Fibrin and atherogenesis--a hypothesis

Correlation Between Hemorheologic Parameters and Carotid Atherosclerosis in Stroke

An evaluation of the hemorheologic parameters and an ultrasonographic study of the extracranial carotid tract were performed on 48 patients, age range: sixty to seventy-five years, affected by acute stroke, 23 of whom had hypertension, while 27 were smokers. None of them was suffering from hyperli pidemia, diabetes mellitus, or symptomatic coronary heart or peripheral artery disease.

The echotomographic analysis, using B-mode real-time echotomography re vealed atherosclerotic lesions in 26 patients. The hemorheologic pattern (hema tocrit, fibrinogen, whole blood filterability, whole blood and plasma viscosity) was determined in all the patients three months after the clinical event.

Statistical analysis of the results indicates a possible link between athero sclerotic lesions of the extracranial carotid tract and age, fibrinogen levels, and whole blood filterability.

Defective Fibrinolysis in Arterial Disease

Defective fibrinolysis is a factor in a variety of vascular diseases both arterial and venous. Therapeutic correction of the defect has been shown to be successful in recurrent venous thrombosis, cutaneous vasculitis and liposclerosis. Therapeutic correction in arterial diseases has yet to be fully evaluated.

Iron-induced fibrin in cardiovascular disease

Accumulating evidence within the last two decades indicates the association between cardiovascular disease (CVD) and chronic inflammatory state. Under normal conditions fibrin clots are gradually degraded by the fibrinolytic enzyme system, so no permanent insoluble deposits remain in the circulation. However, fibrinolytic therapy in coronary and cerebral thrombosis is ineffective unless it is installed within 3-5 hours of the onset. We have shown that trivalent iron (FeIII) initiates a hydroxyl radical-catalyzed conversion of fibrinogen into a fibrin-like polymer (parafibrin) that is remarkably resistant to the proteolytic dissolution and thus promotes its intravascular deposition. Here we suggest that the persistent presence of proteolysis-resistant fibrin clots causes chronic inflammation. We study the effects of certain amphiphilic substances on the iron- and thrombin-induced fibrinogen polymerization visualized using scanning electron microscopy. We argue that the culprit is an excessive accumulation of free iron in blood, known to be associated with CVD. The only way to prevent iron overload is by supplementation with iron chelating agents. However, administration of free radical scavengers as effective protection against persistent presence of fibrin-like deposits should also be investigated to contribute to the prevention of cardiovascular and other degenerative diseases.

Role of platelets in the development of atherosclerotic disease and possible interference with platelet inhibitor drugs

During the last two decades, significant advances have been made in the understanding of atherosclerotic disease. The pathogenesis of atherosclerosis appears to depend on a precise sequence of critical events based on the interaction of blood elements and lipids with the arterial wall. The major critical events and their sequence appears to be as follows: hemodynamic stress and endothelial injury; arterial wall-platelet interaction; smooth muscle cell proliferation; lipid entry and accumulation; significant arterial narrowing with fibrosis and development of thrombi; and complications in the form of calcification, ulceration, aneurysm, acute thrombotic occlusion and embolization. This sequence of critical events starts at a young age and in all geographic racial groups. Their evolution into advanced symptomatic lesions takes many years and varies in incidence and extent among different geographic and ethnic groups. It appears that in promoting and accelerating this process into the advanced stage of the disease, the presence at a young age of the so-called risk factors of atherosclerotic disease is most important. The recent advances in the understanding of the atherosclerotic process will be highlighted in this chapter with particular attention being focused on the role of platelets and thrombosis in the development of the disease and the possible role of platelet inhibitor drugs on the prevention of coronary atherosclerotic disease.

