Is there a role for the tumor cell integrin alpha IIb beta 3 and cytoskeleton in tumor cell-platelet interaction?

Platelets, immune cells and the coagulation cascade; friend or foe of the circulating tumour cell?

Cancer cells that transit from primary tumours into the circulatory system are known as circulating tumour cells (CTCs). These cancer cells have unique phenotypic and genotypic characteristics which allow them to survive within the circulation, subsequently extravasate and metastasise. CTCs have emerged as a useful diagnostic tool using "liquid biopsies" to report on the metastatic potential of cancers. However, CTCs by their nature interact with components of the blood circulatory system on a constant basis, influencing both their physical and morphological characteristics as well as metastatic capabilities. These properties and the associated molecular profile may provide critical diagnostic and prognostic capabilities in the clinic. Platelets interact with CTCs within minutes of their dissemination and are crucial in the formation of the initial metastatic niche. Platelets and coagulation proteins also alter the fate of a CTC by influencing EMT, promoting pro-survival signalling and aiding in evading immune cell destruction. CTCs have the capacity to directly hijack immune cells and utilise them to aid in CTC metastatic seeding processes. The disruption of CTC clusters may also offer a strategy for the treatment of advance staged cancers. Therapeutic disruption of these heterotypical interactions as well as direct CTC targeting hold great promise, especially with the advent of new immunotherapies and personalised medicines. Understanding the molecular role that platelets, immune cells and the coagulation cascade play in CTC biology will allow us to identify and characterise the most clinically relevant CTCs from patients. This will subsequently advance the clinical utility of CTCs in cancer diagnosis/prognosis.

Platelet "first responders" in wound response, cancer, and metastasis

Platelets serve as "first responders" during normal wounding and homeostasis. Arising from bone marrow stem cell lineage megakaryocytes, anucleate platelets can influence inflammation and immune regulation. Biophysically, platelets are optimized due to size and discoid morphology to distribute near vessel walls, monitor vascular integrity, and initiate quick responses to vascular lesions. Adhesion receptors linked to a highly reactive filopodia-generating cytoskeleton maximizes their vascular surface contact allowing rapid response capabilities. Functionally, platelets normally initiate rapid clotting, vasoconstriction, inflammation, and wound biology that leads to sterilization, tissue repair, and resolution. Platelets also are among the first to sense, phagocytize, decorate, or react to pathogens in the circulation. These platelet first responder properties are commandeered during chronic inflammation, cancer progression, and metastasis. Leaky or inflammatory reaction blood vessel genesis during carcinogenesis provides opportunities for platelet invasion into tumors. Cancer is thought of as a non-healing or chronic wound that can be actively aided by platelet mitogenic properties to stimulate tumor growth. This growth ultimately outstrips circulatory support leads to angiogenesis and intravasation of tumor cells into the blood stream. Circulating tumor cells reengage additional platelets, which facilitates tumor cell adhesion, arrest and extravasation, and metastasis. This process, along with the hypercoagulable states associated with malignancy, is amplified by IL6 production in tumors that stimulate liver thrombopoietin production and elevates circulating platelet numbers by thrombopoiesis in the bone marrow. These complex interactions and the "first responder" role of platelets during diverse physiologic stresses provide a useful therapeutic target that deserves further exploration.

Platelets and cancer: a casual or causal relationship: revisited

Human platelets arise as subcellular fragments of megakaryocytes in bone marrow. The physiologic demand, presence of disease such as cancer, or drug effects can regulate the production circulating platelets. Platelet biology is essential to hemostasis, vascular integrity, angiogenesis, inflammation, innate immunity, wound healing, and cancer biology. The most critical biological platelet response is serving as "First Responders" during the wounding process. The exposure of extracellular matrix proteins and intracellular components occurs after wounding. Numerous platelet receptors recognize matrix proteins that trigger platelet activation, adhesion, aggregation, and stabilization. Once activated, platelets change shape and degranulate to release growth factors and bioactive lipids into the blood stream. This cyclic process recruits and aggregates platelets along with thrombogenesis. This process facilitates wound closure or can recognize circulating pathologic bodies. Cancer cell entry into the blood stream triggers platelet-mediated recognition and is amplified by cell surface receptors, cellular products, extracellular factors, and immune cells. In some cases, these interactions suppress immune recognition and elimination of cancer cells or promote arrest at the endothelium, or entrapment in the microvasculature, and survival. This supports survival and spread of cancer cells and the establishment of secondary lesions to serve as important targets for prevention and therapy.

