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

RGD peptides and monoclonal antibodies, antagonists of alpha(v)-integrin, enter the cells by independent endocytic pathways

Cyclic synthetic peptides containing the arginine-glycine-aspartate motif (cRGD) and monoclonal antibodies (mAbs) targeted for individual integrins have been developed as potential therapeutic drugs for the treatment of several diseases. We showed that a cRGD peptide targeted for alpha(v)beta(3) was internalized in alpha(v)-integrin expressing and nonexpressing melanoma cells by an integrin independent fluid-phase endocytosis pathway that does not alter the number of functional integrin receptors at the cell surface. In contrast, a blocking mAb directed to alpha(v) was internalized by an integrin-dependent endocytosis pathway that reduced the number of functional integrin receptors at the cell surface. We prove that melanoma cells pretreated with the mAb do not readhere to the substrate, whereas cells pretreated with cRGD peptide retain their readhesion capacity. Given the growing importance of RGD peptides, knowledge of these cellular mechanisms is required to improve the development of antiangiogenic and anti-inflammatory drugs.

Osteoclast inhibitory peptide 2 inhibits osteoclast formation via its C-terminal fragment

Osteoclast inhibitory peptide 2 (OIP-2) is a novel autocrine/paracrine factor produced by osteoclasts (OCLs) that inhibits bone resorption and OCL formation in vitro and in vivo. It is identical to the asparaginyl endopeptidase legumain. During maturation of OIP-2, a signal peptide and a 17-kDa C-terminal fragment (CTF) are cleaved to produce the mature enzyme. To determine if enzyme activity is required for inhibition of OCL formation or if only the CTF is responsible for these effects, we synthesized His-tagged complementary DNA (cDNA) constructs for the CTF of OIP-2, the proform of OIP-2, and the "mature enzyme" form of OIP-2. The proform or the CTF portion of OIP-2 inhibited OCL formation in a dose-dependent manner in murine bone marrow cultures stimulated with 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The mature form of OIP-2, which was enzymatically active, did not inhibit OCL formation. In addition, OIP-2 inhibited OCL formation in cultures of highly purified human OCL precursor cells or RAW264.7 cells stimulated with 10 ng/ml of receptor activator of NF-kappaB (RANK) ligand. Binding studies with His-tagged OIP-2 showed expression of a putative OIP-2 receptor on RAW264.7 cells treated with RANK ligand for 4 days and human marrow cultures treated with 1,25(OH)2D3 for 3 weeks. These data show that the CTF of OIP-2, rather than the mature enzyme, mediates the inhibitory effects of OIP-2 through a putative receptor on OCL precursors.

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.

Solid-phase synthesis of a nonpeptide RGD mimetic library: new selective alphavbeta3 integrin antagonists

The solid-phase synthesis of a low molecular weight RGD mimetic library is described. Activities of the compounds in inhibiting the interaction of ligands, vitronectin and fibrinogen, with isolated immobilized integrins alphavbeta3 and alphaIIbbeta3 were determined in a screening assay. Highly active and selective nonpeptide alphavbeta3 integrin antagonists with regard to orally bioavailability were developed, based on the aza-glycine containing lead compound 1. An important variation is the substitution of the aspartic amide of 1 by an aromatic residue. Furthermore, different guanidine mimetics have been incorporated to improve the pharmacokinetic profile. Exchange of the beta-amino acid NH by a methylene moiety in one set of RGD mimetics leads to the azacarba analogue compounds representing a novel peptidomimetic approach, which should increase the metabolic stability.

Bone disease in myeloma

The major clinical manifestation of multiple myeloma results from osteolytic bone destruction. The only currently Food and Drug Administration-approved drug for the treatment of the bony complications of multiple myeloma is monthly intravenous pamidronate at a dose of 90 mg infused over 4 hours. Recent studies have shown the safety of 2-hour infusions. A randomized trial comparing pamidronate to placebo continued to show benefits throughout the 21-month trial. Although the duration of therapy has not been firmly determined, it is likely that discontinuation of this drug will be met by enhanced bone loss and an increased risk of bony complications for these patients. Thus, it is recommended that the drug be continued indefinitely. Support for this recommendation also comes from the reduced bone density observed in women with postmenopausal osteoporosis following the withdrawal of bisphosphonate treatment. Recent attempts to give higher doses, more frequent infusions (every 2 weeks or less), or more rapid infusions (1 hour or less) of pamidronate have occasionally been associated with albuminuria and azotemia. These modifications should therefore be avoided. Importantly, the drug can be safely administered at 90 mg monthly to patients with poor renal function. The use of pamidronate for myeloma patients without lytic bone involvement or with Durie-Salmon stages I or II disease has not been evaluated. However, it is recognized that most patients with earlier stages of disease or without lytic bone involvement also develop bony complications. There is no reason to believe that these patients would not benefit from monthly intravenous infusions of pamidronate. The potential antimyeloma effect of this agent is another reason to administer this drug in these types of patients. Thus, it is our practice to administer monthly pamidronate to myeloma patients regardless of stage or bone involvement. However, trials evaluating oral bisphosphonates have produced inconsistent clinical results, probably as a result of the erratic and scanty poor absorption as well as poor oral tolerability of these drugs. Although these oral agents may be useful in some patients, it is impossible to identify which myeloma patients will benefit from orally administered bisphosphonates. The more potent nitrogen-containing bisphosphonate zoledronic acid more effectively reverses hypercalcemia of malignancy than pamidronate, and it appears promising in reducing bone loss in cancer patients. However, its efficacy in preventing skeletal complications is still being evaluated. Many other types of new agents are in early clinical trials, but their efficacy remains unproven at the present time.

A Glanzmann's mutation in beta 3 integrin specifically impairs osteoclast function

Osteoclastic bone resorption requires cell-matrix contact, an event mediated by the alpha v beta 3 integrin. The structural components of the integrin that mediate osteoclast function are, however, not in hand. To address this issue, we generated mice lacking the beta 3 integrin gene, which have dysfunctional osteoclasts. Here, we show the full rescue of beta 3(-/-) osteoclast function following expression of a full-length beta 3 integrin. In contrast, truncated beta 3, lacking a cytoplasmic domain (h beta 3c), is completely ineffective in restoring function to beta 3(-/-) osteoclasts. To identify the components of the beta 3 cytoplasmic domain regulating osteoclast function, we generated six point mutants known, in other circumstances, to mediate beta integrin signaling. Of the six, only the S(752)P substitution, which also characterizes a form of the human bleeding disorder Glanzmann's thrombasthenia, fails to rescue beta 3(-/-) osteoclasts or restore ligand-activated signaling in the form of c-src activation. Interestingly, the double mutation Y(747)F/Y(759)F, which disrupts platelet function, does not affect the osteoclast. Thus similarities and distinctions exist in the mechanisms by which the beta 3 integrin regulates platelets and osteoclasts.

