Defective binding and function of 1,25-dihydroxyvitamin D3 receptors in peripheral mononuclear cells of patients with end-organ resistance to 1,25-dihydroxyvitamin D

Vitamin D-dependent rickets type I and type II

Two distinct hereditary defects, vitamin D-dependent rickets type I (VDDR I) and type II (VDDR II), have been recognized in vitamin D metabolism. VDDR I is suggested to be a deficiency of the renal 25-hydroxyvitamin D (25(OH)D)-1 alpha-hydroxylase. Muscle weakness and rickets are the prominent clinical findings. A normal physiologic dose of 1 alpha-hydroxyvitamin D3 and 1,25-dihydroxyvitamin D3 is sufficient to maintain remission of rickets in this disorder. VDDR II consists of a spectrum of intracellular vitamin D receptor (VDR) defects and is characterized by the early onset of severe rickets and associated alopecia. This can be attributed to mutations in the VDR gene. Massive doses of vitamin D analogs and calcium supplementation is usually required for the treatment; however, the response to therapy is sometimes variable.

Quantification of vitamin D receptor mRNA by competitive polymerase chain reaction in PBMC: lack of correspondence with common allelic variants

It has been recently claimed that polymorphism for the vitamin D receptor (VDR) influences several aspects of calcium and bone metabolism. To evaluate the physiologic plausibility of these claims, we compared the abundance of the VDR mRNA in peripheral blood mononuclear cells (PBMCs) between different VDR genotypes using a quantitative reverse transcribed polymerase chain reaction-based method. The method is based on the coamplification of VDR cDNA and an internal standard consisting of known concentrations of a human VDR CDNA mutated at a BglII restriction site; the interassay coefficient of variation is 11%. To validate the method, we made use of earlier receptor binding studies indicating that normal human monocytes and activated, but not resting, lymphocytes expressed the VDR. The concentration of the VDR mRNA was 10(-8) to 10(-7) g/g of total RNA in cell-sorted monocytes and in in vitro activated lymphocytes, but only 10(-12) g/g of total mRNA in resting lymphocytes, establishing that the VDR mRNA determined by our method in PBMCs is due to constitutive expression in monocytes. Following an initial genotype screening of 85 normal volunteers by polymerase chain reaction or restriction fragment length polymorphism analysis, 14 individuals with the Bb genotype, 12 with the bb genotype, and 12 with the BB genotype were selected. The concentration of the VDR mRNA, corrected for the number of monocytes, was similar among the three genotype groups, as were the other variables examined: serum calcitriol, serum osteocalcin, and vertebral and hip bone density. We conclude that VDR polymorphism does not affect the abundance of the VDR mRNA.

Noncalcemic actions of 1,25-dihydroxyvitamin D3 and clinical applications

Vitamin D is absolutely essential for the maintenance of a healthy skeleton. Without vitamin D, children develop rickets and adults exacerbate their osteoporosis and develop osteomalacia. Casual exposure to sunlight is the major source of vitamin D for most people. During exposure to sunlight, ultraviolet B photons photolyze cutaneous stores of 7-dehydrocholesterol to previtamin D3. Previtamin D3 undergoes a thermal isomerization to form vitamin D3. Increased skin pigmentation, changes in latitude, time of day, sunscreen use, and aging can have a marked influence on the cutaneous production of vitamin D3. Once vitamin D3 is formed in the skin or ingested in the diet, it must be hydroxylated in the liver and kidney to 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. It is now recognized that a wide variety of tissues and cells, both related to calcium metabolism and unrelated to calcium metabolism, are target sites for 1,25(OH)2D3. 1,25(OH)2D3 stimulates intestinal calcium absorption and mobilizes stem cells to mobilize calcium stores from bone. Noncalcemic tissues that possess receptors for 1,25(OH)2D3 respond to the hormone in a variety of ways. Of great interest is that 1,25(OH)2D3 is a potent antiproliferative and prodifferentiation mediator. As a result, 1,25(OH)2D3 and its analogs have wide clinical application in such diverse clinical disorders as rheumatoid and psoriatic arthritis; diabetes mellitus type I; hypertension; cardiac arrhythmias; seizure disorders; cancers of the breast, prostate, and colon; some leukemias and myeloproliferative disorders; chemotherapy-induced hair loss; and skin rejuvenation as well as skin diseases like psoriasis and ichthyosis.

