SIRT6 Protects Against Liver Fibrosis by Deacetylation and Suppression of SMAD3 in Hepatic Stellate Cells

The SIRT6 Activator MDL-800 Inhibits PPARα and Fatty acid Oxidation-Related Gene Expression in Hepatocytes

A histone deacetylase SIRT6 regulates the transcription of various genes involved in lipid metabolism. Fatty acid (FA) oxidation plays a pivotal role in maintaining hepatic lipid homeostasis, and its dysregulation significantly contributes to lipotoxicity and inflammation, driving the progression of steatotic liver disease. While SIRT6 is known to activate peroxisome proliferator-activated receptor-alpha (PPARα), a central regulator of FA oxidation, the development of SIRT6 activators capable of enhancing FA oxidation and mitigating steatotic liver disease has yet to be achieved. This study evaluated the effect of MDL-800, a selective SIRT6 activator, on the expression of PPARα and genes related to FA oxidation. In AML12 mouse hepatocytes, MDL-800 treatment activated SIRT6 but unexpectedly decreased the expression of PPARα and its FA oxidation-associated target genes. Furthermore, OSS128167, a selective SIRT6 inhibitor, did not reverse the suppressive effects of MDL-800 on PPARα, suggesting that MDL-800 downregulates PPARα and FA oxidation-related genes through a mechanism independent of SIRT6 activation. Mechanistic investigations revealed that MDL-800 increased the production of reactive oxygen species and activated stress kinases. The inhibition of PPARα by MDL-800 was reversed by co-treatment with the antioxidant N-acetylcysteine or the JNK inhibitor SP600125. In summary, MDL-800 suppresses PPARα and FA oxidation-related genes primarily through the induction of oxidative stress in hepatocytes, independent of its role as a SIRT6 activator.

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.

Advancing the Metabolic Dysfunction-Associated Steatotic Liver Disease Proteome: A Post-Translational Outlook

Metabolic dysfunction-associated steatotic liver disease (MASLD) is a prevalent liver disorder with limited treatment options. This review explores the role of post-translational modifications (PTMs) in MASLD pathogenesis, highlighting their potential as therapeutic targets. We discuss the impact of PTMs, including their phosphorylation, ubiquitylation, acetylation, and glycosylation, on key proteins involved in MASLD, drawing on studies that use both human subjects and animal models. These modifications influence various cellular processes, such as lipid metabolism, inflammation, and fibrosis, contributing to disease progression. Understanding the intricate PTM network in MASLD offers the potential for developing novel therapeutic strategies that target specific PTMs to modulate protein function and alleviate disease pathology. Further research is needed to fully elucidate the complexity of PTMs in MASLD and translate these findings into effective clinical applications.

Targeting PPARα/γ by icariside II to rescue GalN/LPS-induced acute liver injury in mice: Involvement of SIRT6/NF-κB signaling pathway

BACKGROUND

Peroxisome proliferator-activated receptor α and-γ (PPARα/γ) are known to play crucial roles in acute liver injury (ALI). Icariside II (ICS II), a natural flavonoid compound derived from Herba EpimedII, confers neuroprotection with PPARα/γ induction potency.

PURPOSE

This study was aimed to explore whether ICS II has the capacity to protect against ALI, and the role of PPARα/γ in the beneficial effect of ICS II on ALI.

METHODS

Mice challenged by D-galactosamine (GalN)/lipopolysaccharide (LPS) and Kupffer cells (KCs) upon LPS insult were used as ALI models in vivo and in vitro. PPARα/γ-deficient mice were treated with ICS II to validate the potential targets of ICS II on ALI.

RESULTS

We found that ICS II (5, 10, 20 mg/kg) dose-dependently improved the survival rate and liver histology, decreased ALT and AST in GalN/LPS-treated mice. Furthermore, ICS II directly bound to PPARα/γ and increased their activities. The protective properties of ICS II were counteracted when PPARα/γ were knocked out in GalN/LPS-induced mice and LPS-induced KCs, respectively. Mechanistically, ICS II restored mitochondrial function, reduced oxidative stress and inflammation through activating PPARα/γ, which activated Sirt6 and inhibited NF-κB nuclear translocation.

CONCLUSION

Our findings not only highlight PPARα/γ-SIRT6 signaling as a vital therapeutic target to combat ALI, but also reveal ICS II may serve as a novel dual PPARα/γ agonist to safeguard ALI from the oxidation-inflammation vicious circle by mediating SIRT6/NF-κB.

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.

Myricetin ameliorates airway inflammation and remodeling in asthma by activating Sirt1 to regulate the JNK/Smad3 pathway

BACKGROUND

Myricetin has various biological activities and health benefits; however, its effects on airway remodeling in asthma have not been reported.

PURPOSE

We aimed to investigate the possibility that myricetin improves airway remodeling by activating Sirt1 and has potential as a new treatment for asthma.

METHODS

RAW 264.7 cells were stimulated with lipopolysaccharide and co-cultured with 3T6 cells in vitro to simulate the in vivo effects of inflammation on airway remodeling. Using an ovalbumin-induced chronic asthma mouse model, we compared changes in inflammatory factors and airway remodeling-related factors under treatment with myricetin and/or the Sirt1 inhibitor EX-527 using western blotting and quantitative PCR. Expression plasmids carrying Smad3 site mutations were transfected into 3T6 cells to identify the Sirt1 deacetylation site on Smad3 protein.

RESULTS

Myricetin significantly reduced the infiltration of airway inflammatory cells and the production of interleukin (IL)-6 and IL-5, and inhibited mucus secretion by goblet cells, collagen fiber proliferation, and the increase in inflammatory cells in bronchoalveolar lavage fluid from asthmatic mice. Results of in vitro experiments were consistent with those conducted in vivo. Exploring the mechanism of action of myricetin, we found that myricetin downregulated the levels of phosphorylated (p)-JNK, p-Smad3, and acetylated Smad3 proteins by activating Sirt1 both in vivo and in vitro. K341 was identified as the main deacetylation site of Smad3 by myricetin-activated Sirt1.

CONCLUSION

Myricetin ameliorates airway inflammation and remodeling in asthma by activating Sirt1 to regulate the JNK/Smad3 pathway.

Menin in Cancer

The protein menin is encoded by the MEN1 gene and primarily serves as a nuclear scaffold protein, regulating gene expression through its interaction with and regulation of chromatin modifiers and transcription factors. While the scope of menin's functions continues to expand, one area of growing investigation is the role of menin in cancer. Menin is increasingly recognized for its dual function as either a tumor suppressor or a tumor promoter in a highly tumor-dependent and context-specific manner. While menin serves as a suppressor of neuroendocrine tumor growth, as seen in the cancer risk syndrome multiple endocrine neoplasia type 1 (MEN1) syndrome caused by pathogenic germline variants in MEN1, recent data demonstrate that menin also suppresses cholangiocarcinoma, pancreatic ductal adenocarcinoma, gastric adenocarcinoma, lung adenocarcinoma, and melanoma. On the other hand, menin can also serve as a tumor promoter in leukemia, colorectal cancer, ovarian and endometrial cancers, Ewing sarcoma, and gliomas. Moreover, menin can either suppress or promote tumorigenesis in the breast and prostate depending on hormone receptor status and may also have mixed roles in hepatocellular carcinoma. Here, we review the rapidly expanding literature on the role and function of menin across a broad array of different cancer types, outlining tumor-specific differences in menin's function and mechanism of action, as well as identifying its therapeutic potential and highlighting areas for future investigation.

