Loss of SIRT3 Provides Growth Advantage for B Cell Malignancies

Sirt3 enhances glioma cell viability by stabilizing Ku70-BAX interaction

Background

As one of the most prevalent malignancies, glioma is characterized by poor prognosis and high mortality rate. Glioma patients may show completely distinct clinical outcomes due to their different molecular patterns. Sirtuin 3 (Sirt3) participates in aging, stress resistance, and metabolic regulation. Here we aimed to test the expression and function of Sirt3 in glioma.

Methods

We enrolled 114 patients and tested the protein level of Sirt3 in their glioma tissues. The correlation between prognosis and Sirt3 was evaluated by univariate and multivariate analyses. We also conducted cellular experiments in U87 and U251 glioma cells, including overexpression and knockdown assays.

Results

Sirt3 expression was lower in glioma tissues than normal brain tissues. Higher Sirt3 is significantly correlated to advanced tumor grade (P=0.004), showing its potential role in cancer progression. Consistently, univariate and multivariate analyses identified Sirt3 as an independent prognostic factor (P=0.017). Patients with higher Sirt3 expression showed significantly shorter overall survival time. Moreover, overexpression of Sirt3 in either cell line enhanced cell viability, while silencing Sirt3 attenuated cell growth. Molecular assays showed Sirt3 can deacetylate Ku70 protein, therefore stabilizing the Ku70-BAX interaction. Since Ku70 can help prevent BAX transporting into mitochondria and decrease cell apoptosis, Sirt3 protein may play roles in maintaining cell viability. In addition, silencing Ku70 inhibited the pro-proliferative effect by Sirt3.

Conclusion

Taken together, our results not only identified the prognostic role of Sirt3 in glioma patients but also provided signaling pathway evidence for its functioning mechanisms.

Detection of mitochondria-generated reactive oxygen species in cells using multiple probes and methods: Potentials, pitfalls, and the future

Reactive oxygen and nitrogen species (ROS/RNS) such as superoxide (O₂^̇̄), hydrogen peroxide, lipid hydroperoxides, peroxynitrite, and hypochlorous and hypobromous acids play a key role in many pathophysiological processes. Recent studies have focused on mitochondrial ROS as redox signaling species responsible for promoting cell division, modulating and regulating kinases and phosphatases, and activating transcription factors. Many ROS also stimulate cell death and senescence. The extent to which these processes occur is attributed to ROS levels (low or high) in cells. However, the exact nature of ROS remains unknown. Investigators have used redox-active probes that, upon oxidation by ROS, yield products exhibiting fluorescence, chemiluminescence, or bioluminescence. Mitochondria-targeted probes can be used to detect ROS generated in mitochondria. However, because most of these redox-active probes (untargeted and mitochondria-targeted) are oxidized by several ROS species, attributing redox probe oxidation to specific ROS species is difficult. It is conceivable that redox-active probes are oxidized in common one-electron oxidation pathways, resulting in a radical intermediate that either reacts with another oxidant (including oxygen to produce O₂^̇̄) and forms a stable fluorescent product or reacts with O₂^̇̄ to form a fluorescent marker product. Here, we propose the use of multiple probes and complementary techniques (HPLC, LC-MS, redox blotting, and EPR) and the measurement of intracellular probe uptake and specific marker products to identify specific ROS generated in cells. The low-temperature EPR technique developed to investigate cellular/mitochondrial oxidants can easily be extended to animal and human tissues.