Shear stress induced release of von Willebrand factor and thrombospondin-1 in HUVEC extracellular matrix enhances breast tumour cell adhesion

Endothelial cells in vivo are exposed to blood shear forces and flow perturbations induce their activation. Such modifications of hemodynamic can be observed in patients with cancer. We have submitted endothelial cells (HUVEC) to shear stress (13 dynes/cm(2)) and isolated their extracellular matrix (ECM) prior perfusion with breast adenocarcinoma cells (MDA-MB-231) in whole blood at a shear rate of 1500 s(-1). Exposure of HUVEC to 13 dynes/cm(2) (tau(13)) for 2 h enhanced the secretion of von Willebrand factor (vWF) and thrombospondin-1 (TSP-1) in the ECM. Moreover, MDA-MB-231 cell adhesion was enhanced to such treated-ECM. This over-adhesion was inhibited by pre-incubating the ECM with anti-vWF or anti-TSP-1 antibodies, or by blocking tumour cell alpha(v)beta(3) integrin. Although blood platelets were involved in tumour cell adhesion to ECM, blockade of platelet GPIb or alpha(IIb)beta(3) receptors did not specifically inhibit the enhanced tumour cell adhesion observed on tau(13). ECM. These findings indicate that shear stress can modulate the expression of adhesive proteins in ECM, which favours direct tumour cell adhesion via alpha(v)beta(3) and other receptors.

Platelet-Mimicry of Cancer Cells: Epiphenomenon with Clinical Significance

Stem cell mimicry of cancer cells has been known for a long time and is considered to be responsible for ectopic gene expressions. The stem cell characteristics of tumor cells are shown to be involved in epithelial-mesenchymal transition and in the phenomenon of vascular mimicry. Certain cancer types acquire a geno-phenotype closely resembling the platelets and express several megakaryocytic genes (adhesion receptors alpha IIb beta 3, thrombin receptor and PECAM/CD 31 and/or platelet-type 12-LOX) able to activate the coagulation cascade or the platelets themselves. Here we define these potentials as platelet mimicry of cancer cells typical of pancreatic, breast, prostate, colorectal and urogenital cancers and melanoma. Data all support that platelet mimicry of certain cancer types is an important factor in their hematogenous dissemination and provides an attractive therapeutic target. Besides the long-available preclinical data, clinical trials have only recently provided evidence that targeting platelet mimicry of cancers is an efficient way to prevent tumor progression. The systematic discovery of the markers of platelet mimicry in various cancer types and their molecular targeting may provide new supportive therapeutic modalities for the management of the progressing disease.

Breast adenocarcinoma cell adhesion to the vascular subendothelium in whole blood and under flow conditions: effects of alphavbeta3 and alphaIIbbeta3 antagonists

Tumour cell adhesion to vascular extracellular matrix (ECM), an important step of metastatic progression, is promoted by platelets. The aim of our study was to investigate, in whole blood under venous and arterial shear conditions, the respective role of tumour cell alphavbeta3 and platelet alphaIIbbeta3 integrins in MDA-MB-231 breast adenocarcinoma cell adhesion to human umbilical vein endothelial cell ECM. For that purpose, blood containing MDA-MB-231 cells was incubated with non-peptide antagonists specific for platelet alphaIIbbeta3 (lamifiban) or tumour cell alphavbeta3 (SB-273005). At 300 s(-1), each antagonist used alone did not modify tumour cell adhesion, whereas, at 1500 s(-1), tumour cell adhesion was decreased by 25% in presence of lamifiban indicating a role of platelet alphaIIbbeta3 at higher shear rate. However, a combination of SB-273005 and lamifiban, or c7E3 Fab (a potent inhibitor of both alphaIIbbeta3 and alphavbeta3) inhibited tumour cell adhesion by 40-45%, at either shear rate applied, indicating a cooperation between these two integrins in MDA-MB-231 cell adhesion to ECM, as well as the participation of other adhesive receptors on tumour cells and/or platelets. Thus, efficient anti-metastatic therapy should target multiple receptors on tumour cells and platelets.

