SIRT6 promotes COX-2 expression and acts as an oncogene in skin cancer

Anti-aging Strategies and Topical Delivery of Biopolymer-based Nanocarriers for Skin Cancer Treatment

Environmental factors like UV radiation and epigenetic changes are significant factors for skin cancer that trigger early aging. This review provides essential information on cancer development concerning aging, the receptors involved, and the therapeutic targets. Biopolymers like polysaccharide, polyphenols, proteins, and nucleic acid plays a vital role in the regulation of normal cell homeostasis. Therefore, it is pertinent to explore the role of biopolymers as antiaging formulations and the possibility of these formulations being used against cancer via topical administrations. As UV radiation is one of the predominant factors in causing skin cancer, the association of receptors between aging and cancer indicated that insulin receptor, melatonin receptor, toll-like receptor, SIRT 1 receptor, tumor-specific T cell receptor and mitochondria-based targeting could be used to direct therapeutics for suppression of cancer and prevent aging. Biopolymer-based nanoformulations have tremendously progressed by entrapment of drugs like curcumin and resveratrol which can prevent cancer and aging simultaneously. Certain protein signaling or calcium and ROS signaling pathways are different for cancer and aging. The involvement of mitochondrial DNA mutation along with telomere shortening with a change in cellular energetics leading to genomic instability in the aging process can also induce mitochondrial dysfunction and epigenetic alterations leading to skin cancer. Therefore, the use of biopolymers as a topical supplement during the aging process can result in the prevention of cancer.

The Role of Sirtuin 6 in the Deacetylation of Histone Proteins as a Factor in the Progression of Neoplastic Disease

SIRT6 is a nicotinamide adenine dinucleotide (NAD+)-dependent deacetylase, predominantly located in the nucleus, that is involved in the processes of histone modification, DNA repair, cell cycle regulation, and apoptosis. Disturbances in SIRT6 expression levels have been observed in the development and progression of various types of cancer. Therefore, it is important to better understand the role of SIRT6 in biochemical pathways and assign it specific biological functions. This review aims to summarize the role of SIRT6 in carcinogenesis and tumor development. A better understanding of the factors influencing SIRT6 expression and its biological role in carcinogenesis may help to develop novel anti-cancer therapeutic strategies. Moreover, we discuss the anti-cancer effects and mechanism of action of small molecule SIRT6 modulators (both activators and inhibitors) in different types of cancer.

Epithelial SIRT6 governs IL-17A pathogenicity and drives allergic airway inflammation and remodeling

Dysregulation of IL-17A is closely associated with airway inflammation and remodeling in severe asthma. However, the molecular mechanisms by which IL-17A is regulated remain unclear. Here we identify epithelial sirtuin 6 (SIRT6) as an epigenetic regulator that governs IL-17A pathogenicity in severe asthma. Mice with airway epithelial cell-specific deletion of Sirt6 are protected against allergen-induced airway inflammation and remodeling via inhibiting IL-17A-mediated inflammatory chemokines and mesenchymal reprogramming. Mechanistically, SIRT6 directly interacts with RORγt and mediates RORγt deacetylation at lysine 192 via its PPXY motifs. SIRT6 promotes RORγt recruitment to the IL-17A gene promoter and enhances its transcription. In severe asthma patients, high expression of SIRT6 positively correlates with airway remodeling and disease severity. SIRT6 inhibitor (OSS_128167) treatment significantly attenuates airway inflammation and remodeling in mice. Collectively, these results uncover a function for SIRT6 in regulating IL-17A pathogenicity in severe asthma, implicating SIRT6 as a potential therapeutic target for severe asthma.

SIRT6 pharmacological inhibition delays skin cancer progression in the squamous cell carcinoma

Sirtuin 6 (SIRT6) has a critical role in cutaneous Squamous Cell Carcinoma (cSCC): SIRT6 silencing in skin SCC cells has pro-differentiating effects and SIRT6 deletion abrogated DMBA-TPA-induced skin tumorigenesis in mice. On the other hand, SIRT6 acts as tumor suppressor in SCC by enhancing glycolysis in tumor propagating cells. Herein, pharmacological modulation of SIRT6 deacetylase activity was investigated in cSCC, with S6 (inhibitor) or MDL-800 (activator). In cSCC cells, S6 recreated the pro-differentiating effects of SIRT6 silencing, as the levels of Keratin 1, Keratin 10 and Loricrin were upregulated compared to controls. Next, the effects of SIRT6 pharmacological modulation were evaluated in a DMBA-TPA-induced skin cancer mouse model. Mice treated with the inhibitor S6 in a preventive approach, i.e. at the beginning of the promotion stage, presented reduced number and size of papillomas, compared to the controls. The epidermal hyperproliferation marker Keratin 6 and the cSCC marker Keratin 8 were less abundant when SIRT6 was inhibited. In S6-treated lesions, the Epithelial-Mesenchymal Transition (EMT) markers Zeb1 and Vimentin were less expressed compared to untreated lesions. In a therapeutic approach, i.e. treatment starting after papilloma appearance, the S6 group presented reduced papillomas (number and size), whereas MDL-800-treated mice displayed an opposite trend. In S6-treated lesions, Keratin 6 and Keratin 8 were less expressed, EMT was less advanced, with a higher E-cadherin/Vimentin ratio, indicating a delayed carcinogenesis when SIRT6 was inhibited. Our results confirm that SIRT6 plays a role in skin carcinogenesis and suggest SIRT6 pharmacological inhibition as a promising strategy in cSCC.

