Pulmonary arterial hypertension (PAH), a progressive fatal disease, manifests by remodeling of pulmonary ar- teries (PA), leading to increased PA pressure, right heart failure and death. The key component of PA remodeling is the progressive vessel wall thickening due to hyper-proliferation of PA smooth muscle cells (PASMC), endo- thelial cells (PAEC), and adventitial fibroblasts (PAAF) the mechanisms of which are not completely understood. PASMC in PAH switch to unique disease-specific phenotype, characterized by metabolic shift to glycolysis, au- tonomous proliferation, and apoptosis resistance. PAH pulmonary vascular cells are also highly secretory and support pro-proliferative microenvironment, further amplifying PA remodeling and PAH. Published and our new preliminary data strongly suggest that metabolite L-lactate acts as a central regulator of the molecular and met- abolic processes responsible for pulmonary vascular cell hyper-proliferation, remodeling, and PAH. Our pilot data show that lactate, over-produced by PAH PASMC due to over-expression of lactate dehydrogenase A (LDHA), promotes aberrant lactylation of DNA topoisomerase 1 (TOP1) and EMILIN1, leading to TOP1 up-reg- ulation and EMILIN1 deficiency, consequent up-regulation of pro-proliferative Akt-mTOR, Yap/Taz, TGFβ, in- creased proliferation and survival. Our data also suggest that lactate over-production in PAH PASMC is self- supported via glycolysis and EMILIN1-TGFβ1-HIF1α-LDHA circuit, and that lactate is secreted by PAH PASMC and promotes proliferation of PA endothelial cells (PAEC) and adventitial fibroblasts (PAAF). We further report that suppression of LDHA-lactate axis reduces proliferation and induces apoptosis in human PAH PASMC, re- verses pulmonary vascular remodeling and experimental PH in mice. We propose to elucidate the role and mechanisms of regulation and function of LDHA-lactate signaling in PAH pulmonary vasculature and explore the benefits of targeting this pathway to correct mechanistic abnormalities and reverse PA remodeling and PH. Spe- cifically, we will: (1) critically test the role of LDHA-lactate in PAH PASMC proliferation and survival, pulmonary vascular remodeling and PH using human PAH and non-diseased PASMC and lung tissue samples and SM- specific Ldhaknock-out mice; determine the relationship among LDHA-lactate, lactylation of TOP1 and EMILIN1, and Yap/Taz, Akt-mTOR, and TGFβ1 in regulating PASMC proliferation and survival; (2) investigate whether lactate over-production is supported through up-regulation of glycolysis and EMILIN-TGFβ1-HIF1α-LDHA feed- forward loop, evaluate the metabolic consequences of lactate over-production, and determine the role of PASMC-secreted lactate in the proliferation of PAEC and PAAF; and (3) examine whether targeting lactate sig- naling by LDHA inhibitor oxamate and TOP1 inhibitor indotecan selectively inhibits proliferation and induces apoptosis in vitro in human PAH PASMC, reverses or attenuates experimental pulmon...