Thrombin effects on osteoblastic cells. II. Structure-function relationships

Low Bone Mineral Density in Hemophiliacs

Objective

To review the current knowledge on bone health in patients with hemophilia A and the underlying pathogenetic mechanisms.

Data Sources

Original research articles, meta-analyses, and scientific reviews.

Data Synthesis

Already in childhood, patients with hemophilia A are prone to low bone mineral density, leading to osteopenia and/or osteoporosis. Initially associated with the life style of hemophilia, today we are faced with accumulating evidence that coagulation factor VIII is involved directly or indirectly in bone physiology.

Conclusion

Understanding the role of factor VIII and the mechanisms of decreased bone mineral density in hemophilia A is critically important, especially as non-factor replacement therapies are available, and treatment decisions potentially impact bone health.

Mechanisms of Bone Remodeling Disorder in Hemophilia

Hemophilia is caused by a lack of antihemophilic factor(s), for example, factor VIII (FVIII; hemophilia A) and factor IX (FIX; hemophilia B). Low bone mass is widely reported in epidemiological studies of hemophilia, and patients with hemophilia are at an increased risk of fracture. The detailed etiology of bone homeostasis imbalance in hemophilia is unclear. Clinical and experimental studies show that FVIII and FIX are involved in bone remodeling. However, it is likely that antihemophilic factors affect bone biology through thrombin pathways rather than via their own intrinsic properties. In addition, among patients with hemophilia, there are pathophysiological processes in several systems that might contribute to bone loss. This review summarizes studies on the association between hemophilia and bone remodeling, and might shed light on the challenges facing the care and prevention of osteoporosis and fracture in patients with hemophilia.

Effect of platelet-rich plasma and fibrin glue on healing of critical-size calvarial bone defects

Despite the insufficient number of experimental studies, platelet-rich plasma (PRP) including high amounts of growth factors is introduced to clinical use rapidly. The aim of this study was to compare the effects of PRP and platelet-poor plasma (PPP) on healing of critical-size bone defects.Bilateral full-thickness, critical-size bone defects were created in the parietal bones of 32 rabbits, which had been studied in 4 groups. Saline, thrombin solution, PPP, and PRP were applied to the created defects before closure. Radiologic defect area measurement results at 0, 4, and 16 weeks were compared between the groups. In addition, densities of the newly formed bones at 16th week were studied. Histologic parameters (primary and secondary bone trabecula, neovascularization, and bone marrow and connective tissue formation) were compared between 4- and 16-week groups.More rapid decrease in defect size was observed in groups 3 and 4 than in groups 1 and 2, both in the 4th and 16th weeks. Newly formed bone densities were also found to be higher in these 2 groups. New bone formation was detected to be more rapid considering histologic parameters, in groups 3 and 4 at 4th and 16th weeks.Study demonstrates that PRP and PPP might have favorable effects on bone healing. Although we cannot reveal any statistical difference between these 2 substances considering osteoinductive potential, PRP group has demonstrated superior results compared with fibrin glue group. Higher platelet concentrations may expose beneficial effects of PRP.

Engineering a bioactive matrix by modifications of calcium sulfate

The goal of this study was to define the conditions for the fabrication of a bioactive matrix that induces and supports cell proliferation and tissue regeneration. The proposed hypothesis was that a composite graft could be engineered by the absorption of platelet-rich plasma (PRP) onto calcium sulfate (CS). Evaluation of the biological activity of the engineered grafts was based on osteoblast proliferation studies and scanning electron microscopy (SEM) analyses. Graft samples were created in a standard size and shape so that the surface available for attachment and cell proliferation was always identical. Proliferation data were expressed as counts per minute per group and differences among groups were statistically analyzed by analysis of variance followed by the Scheffé test (alpha = 0.1). SEM analysis showed that the combination of CS and PRP presents a preserved crystalline structure well integrated by organic matrix. This combination showed the highest cell proliferation levels (p < 0.001). Further evaluations demonstrated that PRP is activated when combined with CS. When tested as a possible carrier for biologically active molecules such as platelet-derived growth factor (PDGF), CS showed increased cell proliferation (p < 0.001). SEM revealed adherent osteoblasts with broad flattened edges on CS-PRP. This study proposes CS as an efficient carrier for PRP or PDGF and supports the use of these combinations as bioactive matrices in clinical or laboratory applications.