The Role of NAD+, SIRTs Interactions in Stimulating and Counteracting Carcinogenesis

The World Health Organization has identified oncological diseases as one of the most serious health concerns of the current century. Current research on oncogenesis is focused on the molecular mechanisms of energy-biochemical reprogramming in cancer cell metabolism, including processes contributing to the Warburg effect and the pro-oncogenic and anti-oncogenic roles of sirtuins (SIRTs) and poly-(ADP-ribose) polymerases (PARPs). However, a clear understanding of the interaction between NAD⁺, SIRTs in cancer development, as well as their effects on carcinogenesis, has not been established, and literature data vary greatly. This work aims to provide a summary and structure of the available information on NAD⁺, SIRTs interactions in both stimulating and countering carcinogenesis, and to discuss potential approaches for pharmacological modulation of these interactions to achieve an anticancer effect.

The sirtuin family in health and disease

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

The histone deacetylase SIRT6 promotes glycolysis through the HIF-1α/HK2 signaling axis and induces erlotinib resistance in non-small cell lung cancer

Erlotinib is a first-generation epidermal growth factor receptor tyrosine kinase inhibitor (EGFR-TKI). Overcoming erlotinib resistance is crucial to improve the survival of advanced non-small cell lung cancer (NSCLC) patients with sensitive EGFR mutations. It is also an important clinical problem that urgently needs a solution. In this study, we explored strategies to overcome erlotinib resistance from the perspective of energy metabolism. SIRT6 is a histone deacetylase. Here, we found that high expression of SIRT6 is associated with poor prognosis of lung adenocarcinoma, especially in EGFR-mutated NSCLC patients. The next cell experiment found that SIRT6 expression increased in erlotinib-resistant cells, and SIRT6 expression was negatively correlated with the sensitivity of NSCLC to erlotinib. Inhibition of SIRT6 promoted erlotinib-induced apoptosis in erlotinib-resistant cells, and glycolysis in drug-resistant cells was also inhibited. Functional studies have shown that SIRT6 increases glycolysis through the HIF-1α/HK2 signaling axis in drug-resistant cells and inhibits the sensitivity of NSCLC cells to erlotinib. In addition, the HIF-1α blocker PX478-2HCL attenuated the glycolysis and erlotinib resistance induced by SIRT6. More importantly, we confirmed the antitumor effect of SIRT6 inhibition combined with erlotinib in NSCLC-bearing mice. Our findings indicate that the cancer metabolic pathway regulated by SIRT6 may be a new target for attenuating NSCLC erlotinib resistance and has potential as a biomarker or therapeutic target to improve outcomes in NSCLC patients.

4-phenylpyridine suppresses UVB-induced skin inflammation by targeting c-Src in vitro and in vivo

Acute or repetitive exposure to ultraviolet (UV) cause disruptions to the skin barrier and subsequent inflammatory skin disease. 4‐phenylpyridine (4‐PP) is a constituent of Brassica campestris L. ssp. Pekinensis and its effect on skin inflammation and molecular target remain unclear. The purpose of this study is to confirm the anti‐inflammatory efficacy of 4‐PP on UVB‐induced skin inflammation in human keratinocytes HaCaT and mouse skin and validation of its molecular target. 4‐PP also attenuated UVB‐induced phosphorylation of p38/mitogen‐activated protein kinase kinase (MKK) 3/6, c‐Jun N‐terminal kinase 1/2, MKK 4/7, extracellular‐signal‐regulated kinase 1/2, mitogen‐activated protein kinase 1/2. Additionally, 4‐PP inhibited UVB‐induced phosphorylation of epidermal growth factor receptor (EGFR) Y1068, Y1045 and 854 residues but not the proto‐oncogene tyrosine‐protein kinase c‐Src. Drug affinity responsive target stability assay revealed that 4‐PP directly binds to c‐Src and inhibits pronase c‐proteolysis. Knockdown of c‐Src inhibited UVB‐induced COX‐2 expression and phosphorylation of MAPKs and EGFR in HaCaT cells. Dorsal treatment of 4‐PP prevented UVB (0.5 J/cm²)‐induced skin thickness, phosphorylation of EGFR and COX‐2 expression in mouse skin. Our findings suggest that 4‐PP can be used as anti‐inflammatory agent with an effect of skin inflammation by inhibiting the COX‐2 expression via suppressing the c‐Src/EGFR/MAPKs signalling pathway.

Interleukin-38 promotes skin tumorigenesis in an IL-1Rrp2-dependent manner

Interleukin-38 (IL-38) is strongly associated with chronic inflammatory diseases; however, its role in tumorigenesis is poorly understood. We demonstrated that expression of IL-38, which exhibits high expression in the skin, is downregulated in human cutaneous squamous cell carcinoma and 7,12-dimethylbenzanthracene/12-O-tetradecanoyl phorbol-13-acetate-induced mouse skin tumorigenesis. IL-38 keratinocyte-specific knockout mice displayed suppressed skin tumor formation and malignant progression. Keratinocyte-specific deletion of IL-38 was associated with reduced expression of inflammatory cytokines, leading to reduced myeloid cell infiltration into the local tumor microenvironment. IL-38 is dispensable for epidermal mutagenesis, but IL-38 keratinocyte-specific deletion reduces proliferative gene expression along with epidermal cell proliferation and hyperplasia. Mechanistically, we first demonstrated that IL-38 activates the c-Jun N-terminal kinase (JNK)/activator protein 1 signal transduction pathway to promote the expression of cancer-related inflammatory cytokines and proliferation and migration of tumor cells in an IL-1 receptor-related protein 2 (IL-1Rrp2)-dependent manner. Our findings highlight the role of IL-38 in the regulation of epidermal cell hyperplasia and pro-tumorigenic microenvironment through IL-1Rrp2/JNK and suggest IL-38/IL-1Rrp2 as a preventive and potential therapeutic target in skin cancer.

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.

