Binding proteins for vitamin D metabolites: serum carriers and intracellular receptors

Measurement of circulating 25-hydroxyvitamin D: A historical review

The constantly increasing requests for the measurement of serum 25-hydroxyvitamin D over the last years has led reagent manufacturers to market different automated and semi-automated methods, that being unfortunately not fully harmonized, yield different results. Liquid chromatography coupled to tandem mass spectrometry (LC/MS2) has more recently been introduced. This approach allows the distinction between the two forms of 25-hydroxyvitamin D and to measure other metabolites. This approach also requires harmonization to curtail the differences between the different analytical methods. To meet this requirement, the American National Institutes of Health (NIH), the Centre for Disease Control and Prevention (CDC) in Atlanta, the National Institute of Standards and Technology (NIST) and the vitamin D Reference laboratory of Ghent University have pooled their expertise to develop a standardization program. This article reviews the main elements and the difficulties of the automated and semi-automated methods for 25-hydroxyvitamin D, from sample preparation to the analytical phase, as well as those related to mass spectrometry. It also emphasizes the need for standardization to better define the clinical decision thresholds of vitamin D nutritional status.

Signal transduction: activation of the guanylate cyclase-cyclic guanosine-3'-5' monophosphate system by hormones and free radicals

Intracellular communication and transmission of messages for many hormones and free radicals occur after the hormones and free radicals bind to their receptors by enhancing the activity of guanylate cyclase, the enzyme that catalyzes the conversion of guanosine triphosphate to the intracellular messenger cyclic guanosine-3'-5' monophosphate (cyclic GMP). The guanylate cyclase-linked receptors exist intracellularly (ie, cytoplasmic) and in membrane-bound forms. Enhancement of guanylate cyclase by hormones or free radicals increases intracellular cyclic GMP, which closes cation channels in the kidney while activating cation channels in the retina and olfactory cilia, either directly or by cyclic GMP-dependent protein kinase. Cyclic GMP also has potent blood pressure lowering properties. Cyclic GMP promotes growth by increasing DNA, RNA, and protein synthesis. Overactivity of this system is observed in Traveler's diarrhea, whereas underactivity occurs in Chediak-Higashi syndrome in which lysosomal enzyme release and chemotaxis are defective and can be corrected in vitro by addition of cyclic GMP.

Vitamin D modulates the effects of follicle-stimulating hormone on Sertoli cell function and testicular growth in Siberian hamsters

Sertoli cells from immature (18-20 day old) Siberian hamsters (Phodopus sungorus) raised in a long photoperiod (LD; 16hL:8hD) or a short photoperiod (SD; 6hL:18hD) were cultured and exposed to follicle-stimulating hormone (FSH) and/or vitamin D3 (D3) in vitro. D3 did not stimulate lactate production when added alone, but significantly augmented the stimulatory action of FSH on lactate production by Sertoli cells from both LD- and SD-exposed hamsters. Hamsters raised in LD or SD were injected daily with D3 for 10 days starting at 18 days of age. This treatment resulted in a significant increase in the testicular weight of hamsters raised in LD in which circulating FSH levels are high. In contrast, testicular weight was not altered by D3 treatment in SD hamsters in which circulating levels of FSH are low. We conclude that D3, probably after hydroxylation to 1,25(OH)2 vitamin D3, modulates the action of FSH on testicular growth and Sertoli cell function.

Topical treatment of psoriatic plaques with 1,25-dihydroxyvitamin D3: a cell biological study

Calcitriol, 1 alpha,25 dihydroxycholecalciferol (1 alpha,25 (OH)2 D3) is a natural active vitamin D3 metabolite, which has been shown to have antipsoriatic efficacy. In vitro studies have demonstrated that calcitriol influences various aspects of inflammation, epidermal proliferation and keratinization. The aim of the present study was to determine to what extent calcitriol (3 micrograms/g in white petrolatum) affects these parameters in vivo. Using an immunohistochemical assessment of recruitment of cycling epidermal cells, filaggrin and involucrin expression, T-cell accumulation, polymorphonuclear neutrophil (PMN) accumulation, amount of endothelium and ICAM-1 expression, we demonstrated that: (i) modulation of all these parameters occurred during calcitriol treatment; (ii) there was early reduction of epidermal proliferation and PMN accumulation; (iii) the order of changes was comparable with the response to treatment with calcipotriol. In conclusion, at the cell biological level, calcitriol (3 micrograms/g in white petrolatum) has a substantial effect on various elements of the psoriatic lesion.

The effect of 1,25-dihydroxyvitamin D3 on the growth and differentiation of cultured human outer root sheath cells from normal subjects and patients with vitamin D-dependent rickets type II with alopecia

We examined the effect of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on the growth and differentiation of cultured human hair outer root sheath cells (ORSC) from normal subjects and patients with vitamin D-dependent rickets type II (DDR-II) with alopecia. 1,25(OH)2D3 dose-dependently suppressed the plating efficiency, clonal growth, and DNA synthesis of normal ORSC. It enhanced the cornified envelope formation and caused morphological changes in the cells. All results indicated the existence of specific receptors for 1,25(OH)2D3 in the ORSC, and suggest that 1,25(OH)2D3 is a potent inhibitor of proliferation of ORSC as well as a stimulator of terminal differentiation. However, the cells from DDR-II patients with alopecia did not respond to 1,25(OH)2D3, suggesting a lack of the specific receptors in the cells. The differences in the cellular response to the hormone between the normal ORSC and those from the patients were apparent and easily distinguishable, therefore this experiment may be a rapid and simple diagnostic test for DDR-II patients with alopecia. Large number of hairs were difficult to obtain from patients with alopecia, and we developed a new culture method to accomplish these studies from a few plucked hair follicles. Our system may be useful in the culture of ORSC from limited number of follicles, and could be utilized to analyse the cellular characteristics of ORSC in patients with hair diseases.

