Basal activities and hormone responsiveness of osteoclast-like and osteoblast-like bone cells are regulated by glucocorticoids

Hyaluronan Synthases' Expression and Activity Are Induced by Fluid Shear Stress in Bone Marrow-Derived Mesenchymal Stem Cells

During biomineralization, the cells generating the biominerals must be able to sense the external physical stimuli exerted by the growing mineralized tissue and change their intracellular protein composition according to these stimuli. In molluscan shell, the myosin-chitin synthases have been suggested to be the link for this communication between cells and the biomaterial. Hyaluronan synthases (HAS) belong to the same enzyme family as chitin synthases. Their product hyaluronan (HA) occurs in the bone and is supposed to have a regulatory function during bone regeneration. We hypothesize that HASes’ expression and activity are controlled by fluid-induced mechanotransduction as it is known for molluscan chitin synthases. In this study, bone marrow-derived human mesenchymal stem cells (hMSCs) were exposed to fluid shear stress of 10 Pa. The RNA transcriptome was analyzed by RNA sequencing (RNAseq). HA concentrations in the supernatants were measured by ELISA. The cellular structure of hMSCs and HAS2-overexpressing hMSCs was investigated after treatment with shear stress using confocal microscopy. Fluid shear stress upregulated the expression of genes that encode proteins belonging to the HA biosynthesis and bone mineralization pathways. The HAS activity appeared to be induced. Knowledge about the regulation mechanism governing HAS expression, trafficking, enzymatic activation and quality of the HA product in hMSCs is essential to understand the biological role of HA in the bone microenvironment.

Pathophysiological Mechanism of Bone Loss in Type 2 Diabetes Involves Inverse Regulation of Osteoblast Function by PGC-1α and Skeletal Muscle Atrogenes: AdipoR1 as a Potential Target for Reversing Diabetes-Induced Osteopenia

Type 2 diabetes is associated with increased fracture risk and delayed fracture healing; the underlying mechanism, however, remains poorly understood. We systematically investigated skeletal pathology in leptin receptor-deficient diabetic mice on a C57BLKS background (db). Compared with wild type (wt), db mice displayed reduced peak bone mass and age-related trabecular and cortical bone loss. Poor skeletal outcome in db mice contributed high-glucose- and nonesterified fatty acid-induced osteoblast apoptosis that was associated with peroxisome proliferator-activated receptor γ coactivator 1-α (PGC-1α) downregulation and upregulation of skeletal muscle atrogenes in osteoblasts. Osteoblast depletion of the atrogene muscle ring finger protein-1 (MuRF1) protected against gluco- and lipotoxicity-induced apoptosis. Osteoblast-specific PGC-1α upregulation by 6-C-β-d-glucopyranosyl-(2S,3S)-(+)-5,7,3',4'-tetrahydroxydihydroflavonol (GTDF), an adiponectin receptor 1 (AdipoR1) agonist, as well as metformin in db mice that lacked AdipoR1 expression in muscle but not bone restored osteopenia to wt levels without improving diabetes. Both GTDF and metformin protected against gluco- and lipotoxicity-induced osteoblast apoptosis, and depletion of PGC-1α abolished this protection. Although AdipoR1 but not AdipoR2 depletion abolished protection by GTDF, metformin action was not blocked by AdipoR depletion. We conclude that PGC-1α upregulation in osteoblasts could reverse type 2 diabetes-associated deterioration in skeletal health.

Serum NTX is a practical marker for assessing antiresorptive therapy for glucocorticoid treated patients with chronic kidney disease

INTRODUCTION

In chronic kidney disease (CKD) patients, serum concentration of type I collagen N-telopeptide (S-NTX) increases as renal function declines because of perturbed bone metabolism by renal dysfunction itself and impaired urinary excretion of NTX. Glucocorticoid (GC), which is often used for the treatment of kidney disease, may enhance bone resorption. We studied the bone resorption state in GC-treated CKD patients and the effects of bisphosphonate on S-NTX.

METHODS

We measured S-NTX and creatinine clearance (Ccr) in 48 non-diabetic patients with declining renal function who had never received GC, vitamin D, calcium or bisphosphonate (reference group). Bone markers including S-NTX and BSAP (bone specific alkaline-phosphatase) in 144 patients receiving GC (prednisone equivalent of > or =2.5 mg/day) for more than 6 months (GC-treated group) were measured. We compared the GC-treated group to the reference group in a cross-sectional study. In a longitudinal study, we further followed 55 patients from the GC-treated group, whose Ccr was more than 60 mL/min (CKD stage 1 or 2) for 1 year after 2.5 mg/day of risedronate was commenced.

RESULTS

In the reference group, S-NTX was correlated with Ccr (S-NTX = 456.6/Ccr + 4.5, r = 0.797, P < 0.0001). S-NTX values in the GC-treated group were higher than those found in the reference group at any Ccr. We defined the resorption index (RI) as a ratio of measured S-NTX to estimated NTX given by the correlation curve. Whereas BSAP did not change significantly, S-NTX decreased significantly by risedronate treatment. RI decreased from 1.59 (baseline) to 1.54, 1.25 (P < 0.01, versus baseline) and 1.23 (P < 0.01) at 1, 3 and 6 months after the start of therapy, respectively, which means that S-NTX values approached the correlation curve. Percent changes of S-NTX at 3 months were larger in patients with higher pretreatment S-NTX.

CONCLUSION

Higher S-NTX in the GC-treated group suggests that bone resorption is enhanced by GC. In CKD patients with mild renal dysfunction, S-NTX is a practical and useful marker for monitoring bone resorption during GC treatment.

Dexamethasone induces human spinal ligament derived cells toward osteogenic differentiation

Ossification of spinal ligament is characterized by heterotopic bone formation in the spinal ligaments that are normally composed of fibrous tissues. The pathogenesis of ossification of spinal ligament has been suggested to be associated with osteogenic differentiation of the spinal ligament cells. In order to address this hypothesis, cells derived from human spinal ligament were investigated for their osteogenic potential by the treatment of dexamethasone in vitro. Yellow ligaments were obtained from patients with spinal disorders except ossification of spinal ligament during surgery, and the adhering tissues were removed completely. Most of the ligament cells treated with vehicle exhibited a fibroblast-like spindle shape, while the dexamethasone-treated cells acquired a polygonal morphology. Growth of the ligament cells was suppressed by dexamethasone at a high concentration. Some of the vehicle treated-cells were alkaline phosphatase-positive, and dexamethasone increased the alkaline phosphatase-positive cells and alkaline phosphatase activity in the cells. Northern blot analysis demonstrated that mRNAs expression of pro-alpha1(I) collagen and alkaline phosphatase were promoted by dexamethasone. Analysis by reverse transcription-polymerase chain reaction showed that expression of osteocalcin mRNA was detected in the dexamethasone-treated cells but not in the vehicle-treated cells, and dexamethasone-induced osteocalcin mRNA expression was promoted by 1,25-dihydroxyvitamin D(3). Finally, mineralization of extracellular matrix in the cells was induced by the presence of dexamethasone and 1,25-dihydroxyvitamin D(3). These results suggest for the first time that dexamethasone has a possible involvement in the osteoblastic differentiation of human spinal ligament cells.

