The zinc chelator, N,N,N′,N′-tetrakis (2-pyridylmethyl) ethylenediamine, increases the level of nonfunctional HIF-1α protein in normoxic cells

The hypoxia-inducible factor-1α (HIF-1α) subunit is activated in response to lack of oxygen. HIF-1α-specific prolyl hydroxylase and factor inhibiting HIF-1α (FIH-1) catalyze hydroxylation of the proline and asparagine residues of HIF-1α, respectively. The hydroxyproline then interacts with ubiquitin E3 ligase, the von Hippel-Lindau protein, leading to degradation of HIF-1α by ubiquitin-dependent proteasomes, while the hydroxylation of the asparagine residue prevents recruitment of the coactivator, cAMP-response element-binding protein (CBP), thereby decreasing the transactivation ability of HIF-1α. We found that the Zn-specific chelator, N,N,N′,N′-tetrakis (2-pyridylmethyl) ethylenediamine (TPEN), enhances the activity of HIF-1α-proline hydroxylase 2 but the level of HIF-1α protein does not fall because TPEN also inhibits ubiquitination. Since the Zn chelator does not prevent FIH-1 from hydroxylating the asparagine residue of HIF-1α, its presence leads to the accumulation of HIF-1α that is both prolyl and asparaginyl hydroxylated and is therefore nonfunctional. In hypoxic cells, TPEN also prevents HIF-1α from interacting with CBP, so reducing expression of HIF-1α target genes. As a result, Zn chelation causes the accumulation of nonfunctional HIF-1α protein in both normoxia and hypoxia.