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Triglyceride-rich lipoproteins and the fibrinolytic system
Thursday 13 October 1994: Workshop Abstracts W21 Thrombosis, jibrinolysis and arherosclerosis
and their lipid levels remain abnormal. Even optimal glycemic control by itself does not suffice to control dyslipidemia in some cases. Thus, a significant number of diabetic patients, mostly NIDDM, will need management with hypolipidemic drugs. The most common lipid abnormality in NIDCM is hypertrigliceridemia and low HDL-C, usually associated with small dense LDL. Fibrates are a first-choice therapeutic option since they may cor-
341
rect all three abnormalities. The HMG-CoA reductase inhibitors are indicated in diabetic dyslipidemias with elevated LDLC. Acipimox, a nicotinic acid derivative, has also been tied with success. Indications to use resins are few because of their triglyceride-raising effects and common gastrointestinal intoleran&The effect of hypolipidemic drugs on glycemic control should also be monitored.
W21 THROMBOSIS, FIBRINOLYSIS AND ATHEROSCLEROSIS Blood coagulation: link between thrombosis and atherosclerosis, an overview Mann, Dept. of Biochem., Univ. of Vermont Coil. qf Med., Given Building, Burlington. VT 05405, USA
Response to vascular damage results in the focal generation of athrombin which produces a fibrin/platelet clot at the site of vascular injury. If the damage is caused by a perforating injury to the blood vessel the coagulation response is perceived to be desirable and is termed hemostasis. However, if the vascular injury is in response to intraluminal vascular damage the resulting complete or partial vascular occlusion which occurs as a consequence of the coagulation response is termed thrombosis. The regulated coagulation response derives from the assembly and activity of enzyme complexes which are localized to surfaces presented by the vascular damage. The product of each enzymatic complex provides the serine protease component required for the assembly and activity of each successive enzyme complex, ultimately leading to the formation of thrombin. When one limits attention to the complexes clearly associated with hemostatic or thrombotic risk, the significance of the vitamin K-dependent enzyme complexes becomes apparent. Each of these complexes involves a serine protease and a cofactor protein which assemble on a membrane surface in the presence of Ca2+. The expression of an active complex involves, in addition to the activation of zymogen to an enzyme, the presentation or activation of a cofactor protein and the provision of the appropriate membrane to support the reaction. Two plasma cofactors, factor V and factor VIII, require proteolytic activation, while the cofactors tissue factor and thrombomodulin are membrane-associated proteins, whose presentation is regulated at the cellular level. Since the membrane plays an essential part in the formation and expression of vitamin K-dependent complexes, its regulation is vital in the expression of procoagulant activity.
Immunoassays for blood coagulation activation peptides measure the transitions of specific zymogens to serine proteases or the conversion of fibrinogen to fibrin. Normal individuals exhibit low levels of these species which indicates that the blood coagulation mechanism is continuously active. Prethrombotic states, which are biochemically defined as occurring before the generation of fibrin, show a 2+fold increase in prothrombin fragment F, + 2 levels. The occurrence of overt thrombotic events is associated with rather small elevations in the extent of systemic enzyme generation, which exceed certain inhibitory thresholds. Under basal conditions (i.e. in the absence of thrombosis or provocative stimuli), the factor VII-tissue factor mechanism is the major procoagulant pathway contributing to the continuous generation of factor IXa, factor Xa, and tbrombin. However, factor IXa is unable to convert factor X to factor Xa either because no factor VIIIa is available, an appropriate surface is not available for factor IXa-factor VIIIa-factor X complex assembly, or there is a particularly potent endogenous anticoagulant mechanism. An increase in thrombin production by the factor VII-tissue factor/factor X pathway may be required to activate small amounts of factor VIII or cell surfaces, thereby generating increased amounts of thrombin via the action of the existing factor IXa on factor X. The critical unanswered question is whether the ‘biochemically’ hypercoagulable individuals in whom the blood coagulation mechanism is set closer to the threshold at which the dormant factor IXa-factor VIII-phospholipid membrane complex is ignited are more likely to develop arterial or venous thrombosis. Triglyceride-rich lipoproteins and the fibrinolytic system Tremoli E, Sironi L, Banti C, Prati L, Mannucci L, Mussoni L, Inst. of Pharmacol. Sci., Univ. of Milan, Via Balzaretti 9, 20133 Milan, Italy
Epidemiological follow-up studies have suggested that fibrinogen is a major, primary cardiovascular risk factor. Cross-sectional results also show associations between fibrinogen and cardiovascular risk factors or diseases. Clinical cohort studies demonstrate that fibrinogen might also be a secondary risk factor. The determinants of the plasma level of fibrinogen in health and disease are known only incompletely. Our understanding of the mechanisms involved in the atherogenic actions of fibrinogen is similarly fragmentary. These might include blood coagulation, blood rheology, platelet aggregation, direct effects on the vascular wall, and the acute phase response. Thus, many questions ate yet to be answered. Nevertheless, the data available at present strongly suggest that fibrinogen represents a major, independent cardiovascular risk factor. Fibrinogen should be considered for screening programs aimed at identifying individuals at risk.