Molecular mechanisms of glucocorticoid-induced osteoporosis

Bone loss resulting from long-term glucocorticoid therapy is common and clinically relevant. A number of different glucocorticoid-mediated effects are responsible for the reduction in bone density: (i) glucocorticoid-induced direct impairment of osteoblast, osteocyte, and osteoclast function leads to reduced bone remodeling and diminished repair of microdamage in bone; (ii) the effects of parathyroid hormone (PTH) might be more pronounced in the presence of glucocorticoids, whereas vitamin D plays a lesser role in the pathogenesis of steroid-induced osteoporosis; (iii) glucocorticoids antagonize gonadal function and inhibit the osteoanabolic action of sex steroids; and (iv) increased renal elimination and reduced intestinal absorption of calcium lead to a negative calcium balance that has been suggested to promote secondary hyperparathyroidism. From a mechanistic point of view, all of the aforementioned effects have long been considered to be mediated at the molecular level exclusively by genomic actions. However, there is now increasing evidence for the existence of rapid glucocorticoid effects that are incompatible with this classical mode of action. These rapid effects, termed nongenomic effects, are mediated by glucocorticoid interactions with biological membranes, either through binding to membrane receptors or by physicochemical interactions. It is possible, but has yet to be shown, that these effects play a role in the pathogenesis of glucocorticoid-induced osteoporosis.

Glucocorticoid-induced osteoporosis

Glucocorticoid-induced osteoporosis is a significant problem in patients receiving glucocorticoids after transplantation and for the treatment of parenchymal renal disease and rheumatological disorders. Frequently, patients are not evaluated or treated appropriately for glucocorticoid-induced osteoporosis. Bone loss occurs early after the administration of high-dose glucocorticoid therapy. Elderly patients with low bone densities before the initiation of glucocorticoid therapy are at particular risk of developing significant bone loss that could result in fractures. New information is now available concerning the mechanisms by which glucocorticoid-associated bone disease occurs. New therapies with anti-resorptive agents such as bisphosphonates and with anabolic agents such as parathyroid hormone offer the prospect of effective treatment of glucocorticoid-induced osteoporosis.

Glucocorticoids inhibit developmental stage-specific osteoblast cell cycle. Dissociation of cyclin A-cyclin-dependent kinase 2 from E2F4-p130 complexes

Unique cell cycle control is instituted in confluent osteoblast cultures, driving growth to high density. The postconfluent dividing cells share features with cells that normally exit the cell cycle; p27(kip1) is increased, p21(waf1/cip1) is decreased, free E2F DNA binding activity is reduced, and E2F4 is primarily nuclear. E2F4-p130 becomes the predominant E2F-pocket complex formed on E2F sites, but, unlike the complex that typifies resting cells, cyclin A and CDK2 are also present. Administration of dexamethasone at this, but not earlier stages, results in reduction of cyclin A and CDK2 levels with a parallel decrease in the associated kinase activity, dissociation of cyclin A-CDK2 from the E2F4-p130 complexes, and inhibition of G(1)/S transition. The glucocorticoid-mediated cell cycle attenuation is also accompanied by, but not attributable to, increased p27(kip1) and decreased p21(waf1/cip1) levels. The attenuation of osteoblast growth to high density by dexamethasone is associated with severe impairment of mineralized extracellular matrix formation, unless treatment commences in cultures that have already grown to high density. Both the antimitotic and the antiphenotypic effects are reversible, and both are antagonized by RU486. Thus, glucocorticoids induce premature attenuation of the osteoblast cell cycle, possibly contributing to the osteoporosis induced by these drugs in vivo.

Evaluation of apoptosis and the glucocorticoid receptor in the cartilage growth plate and metaphyseal bone cells of rats after high-dose treatment with corticosterone

A connection has been suggested between glucocorticoid-induced osteopenia and an increase in the apoptosis of bone cells, and between the dimerization of the glucocorticoid receptor (GR) and the development of apoptosis. On this basis, a study has been carried out on the relationships between the occurrence of apoptotic cells and their detectable GR content, and between apoptosis frequency and changes in histomorphometric variables, in the growth plate and secondary spongiosa of rat long bones after the high-dose (10 mg/day) administration of corticosterone (CORT) and after recovery. The main results of the CORT treatment were: a significant increase in apoptotic osteoblasts, and a concomitant decrease in the histomorphometric variables of bone formation, with a reversal of both values during recovery; a nonsignificant increase in the apoptosis of osteoclasts, without changes in the histomorphometric variables of bone resorption; a significant increase in apoptotic terminal hypertrophic chondrocytes; the presence of GR in all types of skeletal cells in control rats, with different (cytoplasmic and/or nuclear) immunohistochemical detection in the same type of cell; a decrease in GR detection in proliferative chondrocytes and osteocytes in CORT and recovery groups, and in the maturative/hypertrophic chondrocytes of the recovery group; a fall in growth cartilage width, possibly due to the reduced proliferation of proliferative chondrocytes and increased apoptosis in terminal hypertrophic chondrocytes. In conclusion, pharmacological doses of CORT reduce bone formation by increasing osteoblast apoptosis; they reduce growth cartilage width, probably by inhibiting chondrocyte proliferation and increasing the apoptosis of terminal hypertrophic chondrocytes, and they reduce osteocyte GR. Although these effects appear to be mediated by the presence of GR in all skeletal cells, no precise correlation between GR immunohistochemical detection and apoptosis induction has been found.

Cytochemical demonstration of the glucocorticoid receptor in skeletal cells of the rat

The in vivo localization of the glucocorticoid receptor (GR) was studied in cartilage and bone cells of femurs of young adult rats. Deparaffinized sections were treated with a polyclonal antibody raised against the amino-terminus of human GR; the immunoreaction was detected with the streptavidin-biotin amplification method. Histomorphometric, computer-assisted analysis of GR-positive cells was performed by counting the percentage of GR-immunostained cells in the proliferative and maturative/hypertrophic zone of the epiphyseal growth plate cartilage, and of the percentage of positive osteoblasts (OBs), osteoclasts (OCLS) and osteocytes (OCs) in the metaphyseal secondary ossification zone. Numbers of OBs and OCLs per mm of metaphyseal endosteal perimeter, and numbers of OCs per mm2 of trabecular area were also counted. Immunopositive cells were found both in cartilage and bone, with variable degree of nuclear and/or cytoplasmic immunostaining; immunonegative cells were present among the positive ones. Histomorphometry showed that about 54% of chondrocytes in the proliferative zone, and 55% of chondrocytes in the maturative/hypertrophic zone of the growth plate were labeled; in metaphyseal bone, 68% of OBs, 65% of OCs, and 98% of OCLs were GR-positive. The density of positive cells was 12.06 OBs/mm, 3.32 OCLs/mm, and 520.40 OCs/mm2. These results, for the first time obtained in vivo, show that GR is present in cartilage and bone cells, and that the degree of GR-immunostaining is variable in the same type of cell. This may be dependent on the cell cycle and stage of differentiation, and may reflect a variable cellular sensitivity to the stimulation of the glucocorticoid hormone.

