Retinoic acid alters the mechanism of attachment of malignant astrocytoma and neuroblastoma cells to thrombospondin-1

Thrombospondin-2 promotes the proliferation and migration of glioma cells and contributes to the progression of glioma

BACKGROUND

Gliomas, especially high-grade gliomas, are highly malignant with a poor prognosis. Although existing treatments have improved the survival rate of patients with glioma, the recurrence and mortality rates are still not ideal. The molecular mechanisms involved in the occurrence and development of glioma are still poorly understood. We previously reported that thrombospondin-2 (TSP2) expression was increased in tumor specimens from rat models, promoting excitatory synapse formation. However, little is known about the effect of TSP2 on the biological characteristics of glioma.

METHODS

Glioma and cerebral cortex tissues were collected from 33 patients, and the expression of TSP2 in them was analyzed. Next, the proliferation and migration of TSP2 on glioma cells were analyzed in vitro. At last, a glioma transplantation model was constructed to explore the growth of TSP2 on glioma in vivo.

RESULTS

The expression of TSP2 in surgical glioma specimens was increased compared to that in the normal cortex. Interestingly, the TSP2 protein level was higher in high-grade glioma (HGG, World Health Organization (WHO) grades 3-4) than in low-grade glioma (LGG, WHO grades 1-2) tissues. Exogenous addition of the TSP2 protein at an appropriate concentration promoted the migration of glioma cells but did not significantly affect their proliferation. Surprisingly, overexpression of TSP2 promoted both the migration and proliferation of cultured glioma cells. Moreover, in vivo experimental data implied that overexpression of TSP2 in C6 cells promoted the malignant growth of gliomas, while knockout of TSP2 slowed glioma growth.

CONCLUSIONS

TSP2 promotes the migration and proliferation of glioma cells, which may provide new ideas for blocking glioma progression.

Expression of dual angiogenic/neurogenic growth factors in human primary brain tumors

Brain tumors, benign or malignant, are characterized by a very high degree of vascularization. Recent accumulating evidence suggests that during development the neuronal wiring follows the same routes as the vasculature and that these two systems may share some of the same factors for guidance. Thus, expression of dual angiogenic/neurogenic growth factors was evaluated by in situ hybridization in human primary brain tumors of three different types, i.e., astrocytomas, oligodendrogliomas, and ependymomas, of increasing grades, in relation with the grade and type of the tumor. For this evaluation we selected vascular endothelial growth factor (VEGF-A) and its receptors VEGF-R1 and VEGF-R2 and the neuropilins 1 and 2 (NRP-1 and NRP-2), which have proangiogenic properties, platelet-derived growth factor (PDGF) receptor-beta (PDGF-Rβ), which is required for the functional maturation of blood vessels, the ephrins and their Eph receptors, angiotensinogen (AGT) and thrombospondin-2 (TSP-2), which have potential antiangiogenic properties, and netrin-1 (Net-1), which regulates vascular architecture. We show that the expression of the VEGF-NRP system, PDGF-Rβ, TSP-2, AGT, and Net-1 are differentially regulated, either increased or decreased, in relation with the type and grade of the tumor, whereas regulation of the ephrinB system does not seem to be relevant in these human brain tumors.

sFRP-1 binds via its netrin-related motif to the N-module of thrombospondin-1 and blocks thrombospondin-1 stimulation of MDA-MB-231 breast carcinoma cell adhesion and migration

Secreted frizzled-related protein (sFRP)-1 is a Wnt antagonist that inhibits breast carcinoma cell motility, whereas the secreted glycoprotein thrombospondin-1 stimulates adhesion and motility of the same cells. We examined whether thrombospondin-1 and sFRP-1 interact directly or indirectly to modulate cell behavior. Thrombospondin-1 bound sFRP-1 with an apparent K(d)=48nM and the related sFRP-2 with a K(d)=95nM. Thrombospondin-1 did not bind to the more distantly related sFRP-3. The association of thrombospondin-1 and sFRP-1 is primarily mediated by the amino-terminal N-module of thrombospondin-1 and the netrin domain of sFRP-1. sFRP-1 inhibited α3β1 integrin-mediated adhesion of MDA-MB-231 breast carcinoma cells to a surface coated with thrombospondin-1 or recombinant N-module, but not adhesion of the cells on immobilized fibronectin or type I collagen. sFRP-1 also inhibited thrombospondin-1-mediated migration of MDA-MB-231 and MDA-MB-468 breast carcinoma cells. Although sFRP-2 binds similarly to thrombospondin-1, it did not inhibit thrombospondin-1-stimulated adhesion. Thus, sFRP-1 binds to thrombospondin-1 and antagonizes stimulatory effects of thrombospondin-1 on breast carcinoma cell adhesion and motility. These results demonstrate that sFRP-1 can modulate breast cancer cell responses by interacting with thrombospondin-1 in addition to its known effects on Wnt signaling.