High levels of plasminogen activator inhibitor type 1 (PAI-I) have been described in patients with known risk factors for atherosclerosis as well as in patients with hypertriglyceridemia. Moreover, the documented correlation between plasma triglyceride levels and PAI-I has raised the question whether TG-rich lipoproteins (VLDL) could influence the synthesis of this inhibitor of the fibrinolytic system in competent cells. To elucidate the mechanisms responsible for the increased levels of PAI-1 observed in hypertriglyceridemia, we investigated the in vitro effects of VLDL on PAI- synthesis by HepG2 cells. VLDL (25-2OOpg protein/ml) concentration-dependently increased the release of PAI- by the cells. The increases in PAI- synthesis were accompanied by concomitant changes in PA-l mRNA. Interestingly, VLDL (lOO@ml ) increased 4-fold the cellular levels of the 2.2 kb PAI- mRNA transcript @ < 0.001, n = 13), whereas levels of the 3.2 kb PAI- mRNA remained essentially unchanged. Interestingly, TG-rich particles mimicked the effect of VLDL on cellular PAI-I biosynthesis. We concluded that VLDL induce PAI-I biosynthesis in HepG2 cells through mechanisms involving triglyceride processing by the cells.
Detection of a prethrombotic state &!ELKA, Rosenberg RD, Dept. of Med., Beth Israel Hosp.,
Risk of arterial thrombosis in the pre-thrombotic state Miller GJ, MRC Epidemiology and Medical Care Unit, Med. Coil.
Fibrinogen, a cardiovascular risk factor EmstE, Postgrad. Med. Sch., Univ. of Exerer, UK
Harvard Med. Sch., Boston, MA 02215, USA
of St Barrholomew’s Hosp., Lxmdon, ECIM 6BQ, UK Alherosclerosis X, Montreal, October I994
and their lipid levels remain abnormal. Even optimal glycemic control by itself does not suffice to control dyslipidemia in some cases. Thus, a significant number of diabetic patients, mostly NIDDM, will need management with hypolipidemic drugs. The most common lipid abnormality in NIDCM is hypertrigliceridemia and low HDL-C, usually associated with small dense LDL. Fibrates are a first-choice therapeutic option since they may cor-
341
rect all three abnormalities. The HMG-CoA reductase inhibitors are indicated in diabetic dyslipidemias with elevated LDLC. Acipimox, a nicotinic acid derivative, has also been tied with success. Indications to use resins are few because of their triglyceride-raising effects and common gastrointestinal intoleran&The effect of hypolipidemic drugs on glycemic control should also be monitored.
W21 THROMBOSIS, FIBRINOLYSIS AND ATHEROSCLEROSIS Blood coagulation: link between thrombosis and atherosclerosis, an overview Mann, Dept. of Biochem., Univ. of Vermont Coil. qf Med., Given Building, Burlington. VT 05405, USA
Response to vascular damage results in the focal generation of athrombin which produces a fibrin/platelet clot at the site of vascular injury. If the damage is caused by a perforating injury to the blood vessel the coagulation response is perceived to be desirable and is termed hemostasis. However, if the vascular injury is in response to intraluminal vascular damage the resulting complete or partial vascular occlusion which occurs as a consequence of the coagulation response is termed thrombosis. The regulated coagulation response derives from the assembly and activity of enzyme complexes which are localized to surfaces presented by the vascular damage. The product of each enzymatic complex provides the serine protease component required for the assembly and activity of each successive enzyme complex, ultimately leading to the formation of thrombin. When one limits attention to the complexes clearly associated with hemostatic or thrombotic risk, the significance of the vitamin K-dependent enzyme complexes becomes apparent. Each of these complexes involves a serine protease and a cofactor protein which assemble on a membrane surface in the presence of Ca2+. The expression of an active complex involves, in addition to the activation of zymogen to an enzyme, the presentation or activation of a cofactor protein and the provision of the appropriate membrane to support the reaction. Two plasma cofactors, factor V and factor VIII, require proteolytic activation, while the cofactors tissue factor and thrombomodulin are membrane-associated proteins, whose presentation is regulated at the cellular level. Since the membrane plays an essential part in the formation and expression of vitamin K-dependent complexes, its regulation is vital in the expression of procoagulant activity.
