Flavonoids-induced accumulation of hypoxia-inducible factor (HIF)-1α/2α is mediated through chelation of iron

Hypoxia-inducible factor-1 alpha (HIF-1 alpha) is the regulatory subunit of the heterodimeric transcription factor HIF-1 that is the key regulator of cellular response to low oxygen tension. Under normoxic conditions, HIF-1 alpha is continuously degraded by the ubiquitin-proteasome pathway through pVHL (von Hippel-Lindau tumor suppressor protein). Under hypoxic conditions, HIF-1 alpha is stabilized and induces the transcription of HIF-1 target genes. Quercetin, a flavonoid with anti-oxidant, anti-inflammatory, and kinase modulating properties, has been found to induce HIF-1 alpha accumulation and VEGF secretion in normoxia. In this study, the molecular mechanisms of quercetin-mediated HIF-1 a accumulation were investigated. Previous studies have shown that, in addition to being induced by hypoxia, HIF-1 a can be induced through the phosphatidylinositol 3-kinase (PI3K)/Akt and p53 signaling pathways. But our study revealed, through p53 mutant-type as well as p53 null cell lines, that neither the PI3K/Akt nor the p53 signaling pathway is required for quercetin-induced HIF-1 a accumulation. And we observed that HIF-1 a accumulated by quercetin is not ubiquitinated and the interaction of HIF-1 alpha with pVHL is reduced, compared with HIF-1 a accumulated by the proteasome inhibitor MG132. The use of quercetin's analogues showed that only quercetin and galangin induce HIF-1/2 alpha accumulation and this effect is completely reversed by additional iron ions. This is because quercetin and galangin are able to chelate cellular iron ions that are cofactors of HIF-1/2 alpha. proline hydroxylase (PHD). These data suggest that quercetin inhibits the ubiquitination of HIF-1/2a in normoxia by hindering PHD through chelating iron ions.