Healing of rachitic lesions in chicks by 24R,25-dihydroxycholecalciferol administered locally into bone

The Impact of Vitamin D Levels in Foot and Ankle Surgery

Hypovitaminosis D has been established as a global health problem. As an important regulator of skeletal health homeostasis throughout one's life, optimal levels are presumed. Debate, however, still exists surrounding the definition of normal vitamin D levels and what affect hypovitaminosis D has on fracture prevention, fracture healing, and successful arthrodesis. A literature search failed to show any level 1 studies examining hypovitaminosis D and union rates in foot and/or ankle arthrodesis procedures. Several retrospective studies do point to some sort of association between nonunion and hypovitaminosis D. Because of lack of high-level studies, a potential study design is proposed.

Effect of modulating dietary vitamin D on the general bone health of rats during posterolateral spinal fusion

Vitamin D plays a significant role in musculoskeletal health by regulating calcium, phosphate, and promoting new bone mineralization. The purpose of this study was to understand the effect of dietary vitamin D on general bone health during peri-operative bone healing via an in vivo dosing study of vitamin D in a rat posterolateral fusion model using autograft. Vitamin D Deficient (DD), vitamin D Insufficient (ID), Control vitamin D (CD), and Hyper-vitamin D (HD) groups were studied. Increasing dietary vitamin D improved quantitative measures of femoral geometry, including femoral strength, stiffness, and density. Femoral biomechanics, cortical thickness, moment of inertia, cross-sectional area, and measures from bone ashing were all greater in the HD group versus the CD. This suggests that additional dietary vitamin D above normal levels during spinal fusion may lead to improvement in bone health. Serum vitamin D levels were also observed to decrease during fusion healing. These results demonstrate that dietary vitamin D improves general bone health in the femur of a rat model during posterolateral spinal fusion. This suggests a role for further clinical evaluation of vitamin D dietary intake during the peri-operative period, with the possibility of avoiding adverse consequences to general bone health. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:1435-1443, 2018.

Vitamin D in Foot and Ankle Fracture Healing: A Literature Review and Research Design

Vitamin D is a generic name for a group of essential vitamins, or secosteroids, important in calcium homeostasis and bone metabolism. Specifically, efficacy of vitamin D with regard to bone healing is in question. A literature review was performed, finding mostly large studies involving vitamin D effects on prevention of fractures and randomized animal model studies consisting of controlled fractures with vitamin D interventions. The prevention articles generally focus on at-risk populations, including menopausal women and osteoporotic patients, and also most often include calcium in the treatment group. Few studies look at vitamin D specifically. The animal model studies often focus more on vitamin D supplementation; however the results are still largely inconclusive. While recent case reports appear promising, the ambiguity of results on the topic of fracture healing suggests a need for more, higher level research. A novel study design is proposed to help determine the efficacy on vitamin D in fracture healing.

LEVELS OF EVIDENCE

Therapeutic, Level IV: Systematic Review.

24, 25-dihydroxycholecalciferol but not 25-hydroxycholecalciferol suppresses apolipoprotein A-I gene expression

AIMS

Ligands for the vitamin D receptor (VDR) regulate apolipoprotein A-I (apo A-I) gene expression in a tissue-specific manner. The vitamin D metabolite 24, 25-dihydroxycholecalciferol (24, 25-(OH)(2)D(3)) has been shown to possess unique biological effects. To determine if 24, 25-(OH)(2)D(3) modulates apo A-I gene expression, HepG2 hepatocytes and Caco-2 intestinal cells were treated with 24, 25-(OH)(2)D(3) or its precursor 25-OHD(3).

MAIN METHODS

Apo A-I protein levels and mRNA levels were measured by Western and Northern blotting, respectively. Changes in apo A-I promoter activity were measured using the chlorampenicol acetytransferase assay.

KEY FINDINGS

Treatment with 24, 25-(OH)(2)D(3), but not 25-OHD(3), inhibited apo A-I secretion in HepG2 and Caco-2 cells and apo A-I mRNA levels and apo A-I promoter activity in HepG2 cells. To determine if 24, 25-(OH)(2)D(3) represses apo A-I gene expression through site A, the nuclear receptor binding element that is essential for VDRs effects on apo A-I gene expression, HepG2 cells were transfected with plasmids containing or lacking site A. While the site A-containing plasmid was suppressed by 24, 25-(OH)(2)D(3), the plasmid lacking site A was not. Likewise, treatment with 24, 25-(OH)(2)D(3) suppressed reporter gene expression in cells transfected with a plasmid containing site A in front of a heterologous promoter. Finally, antisense-mediated VDR depletion failed to reverse the silencing effects of 24, 25-(OH)(2)D(3) on apo A-I expression.

SIGNIFICANCE

These results suggest that the vitamin D metabolite 24, 25-(OH)(2)D(3) is an endogenous regulator of apo A-I synthesis through a VDR-independent signaling mechanism.

Endocrine regulation of the growth plate

Longitudinal bone growth occurs at the growth plate by endochondral ossification. Within the growth plate, chondrocyte proliferation, hypertrophy, and cartilage matrix secretion result in chondrogenesis. The newly formed cartilage is invaded by blood vessels and bone cells that remodel the newly formed cartilage into bone tissue. This process of longitudinal bone growth is governed by a complex network of endocrine signals, including growth hormone, insulin-like growth factor I, glucocorticoid, thyroid hormone, estrogen, androgen, vitamin D, and leptin. Many of these signals regulate growth plate function, both by acting locally on growth plate chondrocytes and also indirectly by modulating other endocrine signals in the network. Some of the local effects of hormones are mediated by changes in paracrine factors that control chondrocyte proliferation and differentiation. Many human skeletal growth disorders are caused by abnormalities in the endocrine regulation of the growth plate. This review provides an overview of the endocrine signals that regulate longitudinal bone growth, their interactions, and the mechanisms by which they affect growth plate chondrogenesis.

