Vitronectin Functions as a Cofactor for Rapid Inhibition of Activated Protein C by Plasminogen Activator Inhibitor-1

Abstract

Activated protein C (APC) is a natural anticoagulant in plasma that down-regulates the coagulation cascade by degrading factors Va and VIIIa. In addition to its anticoagulant function, APC is also known to possess a profibrinolytic property. This property of APC has been attributed to its ability to neutralize PAI-1, thereby increasing the concentration of tissue plasminogen activator in plasma leading to up-regulation of the fibrinolytic cascade. This hypothesis, however, has not been well established, since the concentration of PAI-1 in plasma is low, and its reactivity with APC is very slow in a purified system. Here we demonstrate that vitronectin enhances the reactivity of PAI-1 with APC ϳ300-fold making PAI-1 the most efficient inhibitor of APC thus far reported (k 2 ‫؍‬ 1.8 ؋ 10 5 M ؊1 s ؊1 ). We further show that PAI-1 inhibition of the Glu 192 3 Gln mutant of APC is enhanced ϳ40-fold, independent of vitronectin, suggesting that vitronectin partially overcomes the inhibitory interaction of PAI-1 with Glu 192 . Additionally, we show that PAI-1 inhibition of the Lys 37 -Lys 38 -Lys 39 3 Pro-Gln-Glu mutant of APC is severely impaired, suggesting that, similar to tissue plasminogen activator, the basic 39-loop of APC plays a critical role in the reaction. Together, these results suggest that vitronectin functions as a cofactor to promote the profibrinolytic activity of APC.

The pathogenesis of traumatic coagulopathy

“…As a normal platelet count of 200 9 10 9 l À1 overcame aPC anticoagulant effects even at very high concentrations of aPC, and there was no detectable effect on fibrinolysis with or without platelets [31], it is difficult to envisage how aPC could drive the phenotype described as ATC. Furthermore, though factor V is depleted and PC converted to aPC in ATC, it has been amply demonstrated that thrombin generation potential is dramatically elevated in trauma patients; this is surely inconsistent with the notion that aPC is inhibiting thrombin generation by inactivating factor V [32]. Also, it must be noted that PAI-1 is a potent inhibitor of aPC in the presence of vitronectin [33].…”

Modulation of Serpin Reaction through Stabilization of Transient Intermediate by Ligands Bound to α-Helix F

“…Active PAI-1 circulates in the bloodstream as a high affinity complex with vitronectin (Vn) (38), a cell adhesive glycoprotein, which is present in plasma in micromolar concentrations (39,40), and stabilizes the active conformation of PAI-1 through a decrease in the rate of its spontaneous inactivation (41,42). Although Vn does not significantly affect the SI for the reactions of PAI-1 with uPA and tPA (43)(44)(45), under physiological pH it decreases the k lim for both enzymes and dramatically increases the rate of the reaction with activated protein C (46). On the other hand, the binding of Vn results in an increase in both the rate and SI for the reaction of PAI-1 with thrombin (43,47,48).…”

Hemostatic Soluble Plasma Proteins During Acute-Phase Response and Chronic Inflammation

“…The importance of APC-PAI-1 in vivo association is still disputable because the PAI-1 reactivity with respect to APC is very low in a purified system. Nevertheless, it was demonstrated that vitronectin, a pro-inflammatory protein, enhances the reactivity of PAI-1 with APC about 300 times (Rezaie, 2001). In a study with patients suffering from chronic cardiac failure (CCF) and stable angina pectoris (SAP) we found an abnormality of the ratios between the plasma levels of t-PA, PC, and PAI-1 (Patalakh et al, 2009;Patalakh et al, 2007).…”