Characterization of growth factor activity produced by fetal mouse osteoblasts

Thrombin-stimulated growth factor and cytokine expression in osteoblasts is mediated by protease-activated receptor-1 and prostanoids

Thrombin exerts multiple effects upon osteoblasts including stimulating proliferation, and inhibiting osteoblast differentiation and apoptosis. Some of these effects are believed to be mediated by the synthesis and secretion of autocrine factors such as growth factors and cytokines. Many but not all cellular responses to thrombin are mediated by members of the protease-activated receptor (PAR) family of G protein-coupled receptors. The current study was undertaken to investigate the nature of thrombin's induction of autocrine factors by analysing the expression of twelve candidate genes in thrombin-stimulated primary mouse osteoblasts. Analysis by quantitative reverse transcription polymerase chain reaction (qRT-PCR) demonstrated that thrombin induced transforming growth factor beta, cyclooxygenase-2, tenascin C, fibroblast growth factor-1 and -2, connective tissue growth factor and interleukin-6 expression in wild type osteoblasts, but not PAR-1 null mouse osteoblasts. Induction of all the thrombin-responsive genes was blocked by the presence of the non-selective cyclooxygenase inhibitor indomethacin. Further studies were conducted on interleukin-6, which was the gene that showed the greatest increase in expression following stimulation of osteoblast-like cells with thrombin. A PAR-1-specific activating peptide, but neither a PAR-4-activating peptide nor catalytically inactive thrombin induced release of interleukin-6 by osteoblasts. Furthermore, in the presence of the selective cyclooxygenase-1 and -2 inhibitors SC-560 and NS-398 thrombin-induced interleukin-6 release was prevented. Levels of both prostaglandin E(2) and interleukin-6 in medium conditioned by thrombin-stimulated osteoblast-like cells were found to be significantly increased compared to medium conditioned by non-stimulated cells, however release of prostaglandin E(2) was found to precede release of interleukin-6. Treatment of isolated osteoblast-like cells with a number of synthetic prostanoids stimulated secretion of interleukin-6 with differing potencies. These studies suggest that activation of PAR-1 on osteoblasts by thrombin induces cyclooxygenase activity, which in turn results in the increased expression of multiple secreted factors. The induction of these secreted factors may act in an autocrine fashion to alter osteoblast function, allowing these cells to participate in the earliest stages of bone healing by both autocrine and paracrine mechanisms.

Effects of systemic and local administration of recombinant human IGF-I (rhIGF-I) on de novo bone formation in an aged mouse model

DO was used in an aged mouse model to determine if systemically and/or locally administered rhIGF-I improved osteoblastogenesis and new bone formation. Local and systemic rhIGF-I treatment increased new bone formation. However, only systemic delivery produced measurable concentrations of rhIGF-I in the circulation.

INTRODUCTION

Human and rodent research supports a primary role for IGF-I in bone formation. Significant roles for both endocrine and paracrine/autocrine IGF-I have been suggested for normal osteoblastogenesis and bone formation. We have assessed, using a mouse model of distraction osteogenesis (DO), the impact of continuous administration of recombinant human (rh)IGF-I, delivered either locally to the distraction site or absorbed systemically, on bone formation in an aged mouse model.

MATERIALS AND METHODS

DO was performed in aged mice (18-month-old C57BL/6 male mice), which were distracted at 0.15 mm daily. At the time of osteotomy, miniosmotic pumps were inserted subcutaneously to (1) deliver vehicle or rhIGF-I subcutaneously for systemic delivery or (2) deliver vehicle or rhIGF-I directly to the newly forming bone through infusion tubing routed subcutaneously from the pump to the distraction site. Serum concentrations of mouse IGF-I, human IGF-I, and osteocalcin were determined at the end of the study.

RESULTS

New bone formation observed in DO gaps showed a significant increase in new bone formation in rhIGF-I-treated mice, irrespective of delivery route. However, detectable levels of human IGF-I were found only in the serum of animals receiving rhIGF-I systemically. Osteocalcin levels did not differ between controls and rhIGF-I-treated groups.

CONCLUSIONS

Locally and systemically delivered rhIGF-I both produce significant increases in new bone formed in an aged mouse model in which new bone formation is normally markedly impaired, suggesting that rhIGF-I may improve senile osteoporosis. Because systemic administration of IGF-I can result in untoward side effects, including an increased risk for cancer, the findings that locally delivered IGF-I improves bone regeneration without increasing circulating IGF-I levels suggests that this delivery route may be preferable in an at-risk, aged population.

