Increased bone volume and reduced bone turnover in vitamin D-replete rabbits by the administration of 24R,25-dihydroxyvitamin D3

Effects of 1,25 and 24,25 Vitamin D on Corneal Epithelial Proliferation, Migration and Vitamin D Metabolizing and Catabolizing Enzymes

This study investigated the effects of 1,25(OH)₂D3 and 24R,25(OH)₂D3 on corneal epithelial cell proliferation, migration, and on the vitamin D activating enzyme CYP27B1 (produces 1,25(OH)₂D3) and inactivating enzyme CYP24A1 (produces 24R,25(OH)₂D3). The role of the vitamin D receptor (VDR) was also examined. In VDR wildtype mouse corneal epithelial cells (WT), 1,25(OH)₂D3 increased CYP24A1 protein expression and decreased CYP27B1 expression. In VDR knockout mouse epithelial cells (KO), 1,25(OH)₂D3 increased CYP24A1 and CYP27B1 protein expression. 1,25(OH)₂D3 did not affect WT cell proliferation, but did stimulate VDR KO cell proliferation. In a human corneal epithelial cell line (HCEC), 1,25(OH)₂D3 increased CYP24A1 mRNA and protein expression. 1,25(OH)₂D3 increased CYP27B1 mRNA levels in HCEC, but had no effect on CYP27B1 protein levels. 1,25(OH)₂D3 inhibited HCEC proliferation and stimulated cell migration in primary human epithelial cells. 24,25(OH)₂D3, on the other hand, increased both CYP24A1 and CYP27B1 protein expression in WT and VDR KO cells, and stimulated cell proliferation in both WT and KO cells. In HCEC, 24,25(OH)₂D3 increased CYP24A1 and CYP27B1 mRNA and protein expression, and stimulated cell migration. In human primary corneal epithelial cells, 24,25(OH)₂D3 stimulated migration. We conclude that 24R,25(OH)₂D3 is likely involved in corneal epithelial cell regulation independent of 1,25(OH)₂D3 or VDR.

A comparative meta-analysis on the relationship of faecal calcium and phosphorus excretion in mammals

To investigate the relationship between faecal calcium (Ca) and phosphorus (P) excretion in different mammalian species, a meta-analysis on digestibility data derived from the literature was conducted. Seventy-three studies on carnivores, omnivores, large and small hindgut fermenters, ruminants and hippos (a total of 21 mammalian species, precondition for inclusion dietary Ca/P ratio 1.5/1 - 3.0/1) were analysed for Ca and P digestibility. Dietary Ca/P ratios were lower than faecal Ca/P ratios in carnivores, omnivores, ruminants and hippos. In hindgut fermenters, dietary Ca/P ratios were higher than faecal Ca/P ratios, indicating higher intestinal Ca absorption in these species. In all species investigated, there was a significant positive relationship between Ca intake and faecal Ca excretion and between P intake and faecal P excretion. In the biologically relevant range, these equations predicted lower faecal Ca losses in hindgut fermenters than ruminants, for faecal P vice versa. In all species, faecal Ca and P excretion correlated significantly. In carnivores, this highly linear correlation was exceptionally strong (R² = .92). Yet, the linearity of the correlation was questionable in omnivores and ruminants. Possibly, the strong linear correlation of faecal Ca and P excretion in carnivores is due to the formation of insoluble Ca/P complexes in their relatively short and simple gastrointestinal tract. Another hypothesis is that in carnivores, Ca homeostasis relies on modifying bone turnover to a higher degree than on changes in intestinal Ca absorption. For the formation of bone matrix, a constant ratio of Ca and P absorption is of advantage.

