Upon binding of active TGF-b to the constitutively activated type II receptor, a specific type I receptor is recruited. Activation of this second receptor via phosphorylation initiates intracellular LM11A 31 dihydrochloride signaling through the phosphorylation of a set of receptor-regulated Smads, which subsequently form a complex with co-Smad. The R-Smad/co-Smad complex enters the nucleus, where it modulates the transcription of target genes. In most cells, the TGF-b signaling pathway involves the TbRII/ALK5 complex, which induces Smad2/3 phosphorylation. However, in endothelial cells, TGF-b activates two distinct type I receptors, ALK5 and ALK1, which transmit signals via the ALK5/Smad2/3 and ALK1/Smad1/5 pathways, Monensin sodium salt respectively. ALK5/Smad2/3 inhibits and ALK1/Smad1/5 stimulates endothelial- cell proliferation and migration. ENG, which is a transmembrane accessory receptor for TGF-b signaling, plays a pivotal role in the balance of ALK1 and ALK5 signaling that regulates endothelial cell proliferation. ENG is predominantly expressed on proliferating endothelial cells in vitro and on angiogenic blood vessels in vivo.The TGF-b/ALK1/ENG signaling pathway plays a key role in vessel formation and maintenance. However, its contribution to the pathogenesis of PAH is poorly understood. In the present study, we evaluated the susceptibility of ENG-deficient mice to PAH induced by 3 weeks of chronic hypoxia. We also evaluated pulmonary vascular remodeling and inflammation in these mice comparatively with wild-type mice exposed to chronic hypoxia. We investigated the expression pattern of the TGF-b/ALK1/ENG signaling pathway in lung tissue and in pulmonary-artery smooth-muscle-cells and pulmonary endothelial cells from patients with iPAH comparatively with controls. In a recent study, we found that serum-free medium of quiescent human PECs elicited marked PA-SMC proliferation. We therefore evaluated whether TGFb exposure of cultured PECs from controls increased the growthpromoting activity of the PEC culture medium and modified the expression of growth-promoting factors implicated in PAH development. The main findings from this study are as follows: compared to control specimens, lung tissue and PECs from patients with iPAH expressed increased amounts of ALK1 and ENG located predominantly on endothelial cells; compared to controls, patients with iPAH had higher serum and lung TGF-b levels;