The integrin alphavbeta5 is expressed on avian osteoclast precursors and regulated by retinoic acid

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.

Gingipains promote RANKL-induced osteoclastogenesis through the enhancement of integrin β3 in RAW264.7 cells

As a crucial virulence factor of Porphyromonas gingivalis, gingipains play an important role in periodontal destruction. This study aimed to investigate the effect of gingipains on osteoclastogenesis. We used RAW264.7 cells as osteoclast precursors in our study. In experimental groups, cells were treated with gingipains and/or receptor activator of nuclear factor-κB ligand (RANKL). Tartrate-resistant acid phosphatase (TRAP) activity staining assay showed osteoclast precursors and RANKL-induced mature osteoclasts were increased in a gingipains dose-dependent manner. Real-time reverse transcription polymerase chain reaction analysis demonstrated that gingipains upregulated osteoclastic genes including the protease cathepsin K (Ctsk), matrix metalloprotein 9 (Mmp9), nuclear factor of activated T cells 1 (Nfatc1) and acid phosphatase 5, tartrate resistant (Acp5) in a time-dependent manner. Western blotting assays presented upregulated expressions of TNF receptor-activating factor 6 (TRAF6) and integrin β3 induced by gingipains and RANKL compared to RANKL alone. Enhanced integrin-related signaling was also demonstrated by elevated phosphorylations of FAK and paxillin compared to control. Moreover, the pit resorption assays showed that gingipains augmented bone resorptive function of osteoclasts induced by RANKL. When we used Cilengitide to block integrin αvβ3, gingipains reversed the reduction of formation and resorptive function in RANKL-induced osteoclasts, as they enhanced integrin αvβ3 levels more than RANKL treatment alone. In conclusion, our data suggest that gingipains augmented the differentiation and function of mature osteoclasts induced by RANKL through the increase in integrin αvβ3.

Osteoporosis and skeletal dysplasia caused by pathogenic variants in SGMS2

Mechanisms leading to osteoporosis are incompletely understood. Genetic disorders with skeletal fragility provide insight into metabolic pathways contributing to bone strength. We evaluated 6 families with rare skeletal phenotypes and osteoporosis by next-generation sequencing. In all the families, we identified a heterozygous variant in SGMS2, a gene prominently expressed in cortical bone and encoding the plasma membrane–resident sphingomyelin synthase SMS2. Four unrelated families shared the same nonsense variant, c.148C>T (p.Arg50*), whereas the other families had a missense variant, c.185T>G (p.Ile62Ser) or c.191T>G (p.Met64Arg). Subjects with p.Arg50* presented with childhood-onset osteoporosis with or without cranial sclerosis. Patients with p.Ile62Ser or p.Met64Arg had a more severe presentation, with neonatal fractures, severe short stature, and spondylometaphyseal dysplasia. Several subjects had experienced peripheral facial nerve palsy or other neurological manifestations. Bone biopsies showed markedly altered bone material characteristics, including defective bone mineralization. Osteoclast formation and function in vitro was normal. While the p.Arg50* mutation yielded a catalytically inactive enzyme, p.Ile62Ser and p.Met64Arg each enhanced the rate of de novo sphingomyelin production by blocking export of a functional enzyme from the endoplasmic reticulum. SGMS2 pathogenic variants underlie a spectrum of skeletal conditions, ranging from isolated osteoporosis to complex skeletal dysplasia, suggesting a critical role for plasma membrane–bound sphingomyelin metabolism in skeletal homeostasis.

The identification of 6 families with childhood-onset osteoporosis with mutations in SGMS2 suggests a critical role for plasma membrane–bound sphingomyelin metabolism in skeletal homeostasis.

