Vascular homeostasis in the lung is disturbed in pulmonary arterial hypertension. Jongmin Kim et al. delineate a new signaling axis controlling endothelial cell proliferation and cytokine production that is dysregulated in pulmonary endothelial cells from individuals with this disease. In this axis, the peptide apelin controls expression of the cytokine FGF2, a mitogen for endothelial and vascular smooth-muscle cells, through effects on two microRNAs. The authors also demonstrated the functional importance of these miRNAs in rat models of pulmonary hypertension. Pulmonary arterial hypertension (PAH) is characterized by vascular remodeling associated with obliteration of pulmonary arterioles and formation of plexiform lesions composed of hyperproliferative endothelial and vascular smooth-muscle cells. Here we describe a microRNA (miRNA)-dependent association between apelin (APLN) and fibroblast growth factor 2 (FGF2) signaling in pulmonary artery endothelial cells (PAECs). APLN deficiency in these cells led to increased expression of FGF2 and its receptor FGFR1 as a consequence of decreased expression of miR-424 and miR-503, which directly target FGF2 and FGFR1. miR-424 and miR-503 were downregulated in PAH, exerted antiproliferative effects in PAECs and inhibited the capacity of PAEC-conditioned medium to induce the proliferation of pulmonary artery smooth-muscle cells. Reconstitution of miR-424 and miR-503 in vivo ameliorated pulmonary hypertension in experimental models. These studies identify an APLN-dependent miRNA-FGF signaling axis needed for the maintenance of pulmonary vascular homeostasis.

An endothelial apelin-FGF link mediated by miR-424 and miR-503 is disrupted in pulmonary arterial hypertension