Differential regulation of growth plate chondrocytes by 1alpha,25-(OH)2D3 and 24R,25-(OH)2D3 involves cell-maturation-specific membrane-receptor-activated phospholipid metabolism

This review discusses the regulation of growth plate chondrocytes by vitamin D(3). Over the past ten years, our understanding of how two vitamin D metabolites, 1alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3), exert their effects on endochondral ossification has undergone considerable advances through the use of cell biology and signal transduction methodologies. These studies have shown that each metabolite affects a primary target cell within the endochondral developmental lineage. 1alpha,25-(OH)(2)D(3) affects primarily growth zone cells, and 24R,25-(OH)(2)D(3) affects primarily resting zone cells. In addition, 24R,25-(OH)(2)D(3) initiates a differentiation cascade that results in down-regulation of responsiveness to 24R,25-(OH)(2)D(3) and up-regulation of responsiveness to 1alpha,25-(OH)(2)D(3). 1alpha,25-(OH)(2)D(3) regulates growth zone chondrocytes both through the nuclear vitamin D receptor, and through a membrane-associated receptor that mediates its effects via a protein kinase C (PKC) signal transduction pathway. PKCalpha is increased via a phosphatidylinositol-specific phospholipase C (PLC)-dependent mechanism, as well as through the stimulation of phospholipase A(2) (PLA(2)) activity. Arachidonic acid and its downstream metabolite prostaglandin E(2) (PGE(2)) also modulate cell response to 1alpha,25-(OH)(2)D(3). In contrast, 24R,25-(OH)(2)D(3) exerts its effects on resting zone cells through a separate, membrane-associated receptor that also involves PKC pathways. PKCalpha is increased via a phospholipase D (PLD)-mediated mechanism, as well as through inhibition of the PLA(2) pathway. The target-cell-specific effects of each metabolite are also seen in the regulation of matrix vesicles by vitamin D(3). However, the PKC isoform involved is PKCzeta, and its activity is inhibited, providing a mechanism for differential autocrine regulation of the cell and events in the matrix by these two vitamin D(3) metabolites.

Evidence for distinct membrane receptors for 1 alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) in osteoblasts

1 alpha,25-(OH)(2)D(3) exerts its effects on chondrocytes and enterocytes via nuclear receptors (1,25-nVDR) and a separate membrane receptor (1,25-mVDR) that activates protein kinase C (PKC). 24R,25-(OH)(2)D(3) also stimulates PKC in chondrocytes, but through other membrane mechanisms. This study examined the hypothesis that osteoblasts possess distinct membrane receptors for 1 alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) that are involved in the activation of PKC and that receptor expression varies as a function of cell maturation state. 1 alpha,25-(OH)(2)D(3) stimulated PKC in well differentiated (UMR-106, MC-3T3-E1) and moderately differentiated (ROS 17/2.8) osteoblast-like cells, and in cultures of fetal rat calvarial (FRC) cells and 2T3 cells treated with rhBMP-2 to promote differentiation. 24R,25-(OH)(2)D(3) stimulated PKC in FRC and 2T3 cultures that had not been treated to induce differentiation, and in ROS 17/2.8 cells. MG63 cells, a relatively undifferentiated osteoblast-like cell line, had no response to either metabolite. Ab99, a polyclonal antibody generated to the chick enterocyte 1,25-mVDR, but not a specific antibody to the 1,25-nVDR, inhibited response to 1 alpha,25-(OH)(2)D(3). 1 alpha,25-(OH)(2)D(3) exhibited specific binding to plasma membrane preparations from cells demonstrating a PKC response to this metabolite that is typical of positive cooperativity. Western blots of these membrane proteins reacted with Ab99, and the Ab99-positive protein had an Mr of 64 kDa. There was no cross-reaction with antibodies to the C- or N-terminus of annexin II. The effect of 24,25-(OH)(2)D(3) on PKC was stereospecific; 24S,25-(OH)(2)D(3) had no effect. These results demonstrate that response to 1 alpha,25-(OH)(2)D(3) and 24R,25-(OH)(2)D(3) depends on osteoblast maturation state and suggest that specific and distinct membrane receptors are involved.

Plasma 24,25-dihydroxyvitamin D concentration of Dahl salt-sensitive rats decreases during high salt intake

Dahl salt-sensitive rats, but not salt-resistant rats, develop hypertension in response to high salt intake. We have previously shown an inverse relationship between plasma 25-hydroxyvitamin D (25-OHD) concentration and blood pressure of Dahl salt-sensitive rats during high salt intake. In this study, we report on the relationship between high salt intake and plasma 24,25-dihydroxyvitamin D (24,25-(OH)(2)D) concentration of Dahl salt-sensitive and salt-resistant rats. Rats were fed a high salt diet (8%) and sacrificed at day 2, 7, 14, 21, and 28. Plasma 24,25-(OH)(2)D concentrations of salt-sensitive rats were reduced to 50% of that at baseline at day 2-when blood pressure and plasma 25-OHD concentration were unchanged, but 25-OHD content in the kidney was 81% of that at baseline. Plasma 24,25-(OH)(2)D concentration was reduced further to 10% of that at baseline from day 7 to 14 of high salt intake, a reduction that was prevented in rats switched to a low salt (0.3%) diet at day 7. Exogenous 24,25-dihydroxycholecalciferol (24,25-(OH)(2)D(3)), administered at a level that increased plasma 24,25-(OH)(2)D concentration to five times normal, did not attenuate the salt-induced hypertension of salt-sensitive rats. Plasma 24,25-(OH)(2)D concentration of salt-resistant rats was gradually reduced to 50% of that at baseline at day 14 and returned to baseline value at day 28 of high salt intake. We conclude that the decrease in plasma 24,25-(OH)(2)D concentration in salt-sensitive rats during high salt intake is caused by decreased 25-OHD content in the kidney and also by another unidentified mechanism.