Photoaffinity labeling of human serum vitamin D binding protein and chemical cleavages of the labeled protein: identification of an 11.5-kDa peptide containing the putative 25-hydroxyvitamin D3 binding site

In this paper, we describe photoaffinity labeling and related studies of human serum vitamin D binding protein (hDBP) with 25-hydroxyvitamin D3 3 beta-3'-[N-(4-azido-2-nitrophenyl)amino]propyl ether (25-ANE) and its radiolabeled counterpart, i.e., 25-hydroxyvitamin D3 3 beta-3'-[N-(4-azido-2-nitro-[3,5-3H]phenyl)amino]propyl ether (3H-25-ANE) (Ray et al., 1986, 1991). We have carried out studies to demonstrate that (1) 25-ANE competes with 25-OH-D3 for the binding site of the latter in hDBP and (2) 3H-25-ANE is capable of covalently labeling the hDBP molecule when exposed to UV light. Treatment of a sample of purified hDBP, labeled with 3H-25-ANE, with BNPS-skatole produced two Coomassie Blue stained peptide fragments, and the majority of the radioactivity was associated with the smaller of the two peptide fragments (16.5 kDa). On the other hand, cleavage of the labeled protein with cyanogen bromide produced a peptide (11.5 kDa) containing most of the covalently attached radioactivity. Considering the primary amino acid structure of hDBP, this peptide fragment (11.5 kDa) represents the N-terminus through residue 108 of the intact protein. Thus, our results tentatively identify this segment of the protein containing the binding pocket for 25-OH-D3.

Simultaneous assessment of inflammation and epidermal proliferation in psoriatic plaques during long-term treatment with the vitamin D3 analogue MC903: modulations and interrelations

The influence of topical application of MC903, an analogue of 1 alpha,25-dihydroxyvitamin D3, on psoriatic plaques was investigated during a long-term treatment study. The parameters for epidermal growth and for inflammation were assessed on frozen sections using immunohistochemical methods to elucidate their modulations in time and the interrelations between the different cell types involved during treatment with MC903. Biopsies were taken before and after 1, 2, 4 and 12 weeks of treatment. Monoclonal antibodies against the hyperproliferation-associated keratin 16, against cycling nuclei, and against T lymphocytes, B lymphocytes, Langerhans cells and CD14-positive cells were used in combination with a polyclonal antibody against polymorphonuclear leucocyte (PMN)-elastase. The earliest change was a statistically significant decrease in PMN after 1 week of treatment followed by a decline of cycling nuclei after 2 weeks. These changes preceded a decrease of T lymphocytes which occurred after 4 weeks. Keratin 16 content tended to diminish after 4 weeks of treatment. CD14+ cells decreased slightly during the observation period, whereas Langerhans cells tended to increase. No B lymphocytes were found. These results suggest that MC903 influences the number of PMN and epidermal growth rather than the number of T lymphocytes.

Abnormal vitamin D3 metabolism in patients with primary Sjögren's syndrome

Recent studies have suggested that vitamin D3 may have an immunoregulatory role in vitro. The vitamin D3 metabolism in 35 patients with primary Sjögren's syndrome was investigated by measuring blood concentrations of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) and 25-hydroxyvitamin D3 (25-OHD3), as well as phenotypes and blood concentrations of Gc globulin, the main vitamin D3 binding protein in the blood. 25-OHD3 concentrations were diminished, but those of 1 alpha,25-(OH)2D3 were normal. There was no significant difference between the distribution of Gc phenotypes in the patients with primary Sjögren's syndrome and normal controls. Likewise, blood concentrations of Gc globulin corresponded to normal values. Among patients with increased concentrations of IgM rheumatoid factor there was a significant negative correlation between the serum titres of IgM rheumatoid factor and 25-OHD3 concentrations.

Characterisation of receptors for 1,25-dihydroxyvitamin D3 in the human testis

Specific high affinity receptors for 1,25-dihydroxyvitamin D3 have been demonstrated in the human testes. The mean binding affinity (Kd +/- SD) of the receptor for 1,25-dihydroxyvitamin D3 was 1.75 +/- 0.32 x 10(-10) M but the binding capacity was low (mean Nmax +/- SD = 0.53 +/- 0.18 fmol/mg protein). Binding was time- and temperature-dependent, with a maximum binding achieved after 1 h at 25 degrees C. Although binding also took place at 4 and 37 degrees C, higher and more rapid binding was found at 25 degrees C. Furthermore, the binding between the ligand and the receptor was specific since only unlabelled 1,25-dihydroxyvitamin D3 competed with the labelled ligand. Binding of 1,25-dihydroxyvitamin D3 was abolished by trypsin and heat. Sucrose density gradient centrifugation revealed a sedimentation coefficient of 3.6S.

Enhancement of butyrate-induced differentiation of HT-29 human colon carcinoma cells by 1,25-dihydroxyvitamin D3

The individual and combined effects of sodium butyrate (NaB) and 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) on inhibition of cell growth and initiation of enterocytic differentiation were investigated in established HT-29 human colonic adenocarcinoma cells. 1,25-(OH)2D3 alone caused some growth restriction but failed to induce differentiation of HT-29 carcinoma cells into a more benign enterocytic phenotype, as assessed by the appearance of mucin-producing colonocytes (goblet cells), increased alkaline phosphatase activity, and the generation of flat foci. NaB, in contrast, produced considerable biochemical and morphologic differentiation along the enterocyte maturation pathway. Combined exposure of HT-29 cells to both NaB and 1,25-(OH)2D3, however, significantly augmented the frequency of differentiated colonocytes, growth inhibition, extent of goblet cell maturation attained, in vitro "differentiation" response of human carcinoma cells is a complex process which, like normal cell maturation within the colonic crypts in vivo, is modulatable (both qualitatively and quantitatively) as a function of inducer composition.

Regulation of natural killer cytotoxicity by 1,25-dihydroxyvitamin D3

The steroid hormone 1 alpha, 25-dihydroxyvitamin D3, calcitriol, is crucial in calcium homeostasis. Calcium plays a central role in T, B, and NK cell functions, and calcitriol is a known inhibitor of T cell proliferation and immunoglobulin production. We have analyzed here the immunoregulatory effects of calcitriol on NK cell function. We show that calcitriol specifically specifically inhibits, in a time- and dose-dependent fashion, the generation of cytotoxic activity from cultured CD16+ peripheral blood NK cells. It also suppresses, at similar molar concentrations (1-10 nM), interleukin 2 (IL-2) production by PHA-activated peripheral blood lymphocytes. Calcitriol does not interfere with the cytotoxic function of NK cells, whether fresh or generated in vitro, placing the inhibition at the level of NK cell activation. Interestingly enough, exogenous IL-2 can completely reverse the suppressive effect. These findings suggest that modulation of NK cell activation by control of the internal level of IL-2 may reflect an additional paracrine calcitriol-dependent circuit with immunoregulatory consequences.

