Role of plasma, platelets, and endothelial cells in tumor metastasis

Inhibition of growth and linoleate-enhanced metastasis of a transplantable mouse mammary tumor by indomethacin

The influence of the cyclo-oxygenase inhibitor, indomethacin (IM), on the metastasis, development and prostaglandin E (PGE) levels of line 4526 mammary tumors grown in mice fed high fat (HF, 20%, w/w) diets containing various levels of linoleic acid (18:2) was investigated. Control mice that grew primary tumors and were fed HF diets containing 12% 18:2 (w/w) had 2-3 times the number of lung metastases than mice fed 1%, 4%, or 8% 18:2. Chronic treatment of mice with 10 micrograms/ml IM in drinking water reduced metastasis in 1% and 4% 18:2-fed mice compared to controls and completely inhibited the increased metastasis of mice fed the 12% 18:2 diet. Treatment with IM also increased the latency and decreased the growth rates of primary 4526 tumors of all dietary groups. Treatment of mice with a higher dosage of IM (20 micrograms/ml), decreased tumor metastasis even further compared to controls, but did not decrease tumor growth rate compared to the lower dosage of IM (10 micrograms/ml). Tumor PGE levels, measured by radioimmunoassay (RIA), were decreased by IM treatment. These data provide evidence that arachidonic acid metabolites such as PGE may be involved in the metastasis of 4526 mammary tumors.

Endothelium and underlying membrane reactivity with platelets, leukocytes and tumor cells: regulation by the lipoxygenase-derived fatty acid metabolites, 13-HODE and HETES

We hypothesize that the ratio of intracellular 13-hydroxy-octadeca-dienoic acid (13 HODE) and hydroxy-eicosatetraenoic acid (5-, 12- and/or 15-HETE) influences the expression or presentation of adhesive moieties on platelets, leukocytes, malignant cells and endothelial cells, thereby influencing their subsequent adhesive interactions. Thus, we demonstrate that under unstimulated conditions, these cells preferentially synthesize linoleic acid via their lipoxygenase enzymes into 13-HODE, the intracellular level of which is associated with limited or no cell adhesion, while following stimulation, the same cells preferentially metabolize arachidonic acid via the lipoxygenase enzyme into HETEs, the production of which is associated with enhanced adhesion. Which metabolite is synthesized by these cells and the subsequent adhesivity of these cells appear to be dependent upon both the intracellular level of cAMP and the ratio of linoleic and arachidonic acid substrates. This suggests that manipulation of this ratio will have significant effects on the adhesive events involved in the pathogenesis of thrombosis, inflammation and metastasis.

The biology of melanoma metastasis

The process of cancer metastasis is sequential and selective and contains stochastic elements. The growth of melanoma metastases represents the endpoint of many lethal events that few tumor cells can survive. Primary tumors consist of multiple subpopulations of cells with heterogeneous metastatic properties, and the outcome of metastasis depends on the interplay of metastatic tumor cells with various host factors. This viewpoint is more optimistic than that of metastasis as a random process. A selective biological process is regulated by the interaction of tumor cells with their host, and these complex interactions can now be studied and manipulated.

Binding of 13-HODE and 5-, 12- and 15-HETE to endothelial cells and subsequent platelet, neutrophil and tumor cell adhesion