Sirtuin 3 silencing improves oxaliplatin efficacy through acetylation of MnSOD in colon cancer

Sirtuin 3 (SIRT3) is the major mitochondria deacetylase and regulates ROS levels by targeting several key proteins, such as those involved in mitochondrial function and antioxidant defenses. This way, SIRT3 balances ROS production and scavenging and promotes cell survival. The aim of this study was to analyze the effect of SIRT3 silencing on the antioxidant response in SW620 colon cancer cell line, and whether this intervention could improve efficacy of oxaliplatin, a common drug used to treat colon cancer. For this purpose, we obtained stable clones of SW620 with SIRT3 knockdown and determined parameters such as ROS levels and ROS production, levels of several antioxidant enzymes, cell viability, and apoptosis. Results showed that after SIRT3 silencing, both ROS levels and production were increased, and antioxidant enzymes gene expression was significantly reduced. Furthermore, manganese superoxide dismutase levels and enzymatic activity were reduced. Combination of SIRT3 knockdown with oxaliplatin treatment further increased ROS production and apoptosis, reducing cell viability. Finally, survival curves on colon cancer patients suggested that SIRT3 expression is related to a poorer prognosis. In conclusion, SIRT3 could be a target for colon cancer, since it regulates the antioxidant response and its knockdown improves the efficacy of oxaliplatin treatment.

Insights into the Dichotomous Regulation of SOD2 in Cancer

While loss of antioxidant expression and the resultant oxidant-dependent damage to cellular macromolecules is key to tumorigenesis, it has become evident that effective oxidant scavenging is conversely necessary for successful metastatic spread. This dichotomous role of antioxidant enzymes in cancer highlights their context-dependent regulation during different stages of tumor development. A prominent example of an antioxidant enzyme with such a dichotomous role and regulation is the mitochondria-localized manganese superoxide dismutase SOD2 (MnSOD). SOD2 has both tumor suppressive and promoting functions, which are primarily related to its role as a mitochondrial superoxide scavenger and H₂O₂ regulator. However, unlike true tumor suppressor- or onco-genes, the SOD2 gene is not frequently lost, or rarely mutated or amplified in cancer. This allows SOD2 to be either repressed or activated contingent on context-dependent stimuli, leading to its dichotomous function in cancer. Here, we describe some of the mechanisms that underlie SOD2 regulation in tumor cells. While much is known about the transcriptional regulation of the SOD2 gene, including downregulation by epigenetics and activation by stress response transcription factors, further research is required to understand the post-translational modifications that regulate SOD2 activity in cancer cells. Moreover, future work examining the spatio-temporal nature of SOD2 regulation in the context of changing tumor microenvironments is necessary to allows us to better design oxidant- or antioxidant-based therapeutic strategies that target the adaptable antioxidant repertoire of tumor cells.

The Glycolytic Switch in Tumors: How Many Players Are Involved?

Reprogramming of cellular metabolism is a hallmark of cancers. Cancer cells more readily use glycolysis, an inefficient metabolic pathway for energy metabolism, even when sufficient oxygen is available. This reliance on aerobic glycolysis is called the Warburg effect, and promotes tumorigenesis and malignancy progression. The mechanisms of the glycolytic shift in tumors are not fully understood. Growing evidence demonstrates that many signal molecules, including oncogenes and tumor suppressors, are involved in the process, but how oncogenic signals attenuate mitochondrial function and promote the switch to glycolysis remains unclear. Here, we summarize the current information on several main mediators and discuss their possible mechanisms for triggering the Warburg effect.

A lowered 26S proteasome activity correlates with mantle lymphoma cell lines resistance to genotoxic stress

Background

Mantle cell lymphoma (MCL) is a B-cell hemopathy characterized by the t(11;14) translocation and the aberrant overexpression of cyclin D1. This results in an unrestrained cell proliferation. Other genetic alterations are common in MCL cells such as SOX11 expression, mutations of ATM and/or TP53 genes, activation of the NF-κB signaling pathway and NOTCH receptors. These alterations lead to the deregulation of the apoptotic machinery and resistance to drugs. We observed that among a panel of MCL cell lines, REC1 cells were resistant towards genotoxic stress. We studied the molecular basis of this resistance.

Methods

We analyzed the cell response regarding apoptosis, senescence, cell cycle arrest, DNA damage response and finally the 26S proteasome activity following a genotoxic treatment that causes double strand DNA breaks.

