Requirement of TGF-β Receptor-Dependent Activation of c-Jun N-Terminal Kinases (JNKs)/Stress-Activated Protein Kinases (Sapks) for TGF-β Up-Regulation of the Urokinase-Type Plasminogen Activator Receptor

We have previously demonstrated that activation of the Ras/Mapk pathways is required for transforming growth factor β (TGF-β) induction of TGF-β₁ expression. Here we examined the role of the Ras/Mapk pathways in TGF-β induction of urokinase-type plasminogen activator receptor (uPAR) expression in untransformed intestinal epithelial cells (IECs). TGF-β activated the stress-activated protein kinases (Sapk)/c-Jun N-terminal kinases (JNKs) within 5-10 min, an effect that preceeded TGF-β induction of uPAR expression in these cells. TGF-β induction of both JNK1 activity and JunD phosphorylation was blocked by expression of a dominant-negative mutant of the type II TGF-β receptor (DN TβRII), a dominant-negative mutant of MKK4 (DN MKK4), or a dominant-negative mutant of Ras (RasN17), or by the addition of the JNK inhibitor SP600125. TGF-β also induced AP-1 complex formation at the distal AP-1 site (-184 to -178) of the uPAR promoter within 2 h of TGF-β addition, consistent with the time-dependent up-regulation of uPAR expression. The primary components present in the TGF-β-stimulated AP-1 complex bound to the uPAR promoter were Jun D and Fra-2. Moreover, addition of SP600125, or expression of DN MKK4 or DN TβRII, blocked TGF-β up-regulation of uPAR in IECs. Accordingly, our results indicate that TGF-β activates the Ras/MKK4/JNK1 signaling cascade, leading to induction of AP-1 activity, which, in turn, up-regulates uPAR expression. Our results also indicate that the type II TGF-β receptor (RII) is required for TGF-β activation of JNK1 and the resulting up-regulation of uPAR expression.