Vitamin D metabolites prevent vertebral osteopenia in ovariectomized rats

Vitamin D and Bone: A Story of Endocrine and Auto/Paracrine Action in Osteoblasts

Despite its rigid structure, the bone is a dynamic organ, and is highly regulated by endocrine factors. One of the major bone regulatory hormones is vitamin D. Its renal metabolite 1α,25-OH2D3 has both direct and indirect effects on the maintenance of bone structure in health and disease. In this review, we describe the underlying processes that are directed by bone-forming cells, the osteoblasts. During the bone formation process, osteoblasts undergo different stages which play a central role in the signaling pathways that are activated via the vitamin D receptor. Vitamin D is involved in directing the osteoblasts towards proliferation or apoptosis, regulates their differentiation to bone matrix producing cells, and controls the subsequent mineralization of the bone matrix. The stage of differentiation/mineralization in osteoblasts is important for the vitamin D effect on gene transcription and the cellular response, and many genes are uniquely regulated either before or during mineralization. Moreover, osteoblasts contain the complete machinery to metabolize active 1α,25-OH2D3 to ensure a direct local effect. The enzyme 1α-hydroxylase (CYP27B1) that synthesizes the active 1α,25-OH2D3 metabolite is functional in osteoblasts, as well as the enzyme 24-hydroxylase (CYP24A1) that degrades 1α,25-OH2D3. This shows that in the past 100 years of vitamin D research, 1α,25-OH2D3 has evolved from an endocrine regulator into an autocrine/paracrine regulator of osteoblasts and bone formation.

WY 1048, a 17-methyl 19-nor D-ring analog of vitamin D3, in combination with risedronate restores bone mass in a mouse model of postmenopausal osteoporosis

Bisphosphonates like risedronate inhibit osteoclast-mediated bone resorption and are therefore used in the prevention and treatment of osteoporosis. Also vitamin D₃ and calcium supplementation is commonly used in the prevention or treatment of osteoporosis. Combined therapy of risedronate with 1,25(OH)₂D₃, the active metabolite of vitamin D₃, may be advantageous over the use of either monotherapy, but bears a risk of causing hypercalcemia thereby decreasing the therapeutic window for osteoporosis treatment. In this study, we evaluated the effect on bone mass of the combination of risedronate with the 17-methyl 19-nor five-membered D-ring vitamin D₃ analog WY 1048 in a mouse ovariectomy model for postmenopausal osteoporosis. Ovariectomy-induced bone loss was restored by administration of risedronate or a combination of risedronate with 1,25(OH)₂D_3. However, the combination of WY 1048 with risedronate induced an even higher increase on total body and spine bone mineral density and on trabecular and cortical bone mass. Our data indicate that combination therapy of risedronate with WY 1048 was superior in restoring and improving bone mass over a combination of risedronate with 1,25(OH)₂D₃ with minimal calcemic side effects.

Vitamin D and gene networks in human osteoblasts

Bone formation is indirectly influenced by 1,25-dihydroxyvitamin D3 (1,25D3) through the stimulation of calcium uptake in the intestine and re-absorption in the kidneys. Direct effects on osteoblasts and bone formation have also been established. The vitamin D receptor (VDR) is expressed in osteoblasts and 1,25D3 modifies gene expression of various osteoblast differentiation and mineralization-related genes, such as alkaline phosphatase (ALPL), osteocalcin (BGLAP), and osteopontin (SPP1). 1,25D3 is known to stimulate mineralization of human osteoblasts in vitro, and recently it was shown that 1,25D3 induces mineralization via effects in the period preceding mineralization during the pre-mineralization period. For a full understanding of the action of 1,25D3 in osteoblasts it is important to get an integrated network view of the 1,25D3-regulated genes during osteoblast differentiation and mineralization. The current data will be presented and discussed alluding to future studies to fully delineate the 1,25D3 action in osteoblast. Describing and understanding the vitamin D regulatory networks and identifying the dominant players in these networks may help develop novel (personalized) vitamin D-based treatments. The following topics will be discussed in this overview: (1) Bone metabolism and osteoblasts, (2) Vitamin D, bone metabolism and osteoblast function, (3) Vitamin D induced transcriptional networks in the context of osteoblast differentiation and bone formation.