Immunoassays for blood coagulation activation peptides measure the transitions of specific zymogens to serine proteases or the conversion of fibrinogen to fibrin. Normal individuals exhibit low levels of these species which indicates that the blood coagulation mechanism is continuously active. Prethrombotic states, which are biochemically defined as occurring before the generation of fibrin, show a 2+fold increase in prothrombin fragment F, + 2 levels. The occurrence of overt thrombotic events is associated with rather small elevations in the extent of systemic enzyme generation, which exceed certain inhibitory thresholds. Under basal conditions (i.e. in the absence of thrombosis or provocative stimuli), the factor VII-tissue factor mechanism is the major procoagulant pathway contributing to the continuous generation of factor IXa, factor Xa, and tbrombin. However, factor IXa is unable to convert factor X to factor Xa either because no factor VIIIa is available, an appropriate surface is not available for factor IXa-factor VIIIa-factor X complex assembly, or there is a particularly potent endogenous anticoagulant mechanism. An increase in thrombin production by the factor VII-tissue factor/factor X pathway may be required to activate small amounts of factor VIII or cell surfaces, thereby generating increased amounts of thrombin via the action of the existing factor IXa on factor X. The critical unanswered question is whether the ‘biochemically’ hypercoagulable individuals in whom the blood coagulation mechanism is set closer to the threshold at which the dormant factor IXa-factor VIII-phospholipid membrane complex is ignited are more likely to develop arterial or venous thrombosis. Triglyceride-rich lipoproteins and the fibrinolytic system Tremoli E, Sironi L, Banti C, Prati L, Mannucci L, Mussoni L, Inst. of Pharmacol. Sci., Univ. of Milan, Via Balzaretti 9, 20133 Milan, Italy
Epidemiological follow-up studies have suggested that fibrinogen is a major, primary cardiovascular risk factor. Cross-sectional results also show associations between fibrinogen and cardiovascular risk factors or diseases. Clinical cohort studies demonstrate that fibrinogen might also be a secondary risk factor. The determinants of the plasma level of fibrinogen in health and disease are known only incompletely. Our understanding of the mechanisms involved in the atherogenic actions of fibrinogen is similarly fragmentary. These might include blood coagulation, blood rheology, platelet aggregation, direct effects on the vascular wall, and the acute phase response. Thus, many questions ate yet to be answered. Nevertheless, the data available at present strongly suggest that fibrinogen represents a major, independent cardiovascular risk factor. Fibrinogen should be considered for screening programs aimed at identifying individuals at risk.
High levels of plasminogen activator inhibitor type 1 (PAI-I) have been described in patients with known risk factors for atherosclerosis as well as in patients with hypertriglyceridemia. Moreover, the documented correlation between plasma triglyceride levels and PAI-I has raised the question whether TG-rich lipoproteins (VLDL) could influence the synthesis of this inhibitor of the fibrinolytic system in competent cells. To elucidate the mechanisms responsible for the increased levels of PAI-1 observed in hypertriglyceridemia, we investigated the in vitro effects of VLDL on PAI- synthesis by HepG2 cells. VLDL (25-2OOpg protein/ml) concentration-dependently increased the release of PAI- by the cells. The increases in PAI- synthesis were accompanied by concomitant changes in PA-l mRNA. Interestingly, VLDL (lOO@ml ) increased 4-fold the cellular levels of the 2.2 kb PAI- mRNA transcript @ < 0.001, n = 13), whereas levels of the 3.2 kb PAI- mRNA remained essentially unchanged. Interestingly, TG-rich particles mimicked the effect of VLDL on cellular PAI-I biosynthesis. We concluded that VLDL induce PAI-I biosynthesis in HepG2 cells through mechanisms involving triglyceride processing by the cells.
Detection of a prethrombotic state &!ELKA, Rosenberg RD, Dept. of Med., Beth Israel Hosp.,
Risk of arterial thrombosis in the pre-thrombotic state Miller GJ, MRC Epidemiology and Medical Care Unit, Med. Coil.
Fibrinogen, a cardiovascular risk factor EmstE, Postgrad. Med. Sch., Univ. of Exerer, UK
Harvard Med. Sch., Boston, MA 02215, USA
of St Barrholomew’s Hosp., Lxmdon, ECIM 6BQ, UK Alherosclerosis X, Montreal, October I994