Diabetes mellitus as a prothrombotic condition

Diabetes mellitus (DM) is characterized by fasting hyperglycaemia and a high risk of atherothrombotic disorders affecting the coronary, cerebral and peripheral arterial trees. The risk of myocardial infarction (MI) is 3-5 fold higher in Type 2 DM and a DM subject with no history of MI has the same risk as a non-DM subject with a past history of MI. In total around 70% of deaths are vascular with poorer outcomes to both acute events and cardiological interventions. It was proposed that clustering of vascular risk factors (hyperinsulinaemia, dysglycaemia, dyslipidaemia and hypertension) around insulin resistance (IR) accounted for the increase in risk with Type 2 DM. The importance of this became apparent with the recognition that risk clustering occurs in normoglycaemic and impaired glucose tolerance (IGT) subjects with IR, in total around 25% of the population in addition to long-standing Type 1 subjects with renal disease. Evidence indicates that thrombotic risk clustering also occurs in association with IR, suppression of fibrinolysis due to elevated concentrations of the fibrinolytic inhibitor, plasminogen activator inhibitor-1 (PAI-1) is invariable with IR and there is evidence that this is regulated by the effects of triglyceride on the PAI-1 gene promoter. Other studies indicated that prothrombotic risk (coagulation factors VII, XII and fibrinogen) also associates with the IR syndrome. The development of endothelial cell dysfunction with suppression of nitric oxide and prostacyclin synthesis, combined with platelet resistance to the anti-aggregatory effects of these hormones leads to loss of control over platelet activation. In addition, hyperglycaemia and glycation have marked effects on fibrin structure function, generating a clot which has a denser structure, resistant to fibrinolysis. The combination of increased circulating coagulation zymogens, inhibition of fibrinolysis, changes in fibrin structure/function and alterations in platelet reactivity creates a thrombotic risk clustering which underpins the development of cardiovascular disease.

The influence of type 2 diabetes on fibrin structure and function

AIMS/HYPOTHESIS

The precise mechanisms underlying the increased risk of cardiovascular disease (CVD) in type 2 diabetes are unclear. Fibrin clot structure has been related to CVD risk in the general population. We therefore assessed this in type 2 diabetic patients as a potential mechanism whereby diabetes influences CVD risk.

METHODS

Fibrin clots were formed from fibrinogen purified from 150 subjects with type 2 diabetes and varying degrees of glycaemic control (assessed by HbA1c), and from 50 matched control subjects. Clot structure was assessed by turbidity, permeation and confocal microscopy. The specific effect of glucose itself was assessed by analysing the structure of clots formed from purified fibrinogen in the presence of increasing concentrations of the sugar.

RESULTS

Clots formed by fibrinogen purified from type 2 diabetic subjects had a denser, less porous structure than those from control subjects. The structural changes found were related to the individual's glycaemic control; HbA1c correlated negatively with permeation coefficient (Ks) values (indicates clot pore size) (r = -0.57, p < 0.0001) and positively with maximum absorbance (indicator of fibre size) (r = 0.33, p < 0.0001), branch point number (r = 0.78, p < 0.0001) and fibre density (r = 0.63, p < 0.0001). The ambient glucose level influenced clot structure; hypo- (< 5 mmol) and hyperglycaemia (> or = 10 mmol/l) were both associated with a reduction in Ks values and maximum absorbance, and with increased fibre density and branch point number within clots.

CONCLUSIONS/INTERPRETATION

The structural differences found to occur in type 2 diabetes and in association with hypo- and hyperglycaemia may confer increased resistance to fibrinolysis, and in consequence contribute to the increase in CVD risk in diabetic patients.

The role of the vascular endothelium in inflammatory syndromes, atherogenesis, and the propagation of disease

The vascular endothelium is intimately involved in a wide variety of normal physiological processes, including coagulation/anticoagulation, the maintenance of vascular tone, and pathological processes, including reperfusion injury, inflammatory syndromes, and tumor cell metastasis. This review discusses the importance of increased adhesive molecule expression on the endothelial surface in promoting circulating inflammatory cell-endothelial adherence in inflammatory conditions, as well as the role of the vascular endothelium in reperfusion injury, altered microvascular permeability states, and atherogenesis.

