Summary
Increasing evidence suggests the involvement of leukocytes in the fibrinolytic system.
Monocytes secrete pro-urokinase (Grau, Thromb Res 1989; 53: 145) and it has been shown
that these cells have specific receptors for urokinase and plasminogen (Miles, Thromb
Haemostas 1987; 58: 936). The aim of this study was to analyse the presence of plasminogen
activator inhibitor(s) in platelet-free suspensions of human peripheral blood monocytes
and polymorphonuclear leukocytes (PMN). SDS-PAGE and reverse fibrin autography showed
an inhibitory band of 50 kDa in the monocyte extracts (Triton X-100) but not in the
PMN extracts. Urokinase (u-PA) was mixed with increasing amounts of monocyte extract
for 10 min and the mixtures were added to 125Ifibrin coated wells containing plasminogen.
A dose-dependent decrease in the u-PA fibrinolytic activity was observed. The amount
of inhibition increased when the monocyte releasates were preincubated with u-PA (40%
inhibition after 5 min preincubation and 80% after 15 min), indicating a direct interaction
between this activator and an inhibitor(s). After SDS-PAGE of monocyte extracts, immunoblotting
and peroxidase staining identified both PAI1 and PAI2, with an apparent molecular
weight of 47-50 kDa. Monocyte-associated PAI1 formed complexes with single chain t-PA
with a molecular mass 50 kDa higher than the molecular mass of the free PAI1. However,
a significant amount of PAI remained unbound to t-PA. This inactive PAI1 could have
come from a rapid inactivation of the primary active PAI1. These PAI1 and PAI2 detected
in human monocytes may be transcendent in the regulation of the fibrinolytic system.
Keywords
Fibrinolysis - Plasminogen activator inhibitor - Monocytes - Polymorphonuclear leukocytes
- Immunoblotting
