1,25(OH)2D3 receptor regulation and 1,25(OH)2D3 effects in primary cultures of growth cartilage cells of the rat

Effect of the addition of different concentrations of Solanum glaucophyllum desf. extract on chondrocyte cultures from the growth cartilage of newborn rats

Solanum glaucophyllum Desf. Is a calcinogenic plant responsible for enzootic calcinosis that affects ruminants and causes alterations in bone and cartilaginous tissues, among others. It is believed that changes in cartilage tissue, with reduced bone growth, are due to hypercalcitoninism, caused by excess vitamin D. However, we hypothesized that S. glaucophyllum Desf. Can act directly on chondrocytes and therefore, chondrocyte cultures from the epiphysis of the long bones of newborn rats were used as a model to elucidate the direct effects of S. glaucophyllum Desf. On bone growth. Plant samples were collected from Cañuelas, Argentina. An aliquot of the plant extract was used to quantify vitamin D (1,25(OH)2D3). The effects of the three concentrations of the plant extract were tested in cultures of chondrocytes extracted from the epiphyses of the long bones of 32 three-day-old Wistar rats. A control group (without extract treatment), and three groups treated with different concentrations of plant extract were formed: group 1 (100 μL/L); group 2 (1 mL/L), and group 3 (5 mL/L), containing respectively 1 × 10^-9 M, 1 × 10^-8 M, and 5 × 10^-8 M of 1,25(OH)2D3. After 7, 14, and 21 days of culture, MTT assay for cell viability, alkaline phosphatase activity, and quantification of the percentage of areas with glycosaminoglycans (GAG) stained with periodic acid-Schiff (PAS) were performed. On day 7, all chondrocytes in group 3, that is, those with the highest concentration of plant extract, died. On days 14 and 21, groups 1 and 2 showed a significant reduction in chondrocyte viability compared to the control. At 7, 14, and 21 days, groups 1 and 2 showed significantly lower alkaline phosphatase activity than the control. On day 21, group 2 showed a significant reduction in areas with PAS + GAGs. There were no significant differences between the groups in the expression of gene transcripts for Sox9, Col2, ColX, and aggrecan. The S. glaucophyllum Desf. Extract directly affected growing rat chondrocytes by reducing viability, alkaline phosphatase activity, and GAG synthesis without altering the expression of gene transcripts for Sox9, Col2, ColX, and aggrecan, which may be one of the mechanisms by which there is a reduction in bone growth in animals intoxicated by the plant.

1,25(OH)2D3 Mitigates Oxidative Stress-Induced Damage to Nucleus Pulposus-Derived Mesenchymal Stem Cells through PI3K/Akt Pathway

Intervertebral disc degeneration (IVDD) is one of the main causes of low back pain. The local environment of the degenerated intervertebral disc (IVD) increases oxidative stress and apoptosis of endogenous nucleus pulposus-derived mesenchymal stem cells (NPMSCs) and weakens its ability of endogenous repair ability in degenerated IVDs. A suitable concentration of 1α,25-dihydroxyvitamin D₃ (1,25(OH)₂D₃) has been certified to reduce oxidative stress and cell apoptosis. The current study investigated the protective effect and potential mechanism of 1,25(OH)₂D₃ against oxidative stress-induced damage to NPMSCs. The present results showed that 1,25(OH)₂D₃ showed a significant protective effect on NPMSCs at a concentration of 10⁻¹⁰ M for 24 h. Protective effects of 1,25(OH)₂D₃ were also exhibited against H₂O₂-induced NPMSC senescence, mitochondrial dysfunction, and reduced mitochondrial membrane potential. The Annexin V/PI apoptosis detection assay, TUNEL assay, immunofluorescence, western blot, and real-time quantitative polymerase chain reaction assay showed that pretreatment with 1,25(OH)₂D₃ could alleviate H₂O₂-induced NPMSC apoptosis, including the apoptosis rate and the expression of proapoptotic-related (Caspase-3 and Bax) and antiapoptotic-related (Bcl-2) proteins. The intracellular expression of p-Akt increased after pretreatment with 1,25(OH)₂D₃. However, these protective effects of 1,25(OH)₂D₃ were significantly decreased after the PI3K/Akt pathway was inhibited by the LY294002 treatment. In vivo, X-ray, MRI, and histological analyses showed that 1,25(OH)₂D₃ treatment relieved the degree of IVDD in Sprague–Dawley rat disc puncture models. In summary, 1,25(OH)₂D₃ efficiently attenuated oxidative stress-induced NPMSC apoptosis and mitochondrial dysfunction via PI3K/Akt pathway and is a promising candidate treatment for the repair of IVDD.

Vitamin D status and response to growth hormone treatment in prepubertal children with growth hormone deficiency

PURPOSE

To analyze whether vitamin D deficiency could condition the growth response to GH therapy, as well as to analyze if GH treatment modifies both seasonal variations and vitamin D levels in these patients.

METHODS

Retrospective study in 98 prepubertal children with GH deficiency (GHD), aged 4.1-8.9 years treated with GH. Growth rate and blood testing (calcium, phosphorus, IGF-I, 25(0H)D and PTH) were monitored at diagnostic and every six months until 24 months of treatment. A control group was recruited (247 healthy children, aged 3.8-9.7 years). The criteria of the US Endocrine Society were used for the definition of hypovitaminosis D.

RESULTS

There were no significant differences in vitamin D deficiency among control (12.5%) and GHD groups (15.3%) before starting treatment. Growth rate and IGF-1 and PTH increased (p < 0.05) during GH treatment, but there were no significant differences in calcium, phosphorus and 25(OH)D. There were no significant differences in growth rate and IGF-1, calcium and phosphorus levels in relation to the seasons along GH treatment. There was no correlation between 25(OH)D and IGF-1 during GH therapy. In every programmed control, patients with vitamin D deficiency showed lower growth rate (p < 0.05) compared to patients with vitamin D insufficiency or sufficiency.

CONCLUSION

GH treatment, at least during the first two years, does not modify the vitamin D levels. Vitamin D deficiency could condition the response to GH therapy so vitamin D monitoring should be considered as part of the routine evaluation of children with GH treatment.

Small molecule compounds promote the proliferation of chondrocytes and chondrogenic differentiation of stem cells in cartilage tissue engineering

The application of tissue engineering to generate cartilage is limited because of low proliferative ability and unstable phenotype of chondrocytes. The sources of cartilage seed cells are mainly chondrocytes and stem cells. A variety of methods have been used to obtain large numbers of chondrocytes, including increasing chondrocyte proliferation and stem cell chondrogenic differentiation via cytokines, genes, and proteins. Natural or synthetic small molecule compounds can provide a simple and effective method to promote chondrocyte proliferation, maintain a stable chondrocyte phenotype, and promote stem cell chondrogenic differentiation. Therefore, the study of small molecule compounds is of great importance for cartilage tissue engineering. Herein, we review a series of small molecule compounds and their mechanisms that can promote chondrocyte proliferation, maintain chondrocyte phenotype, or induce stem cell chondrogenesis. The studies in this field represent significant contributions to the research in cartilage tissue engineering and regenerative medicine.

Seasonal variations in vitamin D in relation to growth in short prepubertal children before and during first year growth hormone treatment

PURPOSE

This study investigated the relationship between seasonal variations in 25-hydroxyvitamin D (25(OH)D) levels and growth in prepubertal children during both the pretreatment year and the first year of GH treatment.

METHODS

The study included 249 short prepubertal children with a broad range of GH secretion, GH(max) during a 24 h profile median 23; range 1-127 mU/L, 191 boys (mean age ± SD, 8.6 ± 2.6 years), 58 girls (7.5 ± 1.9 years) receiving GH treatment (mean 43 µg/kg/day; range 17-99 µg/kg/day). Serum 25(OH)D was measured using an automated IDS-iSYS immunoassay.

