Search

The profile of structural preference for the ligand binding domain of the human vitamin D binding protein (DBP) was determined by steroid competition assay of 71 analogs of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3]. The following categories of structural modification were evaluated [values represent fold change; R = reduction, I = increase in binding to the DBP from the reference 1 alpha, 25(OH)2D3]: (1) deletion in the A ring of the 1 alpha-hydroxyl-(20-1600I); (2) conversion of the triene system to the previtamin form (6-40R); (3) addition of substituents to carbon 11 of the C ring (4-14R); (4) inversion of the C/D ring junction (8-20R); (5) unsaturation of the D ring (16-ene; 4-140R); (6) replacement of hydrogen with deuterium atoms (no effect); alteration of the side chain by (7) adding or deleting carbon atoms (5-12R); (8) addition of fluorines (0.2-10R); (9) presence of unsaturation (22-ene, 0-5R; 23-ene, 3R-10I; 23-yne, 5-20R); (10) addition of hydroxyls (2-100R); and (11) addition of an aromatic ring (0-20I). Thus the DBP ligand binding domain could tolerate only modest changes to the structure of 1 alpha,25(OH)2D3 without a reduction in binding of the analog. The increases in binding seen in the aromatic side chain and with a triple bond at carbon-23 may be indicative of a preferred conformation of the flexible 1 alpha, 25(OH)2D3 side chain. In addition, a comparison was made of the DBP ligand binding domain with that of the human HL-60 cell 1 alpha, 25(OH)2D3 nuclear receptor. Both ligand binding domains could equivalently accommodate to the presence of (1) a side-chain cyclopropyl group, (2) 22-ene or 23-yne, (3) lengthening the side chain by two carbons, (4) presence of four to six fluorine atoms, (5) substitution of an oxygen for carbon 22, and (6) presence of a 22-[m-(dimethylhydroxymethyl)phenyl] aromatic group in the side chain. The DPB could tolerate better than the HL-60 cell receptor the presence of a 22-(p-hydroxyphenyl) aromatic group in the side chain and the absence of the 1 alpha-hydroxyl. In contrast, the HL-60 cell receptor could tolerate better than the DBP the following structural modifications: presence of a 16-ene, or 16-ene plus 23-yne unsaturation, and presence of an 11 beta-hydroxyl.

Vitamin D is produced by exposure of 7-dehydrocholesterol in the skin to UV irradiation (UVR) and further converted in the skin to the biologically active metabolite, 1,25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) and other compounds. UVR also results in DNA damage producing cyclobutane pyrimidine dimers (CPD). We previously reported that 1,25(OH)(2)D(3) at picomolar concentrations, protects human skin cells from UVR-induced apoptosis, and decreases CPD in surviving cells. 1,25(OH)(2)D(3) has been shown to generate biological responses via two pathways-the classical steroid receptor/genomic pathway or a rapid, non-genomic pathway mediated by a putative membrane receptor. Whether the rapid response pathway is physiologically relevant is unclear. A cis-locked, rapid-acting agonist 1,25(OH)(2)lumisterol(3) (JN), entirely mimicked the actions of 1,25(OH)(2)D(3) to reduce fibroblast and keratinocyte loss and CPD damage after UVR. The effects of 1,25(OH)(2)D(3) were abolished by a rapid-acting antagonist, but not by a genomic antagonist. Skh:hr1 mice exposed to three times the minimal erythemal dose of solar-simulated UVR and treated topically with 1,25(OH)(2)D(3) or JN immediately after UVR showed reduction in UVR-induced UVR-induced sunburn cells (p

The steroid hormone 1alpha,25(OH)(2)-Vitamin D(3) (1,25D) activates both genomic and non-genomic intracellular signaling cascades. It is also well recognized that co-incubation of 1,25D with its C-1 epimer, 1beta,25D (HL), suppresses the efficiency of the non-genomic signal activated by 1,25D alone and that its C-3 epimer, 3alpha-1,25D (HJ) is nearly as potent as 1,25D in suppressing PTH secretion, believed to be propagated by 1,25D's genomic signaling. Both these sterols lack the hypercalcemic effect induced by pharmacological doses of 1,25D and have reduced VDR affinity compared to 1,25D, as measured in a steroid competition assay. Recent functional studies suggest that the VDR is required for both non-genomic and genomic signaling. Along these lines we have recently proposed a Vitamin D sterol/VDR conformational ensemble model that posits the VDR contains two distinct, yet overlapping ligand binding sites, and that the potential differential stabilities of 1,25D and HL in these two pockets can be used to explain their different non-genomic signaling properties. The overlapping region is predominantly occupied by the sterol's A-ring when it is bound to either the genomic ligand binding pocket (G-pocket), defined by X-ray crystallography, or the alternative ligand binding pocket (A-pocket), discovered using in silico techniques (directed docking). Therefore, to gain further insight into the potential application of this model we docked the other A-ring diastereomer [(1beta,3alpha)=HH] of 1,25D and its 1- and 3-deoxy forms (25D and CF, respectively) to the A- and G-pockets to assess their potential stabilities in the pockets, relative to 1,25D. The models were then used to provide putative mechanistic arguments for their known structure-function experimental results. This model may provide new insights into how Vitamin D sterols that uncouple the unwanted hypercalcemic effect from attractive growth inhibitory/differentiation properties can do so by differentially stabilizing different subpopulations of VDR conformational ensemble members.

Over the past 20 years much has been learned about the cellular actions of the steroid hormone 1alpha,25(OH)2-Vitamin D3 (1,25D). Perhaps most importantly structure-function studies led to the discovery that different chemical and physical features of 1,25D are preferred to initiate either exonuclear, non-genomic or endonuclear, genomic cellular signaling. It is well documented that both a 1alpha-OH and 25-OH, and a 6-s-trans, bowl-shaped, sterol conformation are absolutely required for efficient gene transcription, while 6-s-cis locked analogs and 1-deoxy, 25(OH)D3 metabolites activate a variety of non-genomic, rapid responses. These results and the observation that S237 (helix-3; H3) and R274 (H5) are the most static residues in the human 1,25D-Vitamin D receptor (VDR) X-ray construct (see B-values in pdb: 1DB1) and form H-bonds with the 1alpha-OH of 1,25D in the X-ray, genomic pocket (G-pocket), provided the basis for the molecular modeling experiments that led to the discovery of a putative VDR alternative ligand binding pocket (A-pocket). The conformational ensemble model generated from the in silico results provides an explanation for how the VDR can function as a receptor propagating both genomic and non-genomic signaling events. In this report the theoretical gating properties controlling ligand access to the A- and G-pockets will be compared and the model will be used to provide a molecular explanation for the confusing structure-function results pertaining to 1,25D, its side-chain metabolite, 23S,25R-1alpha,25(OH)2-D3-26,23-lactone (BS), and its synthetic two side-chain analog, 21-(3'-hydroxy-3'-methylbutyl)-1alpha,25(OH)2-D3 (KH or Gemini). In addition, evidence that the model is consistent with the pH requirement for Vitamin D sterol-VDR crystallization will be presented.

The hormonal form of vitamin D, 1α,25-dihydroxyvitamin D 3 (1,25D), generates many biological actions by interactions with its nuclear receptor (VDR). The presence of a carbon-25 hydroxyl group is necessary for optimizing binding to the VDR. To examine the effect of spatial orientation of the 25-hydroxyl, two pairs of 22,23-allene sidechain analogs were studied. The 22R orientation in analogs HR (52 ± 2%) and LA (154 ± 19%) resulted in higher affinity binding than the 22S orientation of analogs HQ (21 ± 3%) and LB (3.5 ± 1.3%; 1,25D = 100%). Limited trypsin proteolysis showed that 22R analogs induced VDR conformational changes better able to protect VDR from digestion than 22S analogs. 22R analogs were also able to induce gene transcription at 10–100-fold lower concentrations than 1,25D; 22S analogs were less effective. Analog LA was at least 10-fold more potent than 1,25D at inducing differentiation, while the other analogs were less potent. None of the analogs were as potent as 1,25D in promoting in vivo intestinal calcium absorption or bone calcium mobilization. LA was the most potent of the analogs but required 20–30-fold higher doses than 1,25D. The 25-hydroxyl orientation combined with the 16,17-ene functionality of analog LA enhances its ability to interact with VDR and induce biological actions.

The steroid hormone 1α,25(OH)2-vitamin D3 (1,25D) regulates gene transcription through a nuclear receptor [vitamin D receptor (VDR)] and initiation of rapid cellular responses through a putative plasma membrane-associated receptor (VDRmem). This study characterized the VDRmem present in a caveolae-enriched membrane fraction (CMF), a site of accumulation of signal transduction agents. Saturable and specific [3H]-1,25D binding in vitro was found in CMF of chick, rat, and mouse intestine; mouse lung and kidney; and human NB4 leukemia and rat ROS 17/2.8 osteoblast-like cells; in all cases the 1,25D KD binding dissociation constant = 1–3 nm. Our data collectively support the classical VDR being the VDRmem in caveolae: 1) VDR antibody immunoreactivity was detected in CMF of all tissues tested; 2) competitive binding of [3H]-1,25D by eight analogs of 1,25D was significantly correlated between nuclei and CMF (r2 = 0.95) but not between vitamin D binding protein (has a different ligand binding specificity) and CMF; 3) confocal immunofluorescence microscopy of ROS 17/2.8 cells showed VDR in close association with the caveolae marker protein, caveolin-1, in the plasma membrane region; 4) in vivo 1,25D pretreatment reduced in vitro [3H]-1,25D binding by 30% in chick and rat intestinal CMF demonstrating in vivo occupancy of the CMF receptor by 1,25D; and 5) comparison of [3H]-1,25D binding in VDR KO and WT mouse kidney tissue showed 85% reduction in VDR KO CMF and 95% reduction in VDR KO nuclear fraction. This study supports the presence of VDR as the 1,25D-binding protein associated with plasma membrane caveolae.

