1 Alpha,25-dihydroxyvitamin D3 rapidly increases cytosolic calcium in clonal rat osteosarcoma cells lacking the vitamin D receptor

A hierarchical regulatory network analysis of the vitamin D induced transcriptome reveals novel regulators and complete VDR dependency in monocytes

The transcription factor vitamin D receptor (VDR) is the high affinity nuclear target of the biologically active form of vitamin D₃ (1,25(OH)₂D₃). In order to identify pure genomic transcriptional effects of 1,25(OH)₂D₃, we used VDR cistrome, transcriptome and open chromatin data, obtained from the human monocytic cell line THP-1, for a novel hierarchical analysis applying three bioinformatics approaches. We predicted 75.6% of all early 1,25(OH)₂D₃-responding (2.5 or 4 h) and 57.4% of the late differentially expressed genes (24 h) to be primary VDR target genes. VDR knockout led to a complete loss of 1,25(OH)₂D₃–induced genome-wide gene regulation. Thus, there was no indication of any VDR-independent non-genomic actions of 1,25(OH)₂D₃ modulating its transcriptional response. Among the predicted primary VDR target genes, 47 were coding for transcription factors and thus may mediate secondary 1,25(OH)₂D₃ responses. CEBPA and ETS1 ChIP-seq data and RNA-seq following CEBPA knockdown were used to validate the predicted regulation of secondary vitamin D target genes by both transcription factors. In conclusion, a directional network containing 47 partly novel primary VDR target transcription factors describes secondary responses in a highly complex vitamin D signaling cascade. The central transcription factor VDR is indispensable for all transcriptome-wide effects of the nuclear hormone.

1alpha,25(OH)2-vitamin D3 membrane-initiated calcium signaling modulates exocytosis and cell survival

1alpha,25(OH)(2)-vitamin D(3) (1,25D) is considered a bone anabolic hormone. 1,25D actions leading to bone formation involve gene transactivation, on one hand, and modulation of cytoplasmic signaling, on the other. In both cases, a functional vitamin D receptor (VDR) appears to be required. Here we study 1,25D-stimulated calcium signaling that initiates at the cell membrane and leads to exocytosis of bone materials and increased osteoblast survival. We found that rapid 1,25D-induction of exocytosis couples to cytoplasmic calcium increase in osteoblastic ROS 17/2.8 cells. In addition, we found that elevation of cytoplasmic calcium concentration is involved in 1,25D anti-apoptotic effects via Akt activation in ROS 17/2.8 cells and non-osteoblastic CV-1 cells. In both cases, 1,25D-stimulated elevation of intracellular calcium is due in part to activation of L-type Ca(2+) channels. We conclude that 1,25D bone anabolic effects that involve increased intracellular Ca(2+) concentration in osteoblasts can be explained at two levels. At the single-cell level, 1,25D promotes Ca(2+)-dependent exocytotic activities. At the tissue level, 1,25D protects osteoblasts from apoptosis via a Ca(2+)-dependent Akt pathway. Our studies contribute to the understanding of the molecular basis of bone diseases characterized by decreased bone formation and mineralization.

Modulation of 1alpha,25-dihydroxyvitamin D3-membrane associated, rapid response steroid binding protein expression in mouse odontoblasts by 1alpha,25-(OH)2D3

The rapid, nongenomic effects of 1alpha,25-dihydroxyvitamin D3 (1alpha,25-(OH)2D3 have been related to a 1,25D3-membrane associated, rapid response steroid binding protein or 1,25D3-[MARRS]bp, with a molecular weight of 65 kDa, in several tissues and species. Currently, no information is available concerning the nongenomic responses to 1alpha,25-(OH)2D3 in dental tissues. In order to investigate the expression of 1,25D3-[MARRS]bp in dental cells, in the presence or absence of 1alpha,25-(OH)2D3, we have used rabbit polyclonal antibodies directed against the N-terminus of the 1,25D3-[MARRS]bp (Ab099) that recognizes the 1alpha,25-(OH)2D3 binding protein in chick intestinal basolateral membranes and a mouse odontoblast-like cell line (MO6-G3). Western blotting and flow cytometric analyses with Ab099 specifically detected 1,25D3-[MARRS]bp in MO6-G3 cells. Moreover, 1,25D3-[MARRS]bp was up-regulated, in vivo, in differentiated dental cells. Electron microscopic analysis confirmed the plasma membrane localization of this binding protein and also showed its intracellular presence. Incubation of MO6-G3 cells with different doses of 1alpha,25-(OH)2D3 for 36 h resulted in an inhibition of 1,25D3-[MARRS]bp expression with a maximal effect at 50 nM steroid. In addition, the culture media of MO6-G3 cells contains immunoreactive 1,25D3-[MARRS]bp. Immunogold positive membrane vesicle-like structures are present in the extracellular matrix of MO6-G3 cells. Altogether, these results indicate that the 1,25D3-[MARRS]bp expression in MO6-G3 cells is modulated by 1alpha,25-(OH)2D3. In conclusion, this 1alpha,25-(OH)2D3 binding protein could play an important role in the rapid, nongenomic responses to 1alpha,25-(OH)2D3 in dental cells.

Nongenomic steroid action: controversies, questions, and answers

Steroids may exert their action in living cells by several ways: 1). the well-known genomic pathway, involving hormone binding to cytosolic (classic) receptors and subsequent modulation of gene expression followed by protein synthesis. 2). Alternatively, pathways are operating that do not act on the genome, therefore indicating nongenomic action. Although it is comparatively easy to confirm the nongenomic nature of a particular phenomenon observed, e.g., by using inhibitors of transcription or translation, considerable controversy exists about the identity of receptors that mediate these responses. Many different approaches have been employed to answer this question, including pharmacology, knock-out animals, and numerous biochemical studies. Evidence is presented for and against both the participation of classic receptors, or proteins closely related to them, as well as for the involvement of yet poorly understood, novel membrane steroid receptors. In addition, clinical implications for a wide array of nongenomic steroid actions are outlined.

Vitamin D receptor is not required for the rapid actions of 1,25-dihydroxyvitamin D3 to increase intracellular calcium and activate protein kinase C in mouse osteoblasts

The rapid, non-genomic actions of 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] have been well described, however, the role of the nuclear vitamin D receptor (VDR) in this pathway remains unclear. To address this question, we used VDR(+/+) and VDR(-/-) osteoblasts isolated from wild-type and VDR null mice to study the increase in intracellular calcium ([Ca(2+)](i)) and activation of protein kinase C (PKC) induced by 1,25(OH)(2)D(3). Within 1 min of 1,25(OH)(2)D(3) (100 nM) treatment, an increase of 58 and 53 nM in [Ca(2+)](i) (n = 3) was detected in VDR(+/+) and VDR(-/-) cells, respectively. By 5 min, 1,25(OH)(2)D(3) caused a 2.1- and 1.9-fold increase (n = 6) in the phosphorylation of PKC substrate peptide acetylated-MBP(4-14) in VDR(+/+) and VDR(-/-) osteoblasts. The 1,25(OH)(2)D(3)-induced phosphorylation was abolished by GF109203X, a general PKC inhibitor, in both cell types, confirming that the secosteroid induced PKC activity. Moreover, 1,25(OH)(2)D(3) treatment resulted in the same degree of translocation of PKC-alpha and PKC-delta, but not of PKC-zeta, from cytosol to plasma membrane in both VDR(+/+) and VDR(-/-) cells. These experiments demonstrate that the 1,25(OH)(2)D(3)-induced rapid increases in [Ca(2+)](i) and PKC activity are neither mediated by, nor dependent upon, a functional nuclear VDR in mouse osteoblasts. Thus, VDR is not essential for these rapid actions of 1,25(OH)(2)D(3) in osteoblasts.

Nongenomic actions of steroid hormones

Steroid hormones modulate many physiological processes. The effects of steroids that are mediated by the modulation of gene expression are known to occur with a time lag of hours or even days. Research that has been carried out mainly in the past decade has identified other responses to steroids that are much more rapid and take place in seconds or minutes. These responses follow nongenomic pathways, and they are not rare.