Maternal vitamin D biomarkers are associated with maternal and fetal bone turnover among pregnant women consuming controlled amounts of vitamin D, calcium, and phosphorus

Vitamin D plays a central role in calcium homeostasis; however, its relationship with bone turnover during pregnancy remains unclear due to a lack of studies that have rigorously controlled for vitamin D and other nutrients known to influence bone metabolism. Similarly, prior investigations of the effect of pregnancy on bone turnover relative to the nonpregnant state may have been confounded by varying intakes of these nutrients. Nested within a controlled intake study, the present investigation sought to quantify associations between maternal vitamin D biomarkers and biochemical markers of bone turnover among pregnant (versus nonpregnant) women and their fetuses under conditions of equivalent and adequate intakes of vitamin D and related nutrients. Changes in markers of bone turnover across the third trimester were also examined. Healthy pregnant (26-29 wk gestation; n=26) and nonpregnant (n=21) women consumed 511IU vitamin D/d, 1.6g calcium/d, and 1.9g phosphorus/d for 10weeks while participating in a controlled feeding study featuring two choline doses. Based on linear mixed models adjusted for influential covariates (e.g., BMI, ethnicity, and season), pregnant women had 50-150% higher (P<0.001) concentrations of bone resorption markers than nonpregnant women. Among pregnant women, increases in maternal 25(OH)D across the study period were associated (P<0.020) with lower osteocalcin and deoxypyridinoline at study-end, and higher fetal osteocalcin. In addition, maternal free 25(OH)D, 1,25(OH)₂D and 24,25(OH)₂D tended to be negatively associated (P≤0.063) with maternal NTx at study-end, and maternal free 25(OH)D and 24,25(OH)₂D were positively associated (P≤0.021) with fetal CTx. Similarly, maternal 3-epi-25(OH)D₃ was negatively related (P≤0.037) to maternal NTx and deoxypyridinoline at study-end. These declines in bone resorption markers resulting from higher vitamin D biomarker concentrations among pregnant women coincided with increases in their albumin-corrected serum calcium concentrations, indicating that calcium transfer to the fetus was uncompromised. Notably, none of these associations achieved statistical significance among nonpregnant women. Overall, our study findings suggest that achieving higher maternal concentrations of vitamin D biomarkers might attenuate third-trimester bone resorption while ensuring sufficient calcium delivery to the fetus.

Mass spectrometric profiling of vitamin D metabolites beyond 25-hydroxyvitamin D

BACKGROUND

The frequency of measurements of vitamin D in the human population has significantly increased over the last decade because vitamin D has now been linked to many diseases, in addition to its established role in bone health. Usually, serum 25-hydroxyvitamin D concentrations are measured to assess the vitamin D status of individuals. Unfortunately, many studies investigating links between vitamin D and disease also use only this single metabolite. Intricate correlations with other vitamin D metabolites or dynamic effects of downstream metabolites may therefore be overlooked. Fortunately, powerful LC-MS/MS approaches have recently become available that can simultaneously quantify the concentrations of multiple vitamin D metabolites. These approaches are challenging, however, because of inherent instrumental problems with detection of vitamin D compounds and the low concentrations of the metabolites in biological fluids.

CONTENT

This review summarizes recent mass spectrometry assays for the quantitative measurement of multiple vitamin D metabolites and their application in clinical research, with a particular focus on the low-abundance downstream metabolic species generated after the initial hydroxylation to 25-hydroxyvitamin D.

SUMMARY

To study the pathobiological effects and function of vitamin D metabolites in disease, in particular in low-abundance species beyond 25-hydroxyvitamin D, we need to know their concentrations. Although detection of these vitamin D species is challenging, a number of recent mass spectrometry assays have successfully demonstrated that LC-MS/MS methods can quantify multiple vitamin D compounds over a wide dynamic range individually or as part of multimetabolite assays.

Serum 24,25-dihydroxyvitamin D concentrations in osteogenesis imperfecta: relationship to bone parameters

BACKGROUND

Several studies suggest that 24,25-dihydroxyvitamin D [24,25(OH)₂D] may have an effect on bone mass and metabolism.

