Association of 1,25-dihydroxyvitamin D3 with cultured 3T6 mouse fibroblasts. Cellular uptake and receptor-mediated migration to the nucleus

The enhanced hypercalcemic response to 20-epi-1,25-dihydroxyvitamin D3 results from a selective and prolonged induction of intestinal calcium-regulating genes

20-Epi-1,25-dihydroxyvitamin D(3) (20-epi-1,25(OH)(2)D(3)) is a vitamin D analog that exhibits unique biologic properties. The mechanism(s) responsible for these activities remains unclear. Here we explore the ability of 20-epi-1,25(OH)(2)D(3) to induce calcemic responses in mice in vivo and identify a potential mechanism. Surprisingly, the levels of calcemia induced at 24 h after single injections of equivalent doses of 1,25(OH)(2)D(3) or 20-epi-1,25(OH)(2)D(3) were similar, suggesting that both compounds were equal in both potency and efficacy. This similarity was also observed at genes involved in calcium homeostasis including, S100g (calbindin D9K), Trpv6, Cldn2 (claudin 2), Trpv5, and Tnfsf11 (Rankl) as well as Cyp24a1. Despite this, the activities of the two compounds at 48 h were strikingly different. Thus, whereas the activity of 1,25-dihydroxyvitamin D(3) declined at this time point, the response to 20-epi-1,25(OH)(2)D(3) was increased. This unique profile was not due to an exaggerated induction of calcium regulating genes in the intestine, kidney, or bone but to a sustained action on these genes in the intestine. This conclusion was supported by studies using in vivo chromatin immunoprecipitation analysis, which revealed a prolonged presence of vitamin D receptor and RNA polymerase II at the Trpv6 and Cyp24a1 promoters and a sustained increase in histone 4 acetylation in these gene regions as well. We conclude that 20-epi-1,25(OH)(2)D(3) displays superagonist properties largely as a result of its duration of action in the intestine. This action is likely due to a decrease in the rate of intestinal-specific degradation of the ligand rather than to an increase in the functional stability of the vitamin D receptor.

Vitamin D-binding protein influences total circulating levels of 1,25-dihydroxyvitamin D3 but does not directly modulate the bioactive levels of the hormone in vivo

Mice deficient in the expression of vitamin D-binding protein (DBP) are normocalcemic despite undetectable levels of circulating 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)]. We used this in vivo mouse model together with cells in culture to explore the impact of DBP on the biological activity of 1,25(OH)(2)D(3). Modest changes in the basal expression of genes involved in 1,25(OH)(2)D(3) metabolism and calcium homeostasis were observed in vivo; however, these changes seemed unlikely to explain the normal calcium balance seen in DBP-null mice. Further investigation revealed that despite the reduced blood levels of 1,25(OH)(2)D(3) in these mice, tissue concentrations were equivalent to those measured in wild-type counterparts. Thus, the presence of DBP has limited impact on the extracellular pool of 1,25(OH)(2)D(3) that is biologically active and that accumulates within target tissues. In cell culture, in contrast, the biological activity of 1,25(OH)(2)D(3) is significantly impacted by DBP. Here, although DBP deficiency had no effect on the activation profile itself, the absence of DBP strongly reduced the concentration of exogenous 1,25(OH)(2)D(3) necessary for transactivation. Surprisingly, analogous studies in wild-type and DBP-null mice, wherein we explored the activity of exogenous 1,25(OH)(2)D(3), produced strikingly different results as compared with those in vitro. Here, the carrier protein had virtually no impact on the distribution, uptake, activation profile, or biological potency of the hormone. Collectively, these experiments suggest that whereas DBP is important to total circulating 1,25(OH)(2)D(3) and sequesters extracellular levels of this hormone both in vivo and in vitro, the binding protein does not influence the hormone's biologically active pool.

