SIRT6 (sirtuin 6) is a member of sirtuin family of deacetylases involved in diverse processes including genome stability, metabolic homeostasis, and tumorigenesis. However, the role of SIRT6 deacetylase activity in its tumor-suppressor functions is not well understood. Here we report that SIRT6 binds to and deacetylates nuclear PKM2 (pyruvate kinase M2) at the lysine 433 residue. PKM2 is a glycolytic enzyme with nonmetabolic nuclear oncogenic functions. SIRT6-mediated deacetylation results in PKM2 nuclear export. We further have identified exportin 4 as the specific transporter mediating PKM2 nuclear export. As a result of SIRT6-mediated deacetylation, PKM2 nuclear protein kinase and transcriptional coactivator functions are abolished. Thus, SIRT6 suppresses PKM2 oncogenic functions, resulting in reduced cell proliferation, migration potential, and invasiveness. Furthermore, studies in mouse tumor models demonstrate that PKM2 deacetylation is integral to SIRT6-mediated tumor suppression and inhibition of metastasis. Additionally, reduced SIRT6 levels correlate with elevated nuclear acetylated PKM2 levels in increasing grades of hepatocellular carcinoma. These findings provide key insights into the pivotal role of deacetylase activity in SIRT6 tumor-suppressor functions.) is a member of the highly conserved sirtuin family of NAD + -dependent enzymes and plays a key role in DNA repair, telomere maintenance, and cellular metabolic processes. It exhibits diverse enzymatic activities including NAD + -dependent deacetylation and mono-ADP ribosylation. SIRT6 deacetylates telomeric histone H3 at lysine 9 (H3K9) and lysine 56 residues (H3K56) (1, 2). SIRT6-mediated deacetylation of telomeric H3K9 is required for the stable association of the Werner syndrome protein with telomeric chromatin for proper telomere function. SIRT6 also interacts with the NF-κB RELA subunit, deacetylates H3K9 at NF-κB target gene promoters, and attenuates NF-κB-mediated apoptosis and senescence (3). Likewise, SIRT6 also binds to hypoxia-inducible factor 1-alpha (HIF1α), deacetylates H3K9 at HIF1α target gene promoters, and regulates glucose homeostasis (4). SIRT6 also interacts with and deacetylates CtIP [C-terminal binding protein (CtBP) interacting protein] to promote the repair of DNA double-strand breaks by homologous recombination (5). SIRT6 mono-ADP ribosylates PARP1 to stimulate the repair of DNA double-strand breaks in response to oxidative stress (6).Several recent studies report that SIRT6 functions as a tumor suppressor. It was observed that loss of SIRT6 promotes tumor formation even without the activation of known oncogenes (7). Furthermore, SIRT6 was found to be down-regulated in pancreatic and colorectal cancers. Down-regulation of SIRT6 also has been reported in hepatocellular carcinoma (8, 9). Elevated c-JUN levels in hepatocellular carcinoma down-regulate SIRT6 expression in a c-FOS-dependent manner (8). Recently, naturally occurring cancer-associated point mutations were identified in SIRT6 that result in its loss of tumor-su...