1alpha,25-dihydroxyvitamin D3 inhibits uncoupling protein 2 expression in human adipocytes

We recently demonstrated that suppressing 1alpha,25-(OH)2-D3 by increasing dietary calcium decreases adipocyte intracellular Ca2+ ([Ca2+]i), stimulates lipolysis, and inhibits lipogenesis. High calcium diets also increase core temperature and white adipose tissue uncoupling protein 2 (UCP2) expression in aP2-agouti transgenic mice. Accordingly, we have evaluated the role of 1alpha,25-(OH)2-D3 in regulating human adipocyte UCP2 expression. Treatment of human adipocytes for 48 h with 1 nM 1alpha,25-(OH)2-D3 inhibited UCP2 mRNA and protein levels by 50% (P<0.002) and completely blocked isoproterenol- or fatty acid-stimulated two- to threefold increases in UCP2 expression. However, a specific agonist for the membrane vitamin D receptor (mVDR), 1alpha,25-dihydroxylumisterol3, was unable to inhibit basal, isoproterenol-stimulated, or fatty acid-stimulated UCP2 expression, whereas a specific mVDR antagonist,1beta,25-dihydroxyvitamin D3, was unable to prevent the 1alpha,25-(OH)2-D3 inhibition of UCP2 expression. In contrast, nuclear vitamin D receptor (nVDR) knockout via antisense oligodeoxynucleotide (ODN) prevented the inhibitory effect of 1alpha,25-(OH)2-D3 on adipocyte UCP2 expression and protein levels. These data indicate that 1a,25-(OH)2-D3 exerts an inhibitory effect on adipocyte UCP2 expression via the nVDR. Thus, suppression of 1alpha,25-(OH)2-D3 and consequent up-regulation of UCP2 may contribute to our previous observation of increased thermogenesis in mice fed with high calcium diets.

Expression of a 1,25-dihydroxyvitamin D3 membrane-associated rapid-response steroid binding protein during human tooth and bone development and biomineralization

The calciotropic hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has been established to control skeletal tissue formation and biomineralization via the regulation of gene expression. This action involves the well-characterized nuclear 1,25(OH)2D3 receptor. However, it has been recognized that several cellular responses to 1,25(OH)2D3 may not to be related to the exclusive nuclear receptor. Indeed, this secosteroid is able to generate rapid responses that have been proposed to be mediated by interactions of the ligand, which is a putative cell membrane-associated rapid-response steroid (MARRS) binding protein for 1,25(OH)2D3 [1,25D3-MARRS]. The nongenomic pathway of 1,25(OH)2D3 was studied here in detail by immunolocalization of the 1,25D3-MARRS during the specific context of human prenatal development. Western blotting with proteins extracted from 4 week- to 27-week-old embryos was performed, evidencing a 65-kDa molecular species recognized by antibody Ab 099 generated against synthetic peptides corresponding to the N terminus of the 1,25D3-MARRS from chick intestinal basolateral membranes. Based on this biochemical conservation of protein in the human species, the temporospatial expression patterns were established in the craniofacial skeleton at the same ages. Comparative analysis was performed in teeth and bones from early morphogenesis to terminal cell differentiation and extracellular biomineralization. The data show the potential implication of 1,25D3-MARRS in the heterogeneous cell population including ameloblasts, odontoblasts, osteoblasts, and osteoclasts. The epithelial-mesenchymal cascade related to odontogenesis was coincident with a sequence of up- and down-regulation of immunoreactive 1,25D3-MARRS. Biomineralization was associated with a striking up-regulation in the adjoining secretory cells in all tissues. Finally, osteoclasts appeared also to express the 1,25D3-MARRS during these early phases of bone modeling. Previously obtained data of the nuclear vitamin D receptor (VDR) expression and this study on 1,25D3-MARRS suggest the existence of cross-talk between the genomic and nongenomic pathways during human development.

1alpha,25-dihydroxyvitamin D(3) and 24R,25-dihydroxyvitamin D(3) modulate growth plate chondrocyte physiology via protein kinase C-dependent phosphorylation of extracellular signal-regulated kinase 1/2 mitogen-activated protein kinase

Membrane-mediated increases in protein kinase C (PKC) activity and PKC-dependent physiological responses of growth plate chondrocytes to vitamin D metabolites depend on the state of endochondral maturation; 1alpha,25-dihydroxyvitamin D(3) [1alpha,25-(OH)(2)D(3)] regulates growth zone (GC) cells, whereas 24R,25-(OH)(2)D(3) regulates resting zone (RC) cells. Different mechanisms, including protein kinase A signaling, mediate the effects of 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) on PKC, suggesting that different mechanisms may also regulate any MAPK involvement in the physiological responses. This study used confluent cultures of rat costochondral chondrocytes as a model. 1alpha,25-(OH)(2)D(3) stimulated MAPK specific activity in GC in a time- and dose-dependent manner, evident within 9 min. 24R,25-(OH)(2)D(3) stimulated MAPK in RC; increases were dose dependent, occurred after 9 min, and were greatest at 90 min. In both cells the effect was due to ERK1/2 activation (p42 > p44 in GC; p42 = p44 in RC). MAPK activation was dependent on PKC, but not protein kinase A. The effect of 1alpha,25-(OH)(2)D(3) required phospholipase C, and the effect of 24R,25-(OH)(2)D(3) required phospholipase D. Inhibition of cyclooxygenase activity reduced the effect of 1alpha,25-(OH)(2)D(3) on MAPK in GC and enhanced the effect of 24R,25-(OH)(2)D(3) in RC. Based on MAPK inhibition with PD98059, ERK1/2 MAPK mediated the effect of 24R,25-(OH)(2)D(3) on [(3)H]thymidine incorporation and [(35)S]sulfate incorporation by RC, but only partially mediated the effect of 1alpha,25-(OH)(2)D(3) on GC. ERK1/2 was not involved in the regulation of alkaline phosphatase specific activity by either metabolite. This paper supports the hypothesis that 1alpha,25-(OH)(2)D(3) regulates the physiology of GC via rapid membrane-mediated signaling pathways, and some, but not all, of the response to 1alpha,25-(OH)(2)D(3) is via the ERK family of MAPKs. In contrast, 24R,25-(OH)(2)D(3) exerts its effects on RC via PKC-dependent MAPK. Whereas 1alpha,25-(OH)(2)D(3) increases MAPK activity via phospholipase C and increased prostaglandin production, 24R,25-(OH)(2)D(3) increases MAPK via phospholipase D and decreased prostaglandin production. The cell specificity, metabolite stereospecificity, and the dependence on PKC argue for the participation of membrane receptors for 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) in the regulation of ERK1/2 in the growth plate.

Histone acetylation in vivo at the osteocalcin locus is functionally linked to vitamin D-dependent, bone tissue-specific transcription

The accessibility of regulatory elements in chromatin represents a principal rate-limiting parameter of gene transcription and is modulated by enzymatic transcriptional co-factors that alter the topology of chromatin or covalently modify histones (e.g. by acetylation). The bone-specific activation and 1,25-dihydroxyvitamin D(3) enhancement of osteocalcin (OC) gene transcription are both functionally linked to modifications in nucleosomal organization. The initiation of tissue-specific basal transcription is accompanied by the induction of two DNase I hypersensitive sites, and this chromatin remodeling event requires binding of the key osteogenic factor RUNX2/CBFA1 to the OC promoter. Here, we analyzed the acetylation status of histones H3 and H4 when the OC gene is active (in osteoblastic ROS17/2.8 cells) or inactive (in fibroblastic ROS24/1 cells) using chromatin immunoprecipitation assays. We find that acetylated histone H3 and H4 proteins are associated with the OC promoter only when the gene is transcriptionally active and that the acetylation status is relatively uniform across the OC locus under basal conditions. Acetylation of H4 at the OC gene is selectively increased following vitamin D(3) enhancement of OC transcription, with the most prominent changes occurring in the region between the vitamin D(3) enhancer and basal promoter. Thus, our results suggest functional linkage of H3 and H4 acetylation in specific regions of the OC promoter to chromatin remodeling that accompanies tissue-specific transcriptional activation and vitamin D enhancement of OC gene expression. These findings provide mechanistic insights into bone-specific gene activation within a native genomic context in response to steroid hormone-related regulatory cues.

The tyrosine kinase c-Src is required for 1,25(OH)2-vitamin D3 signalling to the nucleus in muscle cells

We have recently shown that the hormonal form of vitamin D3, 1,25(OH)2-vitamin D3 (1,25(OH)2D3), stimulates the enzymatic activity of the non-receptor protein tyrosine kinase c-Src in skeletal muscle cells. In this study we show that intracellular and extracellular Ca2+ chelation with BAPTA and EGTA, respectively, blocked hormone stimulation of c-Src activity/dephosphorylation, indicating that the calcium messenger system is an upstream activator of c-Src. Tyrosine phosphorylation and stimulation of the growth-related mitogen-activated protein kinase (MAPK) by 1,25(OH)2D3 was shown to be dependent on activation of c-Src, since pretreatment with the c-Src specific inhibitor PP1 or muscle cell transfection with an antisense oligodeoxynucleotide directed against c-Src mRNA markedly reduced hormone stimulation of MAPK phosphorylation. Evidence was obtained indicating that MAPK is then translocated to the cell nucleus in active phosphorylated form and induces the expression of c-myc oncoprotein, as the MAPK kinase (MEK) inhibitor PD98059 abolished stimulation of c-myc synthesis by 1,25(OH)2D3. In addition, the hormone rapidly stimulated tyrosine phosphorylation of c-myc. In cells pretreated with PP1 (4-amino-5-(4-methylphenyl)-7-(t-butyl)pyrazolo-D3,4-pyrimidine), the 1,25(OH)2D3-induced increase in tyrosine phosphorylation of c-myc was suppressed. Taken together, these results demonstrate that 1,25(OH)2D3 stimulates proliferation-associated signalling pathways in skeletal muscle cells and implicate c-Src kinase as mediator of this response.

