Blockade of GpIIb/IIIa inhibits the release of vascular endothelial growth factor (VEGF) from tumor cell-activated platelets and experimental metastasis

Unlocking the intricacies: Exploring the complex interplay between platelets and ovarian cancer

Ovarian cancer, an aggressive malignancy of the female reproductive tract, is frequently linked to an elevated risk of thrombotic events. This association is manifested by a pronounced rise in platelet counts and activation levels. Current research firmly supports the pivotal role of platelets in the oncogenic processes of ovarian cancer, influencing tumor cell proliferation and metastasis. Platelets influence these processes through direct interactions with tumor cells or by secreting cytokines and growth factors that enhance tumor growth, angiogenesis, and metastasis. This review aims to thoroughly dissect the interactions between platelets and ovarian cancer cells, emphasizing their combined role in tumor progression and associated thrombotic events. Additionally, it summarizes therapeutic strategies targeting platelet-cancer interface which show significant promise. Such approaches could not only be effective in managing the primary ovarian tumor but also play a pivotal role in preventing metastasis and attenuating thrombotic complications associated with ovarian cancer.

Tumor-educated platelets

Beyond traditional roles in homeostasis and coagulation, growing evidence suggests that platelets also reflect malignant transformation in cancer. Platelets are present in the tumor microenvironment where they interact with cancer cells. This interaction results in direct and indirect "education" as evident by platelet alterations in adhesion molecules, glycoproteins, nucleic acids, proteins and various receptors. Subsequently, these tumor-educated platelets (TEPs) circulate throughout the body and play pivotal roles in promotion of tumor growth and dissemination. Accordingly, platelet status can be considered a unique blood-based biomarker that can potentially predict prognosis and therapeutic success. Recently, liquid biopsies including TEPs have received much attention as safe, minimally invasive and sensitive alternatives for patient management. Herein, we provide an overview of TEPs and explore their benefits and limitations in cancer.

Eptifibatide, an Older Therapeutic Peptide with New Indications: From Clinical Pharmacology to Everyday Clinical Practice

Therapeutic peptides are oligomers or short polymers of amino acids used for various medical purposes. Peptide-based treatments have evolved considerably due to new technologies, stimulating new research interests. They have been shown to be beneficial in a variety of therapeutic applications, notably in the treatment of cardiovascular disorders such as acute coronary syndrome (ACS). ACS is characterized by coronary artery wall damage and consequent formation of an intraluminal thrombus obstructing one or more coronary arteries, leading to unstable angina, non-ST elevated myocardial infarction, and ST-elevated myocardial infarction. One of the promising peptide drugs in the treatment of these pathologies is eptifibatide, a synthetic heptapeptide derived from rattlesnake venom. Eptifibatide is a glycoprotein IIb/IIIa inhibitor that blocks different pathways in platelet activation and aggregation. In this narrative review, we summarized the current evidence on the mechanism of action, clinical pharmacology, and applications of eptifibatide in cardiology. Additionally, we illustrated its possible broader usage with new indications, including ischemic stroke, carotid stenting, intracranial aneurysm stenting, and septic shock. Further research is, however, required to fully evaluate the role of eptifibatide in these pathologies, independently and in comparison to other medications.

Structure, signal transduction, activation, and inhibition of integrin αIIbβ3

Integrins are heterodimeric receptors comprising α and β subunits. They are expressed on the cell surface and play key roles in cell adhesion, migration, and growth. Several types of integrins are expressed on the platelets, including αvβ3, αIIbβ3, α2β1, α5β1, and α6β1. Among these, physically αIIbβ3 is exclusively expressed on the platelet surface and their precursor cells, megakaryocytes. αIIbβ3 adopts at least three conformations: i) bent-closed, ii) extended-closed, and iii) extended-open. The transition from conformation i) to iii) occurs when αIIbβ3 is activated by stimulants. Conformation iii) possesses a high ligand affinity, which triggers integrin clustering and platelet aggregation. Platelets are indispensable for maintaining vascular system integrity and preventing bleeding. However, excessive platelet activation can result in myocardial infarction (MI) and stroke. Therefore, finding a novel strategy to stop bleeding without accelerating the risk of thrombosis is important. Regulation of αIIbβ3 activation is vital for this strategy. There are a large number of molecules that facilitate or inhibit αIIbβ3 activation. The interference of these molecules can accurately control the balance between hemostasis and thrombosis. This review describes the structure and signal transduction of αIIbβ3, summarizes the molecules that directly or indirectly affect integrin αIIbβ3 activation, and discusses some novel antiαIIbβ3 drugs. This will advance our understanding of the activation of αIIbβ3 and its essential role in platelet function and tumor development.

Effect of Rho GTPase activating protein 9 combined with preoperative ratio of platelet distribution width to platelet count on prognosis of patients with serous ovarian cancer

Background

This study aimed to investigate the relationship between Rho GTPase activating protein 9 (ARHGAP9) combined with preoperative ratio of platelet distribution width to platelet count (PDW/PLT) and patients prognosis with serous ovarian cancer.

Methods

The clinical data of 80 patients with serous ovarian cancer treated in Jiangsu Cancer Hospital from May 2011 to May 2016 were analyzed retrospectively. We verified ARHGAP9 expression in The Cancer Genome Atlas (TCGA) database, then detected messenger RNA (mRNA) expression encoding ARHGAP9 in ovarian cancer tissue samples using reverse transcription quantitative polymerase chain reaction (RT-qPCR). These patients were divided into an ARHGAP9 low-expression group and an ARHGAP9 high-expression group. The optimal critical value of PDW/PLT was determined by receiver operating characteristic (ROC) curve. The patients were divided into low PDW/PLT group and high PDW/PLT group. Kaplan-Meier method and log-rank test were used for univariate survival analysis, Cox regression method was used for multivariate analysis, and then a nomogram was constructed for internal verification.

