DDAH1 and pulmonary arterial hypertension: Previous studies have shown that miR-21 can inhibit cardiac myocyte/hepatic stellate cell apoptosis through PTEN/PI3K/Akt pathway, protect against neuronal cell death through FasL signaling, attenuate angiogenesis through SMAD7 signaling, and suppress secretion of inflammatory cytokines and chemokine receptor type 7 under hypoxia conditions, whereas it may contribute to pulmonary hypertension through inhibiting DDAH1 and RhoB under hyoxia [24,25,26,27,28].