Inhibition of H3K18 deacetylation of Sirt7 by Myb-binding protein 1a (Mybbp1a)

SIRT7: a novel molecular target for personalized cancer treatment?

The Sirtuin family of NAD+-dependent enzymes assumes a pivotal role in orchestrating adaptive responses to environmental fluctuations and stress stimuli, operating at both genomic and metabolic levels. Within this family, SIRT7 emerges as a versatile player in tumorigenesis, displaying both pro-tumorigenic and tumor-suppressive functions in a context-dependent manner. While other sirtuins, such as SIRT1 and SIRT6, exhibit a similar dual role in cancer, SIRT7 stands out due to distinctive attributes that sharply distinguish it from other family members. Among these are a unique key role in regulation of nucleolar functions, a close functional relationship with RNA metabolism and processing -exceptional among sirtuins- and a complex multienzymatic nature, which provides a diverse range of molecular targets. This review offers a comprehensive overview of the current understanding of the role of SIRT7 in various malignancies, placing particular emphasis on the intricate molecular mechanisms employed by SIRT7 to either stimulate or counteract tumorigenesis. Additionally, it delves into the unique features of SIRT7, discussing their potential and specific implications in tumor initiation and progression, underscoring the promising avenue of targeting SIRT7 for the development of innovative anti-cancer therapies.

Substrates and Cyclic Peptide Inhibitors of the Oligonucleotide-Activated Sirtuin 7

The sirtuins are NAD+ -dependent lysine deacylases, comprising seven isoforms (SIRT1-7) in humans, which are involved in the regulation of a plethora of biological processes, including gene expression and metabolism. The sirtuins share a common hydrolytic mechanism but display preferences for different ϵ-N-acyllysine substrates. SIRT7 deacetylates targets in nuclei and nucleoli but remains one of the lesser studied of the seven isoforms, in part due to a lack of chemical tools to specifically probe SIRT7 activity. Here we expressed SIRT7 and, using small-angle X-ray scattering, reveal SIRT7 to be a monomeric enzyme with a low degree of globular flexibility in solution. We developed a fluorogenic assay for investigation of the substrate preferences of SIRT7 and to evaluate compounds that modulate its activity. We report several mechanism-based SIRT7 inhibitors as well as de novo cyclic peptide inhibitors selected from mRNA-display library screening that exhibit selectivity for SIRT7 over other sirtuin isoforms, stabilize SIRT7 in cells, and cause an increase in the acetylation of H3 K18.

Sirt7 associates with ELK1 to participate in hyperglycemia memory and diabetic nephropathy via modulation of DAPK3 expression and endothelial inflammation

Diabetic nephropathy (DN) is one of the most serious complications of advanced diabetes, and increases patient mortality. Recently, epigenetics-mediated hyperglycemic memory in pathological process of DN has received attention. The purpose of this study was to determine the underlying mechanism by which sirt7 modulates hyperglycemic memory in DN. In glomerular endothelial cells (GECs) cultured in high glucose and glomeruli of DN patients and rats, an increase in p65 phosphorylation and endothelial adhesion molecule levels persisted after glucose normalization but was reversed by glucose normalization associated with death-associated protein kinase-3 (DAPK3) knockout or DAPK3 inhibitor. High glucose-mediated decrease in sirt7, the deacetylase modulating H3K18-acetylation (H3K18ac), was sustained after normoglycemia. Sirt7 overexpression accompanied by glucose normalization suppressed DAPK3 expression and inflammation in GECs. Moreover, sh-sirt7-induced inflammation was inhibited by si-DAPK3. Furthermore, sirt7 and H3K18ac were located at the DAPK3 promoter region. ELK1 was found to combine with sirt7. si-ELK1 supplemented with normoglycemia inhibited high glucose-induced DAPK3 expression and inflammation in GECs. ELK1 overexpression-mediated inflammation was inhibited by si-DAPK3. In addition, ELK1 and sirt7 were located at the same promoter region of DAPK3. ELK1 overexpression enhanced DAPK3 promoter activity, which disappeared after specific binding site mutation. In vivo, sirt7 overexpression decreased inflammation and improved renal function during insulin treatment of DN rats, whereas insulin alone did not work. Our data demonstrated high glucose-mediated mutual inhibition between sirt7 and ELK1 induced DAPK3 transcription and inflammation despite normoglycemia in GECs, thus forming a vicious cycle and participating in the occurrence of hyperglycemic memory in DN.

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.

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.

