Integrin beta3 blockade with abciximab and protection from myocardial ischemic events.
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Abciximab↗
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Identification of critical residues for regulation of integrin activation in the beta6-alpha7 loop of the integrin beta3 I-like domain.
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Amino Acid Motifs↗
The beta1 and beta3 integrins promote T cell receptor-mediated cytotoxic T lymphocyte activation.
Recognition by CD8+ cytotoxic T lymphocytes (CTLs) of antigenic peptides bound to major histocompatibility class (MHC) I molecules on target cells leads to sustained calcium mobilization and CTL degranulation resulting in perforin-dependent killing. We report that beta1 and beta3 integrin-mediated adhesion to extracellular matrix proteins on target cells and/or surfaces dramatically promotes CTL degranulation. CTLs, when adhered to fibronectin but not CTL in suspension, efficiently degranulate upon exposure to soluble MHC.peptide complexes, even monomeric ones. This adhesion induces recruitment and activation of the focal adhesion kinase Pyk2, the cytoskeleton linker paxillin, and the Src kinases Lck and Fyn in the contact site. The T cell receptor, by association with Pyk2, becomes part of this adhesion-induced activation cluster, which greatly increases its signaling.
Animals↗
Recombinant adenovirus type 5 vectors that target DC-SIGN, ChemR23 and alpha(v)beta3 integrin efficiently transduce human dendritic cells and enhance presentation of vectored antigens.
Recombinant adenoviruses (rAds) represent a promising system for vaccine delivery but transduce dendritic cells (DC) relatively poorly. To address this concern, we used a biotin-avidin linkage to conjugate rAd vectors to ligands which bind with high affinity to selected receptors on DC (ChemR23, alpha(v)beta3 integrin, and DC-SIGN). The targeted vectors had an enhanced ability to transduce human monocyte-derived DC compared to untargeted virus. In addition, DC transduced with targeted rAd vectors were more efficient at stimulating cytokine production by autologous memory CD8+ T cells, against a vector-encoded antigen. These results expand the range of cell surface receptors that can be used to target rAd5 vectors to DC, and may facilitate future development of rAd-based vaccines.
Adenoviridae↗
PYK2 in osteoclasts is an adhesion kinase, localized in the sealing zone, activated by ligation of alpha(v)beta3 integrin, and phosphorylated by src kinase.
Osteoclast activation is initiated by adhesion to the bone surface, followed by cytoskeletal rearrangement, the formation of the sealing zone, and a polarized ruffled membrane. This study shows that PYK2/CAKbeta/RAFTK, a cytoplasmic kinase related to the focal adhesion kinase, is highly expressed in rat osteoclasts in vivo. Using murine osteoclast-like cells (OCLs) or their mononuclear precursors (pOCs), generated in a coculture of bone marrow and osteoblastic MB1.8 cells, we show: (a) tyrosine phosphorylation of PYK2 upon ligation of beta3 integrins or adhesion of pOCs to serum, vitronectin, osteopontin, or fibronectin but not to laminin or collagen; (b) coimmunoprecipitation of PYK2 and c-Src from OCLs; (c) PYK2 binding to the SH2 domains of Src; (d) marked reduction in tyrosine phosphorylation and kinase activity of PYK2 in OCLs derived from Src (-/-) mice, which do not form actin rings and do not resorb bone; (e) PYK2 phosphorylation by exogeneous c-Src; (f) translocation of PYK2 to the Triton X-100 insoluble cytoskeletal fraction upon adhesion; (g) localization of PYK2 in podosomes and the ring-like structures in OCLs plated on glass and in the sealing zone in OCLs plated on bone; and (h) activation of PYK2, in the presence of MB1.8 cells, parallels the formation of sealing zones and pit resorption in vitro and is reduced by echistatin or calcitonin and cytochalasin D. Taken together, these findings suggest that Src-dependent tyrosine phosphorylation of PYK2 is involved in the adhesion-induced formation of the sealing zone, required for osteoclastic bone resorption.
Animals↗
Role for beta3 integrins in human melanoma growth and survival.