Dynamic stem cell selection safeguards the genomic integrity of the epidermis

Maintaining genomic integrity and stability is crucial for life; yet, no tissue-driven mechanism that robustly safeguards the epithelial genome has been discovered. Epidermal stem cells (EpiSCs) continuously replenish the stratified layers of keratinocytes that protect organisms against various environmental stresses. To study the dynamics of DNA-damaged cells in tissues, we devised an in vivo fate tracing system for EpiSCs with DNA double-strand breaks (DSBs) and demonstrated that those cells exit from their niches. The clearance of EpiSCs with DSBs is caused by selective differentiation and delamination through the DNA damage response (DDR)-p53-Notch/p21 axis, with the downregulation of ITGB1. Moreover, concomitant enhancement of symmetric cell divisions of surrounding stem cells indicates that the selective elimination of cells with DSBs is coupled with the augmented clonal expansion of intact stem cells. These data collectively demonstrate that tissue autonomy through the dynamic coupling of cell-autonomous and non-cell-autonomous mechanisms coordinately maintains the genomic quality of the epidermis.

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.

Bioinformatics Analysis of the SIRT Family Members and Assessment of Their Potential Clinical Value

Background

Hepatocellular carcinoma (HCC) is a highly malignant and common tumor. Many biomarkers have been identified for HCC. However, the available ones are not accurate enough in term of prognostic value and new markers are needed for the prognosis of this disease. Sirtuins are NAD⁽⁺⁾-dependent histone deacetylases involved in many biological processes of cancers, consisting of family members SIRT1-SIRT7. However, the prognostic value of the SIRTs in HCC remains largely unknown.

Methods

Differential expression of SIRTs and survival analysis were assessed in patients with HCC using Oncomine and UALCAN databases. Gene set enrichment analysis (GSEA) was used for pathway analysis. Metascape software was used to construct gene ontologies, metabolic pathways and protein-protein interaction networks. Moreover, a HCC murine model was used to validate the expression levels of SIRT3/6/7 expression.

Results

Differential expression analysis suggested that SIRT2-7, not SIRT1, were expressed at higher levels in HCC tissues compared to adjacent normal tissues. These SIRTs showed some similarities, as revealed by GO and KEGG pathway. Higher SIRT3/6/7 mRNA expression levels were found to be significantly associated with shorter overall survival (OS) in HCC patients. Both SIRT3/6/7 mRNA and protein levels were highly expressed in HCC. In addition, over-expression of SIRT3/6/7 was associated with tumor stage and grade in HCC patients. Univariate analysis showed that SIRT 6/7 expressions were linked to a shorter OS of HCC patients. Multivariate analysis showed that SIRT7 levels were independently associated with a significantly shorter OS in HCC patients.

Conclusion

Differentially expressed SIRT3/6/7 were significantly associated with tumor stage, grade and OS in HCC patients. In addition, SIRT7 were independently associated with a significantly shorter OS in HCC patients. Thus, SIRT3/6/7 can be used as prognostic biomarkers to predict the survival of HCC patients.

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.

SIRT6 Maintains Redox Homeostasis to Promote Porcine Oocyte Maturation

SIRT6, the sixth member of the sirtuin family proteins, has been characterized as a crucial regulator in multiple molecular pathways related to aging, including genome stability, DNA damage repair, telomere maintenance, and inflammation. However, the exact roles of SIRT6 during female germ cell development have not yet been fully determined. Here, we assessed the acquisition of meiotic competency of porcine oocytes by inhibition of SIRT6 activity. We observed that SIRT6 inhibition led to the oocyte meiotic defects by showing the impairment of polar body extrusion and cumulus cell expansion. Meanwhile, the compromised spindle/chromosome structure and actin dynamics were also present in SIRT6-inhibited oocytes. Moreover, SIRT6 inhibition resulted in the defective cytoplasmic maturation by displaying the disturbed distribution dynamics of cortical granules and their content ovastacin. Notably, we identified that transcript levels of genes related to oocyte meiosis, oxidative phosphorylation, and cellular senescence were remarkably altered in SIRT6-inhibited oocytes by transcriptome analysis and validated that the meiotic defects caused by SIRT6 inhibition might result from the excessive reactive oxygen species (ROS)-induced early apoptosis in oocytes. Taken together, our findings demonstrate that SIRT6 promotes the porcine oocyte meiotic maturation through maintaining the redox homeostasis.

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.

SIRT6 enhances oxidative phosphorylation in breast cancer and promotes mammary tumorigenesis in mice

BACKGROUND

Sirtuin 6 (SIRT6) is a NAD⁺-dependent deacetylase with key roles in cell metabolism. High SIRT6 expression is associated with adverse prognosis in breast cancer (BC) patients. However, the mechanisms through which SIRT6 exerts its pro-oncogenic effects in BC remain unclear. Here, we sought to define the role of SIRT6 in BC cell metabolism and in mouse polyoma middle T antigen (PyMT)-driven mammary tumors.

METHODS

We evaluated the effect of a heterozygous deletion of Sirt6 on tumor latency and survival of mouse mammary tumor virus (MMTV)-PyMT mice. The effect of SIRT6 silencing on human BC cell growth was assessed in MDA-MB-231 xenografts. We also analyzed the effect of Sirt6 heterozygous deletion, of SIRT6 silencing, and of the overexpression of either wild-type (WT) or catalytically inactive (H133Y) SIRT6 on BC cell pyruvate dehydrogenase (PDH) expression and activity and oxidative phosphorylation (OXPHOS), including respiratory complex activity, ATP/AMP ratio, AMPK activation, and intracellular calcium concentration.

RESULTS

The heterozygous Sirt6 deletion extended tumor latency and mouse survival in the MMTV-PyMT mouse BC model, while SIRT6 silencing slowed the growth of MDA-MB-231 BC cell xenografts. WT, but not catalytically inactive, SIRT6 enhanced PDH expression and activity, OXPHOS, and ATP/AMP ratio in MDA-MB-231 and MCF7 BC cells. Opposite effects were obtained by SIRT6 silencing, which also blunted the expression of genes encoding for respiratory chain proteins, such as UQCRFS1, COX5B, NDUFB8, and UQCRC2, and increased AMPK activation in BC cells. In addition, SIRT6 overexpression increased, while SIRT6 silencing reduced, intracellular calcium concentration in MDA-MB-231 cells. Consistent with these findings, the heterozygous Sirt6 deletion reduced the expression of OXPHOS-related genes, the activity of respiratory complexes, and the ATP/AMP ratio in tumors isolated from MMTV-PyMT mice.

