Structure-activity studies of the s-echistatin inhibition of bone resorption

Osteoidosis leads to altered differentiation and function of osteoclasts

In patients with osteomalacia, a defect in bone mineralization leads to changed characteristics of the bone surface. Considering that the properties of the surrounding matrix influence function and differentiation of cells, we aimed to investigate the effect of osteoidosis on differentiation and function of osteoclasts. Based on osteomalacic bone biopsies, a model for osteoidosis in vitro (OIV) was established. Peripheral blood mononuclear cells were differentiated to osteoclasts on mineralized surfaces (MS) as internal control and on OIV. We observed a significantly reduced number of osteoclasts and surface resorption on OIV. Atomic force microscopy revealed a significant effect of the altered degree of mineralization on surface mechanics and an unmasking of collagen fibres on the surface. Indeed, coating of MS with RGD peptides mimicked the resorption phenotype observed in OIV, suggesting that the altered differentiation of osteoclasts on OIV might be associated with an interaction of the cells with amino acid sequences of unmasked extracellular matrix proteins containing RGD sequences. Transcriptome analysis uncovered a strong significant up‐regulation of transmembrane glycoprotein TROP2 in osteoclastic cultures on OIV. TROP2 expression on OIV was also confirmed on the protein level and found on the bone surface of patients with osteomalacia. Taken together, our results show a direct influence of the mineralization state of the extracellular matrix surface on differentiation and function of osteoclasts on this surface which may be important for the pathophysiology of osteomalacia and other bone disorders with changed ratio of osteoid to bone.

64Cu-Labeled Phosphonate Cross-Bridged Chelator Conjugates of c(RGDyK) for PET/CT Imaging of Osteolytic Bone Metastases

OBJECTIVE

The goal of this research was to evaluate c(RGDyK) conjugated to phosphonate-based cross-bridged chelators using Cu-free click chemistry in the 4T1 mouse mammary tumor bone metastasis model in comparison with ⁶⁴Cu-CB-TE2A-c(RGDyK), which previously showed selective binding to integrin αvβ3 on osteoclasts.

EXPERIMENTAL

Two phosphonate-based cross-bridged chelators (CB-TE1A1P and CB-TE1K1P) were conjugated to c(RGDyK) through bio-orthogonal strain-promoted alkyne-azide cycloaddition. In vitro and in vivo evaluation of the ⁶⁴Cu-labeled TE1A1P-DBCO-c(RGDyK) (AP-c(RGDyK)), TE1K1P-PEG4-DBCO-c(RGDyK) (KP-c(RGDyK)), and CB-TE2A-c(RGDyK) were compared in the 4T1 mouse model of bone metastasis. The affinities of the unconjugated and chelator-c(RGDyK) analogs for αvβ3 integrin were determined using a competitive-binding assay. For in vivo evaluation, BALB/c mice were injected with 1 × 10⁵ 4T1/Luc cells in the left ventricle. Formation of metastases was monitored by bioluminescence imaging (BLI) followed by small-animal PET/CT 2 h postinjection of radiotracers.

RESULTS

The chelator-peptide conjugates showed similar affinity to integrin αvβ3, in the low nM range. PET imaging demonstrated a higher uptake in bones having metastases for all ⁶⁴Cu-labeled c(RGDyK) analogs compared with bones in nontumor-bearing mice. The correlation between uptake of ⁶⁴Cu-AP-c(RGDyK) and ⁶⁴Cu-KP-c(RGDyK) in bones with metastases based on PET/CT imaging, and osteoclast number based on histomorphometry, was improved over the previously investigated ⁶⁴Cu-CB-TE2A-c(RGDyK).

CONCLUSION

These data suggest that the phosphonate chelator conjugates of c(RDGyK) peptides are promising PET tracers suitable for imaging tumor-associated osteoclasts in bone metastases.

Effects of Ethyl Acetate Extract of Poncirus trifoliata Fruit for Glucocorticoid-Induced Osteoporosis

Poncirus trifoliata fruit (PTF) affects the digestive and cardiovascular systems, and kidney function. The authors studied the effects of ethyl acetate (EtOAc) extract of PTF on the activities of osteoblasts and in an animal model. The main compounds of the EtOAc extract, naringin and poncirin have been confi rmed by HPLC and NMR analysis. Effects of osteoblastic differentiation were mea-sured by alkaline phosphatase (ALP) activity, osteopontin (OPN) protein expression and osteoprotegerin (OPG) mRNA expression in MC3T3-E1 cells. Also, osteoclast differentiation was measured by multinucleated cells (MNCs) formation through tartrate resistance acid phosphatase (TRAP)-positive staining. Bone mineral density (BMD) was measured before and after treatment with EtOAc extract of PTF in prednisolone-induced osteoporotic mice. Dexamethasone (DEX) decreased OPN and OPG expression level in MC3T3-E1 cells and ALP activity was decreased by DEX dose-dependently. EtOAc extract of PTF recovered the levels of ALP activity, and the expression of OPN and OPG in MC3T3-E1 cells treated with DEX. In osteoclast differentiation, multinucleated TRAP-positive cell formation was significantly suppressed by the EtOAc extract of PTF. Total body BMD was restored by EtOAc extract of PTF in prednisolone-induced osteoporotic mice. In conclusion, EtOAc extract of PTF recovered DEX-mediated deteriorations in osteoblastic and osteoclastic functions, and increased BMD in glucocorticoid-induced osteoporosis.

