Angiostatin II is the predominant glycoform in pleural effusates of rabbit VX-2 lung tumors

Animal models of malignant pleural effusion

PURPOSE OF REVIEW

Malignant pleural effusion (MPE) poses a common and significant clinical problem. Its pathogenesis is poorly understood and therapeutic options are limited. Herein are summarized animal models of MPE and their contributions in unveiling new aspects of the pathobiology of the condition.

RECENT FINDINGS

In recent years, different groups have developed novel models of MPE, including a genetic mouse model of spontaneous mesothelioma development, a model of adenocarcinoma-induced MPE in immunocompetent mice, as well as models of human cancer-induced MPE in immunocompromised animals, all relevant to the human condition to a different extent. Work using these models has yielded novel insights into the pathogenesis of mesothelioma as well as into the mechanisms of intrapleural malignant effusion accumulation and tumor dissemination. The data produced underline the significance of tumor-associated inflammation, angiogenesis, and vascular hyperpermeability in the pathogenesis of MPE.

SUMMARY

In the past few years, novel approaches to induce experimental MPE have yielded new insights into its pathogenesis and have identified possible therapeutic targets to block pleural fluid exudation induced by primary and metastatic cancer cells in the pleural space.

Human ovarian cancer ascites fluid contains a mixture of incompletely degraded soluble products of fibrin that collectively possess an antiangiogenic property

Ovarian cancer ascites fluid (OCAF) displayed an antiangiogenic property in a chick chorioallantoic membrane (CAM) assay. This property was attributed in part to angiostatin although angiostatin-free OCAF retained a net antiangiogenic property. Recently, immunopurified fibrin(ogen) degradation products (FDPs) from malignant effusions of VX2 tumor-burdened rabbits exhibited antiangiogenic activity on the CAM. We questioned whether the FDPs of OCAF were also antiangiogenic. FDPs were immunopurified from individual OCAF samples, characterized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis /western blots, enzyme-linked immunosorbent assays, and CAM assays. FDPs of OCAFs consisted of soluble high molecular weight (MW) fragments (>200 kd; approximately 40% of total FDPs), D-dimer (approximately 180 kd; approximately 37%), fragment D (approximately 90 kd; approximately 15%), and fragment E (approximately 50 kd; approximately 8%); intact fibrinogen was absent. When applied to CAM surfaces (0.5-1.6 mg/10 mL), purified FDPs significantly reduced the area of chorionic capillaries from 90% (in controls) to 47% over a 48-h period; from CAM sections, capillary density was reduced from 60% (controls) to 26%. FDPs prepared from fibrinogen displayed a similar antiangiogenic effect. Further digestion of OCAF FDPs by human plasmin caused degradation of high MW fragments, releasing additional D-dimer, fragment D, and fragment E. Of the fibrinogen-related components, OCAF contained only soluble FDPs (including incompletely digested fibrin fragments). Collectively, these FDPs contributed to the net antiangiogenic property of ascites fluid.

Relationships among tumor burden, tumor size, and the changing concentrations of fibrin degradation products and fibrinolytic factors in the pleural effusions of rabbits with VX2 lung tumors

The VX2 tumor is derived from a papilloma virus-induced rabbit epithelial cell line. If VX2 tumor cells (trapped in a plasma clot) are introduced intravenously into NZW rabbits, the cells lodge in the lung capillary bed and produce tumors. Independently of the tumor burden (ie, the total tumor weight per rabbit), approximately 15% of rabbits with VX2 lung tumors accumulate an effusion in the interpleural space and this pleural effusion contains products of hemostasis. We hypothesized that these products were of intra-tumoral origin and that they changed in concentration as tumor burden increased. Interrelationships among lung-, tumor-weights, and pleural effusion volumes, and the concentrations of fibrinolytic factors, their catabolic products, and other proteins of pleural effusions were measured in rabbits with a wide range of tumor burdens. Positive correlations between tumor burden and total lung weight and between pleural effusion volume and net lung weight suggested that interstitial fluid from the stroma of tumors passed directly into the extravascular space of the lung(s) and into the interpleural space(s). Analyses of pleural effusions indicated that plasminogen-, alpha(2)-antiplasmin-, and plasminogen activator inhibitor-1-related proteins, urokinase-like- and tissue-plasminogen activator activities, and vascular endothelial growth factor increased in concentration up to a tumor burden of approximately 20-25 g. Plasmin activity and intact fibrinogen were absent. The concentration of fibrin(ogen) degradation products did not change significantly up to a tumor burden of approximately 25 g but increased substantially as tumor burdens exceeded 25 g. In conclusion, interstitial fluid from tumors enters the extravascular space of the host and may accumulate with fluid from non-tumor sources as a pleural effusion. The concentrations of fibrinolytic factors and their products in pleural effusions reflect the tumor burden of the rabbit. Conceivably, the components of a malignant effusion contain much information about the extent of tumor growth.

