The vitronectin receptor and its associated CD47 molecule mediates proinflammatory cytokine synthesis in human monocytes by interaction with soluble CD23.
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The vitronectin receptor, alphavbeta3 integrin, plays an important role in tumor cell invasion, angiogenesis, and phagocytosis of apoptotic cells. CD47, a member of the multispan transmembrane receptor family, physically and functionally associates with vitronectin receptor (VnR). Although vitronectin (Vn) is not a ligand of CD47, anti-CD47 and beta3 mAbs suppress Vn, but not fibronectin (Fn) binding and function. Here, we show that anti-CD47, anti-beta3 mAb and Vn, but not Fn, inhibit sCD23-mediated proinflammatory function (TNF-alpha, IL-12, and IFN-gamma release). Surprisingly, anti-CD47 and beta3 mAbs do not block sCD23 binding to alphav+beta3+ T cell lines, whereas Vn and an alphav mAb (clone AMF7) do inhibit sCD23 binding, suggesting the VnR complex may be a functional receptor for sCD23. sCD23 directly binds alphav+beta3+/CD47(-) cell lines, but coexpression of CD47 increases binding. Moreover, sCD23 binds purified alphav protein and a single human alphav chain CHO transfectant. We conclude that the VnR and its associated CD47 molecule may function as a novel receptor for sCD23 to mediate its proinflammatory activity and, as such, may be involved in the inflammatory process of the immune response. (+info)
The thrombospondin receptor integrin-associated protein (CD47) functionally couples to heterotrimeric Gi.
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Integrin-associated protein (IAP; CD47) is a thrombospondin receptor that forms a signaling complex with beta3 integrins resulting in enhanced alphavbeta3-dependent cell spreading and chemotaxis and, in platelets, alphaIIbbeta3-dependent spreading and aggregation. These actions of CD47 are all specifically abrogated by pertussis toxin treatment of cells. Here we report that CD47, its beta3 integrin partner, and Gi proteins form a stable, detergent-soluble complex that can be recovered by immunoprecipitation and affinity chromatography. Gialpha is released from this complex by treatment with GTP or AlF4. GTP and AlF4 also reduce the binding of CD47 to its agonist peptide (4N1K) derived from thrombospondin, indicating a direct association of CD47 with Gi. 4N1K peptide causes a rapid decrease in intraplatelet cyclic AMP levels, a Gi-dependent event necessary for aggregation. Finally, 4N1K stimulates the binding of GTPgamma35S to membranes from cells expressing IAP and alphavbeta3. This functional coupling of CD47 to heterotrimeric G proteins provides a mechanistic explanation for the biological effects of CD47 in a wide variety of systems. (+info)
Cellular entry of hantaviruses which cause hemorrhagic fever with renal syndrome is mediated by beta3 integrins.
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Hantaviruses replicate primarily in the vascular endothelium and cause two human diseases, hemorrhagic fever with renal syndrome (HFRS) and hantavirus pulmonary syndrome (HPS). In this report, we demonstrate that the cellular entry of HFRS-associated hantaviruses is facilitated by specific integrins expressed on platelets, endothelial cells, and macrophages. Infection of human umbilical vein endothelial cells and Vero E6 cells by the HFRS-causing hantaviruses Hantaan (HTN), Seoul (SEO), and Puumala (PUU) is inhibited by antibodies to alphavbeta3 integrins and by the integrin ligand vitronectin. The cellular entry of HTN, SEO, and PUU viruses, but not the nonpathogenic Prospect Hill (PH) hantavirus (i.e., a virus with no associated human disease), was also mediated by introducting recombinant alphaIIbbeta3 or alphavbeta3 integrins into beta3-integrin-deficient CHO cells. In addition, PH infectivity was not inhibited by alphavbeta3-specific sera or vitronectin but was blocked by alpha5beta1-specific sera and the integrin ligand fibronectin. RGD tripeptides, which are required for many integrin-ligand interactions, are absent from all hantavirus G1 and G2 surface glycoproteins, and GRGDSP peptides did not inhibit hantavirus infectivity. Further, a mouse-human hybrid beta3 integrin-specific Fab fragment, c7E3 (ReoPro), also inhibited the infectivity of HTN, SEO, and PUU as well as HPS-associated hantaviruses, Sin Nombre (SN) and New York-1 (NY-1). These findings indicate that pathogenic HPS- and HFRS-causing hantaviruses enter cells via beta3 integrins, which are present on the surfaces of platelets, endothelial cells, and macrophages. Since beta3 integrins regulate vascular permeability and platelet function, these findings also correlate beta3 integrin usage with common elements of hantavirus pathogenesis. (+info)
Cell spreading distinguishes the mechanism of augmentation of T cell activation by integrin-associated protein/CD47 and CD28.
