Modulation of osteoblast-like cell behavior by activation of protease-activated receptor-1

Pharmaceutical treatment of bone loss: From animal models and drug development to future treatment strategies

Animal models are fundamental to advance our knowledge of the underlying pathophysiology of bone loss and to study pharmaceutical countermeasures against it. The animal model of post-menopausal osteoporosis from ovariectomy is the most widely used preclinical approach to study skeletal deterioration. However, several other animal models exist, each with unique characteristics such as bone loss from disuse, lactation, glucocorticoid excess, or exposure to hypobaric hypoxia. The present review aimed to provide a comprehensive overview of these animal models to emphasize the importance and significance of investigating bone loss and pharmaceutical countermeasures from perspectives other than post-menopausal osteoporosis only. Hence, the pathophysiology and underlying cellular mechanisms involved in the various types of bone loss are different, and this might influence which prevention and treatment strategies are the most effective. In addition, the review sought to map the current landscape of pharmaceutical countermeasures against osteoporosis with an emphasis on how drug development has changed from being driven by clinical observations and enhancement or repurposing of existing drugs to today's use of targeted anti-bodies that are the result of advanced insights into the underlying molecular mechanisms of bone formation and resorption. Moreover, new treatment combinations or repurposing opportunities of already approved drugs with a focus on dabigatran, parathyroid hormone and abaloparatide, growth hormone, inhibitors of the activin signaling pathway, acetazolamide, zoledronate, and romosozumab are discussed. Despite the considerable progress in drug development, there is still a clear need to improve treatment strategies and develop new pharmaceuticals against various types of osteoporosis. The review also highlights that new treatment indications should be explored using multiple animal models of bone loss in order to ensure a broad representation of different types of skeletal deterioration instead of mainly focusing on primary osteoporosis from post-menopausal estrogen deficiency.

Protease-activated receptor type 2 activation downregulates osteogenesis in periodontal ligament stem cells

Protease-activated receptor-2 (PAR2) is associated with the pathogenesis of many chronic diseases with inflammatory characteristics, including periodontitis. This study aimed to evaluate how the activation of PAR2 can affect the osteogenic activity of human periodontal ligament stem cells (PDLSCs) in vitro. PDLSCs collected from three subjects were treated in osteogenic medium for 2, 7, 14, and 21 days with trypsin (0.1 U/mL), PAR2 specific agonist peptide (SLIGRL-NH2) (100 nM), and PAR2 antagonist peptide (FSLLRY-NH2) (100 nM). Gene (RT-qPCR) expression and protein expression (ELISA) of osteogenic factors, bone metabolism, and inflammatory cytokines, cell proliferation, alkaline phosphatase (ALP) activity, alizarin red S staining, and supernatant concentration were assessed. Statistical analysis of the results with a significance level of 5% was performed. Activation of PAR2 led to decreases in cell proliferation and calcium deposition (p < 0.05), calcium concentration (p < 0.05), and ALP activity (p < 0.05). Additionally, PAR2 activation increased gene and protein expression of receptor activator of nuclear factor kappa-Β ligand (RANKL) (p < 0.05) and significantly decreased the gene and protein expression of osteoprotegerin (p <0. 05). Considering the findings, the present study demonstrated PAR2 activation was able to decrease cell proliferation, decreased osteogenic activity of PDLSCs, and upregulated conditions for bone resorption. PAR2 may be considered a promising target in periodontal regenerative procedures.

Osteogenesis in human periodontal ligament stem cell sheets is enhanced by the protease-activated receptor 1 (PAR1) in vivo

Human periodontal ligament stem cells (PDLSCs) have been studied as a promising strategy in regenerative approaches. The protease-activated receptor 1 (PAR₁) plays a key role in osteogenesis and has been shown to induce osteogenesis and increase bone formation in PDLSCs. However, little is known about its effects when activated in PDLSCs as a cell sheet construct and how it would impact bone formation as a graft in vivo. Here, PDLSCs were obtained from 3 patients. Groups were divided into control, osteogenic medium and osteogenic medium + PAR₁ activation by TFLLR-NH2 peptide. Cell phenotype was determined by flow cytometry and immunofluorescence. Calcium deposition was quantified by Alizarin Red Staining. Cell sheet microstructure was analyzed through light, scanning electron microscopy and histology and transplanted to Balb/c nude mice. Immunohistochemistry for bone sialoprotein (BSP), integrin β1 and collagen type 1 and histological stains (H&E, Van Giesson, Masson’s Trichrome and Von Kossa) were performed on the ex-vivo mineralized tissue after 60 days of implantation in vivo. Ectopic bone formation was evaluated through micro-CT. PAR₁ activation increased calcium deposition in vitro as well as BSP, collagen type 1 and integrin β1 protein expression and higher ectopic bone formation (micro-CT) in vivo.

Osteoporosis management and falls prevention in patients with haemophilia: Review of haemophilia guidelines

INTRODUCTION

Patients with haemophilia (PWH) have a high prevalence of osteoporosis, falls and fractures at all ages. The role of haemophilia itself may contribute to low bone mineral density (BMD) due to coagulation factor deficiency. Guidelines for the management of osteoporosis, fracture and fall risk may help to reduce fracture and fall risk, and delay osteoporosis onset.

AIM

We aim to review current haemophilia guidelines regarding osteoporosis prevention, screening, diagnosis and management, and fall prevention.

METHOD

A database search (Ovid MEDLINE) revealed two haemophilia guidelines (World and British) published within the last ten years. Local Australian haemophilia guidelines were identified through a manual search.

RESULTS

All haemophilia guidelines were found to contain inadequate recommendations for osteoporosis management and fall prevention due to a lack of evidence in the literature.

CONCLUSION

Further studies are required to assess the trajectory of bone health in PWH, the mechanism of bone loss in PWH, and the effectiveness of weight-bearing exercises, interventions for fall prevention, screening programmes, and use of anti-osteoporosis medications in PWH across the lifecourse.

Deletion of Coagulation Factor IX Compromises Bone Mass and Strength: Murine Model of Hemophilia B (Christmas Disease)

Osteopenia and osteoporosis have increasingly become a recognized morbidity in those persons with hemophilia (PwH) receiving inadequate prophylactic clotting factor replacement. Animal models can control or eliminate genetic and environmental factors and allow for invasive testing not clinically permissible. Here, we describe the skeletal phenotype of juvenile and adult male mice with a genetically engineered deficiency in coagulation factor IX (FIX KO). Although the somatic growth of FIX KO mice matched that of their wild-type (WT) littermates at 10 and 20 weeks of age, the FIX KO mice displayed reduced bone mineral density (BMD), reduced cortical and cancellous bone mass, and diminished whole bone fracture resistance. These findings coupled with parallel observations in a murine model of hemophilia A (FVIII deficiency) point to an effector downstream of the coagulation cascade that is necessary for normal skeletal development. Further study of potential mechanisms underlying the bone disease observed in rare clotting factor deficiency syndromes may lead to new diagnostic and therapeutic insights for metabolic bone diseases in general.