The role of alphaIIbbeta3 integrin in regulating platelet function is well appreciated, whereas its role in tumor progression and metastasis is not. The purpose of our study was to determine a functional relevance to expression of alphaIIbbeta3 integrin in cells derived from human solid tumors. A study of human melanoma biopsies (n = 24) showed that alphaIIbbeta3 expression increased with tumor thickness, which is indicative of metastatic propensity. Expression of alphaIIbbeta3 was 8% (+/-1.8), 33% (+/-10.4) and 62% (+/-5) in melanomas ranging in thickness from 0-1.5 mm, 1.5-4.0 mm and >4 mm, respectively; alphavbeta3 was equally high all categories. To determine biological function, we stably transfected alphaIIbbeta3 into human melanoma cells that express alphavbeta3, but not alphaIIbbeta3. Surface expression of alphavbeta3 remained unaltered between alphaIIbbeta3 (+) and mock transfected counterparts. The alphaIIbbeta3 (+) cells possessed increased ability to adhere, spread and migrate on fibrinogen. They had decreased ability to attach, spread and migrate on vitronectin. Immunocytochemistry showed that expression of alphaIIbbeta3 displaced alphavbeta3 from focal contact points. When implanted subcutaneously into SCID mice, the alphaIIbbeta3 (+) cells developed approximately 4-fold larger tumors when compared to their mock counterparts and the level of apoptosis was reduced within the tumors. Results suggest that co-expression of the 2 beta3 integrins, alphavbeta3 and alphaIIbbeta3, in human melanoma cells enhanced cell survival and promoted growth in vivo.
Animals↗
Critical roles for the COOH-terminal NITY and RGT sequences of the integrin beta3 cytoplasmic domain in inside-out and outside-in signaling.
Bidirectional signaling of integrin alphaIIbbeta3 requires the beta3 cytoplasmic domain. To determine the sequence in the beta3 cytoplasmic domain that is critical to integrin signaling, cell lines were established that coexpress the platelet receptor for von Willebrand factor (vWF), glycoprotein Ib-IX, integrin alphaIIb, and mutants of beta3 with truncations at sites COOH terminal to T741, Y747, F754, and Y759. Truncation at Y759 did not affect integrin activation, as indicated by vWF-induced fibrinogen binding, but affected cell spreading and stable adhesion. Thus, the COOH-terminal RGT sequence of beta3 is important for outside-in signaling but not inside-out signaling. In contrast, truncation at F754, Y747, or T741 completely abolished integrin activation. A point mutation replacing Y759 with alanine also abolished integrin activation. Thus, the T755NITY759 sequence of beta3, containing an NXXY motif, is critical to inside-out signaling, whereas the intact COOH terminus is important for outside-in signaling. In addition, we found that the calcium-dependent protease calpain preferentially cleaves at Y759 in a population of beta3 during platelet aggregation and adhesion, suggesting that calpain may selectively regulate integrin outside-in signaling.
Alanine↗
Phosphorylation sites in the integrin beta3 cytoplasmic domain in intact platelets.
Protein seryl/threonyl phosphatase inhibitors such as calyculin A block inside-out and outside-in platelet signaling. Our studies demonstrate that the addition of calyculin A blocks platelet adhesion and spreading on fibrinogen, responses that depend on integrin alphaIIb beta3 signaling. We hypothesized that this reflects a change in alphaIIb beta3 structure caused by a specific state of phosphorylation. We show that addition of calyculin A leads to increased phosphorylation of the beta3 subunit, and phosphoamino acid analysis reveals that only threonine residues become phosphorylated; sequence analysis by Edman degradation established that threonine 753 became stoichiometrically phosphorylated during inhibition of platelet phosphatases by calyculin A. This region of beta3 is linked to outside-in signaling such as platelet spreading responses. The effect of calyculin A on platelet adhesion and spreading and on the phosphorylation of T-753 in beta3 is reversed by the calcium ionophore A23187, demonstrating that these effects of calyculin A are not generally toxic ones. We propose that phosphorylation of beta3 on threonine 753, a region of beta3 linked to outside-in signaling, may be a mechanism by which integrin alphaIIb beta3 function is regulated.
Blood Platelets↗
The specificity and function of the metal-binding sites in the integrin beta3 A-domain.
The A-domains within integrin beta subunits contain three metal sites termed the metal ion-dependent adhesion site (MIDAS), site adjacent to the metal ion-dependent adhesion site (ADMIDAS), and ligand-induced metal-binding site (LIMBS), and these sites are involved in ligand engagement. The selectivity of these metal sites and their role in ligand binding have been investigated by expressing a fragment corresponding to the beta3 A-domain, beta3-(109-352), and single point mutants in which each of the cation-binding sites has been disabled. Equilibrium dialysis experiments identified three Mn2+- and two Ca2+-binding sites with the LIMBS being the site that did not bind Ca2+. Although the ADMIDAS could bind Ca2+, it did not bind Mg2+. These results indicate that the Ca2+-specific site that inhibits ligand binding is the ADMIDAS. Two different assay systems, surface plasmon resonance and a microtiter plate assay, demonstrated that the beta3 A-domain fragment bound fibrinogen in the presence of 0.1 mm Ca2+ but not in 3 mm Ca2+. This behavior recapitulated the effects of Ca2+ on fibrinogen binding to alphavbeta3 but not alphaIIbbeta3. Disabling any of the three cation-binding sites abrogated fibrinogen binding. These results indicate that the specificities of the three metal-binding sites for divalent cations are distinct and that each site can regulate the ligand binding potential of the beta3 A-domain.
