Impact of cyclic hypoxia on HIF-1 alpha regulation in endothelial cells - new insights for anti-tumor treatments

Heterogeneities in tumor blood flow are associated with cyclic changes in pO(2) or cyclic hypoxia. A major difference from O-2 diffusion-limited or chronic hypoxia is that the tumor vasculature itself may be directly influenced by the fluctuating hypoxic environment, and the reoxygenation phases complicate the usual hypoxia-induced phenotypic pattern. Here, we determined the cyclic hypoxia-driven pathways that modulate hypoxia inducible factor (HIF)-1 alpha abundance in endothelial cells to identify possible therapeutic targets. We found that exposure of endothelial cells to cycles of hypoxia/reoxygenation led to accumulation of HIF-1 alpha during the hypoxic periods and the phosphorylation of protein kinase B (Akt), extracellular regulated kinase (ERK) and endothelial nitric oxide synthase (eNOS) during the reoxygenation phases. We identified stimulation of mitochondrial respiration and activation of the phosphoinositide-3 kinase (PI3K)/Akt pathway during intervening reoxygenation periods as major triggers of the stabilization of HIF-1 alpha. We also found that the NOS inhibitor nitro-l-arginine methyl ester further stimulated the cyclic hypoxia-driven HIF-1 alpha accumulation and the associated gain in endothelial cell survival, thereby mirroring the effects of a PI3K/Akt inhibitor. However, combination of both drugs resulted in a net reduction in HIF-1 alpha and a dramatic in decrease in endothelial cell survival. In conclusion, this study identified cyclic hypoxia, as reported in many tumor types, as a unique biological challenge for endothelial cells that promotes their survival in a HIF-1 alpha-dependent manner through phenotypic alterations occurring during the reoxygenation periods. These observations also indicate the potential of combining Akt-targeting drugs with anti-angiogenic drugs, in particular those interfering with the NO pathway.