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The Fibrinolytic System in Diabetes
Mellitus S.C.L. Gough, P.J. Grant University Department
ot
Medione, Leedi, UK
KEY WORDS
Fibrinolytic system
Introduction Diabetes mellitus i s a heterogeneous condition characterized by fasting hyperglycaemia and the development of chronic vascular complications that cause considerable morbidity and mortality. The cause of increased vascular disease is unknown although abnormal lipid metabolism, hyperglycaemia, glycosylation and changes in the aldose reductase pathway are some of the factors that may be involved. Alterations in the fibrinolytic system have been described in both Type 1 and Type 2 diabetes and although there are qualitative differences between the two groups there is evidence to suggest that these abnormalities may also have a role in the pathogenesis of vascular disease. Type 1 diabetes is associated with normal or increased fibrinolytic activity whereas with i nsu I i n-i nsensitive Type 2 diabetes, profound depression of fibrinolytic activity, due to high circulating levels of an inhibitor, often occurs. It is more than twenty years since the original suggestion that insulin itself may be responsible for the development of vascular disease. Recently this idea has gained further acceptability with the description of Reaven's syndrome (syndrome X) which to a certain extent unifies the aetiology of hypertension in hyperinsulinaemic diabetic and non-diabetic subjects. The observation that insulin releases the major fibrinolytic inhibitor, plasminogen activator inhibitor-1 (PAI-I), from cells of hepatic origin further strengthens the argument that hyperinsulinaemia may lead to vascular damage. In view of the marked depression of fibrinolysis commonly seen in obese diabetic and non-diabetic subjects there is an argument for including this in the description of Reaven's syndrome as an additional factor involved in the pathogenesis of vascular disease.
The Fibrinolytic System
Biochemistry of the Fibrinolytic System The fibrinolytic system i s a complex enzyme cascade that consists of a series of activators and inhibitors that Correspondence to: Dr P.]. Grant, University Department of Medicine, Martin Wing, The General Infirmary, Leeds, LS1 3EX, UK
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Plasminogen activator inhibitor-1
regulate the conversion of plasminogen to plasmin (Figure 1 ) . The production of plasmin leads to the lysis of fibrin and the maintenance of vascular patency. The major activators of plasminogen are tissue-type plasminogen activator (t-PA) and urokinase (u-PA), both of which are inactivated by circulating inhibitors, plasminogen activator inhibitor-I (PAI-l), and PAI-2. Plasmin formed in the circulation i s itself inhibited by alpha-2 anti plasmin. Tissue-type plasminogen activator i s a serine protease of 67000 molecular weight that i s produced by many cell lines in vitro.' The major source of circulating t-PA i s thought to be vascular endothelial cells.2 Urokinase is produced by a number of cell lines and is found in the urine.j PAI-1 is the fast acting inhibitor of both t-PA and u-PA and is found in human plasma, endothelial and hepatic cells, and platelet^.^ The latter account for about 90 % of circulating PAI-1.5 PAI-2 i s the major inhibitor of fibrinolysis during pregnancy. It is localized in the trophoblastic epithelium of the placenta6 and released into the plasma during p r e g n a n ~ y .Various ~ cell lines such as leucocytesU and fibrosarcoma' cells secrete PAI2, but with the exception of some rare hepatic tumours it is uncommon to see increased plasma levels except in pregnancy. l o Much attention has focused on circulating levels of components of the fibrinolytic system in relation to disease. However, it has become clear that the essential reactions that govern clot lysis occur at a cellular level. Both t-PA and plasminogen bind to fibrin and under these circumstances t-PA i s protected from inhibition by PAI-1 to allow local plasmin generation." Similar events take place at the endothelial cell surface.12 The implications of these findings are that measurement of circulating components may barely reflect what is occurring at the molecular and cellular level. Increases in fibrinolytic activity during hypoglycaemia, for example, are not accompanied by evidence of lysis of fibrin.13 In this respect it i s more correct to say that there is an increase in fibrinolytic activity rather than imply that fibrinolysis has occurred.
Regulation of the Fibrinolytic System Clinically it has been known for many years that the fibrinolytic system is activated or inhibited in response
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& Sons, Ltd.
DIABETIC MEDICINE, 1991 ; 8 : 898-YO5
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REVIEW FlBRlNOLYSIS
COAGUIATION
INTRINSIC PATHWAY
EXTRINSIC PATHWAY
EXTRINSIC PATHWAY
\ / Xa
1-PA
FIBRIN
FIBRINOGEN
-
INTRINSIC PATHWAY
I
uPA
FIBRIN DEGRADATION PRODUCTS
Actlvatlon lnhlbltlon
Figure 1. Simplified scheme of the fibrinolytic system and its relationship to the coagulation pathway. t-PA, tissue plasminogen activator; U-PA, urokinase; PAI-1, -2, plasminogen activator inhibitor -1 and -2; Xa, factor Xa
to many stimuli. Conditions such as exercise, the intraoperative period, hypoglycaemia, and most forms of physical stress are accompanied by increases in circulating (ex vivoj fibrinolytic activity. This subject has been recently reviewed.13 In the study of fibrinolysis in man, the venous occlusion test has been developed as an indicator of endothelial cell function. After 10 min venous occlusion, significant increases in t-PA, and to a lesser extent, PAL1 take place. The clinical significance of this test remains uncertain. Post-operatively, marked increases in PAI-1 occur in association with depression of f i b r i n ~ l y s i s , ' a~ response that may be involved in the pathogenesis of deep vein thrombosis. With the relatively recent development of molecular biology as an investigative tool, much interest has focused on the gene regulation of fibrinolytic components. Cytokines, such as tumour necrosis factor and interleukin1 have been shown to suppress t-PA mRNA whilst transcriptionally inducing PAI-1 in endothelial Interestingly, there are recent reports that some patients with diabetes have high circulating levels of cytokines.'u Dexamethasone induces PAI-1 and, to a lesser extent, t-PA whilst suppressing u-PA in fibrosarcoma (HT 1080) cells.'y Of greater interest in the field of diabetes is the effect of insulin on PAI-1 gene expression and protein THE FlBRlNOLYTlC SYSTEM IN DIABETES
secretion. Several papers have now reported that insulin induces PAL1 mRNA expression in both hepatoma (HepG2) cells and hepatocytes to produce a doubling of secreted protein in the culture medium after 24 h.20-2LAs yet, no study has shown that short-term hyperinsulinaemia leads to an increase in PAI-1 in v ~ v o , ~although ~ - ~ ~ several studies have shown a relationship between insulin and PAI-1 in population surveys as discussed later. Work from our own department (unpublished) and from Juhan-Vague indicates that insulin-like growth factor-1, which shares a tyrosine kinase receptor with a high degree of homology to that of insulin, has similar effects on PAIL1 . 2 6 Early in vivo studies in hypopituitary patients treated with growth hormone do not, however, support the idea of a role for IGF-1 as a physiological mediator of PAI-1. Over an 8-week period there was a doubling of circulating IGF-1 in response to growth hormone with no apparent change in any components of the fibrinolytic system (P.J. Grant, unpublished observations). There is substantially more clinical evidence to implicate hyperinsulinaemia in the regulation of PAI-1 and the development of vascular disease. Studies have shown a correlation between insulin and PAI-1 in patients with obesity, angina, and Type 2 diabetes as well as the
