Receptor for 1,25-dihydroxyvitamin D in a vascular smooth muscle cell line derived from rat aorta

The presence of a specific receptor for 1,25-dihydroxyvitamin D (1,25(OH)2D) was investigated in a cell line A7r5 derived from fetal aorta. 1,25(OH)2[3H]D3 binding to cytosol was saturated at 0.6-1 nM, and Scatchard analysis yielded dissociation constant and binding sites, (3.02 +/- 0.4) X 10(-11) M and 33.9 +/- 5.8 fmol/mg protein, respectively. Sucrose density gradient analysis revealed the sedimentation constant 3.2 S. Furthermore, the receptor protein had affinity for DNA-cellulose column and eluted with 0.2 M KCl. These data suggest that vascular smooth muscle cell may be a target tissue of vitamin D.     

DNA binding property of vitamin D3 receptors associated with 26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3

Using [3H]-26,26,26,27,27,27-hexafluoro-1,25-dihydroxyvitamin D3 (F6-1,25-(OH)2D3), we have examined its ability to bind to the 1,25-(OH)2D3 receptor, and the ability of the resulting complex to bind DNA. The binding sites for [3H]F6-1,25-(OH)2D3 in the chick intestinal receptor represented a limited number of saturable sites for which 1,25-(OH)2D3 competes. 1,25-Dihydroxyvitamin D3 is three times more active than F6-1,25-(OH)2D3 in displacing [3H]F6-1,25-(OH)2D3. By affinity chromatography using DNA-Sephadex, the [3H]F6-1,25-(OH)2D3 receptor complex eluted from the column in a single peak at 0.14 M KCl, while [3H]-1,25-(OH)2D3 receptor complex eluted at 0.13 M KCl. These results indicate that F6-1,25-(OH)2D3 and 1,25-(OH)2D3 recognize the same binding site of the receptor and that the F6-1,25-(OH)2D3 receptor complex binds DNA more tightly than the 1,25-(OH)2D3 receptor complex. We suggest that the higher binding affinity for DNA may contribute to the greater biological activity of F6-1,25-(OH)2D3.     

Characterization of the vitamin D receptor from the Caco-2 human colon carcinoma cell line: effect of cellular differentiation.

The human colon carcinoma cell line, Caco-2, is the only intestinal cell line to spontaneously differentiate in culture to a population exhibiting structural and biochemical characteristics of mature enterocytes. We conducted studies to establish the presence of the vitamin D receptor (VDR), determine changes in VDR concentration and affinity with differentiation and determine whether 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) mediates a functional response in this cell line. We found that Caco-2 cells possess a specific 1,25(OH)2D3 binding protein similar to the mammalian VDR. It has an equilibrium dissociation constant (Kd) of 0.72 nM, binds vitamin D analogues in order of their biological activities in vivo (1,25(OH)2D3 greater than 25(OH)D3 greater than 24,25(OH)2D3), sediments as a single peak on sucrose density gradients at 3.7 S, and is eluted from a DNA-cellulose column by 0.16 M KCl. The maximum number of binding sites was 2.6-fold greater in the differentiated cell (Day 15) compared to the preconfluent, undifferentiated (Day 4) cell (23 fmol/mg protein vs 56 fmol/mg protein). Cell growth was reduced 59% when exposed to 10(-7) M 1,25(OH)2D3 for 8 days. Alkaline phosphatase activity significantly increased in cultures incubated with 10(-8) M 1,25(OH)2D3 for up to 4 days when treatment was started in both undifferentiated cells (Day 5) and differentiated cells (Day 11). These findings suggest that the VDR present in undifferentiated and differentiated Caco-2 cells is functional. Caco-2 cells provide a unique in vitro model to study vitamin D-regulated functions in differentiated mammalian enterocytes.     

