The demonstration of urokinase antigen in whole blood

Steady states and dynamics of urokinase-mediated plasmin activation in silico and in vitro

Plasmin (PLS) and urokinase-type plasminogen activator (UPA) are ubiquitous proteases that regulate the extracellular environment. Although they are secreted in inactive forms, they can activate each other through proteolytic cleavage. This mutual interplay creates the potential for complex dynamics, which we investigated using mathematical modeling and in vitro experiments. We constructed ordinary differential equations to model the conversion of precursor plasminogen into active PLS, and precursor urokinase (scUPA) into active urokinase (tcUPA). Although neither PLS nor UPA exhibits allosteric cooperativity, modeling showed that cooperativity occurred at the system level because of substrate competition. Computational simulations and bifurcation analysis predicted that the system would be bistable over a range of parameters for cooperativity and positive feedback. Cell-free experiments with recombinant proteins tested key predictions of the model. PLS activation in response to scUPA stimulus was found to be cooperative in vitro. Finally, bistability was demonstrated in vitro by the presence of two significantly different steady-state levels of PLS activation for the same levels of stimulus. We conclude that ultrasensitive, bistable activation of UPA-PLS is possible in the presence of substrate competition. An ultrasensitive threshold for activation of PLS and UPA would have ramifications for normal and disease processes, including angiogenesis, metastasis, wound healing, and fibrosis.

Source of increased plasminogen activators during pregnancy and puerperium

We investigated the increase of plasminogen activators (tPA and uPA) in the plasma during pregnancy. Both tPA and uPA antigens were found to increase after the third trimester of pregnancy and high levels of PAs persisted through the first stage of labor. The tPA antigen levels rose further for the first few hours post-partum, while the level of uPA antigen returned to normal immediately following childbirth. To clarify whether the uterus and/or placenta are involved in the increased levels of plasma PAs, the levels were measured in uterine venous blood in cases of caesarean sections. During the ante-partum period, the level of uPA antigen in the uterine venous blood was higher than that in the peripheral venous blood, while there was no significant difference between the levels of tPA antigen in peripheral blood and uterine venous blood. The level of tPA antigen in the uterine venous blood rose after delivery. In contrast, the level of uPA antigen declined immediately after delivery. These results suggest that (1) the placenta is the major source of the increased uPA antigen during pregnancy, (2) entire vascular system is involved in the increased tPA antigen during pregnancy, (3) a further increase in tPA after delivery is due to the release of this enzyme from the involuting uterus.

Identification of two types of protein immunochemically related to urinary urokinase occurring in human plasma

Plasma urokinase, a plasminogen activator immunochemically related to urinary urokinase (UK), was removed from human plasma (3.5 ng/ml) by immuno-depletion with antibodies raised against UK. The remaining plasminogen activator activity of the depleted plasma could not be inhibited by anti-UK antibodies and a sensitive ELISA for UK did not detect any UK levels that were higher than the background of the assay (0.1 ng/ml). However, when the depleted plasma was subjected to SDS-PAGE, substantial amounts of protein were found hereafter around 110 and 46 kD which now gave a positive reaction in the ELISA (35-350 ng/ml plasma). From these observations it is concluded that in human plasma two types of UK-related protein occur: Type I, among which the plasma urokinase, has antigenic determinants which are directly accessible to the anti-UK antibodies, Type II has determinants in a latent form. The function of the 110 kD type-II protein is that of a plasminogen activator; that of the 46 kD protein is not yet clear.

Fibrinolytic activity in renal venous blood in man

In 50 patients without renal insufficiency, fibrinolytic activity, as reflected by euglobulin lysis time, was determined in blood obtained from the renal veins, the renal artery and a peripheral vein. Fibrinolytic activity was found to be significantly higher in the renal veins than in the renal artery and the peripheral vein. Other coagulation and fibrinolysis parameters did not show such differences. In addition, a patient with acute oligoanuric renal failure was investigated. This patient demonstrated reduced overall fibrinolytic activity, but there were no differences between the activity in the blood of the renal veins and that of the renal artery or peripheral vein. It seems, therefore, that the kidneys release plasminogen activators into the systemic circulation. This may be decreased in renal failure, probably contributing to the well-known diminished fibrinolysis in some kidney diseases.

The fibrinolytic system in man

The fibrinolytic system comprises a proenzyme, plasminogen, which can be activated to the active enzyme plasmin, that will degrade fibrin by different types of plasminogen activators. Inhibition of fibrinolysis may occur at the level of plasmin or at the level of the activators. Fibrinolysis in human blood seems to be regulated by specific molecular interactions between these components. In plasma, normally no systemic plasminogen activation occurs. When fibrin is formed, small amounts of plasminogen activator and plasminogen adsorb to the fibrin, and plasmin is generated in situ. The formed plasmin, which remains transiently complexed to fibrin, is only slowly inactivated by alpha 2-antiplasmin, while plasmin, which is released from digested fibrin, is rapidly and irreversibly neutralized. The fibrinolytic process, thus, seems to be triggered by and confined to fibrin. Thrombus formation may occur as the result of insufficient activation of the fibrinolytic system and (or) the presence of excess inhibitors, while excessive activation and/or deficiency of inhibitors might cause excessive plasmin formation and a bleeding tendency. Evidence obtained in animal models suggests that tissue-type plasminogen activator, obtained by recombinant DNA technology, may constitute a specific clot-selective thrombolytic agent with higher specific activity and fewer side effects than those currently in use.

Immunological comparison between human and rat plasminogen activators in blood and the vessel wall

To evaluate the rat as an experimental model for plasminogen activator research, the ability of antibodies specific for human tissue type plasminogen activator and urokinase to suppress the plasminogen activator activity in whole plasma and in the vessel wall was studied in both rat and man. Plasminogen activator activity in plasma was assayed on fibrin plates containing plasminogen. Plasminogen activator in the vessel wall was shown by the fibrin side technique. Antibodies against human tissue type melanoma cell activator and urokinase were raised in goats and mixed into the fibrin film or the fibrin plates. In both species antibodies to melanoma cell activator were able to suppress the plasminogen activator activity completely in plasma and in the vessel wall. Anti-urokinase, however, had no suppressing effect. In rat plasma the inhibitory effect on the fibrinolytic activity was seen only with high concentrations of antibodies against melanoma cell activator, which suggests that rat plasminogen activator in plasma and vessel walls is similar to, but not identical with, human tissue type plasminogen activator.

Immunological characterisation of plasminogen activators in the human vessel wall

A histochemical technique was used to identify the activity of the plasminogen activator (PA) in the vessel wall of veins. Antibodies against melanoma cell activator and urokinase (UK), both raised in goats, were mixed into the fibrin film. The PA activity was quenched by the antibodies against melanoma activator but remained unchanged when antibodies against UK, or an IgG preparation of normal goat serum, was mixed in the fibrin film. The results of this study show that the PA activity in the vein vessel wall is immunologically similar to or identical to the PA derived from melanoma cells which has previously been shown to cross-react with the tissue-like PA. No UK-like activity was present in the vessel wall.

Demonstration of urokinase-related fibrinolytic activity in human plasma

Various preparations of anti-urokinase antibodies were found to quench a part of the plasma fibrinolytic activity. This part was also inhibited by a specific, urokinase inhibitor isolated from placenta. These observations indicate that plasma contains a urokinase-related immuno-reactivity. This activity was found to belong to the factor XII-independent proactivator system.