Binding Sites↗
The human integrin beta3 gene is 63 kb and contains a 5'-UTR sequence regulating expression.
The human blood platelet fibrinogen receptor, integrin alphaIIbbeta3 (glycoprotein IIb-IIIa) is an archetypal member of the integrin family of adhesive molecules and is the only integrin encoded by genes physically linked in the genome. Because studies on the normal and abnormal expression of any gene require a thorough understanding of its organization, the initial goals of the current study were to determine the size and complete the genomic organization for the beta3 gene. We now report the isolation of the entire beta3 gene in a single P1 plasmid and for the first time have linked the first and second exons on a contiguous fragment of DNA. Using pulsed-field gel analysis, we determined the full size of the beta3 gene to be 63 kb and show a large (16.7 kb) first intron; based on this information, we propose a uniform numbering system for the beta3 exons. We have completed the 5' genomic structure and generated a long-range restriction map. The promoter and the 5' end of the first intron were found to have approximately 50% sequence identity with a region of the avian beta3 gene known to possess functional transcriptional activity. Analysis of three different homologous regions led to the identification of a sequence in the 5'-UTR of the human gene, CCGCGGGAGG, which shares 90% identity with the avian gene and which bound nuclear proteins in DNaseI and electrophoretic mobility shift assay studies. Mutating this sequence caused a 2.6-fold reduction in reporter gene activity. In these studies we have (1) determined the full length and 5' organization of the beta3 gene, (2) identified a large region of homology between the 5' regions of the avian and human genes, and (3) identified a sequence in the 5'-UTR that augments gene expression. Knowing the genomic structure of beta3 has permitted the uncovering of new mechanisms of mutagenesis causing Glanzmann thrombasthenia (Jin et al, J Clin Invest 98:1745, 1996), and our findings will be valuable for such genetic analyses as well as for studies on the transcriptional regulation of beta3 and other integrin genes.
Antigens, CD↗
The Pro33 isoform of integrin beta3 enhances outside-in signaling in human platelets by regulating the activation of serine/threonine phosphatases.
Integrin beta(3) is polymorphic at residue 33 (Leu(33) or Pro(33)), and the Pro(33)-positive platelets display enhanced aggregation, P-selectin secretion, and shorter bleeding times. Because outside-in signaling is critical for platelet function, we hypothesized that the Pro(33) variant provides a more efficient signaling than the Leu(33) isoform. When compared with Pro(33)-negative platelets, Pro(33)-positive platelets demonstrated significantly greater serine/threonine phosphorylation of extracellular signal-regulated kinase (ERK2) and myosin light chain (MLC) but not cytoplasmic phospholipase A2 upon thrombin-induced aggregation. Tyrosine phosphorylation of integrin beta(3) and the adaptor protein Shc was no different in the fibrinogen-engaged platelets from both genotypes. The addition of Integrilin (alpha(IIb)beta(3)-fibrinogen blocker) or okadaic acid (serine/threonine phosphatase inhibitor) dramatically enhanced ERK2 and MLC phosphorylation in the Pro(33)-negative platelets when compared with Pro(33)-positive platelets, suggesting that integrin engagement during platelet aggregation activates serine/threonine phosphatases. The phosphatase activity of myosin phosphatase (MP) that dephosphorylates MLC is inactivated by phosphorylation of the myosin binding subunit of MP at Thr(696), and aggregating Pro(33)-positive platelets exhibited an increased Thr(696) phosphorylation of MP. These studies highlight a role for the dephosphorylation events via the serine/threonine phosphatases during the integrin outside-in signaling mechanism, and the Leu(33) --> Pro polymorphism regulates this process. Furthermore, these findings support a mechanism whereby the reported enhanced alpha granule secretion in the Pro(33)-positive platelets could be mediated by an increased phosphorylation of MLC, which in turn is caused by an increased phosphorylation and subsequent inactivation of myosin phosphatase.
Blood Platelets↗
Beta3 integrin deficiency promotes atherosclerosis and pulmonary inflammation in high-fat-fed, hyperlipidemic mice.
