Presence of the seven transmembrane thrombin receptor on human tumour cells: effect of activation on tumour adhesion to platelets and tumor tyrosine phosphorylation

Awakening of Dormant Breast Cancer Cells in the Bone Marrow

Up to 40% of patients with breast cancer (BC) have metastatic cells in the bone marrow (BM) at the initial diagnosis of localized disease. Despite definitive systemic adjuvant therapy, these cells survive in the BM microenvironment, enter a dormant state and recur stochastically for more than 20 years. Once they begin to proliferate, recurrent macrometastases are not curable, and patients generally succumb to their disease. Many potential mechanisms for initiating recurrence have been proposed, but no definitive predictive data have been generated. This manuscript reviews the proposed mechanisms that maintain BC cell dormancy in the BM microenvironment and discusses the data supporting specific mechanisms for recurrence. It addresses the well-described mechanisms of secretory senescence, inflammation, aging, adipogenic BM conversion, autophagy, systemic effects of trauma and surgery, sympathetic signaling, transient angiogenic bursts, hypercoagulable states, osteoclast activation, and epigenetic modifications of dormant cells. This review addresses proposed approaches for either eliminating micrometastases or maintaining a dormant state.

Targeting hemostasis-related moieties for tumor treatment

Under normal conditions, the hemostatic system, that includes the involvement of the coagulation response and platelets, is anatomically and functionally inseparable from the vasculature. However, the hemostatic response always occurs in a wide range of tumors because of the high expression of coagulation initiator tissue factor (TF) in many tumor tissues, and due to the leakage of coagulation factors and platelets from the circulation system into the tumor interstitium through abnormal tumor vessels. Therefore, in addition to TF, these coagulation factors, platelets, the central moiety thrombin, the final product fibrin, and fibronectin, which is capable of stabilizing coagulation clots, are also abundant in tumors. These hemostasis-related moieties (HRMs), including TF, thrombin, fibrin, fibronectin, and platelets, are also closely associated with tumor progression, e.g., primary tumor growth and distal metastasis. The hemostatic response only occurs under pathological conditions, such as tumors, thrombosis, and atherosclerosis other than in normal tissues. The HRMs within tumors are also highly specific, establishing functional and therapeutic targets for tumor treatment. Therefore, strategies including active targeting to these moieties, modulation of HRMs deposited in the tumor microenvironment to improve tumor drug delivery, activation of prodrug by the coagulation complex formed during coagulation response, and direct inhibition of the tumor-promoting activity of HRMs could be designed for tumor therapy. In this review, we summarize various strategies that target HRMs for tumor treatment.

Beyond COX-1: the effects of aspirin on platelet biology and potential mechanisms of chemoprevention

After more than a century, aspirin remains one of the most commonly used drugs in western medicine. Although mainly used for its anti-thrombotic, anti-pyretic, and analgesic properties, a multitude of clinical studies have provided convincing evidence that regular, low-dose aspirin use dramatically lowers the risk of cancer. These observations coincide with recent studies showing a functional relationship between platelets and tumors, suggesting that aspirin's chemopreventive properties may result, in part, from direct modulation of platelet biology and biochemistry. Here, we present a review of the biochemistry and pharmacology of aspirin with particular emphasis on its cyclooxygenase-dependent and cyclooxygenase-independent effects in platelets. We also correlate the results of proteomic-based studies of aspirin acetylation in eukaryotic cells with recent developments in platelet proteomics to identify non-cyclooxygenase targets of aspirin-mediated acetylation in platelets that may play a role in its chemopreventive mechanism.

Role of Protease-Activated Receptor-1 in Glioma Growth

Activation of a thrombin receptor, protease-activated receptor-1 (PAR-1), induces angiogenesis, cell proliferation, and invasion in tumors. The present study examined the effect of host PAR-1 gene deletion on glioma growth in a mouse model. F98 glioma cells were implanted into the right caudate of either wild type (WT) or PAR-1 knockout (KO) mice. Mice underwent magnetic resonance imaging (MRI) and the brains were used for measurements of brain water content and tumor mass. Levels of hypoxia-inducible factor-1 alpha (HIF-1α) and vascular endothelial growth factor (VEGF) were also measured by ELISA (enzyme-linked immunosorbent assay). We found brain water content in the ipsilateral hemisphere and the tumor mass were significantly lower in PAR-1 KO than WT mice at day 12 after implantation of F98 cells (p < 0.05). HIF-1α protein levels in the ipsilateral hemisphere were higher in the WT mice (373 ± 25 pg/g brain tissue vs 333 ± 35 pg/g brain tissue in PAR-1 KO mice, p < 0.05) 7 days after F98 cell implantation. VEGF protein levels were also higher in the ipsilateral hemisphere of WT mice (219 ± 21 vs 166 ± 22 pg/g brain tissue in PAR-1 KO mice, p < 0.01). In conclusion, PAR-1 has a role in glioma growth and could be a new therapeutic target for gliomas.

Associated Risk of Malignancy in Patients with Cardiovascular Disease: Evidence and Possible Mechanism

Cardiovascular disease and malignancy are leading causes of morbidity and mortality. Increased risk of malignancy was identified in patients with cardiovascular disease, including patients with heart failure, heart failure after myocardial infarction, patients undergoing cardiac intervention, and patients after a thrombotic event. Common risk factors and biological pathways can explain this association and are explored in this review. Further research is needed to establish the causes of malignancy in this population and direct possible intervention.

Baicalein inhibits agonist- and tumor cell-induced platelet aggregation while suppressing pulmonary tumor metastasis via cAMP-mediated VASP phosphorylation along with impaired MAPKs and PI3K-Akt activation

Recently, the importance of platelet activation in cancer metastasis has become generally accepted. As a result, the development of new platelet inhibitors with minimal adverse effects is now a promising area of targeted cancer therapy. Baicalein is a functional ingredient derived from the root of Scutellaria baicalensis Georgi, a plant used intraditional medicine. The pharmacological effects of this compound including anti-oxidative and anti-inflammatory activities have already been demonstrated. However, its effects on platelet activation are unknown. We therefore investigated the effects of baicalein on ligand-induced platelet aggregation and pulmonary cancer metastasis. In the present study, baicalein inhibited agonist-induced platelet aggregation, granule secretion markers (P-selectin expression and ATP release), [Ca(2+)]i mobilization, and integrin αIIbβ3 expression. Additionally, baicalein attenuated ERK2, p38, and Akt activation, and enhanced VASP phosphorylation. Indeed, baicalein was shown to directly inhibit PI3K kinase activity. Moreover, baicalein attenuated the platelet aggregation induced by C6 rat glioma tumor cells in vitro and suppressed CT26 colon cancer metastasis in mice. These features indicate that baicalein is a potential therapeutic drug for the prevention of cancer metastasis.

