Mechanisms contributing to increased synthesis of plasminogen activator inhibitor type 1 in endothelial cells by constituents of platelets and their implications for thrombolysis

Abstract

We recently hypothesized that after pharmacologically induced coronary thrombolysis, increased activity of plasminogen activator inhibitor type 1 (PAI-1) retards recanalization, contributes to early reocclusion, or both. This hypothesis was based on the increased elaboration of PAI-1 that we observed in cultured liver cells exposed to growth factors releasable from platelets activated at sites of thrombosis in vivo. PAI-1 released locally is particularly likely to attenuate lysis of thrombi that are targets of thrombolytic drugs. Accordingly, the present study was performed to determine whether synthesis of PAI-1 by endothelial cells is augmented by products of platelets. Lysates from platelets (0.5-8.0 × 104/ mm3 media, i.e. <10% of the concentration of platelets in blood) increased synthesis and release of PAI-1 into both the extracellular matrix and conditioned media (by 2.8-fold and 3.3-fold within 6 and 24 hours, respectively). Synthesis of neither tissue-type plasminogen activator nor overall protein increased. Increased synthesis of PAI-1 was confirmed by immunoprecipitation of [35S]PAI-1 after metabolic labeling of cells. The increased elaboration of PAI-1 was consistent with increased transcription as reflected by the observed increase in PAI-1 mRNA of 2.2-fold in 4 hours. Effects of platelet lysates were simulated by transforming growth factor β (TGF-β), known to be present in platelet α-granules and released with platelet activation. Antibody to TGF-β reduced the stimulation of PAI-1 synthesis by TGF-β, as expected, by 82%. In addition, it attenuated PAI-1 synthesis by platelet lysates to a similar extent, by 69%. Other factors released by platelets, including TGF-α, platelet-derived growth factor, histamine, norepinephrine, and serotonin had no effect. These results indicate that products of the platelet release reaction stimulate endothelial synthesis of PAI-1. Accordingly, activation of platelets may attenuate pharmacologically and physiologically induced coronary thrombolysis by augmenting concentrations of PAI-1 locally, a mechanism potentially amenable to pharmacological modification.