Alphavbeta3-integrin imaging: a new approach to characterise angiogenesis?

OVERVIEW

The field of angiogenesis research is one of the most rapidly growing biomedical disciplines. Great efforts are being made to develop anti-angiogenesis drugs for treatment of cancer as well as non-oncological diseases. Thus, imaging techniques allowing non-invasive monitoring of corresponding molecular processes will be of great interest. One target structure involved in the angiogenic process is the integrin alphavbeta3, which mediates the migration of activated endothelial cells during vessel formation.

MATERIALS AND METHODS

A variety of radiolabelled RGD peptides have been introduced for monitoring of alphavbeta3 expression using nuclear medicine tracer techniques.

OBJECTIVES

This review discusses tracer development and highlights some strategies for tracer optimisation. It summarises the preclinical and clinical data and discusses the potential of this class of tracer to characterise angiogenesis.

Imaging of Vx-2 rabbit tumors with alpha(nu)beta3-integrin-targeted 111In nanoparticles

Earlier tumor detection can improve 5-year survival of patients, particularly among those presenting with cancers less than 1 cm in diameter. alpha(nu)beta(3)-Targeted (111)In nanoparticles (NP) were developed and studied for detection of tumor angiogenesis. Studies were conducted in New Zealand white rabbits implanted 12 days earlier with Vx-2 tumor. alpha(nu)beta(3)-Targeted (111)In/NP bearing approximately 10 (111)In/NP vs. approximately 1 (111)In/NP nuclide payloads were compared to nontargeted radiolabeled control particles. In vivo competitive binding studies were used to assess ligand-targeting specificity. alpha(nu)beta(3)-Integrin-targeted NP with approximately 10 (111)In/NP provided better (p < 0.05) tumor-to-muscle ratio contrast (6.3 +/- 0.2) than approximately 1 (111)In/NP (5.1 +/- 0.1) or nontargeted particles with approximately 10 (111)In/NP (3.7 +/- 0.1) over the initial 2-hr postinjection. At 18 hr, mean tumor activity in rabbits receiving alpha(nu)beta(3)-integrin-targeted NP was 4-fold higher than the nontargeted control. Specificity of the NP for the tumor neovasculature was supported by in vivo competition studies and by fluorescence microscopy of alpha(nu)beta(3)-targeted fluorescent-labeled NP. Biodistribution studies revealed that the primary clearance organ in rabbits as a %ID/g tissue was the spleen. Circulatory half-life (t(1/2)beta) was estimated to be approximately 5 hr using a 2-compartment model. alpha(nu)beta(3)-Targeted (111)In perfluorocarbon NP may provide a clinically useful tool for sensitively detecting angiogenesis in nascent tumors, particularly in combination with secondary high-resolution imaging modalities, such as MRI.

Evaluation of a99mTc-Labeled Cyclic RGD Tetramer for Noninvasive Imaging Integrin αvβ3-Positive Breast Cancer

Integrin alphavbeta3 plays a critical role in tumor angiogenesis and metastasis. Radiolabeled RGD peptides that are integrin alphavbeta3-specific are very useful for noninvasive imaging of integrin expression in rapidly growing and metastatic tumors. In this study, we determined the binding affinity of E{E[c(RGDfK)]2}2 (tetramer) and its 6-hydrazinonicotinamide conjugate (HYNIC-tetramer) against the binding of 125I-echistatin to the integrin alphavbeta3-positive MDA-MB-435 breast cancer cells. The athymic nude mice bearing MDA-MB-435 xenografts were used to evaluate the potential of ternary ligand complex [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] (TPPTS = trisodium triphenylphosphine-3,3',3' '-trisulfonate) as a new radiotracer for imaging breast cancer integrin alphavbeta3 expression by single photon emission computed tomography (SPECT). It was found that the binding affinity of tetramer (IC50 = 51 +/- 11 nM) was slightly higher than that of its dimeric analogue (IC50 = 78 +/- 27 nM) and is comparable to that of the HYNIC-tetramer conjugate (IC50 = 55 +/- 11 nM) within the experimental error. Biodistribution data showed that [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] had a rapid blood clearance (4.61 +/- 0.81 %ID/g at 5 min postinjection (p.i.) and 0.56 +/- 0.12 %ID/g at 120 min p.i.) and was excreted mainly via the renal route. [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] had high tumor uptake with a long tumor retention (5.60 +/- 0.87 %ID/g and 7.30 +/- 1.32 %ID/g at 5 and 120 min p.i., respectively). The integrin alphavbeta3-specificity was demonstrated by co-injection of excess E[c(RGDfK)]2, which resulted in a significant reduction in tumor uptake of the radiotracer. The metabolic stability of [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] was determined by analyzing urine and feces samples from the tumor-bearing mice at 120 min p.i. In the urine, about 20% of [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] remained intact while only approximately 15% metabolized species was detected in feces. SPECT images displayed significant radiotracer localization in tumor with good contrast as early as 1 h p.i. The high tumor uptake and fast renal excretion make [99mTc(HYNIC-tetramer)(tricine)(TPPTS)] a promising radiotracer for noninvasive imaging of the integrin alphavbeta3-positive tumors by SPECT.

