Carboxy-terminal phosphorylation of SIRT1 by protein kinase CK2

Redox signaling and skeletal muscle adaptation during aerobic exercise

Redox regulation is a fundamental physiological phenomenon related to oxygen-dependent metabolism, and skeletal muscle is mainly regarded as a primary site for oxidative phosphorylation. Several studies have revealed the importance of reactive oxygen and nitrogen species (RONS) in the signaling process relating to muscle adaptation during exercise. To date, improving knowledge of redox signaling in modulating exercise adaptation has been the subject of comprehensive work and scientific inquiry. The primary aim of this review is to elucidate the molecular and biochemical pathways aligned to RONS as activators of skeletal muscle adaptation and to further identify the interconnecting mechanisms controlling redox balance. We also discuss the RONS-mediated pathways during the muscle adaptive process, including mitochondrial biogenesis, muscle remodeling, vascular angiogenesis, neuron regeneration, and the role of exogenous antioxidants.

CSNK2A1-mediated MAX phosphorylation upregulates HMGB1 and IL-6 expression in cholangiocarcinoma progression

BACKGROUND

We established a novel diethylnitrosamine (DEN) -induced mouse model that reflected the progression of cholangiocarcinoma (CCA) from atypical cystic hyperplasia.

METHODS

BALB/c mice were administered DEN by oral gavage. Cells isolated from livers were analyzed for expression of CSNK2A1, MAX and MAX-interacting proteins. Human CCA cell lines (MzChA-1, HuCCT1), normal human cholangiocyte (H69), human hepatic stellate cells (LX-2), macrophages (RAW 264.7), and primary hepatic cells were used for cellular and molecular biology assays.

RESULTS

Expression of MAX, CSNK2A1, C-MYC, β-catenin, HMGB1, and IL-6 was upregulated in hepatic cells from CCA liver tissue. The half-life of MAX is higher in CCA cells, and this favors their proliferation. Overexpression of MAX increased growth, migration, and invasion of MzChA-1, whereas silencing of MAX had the opposite effect. MAX positively regulated IL-6 and HMGB1 through paracrine signaling in HepG2, LX2, and RAW cells and autocrine signaling in MzChA-1 cells. CSNK2A1-mediated MAX phosphorylation shifts MAX-MAX homodimer to C-MYC-MAX and β-catenin-MAX heterodimers and increases the HMGB1 and IL-6 promoter activities. Increase of MAX phosphorylation promotes cell proliferation, migration, invasion, and cholangiocarcinogenesis. The casein kinase 2 inhibitor CX-4945 induces cell cycle arrest and inhibits cell proliferation, migration, invasion, and carcinogenesis in MzChA-1 cells through the downregulation of CSNK2A1, MAX, and MAX-interaction proteins.

CONCLUSION

C-MYC-MAX and β-catenin-MAX binding to E-box site or β-catenin-MAX bound to TCFs/LEF1 enhanced HMGB1 or IL-6 promoter activities, respectively. IL-6 and HMGB1 secreted by hepatocytes, HSCs, and KCs exert paracrine effects on cholangiocytes to promote cell growth, migration, and invasion and lead to the progression of cholangiocarcinogenesis. CX-4945 provides perspectives on therapeutic strategies to attenuate progression from atypical cystic hyperplasia to cholangiocarcinogenesis.

Role of Post-translational Modification of Silent Mating Type Information Regulator 2 Homolog 1 in Cancer and Other Disorders

Silent mating type information regulator 2 homolog 1 (SIRT1), an NAD⁺-dependent histone/protein deacetylase, has multifarious physiological roles in development, metabolic regulation, and stress response. Thus, its abnormal expression or malfunction is implicated in pathogenesis of various diseases. SIRT1 undergoes post-translational modifications, including phosphorylation, oxidation/reduction, carbonylation, nitrosylation, glycosylation, ubiquitination/deubiquitination, SUMOylation etc. which can modulate its catalytic activity, stability, subcellular localization, and also binding affinity for substrate proteins. This short review highlights the regulation of SIRT1 post-translational modifications and their pathophysiologic implications.

CK2 and the Hallmarks of Cancer

Cancer is a leading cause of death worldwide. Casein kinase 2 (CK2) is commonly dysregulated in cancer, impacting diverse molecular pathways. CK2 is a highly conserved serine/threonine kinase, constitutively active and ubiquitously expressed in eukaryotes. With over 500 known substrates and being estimated to be responsible for up to 10% of the human phosphoproteome, it is of significant importance. A broad spectrum of diverse types of cancer cells has been already shown to rely on disturbed CK2 levels for their survival. The hallmarks of cancer provide a rationale for understanding cancer’s common traits. They constitute the maintenance of proliferative signaling, evasion of growth suppressors, resisting cell death, enabling of replicative immortality, induction of angiogenesis, the activation of invasion and metastasis, as well as avoidance of immune destruction and dysregulation of cellular energetics. In this work, we have compiled evidence from the literature suggesting that CK2 modulates all hallmarks of cancer, thereby promoting oncogenesis and operating as a cancer driver by creating a cellular environment favorable to neoplasia.

Regulation of sclerostin by the SIRT1 stabilization pathway in osteocytes

Osteocytes play a critical role in bone remodeling through the secretion of paracrine factors regulating the differentiation and activity of osteoblasts and osteoclasts. Sclerostin is a key osteocyte-derived factor that suppresses bone formation and promotes bone resorption, therefore regulators of sclerostin secretion are a likely source of new therapeutic strategies for treatment of skeletal disorders. Here, we demonstrate that protein kinase CK2 (casein kinase 2) controls sclerostin expression in osteocytes via the deubiquitinase ubiquitin-specific peptidase 4 (USP4)-mediated stabilization of Sirtuin1 (SIRT1). Deletion of CK2 regulatory subunit, Csnk2b, in osteocytes (Csnk2b^Dmp1) results in low bone mass due to elevated levels of sclerostin. This phenotype in Csnk2b^Dmp1 mice was partly reversed when sclerostin expression was downregulated by a single intravenous injection with bone-targeting adeno-associated virus 9 (AAV9) carrying an artificial-microRNA that targets Sost. Mechanistically, CK2-induced phosphorylation of USP4 is important for stabilization of SIRT1 by suppressing ubiquitin-dependent proteasomal degradation. Upregulated expression of SIRT1 inhibits sclerostin transcription in osteocytes. Collectively, the CK2-USP4-SIRT1 pathway is crucial for the regulation of sclerostin expression in osteocytes to maintain bone homeostasis.

Endothelial SIRT1 as a Target for the Prevention of Arterial Aging: Promises and Challenges

ABSTRACT

SIRT1, a member of the sirtuin family of longevity regulators, possesses potent activities preventing vascular aging. The expression and function of SIRT1 in endothelial cells are downregulated with age, in turn causing early vascular aging and predisposing various vascular abnormalities. Overexpression of SIRT1 in the vascular endothelium prevents aging-associated endothelial dysfunction and senescence, thus the development of hypertension and atherosclerosis. Numerous efforts have been directed to increase SIRT1 signaling as a potential strategy for different aging-associated diseases. However, the complex mechanisms underlying the regulation of SIRT1 have posed a significant challenge toward the design of specific and effective therapeutics. This review aimed to provide a summary on the regulation and function of SIRT1 in the vascular endothelium and to discuss the different approaches targeting this molecule for the prevention and treatment of age-related cardiovascular and cerebrovascular diseases.