Exercise Training in Obese Rats Does Not Induce Browning at Thermoneutrality and Induces a Muscle-Like Signature in Brown Adipose Tissue

Aim: Exercise training elicits diverse effects on brown (BAT) and white adipose tissue (WAT) physiology in rodents housed below their thermoneutral zone (i.e., 28-32°C). In these conditions, BAT is chronically hyperactive and, unlike human residence, closer to thermoneutrality. Therefore, we set out to determine the effects of exercise training in obese animals at 28°C (i.e., thermoneutrality) on BAT and WAT in its basal (i.e., inactive) state. Methods: Sprague-Dawley rats (n = 12) were housed at thermoneutrality from 3 weeks of age and fed a high-fat diet. At 12 weeks of age half these animals were randomized to 4-weeks of swim-training (1 h/day, 5 days per week). Following a metabolic assessment interscapular and perivascular BAT and inguinal (I)WAT were taken for analysis of thermogenic genes and the proteome. Results: Exercise attenuated weight gain but did not affect total fat mass or thermogenic gene expression. Proteomics revealed an impact of exercise training on 2-oxoglutarate metabolic process, mitochondrial respiratory chain complex IV, carbon metabolism, and oxidative phosphorylation. This was accompanied by an upregulation of multiple proteins involved in skeletal muscle physiology in BAT and an upregulation of muscle specific markers (i.e., Myod1, CkM, Mb, and MyoG). UCP1 mRNA was undetectable in IWAT with proteomics highlighting changes to DNA binding, the positive regulation of apoptosis, HIF-1 signaling and cytokine-cytokine receptor interaction. Conclusion: Exercise training reduced weight gain in obese animals at thermoneutrality and is accompanied by an oxidative signature in BAT which is accompanied by a muscle-like signature rather than induction of thermogenic genes. This may represent a new, UCP1-independent pathway through which BAT physiology is regulated by exercise training.

The Tumor Suppressor Roles of MYBBP1A, a Major Contributor to Metabolism Plasticity and Stemness

The MYB binding protein 1A (MYBBP1A, also known as p160) acts as a co-repressor of multiple transcription factors involved in many physiological processes. Therefore, MYBBP1A acts as a tumor suppressor in multiple aspects related to cell physiology, most of them very relevant for tumorigenesis. We explored the different roles of MYBBP1A in different aspects of cancer, such as mitosis, cellular senescence, epigenetic regulation, cell cycle, metabolism plasticity and stemness. We especially reviewed the relationships between MYBBP1A, the inhibitory role it plays by binding and inactivating c-MYB and its regulation of PGC-1α, leading to an increase in the stemness and the tumor stem cell population. In addition, MYBBP1A causes the activation of PGC-1α directly and indirectly through c-MYB, inducing the metabolic change from glycolysis to oxidative phosphorylation (OXPHOS). Therefore, the combination of these two effects caused by the decreased expression of MYBBP1A provides a selective advantage to tumor cells. Interestingly, this only occurs in cells lacking pVHL. Finally, the loss of MYBBP1A occurs in 8%–9% of renal tumors. tumors, and this subpopulation could be studied as a possible target of therapies using inhibitors of mitochondrial respiration.

SIRT7 regulates the nuclear export of NF-κB p65 by deacetylating Ran

Sirtuin 7 (SIRT7) is an NAD+-dependent lysine deacetylase that regulates diverse biological processes. We recently observed that SIRT7 deficiency suppresses the nuclear accumulation of p65, which is a component of nuclear factor kappa B. However, the underlying molecular mechanism remains elusive. In this study, we demonstrated that SIRT7 interacts with a small GTPase, Ras-related nuclear antigen (Ran), and deacetylates Ran at K37. The nuclear export of p65 was facilitated in SIRT7-deficient fibroblast cells, while the nuclear export was inhibited in SIRT7-deficient cells expressing K37R-Ran (deacetylation-mimicking mutant). Additionally, the nuclear export of p65 in wild-type fibroblast cells was promoted by K37Q-Ran (acetylation-mimicking mutant). K37Q-Ran exhibited an increased ability to bind to chromosome region maintenance 1 (CRM1), which is a major nuclear receptor that mediates the export of cargo proteins, and enhanced the binding between p65 and CRM1. These data suggest that SIRT7 is a lysine deacetylase that targets the K37 residue of Ran to suppress the nuclear export of p65.

