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

The role of ubiquitination and deubiquitination in the pathogenesis of non-alcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is one of the most common chronic liver diseases and is closely associated with metabolic abnormalities. The causes of NAFLD are exceedingly complicated, and it is known that a variety of signaling pathways, endoplasmic reticulum stress, and mitochondrial dysfunction play a role in the pathogenesis of NAFLD. Recent studies have shown that ubiquitination and deubiquitination are involved in the regulation of the NAFLD pathophysiology. Protein ubiquitination is a dynamic and diverse post-translational alteration that affects various cellular biological processes. Numerous disorders, including NAFLD, exhibit imbalances in ubiquitination and deubiquitination. To highlight the significance of this post-translational modification in the pathogenesis of NAFLD and to aid in the development of new therapeutic approaches for the disease, we will discuss the role of enzymes involved in the processes of ubiquitination and deubiquitination, specifically E3 ubiquitin ligases and deubiquitinating enzymes that are important in the regulation of NAFLD.

SIRT6 Overexpression Enhances Diabetic Foot Ulcer Healing via Nrf2 Pathway Activation

Sirtuin-6 (SIRT6) has a pivotal role in a wide array of cellular biological functions and is linked to the progression of various diseases. Previous findings have identified SIRT6 as a protective modulator against numerous diabetic complications. However, whether SIRT6 exerts a protective role in diabetic foot ulcer (DFU) remains unstudied. This work established a rat model of DFU and evaluated the possible role of SIRT6 in mediating the wound healing in DFU. Marked reductions in SIRT6 levels were observed in wound samples from DFU patients and rats. Increasing SIRT6 expression in wound tissues remarkably decreased wound area, accelerated epithelialisation, increased collagen deposition and improved angiogenesis. Moreover, up-modulation of SIRT6 relieved the oxidative stress and inflammation in DFU rats. The increase of SIRT6 in cultured vascular endothelial cells restrained cell apoptosis, oxidative stress and inflammation elicited by high glucose (HG). HG-impaired migration capacity and angiogenesis of vascular endothelial cells was also recovered by increasing SIRT6 expression. Mechanism research revealed that SIRT6 overexpression reinforced the activation of the Nrf2 pathway in wound tissues of DFU rats and HG-exposed vascular endothelial cells. Pharmacological suppression of Nrf2 reversed the protective effect of SIRT6 overexpression on HG-triggered endothelial dysfunction. The findings of this work indicate that the positive role of SIRT6 in DFU wound healing is related to Nrf2 activation which contributes to the suppression of oxidative stress and inflammation and the improvement of angiogenesis in vascular endothelial cells. This study highlights the previously unaddressed role of SIRT6 in DFU wound healing, providing novel insights into its protective functions. The findings hold significant clinical value by identifying SIRT6 as a promising therapeutic target for improving DFU wound healing.

The Multifaceted Roles of BACH1 in Disease: Implications for Biological Functions and Therapeutic Applications

BTB domain and CNC homolog 1 (BACH1) belongs to the family of basic leucine zipper proteins and is expressed in most mammalian tissues. It can regulate its own expression and play a role in transcriptionally activating or inhibiting downstream target genes. It has a crucial role in various biological processes, such as oxidative stress, cell cycle, heme homeostasis, and immune regulation. Recent research highlights BACH1's significant regulatory roles in a series of conditions, including stem cell pluripotency maintenance and differentiation, growth, senescence, and apoptosis. BACH1 is closely associated with cardiovascular diseases and contributes to angiogenesis, atherosclerosis, restenosis, pathological cardiac hypertrophy, myocardial infarction, and ischemia/reperfusion (I/R) injury. BACH1 promotes tumor cell proliferation and metastasis by altering tumor metabolism and the epithelial-mesenchymal transition phenotype. Moreover, BACH1 appears to show an adverse role in diseases such as neurodegenerative diseases, gastrointestinal disorders, leukemia, pulmonary fibrosis, and skin diseases. Inhibiting BACH1 may be beneficial for treating these diseases. This review summarizes the role of BACH1 and its regulatory mechanism in different cell types and diseases, proposing that precise targeted intervention of BACH1 may provide new strategies for human disease prevention and treatment.

Rodent model of metabolic dysfunction-associated fatty liver disease: a systematic review

Although significant progress has been made in developing preclinical models for metabolic dysfunction-associated steatotic liver disease (MASLD), few have encapsulated the essential biological and clinical outcome elements reflective of the human condition. We conducted a comprehensive literature review of English-language original research articles published from 1990 to 2023, sourced from PubMed, Embase, and Web of Science, aiming to collate studies that provided a comparative analysis of physiological, metabolic, and hepatic histological characteristics between MASLD models and control groups. The establishment of a robust metabolic dysfunction-associated steatotic liver rodent model hinges on various factors, including animal species and strains, sex, induction agents and methodologies, and the duration of induction. Through this review, we aim to guide researchers in selecting suitable induction methods and animal species for constructing preclinical models aligned with their specific research objectives and laboratory conditions. Future studies should strive to develop simple, reliable, and reproducible models, considering the model's sensitivity to factors such as light-dark cycles, housing conditions, and environmental temperature. Additionally, the potential of diverse in vitro models, including 3D models and liver organ technology, warrants further exploration as valuable tools for unraveling the cellular mechanisms underlying fatty liver disease.

Roles of X-box binding protein 1 in liver pathogenesis

The prevalence of drug-induced liver injury (DILI) and viral liver infections presents significant challenges in modern healthcare and contributes to considerable morbidity and mortality worldwide. Concurrently, metabolic dysfunctionassociated steatotic liver disease (MASLD) has emerged as a major public health concern, reflecting the increasing rates of obesity and leading to more severe complications such as fibrosis and hepatocellular carcinoma. X-box binding protein 1 (XBP1) is a distinct transcription factor with a basic-region leucine zipper structure, whose activity is regulated by alternative splicing in response to disruptions in endoplasmic reticulum (ER) homeostasis and the unfolded protein response (UPR) activation. XBP1 interacts with a key signaling component of the highly conserved UPR and is critical in determining cell fate when responding to ER stress in liver diseases. This review aims to elucidate the emerging roles and molecular mechanisms of XBP1 in liver pathogenesis, focusing on its involvement in DILI, viral liver infections, MASLD, fibrosis/cirrhosis, and liver cancer. Understanding the multifaceted functions of XBP1 in these liver diseases offers insights into potential therapeutic strategies to restore ER homeostasis and mitigate liver damage.

Modulation of SIRT6 related signaling pathways of p-AKT/mTOR and NRF2/HO-1 by memantine contributes to curbing the progression of tamoxifen/HFD-induced MASH in rats

Metabolic dysfunction-associated steatohepatitis (MASH) is a chronic liver disorder marked by hepatic fat accumulation and inflammatory infiltrates which may evolve to cirrhosis. Clinical studies have demonstrated the higher risk of MASH development after tamoxifen (TAM) therapy, especially in obese patients. Therefore, we aimed to evaluate MASH induction by TAM combined with high fat diet (HFD) and the potential interference of memantine (MEMA) with MASH progression via modulation of SIRT6 and its related signaling pathways. MASH was induced in female Wistar rats by co-administration of TAM (25 mg/kg/day, p.o.) and HFD for 5 weeks. Liver function biomarkers, tissue triglyceride and cholesterol, MASH scoring, SIRT6 with its related signals, and lipid synthesis/oxidation markers were estimated. By comparison to MASH group, MEMA improved liver function indices (ALT, AST, ALP, albumin) and reduced the progression of MASH, evidenced by decreased accumulation of lipids in hepatic tissue, improved histological features, and reduced MASH scoring. MEMA enhanced hepatic SIRT6 and downregulated p-AKT/mTOR signaling, that subsequently reduced expressions of the lipid synthesis biomarkers (SREBP1c, SCD), while elevating the lipid oxidation markers (PPAR-α, CPT1). Moreover, MEMA enhanced NRF2/HO-1 signaling, with subsequently improved antioxidant defense and pro-inflammatory/anti-inflammatory cytokines balance. Analysis of SIRT6 correlations with p-AKT/mTOR, NRF2/HO-1, SREBP1c, and PPAR-α further confirmed our results. Consequently, we conclude that MEMA could interfere with MASH progression, at least in part, via enhanced SIRT6 expression and modulation of its related p-AKT/mTOR and NRF2/HO-1 signaling pathways, eventually reducing liver steatosis and inflammation. That could be a promising therapeutic modality for curbing MASH progression.