Calcitriol-resistant rickets due to vitamin D receptor defects

Calcitriol-resistant rickets (CRR) is an autosomal recessive disease due to a defect in the vitamin D receptor (VDR) or a site distal to it. The main characteristics are extreme rickets, with growth attenuation, osteomalacia, secondary hyperparathyroidism, severe dental caries, and alopecia. Serum studies reveal hypocalcemia, hypophosphatemia, very high calcitriol, and increased alkaline phosphatase levels. The clinical and chemical abnormalities do not respond to therapy with high-dose vitamin D, indicating target organ unresponsiveness. Eleven different mutations in the gene-encoding VDR have thus far been reported. They affect either the C-terminal ligand-binding region or the N-terminal DNA binding zinc-fingers sequences, with mutation hot spots identified at conserved sequences among the steroid-thyroid receptors superfamily. These result in impaired calcitriol binding to target organs, signified in vitro as failure of fibroblasts to bind [(3)H]calcitriol or to respond to calcitriol by 24-hydroxylase activity enhancement. Receptor studies and mutational analyses are used for prenatal diagnosis of CRR. Therapy with high-dose calcium overcomes the VDR defect, normalizes serum calcium, and maintains bone remodeling and mineral apposition. These responses to therapy have interesting implications upon our understanding of the potential role of calcium alone and that of vitamin D in bone physiology. Like other hormone-resistant diseases, CRR, with its various mutations, provides the opportunity for investigating the nature of vitamin D and of VDR physiology, which has been only partially explored to date.

1,25-Dihydroxyvitamin D3 receptors in peripheral blood mononuclear cells from patients with primary and secondary hyperparathyroidism

A decreased number of calcitriol (1,25(OH)2D3) receptors has been observed in parathyroid glands of uremic animals. In humans, studies carried out in surgically removed parathyroid glands have shown that calcitriol binding is higher in primary than in secondary hyperparathyroidism. Since specific receptors for calcitriol have been described in peripheral blood mononuclear cells (PBMC), we have investigated the specific uptake of 3H-labelled 1,25(OH)2D3 in PBMC of 12 women with primary hyperparathyroidism (PHP), 8 women with hyperparathyroidism secondary to chronic renal failure (SH), 9 women with renal transplant (RT), and 23 healthy women. The median dissociation constant (Kd) was similar in all three groups of patients and in healthy women (mean +/- S.D. (range): PHP, 1.2 +/- 1.0 (0.2-4) x 10(-10) M; SH, 0.6 +/- 0.4 (0.2-1.2) x 10(-10) M; RT, 1.1 +/- 0.5 (0.4-1.9) x 10(-10) M; controls, 1.0 +/- 0.6 (0.3-2.6) x 10(-10) M). However, the maximal binding capacity (Nmax) was significantly enhanced in PHP (3.9 +/- 1.9 (1.3-7.6) fmol/10(7) cells vs. 2.3 +/- 0.9 (1.1-4.4) fmol/10(7) cells in controls; P = 0.0006) and decreased in SH (0.8 +/- 0.5 (0.2-1.6) fmol/10(7) cells vs. 2.3 +/- 0.9 (1.1-4.4) fmol/10(7) cells in controls; P = 0.0001), whereas no changes were seen in RT (2.3 +/- 0.7 (1.2-3.3) fmol/10(7) cells vs. 2.3 +/- 0.9 (1.1-4.4) fmol/10(7) cells in controls). In three patients with PHP who were subjected to parathyroidectomy, the calcitriol number came down to normal. Changes of calcitriol receptors in primary and secondary hyperparathyroidism could magnify the consequences of disturbances in serum concentration of calcitriol itself and might play an important role in the development of secondary hyperparathyroidism in uremia.

Regulation of lymphocyte calcitonin receptors by interleukin-1 and interleukin-6

We have reported the existence of specific and high-affinity calcitonin (CT) receptors on normal human T-lymphocytes. Because of the increasingly recognized importance of interleukin-1 (IL-1) and IL-6 in the control of bone metabolism, we have examined their influence on the binding parameters of labeled salmon calcitonin (sCT) on lymphocytes. After a 24-hour incubation, IL-1 at 100-5000 U/ml and IL-6, at 1-1000 U/ml, decreased the apparent number of CT binding sites (Bmax) on T-lymphocytes. The effects of IL-6 on purified T-lymphocytes were dose related and 100 U of IL-6/ml reduced sCT binding to 57 +/- 16% (mean +/- SD) of the control values (n = 6). There was no significant change in CT binding affinity (Kd, 0.71 +/- 0.54 x 10(-10) M for controls versus 0.90 +/- 0.55 x 10(-10) M after IL-6) and the decrease in Bmax was reversible after 48 hours. The effects of IL-1 appeared to be mediated through an increased production of IL-6 as they were neutralized by a polyclonal antiserum against IL-6. Added alone, the antiserum caused a slight increase in the apparent number of CT binding sites on T-lymphocytes to 115 +/- 5% of control values (n = 3). In summary, IL-1 and IL-6 can induce a marked apparent loss of CT binding sites on normal T-lymphocytes at concentrations known to be active on bone metabolism. The contributions of our observations to the osteolytic activity of these cytokines deserve further investigation.

