SIRT2 regulates tumour hypoxia response by promoting HIF-1α hydroxylation

Citrullination modulation stabilizes HIF-1α to promote tumour progression

Citrullination plays an essential role in various physiological or pathological processes, however, whether citrullination is involved in regulating tumour progression and the potential therapeutic significance have not been well explored. Here, we find that peptidyl arginine deiminase 4 (PADI4) directly interacts with and citrullinates hypoxia-inducible factor 1α (HIF-1α) at R698, promoting HIF-1α stabilization. Mechanistically, PADI4-mediated HIF-1αR698 citrullination blocks von Hippel-Lindau (VHL) binding, thereby antagonizing HIF-1α ubiquitination and subsequent proteasome degradation. We also show that citrullinated HIF-1αR698, HIF-1α and PADI4 are highly expressed in hepatocellular carcinoma (HCC) tumour tissues, suggesting a potential correlation between PADI4-mediated HIF-1αR698 citrullination and cancer development. Furthermore, we identify that dihydroergotamine mesylate (DHE) acts as an antagonist of PADI4, which ultimately suppresses tumour progression. Collectively, our results reveal citrullination as a posttranslational modification related to HIF-1α stability, and suggest that targeting PADI4-mediated HIF-1α citrullination is a promising therapeutic strategy for cancers with aberrant HIF-1α expression.

Activation and inhibition of sirtuins: From bench to bedside

The sirtuin family comprises seven NAD+-dependent enzymes which catalyze protein lysine deacylation and mono ADP-ribosylation. Sirtuins act as central regulators of genomic stability and gene expression and control key processes, including energetic metabolism, cell cycle, differentiation, apoptosis, and aging. As a result, all sirtuins play critical roles in cellular homeostasis and organism wellness, and their dysregulation has been linked to metabolic, cardiovascular, and neurological diseases. Furthermore, sirtuins have shown dichotomous roles in cancer, acting as context-dependent tumor suppressors or promoters. Given their central role in different cellular processes, sirtuins have attracted increasing research interest aimed at developing both activators and inhibitors. Indeed, sirtuin modulation may have therapeutic effects in many age-related diseases, including diabetes, cardiovascular and neurodegenerative disorders, and cancer. Moreover, isoform selective modulators may increase our knowledge of sirtuin biology and aid to develop better therapies. Through this review, we provide critical insights into sirtuin pharmacology and illustrate their enzymatic activities and biological functions. Furthermore, we outline the most relevant sirtuin modulators in terms of their modes of action, structure-activity relationships, pharmacological effects, and clinical applications.

Revisiting the role of hypoxia-inducible factors and nuclear factor erythroid 2-related factor 2 in regulating macrophage inflammation and metabolism

The recent birth of the immunometabolism field has comprehensively demonstrated how the rewiring of intracellular metabolism is critical for supporting the effector functions of many immune cell types, such as myeloid cells. Among all, the transcriptional regulation mediated by Hypoxia-Inducible Factors (HIFs) and Nuclear factor erythroid 2-related factor 2 (NRF2) have been consistently shown to play critical roles in regulating the glycolytic metabolism, redox homeostasis and inflammatory responses of macrophages (Mφs). Although both of these transcription factors were first discovered back in the 1990s, new advances in understanding their function and regulations have been continuously made in the context of immunometabolism. Therefore, this review attempts to summarize the traditionally and newly identified functions of these transcription factors, including their roles in orchestrating the key events that take place during glycolytic reprogramming in activated myeloid cells, as well as their roles in mediating Mφ inflammatory responses in various bacterial infection models.

Acetylation of ELMO1 correlates with Rac1 activity and colorectal cancer progress

Acetylation, a critical regulator of diverse cellular processes, holds significant implications in various cancer contexts. Further understanding of the acetylation patterns of key cancer-driven proteins is crucial for advancing therapeutic strategies in cancer treatment. This study aimed to unravel the acetylation patterns of Engulfment and Cell Motility Protein 1 (ELMO1) and its relevance to the pathogenesis of colorectal cancer (CRC). Immunoprecipitation and mass spectrometry precisely identified lysine residue 505 (K505) as a central acetylation site in ELMO1. P300 emerged as the acetyltransferase for ELMO1 K505 acetylation, while SIRT2 was recognized as the deacetylase. Although K505 acetylation minimally affected ELMO1's localization and stability, it played a crucial role in mediating ELMO1-Dock180 interaction, thereby influencing Rac1 activation. Functionally, ELMO1 K505 acetylation proved to be a pivotal factor in CRC progression, exerting its influence on key cellular processes. Clinical analysis of CRC samples unveiled elevated ELMO1 acetylation in primary tumors, indicating a potential association with CRC pathologies. This work provides insights into ELMO1 acetylation and its significance in advancing potentially therapeutic interventions in CRC treatment.

Sirt2 inhibition improves gut epithelial barrier integrity and protects mice from colitis

Sirt2 is a nicotinamide adenine dinucleotide (NAD+)-dependent protein lysine deacylase that can remove both acetyl group and long-chain fatty acyl groups from lysine residues of many proteins. It was reported to affect inflammatory bowel disease (IBD) symptoms in a mouse model. However, conflicting roles were reported, with genetic knockout aggravating while pharmacological inhibition alleviating IBD symptoms. These seemingly conflicting reports cause confusion and deter further efforts in developing Sirt2 inhibitors as a potential treatment strategy for IBD. We investigated these conflicting reports and elucidated the role of Sirt2 in the mouse model of IBD. We essentially replicated these conflicting results and confirmed that Sirt2 inhibitors' protective effect is not through off-targets as two very different Sirt2 inhibitors (TM and AGK2) showed similar protection in the IBD mouse model. We believe that the differential effects of inhibitors and knockout are due to the fact that the Sirt2 inhibitors only inhibit some but not all the activities of Sirt2. This hypothesis is confirmed by the observation that a PROTAC degrader of Sirt2 did not protect mice in the IBD model, similar to Sirt2 knockout. Our study provides an interesting example where genetic knockout and pharmacological inhibition do not align and emphasizes the importance of developing substrate-dependent inhibitors. Importantly, we showed that the effect of Sirt2 inhibition in IBD is through regulating the gut epithelium barrier by inhibiting Arf6-mediated endocytosis of E-cadherin, a protein important for the intestinal epithelial integrity. This mechanistic understanding further supports Sirt2 as a promising therapeutic target for treating IBD.

HIF-1α: A potential therapeutic opportunity in renal fibrosis

Renal fibrosis is a common outcome of various renal injuries, leading to structural destruction and functional decline of the kidney, and is also a critical prognostic indicator and determinant in renal diseases therapy. Hypoxia is induced in different stress and injuries in kidney, and the hypoxia inducible factors (HIFs) are activated in the context of hypoxia in response and regulation the hypoxia in time. Under stress and hypoxia conditions, HIF-1α increases rapidly and regulates intracellular energy metabolism, cell proliferation, apoptosis, and inflammation. Through reprogramming cellular metabolism, HIF-1α can directly or indirectly induce abnormal accumulation of metabolites, changes in cellular epigenetic modifications, and activation of fibrotic signals. HIF-1α protein expression and activity are regulated by various posttranslational modifications. The drugs targeting HIF-1α can regulate the downstream cascade signals by inhibiting HIF-1α activity or promoting its degradation. As the renal fibrosis is affected by renal diseases, different diseases may trigger different mechanisms which will affect the therapy effect. Therefore, comprehensive analysis of the role and contribution of HIF-1α in occurrence and progression of renal fibrosis, and determination the appropriate intervention time of HIF-1α in the process of renal fibrosis are important ideas to explore effective treatment strategies. This study reviews the regulation of HIF-1α and its mediated complex cascade reactions in renal fibrosis, and lists some drugs targeting HIF-1α that used in preclinical studies, to provide new insight for the study of the renal fibrosis mechanism.

