Biochemical properties of 1 alpha, 25-dihydroxyvitamin D receptors

Nuclear receptors and their selective pharmacologic modulators

Nuclear receptors are ligand-activated transcription factors and include the receptors for steroid hormones, lipophilic vitamins, sterols, and bile acids. These receptors serve as targets for development of myriad drugs that target a range of disorders. Classically defined ligands that bind to the ligand-binding domain of nuclear receptors, whether they are endogenous or synthetic, either activate receptor activity (agonists) or block activation (antagonists) and due to the ability to alter activity of the receptors are often termed receptor "modulators." The complex pharmacology of nuclear receptors has provided a class of ligands distinct from these simple modulators where ligands display agonist/partial agonist/antagonist function in a tissue or gene selective manner. This class of ligands is defined as selective modulators. Here, we review the development and pharmacology of a range of selective nuclear receptor modulators.

1,25-dihydroxyvitamin D3 and development of tuberculosis in cattle

This report describes the presence and activity of 1,25-dihydroxyvitamin D3 (1,25-D3) in experimental bovine tuberculosis. Animals that went on to develop tuberculous lesions exhibited a rapid transient increase in serum 1,25-D3 within the first 2 weeks following infection with Mycobacterium bovis. 1,25-D3-positive mononuclear cells were later identified in all tuberculous granulomas by immunohistochemical staining of postmortem lymph node tissue. These results suggest a role for 1,25-D3 both at the onset of infection and in the development of the granuloma in these infected animals. Using a monoclonal antibody to the vitamin D receptor (VDR) as a VDR agonist, we confirmed that activation of the vitamin D pathway profoundly depresses antigen-specific, but not mitogenic, bovine peripheral blood T-cell responses (proliferation and gamma interferon production). Investigation of the mechanism of this suppression showed that the VDR antibody modified the expression of CD80 by accessory cells, such that a significant positive correlation between T-cell proliferation and accessory cell CD80 emerged.

Enzymatic studies on the key enzymes of vitamin D metabolism; 1 alpha-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24)

The key enzymes of vitamin D3 metabolism, renal 25-hydroxyvitamin D3 1 alpha-hydroxylase (CYP27B1) and 24-hydroxylase (CYP24) were expressed in Escherichia coli, and their enzymatic properties were revealed. As expected, mouse CYP27B1 and human CYP27B1 showed the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 with the Michaelis constant, Km, value of 2.7 microM. Unexpectedly, both mouse CYP27B1 and human CYP27B1 showed greater Vmax/Km values toward 24,25-dihydroxyvitamin D3 than 25-hydroxyvitamin D3, suggesting that 24, 25-dihydroxyvitamin D3 is a better substrate than 25-hydroxyvitamin D3 for both CYP27B1. Enzymatic studies on substrate specificity of CYP27B1 revealed that 25-hydroxyl group of vitamin D3 was essential for the 1 alpha-hydroxylase activity, and 24-hydroxyl group enhanced the activity, but, 23-hydroxyl group greatly reduced the activity. On rat CYP24, it was demonstrated that CYP24 catalyzed four-step monooxygenation towards 25-hydroxyvitamin D3. Furthermore, in vivo and in vitro metabolic studies on 1 alpha,25-dihydroxyvitamin D3 clearly indicated that CYP24 catalyzed six-step monooxygenation to convert 1 alpha,25-dihydroxyvitamin D3 into calcitroic acid which is known as a final metabolite of 1 alpha,25-dihydroxyvitamin D3 for excretion in bile. These results strongly suggest that CYP24 is highly responsible for the metabolism of both 25-hydroxyvitamin D3 and 1 alpha,25-dihydroxyvitamin D3. In addition, we have succeeded in the construction of mitochondrial P450 electron transport chain consisting of ADR, ADX and each of CYP27B1 and CYP24 in E. coli cells. The coexpression system with CYP27B1 might be useful as a bioreactor to produce 1 alpha,25-dihydroxyvitamin D3. In contrast, the coexpression system with CYP24 would be applied to metabolic studies of vitamin D analogs used as drugs.

