The genomic mechanism of action of 1,25-dihydroxyvitamin D3

1,25(OH)2D3 ameliorates palmitate-induced lipotoxicity in human primary osteoblasts leading to improved viability and function

Contributing to bone loss with aging is a progressive reduction in osteoblast number and function leading to decreased bone formation. In aging bone, mesenchymal stem cells decrease in number and their differentiation potential into osteoblasts is reduced. Instead, there is a shift towards adipogenic differentiation and increased lipid accumulation in the marrow of osteoporotic bones. Bone marrow adipocytes produce palmitic acid (PA), a saturated fatty acid, which is toxic to osteoblasts in vitro. Vitamin D (1,25(OH)₂D₃) stimulates osteoblastogenesis and has known anti-apoptotic effects on osteoblasts, as such it may protect human primary osteoblasts from PA-induced lipotoxicity. Here, the effects of PA (250 μM) or 1,25(OH)₂D₃ (10^-8 M), alone or in combination, on osteoblast differentiation and mineralization, viability and autophagy were investigated. In PA-treated osteoblasts, 1,25(OH)₂D₃ ameliorated the decrease in the mRNA transcript abundance of representative palmitoylation (ZDHHC1, ZDHHC2 and ZDHHC12) and osteogenic (alkaline phosphatase and osteocalcin) genes. Collectively these gene regulate signaling pathways pertinent to osteoblastogenesis. In osteoblasts treated with PA and 1,25(OH)₂D₃, the capacity to undergo differentiation and mineralization was recovered and cell viability was increased when compared to osteoblasts treated with PA alone. 1,25(OH)₂D₃, irrespective of PA treatment, increased the expression of key osteogenic signaling proteins; specifically, SMAD1-3,5, Runx2 and β-catenin. 1,25(OH)₂D₃ also attenuated the high level of impaired autophagy induced by PA and potentiated a shift towards activated, functional autophagy and increased flux through autolysosomes. Altogether, these findings provide in vitro evidence regarding the potential of 1,25(OH)₂D₃ to protect osteoblasts from lipotoxicity by modulating autophagy and facilitating cell differentiation, which may enhance bone formation in an osteoporotic microenvironment with a high level of marrow adipose tissue.

Detection of 1α,25-dihydroxyvitamin D-regulated miRNAs in zebrafish by whole transcriptome sequencing

The sterol hormone, 1α,25-dihydroxyvitamin D₃ (1α,25(OH)₂D₃), regulates gene expression and messenger RNA (mRNA) concentrations in zebrafish in vivo. Since mRNA concentrations and translation are influenced by micro-RNAs (miRNAs), we examined the influence of 1α,25(OH)₂D₃ on miRNA expression in zebrafish in vivo with whole transcriptome RNA sequencing, searched for miRNA binding sites in 1α,25(OH)₂D₃-sensitive genes, and performed correlation analyses between 1α,25(OH)₂D₃-sensitive miRNAs and mRNAs. In vehicle- and 1α,25(OH)₂D₃-treated, 7-day postfertilization larvae, between 282 and 295 known precursor miRNAs were expressed, and in vehicle- and 1α,25(OH)₂D₃-treated fish, between 83 and 122 novel miRNAs were detected. Following 1α,25(OH)₂D₃ treatment, 31 precursor miRNAs were differentially expressed (p<0.05). The differentially expressed miRNAs are predicted to potentially alter mRNAs for metabolic enzymes, transcription factors, growth factors, and Jak-STAT signaling. We verified the role of a 1α,25(OH)₂D₃-sensitive miRNA, miR125b, by demonstrating alterations in the concentrations of the mRNA of a 1α,25(OH)₂D₃-regulated gene, Cyp24a1, following transfection of renal cells with a miR125b miRNA mimic. Changes in the Cyp24a1 mRNA concentration by the miR125b miRNA mimic were associated with changes in the protein for Cyp24a1. Our data show that 1α,25(OH)₂D₃ regulates miRNA in zebrafish larvae in vivo and could thereby influence vitamin D-sensitive mRNA concentrations.

Vitamin D and the kidney

The kidney is essential for the maintenance of normal calcium and phosphorus homeostasis. Calcium and inorganic phosphorus are filtered at the glomerulus, and are reabsorbed from tubular segments by transporters and channels which are regulated by 1α,25-dihydroxyvitamin (1α,25(OH)(2)D) and parathyroid hormone (PTH). The kidney is the major site of the synthesis of 1α,25(OH)(2)D under physiologic conditions, and is one of the sites of 24,25-dihydroxyvitamin D (24,25(OH)(2)D) synthesis. The activity of the 25(OH)D-1α-hydroxylase, the mixed function oxidase responsible for the synthesis of 1α,25(OH)(2)D, is regulated by PTH, 1α,25(OH)(2)D, fibroblast growth factor 23 (FGF23), inorganic phosphorus and other growth factors. Additionally, the vitamin D receptor which binds to, and mediates the activity of 1α,25(OH)(2)D, is widely distributed in the kidney. Thus, the kidney, by regulating multiple transport and synthetic processes is indispensible in the maintenance of mineral homeostasis in physiological states.

Liposomal nanomedicine for breast cancer therapy

Liposomes are well-established nanocarriers for improving the therapeutic index of anticancer agents. A remarkable understanding in the pathophysiology of breast cancer progression has emerged with information on the involved specific biomolecules, which may serve as molecular targets for its therapy. Hormonal and nonhormonal receptors can both be exploited for targeting to breast cancer cells. Targeted delivery of cytotoxic drugs using liposomes is a novel approach for breast cancer therapy. In the present article, we summarize molecular targets present on the breast cancer cells. Recent developments in liposome-based delivery of bioactives for selective treatments of breast cancer are discussed. In addition, utilization of bioenvironmental conditions of tumor for liposome-based targeted delivery is also summed up.

Toxicity and antitumor activity of the vitamin D analogs PRI-1906 and PRI-1907 in combined treatment with cyclophosphamide in a mouse mammary cancer model

PURPOSE

Active and less toxic vitamin D analogs could be useful for clinical applications. In the present study, we evaluated the toxicity and antitumor effect of two new synthetic analogs of vitamin D, namely PRI-1906 [(24E)-24a-Homo-(1S)-1,25-dihydroxyergocalciferol] and its side-chain unsaturated homo analog PRI-1907.

METHODS

The toxicity and calcemic activity, as well as antitumor effect of calcitriol analogs was investigated in vivo. The studies were performed in a mouse mammary 16/C cancer model. Since calcitriol and its analogs inhibited 16/C tumor growth only slightly, we applied them in the combined therapy with cyclophosphamide (CY). Moreover, cell cycle analysis and VDR and p27 expression were investigated.

RESULTS

The LD50 values after five daily subcutaneous (s.c.) injections were 7.8, 10.0 and 2.4 microg/kg per day for calcitriol, PRI-1906 and PRI-1907, respectively. The serum calcium level increased to 40, 23 and 63% over the control for these compounds. We also compare the antitumor activity of the PRI-1906 with the calcitriol and previously studied PRI-2191 (1,24-dihydroxyvitamin D3, tacalcitol). Statistically significant inhibition of tumor growth by calcitriol up to the eighth day was observed in all schedules applied. PRI-1906 inhibited the tumor growth at doses 1 and 5 microg/kg per day, and PRI-2191 only at the dose 5 microg/kg per day.

CONCLUSION

Addition of vitamin D analogs increased the antitumor effect of CY. PRI-1906 exhibited toxicity higher than PRI-2191 but lower than calcitriol and antitumor activity similar to both PRI-2191 and calcitriol. This new analog seems to be a good candidate for the combined treatment of mammary cancer.

