No 1,25-dihydroxyvitamin D3 receptors on osteoclasts of calcium-deficient chicken despite demonstrable receptors on circulating monocytes.

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is known to stimulate osteoclastic bone resorption in vivo and whole organ bone culture systems in vitro. It has not been established whether 1,25(OH)2D3 acts directly on osteoclasts or whether its action on osteoclasts is mediated via other bone cells (e.g., osteoblasts) or recruitment of osteoclast precursor cells. Circulating monocytes have been characterized as osteoclast precursors. In the present study, vitamin D3-replete chicken on a calcium-deficient diet were studied. Circulating monocytes, whole bone cell preparations, and isolated osteoclasts (differential sedimentation) were examined for presence of 1,25(OH)2D3 receptors. Reversible, specific, and saturable binding of [3H]-1,25(OH)2D3 to a 3.5 S macromolecule was demonstrated in nuclear fractions of monocytes (maximal binding capacity, 48 fmol/mg protein; dissociation constant, 1.3 X 10(-10) M) and of whole bone cell preparations. 1,25(OH)2D3 receptors were not demonstrable in osteoclast preparations (70% pure; detection threshold, 2 fmol/mg protein). Data are consistent with indirect action of 1,25(OH)2D3 on osteoclastic bone resorption.

Identification and regulation of 1,25-dihydroxyvitamin D3 receptor activity and biosynthesis of 1,25-dihydroxyvitamin D3. Studies in cultured bovine aortic endothelial cells and human dermal capillaries.

Because 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) has been shown to play roles in both proliferation and differentiation of novel target cells, the potential expression of 1,25(OH)2D3 receptor (VDR) activity was investigated in cultured bovine aortic endothelial cells (BAEC). Receptor binding assays performed on nuclear extracts of BAEC revealed a single class of specific, high-affinity VDR that displayed a 4.5-fold increase in maximal ligand binding (Nmax) in rapidly proliferating BAEC compared with confluent, density-arrested cells. When confluent BAEC were incubated with activators of protein kinase C (PKC), Nmax increased 2.5-fold within 6-24 h and this upregulation was prevented by sphingosine, an inhibitor of PKC, as well as by actinomycin D or cycloheximide. Immunohistochemical visualization using a specific MAb disclosed nuclear localized VDR in venular and capillary endothelial cells of human skin biopsies, documenting the expression of VDR, in vivo, and validating the BAEC model. Finally, additional experiments indicated that BAEC formed the 1,25(OH)2D3 hormonal metabolite from 25(OH)D3 substrate, in vitro, and growth curves of BAEC maintained in the presence of 10(-8) M 1,25(OH)2D3 showed a 36% decrease in saturation density. These data provide evidence for the presence of a vitamin D microendocrine system in endothelial cells, consisting of the VDR and a 1 alpha-hydroxylase enzyme capable of producing 1,25(OH)2D3. That both components of this system are coordinately regulated, and that BAEC respond to the 1,25(OH)2D3 hormone by modulating growth kinetics, suggests the existence of a vitamin D autocrine loop in endothelium that may play a role in the development and/or functions of this pathophysiologically significant cell population.

Synergistic effects of parathyroid hormone and 1,25-dihydroxyvitamin D3 on proliferation and vitamin D receptor expression of rat growth cartilage cells.