Results

MCL cell lines displayed various sensitivity/resistance towards genotoxic stress and, in particular, REC1 cells did not enter apoptosis or senescence after an etoposide treatment. Moreover, the G2/M cell cycle checkpoint was deficient in REC1 cells. We observed that three main actors of apoptosis, senescence and cell cycle regulation (cyclin D1, MCL1 and CDC25A) failed to be degraded by the proteasome machinery in REC1 cells. We ruled out a default of the βTrCP E3-ubiquitine ligase but detected a lowered 26S proteasome activity in REC1 cells compared to other cell lines.

Conclusion

The resistance of MCL cells to genotoxic stress correlates with a low 26S proteasome activity. This could represent a relevant biomarker for a subtype of MCL patients with a poor response to therapies and a high risk of relapse.

Electronic supplementary material

The online version of this article (doi:10.1186/s12885-017-3530-z) contains supplementary material, which is available to authorized users.

Interplay Between SIRT-3, Metabolism and Its Tumor Suppressor Role in Hepatocellular Carcinoma

Sirtuins (SIRT), first described as nicotinamide adenine dinucleotide (NAD+)-dependent type III histone deacetylases, are produced by cells to support in the defense against chronic stress conditions such as metabolic syndromes, neurodegeneration, and cancer. SIRT-3 is one of the most studied members of the mitochondrial sirtuins family. In particular, its involvement in metabolic diseases and its dual role in cancer have been described. In the present review, based on the evidence of SIRT-3 involvement in metabolic dysfunctions, we aimed to provide an insight into the multifaceted role of SIRT-3 in many solid and hematological tumors with a particular focus on hepatocellular carcinoma (HCC). SIRT-3 regulatory effect and involvement in metabolism dysfunctions may have strong implications in HCC development and treatment. Research literature widely reports the relationship between metabolic disorders and HCC development. This evidence suggests a putative bridge role of SIRT-3 between metabolic diseases and HCC. However, further studies are necessary to demonstrate such interconnection.

SIRT3: Oncogene and Tumor Suppressor in Cancer

Sirtuin 3 (SIRT3), the major deacetylase in mitochondria, plays a crucial role in modulating oxygen reactive species (ROS) and limiting the oxidative damage in cellular components. SIRT3 targets different enzymes which regulate mitochondrial metabolism and participate in ROS detoxification, such as the complexes of the respiratory chain, the isocitrate dehydrogenase, or the manganese superoxide dismutase. Thus, SIRT3 activity is essential in maintaining mitochondria homeostasis and has recently received great attention, as it is considered a fidelity protein for mitochondrial function. In some types of cancer, SIRT3 functions as a tumoral promoter, since it keeps ROS levels under a certain threshold compatible with cell viability and proliferation. On the contrary, other studies describe SIRT3 as a tumoral suppressor, as SIRT3 could trigger cell death under stress conditions. Thus, SIRT3 could have a dual role in cancer. In this regard, modulation of SIRT3 activity could be a new target to develop more personalized therapies against cancer.

SIRT3 Enhances Mesenchymal Stem Cell Longevity and Differentiation

Mesenchymal stem cells (MSCs) are multipotent cells that are currently being investigated in a wide variety of clinical trials for their anti-inflammatory and immunomodulatory properties as well as their osteogenic and chondrogenic capabilities. However, there are considerable interdonor variability and heterogeneity of MSC populations, making it challenging to compare different products. Furthermore, proliferation, differentiation, and immunomodulation of MSCs decrease with aging and ex vivo expansion. The sirtuins have emerged as a class of protein deacylases involved in aging, oxidative stress, and metabolism. Sirtuin 3 (SIRT3) is the major mitochondrial deacetylase involved in reducing oxidative stress while preserving oxidative metabolism, and its levels have been shown to decrease with age. This study investigated the role of SIRT3 in MSC differentiation and aging. As MSCs were expanded ex vivo, SIRT3 levels decreased. In addition, SIRT3 depletion reduced MSC differentiation into adipocytes and osteoblasts. Furthermore, overexpression of SIRT3 in later-passage MSCs reduced aging-related senescence, reduced oxidative stress, and enhanced their ability to differentiate. These data suggest that overexpressing SIRT3 might represent a strategy to increase the quality and quantity of MSCs utilized for clinical applications.