Enzymatic properties of human 25-hydroxyvitamin D3 1alpha-hydroxylase coexpression with adrenodoxin and NADPH-adrenodoxin reductase in Escherichia coli

We have cloned human 25-hydroxyvitamin D3 1alpha-hydroxylase cDNAs from normal subjects and patients with pseudovitamin D-deficient rickets (PDDR), and expressed the cDNAs in Escherichia coli JM109 cells. Kinetic analysis of normal 1alpha-hydroxylase in the reconstituted system revealed that Km values for 25(OH)D3 and (24R), 25(OH)2D3 were 2.7 and 1.1 microM, respectively. The lower Km value and higher Vmax/Km value for (24R),25(OH)2D3 indicated that it is a better substrate than 25(OH)D3 for 1alpha-hydroxylase. These results are quite similar to those of mouse 1alpha-hydroxylase. To establish a highly sensitive in vivo system, 1alpha-hydroxylase, adrenodoxin and NADPH-adrenodoxin reductase were coexpressed in E. coli cells. The recombinant E. coli cells showed remarkably high 1alpha-hydroxylase activity, suggesting that the electrons were efficiently transferred from NADPH-adrenodoxin reductase through adrenodoxin to 1alpha-hydroxylase in E. coli cells. Using this system, the activities of four mutants of 1alpha-hydroxylase, R107H, G125E, R335P and P382S, derived from patients with PDDR were examined. Although no significant reduction in expression of these mutants was observed, none showed detectable activity. These results strongly suggest that the mutations found in the patients with PDDR completely abolished 1alpha-hydroxylase activity by replacement of one amino acid residue.

Enzymatic properties of mouse 25-hydroxyvitamin D3 1 alpha-hydroxylase expressed in Escherichia coli

Renal 25-hydroxyvitamin D3 1 alpha-hydroxylase cDNA cloned from the kidneys of mice lacking the vitamin D receptor was expressed in Escherichia coli JM109. As expected, the bacterially-expressed enzyme catalyzes the 1 alpha-hydroxylation of 25-hydroxyvitamin D3 with a Michaelis constant, K(m), value of 2.7 microM. Unexpectedly, the enzyme also hydroxylates the 1 alpha-position of 24,25-dihydroxyvitamin D3 with a K(m) of 1.3 microM, and a fourfold higher Vmax/K(m) compared with the 25-hydroxyvitamin D3 hydroxylase activity, suggesting that 24,25-dihydroxyvitamin D3 is a better substrate than 25-hydroxyvitamin D3 for 1 alpha-hydroxylase. In addition, the enzyme showed 1 alpha-hydroxylase activity toward 24-oxo-25-hydroxyvitamin D3. However, it showed only slight activity towards 23,25-dihydroxyvitamin D3 and 24-oxo-23,25-dihydroxyvitamin D3, and no detectable activity towards vitamin D3 and 24,25,26,27-tetranor-23-hydroxyvitamin D3. These results suggest that the 25-hydroxyl group of vitamin D3 is essential for the 1 alpha-hydroxylase activity and the 24-hydroxyl group enhances the activity, but the 23-hydroxyl group greatly reduced the activity. Another remarkable finding is that living recombinant E. coli cells can convert the substrates into the 1 alpha-hydroxylated products, suggesting the presence of a redox partner of 1 alpha-hydroxylase in E. coli cells.

Homeostatic control of plasma calcium concentration

Due to the importance of Ca2+ in the regulation of vital cellular and tissue functions, the concentration of Ca2+ in body fluids is closely guarded by an efficient feedback control system. This system includes Ca(2+)-transporting subsystems (bone, and kidney), Ca2+ sensing, possibly by a calcium-sensing receptor, and calcium-regulating hormones (parathyroid hormone [PTH], calcitonin [CT], and 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]). In humans and birds, acute Ca2+ perturbations are handled mainly by modulation of kidney Ca2+ reabsorption and by bone Ca2+ flow under PTH and possibly CT regulation, respectively. Chronic perturbations are also handled by the more sluggish but economic regulatory action of 1,25(OH2)D3 on intestinal calcium absorption. Peptide hormone secretion is modulated by Ca2+ and several secretagogues. The hormones' signal is produced by interaction with their respective receptors, which evokes the cAMP and phospholipase C-IP3-Ca2+ signal transduction pathways. 1,25 (OH)2D3 operates through a cytoplasmic receptor in controlling transcription and through a membrane receptor that activates the Ca2+ and phospholipase C messenger system. The calciotropic hormones also influence processes not directly associated with Ca2+ regulation, such as cell differentiation, and may thus affect the calcium-regulating subsystems also indirectly.

