The nicotinamide adenine dinucleotide (NAD)-dependent deacetylase Sir2 (silent information regulator 2) regulates gene silencing in yeast and promotes lifespan extension during caloric restriction. The mammalian homologue of Sir2 (SirT1) regulates p53, NF-kappaB and Forkhead transcription factors, and is implicated in stress response. This report shows that the cell-cycle and apoptosis regulator E2F1 induces SirT1 expression at the transcriptional level. Furthermore, SirT1 binds to E2F1 and inhibits E2F1 activities, forming a negative feedback loop. Knockdown of SirT1 by small interference RNA (siRNA) increases E2F1 transcriptional and apoptotic functions. DNA damage by etoposide causes E2F1-dependent induction of SirT1 expression and knockdown of SirT1 increases sensitivity to etoposide. These results reveal a mutual regulation between E2F1 and SirT1 that affects cellular sensitivity to DNA damage.

The NAD-dependent deacetylase SirT1 regulates factors involved in stress response and cell survival and is a potential drug target of activators and inhibitors. Determination of SirT1 function in tumor cells is important for its targeting in cancer therapy. We found that SirT1 knockdown by short hairpin RNA accelerates tumor xenograft formation by HCT116 cells, whereas SirT1 overexpression inhibits tumor formation. Furthermore, pharmacological inhibition of SirT1 stimulates cell proliferation under conditions of growth factor deprivation. Paradoxically, SirT1 inhibition also sensitizes cells to apoptosis by chemotherapy drugs. Immunohistochemical staining revealed high level SirT1 in normal colon mucosa and benign adenomas. SirT1 overexpression was observed in ϳ25% of stage I/II/III colorectal adenocarcinomas but rarely found in advanced stage IV tumors. Furthermore, ϳ30% of carcinomas showed lower than normal SirT1 expression. This pattern is consistent with SirT1 having pleiotropic effects during cancer development (anti-proliferation and anti-apoptotic). These results suggest a rationale for the use of SirT1 activators and inhibitors in the prevention and treatment of colon cancer.The silent information regulator 2 (Sir2) 2 functions as a nicotinamide adenine dinucleotide (NAD ϩ )-dependent histone deacetylase and regulates chromatin silencing in Saccharomyces cerevisiae (1). Increased Sir2 gene dosage results in the extension of life span in yeast (2). Sir2 is activated by multiple stress signals such as starvation, osmotic stress, and heat shock. In both yeast and C elegans, caloric restriction-induced life span extension is dependent on Sir2 (3, 4). Therefore, Sir2 is a key regulator of cellular homeostasis and survival by connecting stress signals to regulation of gene expression in invertebrate organisms.In addition to its role in extending the lifespan of lower organisms under stressful conditions, mammalian homolog of Sir2 (SirT1) has been shown to regulate glucose homeostasis in mice by deacetylating and activating the transcription factor peroxisome proliferator-activated receptor-␥ coactivator 1␣ (5). Transgenic mice overexpressing SirT1 in pancreatic ␤ cells showed improved glucose tolerance and increased insulin secretion in response to glucose (6, 7). Furthermore, pharmacological activators of SirT1 such as resveratrol can mimic the anti-aging effects of calorie restriction in lower organisms, reduce insulin resistance in mice fed with high fat diet, and prolong survival (8 -10). More recently, a potent activator of SirT1 has shown therapeutic potential in the treatment of type 2 diabetes in animal models by improving insulin sensitivity and lowering plasma glucose level (11).Numerous studies have also suggested a role of SirT1 in tumorigenesis. In general, transient knockdown of SirT1 leads to increased apoptotic response to DNA damage or oxidative stress treatments. SirT1 deacetylates and inhibits the activities of p53, NF-B, Forkhead, Ku, and E2F1 that are critical factors in stress response a...

SUV39H1 is a histone H3K9-specific methyltransferase important for heterochromatin formation, regulation of gene expression, and induction of senescence in premalignant cells. SUV39H1 forms a complex with SirT1, and its activity is stimulated by SirT1 binding. Here we present evidence that the product of the DBC1 (deleted in breast cancer 1) gene disrupts the SUV39H1-SirT1 complex. Furthermore, DBC1 binds to the SUV39H1 catalytic domain and inhibits its ability to methylate histone H3 in vitro and in vivo. Knockdown of endogenous DBC1 increased the level of cellular H3K9 methylation. As expected, DBC1 also binds to SirT1 and inhibits the deacetylase activity of SirT1. These results identify DBC1 as a novel cellular inhibitor of SUV39H1 activity. DBC1 may be an important regulator of heterochromatin formation and genomic stability by disrupting the SUV39H1-SirT1 complex and inactivating both enzymes.