A peptidomimetic antagonist of the alpha(v)beta3 integrin inhibits bone resorption in vitro and prevents osteoporosis in vivo

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Roles of focal adhesion proteins in skeleton and diseases

The skeletal system, which contains bones, joints, tendons, ligaments and other elements, plays a wide variety of roles in body shaping, support and movement, protection of internal organs, production of blood cells and regulation of calcium and phosphate metabolism. The prevalence of skeletal diseases and disorders, such as osteoporosis and bone fracture, osteoarthritis, rheumatoid arthritis, and intervertebral disc degeneration, increases with age, causing pain and loss of mobility and creating a huge social and economic burden globally. Focal adhesions (FAs) are macromolecular assemblies that are composed of the extracellular matrix (ECM), integrins, intracellular cytoskeleton and other proteins, including kindlin, talin, vinculin, paxillin, pinch, Src, focal adhesion kinase (FAK) and integrin-linked protein kinase (ILK) and other proteins. FA acts as a mechanical linkage connecting the ECM and cytoskeleton and plays a key role in mediating cell-environment communications and modulates important processes, such as cell attachment, spreading, migration, differentiation and mechanotransduction, in different cells in skeletal system by impacting distinct outside-in and inside-out signaling pathways. This review aims to integrate the up-to-date knowledge of the roles of FA proteins in the health and disease of skeletal system and focuses on the specific molecular mechanisms and underlying therapeutic targets for skeletal diseases.

Plasma Membrane Receptors Involved in the Binding and Response of Osteoclasts to Noncellular Components of the Bone

Osteoclasts differentiate from hematopoietic cells and resorb the bone in response to various signals, some of which are received directly from noncellular elements of the bone. In vitro, adherence to the bone triggers the reduction of cell–cell fusion events between osteoclasts and the activation of osteoclasts to form unusual dynamic cytoskeletal and membrane structures that are required for degrading the bone. Integrins on the surface of osteoclasts are known to receive regulatory signals from the bone matrix. Regulation of the availability of these signals is accomplished by enzymatic alterations of the bone matrix by protease activity and phosphorylation/dephosphorylation events. Other membrane receptors are present in osteoclasts and may interact with as yet unidentified signals in the bone. Bone mineral has been shown to have regulatory effects on osteoclasts, and osteoclast activity is also directly modulated by mechanical stress. As understanding of how osteoclasts and other bone cells interact with the bone has emerged, increasingly sophisticated efforts have been made to create bone biomimetics that reproduce both the structural properties of the bone and the bone’s ability to regulate osteoclasts and other bone cells. A more complete understanding of the interactions between osteoclasts and the bone may lead to new strategies for the treatment of bone diseases and the production of bone biomimetics to repair defects.

Mechanistic Insight Into the Roles of Integrins in Osteoarthritis

Osteoarthritis (OA), one of the most common degenerative diseases, is characterized by progressive degeneration of the articular cartilage and subchondral bone, as well as the synovium. Integrins, comprising a family of heterodimeric transmembrane proteins containing α subunit and β subunit, play essential roles in various physiological functions of cells, such as cell attachment, movement, growth, differentiation, and mechanical signal conduction. Previous studies have shown that integrin dysfunction is involved in OA pathogenesis. This review article focuses on the roles of integrins in OA, especially in OA cartilage, subchondral bone and the synovium. A clear understanding of these roles may influence the future development of treatments for OA.

Regulatory properties of vitronectin and its glycosylation in collagen fibril formation and collagen-degrading enzyme cathepsin K activity

Vitronectin (VN) is a glycoprotein found in extracellular matrix and blood. Collagen, a major extracellular matrix component in mammals, is degraded by cathepsin K (CatK), which is essential for bone resorption under acidic conditions. The relationship between VN and cathepsins has been unclear. We discovered that VN promoted collagen fibril formation and inhibited CatK activity, and observed its activation in vitro. VN accelerated collagen fibril formation at neutral pH. Collagen fibers formed with VN were in close contact with each other and appeared as scattered flat masses in scanning electron microscopy images. VN formed collagen fibers with high acid solubility and significantly inhibited CatK; the IC₅₀ was 8.1–16.6 nM and competitive, almost the same as those of human and porcine VNs. VN inhibited the autoprocessing of inactive pro-CatK from active CatK. DeN-glycosylation of VN attenuated the inhibitory effects of CatK and its autoprocessing by VN, but had little effect on acid solubilization of collagen and VN degradation via CatK. CatK inhibition is an attractive treatment approach for osteoporosis and osteoarthritis. These findings suggest that glycosylated VN is a potential biological candidate for CatK inhibition and may help to understand the molecular mechanisms of tissue re-modeling.

Securinine suppresses osteoclastogenesis and ameliorates inflammatory bone loss

Securinine (Sec) is a naturally derived compound separated from the roots of Securinega suffruticosa, which has long been used as a herbal medicine. Sec is widely known as a GABA receptor antagonist, it is also known as an innate immune cell agonist and has been reported to increase macrophage activity and promote monocyte maturation. On the basis of these studies, we investigated the effect of Sec on osteoclast differentiation and bone resorbing function. We have found that Sec inhibits RANKL-induced osteoclast differentiation, fusion, actin ring formation, and bone resorbing function by the inhibition of gene expression associated with each stage. Moreover, Sec significantly suppressed osteoclastogenesis by decreasing the phosphorylation of p38, Akt, JNK, IκB, and PLCγ2, in pathways involved in early osteoclastogenesis as well as through the subsequent suppression of c-Fos and NFATc1. Finally, Sec effectively protected bone loss induced by the excessive inflammatory responses and activity of osteoclasts in vivo by a micro-CT and histological analysis. In conclusion, our findings suggest that Sec may be a promising drug for bone metabolic diseases such as osteoporosis, which is associated with the excessive activity of osteoclasts.