1,25-Dihydroxyvitamin D3 receptors in peripheral blood mononuclear cells from patients with postmenopausal osteoporosis

A decrease in intestinal calcium absorption, in spite of normal serum calcitriol levels, has been reported in postmenopausal osteoporotic women, raising the possibility of an intestinal resistance to the hormone. The mechanism responsible for it could lie at the receptor or postreceptor level. Intestinal receptors are difficult to study on clinical settings, but calcitriol receptors have been found in peripheral blood mononuclear cells (PBMC). We have studied the PBMC calcitriol receptors by means of Scatchard analysis in 11 postmenopausal osteoporotic women without any treatment and in 12 normal postmenopausal women of similar age. No differences were found in the dissociation constant (Kd) or the concentration of binding sites (Nmax) (Kd in patients: 0.90 +/- 0.75 x 10(-10) M; Kd in controls: 0.85 +/- 0.40 x 10(-10) M; Nmax in patients: 2.4 +/- 1.2 fmol/10(7) cells; Nmax in controls: 2.1 +/- 0.6 fmol/10(7) cells), supporting the contention that the disorder responsible for the resistance to calcitriol in postmenopausal osteoporotic women is located at the postreceptor level. In addition, our study included five postmenopausal osteoporotic women treated with calcitriol (0.5 microgram/day). The number of calcitriol receptors was increased in this group (Nmax: 3.9 +/- 2.0 fmol/10(7) cells vs. 2.1 +/- 0.6 fmol/10(7) cells; P = 0.02).

Vitamin D receptor quantitation in human blood mononuclear cells in health and disease

Vitamin D receptor (VDR) concentration was quantitated in human peripheral blood mononuclear cells (PBMC) from patients with absorptive hypercalciuria (AH) and patients with high 1,25(OH)2D3 due to acquired or transient disease states and the results compared to those in normal subjects. VDR concentration in resting cells was not different between the three groups and represented constitutive receptor expression of monocytes. Following activation with phytohemagglutinin, patients with hypercalcitriolemia demonstrated significantly greater VDR concentrations. Patients with AH demonstrated a normal value for the group, but 6 patients had significantly greater concentrations of VDR despite normal plasma 1,25(OH)2D3 in four of the patients. Proliferation, as assessed from [3H]thymidine incorporation was inversely correlated with serum 1,25(OH)2D3 and was significant (R = -0.299, p = 0.048). Taken together, the results suggest that PBMC provide a useful system for studying VDR status in transient or acquired states of hypercalcitriolemia. Furthermore, the studies in patients with absorptive hypercalciuria disclosed it to be a heterogeneous disorder, characterized by both vitamin D-dependent and D-independent forms of receptor up-regulation.

1,25(OH)2D3 increases cytotoxicity and exocytosis in lymphokine-activated killer cells

The effect of 1,25-dihydroxyvitamin D3 on lymphokine-activated killer (LAK) cells activity was studied. Treatment of LAK cells with 1,25-dihydroxyvitamin D3 for 24 h increased their cytotoxic activity without affecting cell proliferation. This effect was dose-dependent, detectable already at 10(-11) M attaining 44 +/- 7% increase at 10(-8) M. 1,25-dihydroxyvitamin D3 increased LAK cell content of the cytotoxic granule granzyme A by 21%. Secretion of granzyme A by LAK cells was triggered by 12-O-tetradecanoylphorbol 13-acetate and the calcium ionophore A23187. 1,25-dihydroxyvitamin D3 decreased the lag preceding secretion, increased the rate constant of exocytosis and the fraction of granzyme A cell content secreted. The potentiation of exocytosis was more pronounced at suboptimal calcium ionophore concentration suggesting that 1,25-dihydroxyvitamin D3 affects a calcium-dependent process. Since exocytosis of cytotoxic granules is a pivotal event in the killing of tumor cells by LAK cells, it is plausible that the enhancement of this process underlies the stimulation of LAK cell cytotoxic activity by 1,25-dihydroxyvitamin D3.

Foreign body granulomatous inflammation increases the sensitivity of splenocytes to immunomodulation by 1,25-dihydroxyvitamin D3

1,25-dihydroxyvitamin D3, the active metabolite of vitamin D, partially inhibits antigen and mitogen-driven lymphocyte stimulation. We studied the effect of granulomatous inflammation on the sensitivity of lymphocytes to 1,25-dihydroxyvitamin D3 in vitro, measuring the inhibitory effect of 1,25-dihydroxyvitamin D3 on mitogenesis of splenocytes of mice with chronic inflammation induced by subcutaneous injection of talc. Systemic manifestations of the local inflammation included loss in body weight, splenomegaly, enhanced DNA synthesis by freshly isolated splenocytes and enhanced prostaglandin secretion by activated splenocytes. Splenocytes from animals with local inflammation were more susceptible to inhibition by 1,25-dihydroxyvitamin D3, but not by prostaglandin E2. This increased sensitivity to 1,25-dihydroxyvitamin D3 was abolished by blocking prostaglandin synthesis in splenocyte cultures with indomethacin and was restored by adding prostaglandin E2. This effect cannot be attributed to enhanced prostaglandin synthesis in the presence of 1,25-dihydroxyvitamin D3, but is probably due to a qualitative change in the response of splenocytes from inflamed animals to the combined action of 1,25-dihydroxyvitamin D3 and prostaglandin E2.

