Inhibition of SIRT6 in prostate cancer reduces cell viability and increases sensitivity to chemotherapeutics

Discovery of Novel PROTAC SIRT6 Degraders with Potent Efficacy against Hepatocellular Carcinoma

Sirtuin 6 (SIRT6), a member of the SIRT family, plays essential roles in the regulation of metabolism, inflammation, aging, DNA repair, and cancer development, making it a promising anticancer drug target. Herein, we present our use of proteolysis-targeting chimera (PROTAC) technology to formulate a series of highly potent and selective SIRT6 degraders. One of the degraders, SZU-B6, induced the near-complete degradation of SIRT6 in both SK-HEP-1 and Huh-7 cell lines and more potently inhibited hepatocellular carcinoma (HCC) cell proliferation than the parental inhibitors. In preliminary mechanistic studies, SZU-B6 hampered DNA damage repair, promoting the cellular radiosensitization of cancer cells. Our SIRT6 degrader SZU-B6 displayed promising antitumor activity, particularly when combined with the well-known kinase inhibitor sorafenib or irradiation in an SK-HEP-1 xenograft mouse model. Our results suggest that these PROTACs might constitute a potent therapeutic strategy for HCC.

A Novel Prognostic Model of Hepatocellular Carcinoma per Two NAD+ Metabolic Synthesis-Associated Genes

Hepatocellular carcinoma (HCC) is a formidable challenge to global human health, while recent years have witnessed the important role of NAD+ in tumorigenesis and progression. However, the expression pattern and prognostic value of NAD+ in HCC still remain elusive. Gene expression files and corresponding clinical pathological files associated with HCC were obtained from the Cancer Genome Atlas (TCGA) database, and genes associated with NAD+ were retrieved from the GSEA and differentially analyzed in tumor and normal tissues. A consensus clustering analysis was conducted by breaking down TCGA patients into four distinct groups, while Kaplan-Meier curves were generated to investigate the disparity in clinical pathology and endurance between clusters. A prognostic model based on NAD+-associated genes was established and assessed by combining LASSO-Cox regression, uni- and multi-variate Cox regression, and ROC curve analyses. Investigations were conducted to determine the expression of distinct mRNAs and proteins in both HCC and non-tumor tissues. A novel two-gene signature including poly (ADP-Ribose) polymerase 2 (PARP2) and sirtuin 6 (SIRT6) was obtained through LASSO-Cox regression and was identified to have favorable prognostic performance in HCC patients from TCGA. Analyses of both single and multiple variables showed that the prognostic model was a distinct prognostic factor in the endurance of liver cancer patients in both the training and trial groups. The nomogram also exhibited clinical significance in the prognosis of HCC patients. Immunohistochemistry, qRT-PCR, and Western blotting revealed that HCC samples exhibited higher PARP2 and SIRT6 expression levels than those of normal controls. This study identified a robust prognostic model comprising two NAD+-associated genes using bioinformatic methods, which is accurate in predicting the survival outcome of HCC patients. This model might benefit the early diagnosis of HCC and further facilitate the management of individualized medical service and clinical decision-making.

Activation and inhibition of sirtuins: From bench to bedside

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

Expression analysis and biological regulation of silencing regulatory protein 6 (SIRT6) in cutaneous squamous cell carcinoma

BACKGROUND

Cutaneous squamous cell carcinoma (CSCC) is one of the most common types of skin cancer worldwide. Therefore, the identification of biomarkers associated with CSCC progression could aid in the early detection of high-risk squamous cell carcinoma and the development of novel therapeutic strategies.

OBJECTIVE

This study aimed to investigate the expression patterns of silent mating type Information Regulation 2 homolog 6 (SIRT6) in CSCC and its clinical significance.

METHODS

The protein expression level of SIRT6 in tissues was detected by immunohistochemistry, and the correlation between SIRT6 expression and clinicopathological parameters in CSCC patients was analyzed. The relative expression of SIRT6 in CSCC cell lineage and tissue specimens was determined by western blotting and PCR. The effect of SIRT6 silencing on cell proliferation was evaluated using cell counting kit 8. Wound healing, transwell method, and flow cytometry were used to investigate the migration, invasion, and cell cycle distribution/apoptosis of CSCC cells after SIRT6 silencing, respectively. Western blot was used to detect the expression of EMT (Epithelial-Mesenchymal Transition), cycle, apoptosis, and other related proteins.

RESULTS

The high expression of SIRT6 was correlated with the location of cancer tissue and Broder staging in CSCC patients. Knockdown of SIRT6 inhibited the proliferation, migration, invasion and EMT of CSCC cells, and promoted their apoptosis, with cells blocked in G1 phase.

STUDY LIMITATIONS

No animal experiments were conducted to further verify the results.

CONCLUSION

Decreased expression of SIRT6 can inhibit the occurrence and development of CSCC.

Epigenetic regulation of androgen dependent and independent prostate cancer

Prostate cancer is one of the most common malignancies among men worldwide. Besides genetic alterations, epigenetic modulations including DNA methylation, histone modifications and miRNA mediated alteration of gene expression are the key driving forces for the prostate tumor development and cancer progression. Aberrant expression and/or the activity of the epigenetic modifiers/enzymes, results in aberrant expression of genes involved in DNA repair, cell cycle regulation, cell adhesion, apoptosis, autophagy, tumor suppression and hormone response and thereby disease progression. Altered epigenome is associated with prostate cancer recurrence, progression, aggressiveness and transition from androgen-dependent to androgen-independent phenotype. These epigenetic modifications are reversible and various compounds/drugs targeting the epigenetic enzymes have been developed that are effective in cancer treatment. This chapter focuses on the epigenetic alterations in prostate cancer initiation and progression, listing different epigenetic biomarkers for diagnosis and prognosis of the disease and their potential as therapeutic targets. This chapter also summarizes different epigenetic drugs approved for prostate cancer therapy and the drugs available for clinical trials.

Analysis of the Expression and Prognostic Value of SIRTs in Hepatocellular Carcinoma

Purpose

This study contributes to the evolving understanding of the pivotal involvement of Sirtuins (SIRTs) in various human cancers, with a particular focus on elucidating their expression patterns and clinical relevance within the context of hepatocellular carcinoma (HCC). The investigation involves a comprehensive analysis of mRNA expression and prognostic implications associated with distinct SIRTs in HCC.

Patients and Methods

Initial data pertaining to SIRT expression in HCC patients were collated from publicly accessible databases. Subsequently, the expression levels of select members of the SIRT family were validated using clinicopathological specimens from HCC patients. Additionally, HCC tissue microarray was employed to scrutinize the correlation between SIRT7 expression and HCC prognosis.

Results

The findings indicated a substantial upregulation of SIRT2, SIRT3, SIRT4, SIRT6, and SIRT7 in HCC tissues. Survival analysis underscored a pronounced association between elevated mRNA levels of SIRT3, SIRT6, and SIRT7 and an adverse prognosis for HCC patients. Particularly, SIRT7 emerged as a potential independent risk factor for poor prognosis in HCC patients. Examination of the HCC tissue microarray revealed heightened expression of SIRT7 in 68 cases (54.8%) of HCC tissues. Multivariate analysis established high SIRT7 expression as an independent risk factor for diminished Disease-Free Survival (DFS) and Overall Survival (OS) in HCC patients.

Conclusion

The aberrant expression of SIRT7 presents itself may be as a novel biomarker for predicting the prognosis of HCC patients.