Some studies report that endothelial cells preferentially take up the lipoxygenase-derived arachidonic acid metabolite, 5-hydroxyeicosatetraenoic acid (5-HETE), released from stimulated leukocytes (polymorphonuclear leukocytes, PMNs), whereas others report that endothelial cells preferentially take up 12-HETE released from platelets. The biological relevance of these observations, however, is unknown. Recently, we and others have found that, under basal conditions, endothelial cells, PMNs and tumor cells metabolize linoleic acid via the lipoxygenase enzyme to 13-hydroxyoctadecadienoic acid (13-HODE). We propose that endogenous levels of these metabolites regulate blood-vessel wall cell adhesion. In this study, we have measured (1) the relative binding of 5-, 12- and 15-HETE, and 13-HODE to endothelial cell monolayers, and (2) their effects on endothelial cell adhesivity with platelets, PMNs and tumor cells. There was a dose-related and specific binding of 5-[3H]HETE to endothelial cells but no binding of 12- or 15-HETE or 13-HODE. Platelet or PMN adhesion to endothelial cells was unaffected by the 5-HETE binding, but tumor cell adhesion was blocked by 40% (P less than 0.01). Interestingly, preincubation of endothelial cells with 13-HODE, 12-HETE or 15-HETE decreased platelet adhesion to endothelial cells (P less than 0.05), even though these metabolites did not bind to the endothelial cells. We conclude that 5-HETE preferentially binds to endothelial cells and interferes with a specific receptor for tumor cells, whereas the other metabolites neither bind to cells nor affect cell adhesion.

Organ specificity of tumor metastasis: role of preferential adhesion, invasion and growth of malignant cells at specific secondary sites

The locations of distant secondary tumors in many clinical cancers and animal tumors are nonrandom, and their distributions cannot be explained by simple anatomical or mechanical hypotheses based on the simple lodgment or trapping of tumor cell emboli in the first capillary bed encountered. Evidence from certain experimental tumor systems supports Paget's 'seed and soil' hypothesis on the nonrandom distributions of metastases, in which the unique properties of particular tumor cells ('seeds') and the different characteristics of each organ microenvironment ('soil') collectively determine the organ preference of metastasis. Experimentally, differential tumor cell adhesion to organ-derived microvessel endothelial cells and organ parenchymal cells, differential invasion of basement membranes and organ tissues, and differential responses to organ-derived growth-stimulatory and -inhibitory factors all appear to be important determinants in explaining the organ preference of metastasis. Each tumor system may achieve organ specificity because of its own unique set of multiple metastasis-associated properties and responses to host microenvironments. As neoplasms progress to more highly malignant states multisite metastases are more likely and organ-specific metastases may be masked or circumvented owing to stochastic events, tumor cell diversification, host selection processes, and increased production of tumor autocrine molecules that may modulate adhesion, invasion, growth, and other properties important in metastasis. The importance of each of these properties, however, appears to vary considerably among different metastatic tumor systems. These and other tumor cell and host properties may eventually be used to predict and explain the unique metastatic distributions of certain human malignancies.

Investigation of the biological effects of anti-cell adhesive synthetic peptides that inhibit experimental metastasis of B16-F10 murine melanoma cells

The experimental metastasis of B16-F10 murine melanoma cells is blocked by the anti-cell adhesive pentapeptide Gly-Arg-Gly-Asp-Ser (GRGDS) derived from the central cell-binding domain of fibronectin. In this report, we show that peptide treatment substantially extends the survival time for mice injected intravenously with B16-F10 cells (8/8 vs. 0/8 mice alive at 150 d), thereby demonstrating the potential efficacy of GRGDS treatment in protection against metastatic colonization. We have also examined the specificity of GRGDS activity by testing a series of related homologues for their effects on experimental metastasis. The overall profile of the relative inhibitory activities of these peptides closely matched their previously established capacity to disrupt adhesion in vitro. Lung retention studies with radiolabeled B16-F10 cells revealed an accelerated rate of cell loss from the lung 0-6 h after coinjection with the active peptide GRGDS. This early effect of GRGDS was consistent with its short circulatory half-life, which was found to be 8 min. Taken together, these results suggest that peptide-mediated inhibition of pulmonary colonization is due to interference with B16-F10 cell adhesion to structures in the target organ. Possible peptide interference in tumor cell-blood cell interactions was examined in order to assess (a) possible biological side-effects of peptide treatment and (b) whether such interactions might be an alternative mechanism for GRGDS-mediated inhibition of pulmonary colonization. GRGDS was found to retain full inhibitory activity when coinjected with B16-F10 cells into mice in which platelet function was impaired by acetylsalicylic acid treatment or into thrombocytopenic mice treated with antiplatelet serum (76-93% inhibition of colony formation). These data suggest that platelet involvement in the effects of the peptide is minimal. Similarly, GRGDS was also found to be a potent inhibitor of experimental metastasis in natural killer (NK) cell-deficient beige mice (86% inhibition), thereby discounting the possibility that GRGDS artifactually enhanced NK cell activity. We conclude as a result of these studies that cell-binding fibronectin peptides are specific inhibitors of experimental metastasis that prolong survival, that they appear to function by blocking the adhesion of B16-F10 cells to structures in the target organ, and that they do not appear to act through side effects on certain metastasis-related blood cell functions. In the future, derivatives of fibronectin peptides may be potentially useful prophylactic agents for interfering with the process of metastasis.

