Modulation of endothelial cell proliferation, adhesion, and motility by recombinant heparin-binding domain and synthetic peptides from the type I repeats of thrombospondin

Transcriptomics and proteomics revealed sex differences in human pulmonary microvascular endothelial cells

Marked sexual dimorphism is displayed in the onset and progression of pulmonary hypertension (PH). Females more commonly develop pulmonary arterial hypertension, yet females with pulmonary arterial hypertension and other types of PH have better survival than males. Pulmonary microvascular endothelial cells play a crucial role in pulmonary vascular remodeling and increased pulmonary vascular resistance in PH. Given this background, we hypothesized that there are sex differences in the pulmonary microvascular endothelium basally and in response to hypoxia that are independent of the sex hormone environment. Human pulmonary microvascular endothelial cells (HPMECs) from healthy male and female donors, cultured under physiological shear stress, were analyzed using RNA sequencing and label-free quantitative proteomics. Gene set enrichment analysis identified a number of sex-different pathways in both normoxia and hypoxia, including pathways that regulate cell proliferation. In vitro, the rate of proliferation in female HPMECs was lower than in male HPMECs, a finding that supports the omics results. Interestingly, thrombospondin-1, an inhibitor of proliferation, was more highly expressed in female cells than in male cells. These results demonstrate, for the first time, important differences between female and male HPMECs that persist in the absence of sex hormone differences and identify novel pathways for further investigation that may contribute to sexual dimorphism in pulmonary hypertensive diseases.NEW & NOTEWORTHY There is marked sexual dimorphism in the development and progression of pulmonary hypertension. We show differences in RNA and protein expression between female and male human pulmonary microvascular endothelial cells grown under conditions of physiological shear stress, which identify sex-different cellular pathways both in normoxia and hypoxia. Importantly, these differences were detected in the absence of sex hormone differences. The pathways identified may provide novel targets for the development of sex-specific therapies.

Isthmin-A Multifaceted Protein Family

Isthmin (ISM) is a secreted protein family with two members, namely ISM1 and ISM2, both containing a TSR1 domain followed by an AMOP domain. Its broad expression pattern suggests diverse functions in developmental and physiological processes. Over the past few years, multiple studies have focused on the functional analysis of the ISM protein family in several events, including angiogenesis, metabolism, organ homeostasis, immunity, craniofacial development, and cancer. Even though ISM was identified two decades ago, we are still short of understanding the roles of the ISM protein family in embryonic development and other pathological processes. To address the role of ISM, functional studies have begun but unresolved issues remain. To elucidate the regulatory mechanism of ISM, it is crucial to determine its interactions with other ligands and receptors that lead to the activation of downstream signalling pathways. This review provides a perspective on the gene organization and evolution of the ISM family, their links with developmental and physiological functions, and key questions for the future.

Endothelial Cell Behavior Is Determined by Receptor Clustering Induced by Thrombospondin-1

The thrombospondins (TSPs) are a family of multimeric extracellular matrix proteins that dynamically regulate cellular behavior and response to stimuli. In so doing, the TSPs directly and indirectly affect biological processes such as embryonic development, wound healing, immune response, angiogenesis, and cancer progression. Many of the direct effects of Thrombospondin 1 (TSP-1) result from the engagement of a wide range of cell surface receptors including syndecans, low density lipoprotein receptor-related protein 1 (LRP1), CD36, integrins, and CD47. Different or even opposing outcomes of TSP-1 actions in certain pathologic contexts may occur, depending on the structural/functional domain involved. To expedite response to external stimuli, these receptors, along with vascular endothelial growth factor receptor 2 (VEGFR2) and Src family kinases, are present in specific membrane microdomains, such as lipid rafts or tetraspanin-enriched microdomains. The molecular organization of these membrane microdomains and their constituents is modulated by TSP-1. In this review, we will describe how the presence of TSP-1 at the plasma membrane affects endothelial cell signal transduction and angiogenesis.

Thrombospondin in Tumor Microenvironment

Thrombospondins (TSPs) are multifaceted proteins that contribute to physiologic as well as pathologic conditions. Due to their multiple receptor-binding domains, TSPs display both oncogenic and tumor-suppressive qualities and are thus essential components of the extracellular matrix. Known for their antiangiogenic capacity, TSPs are an important component of the tumor microenvironment. The N- and C-terminal domains of TSP are, respectively, involved in cell adhesion and spreading, an important feature of wound healing as well as cancer cell migration. Previously known for the activation of TGF-β to promote tumor growth and inflammation, TSP-1 has recently been found to be transcriptionally induced by TGF-β, implying the presence of a possible feedback loop. TSP-1 is an endogenous inhibitor of T cells and also mediates its immunosuppressive effects via induction of Tregs. Given the diverse roles of TSPs in the tumor microenvironment, many therapeutic strategies have utilized TSP-mimetic peptides or antibody blockade as anti-metastatic approaches. This chapter discusses the diverse structural domains, functional implications, and anti-metastatic therapies in the context of the role of TSP in the tumor microenvironment.

Nanoparticle exposure driven circulating bioactive peptidome causes systemic inflammation and vascular dysfunction

Background

The mechanisms driving systemic effects consequent pulmonary nanoparticle exposure remain unclear. Recent work has established the existence of an indirect process by which factors released from the lung into the circulation promote systemic inflammation and cellular dysfunction, particularly on the vasculature. However, the composition of circulating contributing factors and how they are produced remains unknown. Evidence suggests matrix protease involvement; thus, here we used a well-characterized multi-walled carbon nanotube (MWCNT) oropharyngeal aspiration model with known vascular effects to assess the distinct contribution of nanoparticle-induced peptide fragments in driving systemic pathobiology.

Results

Data-independent mass spectrometry enabled the unbiased quantitative characterization of 841 significant MWCNT-responses within an enriched peptide fraction, with 567 of these factors demonstrating significant correlation across animal-paired bronchoalveolar lavage and serum biofluids. A database search curated for known matrix protease substrates and predicted signaling motifs enabled identification of 73 MWCNT-responsive peptides, which were significantly associated with an abnormal cardiovascular phenotype, extracellular matrix organization, immune-inflammatory processes, cell receptor signaling, and a MWCNT-altered serum exosome population. Production of a diverse peptidomic response was supported by a wide number of upregulated matrix and lysosomal proteases in the lung after MWCNT exposure. The peptide fraction was then found bioactive, producing endothelial cell inflammation and vascular dysfunction ex vivo akin to that induced with whole serum. Results implicate receptor ligand functionality in driving systemic effects, exemplified by an identified 59-mer thrombospondin fragment, replete with CD36 modulatory motifs, that when synthesized produced an anti-angiogenic response in vitro matching that of the peptide fraction. Other identified peptides point to integrin ligand functionality and more broadly to a diversity of receptor-mediated bioactivity induced by the peptidomic response to nanoparticle exposure.

Conclusion

The present study demonstrates that pulmonary-sequestered nanoparticles, such as multi-walled carbon nanotubes, acutely upregulate a diverse profile of matrix proteases, and induce a complex peptidomic response across lung and blood compartments. The serum peptide fraction, having cell-surface receptor ligand properties, conveys peripheral bioactivity in promoting endothelial cell inflammation, vasodilatory dysfunction and inhibiting angiogenesis. Results here establish peptide fragments as indirect, non-cytokine mediators and putative biomarkers of systemic health outcomes from nanoparticle exposure.

