Here, we demonstrate a role for the mitochondrial NAD-dependent deacetylase Sirt3 in the maintenance of basal ATP levels and as a regulator of mitochondrial electron transport. We note that Sirt3 ؊/؊ mouse embryonic fibroblasts have a reduction in basal ATP levels. Reconstitution with wild-type but not a deacetylase-deficient form of Sirt3 restored ATP levels in these cells. Furthermore in wild-type mice, the resting level of ATP correlates with organ-specific Sirt3 protein expression. Remarkably, in mice lacking Sirt3, basal levels of ATP in the heart, kidney, and liver were reduced >50%. We further demonstrate that mitochondrial protein acetylation is markedly elevated in Sirt3 ؊/؊ tissues. In addition, in the absence of Sirt3, multiple components of Complex I of the electron transport chain demonstrate increased acetylation. Sirt3 can also physically interact with at least one of the known subunits of Complex I, the 39-kDa protein NDUFA9. Functional studies demonstrate that mitochondria from Sirt3 ؊/؊ animals display a selective inhibition of Complex I activity. Furthermore, incubation of exogenous Sirt3 with mitochondria can augment Complex I activity. These results implicate protein acetylation as an important regulator of Complex I activity and demonstrate that Sirt3 functions in vivo to regulate and maintain basal ATP levels.acetylation ͉ sirtuins ͉ complex I ͉ electron transport

SUMMARY The sirtuin gene family (SIRT) is hypothesized to regulate the aging process and play a role in cellular repair. This work demonstrates that SIRT3−/− mouse embryonic fibroblasts (MEFs) exhibit abnormal mitochondrial physiology as well as increases in stress-induced superoxide levels and genomic instability. Expression of a single oncogene (Myc or Ras) in SIRT3−/− MEFs results in in vitro transformation and altered intracellular metabolism. Superoxide dismutase prevents transformation by a single oncogene in SIRT3−/− MEFs and reverses the tumor permissive phenotype as well as stress-induced genomic instability. In addition, SIRT3−/− mice develop ER/PR-positive mammary tumors. Finally, human breast and other human cancer specimens exhibit reduced SIRT3 levels. These results identify SIRT3 as a genomically expressed, mitochondrial localized tumor suppressor.

Summary In lower eukaryotes, Sir2 serves as a histone deacetylase and is implicated in chromatin silencing, longevity and genome stability. Here we mutated the SIRT1 gene, a homolog of yeast Sir2, in mice to study its function. We showed that a majority of SIRT1-null embryos died between E9.5–E14.5, displaying altered histone modification, impaired DNA damage response, and reduced ability to repair DNA damage. We demonstrated that SIRT1+/−;p53+/− mice developed tumors in multiple tissues, whereas activation of SIRT1 by resveratrol treatment reduced tumorigenesis. Finally, we showed that many human cancers exhibited reduced level of SIRT1 than their normal controls. Thus, SIRT1 acts as a tumor suppressor through its role in DNA damage response, genome integrity, and tumor suppression. Significance SIRT1 has diverse roles in various biological processes, including caloric restriction that causes changes in glucose metabolism and lifespan. The role of SIRT1 in cancer is currently under debate due to some recent different findings. It is known that calorie restriction, which activates SIRT1, extends lifespan and inhibits tumorigenesis. On the other hand, SIRT1 deacetylates p53 to decrease its activity. It was therefore hypothesized increased SIRT1 activity, although it extends lifespan, may elevate cancer risk. Here we demonstrate SIRT1 plays an important role in DNA damage response and genome integrity by maintaining proper chromatin structure and DNA damage repair foci formation. We further show that SIRT1 serves as a tumor suppressor in mice and in some types of human cancers.