Post-transcriptional regulation of urokinase mRNA - Identification of a novel urokinase mRNA-binding protein in human lung epithelial cells in vitro

We sought to determine if urokinase expression is regulated at the post-tra nscriptional level in cultured lung epithelial cells. We also sought to det ermine if differences in urokinase expression by cultured human lung carcin oma and non-malignant lung epithelial subtypes were attributable to post-tr anscriptional regulatory mechanisms. Urokinase was expressed by phenotypica lly diverse lung carcinoma cell lines as well as nonmalignant small airway epithelial cells and bronchial epithelial cells. Using gel mobility shift a nd UV crosslinking assays, we identified a 30-kDa urokinase mRNA-binding pr otein that selectively bound to a 66-nucleotide protein-binding fragment of urokinase mRNA. The urokinase mRNA-binding protein is found in the cytosol ic but not nuclear extracts of non-malignant lung epithelial cells; whereas , it is found in the nuclear but not cytosolic extracts of selected maligna nt carcinoma-derived cells that express relatively large amounts of urokina se. Chimeric beta-globin/urokinase cDNA containing the urokinase mRNA-bindi ng protein binding sequence destabilized otherwise stable beta-globin mRNA. Our results demonstrate that urokinase gene expression in lung epithelial and lung carcinoma-derived cells is regulated at the post-transcriptional l evel. The mechanism involves an interaction between a bs-nucleotide sequenc e of the urokinase mRNA 3'-untranslated region with a newly recognized urok inase mRNA-binding protein to regulate urokinase mRNA stability.