Vitamin D endocrine system and osteoblasts

The interaction between vitamin D and osteoblasts is complex. In the current review we will give an overview of the current knowledge of the vitamin D endocrine system in osteoblasts. The presence of the vitamin D receptor in osteoblasts enables direct effects of 1α,25dihydroxyvitamin D3 (1α,25D3) on osteoblasts, but the magnitude of the effects is subject to the presence of many other factors. Vitamin D affects osteoblast proliferation, as well as differentiation and mineralization, but these effects vary with the timing of treatment, dosage and origin of the osteoblasts. Vitamin D effects on differentiation and mineralization are mostly stimulatory in human and rat osteoblasts, and inhibitory in murine osteoblasts. Several genes and mechanisms are studied to explain the effects of 1α,25D3 on osteoblast differentiation and bone formation. Besides the classical VDR, osteoblasts also express a membrane-localized receptor, and in vitro studies have shown that osteoblasts are capable of the synthesis of 1α,25D3.

Effect of intermittent PTH (1-34) on posterolateral spinal fusion with iliac crest bone graft in an ovariectomized rat model

SUMMARY

Intermittent treatment with high-dose parathyroid hormone (PTH) enhances the quantity and quality of the fusion callus and reduces healing time of posterolateral spinal fusion with autologous iliac bone grafts in ovariectomized osteoporotic female Sprague-Dawley rats. Intermittent PTH (1-34) could be an appropriate adjunctive therapy for osteoporotic patients undergoing posterolateral intertransverse process fusion.

INTRODUCTION

The study was designed to test the hypothesis that intermittent administration of PTH improves spinal fusion rates in a randomized controlled, ovariectomized osteoporotic rat spinal fusion model.

METHODS

Thirty-six 10-week-old Sprague-Dawley rats were ovariectomized and underwent bilateral posterolateral L4-L5 spinal fusion with autologous iliac bone graft 6 weeks later. The experimental (PTH) group (18 rats) received daily subcutaneously administered injections of PTH (1-34) at 30 μg/kg/day starting on the day of operation. The control group (18 rats) received a subcutaneously administered injection of normal saline of the same volume. Nine rats from each group were sacrificed at 4 and 6 weeks. After sacrifice, the L4-L5 vertebral segments were removed and analyzed by plain radiographs, μ-CT, histomorphometry, and serum bone metabolism marker.

RESULTS

The PTH group had a significantly higher fusion rate and X-ray fusion score than the control group at 4 and 6 weeks (p < 0.05). μ-CT and histological analysis showed that the fusion bone volume and cortical thickness for the PTH group were significantly higher than those for the control group at 4 and 6 weeks (p < 0.05). Metabolic marker analysis also showed significant difference between the two groups. The serum osteocalcin was significantly higher in the PTH group at 4 and 6 weeks, and levels of N-terminal peptide of type I collagen were significantly higher at 4 weeks (p < 0.05).

CONCLUSION

Intermittent treatment with high-dose PTH enhances the quantity of the fusion callus and reduces the healing time of posterolateral spinal fusion with autologous iliac bone grafts in ovariectomized osteoporotic female Sprague-Dawley rats.

Effects of combined treatment with eldecalcitol and alendronate on bone mass, mechanical properties, and bone histomorphometry in ovariectomized rats: a comparison with alfacalcidol and alendronate

Eldecalcitol (ELD), a 2β-hydroxypropyloxy derivative of 1α,25 (OH) 2D3, inhibits bone resorption more potently than alfacalcidol (ALF) while maintaining osteoblastic function in an ovariectomized (OVX) osteoporosis rat model. Alendronate (ALN), which is the most common bisphosphonate used for the treatment of osteoporosis, increases the bone mineral density (BMD) by suppressing bone resorption. In this study, we investigated the effects of combination treatments with ELD and ALN or with ALF and ALN on bone mass and strength in OVX rats. Seventy female rats, 32 weeks old, were assigned to seven groups: (1) a sham-operated control group; (2) an OVX-control group; (3) an ELD group; (4) an ALF group; (5) an ALN group; (6) an ELD+ALN group; and (7) an ALF+ALN group. OVX rats were orally treated with ELD (0.015 μg/kg), ALF (0.0375 μg/kg), or ALN (0.2mg/kg) daily for 12 weeks. In both the lumbar spine and the femur, ELD and ALF monotherapy significantly increased the BMD, and ELD+ALN and ALF+ALN significantly increased the BMD, compared with ALN monotherapy, as an additive effect. In particular, ELD+ALN resulted in a significantly higher BMD than ALF+ALN in the femur. On mechanical testing of the lumbar spine, ELD and ALF monotherapy significantly increased the ultimate load, and ELD+ALN and ALF+ALN significantly increased the ultimate load compared with ALN monotherapy. In the femur, ELD, ELD+ALN, and ALF+ALN treatment significantly increased the ultimate load, compared with the OVX-control group, and ELD+ALN resulted in a significantly higher ultimate load than ALN monotherapy. A histomorphometric analysis showed that ELD monotherapy and ELD+ALN combination therapy had a potent inhibitory effect on bone resorption parameters (osteoclast surface and eroded surface), while maintaining bone formation parameters (osteoblast surface and osteoid surface). By contrast, ALF and ALF+ALN significantly lowered the histological parameters of both bone resorption and formation. These results suggested that ELD or ALF used in combination with ALN has therapeutic advantages over ALN monotherapy, with ELD+ALN combination treatment producing an especially beneficial anti-osteoporotic effect by inhibiting osteoclastic bone resorption and maintaining osteoblastic function, compared with ALF+ALN combination treatment.

