A one-step procedure for purification of bovine mammary epithelial cell CD36

A simple one-step protocol for the purification of the integral membrane protein CD36 from milk-fat-globule membranes of bovine mammary epithelial cells is described. Nonionic detergent extracts of membrane were chromatographed on hydroxylapatite and pure CD36 was eluted with 1 M NaCl. Other proteins of the milk-fat-globule membrane were eluted after CD36 with phosphate buffer. Human platelet CD36 bound to hydroxylapatite only after neuraminidase treatment. CD36 is an extremely hydrophobic and highly glycosylated protein and previous purification procedures have required multiple steps. Chromatography of CD36 on hydroxylapatite provides a simple and quick method of purification which does not sacrifice yield.

Evidence for novel binding sites on the platelet glycoprotein IIb and IIIa subunits and immobilized fibrinogen

The present study was designed to examine the interaction of the purified platelet glycoprotein IIb-IIIa complex (GP IIb-IIIa or integrin alpha IIb beta 3) and the individual subunits of the complex with immobilized fibrinogen. Although 125I-GP IIb-IIIa binding to fibrinogen immobilized on Sepharose was specific, this interaction exhibited properties distinct from those of reversible fibrinogen binding to platelets: 125I-GP IIb-IIIa binding appeared irreversible, but non-covalent, Ca(2+)-independent, and was inhibited only weakly, or not at all, by the anti-(GP IIb-IIIa) monoclonal antibodies 10E5 and 7E3 and synthetic peptides from known platelet-binding domains of fibrinogen. Reversibly dissociated GP IIb or GP IIIa subunits inhibited 125I-GP IIb-IIIa binding to immobilized fibrinogen and bound directly to the fibrinogen. However, these subunits did not bind to peptides derived from known platelet-binding domains within the fibrinogen alpha- and gamma-chains, although the GP IIb-IIIa complex did. These results show that the complexed form of full-length GP IIb and GP IIIa is required for binding to these synthetic peptides, but not necessarily for binding to immobilized fibrinogen. Thus GP IIb-IIIa can bind to immobilized fibrinogen by a distinct mechanism that appears to involve novel binding sites on each subunit of the GP IIb-IIIa complex and on fibrinogen.

Serum platelet-reactive IgG of autoimmune thrombocytopenic purpura patients is not F(ab')2 mediated and a function of storage

Serum platelet-reactive and glycoprotein (GP) IIb-GPIIIa-reactive IgG and F(ab')2 was examined in 39 patients with classic autoimmune thrombocytopenic purpura (ATP), two patients with anti-PLA1 antibody and 25 control subjects in an enzyme-linked immunosorbent assay. IgG was purified by diethyl aminoethyl chromatography and centrifuged at 100,000g before testing of the supernatant. Significant IgG binding (threefold to fourfold control IgG binding) was noted with 8 of 17 ATP patients' IgG, 2 anti-PLA1 IgGs, and 2 ATP patients with multiple platelet transfusions. However, F(ab')2 fragments of nine of nine positive ATP IgGs were nonreactive; F(ab')2 from the two anti-PLA1 and two multiply transfused ATP IgGs were as reactive as their intact IgG. Antiplatelet or anti-GPIIb-GPIIIa reactivity of ATP IgG could be adsorbed to fixed platelets or solid-phase GPIIb-GPIIIa and eluted with 0.1 mol/L glycine, pH 2.5. However, binding of IgG to GPIIb-GPIIIa could not be inhibited with F(ab')2 of ATP IgG or Fc fragments of control subjects. When platelet- or GPIIb-GPIIIa-reactive ATP IgG was applied to a Sephacryl 300 gel filtration column, no reactivity was noted in the 7S region, whereas anti-PLA1 localized to this region. Antiplatelet or anti-GPIIb-GPIIIa reactivity was noted in the void volume and accompanied by a high molecular weight protein region. An immunoblot of the void volume fraction with goat antihuman IgG (gamma chain) antibody showed high molecular weight bands greater than 250 Kd, which after reduction converted to a 55-Kd heavy-chain band. Fresh samples of ATP and control IgG processed within 1 to 2 days of blood withdrawal had no reactivity for GPIIb-GPIIIa. After storage at -20 degrees C for greater than 3 months, 5 of 19 ATP IgG became reactive, whereas 16 of 16 controls were nonreactive. Thus, platelet-reactive IgG of ATP sera appears to be caused by the development of IgG aggregates held together by disulfide bonds that develop on storage, and is not F(ab')2 mediated.