Impaired SUMOylation of FoxA1 promotes nonalcoholic fatty liver disease through down-regulation of Sirt6

Abnormal SUMOylation is implicated in non-alcoholic fatty liver disease (NAFLD) progression. Forkhead box protein A1 (FoxA1) has been shown to protect liver from steatosis, which was down-regulated in NAFLD. This study elucidated the role of FoxA1 deSUMOylation in NAFLD. NAFLD models were established in high-fat diet (HFD)-induced mice and palmitate acid (PAL)-treated hepatocytes. Hepatic steatosis was evaluated by biochemical and histological methods. Lipid droplet formation was determined by BODIPY and Oil red O staining. Target molecule levels were analyzed by RT-qPCR, Western blotting, and immunohistochemistry staining. SUMOylation of FoxA1 was determined by Ni-NTA pull-down assay and SUMOylation assay Ultra Kit. Protein interaction and ubiquitination were detected by Co-IP. Gene transcription was assessed by ChIP and dual luciferase reporter assays. Liver FoxA1 knockout mice developed severe liver steatosis, which could be ameliorated by sirtuin 6 (Sirt6) overexpression. Nutritional stresses reduced Sumo2/3-mediated FoxA1 SUMOylation at lysine residue K6, which promoted lipid droplet formation by repressing fatty acid β-oxidation. Moreover, Sirt6 was a target gene of FoxA1, and Sirt6 transcription activity was restrained by deSUMOylation of FoxA1 at site K6. Furthermore, nutritional stresses-induced deSUMOylation of FoxA1 promoted the ubiquitination and degradation of FoxA1 with assistance of murine double minute 2 (Mdm2). Finally, activating FoxA1 SUMOylation delayed the progression of NAFLD in mice. DeSUMOylation of FoxA1 at K6 promotes FoxA1 degradation and then inhibits Sirt6 transcription, thereby suppressing fatty acid β-oxidation and facilitating NAFLD development. Our findings suggest that FoxA1 SUMOylation activation might be a promising therapeutic strategy for NAFLD.

Epigenetic and transcriptional control of adipocyte function by centenarian-associated SIRT6 N308K/A313S mutant

BACKGROUND

Obesity is a major health burden. Preadipocytes proliferate and differentiate in mature adipocytes in the adipogenic process, which could be a potential therapeutic approach for obesity. Deficiency of SIRT6, a stress-responsive protein deacetylase and mono-ADP ribosyltransferase enzyme, blocks adipogenesis. Mutants of SIRT6 (N308K/A313S) were recently linked to the in the long lifespan Ashkenazi Jews. In this study, we aimed to clarify how these new centenarian-associated SIRT6 genetic variants affect adipogenesis at the transcriptional and epigenetic level.

METHODS

We analyzed the role of SIRT6 wild-type (WT) or SIRT6 centenarian-associated mutant (N308K/A313S) overexpression in adipogenesis, by creating stably transduced preadipocyte cell lines using lentivirus on the 3T3-L1 model. Histone post-translational modifications (PTM: acetylation, methylation) and transcriptomic changes were analyzed by mass spectrometry (LC-MS/MS) and RNA-Seq, respectively, in 3T3-L1 adipocytes. In addition, the adipogenic process and related signaling pathways were investigated by bioinformatics and biochemical approaches.

RESULTS

Overexpression of centenarian-associated SIRT6 mutant increased adipogenic differentiation to a similar extent compared to the WT form. However, it triggered distinct histone PTM profiles in mature adipocytes, with significantly higher acetylation levels, and activated divergent transcriptional programs, including those dependent on signaling related to the sympathetic innervation and to PI3K pathway. 3T3-L1 mature adipocytes overexpressing SIRT6 N308K/A313S displayed increased insulin sensitivity in a neuropeptide Y (NPY)-dependent manner.

CONCLUSIONS

SIRT6 N308K/A313S overexpression in mature adipocytes ameliorated glucose sensitivity and impacted sympathetic innervation signaling. These findings highlight the importance of targeting SIRT6 enzymatic activities to regulate the co-morbidities associated with obesity.

STUB1 increases adiponectin expression by inducing ubiquitination and degradation of NR2F2, thereby reducing hepatic stellate cell activation and alleviating non-alcoholic fatty liver disease

BACKGROUND

Adiponectin (APN) has exhibited ameliorating effects on non-alcoholic fatty liver disease (NAFLD). This study investigates the roles of APN and its regulatory molecules in hepatic stellate cell (HSC) activation and the progression of NAFLD.

METHODS

Mice were subjected to a high-fat diet (HFD) to establish NAFLD models. Liver tissue was examined for lipid metabolism, fibrosis, and inflammation. Mouse 3T3-L1 adipocytes were exposed to palmitic acid (PA) to mimic a high-fat environment. The conditioned medium (CM) from adipocytes was collected for the culture of isolated mouse HSCs. Gain- or loss-of-function studies of APN, nuclear receptor subfamily 2 group F member 2 (NR2F2), and STIP1 homology and U-box containing protein 1 (STUB1) were performed to analyze their roles in NAFLD and HSC activation in vivo and in vitro.

RESULTS

APN expression was poorly expressed in HFD-fed mice and PA-treated 3T3-L1 adipocytes, which was attributed to the transcription inhibition mediated by NR2F2. Silencing of NR2F2 restored the APN expression, ameliorating liver steatosis, fibrosis, and inflammatory cytokine infiltration in mouse livers and reducing HSC activation. Similarly, the NR2F2 silencing condition reduced HSC activation in vitro. However, these effects were counteracted by artificial APN silencing. STUB1 facilitated the ubiquitination and protein degradation of NR2F2, and its upregulation mitigated NAFLD-like symptoms in mice and HSC activation, effects reversed by the NR2F2 overexpression.

CONCLUSION

This study highlights the role of STUB1 in reducing HSC activation and alleviating NAFLD by attenuating NR2F2-mediated transcriptional repression of APN.

Molecular mechanism and therapeutic significance of essential amino acids in metabolically associated fatty liver disease

Non-alcoholic fatty liver disease (NAFLD), also known as metabolically associated fatty liver disease (MAFLD), is a systemic metabolic disease characterized by lipid accumulation in the liver, lipid toxicity, insulin resistance, intestinal dysbiosis, and inflammation that can progress from simple steatosis to nonalcoholic steatohepatitis (NASH) and even cirrhosis or cancer. It is the most prevalent illness threatening world health. Currently, there are almost no approved drug interventions for MAFLD, mainly dietary changes and exercise to control weight and regulate metabolic disorders. Meanwhile, the metabolic pathway involved in amino acid metabolism also influences the onset and development of MAFLD in the body, and most amino acid metabolism takes place in the liver. Essential amino acids are those amino acids that must be supplemented from outside the diet and that cannot be synthesized in the body or cannot be synthesized at a rate sufficient to meet the body's needs, including leucine, isoleucine, valine (collectively known as branched-chain amino acids), tryptophan, phenylalanine (which are aromatic amino acids), histidine, methionine, threonine and lysine. The metabolic balance of the body is closely linked to these essential amino acids, and essential amino acids are closely linked to the pathophysiological process of MAFLD. In this paper, we will focus on the metabolism of essential amino acids in the body and further explore the therapeutic strategies for MAFLD based on the studies conducted in recent years.