Selective tumor cell targeting using low-affinity, multivalent interactions

This report highlights the advantages of low-affinity, multivalent interactions to recognize one cell type over another. Our goal was to devise a strategy to mediate selective killing of tumor cells, which are often distinguished from normal cells by their higher levels of particular cell surface receptors. To test whether multivalent interactions could lead to highly specific cell targeting, we used a chemically synthesized small-molecule ligand composed of two distinct motifs: (1) an Arg-Gly-Asp (RGD) peptidomimetic that binds tightly (Kd approximately 10(-9)M) to alphavbeta3 integrins and (2) the galactosyl-alpha(1-3)galactose (alpha-Gal epitope), which is recognized by human anti-alpha-galactosyl antibodies (anti-Gal). Importantly, anti-Gal binding requires a multivalent presentation of carbohydrate residues; anti-Gal antibodies interact weakly with the monovalent oligosaccharide (Kd approximately 10(-5)M) but bind tightly (Kd approximately 10(-11) M) to multivalent displays of alpha-Gal epitopes. Such a display is generated when the bifunctional conjugate decorates a cell possessing a high level of alphavbeta3 integrin; the resulting cell surface, which presents many alpha-Gal epitopes, can recruit anti-Gal, thereby triggering complement-mediated lysis. Only those cells with high levels of the integrin receptor are killed. In contrast, doxorubicin tethered to the RGD-based ligand affords indiscriminate cell death. These results highlight the advantages of exploiting the type of the multivalent recognition processes used by physiological systems to discriminate between cells. The selectivity of this strategy is superior to traditional, abiotic, high-affinity targeting methods. Our results have implications for the treatment of cancer and other diseases characterized by the presence of deleterious cells.

Near-infrared fluorescence imaging of tumor integrin alpha v beta 3 expression with Cy7-labeled RGD multimers

PURPOSE

Cell adhesion molecule integrin alpha v beta 3 is an excellent target for tumor interventions because of its unique expression on the surface of several types of solid tumor cells and on almost all sprouting tumor vasculatures. Here, we describe the development of near-infrared (NIR) fluorochrome Cy7-labeled RGD peptides for tumor integrin targeting.

PROCEDURES

Mono-, di-, and tetrameric RGD peptides were synthesized and conjugated with Cy7. The integrin specificity of these fluorescent probes was tested in vitro for receptor binding assay and fluorescence microscopy and in vivo for subcutaneous U87MG tumor targeting.

RESULTS

The tetrameric RGD peptide probe with the highest integrin affinity showed the highest tumor activity accumulation and strongest tumor-to-normal tissue contrast. This uptake is integrin-specific as the signal accumulated in the tumor can be effectively blocked by unconjugated RGD peptide antagonist of integrin alpha v beta 3.

CONCLUSIONS

Noninvasive NIR fluorescence imaging is able to detect and semiquantify tumor integrin expression based upon the highly potent tetrameric RGD peptide probe.

Nanoparticle imaging of integrins on tumor cells

Nanoparticles 10 to 100 nm in size can deliver large payloads to molecular targets, but undergo slow diffusion and/or slow transport through delivery barriers. To examine the feasibility of nanoparticles targeting a marker expressed in tumor cells, we used the binding of cyclic arginine-glycine-aspartic acid (RGD) nanoparticle targeting integrins on BT-20 tumor as a model system. The goals of this study were: 1) to use nanoparticles to image alpha(V)beta3 integrins expressed in BT-20 tumor cells by fluorescence-based imaging and magnetic resonance imaging, and, 2) to identify factors associated with the ability of nanoparticles to target tumor cell integrins. Three factors were identified: 1) tumor cell integrin expression (the alpha(V)beta3 integrin was expressed in BT-20 cells, but not in 9L cells); 2) nanoparticle pharmacokinetics (the cyclic RGD peptide cross-linked iron oxide had a blood half-life of 180 minutes and was able to escape from the vasculature over its long circulation time); and 3) tumor vascularization (the tumor had a dense capillary bed, with distances of <100 microm between capillaries). These results suggest that nanoparticles could be targeted to the cell surface markers expressed in tumor cells, at least in the case wherein the nanoparticles and the tumor model have characteristics similar to those of the BT-20 tumor employed here.

The adhesion molecule L1 (CD171) promotes melanoma progression

The adhesion molecule L1 is expressed in primary melanomas and cutaneous metastases in contrast to melanocytic nevi and melanocytes, and is significantly associated with metastatic spread. Recent studies have demonstrated that in carcinomas L1 expression is associated with sustained activation of the extracellular signal-regulated kinase (ERK) pathway and upregulation of ERK-dependent, motility- and invasion-associated gene products including alphavbeta3 integrin. The objective of this study was to further investigate the role of the adhesion molecule L1 in melanoma progression, and to evaluate whether targeting the L1 adhesion molecule would have therapeutic effects against invasive melanoma growth. Using human melanoma cells from different stages of progression in monolayer and organotypic human skin culture mimicking the pathophysiological environment of cutaneous melanoma, we found that (1) L1 expression mostly correlates with melanoma progression and alphavbeta3 integrin expression, (2) overexpression of L1 in early radial growth phase melanoma cells promotes conversion from radial to vertical growth phase melanoma without upregulation of alphavbeta3 integrin expression, and (3) suppression of L1 function significantly reduces migration and invasion of melanoma cells, but does not completely block invasive melanoma growth. Altogether, L1 plays a critical role in melanoma invasion and progression and offers therapeutic potential in combination with conventional anticancer agents.