Effect of low-dose methylprednisolone on calcium balance and bone composition in rats

Glucocorticoid induced osteoporosis has been associated with high doses and it has been partially attributed to decreased absorption and to increased calcium excretion. The present study examined the effect of low but effective doses of methylprednisolone (MP) on calcium balance and on skeleton in rats. Total duration of the experiment 29 days. Thirty-one male Wistar rats (MP group) were injected with 4mg/kg body weight MP s.c. at the 1st, 11th and 20th day of the experiment and 28 rats (C group) were used as matched controls. The 1st, 11th and 20th day of the experiment rats were placed in individual metabolic cages for three days. Food and water consumption were measured at the 2nd and 3rd days after each injection; urine and faeces were collected at the same days for calcium estimation. Calcium intake and excretion after each injection was significantly lower in the MP group as compared to controls. A statistically significant positive correlation between calcium consumption and calcium excretion was found in both groups resulting in a negative final balance. Rats were killed the 29th day. Adrenal weight was statistically significant lower in MP group (p<0.001). Morphometric properties were evaluated for the right femur. No significant difference was found between the two groups. Mineral and calcium content was slightly increased in the MP group. According to these results, it seems that methylprednisolone while effective on HPA axis did not have any effect on calcium absorption and on bone calcium deposition in rats.

Transcriptional activities of estrogen and glucocorticoid receptors are functionally integrated at the AP-1 response element

Estrogens and glucocorticoids often act in opposition to regulate physiological responses. We investigated whether this might reflect the opposing actions of hormone-bound receptors on target genes regulated by the AP-1 response element. We performed a series of transfection experiments in which transcriptional activation, mediated by the AP-1 response element, was reflected in reporter gene activity. As previously described, we found that estrogens stimulate, whereas the glucocorticoid dexamethasone (Dex) inhibits, transcription through a model promoter from the collagenase gene (-73 to +63). This promoter bears a consensus AP-1 response element. When HeLa cells were treated with both estradiol and Dex, the steroids counteracted each other's transcriptional effects. The amount of transfected estrogen and glucocorticoid receptors (ER and GR) determined the extent to which Dex blunted estrogen stimulation or estrogen prevented Dex inhibition. The ER/GR interaction was observed both in the presence of estradiol and tamoxifen, which has previously been shown to have estrogen-like action at an AP-1 response element. The AP-1 family member c-Jun enhanced Dex inhibition and estradiol stimulation of transcriptional activation. c-Fos potentiated the effect of cotransfected c-Jun on estradiol stimulation but not Dex inhibition. The pattern of steroid responses was retained in the presence of the c-Jun activator phorbol 12-myristate 13-acetate. However, estradiol stimulation was lost in the presence of the c-Jun activator tumor necrosis factor-alpha. The ER/GR/AP-1 response element interaction was present, not only in a cell line originally derived from a uterine cervical adenocarcinoma (HeLa), but also in a cell line derived from the hypothalamus (GT1-1). Lastly, both progesterone receptor types A and B also interacted with the ER at the AP-1 site. These data indicate that opposing steroid influences can be mediated at the level of transcription through the AP-1 site and suggest that the integration of hormone action at this response element may underlie some of the opposing actions of estrogens and glucocorticoids or progestins on physiological responses.

The role of glucocorticoids and prostaglandin E2 in the recruitment of bone marrow mesenchymal cells to the osteoblastic lineage: positive and negative effects

The role of glucocorticoids in bone formation presents a problem because although pharmacological doses in vivo give rise to osteoporosis, physiological concentrations are required for osteoblast (OB) differentiation in vitro. To try and rationalize this dichotomy, we investigated the effect of dexamethasone on the recruitment of OB precursors present in bone marrow. Using the CFU-f assay, we can measure (1) total colony formation; (2) the osteoblastic differentiation of the colonies defined as their ability to express alkaline phosphatase, synthesize collagen, and to calcify; and (3) colony expansion as either average colony surface area or average colony number. In control cultures and in the presence of 10(-10)-10(-9) M dexamethasone, colony formation and total cell number was maximal, but the addition of PGE2 had no effect on colony number and very few colonies expressed the OB phenotype. In the presence of 10(-8)-10(-7) M dexamethasone, colony numbers and total cell numbers were reduced but were increased by the addition of PGE2, the average colony cell number and surface area were relatively unchanged and a proportion of the colonies expressed APase, calcified and synthesized collagen. In cultures containing 10(-6)-10(-5) M dexamethasone, colony numbers were further reduced but were stimulated by the addition of PGE2 and some colonies differentiated; however, colony expansion was dramatically reduced by up to 80%. These results suggest that physiological levels of glucocorticoids are necessary for OB differentiation and allow the control of OB recruitment by PGE2. High levels of glucocorticoids drastically reduce proliferation of the OB precursors leading to glucocorticoid-induced osteoporosis.

Serum bone Gla protein and carboxyterminal cross-linked telopeptide of type I collagen in patients with Cushing's syndrome

Serum bone Gla protein, a marker of bone formation, and carboxyterminal cross-linked telopeptide of type I collagen levels, an index of bone resorption, were evaluated in eight patients with active Cushing's syndrome and in four with 'preclinical' Cushing's syndrome, before and after surgery. In basal conditions, serum bone Gla protein levels were significantly lower (p < 0.0001) in patients with active Cushing's syndrome (1.0 +/- 0.35 ng/ml) than in controls (5.4 +/- 0.15 ng/ml); two out of four patients with the 'preclinical' form had reduced bone Gla protein levels, while in the other two cases levels were in the normal range. Serum levels of carboxyterminal cross-linked telopeptide of type I collagen (3.0 +/- 0.4 ng/ml), although slightly reduced, were similar to those recorded in controls (4.1 +/- 0.3 ng ml), both in patients with active and with preclinical Cushing's syndrome. After surgery serum levels of both marker proteins significantly increased in seven out of eight patients with active Cushing's syndrome; in one patient, who was not cured after surgery, bone Gla proteins levels remained lower than in normals, while levels of carboxyterminal cross-linked telopeptide of type I collagen had a transient increase after six months. In the two patients with a 'preclinical' Cushing's syndrome who underwent surgery, a significant rise of the levels of both marker proteins was observed, similar to that observed in patients with active Cushing's syndrome. It was concluded that serial determinations of these new markers of bone formation and resorption may be usefully employed to follow-up the clinical course of Cushing's syndrome and provide information on the rate of bone turnover in response to medical and/or surgical therapies. Moreover, the evaluation of these markers in preclinical states of Cushing's syndrome might suggest the need for surgery.