Role of Exosomes in Cancer and Aptamer-Modified Exosomes as a Promising Platform for Cancer Targeted Therapy

Exosomes are increasingly recognized as important mediators of intercellular communication in cancer biology. Exosomes can be derived from cancer cells as well as cellular components in tumor microenvironment. After secretion, the exosomes carrying a wide range of bioactive cargos can be ingested by local or distant recipient cells. The released cargos act through a variety of mechanisms to elicit multiple biological effects and impact most if not all hallmarks of cancer. Moreover, owing to their excellent biocompatibility and capability of being easily engineered or modified, exosomes are currently exploited as a promising platform for cancer targeted therapy. In this review, we first summarize the current knowledge of roles of exosomes in risk and etiology, initiation and progression of cancer, as well as their underlying molecular mechanisms. The aptamer-modified exosome as a promising platform for cancer targeted therapy is then briefly introduced. We also discuss the future directions for emerging roles of exosome in tumor biology and perspective of aptamer-modified exosomes in cancer therapy.

Discovery of a potent and highly selective inhibitor of SIRT6 against pancreatic cancer metastasis in vivo

Pancreatic cancer, one of the most aggressive malignancies, has no effective treatment due to the lack of targets and drugs related to tumour metastasis. SIRT6 can promote the migration of pancreatic cancer and could be a potential target for antimetastasis of pancreatic cancer. However, highly selective and potency SIRT6 inhibitor that can be used in vivo is yet to be discovered. Here, we developed a novel SIRT6 allosteric inhibitor, compound 11e, with maximal inhibitory potency and an IC50 value of 0.98 ± 0.13 μmol/L. Moreover, compound 11e exhibited significant selectivity against other histone deacetylases (HADC1‒11 and SIRT1‒3) at concentrations up to 100 μmol/L. The allosteric site and the molecular mechanism of inhibition were extensively elucidated by cocrystal complex structure and dynamic structural analyses. Importantly, we confirmed the antimetastatic function of such inhibitors in four pancreatic cancer cell lines as well as in two mouse models of pancreatic cancer liver metastasis. To our knowledge, this is the first study to reveal the in vivo effects of SIRT6 inhibitors on liver metastatic pancreatic cancer. It not only provides a promising lead compound for subsequent inhibitor development targeting SIRT6 but also provides a potential approach to address the challenge of metastasis in pancreatic cancer.

Binding to nucleosome poises human SIRT6 for histone H3 deacetylation

Sirtuin 6 (SIRT6) is an NAD+-dependent histone H3 deacetylase that is prominently found associated with chromatin, attenuates transcriptionally active promoters and regulates DNA repair, metabolic homeostasis and lifespan. Unlike other sirtuins, it has low affinity to free histone tails but demonstrates strong binding to nucleosomes. It is poorly understood how SIRT6 docking on nucleosomes stimulates its histone deacetylation activity. Here, we present the structure of human SIRT6 bound to a nucleosome determined by cryogenic electron microscopy. The zinc finger domain of SIRT6 associates tightly with the acidic patch of the nucleosome through multiple arginine anchors. The Rossmann fold domain binds to the terminus of the looser DNA half of the nucleosome, detaching two turns of the DNA from the histone octamer and placing the NAD+ binding pocket close to the DNA exit site. This domain shows flexibility with respect to the fixed zinc finger and moves with, but also relative to, the unwrapped DNA terminus. We apply molecular dynamics simulations of the histone tails in the nucleosome to show that in this mode of interaction, the active site of SIRT6 is perfectly poised to catalyze deacetylation of the H3 histone tail and that the partial unwrapping of the DNA allows even lysines close to the H3 core to reach the enzyme.

SIRT6 promotes metastasis and relapse in HER2-positive breast cancer

The histone deacetylase sirtuin 6 (SIRT6) has been endowed with anti-cancer capabilities in many tumor types. Here, we investigate the impact of SIRT6-overexpression (SIRT6-OE) in Delta16HER2 mice, which are a bona fide model of HER2-positive breast cancer. After an initial delay in the tumor onset, SIRT6-OE induces a more aggressive phenotype of Delta16HER2 tumors promoting the formation of higher number of tumor foci and metastases than controls. This phenotype of SIRT6-OE tumors is associated with cancer stem cell (CSC)-like features and tumor dormancy, and low senescence and oxidative DNA damage. Accordingly, a sub-set of HER2-positive breast cancer patients with concurrent SIRT6-OE has a significant poorer relapse-free survival (RFS) probability than patients with low expression of SIRT6. ChIP-seq, RNA-seq and RT-PCR experiments indicate that SIRT6-OE represses the expression of the T-box transcription factor 3 (Tbx3) by deacetylation of H3K9ac. Accordingly, loss-of-function mutations of TBX3 or low TBX3 expression levels are predictive of poor prognosis in HER2-positive breast cancer patients. Our work indicates that high levels of SIRT6 are indicative of poor prognosis and high risk of metastasis in HER2-positive breast cancer and suggests further investigation of TBX3 as a downstream target of SIRT6 and co-marker of poor-prognosis. Our results point to a breast cancer subtype-specific effect of SIRT6 and warrant future studies dissecting the mechanisms of SIRT6 regulation in different breast cancer subtypes.

Caspase-Linked Programmed Cell Death in Prostate Cancer: From Apoptosis, Necroptosis, and Pyroptosis to PANoptosis

Prostate cancer (PCa) is a complex disease and the cause of one of the highest cancer-related mortalities in men worldwide. Annually, more than 1.2 million new cases are diagnosed globally, accounting for 7% of newly diagnosed cancers in men. Programmed cell death (PCD) plays an essential role in removing infected, functionally dispensable, or potentially neoplastic cells. Apoptosis is the canonical form of PCD with no inflammatory responses elicited, and the close relationship between apoptosis and PCa has been well studied. Necroptosis and pyroptosis are two lytic forms of PCD that result in the release of intracellular contents, which induce inflammatory responses. An increasing number of studies have confirmed that necroptosis and pyroptosis are also closely related to the occurrence and progression of PCa. Recently, a novel form of PCD named PANoptosis, which is a combination of apoptosis, necroptosis, and pyroptosis, revealed the attached connection among them and may be a promising target for PCa. Apoptosis, necroptosis, pyroptosis, and PANoptosis are good examples to better understand the mechanism underlying PCD in PCa. This review aims to summarize the emerging roles and therapeutic potential of apoptosis, necroptosis, pyroptosis, and PANoptosis in PCa.

Knockdown of sirtuin6 positively regulates acetylation of DNMT1 to inhibit NOTCH signaling pathway in non-small cell lung cancer cell lines

BACKGROUND AND AIM

Sirtuin proteins (1-7) are nicotinamide adenine dinucleotide (NAD)-dependent deacetylases and ADP-ribosyl transferases (class III histone deacetylase enzymes (HDAC)) mainly involved in the removal of the acetyl group from histone proteins. SIRT6, one of the sirtuins, plays a major role in cancer progression in many types of cancer conditions. We recently reported that SIRT6 acts as an oncogene in NSCLC; thus, silencing of SIRT6 inhibits cell proliferation and induces apoptosis in NSCLC cell lines. NOTCH signaling has been reported to be involved in cell survival and regulates cell proliferation and differentiation. However, recent studies from different groups have converged on the notion that NOTCH1 may be an important oncogene in NSCLC. The abnormal expression of NOTCH signaling pathway members is a relatively frequent event in patients with NSCLC. SIRT6 and the NOTCH signaling pathway might play a critical role in tumorigenesis since they are highly expressed in NSCLC. This study has been performed to explore the exact mechanism by which SIRT6 inhibits cell proliferation and induces the apoptosis of NSCLC cell lines and its correlation with NOTCH signaling.

MAIN METHODS

In vitro experiments with human NSCLC cells have been performed. Immunocytochemistry study was used to analyze the expression of NOTCH1 and DNMT1 in A549 and NCI-H460 cell lines. RT-qPCR, Western Blot, Methylated DNA specific PCR, and Co-Immunoprecipitation were performed to explore the key events in the regulation of NOTCH signaling by silencing SIRT6 in NSCLC cell lines.