Results

The ARHGAP9 protein was highly expressed both in TCGA serous ovarian cancer database and the serous ovarian cancer tumor tissues. There were significant differences in menstrual status, the International Federation of Gynecology and Obstetrics (FIGO) stage and grade between the ARHGAP9 low expression group and ARHGAP9 high expression group (all P<0.05). There were significant differences in FIGO stage, lymph node metastasis, and ascites between the low PDW/PLT group and high PDW/PLT group (all P<0.05). Finally, 80 patients were included, with a mortality rate of 45.0% and a survival rate of 55.0%; the median progression-free survival (PFS) was 19 months, and the median overall survival (OS) was 62.5 months. Cox multivariate analysis showed that PDW/PLT and ARHGAP9 were independent risk factors for tumor progression (P=0.026 and P=0.028, respectively). In the internal validation, the C-index of the nomogram was 0.6518 [95% confidence interval (CI): 0.5685 to 0.7352], and the prediction model had certain accuracy.

Conclusions

ARHGAP9 and PDW/PLT Decrease can significantly prolong OS and PFS in serous ovarian cancer patients. Therefore, ARHGAP9 can be used as a new predictive biomarker and may be related to the immune infiltration of serous ovarian cancer.

Doxorubicin contributes to thrombus formation and vascular injury by interfering with platelet function

In clinical studies, platelet aggregation and risk of thrombosis are increased in patients after doxorubicin treatment. However, the exact role of doxorubicin in platelet functions and thrombus formation in vivo remain unclear. The present study is to investigate the role of doxorubicin in platelet function in relation to thrombus formation and vascular toxicity, as well as the efficacy of antiplatelet therapy. Mice were treated with doxorubicin or vehicle (5 mg/kg iv, 4 wk), and the following parameters were determined: platelet count and size, platelet surface adhesive receptors by flow cytometry, density of granules by electron microscopy, platelet aggregation and degranulation at resting or agonist-stimulated state, platelet adhesion on fibrinogen or endothelial cells, and thrombus formation on collagen matrix. The efficacy of clopidogrel (15 mg·kg^-1·day^-1, followed by 5 mg·kg^-1·day^-1) on doxorubicin-induced changes in the aforementioned parameters as well as vascular injury were also determined. Whereas platelet count and size were similar between doxorubicin-treated and vehicle-treated mice, doxorubicin promoted thrombus formation evidenced by greater platelet aggregation, degranulation, and adhesion to endothelial cells evoked by agonists. Clopidogrel treatment attenuated the enhanced platelet activity and thrombus formation by doxorubicin, as well as vascular platelet infiltration and reactive oxygen species generation. Collectively, this study demonstrates that platelet functions are enhanced after long-term doxorubicin administration, which leads to thrombus formation and vascular toxicity, and that doxorubicin-induced changes in the functionality of platelets can be effectively inhibited by antiplatelet drugs.NEW & NOTEWORTHY Doxorubicin therapy in mice (antitumor dosage) markedly enhanced platelet functions measured as agonist-induced platelet aggregation, degranulation, and adhesion to endothelial cells, actions leading to thrombus formation and thrombosis-independent vascular injury. Clopidogrel treatment ameliorated thrombus formation and vascular toxicity induced by doxorubicin via inhibiting platelet activity.

Platelet-Based Drug Delivery for Cancer Applications

Platelets can be considered as the "guardian of hemostasis" where their main function is to maintain vascular integrity. In pathological conditions, the hemostatic role of platelets may be hijacked to stimulate disease progression. In 1865, Armand Trousseau was a pioneer in establishing the platelet-cancer metastasis relationship, which he eventually termed as Trousseau's Syndrome to describe the deregulation of the hemostasis-associated pathways induced by cancer progression (Varki, Blood. 110(6):1723-9, 2007). Since these early studies, there has been an increase in experimental evidence not only to elucidate the role of platelets in cancer metastasis but also to create novel cancer therapies by targeting the platelet's impact in metastasis. In this chapter, we discuss the contribution of platelets in facilitating tumor cell transit from the primary tumor to distant metastatic sites as well as novel cancer therapies based on platelet interactions.

Aspirin blocks formation of metastatic intravascular niches by inhibiting platelet-derived COX-1/thromboxane A2

Because metastasis is associated with the majority of cancer-related deaths, its prevention is a clinical aspiration. Prostanoids are a large family of bioactive lipids derived from the activity of cyclooxygenase-1 (COX-1) and COX-2. Aspirin impairs the biosynthesis of all prostanoids through the irreversible inhibition of both COX isoforms. Long-term administration of aspirin leads to reduced distant metastases in murine models and clinical trials, but the COX isoform, downstream prostanoid, and cell compartment responsible for this effect are yet to be determined. Here, we have shown that aspirin dramatically reduced lung metastasis through inhibition of COX-1 while the cancer cells remained intravascular and that inhibition of platelet COX-1 alone was sufficient to impair metastasis. Thromboxane A2 (TXA2) was the prostanoid product of COX-1 responsible for this antimetastatic effect. Inhibition of the COX-1/TXA2 pathway in platelets decreased aggregation of platelets on tumor cells, endothelial activation, tumor cell adhesion to the endothelium, and recruitment of metastasis-promoting monocytes/macrophages, and diminished the formation of a premetastatic niche. Thus, platelet-derived TXA2 orchestrates the generation of a favorable intravascular metastatic niche that promotes tumor cell seeding and identifies COX-1/TXA2 signaling as a target for the prevention of metastasis.

Platelet integrin αIIbβ3: signal transduction, regulation, and its therapeutic targeting

Integrins are a family of transmembrane glycoprotein signaling receptors that can transmit bioinformation bidirectionally across the plasma membrane. Integrin αIIbβ3 is expressed at a high level in platelets and their progenitors, where it plays a central role in platelet functions, hemostasis, and arterial thrombosis. Integrin αIIbβ3 also participates in cancer progression, such as tumor cell proliferation and metastasis. In resting platelets, integrin αIIbβ3 adopts an inactive conformation. Upon agonist stimulation, the transduction of inside-out signals leads integrin αIIbβ3 to switch from a low- to high-affinity state for fibrinogen and other ligands. Ligand binding causes integrin clustering and subsequently promotes outside-in signaling, which initiates and amplifies a range of cellular events to drive essential platelet functions such as spreading, aggregation, clot retraction, and thrombus consolidation. Regulation of the bidirectional signaling of integrin αIIbβ3 requires the involvement of numerous interacting proteins, which associate with the cytoplasmic tails of αIIbβ3 in particular. Integrin αIIbβ3 and its signaling pathways are considered promising targets for antithrombotic therapy. This review describes the bidirectional signal transduction of integrin αIIbβ3 in platelets, as well as the proteins responsible for its regulation and therapeutic agents that target integrin αIIbβ3 and its signaling pathways.