24,25-(OH)(2)D(3) regulates cartilage and bone via autocrine and endocrine mechanisms

The purpose of this paper is to summarize recent advances in our understanding of the physiological role of 24(R),25(OH)(2)D(3) in bone and cartilage and its mechanism of action. With the identification of a target cell, the growth plate resting zone (RC) chondrocyte, we have been able to use cell biology methodology to investigate specific functions of 24(R),25(OH)(2)D(3) and to determine how 24(R),25(OH)(2)D(3) elicits its effects. These studies indicate that there are specific membrane-associated signal transduction pathways that mediate both rapid, nongenomic and genomic responses of RC cells to 24(R),25(OH)(2)D(3). 24(R),25(OH)(2)D(3) binds RC chondrocyte membranes with high specificity, resulting in an increase in protein kinase C (PKC) activity. The effect is stereospecific; 24R,25(OH)(2)D(3), but not 24S,25-(OH)(2)D(3), causes the increase, indicating a receptor-mediated response. Phospholipase D-2 (PLD2) activity is increased, resulting in increased production of diacylglycerol (DAG), which in turn activates PKC. 24(R),25(OH)(2)D(3) does not cause translocation of PKC to the plasma membrane, but activates existing PKCalpha. There is a rapid decrease in Ca(2+) efflux, and influx is stimulated. 24(R),25(OH)(2)D(3) also reduces arachidonic acid release by decreasing phospholipase A(2) (PLA(2)) activity, thereby decreasing available substrate for prostaglandin production via the action of cyclooxygenase-1. PGE(2) that is produced acts on the EP1 and EP2 receptors expressed by RC cells to downregulate PKC via protein kinase A, but the reduction in PGE(2) decreases this negative feedback mechanism. Both pathways converge on MAP kinase, leading to new gene expression. One consequence of this is production of new matrix vesicles containing PKCalpha and PKCzeta and an increase in PKC activity. The chondrocytes also produce 24(R),25(OH)(2)D(3), and the secreted metabolite acts directly on the matrix vesicle membrane. Only PKCzeta is directly affected by 24(R),25(OH)(2)D(3) in the matrix vesicles, and activity of this isoform is inhibited. This effect may be involved in the control of matrix maturation and turnover. 24(R),25(OH)(2)D(3) causes RC cells to mature along the endochondral developmental pathway, where they become responsive to 1alpha,25(OH)(2)D(3) and lose responsiveness to 24(R),25(OH)(2)D(3), a characteristic of more mature growth zone (GC) chondrocytes. 1alpha,25(OH)(2)D(3) elicits its effects on GC through different signal transduction pathways than those used by 24(R),25(OH)(2)D(3). These studies indicate that 24(R),25(OH)(2)D(3) plays an important role in endochondral ossification by regulating less mature chondrocytes and promoting their maturation in the endochondral lineage.

Vitamin D receptor gene polymorphism is associated with birth height, growth to adolescence, and adult stature in healthy caucasian men: a cross-sectional and longitudinal study

Vitamin D receptor (VDR) polymorphism has been associated with bone mineral density (BMD), but recent data indicate association to parameters of body constitution and growth. We investigated VDR gene polymorphism, defined by BsmI and TaqI, in 90 healthy Caucasian males and any relation with parameters of body constitution at birth, and to parameters of body constitution, BMD and bone area, at age 16.9 +/- 0.3 yr (mean +/- SD) and at age 19.2 +/- 0.7. Using PCR and the restriction enzyme BsmI and TaqI, the allelic variants BB, Bb, and bb, and TT, Tt, and tt were identified. Height (cm) and weight (kg) were measured using standardized equipment, and BMD of the total body, lumbar spine, and femoral neck, and bone area (cm2) of the total body, humerus, femur was measured using dual-energy x-ray absorptiometry. BsmI and TaqI genotypes were related in 89 of the 90 cases; hence, the same associations were found for both genotypes. Boys with the BB genotype were shorter at birth (P = 0.01) and grew less from birth to age 16.9 +/- 0.3 (P = 0.01) than their Bb and bb counterparts. Both during puberty (age 16.9 +/- 0.3) and after puberty (age 19.3 +/- 0.7), the BB boys were shorter (P = 0.005-0.008) and had lower bone area of the humerus, femur, and total body (P < 0.05) than the Bb and bb boys. The allelic variants were not related to BMD at any site. A prediction model including parental height, birth height, birth weight, and VDR alleles could predict up to 39% of the total variation in adult height in our population. The VDR allelic variants alone contributed to 8% of the total variation.

Effects of 24R,25-dihydroxyvitamin D3 on alkaline phosphatase activity in pig renal epithelial LLC-PK1 cells in culture

1. The effects of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] on alkaline phosphatase activity (ALP) were evaluated in pig kidney LLC-PK1 cells in culture. 2. The vitamin D3 metabolite increased ALP activity in these cells, whereas no effect of the hormone was observed on gamma-glutamyltranspeptidase and acid phosphatase activities. 3. ALP activity was stimulated after 3- to 12-hr incubation in the presence of 10(-9) mol/l 24,25(OH)2D3 with a maximum after 6 hr. 4. The hormonal induction of ALP activity was prevented by pretreatment of cells by actinomycin D. 5. It is proposed that 24,25(OH)2D3 could increase ALP activity by de novo protein synthesis.