Cell adhesion signaling regulates RANK expression in osteoclast precursors

Cells with monocyte/macrophage lineage expressing receptor activator of NF-κB (RANK) differentiate into osteoclasts following stimulation with the RANK ligand (RANKL). Cell adhesion signaling is also required for osteoclast differentiation from precursors. However, details of the mechanism by which cell adhesion signals induce osteoclast differentiation have not been fully elucidated. To investigate the participation of cell adhesion signaling in osteoclast differentiation, mouse bone marrow-derived macrophages (BMMs) were used as osteoclast precursors, and cultured on either plastic cell culture dishes (adherent condition) or the top surface of semisolid methylcellulose gel loaded in culture tubes (non-adherent condition). BMMs cultured under the adherent condition differentiated into osteoclasts in response to RANKL stimulation. However, under the non-adherent condition, the efficiency of osteoclast differentiation was markedly reduced even in the presence of RANKL. These BMMs retained macrophage characteristics including phagocytic function and gene expression profile. Lipopolysaccharide (LPS) and tumor necrosis factor –αTNF-α activated the NF-κB-mediated signaling pathways under both the adherent and non-adherent conditions, while RANKL activated the pathways only under the adherent condition. BMMs highly expressed RANK mRNA and protein under the adherent condition as compared to the non-adherent condition. Also, BMMs transferred from the adherent to non-adherent condition showed downregulated RANK expression within 24 hours. In contrast, transferring those from the non-adherent to adherent condition significantly increased the level of RANK expression. Moreover, interruption of cell adhesion signaling by echistatin, an RGD-containing disintegrin, decreased RANK expression in BMMs, while forced expression of either RANK or TNFR-associated factor 6 (TRAF6) in BMMs induced their differentiation into osteoclasts even under the non-adherent condition. These results suggest that cell adhesion signaling regulates RANK expression in osteoclast precursors.

Mechano-growth factor E peptide inhibits the differentiation and mineralization of osteoblasts

OBJECTIVE

To investigate the effects of mechano-growth factor E (MGF-E) peptide derived from an IGF-1 isoform on the differentiation and mineralization of osteoblasts.

METHODS

MGF-E peptide corresponding to the carboxy terminal 24 amino acid peptide of human MGF was synthesized. MGF-E (1 nM) peptide was then used to treat the pre-osteoblast line MC3T3-E1. At predetermined times, alkaline phosphatase (ALP) activity was quantified using an enzyme activity assay kit. The expression levels of collagen I (Col I) and osteopontin (OPN), and core binding factor 1 (Cbfα-1) were detected by reverse transcription polymerase chain reaction and Western blot analysis. The effect of MGF-E on mineralization was determined by Alizarin Red staining and calcium concentration analysis. The kinase inhibitor PD98059 was used to investigate Erk pathway involvement in the MGF-E role.

RESULTS

In the MGF-E-treated osteoblasts, ALP activity decreased with increased Erk activation. The transcription and translation of Col I were inhibited, but those of OPN were enhanced. PD98059 abolished the inhibitory effect and increased the expression of Col I, but decreased that of OPN. Treatment with MGF-E alone up-regulated the mRNA and total protein levels of Cbfα-1, but decreased the fraction of activated Cbfα-1 in the nucleus. Mineralization was delayed by MGF-E, as shown by the bone nodule staining and calcium concentration analysis. These delayed actions were weakened after treatment with PD98059.

CONCLUSIONS

MGF-E could inhibit osteoblast differentiation and mineralization. The possible mechanisms are increased Erk activity and decreased Cbfα-1 nuclear translocation.

3BP2-deficient mice are osteoporotic with impaired osteoblast and osteoclast functions

A fine balance between bone resorption by osteoclasts and bone formation by osteoblasts maintains bone homeostasis. In patients with cherubism, gain-of-function mutations in 3BP2, which is encoded by SH3-domain binding protein 2 (SH3BP2), cause cystic lesions with activated osteoclasts that lead to craniofacial abnormalities. However, little is known about the function of wild-type 3BP2 in regulating bone homeostasis. Here we have shown that 3BP2 is required for the normal function of both osteoblasts and osteoclasts. Initial analysis showed that Sh3bp2-/-mice developed osteoporosis as a result of reduced bone formation despite the fact that bone resorption was impaired. We demonstrated using reciprocal bone marrow chimeras, a cell-intrinsic defect of the osteoblast and osteoclast compartments in vivo. Further, Sh3bp2-/- osteoblasts failed to mature and form mineralized nodules in vitro, while Sh3bp2-/- osteoclasts spread poorly and were unable to effectively degrade dentine matrix in vitro. Finally, we showed that 3BP2 was required for Abl activation in osteoblasts and Src activation in osteoclasts, and demonstrated that the in vitro defect of each cell type was restored by the respective expression of activated forms of these kinases. These findings reveal an unanticipated role for the 3BP2 adapter protein in osteoblast function and in coordinating bone homeostatic signals in both osteoclast and osteoblast lineages.