Dysregulation of histone deacetylases in ocular diseases

Ocular diseases are a growing global concern and have a significant impact on the quality of life. Cataracts, glaucoma, age-related macular degeneration, and diabetic retinopathy are the most prevalent ocular diseases. Their prevalence and the global market size are also increasing. However, the available pharmacotherapy is currently limited. These diseases share common pathophysiological features, including neovascularization, inflammation, and/or neurodegeneration. Histone deacetylases (HDACs) are a class of enzymes that catalyze the removal of acetyl groups from lysine residues of histone and nonhistone proteins. HDACs are crucial for regulating various cellular processes, such as gene expression, protein stability, localization, and function. They have also been studied in various research fields, including cancer, inflammatory diseases, neurological disorders, and vascular diseases. Our study aimed to investigate the relationship between HDACs and ocular diseases, to identify a new strategy for pharmacotherapy. This review article explores the role of HDACs in ocular diseases, specifically focusing on diabetic retinopathy, age-related macular degeneration, and retinopathy of prematurity, as well as optic nerve disorders, such as glaucoma and optic neuropathy. Additionally, we explore the interplay between HDACs and key regulators of fibrosis and angiogenesis, such as TGF-β and VEGF, highlighting the potential of targeting HDAC as novel therapeutic strategies for ocular diseases.

Epigenetic remodelling under hypoxia

Hypoxia is intrinsic to tumours and contributes to malignancy and metastasis while hindering the efficiency of existing treatments. Epigenetic mechanisms play a crucial role in the regulation of hypoxic cancer cell programs, both in the initial phases of sensing the decrease in oxygen levels and during adaptation to chronic lack of oxygen. During the latter, the epigenetic regulation of tumour biology intersects with hypoxia-sensitive transcription factors in a complex network of gene regulation that also involves metabolic reprogramming. Here, we review the current literature on the epigenetic control of gene programs in hypoxic cancer cells. We highlight common themes and features of such epigenetic remodelling and discuss their relevance for the development of therapeutic strategies.

Acetylation and Phosphorylation in the Regulation of Hypoxia-Inducible Factor Activities: Additional Options to Modulate Adaptations to Changes in Oxygen Levels

O2 is essential for the life of eukaryotic cells. The ability to sense oxygen availability and initiate a response to adapt the cell to changes in O2 levels is a fundamental achievement of evolution. The key switch for adaptation consists of the transcription factors HIF1A, HIF2A and HIF3A. Their levels are tightly controlled by O2 through the involvement of the oxygen-dependent prolyl hydroxylase domain-containing enzymes (PHDs/EGNLs), the von Hippel-Lindau tumour suppressor protein (pVHL) and the ubiquitin-proteasome system. Furthermore, HIF1A and HIF2A are also under the control of additional post-translational modifications (PTMs) that positively or negatively regulate the activities of these transcription factors. This review focuses mainly on two PTMs of HIF1A and HIF2A: phosphorylation and acetylation.

Sirtuin 2 promotes human cytomegalovirus replication by regulating cell cycle progression

IMPORTANCE

This study expands the growing understanding that protein acetylation is a highly regulated molecular toggle of protein function in both host anti-viral defense and viral replication. We describe a pro-viral role for the human enzyme SIRT2, showing that its deacetylase activity supports HCMV replication. By integrating quantitative proteomics, flow cytometry cell cycle assays, microscopy, and functional virology assays, we investigate the temporality of SIRT2 functions and substrates. We identify a pro-viral role for the SIRT2 deacetylase activity via regulation of CDK2 K6 acetylation and the G1-S cell cycle transition. These findings highlight a link between viral infection, protein acetylation, and cell cycle progression.

Functional crosstalk between chromatin and hypoxia signalling

Eukaryotic genomes are organised in a structure called chromatin, comprising of DNA and histone proteins. Chromatin is thus a fundamental regulator of gene expression, as it offers storage and protection but also controls accessibility to DNA. Sensing and responding to reductions in oxygen availability (hypoxia) have recognised importance in both physiological and pathological processes in multicellular organisms. One of the main mechanisms controlling these responses is control of gene expression. Recent findings in the field of hypoxia have highlighted how oxygen and chromatin are intricately linked. This review will focus on mechanisms controlling chromatin in hypoxia, including chromatin regulators such as histone modifications and chromatin remodellers. It will also highlight how these are integrated with hypoxia inducible factors and the knowledge gaps that persist.

Metabolomics Analysis as a Tool to Measure Cobalt Neurotoxicity: An In Vitro Validation

In this study, cobalt neurotoxicity was investigated in human astrocytoma and neuroblastoma (SH-SY5Y) cells using proliferation assays coupled with LC-MS-based metabolomics and transcriptomics techniques. Cells were treated with a range of cobalt concentrations between 0 and 200 µM. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay revealed cobalt cytotoxicity and decreased cell metabolism in a dose and time-dependent manner was observed by metabolomics analysis, in both cell lines. Metabolomic analysis also revealed several altered metabolites particularly those related to DNA deamination and methylation pathways. One of the increased metabolites was uracil which can be generated from DNA deamination or fragmentation of RNA. To investigate the origin of uracil, genomic DNA was isolated and analyzed by LC-MS. Interestingly, the source of uracil, which is uridine, increased significantly in the DNA of both cell lines. Additionally, the results of the qRT-PCR showed an increase in the expression of five genes Mlh1, Sirt2, MeCP2, UNG, and TDG in both cell lines. These genes are related to DNA strand breakage, hypoxia, methylation, and base excision repair. Overall, metabolomic analysis helped reveal the changes induced by cobalt in human neuronal-derived cell lines. These findings could unravel the effect of cobalt on the human brain.

Histone acetyltransferase 1 (HAT1) acetylates hypoxia-inducible factor 2 alpha (HIF2A) to execute hypoxia response

Hypoxic response to low oxygen levels is characteristic of most solid cancers. Hypoxia-inducible factors (HIFs) regulate cellular metabolism, survival, proliferation, and cancer stem cell growth during hypoxia. The genome-wide analysis identified HAT1, a type B histone acetyltransferase, as an upregulated and essential gene in glioblastoma (GBM). GSEA analysis of differentially regulated genes in HAT1 silenced cells identified significant depletion of "hypoxia" gene sets. Hypoxia conditions induced HIF2A, not HIF1A protein levels in glioma cells in a HAT1-dependent manner. HAT1 and HIF2A interacted with each other and occupied the promoter of VEGFA, a bonafide HIF1A/HIF2A target. Acetylation of K512 and K596 residues by HAT1 is essential for HIF2A stabilization under normoxia and hypoxia as HIF2A carrying acetylation mimic mutations at either of these residues (H512Q or K596Q) showed stable expression in HAT1 silenced cells under normoxia and hypoxia conditions. Finally, we demonstrate that the HAT1-HIF2A axis is essential for hypoxia-promoted cancer stem cell maintenance and reprogramming. Thus, our study identifies that the HAT1-dependent acetylation of HIF2A is vital to executing the hypoxia-induced cell survival and cancer stem cell growth, therefore proposing the HAT1-HIF2A axis as a potential therapeutic target.

