Thrombogenic activity of mouse and human tumors: effects on platelets, coagulation, and fibrinolysis, and possible significance for metastases

Primary Thromboprophylaxis in Pancreatic Cancer Patients: Why Clinical Practice Guidelines Should Be Implemented

Exocrine pancreatic ductal adenocarcinoma, simply referred to as pancreatic cancer (PC) has the worst prognosis of any malignancy. Despite recent advances in the use of adjuvant chemotherapy in PC, the prognosis remains poor, with fewer than 8% of patients being alive at 5 years after diagnosis. The prevalence of PC has steadily increased over the past decades, and it is projected to become the second-leading cause of cancer-related death by 2030. In this context, optimizing and integrating supportive care is important to improve quality of life and survival. Venous thromboembolism (VTE) is a common but preventable complication in PC patients. VTE occurs in one out of five PC patients and is associated with significantly reduced progression-free survival and overall survival. The appropriate use of primary thromboprophylaxis can drastically and safely reduce the rates of VTE in PC patients as shown from subgroup analysis of non-PC targeted placebo-controlled randomized trials of cancer patients and from two dedicated controlled randomized trials in locally advanced PC patients receiving chemotherapy. Therefore, primary thromboprophylaxis with a Grade 1B evidence level is recommended in locally advanced PC patients receiving chemotherapy by the International Initiative on Cancer and Thrombosis clinical practice guidelines since 2013. However, its use and potential significant clinical benefit continues to be underrecognized worldwide. This narrative review aims to summarize the main recent advances in the field including on the use of individualized risk assessment models to stratify the risk of VTE in each patient with individual available treatment options.

Integrin β3/Akt signaling contributes to platelet-induced hemangioendothelioma growth

Hemangioendothelioma (HE) is a type of angiomatous lesions that features endothelial cell proliferation. Understanding the mechanisms orchestrating HE angiogenesis can provide therapeutic insights. It has been shown that platelets can support normal and malignant endothelial cells during angiogenesis. Using the mouse endothelial-derived EOMA cell line as a model of HE, we explored the regulatory effect of platelets. We found that platelets stimulated EOMA proliferation but did not mitigate apoptosis. Furthermore, direct platelet-EOMA cell contact was required and the proliferation was mediated via integrin β3/Akt signaling in EOMA cells. SiRNA knockdown of integrin β3 and inhibition of Akt activity significantly abolished platelet-induced EOMA cell proliferation in vitro and tumor development in vivo. These results provide a new mechanism by which platelets support HE progression and suggest integrin β3 as a potential target to treat HE.

Platelet secretion in inflammatory and infectious diseases

Platelets are small, anucleate circulating cells that possess a dynamic repertoire of functions spanning the hemostatic, inflammatory, and immune continuum. Once thought to be merely cell fragments with responses limited primarily to acute hemostasis and vascular wall repair, platelets are now increasingly recognized as key sentinels and effector cells regulating host responses to many inflammatory and infectious cues. Platelet granules, including α-granules and dense-granules, store hundreds of factors and secrete these mediators in response to activating signals. The cargo packaged and stored within platelet granules orchestrates communication between platelets and other circulating cells, augments host defense mechanisms to invading pathogens and tumor cells, and - in some settings - drives dysregulated and injurious responses. This focused review will highlight several of the established and emerging mechanisms and roles of platelet secretion in inflammatory and infectious diseases.

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.

Tissue factor expressed by circulating cancer cell-derived microparticles drastically increases the incidence of deep vein thrombosis in mice

BACKGROUND

The risk of thrombotic complications such as deep vein thrombosis (DVT) during tumor development is well known. Tumors release into the circulation procoagulant microparticles (MPs) that can participate in thrombus formation following vessel injury. The importance of this MP tissue factor (TF) in the initiation of cancer-associated DVT remains uncertain.

OBJECTIVE

To investigate how pancreatic cancer MPs promote DVT in vivo.

METHODS

We combined a DVT mouse model in which thrombosis is induced by flow restriction in the inferior vena cava with one of subcutaneous pancreatic cancer in C57BL/6J mice. We infused high-TF and low-TF tumor MPs to determine the importance of TF in experimental cancer-associated DVT.