On the specificity of vitamin D compounds binding to chick pig intestinal 1,25-dihydroxyvitamin D3 receptor

The binding activity of four vitamin D metabolites and/or analogs for the intestinal 1,25-dihydroxyvitamin D3 receptor was evaluated after incubation at 25 degrees C for 1 h or at 0-4 degrees C for 18 h. The incubation conditions, which had no effect on the binding of 1,25-dihydroxyvitamin D3, had a dramatic effect on the binding of 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3 and a small but reproducible effect on 24,25-dihydroxyvitamin D3 binding to receptor. Affinities 10- to 20-fold higher were obtained for 25-hydroxyvitamin D3 and 1 alpha-hydroxyvitamin D3, and affinities 3-fold higher were obtained for 24,25-dihydroxyvitamin D3 at the 0-4 degrees C/18-h incubation. A comparison of intestinal receptor from chick and pig with nine vitamin D compounds showed no major differences between the two species. The relative affinity of the vitamin D analogs to compete with tritiated 1,25-dihydroxyvitamin D3 for the receptor in pig nuclear extract, expressed as ratios of the molar concentration required for 50% binding of the tritiated 1,25-dihydroxyvitamin D3 compared to nonradioactive 1,25-dihydroxyvitamin D3, are as follows: 1,25-dihydroxyvitamin D3 (1) = 1,25-dihydroxyvitamin D2 = 24-homo-1,25-dihydroxyvitamin D3 greater than 1,24,25-trihydroxyvitamin D3 (4) greater than 25-hydroxyvitamin D3 (21) = 10-oxo-19-nor-25-hydroxyvitamin D3 = 1 alpha-hydroxyvitamin D3 (37) greater than 24,25-dihydroxyvitamin D2 (257) much much greater than vitamin D3 (greater than 10(6)).

The synthesis of a radiolabeled photoaffinity analog of 1,25-dihydroxyvitamin D3

The synthesis of 1 alpha,25-dihydroxyvitamin D3-3 beta-[N-(4-azido-2-nitro-[2,6-3H] phenyl)]glycinate, a radiolabeled photoaffinity analog of 1,25-dihydroxyvitamin D3, is described.

Possible involvement of 1 alpha,25-dihydroxyvitamin D3 in proliferation and differentiation of 3T3-L1 cells

Growth of 3T3-L1 cells was inhibited by 10(-10)-10(-7)M of 1 alpha,25-dihydroxy vitamin D3 [1 alpha,25(OH)2D3] in a dose- and time-dependent manner. The potency of 1 alpha,25(OH)2D3 in inducing differentiation was low, since 3T3-L1 cells cultured with 1 alpha,25(OH)2D3 did not become mature adipocyte-like cells but were changed to slightly rounded cells containing small droplet-like substances in the cytoplasm and glycerophosphate dehydrogenase (sn-glycerol-3-phosphate: NAD+2-oxidoreductase, EC 1.1.1.8), the marker enzyme of differentiation to adipocyte, did not increase. These results together with the natural occurrence of this vitamin indicate that 1 alpha,25(OH)2D3 may play an important role in the cell growth and differentiation besides such known action as intestinal calcium transport and bone mineral mobilization.

Presence of sterol-binding sites in the cytosol of French-bean (Phaseolus vulgaris) roots

Vitamin D3 (cholecalciferol) and stigmasterol have been shown to stimulate Ca2+ uptake and to induce calmodulin synthesis in cultured French-bean (Phaseolus vulgaris) roots. In addition, the appearance of calmodulin in the cultures in response to vitamin D3 could be prevented by RNA-synthesis inhibitors. To investigate the possibility that the sterols affect root DNA transcription through a receptor-mediated mechanism, the existence of sterol-binding sites in P. vulgaris roots was investigated. Specific binding of [3H]vitamin D3 could be demonstrated with intact tissue and the cytosolic fraction obtained therefrom. Equilibrium in the binding reaction with cytosol was attained after 4 h of incubation at 0 degrees C. The [3H]vitamin D3 was reversibly bound, since it could be displaced by an excess of unlabelled sterol. An equilibrium binding constant (KD) of (3.48 +/- 0.09) x 10(-9) M and a maximum binding-site concentration (nmax) of 32 +/- 2.54 (3) pmol/mg of protein could be calculated by Scatchard [(1949) Ann. N.Y. Acad. Sci. 51, 660-672] analysis. In addition to vitamin D3, stigmasterol and sitosterol were effectively able to compete with [3H]vitamin D3 for binding to root cytosol. Cortisol, oestradiol and progesterone displaced bound labelled vitamin D3 to a lesser extent, whereas 5 beta-dihydrotestosterone, lanosterol and diosgenin were ineffective. The affinity and specificity of the root sterol-binding sites are in agreement with the characteristics of tissue responses to the sterols (Ca2+ uptake and calmodulin synthesis).

Regulation of the intestinal 1,25-dihydroxyvitamin D3 receptor during neonatal development in the rat

Monoclonal antibodies have been used to examine the regulation of expression of the rat intestinal receptor for 1,25-dihydroxyvitamin D3 during neonatal development. 1,25-Dihydroxyvitamin D3 binding activity increases dramatically between Days 15 and 28 postpartum in normal neonatal rats. Immunoprecipitation and combined immunoprecipitation-immunoblotting experiments demonstrate that this increase in hormone binding activity is caused by an increase in the amount of receptor protein present in rat intestine between Days 18 and 28 of age. Very little receptor protein, if any, appears to be present in rat intestine prior to Day 18 of life. These results suggest that the expression of the gene for 1,25-dihydroxyvitamin D3 receptor is regulated during neonatal development in the rat.

Effect of 1,25-dihydroxyvitamin D3 on interferon gamma production in vitro

In recent years, receptors for calcitriol (the active form of vitamin D3) have been identified in monocytes and activated, but not resting, human B and T lymphocytes suggesting that it may be involved in immune regulation. Because lymphokines are central in the regulation and modulation of immune or inflammatory responses and since the calcium translocation is involved in the mitogen-induced activation of lymphocytes, we thought it interesting to study the role of calcitriol on interferon gamma (IFN-gamma) production in vitro. In this study, we report that calcitriol inhibits the IFN-gamma production by staphylococcal enterotoxin A-stimulated peripheral blood mononuclear cells (PBMC) in a dose-dependent fashion. The inhibitory effect was less potent in calcium ionophore A23187-stimulated PBMC and was absent in resting PBMC.

