Cobalt-induced oxidant stress in cultured endothelial cells: prevention by ascorbate in relation to HIF-1alpha

Endothelial cells respond to hypoxia by decreased degradation of hypoxia-inducible factor 1alpha (HIF-1alpha), accumulation of which leads to increased transcription of numerous proteins involved in cell growth and survival. Ascorbic acid prevents HIF-1alpha stabilization in many cell types, but the physiologic relevance of such effects is uncertain. Given their relevance for angiogenesis, endothelial cells in culture were used to evaluate the effects of ascorbate on HIF-1alpha expression induced by hypoxia and the hypoxia mimic cobalt. Although EA.hy926 cells in culture under oxygenated conditions did not contain ascorbate, HIF-1alpha expression was very low, showing that the vitamin is not necessary to suppress HIF-1alpha. On the other hand, hypoxia- or cobalt-induced HIF-1alpha expression/stabilization was almost completely suppressed by what are likely physiologic intracellular ascorbate concentrations. Increased HIF-1alpha expression was not associated with significant changes in expression of the SVCT2, the major transporter for ascorbate in these cells. Cobalt at concentrations sufficient to stabilize HIF-1alpha both oxidized intracellular ascorbate and induced an oxidant stress in the cells that was prevented by ascorbate. Whereas the interaction of ascorbate and cobalt is complex, the presence of physiologic low millimolar concentrations of ascorbate in endothelial cells effectively decreases HIF-1alpha expression and protects against cobalt-induced oxidant stress.