Thrombospondins and remodeling of the tumor microenvironment

Vascular remodeling defines cancer growth and aggressiveness. Although cancer cells produce pro-angiogenic signals, the fate of angiogenesis critically depends on the cancer microenvironment. Composition of the extracellular matrix (ECM) and tumor inflammation determine whether a cancer will remain dormant, will be recognized by the immune system and eliminated, or whether the tumor will develop and lead to the spread and metastasis of cancer cells. Thrombospondins (TSPs), a family of ECM proteins that has long been associated with the regulation of angiogenesis and cancer, regulate multiple physiological processes that determine cancer growth and spreading, from angiogenesis to inflammation, metabolic changes, and properties of ECM. Here, we sought to review publications that describe various functions of TSPs that link these proteins to regulation of cancer growth by modulating multiple physiological and pathological events that prevent or support tumor development. In addition to its direct effects on angiogenesis, TSPs have important roles in regulation of inflammation, immunity, ECM properties and composition, and glucose and insulin metabolism. Furthermore, TSPs have distinct roles as regulators of remodeling in tissues and tumors, such that the pathways activated by a single TSP can interact and influence each other. The complex nature of TSP interactions and functions, including their different cell- and tissue-specific effects, may lead to confusing results and controversial conclusions when taken out of the context of interdisciplinary and holistic approaches. However, studies of TSP functions and roles in different systems of the organism offer an integrative view of tumor remodeling and a potential for finding therapeutic targets that would modulate multiple complementary processes associated with cancer growth.

Alternative transcription of a shorter, non-anti-angiogenic thrombospondin-2 variant in cancer-associated blood vessels

Thrombospondin-2 (TSP2) is an anti-angiogenic matricellular protein that inhibits tumor growth and angiogenesis. Tumor-associated blood vascular endothelial cells (BECs) were isolated from human invasive bladder cancers and from matched normal bladder tissue by immuno-laser capture microdissection. Exon expression profiling analyses revealed a particularly high expression of a short TSP2 transcript containing only the last 9 (3′) exons of the full-length TSP2 transcript. Using 5′ and 3′ RACE (rapid amplification of cDNA ends) and Sanger sequencing, we confirmed the existence of the shorter transcript of TSP2 (sTSP2) and determined its sequence which completely lacked the anti-angiogenic thrombospondin type 1 repeats domain. The largest open reading frame predicted within the transcript comprises 209 amino acids and matches almost completely the C-terminal lectin domain of full-length TSP2. We produced recombinant sTSP2 and found that unlike the full-length TSP2, sTSP2 did not inhibit vascular endothelial growth factor-A-induced proliferation of cultured human BECs, but in contrast when combined with TSP2 blocked the inhibitory effects of TSP2 on BEC proliferation. In vivo studies with stably transfected A431 squamous cell carcinoma cells revealed that full-length TSP2, but not sTSP2, inhibited tumor growth and angiogenesis. This study reveals that the transcriptional program of tumor stromal cells can change to transcribe a new version of an endogenous angiogenesis inhibitor that has lost its anti-angiogenic activity.

Thrombospondin-1 and Cutaneous Melanoma

Background:

Thrombospondins (TSPs) are recognized as important glycoproteins that regulate a wide variety of cell functions and interactions. TSPs in malignant tumors can both enhance and inhibit tumor progression, invasion, and metastasis, depending on cell type, stromal interactions, and microenvironment. These proteins are potential targets for anticancer therapy.

Objective:

The aim of our article is to review the role of thrombospondin-1 (TSP1) in cutaneous melanoma.

Conclusions:

TSP1 expression is variable in melanoma cell lines and tumors. Similar to findings in other human cancers, expression of TSP1 by melanoma cells usually inhibits tumor progression via the antiangiogenic effect of TSP1. Conversely, stromal TSP1 overexpression in melanoma is a poor prognostic factor associated with decreased survival. Understanding the interactions of TSP1 with other melanoma- and matrix-associated proteins should provide new prognostic indices and possible therapeutic targets for melanoma treatment.

Calcium-induced conformational changes of Thrombospondin-1 signature domain: implications for vascular disease

CONTEXT

Thrombospondin1 (TSP1) participates in numerous signaling pathways critical for vascular physiology and disease. The conserved signature domain of thrombospondin 1 (TSP1-Sig1) comprises three epidermal growth factor (EGF), 13 calcium-binding type 3 thrombospondin (T3) repeats, and one lectin-like module arranged in a stalk-wire-globe topology. TSP1 is known to be present in both calcium-replete (Holo-) and calcium-depleted (Apo-) state, each with distinct downstream signaling effects.

OBJECTIVE

To prepare a homology model of TSP1-Sig1 and investigate the effect of calcium on its dynamic structure and interactions.

METHODS

A homology model of Holo-TSP1-Sig1 was prepared with TSP2 as template in Swissmodel workspace. The Apo-form of the model was obtained by omitting the bound calcium ions from the homology model. Molecular dynamics (MD) simulation studies (100 ns) were performed on the Holo- and Apo- forms of TSP1 using Gromacs4.6.5.

RESULTS AND DISCUSSION

After simulation, Holo-TSP1-Sig1 showed significant reorientation at the interface of the EGF1-2 and EGF2-3 modules. The T3 wire is predicted to show the maximum mobility and deviation from the initial model. In Apo-TSP1-Sig1 model, the T3 repeats unfolded and formed coils with predicted increase in flexibility. Apo-TSP1-Sig1model also predicted the exposure of the binding sites for neutrophil elastase, integrin and fibroblast growth factor 2. We present a structural model and hypothesis for the role of TSP1-Sig1 interactions in the development of vascular disorders.

CONCLUSION

The simulated model of the fully calcium-loaded and calcium-depleted TSP1-Sig1 may enable the development of its interactions as a novel therapeutic target for the treatment of vascular diseases.

Antiangiogenic mechanisms and factors in breast cancer treatment

Breast cancer is known to metastasize in its latter stages of existence. The different angiogenic mechanisms and factors that allow for its progression are reviewed in this article. Understanding these mechanisms and factors will allow researchers to design drugs to inhibit angiogenic behaviors and control the rate of tumor growth.

Epigenetic profiling of gallbladder cancer and gall stone diseases: Evaluation of role of tumour associated genes

BACKGROUND

As on today, the global mortality rate of gallbladder cancer is still very high. Both genetic and epigenetic alterations play pivotal roles in the development of cancer. We selected seven tumour associated genes, implicated in other cancers, to assess their methylation status in gallbladder cancer and gallstone diseases.

AIM OF STUDY

To study the promoter methylation of certain tumour associated genes in the molecular pathogenesis of gallbladder cancer and gall stone diseases.

MATERIALS AND METHODS

Methylation specific PCR for seven tumour associated genes, viz., MASPIN, 14-3-3 sigma gene, THBS1, FLNC, HLTF, COX-2 and SOCS1, was performed in 50 gallbladder cancer (GBC), 30 gall stone diseases (GSD) and their respective adjacent control tissues. Semi-quantitative PCR and immunohistochemistry was carried out to check the expression level. Student's t-test was carried out to compare the differences in the methylation and expression patterns between cases and control tissues.

RESULTS

We observed methylation of CpG islands in seven of the studied markers, but, the frequency of methylation was found varying among different samples. Of them, 14-33 sigma showed methylation in 45 GBC (90%; p=0.0001) and 25 GSD (86.66%; p=0.001), MASPIN in 35 GBC (70%; p=0.0008) and 18 GSD (51.43%; p=0.040), FLNC in 16 GBC (32%; p=0.0044) and 9 GSD (25.71%; p=ns), THBS1 in 26 GBC (52%; p=0.0009) and 10 GSD (28.57%; p=0.0505), HLTF in 8 GBC (16%; p=ns) and 2 GSD (5.71%; p=ns), COX2 in 10 GBC (20%; p=ns) and 6 GSD (17.14%; p=ns) and SOCS-1 in 3 GBC samples only (6%; p=ns), but not in GSD. Semi-quantitative PCR revealed down regulation in MASPIN, 14-3-3 sigma, THBS1, HLTF, COX2 and SOCS1 in advanced gallbladder cases. Immunohistochemistry further confirmed the down-regulation of SOCS1 in GBC.