Triflavin, an RGD-containing antiplatelet peptide, binds to GpIIIa of ADP-stimulated platelets

Triflavin, an Arg-Gly-Asp-containing snake venom peptide, inhibits platelet aggregation through the blockade of fibrinogen binding to the activated platelets. It binds to fibrinogen receptors associated with the glycoprotein IIb/IIIa complex with a Kd value of 7 x 10(-8) M. In this report, a chemical cross-linking approach was used to further characterize the binding components of triflavin on platelet membrane. 125I-triflavin binding was performed with the aid of a chemical cross-linking reagent, DTSSP. Analysis of the cross-linked products by SDS-PAGE (7.5% gel) and subsequent autoradiogram revealed that 125I-triflavin was cross-linked specifically to a protein with an apparent molecular weight of 1.1 x 10(5), and this reaction was inhibited by GRGDS and excess of non-labeled triflavin. This 110 KDa component was identified to be GpIIIa, recognized by AP3, a mAb against GpIIIa, by immunoblotting technique. These results indicate that the triflavin-binding sites on platelets reside at a site in close proximity to GpIIIa.

Hirudisins. Hirudin-derived thrombin inhibitors with disintegrin activity

Recombinant hirudin variants have been designed which inhibit alpha-thrombin by the hirudin mechanism and which in addition exhibit disintegrin activity. These proteins, called "hirudisins," have been engineered by replacing the Ser-Asp-Gly-Glu sequence at the tip of hirudin's finger-like structure (residues 32-35) by Arg-Gly-Asp-Ser (RGDS) to yield hirudisin and Lys-Gly-Asp-Ser (KGDS) to obtain hirudisin-1. Comparison of thrombin inhibition activities showed that hirudisin is 2-fold more potent (K(i) = 160 +/- 70 fM) than hirudisin-1 (K(i) = 370 +/- 44 fM) and recombinant (r)-hirudin (K(i) = 270 +/- 50 fM). alpha-Thrombin-stimulated platelet aggregation was effectively inhibited by r-hirudin, hirudisin, and hirudisin-1 with IC50 of 5.7 to 6.8 nM. Unlike r-hirudin, hirudisin inhibits ADP-induced platelet aggregation (IC50 = 65 microM) 3- to 5-fold stronger than the linear GRGDS- and RGDS-peptide. Direct interaction of hirudisin with purified glycoprotein IIb-IIIa demonstrated that antiplatelet aggregation activity is due to the integrin-directed RGD motif. Disintegrin activity of hirudisin relative to that of reduced and carboxymethylated hirudisin suggests that the conformational strain favors binding to integrins. On the basis of these results, hirudisins appear to be interesting molecules for the design of potential antithrombotic agents with antithrombin as well as antiplatelet aggregation activities.

Thrombin interaction with platelet glycoprotein Ib: effect of glycocalicin on thrombin specificity

We describe here the alteration of thrombin specificity induced by its interaction with glycocalicin. Glycocalicin is the external part of platelet glycoprotein Ib alpha (GPIb alpha) and contains binding sites for von Willebrand factor and thrombin. Taking advantage of its solubility, we have used glycocalicin in competition assays on various thrombin activities. Glycocalicin did not inhibit chromogenic substrate hydrolysis nor diisopropylfluorophosphate iPr2 (PF) incorporation, indicating that thrombin binding to GPIb does not alter access to or the conformation of the thrombin catalytic site. Glycocalicin competitively inhibited thrombin binding to fibrin (Ki = 0.1 mumol/L) and blocked fibrinogen clotting activity of thrombin. Glycocalicin also inhibited thrombin binding to thrombomodulin in a competitive manner (Ki = 3 to 5 mumol/L), but failed to prevent thrombin interaction with protein C in the absence of thrombomodulin. Previous results have indicated that GPIb binds to thrombin within the anion binding exosite masked by the carboxy-terminal hirudin peptide 54-65. The present results confirm the implication of the anion binding exosite in GPIb recognition, and further indicate that the thrombin binding site for GPIb overlaps with the thrombin binding sites for fibrin and thrombomodulin, whereas it is distinct from the thrombin binding site for protein C. Some of the structural requirements for thrombin binding to GPIb appear to be very similar to those reported for binding to its platelet receptor. However, thrombin-GPIb interaction does not appear to compete with receptor hydrolysis but rather increases the sensitivity and the rate of platelet responses elicited by the receptor.