The effect of ligand type and density on osteoblast adhesion, proliferation, and matrix mineralization

Polystyrene surfaces grafted with a nonfouling interfacial interpenetrating polymer network (IPN) of poly(acrylamide-co-ethylene glycol/acrylic acid) [p(AAm-co-EG/AAc)] were modified with several peptide ligands adapted from bone sialoprotein (BSP). IPNs were modified with both single ligands and ligand blends to study the correlation between a simple metric, ligand-receptor adhesion strength, and the extent of matrix mineralization for osteoblast like cells (rat calvarial osteoblasts). The ligands studied included RGD cell-binding [CGGNGEPRGDTYRAY (l-RGD), CGGEPRGDTYRA (s2-RGD), CGPRGDTYG (lc-RGD), cyclic(CGPRGDTYG) (c-RGD), and CGGPRGDT (s-RGD)], heparin binding (CGGFHRRIKA), and collagen binding (CGGDGEAG) peptides, with the appropriate controls. Adhesion strength scaled with ligand density (1-20 pmol/cm(2)) and was dependent on ligand type with the following trend: l-RGD > s2-RGD approximately c-RGD >> s-RGD approximately lc-RGD >>> FHRRIKA approximately DGEA. Independent of ligand density, % matrix mineralization varied with ligand type resulting in the following trend: lc-RGD > s2-RGD > l-RGD approximately c-RGD >> s-RGD >>> FHRRIKA. The Tyr (Y) residue immediately following the RGD cell-binding domain proved to be critical for stable cell proliferation and mineralization, since removal of this residue resulted in erratic cell attachment and mineralization behavior. The minimum BSP sequence necessary for strong adhesion and extensive mineralization was CGGEPRGDTYRA; the minimal sequence suitable for extensive mineralization but lacking strong adhesion was CGPRGDTYG. The cyclic peptide (c-RGD) had much greater adhesion strength compared to its linear counterpart (lc-RGD). The calculated characteristic adhesion strength (F(70)) obtained using a centrifuge adhesion assay proved to be a poor metric for predicting % mineralized area; however, in general, surfaces possessing a F(70) > 100g promoted extensive matrix mineralization. Percent mineralization and number of mineralized nodules scaled with number of cells seeded suggesting a critical dependence on the initial number of osteoprogenitors in culture. This study demonstrates matrix mineralization dependence on ligand type, ligand density, and adhesion strength. The high-throughput character of these surfaces allowed efficient investigation of multiple ligands at multiple densities providing an excellent tool for studying ligand-receptor interactions under normal cell culture conditions with serum present.

Inhibition of orthodontically induced root resorption with echistatin, an RGD-containing peptide

INTRODUCTION

Induced dental root resorption is a common side effect of orthodontic treatment. It is an unpredictable phenomenon, and its etiology is unknown. Odontoclasts responsible for the resorption of the dental tissues--ie, cementum and dentin--share many cytochemical and morphological characteristics with osteoclasts, which are responsible for bone resorption. The aim of this study was to explore cellular mechanisms that decrease induced root resorption in orthodontically treated teeth.

METHODS

The effects of targeting the alphavbeta3 integrin receptor, expressed by odontoclasts, on induced root resorption surface areas and the number of root resorption lacunae were investigated by using an RGD-containing peptide, echistatin. The effect of echistatin on the number of clast cells in the periodontium was also examined. Tooth movement was achieved in 14 Sprague-Dawley rats by placing elastic bands between the right maxillary first and second molars for 24 hours. The animals were equally divided into 2 groups; the experimental animals received echistatin intravenously for 8 hours (0.8 microg/kg/min), and the controls received sterile water. The specimens obtained were processed for light microscopy. The surface area and the number of root resorption lacunae were measured histomorphometrically by using digital photomicrographs. Echistatin labeled with a fluorescent marker was used to confirm its presence in clast cells with fluorescent microscopy. Cytochemically, tartrate-resistant acid phosphatase was used to quantify mature and committed clast cells. Echistatin was localized in targeted cells in the periodontium.

RESULTS

Echistatin significantly decreased root resorption surface areas (P < .01) and reduced the number of root resorption lacunae (P < .01). There was no statistically significant difference in clast cell numbers.

CONCLUSIONS

Targeting alphavbeta3 integrin receptor expressed by odontoclasts can be effective in reducing root resorption during tooth movement. Further studies are needed to elucidate the mechanism of this inhibition.

Osteopontin is a negative regulator of proliferation and differentiation in MC3T3-E1 pre-osteoblastic cells

Osteopontin (OPN) is an important mediator of bone remodeling. However, the role of OPN in the process of bone formation is not fully understood. In previous studies, we have shown that MC3T3-E1 pre-osteoblastic cells at higher passage number exhibited weakened osteogenic capacity and elevated OPN mRNA expression. In this work, we investigated the role of OPN on proliferation and differentiation of low-passage MC3T3-E1 cells by studying stable cell lines overexpressing either OPN mRNA or its antisense RNA. Overexpression was verified by both Northern and Western blot analyses. Overexpression of OPN markedly inhibited proliferation as determined by daily cell counts, while overexpression of antisense RNA stimulated cellular proliferation. We also examined the effect of OPN level on BMP-2-induced alkaline phosphatase activity. Overexpression of OPN inhibited BMP-2 responsiveness while overexpression of antisense RNA enhanced the effect of BMP-2 on alkaline phosphatase activity. Increased OPN expression also caused decreases in expression of osteocalcin and bone sialoproteins while a reduction of OPN level caused the opposite. Furthermore, endogenous OPN expression in response to BMP-2 exhibited a biphasic pattern, that is, it was initially inhibited and then enhanced by the treatment of BMP-2, indicating that OPN might function as a negative feedback regulator for osteoblastic differentiation. Finally, overexpression of OPN inhibited mineral deposition. In contrast, overexpression of antisense RNA enhanced mineral deposition. These results indicate that OPN is a negative regulator of proliferation and differentiation in MC3T3-E1 cells.