Pathologies at the nexus of blood coagulation and inflammation: thrombin in hemostasis, cancer, and beyond

Thrombin is the protease involved in blood coagulation. Its deregulation can lead to hemostatic abnormalities, which range from subtle subclinical to serious life-threatening coagulopathies, i.e., during septicemia. Additionally, thrombin plays important roles in many (patho)physiological conditions that reach far beyond its well-established role in stemming blood loss and thrombosis, including embryonic development and angiogenesis but also extending to inflammatory processes, complement activation, and even tumor biology. In this review, we will address thrombin's broad roles in diverse (patho)physiological processes in an integrative way. We will also discuss thrombin as an emerging major target for novel therapies.

Protease activated receptor-1 and brain edema formation in glioma models

OBJECTIVE

Our previous studies showed that thrombin contributes to brain edema in gliomas. The present study investigated the role of a thrombin receptor, protease activated receptor-1 (PAR-1), in edema formation in glioma models.

METHODS

These experiments were performed in Fischer 344 rats, PAR-1 knockout mice, and wild-type C57BL/6 mice controls. F98 glioma cells were infused into the right caudate. Animals were euthanized and the brains were used for measurements of brain edema and PAR-1 expression.

RESULTS

In rats, implantation of glioma cells resulted in significant brain edema in the ipsilateral hemisphere (82.6 ± ;1.4 vs. 78.1 ± 0.9 % in the contralateral hemisphere, p < 0.01). By Western blot analysis and RT-PCR, we found that both protein and mRNA levels of PAR-1 were upregulated in the glioma (p < 0.01). In mice, implantation of glioma cells also caused brain edema in the ipsilateral hemisphere (p < 0.05). Glioma-induced brain edema was less in PAR-1 knockout mice (day 12: 79.4 ;± 1.3 vs. 81.5 ± 1.1 % in the wild-type mice, p < ;0.05).

CONCLUSION

PAR-1 plays a role in glioma-induced brain edema. Clarification of the role of PAR-1 in edema formation should help to develop new therapeutic strategies for gliomas.

Megakaryocytes, malignancy and bone marrow vascular niches

Dynamic interactions between hematopoietic cells and their specialized bone marrow microenvironments, namely the vascular and osteoblastic 'niches', regulate hematopoiesis. The vascular niche is conducive for thrombopoiesis and megakaryocytes may, in turn, regulate the vascular niche, especially in supporting vascular and hematopoietic regeneration following irradiation or chemotherapy. A role for platelets in tumor growth and metastasis is well established and, more recently, the vascular niche has also been implicated as an area for preferential homing and engraftment of malignant cells. This article aims to provide an overview of the dynamic interactions between cellular and molecular components of the bone marrow vascular niche and the potential role of megakaryocytes in bone marrow malignancy.

Protease-activated receptors in cancer: A systematic review

The traditional view of the role of proteases in tumor growth, progression and metastasis has significantly changed. Apart from their contribution to cancer progression, it is evident that a subclass of proteases, such as thrombin, serves as signal molecules controlling cell functions through the protease-activated receptors (PARs). Among the four types of PAR (PAR1-4; cloned and named in order of their discovery), PAR1, PAR3 and PAR4 are activated by thrombin, unlike PAR2, which is activated by trypsin-like serine proteases. Thrombin has been proven to be a significant factor in both the behavior of cancer in its involvement in hemostasis and blood coagulation. Thrombin is a key supporter of various cellular effects relevant to tumor growth and metastasis, as well as a potent activator of angiogenesis, which is essential for the growth and development of all solid tumor types. This review presents an overview of the role of PAR-mediated thrombin in angiogenesis and cancer, focusing on the ability of PAR1- and PAR4-mediated thrombin to affect tumorigenesis and angiogenesis.

Integrins and bone metastasis: integrating tumor cell and stromal cell interactions

Integrins on both tumor cells and the supporting host stromal cells in bone (osteoclasts, new blood vessels, inflammatory cells, platelets and bone marrow stromal cells) play key roles in enhancing bone metastasis. Tumor cells localize to specific tissues through integrin-mediated contacts with extracellular matrix and stromal cells. Integrin expression and signaling are perturbed in cancer cells, allowing them to "escape" from cell-cell and cell-matrix tethers, invade, migrate and colonize within new tissues and matrices. Integrin signaling through αvβ3 and VLA-4 on tumor cells can promote tumor metastasis to and proliferation in the bone microenvironment. Osteoclast (OC) mediated bone resorption is a critical component of bone metastasis and can promote tumor growth in bone and αvβ3 integrins are critical to OC function and development. Tumors in the bone microenvironment can recruit new blood vessel formation, platelets, pro-tumor immune cells and bone marrow stromal cells that promote tumor growth and invasion in bone. Integrins and their ligands play critical roles in platelet aggregation (αvβ3 and αIIbβ3), hematopoietic cell mobilization (VLA-4 and osteopontin), neoangiogenesis (αvβ3, αvβ5, α6β4, and β1 integrin) and stromal function (osteopontin and VLA-4). Integrins are involved in the pathogenesis of bone metastasis at many levels and further study to define integrin dysregulation by cancer will yield new therapeutic targets for the prevention and treatment of bone metastasis.

Platelets: linking hemostasis and cancer

Platelets are the main cellular component in blood responsible for maintaining the integrity of the cardiovascular system via hemostasis. Platelet dysfunction contributes to a wide range of obvious pathological conditions, such as bleeding or thrombosis, but normal platelet function is also linked to diseases not immediately associated with hemostasis or thrombosis, such as cancer. Since the description of Trousseau syndrome in 1865, various experimental and clinical studies have detailed the interaction of platelets with primary tumors and circulating metastatic tumor cells. Observations have suggested that platelets not only augment the growth of primary tumors via angiogenesis but endow tumor cells physical and mechanical support to evade the immune system and extravasate to secondary organs, the basis of metastatic disease. Many laboratory and animal studies have identified specific targets for antiplatelet therapy that may be advantageous as adjuncts to existing cancer treatments. In this review, we summarize important platelet properties that influence tumorigenesis, including primary tumor growth and metastasis at the molecular level. The studies provide a link between the well-studied paradigms of platelet hemostasis and tumorigenesis.

Deadly allies: the fatal interplay between platelets and metastasizing cancer cells

The general notion that functional platelets are important for successful hematogenous tumor metastasis has been inaugurated more than 4 decades ago and has since been corroborated in numerous experimental settings. Thorough preclinical investigations have, at least in part, clarified some specifics regarding the involvement of platelet adhesion receptors, such as thrombin receptors or integrins, in the metastasis cascade. Pivotal preclinical experiments have demonstrated that hematogenous tumor spread was dramatically diminished when platelets were depleted from the circulation or when functions of platelet surface receptors were inhibited pharmacologically or genetically. Such insight has inspired researchers to devise novel antitumoral therapies based on targeting platelet receptors. However, several mechanistic aspects underlying the impact of platelet receptors on tumor metastasis are not fully understood, and agents directed against platelet receptors have not yet found their way into the clinic. In addition, recent results suggesting that targeted inhibition of certain platelet surface receptors may even result in enhanced experimental tumor metastasis have demonstrated vividly that the role of platelets in tumor metastasis is more complex than has been anticipated previously. This review gives a comprehensive overview on the most important platelet receptors and their putative involvement in hematogenous metastasis of malignant tumors.

