Differential regulation of insulin-like growth factor-I (IGF-I) and IGF-II release from cultured neonatal mouse calvaria by parathyroid hormone, transforming growth factor-beta, and 1,25-dihydroxyvitamin D3

In a previous study we found that PTH stimulated bone resorption and release of insulin-like growth factor-I (IGF-I) and IGF-II from cultured neonatal mouse calvaria. Since IGF-I and IGF-II stimulate osteoblast proliferation and collagen synthesis, these results suggested that increased release of IGFs during resorption could mediate in part coupling of bone formation to bone resorption. In the present study two other osteolytic agents, transforming growth factor-beta (TGF beta) and 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3 were examined for effects on IGF release from neonatal mouse calvaria. Like PTH, TGF beta stimulated resorption and increased release of IGF-I and IGF-II. 1,25-(OH)2D3, however, stimulated resorption and IGF-II release comparable to PTH, but inhibited release of IGF-I. 1,25-(OH)2D3 (0.1-100 nM) inhibited basal release of IGF-I, and 10 nM 1,25-(OH)2D3 inhibited release of IGF-I induced by PTH or TGF beta. The effects of 1,25-(OH)2D3 were specific to this vitamin D metabolite and did not occur with 25-hydroxyvitamin D3 or 24,25-(OH)2D3 at the same concentration. Calcitonin (50 mU/ml) decreased 1,25-(OH)2D3 stimulation of resorption, but did not affect 1,25-(OH)2D3 stimulation of IGF-II release and inhibition of IGF-I release. This evidence that effects of 1,25-(OH)2D3 on release of the IGFs were independent of bone resorption supports the conclusion that 1,25-(OH)2D3 modulated the production and secretion of IGF-I and IGF-II in calvarial cells. The results of this and the previous study suggest that PTH, TGF beta, and 1,25-(OH)2D3 differentially regulate mouse calvarial cell IGF-I and IGF-II production.     

IGF-I and GH stimulate Phex mRNA expression in lungs and bones and 1,25-dihydroxyvitamin D(3) production in hypophysectomized rats.

OBJECTIVE: X-linked hypophosphatemia, a renal phosphate (Pi)-wasting disorder with defective bone mineralization, is caused by mutations in the PHEX gene (a Pi-regulating gene with homology to endopeptidases on the X chromosome). We wondered whether changes in Phex and neprilysin (NEP) (another member of the family of zinc endopeptidases) mRNA expression could be observed in relation to vitamin D and Pi metabolism during GH- and IGF-I-stimulated growth of hypophysectomized rats. DESIGN: Animals were infused s.c. for 2 days with vehicle, 200 mU (67 microg) GH or 300 microg IGF-I/rat per 24 h. We determined serum osteocalcin and osteocalcin mRNA in bone, Phex mRNA in bone and lungs, serum 1,25-dihydroxyvitamin D(3) (1,25-(OH)(2)D(3)) and serum Pi levels, and renal expression of 25-hydroxyvitamin D(3)-1alpha-hydroxylase (1alpha-hydroxylase), of 25-hydroxyvitamin D(3)-24-hydroxylase (24-hydroxylase) and of the Na-dependent Pi-cotransporter type I and II (Na(d)Pi-I and -II). RESULTS: As compared with vehicle-treated controls, body weight and tibial epiphyseal width significantly increased in GH- and IGF-I-treated animals. Serum osteocalcin and osteocalcin mRNA levels in bone, Phex mRNA in bone and lungs, serum 1,25-(OH)(2)D(3) and renal 1alpha-hydroxylase mRNA rose concomitantly, whereas expression of NEP in lungs was barely affected and renal 24-hydroxylase mRNA decreased. Na(d)Pi-I and -II gene expression in the kidney and serum Pi levels remained unchanged. CONCLUSIONS: Our findings suggest a coordinate regulation of Phex mRNA expression in lungs and bone and vitamin D metabolism during GH- and IGF-I-stimulated growth.     