Enhancement of osteoblast proliferative capacity by growth factor-like molecules in bear serum

The use of animal serum in cell culture is vital for providing the nutrient factors required to promote proliferation and function. Fetal calf serum has become the preferred choice because of its abundance, reasonable cost, and ability to sustain human cells in vitro. Although a wide variety of serum sources have been tested and used, little is known about the ability of serum obtained from the American black bear (Ursus americanus) to support human cell growth in culture. The American black bear, an animal comparable in size to humans, is unique in that it hibernates for mo at a time but does not experience extensive bone loss normally associated with extended immobility. The aim of this study was to analyze the effect of bear serum on human osteoblast cultures. We discovered that three of the eight bear serum samples induced significantly higher proliferation rates in osteoblasts than did fetal calf serum over a 24-h period. Osteoblasts incubated in bear serum displayed higher messenger ribonucleic acid levels for phenotype markers osteocalcin and type I collagen than did those incubated in fetal calf serum. The mitogenic activity of the bear serum was reduced when heated at 56 degrees C for 30 min before use in culture. The molecular weight of the mitogenic factors was found to be primarily greater than 50 kDa. The present work demonstrates the capability of serum from American black bears to support human osteoblast proliferation in vitro.

Growth hormone and bone

It is well known that GH is important in the regulation of longitudinal bone growth. Its role in the regulation of bone metabolism in man has not been understood until recently. Several in vivo and in vitro studies have demonstrated that GH is important in the regulation of both bone formation and bone resorption. In Figure 9 a simplified model for the cellular effects of GH in the regulation of bone remodeling is presented (Fig. 9). GH increases bone formation in two ways: via a direct interaction with GHRs on osteoblasts and via an induction of endocrine and autocrine/paracrine IGF-I. It is difficult to say how much of the GH effect is mediated by IGFs and how much is IGF-independent. GH treatment also results in increased bone resorption. It is still unknown whether osteoclasts express functional GHRs, but recent in vitro studies indicate that GH regulates osteoclast formation in bone marrow cultures. Possible modulations of the GH/IGF axis by glucocorticoids and estrogens are also included in Fig. 9. GH deficiency results in a decreased bone mass in both man and experimental animals. Long-term treatment (> 18 months) of GHD patients with GH results in an increased bone mass. GH treatment also increases bone mass and the total mechanical strength of bones in rats with a normal GH secretion. Recent clinical studies demonstrate that GH treatment of patients with normal GH secretion increases biochemical markers for both bone formation and bone resorption. Because of the short duration of GH treatment in man with normal GH secretion, the effect on bone mass is still inconclusive. Interestingly, GH treatment to GHD adults initially results in increased bone resorption with an increased number of bone-remodeling units and more newly produced unmineralized bone, resulting in an apparent low or unchanged bone mass. However, GH treatment for more than 18 months gives increased bone formation and bone mineralization of newly produced bone and a concomitant increase in bone mass as determined with DEXA. Thus, the action of GH on bone metabolism in GHD adults is 2-fold: it stimulates both bone resorption and bone formation. We therefore propose "the biphasic model" of GH action in bone remodeling (Fig. 10). According to this model, GH initially increases bone resorption with a concomitant bone loss that is followed by a phase of increased bone formation. After the moment when bone formation is stimulated more than bone resorption (transition point), bone mass is increased. However, a net gain of bone mass caused by GH may take some time as the initial decrease in bone mass must first be replaced (Fig. 10). When all clinical studies of GH treatment of GHD adults are taken into account, it appears that the "transition point" occurs after approximately 6 months and that a net increase of bone mass will be seen after 12-18 months of GH treatment. It should be emphasized that the biphasic model of GH action in bone remodeling is based on findings in GHD adults. It remains to be clarified whether or not it is valid for subjects with normal GH secretion. A treatment intended to increase the effects of GH/IGF-I axis on bone metabolism might include: 1) GH, 2) IGF, 3) other hormones/factors increasing the local IGF-I production in bone, and 4) GH-releasing factors. Other hormones/growth factors increasing local IGF may be important but are not discussed in this article. IGF-I has been shown to increase bone mass in animal models and biochemical markers in humans. However, no effect on bone mass has yet been presented in humans. Because the financial cost for GH treatment is high it has been suggested that GH-releasing factors might be used to stimulate the GH/IGF-I axis. The advantage of GH-releasing factors over GH is that some of them can be administered orally and that they may induce a more physiological GH secretion. (ABSTRACT TRUNCATED)

Induction of bone by a demineralized bone matrix gel: a study in a rat femoral defect model

Demineralized bone matrix contains osteoinductive factors and stimulates filling of gaps and defects with bone; however, it is difficult to handle by itself and various preparations have been tested. Demineralized bone matrix with a gel consistency now is available for clinical use. We studied, in a femoral segment defect in the rat, the effects of rat demineralized bone matrix gel with and without a ceramic substratum. This preparation is analogous to the human demineralized bone matrix in the same carrier, used clinically for humans. One hundred adult male Fischer rats were divided into 10 experimental groups. Independent variables included the presence or absence of hydroxyapatite ceramic cylinders, the presence of demineralized bone matrix in carrier or carrier alone (glycerol), and the duration of observation (1, 2, and 4 months). Defects filled with the gel alone had significantly higher radiographic scores for host-graft union at 4 months compared with ceramic with the gel, ceramic alone, or carrier alone. Demineralized bone matrix gel significantly increased the total histologic score for host-graft union, whether ceramic was present or not, and a three-way interaction occurred among ceramic, the gel, and time. Demineralized bone matrix gel was an effective inducer of bone formation in this model. An additional substratum was not required; in fact, significantly more bone was formed in the absence of the ceramic cylinder. Neither the gel nor the ceramic were impediments to revascularization of the defect. Host-graft union was enhanced by demineralized bone matrix gel but not by the ceramic cylinder.

