Soluble nickel inhibits HIF‐prolyl‐hydroxylases creating persistent hypoxic signaling in A549 cells

Abstract

Soluble nickel compounds are carcinogenic to humans although the mechanism by which they cause cancer remains unclear. One major consequence of exposure to nickel is the stabilization of hypoxia inducible factor-1alpha (HIF-1alpha), a protein known to be overexpressed in a variety of cancers. In this study, we report a persistent stabilization of HIF-1alpha by nickel chloride up to 72 h after the removal of nickel from the culture media. In addition, we show that the HIF-prolyl hydroxylases (PHD's) are inhibited when cells are exposed to nickel and that they remain repressed for up to 72 h after nickel is removed. We then show that nickel can inhibit purified HIF-PHD's 2 in vitro, through direct interference with the enzyme. Through theoretical calculations, we also demonstrate that nickel may be able to replace the iron in the active site of this enzyme, providing a plausible mechanism for the persistent inhibition of HIF-PHD's by nickel. The data presented suggest that nickel can interfere with HIF-PHD directly and does not inhibit the enzyme by simply depleting cellular factors, such as iron or ascorbic acid. Understanding the mechanisms by which nickel can inhibit HIF-PHD's and stabilize HIF-1alpha may be important in the treatment of cancer and ischemic diseases.

Nickel Ions Inhibit Histone Demethylase JMJD1A and DNA Repair Enzyme ABH2 by Replacing the Ferrous Iron in the Catalytic Centers

“…In this study, we demonstrate that two distinct members of this enzyme family, histone demethylase JMJD1A and DNA repair enzyme ABH2, are sensitive to nickel ion inhibition. The IC 50 (38). Presumably, this is because all three of these dioxygenases use the same structural motif (His-Asp-His) to bind ferrous iron at their active sites, and Ni(II) competes with Fe(II) and replaces it at the iron-binding site.…”

“…Previously, we calculated the binding constant of various divalent metals to two imidazole ligands that were used to model the two histidines in the His-Asp-His triad (38). The A, assay of demethylase activity in the nuclear extracts of nickel ion-exposed cells.…”

“…In vitro histone demethylase assay was performed as in B. The results of one typical experiment from at least two independent experiments are shown in A-C. logarithm of binding strength of Ni(II) coordination to two imidazole molecules is approximately 3 orders of magnitude greater than that for Fe(II) to bind to the same two molecules (38). However, in this study, the results of ITC analysis show that the binding of nickel to ABH2 is only about three times stronger than iron.…”

“…We have previously reported that HIF prolyl hydroxylase PHD2 was 20 times more sensitive to nickel inhibition when compared with aconitase that binds iron in the form of [4Fe-4S] cluster (38,39). An interesting question is whether these newly identified histone demethylases and the other members in this Fe(II)-and 2-oxoglutarate-dependent dioxygenase family have similar sensitivity to nickel inhibition as PHD2.…”

High Altitude Renal Syndrome (HARS)

“…35,36 One potential explanation for the association of CMS and mining communities could be exposure to heavy metals that can stimulate erythropoiesis, such as cobalt and nickel. 37,38 Cobalt, and to a lesser extent nickel, increase erythropoietin levels by inhibiting HIF-1␣ prolyl hydroxylase, thereby preventing the ubiquitination and degradation of HIF-1␣ and HIF-2␣. This inhibition results in higher levels of HIF-1␣, potentiating erythropoietin production.…”