RESULTS

Both tumor-bearing mice and mice infused with tumor MPs subjected to 3 h of partial flow restriction developed an occlusive thrombus; fewer than one-third of the control mice did. We observed that MPs adhered to neutrophil extracellular traps (NETs), which are functionally important players during DVT, whereas neither P-selectin nor glycoprotein Ib were required for MP recruitment in DVT. The thrombotic phenotype induced by MP infusion was suppressed by hirudin, suggesting the importance of thrombin generation. TF carried by tumor MPs was essential to promote DVT, as mice infused with low-TF tumor MPs had less thrombosis than mice infused with high-TF tumor MPs.

CONCLUSIONS

TF expressed on tumor MPs contributes to the increased incidence of cancer-associated venous thrombosis in mice in vivo. These MPs may adhere to NETs formed at the site of thrombosis.

Platelets guide the formation of early metastatic niches

During metastasis, host cells are recruited to disseminated tumor cells to form specialized microenvironments ("niches") that promote metastatic progression, but the mechanisms guiding the assembly of these niches are largely unknown. Tumor cells may autonomously recruit host cells or, alternatively, host cell-to-host cell interactions may guide the formation of these prometastatic microenvironments. Here, we show that platelet-derived rather than tumor cell-derived signals are required for the rapid recruitment of granulocytes to tumor cells to form "early metastatic niches." Granulocyte recruitment relies on the secretion of CXCL5 and CXCL7 chemokines by platelets upon contact with tumor cells. Blockade of the CXCL5/7 receptor CXCR2, or transient depletion of either platelets or granulocytes prevents the formation of early metastatic niches and significantly reduces metastatic seeding and progression. Thus, platelets recruit granulocytes and guide the formation of early metastatic niches, which are crucial for metastasis.

Inhibition of platelet activation prevents the P-selectin and integrin-dependent accumulation of cancer cell microparticles and reduces tumor growth and metastasis in vivo

Venous thromboembolism constitutes one of the main causes of death during the progression of a cancer. We previously demonstrated that tissue factor (TF)-bearing cancer cell-derived microparticles accumulate at the site of injury in mice developing a pancreatic cancer. The presence of these microparticles at the site of thrombosis correlates with the size of the platelet-rich thrombus. The objective of this study was to determine the involvement of TF expressed by cancer cell-derived microparticles on thrombosis associated with cancer. We observed that pancreatic cancer cell derived microparticles expressed TF, its inhibitor tissue factor pathway inhibitor (TFPI) as well as the integrins αvβ1 and αvβ3. In mice bearing a tumor under-expressing TF, a significant decrease in circulating TF activity associated with an increase bleeding time and a 100-fold diminished fibrin generation and platelet accumulation at the site of injury were observed. This was mainly due to the interaction of circulating cancer cell-derived microparticles expressing TFPI with activated platelets and fibrinogen. In an ectopic model of cancer, treatment of mice with Clopidogrel, an anti-platelet drug, decreased the size of the tumors and restored hemostasis by preventing the accumulation of cancer cell-derived microparticles at the site of thrombosis. In a syngeneic orthotopic model of pancreatic cancer Clopidogrel also significantly inhibited the development of metastases. Together, these results indicate that an anti-platelet strategy may efficiently treat thrombosis associated with cancer and reduce the progression of pancreatic cancer in mice.

Release of angiogenesis regulatory proteins from platelet alpha granules: modulation of physiologic and pathologic angiogenesis

An association between platelets, angiogenesis, and cancer has long been recognized, but the mechanisms linking them remains unclear. Platelets regulate new blood vessel growth through numerous stimulators and inhibitors of angiogenesis by several pathways, including differential exocytosis of angiogenesis regulators. Herein, we investigated the differential release of angiogenesis stimulators and inhibitors from platelets. Activation of human platelets with adenosine diphosphate (ADP) stimulated the release of VEGF, but not endostatin whereas, thromboxane A(2) (TXA(2)) released endostatin but not VEGF. Platelet releasates generated by activation with ADP promoted migration and formation of capillary structures by human umbilical vein endothelial cells (HUV-EC-Cs) in in vitro angiogenesis models. Conversely, TXA(2)-stimulated platelet releasate inhibited migration and formation of capillary structures. Because tumor growth beyond 1-2 mm(3) is angiogenesis-dependent, we hypothesized that cancer cells preferentially stimulate platelets to secrete their pro-angiogenic payload. In support of this, the breast cancer cell line MCF-7 stimulated secretion of VEGF and a pro-angiogenic releasate from platelets. Furthermore, the antiplatelet agent aspirin inhibited platelet-mediated angiogenesis after exposure to ADP or MCF-7 cells providing a potential mechanism for how aspirin may impact malignancy. Manipulation of differentially mediated release of angiogenic factors from platelets may provide a new modality for cancer treatment.

