Thrombospondin Levels in Patients with Malignancy

Thrombospondin (TSP), a large glycoprotein present in platelets, and various normal and tumor tissues, has recently been shown to promote cell adhesion and platelet aggregation. Most importantly because TSP has been shown to promote metastasis of melanoma tumor cells to the lung in a murine model (1) and since thromboembolic events commonly occur in patients afflicted with metastatic tumors, we explored the role of TSP in human cancer by measuring TSP blood levels in patients with various malignant neoplasms. Blood TSP levels were measured by indirect enzyme-linked immunoadsorbent assay (ELISA) from 20 control subjects, 22 patients with gastrointestinal (GI) cancer, 18 patients with breast cancer, and 17 patients with lung cancer. Control subjects consisted both of healthy subjects and acutely ill patients with no malignancies. TSP levels of both healthy and acutely ill controls were found to range between 245-440 μg/ml with a mean of 365 μg/ml. In contrast, elevated levels of TSP greater than the mean value of 400 μg/ml for controls ranging between 590-3,650 μg/ml were found in 20/22 (91%) patients with GI malignancies, 13/18 (72%) patients with breast cancer, and 15/17 (88%) with lung cancer. Mean TSP levels of GI, breast, and lung cancer patients were 3, 2, and 3 fold greater than controls, respectively. Increased blood TSP levels in patients were not due to increased levels of platelets since both control and patient groups had platelet counts within the normal range. These results suggest that TSP may play a role in tumor cell metastasis in man and could serve as a blood marker for metastasis.

Angiocidin induces differentiation of acute myeloid leukemia cells

Acute myeloid leukemia (AML) is a malignant proliferative disorder in which leukemic cells fail to terminally differentiate and accumulate in the blood and bone marrow. Standard AML therapy requires intensive chemotherapy with a low rate of durable remission and is associated with significant treatment-related toxicity, especially in elderly patients. Therefore, new therapeutic options for the treatment of AML are urgently needed. We previously reported that the novel angiogenic inhibitor, angiocidin, induces differentiation of monocytes to macrophages. Here we investigate the effects of angiocidin on AML cells lines and primary AML cells. Differentiation was assessed by flow cytometry measuring the increase in expression of cell surface marker characteristic of normal macrophages. Four AML cell lines (THP-1, Mono-mac-1, HL-60 and MV4-11) and 5 of 10 primary human AML samples showed evidence of differentiation when cultured in vitro for 24 h with 10 μg/mL angiocidin. Additionally, we found that angiocidin promoted secretion of a number of cytokines from the cell lines as well as patient cells. We next evaluated the effect of angiocidin on a xenotransplanted primary human AML sample engrafted in NSG mice. We found angiocidin monotherapy reduced the human AML burden in bone marrow by 63% relative to untreated control. Interestingly, angiocidin+cytosine arabinoside (Ara-C) combination therapy reduced human AML in bone marrow by 79%. We believe the combination of in vitro data supporting the capacity of angiocidin to drive differentiation in multiple AML cell lines and primary human AML samples and its activity in a xenotransplantation model that reproduces the human disease is significant. These observations support the continued evaluation and development of angiocidin as a potential novel, non-toxic therapy for AML.

Abstract 3183: The role of G-protein coupled receptor-associated sorting protein 1 (GASP-1) in breast cancer progression

An innovative “2-D High Performance Liquid Electrophoresis” (2-D HPLE) technology was used to identify serum biomarkers associated with the earliest stage of breast cancer in addition to other more advanced stages. 2-D HPLE is a newly developed electrophoretic technology that separates 100's of serum albumin complexes on a polyvinyl membrane based on their surface charges. Association of cancer proteins (biomarkers) with pre-existing albumin complexes in the blood of cancer patients results in altered mobility on the membrane. Using 2-D HPLE we identified that G-protein coupled receptor-associated sorting protein 1(GASP-1) was present in the sera of patients with early stage disease but absent in sera of normal patients. GASP-1 has been shown to modulate lysosomal sorting and functional down-regulation of a variety of G-protein coupled receptors in neuronal cells. However, no reports have linked GASP-1 to breast cancer pathogenesis. GASP-1 was detected in tumor extracts of 7 cases of stage 2 and stage 3 breast cancer but not in adjacent normal tissue as revealed by Western Blot analysis using an antibody developed against a GASP-1 peptide identified by our 2-D HPLE technology. Using this antibody, we immunohistochemically detected over-expression of GASP-1 in all 59 cases of archived ductal breast carcinoma tumor samples. When GASP-1 was silenced in MB-231 breast carcinoma, the cells grew more than 5 fold slower than corresponding vector controls. These studies identify GASP-1 as a potential new serum and tumor biomarker for breast cancer and suggest that GASP-1 may be a novel target for development of breast cancer therapeutics.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 3183. doi:10.1158/1538-7445.AM2011-3183

Angiocidin inhibits breast cancer proliferation through activation of epidermal growth factor receptor and nuclear factor kappa (NF-ĸB)

Angiocidin, a tumor-associated peptide, has been previously shown to inhibit tumor progression by blocking angiogenesis. We now show that angiocidin has a direct inhibitory effect on tumor cell proliferation. MDA-MB-231 breast cancer cells were inhibited from proliferating in the presence of epidermal growth factor (EGF) and angiocidin. Angiocidin transfected breast cancer cells also displayed growth inhibition in vitro and failed to develop significant tumors in mice as compared to vector controls. The anti-proliferative effect of angiocidin was reversed by treating the cells with the epidermal growth factor receptor (EGFR) inhibitor 4557W, a potent tyrosine kinase inhibitor. Consistent with these results, we found that treatment of breast cancer cells with angiocidin induced a 2.3 fold increase in EGFR tyrosine 845 phosphorylation while no change in phosphorylation was observed in the remaining 16 phosphorylation sites of EGFR and those of its family members as measured by a human EGFR phosphorylation array. Treatment of breast cancer cells with angiocidin also resulted in the activation of nuclear factor ĸB (Nf-ĸB) and the de novo up-regulation of many down-stream genes transcribed by Nf-ĸB, including cytokines, inflammatory mediators and the cell cycle inhibitor p21(waf1). Therefore, angiocidin is a peptide that not only inhibits tumor angiogenesis but also directly induces inhibition of tumor growth progression through the activation of EGFR and down-stream genes transcribed by Nf-ĸB.