Molecular regulation of hypoxia through the lenses of noncoding RNAs and epitranscriptome

Cells maintain homeostasis in response to environmental stress through specific cell stress responses. Hypoxic stress, well known to be associated with diverse solid tumors, is one of the main reasons for cancer-related mortality. Although cells can balance themselves well during hypoxic stress, the underlying molecular mechanisms are not well understood. The enhanced appreciation of diverse roles played by noncoding transcriptome and epigenome in recent years has brought to light the involvement of noncoding RNAs and epigenetic modifiers in hypoxic regulation. The emergence of techniques like deep sequencing has facilitated the identification of large numbers of long noncoding RNAs (lncRNAs) that are differentially regulated in various cancers. Similarly, proteomic studies have identified diverse epigenetic modifiers such as HATs, HDACs, DNMTs, polycomb groups of proteins, and their possible roles in the regulation of hypoxia. The crosstalk between lncRNAs and epigenetic modifiers play a pivotal role in hypoxia-induced cancer initiation and progression. Besides the lncRNAs, several other noncoding RNAs like circular RNAs, miRNAs, and so forth are also expressed during hypoxic conditions. Hypoxia has a profound effect on the expression of noncoding RNAs and epigenetic modifiers. Conversely, noncoding RNAs/epigenetic modifies can regulate the hypoxia signaling axis by modulating the stability of the hypoxia-inducible factors (HIFs). The focus of this review is to illustrate the molecular orchestration underlying hypoxia biology, especially in cancers, which can help in identifying promising therapeutic targets in hypoxia-induced cancers. This article is categorized under: RNA Turnover and Surveillance > Regulation of RNA Stability RNA in Disease and Development > RNA in Disease RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry.

AICAR Ameliorates Non-Alcoholic Fatty Liver Disease via Modulation of the HGF/NF-κB/SNARK Signaling Pathway and Restores Mitochondrial and Endoplasmic Reticular Impairments in High-Fat Diet-Fed Rats

Non-alcoholic fatty liver disease (NAFLD) is a global health problem characterized by altered lipid and redox homeostasis, mitochondrial dysfunction, and endoplasmic reticulum (ER) stress. The AMP-dependent kinase (AMPK) agonist 5-aminoimidazole-4-carboxamide ribonucleoside (AICAR) has been shown to improve the outcome of NAFLD in the context of AMPK activation, yet the underlying molecular mechanism remains obscure. This study investigated the potential mechanism(s) of AICAR to attenuate NAFLD by exploring AICAR's effects on the HGF/NF-κB/SNARK axis and downstream effectors as well as mitochondrial and ER derangements. High-fat diet (HFD)-fed male Wistar rats were given intraperitoneal AICAR at 0.7 mg/g body weight or left untreated for 8 weeks. In vitro steatosis was also examined. ELISA, Western blotting, immunohistochemistry and RT-PCR were used to explore AICAR's effects. NAFLD was confirmed by steatosis score, dyslipidemia, altered glycemic, and redox status. HGF/NF-κB/SNARK was downregulated in HFD-fed rats receiving AICAR with improved hepatic steatosis and reduced inflammatory cytokines and oxidative stress. Aside from AMPK dominance, AICAR improved hepatic fatty acid oxidation and alleviated the ER stress response. In addition, it restored mitochondrial homeostasis by modulating Sirtuin 2 and mitochondrial quality gene expression. Our results provide a new mechanistic insight into the prophylactic role of AICAR in the prevention of NAFLD and its complications.

Shedding light on structure, function and regulation of human sirtuins: a comprehensive review

Sirtuins play an important role in signalling pathways associated with various metabolic regulations. They possess mono-ADP-ribosyltransferase or deacylase activity like demalonylase, deacetylase, depalmitoylase, demyristoylase and desuccinylase activity. Sirtuins are histone deacetylases which depends upon nicotinamide adenine dinucleotide (NAD) that deacetylate lysine residues. There are a total of seven human sirtuins that have been identified namely, SIRT1, SIRT2, SIRT3, SIRT4, SIRT5, SIRT6 and SIRT7. The subcellular location of mammalian sirtuins, SIRT1, SIRT6, and SIRT7 are in the nucleus; SIRT3, SIRT4, and SIRT5 are in mitochondria, and SIRT2 is in cytoplasm. Structurally sirtuins contains a N-terminal, a C-terminal and a Zn+ binding domain. The sirtuin family has been found to be crucial for maintaining lipid and glucose homeostasis, and also for regulating insulin secretion and sensitivity, DNA repair pathways, neurogenesis, inflammation, and ageing. Based on the literature, sirtuins are overexpressed and play an important role in tumorigenicity in various types of cancer such as non-small cell lung cancer, colorectal cancer, etc. In this review, we have discussed about the different types of human sirtuins along with their structural and functional features. We have also discussed about the various natural and synthetic regulators of sirtuin activities like resveratrol. Our overall study shows that the correct regulation of sirtuins can be a good target for preventing and treating various diseases for improving the human lifespan. To investigate the true therapeutic potential of sirtuin proteins and their efficacy in a variety of pathological diseases, a better knowledge of the link between the structure and function of sirtuin proteins would be necessary.

Hypoxia-responsive nanomaterials for tumor imaging and therapy

Hypoxia is an important component of tumor microenvironment and plays a pivotal role in cancer progression. With the distinctive physiochemical properties and biological effects, various nanoparticles targeting hypoxia had raised great interest in cancer imaging, drug delivery, and gene therapy during the last decade. In the current review, we provided a comprehensive view on the latest progress of novel stimuli-responsive nanomaterials targeting hypoxia-tumor microenvironment (TME), and their applications in cancer diagnosis and therapy. Future prospect and challenges of nanomaterials are also discussed.

The Role of Hypoxia-Inducible Factor in the Mechanisms of Aging

Aging is accompanied by a reduction in the oxygen delivery to all organs and tissues and decrease in the oxygen partial pressure in them, resulting in the development of hypoxia. The lack of oxygen activates cell signaling pathway mediated by the hypoxia-inducible transcription factor (HIF), which exists in three isoforms - HIF-1, HIF-2, and HIF-3. HIF regulates expression of several thousand genes and is a potential target for the development of new drugs for the treatment of many diseases, including those associated with age. Human organism and organisms of laboratory animals differ in their tolerance to hypoxia and expression of HIF and HIF-dependent genes, which may contribute to the development of inflammatory, tumor, and cardiovascular diseases. Currently, the data on changes in the HIF expression with age are contradictory, which is mostly due to the fact that such studies are conducted in different age groups, cell types, and model organisms, as well as under different hypoxic conditions and mainly in vitro. Furthermore, the observed discrepancies can be due to the individual tolerance of the studied organisms to hypoxia, which is typically not taken into account. Therefore, the purpose of this review was to analyze the published data on the connection between the mechanisms of aging, basal tolerance to hypoxia, and changes in the level of HIF expression with age. Here, we summarized the data on the age-related changes in the hypoxia tolerance, HIF expression and the role of HIF in aging, which is associated with its involvement in the molecular pathways mediated by insulin and IGF-1 (IIS), sirtuins (SIRTs), and mTOR. HIF-1 interacts with many components of the IIS pathway, in particular with FOXO, the activation of which reduces production of reactive oxygen species (ROS) and increases hypoxia tolerance. Under hypoxic conditions, FOXO is activated via both HIF-dependent and HIF-independent pathways, which contributes to a decrease in the ROS levels. The activity of HIF-1 is regulated by all members of the sirtuin family, except SIRT5, while the mechanisms of SIRT interaction with HIF-2 and HIF-3 are poorly understood. The connection between HIF and mTOR and its inhibitor, AMPK, has been identified, but its exact mechanism has yet to be studied. Understanding the role of HIF and hypoxia in aging and pathogenesis of age-associated diseases is essential for the development of new approaches to the personalized therapy of these diseases, and requires further research.

Gene regulation by histone-modifying enzymes under hypoxic conditions: a focus on histone methylation and acetylation

Oxygen, which is necessary for sustaining energy metabolism, is consumed in many biochemical reactions in eukaryotes. When the oxygen supply is insufficient for maintaining multiple homeostatic states at the cellular level, cells are subjected to hypoxic stress. Hypoxia induces adaptive cellular responses mainly through hypoxia-inducible factors (HIFs), which are stabilized and modulate the transcription of various hypoxia-related genes. In addition, many epigenetic regulators, such as DNA methylation, histone modification, histone variants, and adenosine triphosphate-dependent chromatin remodeling factors, play key roles in gene expression. In particular, hypoxic stress influences the activity and gene expression of histone-modifying enzymes, which controls the posttranslational modification of HIFs and histones. This review covers how histone methylation and histone acetylation enzymes modify histone and nonhistone proteins under hypoxic conditions and surveys the impact of epigenetic modifications on gene expression. In addition, future directions in this area are discussed.