CONCLUSIONS

Via its enzymatic activity, SIRT6 enhances PDH expression and activity, OXPHOS, ATP/AMP ratio, and intracellular calcium concentration, while reducing AMPK activation, in BC cells. Thus, overall, SIRT6 inhibition appears as a viable strategy for preventing or treating BC.

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.

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.

Ultraviolet irradiation-induced inhibition of histone deacetylase 4 increases the expression of matrix metalloproteinase-1 but decreases that of type I procollagen via activating JNK in human dermal fibroblasts

BACKGROUND

Ultraviolet (UV) irradiation is the main contributing factor for skin aging. UV irradiation induces epigenetic changes in skin. It increases the activity of histone acetylases (HATs) but decreases that of histone deacetylases (HDACs).

OBJECTIVE

We aimed to investigate alterations in all classes of HDACs and sirtuins (SIRTs) in response to UV irradiation, and determine the HDACs regulating the expressions of matrix metalloproteinase 1 (MMP-1) and type I procollagen.

METHODS

Primary human dermal fibroblasts were UV irradiated. HDAC4 was knocked-down or overexpressed to investigate its effect on the expression of MMP-1 and type I procollagen. The mRNA and protein levels were analyzed by quantitative real-time polymerase chain reaction and western blotting.

RESULTS

Among 11 HDACs and 7 SIRTs, we found that the expression of HDAC4, HDAC5, HDAC6, HDAC7, HDAC8, HDAC11, SIRT2, and SIRT3 were significantly and consistently reduced by UV at both mRNA and protein levels. Among these, the reduction of HDAC4 was responsible for the basal and UV-induced increase in the expression of MMP-1 and decrease in that of type I procollagen. Furthermore, the reduced HDAC4 could activate c-Jun N-terminal kinase (JNK), resulting in an increase in MMP-1 and decrease in type I procollagen.

CONCLUSIONS

UV treatment decreases the expression of HDACs and SIRTs in dermal fibroblasts; in particular, the UV-induced reduction in the expression of HDAC4 might play an important role in regulating the expression of MMP-1 and type I procollagen.

Long intergenic noncoding RNA smad7 (Linc-smad7) promotes the epithelial-mesenchymal transition of HCC by targeting the miR-125b/SIRT6 axis

Long intergenic noncoding RNA smad7 (Linc-smad7) has been recently identified as a new long non-coding RNA (lncRNA). However, the role of Linc-smad7 in the tumourigenesis of human cancers remains unknown. This study uncovered that Linc-smad7 was increased in HCC samples and HCC cell lines using RT-qPCR assays. Furthermore, the overexpression of Linc-smad7 indicated poor clinicopathological features and outcomes for HCC patients. In addition, Linc-smad7 promoted HCC cells proliferation, migration, invasion and EMT, as determined by MTT, colony formation, Transwell assays and western blot analysis. Functionally, it was demonstrated that Linc-smad7 could bind with microRNA-125b (miR-125b), and the restoration of miR-125b rescued the promoting effects of Linc-smad7 on HCC cells. Finally, it was observed that sirtuin 6 (SIRT6) was positively regulated by Linc-smad7 in HCC as the direct target of miR-125b, and decreased SIRT6 reversed the effects of Linc-smad7 on promoting HCC. In conclusion, the current study first identified Linc-smad7 is increased in HCC, facilitating HCC cells proliferation, migration, invasion and EMT via regulating the miR-125b/SIRT6 axis.

The deacetylase SIRT6 promotes the repair of UV-induced DNA damage by targeting DDB2

The NAD+-dependent deacetylase and mono-ADP-ribosyl transferase SIRT6 stabilizes the genome by promoting DNA double strand break repair, thereby acting as a tumor suppressor. However, whether SIRT6 regulates nucleotide excision repair (NER) remains unknown. Here, we showed that SIRT6 was recruited to sites of UV-induced DNA damage and stimulated the repair of UV-induced DNA damage. Mechanistic studies further indicated that SIRT6 interacted with DDB2, the major sensor initiating global genome NER (GG-NER), and that the interaction was enhanced upon UV irradiation. SIRT6 deacetylated DDB2 at two lysine residues, K35 and K77, upon UV stress and then promoted DDB2 ubiquitination and segregation from chromatin, thereby facilitating downstream signaling. In addition, we characterized several SIRT6 mutations derived from melanoma patients. These SIRT6 mutants ablated the stimulatory effect of SIRT6 on NER and destabilized the genome due to (i) partial loss of enzymatic activity (P27S or H50Y), (ii) a nonsense mutation (R150*) or (iii) high turnover rates (G134W). Overall, we demonstrate that SIRT6 promotes NER by deacetylating DDB2, thereby preventing the onset of melanomagenesis.

The Roles of Histone Deacetylases and Their Inhibitors in Cancer Therapy

Genetic mutations and abnormal gene regulation are key mechanisms underlying tumorigenesis. Nucleosomes, which consist of DNA wrapped around histone cores, represent the basic units of chromatin. The fifth amino group (Nε) of histone lysine residues is a common site for post-translational modifications (PTMs), and of these, acetylation is the second most common. Histone acetylation is modulated by histone acetyltransferases (HATs) and histone deacetylases (HDACs), and is involved in the regulation of gene expression. Over the past two decades, numerous studies characterizing HDACs and HDAC inhibitors (HDACi) have provided novel and exciting insights concerning their underlying biological mechanisms and potential anti-cancer treatments. In this review, we detail the diverse structures of HDACs and their underlying biological functions, including transcriptional regulation, metabolism, angiogenesis, DNA damage response, cell cycle, apoptosis, protein degradation, immunity and other several physiological processes. We also highlight potential avenues to use HDACi as novel, precision cancer treatments.

