Sirtuin 7 promotes cellular survival following genomic stress by attenuation of DNA damage, SAPK activation and p53 response

The dual role of sirtuins in cancer: biological functions and implications

Sirtuins are pivotal in orchestrating numerous cellular pathways, critically influencing cell metabolism, DNA repair, aging processes, and oxidative stress. In recent years, the involvement of sirtuins in tumor biology has garnered substantial attention, with a growing body of evidence underscoring their regulatory roles in various aberrant cellular processes within tumor environments. This article delves into the sirtuin family and its biological functions, shedding light on their dual roles-either as promoters or inhibitors-in various cancers including oral, breast, hepatocellular, lung, and gastric cancers. It further explores potential anti-tumor agents targeting sirtuins, unraveling the complex interplay between sirtuins, miRNAs, and chemotherapeutic drugs. The dual roles of sirtuins in cancer biology reflect the complexity of targeting these enzymes but also highlight the immense therapeutic potential. These advancements hold significant promise for enhancing clinical outcomes, marking a pivotal step forward in the ongoing battle against cancer.

Sirtuin dysregulation in Parkinson's disease: Implications of acetylation and deacetylation processes

Parkinson's disease (PD) is a progressive neurodegenerative condition that primarily affects motor function and is caused by a gradual decline of dopaminergic neurons in the brain's substantia pars compacta (Snpc) region. Multiple molecular pathways are involved in the pathogenesis, which results in impaired cellular functions and neuronal degeneration. However, the role of sirtuins, a type of NAD+-dependent deacetylase, in the pathogenesis of Parkinson's disease has recently been investigated. Sirtuins are essential for preserving cellular homeostasis because they control a number of biological processes, such as metabolism, apoptosis, and DNA repair. This review shed lights on the dysregulation of sirtuin activity in PD, highlighting the role that acetylation and deacetylation processes play in the development of the disease. Key regulators of protein acetylation, sirtuins have been found to be involved in the aberrant acetylation of vital substrates linked to PD pathology when their balance is out of balance. The hallmark characteristics of PD such as neuroinflammation, oxidative stress, and mitochondrial dysfunction have all been linked to the dysregulation of sirtuin expression and activity. Furthermore, we have also explored how the modulators of sirtuins can be a promising therapeutic intervention in the treatment of PD.

Embryonic NIPP1 Depletion in Keratinocytes Triggers a Cell Cycle Arrest and Premature Senescence in Adult Mice

NIPP1 is a ubiquitously expressed regulatory subunit of PP1. Its embryonic deletion in keratinocytes causes chronic sterile skin inflammation, epidermal hyperproliferation, and resistance to mutagens in adult mice. To explore the primary effects of NIPP1 deletion, we first examined hair cycle progression of NIPP1 skin knockouts (SKOs). The entry of the first hair cycle in the SKOs was delayed owing to prolonged quiescence of hair follicle stem cells. In contrast, the entry of the second hair cycle in the SKOs was advanced as a result of precocious activation of hair follicle stem cells. The epidermis of SKOs progressively accumulated senescent cells, and this cell-fate switch was accelerated by DNA damage. Primary keratinocytes from SKO neonates and human NIPP1-depleted HaCaT keratinocytes failed to proliferate and showed an increase in the expression of cell cycle inhibitors (p21, p16/Ink4a, and/or p19/Arf) and senescence-associated-secretory-phenotype factors as well as in DNA damage (γH2AX and 53BP1). Our data demonstrate that the primary effect of NIPP1 deletion in keratinocytes is a cell cycle arrest and premature senescence that gradually progresse to chronic senescence and likely contribute to the decreased sensitivity of SKOs to mutagens.

Loss of hepatic Sirt7 accelerates diethylnitrosamine (DEN)-induced formation of hepatocellular carcinoma by impairing DNA damage repair

The mammalian sirtuin family (SIRT1-SIRT7) has shown diverse biological roles in the regulation and maintenance of genome stability under genotoxic stress. SIRT7, one of the least studied sirtuin, has been demonstrated to be a key factor for DNA damage response (DDR). However, conflicting results have proposed that Sirt7 is an oncogenic factor to promote transformation in cancer cells. To address this inconsistency, we investigated properties of SIRT7 in hepatocellular carcinoma (HCC) regulation under DNA damage and found that loss of hepatic Sirt7 accelerated HCC progression. Specifically, the number, size, and volume of hepatic tumor colonies in diethylnitrosamine (DEN) injected Sirt7-deficient liver were markedly enhanced. Further, levels of HCC progression markers and pro-inflammatory cytokines were significantly elevated in the absence of hepatic Sirt7, unlike those in the control. In chromatin, SIRT7 was stabilized and colocalized to damage site by inhibiting the induction of γH2AX under DNA damage. Together, our findings suggest that SIRT7 is a crucial factor for DNA damage repair and that hepatic loss-of-Sirt7 can promote genomic instability and accelerate HCC development, unlike early studies describing that Sirt7 is an oncogenic factor [BMB Reports 2024; 57(2): 98-103].

SIRT7: the seventh key to unlocking the mystery of aging

Aging is a chronic yet natural physiological decline of the body. Throughout life, humans are continuously exposed to a variety of exogenous and endogenous stresses, which engender various counteractive responses at the cellular, tissue, organ, as well as organismal levels. The compromised cellular and tissue functions that occur because of genetic factors or prolonged stress (or even the stress response) may accelerate aging. Over the last two decades, the sirtuin (SIRT) family of lysine deacylases has emerged as a key regulator of longevity in a variety of organisms. SIRT7, the most recently identified member of the SIRTs, maintains physiological homeostasis and provides protection against aging by functioning as a watchdog of genomic integrity, a dynamic sensor and modulator of stresses. SIRT7 decline disrupts metabolic homeostasis, accelerates aging, and increases the risk of age-related pathologies including cardiovascular and neurodegenerative diseases, pulmonary and renal disorders, inflammatory diseases, and cancer, etc. Here, we present SIRT7 as the seventh key to unlock the mystery of aging, and its specific manipulation holds great potential to ensure healthiness and longevity.

Bioinformatic analysis of SIRT7 sequence and structure

Sirtuins are highly conserved proteins that perform very important functions in different cellular processes. Notably, SIRT7 is the least studied human sirtuin, but it is known to be involved in a wide variety of processes in both health and disease. In this way, SIRT7 activity-regulating molecules could be beneficial for the treatment of relevant diseases such as cardiovascular and bone diseases, where SIRT7 levels are reduced, or obesity and cancer, where they are increased. In this work, using bioinformatic methods, the sequence and structure of SIRT7 orthologs in a wide variety of organisms were analyzed. Thus, the catalytic domain was found to be quite conserved (83.23% identity) and key residues such as D118, Y119, R120, D170, H187, N189, C198, C225, C228, V273, G298, F239 and V237 were identified. Furthermore, a phylogenetic tree was constructed where SIRT7 orthologs from mammals, birds, reptiles, amphibians, fish, insects, and arachnids were found to cluster in different groups. Finally, predicted three-dimensional structures showed a classic structure of the central catalytic region of most sirtuins, while the flanking N- and C-terminal regions were unique to each phylogenetic group. All this helps to understand a little more how SIRT7 works and gives clues for the future design and development of small molecules that benefit human and animal health.Communicated by Ramaswamy H. Sarma.