Influence of vitamin D on mineral metabolism, hormonal status and bone histology in lactating rats and their pups

Mineral, hormonal and skeletal changes were determined in vitamin D-deficient (-D) and vitamin D-replete (+D) mother rats and in their litters on day 20 of lactation. These results were compared with those obtained in -D mothers and pups, after giving the mothers an oral supplement (10 i.u. vitamin D3/day) during the period of lactation (20 days). Compared to +D animals, both -D lactating mothers and their pups exhibited extremely low plasma levels of 25-hydroxyvitamin D3 (25-OH-D3), diminished 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) and increased levels of immunoreactive parathyroid hormone (iPTH). Vitamin D-deficient mothers also had higher levels of calcitonin and lower levels of prolactin than +D mothers. All -D animals (mothers and pups) showed increased osteoclastic bone resorption and severe osteomalacia as shown by decreased bone ash, decreased calcification rate and increased endosteal osteoid surface, volume and thickness. In mothers treated with vitamin D3 during lactation, nearly all the plasma variables measured, as well as bone histomorphometric features, were normal. In contrast, their pups still showed rickets and osteomalacia, despite normal levels of 25-OH-D3 and calcium in the plasma. These pups had raised plasma levels of 1,25(OH)2D3 and iPTH associated with persistent stimulation of bone resorption. This study showed that (1) severe vitamin D deficiency in lactating rats produced marked osteomalacia and secondary hyperparathyroidism in both mothers and pups, and (2) vitamin D treatment of -D mother rats during lactation (10 i.u. vitamin D3/day) reversed the mineral, hormonal and skeletal abnormalities in mothers but not in pups.     

The role of 1,25-dihydroxyvitamin D in the inhibition of bone formation induced by skeletal unloading

Skeletal unloading results in osteopenia. To examine the involvement of vitamin D in this process, the rear limbs of growing rats were unloaded, and alterations in bone calcium and bone histology were related to changes in serum calcium (Ca), inorganic phosphorus, 25-hydroxyvitamin D, 24,25-dihydroxyvitamin D [24,25-(OH)2D], and 1,25-dihydroxyvitamin D [1,25-(OH)2D]. Acute skeletal unloading induced a transitory inhibition of Ca accumulation in unloaded bones. This was accompanied by a transitory rise in serum Ca, a 21% decrease in longitudinal bone growth (P less than 0.01), a 32% decrease in bone surface lined with osteoblasts (P less than 0.05), no change in bone surface lined with osteoclasts, and a decrease in circulating 1,25-(OH)2D from 130 +/- 10 to 53 +/- 11 pg/ml. No significant changes in the serum concentrations of inorganic phosphorus, 25-hydroxyvitamin D, or 24,25-(OH)2D were observed. After 2 weeks of unloading, bone Ca stabilized at approximately 70% of control values, and serum Ca and 1,25-(OH)2D returned to control values. Maintenance of a constant serum 1,25-(OH)2D concentration by chronic infusion of 1,25-(OH)2D (Alza osmotic minipump) throughout the study period did not prevent the bone changes induced by acute unloading. These results suggest that acute skeletal unloading in the growing rat produces a transitory inhibition of bone formation, which, in turn, produces a transitory hypercalcemia, leading to a temporary decrease in serum 1,25-(OH)2D. No evidence could be found for a direct involvement of 1,25-(OH)2D in the bone changes induced by skeletal unloading.     

Vitamin D metabolites regulate osteocalcin synthesis and proliferation of human bone cells in vitro

The effects of six natural vitamin D metabolites of potential biological and therapeutic interest, 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3), 25-hydroxyvitamin D3 (25-OH-D3), 24R,25-dihydroxyvitamin D3 (24R,25-(OH)2D3), 1,24R,25-trihydroxyvitamin D3 (1,24R,25-(OH)3D3), 25S,26-dihydroxyvitamin D3 (25S,26-(OH)2D3) and 1,25S,26-trihydroxyvitamin D3 (1,25S,26-(OH)3D3) on cell replication and expression of the osteoblastic phenotype in terms of osteocalcin production were examined in cultured human bone cells. At a dose of 5 X 10(-12) mol/1, 1,25-(OH)2D3 stimulated cell proliferation, whereas at higher doses (5 X 10(-9)-5 X 10(-6) mol/1) cell growth was inhibited in a dose-dependent manner. The same pattern of effects was seen for the other metabolites in a rank order of potency: 1,25-(OH)2D3 greater than 1,25S,26-(OH)3D3 = 1,24R,25-(OH)3D3 greater than 25S,26-(OH)2D3 = 24R,25-(OH)2D3 = 25-OH-D3. Synthesis of osteocalcin was induced by 1,25-(OH)2D3 in doses similar to those required to inhibit cell proliferation. Biphasic responses were observed for some of the metabolites in terms of osteocalcin synthesis, inhibitory effects becoming apparent at 5 X 10(-6) mol/1. The cells did not secrete osteocalcin spontaneously. These results indicate that vitamin D metabolites may regulate growth and expression of differentiated functions of normal human osteoblasts.     

