Researchers Identify Gene's Role in Suppressing Longevity
Researchers have determined that a gene in mouse cells limits the number of times that a cell can divide. The gene, known as SIRT1, suppresses longevity, and may play a role in regulating the aging process. Their report appears in the July 2005 issue of the journal Cell Metabolism.
SIRT1 is involved in cellular senescence, or limitation of cells' reproductive lifespan, a process thought to ensure that aging cells don't pass on harmful mutations. The researchers, led by Frederick W. Alt, a Howard Hughes Medical Institute investigator at Children's Hospital Boston and Harvard Medical School, became interested in SIRT1 after other groups showed that SIRT1's counterpart in yeast, Sir2, extends the ability of yeast cells to replicate, and that increased Sir2 production in the cells of worms and flies increased these organisms' life span as well. Such studies have led to speculation that enhancing SIRT1 activity might promote longevity in mammals--including humans. For that reason, pharmaceutical companies are exploring drugs that influence SIRT1 activity, said Alt.
In their experiments, Alt and colleagues suppressed SIRT1 activity in cultures of mouse embryonic fibroblasts, or primitive cells that give rise to connective tissue, exploring how SIRT1 deficiency affected the cells' ability to divide.
''We showed that, unlike in yeast, mouse SIRT1 can function to suppress cellular longevity rather than promote it,'' said Alt, who is also scientific director of the CBR Institute for Biomedical Research in Boston. ''Unlike wild-type cells that only undergo a limited number of divisions before they reach senescence, SIRT1-deficient cells continued to grow on and on very well. That was quite surprising, because the findings in yeast and other lower organisms led people in the field to speculate that if you got rid of mammalian SIRT1, the cells would senesce sooner. But in fact we got the opposite result; the cells survived and didn't undergo senescence.''
To confirm the role of SIRT1, the researchers created mouse cells in which they could switch off SIRT1 at will. When SIRT1 was switched off, the cells became ''immortalized,'' undergoing continued division. But when the researchers switched SIRT1 back on, the cells again became subject to senescence. Alt cautioned that senescence is an imperfect model for aging, and that findings in dividing mouse cells in culture are an imperfect model of how aging affects whole organisms, or even human cells. ''The pathways of senescence are potentially different in human and mouse cells,'' he said. There is, however, some indication that suppressing SIRT1 and enabling cells to proliferate beyond senescence could prove valuable to researchers growing cells for study. SIRT1-deficient cells would hold an advantage over other highly proliferative cell types, such as cancer cells, because although they divide indefinitely, they otherwise appear normal.
''The catch, of course, is that we have only shown this in mouse embryonic fibroblasts so far,'' Alt said. ''It needs to be demonstrated in other differentiated cells and in stem cells, and in human cells.''
In further studies, the researchers found that SIRT1 might regulate senescence by down-regulating expression of p19ARF, known to be an important mediator of senescence. Indeed, p19-deficiency in cells similarly eliminates senescence in cell cultures. The researchers also found that SIRT1 affects a particular response pathway to DNA-damaging oxidation: SIRT1-deficient cells, in contrast to normal cells, continued to divide when treated with chronic, low-level doses of hydrogen peroxide (which induces oxidation). However, the SIRT1-deficient cells had a normal senescence response when exposed to high-level oxidation or the activated cancer gene, Ras. Together, these results indicate that SIRT1 has a specific role in the response to chronic oxidative damage.
Alt and his colleagues have been exploring the entire seven-member family of SIRT genes because of their potential regulatory roles in the immune system, genomic stability (an increased tendency to develop gene mutations) and repair of damaged DNA. The enzymes produced by SIRT genes, called deacetylases, activate a wide range of target molecules. In the future, Alt and his colleagues plan to explore the effects of switching off SIRT1 in adult mice, since knocking the gene out completely causes abnormal development. Such studies would enable them to determine whether turning off the gene eliminates senescence in wider range of cells. The studies would also permit Alt's team to explore potential roles for SIRT1 in tumor suppression and other functions.
Katrin Chua and Raul Mostoslavsky in Alt's laboratory were joint first authors of the article, which also included co-authors from the National Institutes of Health and Brigham and Women's Hospital.