Uvaol ameliorates lipid deposition in hyperlipidemic hepatocytes by suppressing protein-tyrosine phosphatase 1B/ER stress signaling

Uvaol (UV), a pentacyclic triterpene found in olives and virgin olive oil, is known for its anti-inflammatory and antioxidant effects in various disease models. While olive oil is reported to reduce obesity and insulin resistance, the specific impact of UV on liver lipid metabolism and its molecular mechanisms are not fully understood. In this study, hepatic lipid accumulation was measured using oil red O staining, and protein expression levels in liver cells were assessed via Western blot analysis. Apoptosis was evaluated through cell viability and caspase 3 activity assays. UV treatment reduced lipid accumulation, fatty acid uptake, apoptosis, and ER stress in palmitate-treated liver cells. Additionally, UV enhanced fatty acid oxidation. Mechanistically, increased SIRT6 expression and autophagy were observed in UV-treated cells. SIRT6-targeted siRNA or 3-methyladenine blocked the effects of UV in hyperlipidemic cells. In conclusion, UV improves SIRT6/autophagy signaling, reducing lipid deposition and apoptosis in liver cells under high lipid conditions. This in vitro study provides strong evidence for potential therapeutic strategies for hepatic steatosis.

Hemoglobin scavenger receptor CD163 as a potential biomarker of hemolysis-induced hepatobiliary injury in sickle cell disease

Sickle cell disease (SCD)-associated chronic hemolysis promotes oxidative stress, inflammation, and thrombosis leading to organ damage, including liver damage. Hemoglobin scavenger receptor CD163 plays a protective role in SCD by scavenging both hemoglobin-haptoglobin complexes and cell-free hemoglobin. A limited number of studies in the past have shown a positive correlation of CD163 expression with poor disease outcomes in patients with SCD. However, the role and regulation of CD163 in SCD-related hepatobiliary injury have not been fully elucidated yet. Here we show that chronic liver injury in SCD patients is associated with elevated levels of hepatic membrane-bound CD163. Hemolysis and increase in hepatic heme, hemoglobin, and iron levels elevate CD163 expression in the SCD mouse liver. Mechanistically we show that heme oxygenase-1 (HO-1) positively regulates membrane-bound CD163 expression independent of nuclear factor erythroid 2-related factor 2 (NRF2) signaling in SCD liver. We further demonstrate that the interaction between CD163 and HO-1 is not dependent on CD163-hemoglobin binding. These findings indicate that CD163 is a potential biomarker of SCD-associated hepatobiliary injury. Understanding the role of HO-1 in membrane-bound CD163 regulation may help identify novel therapeutic targets for hemolysis-induced chronic liver injury.

BACH1 impairs hepatocyte regeneration after hepatectomy with repeated ischemia/reperfusion by reprogramming energy metabolism and exacerbating oxidative stress

BTB and CNC homology 1 (BACH1) regulates biological processes, including energy metabolism and oxidative stress. Insufficient liver regeneration after hepatectomy remains an issue for surgeons. The Pringle maneuver is widely used during hepatectomy and induces ischemia/reperfusion (I/R) injury in hepatocytes. A rat model of two-thirds partial hepatectomy with repeated I/R treatment was used to simulate clinical hepatectomy with Pringle maneuver. Delayed recovery of liver function after hepatectomy with the repeated Pringle maneuver in clinic and impaired liver regeneration in rat model were observed. Highly elevated lactate levels, along with reduced mitochondrial complex III and IV activities in liver tissues, indicated that the glycolytic phenotype was promoted after hepatectomy with repeated I/R. mRNA expression profile analysis of glycolysis-related genes in clinical samples and further verification experiments in rat models showed that high BACH1 expression levels correlated with the glycolytic phenotype after hepatectomy with repeated I/R. BACH1 overexpression restricted the proliferative potential of hepatocytes stimulated with HGF. High PDK1 expression and high lactate levels, together with low mitochondrial complex III and IV activities and reduced ATP concentrations, were detected in BACH1-overexpressing hepatocytes with HGF stimulation. Moreover, HO-1 expression was downregulated, and oxidative stress was exacerbated in the BACH1-overexpressing hepatocytes with HGF stimulation. Cell experiments involving repeated hypoxia/reoxygenation revealed that reactive oxygen species accumulation triggered the TGF-β1/BACH1 axis in hepatocytes. Finally, inhibiting BACH1 with the inhibitor hemin effectively restored the liver regenerative ability after hepatectomy with repeated I/R. These results provide a potential therapeutic strategy for impaired liver regeneration after repeated I/R injury.

BACH1 controls hepatic insulin signaling and glucose homeostasis in mice

Hepatic insulin resistance is central to the metabolic syndrome. Here we investigate the role of BTB and CNC homology 1 (BACH1) in hepatic insulin signaling. BACH1 is elevated in the hepatocytes of individuals with obesity and patients with non-alcoholic fatty liver disease (NAFLD). Hepatocyte-specific Bach1 deletion in male mice on a high-fat diet (HFD) ameliorates hyperglycemia and insulin resistance, improves glucose homeostasis, and protects against steatosis, whereas hepatic overexpression of Bach1 in male mice leads to the opposite phenotype. BACH1 directly interacts with the protein-tyrosine phosphatase 1B (PTP1B) and the insulin receptor β (IR-β), and loss of BACH1 reduces the interaction between PTP1B and IR-β upon insulin stimulation and enhances insulin signaling in hepatocytes. Inhibition of PTP1B significantly attenuates BACH1-mediated suppression of insulin signaling in HFD-fed male mice. Hepatic BACH1 knockdown ameliorates hyperglycemia and improves insulin sensitivity in diabetic male mice. These results demonstrate a critical function for hepatic BACH1 in the regulation of insulin signaling and glucose homeostasis.

The many faces of SIRT6 in the retina and retinal pigment epithelium

Sirtuin 6 (SIRT6) is a member of the mammalian sirtuin family of NAD+-dependent protein deacylases, homologues of the yeast silent information regulator 2 (Sir2). SIRT6 has remarkably diverse functions and plays a key role in a variety of biological processes for maintaining cellular and organismal homeostasis. In this review, our primary aim is to summarize recent progress in understanding SIRT6's functions in the retina and retinal pigment epithelium (RPE), with the hope of further drawing interests in SIRT6 to increase efforts in exploring the therapeutic potential of this unique protein in the vision field. Before describing SIRT6's role in the eye, we first discuss SIRT6's general functions in a wide range of biological contexts. SIRT6 plays an important role in gene silencing, metabolism, DNA repair, antioxidant defense, inflammation, aging and longevity, early development, and stress response. In addition, recent studies have revealed SIRT6's role in macrophage polarization and mitochondrial homeostasis. Despite being initially understudied in the context of the eye, recent efforts have begun to elucidate the critical functions of SIRT6 in the retina and RPE. In the retina, SIRT6 is essential for adult retinal function, regulates energy metabolism by suppressing glycolysis that affects photoreceptor cell survival, protects retinal ganglion cells from oxidative stress, and plays a role in Müller cells during early neurodegenerative events in diabetic retinopathy. In the RPE, SIRT6 activates autophagy in culture and protects against oxidative stress in mice. Taken together, this review demonstrates that better understanding of SIRT6's functions and their mechanisms, both in and out of the context of the eye, holds great promise for the development of SIRT6-targeted strategies for prevention and treatment of blinding eye diseases.

Histone Modifications in NAFLD: Mechanisms and Potential Therapy

Nonalcoholic fatty liver disease (NAFLD) is a progressive condition that encompasses a spectrum of liver disorders, beginning with the simple steatosis, progressing to nonalcoholic steatohepatitis (NASH), and possibly leading to more severe diseases, including liver cirrhosis and hepatocellular carcinoma (HCC). In recent years, the prevalence of NAFLD has increased due to a shift towards energy-dense dietary patterns and a sedentary lifestyle. NAFLD is also strongly associated with metabolic disorders such as obesity and hyperlipidemia. The progression of NAFLD could be influenced by a variety of factors, such as diet, genetic factors, and even epigenetic factors. In contrast to genetic factors, epigenetic factors, including histone modifications, exhibit dynamic and reversible features. Therefore, the epigenetic regulation of the initiation and progression of NAFLD is one of the directions under intensive investigation in terms of pathogenic mechanisms and possible therapeutic interventions. This review aims to discuss the possible mechanisms and the crucial role of histone modifications in the framework of epigenetic regulation in NAFLD, which may provide potential therapeutic targets and a scientific basis for the treatment of NAFLD.