Bone and serum concentrations of osteocalcin as a function of 1,25-dihydroxyvitamin D3 circulating levels in bone disorders in rats

Osteocalcin, the vitamin K-dependent protein in bone containing gamma-carboxyglutamic acid, has been found to be significantly decreased in the osteomalacic bone of chicks made vitamin D deficient for 6 weeks. To evaluate whether this decrease in bone osteocalcin was due directly to the decrease or absence of vitamin D and its metabolites or to the secondary hypocalcemia and osteomalacia or other changes accompanying the deficiency of vitamin D, three experimental groups of Holtzman rats were studied. One group was made rachitic by a diet deficient in vitamin D, and the other groups were made rachitic by diets deficient in inorganic orthophosphate or calcium. The changes in bone and serum osteocalcin, serum 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], and bone mineral content were evaluated, and morphological evaluation of bone was made. In the vitamin D-deficient animals, osteomalacia was evident histologically by 7 weeks, at which time serum 1,25-(OH)2D3 was not detectable, bone osteocalcin was decreased by 50%, and serum osteocalcin was decreased by 20%. In the animals fed a diet deficient in either calcium or inorganic orthophosphate but which were not depleted of vitamin D, the osteocalcin content of osteomalcic bone was normal, and an increase in the concentration of serum osteocalcin accompanied an increase in serum 1,25-(OH)2D3. These data are consistent with the conclusion that the metabolism of osteocalcin is affected by serum 1,25-(OH)2D3 and that the diminished level of osteocalcin in the bone of vitamin D-deficient animals is the result of a direct action of the metabolites and is not secondary to a decrease in the mineralization of bone tissue.     

Serum vitamin D metabolites are not responsible for low turnover osteoporosis in chronic liver disease

We measured the concentrations of vitamin D-binding protein (DBP), total 25-hydroxyvitamin D, total 1,25-dihydroxyvitamin D [1,25-(OH)2D], and free 1,25-(OH)2D in sera of 107 patients with histologically proven chronic liver disease. Bone density measurements and dynamic skeletal histomorphometry were also performed. Osteoporosis, as defined by arbitrary criteria, was found in 42 patients (39%), while no patient had osteomalacia. Serum concentrations of vitamin D-binding protein, 25-hydroxyvitamin D, total 1,25-(OH)2D, and free 1,25-(OH)2D were reduced in patients with cirrhosis, but not in the noncirrhotic patients. Bone formation rates, which were low in 55 patients (51%), were correlated with liver functions, but not with the concentrations of either vitamin D metabolite. A subgroup of 44 patients with low serum 1,25-(OH)2D concentrations and low bone formation rates failed to show an appropriate increase in serum bone Gla protein after 1,25-(OH)2D3 administration even though serum concentrations of 1,25-(OH)2D rose normally. These data suggest that the bone disease in patients with hepatic disorders is not related to the serum concentrations of vitamin D metabolites or the effect of these metabolites on osteoblast function.     

Low 1,25-dihydroxyvitamin D, secondary hyperparathyroidism, and normal osteocalcin in elderly subjects

The relationship among serum vitamin D metabolites, PTH, and osteocalcin concentrations was investigated in 20 elderly subjects. All except 2 had subnormal 25-hydroxyvitamin D concentrations. Eighteen (90%) had subnormal serum 1,25-dihydroxyvitamin D [1,25-(OH)2D] concentrations, while 8 subjects (40%) had elevated PTH concentrations. There was a highly significant inverse relationship between PTH and 1,25-(OH)2D concentrations. Serum osteocalcin concentrations were not elevated in any subject, and in fact, the mean osteocalcin concentration was in the lower part of the normal range. These data indicate no compensatory increase in 1,25-(OH)2D in response to secondary hyperparathyroidism and no increase in osteocalcin in response to hypersecretion of PTH in the elderly. These 2 defects may contribute to the bone disease of the elderly.     

