Loss of <i>Hif-2α</i> Rescues the <i>Hif-1α</i> Deletion Phenotype of Neonatal Respiratory Distress In Mice

Hypoxia is a state of decreased oxygen reaching the tissues of the body. During prenatal development, the fetus experiences localized occurrences of hypoxia that are essential for proper organogenesis and survival. The response to decreased oxygen availability is primarily regulated by hypoxia-inducible factors (HIFs), a family of transcription factors that modulate the expression of key genes involved in glycolysis, angiogenesis, and erythropoiesis. HIF-1α and HIF-2α, two key isoforms, are important in embryonic development, and likely are involved in lung morphogenesis. We have recently shown that the inducible loss of Hif-1α in lung epithelium starting at E4.5 leads to death within an hour of parturition, with symptoms similar to neonatal respiratory distress syndrome (RDS). In addition to Hif-1α, Hif-2α is also expressed in the developing lung, although the overlapping roles of Hif-1α and Hif-2α in this context are not fully understood. To further investigate the independent role of Hif-2α in lung epithelium and its ability to alter Hif-1α-mediated lung maturation, we generated two additional lung-specific inducible Hif-α knockout models (Hif-2α and Hif-1α+Hif-2α). The intrauterine loss of Hif-2α in the lungs does not lead to decreased viability or observable phenotypic changes in the lung. More interestingly, survivability observed after the loss of both Hif-1α and Hif-2α suggests that the loss of Hif-2α is capable of rescuing the neonatal RDS phenotype seen in Hif-1α-deficient pups. Microarray analyses of lung tissue from these three genotypes identified several factors, such as Scd1, Retlnγ, and Il-1r2, which are differentially regulated by the two HIF-α isoforms. Moreover, network analysis suggests that modulation of hormone-mediated, NF-κB, C/EBPα, and c-MYC signaling are central to HIF-mediated changes in lung development.缺氧（hypoxia）指机体组织供氧不足的状态。在产前发育阶段，胎儿会出现局灶性缺氧事件，这类事件对器官发生与机体存活至关重要。机体对氧供不足的应答主要由缺氧诱导因子（hypoxia-inducible factors, HIFs）调控，该家族转录因子可调节糖酵解、血管生成及红细胞生成相关关键基因的表达。HIF-1α与HIF-2α是该家族的两种关键亚型，在胚胎发育中发挥重要作用，且可能参与肺形态发生过程。本研究团队近期研究表明，自胚胎发育第4.5天（E4.5）起诱导肺上皮细胞中Hif-1α基因敲除，会使幼崽在分娩后1小时内死亡，其症状与新生儿呼吸窘迫综合征（neonatal respiratory distress syndrome, RDS）相似。除Hif-1α外，Hif-2α在发育中的肺组织中也有表达，但二者在此过程中的重叠功能尚未完全阐明。为进一步探究Hif-2α在肺上皮细胞中的独立功能，以及其调控Hif-1α介导的肺成熟过程的能力，本研究构建了另外两种肺组织特异性诱导型Hif-α基因敲除模型：Hif-2α单敲除模型与Hif-1α+Hif-2α双敲除模型。宫内肺组织特异性敲除Hif-2α并不会降低幼崽存活率，也未观察到肺组织出现明显表型改变。更值得关注的是，同时敲除Hif-1α与Hif-2α后幼崽的存活率数据表明，敲除Hif-2α可挽救Hif-1α缺陷幼崽出现的新生儿呼吸窘迫综合征表型。对这三种基因型小鼠的肺组织进行微阵列分析，发现Scd1、Retlnγ与Il-1r2等多个因子的表达受两种HIF-α亚型的差异性调控。此外，网络分析结果显示，激素介导的信号通路、NF-κB、C/EBPα及c-MYC信号通路的调控，是HIF介导的肺发育改变的核心环节。