Effect of VP12 and viperistatin on inhibition of collagen-receptor-dependent melanoma metastasis

Viperistatin and VP12 isolated from Vipera paleastinae venom showed a potent inhibitory activity against collagen receptors, alpha1beta1 and alpha2beta1 integrins, respectively. Structurally, viperistatin belongs to the disintegrin family of proteins, whereas VP12 is composed of two subunits VP12A and VP12B displaying amino acid sequence homology with heterodimeric C-lectin type proteins. Viperistatin and VP12 used separately and simultaneously inhibited pro-metastatic activities of melanoma cells lines. The level of inhibition of MV3 and HS.939T human cell lines in cell adhesion and migration assays by both compounds was correlated with expression of alpha1beta1 and alpha2beta1 integrins on the cell surface. MV3 cells express collagen receptors to much higher extent than HS.939T and required the application of higher concentrations of inhibitors to block their adhesion to collagen types I and IV. A melanoma cell transmigration assay through a dHMVEC layer revealed that alpha1beta1 integrin plays a significant role in invasion of HS.939T cells, while alpha2beta1 integrin appears to be more important for MV3 cells. In an animal model of hematogenous metastasis of the mouse B16F10 cell line, the inhibitory effect of viperistatin and VP12 was only partial. These data suggest that collagen receptors may be an interesting target for development of new anti-metastatic therapies.

The novel angiogenic inhibitor, angiocidin, induces differentiation of monocytes to macrophages

We previously showed that angiocidin, a tumor and vascular associated protein, is a potent inhibitor of angiogenesis and tumor growth. Angiocidin is a multidomain protein that exerts its antiangiogenic activity through multiple mechanisms, including effects on cell matrix interaction. Here, we describe another activity of angiocidin that may contribute to its antitumor activity. We show that angiocidin activates monocytes to secrete a mixture of proinflammatory cytokines and induces them to differentiate into macrophage-like cells. Using the monocytic cell line THP-1, we show that angiocidin induces the cells to become adherent and phagocytic, express macrophage markers, and secrete matrix metalloproteinase-9. Microarray analysis of control and angiocidin-treated THP-1 cells revealed that angiocidin up-regulated p105/p50, p100/p52, and rel B, components of the nuclear factor-kappaB (NF-kappaB) pathway. We confirmed the microarray data and showed that angiocidin induced phosphorylation of I kappa beta, p50, and p65 and translocation of p50 and p65 to the nucleus. We also showed that angiocidin activated up-stream mediators of NF-kappaB, such as the mitogen-activated protein kinase (MAPK) pathway and phosphoinositide-3 kinase (PI3K). Blockage of NF-kappaB and MAPK activation with small molecule inhibitors completely prevented angiocidin-mediated secretion of cytokines from THP-1 cells, but did not inhibit their adhesive phenotype. Blocking PI3K inhibited both secretion of cytokines, as well as the adhesive phenotype. These data suggest that angiocidin activates monocytes to secrete cytokines and differentiates them to a macrophage-like phenotype through at least two pathways mediated by MAPK and NF-kappaB, as well as PI3K.

Clinical significance of serum angiocidin levels in hepatocellular carcinoma

Angiocidin, a tumor-secreted protein, was measured in serum of 27 healthy volunteers and 33 hepatocellular carcinoma (HCC) patients. Healthy controls either hepatitis B surface antigen (HBsAg) positive or negative showed undetectable levels. Patients had levels of angiocidin ranging from 15.09 to 195.73 pg/ml. Patients with stages III-IV had higher levels of angiocidin (97+/-13 pg/ml, n=17) compared to those with stages I-II (63+/-37 pg/ml, n=16), p<0.043. Patients with microsatellite tumor nodules had higher average levels (98+/-55 pg/ml, n=17) compared to those without microsatellite nodules (51+/-27 pg/ml, n=20), p<0.032. Our studies suggest that angiocidin predicts advanced stage and intra-hepatic metastasis.

Interaction of alpha9beta1 integrin with thrombospondin-1 promotes angiogenesis

Thrombospondin-1 is a multifunctional protein interacting with several cell surface receptors including integrins. We found that it is a ligand for alpha9beta1 integrin, and has an integrin binding site within its N-terminal domain (NoC1). Interaction of thrombospondin-1 and its recombinant NoC1 domain with alpha9beta1 integrin was confirmed in ELISA and cell adhesion assays. Binding of NoC1 to cells expressing alpha9beta1 integrin activated signaling proteins such as Erk1/2 and paxillin. Blocking of this integrin by monoclonal antibody and the met-leu-asp-disintegrin inhibited dermal human microvascular endothelial cell proliferation and NoC1-induced migration of these cells. Immunohistochemical studies revealed that alpha9beta1 is expressed on microvascular endothelium in several organs including skin, lung, heart and brain. NoC1 induced neovascularization in an experimental quail chorioallantoic membrane system and Matrigel plug formation assay in mice. This proangiogenic activity of NoC1 in vivo was inhibited by alpha9beta1 inhibitors. In summary, our results revealed that alpha9beta1 integrin expressed on microvascular endothelial cells interacts with thrombospondin-1, and this interaction is involved in modulation of angiogenesis.

Reduction of angiocidin expression in human umbilical vein endothelial cells via siRNA silencing inhibits angiogenesis

Angiocidin, a protein over-expressed in many different solid tumors and tumor capillary endothelial cells inhibits angiogenesis and tumor growth [Zhou, J., et al., 2004. Cloning and characterization of angiocidin, a tumor cell binding protein for thrombospondin-1. J Cell Biochem. 92, 125-146]. Since several splice variants of angiocidin have distinct biochemical functions in membrane transport and protein degradation, we sought to evaluate the function of endogenously expressed angiocidin in human umbilical vein endothelial (HUVE) cells using siRNA. We observed a 90% reduction of the target mRNA levels after 24 h. Endogenous angiocidin protein expression was reduced by 80% after three days, as evaluated by Western blot analysis. We also found that anti-angiocidin siRNA down-regulated 90% of the protein expression of matrix metalloproteinase 2 (MMP-2) and 50% of its gelatinolytic activity. Reduction of endogenous angiocidin completely inhibited endothelial cord formation on Matrigel. Cells expressing low levels of angiocidin grew more slowly, were less invasive and less adhesive than control cells. Consistent with the reported function of one of the angiocidin analogues S5a, we found that the expression of polyubiquitinated proteins was higher in anti-angiocidin siRNA-treated cells as compared to normal and control siRNA-treated cells. These results suggest that endogenous angiocidin and its homologues promote endothelial cell invasion, adhesion, and angiogenesis through mechanisms involving polyubiquitin-dependent protein degradation and MMP-2 expression.

Integrin α2β1 mediates the anti-angiogenic and anti-tumor activities of angiocidin, a novel tumor-associated protein

We recently characterized an anti-tumor protein termed angiocidin. Here, we report that angiocidin may inhibit angiogenesis by binding collagen and its receptors. Angiocidin bound purified type I collagen and alpha2beta1 with high affinity. K562 cells expressing alpha2beta1 bound and adhered to angiocidin while K562 cells which only expressed alpha5beta1 integrin showed no binding and adhesion. Binding was specific since a neutralizing antibody against alpha2beta1 inhibited binding but antibodies against alpha5beta1 had no effect. Additionally, angiocidin co-localized with alpha2beta1 on K562 alpha2beta1 transfected cells, pancreatic cancer colo 357 cells, breast cancer MB-231 cells and human umbilical endothelial vein (HUVE) cells. In an alpha2beta1-dependent collagen gel angiogenesis assay, angiocidin showed potent inhibitory activity. We identified a 20-amino-acid amino terminal peptide of angiocidin that bound both alpha2beta1 and type I collagen. This peptide promoted alpha2beta1-dependent cell adhesion and inhibited tumor growth and angiogenesis. Taken together, these results are consistent with the conclusion that the anti-tumor activity of angiocidin arises from its ability to ligate collagen and alpha2beta1 on endothelial cells and tumor cells. Our results provide support for the concept that targeting matrix-cell interactions is a viable strategy for the development of anti-cancer therapeutics.