2-Methylene-19-nor-(20S)-1,25-dihydroxyvitamin D3 potently stimulates gene-specific DNA binding of the vitamin D receptor in osteoblasts

2-Methylene-19-nor-(20S)-1,25-dihydroxyvitamin D3 (2MD) is a highly potent analog of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) whose actions are mediated through the vitamin D receptor (VDR). In this report, we have replicated this increased potency of 2MD in vitro using osteoblastic cells and explored its underlying molecular mechanism. 2MD stimulates the expression of several vitamin D-sensitive genes including 25-hydroxyvitamin D3-24 hydroxylase (Cyp24), osteopontin and receptor activator of NF kappa B ligand and suppresses osteoprotegerin at concentrations two logs lower than that for 1,25(OH)2D3. 2MD is also more potent in stimulating transfected chimeric reporter genes under either Cyp24 or the osteocalcin promoter control. Enhanced potency is retained regardless of medium serum content. Interestingly, the uptake of both 1,25(OH)2D3 and 2MD into cells is similar, as is their rapid association with the VDR. This indicates that comparable levels of occupied VDR do not elicit equivalent levels of transactivation. Using chromatin immunoprecipitation (ChIP), however, we observed a strong correlation between DNA-bound receptor and the level of induced transcription suggesting a 2MD-induced increase in affinity of the VDR for DNA. Additional studies using a mammalian two-hybrid system and ChIP indicate that 2MD is also more potent in promoting interaction with RXR and the coactivators SRC-1 and DRIP205. Finally, protease digestion studies revealed a unique VDR conformation in the presence of 2MD. These studies suggest that the molecular mechanism of 2MD potency is due to its ability to promote enhanced levels of specific DNA binding by the VDR and could suggest possible explanations for the tissue- and gene-selective actions of 2MD.

Subcellular distribution of normal and mutant vitamin D receptors in living cells. Studies with a novel fluorescent ligand

To understand the subcellular localization of the vitamin D receptor (VDR) and to measure VDR content in single cells, we recently developed a fluorescent labeled ligand, 4,4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY)-calcitriol. This tagged hormone has intact biological activity, high affinity and specific binding to the receptor, and enhanced fluorescent emission upon receptor binding. Using BODIPY-calcitriol, here we monitored the subcellular distribution of VDR in living cultured cells by microscopy. Time course studies showed that an equilibrium between the cytoplasmic and nuclear hormone binding developed within 5 min and was maintained thereafter. We found a substantial proportion of VDR residing in the cytoplasm, colocalized with endoplasmic reticulum, the Golgi complex, and microtubules. Confocal microscopy clarified the presence of VDR within discrete regions of the nucleus and along the nuclear envelope. There was no VDR in the plasma membrane. Low affinity BODIPY-calcitriol binding sites were in the mitochondria. Mutations in the VDR gene selectively and specifically altered BODIPY-calcitriol distribution. Defects in the hormone binding region of VDR prevented both nuclear and cytoplasmic hormone binding. Defects in the DNA binding region decreased the nuclear retention of VDR and prevented localization to nuclear foci. These results with BODIPY-calcitriol reveal cytoplasmic VDR localization in living cells and open the possibility of studying the three-dimensional architecture of intranuclear target sites.

Effect of 1,25-dihydroxyvitamin D3 on proliferation in senescent IMR-90 human fibroblasts

The response of IMR-90 human fetal lung fibroblasts at high population doubling level (PDL > 42) to 1,25-dihydroxyvitamin D3[1,25(OH)2D3] was investigated to clarify whether some metabolic and molecular parameters of senescent cells are affected by the hormone treatment. Pyruvate kinase, lactate dehydrogenase and glucose-6-phosphate dehydrogenase activity significantly increased after treatment of confluent-phase cells with 10 nM 1,25(OH)2D3 for 24 h. Steroid specificity was established by the failure of 10 nM levels of 25-hydroxyvitamin D3 to affect the enzyme activities, while estradiol-17 beta and progesterone produced a slight increase in glucose-6-phosphate dehydrogenase and lactate dehydrogenase levels, respectively. 1,25(OH)2D3 also affected fibroblast proliferation, protein content/cell and DNA synthesis. The cell number significantly decreased after a 48 h incubation with 1,25(OH)2D3 at various concentrations (0.01-1 nM) when compared with control fibroblasts, while an increase in the protein content/cell was demonstrated. The same experiment, carried out by protracting the incubation with the hormone for 72 h, showed a similar trend, but 10 nM 1,25(OH)2D3 was also able to inhibit cell proliferation and to stimulate protein synthesis. The incorporation of [3H]thymidine into DNA increased after the treatment of high PDL fibroblasts with 0.01-1 nM of hormone for 48 h in comparison with controls.