JNK-mediated Ser27 phosphorylation and stabilization of SIRT1 promote growth and progression of colon cancer through deacetylation-dependent activation of Snail

Sirtuin 1 (SIRT1), an NAD⁺ -dependent histone/protein deacetylase, has multifaceted functions in various biological events such as inflammation, aging, and energy metabolism. The role of SIRT1 in carcinogenesis, however, is still under debate. Recent studies have indicated that aberrant overexpression of SIRT1 is correlated with metastasis and poor prognosis in several types of malignancy, including colorectal cancer. In the present study, we found that both SIRT1 and SIRT1 phosphorylated on serine 27 were coordinately upregulated in colon cancer patients' tissues and human colon cancer cell lines. This prompted us to investigate a role of phospho-SIRT1 in the context of colon cancer progression. A phosphorylation-defective mutant form of SIRT1, in which serine 27 was substituted by alanine (SIRT1-S27A), exhibited lower protein stability compared to that of wild-type SIRT1. Notably, human colon cancer (HCT-116) cells harboring the SIRT1-S27A mutation showed decreased cell proliferation and reduced capability to form xenograft tumor in athymic nude mice, which was accompanied by diminished transcriptional activity of Snail. HCT-116 cells carrying SIRT1-S27A were less capable of deacetylating the Snail protein, with a concomitant decrease in the levels of interleukin (IL)-6 and IL-8 mRNA transcripts. Taken together, these observations suggest that SIRT1 stabilized through phosphorylation on serine 27 exerts oncogenic effects at least partly through deacetylation-dependent activation of Snail and subsequent transcription of IL-6 and IL-8 in human colon cancer cells.

Anxiety and Cognition in Cre- Collagen Type II Sirt1 K/O Male Mice

Introduction

Using transgenic collagen type II-specific Sirt1 knockout (CKO) mice we studied the role of Sirt1 in nutritional induced catch up growth (CUG) and we found that these mice have a less organized growth plate and reduced efficiency of CUG. In addition, we noted that they weigh more than control (CTL) mice. Studying the reason for the increased weigh, we found differences in activity and brain function.

Methods

Several tests for behavior and activity were used: open field; elevated plus maze, Morris water maze, and home cage running wheels. The level of Glu- osteocalcin, known to connect bone and brain function, was measured by Elisa; brain Sirt1 was analyzed by western blot.

Results

We found that CKO mice had increased anxiety, with less spatial memory, learning capabilities and reduced activity in their home cages. No significant differences were found between CKO and CTL mice in Glu- osteocalcin levels; nor in the level of brain SIRT1.

Discussion/Conclusion

Using transgenic collagen type II-specific Sirt1 knockout (CKO) mice we found a close connection between linear growth and brain function. Using a collagen type II derived system we affected a central regulatory mechanism leading to hypo activity, increased anxiety, and slower learning, without affecting circadian period. As children with idiopathic short stature are more likely to have lower IQ, with substantial deficits in working memory than healthy controls, the results of the current study suggest that SIRT1 may be the underlying factor connecting growth and brain function.

CK2α/CSNK2A1 Induces Resistance to Doxorubicin through SIRT6-Mediated Activation of the DNA Damage Repair Pathway

CK2α/CSNK2A1 is involved in cancer progression by phosphorylating various signaling molecules. Considering the role of CSNK2A1 in cancer progression and the phosphorylation of SIRT6 and the role of SIRT6 in chemoresistance through the DNA damage repair pathway, CSNK2A1 and SIRT6 might be involved in resistance to conventional anti-cancer therapies. We evaluated the expression of CSNK2A1 and phosphorylated SIRT6 in the 37 osteosarcoma patients and investigated the effects of CSNK2A1 and the phosphorylation of SIRT6 on Ser338 on resistance to the anti-cancer effects of doxorubicin. Higher expression of CSNK2A1 and phosphorylated SIRT6 was associated with shorter survival in osteosarcoma patients. U2OS and KHOS/NP osteosarcoma cells with induced overexpression of CSNK2A1 were resistant to the cytotoxic effects of doxorubicin, and the knock-down of CSNK2A1 potentiated the cytotoxic effects of doxorubicin. CSNK2A1 overexpression-mediated resistance to doxorubicin was associated with SIRT6 phosphorylation and the induction of the DNA damage repair pathway molecules. CSNK2A1- and SIRT6-mediated resistance to doxorubicin in vivo was attenuated via mutation of SIRT6 at the Ser338 phosphorylation site. Emodin, a CSNK2A1 inhibitor, potentiated the cytotoxic effects of doxorubicin in osteosarcoma cells. This study suggests that blocking the CSNK2A1-SIRT6-DNA damage repair pathway might be a new therapeutic stratagem for osteosarcomas.

Role of Protein Kinase CK2 in Aberrant Lipid Metabolism in Cancer

Uncontrolled proliferation is a feature defining cancer and it is linked to the ability of cancer cells to effectively adapt their metabolic needs in response to a harsh tumor environment. Metabolic reprogramming is considered a hallmark of cancer and includes increased glucose uptake and processing, and increased glutamine utilization, but also the deregulation of lipid and cholesterol-associated signal transduction, as highlighted in recent years. In the first part of the review, we will (i) provide an overview of the major types of lipids found in eukaryotic cells and their importance as mediators of intracellular signaling pathways (ii) analyze the main metabolic changes occurring in cancer development and the role of oncogenic signaling in supporting aberrant lipid metabolism and (iii) discuss combination strategies as powerful new approaches to cancer treatment. The second part of the review will address the emerging role of CK2, a conserved serine/threonine protein kinase, in lipid homeostasis with an emphasis regarding its function in lipogenesis and adipogenesis. Evidence will be provided that CK2 regulates these processes at multiple levels. This suggests that its pharmacological inhibition combined with dietary restrictions and/or inhibitors of metabolic targets could represent an effective way to undermine the dependency of cancer cells on lipids to interfere with tumor progression.

Improved mass spectrometry-based activity assay reveals oxidative and metabolic stress as sirtuin-1 regulators

Sirtuin-1 (SirT1) catalyzes NAD⁺-dependent protein lysine deacetylation and is a critical regulator of energy and lipid metabolism, mitochondrial biogenesis, apoptosis, and senescence. Activation of SirT1 mitigates metabolic perturbations associated with diabetes and obesity. Pharmacologic molecules, cellular redox, and nutritional states can regulate SirT1 activity.

Technical barriers against measuring endogenous SirT1 activity have limited characterization of SirT1 in disease and its activation by small molecules. Herein, we developed a relative quantitative mass spectrometry-based technique for measuring endogenous SirT1 activity (RAMSSAY/RelAtive Mass Spectrometry Sirt1 Activity assaY) in cell and tissue homogenates using a biotin-labeled, acetylated p53-derived peptide as a substrate.

We demonstrate that oxidative and metabolic stress diminish SirT1 activity in the hepatic cell line HepG2. Moreover, pharmacologic molecules including nicotinamide and EX-527 attenuate SirT1 activity; purported activators of SirT1, the polyphenol S17834, the polyphenol resveratrol, or the non-polyphenolic Sirtris compound SRT1720, failed to activate endogenous SirT1 significantly. Furthermore, we provide evidence that feeding a high fat high sucrose diet (HFHS) to mice inhibits endogenous SirT1 activity in mouse liver.