Osteoclasts-Key Players in Skeletal Health and Disease

The differentiation of osteoclasts (OCs) from early myeloid progenitors is a tightly regulated process that is modulated by a variety of mediators present in the bone microenvironment. Once generated, the function of mature OCs depends on cytoskeletal features controlled by an αvβ3-containing complex at the bone-apposed membrane and the secretion of protons and acid-protease cathepsin K. OCs also have important interactions with other cells in the bone microenvironment, including osteoblasts and immune cells. Dysregulation of OC differentiation and/or function can cause bone pathology. In fact, many components of OC differentiation and activation have been targeted therapeutically with great success. However, questions remain about the identity and plasticity of OC precursors and the interplay between essential networks that control OC fate. In this review, we summarize the key principles of OC biology and highlight recently uncovered mechanisms regulating OC development and function in homeostatic and disease states.

Pathogenesis of Endometriosis: Roles of Retinoids and Inflammatory Pathways

Endometriosis is a nonmalignant, but potentially metastatic, gynecological condition manifested by the extrauterine growth of inflammatory endometrial implants. Ten percent of reproductive-age women are affected and commonly suffer pelvic pain and/or infertility. The theories of endometriosis histogenesis remain controversial, but retrograde menstruation and metaplasia each infer mechanisms that explain the immune cell responses observed around the ectopic lesions. Recent findings from our laboratories and others suggest that retinoic acid metabolism and action are fundamentally flawed in endometriotic tissues and even generically in women with endometriosis. The focus of our ongoing research is to develop medical therapies as adjuvants or alternatives to the surgical excision of these lesions. On the basis of concepts put forward in this review, we predict that the pharmacological actions and anticipated low side-effect profiles of retinoid supplementation might provide a new treatment option for the long-term management of this chronic and debilitating gynecological disease.

Retinoic acid biosynthesis is impaired in human and murine endometriosis

Endometriosis is characterized by the presence of endometrial glands and stroma in extrauterine sites. Our objective was to determine whether endometriotic lesions (ELs) from women with endometriosis have altered retinoid levels compared with their eutopic endometrium, and to test the hypothesis that defects in all-trans retinoic acid (ATRA) biosynthesis in EL is related to reduced expression of cellular retinol-binding protein type 1 (RBP1). Retinoids were evaluated by liquid chromatography-tandem mass spectrometry and high-performance liquid chromatography in eutopic endometrial biopsies (EBs) and ELs from 42 patients with pathologically confirmed endometriosis. The ATRA levels were reduced, whereas the retinol and retinyl ester concentrations were elevated in EL compared with EB tissue. Similar results were found in a mouse model of endometriosis that used green fluorescent protein-positive endometrial tissue injected into the peritoneum of syngeneic hosts to mimic retrograde menses. The ATRA biosynthesis in vitro in retinol-treated primary human endometrial stromal cell (ESC) cultures derived from ELs was reduced compared with that of ESCs derived from patient-matched EBs. Correspondingly, RBP1 expression was reduced in tissue and ESCs derived from EL versus EB. Rbp1(-/-) mice showed reduced endometrial ATRA concentrations compared with wild type, associated with loss of tissue organization and hypercellularity. These findings provide the first quantitative measurements of ATRA in human endometrium and endometriosis, demonstrating reduced ATRA in ectopic tissue and corresponding ESC cultures. Quantitation of retinoids in murine endometriosis and in Rbp1(-/-) mice supports the contention that impaired ATRA synthesis caused by reduced RBP1 promotes an "endometriosis phenotype" that enables cells to implant and grow at ectopic sites.

Analysis of follicular fluid retinoids in women undergoing in vitro fertilization: retinoic acid influences embryo quality and is reduced in women with endometriosis