Ser-752-->Pro mutation in the cytoplasmic domain of integrin beta 3 subunit and defective activation of platelet integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) in a variant of Glanzmann thrombasthenia

Integrins are membrane receptors which mediate cell-cell or cell-matrix adhesion. Integrin alpha IIb beta 3 (glycoprotein IIb-IIIa) acts as a fibrinogen receptor of platelets and mediates platelet aggregation. Platelet activation is required for alpha IIb beta 3 to shift from noncompetent to competent for binding soluble fibrinogen. The steps involved in this transition are poorly understood. We have studied a variant of Glanzmann thrombasthenia, a congenital bleeding disorder characterized by absence of platelet aggregation and fibrinogen binding. The patient's platelets did not bind fibrinogen after platelet activation by ADP or thrombin, though his platelets contained alpha IIb beta 3. However, isolated alpha IIb beta 3 was able to bind to an Arg-Gly-Asp-Ser affinity column, and binding of soluble fibrinogen to the patient's platelets could be triggered by modulators of alpha IIb beta 3 conformation such as the Arg-Gly-Asp-Ser peptide and alpha-chymotrypsin. These data suggested that a functional Arg-Gly-Asp binding site was present within alpha IIb beta 3 and that the patient's defect was not secondary to a blockade of alpha IIb beta 3 in a noncompetent conformational state. This was evocative of a defect in the coupling between platelet activation and alpha IIb beta 3 up-regulation. We therefore sequenced the cytoplasmic domain of beta 3, following polymerase chain reaction (PCR) on platelet RNA, and found a T-->C mutation at nucleotide 2259, corresponding to a Ser-752-->Pro substitution. This mutation is likely to be responsible for the uncoupling of alpha IIb beta 3 from cellular activation because (i) it is not a polymorphism, (ii) it is the only mutation in the entire alpha IIb beta 3 sequence, and (iii) genetic analysis of the family showed that absence of the Pro-752 beta 3 allele was associated with the normal phenotype. Our data thus identify the C-terminal portion of the cytoplasmic domain of beta 3 as an intrinsic element in the coupling between alpha IIb beta 3 and platelet activation.

Isoelectric focusing and immunoblotting of the platelet membrane glycoprotein complex IIb. IIIA following urea solubilization

Effective solubilization of the major platelet membrane component, the glycoprotein IIb.IIIa complex, can be achieved with 8 M urea. By avoiding nonionic detergents in the separation medium it is possible to obtain clear immunoblot patterns without interference from the isoelectric focusing matrix. Upon running on a pH 4.25-5.25 immobilized pH gradient, immunoreactive bands corresponding to the nonreduced IIb.IIIa complex stain between pH 4.5 and 5.0. The method appears of significant potential utility in evaluating glycoprotein IIb.IIIa polymorphisms under different clinical conditions.

Localization of two binding domains for thrombospondin within fibronectin

Thrombospondin is a major glycoprotein of the platelet alpha-granule and is secreted during platelet activation. Several protease-resistant domains of thrombospondin mediate its interactions with components of the extracellular matrix including fibronectin, collagen, heparin, laminin, and fibrinogen. Thrombospondin, as well as fibronectin, is composed of several discretely located biologically active domains. We have characterized the thrombospondin binding domains of plasma fibronectin and determined the binding affinities of the purified domains; fibronectin has at least two binding sites for thrombospondin. Thrombospondin bound specifically to the 29-kDa amino-terminal heparin binding domain of fibronectin as well as to the 31-kDa non-heparin binding domain located within the larger 40-kDa carboxy-terminal fibronectin domain generated by chymotrypsin proteolysis. Platelet thrombospondin interacted with plasma fibronectin in a specific and saturable manner in blot binding as well as solid-phase binding assays. These interactions were independent of divalent cations. Thrombospondin bound to the 29-kDa fibronectin heparin binding domain with a Kd of 1.35 x 10(-9) M. The Kd for the 31-kDa domain of fibronectin was 2.28 x 10(-8) M. The 40-kDa carboxy-terminal fragment bound with a Kd of 1.65 x 10(-8) M. Heparin, which binds to both proteins, inhibited thrombospondin binding to the amino-terminal domain of fibronectin by more than 70%. The heparin effect was less pronounced with the non-heparin binding carboxy-terminal domain of fibronectin. By contrast, the binding affinity of the thrombospondin 150-kDa domain, which itself lacked heparin binding, was not affected by the presence of heparin. Based on these data, we conclude that thrombospondin binds with different affinities to two distinct domains in the fibronectin molecule.