New sequencing technologies are revealing how cells respond to hypoxia, insufficient oxygen, by managing gene activation. In multicellular organisms, gene activation is managed by how tightly a section of DNA is wound around proteins called histones; genes in tightly packed regions are inaccessible and inactive, whereas those in looser regions can be activated. Kyunghwan Kim, Sun-Ju Yi, and co-workers at Chungbuk National University in South Korea have reviewed recent data on how cells regulate gene activity under hypoxic conditions. Advances in sequencing technology have allowed genome-wide studies of how hypoxia affects DNA structure and gene activation, revealing that gene-specific modifications may be more important than genome-wide modifications. Hypoxia is implicated in several diseases, such as cancer and chronic metabolic diseases, and a better understanding of how it affects gene activation may help identify new treatments for hypoxia-related diseases.

Sirtuins and Hypoxia in EMT Control

Epithelial–mesenchymal transition (EMT), a physiological process during embryogenesis, can become pathological in the presence of different driving forces. Reduced oxygen tension or hypoxia is one of these forces, triggering a large number of molecular pathways with aberrant EMT induction, resulting in cancer and fibrosis onset. Both hypoxia-induced factors, HIF-1α and HIF-2α, act as master transcription factors implicated in EMT. On the other hand, hypoxia-dependent HIF-independent EMT has also been described. Recently, a new class of seven proteins with deacylase activity, called sirtuins, have been implicated in the control of both hypoxia responses, HIF-1α and HIF-2α activation, as well as EMT induction. Intriguingly, different sirtuins have different effects on hypoxia and EMT, acting as either activators or inhibitors, depending on the tissue and cell type. Interestingly, sirtuins and HIF can be activated or inhibited with natural or synthetic molecules. Moreover, recent studies have shown that these natural or synthetic molecules can be better conveyed using nanoparticles, representing a valid strategy for EMT modulation. The following review, by detailing the aspects listed above, summarizes the interplay between hypoxia, sirtuins, and EMT, as well as the possible strategies to modulate them by using a nanoparticle-based approach.

Sirtuin1 attenuates acute liver failure by reducing reactive oxygen species via hypoxia inducible factor 1α

BACKGROUND

The occurrence and development of acute liver failure (ALF) is closely related to a series of inflammatory reactions, such as the production of reactive oxygen species (ROS). Hypoxia inducible factor 1α (HIF-1α) is a key factor that regulates oxygen homeostasis and redox, and the stability of HIF-1α is related to the ROS level regulated by Sirtuin (Sirt) family. The activation of Sirt1 will lead to a powerful antioxidant defense system and therapeutic effects in liver disease. However, little is known about the relationship between HIF-1α and Sirt1 in the process of ALF and the molecular mechanism.

AIM

To investigate whether HIF-1α may be a target of Sirt1 deacetylation and what the effects on ALF are.

METHODS

Mice were administrated lipopolysaccharide (LPS)/D-gal and exposed to hypoxic conditions as animal model, and resveratrol was used as an activator of Sirt1. The cellular model was established with L02 cells stimulated by LPS. N-acetyl-L-cysteine was used to remove ROS, and the expression of Sirt1 was inhibited by nicotinamide. Western blotting was used to detect Sirt1 and HIF-1α activity and related protein expression. The possible signaling pathways involved were analyzed by immunofluorescent staining, co-immunoprecipitation, dihydroethidium staining, and Western blotting.

RESULTS

Compared with mice stimulated with LPS alone, the expression of Sirt1 decreased, the level of HIF-1α acetylation increased in hypoxic mice, and the levels of carbonic anhydrase 9 and Bcl-2-adenovirus E1B interacting protein 3 increased significantly, which was regulated by HIF-1α, indicating an increase of HIF-1α activity. Under hypoxia, the down-regulation of Sirt1 activated and acetylated HIF-1α in L02 cells. The inhibition of Sirt1 significantly aggravated this effect and the massive production of ROS. The regulation of ROS was partly through peroxisome proliferator-activated receptor alpha or AMP-activated protein kinase. Resveratrol, a Sirt1 activator, effectively relieved ALF aggravated by hypoxia, the production of ROS, and cell apoptosis. It also induced the deacetylation of HIF-1α and inhibited the activity of HIF-1α.

CONCLUSION

Sirt1 may have a protective effect on ALF by inducing HIF-1α deacetylation to reduce ROS.

Hypoxic Microenvironment-Induced Reduction in PTEN-L Secretion Promotes Non-Small Cell Lung Cancer Metastasis through PI3K/AKT Pathway

Objective

Lung cancer is the leading cause of cancer-related deaths worldwide. The aim of this study was to investigate the effects of hypoxic microenvironment on PTEN-L secretion and the effects of PTEN-L on the metastasis of non-small cell lung cancer (NSCLC) and the potential mechanisms.

Methods

The expression levels of PTEN-L in NSCLC tissues, cells, and cell culture media were detected. The transfection of PTEN-L overexpression construct or HIF-1α-siRNAs was conducted to manipulate the expression of PTEN-L or HIF-1α. NSCLC cells were introduced into 200 μM CoCl2 medium for 72 hours under 37°C to simulate hypoxia. The proliferation and apoptosis of the A549 cells were determined by the Cell Counting Kit-8 assay and Annexin V-FITC/PI-stained flow cytometry assay, respectively. Wound healing assay and transwell invasion assay were used to measure the migration and invasion of A549 cells. The protein expression of PTEN, PTEN-L, PI3K/AKT pathway-related proteins, and HIF-1α was detected by Western blot.

Results

PTEN and PTEN-L are downregulated in lung cancer tissues and cells. The protein expression of PTEN-L in the culture medium of lung cancer cell lines is decreased. The hypoxic microenvironment inhibits PTEN-L secretion. The low level of PTEN-L promotes cell proliferation, migration, and invasion, as well as inhibits apoptosis of A549 cells. The overexpression of PTEN-L attenuated the activation of the PI3K/AKT pathway by the hypoxic microenvironment. The knockdown of HIF-1α upregulates PTEN-L secretion under hypoxia.

Conclusions

The hypoxic microenvironment inhibits PTEN-L secretion and thus activates PI3K/AKT pathway to induce proliferation, migration, and invasion promotion, and apoptosis inhibition in NSCLC cells.

Capsaicin Attenuates Arterial Calcification Through Promoting SIRT6-Mediated Deacetylation and Degradation of Hif1α (Hypoxic-Inducible Factor-1 Alpha)

BACKGROUND

Sustained Hif1α (hypoxic-inducible factor-1 alpha) accumulation plays a central role in osteogenic transdifferentiation and subsequent calcification. Capsaicin, the potent agonist of TRPV1 (transient receptor potential vanilloid type 1), was found to mitigate hypoxic-related injury and reverse phenotypic switch of vascular smooth muscle cells. However, its role in arterial calcification and the underlying mechanisms remain unexplored.

METHODS

We used data from Multi-Ethnic Study of Atherosclerosis to examine the association of coronary artery calcification and chili consumption. Chronic kidney disease mice and high phosphate-induced vascular smooth muscle cells calcification models were established to investigate the anticalcification effect of capsaicin, evaluated by calcium deposition and changes in phenotype markers.

RESULTS

Chili consumption was negatively correlated with coronary artery calcification and conferred a smaller progression burden during follow-up. Capsaicin reduced calcium deposition and osteogenic transdifferentiation both in vivo and in vitro. Using siTRPV1 and capsazepine, the anticalcification effect of capsaicin was abrogated. Hif1α was increased in Pi-treated vascular smooth muscle cells and its degradation was accelerated by capsaicin. Retaining Hif1α stability using CoCl2 or MG132 abolished the protective effect of capsaicin. We further identified an increased expression of SIRT6 in response to capsaicin and confirmed the physical interaction between SIRT6 and Hif1α. Acetylated Hif1α was decreased, whereas hydroxylated Hif1α was increased under capsaicin treatment. Using immunohistochemistry analysis, we observed increased SIRT6 and reduced Hif1α in both SIRT6 transgenic and capsaicin-treated chronic kidney disease mice.