The steroid hormone 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] produces biological responses by interaction with both a well-characterized nuclear receptor (VDR(nuc)) to regulate gene transcription and with an as-yet uncharacterized membrane-associated protein/receptor (VDR(mem)) to generate a variety of rapid, non-genotropic responses. We report for the first time that [3H]1alpha,25(OH)(2)D(3) binds with high affinity to a chick duodenal caveolae-enriched membrane fraction (CMF) isolated without the use of detergents. Caveolae are plasma membrane invaginations implicated in signal transduction and molecular transport processes. Using the CMF fraction as a possible source of VDR(mem), we found that the in vitro binding of [3H]1alpha,25(OH)(2)D(3) was ligand dependent and saturable; the K(D) and B(max) were 1.3+/-0.6nM and 29+/-11fmol 1,25(OH)(2)D(3)/mg protein (n=17), respectively. Immunoblot analysis of the CMF confirms the presence of caveolin-1, a marker protein for membranes with caveolae. Therefore, chick CMF may represent a good source for isolation and characterization of the putative VDR(mem) for 1alpha,25(OH)(2)D(3).

Vitamin D 3 produces biologic responses as a consequence of its metabolism into 1α,25(OH) 2 -vitamin D 3 [1α,25(OH) 2 D 3 ] and 24R,25(OH) 2 -vitamin D 3 . The metabolic production of these two seco steroids and their generation of the plethora of biologic actions that are attributable to the parent vitamin D 3 are orchestrated via the integrated operation of the vitamin D endocrine system. This system is very similar in its organization to that of classic endocrine systems and is characterized by an endocrine gland (the kidney, the source of the two steroid hormones), target cells which possess receptors for the steroid hormones, and a feed-back loop involving changes in serum Ca 2+ that alter the secretion of parathyroid hormone (a stimulator of the renal 1-hydroxylase) which modulates the output by the kidney of the steroid hormones. There are, however, at least two unique aspects to the vitamin D endocrine system. (a) The chemical structures of vitamin D and its steroid hormones dictate that these be highly conformationally flexible molecules present a wide variety of shapes to their biologic environments. (b) It is now believed that 1α,25(OH) 2 D 3 produces biologic responses through two distinct receptors which recognize totally different shapes of the conformationally flexible 1α,25(OH) 2 D 3 . Thus, the classic actions of 1α,25(OH) 2 D 3 to regulate gene transcription occur as a consequence of the stereospecific interaction of a modified 6-s- trans bowl-shape of 1α,25(OH) 2 D 3 with its nuclear receptor (VDR nuc ). The ability of 1α,25(OH) 2 D 3 to generate a variety of rapid (seconds to minutes) biologic responses (opening of chloride channels, activation of PKC and MAP kinases) requires a planar 6-s- cis ligand shape which is recognized by a putative plasma membrane receptor (VDR mem ) to initiate appropriate signal transduction pathways. This report summarizes the evidence for the specificity of different ligand shapes and the operation of the two receptor families for 1α,25(OH) 2 D 3 .

(23S)-25-dehydro-1α-Dihydroxyvitamin D3-26,23-lactone (TEI-9647; MK) has been reported to antagonize the 1α,25-dihydroxyvitamin D3 nuclear receptor (VDR)- mediated increase in transcriptional activity. Using a transient transfection system incorporating the osteocalcin VDRE (vitamin D response element) in Cos-1 cells, we found that 20 nm MK antagonizes VDR-mediated transcription by 50% when driven by 1 nm 1α,25(OH)2D3. Four analogs of 1α,25(OH)2D3, also at 1 nm, were antagonized 25 to 39% by 20 nm MK. However, analogs with 16-ene/23-yne or 20-epi modifications, which have a significantly lower agonist ED50 for the VDR than 1α,25(OH)2D3, were antagonized by 20 nm MK only at 100 pm or 10 pm, respectively. One possible mechanism for antagonism is that the 25-dehydro alkene of MK might covalently bind the ligand-binding site of the VDR rendering it inactive. Utilization of a ligand exchange assay, however, demonstrated that MK bound to VDR is freely exchanged with 1α,25(OH)2D3 in vitro. These data support the apparent correlation between VDR transcriptional activation by agonists and the effective range of MK antagonism by competition. Furthermore, protease sensitivity analysis of MK bound to VDR indicates the presence of a unique conformational change in the VDR ligand-binding domain, showing a novel doublet of VDR fragments centered at 34 kDa, whereas 1α,25(OH)2D3 as a ligand produces only a single 34-kDa fragment. In comparison, the natural metabolite 1α,25dihydroxyvitamin D3-26,23-lactone yields only the 30-kDa fragment that is produced by all ligands to varying degrees. Collectively, these results support that MK is a potent partial antagonist of the VDR for 1α,25(OH)2D3 and its analogs when in appropriate excess of the agonist.

The secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] is metabolized into calcitroic acid through the carbon 24 (C-24) oxidation pathway. It is now well established that the C-24 oxidation pathway plays an important role in the target tissue inactivation of 1alpha,25(OH)2D3. Recently, we reported that 1alpha,25(OH)2D3 is also metabolized into 1alpha,25-dihydroxy-3-epi-vitamin D3 [1alpha,25(OH)2-3-epi-D3] through the carbon 3 (C-3) epimerization pathway in human keratinocytes, human colon carcinoma cells (Caco-2), and bovine parathyroid cells. In a previous study, it was demonstrated that 1alpha,25(OH)2-3-epi-D3 when compared to 1alpha,25(OH)2D3 was less active in stimulating intestinal calcium absorption, calcium mobilization from bone, and induction of calbindin D28k. These findings suggest that the C-3 epimerization pathway, like the C-24 oxidation pathway, may play a role in the target tissue inactivation of 1alpha,25(OH)2D3. In this study, we determined the relationship between the C-24 oxidation and the C-3 epimerization pathways by investigating the metabolism of 1alpha,25(OH)2D3 in two rat osteosarcoma cell lines (UMR 106 and ROS 17/2.8). These two cell lines differ from each other in their ability to metabolize 1alpha,25(OH)2D3 through the C-24 oxidation pathway. It has been previously reported that the C-24 oxidation pathway is expressed only in UMR 106 cells but not in ROS 17/2.8 cells. The results of our present study provide new evidence that both cell lines possess the ability to metabolize 1alpha,25(OH)2D3 into 1alpha,25(OH)2-3-epi-D3 through the C-3 epimerization pathway. Our results also reconfirm the findings of previous studies indicating that UMR 106 cells are the only ones which express the C-24 oxidation pathway out of the two cell lines studied. Furthermore, this study reveals for the first time that the C-3 epimerization pathway may become an alternate metabolic pathway for the target tissue inactivation of 1alpha,25(OH)2D3 in some cells, such as ROS 17/2.8, in which the C-24 oxidation pathway is not expressed.

Recent studies have shown that 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3] actions in cell growth and differentiation are mediated by both its nuclear receptor (VDRnuc) and its rapid membrane-related effects. In the present study, we investigated the effect of 1alpha,25-(OH)2D3 on p42mapk phosphorylation using human acute promyelocytic leukemia cells (NB4). 1Alpha,25-(OH)2D3 (10[-8] M) significantly increased p42mapk phosphorylation in a time- and dose-dependent manner, with the earliest response detectable at 30 sec. Because 1alpha,25-(OH)2D3 is a conformationally flexible molecule, we have used a series of conformationally locked (6-s-cis vs. 6-s-trans) analogs to evaluate which shape is optimal for activation. Four 6-s-cis-locked analogs (HF, JM, JN, and JP) and two 6-s-trans-locked analog (JB and JD) were studied. HF, JM, JN, and JP all increased p42mapk phosphorylation at 1 and 5 min (10[-8] M), but JB and JD had little effect. Analog HL [1beta,25-(OH)2D3], a specific antagonist for only the rapid effects of 1alpha,25-(OH)2D3, attenuated 1alpha,25-(OH)2D3-induced p42mapk phosphorylation 65-90%. To assess the potential involvement of the VDRnuc in mediating the analog's action, the relative abilities of the analogs to compete with [3H]1alpha,25-(OH)2D3 for binding in vitro to the VDRnuc of NB4 cells was measured. All 6-s-cis analogs bound poorly to VDRnuc (relative competitive index, 0.5-2%) compared with 1alpha,25-(OH)2D3 (relative competitive index, 100%). The present studies demonstrate for the first time that in NB4 cells 1alpha,25-(OH)2D3 rapidly activates the p42mapk pathway, and that this effect can be selectively mediated by analogs that can assume a 6-s-cis conformation.

The hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] generates biological responses via both genomic and rapid, nongenomic mechanisms. The genomic responses utilize signal transduction pathways linked to a nuclear receptor (VDRnuc) for 1alpha,25(OH)2D3, while the rapid responses are believed to utilize other signal transduction pathways that may be linked to a putative membrane receptor for 1alpha,25(OH)2D3. The natural seco steroid is capable of facile rotation about its 6,7 single carbon bond, which permits generation of a continuum of potential ligand shapes extending from the 6-s-cis (steroid like) to the 6-s-trans (extended). To identify the shape of conformer(s) that can serve as agonists for the genomic and rapid biological responses, we measured multiple known agonist activities of two families of chemically synthesized analogs that were either locked in the 6-s-cis (6C) or 6-s-trans (6T) conformation. We found that 6T locked analogs were inactive or significantly less active than 1alpha,25(OH)2D3 in both rapid responses (transcaltachia in perfused chick intestine, 45Ca2+ influx in ROS 17/2.8 cells) and genomic (osteocalcin induction in MG-63 cells, differentiation of HL-60 cells, growth arrest of MCF-7 cells, promoter transfection in COS-7 cells) assays. In genomic assays, 6C locked analogs bound poorly to the VDRnuc and were significantly less effective than 1alpha,25(OH)2D3 in the same series of assays designed to measure genomic responses. In contrast, the 6C locked analogs were potent agonists of both rapid response pathways and had activities equivalent to the conformationally flexibile 1alpha,25(OH)2D3; this represents the first demonstration that 6-s-cis locked analogs can function as agonists for vitamin D responses.

The steroid hormone 1α,25-dihydroxyvitamin D 3 [1α,25(OH) 2 D 3 ] stimulates biological responses via both genomic mechanisms and nongenomic mechanisms (opening of voltage-gated Ca 2+ channels). We report here that the two closed B-ring steroid analogs of 1α,25(OH) 2 D 3 , 1α,25(OH) 2 -7-dehydrocholesterol and 1α,25(OH) 2 -lumisterol 3 , are able to generate the nongenomic response, transcaltachia, without the ability to compete with the natural metabolite for binding to its nuclear receptor. We propose that the nongenomic membrane associated receptor can accept the ligand in its closed "6-s- cis " conformation whereas the nuclear receptor prefers the extended "6-s- trans " conformer.

The steroid hormone 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) generates biological responses via both genomic and nongenomic mechanisms. This article reports the biological profile of four A-ring diastereomers of this secosteroid (results are expressed as percentage of the response of 1 alpha,25-(OH)2D3. The activity of the compounds, 1 alpha,25-(OH)2D3, 1 alpha,25-(OH)2-3-epivitamin D3, 1 beta,25-(OH)2D3, 1 beta,25-(OH)2-3-epivitamin D3, for in vivo intestinal Ca2+ absorption and bone Ca2+ mobilization and in vitro binding to the nuclear receptor (genomic responses) was, respectively, 100, 2.8, 1 alpha,25-(OH)2-3-epivitamin D3 > 1 beta,25-(OH)2D3 > 1 beta,25-(OH)2-3-epivitamin D3. 1 beta,25-(OH)2D3 was a potent antagonist of 1 alpha,25-(OH)2D3-mediated transcaltachia and 45Ca2+ uptake in ROS 17/2.8 cells but was unable to block the genomic 1 alpha,25-(OH)2D3 induction of chick calbindin-D28k (in vivo), induction of MG-63 cell osteocalcin, and HL-60 cell differentiation. These results suggest that analogs of 1 alpha,25-(OH)2D3 may be synthesized which are selective agonists or antagonists of genomic or nongenomic responses in the vitamin D endocrine system.

Physiological studies of the Atlantic cod, Gadus morhua, have suggested a role for the vitamin D3 system in this marine teleost similar to that in other vertebrates. Accordingly, the present study was carried out to assess the plasma concentrations of vitamin D3, 25-hydroxyvitamin D3 (25OHD3), and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] in this fish. Additionally, the presence of binding proteins in plasma and target-specific tissue receptors for these vitamin D3 metabolites was studied in organs normally associated with calcium regulation. Plasma levels of 25-OHD3 (undetectable to 148 pg/ml; n = 5) were comparatively low (20-30 ng/ml), whereas the levels of vitamin D3 (approximately 30 ng/ml) and 1,25-(OH)2D3 (approximately 50 pg/ml) were comparable to levels reported in higher vertebrates. Cod plasma contained a protein that bound both 25OHD3 and 1,25-(OH)2D3. This plasma binding protein revealed low affinity for 25OHD3, did not bind G-actin, and had a sedimentation coefficient of 3.4S. High affinity 1,25-(OH)2D3 receptors [Kd, 1.02 +/- 0.36 (n = 6), 1.02 +/- 0.3 (n = 5), and 0.95 +/- 0.51 (n = 5) nM; mean +/- SEM] were found in high salt cytosols from intestine, liver, and gills, respectively, and had sedimentation coefficients (3.6-3.8S in 0.3 M KCl sucrose gradients) similar to those in higher vertebrates. No specific 1,25-(OH)2D3 binding was found in kidney, ultimobranchial glands, corpuscles of Stannius, or bone. The finding of significant quantities of 1,25-(OH)2D3 in the plasma, the presence of plasma binding proteins that bind this seco-steroid, and the localization of specific high affinity receptors for this metabolite in calcium regulatory tissues in teleosts are all consistent with a physiological role for the vitamin D3 system in the calcium regulation of the cod.

The high affinity calcium-binding protein calbindin-D28K is one of the known proteins transcriptionally up-regulated by the hormonally active form of vitamin D3, 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. This regulation is tissue specific, since in the absence of 1,25-(OH)2D3, the expression of calbindin-D28K is virtually abolished in intestine, whereas it is decreased, but clearly detectable, in kidney, and it remains present at its highest level in cerebellum. Several studies have shown that there is a strong correlation between an increase in the sensitivity to nuclease digestion of a given gene locus and its potential for transcription. Furthermore, hypersensitive sites have often been mapped to regions of DNA including or surrounding sequences known to be important for the regulation of gene transcription. In this study we have scanned the 5'-end and flanking DNA of the calbindin-D28K gene for the presence of DNase-I-hypersensitive (DH) sites in order to localize possible regulatory regions involved in the tissue-specific and hormone-dependent regulation of this gene. We have found that in tissues where calbindin is not expressed, such as liver, no DH sites could be detected. In cerebellum, the same set of DH sites was observed in the presence or absence of 1,25-(OH)2D3 treatment, reflecting the vitamin D-independent expression of the calbindin gene in this tissue. A more complex pattern of DH sites was found in intestine, independently of the vitamin D status of the animal.(ABSTRACT TRUNCATED AT 250 WORDS)

Previous studies have demonstrated that human alveolar and bone marrow macrophages when activated in vitro can metabolize 25-hydroxyvitamin[3H]D3 to 1 alpha,25-dihydroxyvitamin[3H]D3; however, to date no animal models to study this system have been available. In the present study, cultured porcine pulmonary alveolar macrophages from two animals were assayed for their capability for metabolism of 25-hydroxyvitamin[3H]D3. The porcine alveolar macrophages constitutively produced a metabolite of 25-hydroxyvitamin[3H]D3 which was identified as 1 alpha,25-dihydroxyvitamin[3H]D3 by high performance liquid chromatography. The apparent KM was in the range of 300 nM. Unlike human macrophages, treatment of porcine alveolar macrophages with lipopolysaccharide did not stimulate 1 alpha,25-dihydroxyvitamin[3H]D3 production. Addition of 1 alpha,25-dihydroxyvitamin-D3 to macrophages cultures led to a sensitive proportional inhibition of 1 alpha,25-dihydroxyvitamin[3H]D3 synthesis.

The syntheses 1α-hydroxy-25-oxo-25-phosphavitamin D 3 , 25-oxo-25-phospha-26,27-dioxa-26,27-dimethylvitamin D 3 , and 1α-hydroxy-25-oxo-25-phospha-26,27-dioxa-26,27-dimethylvitamin D 3 are reported. The biological activity of 25-oxo-25-phosphavitamin D 3 and the above compounds are reported.

The positioning of helix 12 activation domain of nuclear receptor proteins is critically important for gene regulation. Perturbations of the helix 12 by larger analogs may alter interactions with transcriptional machinery which might give rise to selectivity. To explore the topology of the ligand binding pocket and how the bound ligand conceivably gives rise to altered transcriptional efficiencies, we have targeted 4 hydrophobic residues which contact the 25-carbon of the ligand, 1α,25(OH)2-vitamin D3, and made a series of 13 mutants. Substitution of a smaller hydrophobic residue was poorly tolerated compared to a larger one for transactivation. The larger amino acids are likely better tolerated by promoting stronger Van der Waals forces with the ligand. Valine-418 mutants demonstrated an extreme example of this observation with mutation to leucine being transactivationally unaffected with alanine being the most affected of all single mutants. V418L resulted in a 1.3-fold increase in EC50 for 1,25-D mediated transactivation whereas V418A resulted in a 53-fold increase when compared to wildtype VDR. Importantly, this difference is not explained by ligand binding data but by differential VDR protease sensitivity implying that V418L-VDR mutation assumes a better conformational interaction surface for coactivator than V418A. Importantly, the V418 location may accommodate larger sidechains and may even enhance the interaction with specific nuclear coactivators.

Recently, we have developed a Vitamin D sterol (VDS)-VDR conformational ensemble model. This model can be broken down into three individual, yet interlinked parts: (a) the conformationally flexible VDS, (b) the apo/holo-VDR helix-12 (H12) conformational ensemble, and (c) the presence of two VDR ligand binding pockets (LBPs); one thermodynamically favored (the genomic pocket, G-pocket) and the other kinetically favored by VDSs (the alternative pocket, A-pocket). One focus of this study is to use directed VDR mutagenesis to (1) demonstrate H12 is stabilized in the transcriptionally active closed conformation (hVDR-c1) by three salt-bridges that span the length of H12 (cationic residues R154, K264 and R402), (2) to elucidate the VDR trypsin sites [R173 (hVDR-c1), K413 (hVDR-c2) and R402 (hVDR-c3)] and (3) demonstrate the apo-VDR H12 equilibrium can be shifted. The other focus of this study is to apply the model to generate a mechanistic understanding to discrepancies observed in structure–function data obtained with a variety of 1α,25(OH)2-Vitamin D3 (1,25D) A-ring and side-chain analogs, and side-chain metabolites. We will demonstrate that these structure–function conundrums can be rationalized, for the most part by focusing on alterations in the VDS conformational flexibility and the elementary interaction between the VDS and the VDR A- and G-pockets, relative to the control, 1,25D.