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understanding this subject. Additionally, particularly with normal population.27-" Additionally, three prospective respect to diabetes, it remains uncertain what the studies have identified hyperinsulinaemia as being a risk changes mean in relation to the development of vascular factor for the development of myocardial i n f a r ~ t i o n . ~ ~ - ~ ~ complications. The old chestnut in the study of haemoTaken overall, the current position would seem to support stasis, 'cause or effect?' remains unanswered in diabetes, the view that hyperinsulinaemia i s a risk factor involved although there is evidence accumulating that abnormaliin the aetiology of vascular disease and that this process ties in fibrinolysis may have an important role in the may be mediated by the effects of insulin on PAL1 development of vascular disease. synthesis and secretion. Despite this rather unhelpful synopsis, there are Further relationships between PAIL1 and vascular some features beginning to emerge that appear to be disease in diabetes can be found in the complex world characteristic of diabetes. Broadly speaking, it seems of lipid metabolism. Some studies, 35-38 though not that Type 1 diabetic patients have slightly enhanced have demonstrated a correlation between triglyceride fibrinolytic activity whereas Type 2 diabetic patients have levels and PAI-1, although short-term infusions of triglymarkedly depressed activity principally due to an increase ceride have no effect.13 In vitro studies have shown that in plasminogen activator inhibitor-I (Table 1). VLDL increases endothelial cell production of PAIL1 Type 1 diabetic patients are generally younger and whereas LDL induced PAL1 in hepatocytes but not in less likely to have widespread vascular disease. In some endothel ial cel Is."","' ways they seem to be a better group to study, as changes The complex relationship between hyperinsulinaemia, in fibrinolysis could be described as 'cause' rather than insulin insensitivity, obesity, hypertriglyceridaemia, and 'effect'. Co-existent atherosclerosis may cloud the picture vascular disorders may be partly clarified by the recogin Type 2 diabetic patients, but studies in this group are nition that abnormalities of the fibrinolytic system occur equally important because of the different nature of the under these conditions. As described, clinical and disease with different metabolic and clinical profiles. The laboratory studies indicate that both insulin and triglycerremainder of this article will review the evidence to ide may have a role in the regulation of the fibrinolytic support this statement and attempt to clarify the relationinhibitor PAI-1. Obesity i s associated with high PAI-1 ship between these observations and the complications levels that are reduced by weight loss 4 L and prospective of diabetes. studies have shown that depression of the fibrinolytic system is associated with a risk of both myocardial ~-~~ infarction" l-J'i and deep vein t h r o m b o ~ i s . ~The available evidence strongly supports the view that depression of fibrinolysis i s related to obesity and insulin insensitivity and may provide the link between these conditions and the development of vascular disorders in both the diabetic and non-diabetic population.
Diabetes and Fibrinolysis Introduction Diabetes mellitus is associated with a confusing array of abnormalities of coagulation, fibrinolysis, and platelet function that is daunting even to the initiated. Classification of diabetes, the presence of complications, metabolic status, and the sheer number of variables that can be measured all contribute to the problem of
Type 1 Diabetes and the fibrinolytic System In patients with Type 1 diabetes mellitus, authors have reported decreased, normal, and increased fibrinolysis, as assessed by the euglobulin clot lysis time (ECLT), a global test of fibrinolytic activity. The relatively recent development of assays for components for the fibrinolytic system has led to a more complete picture of the status of this pathway in diabetes. Using the ECLT, Sharma4' compared fibrinolytic activity in 26 Type 1 diabetic patients, 36 Type 2 diabetic patients, and 62 control subjects and found increased fibrinolytic activity in the Type 1 diabetic group. In contrast, fibrinolytic activity has been reported as normal in male Type 1 diabetic patients with a significant reduction in female patients, 5o and both basal and post-venous occlusion fibrinolytic activity was found to be no different from control subjects
Table 1. Changes in fibrinolysis in Type 1 and Type 2 diabetes reported in the references cited in the text Type 1 diabetes
Fibrinolytic activity
Increased 4953,54 Unchanged Increased 52,54 Increased "," '
PAI-1 t-PA
900
"
p
S
Type 2 diabetes
Markedly decreased5fl. 52.54,s5 , 5 7 . 5 8 , 7 6
2
Markedly increased Increased "
54,5x,7h
S.C.L. COUCH, P.I. GRANT
DTT7 in 7-ype 1 diabetic patients without r e t i n ~ p a t h y . Auwerx ~’ et a / . reported increased t-PA antigen, PAI-1 activity and fibrinogen with no change in the euglobulin clot lysis timeTL in Type 1 diabetic patients. Nilsson et a / . 5 3 examined 43 Type 1 diabetic patients and found higher t-PA activity levels, both at rest and after venous occlusion. There was a positive correlation between HbA,, and the capacity to release t-PA as assessed by the venous occlusion test. No relationship was found between fibrinolytic activity and vascular complications, although it appeared that endothelial cell production of t-PA decreased with the duration of diabetes. A further significant reduction in t-PA in Type 1 diabetic patients who smoked was seen, making the important point that smoking habits need to be considered in such studies. Further evidence to support the view that Type 1 diabetes is associated with enhanced fibrinolysis comes from a study of patients with and without complication^.^^ In that study, 20 patients with no complications, 17 with laser-treated retinopathy, and 13 with neuropathy were compared with a group of 20 non-diabetic control subjects. Patients with macrovascular disease, hypertension, and smoking were excluded and assays of t-PA and PAI-1 were assessed pre- and post-venous occlusion. Basal t-PA inhibition was higher in the control group than in the diabetic patients and post-venous occlusion t-PA antigen was higher in control subjects than in neuropathic patients. These results indicate that Type 1 diabetic patients have enhanced fibrinolysis, but a diminished response to venous occlusion in the neuropathic patients would be consistent with an endothelial cell defect as in Type 2 diabetes. In the same study, 20 uncomplicated Type 2 patients had diminished basal and post-venous occlusion fibrinolytic activity and high t-PA inhibition, supporting other studies in Type 2 diabetes.