Cloning and characterization of angiocidin, a tumor cell binding protein for thrombospondin-1

Thrombospondin-1 (TSP-1) is a matrix protein that has been implicated in mechanisms of tumor progression. Our laboratory previously showed that the CSVTCG (cys-ser-val-thr-cys-gly) sequence of TSP-1 functioned as a tumor cell adhesion domain and CSVTCG peptides as well as an anti-peptide antibody possessed anti-metastatic activity in a murine model of lung metastasis. In a subsequent study, a putative TSP-1 binding protein from lung carcinoma was isolated by CSVTCG-peptide affinity chromatography. In this study, we present the full-length cDNA of this binding protein isolated from a prostate cancer cell (PC3-NI) cDNA library. The purified recombinant protein, termed angiocidin, is a potent inhibitor of tumor growth of Lewis Lung carcinoma in vivo and tumor invasion and angiogenesis in vitro. In addition, the recombinant protein inhibits tumor and endothelial cell proliferation and induces apoptosis. The activity of angiocidin both in vivo and in vitro is partially dependent on its TSP-1 binding activity, since an angiocidin deletion mutant missing a high affinity-binding site for TSP-1 failed to inhibit tumor growth in vivo and was less active in its anti-tumor and anti-angiogenic activities in vitro. These results suggest that the anti-tumor activity of TSP-1 reported in many studies may be mediated in part by binding proteins such as angiocidin. Such proteins may function as tumor-suppressor proteins, which limit the growth of tumors by inhibiting angiogenesis and cell matrix interaction.

Angiostatin binds to tyrosine kinase substrate annexin II through the lysine-binding domain in endothelial cells

Angiostatin(AS), an internal fragment of plasminogen, is one of the most potent specific inhibitors of angiogenesis. Angiostatin treatment has resulted in the complete regression of human tumors implanted subcutaneously into nude mice and has great therapeutic value (O'Reilly et al., Nat. Med. 2, 689-692, 1996). Despite promising therapeutic value in the treatment of cancer, the mechanism of its action is still unknown. We found that angiostatin binds to a 35-kDa protein in bovine aortic endothelial (BAE) cells (Sharma et al., Proc. Am. Assoc. Cancer Res. 42, 568, A3050, 2002). In an attempt to begin to understand angiostatin's mechanism of action, we have purified and characterized this 35-kDa protein from BAE cells. Internal peptide sequence analysis of purified protein demonstrated (SLYYIQQDTK, SYSPYDMLESIK, and ALLYLXGGDD) 100% sequence identity with tyrosine kinase substrate annexin II. Solid phase binding analysis suggests that angiostatin specifically bound to purified annexin II immobilized on 96-well plastic plates. Hundred-fold molar excess of unlabeled AS and anti-annexin II antibody inhibited bindings 85 and 55%, respectively, suggesting specific interaction. Annexin II is a predominant receptor for angiostatin, since neutralizing the angiostatin by soluble receptor (annexin II) effectively blocks angiostatin's anti-EC activity. Similarly, saturating the annexin II receptor by plasminogen in endothelial cells also blocks angiostatin's activity. Both angiostatin and plasminogen bind to purified annexin II in BAE cells saturably with apparent K(d) values of 101 and 164 nM, respectively, for purified annexin II and K(d) values of 83 and 125 nM, respectively, for BAE cells. Anti-annexin II monoclonal antibody inhibited angiostatin and plasminogen binding to endothelial cells by 68 and 62%, respectively, supporting our in vitro studies that annexin II is a receptor for angiostatin. Angiostatin-binding protein/annexin II specifically expressed in endothelial cells but not in fibroblasts suggests its EC-specific function. Epsilon-aminocaproic acid, a lys analogue, effectively blocks angiostatin and annexin II interaction, indicating that the lysine-binding domain of AS is required for binding to annexin II. These results suggest that the antiangiogenic action of angiostatin may be mediated via interaction with annexin II. Identification of annexin II as a receptor for angiostatin provides further evidence that clotting and fibrinolytic pathways are directly involved in the angiogenic process.

Thrombospondin-1 promotes proliferative healing through stabilization of PDGF

PURPOSE

Thrombospondin-1 (TSP-1) mediates chemotaxis, cell proliferation, angiogenesis, and protease regulation in healing. TSP-1 also binds platelet-derived growth factor (PDGF) and transforming growth factor beta (TGF-beta). This study confirms the role of TSP-1 and defines the relationship between TSP-1 and PDGF in proliferative tissue repair.

METHODS

Purified TSP-1 was analyzed for bound PDGF. Cultured fibroblast growth response to TSP-1 and recombinant PDGF was studied and the effects of antibodies against TSP-1, PDGF, and TGF-beta on this response were evaluated. Levels of TSP-1 and PDGF and relative proteolytic activity in fluid collected from 10 skin graft donor sites were then assessed by ELISA and a protease assay kit. The effect of proteolysis on TSP-bound PDGF and free recombinant PDGF was studied by adding trypsin and measuring the remaining PDGF by ELISA.

RESULTS

TSP-1 promoted dose-dependent fibroblast growth. While antibody to TGF-beta had no effect on promotion, antibody to both TSP-1 and PDGF eliminated this. Since a strong correlation of TSP-1 with PDGF levels was found and strong proteolysis was seen in all samples, we proposed that TSP-1 protected PDGF from proteolysis. Consistent with this, we found PDGF bound to TSP-1 was 33% less degraded than free PDGF upon trypsinization.

CONCLUSIONS

These results suggest that TSP-1 stabilizes PDGF, enhancing the biological effects of PDGF in proliferative tissue repair. This effect of TSP-1 along with its matrix-modulating activities may have important clinical utility regarding topical growth factor therapy in wound healing, since high proteolytic activity is believed to be partially responsible for limiting the efficacy of this treatment.

