Cloning and characterization of a novel integrin beta3 subunit

Comparative studies of vertebrate Beta integrin genes and proteins: ancient genes in vertebrate evolution

Intregins are heterodimeric α- and β-subunit containing membrane receptor proteins which serve various cell adhesion roles in tissue repair, hemostasis, immune response, embryogenesis and metastasis. At least 18 α- (ITA or ITGA) and 8 β-integrin subunits (ITB or ITGB) are encoded on mammalian genomes. Comparative ITB amino acid sequences and protein structures and ITB gene locations were examined using data from several vertebrate genome projects. Vertebrate ITB genes usually contained 13-16 coding exons and encoded protein subunits with ~800 amino acids, whereas vertebrate ITB4 genes contained 36-39 coding exons and encoded larger proteins with ~1800 amino acids. The ITB sequences exhibited several conserved domains including signal peptide, extracellular β-integrin, β-tail domain and integrin β-cytoplasmic domains. Sequence alignments of the integrin β-cytoplasmic domains revealed highly conserved regions possibly for performing essential functions and its maintenance during vertebrate evolution. With the exception of the human ITB8 sequence, the other ITB sequences shared a predicted 19 residue α-helix for this region. Potential sites for regulating human ITB gene expression were identified which included CpG islands, transcription factor binding sites and microRNA binding sites within the 3'-UTR of human ITB genes. Phylogenetic analyses examined the relationships of vertebrate beta-integrin genes which were consistent with four major groups: 1: ITB1, ITB2, ITB7; 2: ITB3, ITB5, ITB6; 3: ITB4; and 4: ITB8 and a common evolutionary origin from an ancestral gene, prior to the appearance of fish during vertebrate evolution. The phylogenetic analyses revealed that ITB4 is the most likely primordial form of the vertebrate β integrin subunit encoding genes, that is the only β subunit expressed as a constituent of the sole integrin receptor 'α6β4' in the hemidesmosomes of unicellular organisms.

Synthesis and evaluation of new iRGD peptide analogs for tumor optical imaging

Recently, a disulfide-based cyclic RGD peptide called iRGD, that is, c(CRGDKGPDC), has been reported to interact with both integrin and neuropilin-1 receptors for cellular and deep tissue penetration to improve the imaging sensitivity and therapeutic efficacy. In this study, two new near-infrared fluorescent iRGD conjugates, that is, Ac-Cys(IRDye®800CW)-iRGD (1), and its dual labeling analog DOTA-Cys(IRDye®800CW)-iRGD (2) were synthesized via the specific mercapto-maleimide reaction for tumor imaging. Both 1 and 2 showed significant tumor localization in optical imaging of MDA-MB-435 tumor-bearing mice. The potential of such iRGD compounds in tumor-targeted imaging and drug delivery deserves further exploration.

Analysis of Fyn function in hemostasis and alphaIIbbeta3-integrin signaling

Recent studies have shown that Src-family kinases (SFKs) play an important role in mediating integrin signalling, and the beta3 subunit of alphaIIbbeta3 integrin has been shown to interact with multiple SFK members. Here, we analyzed the interactions and functional consequences of Fyn and Src binding to alphaIIbbeta3. Fyn associated with the beta3 subunit in resting and thrombin-aggregated platelets, whereas interaction between Src and alphaIIbbeta3 was seen predominantly in resting but not in thrombin-aggregated platelets. We have also observed that Fyn but not Src localized to focal adhesions in CHO cells adherent to fibrinogen through alphaIIbbeta3. On the basis of these differences, we wanted to determine the sequence requirements for the interaction of Fyn and Src within the beta3-cytoplasmic domain. Whereas Src association required the C-terminal region of beta3, Fyn continued to interact with mutants that could no longer associate with Src and that contained as few as 13 membrane-proximal amino acids of the beta3-cytoplasmic tail. Using deletion mutants of beta3-cytoplasmic tails expressed as GST-fusion proteins, we narrowed down the Fyn-binding site even further to the amino acid residues 721-725 (IHDRK) of the beta3-cytoplasmic domain. On the basis of these observations, we explored whether Fyn-/- mice exhibited any abnormalities in hemostasis and platelet function. We found that Fyn-/- mice significantly differed in their second bleeding times compared with wild-type mice, and platelets from Fyn-/- mice exhibited delayed spreading on fibrinogen-coated surfaces. Using mutant forms of Fyn, it appears that its kinase activity is required for its localization to focal adhesions and to mediate alphaIIbbeta3-dependent cell spreading. Our results suggest that Fyn and Src have distinct requirements for interaction with alphaIIbbeta3; and, consequently, the two SFK can mediate different functional responses.