Thrombin, but not bradykinin, stimulates proliferation in isolated human osteoblasts, via a mechanism not dependent on endogenous prostaglandin formation

Osteolysis or osteosclerosis often occurs in bone tissue adjacent to chronic inflammatory processes. Numerous cytokines and inflammatory mediators have been implicated as osteoclast-activating agents, explaining inflammation-induced bone resorption. In many cases, the cause of the sclerosis seen in these lesions is less thoroughly investigated. We have studied the effects of thrombin and bradykinin, 2 inflammatory mediators, on the rate of proliferation in isolated human osteoblasts (hOBs). Thrombin, at and above 1 U/mL, stimulated the rate of thymidine incorporation into hOBs. The absolute cell number also increased, as measured by an assay based on the detection of cell metabolism. A synthetic peptide ligand for the thrombin receptor enhanced the rate of [3H]thymidine incorporation in hOBs, indicating that thrombin-induced proliferation is mediated via the tetheric thrombin receptor. The thrombin-induced proliferation was not affected by indomethacin, excluding prostanoids as mediators of this effect. Bradykinin did not affect either the rate of thymidine incorporation, or number of cells in long-term cultures of hOBs. In conclusion, the inflammatory mediator, thrombin, stimulates proliferation in isolated human osteoblasts probably via the recently described G-protein-coupled tetheric thrombin receptor. Thrombin may therefore be involved as a mediator of inflammation-induced sclerosis and bone formation.

Expression of the thrombin receptor in developing bone and associated tissues

Thrombin, a serine protease with a central role in thrombosis and hemostasis, is also a specific agonist for a variety of cellular responses in osteoblasts and stimulates bone resorption in organ culture. Cultured osteoblast-like cells express the proteolytically activated thrombin receptor, but the significance of this finding in vivo remains unknown. Immunohistochemistry was used to investigate the normal tissue distribution of the proteolytically activated thrombin receptor in developing rat bones and associated tissues. In hind limbs, the receptor was first observed on embryonic day 16 and became more abundant within the limb as gestation progressed. Thrombin receptor staining was detected on osteoblasts, macrophages, muscle cells, and endothelial cells, but not osteoclasts. Similarly, osteoblasts in developing calvariae stained positively for the thrombin receptor. The pattern of receptor expression by primary osteoblast cultures and freshly isolated macrophages and osteoclasts corresponded to that observed in vivo. The observed pattern of thrombin receptor expression in bone cells supports the hypothesis that cell-mediated thrombin-induced bone resorption is mediated by osteoblasts.

A hirudin-sensitive, growth-related proteinase from human fibroblasts

We have identified a hirudin-sensitive proteinase from human fibroblasts. Inhibition of this enzyme results in partial inhibition of cell growth in culture. The enzyme has a molecular weight of about 38000, and can be isolated by affinity chromatography with a hirudin-agarose matrix. The enzyme is apparently not closely related to thrombin, but does show a high level of amino acid sequence homology with fragments of a protein isolated from the synovial fluid of rheumatoid arthritis patients, which acts as a mitogen for human T lymphocytes.

Serum fractions and related agonists with calcium-mobilizing activity in the bovine growth plate chondrocyte

Longitudinal growth of bone involves a complex sequence of cellular events in the cartilaginous epiphysis. Whole blood serum has been shown previously to be a potent stimulus to the cells of the growth plate, as demonstrated by its ability to activate the inositol phosphate-calcium second messenger system, resulting in a rise in intracellular Ca2+. By manipulating the preparation of serum to functionally separate it into its constituent parts, we have shown that the processes of platelet lysis and activation of the clotting cascade are responsible for the generation of factors that stimulate this signaling mechanism in isolated bovine growth plate chondrocytes. Through a subsequent trial of bioactive agents generated in these processes, we identified and partially characterized several novel agonists of growth plate chondrocytes:adenosine triphosphate and adenosine diphosphate, the purine energy substrates, and bradykinin, the bioactive peptide generated in a side reaction of the clotting cascade, each induces a rise in intracellular Ca2+ via release from intracellular ion stores. Additionally, the three distinct isoforms of platelet-derived growth factor (AA, AB, and BB), also released on platelet lysis, were compared with respect to their ability to stimulate the inositol phosphate-calcium second messenger system in growth plate chondrocytes.