Expression of thrombospondin-1 in human pancreatic adenocarcinomas: role in matrix metalloproteinase-9 production

Human pancreatic adenocarcinoma, an aggressive malignant disease, shows a strong desmoplastic reaction characterized by a remarkable proliferation of interstitial connective tissues. Thrombospondin-1 (TSP-1), a 450 kDa platelet and matrix glycoprotein, has been implicated in tumor invasion, angiogenesis and metastasis. TSP-1 and MMP-9 expression in pancreatic adenocarcinoma and control pancreas tissues was measured by immunohistochemistry. TSP-1 expression in pancreatic carcinoma cell lines, fibroblasts, and endothelial cells was measured by a competitive TSP-1 enzyme linked immunosorbent assay (ELISA). The effect of TSP-1 on MMP-9 production in pancreatic carcinoma cell lines was measured by zymography and Western blot analysis. Eighty five per cent (23/27) of cases of pancreatic adenocarcinoma showed increased TSP-1 staining in the desmoplastic stroma adjacent to tumor cells. No specific positive staining for TSP-1 was observed in the normal pancreatic tissues and the inflammatory areas. TSP-1 localized in tumor stroma surrounding the tumor cells expressing MMP-9. Using TSP-1 competitive ELISA, the secretion of TSP-1 by different pancreatic cancer cell lines into culture medium varied from 11.45 plus minus 14.08 to 275.82 plus minus 45.56 ng/10 6 cells/24 hours. The amounts of TSP-1 detected in both culture media and cell extracts from fibroblasts or endothelial cells were at least 2-3 fold higher than those from pancreatic cancer cells. TSP-1 augmented the production of matrix metalloproteinase-9, a matrix degrading enzyme, in pancreatic cancer cells in vitro. Stromally-derived TSP-1 up-regulates the production of MMP-9 by pancreatic adenocarcinoma. These data are consistent with the conclusion that TSP-1-rich stroma is involved in regulating matrix remodeling in tumor invasion.

Tumour cell thrombospondin-1 regulates tumour cell adhesion and invasion through the urokinase plasminogen activator receptor

We have previously shown that platelet-produced thrombospondin-1 up-regulates the urokinase plasminogen activator and its receptor and promotes tumour cell invasion. Although tumour cells produce thrombospondin-1 in vivo, they produce only minimal amounts of thrombospondin-1 in vitro. To determine the effect of tumour cell-produced thrombospondin-1 in the regulation of the plasminogen/plasmin system and tumour cell invasion, we studied THBS-1-transfected MDA-MB-435 breast cancer cells that overexpress thrombospondin-1. The role of urokinase plasminogen receptor in thrombospondin-1-mediated adhesion and invasion was studied by antisense inhibition, enzymatic cleavage and antibody neutralization. Tumour cell adhesion to collagen and laminin was evaluated. Tumour cell invasion was studied in a modified Boyden chamber collagen invasion assay. Tumour cell thrombospondin-1 induced a 2-7 fold increase in urokinase plasminogen activator receptor and cell-associated urokinase plasminogen activator expression and a 50-65% increase in cell-associated urokinase plasminogen activator and plasmin activities. Furthermore, tumour cell thrombospondin-1 promoted tumour cell invasion and decreased tumour cell adhesion through up-regulation of urokinase plasminogen activator receptor-controlled urokinase plasminogen activator and plasmin activities. We conclude that tumour cell-produced thrombospondin-1 may play a critical role in the regulation of tumour cell adhesion and tumour cell invasion.

Thrombospondin 1 and its specific cysteine-serine-valine-threonine-cysteine-clycine receptor in fetal wounds

Thrombospondin 1 (TSP-1), an adhesive glycoprotein, plays an important role in platelet adhesion, inflammation, cell-cell interaction, and angiogenesis. TSP-1 is expressed by endothelial cells, fibroblasts, and macrophages. The unique cysteine-serinevaline-threonine-cysteine-glycine (CSVTCG) binding domain of TSP-1 also plays an important role in cell binding and modulation of cellular processes. The purpose of this study was to evaluate histologically and quantitatively TSP-1 and its CSVTCG receptor in fetal skin wounds over time. Pregnant ewes underwent laparotomy and hysterotomy. At 65 days gestation (term, 145 days), incisional and excisional wounds were created on the fetal back in a similar position on each animal. The uterus and laparotomy were closed. The wounds were harvested on days 1, 3, 7, 21, and 28. Expression of TSP-1 and its CSVTCG receptor was evaluated immunohistochemically and quantitated by computer image analysis in units of absorbance. Immunoglobulin G (negative) controls were performed and subtracted from the TSP-1 sample to eliminate background absorbance readings. Serum (negative) control was used for the CSVTCG receptor. Platelet concentrates were used as the positive control: TSP-1, 63.43; CSVTCG, 58.72. Results are expressed as absorbance+/-SEM. Results of TSP-1 are as follows: day 1, 33.02+/-0.26; day 3, 22.21+/-0.14; day 7, 20.56+/-1.07; day 21, 7.76+/-0.40; and day 28, 5.99+/-0.03. TSP-1 displays an early peak during fetal skin repair, followed by a steep decrease over the viewed time period. Results of CSVTCG receptor are as follows: day 1, 26.19+/-2.43; day 3, 30.20+/-0.64; day 7, 24.56+/-0.80; day 21, 24.70+/-0.40; and day 28, 21.65+/-1.39. Thus, CSVTCG receptor displays a slowed decrease in expression over time during fetal repair. No significant differences were noted between incisional and excisional samples. Temporal and histological differences exist in the localization and expression of TSP-1 and its CSVTCG receptor during fetal wound repair. TSP-1 is upregulated in tissues early. This corresponds with the known role of TSP-1 in cell-cell interaction, including potentiation of growth factor activity. TSP-1 also modulates matrix, allowing scar-free provisional matrix in the earlier stages of repair deposited by platelets. The potentiation of cell-associated protease activity by TSP-1 can support tissue and matrix turnover. This activity of TSP-1 may contribute to the formation of a scarless wound. TSP-1 destabilizes extracellular matrix contacts, and facilitates mitosis and migration. The action of TSP-1 as an adhesive protein allows numerous different cells to adhere to the extracellular membrane. CSVTCG receptor expression decreases during fetal repair as the cells migrate to the epithelial surface, suggesting a significant role of the CSVTCG receptor in keratinocytic maturation, differentiation, and epithelization.

Role of urokinase plasminogen activator receptor in thrombospondin 1-mediated tumor cell invasion

We previously showed that thrombospondin 1 (TSP-1) upregulates the plasminogen/plasmin system and promotes breast tumor cell invasion. Preliminary data from our laboratory using neutralizing antibodies suggested that the upregulation in breast tumor cell invasion seen in response to TSP-1 involved the urokinase plasminogen activator receptor (uPAR). To confirm these findings in MDA-MB-231 breast cancer cells, we developed three other strategies to study the role of uPAR in tumor cell adhesion and TSP-1-mediated tumor cell invasion: (a) enzymatic cleavage of uPAR with glycosylphosphatidylinositol-specific phospholipase C; (b) inhibition at the mRNA level with a uPAR antisense construct (cells named LKAS-MDA); (c) inhibition of plasminogen binding with the lysine analogue epsilon-aminocaproic acid. Adhesion to laminin and type I and type IV collagen with and without the addition of epsilon-aminocaproic acid was studied. Tumor cell invasion was studied in a modified Boyden chamber collagen invasion assay. Antisense uPAR inhibition decreased uPAR expression by 48-66% and cell-associated urokinase plasminogen activator (uPA) by 30-68%. Additionally, antisense uPAR inhibition induced a 68-70% reduction in uPA and plasmin activities. Antisense uPAR transfection increased tumor cell adhesion by 46-53%. A similar effect was observed in epsilon-aminocaproic acid-treated MDA-MB-231 cells. TSP-1-mediated tumor cell invasion was almost completely inhibited by either antisense uPAR inhibition or treatment with phospholipase C or epsilon-aminocaproic acid. We conclude that uPAR plays a crucial role in the regulation of tumor cell adhesion and TSP-1-mediated tumor cell invasion.

