SIRT6 safeguards human mesenchymal stem cells from oxidative stress by coactivating NRF2. Cell Res. 2016;26:190-205. [PMID: 26768768 PMCID: PMC4746611 DOI: 10.1038/cr.2016.4]

The Role of Sirtuins in Antioxidant and Redox Signaling

SIGNIFICANCE

Antioxidant and redox signaling (ARS) events are regulated by critical molecules that modulate antioxidants, reactive oxygen species (ROS) or reactive nitrogen species (RNS), and/or oxidative stress within the cell. Imbalances in these molecules can disturb cellular functions to become pathogenic. Sirtuins serve as important regulators of ARS in cells. Recent Advances: Sirtuins (SIRTs 1-7) are a family of nicotinamide adenine dinucleotide (NAD)-dependent histone deacetylases with the ability to deacetylate histone and nonhistone targets. Recent studies show that sirtuins modulate the regulation of a variety of cellular processes associated with ARS. SIRT1, SIRT3, and SIRT5 protect the cell from ROS, and SIRT2, SIRT6, and SIRT7 modulate key oxidative stress genes and mechanisms. Interestingly, SIRT4 has been shown to induce ROS production and has antioxidative roles as well.

CRITICAL ISSUES

A complete understanding of the roles of sirtuins in redox homeostasis of the cell is very important to understand the normal functioning as well as pathological manifestations. In this review, we have provided a critical discussion on the role of sirtuins in the regulation of ARS. We have also discussed mechanistic interactions among different sirtuins. Indeed, a complete understanding of sirtuin biology could be critical at multiple fronts.

FUTURE DIRECTIONS

Sirtuins are emerging to be important in normal mammalian physiology and in a variety of oxidative stress-mediated pathological situations. Studies are needed to dissect the mechanisms of sirtuins in maintaining redox homeostasis. Efforts are also required to assess the targetability of sirtuins in the management of redox-regulated diseases. Antioxid. Redox Signal. 28, 643-661.

SIRT1 and SIRT6 Signaling Pathways in Cardiovascular Disease Protection

SIGNIFICANCE

Oxidative stress represents the common hallmark of pathological conditions associated with cardiovascular disease (CVD), including atherosclerosis, heart failure, hypertension, aging, diabetes, and other vascular system-related diseases. The sirtuin (SIRT) family, comprising seven proteins (SIRT1-SIRT7) sharing a highly conserved nicotinamide adenine dinucleotide (NAD+)-binding catalytic domain, attracted a great attention for the past few years as stress adaptor and epigenetic enzymes involved in the cellular events controlling aging-related disorder, cancer, and CVD. Recent Advances: Among sirtuins, SIRT1 and SIRT6 are the best characterized for their protective roles against inflammation, vascular aging, heart disease, and atherosclerotic plaque development. This latest role has been only recently unveiled for SIRT6. Of interest, in recent years, complex signaling networks controlled by SIRT1 and SIRT6 common to stress resistance, vascular aging, and CVD have emerged.

CRITICAL ISSUES

We provide a comprehensive overview of recent developments on the molecular signaling pathways controlled by SIRT1 and SIRT6, two post-translational modifiers proven to be valuable tools to dampen inflammation and oxidative stress at the cardiovascular level.

FUTURE DIRECTIONS

A deeper understanding of the epigenetic mechanisms through which SIRT1 and SIRT6 act in the signalings responsible for onset and development CVD is a prime scientific endeavor of the upcoming years. Multiple "omic" technologies will have widespread implications in understanding such mechanisms, speeding up the achievement of selective and efficient pharmacological modulation of sirtuins for future applications in the prevention and treatment of CVD. Antioxid. Redox Signal. 28, 711-732.

Haploinsufficiency of Trp53 dramatically extends the lifespan of Sirt6-deficient mice

Mammalian sirtuin 6 (Sirt6) is a conserved NAD⁺-dependent deacylase and mono-ADP ribosylase that is known to be involved in DNA damage repair, metabolic homeostasis, inflammation, tumorigenesis, and aging. Loss of Sirt6 in mice results in accelerated aging and premature death within a month. Here, we show that haploinsufficiency (i.e., heterozygous deletion) of Trp53 dramatically extends the lifespan of both female and male Sirt6-deficient mice. Haploinsufficiency of Trp53 in Sirt6-deficient mice rescues several age-related phenotypes of Sirt6-deficient mice, including reduced body size and weight, lordokyphosis, colitis, premature senescence, apoptosis, and bone marrow stem cell decline. Mechanistically, SIRT6 deacetylates p53 at lysine 381 to negatively regulate the stability and activity of p53. These findings establish that elevated p53 activity contributes significantly to accelerated aging in Sirt6-deficient mice. Our study demonstrates that p53 is a substrate of SIRT6, and highlights the importance of SIRT6-p53 axis in the regulation of aging.

Almost without exception, mammals age as they grow older. Older mammals are at greater risk of diseases like cancer, and have fewer stem cells that would otherwise help to keep their organs healthy. Some of the proteins that regulate and impact upon aging have been identified. One of these is an enzyme called SIRT6, which is thought to promote longevity. Mice without any SIRT6 suffer from severe premature aging, and rather than living up to two years like normal mice, SIRT6-deficient mice die within one month of their birth. The mutant mice also lose stem cells and exhibit signs of organ degeneration and body wasting.

Another protein called p53 is well known for having the opposite effect to SIRT6: it accelerates aging and helps to prevent tumor growth. However, it was unclear if p53 is also involved in the processes that lead to the premature death of mice without SIRT6.