Osteoblast Ca(2+) permeability and voltage-sensitive Ca(2+) channel expression is temporally regulated by 1,25-dihydroxyvitamin D(3)

The cardiac subtype of the L-type voltage-sensitive Ca(2+) channel (VSCC) Cav1.2 (alpha(1C)) is the primary voltage-sensitive channel responsible for Ca(2+) influx into actively proliferating osteoblasts. This channel also serves as the major transducer of Ca(2+) signals in growth-phase osteoblasts in response to hormone treatment. In this study, we have demonstrated that 24-h treatment of MC3T3-E1 preosteoblasts with 1,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)], a coupling factor for bone resorption, coordinately downregulates Cav1.2 (alpha(1C)) and uniquely upregulates T-type channel Cav3.2 (alpha(1H)). No other voltage-sensitive channel alpha-subunit of the 10 that were surveyed was upregulated by 1,25(OH)(2)D(3). The shift from predominantly L-type to T-type channel expression has been demonstrated to occur at both mRNA and protein levels detected using quantitative PCR and immunohistochemistry with antibodies specific for each channel type. Functional and pharmacological studies using specific inhibitors have revealed that treatment with 1,25(OH)(2)D(3) also alters the Ca(2+) permeability properties of the osteoblast membrane from a state of primarily L-current sensitivity to T-current sensitivity. We conclude that the L-type channel is likely to support proliferation of osteoblast cells, whereas T-type channels are more likely to be involved in supporting differentiated functions after 1,25(OH)(2)D(3)-mediated reversal of remodeling has occurred. This latter observation is consistent with the unique expression of the T-type VSCC Cav3.2 (alpha(1H)) in terminally differentiated osteocytes as we recently reported.

The functional consequences of cross-talk between the vitamin D receptor and ERK signaling pathways are cell-specific

The actions of the active metabolite of 1,25-(OH)2D3 (1,25-D) are mediated primarily by the vitamin D receptor (VDR), a member of the nuclear receptor family of ligand-activated transcription factors. Although their ligands cause transcriptional activation, many of the ligands also rapidly activate cellular signaling pathways through mechanisms that have not been fully elucidated. We find that 1,25-D causes a rapid, but sustained activation of ERK (extracellular signal-regulated kinase) in bone cell lines. However, the effect of ERK activation on VDR transcriptional activity was cell line-specific. Inhibition of ERK activation by the MEK inhibitor, U0126, stimulated VDR activity in MC3T3-E1 cells, but inhibited the activity in MG-63 cells as well as in HeLa cells. VDR is not a known target of ERK. We found that the ERK target responsible for reduced VDR activity in MC3T3-E1 cells is RXRalpha. MC3T3-E1 cells express lower levels of RXRbeta and RXRgamma than either HeLa or MG-63 cells. Although overexpression of RXRalpha in MC3T3-E1 cells increased VDR activity, U0126 further enhanced the activity. In contrast, overexpression of RXRgamma stimulated VDR activity but abrogated the stimulation by U0126. Thus, although 1,25-D treatment activates ERK in many cell types, subsequently inducing changes independent of VDR, the effects of treatment with 1,25-D on the transcriptional activity of VDR are RXR isoform-specific. In cells in which RXRalpha is the VDR partner, the transcriptional activation of VDR by 1,25-D is attenuated by the concomitant activation of ERK. In cells utilizing RXRgamma, ERK activation enhances VDR transcriptional activity.

Vitamin D inhibits Fas ligand-induced apoptosis in human osteoblasts by regulating components of both the mitochondrial and Fas-related pathways

Apoptosis plays an important role in the regulation of bone turnover. Previously, we showed that 1,25(OH)2D3, the active form of vitamin D, may increase osteoblast survival by inhibiting apoptosis induced by serum deprivation. Human osteoblasts express the Fas receptor on their surface and its interaction with Fas ligand has been closely associated with human osteoblast apoptosis. To investigate the mechanism of 1,25(OH)2D3 inhibition of apoptosis in osteoblasts isolated from human calvaria, cells were exposed to Fas antibody. Visualization of apoptotic cells using annexin V revealed a significant decrease in apoptosis at 48 h in the presence of 1,25(OH)2D3 (14 +/- 4%, P < 0.04) compared with non-treated cells (52 +/- 4%). Furthermore, flow cytometric analysis of TUNEL-labeled osteoblasts showed a significant decrease in apoptotic cells in 1,25(OH)2D3-treated cultures (12 +/- 2%) at 48 h compared with non-treated cultures (44 +/- 3%, P < 0.04). Additionally, cells treated with 1,25(OH)2D3 survived longer as found by MTS analysis. To further explore the mechanism of 1,25(OH)2D3-mediated inhibition of apoptosis, we examined the changes in activation of death domain proteins, cleavage of caspases and mitochondrial regulators of apoptosis by Western blot analysis. A significant inhibition of caspase-8 cleavage and activity in 1,25(OH)2D3-treated cells was observed in conjunction with a decrease in the expression of the proapoptotic protein Bax with a significant increase in the expression of antiapoptotic protein Bcl-2. Furthermore, the levels of p21Cip1/WAF1, which inhibits the cleavage of caspase-8, was found to be highly induced in 1,25(OH)2D3-treated cells. In summary, these results demonstrate that the anti-apoptotic effect of 1,25(OH)2D3 in human osteoblasts after the activation of Fas-ligand is mediated by the regulation of components of both the mitochondrial and Fas-related pathways.

Modulation of response to estrogens in cultured human female bone cells by a non-calcemic Vitamin D analog: changes in nuclear and membranal binding

Estradiol17beta (E2) and the phytoestrogens genistein (G), and daidzein (D) increase creatine kinase (CK) specific activity in primary cell cultures of human female to a greater extent in cells from pre-menopausal than post-menopausal women. Pretreatment with the non-calcemic analog of Vitamin D, JK 1624 F2-2 (JKF), upregulated this estrogenic response at all ages. In contrast, biochainin A (BA) and quercertin (Qu) increased CK with no age dependence or modulation by JKF pretreatment. Both ERalpha and ERbeta present in the cells were upregulated by pretreatment with JKF, as measured by Western blot analysis. Real time PCR showed no significant change in ERalpha mRNA but a marked decrease in ERbeta mRNA in both age groups after JKF treatment. Cells from both age groups had surface binding sites for E2, shown by assays using cell impermeable Europium labeled ovalbumin-E2 conjugate (Eu-Ov-E2). Binding of [3H]-E2 to intracellular E2 receptors (ERs) was similar in both age groups with differences in phytoestrogenic competition. JKF pretreatment increased nuclear but decreased membranal binding in both age groups. These results provide evidence for membranal, in addition to nuclear estrogen receptors which are differentially modulated by a Vitamin D analog.

Treatment with non-hypercalcemic analogs of 1,25-dihydroxyvitamin D3 increases responsiveness to 17beta-estradiol, dihydrotestosterone or raloxifene in primary human osteoblasts

Pretreatment with 1 nM 1,25-dihydroxyvitamin D(3) (1,25), or non-hypercalcemic Vitamin D analogs, upregulated the response of creatine kinase (CK) to 17beta-estradiol (30 nM E(2)), raloxifene (3000 nM RAL) or dihydrotestosterone (300 nM DHT) in primary human bone cells. Previously, we reported that these osteoblast-like cells responded to gonadal steroids in a sex specific manner. Bone cells derived from pre-menopausal women showed greater stimulation of CK specific activity by E(2) than bone cells from post-menopausal women; in male-derived cells no age related difference was found. In this study, we treated cells derived from female or male bones, at different ages, with the side chain modified analogs of Vitamin D: CB 1093 (CB), EB 1089 (EB), MC 1288 (MC) and the demonstrably non-calcemic hybrid analog JK 1624 F2-2 (JKF), by daily addition of 1 nM, for 3 days. On day 4, cells were incubated with sex steroids for 4h and cell extracts were prepared. Pretreatment with JKF or CB significantly upregulated the response to 30 nM E(2) in all female-derived cells and to 300 nM DHT in mature male-derived cells. In cells from older males, only JKF caused augmentation of DHT action. Bone cells from pre- or post-menopausal females responded to 3000 nM RAL by increased CK activity to the same extent as to 30 nM E(2); however, RAL and E(2), when applied together, resulted in mutual annihilation of their agonist activities. Vitamin D analogs prevented the antagonistic effect of RAL in the presence of E(2), possibly due to increased numbers of ERs. Both estrogen receptors, alpha (ERalpha) and beta (ERbeta), were expressed in male- as well as in female-derived cells. However, only in female-derived cells were ERalpha and ERbeta upregulated by pretreatment with Vitamin D analogs. This study raises the possibility of testing combined Vitamin D analog and estrogen replacement treatment for post-menopausal women to prevent osteoporosis.