In summary, we introduce a robust, specific and sensitive mass spectrometry-based assay for detecting and quantifying endogenous SirT1 activity using a biotin-labeled peptide in cell and tissue lysates. With this assay, we determine how pharmacologic molecules and metabolic and oxidative stress regulate endogenous SirT1 activity. The assay may also be adapted for other sirtuin isoforms.

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Fast, sensitive, and specific MALDI-TOF based sirtuin-1 activity assay applicable to cell and tissue lysates.

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Oxidative and metabolic stress inhibit Sirtuin-1 deacetylase activity.

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Purported activators of SirT1failed to significantly activate endogenous SirT1.

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The activity assay is adaptable to other sirtuin isoforms using specific synthetic peptides and assay conditions.

Histone deacetylase SIRT6 regulates chemosensitivity in liver cancer cells via modulation of FOXO3 activity

Liver cancer is the leading cause of cancer-related mortality worldwide and its incidence is increasing. Considerable effort has been made in recent decades to improve the diagnosis and treatment of liver cancer. Advanced liver cancer often exhibits a poor response to chemotherapy and poor prognosis due to acquired chemoresistance and tumor recurrence. Understanding the precise molecular mechanisms that are responsible for chemotherapeutic drug-induced cell death could potentially identify novel therapeutic targets and improve liver cancer treatment. In the present study, it was demonstrated that in response to doxorubicin, the most frequently used chemical compound for liver cancer treatment, histone deacetylase sirtuin 6 (SIRT6) is specifically downregulated. This enables forkhead box O3 (FOXO3) upregulation, translocation into the nucleus and increased expression of its target genes p27 and Bim, which further induce apoptosis. Overexpression of SIRT6, but not enzyme-inactivated mutants, prevents FOXO3 translocation into the nucleus and doxorubicin-induced cell death. SIRT6 interacts with FOXO3 and this interaction increases FOXO3 ubiquitination and decreases its stability. Finally, it was identified that the effect of SIRT6 in preventing doxorubicin-induced cell death requires FOXO3. Overexpression of SIRT6 could not prevent doxorubicin-induced cell death in FOXO3-knockdown cells. Therefore, it was concluded that SIRT6 plays a central role in determining doxorubicin-induced cell death via modulation of FOXO3 activity. Therapeutic targeting of SIRT6 and/or FOXO3 may offer novel strategies for treatment of liver cancer.

Lentivirus-mediated down-regulation of CK2α inhibits proliferation and induces apoptosis of malignant lymphoma and leukemia cells

Casein kinase II subunit alpha (CK2α) is highly expressed in many malignant tumor tissues, including lymphomas and leukemia. To investigate the role of CK2α in cell proliferation and apoptosis of malignant lymphomas and leukemia, 2 lymphoma cell lines and one leukemia cell line were infected with CK2α shRNA lentivirus or negative control shRNA lentivirus, and stably infected cell lines were established. Real-time PCR and Western blot results showed that the mRNA and protein levels of CK2α were significantly reduced in CK2α knockdown cells. The tetrazolium-based colorimetric (MTT) assay found that down-regulation of CK2α inhibited the proliferation of these cells. Flow cytometry analysis showed that inhibition of CK2α induced cell cycle arrest and apoptosis of lymphoma and leukemia cells. In accordance with these, down-regulation of CK2α also reduced the protein levels of proliferating cell nuclear antigen (PCNA), cyclinD1, and bcl-2, and increased the protein expression of bax, cleaved caspase-3, cleaved caspase-9, and cleaved poly(ADP ribose) polymerase (PARP). Moreover, knockdown of CK2α impeded the growth of xenograft tumors in vivo. In summary, our study revealed that CK2α may contribute to the development of malignant lymphoma and leukemia, and serve as the therapeutic target of these malignant tumors.

GR-regulating Serine/Threonine Kinases: New Physiologic and Pathologic Implications

Glucocorticoid hormones, end products of the hypothalamic-pituitary-adrenal axis, virtually influence all human functions both in a basal homeostatic condition and under stress. The glucocorticoid receptor (GR), a nuclear hormone receptor superfamily protein, mediates these actions of glucocorticoids by acting as a ligand-dependent transcription factor. Because glucocorticoid actions are diverse and strong, many biological pathways adjust them in local tissues by targeting the GR signaling pathway as part of the regulatory loop coordinating complex human functions. Phosphorylation of GR protein by serine/threonine kinases is one of the major regulatory mechanisms for this communication. In this review, recent progress in research investigating GR phosphorylation by these kinases is discussed, along with the possible physiologic and pathophysiologic implications.

The protective role of Sirt1 in vascular tissue: its relationship to vascular aging and atherosclerosis

Cardiovascular disease (CVD) due to atherosclerosis is the main cause of death in both the elderly and patients with metabolic diseases, including diabetes. Aging processes contribute to the pathogenesis of atherosclerosis. Calorie restriction (CR) is recognized as a dietary intervention for promoting longevity and delaying age-related diseases, including atherosclerosis. Sirt1, an NAD⁺-dependent deacetylase, is considered an anti-aging molecule and is induced during CR. Sirt1 deacetylates target proteins and is linked to cellular metabolism, the redox state and survival pathways. Sirt1 expression/activation is decreased in vascular tissue undergoing senescence. Sirt1 deficiency in endothelial cells (ECs), vascular smooth muscle cells (VSMCs) and monocytes/macrophages contributes to increased oxidative stress, inflammation, foam cell formation, senescences impaired nitric oxide production and autophagy, thereby promoting vascular aging and atherosclerosis. Endothelial dysfunction, activation of monocytes/macrophages, and the functional and phenotypical plasticity of VSMCs are critically implicated in the pathogenesis of atherosclerosis through multiple mechanisms. Therefore, the activation of Sirt1 in vascular tissue, which includes ECs, monocytes/macrophages and VSMCs, may be a new therapeutic strategy against atherosclerosis and the increasing resistance to the metabolic disorder-related causal factors of CVD. In this review, we discuss the protective role of Sirt1 in the pathophysiology of vascular aging and atherosclerosis.

Oxidative stress promotes SIRT1 recruitment to the GADD34/PP1α complex to activate its deacetylase function

Phosphorylation of the eukaryotic translation initiation factor, eIF2α, by stress-activated protein kinases and dephosphorylation by the growth arrest and DNA damage-inducible protein (GADD34)-containing phosphatase is a central node in the integrated stress response. Mass spectrometry demonstrated GADD34 acetylation at multiple lysines. Substituting K³¹⁵ and K³²² with alanines or glutamines did not impair GADD34's ability to recruit protein phosphatase 1α (PP1α) or eIF2α, suggesting that GADD34 acetylation did not modulate eIF2α phosphatase activity. Arsenite (Ars)-induced oxidative stress increased cellular GADD34 levels and enhanced Sirtuin 1 (SIRT1) recruitment to assemble a cytoplasmic complex containing GADD34, PP1α, eIF2α and SIRT1. Induction of GADD34 in WT MEFs paralleled the dephosphorylation of eIF2α (phosphoserine-51) and SIRT1 (phosphoserine-47). By comparison, eIF2α and SIRT1 were persistently phosphorylated in Ars-treated GADD34-/- MEFs. Expressing WT GADD34, but not a mutant unable to bind PP1α in GADD34-/- MEFs restored both eIF2α and SIRT1 dephosphorylation. SIRT1 dephosphorylation increased its deacetylase activity, measured in vitro and in cells. Loss of function of GADD34 or SIRT1 enhanced cellular p-eIF2α levels and attenuated cell death following Ars exposure. These results highlighted a novel role for the GADD34/PP1α complex in coordinating the dephosphorylation and reactivation of eIF2α and SIRT1 to determine cell fate following oxidative stress.