Epigenetic modification in liver fibrosis: Promising therapeutic direction with significant challenges ahead

Liver fibrosis, characterized by scar tissue formation, can ultimately result in liver failure. It's a major cause of morbidity and mortality globally, often associated with chronic liver diseases like hepatitis or alcoholic and non-alcoholic fatty liver diseases. However, current treatment options are limited, highlighting the urgent need for the development of new therapies. As a reversible regulatory mechanism, epigenetic modification is implicated in many biological processes, including liver fibrosis. Exploring the epigenetic mechanisms involved in liver fibrosis could provide valuable insights into developing new treatments for chronic liver diseases, although the current evidence is still controversial. This review provides a comprehensive summary of the regulatory mechanisms and critical targets of epigenetic modifications, including DNA methylation, histone modification, and RNA modification, in liver fibrotic diseases. The potential cooperation of different epigenetic modifications in promoting fibrogenesis was also highlighted. Finally, available agonists or inhibitors regulating these epigenetic mechanisms and their potential application in preventing liver fibrosis were discussed. In summary, elucidating specific druggable epigenetic targets and developing more selective and specific candidate medicines may represent a promising approach with bright prospects for the treatment of chronic liver diseases.

SIRT3/6: an amazing challenge and opportunity in the fight against fibrosis and aging

Fibrosis is a typical aging-related pathological process involving almost all organs, including the heart, kidney, liver, lung, and skin. Fibrogenesis is a highly orchestrated process defined by sequences of cellular response and molecular signals mechanisms underlying the disease. In pathophysiologic conditions associated with organ fibrosis, a variety of injurious stimuli such as metabolic disorders, epigenetic changes, and aging may induce the progression of fibrosis. Sirtuins protein is a kind of deacetylase which can regulate cell metabolism and participate in a variety of cell physiological functions. In this review, we outline our current understanding of common principles of fibrogenic mechanisms and the functional role of SIRT3/6 in aging-related fibrosis. In addition, sequences of novel protective strategies have been identified directly or indirectly according to these mechanisms. Here, we highlight the role and biological function of SIRT3/6 focus on aging fibrosis, as well as their inhibitors and activators as novel preventative or therapeutic interventions for aging-related tissue fibrosis.

LncRNA MEG3: Targeting the Molecular Mechanisms and Pathogenic causes of Metabolic Diseases

BACKGROUND

Non-coding RNA is a type of RNA that does not encode proteins, distributed among rRNA, tRNA, snRNA, snoRNA, microRNA and other RNAs with identified functions, where the Long non-coding RNA (lncRNA) displays a nucleotide length over 200. LncRNAs enable multiple biological processes in the human body, including cancer cell invasion and metastasis, apoptosis, cell autophagy, inflammation, etc. Recently, a growing body of studies has demonstrated the association of lncRNAs with obesity and obesity-induced insulin resistance and NAFLD, where MEG3 is related to glucose metabolism, such as insulin resistance. In addition, MEG3 has been demonstrated in the pathological processes of various cancers, such as mediating inflammation, cardiovascular disease, liver disease and other metabolic diseases.

OBJECTIVE

To explore the regulatory role of lncRNA MEG3 in metabolic diseases. It provides new ideas for clinical treatment or experimental research.

METHODS

In this paper, in order to obtain enough data, we integrate and analyze the data in the PubMed database.

RESULTS

LncRNA MEG3 can regulate many metabolic diseases, such as insulin resistance, NAFLD, inflammation and so on.

CONCLUSION

LncRNA MEG3 has a regulatory role in a variety of metabolic diseases, which are currently difficult to be completely cured, and MEG3 is a potential target for the treatment of these diseases. Here, we review the role of lncRNA MEG3 in mechanisms of action and biological functions in human metabolic diseases.

ATG14 plays a critical role in hepatic lipid droplet homeostasis

BACKGROUND & AIMS

Autophagy-related 14 (ATG14) is a key regulator of autophagy. ATG14 is also localized to lipid droplet; however, the function of ATG14 on lipid droplet remains unclear. In this study, we aimed to elucidate the role of ATG14 in lipid droplet homeostasis.

METHODS

ATG14 loss-of-function and gain-of-function in lipid droplet metabolism were analyzed by fluorescence imaging in ATG14 knockdown or overexpression hepatocytes. Specific domains involved in the ATG14 targeting to lipid droplets were analyzed by deletion or site-specific mutagenesis. ATG14-interacting proteins were analyzed by co-immunoprecipitation. The effect of ATG14 on lipolysis was analyzed in human hepatocytes and mouse livers that were deficient in ATG14, comparative gene identification-58 (CGI-58), or both.

RESULTS

Our data show that ATG14 is enriched on lipid droplets in hepatocytes. Mutagenesis analysis reveals that the Barkor/ATG14 autophagosome targeting sequence (BATS) domain of ATG14 is responsible for the ATG14 localization to lipid droplets. Co-immunoprecipitation analysis illustrates that ATG14 interacts with adipose triglyceride lipase (ATGL) and CGI-58. Moreover, ATG14 also enhances the interaction between ATGL and CGI-58. In vitro lipolysis analysis demonstrates that ATG14 deficiency remarkably decreases triglyceride hydrolysis.

CONCLUSIONS

Our data suggest that ATG14 can directly enhance lipid droplet breakdown through interactions with ATGL and CGI-58.

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.

Interleukin-19 Gene-Deficient Mice Promote Liver Fibrosis via Enhanced TGF-β Signaling, and the Interleukin-19-CCL2 Axis Is Important in the Direction of Liver Fibrosis

IL-19 is a cytokine discovered by homologous searching with IL-10 and is produced by non-immune cells, such as keratinocytes, in addition to immune cells, such as macrophages. Liver fibrosis results from the inflammation and activation of hepatic stellate cells via chronic liver injury. However, the participation of IL-19 in liver fibrosis remains to be sufficiently elucidated. Our group studied the immunological function of IL-19 in a mouse model of carbon tetrachloride (CCl₄)-induced liver fibrosis. IL-19 gene-deficient (KO) mice and body weight-matched wild-type (WT) mice were used. A liver fibrosis mouse model was created via CCl₄ administration (two times per week) for 8 weeks. In CCl₄-induced liver fibrosis, serum analysis revealed that IL-19 KO mice had higher ALT levels compared to WT mice. IL-19 KO mice had worse fibrosis, as assessed by morphological evaluation of total area stained positive with Azan and Masson trichrome. In addition, the expression of α-SMA was increased in liver tissues of IL-19 KO mice compared to WT mice. Furthermore, mRNA expression levels of TGF-β and α-SMA were enhanced in IL-19 KO mice compared to WT mice. In vitro assays revealed that IL-19-high expressing RAW264.7 cells inhibited the migration of NIH3T3 cells via the inhibited expression of CCL2 in the presence of CCl₄ and IL-4. These findings indicate that IL-19 plays a critical role in liver fibrosis by affecting TGF-β signaling and the migration of hepatic stellate cells during liver injury. Enhancement of the IL-19 signaling pathway is a potential treatment for liver fibrosis.

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.