RESULTS

25(OH)D levels showed seasonal variation, and decreased significantly during GH treatment. 25(OH)D levels at start and first year reduction in 25(OH)D, correlated (-) with the first year growth response during treatment. The degree of GH secretion capacity within our study population of mainly non-GH deficient children and 25(OH)D sufficient (67 ± 29 nmol/L) had no influence on 25(OH)D levels. Growth during GH treatment were independent of seasonal variations in 25(OH)D. Multiple regression analysis showed that 25(OH)D levels at treatment start, together with auxological data and IGF-binding protein-(3)SDS, explained 61 % of the variation in first year gain in heightSDS.

CONCLUSION

25(OH)D levels were associated with first year growth response to GH and may be a useful contribution to future growth prediction models.

Relationship between vitamin D receptor gene (VDR) polymorphisms, vitamin D status, osteoarthritis and intervertebral disc degeneration

The vitamin D endocrine system is involved in bony and cartilaginous metabolisms and alterations in the homeostasis of this system could be associated to pathological conditions of cartilaginous tissue. In this context, the presence of polymorphisms in the vitamin D receptor gene (VDR), in association with the susceptibility to common osteochondral diseases, was largely investigated. The aim of this review was to summarize data present in literature, analyzing the association of the VDR polymorphisms, vitamin D status and knee cartilage and intervertebral disc pathologies, trying to suggest links between the different specific pathologies analyzed. Concerning the association between VDR polymorphisms and cartilaginous tissue diseases, we found controversial reports. However, the great majority of papers reported an association with lumbar disc degeneration, whereas about half of the studies found an association with osteoarthritis. A further association between VDR polymorphisms (in linkage disequilibrium) and the presence of specific characteristics of these diseases, in particular the formation of osteophytes, was evidenced. Finally, the influence of vitamin D status on these pathologies was evaluated, trying to evidence the relation between the presence of particular genetic variants in the VDR and vitamin D levels or to show whether a particular vitamin D status could predispose to the development or progression of such diseases, however, no significant associations were found. In the future, given the role of vitamin D system in the cartilaginous tissue metabolism, it could be interesting to perform functional and tissue specific studies to analyze the interplay between the different VDR variants and its ligand.

Metabolic effects of vitamin D active metabolites in monolayer and micromass cultures of nucleus pulposus and annulus fibrosus cells isolated from human intervertebral disc

Intragenic polymorphisms in the vitamin D receptor gene are linked to disc degeneration features, suggesting that alterations in the vitamer-mediated signalling could be involved in the pathophysiology of the disc and that interaction of disc cells with vitamin D metabolites may be critical for disc health. The vitamer-mediated modulation of disc cells proliferation, metabolic activity, extracellular matrix (ECM) genes expression and proteins production was investigated. It was stated that disc cells express vitamin D receptor and are very sensitive to metabolic stimuli. In monolayer cultures, 1,25(OH)(2)D(3), but not 24,25(OH)(2)D(3), determined an inhibition of the proliferation and regulated also the ECM genes expression in nucleus pulposus and annulus fibrosus cells. Micromass cultures induced a more physiologic expression pattern of extracellular matrix genes. Cells Treatment with vitamin D metabolites did not result in relevant modifications of glycosaminoglycans production, except for annulus cells, whose production was reduced after 1,25(OH)(2)D(3) treatment. Moreover, a reduced glycosaminoglycans staining in both cell types and a significant reduced aggrecan gene expression in annulus cells treated with 1,25(OH)(2)D(3) were observed. A reduction of collagen I and II staining in annulus cells 1,25(OH)(2)D(3) treated, in accordance with a downregulation of collagen genes expression, was also registered. Finally, the vitamin D receptor gene expression did not show significant metabolite-mediated modification in monolayer or micromass cultures. These findings could enhance new insights on the biochemical mechanisms regulated by vitamin D in disc cartilage and possibly involved in the development of physiological/pathological modifications of the disc.

Vitamin D deficiency is associated with short stature and may influence blood pressure control in paediatric renal transplant recipients

Vitamin D deficiency is common in adult renal transplant recipients, but data in children are scarce. Vitamin D is shown to have multiple effects on the cardiovascular system, renal function, and maintenance of bone health. We hypothesized that 25(OH)D deficiency is common in pediatric renal transplant recipients, and may be associated with hyperparathyroidism, short stature, renal function, and blood pressure control. We recruited 106 children during the winter/spring season who had a functioning renal transplant for at least 3 months. Twenty-five hydroxyvitamin D [25(OH)D] and 1,25-dihydroxyvitamin D [1,25(OH)(2)D] were measured and correlated with clinical and biochemical parameters. Of the renal transplant patients, 38% were 25(OH)D deficient, 54% had insufficient levels, and only 8% had adequate 25(OH)D levels. Despite alfacalcidol supplementation in 59 (56%) patients, parathyroid hormone was increased in 58 (55%) and showed an inverse correlation with 25(OH)D (p = 0.0003, r = 0.61) but not with 1,25(OH)(2)D levels. Height standard deviation score (SDS) correlated with 25(OH)D (p = 0.007, r = 0.42) and time post transplantation (p = 0.02, r = 0.23); both were significant and independent predictors of height SDS. 25(OH)D inversely correlated with systolic BP SDS (p = 0.02, r =-0.26); this association was lost on multiple regression analysis, but 25(OH)D was the only modifiable risk factor for hypertension. There was no correlation with estimated GFR or proteinuria. In conclusion, 25(OH)D deficiency is common in pediatric renal transplant recipients and correlates with hyperparathyroidism and short stature. 25(OH)D deficiency may be a modifiable risk factor for hypertension in transplant recipients. Further studies are required to test if routine supplementation with ergo or cholecalciferol is safe and effective in children after renal transplantation.

1,25(OH)2-vitamin D3 inhibits proliferation and decreases production of monocyte chemoattractant protein-1, thrombopoietin, VEGF, and angiogenin by human annulus cells in vitro

STUDY DESIGN

Human lumbar anulus tissue and cultured human lumbar anulus cells were used in retrospective studies of the immunocytochemical localization of the vitamin D receptor (VDR) in disc tissue, and of the in vitro effects of the active metabolite of vitamin D, 1,25(OH)2D3, on anulus cell proliferation, cytokine, and proteoglycan (PG) production. 24,25-D3 was also analyzed. Studies were approved by the authors' Human Subjects Institutional Review Board. Discs were obtained from surgical specimens and from control donors.

OBJECTIVES

To determine if human anulus cells express the VDR in vivo, and to test the effect of in vitro exposure to 1,25(OH)2D3 and 24,25-D3 on anulus cell proteoglycan and cytokine production in 3-dimensional culture.

SUMMARY OF BACKGROUND DATA

Intragenic polymorphisms in the VDR gene have been associated with disc degeneration. 1,25(OH)2D3 has well-recognized effects on calcium homeostasis and bone mineralization, and is a negative growth regulator of a variety of normal and tumor cells. Its effects on human disc cells, however, are unexplored.

METHODS

Immunocytochemistry was performed on human lumbar disc anulus tissue from 19 subjects; human disc cells were cultured to test the effect of 1,25(OH)2D3 on proliferation of anulus cells from 5 subjects. A paired experimental design was used to determine proteoglycan production in control or 1,25(OH)2D3-treated cells, or in control or 24,25-D3-treated cells using the dimethylmethylene blue assay. A paired experimental design was also used to identify differences in cytokine production in conditioned media from control or 1,25(OH)2D3-treated cells, or in control or 24,25-D3-treated cells using ELISA assays.

RESULTS

Immunocytochemistry documented expression of the VDR in anulus cells. Young donor discs (aged newborn, 15 years) showed positive localization in all cells of the outer anulus, and some inner anulus cells. In adults (mean age, 38.9 years), some, but not all anulus cells, showed positive localization. Exposure to 10M 1,25(OH)2D3 in monolayer significantly reduced cell proliferation in vitro (P = 0.03). PG production in 3-dimensional was unchanged from control in both 1,25(OH)2D3- and 24,25-D3-treated cells. Cytokine production differed, however. 1,25(OH)2D3-treated cells showed significantly decreased production of vascular endothelial growth factor (VEGF) (P = 0.01), monocyte chemoattractant protein-1 (MCP-1) (P = 0.0006), angiogenin (P = 0.002), and thrombopoietin (P = 0.03) compared with controls. 24,25-D3-treated cells showed significantly elevated vascular endothelial growth factor-D (P = 0.01), beta-fibroblast growth factor (0.03), and significantly decreased interleukin-8, interferon-gamma, leptin, MCP-1, and TIMP-2 (tissue inhibitor of metalloproteinases-2) compared with controls (P <or= 0.01).