Now, Ghosh et al. report that mouse cells and tissues without SIRT6 have more p53 compared to control samples. Biochemical experiments showed that the SIRT6 and p53 proteins physically interact, and that SIRT6 could use its enzymatic activity to remove a chemical modification, called acetylation, from p53. Without this specific acetylation, p53 became less stable and its levels dropped. Consequently, p53 was stabilized in the SIRT6-deficient cells. When Ghosh et al. deleted one copy of the gene that codes for p53 in SIRT6-deficient mice, mutant mice that had before only lived for a month now lived for up to sixteen months. Additionally, the mice were healthier, showing fewer signs of aging: for example, they had more immune cells and stem cells, straighter spines, and showed less gut inflammation and less body wasting.

These findings suggest that SIRT6 does indeed inhibit p53 to counteract the normal aging process. Future experiments may explore if this regulation also holds true in human cells. Detailed knowledge of these molecular interactions could also open up more research into therapies against cancer and aging.

Differential stem cell aging kinetics in Hutchinson-Gilford progeria syndrome and Werner syndrome

Hutchinson-Gilford progeria syndrome (HGPS) and Werner syndrome (WS) are two of the best characterized human progeroid syndromes. HGPS is caused by a point mutation in lamin A (LMNA) gene, resulting in the production of a truncated protein product-progerin. WS is caused by mutations in WRN gene, encoding a loss-of-function RecQ DNA helicase. Here, by gene editing we created isogenic human embryonic stem cells (ESCs) with heterozygous (G608G/+) or homozygous (G608G/G608G) LMNA mutation and biallelic WRN knockout, for modeling HGPS and WS pathogenesis, respectively. While ESCs and endothelial cells (ECs) did not present any features of premature senescence, HGPS- and WS-mesenchymal stem cells (MSCs) showed aging-associated phenotypes with different kinetics. WS-MSCs had early-onset mild premature aging phenotypes while HGPS-MSCs exhibited late-onset acute premature aging characterisitcs. Taken together, our study compares and contrasts the distinct pathologies underpinning the two premature aging disorders, and provides reliable stem-cell based models to identify new therapeutic strategies for pathological and physiological aging.

Hexokinase 2 and nuclear factor erythroid 2-related factor 2 transcriptionally coactivate xanthine oxidoreductase expression in stressed glioma cells

A dynamic network of metabolic adaptations, inflammatory responses, and redox homeostasis is known to drive tumor progression. A considerable overlap among these processes exists, but several of their key regulators remain unknown. To this end, here we investigated the role of the proinflammatory cytokine IL-1β in connecting these processes in glioma cells. We found that glucose starvation sensitizes glioma cells to IL-1β-induced apoptosis in a manner that depended on reactive oxygen species (ROS). Although IL-1β-induced JNK had no effect on cell viability under glucose deprivation, it mediated nuclear translocation of hexokinase 2 (HK2). This event was accompanied by increases in the levels of sirtuin 6 (SIRT6), nuclear factor erythroid 2-related factor 2 (Nrf2), and xanthine oxidoreductase (XOR). SIRT6 not only induced ROS-mediated cell death but also facilitated nuclear Nrf2-HK2 interaction. Recruitment of the Nrf2-HK2 complex to the ARE site on XOR promoter regulated its expression. Importantly, HK2 served as transcriptional coactivator of Nrf2 to regulate XOR expression, indicated by decreased XOR levels in siRNA-mediated Nrf2 and HK2 knockdown experiments. Our results highlight a non-metabolic role of HK2 as transcriptional coactivator of Nrf2 to regulate XOR expression under conditions of proinflammatory and metabolic stresses. Our insights also underscore the importance of nuclear activities of HK2 in the regulation of genes involved in redox homeostasis.

ATF6 safeguards organelle homeostasis and cellular aging in human mesenchymal stem cells

Loss of organelle homeostasis is a hallmark of aging. However, it remains elusive how this occurs at gene expression level. Here, we report that human mesenchymal stem cell (hMSC) aging is associated with dysfunction of double-membrane organelles and downregulation of transcription factor ATF6. CRISPR/Cas9-mediated inactivation of ATF6 in hMSCs, not in human embryonic stem cells and human adipocytes, results in premature cellular aging, characteristic of loss of endomembrane homeostasis. Transcriptomic analyses uncover cell type-specific constitutive and stress-induced ATF6-regulated genes implicated in various layers of organelles’ homeostasis regulation. FOS was characterized as a constitutive ATF6 responsive gene, downregulation of which contributes to hMSC aging. Our study unravels the first ATF6-regulated gene expression network related to homeostatic regulation of membrane organelles, and provides novel mechanistic insights into aging-associated attrition of human stem cells.

Sirtuins and Insulin Resistance

The mammalian Sirtuins (SIRT1-7) are an evolutionarily conserved family of NAD⁺-dependent deacylase and mono-ADP-ribosyltransferase. Sirtuins display distinct subcellular localizations and functions and are involved in cell survival, senescence, metabolism and genome stability. Among the mammalian Sirtuins, SIRT1 and SIRT6 have been thoroughly investigated and have prominent metabolic regulatory roles. Moreover, SIRT1 and SIRT6 have been implicated in obesity, insulin resistance, type 2 diabetes mellitus (T2DM), fatty liver disease and cardiovascular diseases. However, the roles of other Sirtuins are not fully understood. Recent studies have shown that these Sirtuins also play important roles in inflammation, mitochondrial dysfunction, and energy metabolism. Insulin resistance is the critical pathological trait of obesity and metabolic syndrome as well as the core defect in T2DM. Accumulating clinical and experimental animal evidence suggests the potential roles of the remaining Sirtuins in the regulation of insulin resistance through diverse biological mechanisms. In this review, we summarize recent advances in the understanding of the functions of Sirtuins in various insulin resistance-associated physiological processes, including inflammation, mitochondrial dysfunction, the insulin signaling pathway, glucose, and lipid metabolism. In addition, we highlight the important gaps that must be addressed in this field.

