The interaction of activated protein C and thrombin with the plasminogen activator inhibitor released from human endothelial cells.

: The effects of human activated protein C (APC) and thrombin on plasminogen activator inhibitor (PAI-1) released from cultured human umbilical endothelial cells, grown in serum-free 35S-methionine containing medium, were studied in two ways: measurement of PAI-1 activity with an amidolytic assay, and immunoprecipitation of the medium with anti-PAI-1 IgG, anti-protein C IgG or anti-thrombin IgG followed by SDS-PAGE and autoradiography. Addition of APC or thrombin to the endothelial cell conditioned medium results in a time and concentration dependent loss of PAI-1 activity and in the degradation of PAI-1 from 46 kD into a 42 kD product. After incubation of the medium with APC in the presence of cells, an additional band of 95 kD was found, which could be immunoprecipitated with both anti-PAI-1 IgG and anti-protein C IgG, indicating the formation of an APC-PAI-1 complex before degradation occurs. No complex could be detected after incubation of the medium with thrombin in the presence of endothelial cells. Blocking the active sites of APC and thrombin prevented both the formation of APC-PAI-1 complexes and the inactivation and degradation of PAI-1. After removal of the active PAI-1 from the medium, no degradation of the inactive PAI-1 by APC or thrombin could be found. It is concluded that both APC and thrombin react with the active PAI-1, resulting in inactivation and degradation of PAI-1.