Skeletal resistance to 1,25-dihydroxyvitamin D3 in osteopetrotic rats

The osteopetrotic (op/op) rat mutation is a lethal mutation in which decreased osteoclast function (bone resorption) coexists with markedly elevated serum levels of 1 ,25-dihydroxyvitamin D3[1,25(OH)2D3]. Increased circulating levels of 1,25(OH)2D3 have been reported in other osteopetrotic animal mutations and in some osteopetrotic children. This study examined the effects of 1,25(OH)2D3 infusions on serum and skeletal parameters in normal and mutant rats of op stock. We also examined vitamin D receptor expression and binding in bone cells from op normal and mutant animals. Four-week-old normal and mutant rats were infused either with propylene glycol (used as controls) or with 12.5-125 ng of 1,25(OH)2D3/d using osmotic minipumps implanted subcutaneously for 1 wk. Sera were analyzed for calcium, phosphorus, and 1,25(OH)2D3 levels. Histomorphometric analyses of proximal tibiae from treated normal (50 ng/d) and op mutant (125 ng/d) rats and their vehicle-infused controls were performed. Normal animals infused with 1,25(OH)2D3 exhibited a dose-dependent increase in serum calcium levels. Histomorphometric analyses of metaphyseal bone within the primary spongiosae region showed that 1,25(OH)2D3 increased osteoclast number with a reduction in osteoblast surface associated with a decrease in growth plate cartilage thickness. However, similar analyses on secondary spongiosae showed a decrease in osteoclast number and surface associated with an anabolic response. Op mutants infused with 1,25(OH)2D3 did not exhibit any change in serum calcium levels or histomorphometric parameters related to growth plate cartilage and metaphyseal bone compared with mutant controls. Vitamin D mRNA and protein levels were increased twoto threefold in op mutants compared to age-matched normal rats. However, binding affinity of 1,25(OH)2D3 to its receptor was similar between op mutant and normal animals. High dose calcitriol therapy, under the conditions and period of treatment used in this study, failed to stimulate bone turnover in op rats, suggesting that they are resistant to the skeletal effects of 1,25(OH)2D3. The failure of osteoclast activation in response to 1,25(OH)2D3 treatment may be associated with osteoblast incompetence in this mutation.     

Modulation by vitamin D status of the responsiveness of rat bone to gonadal steroids

We have previously demonstrated that gonadal steroids stimulate [3H]thymidine incorporation and creatine kinase specific activity in skeletal tissues. In the present study we report that in 20-day-old vitamin D-deficient Wistar-derived rats, 17 beta-estradiol (E2; 5 micrograms/rat) or testosterone (50 micrograms/rat) failed to stimulate [3H]thymidine incorporation into diaphyses of long bones and that the response to these hormones in terms of increased creatine kinase specific activity was less than half the value in normally fed rats. Two daily ip injections of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3; 0.5 ng/g BW], but not 24,25-(OH)2D3 (5 ng/g BW), partially restored the biological responses to E2 in bone of 21-day-old vitamin D-deficient female rats. Vitamin D deficiency did not impair the responsiveness to gonadal steroids in the epiphysis of long bones, uterus, or prostate, in contrast to its effect on diaphysis. In 21-day-old normally fed female rats, neither vitamin D metabolite enhanced the response to E2. When cultures of rat epiphyseal cells were treated daily for 5 days with either 1,25-(OH)2D3 (1 nM) or 24,25-(OH)2D3 (10 nM), followed by E2 (30 nM) for 24 h, creatine kinase activity was significantly higher than in cultures treated daily for 5 days with vehicle alone, and then with E2. The same treatment of rat embryo calvaria bone cells showed that 1,25-(OH)2D3, but not 24,25-(OH)2D3, significantly increased the creatine kinase activity response to E2. These findings suggest that vitamin D metabolites selectively affect the biological responses of skeletal tissues to gonadal steroids.     

The effects of 1,25-dihydroxyvitamin D3 and dexamethasone on rat osteoblast-like primary cell cultures: receptor occupancy and functional expression patterns for three different bioresponses

