Ginsenoside-Rg1 mediates a hypoxia-independent upregulation of hypoxia-inducible factor-1α to promote angiogenesis

Hypoxia-inducible factor (HIF-1) is the key transcription regulator for multiple angiogenic factors and is an appealing target. Ginsenoside-Rg1, a nontoxic saponin isolated from the rhizome of Panax ginseng, exhibits potent proangiogenic activity and has the potential to be developed as a new angiotherapeutic agent. However, the mechanisms by which Rg1 promotes angiogenesis are not fully understood. Here, we show that Rg1 is an effective stimulator of HIF-1 alpha under normal cellular oxygen conditions in human umbilical vein endothelial cells. HIF-1 alpha steady-state mRNA was not affected by Rg1. Rather, HIF-1 alpha protein synthesis was stimulated by Rg1. This effect was associated with constitutive activation of phosphatidylinositol 3-kinase (PI3K)/Akt and its effector p70 S6 kinase (p70(S6K)), but not extracellular-signal regulated kinase 1/2. We further revealed that HIF-1 alpha induction triggered the expression of target genes, including vascular endothelial growth factor (VEGF). The use of small molecule inhibitors LY294002 or rapamycin to inhibit PI3K/Akt and p70(S6K) activities, respectively, resulted in diminished HIF-1 alpha activation and subsequent VEGF expression. RNA interference-mediated knockdown of HIF-1 alpha suppressed Rg1-induced VEGF synthesis and angiogenic tube formation, confirming that the effect was HIF-1 alpha specific. Similarly, the angiogenic phenotype could be reversed by inhibition of PI3K/Akt and p70(S6K). These results define a hypoxia-independent activation of HIF-1 alpha, uncovering a novel mechanism for Rg1 that could play a major role in angiogenesis and vascular remodeling.