Positron Emission Tomography Using [18F]Galacto-RGD Identifies the Level of Integrin αvβ3 Expression in Man

PURPOSE

The integrin alpha(v)beta3 plays a key role in angiogenesis and tumor cell metastasis and is therefore an important target for new therapeutic and diagnostic strategies. We have developed [18F]Galacto-RGD, a highly alpha(v)beta3-selective tracer for positron emission tomography (PET). Here, we show, in man, that the intensity of [18F]Galacto-RGD uptake correlates with alpha(v)beta3 expression.

EXPERIMENTAL DESIGN

Nineteen patients with solid tumors (musculoskeletal system, n = 10; melanoma, n = 4; head and neck cancer, n = 2; glioblastoma, n = 2; and breast cancer, n = 1) were examined with PET using [18F]Galacto-RGD before surgical removal of the tumor lesions. Snap-frozen specimens (n = 26) were collected from representative areas with low and intense standardized uptake values (SUV) of [18F]Galacto-RGD. Immunohistochemistry was done using the alpha(v)beta3-specific antibody LM609. Intensity of staining (graded on a four-point scale) and the microvessel density of alpha(v)beta3-positive vessels were determined and correlated with SUV and tumor/blood ratios (T/B).

RESULTS

Two tumors showed no tracer uptake (mean SUV, 0.5 +/- 0.1). All other tumors showed tracer accumulation with SUVs ranging from 1.2 to 10.0 (mean, 3.8 +/- 2.3; T/B, 3.4 +/- 2.2; tumor/muscle ratio, 7.7 +/- 5.4). The correlation of SUV and T/B with the intensity of immunohistochemical staining (Spearman's r = 0.92; P < 0.0001) as well as with the microvessel density (Spearman's r = 0.84; P < 0.0001) were significant. Immunohistochemistry confirmed lack of alpha(v)beta3 expression in normal tissue (benign lymph nodes, muscle) and in the two tumors without tracer uptake.

CONCLUSIONS

Molecular imaging of alpha(v)beta3 expression with [18F]Galacto-RGD in humans correlates with alpha(v)beta3 expression as determined by immunohistochemistry. PET with [18F]Galacto-RGD might therefore be used as a new marker of angiogenesis and for individualized planning of therapeutic strategies with alpha(v)beta3-targeted drugs.

Adenoviral-encoded antigens are presented efficiently by a subset of dendritic cells expressing high levels of alpha(v)beta3 integrins

Dendritic cells (DC) play a central role in antigen presentation and are often targeted by adenoviral (Ad)-based gene therapy. However, DC lack the coxsackie-Ad receptor, and little is known about the process by which they acquire and present Ad-encoded antigens. We examined the expression of alpha(v)beta3 integrins (CD51/CD61) on mouse bone marrow-derived DC (BM-DC) and their susceptibility to transduction by Ad vectors. Less than 10% of BM-DC precursors expressed CD51, but expression increased over time in culture with granulocyte macrophage-colony stimulating factor (GM-CSF)/interleukin (IL)-4. After 7 days, 28 +/- 1.7% of CD11c+ DC expressed high levels of CD51 (CD51(hi)), and the remaining DC expressed low levels of CD51 (CD51(lo)). CD51(hi) CD express higher major histocompatibility complex type 1 (MHC I); however, both of the DC subsets expressed similar levels of MHC II and costimulatory molecules. When exposed to a first-generation Ad vector, transgene expression was restricted to the CD51(hi) DC subset and blocked by soluble peptides expressing an arginine, glycine, aspartic acid (RGD) sequence, confirming the role of integrins in viral entry. Consistent with this, a modified Ad expressing an RGD-binding sequence in its fiber knob (Ad-RGD) transduced the CD51(hi) DC subset with significantly higher efficiency. When BM-DC were transduced with an Ad-expressing ovalbumin (Ad-OVA), the CD51(hi) subset proved superior in activating OT-I (T cell receptor-OVA) T cells. Similar to in vitro effects, systemic administration of GM-CSF/IL-4 increased the expression of CD51 on splenic DC and rendered these cells susceptible to Ad transduction. These results suggest that a limited subset of DC expressing high levels of alpha(v)beta3 integrins is preferentially transduced by Ad vectors and activates CD8+ T cell responses against Ad-encoded antigens.

The involvement of osteopontin and its receptors in multiple myeloma cell survival, migration and invasion in the murine 5T33MM model

Multiple myeloma (MM) is a malignancy characterised by the accumulation of monoclonal plasma cells in the bone marrow. Different reports indicate the expression of CD44 variant isoforms (CD44v) by MM cells. Osteopontin (OPN), which is expressed by MM cells, is known to be a ligand for CD44v6. In this study, we investigated the role of OPN with emphasis on a functional correlation between OPN and CD44v in the 5T33MM model. Our group reported the expression of CD44v by 5T33MM cells. Using this model, we have demonstrated the secretion of OPN by 5T33MM cells. OPN affected 5T33MM cell survival by increasing proliferation and inhibiting apoptosis. OPN also stimulated 5T33MM cell migration, transendothelial migration and matrix metalloproteinase-9 activity. We confirmed the proliferative and migratory effects of OPN on human MM cells. By applying inhibiting anti-CD44v6 antibodies, we found that OPN stimulated cell proliferation by engaging this isoform. Anti-CD44v antibodies and RGD peptides both inhibited cell migration, suggesting an involvement of both, CD44v isoforms and integrins. In conclusion, OPN may act as a mediator of MM cell survival by engaging CD44v. The protein is further involved in migration and invasion of MM cells through the activation of either alphavbeta3 integrin or CD44v isoforms.