The effects of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] and dexamethasone to regulate collagen and osteocalcin synthesis and induction of 25-hydroxyvitamin D3-24-hydroxylase (24-hydroxylase) activity were studied in rat osteoblast-like cell primary cultures. In this culture system, the basal levels of collagen and osteocalcin synthesis increased with rising cell density in culture. At maximal doses, both 1,25-(OH)2D3 (8.1 nM) and dexamethasone (130 nM) reduced collagen synthesis to about 50% of the control levels, 1,25-(OH)2D3 affected osteocalcin synthesis in a biphasic manner: stimulatory at low doses, which peaked near 0.33 nM to reach 3- to 5-fold the basal level, followed by a gradual return to the basal level at higher concentrations. Dexamethasone had only a slight stimulatory effect on osteocalcin. 1,25-(OH)2D3 also induced 24-hydroxylase activity in rat osteoblast-like cells, while dexamethasone had no effect on the enzyme. Induction of enzyme activity achieved a 4- to 6-fold rise, but required higher concentrations of 1,25-(OH)2D3 to achieve maximal levels (16 nM). The half-maximal doses (ED50) of 1,25-(OH)2D3 required for each bioresponse were different. The approximate ED50 for the inhibition of collagen synthesis was near the Kin (0.4 nM; apparent dissociation constant of receptor nuclear internalization), while the ED50 for osteocalcin synthesis (0.08 nM) was below the Kin, and the ED50 for 24-hydroxylase induction (20 nM) was greater than the Kin. The ED50 for dexamethasone on collagen synthesis (20 nM) was about 5-fold higher than the Kin (4 nM) of dexamethasone receptor binding. The potencies of various vitamin D3 metabolites in all three functional responses followed their abilities to compete for the 1,25-(OH)2D3 receptor, indicating that these actions were 1,25-(OH)2D3 receptor mediated. In summary, these studies explored bone cell bioresponses to 1,25-(OH)2D3 and dexamethasone and examined the relationship between receptor occupancy and functional expression. Each action exhibited a different dose-response pattern, implying that different levels of control are required for each individual response.     

Evaluation of the total fetomaternal vitamin D relationships at term: evidence for racial differences

The present study assessed the total fetomaternal vitamin D relationship at term in 12 white and 10 black mothers and their infants. Antirachitic sterols were extracted from plasma, chromatographed, and finally quantitated using competitive protein binding assays. Compounds quantitated included vitamins D2 and D3, 25-hydroxyvitamin D2, 25-hydroxyvitamin D3, 24,25-dihydroxyvitamin D2, 24,25-dihydroxyvitamin D3, 25,26-dihydroxyvitamin D3, 1,25-dihydroxyvitamin D2, and 1,25-dihydroxyvitamin D3. There was a strong correlation between maternal and neonatal plasma concentrations of all antirachitic sterols measured with the exception of vitamins D2 and D3. Vitamins D2 and D3, although detectable in maternal plasma, were undetectable in neonatal plasma. Racial comparisons demonstrated that vitamin D3, 25-hydroxyvitamin D3, 24,25-hydroxyvitamin D3, and 25,26-(OH)2-D3 were significantly (P less than 0.05) higher in white than in black mothers. Total 25-hydroxyvitamin D and 24,25-hydroxyvitamin D were also significantly (P less than 0.05) higher in white than in black mothers. A similar pattern was found in black and white infants except for 25,26-(OH)2-D3. Black mothers and their infants had significantly (P less than 0.05) higher 1,25-hydroxyvitamin D3 compared to the white subjects, although total 1,25-hydroxyvitamin D was not different between races. No significant racial (P greater than 0.05) differences were found for any of the vitamin D2 compounds. The results support the concept that fetomaternal vitamin D status are intimately related. Further, they strongly suggest that fetal metabolism begins with 25-hydroxyvitamin D rather than vitamin D. Finally, racial factors appear to influence the overall vitamin D status of both mother and fetus, and may influence antirachitic sterol metabolism.     

1,25-Dihydroxyvitamin D3 [1,25-(OH)2D3] increases insulin-like growth factor I (IGF-I) receptors in clonal osteoblastic cells. Study on interaction of IGF-I and 1,25-(OH)2D3

We previously reported a cooperative effect between insulin-like growth factor I (IGF-I) and 1,25-dihydroxy-vitamin D3 [1,25-(OH)2D3] in murine clonal osteoblastic cells, MCT3T3-E1. In the present study, the possible mechanism of interaction between these hormones was investigated. The effect of IGF-I on 1,25-(OH)2D3 receptors in MC3T3-E1 cells was examined. The affinity and hormone binding capacity of 1,25-(OH)2D3 receptors were not altered by IGF-I. Immunoblot analysis showed about 54 kilodaltons (kDa) 1,25-(OH)2D3 receptors, similar to that observed for mouse fibroblasts. The synthesis of IGF-I by the cells under a serum-free condition was determined by RIA. The assay revealed immunoreactive IGF-I secreted by MC3T3-E1 cells (1.79 +/- 0.04 x 10(-9) M, mean +/- SE, n = 5). Rat GH significantly increased the concentration of IGF-I, but 1,25-(OH)2D3 did not. IGF-I radioligand-receptor assay revealed specific binding of IGF-I to MC3T3-E1 cells. The relative potency of IGF-I-related peptides to bind with the cells was in the order of IGF-I much greater than multiplication-stimulating activity (the rat homologue of IGF-II) greater than insulin, and the receptor protein migrated as a 130-kDa band in autoradiography. Scatchard analysis showed a significant increase in IGF-I binding sites by 50% after 3-day treatment with 5 x 10(-11) M 1,25-(OH)2D3, without any change in affinity. These results indicate that the interaction of IGF-I and 1,25-(OH)2D3 in the culture of MC3T3-E1 cells may be mediated by the effect of 1,25-(OH)2D3 on IGF-I receptors.     