von Willebrand factor expression in osteosarcoma metastasis

A number of genes are implicated in the initiation and progression of osteosarcoma; however, cytogenetic and comparative genomic hybridization studies indicate the involvement of additional unidentified genes. An examination of gene expression profiles in 22 high-grade osteosarcoma tumor specimens from 15 patients (including paired primary and metastatic samples from five patients) indicated that von Willebrand factor (vWF) mRNA expression may increase during tumor progression. vWF, a large glycoprotein previously considered to be expressed exclusively by endothelial cells and megakaryocytes, is involved in platelet aggregation and adhesion to the subendothelial matrix, processes critical to hematogenous tumor cell metastasis to the lung. Analysis of paired primary and metastatic osteosarcoma tumor samples from 10 patients revealed an increase in vWF gene expression in metastases (P=0.005). Immunohistochemistry showed that, in addition to the endothelial cells, vWF protein was also detected in osteosarcoma cells in vivo in 13 of 29 tumor specimens as well as in SAOS2, an osteosarcoma cell line. The tumor cell staining correlated positively with high vWF expression in the sample (P=0.006). Although vascular endothelial cells contribute to the vWF mRNA detected in the tumor samples, there was neither any correlation between vascular density (VD) and vWF mRNA expression nor between VD and clinical outcome. These findings suggest that vWF expression is deregulated in osteosarcoma tumors, potentially contributing to metastasis.

Beta1-integrin-mediated dynamic adhesion of colon carcinoma cells to extracellular matrix under laminar flow

To resist substantial wall shear stress exerted by blood flow metastasizing colon carcinoma cells have to form adhesive contacts with endothelial cells and subendothelial extracellular matrix (ECM). At secondary sites tumor cells have to stabilize these initial adhesive interactions to prevent detachment and recirculation. Previously we found that adhesion of colon carcinoma cells to ECM components under static conditions is mediated, in part, by various beta1-integrins. Since other malignant cells possess adhesive properties that are different under static and dynamic conditions, we analyzed human colon carcinoma cell adhesion under flow by decreasing the flow (wall shear stress, WSS) of cell suspensions and allowing cells to interact with collagen-coated surfaces in a laminar flow chamber. HT-29 colon carcinoma cells were used to study wall shear adhesion threshold (WSAT), dynamic adhesion rate (DAR) and adhesion stabilization rate (ASR). DAR was determined after a low flow period using a WSS set at 50% of WSAT. ASR was calculated 60 sec after reestablishment of high WSS. Glass slides were coated with collagen I (C I) or bovine serum albumin (BSA, negative control). In some experiments cells were pretreated with function-blocking anti-beta1 or nonspecific IgG. Rolling of cells occurred on C I- and BSA-coated surfaces at high WSS. By decreasing WSS cell sticking without definite adhesion was found, and cells stuck to BSA at WSS lower than that found for C I. Further decreasing WSS below WSAT enabled stable cell adhesion to C I, but only a few cells adhered to BSA. ASR was found to be 73% of primarily adherent cells (to C I). Pretreatment with anti-beta1 did not affect cell rolling but did inhibit cell sticking and adhesion completely, whereas nonspecific IgG was without effect. Activation of PKC using phorbol ester resulted in an increase of adhesive interactions under dynamic and static conditions, whereas its inhibition reduced adhesion. Adhesive interactions of HT-29 colon carcinoma cells with ECM-coated surfaces under laminar flow conditions occurred in various steps: (1) rolling, (2) sticking or initial adhesion, and (3) stabilization of adhesion. Under shear flow rolling of tumor cells on ECM-coated surfaces appeared to be mediated mainly by physical/mechanical and nonspecific surface-cell membrane interactions, whereas stabilized adhesion to ECM was specifically mediated by beta1-integrin binding to ECM components. PKC seems to be involved in the regulation of adhesion stabilization under static and flow conditions.

