Environmental salinity regulates receptor expression, cellular effects, and circulating levels of two antagonizing hormones, 1,25-dihydroxyvitamin D3 and 24,25-dihydroxyvitamin D3, in rainbow trout

24R,25-dihydroxyvitamin D3 promotes the osteoblastic differentiation of human mesenchymal stem cells

Although 1α,25-dihydroxyvitamin D3 [1α,25(OH)2D3] is considered the most biologically active vitamin D3 metabolite, the vitamin D3 prohormone, 25-hydroxyvitamin D3 [25(OH)D3], is metabolized into other forms, including 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3]. Herein we show that 24R,25(OH)2D3 is fundamental for osteoblastic differentiation of human mesenchymal stem cells (hMSCs). Our approach involved analyses of cell proliferation, alkaline phosphatase activity, and pro-osteogenic genes (collagen 1A1, osteocalcin, vitamin D receptor [VDR], vitamin D3-hydroxylating enzymes [cytochrome P450 hydroxylases: CYP2R1, CYP27A1, CYP27B1 and CYP24A1]) and assessment of Ca(2+) mineralization of extracellular matrix. 24R,25(OH)2D3 inhibited hMSC proliferation, decreased 1α-hydroxylase (CYP27B) expression, thereby reducing the ability of hMSCs to convert 25(OH)D3 to 1α,25(OH)2D3, and promoted osteoblastic differentiation through increased alkaline phosphatase activity and Ca(2+) mineralization. 24R,25(OH)2D3 decreased expression of the 1α,25(OH)2D3 receptor, VDR. 24R,25(OH)2D3 but not 1α,25(OH)2D3 induced Ca(2+) mineralization dependent on the absence of the glucocorticoid analog, dexamethasone. To elucidate the mechanism(s) for dexamethasone-independent 1α,25(OH)2D3 inhibition/24R,25(OH)2D3 induction of Ca(2+) mineralization, we demonstrated that 1α,25(OH)2D3 increased whereas 24R,25(OH)2D3 decreased reactive oxygen species (ROS) production. 25(OH)D3 also decreased ROS production, potentially by conversion to 24R,25(OH)2D3. Upon inhibition of the vitamin D3-metabolizing enzymes (cytochrome P450s), 25(OH)D3 increased ROS production, potentially due to its known (low) affinity for VDR. We hypothesize that vitamin D3 actions on osteoblastic differentiation involve a regulatory relationship between 24R,25(OH)2D3 and 1α,25(OH)2D3. These results implicate 24R,25(OH)2D3 as a key player during hMSC maturation and bone development and support the concept that 24R,25(OH)2D3 has a bioactive role in the vitamin D3 endocrine system.

Vitamin D: calcium and bone homeostasis during evolution

Vitamin D3 is already found early in the evolution of life but essentially as inactive end products of the photochemical reaction of 7-dehydrocholestol with ultraviolet light B. A full vitamin D (refers to vitamin D2 and D3) endocrine system, characterized by a specific VDR (vitamin D receptor, member of the nuclear receptor family), specific vitamin D metabolizing CYP450 enzymes regulated by calciotropic hormones and a dedicated plasma transport-protein is only found in vertebrates. In the earliest vertebrates (lamprey), vitamin D metabolism and VDR may well have originated from a duplication of a common PRX/VDR ancestor gene as part of a xenobiotic detoxification pathway. The vitamin D endocrine system, however, subsequently became an important regulator of calcium supply for an extensive calcified skeleton. Vitamin D is essential for normal calcium and bone homeostasis as shown by rickets in vitamin D-deficient growing amphibians, reptiles, birds and mammals. From amphibians onward, bone is gradually more dynamic with regulated bone resorption, mainly by combined action of PTH and 1α,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the generation and function of multinucleated osteoclasts. Therefore, bone functions as a large internal calcium reservoir, under the control of osteoclasts. Osteocytes also display a remarkable spectrum of activities, including mechanical sensing and regulating mineral homeostasis, but also have an important role in global nutritional and energy homeostasis. Mineralization from reptiles onward is under the control of well-regulated SIBLING proteins and associated enzymes, nearly all under the control of 1,25(OH)2D3. The vitamin D story thus started as inert molecule but gained an essential role for calcium and bone homeostasis in terrestrial animals to cope with the challenge of higher gravity and calcium-poor environment.