Combination therapy with eldecalcitol and alendronate has therapeutic advantages over monotherapy by improving bone strength

Eldecalcitol (ED-71), a 2β-hydroxypropyloxy derivative of 1α,25(OH)(2)D(3), inhibits bone resorption more potently than does alfacalcidol while maintaining osteoblastic function in an estrogen-deficient, high-turnover osteoporosis rat model. Alendronate (ALN) has been reported to increase bone mass by suppressing bone resorption mainly by inducing apoptosis of osteoclasts. The aim of this study was to clarify the combination effect of ED-71 and ALN on bone loss in ovariectomized rats. Wistar-Imamichi rats (32weeks old) were ovariectomized and randomly assigned to 10 groups (n=9-11); 11 rats were sham-operated. Rats were orally administered either vehicle alone, ALN (0.05, 0.2mg/kg), ED-71 (0.015, 0.03μg/kg), or a combination of ALN and ED-71. The treatment started 2weeks after surgery and continued for 12weeks. ED-71 significantly increased calcium and phosphorus in serum and urine; however, the mean values were within the normal range. Bone mineral density (BMD) and maximum load in both the lumbar spine and femur significantly increased with ED-71 monotherapy, and showed a tendency to increase with ALN monotherapy. Compared with ALN monotherapy, the combination of ALN and ED-71 significantly increased BMD and maximum load in both the lumbar spine and femur, suggesting that the combination therapy is more beneficial than ALN monotherapy in this protocol. The combination treatment had an additive suppressive effect on eroded surface and osteoclast number, with the suppressive effect more potent than either ALN or ED-71 monotherapy. Moreover, the combination therapy partially counteracted the suppressive effects of ALN on bone formation and on the histomorphometric indices of osteoblast number and activity. Interestingly, ALN had no effect on the anabolic action of ED-71. In conclusion, the combination therapy of ALN and ED-71 has therapeutic advantages over ALN monotherapy in terms of improving bone mechanical strength without excessive suppression of bone turnover.

Differential effects of growth hormone and alpha calcidol on trabecular and cortical bones in hypophysectomized rats

Growth hormone (GH) deficiency in children causes severe growth retardation, vitamin D deficiency, and osteopenia. We investigated whether alfacalcidol (1OHD) alone or in combination with GH can improve bone formation. Forty hypophysectomized female rats (HX) at the age of 8 wk were divided into HX, HX + 1OHD (oral 0.25 microg/kg daily), HX+GH (0.666 mg/0.2 mL SC daily) and HX+GH + 1OHD groups for a 4-wk study. Results showed that GH increased body weight, bone area, bone mineral content (BMC), and bone mineral density (BMD), whereas 1OHD only increased BMC and BMD. In cortical bone, GH increased both periosteal and endocortical bone formation resulting in a significant increase in cortical size and area in percentage, whereas 1OHD suppressed endocortical erosion surface per bone surface (ES/BS) without a significant effect on bone formation rate per bone surface (BFR/BS). In trabecular bone, GH mitigated the bone loss by increasing BFR/BS, whereas the 1OHD effect was by suppression of trabecular bone turnover in the HX rats. The combination of GH and 1OHD had no additive effect on increasing trabecular bone mass. In conclusion, GH activates new bone formation and increases bone turnover whereas 1OHD suppresses bone turnover. The combination intervention does not seem to provide any additive benefit.