Silica nanoparticle releases SIRT6-induced epigenetic silencing of follistatin

Follistatin (FST) plays a protective role during silica nanoparticle (SiO₂ NP) exposure. SiO₂ NP treatment induces FST transcription with an unknown mechanism. We herein reported that SIRT6, one of the sirtuin family members, induced epigenetic silencing of FST. The expression of FST was elevated after SIRT6 knockdown while reduced after SIRT6 overexpression. Chromatin immunoprecipitation revealed a direct interaction between SIRT6 with FST promoter. Knockdown of SIRT6 increased both Ac-H3K9 level and Ac-H3K56 level at FST promoter region. SiO₂ NP treatment de-stabilized SIRT6 mRNA and reduced SIRT6 expression, leading to the activation of FST transcription. Finally, over-expression of SIRT6 increased SiO₂ NP-induced apoptosis. Collectively, this study provided evidence that SIRT6 is a negative regulator of FST transcription and participates in the regulation of cell survival during silica nanoparticle exposure.

Cholinergic and cytoprotective signaling cascades mediate the mitigative effect of erythropoietin on acute radiation syndrome

The present investigation aimed to evaluate the radiomitigative efficacy of the recombinant human erythropoietin (EPO) against acute radiation syndrome (ARS) in a rat model. Rats were irradiated with a single sublethal dose of γ-radiation (7 Gy; total body irradiation; TBI) on the 1st day of experimental course, then received EPO (5000 IU/kg; i.p.) 24 h after irradiation, and rats were observed for 30 days of survival analysis. Administration of EPO improved 30-day survival, alleviated TBI-induced myelosuppression and pancytopenia, by augmenting lymphocytes and other white blood cells in the peripheral blood of rats, while bone marrow and spleen cellularity were restored. EPO post-exposure treatment alleviated hepatotoxicity biomarkers and restored splenic function. EPO abrogated radiation-induced oxidative stress through the upregulation of the cholinergic anti-inflammatory nicotinic acetylcholine receptor (α-7-nAChR) and the pro-survival Janus kinase-2 and signal transducers and activators of transcription JAK-2/STAT-3 signaling mediated via enhancing nuclear factor erythroid-2 related factor-2 (Nrf-2) cytoprotective machinery in liver and spleen of irradiated rats. Moreover, EPO treatment prevented hepatic and splenic apoptosis. The present study establishes the implication of α-7-nAChR-JAK-2/STAT-3-Nrf-2 signaling cascade in the radiomitigative potential of EPO against ARS.

Protein sequestration at the nuclear periphery as a potential regulatory mechanism in premature aging

Serebryannyy and Misteli provide a perspective on how protein sequestration at the inner nuclear membrane and nuclear lamina might influence aging.

Despite the extensive description of numerous molecular changes associated with aging, insights into the driver mechanisms of this fundamental biological process are limited. Based on observations in the premature aging syndrome Hutchinson–Gilford progeria, we explore the possibility that protein regulation at the inner nuclear membrane and the nuclear lamina contributes to the aging process. In support, sequestration of nucleoplasmic proteins to the periphery impacts cell stemness, the response to cytotoxicity, proliferation, changes in chromatin state, and telomere stability. These observations point to the nuclear periphery as a central regulator of the aging phenotype.

Sirtuin 6 protects the brain from cerebral ischemia/reperfusion injury through NRF2 activation

Sirtuin 6 (SIRT6), a member of the sirtuin family of NAD(+)-dependent deacetylases, has been shown to produce beneficial effects in myocardial ischemia/reperfusion (I/R). However, the role of SIRT6 in cerebral I/R is largely unclear. In this study, we investigated the effects of SIRT6 overexpression in regulating I/R injury in a mouse cerebral I/R model in vivo and in oxygen-glucose-deprivation/reoxygenation (OGD/R)-stimulated neuro-2a neuroblastoma cells in vitro. We found that cerebral I/R (1 h/24 h) resulted in decreased SIRT6 expression in the cerebral cortex (P < 0.01). SIRT6 overexpression in the brain by in vivo gene transfer enhanced the antioxidant NRF2 signaling (P < 0.05), reduced oxidative stress (P < 0.05), and attenuated cerebral I/R-induced brain tissue damage and neurological deficits (P < 0.05). These neuroprotective effects of SIRT6 overexpression were abolished in NRF2 knockout mice. In neuro-2A neuroblastoma cells, SIRT6 overexpression increased total and nuclear NRF2 levels (P < 0.05), reduced oxidative stress (P < 0.05), and attenuated OGD/R-induced cell death (P < 0.05); these protective effects were blocked by NRF2 knockdown (P < 0.05). Moreover, in OGD/R-stimulated neuro-2A cells, SIRT6 overexpression produced similar protective effects to those induced by the antioxidant NAC, but no added benefits were detected when SIRT6 overexpression was used in combination with NAC (P > 0.05). These findings provide evidence that SIRT6 can protect the brain from cerebral I/R injury by suppressing oxidative stress via NRF2 activation. Thus, SIRT6 may serve as a potential therapeutic target for ischemic stroke.