Type 2 Diabetes and the Fibrinolytic Sys tem In Type 2 diabetes the status of the fibrinolytic system i s much clearer with most studies reporting a profound reduction in activity, which has latterly been shown to be due to an increase in PAI-1.50,52,54It is in this group of patients that high insulin and triglyceride levels are thought to be related to suppression of fibrinolysis and the development of vascular disease, as previously discussed. Almer and Nilsson assessed spontaneous fibrinolytic. activity by the ECLT in 221 diabetic patients compared with 153 age- and sex-matched non-diabetic subjects.55 In this study they found that as a group the diabetic patients had slightly lower fibrinolytic activity than control subjects. This difference was largely attributable to significantly reduced fibrinolytic activity in 21 of the patients who received sulphonylureas. Diabetic patients on diet and those on insulin had activity similar to the control population while overweight patients had lower fibrinolytic activity. Plasminogen activator activity within THE FlBRlNOLYTlC SYSTEM IN DIABETES
REVIEW the vessel wall was assessed using vein biopsy specimens and this was found to be abnormally low in 26 % of Type 2 and 17 % of Type 1 patients. Further work described reduced fibrinolytic activity within endothelial cells of both arteries and veins that was most striking in obese subjects.56 Schneider et a/.57 found resting fibrinolysis to be generally reduced in 16 Type 2 diabetic patients compared with a group of age-matched control subjects. They also found that the increase in fibrinolytic activity associated with exercise was attenuated in the diabetic population. Following a 6-week period of physical training, resting levels of fibrinolytic activity increased to that seen in the control group but the response to exercise remained subnormal. Garcia Frade et a / . T Rstudied 50 patients with Type 2 diabetes mellitus and found reduced fibrinolytic activity with significantly higher levels of t-PA antigen and PAI1 activity compared with control subjects. Increased t-PA antigen with low fibrinolytic activity may result from the existence of inactive tPA-PA1 complexes. No correlation was found between glycosylated haemoglobin and abnormal fibrinolysis.
Fibrinolysis in Relation to the Complications of Diabetes A major complication of diabetes mellitus i s the development of both micro- and macro-vascular complications, and the high mortality associated with acute myocardial infarction in patients with diabetes mellitusS9 may be related to abnormalities of the fibrinolytic system. Evidence exists to suggest that poor metabolic control may prevent clot lysis in that glycosylated fibrin is more resistant to plasmin digestion 6o and accumulation of fibrin is reported to occur in those tissues most affected by diabetic The functional properties of plasminogen activators and plasminogen have been improved by normalization of blood glucose control.65 These observations linked to abnormalities of the circulating components of the fibrinolytic system in diabetes, indicate that fibrin i s more resistant to the actions of the diabetic fibrinolytic system. This tendency towards the maintenance of established fibrin may have a role in the development of complications, and provides further evidence to support the importance of good blood glucose control. The first important study of fibrinolytic activity in diabetes, published in 1963, was carried out in an unclassified group of patients and reported that fibrinolytic activity was generally reduced.66 This early work by Fearnley et a/. in 100 diabetic patients also found that evidence of ischaemia on electrocardiograms occurred twice as frequently in those with markedly reduced fibrinolytic activity. In a small study of 1 3 diabetic and 48 non-diabetic patients, Gray et a/.67 showed that diabetic patients presenting with acute myocardial infarction had higher levels of PAL1 that correlated with glycosylation of haemoglobin and admission glucose.
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REVIEW These findings are in accordance with those of Hamsten et a/.43,44 and Meade et a/.4s that demonstrated high levels of t-PA inhibition in survivors of myocardial infarction compared with control subjects. This latter work adds weight to the importance of abnormal fibrinolytic activity in the development of vascular disease. A number of investigators have examined fibrinolytic activity in relationship to microvascular complications. In an assortment of diabetic patients, those with retinopathy were reported to have lower spontaneous and postvenous occlusion with higher vein wall fibrinolytic activity than patients without complications. It was postulated that this apparent preservation of fibrinolytic activity in the group without complications might prevent the early vascular changes associated with retinopathy."" Type 1 diabetic patients with retinopathy have been described as having higher levels of t-PA both pre- and post-venous occlusion with higher PAI-1 post-venous occlusion than those without complication^.^^ A further study confirmed the higher levels of t-PA in such patients, although there were no differences in PAI-1 .54 Overall, fibrinolytic activity, as assessed by fibrin plates has been reported as normal basally and reduced after venous Type ' 2 occlusion in Type 1 patients with r e t i n ~ p a t h y . ~ patients with retinopathy and hypertension are reported to have higher levels of fibrinolytic inhibition, reflected by PAI-1 activity.'" In that study post-venous occlusion t-PA release was reduced in patients with retinopathy, hypertension, and peripheral vasculopathy. Rydzewski et a/. studied 31 patients with Type 2 diabetes mellitus and reported that the patients without retinopathy did not differ in any measure of fibrinolytic activity from control subjects.'" However, in the group with retinopathy, those with simple retinopathy had significantly higher levels of t-PA than both control subjects and those patients with proliferative retinopathy. Patients with simple retinopathy also had significantly higher levels of t-PA/PAI-l complex and PAI-1 antigen. In that study there were reduced levels of u-PA in the urine in the diabetic group as a whole, and plasma urokinase correlated with glycosylated haemoglobin. These results indicate that fibrinolytic activity may be enhanced in patients with early diabetic microangiopathy but that fibrinolysis i s reduced in patients with more severe disease. Microalbuminuria has been shown to predict increased mortality in diabetic patients, and in non-diabetic subjects is a marker of cardiovascular disease. In a small study of 12 Type 2 diabetic patients with microalbuminuria and 12 with normal albumin excretion compared with 12 non-diabetic control subjects, Collier et a/.'' reported increased levels of t-PA in the group with microalbuminuria. In contrast, a study of 20 newly diagnosed Type 2 diabetic patients indicated a strong correlation between the level of urinary albumin and plasma PAI-1 activity and antigen (S.C. L. Gough, unpublished data). These latter findings may help to explain the increased risk of cardiovascular mortality seen in patients with microalbuminuria.
