MTOR and pulmonary arterial hypertension: Molecular mechanisms of PAH are complex and involve multiple signaling pathways including cell survival and proliferation, such as phosphoinositide 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR), platelet-derived growth factor (PDGF), and transforming growth factor (TGF)-β/bone morphogenetic protein (BMP) (Rabinovitch, 2008; Tang et al., 2018a; Rol et al., 2018).