Sushi Domain Containing 2 (SUSD2) inhibits platelet activation and binding to high-grade serous ovarian carcinoma cells

Platelets play a central role in primary hemostasis affecting tumor survival and metastases. Tumors induce platelets to aggregate and bind to the cancer cells, resulting in protection from immune surveillance and often leading to thrombocytosis. In ovarian cancer (OvCa), one-third of patients present with thrombocytosis, a diagnosis that correlates with shorter survival. SUSD2 (SUShi Domain containing 2), a type I transmembrane protein, shown to inhibit metastatic processes in high-grade serous ovarian carcinoma (HGSOC), is expressed on endothelial cells and thus may influence platelet reactivity. As such, we hypothesized that SUSD2 levels in ovarian cancer-derived cell lines influence platelet activation. We incubated OvCa non-targeting (NT) and SUSD2 knockdown (KD) cell lines with labeled platelets and quantified platelet binding, as well as GPIIb/IIIa integrin activation. The role of GPIIb/IIIa in tumor cell/platelet interaction was also examined by measuring cell-cell adhesion in the presence of eptifibatide. We found that platelets exposed to OvCa cells with low SUSD2 expression display increased tumor cell-platelet binding along with an increase in GPIIb/IIIa receptor activation. As such, platelet activation and binding to HGSOC cells was inversely correlated with the presence of SUSD2. This represents one of the first tumor proteins known to provide differential platelet interaction based on protein status.

Role of platelets and platelet receptors in cancer metastasis

The interaction of tumor cells with platelets is a prerequisite for successful hematogenous metastatic dissemination. Upon tumor cell arrival in the blood, tumor cells immediately activate platelets to form a permissive microenvironment. Platelets protect tumor cells from shear forces and assault of NK cells, recruit myeloid cells by secretion of chemokines, and mediate an arrest of the tumor cell platelet embolus at the vascular wall. Subsequently, platelet-derived growth factors confer a mesenchymal-like phenotype to tumor cells and open the capillary endothelium to expedite extravasation in distant organs. Finally, platelet-secreted growth factors stimulate tumor cell proliferation to micrometastatic foci. This review provides a synopsis on the current literature on platelet-mediated effects in cancer metastasis and particularly focuses on platelet adhesion receptors and their role in metastasis. Immunoreceptor tyrosine-based activation motif (ITAM) and hemi ITAM (hemITAM) comprising receptors, especially, glycoprotein VI (GPVI), FcγRIIa, and C-type lectin-like-2 receptor (CLEC-2) are turned in the spotlight since several new mechanisms and contributions to metastasis have been attributed to this family of platelet receptors in the last years.

Platelet-targeted pharmacologic treatments as anti-cancer therapy

Platelets act as multifunctional cells participating in immune response, inflammation, allergy, tissue regeneration, and lymphoangiogenesis. Among the best-established aspects of a role of platelets in non-hemostatic or thrombotic disorders, there is their participation in cancer invasion and metastasis. The interaction of many different cancer cells with platelets leads to platelet activation, and on the other hand platelet activation is strongly instrumental to the pro-carcinogenic and pro-metastatic activities of platelets. It is thus obvious that over the last years a lot of interest has focused on the possible chemopreventive effect of platelet-targeted pharmacologic treatments. This article gives an overview of the platelet-targeted pharmacologic approaches that have been attempted in the prevention of cancer development, progression, and metastasis, including the application of anti-platelet drugs currently used for cardiovascular disease and of new and novel pharmacologic strategies. Despite the fact that very promising results have been obtained with some of these approaches in pre-clinical models, with the exclusion of aspirin, clinical evidence of a beneficial effect of anti-platelet agents in cancer is however still largely missing. Future studies with platelet-targeted drugs in cancer must carefully deal with design issues, and in particular with the careful selection of patients, and/or explore novel platelet targets in order to provide a solution to the critical issue of the risk/benefit profile of long-term anti-platelet therapy in the prevention of cancer progression and dissemination.

Antiplatelet agents for cancer treatment: a real perspective or just an echo from the past?

The association between coagulation and cancer development has been observed for centuries. However, the connection between inflammation and malignancy is also well-recognized. The plethora of evidence indicates that among multiple hemostasis components, platelets play major roles in cancer progression by providing surface and granular contents for several interactions as well as behaving like immune cells. Therefore, the anticancer potential of anti-platelet therapy has been intensively investigated for many years. Anti-platelet agents may prevent cancer, decrease tumor growth, and metastatic potential, as well as improve survival of cancer patients. On the other hand, there are suggestions that antiplatelet treatment may promote solid tumor development in a phenomenon described as "cancers follow bleeding." The controversies around antiplatelet agents justify insight into the subject to establish what, if any, role platelet-directed therapy has in the continuum of anticancer management.

Activated platelets inhibit hepatocellular carcinoma cell differentiation and promote tumor progression via platelet-tumor cell binding

Lack of differentiation in hepatocellular carcinoma (HCC) is associated with increased circulating platelet size. We measured platelet activation and plasma adenosine diphosphate (ADP) levels in HCC patients based on differentiation status. Local platelet accumulation and platelet-hepatoma cell binding were measured using immunohistochemistry (IHC) or flow cytometry. Using a xenograft assay in NON/SCID mice, we tested the effects of the anti-platelet drug clopidogrel on platelet activation, platelet infiltration, platelet-tumor cell binding and tumor cell differentiation. HCC patients with poor differentiation status displayed elevated platelet activation and higher ADP levels. Platelets accumulated within poorly differentiated tissues and localized at hepatoma cell membranes. Platelet-tumor cell binding was existed in carcinoma tissues, largely mediated by P-selectin on platelets. NOD/SCID mice with xenograft tumors also exhibited increased platelet activation and platelet-tumor cell binding. Clopidogrel therapy triggered hepatoma cell differentiation by attenuating platelet activation and platelet-tumor cell binding. TCF4 knockdown promoted HepG-2 cell differentiation and inhibited tumor formation, and TCF4 could be the potential downstream target for clopidogrel therapy.

