Suppression of HIF2 signalling attenuates the initiation of hypoxia-induced pulmonary hypertension

Abstract

Most published studies addressing the role of HIFs in hypoxia-induced PH development employ models that may not recapitulate the clinical setting, including the use of animals having pre-existing lung/vascular defects secondary to embryonic HIF ablation or activation. Further, critical questions including how and when HIF-signalling contributes to hypoxia-induced PH remains unanswered.Normal adult rodents in which global HIF1 or 2 was inhibited by inducible gene deletion or pharmacological inhibition (antisense oligonucleotides-ASO and small molecule inhibitors) were exposed to short-term (4 days) or chronic (4–5 weeks) hypoxia. Hemodynamic studies were performed, the animals euthanized and lungs and heart obtained for pathologic and transcriptomic analysis. Cell-type specific HIF signals for PH initiation were determined in normal pulmonary vascular cells in vitro and in mice (using cell-type specific HIF deletion).Global HIF1α deletion in mice did not prevent hypoxia-induced PH at 5 weeks. Mice with global HIF2α deletion did not survive long-term hypoxia. Partial HIF2α gene deletion, or HIF2-ASO (but not HIF1-ASO) reduced vessel muscularization, rises in pulmonary artery pressures and right ventricular hypertrophy in mice exposed to 4–5 week hypoxia. A small molecule HIF2 inhibitor (PT2567) significantly attenuated early events (monocyte recruitment and vascular cell proliferation) in rats exposed to 4-day hypoxia as well as vessel musculization, tenascin C accumulation and PH development in rats exposed to 5 week hypoxia. In vitro, HIF2 induced a distinct set of genes in normal pulmonary vascular EC, mediating inflammation and proliferation of EC and SMC. EC HIF2α knockout prevented hypoxia-induced PH in mice.Inhibition of HIF2, not HIF1 can provide a therapeutic approach to prevent the development of hypoxia-induced PH. Future studies are needed to investigate the role of HIFs in PH progression and reversal.

Roles of HIF1 and HIF2 in pulmonary hypertension: it all depends on the context

“…As is frequently the case with interesting studies, the report by HU et al [10] answers some questions while it raises others. HIF2 regulates erythropoietin expression in the kidney [11], so it is not surprising that the systemic antisense knockdown suppressed the polycythaemia that develops during chronic hypoxia.…”

“…However, PHD2 deletion in adult mice does not produce the same phenotype. The study by HU et al [10] shows a more dramatic effect of embryonic endothelial HIF2 knockout (compared with PT2567 or the antisense oligonucleotide interventions in adult mice) in terms of blocking the response to chronic hypoxia. Hence, HIF2 may play an important role in the developing lung, in terms of setting the context for later development of PH.…”

“…Recognising this, HU et al [10] set out to understand the roles of HIF1 and HIF2 in PH. As reported in this issue of the European Respiratory Journal, they first deleted HIF1α systemically in adult mice using Ubc-Cre-ERT2, which causes widespread deletion in cells and tissues.…”

“…To their credit, HU et al [10] strengthened their in vivo analysis by assessing HIF-dependent gene expression patterns in vascular cells cultured under normoxic versus hypoxic conditions. Those results, along with siRNA knockdown studies and conditioned media experiments, underscored the importance of HIF2-dependent gene responses in endothelial cells in controlling the response to hypoxia.…”

Heart of the Matter: Divergent Roles of Hypoxia-Inducible Factors in Hypoxia-induced Right Ventricle Hypertrophy

“…In this setting, HIF-1α in cardiomyocytes appears protective. The role of HIF-1α in hypoxia-induced PH has been the subject of debate, with studies showing protective ( 10 ), detrimental ( 3 , 13 ), or limited ( 8 ) roles. Factors proposed to contribute to differential results across studies have included sex, constitutive versus inducible and/or global versus cell-type–specific deletion, or timing of measurements.…”