Role of transforming growth factor beta in bone remodeling: a review

TGFβ1 induces bone formation from BMP9-activated Bone Mesenchymal Stem Cells, with possible involvement of non-canonical pathways

Reconstruction of bone defects is one of the most substantial and difficult clinical challenges in orthopedics. Transforming growth factor beta 1 (TGFβ1) might play an important role in stimulating osteogenic differentiation of bone morphogenetic protein 9 (BMP9)-induced C3H10T1/2 mesenchymal stem cells. In our current study, we examined the potential synergy between TGFβ1 and BMP9 in promoting the osteogenesis of C3H10T1/2 cells, and whether such effects could contribute to bone formation in vivo. Our experiment data indicated that TGFβ1 could increase the expression of osteogenic markers and the formation of mineralized calcium nodules in, while suppressing the proliferation of, BMP9-induced C3H10T1/2 cells. Furthermore, mice intramuscularly injected with BMP9/TGFβ1-transduced C3H10T1/2 cells into the gastrocnemius muscle on their tibiae developed ectopic bone masses with more mature osteoid structures, compared to those grafted with cells expressing BMP9/RFP. Subsequent mechanistic studies found that TGFβ1-induced enhancement of osteogenesis in BMP9-overexpressing C3H10T1/2 cells was accompanied by augmented expression of heat shock protein 47 (HSP47), a collagen-specific molecular chaperone essential for collagen biosynthesis, and can be attenuated by pirfenidone, a known anti-fibrotic inhibitor. Interestingly, protein microarray analysis suggested that TGFβ1/BMP9-dependent osteogenesis of C3H10T1/2 cells seemed to involve several non-canonical signaling pathways such as Janus kinase-signal transducer and activator of transcription, phosphoinositide-3-kinase-protein kinase B, and mitogen-activated protein kinase. These results provided further evidence that TGFβ1 could promote bone formation from BMP9-induced C3H10T1/2 cells and shed important light on the underlying molecular mechanisms.

Human Spinal Bone Dust as a Potential Local Autograft: In Vitro Potent Anabolic Effect on Human Osteoblasts

STUDY DESIGN

In vitro Study.

OBJECTIVE

To evaluate the effect that factors released from human posterior spinal bone dust have on primary human osteoblast growth and maturation.

SUMMARY OF BACKGROUND DATA

Bone dust, created during spinal fusion surgeries, has the potential to be used as an autologous bone graft by providing a source of viable autologous osteoblasts and mesenchymal stem cells with osteogenic potential. Till date, no information is available on whether bone dust also provides a source of anabolic factors with the potential to enhance osteoblast proliferation and maturation, which would enhance its therapeutic potential.

METHODS

Bone dust was collected from consenting patients undergoing elective posterior spinal fusion surgeries, and primary human osteoblasts were cultured from patients undergoing elective hip or knee arthroplasty. Growth factors and cytokines released by bone dust were quantified using enzyme-linked immunosorbent assay. Primary human osteoblast proliferation and gene expression in response to bone dust were assessed using H-thymidine incorporation and real-time polymerase chain reaction, respectively.

RESULTS

Human bone dust released anabolic cytokines (IL-1β and IL-6) and growth factors (TGF-β, VEGF, FGF-Basic, and PDGF-BB) in increasing concentrations over a 7-day period. In vitro, the anabolic factors released by bone dust increased osteoblast proliferation by 7-fold, compared with osteoblasts cultured alone. In addition, the factors released from bone dust up-regulated a number of osteoblastic genes integral to osteoblast differentiation, maturation, and angiogenesis.

CONCLUSION

This study is the first to demonstrate that human posterior spinal bone dust released anabolic factors that potently enhance osteoblast proliferation and the expression of genes that favor bone healing and bone union. As bone dust is anabolic and its harvest is fast, simple, and safe to perform, spinal surgeons should be encouraged to 'recycle' bone dust and harness the regenerative potential of this free autologous bone graft.

LEVEL OF EVIDENCE

N/A.

In vitro culture of hFOB1.19 osteoblast cells on TGF-β1-SF-CS three-dimensional scaffolds

The aim of the present study was to examine the biocompatibility of transforming growth factor-β1-silk fibroin-chitosan (TGF-β1-SF-CS) scaffolds. In order to provide an ideal scaffold for use in bone tissue engineering, TGF-β1 was introduced into the SF-CS scaffold in order to reconstruct a three dimensional scaffold, following which hFOB1.19 osteoblast cells were seeded onto TGF-β1-SF-CS and SF-CS scaffolds. On the TGF-β1-SF-CS and SF-CS scaffolds, the cell adhesion rate increased in a time-dependent manner. Scanning electron microscopy revealed that the cells grew actively and exhibited normal morphological features with multiple fissions, and granular and filamentous substrates were observed surrounding the cells. In addition, the cell microfilaments were closely connected with the scaffolds. The cells exhibited attached growth on the surfaces of the scaffolds, however, the growth also extended into the scaffolds. Cell Counting Kit-8 and ALP analyses revealed that TGF-β1 significantly promoted the growth and proliferation of the hFOB1.19 osteoblast cells in the SF-CS scaffolds, and the enhancement of osteoblast cell proliferation and activity by TGF-β1 occurred in a time-dependent manner. The TGF-β1-SF-CS composite material may offer potential as an ideal scaffold material for bone tissue engineering.

hBMP-2 and hTGF-β1 expressed in implanted BMSCs synergistically promote the repairing of segmental bone defects

OBJECTIVE

To evaluate the effects of co-expressing hBMP-2 and hTGF-β1 in BMSCs (bone marrow-derived mesenchymal stem cells) on the repairing process of radial segmental defects in rats.