HIPK2 as a Novel Regulator of Fibrosis

Fibrosis is an unmet medical problem due to a lack of evident biomarkers to help develop efficient targeted therapies. Fibrosis can affect almost every organ and eventually induce organ failure. Homeodomain-interacting protein kinase 2 (HIPK2) is a protein kinase that controls several molecular pathways involved in cell death and development and it has been extensively studied, mainly in the cancer biology field. Recently, a role for HIPK2 has been highlighted in tissue fibrosis. Thus, HIPK2 regulates several pro-fibrotic pathways such as Wnt/β-catenin, TGF-β and Notch involved in renal, pulmonary, liver and cardiac fibrosis. These findings suggest a wider role for HIPK2 in tissue physiopathology and highlight HIPK2 as a promising target for therapeutic purposes in fibrosis. Here, we will summarize the recent studies showing the involvement of HIPK2 as a novel regulator of fibrosis.

Epigenetics and mitochondrial dysfunction insights into the impact of the progression of non-alcoholic fatty liver disease

A metabolic problem occurs when regular functions of the body are disrupted due to an undesirable imbalance. Nonalcoholic fatty liver disease (NAFLD) is considered as one of the most common in this category. NAFLD is subclassified and progresses from lipid accumulation to cirrhosis before advancing to hepatocellular cancer. In spite of being a critical concern, the standard treatment is inadequate. Metformin, silymarin, and other nonspecific medications are used in the management of NAFLD. Aside from this available medicine, maintaining a healthy lifestyle has been emphasized as a means of combating this. Epigenetics, which has been attributed to NAFLD, is another essential feature of this disease that has emerged as a result of several sorts of research. The mechanisms by which DNA methylation, noncoding RNA, and histone modification promote NAFLD have been extensively researched. Another organelle, mitochondria, which play a pivotal role in biological processes, contributes to the global threat. Individuals with NAFLD have been documented to have a multitude of alterations and malfunctioning. Mitochondria are mainly concerned with the process of energy production and regulation of the signaling pathway on which the fate of a cell relies. Modulation of mitochondria leads to elevated lipid deposition in the liver. Further, changes in oxidation states result in an impaired balance between the antioxidant system and reactive oxygen species directly linked to mitochondria. Hence mitochondria have a definite role in potentiating NAFLD. In this regard, it is essential to consider the role of epigenetics as well as mitochondrial contribution while developing a medication or therapy with the desired accuracy.

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.

ATL I, Acts as a SIRT6 Activator to Alleviate Hepatic Steatosis in Mice via Suppression of NLRP3 Inflammasome Formation

Accumulating evidence has highlighted that sirtuin-6 (SIRT6) plays an important role in hepatic gluconeogenesis and lipogenesis. We aim to investigate the underlying mechanisms and pharmacological interventions of SIRT6 on hepatic steatosis treatment. Herein, our results showed that atractylenolide I (ATL I) activated the deacetylase activity of SIRT6 to promote peroxisome proliferator-activated receptor alpha (PPARα) transcription and translation, while suppressing nuclear factor NF-kappa-B (NFκB)-induced NACHT, LRR, and PYD domains containing protein 3 (NLRP3) inflammasome formation. Together, these decreased the infiltration of F4/80 and CD11B positive macrophages, accompanied by decreased mRNA expression and serum levels of tumor necrosis factor alpha (TNF-α), interleukin-6 (IL6), and interleukin-1 beta (IL1β). Additionally, these changes decreased sterol regulatory element-binding protein-1c (SREBP-1c) expression, while restoring carnitine O-palmitoyltransferase 1a (Cpt1a) expression, to decrease the size of adipocytes and adipose deposition, which, in turn, reversed high-fat diet (HFD)-induced liver weight and body weight accumulation in C57 mice. SIRT6 knockout or hepatic SIRT6 knockout in C57 mice largely abolished the effect of ATL I on ameliorating hepatic steatosis. Taken together, our results suggest that ATL I acts as a promising compound that activates SIRT6/PPARα signaling and attenuates the NLRP3 inflammasome to ameliorate hepatic inflammation and steatosis.

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.

Ginsenoside Rc Modulates SIRT6-NRF2 Interaction to Alleviate Alcoholic Liver Disease

Alcoholic liver disease (ALD) is a serious worldwide health problem. Ginsenoside Rc is a major active ingredient isolated from Panax ginseng, whose pharmacological effects counteract oxidative stress, inflammation, and lipid accumulation. However, it is still unclear whether ginsenoside Rc might exert beneficial effects on alcohol-induced liver injury. To this aim, mice primary hepatocytes (MPHs) were challenged with alcohol to test ginsenoside Rc's effects on their intracellular alcohol metabolism. C57BL/6J mice or SIRT6alb-/- mice were chronically fed a diet with added alcohol or given a single gavage of alcohol with or without ginsenoside Rc. Analyses of alcohol metabolism, oxidative stress, inflammation, lipid metabolism, and RNaseq expression were conducted to explore potential targets exploited by ginsenoside Rc to protect against ALD. Our results showed that ginsenoside Rc attenuated alcohol-induced liver injury by regulating oxidative stress, inflammation, and lipid accumulation both in vivo and in vitro. Ginsenoside Rc did increase the deacetylase activity of SIRT6, thereby lowering acetylated NRF2 levels, which elevated NRF2's stability, and subsequently exerting an antioxidant effect. In keeping with this, the hepatic knockout of SIRT6 almost abolished the hepatoprotective effects of ginsenoside Rc against ALD. Therefore, our results suggest that ginsenoside Rc attenuated hepatocytes' damage and oxidative stress in ALD by up-regulating the SIRT6/NRF2 pathway. Hence, ginsenoside Rc may be a promising drug to treat or relieve ALD.

Sirtuin 6 protects against hepatic fibrogenesis by suppressing the YAP and TAZ function

Hepatic fibrosis occurs in response to prolonged tissue injury in the liver, which results in abnormal accumulation of extracellular matrix. Hepatic stellate cells (HSCs) have been suggested to play a major role in liver fibrosis. However, the molecular mechanisms remain incompletely understood. Sirtuin 6 (SIRT6), an NAD⁺ -dependent deacetylase, has been previously implicated in the regulation of the transforming growth factor β (TGFβ)-SMAD3 pathway that plays a significant role in liver fibrosis. In this work, we aimed to identify other important players during hepatic fibrogenesis, which are modulated by SIRT6. Yes-associated protein (YAP) and transcriptional coactivator with PDZ-binding motif (TAZ or WWTR1), key players in the Hippo pathway, have been implicated in the promotion of hepatic fibrosis. Our data show that HSC-specific Sirt6 knockout mice are more susceptible to high-fat-cholesterol-cholate diet-induced hepatic fibrosis than their wildtype counterparts. Our signaling analyses suggest that in addition to the TGFβ-SMAD3 pathway, YAP and TAZ are also highly activated in the SIRT6-deficient HSCs. As it is not clear how SIRT6 might regulate YAP and TAZ, we have decided to elucidate the mechanism underlying the regulation of YAP and TAZ by SIRT6 in HSCs. Overexpression or knockdown of SIRT6 corroborates the role of SIRT6 in the negative regulation of YAP and TAZ. Further biochemical analyses reveal that SIRT6 deacetylates YAP and TAZ and reprograms the composition of the TEA domain transcription factor complex to suppress their downstream target genes, particularly those involved in hepatic fibrosis. In conclusion, our data suggest that SIRT6 plays a critical role in the regulation of the Hippo pathway to protect against hepatic fibrosis.