Sirtuin 3 deficiency does not alter host defenses against bacterial and fungal infections

Sirtuin 3 (SIRT3) is the main mitochondrial deacetylase. SIRT3 regulates cell metabolism and redox homeostasis, and protects from aging and age-associated pathologies. SIRT3 may drive both oncogenic and tumor-suppressive effects. SIRT3 deficiency has been reported to promote chronic inflammation-related disorders, but whether SIRT3 impacts on innate immune responses and host defenses against infections remains essentially unknown. This aspect is of primary importance considering the great interest in developing SIRT3-targeted therapies. Using SIRT3 knockout mice, we show that SIRT3 deficiency does not affect immune cell development and microbial ligand-induced proliferation and cytokine production by splenocytes, macrophages and dendritic cells. Going well along with these observations, SIRT3 deficiency has no major impact on cytokine production, bacterial burden and survival of mice subjected to endotoxemia, Escherichia coli peritonitis, Klebsiella pneumoniae pneumonia, listeriosis and candidiasis of diverse severity. These data suggest that SIRT3 is not critical to fight infections and support the safety of SIRT3-directed therapies based on SIRT3 activators or inhibitors for treating metabolic, oncologic and neurodegenerative diseases without putting patients at risk of infection.

Manganese superoxide dismutase (SOD2): is there a center in the universe of mitochondrial redox signaling?

It is becoming increasingly clear that mitochondria drive cellular functions and in vivo phenotypes by directing the production rate and abundance of metabolites that are proposed to function as signaling molecules (Chandel 2015; Selak et al. 2005; Etchegaray and Mostoslavsky 2016). Many of these metabolites are intermediates that make up cellular metabolism, part of which occur in mitochondria (i.e. the TCA and urea cycles), while others are produced "on demand" mainly in response to alterations in the microenvironment in order to participate in the activation of acute adaptive responses (Mills et al. 2016; Go et al. 2010). Reactive oxygen species (ROS) are well suited for the purpose of executing rapid and transient signaling due to their short lived nature (Bae et al. 2011). Hydrogen peroxide (H₂O₂), in particular, possesses important characteristics including diffusibility and faster reactivity with specific residues such as methionine, cysteine and selenocysteine (Bonini et al. 2014). Therefore, it is reasonable to propose that H₂O₂ functions as a relatively specific redox signaling molecule. Even though it is now established that mtH₂O₂ is indispensable, at least for hypoxic adaptation and energetic and/or metabolic homeostasis (Hamanaka et al. 2016; Guzy et al. 2005), the question of how H₂O₂ is produced and regulated in the mitochondria is only partially answered. In this review, some roles of this indispensable signaling molecule in driving cellular metabolism will be discussed. In addition, we will discuss how H₂O₂ formation in mitochondria depends on and is controlled by MnSOD. Finally, we will conclude this manuscript by highlighting why a better understanding of redox hubs in the mitochondria will likely lead to new and improved therapeutics of a number of diseases, including cancer.

Nutrient Sensing and the Oxidative Stress Response

The simplicity and effectiveness of calorie restriction (CR) in lifespan and healthspan extension have fascinated generations searching for the Fountain of Youth. CR reduces levels of oxidative stress and damage, which have been postulated in the free radical theory of aging as a major cause of aging and diseases of aging. This reduction has long been viewed as a result of passive slowing of metabolism. Recent advances in nutrient sensing have provided molecular insights into the oxidative stress response and suggest that CR triggers an active defense program involving a cascade of molecular regulators to reduce oxidative stress. Physiological studies have provided strong support for oxidative stress in the development of aging-associated conditions and diseases but have also revealed the surprising requirement for oxidative stress to support normal physiological functions and, in some contexts, even slow aging and prevent the progression of cancer. Deciphering the molecular mechanisms and physiological implications of the oxidative stress response during CR will increase our understanding of the basic biology of aging and pave the way for the design of CR mimetics to improve healthspan.