Studies on 24R,25-dihydroxyvitamin D3: evidence for a nonnuclear membrane receptor in the chick tibial fracture-healing callus

The effect(s) of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] on fracture healing was studied in a vitamin D-depleted chick model. 24R,25(OH)2D3, together with another hormonally active vitamin D metabolite, 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3], improved bone mechanical strength parameters (torsional strength, angular deformation, and stiffness) and the ash content. The synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25(OH)2D3] was not as potent as the natural 24R,25(OH)2D3. In light of the ability of the fracture-healing callus to discriminate between 24R,25(OH)2D3 and 24S,25(OH)2D3, a search was initiated in fracture-healing callus tissue for the presence of a specific 24R,25(OH)2D3 receptor. No evidence was obtained for a classical nuclear/cytosol receptor for 24R,25(OH)2D3 in the fracture-healing callus. A specific receptor/binding protein for 24R,25(OH)2D3 was found in the callus membrane fraction, which showed different ligand binding affinities [KD = 18.3 +/- 1.9 nmol/L, Bmax = 43.9 +/- 6.0 fmol/mg; relative competitive index (RCI) for 24R,25(OH)2D3/24S,25(OH)2D3/25(OH)D3/1alpha,25(OH)2D3 = 100/37/401/2.0] compared with the ubiquitous serum vitamin D-binding protein (RCI = 100/99/219/5). Also, a callus membrane-binding protein/receptor for 1alpha,25(OH)2D3 was detected with a KD = 0.83 +/- 0.35 nmol/L and a Bmax = 35.5 +/- 5.2 fmol/mg. Thus, we have demonstrated a biological role for 24R,25(OH)2D3 in fracture healing and described the presence of its receptor/binding protein in a callus membrane fraction.

Evidence for a 1 alpha,25-dihydroxyvitamin D3 receptor/binding protein in a membrane fraction isolated from a chick tibial fracture-healing callus

Previous biological studies have implicated two vitamin D metabolites, 1 alpha,25(OH)2-vitamin D3[1 alpha,25(OH)2-D3] and 24R,25(OH)2-vitamin D3 [24R,25(OH)2D3] in the process of skeletal fracture-healing. While a nuclear receptor for 1 alpha,25(OH)2D3 is known to be present in osteoblast and absent in osteoclast cell lines, no systematic study has been carried out on the callus tissue which is formed during fracture-healing. The present report shows that a binding protein/receptor for 1 alpha,25(OH)2D3 resides both in a postnuclear membrane fraction and in a high speed cytosol fraction of the callus tissue obtained 10 days after imposition of a tibial fracture. The dissociation constant, KD, for 1 alpha,25(OH)2D3 was 0.83 +/- 0.34 M and 0.66 +/- 0.38 nM respectively, for the membrane and cytosol fractions. Results from a panel of steroid competition assays indicate that both receptor/binding proteins greatly prefer 1 alpha-hydroxylated ligands as compared to 1 alpha-deoxy or 24-hydroxylated ligands. The presence of 1 alpha,25(OH)2D3 receptors in the fracture-healing callus is consistent with the known biological effects of the metabolite on the fracture-healing process.

24R,25-dihydroxyvitamin D3: an essential vitamin D3 metabolite for both normal bone integrity and healing of tibial fracture in chicks

We tested the hypothesis that 24R,25-dihydroxyvitamin D3 [24R,25-(OH)2D3] is an essential vitamin D metabolite for the development of normal bone integrity and the healing of fractures. The natural 24R,25-(OH)2D3 and its synthetic epimer 24S,25-dihydroxyvitamin D3 [24S,25-(OH)2D3] were tested alone or in combination with 1alpha,25-dihydroxyvitamin D3 [1alpha,25-(OH)2D3], on normal bone development and other related variables of the Ca2+ homeostasis system [serum Ca2+, 25-hydroxyvitamin D3 (25OHD3), 24,25-(OH)2D3, and 1alpha,25-(OH)2D3 levels] in chicks. Mechanical testing of torsional strength was carried out on the femur. 24R,25-(OH)2D3 (80 nmol/kg diet) alone was sufficient for normal bone growth and integrity similar to that achieved by the vitamin D3-replete controls. Next, chicks were fed a 25OHD3-replete diet (75 nmol/kg diet) for 8 days after hatching, and then 25OHD3 was withdrawn to minimize any residual circulating metabolites before the imposition of standardized tibial fractures 14 days later. Vitamin D metabolites were administered for 2 weeks to determine their effects on the mechanical properties of healed tibia. 24S,25-(OH)2D3 combined with 1alpha,25-(OH)2D3 or 1alpha,25-(OH)2D3 alone resulted in poor healing [strength values of 0.158 +/- 0.011 and 0.123 +/- 0.009 Nm (Newton x meter), respectively] compared with that in the 25OHD3-treated control group (0.374 +/- 0.029 Nm). In contrast, the fractured tibia of the birds fed 24R,25-(OH)2D3 in combination with 1alpha,25-(OH)2D3 showed healing equivalent to that in the control group, with strength values of 0.296 +/- 0.043 Nm. These results suggest that when 24R,25-(OH)2D3 is present at normal physiological concentrations, it is an essential vitamin D3 metabolite for both normal bone integrity and healing of fracture in chicks.

