Stabilization of hypoxia-inducible factor-1α by prostacyclin under prolonged hypoxia via reducing reactive oxygen species level in endothelial cells

Hypoxia-inducible factor-1 (HIF-1) takes part in the transcriptional activation of hypoxia-responsive genes. HIF-1 alpha, a subunit of HIF-1, is rapidly degraded under normoxic conditions by the ubiquitin-proteosome system. Hypoxia up-regulates HIF-1 alpha by inhibiting its degradation, thereby allowing it to accumulate to high levels with 3 - 6 h of hypoxia treatment and decreasing thereafter. In vascular tissues, prostacyclin ( prostaglandin I-2 (PGI(2))) is a potent vasodilator and inhibitor of platelet aggregation and is known as a vasoprotective molecule. However, the role of PGI(2) in HIF-1 activation has not been studied. In the present study, we investigated the effect of PGI(2) on HIF-1 regulation in human umbilical vein endothelial cells under prolonged hypoxia ( 12 h). Augmentation of PGI2 via adenovirus-mediated gene transfer of both cyclooxygenase-1 and PGI(2) synthase activated HIF-1 by stabilizing HIF-1 alpha in cells under prolonged hypoxia or the hypoxia-normoxia transition but not under normoxia. Exogenous H2O2 abolished PGI(2)- and catalase-induced HIF-1 alpha up-regulation, which suggests that degradation of HIF-1 alpha under prolonged hypoxia is through a reactive oxygen species-dependent pathway. Moreover, PGI(2) attenuated NADPH oxidase activity by suppressing Rac1 and p47(phox) expression under hypoxia. These data demonstrate a novel function of PGI(2) in down-regulating reactive oxygen species production by attenuating NADPH oxidase activity, which stabilizes HIF-1 alpha in human umbilical vein endothelial cells exposed to prolonged hypoxia.