Interaction of endothelial cells and fibroblasts with modified fibrin networks: role in atherosclerosis

Fibrinogen has been recognised in recent years as an independent risk factor in athero/thrombogenesis. However, the mechanism by which elevated fibrinogen translates into higher incidence of atherosclerosis is not known. One possible mechanism may be through the modification of fibrin. While it is already known that fibrin network is altered in disease states like peripheral vascular disease, diabetes, hypercholesterolaemia and myocardial infarction, the influence of altered fibrin network structure on growth and function of endothelial cells (EC) and fibroblasts (FB) requires investigation. Fibrin network structure in plasma clots was modified by changing pH and characterised using established biophysical methods. PGI(2), von Willebrand Factor (vWF), t-PA and PAI-1 were measured to evaluate changes in cell function induced by modified fibrin structure. In general, networks composed of thin fibres induced growth over their entire layer. Networks composed of thick fibres and open matrix promoted infiltration of cells into gel matrix and growth of macrovascular structures. Furthermore, thin fibres promoted a more prothrombotic environment as observed from changes in cell biochemical function. Fibrin, whilst initially acting as a scaffolding for cellular and biochemical processes, may also alter cell function and determine the progress of atherosclerosis.

Endothelium, fibrinolysis, cardiac risk factors, and prostaglandins: a unified model of atherogenesis

A model of atherogenesis is described in which it is proposed that a state of relative impairment of intravascular fibrinolytic function is the primary defect which makes possible both the initiation and the continued progression of arterial plaques. The key mechanism by which impaired fibrinolysis is atherogenic centers on the unique disruptive effect which fibrin has on the contiguous endothelium of the vascular intimal surface. From this perspective, in areas of spontaneous endothelial injury, impaired fibrinolysis maintains and promotes the gradual enlargement of the area of injury by causing persistently increased intimal permeability and by allowing enhanced fibrin and platelet deposition. This hypothesis thus represents a modification of the response-to-injury hypothesis in which the emphasis has been shifted from the initial endothelial injury to a state of interference with the normal process of healing endothelial injuries. Consistent with this viewpoint, it is noted that all positive risk factors for vascular disease are associated with impairment of fibrinolytic function and, conversely, negative cardiac risk factors enhance fibrinolysis. It is further proposed that one or more prostaglandins, or closely related metabolites, represent the mediators of primary physiologic importance with regard to in vivo regulation of fibrinolysis. By this hypothesis, adequate dietary intake of essential fatty acids, as well as maintenance of unimpaired eicosanoid metabolism, become centrally important in both preventing and reversing arteriosclerosis. This two-tiered model can be used to organize and potentially explain the interrelationship between diverse and apparently divergent sets of epidemiological data which previous models have been unable to accommodate.

The possible role of hemorheology in atherothrombogenesis

Conventional risk factors predict only about 30-50% of incidental cases in cardiovascular diseases, which are still the leading cause of death in western societies. During the last decade, the importance of thrombosis as an essential mechanism in acute myocardial infarction (AMI) and stroke has been established. The introduction of thrombolysis has led to an impressive reduction in AMI case fatality and possibly also to a substantial amelioration of its prognosis. Evidence from experimental, clinical and epidemiological studies suggest, that several hemostatic and hemorheological factors (e.g., fibrinogen, Factor VII, plasma viscosity, hematocrit, red blood cell aggregation, total white cell count) might not only play an important role in the evolution of acute thrombotic events, but may also take part in the pathophysiology of atherosclerosis. An increasing number of studies reports altered hemostatic and hemorheological parameters to be associated with smoking, hyperlipoproteinemia, and high blood pressure, as well as with adverse dietary habits and other life-style factors. To date, their way of interaction with the atherosclerotic process is poorly understood. Hemorheological or hemostatic mechanisms that might promote thromboatherogenesis include the predisposition to thrombosis via a hypercoagulable state, the enhancement of atherosclerosis by fibrinogen and its metabolites, and finally the reduction of blood flow through various rheological effects (e.g., increase in plasma viscosity and red cell aggregation, or leukocyte activation). Future research should focus in more detail on the interrelationship between accepted risk factors and the hemostatic system as well as hemorheological parameters. Deeper insight into the mechanisms involved might lead to new preventive strategies as well as to therapeutic procedures in the management of atherosclerosis and associated thrombotic events.