Differential effects of glucocorticoids on human osteoblastic cell metabolism in vitro

Clinical observations suggest that the onset and severity of glucocorticoid (GC) induced osteoporosis is dependent on the duration of the GC treatment and the applied GC compound. To test whether these in vivo observations are reflected by different in vitro effects of various synthetic GCs on human bone cell metabolism we isolated human osteoblast-like cells (HOC) from bone biopsies of healthy (no clinical symptoms of arthritis or arthrosis) adults who underwent selective orthopedic surgery. HOC were identified as bone cells by 1,25-vitamin D3-stimulated increase of specific alkaline phosphatase (ALP) activity, secretion of osteocalcin and type-I procollagen peptide, and the ability to form mineral in vitro. We investigated the effects of dexamethasone (dexa), methylprednisolone (mpred), prednisolone (pred), and deflazacort (defla) on DNA-synthesis, ALP, and osteocalcin (OC)- and type-I procollagen peptide secretion of HOC in vitro. In summary, (1) GC exposure stimulates DNA synthesis after 6-12 hour treatment periods; (2) dex and mpred strongly inhibit DNA (48-hour treatment) and collagen synthesis but stimulate ALP, whereas pred and defla exhibit smaller effects on DNA synthesis, ALP, and collagen production; and (3) all tested glucocorticoids inhibit OC secretion by HOC in vitro. Thus, the effect of GC on DNA synthesis of HOC varies with the duration of GC exposure, and dex and mpred more potently affect HOC metabolism in vitro than pred and defla.

Prednisone inhibits formation of cortical bone in sham-operated and ovariectomized female rats

Prednisone inhibits bone formation and causes bone loss. To investigate possible mechanisms and sites, the effects of sham operation, ovariectomy, and prednisone were determined on bone and mineral metabolism in 7-week-old growing female rats. Forty animals were divided into groups of 10 each. Sham operation and ovariectomy were performed. One week later, pellets containing 5 mg prednisone or drug free were implanted S.C. at the back of the neck. Four weeks later, animals were sacrificed and tibiae were removed for histomorphometric analysis of the middiaphysis and proximal metaphysis. In both sham-operated and ovariectomized rats, prednisone (1) reduced weight gain (P < 0.02) and did not alter uterine weight; (2) lowered serum magnesium (Mg) (P < 0.001) and did not change serum calcium (Ca), phosphate (P), 25-hydroxyvitamin D (25OHD), or 1,25-dihydroxyvitamin D [1,25(OH)2D]; (3) produced striking increases in calcified cartilage, reduced cross-sectional area (P < 0.05) and cortical area (P < 0.01) and did not change medullary area of the tibial diaphysis; (4) lowered periosteal and endocortical bone formation and apposition rates; and (5) increased mean cancellous bone area (P < 0.05) and cancellous bone perimeter (P < 0.01) of the tibial metaphysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of type I collagen gene expression in bone

The regulation of COL1A1 gene expression in bone was studied by measuring the activity of type I collagen promoter fusion genes (ColCAT) in permanently transfected osteoblastic cells and calvariae from transgenic animals. The basal activity of ColCAT fusion genes in transfected cells is mediated by DNA sequences between -3.5 to -2.3 kb while expression in vivo requires sequences between -2.3 and -1.7 kb. Parathyroid hormone, 1,25-dihydroxyvitamin D3 and interleukin-1 decrease the activity of ColCAT fusion genes in osteoblastic cells and transgenic calvariae. Because there may be differences between the expression of ColCAT fusion genes in cultured cells and intact bone, it will be important to compare data obtained from transfected cells with an in vivo model such as calvariae from transgenic mice.

Formation of mineralized nodules by bone derived cells in vitro: a model of bone formation?

The identification of the factors which regulate the proliferation and differentiation of cells of the osteoblast lineage remains one of the major challenges in the field of bone cell biology. Although considerable progress has been made in the isolation and culture of cells of the osteoblast lineage from both animal and, more recently, human bone, uncertainties have persisted as to the extent to which these cell populations retain the ability to differentiate into functional osteoblasts in vitro. The formation in vitro of mineralized nodules that exhibit the morphological, ultrastructural and biochemical characteristics of embryonic/woven bone formed in vivo, represents the first evidence that the differentiation of functional osteoblasts can occur in cultures of isolated animal bone-derived cell populations. It is clear, however, that the culture conditions employed at present only permit a small number of cells to differentiate to the extent of being capable of organising their extracellular matrix into a structure that resembles that of bone. Moreover, it has generally been found that the reproducible mineralization of this extracellular matrix requires supplementation of the culture medium with mM concentrations of beta-GP, which raises doubts as to the physiological relevance of this process. The formation of nodules has also been observed in cultures of human bone-derived cells. As found in cultures of animal bone-derived cells, reproducible mineralization of these nodules will occur in the presence of beta-GP. We have shown, however, that in the presence of the long acting ascorbate analogue Asc-2-P, the formation and mineralization of nodules can occur in the absence of beta-GP. The nodules formed in human bone-derived cell cultures have yet to be characterized as rigorously as those formed in cultures of animal bone-derived cells and thus it remains to be shown that they resemble bone formed in vivo.

Glucocorticoids promote development of the osteoblast phenotype by selectively modulating expression of cell growth and differentiation associated genes

To understand the mechanisms by which glucocorticoids promote differentiation of fetal rat calvaria derived osteoblasts to produce bone-like mineralized nodules in vitro, a panel of osteoblast growth and differentiation related genes that characterize development of the osteoblast phenotype has been quantitated in glucocorticoid-treated cultures. We compared the mRNA levels of osteoblast expressed genes in control cultures of subcultivated cells where nodule formation is diminished, to cells continuously (35 days) exposed to 10(-7) M dexamethasone, a synthetic glucocorticoid, which promotes nodule formation to levels usually the extent observed in primary cultures. Tritiated thymidine labelling revealed a selective inhibition of internodule cell proliferation and promotion of proliferation and differentiation of cells forming bone nodules. Fibronectin, osteopontin, and c-fos expression were increased in the nodule forming period. Alkaline phosphatase and type I collagen expression were initially inhibited in proliferating cells, then increased after nodule formation to support further growth and mineralization of the nodule. Expression of osteocalcin was 1,000-fold elevated in glucocorticoid-differentiated cultures in relation to nodule formation. Collagenase gene expression was also greater than controls (fivefold) with the highest levels observed in mature cultures (day 35). At this time, a rise in collagen and TGF beta was also observed suggesting turnover of the matrix. Short term (48 h) effects of glucocorticoid on histone H4 (reflecting cell proliferation), alkaline phosphatase, osteopontin, and osteocalcin mRNA levels reveal both up or down regulation as a function of the developmental stage of the osteoblast phenotype. A comparison of transcriptional levels of these genes by nuclear run-on assays to mRNA levels indicates that glucocorticoids exert both transcriptional and post-transcriptional effects. Further, the presence of glucocorticoids enhances the vitamin D3 effect on gene expression. Those genes which are upregulated by 1,25(OH)2D3 are transcribed at an increased rate by dexamethasone, while those genes which are inhibited by vitamin D3 remain inhibited in the presence of dexamethasone and D3. We propose that the glucocorticoids promote changes in gene expression involved in cell-cell and cell-extracellular matrix signaling mechanisms that support the growth and differentiation of cells capable of osteoblast phenotype development and bone tissue-like organization, while inhibiting the growth of cells that cannot progress to the mature osteoblast phenotype in fetal rat calvarial cultures.