Role of Thrombin in Central Nervous System Injury and Disease

Thrombin is a Na+-activated allosteric serine protease of the chymotrypsin family involved in coagulation, inflammation, cell protection, and apoptosis. Increasingly, the role of thrombin in the brain has been explored. Low concentrations of thrombin are neuroprotective, while high concentrations exert pathological effects. However, greater attention regarding the involvement of thrombin in normal and pathological processes in the central nervous system is warranted. In this review, we explore the mechanisms of thrombin action, localization, and functions in the central nervous system and describe the involvement of thrombin in stroke and intracerebral hemorrhage, neurodegenerative diseases, epilepsy, traumatic brain injury, and primary central nervous system tumors. We aim to comprehensively characterize the role of thrombin in neurological disease and injury.

Thrombin inhibitor argatroban modulates bone marrow stromal cells behaviors and promotes osteogenesis through canonical Wnt signaling

AIMS

Coagulation is a common event that play a double-edged role in physiological and pathological process. Anti-coagulation methods were applied in joint surgery or scaffolds implantation to encourage new vascular formation and avoid coagulation block. However, whether anti-coagulation drug perform regulatory roles in bone structure is unknown. This study aims to explore a direct thrombin inhibitor, argatroban, effects on bone marrow stromal cells (BMSCs) and decipher the underlying mechanisms.

MATERIALS AND METHODS

Argatroban effects on BMSCs were investigated in vivo and in vitro. The drug was applied in periodontal disease model mice and bone loss was evaluated by μCT and histology. BMSCs were treated with different doses argatroban or vehicle. Cellular reactions were analyzed using wound healing assay, qRT-PCR, Alizarin Red S staining and western blotting.

KEY FINDINGS

We demonstrated that local injection of argatroban can rescue bone loss in periodontal disease in vivo. To explore the underlying mechanism, we examined that cell proliferation and differentiation capability. Proliferation and migration of BMSCs were both inhibited by applying lower dose of argatroban. Interestingly, without affecting osteoclastogenesis, osteogenic differentiation was significantly induced by argatroban, which were shown by extracellular mineralization and upregulation of early osteoblastic differentiation markers, alkaline phosphatase, Osteocalcin, transcription factors RUNX2 and Osterix. In addition, molecular analysis revealed that argatroban promoted β-catenin nuclear translocation and led to an increase of osteogenesis through activating canonical Wnt signaling.

SIGNIFICANCE

Taken together, our results show the novel application of the anti-coagulation compound argatroban in the commitment of BMSCs-based alveolar bone regeneration and remodeling.

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.

Protease-Activated Receptor Type 1 Activation Enhances Osteogenic Activity in Human Periodontal Ligament Stem Cells

Protease-activated receptor 1 (PAR₁) has been associated to tissue repair and bone healing. The aim of the present study was to evaluate the effect of PAR₁ activation on the osteogenic activity of human periodontal ligament stem cells (PDLSCs). PDLSCs were cultured in the presence of PAR₁-selective agonist peptide (100 nM), thrombin (0.1 U/mL), or PAR₁ antagonist peptide (100 nM). Calcium deposits, calcium concentration (supernatant), alkaline phosphatase activity (ALP), cell proliferation, and gene (qPCR) and protein expression (ELISA assay) of osteogenic factors were assessed at 2, 7, and 14 days. PAR₁ activation led to increased calcium deposits (p < 0.05), calcium concentration (p < 0.05), ALP activity (p < 0.05), and cell proliferation (p < 0.05). Further, PAR₁ activation may increase gene and protein expression of Runx2 (p < 0.05) and OPG (p < 0.05). In conclusion, PAR₁ activation increases osteogenic activity of PDLSCs, providing a possible new strategy for periodontal regenerative therapies.

Protease-Activated Receptor 1 Deletion Causes Enhanced Osteoclastogenesis in Response to Inflammatory Signals through a Notch2-Dependent Mechanism

We found that protease-activated receptor 1 (PAR1) was transiently induced in cultured osteoclast precursor cells. Therefore, we examined the bone phenotype and response to resorptive stimuli of PAR1-deficient (knockout [KO]) mice. Bones and bone marrow-derived cells from PAR1 KO and wild-type (WT) mice were assessed using microcomputed tomography, histomorphometry, in vitro cultures, and RT-PCR. Osteoclastic responses to TNF-α (TNF) challenge in calvaria were analyzed with and without a specific neutralizing Ab to the Notch2-negative regulatory region (N2-NRR Ab). In vivo under homeostatic conditions, there were minimal differences in bone mass or bone cells between PAR1 KO and WT mice. However, PAR1 KO myeloid cells demonstrated enhanced osteoclastogenesis in response to receptor activator of NF-κB ligand (RANKL) or the combination of RANKL and TNF. Strikingly, in vivo osteoclastogenic responses of PAR1 KO mice to TNF were markedly enhanced. We found that N2-NRR Ab reduced TNF-induced osteoclastogenesis in PAR1 KO mice to WT levels without affecting WT responses. Similarly, in vitro N2-NRR Ab reduced RANKL-induced osteoclastogenesis in PAR1 KO cells to WT levels without altering WT responses. We conclude that PAR1 functions to limit Notch2 signaling in responses to RANKL and TNF and moderates osteoclastogenic response to these cytokines. This effect appears, at least in part, to be cell autonomous because enhanced osteoclastogenesis was seen in highly purified PAR1 KO osteoclast precursor cells. It is likely that this pathway is involved in regulating the response of bone to diseases associated with inflammatory signals.

The effect of oral dabigatran etexilate on bone density, strength, and microstructure in healthy mice

Thrombin is a key component in the coagulation cascade where it converts factor V, VIII, XI, and fibrinogen. In addition to the abundant production of thrombin in the liver, osteoclasts synthesize and secrete thrombin as well. Osteoblasts express thrombin receptors, and it has been reported that thrombin stimulates the expression of RANKL relatively to OPG, resulting in greater osteoclast activation and bone degradation. Pradaxa (dabigatran etexilate, DE) is a new anticoagulant, which has recently been approved for clinical use. DE is a direct thrombin inhibitor with potential to modulate the RANKL/OPG ratio and thereby limit osteoclast activation and bone degradation. The purpose of the study was to investigate whether DE can increase bone density, bone strength, and bone microstructure in healthy male and female mice and to investigate whether the effect of DE is sex-dependent. Twenty-eight 14-week-old male C57BL/6 mice were stratified by weight into 4 groups: 1. Control 3 weeks; 2. DE 3 weeks; 3. Control 6 weeks; 4. DE 6 weeks. An identical study design was applied to twenty-four 14-week-old female C57BL/6 mice. Chow mixed with DE was offered ad libitum, resulting in a dose of 1.70 mg DE/g body weight and 1.52 mg DE/g body weight, to female and male mice, respectively. The animals were euthanized after 3 or 6 weeks. Bone mineral density (aBMD) and bone mineral content (BMC) were evaluated with DEXA, 3D microstructural properties were determined with μCT, bone strength was determined with mechanical testing, and bone formation and resorption was evaluated with bone histomorphometry. In female mice, DE resulted in significant higher tibial aBMD values after 6 weeks of intervention. Furthermore, DE significantly increased tibial diaphyseal cortical bone area and tissue area, which was accompanied by significantly increased strength of the tibial shaft. DE had no effect on femoral cortical bone or on femoral and vertebral trabecular 3D microstructure. Finally, bone histomorphometry showed that DE had no effect on MS/BS or Oc.S/BS. In male mice, no bone positive effects of DE were found in any of the parameters investigated. In conclusion, intervention with DE may result in a weak positive site specific effect at tibial cortical bone in female mice, and importantly, no major deleterious effects of DE on bone tissue were seen in either female or male mice despite the relatively high dose of DE used.