Type 2 scavenger receptor CD36 in platelet activation: the role of hyperlipemia and oxidative stress

Platelet hyper-reactivity and a systemic prothrombotic state are associated with atherosclerosis and other inflammatory conditions. CD36, a member of the Type 2 scavenger receptor family, is a multiligand pattern recognition receptor that recognizes specific oxidized phospholipids, molecules expressed on microbial pathogens, apoptotic cells, and cell-derived microparticles. Recent studies have demonstrated that CD36 binding to oxidized LDL or microparticles activates a specific signaling pathway that induces platelet activation. This pathway is activated in vivo in the setting of hyperlipidemia and oxidant stress. Genetic deletion of CD36 protects mice from pathological thrombosis associated with hyperlipidemia without any apparent effect on normal hemostasis. Targeting CD36 or its signaling pathway could potentially lead to the development of novel antithrombotic therapies for patients with atheroinflammatory disorders.

CD36, a scavenger receptor involved in immunity, metabolism, angiogenesis, and behavior

CD36 is a membrane glycoprotein present on platelets, mononuclear phagocytes, adipocytes, hepatocytes, myocytes, and some epithelia. On microvascular endothelial cells, CD36 is a receptor for thrombospondin-1 and related proteins and functions as a negative regulator of angiogenesis. On phagocytes, through its functions as a scavenger receptor recognizing specific oxidized phospholipids and lipoproteins, CD36 participates in internalization of apoptotic cells, certain bacterial and fungal pathogens, and modified low-density lipoproteins, thus contributing to inflammatory responses and atherothrombotic diseases. CD36 also binds long-chain fatty acids and facilitates their transport into cells, thus participating in muscle lipid utilization, adipose energy storage, and gut fat absorption and possibly contributing to the pathogenesis of metabolic disorders, such as diabetes and obesity. On sensory cells, CD36 is involved in insect pheromone signaling and rodent fatty food preference. The signaling pathways downstream of CD36 involve ligand-dependent recruitment and activation of nonreceptor tyrosine kinases, specific mitogen-activated protein kinases, and the Vav family of guanine nucleotide exchange factors; modulation of focal adhesion constituents; and generation of intracellular reactive oxygen species. CD36 in many cells is localized in specialized cholesterol-rich membrane microdomains and may also interact with other membrane receptors, such as tetraspanins and integrins. Identification of the precise CD36 signaling pathways in specific cells elicited in response to specific ligands may yield novel targets for drug development.

New molecular targets in angiogenic vessels of glioblastoma tumours

Antiangiogenesis approaches have the potential to be particularly effective in the treatment of glioblastoma tumours. These tumours exhibit extremely high levels of neovascularisation, which may contribute to their extremely aggressive behaviour, not only by providing oxygenation and nutrition, but also by establishing a leaky vasculature that lacks a blood-brain barrier. This leaky vasculature enables migration of tumour cells, as well as the build up of fluid, which exacerbates tissue damage due to increased intracranial pressure. Here, we discuss the considerable progress that has been made in the identification of the pro- and antiangiogenic factors produced by glioblastoma tumours and the effects of these molecules in animal models of the disease. The safety and efficacy of some of these approaches have now been demonstrated in clinical trials. However, the ability of tumours to overcome these therapies and to re-establish angiogenesis requires further clinical research regarding potential multimodality therapies, as well as basic research into the regulation of angiogenesis by as yet unidentified factors. Optimisation of noninvasive procedures for monitoring of angiogenesis would greatly facilitate such research.

