With every breath, we inhale the oxygen our cells need to sustain metabolism. But whenever we hold our breath, climb a mountain or sprint on a track, the amount of oxygen getting into our cells can fall below normal. When it does, numerous responses ensure that cells keep respiring so that the intracellular machinery keeps working. Biologists know that the hypoxia inducible factor, HIF, initiates many of these responses, including increasing oxygen-free anaerobic metabolism. How oxygen dependent aerobic metabolism is regulated by HIF during hypoxia is poorly understood, however, so Ryo Fukuda and colleagues from Johns Hopkins University decided to explore this area in cultured mammalian cells.
Cytochrome oxidase (COX) is a multi-subunit enzyme in the electron transport system of mitochondria. During aerobic metabolism, COX catalyzes the reaction with oxygen that helps to generate cellular energy in the form of ATP. Subunit 4 of the enzyme, COX4, contains multiple `varieties', or isoforms, and takes part in regulating mitochondrial respiration. The authors suspected that COX4 might somehow be altered in low oxygen conditions, or hypoxia, to help maintain normal cell function. To test this hypothesis, the authors did a series of experiments measuring the expression and regulation of COX4 isoforms 1 and 2, in both normal and low levels of oxygen. The authors saw a switch in the COX4 isoform used during hypoxia: COX4-2 expression increased during low oxygen, replacing the normally predominant COX4-1 subunit which was simultaneously broken down. The team found that activation of HIF directly increased expression of the gene coding for COX4-2, and also increased expression of a mitochondrial protease called LON, which degraded COX4-1. This showed that aerobic metabolism was being regulated specifically by hypoxia.
But what was the purpose of this COX isoform switching? The authors addressed this question with a series of experiments where they added or removed either isoform to cultured cells, and then measured the cells' rates of oxygen consumption. They found that in hypoxia, cells consumed more oxygen with COX4-2 present than when COX4-1 was present, making sense of why this isoform was induced in hypoxia. At normal oxygen levels cells with COX4-1 or COX4-2 consumed oxygen at similar rates, so why then is COX4-1 ever expressed?
The benefit of COX4-1 at normal oxygen levels instead appeared to be that it reduced the production of reactive oxygen species. These are formed when the mitochondrial electron transport system is inefficient, and they damage cellular proteins, lipids and nucleic acids. The COX4-1 subunit therefore helps protect the cell from damage under normal conditions. Conversely, fewer reactive oxygen species were generated with COX4-2 than with COX4-1 in hypoxia, in addition to COX4-2 increasing the cells' rate of oxygen consumption.
The authors concluded from these studies that the COX4 subunit switch induced by HIF in hypoxia is critical to cell function and survival when oxygen levels change. Different isoforms are probably specialized for different cellular oxygen levels, optimizing the efficient balance between aerobic ATP production and reactive oxygen species generation. By initiating this and many other responses in hypoxia, HIF protects our cells so they can function properly; it's enough to make one gasp!
