Thymosin β4 induces the synthesis of plasminogen activator inhibitor 1 in cultured endothelial cells and increases its extracellular expression

Thymosin beta4 (Tbeta4), a 4.9-kDa polypeptide primarily known as a main G-actin-sequestering peptide, is present in high concentrations in various cells and in the circulation. We have found that Tbeta4 upregulates the expression of plasminogen activator inhibitor 1 (PAI-1) in endothelial cells measured both at the level of mRNA and protein synthesis. This effect seems to be cell specific and was not observed when other cells such as human fibroblasts, PC3, and U937 were tested. Tbeta4 significantly activated the PAI-1 promoter in EA.hy 926 cells transiently transfected either with plasmid p800LUC containing PAI-1 promoter fragment (-800 to +71) or the PAI-1 promoter linked with green fluorescent protein. Tbeta4 mediated up-regulation of PAI-1 involved activation of the mitogen-activated protein kinase cascade. Furthermore, Tbeta4 enhanced c-Fos/ c-Jun DNA-binding activity to the activator protein 1 (AP-1)-like element (-59 to -52). The specificity of this binding activity was demonstrated by competition electrophoretic mobility shift assay and after transfection of EA.hy 926 cells with the mutated PAI-1 promoter. Taken together, these data indicate that, in response to Tbeta4 stimulation, AP-1 activity increases to enhance PAI-1 transcription through its unique AP-1-like element at -59 to -52 in the PAI-1 promoter. (C) 2004 by The American Society of Hematology.