Harnessing the Therapeutic Potential of Ginsenoside Rd for Activating SIRT6 in Treating a Mouse Model of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is a prevalent global condition and a common precursor to liver cancer, yet there is currently no specific medication available for its treatment. Ginseng, renowned for its medicinal and dietary properties, has been utilized in NAFLD management, although the precise underlying mechanism remains elusive. To investigate the effectiveness of ginsenoside Rd, we employed mouse and cell models to induce NAFLD using high-fat diets, oleic acid, and palmitic acid. We explored and confirmed the specific mechanism of ginsenoside Rd-induced hepatic steatosis through experiments involving mice with a liver-specific knockout of SIRT6, a crucial protein involved in metabolic regulation. Our findings revealed that administration of ginsenoside Rd significantly reduced the inflammatory response, reactive oxygen species (ROS) levels, lipid peroxide levels, and mitochondrial stress induced by oleic acid and palmitic acid in primary hepatocytes, thereby mitigating excessive lipid accumulation. Moreover, ginsenoside Rd administration effectively enhanced the mRNA content of key proteins involved in fatty acid oxidation, with a particular emphasis on SIRT6 and its target proteins. We further validated that ginsenoside Rd directly binds to SIRT6, augmenting its deacetylase activity. Notably, we made a significant observation that the protective effect of ginsenoside Rd against hepatic disorders induced by a fatty diet was almost entirely reversed in mice with a liver-specific SIRT6 knockout. Our findings highlight the potential therapeutic impact of Ginsenoside Rd in NAFLD treatment by activating SIRT6. These results warrant further investigation into the development of Ginsenoside Rd as a promising agent for managing this prevalent liver disease.

Hepatocyte Sirtuin 6 Protects against Atherosclerosis and Steatohepatitis by Regulating Lipid Homeostasis

Histone deacetylase Sirtuin 6 (SIRT6) regulates many biological processes. SIRT6 is known to regulate hepatic lipid metabolism and inhibit the development of nonalcoholic fatty liver disease (NAFLD). We aimed to investigate the role of hepatocyte SIRT6 in the development of atherosclerosis and further characterize the mechanism underlying SIRT6's effect on NAFLD. Ldlr-/- mice overexpressing or lacking hepatocyte SIRT6 were fed a Western diet for 16 weeks. The role of hepatic SIRT6 in the development of nonalcoholic steatohepatitis (NASH), atherosclerosis, and obesity was investigated. We also investigated whether p53 participates in the pathogenesis of NAFLD in mice overexpressing hepatic SIRT6. Our data show that loss of hepatocyte SIRT6 aggravated the development of NAFLD, atherosclerosis, and obesity in Ldlr-/- mice, whereas adeno-associated virus (AAV)-mediated overexpression of human SIRT6 in the liver had opposite effects. Mechanistically, hepatocyte SIRT6 likely inhibited the development of NAFLD by inhibiting lipogenesis, lipid droplet formation, and p53 signaling. Hepatocyte SIRT6 also likely inhibited the development of atherosclerosis by inhibiting intestinal lipid absorption and hepatic VLDL secretion. Hepatic SIRT6 also increased energy expenditure. In conclusion, our data indicate that hepatocyte SIRT6 protects against atherosclerosis, NAFLD, and obesity by regulating lipid metabolism in the liver and intestine.

SIRT6 overexpression in the nucleus protects mouse retinal pigment epithelium from oxidative stress

Retinal pigment epithelium (RPE) is essential for the survival of retinal photoreceptors. To study retinal degeneration, sodium iodate (NaIO3) has been used to cause oxidative stress-induced RPE death followed by photoreceptor degeneration. However, analyses of RPE damage itself are still limited. Here, we characterized NaIO3-induced RPE damage, which was divided into three regions: periphery with normal-shaped RPE, transitional zone with elongated cells, and center with severely damaged or lost RPE. Elongated cells in the transitional zone exhibited molecular characteristics of epithelial-mesenchymal transition. Central RPE was more susceptible to stresses than peripheral RPE. Under stresses, SIRT6, an NAD+-dependent protein deacylase, rapidly translocated from the nucleus to the cytoplasm and colocalized with stress granule factor G3BP1, leading to nuclear SIRT6 depletion. To overcome this SIRT6 depletion, SIRT6 overexpression was induced in the nucleus in transgenic mice, which protected RPE from NaIO3 and partially preserved catalase expression. These results demonstrate topological differences of mouse RPE and warrant further exploring SIRT6 as a potential target for protecting RPE from oxidative stress-induced damage.

GB1a Activates SIRT6 to Regulate Lipid Metabolism in Mouse Primary Hepatocytes

Lipid accumulation, oxidative stress, and inflammation in hepatocytes are features of nonalcoholic fatty liver disease (NAFLD). Garcinia biflavonoid 1a (GB1a) is a natural product capable of hepatic protection. In this study, the effect of GB1a on anti-inflammatory, antioxidant, and regulation of the accumulation in HepG2 cells and mouse primary hepatocytes (MPHs) was investigated, and its regulatory mechanism was further explored. The result showed that GB1a reduced triglyceride (TG) content and lipid accumulation by regulating the expression of SREBP-1c and PPARα; GB1a reduced reactive oxygen species (ROS) and improved cellular oxidative stress to protect mitochondrial morphology by regulating genes Nrf2, HO-1, NQO1, and Keap1; and GB1a reduced the damage of hepatocytes by inhibiting the expression of inflammatory cytokines interleukin-6 (IL-6), interleukin-1β (IL-1β), tumor necrosis factor-alpha (TNF-α), and nuclear factor kappa B (NF-κB) p65. The activities of GB1a were lost in liver SIRT6-specific knockout mouse primary hepatocytes (SIRT6-LKO MPHs). This indicated that activating SIRT6 was critical for GB1a to perform its activity, and GB1a acted as an agonist of SIRT6. It was speculated that GB1a may be a potential drug for NAFLD treatment.

Pathophysiological role of BACH transcription factors in digestive system diseases

BTB and CNC homologous (BACH) proteins, including BACH1 and BACH2, are transcription factors that are widely expressed in human tissues. BACH proteins form heterodimers with small musculoaponeurotic fibrosarcoma (MAF) proteins to suppress the transcription of target genes. Furthermore, BACH1 promotes the transcription of target genes. BACH proteins regulate physiological processes, such as the differentiation of B cells and T cells, mitochondrial function, and heme homeostasis as well as pathogenesis related to inflammation, oxidative-stress damage caused by drugs, toxicants, or infections; autoimmunity disorders; and cancer angiogenesis, epithelial-mesenchymal transition, chemotherapy resistance, progression, and metabolism. In this review, we discuss the function of BACH proteins in the digestive system, including the liver, gallbladder, esophagus, stomach, small and large intestines, and pancreas. BACH proteins directly target genes or indirectly regulate downstream molecules to promote or inhibit biological phenomena such as inflammation, tumor angiogenesis, and epithelial-mesenchymal transition. BACH proteins are also regulated by proteins, miRNAs, LncRNAs, labile iron, and positive and negative feedback. Additionally, we summarize a list of regulators targeting these proteins. Our review provides a reference for future studies on targeted drugs in digestive diseases.

Sirtuin 6-A Key Regulator of Hepatic Lipid Metabolism and Liver Health

Sirtuin 6 (SIRT6) is an NAD-dependent deacetylase/deacylase/mono-ADP ribosyltransferase, a member of the sirtuin protein family. SIRT6 has been implicated in hepatic lipid homeostasis and liver health. Hepatic lipogenesis is driven by several master regulators including liver X receptor (LXR), carbohydrate response element binding protein (ChREBP), and sterol regulatory element binding protein 1 (SREBP1). Interestingly, these three transcription factors can be negatively regulated by SIRT6 through direct deacetylation. Fatty acid oxidation is regulated by peroxisome proliferator activated receptor alpha (PPARα) in the liver. SIRT6 can promote fatty acid oxidation by the activation of PPARα or the suppression of miR-122. SIRT6 can also directly modulate acyl-CoA synthetase long chain family member 5 (ACSL5) activity for fatty acid oxidation. SIRT6 also plays a critical role in the regulation of total cholesterol and low-density lipoprotein (LDL)-cholesterol through the regulation of SREBP2 and proprotein convertase subtilisin/kexin type 9 (PCSK9), respectively. Hepatic deficiency of Sirt6 in mice has been shown to cause hepatic steatosis, inflammation, and fibrosis, hallmarks of alcoholic and nonalcoholic steatohepatitis. SIRT6 can dampen hepatic inflammation through the modulation of macrophage polarization from M1 to M2 type. Hepatic stellate cells are a key cell type in hepatic fibrogenesis. SIRT6 plays a strong anti-fibrosis role by the suppression of multiple fibrogenic pathways including the transforming growth factor beta (TGFβ)-SMAD family proteins and Hippo pathways. The role of SIRT6 in liver cancer is quite complicated, as both tumor-suppressive and tumor-promoting activities have been documented in the literature. Overall, SIRT6 has multiple salutary effects on metabolic homeostasis and liver health, and it may serve as a therapeutic target for hepatic metabolic diseases. To date, numerous activators and inhibitors of SIRT6 have been developed for translational research.