All-trans-retinoic acid inhibits collapsin response mediator protein-2 transcriptional activity during SH-SY5Y neuroblastoma cell differentiation

Neurons are highly polarized cells composed of two structurally and functionally distinct parts, the axon and the dendrite. The establishment of this asymmetric structure is a tightly regulated process. In fact, alterations in the proteins involved in the configuration of the microtubule lattice are frequent in neuro-oncologic diseases. One of these cytoplasmic mediators is the protein known as collapsin response mediator protein-2, which interacts with and promotes tubulin polymerization. In this study, we investigated collapsin response mediator protein-2 transcriptional regulation during all-trans-retinoic acid-induced differentiation of SH-SY5Y neuroblastoma cells. All-trans-retinoic acid is considered to be a potential preventive and therapeutic agent, and has been extensively used to differentiate neuroblastoma cells in vitro. Therefore, we first demonstrated that collapsin response mediator protein-2 mRNA levels are downregulated during the differentiation process. After completion of deletion construct analysis and mutagenesis and mobility shift assays, we concluded that collapsin response mediator protein-2 basal promoter activity is regulated by the transcription factors AP-2 and Pax-3, whereas E2F, Sp1 and NeuroD1 seem not to participate in its regulation. Furthermore, we finally established that reduced expression of collapsin response mediator protein-2 after all-trans-retinoic acid exposure is associated with impaired Pax-3 and AP-2 binding to their consensus sequences in the collapsin response mediator protein-2 promoter. Decreased attachment of AP-2 is a consequence of its accumulation in the cytoplasm. On the other hand, Pax-3 shows lower binding due to all-trans-retinoic acid-mediated transcriptional repression. Unraveling the molecular mechanisms behind the action of all-trans-retinoic acid on neuroblastoma cells may well offer new perspectives for its clinical application.

The role of the extracellular matrix in angiogenesis in malignant glioma tumors

Angiogenesis is a promising target for the development of effective strategies for the treatment of malignant brain tumors in that it has the potential to starve large tumors and prevent the regrowth of residual margins. Two critical steps in angiogenesis, the proliferation of activated endothelial cells and their migration into the perivascular space (sprouting), require adherence of the endothelial cells to the extracellular matrix (ECM). Thus, the availability of the appropriate ligands within the ECM contributes to the regulation of angiogenesis. In addition, several components of the ECM can act through other mechanisms to further promote angiogenesis or inhibit it. Current evidence suggests that the regulation of angiogenesis is a dynamic process in which the endothelial cells can promote angiogenesis by secreting proteases that remodel the ECM, tumor cells can further promote angiogenesis by secreting ECM components and actively remodeling their environment, and stromal cells may respond to angiogenesis associated with tumors and inflammatory reactions by secreting inhibitory molecules. Here, we provide a critical review of the protein and proteoglycan components of the ECM that have been implicated in angiogenesis with an emphasis on their role in promoting or inhibiting angiogenesis in brain tumors.

Low-density lipoprotein receptor-related protein contributes to the antiangiogenic activity of thrombospondin-2 in a murine glioma model

Host antiangiogenesis factors defend against tumor growth. The matricellular protein, thrombospondin-2 (TSP-2), has been shown to act as an antiangiogenesis factor in a carcinogen-induced model of skin cancer. Here, using an in vivo malignant glioma model in which the characteristics of the tumors formed after intracerebral implantation of GL261 mouse glioma cells are assessed, we found that tumor growth and microvessel density were significantly enhanced in tumors propagated in TSP-2(-/-) mice. Mechanistically, matrix metalloproteinase (MMP)-2 has been associated with neoangiogenesis and it has been proposed that the levels of available MMP-2 may be down-regulated by formation of a complex with TSP-2 that is internalized by low-density lipoprotein receptor-related protein 1 (LRP1). We found elevated expression of MMP-2 and MMP-9 in tumors propagated in TSP-2(-/-) mice, with a preferential localization in the microvasculature. In wild-type mice, MMP-2 was coexpressed with TSP-2 in the tumor microvasculature. In vitro, addition of recombinant (rec) TSP-2 to mouse brain microvessel endothelial cells reduced MMP-2 levels and invasion through mechanisms that could be inhibited by a competitive inhibitor of ligand binding to LRP1 or by siLRP1. Thus, the antiangiogenic activity of TSP-2 is capable of inhibiting the growth of gliomas in part by reducing the levels of MMP-2 in the tumor microvasculature. This mechanism is mediated by LRP1.