1,25-dihydroxyvitamin D3 inhibits proliferation of IMR-90 human fibroblasts and stimulates pyruvate kinase activity in confluent-phase cells

This study tested the hypothesis that 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) plays a role in regulating some aspects of metabolism in IMR-90 normal human fetal lung fibroblasts. Among the enzymes studied, only pyruvate kinase showed a significant increase after treatment of confluent-phase cells with 1,25(OH)2D3 at various concentrations (0.1-100 nM range) for 24 h. A parallel increase in lactate output was observed. Steroid specificity was established by the failure of 10 nM levels of 25-hydroxyvitamin D3, estradiol-17 beta and progesterone to affect pyruvate kinase activity. The determination of the time course of [3H]-2-deoxy-D-glucose transport indicated that the hormone did not influence the transmembrane transport system of D-glucose. The addition of the inhibitors cycloheximide and actinomycin D to the culture medium abolished, at least in part, the 1,25(OH)2D3 stimulation of pyruvate kinase activity, suggesting the probable dependence of the hormone effect on cellular RNA and protein synthesis. 1,25(OH)2D3 also affected fibroblast growth and DNA synthesis. Cell number significantly decreased after 2-5 days treatment with 10 nM hormone in comparison with control fibroblasts, and also the incorporation of [3H]thymidine into DNA decreased after treatment of the cells with 1 and 10 nM hormone for 48 h. In conclusion, these data demonstrate that 1,25(OH)2D3 stimulates pyruvate kinase activity in confluent-phase IMR-90 human fibroblasts by a mechanism probably dependent on de novo protein synthesis, and also affects cell growth and DNA synthesis in sub-confluent-phase cells.

Inhibition of nuclear uptake of calcitriol receptor by uremic ultrafiltrate

The biological action of calcitriol is mediated through a hormone-receptor complex interacting with nuclear chromatin. Interaction of the calcitriol receptor (VDR) with VDR response elements produces bioactive proteins which carry out the physiological actions of calcitriol. Since biological response to calcitriol appears to be diminished in renal failure, we studied the effect of uremic toxins on the interaction of VDR with nuclear chromatin using in vitro nuclear uptake of the 3H-calcitriol labeled VDR by intestinal nuclei. We found that nuclear uptake of the labeled intestinal VDR from renal failure rats was significantly lower than that from the control animals. HPLC fractionated uremic ultrafiltrate directly inhibited nuclear uptake of the labeled VDR when the labeled VDR was incubated with 50% of the ultrafiltrate for various time intervals ranging from 15 minutes to 6 hours. Infusion of uremic ultrafiltrate to normal rats for 20 hours also produced intestinal VDR with a lower binding affinity for intestinal nuclei when compared to the controls infused with normal ultrafiltrate. The latter study suggests that uremic toxins are responsible for the decreased nuclear uptake of VDR of rats with renal failure. Although it is difficult to extrapolate these results directly to the intact cells, our findings suggest that part of the calcitriol resistance in renal failure could be explained by decreased entry of receptor into the nucleus.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

In vitro transcription and translation of the human 1,25-dihydroxyvitamin D3 receptor cDNA

A fragment of the complementary deoxyribonucleic acid to the human 1,25-dihydroxyvitamin D3 receptor protein containing essentially the entire open reading frame was transcribed and translated in vitro. The resulting protein was then demonstrated to exhibit the physical and functional features, i.e. molecular weight, immunoreactivity, 1,25-dihydroxyvitamin D3 binding, and DNA-cellulose binding, of the native human receptor from the T47D cell line. This validates the authenticity of the cDNA in a cell free system and provides a biochemical means of generating this rare and labile macromolecule to use in heretofore difficult structure/function studies.