Thrombospondin-1 and its CSVTCG-specific receptor in wound healing and cancer

Growth factors play a crucial role in the regulation of cellular proliferation and matrix degradation in wound healing and cancer. We have shown that thrombospondin 1 (TSP-1) and its cysteine-serine-valine-threonine-cysteine-glycine (CSVTCG)-specific receptor play a key role in cell invasion and matrix degradation in different carcinomas. The present study was done to determine whether TSP-1 and its receptor show a similar pattern of expression in wound healing and cancer. Expression and localization of TSP-1 and its receptor were determined in fetal wounds, adult burn wounds, and different human malignancies by immunohistochemical staining and computerized image analysis. In healing wounds, TSP-1 was expressed in the stroma early in the process, followed by a steep decline. The TSP-1 receptor localized to neovessels and highly proliferating cells (i.e., fibroblasts, basal cells), its levels remaining relatively constant. Cancer cells and tumor-associated microvessels expressed the TSP-1 receptor, whereas TSP-1 localized predominantly to the tumor-associated stroma. These data suggest a critical role for TSP-1 and its CSVTCG-specific receptor in wound healing and cancer.

Thrombospondin-1 is elevated with both intimal hyperplasia and hypercholesterolemia

BACKGROUND

Thrombospondin-1 (TSP-1) is important in platelet adhesion and aggregation, inflammation, cell to cell interaction, angiogenesis, and smooth muscle cell (SMC) proliferation. TSP-1 expression increases rapidly with injury. Therefore, we hypothesize that TSP-1 may play a role in the development of intimal hyperplasia (IH). The purpose of this study is to examine the interaction between cholesterol and TSP-1 on SMC proliferation and to quantitatively assess TSP-1 expression in an established model of IH, with and without underlying cholesterol-induced atherosclerosis.

MATERIALS AND METHODS

In vitro, rabbit aortic SMC culture studies were performed to see the effect of TSP-1 antibodies on PDGF and, separately, cholesterol-induced SMC proliferation. In vivo, 23 rabbits were fed either a regular or a high-cholesterol diet. Hypercholesterolemia was confirmed by measurement of serum levels. Subsets underwent intraluminal aortic injury. Aortas were harvested 8-10 weeks later. Arterial wall TSP-1 was evaluated immunohistochemically and quantified by computer image analysis.

RESULTS

In vitro, TSP-1 antibodies were able to inhibit PDGF and cholesterol-induced SMC proliferation (P < 0.05). In vivo, TSP-1 was found predominantly in the extracellular matrix in the rabbit aorta. IH was uniformly seen status-post angioplasty. Hyperplasia was more prominent in samples from hypercholesterolemic animals. ANOVA and Student's t test analyses demonstrated significantly more TSP-1 in the high-cholesterol/angioplasty group than in all other groups (P = 0.0006 vs regular diet/no angioplasty group).

CONCLUSIONS

These data are consistent with the hypothesis that TSP-1 contributes to the development of IH. This study suggests that injured arteries in hypercholesterolemic atherosclerotic rabbits overexpress TSP-1.

Histopathology and clinical assessment correlate with the cysteine-serine-valine-threonine-cysteine-glycine (CSVTCG) receptor of thrombospondin-1 in breast tumors

Thrombospondin-1 (TSP-1) is a matrix protein implicated in mechanisms of tumor metastasis. TSP-1 has a characteristic Cysteine-Serine-Valine-Threonine-Cysteine-Glycine (CSVTCG) sequence that functions as a tumor cell adhesion domain. Our laboratory has isolated a novel CSVTCG specific tumor cell receptor. Immunohistochemical staining techniques and computerized image analysis were used to identify and quantitate the CSVTCG receptor of TSP-1 in a wide spectrum of human archival breast tumors. Histopathologic and quantitative examination was correlated with clinical findings two years post operation. Increasing amounts of CSVTCG receptor correlated positively with worsening histopathologic and clinical findings. These findings suggest a role for the TSP-1 CSVTCG receptor in breast tumor progression. This receptor may have utility for the diagnosis, staging, and treatment of this common and deadly disease.

The syndecan family of proteoglycans. Novel receptors mediating internalization of atherogenic lipoproteins in vitro

Cell-surface heparan sulfate proteoglycans have been shown to participate in lipoprotein catabolism, but the roles of specific proteoglycan classes have not been examined previously. Here, we studied the involvement of the syndecan proteoglycan family. First, transfection of CHO cells with expression vectors for several syndecan core proteins produced parallel increases in the cell association and degradation of lipoproteins enriched in lipoprotein lipase, a heparan-binding protein. Second, a chimeric construct, FcR-Synd1, that consists of the ectodomain of the IgG Fc receptor Ia linked to the highly conserved transmembrane and cytoplasmic domains of syndecan-1 directly mediated efficient internalization, in a process triggered by ligand clustering. Third, internalization of lipase-enriched lipoproteins via syndecan-1 and of clustered IgGs via the chimera showed identical kinetics (t1/2 = 1 h) and identical dose-response sensitivities to cytochalasin B, which disrupts microfilaments, and to genistein, which inhibits tyrosine kinases. In contrast, internalization of the receptor-associated protein, which proceeds via coated pits, showed a t1/2 < 15 min, limited sensitivity to cytochalasin B, and complete insensitivity to genistein. Thus, syndecan proteoglycans can directly mediate ligand catabolism through a pathway with characteristics distinct from coated pits, and might act as receptors for atherogenic lipoproteins and other ligands in vivo.