Cancer cell-derived microparticles bearing P-selectin glycoprotein ligand 1 accelerate thrombus formation in vivo

Recent publications have demonstrated the presence of tissue factor (TF)–bearing microparticles (MPs) in the blood of patients suffering from cancer. However, whether these MPs are involved in thrombosis remains unknown. We show that pancreatic and lung cancer cells produce MPs that express active TF and P-selectin glycoprotein ligand 1 (PSGL-1). Cancer cell–derived MPs aggregate platelets via a TF-dependent pathway. In vivo, cancer cell–derived MPs, but not their parent cells, infused into a living mouse accumulate at the site of injury and reduce tail bleeding time and the time to occlusion of venules and arterioles. This thrombotic state is also observed in mice developing tumors. In such mice, the amount of circulating platelet-, endothelial cell–, and cancer cell–derived MPs is increased. Endogenous cancer cell–derived MPs shed from the growing tumor are able to accumulate at the site of injury. Infusion of a blocking P-selectin antibody abolishes the thrombotic state observed after injection of MPs or in mice developing a tumor. Collectively, our results indicate that cancer cell–derived MPs bearing PSGL-1 and TF play a key role in thrombus formation in vivo. Targeting these MPs could be of clinical interest in the prevention of thrombosis and to limit formation of metastasis in cancer patients.

Platelet--cancer interactions: mechanisms and pharmacology of tumour cell-induced platelet aggregation

During haematogenous metastasis, cancer cells migrate to the vasculature and interact with platelets resulting in tumour cell-induced platelet aggregation (TCIPA). We review: 1. The biological and clinical significance of TCIPA; 2. Molecular mechanisms involved in platelet aggregation by cancer cells; 3. Strategies for pharmacological regulation of these interactions. We conclude that pharmacological regulation of platelet-cancer cell interactions may reduce the impact of TCIPA on cancer biology.

Inhibition of metastases by anticoagulants

Metastasis involves several distinct steps, including one in which the tumor cell, after entry into the bloodstream, comes to rest in a capillary located at the distant site where a metastatic tumor will ultimately form. Components of the blood-clotting pathway may contribute to metastasis by trapping cells in capillaries or by facilitating adherence of cells to capillary walls. Conceivably, anticoagulants could interfere with this step in the metastatic process. In this review, we have summarized current knowledge on the interaction of malignant cells, clotting factors, and anticoagulants. We used computerized (MEDLINE) and manual searches to identify studies done in humans, in animals, and in in vitro systems that were published in English between 1952 and 1998. We found many reports that the formation of metastatic tumors could be inhibited by heparin, a vitamin K antagonist (warfarin), and inhibitors of platelet aggregation (prostacyclin and dipyridamole). Despite these encouraging preliminary results and a compelling biochemical rationale, only limited information exists on the clinical use of anticoagulants for the prevention or treatment of metastatic cancer because there have been so few controlled and prospectively randomized studies on this topic. In view of the preliminary results, anticoagulants may hold promise for the prevention and treatment of metastases. We believe that larger controlled investigations are strongly warranted to evaluate the clinical potential of anticoagulants for the prevention and treatment of metastases in humans.

Effects of antiplatelet agents alone or in combinations on platelet aggregation and on liver metastases from a human pancreatic adenocarcinoma in the nude mouse

There is ample evidence to suggest that hematogenous metastasis may be related to the ability of tumor cells to promote aggregation of host platelets. Arachidonic acid metabolism in platelets and vessel walls may also contribute to the metastatic process. Several preliminary trials of platelet inhibitory agents have been performed. Ketoconazole (inhibitor of lipoxygenase and thromboxane synthetase), verapamil (calcium antagonist), forskolin (stimulator of platelet adenylate cyclase), and indomethacin (inhibitor of cyclooxygenase) were examined, alone and in combination, to investigate their effects on platelet aggregation and on hepatic metastases from human pancreatic tumor cells (RWP-2) in nude mice. The tumor cells were injected intrasplenically, and the animals were divided into control, single-drug and combination treatment groups. The agents were administered intraperitoneally 1 hr before and every 24 hr after the tumor cell injections for 6 days. Statistically significant differences were observed between the control and single-treatment groups on the reduction of liver tumor nodules (range P less than 0.001-0.032) and in the liver surface areas occupied by tumor (range P less than 0.001-0.013). Furthermore, when these agents were combined, similar reductions in liver tumor nodules were noted (range P less than 0.001-0.008), while even greater inhibitory effects were seen in the liver surface areas occupied by tumor (P less than 0.001) compared with the single-treatment groups. Also, the combination studies strongly inhibited RWP-2-induced platelet aggregation in human platelet-rich plasma.