A-ring analogues of 1, 25-(OH)2D3 with low affinity for the vitamin D receptor modulate chondrocytes via membrane effects that are dependent on cell maturation

1,25-(OH)2D3 (1,25) and 24,25-(OH)2D3(24,25) mediate their effects on chondrocytes through the classic vitamin D receptor (VDR) as well as through rapid membrane-mediated mechanisms, which result in both nongenomic and genomic effects. In intact cells, it is difficult to distinguish between genomic responses via the VDR and genomic and nongenomic responses via membrane-mediated pathways. In this study, we used two analogues of 1,25 that have been modified on the A-ring (2a, 2b) and are only 0.1% as effective in binding to the VDR as 1,25, to examine the role of the VDR in the response of rat costochondral resting zone (RC) and growth zone (GC) chondrocytes to 1,25 and 24,25. Chondrocyte proliferation ([3H]-thymidine incorporation), proteoglycan production ([35S]-sulfate incorporation), and second messenger activation (activity of protein kinase C) were measured after treatment with 10(-8) M 1,25, 10(-7) M 24,25, or the analogues at 10(-9)-10(-6) M. Both analogues inhibited proliferation of both cell types, as did 1,25 and 24,25. Neither 2a nor 2b had an effect on proteoglycan production by GCs or RCs. 2a caused a dose-dependent stimulation of protein kinase C (PKC) that was not inhibited by cycloheximide or actinomycin D in either GC or RC cells. 2b, on the other hand, had no effect on PKC activity in RCs and only a slight stimulatory effect in GCs. Both cells produce matrix vesicles, extracellular organelles associated with the initial stages of calcification, in culture that are regulated by vitamin D metabolites. Since these organelles contain no DNA or RNA, they provide an excellent model for studying the mechanisms used by vitamin D metabolites to mediate their nongenomic effects. When matrix vesicles were isolated from naive cultures of growth zone cells and treated with 2a, a dose-dependent inhibition of PKC activity was observed that was similar to that found with 1,25-(OH)2D3. Plasma membranes contained increased PKC activity after treatment with 2a, but the magnitude of the effect was less than that seen with 1,25-(OH)2D3. Analogue 2b had no affect on PKC activity in either membrane fraction. When matrix vesicles from resting zone chondrocyte cultures were treated with 24,25-(OH)2D3, a significant decrease in PKC activity was observed. No change in enzyme activity was found for either 1,25-(OH)2D3 or the analogues. PKC activity in the plasma membrane fraction, however, was increased by 24,25-(OH)2D3 as well as by analogue 2a. This study shows that these analogues, with little or no binding to the vitamin D receptor, can affect cell proliferation and PKC activity, but not proteoglycan production. The direct membrane effect is analogue specific and cell maturation dependent. Further, by eliminating the VDR-mediated component of the cellular response, we have provided further evidence for the existence of a membrane receptor(s) involved in mediating nongenomic effects of vitamin D metabolites.

Preferential accumulation in vivo of 24R,25-dihydroxyvitamin D(3) in growth plate cartilage of rats

Vitamin D(3) is metabolized in vivo through 25-(OH)D(3) (25D) to both 1α,25-(OH)(2)D(3) (1,25D) and 24R,25-(OH)(2)D(3) (24,25D). Whereas it is assumed that this metabolism occurs primarily in the kidney, recent studies show that there are extrarenal 1α-and 24R-hydroxylase activities as well, and in chondrocytes, these enzymes are regulated by hormones and growth factors. Furthermore, chondrocytes from the resting zone of growth plate cartilage are a target cell population for 24,25D action, suggesting that this vitamin D metabolite may be targeted to this tissue in vivo. To test this hypothesis, 30 normal male Sprague Dawley rats (120 ±20 g) were divided into three groups of eight animals each, and a control group of six animals, and fed ad libitum for 2 wk, a standard rat chow (Teklad LM-485), which contained 3 IU vitamin D(3)/g. The rats were then injected im daily at 9:00AM: , for 4 consecutive d, with 0.1 mL of either [(3)H]-25D, [(3)H]-1,25D or [(3)H]-24,25D. Each dose contained 13 pmol of hormone (0.36 μCi/dose). The distribution of these metabolites was assessed in tibial bone (B) following ablation of the bone marrow, articular cartilage from the tibia (AC), costochondral growth plate cartilage (GC), serum (S), small intestine (I), and kidney (K). The use of high specific activity tritiated vitamin D metabolites facilitated determining tissue localization and further metabolism without perturbation of the body pools of each major metabolite. Accumulation of [(3)H]-1,25D or [(3)H]-24,25D in each tissue was compared to circulating serum levels. In rats dosed with [(3)H]-25D, the tissue:serum ratios for 1,25D were 4.1 (AC), 35.4 (GC), 1.3 (B), 0.7 (K), and 3.0 (I); and tissue:serum ratios for 24,25D were 1.6 (AC), 9.9 (GC), 0.04 (B), 0.2 (K), and 0.4 (I). In rats dosed with [(3)H]-24,25D alone, GC was the only tissue to accumulate the administered metabolite at a concentration significantly higher than that of serum. Similarly, in rats dosed with [(3)H]-1,25D alone, GC was the only tissue to accumulate 1,25D at a concentration higher than that of serum. These results demonstrate, for the first time, that under in vivo conditions, GC specifically accumulates 24,25D and 1,25D. This suggests that growth plate may be a target organ for these two hormones.

