Thrombospondin modulates alpha v beta 3 function through integrin-associated protein

Amyloid beta peptides trigger CD47-dependent mast cell secretory and phagocytic responses

Mast cells are found in the brain, where they contribute to immune responses. They have been implicated in multiple sclerosis, but their potential role in Alzheimers disease (AD), another inflammatory disease of the central nervous system, remains elusive. In the present study, we examined mast cell responses to amyloid beta (Abeta) peptides 1-40 and 1-42, the major components of the Alzheimer amyloid plaques. Rat peritoneal mast cells were used as experimental model for human brain serosal mast cells. Fibrillar Abeta1-40 and Ami1-42 peptides induced concentration-dependent exocytosis, as assessed by measurement of histamine secretion; exocytosis was reduced by pre-treatment with pertussis toxin and with antibodies against the CD47 receptor and the beta1-integrin subunit. Fibrillar Abeta1-40 and Abeta1- 42 peptides coated on heat-inactivated yeast particles and soluble fibrillar Abeta1-40 and Abeta1-42 peptides were also recognized and phagocyted by mast cells. Uptake of the peptides was decreased in the presence of 4N1, a peptide agonist of the CD47 receptor, but remained unchanged in the presence of 4NGG, a peptide derived from 4N1 which does not bind to CD47. Non-fibrillar forms of Abeta1-40 and 1-42 peptides were unable to elicit mast cell responses. These results show that fibrillar Abeta peptides can trigger mast cells and elicit exocytosis and phagocytosis. The Abeta-induced activation of mast cells operates through a CD47/beta1-integrin membrane complex coupled with Gi-protein. The present data support the hypothesis that mast cells, similarly to microglial cells, could play a major role in AD pathogenesis.

Extracellular interactome of the FGF receptor-ligand system: complexities and the relative simplicity of the worm

Fibroblast growth factors (FGFs) and their receptors (FGFRs) regulate a multitude of biological functions in embryonic development and in adult. A major question is how does one family of growth factors and their receptors control such a variety of functions? Classically, specificity was thought to be imparted by alternative splicing of the FGFRs, resulting in isoforms that bind specifically to a subset of the FGFs, and by different saccharide sequences in the heparan sulfate proteoglycan (HSPG) co-receptor. A growing number of noncanonical co-receptors such as integrins and neural cell adhesion molecule (NCAM) are now recognized as imparting additional complexity to classic FGFR signaling. This review will discuss the noncanonical FGFR ligands and speculate on the possibility that they provide additional and alternative means to determining the functional specificity of FGFR signaling. We will also discuss how invertebrate models such as C. elegans may advance our understanding of noncanonical FGFR signaling.

CD47 regulates bone mass and tumor metastasis to bone

CD47, also called integrin-associated protein, plays a critical role in the innate immune response and is an atypical member of the immunoglobulin superfamily that interacts with and activates beta3 integrins. beta3 integrin(-/-) mice have defective platelet and osteoclast function and are protected from bone metastasis. The role of CD47 in skeletal homeostasis and bone metastasis has not been described. CD47(-/-) mice had increased bone mass and defective osteoclast function in vivo. Although the number of functional osteoclasts formed by differentiating CD47(-/-) bone marrow macrophages was decreased, high doses of RANKL rescued differentiation and function of CD47(-/-) osteoclasts ex vivo and rescued the osteoclast defect in CD47(-/-) mice. Inhibition of nitric oxide (NO) synthase, which is expressed at higher levels in CD47(-/-) osteoclasts, also rescued the osteoclast defect in CD47(-/-) cells. We then examined the consequences of this osteoclast defect in bone metastasis. In a model of tumor metastasis to bone, bone tumor burden was decreased in the CD47(-/-) mice compared with wild-type (WT) controls, with no decrease in s.c. tumor growth in CD47(-/-) mice. There was decreased tumor-associated bone destruction in the CD47(-/-) mice compared with WT controls, consistent with a defect in osteoclast function that was not rescued by the presence of tumor. Our data show that CD47 regulates osteoclastogenesis, in part, via regulation of NO production, and its disruption leads to a decrease in tumor bone metastasis. CD47 is a novel therapeutic target to strengthen bone mass and diminish metastatic tumor growth in bone.

Molecular basis of platelet activation by an alphaIIbbeta3-CHAMPS peptide

BACKGROUND

A novel method, known as computed helical anti-membrane protein (CHAMP), for the design of peptides that bind with high affinity and selectivity to transmembrane helices was recently described and illustrated using peptides that bind alphaIIb- and alphav-integrin subunits, which induce selective activation of integrins alphaIIbbeta3 and alphavbeta3, respectively.

OBJECTIVES

In the present study, we have investigated the ability of an alphaIIb-CHAMPS peptide (termed integrin-activatory-peptide or IAP) to stimulate protein tyrosine phosphorylation and aggregation in human and mouse platelets.

METHODS

The ability of IAP to stimulate platelet aggregation and dense granule secretion was measured in washed preparations of human and mouse platelets. Samples were taken for measurement of tyrosine phosphorylation.

RESULTS

IAP stimulates robust tyrosine phosphorylation of the tyrosine kinase Syk and the FcR gamma-chain, but only weak phosphorylation of PLCgamma2. Aggregation to low but not high concentrations of IAP is reduced in the presence of the Src kinase inhibitor, PP1, or by inhibitors of the two feedback agonists, ADP and thromboxane A(2) (TxA(2)) suggesting that activation is reinforced by Src kinase-driven release of ADP and TxA(2). Unexpectedly, aggregation by IAP is only partially inhibited in human and mouse platelets deficient in integrin alphaIIbbeta3. Further, IAP induces partial aggregation of formaldehyde-fixed platelets.

CONCLUSIONS

The present study demonstrates that the alphaIIb-CHAMPS peptide induces platelet activation through integrin alphaIIbbeta3-dependent and independent pathways with the former mediating tyrosine phosphorylation of FcR gamma-chain and Syk. The use of the alphaIIb-CHAMPS peptide to study integrin alphaIIbbeta3 function is compromised by non-integrin-mediated effects.

The counteradhesive proteins, thrombospondin 1 and SPARC/osteonectin, open the tyrosine phosphorylation-responsive paracellular pathway in pulmonary vascular endothelia

Counteradhesive proteins are a group of genetically and structurally distinct multidomain proteins that have been grouped together for their ability to inhibit cell-substrate interactions. Three counteradhesive proteins that influence endothelial cell behavior include thrombospondin (TSP)1, (SPARC) (Secreted Protein Acidic and Rich in Cysteine), also known as osteonectin, and tenascin. More recently, these proteins have been shown to regulate not only cell-matrix interactions but cell-cell interactions as well. TSP1 increases tyrosine phosphorylation of components of the cell-cell adherens junctions or zonula adherens (ZA) and opens the paracellular pathway in human lung microvascular endothelia. The epidermal growth factor (EGF)-repeats of TSP1 activate the (EGF) receptor (EGFR) and ErbB2, and these two receptor protein tyrosine kinases (PTK)s participate in ZA protein tyrosine phosphorylation and barrier disruption in response to the TSP1 stimulus. For the barrier response to TSP1, EGFR/ErbB2 activation is necessary but insufficient. Protein tyrosine phosphatase (PTP)mu counter-regulates phosphorylation of selected tyrosine residues within the cytoplasmic domain of EGFR. Although tenascin, like TSP1, also contains EGF-like repeats and is known to activate EGFR, whether it also opens the paracellular pathway is unknown. In addition to TSP1, tenascin, and the other TSP family members, there are numerous other proteins that also contain EGF-like repeats and participate in hemostasis, wound healing, and tissue remodeling. EGFR not only responds to direct binding of EGF motif-containing ligands but can also be transactivated by a wide range of diverse stimuli. In fact, several established mediators of increased vascular permeability and/or lung injury, including thrombin, tumor necrosis factor-alpha, platelet-activating factor, bradykinin, angiopoietin, and H(2)O(2), transactivate EGFR. It is conceivable that EGFR serves a pivotal signaling role in a final common pathway for the pulmonary response to selected injurious stimuli. SPARC/Osteonectin also increases tyrosine phosphorylation of ZA proteins and opens the endothelial paracellular pathway in a PTK-dependent manner. The expression of the counteradhesive proteins is increased in response to a wide range of injurious stimuli. It is likely that these same molecules participate in the host response to acute lung injury and are operative during the barrier response within the pulmonary microvasculature.