SIRT6 overexpression retards renal interstitial fibrosis through targeting HIPK2 in chronic kidney disease

Introduction: Renal interstitial fibrosis is a common pathophysiological change in the chronic kidney disease (CKD). Nicotinamide adenine dinucleotide (NAD)-dependent deacetylase sirtuin 6 (SIRT6) is demonstrated to protect against kidney injury. Vitamin B3 is the mostly used form of NAD precursors. However, the role of SIRT6 overexpression in renal interstitial fibrosis of CKD and the association between dietary vitamin B3 intake and renal function remain to be elucidated.

Methods: Wild-type (WT) and SIRT6-transgene (SIRT6-Tg) mice were given with high-adenine diets to establish CKD model. HK2 cells were exposed to transforming growth factor β1 (TGF-β1) in vitro to explore related mechanism. Population data from Multi-Ethnic Study of Atherosclerosis (MESA) was used to examine the association between dietary vitamin B3 intake and renal function decline.

Results: Compared to WT mice, SIRT6-Tg mice exhibited alleviated renal interstitial fibrosis as evidenced by reduced collagen deposit, collagen I and α-smooth muscle actin expression. Renal function was also improved in SIRT6-Tg mice. Homeodomain interacting protein kinase 2 (HIPK2) was induced during the fibrogenesis in CKD, while HIPK2 was downregulated after SIRT6 overexpression. Further assay in vitro confirmed that SIRT6 depletion exacerbated epithelial-to-mesenchymal transition of HK2 cells, which might be linked with HIPK2 upregulation. HIPK2 was inhibited by SIRT6 in the post-transcriptional level. Population study indicated that higher dietary vitamin B3 intake was independently correlated with a lower risk of estimate glomerular filtration rate decline in those ≥65 years old during follow-up.

Conclusion: SIRT6/HIPK2 axis serves as a promising target of renal interstitial fibrosis in CKD. Dietary vitamin B3 intake is beneficial for renal function in the old people.

Nootkatone, a Sesquiterpene Ketone From Alpiniae oxyphyllae Fructus, Ameliorates Metabolic-Associated Fatty Liver by Regulating AMPK and MAPK Signaling

Metabolic-associated fatty liver disease (MAFLD) is becoming more common due to lifestyle changes. A long-term high-fat and high-glucose diet induces glycolipid metabolism disorders in the liver, which results in the development of MAFLD. To date, there is no specific clinically useful therapeutics for this disease. Natural products or synthetic compounds were screened and investigated to find effective agents for treating MAFLD. In this study, nootkatone (Nok), a natural sesquiterpene ketone isolated from Alpiniae oxyphyllae fructus, was explored for its potential to treat MAFLD, and underlying mechanisms were studied. Our results show that Nok dramatically ameliorated the disordered lipid and glucose metabolism in MAFLD mice, decreased fat accumulation in hepatic tissue, and improved liver injury. Inflammation, metabolic disorder, and oxidative stress were ameliorated in liver tissue based on RNA-seq transcriptome comparison between a Nok-treated group and an MAFLD model group. Furthermore, Nok significantly activated AMPK activity and inhibited MAPK activity, especially the p38 and JNK signaling pathways, in vivo based on western blot analysis. The pharmaceutical effects and potential signaling pathways impacted by Nok were also investigated in L02 cells. Nok significantly promoted the consumption of glucose and decreased the deposition of triglycerides in vitro. The p-AMPKα level was notably upregulated by Nok, indicating dramatic AMPK activation. In addition, Nok decreased the levels of p-ERK1/2, p-p38, and p-JNK. Nok also inhibited the activation of MAPK signaling and, thus, alleviated MAFLD development. Our results suggest that Nok may be useful in treating MAFLD. Nok may ameliorate MAFLD by regulating glycolipid metabolism disorders by activating AMPK and inhibiting MAPK activity. Collectively, this study suggests that Nok is an effective compound for the treatment of MAFLD.

Sorafenib Attenuates Fibrotic Hepatic Injury Through Mediating Lysine Crotonylation

Background

Liver fibrosis is an independent contributor of chronic liver diseases, and regressing liver fibrosis is considered a potential therapeutic target for chronic liver diseases. We aimed to explore the effects and mechanism of sorafenib in liver fibrosis.

Methods

Male Sprague Dawley (SD) rats were subjected to subcutaneous injection of carbon tetrachloride (CCl₄) for 8 weeks to induce liver fibrosis and then treated with sorafenib. The degree of liver fibrosis was analyzed by hematoxylin–eosin (H&E) staining, Masson staining, and Picrosirius red (PSR) staining. Serum biochemical indexes were detected by fully automatic biochemical analyzer or enzyme-linked immunosorbent assay (ELISA). Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to detect the expression of pro-fibrotic genes. Immunohistochemical staining and Western blotting were carried out to evaluate the levels of lysine crotonylation.

Results

Liver index was reduced with oral sorafenib in CCl₄-induced rats. Serum liver function (alanine aminotransferase (ALT), aspartate aminotransferase (AST), and total bilirubin (TBIL)) and fibrosis indicators (type III procollagen (PC-III), hyaluronic acid (HA), and laminin (LN)) were attenuated with sorafenib treatment. Sorafenib improved the hepatic structure and fibrotic progression. The expression of fibrosis-related genes was remarkely reduced with sorafenib treatment. Meanwhile, sorafenib inhibited α-SMA and collagen I cumulation induced by CCl₄ injection. Besides, protein lysine crotonylation especially the crotonylated H2BK12 (H2BK12cr) and crotonylated H3K18 (H3K18cr) were reversed by sorafenib, which were decreased in response to CCl₄ treatment. Spearman correlation analysis shown lysine crotonylation expression was negatively correlated with serum fibrotic indicators. Conversely, crotonylation-regulated enzymes, which negatively regulate protein crotonylation, were increased in response to CCl₄ treatment, while sorafenib reduced their expression.

Conclusion

Sorafenib exerts significant anti-fibrotic effects through mediating crotonylation-regulated enzymes and protein crotonylation in fibrotic rats.

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.

Pathogenesis of Liver Fibrosis and Its TCM Therapeutic Perspectives

Liver fibrosis is a pathological process of abnormal tissue proliferation in the liver caused by various pathogenic factors, which will further develop into cirrhosis or even hepatocellular carcinoma if liver injury is not intervened in time. As a diffuse progressive liver disease, its clinical manifestations are mostly excessive deposition of collagen-rich extracellular matrix resulting in scar formation due to liver injury. Hepatic fibrosis can be caused by hepatitis B and C, fatty liver, alcohol, and rare diseases such as hemochromatosis. As the metabolic center of the body, the liver regulates various vital activities. During the development of fibrosis, it is influenced by many other factors in addition to the central event of hepatic stellate cell activation. Currently, with the increasing understanding of TCM, the advantages of TCM with multiple components, pathways, and targets have been demonstrated. In this review, we will describe the factors influencing liver fibrosis, focusing on the effects of cells, intestinal flora, iron death, signaling pathways, autophagy and angiogenesis on liver fibrosis, and the therapeutic effects of herbal medicine on liver fibrosis.

Hepatic SIRT6 Modulates Transcriptional Activities of FXR to Alleviate Acetaminophen-induced Hepatotoxicity

BACKGROUND & AIMS

Excessive acetaminophen (APAP) intake causes oxidative stress and inflammation, leading to fatal hepatotoxicity; however, the mechanism remains unclear. This study aims to explore the protective effects and detailed mechanisms of sirtuin 6 (SIRT6) in the defense against APAP-induced hepatotoxicity.