Mechanism of liver-specific metastatic tumor spread in a murine tumor model

Malignant tumors frequently show an organ-specific metastatic spread, the causes of which are still largely unknown. Using an experimental tumor model, a methylcholanthrene-induced pleomorphic myofibrosarcoma ER 15-P of the C57Bl6J mouse, we wanted to find out whether this phenomenon is due to an adaptation or to a selection of tumor cells. After i.v. injection of tumor cells from the primary ER 15-P into the tail vein of male mice, metastases were regularly found in the lungs, mediastinal lymph nodes, and brain, as well as in the liver and kidneys, and occasionally in the adrenals. The following experimental procedures were used to isolate a tumor cell line with a possible liver preference: (1) Tumor cells from the primary ER 15-P were injected into a mesenteric vein of male mice. Tumor cells from the resulting liver colonies were again injected into the portal system of one group of mice. In a second group, part of the same cell suspension was injected into the tail vein. This procedure was performed four times. (2) Tumor cells from the primary ER 15-P were applied into the tail vein of male mice. Tumor cells from the resulting liver metastases were reinjected directly into the tail vein. This experiment was repeated three times. (3) Tumor cells from the primary ER 15-P were injected into the tail vein of male mice. Tumor cells from liver metastases were then injected, first, into the portal system of one group of male mice, and thereafter into the tail vein of another group of animals. This experiment was repeated twice. The following results were obtained: (1) By a repeated adaptation of tumor cells from the primary ER 15-P to liver tissue, no tumor cell line could be isolated that would show a preferential metastatic spread to this organ after tail-vein injection. (2) Repeated i.v. passages of tumor cells from liver metastases into the tail vein led to the selection of a tumor cell line with a tendency to liver metastasis. (3) Tumor cells selected from liver metastases induced via tail-vein injection showed, after a prolonged stay in the liver and a successive i.v. passage into the tail vein, a marked specificity for this organ. These results indicate that the liver-specific spread of tumor cells in our model is based on the selection of a tumor cell line from the primary ER 15-P influenced by the hepatic microenvironment.

Initiation of coagulation by tissue factor

Tissue factor (TF) is an integral membrane glycoprotein which functions as an initiator of coagulation. Furthermore, it is probably the principal biological initiator of this essential hemostatic process. This article reviews the studies which form the basis for these assertions. The work on TF is traced from the 19th century discovery of the thromboplastic activity of tissues to the recent purification of the protein from bovine and human tissues and the isolation cDNA clones coding from human TF. The features of TF structure and function which tailor it to the role of initiator of the coagulation cascade are considered. For example, cell-surface TF and factor VII, the plasma serine proteases zymogen, form a proteolytic complex without prior proteolysis of either component. In addition, a kinetic model for the molecular mechanism of TF-initiated clotting is reviewed. The factors which control the expression of TF procoagulant activity by cultured cells are examined in light of the hypothesized role of TF in normal hemostasis. Also, the potential pathological consequences of aberrant TF expression, i.e., thrombosis and hemorrhage, are explored.