Inhibition of hepatic metastasis from a human pancreatic adenocarcinoma (RWP-2) in the nude mouse by prostacyclin, forskolin, and ketoconazole

Metastasis is a multistep phenomenon in which platelets appear to play an important role. This study examined several compounds for their effects on experimental hepatic metastasis and on human pancreatic tumor cell-platelet interactions. Prostacyclin (PGI2) and forskolin (stimulators of platelet adenylate cyclase) and ketoconazole (inhibitor of lipoxygenese and thromboxane synthetase) were used in order to investigate their effects on hepatic metastases from a human pancreatic tumor cell (RWP-2) in the nude mouse. The tumor cells were injected intrasplenically and the animals were divided into control, prostacyclin (PGI2 200 micrograms), forskolin (150 micrograms), and ketoconazole (180 micrograms) groups. All three drugs were administered intraperitoneally 30 minutes before and 24 hours after the tumor cell injections. Statistically significant differences were observed between control and treated groups in tumor surface area (P less than 0.001), percentage of liver surface area occupied by tumor (P less than 0.001), and number of tumor colonies (P less than 0.004 for prostacyclin, P less than 0.005 for forskolin, and P less than 0.001 for ketoconazole). These agents also strongly inhibited RWP-2-induced platelet aggregation in human platelet-rich plasma.

Human breast and colon carcinomas express cysteine proteinase activities with pro-aggregating and pro-coagulant properties

We have investigated concomitantly the pro-aggregating and pro-coagulant activities of 11 breast and 2 colon human carcinomas. Tumor tissues, obtained at surgery, were immediately processed to prepare tumor-cell suspensions for the study of aggregating activity and tissue extracts for the study of procoagulant capacity. Nine carcinomas (8 breast and 1 colon) possessed a high, dose-dependent platelet-aggregating activity, which was present in the cell-free supernatant and was inhibited by HgCl2 and iodoacetic acid, specific cysteine proteinase inhibitors, while apyrase and hirudin had no significant effect; in contrast, the other tumors did not aggregate platelets. All the tumor extracts tested from 12 carcinomas (11 breast and 1 colon) were able to activate blood coagulation in both the presence and the absence of F VII. The activity was inhibited by HgCl2 and iodoacetamide, while Con A was less effective. Therefore, these tumors do not aggregate platelets through the production of ADP or thrombin, nor promote blood coagulation through the production and release of tissue factor; a tumor-associated cysteine proteinase plays a major role in both pro-aggregating and pro-coagulant activities.

Characterization of the platelet-aggregating activity of cancer cells with different metastatic potential

We studied the mechanisms of platelet activation by sublines exhibiting different metastatic potential of two murine experimental tumors: sublines M4 and M9 of the benzopyrene-induced mFS6 sarcoma and sublines B77-AA6 and B77-3T3 of RSV-transformed BALB/c 3T3 fibroblasts. The neoplastic cells of both models induced platelet aggregation, secretion and prostaglandin biosynthesis. In the first model but not in the second, all these processes correlated with the in vivo malignancy of cells. Pretreatment of B77-AA6 and B77-3T3 cells with apyrase significantly decreased platelet aggregation, while pretreatment of M4 cells was ineffective. However, pretreatment with trypsin or neuraminidase was effective in reducing platelet aggregation induced by M4 cells, but not that induced by any of the others; furthermore, phospholipase A2 reduced the platelet response by all sublines. Finally, platelet-activating activity was also found in the pellets obtained following centrifugation of culture media. These results suggest that platelets are stimulated by cancer cells through different mechanisms; platelet activation by a sialo-lipo-protein complex of the cellular membrane was found to be characteristic of the model in which the platelet-aggregating activity of neoplastic cells correlated with their in vivo metastatic behavior.