KEY FINDINGS

The findings of this study suggest that silencing of SIRT6 significantly promotes the acetylation status of DNMT1 and stabilizes it. Consequently, acetylated DNMT1 translocates into the nucleus and methylates the NOTCH1 promoter region, resulting in the hindering of NOTCH1-mediated NOTCH signaling.

Aptamers against cancer drug resistance: Small fighters switching tactics in the face of defeat

Discovering novel cancer therapies has attracted extreme interest in the last decade. In this regard, multidrug resistance (MDR) to chemotherapies is the primary challenge in cancer treatment. Cancerous cells are growingly become resistant to existing chemotherapeutics by employing diverse mechanisms, highlighting the significance of discovering approaches to overcome MDR. One promising strategy is utilizing aptamers as unique tools to target elements or signalings incorporated in resistance mechanisms or develop active targeted drug delivery systems or chimeras enabling the precise delivery of novel agents to inhibit the conventionally undruggable resistance elements. Further, due to their advantages over their proteinaceous counterparts, particularly antibodies, including improved targeting action, enhanced thermal stability, easier production, and superior tumor penetration, aptamers are emerging and have frequently been considered for developing cancer therapeutics. Here, we highlighted significant chemoresistance pathways and thoroughly discussed using aptamers as prospective tools to surmount cancer MDR.

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

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

MiR-363 suppresses the tumor growth of natural killer/T-cell lymphoma via the SIRT6/PI3K/AKT axis

Background

Natural killer/T cell lymphoma (NKTCL) is a rare and aggressive tumor of non-Hodgkin's lymphoma. The role of micro ribonucleic acid (RNA) (miR)-363 in NKTCL has not yet been elucidated. The present study aimed to investigate the potential role of miR-363 in NKTCL.

Methods

The expression of the top five differentially expressed microRNAs (miRNAs) as well as sirtuin 6 (SIRT6) in NK normal cells and its tumor cell lines were explored. The clinical tissues of NKTCL patients were collected and analyzed for expression of miR-363 and SIRT6. In addition, human NK/T-cell lymphoma cells (SNK-6) were transfected into different groups to detect cell proliferation and apoptosis abilities through cell counting kit 8 (CCK-8) experiment and flow cytometry analysis. Western blot assay was employed to examine protein expression. NKTCL nude mice models were constructed by subcutaneous injection of stably transfected SNK-6 cells to validate the mechanism of miR-363 in NKTCL via SIRT6 in vivo.

Results

MiR-363 was down-regulated in NKTCL tissues and cell lines. Overexpression of miR-363 inhibited cell proliferation and promoted cell apoptosis. In contrast, SIRT6 was up-regulated in NKTCL and proved to be a downstream target of miR-363. SIRT6 could activate the phosphatidylinositol-3-kinase (PI3K)/protein kinase B (AKT) signaling pathway. Also, miR-363 mimic could suppress the proliferation and induce the apoptosis of NKTCL via the SIRT6/PI3K/AKT axis both in vitro and in vivo.

Conclusions

MiR-363 suppresses the SIRT6/PI3K/AKT pathway to restrain cell proliferation and accelerate cell apoptosis during NKTCL progression.

The Mechanistic Roles of Sirtuins in Breast and Prostate Cancer

Simple Summary

There are diverse reports of the dual role of sirtuin genes and proteins in breast and prostate cancers. This review discusses the current information on the tumor promotion or suppression roles of SIRT1–7 in breast and prostate cancers. Precisely, we highlight that sirtuins regulate various proteins implicated in proliferation, apoptosis, autophagy, chemoresistance, invasion, migration, and metastasis of both breast and prostate cancer. We also provide evidence of the direct regulation of sirtuins by miRNAs, highlighting the consequences of this regulation in breast and prostate cancer. Overall, this review reveals the potential value of sirtuins as biomarkers and/or targets for improved treatment of breast and prostate cancers.

Abstract

Mammalian sirtuins (SIRT1–7) are involved in a myriad of cellular processes, including apoptosis, proliferation, differentiation, epithelial-mesenchymal transition, aging, DNA repair, senescence, viability, survival, and stress response. In this review, we discuss the current information on the mechanistic roles of SIRT1–7 and their downstream effects (tumor promotion or suppression) in cancers of the breast and prostate. Specifically, we highlight the involvement of sirtuins in the regulation of various proteins implicated in proliferation, apoptosis, autophagy, chemoresistance, invasion, migration, and metastasis of breast and prostate cancer. Additionally, we highlight the available information regarding SIRT1–7 regulation by miRNAs, laying much emphasis on the consequences in the progression of breast and prostate cancer.

SIRT6 Widely Regulates Aging, Immunity, and Cancer

SIRT6 is a member of the Sir2-like family in mammals. Recent structural and biochemical studies have characterized SIRT6 as having deacetylation, defatty-acylation, and mono-ADP-ribosylation activities, which determine its important regulatory roles during physiological and pathological processes. This review focuses mainly on the regulatory functions of SIRT6 in aging, cancer, and, especially, immunity. Particular attention is paid to studies illustrating the critical role of SIRT6 in the regulation of immune cells from the viewpoints of immunesenescence, immunometabolism, and tumor immunology. Owing to its role in regulating the function of the immune system, SIRT6 can be considered to be a potential therapeutic target for the treatment of diseases.

An Overview of siRNA Delivery Strategies for Urological Cancers

The treatment of urological cancers has been significantly improved in recent years. However, for the advanced stages of these cancers and/or for those developing resistance, novel therapeutic options need to be developed. Among the innovative strategies, the use of small interfering RNA (siRNA) seems to be of great therapeutic interest. siRNAs are double-stranded RNA molecules which can specifically target virtually any mRNA of pathological genes. For this reason, siRNAs have a great therapeutic potential for human diseases including urological cancers. However, the fragile nature of siRNAs in the biological environment imposes the development of appropriate delivery systems to protect them. Thus, ensuring siRNA reaches its deep tissue target while maintaining structural and functional integrity represents one of the major challenges. To reach this goal, siRNA-based therapies require the development of fine, tailor-made delivery systems. Polymeric nanoparticles, lipid nanoparticles, nanobubbles and magnetic nanoparticles are among nano-delivery systems studied recently to meet this demand. In this review, after an introduction about the main features of urological tumors, we describe siRNA characteristics together with representative delivery systems developed for urology applications; the examples reported are subdivided on the basis of the different delivery materials and on the different urological cancers.

Clinicopathological and prognostic value of SIRT6 in patients with solid tumors: a meta-analysis and TCGA data review

PURPOSES

In addition to its role in cellular progression and cancer, SIRT6, a member of nicotinamide adenine dinucleotide (NAD⁺)-dependent class III deacylase sirtuin family, serves a variety of roles in the body's immune system. In this study, we sought to determine the relationship between the expression of SIRT6 and the clinicopathological outcomes of patients with solid tumours by conducting a meta-analysis of the available data.

METHODS

The databases PubMed and ISI Web of Science were searched for relevant literature, and the results were presented here. Using Stata16.0, a meta-analysis was conducted to determine the impact of SIRT6 on clinicopathological characteristics and prognosis in malignancy patients. The results were published in the journal Cancer Research. The dataset from the Cancer Genome Atlas (TCGA) was used to investigate the prognostic significance of SIRT6 in various types of tumors.

RESULTS

The inclusion and exclusion criteria were met by 15 studies. In patients with solid tumours, reduced SIRT6 expression was found to be related with improved overall survival (OS) (HR = 0.66, 95% CI = 0.45-0.97, P < 0.001) as well as improved disease-free survival (DFS) (HR = 0.48, 95% CI = 0.26-0.91, P < 0.001). Low SIRT6 expression was found to be associated with a better OS in breast cancer (HR = 0.49, 95% CI = 0.27-0.89, P = 0.179), but was found to be associated with a worse OS in gastrointestinal cancer (gastric cancer and colon cancer) (HR = 1.83, 95% CI = 1.20-2.79, P = 0.939) after subgroup analysis. In terms of clinicopathological characteristics, SIRT6 expression was found to be linked with distant metastasis (OR = 2.98, 95% CI = 1.59-5.57, P = 0.694). When the data from the TCGA dataset was compared to normal tissue, it was discovered that SIRT6 expression was significantly different in 11 different types of cancers. Meanwhile, reduced SIRT6 expression was shown to be associated with improved OS (P < 0.05), which was consistent with the findings of the meta-analysis. Aside from that, the expression of SIRT6 was found to be associated with both gender and clinical stage.