A naturally occurring extracellular alpha-beta clasp contributes to stabilization of beta3 integrins in a bent, resting conformation

Control of alphaIIb beta3 and alphav beta3 integrin activation is critical for cardiovascular homeostasis. Mutations that perturb association of integrin alpha and beta subunits in their transmembrane and cytoplasmic regions activate the integrin heterodimer, suggesting that a low-affinity or "off" conformation is the default state, likely corresponding to the bent conformation seen in the crystal structure of alphav beta3. In this bent structure, a segment of alphav (301-308) and beta3 (560-567) are juxtaposed. Here we provide evidence that these regions of alphav/alphaIIb and beta3 function as a novel extracellular clasp to restrain activation. Synthetic peptides representing the alphaIIb and beta3 clasp regions promote integrin activation as judged by cell adhesion, cell spreading, and exposure of epitopes for three beta3 LIBS antibodies. Mutation of the clasp region of alphav or beta3 results in a constitutively activated integrin, confirming the role of the extracellular clasp in restraining integrin activation. Molecular dynamics simulations of the alphav beta3 structure yield a refined model for the alphav beta3 clasp and provide plausible explanations for the effects of the activating mutations.

CD47: a new target in cardiovascular therapy

CD47, originally named integrin-associated protein, is a receptor for thrombospondin-1. A number of important roles for CD47 have been defined in regulating the migration, proliferation, and survival of vascular cells, and in regulation of innate and adaptive immunity. The recent discovery that thrombospondin-1 acts via CD47 to inhibit nitric oxide signaling throughout the vascular system has given new importance and perhaps a unifying mechanism of action to these enigmatic proteins. Here we trace the development of this exciting new paradigm for CD47 function in vascular physiology.

Thrombospondin-1-induced migration is functionally dependent upon focal adhesion kinase

Vascular smooth muscle cell migration is important in vascular disease. Previously, we showed thrombospondin-1 activates focal adhesion kinase in these cells. We hypothesized that focal adhesion kinase is important for thrombspondin-1-induced vascular smooth muscle cell migration. Bovine aortic smooth muscle cells were transfected with FAK397, FAK-wild type, pcDNA, or beta-Gal plasmids. Migration was assessed with thrombospondin-1 or serum-free medium in quiescent transfected cells or quiescent cells pretreated with the focal adhesion kinase inhibitor, geldanamycin. Number of cells migrated per 5 fields (x400) were recorded. Antihemagglutinin immunoprecipitation and Western blot were used to examine thrombospondin-1-induced focal adhesion kinase phosphorylation in transfected cells. FAK397 transfection inhibited thrombospondin-1-induced focal adhesion kinase phosphorylation and migration (P < .05). Geldanamycin inhibited thrombospondin-1-induced smooth muscle cell migration (P < .05). In conclusion, vascular smooth muscle cells transfected with FAK397 inhibited thrombosponin-1-induced migration and tyrosine phosphorylation. Further, geldanamycin also inhibited migration. These results suggest focal adhesion kinase is involved in thrombospondin-1-induced vascular smooth muscle cell migration.

Purification and analysis of thrombospondin-1

Thromboapondin 1 (TSP-1) is a trimeric matricellular protein that is expressed by many cells. It contains several different domains that allow it to participate in cell adhesion, cell migration, and cell signaling. Recently TSP-1 has been shown to activate transforming growth factor beta (TGF-beta) and to inhibit both angiogenesis and tumor growth. This unit contains protocols for the purification of TSP-1 from platelet-rich plasma and the purification of TSP-1 proteolytic fragments.

Thrombospondin-1 and thrombospondin-2 mRNA and TSP-1 and TSP-2 protein expression in uterine fibroids and correlation to the genes COL1A1 and COL3A1 and to the collagen cross-link hydroxyproline

Uterine fibroids are composed of altered collagen fibrils and represent an arrested response to injury-initiating fibrosis. In many tissues, TSP-1 is secreted by adult macrophages and monocytes upon wounding and is involved in the activation of transforming growth factor beta. In the absence of TSP-1, the orchestrated process of wound healing is impaired. The authors obtained tissue from the edge and center of fibroids at the time of hysterectomy and compared them with adjacent myometrium. The pattern of TSP-1 and TSP-2 expression was correlated to that of COL1A1 and COL3A1. Collagen and hydroxyproline were increased in fibroids. Thrombospondin-1 was consistently underexpressed in both the edge and center of the fibroids, while COL1A1 and COL3A1 were consistently overexpressed. However, TSP-2 was inconsistently expressed. These findings lead to the conclusion that the underexpression of TSP-1 may contribute to the overall development of uterine fibroids.

CD47 associates with alpha 5 integrin and regulates responses of human articular chondrocytes to mechanical stimulation in an in vitro model

Background

Recent studies provide evidence of roles for integrins in mechanical signalling in bone and cartilage. Integrin signalling is modulated by various mechanisms, including interaction with other transmembrane proteins. We aimed to identify whether one such protein, integrin-associated protein (CD47/IAP), is expressed by chondrocytes and whether it may regulate integrin-dependent mechanotransduction.

Methods

Chondrocytes, isolated from macroscopically normal and osteoarthritic articular cartilage of human knee joints, were studied in a resting state or following mechanical stimulation at 0.33 Hz. CD47/IAP expression and associations were confirmed by immunohistology, reverse transcription-polymerase chain reaction, Western blotting, and immunoprecipitation. Roles in mechanotransduction were studied by assessing effects of function-blocking antibodies on a range of electrophysiological, cellular, and molecular responses of primary chondrocytes and responses of CD47/IAP-null cell lines transfected with CD47/IAP.

Results

Human articular chondrocytes were shown to express CD47/IAP, predominantly the type 2 isoform. Immunoprecipitation showed association of CD47/IAP with α5 integrin and thrombospondin but not SIRPα (signal-regulatory protein-alpha). The function-blocking anti-CD47/IAP antibody Bric 126 inhibited changes in membrane potential, tyrosine phosphorylation, and elevation of relative levels of aggrecan mRNA induced by mechanical stimulation, whereas in the presence of B6H12, an antibody that has partial agonist activity, a membrane depolarisation rather than a membrane hyperpolarisation response was induced by mechanical stimulation. CD47-null cell lines did not show changes in cell membrane potential following mechanical stimulation. Changes in cell membrane potential following mechanical stimulation were seen when CD47-null cells were transfected with CD47/IAP expression vectors but were not seen following mechanical stimulation of cells transfected with vectors for the extracellular immunoglobulin variable (IgV) domain of CD47/IAP in the absence of the transmembrane and intracellular domains.

Conclusion

CD47/IAP is necessary for chondrocyte mechanotransduction. Through interactions with α5β1 integrin and thrombospondin, CD47/IAP may modulate chondrocyte responses to mechanical signals.

Blocking thrombospondin-1/CD47 signaling alleviates deleterious effects of aging on tissue responses to ischemia

OBJECTIVE

Decreased blood flow secondary to peripheral vascular disease underlies a significant number of chronic diseases that account for the majority of morbidity and mortality among the elderly. Blood vessel diameter and blood flow are limited by the matricellular protein thrombospondin-1 (TSP1) through its ability to block responses to the endogenous vasodilator nitric oxide (NO). In this study we investigate the role TSP1 plays in regulating blood flow in the presence of advanced age and atherosclerotic vascular disease.

METHODS AND RESULTS

Mice lacking TSP1 or CD47 show minimal loss of their resistance to ischemic injury with age and increased preservation of tissue perfusion immediately after injury. Treatment of WT and apolipoprotein E-null mice using therapeutic agents that decrease CD47 or enhance NO levels reverses the deleterious effects of age- and diet-induced vasculopathy and results in significantly increased tissue survival in models of ischemia.

CONCLUSIONS

With increasing age and diet-induced atherosclerotic vascular disease, TSP1 and its receptor CD47 become more limiting for blood flow and tissue survival after ischemic injury. Drugs that limit TSP1/CD47 regulation of blood flow could improve outcomes from surgical interventions in the elderly and ameliorate vascular complications attendant to aging.

Co-expression of angiogenic markers and associations with prognosis in advanced epithelial ovarian cancer: a Gynecologic Oncology Group study

OBJECTIVES

The aim of this study was to explore the co-expression and prognostic relevance of thrombospondin-1 (THBS-1), basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF) and VEGF receptor 1 (VEGFR-1) in epithelial ovarian cancer (EOC).

METHODS

Frozen tumor specimens with defined p53 status were obtained from 67 patients with previously untreated advanced-stage EOC who participated in a Gynecologic Oncology Group specimen-banking protocol and a phase III treatment protocol. Relative expression of the angiogenic markers was quantified by immunoblot analysis and categorized at the median angiogenic marker/actin ratio. p-values are provided as an indication of confidence in the results and to prioritize further testing.