1,25(OH)2vitamin D3 stimulates myogenic differentiation by inhibiting cell proliferation and modulating the expression of promyogenic growth factors and myostatin in C2C12 skeletal muscle cells

Skeletal muscle wasting is an important public health problem associated with aging, chronic disease, cancer, kidney dialysis, and HIV/AIDS. 1,25-Dihydroxyvitamin D (1,25-D3), the active form of vitamin D, is widely recognized for its regulation of calcium and phosphate homeostasis in relation to bone development and maintenance and for its calcemic effects on target organs, such as intestine, kidney, and parathyroid glands. Emerging evidence has shown that vitamin D administration improves muscle performance and reduces falls in vitamin D-deficient older adults. However, little is known of the underlying mechanism or the role 1,25-D3 plays in promoting myogenic differentiation at the cellular and/or molecular level. In this study, we examined the effect of 1,25-D3 on myoblast cell proliferation, progression, and differentiation into myotubes. C(2)C(12) myoblasts were treated with 1,25-D3 or placebo for 1, 3, 4, 7, and 10 d. Vitamin D receptor expression was analyzed by quantitative RT-PCR, Western blottings and immunofluorescence. Expression of muscle lineage, pro- and antimyogenic, and proliferation markers was assessed by immunocytochemistry, PCR arrays, quantitative RT-PCR, and Western blottings. Addition of 1,25-D3 to C(2)C(12) myoblasts 1) increased expression and nuclear translocation of the vitamin D receptor, 2) decreased cell proliferation, 3) decreased IGF-I expression, and 4) promoted myogenic differentiation by increasing IGF-II and follistatin expression and decreasing the expression of myostatin, the only known negative regulator of muscle mass, without changing growth differentiation factor 11 expression. This study identifies key vitamin D-related molecular pathways for muscle regulation and supports the rationale for vitamin D intervention studies in select muscle disorder conditions.     

Osteopetrosis in the rat: coexistence of reductions in osteocalcin and bone resorption

Osteocalcin, one of the vitamin K-dependent bone proteins, has recently been implicated in bone resorption. To explore this hypothesis, bone and serum osteocalcin were measured in three different osteopetrotic rat mutations characterized by reduced bone resorption. These three mutations (ia/ia, tl/tl, and op/op) exhibit heterogeneity with respect to osteoclast number and activity and response to being cured by bone marrow transplantation. Calvarial bone osteocalcin was present in normal amounts, but difficult to extract, in ia/ia rats that have increased numbers of inactive osteoclasts. Bone osteocalcin was greatly decreased in op/op (53-60% of control) and tl/tl (64-73% of control) osteopetrotic rats, in which osteoclasts are both reduced in number and inactive. These decreases in osteocalcin levels in bone coexist with elevated serum levels of osteocalcin in all three mutations. Since osteocalcin synthesis is known to be stimulated by 1,25(OH)2D3, the increase in serum osteocalcin may be a reflection of the elevated blood levels of 1,25(OH)2D3 known to occur in each of these mutations. These findings indicate that the composition of osteopetrotic bone is abnormal with respect to osteocalcin in the two rat osteopetrotic mutations showing decreased osteoclast numbers. Considered together with the emerging evidence that the extracellular matrix in many developing tissues plays a role in cell recruitment and differentiation, these data suggest that osteocalcin abnormalities may be a contributing factor to the spectrum of osteoclast aberrations in osteopetrosis.     