SIRT1-mediated ERβ suppression in the endothelium contributes to vascular aging

SIRT1 has many important molecular functions in aging, and the estrogen receptors (ERs) have a vasculoprotective effect, although the detailed mechanism for the roles of SIRT1 and ERs in vascular aging remains unclear. We found that ERβ expression in the endothelium was reduced in aging mice, and the expression of ERα and SIRT1 did not change, while SIRT1 activity declined. Further investigation showed that the ERβ expression was regulated by SIRT1 through complexes of SIRT1‐PPARγ/RXR‐p300 that bind to a PPRE (PPAR response element) site on the ERβ promoter, and the declined SIRT1 function in aging mice was due to compromised phosphorylation at S154. A single‐mutant SIRT1‐C152(D) restored the reduced ERβ expression in the endothelium with minimized reactive oxygen species generation and DNA damage and increased mitochondrial function and fatty acid metabolism. In high‐fat diet aging mice, the endothelium‐specific delivery of ERβ or SIRT1‐C152(D) on the vascular wall reduced the circulating lipids with ameliorated vascular damage, including the restored vessel tension and blood pressure. We conclude that SIRT1‐mediated ERβ suppression in the endothelium contributes to vascular aging, and the modulation of SIRT1 phosphorylation through a single‐mutant SIRT1‐C152(D) restores this effect.

The interaction between acetylation and serine-574 phosphorylation regulates the apoptotic function of FOXO3

The multispecific transcription factor and tumor suppressor FOXO3 is an important mediator of apoptosis, but the mechanisms that control its proapoptotic function are poorly understood. There has long been evidence that acetylation promotes FOXO3-driven apoptosis and recently a specific JNK (c-Jun N-terminal kinase)-dependent S574 phosphorylated form (p-FOXO3) has been shown to be specifically apoptotic. This study examined whether acetylation and S574 phosphorylation act independently or in concert to regulate the apoptotic function of FOXO3. We observed that both sirtuins 1 and 7 (SIRT1 and SIRT7) are able to deacetylate FOXO3 in vitro and in vivo, and that lipopolysaccharide (LPS) treatment of THP-1 monocytes induced a rapid increase of FOXO3 acetylation, partly by suppression of SIRT1 and SIRT7. Acetylation was required for S574 phosphorylation and cellular apoptosis. Deacetylation of FOXO3 by SIRT activation or SIRT1 or SIRT7 overexpression prevented its S574 phosphorylation and blocked apoptosis in response to LPS. We also found that acetylated FOXO3 preferentially bound JNK1, and a mutant FOXO3 lacking four known acetylation sites (K242, 259, 290 and 569R) abolished JNK1 binding and failed to induce apoptosis. This interplay of acetylation and phosphorylation also regulated cell death in primary human peripheral blood monocytes (PBMs). PBMs isolated from alcoholic hepatitis patients had high expression of SIRT1 and SIRT7 and failed to induce p-FOXO3 and apoptosis in response to LPS. PBMs from healthy controls had lower SIRT1 and SIRT7 and readily formed p-FOXO3 and underwent apoptosis when similarly treated. These results reveal that acetylation is permissive for generation of the apoptotic form of FOXO3 and the activity of SIRT1 and particularly SIRT7 regulate this process in vivo, allowing control of monocyte apoptosis in response to LPS.

SIRT1 Activity Is Linked to Its Brain Region-Specific Phosphorylation and Is Impaired in Huntington's Disease Mice

Huntington’s disease (HD) is a neurodegenerative disorder for which there are no disease-modifying treatments. SIRT1 is a NAD⁺-dependent protein deacetylase that is implicated in maintaining neuronal health during development, differentiation and ageing. Previous studies suggested that the modulation of SIRT1 activity is neuroprotective in HD mouse models, however, the mechanisms controlling SIRT1 activity are unknown. We have identified a striatum-specific phosphorylation-dependent regulatory mechanism of SIRT1 induction under normal physiological conditions, which is impaired in HD. We demonstrate that SIRT1 activity is down-regulated in the brains of two complementary HD mouse models, which correlated with altered SIRT1 phosphorylation levels. This SIRT1 impairment could not be rescued by the ablation of DBC1, a negative regulator of SIRT1, but was linked to changes in the sub-cellular distribution of AMPK-α1, a positive regulator of SIRT1 function. This work provides insights into the regulation of SIRT1 activity with the potential for the development of novel therapeutic strategies.

Silent information regulator 1 (SIRT1) ameliorates liver fibrosis via promoting activated stellate cell apoptosis and reversion

SIRT1 (silent information regulator 1), a conserved NAD+-dependent histone deacetylase, is closely related with various biological processes. Moreover, the important role of SIRT1 in alcoholic liver disease, nonalcoholic fatty liver and HCC had been widely reported. Recently, a novel role of SIRT1 was uncovered in organ fibrosis diseases. Here, we investigated the inhibitory effect of SIRT1 in liver fibrogenesis. SIRT1 protein was dramatically decreased in CCl4-treated mice livers. Stimulation of LX-2 cells with TGF-β1 also resulted in a significant suppression of SIRT1 protein. Nevertheless, TGF-β1-induced LX-2 cell activation was inhibited by SIRT1 plasmid, and this was accompanied by up-regulation of cell apoptosis-related proteins. Overexpression of SIRT1 also attenuated TGF-β1-induced expression of myofibroblast markers α-SMA and COL1a. However, the important characteristic of the recovery of liver fibrosis is not only the apoptosis of activated stellate cells but also the reversal of the myofibroblast-like phenotype to a quiescent-like phenotype. Restoration of SIRT1 protein was observed in the in vivo spontaneously liver fibrosis reversion model and in vitro MDI (isobutylmethylxanthine, dexamethasone, and insulin)-induced reversed stellate cells, and forced expression of SIRT1 also promoted the reversal of activated stellate cells. Furthermore, lncRNA MALAT1 (metastasis-associated lung adenocarcinoma transcript 1) was increased in liver fibrosis. RNAi-mediated suppression of MALAT1 resulted in a decrease of myofibroblast markers and restoration of SIRT1 protein. These observations suggested that SIRT1 contributed to apoptosis and reversion of activated LX-2 cells and SIRT1 might be regulated by MALAT1 in liver fibrosis. Therefore, SIRT1 could be considered as a valuable therapeutic target for translational studies of liver fibrosis.

Sir2 phosphorylation through cAMP-PKA and CK2 signaling inhibits the lifespan extension activity of Sir2 in yeast

Silent information regulator 2 (Sir2), an NAD⁺-dependent protein deacetylase, has been proposed to be a longevity factor that plays important roles in dietary restriction (DR)-mediated lifespan extension. In this study, we show that the Sir2's role for DR-mediated lifespan extension depends on cAMP-PKA and casein kinase 2 (CK2) signaling in yeast. Sir2 partially represses the transcription of lifespan-associated genes, such as PMA1 (encoding an H⁺-ATPase) and many ribosomal protein genes, through deacetylation of Lys 16 of histone H4 in the promoter regions of these genes. This repression is relieved by Sir2 S473 phosphorylation, which is mediated by active cAMP-PKA and CK2 signaling. Moderate DR increases the replicative lifespan of wild-type yeast but has no effect on that of yeast expressing the Sir2-S473E or S473A allele, suggesting that the effect of Sir2 on DR-mediated lifespan extension is negatively regulated by S473 phosphorylation. Our results demonstrate a mechanism by which Sir2 contributes to lifespan extension.