Expression and function of the high affinity alphaIIbbeta3 integrin in murine melanoma cells

In resting platelets integrin alphaIIbbeta3 is constitutively expressed in an inactive state and it does not recognize soluble proteins. Platelet activation results in a conformational change of the low-affinity alphaIIbbeta3 to a high-affinity state which then recognizes plasma fibrinogen. The ectopic expression of alphaIIbbeta3 integrin in rodent and human cells derived from solid tumors is well documented, although little is known about its affinity state in these tumor cells. In this study we analysed expression and function of high-affinity alphaIIbbeta3 in murine metastatic melanoma B16a cells by using a mAb that specifically recognizes high-affinity alphaIIbbeta3 (PAC-1). These tumor cells while in suspension bound PAC-1 and fibrinogen. Immunofluorescent studies of B16a cells indicated that high-affinity alphaIIbbeta3 is associated with the Golgi complex and the cell surface. Stimulation of B16a cells with a PKC-activator, 12(S)-HETE, induced translocation of the high-affinity integrin from an intracellular pool to the plasma membrane, which resulted in increased tumor cell adhesion to fibronectin. In addition to participating in 12(S)-HETE-stimulated adhesion of B16a cells, the high-affinity alphaIIbbeta3 integrin is also involved in tumor cell invasion through a reconstituted basement membrane. In conclusion, results from this study suggest that in non-megakaryocytic lineage B16a cells alphaIIbbeta3 is constitutively expressed in a high-affinity state, and that this conformation participates in tumor cell adhesion and invasion.

Characterization of platelet aggregation induced by the human carcinosarcoma Colo 526: role of platelet activation, tumor cell cytoskeleton and tumor cell plasma membrane

Tumor cell-platelet interactions have been shown to be involved in the process of metastasis. This study characterizes the aggregation of washed platelets induced by the human uterine carcinosarcoma Colo 526. Ultrastructural studies revealed a two-stage process in which the earliest events were the adhesion and degranulation of individual platelets in contact with the tumor cell membrane. The second stage consisted of a wave of aggregation involving all residual platelets. We found that the first stage was initiated by a factor integral to the tumor cell plasma membrane which acted independently of the tumor cell cytoskeleton or metabolic processes. This factor was found to be a glycoprotein or glycolipid with functionally important sialic acid and N-linked carbohydrate residues. The initial stage was not dependent on platelet activation as neither aspirin nor prostacyclin prevented adhesion or degranulation. The second stage was found to be dependent on platelet activation. These results suggest that platelet aggregation induced by Colo 526 involves a distinctive primary stage which is initiated by a factor on the tumor cell plasma membrane resulting in the degranulation and lysis of individual platelets. This process can occur independently of platelet activation or aggregation and thus may have some relevance to the clinical use of platelet antagonists as antimetastatic agents.

Immunoreactivity of glycoprotein IIb is present in metastasized but not in non-metastasized primary malignant melanoma

Glycoprotein IIb-IIIa (integrin alpha IIb beta3) is an adhesive receptor involved in platelet aggregation and adhesion to the extracellular matrix. Previous studies showed the presence of IIb-IIIa-like glycoproteins on cells of melanoma cell lines and on cells of lymph node metastases. This study evaluates the presence of glycoprotein IIb-IIIa subunits on cells of primary cutaneous malignant melanomas with (n = 4) and without (n = 9) metastases over a period of 6 years and on naevus cells (n = 4). Monoclonal antibodies directed against the subunits of the glycoprotein IIb-IIIa receptor were used on paraffin-embedded sections and evaluated by means of immunohistochemistry. The glycoprotein IIb subunit was exclusively present on cells of metastatic melanomas. It was not found on non-metastatic melanomas or benign melanocytes. These data favour the role of the integrin receptor glycoprotein IIb-IIIa in the metastatic behaviour of malignant melanomas.