Clinical disorders of fibrinolysis: a critical review

Much progress has recently been made in understanding the biochemistry and physiology of endogenous fibrinolysis. As a result, a better understanding of the mechanisms and clinical consequences of disordered fibrinolysis has emerged. Increased fibrinolytic activity is an uncommon but important cause of hemorrhagic disease. Congenital disorders of fibrinolysis which cause bleeding include increased plasma plasminogen activator activity and deficiency of alpha-2 antiplasmin. Acquired disorders associated with increased fibrinolytic activity and bleeding include liver cirrhosis, amyloidosis, acute promyelocytic leukemia, some solid tumors, and certain snake envenomation syndromes. Increased fibrinolysis is important to recognize because epsilon-aminocaproic acid (EACA) may be required to prevent or control bleeding. Diminished fibrinolytic activity has been associated with a variety of thrombotic disorders, but a direct cause-and-effect relationship has yet to be established. Congenital abnormalities of fibrinolysis associated with thrombosis include plasminogen deficiency, decreased endothelial generation of plasminogen activator activity, and certain abnormal fibrinogens. Thrombosis in these disorders is effectively managed with warfarin. Diminished fibrinolysis has also been reported in "idiopathic" venous thrombosis, oral contraceptive-induced and post-operative venous thrombosis, coronary artery disease, cerebrovascular disease, systemic lupus erythematosus, and thrombotic thrombocytopenic purpura, but the significance of abnormal fibrinolysis in these disorders is uncertain. Large, prospective studies of fibrinolytic variables as risk factors for vascular and thrombotic disease are needed to determine whether pharmacologic augmentation of impaired fibrinolysis could be useful in the prevention or treatment of these disorders.

Thrombosis in atherogenesis

This review addresses the question of the involvement of fibrin in the development of atherosclerotic plaques. Numerous studies in the older literature demonstrated the presence of fibrinogen and/or fibrin in plaques, but the techniques that were available (mainly immunochemistry and immunohistochemistry with polyclonal antifibrinogen antibodies) did not clearly distinguish fibrinogen from fibrin or fibrinogen/fibrin degradation products. Some of these studies suggested that the fibrinogen-related protein within lesions resulted from incorporation of thrombi into lesions, while other studies suggested that fibrinogen itself entered the vessel wall. Newer studies by the authors and collaborators used specific antibodies for various fibrinopeptides to quantitate fibrinogen, fibrin I, fibrin II, and fragment X in thrombi and different histologic types of plaques. These studies showed that normal aortas contained fibrinogen and that fatty and fibrous plaques contained fibrinogen, fibrin I, and fibrin II, while complicated plaques contained fibrin II and fragment X, indicating a progression from fibrinogen to fibrin and fibrinogen/fibrin degradation products in parallel with increasing severity of the lesions. Later studies by the authors and collaborators used a sensitive immunohistochemical technique with monoclonal antibodies to demonstrate the distribution of fibrinogen-related antigens. Patterns suggesting incorporation of thrombi were seen, as were patterns suggesting formation of fibrin in association with arterial wall monocyte/macrophages and smooth muscle cells. The data from these various studies suggest the possibility that fibrin formation occurs within the arterial wall and contributes to plaque formation.