Immunohistochemical localization of αvβ3 integrin receptor during experimental tooth movement

During orthodontic treatment, multinucleated clast cells carry out the resorption of mineralized tissues. Adhesion of clast cells to the mineralized tissues is mediated by transmembrane cell-surface glycoproteins called integrins, specifically by the alphavbeta3 integrin, which plays an important role in the process of bone resorption. The role of the alphavbeta3 integrin in bone resorption leading to osteoporosis has been demonstrated, but its role in alveolar bone and root resorption during orthodontic tooth movement is unknown. This study examined the expression of the alphavbeta3 integrin during experimental tooth movement. Tooth movement was achieved in 16 male Sprague-Dawley rats (each weighing 120-200 g) with elastic bands between their maxillary first and second molars. The molar-bearing segments were dissected and processed for histologic and immunohistochemical examination. The expression of alphavbeta3 integrin was examined with 2 primary antibodies: a polyclonal anti-alphav integrin subunit antibody and a polyclonal anti-beta3 integrin subunit antibody. Negative controls were similarly processed but without incubation with primary antibodies. The alphavbeta3 integrin was expressed both by osteoclasts associated with alveolar bone resorption and by odontoclasts associated with root resorption during experimental tooth movement. Furthermore, the beta3 integrin subunit was expressed by the epithelial rests of Malassez in the periodontal ligament. Negative controls did not show immunolabeling. The alphavbeta3 integrin adhesion receptor is expressed during experimental tooth movement and might be involved in the process of mineralized tissue resorption and the functions of the epithelial rests of Malassez.

Osteoprotegerin is produced when prostaglandin synthesis is inhibited causing osteoclasts to detach from the surface of mouse parietal bone and attach to the endocranial membrane

Osteoclast differentiation and activation is controlled, at least in part, by the counterbalancing influences of osteoprotegerin ligand (OPGL) and osteoprotegerin (OPG). Nonsteroidal anti-inflammatory drugs have been shown to inhibit bone loss in vivo and bone resorption in vitro, and this is associated with a loss of osteoclasts from the bone surface. We test the hypothesis that OPG mediates the inhibition of osteoclast activity that occurs with indomethacin in the mouse calvaria. Recombinant human OPG, like indomethacin, was found to cause osteoclasts to detach from the bone surface and attach to the adjacent endocranial membrane (periosteum). Recombinant human OPG also inhibited the stimulatory effect of prostaglandin E2 (PGE2), parathyroid hormone (PTH), and 1,25-dihydroxyvitamin D3 (1,25D3) on osteoclast adhesion to bone after an incubation with indomethacin. A function-blocking antibody to OPG and soluble human OPGL both inhibited the effect of indomethacin, leaving active osteoclasts on the bone. OPG activity was detected in the culture medium from indomethacin-treated bones and PTH, PGE2, 1,25D3, and dexamethasone all inhibited the production of OPG activity. We conclude that, in the absence of specific stimulators of bone resorption, OPG is produced by the mouse calvaria in vitro, which inhibits bone resorption by causing osteoclasts to detach from the bone surface.

Osteoprotegerin ligand regulates osteoclast adherence to the bone surface in mouse calvaria

The stimulators of bone resorption, prostaglandin E(2) (PGE(2)) and 1,25-dihydroxyvitamin D(3) (1,25D(3)), act through osteoblast-like cells to activate osteoclasts. One candidate for the intermediary produced by osteoblasts that subsequently stimulates the osteoclast is osteoprotegerin ligand (OPGL). OPGL has been shown to stimulate osteoclast differentiation and activation. The aim of the work reported here was to determine if soluble recombinant extracellular domain of human OPGL would bring about the change in osteoclast adhesion from the periosteum of mouse calvaria to the adjacent bone surface that occurs with the above-mentioned stimulators of resorption. This change in adherence or translocation of osteoclasts onto the bone surface required the expression and functioning of the integrin subunit, beta 3. We show that this soluble OPGL, like PGE(2) and 1,25D(3), stimulated the release of osteoclasts from the periosteum and their adherence to the bone surface accompanied by an increase in staining for immunolocalized integrin subunit beta 3. Recombinant human osteoprotegerin (OPG), which binds strongly to OPGL, inhibited this translocation of osteoclasts that occurred with PGE(2) and 1,25D(3), leaving integrin beta-3-negative osteoclasts on the periosteum. PGE(2) and 1,25D(3) increased the expression of messenger RNA for OPGL compared with indomethacin-treated controls after 6 h exposure. Evidence is presented that the change in the adhesion of osteoclasts from the periosteum to the bone surface, resulting in osteoclast activation, is mediated by OPGL.

