The hypoxia-inducible-factor hydroxylases bring fresh air into hypoxia signalling

Abstract

Metazoans rapidly respond to changes in oxygen availability by regulating gene expression. The transcription factor hypoxia-inducible-factor (HIF), which controls the expression of several genes, 'senses' the oxygen concentration indirectly through the hydroxylation of two proline residues that earmarks the HIF-alpha subunits for proteasomal degradation. We review the expression, regulation and function of the HIF prolyl hydroxylases or prolyl hydroxylases domain proteins, which are genuine oxygen sensors.

Figure 1

Prolyl-hydroxylase-domain proteins regulate hypoxia inducible factor-α in response to O₂ availability. Under aerobic conditions (blue arrows), prolyl-hydroxylase-domain proteins (PHDs) hydroxylate hypoxia inducible factor-α (HIF-α), which allows the von Hippel–Lindau protein (pVHL) to bind and thus target HIF-α to the proteasome. Likewise, by binding to PHD2 and PHD3, OS9 promotes HIF-α hydroxylation. A decrease in O₂ availability (red arrows) inhibits the PHDs; HIF-α accumulates and induces the expression of target genes. In addition, Siah 1 and 2 trigger PHD1 and PHD3 degradation under hypoxic conditions. Interestingly, hypoxia controls PHD2, PHD3, OS9 and Siah 1 and 2 expression by a feedback loop mechanism. Immunofluorescence inserts show expression of HIF-1α in HeLa cells at 20% O₂ (left) and 1–2% O₂ (right). HRE, hypoxia response element.

Figure 2

Catalytic function of the prolyl-hydroxylase-domain proteins. These enzymes need O₂ and 2-oxoglutarate as co-substrates, and Fe²⁺ and ascorbate as co-factors. The hydroxylation reaction forms hydroxyproline and succinate. PHDs, prolyl-hydroxylase-domain proteins.

Figure 3

Schematic model of prolyl-hydroxylase domain regulation and function. Several physiological stimuli, in addition to O₂ availability, regulate prolyl hydroxylase domain (PHD) activity, which promotes fine-tuning adaptation to the microenvironment. PHDs, by modulating hypoxia-inducible factor-α (HIF-α) stability and thus HIF activity, are pivotal in O₂ homeostasis. Nevertheless, PHDs might have a role in other physiological and pathological processes. We still do not know whether PHDs drive these functions through a HIF-dependent mechanism or through hydroxylation of new PHD substrates. HRE, hypoxia response element; ROS, reactive oxygen species.