TGFB1 and pulmonary arterial hypertension: Interestingly, R-SMADs (SMAD1/5) that are up-regulated through TGF-β1 signaling can stimulate miRNA maturation by binding pre-miRNAs and recruiting Drosha, leading to increased levels of several miRNAs important for PAH, such as miR-199a or miR-21 [90].