Fibrin fragment E stimulates the proliferation, migration and differentiation of human microvascular endothelial cells in vitro.

Various factors involved in haemostasis also regulate the development of new blood vessels by a process called angiogenesis. Enzymatic cleavage of fibrin yields a variety of fibrin degradation products, particularly in areas of intense angiogenesis such as in healing wounds and active atherosclerotic plaques. One of these, fibrin fragment E (FnE), is a potent angiogenic factor in the chick chorioallantoic membrane assay of angiogenesis. Here, we extend these studies to show that FnE stimulates the proliferation, migration and differentiation of human dermal microvascular endothelial cells (HuDMECs) in vitro, both in the absence and presence of such additional endothelial growth factors as vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF). We also show that these stimulatory effects occur at concentrations of the protein known to be present in angiogenic tissues in vivo. FnE enhanced the angiogenic effects of VEGF or bFGF, indicating a possible synergy between the signalling pathways used by these three angiogenic factors.

Smooth muscle cell outgrowth stimulated by fibrin degradation products. The potential role of fibrin fragment E in restenosis and atherogenesis.

This study is based on the observation that deposition of thrombus within the arterial wall and on its surface is a consistent response to the vascular injury of angioplasty and of angioplasty lesions at risk of rapid restenosis. Mitogenic activity is stimulated by fibrin degradation products in extracts of human atherosclerotic plaques and plasmin digests of fibrin, and this has been attributed to products that include fibrin fragment E. The effect of human fibrin degradation products on smooth muscle outgrowth from rabbit aortic medial explants now has been explored in culture. Every batch of fibrin degradation products was first tested on the in vivo chick chorioallantoic membrane model for the ability to stimulate cell proliferation, including angiogenesis as shown previously. Increasing concentrations of fibrin degradation products were stimulated significantly earlier and with greater outgrowth of smooth muscle cells than controls, up to an optimum at 92 microg/mL fibrin degradation products. The effect of fibrin degradation products was blocked by the prior admixture of a specific antifragment E antiserum, but not by an antifragment D antiserum. Purified commercial fibrinogen E is inactive, but when treated with thrombin to resemble fibrin E it stimulated smooth muscle cell outgrowth, and this was not seen with comparable dosages of fragment D. We propose that fibrin degradation products, in particular fibrin fragment E, provide an abundant in situ early initiator of smooth muscle cell migration and proliferation in restenosis and atherogenesis.

Locating the active site for angiogenesis and cell proliferation due to fibrin fragment E with a phage epitope display library.

The plasmin-mediated lysis of fibrin present in a wound, or in chronic inflammatory disease, results in the release of fibrin degradation products. One of the two major products is fibrin fragment E, which has been shown to stimulate cell proliferation in many cell types including endothelium, fibroblasts, and smooth muscle cells, and to be angiogenic in the chick chorioallantoic membrane (CAM) system. The activity of fibrin fragment E is dependent on N-terminus thrombin action. Antibodies against fibrin E, which block the cell proliferative activity, were used to locate the active site. Phage epitope display libraries were used to identify the sequence of a peptide, which resembles a region of the N terminus structure. The equivalent synthetic peptide (WTM110) has optimal stimulatory properties at equimolar concentrations to the parent molecule. Such peptide information has therapeutic potential for both stimulating and suppressing angiogenesis and cell proliferation.

Expression of Wnt genes in early wound healing.

The Wnt family of developmental genes has previously been shown to be involved in proliferation, differentiation, and cell to cell signaling during embryogenesis. In addition, several Wnt genes have been shown to be expressed during carcinogenesis. We have investigated these genes during the wound-healing process. Wnt-4 gene expression is found in mouse wounds from 2 hours to 30 hours postwounding. The expression of Wnt-4 is also stimulated by direct trauma to murine fibroblasts in culture, and the expression is greatly enhanced by the addition of a short plasmin digest of fibrin. Therefore the regulation of Wnt-4, appears to be complex, with expression being stimulated both by direct trauma and by the influence of clotting and fibrinolysis products. We propose that the expression of Wnt-4 in the early wound, in response to the provisional fibrin matrix, regulates cell movement and proliferation in the creation of new tissue by mechanisms related to those of embryogenesis.

Presence of growth-stimulating fibrin degradation products containing fragment E in human atherosclerotic plaques.

The key event in the formation of stenosing atherosclerotic lesions is widely thought to be smooth muscle cell proliferation, but the factors primarily responsible for initiating this remain uncertain. Previously we have shown that aqueous extracts of proliferative types of human atherosclerotic plaque stimulate cell proliferation in the chick chorioallantoic membrane (CAM). This has been attributed largely to the fibrin degradation products in the extracts, components removeable by affinity chromatography. We now demonstrate that the fibrinogen content of the extract, removeable by clotting out with thrombin, also makes a contribution to the activity by forming fibrin on the surface of the CAM. Affinity chromatography experiments using anti fragment D and E antisera indicate that activity resides in the E-containing fibrin fragments, consistent with previous work with FDP prepared in vitro.

Fibrin degradation products in growth stimulatory extracts of pathological lesions.