The sirtuin family in health and disease

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

Akt-Dependent Glycolysis-Driven Lipogenesis Supports Proliferation and Survival of Human Pulmonary Arterial Smooth Muscle Cells in Pulmonary Hypertension

Hyper-proliferation of pulmonary arterial vascular smooth muscle cells (PAVSMC) is an important pathological component of pulmonary vascular remodeling in pulmonary arterial hypertension (PAH). Lipogenesis is linked to numerous proliferative diseases, but its role in PAVSMC proliferation in PAH remains to be elucidated. We found that early-passage human PAH PAVSMC had significant up-regulation of key fatty acids synthesis enzymes ATP-citrate lyase (ACLY), acetyl-CoA carboxylase (ACC), and fatty acid synthase (FASN), and increased unstimulated proliferation compared to control human PAVSMC. Treatment with an allosteric ACC inhibitor 5-tetradecyloxy-2-furoic acid (TOFA) significantly decreased proliferation and induced apoptosis of human PAH PAVSMC. Intracellular lipid content and proliferation of PAH PAVSMC were not reduced by incubation in lipid-depleted media but suppressed by a non-metabolizable analog of glucose 2-Deoxy-D-glucose (2-DG) and partially restored by addition of pyruvate. Protein kinase Akt was upregulated in human PAH PAVSMC in a sirtuin 7 (SIRT7)- and c-Jun N-terminal kinase (JNK)-dependent manner. Pharmacological inhibition of Akt down-regulated ACLY and ACC, significantly reduced intracellular lipid content, inhibited proliferation and induced apoptosis of human PAH PAVSMC. Taken together, these data demonstrate that human PAH PAVSMC have up-regulated lipogenesis, which is supported in an Akt- and glycolysis-dependent manner and is required for increased proliferation and survival. Our data suggest that there is a mechanistic link between glycolysis, lipogenesis, and the proliferation of human PAH PAVSMC and call for further studies to determine the potential attractiveness of a SIRT7/JNK-Akt-lipogenesis axis as a target pathway to inhibit PAVSMC hyper-proliferation in PAH.

SIRT7 in the aging process

Aging is the result of the accumulation of a wide variety of molecular and cellular damage over time. This has been associated with a number of features termed hallmarks of aging, including genomic instability, loss of proteostasis, telomere attrition, dysregulated nutrient sensing, mitochondrial dysfunction, cellular senescence, stem cell exhaustion, and impaired intercellular communication. On the other hand, sirtuins are enzymes with an important role in aging and life extension, of which humans have seven paralogs (SIRT1 to SIRT7). SIRT7 is the least studied sirtuin to date, but it has been reported to serve important functions, such as promoting ribosomal RNA expression, aiding in DNA damage repair, and regulating chromatin compaction. Several studies have established a close relationship between SIRT7 and age-related processes, but knowledge in this area is still scarce. Therefore, the purpose of this review was to analyze how SIRT7 is associated with each of the hallmarks of aging, as well as with some of age-associated diseases, such as cardiovascular diseases, obesity, osteoporosis, and cancer.

Sirtuin 7 serves as a promising therapeutic target for cardiorenal diseases

Cardiovascular disorders and associated renal diseases account for the main cause of morbidity and mortality worldwide, necessitating the development of novel effective approaches for the prevention and treatment of cardiorenal diseases. Mammalian sirtuins (SIRTs) function as nicotinamide adenine dinucleotide (NAD+)-dependent protein/histone deacetylases. Seven members of SIRTs share a highly invariant catalytic core domain responsible for the specific enzymatic activity. Intriguingly, the broad distribution of SIRTs and alternative isoforms implicate its distinct functions in diverse cardiac and renal cells and tissue types. Notably, SIRT7 has been shown to exert beneficial effects in cardiorenal physiology and pathophysiology via modulation of senescence, DNA damage repair, ribosomal RNA synthesis, protein biosynthesis, angiogenesis, apoptosis, superoxide generation, cardiorenal metabolism, and dysfunction. Furthermore, SIRT7 has emerged as a critical modulator of a broad range of cellular activities including oxidative stress, inflammation response, endoplasmic reticulum stress, and mitochondrial homeostasis, which are all of great significance in postponing the progression of cardiorenal diseases. More importantly, SIRT7 has been implicated in cardiorenal hypertrophy, fibrosis, remodeling, heart failure, atherosclerosis as well as renal acid-base and electrolyte homeostasis as an essential regulator. In this review, we focus on the involvement in cardiorenal physiology and pathophysiology, diverse actions and underlying mechanisms of the SIRT7 signaling, highlighting its updated research progress in heart failure, atherosclerosis, diabetic nephropathy and other cardiorenal diseases. Targeting SIRT7 signaling could be potentially exploited as a therapeutic strategy aiming to prevent and treat cardiorenal diseases.

SIRT7 Acts as a Guardian of Cellular Integrity by Controlling Nucleolar and Extra-Nucleolar Functions

Sirtuins are key players for maintaining cellular homeostasis and are often deregulated in different human diseases. SIRT7 is the only member of mammalian sirtuins that principally resides in the nucleolus, a nuclear compartment involved in ribosomal biogenesis, senescence, and cellular stress responses. The ablation of SIRT7 induces global genomic instability, premature ageing, metabolic dysfunctions, and reduced stress tolerance, highlighting its critical role in counteracting ageing-associated processes. In this review, we describe the molecular mechanisms employed by SIRT7 to ensure cellular and organismal integrity with particular emphasis on SIRT7-dependent regulation of nucleolar functions.

SIRT7: a sentinel of genome stability

SIRT7 is a class III histone deacetylase that belongs to the sirtuin family. The past two decades have seen numerous breakthroughs in terms of understanding SIRT7 biological function. We now know that this enzyme is involved in diverse cellular processes, ranging from gene regulation to genome stability, ageing and tumorigenesis. Genomic instability is one hallmark of cancer and ageing; it occurs as a result of excessive DNA damage. To counteract such instability, cells have evolved a sophisticated regulated DNA damage response mechanism that restores normal gene function. SIRT7 seems to have a critical role in this response, and it is recruited to sites of DNA damage where it recruits downstream repair factors and directs chromatin regulation. In this review, we provide an overview of the role of SIRT7 in DNA repair and maintaining genome stability. We pay particular attention to the implications of SIRT7 function in cancer and ageing.

SIRT7-dependent deacetylation of NPM promotes p53 stabilization following UV-induced genotoxic stress

Adaptation to different forms of environmental stress is crucial for maintaining essential cellular functions and survival. The nucleolus plays a decisive role as a signaling hub for coordinating cellular responses to various extrinsic and intrinsic cues. p53 levels are normally kept low in unstressed cells, mainly due to E3 ubiquitin ligase MDM2-mediated degradation. Under stress, nucleophosmin (NPM) relocates from the nucleolus to the nucleoplasm and binds MDM2, thereby preventing degradation of p53 and allowing cell-cycle arrest and DNA repair. Here, we demonstrate that the mammalian sirtuin SIRT7 is an essential component for the regulation of p53 stability during stress responses induced by ultraviolet (UV) irradiation. The catalytic activity of SIRT7 is substantially increased upon UV irradiation through ataxia telangiectasia mutated and Rad3 related (ATR)-mediated phosphorylation, which promotes efficient deacetylation of the SIRT7 target NPM. Deacetylation is required for stress-dependent relocation of NPM into the nucleoplasm and MDM2 binding, thereby preventing ubiquitination and degradation of p53. In the absence of SIRT7, stress-dependent stabilization of p53 is abrogated, both in vitro and in vivo, impairing cellular stress responses. The study uncovers an essential SIRT7-dependent mechanism for stabilization of the tumor suppressor p53 in response to genotoxic stress.