Gender-related effects of vitamin D metabolites on cartilage and bone

Sex steroid hormones are known to have gender-dependent effects on bone and cartilage in vivo and in vitro. To investigate whether this is a general property of steroids, or is specific to the sex steroid hormones, we examined whether the effects on bone of 1,25-(OH)2D3 and 24,25(OH)2D3, the two active metabolites of vitamin D, are also gender-dependent. One-month-old male and female rats were treated for 1 month with various doses of 1,25-(OH)2D3, 24,25-(OH)2D3, or a combination of both metabolites. The direct effects of both metabolites on the skeleton of the treated animals were similar in male and female rats. 24,25-(OH)2D3 alone or in combination with 1,25-(OH)2D3 increased bone calcium and phosphorus, while 1,25-(OH)2D3 slightly decreased bone mineral content. 24,25-(OH)2D3 also enhanced the differentiation of cartilage in the growth plate, increasing the size of the hypertrophic zone. In addition, an increased metaphyseal bone volume was observed following 24,25-(OH)2D3 treatment in rats of both sexes, but not with 1,25-(OH)2D3. Vitamin D metabolites affected the weight gain of the experimental animals in a gender-dependent manner; 1,25-(OH)2D3 increased weight gain of male rats and 24,25-(OH)2D3 decreased weight gain of female rats. In addition, 1,25-(OH)2D3 increased bone weight and ash weight in male animals. These gender-dependent effects of vitamin D metabolites may occur indirectly via effects of sex steroid hormones, the latter being a sex-related effect.

Bone-forming ability of 24R,25-dihydroxyvitamin D3 in the hypophosphatemic mouse

To determine whether 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] exerts unique biologic effects on bone, we examined the effects of the vitamin D metabolites, 24R,25(OH)2D3 and 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3], on the hypophosphatemic (Hyp) mouse, a model for X-linked hypophosphatemic rickets in humans. The Hyp mice were administered 1-10,000 micrograms/kg/day of 24R,25(OH)2D3, 0.01-10 micrograms/kg/day of 1 alpha,25(OH)2D3, or vehicle alone, given daily for 28 days by intraperitoneal injection. 24R,25(OH)2D3 at doses of 1-1000 micrograms/kg/day had dose-dependent effects in increasing bone size, dry bone weight, and bone mineral content without causing hypercalcemia. 1 alpha,25(OH)2D3 at doses of 1 or 10 micrograms/kg/day, which we considered to have activity similar to that of 1000 micrograms/kg/day of 24R,25(OH)2D3 with respect to cell differentiation activity, caused severe bone resorption and hypercalcemia. At 0.1 microgram/kg/day, 1 alpha,25(OH)2D3 increased bone size, similarly to a dose of 1000 micrograms/kg/day of 24R,25(OH)2D3, without significantly affecting dry bone weight or bone mineral content, as did 1000 micrograms/kg/day of 24R,25(OH)2D3. These findings suggest that 24R,25(OH)2D3 exerts unique activity in the Hyp mouse rather than merely mimicking the activity of 1 alpha,25(OH)2D3.

Nongenomic regulation of extracellular matrix events by vitamin D metabolites

Vitamin D metabolites appear to regulate chondrocytes and osteoblasts via a combination of genomic and nongenomic mechanisms. Specificity of the nongenomic response to either 1,25-(OH)2D3 or 24,25-(OH)2D3 may be conferred by the chemical composition of the target membrane and its fluid mosaic structure, by the presence of specific membrane receptors, or by the interaction with classic vitamin D receptors. Nongenomic effects have been shown to include changes in membrane fluidity, fatty acid acylation and reacylation, arachidonic acid metabolism and prostaglandin production, calcium ion flux, and protein kinase C activity. Chondrocytes metabolize 25-(OH)D3 to 1,25-(OH)2D3 and 24,25-(OH)2D3; production of these metabolites is regulated by both growth factors and hormones and is dependent on the state of cell maturation. 1,25-(OH)2D3 and 24,25-(OH)2D3 may interact directly with extracellular matrix vesicles to regulate their function in the matrix, including protease activity, resulting in matrix modification and calcification. Isolated matrix vesicles, produced by growth zone chondrocytes, can activate latent transforming growth factor-beta when incubated with exogenous 1,25-(OH)2D3. These observations suggest that nongenomic regulation of matrix vesicle structure and function may be a mechanism by which mesenchymal cells, like osteoblasts and chondrocytes, may modulate events in the extracellular matrix at sites distant from the cell surface.

Modulation of ecto-nucleoside triphosphate pyrophosphatase activity of human osteoblast-like bone cells by 1 alpha,25-dihydroxyvitamin D3, 24R,25-dihydroxyvitamin D3, parathyroid hormone, and dexamethasone

Extracellular inorganic pyrophosphate (PPi) is involved in the regulation of mineralization, and there is evidence that the cell surface enzyme, NTP pyrophosphatase, is a major source of this metabolite in bone. Osteotrophic agents that influence bone turnover may exert their effects, in part, by modulating the activity of ecto-NTP pyrophosphatase in bone cells. We investigated the effect of 1, 25(OH)2 D3, 24, 25(OH)2D3, dexamethasone, and parathyroid hormone (PTH) on the activity of this enzyme in cultured human trabecular bone-derived osteoblast-like cells. 1,25(OH)2D3 at 10(-11)-10(-9) M induced a dose- and time-dependent increase in activity (at 96 h; maximum 10(-9) M, p < 0.001), whereas higher concentrations (10(-8) and 10(-7) M) had no effect. In contrast, 24,25(OH)2D3 was effective only at 10(-8) and 10(-6) M (at 96 h; p < 0.01). Dexamethasone (10(-9)-10(-7) M) caused a dose-dependent decrease in ecto-NTP pyrophosphatase activity (10(-7) M, p < 0.001); concentrations higher than 10(-7) M did not evoke greater inhibition. This effect became apparent by 48 h and was significantly enhanced after 72 h. The response to dexamethasone was attenuated by cycloheximide, indicating a requirement for de novo protein synthesis. Interestingly, the stimulatory effect of 10(-9) M 1,25(OH)2D3 on ecto-NTP pyrophosphatase activity was significantly enhanced in the presence of dexamethasone (10(-9)-10(-7) M). Human PTH(1-34) and bovine PTH(1-34) in the range 10(-10)-10(-7) M had no effect on enzyme activity over a 72 h period.(ABSTRACT TRUNCATED AT 250 WORDS)