CONCLUSION

Data suggest that 1,25(OH)2D3 and 24,25-D3 may play roles as regulators of cell proliferation and production of specific cytokines in the lumbar anulus.

Growth of prepubertal children on dialysis

Growth failure is a common and significant clinical problem for children on dialysis and often remains a major impediment to their rehabilitation. Early referral to a paediatric nephrology centre and appropriate management before the initiation of dialysis may significantly prevent growth deterioration. Growth in children on dialysis can be affected by nutritional, metabolic, and hormonal changes. Early diagnosis of malnutrition and aggressive management should be a priority. Gastrostomy feeding should be used when adequate oral intake to maintain normal height and weight velocity cannot be achieved. Active vitamin D metabolites should be used carefully, to prevent low-turnover bone disease. All children should have an adequate regimen of dialysis and an appropriate management of malnutrition, renal osteodystrophy, metabolic acidosis, salt wasting and anaemia, before recombinant human growth hormone (rhGH) administration is considered. The current challenge of reversing growth impairment in children on dialysis can only be achieved by optimization of their care.

Bone growth during daily or intermittent calcitriol treatment during renal failure with advanced secondary hyperparathyroidism

Calcitriol is a standard therapy for secondary hyperparathyroidism in chronic renal failure. We evaluated whether the effect of daily or intermittent calcitriol administration is more efficient in enhancing bone growth in renal failure with advanced secondary hyperparathyroidism in weanling 5/6 nephrectomized rats loaded with phosphorus to induce severe secondary hyperparathyroidism. The animals were treated daily or three times weekly with calcitriol for 4 weeks but the total weekly dose of calcitriol was the same. Although calcitriol increased the serum calcium, it did not lower parathyroid hormone (PTH) or improve tibia and body length. Animals with renal failure and advanced secondary hyperparathyroidism had decreased PTH/PTHrP, which was accompanied by an increase in the cyclin kinase inhibitor p57(Kip2). Calcitriol treatment upregulated the PTH/PTHrP receptor but also increased inhibitors of cell proliferation such as p21(Waf1/Cip1), IGFBP3, and FGFR3. Calcitriol also enhanced markers of chondrocyte differentiation, such as IGF1, Vitamin D receptor, FGF23, and bone morphogenetic protein-7. Receptor activator of nuclear factor-kappabeta ligand levels improved with calcitriol treatment but without changes in osteoprotegerin suggesting an enhancement of osteo/chondroclastogenesis and mineralization. Overall, both daily and intermittent calcitriol had similar effects on endochondral bone growth in phosphorus-loaded rats with renal failure.

Balanced calcitriol treatment to make children grow

Short stature is an important clinical problem in children with chronic kidney disease. Calcitriol is used as standard therapy to control secondary hyperparathyroidism, but its effect on linear growth remains controversial. Sanchez and He report multiple effects of calcitriol on chondrocyte proliferation and maturation that might help to clarify this controversy.

Insulin-like growth factor (IGF)-I stimulates cell proliferation and induces IGF binding protein (IGFBP)-3 and IGFBP-5 gene expression in cultured growth plate chondrocytes via distinct signaling pathways

The bioactivity of IGF-I in the cellular microenvironment is modulated by both inhibitory and stimulatory IGF binding proteins (IGFBPs), whose production is partially under control of IGF-I. However, little is known on the IGF-mediated regulation of these IGFBPs in the growth plate. We therefore studied the effect of IGF-I on IGFBP synthesis and the involved intracellular signaling pathways in two cell culture models of rat growth plate chondrocytes. In growth plate chondrocytes in primary culture, incubation with IGF-I increased the concentrations of IGFBP-3 and IGFBP-5 in conditioned cell culture medium in a dose- and time-dependent manner. Coincubation of IGF-I with specific inhibitors of the p42/44 MAPK pathway (PD098059 or U0126) completely abolished the stimulatory effect of IGF-I on IGFBP-3 mRNA expression but did not affect increased IGFBP-5 mRNA levels. In contrast, inhibition of the phosphatidylinositol-3 kinase signaling pathway by LY294002 abrogated both IGF-I-stimulated IGFBP-3 and -5 mRNA expression. Comparable results regarding IGFBP-5 were obtained in the mesenchymal chondrogenic cell line RCJ3.1C5.18, which does not express IGFBP-3. The IGF-I-induced IGFBP-5 gene expression required de novo mRNA transcription and de novo protein synthesis. These data suggest that IGF-I modulates its activity in cultured rat growth plate chondrocytes by the synthesis of both inhibitory (IGFBP-3) and stimulatory (IGFBP-5) binding proteins. The finding that IGF-I uses different and only partially overlapping intracellular signaling pathways for the regulation of two IGFBPs with opposing biological functions might be important for the modulation of IGF bioactivity in the cellular microenvironment.

Morphometric changes in the epiphyseal plate of the growing and young adult male rat after long-term salmon calcitonin administration

The function of the epiphyseal plate is related to the differentiation and maturation of the chondrocytes, especially of the hypertrophic zone. Salmon calcitonin exerts a positive effect on chondrocytes of different types of cartilage, e.g., articular cartilage, osteochondral callus formation, and the epiphyseal plate. In the present study, the effect of long-term daily salmon calcitonin treatment upon epiphyseal plate function was examined in 80 male Wistar rats aged 12 weeks at the beginning of the experiment. A daily dose of 6 IU of salmon calcitonin enhanced the number of the chondrocytes of the hypertrophic zone of the upper tibial epiphyseal plate, increased the mean thickness of the epiphyseal plate, and accelerated the longitudinal growth of long bones. It was found that the peripheral growth of the epiphyseal plate was delayed after calcitonin treatment in comparison with the placebo-treated animals. The most effective period for calcitonin treatment on epiphyseal plate function seems to be the late accelerated period of growth, i.e., puberty. In conclusion, long-term salmon calcitonin treatment has a beneficial effect on longitudinal skeletal growth and this effect remains throughout the adult life of the animal. Salmon calcitonin does not enlarge the surface of the epiphyseal plate.

The VDR, COL1A1, PTH, and PTHR1 gene polymorphisms are not associated with bone size and height in Chinese nuclear families

We tested the relationship of the ApaI, Eco31I, BstBI, and (AAAG)n polymorphisms in the vitamin D receptor (VDR), collagen type I alpha-1 (COL1A1), parathyroid hormone (PTH), and parathyroid hormone (PTH)/PTH-related peptide receptor (PTHR1) genes with variations in bone size (BS) and height. Population stratification, total-family association, and within-family association were used to test these relationships in 400 Chinese nuclear families with a total of 1256 individuals. The BS at hip and spine was measured using a Hologic QDR 2000 dual-energy X-ray absorptiometry (DXA) scanner. The minor allele frequencies were 29.2%, 36.0%, and 14.0% for the VDR-ApaI, COL1A1-Eco31I, and PTH-BstBI markers, respectively. (AAAG)5 and (AAAG)6 of the PTHR1 gene are two major alleles in the Chinese people. Significant population stratification was found between the spine BS and PTHR1-(AAAG)5 (P = 0.048) and PTHR1-(AAAG)6 (P = 0.023), as well as between PTHR1-(AAAG)5 and height (P = 0.048), but we did not detect any significant within-family association or total-family association between the VDR, COL1A1, PTH, and PTHR1 gene polymorphisms and the variations in BS and height in our sample. Our results do not support that the VDR, COL1A1, PTH, and PTHR1 genes have an important influence on the variation in BS and height in our Chinese population.