DOI:http://dx.doi.org/10.7554/eLife.09709.001

We know that cutting calorie intake through a restricted diet can slow down the aging process and prolong the lives of many organisms ranging from yeast to mammals. Calorie restriction also has protective effects on various age-related diseases including neurodegenerative disorders, cardiovascular disease, and cancer. Many studies suggest that we may mimic the beneficial effects of calorie restriction by controlling the activities of some proteins involved in the aging process.

An enzyme called Sir2 is required for calorie restriction to be able to increase lifespan. This enzyme modifies proteins called histones, which are used to package DNA inside cells. In yeast, Sir2 modifies the histones in such a way that the genes contained in that section of DNA are inactivated (or ‘silenced’). As the yeast cells age, the activity of Sir2 declines, which allows these genes to become active and contribute to the aging process. However, when yeast cells are grown in the presence of little sugar—which mimics caloric restriction—Sir2 is activated and this restores gene silencing.

It is not clear how Sir2's ability to silence these genes contributes to prolonged lifespan. Kang et al. studied the role of Sir2 in yeast and observed that one of the genes that Sir2 inactivates is called PMA1. This gene encodes a protein that is known to restrict the lifespan of yeast cells. Further experiments show that other proteins attach or remove molecules called phosphate groups from Sir2 to regulate its activity. Sir2 is inactivated when a phosphate group is attached, and active in the absence of phosphate. Under a reduced diet, the proteins that add phosphate to Sir2 are inactive, which allows Sir2 to become active and reduce the expression of the PMA1 gene.

These results show that Sir2 fine-tunes the expression of PMA1 and other age-related genes and that the attachment of phosphate groups to Sir2 by other proteins interferes with this regulation. The next challenges will be to identify the proteins responsible for attaching phosphate groups to Sir2, and to find out how they work.

DOI:http://dx.doi.org/10.7554/eLife.09709.002

P53 and Sirt1: routes of metabolism and genome stability

The tumor suppressor p53 is a transcription factor that regulates key processes. But, the outcomes of the p53 response go beyond its role as a nuclear transcription factor. Sirtuin (SIRT1) regulates p53 functions as transcription factor. At the same time, SIRT1 protects the genome under stress conditions. The link between p53 and SIRT1 responses is unique. Both regulate metabolism, stress signaling, cell survival, cell cycle control and genome stability. Recent studies have proposed cancer as a metabolic disease. This is due to the switch from aerobic to anaerobic metabolism during tumor development. Yet, the complex molecular circuits (in and out of the nucleus) of tumor progression remain elusive. In this review, we will focus on the interplay between p53 and SIRT1. We will discuss their roles as nodes for possible therapeutic intervention.

CK2α phosphorylates DBC1 and is involved in the progression of gastric carcinoma and predicts poor survival of gastric carcinoma patients

CK2α has diverse effects on the tumorigenesis owing to its kinase activity, which phosphorylates various proteins involved in tumorigenesis. We, therefore, investigated the expression and role of CK2α in the phosphorylation of deleted in breast cancer 1 (DBC1) in gastric carcinomas. We used 187 gastric carcinomas and human gastric cancer cells to investigate the roles and relationship between CK2α and DBC1 in gastric carcinomas. Positive expression of CK2α and phospho-DBC1 predicted shorter overall survival and relapse-free survival by univariate analysis. Especially, CK2α expression was an independent prognostic indicator for gastric carcinoma patients. In gastric carcinoma cells, CK2α was bound to DBC1 and phosphorylated DBC1. The phosphorylation of DBC1 by CK2α was evidenced by co-immunoprecipitation of CK2α and DBC1 in a GST pull-down assay, an in vitro kinase assay, and immunofluorescence staining. Inhibition of both CK2α and DBC1 decreased proliferation and invasive activity of cancer cells. Decreased migration and invasive activity was associated with a downregulation of MMP2, MMP9 and the epithelial-mesenchymal transition. A mutation at the phosphorylation site of DBC1 also downregulated the signals related with the epithelial-mesenchymal transition. Our study demonstrated that CK2α is an independent prognostic indicator for gastric carcinoma patients and is involved in tumorigenesis by regulating the phosphorylation of DBC1. In addition, the blocking of CK2α and DBC1 inhibited the proliferation and invasive potential of gastric cancer cells. Therefore, our study suggests that CK2α-DBC1 pathway might be a new therapeutic target for the treatment of gastric carcinoma.

A redox-resistant sirtuin-1 mutant protects against hepatic metabolic and oxidant stress

Sirtuin-1 (SirT1), a member of the NAD(+)-dependent class III histone deacetylase family, is inactivated in vitro by oxidation of critical cysteine thiols. In a model of metabolic syndrome, SirT1 activation attenuated apoptosis of hepatocytes and improved liver function including lipid metabolism. We show in SirT1-overexpressing HepG2 cells that oxidants (nitrosocysteine and hydrogen peroxide) or metabolic stress (high palmitate and high glucose) inactivated SirT1 by reversible oxidative post-translational modifications (OPTMs) on three cysteines. Mutating these oxidation-sensitive cysteines to serine preserved SirT1 activity and abolished reversible OPTMs. Overexpressed mutant SirT1 maintained deacetylase activity and attenuated proapoptotic signaling, whereas overexpressed wild type SirT1 was less protective in metabolically or oxidant-stressed cells. To prove that OPTMs of SirT1 are glutathione (GSH) adducts, glutaredoxin-1 was overexpressed to remove this modification. Glutaredoxin-1 overexpression maintained endogenous SirT1 activity and prevented proapoptotic signaling in metabolically stressed HepG2 cells. The in vivo significance of oxidative inactivation of SirT1 was investigated in livers of high fat diet-fed C57/B6J mice. SirT1 deacetylase activity was decreased in the absence of changes in SirT1 expression and associated with a marked increase in OPTMs. These results indicate that glutathione adducts on specific SirT1 thiols may be responsible for dysfunctional SirT1 associated with liver disease in metabolic syndrome.

SIRT1/PARP1 crosstalk: connecting DNA damage and metabolism

An intricate network regulates the activities of SIRT1 and PARP1 proteins and continues to be uncovered. Both SIRT1 and PARP1 share a common co-factor nicotinamide adenine dinucleotide (NAD+) and several common substrates, including regulators of DNA damage response and circadian rhythms. We review this complex network using an interactive Molecular Interaction Map (MIM) to explore the interplay between these two proteins. Here we discuss how NAD + competition and post-transcriptional/translational feedback mechanisms create a regulatory network sensitive to environmental cues, such as genotoxic stress and metabolic states, and examine the role of those interactions in DNA repair and ultimately, cell fate decisions.

Global screening of CK2 kinase substrates by an integrated phosphoproteomics workflow

Due to its constitutive activity and ubiquitous distribution, CK2 is the most pleiotropic kinase among the individual members of the protein kinase superfamily. Identification of CK2 substrates is vital to decipher its role in biological processes. However, only a limited number of CK2 substrates were identified so far. In this study, we developed an integrated phosphoproteomics workflow to identify the CK2 substrates in large scale. First, in vitro kinase reactions with immobilized proteomes were combined with quantitative phosphoproteomics to identify in vitro CK2 phosphorylation sites, which leaded to identification of 988 sites from 581 protein substrates. To reduce false positives, we proposed an approach by comparing these in vitro sites with the public databases that collect in vivo phosphorylation sites. After the removal of the sites that were excluded in the databases, 605 high confident CK2 sites corresponding to 356 proteins were retained. The CK2 substrates identified in this study were based on the discovery mode, in which an unbiased overview of CK2 substrates was provided. Our result revealed that CK2 substrates were significantly enriched in the spliceosomal proteins, indicating CK2 might regulate the functions of spliceosome.