Calcium therapy for calcitriol-resistant rickets

Ten patients with calcitriol-resistant rickets caused by a defect in the ligand-binding domain of the vitamin D receptor are described. Eight patients, 1.7 to 13.8 years of age, received high doses of elemental calcium (range, 0.4 to 1.4 gm/m2) through indwelling intracaval catheters for periods of 1.8 to 3.8 years. Two other patients, aged 1.1 and 2.2 years, were given oral calcium therapy as the sole mode of treatment. In five of the intravenously treated patients, oral calcium therapy was initiated after radiologic evidence of healing of the rickets. To maintain normal serum calcium concentration, the patients required daily doses of elemental calcium of 3.5 to 9 gm/m2 body surface area. Clinical improvement was observed within a week of the start of intravenous therapy, with disappearance of bone pain; several of the younger patients started to walk for the first time. Growth velocity increased within 2 to 3 months, from a pretreatment rate of -0.8 to -6.3 standard deviation score (SDS), to a posttreatment rate of +0.1 to +5.1 (SDS). Serum calcium, parathyroid hormone, phosphorus, and alkaline phosphatase values returned to normal within a year. Radiologic signs of healing occurred more rapidly in the intravenous treatment groups and in younger patients. Episodes of septicemia occurred frequently in those receiving parenteral therapy and required replacement of the catheter. We recommend that in the treatment of calcitriol-resistant rickets, oral calcium therapy be started at the youngest possible age, in doses to the limit of intestinal tolerance. When rickets is present, calcium should be infused through a large vessel in doses high enough to produce normocalcemia, normophosphatemia, and suppression of parathyroid hormone. Only after radiologic healing has been observed can oral calcium therapy be introduced.

Responsiveness to 1,25-dihydroxyvitamin D3 is reduced in lymphocytes from osteoporotic women

The purpose of this work was to test the hypothesis that reduced responsiveness of target organs to 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] is associated with osteoporosis. Peripheral blood mononuclear (PBM) cells have been previously shown to be a valid model for the action of 1,25(OH)2D3 on its classic target organs in various pathologic and physiologic situations. The responsiveness of lymphocytes to the hormone can be assessed by the extent of inhibition it exerts on the proliferative response to mitogenic lectins. A group of 39 postmenopausal women, at least 10 years after the menopause, participated in the study. Osteoporosis, defined as the presence of at least one nontraumatic vertebral crush fracture, was diagnosed in 19 subjects. Mitogenesis of PBM cells stimulated by phytohemagglutinin and cultured for 72 h in the presence or absence of 1,25-(OH)2D3 (0.03-1 nmol/liter) was assessed by [3H]thymidine incorporation during a 4 h pulse. The maximal inhibitory effect of 1,25-(OH)2D3 at saturating concentration (1 nM/liter) was 74.6 +/- 2.8% (mean +/- SEM) for normal compared to 65.3 +/- 2.9% for osteoporotic women (P = 0.015). The geometric mean of the ED50 values of 1,25-(OH)2D3 was 60% higher in the osteoporotic than in the normal group (P = 0.035). Our data are consistent with the notion that reduced responsiveness of target organs to 1,25-(OH)2D3 is associated with osteoporosis.

1,25-Dihydroxyvitamin D3 potentiates the decreased response of lymphocytes from atopic subjects to agents that increase intracellular cyclic adenosine monophosphate

The inhibitory effect of prostaglandin E2, histamine, isobutylmethylxanthine, and 1,25-dihydroxyvitamin D3 (1,25-[OH]2D3) on the mitogenic stimulation of peripheral blood lymphocytes from normal and atopic subjects was studied. We found that lymphocytes from atopic patients were less susceptible to inhibition by the three agents that elevate intracellular cyclic adenosine monophosphate (cAMP) concentrations and by the active metabolite of vitamin D (inhibition of 27%, 14%, 12%, and 36% for the atopic patients as compared with 40%, 20%, 22%, and 46% for the normal donors, by the four agents, respectively; p less than 0.02). The inhibitory effect of the cAMP-elevating agents was potentiated by the addition of 1,25-(OH)2D3 to the lymphocyte cultures. The potentiation was more pronounced on lymphocytes from the atopic donors, increasing their responsiveness to levels comparable to levels of lymphocytes from normal donors. The synthetic corticosteroid, dexamethasone, had a similar potentiating effect on the inhibitory action of prostaglandin E2. In view of the beneficial action of beta-agonists, phosphodiesterase inhibitors, and corticosteroids in the treatment of allergy, the potentiating effect of 1,25-(OH)2D3 on the action of cAMP-elevating agents may be of therapeutic interest.