Parathyroid hormone-induced bone resorption does not occur in the absence of osteopontin

Osteopontin is an RGDS-containing protein that acts as a ligand for the alpha(v)beta(3) integrin, which is abundantly expressed in osteoclasts, cells responsible for bone resorption in osteopenic diseases such as osteoporosis and hyperparathyroidism. However, the role of osteopontin in the process of bone resorption has not yet been fully understood. Therefore, we investigated the direct function of osteopontin in bone resorption using an organ culture system. The amount of (45)Ca released from the osteopontin-deficient bones was not significantly different from the basal release from wild type bones. However, in contrast to the parathyroid hormone (PTH) enhancement of the (45)Ca release from wild type bones, PTH had no effect on (45)Ca release from organ cultures of osteopontin-deficient bones. Because PTH is located upstream of receptor activator of NF-kappaB ligand (RANKL), that directly promotes bone resorption, we also examined the effect of RANKL. Soluble RANKL with macrophage-colony stimulating factor enhanced (45)Ca release from the bones of wild type fetal mice but not from the bones of osteopontin-deficient mice. To obtain insight into the cellular mechanism underlying the phenomena observed in osteopontin-deficient bone, we investigated the number of tartrate-resistant acid phosphatase (TRAP)-positive cells in the bones subjected to PTH treatment in cultures. The number of TRAP-positive cells was increased significantly by PTH in wild type bone; however, no such PTH-induced increase in TRAP-positive cells was observed in osteopontin-deficient bones. These results indicate that the absence of osteopontin suppressed PTH-induced increase in bone resorption via preventing the increase in the number of osteoclasts in the local milieu of bone.

Cloning and characterization of the murine beta(3) integrin gene promoter: identification of an interleukin-4 responsive element and regulation by STAT-6

Expression of the alpha(v)beta(3) integrin by murine bone marrow macrophages is regulated by cytokines such as IL-4 and GM-CSF through transcriptional activation of the beta(3) subunit gene. To characterize the molecular mechanisms by which such regulation occurs, we isolated the murine beta(3) integrin promoter. To this end, we first cloned a full length beta(3) cDNA and used the 5'UTR and leader peptide coding sequence to identify genomic clones containing the beta(3) promoter region. The transcriptional start site, identified by primer extension and S1 nuclease assay, is 34 nt upstream of the translation initiation codon. A 1.1 kb fragment of the promoter region drives IL-4 responsive transcription in transiently transfected murine bone marrow macrophages. Deletion analysis of the beta(3) promoter indicates the IL-4 responsive element lies between -465 to -678 nt relative to the transcriptional start site. This promoter fragment contains two overlapping STAT consensus recognition sites and nuclear extracts from BMMs contain an IL-4-inducible DNA binding factor, identified by super shift analysis, as STAT-6. Furthermore, an oligonucleotide which includes the two STAT recognition sites residing in the IL-4 responsive region of the beta(3) promoter, competes for STAT-6 binding. Confirming IL-4 induction of the integrin subunit is specifically mediated by STAT-6, beta(3) mRNA is not enhanced in BMMs derived from STAT-6 deleted mice, which however, retain their capacity to respond to GM-CSF.

Inhibition of osteoclast function by adenovirus expressing antisense protein-tyrosine kinase 2

Osteoclast activation is initiated by adhesion to bone, cytoskeletal rearrangement, formation of the sealing zone, and formation of the polarized ruffled membrane. Previous findings suggest that protein-tyrosine kinase 2 (PYK2), a cytoplasmic kinase related to focal adhesion kinase, participates in these events. This study examines the role of PYK2 in adhesion-mediated signaling and osteoclast function, using PYK2 antisense. We produced a recombinant adenovirus containing a 300-base pair reversed 5'-coding region of PYK2 and used full-length PYK2 as a control. Murine osteoclast-like cells or their mononuclear precursors were generated in a co-culture of bone marrow and osteoblasts. Infection with antisense adenovirus significantly reduced the expression of endogenous PYK2 protein relative to uninfected cells or to cells infected with sense PYK2 and caused: 1) a reduction in osteoclast formation in vitro; 2) inhibition of cell spreading and of actin ring formation in osteoclasts plated on glass or bone and of attachment and spreading of osteoclast precursors plated on vitronectin; 3) inhibition of bone resorption in vitro; 4) marked reduction in p130(Cas) tyrosine phosphorylation; and 5) no change in alpha(v)beta(3) integrin expression or c-Src tyrosine phosphorylation. Taken together, these findings support the hypothesis that PYK2 plays a central role in the adhesion-dependent cytoskeletal organization and sealing zone formation required for osteoclastic bone resorption.

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.

An adherent condition is required for formation of multinuclear osteoclasts in the presence of macrophage colony-stimulating factor and receptor activator of nuclear factor κB ligand

Identification of receptor activator of nuclear factor-κB (RANK) and RANK-ligand (RANKL) has provided new insights into the osteoclast differentiation pathway. Osteoclast precursor cells were isolated using monoclonal antibodies against c-Fms and RANK, and the effect of adherence on the in vitro differentiation and proliferation of these cells was examined in 2 different types of stromal-cell–free culture systems: a semisolid culture medium (a nonadherent system) and a liquid culture medium (an adherent system). Osteoclast precursor cells were not able to differentiate into mature osteoclasts efficiently in the semisolid culture system. Trimerized RANKL enhanced osteoclast differentiation in semisolid cultures, but not to the extent seen when cells were allowed to adhere to plastic. Initial precursor cells were capable of differentiating into macrophages or osteoclasts. Once these cells were transferred to adherent conditions, striking differentiation was induced. Multinuclear cells were observed even after they had displayed phagocytic activity, which suggests that cell adhesion plays an important role in the differentiation of osteoclast precursor cells. Integrins, especially the arginine-glycine-aspartic acid (RGD)–recognizing integrins αv and β3, were needed for osteoclast-committed precursor cells to proliferate in order to form multinuclear osteoclasts, and the increase in cell density affected the formation of multinuclear cells. A model of osteoclast differentiation with 2 stages of precursor development is proposed: (1) a first stage, in which precursor cells are bipotential and capable of anchorage-independent growth, and (2) a second stage, in which the further proliferation and differentiation of osteoclast-committed precursor cells is anchorage-dependent.