Effect of combined treatment with alendronate and calcitriol on femoral neck strength in osteopenic rats

Background

Hip fracture is associated with pronounced morbidity and excess mortality in elderly women with postmenopausal osteoporosis. Many drugs have been developed to treat osteoporosis and to reduce the risk of osteoporotic fractures. We investigated the effects of combined alendronate and vitamin D₃ treatment on bone mass and fracture load at the femoral neck in ovariectomized (OVX) rats, and evaluated the relationship between bone mass parameters and femoral neck strength.

Methods

Thirty 12-week-old female rats underwent either a sham-operation (n = 6) or OVX (n = 24). Twenty weeks later, OVX rats were further divided into four groups and received daily doses of either saline alone, 0.1 mg/kg alendronate, 0.1 μg/kg calcitriol, or a combination of both two drugs by continuous infusion via Alzet mini-osmotic pumps. The sham-control group received saline alone. After 12 weeks of treatment, femoral necks were examined using peripheral quantitative computed tomography (pQCT) densitometry and mechanical testing.

Results

Saline-treated OVX rats showed significant decreases in total bone mineral content (BMC) (by 28.1%), total bone mineral density (BMD) (by 9.5%), cortical BMC (by 26.3%), cancellous BMC (by 66.3%), cancellous BMD (by 29.0%) and total cross-sectional bone area (by 30.4%) compared with the sham-control group. The combined alendronate and calcitriol treatments improved bone loss owing to estrogen deficiency. On mechanical testing, although OVX significantly reduced bone strength of the femoral neck (by 29.3%) compared with the sham-control group, only the combined treatment significantly improved the fracture load at the femoral neck in OVX rats to the level of the sham-controls. The correlation of total BMC to fracture load was significant, but that of total BMD was not.

Conclusion

Our results showed that the combined treatment with alendronate and calcitriol significantly improved bone fragility of the femoral neck in OVX osteopenic rats.

Alfacalcidol-stimulated focal bone formation on the cancellous surface and increased bone formation on the periosteal surface of the lumbar vertebrae of adult female rats

PURPOSE

To investigate the skeletal effects of alfacalcidol alone or in combination with exercise in intact adult female rats.

METHODS

Seventy-four 8.5-month-old rats were orally administered 0, 0.005, 0.025, 0.05 or 0.1 microg/kg of alfacalcidol for 12 weeks, alone or in combination with exercise. Cancellous bone histomorphometric measurements were performed on the second lumbar vertebra.

RESULTS

At 0.05 and 0.1 microg/kg, alfacalcidol caused a significant increase in cancellous bone volume, accompanied by an increase in trabecular architecture. Percent eroded surface, bone resorption and formation were suppressed by alfacalcidol treatment. However, mineral apposition rate was significantly increased, indicating osteoblast activity was increased. A positive balance between bone formation and resorption was observed in the rats treated with the highest dose of alfacalcidol. Alfacalcidol induced a unique bone formation site ("bouton") on the cancellous surface. These boutons connected adjacent trabeculae and increased trabecular thickness. They exhibited both smooth and scalloped cement lines, suggesting that they were formed by minimodeling- and remodeling-based bone formation. Furthermore, alfacalcidol at 0.1 microg/kg increased periosteal bone formation of the lumbar transverse processes. Bipedal stance exercise alone did not have an effect on bone balance and bone turnover. There were no interactions between alfacalcidol and bipedal stance exercise except for a decrease in bone resorption.

CONCLUSION

Alfacalcidol exhibited both anti-catabolic and anabolic effects on bone in intact female rats. The effect of combined treatment with alfacalcidol and bipedal stance exercise was no better than that of alfacalcidol alone.

Vitamin D and ibandronate prevent cancellous bone loss associated with binge alcohol treatment in male rats