Tumor cell-induced platelet aggregation in vitro by human pancreatic cancer cell lines

BACKGROUND

Tumor cell-induced platelet aggregation (TCIPA) is considered to be a critical step in hematogenous metastasis.

METHODS

TCIPA was studied in vitro in six human pancreatic carcinoma cell lines (PC 3, PC 44, AsPC1, BxPC3, Capan2, Panc1).

RESULTS

Whereas all cell lines induced aggregation of washed platelets in the presence of minimal amounts of platelet-poor plasma, five cell lines also induced aggregation of platelets in platelet-rich plasma. The thrombin-antagonist hirudin inhibited TCIPA in all cell lines indicating that TCIPA is thrombin-dependent. Since pretreatment of tumor cells with phospholipase A2 or C inhibited TCIPA, the thrombin-generating activity might be confined to the tumor cell surface. Further support for a prothrombinase activity was provided by the observation that all cell lines were able to induce the aggregation of washed platelets after addition of purified coagulation factors II and V.

CONCLUSIONS

Pancreatic carcinoma cells are able to induce platelet aggregation via activation of thrombin. This might support metastasis in pancreatic cancer and possibly explain the incidence of thrombosis in this tumor.

Immunomorphological characterization and effects of 12-(S)-HETE on a dynamic intracellular pool of the alpha IIb beta 3-integrin in melanoma cells

In metastatic B16a murine melanoma cells, alpha IIb beta 3 integrin was shown to be one of the key adhesion molecules responsible for matrix adhesion and spreading. Upon stimulation, alpha IIb beta 3 can be upregulated at the cell surface due to translocation of the receptor to the plasma membrane from an intracellular pool. Here we have characterized this integrin pool as a tubulovesicular structure (TVS) corresponding to endosomes. TVS was found to be associated temporarily with microtubules and intermediate filaments especially after protein kinase C (PKC) stimulation with a lipoxygenase metabolite of arachidonic acid, 12-(S)-hydroxyeicosatetraenoic acid [12-(S)-HETE]. After PKC stimulation, the predominantly vesicular TVS became elongated and alpha IIb beta 3 appeared at the apical plasma membrane and microvilli. Disruption of either the microtubules or intermediate filaments prevented the 12-(S)-HETE effect both on vesicular to tubular transition of TVS as well as on surface expression of this integrin. The connection with the Golgi system of the integrin-containing TVS was proved by a Golgi-inhibitor (brefeldin A) pretreatment, which prevented the PKC-stimulation-induced TVS elongation and subsequent receptor-upregulation at the cell surface. After a soluble ligand binding (mAb to the alpha IIb beta 3 complex) the surface receptor endocytosed back to the TVS indicating the presence of a dynamic, cytoskeleton associated integrin pool in melanoma cells.

