Comparison of the effects of a potent synthetic analog of bovine parathyroid hormone with native bPTH-(1-84) and synthetic bPTH-(1-34) on bone resorption and collagen synthesis

Hormonal regulation of bone growth and remodelling

Many systemic and local hormones influence bone growth and remodelling. These include calcium regulating hormones, systemic growth regulators and local growth factors. Parathyroid hormone (PHT) is a potent stimulator of osteoclastic bone resorption and a direct inhibitor of osteoblastic collagen synthesis. However, intermittent low-dose PTH administration can increase bone formation in vivo. PTH may act indirectly via local factors. It has been shown to increase prostaglandin E2 (PGE2) and transforming growth factor beta (TGF-beta) release from bone. Both PGE2 and TGF-beta have complex effects on bone metabolism and are likely to be physiological regulators of bone remodelling. Oestradiol has been shown to inhibit bone resorption in vivo but not in vitro. While there is evidence for oestrogen receptors in cultured bone cells, the effect could still be indirect. Oestradiol can inhibit bone PGE2 release in an in vivo-in vitro model in the rat. Glucocorticoids are potent inhibitors of bone formation and inhibit PGE2 and interleukin 1 production both in vivo and in vitro. While many regulatory factors affect prostaglandin production in bone, the complex effects of PGE2 on bone metabolism make it difficult to predict the ultimate response. The major effects of PGE2 are stimulation of bone formation and resorption and an increase in bone turnover. However, opposite effects can occur at certain times and concentrations. Interactions among these factors could explain some physiological, pathological, and therapeutic responses in skeletal tissue.

The effect of ultra-high molecular weight polyethylene wear debris on MG63 osteosarcoma cells in vitro

BACKGROUND

Focal osteolysis due to ultra-high molecular weight polyethylene wear debris involves effects on both bone resorption and bone formation.

METHODS

The response of MG63 osteoblast-like osteosarcoma cells to ultra-high molecular weight polyethylene wear debris isolated by enzymatic digestion of granulomatous tissue obtained from the sites of failed total hip arthroplasties was examined. Scanning electron microscopy, particle-size analysis, and Fourier transform infrared spectroscopy were used to characterize the number, morphology, size distribution, and chemical composition of the particles. Cell response was assessed by adding particles at varying dilutions to confluent cultures and measuring changes in cell proliferation (number of cells and [3H]-thymidine incorporation), osteoblast function (alkaline-phosphatase-specific activity and osteocalcin production), matrix production (collagen production and proteoglycan sulfation), and local cytokine production (prostaglandin-E2 production).

RESULTS

The mean size of the particles was 0.60 micrometer, and 95 percent of the particles had a size of less than 1.5 micrometers. The number of particles per gram of tissue ranged from 1.39 to 3.38x10(9). Three of the four batches of particles were endotoxin-free. Exposure of the cells to particles of wear debris significantly increased the number of cells (p<0.05) and the [3H]-thymidine incorporation (p<0.05) in a dose-dependent manner. In contrast, the addition of particles decreased alkaline-phosphatase-specific activity and osteocalcin production. Collagen production and proteoglycan sulfation were also decreased, while prostaglandin-E2 synthesis was increased by the addition of particles.

CONCLUSIONS

Ultra-high molecular weight polyethylene particles isolated from human tissue stimulated osteoblast proliferation and prostaglandin-E2 production and inhibited cell differentiation and matrix production. These results indicate that particles of wear debris inhibit cell functions associated with bone formation and that osteoblasts may produce factors in response to wear debris that influence neighboring cells, such as osteoclasts and macrophages.

CLINICAL RELEVANCE

Particles of wear debris, especially ultra-high molecular weight polyethylene, have been implicated in the loosening of implants and the development of osteolysis. The present study shows that particles of ultra-high molecular weight polyethylene isolated from human tissue inhibit osteoblast functions associated with bone formation. In addition, particles of wear debris induced osteoblasts to secrete factors capable of influencing neighboring cells, such as osteoclasts and macrophages. These results suggest that osteoblasts may play a role in the cascade of events leading to granuloma formation, osteolysis, and failure of orthopaedic implants.

Platelet derived growth factor stimulates chondrocyte proliferation but prevents endochondral maturation

