Regulation of plasminogen activator inhibitor activity in endothelial cells by tissue-type plasminogen activator

Tissue-type plasminogen activator (t-PA) may stimulate the expression of plasminogen activator inhibitor type 1 (PAI-1) mRNA in cultured human umbilical vein endothelial cells. The PAI-1 antigen in the conditioned medium of cells, pretreated with t-PA, was less than the control, probably due to the formation and degradation of the t-PA-PAI-1 complex. However, the PAI activity of the t-PA-pretreated cells reached the same level as control group, 24 h after the residual t-PA activity was rapidly neutralized by the newly synthesized PAI-1. To test the release of PAI-1 from the substratum of the endothelial cells, the cultured cells were metabolically prelabeled with ³⁵S-methionine for 3 h and then treated with t-PA. The ³⁵S-PAI-1 of 46 kDa was found in the substratum and culture medium of cultured endothelial cells as analyzed by the SDS-PAGE after immunoprecipitation. During the treatment of endothelial cells with t-PA, the PAI-1 of 46 kDa in the cell substratum disappeared, and the 110 kDa t-PA-PAI-1 complex, the 81 kDa degraded t-PA-PAI-1, and the 44 kDa degraded PAI-1 products concomitantly appeared in the conditioned medium instead. In summary, t-PA can regulate the fibrinolytic activity of endothelial cells by enhancing PAI-1 mRNA biosynthesis and release PAI-1 from the substratum to neutralize t-PA activity. The PAI-1 which released into the medium was immediately converted to the inactive latent form in the absence of active t-PA.