Effect of 24R,25-dihydroxyvitamin D3 on the formation and function of osteoclastic cells

Previous reports demonstrated that the administration of large doses of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] to animals with normal vitamin D supply causes an increase in bone volume with reduced bone resorption and decreased osteoclast number. The present study was undertaken to clarify if 24R,25(OH)2D3 has any inhibitor effect on the formation and function of osteoclasts. The effect of 24R,25(OH)2D3 on the formation of osteoclastic cells was examined by measuring the number of tartrate-resistant acid phosphatase-positive multinucleated cells (MNCs) formed from hemopoietic progenitor cells obtained from spleens of 5-fluorouracil-treated mice. Treatment with 1,25(OH)2D3 or parathyroid hormone fragment 1-34 [PTH(1-34)] stimulated osteoclast-like MNC formation in a dose-dependent manner. Addition of 24R,25(OH)2D3 alone showed a weak stimulatory effect on MNC formation at 10(-6) M, which appeared to be due to its binding to 1,25(OH)2D3 receptors. In contrast, when 24R,25(OH)2D3 was added together with 1,25(OH)2D3 or PTH(1-34), it inhibited osteoclast-like MNC formation stimulated by these hormones. A significant inhibition of MNC formation was observed with 10(-7) M 24R,25(OH)2D3, and the stimulatory effect of 1,25(OH)2D3 or PTH(1-34) was almost completely eliminated with 10(-6) M 24R,25(OH)2D3. Neither 24S,25(OH)2D3 nor 25(OH)D3 exhibited a similar inhibitory effect. The effect of 24R,25(OH)2D3 on the resorptive function of osteoclasts was examined by measuring the formation of resorption pits by mouse bone cells on dentine slices. Treatment with 24R,25(OH)2D3 also inhibited the resorption pit formation stimulated by 1,25(OH)2D3 or PTH(1-34) with similar dose response.(ABSTRACT TRUNCATED AT 250 WORDS)

Endogenous blockade of 1,25-dihydroxyvitamin D-receptor binding in New World primate cells

When assessed by 1,25-dihydroxyvitamin D3 (1,25(OH)2-D3)-receptor (VDR) binding analysis or 1,25(OH)2-D3-VDR-directed bioresponsiveness, cultured cells from some New World primates (platyrrhines) demonstrate a variable decrement in VDR when compared with Old World primate (catarrhine) cells. To study this difference in VDR expression among primates, we performed immunoblot analysis of the VDR in cultured dermal fibroblasts from platyrrhines in the genera Pithecia and Aotus and from catarrhines in the genus Presbytis; although a platyrrhine, the owl monkey (Aotus) expresses a VDR of the catarrhine (wild type) phenotype. Despite a 10-fold difference in the content of VDR by ligand binding analysis among cells from the three prototypic primate genera, there was a less than or equal to 10% difference in the steady-state level of 50-kD VDR detected by immunoblot analysis of cellular extracts. We investigated this apparent discrepancy in the content of VDR in immunoblots and ligand binding analyses by mixing VDR-containing nuclear extracts of equivalent protein concentration from the various primates. Coincubation of Pithecia and Aotus fibroblast extracts with Presbytis extract diminished specific 1,25(OH)2-D3 binding in the mix by 90% and 95% respectively. Similar results were obtained by mixing nuclear extracts of the owl monkey cell line, OMK, and the vitamin D resistant marmoset B-lymphoblast cell line B95-8. A wild type 1,25(OH)2-D3-binding profile was restored in mixtures after trypsin or heat treatment of the B95-8 extract. These data indicate that some New World primate cells contain a soluble protein that prevents intracellular 1,25(OH)2-D3-VDR binding. It is possible that the quantitative differences in the expression of this protein are responsible for 1,25(OH)2-D3 and other steroid hormone resistant states of variable severity in New World primates.

