UROKINASE-RECEPTOR BIOSYNTHESIS, MESSENGER-RNA LEVEL AND GENE-TRANSCRIPTION ARE INCREASED BY TRANSFORMING GROWTH FACTOR-BETA-1 IN HUMAN A549 LUNG-CARCINOMA CELLS

We have compared the cell-specific expression and regulation of the receptor for urokinase-type plasminogen activator (u-PAR) by transforming growth factor-beta type 1 (TGF-beta-l) in 10 human cell lines derived from both normal and neoplastic tissues. The basal expression of u-PAR mRNA as well as its response to TGF-beta-1 varied strongly between different cell lines; however, five out of the 10 cell lines responded to TGF-beta-1 by an increase in the u-PAR mRNA level. Among these, A549 cells were selected for a detailed elucidation of the molecular mechanism involved in TGF-beta-1 regulation of u-PAR mRNA expression. TGF-beta-1 caused an early increase in u-PAR mRNA level, with a maximal 15-fold enhancement after 24 h of treatment. This was paralleled by an increase in u-PAR protein as detected by crosslinking studies with radiolabeled ligand, and also resulted in an increase in cell surface plasmin generation. The protein synthesis inhibitor cycloheximide also increased the level of u-PAR mRNA in a time-dependent fashion and when both cycloheximide and TGF-beta-1 were used, an additive effect was seen. Nuclear run-on experiments demonstrated only a moderate (3-fold) increase in the u-PAR gene transcription rate after exposure of the cells to TGF-beta-1 for 3 h compared with a 12-fold increase in the mRNA level. TGF-beta-1 also caused an increase of both u-PA and PAI-1 antigens, while there was no detectable effect on t-PA. The tumor promoter phorbol myristate acetate and epidermal growth factor also strongly increased the uPAR mRNA level in both A549 and RD cells, whereas dexamethasone increased the u-PAR mRNA level in the RD cells but had no effect in the A549 cells.