12-lipoxygenases and 12(S)-HETE: role in cancer metastasis

Arachidonic acid metabolites have been implicated in multiple steps of carcinogenesis. Their role in tumor cell metastasis, the ultimate challenge for the treatment of cancer patients, are however not well-documented. Arachidonic acid is primarily metabolized through three pathways, i.e., cyclooxygenase, lipoxygenase, and P450-dependent monooxygenase. In this review we focus our attention on one specific lipoxygenase, i.e., 12-lipoxygenase, and its potential role in modulating the metastatic process. In mammalian cells there exist three types of 12-lipoxygenases which differ in tissue distribution, preferential substrates, and profile of their metabolites. Most of these 12-lipoxygenases have been cloned and sequenced, and the molecular and biochemical determinants responsible for catalysis of specific substrates characterized. Solid tumor cells express 12-lipoxygenase mRNA, possess 12-lipoxygenase protein, and biosynthesize 12(S)-HETE [12(S)-hydroxyeicosatetraenoic acid], as revealed by numerous experimental approaches. The ability of tumor cells to generate 12(S)-HETE is positively correlated to their metastatic potential. A large collection of experimental data suggest that 12(S)-HETE is a crucial intracellular signaling molecule that activates protein kinase C and mediates the biological functions of many growth factors and cytokines such as bFGF, PDGF, EGF, and AMF. 12(S)-HETE plays a pivotal role in multiple steps of the metastatic 'cascade' encompassing tumor cell-vasculature interactions, tumor cell motility, proteolysis, invasion, and angiogenesis. The fact that 12-lipoxygenase is expressed in a wide diversity of tumor cell lines and 12(S)-HETE is a key modulatory molecule in metastasis provides the rationale for targeting these molecules in anti-cancer and anti-metastasis therapeutic protocols.

The importance of blood cell-vessel wall interactions in tumour metastasis

Tumour cell dissemination is a complex process, depending on the ability of malignant cells to escape from the primary tumour and penetrate and flow through the bloodstream. Circulating tumour cells can adhere to the vessel wall, dissolve the basal lamina and extravasate, giving origin to metastases. Interactions between tumour cells, blood platelets and leukocytes favour tumour cell adhesion to the vessel wall, migration in extravascular spaces and growth in secondary sites. The biochemical and molecular mechanisms regulating tumour cell adhesion to the vessel wall and intercellular contacts have been studied extensively in recent years. Moreover, it has been shown that either tumour cells or blood cells release growth factors and inflammatory proteins, such as cytokines and chemokines, that may be involved in tumour cell migration and proliferation. Finally, tumour cells and cells of the surrounding tissue possess procoagulant and fibrinolytic properties that may be important in modulating the extracellular matrix around the tumour, to allow tumour cell invasion and progression. We have described the cell types (i.e. blood platelets, leukocytes, endothelial cells), the matrix components (i.e. fibronectin, thrombospondin and laminin) and the growth factors/cytokines (i.e. platelet-derived growth factor, transforming growth factor beta, tumour necrosis factor) involved in these processes. In particular, we have described cell-cell and cell-matrix interactions, cell migration and release of growth factors, cytokines, chemotactic peptides and proteolytic enzymes. This survey has also considered a few innovative approaches for the prevention and cure of cancer and metastasis that are based on these new concepts.

Integrins: cell adhesives and modulators of cell function

Integrins encompass a family of cell-surface molecules which play a crucial role in cell-cell and cell-extracellular matrix interaction. Of these heterodimeric transmembrane glycoproteins (consisting of an alpha and beta chain) as yet at least 20 different types have been described, all with a different pattern of reactivity with extracellular matrix components. In this review the cell and tissue distribution of the integrins is discussed, with special emphasis on immunohistochemical localization of the beta 1 integrins and the alpha 6 beta 4 integrin. The beta 1 integrins comprise a subfamily in which eight alpha chains combine with one beta (the beta 1) chain. The alpha 2 beta 1, alpha 3 beta 1 and alpha 6 beta 1 and the alpha 6 beta 4 integrins are expressed on a wide variety of epithelia on the basolateral surface or exclusively on the basal surface facing the basement membrane (e.g. alpha 6 beta 1 and alpha 6 beta 4). Leucocyte integrins, which share a common beta 2 chain, occur almost exclusively on white blood cells and their precursors. The vitronectin receptors, which share a common alpha v chain, occur in a wide variety of cell types. Integrins play a major role in the interaction of the cell with the extracellular matrix in order to create and maintain tissue architecture. It has become clear, however, that through integrin-ligand interaction cell function is also modulated. Furthermore, in pathological conditions integrins play a role of some significance. Integrins mediate leucocyte traffic in developing inflammatory processes and function in neoplastic growth when it comes to invasion and metastasis.