METHODS

Hepatocyte-specific SIRT6 knockout mice, farnesoid X receptor (FXR) knockout mice, and mice with genetic or pharmacological activation of SIRT6 were subjected to APAP to evaluate the critical role of SIRT6 in the pathogenesis of acute liver injury. RNA sequences were used to investigate molecular mechanisms underlying this process.

RESULTS

Hepatic SIRT6 expression was substantially reduced in the patients and mice with acute liver injury. The deletion of SIRT6 in mice and mice primary hepatocytes led to high N-acetyl-p-benzo-quinoneimine and low glutathione levels in the liver, thereby enhancing APAP overdose-induced liver injury, manifested as increased hepatic centrilobular necrosis, oxidative stress, and inflammation. Conversely, overexpression or pharmacological activation of SIRT6 enhanced glutathione and decreased N-acetyl-p-benzo-quinoneimine, thus alleviating APAP-induced hepatotoxicity via normalization of liver damage, inflammatory infiltration, and oxidative stress. Our molecular analysis revealed that FXR is regulated by SIRT6, which is associated with the pathological progression of ALI. Mechanistically, SIRT6 deacetylates FXR and elevates FXR transcriptional activity. FXR ablation in mice and mice primary hepatocytes prominently blunted SIRT6 overexpression and activation-mediated ameliorative effects. Conversely, pharmacological activation of FXR mitigated APAP-induced hepatotoxicity in SIRT6 knockout mice.

CONCLUSIONS

Our current study suggests that SIRT6 plays a crucial role in APAP-induced hepatotoxicity, and pharmacological activation of SIRT6 may represent a novel therapeutic strategy for APAP overdose-induced liver injury.

The S100 calcium binding protein A11 promotes liver fibrogenesis by targeting TGF-β signaling

Liver fibrosis is a key transformation stage and also a reversible pathological process in various types of chronic liver diseases. However, the pathogenesis of liver fibrosis still remains elusive. Here, we report that the calcium binding protein A11 (S100A11) is consistently upregulated in the integrated data from GSE liver fibrosis and tree shrew liver proteomics. S100A11 is also experimentally activated in liver fibrosis in mouse, rat, tree shrew, and human with liver fibrosis. While overexpression of S100A11 in vivo and in vitro exacerbates liver fibrosis, the inhibition of S100A11 improves liver fibrosis. Mechanistically, S100A11 activates hepatic stellate cells (HSCs) and the fibrogenesis process via the regulation of the deacetylation of Smad3 in the TGF-β signaling pathway. S100A11 physically interacts with SIRT6, a deacetylase of Smad2/3, which may competitively inhibit the interaction between SIRT6 and Smad2/3. The subsequent release and activation of Smad2/3 promote the activation of HSCs and fibrogenesis. Additionally, a significant elevation of S100A11 in serum is observed in clinical patients. Our study uncovers S100A11 as a novel profibrogenic factor in liver fibrosis, which may represent both a potential biomarker and a promising therapy target for treating liver fibrosis and fibrosis-related liver diseases.

LncRNA MEG3 up-regulates SIRT6 by ubiquitinating EZH2 and alleviates nonalcoholic fatty liver disease

Nonalcoholic fatty liver disease (NAFLD) is a global health threat. Here, we presented the significant role of a novel signaling axis comprising long non-coding RNA maternally expressed gene 3 (MEG3), enhancer of zeste homolog 2 (EZH2), and sirtuin 6 (SIRT6) in controlling lipid accumulation, inflammation, and the progression of NAFLD. Mice fed with high-fat diet (HFD) were established as in vitro and in vivo NAFLD models, respectively. Lipid accumulation was measured by oil red O staining and assays for triglycerides or cholesterol. Inflammation was examined by ELISA for pro-inflammatory cytokines. Gene expressions were examined by RT-qPCR or Western blot. Interactions between key signaling molecules were examined by combining expressional analysis, RNA immunoprecipitation, cycloheximide stability assay, co-immunoprecipitation, and chromatin immunoprecipitation. MEG3 level was reduced in FFA-challenged hepatocytes or liver from HFD-fed mice, and the reduction paralleled the severity of NAFLD in clinic. Overexpressing MEG3 suppressed FFA-induced lipid accumulation or inflammation in hepatocytes. By promoting the ubiquitination and degradation of EZH2, MEG3 upregulated SIRT6, an EZH2 target. SIRT6 essentially mediated the protective effects of MEG3 in hepatocytes. Consistently, overexpressing MEG3 alleviated HFD-induced NAFLD in vivo. By controlling the expressions of genes involved in lipid metabolism and inflammation, the MEG3/EZH2/SIRT6 axis significantly suppressed lipid accumulation and inflammation in vitro, and NAFLD development in vivo. Therefore, boosting MEG3 level may benefit the treatment of NAFLD.

Roles of SIRT6 in kidney disease: a novel therapeutic target

SIRT6 is an NAD+ dependent deacetylase that belongs to the mammalian sirtuin family. SIRT6 is mainly located in the nucleus and regulates chromatin remodeling, genome stability, and gene transcription. SIRT6 extensively participates in various physiological activities such as DNA repair, energy metabolism, oxidative stress, inflammation, and fibrosis. In recent years, the role of epigenetics such as acetylation modification in renal disease has gradually received widespread attention. SIRT6 reduces oxidative stress, inflammation, and renal fibrosis, which is of great importance in maintaining cellular homeostasis and delaying the chronic progression of kidney disease. Here, we review the structure and biological function of SIRT6 and summarize the regulatory mechanisms of SIRT6 in kidney disease. Moreover, the role of SIRT6 as a potential therapeutic target for the progression of kidney disease will be discussed. SIRT6 plays an important role in kidney disease. SIRT6 regulates mitochondrial dynamics and mitochondrial biogenesis, induces G2/M cycle arrest, and plays an antioxidant role in nephrotoxicity, IR, obstructive nephropathy, and sepsis-induced AKI. SIRT6 prevents and delays progressive CKD induced by hyperglycemia, kidney senescence, hypertension, and lipid accumulation by regulating mitochondrial biogenesis, and has antioxidant, anti-inflammatory, and antifibrosis effects. Additionally, hypoxia, inflammation, and fibrosis are the main mechanisms of the AKI-to-CKD transition. SIRT6 plays a critical role in the AKI-to-CKD transition and kidney repair through anti-inflammatory, antifibrotic, and mitochondrial quality control mechanisms. AKI Acute kidney injury, CKD Chronic kidney disease.

Sirtuins at the Service of Healthy Longevity

Sirtuins may counteract at least six hallmarks of organismal aging: neurodegeneration, chronic but ineffective inflammatory response, metabolic syndrome, DNA damage, genome instability, and cancer incidence. Moreover, caloric restriction is believed to slow down aging by boosting the activity of some sirtuins through activating adenosine monophosphate-activated protein kinase (AMPK), thus raising the level of intracellular nicotinamide adenine dinucleotide (NAD⁺) by stimulating NAD⁺ biosynthesis. Sirtuins and their downstream effectors induce intracellular signaling pathways related to a moderate caloric restriction within cells, mitigating reactive oxygen species (ROS) production, cell senescence phenotype (CSP) induction, and apoptosis as forms of the cellular stress response. Instead, it can promote DNA damage repair and survival of cells with normal, completely functional phenotypes. In this review, we discuss mechanisms of sirtuins action toward cell-conserving phenotype associated with intracellular signaling pathways related to moderate caloric restriction, as well as some tissue-specific functions of sirtuins, especially in the central nervous system, heart muscle, skeletal muscles, liver, kidneys, white adipose tissue, hematopoietic system, and immune system. In this context, we discuss the possibility of new therapeutic approaches.

