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Rationale: Pulmonary arterial hypertension is a progressive disease characterized by an elevation in the mean pulmonary artery pressure leading to right heart failure and a significant risk of death. Alterations in two transforming growth factor (TGF) signaling pathways, bone morphogenetic protein receptor II and the TGF-beta receptor I, Alk1, have been implicated in the pathogenesis of pulmonary hypertension (PH). However, the role of TGF-beta family signaling in PH and pulmonary vascular remodeling remains unclear.Objectives: To determine whether inhibition of TGF-beta signaling will attenuate and reverse monocrotaline-induced PH (MCT-PH).Methods: We have used an orally active small-molecule TGF-beta receptor I inhibitor, SD-208, to determine the functional role of this pathway in MCT-PH.Measurements and Main Results: The development of MCT-PH was associated with increased vascular cell apoptosis, which paralleled TGF-beta signaling as documented by psmad2 expression. Inhibition of TGF-beta signaling with SD-208 significantly attenuated the development of the PH and reduced pulmonary vascular remodeling. These effects were associated with decreased early vascular cell apoptosis, adventitial cell proliferation, and matrix metalloproteinase expression. Inhibition of TGF-beta signaling with SD-208 in established MCT-PH resulted in a small but significant improvement in hemodynamic parameters and medial remodeling.Conclusions: These findings provide evidence that increased TGF-beta signaling participates in the pathogenesis of experimental severe PH. [ABSTRACT FROM AUTHOR]