Inflammation and melanoma metastasis

Metastatic melanoma is extremely refractory to existing chemotherapeutic drugs and bioimmune adjuvant therapies, and the life span of patients with metastatic melanoma is often measured in months. Understanding the mechanisms responsible for the development of tumor metastasis is critical for finding successful curative measures. An expending amount of data reveal the importance of inflammatory microenvironment and stroma in cancer initiation and progression, which brings new directions and approaches to cancer treatment. This review will summarize current data on the role of the tumor microenvironment in shaping the metastatic phenotype of melanoma.

Thrombin induces tumor cell cycle activation and spontaneous growth by down-regulation of p27Kip1, in association with the up-regulation of Skp2 and MiR-222

The effect of thrombin on tumor cell cycle activation and spontaneous growth was examined in synchronized serum-starved tumor cell lines and a model of spontaneous prostate cancer development in TRAMP mice. BrdUrd incorporation and propidium iodide staining of prostate LNCaP cells arrested in G(0) and treated with thrombin or serum revealed a 48- and 29-fold increase in S phase cells, respectively, at 8 hours. Similar results were obtained with TRAMP cells and a glioblastoma cell line, T98G. Cell cycle kinases and inhibitors in synchronized tumor cells revealed high levels of p27(Kip1) and low levels of Skp2 and cyclins D1 and A. Addition of thrombin, TFLLRN, or serum down-regulated p27(Kip1) with concomitant induction of Skp2, Cyclin D1, and Cyclin A with similar kinetics. LNCaP p27(Kip1)-transfected cells or Skp2 knockdown cells were refractory to thrombin-induced cell cycle activation. MicroRNA 222, an inhibitor of p27(Kip1), was robustly up-regulated by thrombin. The in vitro observations were tested in vivo with transgenic TRAMP mice. Repetitive thrombin injection enhanced prostate tumor volume 6- to 8-fold (P < 0.04). Repetitive hirudin, a specific potent antithrombin, decreased tumor volume 13- to 24-fold (P < 0.04). Thus, thrombin stimulates tumor cell growth in vivo by down-regulation of p27(Kip1).

APT102, a novel adpase, cooperates with aspirin to disrupt bone metastasis in mice

Platelets contribute to the development of metastasis, the most common cause of mortality in cancer patients, but the precise role that anti-platelet drugs play in cancer treatment is not defined. Metastatic tumor cells can produce platelet alphaIIb beta3 activators, such as ADP and thromboxane A(2) (TXA(2)). Inhibitors of platelet beta3 integrins decrease bone metastases in mice but are associated with significant bleeding. We examined the role of a novel soluble apyrase/ADPase, APT102, and an inhibitor of TXA(2) synthesis, acetylsalicylic acid (aspirin or ASA), in mouse models of experimental bone metastases. We found that treatment with ASA and APT102 in combination (ASA + APT102), but not either drug alone, significantly decreased breast cancer and melanoma bone metastases in mice with fewer bleeding complications than observed with alphaIIb beta3 inhibition. ASA + APT102 diminished tumor cell induced platelet aggregation but did not directly alter tumor cell viability. Notably, APT102 + ASA treatment did not affect initial tumor cell distribution and similar results were observed in beta3-/- mice. These results show that treatment with ASA + APT102 decreases bone metastases without significant bleeding complications. Anti-platelet drugs such as ASA + APT102 could be valuable experimental tools for studying the role of platelet activation in metastasis as well as a therapeutic option for the prevention of bone metastases.

Thrombin up-regulates cathepsin D which enhances angiogenesis, growth, and metastasis

Cathepsin D (CD) up-regulation has been associated with human malignancy and poor prognosis. Thrombin up-regulated CD mRNA and protein in eight tumor cell lines as well as in human umbilical vascular endothelial cells (HUVEC). Thrombin increased the secretion of CD by 3- to 8-fold and enhanced chemotaxis ( approximately 2-fold) in 4T1 murine mammary CA cells, which was completely inhibited with the knockdown of CD. Secreted 4T1 CD induced neoangiogenesis by 2.4-fold on a chick chorioallantoic membrane, which was blocked in CD-KD cells. The addition of pure CD (2 ng) to the chick chorioallantoic membrane increased angiogenesis by 2.1-fold, which was completely inhibited by Pepstatin A (Pep A). CD enhanced human HUVEC chemotaxis and Matrigel tube formation by 2-fold, which was then blocked by Pep A. CD enhanced HUVEC matrix metalloproteinase 9 (MMP-9) activity by approximately 2-fold, which was completely inhibited by Pep A as well as a generic MMP inhibitor, GM6001. The injection of CD-KD 4T1 cells into syngeneic mice inhibited tumor growth by 3- to 4-fold compared with empty vector (EV) cells. Hirudin, a specific thrombin inhibitor, inhibited the growth of wild-type and EV cells by 2- to 3-fold, compatible with thrombin up-regulation of CD. CD and thrombin also contributed to spontaneous pulmonary metastasis; 4-fold nodule inhibition with CD versus EV and 4.6-fold inhibition with hirudin versus EV (P < 0.02). Thus, thrombin-induced CD contributes to the malignant phenotype by inducing tumor cell migration, nodule growth, metastasis, and angiogenesis. CD-induced angiogenesis requires the proteolytic activation of MMP-9.

Twist is required for thrombin-induced tumor angiogenesis and growth

Twist, a master regulator of embryonic morphogenesis, induces functions that are also required for tumor invasion and metastasis. Because thrombin contributes to the malignant phenotype by up-regulating tumor metastasis, we examined its effect on Twist in five different tumor cell lines and two different endothelial cell lines. Thrombin up-regulated Twist mRNA and protein in all seven cell lines. Down-regulation of Twist in B16F10 tumor cell lines led to a approximately 3-fold decrease in tumor growth on a chorioallantoic membrane assay and approximately 2-fold decrease in syngeneic mice. Angiogenesis was decreased approximately 45% and 36%, respectively. The effect of Twist on angiogenesis was further examined and compared with the effect of thrombin. In studies using a Twist-inducible plasmid, several identical vascular growth factors and receptors were up-regulated approximately 2- to 3-fold in tumor cells as well as human umbilical vascular endothelial cells by both Twist as well as thrombin (vascular endothelial growth factor, KDR, Ang-2, matrix metalloproteinase 1, GRO-alpha, and CD31). Thrombin-induced endothelial cell chemotaxis and Matrigel endothelial cell tubule formation were similarly regulated by Twist. Thus, thrombin up-regulates Twist, which is required for thrombin-induced angiogenesis as measured by endothelial cell migration, Matrigel tubule formation, and tumor angiogenesis.