Receptors for 1,25-dihydroxyvitamin D3 in chick pancreas: a partial physical and functional characterization

1,25-Dihydroxyvitamin D3(1,25-(OH)2D3) receptors from the rachitic chick pancreas have been partially characterized. Analyses of these receptors by isokinetic gradient centrifugation and analytical gel filtration reveal a sedimentation coefficient (S) of 3.3-3.7, a molecular weight (Mr) of 58,500-68,000, and a calculated Stokes molecular radius (Rs) of 34-36 A. Polyethylenimine-ammonium sulfate precipitation of pancreatic cytosol partially purifies aporeceptor and reduces nonspecific binding (in part, 5.8S DBP), thus providing material more amenable to kinetic analyses, Binding studies incorporating this fractionated cytosol reveal an equilibrium dissociation constant (K4) of approximately 0.112 nM at 2 degrees C for the 1,25-(OH)2D3-receptor interaction. Competition studies further demonstrate a particular preference for 1,25-(OH)2D3 over 1,24(R),25-trihydroxyvitamin D3, 24(R),25-dihydroxyvitamin C3, and 25-hydroxyvitamin D3. The pancreatic receptor also binds to immobilized group-selective affinity ligands such as DNA, cibacron blue, and heparin, and can be eluted as a single macromolecular species during standard linear KCl gradients. Its interaction with these ligands supports the premise that the 1,25-(OH)2D3 receptors' fundamental mode of action is at the level of the cellular genome. Salt-dependent nuclear uptake and chromatin localization studies with this receptor in vitro also support this potential site of action. Significantly, a physiologic dose of 1,25-(OH)2[3H]D3 to rachitic chicks leads to the in vivo formation of a receptor-hormone complex as identified by DNA-cellulose chromatography. These observations provide further evidence that the pancreatic protein is a biologically relevant component of the chick pancreas which functions to accumulate hormone intracellularly under physiologic situations.     

Induction of specific proteins in cultured skeletal muscle cells by 1,25-dihydroxyvitamin D-3

The presence in myoblasts of an intracellular receptor specific for 1,25-dihydroxyvitamin D-3 [1,25(OH)2D3) and 1,25(OH)2D3-dependent changes in myoblast Ca2+ transport and phospholipid metabolism which are suppressed by RNA and protein synthesis inhibitors have been shown. In agreement with these observations, incubation of chick embryo myoblasts, precultured for 24 h in a medium containing low levels of vitamin D-3 metabolites, with 1,25(OH)2D3 at conditions which induce maximum cell responses (10(-10) M, 24 h) markedly stimulated the incorporation of [3H]leucine into total cell proteins and this effect was abolished when sterol treatment was performed in the presence of cycloheximide or puromycin. To investigate whether 1,25(OH)2D3 selectively stimulates the de novo synthesis of muscle cell proteins, mixtures of myoblast proteins from control and sterol-treated cultures labelled with [14C]leucine and [3H]leucine, respectively, were separated by SDS-polyacrylamide gel electrophoresis and isoelectric focussing. Examination of 3H/14C ratios in gel fractions revealed that 1,25-(OH)2D3 stimulates the production of proteins of molecular masses (isoelectric points) of 9 kDa (4.1 and 8.5), 17 kDa (7.5), 30 kDa (7.2), 40 kDa (5.5), 55 kDa (4.5) and 100 kDa (8.6). Cell fractionation studies showed the following subcellular distribution: 9 kDa (85% cytosol, 15% microsomes); 17 and 100 kDa (100%, 1200 X g pellet); 30 kDa (65% cytosol, 35% mitochondria); 40 kDa (100% microsomes); 55 kDa (65% microsomes, 35% mitochondria). Marker enzyme data indicated that this distribution is not due to cross-contamination between fractions. Affinity chromatography of double-labelled myoblast proteins on an immobilized lectin showed that the 55 kDa protein contains carbohydrate. Labelling of myoblast proteins with 45CaCl2 after their separation on SDS-polyacrylamide gels showed in addition that the 1,25(OH)2D3-dependent proteins of 9, 17, 40 and 100 kDa are major Ca2+-binding components of the cells. Synthesis of these proteins may mediate the effects of the sterol on myoblast calcium metabolism.     

