SMAD1 and pulmonary arterial hypertension: Further investigation demonstrated the promotion of TGF-β to PASMC proliferation in PAH, supported by the fact that the PAH-conditioned medium significantly increased the phosphorylation of Smad2, Smad3, and Smad1/5 and the expression of AKT, ERK1/2, and p38 MAPK in non-diseased PASMCs.