OBJECTIVE

We evaluated the relationship between serum 24,25(OH)₂D levels and bone density and bone metabolism in children with a primary bone disorder-osteogenesis imperfecta (OI).

MATERIALS AND METHODS

The study included 132 patients (age, 1.1 to 17.9 yr; 67 girls) with OI types I, III, or IV who had not received bisphosphonate treatment at the time of analysis.

RESULTS

Serum 24,25(OH)₂D levels were significantly higher in OI type III than in OI type I or IV. Serum 24,25(OH)₂D concentrations were positively correlated with serum 25-hydroxyvitamin D (25OHD) levels and negatively correlated with serum PTH levels, and were not correlated with serum 1α,25-dihydroxyvitamin D [1,25(OH)₂D]. The ratio between serum 24,25(OH)₂D and 25OHD was negatively correlated with age and was independent of serum 25OHD concentrations. Regression analysis revealed that OI severity (P = 0.04), serum 25OHD levels (P < 0.001), and serum PTH concentrations (P = 0.045), but not age, gender, or serum 1,25(OH)₂D, were independent predictors of serum 24,25(OH)₂D levels. No correlation was found between serum 24,25(OH)₂D levels or the ratio between serum 24,25(OH)₂D and 25OHD and lumbar spine bone mineral density z-scores or bone marker levels (serum osteocalcin and urinary collagen type I N-telopeptide) after adjusting for OI type, age, and gender.

CONCLUSION

Patients with more severe OI type had higher 24,25(OH)₂D serum levels and higher serum 24,25(OH)₂D to 25OHD ratios, suggesting an increased 25OHD-24-hydroxylase activity.

Effects of 24R,25- and 1α,25-dihydroxyvitamin D3 on mineralizing growth plate chondrocytes

Time- and dosage-dependent effects of 1,25(OH)(2)D(3) and 24,25(OH)(2)D(3) on primary cultures of pre- and post-confluent avian growth plate (GP) chondrocytes were examined. Cultures were grown in either a serum-containing culture medium designed to closely mimic normal GP extracellular fluid (DATP5) or a commercially available serum-free media (HL-1) frequently used for studying skeletal cells. Hoechst DNA, Lowry protein, proteoglycan (PG), lactate dehydrogenase (LDH), and alkaline phosphatase (ALP) activity and calcium and phosphate mineral deposition in the extracellular matrix were measured. In preconfluent cultures grown in DATP5, physiological levels of 24,25(OH)(2)D(3) (0.10-10 nM) increased DNA, protein, and LDH activity significantly more than did 1,25(OH)(2)D(3) (0.01-1.0 nM). However, in HL-1, the reverse was true. Determining ratios of LDH and PG to DNA, protein, and each other, revealed that 1,25(OH)(2)D(3) specifically increased PG, whereas 24,25(OH)(2)D(3) increased LDH. Post-confluent cells were generally less responsive, especially to 24,25(OH)(2)D(3). The positive anabolic effects of 24,25(OH)(2)D(3) required serum-containing GP-fluid-like culture medium. In contrast, effects of 1,25(OH)(2)D(3) were most apparent in serum-free medium, but were still significant in serum-containing media. Administered to preconfluent cells in DATP5, 1,25(OH)(2)D(3) caused rapid, powerful, dosage-dependent inhibition of Ca(2+) and Pi deposition. The lowest level tested (0.01 nM) caused >70% inhibition during the initial stages of mineral deposition; higher levels of 1,25(OH)(2)D(3) caused progressively more profound and persistent reductions. In contrast, 24,25(OH)(2)D(3) increased mineral deposition 20-50%; it required >1 week, but the effects were specific, persistent, and largely dosage-independent. From a physiological perspective, these effects can be explained as follows: 1,25(OH)(2)D(3) levels rise in hypocalcemia; it stimulates gut absorption and releases Ca(2+) from bone to correct this deficiency. We now show that 1,25(OH)(2)D(3) also conserves Ca(2+) by inhibiting mineralization. The slow anabolic effects of 24,25(OH)(2)D(3)are consistent with its production under eucalcemic conditions which enable bone formation. These findings, which implicate serum-binding proteins and accumulation of PG in modulating accessibility of the metabolites to GP chondrocytes, also help explain some discrepancies previously reported in the literature.