Thrombospondin (TSP1) mediates in vitro proliferation of human MG-63 osteoblastic cells induced by alpha-thrombin

Thrombospondin (TSP) is a 450-kDa glycoprotein synthesized and secreted by human MG-63 osteoblastic cells. In this study, we have first studied the effect of alpha-thrombin on TSP expression by human MG-63 cells. In situ hybridization indicated that TSP mRNA level in thrombin-treated MG-63 cells was increased when compared to unstimulated cells. As judged by immunofluorescence, thrombin-treatment of MG-63 cells resulted in increased cell surface expression of TSP when compared to quiescent cells. Because thrombin stimulates proliferation of osteoblastic cells, the involvement of TSP in proliferation of thrombin-stimulated osteoblastic cells was then investigated using a serum-free mitogenesis assay. Both alpha-thrombin (0.01 to 0.15 U/ml) and TSP (5 to 600 ng/ml) caused a dose-dependent increase in [3H]thymidine incorporation by MG-63 cells. Proliferation of osteoblastic cells induced by alpha-thrombin or TSP was specifically and totally inhibited by anti-TSP monoclonal antibodies (3-10 micrograms/ml) or by indomethacin (1 microM), an inhibitor of prostaglandin synthesis. Anti-TSP antibodies which inhibited cell proliferation also inhibit TSP expression to the surface of these cells. Our experiments support the existence of a mechanism whereby TSP bound to the cell surface of thrombin-treated MG-63 cells stimulates secretion of prostaglandins which, in turn, allow cell proliferation to proceed.

Human platelet factor 4 is a direct inhibitor of human osteoblast-like osteosarcoma cell growth

The effect of purified human platelet factor 4, a platelet alpha-granule protein, on the growth of the human osteoblastic osteosarcoma cell lines Saos-2 and G-292 was investigated. Platelet factor 4 (20 ng/ml to 2 micrograms/ml) caused a significant, dose-dependent inhibition of human osteoblast-like osteosarcoma cell proliferation. Platelet factor 4 exerted its inhibitory effect under all growth conditions tested: serum-free, serum-stimulated and thrombin-stimulated. The platelet factor 4-induced cell inhibition was not associated with a cytotoxic effect on the cells (assessed by lactate dehydrogenase release). The inhibitory effect of platelet factor 4 was not affected by the presence of indomethacin in the cultures, indicating that the effect was prostaglandin-independent. These results suggest that platelet factor 4 has direct antitumor effects and that it may be important in pathological and physiological processes of bone.

Second messenger systems stimulated by bradykinin in osteoblastic cells: evidence for B2 receptors

The effects of bradykinin, analogs and inhibitors on the human osteoblastic osteosarcoma cell lines Saos-2 and G292 and on normal rat calvarial osteoblastic cells were investigated. In all cell types, bradykinin (1 nM-100 microM) caused significant time- and dose-dependent changes in the levels of inositol phosphates. Neomycin inhibited the inositol phosphate response to bradykinin, while indomethacin had no effect. Bradykinin also elicited a dose-dependent increase in free cytosolic calcium concentration. Bradykinin and T-kinin did not affect cyclic AMP levels in these cells. Doses of des-Arg9-bradykinin, a B1 receptor agonist, up to 100 nM did not stimulate the osteoblastic inositol phosphate response. In addition, the bradykinin-stimulated inositol phosphate response was unaffected by des-Arg9-[Leu8]-bradykinin, a B1 receptor antagonist, while it was inhibited by D-Arg-[Hyp3-[beta-(2-thienyl)-Ala]5,8-D-Phe7]-bradykinin, a B2 receptor antagonist. These results suggest that in osteoblastic cells the mechanism of action of bradykinin involves stimulation of the phosphoinositide metabolism and increases in cytosolic calcium levels through activation of B2 receptors.