RESULTS

An association was observed between categorized VEGF and p53 overexpression (p=0.022), and between VEGFR-1 and race (p=0.027) or histologic subtype (p=0.007). Unadjusted Cox regression analyses indicated that high compared with low THBS-1, but not VEGF or VEGFR-1, was associated with an increased risk of disease progression (hazard ratio [HR]=2.19; 95% confidence interval [CI]=1.29-3.71; p=0.004) and death (HR=1.93; 95% CI=1.12-3.32; p=0.018) whereas bFGF was associated with a reduced risk of disease progression (HR=0.60; 95% CI=0.36-0.99; p=0.046) and death (HR=0.54; 95% CI=0.32-0.93; p=0.026). After adjusting for prognostic factors including clinical characteristics and p53 overexpression, THBS-1 but not bFGF, VEGF or VEGFR-1 was associated with progression-free and overall survival.

CONCLUSIONS

These data suggest that high THBS-1 is an independent predictor of worse progression-free and overall survival in women with advanced-stage EOC. A larger prospective study is warranted for validation of these findings.

Increasing Survival of Ischemic Tissue by Targeting CD47

Thrombospondin-1 (TSP1) limits the angiogenic and vasodilator activities of NO. This activity of TSP1 can be beneficial in some disease states, but endogenous TSP1 limits recovery of tissue perfusion following fixed ischemic injury in dorsal skin flaps in mice. Using mice lacking the TSP1 receptors CD36 or CD47, we now show that CD47 is the necessary receptor for limiting NO-mediated vascular smooth muscle relaxation and tissue survival following ischemic injury in skin flaps and hindlimbs. We further show that blocking CD47 or TSP1 using monoclonal antibodies and decreasing CD47 expression using an antisense morpholino oligonucleotide are effective therapeutic approaches to dramatically increase survival of soft tissue subjected to fixed ischemia. These treatments facilitate rapid vascular remodeling to restore tissue perfusion and increase skin and muscle viability. Thus, limiting CD47-dependent antagonism of NO-mediated vasodilation and vascular remodeling is a promising therapeutic modality to preserve tissues subject to ischemic stress.

CD47 regulation of epithelial cell spreading and migration, and its signal transduction

CD47 is an integrin-associated penta-transmembrane protein that possesses an immunoglobulin-like domain in its extracellular region. We have now investigated the role of CD47 in the regulation of epithelial cell spreading and migration. CD47 is colocalized with E-cadherin at cell-cell adhesion sites of epithelial cells. A Ca2+ switch experiment showed that CD47 was endocytosed and then relocalized to cell-cell adhesion sites in a similar manner to E-cadherin. Such polarized localization of CD47 required the multiple spanning region of this protein. Forced expression of CD47 induced cell spreading with marked lamellipodium formation and resulted in both partial disruption of cell-cell adhesion and enhancement of the hepatocyte growth factor-stimulated scattering of Madin-Darby canine kidney cells. The CD47-induced cell spreading was blocked by inhibition of Src and mitogen-activated protein kinase kinase. Thus, these results suggest that CD47 participates in the regulation of cell-cell adhesion and cell migration through reorganization of the actin cytoskeleton in epithelial cells. This function of CD47 is mediated by the activation of Src and mitogen-activated protein kinase kinase.

Txr1: a transcriptional regulator of thrombospondin-1 that modulates cellular sensitivity to taxanes

Using transcripts initiated at a chromosomally integrated retrovirus-based promoter to perturb gene expression randomly in human prostate cancer cells, we isolated cell clones resistant to taxane lethality and discovered the role of a previously uncharacterized gene, txr1, in this phenotype. We show that txr1 impedes taxane-induced apoptosis in tumor cells by transcriptionally down-regulating the production of thrombospondin-1 (TSP-1)--known earlier for both its anti-angiogenic and proapoptotic actions. Decrease of Txr1 or treatment with TSP-1 or TSP-1 mimetic peptide sensitized cells to taxane cytotoxicity by activating signaling through the CD47 receptor (also known as the integrin-associated protein), whereas interference with CD47 function reduced taxane-induced cell death. Cellular abundance of Txr1 and TSP-1 varied inversely, and alteration of the level of both proteins correlated highly with taxol resistance in 13 of 19 NCI-60 cancer cell lines. Our results reveal a hitherto unsuspected mechanism of taxane resistance, elucidate the role of txr1 in this resistance, and identify txr1 as a regulator of TSP-1 production and an agent for its chemotherapeutic modulation.

Melanoma and lymphocyte cell-specific targeting incorporated into a heat shock protein cage architecture

Protein cages, including viral capsids, ferritins, and heat shock proteins (Hsps), can serve as nanocontainers for biomedical applications. They are genetically and chemically malleable platforms, with potential as therapeutic and imaging agent delivery systems. Here, both genetic and chemical strategies were used to impart cell-specific targeting to the Hsp cage from Methanococcus jannaschii. A tumor vasculature targeting peptide was incorporated onto the exterior surface of the Hsp cage. This protein cage bound to alpha(v)beta(3) integrin-expressing cells. Cellular tropism was also imparted by conjugating anti-CD4 antibodies to the exterior of Hsp cages. These Ab-Hsp cage conjugates specifically bound to CD4(+) cells. Protein cages have the potential to simultaneously incorporate multiple functionalities, including cell-specific targeting, imaging, and therapeutic agent delivery. We demonstrate the simultaneous incorporation of two functionalities, imaging and cell-specific targeting, onto the Hsp protein cage.

Endogenous thrombospondin-1 is a cell-surface ligand for regulation of integrin-dependent T-lymphocyte adhesion

Lymphocyte adhesion to cells and extracellular matrix (ECM) via integrins plays a pivotal role for the function of the immune system. We show here that endogenous thrombospondin-1 (TSP-1) is a cell-surface ligand for cis interaction of surface receptors in T lymphocytes controlled by integrins and the T-cell antigen receptor (TCR/CD3). Stimulation of CD3 triggers rapid surface expression of TSP-1 in quiescent T cells, whereas activated cells express TSP-1 constitutively. Endogenous TSP-1 is attached to lipoprotein receptor-related protein 1 (LRP1/CD91) and calreticulin (CRT) on the cell surface through its NH2-terminal domain. Adhesion via integrins to ICAM-1 or ECM components up-regulates TSP turnover dramatically from a low level in nonadherent cells, whereas CD3 stimulation inhibits TSP turnover through interference with CD91/CRT-mediated internalization. Integrin-associated protein (IAP/CD47) is essential for TSP turnover and adhesion through interaction with the C-terminal domain of TSP-1 in response to triggering signals delivered at the NH2-terminal. These results indicate that endogenous TSP-1 connects separate cell-surface receptors functionally and regulates T-cell adhesion.

CD47 is necessary for inhibition of nitric oxide-stimulated vascular cell responses by thrombospondin-1

CD36 is necessary for inhibition of some angiogenic responses by the matricellular glycoprotein thrombospondin-1 and is therefore assumed to be the receptor that mediates its anti-angiogenic activities. Although ligation of CD36 by antibodies, recombinant type 1 repeats of thrombospondin-1, or CD36-binding peptides was sufficient to inhibit nitric oxide (NO)-stimulated responses in both endothelial and vascular smooth muscle cells, picomolar concentrations of native thrombospondin-1 similarly inhibited NO signaling in vascular cells from wild-type and CD36-null mice. Ligation of the thrombospondin-1 receptor CD47 by recombinant C-terminal regions of thrombospondin-1, thrombospondin-1 peptides, or CD47 antibodies was also sufficient to inhibit NO-stimulated phenotypic responses and cGMP signaling in vascular cells. Thrombospondin-1 did not inhibit NO signaling in CD47-null vascular cells or NO-stimulated vascular outgrowth from CD47-null muscle explants in three-dimensional cultures. Furthermore, the CD36-binding domain of thrombospondin-1 and anti-angiogenic peptides derived from this domain failed to inhibit NO signaling in CD47-null cells. Therefore, ligation of either CD36 or CD47 is sufficient to inhibit NO-stimulated vascular cell responses and cGMP signaling, but only CD47 is necessary for this activity of thrombospondin-1 at physiological concentrations.