Activation of the cyclic AMP pathway in cells adhering to biomaterials: regulation by vitronectin- and fibronectin-integrin binding

Our previous studies have shown that cells adhering to biomaterials in serum-free conditions increase their content of cyclic AMP (cAMP) and become aggregated. In cells on an acrylonitrile membrane (AN69), these biochemical and morphological changes are prevented by adding 10% foetal calf serum (FCS) to the medium; cells on the cellulose membrane Cuprophan (CU) remain unaffected. The present study examines the roles of vitronectin (VN)- and/or fibronectin (FN)-integrin binding in this inhibition. Competitively blocking VN- and FN-receptors with echistatin increased intracellular cAMP significantly and caused cells on AN69 to aggregate, but did not modify cAMP-dependent cell aggregation on CU. VN or FN adsorbed onto CU also inhibited cAMP production by attached cells and prevented their aggregation, whereas adsorbed BSA had no effect. Therefore, the binding of VN or FN to cell-surface integrins seems to limit the activation of the cAMP pathway initiated by the substratum itself.

Role of alpha(v)beta(3) integrin in osteoclast migration and formation of the sealing zone

The alpha(v)beta(3) integrin is abundantly expressed in osteoclasts and has been implicated in the regulation of osteoclast function, especially in cell attachment. However, in vivo studies have shown that echistatin, an RGD-containing disintegrin which binds to alpha(v)beta(3), inhibits bone resorption without changing the number of osteoclasts on the bone surface, suggesting inhibition of osteoclast activity. The objective of this study was to examine how occupancy of alpha(v)beta(3) integrins inhibits osteoclast function, using primary rat osteoclasts and murine pre-fusion osteoclast-like cells formed in a co-culture system. We show that: (1) echistatin inhibits bone resorption in vitro at lower concentrations (IC(50)= 0.1 nM) than those required to detach osteoclasts from bone (IC(50) approximately 1 microM); (2) echistatin (IC(50)= 0.1 nM) inhibits M-CSF-induced migration and cell spreading of osteoclasts; (3) alpha(v)beta(3) integrins are localized in podosomes at the leading edge of migrating osteoclasts, whereas, with echistatin treatment (0.1 nM), alpha(v)beta(3) disperses randomly throughout the adhesion surface; and (4) when bone resorption is fully inhibited with echistatin, there is visible disruption of the sealing zone (IC(50)= 13 nM), and alpha(v)beta(3) visualized with confocal microscopy re-distributes from the basolateral membranes to intracellular vesicular structures. Taken together, these findings suggest that alpha(v)beta(3) integrin plays a role in the regulation of two processes required for effective osteoclastic bone resorption: cell migration (IC(50)= 0.1 nM) and maintenance of the sealing zone (IC(50) approximately 10 nM).

Beta1 integrin antisense oligodeoxynucleotides: utility in controlling osteoclast function

The involvement of beta1 integrins in osteoclast function has been investigated by utilising an antisense oligodeoxynucleotide (ODN) approach. 18-mer antisense and control phosphorothioate ODNs were made to a conserved internal region of beta1 integrin sequence (nucleotide positions 1634-1651 of the human beta1 fibronectin receptor). These were tested on rabbit osteoclasts for anti-adhesive and resorptive effects mediated by alphaVbeta3 and alpha2beta1, the major integrins of osteoclasts. Antisense, but not control, beta1 ODNs inhibited osteoclast adhesion to collagen-coated glass (by up to 70%), but not to glass coated with vitronectin, fibronectin or fibrinogen. Adhesion to dentine and subsequent resorption were also inhibited (up to 60%) in a sequence-specific manner. The mechanism of action was verified using both a melanoma cell line, DX3, which expresses multiple integrins at high level including alphaVbeta3 and alpha2beta1, and in a rabbit osteoclast marrow culture (BMC) system. Exposure of DX3 cells to antisense ODN for up to 48 hours reduced adhesion to FCS- and collagen-coated glass, and concomitantly inhibited beta1 protein expression assessed by FACS and Western blot analysis; expression of other integrin subunits, alphaV and beta3, was unaffected. Similarly, the beta1 protein levels in the BMC were reduced by > 75% without any effect on actin expression. These data reveal the utility of antisense ODNs in exploring osteoclast biology and further define the functional role of osteoclastic beta1 integrin(s).

Role of polypeptides in the treatment and diagnosis of osteoporosis

Osteoporosis is a common disorder characterized by reduced bone mineral density, deterioration of the microarchitecture of bone tissue and increased risk of fracture. The aim of treatment of osteoporosis is to maintain and, ideally, to restore bone strength safely. In recent years the role of polypeptide growth factors in bone metabolism has begun to appear. It has been proposed that alterations in the expression or production of growth factor can modulate the proliferation and activity of bone forming cells. In this direction, the role of structurally diverse peptides for the management and diagnosis of osteoporosis has attracted the attention of many investigators. This paper reviews numerous findings concerning the use of polypeptides, hormones, and growth factors, for the management of osteoporosis. Many of the compounds mentioned here are experimental prototypes of new therapeutic classes. Though it is unlikely that some of the compounds may ever be used clinically, development of safe and efficacious agents in each class will define the future course of therapy for osteoporosis.