Steroid hormone receptors

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

Influence of calcitriol on prolactin and prostaglandin production by human decidua

Receptors for calcitriol are described in human decidua. They exhibit a dissociation constant of 35 +/- 6 pM and they are at concentrations similar to those found in other tissues (0.45 +/- 0.04 fmol/micrograms DNA). They are highly specific for calcitriol since neither of the other vitamin D3 derivatives nor the steroid hormone tested displaced labeled calcitriol from the receptor. Also, calcitriol at concentrations of 10(-13) to 10(-11) M stimulates prolactin secretion by dispersed decidual cells. At these concentrations, however, the hormone has no effect on prostaglandin production. The specificity of calcitriol action was further examined by studying the effect of estrogen, progesterone, testosterone, dexamethasone and cortisol, all at 10(-7) M, on the secretion of prolactin and prostaglandins. Under the conditions used in this study, the steroids have no effect on prolactin secretion; but dexamethasone significantly inhibits prostaglandin F2 alpha output by the cells. Taken together with previous studies from our laboratory demonstrating that decidua can synthesize calcitriol the present study indicates that this hormone has an autocrine effect on human decidual cells.

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.

The effect of vitamin D analogs and of vitamin D-binding protein on lymphocyte proliferation

In the absence of vitamin D-binding protein (DBP), 1,25-(OH)2D3 at 10(-12) M significantly inhibited the [3H]thymidine incorporation in human lymphocytes during mixed lymphocyte cultures (MLC) or after phyto-hemaglutinin (PHA) stimulation. In the presence of a physiological concentration of DBP (5 x 10(-6) M), the concentration of 1,25-(OH)2D3 required for inhibition was 10(-10) M (for PHA-cultures) and 10(-9) M (for MLC). Several vitamin D analogs were compared for their inhibitory action on PHA stimulation. In the absence of DBP, the concentration necessary for 50% inhibition of [3H]thymidine incorporation ranged from 10(-12) M [1,25-(OH)2D3 and 24,24-F2-1,25-(OH)2D3], over 10(-10) M [1,24R, 25-(OH)3D3; 1,25S, 26-(OH)3D3 and 26,27-F6-1,25-(OH)2D3] and 10(-8) M [25 OHD3 and 24,25-(OH)2D3] to 10(-6) M [calcitriol-lactone]. This rank order correlates with the binding affinity of the various analogs to the cytoplasmic 1,25-(OH)2D3-receptor. DBP counteracted the inhibitory effect of all analogs and the degree of counteraction was directly proportional to the binding affinity between DBP and the vitamin D analog. DBP thus decreased the in vitro inhibitory action of 1,25-(OH)2D3 and its analogs on lymphocyte proliferation. Of all analogs tested, only 1,25-(OH)2D3 had a significant effect at a physiological concentration.

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.

1,25-Dihydroxyvitamin D3 binds specifically to rat vascular smooth muscle cells and stimulates their proliferation in vitro

Cultured vascular smooth muscle cells (VSMC) derived from rat aorta were found to contain a specific receptor for 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Its Kd (5.0 x 10(-11) M) and capacity (22.9 fmol/mg of cytosol protein) for 1,25-(OH)2D3, its sedimentation coefficient on a sucrose density gradient (3.2 S), its relative affinities for various vitamin D3 metabolites [1,25-(OH)2D3 greater than 25-hydroxyvitamin D3 greater than 24,25-dihydroxyvitamin D3 greater than vitamin D3] and its affinity for DNA-cellulose were similar to those reported for the 1,25-(OH)2D3 receptor in other tissues. 1,25-(OH)2D3 at concentrations of more than 10(-10) M caused dose-dependent enhancement of the proliferation of VSMC in DMEM with 10% FCS. 25-Hydroxyvitamin D3 stimulated the proliferation of VSMC only at its highest concentration tested (10(-6) M). These data show that 1,25-(OH)2D3 stimulates the proliferation of VSMC after its binding to a cytoplasmic receptor of the cells in vitro, and support the possibility that VSMC are target cells of the hormone.