CONCLUSION

The present study infers that accumulation of epigenetic alterations increases poor prognosis of GBC patients. Out of seven genes, MASPIN and THBS1 play key epigenetic role in GBC, but not in GSD. The reason for downregulation of SOCS1 only in GBC, and unaltered expression of 14-3-3 sigma protein in all the GBC and GSD tissue samples is not clear. Further investigation on the expression pattern of these genes in GBC cell lines may elucidate their likely functional role in in association with gallbladder cancer.

Pneumococcal Adhesins PavB and PspC Are Important for the Interplay with Human Thrombospondin-1

The human matricellular glycoprotein thrombospondin-1 (hTSP-1) is released by activated platelets and mediates adhesion of Gram-positive bacteria to various host cells. In staphylococci, the adhesins extracellular adherence protein (Eap) and autolysin (Atl), both surface-exposed proteins containing repeating structures, were shown to be involved in the acquisition of hTSP-1 to the bacterial surface. The interaction partner(s) on the pneumococcal surface was hitherto unknown. Here, we demonstrate for the first time that pneumococcal adherence and virulence factor B (PavB) and pneumococcal surface protein C (PspC) are key players for the interaction of Streptococcus pneumoniae with matricellular hTSP-1. PavB and PspC are pneumococcal surface-exposed adhesins and virulence factors exhibiting repetitive sequences in their core structure. Heterologously expressed fragments of PavB and PspC containing repetitive structures exhibit hTSP-1 binding activity as shown by ELISA and surface plasmon resonance studies. Binding of hTSP-1 is charge-dependent and inhibited by heparin. Importantly, the deficiency in PavB and PspC reduces the recruitment of soluble hTSP-1 by pneumococci and decreases hTSP-1-mediated pneumococcal adherence to human epithelial cells. Platelet activation assays suggested that PavB and PspC are not involved in the activation of purified human platelets by pneumococci. In conclusion, this study indicates a pivotal role of PavB and PspC for pneumococcal recruitment of soluble hTSP-1 to the bacterial surface and binding of pneumococci to host cell-bound hTSP-1 during adhesion.

Resveratrol modulates the angiogenic response to exercise training in skeletal muscles of aged men

In animal studies, the polyphenol resveratrol has been shown to influence several pathways of importance for angiogenesis in skeletal muscle. The aim of the present study was to examine the angiogenic effect of resveratrol supplementation with parallel exercise training in aged men. Forty-three healthy physically inactive aged men (65 ± 1 yr) were divided into 1) a training group that conducted 8 wk of intense exercise training where half of the subjects received a daily intake of either 250 mg trans-resveratrol (n = 14) and the other half received placebo (n = 13) and 2) a nontraining group that received either 250 mg trans-resveratrol (n = 9) or placebo (n = 7). The group that trained with placebo showed a ~20% increase in the capillary-to-fiber ratio, an increase in muscle protein expression of VEGF, VEGF receptor-2, and tissue inhibitor of matrix metalloproteinase (TIMP-1) but unaltered thrombospodin-1 levels. Muscle interstitial VEGF and thrombospodin-1 protein levels were unchanged after the training period. The group that trained with resveratrol supplementation did not show an increase in the capillary-to-fiber ratio or an increase in muscle VEGF protein. Muscle TIMP-1 protein levels were lower in the training and resveratrol group than in the training and placebo group. Both training groups showed an increase in forkhead box O1 protein. In nontraining groups, TIMP-1 protein was lower in the resveratrol-treated group than the placebo-treated group after 8 wk. In conclusion, these data show that exercise training has a strong angiogenic effect, whereas resveratrol supplementation may limit basal and training-induced angiogenesis.

Regulation of soluble guanylate cyclase by matricellular thrombospondins: implications for blood flow

Nitric oxide (NO) maintains cardiovascular health by activating soluble guanylate cyclase (sGC) to increase cellular cGMP levels. Cardiovascular disease is characterized by decreased NO-sGC-cGMP signaling. Pharmacological activators and stimulators of sGC are being actively pursued as therapies for acute heart failure and pulmonary hypertension. Here we review molecular mechanisms that modulate sGC activity while emphasizing a novel biochemical pathway in which binding of the matricellular protein thrombospondin-1 (TSP1) to the cell surface receptor CD47 causes inhibition of sGC. We discuss the therapeutic implications of this pathway for blood flow, tissue perfusion, and cell survival under physiologic and disease conditions.

Effects of the C-terminal of endostatin on the tumorigenic potential of H22 cells

Endostatin is an endogenous angiogenesis inhibitor whose specific functional site has not yet been determined. In the present experiment, 13 amino acids (LCIENSFMTSFSK) were selectively deleted from the C-terminal of endostatin and the resulting mutant endostatin was named EM13. To determine the effect of the C-terminal deletion on the biological activity of endostatin, EM13, wild-type endostatin and empty plasmid were transfected into H22 cells. After 48 h, the three types of transfected cells were harvested and injected into nude mice. The results demonstrated that there was no significant difference in tumor size, as determined by hematoxylin and eosin staining, between the EM13-transfected group and the endostatin and empty plasmid groups, although the nude mice that were injected with EM13-transfected H22 cells exhibited smaller tumors and lower density of blood vessels compared to those injected with endostatin- and empty plasmid-transfected H22 cells. The results suggested that the 13 amino acids of the C-terminal of endostatin do not play an important role in the tumorigenic potential of H22 cells. This experiment was unsuccessful in reproducing the results of several investigators. Therefore, the mechanism underlying the tumorigenesis of H22 cells remains to be elucidated.

The role of thrombospondin 1 on intestinal inflammation and carcinogenesis

Crohn’s disease and ulcerative colitis are inflammatory bowel diseases (IBD) quite common in the United States and other Western countries. Patients suffering IBD are at greater risk of developing colorectal adenocarcinoma than the general population. Both, the adenomacarcinoma and the inflammation-carcinogenesis processes are characterized by active angiogenesis. Recent studies also have shown that anti-angiogenesis might be a novel therapeutic approach for IBD. Thrombospondin 1 (TSP1) is an extracellular protein well known for its anti-angiogenic properties. TSP1 also has key functions in inflammation, which is assumed to be the primary cause for carcinogenesis in IBD. This review is focused on the role of TSP1 in colorectal carcinogenesis. The therapeutic effects of TSP derived-peptides on inhibiting the inflammation-carcinogenesis progression are also discussed.

Emerging roles for the BAI1 protein family in the regulation of phagocytosis, synaptogenesis, neurovasculature, and tumor development

While G-protein-coupled receptors (GPCRs) have received considerable attention for their biological activity in a diversity of physiological functions and have become targets for therapeutic intervention in many diseases, the function of the cell adhesion subfamily of GPCRs remains poorly understood. Within this group, the family of brain angiogenesis inhibitor molecules (BAI1-3) has become increasingly appreciated for their diverse roles in biology and disease. In particular, recent findings suggest emerging roles for BAI1 in the regulation of phenomena including phagocytosis, synaptogenesis, and the inhibition of tumor growth and angiogenesis via the processing of its extracellular domain into secreted vasculostatins. Here we summarize the known biological features of the BAI proteins, including their structure, proteolysis events, and interacting partners, and their recently identified ability to regulate certain signaling pathways. Finally, we discuss the potential of the BAIs as therapeutics or targets for diseases as varied as cancer, stroke, and schizophrenia.