Platelet-derived growth factor (PDGF) is a cytokine released by platelets at sites of injury to promote mesenchymal cell proliferation. Since many bone wounds heal by endochondral bone formation, we examined the response of chondrocytes in the endochondral lineage to PDGF. Confluent cultures of rat costochondral resting zone cartilage cells were incubated with 0-300 ng/mL PDGF-BB for 24 h to determine whether dose-dependent changes in cell proliferation (cell number and [3H]-thymidine incorporation), alkaline phosphatase specific activity, [35S]-sulfate incorporation, or [3H]-proline incorporation into collagenase-digestible protein (CDP) or noncollagenase-digestible protein (NCP), could be observed. Long-term effects of PDGF were assessed in confluent cultures treated for 1, 2, 4, 6, 8, or 10 d with 37.5 or 150 ng/mL PDGF-BB. To determine whether PDGF-BB could induce resting zone chondrocytes to change maturation state to a growth zone chondrocyte phenotype, confluent resting zone cell cultures were treated for 1, 2, 3, or 5 d with 37.5 or 150 ng/ml PDGF-BB and then challenged for an additional 24 h with 1,25-(OH)2D3. PDGF-BB caused a dose-dependent increase in cell number and [3H]-thymidine incorporation at 24 h. The proliferative effect of the cytokine decreased with time. PDGF-BB had no effect on alkaline phosphatase at 24 h, but at later times, the cytokine prevented the normal increase in enzyme activity seen in post-confluent cultures. This effect was primarily on the cells and not on the matrix. PDGF-BB stimulated [35S]-sulfate incorporation at all times examined, but had no effect on [3H]-proline incorporation into either the CDP or NCP pools. Thus, percent collagen production was not changed. Treatment of the cells for up to 5 d with PDGF-BB failed to elicit a 1,25-(OH)2D3 responsive phenotype typical of rat costochondral growth zone cartilage cells. These results show that committed chondrocytes can respond to PDGF-BB with increased proliferation. The effect of the cytokine is to enhance cartilage matrix production, but at the same time to prevent progression of the cells along the endochondral maturation pathway.

Recombinant bone morphogenetic protein (BMP)-2 regulates costochondral growth plate chondrocytes and induces expression of BMP-2 and BMP-4 in a cell maturation-dependent manner

This study examined the effect of recombinant human bone morphogenetic protein-2 on several parameters of growth, differentiation, and matrix synthesis and on the endogenous production of mRNA of bone morphogenetic proteins 2 and 4 by growth plate chondrocytes in culture. Chondrocytes from resting and growth zones were obtained from rat costochondral cartilage and cultured for 24 or 48 hours in medium containing 0.05-100 ng/ml recombinant human bone morphogenetic protein-2 and 10% fetal bovine serum. Incorporation of [3H]thymidine, cell number, alkaline phosphatase specific activity, incorporation of [3H]proline into collagenase-digestible protein and noncollagenase-digestible protein, and incorporation of [35S]sulfate were assayed as indicators of cell proliferation, differentiation, and extracellular matrix synthesis. mRNA levels for bone morphogenetic proteins 2 and 4 were determined by Northern blot analysis. Recombinant human bone morphogenetic protein-2 increased the incorporation of [3H]thymidine by quiescent resting-zone and growth-zone cells in a similar manner, whereas it had a differential effect on nonquiescent cultures. At 24 and 48 hours, 12.5-100 ng/ml recombinant human bone morphogenetic protein-2 caused a dose-dependent increase in cell number and DNA synthesis in resting-zone chondrocytes. No effect was seen in growth-zone cells. Recombinant human bone morphogenetic protein-2 stimulated alkaline phosphatase specific activity in resting-zone chondrocytes in a bimodal manner, causing significant increases between 0.2 and 0.8 ng/ml and again between 25 and 100 ng/ml. In contrast, alkaline phosphatase specific activity in growth-zone chondrocytes was significantly increased only between 12.5 and 100 ng/ml. Recombinant human bone morphogenetic protein-2 increased the production of both collagenase-digestible protein and noncollagenase-digestible protein by resting-zone and growth-zone cells, but incorporation of [35S]sulfate was unaffected. Administration of recombinant human bone morphogenetic protein-2 also increased incorporation of [3H]uridine in both resting-zone and growth-zone chondrocytes; these cells produced mRNA for bone morphogenetic proteins 2 and 4. Bone morphogenetic protein-2 mRNA levels in both resting-zone and growth-zone chondrocytes increased in the presence of recombinant human bone morphogenetic protein-2; however, bone morphogenetic protein-4 mRNA levels in growth-zone cells decreased under its influence, and those in resting-zone cells were upregulated only with a dose of 10 ng/ml. This indicates that recombinant human bone morphogenetic protein-2 regulates chondrocyte proliferation, differentiation, and matrix production, and the effects are dependent on the stage of cell maturation. Resting-zone chondrocytes were more sensitive, suggesting that they are targeted by bone morphogenetic protein-2 and that this growth factor may have autocrine effects on these cells.