1alpha,25-Dihydroxyvitamin D3 modulates human adipocyte metabolism via nongenomic action

We reported recently that suppression of the renal 1alpha,25-dihyroxyvitamin D3 (1lpha,25-(OH)2-D3) production in aP2-agouti transgenic mice by increasing dietary calcium decreases adipocyte intracellular Ca2+ ([Ca2+]i), stimulates lipolysis, inhibits lipogenesis, and reduces adiposity. However, it was not clear whether this modulation of adipocyte metabolism by dietary calcium is a direct effect of inhibition of 1alpha,25-(OH)2-D3-induced [Ca2+]i. Accordingly, we have now evaluated the direct role of 1alpha,25-(OH)2-D3. Human adipocytes exhibited a 1alpha,25-(OH)2-D3 dose-responsive (1-50 nM) increase in [Ca2+]i (P<0.01). This action was mimicked by 1alpha,25-dihyroxylumisterol3 (1alpha,25-(OH)2-lumisterol3) (P<0.001), a specific agonist for a putative membrane vitamin D receptor (mVDR), and completely prevented by 1b,25-dihydroxyvitamin D3 (1beta,25-(OH)2-D3), a specific antagonist for the mVDR. Similarly, 1alpha,25-(OH)2-D3 (5 nM) caused 50%-100% increases in adipocyte fatty acid synthase (FAS) expression and activity (P<0.02), a 61% increase in glycerol-3-phosphate dehydrogenase (GPDH) activity (P<0.01), and an 80% inhibition of isoproterenol-stimulated lipolysis (P<0.001), whereas 1beta,25-(OH)2-D3 completely blocked all these effects. Notably, 1alpha,25-(OH)2-lumisterol3 exerted more potent effects in modulating adipocyte lipid metabolism, with 2.5- to 3.0-fold increases in FAS expression and activity (P<0.001) and a threefold increase in GPDH activity (P<0.001). Also 1alpha,25-(OH)2-lumisterol3 was approximately twice as potent in inhibiting basal lipolysis (P<0.025), whereas 1beta,25-(OH)2-D3 completely blocked all these effects. These data suggest that 1alpha,25-(OH)2-D3 modulates adipocyte Ca2+ signaling and, consequently, exerts a coordinated control over lipogenesis and lipolysis. Thus, a direct inhibition of 1alpha,25-(OH)2-D3-induced [Ca2+]i may contribute to an anti-obesity effect of dietary calcium, and the mVDR may represent an important target for obesity.

Binding of 1alpha,25-dihydroxyvitamin D(3) to annexin II: effect of vitamin D metabolites and calcium

We have recently reported that annexin II serves as a membrane receptor for 1alpha,25-(OH)(2)D(3) and mediates the rapid effect of the hormone on intracellular calcium. The purpose of these studies was to characterize the binding of the hormone to annexin II, determine the specificity of binding, and assess the effect of calcium on binding. The binding of [(14)C]-1alpha,25-(OH)(2)D(3) bromoacetate to purified annexin II was inhibited by 1alpha, 25-(OH)(2)D(3) in a concentration-dependent manner. Binding of the radiolabeled ligand to annexin II was markedly diminished by 1alpha, 25-(OH)(2)D(3) at 24 microM, 18 microM, and 12 microM and blunted by 6 microM and 3 microM. At a concentration of 12 microM, 1beta, 25-(OH)(2)D(3) also diminished the binding of [(14)C]-1alpha, 25-(OH)(2)D(3) bromoacetate to annexin II, but cholecalciferol, 25-(OH)D(3), and 24,25-(OH)(2)D(3) did not. Saturation analyses of the binding of [(3)H]-1alpha,25-(OH)(2)D(3) to purified annexin II showed a K(D) of 5.5 x 10(-9) M, whereas [(3)H]-1beta,25-(OH)(2)D(3) exhibited a K(D) of 6.0 x 10(-9) M. Calcium, which binds to the carboxy terminal domain of annexin II, had a concentration-dependent effect on [(14)C]-1alpha,25-(OH)(2)D(3) bromoacetate binding to annexin II, with 600 nM calcium being able to inhibit binding of the radiolabeled analog. The inhibitory effect of calcium was prevented by EDTA. Homocysteine, which binds to the amino terminal domain of annexin II, had no effect on the binding of the bromoacetate analog to the protein. The data indicate that 1alpha,25-(OH)(2)D(3) binding to annexin II is specific and suggest that the binding site may be located on the carboxy terminal domain of the protein. The ability of 1beta,25-(OH)(2)D(3) to inhibit the binding of [(14)C]-1alpha, 25(OH)(2)D(3) bromoacetate to annexin II provides a biochemical explanation for the ability of the 1beta-epimer to inhibit the rapid actions of the hormone in vitro.

Calcium-channel activation and matrix protein upregulation in bone cells in response to mechanical strain

Femur-derived osteoblasts cultured from rat femora were loaded with Fluo-3 using the AM ester. A quantifiable stretch was applied and [Ca(2+)]i levels monitored by analysis of fluorescent images obtained using an inverted microscope and laser scanning confocal imaging system. Application of a single pulse of tensile strain via an expandable membrane resulted in immediate increase in [Ca(2+)]i in a proportion of the cells, followed by a slow and steady decrease to prestimulation levels. Application of parathyroid hormone (10(-6) M) prior to mechanical stimulation potentiated the load-induced elevation of [Ca(2+)]i. Mechanically stimulating osteoblasts in Ca(2+)-free media or in the presence of either nifedipine (10 microM; L-type Ca(2+)-channel blocker) or thapsigargin (1 microM; depletes intracellular Ca(2+) stores) reduced strain-induced increases in [Ca(2+) ]i. Furthermore, strain-induced increases in [Ca(2+)]i were enhanced in the presence of Bayer K 8644 (500 nm), an agonist of L-type calcium channels. The effects of mechanical strain with and without inhibitors and agonists are described on the total cell population and on single cell responses. Application of strain and strain in the presence of the calcium-channel agonist Bay K 8644 to periosteal-derived osteoblasts increased levels of the extracellular matrix proteins osteopontin and osteocalcin within 24 h postload. This mechanically induced increase in osteopontin and osteocalcin was inhibited by the addition of the calcium-channel antagonist, nifedipine. Our results suggest an important role for L-type calcium channels and a thapsigargin-sensitive component in early mechanical strain transduction pathways in osteoblasts.

Hormonal regulation of [Ca(2+)](i) in periosteal-derived osteoblasts: effects of parathyroid hormone, 1,25(OH)(2)D(3) and prostaglandin E(2)

The effects of hormonal modulators of osteoblast function, parathyroid hormone, 1,25(OH)(2)D(3) and prostaglandins on [Ca(2+)](i) in periosteal-derived osteoblasts from rat femurs have been investigated. Our results show that application of parathyroid hormone PTH (10(-5) M) and prostaglandin E(2) (PGE(2)) (4 microM) result in a rapid heterogeneous elevation in [Ca(2+)](i) that, in the case of PTH, is dependent on both extracellular and intracellular sources of calcium. Variable responses to treatments have been found within populations of cells. The PGE(2) response is dose dependent. Treatment with 1,25(OH)(2)D(3) (10(-8) M) induces a brief (60-90 sec) elevation in [Ca(2+)](i) that is almost totally abolished in EGTA-buffered Ca(2+)-free medium. Interactive effects of multiple hormone treatments have been observed. Pretreatment with 1,25(OH)(2)D(3) results in near-total inhibition of the PTH and PGE(2) responses. In conclusion, modulation of [Ca(2+)](i) appears to play a role not only in the direct effects of osteotropic hormones on osteoblasts but also in the synergistic and antagonistic effects between circulating hormones.

