Hypoxia-inducible transcription factor-1alpha triggers an autocrine survival pathway during embryonic cardiac outflow tract remodeling.

The cardiac outflow tract (OFT) of birds and mammals undergoes complex remodeling in the transition to a dual circulation. We have previously suggested a role of myocardial hypoxia and hypoxia inducible factor (HIF)-1 in the apoptosis-dependent remodeling of the OFT. In the present study, we transduced recombinant adenovirus-mediated HIF-1alpha in embryonic chick OFT myocardium to test its role in OFT remodeling. HIF-1alpha reduced the prevalence of apoptosis in OFT cardiomyocytes at stages 25 and 30, as determined by lysosome accumulation and caspase-3 activity. Associated conotruncal defects included malrotation of the aorta and excessive infundibular myocardium. HIF-1 targets induced in these gain-of-function experiments included vascular endothelial growth factor (VEGF), inducible nitric oxide synthase, and stromal cell-derived factor-1. To test the role of VEGF in this context, an adenovirus expressing secreted Flk1 (VEGF receptor 2) that binds and blocks VEGF signaling was targeted to the OFT myocardium. This caused increased cell death in the OFT myocardium at stages 25 and 30. Associated conotruncal heart defects included malrotation of the aorta, defects in the subpulmonic infundibulum associated with a small right ventricle, and increased OFT mesenchyme with failure of semilunar valve formation. We conclude that hypoxia signaling through HIF-1 and VEGF provides an autocrine survival signal in the developing cardiac OFT and that perturbation in this pathway causes OFT defects that model congenital human conotruncal heart defects.