12(S)-HETE-induced microvascular endothelial cell retraction results from PKC-dependent rearrangement of cytoskeletal elements and alpha V beta 3 integrins

12(S)-HETE, a lipoxygenase metabolite of arachidonic acid, has been demonstrated to induce a reversible retraction of vascular endothelial cells (EC). 12(S)-HETE-induced microvascular EC retraction was blocked by a selective protein kinase C inhibitor, calphostin C, but not by the protein kinase A inhibitor, H8. EC exposed to 12(S)-HETE demonstrated a gradual dissolution of actin microfilaments and a decrease of vinculin-containing focal adhesions. The intermediate filaments, vimentin, also underwent extensive reorganization (i.e., filament bundling and enrichment to the cell filapodia) following 12(S)-HETE treatment. In vivo phosphorylation studies revealed that 12(S)-HETE induced a hyperphosphorylation of several major cytoskeletal proteins including myosin light chain, actin, and vimentin. The increased phosphorylation of these cytoskeletal proteins following 12(S)-HETE stimulation was abolished by calphostin C but not by H8. Confluent EC express alpha v beta 3 in focal adhesions at both the cell body and the cell-cell borders. 12(S)-HETE induced a sequential rearrangement of the alpha v beta 3-containing focal adhesions, resulting in a general decrease in alpha v beta 3 integrin receptors, especially in those retracted EC. 12(S)-HETE-induced rearrangement of alpha v beta 3 was inhibited by calphostin C but not by H8. In contrast to alpha v beta 3, confluent EC enrich alpha 5 beta 1 integrin receptors primarily at the cell-cell borders, colocalizing with extracellular fibronectin and cell cortical microfilaments. 12(S)-HETE treatment also disrupted the cell-border distribution pattern of alpha 5 beta 1 as EC retracted, but no distinct alterations (such as time-related redistribution and quantitative differences) in alpha 5 beta 1 were observed.

Fatty acid modulation of tumor cell-platelet-vessel wall interaction

Prostaglandins and other eicosanoids have been studied extensively in their physical, biochemical, biophysical and pharmacological aspects. However, studies on their role in tumor progression, especially metastases are relatively recent. Following a brief overview of the history of discovery and metabolism of eicosanoids and other fatty acids, we discuss the functions of these fatty acids (with emphasis on prostacyclin, thromboxane A2, 12-hydroxyeicosatetraenoic acid and 13-hydroxyoctadecadienoic acid) in cell transformation, tumor promotion and particularly in tumor cell metastasis. The relation between these monohydroxy fatty acids and tumor cell metastasis is discussed from three different perspectives, i.e., their effects on tumor cells, on platelets and on endothelial cells. The mechanism of these effects are then addressed at cell adhesion molecule, motility, protease, cell cytoskeleton, protein kinase and eicosanoid receptor levels. Finally, regulation of three key enzymes which generate eicosanoids (phospholipase, prostaglandin endoperoxide synthase and lipoxygenase) is explored.

Platelets and cancer metastasis: a causal relationship?

Cancer metastasis is a highly coordinated and dynamic multistep process in which cancer cells undergo extensive interactions with various host cells before they establish a secondary metastatic colony. Ample morphological studies have documented the close association of circulating tumor cells with host platelets. Several lines of evidence provide strong support for the concept that tumor cell-platelet interactions (i.e., TCIPA) significantly contribute to hematogenous metastasis. Clinically, cancer patients with advanced diseases are characterized by a variety of thromboembolic disorders including thrombocytosis. Pharmacologically, various anti-platelet agents/anticoagulants have demonstrated potent inhibitory effects on tumor cell-platelet interactions as well as spontaneous or experimental metastasis. Experimentally, interference with many of the intermediate steps of tumor cell-platelet interactions has resulted in diminished platelet aggregation induced by tumor cells and blocked cancer metastasis. Platelet interaction with tumor cells is a sequential process which involves two general types of mediators, i.e., membrane-bound molecules (adhesion molecules) and soluble release products. alpha IIb beta 3 integrin receptors present on both platelets as well as on tumor cells and 12(S)-HETE, a 12-lipoxygenase metabolite of arachidonic acid, are prototypical examples of each category. Mechanistically, platelets may contribute to metastasis by: (1) stabilizing tumor cell arrest in the vasculature, (2) stimulating tumor cell proliferation, (3) promoting tumor cells extravasation by potentiating tumor cell-induced endothelial cell retraction, and (4) enhancing tumor cell interaction with the extracellular matrix.

