Date Published: April 29, 2019
Publisher: Public Library of Science
Author(s): Rachel J. Hampton-Smith, Briony A. Davenport, Yagnesh Nagarajan, Daniel J. Peet, Sonia Rocha.
The asparaginyl hydroxylase, Factor Inhibiting HIF (FIH), is a cellular dioxygenase. Originally identified as oxygen sensor in the cellular response to hypoxia, where FIH acts as a repressor of the hypoxia inducible transcription factor alpha (HIF-α) proteins through asparaginyl hydroxylation, FIH also hydroxylates many proteins that contain ankyrin repeat domains (ARDs). Given FIH’s promiscuity and the unclear functional effects of ARD hydroxylation, the biological relevance of HIF-α and ARD hydroxylation remains uncertain. Here, we have employed evolutionary and enzymatic analyses of FIH, and both HIF-α and ARD-containing substrates, in a broad range of metazoa to better understand their conservation and functional importance. Utilising Tribolium castaneum and Acropora millepora, we provide evidence that FIH from both species are able to hydroxylate HIF-α proteins, supporting conservation of this function beyond vertebrates. We further demonstrate that T. castaneum and A. millepora FIH homologs can also hydroxylate specific ARD proteins. Significantly, FIH is also conserved in several species with inefficiently-targeted or absent HIF, supporting the hypothesis of important HIF-independent functions for FIH. Overall, these data show that while oxygen-dependent HIF-α hydroxylation by FIH is highly conserved in many species, HIF-independent roles for FIH have evolved in others.
In mammals, communication of oxygen availability within cells is achieved in part by enzymes which directly use dioxygen as a co-substrate . One such enzyme is the oxygen and 2-oxoglutarate (2-OG)-dependent dioxygenase, Factor Inhibiting HIF (FIH), an asparaginyl hydroxylase first characterised through its role in modulation of the Hypoxia-Inducible Factor (HIF) transcription factors [2–5].
The work presented here is the first demonstration of catalytically active FIH homologs beyond the Vertebrata. While the FIH enzyme regulating tcHIF-α is part of a system with strong similarity to the human HIF-1α/FIH/PHD axis, the FIH homolog in the coral, A. millepora, did not appear to modify its corresponding amHIF-α CAD homolog, despite effectively hydroxylating mammalian HIF-1α CAD/Notch ARD substrates. This finding, combined with the intriguing identification of FIH+CAD—species is further evidence for the existence of HIF-α-independent roles for FIH. The nature of these roles, specifically whether they include ARD hydroxylation, how and when they coexist with HIF-α CAD regulation, and their relation to the metabolic function of FIH in mammals are important topics for future study.