Role of class A human thrombospondins in the clearance of dying cells and tolerance induction

Mammalian thrombospondins (TSPs) are a group of large, secreted, calcium-binding glycoproteins of complex spatial structure that mediate a wide range of intercellular activities and participate in cell-matrix interactions. This family includes five proteins, divided into two subfamilies, that possess different roles and tissue expression. TSPs have complex roles in mediating cellular processes. Apoptotic cell and phagocyte interactions show a dynamic structure with expanding complexity. However, a vast majority of the consequences of these interactions can be mediated by a single protein. One of these signaling molecules is TSP-1, which binds to a wide variety of integrin and nonintegrin cell surface receptors and mediates both engulfment and immune modulation. This mechanism is not only important in homeostasis but may also be a major mechanism for inflammation downregulation and in avoiding autoimmunity.

CD36-mediated activation of endothelial cell apoptosis by an N-terminal recombinant fragment of thrombospondin-2 inhibits breast cancer growth and metastasis in vivo

Thus far the clinical benefits seen in breast cancer patients treated with drugs targeting the vascular endothelial growth factor (VEGF) pathway are only modest. Consequently, additional antiangiogenic approaches for treatment of breast cancer need to be investigated. Thrombospondin-2 (TSP-2) has been shown to inhibit tumor growth and angiogenesis with a greater potency than the related molecule TSP-1. The systemic effects of TSP-2 on tumor metastasis and the underlying molecular mechanisms of the antiangiogenic activity of TSP-2 have remained poorly understood. We generated a recombinant fusion protein consisting of the N-terminal region of TSP-2 and the IgG-Fc1 fragment (N-TSP2-Fc) and could demonstrate that the antiangiogenic activity of N-TSP2-Fc is dependent on the CD36 receptor. We found that N-TSP2-Fc inhibited VEGF-induced tube formation of human dermal microvascular endothelial cells (HDMEC) on matrigel in vitro and that concurrent incubation of anti-CD36 antibody with N-TSP2-Fc resulted in tube formation that was comparable to untreated control. N-TSP2-Fc potently induced apoptosis of HDMEC in vitro in a CD36-dependent manner. Moreover, we could demonstrate a CD36 receptor-mediated loss of mitochondrial membrane potential and activation of caspase-3 in HDMEC in vitro. Daily intraperitoneal injections of N-TSP2-Fc resulted in a significant inhibition of the growth of human MDA-MB-435 and MDA-MB-231 tumor cells grown in the mammary gland of immunodeficient nude mice and in reduced tumor vascularization. Finally, increased serum concentrations of N-TSP2-Fc significantly inhibited regional metastasis to lymph nodes and distant metastasis to lung as shown by quantitative real-time alu PCR. These results identify N-TSP2-Fc as a potent systemic inhibitor of tumor metastasis and provide strong evidence for an important role of the CD36 receptor in mediating the antiangiogenic activity of TSP-2.

Extracellular matrix proteins and tumor angiogenesis

Tumor development is a complex process that relies on interaction and communication between a number of cellular compartments. Much of the mass of a solid tumor is comprised of the stroma which is richly invested with extracellular matrix. Within this matrix are a host of matricellular proteins that regulate the expression and function of a myriad of proteins that regulate tumorigenic processes. One of the processes that is vital to tumor growth and progression is angiogenesis, or the formation of new blood vessels from preexisting vasculature. Within the extracellular matrix are structural proteins, a host of proteases, and resident pro- and antiangiogenic factors that control tumor angiogenesis in a tightly regulated fashion. This paper discusses the role that the extracellular matrix and ECM proteins play in the regulation of tumor angiogenesis.

Thrombospondin-1 as a Paradigm for the Development of Antiangiogenic Agents Endowed with Multiple Mechanisms of Action

Uncontrolled neovascularization occurs in several angiogenesis-dependent diseases, including cancer. Neovascularization is tightly controlled by the balance between angiogenic growth factors and antiangiogenic agents. The various natural angiogenesis inhibitors identified so far affect neovascularization by different mechanisms of action. Thrombospondin-1 (TSP-1) is a matricellular modular glycoprotein that acts as a powerful endogenous inhibitor of angiogenesis. It acts both indirectly, by sequestering angiogenic growth factors and effectors in the extracellular environment, and directly, by inducing an antiangiogenic program in endothelial cells following engagement of specific receptors including CD36, CD47, integrins and proteoglycans (all involved in angiogenesis ). In view of its central, multifaceted role in angiogenesis, TSP-1 has served as a source of antiangiogenic tools, including TSP-1 fragments, synthetic peptides and peptidomimetics, gene therapy strategies, and agents that up-regulate TSP-1 expression. This review discusses TSP-1-based inhibitors of angiogenesis, their mechanisms of action and therapeutic potential, drawing our experience with angiogenic growth factor-interacting TSP-1 peptides, and the possibility of exploiting them to design novel antiangiogenic agents.

Thrombospondin-1-N-terminal domain induces a phagocytic state and thrombospondin-1-C-terminal domain induces a tolerizing phenotype in dendritic cells

In our previous study, we have found that thrombospondin-1 (TSP-1) is synthesized de novo upon monocyte and neutrophil apoptosis, leading to a phagocytic and tolerizing phenotype of dendritic cells (DC), even prior to DC-apoptotic cell interaction. Interestingly, we were able to show that heparin binding domain (HBD), the N-terminal portion of TSP-1, was cleaved and secreted simultaneously in a caspase- and serine protease- dependent manner. In the current study we were interested to examine the role of HBD in the clearance of apoptotic cells, and whether the phagocytic and tolerizing state of DCs is mediated by the HBD itself, or whether the entire TSP-1 is needed. Therefore, we have cloned the human HBD, and compared its interactions with DC to those with TSP-1. Here we show that rHBD by itself is not directly responsible for immune paralysis and tolerizing phenotype of DCs, at least in the monomeric form, but has a significant role in rendering DCs phagocytic. Binding of TSP-1-C-terminal domain on the other hand induces a tolerizing phenotype in dendritic cells.

Thrombospondins, metallo proteases and thrombospondin receptors messenger RNA and protein expression in different tumour sublines of the Dunning prostate cancer model

Thrombospondin is a potent inhibitor of angiogenesis and might therefore be important in controlling tumour growth. TSP interacts with a number of proteases and receptors and in this way inhibits stimulation of angiogenesis. An earlier study showed that thrombospondin is expressed in benign prostatic hyperplasia (BPH) and high-grade prostatic intraepithelial neoplasia (PIN) but is absent in prostate cancer. The present study was therefore designed to evaluate the expression of thrombospondin 1 and 2 (TSP-1, TSP-2), TSP receptors CD36 and CD47, and matrix-metalloproteases 2 and 9 (MMP-, MMP-9) in a rat prostate cancer model. By using immunohistochemistry, Western blot, and real-time PCR the expression patterns of TSP-1, TSP-2, CD36, CD47, MMP-2, and MMP-9 were investigated in normal rat prostate tissue and five malignant Dunning sublines tissue. TSP-1 mRNA levels were decreased in all tumours compared with normal prostate. However, there was no difference in expression of TSP-2 and CD36 mRNA in these samples. MMP-2 was increased with malignancy, but no expression of MMP-9 was seen. The CD47 receptor did slightly increase with malignancy except for H3327. The results showed that thrombospondin is expressed in normal prostate but not in prostate tumours in a rat model. Simultaneously, MMP-2 expression increases with malignancy.