The histone deacetylase SIRT6 blocks myostatin expression and development of muscle atrophy

Muscle wasting, also known as cachexia, is associated with many chronic diseases, which worsens prognosis of primary illness leading to enhanced mortality. Molecular basis of this metabolic syndrome is not yet completely understood. SIRT6 is a chromatin-bound member of the sirtuin family, implicated in regulating many cellular processes, ranging from metabolism, DNA repair to aging. SIRT6 knockout (SIRT6-KO) mice display loss of muscle, fat and bone density, typical characteristics of cachexia. Here we report that SIRT6 depletion in cardiac as well as skeletal muscle cells promotes myostatin (Mstn) expression. We also observed upregulation of other factors implicated in muscle atrophy, such as angiotensin-II, activin and Acvr2b, in SIRT6 depleted cells. SIRT6-KO mice showed degenerated skeletal muscle phenotype with significant fibrosis, an effect consistent with increased levels of Mstn. Additionally, we observed that in an in vivo model of cancer cachexia, Mstn expression coupled with downregulation of SIRT6. Furthermore, SIRT6 overexpression downregulated the cytokine (TNFα-IFNγ)-induced Mstn expression in C2C12 cells, and promoted myogenesis. From the ChIP assay, we found that SIRT6 controls Mstn expression by attenuating NF-κB binding to the Mstn promoter. Together, these data suggest a novel role for SIRT6 in maintaining muscle mass by controlling expression of atrophic factors like Mstn and activin.

Regulation of Stem Cell Aging by Metabolism and Epigenetics

Stem cell aging and exhaustion are considered important drivers of organismal aging. Age-associated declines in stem cell function are characterized by metabolic and epigenetic changes. Understanding the mechanisms underlying these changes will likely reveal novel therapeutic targets for ameliorating age-associated phenotypes and for prolonging human healthspan. Recent studies have shown that metabolism plays an important role in regulating epigenetic modifications and that this regulation dramatically affects the aging process. This review focuses on current knowledge regarding the mechanisms of stem cell aging, and the links between cellular metabolism and epigenetic regulation. In addition, we discuss how these interactions sense and respond to environmental stress in order to maintain stem cell homeostasis, and how environmental stimuli regulate stem cell function. Additionally, we highlight recent advances in the development of therapeutic strategies to rejuvenate dysfunctional aged stem cells.

EphB2 signaling-mediated Sirt3 expression reduces MSC senescence by maintaining mitochondrial ROS homeostasis

Disruption of mitochondrial reactive oxygen species (mtROS) homeostasis is a key factor inducing UCB-MSC senescence. Accordingly, preventing mtROS accumulation will help in suppressing the UCB-MSC senescence. In this study, we observed that the expressions of EphrinB2 and EphB2 were inversely regulated by UCB-MSC passage-dependent manner. EphB2 signaling induced mitochondrial translocation of Sirt3. The knockdown of SIRT3 inhibited the effect of EphB2 signaling in UCB-MSCs. Subsequently, EphrinB2-Fc induced the nuclear translocation of Nrf-2 via c-Src phosphorylation dependent manner, and Sirt3 expression was regulated by Nrf-2. Among Sirt3 target genes, EphB2 signaling increased MnSOD and reduced the mtROS level in UCB-MSCs. Furthermore, the deacetylase effect of Sirt3 enhanced the MnSOD activity by deacetylation at the lysine 68 residue and therapeutic effect of UCB-MSCs on skin-wound healing was increased by EphB2 activation. In conclusion, the EphB2 can serve as a novel target for the optimizing the therapeutic use of UCB-MSCs in wound repair by MnSOD-mediated mtROS scavenging through EphB2/c-Src signaling pathway and Nrf-2-dependent Sirt3 expression.

Oxidative stress management in the hair follicle: Could targeting NRF2 counter age-related hair disorders and beyond?

Widespread expression of the transcription factor, nuclear factor (erythroid-derived 2)-like 2 (NRF2), which maintains redox homeostasis, has recently been identified in the hair follicle (HF). Small molecule activators of NRF2 may therefore be useful in the management of HF pathologies associated with redox imbalance, ranging from HF greying and HF ageing via androgenetic alopecia and alopecia areata to chemotherapy-induced hair loss. Indeed, NRF2 activation has been shown to prevent peroxide-induced hair growth inhibition. Multiple parameters can increase the levels of reactive oxygen species in the HF, for example melanogenesis, depilation-induced trauma, neurogenic and autoimmune inflammation, toxic drugs, environmental stressors such as UV irradiation, genetic defects and aging-associated mitochondrial dysfunction. In this review, the potential mechanisms whereby NRF2 activation could prove beneficial in treatment of redox-associated HF disorders are therefore discussed.

Microvesicles as Potential Biomarkers for the Identification of Senescence in Human Mesenchymal Stem Cells

Senescence in human mesenchymal stem cells (MSCs) not only contributes to organism aging and the development of a variety of diseases but also severely impairs their therapeutic properties as a promising cell therapy. Studies searching for efficient biomarkers that represent cellular senescence have attracted much attention; however, no single marker currently provides an accurate cell-free representation of cellular senescence. Here, we studied characteristics of MSC-derived microvesicles (MSC-MVs) that may reflect the senescence in their parental MSCs. We found that senescent late passage (LP) MSCs secreted higher levels of MSC-MVs with smaller size than did early passage (EP) MSCs, and the level of CD105+ MSC-MVs decreased with senescence in the parental MSCs. Also, a substantially weaker ability to promote osteogenesis in MSCs was observed in LP than EP MSC-MVs. Comparative analysis of RNA sequencing showed the same trend of decreasing number of highly-expressed miRNAs with increasing number of passages in both MSCs and MSC-MVs. Most of the highly-expressed genes in LP MSCs and the corresponding MSC-MVs were involved in the regulation of senescence-related diseases, such as Alzheimer's disease. Furthermore, based on the miRNA profiling, transcription factors (TF) and genes regulatory networks of MSC senescence, and the datasets from GEO database, we confirmed that expression of miR-146a-5p in MSC-MVs resembled the senescent state of their parental MSCs. Our findings provide evidence that MSC-MVs are a key factor in the senescence-associated secretory phenotype of MSCs and demonstrate that their integrated characteristics can dynamically reflect the senescence state of MSCs representing a potential biomarker for monitoring MSC senescence.