We investigated possible interaction of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and PTH on: 1) proliferation (monolayer culture) and colony formation (agarose stabilized suspension cultures); 2) expression of 1,25-(OH)2D3 receptor (VDR); and 3) cAMP response to PTH, using primary cultures of chondrocytes from rat tibia proximal epiphysis. 1 alpha,25-(OH)2D3 stereospecifically stimulated DNA synthesis, cell counts, and colony formation at low concentration (10(-12) M). Within 6 h bovine PTH (bPTH)(1-34), human PTH (hPTH)(28-48) (10(-10) M), (Bu)2cAMP (1-2 mM), and 12-O-tetradecanoyl-13-acetate (10(-8) M) increased [3H]thymidine incorporation in the absence and presence of 1,25-(OH)2D3. Both PTH fragments also stimulated chondrocyte growth and colony formation in a Ca-dependent fashion. Prolonged exposure to bPTH(1-34) or hPTH(28-48) did not affect baseline DNA synthesis but increased the stimulatory effect of 1,25-(OH)2D3. This increase was inhibited in the presence of H7 (inhibition of PKC) or the monoclonal hPTH(1-38) antibody A1-70. In subconfluent chondrocyte cultures VDR was up-regulated by bPTH(1-34) and hPTH(28-48) (10(-10) M) or activators of protein kinase C (PKC), but not by (Bu)2cAMP. It was blocked by cycloheximide and actinomycin D and persisted in the presence of Ca-channel blockers. Inhibition of PKC by H7 also blocked the effect of bPTH(1-34) on VDR. The cAMP response to bPTH(1-34) was not affected by 1,25-(OH)2D3. We conclude that: 1) DNA synthesis, cell proliferation, and colony formation in chondrocyte monolayer or suspension cultures is increased by aminoterminal and midregional PTH fragments and by cAMP analogs in a Ca- dependent fashion; 2) bPTH(1-34) and hPTH(28-48) up-regulate VDR by cAMP-independent, PKC-dependent steps requiring transcriptional and translational processes; both PTH fragments also amplify the effect of 1,25-(OH)2D3 on DNA synthesis; and 3) no difference is found between the bPTH(1-34) and hPTH(28-48) fragments with respect to chondrocyte proliferation and VDR up-regulation, although the two differ with respect to stimulation of cAMP production.

Dexamethasone impairs growth hormone (GH)-stimulated growth by suppression of local insulin-like growth factor (IGF)-I production and expression of GH- and IGF-I-receptor in cultured rat chondrocytes.

Growth depression as a side effect of glucocorticoid therapy in childhood is partially mediated by alterations of the somatotropic hormone axis. The mechanisms of interaction between glucocorticoids and somatotropic hormones on the cellular and molecular level are poorly understood. In an experimental model of primary cultured rat growth plate chondrocytes, basal as well as GH (40 ng/ml) or insulin-like growth factor (IGF)-I (60 ng/ml)-stimulated growth was suppressed dose dependently (10(-l2)-10(-7)M) by dexamethasone (Dexa). An IGF-I antibody specifically and dose dependently inhibited the GH- but not the basic fibroblast growth factor (bFGF)-stimulated cell proliferation. GH increased the IGF-I concentration in conditioned serum-free culture medium; this was reversed by concomitant Dexa. Dexa time dependently suppressed the transcription of GH receptor (GHR) messenger RNA (mRNA) and down-regulated the basal and GH-stimulated expression of GHR. Whereas no suppressive effect on basal type I IGF-receptor (IGFR) was observed, Dexa blocked the IGF-I induced increase of IGF binding. These results were confirmed by GHR and IGFR immunostaining. We conclude that Dexa impairs the GH-induced stimulation of local secretion and paracrine action of IGF-I and reduces the homologous increase of IGFR and GHR expression. The above experiments give further insight on the interaction between GH and glucocorticoids on the cellular and molecular level of growth plate chondrocytes.

Nuclear testicular 1,25-dihydroxyvitamin D3 receptors in Sertoli cells and seminiferous tubules of adult rodents.

1,25(OH)2D3 receptors were studied in whole testes, Sertoli cells, seminiferous tubules, Leydig cells and spermatogonia of adult NMRI mice and SD rats. Specific reversible high affinity binding (KD 1.4 x 10(-10)M; Nmax 72 fmol/mg protein) by a 3.5 S macromolecule was demonstrated in whole testes, Sertoli cells and seminiferous tubules. With identical techniques, no receptors were found in Leydig cells despite previous reports of 1,25(OH)2D3 actions on Leydig cell function.