Reduced binding of [3H]1,25-dihydroxyvitamin D3 in the parathyroid glands of patients with renal failure

This study examined the hypothesis that altered binding of 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) to parathyroid receptors might be involved in the pathogenesis of secondary hyperparathyroidism associated with chronic renal failure. The binding of [3H]1,25-(OH)2D3 to hyperplastic parathyroid glands obtained from seven patients with chronic renal failure was measured. These values were compared with those for binding to hyperplastic parathyroid tissue obtained from six patients who had received renal transplants and for binding to parathyroid adenomas removed from five patients who had primary hyperparathyroidism. We found that Nmax (an estimate of the concentration of 1,25-(OH)2D3 receptors) was reduced (42 +/- 15 fmol per milligram of protein) in patients with chronic renal failure as compared with patients with transplanted kidneys (78 +/- 24 fmol per milligram of protein) and patients with primary hyperparathyroidism (114 +/- 30). Nmax correlated inversely with the severity of renal dysfunction, the serum level of phosphorus, and the logarithm of the serum level of immunoreactive parathyroid hormone. These observations suggest that 1,25-(OH)2D3 binding by parathyroid tissue is reduced in chronic renal failure. This may contribute to the pathogenesis of secondary hyperparathyroidism by reducing the inhibition by 1,25-(OH)2D of parathyroid hormone secretion. The low serum levels of 1,25-(OH)2D in chronic renal failure may accentuate this effect.

Biological activity of vitamin D metabolites and analogs: dose-response study of 45Ca transport in an isolated chick duodenum perfusion system

We have previously reported that vascular perfusion of the normal vitamin D3-replete chick duodenum with physiological amounts of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] increases the unidirectional movement of 45Ca from the lumen to the venous effluent under conditions of normal (0.9 mM) Ca2+ concentrations in both the lumen and vascular perfusate [Endocrinology 115: 1476 1984)]. The purpose of the present study was to determine the dose responsivity of this perfused intestinal calcium transport system for 1,25(OH)2D3 and some structurally related congeners. The dose-response curve was biphasic for all compounds studied; for 1,25(OH)2D3 initial stimulation of transport was detected at only 30 pM [the plasma concentration of 1,25(OH)2D3 is normally 125 pM] while maximal stimulation was 154% above control at a concentration of 650 pM. Above 650 pM 1,25(OH)2D3 the stimulation fell off sharply and transport had returned to basal levels by 1.3 nM. The relative potency of the D homologs tested was respectively 1,25(OH)2D3: 10,000; 1-alpha-hydroxyvitamin D3: 400; 25-hydroxyvitamin D3: 200; 24R,25-dihydroxy-vitamin D3: 137; vitamin D3: 34; 5,6-trans-25-hydroxyvitamin D3: 3. These results establish the usefulness of the perfused intestinal calcium transport system to study the nongenomic actions of 1,25(OH)2D3 on intestinal calcium transport.

Effect of 1,25-dihydroxyvitamin D3 on mouse mammary tumor (GR) cells: evidence for receptors, cellular uptake, inhibition of growth and alteration in morphology at physiologic concentrations of hormone

Mammary glands are target tissues for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). We have examined a mouse mammary tumor cell line (GR) for receptors of 1,25(OH)2D3 and have examined alterations in the growth and morphology of these cells in response to 1,25(OH)2D3. GR cells contain a high affinity (Kd approximately 10(-11)), low-capacity receptor with a high specificity for 1,25(OH)2D3. The 1,25(OH)2D3 receptor in GR cells has a sedimentation coefficient of 3.5 and elutes from DEAE cellulose columns with approximately 0.15 M KCl. These properties of the receptor are similar to those reported for other 1,25(OH)2D3 receptors. 1,25(OH)2D3 is internalized by GR cells in situ and specifically bound 1,25(OH)2D3 is found predominantly, if not entirely, in the nucleus as determined by cell fractionation and autoradiographic techniques. The incubation of GR cells in culture for 7 days with 1,25(OH)2D3 markedly alters cell growth. Cell growth is retarded in a dose-dependent manner; physiologic concentrations (10(-10) M) of 1,25(OH)2D3 retard cell growth by approximately 50%. In addition, GR cells incubated with 10(-9) to 10(-8) M 1,25(OH)2D3 undergo marked morphological changes. The incubation of GR cells with other vitamin D metabolites such as 25-hydroxyvitamin D3 (25(OH)D3) at a concentration of 10(-9) M does not significantly alter cell growth or morphology. The presence of high affinity receptors for 1,25(OH)2D3, the specific internalization of 1,25(OH)2D3 predominantly into the nuclei, and the significant effects of physiological concentrations of 1,25(OH)2D3 on cell growth suggest a direct, specific, nuclear effect of 1,25(OH)2D3 on GR cells. The mouse mammary tumor model might be useful in examining the effect of 1,25(OH)2D3 on tumor formation.