The complex role of SIRT7 in p53 stabilization: nucleophosmin joins the debate

Release of nucleophosmin (NPM) from nucleoli following stress promotes rapid stabilization of the tumor suppressor p53 (TP53, best known as p53). Nucleoplasmic NPM binds to the ubiquitin ligase mouse double minute 2 (MDM2) and prevents MDM2-dependent p53 degradation. We recently demonstrated that sirtuin 7 (SIRT7) activates this pathway by directly deacetylating NPM following ultraviolet irradiation, indicating tumor-suppressive functions of SIRT7.

Sirtuin 7 Promotes Mesenchymal to Epithelial Transition by β-Catenin Redistribution and Stabilization

SIRT7 belongs to the family of “NAD+ dependent deacetylases” called Sirtuins. In the present work we report a novel role of SIRT7 in regulating cellular polarity. SIRT7 overexpression in immortalized mouse fibroblasts (NIH3T3) induced epithelial transition. This transition was accompanied by typical N- to E- cadherin transition, stabilization of β-catenin, and the downregulation of transcription factors responsible for maintenance of mesenchymal phenotype (Snail, Slug, and Zeb1). Interestingly, a subpopulation of cells overexpressing SIRT7 exhibited an intermediate stage between mesenchymal and epithelial characters. Transformed epithelial cells showed a loss of heterochromatisation as evidenced by a loss of HP1α and H3K9 dimethylation staining. In conclusion, we report a role of SIRT7 in mesenchymal cells, which may have implications for health and disease.

Sirtuin 7 promotes non‑small cell lung cancer progression by facilitating G1/S phase and epithelial‑mesenchymal transition and activating AKT and ERK1/2 signaling

Increasing evidence has indicated the roles of sirtuin 7 (SIRT7) in numerous human cancers. However, the effects and the clinical significance of SIRT7 in human lung cancer is largely unknown. The present research demonstrated that SIRT7 was increased in human lung cancer tumor tissues. SIRT7 upregulation was associated with clinicopathological characteristics of lung cancer malignancy including positive lymph node metastasis, high pathologic stage and large tumor size. SIRT7 was also upregulated in human non-small cell lung cancer (NSCLC) cell lines. Furthermore SIRT7-overexpressed A549 (A549-SIRT7) and SIRT7-knocked down H292 (H292-shSIRT7) human NSCLC cell lines were established. Using these NSCLC cells and xenograft mouse models, it was revealed that SIRT7 overexpression markedly promoted growth and G1 to S cell cycle phase transition as well as migration, invasion and distant lung metastasis in A549 NSCLC cells, whereas SIRT7 knockdown suppressed these processes in H292 NSCLC cells. Mechanistically, in A549 NSCLC cells, SIRT7 overexpression significantly activated not only protein kinase B (AKT) signaling but also extracellular signal-regulated kinase 1/2 (ERK1/2) signaling. SIRT7 overexpression also significantly downregulated cyclin-dependent kinase (CDK) inhibitors including p21 and p27 as well as upregulated cyclins including cyclin D1 and cyclin E1, and CDKs including CDK2 and CDK4. Notably, the epithelial-mesenchymal transition (EMT) process of A549 NSCLC cells was facilitated by SIRT7 overexpression, as evidenced by E-cadherin epithelial marker downregulation and mesenchymal markers (N-cadherin, vimentin, Snail and Slug) upregulation. In addition, SIRT7 knockdown in H292 NSCLC cells exhibited the opposite regulatory effects. Moreover, inhibition of AKT signaling abated the promoting effects of SIRT7 in NSCLC cell proliferation and EMT progression. The present data indicated that SIRT7 accelerated human NSCLC cell growth and metastasis possibly by promotion of G1 to S-phase transition and EMT through modulation of the expression of G1-phase checkpoint molecules and EMT markers as well as activation of AKT and ERK1/2 signaling. SIRT7 could be an innovative potential target for human NSCLC therapy.

Downregulation of circular RNA circPVT1 restricts cell growth of hepatocellular carcinoma through downregulation of Sirtuin 7 via microRNA-3666

Circular RNAs (circRNAs) have been identified recently as pivotal regulators in the development and progression of cancers, generally by acting as competing endogenous RNAs of microRNAs (miRNAs) to regulate gene expression. The dysregulation of circRNAs in hepatocellular carcinoma (HCC) has attracted much attention, but the precise role of circRNAs in HCC remains largely unknown. In this study, we aimed to investigate the potential role of circular RNA PVT1 (circPVT1), a newly identified cancer-related circRNA, in HCC. Herein, we found that circPVT1 expression was significantly upregulated in HCC tissues and cell lines. Knockdown of circPVT1 significantly reduced the growth and colony formation, and increased cell apoptosis, of HCC cells. Our results further identified circPVT1 as a sponge for miR-3666. Knockdown of circPVT1 significantly increased miR-3666 expression in HCC cells. Moreover, miR-3666 expression was significantly downregulated in HCC tissues and was inversely correlated with circPVT1 expression. In addition, the overexpression of miR-3666 inhibited the growth of HCC cells by targeting Sirtuin 7 (SIRT7). Notably, miR-3666 inhibition or SIRT7 overexpression partially reversed the circPVT1 knockdown-mediated inhibitory effect on HCC cell growth. Overall, these results demonstrate that downregulation of circPVT1 represses HCC cell growth by upregulating miR-3666 to inhibit SIRT7, suggesting circPVT1 as a potential therapeutic target for HCC. Our study highlights the involvement of circPVT1/miR-3666/SIRT7 in regulating HCC cell growth.

SIRT7 antagonizes human stem cell aging as a heterochromatin stabilizer

SIRT7, a sirtuin family member implicated in aging and disease, is a regulator of metabolism and stress responses. It remains elusive how human somatic stem cell populations might be impacted by SIRT7. Here, we found that SIRT7 expression declines during human mesenchymal stem cell (hMSC) aging and that SIRT7 deficiency accelerates senescence. Mechanistically, SIRT7 forms a complex with nuclear lamina proteins and heterochromatin proteins, thus maintaining the repressive state of heterochromatin at nuclear periphery. Accordingly, deficiency of SIRT7 results in loss of heterochromatin, de-repression of the LINE1 retrotransposon (LINE1), and activation of innate immune signaling via the cGAS-STING pathway. These aging-associated cellular defects were reversed by overexpression of heterochromatin proteins or treatment with a LINE1 targeted reverse-transcriptase inhibitor. Together, these findings highlight how SIRT7 safeguards chromatin architecture to control innate immune regulation and ensure geroprotection during stem cell aging.

Transcriptome-Wide Regulation of Key Developmental Pathways in the Mouse Neural Tube by Prenatal Alcohol Exposure

BACKGROUND

Early gestational alcohol exposure is associated with severe craniofacial and CNS dysmorphologies and behavioral abnormalities during adolescence and adulthood. Alcohol exposure during the formation of the neural tube (gestational day [GD] 8 to 10 in mice; equivalent to4th week of human pregnancy) disrupts development of ventral midline brain structures such as the pituitary, septum, and ventricles. This study identifies transcriptomic changes in the rostroventral neural tube (RVNT), the region of the neural tube that gives rise to the midline structures sensitive to alcohol exposure during neurulation.

METHODS

Female C57BL/6J mice were administered 2 doses of alcohol (2.9 g/kg) or vehicle 4 hours apart on GD 9.0. The RVNTs of embryos were collected 6 or 24 hours after the first dose and processed for RNA-seq.

