Protein identified that serves as a switch in a key pathway of programmed cell death

(Press-News.org) Work led by St. Jude Children's Research Hospital scientists identified how cells flip a switch between cell survival and cell death that involves a protein called FLIP.

The findings solve a riddle that has puzzled scientists for more than a decade regarding the dual nature of caspase-8, an enzyme intimately linked to the cell's suicide pathway but also essential for cell survival during embryonic development and the immune response. Researchers identified FLIP and the silencing of another enzyme, named RIPK3, as playing pivotal roles. The study was published in the March 2 advance online edition of Nature.

Douglas Green, Ph.D., the paper's senior author and chair of the St. Jude Department of Immunology, said work is already underway to use the findings to generate new cancer treatment targets and fresh insight into the missteps that give rise to certain tumors as well as evidence of how some virus-infected cells escape the pathways designed to dispatch such threats.

"It is a pretty rare thing to 'cure' a lethal mutation in an animal by removing another gene. When that happens, the biology shouts out to us that this is important. We just have to listen," Green said.

FLIP's role was identified after investigators bred mice that lacked genes for both caspase-8 and RIPK3. Previous research identified RIPK3 as responsible for orchestrating cell death via programmed necrosis. Once viewed as an uncontrolled form of cell death, programmed necrosis is now recognized as a distinct form of cell suicide. The body relies on both programmed necrosis and apoptosis, the more common process, to rid itself of damaged, dangerous or unneeded cells.

While loss of caspase-8 was known to be lethal during embryonic development, in this study investigators showed mice that lacked both caspase-8 and RIPK3 were born at normal rates and appeared developmentally normal early in life.

Investigators went on to show that caspase-8 prevents programmed necrosis by combining with FLIP to form an enzyme complex that disrupts RIPK3 functioning and so prevents death via programmed necrosis. The work also demonstrated that FLIP expression prevents caspase-8 from triggering cell death via apoptosis, although the exact mechanism must still be determined. Apoptosis relies on caspase enzymes and other molecules to ensure the cell self destructs.

Green said the findings provide insight into the mechanisms at work in neuroblastoma and other tumors that suffer a loss of caspase-8. "We are beginning collaborative experiments to examine these tumors to see if RIPK3 is deleted or blocked," he said. Neuroblastoma arises in cells of the sympathetic nervous system. It is the most common solid tumor in children, accounting for up to 10 percent of all childhood cancers.

### Andrew Oberst, a St. Jude postdoctoral fellow, is the study's first author. The other authors are Christopher Dillon, Ricardo Weinlich, Laura McCormick and Patrick Fitzgerald, all of St. Jude; Cristina Pop and Guy Salvesen, of Sanford-Burnham Medical Research Institute, La Jolla; and Razq Hakem, of the University of Toronto.

The research was supported in part by the National Institutes of Health, the Canadian Institutes of Health Research, the Sass Foundation for Medical Research and ALSAC.

St. Jude Children's Research Hospital St. Jude Children's Research Hospital is internationally recognized for its pioneering research and treatment of children with cancer and other catastrophic diseases. Ranked the No. 1 pediatric cancer hospital by Parents magazine and the No. 1 children's cancer hospital by U.S. News & World Report, St. Jude is the first and only National Cancer Institute-designated Comprehensive Cancer Center devoted solely to children. St. Jude has treated children from all 50 states and from around the world, serving as a trusted resource for physicians and researchers. St. Jude has developed research protocols that helped push overall survival rates for childhood cancer from less than 20 percent when the hospital opened to almost 80 percent today. St. Jude is the national coordinating center for the Pediatric Brain Tumor Consortium and the Childhood Cancer Survivor Study. In addition to pediatric cancer research, St. Jude is also a leader in sickle cell disease research and is a globally prominent research center for influenza.

Founded in 1962 by the late entertainer Danny Thomas, St. Jude freely shares its discoveries with scientific and medical communities around the world, publishing more research articles than any other pediatric cancer research center in the United States. St. Jude treats more than 5,700 patients each year and is the only pediatric cancer research center where families never pay for treatment not covered by insurance. St. Jude is financially supported by thousands of individual donors, organizations and corporations without which the hospital's work would not be possible. For more information, go to www.stjude.org.