Snake venoms and the hemostatic system

Snake venoms are complex mixtures containing many different biologically active proteins and peptides. A number of these proteins interact with components of the human hemostatic system. This review is focused on those venom constituents which affect the blood coagulation pathway, endothelial cells, and platelets. Only highly purified and well characterized snake venom proteins will be discussed in this review. Hemostatically active components are distributed widely in the venom of many different snake species, particularly from pit viper, viper and elapid venoms. The venom components can be grouped into a number of different categories depending on their hemostatic action. The following groups are discussed in this review: (i) enzymes that clot fibrinogen; (ii) enzymes that degrade fibrin(ogen); (iii) plasminogen activators; (iv) prothrombin activators; (v) factor V activators; (vi) factor X activators; (vii) anticoagulant activities including inhibitors of prothrombinase complex formation, inhibitors of thrombin, phospholipases, and protein C activators; (viii) enzymes with hemorrhagic activity; (ix) enzymes that degrade plasma serine proteinase inhibitors; (x) platelet aggregation inducers including direct acting enzymes, direct acting non-enzymatic components, and agents that require a cofactor; (xi) platelet aggregation inhibitors including: alpha-fibrinogenases, 5'-nucleotidases, phospholipases, and disintegrins. Although many snake venoms contain a number of hemostatically active components, it is safe to say that no single venom contains all the hemostatically active components described here. Several venom enzymes have been used clinically as anticoagulants and other venom components are being used in pre-clinical research to examine their possible therapeutic potential. The disintegrins are an interesting group of peptides that contain a cell adhesion recognition motif, Arg-Gly-Asp (RGD), in the carboxy-terminal half of their amino acid sequence. These agents act as fibrinogen receptor (integrin GPIIb/IIIa) antagonists. Since this integrin is believed to serve as the final common pathway leading to the formation of platelet-platelet bridges and platelet aggregation, blockage of this integrin leads to inhibition of platelet aggregation regardless of the stimulating agent. Clinical trials suggest that platelet GPIIb/IIIa blockade is an effective therapy for the thrombotic events and restenosis frequently accompanying cardiovascular and cerebrovascular disease. Therefore, because of their clinical poten tial, a large number of disintegrins have been isolated and characterized.

Echistatin inhibits the migration of murine prefusion osteoclasts and the formation of multinucleated osteoclast-like cells

The vitronectin receptor alpha(v)beta3 is highly expressed in osteoclasts and was shown to play a critical role in osteoclast function in vivo. The objective of this study was to examine the role of alpha(v)beta3 integrin in osteoclast formation in vitro using the inhibitory disintegrin echistatin, an RGD-containing snake venom. We documented by immunocytochemistry and Northern blot analysis that during murine osteoclast-like cell (OCL) formation in a coculture of mouse osteoblastic MB1.8 cells and bone marrow cells there is increased expression of the alpha(v) and beta3 integrin subunits. Echistatin binds preferentially to the membrane fraction of isolated enriched OCLs (IC50 = 0.6 nM), and this binding is inhibited by vitronectin receptor-blocking polyclonal antibodies. Additionally, cross-linking of radiolabeled echistatin to OCLs, followed by immunoprecipitation with antibodies to vitronectin or fibronectin receptors, shows that alpha(v)beta3 integrin is the predominant receptor for echistatin in this system. In this coculture, echistatin completely inhibits the formation of multinucleated OCLs, but not that of mononuclear prefusion OCLs (pOCs). This inhibition is RGD and dose dependent (IC50 = 0.7 nM). We tested the hypothesis that inhibition of OCL formation may be due to interference with pOC migration and found that echistatin inhibited macrophage colony-stimulating factor-induced migration and fusion of pOCs (IC50 = 1 and 0.6 nM, respectively). Echistatin inhibition of pOCs migration and fusion is also RGD dependent. These results suggest that the integrin alpha(v)beta3 plays a role in pOC migration, which can explain the inhibitory effect of echistatin on multinucleated osteoclast formation in vitro.

Integrin subunit beta3 plays a crucial role in the movement of osteoclasts from the periosteum to the bone surface

We have shown that, when mouse parietal bones were incubated in culture medium containing indomethacin, the number of tartrate-resistant acid phosphatase-positive osteoclasts (TRAP + OCs) on the bone surface was drastically reduced (down-regulation), and the number on the periosteal membrane adjacent to the resorbing surface was increased. Subsequent incubation of bones with prostaglandin E2 (PGE2) rapidly reversed these changes (up-regulation). In the work reported here, the osteoclast-associated integrin subunit beta3 was stained by immunohistochemistry. The beta3-positive osteoclast (beta3 + OC) population on freshly isolated bone was comprised of about 67% TRAP + OCs and 33% TRAP OCs. Like TRAP + OCs, beta3 + OCs were reduced in number on the surface of bones incubated with indomethacin, but, in contrast to the TRAP + OCs, beta3 + OCs were not seen on the periosteal membrane. Following up-regulation of TRAP + OCs with PGE2, large numbers of beta3 + OCs appeared on the bone surface and, again, were not seen on the periosteal membrane. Echistatin, a peptide that binds to the alphavbeta3 integrin on osteoclasts, was found to inhibit the up-regulation of TRAP + OCs in a dose-dependent manner but had no effect on the down-regulation of TRAP + OCs. Similarly, echistatin inhibited the upregulation of beta3 + OCs on the bone surface, and, under these conditions, beta3 + OCs were observed on the periosteal membrane. The addition of anti-beta3 antibody also inhibited the up-regulation of TRAP + OCs in response to PGE2. The association of beta3 protein expression with the up-regulated osteoclast and the inhibition of up-regulation by echistatin and by anti-beta3 antibody provide strong evidence that beta3 plays an essential role in the movement of osteoclasts from the membrane to the bone.