The molecular basis of hereditary 1,25-dihydroxyvitamin D3 resistant rickets in seven related families

Hereditary 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] resistant rickets (HVDRR) is an autosomal recessive disease caused by target organ resistance to the action of 1,25(OH)2D3, the active form of the hormone. The defect in target cells is heterogenous and commonly appears to be a mutation in the gene encoding the vitamin D receptor (VDR). We have studied cultured skin fibroblasts and Epstein-Barr virus transformed lymphoblasts of seven family branches of an extended kindred having eight children affected with HVDRR. We have previously shown that cells from three affected children in this group contain an "ochre" nonsense mutation coding for a premature stop codon in exon 7 within the steroid-binding domain of the VDR gene. In the current studies, we found that cells from affected children failed to bind [3H]1,25(OH)2D3 and had undetectable levels of VDR as determined by immunoblots using an anti-VDR monoclonal antibody. Measurement of VDR mRNA by hybridization to a human VDR cDNA probe showed undetectable or decreased abundance of steady-state VDR mRNA. Parents, expected to be obligate heterozygotes, showed approximately half the normal levels of [3H]1,25(OH)2D3 binding, VDR protein, and mRNA. The mutation at nucleotide 970 (counting from the mRNA CAP site) results in the conversion of GTAC to GTAA, which eliminates an Rsa I restriction enzyme site and facilitates identification of the mutation. We found that polymerase chain reaction (PCR) amplification of exons 7 and 8 from family members and subsequent Rsa I digestion allows detection of the specific genotype of the individuals. When Rsa I digests of PCR-amplified DNA are subjected to polyacrylamide gel electrophoresis, children with HVDRR exhibit a homozygous banding pattern with loss of an Rsa I site. Parents exhibit a heterozygotic DNA pattern with detection of both normal and mutant alleles. In summary, our data show that the genetic abnormality is a point mutation within the steroid-binding domain of the VDR in all seven related families with HVDRR. Analysis of restriction fragment length polymorphism at the 970 locus of PCR-amplified DNA fragments can be used to diagnose this mutation in both affected children and parents carrying the disease.

Decreased concentration of 1,25-dihydroxyvitamin D3 receptors in peripheral mononuclear cells of patients with X-linked hypophosphatemic rickets: effect of phosphate supplementation

Abnormal renal tubular phosphate transport is considered to be the primary defect in X-linked hypophosphatemic rickets (XLH). However, the resistance to vitamin D treatment in XLH cannot be explained by hypophosphatemia alone. Since most of the actions of vitamin D are mediated by its receptors (VDR), abnormalities of VDR have been postulated in XLH. In order to investigate this possibility, we measured the concentration of VDR in PHA-activated peripheral mononuclear cells from 10 XLH patients. Patients without phosphate supplementation showed significantly lower concentration (21.7 +/- 5.1 fmol/mg protein, mean +/- SEM) compared to the normal controls (60.7 +/- 4.0). On the contrary, there was no significant difference between the phosphate-supplemented patients (58.3 +/- 2.7) and controls. There was a significant positive correlation between VDR concentration and serum phosphate (P less than 0.05). In two patients, VDR was increased after daily phosphate supplementation was started. These results indicate that a decreased concentration of VDR secondary to persistent hypophosphatemia is one of the causes of vitamin D resistance in XLH.

Lymphocyte cell lines from vitamin D-dependent rickets type II show functional defects in the 1 alpha,25-dihydroxyvitamin D3 receptor