IL-19 Contributes to the Development of Nonalcoholic Steatohepatitis by Altering Lipid Metabolism

Interleukin (IL)-19, a member of the IL-10 family, is an anti-inflammatory cytokine produced primarily by macrophages. Nonalcoholic steatohepatitis (NASH) is a disease that has progressed from nonalcoholic fatty liver disease (NAFLD) and is characterized by inflammation and fibrosis. We evaluated the functions of IL-19 in a NAFLD/NASH mouse model using a 60% high fat diet with 0.1% methionine, without choline, and with 2% cholesterol (CDAHFD). Wild-type (WT) and IL-19 gene-deficient (KO) mice were fed a CDAHFD or standard diet for 9 weeks. Liver injury, inflammation, and fibrosis induced by CDAHFD were significantly worse in IL-19 KO mice than in WT mice. IL-6, TNF-α, and TGF-β were significantly higher in IL-19 KO mice than in WT mice. As a mechanism using an in vitro experiment, palmitate-induced triglyceride and cholesterol contents were decreased by the addition of IL-19 in HepG2 cells. Furthermore, addition of IL-19 decreased the expression of fatty acid synthesis-related enzymes and increased ATP content in HepG2 cells. The action of IL-19 in vitro suppressed lipid metabolism. In conclusion, IL-19 may play an important role in the development of steatosis and fibrosis by directly regulating liver metabolism and may be a potential target for the treatment of liver diseases.

SIRT6 controls hepatic lipogenesis by suppressing LXR, ChREBP, and SREBP1

Fatty liver disease is the most prevalent chronic liver disorder, which is manifested by hepatic triglyceride elevation, inflammation, and fibrosis. Sirtuin 6 (Sirt6), an NAD+-dependent deacetylase, has been implicated in hepatic glucose and lipid metabolism; however, the underlying mechanisms are incompletely understood. The aim of this study was to identify and characterize novel players and mechanisms that are responsible for the Sirt6-mediated metabolic regulation in the liver. We generated and characterized Sirt6 liver-specific knockout mice regarding its role in the development of fatty liver disease. We used cell models to validate the molecular alterations observed in the animal models. Biochemical and molecular biological approaches were used to illustrate protein-protein interactions and gene regulation. Our data show that Sirt6 liver-specific knockout mice develop more severe fatty liver disease than wild-type mice do on a Western diet. Hepatic Sirt6 deficiency leads to elevated levels and transcriptional activities of carbohydrate response element binding protein (ChREBP) and sterol regulatory element binding protein 1 (SREBP1). Mechanistically, our data reveal protein-protein interactions between Sirt6 and liver X receptor α (LXRα), ChREBP, or SREBP1c in hepatocytes. Moreover, Sirt6 suppresses transcriptional activities of LXRα, ChREBP, and SREBP1c through direct deacetylation. In conclusion, this work has identified a key mechanism that is responsible for the salutary function of Sirt6 in the inhibition of hepatic lipogenesis by suppressing LXR, ChREBP, and SREBP1.

Vitexin ameliorates glycochenodeoxycholate-induced hepatocyte injury through SIRT6 and JAK2/STAT3 pathways

Objectives

Vitexin, a natural flavonoid, is commonly found in many foods and traditional herbal medicines and has clear health benefits. However, the role of vitexin in cholestasis is presently unclear. This study investigated whether vitexin mitigated glycochenodeoxycholate (GCDC)-induced hepatocyte injury and further elucidated the underlying mechanisms.

Materials and Methods

A cell counting kit-8 (CCK-8) assay was conducted to evaluate cell viability. The mitochondrial membrane potential (MMP, Δψm), reactive oxygen species (ROS) levels, and apoptosis rate of hepatocytes exposed to GCDC were detected by flow cytometry (FCM). We then measured the cytoprotective effects of vitexin against oxidative stress. The molecular signaling pathway was further investigated by using Western blotting and signaling pathway inhibitors.

Results

Here, we showed that vitexin increased cell viability and reduced cell apoptosis, necroptosis, and oxidative stress in a dose-dependent manner in GCDC-treated hepatocytes. In addition, by using selective inhibitors, we further confirmed that inhibition of the JAK2/STAT3 pathway by vitexin was mediated by prolonged activation of Sirtuin 6 (SIRT6).

Conclusion

Vitexin attenuated GCDC-induced hepatocyte injury via SIRT6 and the JAK2/STAT3 pathways.

Novel Therapeutic Targets in Liver Fibrosis

Liver fibrosis is end-stage liver disease that can be rescued. If irritation continues due to viral infection, schistosomiasis and alcoholism, liver fibrosis can progress to liver cirrhosis and even cancer. The US Food and Drug Administration has not approved any drugs that act directly against liver fibrosis. The only treatments currently available are drugs that eliminate pathogenic factors, which show poor efficacy; and liver transplantation, which is expensive. This highlights the importance of clarifying the mechanism of liver fibrosis and searching for new treatments against it. This review summarizes how parenchymal, nonparenchymal cells, inflammatory cells and various processes (liver fibrosis, hepatic stellate cell activation, cell death and proliferation, deposition of extracellular matrix, cell metabolism, inflammation and epigenetics) contribute to liver fibrosis. We highlight discoveries of novel therapeutic targets, which may provide new insights into potential treatments for liver fibrosis.

Mangiferin Ameliorates HFD-Induced NAFLD through Regulation of the AMPK and NLRP3 Inflammasome Signal Pathways

Nonalcoholic fatty liver disease (NAFLD) is closely related to glycolipid metabolism and liver inflammation. And there is no effective drug approved for its clinical therapy. In this study, we focused on mangiferin (Man) and explored its effects and mechanisms on NAFLD treatment based on the regulation of glycolipid metabolism and anti-inflammatory in vivo and in vitro. The results exhibited that Man can significantly attenuate liver injury, insulin resistance, and glucose tolerance in high-fat diet- (HFD-) induced NAFLD mice and significantly reduce fat accumulation and inflammation in hepatic tissue of NAFLD mice. The transcriptome level RNA-seq analysis showed that the significantly different expression genes between the Man treatment group and the HFD-induced NAFLD model group were mainly related to regulation of energy, metabolism, and inflammation in liver tissue. Furthermore, western blots, real-time PCR, and immunohistochemistry experiments confirmed that Man significantly activated the AMPK signal pathway and inhibited NLRP3 inflammasome activation and pyroptosis in NAFLD mice. In in vitro cell experiments, we further confirmed that Man can promote glucose consumption and reduce intracellular triglyceride (TG) accumulation induced by free fatty acids in HepG2 cells and further that it can be blocked by AMPK-specific inhibitors. Western blot results showed that Man upregulated p-AMPKα levels and exhibited a significant AMPK activation effect, which was blocked by compound C. At the same time, Man downregulated the expression of NLRP3 inflammasome-related proteins and inhibited the activation of NLRP3 inflammasome, alleviating cell pyroptosis and inflammation effects. These results indicate that Man anti-NAFLD activity is mediated through its regulation of glucolipid metabolism by AMPK activation and its anti-inflammatory effects by NLRP3 inflammasome inhibition. Our study indicates that Man is a promising prodrug for the therapy of NAFLD patients.