Human colon cell line HT-29: characterisation of 1,25-dihydroxyvitamin D3 receptor and induction of differentiation by the hormone

The human colon carcinoma cell line HT-29 differentiates into functional enterocytes upon replacement of glucose by galactose in the culture medium. Since the differentiation of other types of cells is associated with the modulation of 1,25-dihydroxycholecalciferol (1,25(OH)2D3) receptor concentrations and since enterocytes are classical target cells for 1,25(OH)2D3 we have examined the HT-29 cells to determine whether the differentiated and undifferentiated stages could be directly linked to the presence of 1,25(OH)2D3 receptors. HT-29 cells were grown in Dulbecco's modified medium containing 10% fetal calf serum (FCS) and glucose or galactose. Cell differentiation was assessed by measuring the brush border hydrolase, maltase. 1,25(OH)2D3 receptors were studied in the cells after 48 h without FCS. Nuclear uptake was measured in intact dispersed cells and the receptor protein was further characterized by vitamin D metabolite binding specificity, sucrose density gradient analysis and binding to DNA-cellulose. Maltase activity was 5-fold greater in differentiated HT-29 cells than in undifferentiated cells. Scatchard analysis showed a highly specific saturable (9500 sites per cell) high affinity (2 x 10(-10) M), binding of 1,25(OH)2D3 in undifferentiated cells. This receptor-like protein sedimented at 3.3S, bound to and eluted from DNA-cellulose and had all the characteristics of a 1,25(OH)2D3 receptor. No specific binding was detected in differentiated HT-29 cells. The presence of 1,25(OH)2D3 receptors in undifferentiated HT-29 cells implies that these cells are targets for vitamin D. The maltase activity increased significantly when undifferentiated cells were exposed to 1,25(OH)2D3 for 5-6 days, indicating that the hormone can promote differentiation of HT-29 cells. These results demonstrate that HT-29 cells can provide a new model for studying steroid receptor regulation and cell differentiation.     

Identification of 1,25-dihydroxyvitamin D3 receptors and activities in muscle

Cytosols from cultured myoblast cells (G-8 and H9c2) prepared in high salt (0.3 M KCl) possesses receptor like proteins for 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) that sediment in the 3.2 S region of sucrose gradients. These receptors were characterized as having high affinity (Kd less than 0.1 nM) for 1,25-(OH)2D3 and are in low capacity (less than 80 fmol/mg of cytosol protein). Analog competition for receptor binding revealed that 1,25-(OH)2D3 was more potent than 24,25-(OH)2D3, or 25-(OH)2D3 for displacement of 1,25-(OH)2[3H]D3 from these 3.2 S region sedimenting receptors. Furthermore, the receptor proteins had affinity for DNA and eluted from Sephacryl S-200 as a macromolecule with Stokes radius (Rs) of 32 A. High salt cytosol from collagenase-dispersed skeletal muscle cells was also found to possess a 3.2 S 1,25-(OH)2D3 receptor-like protein. The 1,25-(OH)2D3 receptor concentration in both G-8 and H9c2 myoblast lines was found to down-regulate by 50-70% when cells were stimulated to differentiate to myotubes by lowering fetal calf serum to 5% of the medium. Moreover, we demonstrated that 1,25-(OH)2D3 can inhibit DNA synthesis and cell proliferation of the G-8 myoblast cells in a dose-dependent manner. 1,25-(OH)2D3 was more potent at inhibiting cell proliferation in cells grown in 5% serum than in 20% serum. The data suggest that 1,25-(OH)2D3 can act directly on muscle myoblast via a 1,25-(OH)2D3 receptor that is similar to those found in intestine and bone. The data support the possibility that muscle is a target tissue for 1,25-(OH)2D3 and the hormone may act to initiate terminal differentiation of myoblast cells.     

Self-induction of 1,25-dihydroxyvitamin D3 metabolism limits receptor occupancy and target tissue responsiveness