METHODS

BMSCs were infected with a high titer recombinant adenovirus carrying hTGF-βl and/or hBMP-2 genes. Expression of exogenous genes in BMSCs was confirmed by RT-PCR and ELISA assays. In vitro effects of exogenous genes were assessed by MTT and ALP activity tests. A left radial defect model was created using 120 SD rats. Genetically modified or unmodified BMSCs were implanted with collagen sponge scaffolds into the 5-mm radial defect. The bone repair process was systematically monitored and evaluated by X-ray examinations, gross anatomic examinations, histological analyses, and biomechanical tests.

RESULTS

Expression of hBMP-2 and hTGF-β1 showed synergistic effects on promoting BMSC proliferation and enhancing ALP activity in vitro. Bone repair assays showed that hBMP-2 and hTGF-β1 promoted the production of chondrocytes and osteoblasts. Implanted BMSCs transfected with both hBMP-2 and hTGF-β1 led to the best bone repair outcome.

CONCLUSION

hBMP-2 and hTGF-β1 can synergistically improve the bone repair process. Our results suggest a potential clinical value of combining hBMP-2 and hTGF-β1 in repairing bone defects.

Effects of TGF-β1 and VEGF-A transgenes on the osteogenic potential of bone marrow stromal cells in vitro and in vivo

An exogenous supply of growth factors and bioreplaceable scaffolds may help bone regeneration. The aim of this study was to examine the effects of TGF-β1 and VEGF-A transgenes on the osteogenic potential of bone marrow stromal cells. Rat bone marrow stromal cells were transfected with plasmids encoding mouse TGF-β1 and/or VEGF-A complementary DNAs and cultured for up to 28 days. Furthermore, collagen scaffolds carrying combinations of the plasmids-transfected cells were implanted subcutaneously in rats. The transgenes increased alkaline phosphatase activity, enhanced mineralized nodule formation, and elevated osteogenic gene expressions in vitro. In vivo, messenger RNA expression of osteogenic genes such as BMPs and Runx2 elevated higher by the transgenes. The data indicate that exogenous TGF-β1 and VEGF-A acted synergistically and could induce osteoblastic differentiation of bone marrow stromal cells in both cell culture and an animal model. The results may provide valuable information to optimize protocols for transgene-and-cell-based tissue engineering.

Platelet-rich plasma for the replenishment of bone

This review examines the use of platelet-rich plasma (PRP) in the treatment of bone injuries and to stimulate bone formation. Studies examining both in vivo bone healing and in vitro actions of PRP on osteoblasts are reviewed. Overall, the available literature suggests that PRP does not appreciably impact bone healing or induce bone formation. However, there is some evidence to suggest that PRP might augment recruitment of osteoblast progenitors to injection sites or in sites expected to experience delayed healing. In this capacity PRP might be utilized to initiate repair of an otherwise poorly healing skeletal lesion. The demonstration that PRP is a viable therapy is hindered by a lack of standardized criteria for what constitutes PRP, and more studies are needed to compare the efficacy of PRP to that of transforming growth factor-β or platelet-derived growth factor used as sole agents.

A new method for the preparation of bioactive calcium phosphate films hybridized with 1alpha,25-dihydroxyvitamin D3

The primary goal of this investigation was to develop a calcium phosphate film hybridized with 1alpha,25-dihydroxyvitamin D(3) for the improvement of osteoconductivity of bone substitutes. The hybrid films (hCaP) were prepared at the different concentrations of 1 x 10(-10), 1 x 10(-8), and 1 x 10(-6) M designated as hCaPL, hCaPM, and hCaPH, respectively. The change of the hormone concentration during the preparation of the hybrid films did not cause significant variations on the physical properties of hCaPs, i.e. surface morphology and roughness. On the other hand, X-ray photon spectroscope (XPS) measurements revealed that the concentration change affected the chemical composition of the hybrid films. Recruitment of osteoblast-like MG-63 cells was considerably improved on hCaPs compared to tissue culture plate (TCP). However, cell proliferation on hCaPs was substantially suppressed and inversely proportional to the hormone concentration used. It was observed that bone-like nodules which consisted of bead-like components and well-developed matrix were rapidly formed on hCaPs. Masson's trichrome and safranin-O stainings elucidated that the bead-like components were MG-63 cells. Safranin-O staining showed that proteoglycan was produced actively. These results indicate that the cells cultured on hCaPs were strongly stimulated by the hormone to produce proteoglycan which can be considered as an induction of premature bone formation. The number of the nodules was increased with hormone concentration and most pronounced at the hCaPH. Gene expression patterns of alkaline phosphatase (ALP), transforming growth factor-beta (TGF-beta), and osteopontin (OPN) were strongly modulated by hybridized the hormone. For ALP and OPN, gene expressions were activated earlier on hCaPs than untreated calcium phosphate (CaP) confirming the effect of the hybridization was substantial. The TGF-beta gene expression was immediately activated after seeding but difference between samples was not significant suggesting that the gene expression was modulated not by the hormone hybridization but by CaP itself. As a result, hybridization of 1,25(OH)(2)D(3) with CaP can be a potentially strong candidate to promote osteoconductivity of implant materials.

Biological basis of bone formation, remodeling, and repair-part I: biochemical signaling molecules

The bony biochemical environment is an active and dynamic system that permits and promotes cellular functions that lead to matrix production and ossification. Each component is capable of conveying important regulatory cues to nearby cells, thus effecting gene expression and changes at the cytostructural level. Here, we review the various signaling molecules that contribute to the active and dynamic nature of the biochemical system. These components include hormones, cytokines, and growth factors. We describe their role in regulating bone metabolism. Certain growth factors (i.e., TGF-beta, IGF-1, and VEGF) are described in greater detail because of their potential importance in developing successful tissue-engineering strategies.