Cyclic GMP-dependent protein kinase II is a molecular switch from proliferation to hypertrophic differentiation of chondrocytes

The Komeda miniature rat Ishikawa (KMI) is a naturally occurring mutant caused by an autosomal recessive mutation mri, which exhibits longitudinal growth retardation. Here we identified the mri mutation as a deletion in the rat gene encoding cGMP-dependent protein kinase type II (cGKII). KMIs showed an expanded growth plate and impaired bone healing with abnormal accumulation of postmitotic but nonhypertrophic chondrocytes. Ex vivo culture of KMI chondrocytes reproduced the differentiation impairment, which was restored by introducing the adenovirus-mediated cGKII gene. The expression of Sox9, an inhibitory regulator of hypertrophic differentiation, persisted in the nuclei of postmitotic chondrocytes of the KMI growth plate. Transfection experiments in culture systems revealed that cGKII attenuated the Sox9 functions to induce the chondrogenic differentiation and to inhibit the hypertrophic differentiation of chondrocytes. This attenuation of Sox9 was due to the cGKII inhibition of nuclear entry of Sox9. The impaired differentiation of cultured KMI chondrocytes was restored by the silencing of Sox9 through RNA interference. Hence, the present study for the first time shed light on a novel role of cGKII as a molecular switch, coupling the cessation of proliferation and the start of hypertrophic differentiation of chondrocytes through attenuation of Sox9 function.

Growth factor regulation of human growth plate chondrocyte proliferation in vitro

Linear growth occurs as the result of growth plate chondrocytes undergoing proliferative and hypertrophic phases. Paracrine feedback loops that regulate the entry of chondrocytes into the hypertrophic phase have been shown and similar pathways likely exist for the proliferative phase. Human long-bone growth plate chondrocytes were cultured in vitro. The proliferative effects of a variety of factors were determined by [3H]thymidine uptake and the gene expression profile of these cells was determined by DNA microarray analysis. Serum, insulin-like growth factor (IGF)-I and -II, transforming growth factor-beta (TGF-beta, fibroblast growth factor (FGF)-1, -2, and -18, and platelet-derived growth factor (PDGF)-BB were potent stimulators of proliferation. FGF-10, testosterone, and bone morphogenetic proteins (BMP)-2, -4, and -6 inhibited proliferation. Microarray analysis showed that the genes for multiple members of the IGF-I, TGF-beta, FGF, and BMP pathways were expressed, suggesting the presence of autocrine/paracrine pathways that regulate the proliferative phase of growth plate-mediated growth.

Rat growth plate chondrocytes express low levels of 25-hydroxy-1alpha-hydroxylase

Long standing disturbances of Vitamin D-metabolism as well as null-mutant animals for 25-hydroxy-1alpha-hydroxylase results in disorganised growth plates. Cultured chondrocytes were shown to be target for the hydroxylated Vitamin D-metabolites 1alpha,25(OH)(2)D(3) and 24,25(OH)(2)D(3). Because studies on production of these metabolites were inconclusive in in vitro systems, the expression of the Vitamin D-system was examined in rat growth plate chondrocytes in vitro as well as ex vivo. Gene expression for 25-hydroxy-1alpha-hydroxylase, 25-hydroxy-24-hydroxylase as well as Vitamin D-receptor and collagen II and X were analysed on mRNA level by RT-PCR and quantitative real-time PCR, on protein level by western blotting and by immunohistochemistry in isolated growth plate chondrocytes or intact growth plates. Compared to UMR or CaCo(2) cells and renal homogenates cultured growth plate chondrocytes expressed low levels of 25-hydroxy-1alpha-hydroxylase mRNA and 25-hydroxy-24-hydroxylase mRNA. The expression of both was modulated by 25(OH)D(3), but 1alpha,25(OH)(2)D(3) affected only 25-hydroxy-24-hydroxylase. These data were confirmed by Western blotting. Immunohistochemistry demonstrated predominant staining for 25-hydroxy-1alpha-hydroxylase in chondrocyte nodules and cells embedded in matrix in vitro. Ex vivo, 25-hydroxy-1alpha-hydroxylase was detected predominantly in late proliferative and hypertrophic zone of the growth plate. In conclusion, growth plate chondrocytes express the key components for a paracrine/autocrine Vitamin D-system.

Ligand structure-function relationships in the vitamin D endocrine system from the perspective of drug development (including cancer treatment)

It has become readily apparent to many scientists and pharmaceutical companies that the vitamin D endocrine system offers a wide array of drug development opportunities. There are already successes, as noted by 1alpha,25(OH)2D3 (Roche, and Abbott) for renal osteodystrophy and osteoporosis and 1alpha(OH)D3 (Leo, Chugai, Teijin) for renal osteodystrophy and (in Japan) osteoporosis, 1alpha,24(OH)2-24-cyclopropyl-D3 (Dovonex) and 1alpha,24(OH)2D3 (Teijin) for psoriasis, and 19-nor-1alpha,25(OH)2D2 (Abbott) for renal osteodystrophy, as well as drugs under active development. Yet there are still many important and challenging drug development frontiers, particularly in the area of cancer treatment and immune system disorders where exploration is only in the initial early stages. In addition, the application of vitamin D-related drugs in neurology and brain pathology should not be overlooked. It is to be hoped that the cellular and molecular basis for the vexing problem of analog-induced hypercalcemia will be elucidated. Given that there are believed to be over 2000 analogs of 1alpha,25(OH)2D3 already available for consideration, it is to be expected that over the next decade a significant number of new vitamin D structure-function drug development projects will be brought to conclusion.

Cultured rat growth plate chondrocytes express low levels of 1alpha-hydroxylase

Growth plate chondrocytes are the target of the hydroxylated vitamin D metabolites 1alpha,25(OH)2D3 and 24,25(OH)2D3. Because studies on the production of these polar metabolites were inconclusive in various in vitro systems, the expression of a potential paracrine/autocrine vitamin D system was examined in primary cultures of rat growth plate chondrocytes using real-time RT-PCR. Compared to UMR cells and renal homogenates primary cultures of growth plate chondrocytes expressed low levels of 25-hydroxy-1alpha-hydroxylase as well as 25-hydroxy-24-hydroxylase. The expression of both is modulated by 25 vitamin D3, but 1alpha,25(OH)2D3 affected only 25-hydroxy-24-hydroxylase. If these findings are confirmed in intact growth plates, the polar vitamin D metabolites could act in a paracrine/autocrine fashion within the growth plate.

Intact IGF-binding protein-4 and -5 and their respective fragments isolated from chronic renal failure serum differentially modulate IGF-I actions in cultured growth plate chondrocytes

Impairment of longitudinal growth among children with chronic renal failure (CRF) may be partly attributable to the inhibition of insulin-like growth factor (IGF) activity by an excess amount of high-affinity IGF-binding proteins (IGFBP). Elevated levels of immunoreactive IGFBP-4 in CRF serum are inversely correlated with the standardized heights of these children, whereas levels of IGFBP-5, which circulates mainly as proteolyzed fragments, are positively correlated with growth parameters. To delineate the respective effects of these IGFBP on growth cartilage, the biologic effects of intact and fragmented forms of IGFBP-4 and IGFBP-5 on rat growth plate chondrocytes in primary cultures were characterized. Intact IGFBP-4 and IGFBP-5 and the amino-terminal fragment IGFBP-5(1-169) were recombinant proteins; the carboxy-terminal fragments IGFBP-5(144-252) and IGFBP-4(136-237) and the amino-terminal fragment IGFBP-4(1-122) were purified to homogeneity from CRF hemofiltrates. Intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) inhibited IGF-I-induced cell proliferation. In contrast, intact IGFBP-5 was stimulatory in the absence or presence of exogenous IGF-I, whereas the amino-terminal fragment IGFBP-5(1-169) was inhibitory. Studies on the mechanism by which IGFBP-4 and IGFBP-5 exert opposite effects on chondrocyte proliferation demonstrated that intact IGFBP-4 prevented the binding of (125)I-IGF-I to chondrocytes, whereas intact IGFBP-5 enhanced ligand binding and was able to bind specifically to the cell membrane. These data suggest that intact IGFBP-4 and, to a lesser extent, IGFBP-4(1-122) act exclusively as growth-inhibitory binding proteins in the growth cartilage. IGFBP-5, however, can either stimulate (if it remains intact) or inhibit (if amino-terminal forms predominate) IGF-I-stimulated chondrocyte proliferation.