Emerging roles of PAR-1 and PAFR in melanoma metastasis

Melanoma growth, angiogenesis and metastatic progression are strongly promoted by the inflammatory tumor microenvironment due to high levels of cytokine and chemokine secretion by the recruited inflammatory and stromal cells. In addition, platelets and molecular components of procoagulant pathways have been recently emerging as critical players of tumor growth and metastasis. In particular, thrombin, through the activity of its receptor protease-activated receptor-1 (PAR-1), regulates tumor cell adhesion to platelets and endothelial cells, stimulates tumor angiogenesis, and promotes tumor growth and metastasis. Notably, in many tumor types including melanoma, PAR-1 expression directly correlates with their metastatic phenotype and is directly responsible for the expression of interleukin-8, matrix metalloproteinase-2 (MMP-2), vascular endothelial growth factor, platelet-derived growth factor, and integrins. Another proinflammatory receptor–ligand pair, platelet-activating factor (PAF) and its receptor (PAFR), have been shown to act as important modulators of tumor cell adhesion to endothelial cells, angiogenesis, tumor growth and metastasis. PAF is a bioactive lipid produced by a variety of cells from membrane glycerophospholipids in the same reaction that releases arachidonic acid, and can be secreted by platelets, inflammatory cells, keratinocytes and endothelial cells. We have demonstrated that in metastatic melanoma cells, PAF stimulates the phosphorylation of cyclic adenosine monophosphate response element-binding protein (CREB) and activating transcription factor 1 (ATF-1), which results in overexpression of MMP-2 and membrane type 1-MMP (membrane type 1-MMP). Since only metastatic melanoma cells overexpress CREB/ATF-1, we propose that metastatic melanoma cells are better equipped than their non-metastatic counterparts to respond to PAF within the tumor microenvironment. The evidence supporting the hypothesis that the two G-protein coupled receptors, PAR-1 and PAFR, contribute to the acquisition of the metastatic phenotype of melanoma is presented and discussed.

The biological activity of the histidine-containing diketopiperazines cyclo(His-Ala) and cyclo(His-Gly)

Two cyclic dipeptides, cyclo(His-Ala) and cyclo(His-Gly,) were synthesized from their linear counterparts and their structures elucidated using standard elucidation techniques. Molecular modeling and predictive NMR results indicated that the majority of energetically favourable conformers adopted a boat conformation with respect to the diketopiperazine ring. Cyclo(His-Ala), at concentrations of 100 microM inhibited the growth, in vitro, of various cancer cell lines, including HT-29, MCF-7 and HeLa carcinoma cells while cyclo(His-Gly) inhibited the growth of MCF-7 cells. While the antibacterial potential of these two compounds was limited, both cyclic dipeptides significantly inhibited the growth of C. albicans. Both compounds at a concentration of 100 microM resulted in a decrease in heart rate, coronary flow rate and left ventricular systolic pressure in the isolated rat heart. Inhibition of thrombin, amounting to a 63.3% and 36.7% reduction in the rate of fibrin formation, was noted for cyclo(His-Ala) and cyclo(His-Gly), respectively. While cyclo(His-Ala) showed no notable effects on platelet aggregation, cyclo(His-Gly) significantly inhibited both pathways tested with greatest effects on thrombin-induced platelet aggregation, yielding an IC(50) of 0.0662 mM (R(2)=0.989). The results of the anticancer and hematological studies indicate that histidine-containing diketopiperazines have potential as a novel group of cytotoxic agents with antithrombotic effects.

Thrombin induces tumor growth, metastasis, and angiogenesis: Evidence for a thrombin-regulated dormant tumor phenotype

The association of idiopathic venous thrombosis with occult cancer is generally recognized. However, it has not been fully appreciated that thrombin generated during thrombosis can augment the malignant phenotype. Thrombin activates tumor cell adhesion to platelets, endothelial cells, and subendothelial matrix proteins; enhances tumor cell growth; increases tumor cell seeding and spontaneous metastasis; and stimulates tumor cell angiogenesis. These mechanisms are reviewed. Evidence is also presented to support the hypothesis that thrombin serves to preserve dormant tumor cells in individuals, preventing host eradication. It is proposed that tumor malignancy may be regulated by a procoagulant/anticoagulant axis.

Growth-regulated oncogene is pivotal in thrombin-induced angiogenesis

The mechanism of thrombin-induced angiogenesis is poorly understood. Using a gene chip array to investigate the pro-malignant phenotype of thrombin-stimulated cells, we observed that thrombin markedly up-regulates growth-regulated oncogene-alpha (GRO-alpha) in several tumor cell lines as well as endothelial cells by mRNA and protein analysis. Thrombin enhanced the secretion of GRO-alpha from tumor cells 25- to 64-fold. GRO-alpha is a CXC chemokine with tumor-associated angiogenic as well as oncogenic activation following ligation of its CXCR2 receptor. GRO-alpha enhanced angiogenesis in the chick chorioallantoic membrane assay 2.2-fold, providing direct evidence for GRO-alpha as an angiogenic growth factor. Anti-GRO-alpha antibody completely inhibited the 2.7-fold thrombin-induced up-regulation of angiogenesis, as well as the 1.5-fold thrombin-induced up-regulation of both endothelial cell cord formation in Matrigel and growth in vitro. Thrombin as well as its PAR-1 receptor activation peptide [thrombin receptor activation peptide (TRAP)] as well as GRO-alpha all markedly increased vascular regulatory proteins and growth factors: matrix metalloproteinase (MMP)-1, MMP-2, vascular endothelial growth factor (VEGF), angiopoietin-2 (Ang-2), CD31, and receptors KDR and CXCR2 in human umbilical vein endothelial cells. All of the thrombin/TRAP gene up-regulations were completely inhibited by anti-GRO-alpha antibody and unaffected by irrelevant antibody. Similar inhibition of gene up-regulation as well as thrombin-induced chemotaxis was noted with small interfering RNA (shRNA) GRO-alpha KD 4T1 breast tumor and B16F10 melanoma cells. In vivo tumor growth studies in wild-type mice with shRNA GRO-alpha KD cells revealed 2- to 4-fold impaired tumor growth, metastasis, and angiogenesis, which was not affected by endogenous thrombin. Thus, thrombin-induced angiogenesis requires the up-regulation of GRO-alpha. Thrombin up-regulation of GRO-alpha in tumor cells as well as endothelial cells contributes to tumor angiogenesis.