Anemonin Attenuates RANKL-Induced Osteoclastogenesis and Ameliorates LPS-Induced Inflammatory Bone Loss in Mice via Modulation of NFATc1

Osteoporosis is a metabolic bone disease characterized by insufficient osteoblastic function and/or excessive osteoclastic activity. One promising strategy for treating osteoporosis is inhibiting excessive osteoclast resorbing activity. Previous studies have revealed that anemonin (ANE), isolated from various types of Chinese natural herbs, has anti-inflammatory and anti-oxidative properties. However, whether ANE regulates osteoclastogenesis is unknown. This study aimed to investigate the potential effect of ANE on osteoclastogenesis and inflammatory bone loss in mice. In in vitro studies, ANE suppressed RANKL-stimulated osteoclast differentiation and function by downregulating the expression of osteoclast master transcriptor NFATc1, as well as its upstream transcriptor c-Fos, by decreasing NF-κB and ERK1/2 signaling. Interestingly, ANE did not change the phosphorylation and degradation of IκB-α and activation of JNK and p38 MAPKs. However, ANE repressed the phosphorylation of MSK-1 which is the downstream target of ERK1/2 and p38 MAPK and can phosphorylate NF-κB p65 subunit. These results implicated that ANE might suppress NF-κB activity via modulation of ERK1/2 mediated NF-κB phosphorylation. In addition, ANE directly suppressed NFATc1 transcription by inhibiting Blimp-1 expression, and the subsequent enhancement of the expression of NFATc1 negative regulators, Bcl-6 and IRF-8. Moreover, in vivo studies were conducted using an LPS-induced inflammatory bone loss mice model. Micro-CT and histology analysis showed that ANE treatment significantly improved trabecular bone parameters and bone destruction. These data indicate that ANE can attenuate RANKL-induced osteoclastogenesis and ameliorate LPS-induced inflammatory bone loss in mice through modulation of NFATc1 via ERK1/2-mediated NF-κB phosphorylation and Blimp1 signal pathways. ANE may provide new treatment options for osteoclast-related diseases.

Novel Pure αVβ3 Integrin Antagonists That Do Not Induce Receptor Extension, Prime the Receptor, or Enhance Angiogenesis at Low Concentrations

The integrin αVβ3 receptor has been implicated in several important diseases, but no antagonists are approved for human therapy. One possible limitation of current small-molecule antagonists is their ability to induce a major conformational change in the receptor that induces it to adopt a high-affinity ligand-binding state. In response, we used structural inferences from a pure peptide antagonist to design the small-molecule pure antagonists TDI-4161 and TDI-3761. Both compounds inhibit αVβ3-mediated cell adhesion to αVβ3 ligands, but do not induce the conformational change as judged by antibody binding, electron microscopy, X-ray crystallography, and receptor priming studies. Both compounds demonstrated the favorable property of inhibiting bone resorption in vitro, supporting potential value in treating osteoporosis. Neither, however, had the unfavorable property of the αVβ3 antagonist cilengitide of paradoxically enhancing aortic sprout angiogenesis at concentrations below its IC₅₀, which correlates with cilengitide's enhancement of tumor growth in vivo.

Integrins in Osteocyte Biology and Mechanotransduction

PURPOSE OF REVIEW

Osteocytes are the main mechanosensitive cells in bone. Integrin-based adhesions have been shown to facilitate mechanotransduction, and therefore play an important role in load-induced bone formation. This review outlines the role of integrins in osteocyte function (cell adhesion, signalling, and mechanotransduction) and possible role in disease.

RECENT FINDINGS

Both β₁ and β₃ integrins subunits have been shown to be required for osteocyte mechanotransduction. Antagonism of these integrin subunits in osteocytes resulted in impaired responses to fluid shear stress. Various disease states (osteoporosis, osteoarthritis, bone metastases) have been shown to result in altered integrin expression and function. Osteocyte integrins are required for normal cell function, with dysregulation of integrins seen in disease. Understanding the mechanism of faulty integrins in disease may aid in the creation of novel therapeutic approaches.

Specialized Roles for Actin in Osteoclasts: Unanswered Questions and Therapeutic Opportunities

Osteoclasts are cells of the hematopoietic lineage that are specialized to resorb bone. In osteoclasts, the actin cytoskeleton engages in at least two unusual activities that are required for resorption. First, microfilaments form a dynamic and structurally elaborate actin ring. Second, microfilaments bind vacuolar H⁺-ATPase (V-ATPase) and are involved in forming the V-ATPase-rich ruffled plasma membrane. The current review examines these two specialized functions with emphasis on the identification of new therapeutic opportunities. The actin ring is composed of substructures called podosomes that are interwoven to form a cohesive superstructure. Studies examining the regulation of the formation of actin rings and its constituent proteins are reviewed. Areas where there are gaps in the knowledge are highlighted. Microfilaments directly interact with the V-ATPase through an actin binding site in the B2-subunit of V-ATPase. This binding interaction is required for ruffled membrane formation. Recent studies show that an inhibitor of the interaction blocks bone resorption in pre-clinical animal models, including a model of post-menopausal osteoporosis. Because the unusual actin-based resorption complex is unique to osteoclasts and essential for bone resorption, it is likely that deeper understanding of its underlying mechanisms will lead to new approaches to treat bone disease.

The effect of surface immobilized NBD peptide on osteoclastogenesis of rough titanium plates in vitro and osseointegration of rough titanium implants in ovariectomized rats in vivo

Successful osseointegration in dental implants depends on balanced activation of osteoclasts and osteoblasts. Osteoporosis up-regulates osteoclast activity, so it is desirable to find effective interventions to inhibit osteoclastogenesis and enhance the osseointegration of implants under these conditions. It has been reported that the NF-κB essential modulator (NEMO)-binding domain (NBD) peptide can prevent osteoclast formation and bone resorption. In this study, we conjugated NBD peptide onto the surface of rough pure titanium (Ti) using the layer by layer technique. We analyzed the surface characteristics and determined the successful NBD integration by the presence of trivial granular structures, increased S elements and hydrophilia. Importantly, we first reported that Ti surface-conjugated NBD peptide retained its inhibitory effects on osteoclastogenesis by reducing osteoclast sealing zone formation and function. These effects were mediated by a reduction in NFATc1 expression, which in turn regulated integrin ανβ₃ and MMP9 by targeting the P65 signaling pathway. In vivo TRAP staining suggested NBD-coating decreased osteoclast formation with less pseudopodia. Micro-CT and histomorphometric analysis demonstrated that NBD-coating enhanced pronounced osseointegration in vivo in ovariectomized rats. This study holds great promise for in vivo use of immobilized NBD peptide and offers an effective therapeutic approach to select more suitable Ti-implant surface modifications for improving implant osseointegration in osteoporotic patients.

