TGFB1 and pulmonary arterial hypertension: In addition to the intracellular signaling via VEGF, BMP, PDGF, TGF-β1, Notch, endothelin and TRP channel signaling, mechanical crosstalk and paracrine signaling mediate interactions between PAECs, PASMCs, and PAAFs in response to physical stimuli caused by PAH, including altered fluid shear stress, stretch, ECM stiffness and hypoxia (Fig. 4).