Hyperlipidemia promotes the chronic inflammatory disease atherosclerosis through poorly understood mechanisms. Atherogenic lipoproteins activate platelets, but it is unknown whether platelets contribute to early inflammatory atherosclerotic lesions. To address the role of platelet aggregation in diet-induced vascular disease, we studied beta3 integrin-deficient mice (lacking platelet integrin alphaIIbbeta3 and the widely expressed nonplatelet integrin alphavbeta3) in two models of atherosclerosis, apolipoprotein E (apoE)-null and low-density lipoprotein receptor (LDLR)-null mice. Unexpectedly, a high-fat, Western-type (but not a low-fat) diet caused death in two-thirds of the beta3-/-apoE-/- and half of the beta3-/-LDLR-/- mice due to noninfectious pneumonitis. In animals from both models surviving high-fat feeding, pneumonitis was absent, but aortic atherosclerosis was 2- to 6-fold greater in beta3-/- compared with beta+/+ littermates. Expression of CD36, CD40L, and CD40 was increased in lungs of beta3-/-LDLR-/- mice. Each was also increased in smooth muscle cells cultured from beta3-deficient mice and suppressed by retroviral reconstitution of beta3. These data show that the platelet defect caused by alphaIIbbeta3 deficiency does not impair atherosclerotic lesion initiation. They also suggest that alphavbeta3 has a suppressive effect on inflammation, the loss of which induces atherogenic mediators that are amplified by diet-induced hyperlipidemia.
Animals↗
Adenoviral gene transfer of beta3 integrin subunit induces conversion from radial to vertical growth phase in primary human melanoma.
Expression of the beta3 subunit of the alphavbeta3 vitronectin receptor on melanoma cells is associated with tumor thickness and the ability to invade and metastasize. To address the role of alphavbeta3 in the complex process of progression from the nontumorigenic radial to the tumorigenic vertical growth phase of primary melanoma, we examined the biological consequences of overexpressing alphavbeta3 in early-stage melanoma cells using an adenoviral vector for gene transfer. Overexpression of functional alphavbeta3 in radial growth phase primary melanoma cells 1) promotes both anchorage-dependent and -independent growth, 2) initiates invasive growth from the epidermis into the dermis in three-dimensional skin reconstructs, 3) prevents apoptosis of invading cells, and 4) increases tumor growth in vivo. Thus, alphavbeta3 serves diverse biological functions during the progression from the nontumorigenic radial growth phase to the tumorigenic and-invasive vertical growth phase primary melanoma.
Adenoviridae↗
No association of breast cancer risk with integrin beta3 (ITGB3) Leu33Pro genotype.
To pursue a borderline increased risk of breast cancer for carriers of two integrin beta(3) (ITGB3) 33Pro alleles found in a recent prospective study, we conducted a case-control study of 1088 women with breast cancer and 4815 female controls. Leu33Pro heterozygotes, homozygotes and heterozygotes+homozygotes vs noncarriers had odds ratios for breast cancer of 1.0 (95% confidence interval: 0.8-1.1), 0.8 (0.5-1.2) and 1.0 (0.8-1.1), respectively. After stratification for conventional risk factors, odds ratio for breast cancer in heterozygotes, homozygotes and heterozygotes+homozygotes vs noncarriers were not increased above 1.0 in any of the 14 strata examined. This was also true after stratification for tumour histological subtype and cancer stage at the time of diagnosis.
Adult↗
Antibody to beta3 integrin inhibits osteoclast-mediated bone resorption in the thyroparathyroidectomized rat.
The cell surface integrin, alphaVbeta3, is important for the attachment of osteoclasts to bone matrix and the subsequent resorption of bone. The present study was designed to determine the effects of F11, a monoclonal antibody to the rat beta3 subunit, on calcium mobilization in a rat model of bone resorption. Male Sprague Dawley rats became hypocalcemic within 18 h after thyroparathyroidectomy. Synthetic PTH-related protein (PTHrP(1-34)) administered to control rats caused serum calcium to return to normal. Anti-beta3 treatment of rats after thyroparathyroidectomy inhibited the calcemic response to PTHrP by 65%. Circulating F11 was biologically active as demonstrated by osteoclast retraction and by the inhibition of adenosine diphosphate-induced platelet aggregation via inhibition of the platelet integrin alphaIIbbeta3 in ex vivo assays. F11 antibody was localized by immunohistological staining to osteoclasts in long bones, suggesting that the mechanism of action of the antibody was via a direct effect upon osteoclasts. Echistatin and calcitonin also inhibited calcemic responses to PTHrP in this in vivo model, whereas an isotype-matched, control antibody was ineffective. These studies provide the first direct evidence in vivo that osteoclast-mediated bone resorption is regulated via beta3 integrin.