Quantification of anti-angiogenesis using the capillaries of the chick chorioallantoic membrane demonstrates that the effect of human angiostatin is age-dependent

We present a method whereby en face estimation of the chorionic capillary plexus can be generated in the living chick chorioallantoic membrane (CAM) and confirmed by post-fixation cross section analysis. This value does not alter significantly with age and provides a reliable and simple method to evaluate anti-angiogenesis. Anti-angiogenesis may be induced by an intervention, such as a pharmacological agent, applied to the surface of the CAM. We describe the use of silastic rings that are associated with minimal inflammatory reaction, in this process. By estimating changes in the chorionic capillary plexus to quantify anti-angiogenesis, together with silastic rings, we examined the anti-angiogenic effect of human angiostatin and demonstrated that although there is a significant loss of capillaries en face after exposure from days 7 to 9 of incubation, in contrast there is no significant inhibition after exposure to a similar dose of angiostatin from days 11 to 13 of incubation. This not only demonstrates the important effects on neo-angiogenesis compared to mature vessels, but also illustrates the potential of the CAM to readily provide a means for such a comparison.

Fibrinogen catabolism within the procoagulant VX-2 tumor of rabbit lung in vivo: Effluxing fibrin(ogen) fragments contain antiangiogenic activity

Many types of solid tumors are known to be procoagulant environments. This is partly because a hyperpermeable vascular system within the tumor allows plasma hemostatic factors to accumulate in relatively high concentrations in the stroma, and many solid-tumor cells express tissue factor or a procoagulant factor. These circumstances appear to exist in the VX-2 lung tumor of the New Zealand White (NZW) rabbit, and they sustain a measurable turnover of stromal deposits of fibrin(ogen). We have measured the turnover of fibrinogen within tumors of the VX-2 tumor-burdened rabbit and analysed the catabolic products of fibrin(ogen) and the status of fibrinolysis in tumor-derived interpleural effusate. Using intravenously injected (125)I-labeled rabbit fibrinogen as a marker, we found that fibrinogen (approximate blood concentration 1740 microg/mL) passed from blood to VX-2 tumor stroma, saturating the tumor at a concentration of approximately 348 microg fibrinogen/g in approximately 12 hours. We measured fibrin(ogen) fragments, at a concentration of approximately 292 microg/mL, in interpleural effusates that we recovered from 13% of the VX-2-burdened rabbits. Unreduced fibrin(ogen) fragments consisted of 4 major components with a relative molecular mass of approximately 250,000 (assumed to be fragment X; approximately 9% of total fragments from densitometry of immunoblots), 200,000 (d-dimer; 41%), 110,000 (fragment D; 49%), and 50,000 to 55,000 (fragment E; 1%-2%) kD. Total fibrin(ogen) fragments immunopurified from effusates exhibited an antiangiogenic effect when subjected to a chick embryo chorioallantoic membrane procedure. Interpleural effusates were devoid of plasmin activity or active plasminogen activator inhibitor-1 but contained plasmin complexes and active urokinase-like plasminogen activator (uPA), alpha(2)-antiplasmin, and thrombin-activatable fibrinolysis inhibitor. We speculate that VX-2 cells release uPA to activate fibrinolysis within the tumor stroma. Catabolic products of hemostasis (eg, fibrinolytic fragments, angiostatin) flux from the stroma into the interpleural space, thereby providing a net antiangiogenic property to the effusate and ultimately to the lymphatic and circulatory systems.

Malignant ascites fluid (MAF), including ovarian-cancer-associated MAF, contains angiostatin and other factor(s) which inhibit angiogenesis

OBJECTIVE

The aim was to determine whether human malignant ascites fluid (MAF) associated with abdominal cancer, including ovarian cancer, contained factors which inhibit angiogenesis as well as others which stimulate this process.

METHODS

MAF was collected from six patients, four with ovarian cancer, one with gastric cancer, and one with liver metastases. Using the chick chorioallantoic membrane (CAM) the effect of MAF on 7-day-old CAM capillaries was examined for 48 h. Vascular endothelial growth factor (VEGF) was evaluated by ELISA. Five samples of MAF were fractionated by lysine-Sepharose chromatography and the lysine-bound and -unbound fractions were eluted by epsilon-amino-n-hexanoic acid. Whole MAF, the lysine-bound and -unbound fractions, and human angiostatin were subjected to SDS-PAGE/Western blot analysis and immunostained after exposure to anti-human plasminogen. Human plasminogen was exposed to conditioned medium from ovarian epithelial cancer (HEY) cells and subjected to similar Western blot analysis.

RESULTS

Despite containing VEGF, each MAF sample examined caused a loss of capillaries from the CAM; a similar response was seen using purified human angiostatin. Whole MAF and the lysine-bound fraction contained plasminogen (90 kDa) and a 55-kDa protein which migrated in a similar manner to human angiostatin on Western blot. Both the lysine-bound and -unbound fractions caused a loss of capillaries in the CAM. Human plasminogen exposed to conditioned medium from HEY cells yielded a fragment which was similar in size to angiostatin.

CONCLUSIONS

MAF from patients with various clinical presentations contains angiostatin and VEGF as well as other factors which are capable of inhibiting angiogenesis.