Glutaredoxin regulates vascular development by reversible glutathionylation of sirtuin 1

Embryonic development depends on complex and precisely orchestrated signaling pathways including specific reduction/oxidation cascades. Oxidoreductases of the thioredoxin family are key players conveying redox signals through reversible posttranslational modifications of protein thiols. The importance of this protein family during embryogenesis has recently been exemplified for glutaredoxin 2, a vertebrate-specific glutathione-disulfide oxidoreductase with a critical role for embryonic brain development. Here, we discovered an essential function of glutaredoxin 2 during vascular development. Confocal microscopy and time-lapse studies based on two-photon microscopy revealed that morpholino-based knockdown of glutaredoxin 2 in zebrafish, a model organism to study vertebrate embryogenesis, resulted in a delayed and disordered blood vessel network. We were able to show that formation of a functional vascular system requires glutaredoxin 2-dependent reversible S-glutathionylation of the NAD(+)-dependent protein deacetylase sirtuin 1. Using mass spectrometry, we identified a cysteine residue in the conserved catalytic region of sirtuin 1 as target for glutaredoxin 2-specific deglutathionylation. Thereby, glutaredoxin 2-mediated redox regulation controls enzymatic activity of sirtuin 1, a mechanism we found to be conserved between zebrafish and humans. These results link S-glutathionylation to vertebrate development and successful embryonic angiogenesis.

Stress Inducibility of SIRT1 and Its Role in Cytoprotection and Cancer

Cells must continuously respond to stressful insults via the upregulation of cytoprotective pathways. The longevity factor and deacetylase SIRT1 plays a critical role in coordinating this cellular response to stress. SIRT1 activity and levels are regulated by cellular stressors, including metabolic, genotoxic, oxidative, and proteotoxic stress. As a stress sensor, SIRT1 impacts cell survival by deacetylating substrate proteins to drive the cell towards a cytoprotective pathway. Extreme stress conditions, however, can cause SIRT1 to lead cells down an apoptotic pathway instead. SIRT1 is frequently dysregulated in cancer cells and has been characterized to have a dual role as both an oncogene and a tumor suppressor, likely due to its pivotal function in regulating cytoprotection. Recently, the ability of SIRT1 to regulate HSF1-dependent induction of the heat shock response has highlighted another pathway through which SIRT1 can modulate cytoprotection. Activation of HSF1 results in the production of cytoprotective chaperones that can facilitate the transformed phenotype of cancer cells. In this review, we discuss the stress-dependent regulation of SIRT1. We highlight the role of SIRT1 in stress management and cytoprotection and emphasize SIRT1-dependent activation of HSF1 as a potential mechanism for cancer promotion.

SIRT1: Regulator of p53 Deacetylation

The cellular NAD(+)/NADH level controls Sir2 (silent information regulator 2) deacetylase activity in regulating aging in lower species. Much work has been put forth to identify ways to activate SIRT1, the mammalian ortholog of Sir2. The identification of p53 as a bona fide substrate of SIRT1 deacetylation has linked SIRT1 to a role in tumorigenesis. Here, we review the various SIRT1 endogenous and small molecular activators and inhibitors that regulate p53 acetylation and subsequent activation of p53 tumor suppression activity.

Redox regulation of SIRT1 in inflammation and cellular senescence

Sirtuin 1 (SIRT1) regulates inflammation, aging (life span and health span), calorie restriction/energetics, mitochondrial biogenesis, stress resistance, cellular senescence, endothelial functions, apoptosis/autophagy, and circadian rhythms through deacetylation of transcription factors and histones. SIRT1 level and activity are decreased in chronic inflammatory conditions and aging, in which oxidative stress occurs. SIRT1 is regulated by a NAD(+)-dependent DNA repair enzyme, poly(ADP-ribose) polymerase-1 (PARP1), and subsequent NAD(+) depletion by oxidative stress may have consequent effects on inflammatory and stress responses as well as cellular senescence. SIRT1 has been shown to undergo covalent oxidative modifications by cigarette smoke-derived oxidants/aldehydes, leading to posttranslational modifications, inactivation, and protein degradation. Furthermore, oxidant/carbonyl stress-mediated reduction of SIRT1 leads to the loss of its control on acetylation of target proteins including p53, RelA/p65, and FOXO3, thereby enhancing the inflammatory, prosenescent, and apoptotic responses, as well as endothelial dysfunction. In this review, the mechanisms of cigarette smoke/oxidant-mediated redox posttranslational modifications of SIRT1 and its roles in PARP1 and NF-κB activation, and FOXO3 and eNOS regulation, as well as chromatin remodeling/histone modifications during inflammaging, are discussed. Furthermore, we have also discussed various novel ways to activate SIRT1 either directly or indirectly, which may have therapeutic potential in attenuating inflammation and premature senescence involved in chronic lung diseases.

Redox-regulating sirtuins in aging, caloric restriction, and exercise

The consequence of decreased nicotinamide adenine dinucleotide (NAD(+)) levels as a result of oxidative challenge is altered activity of sirtuins, which, in turn, brings about a wide range of modifications in mammalian cellular metabolism. Sirtuins, especially SIRT1, deacetylate important transcription factors such as p53, forkhead homeobox type O proteins, nuclear factor κB, or peroxisome proliferator-activated receptor γ coactivator 1α (which controls the transcription of pro- and antioxidant enzymes, by which the cellular redox state is affected). The role of SIRT1 in DNA repair is enigmatic, because it activates Ku70 to cope with double-strand breaks, but deacetylation of apurinic/apyrimidinic endonuclease 1 and probably of 8-oxoguanine-DNA glycosylase 1 decreases the activity of these DNA repair enzymes. The protein-stabilizing effects of the NAD+-dependent lysine deacetylases are readily related to housekeeping and redox regulation. The role of sirtuins in caloric restriction (CR)-related longevity in yeast is currently under debate. However, in mammals, it seems certain that sirtuins are involved in many cellular processes that mediate longevity and disease prevention via the effects of CR through the vascular, neuronal, and muscular systems. Regular physical exercise-mediated health promotion also involves sirtuin-regulated pathways including the antioxidant-, macromolecular damage repair-, energy-, mitochondrial function-, and neuronal plasticity-associated pathways. This review critically evaluates these findings and points out the age-associated role of sirtuins.

The ways and means that fine tune Sirt1 activity

Sirt1 is the most evolutionarily conserved mammalian sirtuin. It plays a vital role in the regulation of metabolism, stress responses, genome stability, and ultimately aging. Although much attention has focused on the identification of the cellular targets and functional networks controlled by Sirt1, the mechanisms that regulate Sirt1 activity by biological stimuli have only recently begun to emerge. As an enzyme, the activity of Sirt1 can be controlled by the availability of its substrates, post-translational modifications, interactions with other proteins, or changes in its expression levels. In this review, we briefly discuss the ways and means by which the activity of Sirt1 is fine-tuned under different conditions.