Thrombospondin-1 modulates angiogenesis in vitro by up-regulation of matrix metalloproteinase-9 in endothelial cells

Evidence suggests that thrombospondin-1 (TSP-1), a 450-kDa glycoprotein in platelets and extracellular matrix, is involved in angiogenesis. However, the mechanisms by which TSP-1 regulates angiogenesis are unknown, and the exact role of TSP-1 in angiogenesis has been controversial: both stimulatory and inhibitory effects of TSP-1 have been reported. In this study, we evaluated the effect of TSP-1 on the capacity of bovine aortic endothelial (BAE) cells to both invade and form microvessel-like tubes in collagen gels. BAE cell tube formation was enhanced by exogenous TSP-1 at relatively low concentrations (1-10 microg/ml) but inhibited at higher concentrations of TSP-1 (>15 microg/ml). In addition, we correlated this biphasic effect on tube formation with the capacity of TSP-1 to stimulate the activity of a matrix metalloproteinase-9 (MMP-9) in BAE cell collagen gel cultures. The TSP-1-mediated stimulation of MMP-9 activity was specific and dose- and time-dependent. Furthermore, TSP-1-stimulated BAE cell invasion and tube formation were reversed by antibodies against both TSP-1 and MMP-9, suggesting that TSP-1 modulates endothelial cell invasion and morphogenesis in vitro by a mechanism involving the regulation of MMP-9 activity. These findings support the conclusion that TSP-1 is a multifunctional modulator of angiogenesis and are consistent with the dynamic presence of TSP-1 in remodeling tissues in which matrix degradation is required.

Inhibition of breast cancer progression by an antibody to a thrombospondin-1 receptor

BACKGROUND

Thrombospondin-1 (TSP-1) is a matrix-bound adhesive glycoprotein. Breast carcinoma cells exhibit increased expression of a novel TSP-1 receptor. We evaluated the role of this receptor in breast cancer adhesion and progression.

METHODS

Adhesion assays were performed to evaluate MDA-MB-231 breast cancer cell adhesion to TSP-1 in vitro in the presence of either nonimmune immunoglobulin G(IgG) or anti-TSP-1 receptor IgG. Receptor-mediated tumor cell progression was evaluated in athymic nude mice. Mice were inoculated with MDA-MB-231 breast cancer cells and randomized to treatment with intraperitoneal injections of saline solution, nonspecific IgG antibody, or an anti-TSP-1 receptor antibody every other day for 20 days. Mice were killed at 21 days. The peritoneal cavity was examined grossly for primary tumor implantation. The liver and lungs were examined histologically for micrometastases.

RESULTS

MDA-MB-231 breast cancer cells adhered to TSP-1 in vitro. This adhesion was inhibited to 10% of control by anti-TSP-1 receptor antibody (p < 0.005). Anti-TSP-1 receptor antibody inhibited in vivo breast cancer progression. Mice treated with control IgG antibody or saline solution alone exhibited extensive intraperitoneal seeding. Only one mouse treated with the anti-TSP-1 receptor antibody exhibited any intraperitoneal tumor seeding (p < 0.01).

CONCLUSIONS

These data suggest that TSP-1 and its receptor play an important role in breast cancer progression.

Thrombospondin-1 (TSP-1) promotes the invasive properties of human breast cancer

TSP-1 is a matrix-bound adhesive glycoprotein, which plays a role in tumor cell proliferation and tumor angiogenesis. The purpose of this study was to investigate the effect of TSP-1 on breast tumor cell invasion. Tumor cell invasion assays were performed using a modified Boyden chamber apparatus with collagen-coated membranes. Four breast cell lines were studied in serum-free media: the malignant MDA-MB-231, SKBR-3, and MCF-7 cell lines, and the benign MCF-10A cell line. Invasion was measured as the summation of the number of cells in five representative high power fields (400x) traversing the collagen barrier after a 3-hr incubation period. The effect of an anti-TSP-1 antibody (100 microgram/ml) was also evaluated in the malignant cell lines. Statistical analysis was performed by ANOVA and Student's unpaired t test. TSP-1 promoted a dose-dependent increase in invasion as compared to buffer controls in all three malignant cell lines. TSP-1 (100 nM) promoted a greater than five-fold increase over controls in tumor cell invasion for MDA-MB-231, SKBR-3, and MCF-7 cell lines (P < 0.005). TSP-1 had no effect on the invasiveness of the benign cell type MCF-10A. Anti-TSP-1 antibody inhibited TSP-1 promoted invasion in the MDA-MB-231, SKBR-3, and MCF-7 cell lines by 45, 48, and 39%, respectively (P = 0.003, 0.044, 0.047). TSP-1 promotes tumor cell invasion of collagen by breast cancer cells. Therapy designed to inhibit TSP-1 may prevent invasion and metastasis in breast cancer.

The effect of thrombospondin on oral squamous carcinoma cell invasion of collagen

BACKGROUND

Thrombospondin (TSP), a cell matrix protein, and transforming growth factor beta (TGF-beta), a growth regulatory protein, play roles in tumor progression. The purpose of this study was to investigate the effects of TSP and TGF-beta on tumor cell invasion.

MATERIALS AND METHODS

Tumor cell invasion assays were performed using a modified Boyden chamber apparatus with collagen-coated membranes. The KB oral carcinoma cell line was studied in serum-free media. Invasion was measured as the summation of the number of cells in five representative low-power fields (x 100) traversing the collagen barrier after a 3-hour incubation period. The effects of antibodies against TSP, TGF-beta and the cysteine-serine-valine-threonine-cysteine-glycine (CSVTCG)-specific TSP receptor were also evaluated.

RESULTS

TSP caused a dose-dependent stimulation of tumor cell invasion. Antibodies against TSP, its CSVTCG-specific receptor, and TGF-beta inhibited TSP-promoted invasion by 50% to 71%.

CONCLUSIONS

TSP and its CSVTCG-specific receptor promote KB cell invasion of collagen through the production and/or activation of TGF-beta.

Thrombospondin and transforming growth factor-beta 1 increase expression of urokinase-type plasminogen activator and plasminogen activator inhibitor-1 in human MDA-MB-231 breast cancer cells

BACKGROUND

Thrombospondin is a high molecular weight adhesive glycoprotein that has been shown to function in mechanisms of tumor progression. The authors' previous studies have shown that thrombospondin promotes human lung carcinoma invasion by up-regulation of the plasminogen activator system through a mechanism involving the activation of transforming growth factor-beta 1 (TGF-beta 1). In this study, a similar thrombospondin-mediated mechanism operative in breast carcinoma cells is described.

METHODS

The effect of thrombospondin and TGF-beta 1 on the capacity of a line of breast carcinoma cells to activate plasminogen was measured as well as the physiologic consequences of these activities on cell adhesion and proliferation. Plasminogen activation was assessed by measuring the plasmin activity and plasminogen activator inhibitor-1 (PAI-1) levels in cell-conditioned media and the cell-associated urokinase-type plasminogen activator (uPA) levels.

