Cytoplasmic-nuclear shuttling of the urokinase mRNA binding protein regulates message stability

Treatment of small airway epithelial (SAEC) cells or lung epithelial (Beas2B) cells with TNF-α or PMA induces urokinase-type plasminogen activator (uPA) expression. Treatment of these cells with TNF-α, PMA or cycloheximide but not TGF-β increased steady-state expression of uPA mRNA. TNF-α, PMA or cycloheximide caused 8–10 fold extensions of the uPA mRNA half-life in SAEC or Beas2B cells treated with DRB, a transcriptional inhibitor. These findings suggest that uPA gene expression involves a post-transcriptional regulatory mechanism. Using gel mobility shift and UV cross-linking assays, we identified a 30 kDa uPA mRNA binding protein (uPA mRNABp) that selectively binds to a 66 nt protein binding fragment of uPA mRNA containing regulatory information for message stabilization [1]. Binding of cytoplasmic uPA mRNABp to uPA mRNA was abolished after treatment with TNF-α but not TGF-β. In addition, we found the accumulation of 30 kDa uPA mRNABp in the nuclear extracts of TNF-α but not TGF-β treated cells. The uPA mRNABp starts moving to the nucleus from the cytoplasmic compartment as early as three hours after TNF-α treatment. Complete translocation is achieved between 12–24 h, which coincides with the maximal expression of uPA protein effected by cytokine stimulation. Treatment of Beas2B cells with NaF inhibited TNF-α-mediated translocation of uPA mRNABp from the cytoplasm to the nucleus and concomitant inhibition of uPA expression. TNF-α stabilizes uPA mRNA by translocating the uPA mRNABp from the cytoplasm to the nucleus. Our results demonstrate a novel mechanism governing uPA mRNA stability through shuttling of uPA mRNABp between the nucleus and cytoplasm. This newly identified pathway may have evolved to regulate uPA-mediated functions of the lung epithelium in inflamation or neoplasia.