Mechanisms for the reduction of 24,25-dihydroxyvitamin D3 levels and bone mass in 24-hydroxylase transgenic rats

24-Hydroxylase (CYP24) is an enzyme distributed in the target tissues of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3]. Two functions for this enzyme have been reported: One is production of 24,25-dihydroxyvitamin D3 [24,25(OH)2D3] and the other is inactivation of 1alpha,25(OH)2D3. To elucidate other physiologic roles of CYP24 in vivo, we previously generated rats that constitutively express the CYP24 gene. These transgenic (Tg) rats developed unexpected phenotypes, such as low plasma levels of 24,25(OH)2D3, lipidemia, and albuminuria. In this study, we elucidated the mechanisms for inducing low plasma 24,25(OH)2D3 levels and bone loss. Tg rats excreted massive amounts of vitamin D binding protein (DBP), which coincided with the loss of albumin. In Tg rats, the renal expression pattern of megalin, which serves as an endocytotic receptor responsible for the reuptake of urinary proteins such as DBP and albumin, was identical to that of the wild-type rats. Excreted albumin appeared to compete for the binding and reabsorption of the DBP-25-hydroxyvitamin D3 [25(OH)D3] complex with megalin, resulting in a loss of 25(OH)D3 into the urine and subsequent reduction of plasma 24,25(OH)2D3. In this prominent rat model of nephritis, supplementation of 25(OH)D3 was effective in preventing bone loss in an early stage of renal insufficiency.

Putative basal lateral membrane receptors for 24,25-dihydroxyvitamin D(3) in carp and Atlantic cod enterocytes: characterization of binding and effects on intracellular calcium regulation

The vitamin D metabolite, 24R,25-dihydroxyvitamin D(3) (24R,25(OH)(2)D(3)), was tested for its ability to specifically bind to basal lateral membranes isolated from intestinal epithelium of Atlantic cod (a seawater fish), carp (a freshwater fish), and chicken. Specific saturable binding was demonstrated in membranes from all three species. Membranes from Atlantic cod, carp, and chicken revealed K(d)'s of 7.3 +/- 0.9, 12.5 +/- 0.9 and 7.8 +/- 0.1 nM, and a B(max) for each species estimated to 57.9 +/- 2.9, 195.1 +/- 8.4 and 175 +/- 0.8 fmol/mg protein, respectively. Scatchard analyses indicated a convex curvature and Hill analyses revealed apparent Hill coefficients of 1.84 +/- 0.28, 1.80 +/- 0.29, and 1.78 +/- 0.27 for Atlantic cod, carp and chicken, suggesting a positive cooperative binding in all three species. Basal lateral membranes from Atlantic cod and carp were used to further characterize the binding moiety. In competition studies, basal lateral membranes from Atlantic cod or carp did not discriminate between 24R,25(OH)(2)D(3) and the 24S,25(OH)(2)D(3) isomer, whereas, 1,25(OH)(2)D(3) and 25(OH)D(3), were less effective in competing with [(3)H]24R,25(OH)(2)D(3) for binding to basal lateral membranes in Atlantic cod and carp. In both the Atlantic cod and carp enterocyte basal lateral membranes, the binding activity could be extracted equally well with high salt as with detergent, indicating a peripheral membrane protein rather than an integral membrane binding protein. Finally, isolated Atlantic cod and carp enterocytes were chosen for analyses of signal transduction events mediated by the putative receptor. In both species, 24R,25(OH)(2)D(3) but not 24S,25(OH)(2)D(3), suppressed Ca(2+)-uptake by enterocytes in a dose-dependent manner. Enterocytes from Atlantic cod and carp, acclimated to Ca(2+)-free media, responded by an intracellular Ca(2+)-release within seconds after addition of 24R,25(OH)(2)D(3) or 24S,25(OH)(2)D(3). The effects on intracellular Ca(2+)-release were dose-dependent for both metabolites. 24S,25(OH)(2)D(3) was effective at lower concentrations and triggered a higher response compared to 24R,25(OH)(2)D(3). These results suggest that the binding molecule(s) for 24R,25(OH)(2)D(3) and 24S,25(OH)(2)D(3) is/are capable of acting as a receptor, mediating rapid, non-genomic responses in intestinal cells.