Induction of tissue plasminogen activator secretion from rat heart microvascular cells by fM 1,25(OH)(2)D(3)

We investigated the effects of 1,25-dihydroxyvitamin D(3) [25(OH)(2)D(3)] on tissue plasminogen activator (tPA) secretion from primary cultures of rat heart microvascular cells. After an initial 5-day culture period, cells were treated for 24 h with 1,25(OH)(2)D(3) and several of its analogs. The results showed that 1,25(OH)(2)D(3) induced tPA secretion at 10(-10) to 10(-16) M. A less calcemic analog, Ro-25-8272, and an analog that binds the vitamin D receptor but is ineffective at perturbing Ca(2+) channels, Ro-24-5531, were approximately 10% as active as 1,25(OH)(2)D(3). An analog that binds the vitamin D receptor poorly but is an effective Ca(2+) channel agonist, Ro-24-2287, required approximately 10(-13) M to induce tPA secretion. Combinations of Ro-24-5531 and Ro-24-2287 were approximately as potent as 1,25(OH)(2)D(3). Treatment of the cells with BAY K 8644 or thapsigargin also increased tPA secretion, suggesting that increased cytosolic calcium concentration ([Ca(2+)]) induces tPA secretion. The results suggested that the sensitivity of the tPA secretory response of microvascular cells to 1,25(OH)(2)D(3) was due in part to generation of a vitamin D-depleted state in vitro and in part to synergistic effects of 1,25(OH)(2)D(3) on two different induction pathways of tPA release.

Vitamin D analogs, 20-Epi-22-oxa-24a,26a,27a,-trihomo-1alpha,25(OH)2-vitamin D3, 1,24(OH)2-22-ene-24-cyclopropyl-vitamin D3 and 1alpha,25(OH)2-lumisterol3 prime NB4 leukemia cells for monocytic differentiation via nongenomic signaling pathways, involving calcium and calpain

Side-chain modified vitamin D analogs including 20-Epi-22-oxa-24a,26a,27a-trihomo-1alpha,2 5-dihydroxyvitamin D3 (KH1060), and 1,24-dihydroxy-22-ene-24-cyclopropyl-vitamin D3 (MC903) were originally designed to aid in the treatment of hyperproliferative disorders including psoriasis and cancer. Here we demonstrate that these analogs, as well as the 6-cis-locked conformer, 1alpha,25-dihydroxy-lumisterol3 (JN) prime NB4 cells for monocytic differentiation. Previously, the action of MC903 and KH1060 was presumed to be mediated by the nuclear vitamin D receptor (VDRnuc). Differentiation in response to all analogs was shown to be inhibited by 1beta,25-dihydroxyvitamin D3 (HL), the antagonist to the nongenomic activities of 1,25D3. These data suggest that although MC903 and KH1060 may bind the VDRnuc, that the differentiative activities of these agents requires nongenomic signaling pathways. Here we show that 1alpha,25(OH)2-d5-previtamin D3 (HF), JN, KH1060, and MC903 induce expression of PKC alpha and PKC delta and translocation of both isoforms to the particulate fraction, and PKC alpha to the nuclear fraction. The full differentiation response with combinations of analogs and TPA was inhibited 50% by the membrane permeable Ca2+ chelator, 1,2-bis(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA-AM) or calpain inhibitor I. These data demonstrate that intracellular free calcium and the calcium-dependent protease, calpain play critical roles in monocytic differentiation. Intracellular calcium appears to be most critical in the 1,25D3-priming stage of differentiation, while calpain is essential in the TPA maturation response.

Physiological importance of the 1,25(OH)2D3 membrane receptor and evidence for a membrane receptor specific for 24,25(OH)2D3

We have recently identified a membrane vitamin D receptor (mVDR) specific for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and shown that it mediates the rapid activation of protein kinase C (PKC) in growth zone chondrocytes (GCs). In this study, we examine the role of the 1, 25(OH)2D3-mVDR in chondrocyte physiology and provide evidence for the existence of a specific membrane receptor for 24, 25-dihydroxyvitamin D3 (24,25(OH)2D3-mVDR). Fourth-passage cultures of growth plate chondrocytes at two distinct stages of endochondral development, resting zone (RC) and growth zone (GC) cells, were used to assess the role of the mVDR in cell proliferation, PKC activation, and proteoglycan sulfation. To preclude the involvement of the nuclear vitamin D receptor (nVDR), we used hybrid analogs of 1, 25(OH)2D3 with <0.1% affinity for the nVDR (2a, 1alpha-CH2OH-3beta-25D3; 3a, 1alpha-CH2OH-3beta-20-epi-22-oxa-25D3; and 3b, 1beta-CH2OH-3alpha-20-epi-22-oxa-25D3). To determine the involvement of the mVDR, we used an antibody generated against the highly purified 1,25(OH)2D3 binding protein from chick intestinal basolateral membranes (Ab99). Analog binding to the mVDR was demonstrated by competition with [3H]1,25(OH)2D3 using matrix vesicles (MVs) isolated from cultures of RC and GC cells. Specific recognition sites for 24,25(OH)2D3 in RC MVs were demonstrated by saturation binding analysis. Specific binding of 24,25(OH)2D3 was also investigated in plasma membranes (PMs) from RC and GC cells and GC MVs. In addition, we examined the ability of Ab99 to block the stimulation of PKC by analog 2a in isolated RC PMs as well as the inhibition of PKC by analog 2a in GC MVs. Like 1,25(OH)2D3, analogs 2a, 3a, and 3b inhibit RC and GC cell proliferation. The effect was dose dependent and could be blocked by Ab99. In GC cells, PKC activity was stimulated maximally by analogs 2a and 3a and very modestly by 3b. The effect of 2a and 3a was similar to that of 1, 25(OH)2D3 and was blocked by Ab99, whereas the effect of 3b was unaffected by antibody. In contrast, 2a was the only analog that increased PKC activity in RC cells, and this effect was unaffected by Ab99. Analog 2a had no effect on proteoglycan sulfation in RC cells, whereas analogs 3a and 3b stimulated it and this was not blocked by Ab99. Binding of [3H]1,25(OH)2D3 to GC MVs was displaced completely with 1,25(OH)2D3 and analogs 2a, 3a, and 3b, but 24, 25(OH)2D3 only displaced 51% of the bound ligand. 24,25(OH)2D3 displaced 50% of [3H]1,25(OH)2D3 bound to RC MVs, but 2a, 3a, and 3b displaced <50%. Scatchard analysis indicated specific binding of 24, 25(OH)2D3 to recognition sites in RC MVs with a Kd of 69.2 fmol/ml and a Bmax of 52.6 fmol/mg of protein. Specific binding for 24, 25(OH)2D3 was also found in RC and GC PMs and GC MVs. GC membranes exhibited lower specific binding than RC membranes; MVs had greater specific binding than PMs in both cell types. 2a caused a dose-dependent increase in PKC activity of RC PMs that was unaffected by Ab99; it inhibited PKC activity in GC MVs, and this effect was blocked by Ab99. The results indicate that the 1, 25(OH)2D3 mVDR mediates the antiproliferative effect of 1,25(OH)2D3 on chondrocytes. It also mediates the 1,25(OH)2D3-dependent stimulation of PKC in GC cells, but not the 2a-dependent increase in RC PKC activity, indicating that 24,25(OH)2D3 mediates its effects through a separate receptor. This is supported by the failure of Ab99 to block 2a-dependent stimulation of PKC in isolated PMs. The data demonstrate for the first time the presence of a specific 24, 25(OH)2D3 mVDR in endochondral chondrocytes and show that, although both cell types express mVDRs for 1,25(OH)2D3 and 24,25(OH)2D3, their relative distribution is cell maturation-dependent.