Decreased bone mass and bone strength can result from excess alcohol consumption in humans and alcohol treatment in the rat. Although the specific mechanism is unknown, the damaging effects of alcohol abuse modulate the bone remodeling cycle and increase bone turnover. Chronic alcohol consumption models have shown an inhibition of bone formation. We previously reported that binge alcohol treatment increases bone resorption and that alcohol-induced damage can be prevented by treatments with intermittent parathyroid hormone and bisphosphonates. In this study, we hypothesized that an effective dose of vitamin D (cholecalciferol) or a single dose of ibandronate would prevent bone loss caused by binge alcohol treatment in male rats. Forty-eight adult (450 gram) male Sprague-Dawley rats were randomly assigned to 6 treatment groups (n=8): (a) saline i.p., 3 days/week (C); (b) binge alcohol, 3 g/kg i.p., 3 days/week (A); (c) vitamin D, 5,000 IU/kg daily s.c. (D); (d) binge alcohol and vitamin D (AD); (e) ibandronate (120 microg, given as a single i.p. injection (I)); and (f) alcohol and ibandronate (AI) . After 4 weeks of treatment, proximal tibia and L3 and L4 vertebrae were analyzed for bone mineral density (BMD) by quantitative computerized tomography and compressive strength-to-failure using an Instron materials testing machine. Type I collagen cross-linked c-telopeptide, calcium, and 25-OH vitamin D levels were measured in serum collected at the time of sacrifice. Binge alcohol significantly decreased cancellous BMD by 58% in tibia and 23% in lumbar spine (p<0.05). Binge alcohol treatment decreased L3 and L4 compressive strength-to-failure by 21% (p<.05). Treatment with vitamin D at 5,000 IU/kg/day prevented alcohol-induced bone loss, significantly increasing both tibial and vertebral cancellous BMD values (161% increase in tibia and 40% increase in vertebra, respectively, p<0.05) compared to alcohol alone groups. Pre-treatment with the single dose of 120 microg ibandronate prevented alcohol-induced bone loss, increasing cancellous BMD by 186% in tibiae and by 46% in vertebrae compared to the alcohol alone group (p<0.05). In summary, binge alcohol-induced tibial and vertebral bone loss can be prevented using an effective dose of vitamin D or a single dose of ibandronate even during high blood alcohol concentrations that have been shown to impair osteoblast functions and increase bone resorption.

1 Alpha-hydroxyvitamin D2 and 1 alpha-hydroxyvitamin D3 have anabolic effects on cortical bone, but induce intracortical remodeling at toxic doses in ovariectomized rats

It is well established that vitamin D metabolites have anabolic properties on cancellous bone in rats. However, few data are available on cortical bone effects of vitamin D metabolites. In this study, we examined the effects of the synthetic vitamin D analogs 1alpha-hydroxyvitamin D2 (1alpha(OH)D2) and 1alpha-hydroxyvitamin D3 (1alpha(OH)D3) on cortical bone of the tibial shaft in ovariectomized (OVX) rats using bone histomorphometry. Six-month-old Fischer 344 rats were either OVX or sham-operated (SHAM). OVX rats received vehicle, 1alpha(OH)D2 or 1alpha(OH)D3 orally via the diet in a dose range from 0.025 to 0.2 microg/kg/day. All animals were killed 3 months postsurgery after in vivo fluorochrome labeling. Relative to SHAM rats, vehicle-treated OVX rats showed a reduction in cortical bone area (%) due to expansion of the marrow cavity. Treatment of OVX rats with either 1alpha(OH)D2 or 1alpha(OH)D3 dose-dependently decreased marrow area, and increased cortical area, periosteal perimeter, and periosteal and endocortical bone formation rate compared with OVX vehicle controls. Interestingly, OVX animals receiving the highest doses showed intracortical resorption cavities, a phenomenon only exceptionally observed in rats. The intracortical hole area was significantly lower in 1alpha(OH)D2-treated compared with 1alpha(OH)D3-treated rats. We conclude that 1alpha(OH)D2 and 1alpha(OH)D3 prevent cortical bone loss in OVX rats and have anabolic effects on cortical bone at higher doses. However, very high, toxic doses of both vitamin D analogs induce intracortical remodeling as an untoward side effect.

Prevention of bone loss in ovariectomized rats by combined treatment with risedronate and 1alpha,25-dihydroxyvitamin D3

Bisphosphonates inhibit bone loss through inhibition of osteoclast-mediated bone resorption. At low doses, vitamin D metabolites can prevent bone loss in models of osteopenia in rats by an antiresorptive effect, while at high doses they also stimulate osteoblast activity and show an anabolic effect. Therefore, combined therapy with bisphosphonates and vitamin D analogs might be expected to be more effective than either treatment alone. It was the aim of this study to compare the efficacy of risedronate and of the naturally occurring vitamin D hormone 1alpha,25-dihydroxyvitamin D3 (calcitriol), alone and in combination, for the prevention of ovariectomy-induced bone loss in rats. One hundred ten female 4-month-old Sprague-Dawley rats were used for this experiment. Ninety rats were bilaterally ovariectomized (OVX), 10 rats were sham-operated (SHAM), and 10 rats were killed at the time of surgery as a baseline control. Groups of rats (10 rats/group) received vehicle or daily doses of 0.1 mg or 0.5 mg of risedronate or 0.05 microg or 0.1 microg of calcitriol/kg body weight, alone and in combination. Both compounds were administered orally via gavage, commencing on the day after surgery. Although estrogen deficiency-induced bone loss was prevented by individual prophylactic administration of risedronate or calcitriol, OVX rats treated with a combination of risedronate and calcitriol had higher bone mineral density (BMD), cancellous bone area (B.Ar), and bone strength in long bones and vertebrae compared with rats receiving risedronate alone. Furthermore, calcitriol enhanced the suppressive effects of risedronate on osteoclast number and partially counteracted the suppressive effects of risedronate on bone formation and histomorphometric indices of osteoblast team performance. Risedronate did not reduce the anabolic effect of calcitriol, and at the high dose it normalized hypercalcemia in calcitriol-treated OVX rats. Therefore, this study in OVX rats suggests that combined therapy with bisphosphonates and vitamin D analogs may offer advantages over the treatment with bisphosphonates or vitamin D analogs alone.