Effect of 1,25-(OH)2-vitamin D3 on growth, homologous receptor and c-myc regulation in C3H/10T1/2 cells

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) receptor concentration, cell proliferation, and the steady-state level of c-myc mRNA were examined in the C3H/10T1/2 mouse embryo fibroblasts, before and after exposing the cells to 1,25-(OH)2D3. The non-transformed, logarithmically growing C3H/10T1/2 Cl 8 cells contained a high concentration of 1,25-(OH)2D3 receptor (164 fmol/mg of protein). An up-regulation of the 1,25-(OH)2D3 receptor and a potent inhibition of cell growth were observed by exposing the cells to 10 nM 1,25-(OH)2D3. The concentration of 1,25-(OH)2D3 receptor in the two chemically transformed, tumorigenic cell lines. C3H/10T1/2 Cl 16 and C3H/10T1/2 TPA 482, was 218 and 63 fmol/mg of protein, respectively. In the two transformed cell lines, 10 nM 1,25-(OH)2D3 had only negligible effect on cell growth. In the Cl 16 cells, an up-regulation of the 1,25-(OH)2D3 receptor was demonstrated, but only a weak up-regulation was found in the TPA 482 cells by the 1,25-(OH)2D3 treatment. No major changes were found in c-myc mRNA levels by the 1,25-(OH)2D3 treatment. Despite inhibition of cell growth, the steady-state level of c-myc mRNA was slightly induced (35%, mean) in the Cl 8 cells compared to control cells. In the transformed cells, no consistent change of the c-myc level was found. In contrast to earlier reports, we did not find any correlation between the 1,25-(OH)2D3 receptor and c-myc level, nor did we find any decrease of c-myc mRNA by 1,25-(OH)2D3 treatment in the C3H/10T1/2 fibroblasts.

Agents affecting adenylate cyclase activity modulate the stimulatory action of 1,25-dihydroxyvitamin D3 on the production of osteocalcin by human bone cells

The stimulation of osteocalcin synthesis by human osteoblast-like cells in response to 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is antagonised by several bone regulatory agents. We have shown that agents which activate adenylate cyclase inhibit this action of 1,25(OH)2D3 on human osteoblast-like cells. Activation of adenylate cyclase, either via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic via the stimulatory GTP-binding protein using cholera toxin, or directly at the catalytic subunit using forskolin, results in a suppression of osteocalcin synthesis. Whilst the activation of adenylate cyclase induces this inhibitory response, neither exogenous dibutyryl cyclic AMP nor the phosphodiesterase inhibitor, IBMX, exerted any apparent effect on the production of osteocalcin. The tumour promoting phorbol ester, 4 beta-phorbol 12,13-dibutyrate, also inhibited 1,25(OH)2D3-stimulated osteocalcin production. This was not apparent in response to the non-tumour promoting phorbol ester 4 beta-phorbol suggesting the involvement of protein kinase C.

Demonstration of 1 alpha,25-dihydroxyvitamin D3 receptor-like molecule in ST 13 and 3T3 L1 preadipocytes and its inhibitory effects on preadipocyte differentiation

The active metabolite of vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), inhibited morphologic and enzymatic expression during differentiation of preadipocyte to adipocyte. In the presence of approximately 6.4-20 X 10(-10) M 1,25(OH)2D3, the triacylglycerol accumulation was only 50% of that of fully differentiated control cells. High-affinity binding sites for 1,25-dihydroxyvitamin D3 were detected in two preadipose cell lines. The 1,25(OH)2D3 binding component sediments at 3.3 S in 4-24% (w/v) sucrose gradients prepared in hypertonic buffer. Binding assay revealed that Nmax was 70 fmol/mg protein and 90 fmol/mg protein, and Kd value was 170 pM and 37 pM in cell lines ST 13 and 3T3 L1, respectively. We also found that differentiated adipocytes did not contain specific receptors for 1,25(OH)2D3. 1,25(OH)2D3, 1(OH)D3, 24,25(OH)2D3, and 24(OH)D3 all suppressed differentiation of preadipocytes to adipocytes, and the dose required closely reflected the affinities of the various metabolites and the synthetic derivative for 1,25(OH)2D3 receptor. It is suggested that the action of vitamin D3 on preadipocyte differentiation may result from a receptor-mediated event.