Management of disturbed calcium metabolism in uraemic patients: 1. Use of vitamin D metabolites

Chronic renal failure is characterized by diminished synthesis of, and resistance to, the active vitamin D metabolite 1,25-dihydroxy-vitamin D3 (1,25(OH)2D3, calcitriol). Calcitriol results from the biotransformation of the precursor 25-hydroxy-vitamin D3 (25(OH)D3) to 1,25(OH)2D3. 25(OH)D3 is synthesized in the liver, and 1alpha-hydroxylase, the rate-limiting enzyme for its biotransformation into the most active metabolite, 1,25(OH)2D3, is located in the kidney. The regulation of 1alpha-hydroxylase in renal failure is not well known. Recent work indicates that, in contrast to previous opinion, 1alpha-hydroxylase is predominantly expressed not in the proximal tubule but in the distal tubule [1]. In vivo, the main stimulatory signal is presumably parathyroid hormone (PTH) and the main inhibitory signal hyperphosphataemia. Both signals are altered in renal failure. There is also evidence that the renal 1alpha-hydroxylase becomes substrate-dependent in patients with renal failure. This means that a higher concentration of the precursor 25(OH)2D3 will result in a higher rate of transformation into the active metabolite 1,25(OH)2D3 in renal patients. Calcitriol is not exclusively synthesized in the kidney, but may also be synthesized in extra-renal tissues, e.g. activated monocytes/macrophages [2], particularly in granuloma [3] as shown by anephric uraemic patients who develop hypercalcaemia and elevated calcitriol concentrations when sarcoidosis [4] or tuberculosis [5] supervenes. On the other hand, calcitriol is less effective in uraemia. This may be to some extent due to diminished expression of vitamin D receptors [6], particularly in parathyroid glands when they undergo nodular transformation [7], but there may also be resistance to calcitriol at the post-receptor level [8]. In a series of elegant experiments [9,10], calcitriol resistance has been related to disturbed genomic effects of active vitamin D because the interaction of the vitamin D receptor ligand complex with vitamin D-responsive elements (VDREs) upstream of vitamin D-regulated genes was disturbed by the action of low molecular weight substances in uraemia, which have not been completely characterized. The role of genetically determined polymorphisms of the vitamin D receptor in the genesis of disturbed calcium metabolism of renal failure is currently unclear.

Both retinoic acid and 1,25(OH)2 vitamin D3 inhibit thyroid hormone-induced terminal differentiaton of growth plate chondrocytes

Thyroid hormone has been known for over 50 years to be a potent regulator of skeletal maturation at the growth plate. The receptor for thyroid hormone has been discovered to be a member of the nuclear hormone receptor superfamily. Retinoic acid and 1,25(OH)2 vitamin D3, whose receptors also belong to this nuclear hormone receptor family, have been implicated in the control of chondrocyte proliferation and differentiation at the growth plate. Recent studies demonstrate that the receptors for thyroid hormone, retinoic acid, and vitamin D bind to a similar DNA response element in the promoter region of target genes and may form heterodimers to regulate gene transcription in target cells. These observations led us to hypothesize that the retinoic acid and/or vitamin D signaling pathways may interact with thyroid hormone signaling at the molecular level to modulate growth plate chondrocyte differentiation. Using a chemically defined, serum-free model of growth plate chondrocyte maturation, both all-trans retinoic acid and 1,25(OH)2 vitamin D3 markedly inhibited thyroid hormone-induced terminal differentiation in a dose-dependent manner. In the absence of thyroid hormone, retinoic acid stimulated alkaline phosphatase activity modestly at the highest dose used, however neither retinoic acid nor 1,25(OH)2 vitamin D3 induced expression of type X collagen mRNA. We conclude that retinoic acid and vitamin D are likely to be antagonists of thyroid hormone signaling in the growth plate.

Growth retardation in children with chronic renal failure

Growth retardation is a major obstacle to full rehabilitation of children with chronic renal failure (CRF). Several factors have been identified as contributors to impaired linear growth and they include protein and calorie malnutrition, metabolic acidosis, growth hormone resistance, anemia, and renal osteodystrophy. Although therapeutic interventions such as the use of recombinant human growth hormone, recombinant human erythropoietin, and calcitriol have made substantial contributions, the optimal therapeutic strategy remains to be defined. Indeed, growth failure persists in a substantial proportion of children with renal failure and those treated with maintenance dialysis. In addition, the increasing prevalence of adynamic lesions of renal osteodystrophy and its effect on growth have raised concern about the continued generalized use of calcitriol in children with CRF. Recent studies have shown the critical roles of parathyroid hormone-related protein (PTHrP) and the PTH/PTHrP receptor in the regulation of endochondral bone formation. The PTH/PTHrP receptor mRNA expression has been shown to be down-regulated in kidney and growth plate cartilage of animals with renal failure. Differences in the severity of secondary hyperparathyroidism influence not only growth plate morphology but also the expression of selected markers of chondrocyte proliferation and differentiation in these animals. Such findings suggest potential molecular mechanisms by which cartilage and bone development may be disrupted in children with CRF, thereby contributing to diminished linear growth.

Growth plate cartilage formation and resorption are differentially depressed in growth retarded uremic rats

To characterize the modifications of growth plate in individuals with growth impairment secondary to chronic renal failure, young rats were made uremic by subtotal nephrectomy (NX) and, after 14 d, their tibial growth plates were studied and compared with those of sham-operated rats fed ad libitum (SAL) or pair-fed with NX (SPF). NX rats were growth retarded and severely uremic. Growth plate height (mean +/- SD) was much greater (P<0.05) in NX (868.4+/-85.4 microm) than SAL (570.1+/-93.5 microm) and SPF (551.9+/-99.7 microm) rats as a result of a higher (P<0.05) hypertrophic zone (661.0+/-89.7 versus 362.8+/-71.6 and 353.0+/-93.9 microm, respectively). The increased size of the growth plate was associated with a greater number of chondrocytes and modifications in their structure, particularly in the hypertrophic zone adjacent to bone. In this zone, chondrocytes of NX animals were significantly (P<0.05) smaller (12080.4+/-1158.3 microm3) and shorter (34.1+/-2.5 microm) than those of SAL (16302.8+/-1483.4 microm3 and 37.8+/-2.0 microm) and SPF (14465.8+/-1521.0 microm3 and 36.3+/-1.8 microm). The interface between the growth plate cartilage and the metaphyseal bone appeared markedly irregular in NX rats. Kinetics of chondrocytes was also modified (P<0.05) in the NX rats, which had lower cell turnover per column per day (5.4+/-0.9), longer duration of hypertrophic phase (89.0+/-15.2 h), and reduced cellular advance velocity (7.4+/-2.2 microm/h) compared with SAL (8.0+/-1.6, 32.1+/-6.7 h, and 11.3+/-2.7 microm/h) and SPF (7.2+/-1.1, 34.8+/-5.1 h, and 10.1+/-2.5 microm/h). Cell proliferation was no different among the three groups. Because the growth plates of SPF and SAL rats were substantially not different, modifications observed in the NX rats cannot be attributed to the nutritional deficit associated with renal failure. These findings indicate that chronic renal failure depresses both the activity of the growth plate cartilage by altering chondrocyte hypertrophy and the replacement of cartilage by bone at the metaphyseal end. The two processes are differentially depressed since cartilage resorption is more severely lowered than cartilage enlargement and this leads to an accumulation of cartilage at the hypertrophic zone.

Growth of long bones in renal failure: roles of hyperparathyroidism, growth hormone and calcitriol

BACKGROUND

The treatment of secondary hyperparathyroidism (2 degrees HPT) associated with chronic renal failure adversely affects skeletal growth.

