20-HETE can act as a nonhypoxic regulator of HIF-1α in human microvascular endothelial cells

20-HETE increases the expression of VEGF in human dermal microvascular endothelial cells (ECs). Since VEGF is regulated by hypoxia inducible factor (HIF)-1, we studied whether 20-HETE also upregulates HIF-1α using the stable 20-HETE analog 20-hydroxyeicosa-5(Z),14(Z)dienoic acid (WIT003; 1-10 μM) and found that it induced a marked increase in HIF-1α protein levels. The increases in VEGF after the addition of WIT003 preceded the changes in HIF-1α, and the increases in HIF-1α were prevented by a VEGF neutralizing antibody. This suggests that 20-HETE first causes increases in VEGF, which then, in turn, cause the upregulation of HIF-1α. Stimulation with exogenously added VEGF also led to an upregulation of HIF-1α. Incubation with the MEK1/ERK1/2 inhibitor U-0126 (10 μM) completely abolished the increases in VEGF and thus HIF-1α, suggesting the involvement of ERK1/2 activation. The addition of WIT003 resulted in a rapid and sustained increase in superoxide formation. When WIT003 was added in the presence of the nitric oxide (NO) synthase (NOS) inhibitor N-nitro-L-arginine, no changes in superoxide, VEGF, or HIF-1α were observed. This suggests that NOS is responsible for the early changes in superoxide induced by WIT003. Furthermore, WIT003 induced the expression of the NADPH oxidase subunit p47^phox in ECs before the increases in HIF-1α. Incubation with polyethylene glycol-superoxide dismutase (400 U/ml), apocynin (100 μM), diphenylene iodonium (10 μM), or p47^phox downregulation with small interfering (si)RNA all inhibited the increases in HIF-1α expression. This indicates that the early changes in superoxide lead to VEGF increases and thereby NADPH oxidase-dependent superoxide production, which is required for HIF-1α upregulation. We also found that the higher HIF-1α expression induced by WIT003 was accompanied by higher expression of erythropoietin receptor and angiopoietin-2 proteins. These increases were caused by HIF-1α because their levels were markedly decreased by siRNA downregulation of HIF-1α. 20-HETE may be a novel nonhypoxic regulator of HIF-1α and HIF-1α-regulated genes in ECs.