Lymphocyte cell lines were established from five patients with vitamin D-dependent rickets, type II (VDDR-II). These lines were established by infection with human T-lymphotrophic virus type I (HTLV-I). Binding of [3H]1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) to its receptor in these cell lines was compared to binding studies using a T-lymphocyte cell line (S-LB1) from a normal individual. The 1,25(OH)2D3 receptor of S-LB1 was comparable to the well-characterized chick intestinal 1,25(OH)2D3 receptor in terms of its ligand binding affinity and capacity, its mobility on 5-20% sucrose gradients, and its adsorption to and elution properties from DNA-cellulose. Three cell lines established from patients with VDDR-II (Rh-VDR, Sh-VDR, and Ab-VDR) showed no specific binding of 1,25(OH)2D3 to a receptor and treatment of the cultured cells with 1,25(OH)2D3 did not stimulate production of 24,25-dihydroxy-vitamin D3 (24,25(OH)2D3), a response which is diagnostic of the presence of a functional 1,25(OH)2D3 receptor. In a fourth cell line, A1-VDR, the receptor for 1,25(OH)2D3 had a low binding capacity and 25(OH)D3-24-hydroxylase activity was not detectable. Induction of 24,25-(OH)2D3 synthesis by 1,25(OH)2D3 was observed in the fifth cell line, designated Ro-VDR, although the sensitivity to hormone treatment was lower than in the control cell line from a normal donor. The capacity of the receptor for 1,25(OH)2D3 was low in Ro-VDR. In all cell lines where 1,25(OH)2D3 binding to a receptor was detectable, the receptor had the typical sedimentation coefficient of 3.7 S on sucrose density gradient analysis. Binding and elution properties to DNA-cellulose, however, differed from normal in both Ro-VDR and A1-VDR cells where elution from DNA-cellulose occurred at a lower salt concentration than is typical of the 1,25(OH)2D3 receptor. While Ro-VDR cells showed typical nuclear localization of the unoccupied 1,25(OH)2D3 receptor, neither the unoccupied nor the occupied receptor from A1-VDR cells was completely localized in the nucleus. In a series of functional studies we found that modulation of the level of the mRNAs coding for both the c-myc oncogene and the growth factor known as granulocyte-monocyte colony stimulating activity by 1,25(OH)2D3 correlated with the 1,25(OH)2D3 receptor status of these cells. Use of these cell lines will facilitate further study of the molecular defect(s) in the receptor for 1,25(OH)2D3 in vitamin D-dependent rickets type II and will allow a correlation with impairment of cellular functions.

T lymphocytes and their CD4 subset are direct targets for the inhibitory effect of calcitriol

We studied the direct effects of the hormone calcitriol on the activation and proliferation of pure T lymphocytes and their subsets. Calcitriol inhibited the proliferation of T lymphocytes stimulated in the absence of monocytes with phytohemagglutinin (PHA) and either a monocytic culture supernatant or a combination of monocyte-derived interleukin 1 and interleukin 6. This inhibition was not influenced by the concentration of the stimulating agents. The minimal effective concentration of calcitriol was 10(-10) M. In contrast, the interleukin 2 (10 U/ml)-driven growth of PHA-stimulated T lymphocytes was not significantly altered by calcitriol at 10(-8) M. The hormone had also no influence on the T lymphocyte proliferation induced by a combination of PHA and the anti-CD28 monoclonal antibody 9.3. Pure T lymphocytes, after incubation for 5 days with PHA and monocytic factors, expressed a high level of transferrin receptors. This phenomenon was strongly suppressed on both CD4 and CD8 subsets when 10(-8) M calcitriol had been present during the culture. Moreover, the proliferation of pure CD4 cells was directly inhibited by calcitriol in similar conditions as for unseparated T lymphocytes. We conclude that T lymphocytes and their CD4 subset are direct targets for the inhibitory effect of calcitriol.

1,25-Dihydroxyvitamin D3 acts directly on human lymphocytes and interferes with the cellular response to interleukin-2

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) inhibits mitogen-induced lymphocyte proliferation in peripheral blood mononuclear cell preparations. Receptors for 1,25(OH)2D3 are present in monocytes and are acquired by lymphocytes upon activation. It is not clear whether the hormone inhibits lymphocyte mitogenesis by a direct action on the lymphocyte or only indirectly by affecting the regulatory monocytes. We addressed this question by using highly purified human lymphocyte preparations which contain less than 0.1% monocytes. 1,25(OH)2D3 inhibited the stimulation of purified lymphocytes by phytohemagglutinin and in the presence of fixed accessory cells, which cannot respond to 1,25(OH)2D3. The inhibitory effect attained 35% similar to that obtained in the presence of monocytes. 1,25(OH)2D3 inhibited interleukin 2 (IL-2)-driven stimulation of purified T cells in three different systems: cells treated with a submitogenic concentration of phytohemagglutinin, IL-2-dependent cells which bear receptors for IL-2 and naive cells in the absence of other mitogens. We conclude: (1) the human T lymphocyte is a direct target for 1,25(OH)2D3; (2) 1,25(OH)2D3 inhibits the response of activated, IL-2-receptor-bearing human T cells to interleukin-2.