Glucocorticoid regulation of alkaline phosphatase, osteocalcin, and proto-oncogenes in normal human osteoblast-like cells

In humans, glucocorticoids are known to have marked effects on bone metabolism and function, including the significant regulation of osteoblast cells. To aid in the understanding of the mechanism of glucocorticoid action on normal human osteoblasts (hOB), confluent cells were analyzed for the presence of glucocorticoid receptors (GR) as well as for the effects of the glucocorticoid dexamethasone (Dex) on the expression of both the rapid responding nuclear proto-oncogenes and the late responding structural genes for bone matrix proteins. The interactions between Dex and 1,25 dihydroxy vitamin D3 (1,25 D3) on the gene expression in these cells were also examined. Using a functional receptor assay, a mean of 11,600 functional nuclear bound glucocorticoid receptors (range 6,000-22,000) was measured in fifteen separate cell strains. Northern blot analysis with a cDNA probe to the human GR was used to demonstrate the presence of a 7Kb transcript which is a candidate mRNA for GR in these cells. In agreement with previous studies, treatment of the hOB cells with Dex increased the steady state mRNA levels for alkaline phosphatase (AP) but displayed little or no effect on the mRNA levels for osteocalcin (OC) and glyceraldehyde phosphate dehydrogenase (GAPDH). Interestingly, the 1,25 D3 inductions of mRNA levels for OC were blocked by Dex but enhanced for AP. The above effects of Dex on AP and OC gene expression, including the interaction with 1,25 D3, were also shown to occur at the level of protein. The effect of Dex on the mRNA levels of the nuclear proto-oncogenes c-myc, c-fos, and c-jun was also investigated, since the oncoproteins (Fos/Jun) appear to play a role in the delayed glucocorticoid regulation of structural genes. Interestingly, Dex increased the steady state levels of c-myc, c-fos, and c-jun mRNAs in nonproliferating (confluent) hOB cells by 3.5-, 10-, and 2.0-fold, respectively, over control (untreated cells) values within one h of steroid treatment. The Dex-induced mRNA levels were transient and returned to basal values within 24 h of the steroid treatment. A reduced but qualitatively similar pattern of response was found in proliferating hOB cells. The pattern of response of these genes to glucocorticoids in hOB cells mimics the response in avian liver cells but not in reproductive cells. These results support the theory that hOB cells are target cells for glucocorticoids, and that as a primary event glucocorticoids rapidly regulate the expression of the nuclear oncoproteins Fos/Jun in these cells.

Growth hormone prevents steroid-induced growth depression in health and uremia

Treatment with supraphysiological doses of corticosteroids results in protein wasting and impairment of growth, whereas exogenous growth hormone (GH) causes anabolism and improvement of growth. We wanted to know whether the growth depressing effects of methylprednisolone (MP) are more expressed in an organism which is chronically diseased and whether these effects can be counterbalanced by concomitant treatment with recombinant human growth hormone (rhGH). MP in doses from 1 to 9 mg/kg/day caused a dose dependent reduction of length gain, weight gain and weight gain/food intake ratio in 140 g healthy female Sprague-Dawley rats. Food intake was not affected by MP. This points to a change in food metabolism as a mechanism for growth impairment. In addition, treatment with MP inhibited endogenous GH secretion, documented by serum GH concentration profiles over seven hours, decreased IGF-1 serum concentration and disturbed growth cartilage plate architecture. Concomitant treatment with 2.5 to 20 IU/rhGH/kg/day prevented the negative effects of MP on growth in a dose dependent manner and normalized growth plate architecture. In uremic rats in which food efficiency and growth was already reduced, 6 mg MP/kg/day further decreased length gain and prevented weight gain completely by bringing the weight gain/food conversion ratio to the nadir. All effects of MP including reduction of muscle mass could be prevented by concomitant treatment with 10 IU rhGH/kg/day. The effects of MP and rhGH on food efficiency and growth in uremic animals were numerically nearly identical to those in pair fed ad libitum fed controls, but this may be more relevant in the diseased organism in which basal growth is already suppressed.

Effect of glucocorticoids on bone Gla protein values--BGP as a good marker of osteoporosis

It is important to prevent corticosteroid(CS)-induced osteoporosis, particularly in children. One of the mechanisms is a direct inhibitory effect of CS on osteoblasts. Bone Gla protein (BGP) is produced in osteoblasts, and the serum level of BGP reflects the bone formation rate. The aim of this study is to examine the usefulness of BGP as a marker of CS-induced osteoporosis. In the present study, serum levels of 24 pediatric patients who were given prednisolone (PSL) for long periods were studied in relation to their growth rate. Serum BGP was also determined in 167 healthy children and 16 adults. In healthy children, BGP levels reached a peak at the age of 15 years in boys and 11 years in girls. In patients who were given more than 0.25 mg/kg/day PSL, serum BGP levels were significantly decreased and height growth was remarkably suppressed. In conclusion, the measurement of serum BGP is useful for early detection of CS-induced osteoporosis.

The role of DNA synthesis in the responses of fetal rat calvariae to cortisol

To determine the extent to which the effects of cortisol on collagen synthesis in 21 day fetal rat calvariae are linked to its effects on cell replication, calvariae were cultured for 24-72 h with 0.1 and 1 microM cortisol in the presence or absence of 1 mM hydroxyurea (HU) or 30 microM aphidicolin (APC), inhibitors of DNA synthesis. The incorporation of [3H]proline into collagenase-digestible protein (CDP) and [3H]thymidine into DNA were measured during the last 2 h of culture. At 24 h HU and APC decreased thymidine incorporation by greater than 90%, and this remained low for the duration of culture. In contrast, cortisol reduced thymidine incorporation by only 44% at 72 h. Although cortisol caused a 24 h stimulatory effect and a 48 and 72 h inhibitory effect on CDP labeling and the percentage of collagen being synthesized (PCS), HU, and APC had no effect on basal CDP labeling or PCS over the 72 h culture period. Cortisol caused parallel alterations in the steady-state levels of alpha-1(I) procollagen mRNA, suggesting that its effects occur at the pretranslational level. At 24 h HU and APC did not prevent the stimulatory effect of cortisol on CDP labeling and PCS. At 48 h the inhibitory effects of cortisol on CDP labeling and PCS were observed in the presence of APC but not in the presence of HU. At 72 h the inhibitory effects of cortisol on CDP labeling and PCS were still observed in the presence of HU and APC.(ABSTRACT TRUNCATED AT 250 WORDS)