Expression of Cyclooxygenase-2 in Human Epithelial Skin Lesions: A Systematic Review of Immunohistochemical Studies

Permanent, elevated expression of cyclooxygenase-2 (COX-2) in keratinocytes of epidermis can stimulate its hyperplasia and constitute a factor promoting cancer development, as demonstrated in animal models. Intratumoral level and localization of COX-2 in epithelial lesions of human skin was examined immunohistochemically in 26 studies. In squamous cell carcinomas (SCCs), strong staining was observed with great compatibility. High COX-2 detectability throughout the entire tumor mass could be helpful in the finding of SCC cells. However, in basal cell carcinomas, and precancerous lesions, frequency and detection level of this protein, as well as the type and/or localization of stained cells within the tumor, varied among different research groups. The discrepancies may be due to the heterogeneity of each of these 2 groups of lesions. However, differences in COX-2 staining in normal skin indicate also possible methodological reasons. In general, COX-2 levels were significantly decreased in basal cell carcinomas compared with SCCs, which could be used in the differential diagnosis of these cancers. Reduced, although heterogenous, COX-2 expression in precancerous lesions may suggest its association with SCC development. These observations are consistent with data on the efficacy of preventive and therapeutic effects of nonsteroidal anti-inflammatory drugs that are COX-2 inhibitors.

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.

CRISPR/Cas9-mediated Knockout of SIRT6 Imparts Remarkable Antiproliferative Response in Human Melanoma Cells in vitro and in vivo

Melanoma is one of the most aggressive, potentially fatal forms of skin cancer and has been shown to be associated with solar ultraviolet radiation-dependent initiation and progression. Despite remarkable recent advances with targeted and immune therapeutics, lasting and recurrence-free survival remain significant concerns. Therefore, additional novel mechanism-based approaches are needed for effective melanoma management. The sirtuin SIRT6 appears to have a pro-proliferative function in melanocytic cells. In this study, we determined the effects of genetic manipulation of SIRT6 in human melanoma cells, in vitro and in vivo. Our data demonstrated that CRISPR/Cas9-mediated knockout (KO) of SIRT6 in A375 melanoma cells resulted in a significant (1) decrease in growth, viability and clonogenic survival and (2) induction of G1-phase cell cycle arrest. Further, employing a RT² Profiler PCR array containing 84 key transformation and tumorigenesis genes, we found that SIRT6 KO resulted in modulation of genes involved in angiogenesis, apoptosis, cellular senescence, epithelial-to-mesenchymal transition, hypoxia signaling and telomere maintenance. Finally, we found significantly decreased tumorigenicity of SIRT6 KO A375 cells in athymic nude mice. Our data provide strong evidence that SIRT6 promotes melanoma cell survival, both in vitro and in vivo, and could be exploited as a target for melanoma management.

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.

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.

Structural Insight into the Interactions between Structurally Similar Inhibitors and SIRT6

Sirtuin 6 (SIRT6) is an NAD+-dependent deacetylase with a significant role in 20% of all cancers, such as colon cancers and rectal adenocarcinoma. However, there is currently no effective drug for cancers related to SIRT6. To explore potential inhibitors of SIRT6, it is essential to reveal details of the interaction mechanisms between inhibitors and SIRT6 at the atomic level. The nature of small molecules from herbs have many advantages as inhibitors. Based on the conformational characteristics of the inhibitor Compound 9 (Asinex ID: BAS13555470), we explored the natural molecule Scutellarin, one compound of Huang Qin, which is an effective herb for curing cancer that has been described in the Traditional Chinese Medicine (TCMS) library. We investigated the interactions between SIRT6 and the inhibitors using molecular dynamics (MD) simulations. We illustrated that the structurally similar inhibitors have a similar binding mode to SIRT6 with residues—Leu9, Phe64, Val115, His133 and Trp188. Hydrophobic and π-stacking interactions play important roles in the interactions between SIRT6 and inhibitors. In summary, our results reveal the interactive mechanism of SIRT6 and the inhibitors and we also provide Scutellarin as a new potential inhibitor of SIRT6. Our study provides a new potential way to explore potential inhibitors from TCMS.

Litopenaeus vannamei sirtuin 6 homolog (LvSIRT6) is involved in immune response by modulating hemocytes ROS production and apoptosis

The histone deacetylase, sirtuin 6 (SIRT6), plays an essential role in the regulation of oxidative stress, mitochondrial function and inflammation in mammals. However, the specific role of SIRT6 in invertebrate immunity has not been reported. Here, we characterized for the first time, a sirtuin 6 homolog in Litopenaeus vannamei (LvSIRT6), with full-length cDNA of 2919 bp and 1536 bp open reading frame (ORF) encoding a putative protein of 511 amino acids, which contains a typical SIR2 domain. Sequence and phylogenetic analysis revealed that LvSIRT6 shares a close evolutionary relationship with SIRT6 from invertebrates. Real-time quantitative PCR analysis of LvSIRT6 transcripts revealed that they were ubiquitously expressed in shrimp and induced in hepatopancreas and hemocytes upon challenge with Vibrio parahaemolyticus, Streptococcus iniae, lipopolysaccharide (LPS), and white spot syndrome virus (WSSV), suggesting the involvement of LvSIRT6 in shrimp immune response. Moreover, knockdown of LvSIRT6 decreased mitochondrial membrane potential and increased total ROS level in hemocytes, especially upon V. parahaemolyticus challenge. Depletion of LvSIRT6 also increased hemocytes apoptosis in terms of decreased expression of pro-survival LvBcl-2, but increased expression of pro-apoptotic LvBax and LvCytochrome C, coupled with high LvCaspase3/7 activity. Shrimp were rendered more susceptible to V. parahaemolyticus infection upon LvSIRT6 knockdown. Taken together, our present data suggest that LvSIRT6 plays an important role in shrimp immune response by modulating hemocytes ROS production and apoptosis during pathogen challenge.