Protease-activated receptors: contribution to physiology and disease

Proteases acting at the surface of cells generate and destroy receptor agonists and activate and inactivate receptors, thereby making a vitally important contribution to signal transduction. Certain serine proteases that derive from the circulation (e.g., coagulation factors), inflammatory cells (e.g., mast cell and neutrophil proteases), and from multiple other sources (e.g., epithelial cells, neurons, bacteria, fungi) can cleave protease-activated receptors (PARs), a family of four G protein-coupled receptors. Cleavage within the extracellular amino terminus exposes a tethered ligand domain, which binds to and activates the receptors to initiate multiple signaling cascades. Despite this irreversible mechanism of activation, signaling by PARs is efficiently terminated by receptor desensitization (receptor phosphorylation and uncoupling from G proteins) and downregulation (receptor degradation by cell-surface and lysosomal proteases). Protease signaling in tissues depends on the generation and release of proteases, availability of cofactors, presence of protease inhibitors, and activation and inactivation of PARs. Many proteases that activate PARs are produced during tissue damage, and PARs make important contributions to tissue responses to injury, including hemostasis, repair, cell survival, inflammation, and pain. Drugs that mimic or interfere with these processes are attractive therapies: selective agonists of PARs may facilitate healing, repair, and protection, whereas protease inhibitors and PAR antagonists can impede exacerbated inflammation and pain. Major future challenges will be to understand the role of proteases and PARs in physiological control mechanisms and human diseases and to develop selective agonists and antagonists that can be used to probe function and treat disease.

Formation of tissue factor-factor VIIa-factor Xa complex promotes cellular signaling and migration of human breast cancer cells

Tissue factor (TF) is a transmembrane glycoprotein that initiates blood coagulation when complexed with factor (F)VIIa. Recently, TF has been shown to promote cellular signaling, tumor growth, angiogenesis, and metastasis. In the present study, we examined the pathway by which TF-FVIIa complex induces cellular signaling in human breast cancer cells using the Adr-MCF-7 cell line. This cell line has high endogenous TF expression as measured by flow cytometry and expression of protease-activated receptors 1 and 2 (PAR1 and PAR2) as determined by reverse transcriptase-polymerase chain reaction analysis. Both PAR1 and PAR2 are functionally active as determined by induction of p44/42 mitogen-activated protein kinase (MAPK) phosphorylation using specific agonist peptides. We found that MAPK phosphorylation in this cell line was strongly induced by the combination of FVIIa and factor (F)X, but not by FVIIa alone at a concentration of FVIIa that approaches physiological levels. Induction of MAPK phosphorylation involved the formation of TF-FVIIa-FXa complex and occurred by a pathway that did not require thrombin formation, indicating a critical role for FXa generation. In addition, induction of MAPK phosphorylation was found to be independent of PAR1 activation. We then examined whether TF-FVIIa complex formation could promote tumor cell migration using a modified Boyden chamber chemotaxis assay. The combination of FVIIa and FX, but not FVIIa alone, strongly induced migration of tumor cells by a pathway that probably involves PAR2, but not PAR1 activation. MAPK phosphorylation was found to be required for the induction of cell migration by the combination of FVIIa and FX. These data suggest that TF-FVIIa-mediated signaling in human breast cancer cells occurs most efficiently by formation of the TF-FVIIa-FXa complex. One of the physiological consequences of this signaling pathway is enhanced cell migration that is probably mediated by PAR2, but not PAR1 activation.

Thrombin induces neoangiogenesis in the chick chorioallantoic membrane

Most tumors have constitutively active tissue factor on their surface, capable of generating thrombin in the surrounding environment, and thrombosis is associated with cancer. Thrombin is known to induce a malignant phenotype by enhancing tissue adhesion and cell growth in vitro and in vivo in mice. Because tumors require angiogenesis for growth, we examined whether thrombin induces neoangiogenesis in a physiologically intact in vivo model. Thrombin (0.1 U mL-1) induced neoangiogenesis in the chick chorioallantoic membrane over a 24-72-h period by approximately 2-3-fold. This was inhibited by the potent thrombin inhibitor, hirudin and shown to have its mode of action by ligation of the thrombin protease-activated receptor, PAR-1. The thrombin receptor activation peptide, SFLLRNPNDKYEPF (200 microm) also enhanced neoangiogenesis c. 2-3-fold. Thrombin-induced neoangiogenesis was accompanied by the induction of vascular endothelial growth factor (VEGF) and angiopoietin-2 (Ang-2) mRNA at 24-48 h (approximately 2-fold) as determined by semi-quantitative reverse transcriptase-polymerase chain reaction. Thrombin-induced neoangiogenesis was inhibited to baseline level by the specific angiogenesis receptor inhibitors KDR-Fc (vs. VEGF) and Tie-2-Fc (vs. Ang-1 and Ang-2), as well as the non-specific angiogenesis inhibitor thrombospondin-1. Thrombin-induced neoangiogenesis was also inhibited to baseline level by agents known to inhibit thrombin receptor signaling in other cells: G-coupled protein receptor inhibitor, pertussis toxin (40 pg per egg), protein kinase C inhibitor, bisindolylmaleimide (1 microm per egg), MAP kinase inhibitor, PD980598 (10 microm per egg) and PI3 kinase inhibitor, LY294002 (0.25 microm per egg). Thus angiogenesis is stimulated by thrombosis, which could help explain the enhancement of experimental tumorigenesis by thrombin.

Extracellular mediators in atherosclerosis and thrombosis: lessons from thrombin receptor knockout mice

It is well appreciated that thrombin as well as other proteases can act as signaling molecules that specifically regulate cells by cleaving and activating members of a novel class of protease-activated receptors (PARs). The utility of gene knockout strategies to define and better comprehend the physiological role of specific proteins is perhaps best exemplified in the field of thrombin receptors. The development of PAR knockout mice has provided the unique opportunity to identify and characterize new members of this novel family of GPCRs, evaluate the interaction of PARs jointly expressed in common cells and tissues, and better understand the role of PARs in thrombosis, restenosis, vascular remodeling, angiogenesis, and inflammation. Presently, 4 members of the PAR family have been cloned and identified. In this review, we examine experimental evidence gleaned from PAR-/- mouse models as well as how the use of PAR-/- mice has provided insights toward understanding the physiological role of thrombin in cells of the vascular system and vascular pathology.

Thrombin induces increased expression and secretion of angiopoietin-2 from human umbilical vein endothelial cells

Angiogenesis is required for tumor growth and metastasis. It has recently been suggested that thrombin is a potent promoter of angiogenesis. We therefore examined the possibility that thrombin could be inducing the expression of angiopoietin-2 (Ang-2), necessary for remodeling. Human umbilical vein endothelial cells were incubated with or without thrombin (1 U/mL) for 1 to 24 hours and then examined for messenger RNA (mRNA) by Northern analysis. Enhanced mRNA expression (about 4-fold over baseline) was noted at 4 hours. Enhanced expression of Ang-2 mRNA was secondary to enhanced transcription (about 4-fold), with no effect on stabilization. Enhanced Ang-2 mRNA transcription was inhibited by H7 and PD98059, indicating the requirement of serine/threonine kinases as well as the mitogen-activated protein kinase pathway. Up-regulation of mRNA was associated with enhanced Ang-2 protein synthesis and secretion as assayed by immunoblot. Thrombin-induced secreted Ang-2 inhibited the binding of recombinant (35)S-Ang-1 to its Tie-2-Fc receptor, demonstrating functionality. Hirudin reversed this effect, demonstrating thrombin specificity. Thus, thrombin-induced tumorigenesis and metastasis is associated with enhanced Ang-2 protein synthesis and secretion via enhanced transcription of Ang-2. This could help explain how thrombin promotes angiogenesis.