Carpe low-dose aspirin: the new anti-cancer face of an old anti-platelet drug

Cancer metastasis is a dynamic process during which cancer cells separate from a primary tumor, migrate through the vessel wall into the bloodstream, and extravasate at distant sites to form secondary colonies. During this process, circulating tumor cells are subjected to shear stress forces from blood flow, and in contact with plasma proteins and blood cells of the immune and hemostatic system, including platelets. Many studies have shown an association between high platelet count and cancer metastasis, suggesting that platelets may play an occult role in tumorigenesis. This mini-review summarizes recent and emerging discoveries of mechanisms by which cancer cells activate platelets and the role of activated platelets in promoting tumor growth and metastasis. Moreover, the review discusses how aspirin has the potential for being clinically used as an adjuvant in cancer therapy.

Platelet Integrins in Tumor Metastasis: Do They Represent a Therapeutic Target?

Platelets are small anucleated cell fragments that ensure the arrest of bleeding after a vessel wall injury. They are also involved in non-hemostatic function such as development, immunity, inflammation, and in the hematogeneous phase of metastasis. While the role of platelets in tumor metastasis has been recognized for 60 years, the molecular mechanism underlying this process remains largely unclear. Platelets physically and functionally interact with various tumor cells through surface receptors including integrins. Platelets express five integrins at their surface, namely α2β1, α5β1, α6β1, αvβ3, and αIIbβ3, which bind preferentially to collagen, fibronectin, laminin, vitronectin, and fibrinogen, respectively. The main role of platelet integrins is to ensure platelet adhesion and aggregation at sites of vascular injury. Two of these, α6β1 and αIIbβ3, were proposed to participate in platelet–tumor cell interaction and in tumor metastasis. It has also been reported that pharmacological agents targeting both integrins efficiently reduce experimental metastasis, suggesting that platelet integrins may represent new anti-metastatic targets. This review focuses on the role of platelet integrins in tumor metastasis and discusses whether these receptors may represent new potential targets for novel anti-metastatic approaches.

Platelet GPIIb supports initial pulmonary retention but inhibits subsequent proliferation of melanoma cells during hematogenic metastasis

Platelets modulate the process of cancer metastasis. However, current knowledge on the direct interaction of platelets and tumor cells is mostly based on findings obtained in vitro. We addressed the role of the platelet fibrinogen receptor glycoprotein IIb (integrin αIIb) for experimental melanoma metastasis in vivo. Highly metastatic B16-D5 melanoma cells were injected intravenously into GPIIb-deficient (GPIIb^-/-) or wildtype (WT) mice. Acute accumulation of tumor cells in the pulmonary vasculature was assessed in real-time by confocal videofluorescence microscopy. Arrest of tumor cells was dramatically reduced in GPIIb^-/- mice as compared to WT. Importantly, we found that mainly multicellular aggregates accumulated in the pulmonary circulation of WT, instead B16-D5 aggregates were significantly smaller in GPIIb^-/- mice. While pulmonary arrest of melanoma was clearly dependent on GPIIb in this early phase of metastasis, we also addressed tumor progression 10 days after injection. Inversely, and unexpectedly, we found that melanoma metastasis was now increased in GPIIb^-/- mice. In contrast, GPIIb did not regulate local melanoma proliferation in a subcutaneous tumor model. Our data suggest that the platelet fibrinogen receptor has a differential role in the modulation of hematogenic melanoma metastasis. While platelets clearly support early steps in pulmonary metastasis via GPIIb-dependent formation of platelet-tumor-aggregates, at a later stage its absence is associated with an accelerated development of melanoma metastases.

Curbing tumorigenesis and malignant progression through the pharmacological control of the wound healing process

The prevention of cancer development and its progression is an urgent unmet medical need. Novel knowledge on the biology of cancer has evidenced that genetic changes occurring within cancer cells contribute, but are not sufficient, for tumor promotion and progression. The results of clinical studies and experimental animal models have suggested pursuing new avenues for the prevention of cancer development in the early stages, by using drugs that modulate platelet responses and those interfering with the synthesis and action of the mediators of inflammation. In fact, malignant tumors often develop at sites of chronic injury associated with platelet activation and chronic inflammation. In this review, we cover the evidence supporting this hypothesis and the rationale for the pharmacological treatment with antiplatelet agents, including low-dose aspirin, and antiinflammatory drugs to curb tumorigenesis and malignant progression. The evidence for a chemopreventive effect of low-dose aspirin against colorectal cancer (CRC) has been recently found appropriate by the U.S. Preventive Services Task Force, which recommends the use of the drug for primary prevention of cardiovascular disease and CRC.

The angiogenic responses induced by release of angiogenic proteins from tumor cell-activated platelets are regulated by distinct molecular pathways

There is mounting evidence that tumor angiogenesis can be regulated by platelets (Plts), which serve as major sources and delivery vehicles of many proangiogenic cytokines including transforming growth factor-β and vascular endothelial growth factor. Although considerable progress has been made in understanding the role for Plt secretion in tumor angiogenesis, very little is known about the precise mechanisms underlying cancer cell induction of Plt granule release. Here, we demonstrated that nonsmall cell lung cancer (NSCLC) cells directly induced Plt secretion of several angiogenic regulatory cytokines that promoted angiogenesis in concert. Moreover, we discovered that these Plt-derived angiogenesis modulators were regulated by different molecular pathways and could be largely inhibited by combination of multiple signaling inhibitors. Our present studies indicated that manipulation of Plt secretion of angiogenic cytokines without compromising hemostatic functions could provide a novel option for management of tumor angiogenesis and metastasis in NSCLC patients with thrombocytosis.