Two Functional Active Conformations of the Integrin α2β1, Depending on Activation Condition and Cell Type

For several integrins, the existence of multiple conformational states has been studied intensively. For the integrin alpha2beta1, a major collagen receptor on platelets and other cell types, however, no such experimental data were available thus far. Recently, our group has developed a monoclonal antibody IAC-1 sensitive to the molecular conformation of alpha2beta1 because it only binds to the activated state of alpha2beta1 on platelets, induced upon inside-out signaling. By investigating IAC-1 binding in combination with collagen binding after inside-out stimulation and outside manipulation, we demonstrated the existence of three different conformations of alpha2beta1 on platelets and Chinese hamster ovary cells as follows: (i) a nonactivated, resting state with no collagen nor IAC-1 binding; (ii) an intermediate state, induced by outside manipulation, with collagen but no IAC-1 binding; and (iii) a fully activated state, induced after inside-out stimulation, with both collagen and IAC-1 binding. Moreover, these different conformational states of alpha2beta1 are dependent on the cell type where alpha2beta1 is expressed, as IAC-1 binding to peripheral blood mononuclear cells and Jurkat cells could also be induced by outside manipulation, in contrast to platelets and alpha2beta1-expressing Chinese hamster ovary cells. Finally, we revealed a functional relevance for these different conformational states because the conformation of alpha2beta1, induced after outside manipulation, resulted in significantly more cell spreading on coated collagen compared with nonactivated or inside-out stimulated cells.

Human alpha1 type IV collagen NC1 domain exhibits distinct antiangiogenic activity mediated by alpha1beta1 integrin

Human noncollagenous domain 1 of the alpha1 chain of type IV collagen [alpha1(IV)NC1], or arresten, is derived from the carboxy terminal of type IV collagen. It was shown to inhibit angiogenesis and tumor growth in vivo; however, the mechanisms involved are not known. In the present study we demonstrate that human alpha1(IV)NC1 binds to alpha1beta1 integrin, competes with type IV collagen binding to alpha1beta1 integrin, and inhibits migration, proliferation, and tube formation by ECs. Also, alpha1(IV)NC1 pretreatment inhibited FAK/c-Raf/MEK/ERK1/2/p38 MAPK activation in ECs but had no effect on the PI3K/Akt pathway. In contrast, alpha1(IV)NC1 did not affect proliferation, migration, or the activation of FAK/c-Raf/MEK1/2/p38/ERK1 MAPK pathway in alpha1 integrin receptor knockout ECs. Consistent with this, alpha1(IV)NC1 elicited significant antiangiogenic effects and tumor growth inhibition in vivo but failed to do the same in alpha1 integrin receptor knockout mice. This suggests a highly specific, alpha1beta1 integrin-dependent antiangiogenic activity of alpha1(IV)NC1. In addition, alpha1(IV)NC1 inhibited hypoxia-induced expression of hypoxia-inducible factor 1alpha and VEGF in ECs cultured on type IV collagen by inhibiting ERK1/2 and p38 activation. This unravels a hitherto unknown function of human alpha1(IV)NC1 and suggests a critical role for integrins in hypoxia and hypoxia-induced angiogenesis. Collectively, the above data indicate that alpha1(IV)NC1 is a potential therapeutic candidate for targeting tumor angiogenesis.

Role of phosphatidylinositol 3-kinase-gamma in mediating lung neutrophil sequestration and vascular injury induced by E. coli sepsis

We addressed the in vivo role of phosphatidylinositol 3-kinase-gamma (PI3K-gamma) in signaling the sequestration of polymorphonuclear leukocytes (PMNs) in lungs and in the mechanism of inflammatory lung vascular injury. We studied mice with deletion of the p110 catalytic subunit of PI3K-gamma (PI3K-gamma(-/-) mice). We measured lung tissue PMN sequestration, microvascular permeability, and edema formation after bacteremia induced by intraperitoneal Escherichia coli challenge. PMN infiltration into the lung interstitium in PI3K-gamma(-/-) mice as assessed morphometrically was increased 100% over that in control mice within 1 h after bacterial challenge. PI3K-gamma(-/-) mice also developed a greater increase in lung microvascular permeability after E. coli challenge, resulting in edema formation. The augmented lung tissue PMN sequestration in PI3K-gamma(-/-) mice was associated with increased expression of the PMN adhesive proteins CD47 and beta(3)-integrins. We observed increased association of CD47 and beta(3)-integrins with the extracellular matrix protein vitronectin in lungs of PI3K-gamma(-/-) mice after E. coli challenge. PMNs from these mice also showed increased beta(3)-integrin expression and augmented beta(3)-integrin-dependent PMN adhesion to vitronectin. These results point to a key role of PMN PI3K-gamma in negatively regulating CD47 and beta(3)-integrin expression in gram-negative sepsis. PI3K-gamma activation in PMNs induced by E. coli may modulate the extent of lung tissue PMN sequestration secondary to CD47 and beta(3)-integrin expression. Therefore, the level of PI3K-gamma activation may be an important determinant of PMN-dependent lung vascular injury.

Thrombospondin-1 up-regulates expression of cell adhesion molecules and promotes monocyte binding to endothelium

Expression of cell adhesion molecules (CAM) responsible for leukocyte-endothelium interactions plays a crucial role in inflammation and atherogenesis. Up-regulation of vascular CAM-1 (VCAM-1), intracellular CAM-1 (ICAM-1), and E-selectin expression promotes monocyte recruitment to sites of injury and is considered to be a critical step in atherosclerotic plaque development. Factors that trigger this initial response are not well understood. As platelet activation not only promotes thrombosis but also early stages of atherogenesis, we considered the role of thrombospondin-1 (TSP-1), a matricellular protein released in abundance from activated platelets and accumulated in sites of vascular injury, as a regulator of CAM expression. TSP-1 induced expression of VCAM-1 and ICAM-1 on endothelium of various origins, which in turn, resulted in a significant increase of monocyte attachment. This effect could be mimicked by a peptide derived from the C-terminal domain of TSP-1 and known to interact with CD47 on the cell surface. The essential role of CD47 in the cellular responses to TSP-1 was demonstrated further using inhibitory antibodies and knockdown of CD47 with small interfering RNA. Furthermore, we demonstrated that secretion of endogenous TSP-1 and its interaction with CD47 on the cell surface mediates endothelial response to the major proinflammatory agent, tumor necrosis factor alpha (TNF-alpha). Taken together, this study identifies a novel mechanism regulating CAM expression and subsequent monocyte binding to endothelium, which might influence the development of anti-atherosclerosis therapeutic strategies.

Interleukin-12: an update on its immunological activities, signaling and regulation of gene expression

Interleukin-12 (IL-12) is a heterodimeric cytokine composed of the p35 and p40 subunits. It is produced by antigen-presenting cells and plays a critical role in host defense against intracellular microbial infection and control of malignancy via its ability to stimulate both innate and adaptive immune effector cells. The potency of IL-12 renders itself to stringent regulation of the timing, locality and magnitude of its production during an immune response. Subversion of the delicate control and balance frequently leads to immunologic disorders. In this article, we provide an update, since our last review of the subject four years ago, on recent advances in: (1) uncovering of novel activities of IL-12 and related molecules in various immunological settings and models; and (2) dissection of the physiological pathways involved in the modulation of IL-12 production by pathogens and immune regulators. The increased understanding of IL-12 immunobiology and expression will likely benefit the development of therapeutic modalities to correct immune dysfunctions.

Autocrine Regulation of T Cell Motility by Calreticulin-Thrombospondin-1 Interaction

The mechanisms regulating T lymphocyte migration within the extracellular matrix are not understood. We show in this study that the thrombospondin-1 binding site of calreticulin, spanning aa 19-32, is a major triggering factor for T cell motility and migration within a three-dimensional collagen type 1 matrix, and that exogenous motogenic factors such as chemokines can stimulate migration via a calreticulin-thrombospondin-1 pathway. Endogenous calreticulin binding to the N-terminal domain of endogenous thrombospondin-1 elicited a motogenic signal to the T cells through the C-terminal domain of thrombospondin-1 and its cell surface receptor integrin-associated protein (CD47). Our data further revealed that thrombospondin-1 was expressed on the cell surface with a high turnover, and that PI3K and the Janus family of tyrosine kinases were required for T cell motility mediated through calreticulin, thrombospondin-1, and CD47. These results unveil an autocrine mechanism of calreticulin-thrombospondin-1-CD47 interaction for the control of T cell motility and migration within three-dimensional extracellular matrix substrata.