CONCLUSION

The overall data of the present meta-analysis indicated that low expression of SIRT6 may predict a favorable survival for patients with solid tumors.

Sirtuin 6 Is a Critical Epigenetic Regulator of Cancer

Sirtuin 6 (SIRT6) is a member of the mammalian sirtuin family with deacetylase, deacylase, and mono-ADP-ribosyl-transferase activities. It is a multitasking chromatin-associated protein regulating different cellular and physiological functions in cells. Specifically, SIRT6 dysfunction is implicated in several aging-related human diseases, including cancer. Studies indicate that SIRT6 has a tumor-specific role, and it is considered a tumor suppressor as well as a tumor growth inducer, depending on the type of cancer. In this chapter, we review the role of SIRT6 in metabolism, genomic stability, and cancer. Further, we provide an insight into the interplay of the tumor-suppressing and oncogenic roles of SIRT6 in cancer. Additionally, we discuss the use of small-molecule SIRT6 modulators as potential therapeutics.

Harnessing Reversed Allosteric Communication: A Novel Strategy for Allosteric Drug Discovery

Allostery is a fundamental and extensive mechanism of intramolecular signal transmission. Allosteric drugs possess several unique pharmacological advantages over traditional orthosteric drugs, including greater selectivity, better physicochemical properties, and lower off-target toxicity. However, owing to the complexity of allosteric regulation, experimental approaches for the development of allosteric modulators are traditionally serendipitous. Recently, the reversed allosteric communication theory has been proposed, providing a feasible tool for the unbiased detection of allosteric sites. Herein, we review the latest research on the reversed allosteric communication effect using the examples of sirtuin 6, epidermal growth factor receptor, 3-phosphoinositide-dependent protein kinase 1, and Related to A and C kinases (RAC) serine/threonine protein kinase B and recapitulate the methodologies of reversed allosteric communication strategy. The novel reversed allosteric communication strategy greatly expands the horizon of allosteric site identification and allosteric mechanism exploration and is expected to accelerate an end-to-end framework for drug discovery.

Emerging Therapeutic Potential of SIRT6 Modulators

Sirtuin 6 (SIRT6) is an NAD⁺-dependent protein deacylase and mono-ADP-ribosyltransferase of the sirtuin family with a wide substrate specificity. In vitro and in vivo studies have indicated that SIRT6 overexpression or activation has beneficial effects for cellular processes such as DNA repair, metabolic regulation, and aging. On the other hand, SIRT6 has contrasting roles in cancer, acting either as a tumor suppressor or promoter in a context-specific manner. Given its central role in cellular homeostasis, SIRT6 has emerged as a promising target for the development of small-molecule activators and inhibitors possessing a therapeutic potential in diseases ranging from cancer to age-related disorders. Moreover, specific modulators allow the molecular details of SIRT6 activity to be scrutinized and further validate the enzyme as a pharmacological target. In this Perspective, we summarize the current knowledge about SIRT6 pharmacology and medicinal chemistry and describe the features of the activators and inhibitors identified so far.

Targeted inhibition of SIRT6 via engineered exosomes impairs tumorigenesis and metastasis in prostate cancer

The treatment for metastatic castration-resistant prostate cancer patients remains a great challenge in the clinic and continuously demands discoveries of new targets and therapies. Here, we assess the function and therapeutic value of SIRT6 in metastatic castration-resistant prostate cancer. Methods: The expression of SIRT6 was examined in prostate cancer tissue microarray by immunohistochemistry staining. The functions of SIRT6 and underlying mechanisms were elucidated by in vitro and in vivo experiments. We also developed an efficient method to silence SIRT6 by aptamer-modified exosomes carrying small interfering RNA and tested the therapeutic effect in the xenograft mice models. Results: SIRT6 expression is positively correlated with prostate cancer progression. Loss of SIRT6 significantly suppressed proliferation and metastasis of prostate cancer cell lines both in vitro and in vivo. SIRT6-driven prostate cancer displays activation of multiple cancer-related signaling pathways, especially the Notch pathway. Silencing SIRT6 by siRNA delivered through engineered exosomes inhibited tumor growth and metastasis. Conclusions: SIRT6 is identified as a driver and therapeutic target for metastatic prostate cancer in our findings, and inhibition of SIRT6 by engineered exosomes can serve as a promising therapeutic tool for clinical application.

The Two-Faced Role of SIRT6 in Cancer

Simple Summary

Cancer therapy relies on the employment of different strategies aimed at inducing cancer cell death through different mechanisms, including DNA damage and apoptosis induction. One of the key regulators of these pathways is the epigenetic enzyme SIRT6, which has been shown to have a dichotomous function in cell fate determination and, consequently, cancer initiation and progression. In this review, we aim to summarize the current knowledge on the role of SIRT6 in cancer. We show that it can act as both tumor suppressor and promoter, even in the same cancer type, depending on the biological context. We then describe the most promising modulators of SIRT6 which, through enzyme activation or inhibition, may impair tumor growth. These molecules can also be used for the elucidation of SIRT6 function, thereby advancing the current knowledge on this crucial protein.

Abstract

Sirtuin 6 (SIRT6) is a NAD⁺-dependent nuclear deacylase and mono-ADP-ribosylase with a wide spectrum of substrates. Through its pleiotropic activities, SIRT6 modulates either directly or indirectly key processes linked to cell fate determination and oncogenesis such as DNA damage repair, metabolic homeostasis, and apoptosis. SIRT6 regulates the expression and activity of both pro-apoptotic (e.g., Bax) and anti-apoptotic factors (e.g., Bcl-2, survivin) in a context-depending manner. Mounting evidence points towards a double-faced involvement of SIRT6 in tumor onset and progression since the block or induction of apoptosis lead to opposite outcomes in cancer. Here, we discuss the features and roles of SIRT6 in the regulation of cell death and cancer, also focusing on recently discovered small molecule modulators that can be used as chemical probes to shed further light on SIRT6 cancer biology and proposed as potential new generation anticancer therapeutics.

Emerging roles of SIRT6 in human diseases and its modulators

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

Sirtuin 6 regulates the proliferation and survival of clear cell renal cell carcinoma cells via B-cell lymphoma 2

Sirtuin 6 (SIRT6) is a member of the third family of longevity proteins (SIRTs) that is involved in the development of different types of cancer. However, the potential role of SIRT6 in clear cell renal cell carcinoma (ccRCC) and its molecular mechanism have not yet been fully elucidated. Therefore, the present study aimed to investigate the association between SIRT6 and ccRCC, and to further examine the underlying mechanism of its effect on ccRCC proliferation, using bioinformatics analysis, and in vitro and in vivo experiments. The results of the present study demonstrated that SIRT6 was upregulated in ccRCC tissues. In addition, bioinformatics analysis revealed that high SIRT6 expression was closely associated with poor prognosis of patients with ccRCC. In vitro experiments demonstrated that silencing SIRT6 expression in ccRCC-derived 769-P and 786-O cells significantly inhibited their proliferation, migration and invasion. Consistent with these results, in vivo assays demonstrated that SIRT6 knockdown markedly attenuated tumor growth arising from 769-P cells. Furthermore, depletion of SIRT6 enhanced the sensitivity of ccRCC cells to cisplatin. Notably, silencing SIRT6 expression decreased B-cell lymphoma 2 (Bcl-2) expression and increased Bax expression, respectively. Taken together, these results suggest that SIRT6 acts as a proto-oncogene in ccRCC through the augmentation of the Bcl-2-dependent pro-survival pathway, and may be used as a therapeutic target for patients with ccRCC.