Selective expression of a normal action of the 1,25-dihydroxyvitamin D3 receptor in human skin fibroblasts with hereditary severe defects in multiple actions of that receptor

We evaluated three actions of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] in human skin fibroblasts to test for heterogeneity in hormone-response coupling. In fibroblasts from normal subjects the 1,25-(OH)2D3 concentrations for half-maximal effect (EC50) were: for mitogenic effect 0.0001-0.0005 nM, for antimitogenic effect 1 nM, and for induction of 25-OHD3 24-hydroxylase (24-OHase) 5 nM. To evaluate the effects of mutations presumed to be in the gene for the 1,25-(OH)2D3 receptor we examined cell lines representing four kindreds with hereditary resistance to 1,25-(OH)2D3 ("mutant" cell lines). In one mutant cell line all three 1,25-(OH)2D3 actions were severely abnormal. In one mutant cell line 24-OHase induction and mitogenic action were undetectable, but EC50 and maximal effect were normal for antimitogenic action of 1,25-(OH)2D3. In two mutant cell lines 24-OHase induction and antimitogenic actions were undetectable or severely impaired but mitogenic action were undetectable or severely impaired but mitogenic action was normal in EC50 and normal or increased in maximal effect. The mitogenic and antimitogenic actions in normal cells showed a similar profile of potency ratios for 1,25-(OH)2D3 and six analogues. Whenever a mutant cell showed a normal or even an abnormal mitogenic or antimitogenic effect of 1,25-(OH)2D3, these effects showed potency ratios similar to wild type, suggesting mediation by a similar 1,25-(OH)2D3 receptor. We conclude that three 1,25-(OH)2D3 actions show important differences in hormone response coupling indicated by differences in EC50 for 1,25-(OH)2D3 and by different consequences of receptor mutations.

Long-term intracaval calcium infusion therapy in end-organ resistance to 1,25-dihydroxyvitamin D

Two boys aged six and four with the syndrome of hereditary resistance to 1,25-dihydroxyvitamin D3 with rickets alopecia and growth retardation are presented. After unsuccessful therapeutic trials with pharmacologic doses of vitamin D or its active metabolites, the patients were treated by long-term intracaval infusions of calcium through an implantable catheter. A total of 0.5 to 0.9 g of elemental calcium was infused daily for 18 months and the serum calcium concentration was maintained at 9 to 10 mg/dl. Bone pain subsided within one week of treatment. Serum phosphorus, immunoreactive parathyroid hormone, and 1,25-dihydroxyvitamin D concentrations and alkaline phosphatase activity were normalized within four to nine months. Radiographs of the knees and hands revealed progressive healing of rickets with complete resolution after one year of treatment. The patients gained 12 cm and 8 cm per year in height as compared with 3 cm and 2 cm, respectively, in the previous year. A transilial bone biopsy obtained from one patient prior to treatment revealed severe osteomalacia associated with osteitis fibrosa. A follow-up biopsy examined after 12 months of therapy showed almost complete healing of osteomalacia and normal mineralization. These observations indicate the following: (1) Long-term intracaval calcium infusions are an effective mode of therapy for these patients, and (2) When adequate serum calcium and phosphorus concentrations are maintained, healing of rickets and normal growth rate could be achieved even in the absence of a normal 1,25-dihydroxyvitamin D3 receptor-effector system.

Myeloid progenitors from the bone marrow of patients with vitamin D resistant rickets (type II) fail to respond to 1,25(OH)2D3

The active metabolite of vitamin D, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3), has been shown to enhance the growth of human granulocyte/macrophage haemopoietic progenitors in vitro and to induce these cells to differentiate along the monocyte/macrophage pathway. In order to evaluate the relationship between specific receptors for 1,25(OH)2D3 and the role of 1,25(OH2D3 in the regulation of haemopoietic cell differentiation, we examined the effect of haemopoietic cell differentiation, we examined the effect of 1,25(OH)2D3 on the in vitro growth and differentiation patterns of marrow myeloid progenitor cells from two patients with 1,25(OH)2D3 resistant rickets, resulting from defective receptors to vitamin D. A significant rise in the frequency of myeloid colonies in control marrow cell cultures was induced by 2 X 10(-9) to 2 X 10(-7)M 1,25(OH)2D3. This rise reached a plateau at 2 X 10(-9)_2 X 10(-8) M 1,25(OH)2D3, resulting in a maximal 54 +/- 9% increase in colony numbers. In contrast, no stimulatory effect could be detected when 1,25(OH)2D3 was added to cultured marrow cells from the patients with 1,25(OH)2D3 resistance. Analysis of colony composition revealed that 2 X 10(-8) and 2 X 10(-7) M, 1,25(OH)2D3 induced a 50 +/- 26% increase in the frequency of colonies composed only of monocytes/macrophages in control, but not in the patients' marrow cell cultures. The effect of 2 X 10(-8) and 2 X 10(-7) M 1,25(OH)2D3 on progenitor cell differentiation towards monocytes/macrophages was also observed in marrow cell suspension cultures. Whereas 1,25(OH)2D3 induced a 81-136% increase in the frequency of monocytes in control marrow cells, no effect could be detected on the generation of mature monocytes in marrow cells of the 1,25(OH)2D3 resistant patients. Our results show that marrow granulocyte/macrophage progenitor cells from patients with 1,25(OH)2D3 resistance fail to respond to 1,25(OH)2D3. We thus demonstrate that the effect of 1,25(OH)2D3 on the proliferation and differentiation of haemopoietic progenitor cells is mediated through its binding to specific cytoplasmic receptors.