Induction of monocytic differentiation by calcitriol (1,25-dihydroxyvitamin D3) in the human promyelocytic leukemic cell line (HL-60) in serum-free medium

The effect of calcitriol on the induction of differentiation in human promyelocytic leukemic cell line (HL-60) cultured in serum-free chemically defined medium (SFM) was investigated. The utilization of SFM containing RPMI-1640 basal medium supplemented with insulin (5 micrograms/ml), transferrin (5 micrograms/ml), sodium selenite (5 ng/ml), and bovine serum albumin (0.5 micrograms/ml), transferrin examination of the cellular/molecular mechanism of calcitriol's action in HL-60 cell differentiation without interference of components present in serum. HL-60 cells grown in SFM were induced to differentiate into monocytes/macrophages by calcitriol as indicated by induction of differentiation-associated biological and biochemical parameters: chemiluminescent (CL) responsiveness, lysozyme activity, nonspecific esterase, expression of cell surface antigens, and reduced proliferation. The exposure of HL-60 cells in SFM to calcitriol (from 10(-10) to 10(-8)M) resulted in dose-dependent induction of these parameters, which was similar to those obtained with cells grown in 10% fetal calf serum containing medium (10% SCM). However, calcitriol was 5-fold more potent for HL-60 cells cultured in SFM than those cultured in 10% SCM as indicated by shifts in dose-response curves for induction of CL responsiveness and lysozyme activity. The effect of calcitriol on the proliferation and acquisition of several monocyte-associated cell surface antigens was also more sensitive for HL-60 cells cultured in SFM than for cells grown in 10% SCM. We characterized and quantitated calcitriol receptors in HL-60 cells cultured in SFM in comparison to those in 10% SCM after exposing intact cells to radiolabeled calcitriol. Cells cultured in either SFM or 10% SCM exhibited calcitriol receptors that migrated at 3.4S as a single peak on sucrose gradients and elicited inherent DNA binding ability. There was essentially no difference in the apparent dissociation constants (Kd) nor in the number of calcitriol binding sites per HL-60 cell, that is approximately 6.0 X 10(-11) M and approximately 3000 binding sites/cell respectively. It is concluded that culturing HL-60 cells in SFM results in full expression of calcitriol-induced phenotypic changes excluding the possibility that such changes result from the indirect effect of calcitriol mediated by identified and/or unidentified components present in serum.

1 alpha,25-Dihydroxyvitamin D3 inhibits proliferative response of T- and B-lymphocytes in a serum-free culture

The contribution of 1 alpha,25-dihydroxyvitamin D3 to the proliferative response of human B- and T-lymphocytes was examined in a serum-free culture, in which B-cells were stimulated with Staphylococcus aureus Cowan I, and T-cells with phytohemagglutinin, respectively. 1 alpha, 25-dihydroxyvitamin D3 inhibited mitogen-induced B-cell proliferation at a dose of 10(-7) M (P less than 0.01). T-cell proliferation was inhibited at the lower dose range between 10(-9) M and 10(-7) M. Thus, although 1 alpha, 25-dihydroxyvitamin D3 acts directly on B-cells, it appears that, under physiological circumstance, 1 alpha, 25-dihydroxyvitamin D3 regulates human B-cell growth indirectly through the effect on T-cells.

Monoclonal antibodies to the porcine intestinal receptor for 1,25-dihydroxyvitamin D3: interaction with distinct receptor domains

Monoclonal antibodies to different domains of the porcine intestinal 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptor have been produced. A nuclear extract enriched in the 1,25-(OH)2D3 receptor was prepared from small intestinal mucosa of young pigs. The receptor was purified an additional 6600-fold by chromatography on DNA-cellulose, ammonium sulfate precipitation, gel filtration high-performance liquid chromatography, and DEAE-Sepharose chromatography, with an overall yield of 23% and an average purity of 24%. A BALB/c mouse immunized with this material developed serum polyclonal antibodies to the 1,25-(OH)2D3 receptor, as demonstrated by a change in sedimentation of the porcine receptor on sucrose gradients. Spleen cells from this animal were fused with mouse myeloma cells (P3-NSI/1-Ag4-1, SP2/0-Ag14), and 24 hybridomas secreting antibodies to the 1,25-(OH)2D3 receptor were identified by both a radiometric immunosorbent assay and an immunoprecipitation assay. Twenty-one hybridoma lines were cloned by limiting dilution and further characterized as subclass IgG1 antibodies with the exception of one which is an IgA. All but two of the antibodies cross-react with the 1,25-(OH)2D3 receptor from both mammalian (human, monkey, and rat) and avian (chicken) intestine; two antibodies recognize only porcine intestinal receptor. All antibodies are unreactive to the vitamin D serum transport protein. Eight of the antibodies bind denatured receptor on an immunoblot. A solid-phase competition assay was used to identify four groups of antibodies that bind to distinct epitopes on the 1,25-(OH)2D3 receptor. One antibody from each of the four groups was used to examine the effect of antibody binding on the DNA-binding activity of the receptor-hormone complex. One antibody completely inhibited the binding of the 1,25-(OH)2D3 receptor complex to DNA-cellulose, suggesting that the epitope for this antibody may be located in the polynucleotide binding domain of the protein. Antibodies from two additional groups only slightly perturbed DNA binding, while one had no effect, suggesting that these antibodies bind to receptor epitopes distant from the region of the polypeptide directly involved in polynucleotide binding. These antibodies that are directed to several different binding sites on the 1,25-(OH)2D3 receptor provide important new tools to probe the biochemistry and topology of the 1,25-(OH)2D3 receptor and to investigate its role in mediating target tissue response to hormone.

Effect of 1 alpha,25-dihydroxyvitamin D3 on the morphologic and biochemical differentiation of cultured human epidermal keratinocytes grown in serum-free conditions

The effect of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25-(OH)2-D3] on the proliferation and morphologic and biochemical differentiation of cultured human epidermal keratinocytes grown under defined, serum-free conditions was studied. 1 alpha,25-(OH)2-D3 caused a dose-dependent decrease in proliferation and an increase in the morphologic differentiation of human cultured keratinocytes. The number of attached basal cells decreased when exposed to 1 alpha,25-(OH)2-D3, whereas the number of attached squamous cells, terminally differentiated desquamated cells, and cornified cells increased concurrently. In addition, after incubation with 1 alpha,25-(OH)2-D3, there was a shift to cells of lighter density. In conjunction with its effect on the basal cells, 1 alpha,25-(OH)2-D3 resulted in an inhibition of DNA synthesis. The activity of transglutaminase, the enzyme responsible for cross-linking the proteins of the cornified envelope, was stimulated by 156% with 1 alpha,25-(OH)2-D3, but not with 1 beta,25-(OH)2-D3, a biologically inert isomer. Therefore it appears that 1 alpha,25-(OH)2-D3 is a potent inhibitor of keratinocyte proliferation as well as a stimulator of epidermal terminal differentiation.