Relationship between heterotopic ossification and traumatic brain injury: Why severe traumatic brain injury increases the risk of heterotopic ossification

Heterotopic ossification (HO) is a pathological phenomenon in which ectopic lamellar bone forms in soft tissues. HO involves many predisposing factors, including congenital and postnatal factors. Postnatal HO is usually induced by fracture, burn, neurological damage (brain injury and spinal cord injury) and joint replacement. Recent studies have found that patients who suffered from bone fracture combined with severe traumatic brain injury (S-TBI) are at a significantly increased risk for HO occurrence. Thus, considerable research focused on the influence of S-TBI on fracture healing and bone formation, as well as on the changes in various osteogenic factors with S-TBI occurrence. Brain damage promotes bone formation, but the exact mechanisms underlying bone formation and HO after S-TBI remain to be clarified. Hence, this article summarises the findings of previous studies on the relationship between S-TBI and HO and discusses the probable causes and mechanisms of HO caused by S-TBI. The translational potential of this article: A better understanding of the probable causes of traumatic brain injury-induced HO can provide new perspectives and ideas in preventing HO and may support to design more targeted therapies to reduce HO or enhance the bone formation.

TGF-β induced PAR-1 expression promotes tumor progression and osteoclast differentiation in giant cell tumor of bone

Although protease activated receptor-1 (PAR-1) has been confirmed as an oncogene in many cancers, the role of PAR-1 in giant cell tumor (GCT) of bone has been rarely reported. The mechanism of PAR-1 in tumor-induced osteoclastogenesis still remains unclear. In the present study, we detected that PAR-1 was significantly upregulated in GCT of bone compared to normal tissues, while TGF-β was also overexpressed in GCT tissues and could promote the expression of PAR-1 in a dose and time dependent manner. Using the luciferase reporter assay, we found that two downstreams of TGF-β, Smad3 and Smad4, could activate the promoter of PAR-1, which might explain the mechanism of TGF-β induced PAR-1 expression. Meanwhile, PAR-1 was also overexpressed in microvesicles from stromal cells of GCT (GCTSCs), and might be transported from GCTSCs to monocytes through microvesicles. In addition, knockout of PAR-1 by TALENs in GCTSCs inhibited tumor growth, angiogenesis and osteoclastogenesis in GCT in vitro. Using the chick CAM models, we further showed that inhibition of PAR-1 suppressed tumor growth and giant cell formation in vivo. Using microarray assay, we detected a number of genes involved in osteoclastogenesis as the possible downstreams of PAR-1, which may partly explain the mechanism of PAR-1 in GCT. In brief, for the first time, these results reveal an upstream regulatory role of TGF-β in PAR-1 expression, and PAR-1 expression promotes tumor growth, angiogenesis and osteoclast differentiation in GCT of bone. Hence, PAR-1 represents a novel potential therapeutic target for GCT of bone.

Coagulation and non-coagulation effects of thrombin

Thrombin is a multifunctional serine protease produced from prothrombin, and is a key regulator in hemostatic and non-hemostatic processes. It is the main effector protease in primary hemostasis by activating platelets, and plays a key role in secondary hemostasis. Besides its well-known functions in hemostasis, thrombin also plays a role in various non-hemostatic biological and pathophysiologic processes, predominantly mediated through activation of protease-activated receptors (PARs). Depending on several factors, such as the concentration of thrombin, the duration of activation, the location of PARs, the presence of coreceptors, and the formation of PAR heterodimers, activation of the receptor by thrombin can induce different cellular responses. Moreover, thrombin can have opposing effects in the same cell; it can induce both inflammatory and anti-inflammatory signals. Owing to the complexity of thrombin's signal transduction pathways, the exact mechanism behind the dichotomy of thrombin is yet still unknown. In this review, we highlight the hemostatic and non-hemostatic functions of thrombin, and specifically focus on the non-hemostatic dual role of thrombin under various conditions and in relation to cardiovascular disease.

Protease-activated receptors and their biological role - focused on skin inflammation

OBJECTIVES

For several years, protease-activated receptors (PARs) are targets of science regarding to various diseases and platelet aggregation. In the past, a number of publications related to PARs have been published, which refer to a variety of aspects. An important point of view is the inflammation of the skin, which has not been reported in detail yet. This review will provide an overview of the current knowledge on PARs, and in particular, on the involvement of PARs in terms of skin inflammation.

KEY FINDINGS

Wound healing is an important step after skin injury and is connected with involvement of PARs and inflammation. An important point in skin inflammation is the coagulation-dependent skin inflammation.

SUMMARY

PARs are a special kind of receptors, being activated by proteolytic cleavage or chemical agonists. They may play an important role in various physiological processes. It is shown that the proteases are involved in many diseases for example Parkinson's disease and Alzheimer's disease. The fact, that proteases regulate the coagulation, and are involved in interleukin and cytokine release leads to the conclusion that they are involved in inflammation processes.

Thrombin receptor deficiency leads to a high bone mass phenotype by decreasing the RANKL/OPG ratio

Thrombin and its receptor (TR) are, respectively, expressed in osteoclasts and osteoblasts. However, their physiological roles on bone metabolism have not been fully elucidated. Here we investigated the bone microarchitecture by micro-computed tomography (μCT) and demonstrated increased trabecular and cortical bone mass in femurs of TR KO mice compared to WT littermates. Trabecular thickness and connectivity were significantly enhanced. The physiological role of TR on both inorganic and organic phases of bone is illustrated by a significant increase in BMD and a decrease in urinary deoxypyridinoline (DPD) crosslink concentration in TR KO mice. Moreover, TR KO cortical bone expanded and had a higher polar moment of inertia (J), implying stronger bone. Bone histomorphometry illustrated unaltered osteoblast and osteoclast number and surface in femoral metaphyses, indicating that thrombin/TR regulates osteoblasts and osteoclasts at functional levels. Serum analysis showed a decrease in RANKL and an increase in osteoprotegerin (OPG) levels and reflected a reduced RANKL/OPG ratio in the TR KO group. In vitro experiments using MC3T3 pre-osteoblasts demonstrated a TR-dependent stimulatory effect of thrombin on the RANKL/OPG ratio. This effect was blocked by TR antagonist and p42/p44-ERK inhibitor. In addition, thrombin also intensified p42/p44-ERK expression and phosphorylation. In conclusion, the thrombin/TR system maintains normal bone remodeling by activating RANKL and limiting OPG synthesis by osteoblasts through the p42/44-ERK signaling pathway. Consequently, TR deficiency inhibits osteoclastogenesis, resulting in a high bone mass phenotype.