An adherent condition is required for formation of multinuclear osteoclasts in the presence of macrophage colony-stimulating factor and receptor activator of nuclear factor κB ligand

Identification of receptor activator of nuclear factor-κB (RANK) and RANK-ligand (RANKL) has provided new insights into the osteoclast differentiation pathway. Osteoclast precursor cells were isolated using monoclonal antibodies against c-Fms and RANK, and the effect of adherence on the in vitro differentiation and proliferation of these cells was examined in 2 different types of stromal-cell–free culture systems: a semisolid culture medium (a nonadherent system) and a liquid culture medium (an adherent system). Osteoclast precursor cells were not able to differentiate into mature osteoclasts efficiently in the semisolid culture system. Trimerized RANKL enhanced osteoclast differentiation in semisolid cultures, but not to the extent seen when cells were allowed to adhere to plastic. Initial precursor cells were capable of differentiating into macrophages or osteoclasts. Once these cells were transferred to adherent conditions, striking differentiation was induced. Multinuclear cells were observed even after they had displayed phagocytic activity, which suggests that cell adhesion plays an important role in the differentiation of osteoclast precursor cells. Integrins, especially the arginine-glycine-aspartic acid (RGD)–recognizing integrins αv and β3, were needed for osteoclast-committed precursor cells to proliferate in order to form multinuclear osteoclasts, and the increase in cell density affected the formation of multinuclear cells. A model of osteoclast differentiation with 2 stages of precursor development is proposed: (1) a first stage, in which precursor cells are bipotential and capable of anchorage-independent growth, and (2) a second stage, in which the further proliferation and differentiation of osteoclast-committed precursor cells is anchorage-dependent.

Osteopontin: a versatile regulator of inflammation and biomineralization

Osteopontin is a secreted glycoprotein with a multidomain structure and functions characteristic of a matricellular protein. Osteopontin interacts with cell surface receptors via arginine-glycine-aspartate (RGD)- and non-RGD containing adhesive domains, in addition to binding to components of the structural extracellular matrix. While normally expressed in bone and kidney, osteopontin levels are elevated during wound healing and inflammation in most tissues studied to date. Since 1986, over one thousand studies have been published on osteopontin, including recent experiments in osteopontin-deficient mice. These studies reveal osteopontin as a cell adhesive, signaling, migratory, and survival stimulus for various mesenchymal, epithelial, and inflammatory cells, in addition to being a potent regulator of osseous and ectopic calcification. Based on these reports, a general picture of osteopontin as an important regulator of inflammation and biomineralization is emerging. A common denominator in osteopontin function in these situations is its ability to regulate the function of macrophage and macrophage-derived cells (i.e. osteoclasts). While we have learned much about osteopontin and the processes it appears to regulate over the past decade, many questions regarding this important multifunctional protein remain unanswered and provide important directions for future studies.

Nonpeptide alpha(v)beta(3) antagonists. 1. Transformation of a potent, integrin-selective alpha(IIb)beta(3) antagonist into a potent alpha(v)beta(3) antagonist

Modification of the potent fibrinogen receptor (alpha(IIb)beta(3)) antagonist 1 generated compounds with high affinity for the vitronectin receptor alpha(v)beta(3). Sequential modification of the basic N-terminus of 1 led to the identification of the 5,6,7, 8-tetrahydro[1,8]naphthyridine moiety (THN) as a lipophilic, moderately basic N-terminus that provides molecules with excellent potency and selectivity for the integrin receptor alpha(v)beta(3). The THN-containing analogue 5 is a potent inhibitor of bone resorption in vitro and in vivo. In addition, the identification of a novel, nonpeptide radioligand with high affinity to alpha(v)beta(3) is also reported.

Ligand-integrin alpha v beta 3 interaction determined by photoaffinity cross-linking: a challenge to the prevailing model

Integrin alpha(V)beta(3) plays a crucial role in angiogenesis, apoptosis, and bone remodeling, mainly by interacting with matrix proteins through recognition of an Arg-Gly-Asp (RGD) motif. Recently, a small cyclic RGD-containing alpha(V)beta(3)-ligand possessing a C-terminal photoreactive group was photo-cross-linked within beta(3)[99-118], in the N-terminus of the beta(3) chain [Bitan G et al. (1999) Biochemistry 38, 3414-3420]. In this paper, a photoreactive group at the N-terminus of the RGD-ligand is shown to interact within beta(3)[167-171], approximately 60 residues C-terminal to the previously identified domain. On the basis of these findings, a model of the putative I-like domain of the beta(3) subunit, homologous to alpha(M)-, alpha(L)-, and alpha(2)-I-domains, reveals that the beta(3)[99-118] and beta(3)[167-171] contact sites are close to each other and are on the opposite side relative to the metal ion-dependent adhesion site (MIDAS) motif. These observations contradict the prevailing model that proposes proximity between metal- and RGD-binding sites on the I-like domain. Our data suggest that either the I-like domain structure predicted for beta(3) is incorrect, or there is no spatial proximity between the RGD-binding site and the MIDAS motif in the I-like domain. Our results indicate that the current models for ligand-receptor interaction should be revisited.

Identification and in vivo efficacy of small-molecule antagonists of integrin alphavbeta3 (the vitronectin receptor)

The integrin alphavbeta3 is thought to play a key role in the initiation and/or progression of several human diseases, including osteoporosis, restenosis following percutaneous transluminal coronary angioplasty (PTCA), rheumatoid arthritis, cancer and ocular diseases. Antagonism of integrin alphavbeta3 is therefore expected to provide an approach for the treatment and/or prevention of these diseases. A variety of potent, small-molecule alphavbeta3 antagonists have been identified, several of which are active in disease models, thereby demonstrating the therapeutic potential of alphavbeta3 antagonism. This review will focus on recent advances in the identification of small-molecule alphavbeta3 antagonists, with an emphasis on those studies where small-molecule alphavbeta3 antagonists have been used in proof-of-concept studies in vivo.

Therapeutic approaches to bone diseases

The strength and integrity of our bones depends on maintaining a delicate balance between bone resorption by osteoclasts and bone formation by osteoblasts. As we age or as a result of disease, this delicate balancing act becomes tipped in favor of osteoclasts so that bone resorption exceeds bone formation, rendering bones brittle and prone to fracture. A better understanding of the biology of osteoclasts and osteoblasts is providing opportunities for developing therapeutics to treat diseases of bone. Drugs that inhibit the formation or activity of osteoclasts are valuable for treating osteoporosis, Paget's disease, and inflammation of bone associated with rheumatoid arthritis or periodontal disease. Far less attention has been paid to promoting bone formation with, for example, growth factors or hormones, an approach that would be a valuable adjunct therapy for patients receiving inhibitors of bone resorption.