Effects of surface roughness of titanium implants on bone remodeling activity of femur in rabbits

We investigated the bone remodeling activity on titanium implants with different surface roughnesses using a confocal laser scanning microscope (CLSM). Two kinds of implants were used, the machined smooth-surfaced titanium and the plasma-sprayed rough-surfaced titanium. These implants were randomly inserted in a rabbit's femur from the lateral aspect of the diaphysis bicortically. Rabbits were killed at 6, 16, and 42 weeks after surgery. The implant-bone blocks were embedded in polyester resin, and were prepared to make undecalcified ground sections. Histomorphometric analyses were performed at the cortical bone-implant interface using the image obtained by CLSM. Percentages of direct bone-implant contact and bone volume (BV/TV) around the implant was greater in rough-surfaced titanium compared with the smooth-surfaced titanium at 42 weeks after implantation. On the contrary, the eroded surface (ES/BS) appeared to be less in the rough-surfaced titanium than in the smooth-surfaced titanium at 6 weeks after implantation, but thereafter, no difference was found between the two kinds of implants. Mineralizing surface (MS/BS) and mineral apposition rate (MAR) showed no significant differences throughout the experimental period. These results indicate that increased bone volume in the rabbits of rough-surfaced titanium implants is due to less remodeling activity during the early stage after implantation compared with the smooth-surfaced implants. The surface roughness of titanium is one factor which helps in determining the balance between bone formation and resorption of remodeling at the interface of the bone implants.

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.

Measurement of bone resorption by strontium excretion in prelabelled rats

The body handles strontium (Sr) in a similar way to calcium (Ca) in that Sr is absorbed by the gut, concentrated in bone and excreted in urine and feces. In this study, rats were labelled with Sr during growth and later subjected to various treatments affecting bone resorption and Sr excretion was measured during and after treatment. Six weeks old Wistar rats were repeatedly s.c. injected with SrCl2. After a period of 2 weeks after the last Sr injection the rats were subjected to various treatments. Sr clearance was then measured weekly for 2 weeks. In the first experiment, the Sr labelled rats were sham-operated (sham) or ovariectomized (ovx) and urine collected afterwards. Sham rats were either treated with 4 daily s.c. clodronat injections at the beginning of the urine sampling, fed a low Ca diet (0.08% Ca) during the second sampling week or injected with saline. Urinary Sr excretion was decreased in the clodronate group during the first sampling week and increased in the Ca depleted group during feeding the low Ca diet. Sr excretion by ovx rats was similar to the sham control. In the second experiment, the effect of high-dose treatment with 24,25-dihydroxyvitamin D3 (24,25(OH)2D3) or clodronate on bone resorption induced by Ca depletion was assessed by Sr output in urine and feces. Sr labelled rats were fed a low Ca diet and daily injected with 24,25(OH)2D3 or clodronate for 14 consecutive days. Clodronate significantly decreased Sr output during both sampling weeks. Treatment with 24,25(OH)2D3 resulted in an increased Sr output indicating an increase in bone resorption.

