Abstract
Vitamin D receptor (VDR) mediates the biological function of the steroid hormone calcitriol, which is the metabolically active version of vitamin D. Calcitriol is important for a wide array of physiological functions, including calcium and phosphate homeostasis. In contrast to mammals, which harbor one VDR encoding gene, teleosts possess two orthologous vdr genes encoding Vdr alpha (Vdra) and Vdr beta (Vdrb). Genome mining identified the vdra and vdrb paralogs in the Atlantic cod (Gadus morhua) genome, which were further characterized regarding their phylogeny, tissue-specific expression, and transactivational properties induced by calcitriol. In addition, a selected set of polycyclic aromatic hydrocarbons (PAHs), including naphthalene, phenanthrene, fluorene, pyrene, chrysene, benzo[a]pyrene (BaP), and 7-methylbenzo[a]pyrene, were assessed for their ability to modulate the transcriptional activity of gmVdra and gmVdrb in vitro. Both gmVdra and gmVdrb were activated by calcitriol with similar potencies, but gmVdra produced significantly higher maximal fold activation. Notably, none of the tested PAHs showed agonistic properties towards the Atlantic cod Vdrs. However, binary exposures of calcitriol together with phenanthrene, fluorene, or pyrene, antagonized the activation of gmVdra, while chrysene and BaP significantly potentiated the calcitriol-mediated activity of both receptors. Homology modeling, solvent mapping, and docking analyses complemented the experimental data, and revealed a putative secondary binding site in addition to the canonical ligand-binding pocket (LBP). Calcitriol was predicted to interact with both binding sites, whereas PAHs docked primarily to the LBP. Importantly, our in vitro data suggest that PAHs can interact with the paralogous gmVdrs and interfere with their transcriptional activities, and thus potentially modulate the vitamin D signaling pathway and contribute to adverse effects of crude oil and PAH exposures on cardiac development and bone deformities in fish.
Highlights
The vitamin D receptor (VDR, NR1I1) is a ligand-activated nuclear receptor transcription factor that mediates the biological function of the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3 or calcitriol), the metabolically active form of vitamin D (Reschly and Krasowski, 2006)
The Atlantic cod genome was used for exon-intron mapping of the gmvdr genes, demonstrating that both gmvdra and gmvdrb consist of eight exons, which are located on chromosomes 1 and 13, respectively (Fig. 2A)
Both gmVdra and gmVdrb were transactivated by calcitriol in the luciferase reporter gene assay, but distinct differences in efficacies were observed
Summary
The vitamin D receptor (VDR, NR1I1) is a ligand-activated nuclear receptor transcription factor that mediates the biological function of the steroid hormone 1,25-dihydroxyvitamin D3 (1,25(OH)2D3 or calcitriol), the metabolically active form of vitamin D (Reschly and Krasowski, 2006). The LBD is responsible for the binding of ligands to the ligand-binding pocket (LBP), and it contains a conserved motif important for dimerization to the retinoid X receptor (RXR), as well as the ligand dependent transactivation function (AF-2) for co-activator recruitment and binding (Moras and Gronemeyer, 1998). Following ligand activation by calcitriol, VDR can either form a homo dimer and directly translocate to the nucleus, or heterodimerize with RXR before nuclear translocation (Carlberg et al, 1993, Nishikawa et al, 1994). The conversion of 25(OH)D3 to the metabolically active calcitriol takes place in the liver, as well as the kidney (Takeuchi et al, 1991, Lock et al, 2010, Graff et al, 1999), but some researchers have suggested that the liver is the main organ for synthesis of calcitriol in fish (Sundell et al, 1992, Takeuchi et al, 1991)
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