Use of surface-enhanced Raman spectroscopy for the detection of human integrins

Current research has revealed the importance of a class of cell surface proteins called integrins in various vital physiological functions such as blood clotting, regulation of blood pressure, tissue blood flow, and vascular remodeling. The key to integrin functionality is its ability to mediate force transmission by interacting with the extracellular matrix and cytoskeleton. In addition, they play a role in signal transduction via their connection with the proteins in focal adhesion (FA) points. To understand the complex mechanism of cell-cell and cell-extracellular matrix (ECM) adhesion that is responsible for these diverse biochemical interactions, it is necessary to identify the integrins on cells and monitor their interaction with various ligands. To this end, for the first time, we employ surface-enhanced Raman spectroscopy (SERS) to detect integrins. The results show the capability using SERS to detect the integrins to the nanomolar concentration regime and to distinguish between two different kinds of integrins, alphaVbeta3 and alpha5beta1, that are present in vascular smooth muscle cells (VSMCs). It is anticipated that the SERS approach will potentially help elucidate the mechanism of integrin-ligand interactions in a variety of phenomena of physiological importance.

Near-infrared fluorescent RGD peptides for optical imaging of integrin alphavbeta3 expression in living mice

Near-infrared fluorescence optical imaging is a powerful technique for studying diseases at the molecular level in preclinical models. We recently reported that monomeric RGD peptide c(RGDyK) conjugated to the NIR fluorescent dye specifically targets integrin receptor both in cell culture and in living subjects. In this report, Cy5.5-conjugated mono-, di-, and tetrameric RGD peptides were evaluated in a subcutaneous U87MG glioblastoma xenograft model in order to investigate the effect of multimerization of RGD peptide on integrin avidity and tumor targeting efficacy. The binding affinities of Cy5.5-conjugated RGD monomer, dimer, and tetramer for alpha(v)beta(3) integrin expressed on U87MG cell surface were determined to be 42.9 +/- 1.2, 27.5 +/- 1.2, and 12.1 +/- 1.3 nmol/L, respectively. All three peptide-dye conjugates had integrin specific uptake both in vitro and in vivo. The subcutaneous U87MG tumor can be clearly visualized with each of these three fluorescent probes. Among them, tetramer displayed highest tumor uptake and tumor-to-normal tissue ratio from 0.5 to 4 h postinjection. Tumor-to-normal tissue ratio for Cy5.5-conjugated RGD monomer, dimer, and tetramer were found to be 3.18 +/- 0.16, 2.98 +/- 0.05, and 3.63 +/- 0.09, respectively, at 4 h postinjection. These results suggest that Cy5.5-conjugated monomeric, dimeric, and tetrameric RGD peptides are all suitable for integrin expression imaging. The multmerization of RGD peptide results in moderate improvement of imaging characteristics of the tetramer, compared to that of the monomer and dimeric counterparts.

Limited rescue of osteoclast-poor osteopetrosis after successful engraftment by cord blood from an unrelated donor

We report on a case of osteoclast-poor osteopetrosis who received a hematopoietic stem cell graft and, despite hematological engraftment, showed little signs of response in the skeletal defect. Clinical and laboratory studies supported the concept that the bone microenvironment remained abnormal, thus reducing the clinical response to transplantation.

INTRODUCTION

Osteopetrosis is a rare genetic disorder characterized by severely reduced bone resorption resulting from a defect in either osteoclast development (osteoclast-poor osteopetrosis) or activation (osteoclast-rich osteopetrosis). Patients with osteoclast-rich osteopetrosis can be rescued by allogenic hematopoietic stem cell transplantation; however, little information exists concerning the success of transplantation as a treatment for osteoclast-poor osteopetrosis. We report on a child with osteoclast-poor osteopetrosis whose diagnosis was delayed, consequently receiving a cord blood transplant from an unrelated donor at the age of 8 years. Engraftment was deemed successful by peripheral blood genotyping, although >3 years after transplantation there was little rescue of the skeletal defect and anemia, and extramedullary hematopoiesis persisted.

MATERIALS AND METHODS

Peripheral blood mononuclear cells from the osteopetrosis patient, before and after transplantation, were used to generate osteoclasts in vitro in the presence of macrophage colony-stimulating factor (M-CSF) and RANKL.

RESULTS

Before transplantation few, small mononuclear osteoclasts formed (F-actin ring-positive cells, co-localizing with vitronectin receptor [alphavbeta3 integrin] and TRACP) associated with occasional, small resorption lacunae. Low levels of collagen C-terminal telopeptide (CTx) fragments were released from these cultures as assessed by ELISA (CrossLaps; patient, 12.85 nM; control, 448.6 nM). In contrast, osteoclasts formed in cultures after transplantation formed to a similar degree to control cultures from healthy individuals: large numbers of osteoclasts containing numerous nuclei were present, and approximately 50% of the surface of bone slices was resorbed, associated with intermediate levels of collagen fragment release (116.48 nM). The culture data reflect the histopathology and radiological findings and also support previous studies showing that neither M-CSF nor RANKL rescues osteoclast-poor osteopetrosis.

CONCLUSIONS

This is the first case reported in which a successful hematopoietic engraftment failed to correct an osteopetrotic skeletal defect, and this finding may be credited to the age at which the child was transplanted.