The role of proteases in pain

Proteinase-activated receptors (PARs) are a family of G protein-coupled receptor that are activated by extracellular cleavage of the receptor in the N-terminal domain. This slicing of the receptor exposes a tethered ligand which binds to a specific docking point on the receptor surface to initiate intracellular signalling. PARs are expressed by numerous tissues in the body, and they are involved in various physiological and pathological processes such as food digestion, tissue remodelling and blood coagulation. This chapter will summarise how serine proteinases activate PARs leading to the development of pain in several chronic pain conditions. The potential of PARs as a drug target for pain relief is also discussed.

Protease-activated receptor 2 signalling pathways: a role in pain processing

INTRODUCTION

Pain is a complex biological phenomenon that includes intricate neurophysiological, behavioural, psychosocial and affective components. Despite decades of pain research, many patients continue suffering from chronic pain that may be refractory to current medical regimens. Accumulating evidence has indicated an important role of protease-activated receptor 2 (PAR2) in the pathogenesis of pain, including inflammation, neuropathic and cancer pain.

AREAS COVERED

In this review, the role of the PAR2 signalling pathway in pain processes, basic mechanism of PAR2 activation and expression of PAR2 in the nervous system is covered. Furthermore, intracellular signalling pathways that are activated by PAR2 are also described.

EXPERT OPINION

The role of PAR2 in pain processing is becoming increasingly clear, and although causal implication remains to be established, PAR2 activation has been observed in several disease model systems. Since PAR2 is activated after nerve injury as well as by trypsin and related serine proteases, and PAR2 plays an important role in pain development and maintenance, exploring PAR2 and its corresponding signalling pathways will provide unfathomable knowledge in understanding the molecular basis of pain. This will also help to identify new targets for pharmacological intervention; however, in the context of potential PAR2-directed therapies, several aspects should be clarified.

Thrombin inhibits osteoclast differentiation through a non-proteolytic mechanism

Thrombin stimulates expression of interleukin 6 and cyclooxygenase 2 by osteoblasts, both of which enhance osteoblast-mediated osteoclast differentiation by increasing the ratio of receptor activator of nuclear factor κB ligand (RANKL) expression to that of osteoprotegerin (OPG) in osteoblasts. We hypothesised that thrombin would also increase this ratio and thereby stimulate osteoclast differentiation in mixed cultures of osteoblastic cells and osteoclast precursors. In primary mouse osteoblasts, but not in bone marrow stromal cells, thrombin increased the ratio of RANKL to OPG expression. Thrombin inhibited differentiation of osteoclasts, defined as tartrate-resistant acid phosphatase (TRAP)-positive cells with three or more nuclei, in mouse bone marrow cultures treated with osteoclastogenic hormones; this effect was not mediated by the major thrombin receptor, protease-activated receptor 1, nor did it require thrombin's proteolytic activity. Thrombin also caused a decrease in the number of TRAP-positive cells with fewer than three nuclei. Thrombin (active or inactive) also inhibited osteoclast differentiation and bone resorption, respectively, in cultures of mouse spleen cells and human peripheral blood mononuclear cells induced to undergo osteoclastogenesis by treatment with RANKL and macrophage colony-stimulating factor. Osteoclast differentiation in spleen cells was inhibited when they were exposed to thrombin from days 0 to 3 or 3 to 5 of culture but not days 5 to 7 when most fusion occurred. Thrombin inhibited expression of RANK by spleen cells. These observations indicate that, although thrombin stimulates production of osteoclastogenic factors by osteoblastic cells, it inhibits the early stages of RANKL-induced osteoclast differentiation through a direct effect on osteoclast precursors that does not require thrombin's proteolytic activity.

Divergent upstream osteogenic events contribute to the differential modulation of MG63 cell osteoblast differentiation by MMP-1 (collagenase-1) and MMP-13 (collagenase-3)

Previously we showed that MMP-1 (collagenase-1) and MMP-13 (collagenase-3) differentially regulate the expression of osteoblastic markers in a heterogenous population of primary human periodontal ligament cells. The mechanisms for these differential responses are not known, but may result from divergence in regulation of early osteogenic transcription factors. The purpose of this study was to elucidate where in the hierarchy of osteoblast-specific transcription factors and markers the differences in MMP-1- and -13-mediated regulation of osteoblastic differentiation arise. We found that the overexpression of MMP-1 resulted in significant decreases in BMP-2, Dlx5, AP, OP and BSP and increases in TGF-β1 and MSX2. In contrast, MMP-13 overexpression resulted in significant decreases in Runx2, OP and BSP, and increases in TGF-β1, MSX2 and OC. The knockdown of MMP-1 caused significant increases in all osteoblastic markers. MMP-13 knockdown produced significant increases only in TGF-β1, MSX2 and Osx, but decreases in Runx2 and OC. Suppression of both MMPs together resulted in significant increases of all osteoblastic markers except Runx2. MMP-1 had a more robust and generalized effect in regulating osteoblast transcription factors and markers than MMP-13. Finally, of the markers and transcription factors assayed, Runx2 is the most early stage transcription factor induced by suppression of MMP-1, while Osx and MSX2 are the most early stage transcription factors regulated by MMP-13. These data show that MMP-1's and -13's differential regulation of osteoblastic markers in MG63 cells likely results from their modulation of divergent signaling pathways involved in osteoblastic differentiation.

Activated protein C stimulates osteoblast proliferation via endothelial protein C receptor

INTRODUCTION

Bone is continually remodeled by the action of osteoblasts, osteocytes, and osteoclasts. Resting osteoblasts are able to proliferate and differentiate into mature osteoblasts when physiologically required, as after tissue injury. Activated protein C (APC) is a serine protease that functions in anticoagulation, anti-inflammation, anti-apoptosis, cell proliferation, and wound repair. In this study, we examined the effect of APC on osteoblast proliferation and differentiation.