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.

NQO1 potentiates apoptosis evasion and upregulates XIAP via inhibiting proteasome-mediated degradation SIRT6 in hepatocellular carcinoma

Background

Our previous study has demonstrated that NAD(P)H: quinone oxidoreductase 1 (NQO1) is significantly upregulated in human liver cancer where it potentiates the apoptosis evasion of liver cancer cell. However, the underlying mechanisms of the oncogenic function of NQO1 in HCC have not been fully elucidated.

Methods

Expression of NQO1, SIRT6, AKT and X-linked inhibitor of apoptosis protein (XIAP) protein were measured by western blotting and immunohistochemistry. Additionally, the interaction between NQO1 and potential proteins were determined by immunoprecipitation assays. Furthermore, the effect of NQO1 and SIRT6 on tumor growth was determined in cell model and orthotopic tumor implantation model.

Results

We found that NQO1 overexpression in HCC enhanced SIRT6 protein stability via inhibiting ubiquitin-mediated 26S proteasome degradation. High level of SIRT6 reduced acetylation of AKT which resulted in increased phosphorylation and activity of AKT. Activated AKT subsequently phosphorylated anti-apoptotic protein XIAP at Ser87 which determined its protein stability. Reintroduction of SIRT6 or AKT efficiently rescued NQO1 knock-out-mediated inhibition of growth and induction of apoptosis. In orthotopic mouse model, NQO1 knock-out inhibited tumor growth and induced apoptosis while this effect was effectively rescued by SIRT6 overexpression or MG132 treatment partially.

Conclusions

Collectively, these results reveal an oncogenic function of NQO1 in sustaining HCC cell proliferation through SIRT6/AKT/XIAP signaling pathway.

The Roles of Sirtuin Family Proteins in Cancer Progression

Sirtuin family members are characterized by either mono-ADP-ribosyltransferase or deacylase activity and are linked to various cancer-related biological pathways as regulators of transcriptional progression. Sirtuins play fundamental roles in carcinogenesis and maintenance of the malignant phenotype, mainly participating in cancer cell viability, apoptosis, metastasis, and tumorigenesis. Although sirtuin family members have a high degree of homology, they may play different roles in various kinds of cancer. This review highlights their fundamental roles in tumorigenesis and cancer development and provides a critical discussion of their dual roles in cancer, namely, as tumor promoters or tumor suppressors.

Sirtuins and SIRT6 in Carcinogenesis and in Diet

Sirtuins are a highly conserved family of nicotinamide adenine dinucleotide (NAD)-dependent protein lysine modifying enzymes. They are key regulators for a wide variety of cellular and physiological processes such as cell proliferation, differentiation, DNA damage and stress response, genome stability, cell survival, metabolism, energy homeostasis, organ development and aging. Aging is one of the major risk factors of cancer, as many of the physiological mechanisms and pathologies associated with the aging process also contribute to tumor initiation, growth and/or metastasis. This review focuses on one the mammalian sirtuins, SIRT6, which has emerged as an important regulator of longevity and appears to have multiple biochemical functions that interfere with tumor development and may be useful in cancer prevention and for site-specific treatment. The recent evidence of the role of SIRT6 in carcinogenesis is also discussed, focusing on the potential use of SIRT6 modulators in cancer nanomedicine.

Sirtuin6 (SIRT6) Promotes the EMT of Hepatocellular Carcinoma by Stimulating Autophagic Degradation of E-Cadherin

EMT is a pivotal mechanism involved in tumor metastasis, which is the leading cause of poor prognosis for hepatocellular carcinoma (HCC). Sirtuin family members function as NAD⁺-dependent deacetylases that are essential for tumor metastasis and epithelial-mesenchymal transition (EMT). However, no causal association has been established between Sirtuin6 (SIRT6) and HCC metastasis. SIRT6 expression pattern and its association with HCC metastasis were investigated by informatic analysis, and verified by qRT-PCR and immunochemistry in HCC tissues. Transwell assay, qRT-PCR, and immunofluorescence assay were utilized to assess the effects of SIRT6 on metastasis and E-cadherin expression in vitro and in vivo. Immunoprecipitation assay was performed to observe whether SIRT6 deacetylated Beclin-1 in HCC cells. Immunofluorescence assay and inhibitor treatment rescue experiments were used to clarify the mechanism by which SIRT6 facilitated EMT and metastasis. SIRT6 upregulation was quite prevalent in HCC tissues and closely correlated with worse overall survival, disease-relapse free survival, and HCC metastasis. Furthermore, SIRT6 promoted HCC cell migration, invasion, and EMT. Mechanistically, we found that SIRT6 deacetylated Beclin-1 in HCC cells and this event led to the promotion of the autophagic degradation of E-cadherin. Noticeably, E-cadherin degradation and invasion, migration induced by SIRT6 overexpression could be rescued by dual mutation of Beclin-1 (inhibition of acetylation), CQ (autophagy inhibitor), and knockdown of Atg7. In addition, SIRT6 promoted N-cadherin and Vimentin expression via deacetylating FOXO3a in HCC. These results established a relationship between SIRT6 and HCC EMT and further elucidated the mechanisms underlying HCC metastasis, helping provide a promising approach for the treatment of HCC. IMPLICATIONS: Inhibiting SIRT6 represents a potential therapeutic approach to suppress HCC metastasis partially through reduction of autophagic degradation of E-cadherin.