Animals↗
Unoccupied alpha(v)beta3 integrin regulates osteoclast apoptosis by transmitting a positive death signal.
Cell/matrix detachment is a general inducer of programmed cell death, an event mediated by loss of integrin/ligand association. Because alpha(v)beta3 is the major integrin expressed by the osteoclast, we asked whether its occupancy promotes survival of the resorptive cell. Thus, we generated wild-type preosteoclasts and placed them on selective matrix proteins. Consistent with the posture that alpha(v)beta3 occupancy promotes survival, preosteoclasts plated on native collagen, a matrix not recognized by the integrin, undergo apoptosis 4-fold faster than those on the alpha(v)beta3 ligand, vitronectin. To further explore the role of alpha(v)beta3 in osteoclast apoptosis, wild-type and beta3-/- preosteoclasts were suspended and apoptosis determined, with time. Beta3-/- preosteoclasts, in suspension, undergo a rate of apoptosis only 40-60% of that of their wild-type counterparts, indicating that unoccupied alpha(v)beta3 transmits a positive death signal that we find regulated by caspase-8. Attesting to specificity of the unoccupied integrin-transmitted death signal, apoptosis in the absence of alpha(v)beta3 is mediated by capsase-9. We have shown that the resorptive defect of beta3-/- osteoclasts is rescued by wild-type beta3 cDNA but not by one bearing a S752P mutation. To determine whether the same holds true regarding osteoclast apoptosis, we constructed lentivirus vectors encoding green fluorescent protein, wild-type beta3, or beta3S752P. Once again, native beta3-/- preosteoclasts were protected against apoptosis. Similar to its effect on bone resorption, transduced wild-type beta3 normalizes the apoptotic rate of beta3-/- preosteoclasts. Unexpectedly, however, beta3S752P transductants also die at a rate indistinguishable from wild type. Thus, unoccupied alpha(v)beta3 integrin regulates osteoclast apoptosis via a component of the integrin that is different than that regulating resorption.
Animals↗
Shear stress augments the enhanced adhesive phenotype of cells expressing the Pro33 isoform of integrin beta3.
Adhesion of platelets to the exposed extracellular matrix proteins at sites of vascular injury is partly regulated by the local fluid shear stress. Because the Leu33Pro (Pl(A)) polymorphism of integrin beta(3) confers only a modest increase in adhesion under static conditions, we used CHO and 293 cells expressing the Leu33 or Pro33 isoform of beta(3) in flow chamber experiments to test whether shear forces would alter the Pl(A) adhesive phenotype. We found that shear force augmented the Pro33-mediated enhanced adhesion to fibrinogen. This Pro33-dependent enhancement was aspirin-sensitive and was also observed on immobilized von Willebrand factor and cryoprecipitate, but not fibronectin. Thus, shear stress enhances the adhesive phenotype of the Pro33 cells to multiple physiologic substrates.
Animals↗
In vivo characterization of the integrin beta3 as a receptor for Hantaan virus cellular entry.
Binding of viruses to cell surface molecules is an essential step in viral infection. In vitro studies suggested that the alpha(v)beta(3) integrin receptor is the epithelial cell receptor for Hantaan virus (HTNV). Whether beta(3) is in vivo the only or central cellular receptor for HTNV infection is not known. To investigate the role of beta(3) integrin for cellular entry of HTNV, we established an HTNV infection model in newborn murine pups. Infected pups died at an average age of 14.2 +/- 1.1 days with high levels of viral antigen detected in their brain, lung, and kidney. Pre-injection of blocking monoclonal antibodies (mAb) specific for either beta(3) or av prolonged survival significantly to a maximal average survival of 19.7 +/- 1.5 days (P <0.01) and 18.4 +/- 0.9 days (P < 0.01), respectively. XT-199, a chemical blocker of the alpha(v)beta(3) receptor also prolonged survival to 19.5 +/- 1.3 days (P < 0.01). In contrast to these receptor blockades, anti-HTNV antibody was not only able to prolong survival, but 20% of infected pups achieved long-term survival. An anti-murine beta(1) antibody comparatively prolonged survival (19.0 +/- 1.2 days), suggesting that HTNV infection is partly mediated through integrin beta(1) receptors as well as through beta(3) receptors in vivo. Our data demonstrate that the beta(3) receptor is important for HTNV infection in vivo, but also suggest that HTNV may utilize additional receptors beyond beta(3) for cellular entry within an organism.
Animals↗