Cytokine Release from Osteoblasts in Response to Different Intensities of Pulsed Electromagnetic Field Stimulation

We use an in-vitro osteoblast cell culture model to investigate the effects of low-frequency (7.5 Hz) pulsed electromagnetic field (PEMF) stimulation on osteoblast population, cytokines (prostaglandin E(2) (PGE(2)), transforming growth factor beta1(TGFbeta1), and alkaline phosphatase (ALP) activity to find the optimal intensity of PEMF for osteoblast growth. The results demonstrate that PEMF can stimulate osteoblast growth, release of TGFbeta1, and, in addition, an increase of ALP activity. The synthesis and release of PGE(2) in the culture medium are reduced with increasing numbers of cells. Higher intensity does not necessarily mean increased osteoblast growth, and the most efficient intensity is about 2 mV/cm in this case. Although the lower intensities of the PEMF are yet to be determined, the results of this study can shed light on the mechanisms of PEMF stimulation on non union fracture therapy and osteoporosis prevention in the future.

Controlled delivery of platelet-rich plasma-derived growth factors for bone formation

Platelet-rich plasma (PRP) represents an autologous source of growth factors essential for bone regeneration. The clinical efficacy of PRP is, however, unpredictable, and this is likely due to the inefficient and inconsistent delivery of PRP-derived growth factors. Previous investigations have shown that current methods of PRP preparation result in a premature release of the relevant bone stimulatory factors. As successful bone regeneration requires multiple factors presented in a physiologic temporal and spatial cascade, the objective of this study is to control the bioavailability of PRP-derived growth factors using a hydrogel carrier system. Specifically, the release of platelet-derived growth factor, transforming growth factor beta-1, and insulin-like growth factor from two types of alginate carriers was compared over time. The effects of the released factors on the growth and alkaline phosphatase (ALP) activity of human osteoblast-like cells were also evaluated. It was found that factor release profiles varied as function of carrier type, and binding of growth factors to the alginate matrix also modulated their release. The bioactivity of released factors was maintained in vitro and they promoted cell proliferation and ALP activity. These results demonstrate the potential of this autologous multifactor delivery system for controlling the bioavailability of PRP-derived factors. Future studies will focus on optimizing this system to increase the clinical efficacy of PRP by matching the distribution and temporal sequencing of PRP-derived factors to the bone healing cascade.

Depressed bone mineralization following high dose TGF-beta1 application in an orthopedic implant model

Several previous studies of bone repair have shown 2- to 4-fold increases in bone formation following local delivery of exogenous transforming growth factor-beta (TGF-beta). Here, we use quantitative backscatter electron microscopy to test the effect of TGF-beta1 on mineralization of regenerated bone by examining tissue samples from a previously published canine study in which we found increased bone formation. In the experiment, the proximal humeri of 10 male canines were implanted bilaterally for 28 days with porous-coated implants in the presence of a 3 mm gap between the surface of the implant and the host bone. Implants placed in the left humeri were treated with TGF-beta1 at a dose of either 120 microg (n = 5) or 335 microg (n = 5), and the implants placed in the contralateral humeri served as untreated controls. Quantitative backscatter scanning electron microscopy was used to assess the volume fraction of bone and its degree of mineralization in the 3 mm gaps. The calibrated grayscale mean and median values were depressed compared to the controls in the high dose group (p = 0.048 and p = 0.041, respectively), suggesting that high dose TGF-beta delayed or inhibited mineralization of newly formed osteoid.

Cytokine release from osteoblasts in response to ultrasound stimulation

Bone is a dynamic tissue with a well-balanced homeostasis preserved by both formation and resorption of bone. Normal turnover of bone, however, can be upset by either increased osteoclast activity or decreased osteoblast function; either mechanism alone or both may result in a net loss of bone. Both osteoclasts and osteoblasts could be stimulated by mechanical stimulation in vitro, and it is assumed that this process may occur in vivo as well. In this experiment, we investigated this hypothesis by examining the effects of ultrasound stimulation on osteoblast growth and cytokine release. With this model, we explored the mechanism of low-intensity pulsed ultrasound on osteoblasts growth and upregulation of osteoclasts formation and function by cytokine release. The results showed that specific pulsed ultrasound exposure could enhance osteoblasts population together with increase in TGFbeta1 secretion and decrease in concentration of IL-6 and TNFalpha in the culture medium. Although, animal studies and clinical trial are needed to understand the real process in the whole body, ultrasound stimulation might be a good method for prevention of bone loss due to osteoporosis.

Modulation of growth factor/cytokine synthesis and signaling by 1alpha,25-dihydroxyvitamin D(3): implications in cell growth and differentiation

Distinct from its classic functions in the regulation of calcium and phosphorus metabolism as a systemic hormone, 1alpha,25-dihydroxyvitamin D(3) [1alpha,25(OH)(2)D(3)] is involved in the local control and regulation of cellular growth and differentiation in various tissues, including epidermis (keratinocytes) and bone (osteoblasts and osteoclasts). In this review, the impact of 1alpha,25(OH)(2)D(3) on growth factor/cytokine synthesis and signaling is discussed, particularly as it pertains to bone cells and keratinocytes. 1alpha,25(OH)(2)D(3) not only regulates growth factor/cytokine synthesis but may also alter growth factor signaling. Recently discovered examples for such interactions are the interactions between the vitamin D receptor and the mothers against decapentaplegic-related proteins that function downstream of TGFbeta receptors. Inhibitory effects of 1alpha,25(OH)(2)D(3) on keratinocytes through TGFbeta activation and IL-1alpha, IL-6, and IL-8 suppression may provide a rationale for its beneficial effects in the treatment of hyperproliferative skin disorders, whereas stimulatory effects through the epidermal growth factor-related family members and platelet-derived growth factor may be operative in its beneficial effects in skin atrophy and wound healing. Modulation of cytokines and growth factors by 1alpha,25(OH)(2)D(3) during bone remodeling plays an important role in the coupling of osteoblastic bone formation with osteoclastic resorption to maintain bone mass.