The NAD+-dependent deacetylase, Bifidobacterium longum Sir2 in response to oxidative stress by deacetylating SigH (σH) and FOXO3a in Bifidobacterium longum and HEK293T cell respectively

Silent information regulator 2 (Sir2) enzymes which catalyze NAD+-dependent protein/histone deacetylation. The mammalian sirtuin family SIRT1, SIRT2, SIRT3 and SIRT6 can regulate oxidative stress. The probiotics (Bifidobacterium longum(B.longum) and Lactobacillus acidophilus(L. acidophilus)) have Sir2 gene family and have antioxidant activity in human body. it remains unknown whether probiotics Sir2 has a direct role in regulating oxidative stress. To this end, we knockout BL-sir2(sir2 B. longum) and LA-sir2(sir2 L.acidophilus) in low oxygen level. The antioxidant activities of two sir2 deficient strains was decreased, while when reintroduction of BL-sir2 and LA-sir2, the antioxidant activities were recoveried. In order to understand the regulation mechanism of probiotics Sir2 oxidation response. Then, we screened 65 acetylated protein, and found that SigH (σ^H) was a substrate of BL-Sir2. In addition, the acetylation level of σ^H decreased with the increase of BL-Sir2 level in B. longum. Thus, BL-Sir2 deacetylated σ^H in response to oxidative stress. Next, we transfected BL-Sir2 into H₂O₂-induced oxidative damage of 293T cells, BL-Sir2 increased the activity of manganese superoxide dismutase (MnSOD/SOD2) and catalase (CAT) and reduced reactive oxygen species(ROS). Then, we analyzed the differential gene by RNA sequencing and Gene ontology (GO) and found that BL-Sir2 regulated forkhead transcription factor (FOXO3a) mediated antioxidant genes in overexpressed BL-Sir2 HEK293T cells. Our study is the first to link probiotics Sir2 with oxidative stress and uncover the antioxidant mechanism of BL-Sir2 in B. longum itself and human body.

Hepatocyte-specific sirtuin 6 deletion predisposes to nonalcoholic steatohepatitis by up-regulation of Bach1, an Nrf2 repressor

Sirtuin (Sirt)6 has been implicated in negative regulation of inflammation and lipid metabolism, although its function in the progression from simple steatosis to nonalcoholic steatohepatitis (NASH) remains to be defined. To explore the role of hepatocyte Sirt6 in NASH development, we generated hepatocyte-specific Sirt6-knockout (KO) mice that were fed a high-fat and high-fructose (HFHF) diet for 16 wk. HFHF-fed KO mice had increased hepatic steatosis and inflammation and aggravated glucose intolerance and insulin resistance compared with wild-type mice. HFHF-induced liver fibrosis and oxidative stress and related gene expression were significantly elevated in KO mice. In the livers of KO mice, nuclear factor erythroid 2-related factor 2 (Nrf2) was down-regulated; conversely, BTB domain and CNC homolog 1 (Bach1), a nuclear repressor of Nrf2, were up-regulated. We discovered that Sirt6, which interacts with Bach1 under basal condition, induces its detachment from the antioxidant response element (ARE) region of heme oxygenase 1 promoter. Furthermore, we found that Sirt6 promotes Nrf2 binding to ARE in response to oxidative stimuli, which leads to the expression of phase II/antioxidant enzymes. Finally, we showed that HFHF-induced steatosis, inflammation, and fibrosis were ameliorated by adenoviral Sirt6 overexpression. Sirt6 may be a useful therapeutic target for amelioration of NASH by curbing inflammation and oxidative stress.-Ka, S.-O, Bang, I. H., Bae, E. J., Park, B.-H. Hepatocyte-specific sirtuin 6 deletion predisposes to nonalcoholic steatohepatitis by up-regulation of Bach1, an Nrf2 repressor.

Multiple pathways of SIRT6 at the crossroads in the control of longevity, cancer, and cardiovascular diseases

Sirtuin 6 (SIRT6) is a member of the sirtuin family NAD⁺-dependent deacetylases with multiple roles in controlling organism homeostasis, lifespan, and diseases. Due to its complex and opposite functional roles, this sirtuin is considered a two-edged sword in health and disease. Indeed, SIRT6 improves longevity, similarly to the founding yeast member, silent information regulator-2 (Sir2), and modulates genome stability, telomere integrity, transcription, and DNA repair. Its deficiency is associated with chronic inflammation, diabetes, cardiac hypertrophy, obesity, liver dysfunction, muscle/adipocyte disorders, and cancer. Besides, pieces of evidence showed that SIRT6 is a promoter of specific oncogenic pathways, thus disclosing its dual role regarding cancer development. Collectively, these findings suggest that multiple mechanisms, to date not entirely known, underlie the intriguing roles of SIRT6. Here we provide an overview of the current molecular mechanisms through which SIRT6 controls cancer and heart diseases, and describe its recent implications in the atherosclerotic plaque development.

CRISPR/Cas9-mediated targeted gene correction in amyotrophic lateral sclerosis patient iPSCs

Amyotrophic lateral sclerosis (ALS) is a complex neurodegenerative disease with cellular and molecular mechanisms yet to be fully described. Mutations in a number of genes including SOD1 and FUS are associated with familial ALS. Here we report the generation of induced pluripotent stem cells (iPSCs) from fibroblasts of familial ALS patients bearing SOD1^+/A272C and FUS^+/G1566A mutations, respectively. We further generated gene corrected ALS iPSCs using CRISPR/Cas9 system. Genome-wide RNA sequencing (RNA-seq) analysis of motor neurons derived from SOD1^+/A272C and corrected iPSCs revealed 899 aberrant transcripts. Our work may shed light on discovery of early biomarkers and pathways dysregulated in ALS, as well as provide a basis for novel therapeutic strategies to treat ALS.