Bone resorption by osteoclasts

Osteoporosis, a disease endemic in Western society, typically reflects an imbalance in skeletal turnover so that bone resorption exceeds bone formation. Bone resorption is the unique function of the osteoclast, and anti-osteoporosis therapy to date has targeted this cell. The osteoclast is a specialized macrophage polykaryon whose differentiation is principally regulated by macrophage colony-stimulating factor, RANK ligand, and osteoprotegerin. Reflecting integrin-mediated signals, the osteoclast develops a specialized cytoskeleton that permits it to establish an isolated microenvironment between itself and bone, wherein matrix degradation occurs by a process involving proton transport. Osteopetrotic mutants have provided a wealth of information about the genes that regulate the differentiation of osteoclasts and their capacity to resorb bone.

Parallel solid-phase synthesis of vitronectin receptor (alphavbeta3) inhibitors

A combinatorial approach for rapid optimization of a vitronectin receptor (alphavbeta3) inhibitor lead was accomplished by solid-phase synthesis. Orthogonally bis protected 2,3-diaminopropionic acid was used to immobilize the C-terminus of the molecule. Selective deprotection and functionalization of the alpha-amino group followed by acyl resorcinol scaffold attachment and N-terminus diversification was used to explore structure activity relationship (SAR).

Alpha v integrin antagonists induce the disassembly of focal contacts in melanoma cells

In recent years, several antagonists of alpha(v)beta3 have been used to develop therapeutic approaches to the treatment of melanoma neoplasia. We studied the effects of anti-alpha(v)-integrin-blocking antibodies on attached M21 melanoma cells, the cellular distribution of alpha(v)-integrin and the molecular organization of focal structures. Anti-alpha(v)-integrin-blocking antibodies 17E6 and LM609, and an anti-alpha(v)beta3-integrin antagonist peptide cRGD 85189 induced detachment of M21 melanoma cells cultured for 24 hours on various substrates. cRGD was the most effective antagonist, reducing the number of adherent cells by 80%, while 17E6 reduced adhesion by only 30%. Light- and electron microscopy revealed attached cells with a flat shape and well-formed actin cytoskeleton. After treatment, cells became rounded and detached from the culture dish. alpha(v)-Integrins and focal-contact proteins were observed at adhesion sites in focal structures by immunocytochemistry. After treatment, however, cell rounding was accompanied by disorganization of the actin filaments and redistribution of alpha(v)-integrins and most of the focal proteins studied, except vinculin and tensin. Our results indicate that treatment of M21 melanoma cells with a(v)-integrin antagonists disrupts the actin cytoskeleton, redistributes a(v)-integrin and induces molecular disassembly of focal contacts.

Integrins and signaling in osteoclast function

Integrins are heterodimeric adhesion receptors that mediate cell-matrix and cell-cell interactions. Osteoclasts highly express the alphavbeta3 integrin, which binds to a variety of extracellular matrix proteins including vitronectin, osteopontin and bone sialoprotein. RGD-containing peptides, RGD-mimetics and alphavbeta3 blocking antibodies inhibit bone resorption in vitro and in vivo, suggesting that this integrin plays an important role in osteoclast function. RGD-containing peptides were shown to raise cytosolic calcium in osteoclasts. Furthermore, several signaling and adaptor molecules were found to be involved in alphavbeta3 integrin-dependent signaling pathways, including phosphatidylinositol 3-kinase, c-Src, PYK2 and p130(cas). In addition, cytoskeletal molecules such as paxillin, vinculin, gelsolin and F-actin are recruited to adhesion contacts upon integrin activation. Many of these molecules signaling and cytoskeletal localize to the sealing zone of actively resorbing osteoclasts, suggesting that they play a role in linking the adhesion of osteoclasts to the bone matrix with the cytoskeletal organization and the polarization and activation of these cells for bone resorption.

Adhesion receptors in health and disease

Cell adhesion molecules have been recognized to play a major role in a variety of physiological and pathological phenomena. They determine the specificity of cell-cell binding and the interactions between cells and extracellular matrix proteins. Some of them may also function as receptors that trigger intracellular pathways and participate in cellular processes like migration, proliferation, differentiation, and cell death. The receptors that mediate adhesion between epithelial cells that are discussed in this review include integrins, selectins, the immunoglobulin superfamily members, and cadherins. The intent of this review is to inform the reader about recent advances in cellular and molecular functions of certain receptors, specifically those that are considered important in cell adhesion. We have deliberately not provided all-inclusive detailed information on every molecule, but instead, have presented a generalized overview in order to give the reader a global perspective. This information will be useful in enhancing the reader's understanding of the molecular pathology of diseases and recognizing the potential role of these receptors and ligands as therapeutic agents.

Equine osteoclast-like cells generated in vitro demonstrate similar characteristics to directly isolated mature osteoclasts

We report on novel methods to isolate osteoclasts (OC s) and generate osteoclast-like cells (OCL s) from the bone and bone marrow of the equine femur. OC s were successfully isolated from bone scrapings taken from the endosteal surface of the femurs of three horses. OCL s were generated from bone marrow cells taken from the same animals. The validity of using the formation of OCL s as a method for studying OC differentiation and activity was confirmed by the similar characteristics of these two cells. In particular, they both were multinuclear, expressed the enzyme tartrate resistant acid phosphatase and the vitronectin receptor. Most importantly, both were able to resorb bone as demonstrated by the formation of extensive resorption pits when cultured on dentine slices. The generation of OCL s from bone marrow obtained from the equine femur can therefore be used to study equine OC differentiation and for studies requiring the generation of large numbers of these cells. OC s isolated directly from the same bones may be used to examine the effect of a variety of factors on bone resorption in vitro and to continually reaffirm the validity of using OCL s for large-scale studies on OC biology. Such research is essential for improved understanding of bone turnover and endochondral ossification in the horse.

