Background-Pulmonary arterial hypertension (PAH) is a hyperproliferative vascular disorder observed predominantly in women. Estrogen is a potent mitogen in human pulmonary artery smooth muscle cells and contributes to PAH in vivo; however, the mechanisms attributed to this causation remain obscure. Curiously, heightened expression of the estrogenmetabolizing enzyme cytochrome P450 1B1 (CYP1B1) is reported in idiopathic PAH and murine models of PAH. Methods and Results-Here, we investigated the

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... utative pathogenic role of CYP1B1 in PAH. Quantitative reverse transcriptionpolymerase chain reaction, immunoblotting, and in situ analysis revealed that pulmonary CYP1B1 is increased in hypoxic PAH, hypoxicϩSU5416 PAH, and human PAH and is highly expressed within the pulmonary vascular wall. PAH was assessed in mice via measurement of right ventricular hypertrophy, pulmonary vascular remodeling, and right ventricular systolic pressure. Hypoxic PAH was attenuated in CYP1B1 Ϫ/Ϫ mice, and the potent CYP1B1 inhibitor 2,3Ј,4,5Ј-tetramethoxystilbene (TMS; 3 mg ⅐ kg Ϫ1 ⅐ d Ϫ1 IP) significantly attenuated hypoxic PAH and hypoxicϩSU5416 PAH in vivo. TMS also abolished estrogen-induced proliferation in human pulmonary artery smooth muscle cells and PAH-pulmonary artery smooth muscle cells. The estrogen metabolite 16␣-hydroxyestrone provoked human pulmonary artery smooth muscle cell proliferation, and this mitogenic effect was greatly pronounced in PAH-pulmonary artery smooth muscle cells. ELISA analysis revealed that 16␣-hydroxyestrone concentration was elevated in PAH, consistent with CYP1B1 overexpression and activity. Finally, administration of the CYP1B1 metabolite 16␣-hydroxyestrone (1.5 mg ⅐ kg Ϫ1 ⅐ d Ϫ1 IP) caused the development of PAH in mice. Conclusions-Increased CYP1B1-mediated estrogen metabolism promotes the development of PAH, likely via the formation of mitogens, including 16␣-hydroxyestrone. Collectively, this study reveals a possible novel therapeutic target in clinical PAH. (Circulation. 2012;126:1087-1098.)