Direct Thy-1/alphaVbeta3 integrin interaction mediates neuron to astrocyte communication

Thy-1 is an abundant neuronal glycoprotein of poorly defined function. We recently provided evidence indicating that Thy-1 clusters a beta3-containing integrin in astrocytes to induce tyrosine phosphorylation, RhoA activation and the formation of focal adhesions and stress fibers. To date, the alpha subunit partner of beta3 integrin in DI TNC1 astrocytes is unknown. Similarly, the ability of neuronal, membrane-bound Thy-1 to trigger astrocyte signaling via integrin engagement remains speculation. Here, evidence that alphav forms an alphavbeta3 heterodimer in DI TNC1 astrocytes was obtained. In neuron-astrocyte association assays, the presence of either anti-alphav or anti-beta3 integrin antibodies reduced cell-cell interaction demonstrating the requirement of both integrin subunits for this association. Moreover, anti-Thy-1 antibodies blocked stimulation of astrocytes by neurons but not the binding of these two cell types. Thus, neuron-astrocyte association involved binding between molecular components in addition to the Thy-1-integrin; however, the signaling events leading to focal adhesion formation in astrocytes depended exclusively on the latter interaction. Additionally, wild-type (RLD) but not mutated (RLE) Thy-1 was shown to directly interact with alphavbeta3 integrin by Surface Plasmon Resonance analysis. This interaction was promoted by divalent cations and was species-independent. Together, these results demonstrate that the alphavbeta3 integrin heterodimer interacts directly with Thy-1 present on neuronal cells to stimulate astrocytes.

Beta3 integrins facilitate matrix interactions during transendothelial migration of PC3 prostate tumor cells

BACKGROUND

beta3 integrins play a role in metastatic progression of prostate cancer by mediating adhesion of cancer cells to endothelium and migration through extracellular matrix (ECM). However, the role of beta3 integrins during transendothelial migration (TEM) of prostate tumor cells is poorly understood. We examined the role of beta3 integrins in TEM of PC3 human prostate cancer cells through a monolayer of human lung microvascular endothelial cells (HLMVECs).

METHODS

PC3 cells were challenged with beta3 integrin antibodies or antisense nucleotides and their efficiency to migrate through monolayers of endothelial cells (ECs) was assessed using confocal microscopy.

RESULTS

beta3 integrins in PC3 cells are not localized in focal contacts and their blockade significantly inhibited TEM by over 50% preferentially during late stages of migration. Formation of PC3 cell pseudopodia on matrigel was significantly reduced by beta3 integrin antisense oligonucleotides.

CONCLUSIONS

beta3 integrins play important roles during TEM of PC3 cells while interacting with the matrix underneath the endothelium. These interactions are independent of the ability to cluster beta3 integrins into focal adhesions.

Phenylbutyrates as potent, orally bioavailable vitronectin receptor (integrin alphavbeta3) antagonists

In our continuing efforts to identify small molecule vitronectin receptor antagonists, we have discovered a series of phenylbutyrate derivatives, exemplified by 16, which have good potency and excellent oral bioavailability (approximately 100% in rats). This new series is derived conceptually from opening of the seven-membered ring of SB-265123.