As part of our studies on the membrane-initiated actions of 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] and its localization in caveolae membrane fractions, we used a vitamin D receptor (VDR)-knockout (KO) mouse model to study the binding of [(3)H]-1alpha,25(OH)(2)D(3) in the presumed absence of the VDR. In this mouse model, known as the Tokyo strain, the second exon of the VDR gene, which encodes the first of the two zinc fingers responsible for DNA binding, was removed, and the resulting animals have been considered to be VDR-null mice. To our surprise, several tissues in these KO mice showed significant (5-50% of that seen in wild-type animals) specific binding of [(3)H]-1alpha,25(OH)(2)D(3) in nuclear and caveolae membrane fractions. The dissociation constants of this binding in samples from VDR-KO and wild-type mice were indistinguishable. RT-PCR analysis of intestinal mRNA from the VDR-KO animals revealed an mRNA that lacks exon 2 but contains exons 3-9 plus two 5'-untranslated exons. Western analysis of intestinal extracts from VDR-KO mice showed a protein of a size consistent with the use of Met52 as the translational start site. Transfection of a plasmid construct containing the sequence encoding the human analog of this truncated form of the receptor, VDR(52-C), into Cos-1 cells showed that this truncated form of the receptor retains full [(3)H]-1alpha,25(OH)(2)D(3) binding ability. This same construct was inactive in transactivation assays using the osteocalcin promoter in CV1 cells. Thus, we have determined that this widely used strain of the VDR-KO mouse can express a form of the VDR that can bind ligand but not activate gene transcription.

We previously reported that the natural hormone 1,25dihydroxyvitamin D3 (1,25(OH)(2)D(3)) protects human skin cells from ultraviolet radiation (UVR)-induced apoptosis. UVR-induced pre-mutagenic cyclobutane pyrimidine dimers are diminished in number from 0.5h after cessation of UVR in all skin cell types, by treatment with three different Vitamin D compounds: by 1,25(OH)(2)D(3), by the rapid acting, low calcemic analog, 1alpha,25(OH)(2)lumisterol(3) (JN) and by the low calcemic but transcriptionally active hybrid analog 1alpha-hydroxymethyl-16-ene-24,24-difluoro-25-hydroxy-26,27-bis-homovitamin D3 QW-1624F2-2 (QW), which may explain the enhanced cell survival. The rapid response antagonist analog 1beta,25(OH)(2)D(3) (HL) abolished the photoprotective effects of 1,25(OH)(2)D(3) whilst a genomic antagonist, (23S)-25-dehydro-1alpha-hydroxyvitamin D(3)-26,23-lactone (TEI-9647), had no effect. UVR increased p53 expression in human skin cells, whilst concurrent treatment with 1,25(OH)(2)D(3) further enhanced this effect several fold, at 3 and 6h after UVR. Combined with previously reported lower nitrite levels with 1,25(OH)(2)D(3), this increased p53 expression may favor DNA repair over apoptosis. We now report that topical application of 1,25(OH)(2)D(3) or QW also suppressed solar simulated UV (SSUVR-induced pyrimidine dimers in the epidermis of irradiated hairless Skh:HR1 mice, measured 24h after irradiation. Furthermore, UVR-induced immunosuppression in the mice was markedly reduced by topical application of either 1,25(OH)(2)D(3) or QW. These preliminary results show, for the first time, a protective effect of Vitamin D compounds against DNA photodamage in vivo.

The steroid hormone 1alpha,25(OH)(2)-Vitamin D(3) [1alpha,25(OH)(2)D(3)] exerts a wide variety of biological actions through one or more receptors/binding proteins. The nuclear Vitamin D receptor (VDR) when bound to its natural ligand, 1alpha,25(OH)(2)D(3), can stimulate transcription of a wide variety of genes. The synthesis of 1alpha,25(OH)(2)D(3) analogs allows the study of structure-function relationships between ligand and the VDR. 1alpha,25(OH)(2)D(3) is a conformationally flexible molecule; specifically the side-chain of the hormone can display a large variety of shapes for its receptor. Here, we describe and analyze the properties of 10 1alpha,25(OH)(2)D(3) analogs modified at the side-chain of which five lack carbon-19 (19-nor) but have a novel 20-cyclopropyl functionality. Analog NG [20,21-methylene-23-yne-26,27-F(6)-19-nor-1alpha,25(OH)(2)D(3)] possesses a respectable binding affinity for the VDR and exhibits a high transcriptional activity (EC(50) approximately 10pM), while retaining low induction of hypercalcemia in vivo in the mouse, making it a primary candidate for further analyses of its anti-proliferative and/or cell differentiating properties.

1,25-Dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] is anti-apoptotic in human keratinocytes, melanocytes and fibroblasts after ultraviolet (UV)-exposure. To date, there is no published data on the effects of 1,25(OH)(2)D(3) or its analogs on DNA damage in irradiated skin cells. In these skin cells, 24h pre-treatment with 1,25(OH)(2)D(3) dose-dependently (10(-12) to 10(-8)M) decreased CPD damage by up to 60%. This photoprotective effect was also seen if the 1,25(OH)(2)D(3) was added immediately after irradiation and was mimicked by QW-1624F2-2 (QW), a low-calcemic 1beta-hydroxymethyl-3-epi-16-ene-24,24-difluoro-26,27-bis homo hybrid analog. The well-studied low calcemic, rapid acting agonist analogs 1alpha,25(OH)(2)lumisterol(3) (JN) and 1alpha,25(OH)(2)-7-dehydrocholesterol (JM) also protected skin cells from UV-induced cell loss and CPD damage to an extent comparable with that of 1,25(OH)(2)D(3). In contrast, the rapid response antagonist analog 1beta,25(OH)(2)D(3) (HL) completely abolished the photoprotective effects (reduced cell loss and reduced CPD damage) produced by treatment with 1,25(OH)(2)D(3), JN, JM and QW. Evidence for involvement of the nitric oxide pathway in the protection from CPD damage by 1,25(OH)(2)D(3) was obtained. These data provide further evidence for a role of the vitamin D pathway in the intrinsic skin defenses against UV damage. The data also support the hypothesis that the photoprotective effects of 1,25(OH)(2)D(3) are mediated via the rapid response pathway(s).

The seco-steroid hormone 1alpha,25(OH)(2)-vitamin D(3) (1,25-D(3)) is known to generate biological responses via both genomic and non-genomic rapid signal transduction pathways. The calcium regulated annexin II/p11 heterotetramer (AII(2)/p11(2)] was proposed by Baran and co-authors to be the membrane receptor responsible for mediating non-genomic, rapid actions of 1,25-D(3), based on ligand affinity labeling, competition, and saturation analysis experiments. Given the cytosolic presence of both the monomeric and heterotetrameric form of AII and their functional regulation by intracellular calcium concentrations, which are known to be affected by 1,25-D(3) rapid, non-genomic activities, we investigated in vitro the affinity of [(3)H]1,25-D(3) for the AII monomer and AII(2)/p11(2) in the absence and presence of calcium using saturation analysis and gel-filtration chromatography. Using two different techniques for separating bound from free ligand (perchlorate and hydroxylapatite (HAP)) over a series of 30 experiments, no evidence for specific binding of [(3)H]1,25-D(3) was obtained with or without the presence of 700 nM exogenous calcium, using either the AII monomer or AII(2)/p11(2). However saturable binding of [(3)H]1,25-D(3) to the lipid raft/caveolae enriched rat intestinal fraction was consistently observed (K(d) = 3.0 nM; B(max) = 45 fmols/mg total protein). AII was detected in lipid raft/caveolae enriched fractions from rat and mouse intestine and ROS 17/2.8 and NB4 cells by Western blot, but incubation in the presence of exogenous calcium did not ablate 1,25-D(3) binding as reported by Baran et al. Our results suggest that AII does not bind 1,25-D(3) in a physiologically relevant manner; however, recent studies linking AII(2)/p11(2) phosphorylation to vesicle fusion and its calcium regulated localization may make AII a possible down-stream substrate for 1,25-D(3) induced rapid cellular effects.

The active form of vitamin D, 1α,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ), exerts its effects through regulation of target gene transcription. Configuration at C-20 of 1,25(OH) 2 D 3 is important in determining potency, as shown by the high potency of analogs with inverted configuration at C-20 (20-epi compounds). Gemini analogs of 1,25(OH) 2 D 3 contain two side chains, combining a C-20-normal with a C20-epi side chain. We studied the potency of analogs combining double (Gemini) side chains with a 23-triple bond and a C-26,27-hexafluoro substitution in either the 20-epi (analog 20R) or 20-normal (analog 20S) side chain. These novel Gemini analogs were 8–50-fold more potent than 1,25(OH) 2 D 3 in inducing U937, HL-60G, and THP-1 differentiation and 5–50-fold more potent in inducing transcription from the osteocalcin vitamin D response element or the 25-hydroxyvitamin D 3 -24-hydroxylase (24OHase) promoter. In vivo , following i.p. injection in vitamin D-deficient mice, the 20S analog induced significantly higher levels of calbindin-D 9K mRNA in intestine, and 24OHase and calbindin-D 28K in kidney than 1,25(OH) 2 D 3 or analog 20R. Increased potency did not correlate with ligand-receptor binding affinity. In GST-pull down assays using in vitro translated VDR, Gemini analogs showed equivalent (or even attenuated) potency to 1,25(OH) 2 D 3 in recruiting cofactors DRIP205 and GRIP-1 to VDR. However, Gemini analogs were up to 15-fold more potent than 1,25(OH) 2 D 3 in recruiting the same cofactors to VDR in GST-pull down assays using equal amounts of VDR from nuclear extracts of VDR transfected and hormone treated (24 hr) COS-7 cells. Deletion of C-19 in either 20S or 20R Gemini analogs resulted overall in slightly less potent analogs compared to Gemini itself. We conclude that enhanced potency of the novel Gemini analogs is at least partly due to increased metabolic stability of the analogs, resulting in more cofactor binding and elevated levels of transcription.