The sirtuin family in health and disease

Sirtuins (SIRTs) are nicotine adenine dinucleotide(+)-dependent histone deacetylases regulating critical signaling pathways in prokaryotes and eukaryotes, and are involved in numerous biological processes. Currently, seven mammalian homologs of yeast Sir2 named SIRT1 to SIRT7 have been identified. Increasing evidence has suggested the vital roles of seven members of the SIRT family in health and disease conditions. Notably, this protein family plays a variety of important roles in cellular biology such as inflammation, metabolism, oxidative stress, and apoptosis, etc., thus, it is considered a potential therapeutic target for different kinds of pathologies including cancer, cardiovascular disease, respiratory disease, and other conditions. Moreover, identification of SIRT modulators and exploring the functions of these different modulators have prompted increased efforts to discover new small molecules, which can modify SIRT activity. Furthermore, several randomized controlled trials have indicated that different interventions might affect the expression of SIRT protein in human samples, and supplementation of SIRT modulators might have diverse impact on physiological function in different participants. In this review, we introduce the history and structure of the SIRT protein family, discuss the molecular mechanisms and biological functions of seven members of the SIRT protein family, elaborate on the regulatory roles of SIRTs in human disease, summarize SIRT inhibitors and activators, and review related clinical studies.

Human centenarian-associated SIRT6 mutants modulate hepatocyte metabolism and collagen deposition in multilineage hepatic 3D spheroids

Non-alcoholic fatty liver disease (NAFLD), encompassing fatty liver and its progression into nonalcoholic steatohepatitis (NASH), fibrosis, cirrhosis, and hepatocellular carcinoma (HCC), is one of the rapidly rising health concerns worldwide. SIRT6 is an essential nuclear sirtuin that regulates numerous pathological processes including insulin resistance and inflammation, and recently it has been implicated in the amelioration of NAFLD progression. SIRT6 overexpression protects from formation of fibrotic lesions. However, the underlying molecular mechanisms are not fully delineated. Moreover, new allelic variants of SIRT6 (N308K/A313S) were recently associated with the longevity in Ashkenazi Jews by improving genome maintenance and DNA repair, suppressing transposons and killing cancer cells. Whether these new SIRT6 variants play different or enhanced roles in liver diseases is currently unknown. In this study, we aimed to clarify how these new centenarian-associated SIRT6 genetic variants affect liver metabolism and associated diseases. We present evidence that overexpression of centenarian-associated SIRT6 variants dramatically altered the metabolomic and secretomic profiles of unchallenged immortalized human hepatocytes (IHH). Most amino acids were increased in the SIRT6 N308K/A313S overexpressing IHH when compared to IHH transfected with the SIRT6 wild-type sequence. Several unsaturated fatty acids and glycerophospholipids were increased, and ceramide tended to be decreased upon SIRT6 N308K/A313S overexpression. Furthermore, we found that overexpression of SIRT6 N308K/A313S in a 3D hepatic spheroid model formed by the co-culture of human immortalized hepatocytes (IHH) and hepatic stellate cells (LX2) inhibited collagen deposition and fibrotic gene expression in absence of metabolic or dietary challenges. Hence, our findings suggest that novel longevity associated SIRT6 N308K/A313S variants could favor the prevention of NASH by altering hepatocyte proteome and lipidome.

SIRT6 in Aging, Metabolism, Inflammation and Cardiovascular Diseases

As an important NAD⁺-dependent enzyme, SIRT6 has received significant attention since its discovery. In view of observations that SIRT6-deficient animals exhibit genomic instability and metabolic disorders and undergo early death, SIRT6 has long been considered a protein of longevity. Recently, growing evidence has demonstrated that SIRT6 functions as a deacetylase, mono-ADP-ribosyltransferase and long fatty deacylase and participates in a variety of cellular signaling pathways from DNA damage repair in the early stage to disease progression. In this review, we elaborate on the specific substrates and molecular mechanisms of SIRT6 in various physiological and pathological processes in detail, emphasizing its links to aging (genomic damage, telomere integrity, DNA repair), metabolism (glycolysis, gluconeogenesis, insulin secretion and lipid synthesis, lipolysis, thermogenesis), inflammation and cardiovascular diseases (atherosclerosis, cardiac hypertrophy, heart failure, ischemia-reperfusion injury). In addition, the most recent advances regarding SIRT6 modulators (agonists and inhibitors) as potential therapeutic agents for SIRT6-mediated diseases are reviewed.

Peroxisome proliferator-activated receptor γ coactivator 1α maintains NAD+ bioavailability protecting against steatohepatitis

Hepatic metabolic derangements are pivotal incidences in the occurrence of hepatic steatosis, inflammation, and fibrosis. Peroxisome proliferator-activated receptor-γ, coactivator-1α (PGC-1α), a master regulator that mediates adipose metabolism and mitochondrial biogenesis, its role in hepatic steatosis and progression to steatohepatitis remains elusive. By surveying genomic data on nonalcoholic steatohepatitis (NASH) patients available in the Gene Expression Omnibus, we found that PGC-1α was significantly down-regulated compared with healthy controls, implicating the restoration of PGC-1α may ameliorate the hepatopathy. Using a hepatocyte-specific PGC-1α overexpression (LivPGC1α) mouse model, we demonstrated that PGC-1α attenuated hepatic steatosis induced by methionine-choline-deficient diet (MCD). Biochemical measurements and histological examination indicated less inflammatory infiltration, collagen deposition, NF-kB activation, and less lipid accumulation in LivPGC1α liver fed MCD. Further analyses indicated that the NAD+-dependent deacetylase sirtuin 2 (SIRT2) interacted with and deacetylated PGC-1α. Congruently, ablation of SIRT2 accelerated the NASH progression in mice fed MCD, while NAD+ repletion via its precursor mimicked the beneficial effect of PGC-1α overexpression and was sufficient to alleviate NASH in mice. These findings indicate that hepatic-specific overexpression of PGC-1α exerts a beneficial role in the regulation of steatohepatitis and that pharmacological activation of the SIRT2-PGC-1α-NAD+ axis may help to treat NASH.

Progress in Nonalcoholic Fatty Liver Disease: SIRT Family Regulates Mitochondrial Biogenesis

Nonalcoholic fatty liver disease (NAFLD) is characterized by hepatic steatosis, insulin resistance, mitochondrial dysfunction, inflammation, and oxidative stress. As a group of NAD+-dependent III deacetylases, the sirtuin (SIRT1-7) family plays a very important role in regulating mitochondrial biogenesis and participates in the progress of NAFLD. SIRT family members are distributed in the nucleus, cytoplasm, and mitochondria; regulate hepatic fatty acid oxidation metabolism through different metabolic pathways and mechanisms; and participate in the regulation of mitochondrial energy metabolism. SIRT1 may improve NAFLD by regulating ROS, PGC-1α, SREBP-1c, FoxO1/3, STAT3, and AMPK to restore mitochondrial function and reduce steatosis of the liver. Other SIRT family members also play a role in regulating mitochondrial biogenesis, fatty acid oxidative metabolism, inflammation, and insulin resistance. Therefore, this paper comprehensively introduces the role of SIRT family in regulating mitochondrial biogenesis in the liver in NAFLD, aiming to further explain the importance of SIRT family in regulating mitochondrial function in the occurrence and development of NAFLD, and to provide ideas for the research and development of targeted drugs. Relatively speaking, the role of some SIRT family members in NAFLD is still insufficiently clear, and further research is needed.