Novel role of 1,25(OH)(2)D(3) in induction of erythroid progenitor cell proliferation

OBJECTIVE

Burst-forming unit erythroid and colony-forming unit erythroid growth in vitro is lower in studies of continuous ambulatory peritoneal dialysis patients than healthy controls. Burst-forming unit erythroid growth was potentiated by addition of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and normalized by erythropoietin (Epo) therapy, suggesting an interaction between Epo and 1,25(OH)(2)D(3) at the stem cell level. The objective of this study was to determine the mechanism by which 1,25(OH)(2)D(3) enhances the stimulatory effect of Epo on the growth of erythroid precursor cells.

MATERIALS AND METHODS

We examined the effect of 1,25(OH)(2)D(3) and Epo on stem cell proliferation. Proliferation of TF1 cells of erythroid origin was measured by the XTT method, 3[H] thymidine incorporation, and cell counting by trypan blue exclusion; cord blood (CB) stem cells were counted. Epo receptor (EpoR) quantitation was evaluated by 125I-Epo binding and Scatchard analysis, immunoprecipitation, and Western blotting. Expression of EpoR mRNA was measured by reverse transcriptase polymerase chain reaction.

RESULTS

The stem cell factor-dependent CB stem cells and the TF1 cells responded to Epo and 1,25(OH)(2)D(3) by increased proliferation, while their simultaneous addition potentiated cell proliferation in a synergistic manner (25.67% +/- 4.8% of Epo proliferation at day 10 for CB cells; p < 0.005). 1,25(OH)(2)D(3) produced an up-regulation of EpoR number in TF1 cells and increased the expression of EpoR mRNA (p < 0.01).

CONCLUSIONS

The increase in EpoR expression induced by 1,25(OH)(2)D(3) might explain the synergistic interaction between Epo and 1,25(OH)(2)D(3) in stem cells.

Enhanced activity of osteoblast differentiation factor (PEBP2alphaA2/CBFa1) in affected sutural osteoblasts from patients with nonsyndromic craniosynostosis

OBJECTIVE

Nonsyndromic craniosynostosis is characterized by premature closure of one or more cranial sutures in infants. The purpose of this investigation was to evaluate cellular and molecular events that lead to pathogenesis of nonsyndromic craniosynostosis.

DESIGN

This study utilized discarded samples of normal and affected cranial sutures from 12 patients (7 boys, 5 girls) with nonsyndromic craniosynostosis.

RESULTS

Histological evaluation of affected sutures revealed complete osseous obliteration instead of a zone of connective tissue and osteogenic cells as seen in normal sutures. Although proliferation of normal and affected osteoblasts did not vary substantially, elevated osteocalcin production and increased in vitro bone nodule formation indicated that the differentiation and the bone-forming potential of affected osteoblasts was significantly higher than that of normal cells. We therefore investigated the levels and activity of Cbfa1, a transcription factor that plays an integral role in osteoblast differentiation. Northern blot analysis of messenger RNA from both normal and affected sutural osteoblasts revealed a twofold increase in the expression of Cbfa1 in affected cells. This increase in the level of Cbfa1 transcript correlated with an increase in its transcriptional activity on the osteocalcin gene promoter, as assessed using gene transfer methods.

CONCLUSION

Our results indicated that osteoblasts from synostosed sutures exhibit an increased potential for differentiation and bone formation. The increased level and activity of Cbfa1 could play a vital role in the aberrant function of these affected osteoblasts and may explain their altered behavior compared to the normal cells.

Role of advanced glycation end products in adynamic bone disease in patients with diabetic nephropathy

Adynamic bone disease and elevated serum levels of advanced glycation end products (AGEs) often are found in patients with renal failure caused by diabetic nephropathy. To clarify the role of AGEs in adynamic bone disease, we investigated the effect of these substances on cultured human osteoblasts and parathyroid cells. After 72 hours of incubation with AGEs-bovine serum albumin (BSA) (1,000 microgram/mL), there was significant inhibition of the synthesis of type I collagen and osteocalcin in response to stimulation with 10(-10) to 10(-8) M of 1,25-dihydroxycholecalciferol. In a human osteoblastic cell line (MG 63), AGEs-BSA did not affect human osteocalcin promoter activity. In human parathyroid cells, a receptor for AGEs was detected by reverse-transcriptase polymerase chain reaction. Incubation with AGEs-BSA for 48 hours significantly inhibited parathyroid hormone secretion in response to a low calcium concentration of 0.81 mM (P < 0.01). In HEK-293 cells, expressing calcium-sensing receptors, the same AGE concentration caused a significant potentiation of the extracellular Ca(2+) induced-intracellular calcium concentration after 24 and 48 hours of incubation (P < 0.05 and P < 0.01). These data suggest that AGEs are involved in the pathogenesis of adynamic bone disease by inhibiting osteoblastic activity and by inhibiting parathyroid hormone secretion in response to hypocalcemia.

Contributions of nuclear architecture and chromatin to vitamin D-dependent transcriptional control of the rat osteocalcin gene

The vitamin D response element in the bone tissue-specific osteocalcin gene has served as a prototype for understanding molecular mechanisms regulating physiologic responsiveness of vitamin D-dependent genes in bone cells. We briefly review factors which contribute to vitamin D transcriptional control. The organization of the vitamin D response element (VDRE), the multiple activities of the vitamin D receptor transactivation complex, and the necessity for protein-protein interactions between the VDR-RXR heterodimer activation complex and DNA binding proteins at other regulatory elements, including AP-1 sites and TATA boxes, provide for precise regulation of gene activity in concert with basal levels of transcription. We present evidence for molecular mechanisms regulating vitamin D-dependent mediated transcription of the osteocalcin gene that involve chromatin structure of the gene and nuclear architecture. Modifications in nucleosomal organization, DNase I hypersensitivity and localization of vitamin D receptor interacting proteins in subnuclear domains are regulatory components of vitamin D-dependent gene transcription. A model is proposed to account for the inability of vitamin D induction of the osteocalcin gene in the absence of ongoing basal transcription by competition of the YY1 nuclear matrix-associated transcription factor for TFIIB-VDR interactions. Activation of the VDR-RXR complex at the OC VDRE occurs through modifications in chromatin mediated in part by interaction of OC gene regulatory sequences with the nuclear matrix-associated Cbfa1 (Runx2) transcription factor which is required for osteogenesis.