All biologic responses to vitamin D are now known to arise as a consequence of the metabolism of this seco-steroid into its two principal biologically active metabolites 1alpha,25(OH)(2)-vitamin D(3) (1ALPHA;,25(OH)(2)D(3)) and 24R,25(OH)(2)-vitamin D(3) (24R,25(OH)(2)D(3)). 1alpha,25(OH)(2)D(3) is the dominant metabolite and produces a wide array of biological responses via interacting both with the classical vitamin D nuclear receptor (VDR(nuc)) that regulates gene transcription in over 30 target organs and with a putative cell membrane receptor (VDR(mem1,25)) that mediates rapid (within seconds to minutes) biological responses. Ligand occupancy of VDR(mem1,25) is linked to signal transduction systems that can mediate the opening of Ca(2+) and chloride voltage gated channels as well as activation of MAP-kinase. MAP-kinase activation in some cells containing VDR(mem1,25)+VDR(nuc) then results in "cross-talk" from VDR(mem1,25) to VDR(nuc) which modulates transactivation of 1alpha,25(OH)(2)D(3) responsive gene promoters. The 24R,25(OH)(2)D(3) metabolite has been shown to be an essential hormone for the process of bone fracture healing. The activity of the enzyme responsible for the production of 24R,25(OH)(2)D(3), the renal 25(OH)D-24-hydroxylase, becomes elevated within 4-11 days after imposition of a tibial fracture, thereby increasing the blood concentrations of 24R,25(OH)(2)D(3) by threefold. The 24R,25(OH)(2)D(3) likely initiates its biological responses via binding to the ligand binding domain of a second cell membrane receptor, the VDR(mem24,25), which is stereospecific for 24R,25(OH)(2)D(3) in comparison with 24S,25(OH)(2)D(3) and 1alpha,25(OH)(2)D(3). This report summarizes the status of several current research frontiers in this arena of the vitamin D endocrine system.

The steroid hormone 1 alpha,25(OH)(2)-vitamin D(3) [1 alpha,25(OH)(2)D(3)] mediates through its widely distributed nuclear receptor (VDR(nuc)) regulation of gene transcription (genomic responses) and through a putative membrane receptor (VDR(mem)) a variety of rapid responses. Rapid responses studied in our laboratories include opening of voltage-gated calcium and chloride channels in ROS 17/2.8 osteoblast cells, activation of MAP-kinase in human leukemia NB4 cells and chick intestinal cells, release of insulin by rat pancreatic beta-cells, and in chick duodena transcaltachia (the rapid hormonal stimulation of intestinal Ca(2+) transport). 1 alpha,25(OH)(2)D(3) is conformationally flexible (side chain, seco B-ring and A-ring) and accordingly is able to generate a large array of different shapes to serve as ligands for available receptors (VDR(nuc) and VDR(mem)) in the vitamin D endocrine system. Our laboratories have utilized a number of conformationally restricted analogs of 1 alpha,25(OH)(2)D(3) (from a library of several hundred analogs) to evaluate the preferred shape of the ligands for rapid and genomic responses. The determination of the X-ray structure of the 1 alpha,25(OH)(2)D(3)-occupied VDR(nuc) revealed that the preferred ligand shape was a twisted 6-s-trans bowl shape [Molecular Cell 5 (2000) 173-179]. Optimal agonists for genomic responses include 1 alpha,25(OH)(2)D(3) and other side chain conformationally flexible analogs such as 20-epi-1 alpha,25(OH)(2)D(3) [approximately equal to 200-500-fold more potent than 1 alpha,25(OH)(2)D(3)] and 21-(3'-hydroxy-3-methylbutyl)-1 alpha,25(OH)(2)D(3) [an analog with two side chains] all which can achieve the preferred VDR(nuc) shape. In contrast, rapid responses require a 6-s-cis shape of the agonist ligand such as can be achieved by the natural hormone 1 alpha,25(OH)(2)D(3) or by analogs permanently locked in the 6-s-cis shape such as 1 alpha,25(OH)(2)lumisterol(3) or 1 alpha,25(OH)(2)-7-dehydrocholesterol. Additionally, we have discovered analogs that are specific in their antagonist properties for either rapid or genomic responses. Thus, 1 beta,25(OH)(2)D(3) is an antagonist of only rapid responses [via the VDR(mem)], while 23S-25-dehydro-1 alpha,25(OH)D(3)-26,23-lactone is an antagonist of only nuclear responses [via the VDR(nuc)]. In conclusion, we have presented evidence that 1 alpha,25(OH)(2)D(3) mediated rapid response and genomic response signal transduction pathways utilize differing shapes of ligand, both as agonists and antagonists.

Previous biological studies have implicated two vitamin D metabolites, 1 alpha,25(OH)2-vitamin D3[1 alpha,25(OH)2-D3] and 24R,25(OH)2-vitamin D3 [24R,25(OH)2D3] in the process of skeletal fracture-healing. While a nuclear receptor for 1 alpha,25(OH)2D3 is known to be present in osteoblast and absent in osteoclast cell lines, no systematic study has been carried out on the callus tissue which is formed during fracture-healing. The present report shows that a binding protein/receptor for 1 alpha,25(OH)2D3 resides both in a postnuclear membrane fraction and in a high speed cytosol fraction of the callus tissue obtained 10 days after imposition of a tibial fracture. The dissociation constant, KD, for 1 alpha,25(OH)2D3 was 0.83 +/- 0.34 M and 0.66 +/- 0.38 nM respectively, for the membrane and cytosol fractions. Results from a panel of steroid competition assays indicate that both receptor/binding proteins greatly prefer 1 alpha-hydroxylated ligands as compared to 1 alpha-deoxy or 24-hydroxylated ligands. The presence of 1 alpha,25(OH)2D3 receptors in the fracture-healing callus is consistent with the known biological effects of the metabolite on the fracture-healing process.

1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] is the principal mediator of a wide array of biological responses through the far reaching network of the vitamin D endocine system (VDE). The steroid hormone 1 alpha,25(OH)2D3 is delivered to the various target organs of the VDE via a specific plasma transport protein, the vitamin D binding protein (DBP). Also 1 alpha,25(OH)2D3 is known to initiate biological responses through a nuclear receptor, the nVDR (50 kDa) which regulates selected gene transcription and, in addition in some target tissues, through a second receptor located in the cell membrane, the mVDR (approximately 60 kDa), which is linked to protein kinase C and/or voltage-gated Ca2+ channels so as to generate biological responses very rapidly. 1 alpha,25(OH)2D3 as a ligand is unusually conformationally flexible due to the eight carbon side chain, the seco B-ring which permits rotation about the 6-7 single carbon bond, and the A-ring which undergoes chair-chair conformational interconversion characteristic of cyclohexane rings. This paper reviews the evidence that different shapes of the 1 alpha,25(OH)2D3 satisfy the optimal requirements of the ligand binding domains of the DBP, nVDR and mVDR. The presence of a relatively rigid side chain (composed by the presence of an aromatic ring) enhances ligand interaction 2-3 fold with the DBP, but diminishes ligand affinity for the nVDR by 100 fold. The mVDR responds effectively to analogs of 1 alpha,25(OH)2D3 which are 6-s-cis locked [e.g. 1 alpha,25(OH)2-previtamin D3 or 1 alpha,25(OH)2-provitamin D3], but these same analogs have only 1-2% of the activity of 1 alpha,25(OH)2D3 in regulating gene transcription. Finally the 6-s-trans analog, 1 alpha,25(OH)2-tachysterol3, had

The relative affinities of the 1 alpha,25-dihydroxyvitamin D3 (1,25-D3) analogues 20-epi-1 alpha,25-dihydroxyvitamin D3 (IE) and 20-epi-22-oxa-24a,26a,27a-tri-homo-1 alpha,25-dihydroxyvitamin D3 (ID) to the nuclear vitamin D receptor (VDR) are similar to that of 1,25-D3, but their antiproliferative action is 1000-fold greater. We tested whether the greater antiproliferative effect of these analogues is due to a differential activation of the VDR. In ROS 17/2.8 cells, the effective doses required to produce 50% maximal stimulation (ED50) of transfected reporter genes driven by either the osteocalcin or the osteopontin vitamin D-response elements (VDRE) were 5 x 10(-9) M, 10(-10) M, and 10(-11) M for 1,25-D3, ID, and IE, respectively. Similar results were obtained when recombinant human VDR was cotransfected into CV-1 cells with an osteocalcin VDRE-reporter plasmid. We found that in vitro the sensitivity of 1,25-D3-induced and analogue-induced receptors to proteases was different. The ED50 for binding to VDRE, as determined by electrophoretic mobility shift assays, was significantly higher for 1,25-D3-induced than for analogue-induced VDR. The concentration of retinoid X receptor (RXR) was significantly lower in 1,25-D3-induced than analogue-induced VDR complexes with VDRE. We therefore conclude that IE and ID augment transcriptional activity of VDR more than 1,25-D3 does, by producing conformational changes that enhance dimerization of VDR with RXR. We suggest that these conformational changes are due to differences in the contact sites of the 20-epi analogues and 1,25-D3 with the VDR.