Vitamin D metabolites prevent vertebral osteopenia in ovariectomized rats

The present study investigated the prophylactic effects of vitamin D metabolites and vitamin D metabolite combinations on static and dynamic, tetracycline-based, histomorphometric parameters in the axial skeleton of ovariectomized rats. Forty-three Fischer-344 rats (10 weeks old, 130 g each body weight, BW) were either bilaterally ovariectomized (OVX) or sham-operated (SHAM). The rats were allocated into the following groups: SHAM; OVX; OVX + 7.5 ng 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]/rat/day; OVX + 15 ng 1 alpha,24R,25-trihydroxyvitamin D3 [1,24,25-(OH)3D3]/rat/day; OVX + 75 ng 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3]/rat/day; OVX + 7.5 ng 1,25(OH)2D3/rat/day + 15 ng 1,24,25(OH)3D3/rat/day; OVX + 7.5 ng 1,25(OH)2D3/rat/day + 75 ng 24,25(OH)2D3/rat/day. The vitamin D metabolites were fed orally starting 4 weeks after surgery. Urine and blood samples were collected 12 and 16 weeks postovariectomy, respectively. Sixteen weeks after surgery, all rats were sacrificed, and the first lumbar vertebrae were processed undecalcified for histomorphometric analysis. Ovariectomy induced a highly significant reduction (P less than 0.001) of cancellous bone mass in the secondary spongiosa of the lumbar vertebral body. The bone loss in OVX rats was accompanied by a distinct elevation of all histomorphometric parameters of bone formation and resorption. 1,25(OH)2D3 and both vitamin D metabolite combinations significantly raised serum calcium levels and prevented the bone loss by inhibiting the increased bone resorption in OVX rats. In the applied dosage, 1,24,25(OH)3D3 and 24,25(OH)2D3 alone were ineffective in preserving the cancellous bone of the lumbar vertebra in OVX rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Efficacy of calcium carbonate and low-dose vitamin D/1,25(OH)2D3 in reducing the risk of developing renal osteodystrophy in children on continuous ambulatory peritoneal dialysis

Eight children with terminal renal insufficiency on continuous ambulatory peritoneal dialysis were followed for 12 months to evaluate laboratory parameters of mineral ion and bone metabolism. Calcium carbonate (range 47-295 mg/kg body weight per day) was given in combination with low doses of either vitamin D or 1,25(OH2D3. Blood urea nitrogen and serum phosphate concentrations remained well controlled throughout the observation period. A significant increase in serum calcium levels from 2.35 +/- 0.18 to 2.61 +/- 0.22 mmol/l (mean +/- SD) was observed during the first 6 months. Alkaline phosphatase activity and mid-C-regional parathyroid hormone, both indirect parameters of bone metabolism, revealed no evidence of severe secondary hyperparathyroidism. Our data indicate that calcium carbonate may be sufficient to induce relative hypercalcaemia in uraemic children, and thus reduce the risk of developing renal osteodystrophy. Unwanted side-effects of vitamin D preparations, i.e. increased intestinal phosphate absorption and hypercalcaemia after successful renal transplantation, may thus be avoided.

A model for investigating the local action of bone-acting agents in vivo: effects of hPTH(1-34) on the secondary spongiosa in the rat

Cytokines and other local factors are likely to play an important role in bone remodeling. The purpose of this study was to develop an experimental model for evaluating the local action of various hormones and other agents on rat femur trabecular bone in vivo. Through a 1 mm diameter hole (1 mm deep) on the lateral aspect of the distal cortex of a rat femur (0.9 cm above the joint), a polyethylene tube was inserted and glued onto the bone. This tube was connected to a vascular-access-port which was implanted subcutaneously in the hip area of male Sprague-Dawley rats weighing 190-210 g. To evaluate this model, a single 50 microliters injection of 10(-8), 10(-10), or 10(-12) M hPTH(1-34) was given 1 day after catheter implantation and the number of osteoclasts was estimated 30 hours later. Bones were fixed, embedded, and stained with Masson's Trichrome stain and subjected to histomorphometric analysis. The single local parathyroid hormone (PTH) injection caused a dose-dependent increase in osteoclast number from 1.7 +/- 0.3 for phosphate-buffered saline (PBS) controls to 3.4 +/- 0.5, 5.8 +/- 0.9, and 5.4 +/- 1.0/mm for 10(-12), 10(-10), and 10(-8) M PTH, respectively. There was no increase in osteoclast number in the femurs of PBS-injected control rats, in the femurs of sham-operated rats, or in the untreated contralateral femur of the PTH-injected rat. The local injection of hPTH(1-34) did not change serum calcium, serum phosphate, or the urinary phosphate/creatinine ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term administration of vitamin D3 metabolites alters PTH-responsive osteoblastic adenylate cyclase in rats