Capillary sprout endothelial cells exhibit a CD36 low phenotype: regulation by shear stress and vascular endothelial growth factor-induced mechanism for attenuating anti-proliferative thrombospondin-1 signaling

Endothelial cells acquire distinctive molecular signatures in their transformation to an angiogenic phenotype that are indicative of changes in cell behavior and function. Using a rat mesentery model of inflammation-induced angiogenesis and a panel of known endothelial markers (CD31, VE-cadherin, BS-I lectin), we identified a capillary sprout-specific endothelial phenotype that is characterized by the marked down-regulation of CD36, a receptor for the anti-angiogenic molecule thrombospondin-1 (TSP-1). TSP-1/CD36 interactions were shown to regulate angiogenesis in this model as application of TSP-1 inhibited angiogenesis and blockade of both TSP-1 and CD36 accelerated angiogenesis. Vascular endothelial growth factor, which was up-regulated in the in vivo model, elicited a dose- and time-dependent down-regulation of CD36 (ie, to a CD36 low phenotype) in cultured human umbilical vein endothelial cells. Human umbilical vein endothelial cells that had been conditioned to a CD36 low phenotype with VEGF were found to be refractory to anti-proliferative TSP-1 signaling via a CD36-dependent mechanism. The loss of exposure to wall shear stress, which occurs in vivo when previously quiescent cells begin to sprout, also generated a CD36 low phenotype. Ultimately, our results identified the regulation of endothelial cell CD36 expression as a novel mechanism through which VEGF stimulates and sustains capillary sprouting in the presence of TSP-1. Additionally, CD36 was shown to function as a potential molecular linkage through which wall shear stress may regulate both microvessel sprouting and quiescence.

Reduction of endogenous angiogenesis inhibitors in Bruch's membrane of the submacular region in eyes with age-related macular degeneration

OBJECTIVES

To determine the relative levels of 3 potent inhibitors of angiogenesis (endostatin, pigment epithelium-derived factor, and thrombospondin 1) in the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex in the submacular region in aged control eyes and eyes with age-related macular degeneration (AMD).

METHODS

Immunohistochemical analysis with antibodies against endostatin, pigment epithelium-derived factor, and thrombospondin 1 was performed on the macular region of aged control donor eyes (n = 8; mean age, 79.8 years) and eyes with AMD (n = 12; mean age, 83.9 years). Three independent masked observers scored the reaction product (scored from 0-7). Mean scores from the control eyes and the eyes with AMD were analyzed using 1-way analysis of variance and unpaired t test.

RESULTS

In control eyes, strong immunoreactivity of all 3 inhibitors was observed in the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex. Immunoreactivity for endostatin, pigment epithelium-derived factor, and thrombospondin 1 in Bruch's membrane was significantly lower in eyes with AMD compared with aged control eyes (analysis of variance, P = .003, P = .009, and P < .001, respectively). In the choriocapillaris, a significant reduction was observed in endostatin (analysis of variance, P = .02) and thrombospondin 1 (analysis of variance, P = .005) in eyes with AMD.

CONCLUSIONS

These findings suggest that endogenous angiogenesis inhibitors in the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex may provide a biochemical barrier for choroidal neovascular invasion.

CLINICAL RELEVANCE

Decreased levels of angiogenic inhibitors at the retinal pigment epithelium-Bruch's membrane-choriocapillaris complex in eyes with AMD make Bruch's membrane vulnerable to choroidal neovascularization.

Thrombospondins in cancer

The thrombospondins (TSPs) are a family of five proteins that are involved in the tissue remodeling that is associated with embryonic development, wound healing, synaptogenesis, and neoplasia. These proteins mediate the interaction of normal and neoplastic cells with the extracellular matrix and surrounding tissue. In the tumor microenvironment, TSP-1 has been shown to suppress tumor growth by inhibiting angiogenesis and by activating transforming growth factor beta. TSP-1 inhibits angiogenesis through direct effects on endothelial cell migration and survival, and through effects on vascular endothelial cell growth factor bioavailability. In addition, TSP-1 may affect tumor cell function through interaction with cell surface receptors and regulation of extracellular proteases. Whereas the role of TSP-1 in the tumor microenvironment is the best characterized, the other TSPs may have similar functions. (Part of a Multi-author Review).

Combined effects of the angiogenic genes polymorphisms on prostate cancer susceptibility and aggressiveness

The single-gene approaches in association studies of polygenic diseases are likely to provide limited value in predicting risk. The combined analysis of genetic variants that interact in the same pathway may amplify the effects of individual polymorphisms and enhance the predictive power. To evaluate higher order gene-gene interaction, we have examined the contribution of four angiogenic gene polymorphisms (VEGF-1154G/A; VEGF-634G/C; MMP9-1562C/T and TSP1-8831A/G) in combination to the risk of prostate cancer. For the combined analysis of VEGF and MMP9 SNPs, we found a significant gene-dosage effect for increasing numbers of potential high-risk genotypes. Compared to referent group (low-risk genotypes), individuals with one (OR = 2.79, P = 0.1), two (OR = 4.57, P = 0.02) and three high-risk genotypes (OR = 7.11, P = 0.01) had increasingly elevated risks of prostate cancer. Similarly, gene-gene interaction of VEGF and TSP1 polymorphisms increased risk of prostate cancer in additive manner (OR = 6.00, P = 0.03), although the TSP1 polymorphism itself was not associated with the risk. In addition, we examined the synergistic effect of these polymorphisms in relation to prostate cancer prognosis according to histopathological grade and clinical stage at diagnosis. Cross-classified analysis revealed potential higher order gene-gene interactions between VEGF and TSP1 polymorphisms in increasing the risk of developing an aggressive phenotype disease. Patients carrying three high-risk genotypes showed a 20-fold increased risk of high-grade tumor (OR = 20.75, P = 0.002). These results suggest that the gene-gene interaction of angiogenic gene polymorphisms' increased risk of prostate cancer onset and aggressiveness.

Syndecan-4 contributes to endothelial tubulogenesis through interactions with two motifs inside the pro-angiogenic N-terminal domain of thrombospondin-1

Thrombospondin-1 (TSP-1) is an extracellular matrix protein that modulates focal adhesion in mammalian cells and exhibits dual roles in angiogenesis. In a previous work, we showed that a recombinant 18 kDa protein encompassing the N-terminal residues 1-174 of human TSP-1 (TSP18) induced tubulogenesis of human umbilical vein endothelial cells and protected them from apoptosis. Our results indicated that these effects were possibly mediated by syndecan-4 proteoglycan, since binding of TSP18 to endothelial extracts was inhibited by anti-syndecan-4 antibody. Syndecan-4 is a heparan-sulfate proteoglycan that regulates cell-matrix interactions and is the only member of its family present in focal adhesions. In this report, we demonstrate that a monoclonal antibody against syndecan-4 blocks TSP18-induced tubulogenesis. Furthermore, through 2D adhesion and 3D angiogenic assays, we demonstrate that two sequences, TSP Hep I and II, retain the major pro-angiogenic activity of TSP18. These TSP-1 motifs also compete with the fibronectin Hep II domain for binding to syndecan-4 on endothelial cell surface, indicating that they may exert their effects by interfering with the recognition of fibronectin by syndecan-4. Additionally, TSP18 and its derived peptides activate the PKC-dependent Akt-PKB signaling pathway. Blockage of PKC activation prevented HUVEC spreading when seeded on TSP18 fragment, and on TSP Hep I and TSP Hep II peptides, but not on gelatin-coated substrates. Our results identify syndecan-4 as a novel receptor for the N-terminus of TSP-1 and suggest that TSP-1 N-terminal pro-angiogenic activity is linked to its capacity of interfering with syndecan-4 functions in the course of cell adhesion.