Procoagulant and fibrinolytic activity of human ovarian carcinoma cells in culture

We have investigated here the coordinate expression of both procoagulant (PCA) and fibrinolytic (FA) activity of cells from 16 human ovarian carcinoma cases. To avoid interference of contaminating host cells, we used cells isolated in primary culture from ascitic fluid or from solid tumor. The FA was determined in cellular extracts by an amidolytic assay in the presence of fibrin monomers. FA, which was plasminogen dependent in almost all of the cases, showed a wide range of activity (from less than 0.001 to 2.30 UK units/mg protein). The molecular analysis of plasminogen activator (by SDS-PAGE and fibrin autography) showed a single molecular form of 52,000 daltons, inhibited by an antibody against human urokinase. PCA, studied with a one stage clotting assay in disrupted cells, was of tissue thromboplastin type in all instance and varied from 12.0 to 1300 thromboplastin units/10(4) cells. No simple correlation was found between FA and PCA in the cellular samples studied; moreover, for neither parameter was it possible to find any changes with the staging of the disease.

Inhibition of tumor cell-platelet interactions and tumor metastasis by the calcium channel blocker, nimodipine

Nimodipine, a dihydropyridine calcium channel blocker, was evaluated in vitro for its ability to inhibit platelet aggregation induced by B16 amelanotic melanoma (B16a) and Walker 256 carcinosarcoma (W256) cells, and for its ability to inhibit platelet-enhanced B16a and W256 adhesion to rat microvascular endothelial cells. Nimodipine produced a dose-dependent inhibition of tumor-cell-induced platelet aggregation (TCIPA). Platelets enhanced tumor cell adhesion to endothelium both in the presence and absence of overt platelet aggregation. However, the greatest enhancement of adhesion occurred under aggregatory conditions. Nimodipine at a dose of 40 micrograms/ml inhibited platelet-enhanced adhesion to endothelium under aggregatory and nonaggregatory conditions. Nimodipine was tested in vivo for its ability to inhibit both "experimental' and spontaneous metastasis. Nimodipine produced a 46 per cent inhibition of lung colony formation at a dose of 5 mg/kg body-weight. Over a dose range of 0.1-80 mg/kg, nimodipine produced a significant dose-dependent inhibition in the formation of lung metastases from a subcutaneous tumor. The in vitro results demonstrate that a dihydropyridine calcium channel blocker can inhibit tumor cell-platelet-endothelial cell interactions. The in vivo results suggest that these compounds may be a new class of antimetastatic agent.

Influence of migratory blood cells on the attachment of tumor cells to vascular endothelium

The purpose of this study was to determine whether the presence of migratory blood cells in association with tumor emboli had the capacity to alter the attachment of tumor cells to vascular endothelium. Highly metastatic RT7-4bs rat hepatocarcinoma cells were labelled with [125I]UdR before being allowed to form mixed cellular spheroids incorporating resident peritoneal macrophages, activated peritoneal macrophages, polymorphonuclear leukocytes, splenic T lymphocytes, or splenic B lymphocytes derived from both normal and tumor-bearing animals. The presence of polymorphonuclear leukocytes or activated macrophages led to a considerable increase in the number of tumor cells attaching to endothelial cell monolayers in vitro. The presence of T or B lymphocytes from either normal or tumor-bearing rats was without effect on tumor attachment to endothelium. Increased tumor cell retention in the lungs was evident for mixed spheroids containing tumor cells and polymorphonuclear leukocytes compared to homotypic tumor spheroids composed of tumor cells alone. Furthermore, preinjection of polymorphonuclear leukocytes intravascularly or inoculation of tumor cells as heterotypic spheroids containing polymorphonuclear leukocytes increased lung colony formation over that obtained after inoculation of tumor cells alone. Several simple sugars were tested for their ability to block tumor cell, polymorphonuclear leukocyte or activated macrophage binding to endothelium in vitro. The results indicate that the glycosylated cell surface components mediating tumor cell attachment to endothelium are not identical with those mediating attachment of either polymorphonuclear leukocytes or activated macrophages. Medium conditioned during mixed spheroid formation was without effect on tumor cell attachment to endothelium. We conclude that the presence of some, but not all classes of leukocytes can modulate tumor cell attachment to vascular endothelium, an effect most likely mediated by a mechanism involving direct contact between the leukocytes and the endothelial cell monolayer.