H3K18Ac as a Marker of Cancer Progression and Potential Target of Anti-Cancer Therapy

Acetylation and deacetylation are posttranslational modifications (PTMs) which affect the regulation of chromatin structure and its remodeling. Acetylation of histone 3 at lysine placed on position 18 (H3K18Ac) plays an important role in driving progression of many types of cancer, including breast, colon, lung, hepatocellular, pancreatic, prostate, and thyroid cancer. The aim of this review is to analyze and discuss the newest findings regarding the role of H3K18Ac and acetylation of other histones in carcinogenesis. We summarize the level of H3K18Ac in different cancer cell lines and analyze its association with patients’ outcomes, including overall survival (OS), progression-free survival (PFS), and disease-free survival (DFS). Finally, we describe future perspectives of cancer therapeutic strategies based on H3K18 modifications.

Targeting Mybbp1a suppresses HCC progression via inhibiting IGF1/AKT pathway by CpG islands hypo-methylation dependent promotion of IGFBP5

BACKGROUND

Myb-binding protein 1A (Mybbp1a) is a nucleolar protein that can regulate rRNA metabolism, the stress response and carcinogenesis. However, the function of Mybbp1a in the progression of hepatocellular carcinoma (HCC) is unclear. We aimed to determine the role of Mybbp1a in HCC and the underlying mechanism.

METHODS

We investigated the function of Mybbp1a in HCC cell models and the xenograft mouse model. The relationship between Mybbp1a and IGFBP5 was found through expression profile chip. The molecular mechanism of Mybbp1a regulating IGFBP5 was proved through CO-IP, CHIP, Bisulfite Sequencing and Pyrosequencing.

FINDINGS

In this study, we observed that Mybbp1a was overexpressed in HCC tissues and associated with the poor prognosis of HCC patients. Suppression of Mybbp1a led to a reduction in the proliferation and migration ability of HCC cells through inhibiting the IGF1/AKT signaling pathway. Further study found that Mybbp1a could form a complex with DNMT1 and induce aberrant hyper-methylation of CpG islands of IGFBP5, which inhibits secretion of IGFBP5 and then activates IGF1/AKT signaling pathway.

INTERPRETATION

These findings extend our understanding of the function of Mybbp1a in the progression of HCC. The newly identified Mybbp1a may provide a novel biomarker for developing potential therapeutic targets of HCC. FUND: Science Technology Department of Zhejiang Province (No. 2015C03034), National Health and Family Planning Commission of China (No. 2016138643), Innovative Research Groups of National Natural Science Foundation of China (No. 81721091), Major program of National Natural Science Foundation of China (No. 91542205).

SIRT7 has a critical role in bone formation by regulating lysine acylation of SP7/Osterix

SP7/Osterix (OSX) is a master regulatory transcription factor that activates a variety of genes during differentiation of osteoblasts. However, the influence of post-translational modifications on the regulation of its transactivation activity is largely unknown. Here, we report that sirtuins, which are NAD(+)-dependent deacylases, regulate lysine deacylation-mediated transactivation of OSX. Germline Sirt7 knockout mice develop severe osteopenia characterized by decreased bone formation and an increase of osteoclasts. Similarly, osteoblast-specific Sirt7 knockout mice showed attenuated bone formation. Interaction of SIRT7 with OSX leads to the activation of transactivation by OSX without altering its protein expression. Deacylation of lysine (K) 368 in the C-terminal region of OSX by SIRT7 promote its N-terminal transactivation activity. In addition, SIRT7-mediated deacylation of K368 also facilitates depropionylation of OSX by SIRT1, thereby increasing OSX transactivation activity. In conclusion, our findings suggest that SIRT7 has a critical role in bone formation by regulating acylation of OSX.

SP7/Osterix is a transcription factor involved in osteoblast differentiation. Here, the authors show that Sirtuin 7 activates Osterix posttranslationally by regulating its lysine acylation, and that mice lacking Sirtuin 7 in osteoblasts show reduced bone formation.

SIRT7 is an important regulator of cartilage homeostasis and osteoarthritis development

Sirtuins (SIRT1-7) are NAD+-dependent deacetylase/deacylases that regulate a wide variety of biological functions. Although the roles of sirtuins in cartilage homeostasis and cartilage diseases have been well studied, there is no information on the contribution of SIRT7 to cartilage homeostasis and osteoarthritis (OA) pathologies. Here, we demonstrate that Sirt7 knockout mice are resistant to the development of aging-associated OA and forced exercise-induced OA. Attenuation of Sirt7 in the murine chondrogenic cell line ATDC5 increased the deposition of a glycosaminoglycan-rich extracellular matrix and the mRNA expression of extracellular matrix components such as Col2a1 and Acan. Mechanistically, we found that SIRT7 suppressed the transcriptional activity of SOX9, which is an important transcription factor in chondrocytes, and that the enzymatic activity of SIRT7 was required for its function. Our results indicate that SIRT7 is a novel important regulator of cartilage homeostasis and OA development.