RGD-based strategies for selective delivery of therapeutics and imaging agents to the tumour vasculature

During the past decade, RGD-peptides have become a popular tool for the targeting of drugs and imaging agents to alphavbeta3-integrin expressing tumour vasculature. RGD-peptides have been introduced by recombinant means into therapeutic proteins and viruses. Chemical means have been applied to couple RGD-peptides and RGD-mimetics to liposomes, polymers, peptides, small molecule drugs and radiotracers. Some of these products show impressive results in preclinical animal models and a RGD targeted radiotracer has already successfully been tested in humans for the visualization of alphavbeta3-integrin, which demonstrates the feasibility of this approach. This review will summarize the structural requirements for RGD-peptides and RGD-mimetics as ligands for alphavbeta3. We will show how they have been introduced in the various types of constructs by chemical and recombinant techniques. The importance of multivalent RGD-constructs for high affinity binding and internalization will be highlighted. Furthermore the in vitro and in vivo efficacy of RGD-targeted therapeutics and diagnostics reported in recent years will be reviewed.

Systemic Activation of Integrin αVβ3 in Donors with Spontaneous Intracerebral Hemorrhage is Associated with Subsequent Development of Vasculopathy in the Heart Transplant Recipient

BACKGROUND

Recipients of hearts from donors with spontaneous intracerebral hemorrhage (ICH) are at increased risk of allograft vasculopathy compared with trauma donors. We have recently shown that the vitronectin receptor (integrin alpha(V)beta3) is upregulated in transplant vasculopathy. We hypothesized that donor ICH is associated with systemic activation of alpha(V)beta3 in the donor before transplantation.

METHODS

We evaluated mRNA expressions of alpha(V)beta3 (TaqMan PCR) in endomyocardial biopsy samples at 1-week post-transplant in 20 recipients from ICH donors and 20 recipients from trauma donors. To investigate whether systemic activation of alpha(V)beta3 was present in the donor before transplantation, alpha(V)beta3 expression was also evaluated in the corresponding donor spleen lymphocytes. All patients underwent serial coronary intravascular ultrasound to evaluate for coronary vasculopathy. The baseline characteristics were similar except for increased donor age in the ICH Group.

RESULTS

The ICH Group showed significant increased mRNA expression of alpha(V)beta3 in the heart biopsy samples (3.8-fold, p = 0.012) and in the corresponding donor spleen lymphocytes (3.5-fold, p = 0.014) compared with the Trauma Group. At 1 year, the ICH Group also showed increased progression of coronary vasculopathy. Multivariate regression analysis found that donor lymphocytic alpha(V)beta3 mRNA expression was independently associated with increased risk of vasculopathy (odds ratio, 1.9; 95% CI, 1.21-3.98, p = 0.03).

CONCLUSIONS

Our report demonstrates the presence of systemic activation of alpha(V)beta3 in donors with spontaneous intracerebral hemorrhage and its association with the subsequent development of allograft vasculopathy in the recipient.

Noninvasive Imaging of Angiogenesis With a 99m Tc-Labeled Peptide Targeted at α v β 3 Integrin After Murine Hindlimb Ischemia

Background— Noninvasive imaging strategies play a critical role in assessment of the efficacy of angiogenesis therapies. The α v β 3 integrin is activated in angiogenic vessels and represents a potential target for noninvasive imaging of angiogenesis.

Methods and Results— We evaluated a 99m Tc-labeled peptide (NC100692) targeted at α v β 3 integrin for imaging in an established murine model of angiogenesis induced by hindlimb ischemia. Control mice (n=9) or mice with surgical right femoral artery occlusion (n=29) were injected with NC100692 (1.5±0.2 mCi IV) at different times after femoral occlusion (1, 3, 7, and 14 days) for in vivo pinhole planar gamma camera imaging. Tissue from hindlimb proximal and distal to occlusion was excised for gamma well counting and for immunostaining. On in vivo pinhole images, increased focal NC100692 activity was seen distal to the occlusion at days 3 and 7. This increase in relative NC100692 activity was confirmed by gamma well counting. Lectin staining confirmed increased angiogenesis in the ischemic hindlimb at these time points. A fluorescent analogue of NC100692 was used to confirm specificity and localization of the targeted tracer in cultured endothelial cells. In addition, endothelial cell specificity was confirmed on tissue sections with the use of dual immunofluorescent staining of endothelium and the fluorescent analogue targeted at the α v β 3 integrin.

Conclusions— A 99m Tc-labeled peptide (NC100692) targeted at α v β 3 integrin selectively localized to endothelial cells in regions of increased angiogenesis and could be used for noninvasive serial “hot spot” imaging of angiogenesis. This targeted radiotracer imaging approach is a major advance in tracking therapeutic myocardial angiogenesis and has an important clinical potential.

Ongoing angiogenesis in blood vessels of the abdominal aortic aneurysm

Pathogenesis of the abdominal aortic aneurysm has been attributed to neovascularization of the aortic wall. However, it is not clear whether angiogenesis persists in the aneurysm. In sections of aneurysms, we determined the immunohistochemical distributions of the avb3 integrin, tenascin and endothelial nitric oxide synthase (eNOS), which are markers respectively, of angiogenesis, matrix remodeling and vasoregulatory function. In addition, we used reverse transcription followed by in situ PCR, to determine the distribution of av mRNA. All aneurysm specimens exhibited extensive increases of wall vascularization as compared with the control aortic wall, and showed the presence of perivascular inflammatory exudates containing macrophages and lymphocytes. The neovascularization consisted of thick-walled vessels in the media and adventitia, and capillaries in the subintima. The majority of vessels stained positively for the avb3 antigen and eNOS. Tenascin was deposited as bands that circumscribed thick-walled vessels. The distribution of av mRNA was extensive and was positive even in those vessels that failed to stain for the avb3 protein. No staining was evident in control aortas for the avb3 antigen, tenascin or av mRNA. The upregulation of av mRNA and the avb3 integrin in blood vessels surrounded by a matrix expressing tenascin, indicates that angiogenesis is an ongoing process in the mature aortic aneurysm.