Osteoblast gene expression is differentially regulated by TGF-beta isoforms

The transforming growth factor beta (TGF-beta) superfamily encompasses a number of important growth factors including several TGF-beta isoforms, the bone morphogenetic proteins, activins, inhibins, and growth and differentiation factors. TGF-beta 1, -beta 2, and -beta 3 are three closely related isoforms that are widely expressed during skeletal morphogenesis and bone repair. Numerous studies suggest that each isoform has unique in vivo functions; however, the effects of these TGF-beta isoforms on osteoblast gene expression and maturation have never been directly compared. In the current study, we treated undifferentiated neonatal rat calvaria osteoblast-enriched cell cultures with 2.5 ng/ml of each TGF-beta isoform and analyzed gene expression at 0, 3, 6, and 24 hours. We demonstrated unique isoform-specific regulation of endogenous TGF-beta 1 and type I collagen mRNA transcription. To assess the effects of extended TGF-beta treatment on osteoblast maturation, we differentiated osteoblast cultures in the presence of 2.5 ng/ml of each TGF-beta isoform. Analysis of collagen I, alkaline phosphatase, and osteocalcin demonstrated that each TGF-beta isoform uniquely suppressed the transcription of these osteoblast differentiation markers. Interestingly, TGF-beta isoform treatment increased osteopontin expression in primary osteoblasts after 4 and 10 days of differentiation. To our knowledge, these data provide the first direct comparison of the effects of the TGF-beta isoforms on osteoblast gene expression in vitro. Furthermore, these data suggest that TGF-beta isoforms may exert their unique in vivo effects by differentially regulating osteoblast cytokine secretion, extracellular matrix production, and the rate of cellular maturation.

The effects of ionizing radiation on osteoblast-like cells in vitro

The well-described detrimental effects of ionizing radiation on the regeneration of bone within a fracture site include decreased osteocyte number, suppressed osteoblast activity, and diminished vascularity. However, the biologic mechanisms underlying osteoradionecrosis and the impaired fracture healing of irradiated bone remain undefined. Ionizing radiation may decrease successful osseous repair by altering cytokine expression profiles resulting from or leading to a change in the osteoblastic differentiation state. These changes may, in turn, cause alterations in osteoblast proliferation and extracellular matrix formation. The purpose of this study was to investigate the effects of ionizing radiation on the proliferation, maturation, and cytokine production of MC3T3-E1 osteoblast-like cells in vitro. Specifically, the authors examined the effects of varying doses of ionizing radiation (0, 40, 400, and 800 cGy) on the expression of transforming growth factor-beta1 (TGF-beta1), vascular endothelial growth factor (VEGF), and alkaline phosphatase. In addition, the authors studied the effects of ionizing radiation on MC3T3-E1 cellular proliferation and the ability of conditioned media obtained from control and irradiated cells to regulate the proliferation of bovine aortic endothelial cells. Finally, the authors evaluated the effects of adenovirus-mediated TGF-beta1 gene therapy in an effort to "rescue" irradiated osteoblasts. The exposure of osteoblast-like cells to ionizing radiation resulted in dose-dependent decreases in cellular proliferation and promoted cellular differentiation (i.e., increased alkaline phosphatase production). Additionally, ionizing radiation caused dose-dependent decreases in total TGF-beta1 and VEGF protein production. Decreases in total TGF-beta1 production were due to a decrease in TGF-beta1 production per cell. In contrast, decreased total VEGF production was secondary to decreases in cellular proliferation, because the cellular production of VEGF by irradiated osteoblasts was moderately increased when VEGF production was corrected for cell number. Additionally, in contrast to control cells (i.e., nonirradiated), conditioned media obtained from irradiated osteoblasts failed to stimulate the proliferation of bovine aortic endothelial cells. Finally, transfection of control and irradiated cells with a replication-deficient TGF-beta1 adenovirus before irradiation resulted in an increase in cellular production of TGF-beta1 protein and VEGF. Interestingly, this intervention did not alter the effects of irradiation on cellular proliferation, which implies that alterations in TGF-beta1 expression do not underlie the deficiencies noted in cellular proliferation. The authors hypothesize that ionizing radiation-induced alterations in the cytokine profiles and differentiation states of osteoblasts may provide insights into the cellular mechanisms underlying osteoradionecrosis and impaired fracture healing.

Osteogenesis in cranial defects: reassessment of the concept of critical size and the expression of TGF-beta isoforms