Retinol (ROL) and its biologically active metabolite, all-trans retinoic acid (ATRA), are essential for a number of reproductive processes. However, there is a paucity of information regarding their roles in ovarian folliculogenesis, oocyte maturation, and early embryogenesis. The objectives of this study were to quantify and compare peripheral plasma (PP) and follicular fluid (FF) retinoid levels, including ATRA in women undergoing in vitro fertilization (IVF) and to investigate the relationship between retinoid levels and embryo quality. Retinoid levels were evaluated in PP and FF from 79 women undergoing IVF at the time of oocyte retrieval and corresponding embryo quality assessed on a daily basis after retrieval for 3 days until uterine transfer. Analysis compared the retinoid levels with day 3 embryo grades and between endometriosis versus control patients. Results demonstrated distinctive levels of retinoid metabolites and isomers in FF versus PP. There was a significantly larger percentage of high-quality grade I embryos derived from the largest versus smallest follicles. An increase in follicle size also correlated with a >50% increase in FF ROL and ATRA concentrations. Independent of follicle size, FF yielding grade I versus nongrade I embryos showed higher mean levels of ATRA but not ROL. In a nested case-control analysis, control participants had 50% higher mean levels of ATRA in their FF and PP than women with endometriosis. These findings strongly support the proposition that ATRA plays a fundamental role in oocyte development and quality, and that reduced ATRA synthesis may contribute to decreased fecundity of participants with endometriosis.

Osteopetrosis with micro-lacunar resorption because of defective integrin organization

In vitro differentiated monocytes were used to characterize the cellular defect in a type of osteopetrosis with minimally functional osteoclasts, in which defects associated with common causes of osteopetrosis were excluded by gene sequencing. Monocytes from the blood of a 28-year-old patient were differentiated in media with RANKL and CSF-1. Cell fusion, acid compartments within cells, and tartrate resistant acid phosphatase (TRAP) activity were normal. However, the osteoclasts made abnormally small pits on the dentine. Phalloidin labeling showed that the cell attachments lacked the peripheral ring structure that supports lacunar resorption. Instead, the osteoclasts had clusters of podosomes near the center of cell attachments. Antibody to the alphavbeta3 integrin pair or to the C-terminal of beta3 did not label podosomes, but antibody to alphav labeled them. Western blots using antibody to the N-terminal of beta3 showed a protein of reduced size. Integrins beta1 and beta5 were upregulated, but, in contrast to observations in beta3 defects, alpha2 had not increased. The rho-GTP exchange protein Vav3, a key attachment organizing protein, did not localize normally with peripheral attachment structures. Vav3 forms of 70 kD and 90 kD were identified on western blots. However, the proteins beta3 integrin, Vav3, Plekhm1, and Src, implicated in attachment defects, had normal exon sequences. In this new type of osteopetrosis, the integrin-organizing complex is dysfunctional, and at least two attachment proteins may be partially degraded.

Chemical and Biochemical Basis of Cell-Bone Matrix Interaction in Health and Disease

Bone, a calcified tissue composed of 60% inorganic component (hydroxyapatite), 10% water and 30% organic component (proteins), has three functions: providing mechanical support for locomotion, protecting vital organs, and regulating mineral homeostasis. A lifelong execution of these functions depends on a healthy skeleton, which is maintained by constant bone remodeling in which old bone is removed by the bone-resorbing cell, osteoclasts, and then replaced by new bone formed by the bone-forming cell, osteoblasts. This remodeling process requires a physical interaction of bone with these bone cells. Moreover, numerous cancers including breast and prostate have a high tendency to metastasize to bone, which is in part attributable to the capacity of the tumor cells to attach to bone. The intensive investigation in the past two decades has led to the notion that the cell-bone interaction involves integrins on cell surface and bone matrix proteins. However, the biochemical composition of bone and emerging evidence are inconsistent with this belief. In this review, I will discuss the current understanding of the molecular mechanism underlying the cell-bone interaction. I will also highlight the facts and new findings supporting that the inorganic, rather than the organic, component of bone is likely responsible for cellular attachment.

Identification and Characterization of the Precursors Committed to Osteoclasts Induced by TNF-Related Activation-Induced Cytokine/Receptor Activator of NF-κB Ligand