The prothrombotic activity of cancer cells in the circulation

The hemostatic system is often subverted in patients with cancer, resulting in life-threatening venous thrombotic events. Despite the multifactorial and complex etiology of cancer-associated thrombosis, changes in the expression and activity of cancer-derived tissue factor (TF) - the principle initiator of the coagulation cascade - are considered key to malignant hypercoagulopathy and to the pathophysiology of thrombosis. However, many of the molecular and cellular mechanisms coupling the hemostatic degeneration to malignancy remain largely uncharacterized. In this review we discuss some of the tumor-intrinsic and tumor-extrinsic mechanisms that may contribute to the prothrombotic state of cancer, and we bring into focus the potential for circulating tumor cells (CTCs) in advancing our understanding of the field. We also summarize the current status of anti-coagulant therapy for the treatment of thrombosis in patients with cancer.

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.

The functional role of platelets in the regulation of angiogenesis

Functionally, platelets are primarily recognized as key regulators of thrombosis and hemostasis. Upon vessel injury, the typically quiescent platelet interacts with subendothelial matrix to regulate platelet adhesion, activation and aggregation, with subsequent induction of the coagulation cascade forming a thrombus. Recently, however, newly described roles for platelets in the regulation of angiogenesis have emerged. Platelets possess an armory of pro- and anti-angiogenic proteins, which are actively sequestered and highly organized in α-granule populations. Platelet activation facilitates their release, eliciting potent angiogenic responses through mechanisms that appear to be tightly regulated. In conjunction, the release of platelet-derived phospholipids and microparticles has also earned merit as synergistic regulators of angiogenesis. Consequently, platelets have been functionally implicated in a range of angiogenesis-dependent processes, including physiological roles in wound healing, vascular development and blood/lymphatic vessel separation, whilst facilitating aberrant angiogenesis in a range of diseases including cancer, atherosclerosis and diabetic retinopathy. Whilst the underlying mechanisms are only starting to be elucidated, significant insights have been established, suggesting that platelets represent a promising therapeutic strategy in diseases requiring angiogenic modulation. Moreover, anti-platelet therapies targeting thrombotic complications also exert protective effects in disorders characterized by persistent angiogenesis.

Pharmacological regulation of platelet factors that influence tumor angiogenesis

In addition to maintaining hemostasis, platelets play an important pathological role driving tumor growth and metastasis. One mechanism by which platelets contribute to tumor growth and metastasis is their potent promotion of angiogenesis. This is accomplished in large part by the numerous factors stored, generated, and released by platelets that have the potential to influence every stage of angiogenesis. In this review, we provide an overview of the many platelet-secreted pro- and anti-angiogenic factors. We examine the basic science and clinical evidence supporting their contributions to tumor angiogenesis. Finally, we review the pharmacological regulation of their release from platelets and discuss the potential of anti-platelet drugs as adjuvant anti-angiogenesis therapy.

Platelets, selectins, and the control of tumor metastasis

The significant role of platelets and P-selectin in assisting tumor cell metastasis to the lungs has been frequently reported and reviewed. However, evidence recently has come to light on other pro-metastatic mechanisms of platelets beyond that of tumor cell protection from immune cell attack and aiding extravasation, such as promoting epithelial to mesenchymal transition in tumor cells and conveying signals from the primary tumor to distant tissues that optimize conditions for metastasis. Moreover, the role of platelets and selectins in hematogenous metastasis to frequently targeted organs other than the lungs has been less well examined. This review aims to summarize the literature on the roles of platelets in all stages of the metastatic process and to examine the participation of platelets and selectins in hematogenous metastasis to the lungs, liver, bone, and brain. In the light of the available evidence, potential therapeutic avenues for the control of metastasis are also discussed.

Strategies to inhibit tumor associated integrin receptors: rationale for dual and multi-antagonists

The integrins are a family of 24 heterodimeric transmembrane cell surface receptors. Involvement in cell attachment to the extracellular matrix, motility, and proliferation identifies integrins as therapeutic targets in cancer and associated conditions: thrombosis, angiogenesis, and osteoporosis. The most reported strategy for drug development is synthesis of an agent that is highly selective for a single integrin receptor. However, the ability of cancer cells to change their integrin repertoire in response to drug treatment renders this approach vulnerable to the development of resistance and paradoxical promotion of tumor growth. Here, we review progress toward development of antagonists targeting two or more members of the Arg-Gly-Asp (RGD) binding integrins, notably αvβ3, αvβ5, αvβ6, αvβ8, α5β1, and αIIbβ3, as anticancer therapeutics.

A novel and essential role for FcγRIIa in cancer cell-induced platelet activation

Platelets play a role in cancer by acting as a dynamic reservoir of effectors that facilitate tumor vascularization, growth, and metastasis. However, little information is available about the mechanism of tumor cell-induced platelet secretion (TCIPS) or the molecular machinery by which effector molecules are released from platelets. Here we demonstrate that tumor cells directly induce platelet secretion. Preincubation of platelets with human colon cancer (Caco-2), prostate cancer (PC3M-luc), or breast cancer cells (MDA-MB-231;MCF-7) resulted in a marked dose-dependent secretion of dense granules. Importantly, TCIPS preceded aggregation which always displayed a characteristic lag time. We investigated the role of platelet receptors and downstream molecules in TCIPS. The most potent modulators of TCIPS were the pharmacologic antagonists of Syk kinase, phospholipase C and protein kinase C, all downstream mediators of the immunoreceptor tyrosine-based activation motif (ITAM) cascade in platelets. Supporting this, we demonstrated a central role for the immune Fcγ receptor IIa (FcγRIIa) in mediating platelet-tumor cell cross-talk. In conclusion, we demonstrate that cancer cells can promote platelet dense-granule secretion, which is required to augment platelet aggregation. In addition, we show a novel essential role for FcγRIIa in prostate cancer cell-induced platelet activation opening the opportunity to develop novel antimetastatic therapies.