Disruption of C-terminal cytoplasmic domain of betaPS integrin subunit has dominant negative properties in developing Drosophila

We have analyzed a set of new and existing strong mutations in the myospheroid gene, which encodes the betaPS integrin subunit of Drosophila. In addition to missense and other null mutations, three mutants behave as antimorphic alleles, indicative of dominant negative properties. Unlike null alleles, the three antimorphic mutants are synthetically lethal in double heterozygotes with an inflated (alphaPS2) null allele, and they fail to complement very weak, otherwise viable alleles of myospheroid. Two of the antimorphs result from identical splice site lesions, which create a frameshift in the C-terminal half of the cytoplasmic domain of betaPS. The third antimorphic mutation is caused by a stop codon just before the cytoplasmic splice site. These mutant betaPS proteins can support cell spreading in culture, especially under conditions that appear to promote integrin activation. Analyses of developing animals indicate that the dominant negative properties are not a result of inefficient surface expression, or simple competition between functional and nonfunctional proteins. These data indicate that mutations disrupting the C-terminal cytoplasmic domain of integrin beta subunits can have dominant negative effects in situ, at normal levels of expression, and that this property does not necessarily depend on a specific new protein sequence or structure. The results are discussed with respect to similar vertebrate beta subunit cytoplasmic mutations.

Gingival, dermal, and periodontal ligament fibroblasts express different extracellular matrix receptors

BACKGROUND

Fibroblasts are the predominant cells of the periodontal ligament and the gingiva and have important roles in the function and regeneration of the tooth support apparatus. The goal of this study was to investigate the possible differences in the adhesion properties and expression of extracellular matrix (ECM) receptors among different fibroblast populations.

METHODS

The adhesion of gingival (GF), dermal (DF), and periodontal ligament fibroblast (PDLF) cultures to ECM proteins (fibronectin, laminin, vitronectin, RGD peptide, collagen type I, and collagen type IV) adsorbed to tissue culture plastic was evaluated fluorometrically. Quantitative reverse transcription-polymerase chain reactions (RT-PCR) were performed using primers specific for 19 integrin subunits to quantify ECM receptor transcript expression.

RESULTS

Our data demonstrated that GF and PDLF adhere to vitronectin and collagen types I and IV more avidly than do DF. PDLF adhered well to laminin, whereas GF and DF did not. Quantitation of integrin expression demonstrated that the different fibroblast types expressed different integrin transcripts, further demonstrating their innate differences.

CONCLUSIONS

The 3 fibroblast types studied behave differently and expressed different ECM receptors. However, gingival fibroblasts and periodontal ligament fibroblasts are more similar in their attachment and integrin expression than either is to dermal fibroblasts. Therefore, experiments using DF will not necessarily be valid for oral tissues.

The NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform

Beta3 integrin adhesion molecules play important roles in wound repair and the regulation of vascular development and three beta3 integrin isoforms (beta3-A, -B, -C) have been described so far. Surface expression of beta3 integrins is dynamically regulated through internalization of beta3 integrins, however, the molecular mechanisms are understood incompletely. To evaluate the role of the cytoplasmic domain of beta3 integrins for internalization, we have generated single chain chimeras with variant and mutated forms of beta3 cytoplasmic domains. Upon transient transfection into chinese hamster ovary cells, it was found that the beta3-A chimera had strongly reduced cell surface expression compared with the corresponding beta3-B, or beta3-C fusion proteins, or the tail-less constructs, whereas steady state levels of all chimeras were near identical. Studies employing cytoplasmic domain mutants showed that the NITY motif at beta3-A 756–759 is critical for plasma membrane expression of beta3-A. Furthermore, delivery of beta3-A to the cell surface was specifically modulated by the cytoplasmic protein beta3-endonexin, a previously described intracellular protein. Coexpression of the native, long form of beta3-endonexin, which does not interact with the beta3 tail, acted as a dominant negative inhibitor of beta3-A-internalization and enhanced steady-state surface expression of the beta3-A-chimera. Furthermore, anti-beta3 antibody-induced internalization of the native beta3 integrin (alpha(IIb)beta3 was dramatically reduced for the Tyr(759)-Ala substitution mutant (alpha(IIb)beta3) (Y759A) and expression of the long isoform of beta3-endonexin substantially decreased the internalization of wild-type alpha(IIb)beta3. Thus, the NITY motif of the beta-chain cytoplasmic domain is involved in stimulated internalization of the beta3 integrin A isoform and beta3-endonexin appears to couple the beta3-A isoform to a specific receptor-recycling pathway.

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.

