Lei Zhong

The Histone Deacetylase Sirt6 Regulates Glucose Homeostasis via Hif1α

SIRT6 is a member of a highly conserved family of NAD(+)-dependent deacetylases with various roles in metabolism, stress resistance, and life span. SIRT6-deficient mice develop normally but succumb to a lethal hypoglycemia early in life; however, the mechanism underlying this hypoglycemia remained unclear. Here, we demonstrate that SIRT6 functions as a histone H3K9 deacetylase to control the expression of multiple glycolytic genes. Specifically, SIRT6 appears to function as a corepressor of the transcription factor Hif1alpha, a critical regulator of nutrient stress responses. Consistent with this notion, SIRT6-deficient cells exhibit increased Hif1alpha activity and show increased glucose uptake with upregulation of glycolysis and diminished mitochondrial respiration. Our studies uncover a role for the chromatin factor SIRT6 as a master regulator of glucose homeostasis and may provide the basis for novel therapeutic approaches against metabolic diseases, such as diabetes and obesity.

The Histone Deacetylase SIRT6 Is a Tumor Suppressor that Controls Cancer Metabolism

Reprogramming of cellular metabolism is a key eventxa0during tumorigenesis. Despite being known for decades (Warburg effect), the molecular mechanisms regulating this switch remained unexplored. Here, we identify SIRT6 as a tumor suppressor thatxa0regulates aerobic glycolysis in cancer cells. Importantly, loss of SIRT6 leads to tumor formation without activation of known oncogenes, whereas transformed SIRT6-deficient cells display increased glycolysis and tumor growth, suggesting that SIRT6 plays a role in both establishment and maintenance of cancer. By using a conditional SIRT6 allele, we show that SIRT6 deletion inxa0vivo increases the number, size, and aggressiveness of tumors. SIRT6 also functions as a regulator of ribosome metabolismxa0by corepressing MYC transcriptional activity. Lastly,xa0Sirt6 is selectively downregulated in several human cancers, and expression levels of SIRT6 predict prognosis and tumor-free survival rates, highlighting SIRT6 as a critical modulator of cancerxa0metabolism. Our studies reveal SIRT6 to be a potent tumor suppressor acting to suppress cancer metabolism.

The sirtuin SIRT6 blocks IGF-Akt signaling and development of cardiac hypertrophy by targeting c-Jun.

Abnormal activation of insulin-like growth factor (IGF)-Akt signaling is implicated in the development of various diseases, including heart failure. However, the molecular mechanisms that regulate activation of this signaling pathway are not completely understood. Here we show that sirtuin 6 (SIRT6), a nuclear histone deacetylase, functions at the level of chromatin to directly attenuate IGF-Akt signaling. SIRT6-deficient mice developed cardiac hypertrophy and heart failure, whereas SIRT6 transgenic mice were protected from hypertrophic stimuli, indicating that SIRT6 acts as a negative regulator of cardiac hypertrophy. SIRT6-deficient mouse hearts showed hyperactivation of IGF signaling–related genes and their downstream targets. Mechanistically, SIRT6 binds to and suppresses the promoter of IGF signaling–related genes by interacting with c-Jun and deacetylating histone 3 at Lys9 (H3K9). We also found reduced SIRT6 expression in human failing hearts. These findings disclose a new link between SIRT6 and IGF-Akt signaling and implicate SIRT6 in the development of cardiac hypertrophy and failure.

SIRT6 Recruits SNF2H to DNA Break Sites, Preventing Genomic Instability through Chromatin Remodeling

DNA damage is linked to multiple human diseases, such as cancer, neurodegeneration, and aging. Little is known about the role of chromatin accessibility in DNA repair. Here, we find that the deacetylase sirtuin 6 (SIRT6) is one of the earliest factors recruited toxa0double-strand breaks (DSBs). SIRT6 recruits the chromatin remodeler SNF2H to DSBs and focally deacetylates histone H3K56. Lack of SIRT6 and SNF2H impairs chromatin remodeling, increasing sensitivity to genotoxic damage and recruitment of downstream factors such as 53BP1 and breast cancer 1 (BRCA1). Remarkably, SIRT6-deficient mice exhibit lower levels of chromatin-associated SNF2H in specific tissues, a phenotype accompanied by DNA damage. We demonstrate that SIRT6 is critical for recruitment of a chromatin remodeler as an early step in the DNA damage response, indicating that proper unfolding of chromatin plays a rate-limiting role. We present a unique crosstalk between a histone modifier and a chromatin remodeler, regulating a coordinated response to prevent DNA damage.