MATERIALS AND METHODS

We examined the presence of protein C in human fracture hematoma by immunohistochemical staining. We then evaluated the effect of APC, diisopropyl fluorophosphate-inactivated APC (DIP-APC) or protein C zymogen on normal human osteoblast (NHOst) proliferation using tetrazolium salt assay in the presence or absence of aprotinin, hirudin, protein C, antibody against protein C, endothelial protein C receptor (EPCR) or protease-activated receptor (PAR)-1. Finally, activation of p44/42 MAP kinase was evaluated by Western blot analysis.

RESULTS

Both APC and DIP-APC increased osteoblast proliferation in a dose-dependent manner, while protein C did not. The APC-induced increased proliferation of osteoblast was not affected by aprotinin, hirudin, and anti-protein C antibody which inhibits the protease activity of APC. Treatment with protein C or anti-EPCR antibody which inhibits APC binding to EPCR inhibited APC-mediated osteoblast proliferation, while treatment with anti-PAR-1 antibody did not. APC promoted the phosphorylation of p44/42 MAP kinase within osteoblasts; this effect was inhibited by the anti-EPCR antibody.

CONCLUSIONS

APC stimulates osteoblast proliferation by activating p44/42 MAP kinase through a mechanism that requires EPCR but not PAR-1 or the proteolytic activity of APC. APC generated at fracture sites may contribute to fracture healing by promoting osteoblast proliferation.

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.

Thrombin induces osteoprotegerin synthesis via phosphatidylinositol 3'-kinase/mammalian target of rapamycin pathway in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Thrombin influences the biological behavior of periodontal ligament cells and plays multiple roles in the early stages of bone healing. Osteoprotegerin (OPG) is one of the key molecules that regulate bone homeostasis and prevent osteoclastogenesis. The purpose of this study was to evaluate the biological effects of thrombin on OPG synthesis in human periodontal ligament (HPDL) cells in vitro.

MATERIAL AND METHODS

Cells were treated with various concentrations (0.001, 0.01 and 0.1 U/mL) of thrombin. The mRNA expression and protein synthesis of OPG, as well as of receptor activator of nuclear factor kappaB ligand (RANKL), were determined by reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot analysis, respectively. The influence of thrombin on OPG synthesis and its signaling pathway were investigated using inhibitors.

RESULTS

Thrombin profoundly induces protein synthesis of OPG at 0.1 U/mL. The inductive effect was inhibited by cycloheximide, but not by indomethacin. The phosphatidylinositol 3'-kinase (PI3K) inhibitor, LY294002, and the mammalian target of rapamycin (mTOR) inhibitor, rapamycin, exerted an inhibitory effect on the thrombin-induced OPG synthesis. In addition, the effect was inhibited by protease-activated receptor (PAR)-1 antagonist. Activation of phospho-Akt (p-Akt) was observed and the effect was abolished by LY294002.

CONCLUSION

Thrombin induces OPG synthesis in HPDL cells post-transcriptionally, possibly through PAR-1. The regulation was through the PI3K/Akt and mTOR pathway. This finding suggests that thrombin may play a significant role in alveolar bone repair and homeostasis of periodontal tissue, partly through the OPG/RANKL system.

Kallikrein-related peptidase 4 (KLK4) initiates intracellular signaling via protease-activated receptors (PARs). KLK4 and PAR-2 are co-expressed during prostate cancer progression

Kallikrein-related peptidase 4 (KLK4) is one of the 15 members of the human KLK family and a trypsin-like, prostate cancer-associated serine protease. Signaling initiated by trypsin-like serine proteases are transduced across the plasma membrane primarily by members of the protease-activated receptor (PAR) family of G protein-coupled receptors. Here we show, using Ca(2+) flux assays, that KLK4 signals via both PAR-1 and PAR-2 but not via PAR-4. Dose-response analysis over the enzyme concentration range 0.1-1000 nM indicated that KLK4-induced Ca(2+) mobilization via PAR-1 is more potent than via PAR-2, whereas KLK4 displayed greater efficacy via the latter PAR. We confirmed the specificity of KLK4 signaling via PAR-2 using in vitro protease cleavage assays and anti-phospho-ERK1/2/total ERK1/2 Western blot analysis of PAR-2-overexpressing and small interfering RNA-mediated receptor knockdown cell lines. Consistently, confocal microscopy analyses indicated that KLK4 initiates loss of PAR-2 from the cell surface and receptor internalization. Immunohistochemical analysis indicated the co-expression of agonist and PAR-2 in primary prostate cancer and bone metastases, suggesting that KLK4 signaling via this receptor will have pathological relevance. These data provide insight into KLK4-mediated cell signaling and suggest that signals induced by this enzyme via PARs may be important in prostate cancer.

Osteoclast size heterogeneity in rat long bones is associated with differences in adhesive ligand specificity

Prothrombin (PT) is an RGD-containing bone-residing precursor to the serine protease thrombin (TH), which acts as an agonist for a variety of cellular responses in osteoblasts and osteoclasts. We show here that PT, TH, osteopontin (OPN) and fibronectin (FN) promoted adhesion of isolated neonatal rat long bone osteoclasts. However, the cells that adhered to PT and TH were smaller in size, rounded and contained 3-4 nuclei, in comparison to the cells adhering to OPN and FN, which were larger with extended cytoplasmic processes and 6-7 nuclei. Attachment of the larger osteoclasts to OPN and FN was inhibited by antibodies towards beta 3 and beta 1 integrin subunits, respectively. Whereas an RGD-containing peptide inhibited adhesion of the smaller osteoclasts to PT and TH, this was not seen with the beta 3 or beta 1 antibodies. In contrast, the beta 1 antibody augmented osteoclast adhesion to PT and TH in an RGD-dependent manner. Small osteoclasts were less efficient in resorbing mineralized bovine bone slices, as well as expressed lower mRNA levels of MMP-9 and the cathepsins K and L compared to large osteoclasts. The small osteoclast adhering to PT and TH may represent either an immature, less functional precursor to the large osteoclast or alternatively constitute a distinct osteoclast population with a specific role in bone.

Overexpression of protease-activated receptors-1,-2, and-4 (PAR-1, -2, and -4) in prostate cancer

BACKGROUND

Although protease-activated receptors (PARs) have been described to play a role in different malignancies, their expression and biological activity in prostate cancer are mostly unknown.

METHODS

PAR expression in radical prostatectomy specimens was investigated by immunohistochemistry (IHC, 40 patients) and RT-PCR. Their role in LNCaP prostate cancer cell migration and Rac1/Cdc42 signaling was assessed with Boyden chamber analysis and Western blot, respectively.

RESULTS

PAR mRNA expression was higher in cancer, and protein expression was increased in PAR-1 (45%), PAR-2 (42%), and PAR-4 (68%), compared to normal glands. Increased PAR-1 (periglandular stroma) was associated with higher rates of biochemical recurrence (median follow-up, 5 years; P = 0.006). LNCaP migration was enhanced twofold and Rac1/Cdc42 signaling was activated by stimulation of PAR-1 and PAR-2.

