Inhibition of the activation of Hageman factor (factor XII) by human vascular endothelial cell culture supernates

Rapid and cooperative binding of factor XII to human umbilical vein endothelial cells

When activated, factor XII (FXII) has been shown to play a role in a series of proteolytic cascades including systems as the fibrinolytic, the coagulation, the kallikrein-kinin and the complement. How FXII is activated in vivo remains poorly understood as the concentration and density of surface bound negative charges known to trigger the activation in vitro is far from sufficient in vivo. Specific binding of FXII to cellular receptors in the blood stream may, however, solve this problem which may be a question of inter molecular vicinity enhanced by binding to any surface. Here we report that the Zn(2+)-dependent binding of FXII to endothelial cells is rapid, saturable, specific and cooperative. Each endothelial cell from the human umbilical veins was found to bind (417 +/- 202) x 10(3) molecules of FXII with a Kd of (65 +/- 23) nM and a Hill coefficient of 2.1. The binding was inhibited by alpha-FXIIa but not by beta-FXIIa. The Kd for binding alpha-FXIIa was (50 +/- 27) nM. The rate of association was found to be 1.9 x 10(5) M(-1). min(-1). A confirmed inhibition by HK increased the Kd without affecting the maximal number of binding sites and the Hill coefficient. The concentration of HK in serum did not prevent binding of FXII/FXIIa to cells incubated with serum supplemented with Zn2+. The optimal concentration of Zn(2+) was 15 microM for binding factor XII/FXIIa whether purified or in serum.

Activation of the plasma kallikrein/kinin system on endothelial cell membranes

For more than three decades, it has been known that the plasma kallikrein/kinin system becomes activated when exposed to artificial, negatively charged surfaces. The existence of an encompassing in vivo, negatively charged surface capable of activation of the plasma kallikrein/kinin system has, however, never been convincingly demonstrated. In this report, we describe current knowledge on how the proteins of the plasma kallikrein/kinin system assemble to become activated on cell membranes. On endothelial cells, the activation of the plasma kallikrein/kinin system is not initiated by factor XII autoactivation as seen on artificial surfaces. On endothelial cells, prekallikrein is activated by an antipain sensitive protease. Prekallikrein activation is dependent on the presence of high molecular weight kininogen and an optimal free Zn2+ concentration. Kallikrein generated on the surface of endothelial cell is capable of activating factor XII. Further, kallikrein formed on endothelial cell membranes is capable of cleaving its receptor and native substrate, high molecular weight kininogen, liberating bradykinin and the HK PK complex from the endothelial cell surface. Endothelial cell-associated kallikrein also is capable of kinetically favorable pro-urokinase and, subsequent, plasminogen activation.

Activation of the plasma kallikrein/kinin system on endothelial cells

For more than two decades, it has been known that activation of the plasma kallikrein/kinin system only occurs when it is exposed to artificial, negatively charged surfaces. The existence of physiological, negatively charged surfaces has, however, never been demonstrated in vivo. In this report, we describe current knowledge about how the proteins of the plasma kallikrein/kinin system interact with and become activated on cell membranes. In this model, activation of the plasma kallikrein/kinin system on endothelial cells is not initiated by factor XII autoactivation, as seen on artificial surfaces. On endothelial cells, plasma prekallikrein is activated by a membrane-associated cysteine protease. This activation is dependent on the presence of high molecular weight kininogen and an optimal zinc (Zn2+) concentration. Although the initiation of activation of plasma prekallikrein is independent of factor XII, kallikrein-mediated factor XIIa generation, in turn, accelerates the activation of the system. Further kallikrein formed on endothelial cell membranes is capable of cleaving its receptor and native substrate, high molecular weight kininogen, liberating bradykinin and terminating activation. In addition, the kallikrein formed on the surface of endothelial cells results in kinetically favorable activation of prourokinase and, subsequently, plasminogen. Activation of the plasma kallikrein/kinin system on endothelial cells proceeds by a physiological mechanism to initiate cellular fibrinolysis independent of plasmin, fibrin, and tissue-type plasminogen activator.

Inhibition of the activation of hageman factor (factor XII) by eosinophils and eosinophilic constituents

Several syndromes characterized by striking eosinophilia may be complicated by thrombosis. The experiments described indicate that, paradoxically, eosinophils and certain of their constituents inhibit the activation of Hageman factor (HF, factor XII). In earlier studies, suspensions of mixed types of granulocytes, other nucleated peripheral blood cells, and platelets inhibited activation of Hageman factor by ellagic acid, glass, and sulfatides. After these cells were sedimented by centrifugation, the supernatant fluids were also inhibitory. No attempt had been made earlier to distinguish among different granulocytic species. In the present study, suspensions of eosinophils and the supernatant fluid after eosinophils had been separated by centrifugation inhibited activation of Hageman factor by ellagic acid. The protein concentration of that amount of supernatant fluid that inhibited activation by about half was 16 micrograms/ml, approximately the same as had been described for suspensions of peripheral blood mononuclear cells. Activation of Hageman factor by ellagic acid was also inhibited by certain constituents of eosinophils, including eosinophil peroxidase, eosinophil major basic protein and eosinophil cationic protein. Inhibition was not specific for ellagic acid-induced activation of Hageman factor, as inhibition was also observed with sulfatide-induced activation. Inhibition was presumably related to neutralization of the negative charge of activators of Hageman factor. Thus, bismuth subgallate, a particulate activator of Hageman factor, was no longer effective after it had been exposed to eosinophil cationic protein. The observations reported here raise the question of whether in vivo eosinophils modulate certain of the defense reactions ascribed to Hageman factor.

Endothelial cells produce a substance that inhibits contact activation of coagulation by blocking the activation of Hageman factor

Human umbilical vein endothelial cells (HUVECs) produce a property that impairs the generation of coagulant and amidolytic activity initiated when normal human plasma is exposed to glass. This inhibitory property blocks the adsorption of Hageman factor (factor XII) to glass, thereby preventing the activation of Hageman factor, but does not impair the coagulant or amidolytic activity of already activated Hageman factor (factor XIIa). This property in HUVEC lysates could be neutralized by a purified preparation of Hageman factor but not by purified prekallikrein or high molecular mass kininogen. A partially purified inhibitory fraction from cell lysates exhibited a single homogeneous band in SDS/PAGE of approximately 22.5 kDa. Inhibitory activity was also found in concentrates of conditioned media from HUVECs, which also impaired the binding of Hageman factor to a surface; it may not be identical with that found in cell lysates.