Production of 1alpha,25-dihydroxy-3-epi-vitamin D3 in two rat osteosarcoma cell lines (UMR 106 and ROS 17/2.8): existence of the C-3 epimerization pathway in ROS 17/2.8 cells in which the C-24 oxidation pathway is not expressed

The secosteroid hormone 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] is metabolized into calcitroic acid through the carbon 24 (C-24) oxidation pathway. It is now well established that the C-24 oxidation pathway plays an important role in the target tissue inactivation of 1alpha,25(OH)2D3. Recently, we reported that 1alpha,25(OH)2D3 is also metabolized into 1alpha,25-dihydroxy-3-epi-vitamin D3 [1alpha,25(OH)2-3-epi-D3] through the carbon 3 (C-3) epimerization pathway in human keratinocytes, human colon carcinoma cells (Caco-2), and bovine parathyroid cells. In a previous study, it was demonstrated that 1alpha,25(OH)2-3-epi-D3 when compared to 1alpha,25(OH)2D3 was less active in stimulating intestinal calcium absorption, calcium mobilization from bone, and induction of calbindin D28k. These findings suggest that the C-3 epimerization pathway, like the C-24 oxidation pathway, may play a role in the target tissue inactivation of 1alpha,25(OH)2D3. In this study, we determined the relationship between the C-24 oxidation and the C-3 epimerization pathways by investigating the metabolism of 1alpha,25(OH)2D3 in two rat osteosarcoma cell lines (UMR 106 and ROS 17/2.8). These two cell lines differ from each other in their ability to metabolize 1alpha,25(OH)2D3 through the C-24 oxidation pathway. It has been previously reported that the C-24 oxidation pathway is expressed only in UMR 106 cells but not in ROS 17/2.8 cells. The results of our present study provide new evidence that both cell lines possess the ability to metabolize 1alpha,25(OH)2D3 into 1alpha,25(OH)2-3-epi-D3 through the C-3 epimerization pathway. Our results also reconfirm the findings of previous studies indicating that UMR 106 cells are the only ones which express the C-24 oxidation pathway out of the two cell lines studied. Furthermore, this study reveals for the first time that the C-3 epimerization pathway may become an alternate metabolic pathway for the target tissue inactivation of 1alpha,25(OH)2D3 in some cells, such as ROS 17/2.8, in which the C-24 oxidation pathway is not expressed.

Nongenomic effects of 1alpha,25-dihydroxyvitamin D(3)

The hormonally active form of vitamin D, 1alpha,25-dihydroxyvitamin D(3), is the key molecule of the vitamin D endocrine system, which produces biological effects in about 30 target cell systems. Growing experimental evidence supports the hypothesis that these biological effects can be generated both by a signal transduction mechanism involving a nuclear receptor (nVDR) that modulates gene transcription, and via a nongenomic receptor located in the plasma membrane (mVDR), which modulates a complex signaling system involving the rapid opening of Ca(2+) channels. Some data reviewed herein also indicate that crosstalk between genomic and nongenomic pathways operates in several cell types, and suggest that the physiological role of the rapid, nongenomic actions might involve the regulation of hormone-mediated gene activation.

Membrane receptors for steroid hormones: a case for specific cell surface binding sites for vitamin D metabolites and estrogens

Steroid hormones, including vitamin D metabolites and estrogens, activate target cells through specific receptors that discriminate among ligands based upon recognition of distinct structural features. For both classes of ligands, cell surface and nuclear receptors co-exist in many target cells. Upon ligand binding, these receptors generate both rapid and long lasting responses. While the structure of the nuclear receptors and their function as transcriptional activators of specific target genes is generally understood, the identity of the membrane receptors remains elusive. Using pharmacological, functional and biochemical approaches, new insights are being gained into nature of the cell surface receptors for both vitamin D metabolites and estrogens.

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.

Up-regulation of Ca2+ influx mediated by store-operated channels in HL60 cells induced to differentiate by 1 alpha,25-dihydroxyvitamin D3

The physiologically active form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 (1,25D3), induces promyelocytic HL60 cells to differentiate towards monocyte-like cells. During this differentiation increased cytosolic calcium (Cai2+) and expression of surface receptors for chemotactic factors "prime" the cell for the activation of monocyte functions and the triggering of the respiratory burst pathway. We examined whether the Ca2+ influx mediated by store-operated channels (SOC) contributed to the increased Cai2+ following exposure of HL60 cells to 10(-7) M 1,25D3. Cells treated with 1,25D3 for 72 hr demonstrated a rapid transient rise in Cai2+ followed by a second, phasic, increase in Cai2+ in response to the purinergic agonist ATP. This second Cai2+ transient was blocked by Ni2+, SKF 96365, or withdrawal of extracellular Ca2+. In cells suspended in Ca(2+)-free medium, peak changes (delta) in [Ca2+]i elicited by ATP-induced Ca2+ mobilization occurred with similar EC50 values in differentiated and vehicle (EtOH)-treated cells; however, peak [Ca2+]i was reduced by 55% in 1,25D3-treated cells. Decreased Ca2+ mobilization was associated with a 25-35% reduction in intracellular Ca2+ stores (determined with ionomycin). 1,25D3-treated cells exposed to ATP or thapsigargin (Tg) in Ca(2+)-free medium for 3 min with subsequent addition of 1 mM Ca2+ exhibited a respective 80% or 120% stimulation in peak [Ca2+]i compared to EtOH-treated cells. Enhanced Ca2+ influx mediated by SOC was also seen in these cells as an increase in the rate of Mn2+ entry after exposure to ATP or Tg. At 96 hr after addition of 1,25D3, when differentiated phenotype was established, basal Ca2+i and Ca2+ entry mediated by SOC returned to control values, but Ca2+ store size remained reduced. Up-regulation of Ca2+ influx via the SOC pathway during 1,25D3-induced differentiation may contribute to the functional properties of the maturing monocyte, or to the resetting of molecular programs responsible for the changing phenotype.

Tissue-specific regulation by vitamin D3 of a novel protein containing ankyrin-like repeats

Vitamin D3 is the precursor of the steroid hormone 1,25-dihydroxyvitamin D3 which is involved in the regulation of calcium metabolism, growth and differentiation. We used differential display of mRNA populations from kidney and intestine of vitamin D3-deficient and -replete chicks to determine the steady-state abundance of approximately 5000 mRNAs. One of these sequences, whose differential expression in kidney and down-regulation by vitamin D3 was confirmed by Northern analysis, was used to screen a cDNA library from vitamin D3-deficient chick kidney in order to obtain a full length cDNA. Subcloning and sequencing revealed that this cDNA encodes a novel protein containing ankyrin-like repeats and a C-terminal Fe-S binding region signature. The encoded protein consists of 617 amino acids and contains two sets of four ankyrin-like repeats separated by 146 amino acids. This motif consists of approximately 33 amino acids containing a highly conserved central hydrophobic alpha helix and is abundant in a wide variety of proteins, particularly those participating in the protein-protein or protein-membrane interactions involved in signal transduction, regulation of the cell cycle and control of transcription. Outside of the ankyrin-like domains, no homologies with other proteins in existing data bases were found. Our results have revealed a novel protein containing ankyrin-like repeats tissue-specifically down-regulated by vitamin D3 in the kidney.

Effects of the vitamin D3 analog 1 alpha, 25-dihydroxyvitamin D3-3 beta-bromoacetate on rat osteosarcoma cells: comparison with 1 alpha, 25-dihydroxyvitamin D3

The actions of the hormonal form of vitamin D, 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2 D3], are mediated by both genomic and nongenomic mechanisms. Several vitamin D synthetic analogs have been developed in order to identify and characterize the site(s) of action of 1 alpha, 25-(OH)2D3 in many cell types including osteoblastic cells. We have compared the effects of 1 alpha, 25-(OH)2D3 and a novel 1 alpha, 25-(OH)2D3 bromoester analog (1,25-(OH)2-BE) that covalently binds to vitamin D receptors. Rat osteosarcoma cells that possess (ROS 17/2.8) or lack (ROS 24/1) the classic intracellular vitamin D receptor were studied to investigate genomic and nongenomic actions. In ROS 17/2.8 cells plated at low density, the two vitamin D compounds (1 x 10(-8) M) caused increased cell proliferation, as assessed by DNA synthesis and total cell counts. Northern blot analysis revealed that the mitogenic effect of both agents was accompanied by an increase in steady-state osteocalcin mRNA levels, but neither agent altered alkaline phosphatase mRNA levels in ROS 17/2.8 cells. ROS 17/2.8 cells responded to 1,25-(OH)2-BE but not the natural ligand with a significant increase in osteocalcin secretion after 72, 96, 120, and 144 hr of treatment. Treatment of ROS 17/2.8 cells with the bromoester analog also resulted in a significant decrease in alkaline phosphatase-specific activity. To compare the nongenomic effects of 1 alpha, 25-(OH)2D3 and 1,25-(OH)2-BE intracellular calcium was measured in ROS 24/1 cells loaded with the fluorescent calcium indicator Quin 2. At 2 x 10(-8) M, both 1 alpha,25-(OH)2D3 and 1, 25-(OH)2-BE increased intracellular calcium within 5 min. Both the genomic and nongenomic actions of 1,25-(OH)2-BE are similar to those of 1 alpha,25-(OH)2D3, and since 1,25-(OH)2-BE has more potent effects on osteoblast function than the naturally occurring ligand due to more stable binding, this novel vitamin D analog may be useful in elucidating the structure and function of cellular vitamin D receptors.