Regulation of gene expression by SPARC during angiogenesis in vitro. Changes in fibronectin, thrombospondin-1, and plasminogen activator inhibitor-1

Angiogenesis in vitro, the formation of capillary-like structures by cultured endothelial cells, is associated with changes in the expression of several extracellular matrix proteins. The expression of SPARC, a secreted collagen-binding glycoprotein, has been shown to increase significantly during this process. We now show that addition of purified SPARC protein, or an N-terminal synthetic peptide (SPARC4-23), to strains of bovine aortic endothelial cells undergoing angiogenesis in vitro resulted in a dose-dependent decrease in the synthesis of fibronectin and thrombospondin-1 and an increase in the synthesis of type 1-plasminogen activator inhibitor. SPARC decreased fibronectin mRNA by 75% over 48 h, an effect that was inhibited by anti-SPARC immunoglobulins. Levels of thrombospondin-1 mRNA were diminished by 80%. Over a similar time course, both mRNA and protein levels of type 1-plasminogen activator inhibitor (PAI-1) were enhanced by SPARC and the SPARC4-23 peptide. The effects were dose-dependent with concentrations of SPARC between 1 and 30 micrograms/ml. In contrast, no changes were observed in the levels of either type I collagen mRNA or secreted gelatinases. Half-maximal induction of PAI-1 mRNA or inhibition of fibronectin and thrombospondin mRNAs occurred with 2-5 micrograms/ml SPARC and approximately 0.05 mM SPARC4-23. Strains of endothelial cells that did not form cords and tubes in vitro had reduced or undetectable responses to SPARC under identical conditions. These results demonstrate that SPARC modulates the synthesis of a subset of secreted proteins and identify an N-terminal acidic sequence as a region of the protein that provides an active site. SPARC might therefore function, in part, to achieve an optimal ratio among different components of the extracellular matrix. This activity would be consistent with known effects of SPARC on cellular morphology and proliferation that might contribute to the regulation of angiogenesis in vivo.

Examination of the platelet membrane glycoprotein IIb-IIIa complex and its interaction with fibrinogen and other ligands by electron microscopy

The platelet integrin, glycoprotein IIb-IIIa (GPIIb-IIIa), is a calcium-dependent heterodimer that binds fibrinogen, von Willebrand factor, and fibronectin after platelet activation. We examined GPIIb-IIIa alone and bound to these ligands by electron microscopy after rotary shadowing with platinum/tungsten. We found, as observed previously, that in the presence of detergent and 2 mM Ca2+, GPIIb-IIIa consists of an 8 x 12-nm globular head with two 18-nm flexible tails extending from one side. We also found that in the presence of EDTA, GPIIb-IIIa dissociates into two similar comma-shaped subunits, each containing a portion of the globular head and a single tail. Using monoclonal antibodies to GPIIb, GPIIIa, and the GPIIb-IIIa heterodimer, we found that the tails contained the carboxyl termini of each subunit, while the nodular head was composed of amino-terminal segments of both subunits. Electron microscopy of GPIIb-IIIa bound to fibrinogen revealed a highly specific interaction of the nodular head of GPIIb-IIIa with the distal end of the trinodular fibrinogen molecule and with the tails of GPIIb-IIIa extended laterally at an angle of approximately 98 degrees with respect to the long axis of fibrinogen. When a GPIIb-IIIa was bound to each end of a single fibrinogen, the tails were oriented to opposite sides of fibrinogen, enabling fibrinogen to bridge two adjacent platelets. Electron microscopy of GPIIb-IIIa bound to fibronectin revealed GPIIb/IIIa-binding sites approximately two-thirds of the distance from the amino terminus of each end of the fibronectin molecule, while GPIIb-IIIa was found to bind to von Willebrand factor protomers along a rod-like region near the central nodule of the molecule.