RESULTS

Treatment of MDA-MB-231 breast carcinoma cells with either thrombospondin or TGF-beta 1 caused increased secretion of PAI-1 with a concomitant decrease in plasmin activity, whereas cell-associated uPA expression was increased with respect to controls. Thrombospondin (40 micrograms/ml) or TGF-beta 1 (5 ng/ml) stimulated the cells to secrete 5.5- and 6.7-fold more PAI-1 than controls, respectively, and caused decreased plasmin activity in the cell culture medium. Conversely, either thrombospondin (40 micrograms/ml) or TGF-beta 1 (5 ng/ml) caused the cells to express 4.55- and 5.38-fold more uPA than controls, respectively. Thrombospondin and TGF-beta 1 induced a more flattened and spread appearance in the cells with no effect on proliferation. These effects could be reversed with antibodies to either thrombospondin or TGF-beta 1 and were not due to contamination of thrombospondin with active TGF-beta 1.

CONCLUSIONS

Thrombospondin and TGF-beta 1 function similarly to increase cell-associated uPA and cell-secreted PAI-1. These data suggest that thrombospondin may not only function as an adhesive molecule, but through a mechanism involving the activation of TGF-beta 1, may modulate cell surface protease expression. In addition, these observations suggest that thrombospondin and TGF-beta 1 could promote metastasis by increasing uPA-mediated cell invasion, whereas through the action of PAI-1, also protect blood-born tumor emboli from destruction by host fibrinolytic enzymes.

Computer-assisted image analysis of tumor sections for a new thrombospondin receptor

BACKGROUND

A cell surface receptor (50 kd) has been recently identified in malignant cells that recognizes the tumor cell adhesive domain (ie, cysteine-serine-valine-threonine-cysteine-glycine [CSVTCG]) of thrombospondin (TSP). This CSVTCG-specific TSP receptor can be considered as a new tumor marker, and its concentration on the cell surface may correlate directly with the capacity of tumor cells to invade and metastasize.

MATERIALS AND METHODS

Six patients with primary, stages III and IV squamous cell carcinomas of the head and neck were studied. Tumor sections were specifically stained for this receptor with immunohistochemical techniques. The stained specimens were then subjected to computer-assisted image analysis. The area of positive staining and the heterogeneity of the pattern of staining were compared to peritumoral angiogenesis and clinical outcome of the patients.

RESULTS

The results indicate that those patients with a high and homogenous positive stain score (mean +/- standard error [SE] 78 +/- 5%) for the CSVTCG-specific TSP receptor had high microvessel density and died from metastatic disease within 12 months of initial treatment (correlation coefficients = 0.95 and 1, respectively). Patients with a low and heterogenous positive stain score for receptor (mean +/- SE 8 +/- 2%; P < 0.001) had low microvessel counts and remained disease-free for at least 2 years. There was no relationship between receptor density and histologic classification of the primary tumors.

CONCLUSION

The CSVTCG-specific TSP receptor, quantified through image analysis of immunohistochemical stained tissue sections, is highly predictive of clinical outcome in patients with squamous cell carcinomas of the head and neck.

Thrombospondin (TSP) and transforming growth factor beta 1 (TGF-beta) promote human A549 lung carcinoma cell plasminogen activator inhibitor type 1 (PAI-1) production and stimulate tumor cell attachment in vitro

A growing body of evidence has recently implicated TSP and TGF-beta in the process of malignancy, such as tumor cell proliferation, tumor angiogenesis, and metastasis. The purpose of the present study was to evaluate potential mechanisms of TSP and TGF-beta in tumor cell attachment and invasion. Our results indicate that both TSP and TGF-beta promoted tumor cell attachment and spreading in the presence of plasminogen. The mechanism for these effects appeared to be due, in part, to the capacity of TSP and TGF-beta to induce tumor cell production of (PAI-1). PAI-1, which is a natural inhibitor of tumor-cell associated urokinase-type plasminogen activator (uPA) activity, inhibited activation of plasminogen to plasmin in the growth media, thereby preventing plasmin-induced detachment of cells. The TSP-promoted production of PAI-1 could be inhibited not only by anti-TSP antibodies but also by a neutralizing antibody against TGF-beta. These results suggest that TSP by a mechanism involving TGF-beta can promote cell adhesion through stimulation of tumor cell secretion of PAI-1. These data provide evidence that TSP not only has the capacity of functioning as a matrix protein to directly promote cell-substratum adhesion but that TSP can also stimulate cell adhesion and spreading by modulating cell surface protease expression through stimulation of tumor-associated production of PAI-1.

Identification and characterization of a tumor cell receptor for CSVTCG, a thrombospondin adhesive domain

We have previously shown that peptides derived from the thrombospondin sequence, CSVTCG, promoted tumor cell adhesion. To further investigate this observation, the CSVTCG-tumor cell adhesion receptor from A549 human lung adenocarcinoma cells was isolated and characterized. A single protein peak was isolated by CSVTCG affinity chromatography which also analyzed as a single peak by anion exchange chromatography. The purified protein had a pI of 4.7 and analyzed on SDS-gels as a single band of M(r) = 50,000 under nonreducing conditions and as two protein bands of M(r) = 50,000, and 60,000 under reducing conditions. Purified CSVTCG binding protein (CBP) bound either CSVTCG- or TSP-Sepharose but showed little interaction with either VCTGSC- or BSA-Sepharose. CBP was cell surface exposed. CSVTCG derivatized with [125I] Bolton-Hunter reagent was taken up by cells in a dose-dependent manner and the cell association was inhibited with a monospecific polyclonal anti-CBP antibody. Examination of the cell proteins crosslinked to labeled CSVTCG by SDS-gel electrophoresis revealed one band that comigrated with purified CPB. Using an in vitro binding assay, purified CBP bound mannose, galactose, and glucosamine-specific lectins. CBP bound TSP saturably and reversibly. The binding was Ca+2/Mg+2 ion dependent and inhibited with fluid phase TSP and anti-CBP. Little or no binding was observed on BSA, fibronectin, GRGES, and GRGDS. Heparin, but not lactose, inhibited binding. Anti-CBP IgG and anti-CSVTCG peptide IgG inhibited A549 cell spreading and adhesion on TSP but not on fibronectin and laminin. These results indicate that CBP and the CSVTCG peptide domain of TSP can mediate TSP-promoted tumor cell adhesion.

Thrombospondin levels in patients with malignancy

Thrombospondin (TSP), a large glycoprotein present in platelets, and various normal and tumor tissues, has recently been shown to promote cell adhesion and platelet aggregation. Most importantly because TSP has been shown to promote metastasis of melanoma tumor cells to the lung in a murine model (1) and since thromboembolic events commonly occur in patients afflicted with metastatic tumors, we explored the role of TSP in human cancer by measuring TSP blood levels in patients with various malignant neoplasms. Blood TSP levels were measured by indirect enzyme-linked immunoadsorbent assay (ELISA) from 20 control subjects, 22 patients with gastrointestinal (GI) cancer, 18 patients with breast cancer, and 17 patients with lung cancer. Control subjects consisted both of healthy subjects and acutely ill patients with no malignancies. TSP levels of both healthy and acutely ill controls were found to range between 245-440 ng/ml with a mean of 365 ng/ml. In contrast, elevated levels of TSP greater than the mean value of 400 ng/ml for controls ranging between 590-3,650 ng/ml were found in 20/22 (91%) patients with GI malignancies, 13/18 (72%) patients with breast cancer, and 15/17 (88%) with lung cancer. Mean TSP levels of GI, breast, and lung cancer patients were 3, 2, and 3 fold greater than controls, respectively. Increased blood TSP levels in patients were not due to increased levels of platelets since both control and patient groups had platelet counts within the normal range. These results suggest that TSP may play a role in tumor cell metastasis in man and could serve as a blood marker for metastasis.

