Transcriptional and posttranscriptional regulation of urokinase‐type plasminogen activator expression in endothelial cells by basic fibroblast growth factor

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Abstract
The mechanism of induction of urokinase‐type plasminogen activator (uPA) by basic fibroblast growth factor (bFGF) was explored in fetal bovine aortic endothelial GM 7373 cells. A three‐ to four‐fold increase in the steady‐state levels of uPA mRNA was observed after 6 h of incubation of the cell cultures with bFGF. Accordingly, nuclear run‐on experiments showed a 2–2.4‐fold increase in the rate of uPA gene transcription during the first 4 h of treatment with the growth factor. bFGF did not affect uPA mRNA stability, as evaluated by chase experiments with the mRNA synthesis inhibitor actinomycin D. Upregulation of uPA mRNA was followed by a delayed increase in uPA protein synthesis paralleled by an increase in secreted and cell‐associated uPA activity. Twelve h were required before accumulated uPA mRNA was translated into the corresponding protein. During this time interval, the continuous presence of biologically active bFGF in the extracellular environment represented an absolute requirement for uPA mRNA translation. Substitution of residues Lys‐27, Lys‐30, and Arg‐31 to glutamine residues in the bFGF molecule resulted in a mutant (M1Q‐bFGF) that caused uPA mRNA accumulation in the absence of a significant increase in cell‐associated uPA activity. M1Q‐bFGF also induced an increase in cell‐associated uPA activity only when added to the cell cultures in the presence of soluble heparin. These results provide evidence that bFGF can affect uPA expression in endothelial GM 7373 cells both at transcriptional and posttranscriptional/translational levels. They also show the possibility to dissociate upregulation of uPA mRNA from upregulation of uPA activity by mutagenesis of the bFGF molecule.

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