Identification of the porcine intestinal 1,25-dihydroxyvitamin D3 receptor on sodium dodecyl sulfate/polyacrylamide gels by renaturation and immunoblotting

Identification of the porcine 1,25-dihydroxyvitamin D3 receptor protein on NaDodSO4/polyacrylamide slab gels was accomplished by two separate techniques: (i) assay of the specific binding activity of tritiated 1,25-dihydroxyvitamin D3 to protein eluted from NaDodSO4/polyacrylamide gels and renatured and (ii) immunoblotting of the partially purified receptor using two anti-receptor monoclonal antibodies. The porcine receptor preparation used in these studies was isolated from a crude nuclear extract of intestinal mucosa followed by chromatography on DNA-cellulose, ammonium sulfate precipitation, gel filtration HPLC, and DEAE-Sepharose chromatography. These receptor fractions were then electrophoresed on NaDodSO4/polyacrylamide gels. The receptor was eluted from the gel, renatured, and assayed for its ability to bind tritiated 1,25-dihydroxyvitamin D3. The renatured receptor appears as a single peak of specific tritiated 1,25-dihydroxyvitamin D3 binding activity. This binding activity corresponds to a band on a silver-stained gel that correlates with the receptor peak eluted from the DEAE-Sepharose column. It also corresponds to the highest molecular weight species identified on an immunoblot with anti-receptor monoclonal antibodies. The 1,25-dihydroxyvitamin D3 receptor protein has a molecular weight of 55,000 as deduced from its migration on NaDodSO4/polyacrylamide gels.

Monoclonal antibodies to human vitamin D-binding protein

Monoclonal antibodies to vitamin D-binding protein isolated from human serum have been produced. The antibodies obtained have been shown to be specific for human vitamin D-binding protein by three independent assays. The antibodies recognize human vitamin D-binding protein specifically in an enzyme-linked immunosorbent assay. Human vitamin D-binding protein is detected specifically in both pure and crude samples by a radiometric immunosorbent assay (RISA) and by an immunoprecipitation assay. The anti-human vitamin D-binding protein antibodies cross-react with monkey and pig vitamin D-binding protein, but not with vitamin D-binding protein from rat, mouse, or chicken, as determined by the RISA and immunoprecipitation assays.

Comparison of a cytosol radioreceptor assay with a radioimmunoassay for 1,25-dihydroxyvitamin D in serum or plasma

Human plasma and serum levels for 1 alpha,25-dihydroxyvitamin D were determined by a cytosol radioreceptor assay (RRA) and a radioimmunoassay (RIA). For both assays, 1.5 ml of human serum or plasma is used. Prior to RRA or RIA, extraction with benzene is performed followed by 'high-performance' liquid chromatography (HPLC) on a silica column (25 X 0.46 cm) with hexane/isopropanol (9/1 by vol), to isolate 1 alpha,25-dihydroxyvitamin D from the other vitamin D metabolites. The cytosol receptor was isolated from the intestine of healthy chickens. The antisera were raised in rabbits to 1 alpha,25-dihydroxyvitamin D3-3-hemisuccinate coupled to bovine serum albumin. The standard curves for RRA and RIA are prepared with 1 alpha,25-dihydroxyvitamin D3. 1 alpha,25-dihydroxy[3H]vitamin D3 of high spec act (158 kCi/mol) is used as tracer. The reactants are incubated for 16 h at 4 degrees C. Then, bound and free ligand are separated after the addition of dextran-coated charcoal. Both assays have a sensitivity of 2 pg/tube. The cytosol receptor and the antibodies have about the same absolute affinity for 1 alpha,25-dihydroxyvitamin D3 but the cytosol receptor has a higher relative affinity for 1 alpha,25-dihydroxyvitamin D3 (compared with other vitamin D metabolites). Reproducibility and precision are better for the RIA. The between- and within-assay CVs are 16.0% (mean = 58.7 ng/l, n = 16) and 11.2% (mean = 52.1 ng/l, n = 15), respectively, for RRA and 12.6% (mean = 61.8 ng/l, n = 27) and 7.4% (mean = 61.8 ng/l, n = 15), respectively using RIA. Reference values obtained by both assays on healthy males and healthy premenopausal females are the same for both sexes; 53.9 +/- 31.0 ng/l (n = 46) using RRA and 51.8 +/- 30.2 ng/l (n = 91) for RIA (mean +/- 2 SD).

1H nuclear magnetic resonance studies of the conformations of vitamin D compounds in various solvents

Using proton NMR, the solution conformation of the A ring of vitamin D3 and its analogs has been studied by application of the Karplus relation to the observed coupling constants. The A-ring conformations of vitamins D3, D2, and 25-hydroxyvitamin D3 were found to be solvent dependent, with a clear preference for an equatorial hydroxyl group in polar solvents such as methanol and dimethyl sulfoxide. Conversion of the hydroxyl group to an acetate did not affect solution conformation appreciably, but the corresponding t-butyl-dimethylsilyl ether derivative of vitamin D3 showed a strong preference for the 3 beta-equatorial conformer. The A-ring conformation of the active hormone, 1,25-dihydroxyvitamin D3, which has two hydroxyl groups competing for the equatorial position, was found not to be solvent-dependent.

"Differential displacement"--a simple method to study receptors for 1,25-dihydroxyvitamin D3 in the presence of contaminating serum vitamin D binding protein

This report describes a simple procedure to quantitate receptors for 1,25-dihydroxyvitamin D3 in the presence of contaminating serum vitamin D binding protein. The method ("differential displacement") takes advantage of the greatly different rates of dissociation of 1,25-dihydroxyvitamin D3 from the serum vitamin D binding protein (t 1/2 less than 5 min) and from the receptor (t 1/2 greater than 120 h) at 0 degrees C. The principle of "differential displacement" can be used for other steroid receptors as well, and in combination with a variety of different binding assays, provided they are performed under conditions where the dissociation of the steroid from the receptor is slow.

Detection and developmental changes of the 1,25-(OH)2-D3 receptor concentration in mouse skin and intestine

We investigated [3H]1,25-dihydroxyvitamin D3-specific binding activity in fetal, neonatal, and adult mouse skin to determine (a) during which stage in development the skin develops the capacity to respond to this hormone and (b) whether the hormone binding activity changed during development and maturation. A macromolecule with properties similar to those of the chick intestinal 1,25-dihydroxyvitamin D3 receptor was detected in the skin and intestine of mouse pups at 17 days of fetal life. 1,25-Dihydroxyvitamin D3-specific binding activity from both tissues sedimented on linear sucrose gradients at 3.5-3.7S and eluted from DNA cellulose at 0.22 M KCl. At earlier stages of fetal life (12-14 days) receptor-like activity was detected in cytosols prepared from whole-mouse fetuses. 1,25-Dihydroxyvitamin D3-specific binding activity was quantitated in the skin and intestine throughout development using a chromatin binding assay. Scatchard analysis of saturation binding data showed that the concentration of binding activity in skin increased rapidly after birth and reached a maximum when the mice were 10-19 days old. By contrast, the binding activity that was detected in the fetal and neonatal whole intestine remained low until the mice were weaned. The affinity (Kd) of the binding activity was similar in skin and intestine at all ages studied. It is concluded that 1,25-dihydroxyvitamin D3-specific binding activity appears in both skin and intestine of the mouse prior to birth and increases in these two tissues during different stages in development.

