Abstract Pulmonary arterial hypertension (PAH) is a devastating disease with 5-year survival under 60%. Recent attention has focused on a role for pulmonary endothelial cells (EC) injury and proliferation in the initiation of vascular remodeling in PAH. However, the mechanisms in pulmonary ECs driving proliferation and remodeling is poorly understood limiting therapeutic options. Our laboratory has implicated a role for NADPH oxidase 1 (Nox1) in human pulmonary artery EC (hPAEC) proliferation. However, the key downstream effectors of this pathway are still poorly defined. Recently elevations in the chemokine CXCL12 has been associated with PAH and disease severity. CXCL12 is well known in other pathologies to induce alterations in cellular metabolism, migration and proliferation, all hallmarks of PAH vascular remodeling. Additionally, transcriptional regulation of CXCL12 described in other cell types has the potential to be modulated by Nox1-mediated signaling. However, the mechanisms of CXCL12 upregulation and signaling pathways in hPAECs remain poorly understood. We will therefore examine the hypothesis that hypoxia-mediated Nox1-signaling increases CXCL12, leading to alterations in migration and proliferation in PAEC. To explore this hypothesis, we have designed in vitro and in vivo experiments utilizing pharmacological agents and genetic perturbations. Our preliminary data shows trending elevation of CXCL12 in our local PAH population measured in total lung lysates and that hypoxia induces a >2-fold increase in CXCL12 expression in hPAECs. Using both gain and loss of function approaches our preliminary data implicates two Nox1-signaling pathways leading to the upregulation of CXCL12 in hypoxic hPAECs. This elevation in CXCL12 results in altered migration and proliferation of hPAECs. To investigate these pathways in vivo we have created a novel EC-specific inducible Nox1 knockout mouse. We hypothesize the loss of EC Nox1 will prevent PAH development while exogenous administration of CXCL12 will restore PAH in the knockout mice. These studies will advance our knowledge of a causal role for CXCL12 in PAH pathology, identify Nox1-mediated signaling pathways as key regulators of CXCL12, and suggest Nox1 as a potential therapeutic target in PAH.