Temporal and pharmacological characterization of angiostatin release and generation by human platelets: implications for endothelial cell migration

Platelets play an important role in thrombosis and in neo-vascularisation as they release and produce factors that both promote and suppress angiogenesis. Amongst these factors is the angiogenesis inhibitor angiostatin, which is released during thrombus formation. The impact of anti-thrombotic agents and the kinetics of platelet angiostatin release are unknown. Hence, our objectives were to characterize platelet angiostatin release temporally and pharmacologically and to determine how angiostatin release influences endothelial cell migration, an early stage of angiogenesis. We hypothesized anti-platelet agents would suppress angiostatin release but not generation by platelets. Human platelets were aggregated and temporal angiostatin release was compared to vascular endothelial growth factor (VEGF). Immuno-gold electron microscopy and immunofluorescence microscopy identified α-granules as storage organelles of platelet angiostatin. Acetylsalicylic acid, MRS2395, GPIIb/IIIa blocking peptide, and aprotinin were used to characterize platelet angiostatin release and generation. An endothelial cell migration assay was performed under hypoxic conditions to determine the effects of pharmacological platelet and angiostatin inhibition. Compared to VEGF, angiostatin generation and release from α-granules occurred later temporally during platelet aggregation. Consequently, collagen-activated platelet releasates stimulated endothelial cell migration more potently than maximally-aggregated platelets. Platelet inhibitors prostacyclin, S-nitroso-glutathione, acetylsalicylic acid, and GPIIb/IIIa blocking peptide, but not a P2Y12 inhibitor, suppressed angiostatin release but not generation. Suppression of angiostatin generation in the presence of acetylsalicylic acid enhanced platelet-stimulated endothelial migration. Hence, the temporal and pharmacological modulation of platelet angiostatin release may have significant consequences for neo-vascularization following thrombus formation.

A humanized single-chain antibody against beta 3 integrin inhibits pulmonary metastasis by preferentially fragmenting activated platelets in the tumor microenvironment

Platelets play a supportive role in tumor metastasis. Impairment of platelet function within the tumor microenvironment may provide a clinically useful approach to inhibit metastasis. We developed a novel humanized single-chain antibody (scFv Ab) against integrin GPIIIa49-66 (named A11) capable of lysing activated platelets. In this study, we investigate the effect of A11 on the development of pulmonary metastases. In the Lewis lung carcinoma (LLC) metastatic model, A11 decreases the mean number of surface nodules and mean volume of pulmonary nodules. It protects against lung metastases in a time window that extended 4 hours before and 4 hours after the IV injection of LLCs. Coinjection of GPIIIa49-66 albumin reverses the antimetastatic activity of A11 in the B16 melanoma model, consistent with the pathophysiologic relevance of the platelet GPIIIa49-66 epitope. Significantly, A11 had no effect on angiogenesis using both in vitro and in vivo assays. The underlying molecular mechanisms are a combination of inhibition of each of the following interactions: between activated platelets and tumor cells, platelets and endothelial cells, and platelets and monocytes, as well as disaggregation of an existing platelet/tumor thrombus. Our observations may provide a novel antimetastatic strategy through lysing activated platelets in the tumor microenvironment using humanized anti-GPIIIa49-66 scFv Ab.

Platelet-associated angiogenesis regulating factors: a pharmacological perspective

Platelets, in addition to maintaining hemostasis, also stimulate angiogenesis by generating and releasing, upon activation, factors that promote the growth of new blood vessels. To date, at least 20 angiogenesis-regulating factors have been identified in platelets, including both promoters and inhibitors. Platelet-derived angiogenesis regulators promote angiogenesis during wound healing, tumor growth, and in response to ischemia. Within platelets, angiogenesis regulators are primarily stored in α-granules, but are also found in the cytosol or derived from membrane lipids. Their release can be inhibited pharmacologically by anti-platelet agents, which consequently suppress platelet-stimulated angiogenesis. Several years ago, our research group discovered that platelets generate the angiogenesis inhibitor angiostatin independent of the activation state of platelets, and that platelet-derived angiostatin serves to limit the angiogenesis-stimulating effects of platelets. In this review, we summarize the current knowledge of platelet-associated angiogenesis regulators, how they impact angiogenesis, and how they are controlled pharmacologically.

Effect of statins and anticoagulants on prostate cancer aggressiveness

PURPOSE

Statins and anticoagulants (ACs) have both been associated with a less-aggressive prostate cancer (PCa) and a better outcome after treatment of localized PCa. The results of these studies might have been confounded because patients might often take both medications. We examined their respective influence on PCa aggressiveness at initial diagnosis.

MATERIALS AND METHODS

We analyzed 381 patients treated with either external beam radiotherapy or brachytherapy for low-risk (n = 152), intermediate-risk (n = 142), or high-risk (n = 87) localized PCa. Univariate and multivariate logistic regression analyses were used to investigate an association between these drug classes and prostate cancer aggressiveness. We tested whether the concomitant use of statins and ACs had a different effect than that of either AC or statin use alone.

RESULTS

Of the 381 patients, 172 (45.1%) were taking statins and 141 (37.0%) ACs; 105 patients (27.6%) used both. On univariate analysis, the statin and AC users were associated with the prostate-specific antigen (PSA) level (p = .017) and National Comprehensive Cancer Network risk group (p = .0022). On multivariate analysis, statin use was associated with a PSA level <10 ng/mL (odds ratio, 2.9; 95% confidence interval, 1.3-6.8; p = .012) and a PSA level >20 ng/mL (odds ratio, 0.29; 95% confidence interval, 0.08-0.83; p = .03). The use of ACs was associated with a PSA level >20 ng/mL (odds ratio, 0.13; 95% confidence interval, 0.02-0.59, p = .02).

CONCLUSION

Both AC and statins have an effect on PCa aggressiveness, with statins having a more stringent relationship with the PSA level, highlighting the importance of considering statin use in studies of PCa aggressiveness.

