Tissue-type plasminogen activator binding to human endothelial cells. Evidence for two distinct binding sites

Induction of morphological differentiation of human neuroblastoma cells is accompanied by induction of tissue-type plasminogen activator

Human SH-SY5Y neuroblastoma cells treated with retinoic acid, 12-O-tetradecanoyl-phorbol-13-acetate (TPA) or nerve growth factor differentiated morphologically to neuronlike cells with increased amounts of neurofilament protein and mRNA. All three effectors induced an increase in the amount of relative molecular weight (Mr) 70,000 tissue-type plasminogen activator (t-PA) and its mRNA, as determined by immunocapture, enzyme activity, and Northern blotting analyses. About 90% of the t-PA activity was secreted to the culture medium. In contrast, of the three effectors studied, only TPA induced transcription of the proto-oncogene c-fos, studied as a control gene responsive to various stimuli, and induced a rapid increase in urokinase-type PA (u-PA). Most of the u-PA activity induced by TPA remained cell-associated. Because induction of differentiation correlated closely with induction of t-PA, and not u-PA, the authors propose that t-PA may have a functional role in the morphological differentiation of neuronal cells.

Lipoprotein(a) modulation of endothelial cell surface fibrinolysis and its potential role in atherosclerosis

Endothelial cells play a critical role in thromboregulation by virtue of a surface-connected fibrinolytic system. Cultured endothelial cells synthesize and secrete tissue-type plasminogen activator (t-PA) which can bind to at least two discrete sites on the cell surface. These binding sites preserve the catalytic activity of t-PA and protect it from its physiological inhibitor (PAI-1). N-terminal glutamic acid plasminogen (Glu-PLG), the main circulating fibrinolytic zymogen, also interacts specifically with the endothelial cell surface. Binding is associated with a 12-fold increase in catalytic efficiency of plasmin generation by t-PA which may reflect conversion of Glu-PLG to its plasmin-modified form, N-terminal lysine plasminogen (Lys-PLG). Lipoprotein(a) is an atherogenic lipoprotein particle which contains the plasminogen-like apolipoprotein(a) bound to low density lipoprotein. We report here that lipoprotein(a) interferes with endothelial cell fibrinolysis by inhibiting plasminogen binding and hence plasmin generation. In addition, we demonstrate lipoprotein(a) accumulation in atherosclerotic lesions. These findings may provide a link between impaired cell surface fibrinolysis and progressive atherosclerosis.

Effect of fibrin on endothelial cell production of prostacyclin and tissue plasminogen activator

Fibrin formed on endothelial cells has previously been shown to have deleterious effects on the cells. Additionally, substances that cause endothelial cell damage have been reported to induce cultured endothelial cells to synthesize prostacyclin and tissue plasminogen activator (t-PA). The present studies were undertaken to determine whether fibrin formed on cultured human umbilical vein endothelial cells would alter synthesis of prostacyclin and t-PA by the cells. Fibrin was found to increase synthesis of both prostacyclin and t-PA in a dose and time dependent manner. Stimulation of prostacyclin synthesis was completely inhibited by indomethacin; partially inhibited by actinomycin D, cycloheximide, and trifluoperazine; and not affected by cytochalasin D or vinblastine. In contrast, stimulation of t-PA synthesis was completely inhibited by actinomycin D and cycloheximide; partially inhibited by cytochalasin D, vinblastine, and trifluoperazine; and not affected by indomethacin. Fibrin I, formed with Reptilase, caused only slight stimulation of t-PA production, but virtually no stimulation of prostacyclin synthesis. Neither collagen polymerization on the cells nor thrombin added in concentrations that did not induce fibrin polymer formation stimulated production of either substance. Furthermore, soluble fibrin II generated in the presence of the fibrin polymerization inhibitor gly-pro-arg-pro also failed to stimulate either prostacyclin or t-PA production. The presence of platelets in the plasma from which the fibrin was formed did not affect the amount of stimulation of the cells. Fibrin-induced stimulation of endothelial cell production of prostacyclin and t-PA could act to limit vascular occlusion in vivo by inhibiting platelet function and by stimulating fibrinolysis via t-PA.

New strategies in the development of thrombolytic agents

Recombinant DNA technology has allowed large-scale production of the physiological, fibrin-specific, plasminogen activators tissue-type plasminogen activator (t-PA) and single-chain urokinase-type plasminogen activator (scu-PA). The results of clinical trials with these agents, mainly for the treatment of acute myocardial infarction, have revealed a limited fibrin specificity at the large therapeutic doses required for efficient thrombolysis. Mutants and variants of t-PA and scu-PA have given important information on structure-function relationships in these proteins and have resulted in rt-PA variants with significantly prolonged half-lives in vivo. Construction of chimaeric plasminogen activators containing various portions of t-PA and scu-PA has produced functionally active enzymes, however with a lower fibrin-affinity than wild-type t-PA. The promise of antibody targeting and the use of synergistic combinations of thrombolytic agents remains to be further investigated. We anticipate that eventually these research lines will yield artificial plasminogen activators with improved efficacy, risk/benefit and cost/benefit ratios.