Successful osseointegration in dental implants depends on balanced activation of osteoclasts and osteoblasts.

Isolation and screening of rare Actinobacteria, a new insight for finding natural products with antivascular calcification activity

AIM

Vascular calcification (VC) is a significant pathological process in some life-threatening diseases. Several pathological mechanisms, including transdifferentiation of vascular smooth muscle cells to osteoblast-like cells and apoptosis are involved in VC. Compounds with an inhibitory effect on these processes are potentially efficient medications. In consideration of the multiple biological activities of Actinobacteria, this research was aimed at finding anti-VC metabolite-producing Actinobacteria.

METHODS AND RESULTS

After the isolation and identification of Actinobacteria, the effect of their fermentation broth extracts on the apoptosis rate was measured using various methods, for example, ethidium bromide/acridine orange staining, DNA laddering and diphenylamine assays. The effect of the most effective fermentation broth extract of Actinobacteria (FBEA) on the mRNA expression of runt-related transcription factor 2 (Runx2) and osteopontin (OPN) was examined. Finally, the most effective FBEA was fractionated and the chemical composition of anti-VC fractions was analysed using GC-MS. Various VC inhibition rates were observed in the tested FBEA (20 μg ml^-1 ; 17·9-60·15%). The inhibition of DNA fragmentation was 7-48%. The FBE with the greatest anticalcification activity belonged to Kribbella sp. UTMC 267 and, according to 16S rRNA analysis, Kribbella sancticallisti with a similarity of 98·53% is its nearest neighbour. The FBE of Kribbella sp. UTMC 267 reduced Runx2 mRNA expression by 2·95-fold and OPN mRNA expression by 28·57-fold, both of which are considered significant (P < 0·05). Finally, GC-MS analysis showed the existence of potent anti-oxidative and anti-inflammation agents in FBE of Kribbella sp. UTMC 267.

CONCLUSIONS

Actinobacterial metabolites can provide a new strategy for treating VC diseases by reducing the expression of osteogenic genes, the apoptosis rate and oxidative stress.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the therapeutic potential of Kribbella sp. metabolites and Actinobacteria as a new natural source for drug discovery programs in the nonantibiotic bioactivity field.

Macrophage integrins modulate response to ultra-high molecular weight polyethylene particles and direct particle-induced osteolysis

Aseptic loosening due to peri-prosthetic osteolysis is one of the primary causes for failure of artificial joint replacements. Implant-derived wear particles, often ultra-high molecular weight polyethylene (UHMWPE) microparticles, initiate an inflammatory cascade upon phagocytosis by macrophages, which leads to osteoclast recruitment and activation, ultimately resulting in osteolysis. Investigation into integrin receptors, involved in cellular interactions with biomaterial-adsorbed adhesive proteins, is of interest to understand and modulate inflammatory processes. In this work, we investigate the role of macrophage integrins Mac-1 and RGD-binding integrins in response to UHMWPE wear particles. Using integrin knockout mice as well as integrin blocking techniques, reduction in macrophage phagocytosis and inflammatory cytokine secretion is demonstrated when these receptors are either absent or blocked. Along this line, various opsonizing proteins are shown to differentially modulate microparticle uptake and macrophage secretion of inflammatory cytokines. Furthermore, using a calvarial osteolysis model it is demonstrated that both Mac-1 integrin and RGD-binding integrins modulate the particle induced osteolysis response to UHMWPE microparticles, with a 40% decrease in the area of osteolysis by the absence or blocking of these integrins, in vivo. Altogether, these findings indicate Mac-1 and RGD-binding integrins are involved in macrophage-directed inflammatory responses to UHMWPE and may serve as therapeutic targets to mitigate wear particle induced peri-prosthetic osteolysis for improved performance of implanted joints.

The osteoclasts attach to the bone surface where the extracellular calcium concentration decreases

Osteoclasts demineralize and resorb bone once they attach to its surface. However, it's still unclear how the osteoclasts choose the specific sites for their attachments. It is postulated in this article that the decreased extracellular free ionized calcium concentration (Ca²⁺_[e]) can provide a microenvironment for osteoclasts to recognize and then initiate the attachment process. The osteoclasts initially attach to the bone surface via integrating its integrin α_vβ₃ and RGD containing ligands in bone matrix. Through the interaction with RGD-containing ligand, the integrin α_vβ₃ forms carboxylate oxygen noncovalent, which is further stabilized by accompanied electrostatic interaction between the Ca²⁺ and the β₃ subunit. There are two types of cation-binding sites on the β₃ subunit: the high affinity Ca²⁺ binding site ("LC" site) that activates the osteoclasts by promoting the combination; the low affinity Ca²⁺ binding site ("I" site, also named ADMIDAS) that deactivates the osteoclasts by dissociating the combination and it can override the "LC" site's positive effect on osteoclasts when necessary. Normally the Ca²⁺ concentration of bone extracellular fluid is maintained within a normal range by osteocytes, keeping the "I" sites activated. When the osteocytes' function stalls and the ambient Ca²⁺ concentration falls below the affinity discrimination threshold between the "I" site and the "LC" site, the "LC" site's promoting function starts to out compete the "I" site in its inhibitory effect, in which case the inactive integrin α_vβ₃ turns into an extended active form and the osteoclasts start to attach, signifying the initiation of bone resorption.