CONCLUSIONS

Capsaicin facilitates deacetylation and degradation of Hif1α by upregulating SIRT6, which inhibits osteogenic transdifferentiation and protects against arterial calcification. These data highlight a promising therapeutic target for the management of arterial calcification.

Development of a NanoBRET assay to validate dual inhibitors of Sirt2-mediated lysine deacetylation and defatty-acylation that block prostate cancer cell migration

Sirtuin2 (Sirt2) with its NAD⁺-dependent deacetylase and defatty-acylase activities plays a central role in the regulation of specific cellular functions. Dysregulation of Sirt2 activity has been associated with the pathogenesis of many diseases, thus making Sirt2 a promising target for pharmaceutical intervention. Herein, we present new high affinity Sirt2 selective Sirtuin-Rearranging Ligands (SirReals) that inhibit both Sirt2-dependent deacetylation and defatty-acylation in vitro and in cells. We show that simultaneous inhibition of both Sirt2 activities results in strongly reduced levels of the oncoprotein c-Myc and an inhibition of cancer cell migration. Furthermore, we describe the development of a NanoBRET-based assay for Sirt2, thereby providing a method to study cellular target engagement for Sirt2 in a straightforward and accurately quantifiable manner. Applying this assay, we could confirm cellular Sirt2 binding of our new Sirt2 inhibitors and correlate their anticancer effects with their cellular target engagement.

Sirt2 inhibitors that show simultaneous inhibition of Sirt2 deacetylase and defatty-acylase activity block prostate cancer cell migration and their target engagement is shown by a newly developed NanoBRET assay.

Potential Synthetic Lethality for Breast Cancer: A Selective Sirtuin 2 Inhibitor Combined with a Multiple Kinase Inhibitor Sorafenib

Sorafenib is a clinically useful multiple kinase inhibitor for the treatment of kidney cancer, liver cancer and acute myelocytic leukemia, while it has shown weak efficacy in suppressing breast cancer. Since sirtuin2 (SIRT2) is an important epigenetic regulator and associated with several cancer types including breast cancer, development and evaluation of new SIRT2 inhibitors to probe their therapeutic potentials is currently desirable. A highly selective SIRT2 inhibitor named I was previously developed by us, which showed activity to inhibit non-small cell lung cancer cell lines in vitro. We herein report expanded screening of I and its structurally similar inactive compound II against other cancer cell lines, and found that I had a wide spectrum of anticancer activity while II had no such effects. The I-sorafenib combination treatment exerted obvious synergistic reduction on cell viability of MCF-7 cells. We observed that the combination treatment could suppress cell proliferation, survival and migration, arrest cell cycle at G0/G1 phase, and induce apoptosis in MCF-7 cells, when compared with the single treatment. In vivo studies revealed that the combination treatment showed stronger tumor growth inhibition (87%), comparing with I-(42.8%) or sorafenib-solely-treated groups (61.1%) in MCF-7 xenograft model. In conclusion, this work clearly revealed a potential synthetic lethality effect for I combined with sorafenib, and will probably offer a new strategy at least for breast cancer treatment.

Regulation of Hypoxic Signaling and Oxidative Stress via the MicroRNA-SIRT2 Axis and Its Relationship with Aging-Related Diseases

The sirtuin family of nicotinamide adenine dinucleotide-dependent deacetylase and ADP-ribosyl transferases plays key roles in aging, metabolism, stress response, and aging-related diseases. SIRT2 is a unique sirtuin that is expressed in the cytosol and is abundant in neuronal cells. Various microRNAs were recently reported to regulate SIRT2 expression via its 3'-untranslated region (UTR), and single nucleotide polymorphisms in the miRNA-binding sites of SIRT2 3'-UTR were identified in patients with neurodegenerative diseases. The present review highlights recent studies into SIRT2-mediated regulation of the stress response, posttranscriptional regulation of SIRT2 by microRNAs, and the implications of the SIRT2-miRNA axis in aging-related diseases.

Chronic and Cycling Hypoxia: Drivers of Cancer Chronic Inflammation through HIF-1 and NF-κB Activation: A Review of the Molecular Mechanisms

Chronic (continuous, non-interrupted) hypoxia and cycling (intermittent, transient) hypoxia are two types of hypoxia occurring in malignant tumors. They are both associated with the activation of hypoxia-inducible factor-1 (HIF-1) and nuclear factor κB (NF-κB), which induce changes in gene expression. This paper discusses in detail the mechanisms of activation of these two transcription factors in chronic and cycling hypoxia and the crosstalk between both signaling pathways. In particular, it focuses on the importance of reactive oxygen species (ROS), reactive nitrogen species (RNS) together with nitric oxide synthase, acetylation of HIF-1, and the action of MAPK cascades. The paper also discusses the importance of hypoxia in the formation of chronic low-grade inflammation in cancerous tumors. Finally, we discuss the effects of cycling hypoxia on the tumor microenvironment, in particular on the expression of VEGF-A, CCL2/MCP-1, CXCL1/GRO-α, CXCL8/IL-8, and COX-2 together with PGE₂. These factors induce angiogenesis and recruit various cells into the tumor niche, including neutrophils and monocytes which, in the tumor, are transformed into tumor-associated neutrophils (TAN) and tumor-associated macrophages (TAM) that participate in tumorigenesis.

Emerging roles of Sirtuin 2 in cardiovascular diseases

Sirtuins are a family of NAD⁺ -dependent deacetylases implicated in a wide variety of age-associated pathologies, including cardiovascular disorders. Among the seven mammalian sirtuins, SIRT2 modulates various cellular processes through the deacetylation or deacylation of their target proteins. Notably, the levels of SIRT2 in the heart decline with age and other pathological conditions, leading to cardiovascular dysfunction. In the present review, we discuss the emerging roles of SIRT2 in cardiovascular dysfunction and heart failure associated with factors like age, hypertension, oxidative stress, and diabetes. We also discuss the potential of using inhibitors to study the unexplored role of SIRT2 in the heart. While SIRT2 undoubtedly plays a crucial role in the cardiovascular system, its functions are only beginning to be understood, making it an attractive candidate for further research in the field.

Post-translational lysine ac(et)ylation in health, ageing and disease

The acetylation/acylation (ac(et)ylation) of lysine side chains is a dynamic post-translational modification (PTM) regulating fundamental cellular processes with implications on the organisms' ageing process: metabolism, transcription, translation, cell proliferation, regulation of the cytoskeleton and DNA damage repair. First identified to occur on histones, later studies revealed the presence of lysine ac(et)ylation in organisms of all kingdoms of life, in proteins covering all essential cellular processes. A remarkable finding showed that the NAD⁺-dependent sirtuin deacetylase Sir2 has an impact on replicative lifespan in Saccharomyces cerevisiae suggesting that lysine acetylation has a direct role in the ageing process. Later studies identified sirtuins as mediators for beneficial effects of caloric/dietary restriction on the organisms' health- or lifespan. However, the molecular mechanisms underlying these effects are only incompletely understood. Progress in mass-spectrometry, structural biology, synthetic and semi-synthetic biology deepened our understanding of this PTM. This review summarizes recent developments in the research field. It shows how lysine ac(et)ylation regulates protein function, how it is regulated enzymatically and non-enzymatically, how a dysfunction in this post-translational machinery contributes to disease development. A focus is set on sirtuins and lysine acyltransferases as these are direct sensors and mediators of the cellular metabolic state. Finally, this review highlights technological advances to study lysine ac(et)ylation.