Transforming growth factor-betas (TGF-beta) have been demontstrated to be upregulated during osteoblast function in vitro and during cranial suture fusion in vivo. The authors hypothesized that spontaneous reossification of calvarial defects was also associated with upregulation of TGF-beta. The present study was designed to (1) evaluate the concept of a critical-size defect within the calvaria in an adult guinea pig model and (2) investigate the association between the ossification of calvarial defects and TGF-beta upregulation. Paired circular parietal defects with diameters of 3 and 5 mm and single parietal defects with diameters of 8 or 12 mm were made in 45 six-month-old skeletally mature guinea pigs. Three animals per defect size were killed after survival periods of 3 days, 1 week, 4 weeks, 8 weeks, or 12 weeks. New bone ingrowth was evaluated by assessing for linear closure by a traditional linear method and by a modified cross-sectional area method using an image analysis system in which the thickness of new bone was taken into account. Immunohistochemistry was performed using rabbit polyclonal antibodies to localize TGF-beta1, -beta2, and -beta3. All specimens were photographed, and the intensity of immunostaining was graded based on subjective photographic assessment by three independent reviewers. No defect demonstrated any measurable bone replacement after a survival period of 3 days. All 3- and 5-mm defects were completely reossified after 12 weeks based on the linear analysis of new bone, indicating these defects to be less than critical size. However, new bone formation in the 5-mm defects never exceeded a mean of 40 percent by cross-sectional area of new bone. Percent of new bone formation by cross-sectional area was significantly higher within 3-mm defects than in all larger defects 4 weeks after the craniotomy, reaching a mean of 89 percent new bone by 12 weeks. Persistent gaps were noted on linear analysis of the 8- and 12-mm wounds by 12 weeks, and mean percent new bone by cross-sectional area remained below 30 percent. Immunolocalization demonstrated osteogenic fronts at the advancing bone edge and the endocranial side, in which the osteoblasts stained strongly for all isoforms of TGF-beta. The intensity of osteoblast expression waned considerably after the majority of the defect had reossified. These data indicate that histometric analysis based on cross-sectional area more accurately reflects the osteogenic potential of a cranial defect than does linear inspection of defect closure. Although the interpretation of immunolocalization studies is highly subjective, independent assessment by three reviewers indicates that isoforms of TGF-beta were upregulated during a limited "window" of time corresponding to the period of active calvarial reossification, and expression of TGF-beta corresponded to osteoblast activity within osteogenic fronts.

Mechanisms of fibroblast growth factor-2 modulation of vascular endothelial growth factor expression by osteoblastic cells

Normal bone growth and repair is dependent on angiogenesis. Fibroblast growth factor-2 (FGF-2), vascular endothelial growth factor (VEGF), and transforming growth factor-beta (TGFbeta) have all been implicated in the related processes of angiogenesis, growth, development, and repair. The purpose of this study was to investigate the relationships between FGF-2 and both VEGF and TGFbeta in nonimmortalized and clonal osteoblastic cells. Northern blot analysis revealed 6-fold peak increases in VEGF mRNA at 6 h in fetal rat calvarial cells and MC3T3-E1 osteoblastic cells after stimulation with FGF-2. Actinomycin D inhibited these increases in VEGF mRNA, whereas cycloheximide did not. The stability ofVEGF mRNA was not increased after FGF-2 treatment. Furthermore, FGF-2 induced dose-dependent increases in VEGF protein levels (P < 0.01). Although in MC3T3-E1 cells, TGFbeta1 stimulates a 6-fold peak increase in VEGF mRNA after 3 h of stimulation, we found that both TGFbeta2 and TGFbeta3 yielded 2- to 3-fold peak increases in VEGF mRNA levels noted after 6 h of stimulation. Similarly, both TGFbeta2 and TGFbeta3 dose dependently increased VEGF protein production. To determine whether FGF-2-induced increases in VEGF mRNA may have occurred independently of TGFbeta, we disrupted TGFbeta signal transduction (using adenovirus encoding a truncated form of TGFbeta receptor II), which attenuated TGFbeta1 induction of VEGF mRNA, but did not impede FGF-2 induction ofVEGF mRNA. In summary, FGF-2-induced VEGF expression by osteoblastic cells is a dose-dependent event that may be independent of concomitant FGF-2-induced modulation of TGFbeta activity.

Effects of transforming growth factor-beta and mechanical strain on osteoblast cell counts: an in vitro model for distraction osteogenesis

Factors known to regulate bone production during distraction osteogenesis include mechanical strain on bone forming cells and up-regulation of transforming growth factor-beta (TGF-beta) during the distraction, or strain phase of distraction osteogenesis. In the present study, an in vitro model was used to evaluate the functional effect of exogenous TGF-beta1 on mitogenesis in murine-derived MC3T3 osteoblasts during the period of active mechanical strain. The first hypothesis to be tested was that mitogenic suppression of MC3T3 osteoblasts by TGF-beta1 is further enhanced when these cells are also subjected to mechanical strain. To test this hypothesis, MC3T3 osteoblasts were seeded on flexible and rigid membranes. These were subjected to cyclic, vacuum-induced strain, simulating physiologic stress loads. After 24 hours, all cells were transferred to media containing TGF-beta1, and strain was continued for an additional 48 hours. The study was repeated by using two doses of TGF-beta1. This study demonstrated that final cell counts were significantly decreased in the presence of TGF-beta1 in both the nonstrained and strained groups (p < 0.0001). The final cell count in the strained group was significantly less than that in the nonstrained group (p < 0.0001) for both concentrations of TGF-beta1 tested, confirming the initial hypothesis. The second hypothesis to be tested was that alteration in the mitogenic response of MC3T3 osteoblasts after strain is not directly due to autocrine factors produced by the strained osteoblasts. To test this hypothesis, a proliferation assay was performed on nonconfluent MC3T3 osteoblasts by using conditioned media collected from strained and nonstrained osteoblasts. This study demonstrated no significant differences in cell counts after addition of conditioned media collected from strained versus nonstrained cells, confirming the latter hypothesis. The present study demonstrates the functional significance of mechanical strain on osteoblast cell counts. Furthermore, this may help to explain the temporal relationship observed during the early distraction (strain) phase of distraction osteogenesis in rodent models in which peak up-regulation of TGF-beta1 gene expression correlates with peak suppression of osteoblast function as measured by gene expression of extracellular matrix proteins.