Research resource: whole transcriptome RNA sequencing detects multiple 1α,25-dihydroxyvitamin D(3)-sensitive metabolic pathways in developing zebrafish

The biological role of vitamin D receptors (VDR), which are abundantly expressed in developing zebrafish (Danio rerio) as early as 48 h after fertilization, and before the development of a mineralized skeleton and mature intestine and kidney, is unknown. We probed the role of VDR in developing zebrafish biology by examining changes in expression of RNA by whole transcriptome shotgun sequencing (RNA-seq) in fish treated with picomolar concentrations of the VDR ligand and hormonal form of vitamin D(3), 1α,25-dihydroxyvitamin D(3) [1α,25(OH)(2)D(3))].We observed significant changes in RNAs of transcription factors, leptin, peptide hormones, and RNAs encoding proteins of fatty acid, amino acid, xenobiotic metabolism, receptor-activator of NFκB ligand (RANKL), and calcitonin-like ligand receptor pathways. Early highly restricted, and subsequent massive changes in more than 10% of expressed cellular RNA were observed. At days post fertilization (dpf) 2 [24 h 1α,25(OH)(2)D(3)-treatment], only four RNAs were differentially expressed (hormone vs. vehicle). On dpf 4 (72 h treatment), 77 RNAs; on dpf 6 (120 h treatment) 1039 RNAs; and on dpf 7 (144 h treatment), 2407 RNAs were differentially expressed in response to 1α,25(OH)(2)D(3). Fewer RNAs (n = 481) were altered in dpf 7 larvae treated for 24 h with 1α,25(OH)(2)D(3) vs. those treated with hormone for 144 h. At dpf 7, in 1α,25(OH)(2)D(3)-treated larvae, pharyngeal cartilage was larger and mineralization was greater. Changes in expression of RNAs for transcription factors, peptide hormones, and RNAs encoding proteins integral to fatty acid, amino acid, leptin, calcitonin-like ligand receptor, RANKL, and xenobiotic metabolism pathways, demonstrate heretofore unrecognized mechanisms by which 1α,25(OH)(2)D(3) functions in vivo in developing eukaryotes.

Bioconcentration of perfluorinated compounds in blackrock fish, Sebastes schlegeli, at different salinity levels

Bioconcentration of perfluorinated compounds (PFCs) was studied in a biphasic (uptake and elimination) study with blackrock fish, Sebastes schlegeli. The blackrock fish was acclimated to varying salinities over a two-week period before the present study. Among the four selected PFCs: perfluorooctane sulfonate (PFOS), perfluoro-octanoic acid (PFOA), perfluorodecanoic acid (PFDA), and perfluoroundecanoic acid (PFUnDA), PFUnDA accumulated significantly in serum, followed by PFDA, PFOS, and PFOA, in that order, while the accumulation profile in liver was PFOS > PFUnDA > PFDA > PFOA. Total PFC levels in plasma were approximately four times greater than those found in liver. The uptake and elimination rate constants (K(u) and K(e)) of PFCs decreased as salinity decreased, suggesting delayed diffusion of PFCs between water and fish, possibly associated with the osmolality gradient. A significant correlation was found between bioconcentration factors (BCF) of PFCs and salinity, except for PFOA, possibly resulting from the effects of salinity on biological responses and chemical activity of PFCs. Even though salinity did not affect the kinetics of PFC accumulation in serum and liver, the results provide useful information on the toxicokinetics of PFCs for saltwater fish.