Role of NK cells in the antimetastatic effect of anticoagulant drugs

The antimetastatic effects of heparin (40 units) and prostacyclin (PGI2, 100 microgram)1 were investigated in normal mice and in mice with depressed or activated natural killer (NK) cell activity. Both anticoagulants inhibited the formation of lung metastases after inoculation of the FI or F10 sublines of B16 melanoma. Inhibition of NK activity by treatment of mice with anti-asialo GM1 serum abrogated the antimetastatic effects of PGI2 or heparin. Conversely, augmentation of NK-cell activity by poly I:C plus treatment with anticoagulants produced synergistic antimetastatic effects. A similar pattern of results was obtained with heparin treatment of mice challenged with the Madison lung carcinoma (M109), but PGI2 alone or in combination with theophylline had little or no detectable antimetastatic effect on M109 or on the parental B16 melanoma. Studies of the mechanism of the interaction between heparin nd NK cells revealed that the anticoagulant treatment did not affect splenic NK activity in vitro. However, heparin treatment caused a significant increase in the clearance of radiolabelled tumor cells from the lungs of normal mice. Combined treatment of mice with poly I:C and heparin synergistically accelerated the elimination of radiolabelled tumor cells. In contrast, heparin did not affect the clearance of tumor cells from the lungs of mice with depressed NK activity. Thus the antimetastatic effects of heparin and PGI2 are dependent on levels of NK activity in the host. Platelet aggregation and fibrin coating of the surface of tumor cells may be among the mechanisms by which hematogenously spread tumor cells are protected from destruction by NK cells. Anticoagulant drugs may exert antimetastatic effects by making tumor cells more vulnerable to the cytotoxic effects of NK cells, rather than by blocking adherence of tumor cells to vascular endothelium.

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

This review studies interactions of tumor cells with a particular host system which is normally responsible for hemostasis and the physiological integrity of the blood vessel luminal surface. With malignancy components of this system are frequently activated, producing abnormalities of blood coagulation, increased platelet responses, and conditions favoring tumor growth and metastasis. Activation of the clotting cascade is mediated by tumor and macrophage procoagulants, acting via Factor X or VII. Thrombin and fibrin are formed. Thrombin also interacts with platelets and the endothelium, potentiating or decreasing coagulation. Generation of thrombin or other tumor mechanisms activate platelets, leading to direct aggregation or secretion of ADP, serotonin, and/or intermediates of the arachidonate metabolism. Vascular lesions caused by tumor attack, platelet secretion, or exogenous agents promoting metastasis may also activate the hemostatic system. It is not yet fully understood how activation of the clotting system, including platelets, contributes to metastasis. Secretion of platelet products appears, however, to be heavily involved. Based on putative mechanisms of action, anticoagulants, platelet inhibitors, thrombocytopenic or vascular repairing agents have been used to control tumor spread. Results depended on the agent and experimental model of metastasis used. Except for coumarin, which was beneficial even against spontaneous metastases, other anticoagulants and platelet inhibitors, excluding perhaps Nafazatrom, gave equivocal results. Thrombocytopenic agents, however, were effective in every tumor system and with any experimental model of metastasis, indicating that platelets play a role in this process. Also consistent were the inhibitory effects of leech salivary gland extract (probably a vascular repairing agent) against lung tumor colonization promoted by ionizing radiation, cyclophosphamide, and cortisone.

Transmission and scanning-electronmicroscopy of experimental liver metastases derived from intrasplenically growing primary tumor

Liver metastasis formation from intrasplenically growing Lewis lung tumor was studied with transmission and scanning electron microscopy. Tumor cells arrested in the sinusoids formed desmosome-like junctions with endothelial cells and crossed the endothelial lining at intercellular gaps and by transcellular diapedesis. The metastatic foci had no newly formed vessels, and the stroma was provided by non-parenchymatous liver cells. Morphology suggested different outcome for tumor cell-host cell interactions at different stages of tumor growth. The host cell activity to destroy tumor cells was present only at the early stage but disappeared later, when the tumor cells were ready to phagocyte normal cells. The reason for this could be the immunosuppressive effect of the primary tumor. Results emphasize the importance to study metastasis formation in primary tumor-bearing hosts.

Interactions between neoplastic cells with different metastasizing capacity and platelet function

We have examined the effects on platelet function of two sublines (M4 and M9) derived from spontaneous lung nodules of a benzopyrene-induced murine fibrosarcoma (m FS6). The subline M4 was more metastatic and the subline M9 less metastatic than the primary tumour. Only the more malignant cells were able to induce irreversible aggregation of human platelets; this effect was concentration-dependent and was associated with the release of serotonin by platelets. Both aggregation and release were inhibited by preincubation of platelets with ASA, not by preincubation of the cells. The supernatants of cell suspensions had no aggregating activity. However, the neoplastic cells in culture media released an activity directly stimulating platelet aggregation and potentiating the platelet response to ADP; again, this activity was higher for the more malignant cells and the effects were inhibited by preincubation of platelets with ASA. These results suggest a role for platelets in the development of tumour metastases.