Redox control of senescence and age-related disease

The signaling networks that drive the aging process, associated functional deterioration, and pathologies has captured the scientific community's attention for decades. While many theories exist to explain the aging process, the production of reactive oxygen species (ROS) provides a signaling link between engagement of cellular senescence and several age-associated pathologies. Cellular senescence has evolved to restrict tumor progression but the accompanying senescence-associated secretory phenotype (SASP) promotes pathogenic pathways. Here, we review known biological theories of aging and how ROS mechanistically control senescence and the aging process. We also describe the redox-regulated signaling networks controlling the SASP and its important role in driving age-related diseases. Finally, we discuss progress in designing therapeutic strategies that manipulate the cellular redox environment to restrict age-associated pathology.

Visualization of aging-associated chromatin alterations with an engineered TALE system

Visualization of specific genomic loci in live cells is a prerequisite for the investigation of dynamic changes in chromatin architecture during diverse biological processes, such as cellular aging. However, current precision genomic imaging methods are hampered by the lack of fluorescent probes with high specificity and signal-to-noise contrast. We find that conventional transcription activator-like effectors (TALEs) tend to form protein aggregates, thereby compromising their performance in imaging applications. Through screening, we found that fusing thioredoxin with TALEs prevented aggregate formation, unlocking the full power of TALE-based genomic imaging. Using thioredoxin-fused TALEs (TTALEs), we achieved high-quality imaging at various genomic loci and observed aging-associated (epi) genomic alterations at telomeres and centromeres in human and mouse premature aging models. Importantly, we identified attrition of ribosomal DNA repeats as a molecular marker for human aging. Our study establishes a simple and robust imaging method for precisely monitoring chromatin dynamics in vitro and in vivo.

The role of mitochondria in osteogenic, adipogenic and chondrogenic differentiation of mesenchymal stem cells

Mesenchymal stem cells (MSCs) are progenitors of connective tissues, which have emerged as important tools for tissue engineering due to their differentiation potential along various cell types. In recent years, accumulating evidence has suggested that the regulation of mitochondria dynamics and function is essential for successful differentiation of MSCs. In this paper, we review and provide an integrated view on the role of mitochondria in MSC differentiation. The mitochondria are maintained at a relatively low activity level in MSCs, and upon induction, mtDNA copy number, protein levels of respiratory enzymes, the oxygen consumption rate, mRNA levels of mitochondrial biogenesis-associated genes, and intracellular ATP content are increased. The regulated level of mitochondrial ROS is found not only to influence differentiation but also to contribute to the direction determination of differentiation. Understanding the roles of mitochondrial dynamics during MSC differentiation will facilitate the optimization of differentiation protocols by adjusting biochemical properties, such as energy production or the redox status of stem cells, and ultimately, benefit the development of new pharmacologic strategies in regenerative medicine.

SIRT6: Novel Mechanisms and Links to Aging and Disease

SIRT6, a member of the Sirtuin family of NAD⁺-dependent enzymes, has established roles in chromatin signaling and genome maintenance. Through these functions, SIRT6 protects against aging-associated pathologies including metabolic disease and cancer, and can promote longevity in mice. Research from the past few years revealed that SIRT6 is a complex enzyme with multiple substrates and catalytic activities, and uncovered novel SIRT6 functions in the maintenance of organismal health span. Here, we review these new discoveries and models of SIRT6 biology in four areas: heterochromatin stabilization and silencing; stem cell biology; cancer initiation and progression; and regulation of metabolic homeostasis. We discuss the possible implications of these findings for therapeutic interventions in aging and aging-related disease processes.

Sirtuin 6 suppresses hypoxia-induced inflammatory response in human osteoblasts via inhibition of reactive oxygen species production and glycolysis-A therapeutic implication in inflammatory bone resorption

Elevated glycolytic activity and redox imbalance induced by tissue hypoxia are common phenomena of chronic inflammation, including inflammatory bone diseases such as arthritis. However, relation between glycolysis and redox signaling in the inflammatory milieu is unclear. The histone deacetylase sirtuin 6 (SIRT6) is a crucial modulator of inflammation and glucose metabolism, and it is also involved in cellular protection against oxidative injury. The aims of the study were to examine the connection between glycolysis and reactive oxygen species (ROS) production in human osteoblastic cells (HOB) and whether SIRT6 modulates inflammatory response via regulation of glycolytic activity and ROS generation. In HOB cultured under hypoxia, expression of lactate dehydrogenase A (LDHA), lactate production and ROS generation were examined. The reciprocal effects between lactate and ROS production and their impact on inflammatory cytokine induction were assessed. The action of SIRT6 on the above reactions was determined. In a rat model of collagen-induced arthritis (CIA), the relation between inflammatory activity and osteoblastic expression of LDHA, level of oxidative lesions, Cyr61 synthesis and macrophage recruitment were examined in joints with or without lentiviral-SIRT6 gene therapy. Results showed that hypoxia stress enhanced lactate and LDHA production in HOB. ROS generation was also increased, and there was a positive feedback between glycolysis and ROS formation. Overexpression of SIRT6 attenuated hypoxia-enhanced glycolysis and ROS generation. Hypoxia-induced expressions of Cyr61, TNF-α, IL-1β, and IL-6 were suppressed by SIRT6 and the inhibitory effects overlapped with antiglycolytic and antioxidation mechanisms. In the model of CIA, forced expression of SIRT6 ameliorated disease progression, osteoblastic synthesis of Cyr61, and macrophage recruitment. More importantly, expression of LDHA and oxidative lesions were decreased in osteoblasts of SIRT6-treated joints. Our findings suggest that SIRT6 suppresses inflammatory response in osteoblasts via modulation of glucose metabolism and redox homeostasis. SIRT6-based strategy may possess therapeutic potential for inflammatory bone resorption. © 2016 BioFactors, 43(2):170-180, 2017.