1alpha-hydroxyvitamin D2 is less toxic but not bone selective relative to 1alpha-hydroxyvitamin D3 in ovariectomized rats

Identification of bone selective vitamin D analogues would provide an interesting substance class for the treatment of osteoporosis. The synthetic prodrug 1alpha-hydroxyvitamin D2 [1alpha(OH)D2] has been shown to combine equal bone-preserving activity with distinctly reduced calcemic effects relative to 1alpha-hydroxyvitamin D3 [1alpha(OH)D3] in 3-month-old ovariectomized (OVX) rats. Therefore, 1alpha(OH)D2 may be a bone-selective compound. The aim of this study was to compare the bone protective and the calcemic activities of chronically administered 1alpha(OH)D2 and 1alpha(OH)D3 in 6-month-old OVX rats over a broad dose range from ineffective to toxic doses. Ninety-six female 6-month-old Fischer-344 rats were used for this experiment. Eighty rats were bilaterally OVX, 8 rats were sham-operated (SHAM), and 8 rats were killed at the time of surgery as a baseline control. Groups of OVX rats received vehicle alone (n = 16) or daily doses in the diet of 0.025, 0.05, 0.1, and 0.2 microg of 1alpha(OH)D2 or 1alpha(OH)D3 per kg body weight (BW) per day (n = 8 each). After calcein double-labeling, all animals were killed 3 months post-OVX. Orally administered 1alpha(OH)D2 was significantly less toxic compared with 1alpha(OH)D3 in terms of BW gain and kidney calcium content. The effects of 1alpha(OH)D2 and 1alpha(OH)D3 on serum calcium and urinary calcium excretion were generally similar at all doses in this study. Both 1alpha(OH)D2 and 1alpha(OH)D3 prevented the estrogen deficiency-induced bone loss in OVX rats, and induced profound bone anabolic effects at high dosages. 1alpha(OH)D3 and 1alpha(OH)D2 also dose-dependently increased total bone mineral density (BMD), cortical area, and cortical thickness in the tibial diaphysis of OVX rats. Bone resorption as assessed by osteoclast numbers (Oc.Ns) in vertebral cancellous bone and urinary excretion of deoxypyridinoline (DPD) was dose-dependently suppressed by 1alpha(OH)D2 and 1alpha(OH)D3. These data show that although 1alpha(OH)D2 was slightly but significantly less toxic compared with 1alpha(OH)D3, it did not have increased skeletal effects at any dose. Taken together, our findings argue against selective metabolic activation of 1alpha(OH)D2 in bone.

24,25-Dihydroxyvitamin D(3) and bone metabolism

The 1alpha-hydroxylated metabolite of 25-hydroxyvitamin D(3), 1,25-dihydroxyvitamin D(3), is the biologically most active metabolite of vitamin D. The 24-hydroxylated metabolites were generally considered as degradation products of a catabolic pathway finally leading to excretion of calcitroic acid. Studies with analogues fluorinated at the C-24 position did not indicate a physiological function for 24R,25(OH)(2)D(3). Nevertheless throughout the years various studies showed biologic effects of other metabolites than 1alpha,25(OH)(2)D(3). In particular the metabolite 24R,25(OH)(2)D(3) has been functionally analyzed, e.g. with respect to a role in normal chicken egg hatchability and effects on chondrocytes in the resting zone of cartilage. Numerous studies have shown the presence of the vitamin D receptor in bone cells and effects of 1alpha,25(OH)(2)D(3) on bone and bone cells. Also for 24R,25(OH)(2)D(3) studies have been performed focusing on effects on bone and bone cells. The purpose of this review is to summarize the data regarding 24R,25(OH)(2)D(3) and bone and to evaluate its role in bone biology.