The sirtuin family in cancer

Sirtuins are a family of protein deacylases and ADP-ribosyl-transferases, homologs to the yeast SIR2 protein. Seven sirtuin paralogs have been described in mammals, with different subcellular locations, targets, enzymatic activities, and regulatory mechanisms. All sirtuins share NAD+ as substrate, placing them as central metabolic hubs with strong relevance in lifespan, metabolism, and cancer development. Much effort has been devoted to studying the roles of sirtuins in cancer, providing a wealth of data on sirtuins roles in mouse models and humans. Also, extensive data are available on the effects of pharmacological modulation of sirtuins in cancer development. Here, we present a comprehensive and organized resume of all the existing evidence linking every sirtuin with cancer development. From our analysis, we conclude that sirtuin modulation after tumor initiation results in unpredictable outcomes in most tumor types. On the contrary, all genetic and pharmacological models indicate that sirtuins activation prior to tumor initiation can constitute a powerful preventive strategy.

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.

Phosphorylation of xeroderma pigmentosum group C regulates ultraviolet-induced DNA damage repair

Nucleotide excision repair (NER) is the most versatile DNA repair system that removes bulky DNA damage induced by various endogenous and exogenous factors, including UV radiation. Defects in NER can lead to the xeroderma pigmentosum (XP) syndrome, mainly characterized by increased carcinogenesis in the skin. The function of NER factors, including xeroderma pigmentosum group C (XPC), can be regulated by post-translational modifications such as ubiquitination. However, the role of phosphorylation in XPC function remains unknown. Here, we show that phosphorylation of XPC acts as a novel post-translational regulatory mechanism of the NER pathway. We show that XPC is phosphorylated at serine 94. Moreover, after UVB irradiation, XPC phosphorylation regulates recruitment of ubiquitinated XPC and its downstream NER factors to the chromatin. In addition, upon evaluating the predicted kinases for XPC phosphorylation, we found that casein kinase II (CK2) promotes NER. Furthermore, CK2 kinase mediates XPC phosphorylation at serine 94, and also promotes recruitment of ubiquitinated XPC to the chromatin after UVB irradiation. Our findings have identified XPC phosphorylation as a new mechanism for regulating NER following UV-induced DNA damage.

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.

Differential modulation of SIRT6 deacetylase and deacylase activities by lysine-based small molecules

Sirtuin 6 (SIRT6) is an NAD⁺-dependent deacetylase regulating important functions: modulators of its enzymatic activity have been considered as possible therapeutic agents. Besides the deacetylase activity, SIRT6 also has NAD⁺-dependent deacylase activity, whereby it regulates the secretion of cytokines and proteins. We identified novel SIRT6 modulators with a lysine-based structure: compound 1 enhances SIRT6 deacylase while inhibiting the deacetylase activity. As expected based on the biological effects of SIRT6 deacetylase activity, compound 1 increased histone 3 lysine 9 acetylation and the activity of glycolytic enzymes. Moreover, the fact that compound 1 enhanced SIRT6 deacylase activity was accompanied by an increased TNF-α release. In conclusion, new SIRT6 modulators with a lysine-like structure were identified, with differential effects on specific SIRT6 activities. The novel SIRT6 modulator concomitantly inhibits deacetylase and enhances deacylase activity.

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.

Jie Du Tong Ye San Prevents N-Nitrosomethylbenzylamine-Induced Esophageal Carcinogenesis via Inhibition of Inflammation and Proliferation

Jie du tong ye san (JDTYS), a traditional Chinese herbal formula, has been used for cancer adjuvant therapy in clinical use and has been shown to be effective in cancer patients. However, the mechanism of JDTYS is still unclear. Therefore, the aim of the present study is to investigate the chemopreventive effects of JDTYS for esophageal squamous cell carcinoma (ESCC) and to clarify the potential mechanism. N-nitrosomethylbenzylamine (NMBA)-induced rat esophageal carcinogenesis was used to evaluate the effect of JDTYS in vivo. Rats were treated with NMBA 3 times per week, for a total of 5 weeks. Rats in the treated groups were given JDTYS for 35 weeks. When rats were euthanized, esophageal tissue and blood were collected to evaluate the effects of JDTYS. The pathological grading of the rat esophageal preneoplastic lesions was classified and statistically analyzed. The protein levels of c-Jun and Ki67 were determined by immunohistochemistry. In addition, inflammation markers nuclear factor kappa B (NF-κB), cyclooxygenase-2 (COX-2), and the cluster of differentiation molecule 11B (CD11B) were also determined by immunohistochemistry. Moreover, the expression of COX-2 and Pentraxin 3 (PTX3) in rat serum was determined by enzyme-linked immunosorbent assay (ELISA). JDTYS could inhibit the formation of NMBA-induced esophageal preneoplastic lesions. JDTYS could downregulate the expression of proliferation related proteins Ki67 and c-Jun. Moreover, inflammation related proteins NF-κB, COX-2, and CD11B were inhibited and PTX3 was increased by JDTYS. In all, JDTYS is a promising chemopreventive formula against esophageal carcinogenesis by regulating inflammation and inhibiting cell proliferation.

RNA-Seq analysis revealed genes associated with UV-induced cell necrosis through MAPK/TNF-α pathways in human dermal fibroblast cells as an inducer of premature photoaging

Exposing the skin to solar UV radiation induces cascades of signaling pathways and biological alterations such as redox imbalance, suppression of antioxidant genes and programmed cell death. Therefore, the aim of this study was to use RNA-Seq to unravel the effects of UV radiation on Normal Human Adult Fibroblast cells (NHDF). Cells were exposed to UV (20 mJ/cm2 for 3 mins) and incubated for 24 h. Total mRNA from the cells generated libraries of 72,080,648 and 40,750,939 raw reads from UV-treated and control cells respectively. Of the differentially expressed genes (DEGs) produced 2,007 were up-regulated and 2,791 were down-regulated (fold change ≥2, p < 0.05). The expression of 4 genes was validated with RT-qPCR. Chemokine signaling pathways in cancer were significantly activated and antioxidant genes were down-regulated. This study applied Next Generation Sequencing technology to reveal the genes and pathways involved in UV-induced human dermal fibroblast cells necrosis.

