Activation of Smad and ERK1/2 pathways by TGF-β1. A549 cells were pre-incubated in the presence or the absence of 10 μM of U0126, a potent MEK inhibitor, for 1 h prior to TGF-β1 stimulation. 5 ng/ml of TGF-β1 was used as a stimulus. TGF-β1 activates Erk1/2 and Smad2 pathways within 5 min after stimulation. The MEK inhibitor blocks Erk1/2 phosphorylation, but does not influence Smad2 phosphorylation. Equal amounts of total protein are loaded in each lane.