Influence of nutritional factors on bone collagen fibrils in ovariectomized rats

The influence of both vitamin D(3) and Ca:P ratio on bone collagen fibrils was investigated in ovariectomized rats. Six weeks after ovariectomy the rats were maintained for 80 days with diets containing vitamin D(3) and calcium supplementation. Age-matched ovariectomized animals were fed a normal diet. When vitamin D(3) was increased in the diet, although no effect in fibril organization was observed in relation to that from ovariectomized rats with the normal diet, a highly significant effect in fibril diameter was detected. When the calcium:phosphorus (Ca:P) ratio was increased from 1:1 to 2:1 (without vitamin D(3) supplementation) both structural fiber parameters were significantly affected. The results were closer to normal (i.e., collagen fibrils from animals without ovariectomy) when vitamin D(3) and Ca:P ratios were combined.

Alfacalcidol inhibits bone resorption and stimulates formation in an ovariectomized rat model of osteoporosis: distinct actions from estrogen

Although alfacalcidol has been widely used for the treatment of osteoporosis in certain countries, its mechanism of action in bone, especially in the vitamin D-replete state, remains unclear. Here we provide histomorphometric as well as biochemical evidence that alfacalcidol suppresses osteoclastic bone resorption in an ovariectomized rat model of osteoporosis. Furthermore, when compared with 17beta-estradiol, a representative antiresorptive drug, it is evident that alfacalcidol causes a dose-dependent suppression of bone resorption, and yet maintains or even stimulates bone formation, as reflected in increases in serum osteocalcin levels and bone formation rate at both trabecular and cortical sites. 17beta-Estradiol, which suppresses bone resorption to the same extent as alfacalcidol, causes a parallel reduction in the biochemical and histomorphometric markers of bone formation. As a final outcome, treatment with alfacalcidol increases bone mineral density and improves mechanical strength more effectively than 17beta-estradiol, with a more pronounced difference in cortical bone. We conclude that estrogens depress bone turnover primarily by suppressing bone resorption and, as a consequence, bone formation as well, whereas alfacalcidol "supercouples" these processes, in that it suppresses bone resorption while maintaining or stimulating bone formation.

Short-term treatment of rats with high dose 1,25-dihydroxyvitamin D3 stimulates bone formation and increases the number of osteoblast precursor cells in bone marrow

Using an experimental rat model, this study was undertaken to assess the effects of a short-term application of high dose 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] on calcium homeostasis, cancellous bone formation, and numbers of osteoblast precursors in ex vivo bone marrow cultures. For Exp 1 and 2, 6-month-old female rats were sc injected with either 0.2 microg 1,25-(OH)2D3/kg x day or vehicle on days 1, 2, and 3 of the studies. Serum calcium and urinary excretion of calcium were monitored for 12 days in Exp 1. In Exp 2, the rats were ip labeled with five different fluorochromes on days 0, 5, 10, 15, and 20, respectively. Half of the rats in each group were killed on day 7, the rest of the rats were killed on day 24, and the first lumbar vertebrae were processed for histomorphometry. In Exp 3, 0.2 microg 1,25-(OH)2D3/kg BW or vehicle was sc administered to 6-month-old male rats on days 1, 2, and 3, and the number of colony-forming units with the ability to express alkaline phosphatase, to calcify, and/or to synthesize collagen were enumerated sequentially on days 4, 6, 8, 10, 12, and 14 in bone marrow cultures. Short-term 1,25-(OH)2D3 treatment resulted in increased values for serum and urinary calcium during the treatment phase in Exp 1, depressed osteoclast numbers and strongly elevated osteoblast perimeter by day 7, and stimulated mineral apposition rate and bone formation rate in the interval between days 5-15 of Exp 2. Moreover, 1,25-(OH)2D3 administration to rats significantly enhanced the number of mesenchymal precursor cells in bone marrow with the ability to differentiate into an osteoblastic phenotype in ex vivo bone marrow cultures on day 4 of Exp 3. These studies provide evidence that short-term 1,25-(OH)2D3 treatment creates new bone remodeling units and augments osteoblast recruitment and osteoblast team performance in rat cancellous bone.

Trabecular and endocortical bone surfaces in the rat: modeling or remodeling?

BACKGROUND

There is conflicting evidence as to whether bone resorption and bone formation are coupled in the site-specific manner that is typical of bone remodeling in the rat. The aim of this study was to elucidate this controversy further by analysis of tibial and vertebral cancellous and endocortical bone in rats of different age groups with a combination of in vivo fluorochrome labeling with cement line staining.