Transcriptional induction of matrix metalloproteinase-13 (collagenase-3) by 1alpha,25-dihydroxyvitamin D3 in mouse osteoblastic MC3T3-E1 cells

The removal of unmineralized matrix from the bone surface is essential for the initiation of osteoclastic bone resorption because osteoclasts cannot attach to the unmineralized osteoid. Matrix metalloproteinases (MMPs) are known to digest bone matrix. We recently reported that among the MMPs expressed in mouse osteoblastic cells, MMP-13 (collagenase-3) was the one most predominantly up-regulated by bone resorbing factors including 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. In this study, we examined the mechanism of regulation of MMP-13 expression by 1alpha,25(OH)2D3 in mouse osteoblastic MC3T3-E1 cells. 1Alpha,25(OH)2D3 increased steady-state messenger RNA (mRNA) and protein levels of MMP-13. De novo protein synthesis was essential for the induction because cycloheximide (CHX) decreased the effect of 1alpha,25(OH)2D3 on the MMP-13 mRNA level. 1Alpha,25(OH)2D3 did not alter the decay of MMP-13 mRNA in transcriptionally arrested MC3T3-E1 cells; however, it increased the MMP-13 heterogeneous nuclear RNA (hnRNA) level and MMP-13 transcriptional rate. The binding activity of nuclear extracts to the AP-1 binding site, but not to the Cbfa1 binding site, in the MMP-13 promoter region was up-regulated by 1alpha,25(OH)2D3, suggesting the mediation of AP-1 in this transcriptional induction. To determine the contribution of MMPs to bone resorption by 1alpha,25(OH)2D3, the inhibitory effect of BB94, an MMP inhibitor, on resorbed pit formation by mouse crude osteoclastic cells was examined on either an uncoated or collagen-coated dentine slice. BB94 did not prevent resorbed pit formation on uncoated dentine whereas it did on collagen-coated dentine. We therefore propose that the transcriptional induction of MMP-13 in osteoblastic cells may contribute to the degradation of unmineralized matrix on the bone surface as an early step of bone resorption by 1alpha,25(OH)2D3.

Rat oligodendrocytes express the vitamin D(3) receptor and respond to 1,25-dihydroxyvitamin D(3)

The present study investigates the presence of vitamin D receptor (VDR) in cells of the rat oligodendrocyte (OL) lineage. VDR transcripts were detected by in situ hybridization in a fraction of rat OL in secondary cultures. The VDR protein was shown to be co-localized in cells that are also recognized by an anti-myelin basic protein (MBP) antibody. Likewise, in vivo, VDR-positive cells were found in the brain white matter, such as the internal capsule of the striatum or the corpus callosum but also in the spinal cord. At least part of these positive cells in vivo correspond to OL, since they were co-stained by an anti-carbonic anhydrase II antiserum. Northern blot analyses of the CG-4 OL cell line demonstrated that the VDR transcripts are already found in the O-2A precursors. There was a two-fold increase in the relative abundance of these transcripts in differentiated OL or in type-2 astrocytes. 1, 25-dihydroxyvitamin D(3) [1,25-(OH)(2)D(3)] increased the pool of transcripts encoding its own receptor, the VDR. The hormone also enhanced the abundance of the mRNA of the nerve growth factor (NGF) and of its low-affinity receptor, the p75(NTR) protein. By contrast, the hormone had no effect on the levels of MBP or proteolipid protein (PLP) mRNA. This finding suggests that unlike retinoic acid (RA) or thyroid hormone, 1,25-(OH)(2)D(3) has no regulatory action on the synthesis of myelin proteins.

Effect of cyclic adenosine 3',5'-monophosphate and protein kinase A on ligand-dependent transactivation via the vitamin D receptor

We examined the effects of cyclic adenosine 3',5'-monophosphate (cAMP) and protein kinase A (PKA) on the ligand-dependent transactivation mediated via the 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) receptor (VDR). A human VDR expression plasmid was transfected into HeLa, Saos-2 and MG63 cells with a luciferase reporter gene construct containing the vitamin D responsive element. With the addition of 0.5 mM 8 bromo-cAMP, the response to 1,25(OH)2D3 was suppressed to 61 and 78% in the HeLa and Saos-2 cells, respectively. The suppressive effect of 8 bromo-cAMP was observed without the introduction of the VDR expression plasmid in the MG63 cells. In the HeLa cells the co-expression of PKA reduced the ligand-inducible transactivation to 61% and the fold induction by 1,25(OH)2D3 to 89% of that without PKA. The CREB binding protein (CBP) was recently reported to integrate the intracellular signals via the cAMP/PKA cascade and nuclear hormone receptors. However, the suppressive effect of cAMP was not influenced by the co-expression of CBP. Lastly, we introduced point mutations at possible PKA phosphorylation sites into the VDR expression vector at serine-172 and threonine-175, but both mutant receptors still exhibited reduced transactivation with the co-expression of PKA. These results indicate that the phosphorylation of proteins other than the VDR may also be involved in the inhibitory effect mediated by the cAMP/PKA cascade.

In vivo and in vitro pharmacokinetics and metabolism studies of 26,26,26,27,27,27-F6-1,25(OH)2 vitamin D3 (Falecalcitriol) in rat: induction of vitamin D3-24-hydroxylase (CYP24) responsible for 23S-hydroxylation in target tissues and the drop in serum levels

1. 26,26,26,27,27,27-F6,-1,25(OH)2 vitamin D3, Falecalcitriol, the hexafluorinated analogue of 1,25(OH)2 vitamin D3, has been reported to be several times more potent than the parent compound regarding some vitamin D actions. The reason for enhanced biological activity appears related to F6-1,25(OH)2 vitamin D3 metabolism to F6-1,23S,25(OH)3 vitamin D3, a bioactive 23S-hydroxylated form which is resistant to further metabolism. 2. In the present in vivo studies, the repeated oral administration of [3H]F6-1,25(OH)2 vitamin D3 to rat resulted in a significant reduction of the radioactivity and the F6-1,25(OH)2 vitamin D3 concentrations in serum, especially at the 2 h maximum point after each dosing. Additionally, F6-1,23S,25(OH)3 vitamin D3 in the serum and small intestine was increased by the prior administration of F6-1,25(OH)2 vitamin D3. 3. Further in vitro investigation showed [3H]F6-1,25(OH)2 vitamin D3 to be metabolized to F6-1,23S,25(OH)3 vitamin D3 by kidney and small intestine homogenates of rat, the reaction being increased by the prior administration of F6-1,25(OH)2 vitamin D3. Moreover, this latter treatment was associated with a marked increase of CYP24 mRNA in the small intestine within 4 h after dosing. 4. The results indicate that in vivo metabolism of F6-1,25(OH)2 vitamin D3 to F6-1,23S,25(OH)3 vitamin D3 is catalysed by CYP24, the enzyme being induced by prior substrate exposure.

Novel nonsecosteroidal vitamin D mimics exert VDR-modulating activities with less calcium mobilization than 1,25-dihydroxyvitamin D3

BACKGROUND

The secosteroid 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) acts through the vitamin D receptor (VDR) to elicit many activities that make it a promising drug candidate for the treatment of a number of diseases, including cancer and psoriasis. Clinical use of 1,25(OH)2D3 has been limited by hypercalcemia elicited by pharmacologically effective doses. We hypothesized that structurally distinct, nonsecosteroidal mimics of 1,25(OH)2D3 might have different activity profiles from vitamin D analogs, and set out to discover such compounds by screening small-molecule libraries.

RESULTS

A bis-phenyl derivative was found to activate VDR in a transactivation screening assay. Additional related compounds were synthesized that mimicked various activities of 1,25(OH)2D3, including growth inhibition of cancer cells and keratinocytes, as well as induction of leukemic cell differentiation. In contrast to 1, 25(OH)2D3, these synthetic compounds did not demonstrate appreciable binding to serum vitamin D binding protein, a property that is correlated with fewer calcium effects in vivo. Two mimics tested in mice showed greater induction of a VDR target gene with less elevation of serum calcium than 1,25(OH)2D3.

CONCLUSIONS

These novel VDR modulators may have potential as therapeutics for cancer, leukemia and psoriasis with less calcium mobilization side effects than are associated with secosteroidal 1,25(OH)2D3 analogs.

