Factors relevant to stimulatory activity of fibrin degradation products in vivo

The role of fibrinogen and related fragments in tumour angiogenesis and metastasis

Angiogenesis, the development of new blood vessels from existing vasculature, involves the migration, proliferation and differentiation of endothelial cells and is crucial for the growth and mestastasis of tumours. A specific association between cancer and the haemostatic system has long been recognised. Haemostatic mechanisms regulate blood flow by controlling platelet adhesion and fibrin deposition, and a number of haemostatic proteins have been shown to regulate angiogenesis, either directly, by interacting with endothelial cells themselves, or indirectly, by interacting with other regulators of angiogenesis. The polypeptide fibrinogen is the central protein in the haemostasis pathway and is found deposited in the majority of human and experimental animal tumours. In this review, the evidence for the ability of fibrinogen and various protein/peptide fragment derivatives to modulate angiogenic mechanisms in vitro and to affect tumour growth and metastasis in vivo is discussed.

Angiogenic activity of fibrin degradation products is located in fibrin fragment E

The source of angiogenic activity of fibrin degradation products has been sought in a series of experiments, applying degradation products from different types of fibrin and fibrinogen to the chick chorioallantoic membrane. The presence of platelets or fibronectin during clotting was not essential for activity, and neither was crosslinking. Fibrinogen degradation products were non-stimulatory, as was serum. Molecular sieve column chromatography indicated a range of active fragments. Admixture of active fibrin degradation products with antifibrin fragment E, but not D, antiserum neutralized activity. Preparations containing only fibrin fragment E retained activity. A commercial preparation of fibrinogen fragment E was inactive until treated with thrombin. These experiments point to fibrin fragment E being the source of angiogenic activity, with thrombin cleavage being the essential step in generating activity.

Fibrinogen/fibrin in atherogenesis

Fibrin is a major component of many atherosclerotic plaques. Within the intima there is continuous formation of fibrin, and continuous fibrinolysis. In aortic lesions, a lipoprotein bound to fibrin can be released by incubation with plasmin. Most of this lipoprotein is accounted for by Lp(a). The atherogenicity of Lp(a) may be more associated with lipid deposition than with inhibition of fibrinolysis. Fibrin degradation products may be chemotactic to monocyte-macrophages and stimulate smooth muscle cell proliferation.

Fibrinolysis and angiogenesis in wound healing

The healing wound offers a clear example of the sequence of events in chronic inflammation leading to repair. Although angiogenesis has an obvious and essential role in this process, it has been little studied. For an angiogenic factor to seem relevant, it would have to be shown to precede the peak of increased vascularity. To define this peak, the vessel content of simple, incised mouse wounds was estimated using morphometry of histological sections, and found to rise to a maximum at days 5 and 6. Total angiogenic activity of aqueous extracts was found to reach a peak at day 3. The detection of such activity on the chick chorioallantoic membrane is very dependent on the preparation technique and the choice of proteinase inhibitors. Previous in vitro work by us using purified material has shown fibrin degradation products to be effective in stimulating angiogenesis. Fibrin degradation products are prominent on immunoblotting from day 3, when macrophages are plentiful, with a similar band pattern to human granulation tissue.