METHODS

We assessed epiphyseal growth plate morphology by quantitative histology and measured mRNA levels for selected markers of chondrocyte proliferation and differentiation by in situ hybridization in the growth plate cartilage of subtotally nephrectomized rats with either mild or advanced 2 degrees HPT.

RESULTS

The width of the growth plate cartilage in the proximal tibia and mRNA levels for PTH/PTHrP receptor were unchanged in rats with mild 2 degrees HPT, however, they were markedly less in nephrectomized rats with advanced 2 degrees HPT than in intact controls. Treatment with growth hormone 10 IU/kg/day increased growth plate thickness both in mild and in advanced 2 degrees HPT and raised mRNA levels for type II and type X collagen in rats with advanced 2 degrees HPT. The administration of calcitriol 50 ng/kg/day attenuated these responses in animals with advanced 2 degrees HPT. Overall, PTH/PTHrP receptor mRNA levels did not correspond to the serum levels of PTH indicating that PTH/PTHrP receptor expression is down-regulated in renal failure by a PTH-independent mechanism.

CONCLUSION

Calcitriol counteracts the trophic actions of growth hormone on epiphyseal growth plate cartilage and modifies chondrocyte differentiation in vivo, and these mechanisms may contribute to disturbances in longitudinal bone growth in renal failure.

Difference in height associated with a translation start site polymorphism in the vitamin D receptor gene

The function of 1,25-dihydroxyvitamin D3 in bone development is to regulate the differentiation or proliferation of osteoblastic, osteoclastic, and chondrocytic lineage, affecting bone mineralization and linear bone growth. An A(T/C)G substitution exists in the first of the two putative translation initiation sites in exon 2 of the vitamin D receptor (VDR) gene. We studied the relationship between exon 2 polymorphism and height in 90 healthy Japanese female subjects aged 18-20 y, who had attained final height, in 159 healthy Japanese aged 13 y, and 24 children with constitutional short stature aged 6-10 y less than -1.5 SD in height, mostly with parents of normal height. Exon 2 polymorphism was analyzed by PCR-single strand conformation polymorphism, PCR-direct sequencing, and PCR-restriction fragment length polymorphism. The frequency of the exon 2 polymorphism was genotype CC (ACG/ACG), 0.37; genotype CT (ACG/ATG), 0.51; and genotype TT (ATG/ATG), 0.12 in 249 normal subjects. The mean height of female subjects aged 18-20 y with the genotype CT was 160.3 +/- 4.3 cm (mean +/- SD), whereas that with CC was 4.4 cm lower (p < 0.0001), and that with TT was 2.7 cm lower (p = 0.0302). At age 13 y, this trend was observed, and the mean height SD score of subjects with the genotype CT was taller than homozygotes (CC/CT; p = 0.0363 and CT/TT; p = 0.0208) in 159 subjects. Genotype CT in 24 children with constitutional short stature was less frequent than other genotypes (CC, 0.62; CT, 0.21; TT. 0.17) (chi 2 p value, 0.0171). In conclusion, the exon 2 polymorphism affecting the VDR mRNA and protein is one of the major determinants of adult height at least in female Japanese, which was noted at the age of 13 y examined in both sexes.

Dexamethasone impairs growth hormone (GH)-stimulated growth by suppression of local insulin-like growth factor (IGF)-I production and expression of GH- and IGF-I-receptor in cultured rat chondrocytes

Growth depression as a side effect of glucocorticoid therapy in childhood is partially mediated by alterations of the somatotropic hormone axis. The mechanisms of interaction between glucocorticoids and somatotropic hormones on the cellular and molecular level are poorly understood. In an experimental model of primary cultured rat growth plate chondrocytes, basal as well as GH (40 ng/ml) or insulin-like growth factor (IGF)-I (60 ng/ml)-stimulated growth was suppressed dose dependently (10(-l2)-10(-7)M) by dexamethasone (Dexa). An IGF-I antibody specifically and dose dependently inhibited the GH- but not the basic fibroblast growth factor (bFGF)-stimulated cell proliferation. GH increased the IGF-I concentration in conditioned serum-free culture medium; this was reversed by concomitant Dexa. Dexa time dependently suppressed the transcription of GH receptor (GHR) messenger RNA (mRNA) and down-regulated the basal and GH-stimulated expression of GHR. Whereas no suppressive effect on basal type I IGF-receptor (IGFR) was observed, Dexa blocked the IGF-I induced increase of IGF binding. These results were confirmed by GHR and IGFR immunostaining. We conclude that Dexa impairs the GH-induced stimulation of local secretion and paracrine action of IGF-I and reduces the homologous increase of IGFR and GHR expression. The above experiments give further insight on the interaction between GH and glucocorticoids on the cellular and molecular level of growth plate chondrocytes.

Effect of 1,25 (OH)2 vitamin D3 on glomerulosclerosis in subtotally nephrectomized rats

In the past, there has been considerable concern that treatment with active vitamin D might accelerate progression independent of hypercalcemia and hypercalcuria. Nevertheless, 1,25(OH)2D3 has known antiproliferative properties and has also been shown to inhibit renal growth. Since glomerular growth is a permissive factor for the development of glomerulosclerosis, we reasoned that 1,25(OH)2D3 might even attenuate progression. To test this working hypothesis we performed two experiments of 8 and 16 weeks duration, respectively, to compare subtotally nephrectomized (SNX) rats treated with ethanol and SNX treated with 1,25(OH)2D3. Control animals were sham operated and pair-fed with SNX animals. 1,25(OH)2D3 (3 ng/100 g body wt/day) was administered by osmotic minipump. 1,25(OH)2D3 had no significant effect on systolic blood pressure and only a transient effect on weight gain. SNX reduced the number of glomeruli (left kidney) from an average of 3.3 x 10(4) to 1.2 x 10(4) per kidney. Mean glomerular volume was 3.87 +/- 0.71 x 10(6) microns 3 in sham operated animals and significantly (P < 0.05) higher (10.1 +/- 1.75 x 10(6) microns 3) in untreated animals 16 weeks after SNX. Glomerular volume was significantly (P < 0.05) less in 1,25(OH)2D3 treated SNX [10.1 +/- 1.75 in ethanol vs. 7.04 +/- 1.78 in 1,25(OH)2D3 treated SNX]. In parallel, there was significantly (P < 0.01) less glomerulosclerosis [glomerulosclerosis index 1.16 +/- 0.14 in the ethanol treated SNX vs. 0.80 +/- 0.16 in SNX treated with 1,25(OH)2D3] in the eight week experiment. Albuminuria was significantly (P < 0.01) lower in 1,25(OH)2D3 treated than in ethanol treated SNX (mean 0.785 mg/24 hr, range 0.43 to 1.80, vs. 3.75 mg/24 hr, 1.29 to 14.2). The morphological data were directionally analogous in a second 16 week experiment. Only slight changes of the vascular sclerosis index and tubulointerstitial index were seen in SNX and were not affected by 1,25(OH)2D3 further. To prove that the effect of 1,25(OH)2D3 was independent of PTH, parathyreoidectomized SNX rats without or with 1,25(OH)2D3 treatment were examined seven days post-SNX. PCNA staining showed suppression of cell proliferation. Furthermore, in situ hybridization for transforming growth factor-B (TGF-beta) showed less vascular and tubular expression in 1,25(OH)2D3 treated rats. We conclude that 1,25(OH)2D3 has antiproliferative actions during the compensatory growth of nephrons in response to subtotal nephrectomy. These effects are independent of PTH. The data document that 1,25(OH)2D3 reduces renal cell proliferation and glomerular growth as well as glomerulosclerosis and albuminuria as indicators of progressive glomerular damage.