Identification and characterisation of a platelet GPIb/V/IX-like complex on human breast cancers: implications for the metastatic process

The glycoprotein (GP) Ib /V/IX receptor complex is an important adhesion molecule, originally thought to be unique to the megakaryocytic lineage. Recent evidence now indicates that GPIb /V/IX may be more widely expressed. In this study we report the presence of all subunits of the complex on four breast cancer cell lines, and 51 / 80 primary breast tumours. The surface expression of GPIb /V/IX was confirmed by flow cytometry, and by immunoprecipitation of biotin surface-labelled tumour cells. Western blotting of cell lysates under reducing conditions revealed that tumour cell-GPIb alpha had a relative molecular weight of 95 kDa as compared to 135 kDa on platelets. Despite the discrepant protein size, molecular analyses on the tumour cell-GPIb alpha subunit using RT-PCR and DNA sequencing revealed 100% sequence homology to platelet GPIb alpha. Tumour cell-GPIb /V/IX was capable of binding human von Willebrand factor (vWf), and this binding caused aggregation of tumour cells in suspension. Tumour cells bound to immobilised vWf in the presence of EDTA and demonstrated prominent filapodial extensions indicative of cytoskeletal reorganisation. Furthermore, in a modified Boyden chamber assay, prior exposure to vWf or a GPIb alpha monoclonal antibody, AK2, enhanced cell migration. The presence of a functional GPIb /V/IX-like complex in tumour cells suggests that this complex may participate in the process of haematogenous breast cancer metastasis.

A novel PAR-1-type thrombin receptor signaling pathway: cyclic AMP-independent activation of PKA in SNB-19 glioblastoma cells

Cellular effects of thrombin are mediated by members of a new subfamily of G protein-coupled receptors designated proteinase-activated receptors (PARs) with the prototype PAR-1. Investigation of PAR-1-induced signaling has been shown to be very important in clarifying thrombin's role in cell metabolism, differentiation, and growth. We evaluated connection of PAR-1 with the cAMP/PKA pathway in SNB-19 glioblastoma cells. Alpha-thrombin and the synthetic PAR-1 agonist SFLLRN stimulated PKA as shown by increased PKA activity and translocation of the catalytic PKA alpha subunits (PKA(cat)alpha) into the nucleus. However, no effect on cAMP could be observed. PKA(cat)alpha was found to be associated with nuclear factor-kappa B (NF-kappaB) p65 and its inhibitor protein IkappaB in SNB-19 cells. After PAR-1 stimulation, this association was markedly diminished. We conclude that PAR-1 mediates PKA activation without altering cAMP levels but includes NF-kappaB-associated PKA(cat)alpha in SNB-19 glioblastoma cells. This is the first evidence for a cAMP-independent PKA signaling by a G protein-coupled receptor.

Differential expression of protease-activated receptors-1 and -2 in stromal fibroblasts of normal, benign, and malignant human tissues

The serine proteases thrombin and trypsin are among many factors that malignant cells secrete into the extracellular space to mediate metastatic processes such as cellular invasion, extracellular matrix degradation, angiogenesis, and tissue remodeling. The degree of protease secretion from malignant cells has been correlated to their metastatic potential. Protease activated receptors (PAR)-1 and -2, which are activated by thrombin and trypsin respectively, have not been extensively characterized in human tumors in situ. We investigated the presence of PAR-1 and PAR-2 in human normal, benign and malignant tissues using immunohistochemistry and in situ hybridization. Our results demonstrate PAR-1 and PAR-2 expression in the tumor cells, mast cells, macrophages, endothelial cells, and vascular smooth muscle cells of the metastatic tumor microenvironment. Most notably, an up-regulation of PAR-1 and PAR-2 observed in proliferating, smooth muscle actin (SMA)-positive stromal fibroblasts surrounding the carcinoma cells was not observed in normal or benign conditions. Furthermore, in vitro studies using proliferating, SMA-positive, human dermal fibroblasts, and scrape-wounded human dermal fibroblasts demonstrated the presence of PAR-1 and PAR-2 not detected in quiescent, SMA-negative cultures. PAR-1 and PAR-2 in the cells forming the tumor microenvironment suggest that these receptors mediate the signaling of secreted thrombin and trypsin in the processes of cellular metastasis.

Concentration-dependent dual effect of thrombin on impaired growth/apoptosis or mitogenesis in tumor cells

Because thrombin-treated tumor cell-induced metastasis increases tumor nodule volume12 greater than nodule number, we studied the effect of thrombin on tumor cell growth in vitro and in vivo (murine B16F10 melanoma, human HCT8 colon carcinoma, DU145 prostate carcinoma). Tumor cell growth was measured after 3 to 7 days in 1% fetal calf serum (FCS) + RPMI 1640. We found that, whereas relatively low concentrations of thrombin, 0.1 to 0.5 U/mL (1-5 nmol/L) enhance tumor cell growth in vitro approximately 2- to 3-fold, higher concentrations, 0.5 to 1 U/mL (5-10 nmol/L) impaired cell growth approximately 2- to 4-fold. Impaired cell growth was associated with cell cycle arrest at G2M and increased pre-GoDNA, as well as apoptosis, measured by tumor cell binding to Annexin V and propidium iodide. Apoptosis was reversed with the general caspase inhibitor, FK-011. The enhancing and inhibiting effects were specific for thrombin (reversed with inactive diisopropyl-fluorophosphate [DFP]-thrombin) and mediated via the protease-activated receptor 1 (PAR-1). PAR-1 activation was demonstrated by (1) use of a cell line, B16F10, devoid of the 3 other thrombin receptors, PAR-3, PAR-4, and GPIb; and (2) greater sensitivity of PAR-1 transfected B16F10 and HCT8 cells to impaired cell growth/apoptosis, 3- and 14-fold, respectively. Thus, thrombin has a bimodal effect on PAR-1 in tumor cells: enhanced growth at low concentration, impaired growth/apoptosis at higher concentration.

