A Deacetylase-Deficient SIRT1 Variant Opposes Full-Length SIRT1 in Regulating Tumor Suppressor p53 and Governs Expression of Cancer-Related Genes

Abstract

SIRT1 is an NAD-dependent deacetylase and epigenetic regulator essential for normal mammalian development and homeostasis. Here we describe a human SIRT1 splice variant, designated SIRT1-⌬2/9, in which the deacetylase coding sequence is lost due to splicing between exons 2 and 9. This work aimed to determine if SIRT1-⌬2/9 is a novel functional product of the SIRT1 gene. Endogenous SIRT1-⌬2/9 protein was identified in human cell lysate by immunoblotting and splice variant-specific RNA interference (RNAi). SIRT1-⌬2/9 mRNA is bound by CUGBP2, which downregulates its translation. Using pulldown assays, we demonstrate that SIRT1-⌬2/9 binds p53 protein. SIRT1-⌬2/9 maintains basal p53 protein levels and supports p53 function in response to DNA damage, as evidenced by RNAi-mediated depletion of SIRT1-⌬2/9 prior to damage. In turn, basal p53 downregulates SIRT1-⌬2/9 RNA levels, while stress-activated p53 eliminates SIRT1-⌬2/9. Loss of wild-type (wt) p53 has been correlated with overexpression of SIRT1-⌬2/9 in a range of human cancers. Exogenous SIRT1-⌬2/9 protein associates with specific promoters in chromatin and can regulate cancer-related gene expression, as evidenced by chromatin immunoprecipitation analysis and RNAi/genomic array data. SIRT1 is of major therapeutic importance, and potential therapeutic drugs are screened against SIRT1 deacetylase activity. Our discovery of SIRT1-⌬2/9 identifies a new, deacetylase-independent therapeutic target for SIRT1-related diseases, including cancer. Mammalian SIRT1 belongs to the sirtuin family of proteins that was first identified and characterized in yeast and subsequently found to be highly conserved through evolution (2,23,43,47,51). The Saccharomyces cerevisiae homologue of SIRT1 is Sir2, which stabilizes yeast chromosomes and impacts yeast aging. In mammals, SIRT1 is an epigenetic regulator of normal development, gametogenesis, homeostasis, and aging-related processes (3,22,30,34,41,54). Mammalian genes that fall within the scope of SIRT1 regulation include key genes linked, for example, with hormonal control of metabolism and insulin signaling (e.g., PGC-1␣), the ability of cells to respond to stress (e.g., p53, Foxo, and p300), and the processing of amyloid precursor protein in neuronal cells of the brain (ADAM10) (6,15,17,20,31,37,42,52,53). These genes in turn link SIRT1 with disease processes, including diabetes, cancer, and neurodegeneration (4,17,54).Given the multifunctional roles of SIRT1 in health and disease, it is not surprising that SIRT1 is now recognized as an important therapeutic target across a range of age-related diseases, and this is a strong driving force for understanding the pathways subject to SIRT1 activity. For example, with a mouse model of Alzheimer's disease, Guarente's group recently demonstrated that SIRT1 suppresses the production of ␤-amyloid protein and the formation of amyloid plaques in the brain. This is achieved via SIRT1-dependent transcriptional activation of ␣-secretase ADAM10, which is involved in the cellular cleavage of amyloi...

Recreational scuba diving: negative or positive effects of oxidative and cardiovascular stress?

“…Redox targets whose activity is regulated by SIRT1 include p53 (21) and fork-head box O (FOXO) transcription factor family (22). SIRT1 deacetylates p53 and thus reduces its transcriptional activity and preventing the induction of a p53-mediated apoptosis program (23).…”

Identification of a Tissue-Restricted Isoform of SIRT1 Defines a Regulatory Domain that Encodes Specificity

“…Naturally occurring variants generate diversity of functions for any given protein and also provide insights into domain-specific molecular mechanisms. Two isoforms of SIRT1 have been described earlier, which have differential effects on p53-dependent functions (Lynch et al, 2010; Shah et al, 2012). However, they have not been studied in the context of encoding molecular, physiological, or tissue-specific functions.…”

p53 inhibits the upregulation of sirtuin 1 expression induced by c-Myc

“…p53 has been reported to repress the expression of Sirt1 and its removal by forkhead box O3 activated Sirt1 transcription in PC12 neuronal cancer cells (8,9). This model is further supported by studies investigating transformation-associated p53 knockout mice, which expressed constitutively higher Sirt1 mRNA expression levels in numerous tissues (10).…”