The hormonally active form of vitamin D is 1,25(OH)2-vitamin D3 [1,25(OH)2D3]. This seco-steroid is the key mediator of the vitamin D endocrine system which produces biological effects in over 28 target tissues. In these target tissues, the biological responses may be generated both by a signal transduction mechanism which involves a nuclear receptor for 1,25(OH)2D3 that modulates gene transcription or a signal transduction pathway which involves rapid opening of Ca2+ channels which are externally located on the plasma membrane. This paper reviews the evidence in support of the pleiotropic effects of this steroid hormone and presents evidence that the receptor of the genomic effects is likely to be separate from the receptor/membrane recognition element which initiates the rapid nongenomic biological effects.

Lymphocyte cell lines were established from five patients with vitamin D-dependent rickets, type II (VDDR-II). These lines were established by infection with human T-lymphotrophic virus type I (HTLV-I). Binding of [3H]1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) to its receptor in these cell lines was compared to binding studies using a T-lymphocyte cell line (S-LB1) from a normal individual. The 1,25(OH)2D3 receptor of S-LB1 was comparable to the well-characterized chick intestinal 1,25(OH)2D3 receptor in terms of its ligand binding affinity and capacity, its mobility on 5-20% sucrose gradients, and its adsorption to and elution properties from DNA-cellulose. Three cell lines established from patients with VDDR-II (Rh-VDR, Sh-VDR, and Ab-VDR) showed no specific binding of 1,25(OH)2D3 to a receptor and treatment of the cultured cells with 1,25(OH)2D3 did not stimulate production of 24,25-dihydroxy-vitamin D3 (24,25(OH)2D3), a response which is diagnostic of the presence of a functional 1,25(OH)2D3 receptor. In a fourth cell line, A1-VDR, the receptor for 1,25(OH)2D3 had a low binding capacity and 25(OH)D3-24-hydroxylase activity was not detectable. Induction of 24,25-(OH)2D3 synthesis by 1,25(OH)2D3 was observed in the fifth cell line, designated Ro-VDR, although the sensitivity to hormone treatment was lower than in the control cell line from a normal donor. The capacity of the receptor for 1,25(OH)2D3 was low in Ro-VDR. In all cell lines where 1,25(OH)2D3 binding to a receptor was detectable, the receptor had the typical sedimentation coefficient of 3.7 S on sucrose density gradient analysis. Binding and elution properties to DNA-cellulose, however, differed from normal in both Ro-VDR and A1-VDR cells where elution from DNA-cellulose occurred at a lower salt concentration than is typical of the 1,25(OH)2D3 receptor. While Ro-VDR cells showed typical nuclear localization of the unoccupied 1,25(OH)2D3 receptor, neither the unoccupied nor the occupied receptor from A1-VDR cells was completely localized in the nucleus. In a series of functional studies we found that modulation of the level of the mRNAs coding for both the c-myc oncogene and the growth factor known as granulocyte-monocyte colony stimulating activity by 1,25(OH)2D3 correlated with the 1,25(OH)2D3 receptor status of these cells. Use of these cell lines will facilitate further study of the molecular defect(s) in the receptor for 1,25(OH)2D3 in vitamin D-dependent rickets type II and will allow a correlation with impairment of cellular functions.

The data presented herein indicate that the chick intestinal 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptor which is localized in the chromatin fraction of a low salt homogenate (Walters, M. R., Hunziker, W., and Norman, A. W. (1980) J. Biol. Chem. 255, 6799-6805) can exist in three distinct biochemical forms. The three 1,25(OH)2D3 receptor forms depended on the absence or presence of ligand and additionally whether the ligand was acquired in vitro (4 degrees C incubation for 3-4 h) or in vivo (13 nmol of 1,25(OH)2D3 administered intramuscularly 2 h prior to sacrifice). The receptor forms were distinguished by their relative KCl extractabilities from the target tissue chromatin preparation and from a reconstituted nontarget tissue (liver) chromatin preparation, as well as their relative elution from DNA-cellulose columns when applied as a mixture. In all cases the rank order "affinity" of the receptor for chromatin or DNA was: unoccupied 1,25(OH)2D3 receptors less than in vivo occupied receptors less than in vitro occupied receptors. These changes in DNA-binding "affinity" occurred without a major change in overall surface charge of the receptor molecules as evaluated by co-elution of all three receptor forms from DEAE-Sepharose columns. Similarly, these changes in DNA-binding characteristics were not accompanied by changes in the apparent molecular weights of these receptor species (91,900 +/- 3300; 99,700 +/- 9400; 93,100 +/- 5600, respectively) as assessed by Sephacryl S-200 gel filtration chromatography in the presence of the protease inhibitor phenylmethylsulfonyl fluoride, included throughout these experiments to protect from proteolytic damage. These results represent the first demonstration of biochemical heterogeneity in the 1,25(OH)2D3 receptor system and suggest the existence of a two-step transformation process for the 1,25(OH)2D3 receptor.

The increasing serum concentrations of various hormones (PTH, PRL, estrogens, and human placental lactogen) are hypothesized to regulate 1,25-dihydroxyvitamin D [1,25(OH)2D] and possibly 24,25-dihydroxyvitamin D [24,25(OH)2D] production during pregnancy. We examined the correlation between the serum levels of 1,25(OH)2D and the pregnancy-related hormones in 25 normal pregnant women, followed throughout gestation and postpartum. Maternal serum levels of 1,25(OH)2D were high during the first trimester (mean +/- SE, 74 +/- 8 pg/ml), remained high until the time of delivery (95 +/- 14 pg/ml), and then fell to almost normal levels (50 +/- 9 pg/ml) on the third postpartum day. The serum levels of 1,25(OH)2D do not correlate with the serum levels of any of the aforementioned hormones. The increase in serum 1,25(OH)2D in pregnancy has been postulated to be related to the stressed calcium homeostatic mechanisms known to occur in the mother. In twin pregnancy, this maternal calcium homeostatic mechanism(s) conceivably may be stressed to a greater extent. However, serum 1,25(OH)2D levels, measured in 27 women with a twin pregnancy in both the second and third trimesters as well as at delivery, did not differ from the levels observed in women with a singleton pregnancy. There were no significant changes in the serum levels of 24,24(OH)2D or 25-hydroxyvitamin D as pregnancy progressed. However, serum 24,25(OH)2D correlated significantly with both serum 1,25(OH)2D (r - 0.51; p less than 0.001, n = 83) and serum 25-hydroxyvitamin D (r = 0.37; P less than 0.001, n = 94). In conclusion, serum levels of 1,25(OH)2D rise early in the first trimester of pregnancy, fall acutely to normal levels soon after delivery, and are similar in singleton and twin pregnancies. The changes in the serum levels of 1,25(OH)2D do not relate to the changes in the serum levels of any of the pregnancy-related hormones.

The serum levels of 1.25-dihydroxycholecalciferol [1,25(OH)2D3] were increased in five patients with primary hyperparathyroidism [60 +/- 13 (SD) pg/ml; normal value, 33 +/- 15 (SD) pg/ml] but fell rapidly after parathyroidectomy to values of 23 +/- 9 (SD) pg/ml. This was accompanied by parallel decreases in the serum concentrations of calcium and immunoreactive parathyroid hormone. Over the following 5--35 days, the serum 1,25(OH)2D3 concentrations increased markedly to levels of 59 +/- 17 (SD) pg/ml, which could most likely be explained by a stimulatory effect of the hypocalcemia per se on the renal production of 1,25(OH)2D3.

We describe herein two different effects of protease inhibitors and substrates on receptors for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) obtained from the intestinal mucosa of vitamin D-deficient chicks: inhibition of binding of 1,25(OH)2D3 to its receptor and stabilization of the receptor. Both L-1-tosylamido-2-phenylethyl chloromethyl ketone (TPCK), a chymotrypsin inhibitor, and N alpha-p-tosyl-L-lysine chloromethyl ketone (TLCK), a trypsin inhibitor, block [3H]1,25(OH)2D3 binding to the receptor. Fifty per cent inhibition of binding occurs at 20 microM TPCK, and 100% inhibition at 100-200 microM; TLCK is about 25-fold less effective. At higher concentrations (10-100 mM), the chymotrypsin substrates N alpha-p-tosyl-L-arginine methyl ester and tryptophan methyl ester and the cathepsin B inhibitor leupeptin also inhibit [3H] 1,25(OH)2D3 binding to its receptor. Different inhibitors and substrates interact with the receptor differently: TPCK (20 microM) and N alpha-p-tosyl-L-arginine methyl ester (10 mM) are reversible, noncompetitive inhibitors, L-tryptophan methyl ester (20 mM) is a reversible competitive inhibitor, and phenylmethylsulfonyl fluoride (300 microM) shows no effect on [3H]1,25(OH)2D3 binding to its receptor. The most stable form of unoccupied 1,25(OH)2D3 receptors from chick intestinal mucosa was that obtained from a low salt chromatin preparation (t 1/2 = 6.0 h). The presence of KCl drastically decreased receptor stability (t 1/2 = 1.8 h); and the addition of 2.5 mM CaCl2 further reduced their stability. Phenylmethylsulfonyl fluoride and Trasylol inhibited the KCl-induced receptor instability, but did not prevent the additional instability in the presence of CaCl2. In summary, TPCK and TLCK exert direct effects on the 1,25(OH)2D3 receptor molecule, independent of their protease inhibitor function. These compounds may prove useful as covalent affinity labels for the receptor. On the other hand, phenylmethylsulfonyl fluoride and Trasylol stabilize 1,25(OH)2D3 receptors, probably via inhibition of KCl-activated nuclear protease(s). This receptor stabilization will be advantageous in receptor assays and/or purification procedures.