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Integrin-associated protein (IAP/CD47) is a 50 kDa transmembrane protein initially defined as a regulator of beta3 integrin-mediated functions in neutrophils. IAP also can synergize with the TCR in T cell activation independent of beta3 integrins. To analyze the mechanism for IAP synergy with TCR, we expressed in Jurkat cells a chimeric molecule, consisting of the CD16 extracellular domain, the CD7 transmembrane domain and the TCR zeta chain cytoplasmic tail (CD16-7-zeta), which on its own is unable to induce IL-2 production. Ligation of IAP acted in synergy with TCR to induce IL-2 transcription and synthesis, but failed to synergize with the signal generated by CD16-7-zeta, while CD28 was a potent co-stimulator with both TCR and CD16-7-zeta. The failure of IAP to activate Jurkat together with CD16-7-zeta correlated with a lack of c-Jun N-terminal kinase, but not extracellular-signal-regulated kinase activation. Jurkat adhesion to anti-IAP, but not anti-CD28, induced cell spreading and the same domains of IAP required for augmentation of T cell activation were required to induce cell spreading. IAP synergy with TCR signaling likely results from its ability to stimulate adhesion to a ligand-expressing surface or antigen-presenting cell (APC), rather than from initiation of a novel signaling cascade. We conclude that a major role for ligation of IAP in T cell activation is to enhance the efficiency of TCR signaling by causing T cells to spread on an APC or surface. (+info)
CD47 signals T cell death.
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Activation-induced death of T cells regulates immune responses and is considered to involve apoptosis induced by ligation of Fas and TNF receptors. The role of other receptors in signaling T cell death is less clear. In this study we demonstrate that activation of specific epitopes on the Ig variable domain of CD47 rapidly induces apoptosis of T cells. A new mAb, Ad22, to this site induces apoptosis of Jurkat cells and CD3epsilon-stimulated PBMC, as determined by morphological changes, phosphatidylserine exposure on the cell surface, uptake of propidium iodide, and true counts by flow cytometry. In contrast, apoptosis was not observed following culture with anti-CD47 mAbs 2D3 or B6H12 directed to a distant or closely adjacent region, respectively. CD47-mediated cell death was independent of CD3, CD4, CD45, or p56lck involvement as demonstrated by studies with variant Jurkat cell lines deficient in these signaling pathways. However, coligation of CD3epsilon and CD47 enhanced phosphatidylserine externalization on Jurkat cells with functional CD3. Furthermore, normal T cells required preactivation to respond with CD47-induced apoptosis. CD47-mediated cell death appeared to proceed independent of Fas or TNF receptor signaling and did not involve characteristic DNA fragmentation or requirement for IL-1beta-converting enzyme-like proteases or CPP32. Taken together, our data demonstrate that under appropriate conditions, CD47 activation results in very rapid T cell death, apparently mediated by a novel apoptotic pathway. Thus, CD47 may be critically involved in controlling the fate of activated T cells. (+info)
Thrombospondin-1 acts via IAP/CD47 to synergize with collagen in alpha2beta1-mediated platelet activation.
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Integrin-associated protein (IAP; or CD47) is a receptor for the cell binding domain (CBD) of thrombospondin-1 (TS1). In platelets, IAP associates with and regulates the function of alphaIIbbeta3 integrin (Chung et al, J Biol Chem 272:14740, 1997). We test here the possibility that CD47 may also modulate the function of platelet integrin alpha2beta1, a collagen receptor. The CD47 agonist peptide, 4N1K (KRFYVVMWKK), derived from the CBD, synergizes with soluble collagen in aggregating platelet-rich plasma. 4N1K and intact TS1 also induce the aggregation of washed, unstirred platelets on immobilized collagen with a rapid increase in tyrosine phosphorylation. The effects of TS1 and 4N1K on platelet aggregation are absolutely dependent on IAP, as shown by the use of platelets from IAP-/- mice. Prostaglandin E1 (PGE1) prevents 4N1K-dependent aggregation on immobilized collagen but does not inhibit the 4N1K peptide stimulation of alpha2beta1-dependent platelet spreading. Finally, a detergent-stable, physical association of IAP and alpha2beta1 integrin is detected by coimmunoprecipitation. These results imply a role for IAP and TS1 in the early activation of platelets upon adhesion to collagen. (+info)
Role of cholesterol in formation and function of a signaling complex involving alphavbeta3, integrin-associated protein (CD47), and heterotrimeric G proteins.