Osteocalcin during the reproductive cycle in normal and diabetic rats

Concentrations of osteocalcin were measured in plasma and bone of normal and diabetic rats during the reproductive cycle and compared with plasma 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) concentrations. The age-dependence of osteocalcin was also examined. Plasma concentrations of osteocalcin levels were low but detectable in 21-day-old fetuses (3.7 +/- 0.3 nmol/l); osteocalcin concentrations were highest in weaning rats (104 +/- 9 nmol/l) and decreased thereafter. In adult rats, plasma concentrations of both osteocalcin and 1,25-(OH)2D3 increased during the last days of normal pregnancy, and even more so in rats fed a diet low in calcium and phosphate. After an early post-partum decline, osteocalcin concentrations in plasma remained at non-pregnant levels in lactating rats fed a high calcium/phosphate diet while their 1,25-(OH)2D3 concentrations were higher than in non-pregnant rats; however, lactating rats fed a low calcium/phosphate diet showed increasing osteocalcin concentrations. In spontaneously diabetic BB rats, plasma osteocalcin concentrations were severely decreased compared with those in non-diabetic rats, more than would have been expected from their decreased 1,25-(OH)2D3 concentrations. Moreover, plasma osteocalcin did not increase during pregnancy or lactation in diabetic rats, even when fed a low calcium/phosphate diet. Fetuses of diabetic rats also had lower plasma osteocalcin levels than fetuses from non-diabetic rats or than weight-matched fetuses from semistarved rats. In contrast to plasma osteocalcin concentrations, bone osteocalcin concentrations and content were not altered by pregnancy, lactation, low calcium/phosphate diet or diabetes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Calcitropic hormones and IGF-I are influenced by dietary protein

Elderly men and women with protein deficiencies have low levels of circulating IGF-I, and it is likely this contributes to reduced bone formation and increased bone resorption. We hypothesized that calcitropic hormones are involved in this effect and are affected by dietary protein. We therefore investigated the influence of a low-protein diet on the PTH-1,25-dihydroxyvitamin D3 [1,25(OH)?D?] axis and IGF-I in rats, using pamidronate to block resorption that normally contributes to mineral homeostasis. We fed 6-month-old Sprague Dawley female rats isocaloric diets containing 2.5% or 15% casein for 2 wk. Pamidronate was then administered sc (0.6 mg/kg/) for 5 d. Blood samples were collected at different time points. Serum 1,25(OH)?D?, IGF-I, PTH, calcium, and phosphorus were determined in all rats; vertebral bone strength and histomorphometric analysis were performed in rats subject to the longest low-protein diets. We found 2 wk of low protein increased PTH levels, decreased 1,25(OH)?D?, calcium, and IGF-I, suggesting that increased PTH compensates for low-protein-induced decreases in 1,25(OH)?D?. Pamidronate augmented the increased PTH after 8 wk of low protein and prevented the 1,25(OH)?D? decrease. IGF-I remained low. Protein malnutrition induced decreases in relative bone volume and trabecular thickness, which was prevented by pamidronate. Maximal load was reduced by protein restriction, but rescued by pamidronate. In summary, the low protein diet resulted in hyperparathyroidism, a reduction in circulating levels of IGF-I, and reduced 1,25(OH)?D? despite hyperparathyroidism. Blocking resorption resulted in further increases in PTH and improved microarchitecture and biomechanical properties, irrespective of vitamin D status or protein intake.     

Dexamethasone increases 1,25-dihydroxyvitamin D3 receptor levels and augments bioresponses in rat osteoblast-like cells

Glucocorticoids increase the level of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3] receptors in primary cultures of rat calvarial osteoblast-like (OB) cells. The present study investigated how this dexamethasone (DEX) up-regulation of 1,25-(OH)2D3 receptors modulates three 1,25-(OH)2D3 bioresponses: inhibition of collagen synthesis, stimulation of osteocalcin synthesis, and induction of 25-hydroxyvitamin D3-24-hydroxylase activity. Pretreatment of OB cells with 13 nM DEX for 24 h doubled the 1,25-(OH)2D3 receptor level without changing receptor affinity for 1,25-(OH)2D3 to study bioresponses. After DEX treatment to increase the 1,25-(OH)2D3 receptor level, the magnitude and sensitivity of all three 1,25-(OH)2D3 bioresponses were enhanced. The maximal 1,25-(OH)2D3 inhibition of collagen synthesis was increased by DEX pretreatment compound to control values: 30 to 50% (1 day treatment) and 50 to 70% (2 day treatment). The sensitivity to 1,25-(OH)2D3, as measured by reduction of the half-maximal inhibitory dose (ED50), was increased 50%. This potentiation of 1,25-(OH)2D3 inhibitory action on collagen synthesis was still evident after correction for the inhibitory effect on collagen synthesis by DEX alone. The maximal stimulation of osteocalcin by 1,25-(OH)2D3 was also enhanced from 2- to 3-fold in controls to over 4- to 5-fold by DEX pretreatment. Similarly, the ED50 of the response was reduced 50%. For the induction of 25-hydroxyvitamin D3-24-hydroxylase activity, DEX doubled the enzyme activity over that seen with 1,25-(OH)2D3 alone, but only slightly affected the sensitivity of the enzyme induction. In conclusion, after DEX up-regulation of 1,25-(OH)2D3 receptor levels, there was a general potentiation of 1,25-(OH)2D3 bioresponses in rat OB cells. However, the detailed patterns of the augmented responses were different for each of the three biological functions we studied.     

