Endothelial Hif-2 Alpha Contributes To Severe Pulmonary Hypertension Due To Endothelial-To-Mesenchymal Transition

Pulimonary vascular remodeling characterized by concentric wall thickening and intraluminal obliteration is a major contributor to the elevated pulmonary vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Here we report that increased hypoxia-inducible factor 2 alpha (HIF-2 alpha) in lung vascular endothelial cells (LVECs) under normoxic conditions is involved in the development of pulmonary hypertension (PH) by inducing endothelial-to-mesenchymal transition (EndMT), which subsequently- results in vascular remodeling and occlusive lesions. We observed significant EndMT and markedly increased expression of SNAL an inducer of EndMT, in LVECs from patients with IPAH and animals with experimental PH compared with normal controls. LVECs isolated from IPAH patients had a higher level of HIF-2 alpha than that from normal subjects, whereas HIF-l alpha\ was upregulated in pulmonary arterial smooth muscle cells (PASMCs) from IPAH patients. The increased HIF-2 alpha level, due to downregulated prolyl hydroxylase domain protein 2 (PHD2), a prolyl hydroxylase that promotes HIF-2 alpha degradation, was involved in enhanced EndMT and upregulated SNAI1/2 in LVECs from patients with IPAH. Moreover, knockdown of HIF-2 alpha (but not HIP-l alpha) with siRNA decreases both SNAH and SNAI2 expression in IPAH-LVECs. Mice with endothelial cell ( EC) -specific knockout (KO) of the PHD)2gene, egInI (eglnlEc), developed severe PH under normoxic conditions, whereas Snail/2 and EndMT were increased in LVECs of mice. EC -specific KO of the HIF-a alpha gene, hif-2 alpha, prevented mice from developing hypoxia-induced PH, whereas EC -specific deletion of the HIP-la gene, hifla, or smooth muscle cell (SMC)-specific deletion of hif2a, negligibly affected the development of PH. Also, exposure to hypoxia for 48-72 h increased protein level of HIF-la in normal human PASMCs and HIF-2 alpha in normal human LVECs. These data indicate that increased 11:1F -2 alpha in LVECs plays a pathogenic role in the development of severe PI1 by upregulating SNAI1/2, inducing EndMT, and causing obliterative pulmonary vascular lesions and vascular remodeling.