RESULTS

Six hours following GD 9.0 alcohol exposure (GD 9.25), over 2,300 genes in the RVNT were determined to be differentially regulated by alcohol. Enrichment analysis determined that PAE affected pathways related to cell proliferation, p53 signaling, ribosome biogenesis, and immune activation. In addition, over 100 genes involved in primary cilia formation and function and regulation of morphogenic pathways were altered 6 hours after alcohol exposure. The changes to gene expression were largely transient, as only 91 genes identified as differentially regulated by prenatal alcohol at GD 10 (24 hours postexposure). Functionally, the differentially regulated genes at GD 10 were related to organogenesis and cell migration.

CONCLUSIONS

These data give a comprehensive view of the changing landscape of the embryonic transcriptome networks in regions of the neural tube that give rise to brain structures impacted by a neurulation-stage alcohol exposure. Identification of gene networks dysregulated by alcohol will help elucidate the pathogenic mechanisms of alcohol's actions.

SIRT7 Deacetylates STRAP to Regulate p53 Activity and Stability

Serine-threonine kinase receptor-associated protein (STRAP) functions as a regulator of both TGF-β and p53 signaling that participates in the regulation of cell proliferation and cell death in response to various stresses. Here, we demonstrate that STRAP acetylation plays an important role in p53-mediated cell cycle arrest and apoptosis. STRAP is acetylated at lysines 147, 148, and 156 by the acetyltransferases CREB-binding protein (CBP) and that the acetylation is reversed by the deacetylase sirtuin7 (SIRT7). Hypo- or hyperacetylation mutations of STRAP at lysines 147, 148, and 156 (3KR or 3KQ) influence its activation and stabilization of p53. Moreover, following 5-fluorouracil (5-FU) treatment, STRAP is mobilized from the cytoplasm to the nucleus and promotes STRAP acetylation. Our finding on the regulation of STRAP links p53 with SIRT7 influencing p53 activity and stability.

Granulosa-Lutein Cell Sirtuin Gene Expression Profiles Differ between Normal Donors and Infertile Women

Sirtuins are a family of deacetylases that modify structural proteins, metabolic enzymes, and histones to change cellular protein localization and function. In mammals, there are seven sirtuins involved in processes like oxidative stress or metabolic homeostasis associated with aging, degeneration or cancer. We studied gene expression of sirtuins by qRT-PCR in human mural granulosa-lutein cells (hGL) from IVF patients in different infertility diagnostic groups and in oocyte donors (OD; control group). Study 1: sirtuins genes’ expression levels and correlations with age and IVF parameters in women with no ovarian factor. We found significantly higher expression levels of SIRT1, SIRT2 and SIRT5 in patients ≥40 years old than in OD and in women between 27 and 39 years old with tubal or male factor, and no ovarian factor (NOF). Only SIRT2, SIRT5 and SIRT7 expression correlated with age. Study 2: sirtuin genes’ expression in women poor responders (PR), endometriosis (EM) and polycystic ovarian syndrome. Compared to NOF controls, we found higher SIRT2 gene expression in all diagnostic groups while SIRT3, SIRT5, SIRT6 and SIRT7 expression were higher only in PR. Related to clinical parameters SIRT1, SIRT6 and SIRT7 correlate positively with FSH and LH doses administered in EM patients. The number of mature oocytes retrieved in PR is positively correlated with the expression levels of SIRT3, SIRT4 and SIRT5. These data suggest that cellular physiopathology in PR’s follicle may be associated with cumulative DNA damage, indicating that further studies are necessary.

G Protein-Coupled Receptor Systems and Their Role in Cellular Senescence

Aging is a complex biological process that is inevitable for nearly all organisms. Aging is the strongest risk factor for development of multiple neurodegenerative disorders, cancer and cardiovascular disorders. Age-related disease conditions are mainly caused by the progressive degradation of the integrity of communication systems within and between organs. This is in part mediated by, i) decreased efficiency of receptor signaling systems and ii) an increasing inability to cope with stress leading to apoptosis and cellular senescence. Cellular senescence is a natural process during embryonic development, more recently it has been shown to be also involved in the development of aging disorders and is now considered one of the major hallmarks of aging. G-protein-coupled receptors (GPCRs) comprise a superfamily of integral membrane receptors that are responsible for cell signaling events involved in nearly every physiological process. Recent advances in the molecular understanding of GPCR signaling complexity have expanded their therapeutic capacity tremendously. Emerging data now suggests the involvement of GPCRs and their associated proteins in the development of cellular senescence. With the proven efficacy of therapeutic GPCR targeting, it is reasonable to now consider GPCRs as potential platforms to control cellular senescence and the consequently, age-related disorders.

Knockout of SIRT4 decreases chemosensitivity to 5-FU in colorectal cancer cells

Previous studies demonstrated that sirtuin (SIRT) 4 is aberrantly expressed in human malignant tumors and is associated with poor prognosis in patients with colorectal cancer. However, the role of SIRT4 in the progression of human colorectal cancer (CRC) and in chemotherapy remains unclear. In the present study, the expression of SIRT4 in CRC tissues and the effect of SIRT4 on colorectal cancer proliferation, migration and invasion was investigated. Additionally, the effects of SIRT4 on the chemosensitivity in colorectal cancer cells and the underlying molecular mechanisms were also explored. The results demonstrated that SIRT4 expression is significantly downregulated in CRC tissues and cell lines. Downregulation of SIRT4 significantly increased tumor proliferation, migration and invasion. Additionally, downregulation of SIRT4 decreased the chemosensitivity of CRC cells by inhibiting cell apoptosis. Thus, these results suggest that SIRT4 may be a promising therapeutic target in CRC.

SIRT7 regulates the TGF-β1-induced proliferation and migration of mouse airway smooth muscle cells by modulating the expression of TGF-β receptor I

Accumulating evidence shows that sirtuin 7 (SIRT7), a key mediator of many cellular activities, plays an important role in the pathogenesis of various diseases; however, little is known about the role of SIRT7 in asthma, which is characterized by airway remodeling. This study investigated the potential role of SIRT7 in regulating the proliferation and migration of airway smooth muscle (ASM) cells, which are critical events during airway remodeling in asthmatic conditions. The results demonstrated that SIRT7 expression was significantly upregulated in ASM cells treated with transforming growth factor-beta 1 (TGF-β1). Knockdown of SIRT7 inhibited the proliferation, promoted the apoptosis, and suppressed the migration of TGF-β1-treated ASM cells, while overexpression of SIRT7 had the opposite effect. Moreover, knockdown of SIRT7 inhibited protein expression of the TGF-β receptor I (TβRI), whilst overexpression of SIRT7 promoted the expression of TβRI. Importantly, knockdown of TβRI partially reversed the stimulatory effect of SIRT7 overexpression on the TGF-β1-induced proliferation and migration of ASM cells. Taken together, these results demonstrate that SIRT7 is involved in regulating TGF-β1-induced ASM cell proliferation and migration by regulating the expression of TβRI, thus indicating an important role of SIRT7 during airway remodeling in asthma.