Nonalcoholic steatohepatitis and mechanisms by which it is ameliorated by activation of the CNC-bZIP transcription factor Nrf2

Non-alcoholic steatohepatitis (NASH) represents a global health concern. It is characterised by fatty liver, hepatocyte cell death and inflammation, which are associated with lipotoxicity, endoplasmic reticulum (ER) stress, mitochondrial dysfunction, iron overload and oxidative stress. NF-E2 p45-related factor 2 (Nrf2) is a transcription factor that combats oxidative stress. Remarkably, Nrf2 is downregulated during the development of NASH, which probably accelerates disease, whereas in pre-clinical studies the upregulation of Nrf2 inhibits NASH. We now review the scientific literature that proposes Nrf2 downregulation during NASH involves its increased ubiquitylation and proteasomal degradation, mediated by Kelch-like ECH-associated protein 1 (Keap1) and/or β-transducin repeat-containing protein (β-TrCP) and/or HMG-CoA reductase degradation protein 1 (Hrd1, also called synoviolin (SYVN1)). Additionally, downregulation of Nrf2-mediated transcription during NASH may involve diminished recruitment of coactivators by Nrf2, due to increased levels of activating transcription factor 3 (ATF3) and nuclear factor-kappaB (NF-κB) p65, or competition for promoter binding due to upregulation of BTB and CNC homology 1 (Bach1). Many processes that downregulate Nrf2 are triggered by transforming growth factor-beta (TGF-β), with oxidative stress amplifying its signalling. Oxidative stress may also increase suppression of Nrf2 by β-TrCP through facilitating formation of the DSGIS-containing phosphodegron in Nrf2 by glycogen synthase kinase-3. In animal models, knockout of Nrf2 increases susceptibility to NASH, while pharmacological activation of Nrf2 by inducing agents that target Keap1 inhibits development of NASH. These inducing agents probably counter Nrf2 downregulation affected by β-TrCP, Hrd1/SYVN1, ATF3, NF-κB p65 and Bach1, by suppressing oxidative stress. Activation of Nrf2 is also likely to inhibit NASH by ameliorating lipotoxicity, inflammation, ER stress and iron overload. Crucially, pharmacological activation of Nrf2 in mice in which NASH has already been established supresses liver steatosis and inflammation. There is therefore compelling evidence that pharmacological activation of Nrf2 provides a comprehensive multipronged strategy to treat NASH.

Sirtuins and cognition: implications for learning and memory in neurological disorders

Sirtuins are an evolutionarily conserved family of regulatory proteins that function in an NAD+ -dependent manner. The mammalian family of sirtuins is composed of seven histone deacetylase and ADP-ribosyltransferase proteins (SIRT1-SIRT7) that are found throughout the different cellular compartments of the cell. Sirtuins in the brain have received considerable attention in cognition due to their role in a plethora of metabolic and age-related diseases and their ability to induce neuroprotection. More recently, sirtuins have been shown to play a role in normal physiological cognitive function, and aberrant sirtuin function is seen in pathological cellular states. Sirtuins are believed to play a role in cognition through enhancing synaptic plasticity, influencing epigenetic regulation, and playing key roles in molecular pathways involved with oxidative stress affecting mitochondrial function. This review aims to discuss recent advances in the understanding of the role of mammalian sirtuins in cognitive function and the therapeutic potential of targeting sirtuins to ameliorate cognitive deficits in neurological disorders.

Impaired reciprocal regulation between SIRT6 and TGF-β signaling in fatty liver

Dysregulated transforming growth factor-beta (TGF-β) signaling contributes to fibrotic liver disease and hepatocellular cancer (HCC), both of which are associated with fatty liver disease. SIRT6 limits fibrosis by inhibiting TGF-β signaling through deacetylating SMAD2 and SMAD3 and limits lipogenesis by inhibiting SREBP1 and SREBP2 activity. Here, we showed that, compared to wild-type mice, high-fat diet-induced fatty liver is worse in TGF-β signaling-deficient mice (SPTBN1+/- ) and the mutant mice had reduced SIRT6 abundance in the liver. Therefore, we hypothesized that altered reciprocal regulation between TGF-β signaling and SIRT6 contributes to these liver pathologies. We found that deficiency in SMAD3 or SPTBN1 reduced SIRT6 mRNA and protein abundance and impaired TGF-β induction of SIRT6 transcripts, and that SMAD3 bound to the SIRT6 promoter, suggesting that an SMAD3-SPTBN1 pathway mediated the induction of SIRT6 in response to TGF-β. Overexpression of SIRT6 in HCC cells reduced the expression of TGF-β-induced genes, consistent with the suppressive role of SIRT6 on TGF-β signaling. Manipulation of SIRT6 abundance in HCC cells altered sterol regulatory element-binding protein (SREBP) activity and overexpression of SIRT6 reduced the amount of acetylated SPTBN1 and the abundance of both SMAD3 and SPTBN1. Furthermore, induction of SREBP target genes in response to SIRT6 overexpression was impaired in SPTBN1 heterozygous cells. Thus, we identified a regulatory loop between SIRT6 and SPTBN1 that represents a potential mechanism for susceptibility to fatty liver in the presence of dysfunctional TGF-β signaling.

Hepatic SIRT6 deficit promotes liver tumorigenesis in the mice models

SIRT6 belongs to class III sirtuin family with NAD+-dependent histone deacetylase activities and controls multiple processes including aging, metabolism and inflammation. In recent years, increasing studies showed tumor suppressor role of SIRT6 in HCC development. We established a two-stage DEN followed CCl4 induced liver carcinogenesis in the hepatic-specific SIRT6 HKO mice models and found that hepatic SIRT6 deficit significantly promotes liver injury and liver cancer through inhibition of the ERK1/2 pathway. SIRT6 was compensatory upregulated in mice tumor tissues and human HCC cells and overexpressed SIRT6 inhibits tumor growth both in vitro and in vivo. Taken together, we provide a useful mouse model for delineating the molecular pathways involved in chronic liver diseases and primary liver cancer and suggest that SIRT6 can be a promising target for HCC therapies.

Sirtuin 6 is a negative regulator of FcεRI signaling and anaphylactic responses

BACKGROUND

Binding IgE to a cognate allergen causes aggregation of Fcε receptor I (FcεRI) in mast cells, resulting in activation of receptor-associated Src family tyrosine kinases, including Lyn and Syk. Protein tyrosine phosphatase, receptor type C (PTPRC), also known as CD45, has emerged as a positive regulator of FcεRI signaling by dephosphorylation of the inhibitory tyrosine of Lyn.

OBJECTIVE

Sirtuin 6 (Sirt6), a NAD+-dependent deacetylase, exhibits an anti-inflammatory property. It remains to be determined, however, whether Sirt6 attenuates mast cell-associated diseases, including anaphylaxis.

METHODS

FcεRI signaling and mast cell degranulation were measured after IgE cross-linking in murine bone marrow-derived mast cells (BMMCs) and human cord blood-derived mast cells. To investigate the function of Sirt6 in mast cell activation in vivo, we used mast cell-dependent animal models of passive systemic anaphylaxis (PSA) and passive cutaneous anaphylaxis (PCA).

RESULTS

Sirt6-deficient BMMCs augmented IgE-FcεRI-mediated signaling and degranulation compared to wild-type BMMCs. Reconstitution of mast cell-deficient KitW-sh/W-sh mice with BMMCs received from Sirt6 knockout mice developed more severe PSA and PCA compared to mice engrafted with wild-type BMMCs. Similarly, genetic overexpression or pharmacologic activation of Sirt6 suppressed mast cell degranulation and blunted responses to PCA. Mechanistically, Sirt6 deficiency increased PTPRC transcription via acetylating histone H3, leading to enhanced aggregation of FcεRI in BMMCs. Finally, we recapitulated the Sirt6 regulation of PTPRC and FcεRI signaling in human mast cells.

CONCLUSIONS

Sirt6 acts as a negative regulator of FcεRI signaling cascade in mast cells by suppressing PTPRC transcription. Activation of Sirt6 may therefore represent a promising and novel therapeutic strategy for anaphylaxis.

Sirtuin 6: linking longevity with genome and epigenome stability

Sirtuin 6 (SIRT6) has been in the spotlight of aging research because progeroid phenotypes are associated with SIRT6 deficiency. SIRT6 has multiple molecular functions, including DNA repair and heterochromatin regulation, which position SIRT6 as a hub that regulates genome and epigenome stability. Genomic instability caused by persistent DNA damage and accumulating mutations, together with alterations in the epigenetic landscape and derepression of repetitive genetic elements, have emerged as mechanisms driving organismal aging. Enhanced levels of SIRT6 expression or activity provide avenues for rejuvenation strategies. This review focuses on the role of SIRT6 in the maintenance of genome and epigenome stability and its link to longevity. We propose a model where SIRT6 together with lamins control aging and rejuvenation by maintaining epigenetic silencing of repetitive elements.

