Selective Inhibition of Hypoxia-Inducible Factor (HIF) Prolyl-Hydroxylase 1 Mediates Neuroprotection against Normoxic Oxidative Death via HIF- and CREB-Independent Pathways

Abstract

Oxidative stress contributes to tissue injury in conditions ranging from cardiovascular disease to stroke, spinal cord injury, neurodegeneration, and perhaps even aging. Yet the efficacy of antioxidants in human disease has been mixed at best. We need a better understanding of the mechanisms by which established antioxidants combat oxidative stress. Iron chelators are well established inhibitors of oxidative death in both neural and non-neural tissues, but their precise mechanism of action remains elusive. The prevailing but not completely substantiated view is that iron chelators prevent oxidative injury by suppressing Fenton chemistry and the formation of highly reactive hydroxyl radicals. Here, we show that iron chelation protects, rather unexpectedly, by inhibiting the hypoxia-inducible factor prolyl 4-hydroxylase isoform 1 (PHD1), an iron and 2-oxoglutarate-dependent dioxygenase. PHD1 and its isoforms 2 and 3 are best known for stabilizing transcriptional regulators involved in hypoxic adaptation, such as HIF-1␣ and cAMP response element-binding protein (CREB). Yet we find that global hypoxia-inducible factor (HIF)-PHD inhibition protects neurons even when HIF-1␣ and CREB are directly suppressed. Moreover, two global HIF-PHD inhibitors continued to be neuroprotective even in the presence of diminished HIF-2␣ levels, which itself increases neuronal susceptibility to oxidative stress. Finally, RNA interference to PHD1 but not isoforms PHD2 or PHD3 prevents oxidative death, independent of HIF activation. Together, these studies suggest that iron chelators can prevent normoxic oxidative neuronal death through selective inhibition of PHD1 but independent of HIF-1␣ and CREB; and that HIF-2␣, not HIF-1␣, regulates susceptibility to normoxic oxidative neuronal death.

Prophylactic neuroprotection against stroke: Low-dose, prolonged treatment with deferoxamine or deferasirox establishes prolonged neuroprotection independent of HIF-1 function

“…Instead, siRNA directed against prolyl 4-hydroxylase isoform 1 diminished neuronal death induced by oxidative stress through HIF-independent mechanisms (Siddiq et al, 2009). In addition, structurally diverse PHIs (including DFO) protect neurons in an in vitro model of Huntington's disease through a mechanism independent of HIF-1 function (Niatsetskaya et al, 2010).…”

Hypoxia-Inducible Factor Prolyl Hydroxylase Inhibition: Robust New Target or Another Big Bust for Stroke Therapeutics?

“…Our laboratory used a cell permeant peptide inhibitor of all three HIF PHD isoforms but not the collagen PHDs to prevent oxidative death of cortical neurons (Siddiq et al, 2005). RNA interference showed that deletion of HIF PHD1 but not deletion of HIF PHD2 or HIF PHD3 can provide protection against oxidative death of neurons (Siddiq et al, 2009). Conditional deletion of all HIF PHDs has been shown to affect cell viability in response to developmental stimuli in sympathetic neurons (HIF PHD3; Bishop et al, 2008), ischemia in muscle (HIF PHD1; Aragones et al, 2008), and ischemia in the heart (HIF PHD2; Holscher et al, 2011), but these studies while promising, seem to have limited import for posttreatment protection.…”

Antihelminthic Benzimidazoles Are Novel HIF Activators That Prevent Oxidative Neuronal DeathviaBinding to Tubulin

“…Increases in HIF-1a transcription could overwhelm the ability of neurons to degrade HIF-1a and lead to its stabilization. In this scheme, HIF-1a induction may be a biomarker of other genes that are known to abrogate oxidative stress, such as MnSOD (9), rather than being responsible for protection itself (49). Of course, it is also possible that benzimidazoles act post-transcriptionally rather than transcriptionally to enhance the interaction of HIF-1a (and other proteins) with Septins, a group of microtubule polymerizing guanine nucleotide-binding proteins which lead to HIF-1a stabilization in tumors (2).…”