Endogenous inhibitors of angiogenesis in malignant gliomas: nature's antiangiogenic therapy

Angiogenesis is necessary for tumor growth beyond a volume of approximately 2 mm(3). This observation, along with the accessibility of tumor vessels to therapeutic targeting, has resulted in a research focus on inhibitors of angiogenesis. A number of endogenous inhibitors of angiogenesis are found in the body. Some of these are synthesized by specific cells in different organs, and others are created by extracellular proteolytic cleavage of plasma-derived or extracellular matrix-localized proteins. In this review, we focus on angiostatin, endostatin, PEX, pigment epithelial-derived factor, and thrombospondin (TSP)-1 and -2, either because these molecules are expressed in malignant glioma biopsies or because animal studies in malignant glioma models have suggested that their therapeutic administration could be efficacious. We review the known mechanisms of action, potential receptors, expression in glioma biopsy samples, and studies testing their potential therapeutic efficacy in animal models of malignant glioma. Two conclusions can be made regarding the mechanisms of action of these inhibitors: (1) Several of these inhibitors appear to mediate their antiangiogenic effect through multiple protein-protein interactions that inhibit the function of proangiogenic molecules rather than through a specific receptor-mediated signaling event, and (2) TSP-1 and TSP-2 appear to mediate their antiangiogenic effect, at least in part, through a specific receptor, CD36, which initiates the antiangiogenic signal. Although not proven in gliomas, evidence suggests that expression of specific endogenous inhibitors of angiogenesis in certain organs may be part of a host antitumor response. The studies reviewed here suggest that new antiangiogenic therapies for malignant gliomas offer exciting promise as nontoxic, growth-inhibitory agents.

Quantification of thrombospondin-1 secretion and expression of alphavbeta3 and alpha3beta1 integrins and syndecan-1 as cell-surface receptors for thrombospondin-1 in malignant glioma cells

Malignant glioma cells secrete thrombospondin-1 (TSP-1) which participates in the motility of glioma cells, and binds to cell surface alphavbeta3 and alpha3beta1 integrins, and syndecan-1. This study evaluated the amount of TSP-1 secretion from malignant glioma cells, and the expression of alphavbeta3 and alpha3beta1 integrins, and syndecan-1. The amounts of TSP-1 in the supernatants from 10 malignant glioma cell lines and eight non-glioma malignant tumor cell lines were measured by enzyme-linked immunosorbent assay. Expression of alphavbeta3 and alpha3beta1 integrins, and syndecan-1 were examined by flow cytometry. The amounts of TSP-1 secreted by malignant glioma cells were 43 to 2431 ng/l x 10(6) cells/24 h (mean +/- SD = 626 +/- 792). Seven of 10 glioma cell lines secreted more than 100 ng of TSP-1 and three of these cell lines secreted more than 1 microg. Seven of eight non-glioma cell lines secreted less than 100 ng of TSP-1. All glioma cell lines expressed alpha3beta1 integrin and syndecan-1, and seven of 10 glioma cell lines expressed alphavbeta3 integrin. Treatment of the glioma cell lines with TGF-beta2 did not change the expression of alphavbeta3 integrin. These results suggest that malignant glioma cells secrete high levels of TSP-1, which may be important in the migration of glioma cells via interactions with alphavbeta3 and alpha3beta1 integrins, and syndecan-1.

Functions of the conserved thrombospondin carboxy-terminal cassette in cell-extracellular matrix interactions and signaling

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that function at cell surfaces, in extracellular matrix (ECM) and as bridging molecules in cell-cell interactions. TSPs are multifunctional and modulate cell behavior during development, wound-healing, immune response, tumor growth and in the homeostasis of adult tissues. TSPs are assembled as oligomers that are composed of homologous polypeptides. In all the TSP polypeptides, the most highly-conserved region is the carboxyl-region, which contains a characteristic set of domains comprising EGF domains, TSP type 3 repeats and a globular carboxy-terminal domain. This large region is termed here the thrombospondin carboxy-terminal cassette (TSP-CTC). The strong conservation of the TSP-CTC suggests that it may mediate ancestral functions that are shared by all TSPs. This review summarizes the current knowledge of the TSP-CTC and areas of future interest.