Total Sesquiterpene Glycosides from Loquat Leaves Ameliorate HFD-Induced Insulin Resistance by Modulating IRS-1/GLUT4, TRPV1, and SIRT6/Nrf2 Signaling Pathways

Loquat (Eriobotrya japonica Lindl.), a subtropical fruit tree native to Asia, is not only known to be nutritive but also beneficial for the treatment of diabetes in the south of China. To expand its development, this study was undertaken concerning the potential therapeutic role of total sesquiterpene glycosides (TSGs) from loquat leaves in insulin resistance (IR), the major causative factor of type 2 diabetes mellitus (T2DM). Male C57BL/6 mice were fed on high-fat diet (HFD) to induce IR and then were given TSG by oral administration at 25 and 100 mg/kg/day, respectively. TSG notably improved metabolic parameters including body weight, serum glucose, and insulin levels and prevented hepatic injury. Moreover, inflammatory response and oxidative stress were found to be remarkably alleviated in IR mice with TSG supplement. Further research in liver of IR mice demonstrated that TSG repaired the signalings of insulin receptor substrate-1 (IRS-1)/glucose transporter member 4 (GLUT4) and AMP-activated protein kinase (AMPK), which improved glucose and lipid metabolism and prevented lipid accumulation in liver. It was also observed that TSG suppressed the expression of transient receptor potential vanilloid 1 (TRPV1), whereas the signaling pathway of sirtuin-6 (SIRT6)/nuclear factor erythroid 2-related factor 2 (Nrf2) was significantly promoted. Based on the results, the current study demonstrated that TSG from loquat leaves potentially ameliorated IR in vivo by enhancing IRS-1/GLUT4 signaling and AMPK activation and modulating TRPV1 and SIRT6/Nrf2 signaling pathways.

SIRT6 inhibits inflammatory response through regulation of NRF2 in vascular endothelial cells

Emerging evidence suggests that inflammation plays a pivotal role in Atherosclerosis. Sirtuin 6 (SIRT6), a member of NAD+-dependent protein lysine deacylases of the sirtuin family, plays an important role in the regulation of metabolism, aging and stress resistance. However, the role of SIRT6 in vascular inflammation and its molecular mechanism is unknown. The present study showed that TNF-α significantly reduced the expression of SIRT6 protein and mRNA in a concentration- and time-dependent manner and increased the expression of monocyte chemotactic protein 1 (MCP-1), interleukin (IL) -6 and IL-1β in human umbilical vein endothelial cells (HUVECs). Overexpression of SIRT6 but not its catalytically inactive mutant inhibited TNF-α-induced expression of MCP-1, IL-6 and IL-1β. Knockdown of SIRT6 significantly enhanced TNF-α-induced expression of MCP-1, IL-6 and IL-1β. Moreover, knockdown of SIRT6 reduced TNF-α-induced nuclear factor erythroid 2 related factor 2 (NRF2) nucleus protein expression, whereas knockdown of NRF2 significantly enhanced TNF-α-induced expression of MCP-1, IL-6 and IL-1β. In addition, overexpression of SIRT6 increased NRF2 and its target genes expression, and knockdown of SIRT6 decreased NRF2 and its target genes expression. Meanwhile, knockdown of SIRT6 inhibited NRF2 nucleus protein expression. Further, knockdown of SIRT6 decreased phosphorylation of NRF2, overexpression of SIRT6 increased phosphorylation of NRF2. SIRT6 interacted with NRF2. In vivo, the levels of TNF-α and IL-1β were increased in the serum of hyperlipidemia mice. Hyperlipidemia-induced production of MCP-1, IL-6 and IL-1β was significantly augmented in the endothelium specific SIRT6 knockout mice. In contrast, the expression of NRF2 and its target genes was reduced. Taken together, these results indicate that SIRT6 protects against vascular inflammation via its deacetylase activity and the NRF2-dependent signaling pathway.

Signal Transduction and Molecular Regulation in Fatty Liver Disease

Significance: Fatty liver disease is a major liver disorder in the modern societies. Comprehensive understanding of the pathophysiology and molecular mechanisms is essential for the prevention and treatment of the disease. Recent Advances: Remarkable progress has been made in the recent years in basic and translational research in the field of fatty liver disease. Multiple signaling pathways have been implicated in the development of fatty liver disease, including AMP-activated protein kinase, mechanistic target of rapamycin kinase, endoplasmic reticulum stress, oxidative stress, inflammation, transforming growth factor β, and yes1-associated transcriptional regulator/transcriptional coactivator with PDZ-binding motif (YAP/TAZ). In addition, critical molecular regulations at the transcriptional and epigenetic levels have been linked to the pathogenesis of fatty liver disease. Critical Issues: Some critical issues remain to be solved so that research findings can be translated into clinical applications. Robust and reliable biomarkers are needed for diagnosis of different stages of the fatty liver disease. Effective and safe molecular targets remain to be identified and validated. Prevention strategies require solid scientific evidence and population-wide feasibility. Future Directions: As more data are generated with time, integrative approaches are needed to comprehensively understand the disease pathophysiology and mechanisms at multiple levels from population, organismal system, organ/tissue, to cell. The interactions between genes and environmental factors require deeper investigation for the purposes of prevention and personalized treatment of fatty liver disease. Antioxid. Redox Signal. 35, 689-717.

Loss of Sirt6 in adipocytes impairs the ability of adipose tissue to adapt to intermittent fasting

Intermittent fasting (IF) is gaining popularity for its effectiveness in improving overall health, including its effectiveness in achieving weight loss and euglycemia. The molecular mechanisms of IF, however, are not well understood. This study investigated the relationship between adipocyte sirtuin 6 (Sirt6) and the metabolic benefits of IF. Adipocyte-specific Sirt6-knockout (aS6KO) mice and wild-type littermates were fed a high-fat diet (HFD) ad libitum for four weeks and then subjected to 12 weeks on a 2:1 IF regimen consisting of two days of feeding followed by one day of fasting. Compared with wild-type mice, aS6KO mice subjected to HFD + IF exhibited a diminished response, as reflected by their glucose and insulin intolerance, reduced energy expenditure and adipose tissue browning, and increased inflammation of white adipose tissue. Sirt6 deficiency in hepatocytes or in myeloid cells did not impair adaptation to IF. Finally, the results indicated that the impaired adipose tissue browning and reduced expression of UCP1 in aS6KO mice were accompanied by downregulation of p38 MAPK/ATF2 signaling. Our findings indicate that Sirt6 in adipocytes is critical to obtaining the improved glucose metabolism and metabolic profiles conferred by IF and that maintaining high levels of Sirt6 in adipocytes may mimic the health benefits of IF.

A comprehensive review of Sirtuins: With a major focus on redox homeostasis and metabolism

Sirtuins are Class III protein deacetylases with seven conserved isoforms. In general, Sirtuins are highly activated under cellular stress conditions in which NAD⁺ levels are increased. Nevertheless, regulation of Sirtuins extends far beyond the influences of cellular NAD⁺/NADH ratio and a rapidly expanding body of evidence currently suggests that their expression and catalytic activity are highly kept under control at multiple levels by various factors and processes. Owing to their intrinsic ability to enzymatically target various intracellular proteins, Sirtuins are prominently involved in the regulation of fundamental biological processes including inflammation, metabolism, redox homeostasis, DNA repair and cell proliferation and senescence. In fact, Sirtuins are well established to regulate and reprogram different redox and metabolic pathways under both pathological and physiological conditions. Therefore, alterations in Sirtuin levels can be a pivotal intermediary step in the pathogenesis of several disorders. This review first highlights the mechanisms involved in the regulation of Sirtuins and further summarizes the current findings on the major functions of Sirtuins in cellular redox homeostasis and bioenergetics (glucose and lipid metabolism).

Oxidative Stress-A Key Player in the Course of Alcohol-Related Liver Disease

Oxidative stress is known to be an inseparable factor involved in the presentation of liver disorders. Free radicals interfere with DNA, proteins, and lipids, which are crucial in liver metabolism, changing their expression and biological functions. Additionally, oxidative stress modifies the function of micro-RNAs, impairing the metabolism of hepatocytes. Free radicals have also been proven to influence the function of certain transcriptional factors and to alter the cell cycle. The pathological appearance of alcohol-related liver disease (ALD) constitutes an ideal example of harmful effects due to the redox state. Finally, ethanol-induced toxicity and overproduction of free radicals provoke irreversible changes within liver parenchyma. Understanding the underlying mechanisms associated with the redox state in the course of ALD creates new possibilities of treatment for patients. The future of hepatology may become directly dependent on the effective action against reactive oxygen species. This review summarizes current data on the redox state in the natural history of ALD, highlighting the newest reports on this topic.