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.

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

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

Demonstration and characterization of a 1 alpha,25-(OH)2D3 receptor-like macromolecule in cultured rat pituitary cells

Recent studies have demonstrated the importance of prolactin (PRL) and growth hormone (GH) in the regulation of 25-hydroxycholecalciferol-1 alpha-hydroxylase activity. We have previously shown that 1 alpha,25-dihydroxycholecalciferol (1 alpha,25-(OH)2D3) reduces PRL and GH production by a clonal strain of rat pituitary tumour cells (GH3). The biologically active form of vitamin D3, 1 alpha,25-(OH)2D3, acts via an initial binding to cytoplasmic receptor proteins in target cells, and we demonstrate in this study the presence of specific receptors for 1 alpha,25-(OH)2D3 in the GH3 cells. GH3 cell cytosol was incubated with [3H]1 alpha,25-(OH)2D3 at 0-4 degrees C. Maximal binding was obtained between 2 and 6 h, and Scatchard analysis showed one single class of binding sites with Kd of 0.33 +/- 0.05 nM (mean + SD) and a Bmax of 103 +/- 26 fmol/mg cytosol protein. Competitive binding experiments revealed the following potency order: 1 alpha,25-(OH)2D3 greater than 25-OHD3 greater than 1 alpha-OHD3, 24,25-(OH)2D3. In contrast, corticosterone, testosterone, progesterone and oestradiol showed negligible ability to displace [3H]1 alpha,25-(OH)2D3 from its receptor. Sucrose gradient ultracentrifugation in high salt concentration revealed that GH3 cell cytosol possessed at 3.7S [3H]1 alpha,25-(OH)2D3 receptor protein which was inactivated by heating and protease treatment, but not after incubation with DNase or RNase. The receptor protein aggregated in salt-free sucrose gradients since the 3.7S complex was shifted reversibly to a approximately 6S form. Isoelectric focussing localized most of the [3H]1 alpha,25-(OH)2D3 to a protein peak with an isoelectric point of approximately 6 (pI 5.8-6.2). Since this 1 alpha,25-(OH)2D3 receptor protein has similar properties as the corresponding 1 alpha,25-(OH)2D3 receptors found in normal rat tissues, we suggest that lactotropes and somatotropes represent true target cells for 1 alpha,25-(OH)2D3.

1,25-Dihydroxyvitamin D3 regulates proliferation of activated T-lymphocyte subsets

The biologically active vitamin D metabolite, 1,25-(OH) 2D3 suppressed phytohaemagglutinin (PHA)-induced lymphocyte proliferation dose-dependently (0.1 nM-100 nM), and decreased the OKT4+/OKT8+ ratio and transferrin-receptor-positive (OKT9+) cells. A possible parallelism between expression of 1,25-(OH) 2D3 receptors and interleukin 2 (IL2)-receptors recognized by anti-Tac antibody was not confirmed in this study. However, the addition of exogenous IL2 abolished the inhibitory effects of 1,25-(OH) 2D3 on PHA-stimulated T-cell proliferation, and the decrease of OKT4+ and OKT9+ T-cell in this population. Among various vitamin D3 analogues examined, 1,25-(OH) 2D3 was the most potent anti-proliferative effect, followed in order by 1,24S-(OH) 2D3, 1 alpha OH D3, 25 OH D3 and 24,25-(OH) 2D3.

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.

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

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

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.

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.

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.

Phenotypic differentiation-linked growth inhibition in human leukemia cells by active vitamin D3 analogues

Active vitamin D3 induced phenotypic differentiation of the human promyelocytic leukemia cell line, HL-60, cultured in serum-free medium. Upon exposure to 10(-10)-10(-7) M 1,25-(OH)2D3, 1,24S-(OH)2D3 or 1,24R(OH)2D3, monocyte-granulocyte-associated plasma membrane antigens of HL-60 cells detected by monoclonal antibodies, OKMI, 63D3 and Mo2, quantitated by fluorescence-activated cell sorter analysis, were increased time- and dose-dependently. After expose to 1,25-(OH)2D3, promotion of this antigenic expression was detected within 16 h, and the induction of differentiation continued until 96 h. The number of cells bearing transferrin receptors recognized by the monoclonal antibody, OKT9, and its density on the surface of HL-60 cells were decreased symmetrically. These effects appeared in parallel with the inhibition of cell growth, poly(ADP-ribose) content and de novo DNA-RNA synthesis. These findings indicate that 1,24S-(OH)2D3 stimulates differentiating induction of HL-60 like 1,25-(OH)2D3 in vitro and that the decrease of transferrin receptor is apparently correlated with the inhibition of proliferation.