Autocrine and paracrine nitric oxide regulate attachment of human osteoclasts

Nitric oxide (NO) can reduce bone loss in chronic bone diseases. NO inhibits or kills osteoclasts, but the mechanism of action of NO in human bone turnover is not clear. To address this, we studied effects of NO on attachment and motility of human osteoclasts on mineralized and tissue culture substrates under defined conditions. Osteoclasts were differentiated in vitro from CD14 selected monocytes in RANKL and CSF-1, and characterized by cathepsin K expression, tartrate-resistant acid phosphatase (TRAP) activity, acid secretion, and lacunar resorption. Cell attachment was labeled with monoclonal antibody 23C6, specific for a binding domain of a key osteoclast attachment protein, the CD51/CD61 integrin dimer (alpha(v)beta(3)), with or without cell permeabilization. A ring of integrin attachment during bone degradation delimits an extracellular acid compartment, while alpha(v)beta(3) forms focal attachments on non-resorbable substrates. On resorbable substrate but not non-resorbable substrate, alpha(v)beta(3) labeling required cell permeabilization, in keeping with the membrane-matrix apposition that excludes large molecules and allows extracellular acidification. Acid secretion was labeled with the fluorescent weak base indicator lysotracker. NO donors, S-nitroso-N-acetyl penicillamine (SNAP) or sodium nitroprusside (SNP), downmodulated acid secretion simultaneously with cytoskeletal rearrangement, with alpha(v)beta(3) redistributed to a discontinuous pattern that labeled, on bone substrate, without membrane permeabilization. These effects were reversible, and an inhibitor of NO synthesis, N(G)-monomethyl-L-arginine (l-NMMA), increased acid secretion and decreased heterogeneity of attachment structures, showing that NO is an autocrine regulator of attachment. A hydrolysis-resistant activating cGMP analog 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphate replicated effects of NO donors, while an inhibiting analog, 8-(4-chlorophenylthio)guanosine-3',5'-cyclic monophosphorothioate, Rp-isomer, opposed them. On tissue culture or mineralized substrates, NO or cGMP analogs directly regulated motility; after washout cells reattached and survived for days. We conclude that NO is produced by human osteoclasts and regulates acid secretion and cellular motility, in keeping with autocrine and paracrine NO regulation of the resorption cycle.

Noninvasive visualization of the activated alphavbeta3 integrin in cancer patients by positron emission tomography and [18F]Galacto-RGD

BACKGROUND

The integrin alphavbeta3 plays an important role in angiogenesis and tumor cell metastasis, and is currently being evaluated as a target for new therapeutic approaches. Several techniques are being studied to enable noninvasive determination of alphavbeta3 expression. We developed [(18)F]Galacto-RGD, a (18)F-labeled glycosylated alphavbeta3 antagonist, allowing monitoring of alphavbeta3 expression with positron emission tomography (PET).

METHODS AND FINDINGS

Here we show by quantitative analysis of images resulting from a small-animal PET scanner that uptake of [(18)F]Galacto-RGD in the tumor correlates with alphavbeta3 expression subsequently determined by Western blot analyses. Moreover, using the A431 human squamous cell carcinoma model we demonstrate that this approach is sensitive enough to visualize alphavbeta3 expression resulting exclusively from the tumor vasculature. Most important, this study shows, that [(18)F]Galacto-RGD with PET enables noninvasive quantitative assessment of the alphavbeta3 expression pattern on tumor and endothelial cells in patients with malignant tumors.

CONCLUSIONS

Molecular imaging with [(18)F]Galacto-RGD and PET can provide important information for planning and monitoring anti-angiogenic therapies targeting the alphavbeta3 integrins and can reveal the involvement and role of this integrin in metastatic and angiogenic processes in various diseases.

Cell Membrane-Associated MT1-MMP-Dependent Activation of Pro-MMP-2 in A375 Melanoma Cells

Matrix metalloproteinases (MMPs), a family of zinc-dependent endopeptidases, can degrade extracellular matrix components under physiological conditions and during cancer invasion and metastasis. Among the MMPs, the 72 kDa type IV collagenase MMP-2 (gelatinase A) is activated in a membrane-associated manner by an activation complex composed of membrane type 1 matrix metalloproteinase (MT1-MMP), tissue inhibitor of matrixmetalloproteinase-2 (TIMP-2), and pro-MMP-2 in the presence of alphavbeta3 integrin receptor. The activation of pro-MMP-2 correlates with increased occurrence of metastases. Increased MMP-2 activity has been demonstrated in many human tumors. In the present communication, we studied cell surface-associated activation of MMP-2 (72 kDa collagenase type IV) in the moderately metastatic human melanoma cell line A375.