Evidence that interleukin-1, but not interleukin-6, affects costochondral chondrocyte proliferation, differentiation, and matrix synthesis through an autocrine pathway

Although the effects of interleukin-1 (IL-1) and interleukin-6 (IL-6) on articular cartilage chondrocytes have been reported, little is known concerning the effects of these cytokines on growth plate chondrocytes. In this study, we examined the effect of IL-1 alpha, IL-1 beta, and IL-6 on growth plate chondrocyte proliferation, differentiation, and matrix production as a function of cell maturation and examined the ability of these cells to produce IL-1 alpha and IL-1 beta. Confluent fourth passage cultures of rat costochondral resting zone and growth zone chondrocytes were treated with 0-100 ng/ml of IL-1 alpha, IL-1 beta, or IL-6 for 24 h and then assayed for [3H]-thymidine incorporation, alkaline phosphatase specific activity, [35S]-sulfate incorporation, and percent collagen production. Neutralizing polyclonal antibodies were used to confirm the specificity of response to each cytokine. Treatment of resting zone cells with IL-1 alpha produced a significant, dose-dependent decrease in [3H]-thymidine incorporation, while similarly treated growth zone cells were unaffected by treatment with this cytokine. IL-1 alpha also stimulated alkaline phosphatase specific activity and inhibited [35S]-sulfate incorporation by resting zone chondrocytes, but had no affect on growth zone chondrocytes. When collagen production was examined, it was observed that IL-1 alpha had a stimulatory affect on growth zone cells but no affect on resting zone cells. When the effect of IL-1 beta was examined, it was observed that this cytokine inhibited [3H]-thymidine incorporation by resting zone cells and stimulated isotope incorporation in growth zone cells. IL-1 beta also stimulated alkaline phosphatase specific activity and inhibited [35S]-sulfate incorporation by resting zone chondrocytes but had no affect on growth zone chondrocytes. In contrast to IL-1 alpha, IL-1 beta stimulated collagen production by resting zone cells but not growth zone cells. IL-6 had no affect on any of the parameters measured in either cell type. When cytokine production was measured, it was found that IL-1 alpha was produced by both cell types, while IL-1 beta was produced only by resting zone cells. Resting zone cells secreted both IL-1 alpha and IL-1 beta into the media, but 75% of the total cytokine produced by these cells was retained in the cell layer. In contrast, growth zone cells did not secrete measurable IL-1 alpha into the media. These results suggest that IL-1 alpha and IL-1 beta target resting zone cells, inducing them to differentiate and acquire a phenotype characteristic of the more mature growth zone cells. Moreover, resting zone chondrocytes produce both IL-1 alpha and IL-1 beta, suggesting the possibility of an autocrine effect of these cytokines on the cells.

Effect of titanium surface roughness on chondrocyte proliferation, matrix production, and differentiation depends on the state of cell maturation

Although it is well accepted that implant success is dependent on various surface properties, little is known about the effect of surface roughness on cell metabolism or differentiation, or whether the effects vary with the maturational state of the cells interacting with the implant. In the current study, we examined the effect of titanium (Ti) surface roughness on chondrocyte proliferation, differentiation, and matrix synthesis using cells derived from known stages of endochondral development. Chondrocytes derived from the resting zone (RCs) and growth zone (GCs) of rat costochondral cartilage were cultured on Ti disks that were prepared as follows: HF-HNO3-treated and washed (PT); PT-treated and electropolished (EP); fine sand-blasted, HCl-H2SO4-etched, and washed (FA); coarse sand-blasted, HCl-H2SO4-etched, and washed (CA); or Ti plasma-sprayed (TPS). Based on surface analysis, the Ti surfaces were ranked from smoothest to roughest: EP, PT, FA, CA, and TPS. Cell proliferation was assessed by cell number and [3H]-thymidine incorporation, and RNA synthesis was assessed by [3H]-uridine incorporation. Differentiation was determined by alkaline phosphatase specific activity (AL-Pase). Matrix production was measured by [3H]-proline incorporation into collagenase-digestible (CDP) and noncollagenase-digestible (NCP) protein and by [35S]-sulfate incorporation into proteoglycan. GCs required two trypsinizations for complete removal from the culture disks; the number of cells released by the first trypsinization was generally decreased with increasing surface roughness while that released by the second trypsinization was increased. In RC cultures, cell number was similarly decreased on the rougher surfaces; only minimal numbers of RCs were released by a second trypsinization. [3H]-thymidine incorporation by RCs decreased with increasing surface roughness while that by GCs was increased. [3H]-Uridine incorporation by both GCs and RCs was greater on rough surfaces. Conversely, ALPase in the cell layer and isolated cells of both cell types was significantly decreased. GC CDP and NCP production was significantly decreased on rough surfaces while CDP production by RC cells was significantly decreased on smooth surfaces. [35S]-sulfate incorporation by RCs and GCs was decreased on all surfaces compared to tissue culture plastic. The results of this study indicate that surface roughness affects chondrocyte proliferation, differentiation, and matrix synthesis, and that this regulation is cell maturation dependent.