SIRT6 Through the Brain Evolution, Development, and Aging

During an organism's lifespan, two main phenomena are critical for the organism's survival. These are (1) a proper embryonic development, which permits the new organism to function with high fitness, grow and reproduce, and (2) the aging process, which will progressively undermine its competence and fitness for survival, leading to its death. Interestingly these processes present various similarities at the molecular level. Notably, as organisms became more complex, regulation of these processes became coordinated by the brain, and failure in brain activity is detrimental in both development and aging. One of the critical processes regulating brain health is the capacity to keep its genomic integrity and epigenetic regulation-deficiency in DNA repair results in neurodevelopmental and neurodegenerative diseases. As the brain becomes more complex, this effect becomes more evident. In this perspective, we will analyze how the brain evolved and became critical for human survival and the role Sirt6 plays in brain health. Sirt6 belongs to the Sirtuin family of histone deacetylases that control several cellular processes; among them, Sirt6 has been associated with the proper embryonic development and is associated with the aging process. In humans, Sirt6 has a pivotal role during brain aging, and its loss of function is correlated with the appearance of neurodegenerative diseases such as Alzheimer's disease. However, Sirt6 roles during brain development and aging, especially the last one, are not observed in all species. It appears that during the brain organ evolution, Sirt6 has gained more relevance as the brain becomes bigger and more complex, observing the most detrimental effect in the brains of Homo sapiens. In this perspective, we part from the evolution of the brain in metazoans, the biological similarities between brain development and aging, and the relevant functions of Sirt6 in these similar phenomena to conclude with the evidence suggesting a more relevant role of Sirt6 gained in the brain evolution.

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.

The Role of Epigenetic Changes in the Progression of Alcoholic Steatohepatitis

Alcoholic steatohepatitis (ASH) is a progression hepatitis with severe fatty liver and its mortality rate for 30-days in patients are over 30%. Additionally, ASH is well known for one-fifth all alcoholic related liver diseases in the world. Excessive chronic alcohol consumption is one of the most common causes of the progression of ASH and is associated with poor prognosis and liver failure. Alcohol abuse dysregulates the lipid homeostasis and causes oxidative stress and inflammation in the liver. Consequently, metabolic pathways stimulating hepatic accumulation of excessive lipid droplets are induced. Recently, many studies have indicated a link between ASH and epigenetic changes, showing differential expression of alcohol-induced epigenetic genes in the liver. However, the specific mechanisms underlying the pathogenesis of ASH remain elusive. Thus, we here summarize the current knowledge about the roles of epigenetics in lipogenesis, inflammation, and apoptosis in the context of ASH pathophysiology. Especially, we highlight the latest findings on the roles of Sirtuins, a conserved family of class-III histone deacetylases, in ASH. Additionally, we discuss the involvement of DNA methylation, histone modifications, and miRNAs in ASH as well as the ongoing efforts for the clinical translation of the findings in ASH-related epigenetic changes.

Fibrosis: Sirtuins at the checkpoints of myofibroblast differentiation and profibrotic activity

Fibrotic diseases are still a serious concern for public health, due to their high prevalence, complex etiology and lack of successful treatments. Fibrosis consists of excessive accumulation of extracellular matrix components. As a result, the structure and function of tissues are impaired, thus potentially leading to organ failure and death in several chronic diseases. Myofibroblasts represent the principal cellular mediators of fibrosis, due to their extracellular matrix producing activity, and originate from different types of precursor cells, such as mesenchymal cells, epithelial cells and fibroblasts. Profibrotic activation of myofibroblasts can be triggered by a variety of mechanisms, including the transforming growth factor-β signalling pathway, which is a major factor driving fibrosis. Interestingly, preclinical and clinical studies showed that fibrotic degeneration can stop and even reverse by using specific antifibrotic treatments. Increasing scientific evidence is being accumulated about the role of sirtuins in modulating the molecular pathways responsible for the onset and development of fibrotic diseases. Sirtuins are NAD⁺ -dependent protein deacetylases that play a crucial role in several molecular pathways within the cells, many of which at the crossroad between health and disease. In this context, we will report the current knowledge supporting the role of sirtuins in the balance between healthy and diseased myofibroblast activity. In particular, we will address the signalling pathways and the molecular targets that trigger the differentiation and profibrotic activation of myofibroblasts and can be modulated by sirtuins.

FOXO3a Protects against Kidney Injury in Type II Diabetic Nephropathy by Promoting Sirt6 Expression and Inhibiting Smad3 Acetylation

Diabetic nephropathy (DN) is the most common cause of end-stage renal disease. Although numerous reports have demonstrated a correlation between epithelial-mesenchymal transition (EMT) and renal fibrosis, how these processes lead to tubular dysfunction remains unclear. Here, we show that FOXO3a protects kidneys from injury in type II DN by increasing Sirt6 expression, which deacetylates Smad3 and inhibits its transcriptional activity. The results showed that progressive EMT in the kidneys from db/db mice is associated with Sirt6 downregulation and involved in tubular injury and dysfunction. The reduction of Sirt6 levels in db/db mice resulted in progressive kidney injury, indicating the protective role of Sirt6. Furthermore, Sirt6 was shown to directly bind to Smad3, a key downstream mediator of TGF-β, and could deacetylate it to inhibit its nuclear accumulation and transcriptional activity in HK2 cells. Besides, we demonstrate that FOXO3a activates Sirt6 expression by binding to its promoter. shRNA-induced FOXO3a knockdown in the kidneys of db/db mice exacerbated tubular injury and renal function loss. Mechanistically, FOXO3a protects against kidney injury in type II DN through the Sirt6/Smad3 axis. Thus, the pharmacological targeting of FOXO3a-mediated Sirt6/Smad3 signaling pathways may provide a novel strategy for treating type II DN.

Understanding the Function of Mammalian Sirtuins and Protein Lysine Acylation

Protein lysine acetylation is an important posttranslational modification that regulates numerous biological processes. Targeting lysine acetylation regulatory factors, such as acetyltransferases, deacetylases, and acetyl-lysine recognition domains, has been shown to have potential for treating human diseases, including cancer and neurological diseases. Over the past decade, many other acyl-lysine modifications, such as succinylation, crotonylation, and long-chain fatty acylation, have also been investigated and shown to have interesting biological functions. Here, we provide an overview of the functions of different acyl-lysine modifications in mammals. We focus on lysine acetylation as it is well characterized, and principles learned from acetylation are useful for understanding the functions of other lysine acylations. We pay special attention to the sirtuins, given that the study of sirtuins has provided a great deal of information about the functions of lysine acylation. We emphasize the regulation of sirtuins to illustrate that their regulation enables cells to respond to various signals and stresses.

The Epigenetic Regulation of Microenvironment in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is a highly lethal and complex malignancy strongly influenced by the surrounding tumor microenvironment. The HCC microenvironment comprises hepatic stellate cells (HSCs), tumor-associated macrophages (TAMs), stromal and endothelial cells, and the underlying extracellular matrix (ECM). Emerging evidence demonstrates that epigenetic regulation plays a crucial role in altering numerous components of the HCC tumor microenvironment. In this review, we summarize the current understanding of the mechanisms of epigenetic regulation of the microenvironment in HCC. We review recent studies demonstrating how specific epigenetic mechanisms (DNA methylation, histone regulation, and non-coding RNAs mediated regulation) in HSCs, TAMs, and ECM, and how they contribute to HCC development, so as to gain new insights into the treatment of HCC via regulating epigenetic regulation in the tumor microenvironment.

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.