Functional characterization of N-methyl-D-aspartic acid-gated channels in bone cells

Our recent identification of glutamate receptors in bone cells suggested a novel means of paracrine communication in the skeleton. To determine whether these receptors are functional, we investigated the effects of the excitatory amino acid, glutamate, and the pharmacological ligand, N-methyl-D-aspartic acid (NMDA), on glutamate-like receptors in the human osteoblastic cell lines MG63 and SaOS-2. Glutamate binds to osteoblasts, with a Kd of approximately 10(-4) mol/L and the NMDA receptor antagonist, D(L)-2-amino-5-phosphonovaleric acid (D-APV), inhibits binding. Using the patch-clamp technique, we measured whole-cell currents before and after addition of L-glutamate or NMDA and investigated the effects of the NMDA channel blockers, dizolcipine maleate (MK801), and Mg2+, and the competitive NMDA receptor antagonist, 3-((R)-2-carboxypiperazin-4-yl)-propyl-1-phosphoric acid (R-CPP), on agonist-induced currents. Both glutamate and NMDA induced significant increases in membrane currents. Application of Mg2+ (200 micromol/L) and MK801 (100 micromol/L) caused a significant decrease in inward currents elicited in response to agonist stimulation. The competitive NMDA receptor antagonist, R-CPP (100 micromol/L), also partially blocked the NMDA-induced currents in MG63 cells. This effect was reversed by addition of further NMDA (100 micromol/L). In Fura-2-loaded osteoblasts, glutamate induced elevation of intracellular free calcium, which was blocked by MK801. These results support the hypothesis that glutamate plays a role in bone cell signaling and suggest a possible role for glutamate agonists/antagonists in the treatment of bone diseases.

Transforming growth factor-beta1 modulates the expression of vascular endothelial growth factor by osteoblasts

Angiogenesis is essential to both normal and pathological bone physiology. Vascular endothelial growth factor (VEGF) has been implicated in angiogenesis, whereas transforming growth factor-beta1 (TGF-beta1) modulates bone differentiation, matrix formation, and cytokine expression. The purpose of this study was to investigate the relationship between TGF-beta1 and VEGF expression in osteoblasts and osteoblast-like cells. Northern blot analysis revealed an early peak of VEGF mRNA (6-fold at 3 h) in fetal rat calvarial cells and MC3T3-E1 osteoblast-like cells after stimulation with TGF-beta1 (2.5 ng/ml). The stability of VEGF mRNA in MC3T3-E1 cells was not increased after TGF-beta1 treatment. Actinomycin D inhibited the TGF-beta1-induced peak in VEGF mRNA, whereas cycloheximide did not. Blockade of TGF-beta1 signal transduction via a dominant-negative receptor II adenovirus significantly decreased TGF-beta1 induction of VEGF mRNA. Additionally, TGF-beta1 induced a dose-dependent increase in VEGF protein expression by MC3T3-E1 cells (P < 0.01). Dexamethasone similarly inhibited VEGF protein expression. Both TGF-beta1 mRNA and VEGF mRNA were concurrently present in rat membranous bone, and both followed similar patterns of expression during rat mandibular fracture healing (mRNA and protein). In summary, TGF-beta1-induced VEGF expression by osteoblasts and osteoblast-like cells is a dose-dependent event that may be intimately related to bone development and fracture healing.

Expression of transforming growth factor beta isoforms in osteosarcoma variants: association of TGF beta 1 with high-grade osteosarcomas

Studies on osteosarcoma cell lines point to the potential importance of transforming growth factor beta (TGF beta) as an autocrine factor which controls the growth of human osteosarcomas. To define further the role of TGF beta isoforms in these neoplasms, a series of 27 osteosarcomas was studied using immunohistochemical, mRNA in situ hybridization, and reverse transcriptase-polymerase chain reaction (RT-PCR) techniques. All 14 central high-grade osteosarcomas, two telangiectatic osteosarcomas, and one high-grade surface osteosarcoma showed cytoplasmic immunoreactivity for TGF beta 1, -2, and -3. The expression of TGF beta 1 was moderate or diffuse in 14 cases (82.3 per cent), while low expression was detected in only three cases (17.7 per cent). For TGF beta 2 and -3, only moderate or diffuse staining was observed. Low-grade parosteal and periosteal osteosarcomas showed low or undetectable levels of TGF beta 1, while TGF beta 2 and -3 were moderately or diffusely expressed. Finally, three dedifferentiated parosteal osteosarcomas were diffusely positive for TGF beta 1, -2, and -3 in the high-grade component, while in the low-grade component, available for analysis in two of these cases, TGF beta 1 was demonstrated in a few neoplastic cells, and TGF beta 2 and -3 maintained a diffuse distribution. Statistical analysis of these data showed that high-grade osteosarcomas had a significantly higher expression of TGF beta 1 than low-grade osteosarcomas, while levels of TGF beta 2 and -3 were comparable in the two groups (p < 0.001; p = 0.3; p = 0.3, respectively; Fisher's exact test). Similarly, mRNA levels of TGF beta 1 detected by in situ hybridization were significantly higher (p = 0.04, Fisher's exact test) in high-grade osteosarcoma variants, while no differences were found for TGF beta 2 and -3 mRNA (p = 1.0; p = 0.2, respectively; Fisher's exact test). In addition, mRNA analysis performed by RT-PCR in seven cases (five high-grade and two low-grade osteosarcomas) confirmed the presence of high levels of TGF beta 1 in high-grade osteosarcomas, while low-grade tumours had low or absent mRNA expression. In conclusion, this positive association suggests that TGF beta 1 may be involved in determining the aggressive clinical behaviour of high-grade osteosarcomas.