Osteoclasts are terminally differentiated from cells of monocyte/macrophage lineage by stimulation with TNF-related activation-induced cytokine (TRANCE) (receptor activator of NF-kappaB ligand/osteoprotegerin ligand/osteoclast differentiation factor/TNFSF11/CD254). In the present study, we attempted to determine when and how the cell fate of precursors becomes committed to osteoclasts following TRANCE stimulation. Although mouse bone marrow-derived macrophages (BMMs) were able to differentiate into either osteoclasts or dendritic cells, the cells no longer differentiated into dendritic cells after treatment with TRANCE for 24 h, indicating that their cell fate was committed to osteoclasts. Committed cells as well as BMMs were still quite weak in tartrate-resistant acid phosphatase activity, an osteoclast marker, and incorporated zymosan particles by phagocytosis. Interestingly, committed cells, but not BMMs, could still differentiate into osteoclasts even after incorporation of the zymosan particles. Furthermore, IL-4 and IFN-gamma, potent inhibitors of osteoclast differentiation, failed to inhibit osteoclast differentiation from committed cells, and blocking of TRANCE stimulation by osteoprotegerin resulted in cell death. Adhesion to culture plates was believed to be essential for osteoclast differentiation; however, committed cells, but not BMMs, differentiated into multinucleated osteoclasts without adhesion to culture plates. Although LPS activated the NF-kappaB-mediated pathway in BMMs as well as in committed cells, the mRNA expression level of TNF-alpha in the committed cells was significantly lower than that in BMMs. These results suggest that characteristics of the committed cells induced by TRANCE are distinctively different from that of BMMs and osteoclasts.

Design, synthesis, and evaluation of near infrared fluorescent multimeric RGD peptides for targeting tumors

Molecular interactions between RGD peptides and integrins are known to mediate many biological and pathological processes. This has led to an increased interest in the development of RGD compounds with high affinity and improved selectivity for integrin receptors. In this study, we synthesized and evaluated a series of multimeric RGD compounds constructed on a dicarboxylic acid-containing near-infrared (NIR) fluorescent dye (cypate) for tumor targeting. An array of NIR fluorescent RGD compounds was prepared efficiently, including one RGD monomer (cypate-(RGD)(2)-NH(2)), two RGD dimers (cypate-(RGD)(2)-NH(2) and cypate-(RGD-NH(2))(2)), one trimer (cypate-(RGD)(3)-NH(2)), two tetramers (cypate-(RGD)(4)-NH(2) and cypate-[(RGD)(2)-NH(2)](2)), one hexamer (cypate-[(RGD)(3)-NH(2)](2)), and one octamer (cypate-[(RGD)(4)-NH(2)](2)). The binding affinity of the multimeric RGD compounds for alpha(v)beta(3) integrin receptor (ABIR) showed a remarkable increase relative to the monomer cypate-RGD-NH(2). Generally, the divalent linear arrays of the multimeric RGD units bound the ABIR with slightly higher affinity than their monovalent analogues. These results suggest that the receptor binding affinity was not only dependent on the number of RGD moieties but also on the spatial alignments of the pendant peptides. Internalization of the compounds by ABIR-positive tumor cells (A549) was monitored by NIR fluorescence microscopy. The data showed that endocytosis of the octameric RGD derivative was significantly higher by comparison to other compounds in this study. In vivo noninvasive optical imaging and biodistribution data showed that the compounds were retained in A549 tumor tissue. These results clearly demonstrated that an array of simple RGD tripeptides on a NIR fluorescent dye core can be recognized by ABIR. Optimization of the spatial alignment of the RGD moieties through careful molecular design and library construction could induce multivalent ligand-receptor interactions useful for in vivo tumor imaging and tumor-targeted therapy.

Rapid inhibition of thyroxine-induced bone resorption in the rat by an orally active vitronectin receptor antagonist