We have previously shown that similar patterns of fibrin degradation products (FbDP) by gel electrophoresis and immunoblotting are present in extracts of human atherosclerotic plaques, human and experimental wounds and breast cancers. Such extracts were also shown to stimulate cell proliferation including angiogenesis in the chick chorioallantoic membrane, now shown also for breast cancers. Removal of FbDP from plaque extracts by an anti-fibrinogen affinity column, or by an anti-fragment E column, reduced activity. Human FbDP prepared in vitro were active, but not FgDP. Fibrin fragment E was active, and we also showed that admixture of FbDP with a polyclonal rabbit anti-fibrin E but not anti-fibrin D neutralized activity. However attempts to raise comparable monoclonal blocking antibodies were hindered by species similarities. The response of the Balb/c mouse was predominantly directed at minor D contaminants, in contrast to the Sprague-Dawley rat which responded to fibrin fragment E in our antigen preparation.

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.

Artificial exudate: stimulation of cell proliferation by plasma not serum is associated with fibrinolysis.

Despite its abundance of cell culture growth factors, serum does not enhance growth of the in vivo test system, the chick chorioallantoic membrane (CAM). Previously we have also shown that fibrinogen (Kabi) and its degradation products do not display mitogenic activity on the CAM, but have now been surprised to find stimulation of DNA synthesis (up to 2.5-fold) 18 h after application of human plasma [incorporation of [3H]thymidine control mean +/- SEM 8442 +/- 1338 dpm/CAM (n = 8); test--20 199 = 3491 (n = 6); t-test P less than 0.01)]. Plasma (platelet-rich and platelet-poor) was freshly prepared by minicolumn removal of citrate at 16 degrees C; 0.3 ml aliquots of 10% human plasma were applied to the CAM and groups were fixed at various time intervals. Cross-sections were examined histologically, including immunoperoxidase studies with antihuman fibrinogen which does not cross react with chick fibrinogen, and autoradiography with [3H]thymidine. Initially, a layer of plasma was observed on the surface of the CAM. After 1 h a condensed fibrillar layer of fibrin was formed and was still present at 6 h. This was associated with increased [3H]thymidine labelling of the surface layer of the CAM. By 18 h the fibrin had disappeared, indicating that it had been lysed by the CAM, and widespread [3H]thymidine labelling was observed. No inflammatory cell infiltrate was apparent, but marked oedema developed. Oedema alone was observed in controls receiving serum or Dulbecco buffer. Both platelet-rich and platelet-free plasma gave stimulation of mitogenic activity, implying that platelet-derived growth factor is not involved in the stimulation of the CAM.(ABSTRACT TRUNCATED AT 250 WORDS)

Atherosclerotic plaque growth: presence of stimulatory fibrin degradation products.

Focal smooth muscle cell proliferation is widely perceived as a key event in the formation of stenosing atherosclerotic lesions, but the stimuli for this remain uncertain. Soluble extracts of human aortic intima from proliferative gelatinous and transitional lesions, as well as surface encrusted thrombi, have been shown by us to be mitogenic for the chick chorioallantoic membrane (CAM). They have also been shown to stimulate increase in vascularity of the CAM. When active samples were passed through anti-albumin and anti-whole-serum affinity columns, mitogenic activity in the unabsorbed, fibrin related antigen fraction remained close to the original whole extract level. In contrast, when the unabsorbed fractions from anti-whole-serum columns were passed through an antifibrinogen affinity columns, the activity was reduced to insignificant levels. Similarly, whole extracts lost activity after passing through an antifibrinogen column. This has been taken one stage further by dividing the unabsorbed fraction from an anti-whole-serum column into two equal volumes and passing one half through an antifibrinogen fragment D affinity column, and the other through an antifibrinogen fragment E affinity column. The activity of the unabsorbed fraction from the fragment D column remained the same, but that from the fragment E column was significantly reduced. Most of the fibrin degradation products (FbDP) in lesion extracts are derived from fibrin, not fibrinogen, and clotting out fibrinogen and fragment X with thrombin did not remove the activity. Whole extracts of atherosclerotic lesions clotted on the CAM surface as has previously been shown with plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors relevant to stimulatory activity of fibrin degradation products in vivo.

Extracts of atherosclerotic lesions contain a range of fibrin degradation products (FbDP), similar fragments have been detected in extracts from human and mouse healing skin wounds and from the invasive edge of human breast carcinomas, which are all proliferating systems. We have previously shown that FbDP stimulate cell proliferation including angiogenesis in the chick chorioallantoic membrane (CAM), and sought to characterize further the active components. Fibrin prepared from platelet-rich and platelet-free plasma, and purified Kabi fibrinogen, was treated with plasmin, and the digests were all active. FbDP from platelet-rich plasma clots also increased vascularity of the CAM. Prior removal of fibronectin from plasma by gelatin-Sepharose affinity chromatography did not affect proliferative activity. Current studies showed that long digests of fibrin, in which the only major band detectable is fibrin fragment E are active. Commercial fibrinogen derived fragment E, itself inactive on the CAM, becomes active after exposure to thrombin cleavage of fibrinopeptides. Recently fragment E has been isolated from shorter digests, by simple filtration through a Millipore 0.2 microns centrifuge filter. It displayed similar activity to the fragment E obtained from long digests. Fragment E in plaque extracts has been shown consistently to lack fibrinopeptide A indicating it is of fibrin origin.