Diminished parathyroid 1,25(OH)2D3 receptors in experimental uremia.

In Sprague Dawley rats, six days after subtotal nephrectomy, serum 1,25(OH)2D3 concentration was diminished (59.8 +/- 17.5 pg/ml vs. 121 +/- 48; P less than 0.01). Despite low circulating 1,25(OH)2D3 levels, maximal specific binding capacity for 1,25(OH)2D3 in parathyroid glands was diminished (Nmax 87.5 fmol/mg protein and 3.52 fmol/mg DNA vs. 143 fmol/mg protein and 4.75 fmol/mg DNA, respectively). There was no change of KD, apparent molecular size (sucrose density gradient) and DNA binding affinity (DNA cellulose chromatography) pointing to intactness of the receptor. Since 1,25(OH)2D3 is a potent negative feedback signal for parathyroids, the data are potentially relevant for the genesis of secondary renal hyperparathyroidism.

Intermittent and continuous exposure to 1,25(OH)2D3 have different effects on growth plate chondrocytes in vitro.

Intermittent 1,25(OH)2D3 administration is widely used to suppress parathyroid glands in secondary (renal) hyperparathyroidism. It is unknown whether the effects of continuous and intermittent 1,25(OH)2D3 differ on vitamin D target organs other than parathyroids. Using primary cultures of rat chondrocytes (tibia) we compared the effects of continuous versus intermittent exposure to physiologic concentrations of 1 alpha,25(OH)2D3 on proliferation (radiothymidine incorporation), cell count, protein synthesis ([3H]-leucine incorporation), alkaline phosphatase activity (as a marker of differentiation) and 1 alpha,25(OH)2D3 receptor (VDR) regulation. Cells were synchronized and then exposed for variable periods to a medium containing 10% delipidated FCS and 10(-8) M to 10(-12) M 1 alpha,25(OH)2D3 (or 1 beta,25(OH)2D3 as specificity control). Intermittent (8 hr exposure every 48 hr) as well as continuous (sham washing) administration of 1 alpha,25(OH)2D3 had a biphasic effect on proliferation, that is, stimulation at low (10(-12) M) and inhibition at high (10(-8) M) concentrations. At 10(-12) M intermittent 1 alpha,25(OH)2D3 yielded higher cell counts than continuous 1,25(OH)2D3. This was seen in the log phase, which was day 3 (continuous 141 +/- 2.3% of solvent control; intermittent 185 +/- 2.0%) and in the plateau phase of growth, which was day 6 (128 +/- 2.6 vs. 169 +/- 2.7% of solvent control). Dependence on extracellular Ca is suggested by the effects of varying nominal Ca concentrations in the medium and of Ca channel blockers. Even two hours of exposure to 1 alpha,25(OH)2D3 (10(-12) M) yielded maximal activation of AP during postincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for in vivo upregulation of 1,25(OH)2 vitamin D3 receptor in human monocytes.

Mammalian cells increase net expression of 1,25(OH) 2D3 receptors after exposure to physiological concentrations of 1,25(OH) 2D3 in vitro. We examined specific binding of 1,25(OH) 2D3 by human monocytes before and after daily administration of 1.5-2 micrograms 1,25(OH) 2D3 p.o. for 3 days in 5 healthy normal D-replete probands. Median specific binding (Nmax) at baseline was 793 molecules/cell and 2052 or 2828 at 24h and 72h of 1,25(OH) 2D3 treatment respectively. The results suggest (a) upregulation of 1,25(OH) 2D3 receptors occurs in man and (b) monocyte preparations can be used to assess receptor regulation in vivo.

Hyperparathyroidism and abnormal 1,25(OH)2 vitamin D3 metabolism in experimental lead intoxication.