RUNX2 promotes breast cancer bone metastasis by increasing integrin α5-mediated colonization

Runt-related transcription factor 2 (RUNX2) is regarded as an important contributor to breast cancer bone metastasis. However, previous studies did not provide direct clinical evidence for a role of RUNX2 in bone-specific metastasis in breast cancer, and the mechanism of RUNX2 in cancer cell recruitment and adhesion to the bone remains unclear. In this study, we showed that RUNX2 expression is positively correlated with the risk of bone-specific metastasis in lymph node-negative breast cancer patients. Then, we identified ITGA5 as a transcriptional target of RUNX2 from multiple candidate genes encoding adhesion molecules or chemokine receptors. We further provided experimental and clinical evidence that RUNX2, in an integrin α5-dependent manner, promotes the attraction and adhesion of breast cancer cells to the bone and confers cancer cell survival and bone colonization advantages. Overall, our findings clarify an adhesion-dependent mechanism of RUNX2 for the osteotropism and bone colonization of breast cancer cells and implicate RUNX2 and integrin α5 as potential molecular markers for the prediction of bone metastasis and therapeutic targets for the treatment of breast cancer bone metastasis.

A dose escalating phase I study of GLPG0187, a broad spectrum integrin receptor antagonist, in adult patients with progressive high-grade glioma and other advanced solid malignancies

BACKGROUND

Integrin signaling is an attractive target for anti-cancer treatment. GLPG0187 is a broad spectrum integrin receptor antagonist (IRA). GLPG0187 inhibited tumor growth and metastasis in mouse models.

METHODS

We aimed to determine the Recommended Phase II Dose (RP2D) and to assess safety and tolerability of continuous i.v. infusion in patients with advanced malignant solid tumors. Anticipated dose levels were 20, 40, 80, 160, 320, and 400 mg/day in a modified 3 + 3 design. Plasma concentrations of GLPG0187 were assessed to characterize the pharmacokinetics (PK). C-terminal telopeptide of type I collagen (CTX) was used as pharmacodynamics marker.

RESULTS

Twenty patients received GLPG0187. No dose limiting toxicities (DLTs) were observed. The highest possible and tested dose was 400 mg/day. Fatigue was the most frequently reported side effect (25%). Recurrent Port-A-Cath-related infections and skin toxicity suggest cutaneous integrin inhibition. No dose-dependent toxicity could be established. PK analysis showed a short average distribution (0.16 h) and elimination (3.8 h) half-life. Continuous infusion resulted in dose proportional PK profiles. We observed decreases in serum CTX levels independent of the dose given, suggesting target engagement at the lowest dose level tested. Single agent treatment did not result in tumor responses.

CONCLUSIONS

GLPG0187 was well tolerated with a dose-proportional PK profile upon continuous infusion. No formal maximal tolerated dose could be established. GLPG0187 showed signs of target engagement with a favourable toxicity profile. However, continuous infusion of GLPG0187 failed to show signs of monotherapy efficacy.

Differential β3 and β1 Integrin Expression in Bone Marrow and Cortical Bone of Estrogen Deficient Rats

Integrin-based (β3 ) attachments to the extracellular matrix (ECM) on osteocyte cell processes have recently been proposed to play an important role in facilitating osteocyte mechanosensation. However, it is not yet known whether integrin expression is altered in the mechanoregulatory osteocytes during osteoporosis. The objective of this study was to test the hypothesis that the expression of integrin-based mechanosensory complexes (β1 and β3 integrins) is altered as a direct response to estrogen deficiency, in an estrogen deficient animal model of osteoporosis. Four weeks post-operatively, immunohistochemistry was used to detect for β1 and β3 integrin subunits in bone tissue and marrow of ovariectomized (OVX; N = 4) and SHAM (N = 4) operated animals. A tartrate resistant acid phosphatase (TRAP) control stain was performed to quantify the presence of osteoclasts in the bone marrow and bone surfaces. Image analysis was performed to quantify expression patterns in different biological compartments, that is, bone marrow, endosteum, and cortical bone. Our results showed that β1 integrins were ubiquitously expressed throughout the bone and marrow, for both OVX and SHAM groups. β3 integrin subunit expression was lower in bone cells from osteoporotic animals compared to controls, whereas β3 expression in marrow cells did not differ significantly between groups. At the endosteum no difference was observed in β3 integrin subunit expression. As expected, the number of osteoclasts was higher in the OVX group validating an imbalance in bone remodeling. We propose that a reduction in β3 integrin expression in osteocytes might impair mechanosensation by bone cells during estrogen deficiency.

Proteomic Analysis of Estrogen-Mediated Signal Transduction in Osteoclasts Formation

Estrogen plays an important role in inhibiting osteoclast differentiation and protecting against bone loss from osteoporosis, especially in postmenopausal women. However, the precise mechanisms underlying the effect of estrogen on osteoclasts are not well known. In the present study, we performed proteomics analysis and bioinformatics analysis to comprehensively compare the differential expression of proteins in receptor activator of nuclear factor-κB ligand RANKL-induced osteoclasts in the presence and absence of estrogen. We identified 6403 proteins, of which 124 were upregulated and 231 were downregulated by estrogen. Bioinformatics analysis showed that estrogen treatment interfered with 77 intracellular pathways, including both confirmed canonical and unconfirmed pathways of osteoclast formation. Our findings validate the inhibitory effect of estrogen on osteoclasts via the promotion of apoptosis and suppression of differentiation and polarization and suggest that estrogen might inhibit osteoclast formation via other pathways, which requires further investigation and verification.

Strategies to inhibit tumor associated integrin receptors: rationale for dual and multi-antagonists

The integrins are a family of 24 heterodimeric transmembrane cell surface receptors. Involvement in cell attachment to the extracellular matrix, motility, and proliferation identifies integrins as therapeutic targets in cancer and associated conditions: thrombosis, angiogenesis, and osteoporosis. The most reported strategy for drug development is synthesis of an agent that is highly selective for a single integrin receptor. However, the ability of cancer cells to change their integrin repertoire in response to drug treatment renders this approach vulnerable to the development of resistance and paradoxical promotion of tumor growth. Here, we review progress toward development of antagonists targeting two or more members of the Arg-Gly-Asp (RGD) binding integrins, notably αvβ3, αvβ5, αvβ6, αvβ8, α5β1, and αIIbβ3, as anticancer therapeutics.

