Effect of chemical stabilizers of hypoxia-inducible factors on early lung development

Low oxygen stimulates pulmonary vascular development and airway branching and involves hypoxia-inducible factor (HIF). HIF is stable and initiates expression of angiogenic factors under hypoxia, whereas normoxia triggers hydroxylation of the HIF- 1 alpha subunit by prolyl hydroxylases (PHDs) and subsequent degradation. Herein, we investigated whether chemical stabilization of HIF- 1 alpha under normoxic (20% O-2) conditions would stimulate vascular growth and branching morphogenesis in early lung explants. Tie2-LacZ ( endothelial LacZ marker) mice were used for visualization of the vasculature. Embryonic day 11.5 (E11.5) lung buds were dissected and cultured in 20% O-2 in the absence or presence of cobalt chloride (CoCl2, a hypoxia mimetic), dimethyloxalylglycine (DMOG; a nonspecific inhibitor of PHDs), or desferrioxamine (DFO; an iron chelator). Vascularization was assessed by X-gal staining, and terminal buds were counted. The fine vascular network surrounding the developing lung buds seen in control explants disappeared in CoCl2- and DFO-treated explants. Also, epithelial branching was reduced in the explants treated with CoCl2 and DFO. In contrast, DMOG inhibited branching but stimulated vascularization. Both DFO and DMOG increased nuclear HIF- 1 alpha protein levels, whereas CoCl2 had no effect. Since HIF- 1 alpha induces VEGF expression, the effect of SU-5416, a potent VEGF receptor (VEGFR) blocker, on early lung development was also investigated. Inhibition of VEGFR2 signaling in explants maintained under hypoxic (2% O2) conditions completely abolished vascularization and slightly decreased epithelial branching. Taken together, the data suggest that DMOG stabilization of HIF- 1 alpha during early development leads to a hypervascular lung and that airway branching proceeds without the vasculature, albeit at a slower rate.