Sirtuin1 single nucleotide polymorphism (A2191G) is a diagnostic marker for vibration-induced white finger disease

Background

Vibration-induced white finger disease (VWF), also known as hand-arm vibration syndrome, is a secondary form of Raynaud’s disease, affecting the blood vessels and nerves. So far, little is known about the pathogenesisof the disease. VWF is associated with an episodic reduction in peripheral blood flow. Sirtuin 1, a class III histone deacetylase, has been described to regulate the endothelium dependent vasodilation by targeting endothelial nitric oxide synthase. We assessed Sirt1single nucleotide polymorphisms in patients with VWF to further elucidate the role of sirtuin 1 in the pathogenesis of VWF.

Methods

Peripheral blood samples were obtained from 74 patients with VWF (male 93.2%, female 6.8%, median age 53 years) and from 317 healthy volunteers (gender equally distributed, below 30 years of age). Genomic DNA was extracted from peripheral blood mononuclear cells and screened for potential Sirt1single nucleotide polymorphisms. Four putative genetic polymorphisms out of 113 within the Sirt1 genomic region (NCBI Gene Reference: NM_012238.3) were assessed. Allelic discrimination was performed by TaqMan-polymerasechainreaction-based allele-specific genotyping single nucleotide polymorphism assays.

Results

Sirt1single nucleotide polymorphism A2191G (Assay C_25611590_10, rs35224060) was identified within Sirt1 exon 9 (amino acid position 731, Ile → Val), with differing allelic frequencies in the VWF population (A/A: 70.5%, A/G: 29.5%, G/G: 0%) and the control population (A/A: 99.7%, A/G: 0.3%, G/G: 0.5%), with significance levels of P < 0.001 (Mann–Whitney U test (two-tailed) P <0.001; F-exact t-test and Chi-square test with Yates correction (all two-tailed): P <0.0001). The heterogeneous A/G genotype in base pair position 2191 is significantly overrepresented in the VWF patient population when compared with healthy controls.

Conclusion

We identified theSirt1_A2191Gsingle nucleotide polymorphism as a diagnostic marker for VWF.

Suppression of centrosome duplication and amplification by deacetylases

Centrosome duplication is controlled both negatively and positively by a number of proteins. The activities and stabilities of those regulatory proteins are in many cases controlled by posttranslational modifications. Although acetylation and deacetylation are highly common posttranslational modifications, their roles in the regulation of centrosome duplication had not been closely examined. Here, through focusing on the deacetylases, we investigated the role of acetylation/deacetylation in the regulation of centrosome duplication and induction of abnormal amplification of centrosomes. We found that the deacetylation event negatively controls centrosome duplication and amplification. Of the 18 total known deacetylases (HDAC1-11, SIRT1-7), ten deacetylases possess the activity to suppress centrosome amplification, and their centrosome amplification suppressing activities are strongly associated with their abilities to localize to centrosomes. Among them, HDAC1, HDAC5 and SIRT1 show the highest suppressing activities, but each of them suppresses centrosome duplication and/or amplification with its unique mechanism.

Cyclin-dependent kinase 5-mediated hyperphosphorylation of sirtuin-1 contributes to the development of endothelial senescence and atherosclerosis

BACKGROUND

Endothelial senescence represents one of the major characteristics of vascular aging and promotes the development of atherosclerosis. Sirtuin-1 (SIRT1) is an NAD-dependent deacetylase possessing antiaging activities. During the occurrence of endothelial senescence, both the expression and activity of SIRT1 are downregulated. The present study was designed to investigate the molecular mechanisms contributing to the loss-of-SIRT1 function in senescent endothelial cells.

METHODS AND RESULTS

After repetitive passages, primary cultures of porcine aortic endothelial cells exhibited a severe senescence phenotype. Western blotting revealed that phosphorylation of SIRT1 at serine 47 (S47) was significantly enhanced in senescent endothelial cells. S47 phosphorylation was stimulated by agents promoting senescence and attenuated by drugs with antisenescence properties. Mutation of S47 to nonphosphorable alanine (S47A) enhanced whereas replacing S47 with phospho-mimicking aspartic acid (S47D) abolished the antisenescent, growth-promoting, and LKB1-downregulating actions of SIRT1. Phosphorylation at S47 was critically involved in the nuclear retention of SIRT1 but abolished its association with the telomeric repeat-binding factor 2-interacting protein 1. Cyclin-dependent kinase 5 (CDK5) was identified as an SIRT1 kinase modulating S47 phosphorylation. Knockdown or inhibition of CDK5 reduced the number of senescent endothelial cells, promoted nuclear exportation of SIRT1, and attenuated the expression of inflammatory genes in porcine aortic endothelial cells. The truncated regulatory subunit of CDK5, P25, accumulated in senescent porcine aortic endothelial cells and atherosclerotic aortas. Long-term treatment with roscovitine, a CDK5 inhibitor, blocked the development of cellular senescence and atherosclerosis in aortas of hypercholesterolemic apolipoprotein E-deficient mice.

CONCLUSION

CDK5-mediated hyperphosphorylation of SIRT1 facilitates the development of endothelial senescence and atherosclerosis.

Role of deleted in breast cancer 1 (DBC1) protein in SIRT1 deacetylase activation induced by protein kinase A and AMP-activated protein kinase

The NAD(+)-dependent deacetylase SIRT1 is a key regulator of several aspects of metabolism and aging. SIRT1 activation is beneficial for several human diseases, including metabolic syndrome, diabetes, obesity, liver steatosis, and Alzheimer disease. We have recently shown that the protein deleted in breast cancer 1 (DBC1) is a key regulator of SIRT1 activity in vivo. Furthermore, SIRT1 and DBC1 form a dynamic complex that is regulated by the energetic state of the organism. Understanding how the interaction between SIRT1 and DBC1 is regulated is therefore essential to design strategies aimed to activate SIRT1. Here, we investigated which pathways can lead to the dissociation of SIRT1 and DBC1 and consequently to SIRT1 activation. We observed that PKA activation leads to a fast and transient activation of SIRT1 that is DBC1-dependent. In fact, an increase in cAMP/PKA activity resulted in the dissociation of SIRT1 and DBC1 in an AMP-activated protein kinase (AMPK)-dependent manner. Pharmacological AMPK activation led to SIRT1 activation by a DBC1-dependent mechanism. Indeed, we found that AMPK activators promote SIRT1-DBC1 dissociation in cells, resulting in an increase in SIRT1 activity. In addition, we observed that the SIRT1 activation promoted by PKA and AMPK occurs without changes in the intracellular levels of NAD(+). We propose that PKA and AMPK can acutely activate SIRT1 by inducing dissociation of SIRT1 from its endogenous inhibitor DBC1. Our experiments provide new insight on the in vivo mechanism of SIRT1 regulation and a new avenue for the development of pharmacological SIRT1 activators targeted at the dissociation of the SIRT1-DBC1 complex.