Whole cell 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) receptor (VDR) binding assays, which measure VDR in the presence of the metabolic machinery of the cell, were used in conjunction with a cytosol binding assay for VDR to determine if self-induced metabolism of 1,25-(OH)2D3 limits VDR occupancy, total VDR levels, and target cell responsiveness. Treatment of cells with 0.5 nM 1,25-(OH)2[3H]D3 for 16 h results in up-regulation of total cell VDR from 82 to 170 fmol/mg protein as measured in a cytosol binding assay. Conversely, whole cell binding assays of VDR showed a 1,25-(OH)2D3-mediated apparent down-regulation of VDR from 90 to 40 fmol/mg protein. Scatchard analysis using the cytosol binding assay demonstrated that 1,25-(OH)2D3 treatment increased total cell VDR from 93 to 154 fmol/mg protein. In contrast, Scatchard analysis with the whole cell binding assay demonstrated that 1,25-(OH)2D3 treatment resulted in reduction in total cell VDR from 100 to 64 fmol/mg protein. Initial Kd estimates with the whole cell binding assay suggested that 1,25-(OH)2D3 treatment resulted in a reduction in VDR Kd from 0.6 to 6.2 nM. This apparent reduction in the affinity of VDR for 1,25-(OH)2D3 was due to degradation of free 1,25-(OH)2[3H]D3 which occurred during whole cell saturation assay. Competitive inhibitors of 1,25-(OH)2D3 metabolism were found to reverse the apparent receptor down-regulation observed in whole cell binding assays of treated cells. In addition, the presence of competitive inhibitors amplified responses of cells to 1,25-(OH)2[3H]D3 treatment as measured by an increased occupancy of VDR by 1,25-(OH)2[3H]D3 and increased up-regulation of VDR over that observed without metabolism inhibitors. These data demonstrate that self-induced target tissue deactivation of 1,25-(OH)2D3 regulates 1,25-(OH)2D3 occupancy of VDR and ultimately the biopotency of 1,25-(OH)2D3 in target cells.     

1,25-Dihydroxyvitamin D3 regulation of phorbol ester receptors in HL-60 leukemia cells

In this study the relationship between cell binding of phorbol 12,13-dibutyrate (PDBu) and induction of differentiation by 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) was examined. Binding of [3H]PDBu increased within 12 h of 1,25-(OH)2D3 treatment, and a 60-130% increase in [3H]PDBu receptor levels was observed within 24 h. By 48 h, however, [3H]PDBu binding was not different from control. Scatchard analysis of [3H]PDBu binding showed no statistical differences in Kd value (Kd approximately equal to 30 nM) between 1,25-(OH)2D3-treated and control cells 22 h post-treatment; however, a 2-fold increase in Bmax was observed in treated (338 +/- 24 pmol/10(9) cells) compared to control cultures (170 +/- 14 pmol/10(9) cells). Stimulation of [3H]PDBu binding was dependent on 1,25-(OH)2D3 concentrations over a range of 1-100 nM. Homogenates from 1,25-(OH)2D3-treated HL-60 cells also demonstrated an increase (70%) in [3H]PDBu binding to the Ca2+/phospholipid-dependent enzyme protein kinase C as assessed by incubation of cell homogenates with [3H]PDBu in the presence of saturating phosphatidylserine and calcium concentrations. This suggests that the increase in [3H]PDBu binding cannot be entirely explained by modulation of the latter two agents. Cycloheximide (5 microM), an inhibitor of protein synthesis, ablated the 1,25-(OH)2D3-stimulated increase in [3H]PDBu binding to intact HL-60 cells. These data demonstrate that an increase in [3H]PDBu binding occurs early in the course of 1,25-(OH)2D3-induced differentiation, results from an increased number of [3H]PDBu-binding site, and is dependent on protein synthesis.     

A receptor-like binding macromolecule for 1 alpha, 25-dihydroxycholecalciferol in cultured mouse bone cells.

1alpha, 25-Dihydroxycholecalciferol (1,25-(OH)2D3), the active form of vitamin D, like other steroid hormones, initiates its action by binding to cytoplasmic receptors in target cells. Although the 1,25-(OH)2D3 receptor has been well studied in intestine, little information beyond sucrose gradient analyses is presently available from mammalian bone. We, therefore, employed primary cultures of mouse calvarial cells to characterize the mammalian receptor in bone. A hypertonic molybdate-containing buffer was found to protect receptor binding. On hypertonic sucrose gradients, the 1,25-(OH)2-[3H]D3 binder sedimented at 3.2 S. Scatchard analysis of specific 1,25-(OH)2[3H]D3 binding sites at 0 degrees C yielded an apparent Kd of 0.26 nM and an Nmax of 75 fmol/mg of cytosol protein. Competitive binding experiments revealed the receptor to prefer 1,25-(OH)2D3 greater than 25-(OH)-D3 = 1 alpha-(OH)-D3 greater than 24R,25-(OH)2D3; vitamin D3, dihydrotachysterol, sex steroids, and glucocorticoids exhibited negligible binding. As shown in other systems, the receptor could be distinguished from a 25-(OH)-[3H]D3 binder which sedimented at approximately 6 S. In summary, cultured mouse calvarial cells possess a macromolecule with receptor-like properties. This system appears to be an ideal model for the investigation of 1,25-(OH)2D3 receptor binding and action in mammalian bone.     

