Nitric oxide inhibits HIF-1alpha protein accumulation under hypoxic conditions: implication of 2-oxoglutarate and iron

Cells exposed to low oxygen conditions respond by initiating defense mechanisms, including the stabilization of hypoxia-inducible factor (HIF) 1alpha, a transcription factor that upregulates genes such as those involved in angiogenesis and glycolysis, which also plays a pivotal role in the regulation of cellular utilization of oxygen and is an essential regulator of angiogenesis in solid tumor and ischemic disorders. Nitric oxide and other inhibitors of mitochondrial respiration prevent the stabilization of HIF-1alpha during hypoxia. In the present study we found that nitric oxide inhibits HIF-1alpha accumulation under low oxygen (1%) conditions. The effect is supported by an increase in 3-nitrotyrosine and is more likely caused by the formation of peroxynitrite in the cells, which leads to the damage of mitochondria and their respiratory chain followed by the increase in 2-oxoglutarate (2-OG) and iron (the components needed to activate HIF-1alpha proline hydroxylases) concentrations in cell cytosol. The inhibiting effect of NO on HIF-1alpha accumulation was not observed in the cells lacking mitochondria. On the other hand the depletion of intracellular glutathione (GSH) was observed upon cell treatment with nitric oxide donors under hypoxic conditions. Treatment of those cells with N-acetyl cysteine (NAC) increased the amount of intracellular GSH and attenuated the NO effect and abolished the damage of mitochondria as well as 2-OG/iron release.