Pharmacological Advantage of SIRT2-Selective versus pan-SIRT1-3 Inhibitors

Because of their involvement in various biological pathways, the sirtuin enzyme family members SIRT1, SIRT2, and SIRT3 play both tumor-promoting and tumor-suppressing roles, based on the context and experimental conditions. Thus, an interesting question is whether inhibiting one of them or inhibiting all of them would be better for treating cancers. Pharmacologically, this is difficult to address, due in part to potential off-target effects of different compounds. Compounds with almost identical properties but differing in SIRT1-3 selectivity will be useful for addressing this question. Here, we have developed a pan SIRT1-3 inhibitor (NH4-6) and a SIRT2-selective inhibitor (NH4-13) with very similar chemical structures, with the only difference being the substitution of an ester bond to an amide bond. Such a minimal difference allows us to accurately compare the anticancer effect of pan SIRT1-3 inhibition and SIRT2-selective inhibition in cellular and mouse models. NH4-6 showed stronger cytotoxicity than NH4-13 in cancer cell lines. In mice, both inhibitors showed similar anticancer efficacy. However, NH4-6 is toxic to mice, which hinders the use of higher dosages. These results highlight the advantage of SIRT2-selective inhibitors as potential anticancer therapeutics.

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).

Histone Deacetylase 5 Is an Early Epigenetic Regulator of Intermittent Hypoxia Induced Sympathetic Nerve Activation and Blood Pressure

Intermittent hypoxia (IH) is a hallmark manifestation of obstructive sleep apnea (OSA). Long term IH (LT-IH) triggers epigenetic reprogramming of the redox state involving DNA hypermethylation in the carotid body chemo reflex pathway resulting in persistent sympathetic activation and hypertension. Present study examined whether IH also activates epigenetic mechanism(s) other than DNA methylation. Histone modification by lysine acetylation is another major epigenetic mechanism associated with gene regulation. Equilibrium between the activities of histone acetyltransferases (HATs) and histone deacetylases (HDACs) determine the level of lysine acetylation. Here we report that exposure of rat pheochromocytoma (PC)-12 cells to IH in vitro exhibited reduced HDAC enzyme activity due to proteasomal degradation of HDAC3 and HDAC5 proteins. Mechanistic investigations showed that IH-evoked decrease in HDAC activity increases lysine acetylation of α subunit of hypoxia inducible factor (HIF)-1α as well as Histone (H3) protein resulting in increased HIF-1 transcriptional activity. Trichostatin A (TSA), an inhibitor of HDACs, mimicked the effects of IH. Studies on rats treated with 10 days of IH or TSA showed reduced HDAC activity, HDAC5 protein, and increased HIF-1 dependent NADPH oxidase (NOX)-4 transcription in adrenal medullae (AM) resulting in elevated plasma catecholamines and blood pressure. Likewise, heme oxygenase (HO)-2 null mice, which exhibit IH because of high incidence of spontaneous apneas (apnea index 72 ± 1.2 apnea/h), also showed decreased HDAC activity and HDAC5 protein in the AM along with elevated circulating norepinephrine levels. These findings demonstrate that lysine acetylation of histone and non-histone proteins is an early epigenetic mechanism associated with sympathetic nerve activation and hypertension in rodent models of IH.

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.

Surtuin 1 as a potential prognostic biomarker in very elderly patients with colorectal cancer

BACKGROUND/AIMS

Colorectal cancer (CRC) rate increases with aging. Aging-related proteins, such as sirtuins (SIRTs) may be a potential therapeutic target in the elderly patients with CRC. The clinical implications of SIRT1 and SIRT2 have not been reported for elderly patients with cancer. The aim of this study was to evaluate the impact of expression of SIRT1 and SIRT2 on clinical outcome in two extreme age groups of patients with CRC.

METHODS

The expression of SIRT1 and SIRT2 were evaluated in CRC tissues of 101 patients aged ≥ 80 years and 29 patients aged ≤ 40 years by immunohistochemistry. We defined the patients aged ≥ 80 years as the very elderly and patients aged ≤ 40 years as the young patients. Correlations between the expression of these proteins and clinicopathological features were analyzed.

RESULTS

The prognosis for the very elderly patients with high expressions of SIRT1 was significantly worse than that for patients showing low expression (median survival, 24.9 months vs. 38.6 months, p = 0.027) whereas high expression of SIRT2 better prognosis (median survival, 37.9 months vs. 17.3 months, p = 0.006). However, the young patients did not show any difference in prognosis according to expression of SIRT1 and SIRT2. In multivariate analysis, high SIRT1 expression retained statistical significance as a poor prognostic factor in the very elderly patients with CRC.

CONCLUSION

The results suggest that high SIRT1 expression could be predictive of a poor outcome for very elderly patients with CRC.

Is nuclear sirtuin SIRT6 a master regulator of immune function?

The ability to ward off pathogens with minimal damage to the host determines the immune system's robustness. Multiple factors, including pathogen processing, identification, secretion of mediator and effector molecules, and immune cell proliferation and differentiation into various subsets, constitute the success of mounting an effective immune response. Cellular metabolism controls all of these intricate processes. Cells utilize diverse fuel sources and switch back and forth between different metabolic pathways depending on their energy needs. The three most critical metabolic pathways on which immune cells depend to meet their energy needs are oxidative metabolism, glycolysis, and glutaminolysis. Dynamic switching between these metabolic pathways is needed for optimal function of the immune cells. Moreover, switching between these metabolic pathways needs to be tightly regulated to achieve the best results. Immune cells depend on the Warburg effect for their growth, proliferation, secretory, and effector functions. Here, we hypothesize that the sirtuin, SIRT6, could be a negative regulator of the Warburg effect. We also postulate that SIRT6 could act as a master regulator of immune cell metabolism and function by regulating critical signaling pathways.

NAD+ metabolism, stemness, the immune response, and cancer

NAD⁺ was discovered during yeast fermentation, and since its discovery, its important roles in redox metabolism, aging, and longevity, the immune system and DNA repair have been highlighted. A deregulation of the NAD⁺ levels has been associated with metabolic diseases and aging-related diseases, including neurodegeneration, defective immune responses, and cancer. NAD⁺ acts as a cofactor through its interplay with NADH, playing an essential role in many enzymatic reactions of energy metabolism, such as glycolysis, oxidative phosphorylation, fatty acid oxidation, and the TCA cycle. NAD⁺ also plays a role in deacetylation by sirtuins and ADP ribosylation during DNA damage/repair by PARP proteins. Finally, different NAD hydrolase proteins also consume NAD⁺ while converting it into ADP-ribose or its cyclic counterpart. Some of these proteins, such as CD38, seem to be extensively involved in the immune response. Since NAD cannot be taken directly from food, NAD metabolism is essential, and NAMPT is the key enzyme recovering NAD from nicotinamide and generating most of the NAD cellular pools. Because of the complex network of pathways in which NAD⁺ is essential, the important role of NAD⁺ and its key generating enzyme, NAMPT, in cancer is understandable. In the present work, we review the role of NAD⁺ and NAMPT in the ways that they may influence cancer metabolism, the immune system, stemness, aging, and cancer. Finally, we review some ongoing research on therapeutic approaches.

The Role of Hypoxia-Inducible Factor Post-Translational Modifications in Regulating Its Localisation, Stability, and Activity

The hypoxia signalling pathway enables adaptation of cells to decreased oxygen availability. When oxygen becomes limiting, the central transcription factors of the pathway, hypoxia-inducible factors (HIFs), are stabilised and activated to induce the expression of hypoxia-regulated genes, thereby maintaining cellular homeostasis. Whilst hydroxylation has been thoroughly described as the major and canonical modification of the HIF-α subunits, regulating both HIF stability and activity, a range of other post-translational modifications decorating the entire protein play also a crucial role in altering HIF localisation, stability, and activity. These modifications, their conservation throughout evolution, and their effects on HIF-dependent signalling are discussed in this review.