Adhesion molecules and tumor cell interaction with endothelium and subendothelial matrix

Cancer metastasis poses the greatest challenge to the eradication of malignancy. The majority of clinical and experimental evidence indicates that metastasis is a non-random, organ-specific process. Tumor cell interaction with endothelium and subendothelial matrix constitutes the most crucial factor in determining the organ preference of metastasis. A plethora of cell surface adhesion molecules, which encompass four major families (i.e., integrins, cadherins, immunoglobulins and selectins) and many other unclassified molecules, mediate tumor-host interactions. Adhesion molecules and adhesion processes are involved in most, if not all, of the intermediate steps of the metastatic cascade. Decreased E-cadherin expression and increased CD44 expression are clearly correlated with the acquisition of the invasive capacity of primary tumor cells. Similarly, altered expression pattern of many other adhesion molecules such as upregulated expression of the laminin receptors and depressed expression of fibronectin receptors (alpha 5 beta 1) appears to be involved in tumor cell invasion into the subendothelial matrix. Tumor cell-endothelium interactions involve several well-defined sequential steps that can be analyzed by the 'Docking and Locking' hypothesis at the molecular level. Tumor cell-matrix interactions are determined by the repertoire of adhesion receptors of tumor cells and the unique composition of organ-specific matrices. Our experimental data, together with others', suggest that the integrin alpha IIb beta 3 is one of the major players in these tumor-host interactions. Tumor-host interaction is a dynamic process which is constantly modulated by a host of factors including various cytokines, growth factors and arachidonate metabolites such as 12(S)-HETE. Delineation of the molecular mechanisms of tumor-host interactions may provide additional means to intervene in the metastatic process.

Increased expression of alpha IIb beta 3 integrin in subpopulations of murine melanoma cells with high lung-colonizing ability

Four subpopulations of B16 amelanotic melanoma cells, possessing different abilities to induce platelet aggregation (TCIPA) and to form lung colonies, were isolated by centrifugal elutriation. The expression of alpha IIb beta 3, alpha v beta 3 and alpha 5 beta 1 integrins was examined in the 4 subpopulations in order to determine the relationship between integrin receptor expression and tumor-cell metastatic potential. The mRNA of alpha IIb, alpha 5, beta 1 and beta 3 was detectable in the 4 subpopulations by Northern blotting. A gradual increase in mRNAs and cell-surface immunoreactivity of the alpha IIb beta 3 receptor, but not in their gene copies, was observed from the low to the high metastatic subpopulations. The ability of tumor cells to adhere to fibronectin and subendothelial matrix (SEM) increased in parallel. In the high metastatic cells, the alpha IIb beta 3 receptors, but not the alpha 5 beta 1 receptors, were localized to focal adhesion plaques. Incubation of the high metastatic cells with alpha IIb beta 3-specific antibodies reduced their matrix adhesion, TCIPA and lung-colonizing abilities. In contrast, in the low met- astatic cells, SEM adhesion and lung-colony formation were not affected by anti-alpha IIb beta 3 antibody treatment. Incubation of either the low or the high metastatic subpopulation with an alpha 5 beta 1-specific antibody had no effect in vitro and showed a slight inhibition of lung colonization in vivo. Our results suggest that several phenotypic characteristics of the enhanced metastatic potential of B16a subpopulations may be mediated by increased expression of alpha IIb beta 3 receptors and that expression of these receptors may be regulated at the transcriptional level.