Osteoporosis associated with rheumatoid arthritis: pathogenesis and management

Rheumatoid arthritis is associated with both localized and generalized osteoporosis. Localized osteoporosis can be considered to be caused by local disease mechanisms, including the generation of factors from activation of the cytokine pathway. The etiology of generalized osteoporosis has been difficult to elucidate, particularly because of the lack of sensitive techniques to measure bone mineral density. The introduction of single- and dual-photon absorptiometry and quantitative computed tomography has allowed more accurate assessment of bone mineral density. In general, bone mineral density loss at appendicular sites does not correlate well with axial bone density loss. Corticosteroid treatment exaggerates the development of osteoporosis in up to 40% of patients with rheumatoid arthritis. Sex hormone status, physical activity, disease duration, and functional class are all significant predictors for the development of osteoporosis. Current therapy for prevention and treatment is based largely on theoretical considerations. Physical activity should be encouraged once acute joint inflammation has settled. Postmenopausal women and amenorrheic premenopausal women will benefit from cyclical estrogen replacement. Patients with low serum 1,25-dihydroxy vitamin D3 levels, and males with low serum testosterone levels, are candidates for replacement therapy with the appropriate hormones. In patients who are receiving corticosteroids the dose should be limited, and oral calcium supplements are of benefit. The use of the newer corticosteroid deflazacort, and disease-modifying immunosuppressive drugs, are discussed. Other therapeutic options which should be considered, although published trials are scarce, are calcitonin and the diphosphonates. Further studies are awaited concerning the optimum prevention and treatment of osteoporosis associated with rheumatoid arthritis.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell density-dependent decrease in cytoskeletal actin and myosin in cultured osteoblastic cells: correlation with cyclic AMP changes

During bone development, osteoblasts form a contiguous layer along recently deposited osteoid and their morphology changes from fibroblast-like to cuboidal. In culture, similar changes occur with increased cell density. We examined the possible role of cyclic AMP in this process since cyclic AMP was reported to increase in fibroblasts with increased cell density and similar shape changes were seen in response to parathyroid hormone, which also increases cellular cyclic AMP in osteoblastic cells. Osteoblast-enriched rat calvaria cells were seeded at increasing density. The distribution between Triton X-100 extractable and nonextractable actin and myosin was estimated by polyacrylamide gel electrophoresis. Intracellular cyclic AMP was estimated by prelabeling the cellular ATP pool with 3H-adenine, followed by extraction and separation of 3H-cAMP by high-performance liquid chromatography. We found that osteoblastic cells contain about 40 pg actin and 5.3 pg myosin per cell. Around 60% of the actin and 70% of the myosin were in the nonextractable (crosslinked) form at cell densities of 10,000 to 50,000 cells per cm2. Above 50,000 cells/cm2, there was a cell density-dependent reduction in crosslinked actin and myosin and a concomitant increase in cellular cyclic AMP. A comparable rise in cyclic AMP, produced by incubation with phosphodiesterase inhibitors, and treatment with other agents that increase cyclic AMP produced a similar decrease in the level of cytoskeletal actin and myosin. Cytochalasin B treatment, through its effect on actin polymerization, produced similar changes in cell shape and cytoskeletal actin. The findings suggest that an elevation in intracellular cyclic AMP may play a role in the density-dependent changes in cell shape and microfilament organization observed in osteoblasts.

Physiological and pharmacological regulation of biological calcification

Biological calcification is a highly regulated process which occurs in diverse species of microorganisms, plants, and animals. Calcification provides tissues with structural rigidity to function in support and protection, supplies the organism with a reservoir for physiologically important ions, and also serves in a variety of specialized functions. In the vertebrate skeleton, hydroxyapatite crystals are laid down on a backbone of type I collagen, with the process being controlled by a wide range of noncollagenous proteins present in the local surroundings. In bone, cells of the osteoblast lineage are responsible for the synthesis of the bone matrix and many of these regulatory proteins. Osteoclasts, on the other hand, are continually resorbing bone to both produce changes in bone shape and maintain skeletal integrity, and to establish the ionic environment needed by the organism. The proliferation, differentiation, and activity of these cells is regulated by a number of growth factors and hormones. While much has already been discovered over the past few years about the involvement of various regulators in the process of mineralization, the identification and functional characterization of these factors remains an area of intense investigation. As with any complex, biological system that is in a finely tuned equilibrium under normal conditions, problems can occur. An imbalance in the processes of formation and resorption can lead to calcification disorders, and the resultant diseases of the skeletal system have a major impact on human health. A number of pharmacological agents have been, and are being, investigated for their therapeutic potential to correct these defects.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucocorticoids stimulate resorption in fetal rat parietal bones in vitro

The effect of glucocorticoids on bone resorption was examined in a serum-free mineralizing organ culture system derived from 20 day fetal rat parietal bones. Bone resorption was assessed by prelabeling the fetal rats in utero with 45Ca and determining the daily release of 45Ca into the medium of cultured bones. During the first 24 h of treatment a transient stimulation of bone resorption was found; 4.5 +/- 0.3% of the total 45Ca was released into the medium with 1 nM corticosterone and 4.1 +/- 0.2% with 10 nM corticosterone compared to 2.9 +/- 0.2% in control bones. Treatment with 1 and 10 nM dexamethasone for 24 h also showed an increase in 45Ca release compared to control bones. During the same time period 45Ca release was 6.9 +/- 1.4% with 10 nM parathyroid hormone. At later time points 100 and 1000 nM corticosterone inhibited 45Ca release, but 1 and 10 nM corticosterone values were similar to controls. At 24 h the number of osteoclasts per mm2 tissue in bone lacunae was significantly elevated with 1-100 nM corticosterone and 10 nM parathyroid hormone compared to control bones. In control bones 0.10 +/- 0.05 osteoclasts per mm2 of tissue were found, but 0.59 +/- 0.21 osteoclasts per mm2 were seen with 10 nM corticosterone and 1.50 +/- 0.34 with 10 nM parathyroid hormone. An additional assay of bone resorption, the release of lysosomal beta-glucuronidase into the medium was also elevated in glucocorticoid and parathyroid hormone-treated cultures.(ABSTRACT TRUNCATED AT 250 WORDS)

Long-term effects of physiologic concentrations of dexamethasone on human bone-derived cells

Bone cells derived from human trabecular explants display osteoblastic features. We examined the modulation of alkaline phosphatase activity and cAMP production as the result of exposing trabecular explants to physiologic concentrations of dexamethasone for 4 weeks during cellular outgrowth and subculture. Cells treated with dexamethasone were observed to grow generally more slowly than control cells. Cells appeared larger and more polygonal, and staining for alkaline phosphatase was more intense in the dexamethasone-exposed cultures. There was a progressive increase in cellular PTH responsiveness with increasing duration of exposure of cells to dexamethasone. Cells grown for 6 weeks in 3 x 10(-8) M dexamethasone had a 10-fold increase in PTH-stimulated cyclic AMP accumulation. Dexamethasone-treated cells also had a significantly increased alkaline phosphatase activity. 1,25-(OH)2D3-stimulated alkaline phosphatase activity was increased approximately 20-fold. cAMP responses were significantly increased to PTH (21.7-fold), PGE1 (2.67-fold), and forskolin (4.81-fold), but not to cholera toxin. Dexamethasone-treated cells also had a mean decrease in 1,25-(OH)2D3-stimulated osteocalcin production to 26.2% of control values (p less than 0.001). Hydrocortisone treatment gave rise to similar effects but of smaller magnitude than those of dexamethasone. Testosterone did not have a significant effect on alkaline phosphatase activity or cAMP production. Skin fibroblasts showed a significant enhancement of alkaline phosphatase activity in response to dexamethasone, but of a much smaller magnitude than in bone cells. The phenotypic changes induced by long-term culture in dexamethasone are consistent with the promotion of a more differentiated osteoblastic phenotype.