Regulation of Nrf2 signaling pathway in heart failure: Role of extracellular vesicles and non-coding RNAs

The balance between pro- and antioxidant molecules has been established as an important driving force in the pathogenesis of cardiovascular disease. Chronic heart failure is associated with oxidative stress in the myocardium and globally. Redox balance in the heart and brain is controlled, in part, by antioxidant proteins regulated by the transcription factor Nuclear factor erythroid 2-related factor 2 (Nrf2), which is reduced in the heart failure state. Nrf2 can, in turn, be regulated by a variety of mechanisms including circulating microRNAs (miRNAs) encapsulated in extracellular vesicles (EVs) derived from multiple cell types in the heart. Here, we review the role of the Nrf2 and antioxidant enzyme signaling pathway in mediating redox balance in the myocardium and the brain in the heart failure state. This review focuses on Nrf2 and antioxidant protein regulation in the heart and brain by miRNA-enriched EVs in the setting of heart failure. We discuss EV-mediated intra- and inter-organ communications especially, communication between the heart and brain via an EV pathway that mediates cardiac function and sympatho-excitation in heart failure. Importantly, we speculate how engineered EVs with specific miRNAs or antagomirs may be used in a therapeutic manner in heart failure.

Exercise ameliorates insulin resistance and improves SIRT6-mediated insulin signaling transduction in liver of obese rats

Physical exercise is essential for the amelioration of insulin resistance (IR). The mechanisms in charge of improved IR, regulated by exercise, are insufficiently studied. Previous research revealed that Sirtuin 6 (SIRT6) - mediated insulin signaling acts a crucial element in hepatic IR. The objective of our research was to determine the effects of exercise on SIRT6-mediated insulin signaling in liver of IR rats. Forty male Sprague Dawley rats were randomly assigned to four groups (n = 10 rats each): control rats fed with standard chow (Lean group); sedentary rats fed with a high-fat diet (HFD-SED); rats fed with HFD and submitted to 8 week chronic swimming exercise training (HFD-CE); and rats fed HFD and submitted to one acute swimming exercise training (HFD-AE). HFD feeding lead to increased body weight, accumulation of hepatic triglyceride and serum free fatty acids, and enhanced gluconeogenesis. Besides, HFD feeding decreased body insulin sensitivity. Hepatic USP10 and SIRT6 protein levels decreased under obese status. Both chronic and acute exercise intervention alleviated physiological and metabolic status, increased hepatic USP10 and SIRT6 levels, improved insulin signaling transduction, and inhibited gluconeogenesis. These results showed that exercise intervention regulated SIRT6-mediated insulin signaling, which contributes to our understanding of the molecular mechanisms behind IR, in that a regular exercise can mitigate the effects of IR.

SIRT6 in Senescence and Aging-Related Cardiovascular Diseases

SIRT6 belongs to the nicotinamide adenine dinucleotide (NAD+)-dependent deacetylases and has established diverse roles in aging, metabolism and disease. Its function is similar to the Silent Information Regulator 2 (SIR2), which prolongs lifespan and regulates genomic stability, telomere integrity, transcription, and DNA repair. It has been demonstrated that increasing the sirtuin level through genetic manipulation extends the lifespan of yeast, nematodes and flies. Deficiency of SIRT6 induces chronic inflammation, autophagy disorder and telomere instability. Also, these cellular processes can lead to the occurrence and progression of cardiovascular diseases (CVDs), such as atherosclerosis, hypertrophic cardiomyopathy and heart failure. Herein, we discuss the implications of SIRT6 regulates multiple cellular processes in cell senescence and aging-related CVDs, and we summarize clinical application of SIRT6 agonists and possible therapeutic interventions in aging-related CVDs.

Role of HO-1 against Saturated Fatty Acid-Induced Oxidative Stress in Hepatocytes

Increased circulating levels of free fatty acids, especially saturated ones, are involved in disease progression in the non-alcoholic fatty liver. Although the mechanism of saturated fatty acid-induced toxicity in the liver is not fully understood, oxidative stress may be deeply involved. We examined the effect of increased palmitic acid, the most common saturated fatty acid in the blood, on the liver of BALB/c mice via tail vein injection with palmitate. After 24 h, among several anti-oxidative stress response genes, only heme oxygenase-1 (HO-1) was significantly upregulated in palmitate-injected mice compared with that in vehicle-injected mice. Elevation of HO-1 mRNA was also observed in the fatty liver of high-fat-diet-fed mice. To further investigate the role of HO-1 on palmitic acid-induced oxidative stress, in vitro experiments were performed to expose palmitate to HepG2 cells. SiRNA-mediated knockdown of HO-1 significantly increased the oxidative stress induced by palmitate, whereas pre-treatment with SnCl₂, a well-known HO-1 inducer, significantly decreased it. Moreover, SB203580, a selective p38 inhibitor, reduced HO-1 mRNA expression and increased palmitate-induced oxidative stress in HepG2 cells. These results suggest that the HO-1-mediated anti-oxidative stress compensatory reaction plays an essential role against saturated fatty acid-induced lipotoxicity in the liver.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

Sirt6 Alleviated Liver Fibrosis by Deacetylating Conserved Lysine 54 on Smad2 in Hepatic Stellate Cells

BACKGROUNDS AND AIMS

Activation of hepatic stellate cells (HSCs) is a central driver of fibrosis. This study aimed to elucidate the role of the deacetylase sirtuin 6 (Sirt6) in HSC activation and liver fibrosis.

APPROACH AND RESULTS

Gain-of-function and loss-of-function models were used to study the function of Sirt6 in HSC activation. Mass spectrometry was used to determine the specific acetylation site. The lecithin retinol acyltransferase-driven cyclization recombination recombinase construct (CreERT2) mouse line was created to generate HSC-specific conditional Sirt6-knockout mice (Sirt6^△HSC ). We found that Sirt6 is most abundantly expressed in HSCs as compared with other liver cell types. The expression of Sirt6 was decreased in activated HSCs and fibrotic livers of mice and humans. Sirt6 knockdown and Sirt6 overexpression increased and decreased fibrogenic gene expression, respectively, in HSCs. Mechanistically, Sirt6 inhibited the phosphorylation and nuclear localization of mothers against decapentaplegic homolog (Smad) 2. Further study demonstrated that Sirt6 could directly interact with Smad2, deacetylate Smad2, and decrease the transcription of transforming growth factor β/Smad2 signaling. Mass spectrometry revealed that Sirt6 deacetylated conserved lysine 54 on Smad2. Mutation of lysine 54 to Arginine in Smad2 abolished the regulatory effect of Sirt6. In vivo, specific ablation of Sirt6 in HSCs exacerbated hepatocyte injury and cholestasis-induced liver fibrosis in mice. With targeted delivery of the Sirt6 agonist MDL-800, its concentration was 9.28-fold higher in HSCs as compared with other liver cells and alleviated hepatic fibrosis.

CONCLUSIONS

Sirt6 plays a key role in HSC activation and liver fibrosis by deacetylating the profibrogenic transcription factor Smad2. Sirt6 may be a potential therapeutic target for liver fibrosis.

Sirtuin 6 supra-physiological overexpression in hypothalamic pro-opiomelanocortin neurons promotes obesity via the hypothalamus-adipose axis

Sirtuin 6 (Sirt6), a member of the Sirtuin family, has important roles in maintaining glucose and lipid metabolism. Our previous studies demonstrated that the deletion of Sirt6 in pro-opiomelanocortin (POMC)-expressing cells by the loxP-Cre system resulted in severe obesity and hepatic steatosis. However, whether overexpression of Sirt6 in hypothalamic POMC neurons could ameliorate diet-induced obesity is still unknown. Thus, we generated mice specifically overexpressing Sirt6 in hypothalamic POMC neurons (PSOE) by stereotaxic injection of Cre-dependent adeno-associated viruses into the arcuate nucleus of Pomc-Cre mice. PSOE mice showed increased adiposity and decreased energy expenditure. Furthermore, thermogenesis of BAT and lipolysis of WAT were both impaired, caused by reduced sympathetic nerve innervation and activity in adipose tissues. Mechanistically, Sirt6 overexpression decreasing STAT3 acetylation, thus lowering POMC expression in the hypothalamus underlined the observed phenotypes in PSOE mice. These results demonstrate that Sirt6 overexpression specifically in the hypothalamic POMC neurons exacerbates diet-induced obesity and metabolic disorders via the hypothalamus-adipose axis.