RESULTS

Activation of purified 72 kDa collagenase type IV, pro-MMP-2 from cervical cancer tissue homogenate and from serum-free culture medium of HT1080 cells grown in presence of concanavalin A, by A375 cells, was shown by gelatin zymography. A375 cells activated only pro-MMP-2 from purified MMP-9/MMP-2 mixture indicating that the activation is specific for MMP-2. Activation of MMP-2 and purified collagenase type IV by A375 membrane fraction and membrane extract was also demonstrated by gelatin zymography. When A375 cells were first incubated with anti-MT1-MMP polyclonal antibody, activation of collagenase type IV was significantly decreased, indicating that membrane-associated MMP-2 activation is MT1-MMP-mediated. Immunocytochemistry showed MT1-MMP localization at focal adhesion sites. The presence of the components of activation complex-MT1-MMP and integrin alphavbeta3-were confirmed by Western blot, cell adhesion assay, and integrin subunit assay.

CONCLUSION

Our experimental findings furnish another example of the unique membrane-associated MMP-2 activation mechanism in A375 melanoma cells and clearly indicate the role of MT1-MMP in MMP-2 activation. The information could help in developing new therapies designed to interfere with MMP activation and management of cancer and metastases.

Oxidation of Titanium, RGD Peptide Attachment, and Matrix Mineralization of Rat Bone Marrow Stromal Cells

The aim of this study was to compare the efficacy of attachment of arginine-glycine-aspartic acid (RGD) peptide to titanium surfaces oxidized by different methods. Titanium surfaces were treated as follows: (1) treatment A: passivation in nitric acid, (2) treatment B: heated in air at 400°C for 1 hour, (3) treatment C: immersed in 8.8 M H2O2/0.1 M HCl at 80°C for 30 minutes, and (4) treatment D: treated as in treatment C and then heated at 400°C for 1 hour. RGD was attached to titanium samples treated as in treatments A through D. The quantity of attached RGD was determined by an enzyme-linked immunoabsorbent assay. Mineralization of a rat bone marrow stromal cell (RMSC) culture on the titanium surfaces after 21 days was determined y atomic absorption spectroscopy. The treatments were ranked according to quantity of RGD attached as C, A, B, and D. Twenty-one days after RMSC culture, the degree of mineralization was significantly higher for treatment C than for treatments A, B, and D and for controls. The efficacy of RGD attachment varies with the oxidation treatment given to titanium. Oxidation in H2O2/0.1 M HCl at 80°C provided the best overall surface for RGD attachment as well as calcified matrix formation of RMSCs.

Plasmon-waveguide resonance spectroscopy applied to three potential drug targets: cyclooxygenase-2, hepatitis C virus RNA polymerase and integrin αVβ3

Plasmon-waveguide resonance (PWR) spectroscopy has been used to study the interactions between ligands that correspond to inhibitors, activators or substrates and three integral membrane proteins representing potential drug targets; cyclooxygenases 1 and 2 (COX-1 and -2), integrin alphaVbeta3, and hepatitis C virus RNA polymerase. The proteins were incorporated into an egg phosphatidylcholine bilayer deposited onto the surface of the PWR resonator, and changes in the amplitude and position of the PWR spectra due to mass density increases and conformational transitions have been used to characterize the kinetics and binding affinities corresponding to these interactions. Although the partition of COX-2 into the bilayer was not as efficient as was the case with the other two proteins, sufficient protein could be incorporated to allow ligand binding to be observed. It was also possible to incorporate COX-1 into a lipid bilayer by adding a suspension of microsomal membrane fragments containing this enzyme to a preformed bilayer, and to observe binding of an inhibitory ligand. The interactions between integrin alphaVbeta3 and two ligands with different in vivo efficacies could be distinguished by both spectral measurements and binding kinetics. In the case of the RNA polymerase, the kinetics of PWR spectral changes upon adding a substrate solution to an enzyme-template complex were consistent with those obtained from direct measurements of enzymatic turnover. These experiments demonstrate the utility of PWR spectroscopy to provide novel information regarding drug interactions with membrane proteins in a lipid environment and to distinguish conformational changes induced by binding of various drug molecules.

Evaluation of 64Cu- and 125I-Radiolabeled Bitistatin as Potential Agents for Targeting αvβ3 Integrins in Tumor Angiogenesis