Granulocyte-macrophage colony-stimulating factor. Sensitive and receptor-mediated regulation by 1,25-dihydroxyvitamin D3 in normal human peripheral blood lymphocytes

We show that 1,25-dihydroxyvitamin D3 (1,25[OH]2D3), the most hormonally active metabolite of vitamin D3, modulates sensitively and specifically both the protein and messenger RNA accumulation of the multilineage growth factor granulocyte-macrophage colony-stimulating factor (GM-CSF). The regulation of GM-CSF expression is seen in both normal human mitogen-activated T lymphocytes and T lymphocytes from a line (S-LB1) transformed with human T cell lymphotropic virus 1 (HTLV-1). In contrast, cells from a HTLV-1 transformed T lymphocyte line (Ab-VDR) established from a patient with vitamin D-resistant rickets type II with undetectable 1,25(OH)2D3 cellular receptors are resistant to the action of 1,25(OH)2D3. Inhibition of GM-CSF expression by 1,25(OH)2D3 can occur independently of interleukin 2 regulation and is probably mediated through cellular 1,25(OH)2D3 receptors. We conclude that 1,25(OH)2D3 may be important in the physiology of hematopoiesis.

Vitamin D and the immune system

The investigation of the potential influence of 1,25-(OH)2D3 on immune cells has expanded our understanding of hormone-cytokine interactions. 1,25-(OH)2D3 stimulates phenotypic and function changes in immature monocytes, alters protein synthesis, increases adherence, and augments interleukin-1 secretion. T lymphocyte proliferation and B cell immunoglobulin production are inhibited by the hormone. 1,25-(OH)2D3 decreases IL 2 and IFN-gamma synthesis by activated T lymphocytes in association with decreases in mRNA for these proteins. The step from the investigation of in vitro interactions to an understanding of in vivo effects of 1,25-(OH)2D3 on immune cells requires further study. On the basis of information at hand, such as the potential for macrophage conversion of 25-OH-D3 to 1,25-(OH)2D3, decreased or increased macrophage function in association with vitamin D3 status in vitro and in vivo, as well as altered T cell subset ratios and proliferative responses with administration of the hormone, it is tempting to speculate that 1,25-(OH)2D3 exerts an influence on immune cell function in concert with other recognized soluble mediators of monocyte and lymphocyte origin. The primary influence of 1,25-(OH)2D3 may vary with the tissue site. Systemic levels of hormone may aid in maintaining tonic immunosuppression and thus prevent trivial antigenic stimuli from initiating an immune response. Upon initiation of an immune response to a significant antigenic challenge 1,25-(OH)2D3 may, in concert with other suppressor mechanisms, limit the extent of the host response by inhibition of IL 2 and IFN-gamma production. At local sites of chronic inflammation concentrations of 1,25-(OH)2D3 may be elevated and may act in an autocrine or paracrine fashion to alter the immune response, for example, by increasing IL 1 production and antigen presentation by tissue monocyte/macrophages. The activation of T cells is associated with the synthesis of 1,25-(OH)2D3 receptors, thus potentially limiting T cell proliferation in the presence of the hormone. Other biological actions of IL 1, however, including effects on cells in bone, joint, and brain may be augmented. Thus, the end result of the opposing effects of 1,25-(OH)2D3 on immune cells and their secretory products may vary with the specific cells involved, their state of maturation and activation, and the local concentrations of the hormone. Studies to date support the concept of an expanded role for 1,25-(OH)2D3 in immune cell biology.

1,25-Dihydroxyvitamin D3 enhances prostaglandin E2 production by monocytes. A mechanism which partially accounts for the antiproliferative effect of 1,25(OH)2D3 on lymphocytes

Partial removal of monocytes from human peripheral blood mononuclear cells, or the addition of indomethacin, reduced the antiproliferative effect of 1,25(OH)2D3 on mitogen-stimulated mononuclear cells. Addition of 1,25(OH)2D3 (1 nM) to mitogen-stimulated mononuclear cells caused a 2-4-fold increase in prostaglandin E2 production during the second day of culture. The inhibitory effect of 1,25(OH)2D3 on lymphocyte proliferation is greatly augmented up to 7-fold in the presence of prostaglandin E2. We conclude that monocytes are involved in the inhibitory effect of 1,25(OH)2D3 on the mitogenic stimulation of human lymphocytes and that their action is probably mediated by prostaglandins.