Sci. Aging Knowl. Environ., 29 September 2004
Off With Your (Fork)Head
One stress-defusing pathway shuts down another
To win a crucial game, a coach might decide to keep some good players on the bench. Similarly, cells sideline a key protective pathway to take on DNA-breaking stress, according to new research. The finding could help illuminate how cells decide when they can recover from damage and when their prospects are hopeless.
The p53 protein helps cells cope with harsh conditions. Some of the FOXO proteins, which belong to the forkhead molecular family, perform similar functions. Both kinds of stress fighters can induce suicide or stop division and allow the cell to mend broken DNA. The worm and fly versions of FOXO also stretch longevity. Although FOXO and p53 are vital when cells are suffering, nobody had checked whether they interact, says molecular biologist Tak Mak of the University of Toronto in Canada. He and colleagues decided to gauge p53's effects on a mammalian version of FOXO called FKHRL1.
First, the researchers zapped mouse connective tissue cells with radiation or DNA-wounding chemicals. Then the team measured how many FKHRL1 molecules carried phosphate tags, a sign of inactivation. In control cells, which hold two copies of the p53 gene, the assault boosted the amount of phosphate-toting protein. But in cells that lack the gene for p53, the treatment barely increased the value, suggesting that FKHRL1 remains active. Phosphates neutralize FKHRL1 by encouraging it to exit the nucleus. The researchers mapped the location of FKHRL1 proteins in cells with normal p53 and in two cell lines carrying faulty p53 variants. Only in cells with intact p53 did radiation or harsh chemicals increase the quantity of FKHRL1 loitering outside the nucleus. Together, these results suggest that p53 shuts off FKHRL1 and exiles it from the vicinity of the DNA.
To find out how, the researchers measured amounts of activated SGK1, a protein that slaps a phosphate onto FKHRL1 (see "Fork in the Road"). DNA-damaging treatments hiked SGK1 quantities in cells with normal p53 but not in cells lacking the molecule. To confirm that SGK1 was p53's proxy, the team added to the p53-making cells snippets of RNA that diminish SGK1 output. The amount of phosphate-tagged FKHRL1 plunged. "We've known that p53 and FOXO are involved in aging, cell death, and cancer, but we've never known that they 'talk' to each other," says Mak.
Communication between these protective systems makes sense, says molecular biologist Toren Finkel of the National Heart, Lung, and Blood Institute in Bethesda, Maryland. The cell has to integrate its responses to stress. But no one knows how shutting off one apparently beneficial pathway helps the organism, he says. Previous work indicated that p53 consorts with mouse and human versions of Sir2, a protein that boosts longevity in worms and yeast (see "Death and Aging, Together at Last"). Sir2 indirectly spurs a worm version of FKHRL1 called DAF-16, and other recent evidence implies that the human form of Sir2 controls another FOXO protein, suggesting "a complicated regulatory network ... where each protein can regulate the activity of the other two proteins," says Finkel. Teasing out the interactions among pathways might help researchers better understand the cell's game plan against stress.
September 29, 2004
Science of Aging Knowledge Environment. ISSN 1539-6150