Distribution and metabolism of F6-1,25(OH)2 vitamin D3 and 1,25(OH)2 vitamin D3 in the bones of rats dosed with tritium-labeled compounds

26,26,26,27,27,27-Hexafluo-1,25(OH)2 vitamin D3, the hexafluorinated analog of 1,25(OH)2 vitamin D3, has been reported to be several times more potent than the parent compound regarding some vitamin D actions. The reason for enhanced biologic activity in the kidneys and small intestine appears to be related to F6-1,25(OH)2 vitamin D3 metabolism to ST-232, 26,26,26,27,27,27-hexafluoro-1 alpha, 23S,25-trihydroxyvitamin D3, a bioactive 23S-hydroxylated form that is resistant to further metabolism. Since F6-1,25(OH)2 vitamin D3 is considered to prevent osteoporotic decrease in bone mass by suppressing bone turnover, we here compared the distribution and metabolism of [1 beta-3H]F6-1,25(OH)2 vitamin D3 and [1 beta-3H]1,25(OH)2 vitamin D3 in bones of rats by autoradiography and radio-HPLC. In the dosed groups, radioactivity was detected locally in the metaphysis, the modeling site in bones. As compared with the [1 beta-3H]1,25(OH)2 vitamin D3 case, [1 beta-3H]F6-1,25(OH)2 vitamin D3 was significantly retained in this site, and moreover, it mainly persisted as unchanged compound and ST-232. These findings indicate that the reason for the higher potency of F6-1,25(OH)2 vitamin D3 than 1,25(OH)2 vitamin D3 in bones are linked with increased distribution and reduced metabolism.

1alpha,25-dehydroxyvitamin D3 synergism toward transforming growth factor-beta1-induced AP-1 transcriptional activity in mouse osteoblastic cells via its nuclear receptor

The present study demonstrates 1alpha,25-dehydroxyvitamin D3 (1alpha-25-(OH)2D3) synergism toward transforming growth factor (TGF)-beta1-induced activation protein-1 (AP-1) activity in mouse osteoblastic MC3T3-E1 cells via the nuclear receptor of the vitamin. 1alpha-25-(OH)2D3 synergistically stimulated TGF-beta1-induced expression of the c-jun gene in the cells but not that of the c-fos gene. We actually showed by a gel mobility shift assay 1alpha-25-(OH)2D3 synergism of TGF-beta1-induced AP-1 binding to the 12-(O-tetradecanoylphorbol-13-acetate response element (TRE). 1alpha-25-(OH)2D3 markedly stimulated the transient activity of TGF-beta1-induced AP-1 in the cells transfected with a TRE-chloramphenicol acetyltransferase (CAT) reporter gene. Also, a synergistic increase in TGF-beta1-induced CAT activity was observed in the cells cotransfected with an expression vector encoding vitamin D3 receptor (VDR) and the reporter gene. However, the synergistic CAT activity was inhibited by pretreatment with VDR antisense oligonucleotides. In addition, in a Northern blot assay, we observed 1alpha-25-(OH)2D3 synergism of TGF-beta1-induced expression of the c-jun gene in the cells transfected with the VDR expression vector and also found that the synergistic action was clearly blocked by VDR antisense oligonucleotide pretreatment. The present study strongly suggests a novel positive regulation by 1alpha-25-(OH)2D3 of TGF-beta1-induced AP-1 activity in osteoblasts via "genomic action."

Characterization of the activation function-2 domain of the human 1,25-dihydroxyvitamin D3 receptor

In this study, we determined the ligand-dependent activation function domain 2 (AF-2) of the human vitamin D receptor (hVDR) and characterized it using site-directed mutagenesis. A single mutation at glutamic acid-420 (E420Q) and an additional mutation at leucine-417 (L417A-E420Q) eliminated ligand-dependent transcriptional activation. In addition, lysine-264 was also demonstrated to be vital for ligand-induced transactivation. However, bacterial-overexpressed transcriptional factor IIB (TFIIB) was able to bind to both AF-2 and lysine-264 mutant hVDRs in vitro. The ligand-dependent transactivation via wild type hVDR was interfered with weakly only when a 10-fold molar excess of L417A-E420Q plasmid was co-transfected. This suppressive effect was diminished by introducing an additional mutation at a cysteine residue in the DNA binding domain. Thus, we conclude that the AF-2 domain of the hVDR located between amino acids 417 and 420, as well as lysine-264, are essential for ligand-dependent transactivation, and that TFIIB was not necessary for the function of these two regions of the hVDR. Our finding that AF-2 mutant hVDRs exhibit only very weak suppressive effect may indicate a difference in the molecular mechanism of the VDR-mediated transactivation from other nuclear receptors.

Differentiation of myeloid cells and 1,25-dihydroxyvitamin D3

It is well established that 1,25(OH)2D3 induces monocyte/macrophage (Mo/Mphi) colonies when added to culture of granulocyte/macrophage progenitors. Recently, we demonstrated that one of the target cells of 1,25(OH)2D3 in Mo/Mphi differentiation is the neutrophilic promyelocyte that is believed to belong to the neutrophilic lineage. This fact overthrows the established theory that normal hematopoietic precursors are committed to respective cell lineages and do not deviate from their own lineage. The lineage switching from the promyelocyte to Mo/Mphi was suggested to be operating in vivo because 1,25(OH)2D3 is a physiological substance produced by Mphi. More recently, we have shown that transient exposure (24 h) of promyelocytes to 1,25(OH)2D3 causes Mo/Mphi differentiation. This strategy could be useful for examining the effects of 1,25(OH)2D3 on the growth and differentiation of normal myeloblasts and myeloid progenitor cells. Recent advances in molecular biology have enabled investigators to identify a number of genes involved in Mo/Mphi differentiation induced by 1,25(OH)2D3. Some of these may be the determinant genes for Mo/Mphi differentiation; however, further studies are required to determine the underlying mechanisms of Mo/Mphi differentiation.

The osteocalcin gene: a model for multiple parameters of skeletal-specific transcriptional control

Influences of promoter regulatory elements that are responsive to basal and tissue-restricted transactivation factors, steroid hormones, growth factors and other physiologic mediators has provided the basis for understanding regulatory mechanisms contributing to developmental expression of osteocalcin, tissue specificity and biological activity (reviewed in [1-3]). These regulatory elements and cognate transcription factors support postproliferative transcriptional activation and steroid hormone (e.g. vitamin D) enhancement at the onset of extracellular matrix mineralization during osteoblast differentiation. Three parameters of nuclear structure contribute to osteocalcin gene transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of developmental as well as phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented which are consistent with involvement of multiple levels of nuclear architecture in tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization and the nuclear matrix are considered which influence transcription of the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype.

Vitamin D-dependent rickets type I and type II

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

A 65-kilodalton nuclear protein binds to the human vitamin D receptor: a bacterial-expressed histidine-tagged receptor study

We report here that the human 1,25-dihydroxyvitamin D3 receptor (hVDR) binds to a 65 kD nuclear protein in a ligand-dependent manner. Histidine-tagged full-length hVDR was overexpressed in E.coli and purified to near homogeneity using Ni-NTA and gel filtration columns without denature/renature procedures. Nuclear extract from the osteoblastic cell line MG-63 was incubated with the recombinant hVDR and Ni-NTA agar in the presence of a double-stranded DNA fragment containing vitamin D responsive element. Proteins bound to the hVDR were eluted and analyzed on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A 65 kD protein was detected with full-length hVDR in the presence of 100 nM 1,25(OH)2D3, while this interaction was not observed in the absence of the ligand nor with carboxyl-terminally truncated hVDR, which lacks an activation function-2 domain. Therefore, this nuclear protein may be involved in the ligand-dependent transcriptional regulation via the hVDR.