Insulin-like growth factor binding protein-5 (IGFBP-5) interacts with thrombospondin-1 to induce negative regulatory effects on IGF-I actions

Insulin-like growth factor binding protein-5 (IGFBP-5) and thrombospondin-1 (TS-1) are both present in extracellular matrix (ECM). Both proteins have been shown to bind to one another with high affinity. The purpose of these studies was to determine how the interaction between IGFBP-5 and TS-1 modulates IGF-I actions in porcine aortic smooth muscle cells (pSMC) in culture. The addition of increasing concentrations of TS-1 to pSMC cultures enhanced the protein synthesis and cell migration responses to IGF-I; whereas the addition of IGFBP-5 alone resulted in minimal changes. In contrast, the addition of IGFBP-5 to cultures that were also exposed to IGF-I and TS-1 resulted in inhibition of protein synthesis. When the cell migration response was assessed, the response to IGF-I plus TS-1 was also significantly inhibited by the addition of IGFBP-5, whereas 1.0 microg/ml of IGFBP-5 alone had no effect on the response to IGF-I. To determine the molecular mechanism by which this inhibition occurred, a mutant form of IGFBP-5 that does not bind to IGF-I was tested. This mutant was equipotent compared to native IGFBP-5 in its ability to inhibit both protein synthesis and cell migration responses to IGF-I plus TS-1 thus excluding the possibility that IGFBP-5 was inhibiting the response to TS-1 and IGF-I by inhibiting IGF-I binding to the IGF-I receptor. To determine if an interaction between TS-1 and IGFBP-5 was the primary determinant of the inhibitory effect of IGFBP-5, an IGFBP-5 mutant that bound poorly to TS-1 was utilized. The addition of 1.0 microg/ml of this mutant did not inhibit the protein synthesis or cell migration responses to IGF-I plus TS-1. To determine the mechanism by which IGFBP-5 binding to TS-1 inhibited cellular responses to TS-1 plus IGF-I, TS-1 binding to integrin associated protein (IAP) was assessed. The addition of IGFBP-5 (1.0 microg/ml) inhibited TS-1-IAP association. In contrast, a mutant form of IGFBP-5 that bound poorly to TS-1 had a minimal effect on TS-1 binding to IAP. Further analysis showed that IGFBP-5 addition altered the ability of TS-1 to modulate the SHPS-1/IAP interaction. When the IGFBP-5 mutant that did not bind to IGF-I was incubated with TS-1 and IGF-I, it inhibited the capacity of TS-1 to enhance the IGF-I receptor phosphorylation and MAP kinase activation in response to IGF-I. In contrast, the IGFBP-5 mutant that did not bind to TS-1 had no effect on IGF-I stimulated IGF-I receptor phosphorylation or MAP kinase activation. These results indicate that IGFBP-5 inhibits the binding of TS-1 to IAP, and this results in an alteration of the ability of TS-1 to modulate the disruption of the IAP/SHPS-1 interaction which leads to attenuation of the ability of TS-1 to enhance cellular responsiveness to IGF-I.

The syndecan-1 ectodomain regulates alphavbeta3 integrin activity in human mammary carcinoma cells

The α_vβ₃ integrin participates in cell morphogenesis, growth factor signaling, and cell survival. Activation of the integrin is central to these processes and is influenced by specific ECM components, which engage both integrins and syndecans. This paper demonstrates that the α_vβ₃ integrin and syndecan-1 (S1) are functionally coupled. The integrin is dependent on the syndecan to become activated and to mediate signals required for MDA-MB-231 and MDA-MB-435 human mammary carcinoma cell spreading on vitronectin or S1-specific antibody. Coupling of the syndecan to α_vβ₃ requires the S1 ectodomain (ED), as ectopic expression of glycosylphosphatidylinositol-linked S1ED enhances α_vβ₃ recognition of vitronectin; and treatments that target this domain, including competition with recombinant S1ED protein or anti-S1ED antibodies, mutation of the S1ED, or down-regulation of S1 expression by small-interfering RNAs, disrupt α_vβ₃-dependent cell spreading and migration. Thus, S1 is likely to be a critical regulator of many cellular behaviors that depend on activated α_vβ₃ integrins.

Interaction between insulin-like growth factor-I receptor and alphaVbeta3 integrin linked signaling pathways: cellular responses to changes in multiple signaling inputs

Integrins are heterodimeric transmembrane proteins that mediate cell attachment to extracellular matrix, migration, division, and inhibition of apoptosis. Because growth factors are also important for these processes, there has been interest in cooperative signaling between growth factor receptors and integrins. IGF-I is an important growth factor for vascular cells. One integrin, alphaVbeta3, that is expressed in smooth muscle cells modulates IGF-I actions. Ligand occupancy of alphaVbeta3 is required for IGF-I to stimulate cell migration and division. Src homology 2 containing tyrosine phosphatase (SHP-2) is a tyrosine phosphatase whose recruitment to signaling molecules is stimulated by growth factors including IGF-I. If alphaVbeta3 ligand occupancy is inhibited, there is no recruitment of SHP-2 to alphaVbeta3 and its transfer to downstream signaling molecules is blocked. Ligand occupancy of alphaVbeta3 stimulates tyrosine phosphorylation of the beta3-subunit, resulting in recruitment of SHP-2. This transfer is mediated by an insulin receptor substrate-1-related protein termed DOK-1. Subsequently, SHP-2 is transferred to another transmembrane protein, SHPS-1. This transfer requires IGF-I receptor-mediated tyrosine phosphorylation of SHPS-1, which contains two YXXL motifs that mediate SHP-2 binding. The transfer of SHP-2 to SHPS-1 is also required for recruitment of Shc to SHPS-1. Ligand occupancy of alphaVbeta3 results in sustained Shc phosphorylation and enhanced Shc recruitment. Shc activation results in induction of MAPK. Inhibition of the Shc/SHPS-1 complex formation results in failure to achieve sustained MAPK activation and an attenuated mitogenic response. Thus, within the vessel wall, a mechanism exists whereby ligand occupancy of the alphaVbeta3 integrin is required for assembly of a multicomponent membrane signaling complex that is necessary for cells to respond optimally to IGF-I.

Whole genome expression profiling of advance stage papillary serous ovarian cancer reveals activated pathways

Ovarian cancer is the most lethal type of gynecologic cancer in the Western world. The high case fatality rate is due in part because most ovarian cancer patients present with advanced stage disease which is essentially incurable. In order to obtain a whole genome assessment of aberrant gene expression in advanced ovarian cancer, we used oligonucleotide microarrays comprising over 40,000 features to profile 37 advanced stage papillary serous primary carcinomas. We identified 1191 genes that were significantly (P < 0.001) differentially regulated between the ovarian cancer specimens and normal ovarian surface epithelium. The microarray data were validated using real time RT-PCR on 14 randomly selected differentially regulated genes. The list of differentially expressed genes includes ones that are involved in cell growth, differentiation, adhesion, apoptosis and migration. In addition, numerous genes whose function remains to be elucidated were also identified. The microarray data were imported into PathwayAssist software to identify signaling pathways involved in ovarian cancer tumorigenesis. Based on our expression results, a signaling pathway associated with tumor cell migration, spread and invasion was identified as being activated in advanced ovarian cancer. The data generated in this study represent a comprehensive list of genes aberrantly expressed in serous papillary ovarian adenocarcinoma and may be useful for the identification of potentially new and novel markers and therapeutic targets for ovarian cancer.

Expression of the type-1 repeats of thrombospondin-1 inhibits tumor growth through activation of transforming growth factor-beta

In the present study, the type-1 repeats of thrombospondin-1 (TSP-1) were transfected into A431 cells. Expression of all three type-1 repeats (3TSR) and expression of just the second type-1 repeat containing the transforming growth factor (TGF)-beta activating sequence KRFK (TSR2 + KRFK) significantly inhibited in vivo tumor angiogenesis and growth in nude mice. These tumors expressed increased levels of both active and total TGF-beta. A431 cells expressing the second type-1 repeat without the KRFK sequence (TSR2 - KRFK) produced tumors that were slightly larger than the 3TSR and TSR2 + KRFK tumors. These tumors expressed elevated levels of active TGF-beta but levels of total TGF-beta were not different from control tumors. Injection of the peptide, LSKL, which blocks TSP-1 activation of TGF-beta, reversed the growth inhibition observed with cells expressing TSR2 + KRFK to a level comparable to controls. Various residues in the WSHWSPW region and the VTCG sequence of both TSR2+/- KRFK were mutated. Although mutation of the VTCG sequence had no significant effect on tumor growth, mutation of the WSHWSPW sequence reduced inhibition of tumor growth. These findings suggest that the inhibition of tumor angiogenesis and growth by endogenous TSP-1 involves regulation of both active and total TGF-beta and the sequences KRFK and WSHWSPW in the second type-1 repeat.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

Neuronal roles of the integrin-associated protein (IAP/CD47) in developing cortical neurons

Little is known about the role of the integrin-associated protein (IAP, or CD47) in neuronal development and its function in the central nervous system. We investigated neuronal responses in IAP-overexpressing cortical neurons using a virus-gene transfer system. We found that dendritic outgrowth was significantly enhanced in IAP (form 4)-transfected neurons. Furthermore, synaptic proteins including synaptotagmin, syntaxin, synapsin I, and SNAP25 (25-kDa synaptosomal associated protein) were up-regulated. In accordance with this finding, the release of the excitatory transmitter glutamate and the frequencies of Ca2+ oscillations (glutamate-mediated synaptic transmission) were increased. Interestingly, the overexpression of IAP activated mitogen-activated protein kinase (MAPK), and this activation was required for the IAP-dependent biological effects. After down-regulation of the endogenous IAP by small interfering RNA, MAPK activity, synaptic protein levels, and glutamate release decreased. These observations suggest that the IAP plays important roles in dendritic outgrowth and synaptic transmission in developing cortical neurons through the activation of MAPK.