Effect of titanium surface roughness on chondrocyte proliferation, matrix production, and differentiation depends on the state of cell maturation

Although it is well accepted that implant success is dependent on various surface properties, little is known about the effect of surface roughness on cell metabolism or differentiation, or whether the effects vary with the maturational state of the cells interacting with the implant. In the current study, we examined the effect of titanium (Ti) surface roughness on chondrocyte proliferation, differentiation, and matrix synthesis using cells derived from known stages of endochondral development. Chondrocytes derived from the resting zone (RCs) and growth zone (GCs) of rat costochondral cartilage were cultured on Ti disks that were prepared as follows: HF-HNO3-treated and washed (PT); PT-treated and electropolished (EP); fine sand-blasted, HCl-H2SO4-etched, and washed (FA); coarse sand-blasted, HCl-H2SO4-etched, and washed (CA); or Ti plasma-sprayed (TPS). Based on surface analysis, the Ti surfaces were ranked from smoothest to roughest: EP, PT, FA, CA, and TPS. Cell proliferation was assessed by cell number and [3H]-thymidine incorporation, and RNA synthesis was assessed by [3H]-uridine incorporation. Differentiation was determined by alkaline phosphatase specific activity (AL-Pase). Matrix production was measured by [3H]-proline incorporation into collagenase-digestible (CDP) and noncollagenase-digestible (NCP) protein and by [35S]-sulfate incorporation into proteoglycan. GCs required two trypsinizations for complete removal from the culture disks; the number of cells released by the first trypsinization was generally decreased with increasing surface roughness while that released by the second trypsinization was increased. In RC cultures, cell number was similarly decreased on the rougher surfaces; only minimal numbers of RCs were released by a second trypsinization. [3H]-thymidine incorporation by RCs decreased with increasing surface roughness while that by GCs was increased. [3H]-Uridine incorporation by both GCs and RCs was greater on rough surfaces. Conversely, ALPase in the cell layer and isolated cells of both cell types was significantly decreased. GC CDP and NCP production was significantly decreased on rough surfaces while CDP production by RC cells was significantly decreased on smooth surfaces. [35S]-sulfate incorporation by RCs and GCs was decreased on all surfaces compared to tissue culture plastic. The results of this study indicate that surface roughness affects chondrocyte proliferation, differentiation, and matrix synthesis, and that this regulation is cell maturation dependent.

Effect of titanium surface roughness on proliferation, differentiation, and protein synthesis of human osteoblast-like cells (MG63)

The effect of surface roughness on osteoblast proliferation, differentiation, and protein synthesis was examined. Human osteoblast-like cells (MG63) were cultured on titanium (Ti) disks that had been prepared by one of five different treatment regimens. All disks were pretreated with hydrofluroic acid-nitric acid and washed (PT). PT disks were also: washed, and then electropolished (EP); fine sandblasted, etched with HCl and H2SO4, and washed (FA); coarse sandblasted, etched with HCl and H2SO4, and washed (CA); or Ti plasma-sprayed (TPS). Standard tissue culture plastic was used as a control. Surface topography and profile were evaluated by brightfield and darkfield microscopy, cold field emission scanning electron microscopy, and laser confocal microscopy, while chemical composition was mapped using energy dispersion X-ray analysis and elemental distribution determined using Auger electron spectroscopy. The effect of surface roughness on the cells was evaluated by measuring cell number, [3H]thymidine incorporation into DNA, alkaline phosphatase specific activity, [3H]uridine incorporation into RNA, [3H]proline incorporation into collagenase digestible protein (CDP) and noncollagenase-digestible protein (NCP), and [35S]sulfate incorporation into proteoglycan. Based on surface analysis, the five different Ti surfaces were ranked in order of smoothest to roughest: EP, PT, FA, CA, and TPS. A TiO2 layer was found on all surfaces that ranged in thickness from 100 A in the smoothest group to 300 A in the roughest. When compared to confluent cultures of cells on plastic, the number of cells was reduced on the TPS surfaces and increased on the EP surfaces, while the number of cells on the other surfaces was equivalent to plastic. [3H]Thymidine incorporation was inversely related to surface roughness. Alkaline phosphatase specific activity in isolated cells was found to decrease with increasing surface roughness, except for those cells cultured on CA. In contrast, enzyme activity in the cell layer was only decreased in cultures grown on FA- and TPS-treated surfaces. A direct correlation between surface roughness and RNA and CDP production was found. Surface roughness had no apparent effect on NCP production. Proteoglycan synthesis by the cells was inhibited on all the surfaces studied, with the largest inhibition observed in the CA and EP groups. These results demonstrate that surface roughness alters osteoblast proliferation, differentiation, and matrix production in vitro. The results also suggest that implant surface roughness may play a role in determining phenotypic expression of cells in vivo.

Culture surfaces coated with various implant materials affect chondrocyte growth and metabolism