Interaction of IGF-I and 1 alpha, 25(OH)2D3 on receptor expression and growth stimulation in rat growth plate chondrocytes

Growth plate cartilage cell express receptors for, and are affected by both IGF-I and 1 alpha, 25(OH)2D3. The studies were undertaken to investigate interaction between these two hormone systems, that is, (i) to study effects of 1 alpha, 25(OH)2D3 on IGF-type 1 receptors (IGFIR), on IGF-I stimulated cell replication, colony formation, and on alkaline phosphatase activity (AP), and conversely, (ii) to study the effect of IGF-I on vitamin D receptor (VDR) expression on 1 alpha, 25(OH)2D3 stimulated growth parameters and on AP activity. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures, (serum-free) or in agarose stabilized suspension cultures (0.1% FCS). Vitamin D receptor and IGFIR were visualized by immunostaining with the monoclonal antibody (mAb) 9A7 gamma and mAb alpha IR3, respectively, and quantitated by RT-PCR for mRNA and by Scatchard analysis using [3H]-1,25(OH)2D3 and [125I]-alpha IR3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures, and by colony formation in agarose-stabilized suspension cultures. IGF-I dose-dependently increased [3H]-thymidine incorporation. 1 alpha, 25(OH)2D3, but not 1 beta, 25(OH)2D3 was stimulatory at low ((10-12 M) and slightly inhibitory at high (10-8 M) concentrations. The effect of IGF-I was additive to that of 1 alpha, 25 (OH)2D3 [IGF-I 60 ng/ml, 181 +/- 12.7; 1 alpha, 25(OH)2D3 10(-12) M, 181 +/- 9.8%, IGF-I + 1 alpha, 25(OH)2D3, 247 +/- 16.7%, P < 0.05 by ANOVA] and specifically obliterated by polyclonal IGF-I antibody (AB-1). Interaction could also be confirmed in suspension cultures. IGFIR mRNA and [125I]-alphaIR3 binding was increased by low (10(-12) m) but not by high (10(-8) M) concentrations of 1 alpha, 25(OH)2D3. Homologous up-regulation by IGF-I (60 ng/ml) was specifically inhibited by AB-1 and markedly amplified by coincubation with 1 alpha, 25(OH)2D3 (10(-12)m). Immunostaining with alpha IR3 showed specific IGFIR expression in rat growth cartilage, but not liver tissue. Stimulation of chondrocytes with 1 alpha, 25(OH)2D3 or IGF-I suggested some increase of receptor expression in single cells, but the predominant effect was increased recruitment of receptor positive cells, Vitamin D receptor expression was markedly stimulated (fourfold) by IGF-I (60 ng/ml), but not IGF-II and inhibited by actinomycin D. This study shows that IGF-I and 1 alpha, 25(OH)2D3 mutually up-regulate their respective receptors in growth plate chondrocytes. In parallel, they have additive effects on cell proliferation and colony formation suggesting independent effector pathways.

Serum 1alpha,25-dihydroxyvitamin D3 accumulates into the fracture callus during rat femoral fracture healing

1,25-dihydroxyvitamin D3 (1,25(OH)2D3) is thought to be an important systemic factor in the fracture repair process, but the mechanism of action of 1,25(OH)2D3 has not been clearly defined. In this study, the role of 1,25(OH)2D3 in the fracture repair process was analyzed in a rat closed femoral fracture model. The plasma concentration of 1,25(OH)2D3 rapidly decreased on day 3 and continued to decrease to 10 days after fracture. We assessed whether this decrease was based on the accelerated degradation or retardation of the synthesis rate of 1,25(OH)2D3, from 25(OH)D3. After radiolabeled 3H-1,25(OH)2D3 or 3H-25(OH)D3 was injected i.v. into fractured or control (unfractured) rats, the concentrations of 25(OH)D3 and 1,25(OH)2D3 metabolites were measured by HPLC. The plasma concentrations of these radiolabeled metabolites in fractured group were similar to those in control rats early after operation. However, radioactivity in the femurs of fractured rats was higher than that of the control group. Furthermore, the radioactivity was concentrated in the callus of the fractured group analyzed by autoradiography. 1,25(OH)2D3 receptor gene expression was detected early after fracture and, additionally, both in the soft and hard callus on days 7 and 13 after fracture. These results showed that the rapid disappearance of 1,25(OH)2D3 in the early stages after fracture was not due to either increased degradation or decreased synthesis of 1,25(OH)2D3, but rather to increased consumption. Further, these results suggest the possibility that plasma 1,25(OH)2D3 becomes localized in the callus and may regulate cellular events in the process of fracture healing.

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

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

Parathyroid hormone prevents 1,25 (OH)2D3 induced down-regulation of the vitamin D receptor in growth plate chondrocytes in vitro

1,25(OH)2D3 has an antiproliferative effect on growth plate chondrocytes when given in high doses, whereas low doses stimulate chondrocyte proliferation. In the present in vitro study we investigated the effects of parathyroid hormone (PTH) when given concomitantly with 1,25(OH)2D3 on cell proliferation and vitamin D receptor (VDR) regulation. Freshly isolated rat tibial chondrocytes were grown in monolayer cultures or in agarose stabilized suspension cultures (10% charcoal-treated FCS). VDR expression was determined by RT-PCR generating a 297 bp fragment and by binding assays (Scatchard analysis) with [3H]-1,25(OH)2D3. Cell proliferation was measured by [3H]-thymidine incorporation, growth curves in monolayer cultures and by colony formation in agarose-stabilized suspension cultures. Optimal concentration of 1,25(OH)2D3 (10(-12) M) and of PTH fragments [bPTH(1-34) or hPTH(28-48), 10(-10)M] showed additive effects on DNA synthesis of and colony formation by growth plate chondrocytes. This may be explained in part by an up-regulation of VDR by PTH: PTH increased both mRNA expression of VDR and binding capacity. 1,25(OH)2D3 (10(-12) M) induced an up-regulation of the VDR within 24 hours followed by a down-regulation after incubation for more than 24 hours. PTH fragments added concomitantly prevented the down-regulation seen with 1,25(OH)2D3. These findings provide evidence that PTH is a growth promoting hormone that also modulates the effects of 1,25(OH)2D3 by regulating the VDR status of 1,25(OH)2D3 target cells.

Growth plate chondrocyte vitamin D receptor number and affinity are reduced in avian tibial dyschondroplastic lesions

Tibial dyschondroplasia (TD) is a condition of rapidly growing poultry in which a mass of unmineralized cartilage extends distally from the tibiotarsal growth plate, leading to deformity and lameness. The lesion is characterized by the accumulation of prehypertrophic chondrocytes, probably because the maturing chondrocytes are unable to differentiate fully. The condition can be prevented by feeding 1,25-(OH)2D3. We have investigated the possibility that vitamin D receptors (VDR), through which 1,25-(OH)2D3 exerts its differentiating effects on chondrocytes, may be defective in TD birds. Chondrocytes were isolated from the proliferating and hypertrophic zones of normal tibiotarsi and from the proliferating zone and lesion of affected birds and receptors were characterized by Scatchard analysis. Results showed that, while cells from the proliferating zone in TD birds had normal receptors, those from the TD lesion had significantly lower numbers and affinity for 1,25-(OH)2D3 compared to all other zones. Lesion VDR had low affinity; Kd 83.9 +/- 20.6 pM compared to 30.0 +/- 2.8, 37.8 +/- 3.1, and 33.0 +/- 4.0 pM (p < 0.001), and low receptor number per cell, 920 +/- 74, compared to 1329 +/- 151, 1664 +/- 167, and 1360 +/- 104 (p < 0.01) in the normal proliferating, normal hypertrophic, and TD proliferating cells, respectively. These findings were confirmed by immunohistochemical localization of VDR in sections of normal and TD growth plates using monoclonal antibody 9A7 gamma. In normal growth plate, most cells were VDR positive with intense staining in the mature hypertrophic chondrocytes; in TD growth plates, proliferating zone cells stained well but signal was largely absent from chondrocytes in the lesion. Image analysis showed integrated nuclear staining density per cell of 168.2 +/- 36.9 arbitrary units in normal hypertrophic cartilage compared to 98.8 +/- 60.2 units in the top of the lesion and 2.2 +/- 2.0 units in the midlesion. We conclude that both numbers and affinity of VDR are reduced in TD and this may explain the failure of chondrocytes to differentiate to the mature form. The adverse consequences of defective receptors may be partly overcome by treatment with 1,25-(OH)2D3.