Sirt1 Is a Regulator of Bone Mass and a Repressor of Sost Encoding for Sclerostin, a Bone Formation Inhibitor

Sirt1, the mammalian ortholog of the yeast Sir2 (silent information regulator 2), was shown to play an important role in metabolism and in age-associated diseases, but its role in skeletal homeostasis and osteoporosis has yet not been studied. Using 129/Sv mice with a germline mutation in the Sirt1 gene, we demonstrate that Sirt1 haplo-insufficient (Sirt1(+/-)) female mice exhibit a significant reduction in bone mass characterized by decreased bone formation and increased marrow adipogenesis. Importantly, we identify Sost, encoding for sclerostin, a critical inhibitor of bone formation, as a novel target of Sirt1. Using chromatin immunoprecipitation analysis, we reveal that Sirt1 directly and negatively regulates Sost gene expression by deacetylating histone 3 at lysine 9 at the Sost promoter. Sost down-regulation by small interfering RNA and the administration of a sclerostin-neutralizing antibody restore gene expression of osteocalcin and bone sialoprotein as well as mineralized nodule formation in Sirt1(+/-) marrow-derived mesenchymal stem cells induced to osteogenesis. These findings reveal a novel role for Sirt1 in bone as a regulator of bone mass and a repressor of sclerostin, and have potential implications suggesting that Sirt1 is a target for promoting bone formation as an anabolic approach for treatment of osteoporosis.

MicroRNAs in Metabolism and Metabolic Diseases

Aberrant cholesterol/lipid homeostasis is linked to a number of diseases prevalent in the developed world, including metabolic syndrome, type II diabetes, and cardiovascular disease. We have previously uncovered gene regulatory mechanisms of the sterol regulatory element-binding protein (SREBP) family of transcription factors, which control the expression of genes involved in cholesterol and lipid biosynthesis and uptake. Intriguingly, we recently discovered conserved microRNAs (miR-33a/b) embedded within intronic sequences of the human SREBF genes that act in a concerted manner with their host gene products to regulate cholesterol/lipid homeostasis. Indeed, miR-33a/b control the levels of ATP-binding cassette (ABC) transporter ABCA1, a cholesterol efflux pump critical for high-density lipoprotein (HDL) synthesis and reverse cholesterol transport from peripheral tissues. Importantly, antisense inhibition of miR-33 in mice results in elevated HDL and decreased atherosclerosis. Interestingly, miR-33a/b also act in the fatty acid/lipid homeostasis pathway by controlling the fatty acid β-oxidation genes carnitine O-octanoyltransferase (CROT), hydroxyacyl-coenzyme A-dehydrogenase (HADHB), and carnitine palmitoyltransferase 1A (CPT1A), as well as the energy sensor AMP-activated protein kinase (AMPKα1), the NAD(+)-dependent sirtuin SIRT6, and the insulin signaling intermediate IRS2, key regulators of glucose and lipid metabolism. These results have revealed a highly integrated microRNA (miRNA)-host gene circuit governing cholesterol/lipid metabolism and energy homeostasis in mammals that may have important therapeutic implications for the treatment of cardiometabolic disorders.

Fine Tuning our Cellular Factories: Sirtuins in Mitochondrial Biology

Sirtuins have emerged in recent years as critical regulators of metabolism, influencing numerous facets of energy and nutrient homeostasis. Here, we review recent advances on the role of this fascinating family of mammalian proteins and their well-orchestrated function in modulating mitochondrial activity.