Concentration-dependent dual effect of thrombin on impaired growth/apoptosis or mitogenesis in tumor cells

Because thrombin-treated tumor cell-induced metastasis increases tumor nodule volume12 greater than nodule number, we studied the effect of thrombin on tumor cell growth in vitro and in vivo (murine B16F10 melanoma, human HCT8 colon carcinoma, DU145 prostate carcinoma). Tumor cell growth was measured after 3 to 7 days in 1% fetal calf serum (FCS) + RPMI 1640. We found that, whereas relatively low concentrations of thrombin, 0.1 to 0.5 U/mL (1-5 nmol/L) enhance tumor cell growth in vitro approximately 2- to 3-fold, higher concentrations, 0.5 to 1 U/mL (5-10 nmol/L) impaired cell growth approximately 2- to 4-fold. Impaired cell growth was associated with cell cycle arrest at G2M and increased pre-GoDNA, as well as apoptosis, measured by tumor cell binding to Annexin V and propidium iodide. Apoptosis was reversed with the general caspase inhibitor, FK-011. The enhancing and inhibiting effects were specific for thrombin (reversed with inactive diisopropyl-fluorophosphate [DFP]-thrombin) and mediated via the protease-activated receptor 1 (PAR-1). PAR-1 activation was demonstrated by (1) use of a cell line, B16F10, devoid of the 3 other thrombin receptors, PAR-3, PAR-4, and GPIb; and (2) greater sensitivity of PAR-1 transfected B16F10 and HCT8 cells to impaired cell growth/apoptosis, 3- and 14-fold, respectively. Thus, thrombin has a bimodal effect on PAR-1 in tumor cells: enhanced growth at low concentration, impaired growth/apoptosis at higher concentration.

Trypsin stimulates integrin alpha(5)beta(1)-dependent adhesion to fibronectin and proliferation of human gastric carcinoma cells through activation of proteinase-activated receptor-2

Trypsin is widely expressed in various non-pancreatic tissues at low levels and overexpressed in some types of human cancers. In the present study, we found that trypsin stimulates integrin-dependent adhesion and growth of MKN-1 human gastric carcinoma cells. MKN-1 cells expressed both proteinase-activated receptor-1 (PAR-1) and PAR-2, which are activated by thrombin and trypsin, respectively. Both trypsin and the PAR-2 ligand SLIGKV promoted integrin alpha(5)beta(1)-mediated adhesion of MKN-1 cells to fibronectin, and less effectively integrin alpha(v)beta(3)-mediated cell adhesion to vitronectin, but not that to type IV collagen or laminin-1 at all. Thrombin and the PAR-1 ligand SFLLRN promoted the cell adhesion to vitronectin more strongly than trypsin or the PAR-2 ligand, but not the cell adhesion to fibronectin at all. The cell adhesion-stimulating effect of the PAR-2 ligand was significantly reduced by the pre-treatment of cells with trypsin, indicating that the effect of trypsin is mediated by PAR-2 activation. The trypsin-stimulated cell adhesion to vitronectin, but not to fibronectin, was effectively inhibited by the G(i) protein blocker pertussis toxin, and both cell adhesions were completely inhibited by the Src kinase inhibitor herbimycin A. Furthermore, trypsin and the PAR-2 ligand stimulated growth of MKN-1 cells more strongly than thrombin or the PAR-1 ligand. These results show that trypsin regulates cellular adhesion and proliferation by inducing PAR-2/G protein signalings, and that the integrin alpha(5)beta(1)- and integrin alpha(v)beta(3)-dependent cell adhesions are regulated by different PAR/G protein signalings.

PAR 1-type thrombin receptors are involved in thrombin-induced calcium signaling in human meningioma cells

Thrombin is known to play a role as regulator in tumor spreading and tumor growth. Proteinase-activated receptor 1 (PAR 1)-type thrombin receptors were identified in different cancer cells including human glioblastoma cells. Thus a function of PAR 1 in brain tumors may be suggested. In this study, the presence of PAR 1-type thrombin receptors was investigated in primary cell cultures established from operated human meningiomas from two 59- and 79-year-old women. Characterization of PAR 1 on binding level was performed using immunofluorescence studies with the monoclonal anti-PAR 1 antibody Mab 61-1 directed against a domain in the NH2-terminus of PAR 1. These binding sites constitute functional thrombin receptors that are involved in thrombin-induced signaling in human meningioma cells as demonstrated by investigation of alpha-thrombin- and PAR 1-activating hexapeptide (TRAP-6)-induced [Ca2+]i mobilization. To our knowledge, this is the first report demonstrating thrombin-induced intracellular signaling in human meningioma cells mediated by the PAR 1-type thrombin receptor.

Role of thrombin receptor in breast cancer invasiveness

Invasion, the ability of an epithelial cancer cell to detach from and move through a basement membrane, is a central process in tumour metastasis. Two components of invasion are proteolysis of extracellular matrix and cellular movement through it. A potential promoter of these two processes is thrombin, the serine proteinase derived from the ubiquitous plasma protein prothrombin. Thrombin promotes the invasion of MDA-MB231 breast tumour cells (a highly aggressive cell line) in an in vitro assay. Invasion by MDA-MB436 and MCF-7 cells, less aggressive cell lines, is not promoted by thrombin. Thrombin, added to the cells, is a stimulator of cellular movement; fibroblast-conditioned medium is the chemotaxin. Thrombin-promoted invasion is inhibited by hirudin. Stimulation of invasion is a receptor-mediated process that is mimicked by a thrombin receptor-activating peptide. Thrombin has no effect on chemotaxis in vitro. Thrombin receptor is detectable on the surface of MDA-MB231 cells, but not on the other two cell lines. Introduction of oestrogen receptors into MDA-MB231 cells by transfection with pHEO had no effect on thrombin receptor expression, in the presence or absence of oestradiol. This paper demonstrates that thrombin increases invasion by the aggressive breast cancer cell line MDA-MB231 by a thrombin receptor-dependent mechanism.

Protease-Activated Receptor 1 (PAR-1) Is Required and Rate-Limiting for Thrombin-Enhanced Experimental Pulmonary Metastasis

Thrombin-treated tumor cells induce a metastatic phenotype in experimental pulmonary murine metastasis. Thrombin binds to a unique protease-activated receptor (PAR-1) that requires N-terminal proteolytic cleavage for activation by its tethered end. A 14-mer thrombin receptor activation peptide (TRAP) of the tethered end induces the same cellular changes as thrombin. Four murine tumor cells (Lewis lung, CT26 colon CA, B16F10 melanoma, and CCL163 fibroblasts) contain PAR-1, as detected by reverse transcriptase-polymerase chain reaction (RT-PCR). B16F10 cells did not contain the two other thrombin receptors, PAR-3 and glycoprotein Ib. TRAP-treated B16F10 tumor cells enhance pulmonary metastasis 41- to 48-fold (n = 17). Thrombin-treated B16F10 cells transfected with full-length murine PAR-1 sense cDNA (S6, S7, S14, and S22) enhanced their adhesion to fibronectin 1.5- to 2.4-fold (n = 5, P &lt; .04), whereas thrombin-treated wild-type cells do not. S6 (adhesion index, 1.5-fold) and S14 (index, 2.4-fold) when examined by RT-PCR and Northern analysis showed minimal expression of PAR-1 for S6 over wild-type and considerable expression for S14. Immunohistochemistry showed greater expression of PAR-1 for S14 compared with wild-type or empty-plasmid transfected cells. In vivo experiments with the thrombin-treated S14 transfectant showed a fivefold to sixfold increase in metastases compared with empty-plasmid transfected thrombin-treated naive cells or S6 cells (n = 20, P = .0001 to .02). Antisense had no effect on thrombin-stimulated tumor mass. Thus, PAR-1 ligation and expression enhances and regulates tumor metastasis.