Fibrinogen binding to purified platelet glycoprotein IIb-IIIa (integrin alpha IIb beta 3) is modulated by lipids

Soluble fibrinogen binding to the glycoprotein IIb-IIIa complex (integrin alpha IIb beta 3) requires platelet activation. The intracellular mediator(s) that convert glycoprotein IIb-IIIa into an active fibrinogen receptor have not been identified. Because the lipid composition of the platelet plasma membrane undergoes changes during activation, we investigated the effects of lipids on the fibrinogen binding properties of purified glycoprotein IIb-IIIa. Anion exchange chromatography of lipids extracted from platelets exposed to thrombin or other platelet agonists resolved an activity that increased fibrinogen binding to glycoprotein IIb-IIIa. A monoester phosphate was important for activity, and phosphatidic acid coeluted with the peak of activity. Purified phosphatidic acid dose-dependently promoted a specific interaction between glycoprotein IIb-IIIa and fibrinogen which possessed many but not all of the properties of fibrinogen binding to activated platelets. Phosphatidic acid appeared to increase the proportion of fibrinogen binding-competent glycoprotein IIb-IIIa complexes without altering their affinity for fibrinogen. The effects of phosphatidic acid were a result of specific structural properties of the lipid and were not mimicked by other phospholipids. Lysophosphatidic acid, however, was a potent inducer of fibrinogen binding to glycoprotein IIb-IIIa. These results demonstrate that specific lipids can affect fibrinogen binding to purified glycoprotein IIb-IIIa and suggest that the lipid environment has the potential to influence fibrinogen binding to its receptor.

Polymorphism of human glycoprotein Ib alpha results from a variable number of tandem repeats of a 13-amino acid sequence in the mucin-like macroglycopeptide region. Structure/function implications

Four polymorphic variants of the platelet receptor for von Willebrand factor, glycoprotein Ib, have been described that differ in molecular weight on sodium dodecyl sulfate-polyacrylamide gels (Moroi, M., Jung, S. M., and Yoshida, N. (1984) Blood 64, 622-629). A recent report localized the polymorphic site to the heavily O-glycosylated region of the glycoprotein Ib alpha-chain known as the macroglycopeptide (Meyer, M., and Schellenberg, I. (1990) Thromb. Res. 58, 233-242). This region contains several tandem repeats of a mucin-like sequence, which appeared to be a likely site for polymorphic variation. We amplified genomic DNA corresponding to the macroglycopeptide from 206 individuals from four ethnic groups and identified three length variants based on the migration of the amplified DNA on denaturing polyacrylamide gels. DNA sequencing revealed that the three variants represented four alleles, two of which varied by only one base pair, a difference that did not result in an amino acid change. The three length variants differed in the number of tandem repeats of a 39-base pair sequence that results in perfect duplication of a 13-amino acid sequence that originated within a region flanked by Glu-396 and Thr-411. The smallest isoform contained one such sequence; the next largest, two repeats; and the largest, three repeats. The DNA sequence containing the tandem repeats was flanked by direct repeats typical of the target site duplications found flanking transposed DNA, suggesting a mechanism for acquisition of this region by the primordial glycoprotein Ib alpha precursor. The amino acid sequence of the repeated element that accounts for the polymorphism contained five sites for potential O-glycosylation, which together with the repeated amino acids would result in incremental differences in molecular weight of approximately 6,000 between the different isoforms. The addition of repeats to the macroglycopeptide is predicted to increase the length of this elongated glycosylated region and extend the distance between the ligand-binding domain of glycoprotein Ib and the platelet plasma membrane, an effect that would project the ligand-binding domain farther into the bloodstream. Such a change may alter the susceptibility of platelets to shear-induced activation, a process that requires an interaction between glycoprotein Ib and von Willebrand factor.

Effects of sized heparin oligosaccharide on the interactions of Chinese hamster ovary cell with thrombospondin

Binding and degradation of TSP by CHO cells and adhesion of CHO cells to substrate-adsorbed TSP are mediated by cell surface PGs and inhibitable by heparin. In order to learn how these three processes are related, we studied the effects of defined heparin oligosaccharides up to 18-mer produced by nitrous acid digestion. There was a complex correlation among oligosaccharide chain length, affinity of oligosaccharide for TSP in a solid phase binding assay, and potencies of oligosaccharide in inhibition of the three cellular processes. Inhibition of degradation was more sensitive to shorter oligosaccharides than inhibition of binding. For instance, the 10-mer inhibited binding of TSP to cells by 10% and degradation by 70%. Punctate immunofluorescence of cell surface bound TSP was replaced by a diffuse pattern after incubation in the presence of the 10-mer. These results suggest that the clustering of TSP on the cell surface may trigger endocytosis and degradation. Inhibition of binding of TSP to cells, in turn, was more sensitive to midsized oligosaccharides than inhibition of cell adhesion to adsorbed TSP. Inhibition of adhesion correlated with the ability of oligosaccharides to block binding of 125I-heparin to adsorbed TSP.