Differing shapes of 1 alpha,25-dihydroxyvitamin D3 function as ligands for the D-binding protein, nuclear receptor and membrane receptor: a status report

1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3] is the principal mediator of a wide array of biological responses through the far reaching network of the vitamin D endocine system (VDE). The steroid hormone 1 alpha,25(OH)2D3 is delivered to the various target organs of the VDE via a specific plasma transport protein, the vitamin D binding protein (DBP). Also 1 alpha,25(OH)2D3 is known to initiate biological responses through a nuclear receptor, the nVDR (50 kDa) which regulates selected gene transcription and, in addition in some target tissues, through a second receptor located in the cell membrane, the mVDR (approximately 60 kDa), which is linked to protein kinase C and/or voltage-gated Ca2+ channels so as to generate biological responses very rapidly. 1 alpha,25(OH)2D3 as a ligand is unusually conformationally flexible due to the eight carbon side chain, the seco B-ring which permits rotation about the 6-7 single carbon bond, and the A-ring which undergoes chair-chair conformational interconversion characteristic of cyclohexane rings. This paper reviews the evidence that different shapes of the 1 alpha,25(OH)2D3 satisfy the optimal requirements of the ligand binding domains of the DBP, nVDR and mVDR. The presence of a relatively rigid side chain (composed by the presence of an aromatic ring) enhances ligand interaction 2-3 fold with the DBP, but diminishes ligand affinity for the nVDR by 100 fold. The mVDR responds effectively to analogs of 1 alpha,25(OH)2D3 which are 6-s-cis locked [e.g. 1 alpha,25(OH)2-previtamin D3 or 1 alpha,25(OH)2-provitamin D3], but these same analogs have only 1-2% of the activity of 1 alpha,25(OH)2D3 in regulating gene transcription. Finally the 6-s-trans analog, 1 alpha,25(OH)2-tachysterol3, had <0.1% of the activity of 1 alpha,25(OH)2D3 in regulating gene transcription.

1 alpha,25-Dihydroxyvitamin D3 rapidly alters phospholipid metabolism in the nuclear envelope of osteoblasts

1 alpha,25-Dihydroxyvitamin D3 (1 alpha,25-(OH)2D3) has been shown to increase cytosolic calcium and inositol triphosphate levels in rat osteosarcoma cells (ROS 17/2.8) and to increase nuclear calcium in these cells. To determine the mechanism(s) of 1 alpha,25-(OH)2D3-induced changes in nuclear calcium, the effect of the hormone on phospholipid metabolism in isolated osteoblast nuclei was assessed. 1 alpha,25(OH)2D3, 20 nM, increased inositol triphosphate levels in the nuclei after 5 min of treatment. The biologically inactive epimer, 1 beta,25-(OH)2D3, had no significant effect on inositol triphosphate levels. ATP, 1 mM, also increased inositol triphosphate levels in the isolated nuclei after 5 min. 1 alpha,25-(OH)2D3, 20 nM, increased calcium in the isolated nuclei in the presence but not in the absence of extranuclear calcium within 5 min. Nuclear calcium was also increased within 5 min by ATP, 1 mM, and inositol triphosphate, 1 mM. The effect of ATP on nuclear calcium was not additive with 1 alpha,25-(OH)2D3, suggesting that these two agents increase nuclear calcium in these osteoblast-like cells by similar mechanisms. In summary, 1 alpha,25-(OH)2D3 and ATP rapidly increase inositol triphosphate levels in nuclei isolated from ROS 17/2.8 cells. The hormone, the nucleotide, and the inositol phospholipid increase nuclear calcium. Thus, the 1 alpha,25-(OH)2D3 and ATP effects on nuclear calcium may be mediated by changes in phospholipid metabolism in the nuclei of these osteoblast-like cells.

In vivo occupancy of the vitamin D responsive element in the osteocalcin gene supports vitamin D-dependent transcriptional upregulation in intact cells

The steroid hormone vitamin D is a principal mediator of skeletal homeostasis. 1,25-Dihydroxyvitamin D3 treatment of ROS 17/2.8 osteoblast-like cells results in a ligand-dependent increase in transcription of the bone-specific osteocalcin gene. This transcriptional upregulation requires the positive cis-acting vitamin D responsive element (VDRE). We have used the ligation-mediated polymerase chain reaction to demonstrate that protein occupancy of the VDRE within the intact cell correlates with increased synthesis of osteocalcin transcripts. These protein-DNA contacts were not present in the absence of vitamin D or in osteosarcoma cells (ROS 24.1) lacking the vitamin D receptor. Our results establish in intact cells the requirement for both ligand- and receptor-dependent occupancy of the VDRE for vitamin D responsive enhancement of osteocalcin gene transcription.

Binding characteristics of a membrane receptor that recognizes 1 alpha,25-dihydroxyvitamin D3 and its epimer, 1 beta,25-dihydroxyvitamin D3

The steroid hormone 1 alpha,25-dihydroxyvitamin D3 has been shown to exert rapid effects (15 s to 5 min) in osteoblasts. These effects occur in osteoblast-like cells lacking the nuclear vitamin D receptor, ROS 24/1, suggesting that a separate signalling system mediates the rapid actions. These non-genomic actions include rapid activation of phospholipase C and opening of calcium channels, pointing to a membrane localization of this signalling system. Previous studies have shown that the 1 beta epimer of 1 alpha,25-dihydroxyvitamin D3 can block these rapid actions, indicating that the 1 beta epimer may bind to the receptor responsible for the rapid actions in a competitive manner. We have assessed the displacement of 3H-1 alpha,25-dihydroxyvitamin D3 by vitamin D compounds, as well as the apparent dissociation constant of 1 alpha,25-dihydroxyvitamin D3 and its 1 beta epimer for the membrane receptor in membrane preparations from ROS 24/1 cells. Increasing concentrations of 1 alpha,25-dihydroxyvitamin D3, 7.25 nM to 725 nM, displaced 3H-1 alpha,25-dihydroxyvitamin D3 from the membranes with 725 nM of the hormone displacing 40-49% of the radioactivity. Similarly, 1 beta,25-dihydroxyvitamin D3, 7.25 nM and 72.5 nM, displaced 1 alpha,25-dihydroxyvitamin D3 binding while 25-hydroxyvitamin D3, 72.5 nM and 725 nM, did not. The apparent dissociation constant (KD) for 1 alpha,25-dihydroxyvitamin D3 was determined from displacement of 3H-1 alpha,25-dihydroxyvitamin D3 yielding a value of 8.1 x 10(-7) M by Scatchard analysis. The KD for the 1 beta epimer determined from displacement of 3H-1 beta,25-dihydroxyvitamin D3 was 4.8 x 10(-7) M.(ABSTRACT TRUNCATED AT 250 WORDS)

Nongenomic actions of the steroid hormone 1 alpha,25-dihydroxyvitamin D3

Recent studies indicate that the vitamin D hormone, 1 alpha,25-dihydroxyvitamin D3 exerts rapid effects (seconds to minutes) in a variety of cell types. These rapid nongenomic actions in osteoblasts include effects on membrane voltage-gated calcium channels, phospholipase C activity, and the sodium/hydrogen antiport. Since the rapid effects occur in osteoblasts that lack the nuclear vitamin D receptor, it is postulated that the nongenomic responses to the hormone reflect interaction with a separate, membrane localized signalling system. Preliminary studies demonstrate the presence of a receptor on the membranes of osteoblasts that lack the nuclear vitamin D receptor. This membrane receptor recognizes 1 alpha,25-dihydroxyvitamin D3 and its inaction 1 beta epimer, but not 25-hydroxyvitamin D3. These rapid nongenomic actions generated by interaction with the membrane receptor modulate the effects of the hormone on gene transcription. Thus, the rapid nongenomic pathway may play a regulatory function in modulating the genomic pathways affected by 1 alpha,25-dihydroxyvitamin D3.

Lead perturbs 1,25 dihydroxyvitamin D3 modulation of intracellular calcium metabolism in clonal rat osteoblastic (ROS 17/2.8) cells

1,25-Dihydroxyvitamin D3 (1,25(OH)2D3) is known to modulate Ca2+ metabolism in several cell types. 1,25(OH)2D3 causes an increase in Ca2+ influx and probably exerts many of its effects via the Ca2+ messenger system. Lead (Pb2+) interacts with and perturbs normal Ca2+ signalling pathways; hence, the purpose of this work was to determine if Pb2+ perturbs 1,25(OH)2D3 modulation of Ca2+ metabolism in ROS 17/2.8 cells, which express receptors for and respond to 1,25(OH)2D3, and to determine the effect of 1,25(OH)2D3 on Pb2+ metabolism in these cells. In both cases three kinetic compartments described the intracellular metabolism of the isotope. These data show that 1 microM Pb2+ inhibits 1,25(OH)2D3 modulated increases in Ca2+ flux, whereas 5 microM Pb2+ increases membrane fluxes, all intracellular Ca2+ pools, and total cell Ca2+. In the Pb2+ metabolism studies it was found that 10 nM 1,25(OH)2D3 increases intracellular Pb2+. Pb2+ appears to disrupt the modulation of intracellular steady-state Ca2+ homeostasis by 1,25(OH)2D3 in a complex, biphasic manner and may therefore perturb functions that are modulated by 1,25(OH)2D3 via the Ca2+ messenger system.