Targeting integrins to promote bone formation and repair

The ageing skeleton experiences a progressive decline in the rate of bone formation, which can eventually result in osteoporosis--a common disease characterized by reduced bone mass and altered bone microarchitecture which can result in fractures. One emerging therapy involves the identification of molecules that target bone-marrow mesenchymal stromal cells (MSCs) and promote their differentiation into osteoblasts, thereby counteracting bone loss. This Review highlights the discovery that some integrins, a family of heterodimeric transmembrane proteins that can interact with matrix proteins and generate intracellular signals, can be targeted to promote homing of MSCs to bone, osteogenic differentiation and bone formation. Specifically, priming of the α(5)β(1) integrin, which is required for osteoblastic differentiation of MSCs, leads to increased bone formation and improved bone repair in mice. Additionally, treatment with a peptidomimetic ligand of the α(4)β(1) integrin coupled to an agent with a high affinity for bone improves the homing of MSCs to bone and promotes osteoblast differentiation and bone formation, leading to increased bone mass in osteopenic mice. Strategies that target key integrins expressed by MSCs might, therefore, translate into improved therapies for age-related bone loss and possibly other disorders.

Elevation of fibrinogen due to loss of ovarian function enhances actin ring formation and leads to increased bone resorption

The aim of the present study was to evaluate the effect of fibrinogen on number and function of osteoclasts (OC) consequently resulting in bone loss. It was hypothesized that the enhanced level of released fibrinogen due to loss of ovarian function caused bone loss by acting on OCs. Bone loss was induced by ovariectomy (OVX) in mice and analyzed by micro-CT. The effect of fibrinogen on OCs was evaluated by tartrate-resistant acid phosphatase, annexin V, actin staining, pit formation observed on dentine slices, and Western blotting. Exogenous fibrinogen increased OC survival, actin ring formation, and bone resorption in vitro. The effect of fibrinogen was dependent on β(3)-integrin, which is a marker for mature OCs. Fibrinogen induced the activation of transforming oncogene from Ak strain (Akt), Ras-related C3 botulinum toxin substrate 1 (Rac1), and Rho family of GTPase (Rho) and the degradation of the Bcl-2 interacting mediator of cell death (Bim) in a manner similar to macrophage colony-stimulating factor (M-CSF). OVX increased plasma fibrinogen and serum M-CSF together with elevated actin ring formation and bone loss. The increased fibrinogen level due to loss of ovarian function may contribute, at least partly, to bone loss through the enhanced number and activity of OCs.

RNA therapeutics targeting osteoclast-mediated excessive bone resorption

RNA interference (RNAi) is a sequence-specific post-transcriptional gene silencing technique developed with dramatically increasing utility for both scientific and therapeutic purposes. Short interfering RNA (siRNA) is currently exploited to regulate protein expression relevant to many therapeutic applications, and commonly used as a tool for elucidating disease-associated genes. Osteoporosis and their associated osteoporotic fragility fractures in both men and women are rapidly becoming a global healthcare crisis as average life expectancy increases worldwide. New therapeutics are needed for this increasing patient population. This review describes the diversity of molecular targets suitable for RNAi-based gene knock down in osteoclasts to control osteoclast-mediated excessive bone resorption. We identify strategies for developing targeted siRNA delivery and efficient gene silencing, and describe opportunities and challenges of introducing siRNA as a therapeutic approach to hard and connective tissue disorders.

Anti-RANKL therapy for bone tumours: Basic, pre-clinical and clinical evidences

Bone remodelling is related to coordinated phases of bone resorption and bone apposition allowing the maintenance of bone integrity, the phosphocalcic homoeostasis all along the life and consequently the bone adaptation to mechanical constraints or/and to endocrine fluctuations. Unfortunately, bone is a frequent site of tumour development originated from bone cell lineages (primary bone tumours: bone sarcomas) or from nonosseous origins (bone metastases: carcinomas). These tumour cells disrupt the balance between osteoblast and osteoclast activities resulting in a disturbed bone remodelling weakening the bone tissue, in a strongly altered bone microenvironment and consequently facilitating the tumour growth. At the early stage of tumour development, osteoclast differentiation and recruitment of mature osteoclasts are strongly activated resulting in a strong bone matrix degradation and release of numerous growth factors initially stored into this organic/calcified matrix. In turn these soluble factors stimulate the proliferation of tumour cells and exacerbate their migration and their ability to initiate metastases. Because Receptor Activator of NFκB Ligand (RANKL) is absolutely required for in vivo osteoclastogenesis, its role in the bone tumour growth has been immediately pointed out and has consequently allowed the development of new targeted therapies of these malignant diseases. The present review summarises the role of RANKL in the bone tumour microenvironment, the most recent pre-clinical and clinical evidences of its targeting in bone metastases and bone sarcomas. The following sections position RANKL targeted therapy among the other anti-resorptive therapies available and underline the future directions which are currently under investigations.

How are osteoclasts induced to resorb bone?

Although much is known about how osteoclasts are formed, we know little about how they are activated, or how they recognize bone as the substrate appropriate for resorption. Bone mineral is considered to be essential to this recognition process, but a "mineral receptor" has never been identified. Recently, we found that resorptive behavior, as judged by the formation of ruffled borders and actin rings, occurs on ordinary tissue culture substrates if they are first coated with vitronectin. Similarly, vitronectin-coated substrates induce osteoclasts to secrete tartrate-resistant acid phosphatase and to form podosome belts, and to make resorption trails in the protein that coat the substrate. The same applies to bone mineral, which only induces resorptive behavior if coated with vitronectin. In contrast, fibronectin has none of these effects, despite inducing adhesion and spreading. It appears that osteoclasts recognize bone as the substrate appropriate for resorption through the high affinity of vitronectin-receptor ligands for bone mineral.

The type II collagen N-propeptide, PIIBNP, inhibits cell survival and bone resorption of osteoclasts via integrin-mediated signaling

OBJECTIVE

Type IIB procollagen is characteristic of cartilage, comprising 50% of the extracellular matrix. The NH(2)-propeptide of type IIB collagen, PIIBNP, can kill tumor cells via binding to integrins α(V)β(3) and α(V)β(5). As osteoclasts rely on α(V)β(3) integrins for function in bone erosion, we sought to determine whether PIIBNP could inhibit osteoclast function.