An excess of thyroid hormone results in increased bone turnover and loss of bone mass in humans. Exogenous administration of thyroid hormone to rats has served as a model of human hyperthyroidism in which antiresorptive therapies have been tested. We have further refined this model of thyroxine (T4)-induced turnover in the rat. Daily administration of T4 to aged rats for as short as 1 week resulted in elevated bone resorption determined by significantly higher urinary deoxypyridinoline (Dpd) compared with vehicle controls or animals receiving T4 plus estradiol. Three weeks of daily administration of T4 led to significantly lower bone mineral density compared with untreated controls or animals receiving T4 plus estradiol. In a follow-up study, a depot formulation of T4 caused an increase in Dpd identical to that achieved with a bolus dose. SB-273005 [(4S)-2,3,4,5-tetrahydro-8-[2-[6-(methylamino)-2-pyridinyl] ethoxy]-3-oxo-2-(2,2,2-trifluoroethyl)-1H-2-benzazepine-4- acetic acid] a potent antagonist of the integrins alpha(v)beta(3) and alpha(v)beta(5), has been shown previously to inhibit bone resorption in cultures of human osteoclasts and to protect bone in ovariectomized rats. The effect of SB-273005 by oral administration was evaluated in this thyroxine-induced turnover model. Dose-dependent inhibition of resorption was seen with SB-273005 after 7 days of dosing using Dpd as a measure of bone resorption. In summary, it has been demonstrated that the antiresorptive activity of a vitronectin receptor antagonist can be measured after only 7 days of treatment in this refined rat model of thyroxine-induced bone turnover. These data suggest that SB-273005 may be useful for the treatment of metabolic bone diseases, including those resulting from hyperthyroidism.

Emerging therapies in osteoporosis

The approval of alendronate in 1994 marked a watershed in the treatment of osteoporosis. Before that time there was no therapy for which unequivocal proof of efficacy existed. Since then several more agents, all from the anti-resorptive class, have also been approved for use in the treatment of this disease and the range of indications for alendronate has been extended to include the prevention of osteoporosis in women with lesser degrees of bone loss, the treatment of glucocorticoid-induced osteoporosis and, most recently, the treatment of male osteoporosis. Despite this there are still several areas of unmet medical need in this disease, including the availability of well tolerated and convenient therapies and treatments that will go beyond the levels of efficacy offered by current therapies. An intense effort is now being directed towards meeting these unmet needs with the improvement of existing therapies and the development of novel agents that will provide superior long term benefit. Important and exciting drug targets are yielding novel compounds with anti-resorptive activity or anabolic effects to complement current anti-resorptives. Despite this effort considerable obstacles to the successful development of these compounds remain, not least the stringent safety requirements needed to provide an acceptable risk-to-benefit profile and the increasing difficulties of conducting placebo controlled studies in patients at high risk of fracture.

Antagonism of the osteoclast vitronectin receptor with an orally active nonpeptide inhibitor prevents cancellous bone loss in the ovariectomized rat

An orally active, nonpeptide Arg-Gly-Asp (RGD) mimetic alpha(v)beta3 antagonist, (S)-3-Oxo-8-[2-[6-(methylamino)-pyridin-2-yl]-1-ethoxy]-2-(2,2,2-trifluoroethyl)-2,3,4,5-tetrahydro-1H-2-benzazepine-4-acetic acid (compound 1), has been generated, which prevented net bone loss and inhibited cancellous bone turnover in vivo. The compound binds alpha(v)beta3 and the closely related integrin alpha(v)beta5 with low nanomolar affinity but binds only weakly to the related integrins alpha(IIb)beta3, and alpha5beta1. Compound 1 inhibited alpha(v)beta3-mediated cell adhesion with an IC50 = 3 nM. More importantly, the compound inhibited human osteoclast-mediated bone resorption in vitro with an IC50 = 11 nM. In vivo, compound 1 inhibited bone resorption in a dose-dependent fashion, in the acute thyroparathyroidectomized (TPTX) rat model of bone resorption with a circulating EC50 approximately 20 microM. When dosed orally at 30 mg/kg twice a day (b.i.d.) in the chronic ovariectomy (OVX)-induced rat model of osteopenia, compound 1 also prevented bone loss. At doses ranging from 3 to 30 mg/kg b.i.d., compound 1 partially prevented the OVX-induced increase in urinary deoxypyridinoline. In addition, the compound prevented the OVX-induced reduction in cancellous bone volume (BV), trabecular number (Tb.N), and trabecular thickness (Tb.Th), as assessed by quantitative microcomputerized tomography (microCT) and static histomorphometry. Furthermore, both the 10-mg/kg and 30-mg/kg doses of compound prevented the OVX-induced increase in bone turnover, as measured by percent osteoid perimeter (%O.Pm). Together, these data indicate that the alpha(v)beta3 antagonist compound 1 inhibits OVX-induced bone loss. Mechanistically, compound 1 prevents bone loss in vivo by inhibiting osteoclast-mediated bone resorption, ultimately preventing cancellous bone turnover.