Growth hormone signal transduction

Growth hormone (GH) promotes animal growth by stimulating bone and cartilage cell proliferation, and influences carbohydrate and lipid metabolism. Some of these effects are brought about indirectly via somatomedin induction in hepatocytes, others by acting directly on the target cells. In either case, GH first binds to specific receptors on cells to trigger a sequence of biochemical events culminating in a biological response. Recently much has been learnt about the molecular structure of GH receptor, its binding to ligand, and the ensuing signal transduction events.

Autocrine regulators of MC3T3-E1 cell proliferation

MC3T3-E1 cells, a clonal osteoblast-like mouse calvarial cell line, secrete several growth regulating factors. These regulators include insulin-like growth factor (IGF) type I, transforming growth factor beta (TGF-beta), and IGF-II in descending order of abundance. MC3T3-E1 cells in culture also produce two IGF binding proteins (IGFBP), M(r) 25 and 32 kD, having sequence identity with IGFBP-4 and IGFBP-6, respectively. In addition, this is the first observation that osteoblast-like bone cells in culture produce IGFBP-6. To determine if growth factors produced by MC3T3-E1 cells have autocrine actions on these cells, the effects of IGF-I, IGF II, TGF-beta 1, and IGFBP-4 on MC3T3-E1 cell proliferation were determined. Exogenous addition of IGF-I and IGF-II stimulated MC3T3-E1 cell proliferation, but TGF-beta 1 and IGFBP-4 inhibited MC3T3-E1 cell proliferation. Based on these findings, we conclude that MC3T3-E1 cells in culture produce autocrine regulators of MC3T3-E1 cell proliferation and that the actions of IGFs may also be regulated by IGFBPs produced by these same cells.

Insulin-like growth factors-I and -II stimulate chemotaxis of osteoblasts isolated from fetal rat calvaria

Repair and regeneration of damaged bone is believed to be regulated in part by growth factors stored in the bone matrix. These growth factors are synthesized and secreted by osteoblasts and are incorporated into the developing bone. This pool of stored growth factors is then released into the immediate area following resorption of the matrix. One of the initial steps in bone repair is the recruitment of osteoblasts to the repair site. Growth factors, such as TGF-beta and PDGF, which are present in bone matrix, have been shown to be chemotactic for osteoblasts. In this study, primary cultures of osteoblasts isolated from fetal rat calvaria were examined for chemotaxis in response to IGF-I and IGF-II. IGF-I stimulated a dose-dependent increase in osteoblast chemotaxis, while IGF-II stimulated chemotaxis maximally at the lowest concentration studied (0.1 ng/ml), and had no effect at the highest concentration studied (100 ng/ml). IGF-I and -II had no effect on osteoblast proliferation at any of the concentrations examined. These results indicate that IGFs may be playing an important role in the early stages of bone repair by stimulating osteoblast chemotaxis to the repair site.

Maturation-associated changes in the cellular composition of mouse calvariae and in the biochemical characteristics of calvarial cells separated into subclasses on Percoll density gradients

The effects of tissue maturation on the cellular composition and biochemical characteristics of bone were studied in neonatal, young adult, and aging mice. Osteoblast subclasses were isolated on Percoll density gradients. Neonatal calvariae consisted almost exclusively of cells banding at low and intermediate buoyant density. High buoyant density cells constituted 5-10% of total cells at 10 days of age but increased to 50-60% by 5 weeks of age. These latter cells were released late during collagenase digestion. This indicates that they arise from the deeper layer of bone. For this reason, we consider them putative osteocytes. We established that constitutive secretion of IGF-I and TGF-beta and activities of cellular alkaline phosphatase paralleled those of the tissue of origin in all cell groups and was highest in cells of intermediate buoyant density. These activities declined rapidly after cessation of growth at 5 weeks of age in both bone and isolated cells. Between 5 and 8 weeks of age, the hormonal response to PTH also declined dramatically. The maximum cAMP induced by PTH declined by about 70% in highly responsive cells of intermediate buoyant density and fell to insignificant levels in cells of high buoyant density. We found that a cyclic AMP response to PTH was positively correlated with stimulated secretion of IGF-I by this hormone in cells from animals of all ages. Despite their inability to respond to PTH with increases in cAMP and IGF-I, adult bone cells of high buoyant density continued to respond to PTH with increases in the secretion of TGF-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of biological assays for detection of polypeptide growth factors

Polypeptide growth factors can be identified and quantified with high accuracy by the use of specific biological assays. In general these bioassays are highly sensitive for detection of growth factor activity, and enhanced specificity can be obtained by a proper choice of selective culture conditions for the target cells involved. In this paper sensitive and selective bioassays are described for growth factors acting on substrate-attached cells, in particular members of the epidermal growth factor, transforming growth factor beta, platelet-derived growth factor, insulin-like growth factor, and heparin-binding growth factor families. A cross-reactivity scheme has been worked out to identify possible contaminations in growth factor preparations.