We have examined the effect of induced hyper D3 vitaminosis on bone-related variables in the rat with special reference to the parathyroid (PTH)-sensitive adenylate cyclase (AC) in rat calvariae. Subcutaneous injections three times a week of doses theoretically corresponding to about 10 times the average physiological serum levels of either 25 hydroxyvitamin D3 (25OHD3), 1,25 dihydroxyvitamin D3 (1,25(OH)2D3), or 24,25 dihydroxyvitamin D3 (24,25(OH)2D3) for 12 weeks gave the following results: At 12 weeks of treatment, 24,25(OH)2D3 levels in the groups receiving 25OHD3 or 24,25(OH)2D3 increased significantly, whereas 1,25(OH)2D3 levels remained unaffected. Correspondingly, PTH-sensitive AC activities in crude calvarial membrane fractions from 25OHD3- and 24,25(OH)2D3-treated animals were obliterated. This effect was apparent after 4 weeks of treatment. In the group receiving 25OHD3, both basal, plus Gpp(NH)p-, and forskolin-sensitive AC activities were significantly reduced after 4 weeks of treatment. Similar effects in crude kidney membrane fractions were, however, not observed. Liver membranes from 25OHD3- or 24,25(OH)2D3-treated animals showed insignificant changes in the isoprenalin-, PGE1-, Gpp(NH)p-, or forskolin-sensitive AC activities. Finally, the significance of reduced PTH-sensitive bone AC activity has been assessed. 25OHD3 treatment yielded normocalcemic and hypercalciuric rats, whereas 1,25(OH)2D3 enhanced both serum and urine Ca2+ levels. 24,25(OH)2D3-treated and control animals were undiscernible in this respect. However, the 24,25-(OH)2D3 treatment caused reductions in both serum alkaline phosphatase levels and urinary hydroxyproline/creatinine ratio.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum vitamin D metabolites in cadmium-exposed persons with renal damage

Serum concentrations of 25-hydroxyvitamin D [25(OH)D], 24,25-dihydroxyvitamin D [24,25(OH)2D], and 1 alpha,25-dihydroxyvitamin D [1 alpha,25(OH)2D] were measured in ten cadmium (Cd)-exposed subjects and five non exposed subjects. The Cd-exposed subjects were divided into two groups according to serum 1 alpha,25(OH)2D levels. No significant differences for 25(OH)D were found between the Cd-exposed group with low or normal serum 1 alpha,25(OH)2D and the non exposed group. The concentrations of 24,25(OH)2D were the lowest in the Cd-exposed group with low serum 1 alpha,25(OH)2D, highest in the non exposed group, and significantly lower in the Cd-exposed group with normal serum 1 alpha,25(OH)2D than in the non exposed group. Renal function was much worse in the Cd-exposed group with low serum 1 alpha,25(OH)2D than in the group with normal serum 1 alpha,25(OH)2D. These findings indicate that Cd initially disturbs hydroxylation from 25(OH)D to 24,25(OH)2D and then disturbs hydroxylation from 25(OH)D to 1 alpha,25(OH)2D. The decrease of serum 24,25(OH)2D and 1 alpha,25(OH)2D in Cd-exposed subjects is not due to a decrease of the serum 25(OH)D level.

Cartilage macromolecules and the calcification of cartilage matrix

The calcification of cartilage matrix in endochondral bone formation occurs in an extracellular matrix composed of fibrils of type II collagen with which type X collagen is closely associated. Also present within this matrix are the large proteoglycans containing chondroitin sulfate which aggregate with hyaluronic acid. In addition, the matrix contains matrix vesicles containing alkaline phosphatase. There is probably a concentration of calcium as a result of its binding to the many chondroitin sulfate chains. At the time of calcification, these proteoglycans become focally concentrated in sites where mineral is deposited. This would result in an even greater focal concentration of calcium. Release of inorganic phosphate, as a result of the activity of alkaline phosphatase, can lead to the displacement of proteoglycan bound calcium and its precipitation. The C-propeptide of type II collagen becomes concentrated in the mineralizing sites, prior to which it is mainly associated with type II collagen fibrils and is present in dilated cisternae of the enlarged hypertrophic chondrocytes. The synthesis of type II collagen and the C-propeptide, together with alkaline phosphatase, are regulated by the vitamin D metabolites 24,25(OH)2 cholecalciferol and 1,25 (OH)2 cholecalciferol. At the time of calcification, type X collagen remains associated with type II collagen fibrils. It may play a role in preventing the initial calcification of these fibrils focusing mineral formation in focal interfibrillar sites. This process of calcification is clearly very complex, and involves different interacting matrix molecules and is carefully regulated at the cellular level.

Proteoglycan synthesis in vitamin D-deficient cartilage: recovery from vitamin D deficiency

Vitamin D appears to be required for mineralization of skeletal elements. There is also evidence that cartilage proteoglycans may be involved in the regulation of mineralization. Previous studies have shown an alteration in the structure of the proteoglycans of the epiphyseal growth cartilage as a result of the decrease in serum calcium related to deficiency of dietary vitamin D. Vitamin D deficiency also induces a thickening of the epiphyseal growth plate presumably because of the inhibition of maturation of the growth plate chondrocytes. In order to compare the effect on proteoglycan structure with that on growth plate morphology, the proteoglycans of healing epiphyseal cartilage were characterized. The results indicate that, consistent with previous data, in vitamin D-deficient hatching chicks, the proteoglycans of the growth cartilage, but not of the articular cartilage, are smaller in monomer size with slightly smaller chondroitin sulfate chains whose sulfation pattern is unaltered. Sternal cartilage proteoglycans are unaffected. During recovery from vitamin D deficiency, the proteoglycans isolated from the growth cartilage are still not completely normal one day after supplementation with vitamin D, but are indistinguishable from normal by four days. In addition, the results conflict with those of a previous study in which only growth cartilage of hatchling chicks, not sternal or articular cartilage, was reported to synthesize large proteoglycans. Instead, all of these cartilages in the normal chicken have been found in this study to produce large proteoglycans of a size typical for mammalian cartilage and embryonic chick cartilage.