Sci. Aging Knowl. Environ., 3 August 2005
Detour to Death
Protein kills cells by diverting electrons and crafting free radicals
R. John Davenporthttp://sageke.sciencemag.org/cgi/content/full/2005/31/nf62
Construction signs steer traffic around trouble, but new work reveals a diversion that routes cells into danger. A protein named p66Shc funnels electrons down an unusual path and uses them to make cell-killing compounds instead of energy. The results might help researchers understand why mice without the protein survive unusually well.
Mice lacking p66Shc live 30% longer than normal and fend off age-related diseases, and their cells resist stress (see "Stay Mellow, Stay Young" and "Acing the Stress Test"). p66Shc induces cells to commit suicide in response to various kinds of insults, apparently by increasing quantities of reactive oxygen species (ROS). Some researchers have suggested that p66Shc squelches proteins that disarm ROS (see Martin and Friedman Perspective), but how the protein operates remains unclear. Mitochondria, the cell's powerhouses, make most of a cell's ROS, and p66Shc lurks there (see "Lethal Leak"). So geneticist Marco Giorgio of the Experimental Oncology Department at the European Institute of Oncology in Milan, Italy, and colleagues probed how the protein might control oxidant amounts in mitochondria.
The team treated normal or p66Shc-less mice with carbon tetrachloride, a chemical that prompts cell suicide, and then isolated mitochondria from the rodents' livers. Mitochondria from genetically intact mice held larger amounts of ROS than did those from p66Shc-deficient animals. But the two types of animals displayed similar amounts of ROS-quenching molecules, and such scavengers performed equally well, suggesting that p66Shc prompts the production of ROS rather than preventing their destruction.
Further experiments suggested that p66Shc fuels production of a certain ROS, hydrogen peroxide, and that its manufacture requires an intact respiratory chain. This set of mitochondrial proteins passes electrons down the line, eventually turning oxygen into water and generating energy. Because electrons also help craft hydrogen peroxide, the researchers wondered whether p66Shc diverts electrons from this path. First, they measured the current through an electrode coated with p66Shc and through an uncoated electrode. They discovered that p66Shc can transfer electrons. Next, the team coated an electrode with p66Shc and cytochrome c, an electron-moving component of the respiratory chain. Measurements revealed that electrons pass between the two proteins.
Additional test tube experiments showed that combining cytochrome c and p66Shc produces hydrogen peroxide. And in mitochondria, shutting down all parts of the respiratory chain other than cytochrome c allows p66Shc to prompt peroxide production, supporting the idea that p66Shc robs cytochrome c of electrons and uses them to craft the oxidant. Next, the team homed in on the portion of p66Shc that gives and receives electrons; altering that section obliterates p66Shc's capacity to spur peroxide generation and prompt cell death.
"It's an incredibly creative paper," says oxidative stress researcher Jeffrey Friedman of the Scripps Research Institute in La Jolla, California. "This takes us in a direction that is largely unexpected. [The results suggest that] p66Shc is ... a direct contributor to ROS production." Next, the researchers want to investigate how the version of the protein that can't transfer electrons alters life span, ROS production, and stress resistance of mice, says Giorgio. Such efforts might reveal how to keep aging animals on the right path.
August 3, 2005
Suggested by Arlan Richardson.
Science of Aging Knowledge Environment. ISSN 1539-6150