Clinical evidence points to disturbed calcium metabolism in lead (Pb) intoxication. To further clarify the mechanisms involved, serum levels of 1,25(OH)2D3, receptors for 1,25(OH)2D3 as well as size and ultrastructure of parathyroid glands were examined in Wistar Kyoto rats exposed to 1% lead (Pb) acetate in drinking water for 10 weeks (short-term study) or 0.001-1% Pb acetate for 24 weeks (long-term study). After administration of Pb for 10 weeks, bone Pb was significantly increased (641 +/- 66.9 (SD) vs. 0.648 +/- 0.39 mg kg-1 ash in controls). Total serum calcium and ionized Ca2+ (1.15 +/- 0.031 vs. 1.25 +/- 0.03 mmol l-1) were significantly decreased. Renal function (Ccr) was unchanged, but urinary cAMP excretion and circulating 1,25(OH)2D3 (177 +/- 10.9 vs. 232 +/- 18.9 pmol l-1) were diminished. Specific binding of 1,25(OH)2D3 was increased in parathyroids (Bmax 128 +/- 4.7 vs. 108 +/- 0.6 fmol mg-1 protein) and intestinal muscosa; Bmax failed to adequately rise in response to pretreatment with 1,25(OH)2D3 (2 x 10 ng day-1 for 4 d) in Pb-exposed animals. Receptor characteristics (sedimentation constant, KD, DNA affinity) were unchanged. Parathyroid weight was significantly increased (178 +/- 25 vs. 96 +/- 34 micrograms) with no change of estimated nuclear volume, cell volume or cell ultrastructure. After 24 weeks of Pb exposure, a dose-dependent but non-linear increase of parathyroid weight was noted between 0.001% and 1% Pb in drinking fluid. The present study documents secondary hyperparathyroidism associated with, and presumably caused by, hypocalcaemia and low 1,25(OH)2D3 levels, in experimental Pb intoxication.

1,25(OH)2D3 and dihydrotestosterone interact to regulate proliferation and differentiation of epiphyseal chondrocytes.

Growth plate chondrocytes are affected by 1,25(OH)2D3 and androgens, which may critically interact to regulate proliferation and differentiation during the male pubertal growth spurt. We investigated possible interactions of 1,25(OH)2D3 and the non-aromatizable androgen dihydrotestosterone (DHT) in primary chondrocyte cultures from young male rats. DHT and 1,25(OH)2D3 independently stimulated DNA synthesis and cell proliferation in a dose-dependent manner with maximally effective doses of [10(-8) M] and [10(-12) M], respectively. Both DHT and 1,25(OH)2D3 stimulated the expression and release of IGF-I, and the proliferative effects of each hormone were prevented by an IGF-I antibody. DHT and 1,25(OH)2D3 increased messenger RNAs (mRNAs) of their cognate receptors and of IGF-I receptor mRNA (IGF-I-R). 1,25(OH)2D3 also stimulated mRNA of the androgen receptor (AR), whereas DHT did not affect mRNA of the vitamin-D receptor (VDR). Coincubation with both steroid hormones did not stimulate receptor mRNAs more than either hormone alone. The proliferative effects of DHT and 1,25(OH)2D3 were completely inhibited by simultaneous incubation with both hormones, despite potentiation of IGF-I synthesis. In contrast, both hormones synergistically stimulated cell differentiation as judged by alkaline phosphatase activity, collagen X mRNA, and matrix calcification in long-term experiments. We conclude that DHT and 1,25(OH)2D3 interact with respect to chondrocyte proliferation and cell differentiation. The proliferative effects of both hormones are mediated by local IGF-I synthesis. Simultaneous coincubation with both hormones blunts the proliferative effect exerted by either hormone alone, in favor of a more marked stimulation of cell differentiation.

1,25(OH)2D3 receptors and endorgan response in experimental aluminium intoxication.