Mice deficient in Abl are osteoporotic and have defects in osteoblast maturation

The c-Abl protein is a non-receptor tyrosine kinase involved in many aspects of mammalian development. c-Abl kinase is widely expressed, but high levels are found in hyaline cartilage in the adult, bone tissue in newborn mice, and osteoblasts and associated neovasculature at sites of endochondrial ossification in the fetus. Mice homozygous for mutations in the gene encoding c-Abl (AIM) display increased perinatal mortality, reduced fertility, foreshortened crania and defects in the maturation of B cells in bone marrow. Here we demonstrate that Abl-/- mice are also osteoporotic. The long bones of mutant mice contain thinner cortical bone and reduced trabecular bone volume. The osteoporotic phenotype is not due to accelerated bone turnover--both the number and activity of osteoclasts are similar to those of control littermates--but rather to dysfunctional osteoblasts. In addition, the rate of mineral apposition in the mutant animals is reduced. Osteoblasts from both stromal and calvarial explants showed delayed maturation in vitro as measured by expression of alkaline phosphatase (ALP), induction of mRNA encoding osteocalcin and mineral deposition.

Design and evaluation of benzophenone-containing conformationally constrained ligands as tools for photoaffinity scanning of the integrin alphaVbeta3-ligand bimolecular interaction

Integrins are cell-surface adhesion molecules involved in mediating cell-extracellular matrix interactions. High-resolution structural data are not available for these heterodimeric receptors. In order to generate tools for photoaffinity scanning of the RGD-binding site of human integrin alphaVbeta3. new conformationally constrained ligands were designed. The ligands were based on five different cyclic peptidic or peptidomimetic scaffolds with high affinity for alphaVbeta3. A single photoreactive group (a benzophenone moiety) was introduced at different positions relative to the RGD triad. In addition, an 125I or a biotin group was introduced as a reporting tag. Twenty-four cyclic ligands were prepared and their binding affinity for alphaVbeta3 was determined. In most cases, the modifications resulted in a 5- to 500-fold decrease in affinity relative to the unmodified scaffold. Analogs representing three of the five families were screened for their cross-linking efficiency. Ligands with submicromolar affinities cross-linked efficiently and specifically to the integrin receptor, whereas ligands with weaker affinities gave specific cross-linking, but with lower efficiency. Almost all of the screened ligands cross-linked predominantly to the beta3 subunit.

Mice lacking beta3 integrins are osteosclerotic because of dysfunctional osteoclasts

Osteoclasts express the alphavbeta3 integrin, an adhesion receptor that has been implicated in bone resorption and that is therefore a potential therapeutic target. To assess the role of this heterodimer in skeletal development in vivo, we engineered mice in which the gene for the beta3 integrin subunit was deleted. Bone marrow macrophages derived from these mutants differentiate in vitro into numerous osteoclasts, thus establishing that alphavbeta3 is not necessary for osteoclast recruitment. Furthermore, the closely related integrin, alphavbeta5, does not substitute for alphavbeta3 during cytokine stimulation or authentic osteoclastogenesis. beta3 knockout mice, but not their heterozygous littermates, develop histologically and radiographically evident osteosclerosis with age. Despite their increased bone mass, beta3-null mice contain 3.5-fold more osteoclasts than do heterozygotes. These mutant osteoclasts are, however, dysfunctional, as evidenced by their reduced ability to resorb whale dentin in vitro and the significant hypocalcemia seen in the knockout mice. The resorptive defect in beta3-deficient osteoclasts may reflect absence of matrix-derived intracellular signals, since their cytoskeleton is distinctly abnormal and they fail to spread in vitro, to form actin rings ex vivo, or to form normal ruffled membranes in vivo. Thus, although it is not required for osteoclastogenesis, the integrin alphavbeta3 is essential for normal osteoclast function.

Isoxazolines as potent antagonists of the integrin alpha(v)beta(3)

Starting with lead compound 2, we sought to increase the selectivity for alpha(v)beta(3)-mediated cell adhesion by examining the effects of structural changes in both the guanidine mimetic and the substituent alpha to the carboxylate. To prepare some of the desired aminoimidazoles, a novel reductive amination utilizing a trityl-protected aminoimidazole was developed. It was found that guanidine mimetics with a wide range of pK(a)'s were potent antagonists of alpha(v)beta(3). In general, it appeared that an acylated 2-aminoimidazole guanidine mimetic imparted excellent selectivity for alpha(v)beta(3)-mediated adhesion versus alpha(IIb)beta(3)-mediated platelet aggregation, with selectivity of approximately 3 orders of magnitude observed for compounds 3g and 3h. It was also found in this series that the alpha-substituent was required for potent activity and that 2,6-disubstituted arylsulfonamides were optimal. In addition, the selective alpha(v)beta(3) antagonist 3h was found to be a potent inhibitor of alpha(v)beta(3)-mediated cell migration.

Multiple Myeloma: New Insights and Therapeutic Approaches

This review discusses the evolution of novel diagnostic and treatment strategies for multiple myeloma based upon increased understanding of basic disease pathogenesis. Although myeloma has remained an incurable illness to date, these new developments will derive treatments to improve outcome and achieve eventual cure.

In Section I, Dr. Kyle reviews the results of current therapy for multiple myeloma, including high dose therapy and stem cell transplantation which have proven to achieve improved response rates, event-free, and overall survival. Supportive therapy, such as erythropoietin to treat disease-related anemia, and methods of prophylaxis against infection, which both lessen toxicities of treatment and improve quality of life for patients, are also addressed.

In Section II, Dr. Dalton with Drs. Landowski, Shain, Jove and Hazlehurst discusses mechanisms of drug resistance in myeloma, with emphasis on novel treatment approaches to prevent development of drug resistance and to overcome drug resistance. Laboratory studies delineating mechanisms whereby myeloma cells resist drug-induced apoptosis provide the framework for related treatment protocols for patients with refractory disease.

In Section III, Dr. Berenson reviews the management of complications in bone, which occur in the majority of patients with myeloma and are the major cause of decreased quality of life. New insights into the mediators of bone resorption and new bone formation in the marrow milieu have already derived effective bisphosphonate therapy. These drugs not only reduce bone complications and related pain, thereby improving quality of life, but also may have intrinsic anti-tumor activity by virtue of inducing tumor cell adherence to marrow, reducing interleukin-6 secretion, inducing tumor cell apoptosis, or inhibiting angiogenesis.