Post-Translational Modifications That Drive Prostate Cancer Progression

While a protein primary structure is determined by genetic code, its specific functional form is mostly achieved in a dynamic interplay that includes actions of many enzymes involved in post-translational modifications. This versatile repertoire is widely used by cells to direct their response to external stimuli, regulate transcription and protein localization and to keep proteostasis. Herein, post-translational modifications with evident potency to drive prostate cancer are explored. A comprehensive list of proteome-wide and single protein post-translational modifications and their involvement in phenotypic outcomes is presented. Specifically, the data on phosphorylation, glycosylation, ubiquitination, SUMOylation, acetylation, and lipidation in prostate cancer and the enzymes involved are collected. This type of knowledge is especially valuable in cases when cancer cells do not differ in the expression or mutational status of a protein, but its differential activity is regulated on the level of post-translational modifications. Since their driving roles in prostate cancer, post-translational modifications are widely studied in attempts to advance prostate cancer treatment. Current strategies that exploit the potential of post-translational modifications in prostate cancer therapy are presented.

Sirtuins as Important Factors in Pathological States and the Role of Their Molecular Activity Modulators

Sirtuins (SIRTs), enzymes from the family of NAD⁺-dependent histone deacetylases, play an important role in the functioning of the body at the cellular level and participate in many biochemical processes. The multi-directionality of SIRTs encourages scientists to undertake research aimed at understanding the mechanisms of their action and the influence that SIRTs have on the organism. At the same time, new substances are constantly being sought that can modulate the action of SIRTs. Extensive research on the expression of SIRTs in various pathological conditions suggests that regulation of their activity may have positive results in supporting the treatment of certain metabolic, neurodegenerative or cancer diseases or this connected with oxidative stress. Due to such a wide spectrum of activity, SIRTs may also be a prognostic markers of selected pathological conditions and prove helpful in assessing their progression, especially by modulating their activity. The article presents and discusses the activating or inhibiting impact of individual SIRTs modulators. The review also gathered selected currently available information on the expression of SIRTs in individual disease cases as well as the biological role that SIRTs play in the human organism, also in connection with oxidative stress condition, taking into account the progress of knowledge about SIRTs over the years, with particular reference to the latest research results.

SIRT6 inhibits metastasis by suppressing SNAIL expression in nasopharyngeal carcinoma cells

Nasopharyngeal carcinoma (NPC) is a head and neck cancer with severe local invasion and early distant metastasis. SIRT6 serves as a critical modulator of the development and metastasis of multiple types of cancer; however, the roles and underlying mechanisms of SIRT6 in regulating NPC metastasis remain largely unknown. Here, the expression of SIRT6 in high metastatic 5-8F cells and low metastatic 6-10B cells was analyzed. SIRT6 expression was found to be negatively associated with the metastatic capability of NPC cells. Moreover, we identified that SIRT6 inhibited NPC cell metastasis through suppression of SNAIL expression. Mechanistically, we demonstrated that SIRT6 interacted with transcription factor p65 (NF-kB subunit) and deacetylated histone H3 lysine 9 (H3K9) and lysine 56 (H3K56) at the promoter of SNAIL, leading to reduced transcription of SNAIL. In summary, SIRT6 functions as a metastasis suppressor in NPC cells through epigenetic regulation of SNAIL gene expression.

MDL-800, an allosteric activator of SIRT6, suppresses proliferation and enhances EGFR-TKIs therapy in non-small cell lung cancer

Sirtuin 6 (SIRT6), a member of the sirtuin family, is a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase that is involved in various physiological and pathological processes. SIRT6 is generally downregulated and linked to tumorigenesis in non-small cell lung carcinoma (NSCLC), thus regarded as a promising therapeutic target of NSCLC. In this study, we investigated whether MDL-800, an allosteric activator of SIRT6, exerted antiproliferation effect against NSCLC cells in vitro and in vivo. We showed that MDL-800 increased SIRT6 deacetylase activity with an EC50 value of 11.0 ± 0.3 μM; MDL-800 (10-50 μM) induced dose-dependent deacetylation of histone H3 in 12 NSCLC cell lines. Treatment with MDL-800 dose dependently inhibited the proliferation of 12 NSCLC cell lines with IC50 values ranging from 21.5 to 34.5 μM. The antiproliferation effect of MDL-800 was significantly diminished by SIRT6 knockout. Treatment with MDL-800 induced remarkable cell cycle arrest at the G0/G1 phase in NSCLC HCC827 and PC9 cells. Furthermore, MDL-800 (25, 50 μM) enhanced the antiproliferation of epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) in osimertinib-resistant HCC827 and PC9 cells as well as in patient-derived primary tumor cells, and suppressed mitogen-activated protein kinase (MAPK) pathway. In HCC827 cell-derived xenograft nude mice, intraperitoneal administration of MDL-800 (80 mg · kg-1 · d-1, for 14 days) markedly suppressed the tumor growth, accompanied by enhanced SIRT6-dependent histone H3 deacetylation and decreased p-MEK and p-ERK in tumor tissues. Our results provide the pharmacological evidence for future clinical investigation of MDL-800 as a promising lead compound for NSCLC treatment alone or in combination with EGFR-TKIs.

The SIRT3 and SIRT6 Promote Prostate Cancer Progression by Inhibiting Necroptosis-Mediated Innate Immune Response

The sirtuins (SIRTs), including seven family members, belong to class III histone deacetylase (HDAC) enzymes, which have been intensively investigated in cancers. Although the function of SIRTs in the cancer immunology is explored, SIRT-specific mechanisms regulating necroptosis-related innate immune response are not clear. In our present study, we found that both the mRNA and protein expression levels of SIRT3 and SIRT6 are significantly increased in the PCa tissues (HR, CI P = 3.30E - 03; HR, CI P = 2.35E - 08; and HR, CI P = 9.20E - 08) and were associated with patients' Gleason score and nodal metastasis. Furthermore, multivariate analysis showed that the PCa patients with higher expression levels of SIRT3 and SIRT6 had shorter overall survival (OS). Mechanistically, we found that SIRT3 and SIRT6 promote prostate cancer progress by inhibiting RIPK3-mediated necroptosis and innate immune response. Knockdown of both SIRT3 and SIRT6 not only activates TNF-induced necroptosis but also refreshes the corresponding recruitment of macrophages and neutrophils. Overall, our study identified that SIRT3 and SIRT6 are key regulators of necroptosis during prostate cancer progression.

How Dysregulated Ion Channels and Transporters Take a Hand in Esophageal, Liver, and Colorectal Cancer

Over the last two decades, the understanding of how dysregulated ion channels and transporters are involved in carcinogenesis and tumor growth and progression, including invasiveness and metastasis, has been increasing exponentially. The present review specifies virtually all ion channels and transporters whose faulty expression or regulation contributes to esophageal, hepatocellular, and colorectal cancer. The variety reaches from Ca²⁺, K⁺, Na⁺, and Cl^- channels over divalent metal transporters, Na⁺ or Cl^- coupled Ca²⁺, HCO₃^- and H⁺ exchangers to monocarboxylate carriers and organic anion and cation transporters. In several cases, the underlying mechanisms by which these ion channels/transporters are interwoven with malignancies have been fully or at least partially unveiled. Ca²⁺, Akt/NF-κB, and Ca²⁺- or pH-dependent Wnt/β-catenin signaling emerge as cross points through which ion channels/transporters interfere with gene expression, modulate cell proliferation, trigger epithelial-to-mesenchymal transition, and promote cell motility and metastasis. Also miRs, lncRNAs, and DNA methylation represent potential links between the misexpression of genes encoding for ion channels/transporters, their malfunctioning, and cancer. The knowledge of all these molecular interactions has provided the basis for therapeutic strategies and approaches, some of which will be broached in this review.