Increased sensitivity to 1,25(OH)2D3 in bone from genetic hypercalciuric rats

As a model of human hypercalciuria, we have selectively inbred genetic hypercalciuric stone-forming (GHS) Sprague-Dawley rats whose mean urine calcium excretion is eight to nine times greater than that of controls. A large component of this excess urine calcium excretion is secondary to increased intestinal calcium absorption, which is not due to an elevation in serum 1,25(OH)2D3, but appears to result from an increased number of intestinal 1,25(OH)2D3 receptors (VDR). When GHS rats are fed a low-calcium diet, the hypercalciuria is only partially decreased and urine calcium excretion exceeds intake, suggesting that an additional mechanism contributing to the hypercalciuria is enhanced bone demineralization. To determine if GHS rat bones are more sensitive to exogenous 1,25(OH)2D3, we cultured calvariae from neonatal (2- to 3-day-old) GHS and control rats with or without 1,25(OH)2D3 or parathyroid hormone (PTH) for 48 h at 37 degrees C. There was significant stimulation of calcium efflux from GHS calvariae at 1 and 10 nM 1,25(OH)2D3, whereas control calvariae showed no significant response to 1,25(OH)2D3 at any concentration tested. In contrast, PTH induced similar bone resorption in control and GHS calvariae. Immunoblot analysis demonstrated a fourfold increase in the level of VDR in GHS calvariae compared with control calvariae, similar to the increased intestinal receptors described previously. There was no comparable change in VDR RNA levels as measured by slot blot analysis, suggesting the altered regulation of the VDR occurs posttranscriptionally. That both bone and intestine display an increased amount of VDR suggests that this may be a systemic disorder in the GHS rat and that enhanced bone resorption may be responsible, in part, for the hypercalciuria in the GHS rat.

Demonstration of vitamin D receptor expression in a human megakaryoblastic leukemia cell line: regulation of vitamin D receptor mRNA expression and responsiveness by forskolin

We have shown earlier that 1,25-dihydroxyvitamin D3 [1,25(OH)2 D3] induces cell growth suppression and cell differentiation of a human megakaryoblastic leukemia cell line, HIMeg. However, the molecular mechanism of 1,25(OH)2 D3 action is still unknown. Prompted by this, we have searched here for the presence of 1,25(OH)2 D3 receptor (VDR) expression in HIMeg cells by reverse transcription-polymerase chain reaction (RT-PCR). The amplified product showed an identical size to the product amplified from the control human VDR cDNA and hybridized specifically with the digoxigenin-labeled human VDR cDNA fragment. As expected, VDR mRNA is also expressed in HOS-8603, a human osteosarcoma cell line. These results represent the first reported evidence that VDR mRNA is expressed in megakaryoblastic cells. In addition, the regulation of VDR mRNA expression in HIMeg cells was studied by quantitative RT-PCR. It was found that [correction of the] VDR mRNA expression in HIMeg cells could be down-regulated rapidly by 1,25(OH)2 D3 (10 nM) in a time-dependent manner, reaching a maximal reduction to about 15% of control. However, VDR mRNA expression in HOS-8603 cells was not regulated by 1,25(OH)2 D3 at any time-point tested. Treatment of HIMeg cells with forskolin (1 microM), an activator of adenylate cyclase, caused an increase in VDR mRNA levels. Similarly, VDR mRNA expression in HOS-8603 cells was also up-regulated by forskolin. Consistent with the functionality of the VDR in other target cells, we found that the up-regulation of VDR expression in HIMeg cells by forskolin was accompanied by an increased responsiveness of HIMeg cells to 1,25(OH)2 D3 even though forskolin alone had no effects. Exposure to 1,25(OH)2 D3 in combination with forskolin resulted in a much more significant inhibition of cell proliferation than to 1,25(OH)2 D3 alone. Similarly, forskolin could also augment the differentiation induced by 1,25(OH)2 D3 reflected by a more evident morphological change and a higher percentage of development of cells with multilobular nuclei. These alterations were accompanied by a loss of clonogenic capacity and a decrease in the number of cells in the S phase. These data establish that HIMeg cells express functional VDR, which served to mediate actions of its ligand on the proliferation and differentiation of these cells.

Liposomal incorporation changes the effect of 1.25-dihydroxyvitamin D3 on the phospholipase C signal transduction pathway and the eicosanoid cascade on keratinocytes in vitro

1.25-dihydroxyvitamin D3 is of clinical importance (e.g. in the treatment of psoriasis) given its ability to regulate the proliferation and differentiation of human keratinocytes. 1.25-Dihydroxyvitamin D3 mediates its action via genomic and nongenomic pathways. The nongenomic actions begin with the activation of phospholipase C and the subsequent rapid rise in calcium within the cells. We incorporated 1.25-dihydroxyvitamin D3 in liposomes of varying compositions in an attempt to improve their effect/negative side effect ratio. The influence of empty liposomes (1 mM) and free and liposomally incorporated 1.25-dihydroxyvitamin D3 (10 nM) on the rapid release of sulfidoleucotrien and inositole 1,4,5 triphosphate was examined in keratinocytes in vitro. Free 10 nM 1.25-dihydroxyvitamin D3 provoked a rapid rise in sulfidoleucotriens within 30 seconds, followed by a swift decrease in sulfidoleucotrien and inositole 1,4,5-triphosphate concentration after 10 minutes. Empty liposomes and liposomal-incorporated 1.25-dihydroxyvitamin D3 did not show such a strong effect. These results suggest the occurrence of specific binding sites for 1.25-dihydroxyvitamin D3 on the membrane level that are incapable of recognizing 1.25-dihydroxyvitamin D3 trapped within liposomal membrane.

A composite element binding the vitamin D receptor, retinoid X receptor alpha, and a member of the CTF/NF-1 family of transcription factors mediates the vitamin D responsiveness of the c-fos promoter

The hormonal form of vitamin D, 1 alpha,25-dihydroxyvitamin D3 [1,25- (OH)2D3], transiently stimulates the transcription of the c-fos proto-oncogene in osteoblastic cells. We have identified and characterized a vitamin D response element (VDRE) in the promoter of c-fos. The 1,25-(OH)2D3-responsive region was delineated between residues -178 and -144 upstream of the c-fos transcription start site. A mutation that inhibited binding to the sequence concomitantly abolished 1,25-(OH)2D3-induced transcriptional responsiveness; similarly, cloning to the site upstream of a heterologous promoter conferred copy-number-dependent vitamin D responsiveness to a reporter gene, demonstrating that we have identified a functional response element. The structure of the c-fos VDRE was found to be unusual. Mutational analysis revealed that the c-fos VDRE does not conform to the direct repeat configuration in which hexameric core-binding sites are spaced by a few nucleotide residues. In contrast, the entire 36-bp sequence was essential for binding. We identified the vitamin D receptor and the retinoid X receptor alpha as components of the complex that bound the c-fos VDRE. However, our results also show that a putative CCAAT-binding transcription factor/nuclear factor 1 (CTF/NF-1) family member bound the response element in conjunction with the nuclear hormone receptors. The expression of this CTF/NF-1 family member appeared restricted to bone cells. These data hint at new molecular mechanisms of action for vitamin D.

Homeostatic control of plasma calcium concentration

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

Contributions of nuclear architecture to transcriptional control

Three parameters of nuclear structure contribute to transcriptional control. The linear representation of promoter elements provides competency for physiological responsiveness within the contexts of development as well as cycle- and phenotype-dependent regulation. Chromatin structure and nucleosome organization reduce distances between independent regulatory elements providing a basis for integrating components of transcriptional control. The nuclear matrix supports gene expression by imposing physical constraints on chromatin related to three-dimensional genomic organization. In addition, the nuclear matrix facilitates gene localization as well as the concentration and targeting of transcription factors. Several lines of evidence are presented that are consistent with involvement of multiple levels of nuclear architecture in cell growth and tissue-specific gene expression during differentiation. Growth factor and steroid hormone responsive modifications in chromatin structure, nucleosome organization, and the nuclear matrix that influence transcription of the cell cycle-regulated histone gene and the bone tissue-specific osteocalcin gene during progressive expression of the osteoblast phenotype are considered.