CONCLUSIONS

PARs are overexpressed in prostate cancer and may serve as potential predictors of recurrence. The data suggest potential role of PARs in autocrine and paracrine mechanisms of prostate cancer.

Functional responses of bone cells to thrombin

Cells responsible for the formation and maintenance of bone express thrombin-responsive members of the protease-activated receptor family of G protein-coupled receptors. Thrombin has been shown to elicit a number of functional responses in these cells, including proliferation and cytokine production in osteoblasts. Many, but not all, of the effects of thrombin on bone cells are initiated by activation of protease-activated receptor-1. A combination ofin vitroobservations and results ofin vivostudies in protease-activated receptor-1-null mice suggest that thrombin plays multiple roles in the early stages of bone healing.

The modulation of gene expression in osteoblasts by thrombin coated on biphasic calcium phosphate ceramic

For many years, fibrin sealants were associated with bone substitutes to promote bone healing. However, the osteoblastic response to fibrin sealant components remains poorly documented. In this study, MC3T3-E1 osteoblastic cells were cultured on biphasic calcium phosphate ceramic (MBCP) coated with Tissucol components (thrombin and fibrinogen). Analysis of osteoblastic differentiation markers by RT-PCR revealed that MBCP coated with Tissucol stimulated mRNA levels for osteocalcin and alkaline phosphatase (ALP). Of all the components of Tissucol, thrombin has been reported to affect osteoblastic behavior. Our results demonstrated that low thrombin concentrations (0.5-5 U/ml) stimulated mRNA levels for ALP, whereas high thrombin concentrations (50-100 U/ml) decreased mRNA levels for ALP and PTH/PTHrP receptor and also increased mRNA level for the osteoclastogenesis inhibitor OPG. As thrombin stimulated angiogenesis, we then wondered whether thrombin could influence the expression of angiogenic factors. Low thrombin concentrations were shown to up-regulate mRNA levels for VEGF-B and VEGF-R1, suggesting an autocrine/paracrine role for VEGF-B. Higher thrombin concentrations also up-regulated mRNA for VEGF-A and neuropilin-1. In conclusion, the association of MBCP with thrombin and fibrinogen appears to be a convenient scaffold for bone cell differentiation. Thrombin could also acts at the cellular level by increasing the angiogenic potential of osteoblasts as well as their responsiveness to thrombin and VEGF.

Thrombin mediated migration of osteogenic cells

Given that thrombin is ubiquitously expressed at sites of vascular injury, and that osteogenic cells express receptors for thrombin, we questioned whether thrombin could attract osteogenic cells to a bony wound. Using a scratch wound assay, thrombin stimulated a significant increase in migration of osteogenic cultures of primary marrow cells. This effect was dependent on thrombin proteolytic activity; however, thrombin was unable to stimulate the migration of a more differentiated marrow-derived osteogenic cell line. To better understand the role of thrombin in osteoprogenitor migration, we developed an osteoprogenitor migration assay that combines a modified Boyden chamber with a bone nodule assay. Primary cells that migrated through the transwell filter in the presence of thrombin formed significantly more bone nodules compared to the condition without thrombin. This was not due to proliferation or differentiation effects of thrombin. In contrast, thrombin was unable to stimulate an increase in the number of nodules for the more differentiated osteogenic cell line. Thus, our results suggest that thrombin exhibits differential motogenic effects on osteogenic cells depending on their differentiation state. The cell migration/bone nodule assay described here is the first assay that can be specifically used to examine the effects of factors on the migration of osteoprogenitor cells, particularly those derived from primary populations.

Thrombin Is a Pro-Fibrotic Factor for Rat Renal Fibroblasts in vitro

BACKGROUND

Generation of thrombin occurs in response to parenchymal injury. Thrombin not only converts plasma fibrinogen into an insoluble fibrin clot, but also potentially augments inflammation through receptor-mediated activity. This study examines whether thrombin may potentially exacerbate fibrosis by upregulating the function of interstitial fibroblasts in vitro.

METHODS

Fibroblasts were isolated by explant outgrowth culture of rat kidneys. Subcultured cells were grown in DMEM+10% FCS supplemented with 0.1-0.5 U/ml thrombin. Functional parameters examined included kinetics (thymidine incorporation and change in cell number), differentiation (Western blotting for alpha-smooth muscle actin; alphaSMA), expression of procollagen alpha1(I) (Northern blotting) and contraction of collagen I lattices. RT-PCR was used to characterise expression of protease-activated receptors (PAR) previously implicated in thrombin's cellular effects.

RESULTS

Cell population growth was increased 66 +/- 41 and 47 +/- 41% by 0.1 and 0.5 U/ml thrombin respectively (both p < 0.05 vs. basal). Likewise, 0.5 U/ml thrombin increased corrected procollagen alpha1(I) expression 2.4-fold (p < 0.05 vs. basal) and exacerbated the ability of fibroblasts to contract collagen matrix (p < 0.05 vs. basal). These effects were not associated with any change in expression of the myofibroblast marker alphaSMA. Effects on cell number were inhibited by treatment with (D)-Phe-Pro-Arg-chloromethylketone HCl (PPACK) suggesting that functional effects were mediated by serine protease activity. PAR-1 was the only fully functional known thrombin receptor expressed by these cells.

CONCLUSION

Thrombin is a potential unrecognised fibroblast agonist in renal disease. Further studies of thrombin and its receptors may yield valuable insights into the pathogenesis of interstitial fibrosis.

The role of protease-activated receptor-1 in bone healing

Protease-activated receptor (PAR)-1, a G-protein-coupled receptor activated by thrombin, mediates thrombin-induced proliferation of osteoblasts. The current study was undertaken to define the role of PAR-1 in bone repair. Holes were drilled transversely through the diaphysis of both tibiae of PAR-1-null and wild-type mice. Three days later, fewer cells had invaded the drill site from adjacent bone marrow in PAR-1-null mice than in wild-type mice, and a lower percentage of cells were labeled with [(3)H]thymidine in PAR-1-null drill sites. More osteoclasts were also observed in the drill site of PAR-1-null mice than in wild-type mice 7 days after drilling. New mineralized bone area was less in the drill site and on the adjacent periosteal surface in PAR-1-null mice than in wild-type mice at day 9. From day 14, no obvious differences could be seen between PAR-1-null and wild-type tibiae. In vitro thrombin caused a dose-dependent increase in proliferation of bone marrow stromal cells isolated from wild-type mice but not PAR-1-null mice. Thrombin stimulated survival of bone marrow stromal cells from both wild-type and PAR-1-null mice, but it did not affect bone marrow stromal cell migration in either wild-type or PAR-1-null cells. The results indicate that PAR-1 plays an early role in bone repair.