Salt-induced activation of 1,25-dihydroxyvitamin D3 receptors to a DNA binding form

In this report we examine the DNA-cellulose binding and sedimentation properties of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptors from rat intestine and cultured human mammary cancer cells (MCF-7) extracted in nonactivating (low salt) buffers. Receptors prepared in hypotonic buffer had low DNA binding (13%) compared to receptors extracted with 0.3 M KCl (50%). Treatment of low salt receptor preparations with KCl significantly increased (approximately 3-fold) DNA-binding (activation), demonstrating that receptors can be "activated" in vitro. Activated receptors eluted from DNA-cellulose at 0.18 M KCl. Sedimentation analysis followed by DNA-cellulose binding indicated that activated receptors are approximately 3.2 S and unactivated receptors 5.5 S in size. These results suggest that dissociation of an aggregated moiety may lead to receptor activation. Treatment of unactivated receptor with RNase did not alter DNA binding or sedimentation properties of the aggregated receptor. Treatment of unactivated receptor complexes with heat did not increase DNA binding, and molybdate did not block subsequent salt activation. In summary these results suggest that 1,25(OH)2D3 receptors undergo a salt-induced activation step similar to that described for other steroid receptor systems. However, 1,25(OH)2D3 receptors differ from other steroid receptors in not exhibiting heat activation nor having salt activation blocked by molybdate.     

Specific binding of 24R,25-dihydroxyvitamin D3 to chick intestinal mucosa: 24R,25-dihydroxyvitamin D3 is an allosteric effector of 1,25-dihydroxyvitamin D3 binding

We have previously described a significant decrease in the positive cooperativity level and affinity of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] binding to its chick intestinal chromatin receptor induced in vitro by a physiological 10-fold molar excess of (24R)-25-dihydroxyvitamin D3 [24R,25(OH)2D3] [F. Wilhelm and A. W. Norman (1985) Biochem. Biophys. Res. Commun. 126, 496-501]. In this report, we have initiated a comparative study of the binding of 24R,25(OH)2[3H]D3 and 1,25(OH)2[3H]D3 to the the intestinal chromatin fraction obtained from vitamin D-replete birds. 24R,25(OH)2[3H]D3 specific binding to this chromatin fraction was characterized by a dissociation constant (Kd) of 34.0 +/- 6.4 nM, a positive cooperativity level (nH) of 1.40 +/- 0.13, and a capacity (Bmax) of 47 +/- 8 fmol/mg protein. The very low relative competitive index (RCI) of 24R,25(OH)2D3 (0.11 +/- 0.03%) for the 1,25(OH)2D3 binding site/receptor, as well as the inability of 1,25(OH)2D3 to displace 24R,25(OH)2D3 from its binding site at a physiological molar ratio of 1:10, strongly suggest the independence of 24R,25(OH)2D3 and 1,25(OH)2D3 binding sites. Stereospecificity of the 24R,25(OH)2D3 binding sites was attested by the displacement of only 45 +/- 6% of 24R,25(OH)2D3 specific binding by equimolar concentrations of 24S,25(OH)2D3. Collectively these results suggest the existence of a binding domain/receptor for 24,25(OH)2D3 in the chick intestine which is independent of the 1,25(OH)2D3 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.     

1,25-Dihydroxyvitamin D3-induced differentiation in a human promyelocytic leukemia cell line (HL-60): receptor-mediated maturation to macrophage-like cells