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.

Microregional extracellular matrix heterogeneity in brain modulates glioma cell invasion

The invasion of neoplastic cells into healthy brain tissue is a pathologic hallmark of gliomas and contributes to the failure of current therapeutic modalities (surgery, radiation and chemotherapy). Transformed glial cells share the common attributes of the invasion process, including cell adhesion to extracellular matrix (ECM) components, cell locomotion, and the ability to remodel extracellular space. However, glioma cells have the ability to invade as single cells through the unique environment of the normal central nervous system (CNS). The brain parenchyma has a unique composition, mainly hyaluronan and is devoid of rigid protein barriers composed of collagen, fibronectin and laminin. The integrins and the hyaluronan receptor CD44 are specific adhesion receptors active in glioma-ECM adhesion. These adhesion molecules play a major role in glioma cell-matrix interactions because the neoplastic cells use these receptors to adhere to and migrate along the components of the brain ECM. They also interact with the proteases secreted during glioma progression that degrade ECM allowing tumor cells to spread and diffusely infiltrate the brain parenchyma. The plasminogen activators (PAs), matrix metalloproteinases (MMPs) and lysosomal cysteine peptidases called cathepsins are also induced during the invasive process. Understanding the mechanisms of tumor cell invasion is critical as it plays a central role in glioma progression and failure of current treatment due to tumor recurrence from micro-disseminated disease. This review will focus on the impact of microregional heterogeneity of the ECM on glioma invasion in the normal adult brain and its modifications in tumoral brain.

The pattern of enhancement of Src kinase activity on platelet-derived growth factor stimulation of glioblastoma cells is affected by the integrin engaged

Enhanced expression of both integrin alpha v beta 3 and platelet-derived growth factor receptor (PDGFr) has been described in glioblastoma tumors. We therefore explored the possibility that integrin alpha v beta 3 cooperates with PDGFr to promote cell migration in glioblastoma cells, and extended the study to identify the Src family members that are activated on PDGF stimulation. Glioblastoma cells utilize integrins alpha v beta 3 and alpha v beta 5 to mediate vitronectin attachment. We found that physiologic PDGF stimulation (83 pm, 10 min) of vitronectin-adherent cells promoted the specific recruitment of integrin alpha v beta 3-containing focal adhesions to the cell cortex and alpha v beta 3-mediated cell motility. Analysis of PDGFr immunoprecipitates indicated an association of the PDGFr beta with integrin alpha v beta 3, but not integrin alpha v beta 5. Cells plated onto collagen or laminin, which engage different integrins, exhibited significantly less migration on PDGF stimulation, indicating a cooperation of alpha v beta 3 and the PDGFr beta in glioblastoma cells that promotes migration. Further analysis of the cells plated onto vitronectin indicated that PDGF stimulation caused an increase in Src kinase activity, which was associated with integrin alpha v beta 3. In the vitronectin-adherent cells, Lyn was associated preferentially with alpha v beta 3 both in the presence and absence of PDGF stimulation. In contrast, Fyn was associated with both alpha v beta 3 and alpha v beta 5. Moreover, PDGF stimulation increased the activity of Lyn, but not Fyn, in vitronectin-adherent cells, and the activity of Fyn, but not Lyn, in laminin-adherent cells. Using cells attached to mAb anti-alpha v beta 3 or mAb anti-integrin alpha 6, we confirmed the activation of specific members of the Src kinase family with PDGF stimulation. Down-regulation of Lyn expression by siRNA significantly inhibited the cell migration mediated by integrin alpha v beta 3 in PDGF-stimulated cells, demonstrating the PDGFr beta cooperates with integrin alpha v beta 3 in promoting the motility of vitronectin-adherent glioblastoma cells through a Lyn kinase-mediated pathway. Notably, the data indicate that engagement of different integrins alters the identity of the Src family members that are activated on stimulation with PDGF.