Adhesion of human breast cancer cell line MCF-7 to human vascular endothelial cells in culture. Enhancement by activated platelets

The interactions of MCF-7 tumor cells with human vascular endothelial cells (EC) and subendothelial extracellular matrices (ECM) were morphologically observed by electron microscopy and quantitatively evaluated by labelling tumor cells with 111Indium-oxine. MCF-7 tumor cells adhered more rapidly to ECM than to the apical surface of a confluent monolayer of EC. The affinity of MCF-7 cells for type-IV collagen was greater than for fibronectin, suggesting that type-IV collagen contributes to the higher rate of adhesion of MCF-7 cells to the subendothelial ECM. Otherwise, the attachment of tumor cells to EC was increased in the presence of both washed platelets and 0.1% citrated platelet-poor plasma (cPPP), a condition accelerating platelet aggregation by tumor cells. The enhancement of MCF-7 adhesion to EC in the presence of platelets and cPPP was completely blocked by the addition of prostacyclin, or hirudin, a specific thrombin inhibitor. In ultrastructural studies, MCF-7 initiated EC retraction, and firm attachment and flattening occurred on exposed ECM. When MCF-7 cells were incubated with platelets and cPPP, most of the tumor cells adhering to the EC and inducing disruption of endothelial monolayer were closely packed and associated with platelet aggregates. MCF-7 cells appeared to adhere more efficiently to exposed subendothelial ECM when they were associated into multicellular aggregates containing platelets and trapped in a fibrin thrombus. Thus, this homologous human system of cultured vascular EC and breast carcinoma line MCF-7 cells may be used to assess anti-aggregant compounds for their ability to alter tumor-cell implantation on EC-lined surfaces.

Tumor cell-platelet interactions in vitro and their relationship to in vivo arrest of hematogenously circulating tumor cells

Aggregation of rat platelets was induced in vitro by homologous rat Walker 256 carcinosarcoma cells and the extent of tumor cell-platelet interactions examined ultrastructurally. By 30s there was surface contact between unstimulated platelets and tumor cell microvilli. By midphase aggregation (2-3 min) tumor cells became enmeshed within expanding platelet aggregates. Tumor cell microvilli and platelet pseudopodia interdigitated as aggregation progressed. During the later stages of aggregation (6-10 min) tumor cells formed large processes which penetrated deep into the aggregate. Platelet activation (i.e. degranulation) occurred in gradient fashion and was concentrated near tumor cell membrane sites involved in process formation. At these later stages tumor cells near the aggregate periphery were found to have engulfed platelets or platelet fragments. Tumor cell-platelet interactions in the pulmonary microvasculature were also studied in vivo following injection of murine Lewis lung carcinoma, 16C mammary adenocarcinoma, and B16 amelanotic melanoma tumor cells into the tail vein. Platelets demonstrated a biphasic association with arrested tumor cells with peak interactions occurring at 10-30 min and 4-24 h. Ultrastructurally, tumor cells exhibited newly formed processes which interdigitated with the platelet aggregate. Such processes formed only in areas of contact with platelets and not in areas of contact with endothelial cells or other blood elements (i.e. erythrocytes, polymorphonuclear leukocytes). Numerous tumor cell mitochondria were concentrated in the areas of greatest platelet-tumor cell process activity. At early time intervals (2-10 min), intravascular platelet degranulation was observed primarily in platelets associated with tumor cell processes. Tumor cells also were found to have engulfed platelet fragments in vivo.

Effects of injury and repair of the pulmonary endothelium on lung metastasis after bleomycin

Acute endothelial injury induced by bleomycin has been shown to enhance the localization and metastasis of circulating tumour cells. In the present study we wished to determine whether increased metastases to the lung is related to the degree of endothelial damage as indicated by morphology and protein leakage to alveoli and whether the progression to repair with pulmonary fibrosis also effects metastatic tumour growth. C57b1/6 mice were injected with a single intravenous dose of bleomycin (120 mg/kg). After 5 days, severe enothelial injury was demonstrated by morphology and by increased levels of protein in lung lavage fluid. When [131I]-iododeoxyuridine labeled syngeneic fibrosarcoma cells were injected intravenously at this time, a 9-fold increase in their localization was detected 24 h later in bleomycin-treated lungs compared with saline controls. By electron microscopy tumour cells were observed at sites of denuded vascular basement membrane. There was also a significant increase in the number of gross metastases which developed subsequently and in the percentage of lung occupied by tumour in the bleomycin group. Animals examined 10 days after bleomycin showed less endothelial damage and a smaller increase in tumour cell localization and metastases. At 21 days, when endothelial structure and alveolar protein levels had returned to normal, and at 6 weeks, when there was focal fibrosis, no increase in tumour cell localization or metastases was found. It is concluded that damage to the pulmonary endothelium is a key factor in enhancing the trapping of circulating tumour cells and increasing metastatic tumour growth after bleomycin.

