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

Photoactivable analogs for labeling 25-hydroxyvitamin D3 serum binding protein and for 1,25-dihydroxyvitamin D3 intestinal receptor protein

3-Azidobenzoates and 3-azidonitrobenzoates of 25-hydroxyvitamin D3 as well as 3-deoxy-3-azido-25-hydroxyvitamin D3 and 3-deoxy-3-azido-1,25-dihydroxyvitamin D3 were prepared as photoaffinity labels for vitamin D serum binding protein and 1,25-dihydroxyvitamin D3 intestinal receptor protein. The compounds prepared were easily activated by short- or long-wavelength uv light, as monitored by uv and ir spectrometry. The efficacy of the compounds to compete with 25-hydroxyvitamin D3 or 1,25-dihydroxyvitamin D3 for the binding site of serum binding protein and receptor, respectively, was studied to evaluate the vitamin D label with the highest affinity for the protein. The presence of an azidobenzoate or azidonitrobenzoate substituent at the C-3 position of 25-OH-D3 significantly decreased (10(4)- to 10(6)-fold) the binding activity. However, the labels containing the azido substituent attached directly to the vitamin D skeleton at the C-3 position showed a high affinity, only 20- to 150-fold lower than that of the parent compounds with their respective proteins. Therefore, 3-deoxy-3-azidovitamins present potential ligands for photolabeling of vitamin D proteins and for studying the structures of the protein active sites.

Mechanisms and functions of vitamin D

The vitamin D hormone, 1,25-dihydroxyvitamin D3, functions by way of a nuclear receptor (vitamin D receptor [VDR]) in a manner analogous to the other members of the steroid-thyroid hormone superfamily. Although the vitamin D receptor has been cloned, its three-dimensional structure remains unknown. The VDR binds to the direct repeat response elements called DR-3 in the promoter region of target genes to stimulate or suppress transcription of those genes encoding for proteins that carry out a wide variety of functions. The binding of the VDR to a DR-3 requires the presence of its ligand and a companion protein, namely the RXR group of retinoid receptors. The RXR binds to the 5' arm of the response element while the VDR binds to the 3' arm. In addition, the transcription factor TFIIB has been shown to bind VDR but there is currently no evidence that a co-repressor or co-activator of VDR is also involved. Phosphorylation of VDR in the transcription complex occurs as does bending of the DNA prior to the initiation or suppression of transcription. As VDR has been detected in cells not previously thought to be target organs, scientists continue to discover new functions of vitamin D. Among these new functions are those noted in the immune system. Experiments in mice have illustrated that the autoimmune diseases of multiple sclerosis and rheumatoid arthritis can be successfully treated with the vitamin D hormone and its analogs. New experiments illustrating the use of the vitamin D hormone and its analogs in suppressing transplant rejection indicate that these compounds may be superior to cyclosporin and may not have the side effects attributed to the cyclosporin immunosuppression therapies.

Osteoporosis and the metabolites of vitamin D

Vitamin D is metabolized by sequential steps in the liver and kidney to its active form, a process that is strongly feedback-regulated. In old age, the activity of the enzyme, 25-hydroxyvitamin D 1 hydroxylase, which produces the vitamin D hormone, is diminished. The activities of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) extend beyond increasing intestinal absorption of calcium. The vitamin D hormone (or its analogs) is useful in the treatment of osteoporosis because it not only stimulates intestinal calcium absorption, but also is required for the stimulation of osteoblasts and many other cells in the body. Recent work demonstrates that it is possible to chemically synthesize analogs selective for specific actions of the vitamin D hormone, especially in inducing differentiation of promyelocytes and keratinocytes.

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)).

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.

Cleavage of the rat intestinal 1,25-dihydroxyvitamin D3 receptor by an endogenous protease to a form with defective DNA binding

In this report we describe a form of the 1,25(OH)2D3 receptor which no longer binds to DNA. The defective form of the receptor was produced by the action of an endogenous protease. Rat intestinal receptors, obtained by a two-step procedure of a low salt homogenization followed by extraction of the chromatin pellet with high salt, fail to bind to DNA-cellulose. Inclusion of various serine protease inhibitors during the preparation protects against the loss of DNA binding. Sedimentation analysis in sucrose gradients indicates that the defective receptor is measurably smaller than the native receptor and is unable to aggregate normally under low salt conditions. The size difference, as determined by gel chromatography, is approximately 9,000 Da (56,000 for the protected receptor, 47,000 for the cleaved form). The elution from DEAE-cellulose indicates that the overall charge of both intact and cleaved receptor forms is very similar. Cell fractionation and mixing experiments suggest the enzyme may be located in the lysosomal compartment, organelles which are susceptible to breakage during the extraction procedure. The results demonstrate that an endogenous enzyme preferentially cleaves the 1,25(OH)2D3 DNA binding site resulting in a receptor with altered characteristics. Such an enzymatic activity has not been previously described for the 1,25(OH)2D3 receptor from other tissues or species. Since rat intestine is a classically studied target organ, these findings have additional relevance in receptor purification or other studies to characterize the receptor.

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.

The relationship between the vitamin D system and cancer

The classic function of 1,25-dihydroxyvitamin D3, the hormonally active form of vitamin D, is the maintenance of normal levels of calcium and phosphorus in the blood. 1,25-Dihydroxyvitamin D3 binds to a specific receptor protein and exerts its biologic action by a mechanism analogous to that proposed for other steroid hormones, that is, the receptor-ligand complex acts on the chromatin to induce transcription of specific genes. Intracellular receptors that bind 1,25-dihydroxyvitamin D3 with high affinity have been found in a large number of tumor cell lines examined as melanoma, osteosarcoma, and human breast and colonic carcinoma cells. The 1,25-dihydroxyvitamin D3 receptor in these cells has characteristics similar to the receptor in bone and intestine, the known target tissues of the hormone. In fact, 1,25-dihydroxyvitamin D3 inhibits the proliferation of melanoma, osteosarcoma, and breast carcinoma cells. More recently, 1,25-dihydroxyvitamin D3 has been shown to suppress the growth and induce monocytic differentiation of murine and human myeloid leukemia cells in vitro. These results point to a previously unsuspected involvement of vitamin D in cell proliferation and differentiation and suggest that analogs of the vitamin D hormone may be of interest as possible therapeutic agents in the treatment of malignancy.

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.

Subcellular distribution of DNA-binding and non-DNA-binding 1,25-dihydroxyvitamin D receptors in chicken intestine

A comparison of 1,25-dihydroxyvitamin D3 receptor concentration and equilibrium dissociation constants of whole tissue, nuclear, and cytosol extracts of vitamin D-deficient chicken intestine has been carried out. The extracts had the following order of receptor concentration: nuclei greater than whole tissue greater than cytosol. The receptors in each preparation had identical equilibrium dissociation constants (Kd) for 1,25-dihydroxyvitamin D3. However, the receptor in the cytosol fraction did not bind to DNA-cellulose, while a large fraction of the receptor population in crude nuclear extract and whole tissue extract bound to DNA-cellulose and could be eluted with 0.22 M KCl, suggesting that the cytosolic form of the receptor does not possess a DNA binding site.