MicroRNA-340 inhibits the growth and invasion of angiosarcoma cells by targeting SIRT7

MicroRNAs (miRNAs) are emerging as critical regulators for the development and progression of various cancers, including angiosarcoma. Accumulating evidence suggests that miRNA-340 (miR-340) is an important cancer-associated miRNA. However, little is known about the role of miR-340 in angiosarcoma. In this study, we aimed to investigate the potential biological functions of miR-340 and its potential target gene in angiosarcoma. Our results showed that miR-340 expression was significantly decreased in angiosarcoma compared with normal controls. The overexpression of miR-340 inhibited the growth and invasion of angiosarcoma cells, while the inhibition of miR-340 showed the opposite effect. Bioinformatics analysis predicted that Sirtuin 7 (SIRT7) was a potential target gene of miR-340. The binding relationship between miR-340 and the SIRT7 3'-untranslated region was verified by dual-luciferase reporter assay. Moreover, our results showed that miR-340 negatively regulated SIRT7 expression in angiosarcoma cells and an inverse correlation between miR-340 and SIRT7 expression was shown in clinical angiosarcoma tissues. We found that silencing SIRT7 significantly inhibited the proliferation and invasion of angiosarcoma cells. Notably, the overexpression of SIRT7 promoted the proliferation and invasion of angiosarcoma cells and also partially reversed the antitumor effect of miR-340 on angiosarcoma cell proliferation and invasion. Taken together, our results demonstrate that miR-340 inhibits the growth and invasion of angiosarcoma cells by targeting SIRT7. Our study provides evidence that the miR-340/SIRT7 axis may play an important role in the molecular pathogenesis of angiosarcoma and suggests that miR-340 and SIRT7 may be used as potential and novel therapeutic targets for the treatment of angiosarcoma.

SIRT7 depletion inhibits cell proliferation, migration, and increases drug sensitivity by activating p38MAPK in breast cancer cells

SIRT7 is a member of the sirtuin family of proteins that are known to be associated with tumor development. However, the functional roles and molecular mechanisms underlying the function of SIRT7 in breast cancer cell survival and tumor development remain unclear. Recent studies demonstrated that SIRT7 is upregulated in breast cancer cells and tissues. In the present study, we systematically explored the roles of SIRT7 in the growth of breast cancer cells and tumors both in vitro and in vivo. Our results showed that SIRT7 plays a major role in facilitating cell survival by promoting cell proliferation and inhibiting apoptosis. SIRT7 depletion significantly inhibited cell invasion and wound healing by blocking cell cycle progression and inducing cell apoptosis. Meanwhile, SIRT7 depletion can increase the sensitivity of breast cancer cells to doxorubicin (DOX). Xenograft model studies showed that stable silencing of SIRT7 inhibited tumor growth and enhanced tumor sensitivity to DOX. Further research revealed that p38MAPK is involved in SIRT7-mediated regulation of breast cancer cell proliferation and tumor growth. Taken together, our results showed that SIRT7 plays a critical role in breast cancer cell survival, migration, and tumor growth, and increased the efficiency of DOX treatment both in vitro and in vivo. Therefore, SIRT7 is a promising therapeutic target in breast cancer treatment.

Sirtuins and Accelerated Aging in Scleroderma

PURPOSE OF REVIEW

Premature activation of aging-associated molecular mechanisms is emerging as an important contributor to many diseases, including scleroderma. Among central regulators of the aging process are a group of histone deacetylases called sirtuins (SIRTs). Recent findings implicate these molecules as pathophysiological players in scleroderma skin and lung fibrosis. The goal of this article is to review recent studies on the involvement of SIRTs in scleroderma from the perspective of aging-related molecular mechanisms.

RECENT FINDINGS

Despite a degree of controversy in this rapidly developing field, the majority of data suggest that SIRT levels are decreased in tissues from patients with scleroderma compared to healthy controls as well as in animal models of scleroderma. Molecular studies reveal several mechanisms through which declining SIRT levels contribute to fibrosis, with the most attention given to modulation of the TGF-β signaling pathway. Activation of SIRTs in cell culture and in animal models elicits antifibrotic effects. Declining SIRT levels and activity are emerging as pathophysiological contributors to scleroderma. Restoration of SIRTs may be therapeutic in patients with scleroderma.

Sirtuin7 has an oncogenic potential via promoting the growth of cholangiocarcinoma cells

Accumulating evidence indicates that sirtuin7 (SIRT7) plays an oncogenic role in the main types of liver cancer, hepatocellular carcinoma (HCC). Nevertheless, the clinical significance of SIRT7 and its role in cholangiocarcinoma (CCA) is largely undiscovered. Here, we found that SIRT7 displayed higher expression in CCA tissues compared to intrahepatic normal bile duct and surrounding liver tissues based on The Cancer Genome Atlas (TCGA) data. Our data further confirmed that SIRT7 was overexpressed in CCA patient tissues and cell lines. Clinical analysis revealed that high SIRT7 expression was correlated with large tumor size and advanced tumor-node-metastasis (TNM) stage. Furthermore, SIRT7 overexpression independently predicted poor prognosis of CCA patients. Functionally, we demonstrated that SIRT7 knockdown suppressed proliferation and cell cycle progression of HUCCT1 cells in vitro and in vivo. SIRT7 restoration promoted the growth of QBC-939 cells. Mechanistically, SIRT7 reduced p21 expression and increased the levels of Cyclin D1 and cyclin dependent kinase 2 (CDK2) in CCA cells. Furthermore, microRNA-125b-5p (miR-125b-5p) was recognized as a direct negative regulator of SIRT7 and reduced SIRT7 abundance in CCA cells. Notably, miR-125b-5p restoration showed similar effects to SIRT7 knockdown on the growth of CCA cells. Taken together, we demonstrate for the first time that miR-125b-5p regulation of SIRT7 functions as an oncogene and a potential prognostic biomarker in CCA.

MicroRNA-519d inhibits proliferation and induces apoptosis of human hypertrophic scar fibroblasts through targeting Sirtuin 7

MicroRNAs (miRNAs) play critical roles in various pathological processes, including hypertrophic scar (HS) formation. However, the precise role of miRNAs in HS formation remains largely unknown. In this study, we aimed to investigate the role of miR-519d in HS formation. We found that miR-519d expression was significantly downregulated in HS tissues and fibroblasts. Overexpression of miR-519d inhibited the expression of type I collagen (Col I), type III collagen (Col III) and α-smooth muscle actin (α-SMA) in HS fibroblasts. Moreover, overexpression of miR-519d reduced the proliferation and induced the apoptosis of HS fibroblasts. In contrast, suppression of miR-519d showed the opposite effects. Interestingly, Sirtuin 7 (SIRT7) was identified as a target gene of miR-519d. The results showed that miR-519d directly targeted the 3'-untranslated region of SIRT7 and negatively regulated its expression. Furthermore, miR-519d regulated the expression of TGF-β type I receptor (TGFBRI) and the phosphorylation of Smad2. Knockdown of SIRT7 by siRNA inhibited the expression of Col I, Col III and α-SMA, and reduced the proliferation and induced the apoptosis of HS fibroblasts. Overexpression of SIRT7 abrogated the effects mediated by miR-519d overexpression in HS fibroblasts. Overall, these results suggest that miR-519d inhibits the expression of extracellular matrix-associated genes, reduces the proliferation and induces the apoptosis of HS fibroblasts by targeting SIRT7, implying a suppressive role of miR-519d in HS formation. This study suggests that miR-519d may serve as a promising therapeutic target for treatment of human HS.

Programming of Cell Resistance to Genotoxic and Oxidative Stress

Different organisms, cell types, and even similar cell lines can dramatically differ in resistance to genotoxic stress. This testifies to the wide opportunities for genetic and epigenetic regulation of stress resistance. These opportunities could be used to increase the effectiveness of cancer therapy, develop new varieties of plants and animals, and search for new pharmacological targets to enhance human radioresistance, which can be used for manned deep space expeditions. Based on the comparison of transcriptomic studies in cancer cells, in this review, we propose that there is a high diversity of genetic mechanisms of development of genotoxic stress resistance. This review focused on possibilities and limitations of the regulation of the resistance of normal cells and whole organisms to genotoxic and oxidative stress by the overexpressing of stress-response genes. Moreover, the existing experimental data on the effect of such overexpression on the resistance of cells and organisms to various genotoxic agents has been analyzed and systematized. We suggest that the recent advances in the development of multiplex and highly customizable gene overexpression technology that utilizes the mutant Cas9 protein and the abundance of available data on gene functions and their signal networks open new opportunities for research in this field.

