Reactive oxygen species and cellular oxygen sensing

Abstract

Many organisms activate adaptive transcriptional programs to help them cope with decreased oxygen (O(2)) levels, or hypoxia, in their environment. These responses are triggered by various O(2) sensing systems in bacteria, yeast and metazoans. In metazoans, the hypoxia inducible factors (HIFs) mediate the adaptive transcriptional response to hypoxia by upregulating genes involved in maintaining bioenergetic homeostasis. The HIFs in turn are regulated by HIF-specific prolyl hydroxlase activity, which is sensitive to cellular O(2) levels and other factors such as tricarboxylic acid cycle metabolites and reactive oxygen species (ROS). Establishing a role for ROS in cellular oxygen sensing has been challenging since ROS are intrinsically unstable and difficult to measure. However, recent advances in fluorescence energy transfer resonance (FRET)-based methods for measuring ROS are alleviating some of the previous difficulties associated with dyes and luminescent chemicals. In addition, new genetic models have demonstrated that functional mitochondrial electron transport and associated ROS production during hypoxia are required for HIF stabilization in mammalian cells. Current efforts are directed at determining how ROS mediate prolyl hydroxylase activity and hypoxic HIF stabilization. Progress in understanding this process has been enhanced by the development of the FRET-based ROS probe, an vivo prolyl hydroxylase reporter and various genetic models harboring mutations in components of the mitochondrial electron transport chain.

Figure 1. The hydroxylase reaction

PHD2 utilizes cellular O₂ and 2-oxoglutarate as co-substrates to hydroxylate two proline residues on HIFα. PHD2 also requires ferrous iron for its activity and ascorbate to recycle oxidized iron during uncoupled reactions.

Figure 2. Plausible mechanisms by which ROS regulate PHD2 activity

ROS may trigger a signal transduction cascade which results in post-translational modifications on PHD2, may oxidize bound iron, or may alter PHD2 disulfide bond structure to regulate PHD2 activity.