Promotion of neurite and filopodium formation by CD47: roles of integrins, Rac, and Cdc42

Axon extension during development is guided by many factors, but the signaling mechanisms responsible for its regulation remain largely unknown. We have now investigated the role of the transmembrane protein CD47 in this process in N1E-115 neuroblastoma cells. Forced expression of CD47 induced the formation of neurites and filopodia. Furthermore, an Fc fusion protein containing the extracellular region of the CD47 ligand SHPS-1 induced filopodium formation, and this effect was enhanced by CD47 overexpression. SHPS-1-Fc also promoted neurite and filopodium formation triggered by serum deprivation. Inhibition of Rac or Cdc42 preferentially blocked CD47-induced formation of neurites and filopodia, respectively. Overexpression of CD47 resulted in the activation of both Rac and Cdc42. The extracellular region of CD47 was sufficient for the induction of neurite formation by forced expression, but the entire structure of CD47 was required for enhancement of filopodium formation by SHPS-1-Fc. Neurite formation induced by CD47 was also inhibited by a mAb to the integrin beta3 subunit. These results indicate that the interaction of SHPS-1 with CD47 promotes neurite and filopodium formation through the activation of Rac and Cdc42, and that integrins containing the beta3 subunit participate in the effect of CD47 on neurite formation.

CD47 mediates killing of breast tumor cells via Gi-dependent inhibition of protein kinase A

Thrombospondins (TSPs) have been implicated as antitumor and antimetastasis factors in breast cancer. Although this effect has been attributed to the antiangiogenic activity of TSPs, recent observations suggest other mechanisms may be at work. The TSP receptor CD47 (integrin-associated protein) has recently been reported to mediate a novel form of apoptosis. Here, we have studied the response of breast cancer cells to CD47 ligands TSP-1, the CD47 agonist peptide 4N1K derived from TSP-1, and the anti-CD47 monoclonal antibody 1F7. All of these ligands killed four different breast cancer cell lines. This CD47-mediated cell death did not require active caspases or Bcl-2 degradation and did not cause DNA laddering or cytochrome c release. Pertussis toxin (PTX) prevented CD47-mediated death, indicating the involvement of Gi alpha. 4N1K dramatically reduced intracellular cAMP levels, an effect reversed with PTX. Forskolin, 8-bromo cAMP, and isobutylmethylxanthine (IBMX) all prevented CD47-mediated apoptosis, indicating the involvement of cAMP. H89 and protein kinase A (PKA) inhibitor peptide prevented rescue of breast cancer cells by PTX, 8-Br-cAMP, and forskolin, suggesting that the effects of cAMP are mediated via PKA-dependent phosphorylation events. Epidermal growth factor also inhibited CD47-induced apoptosis via a PKC-dependent but ERK-independent pathway. Thus, CD47-mediated killing of breast cancer cells occurs by a novel pathway involving regulation of cAMP levels by heterotrimeric Gi with subsequent effects mediated by PKA.

Cholesterol-independent Interactions with CD47 Enhance αvβ3 Avidity

Expression in OV10 cells of either wild-type CD47 or its extracellular IgV domain linked to a glycosylphosphatidylinositol anchor-(IgV-GPI) enhanced ligand-induced alpha(v)beta(3) activation as detected by the binding of LIBS1 and LIBS6 mAbs. The amplitude of LIBS binding was greater with both CD47 and IgV-GPI expression, indicating an increase in the population of "activable" integrin molecules. Expression of either CD47 species also increased alpha(v)beta(3)-mediated adhesion to vitronectin, and to surfaces coated with the anti-beta(3) antibody AP3, because of enhanced clustering of alpha(v)beta(3) as confirmed by chemical cross-linking. Cholesterol depletion with methyl-beta-cyclodextrin did not prevent the increase in anti-LIBS binding, but reduced cell adhesion to vitronectin and AP3. However, cells expressing CD47 were partially insulated against this disruption, and IgV-GPI was even more effective. Both CD47 and IgV-GPI were found in cholesterol-rich rafts prepared in the absence of detergent, but only CD47 could recruit alpha(v)beta(3) and its associated signaling molecules to these domains. Thus CD47-alpha(v)beta(3) complexes in cholesterol-rich raft domains appear to engage in G(i)-dependent signaling whereas CD47-alpha(v)beta(3) interactions that lead to integrin clustering are also detergent resistant, but are insensitive to cholesterol depletion and do not require the transmembrane region of CD47.

Positional control of cell fate through joint integrin/receptor protein kinase signaling

Cells adhere to the extracellular matrix throughout most of their lifetime. This close, intimate contact with the matrix exerts an extraordinary control on the behavior of cells, determining whether they move or stay put, proliferate or remain quiescent, and even live or die. Attachment to the matrix not only enables cells to respond to soluble growth factors and cytokines but also determines the nature of the response. The integrins are a large family of receptors that attach cells to the matrix, organize their cytoskeleton, and cooperate with receptor protein tyrosine kinases to regulate cell fate. Research on integrin signaling is beginning to explain the complex and specific effects that the extracellular matrix exerts on cells.

Ligation of CD47 During Monocyte Differentiation into Dendritic Cells Results in Reduced Capacity for Interleukin-12 Production

We have previously shown that the CD47-binding thrombospondin-1 (TSP-1)-derived peptide 4N1K induces a rapid apoptosis-like death of human monocytes and dendritic cells (DCs). However, not all cells were susceptible to the peptide-induced cell death and here, we have investigated whether surviving monocytes could differentiate into functionally normal DCs. We found that the cell-surface phenotype, the T-cell stimulatory capacity and the ability to undergo lipopolysaccharide (LPS)-induced maturation into CD83+ DCs were essentially identical in 4N1K-derived and control DCs. Interleukin-10 (IL-10) production was also normal, but a significant downregulation of the pro-inflammatory cytokines IL-12 and tumour necrosis factor-alpha (TNF-alpha) was observed in the 4N1K-derived DCs. To the contrary, simultaneous stimulation of control DCs with 4N1K and LPS + interferon-gamma did not alter IL-12 production. These results indicate that although activation of the TSP-1-binding region of CD47 on monocytes induces apoptosis in a large proportion of the cells, it does not hamper the overall capacity of the surviving cells to differentiate into DCs. Such DCs, however, have a reduced capacity for IL-12 and TNF-alpha production, and the possibility that this is linked to the uptake of apoptotic cells is discussed.

CD47 Ligation Induces a Rapid Caspase-Independent Apoptosis-Like Cell Death in Human Monocytes and Dendritic Cells

CD47 is a versatile cell-surface molecule expressed on nearly all haematopoietic cells. In its capacity as a thrombospondin-1 (TSP-1) receptor, CD47 has recently been shown to mediate cell death in certain cells, for example, activated but not resting T cells. Here, we have investigated the possibility that human monocytes and dendritic cells (DCs) undergo cell death, following CD47 ligation. Using the TSP-1-derived CD47-binding peptide 4N1K, we found that both freshly isolated monocytes and monocyte-derived DCs underwent a rapid, caspase-independent cell death. This was characterized by the simultaneous presence of phosphatidylserine exposure, plasma membrane permeability, reduced mitochondrial membrane potential and highly fragmented DNA. Not all cells were sensitive to 4N1K-induced apoptosis; a plateau of cell death reached at an average of 38% of the monocyte and DCs populations. The results presented here, thus, show that CD47 can mediate a rapid apoptosis-like cell death of human monocytes and DCs.

