SIRT1 protein levels in cancer: Tuning SIRT1 to the needs of a cancer cell

Abstract

SIRT1 (silent information regulator 1) is a member of the highly conserved gene family of sirtuins, discovered in yeast as factors required for gene silencing, and classified as enzymes harboring NAD+-dependent deacetylase and ADP-ribosyltransferase activities. SIRT1 is able to deacetylate histones and histone methyl-transferase SUV39H1, thereby initiating and coordinating facultative heterochromatin formation. SIRT1 also deacetylates a number of non-histone target proteins, including the tumor suppressor p53, the retinoblastoma protein, the transcription factors of the FoxO family, MyoD and RelA/p65 (NFκB), the DNA repair protein Ku70, and the transcriptional co-activators PCG-1α and p300. SIRT1 co-ordinates cellular responses to caloric restriction (CR) including enhanced metabolic efficiency, is involved in the biogenesis of ribosomes and mitochondria, and in metabolic endocrine signaling 1. In this way, SIRT1 regulates important cellular processes like aging and longevity, acts anti-apoptotic and protects neurons from degeneration. Given the diversity of SIRT1 functions, it is not surprising that SIRT1 is also implicated in a variety of pathologic conditions, including diabetes, neurodegeneration, Parkinson’s disease, and cancer 2. The beneficial or detrimental roles of SIRT1 strongly depend on SIRT1 activity levels; therefore regulation of its activity is of pivotal importance. SIRT1 deacetylase activity can be regulated by interacting proteins, like AROS which enhances 3, and DBC1 (deleted in breast cancer-1) which inhibits SIRT1 activity 4. Alternatively or in addition, changes in SIRT1 protein levels will also change SIRT1 activity.In cancer cells, SIRT1 protein levels are abnormally elevated, indicative for higher SIRT1 activity in such cells. SIRT1 functions as a survival factor in cancer cells, as demonstrated by RNAi-induced silencing of SIRT1, as SIRT1 depletion resulted in apoptosis of epithelial cancer cells, but not of non-cancer epithelial cells 5. In this issue of Cell Cycle, Milner and her colleagues provide convincing evidence that elevated SIRT1 protein levels in cancer cells are not attributable to increased SIRT1 mRNA levels, but instead correlate with increased SIRT1 protein stability. Protein stability often is regulated by post-translational modification. SIRT1 is a phosphoprotein, with two potential phosphorylation sites at serines 27 (S27P) and 47 (S47P). Using phosphopeptide-specific antibodies, Milner and colleagues found that SIRT1 protein levels in normal and in cancer cells strongly correlated with the presence of S47P, indicating that S47 is constitutively phosphorylated. In contrast, S27P levels were variable and did not correlate with SIRT1 proteins levels: S27P was hardly detectable in normal cells, but strongly present in cancer cells. Thus S27P phosphorylation correlated with the enhanced SIRT1 protein levels in cancer cells, and indicated that phosphorylation of SIRT1 at S27 enhances its metabolic stability. Increased SIRT1 protein stability is dependent, directly or indirectly, upon the presence of JNK2, as silencing of JNK2, but not of JNK1, with RNAi decreased SIRT1 protein stability while SIRT1 mRNA levels remained unaffected. The JNK2 connection links SIRT1 with the MAP kinase system.A very interesting effect was observed, when colon cancer HCT116 cells were treated with the commonly used anti-cancer drug 5-fluorouracil (5FU). 5FU treatment also induced a decline in SIRT1 protein levels, but only in parental HCT116 cells expressing wild-type p53 and not in their p53-negative derivatives (HCT116 p53-/- cells). Thus control of SIRT1 levels is also dependent on a functional wild-type p53, creating yet another link between SIRT1 and p53. However, p53 influences SIRT1 protein levels not via SIRT1 turnover, but via control of SIRT1 translation. In a very recent paper Yamakuchi et al. 6 demonstrated that miR-34a, a bona fide transcriptional target of p53, represses SIRT1 expression. As p53 loss of function is common to many cancer cells, cancer cells can achieve increased SIRT1 protein levels by two independent means. The finding that SIRT1 protein levels can be regulated at the translational (via miR-34a) and at the post-translational level (via JNK2), and that both effects are additive, should provide novel approaches for a combinatorial targeting of SIRT1 not only in cancer therapy. Given the diverse roles of SIRT1 and its links with diseases including cancer understanding the various means of regulating SIRT1 protein levels and activity are extremely important.ReferencesJiang J, Huang Z, Zhao Q, Feng W, Belikova NA, Kagan VE. Interplay between bax, reactive oxygen species production, and cardiolipin oxidation during apoptosis. Biochem Biophys Res Commun 2008; 368:145-50.Guarente L. Sirtuins in aging and disease. Cold Spring Harb Symp Quant Biol 2007; 72:483-8.Verdin E. AROuSing SIRT1: identification of a novel endogenous SIRT1 activator. Mol Cell 2007; 28:354-6.Zhao W, Kruse JP, Tang Y, Jung SY, Qin J, Gu W. Negative regulation of the deacetylase SIRT1 by DBC1. Nature 2008; 451:587-90.Ford J, Jiang M, Milner J. Cancer-specific functions of SIRT1 enable human epithelial cancer cell growth and survival. Cancer Res 2005; 65:10457-63.Yamakuchi M, Ferlito M, Lowenstein CJ. miR-34a repression of SIRT1 regulates apoptosis. Proc Natl Acad Sci USA 2008; 105:13421-6.