Protein Hydroxylation by Hypoxia-Inducible Factor (HIF) Hydroxylases: Unique or Ubiquitous?

Abstract

All metazoans that utilize molecular oxygen (O2) for metabolic purposes have the capacity to adapt to hypoxia, the condition that arises when O2 demand exceeds supply. This is mediated through activation of the hypoxia-inducible factor (HIF) pathway. At physiological oxygen levels (normoxia), HIF-prolyl hydroxylases (PHDs) hydroxylate proline residues on HIF-α subunits leading to their destabilization by promoting ubiquitination by the von-Hippel Lindau (VHL) ubiquitin ligase and subsequent proteasomal degradation. HIF-α transactivation is also repressed in an O2-dependent way due to asparaginyl hydroxylation by the factor-inhibiting HIF (FIH). In hypoxia, the O2-dependent hydroxylation of HIF-α subunits by PHDs and FIH is reduced, resulting in HIF-α accumulation, dimerization with HIF-β and migration into the nucleus to induce an adaptive transcriptional response. Although HIFs are the canonical substrates for PHD- and FIH-mediated protein hydroxylation, increasing evidence indicates that these hydroxylases may also have alternative targets. In addition to PHD-conferred alterations in protein stability, there is now evidence that hydroxylation can affect protein activity and protein/protein interactions for alternative substrates. PHDs can be pharmacologically inhibited by a new class of drugs termed prolyl hydroxylase inhibitors which have recently been approved for the treatment of anemia associated with chronic kidney disease. The identification of alternative targets of HIF hydroxylases is important in order to fully elucidate the pharmacology of hydroxylase inhibitors (PHI). Despite significant technical advances, screening, detection and verification of alternative functional targets for PHDs and FIH remain challenging. In this review, we discuss recently proposed non-HIF targets for PHDs and FIH and provide an overview of the techniques used to identify these.

Prolyl hydroxylase domain 2 reduction enhances skeletal muscle tissue regeneration after soft tissue trauma in mice

“…In contrast to established mechanisms of non-HIF hydroxylation targets of FIH, like Ovarian tumor domain containing ubiquitin aldehyde binding protein 1 (OTUB1), non-HIF hydroxylation targets of PHDs are less well characterised. It is known that molecules of the NF-κB pathway and zinc fingers and homeoboxes 2 (ZHX2) are hydroxylated by PHDs but the exact mechanisms and especially a role in muscle regeneration are yet unknown [40]. Regarding the NF-κB pathway, we and others have shown that inhibition of PHDs (although this is mostly directed via PHD1 in the so far analysed settings) induces an activation of the classical NFκB pathway [41].…”

Inhibition of prolyl hydroxylases increases hepatic insulin and decreases glucagon sensitivity by an HIF-2α-dependent mechanism

“…Indeed, PHDs are multifunctional proteins. Besides HIF-αs, PHDs interact with some proteins, including GCN5, IKKβ, MAPK6, p53, and FOXO3a, and regulate their functions by hydroxylase activity-dependent and independent mechanisms [ 35 , 36 ]. For example, PHD3 suppresses IKKβ/NF-kB signaling in a mechanism not requiring its hydroxylase activity [ 36 ], and PHD-dependent regulation of HIF-αs is dependent of hydroxylase activity.…”

Lysophosphatidic Acid and Hematopoiesis: From Microenvironmental Effects to Intracellular Signaling

“…Inhibitors of the hypoxia-inducible factor (HIF) prolyl hydroxylase (PH) enzyme are a new class of agents for the treatment of anemia [142]. HIF is hydroxylated by HIF-PH to cause its degradation [143]. On the other hand, HIF is reported to play important roles in cell-cycle regulation of HSC and production of EPO.…”