Disease-modifying activity of SB 273005, an orally active, nonpeptide alphavbeta3 (vitronectin receptor) antagonist, in rat adjuvant-induced arthritis

OBJECTIVE

To evaluate the effects of SB 273005, a potent, orally active nonpeptide antagonist of the integrin avbeta3 vitronectin receptor, on joint integrity in rats with adjuvant-induced arthritis (AIA).

METHODS

Male Lewis rats with AIA were orally dosed either prophylactically (days 0-20) or therapeutically (days 10-20) with SB 273005. Efficacy was determined by measurement of paw inflammation, assessment of bone mineral density using dual-energy x-ray absorptiometry (DEXA), magnetic resonance imaging (MRI), and histologic evaluation.

RESULTS

SB 273005 is a potent antagonist of the closely related integrins, avbeta3 (Ki = 1.2 nM) and alphavbeta5 (Ki = 0.3 nM). When SB 273005 was administered prophylactically to AIA rats twice per day, it inhibited paw edema at doses of 10, 30, and 60 mg/kg, by 40%, 50%, and 52%, respectively. Therapeutic administration twice daily was also effective, and a reduction in paw edema was observed at 30 mg/kg and 60 mg/kg of the antagonist (by 36% and 48%, respectively). SB 273005 was also effective when administered once per day, both prophylactically and therapeutically. Significant improvement in joint integrity in treated rats was shown using DEXA and MRI analyses. These findings were confirmed histologically, and significant protection of bone, cartilage, and soft tissue was observed within the joint.

CONCLUSION

Symptoms of AIA in rats were significantly reduced by either prophylactic or therapeutic treatment with the alphavbeta3 antagonist, SB 273005. Measurements of paw inflammation and of bone, cartilage, and soft tissue structure indicated that this compound exerts a protective effect on joint integrity and thus appears to have disease-modifying properties.

Oligodeoxynucleotide targeted to the alphav gene inhibits alphav integrin synthesis, impairs osteoclast function, and activates intracellular signals to apoptosis

The alphav integrin subunit is highly expressed in osteoclasts where it dimerizes with beta1 and beta3 subunits to form receptors for vitronectin and bone sialoproteins. Inhibition of osteoclast adhesion and function has previously been achieved by alphavbeta3 antibodies or Arg-Gly-Asp-containing peptides which have the disadvantages of blocking a single receptor type, or of being rather nonspecific, respectively. Here we show that alphav integrin expression in rabbit osteoclasts can be inhibited by partially phosphorothioated antisense oligodeoxynucleotide (ODN) spanning the adenine-uracil-guanine (AUG) translational start site of the human/rabbit alphav gene, a procedure which offers the advantage of affecting all the alphav receptors with high efficiency. The alphav antisense ODN caused a dose-dependent, substrate-specific reduction of osteoclast adhesion and bone resorption. Control ODNs, such as sense, inverted, and mismatch, were without effect, providing evidence of specificity of the antisense reagent. It is likely as a consequence of loss of substrate interaction, the antisense ODN induced osteoclast retraction and apoptosis, increase of the cyclin/cyclin-dependent kinase complex inhibitor p21WAF1/CIP1, and inhibition of the cell survival gene, bcl-2. Although the expression of the cell death-promoting gene, bax, remained unchanged, a reduction of the bcl-2/bax ratio, known to underlie the intracellular signal to apoptosis, was observed. This finding led us to hypothesize that these changes could provide a link between reduction of alphav synthesis and osteoclast programmed death. In conclusion, this study provides novel insights into the use of alphav antisense ODN as an efficacious mechanism for blocking osteoclast function and underscores for the first time the involvement of integrins in bone cell apoptosis. In vivo studies may verify potential application of this ODN as alternative therapy for bone diseases.