Glucocorticoids increase parathyroid hormone receptors in rat osteoblastic osteosarcoma cells (ROS 17/2)

The effects of glucocorticoids on parathyroid hormone (PTH) receptors was studied using rat osteosarcoma-derived cells (ROS 17/2), which have an osteoblastic phenotype, and [125I][Nle8,Nle18,Tyr34]bovine(b)PTH-(1-34)amide as the radioligand. Treatment of cells with physiologic concentrations of hydrocortisone resulted in a time and dose-dependent increase in PTH binding. The increase in PTH binding could be observed by 10 h of exposure to hydrocortisone (2 x 10(-7) M), was maximally enhanced by 48 h, and was maintained for the subsequent 7 days of continuous exposure to the steroid. With removal of hydrocortisone, PTH receptor binding promptly returned toward control levels. The increase in PTH binding was attributed to an increase in the availability of receptor binding sites, not to altered receptor binding affinity, and was blocked by cycloheximide. PTH-stimulated adenylate cyclase was also enhanced by glucocorticoids, and a close correlation was observed between PTH binding and PTH-stimulated adenylate cyclase. However, hydrocortisone not only increased PTH binding but also enhanced the efficiency of postreceptor signaling: 5'-guanylimidodiphosphate [Gpp(NH)p]- and forskolin-stimulated adenylate cyclase activities were also increased. Thus, enhanced PTH stimulation of adenylate cyclase by glucocorticoids resulted from at least two effects--increased receptor availability and enhanced postreceptor efficiency of transmembranous signaling.

The adenylate cyclase response to parathyroid hormone in fetal lung fibroblasts is enhanced by cortisol

Parathyroid hormone (PTH, less than 10(-8) M) stimulated adenylate cyclase in fibroblasts, but not epithelial cells, isolated from fetal rat lung. In contrast to osteosarcoma cells (UMR 106), the response of fibroblasts to PTH was increased by pretreatment with cortisol (less than 10(-8)-10(-7) M).

Altered parathyroid hormone- or calcitonin-stimulated adenosine 3', 5'-monophosphate release by isolated perfused bone from glucocorticoid-treated rats

The present studies were designed to examine in vivo effects of glucocorticoid on PTH-or calcitonin (CT)-stimulated adenosine 3',5'-monophosphate (cAMP) release from the isolated perfused bone of rat and to test whether the duration of glucocorticoid administration influenced such effects. We assessed the ability of acute (24 hour) or chronic (2 week) dexamethasone administration to modulate the cAMP response to 5 micrograms human PTH-(1-34) or 1 micrograms eel CT. Acute treatment with dexamethasone (1 mg/100 g body wt) increased the cAMP response to PTH, but decreased the response to CT. This enhanced effect on PTH-stimulated cAMP release was not apparent in the presence of phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (IBMX, ImM). In contrast, chronic dexamethasone treatment (0.2 mg daily for 2 weeks) led to a decrease in both PTH- and CT-stimulated cAMP release. Such impaired response of the dexamethasone-treated bones to PTH was also found in rats that underwent parathyroidectomy 24 hours before sacrifice. These data indicate that 1) the duration of glucocorticoid administration may influence the effect of PTH on bone and 2) glucocorticoid may decrease cAMP-mediated CT function, regardless of the duration of treatment.

The effects of hydrocortisone, parathyroid hormone and the bisphosphonate, APD, on bone resorption in neonatal mouse calvaria

The effects of hydrocortisone and parathyroid hormone (PTH) upon bone resorption rates in neonatal mouse calvaria have been studied. Bone resorption (measured as 45Ca release) was significantly increased by hydrocortisone (10(-7) M and 10(-6) M) and there was a dose-dependent rise with PTH (0.3-0.9 micrograms/liter). When both PTH 0.3 micrograms/liter and hydrocortisone 10(-8) M were present in the incubating medium, bone resorption did not differ from control, but increasing the hydrocortisone concentration to 10(-7) M augmented 45Ca release by 25% (P less than 0.02) and doubling of the PTH level was associated with a 10% increase (nonsignificant). When both PTH and hydrocortisone were present in the higher concentrations (0.6 micrograms/liter and 10(-7) M, respectively) 45Ca release increased by 39% (P less than 0.005) above that resulting from the lower levels of both hormones (0.3 micrograms/l and 10(-8) M, respectively). (3-Amino-1-hydroxypropylidene)-1,1-bisphosphonate (APD) in concentrations of 3 X 10(-5) M and 10(-4) M, produced inhibition of basal and hydrocortisone/PTH-stimulated bone resorption without evidence of toxicity. These results indicate that hydrocortisone stimulates bone resorption in neonatal mouse calvaria in vitro, in contrast to the results found in fetal rat bone culture systems. PTH has a similar effect, which is additive to that of hydrocortisone and the combined stimulation can be overcome by APD. The possible relevance of these results to the development and prevention of glucocorticoid-induced osteoporosis is discussed.

Vitamin D and skeletal tissues

It is now accepted that vitamin D is an integral part of a complex endocrine system, one with far-reaching implications in mineral metabolism. Reviews of the sources, functions and metabolism of vitamin D, as currently understood, are presented as a prelude to discussions of the role of vitamin D in calcium and phosphorous homeostatis and possible specific roles for vitamin D in mineralized tissues. Data describing a possible regulatory function for vitamin D in bone and bone protein metabolism are presented. Some of the controversy which presently exists regarding the biochemical mechanism of the action of this vitamin is discussed. Finally, the possible relationship of vitamin D and disorders of skeletal tissues is described.

Effect of oral 1,25-dihydroxyvitamin D and calcium on glucocorticoid-induced osteopenia in patients with rheumatic diseases

Twenty-three rheumatic disease patients with glucocorticoid-induced osteopenia (defined by measurement of forearm bone mass) completed an 18-month double-blind, randomized study to assess the effect of oral calcium and 1,25-dihydroxyvitamin D (1,25-OH2D) or calcium and placebo on bone and mineral metabolism. Intestinal 47Ca absorption was increased (P less than 0.05) and serum parathyroid hormone levels were suppressed (P less than 0.01) by 1,25-OH2D (mean dose 0.4 micrograms/day); however, no significant gain in forearm bone mass occurred, and bone fractures were frequent in both groups. In the 1,25-OH2D group, histomorphometric analysis of iliac crest biopsy specimens demonstrated a decrease in osteoclasts/mm2 of trabecular bone (P less than 0.05) and parameters of osteoblastic activity (P less than 0.05), indicating that 1,25-OH2D reduced both bone resorption and formation. We conclude that 1,25-OH2D should not be used for treatment of glucocorticoid-induced osteopenia. Since patients receiving calcium and placebo did not exhibit a loss of forearm bone mass, elemental calcium supplementation of 500 mg daily might be useful to maintain skeletal mass in patients receiving long-term glucocorticord therapy.