Apoptotic cell thrombospondin-1 and heparin-binding domain lead to dendritic-cell phagocytic and tolerizing states

Apoptotic cells were shown to induce dendritic cell immune tolerance. We applied a proteomic approach to identify molecules that are secreted from apoptotic monocytes, and thus may mediate engulfment and immune suppression. Supernatants of monocytes undergoing apoptosis were collected and compared using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), and differentially expressed proteins were identified using tandem mass spectrometry. Thrombospondin-1 (TSP-1) and its cleaved 26-kDa heparin-binding domain (HBD) were identified. We show that TSP-1 is expressed upon induction of monocyte apoptosis in a caspase-dependent pattern and the HBD is cleaved by chymotrypsin-like serine protease. We further show that CD29, CD36, CD47, CD51, and CD91 simultaneously participate in engulfment induction and generation of an immature dendritic cell (iDC) tolerogenic and phagocytic state. We conclude that apoptotic cell TSP-1, and notably its HBD, creates a signalosome in iDCs to improve engulfment and to tolerate engulfed material prior to the interaction with apoptotic cells.

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.

Thrombospondin 1, thrombospondin 2 and the eye

Thrombospondin 1 and thrombospondin 2 (TSP1 and TSP2), which comprise the subgroup A thrombospondins, are matricellular proteins. As matricellular proteins, they modulate interactions between cells and the cellular environment, regulate cell adhesion and typically are expressed during tissue formative processes. In general, TSP1 and TSP2 counter angiogenesis (including tumour angiogenesis) and play important but contrasting roles during cutaneous repair. The two proteins are involved in development, including that of the eye, although evidence suggests that they have their greatest impact during tissue production in the adult. In the normal adult eye, they tend to be found at sites of ongoing matrix synthesis or cell-matrix interactions. At these sites, the two proteins possibly influence cellular differentiation and/or basement membrane deposition. TSP1 is also present in the intraocular fluids and drainage pathway, where it may function in maintaining the anti-angiogenic environment and in intraocular pressure control, respectively. TSP1 could also be involved in ocular immune privilege. Unlike in skin wounds, where TSP1 is derived from the blood and is present only in the early phases of repair, ocular tissue damage appears to lead to protacted TSP1 synthesis by local cells. This response might help suppress angiogenesis in the transparent tissues of the eye and so lessen visual axis opacification following injury. However, TSP2, which is also produced by damaged ophthalmic tissue and may be especially important in matrix organisation, seems to augment contraction in anomalous intraocular fibrosis. Elucidating the roles of TSP1 and TSP2 in ocular physiology and pathobiology may lead to improved therapies for neovascular, neoplastic, reparative and other ophthalmic diseases.

Regulation of tumor angiogenesis by thrombospondin-1

Angiogenesis plays a critical role in the growth and metastasis of tumors. Thrombospondin-1 (TSP-1) is a potent angiogenesis inhibitor, and down-regulation of TSP-1 has been suggested to alter tumor growth by modulating angiogenesis in a variety of tumor types. Expression of TSP-1 is up-regulated by the tumor suppressor gene, p53, and down-regulated by oncogenes such as Myc and Ras. TSP-1 inhibits angiogenesis by inhibiting endothelial cell migration and proliferation and by inducing apoptosis. In addition, activation of transforming growth factor beta (TGF-beta) by TSP-1 plays a crucial role in the regulation of tumor progression. An understanding of the molecular basis of TSP-1-mediated inhibition of angiogenesis and tumor progression will aid in the development of novel therapeutics for the treatment of cancer.

Sodium/potasium ATPase (Na+, K+-ATPase) and ouabain/related cardiac glycosides: a new paradigm for development of anti- breast cancer drugs?

Prolonged exposure to 17beta-estradiol (E2) is a key etiological factor for human breast cancer. The biological effects and carcinogenic effects of E2 are mediated via estrogen receptors (ERs), ERalpha and ERbeta. Anti-estrogens, e.g. tamoxifen, and aromatase inhibitors have been used to treat ER-positive breast cancer. While anti-estrogen therapy is initially successful, a major problem is that most tumors develop resistance and the disease ultimately progresses, pointing to the need of developing alternative drugs targeting to other critical targets in breast cancer cells. We have identified that Na+, K+-ATPase, a plasma membrane ion pump, has unique/valuable properties that could be used as a potentially important target for breast cancer treatment: (a) it is a key player of cell adhesion and is involved in cancer progression; (b) it serves as a versatile signal transducer and is a target for a number of hormones including estrogens and (d) its aberrant expression and activity are implicated in the development and progression of breast cancer. There are several lines of evidence indicating that ouabain and related digitalis (the potent inhibitors of Na+, K+-ATPase) possess potent anti-breast cancer activity. While it is not clear how the suggested anti-cancer activity of these drugs work, several observations point to ouabain and digitalis as being potential ER antagonists. We critically reviewed many lines of evidence and postulated a novel concept that Na+, K+-ATPase in combination with ERs could be important targets of anti-breast cancer drugs. Modulators, e.g. ouabain and related digitalis could be useful to develop valuable anti-breast cancer drugs as both Na+, K+-ATPase inhibitors and ER antagonists.

The response of VEGF-stimulated endothelial cells to angiostatic molecules is substrate-dependent

Background

The microenvironment surrounding cells can exert multiple effects on their biological responses. In particular the extracellular matrix surrounding cells can profoundly influence their behavior. It has been shown that the extracellular matrix composition in tumors is vastly different than that found in normal tissue with increased amounts of certain matrices such as collagen I. It has been previously demonstrated that VEGF stimulation of endothelial cells growing on type I collagen results in the induction of bcl-2 expression and enhanced endothelial cell survival. We sought to investigate whether this increased endothelial cell survival resulted in the failure of angiostatic molecules to inhibit angiogenesis.

Results

We now demonstrate that VEGF-induced survival on collagen I impairs the ability of three known angiostatic molecules, TSP-1, IP-10 and endostatin to inhibit endothelial cell proliferation. Apoptosis of endothelial cells, growing on collagen I, induced by TSP-1 and IP-10 was also inhibited following VEGF stimulation. In contrast, endostatin induced apoptosis in these same cells. Further analysis determined that endostatin did not decrease the expression of bcl-2 nor did it increase activation of caspase-3 in the presence of VEGF. Alternatively, it appeared that in the presence of VEGF, endostatin induced the activation of caspase-8 in endothelial cells grown on collagen I. Furthermore, only endostatin had the ability to inhibit VEGF-induced sprout formation in collagen I gels.

Conclusion

These data suggest that TSP-1, IP-10 and endostatin inhibit endothelial cells via different mechanisms and that only endostatin is effective in inhibiting angiogenic activities in the presence of collagen I. Our results suggest that the efficacy of angiostatic treatments may be impaired depending on the context of the extracellular matrix within the tumor environment and thus could impede the efficacy of angiostatic therapies.

Thrombospondin-1 inhibits endothelial cell responses to nitric oxide in a cGMP-dependent manner

Redox signaling plays an important role in the positive regulation of angiogenesis by vascular endothelial growth factor, but its role in signal transduction by angiogenesis inhibitors is less clear. Using muscle explants in 3D culture, we found that explants from mice lacking the angiogenesis inhibitor thrombospondin-1 (TSP1) exhibit exaggerated angiogenic responses to an exogenous NO donor, which could be reversed by providing exogenous TSP1. To define the basis for inhibition by TSP1, we examined the effects of TSP1 on several proangiogenic responses of endothelial cells to NO. NO has a biphasic effect on endothelial cell proliferation. The positive effect at low doses of NO is sensitive to inhibition of cGMP signaling and picomolar concentrations of TSP1. NO stimulates both directed (chemotactic) and random (chemokinetic) motility of endothelial cells in a cGMP-dependent manner. TSP1 potently inhibits chemotaxis stimulated by NO. Low doses of NO also stimulate adhesion of endothelial cells on type I collagen in a cGMP-dependent manner. TSP1 potently inhibits this response both upstream and downstream of cGMP. NO-stimulated endothelial cell responses are inhibited by recombinant type 1 repeats of TSP1 and a CD36 agonist antibody but not by the N-terminal portion of TSP1, suggesting that CD36 or a related receptor mediates these effects. These results demonstrate a potent antagonism between TSP1 and proangiogenic signaling downstream of NO. Further elucidation of this inhibitory signaling pathway may identify new molecular targets to regulate pathological angiogenesis.