Fibrin as a component of the tumor stroma: origins and biological significance

An association between cancer and the coagulation system was suggested by Trousseau more than a century ago and initial reports of fibrin deposition in the stroma of solid tumors date back some 25 years. However, the validity and generality of these observations have only quite recently been established, and their implications for an understanding of tumor biology, metastasis, and therapy are only now coming to be appreciated by investigators in the mainstream of cancer research. This article reviews the current status of fibrin's role in the biology of tumor growth, considering in turn: (1) the evidence that fibrin is present in tumors, the nature of such fibrin, and its relation to plasma fibronectin; (2) the mechanisms by which fibrin may come to be deposited in tumors; and (3) the potential biological and medical significance of tumor-associated fibrin deposition and degradation. Among the last are such important possibilities as a barrier function to the immune response and possible roles in angiogenesis, desmoplasia, and metastasis.

Inhibition of the arrest of hematogenously disseminated tumor cells

Most metastases in patients occur as a result of hematogenous dissemination of tumor cells. This process of metastasis is complex and consists of several steps, foremost of which is the arrest of circulating emboli in capillary beds and the formation of a thrombus at that site. Thrombus formation in the metastasis of human cancer was described first by Billroth in 1878. It was reported that the organization of tumor cell emboli, and the subsequent penetration of tumor cells into the capillary wall, was the first stage of metastasis. Since then, many investigations and observations have been made clinically as well as experimentally to clarify the process (or mechanisms) of tumor cell arrest and how to inhibit it. Coagulative and fibrinolytic pathways were believed to have a main role in thrombus formation. However, other factors responsible for the relationship between tumor cells and the host must be also considered. Elegant and extensive studies by Fidler and Kripke demonstrated that development of metastasis is not a random process, but a selection process of specialized subpopulations of highly metastatic cells within the primary tumors. Biochemical constituents and ionic properties on cell surfaces, deformability or locomotive activities of tumor cells, as well as thrombo-plastic-fibrinolytic activities, are also important factors determining the arrest patterns of circulating tumor cells. On the other hand, host defense factors against tumor cells in the bloodstream have been attracting much attention recently in tumor immunology. Host defense factors relating the arrest of tumor cells to the establishment of metastatic foci seemed difficult to define, since many studies showed contradictory data concerning the influence of immune response on tumor cell arrest. Hemodynamic abnormality may also influence the arrest of tumor cells in the circulation. Hypercoagulability induced from host tissues is greatly associated with the arrest patterns. Platelet activities might affect thrombus formation. Nevertheless, exact explanations of the process or mechanisms inhibiting or enhancing the arrest of tumor cells after hematogenous dissemination have not been obtained. In any event, for cancer treatment, it is important to determine which substances inhibit the arrest of circulating tumor cells and how to prevent hematogenous metastasis. In this review, we will focus upon coagulative and fibrinolytic processes and then upon substances that inhibit the arrest of circulating tumor cells. Furthermore, some comments on the possible clinical applications of inhibitory substances for prevention of cancer metastasis are added.

The effect of thrombocytopenia and antiserotonin treatment on the lodgement of circulating tumor cells. A vital fluorescence microscopic, electron microscopic and isotope study in the rat

The hepatic lodgement of intraportally injected fibrosarcoma cells was analysed with a combination of vital fluorescence microscopic, electron microscopic and isotope techniques in normal, thrombocytopenic and antiserotonin treated (Ketanserin) rats. Ketanserin had no effect on the initial arrest of the tumor cells, as measured 5 min after tumor cell injection, which is in analogy with our previous results on thrombocytopenia. Three hours after injection, the number of lodged tumor cells was significantly reduced by both thrombocytopenia and Ketanserin treatment. Thrombocytopenia was more efficient in reducing tumor cell lodgement than antiserotonin treatment. The in vivo and electron microscopic observations indicated that the reduction of tumor cell lodgement was due to an increased destruction of tumor cells. The similar effects of Ketanserin and thrombocytopenia suggest that serotonin, when released from platelets activated in the presence of tumor cells, increases the survival of fibrosarcoma cells lodged in the liver.