METHODS

We undertook in vitro and in vivo experiments to evaluate both osteoblast and osteoclast functions in the presence of recombinant PIIBNP. Adhesion of osteoclasts to PIIBNP was analyzed by staining of attached cells with crystal violet. PIIBNP-induced cell death was evaluated by counting Trypan Blue stained cells. The mechanism of cell death was evaluated by DNA fragmentation, TUNEL staining and western blotting to detect cleaved caspases. To determine the role of α(V)β(3) integrin, osteoclasts were pretreated with α(V) or β(3) integrin specific siRNA before the treatment with PIIBNP. To explore PIIBNP function in vivo, a lipopolysaccharide-induced mouse calvaria lysis model was employed.

RESULTS

Osteoclasts adhered to PIIBNP via an RGD-mediated mechanism. When osteoclasts were plated on extracellular matrix proteins, PIIBNP induced apoptosis of osteoclasts via caspase 3/8 activation. Osteoblasts and macrophages were not killed. Reduction of α(V) or β(3) integrin levels on osteoclasts by siRNA reduced cell death in a dose-dependent manner. In vivo, PIIBNP could inhibit bone resorption.

CONCLUSION

We conclude that PIIBNP can inhibit osteoclast survival and bone resorption via signal transduction through the α(V)β(3) integrins. Because of this property and the cell specificity, we propose that PIIBNP may play a role in vivo in protecting cartilage from osteoclast invasion and also could be a new therapeutic strategy for decreasing bone loss.

Regulation of osteoclast function

Osteoclasts are terminally differentiated multinucleated cells that are the principal resorptive cells of bone, playing a central role in the formation of the skeleton and regulation of its mass. The molecular events involved in the differentiation and function of osteoclasts had not been clarified for a long time. Over the past two decades, several novel approaches have been developed and adopted to investigate osteoclast biology. In the present review, we would like to update recent progress in the elucidation of the molecular mechanism of osteoclast activation and function.

Contribution of bone tissue modulus to breast cancer metastasis to bone

Certain tumors, such as breast, frequently metastasize to bone where they can induce bone destruction. Currently, it is well-accepted that the tumor cells are influenced by other cells and growth factors present in the bone microenvironment that lead to tumor-induced bone disease. Over the past 20 years, many groups have studied this process and determined the major contributing factors; however, these results do not fully explain the changes in gene expression and cell behavior that occur when tumor cells metastasize to bone. More recently, groups studying metastasis from soft tissue sites have determined that the rigidity of the microenvironment, which increases during tumor progression in soft tissue, can regulate tumor cell behavior and gene expression. Therefore, we began to investigate the role of the rigid bone extracellular matrix in the regulation of genes that stimulate tumor-induced bone disease. We found that the rigidity of bone specifically regulates parathyroid hormone-related protein (PTHrP) and Gli2 expression in a transforming growth factor β (TGF-β) and mechanotransduction-dependent mechanism. In this review, we summarize the mechanotransduction signaling pathway and how this influences TGF-β signaling and osteolytic gene expression.

New approaches to treating and preventing bone metastases

PURPOSE OF REVIEW

Treatment and prevention of bone metastases is a major problem in patients with cancer. New treatment of bone metastases are needed to maintain the quality of life of our patients with metastastic bone disease. In addition, promising preliminary results suggest that bone-directed therapies may be able to prevent both skeletal and extraskeletal metastases

RECENT FINDINGS

For the past decade intravenous bisphosphonates have been the mainstay of treatment of patients with bone metastases. New therapies such as the antibody to RANKL (denosumab) are undergoing phase III clinical testing. In addition, confirmatory studies suggesting that bisphosphonates can prevent metastatic disease are underway.

SUMMARY

Understanding the biology of bone metastases has uncovered many new potential therapies for the treatment and prevention of bone metastases. Many of these potential new approaches are discussed in the enclosed article.

Are bone defects in rare patients with Glanzmann's thrombasthenia associated with ITGB3 or ITGA2B mutations?

The question as to whether Glanzmann thrombasthenia patients with ITGB3 defects and deficiencies of both αIIbβ3 and αvβ3 show phenotypic differences to those with abnormalities exclusive to αIIbβ3 is unresolved. Studies on β3-deficient mice have shown an increased bone mass. Here we review the literature on bone defects in thrombasthenia patients and report the molecular analysis of a patient associating a lifelong thrombasthenia-like syndrome with skeletal defects. We show that the patient is compound heterozygote for Arg327His and Gly391Arg mutations in αIIb, with one mutation inherited from each parent. Modelling strongly suggested that both mutations act by destabilizing the αIIb beta propeller. So it appears likely that this patient has a combination of co-expressed genetic defects.

Integrins and bone metastasis: integrating tumor cell and stromal cell interactions

Integrins on both tumor cells and the supporting host stromal cells in bone (osteoclasts, new blood vessels, inflammatory cells, platelets and bone marrow stromal cells) play key roles in enhancing bone metastasis. Tumor cells localize to specific tissues through integrin-mediated contacts with extracellular matrix and stromal cells. Integrin expression and signaling are perturbed in cancer cells, allowing them to "escape" from cell-cell and cell-matrix tethers, invade, migrate and colonize within new tissues and matrices. Integrin signaling through αvβ3 and VLA-4 on tumor cells can promote tumor metastasis to and proliferation in the bone microenvironment. Osteoclast (OC) mediated bone resorption is a critical component of bone metastasis and can promote tumor growth in bone and αvβ3 integrins are critical to OC function and development. Tumors in the bone microenvironment can recruit new blood vessel formation, platelets, pro-tumor immune cells and bone marrow stromal cells that promote tumor growth and invasion in bone. Integrins and their ligands play critical roles in platelet aggregation (αvβ3 and αIIbβ3), hematopoietic cell mobilization (VLA-4 and osteopontin), neoangiogenesis (αvβ3, αvβ5, α6β4, and β1 integrin) and stromal function (osteopontin and VLA-4). Integrins are involved in the pathogenesis of bone metastasis at many levels and further study to define integrin dysregulation by cancer will yield new therapeutic targets for the prevention and treatment of bone metastasis.