Severe aluminium-induced osteomalacia is refractory to treatment with 1,25(OH)2D3 which frequently causes hypercalcemia. To further explore the mechanisms involved, we have utilized a model of short-term aluminium intoxication in the rat (total: 11 mg elemental aluminium in 3 weeks) to study (a) 1,25(OH)2D3 receptor status in a variety of classical and non-classical target organs for 1,25(OH)2D3; (b) circulating 1,25(OH)2D3 levels; (c) baseline duodenal calcium transport, utilising the Ussing chamber, to investigate the functional significance of receptor status in a classical target organ; and (d) duodenal calcium transport response to exogenously administered 1,25(OH)2D3. Both in the three week model and in the 16 week model (total: 41 mg elemental calcium) increased maximal specific binding capacity for 1,25(OH)2D3 (Nmax), that is, number of unoccupied receptors, was observed in nuclear fractions of all tissues studied. Receptor affinity, the apparent dissociation constant KD, was unchanged. Total binding capacity, measured after displacement of endogenous ligand by Mersalyl, that is, the sum of occupied plus non-occupied receptors, was also increased. Both circulating 1,25(OH)2D3, mucosa-to-serosa calcium flux (Jms) and net calcium flux (Jnet) were reduced under baseline conditions, suggesting the lack of a direct relationship between receptor expression and endorgan response. Following exogenous 1,25(OH)2D3 administration, calcium Jms and Jnet were significantly lower in the aluminium intoxicated animals, with the increment induced in Jnet in aluminium intoxicated animals being 63% of that induced in controls. Our data suggest that resistance to the action of 1,25(OH)2D3 in aluminium intoxication is postreceptor in nature.

1,25(OH)2D3 receptor regulation and 1,25(OH)2D3 effects in primary cultures of growth cartilage cells of the rat.

Vitamin D deficiency leads to disturbed calcification of growth cartilage and enlargement of growth plate, illustrating that chondrocytes are a target for vitamin D. This observation prompted an investigation of 1,25(OH)2D3 receptor expression and action of vitamin D metabolites on chondrocyte proliferation. In primary cultures of tibial growth cartilage of male SD rats (80 g), specific binding of [3H]-1,25(OH)2D3 is noted in both the logarithmic growth phase and at confluence (Nmax 12780 molecules/cell versus 4368 molecules/cell). Scatchard analysis revealed the presence of a single class of noninteracting binding sites. KD was 10(-11) M irrespective of growth phase. The binding macromolecule had a sedimentation coefficient of 3.5 S. Interaction with DNA was demonstrated by DNA cellulose affinity chromatography. In immunohistology, growth cartilage cells (rabbit tibia) expressed nuclear 1,25(OH)2D3 receptors most prominently in the proliferative and hypertrophic zone. This corresponds to binding data which showed highest Nmax in the proliferating cartilage. 1,25(OH)2D3 in the presence of delipidated fetal calf serum (FCS) had a biphasic effect on cell proliferation and density, i.e., stimulation at 10(-12) M and dose-dependent inhibition at 10(-10) M and below. Inhibition was specific and not seen with 24,25(OH)2D3 or dexamethasone. Growth phase-dependent 1,25(OH)2D3 receptor expression and effects of 1,25(OH)2D3 on chondrocyte proliferation point to a role of vitamin D in the homeostasis of growth cartilage.

Parathyroid hormone prevents 1,25 (OH)2D3 induced down-regulation of the vitamin D receptor in growth plate chondrocytes in vitro.