Astaxanthin attenuates total body irradiation-induced hematopoietic system injury in mice via inhibition of oxidative stress and apoptosis

Background

The hematopoietic system is especially sensitive to total body irradiation (TBI), and myelosuppression is one of the major effects of TBI. Astaxanthin (ATX) is a powerful natural anti-oxidant with low toxicity. In this study, the effect of ATX on hematopoietic system injury after TBI was investigated.

Methods

Flow cytometry was used to detect the proportion of hematopoietic progenitor cells (HPCs) and hematopoietic stem cells (HSCs), the level of intracellular reactive oxygen species (ROS), expression of cytochrome C, cell apoptosis, and NRF2-related proteins. Immunofluorescence staining was used to detect Nrf2 translocation. Western blot analysis was used to evaluate the expression of apoptotic-related proteins. Enzymatic activities assay kits were used to analyze SOD2, CAT, and GPX1 activities.

Results

Compared with the TBI group, ATX can improve radiation-induced skewed differentiation of peripheral blood cells and accelerate hematopoietic self-renewal and regeneration. The radio-protective effect of ATX is probably attributable to the scavenging of ROS and the reduction of cell apoptosis. These changes were associated with increased activation of Nrf2 and downstream anti-oxidative proteins, and regulation of apoptotic-related proteins.

Conclusions

This study suggests that ATX could be used as a potent therapeutic agent to protect the hematopoietic system against TBI-induced bone marrow suppression.

Hydrogen-Rich Water Ameliorates Total Body Irradiation-Induced Hematopoietic Stem Cell Injury by Reducing Hydroxyl Radical

We examined whether consumption of hydrogen-rich water (HW) could ameliorate hematopoietic stem cell (HSC) injury in mice with total body irradiation (TBI). The results indicated that HW alleviated TBI-induced HSC injury with respect to cell number alteration and to the self-renewal and differentiation of HSCs. HW specifically decreased hydroxyl radical (^∙OH) levels in the c-kit⁺ cells of 4 Gy irradiated mice. Proliferative bone marrow cells (BMCs) increased and apoptotic c-kit⁺ cells decreased in irradiated mice uptaken with HW. In addition, the mean fluorescence intensity (MFI) of γ-H2AX and percentage of 8-oxoguanine positive cells significantly decreased in HW-treated c-kit⁺ cells, indicating that HW can alleviate TBI-induced DNA damage and oxidative DNA damage in c-kit⁺ cells. Finally, the cell cycle (P21), cell apoptosis (BCL-XL and BAK), and oxidative stress (NRF2, HO-1, NQO1, SOD, and GPX1) proteins were significantly altered by HW in irradiated mouse c-kit⁺ cells. Collectively, the present results suggest that HW protects against TBI-induced HSC injury.

NGF protects against oxygen and glucose deprivation-induced oxidative stress and apoptosis by up-regulation of HO-1 through MEK/ERK pathway

Both nerve growth factor (NGF) and heme oxygenases-1 (HO-1) promotes neuron survival from cerebral ischemic lesions. NGF protects neurons from oxygen-glucose deprivation (OGD), and HO-1 expression can be induced by some growth factors like NGF. This work attempted to identify the contribution of HO-1 on the neuroprotection role of NGF in OGD model, which is an injury simulation of ischemic neuron in vitro. The viability of cortical neurons cells treated with OGD restored significantly by pretreatment with NGF in a dose dependent manner. Moreover, NGF provided obvious protective effects against OGD-induced neurons apoptosis. It identified that NGF could prevent apoptosis and ROS (reactive oxygen species) accumulation in the primary cortical neurons exposed to OGD. NGF could up-regulate the expression level of HO-1, and then afford neuroprotection against OGD insult. In addition, we found that MEK/ERK pathway participated NGF-induced over-expression of HO-1, and was involved in the transcriptional activity or neuroprotection effect of NGF.

Repression of the Antioxidant NRF2 Pathway in Premature Aging

Hutchinson-Gilford progeria syndrome (HGPS) is a rare, invariably fatal premature aging disorder. The disease is caused by constitutive production of progerin, a mutant form of the nuclear architectural protein lamin A, leading, through unknown mechanisms, to diverse morphological, epigenetic, and genomic damage and to mesenchymal stem cell (MSC) attrition in vivo. Using a high-throughput siRNA screen, we identify the NRF2 antioxidant pathway as a driver mechanism in HGPS. Progerin sequesters NRF2 and thereby causes its subnuclear mislocalization, resulting in impaired NRF2 transcriptional activity and consequently increased chronic oxidative stress. Suppressed NRF2 activity or increased oxidative stress is sufficient to recapitulate HGPS aging defects, whereas reactivation of NRF2 activity in HGPS patient cells reverses progerin-associated nuclear aging defects and restores in vivo viability of MSCs in an animal model. These findings identify repression of the NRF2-mediated antioxidative response as a key contributor to the premature aging phenotype.