1 alpha,25-dihydroxyvitamin D3-induced changes in intracellular pH in osteoblast-like cells modulate gene expression

1 alpha,25-Dihydroxyvitamin D3 exerts rapid nongenomic effects on rat osteoblast-like cells independent of the classic nuclear receptor. These effects include changes in phospholipid metabolism and cell calcium. Intracellular calcium itself has been proposed to regulate intracellular pH in osteoblast cell lines. The purpose of this study was to determine the effect of 1 alpha,25-dihydroxyvitamin D3 on intracellular pH, the relationship of changes in calcium to changes in pH, and the role of pH changes in genomic activation. 1 alpha,25-Dihydroxyvitamin D3 increased intracellular pH within 10 min in rat osteoblast-like cells, an effect that was inhibited by removal of extracellular sodium and by the biologically inactive epimer 1 beta,25-dihydroxyvitamin D3. The hormone increased intracellular calcium in Quin 2 loaded cells in the presence and absence of extracellular sodium. The 1 alpha,25-dihydroxyvitamin D3-induced increments in osteocalcin and osteopontin mRNA levels were abolished in sodium-free medium. The results indicate that 1 alpha,25-dihydroxyvitamin D3-induced increments in cellular calcium precede cell alkalinization and that these changes in intracellular pH may modulate steady-state mRNA levels of genes induced by vitamin D.

Rapid stimulation of Na+/H+ exchange by 1,25-dihydroxyvitamin D3; interaction with parathyroid-hormone-dependent inhibition

We have examined the rapid effect of 1,25-dihydroxyvitamin-D3 [1,25(OH)2D3] on apical Na+/H+ exchange activity in opossum kidney (OK) cells and in MCT cells (a culture of simian-virus-40-immortalized mouse cortical tubule cells) grown on filter support. Addition of 1,25(OH)2D3 (10 nM) for 1 min increased apical Na+/H+ exchange activity [recovery from an acid load; measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein] in OK cells (by 56%) and in MCT cells (by 36%). The cellular mechanisms involved in 1,25(OH)2D3-dependent stimulation of Na+/H+ exchange were analysed in OK cells; stimulation of Na+/H+ exchange by 1,25(OH)2D3 was not prevented by actinomycin D. Applying parathyroid hormone (PTH) reduced Na+/H+ exchange activity in OK cells (by 34% at 10 nM, 5 min); 1,25(OH)2D3 "reversed" PTH-induced inhibition, either when PTH was added prior to 1,25(OH)2D3 or when the two agonists were applied together. 1,25(OH)2D3 had no effect on basal OK cell cAMP content or on [Ca2+]i (fura-2). 1,25(OH)2D3 attenuated PTH-induced cAMP accumulation and had no effect on the PTH-dependent increase in [Ca2+]i. These data suggest a regulatory control (stimulation) of proximal tubular brush-border Na+/H+ exchange by 1,25(OH)2D3. This effect is non-genomic and might in part be explained by a release from cAMP-dependent control of transport activity.

Tumor necrosis factor alpha decreases 1,25-dihydroxyvitamin D3 receptors in osteoblastic ROS 17/2.8 cells

Bone remodeling is a complex process regulated by systemic hormones, local cytokines, and growth factors. One cytokine, tumor necrosis factor alpha (TNF-alpha), is known to have potent inhibitory effects on osteoblast matrix protein production and to stimulate osteoclast recruitment. We have previously shown that TNF-alpha inhibits 1,25-(OH)2D3-stimulated synthesis of bone gla protein (BGP), an abundant and osteoblast-specific matrix constituent. We hypothesized that the mechanism of TNF-alpha action included inhibition of intracellular 1,25-(OH)2D3 receptor (VDR) number or function. To test this, the osteoblastic cell line ROS 17/2.8 was cultured in the presence or absence of TNF-alpha (100 ng/ml), and binding of [3H]1,25-(OH)2D3 to 0.3 M KCl extracts of cytosol was measured by equilibrium assay. Specific [3H]1,25-(OH)2D3 binding decreased 70%, 25 h after addition of TNF-alpha. The decrease in [3H]1,25-(OH)2D3 binding was seen by 18 h, was sustained throughout the 72 h culture period, and was greater in low-density cultures. Scatchard analysis confirmed that TNF-alpha (100 ng/ml for 24 h) caused a decrease in the number of binding sites without change in VDR affinity. Northern analysis with a VDR riboprobe revealed that the decrease in VDR occurred without a change in the 4.4 kb steady-state VDR mRNA [VDR/cyclophilin mRNA signal ratio: control, 2.25; TNF-alpha, 2.24 (24 h), 2.17 (40 h), n = 2 flasks/time point]. These results suggest that TNF-alpha action on osteoblastic cells includes an inhibitory effect on VDR number at a point distal to the synthesis of VDR mRNA.

Effect of gallium nitrate in vitro and in normal rats

Gallium nitrate (GN) is an inhibitor of bone resorption and thereby may result in a change in coupled bone formation. In the present investigation the effects of GN on bone formation were studied in the rat osteosarcoma (ROS) 17/2.8 cell line and normal diploid rat osteoblasts (ROB) in vitro and the femur of rats treated in vivo, measuring mRNA levels for two osteoblast parameters, type I collagen, a marker of matrix formation, and osteocalcin, a bone specific protein and also histone H4, a marker of cell proliferation. GN, at 50 microM for 3 h, increased type I collagen mRNA levels by 132% in ROS 17/2.8 cells and by 122% in proliferating ROB cells. Osteocalcin (OC) mRNA levels were decreased by 61% in ROS 17/2.8 cells and by 97% in differentiated ROB cells. These changes occurred in the absence of any effects on cell proliferation. Seventy-day-old female rats were then treated with GN, 0.5 mg/kg/day, for 3 weeks. As previously reported, GN decreased serum calcium levels, but had no effect on lumbar or femoral bone density. In contrast to the in vitro effects, GN had no effect on type I collagen steady-state mRNA levels in the femur; however, it decreased OC steady-state mRNA levels in the femur by 58%. These results suggest that GN has similar in vitro effects in transformed and normal osteoblasts, while the collagen-stimulatory effects observed in vitro cannot be extrapolated to in vivo models. The consistent inhibition of osteocalcin in vitro and in vivo suggests a more specific target for GN that may relate to its effects in inhibiting bone resorption in normal rats.

Potassium currents and effects of vitamin D-3 metabolites and cyclic GMP in rat osteoblastic cells

A K+ current (IK1), activated by depolarization above -20 mV, showing voltage-dependent inactivation within a few seconds and reduced by 40% by 1 mM TEA, was observed in all cells. In a few cells, we also observed a progressive K(+)-current increase during cell dialysis. The developing current (IK2) was not sensitive to 1 mM TEA and did not inactivate. It was detectable over the whole voltage range and slowly increased during 10 s depolarizations. 1,25-(OH)2D3 and 24,25-(OH)2D3 did not affect IK1, but induced a small K(+)-current increase in some cells showing no IK2. This effect was not mimicked by cyclic GMP analogs which, on the contrary, induced a K(+)-current decrease. 24,25-(OH)2D3 (even at 10(-11)M, but not 1,25-(OH)2D3, strongly reduced IK2. The results further document the diversity of voltage-gated currents of osteoblastic cells, confirm the existence of immediate effects of vitamin D-3 metabolites, which are independent of classical 1,25-(OH)2D3 receptors.

Effects of vitamin D3 on signaling by prostaglandin E2 in osteoblast-like cells

We investigated the effects of vitamin D3 on the signaling pathways by prostaglandin E2 (PGE2) in osteoblast-like MC3T3-E1 cells. The pretreatment with 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), an active form of vitamin D3, significantly inhibited cAMP accumulation induced by 10 microM PGE2 in a dose-dependent manner in the range between 1 pM and 1 nM. This effect of 1,25-(OH)2D3 was dependent on the time of pretreatment up to 8h. 1,25-(OH)2D3 also inhibited the cAMP accumulation induced by NaF, a GTP-binding protein activator, or forskolin which directly activates adenylate cyclase. On the other hand, 1,25-(OH)2D3 significantly inhibited PGE2-induced IP3 formation in a dose-dependent manner between 10 pM and 1 nM. However, 1,25-(OH)2D3 had little effect on NaF-induced IP3 formation. The pretreatment with 24,25-dihydroxyvitamin D3, an inactive form of vitamin D3, affected neither cAMP accumulation nor IP3 formation induced by PGE2. These results strongly suggest that 1,25-(OH)2D3 modulates the signaling by PGE2 in osteoblast-like cells as follows: the inhibitory effect on the cAMP production is exerted at a point downstream from adenylate cyclase and the inhibitory effect on the phosphoinositide hydrolysis is exerted at the point between the PGE2 receptor and GTP-binding protein, probably Gi2.