TGF-beta latency: biological significance and mechanisms of activation

Transforming growth factor (TGF-) beta is secreted as a latent complex in which the mature growth factor remains associated with its propeptide. In order to elicit a biological response, the cytokine must be released from the latent complex, a process termed latent TGF-beta activation or TGF-beta formation. Although latent TGF-beta activation is a critical step in the regulation of its activity, little is known about the molecular mechanisms that lead to the production of active TGF-beta. In this article, we present an overview of the data available on this topic, and we propose a tentative model for the mechanism of TGF-beta formation based upon the observations with different cell systems and on recent findings on the structure of the latent TGF-beta complex.

An age-related decrease in the concentration of insulin-like growth factor binding protein-5 in human cortical bone

The skeletal contents of insulin-like growth factor-2 (IGF-II), insulin-like growth factor binding protein-5 (IGFBP-5), and insulin-like growth factor binding protein-3 (IGFBP-3) were determined in duplicate samples of human femoral cortical bone obtained from 64 subjects (44 males and 20 females) between the ages of 20 and 64 years. The results of these quantitative measurements revealed an age-related decrease in the femoral cortical content of IGFBP-5 (r = -0.272, P = 0.031) in the total population. Although the femoral cortical content of IGF-II did not show a similar decrease with age, it could be correlated to the femoral cortical content of IGFBP-5 (r = 0.442, P < 0.001). In contrast, the femoral cortical content of IGFBP-3 did not decrease with age and could not be correlated to the femoral cortical contents of either IGFBP-5 or IGF-II. Comparisons of these results with previous measurements of insulin-like growth factor-1 (IGF-I) and transforming growth factor-beta (TGF-beta), in extracts of the same bones, showed significant cross-correlations between the femoral cortical contents of each of these growth factors and the femoral cortical contents of IGFBP-5 (r = 0.625 for IGF-I versus IGFBP-5, r = 0.554 for TGF-beta versus IGFBP-5, P < 0.001 for each) but not IGFBP-3.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum IGF 1 is low and correlated with osteoblastic surface in idiopathic osteoporosis

We have previously reported that bone formation is impaired on histomorphometric analysis of bone in patients with idiopathic osteoporosis. In the present study, 30 patients with idiopathic osteoporosis (18 men and 12 women mean age 44 +/- 12 years) and spinal and/or appendicular fractures were studied. Compared with control values, bone biopsy analysis demonstrated reduced bone volume (13.0 +/- 4.4 vs. 23.2 +/- 4.4, p < 0.0001), osteoid volume (0.13 +/- 0.13 vs. 0.32 +/- 0.19, p = 0.001), osteoid surface (5.9 +/- 4.3 vs. 12.1 +/- 4.6, p = 0.0004), and diminished double-labeled mineralizing surface (MS/BS 2.0 +/- 2.1 vs. 5.1 +/- 2.7%, p = 0.0001) in the patients. Since insulin-like growth factor 1 (IGF-1) is one of the major determinants of bone growth and remodeling, we measured the circulating level of this growth factor in these patients. The mean serum IGF-1 concentration was lower in patients as compared with 33 healthy age-matched controls (193 +/- 59 SD ng/ml vs. 232 +/- 79). A significant difference was noted between the two groups only in subjects younger than 36 years. In patients with idiopathic osteoporosis, regression analysis of serum IGF-1 against the various histological parameters measured from the bone biopsy disclosed significant correlation's between serum IGF-1 and osteoblastic surface (r = 0.429, p = 0.032), mineralizing bone surface with a double label (r = 0.480, p = 0.015), and the bone formation rate (r = 0.457, p = 0.021). These findings suggest that in young eugonadal individuals with idiopathic osteoporosis, reduced IGF-1 concentrations may have an etiological role in the development of this disease.

Effects of transforming growth factor beta on cells derived from bone and callus of patients with osteogenesis imperfecta

We studied the influence of transforming growth factor beta (TGF-beta) on cultured bone cells derived from two patients with osteogenesis imperfecta (OI) and from human controls. Additionally, cells from a hyperplastic callus that had developed spontaneously at the femur of the patient in Case 1 and cells from a normal fracture callus were included in the study. TGF-beta increased the synthesis of total protein and collagen of all cells without changing the pattern of interstitial collagens. Proliferation was stimulated by TGF-beta in the OI bone cells from Case 1, in cells from the central part of the hyperplastic callus, and in cells from the fracture callus. In Case 2, proliferation of bone cells was decreased by low concentrations of TGF-beta. Alkaline phosphatase (AP) activity was enhanced by TGF-beta in normal human bone cells, not affected in bone cells from the patient in Case 2 or in cells from the central part of the hyperplastic callus, and inhibited in bone cells and cells from the peripheral part of the hyperplastic callus of Case 1 and in cells from the fracture callus. We conclude that TGF-beta has common and specific effects on cultured human cells derived from different types of skeletal tissues. Simultaneous stimulation of collagen synthesis and AP activity by TGF-beta was restricted to normal human bone cells and might reflect their mature state of osteoblastic differentiation. Cells derived from bone of both patients with OI, from the hyperplastic callus, and from the fracture callus showed a different response pattern to TGF-beta.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversal of acute and chronic synovial inflammation by anti-transforming growth factor beta

Transforming growth factor beta (TGF-beta) induces leukocyte recruitment and activation, events central to an inflammatory response. In this study, we demonstrate that antagonism of TGF-beta with a neutralizing antibody not only blocks inflammatory cell accumulation, but also tissue pathology in an experimental model of chronic erosive polyarthritis. Intraarticular injection of monoclonal antibody 1D11.16, which inhibits both TGF-beta 1 and TGF-beta 2 bioactivity, into animals receiving an arthropathic dose of bacterial cell walls significantly inhibits arthritis. Inhibition was observed with a single injection of 50 micrograms antibody, and a 1-mg injection blocked acute inflammation > 75% compared with the contralateral joints injected with an irrelevant isotype control antibody (MOPC21) as quantitated by an articular index (AI = 0.93 +/- 0.23 for 1D11.16, and AI = 4.0 +/- 0 on day 4; p < 0.001). Moreover, suppression of the acute arthritis achieved with a single injection of antibody was sustained into the chronic, destructive phase of the disease (on day 18, AI = 0.93 +/- 0.07 vs. AI = 2.6 +/- 0.5; p < 0.01). The decreased inflammatory index associated with anti-TGF-beta treatment was consistent with histopathologic and radiologic evidence of a therapeutic response. These data implicate TGF-beta as a profound agonist not only in the early events responsible for synovial inflammation, but also in the chronicity of streptococcal cell wall fragment-induced inflammation culminating in destructive pathology. Interrupting the cycle of leukocyte recruitment and activation with TGF-beta antagonists may provide a mechanism for resolution of chronic destructive lesions.