Studies on the receptors mediating responses of osteoblasts to thrombin

The serine protease thrombin stimulates proliferation in osteoblasts, but decreases alkaline phosphatase (ALP) activity, a marker of osteoblast differentiation. Three thrombin receptors have been identified, protease activated receptor (PAR)-1, PAR-3 and PAR-4; we have previously demonstrated that mouse osteoblasts express PAR-1 and PAR-4. The effect of thrombin on osteoblast proliferation and differentiation was studied to determine which of the thrombin receptors is responsible for the primary effects of thrombin. Primary mouse calvarial osteoblasts from PAR-1-null and wild-type mice, and synthetic peptides that specifically activate PAR-1 (TFFLR-NH2) and PAR-4 (AYPGKF-NH2) were used. Both the PAR-1-activating peptide and thrombin stimulated incorporation of 5-bromo-2'-deoxyuridine (two to four-fold, P < 0.001) and reduced alkaline phosphatase activity (approximately three-fold, P < 0.05) in cells from wild-type mice. The PAR-4-activating peptide, however, had no effect on either alkaline phosphatase activity or proliferation in these cells. Neither thrombin nor PAR-4-activating peptide was able to affect osteoblast proliferation or alkaline phosphatase activity in cells isolated from PAR-1-null mice. The results demonstrate that thrombin stimulates proliferation and inhibits differentiation of osteoblasts through activation of PAR-1. No other thrombin receptor appears to be involved in these effects.

Proteinase-activated receptors: transducers of proteinase-mediated signaling in inflammation and immune response

Serine proteinases such as thrombin, mast cell tryptase, trypsin, or cathepsin G, for example, are highly active mediators with diverse biological activities. So far, proteinases have been considered to act primarily as degradative enzymes in the extracellular space. However, their biological actions in tissues and cells suggest important roles as a part of the body's hormonal communication system during inflammation and immune response. These effects can be attributed to the activation of a new subfamily of G protein-coupled receptors, termed proteinase-activated receptors (PARs). Four members of the PAR family have been cloned so far. Thus, certain proteinases act as signaling molecules that specifically regulate cells by activating PARs. After stimulation, PARs couple to various G proteins and activate signal transduction pathways resulting in the rapid transcription of genes that are involved in inflammation. For example, PARs are widely expressed by cells involved in immune responses and inflammation, regulate endothelial-leukocyte interactions, and modulate the secretion of inflammatory mediators or neuropeptides. Together, the PAR family necessitates a paradigm shift in thinking about hormone action, to include proteinases as key modulators of biological function. Novel compounds that can modulate PAR function may be potent candidates for the treatment of inflammatory or immune diseases.

Platelet-rich plasma for bone graft enhancement in sinus floor augmentation with simultaneous implant placement: patient series study

The use of autologous platelet-rich plasma (PRP) as a source for growth factors in bone grafting is a relatively new and promising technique. Early controlled studies indicate that combining PRP with autologous bone grafts significantly enhances the rate of bone formation and maturation. The study consisted of 105 patients who required sinus augmentation with crestal bone height of less than 5 mm in the posterior maxilla. All patients received a composite bone graft that consisted of 30% to 40% autogenous bone harvested from the lateral wall of the maxilla zygomatic-maxillary buttress and the tuberosity and 60% to 70% xenograft. A total of 50 mL of blood was obtained from each patient before the surgical treatment for preparation of 10 mL of PRP. The graft-PRP mixture was activated by human thrombin. All sinus augmentations were carried out simultaneously with dental implants. At 6 months postoperatively, implants were exposed showing no clinical evidence of crestal bone loss around the implants both clinically and radiographically. All implants were clinically osseointegrated and loaded with fixed porcelain fused to metal prosthesis. The use of PRP in augmenting the severely atrophic posterior maxilla has obvious clinical benefits in terms of reducing the healing period of bone maturation, better graft handling, and accelerated soft tissue healing.

Activation of protease-activated receptor-2 leads to inhibition of osteoclast differentiation

PAR-2 is expressed by osteoblasts and activated by proteases present during inflammation. PAR-2 activation inhibited osteoclast differentiation induced by hormones and cytokines in mouse bone marrow cultures and may protect bone from uncontrolled resorption.

INTRODUCTION

Protease-activated receptor-2 (PAR-2), which is expressed by osteoblasts, is activated specifically by a small number of proteases, including mast cell tryptase and factor Xa. PAR-2 is also activated by a peptide (RAP) that corresponds to the "tethered ligand" created by cleavage of the receptor's extracellular domain. The effect of activating PAR-2 on osteoclast differentiation was investigated.

MATERIALS AND METHODS

Mouse bone marrow cultures have been used to investigate the effect of PAR-2 activation on osteoclast differentiation induced by parathyroid hormone (PTH), 1,25 dihydroxyvitamin D3 [1,25(OH)2D3], and interleukin-11 (IL-11). Expression of PAR-2 by mouse bone marrow, mouse bone marrow stromal cell-enriched cultures, and the RAW264.7 osteoclastogenic cell line was demonstrated by RT-PCR.

RESULTS

RAP was shown to inhibit osteoclast differentiation induced by PTH, 1,25(OH)2D3, or IL-11. Semiquantitative RT-PCR was used to investigate expression of mediators of osteoclast differentiation induced by PTH, 1,25(OH)2D3, or IL-11 in mouse bone marrow cultures and primary calvarial osteoblast cultures treated simultaneously with RAP. In bone marrow and osteoblast cultures treated with PTH, 1,25(OH)2D3, or IL-11, RAP inhibited expression of RANKL and significantly suppressed the ratio of RANKL:osteoprotegerin expression. Activation of PAR-2 led to reduced expression of prostaglandin G/H synthase-2 in bone marrow cultures treated with PTH, 1,25(OH)2D3, or IL-11. RAP inhibited PTH- or 1,25(OH)2D3-induced expression of IL-6 in bone marrow cultures. RAP had no effect on osteoclast differentiation in RANKL-treated RAW264.7 cells.

CONCLUSION

These observations indicate that PAR-2 activation inhibits osteoclast differentiation by acting on cells of the osteoblast lineage to modulate multiple mediators of the effects of PTH, 1,25(OH)2D3, and IL-11. Therefore, the role of PAR-2 in bone may be to protect it from uncontrolled resorption by limiting levels of osteoclast differentiation.

Proliferation of dental pulp fibroblasts in response to thrombin involves mitogen-activated protein kinase signalling

AIM

To examine the involvement of mitogen-activated protein kinases (MAPK) signalling on thrombin-stimulated human dental pulp fibroblasts (DPF).

METHODOLOGY

Dental pulp fibroblasts were isolated from dental pulp connective tissue of third molars and expanded in vitro. Expression of thrombin receptors was analysed by RT-PCR, and cell proliferation was measured by 3[H]-thymidine incorporation assay. Phosphorylation levels of MAPK were determined by Western blot analysis, and alkaline phosphatase activity was measured to serve as a marker for odontogenic differentiation. Statistical analysis was performed by Student's t-test.