Effects of 24R,25-dihydroxyvitamin D3 in combination with 1 alpha-hydroxyvitamin D3 in predialysis renal insufficiency: biochemistry and histomorphometry of cancellous bone

The effect of combined administration of 24R,25-dihydroxyvitamin D3 (24,25-(OH)2D3) and 1 alpha-hydroxyvitamin D3 (1 alpha-(OH)D3) was studied in 24 non-dialyzed patients with chronic renal insufficiency (CRI), matched pairwise as to age, sex, and creatinine clearance (Cr.cl). Low Ca intake had been supplemented beforehand. Then, 1 alpha-(OH)D3 (mean dose 0.55 micrograms daily) was given orally to all patients for 3 months (T0 to T3). Subsequently, patients were assigned randomly to 6 months further treatment either with 1 alpha-(OH)D3 alone (Group A) or with 1 alpha-(OH)D3 plus a high dosage of 24,25-(OH)2D3 (50 micrograms orally, twice weekly) (Group B). Histomorphometry was performed at T0, T3, and T9. In both groups iPTH was equally suppressed, into the lower normal range. Whereas in Group A, serum Ca rose steadily and Cr.cl declined, in Group B both parameters levelled off between T6 and T9. At T9, in Group A the elevated resorption and osteoid indices had normalized markedly, but osteoblasts (Ob.Pm) and mineralizing boundaries (M.Bd) were depressed considerably between T3 and T9. In contrast, in Group B, preservation of Ob.Pm and improved mineralizing activity were observed (M.Bd at T9 > T3 > T0). Resorption indices hardly changed. In the patients with high Ob.Pm at T0, cancellous bone area increased significantly. This was not observed in Group A. Thus, in Group B, osteoblast recruitment appeared maintained and M.Bd appeared normalized. Decline of remodeling toward an adynamic state with an increased risk of hypercalcemia appeared prevented.

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.

Gas chromatography/mass spectrometric analysis of 24R,25-dihydroxyvitamin D3 using 24R,25-dihydroxy[6,19,19-2H]vitamin D3 as internal standard

Gas chromatography/mass spectrometric (GC/MS) analysis of 24R,25-dihydroxyvitamin D3 (24,25-(OH)2D3, 1a) in rat serum is examined using 6,19,19-trideuterated derivative (24,25-(OH)2[6,19,19-2H]D3, 1c) as an internal standard. Pyro- and isopyro-24,25-(OH)2D3 (2a and 3a) are synthesized and the structures are determined unambiguously by their spectral data including nuclear overhauser enhancement study of the NMR spectra. The two peaks which appear on the total ion chromatogram of 24,25-cyclic n-butyl boronate-3-trimethylsilyl derivative of 24,25-(OH)2D3 are identified to be those of the pyro and isopyro isomers (2a and 3a) by direct comparison with the synthetic standards. Analysis of the GC/MS spectrum of 24,25-(OH)2[6,19,19-2H]D3 (1c) indicated that the CH3(19) is preferentially eliminated in the fragmentation giving rise to a base peak (M-TMSOH-Me)+. Thus the molecular ion peak, which is about 20% of the base peak, rather than the base peak is used for GC/MS assay of 24,25-(OH)2D3 in serum.

Calcium regulating activity of 26,27-dimethyl analog of 24R,25-dihydroxyvitamin D3