Inhibition of endothelial cell migration by thrombospondin-1 type-1 repeats is mediated by beta1 integrins

The anti-angiogenic effect of thrombospondin-1 has been shown to be mediated through binding of the type-1 repeat (TSR) domain to the CD36 transmembrane receptor. We now report that the TSR domain can inhibit VEGF-induced migration in human umbilical vein endothelial cells (HUVEC), cells that lack CD36. Moreover, we identified β₁ integrins as a critical receptor in TSR-mediated inhibition of migration in HUVEC. Using pharmacological inhibitors of downstream VEGF receptor effectors, we found that phosphoinositide 3-kinase (PI3k) was essential for TSR-mediated inhibition of HUVEC migration, but that neither PLCγ nor Akt was necessary for this response. Furthermore, β₁ integrins were critical for TSR-mediated inhibition of microvascular endothelial cells, cells that express CD36. Together, our results indicate that β₁ integrins mediate the anti-migratory effects of TSR through a PI3k-dependent mechanism.

The N-terminus of thrombospondin: the domain stands apart

Thrombospondin 1 (TSP1) was first recognized as a thrombin-sensitive protein associated with platelet membranes. It is secreted by numerous cell types and its expression is predominant in areas of active tissue remodeling. Thrombospondins 1 and 2 are large, trimeric, matricellular proteins, composed of multiple structural motifs which interact with a diverse array of receptors and molecules. Thrombospondin's capacity to bind multiple receptors renders it multifunctional. The functions of its isolated domains can be overlapping or contradictory. In this review, we focus on the N-terminus of the molecule, first recognized for its strong heparin binding properties and characterized by its susceptibility to proteolytic cleavage from the stalk region of thrombospondin. The N-terminus, called the heparin binding domain (HBD), interacts with a variety of macromolecules including heparan sulfate proteoglycans at the membrane and in the matrix, LDL receptor-related protein (LRP), sulfated glycolipids, calreticulin, and integrins. The HBD mediates endocytosis of thrombospondin. It functions both as a soluble and an insoluble modulator of cell adhesion and motility. In contrast to thrombospondin, the HBD has pro-angiogenic activity. We propose that the HBD of thrombospondins 1 and 2 are found primarily in the cellular microenvironment in conditions of cellular injury, stress and tissue remodeling and that the HBD conveys multiple signals involved in cellular adaptation to injury.

Thrombospondin 1: a multifunctional protein implicated in the regulation of tumor growth

Thrombospondins belong to a family of extracellular matrix (ECM) proteins widely found from embryonic to adult tissues. The modular structure of thrombospondins contains a series of peptide sequences implicated in a multiplicity of biological functions. Extracellular matrix undergoes important alterations under proteolysis that occurs in pathological processes like tumorigenesis. An elevated secretion of thrombospondin 1 (TSP1) is often observed in tumors and is sometimes considered as a predictive factor. However, the role of TSP1 in cancer progression remains controversial and must be carefully apprehended. The regulation of cell adhesion, proliferation, apoptosis by TSP1 is examined in the present review and it is clear from the literature and from our investigations that TSP1 presents both stimulatory and inhibitory effects. The exposition of cryptic sites upon conformational changes can partially explain this contradiction. More interestingly, the analysis of TSP1-directed intracellular signaling pathways activated through specific receptors or supramolecular receptors docking systems may be useful to discriminate the precise function of TSP1 in tumor progression. The central role played by TSP1 in the control of matrix-degrading enzyme activation and catabolism reveals attractive tracks of research and highlights the involvement of the lipoprotein receptor-related protein (LRP) receptor in these events. Therefore, TSP1-derived peptides constitute a source of potentially active matrikins which could provide essential tools in cancer therapy.

The carboxyl-terminal half region of ADAMTS-1 suppresses both tumorigenicity and experimental tumor metastatic potential

ADAMTS-1 is an ECM-anchored metalloproteinase with proteoglycan-degrading activity as well as an angiogenesis inhibiting activity. Here, we examined the effects of ADAMTS-1 overexpression on in vivo tumor growth and tumor metastasis. Overexpression of only the C-terminal half region of ADAMTS-1, consisting of TSP type I motifs and the spacer region, suppressed Chinese hamster ovary (CHO) tumor growth in mice. In addition, a significant reduction in tumor metastatic potential was observed in ADAMTS-1-transfected CHO cells in an experimental metastasis assay. Furthermore, deletional analyses revealed that the C-terminal half region of ADAMTS-1 is responsible for its experimental metastasis-inhibitory activity. Our data suggest that the C-terminal half region of ADAMTS-1 has therapeutic potential as an inhibitor of tumor growth and metastasis.

The expression of thrombospondin-1 in benign prostatic hyperplasia and prostatic intraepithelial neoplasia is decreased in prostate cancer

OBJECTIVE

To evaluate the immunohistochemical expression of thrombospondin (TSP), a potent inhibitor of angiogenesis, in human benign prostatic hyperplasia (BPH) and prostate cancer.

MATERIALS AND METHODS

The expression of TSP-1, TSP-2 and CD36 receptor was assessed in 73 tissue specimens using immunohistochemistry; specimens were from 32 patients with BPH, seven with prostatic intraepithelial neoplasia (PIN) and 34 with cancer.

RESULTS

Immunohistochemistry showed that all 39 patients with BPH and PIN had TSP-1-positive glands. In contrast, none of the 34 patients with cancer had positive TSP-1 staining in the cancer tissue. All 73 patients were positive for TSP receptor CD36 and negative for TSP-2.

CONCLUSIONS

TSP is expressed in BPH, down-regulated in PIN and absent in prostate cancer tissue. This may indicate that TSP is important in prostate cancer progression. Further studies are needed to understand the significance of these findings for the malignant transformation of the prostate gland.

Thombospondin-1 Disrupts Estrogen-Induced Endothelial Cell Proliferation and Migration and Its Expression Is Suppressed by Estradiol

The natural hormone 17β-estradiol (17β-E2) is known to induce tumor angiogenesis in various target organs by activating positive regulators of angiogenesis. In this study, we show for the first time that in human umbilical vein endothelial cells (HUVECs), 17β-E2 transiently down-regulates the expression and secretion of a potent negative regulator of angiogenesis, thrombospondin-1 (TSP-1). This inhibitory effect of 17β-E2 is mediated through nongenomic estrogen receptor (ER)/mitogen-activated protein kinase (MAPK)/extracellular-regulated kinase (ERK) 1/2 and c-Jun NH2-terminal kinase (JNK)/stress-activated protein kinase (SAPK) signaling pathways, because this effect can be abolished by a pure ER antagonist (ICI 182,780) and inhibitors of downstream signaling proteins of MAPK signaling cascades, including MAPK kinase 1/2 and ERK1/2 inhibitor and JNK/SAPK inhibitor. To understand the functional role(s) of TSP-1 during estradiol-induced angiogenesis, we examined the growth and migration of endothelial cells in different experimental environments. Using a recombinant protein, we show that increments of TSP-1 protein concentration in culture medium significantly reduce the migration and proliferation of HUVECs stimulated by 17β-E2. Together, these studies suggest that TSP-1 can be considered an important negative factor in understanding the increased angiogenesis in response to estrogens.

