Pulmonary arterial hypertension (PAH) is a rare disease characterized by the obstructive elevation in pulmonary vascular resistance (PVR) and subsequent right-heart failure and death. There is currently no cure for PAH that has a very poor prognosis with a mean survival of 2.8 yr. Although the exact mechanisms leading to the onset and progression of PAH are still largely unclear, many predisposing and contributing factors have been identified, such as mutations in the bone morphogenetic protein-receptor (BMP-R)2 gene, inflammation and endothelial dysfunction, characterized by increased vasoconstriction, remodeling of pulmonary arterial wall of distal vessels and thrombosis. My primary research for the past several years seeks to provide a better understanding of the role played by pulmonary endothelial cells (P-ECs) in idiopathic PAH (iPAH) pathophysiology and identify target genes potentially involved in endothelial dysfunction. During my postdoctoral research, I have found that a selective disruption of the endothelial peroxisome proliferators activated receptor (PPAR)-gamma signaling in mice is sufficient to cause mild PAH and impair recovery from chronic hypoxia-induced PAH. More recently, we have observed paracrine overproduction of serotonin, endothelin-1 and fibroblast growth factor (FGF)-2 from iPAH P-ECs that contribute to increased smooth muscle cell proliferation. Moreover, our data have revealed that iPAH P-ECs display phenotypic abnormalities including a greater proliferative response to growth factors and apoptosis resistance. Although our observations support the hypothesis that pulmonary endothelial dysfunction is a major participant in the pathogenesis of PAH, its exact contribution to the disease remains unknown. CD74 (invariant chain, Ii) is an integral membrane protein which, beside its role as an MHC class II chaperone, has recently been found to play an additional role as an accessory-signaling molecule. CD74 demonstrates high-affinity binding to the proinflammatory cytokine macrophage migration-inhibitory factor (MIF), and the MIF homologue D-dopachrome tautomerase (DDT) that initiates a signaling cascade leading to cell proliferation and survival. Signal transduction from CD74/MIF or CD74/DDT occurs through two different modes: a) in a CD44/Src dependent pathway in which CD74 interacts with CD44. CD44 is a transmembrane protein whose phosphorylation leads to activation of Src-family kinase and MAPK/ERK, PI3K/Akt and NF-kappa B pathway and to apoptotic resistance by increasing the anti-apoptotic factors BCL2, BCL-xL and by inhibiting p53; b) in a CD44 independent pathway in which CD74 cytosolic region is cleaved by a two-step process: translocation of CD74 cytosolic fragment (CD74-ICD) to the cell nucleus resulting in NF-kappa B activation, and induction of a survival cascade via up-regulation of BCL2. Moreover, CD74 is known to interact with angiotensin AT1-receptor and nitric-oxide synthase 2. Taken together, these results strongly suggest that endothelial CD74 is a key molecule at the crossroad of inflammation and endothelial dysfunction, and thus may play an important role in the pathogenesis of PAH. Some of our recent data have shown that endothelial CD74 and MIF protein levels are markedly increased in the endothelium of muscularized pulmonary arteries from PAH patients compared to control subjects (unpublished results). Interestingly, in vitro exposure of P-ECs to MIF stimulation resulted in a 2.5-fold increase in IL-6 levels. This effect was significantly abolished by the MIF antagonist ISO-1. To expand our knowledge on this topic in iPAH, we propose to: •determine whether endothelial CD74 is involved in the acquisition/maintenance of endothelial dysfunction (Aim 1) •study the relationship between CD74 and inflammation (Aim 2) •investigate the mechanism(s) underlying the over-expression of endothelial CD74 (Aim 3)