Expression of adhesion and extracellular matrix molecules in the developing human brain

Cell adhesion molecules and extracellular matrix molecules have important roles in cell migration and connection. Their developmental expression has not been fully described in humans. In this report, these molecules were examined by immunohistochemistry in frontal tissue samples from 14- to 28-week-old fetuses aborted for obstetric reasons (n = 20) and four fetuses with nervous system abnormalities. Neural cell adhesion molecule (NCAM), tenascin, and laminin were expressed after 17 weeks. Neural cell adhesion molecule was observed in the neuropil, whereas tenascin and laminin also had cellular and vascular expression. Thrombospondin and fibronectin, apparent after 14 weeks, showed a redistribution from periventricular to outer cortical layers after midgestation. N-cadherin and integrin were observed in mid- and late gestation. Maternal or environmental conditions seemed to influence the pattern of expression. Fetuses with nervous system abnormalities had altered expression of several molecules. The descriptive data obtained in this study might constitute a basis for further studies investigating the role of these molecules in developmental abnormalities of the brain.

TGF-beta1 up-regulates paxillin protein expression in malignant astrocytoma cells: requirement for a fibronectin substrate

Cytokines can influence the interactions between members of the integrin cell adhesion receptor family and the extracellular matrix thereby potentially affecting cell function and promoting cell adhesion, growth and migration of malignant astrocytoma tumor cells. As malignant astrocytoma cells synthesize TGF-beta1 in vivo, we analysed the effects of TGF-beta1 on signaling events associated with integrin receptor ligation, focusing on the effects on paxillin, a phosphorylated adaptor protein, that acts as a scaffold for signaling molecules recruited to focal adhesions. TGF-beta1-stimulation of primary astrocytes and serum-starved U-251MG malignant astrocytoma cells attached to fibronectin induced a substantial increase in the levels of paxillin protein (fivefold increase at 2.0 ng/ml) in a dose- and time-dependent manner compared to the levels observed on plating onto fibronectin in the absence of stimulation. In the astrocytoma cells, this resulted in an increase in the pool of tyrosine-phosphorylated paxillin, although it did not appear to alter the extent of phosphorylation of the paxillin molecules. In contrast, in primary astrocytes the protein levels were upregulated in the absence of a parallel increase in phosphorylation. The TGF-beta1-stimulated increase in paxillin levels required ligation of the fibronectin receptor, as it was not induced when the cells were plated onto vitronectin, collagen or laminin. The increase in the pool of paxillin on TGF-beta1 stimulation of the fibronectin-plated astrocytoma cells was associated with an increase in translation, but was not associated with an increase in the steady-state levels of paxillin mRNA. Stimulation with TGF-beta1 on a fibronectin substrate increased subsequent attachment and spreading of U-251MG cells onto fibronectin and, to a lesser extent, vitronectin, but not collagen. Our results indicate that physiologic levels of TGF-beta1 stimulate the expression of paxillin protein at the level of translation through a process that requires engagement of the fibronectin receptor, and promotes attachment and spreading of malignant astrocytoma cells on fibronectin.

A possible role for CXCR4 and its ligand, the CXC chemokine stromal cell-derived factor-1, in the development of bone marrow metastases in neuroblastoma

The homing of hemopoietic stem cells to the bone marrow is mediated by specific interactions occurring between CXCR4, which is expressed on hemopoietic stem cells, and its ligand, stromal cell-derived factor-1 (SDF-1), a CXC chemokine secreted by bone marrow stromal cells. In the present study we evaluated the possibility that neuroblastoma cells use a mechanism similar to that used by hemopoietic stem cells to home to the bone marrow and adhere to bone marrow stromal cells. Our study suggests that CXCR4 expression may be a general characteristic of neuroblastoma cells. SH-SY5Y neuroblastoma cells express not only CXCR4, but also its ligand, SDF-1. CXCR4 expression on SH-SY5Y neuroblastoma cells is tightly regulated by tumor cell-derived SDF-1, as demonstrated by the ability of neutralizing Abs against human SDF-1alpha to up-regulate CXCR4 expression on the tumor cells. The reduction in CXCR4 expression following short term exposure to recombinant human SDF-1alpha can be recovered as a result of de novo receptor synthesis. Recombinant human SDF-1alpha induces the migration of CXCR4-expressing SH-SY5Y neuroblastoma cells in CXCR4- and heterotrimeric G protein-dependent manners. Furthermore, SH-SY5Y cells interact at multiple levels with bone marrow components, as evidenced by the fact that bone marrow-derived constituents promote SH-SY5Y cell migration, adhesion to bone marrow stromal cells, and proliferation. These results suggest that SH-SY5Y neuroblastoma cells are equipped with adequate machinery to support their homing to the bone marrow. Therefore, the ability of neuroblastoma tumors to preferentially form metastases in the bone marrow may be influenced by a set of complex CXCR4-SDF-1 interactions.

