calcium homeostasis and skeletal maintenance. Additionally, VDR has received an increased amount of attention due to the discovery of numerous non-calcemic functions, including the detoxification of lithocholic acid. Lithocholic acid is a toxic metabolite of chenodeoxycholic acid, a primary bile acid. The partnership between the VDR and lithocholic acid has been hypothesized to be a recent adaptation that evolved to mediate the detoxification and elimination of lithocholic acid from the gut. This partnership is speculated to be limited to higher vertebrates (birds and mammals), as lower vertebrates do not synthesize the parent compound of lithocholic acid. However, the molecular functions associated with the observed insensitivity of basal VDRs to lithocholic acid have not been explored. Here we characterize canonical nuclear receptor functions of VDRs from select species representing key nodes in vertebrate evolution and span a range of bile salt phenotypes. Competitive ligand binding assays revealed that the receptor’s affinity for lithocholic acid is highly conserved across species, suggesting that lithocholic acid affinity is an ancient and non-adaptive trait. However, transient transactivation assays revealed that lithocholic acid-mediated VDR activation might have evolved more recently, as the non-mammalian receptors did not respond to lithocholic acid unless exogenous coactivator proteins were co-expressed. Subsequent functional assays indicated that differential lithocholic acid-mediated receptor activation is potentially driven by differential protein-protein interactions between VDR and nuclear receptor coregulator proteins. We hypothesize that the vitamin D receptor-lithocholic acid partnership evolved as a by-product of natural selection on the ligand-receptor partnership between the vitamin D receptor and the native VDR ligand: 1α,25-dihydroxyvitamin D3, the biologically active metabolite of vitamin D3.
Highlights
Conjugated bile alcohols and bile acids are multi-functional end products of cholesterol metabolism
We hypothesized that Lithocholic acid (LCA) would not function as a vitamin D receptor (VDR) ligand in the majority of the species examined in this study
We found that nuclear receptors (NRs) activation steps subsequent to ligand binding varied between species, and the ability of the VDRs to respond to LCA may be driven through increasingly sensitive protein-protein interactions
Summary
Conjugated bile alcohols and bile acids (collectively referred to as bile salts) are multi-functional end products of cholesterol metabolism. They function as water-soluble amphipathic detergents that facilitate the solubilization and absorption of lipids, vitamins, and proteins in the small intestine [1]. All vertebrates synthesize bile salts, studies have revealed substantial diversity in chemical structure across species. This diversity is hypothesized to be due to the evolution of an increasingly complex biosynthesis pathway that parallels vertebrate evolution [2,3]. Unlike the C27 bile acids, the cholesterol side doi:10.1371/journal.pone.0168278.g001
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