Identification of novel mechanisms regulating the NAD+ -dependent deacetylase SIRT1

Sirtuins comprise a highly conserved protein family which catalyse the deacetylation of proteins in an NAD+-dependent manner. In S. cerevisiae and in C. elegans sirtuins have been shown to mediate the beneficial effects of caloric restriction on organismal longevity thus giving rise to the notion that may function as key regulators of the ageing process. SIRT1 is the best characterised member of the mammalian sirtuin family which comprises seven homologues. It has been shown to associate with and deacetylate several proteins, predominantly transcriptional regulators such as p53, NFκB, MyoD and FOXO, thus functioning in processes as diverse as development, differentiation, senescence, survival, proliferation and metabolic regulation. Understanding the dynamics of intracellular SIRT1 function has been hindered by the limitation of methods for accurately quantifying free intracellular [NAD+] levels and thus SIRT1 activity. Furthermore, nothing is known about potential post-translational mechanisms involved in SIRT1 regulation. During the course of this work, the task of identifying novel mechanisms that regulate SIRT1 function was undertaken. SIRT1 was found to be specifically phosphorylated in mitosis as well as in interphase. In addition, its subcellular localisation and turnover are sensitive to UV irradiation. Finally, various stresses induce caspase-mediated SIRT1 cleavage which has an impact on overall protein stability. These data provide the first glimpse into the molecular regulatory mechanisms that dictate SIRT1 function. The implications of this work are discussed in the context of current knowledge as well as proposed novel functions of SIRT1.