Inhibitors of Trypanosoma cruzi Sir2 related protein 1 as potential drugs against Chagas disease

Chagas disease remains one of the most neglected diseases in the world despite being the most important parasitic disease in Latin America. The characteristic chronic manifestation of chagasic cardiomyopathy is the region’s leading cause of heart-related illness, causing significant mortality and morbidity. Due to the limited available therapeutic options, new drugs are urgently needed to control the disease. Sirtuins, also called Silent information regulator 2 (Sir2) proteins have long been suggested as interesting targets to treat different diseases, including parasitic infections. Recent studies on Trypanosoma cruzi sirtuins have hinted at the possibility to exploit these enzymes as a possible drug targets. In the present work, the T. cruzi Sir2 related protein 1 (TcSir2rp1) is genetically validated as a drug target and biochemically characterized for its NAD⁺-dependent deacetylase activity and its inhibition by the classic sirtuin inhibitor nicotinamide, as well as by bisnaphthalimidopropyl (BNIP) derivatives, a class of parasite sirtuin inhibitors. BNIPs ability to inhibit TcSir2rp1, and anti-parasitic activity against T. cruzi amastigotes in vitro were investigated. The compound BNIP Spermidine (BNIPSpd) (9), was found to be the most potent inhibitor of TcSir2rp1. Moreover, this compound showed altered trypanocidal activity against TcSir2rp1 overexpressing epimastigotes and anti-parasitic activity similar to the reference drug benznidazole against the medically important amastigotes, while having the highest selectivity index amongst the compounds tested. Unfortunately, BNIPSpd failed to treat a mouse model of Chagas disease, possibly due to its pharmacokinetic profile. Medicinal chemistry modifications of the compound, as well as alternative formulations may improve activity and pharmacokinetics in the future. Additionally, an initial TcSIR2rp1 model in complex with p53 peptide substrate was obtained from low resolution X-ray data (3.5 Å) to gain insight into the potential specificity of the interaction with the BNIP compounds. In conclusion, the search for TcSir2rp1 specific inhibitors may represent a valuable strategy for drug discovery against T. cruzi.

Trypanosoma cruzi is a protozoan parasite belonging to the Kinetoplastida class responsible for Chagas disease, a neglected tropical illness that affects an estimated 6 to 8 million people in Latin America and some Southern regions of the USA, with another 25 million at risk of acquiring the disease and a death toll of 12,000 every year. Commonly transmitted from the feces of the kissing bug, the disease is characterized by a nearly asymptomatic acute phase but a problematic chronic phase in which 20–30% of individuals develop serious cardiac and/or intestinal problems. The therapies currently in use were introduced more than forty years ago, and there are important concerns about adverse effects and lower effectiveness with disease progression. There is, therefore, an urgent need to find better alternatives. In this study, we evaluate the potential of a Trypanosoma cruzi sirtuin protein as a novel drug target and its inhibition by novel members of a known class of sirtuin compound inhibitors.

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.

SIRT7-dependent deacetylation of CDK9 activates RNA polymerase II transcription

SIRT7 is an NAD⁺-dependent protein deacetylase that regulates cell growth and proliferation. Previous studies have shown that SIRT7 is required for RNA polymerase I (Pol I) transcription and pre-rRNA processing. Here, we took a proteomic approach to identify novel molecular targets and characterize the role of SIRT7 in non-nucleolar processes. We show that SIRT7 interacts with numerous proteins involved in transcriptional regulation and RNA metabolism, the majority of interactions requiring ongoing transcription. In addition to its role in Pol I transcription, we found that SIRT7 also regulates transcription of snoRNAs and mRNAs. Mechanistically, SIRT7 promotes the release of P-TEFb from the inactive 7SK snRNP complex and deacetylates CDK9, a subunit of the elongation factor P-TEFb, which activates transcription by phosphorylating serine 2 within the C-terminal domain (CTD) of Pol II. SIRT7 counteracts GCN5-directed acetylation of lysine 48 within the catalytic domain of CDK9, deacetylation promoting CTD phosphorylation and transcription elongation.