Zeolite A increases proliferation, differentiation, and transforming growth factor beta production in normal adult human osteoblast-like cells in vitro

Silicon in trace amounts enhances bone formation, and the silicon-containing compound zeolite A (ZA) increases eggshell thickness in hens. In the studies reported here, treatment of nearly homogeneous strains of normal human osteoblast-like cells for 48 h with ZA at 0.1-100 micrograms/ml induced a dose-dependent increase (r = 0.35, P < 0.001) in DNA synthesis (n = 31) to 162 +/- 16% (mean +/- SEM) of control and in the proportion of cells in mitosis (n = 4) from 9.1 +/- 1.8 to 27.0 +/- 4.5% (r = 0.69, P < 0.005). ZA treatment also increased alkaline phosphatase activity (P < 0.05) and osteocalcin release (P < 0.05) but did not significantly affect collagen production per individual cell. The mitogenic action of ZA was dependent on cell seeding density over the range of 1250-40,000 cells per cm2, which is consistent with induction of an autocrine factor(s). TGF-beta is a potent mitogen for osteoblasts. ZA treatment increased the steady-state mRNA levels of transforming growth factor beta 1 (TGF-beta 1) and induced the release of the latent form of TGF-beta protein into the conditioned medium within 6 h. We conclude that ZA induces the proliferation and differentiation of cells of the osteoblast lineage.

Mesoderm and jaw development in vertebrates: the role of growth factors

The head and neck arise during development as the result of a complex series of cellular and molecular interactions that begin in the fertilized egg. In this article, the role of an important class of molecules, growth factors, is examined in two main steps of the developmental sequence: the initial induction of mesoderm and the later induction of jaw cartilage and bone. The article focuses particularly on the roles of members of the transforming growth factor-beta (TGF-beta), fibroblast growth factor (FGF), and epithelial growth factor (EGF) families in these processes and current models of growth factor involvement. Possible experiments for the future are discussed.

Identification and analysis of transforming growth factor beta receptors on primary osteoblast-enriched cultures derived from adult human bone

Primary human osteoblast-enriched (PHO) cultures derived from adult trabecular bone were analyzed to determine the presence or absence of transforming growth factor beta (TGF-beta) receptors. Saturation binding studies were performed with 125I-TGF-beta in the absence or presence of 200-fold excess cold TGF-beta. Cross-linking experiments utilizing 125-I-TGF-beta were performed to identify specific cell surface binding proteins for TGF-beta. The saturation binding studies demonstrated saturable binding for TGF-beta on PHO cells. TGF-beta was cross-linked to cell surface binding proteins of 50 to 110 KDa and a high molecular weight component. Thus, these receptors appear to be similar in affinity, number per cell, and molecular weight to those previously identified with other cell types. The potential biological effects of TGF-beta on the growth of PHO cultures were evaluated by both 3H-thymidine incorporation and cell number determination. Growth of PHO cells in the presence of TGF-beta resulted in an approximately two-fold stimulation in cell number as compared to control cells while the 3H-thymidine experiments demonstrated a two to four-fold increase in thymidine uptake in the presence of TGF-beta. Radiographic emulsion studies revealed that the alkaline phosphatase positive and negative cell populations were responsive to the TGF-beta mitogenic stimulation. The cumulative findings of saturable binding, specific cell surface binding proteins, and biological effects suggest that functional TGF-beta cell surface receptors are present on primary osteoblast-enriched cultures derived from adult human trabecular bone.

Control of transforming growth factor-beta activity: latency vs. activation

Transforming growth factor-beta is a pluripotent regulator of cell growth and differentiation. The growth factor is expressed as a latent complex that must be converted to an active form before interacting with its ubiquitous high affinity receptors. This conversion involves the release of the mature growth factor through disruption of the non-covalent interactions with its pro-peptide or latency associated peptide. The mechanisms for this release in vivo have not been fully characterized but appear to be cell specific and might involve processes such as acidification or proteolysis. Although several factors including transcriptional regulation, receptor modulation and scavenging of the active growth factor have been implicated, the critical step controlling the biological effects of transforming growth factor-beta may be the activation of the latent molecule.

Pathogenesis of osteoporosis

The final clinical outcome of the osteoporotic process is a fracture, which can occur as a result of minimal trauma or even spontaneously. At present low bone mass is regarded as the main contributor to bone fragility, but possible qualitative changes in the bone matrix must also be considered. Two factors which determine the level of bone mass at any age are the obtained peak bone mass and duration and rate of bone loss. Peak bone mass is achieved during the first three decades of life. Genetic and nutritional factors as well as mechanical stress on the skeleton obviously play crucial roles in determining peak bone mass. Two phases of bone loss--age-related and menopause-related--dictate the final bone mass at old age. Postmenopausal osteoporosis is a particular example of unbalanced bone resorption leading to net bone loss. An increasing number of systemic and local factors have been found to participate in the regulation of bone remodeling.