Sirt6 inhibition delays the onset of experimental autoimmune encephalomyelitis by reducing dendritic cell migration

Background

Experimental autoimmune encephalomyelitis (EAE) is the most common animal model of multiple sclerosis (MS), a neuroinflammatory and demyelinating disease characterized by multifocal perivascular infiltrates of immune cells. Although EAE is predominantly considered a T helper 1-driven autoimmune disease, mounting evidence suggests that activated dendritic cells (DC), which are the bridge between innate and adaptive immunity, also contribute to its pathogenesis. Sirtuin 6 (SIRT6), a NAD⁺-dependent deacetylase involved in genome maintenance and in metabolic homeostasis, regulates DC activation, and its pharmacological inhibition could, therefore, play a role in EAE development.

Methods

EAE was induced in female C57bl/6 mice by MOG35-55 injection. The effect of treatment with a small compound SIRT6 inhibitor, administered according to therapeutic and preventive protocols, was assessed by evaluating the clinical EAE score. SIRT6 inhibition was confirmed by Western blot analysis by assessing the acetylation of histone 3 lysine 9, a known SIRT6 substrate. The expression of DC activation and migration markers was evaluated by FACS in mouse lymph nodes. In addition, the expression of inflammatory and anti-inflammatory cytokines in the spinal cord were assessed by qPCR. T cell infiltration in spinal cords was evaluated by immunofluorescence imaging. The effect of Sirt6 inhibition on the migration of resting and activated bone marrow-derived dendritic cells was investigated in in vitro chemotaxis assays.

Results

Preventive pharmacological Sirt6 inhibition effectively delayed EAE disease onset through a novel regulatory mechanism, i.e., by reducing the representation of CXCR4-positive and of CXCR4/CCR7-double-positive DC in lymph nodes. The delay in EAE onset correlated with the early downregulation in the expression of CD40 on activated lymph node DC, with increased level of the anti-inflammatory cytokine IL-10, and with a reduced encephalitogenic T cell infiltration in the central nervous system. Consistent with the in vivo data, in vitro pharmacological Sirt6 inhibition in LPS-stimulated, bone marrow-derived DC reduced CCL19/CCL21- and SDF-1-induced DC migration.

Conclusions

Our findings indicate the ability of Sirt6 inhibition to impair DC migration, to downregulate pathogenic T cell inflammatory responses and to delay EAE onset. Therefore, Sirt6 might represent a valuable target for developing novel therapeutic agents for the treatment of early stages of MS, or of other autoimmune disorders.

Sirt6 promotes tumorigenesis and drug resistance of diffuse large B-cell lymphoma by mediating PI3K/Akt signaling

Background

Sirtuin 6 (Sirt6) is a highly conserved ADP-ribosylase and NAD+ dependent deacylase, involved in broad cellular processes. This molecule possesses contradictory roles in carcinogenesis, as it has been documented to both suppressing and augmenting tumor growth. This project aimed to explore the expression and functions of Sirt6 in diffuse large B-cell lymphoma (DLBCL), especially with regards to the regulatory role of OSS_128167, a novel small molecular inhibitor targeting Sirt6.

Methods

Immunohistochemistry (IHC) was conducted to assess the expression of Sirt6 on paraffin-embedded tissues. Microarray dataset GSE32918 and GSE83632 were obtained from Gene Expression Omnibus and survival analysis was performed. Lentivirus vectors either encoding shSirt6, lvSirt6 or empty lentiviral vector were stably transfected into DLBCL cells. LY1 cell transfected with shSirt6 were performed RNA-sequencing (RNA-seq) analysis, functional enrichment analyses of gene ontology (GO) and gene set enrichment analysis (GSEA). DLBCL cells were subcutaneously injected to SCID beige mice to establish xenograft models.

Results

Sirt6 is found to be overexpressed in DLBCL, and is related to poor prognosis. Sirt6-deprived DLBCL cells displayed augmented sensitivity towards chemotherapy, higher rates of apoptosis, dysfunctional cell proliferation, and arrested cell cycle progression between the G2 and M phases. Selective OSS_128167-mediated Sirt6 blockage resulted in similar anti-lymphoma effects when compared to Sirt6 knocked-down DLBCL cells. PI3K signaling along with phosphorylation of its downstream targets was reduced upon Sirt6 downregulation. Xenograft models subjected to either OSS_128167 treatment or Sirt6-knockdown showed suppressed tumor growth and lower Ki-67 level.

Conclusions

These findings provide mechanistic insights into the oncogenic activity of Sirt6 in DLBCL for the first time and highlighted the potency of OSS_128167 for novel therapeutic strategies in DLBCL.

Sirtuins and their Biological Relevance in Aging and Age-Related Diseases

Sirtuins, initially described as histone deacetylases and gene silencers in yeast, are now known to have many more functions and to be much more abundant in living organisms. The increasing evidence of sirtuins in the field of ageing and age-related diseases indicates that they may provide novel targets for treating diseases associated with aging and perhaps extend human lifespan. Here, we summarize some of the recent discoveries in sirtuin biology that clearly implicate the functions of sirtuins in the regulation of aging and age-related diseases. Furthermore, human sirtuins are considered promising therapeutic targets for anti-aging and ageing-related diseases and have attracted interest in scientific communities to develop small molecule activators or drugs to ameliorate a wide range of ageing disorders. In this review, we also summarize the discovery and development status of sirtuin-targeted drug and further discuss the potential medical strategies of sirtuins in delaying aging and treating age-related diseases.

LncRNA HMMR-AS1 promotes proliferation and metastasis of lung adenocarcinoma by regulating MiR-138/sirt6 axis

Purpose: Long noncoding RNAs (lncRNA) play critical roles in cancer development. In this study, we aimed to explore the function and possible molecular mechanism of HMMR-AS1 involved in lung adenocarcinoma (LUAD).

Experimental Design: Firstly, we analyzed HMMR-AS1 expression in LUAD tissues with the sequencing data from The Cancer Genome Atlas (TCGA). Next, we evaluated the effects of HMMR-AS1 on LUAD cell proliferation and apoptosis, and its regulation of miR-138 by acting as a ceRNA. The animal model was used to support the in vitro experimental findings.

Results: HMMR-AS1 expression was significantly upregulated in LUAD tissues and was associated with larger tumor diameter, advanced TNM stage, lymph node metastasis, and shorter survival. Knockdown of HMMR-AS1 induced apoptosis and growth arrest in vitro and inhibited tumorigenesis in mouse xenografts. Mechanistically, HMMR-AS1 functioned as a ceRNA of miR-138, thereby leading to repression of its endogenous target sirt6. Moreover, knockdown of HMMR-AS1 dramatically inhibited tumor growth and metastasis of LUAD in vivo.

Conclusions: Taken together, HMMR-AS1 is significantly over-expressed in LUAD, and HMMR-AS1–miR-138–sirt6 axis play a critical role in LUAD tumorigenesis. Our findings highlight an oncogenic role of HMMR-AS1 in LUAD.