Blood rheology associated with cardiovascular risk factors and chronic cardiovascular diseases: results of an epidemiologic cross-sectional study

As part of an epidemiologic cross-sectional study to determine cardiovascular (CV) risk factors in the population (total serum cholesterol, smoking, blood pressure, and body weight) hemoglobin (Hb) and plasma viscosity (PV) were measured. A two-stage cluster sample of 5,312 persons, aged twenty-five to sixty-four (available 5,069) was selected from a mixed urban-rural target population of 282,279 inhabitants, from which 4,022 (79.3%) participated in the study. Patients with chronic myocardial infarction (MI), cerebral infarction (CI), angina pectoris (AP), and peripheral arterial disease (PAD) were identified by questionnaire. The results show that there is no age or sex dependency of PV in healthy participants, while hemoglobin shows the well-known sex difference. In contrast, PV increases continuously with age in the total population. In men, increased PV is found in untreated hypertension, in hypercholesterolemia, and in smokers. In women, it is raised in hypercholesterolemia and in gross obesity. Male MI patients and patients of both sexes after CI in particular show statistically significantly elevated PV. Finally, in male patients with chronic AP or patients of both sexes with PAD, PV is elevated and a tendency to higher Hb values is seen. These results confirm smaller clinical trials suggesting that blood fluidity is pathologically altered in patients with CV risk factors or diseases. Since impaired blood fluidity may worsen the hemodynamic situation, in particular in patients with limited vasomotor reserve, hemorheologic parameters may be of prognostic relevance. Therapeutic implications of these findings should be considered.

The effect of endothelial cell proliferation in vitro

The proliferation of bovine aortic endothelial cells is highly stimulated by using a plasma clot instead of glass or plastic as a substrate. The cells form a dense monolayer which is usually under grown by a second layer of endothelial cells showing a more reticular growth pattern. When endothelial cells attached to a plasma clot are covered with a second layer of coagulated plasma, the cells arrange to shapes which bear a great resemblance to capillary structures. It seems that this endothelial cell property is only expressed when the entire cell surface is forced to interact with a surrounding adhesive matrix.

Role of the altered transmural permeability in the pathomechanism of arteriosclerosis. History of arteriosclerosis theories. Role of the altered permeability in experimental arteriosclerosis models

After presenting an overview on classification and history of arteriosclerosis theories, the physiological factors involved in the transmural permeability of the arteries are discussed in detail. The development and characteristic features of the altered transmural permeability were studied in various experimental models such as in rat's hypercholesterolemia, local aortic hypoxia, lymphedema of the vascular wall and in hypertension. Results appear to show that alterations in permeability invariably developed in all of the pathological conditions examined, they were transient in nature and preceded the onset of intimal proliferation(s). The disturbance of transmural permeability might be the common pathologic clue which initiates uniform vascular responses to injuries produced by a variety of noxious stimuli. The possible role of the altered transmural permeability in the induction of smooth muscle cell proliferation is also discussed and evidence is provided that after withdrawal of stimulus for vascular injury intimal proliferation will not develop despite the manifest disorders in permeability.

Biochemical changes in endothelial cell monolayers induced by fibrin deposition in vitro

Endothelial cells in vivo come into contact with fibrin during numerous physiological and pathological processes, but the nature of this interaction is poorly understood. The interaction of fibrin and endothelial cells was examined by incubating bovine aortic endothelial cell monolayers in vitro with fibrinogen (100 micrograms/ml) in medium supplemented with 10% calf serum. This treatment produced a layer of fibrin above the endothelial cell monolayer, which induced disorganization of the monolayer and remained associated with the monolayer even after washing. Endothelial cell pinocytosis of 14C-sucrose was elevated by 25% and 60% after incubation with fibrin for 8 hours and 2 days, respectively. Fibrin deposition on endothelial cell monolayers increased the rate of DNA synthesis (3H-thymidine incorporation) for 1 to 3 days by as much as 60% over controls. Concomitant with the lysis of fibrin above endothelial cells and the reorganization of the monolayer at Days 4 or 5 was a reduction of endothelial cell pinocytosis and DNA synthesis to control values. In contrast, there was no effect on the cell density at any time after incubation with fibrin. These results indicate that fibrin interacts with endothelial cells and induces a disorganization of the monolayer that is accompanied by changes in the rate of pinocytosis and in the rate of DNA synthesis.