1 alpha,25-Dihydroxyvitamin D3 rapidly increases nuclear calcium levels in rat osteosarcoma cells

1 alpha,25-Dihydroxyvitamin D3 increases intracellular calcium in rat osteoblast-like cells that possess the classic receptor (ROS 17/2.8) as well as those that lack the classic receptor (ROS 24/1), indicating that a separate signalling system mediates this rapid nongenomic action. To determine the intracellular sites of this calcium increase, cytosolic and nuclear fluorescence (340 nm/380 nm ratio) were measured in Fura 2AM loaded ROS 17/2.8 cells using digital microscopy. Within 5 min, cytosolic fluorescence increased by 29% (P < 0.05) and nuclear fluorescence by 30% (P < 0.01) after exposure to 1 alpha,25-dihydroxyvitamin D3 (20 nM). This effect was blocked by the inactive epimer 1 beta,25-dihydroxyvitamin D3. In an individual cell, cytosolic and nuclear fluorescence increased gradually after 1, 3, and 5 min exposure to vitamin D. Nuclei were then isolated from ROS 17/2.8 cells to directly measure the hormone's effect on nuclear calcium. The calcium content of Fura 2AM loaded nuclei was not affected by increasing the calcium concentration in the incubation buffer from 50 nM to 200 nM. After 5 min, 1 alpha,25-dihydroxyvitamin D3, 20 nM, increased the calcium of isolated nuclei in medium containing 50 nM calcium and 200 nM calcium. 1 beta,25-dihydroxyvitamin D3, 20 nM, had no effect on nuclear calcium but blocked the 1 alpha,25-dihydroxyvitamin D3 induced rise in the isolated nuclei. The results indicate that the nuclear membrane of the ROS 17/2.8 cells contain calcium permeability barriers and transport systems that are sensitive to and specific for 1 alpha,25-dihydroxyvitamin D3.(ABSTRACT TRUNCATED AT 250 WORDS)

1 beta, 25 (OH)2-vitamin D3 is an antagonist of 1 alpha,25 (OH)2-vitamin D3 stimulated transcaltachia (the rapid hormonal stimulation of intestinal calcium transport)

The steroid hormone 1 alpha,25-dihydroxyvitamin-D3 [1 alpha,25(OH)2D3] stimulates biological responses via both genomic mechanisms and nongenomic mechanisms (opening of voltage-gated Ca2+ channels). We report here that 1 beta, 25(OH)2-vitamin-D3 (a) is devoid of activity as an agonist for transcaltachia, (b) is a potent stereospecific antagonist of 1 alpha,25 (OH)2D3 stimulation of the nongenomic transcaltachia response and also (c) has less than 1% the ability of 1 alpha,25(OH)2D3 to bind to the chick intestinal nuclear 1 beta,25(OH)2D3 receptor. We conclude that the membrane response element(s) which generates the nongenomic response of transcaltachia has a different ligand specificity than the classic nuclear 1 alpha, 25(OH)2D3 receptor.

1,25(OH)2D3 blunts hormone-elevated cytosolic Ca2+ in osteoblast-like cells

Cytosolic free calcium ([Ca2+]i) is an important regulator of bone cell physiology. We studied the interaction of vitamin D metabolites on the hormonal-activated Ca message system in the osteoblastic cell line UMR-106. The acute rise in [Ca2+]i induced by different calciotropic hormones [parathyroid hormone, prostaglandin E2 (PGE2)] was dose dependently blunted by 1,25-dihydroxyvitamin D [1,25(OH)2D3; half-maximal inhibitory concn approximately 5 x 10(-11) M] and was initially observed after 8 h of preincubation. The 1,25(OH)2D3 metabolite of vitamin D was two orders of magnitude more potent than 24,25(OH)2D3 and 25(OH)D3. To discern between an effect of 1,25(OH)2D3 on hormonal-induced Ca2+ entry through the plasma membrane channel vs. release of Ca2+ from internal stores, we suspended fura-2-loaded cells in Mn2+ rather than Ca2+ buffers. In cells preincubated with 1,25(OH)2D3, [Ca2+]i release (indicated by [Ca2+]i transient) was significantly blunted, whereas Mn2+ influx (indicating Ca2+ flux across the plasma membrane) was unaltered, suggesting a selective effect of 1,25(OH)2D3 on hormonal-activated release of Ca2+ from intracellular stores. 1,25(OH)2D3 also inhibited the PGE2-induced production of inositol 1,4,5-trisphosphate. We conclude that, in osteoblasts, chronic (hours) incubation with 1,25(OH)2D3 leads to attenuated stimulation of the [Ca2+]i transduction pathway by calciotropic hormones. This effect of 1,25(OH)2D3 may provide a cellular basis for the synergism between the effects of vitamin D and calciotropic hormones at the bone level.

Covalent modification of proteins by ligands of steroid hormone receptors

Retinoylation, acylation with retinoic acid (RA), is a covalent modification of proteins occurring in a variety of eukaryotic cell lines. In this study, we found that proteins in HL-60 cells were labeled by 17 beta-[3H]estradiol (E2), [3H]progesterone (Pg), 1 alpha,25-dihydroxy[3H]vitamin D3 [1,25(OH)2D3], [125I]triiodothyronine (T3), [125I]thyroxine (T4), and [3H]prostaglandin E2 (PGE2). All of these hormones, except PGE2, are ligands of the steroid hormone receptor family. Addition to the growth medium of 5 microM ketoconazole, an inhibitor of cytochrome P450-dependent enzymes, increased about 2-fold the labeling of proteins by T3, T4, 1,25(OH)2D3, and PGE2. In contrast, ketoconazole did not change markedly the extent of labeling by RA, E2, or Pg. Alkaline methanolysis, which cleaves ester bonds, released variable percentages of the radioactive ligands bound to protein. These values were about 80% for RA and PGE2; 50% for T3, T4, and Pg; and 20% for E2 and 1,25(OH)2D3. Treatment with thioether-cleavage reagents, iodomethane or Raney nickel catalyst, released < 2% of the covalently bound ligands. Two-dimensional polyacrylamide gel electrophoresis patterns of labeled proteins were unique for each ligand. Proteins of M(r) 47,000 and 51,000 were labeled by RA, E2, T3, and T4. These proteins had the same mobilities as RI and RII, the cAMP-binding regulatory subunits of type I and type II cAMP-dependent protein kinases. 1,25(OH)2D3 also bound to proteins of M(r) 47,000 and 51,000. However, these proteins had pI values different from those of RI or RII. These results suggest that some activities of ligands of the steroid hormone receptor family and of PGE2 may be mediated by their covalent modification of proteins.

The rapid nongenomic actions of 1 alpha,25-dihydroxyvitamin D3 modulate the hormone-induced increments in osteocalcin gene transcription in osteoblast-like cells

We have previously shown that one of the rapid nongenomic actions of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25-(OH)2D3), the increase in intracellular calcium (Ca2+), accompanies the increased osteocalcin (OC) mRNA steady-state levels in rat osteosarcoma cells. To determine the functional significance of the nongenomic actions, we have measured changes in intracellular Ca2+ as an indicator of the rapid effects and have assessed the effect of inhibition of the rapid increase in cellular Ca2+ by the inactive epimer, 1 beta, 25-dihydroxyvitamin D3 (1 beta,25-(OH)2D3), on OC mRNA steady-state levels and transcription. 1 beta,25-dihydroxyvitamin D3 inhibited 1 alpha,25-(OH)2D3 induced increases in intracellular Ca2+ and OC mRNA transcription at 1 hr and OC mRNA steady state levels at 3 hr. 1 beta,25-Dihydroxyvitamin D3 did not alter the binding of the vitamin D receptor complex to the vitamin D responsive element of the OC gene. The results demonstrate the functional importance of the rapid, nongenomic actions of 1 alpha,25-(OH)2D3 in the genomic activation of the OC gene by the hormone in rat osteoblast-like cells, perhaps by modifying subtle structural and/or functional properties of the vitamin D-receptor DNA complex or by affecting other protein DNA interactions that support OC gene transcription.