ABSTRACT: Lung development occurs under relative hypoxia and the most important oxygen-sensitive response pathway is driven by Hypoxia Inducible Factors (HIF). HIFs are heterodimeric transcription factors of an oxygen-sensitive subunit, HIF?, and a constitutively expressed subunit, HIF1?. HIF1? and HIF2?, encoded by two separate genes, contribute to the activation of hypoxia inducible genes. A third HIF? gene, HIF3?, is subject to alternative promoter usage and splicing, leading to three major isoforms, HIF3?, NEPAS and IPAS. HIF3? gene products add to the complexity of the hypoxia response as they function as dominant negative inhibitors (IPAS) or weak transcriptional activators (HIF3?/NEPAS). Previously, we and others have shown the importance of the Hif1? and Hif2? factors in lung development, and here we investigated the role of Hif3? during pulmonary development. Therefore, HIF3? was conditionally expressed in airway epithelial cells during gestation and although HIF3? transgenic mice were born alive and appeared normal, their lungs showed clear abnormalities, including a post-pseudoglandular branching defect and a decreased number of alveoli. The HIF3? expressing lungs displayed reduced numbers of Clara cells, alveolar epithelial type I and type II cells. As a result of HIF3? expression, the level of Hif2? was reduced, but that of Hif1? was not affected. Two regulatory genes, Rar?, involved in alveologenesis, and Foxp2, a transcriptional repressor of the Clara cell specific Ccsp gene, were significantly upregulated in the HIF3? expressing lungs. In addition, aberrant basal cells were observed distally as determined by the expression of Sox2 and p63. We show that Hif3? binds a conserved HRE site in the Sox2 promoter and weakly transactivated a reporter construct containing the Sox2 promoter region. Moreover, Hif3? affected the expression of genes not typically involved in the hypoxia response, providing evidence for a novel function of Hif3? beyond the hypoxia response.