The human-derived promyelocytic leukemia cell line, HL-60, is known to differentiate into mature myeloid cells in the presence of 1,25-dihydroxyvitamin D3 (1,25[OH]2D3). We investigated differentiation by monitoring 1,25(OH)2D3-exposed HL-60 cells for phagocytic activity, ability to reduce nitroblue tetrazolium, binding of the chemotaxin N-formyl-methionyl-leucyl-[3H]phenylalanine, development of nonspecific acid esterase activity, and morphological maturation of Wright-Giemsa-stained cells. 1,25(OH)2D3 concentrations as low as 10(-10) M caused significant development of phagocytosis, nitroblue tetrazolium reduction, and the emergence of differentiated myeloid cells that had morphological characteristics of both metamyelocytes and monocytes. These cells were conclusively identified as monocytes/macrophages based upon their adherence to the plastic flasks and their content of the macrophage-characteristic nonspecific acid esterase enzyme. The estimated ED50 for 1,25(OH)2D3-induced differentiation based upon nitroblue tetrazolium reduction and N-formyl-methionyl-leucyl-[3H]phenylalanine binding was 5.7 X 10(-9) M. HL-60 cells exhibited a complex growth response with various levels of 1,25(OH)2D3: less than or equal to 10(-10) M had no detectable effect, 10(-9) M stimulated growth, and greater than or equal to 10(-8) M sharply inhibited proliferation. We also detected and quantitated the specific receptor for 1,25(OH)2D3 in HL-60 and HL-60 Blast, a sub-clone resistant to the growth and differentiation effects of 1,25(OH)2D3. The receptor in both lines was characterized as a DNA-binding protein that migrated at 3.3S on high-salt sucrose gradients. Unequivocal identification was provided by selective dissociation of the 1,25(OH)2D3-receptor complex with the mercurial reagent, p-chloromercuribenzenesulfonic acid, and by a shift in its sedimentation position upon complexing with anti-receptor monoclonal antibody. On the basis of labeling of whole cells with 1,25(OH)2[3H]D3 in culture, we found that HL-60 contains approximately 4,000 1,25(OH)2D3 receptor molecules per cell, while the nonresponsive HL-60 Blast is endowed with approximately 8% of that number. The concentration of 1,25(OH)2D3 (5 X 10(-9) M) in complete culture medium, which facilitates the saturation of receptors in HL-60 cells, is virtually identical to the ED50 for the sterol's induction of differentiation. This correspondence, plus the resistance of the relatively receptor-poor HL-60 Blast, indicates that 1,25(OH)2D3-induced differentiation of HL-60 cells to monocytes/macrophages is occurring via receptor-mediated events.     

24R,25-dihydroxyvitamin D3 regulates 1,25-dihydroxyvitamin D3 binding to its chick intestinal receptor

We have studied the binding of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] to its crude chromatin chick intestinal receptor in the absence or presence of a ten-fold excess of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] for each concentration of [3H]-1,25(OH)2D3 studied. We have found a significant shift to the right in the binding of 1,25(OH)2D3 to its receptor in the presence of this excess of 24R,25(OH)2D3. As a result, the affinity was found to be significantly reduced, the apparent dissociation constants varied from 0.97 +/- 0.09 (n = 5) to 1.36 +/- 0.04 nM (p less than 0.01). This reduction was related to a significant decrease in the positive cooperativity for the apparent Hill coefficient from nH = 1.49 +/- 0.06 to nH = 1.26 +/- 0.06 (p less than 0.03) in the binding of 1,25(OH)2D3 to its receptor. There was no significant change in the capacity of the receptor (189 +/- 11 compared to 200 +/- 9 fmoles/mg protein). These results suggest that the intestinal 1,25(OH)2D3 receptor must also have a binding recognition site for 24R,25(OH)2D3 which is postulated to play a regulatory role in the 1,25(OH)2D3 receptor's ligand binding properties.     

A specific binding protein for 1 alpha,25-dihydroxyvitamin D in the chick embryo chorioallantoic membrane