Emerging roles of SIRT6 in human diseases and its modulators

The biological functions of sirtuin 6 (SIRT6; e.g., deacetylation, defatty-acylation, and mono-ADP-ribosylation) play a pivotal role in regulating lifespan and several fundamental processes controlling aging such as DNA repair, gene expression, and telomeric maintenance. Over the past decades, the aberration of SIRT6 has been extensively observed in diverse life-threatening human diseases. In this comprehensive review, we summarize the critical roles of SIRT6 in the onset and progression of human diseases including cancer, inflammation, diabetes, steatohepatitis, arthritis, cardiovascular diseases, neurodegenerative diseases, viral infections, renal and corneal injuries, as well as the elucidation of the related signaling pathways. Moreover, we discuss the advances in the development of small molecule SIRT6 modulators including activators and inhibitors as well as their pharmacological profiles toward potential therapeutics for SIRT6-mediated diseases.

AGER1 downregulation associates with fibrosis in nonalcoholic steatohepatitis and type 2 diabetes

Type 2 diabetes is clinically associated with progressive necroinflammation and fibrosis in nonalcoholic steatohepatitis (NASH). Advanced glycation end-products (AGEs) accumulate during prolonged hyperglycemia, but the mechanistic pathways that lead to accelerated liver fibrosis have not been well defined. In this study, we show that the AGEs clearance receptor AGER1 was downregulated in patients with NASH and diabetes and in our NASH models, whereas the proinflammatory receptor RAGE was induced. These findings were associated with necroinflammatory, fibrogenic, and pro-oxidant activity via the NADPH oxidase 4. Inhibition of AGEs or RAGE deletion in hepatocytes in vivo reversed these effects. We demonstrate that dysregulation of NRF2 by neddylation of cullin 3 was linked to AGER1 downregulation and that induction of NRF2 using an adeno-associated virus-mediated approach in hepatocytes in vivo reversed AGER1 downregulation, lowered the level of AGEs, and improved proinflammatory and fibrogenic responses in mice on a high AGEs diet. In patients with NASH and diabetes or insulin resistance, low AGER1 levels were associated with hepatocyte ballooning degeneration and ductular reaction. Collectively, prolonged exposure to AGEs in the liver promotes an AGER1/RAGE imbalance and consequent redox, inflammatory, and fibrogenic activity in NASH.

Chemical Activators of the Nrf2 Signaling Pathway in Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is paralleling the insulin resistance and obesity epidemic and is regarded as liver metabolic syndrome, and its prevalence rate is increasing rapidly. The best explanation for the occurrence and development of NAFLD is the “multiple hit” hypothesis instead of the “two-hit” hypothesis. At present, NAFLD therapies are limited. The nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway is a key pathway in oxidative stress. Its downstream proteins/enzymes are regulated. Metabolic enzymes and antioxidant proteins/enzymes play a vital role in cell defense protection and have attracted attention in the field of antioxidant research in recent years. This paper summarizes the regulatory mechanism of the Nrf2 signaling pathway and the research progress of Nrf2 activators in NAFLD to provide guidance for NAFLD therapy in the future.

Downregulation of FOXO6 alleviates hypoxia-induced apoptosis and oxidative stress in cardiomyocytes by enhancing Nrf2 activation via upregulation of SIRT6

Forkhead box protein O6 (FOXO6) has been recently identified as a novel regulator of oxidative stress in multiple pathological processes. However, whether FOXO6 participates in the regulation of oxidative stress of myocardial infarction is unclear. The present study was performed to evaluate the potential role of FOXO6 in regulating hypoxia-induced apoptosis and oxidative stress in cardiomyocytes in vitro. Our results demonstrated that FOXO6 expression was highly elevated in cardiomyocytes exposed to hypoxia. Downregulation of FOXO6 expression by the siRNA-mediated gene knockdown in hypoxia-exposed cardiomyocytes increased cell viability, while repressing apoptosis and reactive oxygen species (ROS) production. In contrast, overexpression of FOXO6 enhanced the sensitivity of cardiomyocytes to hypoxia-induced injury. Further, in-depth research revealed that knockdown of FOXO6 promoted the expression of sirtuin6 (SIRT6) and enhanced the activation of nuclear factor erythroid 2-related factor 2 (Nrf2)-mediated antioxidant signaling. Moreover, SIRT6 inhibition markedly blocked the FOXO6 knockdown-induced promotion effect on Nrf2 activation. In addition, Nrf2 inhibition partially reversed the FOXO6 knockdown-mediated protective effect against hypoxia-induced cardiomyocyte injury. Taken together, the findings of our study demonstrate that knockdown of FOXO6 is capable of protecting cardiomyocytes from hypoxia-induced apoptosis and oxidative stress by enhancing Nrf2 activation via upregulation of SIRT6. Our study highlights a potential role of FOXO6 in myocardial infarction and suggests it as an attractive target for cardioprotection.

Statin suppresses sirtuin 6 through miR-495, increasing FoxO1-dependent hepatic gluconeogenesis

Rationale: Statin, the most widely used medication in lowering cholesterol, is also associated with increased risk of type 2 diabetes, but its molecular basis remains unclear.

Methods: Mice were injected intraperitoneally with statins alone or in combination with sirtuin (Sirt) 6 activator, and blood glucose levels were measured. Liver tissues from patients with statin use were analyzed for the expression of Sirt6.

Results: Statin treatment up-regulated the hepatic expression of phosphoenolpyruvate carboxykinase and glucose-6-phosphatase, which was prevented by Sirt6 overexpression. Mechanistically, statin directly repressed Sirt6 expression by induction of microRNA (miR)-495, a novel inhibitor of Sirt6. Pathway analysis for predicted target genes of miR-495 recognized forkhead box protein (Fox)O1 as a key downstream signaling of Sirt6. Statin treatment increased the acetylation and protein stability of FoxO1, which was suppressed by Sirt6 overexpression. Inhibiting miR-495 recovered Sirt6 levels, blocking the ability of statin to increase FoxO1 mediated gluconeogenesis, and thus confirming the role of the miR-495/Sirt6/FoxO1 pathway in controlling gluconeogenesis. Moreover, the Sirt6 activator MDL801 prevented gluconeogenesis and hyperglycemia induced by statin in mice. Equally noteworthy was that human liver tissues obtained from statin users showed a significant decrease in Sirt6 protein levels compared to those of non-users.

Conclusion: Statin induces miR-495 to suppress Sirt6 expression, which leads to enhancement of FoxO1-mediated hepatic gluconeogenesis. Thus, Sirt6 activation may offer a promising strategy for preventing statin-induced hyperglycemia.

ERRγ suppression by Sirt6 alleviates cholestatic liver injury and fibrosis

Orphan nuclear receptor estrogen-related receptor γ (ERRγ) stimulates bile acid production; however, the role and the regulatory mechanism of ERRγ in cholestatic liver disease are largely unknown. This study identifies that Sirt6 is a deacetylase of ERRγ and suggests a potentially novel mechanism by which Sirt6 activation alleviates cholestatic liver damage and fibrosis through regulating ERRγ. We observed that hepatic expression of Sirt6 is repressed, whereas hepatic expression of ERRγ is upregulated in murine cholestasis models. Hepatocyte-specific Sirt6-KO mice were more severely injured after a bile duct ligation (BDL) than WT mice, and adenoviral reexpression of Sirt6 reversed liver damage and fibrosis as demonstrated by biochemical and histological analyses. Mechanistically, Sirt6 deacetylated ERRγ, thereby destabilizing ERRγ and inhibiting its transcriptional activity. Elimination of hepatic ERRγ using Ad-shERRγ abolished the deleterious effects of Sirt6 deficiency, whereas ERRγ overexpression aggravated cholestatic liver injury. Administration of a Sirt6 deacetylase activator prevented BDL-induced liver damage and fibrosis. In patients with cholestasis, Sirt6 expression was decreased, whereas total ERRγ and acetylated ERRγ levels were increased, confirming negative regulation of ERRγ by Sirt6. Thus, Sirt6 activation represents a potentially novel therapeutic strategy for treating cholestatic liver injury.

Sirt6 activation alleviates cholestatic liver damage and fibrosis in murine cholestasis models by deacetylation of ERRγ.