Pulmonary arterial hypertension (PAH) is a life-threatening disease characterized by abnormally elevated pulmonary pressures and right heart failure resulting in high morbidity and mortality. The pathologic hallmark of PAH is progressive loss and obstructive remodeling of lung microvessels unresponsive to available therapies. Studies by our group and others have shown that pulmonary microvascular endothelial cells (PMVECs) derived from PAH patients are highly susceptible to apoptosis and have a lower capacity to form blood vessels (i.e., angiogenesis). Fatty acid oxidation (FAO) is an essential energy source for PMVECs that requires metabolic coupling of mitochondria and endoplasmic reticulum (ER). Metabolic reprogramming characterized by high glycolysis, reduced FAO, and mitochondrial/ER dysfunction is a key pathological feature of PAH PMVECs linked to oxidative stress, endothelial dysfunction, and reduced angiogenesis. Our group has shown that reduced activity of bone morphogenetic protein receptor 2 (BMPR2), the most common genetic cause of hereditary and sporadic PAH, promotes metabolic reprogramming but, given the low penetrance of BMPR2 mutations, alterations in other genes (i.e., “second hit”) are likely necessary for PAH development. In this proposal, we will show that carboxylesterase 1 (CES1), a lipolytic enzyme responsible for releasing free fatty acids from the ER to the mitochondria, is required for FAO and metabolic homeostasis in PMVECs. Our preliminary studies show that CES1 knockdown in healthy PMVECs results in 1) high glycolysis, 2) reduced FAO, 3) mitochondrial/ER dysfunction, and 4) oxidative stress. Furthermore, loss of CES1 exacerbates metabolic reprogramming associated with BMPR2 insufficiency, and restoring CES1 expression improves PAH PMVEC functional status. Based on our preliminary studies, we hypothesize that loss of CES1 in PAH leads to endothelial dysfunction in PAH through metabolic reprogramming, lipotoxicity, and oxidative stress. To test this, we propose the following aims: (1): Determine the mechanisms by which loss of CES1 results in metabolic reprogramming and lipotoxicity in PMVECs, (2) Determine whether loss of CES1 promotes the development and severity of pulmonary hypertension and vasculopathy in mice, and (3) Determine the contribution of BMPR2 insufficiency and epigenetic repression to reduced CES1 expression in PAH PMVECs. Using the proposed approach, we will demonstrate that CES1 is essential for properly maintaining and repairing the pulmonary endothelium and acts as a key modifier of BMPR2 signaling. Given the limited capacity of current therapies to reverse endothelial dysfunction and prevent small vessel loss, therapeutic interventions that can restore CES1 expression could serve as a novel treatment approach for PAH.