The effect on chondrocyte metabolism of culture surfaces sputter-coated with various materials used for orthopaedic implants was studied and correlated with the stage of cartilage cell maturation. Confluent, fourth-passage chondrocytes from the costochondral resting zone and growth zone of rats were cultured for 6 or 9 days on 24-well plates sputter-coated with ultrathin films of titanium, titanium dioxide, aluminum oxide, zirconium oxide, and calcium phosphate (1.67:1). Corona-discharged tissue culture plastic served as the control. The effect of surface material was examined with regard to cell morphology; cell proliferation (cell number) and DNA synthesis ([3H]thymidine incorporation); RNA synthesis ([3H]uridine incorporation); collagenase-digestible protein, noncollagenase-digestible protein, and percentage of collagen production; and alkaline phosphatase-specific activity, both in the cell layer and in trypsinized chondrocytes. Cell morphology was dependent on surface material; only cells cultured on titanium had an appearance similar to that of cells cultured on plastic. While titanium or titanium dioxide surfaces had no effect on cell number or [3H]thymidine incorporation, aluminum oxide, calcium phosphate, and zirconium oxide surfaces inhibited both parameters. Cells cultured on aluminum oxide, calcium phosphate, zirconium oxide, and titanium dioxide exhibited decreased collagenase-digestible protein, noncollagenase-digestible protein, and percentage of collagen production, but [3H]uridine incorporation was decreased only in those chondrocytes cultured on aluminum oxide, calcium phosphate, or zirconium oxide. Chondrocytes cultured on titanium had greater alkaline phosphatase-specific activity than did cells cultured on plastic, but the incorporation of [3H]uridine and production of collagenase-digestible protein, noncollagenase-digestible protein, and percentage of collagen was comparable. The response of chondrocytes from the growth zone and resting zone to culture surface was comparable, differing primarily in magnitude. Cell maturation-dependent effects were evident when enzyme activity in trypsinized and scraped cells was compared. These results indicate that different surface materials affect chondrocyte metabolism and phenotypic expression in vitro and suggest that implant materials may modulate the phenotypic expression of cells in vivo.

Sex-dependent effects of 17-beta-estradiol on chondrocyte differentiation in culture

This study examined the effects of 17-beta-estradiol (E2) on chondrocyte differentiation in vitro. Cells derived from male or female rat costochondral growth zone and resting zone cartilage were used to determine whether the effects of E2 were dependent on the stage of chondrocyte maturation and whether they were sex-specific. [3H]-Thymidine incorporation, cell number, alkaline phosphatase specific activity, and percent collagen production were used as indicators of differentiation. Alkaline phosphatase specific activity in matrix vesicles and plasma membranes isolated from female chondrocyte cultures was measured to determine which membrane fraction was targeted by the hormone. Specificity of the E2 effects was assessed using 17-alpha-estradiol. The role of fetal bovine serum and phenol red in the culture medium was also addressed. The results demonstrated that E2 decreases cell number and [3H]-thymidine incorporation in female chondrocytes, indicating that it promotes differentiation of these cells. Alkaline phosphatase specific activity is stimulated in both growth zone and resting zone cells, but the effect is greater in the less mature resting zone chondrocytes. The increase in enzyme activity is targeted to the matrix vesicles in both cell types, but the fold increase is greater in the growth zone cells. In male chondrocytes, there was a decrease in [3H]-thymidine incorporation at high E2 concentrations in resting zone cells at the earliest time point examined (12 hours) and a slight stimulation in alkaline phosphatase activity in growth zone cells at 24 hours. Cells cultured in serum-free medium exhibited a dose-dependent inhibition in alkaline phosphatase activity when cultured with E2, even in the presence of phenol red. E2-dependent stimulation of enzyme activity is seen only in the presence of serum, suggesting that serum factors are also necessary. E2 increased percent collagen production in female cells only; the magnitude of the effect was greatest in the resting zone chondrocyte cultures. The results of this study indicate that the effects of E2 are dependent on time of exposure, presence of serum, and the sex and state of maturation of the chondrocytes. E2-dependent stimulation of alkaline phosphatase specific activity is targeted to matrix vesicles.

In vitro studies on bone resorption in neonatal mouse calvariae using a modified dissection technique giving four samples of bone from each calvaria

Bone resorption in a modified bone culture system, based on incubation of small fragments from neonatal mouse calvarial bones, has been studied. Four bone fragments were dissected out from each mouse calvaria and were thereafter cultured in CMRL 1066 medium in plastic multiwell dishes. Bone resorption was assessed by 45Ca release from prelabeled bones. The rate of bone resorption in response to parathyroid hormone (PTH) was less in the anterior part of the calvaria compared to the posterior part. After removing the anterior region, four parietal bone fragments that showed identical basal and PTH-stimulated release of 45Ca could be dissected out from each mouse. Excretion of lactate dehydrogenase and beta-glucuronidase was the same in bones cultured submerged or on grids. Uptake of [3H]thymidine in bones cultured submerged was 54% of [3H]thymidine uptake in bones cultured on grids. Dose-response curves, established by using parietal bone fragments, showed that the sensitivity and the magnitude of the increase in 45Ca release seen after stimulation with PTH, prostaglandin E2, and 1 alpha-hydroxyvitamin D3 were the same for bones cultured submerged or on grids. The 45Ca release in response to stimulation with PTH, prostaglandin E2, and 1 alpha-OHD3 was the same in calvarial fragments cultured submerged and those previously obtained with calvarial halves cultured on grids. Thus, even though the rate of DNA synthesis was slower in bones cultured submerged, the rate and the magnitude of resorption were the same in bones cultured on grids or submerged. These data show that it is possible to perform studies on bone resorption with small fragments of neonatal mouse parietal bones.