The vitamin D receptor: a primitive steroid receptor related to thyroid hormone receptor

We have previously reported the cloning and sequencing of both the chicken and human vitamin D3 receptor cDNAs. A comparison of their deduced amino acid sequence with that of the other classic steroid hormone receptors and the receptor for thyroid hormone indicates that there are two regions of conservation between these molecules. The first is a 70 amino acid, cysteine-rich sequence (C1), the second region (C2) is a 62 amino acid region located towards the carboxyl terminus of the proteins. In other systems the former has been identified as a region responsible for DNA binding activity, whereas the latter represents the NH2-terminal boundary of the hormone binding domain. We present here evidence utilizing eucaryotic expression of cDNA encoding the hVDR C1 domain, followed by a DNA cellulose chromatography assay, which confirms that the DNA binding activity resides in this region of the receptor for vitamin D3. Additionally, the vitamin D3 receptor contains a 60 amino acid portion at its carboxyl terminus (C3) which exhibits homology with the receptor for thyroid hormone. Conservation in this region of the molecule is found only between homologous or closely related receptors. This indicates a relationship between the vitamin D3 receptor and the receptor for thyroid hormone and may suggest that they evolved from a single primordial gene.

Molecular cloning of complementary DNA encoding the avian receptor for vitamin D

Vitamin D3 receptors are intracellular proteins that mediate the nuclear action of the active metabolite 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. Two receptor-specific monoclonal antibodies were used to recover the complementary DNA (cDNA) of this regulatory protein from a chicken intestinal lambda gt11 cDNA expression library. The amino acid sequences that were deduced from this cDNA revealed a highly conserved cysteine-rich region that displayed homology with a domain characteristic of other steroid receptors and with the gag-erbA oncogene product of avian erythroblastosis virus. RNA selected via hybridization with this DNA sequence directed the cell-free synthesis of immunoprecipitable vitamin D3 receptor. Northern blot analysis of polyadenylated RNA with these cDNA probes revealed two vitamin D receptor messenger RNAs (mRNAs) of 2.6 and 3.2 kilobases in receptor-containing chicken tissues and a major cross-hybridizing receptor mRNA species of 4.2 kilobases in mouse 3T6 fibroblasts. The 4.2-kilobase species was substantially increased by prior exposure of 3T6 cells to 1,25(OH)2D3. This cDNA represents perhaps the rarest mRNA cloned to date in eukaryotes, as well as the first receptor sequence described for an authentic vitamin.

Emerging concepts on the biologic role and mechanism of action of 1,25-dihydroxyvitamin D3

The biologic actions of 1,25-(OH)2D3 are diverse, ranging from a major role in the regulation of mineral homeostasis in intestine, kidney, and bone to the control of such fundamental processes as myeloid progenitor cell differentiation. The central character in this action is the 1,25-(OH)2D3 receptor, a protein whose activity is focused at the level of the genome. The function of this polypeptide, by analogy with other steroid receptors, is to interact in a sequence-specific manner with unique regulatory elements of DNA, which serve to modify the activity of their respective promoters. The exact manner in which receptor binding to these sequences precipitates promoter activity is unclear. It is, however, a direct result of the structural organization of the steroid receptors, which represent a class of transcriptional controlling proteins. The deduced primary sequences emanating from the molecular cloning of estrogen, progesterone, glucocorticoid, and 1,25-(OH)2D3 receptors has revealed several important structure-function relationships. These include the identification of a highly conserved cysteine-rich domain that may interact with DNA and a steroid-binding domain that is hydrophobic and is located at the carboxy terminus of the protein. The similarity of this domain among heterologous steroid receptor species implies that each of these proteins belongs to a common gene family whose functional activities are similar if not identical. It is this structure within the 1,25-(OH)2D3 receptor that provides conclusive evidence that 1,25-(OH)2D3 is a steroid hormone that via its receptor modifies the activity of hormone-sensitive genes.

The metabolism and mechanism of action of 1,25-dihydroxyvitamin D3

Much has been learned about the formation of the active metabolite of vitamin D3, 1,25-dihydroxyvitamin D3. Information concerning its formation and catabolism has allowed a clear understanding of factors involved in the maintenance of plasma concentrations of the hormone. The effects of 1,25-dihydroxyvitamin D3 on calcium transporting cells in the intestine are marked and well defined. The tissue (intestinal tissue) is easily isolated and manipulated and hence, this is an ideal tissue in which to examine the mechanism of divalent cation transport. The mechanism by which 1,25-dihydroxyvitamin D3 brings about this effect should help in understanding sterol hormone action.