Conformational regulation of the fibronectin binding and alpha 3beta 1 integrin-mediated adhesive activities of thrombospondin-1

The recognition of extracellular matrix components can be regulated by conformational changes that alter the activity of cell surface integrins. We now demonstrate that conformational regulation of the matrix glycoprotein thrombospondin-1 (TSP1) can also modulate its binding to an integrin receptor. F18 1G8 is a conformation-sensitive TSP1 antibody that binds weakly to soluble TSP1 in the presence of divalent cations. However, binding of the antibody to melanoma cells was strongly stimulated by adding exogenous TSP1 in the presence of calcium, suggesting that TSP1 undergoes a conformational change following its binding to the cell surface. This conformation was not induced by known cell surface TSP1 receptors, whereas binding of F18 was stimulated when TSP1 bound to fibronectin but not to heparin or fibrinogen. Conversely, binding of F18 to TSP1 enhanced TSP1 binding to fibronectin. Exogenous fibronectin also stimulated TSP1-dependent binding of F18 to melanoma cells. Binding of the fibronectin-TSP1 complex to melanoma cells was mediated by alpha4beta1 and alpha5beta1 integrins. Furthermore, binding to F18 or fibronectin strongly enhanced the adhesive activity of immobilized TSP1 for some cell types. This enhancement of adhesion was mediated by alpha3beta1 integrin and required that the alpha3beta1 integrin be in an active state. Fibronectin also enhanced TSP1 binding to purified alpha3beta1 integrin. Therefore, both fibronectin and the F18 antibody induce conformational changes in TSP1 that enhance the ability of TSP1 to be recognized by alpha3beta1 integrin. The conformational and functional regulation of TSP1 activity by fibronectin represents a novel mechanism for extracellular signal transduction.

Histidine-rich glycoprotein inhibits the antiangiogenic effect of thrombospondin-1

Angiogenesis is critical for the growth and proliferation of tumors as well as for normal development. We now describe a novel role for histidine-rich glycoprotein (HRGP) in the modulation of angiogenesis. HRGP is a plasma protein that circulates in relatively high concentrations (1.5 microM), but has no known function in vivo. We have shown previously that HRGP binds with high affinity to thrombospondin-1 (TSP-1), a homotrimeric glycoprotein that is a potent inhibitor of angiogenesis. The antiangiogenic activity of TSP-1 is mediated by the binding of properdin-like type I repeats to the receptor CD36. We found that binding of HRGP to TSP-1 was similarly mediated by TSP type I repeats. HRGP colocalized with TSP-1 in the stroma of human breast cancer specimens, and this interaction masked the antiangiogenic epitope of TSP-1. In assays performed in vitro of endothelial cell migration and tube formation, and in vivo corneal angiogenesis assays, HRGP inhibited the antiangiogenic effect of TSP-1. These studies suggest that HRGP can modulate the antiangiogenic activity of TSP-1, and identify a potential mechanism of resistance to the antiangiogenic effect of TSP-1.

p125 focal adhesion kinase promotes malignant astrocytoma cell proliferation in vivo

p125 focal adhesion kinase (p125FAK) is a cytoplasmic tyrosine kinase that is activated upon engagement of integrin cell adhesion receptors, and initiates several signaling events that modulate cell function in vitro. To determine the biologic role of p125FAK in malignant astrocytic tumor cells, U-251MG human malignant astrocytoma cells were stably transfected with p125FAK cDNA using the TET-ON system, and stable clones isolated that exhibited an estimated 5- or 20-fold increase in p125FAK expression on administration of 0.1 or 2.0 microg/ml doxycycline, respectively. In vitro studies demonstrated that induction of p125FAK resulted in a 2- to 3-fold increase in cell migration, increased p130CAS phosphorylation, localization of exogenous p125FAK to focal adhesions, and a 2-fold increase in soft agar growth. To determine the role of p125FAK in vivo, clones were injected stereotactically into the brains of scid mice. A 4.5-fold estimated increase in p125FAK expression was induced by administration of doxycycline in the drinking water. Analysis of xenograft brains demonstrated that, upon induction of p125FAK, there was a 1.6- to 2.8-fold increase in tumor cell number, and an increase in mAb PCNA-labeling of tumor cells in the absence of a change in the apoptotic index. Compared to normal brain, the expression of p125FAK was elevated in malignant astrocytic tumor biopsies from patient samples. These data demonstrate for the first time that p125FAK promotes tumor cell proliferation in vivo, and that the underlying mechanism is not associated with a reduction in apoptosis.