Advances in Cellular Characterization of the Sirtuin Isoform, SIRT7

SIRT7 is one of seven mammalian sirtuins that functions as an NAD⁺-dependent histone/protein deacetylase. SIRT7 is the least well-known member of the sirtuin family, but recent efforts have identified its involvement in various cellular processes, such as ribosome biogenesis, gene expression, cellular metabolism and cancer. Here we provide an update on the functions and mechanisms of SIRT7 in cellular regulation and disease.

Quantitative proteome-based systematic identification of SIRT7 substrates

SIRT7 is a class III histone deacetylase that is involved in numerous cellular processes. Only six substrates of SIRT7 have been reported thus far, so we aimed to systematically identify SIRT7 substrates using stable-isotope labeling with amino acids in cell culture (SILAC) coupled with quantitative mass spectrometry (MS). Using SIRT7^+/+ and SIRT7^-/- mouse embryonic fibroblasts as our model system, we identified and quantified 1493 acetylation sites in 789 proteins, of which 261 acetylation sites in 176 proteins showed ≥2-fold change in acetylation state between SIRT7^-/- and SIRT7^+/+ cells. These proteins were considered putative SIRT7 substrates and were carried forward for further analysis. We then validated the predictive efficiency of the SILAC-MS experiment by assessing substrate acetylation status in vitro in six predicted proteins. We also performed a bioinformatic analysis of the MS data, which indicated that many of the putative protein substrates were involved in metabolic processes. Finally, we expanded our list of candidate substrates by performing a bioinformatics-based prediction analysis of putative SIRT7 substrates, using our list of putative substrates as a positive training set, and again validated a subset of the proteins in vitro. In summary, we have generated a comprehensive list of SIRT7 candidate substrates.

Long noncoding RNA and mRNA profiling in MDA-MB-231 cells following RNAi-mediated knockdown of SIRT7

Breast cancer is one of the most common malignant cancers among women and a major clinical obstacle. Although studies have reported the abnormal expression of SIRT7 in breast cancer, whether the function of SIRT7 regulates the expression of long noncoding RNAs (lncRNAs) in breast cancer remains unknown. We aimed to determine the differential expressions of mRNAs and lncRNAs associated with SIRT7 and understand the regulatory mechanism of SIRT7 in breast cancer. RNA sequencing was performed to explore the transcriptome in MDA-MB-231 cells after SIRT7 depletion, and a total of 50,634 different transcripts were identified. In comparison with the negative control, siSIRT7 groups showed 240 differentially expressed mRNAs and 26 differentially expressed lncRNAs. Gene ontology analysis revealed that the differentially expressed mRNAs mainly regulated DNA replication, CXCR chemokine receptor binding, and maturation of large subunit rRNA from tricistronic rRNA transcript, nucleoplasm, mitochondrion, and NAD⁺ ADP-ribosyltransferase activity. Kyoto Encyclopedia of Genes and Genomes analysis showed that the differentially expressed mRNAs were mainly involved in pathways associated with MAPK signaling pathway, tumor necrosis factor signaling pathway, hepatitis B, and cancer. Moreover, the target genes of the differentially expressed lncRNAs mainly regulated the carboxylic acid metabolic processes and were involved in glycolysis pathway. The mRNA-lncRNA coexpression network comprised 186 mRNAs and 23 lncRNAs. Our results provide essential data regarding differentially expressed lncRNAs and mRNAs after the depletion of SIRT7 in breast cancer cells, which may be useful to elucidate the role of SIRT7 in breast cancer development.

Dicer regulates non-homologous end joining and is associated with chemosensitivity in colon cancer patients

DNA double-strand break (DSB) repair is an important mechanism underlying chemotherapy resistance in human cancers. Dicer participates in DSB repair by facilitating homologous recombination. However, whether Dicer is involved in non-homologous end joining (NHEJ) remains unknown. Here, we addressed whether Dicer regulates NHEJ and chemosensitivity in colon cancer cells. Using our recently developed NHEJ assay, we found that DSB introduction by I-SceI cleavage leads to Dicer upregulation. Dicer knockdown increased SIRT7 binding and decreased the level of H3K18Ac (acetylated lysine 18 of histone H3) at DSB sites, thereby repressing the recruitment of NHEJ factors to DSB sites and inhibiting NHEJ. Dicer overexpression reduced SIRT7 binding and increased the level of H3K18Ac at DSB sites, promoting the recruitment of NHEJ factors to DSBs and moderately enhancing NHEJ. Dicer knockdown and overexpression increased and decreased, respectively, the chemosensitivity of colon cancer cells. Dicer protein expression in colon cancer tissues of patients was directly correlated with chemoresistance. Our findings revealed a function of Dicer in NHEJ-mediated DSB repair and the association of Dicer expression with chemoresistance in colon cancer patients.

MicroRNA‑3666 inhibits breast cancer cell proliferation by targeting sirtuin 7

The abnormal expression of microRNAs (miRNAs) is associated with cancer initiation and progression. miRNAs functioning as oncogenes or tumor suppressors represent novel biomarkers for cancer diagnosis, prognosis, and serve as therapeutic tools. MiR‑3666 has been reported as a tumor suppressor in various types of cancer; however, its role in breast cancer remains unknown. In the current study, the aim was to investigate the potential role of miR‑3666 in breast cancer. It was identified that miR‑3666 was decreased in breast cancer cell lines and that the overexpression of miR‑3666 inhibited breast cancer cell proliferation. Furthermore, miR‑3666 promotes cell apoptosis of breast cancer cells. Bioinformatics analysis and dual‑luciferase reporter assay demonstrated that miR‑3666 targeted the 3'‑untranslated region of sirtuin 7 (SIRT7) which was recognized as an oncogene. Overexpression of miR‑3666 decreased SIRT7 expression levels, and knockdown of SIRT7 suppressed proliferation and promoted apoptosis of breast cancer cells. A rescue assay demonstrated that the restoration of SIRT7 expression markedly reversed the miR‑3666‑induced anti‑tumor effects. Thus, the current study indicates that miR‑3666 suppresses breast cancer cell proliferation by targeting SIRT7, and propose miR‑3666 as a potential candidate for breast cancer therapy.

Sirtuin 7: a new marker of aggressiveness in prostate cancer

Predictive biomarkers for advanced prostate cancer (PCa) are still missing. The sirtuin 7 (SIRT7) has been linked to tumorogenesis but its role in prostate cancer is poorly documented. To determine if SIRT7 can be a biomarker for aggressive prostate cancer and plays a role in PCa aggressiveness. We analyzed the expression of SIRT7 by immunohistochemistry in 57 patients comparing healthy with adjacent cancer tissue. SIRT7 levels were significantly elevated in tumors and its expression was positively associated with the grade. We also demonstrated that the knock down of SIRT7 decreased the migration of DU145 and PC3 cells (two androgen-independent prostate cancer cell lines) whereas the overexpression of the native protein but not the mutated form increased the cell migration and the invasion of the poorly aggressive prostate cancer cell line LNCaP. Finally, we also showed that SIRT7 overexpression induced the resistance to docetaxel. Our results demonstrate that SIRT7 promotes prostate cancer cell aggressiveness and chemoresistance and suggest that SIRT7 is a good predictive biomarker of PCa aggressiveness.