1 alpha, 25-dihydroxyvitamin D3 specific regulation of growth, morphology, and fibronectin in a human osteosarcoma cell line

The ability of the hormonally active vitamin D metabolite, 1 alpha, 25-dihydroxyvitamin D3, to affect cell growth, morphology and fibronectin production has been examined using the MG-63 human osteosarcoma cell line. Hormone treatment reduced cell growth rate, saturation density and [3H]thymidine incorporation. Inhibition was specific for 1 alpha, 25-dihydroxyvitamin D3 relative to other vitamin D metabolites (1 alpha, 25-dihydroxyvitamin D3 greater than 25-dihydroxyvitamin D3 greater than 24R,25-dihydroxyvitamin D3 greater than D3), antagonized by high concentrations of serum and readily reversed by removal of 1 alpha, 25-dihydroxyvitamin D3 from the culture medium. Hormone treatment also increased cell associated alkaline phosphatase activity up to twofold and altered morphology such that treated cells were more spread out on the culture dish and contained more cytoplasmic processes. Significantly, 1 alpha, 25-dihydroxyvitamin D3 increased cellular and medium concentrations of fibronectin, a glycoprotein known to be involved in cellular adhesiveness. MG-63 cells contain a specific 1 alpha, 25-dihydroxyvitamin D3 receptor which may mediate these responses.

The appearance of 1,25-dihydroxyvitamin D3 receptor during chick embryo development

The appearance of the 1,25-dihydroxyvitamin D3 receptor in intestine, kidney, and chorioallantoic membrane of chick embryo was followed by sucrose density gradient sedimentation analysis and Scatchard plot analysis. The receptor from each of these organs sediments as a single 3.7S component. At 19 days of embryonic life, intestine had the highest specific 1,25-dihydroxyvitamin D3 binding activity followed by kidney and chorioallantoic membrane. The 1,25-dihydroxyvitamin D3 binding activity increased gradually at 12-15 days and rapidly until 20 days in intestine. In kidney, this protein increased rapidly from 12 to 16 days and did not change subsequently. In chorioallantoic membrane, the receptor increased slowly from 8 through 15 days, rapidly until 19 days, and decreased at 20 days. The injection of hydrocortisone into the chick embryo at 10 days increased receptor number in intestine, kidney, and chorioallantoic membrane by a factor of 2 at 12 days. Injection of this hormone after this time had little or no effect.

Properties and compartmentalization of the testicular receptor for 1,25-dihydroxyvitamin D3

Adult rat testis contains a specific, high-affinity, low-capacity binding protein for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) with properties similar to 1,25-(OH)2D3 receptors in other tissues. The receptor sediments at 3.5 +/- 0.2 S20,w in high-salt sucrose density gradients, but aggregates in low-salt gradients. Binding of 1,25-(OH)2D3 was abolished by trypsin, but not by DNase or RNase. Binding was also heavily reduced by the sulfhydryl alkylating agent, N-ethylmaleimide, and by the mercurial reagent, mersalyl, showing that free, reduced SH-groups are necessary for hormone-binding activity. The receptor shows high affinity for 1,25-(OH)2D3 (Kd = 3 X 10(-11) M), but low capacity (Nmax = 8 fmol/mg protein) and is specific for 1,25-(OH)2D3 (Affinity: 1,25-(OH)2D3 greater than 1,24(R),25-(OH)3D3 greater than 25-OH-D3 greater than 1 alpha-OH-D3 greater than 24(R),25-(OH)2D3 much greater than 17 beta-estradiol, testosterone, dexamethasone, R5020, progesterone). With 0.6 nM [3H]1,25-(OH)2D3 and at 0 degrees C, maximum specific binding was achieved after 4 h, and the occupied receptors were stable for more than 24 h. The dissociation of hormone-receptor complexes was temperature-dependent and very slow at low temperature (t1/2 (0 degrees C) much greater than 48 h). At 0 degrees C, the second order association rate constant and the pseudo-first order dissociation rate constant were 2.7 X 10(7) M-1 min-1 and 2 X 10(-5) min-1, respectively. Receptors for 1,25-(OH)2D3 are present in similar amounts in isolated seminiferous tubules and interstitial tissue of adult rats. No specific binding of [3H]1,25-(OH)2D3 could be detected in cultured immature Sertoli cells, cultured immature peritubular (myoid) cells or crude germ cells.

Effect of vitamin D and its metabolites on developing myogenic cultures

Growth, protein synthesis and expression of creatine kinase (CK) by embryonic chick myogenic cells are inhibited by vitamin D and certain of its metabolites. 25-OH cholecalciferol was most active in concentrations of 10(-5) - 10(-6) M, with cholecalciferol and ergocalciferol less active in that order. Ergosterol had no activity of this sort. Inhibition of CK was most marked on the 4th and 5th day of culture and was due to suppression of the appearance of CK-NM and MB. CK-BB was not affected and CK-MB was more affected than CK-BB. Skin fibroblasts by comparison were slightly stimulated in growth at 10(-6) M and much less affected at 10(-5) M than the myogenic cells. It is suggested that vitamin D has a direct effect upon the muscle cell, to cause a selective diminution in the production of certain polypeptides.

1 alpha,25-dihydroxyvitamin D3 suppresses proliferation and immunoglobulin production by normal human peripheral blood mononuclear cells

Activated B and T lymphocytes from normal human subjects are known to have the specific high-affinity receptor for 1,25-dihydroxyvitamin D3 (1,25-(OH)2-D3). In an attempt to determine a functional role for the sterol in such cells, we studied the effect of 1,25-(OH)2-D3 on DNA synthesis and Ig production by normal human peripheral blood mononuclear (PBM) cells activated in vitro by the polyclonal lymphocyte activators pokeweed mitogen and phytohemagglutinin, and the specific antigen dermatophyton O. A dose-dependent inhibition of [3H]thymidine incorporation was observed in cells incubated with 1,25-(OH)2-D3 in concentrations ranging from 10(-10) to 10(-7) M. Production of IgG and IgM, determined by enzyme-linked immunosorbent assay, was similarly inhibited by increasing concentrations of 1,25-(OH)2-D3. Half-maximal inhibition of DNA and Ig synthesis was found at 10(-10) to 10(-9) M 1,25-(OH)2-D3. This suppressive effect was specific for 1,25-(OH)2-D3; of the other vitamin D metabolites examined, only 10(-7) M 24R,25 dihydroxyvitamin D3 (24,25-(OH)2-D3) had a similar inhibitory effect. 1,25-(OH)2-D3 was not cytotoxic and did not affect unactivated PBMs. These data demonstrate that 1,25-(OH)2-D3 is a potent inhibitor of human PBM Ig production in vitro and suggest that this action is mediated through the hormone's antiproliferative effect on Ig-producing B cells and/or helper T cells.