Contortrostatin, a homodimeric snake venom disintegrin, is a potent inhibitor of osteoclast attachment

Disintegrins are small disulfide-rich proteins containing an Arg-Gly-Asp (RGD) sequence near their carboxyl terminus. These polypeptides inhibit binding of adhesion molecules to their receptors (integrins) on the surface of cells. Osteoclasts express integrins, heterodimeric cell surface adhesion receptors, that have been shown to be involved in interactions with the extracellular matrix (ECM), including attachment to bone and bone resorption. It has recently been shown that disintegrins effectively inhibit attachment of osteoclasts to components of the ECM and also disrupt osteoclast-mediated bone resorption. Here we characterize the effects of contortrostatin (CTS), a novel homodimeric snake venom disintegrin, on osteoclast attachment. Plastic dishes coated with CTS were able to support osteoclast attachment with a high affinity (EC50,CTS = 86 +/- 6.7 nM) similar to that of vitronectin (VTN; EC50,VTN = 80 +/- 20 nM). Further, CTS was observed to inhibit completely osteoclast attachment to fetal bovine serum (FBS; IC50,FBS = 0.36 +/- 0.04 nM) and VTN (IC50,VTN = 0.85 +/- 0.13 nM). We used monoclonal antibodies directed against the beta1 (monoclonal antibody [MAb] CD29) and beta3 (MAb F11) integrin subunits to explore the mechanism of osteoclast attachment to immobilized CTS. Only MAb F11 inhibited attachment to immobilized CTS (IC50 = 0.41 +/- 0.12 microg/ml), suggesting that binding to CTS is mediated in part by a beta3 integrin, presumably the alpha(v)beta3 VTN receptor. In further support of an integrin-mediated mechanism, binding of osteoclasts to CTS is inhibited by the RGD peptide, GRGDSP. CTS was also more potent (IC50,FBS = 0.36 +/- 0.04 nM) at inhibiting osteoclast attachment to FBS-coated wells than the monomeric snake venom disintegrin echistatin (IC50,FBS = 8.9 +/- 1.5 nM) or VTN (IC50FBS = 97.5 +/- 25.5 nM). Taken together, these data suggest that the snake venom disintegrin CTS is a potent inhibitor of beta3 integrin-mediated osteoclast attachment, presumably involving the VTN receptor (an alpha[v]beta3 integrin). Further studies of the mechanism of CTS-osteoclast interactions may aid in the design of peptide mimetics to act as antiresorptive agents for the treatment of osteoporosis and other skeletal pathology.

Histomorphometric evidence for echistatin inhibition of bone resorption in mice with secondary hyperparathyroidism

Echistatin, an RGD-containing peptide, was shown to inhibit the acute calcemic response to exogenous PTH or PTH-related protein (PTH-rP) in thyroparathyroidectomized rats, suggesting that echistatin inhibits bone resorption. In this study: 1) we present histological evidence for echistatin inhibition of bone resorption in mice with secondary hyperparathyroidism, and show that 2) echistatin binds to osteoclasts in vivo, 3) increases osteoclast number, and 4) does not detectably alter osteoclast morphology. Infusion of echistatin (30 microg/kg x min) for 3 days prevented the 2.6-fold increase in tibial cancellous bone turnover and the 36% loss in bone volume, produced by a low calcium diet. At the light microscopy level, echistatin immunolocalized to osteoclasts and megakaryocytes. Echistatin treatment increased osteoclast-covered bone surface by about 50%. At the ultrastructural level, these osteoclasts appeared normal, and the fraction of cells containing ruffled borders and clear zones was similar to controls. Echistatin was found on the basolateral membrane and in intracellular vesicles of actively resorbing osteoclasts. Weak labeling was found in the ruffled border, and no immunoreactivity was detected at the clear zone/bone surface interface. These findings provide histological evidence for echistatin binding to osteoclasts and for inhibition of bone resorption in vivo, through reduced osteoclast efficacy, without apparent changes in osteoclast morphology.

Cloning and characterization of a novel integrin beta3 subunit

We have identified a novel integrin beta3 subunit, termed beta3C, from a human osteoclast cDNA library. The COOH-terminal sequence and 3'-untranslated region of the beta3C subunit differs from the previously reported beta3A (platelet) and beta3B (placenta) sequences, while the regions coding for the transmembrane and extracellular domains are identical. The beta3C cytoplasmic domain contains 37 amino acids, the last 17 of which are encoded by a novel exon located about 6 kilobase pairs downstream of exon 14 of the beta3A gene. HEK 293 cells were stably co-transfected with alphaV and either beta3C (HEKbeta3C) or beta3A (HEKbeta3A). The viability of HEKbeta3C cells was lower than that of HEKbeta3A cells, and HEKbeta3C cells in culture grew as clusters rather than as a monolayer. The novel cytoplasmic domain did not affect receptor binding affinity; both alphaVbeta3A and alphaVbeta3C isoforms exhibited high affinity binding to 125I-echistatin and cyclic and linear RGD peptides. However, in contrast to HEKbeta3A, HEKbeta3C cells failed to adhere to osteopontin, an alphaVbeta3 matrix protein. The data provide further support for the key role of the cytoplasmic domain of the beta3 integrin in cell adhesion and suggest a potential role for the beta3C integrin subunit in modulating cell-matrix interactions.