To determine the possibility that methyl substitution in 26- and 27-positions of 24R,25-dihydroxyvitamin D3 [24,25(OH)2D3] alters activities of the original compound, the effects of 24,25(OH)2D3 on calcium (Ca) regulating activity were compared with those of its methyl analog [24,25(OH)2(CH3)2D3] in addition to 1 alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. 24,25(OH)2D3 at 10(-6) M and 24,25(OH)2(CH3)2D3 at 10(-7) M and above significantly stimulated both bone resorption in neonatal mouse calvaria cultures and formation of osteoclast-like multinucleated cells (MNC) in mouse bone marrow cultures. A stimulative effect of 1,25(OH)2D3 on bone resorption and MNC formation was recognized in very low concentrations (10(-11) M and above). Although a potency of 24,25(OH)2(CH3)2D3 in stimulating bone calcium (Ca) mobilization and intestinal Ca transport was higher than that of 24,25(OH)2D3, the potencies of both compounds were similar to that of 1,25(OH)2D3 unlike in vitro experiments. As 1,24R,25-trihydroxy-26,27-dimethylvitamin D3 showed almost the same effect as 24,25(OH)2(CH3)2D3, the dihydroxy form is suggested to be hydroxylated at 1 alpha position and converted to trihydroxy form in vitamin D-deficient rats. From these results, methyl substitution in 26- and 27-position of 24,25(OH)2D3 was found to elevate Ca regulating activity of the original compound. In addition, it is suggested that the basis for a similarity in potency between 1,25(OH)2D3 and 24,25(OH)2D3 or its dimethyl analog in vitamin D-deficient rats is likely the result of 1 alpha-hydroxylation.

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)

A novel synthetic vitamin D analogue, 2 beta-(3-hydroxypropoxy)1 alpha, 25-dihydroxyvitamin D3 (ED-71), increases bone mass by stimulating the bone formation in normal and ovariectomized rats

We performed dosing experiments to evaluate the bone mass increasing action of a novel, synthetic vitamin D derivative, 2 beta-(3-hydroxypropoxy)-1 alpha,25(OH)2D3 (ED-71), in normal and estrogen-deficient rats. The first experiment consisted of 31 Sprague-Dawley rats, 28 weeks of age. The second experiment consisted of 44 animals who were ovariectomized (OVX) or sham operated at the age of 12 weeks. ED-71 was given twice a week for the duration of 12 weeks. At the end of the experiments, serum chemistries were examined and lumbar vertebrae were assessed histomorphometrically. Serum alkaline-phosphatase levels tended to decrease by ED-71 administration in the first experiment and their elevated values after ovariectomy were also depressed by ED-71 in the second experiment. Serum osteocalcin levels, however, increased by the agent. In the first experiment, cancellous bone volume (BV/TV) increased dose dependently. Bone formation rates (BFR/BS) also increased. In the second experiment, BV/TV significantly decreased by ovariectomy and it increased in ED-71-treated groups, but not in 1 alpha-(OH)D3-treated group. BFR/BS increased by ED-71. Activation frequency did not decrease by ED-71 in either experiment. These data clearly demonstrated that ED-71 administration was capable of increasing the bone mass by stimulating bone formation in normal and estrogen-deficient rats.

Thyroid hormone action on rat calvarial sutures

To clarify the effect of thyroid hormone on the calvarial sutures, the morphology, histology, and bone mineral density were analyzed in thyroid hormone-injected rats. A total of 80 female Wistar rats at the age of 10 days were divided into 2 groups; the triiodothyronine (T3)-treated rats (n = 40, T3 100 micrograms/kg body weight/day) were maintained under the same conditions as controls (n = 40, saline-vehicle only), and both were sacrificed at 50, 80, and 200 experimental days, respectively. T3-treated animals showed smaller values of lambda-asterion and pterion-bregma distances at the various periods of examination, resulting in smaller sizes in right-left direction of the skull. The sagittal suture distance in the skull specimens observed by Goldner's staining (fibrous tissue content) appeared smaller and the osseous margin widths greatly increased in the T3-treated rats. Fluorescent microscopy revealed that the tetracycline-calcein double labelings were not interrupted from the periosteal to the endosteal areas in any specimens indicating the scantiness of osteoclastic activity in the suture areas. Mineral apposition rates at the osseous edges of the suture were significantly increased in the T3-treated rats (p < 0.01 at the age of 90 days). The distance between the second labels and the distance between the first labels were significantly larger in the T3-treated rats. Bone mineral density (BMD) measurement showed that the T3 administration decreased only the calvarial BMD at the age of 210 days (p < 0.01), but did not decrease lumbar or femur BMD. This experiment demonstrated clear evidence of excess thyroid hormone action on the early narrowing of the sagittal suture in infantile hyperthyroid rats, probably due to the enhanced osteogenic activity caused by the hormone.

