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. Key components of PA remodeling are increased proliferation and survival of PA vascular smooth muscle cells (PAVSMC) associated with exces- sive extracellular matrix (ECM) production. We recently identified dysfunction of LATS1, a member of growth- suppressor HIPPO cassette, as a key event supporting proliferative/apoptosis-resistant PAH PAVSMC pheno- type, dissected the mechanism of LATS1 inactivation via up-regulation of Yap/Taz-ILK1, and demonstrated ben- efits of ILK1 inhibition to restore LATS1 function, reverse pulmonary vascular remodeling and reduce PH. The role of other HIPPO components, protein kinases MST1/2, in PAH remains to be established. We have found that MST1/2 in PAH play a unique pro-proliferative/pro-survival role and support increased proliferation and sur- vival of human PAH PAVSMC, pulmonary vascular remodeling and PH in mice. We report that this function of MST1/2 depend on Akt and unique PAH-specific interaction with cell cycle protein BUB3. Further, we show that MST1/2 and BUB3 act via supporting overexpression of cell-division cycle protein CDC20, consequent deficiency of pro-apoptotic Bim, activation of Akt, mTORC1, and Yap/Taz, increased proliferation, survival, and over-pro- duction of ECM. In turn, ECM, produced by human PAH PAVSMC, up-regulates BUB3, Akt-mTORC1, Yap/Taz, and PAVSMC proliferation, and induces growth of PA endothelial cells (PAEC) and adventitial fibroblasts (PAAF). Lastly, our data show that pharmacological inhibition of MST1/2 reverses PAH-specific signaling abnor- malities and selectively inhibits proliferation and/or promotes apoptosis in human PAH PAVSMC without affecting non-diseased cells. During the next funding period, we propose to elucidate roles, mechanisms of regulation and function of MST1/2 in PAH, and explore the benefits of targeting MST1/2 signaling to correct mechanistic abnor- malities and reverse PA remodeling and PH. Specifically, we will: (1) critically evaluate roles of MST1 and 2, its interacting partner BUB3 and downstream effector CDC20 in regulating Akt, mTORC1 and Yap/Taz networks, PAVSMC proliferation and survival, pulmonary vascular remodeling and PH using human PAH and non-diseased tissues and PAVSMC and mice with VSM-specific Mst1/2 knockout; (2) determine whether pathological function of MST1/2 in PAH PAVSMC is supported by Akt directly or via Akt-BUB3-Yap/Taz fed-forward loop, and evaluate the role of MST1/2 in ECM remodeling and ECM-dependent proliferation of PAEC and PAAF; and (3) test po- tential benefits of targeting MST1/2 signaling by pharmacological inhibition of MST1/2 kinase, Akt, or (alterna- tively) CDC20 to selectively suppress proliferation, induce apoptosis and reduce pathological ECM production by human PAH PAVSMC, reduce consequent hyper-proliferation of PAEC and PA...