A molecule bearing an immunoglobulin-like V region of the CTX subfamily in amphioxus

An expressed sequence tag with significant similarity to a vertebrate T-cell receptor (Tcr) sequence was found in a cDNA library prepared from the posterior part of the adult amphioxus, Branchiostoma lanceolatum ( Brla). Characterization of the corresponding cDNA clone revealed the presence of an open reading frame encoding a 351 amino acid residue-long polypeptide. The putative protein, tentatively designated Brla-VDB for "V-domain bearing", appears to consist of two domains. The N-terminal domain begins with a putative leader peptide followed by a sequence resembling the V domain of the CTX protein originally found to be expressed in the cortical thymocytes of the clawed frog Xenopus. The C-terminal domain of the VDB protein does not show significant sequence similarity to any entry in the databases and contains five hydrophobic segments separated by short intervening hydrophilic stretches. It may therefore belong to a protein that crosses the plasma membrane five times. These findings support the notion that V domains resembling those found in Tcrs evolved in nonvertebrates before the emergence of the adaptive immune system and may have participated in functions not involved directly in immunity.

Thrombospondin-bound integrin-associated protein (CD47) physically and functionally modifies integrin alphaIIbbeta3 by its extracellular domain

Integrin-associated protein (IAP/CD47) is a receptor for the C-terminal cell binding domain of thrombospondin (TS). A peptide from the C-terminal cell binding domain, KRFYVVMWKK (4N1K) binds to IAP and stimulates the integrin-dependent cell functions, including platelet aggregation. We investigated the mechanism by which TS-bound IAP modulates the affinity of platelet integrin, alphaIIbbeta3. Platelet aggregation induced by 4N1K was not completely inhibited by energy depletion with sodium azide and 2-deoxy-d-glucose, although ADP or collagen-induced platelet response was completely inhibited. The binding of ligand-mimetic antibody PAC1 to alphaIIbbeta3 was also induced in the energy-depleted platelets. In the transfected Namalwa cells, 4N1K induced activation of the alphaIIbbeta3 with mutated beta3 (Ser-752 to Pro), which is a non-responsive form to inside-out signaling, as well as wild type alphaIIbbeta3. The truncated form of IAP with only the extracellular immunoglobulin-like (Ig) domain was sufficient for the activation of alphaIIbbeta3 in Chinese hamster ovary cells, although the IAP-mediated intracellular signaling was abolished, which was monitored by the absence of down-regulation of mitogen-activated protein kinase phosphorylation. Furthermore, the soluble recombinant Ig domain of IAP induced PAC1 binding to alphaIIbbeta3 on Chinese hamster ovary cells when added with 4N1K. Physical association between the soluble recombinant Ig domain of IAP and purified alphaIIbbeta3 was detected in the presence of 4N1K. These data indicate that the extracellular Ig domain of IAP, when bound to TS, interacts with alphaIIbbeta3 and can change alphaIIbbeta3 in a high affinity state without the requirement of intracellular signaling. This extracellular event would be a novel mechanism of affinity modulation of integrin.

Single-chain antibody fragments derived from a human synthetic phage-display library bind thrombospondin and inhibit sickle cell adhesion

The enhanced adhesion of sickle red blood cells (RBCs) to the vascular endothelium and subendothelial matrix likely plays a significant role in the pathogenesis of vaso-occlusion in sickle cell disease. Sickle RBCs have enhanced adhesion to the plasma and extracellular matrix protein thrombospondin-1 (TSP) under conditions of flow in vitro. In this study, we sought to develop antibodies that bind TSP from a highly diverse library of human single-chain Fv fragments (scFvs) displayed on filamentous phage. Following 3 rounds of phage selection of increasing stringency 6 unique scFvs that bound purified TSP by enzyme-linked immunosorbent assay were isolated. Using an in vitro flow adhesion assay, 3 of the 6 isolated scFvs inhibited the adhesion of sickle RBCs to immobilized TSP by more than 40% compared with control scFvs (P <.001). Furthermore, scFv TSP-A10 partially inhibited sickle RBC adhesion to activated endothelial cells (P <.005). Using TSP proteolytic fragments to map the binding site, we showed that 2 of the inhibitory scFvs bound an epitope in the calcium-binding domain or proximal cell-binding domain of TSP, providing evidence for the role of these domains in the adhesion of sickle RBCs to TSP. In summary, we have isolated a panel of scFvs that specifically bind to TSP and differentially inhibit sickle RBC adhesion to surface-bound TSP under flow conditions. These scFvs will be useful reagents for investigating the role of the calcium and cell-binding domains of TSP in sickle RBC adhesion.

CD47 and the 19 kDa interacting protein-3 (BNIP3) in T cell apoptosis

CD47 is a surface receptor that induces either coactivation or apoptosis in lymphocytes, depending on the ligand(s) bound. Interestingly, the apoptotic pathway is independent of caspase activation and cytochrome c release and is accompanied by early mitochondrial dysfunction with suppression of mitochondrial membrane potential (Deltapsim). Using CD47 as bait in a yeast two-hybrid system, we identified the Bcl-2 homology 3 (BH3)-only protein 19 kDa interacting protein-3 (BNIP3), a pro-apoptotic member of the Bcl-2 family, as a novel partner. Interaction between CD47 and the BH3-only protein was confirmed by immunoprecipitation analysis, and CD47-induced apoptosis was inhibited by attenuating BNIP3 expression with antisense oligonucleotides. Finally, we showed that the C-terminal domain of thrombospondin-1 (TSP-1), but not signal-regulatory protein (SIRPalpha1), is the ligand for CD47 involved in inducing cell death. Immunofluorescence analysis of CD47 and BNIP3 revealed a partial colocalization of both molecules under basal conditions. After T cell stimulation via CD47, BNIP3 translocates to the mitochondria to induce apoptosis. These results show that the BH3-dependent apoptotic pathways, previously shown to be activated by intracellular pro-apoptotic events, can also be turned on by surface receptors. This new pathway results in a fast induction of cell death resembling necrosis, which is likely to play an important role in lymphocyte regulation at inflammatory sites and/or in the vicinity of thrombosis.

Involvement of a CD47-dependent pathway in platelet adhesion on inflamed vascular endothelium under flow

Resting platelet adhesion to inflammatory vascular endothelium is thought to play a causal role in secondary thrombus formation or microcirculatory disturbance after vessel occlusion. However, though adhesion receptors involved in platelet-matrix interactions have been extensively studied, the molecular mechanisms involved in platelet-endothelium interactions are incompletely characterized and have been mainly studied under static conditions. Using human platelets or platelets from wild-type and CD47-/- mice in whole blood, we demonstrated that at low shear rate, CD47 expressed on human and mouse platelets significantly contributes to platelet adhesion on tumor necrosis factor-alpha (TNF-alpha)-stimulated vascular endothelial cells. Using the CD47 agonist peptide 4N1K and blocking monoclonal antibodies (mAbs), we showed that CD47 binds the cell-binding domain (CBD) of endothelial thrombospondin-1 (TSP-1), inducing activation of the platelet alphaIIbbeta3 integrin that in turn becomes able to link the endothelial receptors intercellular adhesion molecule 1 (ICAM-1) and alphavbeta3. Platelet CD36 and GPIbalpha are also involved because platelet incubation with blocking mAbs directed against each of these 2 receptors significantly decreased platelet arrest. Given that anti-CD47 treatment of platelets did not further decrease the adhesion of anti-CD36-treated platelets and CD36 is a TSP-1 receptor, it appears that CD36/TSP-1 interaction could trigger the CD47-dependent pathway. Overall, CD47 antagonists may be potentially useful to inhibit platelet adhesion on inflamed endothelium.

Thrombospondin-1-induced vascular smooth muscle cell chemotaxis: the role of the type 3 repeat and carboxyl terminal domains

Thrombospondin-1 (TSP-1), an acute phase reactant implicated in vascular disease, is a matricellular glycoprotein with six domains that confer different functions. The authors have shown TSP-1 induces vascular smooth muscle cell (VSMC) chemotaxis via extracellular signal-regulated kinases-1 and -2 (ERK) and p38 kinase (p38) and that a fusion protein of the carboxyl terminal (COOH) and type 3 repeat (T3) domains independently induce VSMC chemotaxis. The purpose of this study was to determine whether COOH-, T3-induced VSMC chemotaxis, or both, is dependent upon ERK or p38 activation. To determine if the T3, COOH, or type 2 repeat domain (T2, control domain not associated with chemotaxis) activate ERK, p38, or both, VSMCs were exposed to each fusion protein (20 microg/ml for 15, 30, 60, or 120 min), serum-free media (SFM, negative control), or TSP-1 (20 microg/ml for 30 min, positive control). Western immunoblotting was performed for activation studies. Using a microchemotaxis chamber, VSMCs pre-incubated in SFM, DMSO (vehicle control), PD98059 (10 microM), or SB202190 (10 microM) were exposed to each domain, TSP-1, or SFM. After 4 h (37 degrees C), migrated VSMCs were recorded as cells/five fields (400 x) and analyzed by paired t-test. ERK was activated by T2, T3, and COOH. However, p38 was activated by T3 and COOH, but not T2. T3 and COOH-induced VSMC chemotaxis were inhibited by PD98059 or SB202190, but more completely by SB202190. The T2 domain had no effect on VSMC chemotaxis. These results suggest activation of the p38 pathway may be more specific than ERK for COOH- and T3-induced VSMC chemotaxis.