Inhibitory action of aurintricarboxylic acid and rifamycin AF/013 at the polynucleotide domain of 1,25-dihydroxyvitamin D3-receptor complexes

The binding of 1,25-dihydroxyvitamin D3-receptor complexes from chicken intestine to DNA-cellulose and isolated intestinal nuclei is inhibited in a dose-dependent manner by aurintricarboxylic acid and rifamycin AF/013. Since both nuclear- and cytoplasmic-associated receptors have been identified, some experiments were carried out on both populations of receptors. Concentrations resulting in 50% displacement of cytoplasmic receptor complexes were 3.2 X 10(-6) M and 1.2 X 10(-4) M for aurintricarboxylic acid and rifamycin AF/013 respectively. Moreover, rifamycin AF/013 was approximately nine times more potent at inhibiting nuclear receptor binding to DNA-cellulose compared to cytoplasmic receptors. Contrary to these findings, rifampicin, which does not inhibit eukaryotic RNA or DNA polymerases, did not cause a loss of receptor complex binding to DNA-cellulose at the doses tested. Neither aurintricarboxylic acid, rifampicin, nor rifamycin AF/013 resulted in any significant loss of sterol binding. Inhibition of receptor binding to DNA-cellulose by these polymerase inhibitors was not due to alteration of the DNA and was reversed by dialysis. Incubation of receptor complexes with aurintricarboxylic acid or rifamycin AF/013 inhibited binding to Cibacron blue-agarose and phosphocellulose. Furthermore, these polymerase inhibitors were utilized specifically to desorb receptor complexes from Cibacron blue-agarose columns. Sucrose density gradient analysis of inhibitor treated and untreated receptor revealed that rifamycin AF/013 treatment resulted in the appearance of a broadened 3.7 S sedimenting receptor in addition to specific bound 1,25-dihydroxyvitamin D3 in the 6.0 S region and in the pellet of the gradient.

Specific internalization of 1,25-dihydroxyvitamin D3 by cultured intestinal epithelial cells

A rapid and sensitive method for evaluating the specific internalization of [3H]1,25-(OH)2-D3 by intact, cultured intestinal epithelial (IEC) cells was developed. The uptake of hormone was determined by incubation of suspensions of IEC cells in medium containing [3H]1,25-(OH)2-D3 and 1% fetal bovine serum. Saturation of the high affinity intracellular receptor with 1,25-(OH)2-D3 was reflected in an analysis of the saturable uptake of [3H]1,25-(OH)2-D3; the apparent dissociation constant for labelled hormone was 0.64 nM which is in the range of that reported for hormone binding to receptor extracted from known target tissues. The uptake of [3H]1,25-(OH)2-D3 was dependent on both the cellular content of receptor and the availability of unbound hormone in the extracellular medium. The latter phenomenon was demonstrated in competitive internalization studies with 25-OH-D3 and 24,25-(OH)2-D3. At extracellular concentrations of 25-OH-D3 and 24,25-(OH)2-D3 ranging from 10(-8) to 10(-7) M there was competitive inhibition of [3H]1,25-(OH)2-D3, uptake by IEC cells. However, when cells were exposed to a higher concentration (10(-6) M) of these metabolites, label uptake paradoxically increased owing to probable saturation of the sterol binding sites on the vitamin D binding protein present in the serum-supplemented incubation medium.

Specific binding of 1,25 dihydroxyvitamin D3 in lymphocytes

We examined ten cellular or tissue sources of lymphocytes for specific binding of 1,25(OH)2D3, the hormonally active form of vitamin D3. A specific-binding protein was found in three of these sources. Scatchard analysis of cytosol from a follicular lymphoma cell line revealed binding sites with a Kd of 7.0 X 10(-11) and a receptor concentration of 6.6 fmol/mg protein. Sucrose density centrifugation of 3H-1,25(OH)2D3 labeled cytosol showed a 3.75 peak which was absent in cytosols incubated with excess nonradioactive 1,25(OH)2D3. The relative amounts of vitamin D3 metabolites required to displace 50% of the specifically bound 3H-1,25(OH)2D3 were 1,25(OH)2D3: 1,24,25(OH)3D3: 25(OH)D3: 24,25(OH)2D3 = 1: 180: 1000: 2700. Excess vitamin D3, cortisol, and estradiol failed to displace 3H-1,25(OH)2D3. Scatchard analysis of spleen cytosol from a patient with prolymphocytic transformation of chronic lymphocytic leukemia demonstrated a binding protein with a Kd of 1.2 X 10(-10) and a receptor concentration of 0.2 fmol/mg protein. DNA cellulose binding confirmed the presence of the specific-binding protein in this cytosol. Specific binding of 3H-1,25(OH)2D3 was also quantitated in a cell line from a patient with Burkitt's lymphoma with a Kd of 0.3 X 10(-10) and a receptor concentration of 29.6 fmol/mg protein. No specific binding of 3H-1,25(OH)2D3 was observed in lymphocytes from seven other malignant and nonmalignant sources. These results are the first to demonstrate a specific-binding protein for 1,25(OH)2D3 in lymphocytes from tissue and from these specific cell lines. The presence of this protein in some lymphocytes but not others may reflect the state of activation of the lymphocytes.