RESULTS

Dental pulp fibroblasts express the thrombin receptors protease-activated receptor-1 (PAR-1), PAR-3 and PAR-4. Measurement of 3[H]-thymidine incorporation revealed a dose-dependent increase of DNA synthesis in response to thrombin treatment. The thrombin-induced mitogenic activity was decreased by the extracellular signal-regulated protein kinase (ERK) signalling inhibitor PD98059 (P < 0.05), and by SB203580 (P < 0.05), a p38 MAPK inhibitor. Western blot analysis demonstrated increased phosphorylation of ERK in DPF following stimulation with thrombin, while p38 MAPK and c-Jun NH2-terminal kinase (JNK) were not activated. Alkaline phosphatase activity of DPF remained unchanged upon incubation with thrombin.

CONCLUSIONS

These results suggest that signalling via MAPK mediates the mitogenic activity of thrombin on DPF and may thus play a role during the early stages of pulp repair.

Inhibition of osteoblast apoptosis by thrombin

The multifunctional serine protease thrombin has been shown to be a specific agonist for a variety of functional responses of cells including osteoblasts. The current study was conducted to determine if thrombin was capable of inhibiting apoptosis in osteoblasts, and if so, to examine the mechanism by which this occurred. Thrombin (20-100 nM) significantly inhibited apoptosis in serum-starved cultures of the human osteoblast-like Saos-2 cell line and cultures of primary osteoblasts isolated from mouse calvariae, as well as dexamethasone-treated primary mouse osteoblasts. Inhibition of serum deprivation-induced apoptosis was shown to require thrombin's specific proteolytic activity. Primary mouse osteoblasts were found to express two functional thrombin receptors, PAR-1 and PAR-4. Thrombin inhibited serum deprivation-induced apoptosis in osteoblasts isolated from PAR-1 null mice to the same degree as in osteoblasts isolated from wild-type mice. Treatment of serum-deprived osteoblasts, isolated from either PAR-1 null or wild-type mice, with a PAR-4-activating peptide failed to significantly inhibit apoptosis compared to the relevant control. Medium conditioned by thrombin-treated osteoblasts, in which thrombin had been inactivated, was able to inhibit serum deprivation-induced osteoblast apoptosis almost as well as thrombin itself. Blocking protein synthesis, by cycloheximide pretreatment of the conditioning cells, prevented this action. The ability of known osteoblast survival factors, such as transforming growth factor beta1, fibroblast growth factor-2, insulin-like growth factor-II, and interleukin-6, to inhibit serum deprivation-induced osteoblast apoptosis was also tested. None of these factors was able to inhibit serum deprivation-induced osteoblast apoptosis to the same extent as thrombin. The results presented here demonstrate that thrombin treatment of osteoblasts inhibits apoptosis induced either by dexamethasone or by serum deprivation. Furthermore, it does so independently of the known thrombin receptors by bringing about the synthesis and/or secretion of an unknown survival factor or factors, which then act in an autocrine fashion to inhibit apoptosis.

Protease-activated receptor mediated RhoA signaling and cytoskeletal reorganization in LNCaP cells

Thrombin and trypsin induce cell signaling through a subclass of G-protein-coupled receptors called the protease-activated receptors (PARs). In many cells, PAR signaling results in the activation of RhoA and other members of the Rho family of small GTPases which are involved in cytoskeletal reorganization. The expression of PARs and their role in the activation of Rho GTPases in prostate cancer cells are not clearly known. FACS analysis demonstrated that the androgen-dependent LNCaP cells express PAR1, PAR2, and PAR4 but not PAR3. Stimulation with thrombin and trypsin resulted in the rapid activation of RhoA in a dose-dependent manner with an EC(50) of 1.0 and 5 nM, respectively. Activation of RhoA was enhanced by, but not dependent on, the presence of 1 nM dihydrotestosterone. Inhibition of the proteolytic properties of thrombin by hirudin and trypsin by diisopropyl fluorophosphate abolished the observed RhoA activation. Stimulation with 150 microM PAR-activating peptides TFFLRN (PAR1), SLIGKV (PAR2), and AYPGKF (PAR4) demonstrated that PAR1 and PAR2 mediated protease-activated RhoA signaling. Fluorescent microscopy studies showed that LNCaP cells treated with either thrombin (10 nM) or trypsin (10 nM) developed an increased number of filopodia, stress fibers, and focal adhesions relative to untreated cells. These observations represent the first report of PAR signaling in prostate cancer cells as well as the ability of PAR2 to mediate RhoA activation. Since the activation of RhoA is important for cytoskeletal reorganization, we postulate that PAR-mediated RhoA activation may be a major signaling pathway in the biology of prostate cancer.

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.

Expression of protease-activated receptor-2 by osteoblasts

Osteoblasts express protease-activated receptor-1 (PAR-1), which is activated by thrombin or by synthetic peptides corresponding to the new "tethered ligand" N-terminus of PAR-1 created by receptor cleavage. Both thrombin and human PAR-1-activating peptide stimulate an elevation of [Ca2+]i in the human SaOS-2 osteoblast-like cell line, but the peptide stimulates receptor-mediated Ca+ entry, whereas thrombin does not. Stimulation of proliferation in rat primary osteoblast-like cells is greater in response to rat PAR-1-activating peptide than to thrombin. Because the PAR-1-activating peptides are now known to activate PAR-2, the current study was undertaken to investigate whether osteoblasts express this receptor and, if so, whether this could account for the observed discrepancies between responses of osteoblasts to thrombin and to PAR-1-activating peptides. Reverse transcriptase-polymerase chain reaction (RT-PCR) and immunocytochemical studies demonstrated expression of PAR-2 by primary cultures of rat calvarial osteoblast-like cells. In immunohistochemical studies of embryonic mouse bones, osteoblasts showed positive staining for the presence of PAR-2. Activators of PAR-2 include trypsin, mast cell tryptase, gingipain-R, and synthetic peptides corresponding to the PAR-2 tethered ligand sequence. Treatment of primary rat osteoblast-like cells with rat PAR-2-activating peptide (SLIGRL), or SaOS-2 cells with human PAR-2-activating peptide (SLIGKV), caused a dose-dependent increase in [Ca2+]i. Trypsin or gingipain-R also induced an increase in intracellular calcium concentration, and caused reciprocal cross desensitization. Activators of PAR-2 caused a sharp peak in [Ca2+]i followed by a sustained plateau; [Ca2+]i returned to baseline levels upon treatment with ethylene-glycol tetraacetic acid (EGTA). Treatment of rat osteoblast-like cells in vitro with SLIGRL did not affect thymidine incorporation or endogenous alkaline phosphatase activity. The results presented here demonstrate that osteoblasts express PAR-2, and that such expression is able to account for the observed discrepancies between thrombin and PAR-1-activating peptides in their ability to evoke calcium entry, but not proliferative responses.