A 3.7 S binding protein for the steroid hormone and vitamin D metabolite 1 alpha-25-dihydroxyvitamin D (1,25-(OH)2-D) was observed in high salt cytosol extracts of chick embryo chorioallantoic membrane. The binding protein was characterized after partial purification of cytosol extracts by ammonium sulfate fractionation. The binding of 1,25-(OH)2-D was saturable, had a high affinity (Kd = 0.16 nM), and was specific for hormonally active vitamin D metabolites. Analysis of the displacement of [3H]1,25-(OH)2-D by unlabeled analogues showed the affinities of vitamin D metabolites to be in the order of 1,25-(OH)2-D = 1,24R,25-(OH)3-D much greater than 25-OH-D = 1-OH-D greater than 24R,25-(OH)2-D. Hormone binding was sensitive to pretreatment with sulfhydryl-blocking reagents. The chorioallantoic membrane 1,25-(OH)2-D-binding protein associated with the chromatin fraction after homogenization of membranes in low salt buffer, and bound to DNA-cellulose columns, eluting as a single peak at 0.215 M KCl. These findings support identification of this 1,25-(OH)2-D-binding protein as a steroid hormone receptor, with properties indistinguishable from 1,25-(OH)2-D receptors in other chick tissues. The chorioallantoic membrane functions in the last third of embryonic development to reabsorb calcium from the eff shell for deposition in embryonic bone. 1,25-(OH)2-D binding activity in the chorioallantoic membrane increased 4- to 5-fold from day 12 to day 16 of incubation, immediately preceding the onset of shell reabsorption. This finding suggests that 1,25-(OH)2-D may act to regulate shell mobilization and transepithelial calcium transport by the chorioallantoic membrane. Finally, the similarity of shell mobilization to bone resorption, which is also stimulated by 1,25-(OH)2-D, suggests that the chorioallantoic membrane is a useful alternate model for the study of 1,25-(OH)2-D action on bone mineral metabolism.     

Biochemical evidence for 1, 25-dihydroxyvitamin D receptor macromolecules in parathyroid,pancreatic, pituitary,and placental tissues

DNA-cellulose chromatography has been recently employed in our laboratory as an extremely effective and sensitive technique with which to identify macromolecules which specifically bind 1,25-dihydroxyvitamin D (1,25-(OH)₂D). Chromatography of cytosols prepared from rachitic chick intestine, parathyroid gland, pancreas, pituitary gland, and normal rat placenta all demonstrate binding components for 1,25-(OH)₂D which interact with the DNA affinity ligand under low salt conditions (< 0.15M), and can be eluted as a single macromolecular peak during a linear KCl gradient between 0.25–0.30M. Further, sucrose gradient analysis of these DNA-cellulose purified components under high salt conditions (0.3M KCl) indicates a common sedimentation coefficient of 3.3S. Since the receptor properties of this macromolecule in rachitic chick intestine have been previously characterized, it seems likely that these components in the parathyroid, pituitary, pancreas, and placenta represent typical receptors for 1,25-(OH)₂D.     

Dibutyryl cAMP enhances the effect of 1,25-dihydroxyvitamin D3 on a human promyelocytic leukemia cell, HL-60, at both the receptor and the postreceptor steps.

1,25-Dihydroxyvitamin D3 (1,25-(OH)2D3) induces differentiation of a human promyelocytic leukemia cell line, HL-60, into monocytes/macrophages, and 25-hydroxyvitamin D3- and 1,25-(OH)2D3-24-hydroxylase activities in HL-60 mitochondria via a steroid-hormone receptor mechanism. Dibutyryl cyclic adenosine monophosphate (dbcAMP), a granulocyte inducer, significantly augmented the differentiation-inducing effect of 1,25-(OH)2D3 along the monocyte/macrophage pathway. Furthermore, dbcAMP significantly potentiated the effect of 1,25-(OH)2D3 on HL-60 cells to hydroxylate 1,25-(OH)2[26,27-3H]D3 to form 1,24,25-(OH)3[26,27-3H]D3. DbcAMP seemed to augment the effect of 1,25-(OH)2D3 in part through upregulation of the 1,25-(OH)2D3 receptor, because 10(-7) M dbcAMP increased 1,25-(OH)2D3 receptor levels approximately 2.3-fold, which was similar to a 1.9-fold augmentation by the same concentrations of dbcAMP of 1,25-(OH)2D3-induced cell characteristics to hydroxylate C-24 of 1,25-(OH)2[26,27-3H]D3. However, dbcAMP is also known to enhance HL-60 cell differentiation caused by other differentiation inducers. We have established another HL-60 clone which acquires resistance to 1,25-(OH)2D3 in the induction of cell differentiation by a defect at the postreceptor step, as reflected by resistance to other differentiation inducers, such as retinoic acid and dimethyl sulfoxide. Even in this resistant clone, dbcAMP significantly enhanced the differentiation-inducing effect of 1,25-(OH)2D3. Of interest, this clone showed resistance to dbcAMP in the induction of cell differentiation. Furthermore, we have demonstrated that intracellular cAMP levels were significantly lower in uremic serum-treated cells than in cells treated with normal human serum and that a significant positive correlation was found between intracellular cAMP levels and 1,25-(OH)2D3-induced cell differentiation. These data indicated that the intracellular cAMP level is one of the major determinants of 1,25-(OH)2D3-induced HL-60 cell differentiation and that dbcAMP could enhance the effects of 1,25-(OH)2D3 on HL-60 cells not only by increasing 1,25-(OH)2D3 receptor levels but also at the postreceptor step.     