A previous report [Science 201, 835-837 (1978)] presented evidence that the combined and simultaneous administration of the cholecalciferol (D3) metabolites 1 alpha,25-dihydroxycholecalciferol [1 alpha,25(OH)2D3] and 24R,25-dihydroxycholecalciferol [24R,25(OH)2D3] to White Leghorn hens was necessary for embryo development and normal egg hatchability; in the absence of 24R,25(OH)2D3 none of the fertile eggs hatched. The present study extends this fundamental observation to a second species, the Japanese quail, Coturnix coturnix japonica and compares the biological actions of the two stereoisomers of the 24,25(OH)2 metabolite, namely the naturally occurring 24R,25(OH)2D3 and its unnatural epimer 24S,25(OH)2D3. Groups of 12-14 vitamin D-depleted adult female Japanese quail were mated with normal male quail and eight consecutive batches of eggs (25-41 eggs from each group) were placed in an egg incubator, and egg hatchability for the fertile eggs monitored on days 21 and 22. The egg hatchability (in percent +/- SD) for each group was: D3 (56.5% +/- 12.8); 1 alpha,25(OH)2D3 (1.3% +/- 2.5); 24R,25(OH)2D3 (29.6% +/- 3.1); 24R25(OH)2D3 + 1 alpha,25-(OH)2D3 (32.8%); and 24S,25(OH)2D3 + 1 alpha,25(OH)2D3 (7.2%). Also for all treatment groups the blood level of the expected vitamin D metabolites were in the normal range, and there were no significant differences in the embryo weights and eggshell thickness (of both hatched and unhatched eggs). These results indicate that the Japanese quail have the inherent capability to discriminate between the stereoisomers of 24,25(OH)2D3 and therefore strongly support the concept that only the naturally occurring 24R,25(OH)2D3 has an identifiable, unique biological role which is different from that of 1 alpha,25(OH)2D3.

Human promyelocytic leukemia cells incubated with 25-hydroxy[26,27-methyl-3H] cholecalciferol (1 microCi) or non-radioactive 25-hydroxycholecalciferol (550 micrograms) produced significant quantities of two vitamin D3 metabolites. The two metabolites were isolated and purified by methanol chloroform extraction and a series of chromatographic procedures. The metabolite purification and elution positions on these columns were followed by radioactivity and their ultraviolet absorption at 310 nm. The two metabolites have been unequivocally identified as (5Z)- and (5E)-19-nor-10-oxo-25-hydroxycholecalciferol by ultraviolet absorption spectrophotometry, mass spectrometry, Fourier-transform infrared spectrophotometry and co-chromatography with synthetic compounds on a high-performance liquid chromatograph. (5E)- but not (5Z)-19-nor-10-oxo-25-hydroxycholecalciferol was able to induce HL-60 cell phenotypic and functional differentiation. However, these two metabolites of 25-hydroxycholecalciferol did not bind specifically to the chick intestinal 3.7 S. receptor protein for 1 alpha,25-dihydroxycholecalciferol. The precise biological role of these metabolites is as yet unclear.

The present study examined the interactions of the synthetic glucocorticoid, dexamethasone, with the regulation of chick intestinal calbindin-D 28K (a 28 000 Da vitamin D-dependent calcium binding protein, CaBP) and its mRNA by 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ). Dexamethasone (0–500 nmol) had a neutral impact on calbindin levels in the rachitic chick intestine when measured 12 h later. However, dexamethasone appeared to exert a significant, though modest, stimulatory influence upon calbindin-mRNA accumulation in the vitamin D-deficient (−D) intestine when measured 12 h after administration. 1,25(OH) 2 D 3 also stimulated calbindin-mRNA accumulation in the −D chick intestine; half-maximal (ED 50 ) doses were 1.1 nmol (7.6-fold) and 12.6 nmol (4.3-fold stimulation) for 1,25(OH) 2 D 3 and dexamethasone respectively. In contrast, when both 1,25(OH) 2 D 3 and dexamethasone were administered simultaneously, the stimulatory effect of 1,25(OH) 2 D 3 (and that of the glucocorticoid) was lost in terms of calbindin and calbindin-mRNA accumulation. Dexamethasone treatment of vitamin D-replete (+D) chicks resulted in a depression of calbindin-mRNA accumulation; levels were depressed to baseline with 250 nmol/bird. Dexamethasone (1.25 μmol per day for 3 days) also induced an apparent ‘down-regulation’ of the 1,25(OH) 2 D 3 receptor population in the −D chick intestine but failed to influence the binding of 1,25(OH) 2 D 3 to its receptor in vitro. Taken collectively, these data indicate that glucocorticoids are able to influence the receptor-mediated action of 1,25(OH) 2 D 3 , possibly at the level of calbindin-D 28K gene expression.

Two new vitamin D metabolites were isolated in pure form from separate incubations of homogenates of chick small intestinal mucosa or rat kidney employing either 1 alpha,25-dihydroxyvitamin D3 (28 microM) or 1 alpha,24R,25-trihydroxyvitamin D3 as substrate (0.17-1.3 microM). The newly characterized compounds and the amounts isolated in pure form from separate isolations are, respectively: 1 alpha,25-dihydroxy-24-oxo-vitamin D3 (1,25(OH)2-24-oxo-D3), 147 micrograms from kidney and 4.2 and 40 micrograms from intestine; 1 alpha,23,25-trihydroxy-24-oxo-vitamin D3 (1,23,25(OH)3-24-oxo-D3), 155 micrograms from kidney and 5.9 and 34 micrograms from intestine. Their structures were identified after extensive high pressure liquid chromatography by means of ultraviolet absorption spectrometry, mass spectrometry of the free compounds and their trimethylsilylated derivatives, proton nuclear magnetic resonance spectrometry, specific chemical reduction of the 24-oxo functionality with sodium borohydride, as well as direct comparison with synthetic 1,25(OH)2-24-oxo-D3. These structural assignments for both compounds correct previous determinations which had been proposed (Ohnuma, N., Kruse, J. R., Popjak, G., and Norman, A. W. (1982) J. Biol. Chem. 257, 5097-5102). The activity of the C-24 oxidation pathway used for the production of the 1,25(OH)2-24-oxo-D3 and 1,23,25(OH)3-24-oxo-D3 can be enhanced 10-fold by prior priming of the chicks or rats with a single intravenous dose of 1,25(OH)2D3 (1-12 nmol/100 g body weight); the induction of the enzyme activity is maximal by 3-6 h and returns to basal levels within 12 h. Further, 1,25(OH)2D3, 1,24,25(OH)3D3, and 1,25(OH)2-24-oxo-D3 all were found to be capable of serving as a precursor with chick intestine and rat kidney homogenates of 1,23,25(OH)3-24-oxo-D3. Collectively these results suggest the existence of a C-24 oxidation pathway for metabolism of 1,25(OH)2D3 by the target intestinal mucosa and kidney to 1,23,25(OH)3-24-oxo-D3. The pathway may play an important role in controlling the tissue levels of this hormonally active form of vitamin D3.

It is known that after birth of a vertebrate there is a requirement for the metabolism of Vitamin D 3 (cholecalciferol) to 1,25-(OH) 2 -Vitamin D 3 to produce the hormonally active form essential for calcium homeostasis. However it is not known whether the enzymatic capability to produce 1,25-(OH) 2 -D 3 only appears after birth or whether it is generated in the embryo. Presented in this paper are results of studies designed to measure the production and localization of 1,25-(OH) 2 -D 3 in the embryo. It was found that the renal enzyme, 25-OH-cholecalciferol-1-hydroxylase, which is capable of producing 1,25-(OH) 2 -D 3 , is present as early as day 9 of incubation (12 days before hatch) in White Leghorn chicks. Further, the enzyme activity increases 6-fold to a maximal level which occurs on the day of hatching. 1,25-(OH) 2 -D 3 was shown to be produced in vivo at day 17 and was found then in low levels in the embryonic intestine and kidney. Thus we have shown that 1,25-(OH) 2 -D 3 is made by embryonic chick kidneys and is found in low levels in embryonic chick intestine and kidney significantly before hatch.