SUMOylation is essential for Sirt2 tumor-suppressor function in neuroblastoma

SUMOylation is an important post-translational modification that participates in a variety of cellular physiological and pathological processes in eukaryotic cells. Sirt2, a NAD⁺-dependent deacetylase, usually exerts a tumor-suppressor function. However, the role of SUMOylation in cancer cells is not fully known. In this study, we found that SUMOylation can occur in the Sirt2 protein at both lysine 183 and lysine 340 sites. SUMOylation did not affect Sirt2 localization or stability but was involved in P38-mTORC2-AKT cellular signal transduction via direct deacetylation on a new substrate MAPK/P38. SUMOylation-deficient Sirt2 lost the capability of suppressing tumor processes and showed resistance to the Sirt2-specific inhibitor AK-7 in neuroblastoma cells. Here, we revealed the important function of Sirt2-SUMOylation, which is closely associated with cellular signal transduction and is essential for suppressing tumorigenesis in neuroblastoma.

Sirtuins' control of autophagy and mitophagy in cancer

Mammalian cells use a specialized and complex machinery for the removal of altered proteins or dysfunctional organelles. Such machinery is part of a mechanism called autophagy. Moreover, when autophagy is specifically employed for the removal of dysfunctional mitochondria, it is called mitophagy. Autophagy and mitophagy have important physiological implications and roles associated with cellular differentiation, resistance to stresses such as starvation, metabolic control and adaptation to the changing microenvironment. Unfortunately, transformed cancer cells often exploit autophagy and mitophagy for sustaining their metabolic reprogramming and growth to a point that autophagy and mitophagy are recognized as promising targets for ongoing and future antitumoral therapies. Sirtuins are NAD+ dependent deacylases with a fundamental role in sensing and modulating cellular response to external stresses such as nutrients availability and therefore involved in aging, oxidative stress control, inflammation, differentiation and cancer. It is clear, therefore, that autophagy, mitophagy and sirtuins share many common aspects to a point that, recently, sirtuins have been linked to the control of autophagy and mitophagy. In the context of cancer, such a control is obtained by modulating transcription of autophagy and mitophagy genes, by post translational modification of proteins belonging to the autophagy and mitophagy machinery, by controlling ROS production or major metabolic pathways such as Krebs cycle or glutamine metabolism. The present review details current knowledge on the role of sirtuins, autophagy and mitophagy in cancer to then proceed to discuss how sirtuins can control autophagy and mitophagy in cancer cells. Finally, we discuss sirtuins role in the context of tumor progression and metastasis indicating glutamine metabolism as an example of how a concerted activation and/or inhibition of sirtuins in cancer cells can control autophagy and mitophagy by impinging on the metabolism of this fundamental amino acid.

Simultaneous Inhibition of SIRT2 Deacetylase and Defatty-Acylase Activities via a PROTAC Strategy

As a member of the sirtuin family of enzymes, SIRT2 promotes tumor growth and regulates various biological pathways through lysine deacetylation and defatty-acylation. In the past few years, many SIRT2-selective small molecule inhibitors have been developed, but none have demonstrated simultaneous inhibition of both SIRT2 activities in cells. To further scrutinize the physiological importance and significance of SIRT2 deacetylase and defatty-acylase activities, small molecules that can selectively inhibit both activities of SIRT2 in living cells are needed. Here, we have applied the Proteolysis Targeting Chimera (PROTAC) strategy and synthesized a new SIRT2 inhibitor (TM-P4-Thal) to degrade SIRT2 selectively, which led to simultaneous inhibition of its deacetylase and defatty-acylase activities in living cells. Additionally, this compound exemplifies the advantage of the PROTAC strategy that allows complete eradication of an enzyme and its activity in biological settings.

Hypoxic Regulation of Gene Transcription and Chromatin: Cause and Effect

Cellular responses to low oxygen (hypoxia) are fundamental to normal physiology and to the pathology of many common diseases. Hypoxia-inducible factor (HIF) is central to this by enhancing the transcriptional activity of many hundreds of genes. The cellular response to HIF is cell-type-specific and is largely governed by the pre-existing epigenetic landscape. Prior to activation, HIF-binding sites and the promoters of HIF-target genes are already accessible, in contact with each other through chromatin looping and display markers of activity. However, hypoxia also modulates the epigenetic environment, both in parallel to and as a consequence of HIF activation. This occurs through a combination of oxygen-sensitive changes in enzyme activity, transcriptional activation of epigenetic modifiers, and localized recruitment to chromatin by HIF and activated RNApol2. These hypoxic changes in the chromatin environment may both contribute to and occur as a consequence of transcriptional regulation. Nevertheless, they have the capacity to both modulate and extend the transcriptional response to hypoxia.

Romance of the three kingdoms in hypoxia: HIFs, epigenetic regulators, and chromatin reprogramming

Hypoxia is a hallmark of cancer. To cope with hypoxic conditions, tumor cells alter their transcriptional profiles mainly through hypoxia-inducible factors (HIFs) and epigenetic reprogramming. Hypoxia, in part through HIF-dependent mechanisms, influences the expression or activity of epigenetic regulators to control epigenetic reprogramming, including DNA methylation and histone modifications, which regulate hypoxia-responsive gene expression in cells. Conversely, epigenetic regulators and chromatin architecture can modulate the expression, stability, or transcriptional activity of HIF. Understanding the complex networks between HIFs, epigenetic regulators, and chromatin reprogramming in response to hypoxia will provide insight into the fundamental mechanism of transcriptional adaptation to hypoxia, and may help identify novel targets for future therapies. In this review, we will discuss the comprehensive relationship between HIFs, epigenetic regulators, and chromatin reprogramming under hypoxic conditions.

Hypoxia-induced NAD+ interventions promote tumor survival and metastasis by regulating mitochondrial dynamics

Hypoxia, an important feature of the tumor microenvironment, is responsible for the chemo-resistance and metastasis of malignant solid tumors. Recent studies indicated that mitochondria undergo morphological transitions as an adaptive response to maintain self-stability and connectivity under hypoxic conditions. NAD⁺ may not only provide reducing equivalents for biosynthetic reactions and in determining energy production, but also functions as a signaling molecule in mitochondrial dynamics regulation. In this review, we describe the upregulated KDAC deacetylase expression in the mitochondria and cytoplasm of tumor cells that results from sensing the changes in NAD⁺ to control mitochondrial dynamics and distribution, which is responsible for survival and metastasis in hypoxia.

AGK2 ameliorates mast cell-mediated allergic airway inflammation and fibrosis by inhibiting FcεRI/TGF-β signaling pathway

Asthma is characterized by airway hyperresponsiveness and allergic inflammation, detrimentally affecting the patients' quality of life. The development of new drugs for the treatment of asthma is warranted to alleviate these issues. Recent studies have demonstrated that sirtuin2 (SIRT2) aggravates asthmatic inflammation by up-regulation of T-helper type 2 responses and macrophage polarization. However, effects of SIRT2 on mast cell activation remain obscure. In this study, we investigated the effects of AGK2, an inhibitor for SIRT2, on mast cell-mediated allergic airway inflammation. Pre-treatment with AGK2 inhibited degranulation of mast cells by suppressing the FcεRI signaling pathway and intracellular calcium influx. The expression of pro-inflammatory cytokines, such as tumor necrosis factor (TNF)-α, interleukin (IL)-1β, IL-4, IL-5, IL-6, and IL-8, was inhibited via regulation of transcription factors such as NF-κB and NRF2. These effects of AGK2 were verified in passive cutaneous anaphylaxis and acute lung injury animal models. AGK2 attenuated Evans blue pigmentation by inhibiting mast cell activation and lung barrier dysfunction by inhibiting inflammatory responses in these animal models. In the ovalbumin (OVA)-induced allergic airway inflammation murine model, AGK2 alleviated allergic asthma symptoms such as lung histological changes (immune cell and mast cell infiltration, collagen deposition, and α-smooth muscle actin expression) and serum immunoglobulins (Ig) levels (IgE, OVA-specific IgE, IgG1, and IgG2a). Moreover, AGK2 reduced the levels of pro-inflammatory cytokines (TNF-α, IL-1β, IL-4, IL-5, and IL-6) and inflammatory mediators (myeloperoxidase, eosinophil peroxidase, and tumor growth factor-α) in the bronchoalveolar lavage fluid and lung tissues. In addition, the anti-fibrotic effects of AGK2 were verified using lung epithelial cells and TGF-β/Smad reporter stable cells. In conclusion, our findings suggest that SIRT2 plays a role in mast cell-mediated airway inflammatory disease. Therefore, AGK2 is a good potential candidate for treating allergic asthma and lung inflammation.