Bioaccumulation of perfluorochemicals in Pacific oyster under different salinity gradients

Despite the reports of widespread occurrence of perfluorinated compounds (PFCs) in estuarine and coastal waters and open seas, little is known on the effect of salinity on bioaccumulation. In this study, effects of salinity on bioaccumulation of PFCs in Pacific oysters (Crassostrea gigas) were investigated. Furthermore, partitioning of PFCs between water and particles (oysters' food) was examined at different salinities. The distribution coefficients (K(d); partitioning between water and particles) for selected PFCs, that is, PFOS, PFOA, PFDA, and PFUnDA, increased by 2.1- to 2.7-fold with the increase in water salinity from 10 to 34 psu, suggesting "salting-out" effect, and the salting constant (delta) was estimated to range from 0.80 to 1.11. The nonlinear regression analysis of bioaccumulation suggested increase in aqueous and dietary uptake rates (K(w) and K(f)), with the increase in salinity, which resulted in elevated bioaccumulation, although the depuration rates (K(e)) also increased. The relative abundance of long carbon chain length PFCs (i.e., PFDA and PFUnDA) increased as salinity increased, while the proportion of PFOS and PFOA decreased, which is explained by the positive relationship between delta and carbon chain length. The contribution of diet to bioaccumulation in oysters ranged from 18 to 92%. Overall, salinity not only affected the chemistry of PFCs, but also the physiology of oysters, contributing to sorption and bioaccumulation of perfluorochemicals in oysters.

Paralogous vitamin D receptors in teleosts: transition of nuclear receptor function

The availability of multiple teleost (bony fish) genomes is providing unprecedented opportunities to understand the diversity and function of gene duplication events using comparative genomics. Here we describe the cloning and functional characterization of two novel vitamin D receptor (VDR) paralogs from the freshwater teleost medaka (Oryzias latipes). VDR sequences were identified through mining of the medaka genome database in which gene organization and structure was determined. Two distinct VDR genes were identified in the medaka genome and mapped to defined loci. Each VDR sequence exhibits unique intronic organization and dissimilar 5' untranslated regions, suggesting they are not isoforms of the same gene locus. Phylogenetic comparison with additional teleosts and mammalian VDR sequences illustrate that two distinct clusters are formed separating aquatic and terrestrial species. Nested within the teleost cluster are two separate clades for VDRalpha and VDRbeta. The topology of teleost VDR sequences is consistent with the notion of paralogous genes arising from a whole genome duplication event prior to teleost radiation. Functional characterization was conducted through the development of VDR expression vectors including Gal4 chimeras containing the yeast Gal4 DNA binding domain fused to the medaka VDR ligand binding domain and full-length protein. The common VDR ligand 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)(2)D(3)] resulted in significant transactivation activity with both the Gal4 and full-length constructs of medaka (m) VDRbeta. Comparatively, transactivation of mVDRalpha with 1alpha,25(OH)(2)D(3) was highly attenuated, suggesting a functional divergence between these two nuclear receptor paralogs. We additionally demonstrate through coactivator studies that mVDRalpha is still functional; however, it exhibits a different sensitivity to 1alpha,25(OH)(2)D(3), compared with VDRbeta. These results suggest that in mVDRalpha and VDRbeta have undergone a functional divergence through a process of sub- and/or neofunctionalization of VDR nuclear receptor gene pairs.

Environmental salinity regulates the in vitro production of [3H]-1,25-dihydroxyvitamin D3 and [3H]-24,25 dihydroxyvitamin D3 in rainbow trout (Oncorhynchus mykiss)

Previous studies have shown that specific binding of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) to enterocyte basolateral membranes (BLM), as well as circulating concentrations, is affected in response to changes in environmental salinity. It is not known if the production of 1,25(OH)2D3 and 24,25(OH)2D3 is affected by environmental salinity. The aim of the present study was to measure the in vitro production of [3H]-1,25(OH)2D3 and [3H]-24,25(OH)2D3 in fresh water (FW) and after 1, 2, 3, and 7 days after transfer to seawater (SW). Pooled sub-cellular fractions (mitochondria and microsomes) from liver or kidney was incubated with [3H]-25(OH)D3 and the produced metabolites were separated using HPLC. Hepatic production of [3H]-1,25(OH)2D3 was decreased after 24h in SW. This was followed by an up-regulation after 48h and a second, slower decrease in production rate which leveled out after 7 days in SW. The production rate in SW was lower than the original rate in FW-adapted fish. For hepatic [3H]-24,25(OH)2D3 production the pattern was reversed. Renal production of [3H]-24,25(OH)2D3 increased significantly during the period of SW acclimation. These results suggest that environmental salinity regulates the production rate of the two antagonizing calcium regulatory hormones; 1,25(OH)2D3 and 24,25(OH)2D3. This gives further evidence to the hypothesis that there is a physiological regulation and a differentiated importance of 1,25(OH)2D3 and 24,25(OH)2D3 in relation to environmental calcium concentrations.