In the last section, Dr. Anderson explores the potential for future therapies which offer great promise to improve patient outcomes. First, drugs which alter the marrow microenvironment include thalidomide and its derivative immunomodulatory drugs, which act directly on tumor cells to induce apoptosis or G1 growth arrest, alter tumor cell adhesion to marrow stroma, inhibit angiogenesis, and trigger a cellular anti-tumor response. The proteasome inhibitors both act directly on tumor cells and also inhibit the transcription factor NFκB-dependent upregulation of IL-6 secretion triggered by tumor cell adhesion. Second, delineation of both growth and apoptotic pathways has derived novel treatment strategies. Third, the preclinical basis and early clinical trial results using vaccination and adoptive immunotherapy to harness autoimmune and alloimmune anti-myeloma responses are presented. This review sets the stage for an evolving new biologically based treatment paradigm in myeloma targeting both the tumor and its microenvironment to improve outcome and achieve eventual cure.

Tumor necrosis factor alpha regulates alpha(v)beta5 integrin expression by osteoclast precursors in vitro and in vivo

Early osteoclast precursors, in the form of murine bone marrow macrophages (BMMs), while expressing no detectable alpha(v)beta3 integrin, contain abundant alpha(v)beta5 and attach to matrix in an alpha(v) integrin-dependent manner. Furthermore, alpha(v)beta5 expression by osteoclast precursors progressively falls as they assume the resorptive phenotype. We find the osteoclastogenic agent, tumor necrosis factor-alpha, (TNF) down-regulates alpha(v)beta5 expression by BMMS via attenuation of beta5 messenger RNA (mRNA) t1/2. Using BMMs from TNF receptor knockout mice we establish the p55 receptor transmits the beta5 suppressive effect. The functional implications of TNF-mediated alpha(v)beta5 down-regulation are underscored by the capacity of an alpha(v) inhibitory peptide mimetic to prevent spreading by BMMs expressing abundant alpha(v)beta5 while failing to impact those in which the integrin has been diminished by TNF. Finally, beta5 mRNA in BMMs of wild-type mice administered lipopolysaccharide (LPS) progressively falls with time of in vivo treatment. Alternatively, beta5 mRNA does not decline in BMMs of LPS-treated mice lacking both TNF receptors, documenting down-regulation of the beta5 integrin subunit, in vivo, is mediated by TNF. Thus, matrix attachment of osteoclast precursors and mature osteoclasts are governed by distinct alpha(v) integrins which are differentially regulated by specific cytokines.

Multiple Myeloma: New Insights and Therapeutic Approaches

This review discusses the evolution of novel diagnostic and treatment strategies for multiple myeloma based upon increased understanding of basic disease pathogenesis. Although myeloma has remained an incurable illness to date, these new developments will derive treatments to improve outcome and achieve eventual cure.

In Section I, Dr. Kyle reviews the results of current therapy for multiple myeloma, including high dose therapy and stem cell transplantation which have proven to achieve improved response rates, event-free, and overall survival. Supportive therapy, such as erythropoietin to treat disease-related anemia, and methods of prophylaxis against infection, which both lessen toxicities of treatment and improve quality of life for patients, are also addressed.

In Section II, Dr. Dalton with Drs. Landowski, Shain, Jove and Hazlehurst discusses mechanisms of drug resistance in myeloma, with emphasis on novel treatment approaches to prevent development of drug resistance and to overcome drug resistance. Laboratory studies delineating mechanisms whereby myeloma cells resist drug-induced apoptosis provide the framework for related treatment protocols for patients with refractory disease.

In Section III, Dr. Berenson reviews the management of complications in bone, which occur in the majority of patients with myeloma and are the major cause of decreased quality of life. New insights into the mediators of bone resorption and new bone formation in the marrow milieu have already derived effective bisphosphonate therapy. These drugs not only reduce bone complications and related pain, thereby improving quality of life, but also may have intrinsic anti-tumor activity by virtue of inducing tumor cell adherence to marrow, reducing interleukin-6 secretion, inducing tumor cell apoptosis, or inhibiting angiogenesis.

In the last section, Dr. Anderson explores the potential for future therapies which offer great promise to improve patient outcomes. First, drugs which alter the marrow microenvironment include thalidomide and its derivative immunomodulatory drugs, which act directly on tumor cells to induce apoptosis or G1 growth arrest, alter tumor cell adhesion to marrow stroma, inhibit angiogenesis, and trigger a cellular anti-tumor response. The proteasome inhibitors both act directly on tumor cells and also inhibit the transcription factor NFκB-dependent upregulation of IL-6 secretion triggered by tumor cell adhesion. Second, delineation of both growth and apoptotic pathways has derived novel treatment strategies. Third, the preclinical basis and early clinical trial results using vaccination and adoptive immunotherapy to harness autoimmune and alloimmune anti-myeloma responses are presented. This review sets the stage for an evolving new biologically based treatment paradigm in myeloma targeting both the tumor and its microenvironment to improve outcome and achieve eventual cure.

Reduction in atherosclerotic lesion size in pigs by alphaVbeta3 inhibitors is associated with inhibition of insulin-like growth factor-I-mediated signaling

Insulin-like growth factor-I (IGF-I) is a potent stimulant of smooth muscle cell (SMC) migration and proliferation and has been implicated in the development of experimental atherosclerotic lesions. Because optimal stimulation of SMC in vitro by IGF-I requires ligand occupancy of alphaVbeta3, these studies were conducted to determine whether alphaVbeta3 antagonists would result in a change in lesion size and whether they could alter IGF-I-mediated actions. Clamps were placed on the carotid and femoral arteries of normal pigs that had been fed a high-cholesterol diet for 4 weeks. alphaVbeta3 inhibitors (SC-69000, SC-65811) (10(-6) mol/L) or saline were infused for 2 weeks into the peristenotic area. Lesion area, the number of SMC layers, and proliferating cell nuclear antigen positive cells were determined in a 1.2-mm segment of each artery. Lesion areas were 304 788+/-113 453 micron(2) (saline), compared with 149 799+/-35 456 micron(2) (SC-69000) (P<0.01). Lesion areas in arteries treated with SC-64258, a compound that does not bind to alphaVbeta3, were 310 284+/-160 467 micron(2), P=not significant. In a second experiment, lesion areas were 110 391+/-17 347 micron(2) (saline) and 59 533+/-17 568 micron(2) (SC-65811, P<0.001). Neointimal SMC layers were reduced by SC-65811 from 7.4+/-4.5 to 3.0+/-0.4 (P<0.001). To determine whether IGF-I action was altered, IGF binding protein-5, which is synthesized in response to IGF-I, was analyzed. IGF-I binding protein-5 mRNA abundance was reduced by 67+/-8% in the 6 lesions treated with SRL-69000 compared with saline controls (P<0.001). We conclude that alphaVbeta3 antagonists block the development of lesions in pigs that have been induced by a high-cholesterol diet and stenosis, and the effect of these compounds is associated with their ability to inhibit IGF-I-mediated signaling.