The platelet contribution to cancer progression

Traditionally viewed as major cellular components in hemostasis and thrombosis, the contribution of platelets to the progression of cancer is an emerging area of research interest. Complex interactions between tumor cells and circulating platelets play an important role in cancer growth and dissemination, and a growing body of evidence supports a role for physiologic platelet receptors and platelet agonists in cancer metastases and angiogenesis. Platelets provide a procoagulant surface facilitating amplification of cancer-related coagulation, and can be recruited to shroud tumor cells, thereby shielding them from immune responses, and facilitate cancer growth and dissemination. Experimental blockade of key platelet receptors, such as GP1b/IX/V, GPIIbIIIa and GPVI, has been shown to attenuate metastases. Platelets are also recognized as dynamic reservoirs of proangiogenic and anti-angiogenic proteins that can be manipulated pharmacologically. A bidirectional relationship between platelets and tumors is also seen, with evidence of 'tumor conditioning' of platelets. The platelet as a reporter of malignancy and a targeted delivery system for anticancer therapy has also been proposed. The development of platelet inhibitors that influence malignancy progression and clinical testing of currently available antiplatelet drugs represents a promising area of targeted cancer therapy.

Platelet adhesion and fusion to endothelial cell facilitate the metastasis of tumor cell in hypoxia-reoxygenation condition

To investigate the relevant molecular mechanisms of platelet in promoting metastasis of tumor cell. The adhesion of fluorescence dye labeled-platelet to human liver sinusoidal endothelial cell (LSEC) line and tumor cell lines were detected by fluorescence microscope and fluorescence plate reader or laser scanning confocal microscope. The relevant adhesion molecules were analyzed by the antibody blockage experiment. The immune colloidal gold transmission electron microscope (TEM), flow cytometry and dye transfer were used to decipher the adhesion and fusion of platelet and LSEC. The tumor cells adhesion to vessels in ischemia condition was analyzed on mouse mesenteric vessels and the metastasis and neovascularization of metastatic foci in pulmonary tissue were also detected after tumor cells injected into nude mice via tail veil. After hypoxia-reoxygenation, tumor cell or LSEC markedly increased its adhesion with platelet, which could be blocked by different antibodies to platelet adhesion molecules. Platelet increased adhesion of tumor cell to LSEC in dose-dependent manner. The fusion of platelet and LSEC was demonstrated by translocation of fluorescent dye from platelet into the adherent LSEC; gpIIb emerged on the LSEC; and confirmed by TEM. The morphological examination found platelet presented between tumor cell and LSEC. Animal experiment indicated that the tumor adhesion to vessels was seldom in normal condition, but increased in ischemia-reperfusion condition, and further significantly enhanced by platelets. The number of tumor metastatic foci and the density of blood vessels within metastatic foci in lung were markedly increased by tumor cell pre-adhered with platelet. The adhesion or fusion of platelet to endothelial cell mediated by platelet surface adhesion molecules, which could promote the adhesion of tumor cell with endothelial cells and the tumor metastasis.

Serum VEGF levels and tissue activation of VEGFR2/KDR receptors in patients with breast and gynecologic cancer

OBJECTIVES

Vascular endothelial cell growth factor (VEGF) plays an important role in the biology of gynecological cancer, usually linked with aggressive tumour behaviour and a poor postoperative outcome. Yet, its role in benign breast/gynecological conditions is less clear.

METHODS

Serum VEGF was analysed in a series of 49 patients with gynecological cancer and 61 patients with benign disease and compared to those of 12 normal female subjects. In addition, the activation status of VEGFR2/KDR receptors was investigated in formalin-fixed paraffin embedded tissues and related to VEGF.

RESULTS

Mean serum levels of VEGF were significantly higher in patients with breast, endometrial and ovarian cancer compared to healthy controls and those with benign breast/gynecologic disease in the respective organs. A similar trend was noted in some cases of simple endometrial hyperplasia, fibroadenoma and fibrocystic disease of the breast. The expression of phosphorylated VEGFR2/KDR receptors was higher in breast, endometrial, ovarian cancer in patients with high VEGF serum levels and this reached a level of statistical significance when all malignancies were combined.

CONCLUSIONS

Serum VEGF levels are increased in patients with breast and gynecological malignancies, but this can not be considered pathognomonic for cancer as it is also increased in certain benign conditions, including cases of fibroadenoma, fibrocystic disease of breast and simple endometrial hyperplasia. Furthermore, high serum VEGF levels are closely related to the activation status of the VEGFR2/KDR receptor in cancer cells, indicating a stimulatory effect of serum VEGF on the VEGF pathway contributing to tumor progression.

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.

The use of anticoagulants improves biochemical control of localized prostate cancer treated with radiotherapy

BACKGROUND

: Substantial experimental evidence suggests that anticoagulants (ACs) may inhibit cancer growth and metastasis, although the limited data from clinical trials have been inconsistent. The potential antineoplastic effect of ACs was investigated in patients who received radiotherapy for localized prostate cancer.

METHODS

: The study cohort consisted of 662 patients with adenocarcinoma of the prostate who received radiotherapy (RT) with curative intent. Among those 622 men, 243 (37%) were receiving ACs (warfarin, clopidogrel, and/or aspirin). All patients received external-beam RT, permanent seed implantation, or a combination of both. Prostate-specific antigen (PSA) values were monitored for biochemical control of disease.

RESULTS

: At a median follow-up of 49 months, the biochemical control rate at 4-years was significantly better in patients who received ACs at 91% compared with 78% in patients who did not receive ACs (P = .0002). The distant metastasis rate at 4 years also was reduced in the AC group compared with the non-AC group (1% vs 5%; P = .0248). In subgroup analysis, the improvement in biochemical control was significant only for patients with high-risk disease. Along with Gleason score, T classification, and initial PSA, the use of AC therapy was associated independently with improved biochemical control in multivariate analysis.

CONCLUSIONS

: AC therapy was associated with an improvement in biochemical control in patients with prostate cancer who received RT with curative intent. The effect was most prominent in patients who had high-risk disease. Cancer 2010. (c) 2010 American Cancer Society.