Somatomedin-C/insulin-like growth factor I and vitamin D-induced growth

We measured serum somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) concentrations in vitamin D-deficient (-D) rats from 3-7 weeks of age to evaluate the role of Sm-C/IGF-I in the growth effects of vitamin D. To exclude effects of alterations in food intake, feeding patterns, and plasma minerals, controls included the following groups: pair fed (PFC; n = 10), low calcium diet (LCa; n = 10), meal fed (MFC; n = 10), and ad libitum fed (AdLC; n = 5). The -D, LCa, and PFC groups had similarly depressed growth rates and Sm-C/IGF-I concentrations. The growth and Sm-C/IGF-I concentrations of the MFC group were less than those in the AdLC group, but greater than those in the -D, PFC, and LCa groups. In all groups Sm-C/IGF-I concentrations correlated well with food intake, indicating that calorie intake, and not vitamin D deficiency or hypocalcemia, was the primary factor in reducing Sm-C/IGF-I. In the final 4 days of the study, half of the -D rats were given 1,25-dihydroxyvitamin D3 (2 ng/g BW;-D/repleted), while half of the PFC rats were given excess food during the meal period (PFC/refed). The weight gain of -D/repleted rats surpassed that of the -D rats treated with vehicle (P less than 0.05); similarly, the weight gain of PFC/refed rats exceeded that of the PFC rats (P less than 0.01). In contrast, the food intake and Sm-C/IGF-I levels of PFC/refed rats were greater than those in PFC rats (P less than 0.01), while the -D/repleted rats did not have significantly altered food intake or Sm-C/IGF-I levels. We conclude 1) that alterations in Sm-C/IGF-I concentrations of young growing -D rats are not directly related to lack of vitamin D, but, rather, to poor nutrition resulting from reduced food intake; and 2) that serum Sm-C/IGF-I is not a primary mediator of growth in -D/repleted rats since improved weight gain resulting from 1,25-dihydroxyvitamin D3 treatment can occur, acutely, without changes in serum Sm-C/IGF-I concentrations.     

The effect of vitamin D on bone in vivo

The linear rate of bone mineral apposition (BMAR) was measured in vitamin D-deficient and vitamin D-sufficient adult rats before and during treatment with either 25-hydroxyvitamin D3 (25OHD3), 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3], or 24,25-dihydroxyvitamin D3 [24,25-(OH)2D3]. Dietary vitamin D restriction caused a fall in BMAR which began after 1 week and fell progressively to a value of 35-50% of control values by 4 weeks. The fall in BMAR was related to a fall in the serum concentrations of 25(OH)D3 and 24,25-(OH)2D3, without a fall in the 1,25-(OH)2D3 concentration. Dietary supplementation of the D-deficient animals with either 25OHD3 or 24,25-(OH)2D3 at doses of 200 ng/day restored BMAR. If vitamin D-deficient animals were thyroparathyroid-ectomized before supplementation with vitamin D metabolites, 24,25-(OH)2D3 administration was without effect on BMAR. The combined administration of PTH and 24,25-(OH)2D3 to such animals led to a restoration of the BMAR to normal. In vitamin D-sufficient animals, parathyroidectomy led to a 50% reduction in BMAR, which could be restored by treatment with PTH alone but not with 24,25-(OH)2D3. Simultaneous treatment of these animals with PTH and 24,25-(OH)2D3 led to a greater than normal increase in BMAR (130% of control) in these animals. These data support the concept that 24,25-(OH)2D3 has a role in the regulation of bone formation and/or mineralization, and demonstrate the interrelation between the effects of PTH and 24,25-(OH)2D3 on bone.