Differing platelet aggregating effects by two tumor cell lines: absence of role for platelet-derived ADP

Two different mechanisms of aggregation of heparinized human platelet-rich plasma have been identified with two tumor cell lines: In neither case are these mechanisms dependent on platelet-derived ADP. U87MG cells from a glioblastoma line of human origin caused a single irreversible wave of aggregation simultaneously with the onset of platelet secretion, and this was inhibited by heparin and hirudin but not by apyrase or phospholipase D. In contrast, Hut 20 cells from an undifferentiated tumor cell line of murine origin gave an initial reversible wave followed by a second irreversible wave, which then led to secretion. The first wave of platelet aggregation was unaffected by heparin or hirudin but was inhibited by apyrase, and the second wave was inhibited by phospholipase D. Citrate caused irreversible inhibition with either cell line, and aggregation did not occur with gel filtered platelets. These results suggest that platelet aggregation by the Hut 20 line is initially dependent on ADP released from the tumor cells, whereas aggregation induced by the U87MG line is dependent on a procoagulant activity of the tumor cell surface.

Prostacyclin: a potent antimetastatic agent

Metastasis is the principal cause of failures to cure human cancers. Prostacyclin is a powerful antimetastatic agent against B16 amelanotic melanoma cells. This effect, which may result from the platelet antiaggregatory action of prostacyclin, is potentiated by a phosphodiesterase inhibitor. Inhibitors of prostacyclin synthesis increase metastasis. Prostacyclin and agents that may increase endogenous prostacyclin production or prolong its activity are suggested as new antimetastatic agents.

In vitro characteristics of metastatic variant subclones of restricted genetic origin

We have studied several metastatic variant cell lines derived from a common clonal origin and their transformed and untransformed parental cell lines. A number of in vitro characteristics were examined for each tumor line and these properties were correlated with the ability of the tumor cells to form pulmonary nodules in an experimental metastasis assay. Direct correlations with metastatic behavior in the lung colony assay were found to exist with the amount of cell-bound Concanavalin A and the procoagulant activities of cell lysates. In vitro parameters that did not correlate with the metastatic phenotype were: population doubling times in culture, saturation density achieved in culture, the number of colony-forming cells shed from confluent cultures, rates of cellular attachment to homotypic or heterotypic cell monolayers, plasminogen-activator production and procoagulant activity produced in serum-free conditioned medium.

Thromboplastic and fibrinolytic activities of cultured human cancer cell lines

Thromboplastic and fibrinolytic activities of 14 lines of cultured human cancer cells were estimated by modified Astrup's methods. High tissue thromboplastic activity was found in one line of urinary-bladder cancer, 2 lines of gastric cancer and one line of lung cancer, but no activity was found in 6 lines of lung cancer. High fibrinolytic activity was noted in one line of gastric cancer and 2 lines of lung cancer, but no activity was seen in 6 lines of lung cancer and one line of gastric cancer. No plasmin activity was found. The tumour cell lines could be classified into 3 groups on the basis of the 2 activities. Cancer cell lines could also be classified into 2 groups: with high or low release of thromboplastin into culture media. Fibrinolytic activity was found in the culture media of all cell lines with high fibrinolytic activity. Fibrinolytic activity, but not thromboplastic activity, seemed to be influenced by the constituents of culture media. No definite correlation was found between the 2 activities and the histological types of the parent tumours of the cultured cells.

Effect of Corynebacterium parvum on peripheral blood platelets

The level of peripheral blood platelets was determined after i.v. injection of Corynebacterium parvum in normal C57BL mice and in those bearing the Lewis lung carcinoma. Twenty minutes after injection of a formalin-killed active strain (CN6134, (CN6134, which inhibited tumour metastases) or a killed inactive strain (CN 5888, which did not inhibit metastases) the number of circulating blood platelets was reduced by 50%. The level of platelets returned to control values by 8 h after the active, and by approximately 3 days after the inactive strain. The active strain alone caused a second and prolonged fall in platelet numbers, from approximately 16 h to 21 days after injection. Heparin given 3 X weekly to these mice restored the platelet count to normal values by 10 days after injection of active-strain C. parvum. The level of platelets in tumour-bearing mice was essentially similar to that in normal mice. Possible causes of the thrombocytopenia and the significance of platelets in metastasis are discussed.

The spread of cancer in the organism. Facts and problems

In this review, cancer is conceived as an alteration of the suface-monitored social behavior of cells. Apart from impaired growth controls, loss of residency (tissue affiliation) is the most important consequence of this homeostatic disorder. It results in local spread (penetration) which is initiated by locomotive and/or desctructive activities of the neoplastic cells. Access of cancer elements to the circulation possibly leads to distant spread (metastasis). Penetration and metastasis largely depend upon reactions of the organism, which are of an ill-understood, ambiguous nature favoring both the tumor and the host.