Characterization of the DNA-binding properties of the receptor for 2,3,7,8-tetrachlorodibenzo-p-dioxin

The DNA-binding properties of the receptor for 2,3,7, 8-tetrachlorodibenzo-p-dioxin (TCDD) were investigated using chromatography on DNA-cellulose columns. A maximal binding of about 40% of the total receptor complex to DNA-cellulose was observed. In order to interact with DNA, the receptor must first bind TCDD. A heat-activation step followed by gel permeation chromatography using Sephadex G-25 increased the binding of the cytosolic receptor to DNA. The DNA-binding ability of the receptor was almost lost following mild proteolysis using trypsin or alpha-chymotrypsin, although these treatments did not reduce its ligand binding capacity and had no apparent effect on its size. Furthermore, pre-treatment of the DNA-cellulose column with an intercalating drug, ethidium bromide, resulted in inhibition of the binding of the TCDD-receptor complex to DNA, indicating that not only electrostatic interactions but also the configuration of DNA are of importance in receptor-DNA interactions.

1,25-Dihydroxyvitamin D3 receptors: altered functional domains are associated with cellular resistance to vitamin D3

1,25-Dihydroxyvitamin D3 receptors are cytosoluble proteins detectable in a variety of tissues responsive to 1,25(OH)2D3. They are DNA binding-proteins analogous to other steroid receptors and it is this functional property which is likely involved in the activation of hormone-sensitive genes. Utilizing 1,25(OH)2D3 and DNA binding assays, as well as anti-receptor monoclonal antibodies, we have probed the relationship between the 1,25(OH)2D3 receptor binding domains after selective cleavage with trypsin. These studies reveal that the hormone and DNA binding regions are separable, and are consistent with the finding that tissue resistance to 1,25(OH)2D3 is a result of structural defects in these domains. Recently, a primate model, the LLC-MK2 monkey kidney line, has been uncovered which may exemplify a hormone-binding defect. Here, 25-hydroxyvitamin D3-24-hydroxylase induction, a 1,25(OH)2D3 bioresponse, requires 100-fold higher concentrations of the hormone for maximal response. Concomitantly, this cell contains a variant receptor form which displays a correspondingly lowered apparent affinity for the hormone despite its seemingly normal DNA binding characteristics. Taken together, these studies suggest that the 1,25(OH)2D3 receptor is a macromolecule with multiple domains each of which may produce modified cellular resistance to 1,25(OH)2D3 if structurally altered.

Hormone-dependent phosphorylation of the 1,25-dihydroxyvitamin D3 receptor in mouse fibroblasts

Experimental results, employing several immunologic techniques, suggest that the mouse receptor for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) undergoes hormone-dependent phosphorylation in intact cells. Treatment of monolayer cultures of mouse 3T6 fibroblasts with 1,25(OH)2D3 reveals that the occupied 1,25(OH)2D3 receptor displays a minor reduction in electrophoretic mobility as compared to its unoccupied 54,500 dalton counterpart, a change consistent with covalent modification. Similar results were obtained by immunoprecipitation of metabolically-labeled receptors after incubation of 3T6 cells with [35S]methionine. This technique also provided greater insight into the precursor-product relationship between the two receptor forms. [32P]Orthophosphate-labeling of 3T6 cells, followed by immunoprecipitation indicated that only the form exhibiting covalent modification was phosphorylated. The temporal correspondence between the binding of 1,25(OH)2D3 to its cellular receptor and its phosphorylation suggests that the biochemical role of 1,25(OH)2D3 may be to induce a conformational change susceptible to phosphorylation and possibly functional activation.

Demonstration and characterization of 1,25-dihydroxyvitamin D3 receptors in basal cells of epidermis of neonatal and adult mice