E3 ubiquitin ligases and deubiquitylating enzymes (DUBs) are the key components of ubiquitin proteasome system which plays a critical role in cellular protein homeostasis. Any shortcoming in their biological roles can lead to various diseases including cancer. The dynamic interplay between ubiquitylation and deubiquitylation determines the level and activity of several proteins including p53, which is crucial for cellular stress response and tumor suppression pathways. In this review, we describe the different types of E3 ubiquitin ligases including those targeting tumor suppressor p53, SCF ligases and RING type ligases and accentuate on biological functions of few important E3 ligases in the cellular regulatory networks. Tumor suppressor p53 level is tightly regulated by multiple E3 ligases including Mdm2, COP1, Pirh2, etc. SCF ubiquitin ligase complexes are key regulators of cell cycle and signal transduction. BRCA1 and VHL RING type ligases function as tumor suppressors and play an important role in DNA repair and hypoxia response respectively. Further, we discuss the biological consequences of deregulation of the E3 ligases and the implications for cancer development. We also describe deubiquitylases which reverse the process of ubiquitylation and regulate diverse cellular pathways including metabolism, cell cycle control and chromatin remodelling. As the E3 ubiquitin ligases and DUBs work in a substrate specific manner, an improved understanding of them can lead to better therapeutics for cancer.Cancer is best characterized at molecular level either by gainof-function of oncoproteins and/or loss-of-function of tumor suppressor proteins. The turnover and the biological activity of several oncoproteins and tumor suppressors is governed by the ubiquitin proteasome system (UPS). The UPS comprises two distinct steps: the covalent attachment of multiple ubiquitin molecules to the protein substrate and degradation of the polyubiquitylated protein by the 26S proteasome complex. 1 A ubiquitin molecule consists of a highly conserved 76-amino-acid polypeptide that is conjugated through an isopeptide linkage to other proteins. Key structural features of ubiquitin include its b-grasp fold, its C-terminal tail and seven lysine residues through which polyubiquitin chains are linked. The most important lysine residues of ubiquitin are K48 and K63. K48-linked polyubiquitylation targets the protein for proteasomal degradation while linear ubiquitin chains, monoubiquitylation and K63-linked polyubiquitylation play a role in signalling.The first step of UPS is cascaded by at least three enzymes: the ubiquitin-activating enzyme E1, ubiquitin-conjugating enzyme E2 and ubiquitin ligase E3. A role for E1 in cancer has not been described, and only a few reports have correlated E2 to tumorigenesis. 2 In contrast, cumulative evidence suggests alterations in the activity of many E3 ligases in the etiology of cancer development. 3,4 Because E3 ligases are the recognition element of the system, thus by providing the substrate specificit...

UBE3A is an E3 ubiquitin ligase well known for its role in the proteasomal degradation of p53 in human papillomavirus (HPV)-associated cancers. Here we report that UBE3A ubiquitylates and triggers degradation of the tumor-suppressive sirtuin SIRT6 in hepatocellular carcinoma. UBE3A ubiquitylated the highly conserved Lys160 residue on SIRT6. FOXO1-mediated transcriptional repression of was sufficient to stabilize SIRT6 and to epigenetically repress, a key mediator of UBE3A oncogenic function. Thus, UBE3A-mediated SIRT6 degradation promoted the proliferative capacity, migration potential, and invasiveness of cells. In mouse models of hepatocellular carcinoma, SIRT6 downregulation and consequent induction of ANXA2 were critical for UBE3A-mediated tumorigenesis. Furthermore, in clinical specimens, increased UBE3A levels correlated with reduced SIRT6 levels and elevated ANXA2 levels in increasing tumor grades. Overall, our findings show how the tumor suppressor SIRT6 is regulated in hepatocellular carcinoma and establish the mechanism underlying UBE3A-mediated tumorigenesis in this disease. These findings provide mechanistic insights into regulation of the tumor suppressive sirtuin SIRT6 and its implications for the development of hepatocellular carcinoma. .

Single-nucleotide polymorphisms (SNPs) in RNF213, which encodes a 591-kD protein with AAA+ ATPase and RING E3 domains, are associated with a rare, autosomal dominant cerebrovascular disorder, moyamoya disease (MMD). MMD-associated SNPs primarily localize to the C-terminal region of RNF213, and some affect conserved residues in the RING domain. Although the autosomal dominant inheritance of MMD could most easily explained by RNF213 gain-of-function, the type of ubiquitylation catalyzed by RNF213 and the effects of MMD-associated SNPs on its E3 ligase activity have remained unclear. We found that RNF213 uses the E2-conjugating enzymes UBE2D2 and UBE2L3 to catalyze distinct ubiquitylation events. RNF213-UBED2 catalyzes K6 and, to a lesser extent, K48-dependent poly-ubiquitylation in vitro, whereas RNF213-UBE2L3 catalyzes K6-, K11-, and K48-dependent poly-ubiquitylation events. MMD-associated SNPs encode proteins with decreased E3 activity, and the most frequent MMD allele, RNF213R4810K, is a dominant-negative mutant that decreases ubiquitylation globally. By contrast, MMD-associated RNF213 SNPs do not affect ATPase activity. Our results suggest that decreased RNF213 E3 ligase activity is central to MMD pathogenesis.

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