Influence of fibrin, fibrinogen and fibrinogen degradation products on cultured endothelial cells

Thin films of a fibrin-agar mixture induced significant morphological changes in porcine aortic endothelial cells in vitro. The cells became spindle shaped, piled up and consequently the intercellular spaces widened. These morphological changes became more significant with higher concentrations of fibrin. Films of fibrinogen-agar or thrombin-agar mixtures induced no such morphological changes thereby suggesting that fibrin itself injures the endothelial cells. Despite the lack of morphological effects of fibrinogen degradation product D or E on cultured endothelial cells, lower molecular weight fibrinogen degradation products separated by Sephadex G-25 chromatography did induce injury in the endothelial cells. As damage to the endothelium is a key event in initiation of atherosclerosis, fibrinogen and its metabolites probably play important roles.

A comparison of coronary and internal mammary arteries and implications of the results in the etiology of arteriosclerosis

Comparison was made between the intimal thickening of the anterior descending branch of the left coronary artery and the internal mammary artery in 352 necropsy examinations. The coronary arteries showed severe intimal thickening, progressing in severity throughout life, whereas the internal mammary showed no more than slight changes at any age. These observations, together with the variation in severity of the changes in different portions of the same vessel, and the freedom from this disease of the smaller arteries throughout the body, strongly suggest that a local or anatomic factor is the dominant influence in coronary artery disease.

The effect of fibrin on endothelial cell migration in vitro

Endothelial cells are known to migrate and come into contact with fibrin during numerous physiological processes, such as in wound healing and in tumor growth. The present study was initiated to investigate the effect of fibrin on endothelial cell migration in vitro. Endothelial cell migration was assayed by wounding confluent monolayers of bovine aortic endothelial cells with a razor blade and counting the number of cells crossing the wound per unit time. Wound-induced proliferation of endothelial cells was inhibited by mitomycin C-treatment without affecting endothelial cell migration, indicating that in this assay migration could be measured independent of proliferation. Migration of endothelial cells in vitro was inhibited by fibrin in a concentration dependent manner. Endothelial cell migration under fibrin was further reduced by plasminogen depletion of the serum, and fibrin still inhibited the migration of mitomycin C-treated endothelial cells. Kadish et al. (Tissue and Cell, 11, 99, 1979) previously reported that fibrin did not affect EC migration in vitro. The inability to inhibit EC migration with fibrin appears to be due to their assay system which employed agarose, since pre-treating the wounded monolayer with agarose eliminated the inhibition of EC migration by fibrin. The present results indicate that EC migration in vitro can be used as a model system for studying the interaction of fibrin with EC.

Haemostatic factors in human aortic intima

Accumulation of lipid in developing fibrous atherosclerotic plaques is associated with high concentrations of fibrin and accumulation of a tightly bound lipoprotein fraction that can be released by incubation with fibrinolytic enzymes, suggesting that fibrin may play a key role in lesion development. It is not known whether this fibrin represents incorporated mural thrombus or is produced in situ by clotting of the fibrinogen that is present in intima in high concentration. Immunoelectrophoresis was used to measure the concentrations of fibrinogen and components of the clotting system in human aortic intima and lesions, and in mural thrombi. In the lipid and fibrin rich plaque centres prothrombin concentration was almost twice that in normal intima, but concentrations of the thrombin inhibitors antithrombin III and alpha 2-macroglobulin fell, so that the molar ratios of inhibitor/prothrombin fell from 3:1 in normal intima to 1:1 in the plaque centre. In mural thrombi there was preferential sequestration of fibrinogen-like antigen and prothrombin. Distribution of factor-VIII-related antigen was highly unpredictable; in both normal intima and all types of intimal lesion substantial amounts were recovered from some samples, whereas none was recovered from others. The highest concentrations were found in samples free of endothelium from the deep layers of lesions. The results are compatible with the idea that fibrinogen may be converted to fibrin within lesions; once some fibrin has accumulated within the intima it seems to bind low-density lipoprotein and sequester fibrinogen and clotting factors, thereby producing a self-amplifying system.