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).

Synthetic alphaVbeta3 antagonists inhibit insulin-like growth factor-I-stimulated smooth muscle cell migration and replication

Porcine aortic smooth cells respond to insulin-like growth factor-I (IGF-I) with increases in DNA synthesis and cell migration. Because ligand occupancy of the alphaVbeta3 integrin has been shown to be necessary for IGF-I to stimulate maximal increases in both processes, we determined whether synthetic alphaVbeta3 antagonists could inhibit IGF-I-stimulated actions on this cell type. Low-molecular-weight compounds that had been selected based on their ability to compete with vitronectin for binding to purified human alphaVbeta3 in vitro were analyzed for their ability to compete with 125I-kistrin (a known ligand for porcine alphaVbeta3) for binding to porcine alphaVbeta3. Nine compounds were screened, and five were found to be potent competitive inhibitors. The most potent compound, SC-69000, resulted in 88% competition at 10(-7) M and was nearly equipotent with echistatin. The compounds that were the most potent inhibitors of kistrin binding were tested for their capacity to inhibit the cell migration response to IGF-I. Three compounds caused between 81-88% inhibition of IGF-I-stimulated migration at 10(-7) M. To determine whether these compounds could inhibit other IGF-I-stimulated actions, their ability to inhibit IGF-I-stimulated [3H]-thymidine incorporation into DNA was analyzed. The four compounds that were the most potent inhibitors of cell migration also inhibited IGF-I-stimulated DNA replication. IGF-I stimulates the synthesis of IGF binding protein-5 by these cells. Preincubation with the four most active compounds also resulted in significant inhibition of the ability of IGF-I to stimulate IGF binding protein-5 synthesis. AlphaVbeta3 occupancy by the ligand vitronectin has been shown to enhance the capacity of IGF-I to activate its receptor tyrosine kinase. The four most active compounds were shown to inhibit IGF-I-stimulated IGF-I receptor autophosphorylation. These findings suggest that blockade of ligand occupancy of the alphaVbeta3 integrin globally inhibits several IGF-I-stimulated biologic actions and that synthetic inhibitors are very active in this regard. Because these compounds can be administered to whole animals, they should be very useful in determining whether blocking alphaVbeta3 occupancy in vivo results in alteration in responsiveness to IGF-I.

Regulation of osteoclast activity

Osteoclasts are the primary cell type responsible for bone resorption. This paper reviews many of the known regulators of osteoclast activity, including hormones, cytokines, ions, and arachidonic acid metabolites. Most of the hormones and cytokines that inhibit osteoclast activity act directly on the osteoclasts. In contrast, most of the hormones and cytokines that stimulate osteoclast activity act indirectly through osteoblasts. Particularly interesting in this regard are agents that directly inhibit activity of highly purified osteoclasts yet stimulate activity of osteoclasts that are co-cultured with osteoblasts. Recent studies have demonstrated that the primary mechanism by which bone resorptive agents stimulate osteoclast activity indirectly is likely to be up-regulation of production of osteoclast differentiation factor/osteoprotegerin ligand (ODF/OPGL) by the osteoblasts. In addition to discussing regulators of osteoclast activity per se, this paper also reviews the role of osteoclast apoptosis to limit the extent of bone resorption.

Cloning, sequence analysis, and chromosomal localization of the novel human integrin alpha11 subunit (ITGA11)

The integrins are a large family of cell adhesion molecules consisting of noncovalently associated alphabeta heterodimers. We have cloned and sequenced the cDNA of a novel human integrin alpha-subunit, designated alpha11. The alpha11 cDNA encodes a mature protein with a large 1120-residue extracellular domain that contains an I-domain of 207 residues and is linked by a transmembrane domain to a short cytoplasmic domain of 24 amino acids. The deduced alpha11 protein shows the typical structural features of integrin alpha-subunits and is similar to a distinct group of alpha-subunits from collagen-binding integrins. However, it differs from most integrin alpha-chains by an incompletely preserved cytoplasmic GFFKR motif. The human ITGA11 gene was localized to bands q22.3-q23 on chromosome 15, and its transcripts were found in a variety of tissues, but predominantly in bone, cartilage, cardiac muscle, and skeletal muscle. Expression of a 5.5-kb alpha11 mRNA was detectable in small intestine.

Mechanisms and consequences of affinity modulation of integrin alpha(V)beta(3) detected with a novel patch-engineered monovalent ligand

Integrin alpha(V)beta(3) mediates diverse responses in vascular cells, ranging from cell adhesion, migration, and proliferation to uptake of adenoviruses. However, the extent to which alpha(V)beta(3) is regulated by changes in receptor conformation (affinity), receptor diffusion/clustering (avidity), or post-receptor events is unknown. Affinity regulation of the related integrin, alpha(IIb)beta(3), has been established using a monovalent ligand-mimetic antibody, PAC1 Fab. To determine the role of affinity modulation of alpha(V)beta(3), a novel monovalent ligand-mimetic antibody (WOW-1) was created by replacing the heavy chain hypervariable region 3 of PAC1 Fab with a single alpha(V) integrin-binding domain from multivalent adenovirus penton base. Both WOW-1 Fab and penton base bound selectively to activated alpha(V)beta(3), but not to alpha(IIb)beta(3), in receptor and cell binding assays. alpha(V)beta(3) affinity varied with the cell type. Unstimulated B-lymphoblastoid cells bound WOW-1 Fab poorly (apparent K(d) = 2.4 microM), but acute stimulation with phorbol 12-myristate 13-acetate increased receptor affinity >30-fold (K(d) = 80 nM), with no change in receptor number. In contrast, alpha(V)beta(3) in melanoma cells was constitutively active, but ligand binding could be suppressed by overexpression of beta(3) cytoplasmic tails. Up-regulation of alpha(V)beta(3) affinity had functional consequences in that it increased cell adhesion and spreading and promoted adenovirus-mediated gene transfer. These studies establish that alpha(V)beta(3) is subject to rapid regulated changes in affinity that influence the biological functions of this integrin.