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Therapy Although abnormalities of the fibrinolytic system occur in patients with diabetes mellitus, an important question is whether these can be favourably influenced by treatment. Unfortunately, no long-term studies exist that show improvement in fibrinolytic activity with improved blood glucose control. In a small study of 1 5 patients, Alm6r 7 2 compared fibrinolytic activity in patients initially on chlorpropamide who were than changed to gliclazide. He found seven out of 1 5 patients to have abnormally low plasminogen activator activity on chlorpropamide but after 6 months on gliclazide all patients had normal plasminogen activator activity. After 24 and 48 months the results remained the same although by 48 months only eight patients remained in the study. The normalization of fibrinolytic activity could not be explained by improvement in blood glucose control as this remained the same throughout the study. Metformin has recently gained new interest as it has been found to have a favourable effect on f i b r i n ~ l y s i s , ~ ~ lipid metabolism, and blood pressure in addition to its traditional effects on blood glucose control. Vague et a/.74described a decrease in high plasminogen activator inhibition capacity, plasmin, insulin, and triglyceride levels in non-diabetic obese patients receiving metformin, although a recent study by Tengborn 7 5 looking at nonobese, non-diabetic hypertensive patients found that metformin did not affect fibrinolytic variables. His group of patients, in contrast to those of Vague, had only slightly elevated levels of plasma insulin. Tengborn still found a decrease in blood pressure as well as normalization of lipid and glucose metabolism. Our own ~ o r k , ' looking ~ at the effects of metformin in 38 Type 2 diabetic patients in a double-blind, placebo-controlled trial, showed that over a 6-week period metformin caused a reduction in both basal and post-venous occlusion levels of PAIL1 antigen. However, there was no change in plasminogen activator activity and metformin did not affect insulin levels.
Conclusion Patients with diabetes mellitus clearly have abnormalities of the fibrinolytic system although, at first glance, authors describe abnormalities which may be conflicting, with fibrinolysis being described as increased, normal or decreased. Most studies point to a reduced fibrinolytic activity in patients with Type 2 diabetes mellitus which i s mainly due to increased PAI-1 levels and either normal or increased fibrinolytic activity in patients with Type 1 diabetes mellitus. It i s not clear whether abnormalities of fibrinolytic activity are related to blood glucose control and, although certain agents affecting blood glucose control may have favourable effects on fibrinolytic activity, there are no long-term studies available for analysis and, clearly, these are needed in this area. Fibrinolytic activity may be increased in those with early changes of microangiopathy and depressed when 5 C L G O U C H , P J GRANT
DTT7 microangiopathy i s more severe. Whether this represents cause or effect in diabetic patients i s uncertain. However, evidence exists to suggest that depression of fibrinolysis occurs in patients with venous and arterial thromboembolic disease and further depression in patients with diabetes mellitus, particularly those with Type 2 diabetes, may contribute to the high incidence of vascular disease seen in this condition.
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Ducimetiere P, Eschwege E, Papoz L, Richard JL, Claude JR, Rosseling G.Relationship of plasma insulin levels to the incidence of myocardial infarction and coronary heart disease mortality in a middle-aged population. Diabetologia 1980; 19: 205-21 0. Mehta J, Mehta P, Lawson D, Saldeen T. Plasma tissue plasminogen activator inhibitor levels in coronary artery disease; correlation with age and serum triglyceride levels. / Am Coll Cardiol 1987; 9: 263-268. Nilsson TK, Johnson 0. The extrinsic fibrinolytic system in survivors o i myocardial infarction. Thromb Res 1987; 48: 621-630. Brommer EJP, Gevers Leuven JA, Kluft C, Wijngaards G. Fibrinolytic inhibitor in type II hyperlipoproteinaemia. Lancet 1982; i: 1066. Brommer EJP, Verheijen JH, Chang CTG, Rijken DC. Masking of fibrinolytic response to stimulation by an inhibitor of tissue-type plasminogen activator in plasma. Thromb Haemost 1984; 52: 154-1 56. Huber K, Rose D, Resch I, et a / . Circadian fluctuations of plasminogen activator inhibitor and tissue plasrninogen activator levels in plasma with unstable coronary artery disease and acute myocardial infarction. Thromb Haemost 1988; 60: 372-376. Stiko-Rahm A, Wiman B, Hamsten A, Nilsson J. Secretion of plasrninogen activator inhibitor 1 from cultured human umbilical vein endothelial cells is induced by very low density lipoprotein. Arteriosclerosis 1990; 10: 1067-1073. Latron Y, Alessi MC, Anfosso F, Nalbone G, Lafont H, Juhan-Vague I. Effect of low density lipoproteins on secretion of plasminogen activator inhibitor (PAI-1) by human endothelial cells and hepatoma cells. Fibrinolysis 1990; 4(suppl): 82-83. Sundell JB, Dahlgren 5, Ranby M, Lundin E, Stenling R, Nilsson TK. Reduction of elevated plasminogen activator inhibitor levels during modest weight loss. Fibrinolysis 1989; 3: 51-53. Hamsten A, Wiman B, deFaire U, Blomback M. Increased plasma levels of a rapid inhibitor of tissue plasminogen activator in young survivors of myocardial infarction. N Engl I Med 1985; 31 3: 1557-1 563. Hamsten A, De Faire U,Walldius G, Dahlen G,Szamosi A, Landou C, et a/. Plasminogen activator inhibitor in plasma: risk factor for recurrent myocardial infarction. Lancet 1987; iv: 3-9. Meade TW, Chakrabarti R, Haines AP, North WRS, Stirling Y, Thompson SG,et a / . Haemostatic function and cardiovascular death: early results of a prospective study. Lancet 1980; i: 1050-1053. Nilsson IM, Ljungner H, Tengborn L. Two different mechanisms in patients with venous thrombosis and defective fibrinolysis: low concentrations of plasminogen activator or increased concentration of plasminogen activator inhibitor. Br Med 1985; 290: 1453-1456. Juhan-Vague I, Valadier J, Alessi MC, Aillaud MF,Ansaldi J, Philip-Joet C, et a/. Deficient t-PA release and elevated PA inhibitor levels in patients with spontaneous or recurrent deep vein thrombosis. Thromb Haemost 1987; 57: 67-72. Sue-Ling HM, Johnston D, McMahon MJ, Philips PR, Davies JA. Preoperative identification of patients at high risk of deep vein thrombosis after elective abdominal surgery. Lancet 1986; i: 1 173-1 176. Sharma SC. Platelet adhesiveness, plasma fibrinogen and fibrinolytic activity in juvenile-onset and maturity-onset diabetes rnellitus. J Clin Pathol 1981; 34: 501-503. Fuller JH, Keen H, Jarrett RJ, Omer T, Meade TW, Chakrabarti R, et a / . Haemostatic variables associated with diabetes and its complications. Br M e d 1 1979; ii: 964-966.
51.