1,25(OH)2D3 has an antiproliferative effect on growth plate chondrocytes when given in high doses, whereas low doses stimulate chondrocyte proliferation. In the present in vitro study we investigated the effects of parathyroid hormone (PTH) when given concomitantly with 1,25(OH)2D3 on cell proliferation and vitamin D receptor (VDR) regulation. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures or in agarose stabilized suspension cultures (10% charcoal-treated FCS). VDR expression was determined by RT-PCR generating a 297 bp fragment and by binding assays (Scatchard analysis) with [3H]-1,25(OH)2D3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures and by colony formation in agarose-stabilized suspension cultures. Optimal concentration of 1,25(OH)2D3 (10(-12) M) and of PTH fragments [bPTH(1-34) or hPTH(28-48), 10(-10)M] showed additive effects on DNA synthesis of and colony formation by growth plate chondrocytes. This may be explained in part by an up-regulation of VDR by PTH: PTH increased both mRNA expression of VDR and binding capacity. 1,25(OH)2D3 (10(-12) M) induced an up-regulation of the VDR within 24 hours followed by a down-regulation after incubation for more than 24 hours. PTH fragments added concomitantly prevented the down-regulation seen with 1,25(OH)2D3. These findings provide evidence that PTH is a growth promoting hormone that also modulates the effects of 1,25(OH)2D3 by regulating the VDR status of 1,25(OH)2D3 target cells.

Interaction of IGF-I and 1 alpha, 25(OH)2D3 on receptor expression and growth stimulation in rat growth plate chondrocytes.

Growth plate cartilage cell express receptors for, and are affected by both IGF-I and 1 alpha, 25(OH)2D3. The studies were undertaken to investigate interaction between these two hormone systems, that is, (i) to study effects of 1 alpha, 25(OH)2D3 on IGF-type 1 receptors (IGFIR), on IGF-I stimulated cell replication, colony formation, and on alkaline phosphatase activity (AP), and conversely, (ii) to study the effect of IGF-I on vitamin D receptor (VDR) expression on 1 alpha, 25(OH)2D3 stimulated growth parameters and on AP activity. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures, (serum-free) or in agarose stabilized suspension cultures (0.1% FCS). Vitamin D receptor and IGFIR were visualized by immunostaining with the monoclonal antibody (mAb) 9A7 gamma and mAb alpha IR3, respectively, and quantitated by RT-PCR for mRNA and by Scatchard analysis using [3H]-1,25(OH)2D3 and [125I]-alpha IR3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures, and by colony formation in agarose-stabilized suspension cultures. IGF-I dose-dependently increased [3H]-thymidine incorporation. 1 alpha, 25(OH)2D3, but not 1 beta, 25(OH)2D3 was stimulatory at low ((10-12 M) and slightly inhibitory at high (10-8 M) concentrations. The effect of IGF-I was additive to that of 1 alpha, 25 (OH)2D3 [IGF-I 60 ng/ml, 181 +/- 12.7; 1 alpha, 25(OH)2D3 10(-12) M, 181 +/- 9.8%, IGF-I + 1 alpha, 25(OH)2D3, 247 +/- 16.7%, P < 0.05 by ANOVA] and specifically obliterated by polyclonal IGF-I antibody (AB-1). Interaction could also be confirmed in suspension cultures. IGFIR mRNA and [125I]-alphaIR3 binding was increased by low (10(-12) m) but not by high (10(-8) M) concentrations of 1 alpha, 25(OH)2D3. Homologous up-regulation by IGF-I (60 ng/ml) was specifically inhibited by AB-1 and markedly amplified by coincubation with 1 alpha, 25(OH)2D3 (10(-12)m). Immunostaining with alpha IR3 showed specific IGFIR expression in rat growth cartilage, but not liver tissue. Stimulation of chondrocytes with 1 alpha, 25(OH)2D3 or IGF-I suggested some increase of receptor expression in single cells, but the predominant effect was increased recruitment of receptor positive cells, Vitamin D receptor expression was markedly stimulated (fourfold) by IGF-I (60 ng/ml), but not IGF-II and inhibited by actinomycin D. This study shows that IGF-I and 1 alpha, 25(OH)2D3 mutually up-regulate their respective receptors in growth plate chondrocytes. In parallel, they have additive effects on cell proliferation and colony formation suggesting independent effector pathways.