Effects of 1 alpha,25-dihydroxyvitamin D3 and glucocorticoids on the growth of rat and mouse osteoblast-like bone cells

The effects of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its interaction with glucocorticoids to regulate bone cell growth were studied in osteoblast-like (OH) cell cultures. Owing to our earlier findings that species difference and cell density at the time of treatment modified hormonal responses, comparisons were made between rat and mouse cells and sparse and dense cultures. 1,25(OH)2D3 inhibited cell proliferation in both species regardless of cell density. The magnitude of inhibition was larger in mouse cells, but the sensitivity to 1,25(OH)2D3 was the same for both species. Other metabolites, 25(OH)D3 and 24R,25(OH)2D3, were greater than 100-fold less potent than 1,25(OH)2D3 even in serum-free medium, which is similar to their ratio of affinity for the 1,25(OH)2D3 receptor. Dexamethasone, as previously shown, inhibited sparse and dense mouse cell cultures and sparse rat cell cultures while stimulating dense rat cell cultures to grow. The inhibitory actions of 1,25(OH)2D3 were not additive to the inhibitory dexamethasone effects. However, 1,25(OH)2D3 addition resulted in attenuation of the stimulatory effect of dexamethasone. These responses to 1,25(OH)2D3 and dexamethasone were dependent on cell density and not selective attachment of certain cell types at either plating density. In conclusion, the findings demonstrated that 1,25(OH)2D3 exerts an inhibiting action on both mouse and rat bone cell proliferation. This effect must be reconciled with the in vivo beneficial actions of 1,25(OH)2D3 on bone metabolism. Also, the likelihood of decreased cell number must be considered when biochemical activities are assessed after vitamin D treatment in vitro.

Possible significance of new target tissues for 1,25-dihydroxyvitamin D3

A sensitive sucrose gradient procedure provided evidence for specific 3.6S 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] receptor-like binding components with low affinity for 25(OH)D3 in several reproductive target tissues in the rat, including testis, uterus and probably epididymis. Thus, the presence of 1,25(OH)2D3 receptors in non-vitamin D endocrine tissues is not restricted to the hormone secreting glands. Estrogen-stimulated uterine growth paralleled increased levels of the putative 1,25(OH)2D3 receptors, providing in vivo evidence for the concept of a relationship between these phenomena. However, a similar correlation was not observed in the growing testis (prepubertal vs mature rats). Whether this result stems from receptor localization in a cell type prevalent in the adult or a true dissociation between testis growth and receptor levels is unclear. Finally, significant levels (10.3 +/- 1.0% vs intestinal mucosa) of the putative 1,25(OH)2D3 receptors were found in the heart, an organ in which calcium mediates many specialized functions. Taken collectively, these observations on new target tissues of quite different overall function suggest that 1,25(OH)2D3 and its receptors may play a role in intracellular calcium homeostasis and possibly in regulating specialized intracellular functions of calcium.

Purification of the receptor for 1 alpha,25-dihydroxyvitamin D3 from chicken intestine

Methods were investigated for use in the purification of the chicken intestinal receptor for 1 alpha,25-dihydroxyvitamin D3. The techniques investigated include column isoelectric focusing, gel exclusion, polyacrylamide gel electrophoresis, and DNA-cellulose, DEAE-cellulose, and hydroxylapatite chromatography. For the starting receptor preparation, a nuclear extract of chicken intestinal mucosa was found to be enriched above cytosol preparations and a plentiful source of receptor. A five-step purification scheme that resulted in the purification of the receptor protein by 5800-fold with 8% yield has been described. Analysis of the purified proteins on polyacrylamide gel electrophoresis containing sodium dodecyl sulfate suggests homogeneity. Analysis using two-dimensional polyacrylamide electrophoresis characterized the purified protein as having a molecular weight of approximately 63 000 and a pI of 6.0-6.2. Furthermore, assessment of protein purity by 125I iodination followed by sucrose gradient analysis revealed that approximately 90% of the iodinated macromolecules have the same sedimentation coefficient as the titrated 1 alpha,25-dihydroxyvitamin D3 receptor complex. The final purified receptor that bound tritiated 1 alpha,25-dihydroxyvitamin D3 retained affinity for DNA-cellulose and possesses a 3.7S sedimentation coefficient. The receptor has an estimated Stokes radius of 37 A.

Quantitation of endogenously occupied and unoccupied binding sites for 1,25-dihydroxyvitamin D3 in rat intestine

The quantitative reversible dissociation of the 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]-receptor complex by the mercurial reagent mersalyl was used to develop an assay for endogenously occupied and unoccupied 1,25-(OH)2D3 binding sites. Incubation of intestinal cytosol preparations in buffer containing 50 mM Tris . HCl, 300 mM KCl, and 1.5 mM EDTA, pH 7.4, with 1 mM mersalyl for 60 min was effective in inhibiting 98% of 1,25-(OH)2D3 specific binding activity. Dissociation of bound 1,25-(OH)2[26,27-3H]D3 from the hormone-receptor complex approached completion by 180 min. In cytosol incubated with saturating levels of nonradioactive hormone, 96% of total binding activity was measurable with the hormone binding assay after displacement of bound nonradioactive ligand with 1 mM mersalyl. Endogenously occupied 1,25-(OH)2D3 binding sites contributed 0, 9, and 19% of total binding activity in rats with plasma 1,25-(OH)2D3 levels averaging 2, 121 +/- 36 and 516 +/- 92 pg/ml, respectively. Therefore, the major fraction of cytosolic 1,25-(OH)2D3 specific binding activity is unoccupied in rat intestine. The results suggest that only a small proportion of the measurable receptors are in the bound form to provide maximal 1,25-(OH)2D3-induced calcium transport.

1- but not 24-hydroxylation of vitamin D is required for growth and reproduction in rats

This study examines whether 1,25-dihydroxyvitamin D3 or 24,24-difluoro-25-hydroxyvitamin D3, an analogue of 25-hydroxyvitamin D3 blocked from undergoing 24-hydroxylation, can maintain normal growth and reproduction in the female rat. Vitamin D-deficient weanling rats were maintained from weaning through mating, pregnancy, and lactation with either 1,25-dihydroxyvitamin D3 (given by continuous subcutaneous infusion), 24,24-difluoro-25-hydroxyvitamin D3, 25-hydroxyvitamin D3, or vehicle. Body weight, plasma calcium levels, estrous cycling time, ability to give birth to live pups, litter weight, number of pups per litter, dam plasma calcium level during lactation, and pup growth to 9 wk of age were recorded. No striking differences were observed between the 25-hydroxyvitamin D3 groups and either the 1,25-dihydroxyvitamin D3 group or the 24,24-difluoro-25-hydroxyvitamin D3 group. However, significant differences in most parameters were observed between the vitamin D-deficient and metabolite- or analogue-dosed rats. The results demonstrate that 1,25-dihydroxyvitamin D3 and/or one of its metabolites is sufficient to maintain normal growth, development, and reproductive functions in the female rat. Because 24,24-difluoro-25-hydroxyvitamin D3 cannot be hydroxylated at C-24, the 24-hydroxylation of 25-hydroxyvitamin D3 is not essential for normal growth, development, and reproduction in the female rat.