Plasma Levels of Activated Protein C in Healthy Subjects and Patients With Previous Venous Thromboembolism

Abstract

Abstract-The proteolytic enzyme activated protein C (APC) is a normal plasma component, indicating that protein C (PC) is continuously activated in vivo. High concentrations of homocysteine (Hcy) inhibit the activation of PC in vitro; this effect may account for the high risk for thrombosis in patients with hyperhomocysteinemia (HyperHcy). We measured the plasma levels of APC in 128 patients with previous venous thromboembolism (VTE) and in 98 age-and sex-matched healthy controls and correlated them with the plasma levels of total Hcy (tHcy) measured before and after an oral methionine loading (PML). Forty-eight patients had HyperHcy and 80 had normal levels of tHcy. No subject was known to have any of the congenital or acquired thrombophilic states at the time of the study. Because the plasma levels of APC and PC were correlated in healthy controls, the APC/PC ratios were also analyzed. Plasma APC levels and APC/PC ratios were significantly higher in VTE patients than in controls (Pϭ0.03 and 0.0004, respectively). Most of the increase in APC levels and APC/PC ratios were attributable to patients with HyperHcy. Patients with normal tHcy had intermediate values, which did not differ significantly from those of healthy controls. There was no correlation between the plasma levels of tHcy or its PML increments and APC or APC/PC ratios in controls. The fasting plasma levels of APC and APC/PC ratios of 10 controls did not increase 4 hours PML, despite a 2-fold increase in tHcy. This study indicates that APC plasma levels are sensitive markers of activation of the hemostatic system in vivo and that Hcy does not interfere with the activation of PC in vivo. (Arterioscler Thromb Vasc Biol. 1998;18:1371-1375.)

Impaired Homocysteine Metabolism and Atherothrombotic Disease

“…To confirm the onset of a prethrombotic state, clinical studies have been performed in hyperhomocysteinemic patients. Some abnormal plasma elevations of hemostatic parameters, including factor VIII (Freyburger et al, 1997), von Willebrand factor (van den Berg et al, 1995), fibrinogen (von Eckardstein et al, 1994), thrombomodulin (van den Berg et al, 1995), thrombin/antithrombin III complex and activated protein C (Cattaneo, 1999;Cattaneo et al, 1998), and type 1 plasminogen activation inhibitor (Freyburger et al, 1997) have been reported. However, the increase of the anticoagulant effectors is inconsistent with a prothrombotic environment.…”

Molecular bases of defective signal transduction in the platelet P2Y 12 receptor of a patient with congenital bleeding

“…In some of these patients, platelets that produce normal amounts of thromboxane A 2 fail to respond to the agonist because of defects in the corresponding receptor, but in many others the pathogenetic mechanism is poorly defined (12). Thus, congenital P2Y 12 defects may be more common than presently recognized, but they may be mistakenly identified with other disorders of platelet function (13).The P2Y 12 locus has been analyzed in some patients in whom the platelet binding sites for ADP and the nonhydrolyzable ADP analogue 2-methylthioadenosine 5Ј-diphosphate (2MeS-ADP) were reduced by 70-80% (7,11,14). In two instances, different homozygous frameshift mutations were found to cause premature termination of translation.…”

“…The other, P2Y 12 , is negatively coupled to adenylyl cyclase through G i ; it mediates progressive and sustained platelet aggregation in the absence of shape change (2, 3, 5) and plays an important role in the potentiation of secretion induced by several agonists (6, 7). P2Y 12 is the therapeutic target of ticlopidine and clopidogrel (8), two platelet aggregation inhibitors used for the prevention and treatment of arterial thrombosis (9), and its congenital deficiency results in a bleeding disorder (7,10,11). Platelets deficient in P2Y 12 exhibit normal ADP-induced shape change but only slight and rapidly reversible aggregation, as well as a failure of ADP to inhibit the rise of cAMP levels after stimulation with prostaglandin E 1 (PGE 1 ) (1).…”

“…The P2Y 12 locus has been analyzed in some patients in whom the platelet binding sites for ADP and the nonhydrolyzable ADP analogue 2-methylthioadenosine 5Ј-diphosphate (2MeS-ADP) were reduced by 70-80% (7,11,14). In two instances, different homozygous frameshift mutations were found to cause premature termination of translation.…”

“…One receptor, P2Y 1 , is coupled to the heterotrimeric GTP-binding protein G q and to phospholipase C-␤; it induces mobilization of cytoplasmic calcium and mediates shape change followed by an initial wave of rapidly reversible aggregation (3,4). The other, P2Y 12 , is negatively coupled to adenylyl cyclase through G i ; it mediates progressive and sustained platelet aggregation in the absence of shape change (2,3,5) and plays an important role in the potentiation of secretion induced by several agonists (6,7). P2Y 12 is the therapeutic target of ticlopidine and clopidogrel (8), two platelet aggregation inhibitors used for the prevention and treatment of arterial thrombosis (9), and its congenital deficiency results in a bleeding disorder (7,10,11).…”