Transcriptional regulation of the human osteopontin promoter: functional analysis and DNA-protein interactions

Synthesis of cell attachment proteins and cytokines, such as osteopontin (OPN), can promote tumor cell remodeling of the extracellular matrix into an environment that promotes tumor cell attachment and migration. We investigated the transcriptional regulation of OPN in the U-251MG and U-87MG human malignant astrocytoma cell lines. Deletion and mutagenesis analyses of the OPN promoter region identified a proximal promoter element (-24 to -94 relative to the transcription initiation site) that is essential for maintaining high levels of OPN expression in the tumor cells. This element, designated RE-1, consists of two cis-acting elements, RE-1a (-55 to -86) and RE-1b (-22 to -45), which act synergistically to regulate the activity of the OPN promoter. Gel shift assays using nuclear extracts of U-251MG cells demonstrated that RE-1a contains binding sites for transcription factors Sp1, the glucocorticoid receptor, and the E-box-binding factors, whereas RE-1b contains a binding site for the octamer motif-binding protein (OCT-1/OCT-2). Inclusion of antibodies directed toward Myc and OCT-1 in the gel shift assays indicated that Myc and OCT-1 participate in forming DNA-protein complexes on the RE-1a and RE-1b elements, respectively. Our results identify two previously unrecognized elements in the OPN promoter that act synergistically to promote upregulation of OPN synthesis by tumor cells but are regulated by different transcription factors.

The thrombospondin type 1 repeat (TSR) superfamily: diverse proteins with related roles in neuronal development

The thrombospondins are a family of proteins found widely in the embryonic extracellular matrix. Like most matrix proteins, thrombospondins are modular and contain a series of repeated domains arrayed between globular amino and carboxyl terminal domains. In recent years, other proteins that share thrombospondin type 1 repeats, or TSRs, have been identified. These include the F-spondin gene family, the members of the semaphorin 5 family, UNC-5, SCO-spondin, and others. Most of these are expressed in the developing nervous system, and many have expression patterns and in vitro properties that suggest potential roles in the guidance of cell and growth cone migration. Both cell- and matrix-binding motifs have been identified in the TSRs of thrombospondin-1, so it has been hypothesized that the properties of these diverse proteins may also depend on the presence of these repeats. Here, we review the cell biology of the TSR module, the extensive literature regarding the distribution and functions of thrombospondins and other TSR superfamily proteins, and evaluate their possible roles during the development of the nervous system.

Attachment of primary neonatal rat astrocytes to vitronectin is mediated by integrins alphavbeta5 and alpha8beta1: modulation by the type 1 plasminogen activator inhibitor

Vitronectin is expressed in a cell-specific manner in the developing brain and concentrated in the brain during disease processes, such as germinal matrix hemorrhage and infarction, in which there is breakdown of the blood-brain barrier. In this study, we identified the integrin receptors that mediate attachment of primary neonatal rat astrocytes to vitronectin. Using fluorescent activated cell sorter and immunoprecipitation analyses, we established that the vitronectin receptor integrins alphavbeta5 and alpha8beta1, but not alphavbeta3, are expressed on neonatal rat astrocytes. Attachment of the neonatal astrocytes to vitronectin was inhibited (85%) in an additive manner by neutralizing anti-alphavbeta5 and anti-beta1 antibodies. Attachment to vitronectin was also inhibited in a dose-dependent manner by the type I plasminogen activator inhibitor (PAI-1), a serine protease inhibitor. Our data demonstrate that unstimulated primary neonatal rat astrocytes attach to vitronectin, utilizing integrins alphavbeta5 and alpha8beta1, and that this attachment is regulated by PAI-1.