Suppression of microRNA-142-5p attenuates hypoxia-induced apoptosis through targeting SIRT7

Increasing study has suggested that microRNAs (miRNAs) are pivotal regulators in regulating hypoxia-induced injury. miR-142-5p has been suggested as a critical regulator for cellular survival. However, the role of miR-142-5p in regulating hypoxia-induced injury remains unknown. In this study, we aimed to investigate the mechanistic roles of miR-142-5p in regulating cell survival during hypoxia treatment using H9C2 cardiomyoblasts and primary cardiomyocytes. We showed that miR-142-5p expression level was significantly repressed by hypoxia treatment. Overexpression of miR-142-5p during hypoxia induced extensive cell injury and apoptosis whereas suppression of miR-142-5p significantly promoted cell viability and attenuated cell apoptosis with hypoxia treatment. Sirtuin7 (SIRT7) was identified as a direct target gene of miR-142-5p by bioinformatics analysis and dual-luciferase reporter assays. Overexpression of miR-142-5p significantly decreased SIRT7 expression, while suppression of miR-142-5p increased SIRT7 expression. Furthermore, overexpression of SIRT7 protected H9C2 cardiomyoblasts and primary cardiomyocytes against hypoxia-induced injury and apoptosis. The silencing of SIRT7 markedly abrogated the protective effect induced by miR-142-5p suppression. Taken together, these results suggest that downregulation of miR-142-5p alleviates hypoxia-induced injury through upregulation of SIRT7. Our study suggests miR-142-5p/SIRT7 as potential therapeutic targets for ischemic heart disease.

Sirtuin7 is involved in protecting neurons against oxygen-glucose deprivation and reoxygenation-induced injury through regulation of the p53 signaling pathway

Sirtuin7 (SIRT7) is known to regulate apoptosis and stress responses. So far, very little is known about the role of SIRT7 in cerebral ischemia/reperfusion injury. In this study, we aimed to investigate the potential role of SIRT7 in regulating oxygen-glucose deprivation and reoxygenation (OGD/R)-induced injury in neurons. We found a significant increase of SIRT7 expression in neurons in response to OGD/R treatment. Knockdown of SIRT7 aggravated OGD/R-induced injury. Knockdown of SIRT7 augmented the levels of total and acetylated p53 protein. Moreover, knockdown of SIRT7 markedly increased the transcriptional activity of p53 toward apoptosis and activated the p53-mediated proapoptotic signaling pathway. By contrast, overexpression of SIRT7 showed the opposite effects. Taken together, the results of our study suggest that SIRT7 is involved in protecting neurons against OGD/R-induced injury, possibly through regulation of the p53-mediated proapoptotic signaling pathway, indicating a potential therapeutic target for cerebral ischemia/reperfusion injury.

Ubiquitin-specific peptidase 7 (USP7)-mediated deubiquitination of the histone deacetylase SIRT7 regulates gluconeogenesis

Sirtuin 7 (SIRT7), a member of the NAD⁺-dependent class III histone deacetylases, is involved in the regulation of various cellular processes and in resisting various stresses, such as hypoxia, low glucose levels, and DNA damage. Interestingly, SIRT7 is linked to the control of glycolysis, suggesting a role in glucose metabolism. Given the important roles of SIRT7, it is critical to clarify how SIRT7 activity is potentially regulated. It has been reported that some transcriptional and post-transcriptional regulatory mechanisms are involved. However, little is known how SIRT7 is regulated by the post-translational modifications. Here, we identified ubiquitin-specific peptidase 7 (USP7), a deubiquitinase, as a negative regulator of SIRT7. We showed that USP7 interacts with SIRT7 both in vitro and in vivo, and we further demonstrated that SIRT7 undergoes endogenous Lys-63-linked polyubiquitination, which is removed by USP7. Although the USP7-mediated deubiquitination of SIRT7 had no effect on its stability, the deubiquitination repressed its enzymatic activity. We also showed that USP7 coordinates with SIRT7 to regulate the expression of glucose-6-phosphatase catalytic subunit (G6PC), a gluconeogenic gene. USP7 depletion by RNA interference increased both G6PC expression and SIRT7 enzymatic activity. Moreover, SIRT7 targeted the G6PC promoter through the transcription factor ELK4 but not through forkhead box O1 (FoxO1). In summary, SIRT7 is a USP7 substrate and has a novel role as a regulator of gluconeogenesis. Our study may provide the basis for new clinical approaches to treat metabolic disorders related to glucose metabolism.

Sirtuin 7 is decreased in pulmonary fibrosis and regulates the fibrotic phenotype of lung fibroblasts

Pulmonary fibrosis is a severe condition with no cure and limited therapeutic options. A better understanding of its pathophysiology is needed. Recent studies have suggested that pulmonary fibrosis may be driven by accelerated aging-related mechanisms. Sirtuins (SIRTs), particularly SIRT1, SIRT3, and SIRT6, are well-known mediators of aging; however, limited data exist on the contribution of sirtuins to lung fibrosis. We assessed the mRNA and protein levels of all seven known sirtuins in primary lung fibroblasts from patients with idiopathic pulmonary fibrosis (IPF) and systemic sclerosis-associated interstitial lung disease (SSc-ILD) in comparison with lung fibroblasts from healthy controls. These unbiased tests revealed a tendency for all sirtuins to be expressed at lower levels in fibroblasts from patients compared with controls, but the greatest decrease was observed with SIRT7. Similarly, SIRT7 was decreased in lung tissues of bleomycin-challenged mice. Inhibition of SIRT7 with siRNA in cultured lung fibroblasts resulted in an increase in collagen and α-smooth muscle actin (α-SMA). Reciprocally, overexpression of SIRT7 resulted in lower basal and TGF-β-induced levels of COL1A1, COL1A2, COL3A1, and α-SMA mRNAs, as well as collagen and α-SMA proteins. Induced changes in SIRT7 had no effect on endogenous TGF-β mRNA levels or latent TGF-β activation, but overexpression of SIRT7 reduced the levels of Smad3 mRNA and protein. In conclusion, the decline in SIRT7 in lung fibroblasts has a profibrotic effect, which is mediated by changes in Smad3 levels.

Downregulation of SIRT7 by 5-fluorouracil induces radiosensitivity in human colorectal cancer

5-Fluorouracil (5-FU) combined with radiotherapy is a common treatment strategy to treat human cancers, but the underlying mechanisms of this combination treatment remain unclear. Here, we report that NAD⁺-dependent deacetylase sirtuin-7 (SIRT7) protein levels were decreased due to 5-FU exposure rendering colorectal cancer cells sensitive to radiation. We found that SIRT7 downregulation was mediated via a Tat-binding Protein 1 (TBP1) proteasome-dependent pathway. Specifically, TBP1 was dephosphorylated at tyrosine 381 upon 5-FU treatment, which enhanced its direct interaction with SIRT7 and targeted it for degradation. Depletion of SIRT7 in cultured colorectal cancer cells induced radiosensitivity triggering cell death. Interestingly, decreased levels of SIRT7 mediated by 5-FU correlated well with improved therapeutic effect in patients with rectal cancer and with inhibited tumor growth in immune-compromised mice post-irradiation. Taken together, these data suggest that 5-FU induces radiosensitivity via SIRT7 degradation to favor a cell death pathway in targeted cancer cells. Thus, downregulation of SIRT7 could be a promising pharmacologic strategy to increase the effectiveness of chemoradiation therapy in cancer patients.