Immunochemical studies on the putative plasmalemmal receptor for 1, 25(OH)(2)D(3). I. Chick intestine

Antisera were raised against the NH(2)-terminus of the putative basal lateral membrane (BLM) receptor for 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3); BLM-VDR]. In Western analyses of BLM proteins, antibody (Ab) 099 was monospecific for a 64.5-kDa band. A protein of 64.5 kDa was also labeled by the affinity ligand [(14)C]1, 25(OH)(2)D(3)-bromoacetate; label was diminished in the presence of excess unlabeled secosteroid. The monoclonal antibody against the nuclear VDR (9A7) failed to detect an appropriate band in BLM fractions. Preincubation of isolated intestinal cells with Ab 099, but not 9A7, affected the following two 1,25(OH)(2)D(3)-mediated signal transduction events: augmented intracellular calcium and protein kinase C activity. Subcellular distribution of Ab 099 reactivity by Western analyses and fluorescence microscopy revealed the highest concentrations in BLM followed by the endoplasmic reticulum. Exposure of isolated intestinal cells to 1,25(OH)(2)D(3) for 10 s or vascular perfusion of duodena for 5 min resulted in a time-dependent increase in nuclear localization of the BLM-VDR antigen, as judged by electron microscopy, whereas 24, 25-dihydroxyvitamin D(3) failed to increase antigenic labeling in nuclei. Densitometric quantitation of Western blots of subcellular fractions prepared from isolated intestinal cells treated with vehicle or 1,25(OH)(2)D(3) confirmed a hormone-induced increase of putative BLM-VDR in the nucleus. It is concluded that a novel cell surface binding protein for 1,25(OH)(2)D(3) has been identified.     

Biological activity assessment of the 26,23-lactones of 1,25-dihydroxyvitamin D3 and 25-hydroxyvitamin D3 and their binding properties to chick intestinal receptor and plasma vitamin D binding protein

The binding of the natural and unnatural diastereoisomers 25-hydroxyvitamin D3-26,23-lactone and 1,25 dihydroxyvitamin D3-26,23-lactone to the vitamin D-binding protein (DBP) and 1,25 dihydroxyvitamin D3 [1,25(OH)2D3] chick intestinal receptor have been investigated. Also, the biological activities, under in vivo conditions, of these compounds, in terms of intestinal calcium absorption (ICA) and bone calcium mobilization (BCM), in the chick are reported. The presence of the lactone ring in the C23-C26 position of the seco-steroid side chain increased two to three times the ability of both 25(OH)D3 and 1,25(OH)2D3 to displace 25(OH)[3H]D3 from the D-binding protein; however, the DBP could not distinguish between the various diastereoisomers. In contrast, the unnatural form (23R,25S) of the 25-hydroxy-lactone was found to be 10-fold more potent than the natural form, and the unnatural (23R,25S)1,25(OH)2D3-26,23-lactone three times more potent than the natural 1,25-dihydroxy-lactone in displacing 1,25(OH)2[3H]D3 from its intestinal receptor. While studying the biological activity of these lactone compounds, it was found that the natural form of the 25-hydroxy-lactone increased the intestinal calcium absorption 48 h after injection (16.25 nmol), while bone calcium mobilization was decreased by the same dose of the 25-hydroxy-lactone. The 1,25-dihydroxyvitamin D3-26,23-lactone in both its natural and unnatural forms was found to be active in stimulating ICA and BCM. These results suggest that the 25-hydroxy-lactone has some biological activity in the chick and that 1,25(OH)2D3-26,23-lactone can mediate ICA and BCM biological responses, probably through an interaction with 1,25-(OH)2D3 specific receptors in these target tissues.