Biological activity assessment of 1 alpha,25-dihydroxyvitamin D3-26,23-lactone in the rat

1 alpha,25-Dihydroxyvitamin D3-26,23-lactone [1 alpha,25(OH)2D3-26,23-lactone] was compared to 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] in terms of their stimulation, in vivo, of intestinal calcium transport and mobilization of calcium from bone in the rat (the two classic vitamin D-mediated responses), and their relative binding to the chick intestinal receptor for 1 alpha,25(OH)2D3, 1 alpha,25-(OH)2D3-26,23-lactone was found to be only one-thirtieth as active as 1 alpha,25-(OH)2D3 in the stimulation of intestinal calcium transport and was found to mediate a significant reduction in the steady-state serum calcium levels. Associated with the reduction in serum calcium was a significant increase in urinary calcium excretion for 24 h after the administration of the steroid. Prior administration of 1 alpha,25(OH)2D3-26,23-lactone partially blocked the actions of a subsequently administered dose of 1 alpha,25(OH)2D3 in increasing serum calcium levels, but did not affect the action of 1 alpha,25(OH)2D3 in stimulating intestinal calcium transport. The binding affinity of 1 alpha,25(OH)2D3-26,23-lactone to the chick intestinal cytosol receptor protein was observed to be 670 times lower than that of 1,25-(OH)2D3 which indicates that perturbation of the 25-hydroxylated side chain by formation of the 26,23-lactone causes a significant reduction in ligand affinity for the receptor.

Resolution of the effects of sulfhydryl-blocking reagents on hormone- and DNA-binding activities of the chick oviduct progesterone receptor

The differential effects of sulfhydryl (SH)-blocking agents on hormone and DNA binding by the chick oviduct progesterone receptor were investigated. Previous studies have demonstrated inhibition of steroid-receptor interaction by SH-blocking agents and protection against inhibition by bound hormone. The present results indicate that the SH group required for steroid binding is within or near the hormone-binding site itself, and that a second SH group (or groups) is involved in the binding of receptor to DNA. Three findings relate to the site of action of SH-blocking agents on hormone binding. First, glycerol decreased the rate of hormone dissociation and the rate of hormone displacement by mercurial reagents by 75 to 90%. Second, mercurial reagents displaced [3H]progesterone bound to the mero-receptor, a Mr 23,000 proteolytic fragment containing the hormone-binding site, but not the site of interaction with DNA. Third, hormone displacement was still present after a 10,000-fold purification of the progesterone receptor. Mercurial reagents also inhibited binding of progesterone receptor to DNA, whereas the SH-alkylating agents N-ethylmaleimide and iodoacetamide had no effect. It is likely that distinct sulfhydryl groups are required for steroid receptor interaction with hormone and with DNA, since brief treatment with mercurial reagents blocked DNA binding, but caused only a slight displacement of bound hormone. The SH group required for hormone binding probably lies within or near the hormone-binding site, is sensitive to mercurials, alkylating agents, and 5,5'-dithiobis(2-nitrobenzoate) (DTNB), and is protected by bound hormone. The SH group required for DNA binding, in contrast, is sensitive to mercurials but not to alkylating agents, is only partially sensitive to DTNB, and is not protected by bound hormone.