A synthetic peptide from fibronectin inhibits experimental metastasis of murine melanoma cells

Adhesive interactions between cells and the extracellular matrix occur at several stages of metastasis. Such interactions might be inhibited by synthetic peptide probes derived from the cell-binding regions of matrix molecules. Gly-Arg-Gly-Asp-Ser (GRGDS) is a pentapeptide sequence that appears to be critical for cell interaction with fibronectin. Coinjection of GRGDS with B16-F10 murine melanoma cells dramatically inhibited the formation of lung colonies in C57BL/6 mice. Two closely related control peptides, in which specific amino acids within the GRGDS sequence were transposed or substituted, displayed little or no activity. Inhibition by GRGDS was dose-dependent, noncytotoxic, and did not result from an impairment of cellular tumorigenicity. GRGDS may function by inhibiting tumor cell retention in the lung since radiolabeled B16-F10 tumor cells injected with the peptide were lost at a substantially greater rate than control cells.

Dynamic heterogeneity: isolation of murine tumor cell populations enriched for metastatic variants and quantification of the unstable expression of the phenotype

Recent studies have indicated that KHT fibrosarcoma or B16 melanoma cell variants capable of forming experimental metastases in the lungs of mice after i.v. injection are created stochastically at high rates (approximately 10(-5)/cell/generation). Expression of this phenotype is unstable and hence expanding populations of tumor cells establish a dynamic equilibrium between a small subpopulation of metastatic variants and a large compartment of nonmetastatic cells. In the present experiments, cell suspensions were prepared from the lungs of mice bearing 'experimental' metastases and the tumor cells contained in them were tested for their metastatic efficiency (ME) using the lung colony assay. The ME of the recovered tumor cell populations was found to be a function of the time of metastatic growth in the animal. Tumor cells isolated soon after the initial i.v. injection, i.e. derived from micrometastases, are highly metastatic while populations recovered from macroscopic nodules are similar to parental lines in their ability to colonize the lung. These results are consistent with the prediction of the above 'dynamic heterogeneity' model that nascent lung metastases should be composed largely of tumor cells expressing the variant metastatic phenotype, but that the proportion of such variants should decline during growth to the equilibrium (parental population) level. Mathematical analysis of the results indicates that the effective rate of reversion of the variant phenotype is approximately 10(-1)/cell/generation.

Promotion of lung tumor colonization in mice by the synthetic thrombin inhibitor (no. 805) and its reversal by leech salivary gland extracts

The role of anticoagulation per se in the reduction of experimental or spontaneous metastasis still remains to be determined, as shown by the conflicting results reported by the literature using different conventional anticoagulants. A new compound has been synthesized (compound no. 805) which prolongs or suppresses coagulation via specific inhibition of thrombin and its possible use in a model of experimental metastasis to clarify the role of anticoagulants in tumor spread was investigated. Contrary to our expectations, this compound increased rather than decreased the number of lung colonies induced by intravenous injections of a variety of murine neoplasias. Studies of the mechanism of this effect indicated that the compound increases retention of tumor cells by the lung without apparent impairment of the natural cell immune system, suggesting that the synthetic thrombin inhibitor may enhance vascular attachment of tumor cells. The promoting effect of compound no. 805 on metastasis was totally reversed by the administration of leech salivary gland extracts, which appear to protect capillaries from damage produced by cyclophosphamide, as revealed by other studies.

