Molecular Physiology: Hypoxia, HIF-1, VEGF, and Vascularization
Analysis of the human, mouse and rat VEGF genes revealed the presence of a hypoxia response element in the -flanking region of the gene approximately 1 kb upstream of the transcription start site. The following evidence indicated that HIF-1 activated VEGF transcription in hypoxic cells: (1) forced expression of recombinant HIF-la and HIF-lp increased transcription of a reporter gene containing a 47-bp hypoxia-response element from the human VEGF promoter in cells exposed to 20% or 1% 02 in a dose-dependent manner; (2) an HIF-1 binding site was identified in the VEGF hypoxia-response element; (3) a mutation in the hypoxia-response element that eliminated HIF-1 binding also eliminated transcriptional responses to hypoxia and recombinant HIF-1; (4) overexpression of a dominant negative form of HIF-la that could dimerize with HIF-1 P but could not bind to DNA inhibited in a dose-dependent manner the transcriptional response to hypoxia mediated by the VEGF hypoxia-response element; and (5) mouse hepatoma cells that lacked HIF-1 activity, due to a mutation that prevented expression of HIF-1 P protein, showed a markedly reduced induction of VEGF mRNA in response to hypoxia compared with wild-type cells or to mutant cells corrected by transfection of a HIF-1 P expression vector.
To link these molecular observations in tissue culture cells to physiologic responses in vivo, we utilized a fetal sheep model of chronic anemia developed by Davis and Hohimer health and care mall. In this model, near-term fetal sheep are instrumented in utero and subjected to isovolemic hemorrhage on a daily basis for 6 days, which results in 50 to 60% reduction in hematocrit and a threefold to fourfold decrease in arterial oxygen content. These fetuses undergo remarkable physiologic adaptations. After 1 week, biventricular cardiac output is increased by 50%, associated with myocardial hypertrophy as reflected by a 30% increase in the heart:body weight ratio. Myocardial blood flow is increased fivefold in anemic fetuses, with approximately half of the increase due to decreased blood viscosity.