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Integrin-associated protein (CD47) is a multiply membrane spanning member of the immunoglobulin superfamily that regulates some adhesion-dependent cell functions through formation of a complex with alphavbeta3 integrin and trimeric G proteins. Cholesterol is critical for the association of the three protein components of the supramolecular complex and for its signaling. The multiply membrane spanning domain of IAP is required for complex formation because it binds cholesterol. The supramolecular complex forms preferentially in glycosphingolipid-enriched membrane domains. Binding of mAb 10G2 to the IAP Ig domain, previously shown to be required for association with alphavbeta3, is affected by both the multiply membrane spanning domain and cholesterol. These data demonstrate that cholesterol is an essential component of the alphavbeta3/IAP/G protein signaling complex, presumably acting through an effect on IAP conformation. (+info)
Identification of CD47/integrin-associated protein and alpha(v)beta3 as two receptors for the alpha3(IV) chain of type IV collagen on tumor cells.
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Previous studies from our laboratories demonstrated that a peptide from the noncollagenous domain of the alpha3 chain of basement membrane collagen (COL IV), comprising residues 185-203, inhibits polymorphonuclear leukocyte activation and melanoma cell proliferation independently of its ability to promote cell adhesion; these properties require the presence of the triplet -SNS- at residues 189-191 (J. C. Monboisse et al., J. Biol. Chem., 269: 25475-25482, 1994; J. Han et al., J. Biol. Chem., 272: 20395-20401, 1997). More recently, we demonstrated that native COL IV and -SNS-containing synthetic peptides (10 microg/ml) added to culture medium inhibit the proliferation of not only melanoma cells but also breast, pancreas, and stomach tumor cells up to 82% and prostate tumor cells by 15%. This inhibition was shown to be dependent on a COL IV- or peptide-induced increase in intracellular cAMP (T. A. Shahan et al., Connect. Tissue Res., 40: 221-232, 1999). Attempts to identify the putative receptor(s) on tumor cells led to the isolation of five proteins (Mr 33,000, 52,000, 72,000, 95,000, and 250,000) from melanoma and prostate cells by affinity purification with the alpha3(IV)179-208 peptide. The Mr 52,000, 95,000, and 250,000 proteins were shown to be CD47/integrin-associated protein(IAP), the integrin beta3 subunit, and the alpha(v)beta3 integrin complex, respectively. The Mr 33,000 and 72,000 proteins have not yet been identified. To confirm the specificity of ligand binding to the receptors, cell membranes from either melanoma or prostate tumor cells were pretreated with the unlabeled ligand alpha3(IV)187-191 (-YYSNS-); alternatively, the peptide was pretreated with a peptide-reactive monoclonal antibody (A5D7) before receptor isolation. These treatments inhibited the purification of CD47/IAP, the integrin beta3 subunit, and the alpha(v)beta3 integrin complex from tumor cells. Furthermore, cells treated with CD47/IAP- or the alpha(v)beta3 integrin-reactive antibodies prevented the alpha3(IV)185-203 peptide from inhibiting cell proliferation and the subsequent rise in intracellular cAMP. Pretreating cells with the alpha3(IV)187-191 (-YYSNS-) peptide also inhibited their adhesion to the alpha3(IV)185-203 peptide substrate, whereas the inactive alpha1(IV)185-203 peptide, from the same region of the alpha1 chain as the alpha3(IV)185-203 peptide, had no effect. Incubation of cells with either CD47/IAP and/or alpha(v)beta3 integrin-reactive antibodies inhibited their adhesion to the alpha3(IV)185-203 peptide, whereas antibodies to the beta1 and beta2 integrin subunits were without effect. These data suggest that ALC-COL IV, through its alpha3(IV) chain, inhibits tumor cell proliferation using the receptors CD47/IAP and the alpha(v)beta3 integrin. (+info)