Biological and catalytic functions of sirtuin 6 as targets for small-molecule modulators

Sirtuin 6 (SIRT6) is a nuclear NAD⁺-dependent deacetylase of histone H3 that regulates genome stability and gene expression. However, nonhistone substrates and additional catalytic activities of SIRT6, including long-chain deacylation and mono-ADP-ribosylation of other proteins, have also been reported, but many of these noncanonical roles remain enigmatic. Genetic studies have revealed critical homeostatic cellular functions of SIRT6, underscoring the need to better understand which catalytic functions and molecular pathways are driving SIRT6-associated phenotypes. At the physiological level, SIRT6 activity promotes increased longevity by regulating metabolism and DNA repair. Recent work has identified natural products and synthetic small molecules capable of activating the inefficient in vitro deacetylase activity of SIRT6. Here, we discuss the cellular functions of SIRT6 with a focus on attributing its catalytic activity to its proposed biological functions. We cover the molecular architecture and catalytic mechanisms that distinguish SIRT6 from other NAD⁺-dependent deacylases. We propose that combining specific SIRT6 amino acid substitutions identified in enzymology studies and activity-selective compounds could help delineate SIRT6 functions in specific biological contexts and resolve the apparently conflicting roles of SIRT6 in processes such as tumor development. We further highlight the recent development of small-molecule modulators that provide additional biological insight into SIRT6 functions and offer therapeutic approaches to manage metabolic and age-associated diseases.

Sodium Tanshinone IIA Sulfonate Protects Against Cerebral Ischemia-reperfusion Injury by Inhibiting Autophagy and Inflammation

Sodium tanshinone IIA sulfonate (STS) can protect against brain damage induced by stroke. However, the neural protection mechanism of STS remains unclear. We investigated whether STS performs its protective function by suppressing autophagy and inflammatory activity during brain injury. We established a transient middle cerebral artery occlusion and reperfusion (MCAO/R) model by blocking the left middle cerebral artery with a thread inserted through the internal carotid artery for 1 h, followed by reperfusion for 48 h either with or without STS and the autophagy inhibitor 3-methyladenine (3-MA). Neuroprotective effects were determined by evaluating infarction, brain edema, and neurological deficits. The numbers of microglia-derived macrophages, monocyte-derived microglia, T cells, and B cells in the brains were measured, based on the surface marker analyses of CD45, CD11b, B220, CD3, and CD4 using fluorescence-assisted cell sorting. STS (10, 20, 40 mg/kg) was able to significantly reduce infarct volumes, improve neurological deficits, and reduce brain water contents. STS treatment reduced neuroinflammation, as assessed by the infiltration of macrophages and neutrophils, corresponding with reduced numbers of macrophages, T cells, and B cells in ischemia/reperfusion (I/R) brains. In addition, STS treatment also attenuated the upregulation of autophagy associated proteins, such as LC3-II, Beclin-1 and Sirt 6, which was induced by MCAO. These results demonstrated that STS can provide remarkable protection against ischemic stroke, possibly via the inhibition of autophagy and inflammatory activity.

NAD metabolism in aging and cancer

IMPACT STATEMENT

NAD is a central metabolite connecting energy balance and organismal growth with genomic integrity and function. It is involved in the development of malignancy and has a regulatory role in the aging process. These processes are mediated by a diverse series of enzymes whose common focus is either NAD's biosynthesis or its utilization as a redox cofactor or enzyme substrate. These enzymes include dehydrogenases, cyclic ADP-ribose hydrolases, mono(ADP-ribosyl)transferases, poly(ADP-ribose) polymerases, and sirtuin deacetylases. This article describes the manifold pathways that comprise NAD metabolism and promotes an increased awareness of how perturbations in these systems may be important in disease prevention and/or progression.

Small-molecule activating SIRT6 elicits therapeutic effects and synergistically promotes anti-tumor activity of vitamin D3 in colorectal cancer

Colorectal cancer (CRC) is the leading cause of cancer death; however, targets with broad anti-CRC effects are limited. Sirtuin6 (SIRT6) is a conserved nicotinamide adenine dinucleotide (NAD⁺)-dependent deacetylase that is widely pathologically downregulated in CRC, but its pharmacological effect in CRC remains undefined due to the lack of small-molecule SIRT6 activators. We searched for a compound activating SIRT6 and investigated its anti-CRC effect in various models. Methods: We identified an allosteric SIRT6 activator, MDL-811. Its ability to enhance SIRT6 deacetylation at protein and cellular levels was evaluated by Fluor de Lys (FDL) and western blots. We assessed the proliferation of 26 CRC cell lines and patient-derived organoids (PDOs) treated with MDL-811. In vivo efficacy of MDL-811 was evaluated in HCT116 cell line- and patient-derived xenografts as well as a spontaneous CRC model. RNA sequencing and real-time quantitative PCR assays were performed to analyze gene expression changes in MDL-811-treated HCT116 cells. Along with controls in SIRT6-overexpressing HCT116 cells, the SIRT6-mediated histone H3 deacetylation at the Cytochrome P450 family 24 subfamily A member 1 (CYP24A1) gene locus was assessed by chromatin immunoprecipitation (ChIP) in MDL-811-treated HCT116 cells. A combination therapy against CRC based on the downstream gene of SIRT6 activation was evaluated in cells and mouse models. Results: MDL-811 significantly activated SIRT6 histone H3 deacetylation (H3K9Ac, H3K18Ac, and H3K56Ac) in vitro and had broad antiproliferative effects on diverse CRC cell lines and PDOs. More importantly, the in vivo anti-tumor efficacy of MDL-811 was demonstrated across cell line- and patient-derived xenografts and in the APC^min/+ spontaneous CRC model. Mechanically, we identified a new downstream target gene of SIRT6 in CRC, CYP24A1. Based on these findings, a combination drug strategy with MDL-811 to synergistically enhance the anti-CRC effect of vitamin D₃ was validated in vitro and in vivo. Conclusions: Our data provide proof of concept that targeting SIRT6 using a small-molecule activator is an attractive therapeutic strategy for CRC and that MDL-811 could be a promising lead compound for further preclinical and clinical studies of treatments for CRC.

SIRT6, a Mammalian Deacylase with Multitasking Abilities

Mammalian sirtuins have emerged in recent years as critical modulators of multiple biological processes, regulating cellular metabolism, DNA repair, gene expression, and mitochondrial biology. As such, they evolved to play key roles in organismal homeostasis, and defects in these proteins have been linked to a plethora of diseases, including cancer, neurodegeneration, and aging. In this review, we describe the multiple roles of SIRT6, a chromatin deacylase with unique and important functions in maintaining cellular homeostasis. We attempt to provide a framework for such different functions, for the ability of SIRT6 to interconnect chromatin dynamics with metabolism and DNA repair, and the open questions the field will face in the future, particularly in the context of putative therapeutic opportunities.

The sirtuin 6: An overture in skin cancer

In the recent past, the sirtuins have been under intense investigation for their roles in biology and disease, including cancer. The sirtuin SIRT6 is comparatively a lesser studied member of this family of seven proteins. Like certain other sirtuins, SIRT6 is emerging to have an oncogenic function as well as tumor suppressor roles in cancer. Limited studies have been conducted assessing the role and functional significance of SIRT6 in melanoma and non-melanoma skin cancers. In this review, we have attempted to critically dissect the potential role and significance of SIRT6 in skin carcinogenesis. With limited available information to date, SIRT6 appears to have a pro-proliferative function in non-melanoma skin cancers (NMSCs), including squamous cell carcinoma (SCC) and basal cell carcinoma (BCC). In addition, SIRT6 is also emerging to have an oncogenic function in melanoma. Moreover, we have provided information regarding the available SIRT6 inhibitors. Conclusively, it appears that additional comprehensive studies are needed to establish the role of SIRT6 in skin biology and skin diseases, including cancer. Further, concerted efforts are needed to characterize the stage-specific role of SIRT6 in skin cancers.

Targeting epigenetic regulators for cancer therapy: mechanisms and advances in clinical trials

Epigenetic alternations concern heritable yet reversible changes in histone or DNA modifications that regulate gene activity beyond the underlying sequence. Epigenetic dysregulation is often linked to human disease, notably cancer. With the development of various drugs targeting epigenetic regulators, epigenetic-targeted therapy has been applied in the treatment of hematological malignancies and has exhibited viable therapeutic potential for solid tumors in preclinical and clinical trials. In this review, we summarize the aberrant functions of enzymes in DNA methylation, histone acetylation and histone methylation during tumor progression and highlight the development of inhibitors of or drugs targeted at epigenetic enzymes.