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Haitas B, Barnes AJ, Cederholm-Williams SA, Moore J, Shogry MEC, Turner RC. Abnormal endothelial release of fibrinolytic activity and fibronectin in diabetic microangiopathy. Diabetologia 1984; 27: 493-496. Auwerx J, Bouillon R, Collen D, Ceboers J. Tissue-type plasminogen activator antigen and plasminogen activator inhibitor in diabetes mellitus. Arteriosclerosis 1988; 8: 68-72. Nilsson TK, Lithner F. Glycaemic control, smoking habits and diabetes duration affect the extrinsic fibrinolytic system in type 1 diabetic patients but microangiopathy does not. Acta M e d Scand 1988; 224: 123-1 29. Walmsley D, Hampton KK, Grant PI. Contrasting fibrinolytic responses in Type 1 (insulin-dependent) and Type 2 (non-insulin-dependent) diabetes. Diabetic Med 1991; 8 : 954-959. Almer LO, Nilsson IM. O n fibrinolysis in diabetes mellitus. Acta M e d Scand 1975; 1980: 101-106. Almer LO, Pandolfi M, Aberg M. The plasminogen activator activity of arteries and veins in diabetes mellitus. Thromb Res 1975; 6: 177-1 82. Schneider SH, Kein HC, Khachdurian AK, Ruderman NB. Impaired fibrinolytic response to exercise in type 2 diabetes: effects of exercise and physical training. Metabolism 1988; 37: 924-929. Garcia Frade LJ, de la Calle H, Torade MC, Lara JI, Cuellar L, Garcia Avello A. Hypofibrinolysis associated with vasculopathy in non-insulin dependent diabetes mellitus. Thromb Res 1990; 59: 51-59. Gwilt DJ. Outcome of acute myocardial infarction in diabetic patients: relation to metabolic control. Practical Cardiology 1986; 12: 95-1 10. Brownlee M, Vlassara H, Cerami A. Nonenzymatic glycosylation reduced the susceptibility of fibrin to degradation by plasmin. Diabetes 1983; 32: 680-684. Ireland JT, Viberti GC, Watkins PJ. The kidney and renal tract. In: Keen H, JarrettJ, eds. Complications of Diabetes. London: Edward Arnold, 1982: 137-1 78. Cunah-Vaz JG. Pathophysiology of diabetic retinopathy. Br J Ophthalmol 1978; 62: 351-355. Timperly WR, Ward JD, Preston FE, et a/. Clinical and histological studies in diabetic neuropathy. Diabetologia 1976; 12: 237-243. Haust MD, Wyliss JC, More RH. Electron microscopy of fibrin in human atherosclerotic lesions. Exp M o l Pathol 1965; 4 : 205-216. Geiger M, Binder BR. Plasminogen activation in diabetes mellitus: normalization of blood sugar levels improves impaired enzyme kinetics in vitro. Thromb Haemost 1985; 54: 413-414. Fearnley GR, Chakrabarti R, Avis PRD. Blood fibrinolytic activity in diabetes mellitus and its bearing on ischaemic heart disease and obesity. Br M e d 1 1963; i: 921-923. Gray R, Yudkin JS, Patterson D. Impaired fibrinolytic activity relates to glycaemia in patients with acute myocardial infarction (Abstract). Clin Sci 1991 ; 8O(suppl 24): 23p. Almer LO, Pandolfi M, Nilsson IM. Diabetic retinopathy and the fibrinolytic system. Diabetes 1975; 24: 529-534. McLaren M, Jennings PE, ForbesCD, Belch JJF. Fibrinolytic response to venous occlusion in diabetic with and without rnicroangiopathy compared to normal age and sex matched controls. Fibrinolysis 1990; 4: 95-96. Rydezewki A, Kawamura H, Watanabe Y, Takada TA. Plasminogen activators and plasrninogen activator inhibitor (PAI-1) in type 1 diabetes mellitus. Fibrinolysis 1990; 4: 183-188. Collier A, Rumley A, Leach JP, Lowe GDO, Small M. Abnormalities of haemostasis in type 2 diabetes with microalbuminuria (Abstract). Diabetic Med 1991 ; 8 (suppl): 36A. S.C.L. COUCH, P.J. GRANT
Drn 72.
73.
74.
Alnier LO. Effects of chlorpropamide and gliclazide on plasminogen activator activity in vascular walls in patients with maturity onset diabetes. Thromb Res 1984; 35: 19-25. Grant PI. The effects of metformin on the fibrinolytic system in diabetic and non-diabetic subjects. Diabete Metdbol 1991 ; 17: 168-1 73. Vague PH, Juhan-Vague I, Alessi MC, Badier C, Valadier I. Metformin decreases the high plasminogen activator inhibition capacity, plasma insulin and triglyceride levels
THE FlBKlNOLYTlC SYSTEM IN DIABETES
REVIEW 75.
76.
in non-diabetic obese subjects. Jhromb Haemost 1987; 57: 326-328. Tengborn L, Landin K, Anderson I, Chruidewska J, Smith U. Influence of metformin on the fibrinolytic system and insulin resistance in patients with hypertension. Fibrinolysis 1990; 4(suppl 2): 91-92. Grant PJ, Stickland M H , Booth NA, Prentice CRM. Metformin causes a reduction in basal and post-venous occlusion plasminogen activator inhibitor-1 in type 2 diabetic patients. Diabetic M e d 1991 ; 8 : 361-365.
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The Fibrinolytic System in Diabetes
Mellitus S.C.L. Gough, P.J. Grant University Department
ot
Medione, Leedi, UK
KEY WORDS
Fibrinolytic system
Introduction Diabetes mellitus i s a heterogeneous condition characterized by fasting hyperglycaemia and the development of chronic vascular complications that cause considerable morbidity and mortality. The cause of increased vascular disease is unknown although abnormal lipid metabolism, hyperglycaemia, glycosylation and changes in the aldose reductase pathway are some of the factors that may be involved. Alterations in the fibrinolytic system have been described in both Type 1 and Type 2 diabetes and although there are qualitative differences between the two groups there is evidence to suggest that these abnormalities may also have a role in the pathogenesis of vascular disease. Type 1 diabetes is associated with normal or increased fibrinolytic activity whereas with i nsu I i n-i nsensitive Type 2 diabetes, profound depression of fibrinolytic activity, due to high circulating levels of an inhibitor, often occurs. It is more than twenty years since the original suggestion that insulin itself may be responsible for the development of vascular disease. Recently this idea has gained further acceptability with the description of Reaven's syndrome (syndrome X) which to a certain extent unifies the aetiology of hypertension in hyperinsulinaemic diabetic and non-diabetic subjects. The observation that insulin releases the major fibrinolytic inhibitor, plasminogen activator inhibitor-1 (PAI-I), from cells of hepatic origin further strengthens the argument that hyperinsulinaemia may lead to vascular damage. In view of the marked depression of fibrinolysis commonly seen in obese diabetic and non-diabetic subjects there is an argument for including this in the description of Reaven's syndrome as an additional factor involved in the pathogenesis of vascular disease.
