Postulated mechanisms of TGF-β1-induced EMT in RLE cells and the essential function of DA-Raf for EMT.

(A) TGF-β1-induced Smad2/3 signaling is essential for EMT from RLE cells to myofibroblasts expressing αSMA, collagen 1, and fibronectin. TGF-β1-induced Ras activity is weak, and intrinsic DA-Raf is sufficient to suppress the ERK pathway by binding to activated Ras. (B) FGF2 stimulation (as well as FGF1 or HGF stimulation) or overexpression of the constitutively active Ras, Raf, or MEK intensifies the activity of the Ras—ERK pathway activated by TGF-β1. Intrinsic DA-Raf is not sufficient to overcome the strong Ras—ERK pathway activity. Activated ERK might interfere with the nuclear translocation of Smad2/3, which is required to induce EMT.