SIRT3-Mediated Dimerization of IDH2 Directs Cancer Cell Metabolism and Tumor Growth

The isocitrate dehydrogenase IDH2 produces α-ketoglutarate by oxidizing isocitrate, linking glucose metabolism to oxidative phosphorylation. In this study, we report that loss of SIRT3 increases acetylation of IDH2 at lysine 413 (IDH2-K413-Ac), thereby decreasing its enzymatic activity by reducing IDH2 dimer formation. Expressing a genetic acetylation mimetic IDH2 mutant (IDH2^K413Q) in cancer cells decreased IDH2 dimerization and enzymatic activity and increased cellular reactive oxygen species and glycolysis, suggesting a shift in mitochondrial metabolism. Concurrently, overexpression of IDH2^K413Q promoted cell transformation and tumorigenesis in nude mice, resulting in a tumor-permissive phenotype. IHC staining showed that IDH2 acetylation was elevated in high-risk luminal B patients relative to low-risk luminal A patients. Overall, these results suggest a potential relationship between SIRT3 enzymatic activity, IDH2-K413 acetylation-determined dimerization, and a cancer-permissive phenotype. Cancer Res; 77(15); 3990-9. ©2017 AACR.

Development of Inhibitors Targeting Hypoxia-Inducible Factor 1 and 2 for Cancer Therapy

Hypoxia is frequently observed in solid tumors and also one of the major obstacles for effective cancer therapies. Cancer cells take advantage of their ability to adapt hypoxia to initiate a special transcriptional program that renders them more aggressive biological behaviors. Hypoxia-inducible factors (HIFs) are the key factors that control hypoxia-inducible pathways by regulating the expression of a vast array of genes involved in cancer progression and treatment resistance. HIFs, mainly HIF-1 and -2, have become potential targets for developing novel cancer therapeutics. This article reviews the updated information in tumor HIF pathways, particularly recent advances in the development of HIF inhibitors. These inhibitors interfere with mRNA expression, protein synthesis, protein degradation and dimerization, DNA binding and transcriptional activity of HIF-1 and -2, or both. Despite efforts in the past two decades, no agents directly inhibiting HIFs have been approved for treating cancer patients. By analyzing results of the published reports, we put the perspectives at the end of the article. The therapeutic efficacy of HIF inhibitors may be improved if more efforts are devoted on developing agents that are able to simultaneously target HIF-1 and -2, increasing the penetrating capacity of HIF inhibitors, and selecting suitable patient subpopulations for clinical trials.

Sirtuin 3: A Janus face in cancer (Review)

The NAD-dependent protein deacetylase sirtuin 3 (SIRT3) is an enzyme localized primarily in the mitochondrion, where it modulates cellular functions such as nutrient metabolism, ATP balance, antioxidant machinery, and other mechanisms fundamental to mitochondria. SIRT3 is closely associated with the pathogenesis of diverse disorders. In particular, it plays a dual role in the development and progression of cancer. In many cancers, SIRT3 acts as an oncogene by promoting or maintaining the malignant phenotypes of neoplastic cells, including uncontrolled proliferation, resistance to apoptosis, and increased motility or invasiveness; however, SIRT3 suppresses these phenotypes in certain types of malignancy. The underlying mechanisms involve depletion of intracellular reactive oxygen species, modulation of metabolic reprogramming, and regulation of intracellular signaling responsible for cell growth and death. This review summarizes recent findings concerning the characteristics and substrates/interacting partners of SIRT3, with particular emphasis on emerging mechanisms responsible for fine-tuning cellular behaviors that potentially underlie its conflicting roles in carcinogenesis.