The Fibrinolytic System
Biochemistry of the Fibrinolytic System The fibrinolytic system i s a complex enzyme cascade that consists of a series of activators and inhibitors that Correspondence to: Dr P.]. Grant, University Department of Medicine, Martin Wing, The General Infirmary, Leeds, LS1 3EX, UK
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Plasminogen activator inhibitor-1
regulate the conversion of plasminogen to plasmin (Figure 1 ) . The production of plasmin leads to the lysis of fibrin and the maintenance of vascular patency. The major activators of plasminogen are tissue-type plasminogen activator (t-PA) and urokinase (u-PA), both of which are inactivated by circulating inhibitors, plasminogen activator inhibitor-I (PAI-l), and PAI-2. Plasmin formed in the circulation i s itself inhibited by alpha-2 anti plasmin. Tissue-type plasminogen activator i s a serine protease of 67000 molecular weight that i s produced by many cell lines in vitro.' The major source of circulating t-PA i s thought to be vascular endothelial cells.2 Urokinase is produced by a number of cell lines and is found in the urine.j PAI-1 is the fast acting inhibitor of both t-PA and u-PA and is found in human plasma, endothelial and hepatic cells, and platelet^.^ The latter account for about 90 % of circulating PAI-1.5 PAI-2 i s the major inhibitor of fibrinolysis during pregnancy. It is localized in the trophoblastic epithelium of the placenta6 and released into the plasma during p r e g n a n ~ y .Various ~ cell lines such as leucocytesU and fibrosarcoma' cells secrete PAI2, but with the exception of some rare hepatic tumours it is uncommon to see increased plasma levels except in pregnancy. l o Much attention has focused on circulating levels of components of the fibrinolytic system in relation to disease. However, it has become clear that the essential reactions that govern clot lysis occur at a cellular level. Both t-PA and plasminogen bind to fibrin and under these circumstances t-PA i s protected from inhibition by PAI-1 to allow local plasmin generation." Similar events take place at the endothelial cell surface.12 The implications of these findings are that measurement of circulating components may barely reflect what is occurring at the molecular and cellular level. Increases in fibrinolytic activity during hypoglycaemia, for example, are not accompanied by evidence of lysis of fibrin.13 In this respect it i s more correct to say that there is an increase in fibrinolytic activity rather than imply that fibrinolysis has occurred.
Regulation of the Fibrinolytic System Clinically it has been known for many years that the fibrinolytic system is activated or inhibited in response
0742-3071/91/100898-08$05.00
0 1991 by John Wiley
& Sons, Ltd.
DIABETIC MEDICINE, 1991 ; 8 : 898-YO5
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REVIEW FlBRlNOLYSIS
COAGUIATION
INTRINSIC PATHWAY
EXTRINSIC PATHWAY
EXTRINSIC PATHWAY
\ / Xa
1-PA
FIBRIN
FIBRINOGEN
-
INTRINSIC PATHWAY
I
uPA
FIBRIN DEGRADATION PRODUCTS
Actlvatlon lnhlbltlon
Figure 1. Simplified scheme of the fibrinolytic system and its relationship to the coagulation pathway. t-PA, tissue plasminogen activator; U-PA, urokinase; PAI-1, -2, plasminogen activator inhibitor -1 and -2; Xa, factor Xa
to many stimuli. Conditions such as exercise, the intraoperative period, hypoglycaemia, and most forms of physical stress are accompanied by increases in circulating (ex vivoj fibrinolytic activity. This subject has been recently reviewed.13 In the study of fibrinolysis in man, the venous occlusion test has been developed as an indicator of endothelial cell function. After 10 min venous occlusion, significant increases in t-PA, and to a lesser extent, PAL1 take place. The clinical significance of this test remains uncertain. Post-operatively, marked increases in PAI-1 occur in association with depression of f i b r i n ~ l y s i s , ' a~ response that may be involved in the pathogenesis of deep vein thrombosis. With the relatively recent development of molecular biology as an investigative tool, much interest has focused on the gene regulation of fibrinolytic components. Cytokines, such as tumour necrosis factor and interleukin1 have been shown to suppress t-PA mRNA whilst transcriptionally inducing PAI-1 in endothelial Interestingly, there are recent reports that some patients with diabetes have high circulating levels of cytokines.'u Dexamethasone induces PAI-1 and, to a lesser extent, t-PA whilst suppressing u-PA in fibrosarcoma (HT 1080) cells.'y Of greater interest in the field of diabetes is the effect of insulin on PAI-1 gene expression and protein THE FlBRlNOLYTlC SYSTEM IN DIABETES
secretion. Several papers have now reported that insulin induces PAL1 mRNA expression in both hepatoma (HepG2) cells and hepatocytes to produce a doubling of secreted protein in the culture medium after 24 h.20-2LAs yet, no study has shown that short-term hyperinsulinaemia leads to an increase in PAI-1 in v ~ v o , ~although ~ - ~ ~ several studies have shown a relationship between insulin and PAI-1 in population surveys as discussed later. Work from our own department (unpublished) and from Juhan-Vague indicates that insulin-like growth factor-1, which shares a tyrosine kinase receptor with a high degree of homology to that of insulin, has similar effects on PAIL1 . 2 6 Early in vivo studies in hypopituitary patients treated with growth hormone do not, however, support the idea of a role for IGF-1 as a physiological mediator of PAI-1. Over an 8-week period there was a doubling of circulating IGF-1 in response to growth hormone with no apparent change in any components of the fibrinolytic system (P.J. Grant, unpublished observations). There is substantially more clinical evidence to implicate hyperinsulinaemia in the regulation of PAI-1 and the development of vascular disease. Studies have shown a correlation between insulin and PAI-1 in patients with obesity, angina, and Type 2 diabetes as well as the
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understanding this subject. Additionally, particularly with normal population.27-" Additionally, three prospective respect to diabetes, it remains uncertain what the studies have identified hyperinsulinaemia as being a risk changes mean in relation to the development of vascular factor for the development of myocardial i n f a r ~ t i o n . ~ ~ - ~ ~ complications. The old chestnut in the study of haemoTaken overall, the current position would seem to support stasis, 'cause or effect?' remains unanswered in diabetes, the view that hyperinsulinaemia i s a risk factor involved although there is evidence accumulating that abnormaliin the aetiology of vascular disease and that this process ties in fibrinolysis may have an important role in the may be mediated by the effects of insulin on PAL1 development of vascular disease. synthesis and secretion. Despite this rather unhelpful synopsis, there are Further relationships between PAIL1 and vascular some features beginning to emerge that appear to be disease in diabetes can be found in the complex world characteristic of diabetes. Broadly speaking, it seems of lipid metabolism. Some studies, 35-38 though not that Type 1 diabetic patients have slightly enhanced have demonstrated a correlation between triglyceride fibrinolytic activity whereas Type 2 diabetic patients have levels and PAI-1, although short-term infusions of triglymarkedly depressed activity principally due to an increase ceride have no effect.13 In vitro studies have shown that in plasminogen activator inhibitor-I (Table 1). VLDL increases endothelial cell production of PAIL1 Type 1 diabetic patients are generally younger and whereas LDL induced PAL1 in hepatocytes but not in less likely to have widespread vascular disease. In some endothel ial cel Is."","' ways they seem to be a better group to study, as changes The complex relationship between hyperinsulinaemia, in fibrinolysis could be described as 'cause' rather than insulin insensitivity, obesity, hypertriglyceridaemia, and 'effect'. Co-existent atherosclerosis may cloud the picture vascular disorders may be partly clarified by the recogin Type 2 diabetic patients, but studies in this group are nition that abnormalities of the fibrinolytic system occur equally important because of the different nature of the under these conditions. As described, clinical and disease with different metabolic and clinical profiles. The laboratory studies indicate that both insulin and triglycerremainder of this article will review the evidence to ide may have a role in the regulation of the fibrinolytic support this statement and attempt to clarify the relationinhibitor PAI-1. Obesity i s associated with high PAI-1 ship between these observations and the complications levels that are reduced by weight loss 4 L and prospective of diabetes. studies have shown that depression of the fibrinolytic system is associated with a risk of both myocardial ~-~~ infarction" l-J'i and deep vein t h r o m b o ~ i s . ~The available evidence strongly supports the view that depression of fibrinolysis i s related to obesity and insulin insensitivity and may provide the link between these conditions and the development of vascular disorders in both the diabetic and non-diabetic population.