Single cell enzyme activity and proliferation in the growth plate: effects of growth hormone

Longitudinal growth is a result of proliferation and differentiation of chondrocytes in the growth plate. Growth hormone (GH) stimulates longitudinal growth, and GH receptors have been shown on growth plate chondrocytes, but the effects of GH on chondrocytes of different cell layers are not clear. To study the effect of GH on chondrocyte activity, in situ biochemical techniques were used to measure enzyme activities, which are associated with cell differentiation (alkaline phosphatase [ALP]) and osteoclast activity (tartrate-resistant acid phosphatase [TRAP]), within single cells of the growth plate. Uptake of bromodeoxyuridine (BrdU) was used as a parameter for proliferative activity. In addition, glucose-6-phosphate dehydrogenase (G6PD) was measured since increased proliferation has been associated with increased G6PD activity. The role of GH was studied in a model of isolated GH deficiency (dwarf rat) and complete pituitary deficiency (hypophysectomized rat). Groups of GH-deficient dwarf rats were infused with recombinant human GH in either a continuous or a pulsatile manner, since the pattern of GH secretion is an important regulator of growth in the rat. After 7 days, G6PD activity in proliferative chondrocytes and TRAP activity in osteoclasts was increased, while ALP activity in hypertrophic chondrocytes was decreased. GH not only increased the number of chondrocytes that incorporated BrdU but also the total number of chondrocytes in the proliferative zone; therefore, its ratio, the labeling index (an indicator of proliferative rate), was not increased. The widths of the proliferative and hypertrophic zones were increased by both patterns of GH administration. The width of the resting zone was unaffected by continuous GH but decreased by pulsatile GH. ALP and TRAP activities were, respectively, higher and lower in hypophysectomized rats compared with the GH-deficient animals. Hypophysectomized rats had smaller growth plates than dwarf rats with a disproportionally wide resting zone, which, like BrdU uptake, was not affected by GH. GH treatment resulted in increased TRAP and decreased ALP activity. These results indicate that GH stimulates the commitment of chondrocytes within the resting/germinal layer to a proliferative phenotype (as opposed to stimulating the rate of chondrocyte proliferation) but only in the presence of other pituitary hormones. Furthermore, this study shows that enzyme activities within single chondrocytes and osteoclasts are GH-sensitive. The extent to which these effects are direct or mediated by systemic or local growth factors remains to be clarified.

Calbindin-D28K and -D9K and 1,25(OH)2 vitamin D3 receptor immunolocalization and mineralization induction in long-term primary cultures of rat epiphyseal chondrocytes

Rat epiphyseal plat chondrocytes were grown on glass slides, as nonadhering monolayer cultures for up to 6 weeks. Chondrocyte growth, differentiation and maturation, matrix formation and mineralization, and the temporospatial distribution of the vitamin D-dependent calcium-binding proteins, calbindin-D9K and -D28K, and the 1,25(OH)2D3 receptor (VDR), were all monitored. Chondrocytes became confluent in 2.5 weeks, differentiated to acquire a chondrocyte (polygonal) morphology, produced extracellular matrix, and finally formed a true monolayer mineralizing cartilaginous tissue, with all the stages of chondrocyte development within a single culture. beta-Glycerophosphate promoted initial matrix mineralization in 4 weeks and accelerated cell differentiation. High nominal calcium and ascorbic acid were needed for abundant matrix formation. VDR occurred at all differentiation stages, in the nuclei and nucleoli and in the cytoplasm. Calbindin-D28K and -D9K were not coexpressed. Calbindin-D28K was found in prechondroblasts, chondroblasts, and in newly differentiated chondrocytes. It was cytoplasmic in prechondroblasts and subsequently also in the perinuclear region and in nuclei, suggesting migration to the nuclear chromatin. Calbindin-D28K was nuclear only in newly differentiated chondrocytes in vitro and was not found in mature chondrocytes. In contrast, calbindin-D9K was present in the cytoplasm of mature and hypertrophic chondrocytes only. It was first in the cell body and eventually migrated within and to the far end of long cell processes with a decreasing cytoplasmic concentration showed by decreased immunostaining intensity, and ultimately hypertrophy of chondrocytes in culture. These in vitro patterns of calbindins-D and VDR accurately reflect their in vivo distributions. The genomic action of vitamin D, in vitro, resulted in the synthesis of nuclear VDR and calbindins-D.(ABSTRACT TRUNCATED AT 250 WORDS)

Intermittent and continuous exposure to 1,25(OH)2D3 have different effects on growth plate chondrocytes in vitro

Intermittent 1,25(OH)2D3 administration is widely used to suppress parathyroid glands in secondary (renal) hyperparathyroidism. It is unknown whether the effects of continuous and intermittent 1,25(OH)2D3 differ on vitamin D target organs other than parathyroids. Using primary cultures of rat chondrocytes (tibia) we compared the effects of continuous versus intermittent exposure to physiologic concentrations of 1 alpha,25(OH)2D3 on proliferation (radiothymidine incorporation), cell count, protein synthesis ([3H]-leucine incorporation), alkaline phosphatase activity (as a marker of differentiation) and 1 alpha,25(OH)2D3 receptor (VDR) regulation. Cells were synchronized and then exposed for variable periods to a medium containing 10% delipidated FCS and 10(-8) M to 10(-12) M 1 alpha,25(OH)2D3 (or 1 beta,25(OH)2D3 as specificity control). Intermittent (8 hr exposure every 48 hr) as well as continuous (sham washing) administration of 1 alpha,25(OH)2D3 had a biphasic effect on proliferation, that is, stimulation at low (10(-12) M) and inhibition at high (10(-8) M) concentrations. At 10(-12) M intermittent 1 alpha,25(OH)2D3 yielded higher cell counts than continuous 1,25(OH)2D3. This was seen in the log phase, which was day 3 (continuous 141 +/- 2.3% of solvent control; intermittent 185 +/- 2.0%) and in the plateau phase of growth, which was day 6 (128 +/- 2.6 vs. 169 +/- 2.7% of solvent control). Dependence on extracellular Ca is suggested by the effects of varying nominal Ca concentrations in the medium and of Ca channel blockers. Even two hours of exposure to 1 alpha,25(OH)2D3 (10(-12) M) yielded maximal activation of AP during postincubation.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo effect of 1,25-dihydroxycholecalciferol on the proliferation and differentiation of avian chondrocytes

A combination of immunocytochemistry and in situ biochemistry has been used to determine the in vivo effects of 1,25-dihydroxycholecalciferol [1,25-(OH)2D3] on the proliferation and differentiation of chondrocytes. Chicks were fed a diet supplemented with 1,25-(OH)2D3 (2.5, 5, or 10 micrograms/kg diet) for 3 weeks, and measurements were made in sections of growth plate of chondrocyte proliferation and rate of maturation through the growth plate [using bromodeoxyuridine (BrdUrd) labeling] and also chondrocyte differentiation [assessed by alkaline phosphatase (ALP) activity]. The labeling indices of the control and supplemented chicks were similar (23.1 +/- 1.3 versus 23.2 +/- 1.6%); however, within a 21 h period the BrdUrd-positive cells of the supplemented chicks had moved down the growth plate significantly farther than in the control chicks (71.0 +/- 2.8 versus 52.6 +/- 1.8%). Greater ALP (mean integrated absorbance) activity higher up the growth plate of the supplemented chicks indicated a more differentiated phenotype in cells closer to the epiphyseal junction. Within individual transitional chondrocytes ALP activity in the 10 micrograms/kg supplemented chicks was 26.6 +/- 0.85, which was significantly higher (p < 0.01) than that of the control chicks (19.2 +/- 0.9). These results suggest that 1,25-(OH)2D3 in vivo does not increase the rate of chondrocyte proliferation but accelerates the onset of maturation.