The biphasic effect of triiodothyronine compared to bone resorbing effect of PTH on bone modelling of mouse long bone in vitro

To examine the effects of T3 on fetal long bone modelling the radii and ulnae of 16 day old fetal mice were grown in vitro for two days. Their growth, mineralization, and resorption were assessed by measuring diaphyseal length, calcium and phosphorus content, hydroxyproline content, and the release of incorporated 45Ca. The effects of T3 were compared to the effects of 1-34 PTH, a known resorbing agent, on the same system. Devitalized bones were used as a control. The results showed that T3 had a biphasic effect. At high concentrations (10(-5) M-10(-6) M) T3 inhibited the growth of the bones as indicated by their diaphyseal length and hydroxyproline content. Calcium and phosphorus content were significantly decreased while 45Ca release was increased. Similar effects were also found after the addition of 1-34 PTH to the media. However, T3, at lower concentrations (10(-7) M-10(-9) M), stimulated the growth and calcification of the bones as indicated by an increase in diaphyseal length and the hydroxyproline, calcium, and phosphorus content. 45Ca release was significantly decreased at these concentrations. Neither T3 nor 1-34 PTH affected devitalized bones in the same system. The results suggest that at physiological concentrations, T3 has a direct, anabolic effect on bone, which may explain its major role in the growth process of various species. At high doses, however, T3 stimulates bone resorption in a way similar to PTH.

Effects of vitamin D metabolites on collagen production and cell proliferation of growth zone and resting zone cartilage cells in vitro

Previous studies have suggested that vitamin D metabolites directly influence the differentiation and maturation of chondrocytes in calcifying cartilage. Recently, this laboratory has shown that the response of chondrocyte plasma membrane and matrix vesicle enzymes to 1,25-(OH)2D3 and 24,25-(OH)2D3 is both cell and membrane specific. The current study demonstrates that cell replication and matrix protein synthesis are also modulated by vitamin D. Confluent, third-passage growth zone (GC) and resting zone (RC) costochondral chondrocytes were incubated in medium containing 10(-13)-10(-7) M 1,25-(OH)2D3 or 10(-12)-10(-6) M 24,25-(OH)2D3. The amount of collagenase-digestible protein (CDP) secreted into the media was inversely proportional to the concentration of fetal bovine serum (FBS). At 10% FBS, greater than 80% of the CDP was incorporated into the matrix. 1,25-(OH)2D3 stimulated CDP and percentage collagen synthesis by GC cells but had no effect on the synthesis of noncollagenous protein (NCP). 1,25-(OH)2D3 inhibited CDP and percentage collagen synthesis by RC cells but did not alter NCP synthesis. [3H]thymidine incorporation was inhibited in both cell types, whether confluent or subconfluent cultures were examined. At 10(-6) and 10(-7) M 24,25-(OH)2D3, there was a significant decrease in CDP production and percentage collagen synthesis by RC cells but no effect on NCP. However, at 10(-9) and 10(-10) M hormone there was an increase in NCP production but no effect on CDP, resulting in a decrease in percentage collagen synthesis. CDP and NCP production were unaffected by 24,25-(OH)2D3 in GC cells. High concentrations of hormone inhibited [3H]thymidine incorporation in both cell types.(ABSTRACT TRUNCATED AT 250 WORDS)

Forskolin sensitizes parathyroid hormone-induced cyclic AMP response, but not the bone resorptive effect, in mouse calvarial bones

The effect of forskolin on parathyroid hormone (PTH) stimulated bone resorption, as assessed in vitro by the release of 45Ca from prelabelled neonatal mouse calvarial bones, and cyclic AMP formation in mouse calvarial bones and osteoblast-like cells was investigated. Much higher concentrations (100-300-times) of PTH were required to stimulate cyclic AMP accumulation than to stimulate mineral mobilization in murine calvarial bones. PTH, in the absence of phosphodiesterase inhibitor, stimulated cyclic AMP formation in mouse calvarial bones at and above concentrations of 3-10 nmol/l with EC50 at 10-15 nmol/l. In the presence of forskolin (1 or 10 mumol/l) the minimal concentration required to obtain a cyclic AMP response to PTH was decreased by a factor of 30-100 and the EC50 value was decreased to 1-2 nmol/l. Similar results were seen in osteoblast-enriched cells. In addition, the magnitude of the PTH-induced cyclic AMP response was substantially potentiated by forskolin, both in calvarial bones and in isolated osteoblasts. Forskolin, in the absence of PTH, stimulated cyclic AMP levels in mouse calvaria at and above 1 mumol/l. In the presence of PTH, the response to forskolin was potentiated over the whole dose-response curve with apparent EC50 value at 1-2 mumol/l of forskolin. Forskolin (1 mumol/l) did not affect the magnitude of the 45Ca release response to PTH in 24 or 48 h cultures. In 96 h cultures, forskolin, in an additive manner, potentiated the effect of PTH on calcium mobilization. These results show that forskolin, in mouse calvarial bones and in isolated osteoblasts, in addition to directly stimulating cyclic AMP, can enhance receptor-mediated activation of adenylate cyclase. The finding that forskolin did not synergistically potentiate PTH-induced bone resorption suggests that there is no simple relationship between PTH-induced cyclic AMP formation and stimulation of bone resorption.

