RESEARCH COMMUNICATION Tissue-specific regulation

Calorie restriction (CR) has been reported to increase SIRT1 protein levels in mice, rats, and humans, and el-evated activity of SIRT1 orthologs extends life span in yeast, worms, and flies. In this study, we challenge the paradigm that CR induces SIRT1 activity in all tissues by showing that activity of this sirtuin in the liver is, in fact, reduced by CR and activated by a high-caloric diet. We demonstrate this change both by assaying levels of SIRT1 and its small molecule regulators, NAD and NADH, as well as assessing phenotypes of a liver-spe-cific SIRT1 knockout mouse on various diets. Our find-ings suggest that designing CR mimetics that target SIRT1 to provide uniform systemic benefits may be more complex than currently imagined. Supplemental material is available a