Inhibition of Sirt6 suppresses tumor growth by inducing G1/S phase arrest in renal cancer cells

Sirt6 is a vital member of the Sirtuin family that plays a key role in cellular apoptosis, aging, DNA damage repair, telomere homeostasis and integrity, energy metabolism, glucose homeostasis, and gene regulation. In recent studies, Sirt6 is down-regulated in several cancers and predicted to be a tumor suppressor, but as a tumor oncogene in other cancers. In this study, we explored the specific role of Sirt6 in human renal cell carcinoma (RCC). We found that Sirt6 was up-expressed in renal tumor tissues and cells. Sirt6 silence in RCC led to G1/S phase arrest, a rise in apoptosis and a decrease in cell viability, as well as an enhancement of chemotherapeutic sensitivity. In conclusion, these findings suggest that Sirt6 acted as an oncogene in human RCC and it could be a potential target for RCC treatment.

SIRT6 regulates the proliferation and apoptosis of hepatocellular carcinoma via the ERK1/2 signaling pathway

Hepatocellular carcinoma (HCC) is the most common type of liver cancer, and exhibits a high mortality rate. Sirtuin (SIRT)6 is a member of the sirtuin family, which may be useful targets in the treatment of tumors. The present study aimed to explore the expression of SIRT6 in numerous HCC cell lines and investigate the role of SIRT6 in the proliferation and apoptosis of the HCC cells, and the underlying mechanisms. Overexpression and silencing of SIRT6 were performed by transfection of Huh‑7 cells with synthetic overexpression and small interfering RNA (siRNA) plasmids. Cell proliferation was evaluated using a Cell Counting Kit‑8 assay. The apoptosis rate was measured via flow cytometry. Cloning efficiency was assessed using plate clone formation assays. The expression of mRNAs and proteins were determined via reverse transcription‑quantitative PCR and western blot analyses, respectively. SIRT6 was overexpressed in Hep3B, Huh‑7, MHCC‑97H, MHCC‑97L, MHCC‑LM6, MHCC‑LM3, YY‑8103 and SK‑hep‑1 cell lines, compared with MIHA and HL‑7702 normal liver cell lines. Overexpression of SIRT6 increased the proliferation of Huh‑7 cells, upregulated the expression of Bcl‑2 and phosphorylation of extracellular‑signal regulated protein kinase (ERK), and decreased the expression of cleaved‑caspase‑3 and Bcl‑2‑associated X protein (Bax) in Huh‑7 cells. siRNA‑mediated silencing of SIRT6 decreased the proliferation and increased the apoptosis of Huh‑7 cells, downregulated the expression of Bcl‑2 and phosphorylated‑ERK, and promoted the expression of cleaved‑caspase‑3 and Bax. The proliferation of Huh‑7 cells was decreased using the ERK1/2 inhibitor U0126. The results suggested that SIRT6 affected the proliferation and apoptosis of HCC cells via the regulation of the ERK1/2 pathway, altering the activation of the intrinsic apoptosis pathway. SIRT6 may be a potential target for the treatment of HCC; however, its role requires further investigation.

SIRT6, a novel direct transcriptional target of FoxO3a, mediates colon cancer therapy

SIRT6, NAD+-dependent deacetylase sirtuin 6, has recently shown to suppress tumor growth in several types of cancer. Colon cancer is a challenging carcinoma associated with high morbidity and death. However, whether SIRT6 play a direct role in colon tumorigenesis and the underlying mechanism are not understood.

Methods: To investigate the role of SIRT6 in colon cancer, we firstly analyzed the specimens from 50 colorectal cancer (CRC) patients. We generated shSIRT6 LoVo cells and xenograft mouse to reveal the essential role of SIRT6 in cell apoptosis and tumor growth. To explore the underlying mechanism of SIRT6 regulation, we performed FRET and real-time fluorescence imaging in living cells, real-time PCR, immunoprecipitaion, immunohistochemistry, flow cytometry and luciferase reporter assay.

Results: The expression level of SIRT6 in patients' specimens is lower than that of normal controls, and patients with higher SIRT6 level have a better prognosis. Here, we identified that transcriptional factor FoxO3a is a direct up-stream of SIRT6 and positively regulated SIRT6 expression, which in turn, promotes apoptosis by activating Bax and mitochondrial pathway. Functional studies reveal that Akt inactivation increases FoxO3a activity and augment its binding to SIRT6 promoter, leading to elevated SIRT6 expression. Knocking down SIRT6 abolished apoptotic responses and conferred resistance to the treatment of BKM120. Combinational therapies with conventional drugs showed synergistic chemosensitization, which was SIRT6-dependent both in vitro and in vivo.

Conclusion: The results uncover SIRT6 as a new potential biomarker for colon cancer, and its unappreciated mechanism about transcription and expression via Akt/FoxO3a pathway.

SIRT6 overexpression induces apoptosis of nasopharyngeal carcinoma by inhibiting NF-κB signaling

BACKGROUND

Previous reports show that SIRT6 serves as a critical modulator of the development of multiple malignancies as well as other disorders. However, its role in nasopharyngeal carcinoma (NPC) is unknown. Thus, we elucidated the effects of SIRT6 on the survival of NPC cells, and modulation of cell death.

METHODS

We found that expression of SIRT6 is downregulated in ten human NPC specimens as well as in the human NPC cell lines, 5-8 F and CNE1, as compared with that in healthy tissues and normal nasopharyngeal NP69 cells. The MTT assay and colony formation assay revealed that upregulation of SIRT6 impaired the proliferation, as well as the survival of 5-8 F and CNE1 cells. The TUNEL assay, annexin V-FITC/propidium iodide, and flow cytometry were performed to detect apoptosis. The results revealed that the expression of SIRT6 resulted in increased apoptosis.

RESULTS

Western blotting results showed that SIRT6 overexpression decreased anti-apoptotic Bcl-2 levels, whereas it promoted an increase in pro-apoptotic Bax and cleaved caspase-3 levels. Moreover, NF-κB levels were markedly reduced in cells expressing SIRT6, whereas they were increased in cells transfected with shRNA-SIRT6. Recovery of NF-κB expression was found to counter the suppressive influence of SIRT6 on NPC cell survival, whereas, NF-κB knockdown increased apoptosis of NPC cells.

CONCLUSION

Thus, the findings of our study offer insight into the biological and molecular mechanisms underlying the development of NPC and may lead to the development of new and innovative strategies for the treatment of NPC.

The clinical significance of PINX1 expression in papillary thyroid carcinoma

PIN2/TERF1 interacting telomerase inhibitor 1 (PINX1) is a telomerase inhibitor located on human chromosome 8p23 and also acts as a tumor suppressor in several types of cancers, including breast, gastric, ovarian, and bladder cancer. However, the role of PINX1 expression in papillary thyroid carcinoma (PTC) has not been defined. Therefore, we investigated the role of PINX1 expression in PTC by analyzing the correlation between PINX1 expression and various clinicopathological factors. Immunohistochemistry for PINX1 was performed using a tissue microarray of samples taken from the 160 patients with PTC. We also assessed mRNA and protein expression for PINX1 via quantitative real-time polymerase chain reaction and immunohistochemical analysis. Positive staining for PINX1 was found in 16.3% of PTC cases. PINX1 expression was significantly associated with tumor size, lymph node metastasis, telomerase reverse transcriptase, promoter mutation and recurrence. PINX1 mRNA expression was more pronounced in the recurrent group than in the nonrecurrent group. In addition, results of the binary logistic regression model showed that PINX1 protein expression had a significant influence on recurrence. We concluded that PINX1 expression was associated with several clinicopathological factors and had a significant influence on recurrence in patients with PTC. Therefore, PINX1 expression could be a useful prognostic marker in PTC patients.