Constitutive expression of calreticulin in osteoblasts inhibits mineralization

Recent studies have shown that the multifunctional protein calreticulin can localize to the cell nucleus and regulate gene transcription via its ability to bind a protein motif in the DNA-binding domain of nuclear hormone receptors. A number of known modulators of bone cell function, including vitamin D, act through this receptor family, suggesting that calreticulin may regulate their action in bone cells. We have used a gain-of-function strategy to examine this putative role of calreticulin in MC3T3-E1 osteoblastic cells. Purified calreticulin inhibited the binding of the vitamin D receptor to characterized vitamin D response elements in gel retardation assays. This inhibition was due to direct protein-protein interactions between the vitamin D receptor and calreticulin. Expression of calreticulin transcripts declined during MC3T3-E1 osteoblastic differentiation. MC3T3-E1 cells were transfected with calreticulin expression vectors; stably transfected cell lines overexpressing recombinant calreticulin were established and assayed for vitamin D-induced gene expression and the capacity to mineralize. Constitutive calreticulin expression inhibited basal and vitamin D-induced expression of the osteocalcin gene, whereas osteopontin gene expression was unaffected. This pattern mimicked the gene expression pattern observed in parental cells before down-regulation of endogenous calreticulin expression. In long-term cultures of parental or vector-transfected cells, 1 alpha,25-dihydroxyvitamin D3 (1,25[OH]2D3) induced a two- to threefold stimulation of 45Ca accumulation into the matrix layer. Constitutive expression of calreticulin inhibited the 1,25(OH)2D3-induced 45Ca accumulation. This result correlated with the complete absence of mineralization nodules in long-term cultures of calreticulin-transfected cells. These data suggest that calreticulin can regulate bone cell function by interacting with specific nuclear hormone receptor-mediated pathways.

Comparative effects of 1,25-dihydroxyvitamin D3 and EB 1089 on mouse renal and intestinal 25-hydroxyvitamin D3-24-hydroxylase

EB 1089 is a vitamin D analog that is less potent than 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) in its calcemic action but more potent in its antiproliferative action. We characterized the interaction of 1,25(OH)2D3 and EB 1089 with renal 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase), the first enzyme in the C-24 oxidation pathway, and compared the effects of 1,25(OH)2D3 and EB 1089 on induction of 24-hydroxylase mRNA in mouse kidney and intestine. 1,25(OH)2D3 and EB 1089 were competitive inhibitors of 24-hydroxylase activity. However, the Ki for 1,25(OH)2D3 (5.2 +/- 2.5 nM) was significantly lower than that for EB 1089 (286 +/- 59 nM). In the kidney, the time course and extent of 24-hydroxylase mRNA induction, relative to 18S rRNA, was similar for 1,25(OH)2D3 and EB 1089 with a peak response at approximately equal to 6 h that was sustained for at least 16 h. In the intestine, however, induction of 24-hydroxylase mRNA, relative to 18S rRNA, was approximately 50% lower for EB 1089 than for 1,25(OH)2D3 at 3 h (p < 0.05) and 6 h (p < 0.05) while at 16 h 24-hydroxylase mRNA was no longer detectable. Moreover, while both 1,25(OH)2D3 and EB 10898 elicited a similar dose-dependent induction of 24-hydroxylase mRNA in the kidney (EC50 = 0.4 +/- 0.13 and 0.3 +/- 0.08 ng/g for EB 1089 and 1,25(OH)2D3, respectively), the EC50 for EB 1089 (6.6 +/- 1.7 ng/g) was significantly higher than that for 1,25(OH)2D3 (0.9 +/- 0.32 ng/g) in the intestine (p < 0.01). EB 1089 was also less effective than 1,25(OH)2D3 in the induction of intestinal but not renal calbindin-D9k mRNA. To determine the mechanism for tissue-specific differences in potency, we determined the binding affinity of 1,25(OH)2D3 and EB 1089 for the vitamin D receptor. In the kidney, Kd values for 1,25(OH)2D3 (0.40 +/- 0.95 nM) and EB 1089 (0.48 +/- 0.04 nM) were not different. However, in the intestine, the Kd for EB 1089 (1.43 +/- 0.19 nM) was significantly higher than that for 1,25(OH)2D3 (0.85 +/- 0.06 nM; p < 0.05). Our results demonstrate that: (i) EB 1089 has a 50-fold lower affinity than 1,25(OH)2D3 for renal 24-hydroxylase, suggesting that it is more resistant to catabolism by the C-24 oxidation pathway; and (ii) EB 1089 and 1,25(OH)2D3 exhibit tissue-specific differences in vitamin D receptor-mediated responses in vivo that may be ascribed, at least in part, to differences in binding affinities for the vitamin D receptor.

Studies on two new vitamin D analogs, EB 1089 and KH 1060: effects on bone resorption and osteoclast recruitment in vitro

We have investigated the effects on bone resorption of two new potent antiproliferative vitamin D3 analogs, EB 1089 and KH 1060, by studying recruitment of osteoclasts in murine bone marrow cultures and 45Ca release from prelabeled neonatal mouse calvarial bones. Binding studies to vitamin D receptor protein, from human osteosarcoma MG-63 cells, demonstrated kd values of 8.5 x 10(-11) for 1 alpha,25(OH)2D3, 6.5 x 10(-11) for KH 1060, and 2.7 x 10(-10) for EB 1089. 1 alpha,25(OH)2D3 and EB 1089 were equipotent stimulators of osteoclast recruitment in murine bone marrow cultures, with EC50 at 10(-10) mol/L, whereas KH 1060 was about tenfold more potent with an EC50 at 10(-11) mol/L. In serum-free media, 1 alpha,25(OH)2D3 enhanced 45Ca release from neonatal mouse calvarial bones with EC50 at 10(-11) mol/L, but in the presence of 10% fetal calf serum (FCS) the stimulatory effect was significantly diminished, with a threshold value at 10(-10) mol/L. EB 1089 stimulated bone resorption with an estimated EC50 at 3 x 10(-11) mol/L, whereas KH 1060 was about tenfold more potent than 1 alpha,25(OH)2D3, and stimulated bone resorption with an EC50 at 10(-12) mol/L. The effects of EB 1089 and KH 1060 on 45Ca release were not significantly affected by the addition of 10% FCS. Addition of vitamin D binding protein to serum-free incubations of neonatal mouse calvarial bones, significantly inhibited the bone resorbing effect of 1 alpha,25(OH)2D3, but did not affect EB 1089 and KH 1060 induced 45Ca release.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcipotriol: a new drug for topical psoriasis treatment

Vitamin D is best known for its role in the regulation of calcium and bone metabolism. The effects of the biologically active form of vitamin D, 1,25-dihydroxyvitamin D3 (1,25 (OH)2D3), are mediated by binding to a specific intracellular vitamin D receptor, which is present in most tissues including the skin where it regulates the growth of epidermal cells. Calcipotriol is a synthetic analogue of 1,25(OH)2D3. In vitro the activity of calcipotriol is comparable to that of 1,25(OH)2D3. In vivo, however, the risk of calcipotriol changing calcium metabolism is greatly reduced. Animal studies have established that calcipotriol is 100-200 times less calcaemic than 1,25(OH)2-D3. This low calcaemic activity is mainly due to the rapid metabolism of calcipotriol. This pharmacological profile makes calcipotriol an ideal candidate for topical treatment of hyperproliferative skin disorders, such as psoriasis. This paper reviews the clinical experience with calcipotriol in psoriasis patients.