The formation of new blood vessels (angiogenesis) is a feature common to all solid tumors. The integrin receptor alpha(V)beta(3), which is found on endothelial cells lining newly growing blood vessels at a higher density than on mature blood vessels, is being explored as a marker for tumor angiogenesis. Bitistatin, a member of the disintegrin family of polypeptides, has affinity for alpha(V)beta(3) integrins. To determine whether radiolabeled bitistatin could target tumors, its biodistribution was tested in tumor-bearing mice. For initial validation studies, (125)I-bitistatin was injected into BALB/c mice bearing EMT-6 mouse mammary carcinoma tumors, a model that is highly vascular but which lacks alpha(V)beta(3) directly on tumor cells. Tumor uptake reached maximal values (11.7 +/- 4.6 %ID/g) at 2 h. Co-injection of 200 microg of unlabeled bitistatin reduced tumor uptake 62%, suggesting that the binding of (125)I-bitistatin is receptor-mediated. This work was extended to include the beta(+)-emitting radionuclide (64)Cu, which was attached to bitistatin via 1,4,7,10-tetraazacyclododecane-N,N',N' ',N' "-tetraacetic acid (DOTA). This modification did not significantly alter receptor binding in vitro. MicroPET images obtained with (64)Cu-DOTA-bitistatin showed that the tumor could easily be identified 4 h after administering the radiopharmaceutical. The biodistribution of (64)Cu-DOTA-bitistatin differed from the (125)I analogue, in that maximum tumor uptake was nearly 8-fold lower and took at least 6 h to reach maximal binding (1.6 +/- 0.2 %ID/g). As with (125)I-labeled bitistatin, the (64)Cu conjugate showed a 50% reduction in tumor uptake with the co-injection of 200 microg of unlabeled bitistatin (0.8 +/- 0.2 %ID/g). Competition studies with integrin-specific peptides indicated that the tumor uptake was related to both alpha(v)beta(3) and alpha(IIb)beta(3) integrin binding. To see if tumor uptake could be improved upon, (64)Cu was tethered to bitistatin using bromoacetamidobenzyl-1,4,7,10-tetraazacyclododecane-N,N',N' ',N' "-tetraacetic acid (BAD). Tumor uptake for (64)Cu-BAD-2IT-bitistatin was higher than the DOTA conjugate at all time points, reaching a maximum at least 6 h postinjection (5.2 +/- 0.6 %ID/g); however, this was accompanied by higher uptake in nontarget organs at all time points. Radiolabeled ligands of this type may be useful in the targeting of tumor angiogenesis, but the choice of radiolabeling approach has a significant impact on the in vivo properties of the radioligand.

In vitro differentiation of CD14 cells from osteopetrotic subjects: contrasting phenotypes with TCIRG1, CLCN7, and attachment defects

We studied osteoclastic differentiation from normal and osteopetrotic human CD14 cells in vitro. Defects in acid transport, organic matrix removal, and cell fusion with deficient attachment were found. Analysis of genotypes showed that TCIRG1 anomalies correlated with acid transport defects, but surprisingly, organic matrix removal failure correlated with CLCN7 defects; an attachment defect had normal TCIRG1 and CLCN7.

INTRODUCTION

Osteopetrotic subjects usually have normal macrophage activity, and despite identification of genetic defects associated with osteopetrosis, the specific developmental and biochemical defects in most cases are unclear. Indeed, patients with identical genotypes often have different clinical courses. We classified defects in osteoclast differentiation in vitro using four osteopetrotic subjects without immune or platelet defects, three of them severe infantile cases, compared with normals.

MATERIALS AND METHODS

Osteoclast differentiation used isolated CD14 cells; results were correlated with independent analysis of two key genes, CLCN7 and TCIRG1. CD14 cell attachment and cell surface markers and extent of differentiation in RANKL and colony-stimulating factor (CSF)-1 were studied using acid secretion, bone pitting, enzyme, and attachment proteins assays.

RESULTS AND CONCLUSIONS

CD14 cells from all subjects had similar lysosomal and nonspecific esterase activity. With the exception of cells from one osteopetrotic subject, CD14 cells from osteopetrotic and control monocytes attached similarly to bone or tissue culture substrate. Cells from one osteopetrotic subject, with normal CLCN7 and TCIRG1, did not attach to bone, did not multinucleate, and formed no podosomes or actin rings in RANKL and CSF-1. Attachment defects are described in osteopetrosis, most commonly mild osteopetrosis with Glantzman's thrombasthenia. However, this case, with abnormal integrin alphavbeta3 aggregates and no osteoclasts, seems to be unique. Two subjects were compound heterozygotes for TCIRG1 defects; both had CD14 cells that attached to bone but did not acidify attachments; cell fusion and attachment occurred, however, in RANKL and CSF-1. This is consistent with TCIRG1, essential for H+-ATPase assembly at the ruffled border. A compound heterozygote for CLCN7 defects had CD14 cells that fused in vitro, attached to bone, and secreted acid, TRACP, and cathepsin K. However, lacunae were shallow and retained demineralized matrix. This suggests that CLCN7 may not limit H+-ATPase activity as hypothesized, but may be involved in control of organic matrix degradation or removal.

Haemangiomas are formed by cells expressing high levels of alphavbeta3 integrin and lacking acetylated LDL uptake

Haemangiomas are benign tumours occurring in up to 12% of Caucasians, particularly in infancy and childhood. In the present study, two variant cell lines were isolated from murine endothelioma cells. One variant, named t.End.1V(high), represented 16.9% of the parental cell population and was selected by virtue of high expression levels of integrin alphavbeta3 and reduced capacity to endocytose acetylated low-density lipoproteins (Ac-LDLs). A second variant, named t.End.1V(low), represented 38.8% of the parental endothelioma cell line, expressed low levels of alphavbeta3 integrin, and was able to endocytose Ac-LDL. These phenotypic modifications were stable and correlated with specific morphological and functional properties of the two variant cell lines. While the t.End.1V(high) cells induced the formation of large haemangiomas when injected subcutaneously into mice, the t.End.1V(low) cells formed haemangiocytomas. When compared with t.End.1V(low) cells, the t.End.1V(high) cells showed increased migratory capacity, lacked an inflammatory response, and formed cord-like structures in fibrin gels. In contrast, the t.End.1V(low) cells organized into cysts with a lumen in fibrin gels. They rarely formed blood-filled haemangiomas in vivo and recruited host smooth muscle cells, a phenomenon typical for vessel wall maturation of resting cells. These data suggest that Ac-LDL uptake and the level of alphavbeta3 integrin expression are linked to the ability of endothelial cells to form large haemangiomas in vivo.