Abstract
(Englisch)
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Thrombotic and arterial disease are among the main causes of morbidity and mortality in the western world. They are caused by imbalances in the natural defence mechanisms against bleeding and thrombosis, in particular a hypercoagulable state (an increased tendency to form intravascular blood clots) or a diminished fibrinolytic capacity (ability of the vascular endothelium to initiate thrombus degradation). Existing animal models, especially in rodents, are, in various aspects, too far removed from the situation in man to allow a reliable extrapolation from animal to man. The European concerted action program PRIMEFITT ('Primate models for studying fibrinolysis, thrombosis and tissue remodelling') was initiated to develop suitable models in primates (Macaca mulatta).
In the reporting period, the partners met once, during the international Congress en Fibrimlysis aid Thrembolysis in Ljubljana, Slovenia (June 22-25). During this meeting scientific results d the past year were presented, and the program for the next was discussed.
The Swiss contribution to the programme is aimed at better understanding the mechanisms that regulate the plasma concentrations of tissue-type plasminogen activator (t-PA), a major contributor to the fibrinolytic capacity in vivo. Two mechanisms are of major importance: the regulated release of t-PA from the vascular endothelium and the rapid hepatic clearance of t-PA that is mediated by two receptors. One, the low density lipoprotein receptor-related protein (LRP), is expressed by hepatocytes and is a multiligand clearance receptor. In rodents, the LRP is the principal t-PA clearance receptor. The other clearance receptor, the mannose receptor, is expressed by liver endothelial cells contributes significantly to t-PA clearance.
In the period from October 1997 to October 1998 we have investigated in vitro the mechanisms that contribute to the release of t-PA from endothelial cells. We observed that t-PA is stored in endothelial cells in specific granules, called Weibel-Palade bodies, and is released from these cells by thrombogenic and vasoactive stimuli. By adenovirus mediated gene transfer we were able to increase t-PA production by human endothelial cells approximately hundred4old. Overexpressed t-PA was also found to be stored in Weibel-Palade bodies. A manuscript describing these in vitro results is in press. We have produced a large batch of adenovirus t-PA construct to be able to study in vivo in primates whether gene transfer can be used to increase plasma levels of t-PA and whether in vivo t-PA is also stored in Weibel-Palade bodies. Such in vivo experiments are now planned to start in the first half of 1999.
Our studies on the clearance of t-PA by hepatocytes in vitro have shown that clearance by these cells is mediated by LRP and that clearance can be fully inhibited by a receptor-associated protein (RAP) and by an amino-terminal fragment of t-PA. Recently, we obtained evidence that the LRP-mediated clearance requires the presence of an as yet unknown co-receptor. The observation that the clearance of t-PA by hepatocytes can be completely blocked by RAP will be used to establish to what extent t-PA clearance in primates is mediated by LRP. To study this, we will use adenovirus mediated gene transfer to generate high plasma levels, which then will inhibit the LRP-mediated clearance of t-PA in primates. These in vivo experiments are planned for the first half of 1999.
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