Oxygen-dependent ATF-4 stability is mediated by the PHD3 oxygen sensor

Abstract

IntroductionWhen O 2 delivery is impaired, the resulting hypoxia activates homeostatic mechanisms at the systemic and cellular level. 1 This response involves changes in gene expression mediated by hypoxiainducible factor-1 (HIF-1), the master transcription factor of oxygen-regulated genes. HIF-1 is a heterodimeric protein comprising the oxygen-sensitive ␣-subunit (HIF-1␣, or the more cell-typespecifically expressed HIF-2␣) and the oxygen-insensitive ␤-subunit. 2 Oxygen-regulated gene expression involves binding of HIF to cis-regulatory hypoxia response elements (HREs) of HIF target genes such as erythropoietin or vascular endothelial growth factor. 3 The molecular basis for the hypoxia-induced stability and activity of HIF-1␣ and HIF-2␣ is the O 2 -dependent hydroxylation of distinct prolyl residues. [4][5][6] A family of oxygen-, iron-and 2-oxoglutarate-dependent prolyl-4-hydroxylases has been described recently to hydroxylate the oxygen-labile ␣ subunits of HIF-1 and HIF-2. 5,7,8 This family consists of 3 members called prolyl-4-hydroxylase domain (PHD) 1, PHD2, PHD3, or HIF prolyl hydroxylase (HPH) 3, HPH2, and HPH1, respectively. 4,5 Following prolyl-4-hydroxylation of the critical prolyl residues under normoxic conditions, the ubiquitin ligase von Hippel-Lindau tumor suppressor protein (pVHL) recognizes HIF␣ subunits and targets them for rapid ubiquitination and proteasomal degradation. 9 Binding of pVHL strictly requires prior modification of human HIF-1␣ and HIF-2␣ by prolyl-4-hydroxylation at prolines 402 and 564 or prolines 405 or 531, respectively. 10,11 Limited oxygen supply prevents HIF␣ hydroxylation and degradation. 12 In addition to protein stability, oxygen-dependent C-terminal asparagine hydroxylation of HIF␣ by factor-inhibiting HIF (FIH) prevents transcriptional cofactor recruitment, thereby fine-tuning HIF-1 activity after a further decrease in oxygen availability. 13 Most interestingly, in addition to HIF␣, ankyrin repeats present in IB and NF-B family members have recently been described to be hydroxylated by FIH, demonstrating that hydroxylation is not restricted to the HIF signaling pathway. 14 Besides similarities in the hydroxylation reaction in vitro, the 3 PHDs differ in their ability to hydroxylate HIF-1␣ in vivo and in their organ-specific expression pattern. [15][16][17] Moreover, the phenotypes of PHD knock-out mice demonstrate divergent roles of the 3 PHDs during embryonic development. 18 These data indicate that under physiologic conditions, PHD1, PHD2, and PHD3 mediate different, probably even HIF-independent, oxygen-regulated signal transduction pathways.By searching for novel targets of PHD3 using yeast 2-hybrid technology, we identified activating transcription factor-4 (ATF-4) as a novel interaction partner, and we found that PHD3 confers oxygen-dependent ATF-4 protein stability in a pVHL-independent manner. ATF-4-deficient mice are severely anemic during fetal development, apparently because of an impairment in definitive hematopoiesis. 19 In addition, overexpression of A...

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

“…ATF4 is a leucine zipper transcription factor that has been shown to have prodeath and prosurvival roles in response to oxidative stress or endoplasmic reticulum stress in neurons (Lange et al, 2008). Prolyl hydroxylase-3 appears not to be necessary for oxidative death of cortical neurons, and given the sympathoadrenal dysfunction associated with germline deletion of PHD3, conditional knockouts will need to be used to assess the role of this PHD in ischemic or hemorrhagic stroke (Koditz et al, 2007;Xie et al, 2009) (Figure 2). …”

Onconeuronal cerebellar degeneration-related antigen, Cdr2, is strongly expressed in papillary renal cell carcinoma and leads to attenuated hypoxic response

“…Although direct mass spectroscopic evidence for hydroxylation is missing, PHD1 and PHD3 have recently been shown to regulate IkB kinase-b and activating transcription factor-4, respectively (Cummins et al, 2006;Koditz et al, 2007). Apart from mediating HIF-a protein stability, these data indicate that PHDs might regulate additional oxygen-dependent signaling pathways.…”

Circulating tumour cells demonstrate an altered response to hypoxia and an aggressive phenotype

“…This same anoxia pathway can also be activated by nutritional deficiencies such as glucose and/or amino acid deprivation, and can lead to ERSR and translational increase of ATF4 (Harding et al, 2000). In addition to an increase in mRNA translation of factors related to the unfolded protein response pathway, such as ATF4, cancer cells also respond to anoxia via other multiple HIF1a-independent pathways, including the mitogen-activated protein kinase pathway and protein stabilisation pathways, which result in the induction of factors such as ATF3 (Ameri et al, 2007;Nemetski and Gardner, 2007) and ATF4 (Ameri et al, 2004;Koditz et al, 2007;Rzymski et al, 2008).…”