Diabetes and Fibrinolysis Introduction Diabetes mellitus is associated with a confusing array of abnormalities of coagulation, fibrinolysis, and platelet function that is daunting even to the initiated. Classification of diabetes, the presence of complications, metabolic status, and the sheer number of variables that can be measured all contribute to the problem of
Type 1 Diabetes and the fibrinolytic System In patients with Type 1 diabetes mellitus, authors have reported decreased, normal, and increased fibrinolysis, as assessed by the euglobulin clot lysis time (ECLT), a global test of fibrinolytic activity. The relatively recent development of assays for components for the fibrinolytic system has led to a more complete picture of the status of this pathway in diabetes. Using the ECLT, Sharma4' compared fibrinolytic activity in 26 Type 1 diabetic patients, 36 Type 2 diabetic patients, and 62 control subjects and found increased fibrinolytic activity in the Type 1 diabetic group. In contrast, fibrinolytic activity has been reported as normal in male Type 1 diabetic patients with a significant reduction in female patients, 5o and both basal and post-venous occlusion fibrinolytic activity was found to be no different from control subjects
Table 1. Changes in fibrinolysis in Type 1 and Type 2 diabetes reported in the references cited in the text Type 1 diabetes
Fibrinolytic activity
Increased 4953,54 Unchanged Increased 52,54 Increased "," '
PAI-1 t-PA
900
"
p
S
Type 2 diabetes
Markedly decreased5fl. 52.54,s5 , 5 7 . 5 8 , 7 6
2
Markedly increased Increased "
54,5x,7h
S.C.L. COUCH, P.I. GRANT
DTT7 in 7-ype 1 diabetic patients without r e t i n ~ p a t h y . Auwerx ~’ et a / . reported increased t-PA antigen, PAI-1 activity and fibrinogen with no change in the euglobulin clot lysis timeTL in Type 1 diabetic patients. Nilsson et a / . 5 3 examined 43 Type 1 diabetic patients and found higher t-PA activity levels, both at rest and after venous occlusion. There was a positive correlation between HbA,, and the capacity to release t-PA as assessed by the venous occlusion test. No relationship was found between fibrinolytic activity and vascular complications, although it appeared that endothelial cell production of t-PA decreased with the duration of diabetes. A further significant reduction in t-PA in Type 1 diabetic patients who smoked was seen, making the important point that smoking habits need to be considered in such studies. Further evidence to support the view that Type 1 diabetes is associated with enhanced fibrinolysis comes from a study of patients with and without complication^.^^ In that study, 20 patients with no complications, 17 with laser-treated retinopathy, and 13 with neuropathy were compared with a group of 20 non-diabetic control subjects. Patients with macrovascular disease, hypertension, and smoking were excluded and assays of t-PA and PAI-1 were assessed pre- and post-venous occlusion. Basal t-PA inhibition was higher in the control group than in the diabetic patients and post-venous occlusion t-PA antigen was higher in control subjects than in neuropathic patients. These results indicate that Type 1 diabetic patients have enhanced fibrinolysis, but a diminished response to venous occlusion in the neuropathic patients would be consistent with an endothelial cell defect as in Type 2 diabetes. In the same study, 20 uncomplicated Type 2 patients had diminished basal and post-venous occlusion fibrinolytic activity and high t-PA inhibition, supporting other studies in Type 2 diabetes.
Type 2 Diabetes and the Fibrinolytic Sys tem In Type 2 diabetes the status of the fibrinolytic system i s much clearer with most studies reporting a profound reduction in activity, which has latterly been shown to be due to an increase in PAI-1.50,52,54It is in this group of patients that high insulin and triglyceride levels are thought to be related to suppression of fibrinolysis and the development of vascular disease, as previously discussed. Almer and Nilsson assessed spontaneous fibrinolytic. activity by the ECLT in 221 diabetic patients compared with 153 age- and sex-matched non-diabetic subjects.55 In this study they found that as a group the diabetic patients had slightly lower fibrinolytic activity than control subjects. This difference was largely attributable to significantly reduced fibrinolytic activity in 21 of the patients who received sulphonylureas. Diabetic patients on diet and those on insulin had activity similar to the control population while overweight patients had lower fibrinolytic activity. Plasminogen activator activity within THE FlBRlNOLYTlC SYSTEM IN DIABETES
REVIEW the vessel wall was assessed using vein biopsy specimens and this was found to be abnormally low in 26 % of Type 2 and 17 % of Type 1 patients. Further work described reduced fibrinolytic activity within endothelial cells of both arteries and veins that was most striking in obese subjects.56 Schneider et a/.57 found resting fibrinolysis to be generally reduced in 16 Type 2 diabetic patients compared with a group of age-matched control subjects. They also found that the increase in fibrinolytic activity associated with exercise was attenuated in the diabetic population. Following a 6-week period of physical training, resting levels of fibrinolytic activity increased to that seen in the control group but the response to exercise remained subnormal. Garcia Frade et a / . T Rstudied 50 patients with Type 2 diabetes mellitus and found reduced fibrinolytic activity with significantly higher levels of t-PA antigen and PAI1 activity compared with control subjects. Increased t-PA antigen with low fibrinolytic activity may result from the existence of inactive tPA-PA1 complexes. No correlation was found between glycosylated haemoglobin and abnormal fibrinolysis.
