A Urokinase Receptor mRNA Binding Protein–mRNA Interaction Regulates Receptor Expression and Function in Human Pleural Mesothelioma Cells

Human pleural malignant mesothelioma (MS-1) or mesothelial (MeT5A) cells express the multifunctional urokinase receptor (uPAR) which influences neoplastic propagation via contributions to cellular proteolysis, migration, and mitogenesis. Recently, we reported that a 51-nucleotide fragment of the uPAR mRNA coding region contains regulatory information for uPAR message stability and that a cytoplasmic uPAR mRNA binding protein (uPAR mRNABp) specifically bound to this sequence in temporal association with uPAR mRNA destabilization in MS-1 cells. To determine if the uPAR mRNA–uPAR mRNABp interaction is a determinant of uPAR message stability as well as uPAR expression, we further characterized thiscis–transinteraction and created stable transfected cell lines designed to exploit the interaction and to increase uPAR at the cell surface. The uPAR mRNABp was purified from MS-1 cells, has an apparent molecular mass of 50 kDa, selectively binds to the 51-nt fragment of the uPAR coding region, and does not degrade uPAR mRNA. To determine the role of the uPAR mRNABp on receptor expression, we overexpressed a chimeric β-globin/uPAR/β-globin mRNA containing the 51-nt binding fragment of uPAR mRNA in MS-1 cells and found that uPAR at the cell surface increased by twofold as measured by [125I]uPA binding or ligand blotting. Cellular proliferation of uPA-treated cells and invasiveness was similarly increased. The increase in cell surface uPAR was due to commensurately increased uPAR mRNA. The results suggest that competition between the overexpressed 51-nt fragment of the uPAR coding region and the wild-type uPAR mRNA transcript for uPAR mRNABp binding enables the cells to translate and express more uPAR at the cell surface. The interaction between the uPAR mRNABp and uPAR mRNA regulates message stability as well as uPAR expression by MS-1 cells.