Calcium signaling in osteoclasts

It has long been known that many bone diseases, including osteoporosis, involve abnormalities in osteoclastic bone resorption. As a result, there has been intense study of the mechanisms that regulate both the differentiation and bone resorbing function of osteoclast cells. Calcium (Ca(2+)) signaling appears to play a critical role in the differentiation and functions of osteoclasts. Cytoplasmic Ca(2+) oscillations occur during RANKL-mediated osteoclastogenesis. Ca(2+) oscillations provide a digital Ca(2+) signal that induces osteoclasts to up-regulate and autoamplify nuclear factor of activated T cells c1 (NFATc1), a Ca(2+)/calcineurin-dependent master regulator of osteoclastogenesis. Here we review previous studies on Ca(2+) signaling in osteoclasts as well as recent breakthroughs in understanding the basis of RANKL-induced Ca(2+) oscillations, and we discuss possible molecular players in this specialized Ca(2+) response that appears pivotal for normal bone function. This article is part of a Special Issue entitled: 11th European Symposium on Calcium.

Osteoclast-specific inactivation of the integrin-linked kinase (ILK) inhibits bone resorption

Bone resorption requires the adhesion of osteoclasts to extracellular matrix (ECM) components, a process mediated by the alpha(v)beta(3) integrin. Following engagement with the ECM, integrin receptors signal via multiple downstream effectors, including the integrin-linked kinase (ILK). In order to characterize the physiological role of ILK in bone resorption, we generated mice with an osteoclast-specific Ilk gene ablation by mating mice with a floxed Ilk allele with TRAP-Cre transgenic mice. The TRAP-Cre mice specifically excised floxed alleles in osteoclasts, as revealed by crossing them with the ROSA26R reporter strain. Osteoclast-specific Ilk mutant mice appeared phenotypically normal, but histomorphometric analysis of the proximal tibia revealed an increase in bone volume and trabecular thickness. Osteoclast-specific Ilk ablation was associated with an increase in osteoclastogenesis both in vitro and in vivo. However, the mutant osteoclasts displayed a decrease in resorption activity as assessed by reduced pit formation on dentin slices in vitro and decreased serum concentrations of the C-terminal telopeptide of collagen in vivo. Interestingly, compound heterozygous mice in which one allele of Ilk and one allele of the beta(3) integrin gene were inactivated (ILK(+/-); beta(3) (+/-)) also had increased trabecular thickness, confirming that beta(3) integrin and Ilk form part of the same genetic cascade. Our results show that ILK is important for the function, but not the differentiation, of osteoclasts.

Hypoxia and hypoxia-inducible factors: master regulators of metastasis

Hypoxia is a common condition found in a wide range of solid tumors and is often associated with poor prognosis. Hypoxia increases tumor glycolysis, angiogenesis, and other survival responses, as well as invasion and metastasis by activating relevant gene expressions through hypoxia-inducible factors (HIF). HIF-1α and HIF-2α undergo oxygen-dependent regulation, and their overexpression is frequently associated with metastasis and poor clinical outcomes. Recent studies show that each step of the metastasis process, from the initial epithelial-mesenchymal transition to the ultimate organotropic colonization, can potentially be regulated by hypoxia, suggesting a master regulator role of hypoxia and HIFs in metastasis. Furthermore, modulation of cancer stem cell self-renewal by HIFs may also contribute to the hypoxia-regulated metastasis program. The hypoxia-induced metastatic phenotype may be one of the reasons for the modest efficacy of antiangiogenic therapies and may well explain the recent provocative findings that antiangiogenic therapy increased metastasis in preclinical models. Multiple approaches to targeting hypoxia and HIFs, including HIF inhibitors, hypoxia-activated bioreductive prodrugs, and gene therapies may become effective treatments to prevent or reduce metastasis.

Bone is not essential for osteoclast activation

BACKGROUND

The mechanism whereby bone activates resorptive behavior in osteoclasts, the cells that resorb bone, is unknown. It is known that α(v)β(3) ligands are important, because blockade of α(v)β(3) receptor signaling inhibits bone resorption, but this might be through inhibition of adhesion or migration rather than resorption itself. Nor is it known whether α(v)β(3) ligands are sufficient for resorption the consensus is that bone mineral is essential for the recognition of bone as the substrate appropriate for resorption.

METHODOLOGY/PRINCIPAL FINDINGS

Vitronectin- but not fibronectin-coated coverslips induced murine osteoclasts to secrete tartrate-resistant acid phosphatase, as they do on bone. Osteoclasts incubated on vitronectin, unlike fibronectin, formed podosome belts on glass coverslips, and these were modulated by resorption-regulating cytokines. Podosome belts formed on vitronectin-coated surfaces whether the substrates were rough or smooth, rigid or flexible. We developed a novel approach whereby the substrate-apposed surface of cells can be visualized in the scanning electron microscope. With this approach, supported by transmission electron microscopy, we found that osteoclasts on vitronectin-coated surfaces show ruffled borders and clear zones characteristic of resorbing osteoclasts. Ruffles were obscured by a film if cells were incubated in the cathepsin inhibitor E64, suggesting that removal of the film represents substrate-degrading behavior. Analogously, osteoclasts formed resorption-like trails on vitronectin-coated substrates. Like bone resorption, these trails were dependent upon resorbogenic cytokines and were inhibited by E64. Bone mineral induced actin rings and surface excavation only if first coated with vitronectin. Fibronectin could not substitute in any of these activities, despite enabling adhesion and cell spreading.

CONCLUSIONS/SIGNIFICANCE

Our results show that ligands α(v)β(3) are not only necessary but sufficient for the induction of resorptive behavior in osteoclasts; and suggest that bone is recognized through its affinity for these ligands, rather than by its mechanical or topographical attributes, or through a putative 'mineral receptor'.