Isolation and amino acid sequence of flavostatin, a novel disintegrin from the venom of Trimeresurus flavoviridis

Flavostatin, a novel disintegrin purified from the venom of Trimeresurus flavoviridis, consists of 68 amino acids, including an Arg-Gly-Asp sequence and 12 Cys residues at positions highly conserved among disintegrins. The N-terminal sequence of flavostatin was identical to those of triflavin and flavoridin, previously reported disintegrins from the Trimeresurus flavoviridis venom. Differences among the C-terminal sequences of these disintegrins are considered to affect their biological potencies. Isolated flavostatin inhibited ADP collage, and thrombin receptor agonist peptide-induced platelet aggregation in human platelet-rich plasma with an IC50 range of 59 to 98 nM. Contrary to expectations, these values were similar to those for triflavin.

Human osteoclasts, not osteoblasts, deposit osteopontin onto resorption surfaces: an in vitro and ex vivo study of remodeling bone

Osteopontin is a phosphorylated glycoprotein believed to be secreted by osteoblasts and deposited into the bone matrix to facilitate osteoclasts adhesion or to initiate osteoid mineralization. Previously we have presented contradictory evidence that osteoclasts express osteopontin mRNA in human remodeling bone. The aim of this study was to ascertain whether osteoclasts synthesize and deposit osteopontin in resorption lucunae. We characterized expression of osteopontin mRNA and protein expression in both intramembranous and endochondral ossification, as well as remodeling bone, in the human osteophyte. Osteopontin mRNA was expressed in osteoclast with tartrate-resistant acid phosphatase (TRAP) positivity within resorption lacunae. The osteoclasts and immediate resorption surfaces also expressed osteopontin. However, osteopontin mRNA and protein were weak (transient) or undetectable in osteoblasts at adjacent bone formation sites; no osteopontin expression was observed in the osteoid, although occasional reactivity was observed in osteocytes and the mineral-osteoid interface. In contrast, osteopontin was highly expressed in the osteoblasts and matrix of woven bone during intramembranous and endochondral ossification. The matrix expression correlated with mineralization; however, in some instances osteopontin deposition was observed prior to mineralization. Similarly, osteopontin expression was evident in cartilage matrix, solely at foci of mineralization. Chondroclasts expressed osteopontin mRNA and protein: the surfaces of resorbed calcified cartilage also expressed osteopontin. Abnormal, unmineralized matrices apparently lacked deposited osteopontin, but were nevertheless resorbed by osteoclasts; the osteoclasts and resorbed surfaces expressed no osteopontin protein. That osteoclasts are responsible for the deposition of osteopontin was confirmed in vitro, whereby resorption pits in whale dentine and bovine bone slices, produced by isolated human osteoclasts, contained deposited osteopontin. Osteopontin may facilitate the adhesion (or detachment) of the osteoclast to the bone surface. Alternatively, the possibility that osteopontin may act as a postresorptive signal to recruit osteoblasts, or to polarize and direct the mineralization of the formed osteoid, is discussed.

Differentiation dependent expression of tensin and cortactin in chicken osteoclasts

The expression and localization of tensin and cortactin were examined in osteoclast precursors in comparison with isolated osteoclasts on various substrates. Initially, the ability of hen monocytes to differentiate into osteoclasts was evaluated on plastic or glass, and compared to differentiation on bone. Specifically, monocytes were isolated from the medullary bones of egg-laying hens maintained on a Ca-deficient diet. Differentiation was monitored morphologically and by quantitation of the ability to form Howship's lacunae in bone slices or resorb radiolabeled bone particles of 20-53 microns diameter. These cells differentiated into tartrate resistant acid phosphatase (TRAP)-positive, bone resorbing, multinucleated syncytia in the presence of cytosine-1-beta-D-arabinofuranoside in a time dependent manner (day 1-6). Differentiation into osteoclast-like cells was similar whether cultured on plastic, on glass, or on bone. When compared to GAP-DH control levels, tensin and cortactin mRNA levels increased by 7- and 10-fold, respectively, by day 6. Tensin and cortactin protein levels also increased by 6- and 15-fold, respectively, by day 6. Immunofluorescence of differentiating precursors showed that tensin localized between regions of cell to cell contact and colocalized with vinculin in podosomes of osteoclast-like cells and of real osteoclasts. Cortactin immunofluorescence was not detectable in monocytes but localized inside tensin/vinculin podosome structures after fusion into osteoclast-like cells and in freshly isolated osteoclasts. Both tensin and cortactin were associated with attachment complexes used by osteoclast-like cells and osteoclasts to resorb bone. Specifically, punctate cortactin staining was observed inside tensin staining which formed a double ring structure at the membrane/bone interface of resorbing osteoclasts. These data showed that tensin and cortactin can be used as osteoclast differentiation markers, that participate in attachment complexes used to resorb bone, and that tensin may participate in the fusion process of osteoclast precursors.