The mechanism of CD47-dependent killing of T cells: heterotrimeric Gi-dependent inhibition of protein kinase A

CD47 has been implicated in both positive and negative regulation of T cells as well as in T cell death. To clarify the role of CD47 in T cell function, we have studied the mechanism of T cell death in response to CD47 ligands, including mAb 1F7, thrombospondin-1, and a CD47 agonist peptide derived from it. CD47(-/-) Jurkat T cells (JINB8) were resistant to killing by all three ligands, indicating the essential role of CD47. Primary human T cells were also killed by CD47 ligands, but only after activation with anti-CD3. CD47-mediated cell death occurred without active caspases, DNA fragmentation, or Bcl-2 degradation. Pretreatment of Jurkat and primary T cells with pertussis toxin (PTX) prevented CD47-mediated death, indicating the involvement of G((i)alpha). Pretreatment of T cells with 8-bromo cAMP, forskolin, or 3-isobutyl-1-methylxanthine prevented the CD47-mediated apoptosis, and 1F7 dramatically reduced intracellular cAMP levels, an effect reversed with PTX. H89 and protein kinase A (PKA) inhibitor peptide, a specific PKA inhibitor, prevented rescue of T cells by PTX, 8-bromo cAMP, and forskolin, indicating a direct role for one or more PKA substrates. Thus, CD47-mediated killing of activated T cells occurs by a novel pathway involving regulation of cAMP levels by heterotrimeric G((i)alpha) with subsequent effects mediated by PKA.

Thrombospondin-1 differentially induces chemotaxis and DNA synthesis of human venous smooth muscle cells at the receptor-binding level

Thrombospondin-1 is a large matricellular protein that acts as a pleiotropic growth factor for human vascular smooth muscle cells, and may play a role in the progression of vascular disease. Although we have previously demonstrated the dependence of both thrombospondin-1-stimulated cell chemotaxis and proliferation on tyrosine kinases, the receptor mechanisms involved remain obscure. This investigation aims to determine the nature of the receptor(s) involved in the cellular responses to thrombospondin-1. Cellular signals were identified by western blotting following cell stimulation, while cellular responses were assessed by measuring DNA synthesis and chemotaxis. These data demonstrate that thrombospondin-1-induced cell chemotaxis can be inhibited by a peptide containing the Arg-Gly-Asp motif, a function-blocking alpha(v)beta(3) antibody, a function-blocking integrin-associated protein (IAP) antibody and pertussis toxin, while thrombospondin-1-stimulated DNA synthesis is inhibited by a function-blocking alpha(3)beta(1) antibody. Similarly the Arg-Gly-Asp-containing peptide inhibits tyrosine phosphorylation of focal adhesion kinase and the p85 regulatory subunit of phosphatidylinositol 3-kinase, but does not significantly affect tyrosine phosphorylation, or activation, of extracellular-regulated kinase. These data suggest that soluble thrombospondin-1 interacts with human vascular smooth muscle cells via two independent and separable receptor-binding sites, to differentially stimulate cell chemotaxis and DNA synthesis.

Regulation of integrin function by CD47 ligands. Differential effects on alpha vbeta 3 and alpha 4beta1 integrin-mediated adhesion

We examined the regulation of alpha4beta1 integrin function in melanoma cells and T cells by ligands of CD47. A CD47 antibody (B6H12) that inhibited alphavbeta3-mediated adhesion of melanoma cells induced by CD47-binding peptides from thrombospondin-1 directly stimulated alpha4beta1-mediated adhesion of the same cells to vascular cell adhesion molecule-1 and N-terminal regions of thrombospondin-1 or thrombospondin-2. B6H12 also stimulated alpha4beta1- as well as alpha2beta1- and alpha5beta1-mediated adhesion of CD47-expressing T cells but not of CD47-deficient T cells. alpha4beta1 and CD47 co-purified as a detergent-stable complex on a CD47 antibody affinity column. CD47-binding peptides based on C-terminal sequences of thrombospondin-1 also specifically enhanced adhesion of melanoma cells and T cells to alpha4beta1 ligands. Unexpectedly, activation of alpha4beta1 function by the thrombospondin-1 CD47-binding peptides also occurred in CD47-deficient T cells. CD47-independent activation of alpha4beta1 required the Val-Val-Met (VVM) motif of the peptides and was sensitive to inhibition by pertussis toxin. These results indicate that activation of alpha4beta1 by the CD47 antibody B6H12 and by VVM peptides occurs by different mechanisms. The antibody directly activates a CD47-alpha4beta1 complex, whereas VVM peptides may target an unidentified Gi-linked receptor that regulates alpha4beta1.

Thrombospondin-1 mediates smooth muscle cell proliferation induced by interaction with human platelets

OBJECTIVES

Platelet adherence and activation are associated with smooth muscle cell (SMC) proliferation and arterial restenosis. This study examined platelet-SMC interaction on fibrillar type I collagen and analyzed the role of thrombospondin (TSP)-1 in platelet-induced SMC proliferation.

METHODS AND RESULTS

When SMCs cultured on fibrillar collagen were treated with human platelets (5 preparations), 7.45+/-2.94% of the cells passed through S phase within 24 hours, as determined by bromodeoxyuridine nuclear labeling. The addition of platelets markedly induced SMC TSP-1 mRNA expression and cell surface protein accumulation, which colocalized with adhered platelets, as determined by alpha(IIb) integrin immunostaining. Direct interaction of platelets with SMCs was necessary for its effect on proliferation and TSP-1 accumulation, as determined in the transwell culture system. The anti-TSP-1 blocking antibody strongly inhibited platelet-induced SMC proliferation by approximately 60%. Analysis of the receptors for TSP-1 accumulation on the SMC surface revealed that beta1 integrins are mainly involved. The anti-beta1 integrin blocking antibody, which potently suppressed TSP-1 accumulation on SMCs, also markedly inhibited platelet-stimulated SMC proliferation.

CONCLUSIONS

TSP-1 and beta1 integrin interaction is involved in platelet-stimulated SMC proliferation. This in vitro coculture system could prove useful for examining the molecular mechanism underlying platelet-induced vascular remodeling and for studying the mechanism of a tested drug for restenosis.

A recombinant NH(2)-terminal heparin-binding domain of the adhesive glycoprotein, thrombospondin-1, promotes endothelial tube formation and cell survival: a possible role for syndecan-4 proteoglycan

Thrombospondin-1 (TSP-1) is a multifunctional protein known to modulate angiogenesis, endothelial cell adhesion and apoptosis. In this study, we have demonstrated that TSP18, a recombinant 18 kDa protein encompassing the N-terminal residues 1-174 of human TSP-1, accelerated the process of tube-like structures formation by human umbilical vein endothelial cells (HUVECs) when included in fibrin matrices at 0.55-2.2 microM concentrations, for times ranging from 24 to 72 h. This effect was specifically inhibited by V58A4, a Mab raised against TSP18. Whole TSP-1 showed a dual effect, weakly enhancing tube formation at 22 nM (10 microg/ml), but causing inhibition at 45 and 90 nM (20 and 40 microg/ml, respectively). In order to investigate the possible effects of TSP18 on cell adhesion and viability, we performed adhesion assays on different protein supports. HUVECs adhered more weakly on TSP-1-coated surfaces, remaining round-shaped, as compared to the well-spread phenotype displayed on fibronectin and gelatin. Cells adhering on TSP18-coated surfaces displayed a well spread phenotype, with this adhesion strongly inhibited by heparin. The binding of TSP18 to endothelial membrane extracts was blocked by a monoclonal IgG directed against the cell surface proteoglycan syndecan-4. The DNA fragmentation patterns and the nuclear morphology were comparable for HUVECs adhering on all proteins, including TSP18, showing minimal cell apoptosis. Our results indicate that the N-terminal region of TSP-1 constitutes a suitable adhesive support for HUVECs, protecting them from apoptosis, possibly mediated by syndecan-4 proteoglycan.