Emerging therapeutic roles for NAD(+) metabolism in mitochondrial and age-related disorders

Nicotinamide adenine dinucleotide (NAD⁺) is a central metabolic cofactor in eukaryotic cells that plays a critical role in regulating cellular metabolism and energy homeostasis. NAD⁺ in its reduced form (i.e. NADH) serves as the primary electron donor in mitochondrial respiratory chain, which involves adenosine triphosphate production by oxidative phosphorylation. The NAD⁺/NADH ratio also regulates the activity of various metabolic pathway enzymes such as those involved in glycolysis, Kreb’s cycle, and fatty acid oxidation. Intracellular NAD⁺ is synthesized de novo from l-tryptophan, although its main source of synthesis is through salvage pathways from dietary niacin as precursors. NAD⁺ is utilized by various proteins including sirtuins, poly ADP-ribose polymerases (PARPs) and cyclic ADP-ribose synthases. The NAD⁺ pool is thus set by a critical balance between NAD⁺ biosynthetic and NAD⁺ consuming pathways. Raising cellular NAD⁺ content by inducing its biosynthesis or inhibiting the activity of PARP and cADP-ribose synthases via genetic or pharmacological means lead to sirtuins activation. Sirtuins modulate distinct metabolic, energetic and stress response pathways, and through their activation, NAD⁺ directly links the cellular redox state with signaling and transcriptional events. NAD⁺ levels decline with mitochondrial dysfunction and reduced NAD⁺/NADH ratio is implicated in mitochondrial disorders, various age-related pathologies as well as during aging. Here, I will provide an overview of the current knowledge on NAD⁺ metabolism including its biosynthesis, utilization, compartmentalization and role in the regulation of metabolic homoeostasis. I will further discuss how augmenting intracellular NAD⁺ content increases oxidative metabolism to prevent bioenergetic and functional decline in multiple models of mitochondrial diseases and age-related disorders, and how this knowledge could be translated to the clinic for human relevance.

Effects of Oxidative Stress on Mesenchymal Stem Cell Biology

Mesenchymal stromal/stem cells (MSCs) are multipotent stem cells present in most fetal and adult tissues. Ex vivo culture-expanded MSCs are being investigated for tissue repair and immune modulation, but their full clinical potential is far from realization. Here we review the role of oxidative stress in MSC biology, as their longevity and functions are affected by oxidative stress. In general, increased reactive oxygen species (ROS) inhibit MSC proliferation, increase senescence, enhance adipogenic but reduce osteogenic differentiation, and inhibit MSC immunomodulation. Furthermore, aging, senescence, and oxidative stress reduce their ex vivo expansion, which is critical for their clinical applications. Modulation of sirtuin expression and activity may represent a method to reduce oxidative stress in MSCs. These findings have important implications in the clinical utility of MSCs for degenerative and immunological based conditions. Further study of oxidative stress in MSCs is imperative in order to enhance MSC ex vivo expansion and in vivo engraftment, function, and longevity.

Mitochondrial Sirtuins in Cancer: Emerging Roles and Therapeutic Potential

The past few decades have witnessed a furious attention of scientific community toward identifying novel molecular factors and targets that could be exploited for drug development for cancer management. One such factor is the sirtuin (SIRT) family of nicotinamide adenine dinucleotide (NAD(+))-dependent deacetylases. The role of SIRTs in cancer is extremely complex, with dichotomous functions depending on cell contexts. Mammalian SIRTs (SIRT1-7) differ in their cellular localization and biologic functions. Among these, SIRT -3, -4, and -5 are located in the mitochondria and are being carefully investigated. These mitochondrial SIRTs (mtSIRT) regulate multiple cellular and physiologic processes, including cell cycle, gene expression, cell viability, stress response, metabolism, and energy homeostasis. Recent research suggests that mtSIRTs influence tumors by regulating the metabolic state of the cell. Although the research on the role of mtSIRTs in cancer is still in its infancy, studies have suggested tumor suppressor as well as tumor promoter roles for them. This review is focused on discussing up-to-date information about the roles and functional relevance of mtSIRTs (SIRT -3, -4, -5) in cancers. We have also provided a critical discussion and our perspective on their dual roles, as tumor promoter versus tumor suppressor, in cancer. Cancer Res; 76(9); 2500-6. ©2016 AACR.