Modifications of the mouse calvarial technique improve the responsiveness to stimulators of bone resorption

The effect of a preincubation period, in basic medium or in medium with inhibitors of prostaglandin biosynthesis, on the response to different stimulators of bone resorption has been studied in an organ culture system using calvarial bones from neonatal mice. Bone resorption was assessed either by the release of 45Ca or by the release of 3H from [3H]-proline labeled bones. Preincubated bones were cultured for 18-24 hr in medium, with and without indomethacin, hydrocortisone, and dexamethasone, and then extensively washed before being transferred to culture medium containing different stimulators of bone resorption. Preincubation in medium containing indomethacin or corticosteroids resulted in an increased response to parathyroid hormone (PTH), prostaglandin E2 (PGE2), 1-alpha-hydroxyvitamin-D3 and thrombin as compared to the response in bones which were exposed to the stimulants directly after dissection. Preincubation in basic medium did not enhance the subsequent response to PTH. By using a preincubation period in indomethacin, the dose-response curves for the stimulatory effect of PTH and PGE2 on mineral mobilization could be sensitized as compared to the curves obtained with fresh bones. Thus, the concentration of agonists causing 50% stimulation of 45Ca release was decreased by a factor of 10. The threshold for actions of PTH and PGE2 on 45Ca release was 0.01-0.03 and 1-3 nmol/l, respectively.

Effects of DNA and prostaglandin synthesis inhibitors on the stimulation of bone resorption by epidermal growth factor in fetal rat long-bone cultures

We examined two inhibitors of DNA synthesis, hydroxyurea (HU) and aphidicholin (APC), and two inhibitors of prostaglandin cyclooxygenase, indomethacin and flufenamic acid, for their effects on the resorptive responses of fetal rat long-bone cultures to epidermal growth factor (EGF) and parathyroid hormone (PTH). As we have previously found, HU decreased unstimulated 45Ca release but had little effect on the resorptive response to PTH. HU also did not block resorption stimulated by EGF. Addition of the cyclooxygenase inhibitor, indomethacin, did not alter the resorptive responses of unstimulated or PTH-treated cultures in either the presence or absence of HU or the resorptive response of bones cultured with EGF alone. However, indomethacin completely blocked the resorptive response to EGF of bones that were cultured with HU. The effects of indomethacin on EGF-mediated resorption in HU-treated cultures appeared to be related to an inhibition of prostaglandin synthesis since flufenamic acid had similar effects. However, the effects of HU on the resorptive response to EGF may not have resulted solely from its inhibitory action on DNA synthesis since APC, in the absence of cyclooxygenase inhibitors, completely blocked EGF-mediated resorption without significantly affecting the response to PTH. These results demonstrate that the mechanisms regulating PTH- and EGF-mediated resorption in fetal rat long-bone cultures differ, and imply that a component of EGF-mediated resorption in these cultures is dependent on sustained DNA synthesis.

Effects of parathyroid hormone and cortisol on prostaglandin production by neonatal rat calvaria in vitro

We have examined the effects of parathyroid hormone (PTH) and cortisol on the production of prostaglandins, particularly PGE2, by neonatal rat calvaria cultured in a chemically-defined medium. Although there was considerable variability, calvaria produced large amounts of PGE2 in control cultures, reaching medium concentrations of 40 to 200 nM. PGE2 release was partially inhibited by cortisol at 10 nM and markedly inhibited at 100 nM. Bovine 1-34 synthetic PTH produced an increase in PGE2 concentration which was most striking in the presence of a low concentration of cortisol (10 nM). The medium also contained large amounts of 6-keto PGF1 alpha, the metabolite of prostacyclin, which showed similar changes in response to PTH and cortisol. Thromboxane B2 concentrations were low and unaffected by these hormones. 1,25-dihydroxyvitamin D did not increase medium PGE2 concentration. Since PGE2 is a potent stimulator of bone resorption and formation, some of the effects of PTH as well as cortisol may be mediated by their ability to alter PGE2 production in skeletal tissue.

