As a rule, animal cells require molecular oxygen to support mitochondrial energy production, and have thus evolved elaborate mechanisms to sense and respond to changes in ambient oxygen concentrations. The HIF prolyl hydroxylases (PHD1–3) are enzymes that use molecular oxygen as a substrate, and thus their activity is dependent on the availability of oxygen,which makes them highly sensitive oxygen sensors. When oxygen is high, these enzymes hydroxylate proline residues on the Hif-1α and Hif-2αproteins, which marks these proteins for degradation and thus keeps their activity low. When oxygen becomes limiting, hydroxylation is inhibited, which leads to Hif protein stabilization and the subsequent activation of the HIF-1 transcription factor. HIF-1 activates the transcription of a number of genes that collectively initiate a cellular response to hypoxia. Julián Aragonés and colleagues investigated the role of Phd1 in regulating metabolism and the cellular response to hypoxia in their 2008 Nature Genetics paper.
To investigate the function of Phd1, the group created a Phd1knockout (Phd1–/–) line of mice. They then characterized a battery of physiological, metabolic and biochemical parameters in skeletal muscle tissue and isolated myofibers from these mice including oxygen consumption, glucose utilization, levels of oxidative stress and hypoxia tolerance. The team hoped to find a link between oxygen sensing and the changes in metabolism that are associated with survival of hypoxia.
They found that loss of Phd1 function caused a reduction in oxygen consumption in cells exposed to normal levels of oxygen. This decreased oxygen consumption was associated with decreased oxidation of glucose, while oxidation of lipids was not affected. In addition, anaerobic utilization of glucose increased in Phd1–/– mice, indicating a shift towards increased anaerobic capacity even under aerobic conditions. Taken together, these findings suggest a reorganization of basal metabolism in mice lacking the Phd1 gene.
The group also discovered that isolated myofibers from Phd1–/– mice were protected from damage normally associated with a lack of blood flow (ischemia) and exhibited an increased tolerance of hypoxia. These traits are probably due to changes in basal metabolism that are mediated by activation of two potent transcription factors Pparα and Hif-2α due to the loss of Phd1. Increased tolerance of hypoxia in Phd1–/– mice was associated with decreased oxygen consumption, reduced oxidative stress and reduced mitochondrial damage compared with hypoxic wild-type myofibers. Importantly, the muscle fibres of mice lacking the Phd1 gene were able to continue producing ATP at low oxygen levels when ATP production failed in normal mice. Interestingly, Phd1-deficient mice showed a reduced exercise endurance compared with wild-type mice when forced to run uphill on a treadmill, indicating that increased hypoxia tolerance may come at the cost of decreased exercise performance.
Loss of Phd1 activity appears to pre-adapt myofibers for increased tolerance of hypoxia. These findings are the first report of a mechanistic link between cellular oxygen sensing and the rate of oxygen consumption in cells. This reprogramming of metabolism and the associated increase in hypoxia tolerance may help to explain changes in metabolism associated with metabolic dormancy in a variety of animals, and the baseline differences in the metabolism of hypoxia-tolerant and -intolerant species. In addition, this study establishes the possibility that inhibition of Phd1 may be one avenue for reducing damage during ischemic events in mammalian tissues.