Sirtuins and Their Roles in Brain Aging and Neurodegenerative Disorders

Sirtuins (SIRT1-SIRT7) are unique histone deacetylases (HDACs) whose activity depends on NAD⁺ levels and thus on the cellular metabolic status. SIRTs regulate energy metabolism and mitochondrial function. They orchestrate the stress response and damage repair. Through these functions sirtuins modulate the course of aging and affect neurodegenerative diseases. SIRTSs interact with multiple signaling proteins, transcription factors (TFs) and poly(ADP-ribose) polymerases (PARPs) another class of NAD⁺-dependent post-translational protein modifiers. The cross-talk between SIRTs TFs and PARPs is a highly promising research target in a number of brain pathologies. This review describes updated results on sirtuins in brain aging/neurodegeneration. It focuses on SIRT1 but also on the roles of mitochondrial SIRTs (SIRT3, 4, 5) and on SIRT6 and SIRT2 localized in the nucleus and in cytosol, respectively. The involvement of SIRTs in regulation of insulin-like growth factor signaling in the brain during aging and in Alzheimer's disease was also focused. Moreover, we analyze the mechanism(s) and potential significance of interactions between SIRTs and several TFs in the regulation of cell survival and death. A critical view is given on the application of SIRT activators/modulators in therapy of neurodegenerative diseases.

SIRT6 protects against palmitate-induced pancreatic β-cell dysfunction and apoptosis

Chronic exposure of pancreatic β-cells to abnormally elevated levels of free fatty acids can lead to β-cell dysfunction and even apoptosis, contributing to type 2 diabetes pathogenesis. In pancreatic β-cells, sirtuin 6 (SIRT6) has been shown to regulate insulin secretion in response to glucose stimulation. However, the roles played by SIRT6 in β-cells in response to lipotoxicity remain poorly understood. Our data indicated that SIRT6 protein and mRNA levels were reduced in islets from diabetic and aged mice. High concentrations of palmitate (PA) also led to a decrease in SIRT6 expression in MIN6 β-cells and resulted in cell dysfunction and apoptosis. Knockdown of Sirt6 caused an increase in cell apoptosis and impairment in insulin secretion in response to glucose in MIN6 cells even in the absence of PA exposure. Furthermore, overexpression of SIRT6 alleviated the palmitate-induced lipotoxicity with improved cell viability and increased glucose-stimulated insulin secretion. In summary, our data suggest that SIRT6 can protect against palmitate-induced β-cell dysfunction and apoptosis.

Dangerous Entrapment for NRF2

Progerin, a mutated lamin A, causes the severe premature-aging syndrome Hutchinson-Gilford progeria (HGPS). Kubben et al. present a driving mechanism for HGPS involving trapping of NRF2 at the nuclear periphery by progerin. This local restriction results in impaired NRF2 signaling and chronic oxidative stress.

Adipose-derived mesenchymal stem cells promote the survival of fat grafts via crosstalk between the Nrf2 and TLR4 pathways

Autologous fat grafting is an effective reconstructive surgery technique; however, its success is limited by inconsistent graft retention and an environment characterized by high oxidative stress and inflammation. Adipose-derived stem cells (ADSCs) increase the survival of fat grafts, although the underlying mechanisms remain unclear. Here, TLR4^−/− and Nrf2^−/− mice were used to explore the effects of oxidative stress and inflammation on the viability and function of ADSCs in vitro and in vivo. Enrichment of fat grafts with ADSCs inhibited inflammatory cytokine production, enhanced growth factor levels, increased fat graft survival, downregulated NADPH oxidase (NOX)1 and 4 expression, increased vascularization and reduced ROS production in a manner dependent on toll-like receptor (TLR)-4 and nuclear factor erythroid 2-related factor 2 (Nrf2) expression. Immunohistochemical analysis showed that exposure to hypoxia enhanced ADSC growth and promoted the differentiation of ADSCs into vascular endothelial cells. Hypoxia-induced inflammatory cytokine, growth factor and NOX1/4 upregulation, as well as increased ROS production and apoptosis in ADSCs were dependent on TLR4 and Nrf2, which also modulated the effect of ADSCs on promoting endothelial progenitor cell migration and angiogenesis. Western blot analyses showed that the effects of hypoxia on ADSCs were regulated by crosstalk between Nrf2 antioxidant responses and NF-κB- and TLR4-mediated inflammatory responses. Taken together, our results indicate that ADSCs can increase the survival of fat transplants through the modulation of inflammatory and oxidative responses via Nrf2 and TLR4, suggesting potential strategies to improve the use of ADSCs for cell therapy.

Vitamin C alleviates aging defects in a stem cell model for Werner syndrome

Werner syndrome (WS) is a premature aging disorder that mainly affects tissues derived from mesoderm. We have recently developed a novel human WS model using WRN-deficient human mesenchymal stem cells (MSCs). This model recapitulates many phenotypic features of WS. Based on a screen of a number of chemicals, here we found that Vitamin C exerts most efficient rescue for many features in premature aging as shown in WRN-deficient MSCs, including cell growth arrest, increased reactive oxygen species levels, telomere attrition, excessive secretion of inflammatory factors, as well as disorganization of nuclear lamina and heterochromatin. Moreover, Vitamin C restores in vivo viability of MSCs in a mouse model. RNA sequencing analysis indicates that Vitamin C alters the expression of a series of genes involved in chromatin condensation, cell cycle regulation, DNA replication, and DNA damage repair pathways in WRN-deficient MSCs. Our results identify Vitamin C as a rejuvenating factor for WS MSCs, which holds the potential of being applied as a novel type of treatment of WS.

SIRT6, oxidative stress, and aging

The role of oxidative stress in the aging process has been highly debated for decades and remains equivocal. A new study published in Cell Research reports a novel role for the aging-associated SIRT6 deacetylase in the control of oxidative homeostasis in human mesenchymal stem cells.