Protease-Activated Receptor 1 (PAR-1) Is Required and Rate-Limiting for Thrombin-Enhanced Experimental Pulmonary Metastasis

Thrombin-treated tumor cells induce a metastatic phenotype in experimental pulmonary murine metastasis. Thrombin binds to a unique protease-activated receptor (PAR-1) that requires N-terminal proteolytic cleavage for activation by its tethered end. A 14-mer thrombin receptor activation peptide (TRAP) of the tethered end induces the same cellular changes as thrombin. Four murine tumor cells (Lewis lung, CT26 colon CA, B16F10 melanoma, and CCL163 fibroblasts) contain PAR-1, as detected by reverse transcriptase-polymerase chain reaction (RT-PCR). B16F10 cells did not contain the two other thrombin receptors, PAR-3 and glycoprotein Ib. TRAP-treated B16F10 tumor cells enhance pulmonary metastasis 41- to 48-fold (n = 17). Thrombin-treated B16F10 cells transfected with full-length murine PAR-1 sense cDNA (S6, S7, S14, and S22) enhanced their adhesion to fibronectin 1.5- to 2.4-fold (n = 5, P < .04), whereas thrombin-treated wild-type cells do not. S6 (adhesion index, 1.5-fold) and S14 (index, 2.4-fold) when examined by RT-PCR and Northern analysis showed minimal expression of PAR-1 for S6 over wild-type and considerable expression for S14. Immunohistochemistry showed greater expression of PAR-1 for S14 compared with wild-type or empty-plasmid transfected cells. In vivo experiments with the thrombin-treated S14 transfectant showed a fivefold to sixfold increase in metastases compared with empty-plasmid transfected thrombin-treated naive cells or S6 cells (n = 20, P = .0001 to .02). Antisense had no effect on thrombin-stimulated tumor mass. Thus, PAR-1 ligation and expression enhances and regulates tumor metastasis.

Proteinase-activated receptors: novel mechanisms of signaling by serine proteases

Although serine proteases are usually considered to act principally as degradative enzymes, certain proteases are signaling molecules that specifically regulate cells by cleaving and triggering members of a new family of proteinase-activated receptors (PARs). There are three members of this family, PAR-1 and PAR-3, which are receptors for thrombin, and PAR-2, a receptor for trypsin and mast cell tryptase. Proteases cleave within the extracellular NH2-terminus of their receptors to expose a new NH2-terminus. Specific residues within this tethered ligand domain interact with extracellular domains of the cleaved receptor, resulting in activation. In common with many G protein-coupled receptors, PARs couple to multiple G proteins and thereby activate many parallel mechanisms of signal transduction. PARs are expressed in multiple tissues by a wide variety of cells, where they are involved in several pathophysiological processes, including growth and development, mitogenesis, and inflammation. Because the cleaved receptor is physically coupled to its agonist, efficient mechanisms exist to terminate signaling and prevent uncontrolled stimulation. These include cleavage of the tethered ligand, receptor phosphorylation and uncoupling from G proteins, and endocytosis and lysosomal degradation of activated receptors.

Thrombin promotes platelet-mediated melanoma cell adhesion to endothelial cells under flow conditions: role of platelet glycoproteins P-selectin and GPIIb-IIIA

We investigated the role of platelets in human melanoma cell (line 397) interaction with vascular endothelial cells (ECs) under flow conditions. The ability of the tumour cells to adhere to the EC monolayer was significantly reduced by application of flow at a shear rate of 250 s(-1). A 2.2-fold increase in tumour cell adhesion to ECs under flow was observed upon addition of thrombin receptor agonist peptide (TRAP)-activated platelets but not resting platelets. A similar increase (2.5-fold) in tumour cell adhesion to ECs under flow was observed when the tumour cells were incubated with resting platelets on thrombin-treated ECs. However, thrombin treatment of the ECs alone had no effect on tumour cell adhesion in the absence of platelets. The enhancement of tumour cell adhesion to ECs by TRAP-activated platelets was virtually abolished by blockade of the platelet glycoproteins P-selectin and GPIIb-IIIa by monoclonal antibodies. Blockade of P-selectin also inhibited the direct adhesion of TRAP-activated platelets to ECs, but did not affect the interaction of the tumour cells with platelets immobilized on subendothelial extracellular matrix (ECM). Blockade of GPIIb-IIIa inhibited both platelet-EC and platelet-tumor cell interactions. Our results indicate that tumour cell adhesion to the endothelium under flow is enhanced by platelets under conditions that allow platelet adhesion to ECs. Inhibition studies suggest that activated platelet adhesion to ECs is mediated by P-selectin and GPIIb-IIIA, and tumour cell adhesion to EC-bound platelets--mainly by GPIIb-IIIa.

Redistribution of von Willebrand factor in porcine carotid arteries after balloon angioplasty

von Willebrand factor (VWF) is a well-characterized multimeric glycoprotein present in platelets and plasma and synthesized by vascular endothelial cells and megakaryocytes. Its role in platelet-vessel wall interactions has been studied extensively, but its involvement in intravascular events after balloon angioplasty has not been clarified. VWF antigen is not present in porcine arterial endothelium (except for the pulmonary artery) but is readily detected in porcine venous endothelial cells. We have examined the localization of VWF in porcine vessel walls during neointima formation after bilateral carotid balloon-angioplasty. Endothelium was denuded by balloon injury but regenerated by 7 days and was fully confluent by 42 days. VWF was detected at the site of injury in localized, adherent platelet aggregates at 10 minutes after angioplasty that were not present at later time points. A well-demarcated homogeneous layer of VWF was observed on the luminal surface from 30 minutes to day 7, but there was a progressive shift of positive staining from the lumen to the outer media from days 1 to 7. VWF was also strongly detected at sites proximal and distal to the balloon injury from 30 minutes to day 7, although endothelial disruption was minimal and the monolayer remained substantially intact at these sites. Regrowing endothelial cells appeared to contain granular VWF from days 12 to 21, but this was not readily evident at later time points. The results suggest that balloon injury is associated with deposition and medial absorption of plasma or platelet VWF in this porcine model over a time period that precedes and overlaps vascular smooth muscle proliferation and endothelial recoverage. The findings provide evidence to support the concept of a wider role for VWF in tissue injury responses.