1,25(OH)2-vitamin D-3 stimulates phospholipase A2 activity via a guanine nucleotide-binding protein in chick myoblasts

The steroid hormone 1,25(OH)2-vitamin D-3 [1,25(OH)2D3] stimulated phospholipase A2 (PLA2) activity in embryonic chick myoblasts releasing [3H]arachidonic acid from the sn-2 position of phospholipids. GTP-binding protein mediation of 1,25(OH)2D3-dependent PLA2 activity was investigated in cells prelabeled with [3H]arachidonic acid. AIF4-, a G-protein activator, mimicked 1,25(OH)2D3-stimulated arachidonic acid release from myoblasts in a dose-dependent manner. Consistent with the involvement of a G-protein in the activation of PLA2 by the hormone, guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S), a stable GTP analogue which activates G-protein mediated signals, strongly enhanced arachidonic acid release in myoblasts. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S), which competitively inhibits G-protein activation by GTP and its analogues, abolished 1,25(OH)2D3-dependent arachidonic acid release. Bordetella pertussis toxin pretreatment significantly suppressed the hormone action whereas cholera toxin had minor effects on 1,25(OH)2D3 action. Hormone-induced activation of PLA2 was mimicked by the Ca2+ ionophore A23187 and blocked by nifedipine, but was unaffected by neomycin, a phospholipase C inhibitor, ruling out the contribution of phosphoinositide metabolism to arachidonic acid release. These results suggest that 1,25(OH)2D3-stimulation of PLA2 activity in embryonic chick myoblasts is mediated by a pertussis toxin-sensitive GTP-binding protein coupled to influx of extracellular calcium.

Biological activity of vitamin D analogues in the skin, with special reference to antipsoriatic mechanisms

Active vitamin D3 modulates epidermal growth, keratinization and inflammation, and various vitamin D3 analogues have been shown to be effective in the treatment of psoriasis. These analogues now provide a useful addition to the therapeutic modalities available for the treatment of psoriasis. Epidermal hyperproliferation, abnormal keratinization and inflammation are the well-established hallmarks of the psoriatic plaque. The aim of this review is to provide an update of information on the cell biological effects of vitamin D3, and the influence of vitamin D analogues on the pathomechanisms of psoriasis.

Interactions of 1,25(OH)(2)D (3) and retinoic acid in the regulation of IEC-6 cell alkaline phosphatase activity

The goal of the present work was to use IEC-6 cells to investigate the possible mechanisms underlying the regulation of alkaline phosphatase (ALP) activity (ALPA) by 1α, 25-dihydroxyvitamin D(3) (1,25(OH)(2)D(3)) and retinoids. Here we demonstrate that the vitamin D analogs, 25(OH)(2)-16-ene-23-yne-D(3) and 1α, 24S-(OH)(2)-22-en-26, 27-dehydro-vitamin D(3), which have been shown by others to bind to the intracellular vitamin D receptor (VDR), have similar effects to 1, 25(OH)(2)D(3) in increasing ALPA of IEC-6 cells. A third vitamin D analog, 25-(OH)-16-ene-23-yne-D(3) (AT), which activates membrane 1,25(OH)(2)D(3) effects, but binds poorly to the intracellular VDR, did not stimulate ALPA of IEC-6 cells. These data suggest that the effects of 1,25(OH)(2)D(3) to increase ALPA are mediated by intracellular VDR rather than by membrane actions of the hormone. The all-trans and 9-cis retinoic acids alone each caused increased ALPA of IEC-6 cells without altering steady-state levels of ALP mRNA, suggesting that retinoic acids may regulate ALPA of IEC-6 cells at a posttranscriptional level. Vitamin D analogs which bind intracellular receptors showed synergistic effects with either retinoid to increase ALPA, but there was no interaction with AT. Although the retinoids alone did not alter ALP mRNA levels, addition of 1,25(OH)(2)D(3) in combination with either retinoid increased ALP mRNA more than did 1,25(OH)(2)D(3) alone. These data suggest that the synergistic effects of 1,25(OH)(2)D(3) and retinoids on IEC-6 cell ALPA are mediated by intracellular VDR. The results of these experiments indicate that 1,25(OH)(2)D(3) alters IEC-6 cell ALPA via increased mRNA levels, while retinoids appear to both have post-transcriptional effects and the capacity to interact with 1,25(OH)(2)D(3) in altering ALP mRNA levels.

1,25-Dihydroxyvitamin D3 and retinoid X receptor expression in human colorectal neoplasms

Epidemiological studies suggest that 1,25-dihydroxyvitamin D3 (D3) protects against colorectal carcinogenesis. Animal and in vitro studies show an antiproliferative effect of D3 in a variety of tumours including those of large bowel origin. D3 actions are mediated by D3 receptors (VDR) alone or by VDR in conjunction with retinoid X receptors (RXRs) in all D3 responsive tissues. The expression of mRNAs encoding VDR and RXRs in normal and malignant human colorectum was determined. Full length VDR (4.6 kB), RXR alpha (5.5 kB), and RXR gamma (3.5 and 7 kB) mRNAs were expressed in all tissues, but RXR beta mRNA was not expressed in any. VDR expression was reduced in 12 carcinomas relative to paired normal mucosa, and RXR alpha expression was reduced in nine. There was no correlation between VDR or RXR alpha expression and the site, grade of differentiation, or Dukes's staging of the tumour. The finding of persistent VDR and RXR coexpression in all colorectal tumours provides a rational basis for exploring a role for D3 in the treatment of colorectal malignancy.

Regulation of type I collagen gene expression in bone

The regulation of COL1A1 gene expression in bone was studied by measuring the activity of type I collagen promoter fusion genes (ColCAT) in permanently transfected osteoblastic cells and calvariae from transgenic animals. The basal activity of ColCAT fusion genes in transfected cells is mediated by DNA sequences between -3.5 to -2.3 kb while expression in vivo requires sequences between -2.3 and -1.7 kb. Parathyroid hormone, 1,25-dihydroxyvitamin D3 and interleukin-1 decrease the activity of ColCAT fusion genes in osteoblastic cells and transgenic calvariae. Because there may be differences between the expression of ColCAT fusion genes in cultured cells and intact bone, it will be important to compare data obtained from transfected cells with an in vivo model such as calvariae from transgenic mice.

Analysis of regulatory regions in the COL1A1 gene responsible for 1,25-dihydroxyvitamin D3-mediated transcriptional repression in osteoblastic cells

The synthesis of type I collagen in bone cells is inhibited by the calcium-regulating hormone 1,25-dihydroxyvitamin D3. Earlier work from our laboratories has indicated that vitamin D regulation is at the level of transcription, based on results from both nuclear run-off assays and functional promoter analysis of a hybrid gene consisting of a 3.6 kb COL1A1 promoter fragment fused to the chloramphenicol acetyltransferase reporter gene. In the present study, we investigated the molecular basis for vitamin D-mediated transcriptional repression of the COL1A1 gene and report the identification of a region within the COL1A1 upstream promoter (the HindIII-Pstl restriction fragment between nucleotides -2295 and -1670) which is necessary for 1,25-dihydroxyvitamin D3 responsiveness in osteoblastic cells. This hormone-mediated inhibitory effect on the marker gene parallels the inhibition of the endogenous collagen gene. A 41 bp fragment from this region (between nucleotides -2256 and -2216) contains a sequence which is very similar to vitamin D-responsive elements identified in the osteocalcin gene. Extracts from cultured cells which express a high level of vitamin D receptor contain a hormone:receptor complex that binds specifically to this 41 bp fragment, as demonstrated by bandshift analysis. However, deletion of this vitamin D receptor binding region from either a -3.5 kb or a -2.3 kb promoter fragment did not abolish vitamin D responsiveness. These results indicate that a vitamin D response element similar to that described for other vitamin D responsive genes (osteocalcin and osteopontin) does not alone mediate the repression of COL1A1 by 1,25-dihydroxyvitamin D3.