SIRT2 suppresses expression of inflammatory factors via Hsp90-glucocorticoid receptor signalling

SIRT2 is a NAD⁺‐dependent deacetylase that deacetylates a diverse array of protein substrates and is involved in many cellular processes, including regulation of inflammation. However, its precise role in the inflammatory process has not completely been elucidated. Here, we identify heat‐shock protein 90α (Hsp90α) as novel substrate of SIRT2. Functional investigation suggests that Hsp90 is deacetylated by SIRT2, such that overexpression and knock‐down of SIRT2 altered the acetylation level of Hsp90. This subsequently resulted in disassociation of Hsp90 with glucocorticoid receptor (GR), and translocation of GR to the nucleus. This observation was further confirmed by glucocorticoid response element (GRE)‐driven reporter assay. Nuclear translocation of GR induced by SIRT2 overexpression repressed the expression of inflammatory cytokines, which were even more prominent under lipopolysaccharide (LPS) stimulation. Conversely, SIRT2 knock‐down resulted in the up‐regulation of cytokine expression. Mutation analysis indicated that deacetylation of Hsp90 at K294 is critical for SIRT2‐mediated regulation of cytokine expression. These data suggest that SIRT2 reduces the extent of LPS‐induced inflammation by suppressing the expression of inflammatory factors via SIRT2‐Hsp90‐GR axis.

Silencing Sirtuin 6 induces cell cycle arrest and apoptosis in non-small cell lung cancer cell lines

Sirtuins (SIRT1-7), are NAD-dependent deacetylases and ADP-ribosyl transferases, plays a major part in carcinogenesis. The previous report suggests that in cancer, sirtuins gained tremendous interest and critical regulators of the unusual processes. In carcinogenesis, sirtuins possess either tumor suppressor or promoter. However, in lung cancer condition the studies of sirtuins are less studied. Hence, this designed study investigates the impact of multifaceted sirtuins in NSCLC cells. We evaluated the mRNA and protein expressions of sirtuins by RTPCR and western blot. We found SIRT6 significantly overexpressed in NCI-H520, A549, and NCI-H460 compared with the normal BEAS-2B cell line. Silencing of SIRT6 by siRNA in NSCLC cells caused activation of p53/p21 mediated inhibition of cell proliferation leading to arrest in cell cycle and apoptosis induction. Our results implied that SIRT6 is a tumor promoter in NSCLC development, progression, and regulation. The silencing of SIRT6 to be a novel therapy for lung cancer.

Host NAD+ metabolism and infections: therapeutic implications

Nicotinamide adenine dinucleotide (NAD+) is both a crucial coenzyme and a cosubstrate for various metabolic reactions in all living cells. Maintenance of NAD+ levels is essential for cell energy homeostasis, survival, proliferation and function. Mounting evidence points to NAD+ as one of the major modulators of immuno-metabolic circuits, thus regulating immune responses and functions. Recent studies delineate impaired host NAD+ metabolism during chronic infections and inflammation, suggesting NAD+ replenishment as an avenue to ameliorate deleterious inflammatory responses. Here, we discuss aspects of NAD+ biosynthesis and consumption, NAD+ biology during infections and how NAD+ metabolism can be intervened with pharmacologically to enhance the host's immunological fitness against pathogens.

Dual Deletion of the Sirtuins SIRT2 and SIRT3 Impacts on Metabolism and Inflammatory Responses of Macrophages and Protects From Endotoxemia

Sirtuin 2 (SIRT2) and SIRT3 are cytoplasmic and mitochondrial NAD-dependent deacetylases. SIRT2 and SIRT3 target proteins involved in metabolic, proliferation and inflammation pathways and have been implicated in the pathogenesis of neurodegenerative, metabolic and oncologic disorders. Both pro- and anti-inflammatory effects have been attributed to SIRT2 and SIRT3, and single deficiency in SIRT2 or SIRT3 had minor or no impact on antimicrobial innate immune responses. Here, we generated a SIRT2/3 double deficient mouse line to study the interactions between SIRT2 and SIRT3. SIRT2/3^−/− mice developed normally and showed subtle alterations of immune cell populations in the bone marrow, thymus, spleen, blood and peritoneal cavity that contained notably more anti-inflammatory B-1a cells and less NK cells. In vitro, SIRT2/3^−/− macrophages favored fatty acid oxidation (FAO) over glycolysis and produced increased levels of both proinflammatory and anti-inflammatory cytokines. In line with metabolic adaptation and increased numbers of peritoneal B-1a cells, SIRT2/3^−/− mice were robustly protected from endotoxemia. Yet, SIRT2/3 double deficiency did not modify endotoxin tolerance. Overall, these data suggest that sirtuins can act in concert or compensate each other for certain immune functions, a parameter to be considered for drug development. Moreover, inhibitors targeting multiple sirtuins developed for clinical purposes may be useful to treat inflammatory diseases.

Enzymatic and nonenzymatic protein acetylations control glycolysis process in liver diseases

Impaired glycolysis has pathologic effects on the occurrence and progression of liver diseases, and it appears that glycolysis is increased to different degrees in different liver diseases. As an important post-translational modification, reversible lysine acetylation regulates almost all cellular processes, including glycolysis. Lysine acetylation can occur enzymatically with acetyltransferases or nonenzymatically with acetyl-coenzyme A. Accompanied by the progression of liver diseases, there seems to be a temporal and spatial variation between enzymatic and nonenzymatic acetylations in the regulation of glycolysis. Here, we summarize the most recent findings on the functions and targets of acetylation in controlling glycolysis in the different stages of liver diseases. In addition, we discuss the differences and causes between enzymatic and nonenzymatic acetylations in regulating glycolysis throughout the progression of liver diseases. Then, we review these new discoveries to provide the potential implications of these findings for therapeutic interventions in liver diseases.-Li, J., Wang, T., Xia, J., Yao, W., Huang, F. Enzymatic and nonenzymatic protein acetylations control glycolysis process in liver diseases.

SIRT2: Controversy and multiple roles in disease and physiology

Sirtuin 2 (SIRT2) is an NAD+-dependent deacetylase that was under studied compared to other sirtuin family members. SIRT2 is the only sirtuin protein which is predominantly found in the cytoplasm but is also found in the mitochondria and in the nucleus. Recently, accumulating evidence has uncovered a growing number of substrates and additional detailed functions of SIRT2 in a wide range of biological processes, marking its crucial role. Here, we give a comprehensive profile of the crucial physiological functions of SIRT2 and its role in neurological diseases, cancers, and other diseases. This review summarizes the functions of SIRT2 in the nervous system, mitosis regulation, genome integrity, cell differentiation, cell homeostasis, aging, infection, inflammation, oxidative stress, and autophagy. SIRT2 inhibition rescues neurodegenerative disease symptoms and hence SIRT2 is a potential therapeutic target for neurodegenerative disease. SIRT2 is undoubtedly dysfunctional in cancers and plays a dual-faced role in different types of cancers, and although its mechanism is unresolved, SIRT2 remains a promising therapeutic target for certain cancers. In future, the continued rapid growth in SIRT2 research will help clarify its role in human health and disease, and promote the progress of this target in clinical practice.