A multiresponse parathyroid hormone assay: an inhibitor has agonist properties in vivo

Vitamin D-deficient rats subjected to thyroparathyroidectomy (TPTX) were used to evaluate in vivo the biological properties of native bovine parathyroid hormone (bPTH) and chemically synthesized fragments and analogues of the hormone on several parameters of hormone action: calcium and phosphorus fluxes, generation of cyclic adenosine 3',5'-monophosphate (cAMP), and the metabolism of 25-hydroxyvitamin D3 [25(OH)D3]. Vitamin D-deficient rats, after TPTX or sham operation, were intravenously infused with a nutrient containing 7.5 mM CaCl2 for 30 h. During the last 7 h, PTH or one of its analogues was infused intravenously at rates between 0.04 and 20 nmol/h. One hour after the start of the peptide infusion, tritiated 25(OH)D3 was injected. Urine was collected hourly for phosphate and cAMP determinations and, at the end of the experiment, blood was obtained to determine the relative accumulation of tritiated 1,25-dihydroxyvitamin D3 ([3H]1,25(OH)2D3). Infusion of bPTH-(1--84), bPTH-(1--34), human (h)PTH-(1--34), or [Nle8, Nle18, Tyr34]bPTH-(1--34) amide was accompanied by a comparable dose-dependent decrease in plasma phosphate and a dose-dependent increase in plasma calcium and [3H]-1,25(OH)2D3, and urinary excretion of phosphate and cAMP. An evaluation of [Nle8, Nle18, Tyr34]bPTH-(3--34) amide, a potent inhibitor of PTH action in vitro in the renal adenylate cyclase assay, revealed that the analogue possessed weak agonist properties in vivo. The analogue increased excretion of both cAMP and phosphate in the urine, decreased plasma phosphate levels, and increased the accumulation of [3H]-1,25(OH)2D3 in the plasma. This multiparameter model system should aid in the elucidation of the in vivo biological effects of PTH and its analogues.

Use of osmotic minipumps for delivery of parathyroid hormone

To determine if osmotic minipumps can be used for the local delivery of parathyroid hormone (PTH), we examined the bone resorbing activity of PTH in minipumps, either removed and assayed in bone organ cultures or released over rat calvaria. Biological activity of PTH was maintained for up to 6 days when the hormone solution contained serum and the minipumps and tubing were siliconized and flushed with diluent prior to use. Addition of cysteine did not enhance activity.

Alkaline phosphatase inhibition by parathyroid hormone and isoproterenol in a clonal rat osteosarcoma cell line. Possible mediation by cyclic AMP

The effect of parathyroid hormone (PTH 1-34 bovine) on alkaline phosphatase activity was investigated in an osteoblast-like clonal cell line derived from rat osteosarcoma (ROS 17/2). ROS 17/2 alkaline phosphatase resembled the bone enzyme in levamisole sensitivity and electrophoretic mobility but differed in heat stability. The specific activity of ROS 17/2 alkaline phosphatase increased with time in culture. This increase was inhibited by PTH (1-34) and (-)-isoproterenol in a dose-dependent manner starting at near-physiological hormone concentrations. The ID50 values were 0.02 nM for PTH (1-34) and 1.7 nM for isoproterenol. The two hormones stimulated ROS 17/2 adenylate cyclase, albeit at higher concentrations: Km values were 13 nM for PTH (1-34) and 16 nM for isoproterenol. The rise in alkaline phosphatase was also inhibited by dibutyryl cyclic AMP and 8-bromocyclic AMP (0.1 mM). These findings further document the osteoblastic properties of the ROS 17/2 osteosarcoma cell line, suggest that PTH inhibition of alkaline phosphatase represents a physiological response to the hormone in these cells, and implicate cyclic AMP as a mediator of this PTH effect.

Parathyroid hormone stimulates bone formation and resorption in organ culture: evidence for a coupling mechanism

We have developed an in vitro system, using embryonic chicken tibiae grown in a serum-free medium, which exhibits simultaneous bone formation and resorption. Tibiae from 8-day embryos increased in mean (+/- SD) length (4.0 +/- 0.4 to 11.0 +/- 0.3 mm) and dry weight (0.30 +/- 0.04 to 0.84 +/- 0.04 mg) during 12 days in vitro. There was increased incorporation of [3H]proline into hydroxyproline (120 +/- 20 to 340 +/- 20 cpm/mg of bone per 24 hr) as a measure of collagen synthesis, as well as a 62 +/- 5% increase in total calcium and 45Ca taken up as an indication of active mineralization. A physiologic concentration (1 pM) of parathyroid hormone was found to stimulate bone resorption over control levels in this system. Parathyroid hormone stimulated the release of [3H]hydroxyproline from the bone shafts but not from the cartilage ends, indicating the specificity of the response. With 1 pM parathyroid hormone we observed an acute inhibition of bone formation, followed (after 12-16 hr) by a chronic stimulation of bone formation during the 12-day incubation. Both mineral uptake and matrix formation were enhanced at approximately the same rate during the 12-day incubation. The chronic enhancement of formation required parathyroid hormone only for the initial 8-10 hr of incubation. These results could be explained by the production or release of a factor from bone to stimulate formation in response to the acute increase in resorption--a "coupling factor." Indeed, dialyzed culture medium conditioned by actively resorbing bones stimulated bone formation over controls when added to organ cultures at a 1:20 dilution. The factor is larger than 12,000 daltons as determined by dialysis. The factor is specific for the bone shaft and did not affect the cartilage ends.