Regulation of sirtuin function by posttranslational modifications

Sirtuins are homologs of the yeast silencing information regulator 2 protein, an NAD⁺-dependent (histone) deacetylase. In mammals seven different sirtuins, SIRT1–7, have been identified, which share a common catalytic core domain but possess distinct N- and C-terminal extensions. This core domain elicits NAD⁺-dependent deacetylase and in some cases also ADP-ribosyltransferase, demalonylase, and desuccinylase activities. Sirtuins have been implicated in key cellular processes, including cell survival, autophagy, apoptosis, gene transcription, DNA repair, stress response, and genome stability. In addition some sirtuins are associated with disease, including cancer and neurodegeneration. These findings suggest strongly that sirtuins are tightly controlled and potentially responsive to different signal transduction pathways. Here, we review the posttranslational regulation mechanisms of mammalian sirtuins and discuss their relevance regarding the physiological processes, with which the different sirtuins are associated. The available data suggest that the N- and C-terminal extensions are the targets of posttranslational modifications (PTM) that can affect the functions of sirtuins. Mechanistically this can be explained by the interaction of these extensions with the catalytic core domain, which appears to be controlled by PTM at least in some cases. In contrast little is known about PTM and regulation of the catalytic domain itself. Together these findings point to key regulatory roles of the N- and C-terminal extensions in controlling sirtuin functions, thus connecting these regulators to different signaling pathways.

Targeting sirtuin 1 to improve metabolism: all you need is NAD(+)?

Sirtuin 1 (SIRT1) is an evolutionarily conserved NAD(+)-dependent deacetylase that is at the pinnacle of metabolic control, all the way from yeast to humans. SIRT1 senses changes in intracellular NAD(+) levels, which reflect energy level, and uses this information to adapt the cellular energy output such that it matches cellular energy requirements. The changes induced by SIRT1 activation are generally (but not exclusively) transcriptional in nature and are related to an increase in mitochondrial metabolism and antioxidant protection. These attractive features have validated SIRT1 as a therapeutic target in the management of metabolic disease and prompted an intensive search to identify pharmacological SIRT1 activators. In this review, we first give an overview of the SIRT1 biology with a particular focus on its role in metabolic control. We then analyze the pros and cons of the current strategies used to activate SIRT1 and explore the emerging evidence indicating that modulation of NAD(+) levels could provide an effective way to achieve such goals.

Deacetylation of PGC-1α by SIRT1: importance for skeletal muscle function and exercise-induced mitochondrial biogenesis

Activation of peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α)-mediated transcription is important for both the determination of mitochondrial content and the induction of mitochondrial biogenesis in skeletal muscle. SIRT1 (silent mating type information regulator 2 homolog 1) deactetylation is proposed as a potential activator of PGC-1α transcriptional activity. The current review examines the importance of SIRT1 deacetylation of PGC-1α in skeletal muscle. Models of SIRT1 overexpression and pharmacological activation are examined, but changes in SIRT1 expression and deacetylase activity following acute and chronic contractile activity will be emphasized. In addition, potential mechanisms of SIRT1 activation in skeletal muscle will be examined. The importance of the PGC-1α acetyltransferase GCN5 will also be briefly discussed. The current evidence supports the contribution of SIRT1 deacetylation of PGC-1α to exercise-induced mitochondrial biogenesis. Further research examining exercise-mediated activation of SIRT1 and the role of GCN5 in regulating PGC-1α transcriptional activity in skeletal muscle is required.

Protective effects and mechanisms of sirtuins in the nervous system

Silent information regulator two proteins (sirtuins or SIRTs) are a group of histone deacetylases whose activities are dependent on and regulated by nicotinamide adenine dinucleotide (NAD(+)). They suppress genome-wide transcription, yet upregulate a select set of proteins related to energy metabolism and pro-survival mechanisms, and therefore play a key role in the longevity effects elicited by calorie restriction. Recently, a neuroprotective effect of sirtuins has been reported for both acute and chronic neurological diseases. The focus of this review is to summarize the latest progress regarding the protective effects of sirtuins, with a focus on SIRT1. We first introduce the distribution of sirtuins in the brain and how their expression and activity are regulated. We then highlight their protective effects against common neurological disorders, such as cerebral ischemia, axonal injury, Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis. Finally, we analyze the mechanisms underlying sirtuin-mediated neuroprotection, centering on their non-histone substrates such as DNA repair enzymes, protein kinases, transcription factors, and coactivators. Collectively, the information compiled here will serve as a comprehensive reference for the actions of sirtuins in the nervous system to date, and will hopefully help to design further experimental research and expand sirtuins as therapeutic targets in the future.

Sirtuins: molecular traffic lights in the crossroad of oxidative stress, chromatin remodeling, and transcription

Transcription is regulated by acetylation/deacetylation reactions of histone and nonhistone proteins mediated by enzymes called KATs and HDACs, respectively. As a major mechanism of transcriptional regulation, protein acetylation is a key controller of physiological processes such as cell cycle, DNA damage response, metabolism, apoptosis, and autophagy. The deacetylase activity of class III histone deacetylases or sirtuins depends on the presence of NAD⁺ (nicotinamide adenine dinucleotide), and therefore, their function is closely linked to cellular energy consumption. This activity of sirtuins connects the modulation of chromatin dynamics and transcriptional regulation under oxidative stress to cellular lifespan, glucose homeostasis, inflammation, and multiple aging-related diseases including cancer. Here we provide an overview of the recent developments in relation to the diverse biological activities associated with sirtuin enzymes and stress responsive transcription factors, DNA damage, and oxidative stress and relate the involvement of sirtuins in the regulation of these processes to oncogenesis. Since the majority of the molecular mechanisms implicated in these pathways have been described for Sirt1, this sirtuin family member is more extensively presented in this paper.

Pathways for ischemic cytoprotection: role of sirtuins in caloric restriction, resveratrol, and ischemic preconditioning

Caloric restriction (CR), resveratrol, and ischemic preconditioning (IPC) have been shown to promote protection against ischemic injury in the heart and brain, as well as in other tissues. The activity of sirtuins, which are enzymes that modulate diverse biologic processes, seems to be vital in the ability of these therapeutic modalities to prevent against cellular dysfunction and death. The protective mechanisms of the yeast Sir2 and the mammalian homolog sirtuin 1 have been extensively studied, but the involvement of other sirtuins in ischemic protection is not yet clear. We examine the roles of mammalian sirtuins in modulating protective pathways against oxidative stress, energy depletion, excitotoxicity, inflammation, DNA damage, and apoptosis. Although many of these sirtuins have not been directly implicated in ischemic protection, they may have unique roles in enhancing function and preventing against stress-mediated cellular damage and death. This review will include in-depth analyses of the roles of CR, resveratrol, and IPC in activating sirtuins and in mediating protection against ischemic damage in the heart and brain.

Hypoxia increases sirtuin 1 expression in a hypoxia-inducible factor-dependent manner

Hypoxia-inducible factors (HIFs) are stress-responsive transcriptional regulators of cellular and physiological processes involved in oxygen metabolism. Although much is understood about the molecular machinery that confers HIF responsiveness to oxygen, far less is known about HIF isoform-specific mechanisms of regulation, despite the fact that HIF-1 and HIF-2 exhibit distinct biological roles. We recently determined that the stress-responsive genetic regulator sirtuin 1 (Sirt1) selectively augments HIF-2 signaling during hypoxia. However, the mechanism by which Sirt1 maintains activity during hypoxia is unknown. In this report, we demonstrate that Sirt1 gene expression increases in a HIF-dependent manner during hypoxia in Hep3B and in HT1080 cells. Impairment of HIF signaling affects Sirt1 deacetylase activity as decreased HIF-1 signaling results in the appearance of acetylated HIF-2α, which is detected without pharmacological inhibition of Sirt1. We also find that Sirt1 augments HIF-2 mediated, but not HIF-1 mediated, transcriptional activation of the isolated Sirt1 promoter. These data in summary reveal a bidirectional link of HIF and Sirt1 signaling during hypoxia.