24,25-dihydroxyvitamin D3 binds to catalase

There is increasing evidence that the vitamin D metabolite, 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) has endocrine actions. In the current work, we report that an endogenous binding protein for 24,25(OH)2D3 is catalase, based on sequence analysis of the isolated protein. An antibody (Ab 365) generated against equivalent protein recognized bovine catalase and a 64 kDa band in subcellular fractions of chick intestine. A commercially available anti-catalase antibody reduced specific [3H]24,25(OH)2D3 binding in subcellular fractions of chick intestine by greater than 65%, relative to the same fractions treated with an unrelated antibody (Ab 099). The same commercially available anti-catalase was able to block the inhibitory actions of 24,25(OH)2D3 on 32P uptake in isolated intestinal epithelial cell suspensions. We subsequently characterized binding of steroid to commercially available catalase, and found that between 0 and 5 nM of enzyme added to subcellular fraction P2 (20,000g, 10-min post-nuclear pellet) resulted in a linear increase in the amount of [3H]24,25(OH)2D3 specifically bound. Additional studies indicated that 25(OH)D3 was an effective competitor for binding, whereas 1,25(OH)2D3 only poorly displaced [3H]24,25(OH)2D3. Saturation analyses with added catalase yielded a physiologically relevant affinity constant (KD=5.6+/-2.7 nM) and a Bmax=209+/-34 fmols/mg protein, comparable to previous studies using purified basal lateral membranes or vesicular fractions. Moreover, in a study on subcellular fractions isolated from chickens of varying ages, we found that in females, both specific [3H]24,25(OH)2D3 binding and catalase activity increased from 7- to 58-week-old birds, whereas in males, elevated levels of both parameters were expressed in preparations of 7- and 58-week-old birds. The data suggest that signal transduction may occur through modulation of hydrogen peroxide production.

Identification and characterization of two parathyroid hormone-like molecules in zebrafish

Zebrafish (Danio rerio) have receptors homologous to the human PTH (hPTH)/PTHrP receptor (PTH1R) and PTH-2 receptor (PTH2R) and an additional receptor (PTH3R) with high homology to the PTH1R. To find natural ligands for zPTH1R and zPTH3R, we searched the zebrafish genomic database and discovered two distinct regions that, when translated (zPTH1 and zPTH2), showed high homology to hPTH. Isolation of cDNAs and determination of the intron/exon boundaries revealed genomic structures which were similar to known PTHs. Peptides consisting of the first 34 amino acids after the pre- and prosequences of the zebrafish PTHs (zPTHs) were synthesized and were shown to be fully active at the hPTH1R. zPTH2(1-34) was, however, approximately 30-fold less potent at the zPTH1R than hPTH(1-34), hPTHrP(1-36), and zPTH1(1-34). When tested with zPTH3R, zPTH1(1-34) and hPTHrP(1-36) showed similar potencies, whereas the potency of zPTH2(1-34) was moderately (3-fold) reduced. To determine whether other fishes have multiple PTHs, we searched the genomic database of the Japanese pufferfish (Takifugu rubripes) and identified zPTH1 and zPTH2 homologs. Phylogenetic analysis showed that PTHs from zebrafish and pufferfish are more closely related to each other than to known mammalian PTH homologs or to PTHrP and tuberoinfundibular peptide of 39 residues. This is consistent with evolution of two teleost PTH-like peptides occurring after the evolutionary divergence between fishes and mammals. Overall, the PTH system appears more complex in fishes than in mammals, providing evidence of continued evolution in nontetrapod species. The availability of multiple forms of fish PTH and their receptors provide additional tools for PTH ligand/receptor structure-function studies.