Biological activities of peptides and peptide analogues derived from common sequences present in thrombospondin, properdin, and malarial proteins

Thrombospondin (TSP), a major platelet-secreted protein, has recently been shown to have activity in tumor cell metastasis, cell adhesion, and platelet aggregation. The type 1 repeats of TSP contain two copies of CSVTCG and one copy of CSTSCG, per each of the three polypeptide chains of TSP and show homology with peptide sequences found in a number of other proteins including properdin, malarial circumsporozoite, and a blood-stage antigen of Plasmodium falciparum. To investigate whether these common sequences functioned as a cell adhesive domain in TSP, we assessed the effect of peptides corresponding to these sequences and an antibody raised against one of these sequences, CSTSCG, in three biological assays which depend, in part, on the cell adhesive activity of TSP. These assays were TSP-dependent cell adhesion, platelet aggregation, and tumor cell metastasis. We found that a number of peptides homologous to CSVTCG promoted the adhesion of a variety of cells including mouse B16-F10 melanoma cells, inhibited platelet aggregation and tumor cell metastasis, whereas control peptides had no effect. Anti-CSTSCG, which specifically recognized TSP, inhibited TSP-dependent cell adhesion, platelet aggregation, and tumor cell metastasis, whereas control IgG had no effect. These results suggest that CSVTCG and CSTSCG present in the type I repeats function in the adhesive interactions of TSP that mediate cell adhesion, platelet aggregation, and tumor cell metastasis. Peptides, based on the structure of these repeats, may find wide application in the treatment of thrombosis and in the prevention of cancer spread.

The interaction of thrombospondin with platelet glycoprotein GPIIb-IIIa

The interaction of human platelet thrombospondin (TSP) with human platelet glycoproteins GPIIb-IIIa was studied using a solid-phase binding assay. Polystyrene test tubes were coated with TSP, and 125I-labeled GPIIb-IIIa was added, allowed to bind, and the bound radioactivity was measured. After 90 min, the binding became time independent, and in most experiments, more than 10% of the exogenously added radioactivity was bound to the tube. Analysis of the bound radioactivity by polyacrylamide gel electrophoresis and autoradiography indicated that it was from labeled GPIIb-IIIa. Several lines of evidence indicate that the binding of GPIIb-IIIa to TSP was specific. (a) TSP immobilized on plastic or Sepharose bound 3-10-fold more GPIIb-IIIa than immobilized bovine serum albumin. (b) Addition of unlabeled excess GPIIb-IIIa reversed the binding of 125I-labeled GPIIb-IIIa to immobilized TSP. (c) Addition of EDTA inhibited the binding of GPIIb-IIIa to TSP by more than 90%, whereas addition of 1 mM CaCl2 and 1 mM MgCl2 potentiated the binding by more than 100%. (d) Monoclonal antibodies against TSP and GPIIb-IIIa inhibited the binding by 30-70% as compared with control and polyclonal anti-fibrinogen anti-serum. (e) A plot of GPIIb-IIIa bound versus GPIIb-IIIa added was best described as a rectangular hyperbola by regression analysis with half-saturation at 60 ng/ml GPIIb-IIIa. Similar results were obtained when labeled TSP was added to tubes coated with GPIIb-IIIa. These results show that TSP and GPIIb-IIIa can specifically interact in vitro and suggest that GPIIb-IIIa may function as a platelet TSP receptor during platelet aggregation.

The GPIIB-IIIa-like complex may function as a human melanoma cell adhesion receptor for thrombospondin

The purpose of this study was to determine whether a heterodimeric complex immunologically related to the fibrinogen receptor could function as a thrombospondin (TSP) receptor in TSP-mediated cell-substratum adhesion of human melanoma cells. We found that polyclonal antibodies to the platelet GPIIb-IIIa complex, GPIIIa, and the human vitronectin receptor inhibited TSP-mediated cell adhesion by 63-68%. Immunoprecipitation of detergent extracts of 125I-surface-labeled melanoma cells using either anti-human platelet GPIIb-IIIa or anti-human vitronectin receptor antibody revealed the presence of a single heterodimeric complex, suggesting that both antisera recognize the same integrin receptor, GPIIb-IIIa-like antigen. Adhesion of cells to TSP is likely mediated through a region of the TSP molecule containing the arginine-glycine-aspartic (RGD) peptide sequence, since cell attachment to TSP was inhibited 50-66% in the presence of peptides containing RGD. These results strongly suggest that a GPIIb-IIIa-like/vitronectin receptor can serve as a cell binding site for TSP in mediating cell-substratum adhesion.

Thrombospondin promotes platelet aggregation

Thrombospondin (TSP), isolated from human platelets, promotes aggregation of both nonstimulated platelets and platelets stimulated with thrombin or ADP. The TSP-promoted aggregation is specific since a monoclonal antibody against TSP inhibits the effect of exogenously added TSP and inhibits thrombin-induced platelet aggregation in the absence of added TSP. Several lines of evidence suggest that TSP mediates its effect on aggregation of nonstimulated and stimulated platelets through different platelet-surface receptor systems. The TSP-promoted aggregation of nonstimulated platelets was inhibited by a monoclonal antibody to platelet glycoprotein IV (GPIV), but not by a monoclonal antibody to the fibrinogen receptor, GPIIb-IIIa. In contrast, the antibody to GPIIb-IIIa totally inhibited the TSP-potentiated aggregation of thrombin-stimulated platelets, whereas the antibody to GPIV has no effect. Thus, these studies suggest that TSP promotes platelet aggregation by at least two mechanisms--one dependent on and one independent of the platelet fibrinogen receptor system.

Thrombospondin, a potentiator of tumor cell metastasis

The platelet-secreted protein thrombospondin (TSP) potentiates tumor cell metastasis. Human TSP injected i.v. into mice 5 min prior to i.v. injection of T241 sarcoma cells potentiates lung tumor colony formation. Several lines of evidence suggest that the TSP-enhancing effect involves both TSP-mediated tumor cell adhesion and the host's hemostatic system: TSP potentiates the initial, rapid sequestering of tumor cells in the lung; TSP promotes the adhesion of tumor cells in vitro; the effect of TSP on tumor cell metastasis is dependent on the presence of platelets and a normal plasma clotting system, since TSP does not potentiate lung tumor colony formation in either thrombocytopenic mice or mice anticoagulated with Coumadin. Our results suggest a central role for TSP in the metastatic process.