New treatment targets in osteoporosis

Postmenopausal osteoporosis is characterized by bone remodeling alterations with an imbalance between excessive bone resorption and inadequate bone formation. At present, osteoporosis treatment rests on bone resorption inhibitors and, more specifically, on bisphosphonates. However, the introduction of anabolic agents such as parathyroid hormone that stimulate bone formation has expanded the range of treatment options. New treatment targets have been identified via improved knowledge on bone pathophysiology, bone remodeling, bone cells and intracellular signaling pathways. RANKL inhibition by anti-RANKL antibodies is undergoing considerable development as a treatment for osteoporosis. Also under development are anti-catabolic drugs that target the molecular mechanisms involved in bone resorption, including cathepsin K inhibitors and integrin alpha(v)beta(3) antagonists. The identification of new pathways involved in bone formation is directing clinical research efforts toward the development of anabolic agents. The signaling pathways involved in bone formation, most notably the Wnt-pathway, hold considerable promise as treatment targets in conditions characterized by insufficient bone formation. Current focuses of interest include antibodies against naturally occurring Wnt-pathway antagonists (e.g., sclerostin and Dkk1) and modulators of parathyroid hormone production (calcilytic agents). Thus, active research is ongoing to improve the treatment of osteoporosis, a disease whose high prevalence and considerable functional and socioeconomic impact will raise formidable challenges in the near future.

In vitro evaluation of functional interaction of integrin alphavbeta3 and matrix metalloprotease-2

Integrin alphavbeta3 and matrix metalloprotease-2 (MMP-2) are two established molecular targets of angiogenesis. Basic understanding of various forms of functional interaction of integrin alphavbeta3 and active MMP-2 may be used to develop therapeutic approaches. Based upon the idea that integrins are present on the surface of invasive cells and MMP-2 may be localized to this and other cell-surface receptors, we investigated the hypothesis that integrin binding will alter cleavage of MMP-2 substrates. To investigate this hypothesis, integrin-binding and MMP-2 cleavable motifs were combined in a single peptide, MMP-RGD, designed with fluorescent probes for monitoring peptide cleavage. MMP-RGD was bound to integrin alphavbeta3 with equal affinity compared to the integrin-binding motif and was cleaved with equal specificity by active MMP-2. MMP-RGD bound to human umbilical vein endothelial cells (HUVECs). MMP-2 from HUVECs cleaved MMP-RGD, but the cleavage was not altered due to integrin binding. Our results indicate that integrin alphavbeta3 and active MMP-2 may not be as functionally collaborative for substrate cleavage as expected based on the current knowledge of their cell surface colocalization.

Alphav and beta1 integrins regulate dynamic compression-induced proteoglycan synthesis in 3D gel culture by distinct complementary pathways

OBJECTIVE

Our goal was to test the hypothesis that specific integrin receptors regulate chondrocyte biosynthetic response to dynamic compression at early times in 3D gel culture, during initial evolution of the pericellular matrix, but prior to significant accumulation of further-removed matrix. The study was motivated by increased use of dynamic loading, in vitro, for early stimulation of tissue engineered cartilage, and the need to understand the effects of loading, in vivo, at early times after implantation of constructs.

METHODS

Bovine articular chondrocytes were seeded in 2% agarose gels (15x10(6)cells/mL) and incubated for 18 h with and without the presence of specific integrin blockers (small-molecule peptidomimetics, function-blocking antibodies, and RGD-containing disintegrins). Samples were then subjected to a 24-h dynamic compression regime found previously to stimulate chondrocyte biosynthesis in 3D gel as well as cartilage explant culture (1 Hz, 2.5% dynamic strain amplitude, 7% static offset strain). At the end of loading, proteoglycan (PG) synthesis ((35)S-sulfate incorporation), protein synthesis ((3)H-proline incorporation), DNA content (Hoechst dye 33258) and total glycosaminoglycan (GAG) content (dimethyl methylene blue (DMMB) dye binding) were assessed.

RESULTS

Consistent with previous studies, dynamic compression increased PG synthesis and total GAG accumulation compared to free-swelling controls. Blocking alphavbeta3 abolished this response, independent of effects on controls, while blocking beta1 abolished the relative changes in synthesis when changes in free-swelling synthesis rates were observed.

CONCLUSIONS

This study suggests that both alphavbeta3 and beta1 play a role in pathways that regulate stimulation of PG synthesis and accumulation by dynamic compression, but through distinct complementary mechanisms.

Unraveling metalloproteinase function in skeletal biology and disease using genetically altered mice

The metalloproteinase family includes MMP, ADAM and ADAMTS proteases. Mice deficient in individual or pairs of metalloproteinases have been generated, and a number of these genetic models spontaneously develop skeletal abnormalities. Here we review metalloproteinase function in endochondral and intramembranous ossification, as well as in postnatal bone remodeling. We highlight how metalloproteinases enable interactions between distinct bone cell types and how this communication contributes to the skeletal phenotypes observed in knockout mice. In addition to the physiological actions of metalloproteinases in the skeletal system, the experimental manipulation of metalloproteinase-deficient mice has revealed substantial roles for these enzymes in osteoarthritis and rheumatoid arthritis. MMP, ADAM and ADAMTS proteases thus emerge as key players in the development and homeostasis of the skeletal system.

Integrins: Signaling, disease, and therapy

BACKGROUND

Integrins are a family of transmembrane receptors that mediate cell-cell and cell-matrix adhesion. They are involved in stable cell adhesion and migration of cells. In addition, integrin-mediated interactions modulate the response to most, if not all growth factors, cytokines, and other soluble factors.

PURPOSE

In this review, we briefly explain how integrins can affect the multitude of signal transduction cascades in control of survival, proliferation, and differentiation. Subsequently, we primarily focus on targeting integrins alpha5beta1 and alphanubeta3 in disease and we discuss how antagonists of these integrins, including disintegrins, RGD peptides, small molecules, and function blocking antibodies, may be of therapeutical value either alone or, especially in the treatment of cancer, in combination with existing therapeutical strategies.