Acetylation of p53 leads to enhanced transcriptional activation, whereas by deacetylating p53, SIRT1 may turn off p53 to promote cellular proliferation (7-10,22). protein, mRNA, and enzymatic activity levels. Voreloxin SIRT1 was also found to be overexpressed in human PCa tissues compared with adjacent normal prostate tissue. Interestingly, our data demonstrated that SIRT1 inhibition via nicotinamide and sirtinol (at the activity level) as well as via short hairpin RNA-mediated RNA interference (at the genetic level) resulted in a significant inhibition Voreloxin in the growth and viability of human PCa cells while having no effect on normal prostate epithelial cells. EIF4G1 Further, we found that inhibition of SIRT1 caused an increase in FOXO1 acetylation and transcriptional activation in PCa cells. Our data suggested that SIRT1, via inhibiting FOXO1 activation, could contribute to the development of PCa. We suggest that SIRT1 could serve as a target toward developing novel strategies for PCa management. Prostate cancer (PCa)2is a major age-related malignancy and is rarely seen in men younger than 40 years; the incidence rises rapidly with each decade thereafter. Because the present life expectancy has significantly improved and Americans are living longer, it is believed that more cases of PCa will be diagnosed in the future. According to one prediction, by year 2010, the number of annual PCa cases will skyrocket to 330,000. Thus, it will be immensely useful to better understand the molecular mechanism and connection between aging and PCa. Unraveling determinants such as genes and gene-products involved in aging and that have a connection with PCa could be exploited in designing novel targets and approaches for the management of this age-related neoplasm. We hypothesized that sirtuins (Sirt proteins), which are nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases, could serve as a connection between aging and cancer. Originally discovered in yeast, sirtuins are a unique class of type III histone deacetylases that utilize NAD+as a cofactor for their functions (1-4). Seven homologs of yeast Sir2 have been identified in the human genome. Called SIRT1 to -7, they all contain a highly conserved catalytic domain, and despite their enzymatic activity on histone substratesin vitro, Sirt proteins predominantly target nonhistone proteins for deacetylation, in both the nucleus and the cytoplasm (1-4). Each sirtuin is characterized by a conserved 275-amino acid catalytic core domain and unique N-terminal and/or C-terminal sequences of variable length. The catalytic core domain may act preferentially as a mono-ADP-ribosyltransferase or NAD+-dependent deacetylase (3). Their functions and locations differ greatly, and SIRT1 is the best characterized member among the mammalian sirtuins (1-4). SIRT1 has been reported to be a nuclear as well as cytoplasmic protein and demonstrated to be involved in a number of cellular processes, including gene silencing at telomere and mating loci, DNA repair, recombination, and aging (1-6). Recent studies have demonstrated that SIRT1 plays an important role in the regulation Voreloxin of cell death/survival and stress response in mammals. SIRT1 promotes cell survival by inhibiting apoptosis or cellular senescence induced by stresses, including DNA damage and oxidative stress (1-6). An increasing number of proteins have been identified as substrates of SIRT1, including p53 (7-10), forkhead (FOXO) transcription factors (11-16), repair protein Ku70 (11,17,18), p300 (19), Rb (19,20), and p73 (19,21) just to name a few. Interestingly, SIRT1 has been shown to negatively regulate proliferative signaling via regulating (i) p53 function (7-10,19,22), (ii) FOXO pathway (1,11-13,23), and (iii) MAPK signaling (24). Improper regulation of sirtuin proteins has been reported in a number of diseases, including Bowen’s disease (25), type I diabetic nephropathy (26), Alzheimer disease and amyotrophic lateral sclerosis (27), and nonalchoholic fatty liver disease (28). There are four reported human forkhead family members, FOXO1 (FKHR) (29), FOXO3a (FKHRL1) (30), FOXO4 (ARX) (31), and FOXO6 (32), which have been shown to regulate a variety of cellular processes, including cell differentiation, transformation, and metabolism (11-13,15,33). The activities and localization of the FOXOs have been shown to be dependent on their phosphorylation and acetylation status. Phosphorylation of FOXO factors, with subsequent ubiquitination, has been shown to modulate a variety of downstream target genes (34). FOXO acetylation is believed to have multiple and opposing effects, depending on the specific FOXO factor and model system (11,34). Regardless, the various FOXO factors have been shown to be deacetylated by SIRT1 (11,13,15,35,36); however, the connection between SIRT1 and the FOXO.