SIRT6: a master epigenetic gatekeeper of glucose metabolism.

Sirtuins are the mammalian homologs of the yeast histone deacetylase Sir2. In recent years, an ever-expanding picture has emerged indicating that these proteins (SIRT1-7) play broad functions in cellular stress resistance, genomic stability, energy metabolism, aging and tumorigenesis. Among members of this family, SIRT6 appears to have particular significance in regulating metabolism, DNA repair and lifespan. In this context, new research from our lab has established SIRT6 as a key regulator of glucose homeostasis. In this Point of View article, we will first highlight our recent findings, and then provide an in-depth discussion of their implications in cancer and aging.Sirtuins are the mammalian homologs of the yeast histone deacetylase Sir2. In recent years, an ever-expanding picture has emerged indicating that these proteins (SIRT1-7) play broad functions in cellular stress resistance, genomic stability, energy metabolism, aging and tumorigenesis. Among members of this family, SIRT6 appears to have particular significance in regulating metabolism, DNA repair and lifespan. In this context, new research from our lab has established SIRT6 as a key regulator of glucose homeostasis. In this Point of View article, we will first highlight our recent findings, and then provide an in-depth discussion of their implications in cancer and aging.

Chromatin Protein L3MBTL1 Is Dispensable for Development and Tumor Suppression in Mice

L3MBTL1, a paralogue of Drosophila tumor suppressor lethal(3)malignant brain tumor (l(3)mbt), binds histones in a methylation state-dependent manner and contributes to higher order chromatin structure and transcriptional repression. It is the founding member of a family of MBT domain-containing proteins that has three members in Drosophila and nine in mice and humans. Knockdown experiments in cell lines suggested that L3MBTL1 has non-redundant roles in the suppression of oncogene expression. We generated a mutant mouse strain that lacks exons 13–20 of L3mbtl1. Markedly reduced levels of a mutant mRNA with an out-of-frame fusion of exons 12 and 21 were expressed, but a mutant protein was undetectable by Western blot analysis. L3MBTL1−/− mice developed and reproduced normally. The highest expression of L3MBTL1 was detected in the brain, but its disruption did not affect brain development, spontaneous movement, and motor coordination. Despite previous implications of L3mbtl1 in the biology of hematopoietic transcriptional regulators, lack of L3MBTL1 did not result in deficiencies in lymphopoiesis or hematopoiesis. In contrast with its demonstrated biochemical activities, embryonic stem (ES) cells lacking L3MBTL1 displayed no abnormalities in H4 lysine 20 (H4K20) mono-, di-, or trimethylation; had normal global chromatin density as assessed by micrococcal nuclease digests; and expressed normal levels of c-myc. Embryonic fibroblasts lacking L3MBTL1 displayed unaltered cell cycle arrest and down-regulation of cyclin E expression after irradiation. In cohorts of mice followed for more than 2 years, lack of L3MBTL1 did not alter normal lifespan or survival with or without sublethal irradiation.

Neuroprotective Functions for the Histone Deacetylase SIRT6

The histone deacetylase SIRT6 promotes DNA repair, but its activity declines with age with a concomitant accumulation of DNA damage. Furthermore, SIRT6 knockout mice exhibit an accelerated aging phenotype and die prematurely. Here, we report that brain-specific SIRT6-deficient mice survive but present behavioral defects with major learning impairments by 4xa0months of age. Moreover, the brains of these mice show increased signs of DNA damage, cell death, and hyperphosphorylated Tau-a critical mark in several neurodegenerative diseases. Mechanistically, SIRT6 regulates Tau protein stability and phosphorylation through increased activation of the kinase GSK3α/β. Finally, SIRT6 mRNA and protein levels are reduced in patients with Alzheimers disease. Taken together, our results suggest that SIRT6 is critical to maintain genomic stability in the brain and that its loss leads to toxic Tau stability and phosphorylation. Therefore, SIRT6 and its downstream signaling could be targeted in Alzheimers disease and age-related neurodegeneration.