Probing chemical and conformational differences in the resting and active conformers of platelet integrin alpha(IIb)beta(3)

Integrin alpha(IIb)beta(3) is the fibrinogen receptor that mediates platelet adhesion and aggregation. The ligand binding function of alpha(IIb)beta(3) is "activated" on the platelet surface by physiologic stimuli. Two forms of alpha(IIb)beta(3) can be purified from platelet lysates. These forms are facsimiles of the resting (Activation State-1 or AS-1) and the active (Activation State-2 or AS-2) conformations of the integrin found on the platelet surface. Here, the differences between purified AS-1 and AS-2 were examined to gain insight into the mechanism of activation. Four major findings are put forth. 1) The association rate (k(1)) between fibrinogen and the integrin is a key difference between AS-1 and AS-2. 2) Although the divalent ion Mn(2+) enhances the ligand binding function of AS-1, this ion is unable to convert AS-1 to AS-2. Therefore, its effect on integrin is unrelated to activation. 3) Peptide mass fingerprints indicate that the chemical structure of AS-1 and AS-2 are virtually identical, calling into question the idea that post-translational modifications are necessary for activation. 4) The two forms of alpha(IIb)beta(3) have significant conformational differences at three positions. These include the junction of the heavy and light chain of alpha(IIb), the divalent ion binding sites on alpha(IIb), and at a disulfide-bonded knot linking the amino terminus of beta(3) to the cysteine-rich domain. These observations indicate that integrin is activated by a series of specific conformational rearrangements in the ectodomain that increase the rate of ligand association.

Development and characterization of a human in vitro resorption assay: demonstration of utility using novel antiresorptive agents

A human in vitro resorption assay has been developed using osteoclastoma-derived osteoclasts and used to evaluate novel antiresorptive agents including antagonists of the alphavbeta3 integrin, and inhibitors of cathepsin K and the osteoclast ATPase. The potency of novel compounds in the in vitro resorption assay correlates with functional assays for each class of inhibitor: the human alphavbeta3-mediated cell adhesion assay for the vitronectin receptor antagonists (r2 = 0.82), the chick osteoclast vacuolar ATPase enzyme assay for the H+-ATPase inhibitors (r2 = 0.77) and the recombinant human cathepsin K enzyme assay for the cathepsin K inhibitors (r2 = 0.80). Cell suspensions, rich in osteoclasts, are prepared by collagenase digestion of the tumor tissue. These cells can be stored long-term in liquid nitrogen and upon thawing maintain their bone-resorbing phenotype. The cryopreserved cells can be cultured on bovine cortical bone for 24-48 h and resorption can be measured by either confocal microscopy or biochemical assays. The resorptive activity of osteoclasts derived from a number of tumors can be inhibited reproducibly using a number of mechanistically unique antiresorptive compounds. In addition, the measurement of resorption pits by laser confocal microscopy correlates with the release of type I collagen C-telopeptides or N-telopeptides, as measured by enzyme-linked immunosorbent assay. Resorption can be measured reproducibly using a 48-h incubation of osteoclasts on bone slices, or a 24-h incubation with bone particles. This in vitro human osteoclast resorption assay provides a robust system for the evaluation of inhibitors of osteoclastic function that may be developed for the treatment of metabolic bone diseases such as 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.

Integrins: alternative splicing as a mechanism to regulate ligand binding and integrin signaling events

Integrins are a family of transmembrane proteins composed of heterodimers of alpha and beta subunits. With their extracellular domain they bind extracellular matrix proteins or other cell surface molecules, and their cytoplasmic domain binds to cytoskeletal and signaling proteins. Thus, they are in an ideal position to transfer information from the extracellular environment to the interior of the cell and vice versa. For several integrin subunits, alternative splicing of mRNA leads to variations in the sequence of both extracellular and cytoplasmic domains. Many integrin splice variants have specific expression patterns, but for some time, functional differences between these variants were not evident. Recent experiments using transfected cell lines and gene targeting of specific splice variants have contributed significantly to our understanding of the function of these splice variants. The results indicate that alternative splicing is a mechanism to subtly regulate the ligand binding and signaling activity of integrins.

Benzimidazole derivatives as arginine mimetics in 1,4-benzodiazepine nonpeptide vitronectin receptor (alpha v beta 3) antagonists

In a 3-oxo-1,4-benzodiazepine-2-acetic acid series of vitronectin receptor (alpha v beta 3) antagonists containing a benzimidazole as a novel arginine mimetic, we examined the effects of benzimidazole modifications and amide substitutions on both activity and pharmacokinetics.

