TGFB1 and pulmonary arterial hypertension: Therefore, we utilized cultured human umbilical vein endothelial cells (HUVECs) to test the suppressive effect of preclinical and clinical PAH drug candidates such as propylthiouracil (PTU) (Sigma, Saint Louis, MO, USA) [26], N-[N-(3,5-difluorophenacetyl)-l-alanyl]-S-phenylglycine t-butyl ester (DAPT, a γ-secretase inhibitor; Calbiochem, La Jolla, CA, USA) [27], riociguat (Cayman Chemical, Ann Arbor, MI, USA) [28], sildenafil (Cayman Chemical) [29], and L-902,688 (Cayman Chemical) at 1 μM against 5 ng/mL TGF-β-induced EndMT.