Alpha4beta1 integrin mediates selective endothelial cell responses to thrombospondins 1 and 2 in vitro and modulates angiogenesis in vivo

We examined the function of alpha4beta1 integrin in angiogenesis and in mediating endothelial cell responses to the angiogenesis modulators, thrombospondin-1 and thrombospondin-2. Alpha4beta1 supports adhesion of venous endothelial cells but not of microvascular endothelial cells on immobilized thrombospondin-1, vascular cell adhesion molecule-1, or recombinant N-terminal regions of thrombospondin-1 and thrombospondin-2. Chemotactic activities of this region of thrombospondin-1 and thrombospondin-2 are also mediated by alpha4beta1, whereas antagonism of fibroblast growth factor-2-stimulated chemotaxis is not mediated by this region. Immobilized N-terminal regions of thrombospondin-1 and thrombospondin-2 promote endothelial cell survival and proliferation in an alpha4beta1-dependent manner. Soluble alpha4beta1 antagonists inhibit angiogenesis in the chick chorioallantoic membrane and neovascularization of mouse muscle explants. The latter inhibition is thrombospondin-1-dependent and not observed in explants from thrombospondin-1-/- mice. Antagonizing alpha4beta1 may in part block proangiogenic activities of thrombospondin-1 and thrombospondin-2, because N-terminal regions of thrombospondin-1 and thrombospondin-2 containing the alpha4beta1 binding sequence stimulate angiogenesis in vivo. Therefore, alpha4beta1 is an important endothelial cell receptor for mediating motility and proliferative responses to thrombospondins and for modulation of angiogenesis.

An N-Terminal 80 kDa Recombinant Fragment of Human Thrombospondin-2 Inhibits Vascular Endothelial Growth Factor Induced Endothelial Cell Migration In Vitro and Tumor Growth and Angiogenesis In Vivo

We have previously shown that stable overexpression of the thrombospondin-2 (TSP-2) gene inhibited the tumor growth and angiogenesis of human squamous cell carcinoma xenotransplants. To investigate the potential antitumoral efficacy of systemic TSP-2 therapy, we expressed a recombinant 80 kDa fragment of human TSP-2 (TSP-2/NTF), encompassing the N-terminal globular region through the three type 1 repeats, in human kidney 293 EBNA cells, using a modified pCEP4 expression vector. Daily intraperitoneal injections of TSP-2/NTF resulted in a significant inhibition of the growth of human A431 squamous cell carcinomas in vivo and in reduced tumor vascularization. To further investigate possible mechanisms of the antiangiogenic activity of TSP-2/NTF, several in vitro angiogenesis assays were performed in human dermal microvascular endothelial cells. TSP-2/NTF inhibited vascular endothelial growth factor induced migration of human dermal microvascular endothelial cells and inhibited tube formation on Matrigel in vitro. TSP-2/NTF also inhibited vascular endothelial growth factor induced angiogenesis in an in vivo Matrigel assay. Moreover, TSP-2/NTF potently induced human dermal microvascular endothelial cell apoptosis in vitro but did not affect A431 tumor cell proliferation or apoptosis. These findings identify TSP-2/NTF as a potent systemic inhibitor of tumor growth and angiogenesis, acting by direct inhibition of several endothelial cell functions involved in neovascularization.

Recognition of the N-terminal modules of thrombospondin-1 and thrombospondin-2 by alpha6beta1 integrin

In addition to its recognition by alpha3beta1 and alpha4beta1 integrins, the N-terminal pentraxin module of thrombospondin-1 is a ligand for alpha6beta1 integrin. alpha6beta1 integrin mediates adhesion of human microvascular endothelial and HT-1080 fibrosarcoma cells to immobilized thrombospondin-1 and recombinant N-terminal regions of thrombospondin-1 and thrombospondin-2. alpha6beta1 also mediates chemotaxis of microvascular cells to thrombospondin-1 and thrombospondin-2. Using synthetic peptides, LALERKDHSG was identified as an alpha6beta1-binding sequence in thrombospondin-1. This peptide inhibited alpha6beta1-dependent cell adhesion to thrombospondin-1, thrombospondin-2, and the E8 fragment of murine laminin-1. The Glu residue in this peptide was required for activity, and the corresponding residue (Glu90) in the N-terminal module of thrombospondin-1 was required for its recognition by alpha6beta1, but not by alpha4beta1. alpha6beta1 was also expressed in human umbilical vein endothelial cells; but in these cells, only certain agonists could activate the integrin to recognize thrombospondins. Selective activation of alpha6beta1 integrin in microvascular endothelial cells by the anti-beta1 antibody TS2/16 therefore accounts for their adhesion responses to thrombospondins and explains the distinct functions of alpha4beta1 and alpha6beta1 integrins as thrombospondin receptors in microvascular and large vessel endothelial cells.

Thrombospondin signaling through the calreticulin/LDL receptor-related protein co-complex stimulates random and directed cell migration

The matricellular extracellular matrix protein thrombospondin-1 (TSP1) stimulates focal adhesion disassembly through a sequence (known as the hep I peptide) in its heparin-binding domain. This mediates signaling through a receptor co-complex involving calreticulin and low-density lipoprotein (LDL) receptor-related protein (LRP). We postulate that this transition to an intermediate adhesive state enhances cellular responses to dynamic environmental conditions. Since cell adhesion dynamics affect cell motility, we asked whether TSP1/hep I-induced intermediate adhesion alters cell migration. Using both transwell and Dunn chamber assays, we demonstrate that TSP1 and hep I gradients stimulate endothelial cell chemotaxis. Treatment with focal adhesion-labilizing concentrations of TSP1/hep I in the absence of a gradient enhances endothelial cell random migration, or chemokinesis, associated with an increase in cells migrating, migration speed, and total cellular displacement. Calreticulin-null and LRP-null fibroblasts do not migrate in response to TSP1/hep I, nor do endothelial cells treated with the LRP inhibitor receptor-associated protein (RAP). Furthermore, TSP1/hep I-induced focal adhesion disassembly is associated with reduced chemotaxis to basic fibroblast growth factor (bFGF) but enhanced chemotaxis to acidic (a)FGF, suggesting differential modulation of growth factor-induced migration. Thus, TSP1/hep I stimulation of intermediate adhesion regulates the migratory phenotype of endothelial cells and fibroblasts, suggesting a role for TSP1 in remodeling responses.

Thrombospondins 1 and 2 function as inhibitors of angiogenesis

Thrombospondins (TSPs) 1 and 2 are matricellular proteins with the well-characterized ability to inhibit angiogenesis in vivo, and the migration and proliferation of cultured microvascular endothelial cells (ECs). Angiogenesis in developing tumors and in various models of wound healing is diminished or delayed by the presence of TSP1 or 2. Sequences within the type I repeats of TSP1 and 2 have been demonstrated to mediate the anti-migratory effects of TSPs on microvascular EC, although, paradoxically, sequences in the N- and C-terminal domains have pro-angiogenic effects. A scavenger receptor, CD36, recognizes the active sequences in the type I repeats, and is required for the anti-angiogenic effects of TSP1 in the corneal neovascularization assay. However, interactions of TSPs with growth factors, proteases, histidine-rich glycoprotein, and other cell-surface receptors on EC have the potential to modulate CD36-mediated effects. Binding of TSP1 to CD36 has been shown to activate apoptosis by inducing p38 and Jun N-terminal kinase, members of the mitogen-activated protein kinase superfamily, and subsequently the cell-surface expression of FasL. Ligation of Fas by FasL then induces a caspase cascade and apoptotic cell death. However, we have recently shown that inhibition of proliferation of microvascular EC by TSPs can occur in the absence of cell death. This finding raises the possibility that TSPs can activate separate cell death and anti-proliferative pathways.