Discovery of an imidazopyridine-containing 1,4-benzodiazepine nonpeptide vitronectin receptor (alpha v beta 3) antagonist with efficacy in a restenosis model

In the 3-oxo-1,4-benzodiazepine-2-acetic acid series of vitronectin receptor (alpha v beta 3) antagonists, a compound containing an imidazopyridine arginine mimetic was discovered which had sufficient potency and i.v. pharmacokinetics for demonstration of efficacy in a rat restenosis model.

Cre-loxP-mediated inactivation of the alpha6A integrin splice variant in vivo: evidence for a specific functional role of alpha6A in lymphocyte migration but not in heart development

Two splice variants of the alpha6 integrin subunit, alpha6A and alpha6B, with different cytoplasmic domains, have previously been described. While alpha6B is expressed throughout the development of the mouse, the expression of alpha6A begins at 8.5 days post coitum and is initially restricted to the myocardium. Later in ontogeny, alpha6A is found in various epithelia and in certain cells of the immune system. In this study, we have investigated the function of alpha6A in vivo by generating knockout mice deficient for this splice variant. The Cre- loxP system of the bacteriophage P1 was used to specifically remove the exon encoding the cytoplasmic domain of alpha6A in embryonic stem cells, and the deletion resulted in the expression of alpha6B in all tissues that normally express alpha6A. We show that alpha6A-/- mice develop normally and are fertile. The substitution of alpha6A by alpha6B does not impair the development and function of the heart, hemidesmosome formation in the epidermis, or keratinocyte migration. Furthermore, T cells differentiated normally in alpha6A-/- mice. However, the substitution of alpha6A by alpha6B leads to a decrease in the migration of lymphocytes through laminin-coated Transwell filters and to a reduction of the number of T cells isolated from the peripheral and mesenteric lymph nodes. Lymphocyte homing to the lymph nodes, which involves various types of integrin-ligand interactions, was not affected in the alpha6A knockout mice, indicating that the reduced number of lymph node cells could not be directly attributed to defects in lymphocyte trafficking. Nevertheless, the expression of alpha6A might be necessary for optimal lymphocyte migration on laminin in certain pathological conditions.

Casein kinase 2 phosphorylation of recombinant rat osteopontin enhances adhesion of osteoclasts but not osteoblasts

Osteopontin (OP) is a highly phosphorylated bone matrix protein and contains the RGD cell-binding motif, which mediates cell adhesion through integrin receptors that include alpha(v)beta3. Casein kinase 2 (CK2) is a factor-independent serine/threonine kinase, which may be the predominant physiologically relevant kinase for OP phosphorylation. This study was designed to examine the effects of unphosphorylated recombinant rat OP, and CK2-phosphorylated OP (P-OP), on the adhesion and function of mouse osteoclasts (OC) and osteoblast-like cells (UMR 201-10B and UMR 106-06) in vitro. OP significantly increased OC adhesion compared to plastic alone, and cell attachment was further increased at least twofold on OP phosphorylated with CK2. Attachment was dependent on the integrity of the RGD domain and was completely abolished in the presence of 1 mM RGD peptide. Neither CK2 phosphorylation of mutant OP, in which the RGD was converted to RGE or RAD, nor protein kinase C (PKC) phosphorylation of wild-type OP enhanced OC attachment. An antibody to the beta3 integrin subunit, but not anti-mouse CD44 antibody, specifically blocked the proportion of attachment due to phosphorylation of OP. Actin ring formation in OC was increased by plating cells onto OP, with no further increase by phosphorylation. Both OP and CK2-phosphorylated OP enhanced attachment of the two osteoblastic cell lines, compared to plastic, but in contrast to OCs, there was no significant difference with phosphorylation. Osteoblast attachment was totally blocked by 1 mM RGD peptide, but was not influenced by the beta3 integrin antibody. Plating of UMR 201-10B cells onto OP further increased retinoic acid-induced alkaline phosphatase expression. The results suggest that specific phosphorylation of OP is important for interaction with OCs, compared with osteoblastic cells, and that alternative integrins may be important in the interaction between osteoblastic cells and OP compared with OCs.