Thrombospondin promotes process outgrowth in neurons from the peripheral and central nervous systems

Thrombospondin (TSP) is a prominent constituent of the extracellular matrix of the developing nervous system. We have examined the effects of TSP on the morphological differentiation of neurons. In short-term cultures (less than or equal to 24 hr) of embryonic rat sympathetic neurons, TSP stimulated neurite outgrowth, causing significant increase in the number of processes and their length. Similar effects were observed in cultures of rat dorsal root ganglion, hippocampal, and cerebral cortical neurons. Moreover, in cultures of central neurons, TSP was more effective than laminin in enhancing process extension. Analysis of long-term (5-7 days) cultures of sympathetic neurons indicated that processes formed in the presence of TSP had the cytochemical characteristics of axons. Thus, TSP can influence neuronal development by selectively enhancing axonal growth. The neurite-promoting region of the molecule was identified using a panel of monoclonal antibodies targeted to different regions of the protein. Process outgrowth could be totally inhibited with antibody A4.1, which recognizes the stalk region of TSP. These data suggest that the neurite-promoting activity is localized to a single region of the TSP molecule.

Cytoplasmic domain of P-selectin (CD62) contains the signal for sorting into the regulated secretory pathway

P-selectin (CD62), formerly called GMP-140 or PADGEM, is a membrane protein located in secretory storage granules of platelets and endothelial cells. To study the mechanisms responsible for the targeting of P-selectin to storage granules, we transfected its cDNA into COS-7 and CHO-K1 cells, which lack a regulated exocytic pathway, or into AtT20 cells, which are capable of regulated secretion. P-selectin was expressed on the plasma membrane of COS-7 and CHO-K1 cells but was concentrated in storage granules of AtT20 cells. Immunogold electron microscopy indicated that the electron-dense granules containing P-selectin in AtT20 cells also stored the endogenous soluble hormone ACTH. Activation of AtT20 cells with 8-Br-cAMP increased the surface expression of P-selectin, consistent with agonist-induced fusion of granule membranes with the plasma membrane. Deletion of the last 23 amino acids of the 35-residue cytoplasmic domain resulted in delivery of P-selectin to the plasma membrane of AtT20 cells. Replacement of the cytoplasmic tail of tissue factor, a plasma membrane protein, with the cytoplasmic domain of P-selectin redirected the chimeric molecule to granules. We conclude that the cytoplasmic domain of P-selectin is both necessary and sufficient for sorting of membrane proteins into the regulated pathway of secretion.

Transforming growth factor-beta complexes with thrombospondin

Thrombospondin (TSP) was demonstrated to inhibit the growth of bovine aortic endothelial cells, an activity that was not neutralized by antibodies to TSP or by other agents that block TSP-cell interactions but that partially was reversed by a neutralizing antibody to transforming growth factor-beta (TGF-beta). Similar to TGF-beta, TSP supported the growth of NRK-49F colonies in soft agar in a dose-dependent manner, which required epidermal growth factor and was neutralized by anti-TGF-beta antibody. Chromatography of a TSP preparation did not separate the TGF-beta-like NRK colony-forming activity from high molecular weight protein. However, when chromatography was performed at pH 11, this activity was dissociated from TSP. These results suggest that at least some growth modulating activities of TSP are due to TGF-beta associated with TSP by strong non-covalent forces. Most of the active TGF-beta released from platelets after degranulation was associated with TSP, as demonstrated by anti-TSP immunoaffinity and gel permeation chromatography. 125I-TGF-beta binds to purified TSP in an interaction that is specific in the sense that bound TGF-beta could be displaced by TGF-depleted TSP but not significantly by native TSP, heparin, decorin, alpha 2-macroglobulin, fibronectin, or albumin. Hence, TGF-beta can bind to TSP, and the complex forms under physiological conditions. Furthermore, TSP-associated TGF-beta is biologically active, and the binding of TGF-beta to TSP may protect TGF-beta from extracellular inactivators.

Thrombospondin modulates basic fibroblast growth factor activities on endothelial cells

We previously reported that thrombospondin (TSP) induces endothelial cell (EC) adhesion, spreading and motility, suggesting that it can play a role in angiogenesis. We then studied whether TSP might modulate EC response to known angiogenic stimuli in vitro. Here we describe that TSP inhibits EC chemotactic response to basic fibroblast growth factor (bFGF). Furthermore, TSP and its 140 kD fragment reduce EC proliferative response to serum and bFGF. These data support the indicated role of TSP and its 140 kD fragment in angiogenesis and in related pathologies including tumor malignancy.