HIF2α is required for the development of pulmonary vascular fibrosis associated with pulmonary arterial hypertension
Authors: Tarjus Antoine , Dai Zhiyu , Zhao Youyang 
Published: November 2023

BACKGROUNDPulmonary arterial hypertension (PAH) is characterized by progressive increase of pulmonary vascular resistance and obliterative pulmonary vascular remodeling that result in right heart hypertrophy, failure and premature death. One of the hallmarks of PAH is vascular fibrosis, an excessive deposition of extracellular matrix components (most notably collagen) in the vessel wall, which has been shown to have an important role in the pathogenesis of severe vascular remodeling. Our goal is to gain a better understanding of the mechanisms underlying pulmonary vascular fibrosis seen in IPAH patients in order to identify potential new drug target(s) to inhibit pulmonary vascular fibrosis and thereby prevent/reverse vascular remodeling for treatment of PAH.METHODSEmploying the mice with Tie2Cre‐mediated deletion of Egln1 which encodes hypoxia‐inducible factor prolyl hydroxylase 2, PHD2 (Egln1Tie2Cre), and double knockout mice with Tie2Cre‐mediated genetic deletions of both Egln1 and Hif1a (EH1) or Egln1 and Hif2a (EH2), we studied the role of PHD2 deficiency and HIF activation in inducing pulmonary vascular fibrosis. Trichrome staining was used to quantify pulmonary vascular fibrosis. Quantitative RT‐PCR analysis and immunostaining were carried out to define the molecular mechanisms of vascular fibrosis associated with PAH in Egln1Tie2Cre mice.RESULTSSimilar to IPAH lungs, mice inactivated for PHD2 in endothelial cells exhibited obliterative vascular remodeling with massive collagen deposition in the vessel wall as observed in Figure 1A. There was however no marked increase of interstitial fibrosis in Egln1Tie2 lungs. Intriguingly, pulmonary vascular fibrosis was inhibited by genetic deletion of HIF2α but not HIF1α, suggesting an important role of activation of HIF2α secondary to PHD2 deficiency in mediating pulmonary vascular fibrosis. Quantitative RT‐PCR analysis revealed a marked increase of fibrotic markers such as collagen 1α1, 1α2 and 3α1 in Egln1Tie2Cre mouse lungs, which was inhibited in EH2 mouse lungs. Another important feature of vascular fibrosis is the presence of myofibroblasts, which are responsible for extracellular matrix (ECM) production and accumulation. Immunostaining demonstrated prominent expression of periostin, a marker for myofibroblasts in Egln1Tie2Cre mouse lungs. The number of cells co‐stained with α‐smooth muscle actin and periostin was drastically increased in the vessel wall of Egln1Tie2 mouse lungs.CONCLUSIONSOur results demonstrate that endothelial PHD2 deficiency and resultant activation of HIF2α are responsible for pulmonary vascular fibrosis in the newly discovered mouse model of PAH with obliterative vascular remodeling and progressive right heart failure. We also showed that in vivo PHD2 deletion increases the number of myofibroblasts in the vessel wall. These studies defined an unexpected role of PHD2 deficiency in the mechanisms of vascular fibrosis associated with severe PAH. Thus, targeting PHD2/HIF‐2α signaling is a promising strategy to reverse vascular fibrosis for treatment of severe PAH and promote survival.