Effect of 24R,25-dihydroxyvitamin D3 on the formation and function of osteoclastic cells

Previous reports demonstrated that the administration of large doses of 24R,25-dihydroxyvitamin D3 [24R,25(OH)2D3] to animals with normal vitamin D supply causes an increase in bone volume with reduced bone resorption and decreased osteoclast number. The present study was undertaken to clarify if 24R,25(OH)2D3 has any inhibitor effect on the formation and function of osteoclasts. The effect of 24R,25(OH)2D3 on the formation of osteoclastic cells was examined by measuring the number of tartrate-resistant acid phosphatase-positive multinucleated cells (MNCs) formed from hemopoietic progenitor cells obtained from spleens of 5-fluorouracil-treated mice. Treatment with 1,25(OH)2D3 or parathyroid hormone fragment 1-34 [PTH(1-34)] stimulated osteoclast-like MNC formation in a dose-dependent manner. Addition of 24R,25(OH)2D3 alone showed a weak stimulatory effect on MNC formation at 10(-6) M, which appeared to be due to its binding to 1,25(OH)2D3 receptors. In contrast, when 24R,25(OH)2D3 was added together with 1,25(OH)2D3 or PTH(1-34), it inhibited osteoclast-like MNC formation stimulated by these hormones. A significant inhibition of MNC formation was observed with 10(-7) M 24R,25(OH)2D3, and the stimulatory effect of 1,25(OH)2D3 or PTH(1-34) was almost completely eliminated with 10(-6) M 24R,25(OH)2D3. Neither 24S,25(OH)2D3 nor 25(OH)D3 exhibited a similar inhibitory effect. The effect of 24R,25(OH)2D3 on the resorptive function of osteoclasts was examined by measuring the formation of resorption pits by mouse bone cells on dentine slices. Treatment with 24R,25(OH)2D3 also inhibited the resorption pit formation stimulated by 1,25(OH)2D3 or PTH(1-34) with similar dose response.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of bone turnover and prevention of bone atrophy in ovariectomized beagle dogs by the administration of 24R,25(OH)2D3

In order to determine whether the administration of 24R,25(OH)2D3 had any beneficial effect on the regulation of bone turnover and the prevention of bone atrophy, we examined beagles for 31 months after ovariectomy (OVX). Fourteen beagle dogs (8.54 +/- 1.22 kg body wt-b.w.) were divided into four groups. Group 1 (n = 3) was the sham, and Group 2 (n = 3) served as the OVX control. In Group 3 (n = 4) and Group 4 (n = 4), 24,25-dihydroxyvitamin D3(24R,25(OH)2D3) was given daily at dose levels of 2 and 10 mcg/kg B.W., respectively. In Group 4, the dose level was increased to 100 mcg/kg by 17 months. During the experiments, urinary hydroxyproline (U-HPr), serum chemistry, serum bone gla-protein (BGP), and vitamin D metabolite levels were monitored. At the end of the experiment, bone mineral content (BMC) in the 6th and 7th lumbar vertebrae and right femur was determined by single photon absorptiometry. The left iliac bone sample was obtained after tetracycline labeling, and undecalcified sections were observed. In Group 2, excretion of U-HPr increased after OVX and had reached a level of approximately twice the baseline values by 10 months; then it gradually came down to the original level. In Group 3, however, U-HPr excretion remained at the same level as the baseline value, as it did in Group 1. In Group 4, it was remarkably reduced down to 50-60% of the baseline values. Serum BGP level was markedly reduced in Group 4. Serum 24,25(OH)2D levels were markedly increased in Groups 3 and 4.(ABSTRACT TRUNCATED AT 250 WORDS)