Platelet fibronectin release induced by Walker 256 rat carcinoma tumor cells

This study examined gel filtered rat platelet activation by Walker 256 rat carcinoma cells and characterized fibronectin release. Using aggregometry measurements, a biphasic platelet response was characterized and the timing of alpha granule release was determined. The localization and association of released platelet fibronectin with tumor cell-platelet aggregates was determined by immunofluorescent and immunocytochemical methods. The immunofluorescent studies showed that the platelet fibronectin was released and became associated with the peripheries of the tumor cells following alpha granule discharge. The ultrastructural immunocytochemical data revealed that this fibronectin was associated with a fibrin-like material, enmeshing the tumor cell-platelet aggregates. The significance of the release and localization of platelet fibronectin to tumor cell metastasis is discussed.

Involvement of platelet-released 5-HT in tumor cell lodgement

Adherence of circulating tumor cells in the microvasculature (lodgement) is a key event in metastasis formation, once cells released from a primary neoplasm have reached the bloodstream. Platelets appear to be involved in tumor cell lodgement, since thrombocytopenia significantly reduces the number of lodged tumor cells. Platelets activated by tumor cells are known to release 5-HT, and therefore the present study was focused on the influence of this amine on hepatic lodgement of intraportally injected fibrosarcoma cells in rats. Lodgement was quantified by labeling of tumor cells with [5-125I]iodo-2-deoxy-uridine. 5-HT levels were simultaneously determined using liquid chromatography with electrochemical detection (LCED) from hepatic areas with lodged tumor cells, as seen in vital microscopy. Injection of tumor cells increased hepatic 5-HT levels twofold (614 +/- 64 micrograms/g) compared with the levels found in rapidly killed (284 +/- 22 micrograms/g) or saline-injected animals (231 +/- 75 micrograms/g). In thrombocytopenia lodgement was reduced by 47% and hepatic 5-HT levels by 52% in comparison with tumor cell-injected controls. Peripheral blockade of 5-HT2 receptors with ketanserin (0.6 mg/kg sc) also reduced lodgement by 28% and hepatic 5-HT levels by 36%, strongly indicating involvement of platelet-released 5-HT in the lodgement process. Immunocytochemical studies of hepatic tissue in untreated rats demonstrated presence of 5-HT only in mast cells, while as early as 15 min after tumor cell injection 5-HT immunopositive nerves were also seen. This implies an active uptake of circulating 5-HT by neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of cell adhesion proteins--laminin and fibronectin--in the movement of malignant and metastatic cells

Metastasizing tumor cells must traverse diverse extracellular matrices during dissemination. Extracellular matrices consist of two basic types, interstitial stroma and basement membranes. Extracellular matrices are chemically complex structures that interact with cell surfaces by a number of mechanisms. There has been a great deal of effort in recent years to understand the molecular nature of extracellular matrices, especially as it relates to the adhesion of normal and malignant cell types. Adhesive noncollagenous glycoproteins, such as laminin and fibronectin, serve pivotal roles in basement membrane and stromal matrices, respectively. These proteins participate in establishing the architecture of extracellular matrices as well as in attaching to the surface of cells and affecting cellular phenotype. This phenotypic effect ranges from adhesion and motility to growth and differentiation. Changes in adhesive characteristics and motility of cells have long been suspected to play a role in mediating the spread of malignant neoplasms. This article is designed to review extracellular matrix constituents that are currently known that can mediate the adhesion and motility of malignant neoplasms. The adhesion of normal and malignant cells to matrices is a complex process mediated by several distinct mechanisms which are initially manifested by changes in cytoskeletal architecture. The topic of normal and malignant cell adhesion to matrices will also be discussed in this regard, since any explanation of tumor cell migration must account for the complex dynamic interactions of the cell surface with the substratum as well as with the cytoskeleton. Finally, current efforts designed to understand the molecular nature of tumor cell:matrix interactions that contribute to metastatic behavior will also be discussed. The rationale behind these studies is that selective inhibition of specific tumor:extracellular matrix interactions can provide an avenue for therapeutic intervention of metastatic cancer.