METHODS

After multiple in vivo fluorochrome labeling, groups of female Fischer-344 rats were killed at 3, 6, 9, and 12 months of age, and the first lumbar vertebrae and the proximal tibiae were processed undecalcified for bone histomorphometry. By comparing fluorochrome labeling and the contour of cement lines in serial sections, cancellous and endocortical bone formation sites were classified as "remodeling," "modeling," or "uncertain."

RESULTS

In vertebral cancellous bone, remodeling was the main turnover activity in all age groups, increasing from 70.4 +/- 2.2% (mean +/- SEM) in 3-month-old rats to 91.0 +/- 2.4% in 12-month-old rats. The percentage of modeling sites decreased from 17.1 +/- 1.7% at age 3 months to 4.67 +/- 1.84% at age 12 months. In the proximal tibial metaphysis of 3-month-old rats, 61.6 +/- 3.6% of all trabecular bone-forming sites were classified as modeling and 21.1 +/- 3.1% as remodeling sites. In the 12-month-old rats, 66.3 +/- 3.4% were classified as remodeling and 16.0 +/- 3.1% as modeling sites. A similar trend toward augmented portions of remodeling with increasing age was observed in tibial and vertebral endocortical bone-formation sites.

CONCLUSIONS

The present study suggests that, similar to higher mammals, the prevailing activity in vertebral and tibial cancellous bone of aged rats is remodeling. In the rapidly growing proximal tibia of 3-month-old rats, however, most of the cancellous bone-forming sites were minimodeling sites.

Calcium and vitamin D enriched diets increase and preserve vertebral mineral content in aging laboratory rats

To assess the long-term effect of vitamin D or calcium supplementation on the skeletal metabolism of aging laboratory rodents, 1.5-month-old female Wistar rats were fed with diets containing twice the concentration of vitamin D (group 2) and of calcium (group 3) as in the usual rat chow. Follow-up to 24 months of age did not show significant differences between the enriched-diet groups and the controls (group 1) in terms of the vertebral body weight and protein content. Significantly higher bone mineral contents were found in groups 2 and 3 than were found in controls, as revealed by an increased bone mineral density (BMD: +62%, group 2; +48%, group 3) and vertebral calcium content (+73%, group 2; +84%, group 3). The vertebral alkaline phosphatase enzymatic activity was significantly lower in the enriched diet groups than in controls (-47%, group 2; -45%, group 3). The ratio alkaline phosphatase/acid phosphatase activity was markedly reduced in groups 2 and 3 (-57% and -59%, respectively), which might indicate a diminished rate of bone turnover. The trabecular bone volume (BV/TV) decreased in all groups during senescence, being significantly elevated in group 3 as compared to controls. Vitamin D and calcium dietary supplementations increase the axial mineral bone content in laboratory rats and might reduce the bone turnover. Their influence on the trabecular bone volume has yet to be examined.

Bone cell biology: new approaches and unanswered questions

In an effort to define the major unanswered questions in bone cell biology and suggest new approaches to answering these questions, I have outlined the bone remodeling cycle and briefly described the major local and systemic factors that regulate bone cell function. These factors include calcium-regulating and systemic hormones as well as locally produced prostaglandins, cytokines, and growth factors. To understand the individual roles of this large number of regulators, it will be necessary to develop new approaches to measure their production and activity in bone under physiologic and pathologic conditions. Quantitative methods in molecular and cellular biology have been developed that should make this identification possible.

Effects of surgical ovariectomy on rat salivary gland function

Studies sought to determine whether there are specific changes in salivary gland protein synthesis and secretion in response to hormone deficiency caused by ovariectomy of female rats. After 50 days, the wet weights of the parotid and submandibular glands did not change with hormone loss while that of the sublingual gland increased by 26% when compared to sham-operated controls. Amylase activity in the parotid declined, as did the level of enzyme activity present in saliva. The amount of the acidic proline-rich protein in the parotid was not altered after ovariectomy when compared to control sham-operated animals, using constant quantities of lysate protein. The total of secreted protein per unit volume did not change with ovariectomy. However, sodium dodecylsulphate-polyacrylamide gel electrophoresis of whole saliva showed the loss of a substantial number of proteins, including amylase and the acidic proline-rich proteins, from the experimental group. Epidermal growth factor concentrations were not significantly altered in the submandibular gland, while again showing a decrease in the concentration from saliva in ovariectomized rats.