Osteopontin-deficient mice are resistant to ovariectomy-induced bone resorption

Osteopontin is one of the major noncollagenous bone matrix proteins produced by osteoblasts and osteoclasts, bone cells that are uniquely responsible for the remodeling of mineralized tissues. Osteoclasts express the alphavbeta3 integrin, which is one of the receptors for osteopontin. Recent knockout studies revealed that noncollagenous bone matrix proteins are functionally important in regulation of bone metabolism. However, the significance of the presence of osteopontin in in vivo has not been known. We report here that osteopontin knockout mice are resistant to ovariectomy-induced bone resorption compared with wild-type mice. Microcomputed tomography analysis indicated about 60% reduction in bone volume by ovariectomy in wild-type mice, whereas the osteopontin-deficient mice exhibited only about 10% reduction in trabecular bone volume after ovariectomy. Reduction in uterine weight was observed similarly in both wild-type and osteopontin-deficient mice, indicating the specificity of the effect of osteopontin deficiency on bone metabolism. We propose that osteopontin is essential for postmenopausal osteoporosis in women. Strategies to counteract osteopontin's action may prove effective in suppressing osteoporosis.

Orally bioavailable nonpeptide vitronectin receptor antagonists with efficacy in an osteoporosis model

A new series of potent nonpeptide vitronectin receptor antagonists, based on a novel carbocyclic Gly-Asp mimetic, has been discovered. A representative of this series, SB 265123 (4), has 100% oral bioavailability in rats, and is orally active in vivo in the ovariectomized rat model of osteoporosis.

Rapid synthesis of RGD mimetics with isoxazoline scaffolds on solid phase: identification of alphavbeta3 antagonists lead compounds

Isoxazoline containing RGD mimetics were rapidly synthesized on a solid phase to optimize linkers, regioisomers of isoxazoline scaffolds, and exosite binding groups to yield lead alphavbeta3 antagonists.

Mapping the integrin alpha V beta 3-ligand interface by photoaffinity cross-linking

Integrins are cell surface adhesion molecules involved in mediating cell-extracellular matrix interactions. High-resolution structural data are not available for these heterodimeric receptors. Previous cross-linking studies of integrins aimed at elucidating the nature of the receptor-ligand interface have been limited to identification of relatively large binding domains. To create reagents for "photoaffinity scanning" of the RGD-binding site of human integrin alpha V beta 3, new conformationally constrained ligands were designed. These photoreactive ligands are based on cyclo Ac-[Cys-Asn-Dmt-Arg-Gly-Asp-Cys]-OH, which displays an affinity of 50 nM for alpha V beta 3. This molecular scaffold was modified at the C-terminus by a benzophenone-containing amino acid residue, L-4-benzoylphenylalanine (Bpa). At the N-terminus, a molecular tag was introduced in the form of radioactive iodine or biotin. The newly designed tagged photoreactive RGD-containing ligands display an affinity of 0.5-0.7 microM for alpha V beta 3, and cross-link efficiently and specifically to the receptor. A 100 kDa band corresponding to the beta 3 subunit-ligand conjugate was detected as the major cross-linking product. Cross-linking was dependent upon the presence of Ca2+ and Mg2+ ions, and was competitively inhibited by a nonphotoreactive ligand. Enzymatic and chemical digestions of the radiolabeled photoconjugate enabled identification of a 20-amino acid fragment between positions 99 and 118 in the beta 3 chain of the integrin as the contact domain for ligand at a site adjacent to the C-terminal portion of the RGD triad.

Bone resorption induced by parathyroid hormone is strikingly diminished in collagenase-resistant mutant mice

Parathyroid hormone (PTH) stimulates bone resorption by acting directly on osteoblasts/stromal cells and then indirectly to increase differentiation and function of osteoclasts. PTH acting on osteoblasts/stromal cells increases collagenase gene transcription and synthesis. To assess the role of collagenase in the bone resorptive actions of PTH, we used mice homozygous (r/r) for a targeted mutation (r) in Col1a1 that are resistant to collagenase cleavage of type I collagen. Human PTH(1-34) was injected subcutaneously over the hemicalvariae in wild-type (+/+) or r/r mice four times daily for three days. Osteoclast numbers, the size of the bone marrow spaces and periosteal proliferation were increased in calvariae from PTH-treated +/+ mice, whereas in r/r mice, PTH-induced bone resorption responses were minimal. The r/r mice were not resistant to other skeletal effects of PTH because abundant interstitial collagenase mRNA was detected in the calvarial periosteum of PTH-treated, but not vehicle-treated, r/r and +/+ mice. Calcemic responses, 0.5-10 hours after intraperitoneal injection of PTH, were blunted in r/r mice versus +/+ mice. Thus, collagenase cleavage of type I collagen is necessary for PTH induction of osteoclastic bone resorption.

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.

Beta3-integrin-deficient mice are a model for Glanzmann thrombasthenia showing placental defects and reduced survival

beta3 integrins have been implicated in a wide variety of functions, including platelet aggregation and thrombosis (alphaIIbbeta3) and implantation, placentation, angiogenesis, bone remodeling, and tumor progression (alphavbeta3). The human bleeding disorder Glanzmann thrombasthenia (GT) can result from defects in the genes for either the alphaIIb or the beta3 subunit. In order to develop a mouse model of this disease and to further studies of hemostasis, thrombosis, and other suggested roles of beta3 integrins, we have generated a strain of beta3-null mice. The mice are viable and fertile, and show all the cardinal features of GT (defects in platelet aggregation and clot retraction, prolonged bleeding times, and cutaneous and gastrointestinal bleeding). Implantation appears to be unaffected, but placental defects do occur and lead to fetal mortality. Postnatal hemorrhage leads to anemia and reduced survival. These mice will allow analyses of the other suggested functions of beta3 integrins and we report that postnatal neovascularization of the retina appears to be beta3-integrin-independent, contrary to expectations from inhibition experiments.

Decreased angiogenesis and arthritic disease in rabbits treated with an alphavbeta3 antagonist

Rheumatoid arthritis (RA) is an inflammatory disease associated with intense angiogenesis and vascular expression of integrin alphavbeta3. Intra-articular administration of a cyclic peptide antagonist of integrin alphavbeta3 to rabbits with antigen-induced arthritis early in disease resulted in inhibition of synovial angiogenesis and reduced synovial cell infiltrate, pannus formation, and cartilage erosions. These effects were not associated with lymphopenia or impairment of leukocyte function. Furthermore, when administered in chronic, preexisting disease, the alphavbeta3 antagonist effectively diminished arthritis severity and was associated with a quantitative increase in apoptosis of the angiogenic blood vessels. Therefore, angiogenesis appears to be a central factor in the initiation and persistence of arthritic disease, and antagonists of integrin alphavbeta3 may represent a novel therapeutic strategy for RA.