Fibrinolysis in Relation to the Complications of Diabetes A major complication of diabetes mellitus i s the development of both micro- and macro-vascular complications, and the high mortality associated with acute myocardial infarction in patients with diabetes mellitusS9 may be related to abnormalities of the fibrinolytic system. Evidence exists to suggest that poor metabolic control may prevent clot lysis in that glycosylated fibrin is more resistant to plasmin digestion 6o and accumulation of fibrin is reported to occur in those tissues most affected by diabetic The functional properties of plasminogen activators and plasminogen have been improved by normalization of blood glucose control.65 These observations linked to abnormalities of the circulating components of the fibrinolytic system in diabetes, indicate that fibrin i s more resistant to the actions of the diabetic fibrinolytic system. This tendency towards the maintenance of established fibrin may have a role in the development of complications, and provides further evidence to support the importance of good blood glucose control. The first important study of fibrinolytic activity in diabetes, published in 1963, was carried out in an unclassified group of patients and reported that fibrinolytic activity was generally reduced.66 This early work by Fearnley et a/. in 100 diabetic patients also found that evidence of ischaemia on electrocardiograms occurred twice as frequently in those with markedly reduced fibrinolytic activity. In a small study of 1 3 diabetic and 48 non-diabetic patients, Gray et a/.67 showed that diabetic patients presenting with acute myocardial infarction had higher levels of PAL1 that correlated with glycosylation of haemoglobin and admission glucose.
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REVIEW These findings are in accordance with those of Hamsten et a/.43,44 and Meade et a/.4s that demonstrated high levels of t-PA inhibition in survivors of myocardial infarction compared with control subjects. This latter work adds weight to the importance of abnormal fibrinolytic activity in the development of vascular disease. A number of investigators have examined fibrinolytic activity in relationship to microvascular complications. In an assortment of diabetic patients, those with retinopathy were reported to have lower spontaneous and postvenous occlusion with higher vein wall fibrinolytic activity than patients without complications. It was postulated that this apparent preservation of fibrinolytic activity in the group without complications might prevent the early vascular changes associated with retinopathy."" Type 1 diabetic patients with retinopathy have been described as having higher levels of t-PA both pre- and post-venous occlusion with higher PAI-1 post-venous occlusion than those without complication^.^^ A further study confirmed the higher levels of t-PA in such patients, although there were no differences in PAI-1 .54 Overall, fibrinolytic activity, as assessed by fibrin plates has been reported as normal basally and reduced after venous Type ' 2 occlusion in Type 1 patients with r e t i n ~ p a t h y . ~ patients with retinopathy and hypertension are reported to have higher levels of fibrinolytic inhibition, reflected by PAI-1 activity.'" In that study post-venous occlusion t-PA release was reduced in patients with retinopathy, hypertension, and peripheral vasculopathy. Rydzewski et a/. studied 31 patients with Type 2 diabetes mellitus and reported that the patients without retinopathy did not differ in any measure of fibrinolytic activity from control subjects.'" However, in the group with retinopathy, those with simple retinopathy had significantly higher levels of t-PA than both control subjects and those patients with proliferative retinopathy. Patients with simple retinopathy also had significantly higher levels of t-PA/PAI-l complex and PAI-1 antigen. In that study there were reduced levels of u-PA in the urine in the diabetic group as a whole, and plasma urokinase correlated with glycosylated haemoglobin. These results indicate that fibrinolytic activity may be enhanced in patients with early diabetic microangiopathy but that fibrinolysis i s reduced in patients with more severe disease. Microalbuminuria has been shown to predict increased mortality in diabetic patients, and in non-diabetic subjects is a marker of cardiovascular disease. In a small study of 12 Type 2 diabetic patients with microalbuminuria and 12 with normal albumin excretion compared with 12 non-diabetic control subjects, Collier et a/.'' reported increased levels of t-PA in the group with microalbuminuria. In contrast, a study of 20 newly diagnosed Type 2 diabetic patients indicated a strong correlation between the level of urinary albumin and plasma PAI-1 activity and antigen (S.C. L. Gough, unpublished data). These latter findings may help to explain the increased risk of cardiovascular mortality seen in patients with microalbuminuria.
902
Therapy Although abnormalities of the fibrinolytic system occur in patients with diabetes mellitus, an important question is whether these can be favourably influenced by treatment. Unfortunately, no long-term studies exist that show improvement in fibrinolytic activity with improved blood glucose control. In a small study of 1 5 patients, Alm6r 7 2 compared fibrinolytic activity in patients initially on chlorpropamide who were than changed to gliclazide. He found seven out of 1 5 patients to have abnormally low plasminogen activator activity on chlorpropamide but after 6 months on gliclazide all patients had normal plasminogen activator activity. After 24 and 48 months the results remained the same although by 48 months only eight patients remained in the study. The normalization of fibrinolytic activity could not be explained by improvement in blood glucose control as this remained the same throughout the study. Metformin has recently gained new interest as it has been found to have a favourable effect on f i b r i n ~ l y s i s , ~ ~ lipid metabolism, and blood pressure in addition to its traditional effects on blood glucose control. Vague et a/.74described a decrease in high plasminogen activator inhibition capacity, plasmin, insulin, and triglyceride levels in non-diabetic obese patients receiving metformin, although a recent study by Tengborn 7 5 looking at nonobese, non-diabetic hypertensive patients found that metformin did not affect fibrinolytic variables. His group of patients, in contrast to those of Vague, had only slightly elevated levels of plasma insulin. Tengborn still found a decrease in blood pressure as well as normalization of lipid and glucose metabolism. Our own ~ o r k , ' looking ~ at the effects of metformin in 38 Type 2 diabetic patients in a double-blind, placebo-controlled trial, showed that over a 6-week period metformin caused a reduction in both basal and post-venous occlusion levels of PAIL1 antigen. However, there was no change in plasminogen activator activity and metformin did not affect insulin levels.
Conclusion Patients with diabetes mellitus clearly have abnormalities of the fibrinolytic system although, at first glance, authors describe abnormalities which may be conflicting, with fibrinolysis being described as increased, normal or decreased. Most studies point to a reduced fibrinolytic activity in patients with Type 2 diabetes mellitus which i s mainly due to increased PAI-1 levels and either normal or increased fibrinolytic activity in patients with Type 1 diabetes mellitus. It i s not clear whether abnormalities of fibrinolytic activity are related to blood glucose control and, although certain agents affecting blood glucose control may have favourable effects on fibrinolytic activity, there are no long-term studies available for analysis and, clearly, these are needed in this area. Fibrinolytic activity may be increased in those with early changes of microangiopathy and depressed when 5 C L G O U C H , P J GRANT
DTT7 microangiopathy i s more severe. Whether this represents cause or effect in diabetic patients i s uncertain. However, evidence exists to suggest that depression of fibrinolysis occurs in patients with venous and arterial thromboembolic disease and further depression in patients with diabetes mellitus, particularly those with Type 2 diabetes, may contribute to the high incidence of vascular disease seen in this condition.
REVIEW 17.
18.
19.
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7.
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