June 7, 2005

Studies Reveal How Cells Sense Oxygen

Three studies in the June issue of Cell Metabolism offer additional insight into how the cells of mammals sense oxygen. Oxygen plays a central role in fueling cells, and oxygen deficiency underlies many disease conditions, including heart attack, stroke, inflammation, and cancer.

The findings should help to resolve a long-standing controversy over the identity of mammalian oxygen sensors, according to the researchers.

The studies report genetic evidence that the cellular powerhouses known as mitochondria are required for cells to detect and respond to changes in the availability of oxygen. Byproducts of mitochondrial activity, called reactive oxygen species or free radicals, signal low oxygen conditions, launching a cascade of effects that allow cells to adapt, their research shows.

"Oxygen is so central for life," said M. Celeste Simon of the University of Pennsylvania, an author on two of the three studies. "Oxygen deprivation is behind stroke, heart attack, inflammation, and tumor physiology. Even brief oxygen limitation can render someone unable to function.

"While cellular responses to low oxygen have been studied extensively, the precise identity of mammalian oxygen sensors remains controversial. The current studies address the controversy about the role that mitochondria play in oxygen sensing, which should allow the field to move forward."

Mammals require oxygen to obtain energy and respond to decreases in oxygen supply by activating genes that preserve oxygen in tissues. Earlier findings suggested that reactive oxygen species generated by mitochondria under low oxygen conditions regulate a variety of hypoxic responses, including the activation of hypoxia-inducible factor 1 (HIF-1). HIF-1 is a master regulator of local cellular and systemic responses to oxygen starvation.

In the new studies, the groups used a variety of genetic methods to specifically disable the function of mitochondrial components, thereby inhibiting mitochondrial activity and the production of its free radical byproducts, including hydrogen peroxide. They then examined the consequences of that inactivation for the cell's defenses against low oxygen.

In mouse cells lacking mitochondrial activity, oxygen sensing fails and a component of element of hypoxia response, HIF-1a, is degraded as it is under normal oxygen, Simon's team reported. Treatment with hydrogen peroxide stabilized the defense protein.

Two companion studies in the same journal issue, led by Paul Schumacker at The University of Chicago and Navdeep Chandel at Northwestern University, provide additional support that reactive oxygen species produced by mitochondria under conditions of low oxygen stabilize HIF-1a.

The new findings bolster an earlier report by the studies' senior authors, which implicated mitochondria as potential oxygen sensors. That report had remained controversial because experiments designed to test the idea by limiting mitochondrial function with nonspecific drugs yielded conflicting results.

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