Nuclear and cytosolic receptors for 1,25-dihydroxycholecalciferol [1,25(OH)2D3] were demonstrated in the epidermis of neonatal and adult mice. The macromolecular binding protein sedimented at 3.5 S (sucrose density gradient) and was distinct from the 6.0 S binding protein for 25-hydroxycholecalciferol [25(OH)D3]. Analysis at different ionic strengths suggested the presence of unoccupied nuclear receptors. Digestion with proteases or nucleases, respectively, and inactivation with alkylating agents demonstrated that the binding macromolecule is a protein with SH groups at the active site. Binding of 1,25(OH)2D3 was specific and reversible. In neonatal mice KD was 1.6 X 10(-10) M for both cytosolic and nuclear fractions, binding capacity was 54 fmol/mg protein in the cytosolic and 108 in the nuclear fractions, respectively. The phenotypic expression of the 1,25(OH)2D3 receptor (dissociation constant, binding capacity) was identical in neonatal and adult epidermis. Half maximal displacement of 1,25(OH)2D3 was achieved with an 80-fold and 200-fold molar excess of 25(OH)D3 and 1-alpha-hydroxycholecalciferol [1(OH)D3], respectively. Using Percoll density gradient centrifugation, 1,25(OH)2D3 receptors could be localized in the basal cell fraction. DNA cellulose chromatography with 1,25(OH)2D3 receptor elution from DNA at 0.25 M KCl (linear gradient) points to a possible role in gene transcription. In mouse primary epidermal cell cultures, 1,25(OH)2D3, but not 25(OH)D3, 24,25(OH)2D3, and 1(OH)D3 influenced [3H]thymidine incorporation (at physiological concentrations); the magnitude of change depending on the concentration of 1,25(OH)2D3 and the time of incubation. These data demonstrate that skin is a target organ for the active vitamin D secosterol.

Selective, rapid removal of the vitamin D-binding protein and its sterol ligands from human and bovine plasma

Rapid and selective removal of plasma vitamin D-binding protein was effected by the serial passage of plasma over four columns of agarose containing covalently linked skeletal muscle G-actin. By maintaining an actin-to-binding protein molar ratio of at least 4 to 1 throughout, greater than 99% of the binding protein was removed from the fourth column's eluate. In contrast, 87% of the total plasma or serum protein applied was recovered, and electrophoretic analyses of human and bovine sera that had undergone these affinity chromatography steps revealed no major alterations in protein distribution. The procedure also removes vitamin D sterols selectively, with preference for 25-hydroxycalciferol (90% removal) over 1,25-dihydroxycalciferol (65-70% removal) and calciferol (70% removal), in accordance with the known affinity displayed by the binding protein for these sterol ligands. Recovery of other serum constituents (cortisol, proteins, peptide hormones, calcium and alkaline phosphatase) was excellent, further confirming the selectivity of the technique. Utilizing vitamin D-deficient serum, serum depleted of the vitamin D-binding protein was not distinguishable from control serum in supporting the growth of human fibroblasts in vitro. In comparison with other methods to remove serum-binding protein or sterols, the present technique is more selective and can be used for mammalian and avian sera. Material so prepared could prove useful for studies of the cellular access, metabolism, and effects of vitamin D sterols in vitro.

Activation of the 1,25-dihydroxyvitamin D3 receptor in cultured rat osteogenic sarcoma cells

Activation of the 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor-hormone complex was studied in-vitro using cytosolic preparations of rat osteogenic sarcoma cell ROS 17/2-8 and a DNA-cellulose assay. We found that salt was required for extraction of the unoccupied receptor indicating its possible nuclear localization. The 1,25(OH)2D3 receptor underwent an activation process similar to other steroid hormones which could be stimulated by heat and salt. At physiological ionic strength 100% of the complexes were, however, activated at 2 degrees C, indicating that the activation process is not absolutely temperature dependent. In contrast to other steroid hormones, 30-50% of the complexes were in an activated state in the absence of heat and salt moreover, alkaline phosphatase and ammonium sulphate had no effect on activation. Activation was also stimulated by ATP and ATP plus 8BrcAMP indicating the possible role of phosphorylation in the activation process; however, further work is required to clarify this point.

Vitamin D3--resistant fibroblasts have immunoassayable 1,25-dihydroxyvitamin D3 receptors

Cultured fibroblasts obtained from patients with tissue resistance to 1,25-dihydroxyvitamin D3 (vitamin D3--dependent rickets, type II) contain normal, low, or undetectable concentrations of this hormone's receptor protein as measured by a ligand-binding assay. Extracts from these cells were evaluated for receptors by immunoassay with a recently developed monoclonal antibody to the chick receptor. The results show that a protein sedimenting at 3.7S and recognizable by the antibody exists in comparable concentrations in cells from both normal and resistant patients, irrespective of the hormone-binding abnormalities of the cells. This implies that deficiencies in hormone binding associated with inherited tissue resistance to 1,25-dihydroxyvitamin D3 probably arise from structural variations in the receptor molecule and not from defective receptor synthesis.