Assessing sirtuin expression in endometrial carcinoma and non-neoplastic endometrium

Sirtuins participate in hormone imbalance, metabolism and aging, which are important processes for endometrial cancer (EC) development. Sirtuins mRNA expression (SIRT1 to 7) was determined in 76 ECs (63 Type I, 12 Type II and one mixed EC), and 30 non-neoplastic endometria (NNE) by quantitative real-time PCR. SIRT1 and SIRT7 protein expression was evaluated by immunohistochemistry using Allred score. Compared to NNE, ECs showed SIRT7 (p < 0.001) mRNA overexpression, whereas SIRT1 (p < 0.001), SIRT2 (p < 0.001), SIRT4 (p < 0.001) and SIRT5 (p < 0.001) were underexpressed. No significant differences were observed for SIRT3 and SIRT6. Type II ECs displayed lower SIRT1 (p = 0.032) and SIRT3 (p = 0.016) transcript levels than Type I ECs. Concerning protein expression, SIRT1 immunostaining median score was higher in ECs compared to NNE epithelium (EC = 5 vs. NNE = 2, p < 0.001), while SIRT7 was lower in ECs (EC = 6 vs. NNE = 7, p < 0.001). No significant associations were found between SIRT1/7 immunoexpression and histological subtype, grade, lymphovascular invasion or stage. Our data shows that sirtuins are deregulated in EC. The diversity of expression patterns observed suggests that sirtuins may have distinctive roles in endometrial cancer similarly to what has been described in other cancer models.

Sirtuins in metabolism, DNA repair and cancer

The mammalian sirtuin family has attracted tremendous attention over the past few years as stress adaptors and post-translational modifier. They have involved in diverse cellular processes including DNA repair, energy metabolism, and tumorigenesis. Notably, genomic instability and metabolic reprogramming are two of characteristic hallmarks in cancer. In this review, we summarize current knowledge on the functions of sirtuins mainly regarding DNA repair and energy metabolism, and further discuss the implication of sirtuins in cancer specifically by regulating genome integrity and cancer-related metabolism.

Downregulation of B-myb promotes senescence via the ROS-mediated p53/p21 pathway, in vascular endothelial cells

OBJECTIVES

To reveal whether B-myb is involved in preventing senescence of vascular endothelial cells, and if so, to identify possible mechanisms for it.

MATERIALS AND METHODS

C57/BL6 male mice and primary human aortic endothelial cells (HAECs) were used. Bleomycin was applied to induce stress-related premature senescence. B-myb knockdown was achieved using an siRNA technique and cell senescence was assessed using the senescence-associated β-galactosidase (SA-β-gal) assay. Intracellular reactive oxygen species (ROS) production was analysed using an ROS assay kit and cell proliferation was evaluated using KFluor488 EdU kit. Capillary tube network formation was determined by Matrigel assay. Expressions of mRNA and protein levels were detected by real-time PCR and western blotting.

RESULTS

B-myb expression significantly decreased, while p53 and p21 expressions increased in the aortas of aged mice. This expression pattern was also found in replicative senescent HAECs and senescent HAECs induced by bleomycin. B-myb knockdown resulted in upregulation of p22^phox , ROS accumulation and cell senescence of HAECs. Downregulation of B-myb significantly inhibited cell proliferation and capillary tube network formation and activated the p53/p21 signalling pathway. Blocking ROS production or inhibiting p53 activation remarkably attenuated SA-β-gal activity and delayed cell senescence induced by B-myb-silencing.

CONCLUSION

Downregulation of B-myb induced senescence by upregulation of p22^phox and activation of the ROS/p53/p21 pathway, in our vascular endothelial cells, suggesting that B-myb may be a novel candidate for regulating cell senescence to protect against endothelial senescence-related cardiovascular diseases.

Regulation of energy homeostasis by the ubiquitin-independent REGγ proteasome

Maintenance of energy homeostasis is essential for cell survival. Here, we report that the ATP- and ubiquitin-independent REGγ-proteasome system plays a role in maintaining energy homeostasis and cell survival during energy starvation via repressing rDNA transcription, a major intracellular energy-consuming process. Mechanistically, REGγ-proteasome limits cellular rDNA transcription and energy consumption by targeting the rDNA transcription activator SirT7 for ubiquitin-independent degradation under normal conditions. Moreover, energy starvation induces an AMPK-directed SirT7 phosphorylation and subsequent REGγ-dependent SirT7 subcellular redistribution and degradation, thereby further reducing rDNA transcription to save energy to overcome cell death. Energy starvation is a promising strategy for cancer therapy. Our report also shows that REGγ knockdown markedly improves the anti-tumour activity of energy metabolism inhibitors in mice. Our results underscore a control mechanism for an ubiquitin-independent process in maintaining energy homeostasis and cell viability under starvation conditions, suggesting that REGγ-proteasome inhibition has a potential to provide tumour-starving benefits.

In conditions of energy stress cells reduce transcription of ribosomal RNA (rRNA) to maintain cell survival. Here, the authors show that energy stress induces an AMPK-dependent phosphorylation of Sirt7, which promotes its ubiquitin-independent degradation by REGγ, resulting in the down-regulation of rRNA transcription and cell survival.

SIRT1 and SIRT7 expression in adipose tissues of obese and normal-weight individuals is regulated by microRNAs but not by methylation status

BACKGROUND/OBJECTIVE

Given their importance in the regulation of metabolism, sirtuins (SIRTs) constitute promising subjects of research on the pathogenesis of obesity and the metabolic syndrome. The aim of this study was to assess whether obesity in humans is associated with changes in the expression of SIRT genes in adipose tissue and whether epigenetic mechanisms, DNA methylation and microRNA (miRNA) interference, mediate in this phenomenon.

SUBJECTS/METHODS

The expression of SIRTs and of SIRT1 and SIRT7 mRNA-interacting miRNAs was evaluated by real-time PCR in visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT) of 58 obese (body mass index (BMI) >40 kg m^-2) and 31 normal-weight (BMI 20-24.9 kg m^-2) individuals. The methylation status of SIRTs was studied by the methylation-sensitive digestion/real-time PCR method.

RESULTS

SIRT1 mRNA levels were lower in adipose tissues of obese patients than of normal-weight controls (VAT: P=0.0002, SAT: P=0.008). In contrast, expression of SIRT7 was higher in adipose tissues of obese patients than in the control group (VAT: P=0.001, SAT: P=0.008). The mean methylation of the SIRT1 and SIRT7 CpG islands was similar in tissues with high and low expression of these genes, and there was no correlation between the level of expression and the level of methylation. On the other hand, expression of SIRT1 in VAT of obese subjects correlated negatively with the expression of miR-22-3p (P<0.0001, r_s=-0.514), miR-34a-5p (P=0.01, r_s=-0.326) and miR-181a-3p (P<0.0001, r_s=-0.536). In turn, expression of SIRT7 in VAT of slim individuals correlated negatively with the expression of miR-125a-5p (P=0.003, r_s=-0.562) and miR-125b-5p (P=0.018, r_s=-0.460).

CONCLUSIONS

We observed obesity-associated downregulation of SIRT1 and upregulation of SIRT7 mRNA levels that were not associated with the methylation status of their promoters. We found a negative correlation between mRNA levels of SIRT1 in VAT of obese individuals and SIRT7 in VAT of the normal-weight subjects and expression of the relevant miRNAs.