HIF prolyl-hydroxylase 2 is the key oxygen sensor setting low steady-state levels of HIF-1  in normoxia

Abstract

² Deceased during the course of this workHypoxia-inducible factor (HIF), a transcriptional complex conserved from Caenorhabditis elegans to vertebrates, plays a pivotal role in cellular adaptation to low oxygen availability. In normoxia, the HIF-a subunits are targeted for destruction by prolyl hydroxylation, a speci®c modi®cation that provides recognition for the E3 ubiquitin ligase complex containing the von Hippel±Lindau tumour suppressor protein (pVHL). Three HIF prolyl-hydroxylases (PHD1, 2 and 3) were identi®ed recently in mammals and shown to hydroxylate HIF-a subunits. Here we show that speci®c`silencing' of PHD2 with short interfering RNAs is suf®cient to stabilize and activate HIF-1a in normoxia in all the human cells investigated.`Silencing' of PHD1 and PHD3 has no effect on the stability of HIF-1a either in normoxia or upon re-oxygenation of cells brie¯y exposed to hypoxia. We therefore conclude that, in vivo, PHDs have distinct assigned functions, PHD2 being the critical oxygen sensor setting the low steady-state levels of HIF-1a in normoxia. Interestingly, PHD2 is upregulated by hypoxia, providing an HIF-1-dependent auto-regulatory mechanism driven by the oxygen tension. Keywords: angiogenesis/HIF prolyl-hydroxylases/ hypoxia signalling/oxygen sensor/small interfering RNA IntroductionAll organisms possess mechanisms to maintain oxygen homeostasis, which are essential for survival. The hypoxia-inducible factor-1 (HIF-1), conserved during evolution from worms to¯ies to vertebrates, is central to adaptation to low oxygen availability. HIF-1 in turn regulates transcription of many genes involved in cellular and systemic responses to hypoxia, including breathing, vasodilation, anaerobic metabolism, erythropoiesis and angiogenesis. Therefore, hif represents a`master' gene in oxygen homeostasis during embryonic development and postnatal life in both physiological and pathophysiological processes such as tumour growth and metastasis (for a review, see Semenza, 1998).HIF-1 is a heterodimer consisting of one of three a-subunits (HIF-1a, HIF-2a or HIF-3a) and the b-subunit (HIF-1b, also called aryl hydrocarbon nuclear translocator, or ARNT) (Wang et al., 1995;Ema et al., 1997;Tian et al., 1997;Gu et al., 1998). HIF-1b is a constitutive nuclear protein, which also participates in the cellular response to environmental toxins such as aryl hydrocarbons, whereas HIF-a is speci®c to the response to hypoxia (Hoffman et al., 1991). Although oxygen availability regulates multiple steps on HIF-1 transcriptional activation, the dominant control mechanism occurs through oxygen-dependent proteolysis of HIF-a (Huang et al., 1996). The most extensively studied isoform of the a-subunits is the ubiquitous HIF-1a.In normoxia, HIF-1a is constitutively synthesized and sent to destruction by the ubiquitin±proteasome pathway (half-life <5 min) (Salceda and Caro, 1997;Huang et al., 1998;Kallio et al., 1999). This process is mediated by the speci®c binding of pVHL, the product of the von Hippel± Lindau tumour suppressor gene, which...

Understanding complexity in the HIF signaling pathway using systems biology and mathematical modeling

“…This observation is further supported by doseresponse simulations showing that the steady-state level of HIF-1 exponentially depends on the PHD2 mediated feedback strength while it is relatively insensitive to change in feedback strength mediated by PHD3 ( Figure 2F). These simulations are in agreement with the conclusions in previous reports suggesting PHD2 to be the more dominant isoform in driving HIF-1 transient response [34,35]. Further questions could be asked.…”

A positive feedback loop promotes HIF‐1α stability through miR‐210‐mediated suppression of RUNX3 in paraquat‐induced EMT

“…The PHDs family include PHD1, PHD2 and PHD3. PHD2 is believed to be a key oxygen sensor that hydroxylates HIF‐1α and promotes its degradation 36, 37. OH‐HIF‐1α levels were markedly decreased in PQ group.…”

Affinity-Based Fluorescence Polarization Assay for High-Throughput Screening of Prolyl Hydroxylase 2 Inhibitors

“…7 It is currently believed that PHD2 plays a dominant role in controlling the cellular HIFα levels. 8 Inhibitor of PHD2 has been pursued as a promising therapy for conditions including anemia and ischemic disease.To discover small molecules that can regulate PHD2 activity, many activity-based assays have been developed. The development of activity-based assay was based on the catalytic activity of PHD2, which utilizes 2-OG and oxygen as cosubstrates to catalyze the prolyl hydroxylations.…”