The dynamic equilibrium between in vivo occupied and unoccupied 1,25-dihydroxyvitamin D(3)[1,25(OH)(2)D(3)] receptors of the chick intestinal mucosa was investigated by the exchange assay previously reported [(1980). J. Biol. Chem.255: 9534-9537]. These parameters and their correlation to biological response, i.e., the levels of intestinal vitamin D-dependent calcium binding protein (CaBP), were assessed under different physiological conditions. After a single 1,25(OH)(2)D(3) injection (3.25 nmol), occupied receptor levels increased sharply to a maximum between 1 and 2 h, followed by a rapid decline. A single dose of 1alpha-hydroxy-vitamin D(3) [1alpha(OH)D(3)], an analog that requires 25-hydroxylation for biological activity, resulted in a protracted, albeit lower, response with maximal receptor occupancy at 6 h and half maximal levels 24 h after injection. The intestinal receptor occupancy patterns mirrored the serum 1,25(OH)(2)D(3) levels after either 1,25(OH)(2)D(3) or 1alpha(OH)D(3) treatment. Additionally, time-course (half-life) of blood disappearance of 1,25(OH)(2)D(3) and occupied receptor levels were similar (1.9 and 2.3 h, respectively), suggesting that the amount of occupied 1,25(OH)(2)D(3) receptor is determined by a simple equilibrium between serum 1,25(OH)(2)D(3) and unoccupied receptors. A dose-response study after intramuscular 1,25(OH)(2)D(3) injection yielded a hyperbolic curve with an apparent plateau at 70% receptor occupancy, corresponding to 5 nmol 1,25(OH)(2)D(3) injected. Half-maximal occupancy was reached after a dose of 1 nmol 1,25(OH)(2)D(3), corresponding to 1.5 ng 1,25(OH)(2)D(3)/ml serum. From this value the apparent K(d) in vivo is 3.7 nM, which is similar to that determined in vitro. A 10-fold increase in the 1alpha(OH)D(3) dose resulted in less than a doubling of the levels of serum 1,25(OH)(2)D(3), occupied 1,25(OH)(2)D(3) receptors, or CaBP. Under all experimental conditions, there was a positive correlation between occupied receptor and CaBP levels; however, the slope of the lines depended on the times chosen for the assays due in part to the lag period for CaBP induction and its accumulation within the cell. Conversely, the correlation between serum 1,25-(OH)(2)D(3) levels and occupied receptor levels yielded a single regression line independent of the observation time. Short and long-term treatment with different vitamin D metabolites, estrogen, progesterone, or cortisol did not affect the levels of total intestinal 1,25(OH)(2)D(3) receptor. Under normal physiological conditions, only 10-15% of the total 1,25(OH)(2)D(3) receptor population was occupied by ligand. These studies provide a basis for further investigations of physiological and biochemical parameters of the vitamin D endocrine system and their clinical applications.

Patients with rhabdomyolysis (RBD) and acute renal failure (ARF) are hypocalcemic during the oliguric phase of ARF and over 30% develop hypercalcemia during the diuretic phase. The present study examined the factors underlying these derangements in calcium metabolism in 15 patients: 7 with RBD and ARF, 4 with RBD only, and 4 with ARF only. All patients had hypocalcemia on admission and the hypocalcemia was more pronounced in those with RBD and ARF. All patients with RBD independent of the presence or absence of ARF had calcium deposition in soft tissues as documented by technetium-99 scan. In 4 patients with RBD and ARF, hypercalcemia developed during the diuretic phase at a time when Serum PTH levels were undetectable. Only patients with RBD and ARF had a significant increase in serum levels of 25-hydroxyvitamin D and 1,25-dihydroxyvitamin D [1,25(OH)2D] during the diuretic phase and both the increments in and the levels of 1,25(OH)2D were significantly greater in those who were hypercalcemic. The data indicate that 1) hypocalcemia occurs in RBD independent of ARF and is most likely related to calcium deposition in injured tissues, and 2) elevation in serum levels of 1,25(OH)2D plays an important role in the genesis of hypercalcemia during the diuretic phase of patients with RBD and ARF. Our observations suggest that extrarenal production of 1,25(OH)2D may occur in these patients, and/or that the renal production of 1,25(OH)2D may not be so tightly controlled as it is in normal subjects.

This study was carried out to evaluate the effects of an iv injection of parathyroid extract on serum levels of 1,25-dihydroxyvitamin D [1,25-(OH)2D3] in elderly osteopenic patients and age-matched nonosteopenic controls. Serum concentrations of 1,25-(OH)2D were reduced in elderly osteopenic subjects (mean ± SEM, 20 ± 3 pg/ml) compared with values in agematched nonosteopenic controls (35 ± 3 pg/ml), whereas no differences were found in serum 24,25-dihydroxyvitamin D levels (1.5 ± 0.3 and 2.2 ± 0.5 ng/ml, respectively). An iv injection of parathyroid extract was followed by a significant increase in serum 1,25-(OH)2D levels in both osteopenic patients (16 ± 6 pg/ml) and controls (15 ± 5 pg/ml). The mean 4-h increases in serum 1,25-(OH2D of 11–18 pg/ml were not significantly different in the two groups. The results indicate that the reduced 1,25-(OH)2D concentrations in the osteopenic patients are secondary to changes in factors that normally stimulate this enzyme system. (J Clin Endocrinol Metab 54: 1258, ...

Fifteen acromegalic subjects were found to have elevated plasma levels of both 1,25(OH)2-vitamin D, 65 +/- 23 (SD) pg/ml [normal 33 + 15 pg/ml (SD)] and 24,25(OH)2-vitamin D, 6.8 +/- 1.6 (SD) ng/ml [normal 3.4 + 1.2 ng/ml (SD)]. Treatment with bromocriptine for 6 months reduced the plasma 1,25(OH)2-vitamin D3 level to 40 +/- 13 (SD) pg/ml, p less than 0.01 and the 24,25(OH)2-vitamin D level to 5.4 +/- 1.7 (SD) ng/ml, p less than 0.05.

Publisher Summary This chapter elaborates why vitamin D is essential for the maintenance of calcium homeostasis, its primary function being to maintain normal serum calcium and phosphate levels through a direct stimulation of intestinal transport and mobilization of mineral from bone. The genomic action of 1,25(OH)2D3-receptor complex involves binding to specific DNA sequences of calbindin-D28K gene, but this remains to be experimentally determined. Vitamin D plays a physiological role in pancreatic function and particularly in insulin secretion. In the model for calcium transport in intestinal epithelial ceils, calcium is internalized at the brush border membrane in endocytic vesicles that subsequently fuse with lysosomes. Calcium-bearing lysosomes are then transported to the basal lateral membrane along microtubules, where exocytosis of the vesicular contents completes the transport process. 1,25(OH)2D3 from Bone Marrow Macrophages (BMM) and Pulmonary Alveolar Macrophages (PAM) may be effective locally in a paracrine sense because both bone marrow cells and activated lymphocytes have been recognized as target cells for 1,25(OH)2D3. The 1,25(OH)2D3 produced in the bone marrow may influence myeloid stem cell differentiation toward mature macrophages.

Aluminum (Al) may cause vitamin D-resistant osteomalacia and depress the serum levels of immunoreactive parathyroid hormone (iPTH) in patients treated with maintenance dialysis and those on total parenteral nutrition (TPN). Both conditions have been associated with low serum levels of 1,25(OH) 2 -vitamin D (1,25(OH) 2 D). Al may inhibit PTH secretion in vitro ; however, induction of hypocalcemia can enhance endogenous PTH secretion in Al-loaded dogs and TPN patients. Despite hypocalcemia and/or increased endogenous iPTH levels, Al-loaded TPN patients fail to show the expected rise in serum 1,25(OH) 2 D levels. Such observations suggest that Al may impair the renal response to PTH. We studied vitamin D-replete rats given Al or saline vehicle IP for 5 days. Al and control rats then received a saline infusion with an IV bolus of PTH 1–34. Urinary cyclic AMP and P excretion rose in Al and control rats by 1 hr post-PTH, without differences between the groups. Serum P and ionized Ca levels were not different between Al and control rats. In other Al and control rats, serum 1,25(OH) 2 D levels were measured after saline without PTH. Serum 1,25(OH) 2 D levels were higher in controls given PTH than in those without, but 1,25(OH) 2 D levels were not different between Al rats given PTH and those with none. Thus, aluminum does not affect cyclic AMP or P excretion but may impair 25(OH)D-1α-hydroxylase activity in response to PTH.

Two new metabolites of 1,25-dihydroxyvitamin D 3 [1,25(OH) 2 D 3 ], namely 1,25(OH) 2 -24-oxo-vitamin D 3 and 1,23,25(OH) 3 -24-oxo-vitamin D 3 , have been prepared in vitro using chick intestinal mucosal homogenates. To investigate the binding of 1,25(OH) 2 -[23- 3 H]-24-oxo-D 3 and 1,23,25(OH) 3 -[23- 3 H]-24-oxo-D 3 to the chick intestinal receptor we have isolated both metabolites in radioactive form using an incubation system containing 1,25(OH) 2 -[23,24- 3 H]-D 3 with a specific radioactivity of 5.6 Ci/mmol. Both metabolites were highly purified by using Sephadex LH-20 chromatography followed by high-pressure liquid chromatography (HPLC). Sucrose density gradient sedimentation analysis showed specific binding of both tritium-labeled metabolites to the chick intestinal cytosol receptor. Experiments were carried out to determine the relative effectiveness of binding to the chick intestinal mucosa receptor for 1,25(OH) 2 D 3 . The results are expressed as relative competitive index (RCI), where the RCI is defined as 100 for 1,25(OH) 2 D 3 . Whereas the RCI obtained for 1,25(OH) 2 -24-oxo-D 3 was 98 ± 2 (SE), the RCI for 1,23,25(OH) 3 -24-oxo-D 3 was only 28 ± 6 (SE). Also, the biological activity of both new metabolites was assessed in vivo in the chick. In our assay for intestinal calcium absorption, 1,25(OH) 2 -24-oxo-D 3 was active at a dose level of 1.63 and 4.88 nmol/bird (at 14 h), whereas 1,23,25(OH) 3 -24-oxo-D 3 showed only weak biological activity in this system. In our assay for bone calcium mobilization, administration of both new metabolites showed modest activity at the 4.88-nmol dose level, which was reduced at the 1.63-nmol dose level. The results indicate that biological activity declines as 1,25(OH) 2 D 3 is metabolized to 1,24 R ,25(OH) 3 D 3 ) 1,25(OH) 2 -24-oxo-D 3 , and then 1,23,25(OH) 3 -24-oxo-D 3 .