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-dependent deacetylation of DDB1 regulates the expression of nuclear receptor TR4

Sirtuin 7 (SIRT7) is an NAD⁺-dependent deacetylase/deacylase, but only a limited number of SIRT7 substrates have been identified. Recently, we found that Sirt7 knockout mice are resistant to high-fat diet-induced fatty liver, and that SIRT7 positively regulates the protein level of TR4, a nuclear receptor involved in lipid metabolism, by inhibiting the CUL4B/DDB1/DCAF1 E3 ubiquitin ligase complex. However, the mechanism by which SIRT7 inhibits the E3 ubiquitin ligase complex was not identified. Here, we demonstrate that SIRT7 binds directly to DDB1 and deacetylates DDB1 at Lys1121. K1121R-DDB1 (a deacetylation-mimicking mutant) displayed reduced binding with DCAF1. The expression of TR4 protein and TR4 target genes, including Cd36, Cidea, Cidec and Pparg1, was increased in K1121R-DDB1-overexpressing Hepa1-6 cells compared to WT-DDB1-overexpressing cells. Our results indicate that the SIRT7-mediated deacetylation of K1121 attenuates the activity of the CUL4B/DDB1/DCAF1 E3 ubiquitin ligase complex by reducing binding between DDB1 and DCAF1, leading to the increased expression of TR4.

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.

SIRT7 Is Activated by DNA and Deacetylates Histone H3 in the Chromatin Context

Mammalian sirtuins (SIRT1-7) are members of a highly conserved family of nicotinamide adenine dinucleotide (NAD(+))-dependent protein deacetylases that regulate many biological processes including metabolism, genome stability, and transcription. Among the seven human sirtuins, SIRT7 is the least understood, to a large extent due to the lack of enzymatic activity in vitro. Here, we reported that SIRT7 can be activated by DNA to hydrolyze the acetyl group from lysine residues in vitro on histone peptides and histones in the chromatin context. Both N- and C- termini of SIRT7 are important for the DNA-activated deacetylase activity. The regulatory mechanism of SIRT7 is different from that of SIRT6, which also showed increased activity on chromatin substrates, but the deacetylase activity of SIRT6 on a peptide substrate cannot be activated by DNA. This finding provides an improved enzymatic activity assay of SIRT7 that will promote the development of SIRT7 modulators. Further investigation into the activation mechanism of SIRT7 by DNA could provide new insights into its biological function and help the development of sirtuin activators.

Targeting aberrant cancer metabolism - The role of sirtuins

Cancer cells, as opposed to normal cells, generate energy by increasing aerobic glycolysis, which is a phenomenon called "the Warburg effect". An altered energy metabolism supporting continuous cell growth and proliferation was pointed to as the new "hallmark" of cancer cells. Several hypotheses have been proposed to explain the maintenance of this seemingly wasteful catabolic state. The epigenetic mechanisms which depend on the covalent modifications of both DNA and histones have recently emerged as important players in the regulation of glucose metabolism. The sirtuin family of histone deacetylases has emerged as important regulators of diverse physiological and pathological events, including cancer metabolism. Sirtuins 1-7 (SIRT1-7) belong to class III of histone deacetylase enzymes which are dependent on NAD(+) for activity. It was recently demonstrated that SIRT6 is a tumor suppressor that modulates aerobic glycolysis by repressing HIF1 transcription. Members of this family of enzymes are considered promising pharmaceutical targets for cancer treatment. This review highlights the major functions of sirtuins in relation to cancer metabolism and the possibilities of their activation and inhibition by small molecule drugs.

Sirtuin 7 in cell proliferation, stress and disease: Rise of the Seventh Sirtuin!

Sirtuin 7 is a member of the sirtuin family of proteins. Sirtuins were originally discovered in yeast for its role in prolonging replicative lifespan. Until recently SIRT7 happened to be the least studied sirtuin of the seven mammalian sirtuins. However, a number of recent breakthrough reports have provided significant clarity to SIRT7 biology. SIRT7 is now seen as a vital regulator of rRNA and protein synthesis for maintenance of normal cellular homeostasis. Proteins like p53, H3K18, PAF53, NPM1 and GABP-β1 are the known substrates for the deacetylase activity of SIRT7, thereby making it a key mediator of many cellular activities. Studies using in vitro based assays and also knockout mice have revealed a role of SIRT7 in certain disease pathologies as well. High expression of SIRT7 has been reported in few cancer types and is steadily propelling SIRT7 towards an oncogene status. The role of SIRT7 as a pro-survival adaptor molecule in conditions of cellular stress has recently emerged in view of the fact that SIRT7 can regulate molecules like HIF and IRE1α. Additionally, SIRT7 plays a key role in maintenance of the epigenome as it caused the deacetylation of histone (H3K18) and global proteomics studies have shown its interaction with many chromatin remodelling complexes such as B-WICH and other proteins. Lately, the role of SIRT7 in hepatic lipid metabolism has been debated. This review attempts to summarize these recent findings and present the role of SIRT7 as an important cellular regulator.