1,25-Dihydroxyvitamin D3 receptor from Japanese quail intestine; kinetic studies

The cytosolic receptor from Japanese quail intestine has been partially purified and selected kinetic properties of the receptor preparation have been characterized. The receptor--chromatin complex technique developed by Haussler and his associates for separation of the bound from free 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] was used with modifications. Examination of the conditions for incubation of the receptor--chromatin complex with tritiated 1,25-(OH)2D3 led to the selection of 25 degrees for 25 min at pH 7.4. Under these conditions the following kinetic characteristics prevailed. Specific binding was greater than 90% and the affinity of the receptor for 1,25-(OH)2D3 was 100 times greater than that for 25-(OH)D3 and 24,25-(OH)2D3 and 1.0 x 10(5) times greater than that for vitamin D3. Scatchard analysis revealed an equilibrium dissociation constant of 1.1 nM. Determination of rate constants yielded an association rate constant (ka) of 4.2 x 10(7) M-1 min-1 and a dissociation rate constant (kd) of 4.1 x 10(-2) min-1. The equilibrium dissociation constant, calculated from the ratio kd/ka, is 1.0 nM. When used as a radioreceptor assay the preparation has a sensitivity of 5 pg of 1,25-(OH)2D3 per tube and a working range of 5 to 80 pg per tube.

Parathyroid hormone and vitamin D receptors

Parathyroid hormone (PTH), an 84-amino-acid polypeptide hormone, interacts with plasma membrane-bound receptors on bone and kidney cells, thereby increasing intracellular cAMP. Although the synthetic amino terminal 1-34 fragment of PTH possesses full biological activity, there is no firm evidence that hormone cleavage is a requisite step in the biological action of PTH. In contrast, vitamin D must undergo hydroxylations in the 1 and 25 positions in order to become fully active on target organs. 1,25(OH)2D, a steroid-like hormone, interacts with cytoplasmic receptors in small intestinal and bone cells. The 1,25(OH)2D-receptor complex (in the same manner as other steroid hormones) directs the synthesis of specific proteins. The integrated actions of PTH and vitamin D maintain serum calcium within narrow limits, thereby permitting normal neuromuscular and secretory function, as well as normal bone mineralization. Abnormalities in hormone secretion (PTH) or metabolic activation (vitamin D) lead to clinical disturbances in hormone action. Abnormalities at the level of the receptor-effector system for PTH and vitamin D also lead to clinical disturbances characterized by resistance to hormone action. Specific examples of the latter include pseudohypoparathyroidism, in which deficient activity of a component (the G unit) of the adenylate cyclase complex may lead to resistance to PTH, and hereditary vitamin D-dependent rickets type II, in which abnormalities in the nuclear uptake of 1,25(OH)2D may lead to impaired response to 1,25(OH)2D.

Resistance to 1,25-dihydroxyvitamin D. Association with heterogeneous defects in cultured skin fibroblasts

We evaluated the interaction of [3H]1,25(OH)2D3 with skin fibroblasts cultured from normal subjects or from affected members of six kindreds with rickets and resistance to 1-alpha, 25(OH)2D [1,25(OH)2D]. We analyzed two aspects of the radioligand interaction; nuclear uptake with dispersed, intact cells at 37 degrees C and binding at 0 degrees C with soluble extract ("cytosol") prepared from cells disrupted in buffer containing 300 mM KCl and 10 mM sodium molybdate. With normal fibroblasts the affinity and capacity of nuclear uptake of [3H]1,25(OH)2D3 were 0.5 nM and 10,300 sites per cell, respectively; for binding with cytosol these were 0.13 nM and 8,900 sites per cell, respectively. The following four patterns of interaction with [3H]1,25(OH)2D3 were observed with cells cultured from affected patients: (a) two kindreds; cytosol binding and whole-cell nuclear uptake both unmeasurable; (b) one kindred, decreased capacity and normal affinity both for binding in cytosol and for nuclear uptake in whole cells; (c) two kindreds, normal or nearly normal capacity and affinity of binding in cytosol but unmeasurable whole-cell nuclear uptake; and (d) one kindred, normal capacity and affinity of both cytosol binding and whole-cell nuclear uptake. In all cases where the radioligand bound with high affinity in nucleus or cytosol, the nucleus- or cytosol-associated radioligand exhibited normal sedimentation velocity on sucrose density gradients. When two kindreds exhibited similar patterns (i.e. pattern a or c) with the analyses of cultured fibroblasts, clinical features in affected members suggested that the underlying genetic defects were not identical.

IN CONCLUSION

(a) Fibroblasts cultured from human skin manifest nuclear uptake and cytosol binding of [3H]1,25(OH)2D3 that is an expression of the genes determining these processes in target tissues. (b) Based upon data from clinical evaluations and from analyses of cultured fibroblasts, severe resistance to 1,25(OH)2D resulted from five or six distinct genetic mutations in six kindreds.