Modulation of PTH-stimulated cyclic AMP in cultured rodent bone cells: the effects of 1,25(OH)2 vitamin D3 and its interaction with glucocorticoids

Parathyroid hormone (PTH)-stimulated cyclic adenosine monophosphate (cAMP) in rat osteoblastlike (OB) cells has been shown to be modulated by steroid hormones; glucocorticoids are known to increase the level, while the effects of 1,25(OH)2D3 are inhibitory. In the present study, we found that the PTH-stimulated cAMP responses are similar in neonatal mouse and fetal rat OB cells. Dexamethasone (0.13-13 nM) augmented PTH-stimulated cAMP in both species. Mouse cells showed a higher maximal response to dexamethasone (100% increment) than rat cells (60-70% increment) with similar sensitivity to dexamethasone (ED50 approximately 1.0 nm). On the other hand, 1,25(OH)2D3 decreased PTH-stimulated cAMP, but the effect required pharmacological levels of hormone; mouse cells responded at a lower dose (1.3 nM) and were more sensitive than rat cells (responded at 13 nM) to 1,25(OH)2D3 treatment. Introduction of physiological concentrations of 1,25(OH)2D3 (0.013-1.3 nm) in addition to dexamethasone (13 nM) resulted in a synergistic enhancement of PTH-stimulated cAMP in rat cells. In contrast, a dose-dependent antagonistic effect was observed in mouse cells. In summary, our findings demonstrate species and concentration-dependent differences in hormonal responses to 1,25(OH)2D3 and a complex interplay among PTH, dexamethasone, and 1,25(OH)2D3.

Comparative study of deflazacort, a new synthetic corticosteroid, and dexamethasone on the synthesis of collagen in different rat bone cell populations and rabbit articular chondrocytes

Deflazacort is a new synthetic glucocorticoid which is an oxazoline derivative of prednisolone. In previous studies, it was shown that deflazacort, depending on the test model used, not only showed considerably more antiinflammatory potency than prednisolone in animals but also caused less deleterious effects on bone mineral metabolism than equivalent amounts of other glucocorticoids in man. In this study, we have compared the effects of deflazacort with those of dexamethasone on the synthesis of collagen in various rat bone cell populations and chondrocytes. Three bone cell populations were prepared by sequential time-dependent collagenase treatment of 1-day-old rat calvaria. Each cell population was further purified on a Percoll gradient (10-90%) yielding three populations of which two are different in alkaline and acid phosphatase and response to parathyroid hormone. A 3-day treatment of bone cell populations with deflazacort and dexamethasone (10(-11)-10(-5) M) revealed that both glucocorticoids, although at different concentrations, inhibited collagen synthesis. 21-desacetyl-deflazacort (5 beta, 11 beta, 16 beta)-11,21-dihydroxy-2'-methyl-5-H-pregna-1-enol [17,16-d]oxazole-3,20-dione), the presumably active form of the steroid, which is formed in vivo after administration, produced nearly identical results as its precursor. Glucocorticoid concentrations at which inhibition was initially observed were 10(-9) M and 10(-7) M for dexamethasone and deflazacort respectively. Inhibition of collagen synthesis was significantly impaired only in cells isolated from bone during early tissue digestion, and not in those obtained during extensive collagenase treatment. Chondrocytes isolated from articular cartilage of 3-month-old rabbits and grown in primary cultures did not respond to either steroid.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of cortisol on periosteal and nonperiosteal collagen and DNA synthesis in cultured rat calvariae

The effects of cortisol on bone formation are complex and may be modulated by the presence of periosteal cells or by factors released by the periosteal tissue. To test these possibilities, cortisol was examined for its effects on the incorporation of 3H-proline into collagenase-digestible protein (CDP) and noncollagen protein (NCP), on DNA synthesis and on alkaline phosphatase activity in intact and in the periosteum and nonperiosteal bone of dissected calvariae from 21-day-old fetal rats. After 24 h of treatment, cortisol increased the incorporation of 3H-proline into CDP in intact bones and in the nonperiosteal bone of calvariae dissected after the culture. Cortisol inhibited the incorporation of 3H-thymidine into calvarial DNA but it caused a small increase in nonperiosteal DNA content. Cortisol did not affect the incorporation of 3H-proline into CDP in calvariae dissected prior to the culture if the periosteum and nonperiosteal central bone were incubated separately; the stimulatory effect was observed only if the two tissues were cultured in the same vial and were in contact. In contrast, cortisol stimulated alkaline phosphatase activity in the central nonperiosteal bone of calvariae dissected before or after the culture. After 72-96 h of treatment, cortisol inhibited the labeling of CDP, NCP, and DNA and the DNA content in intact bones and in both periosteal and nonperiosteal central bone of calvariae dissected after the culture. In contrast, when the periosteum was removed before the incubation, these inhibitory effects were observed in the periosteum and not in the nonperiosteal bone.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of 1 alpha,25-dihydroxyvitamin D3 and glucocorticoids on the growth of rat and mouse osteoblast-like bone cells

The effects of 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3) and its interaction with glucocorticoids to regulate bone cell growth were studied in osteoblast-like (OH) cell cultures. Owing to our earlier findings that species difference and cell density at the time of treatment modified hormonal responses, comparisons were made between rat and mouse cells and sparse and dense cultures. 1,25(OH)2D3 inhibited cell proliferation in both species regardless of cell density. The magnitude of inhibition was larger in mouse cells, but the sensitivity to 1,25(OH)2D3 was the same for both species. Other metabolites, 25(OH)D3 and 24R,25(OH)2D3, were greater than 100-fold less potent than 1,25(OH)2D3 even in serum-free medium, which is similar to their ratio of affinity for the 1,25(OH)2D3 receptor. Dexamethasone, as previously shown, inhibited sparse and dense mouse cell cultures and sparse rat cell cultures while stimulating dense rat cell cultures to grow. The inhibitory actions of 1,25(OH)2D3 were not additive to the inhibitory dexamethasone effects. However, 1,25(OH)2D3 addition resulted in attenuation of the stimulatory effect of dexamethasone. These responses to 1,25(OH)2D3 and dexamethasone were dependent on cell density and not selective attachment of certain cell types at either plating density. In conclusion, the findings demonstrated that 1,25(OH)2D3 exerts an inhibiting action on both mouse and rat bone cell proliferation. This effect must be reconciled with the in vivo beneficial actions of 1,25(OH)2D3 on bone metabolism. Also, the likelihood of decreased cell number must be considered when biochemical activities are assessed after vitamin D treatment in vitro.