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.

Assessment of anti-metastatic effects of anticoagulant and antiplatelet agents using animal models of experimental lung metastasis

It is well established that the blood coagulation system is activated in cancer. In addition, there is considerable evidence to suggest that clotting activation plays an important role in the biology of malignant tumors, including the process of blood-borne metastasis. For many years our laboratory has used experimental models of lung metastasis to study the events that follow the introduction of procoagulant-bearing tumor cells into circulating blood. This chapter focuses on the basic methods involved in assessing the anti-metastatic effects of anticoagulants and anti-platelet agents using rodent models of experimental metastasis. In addition, it summarizes our experience with these models, which collectively suggests that intravascular coagulation and platelet activation are a necessary prelude to lung tumor formation and that interruption of coagulation pathways or platelet aggregation may be an effective anti-metastatic strategy.

Impact of venous thromboembolism and anticoagulation on cancer and cancer survival

Changes in the hemostatic system and chronic hemostatic activation are frequently observed in patients with cancer, even in the absence of venous thromboembolism (VTE). VTE is a leading cause of death among patients with cancer and contributes to long-term mortality in patients with early as well as advanced-stage cancer. Mounting evidence suggests that components of the clotting cascade and associated vascular factors play an integral part in tumor progression, invasion, angiogenesis, and metastasis formation. Furthermore, there are intriguing in vitro and animal findings that anticoagulants, in particular the low molecular weight heparins (LMWHs), exert an antineoplastic effect through multiple mechanisms, including interference with tumor cell adhesion, invasion, metastasis formation, angiogenesis, and the immune system. Several relatively small randomized controlled clinical trials of anticoagulation as cancer therapy in patients without a VTE diagnosis have been completed. These comprise studies with LMWH, unfractionated heparin, and vitamin K antagonists, with overall encouraging but nonconclusive results and some limitations. Meta-analyses performed for the American Society of Clinical Oncology VTE Guidelines Committee and the Cochrane Collaboration suggest overall favorable effects of anticoagulation on survival of patients with cancer, mainly with LMWH. However, definitive clinical trials have been elusive and questions remain regarding the importance of tumor type and stage on treatment efficacy, the impact of fatal thromboembolic events, optimal anticoagulation therapy, and safety with differing chemotherapy regimens. Although the LMWHs and related agents hold promise for improving outcomes in patients with cancer, additional studies of their efficacy and safety in this setting are needed.

Antitumor and antimetastatic effect of warfarin and heparins

Experimental and clinical studies have shown an anticancer effect of anticoagulant drugs. The aim of this study is to review the mechanisms by which the common types of anticoagulants influence the primary tumor and metastatic processes of solid tumors. The review evaluates the interference of unfractionated heparin (UFH), low molecular weight heparin (LMWH) and warfarin on the growth of primary tumors and on the development of metastases. The first part of the review evaluates the effect on the growth and development of primary tumors. Attention is paid to the interference with proliferation of cancer cells, tumor angiogenesis and to the interference with the immune system. The second part of the review describes the metastatic process and the effect of anticoagulants on the cell motility and cancer cell adhesion. The third part refers to the outcomes of clinical studies with anticoagulant treatment in patients with cancer. The problem of thromboembolic disease in patients with advanced cancer is also mentioned. The anticoagulants are more effective in inhibition of stages of the metastatic cascade than in the influence on primary tumors. They can interfere with tumor angiogenesis, immunity system, cancer cell motility and adhesion. The first clinical trials showed an effect on the development of primary tumors and survival of patients namely with lung cancer.

Platelet and osteoclast beta3 integrins are critical for bone metastasis

Mice with a targeted deletion of beta3 integrin were used to examine the process by which tumor cells metastasize and destroy bone. Injection of B16 melanoma cells into the left cardiac ventricle resulted in osteolytic bone metastasis in 74% of beta3+/+ mice by 14 days. In contrast, only 4% of beta3-/- mice developed bone lesions. Direct intratibial inoculation of tumor resulted in marrow replacement by tumor in beta3-/- mice, but no associated trabecular bone resorption as seen inbeta3+/+ mice. Bone marrow transplantation studies showed that susceptibility to bone metastasis was conferred by a bone marrow-derived cell. To dissect the roles of osteoclast and platelet beta3 integrins in this model of bone metastasis, osteoclast-defective src-/- mice were used. Src-null mice were protected from tumor-associated bone destruction but were not protected from tumor cell metastasis to bone. In contrast, a highly specific platelet aggregation inhibitor of activated alphaIIbbeta3 prevented B16 metastases. These data demonstrate a critical role for platelet alphaIIbbeta3 in tumor entry into bone and suggest a mechanism by which antiplatelet therapy may be beneficial in preventing the metastasis of solid tumors.

Potential future clinical applications for the GPIIb/IIIa antagonist, abciximab in thrombosis, vascular and oncological indications

Abciximab (ReoPro) is a mouse-human chimeric monoclonal antibody Fab fragment of the parent murine monoclonal antibody 7E3, and was the first of these agents approved for use as adjunct therapy for the prevention of cardiac ischemic complications in patients undergoing percutaneous coronary intervention (PCI). Abciximab binds with high avidity to both the non-activated and activated form of the GPIIb/IIIa receptor of platelets, the major adhesion receptor involved in aggregation. Additional cardiovascular indications for abciximab are unstable angina, carotid stenting, ischemic stroke and peripheral vascular diseases. Abciximab also interacts with two other integrin receptors; the a av b b3 receptor, which is present in low numbers on platelets but in high density on activated endothelial and smooth muscle cells, and a aMb b2 integrin which is present on activated leukocytes. Cell types that express integrins GPIIb/IIIa and a av b b3 such as platelets, endothelial and tumor cells have been implicated in angiogenesis, tumor growth and metastasis. Since abciximab interacts with high avidity to integrins GPIIb/IIIa and a av b b3, it is reasonable to assume that it may possess anti-angiogenic properties in angiogenesis-related diseases, as well as anti-metastastatic properties in case of disseminating tumors expressing the target integrin receptors.