MYC upregulated LINC00319 promotes human acute myeloid leukemia (AML) cells growth through stabilizing SIRT6

Long non-coding RNAs (lncRNAs) have been identified by accumulating studies as critical regulator in tumorigenesis and tumor development in human cancers, including in acute myeloid leukemia (AML). This study investigated the function and the underlying mechanism of LINC00319 in AML progression. Firstly, the low expression level of LINC00319 in whole blood of healthy individuals was obtained from UCSC, and its upregulation was detected in AML patients as well as AML cell lines. Besides, the prognostic significance of LINC00319 was revealed in AML patients. Functionally, the loss-of-function assays revealed that LINC00319 silence restrained proliferation but stimulated apoptosis in AML cells. Furthermore, LINC00319 expression was demonstrated proportional to MYC level in AML samples and transcriptionally regulated by MYC. Mechanistically, we identified FUS as a shared RNA binding protein (RBP) interacting with both LINC00319 and SIRT6. And LINC00319 regulated SIRT6 expression at post-transcriptional level through FUS-dependent pathway. Last but not least, SIRT6 overexpression rescued the suppressive effect of LINC00319 knockdown on AML cells growth. Overall, our findings unveiled that LINC00319 contributed to AML leukemogenesis via elevating SIRT6 expression, indicating a possible molecular target of LINC00319 for AML treatment.

SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells

Nicotinamide phosphoribosyltransferase (NAMPT) is the rate-limiting enzyme in the NAD⁺ salvage pathway from nicotinamide. By controlling the biosynthesis of NAD⁺, NAMPT regulates the activity of NAD⁺-converting enzymes, such as CD38, poly-ADP-ribose polymerases, and sirtuins (SIRTs). SIRT6 is involved in the regulation of a wide number of metabolic processes. In this study, we investigated the ability of SIRT6 to regulate intracellular NAMPT activity and NAD(P)(H) levels. BxPC-3 cells and MCF-7 cells were engineered to overexpress a catalytically active or a catalytically inactive SIRT6 form or were engineered to silence endogenous SIRT6 expression. In SIRT6-overexpressing cells, NAD(H) levels were up-regulated, as a consequence of NAMPT activation. By immunopurification and incubation with recombinant SIRT6, NAMPT was found to be a direct substrate of SIRT6 deacetylation, with a mechanism that up-regulates NAMPT enzymatic activity. Extracellular NAMPT release was enhanced in SIRT6-silenced cells. Also glucose-6-phosphate dehydrogenase activity and NADPH levels were increased in SIRT6-overexpressing cells. Accordingly, increased SIRT6 levels reduced cancer cell susceptibility to H₂O₂-induced oxidative stress and to doxorubicin. Our data demonstrate that SIRT6 affects intracellular NAMPT activity, boosts NAD(P)(H) levels, and protects against oxidative stress. The use of SIRT6 inhibitors, together with agents inducing oxidative stress, may represent a promising treatment strategy in cancer.-Sociali, G., Grozio, A., Caffa, I., Schuster, S., Becherini, P., Damonte, P., Sturla, L., Fresia, C., Passalacqua, M., Mazzola, F., Raffaelli, N., Garten, A., Kiess, W., Cea, M., Nencioni, A., Bruzzone, S. SIRT6 deacetylase activity regulates NAMPT activity and NAD(P)(H) pools in cancer cells.

Pharmacological activation of SIRT6 triggers lethal autophagy in human cancer cells

Sirtuin 6 (SIRT6) is a member of the NAD⁺-dependent class III deacetylase sirtuin family, which plays a key role in cancer by controlling transcription, genome stability, telomere integrity, DNA repair, and autophagy. Here we analyzed the molecular and biological effects of UBCS039, the first synthetic SIRT6 activator. Our data demonstrated that UBCS039 induced a time-dependent activation of autophagy in several human tumor cell lines, as evaluated by increased content of the lipidated form of LC3B by western blot and of autophagosomal puncta by microscopy analysis of GFP-LC3. UBCS039-mediated activation of autophagy was strictly dependent on SIRT6 deacetylating activity since the catalytic mutant H133Y failed to activate autophagy. At the molecular level, SIRT6-mediated autophagy was triggered by an increase of ROS levels, which, in turn, resulted in the activation of the AMPK-ULK1-mTOR signaling pathway. Interestingly, antioxidants were able to completely counteract UBCS039-induced autophagy, suggesting that ROS burst had a key role in upstream events leading to autophagy commitment. Finally, sustained activation of SIRT6 resulted in autophagy-related cell death, a process that was markedly attenuated using either a pan caspases inhibitor (zVAD-fmk) or an autophagy inhibitor (CQ). Overall, our results identified UBCS039 as an efficient SIRT6 activator, thereby providing a proof of principle that modulation of the enzyme can influence therapeutic strategy by enhancing autophagy-dependent cell death.

Cytotoxic Effects of 24-Methylenecyloartanyl Ferulate on A549 Nonsmall Cell Lung Cancer Cells through MYBBP1A Up-Regulation and AKT and Aurora B Kinase Inhibition

Lung cancer is the second most prevalent cancer. Nonsmall cell lung cancer (NSCLC) is the most common type of lung cancer. The low efficacy in current chemotherapies impels us to find new alternatives to prevent or treat NSCLC. Rice bran oil is cytotoxic to A549 cells, a NSCLC cell line. Here, we identified 24-methylenecyloartanyl ferulate (24-mCAF) as the main component responsible for the cytotoxicity in A549 cells. An iTRAQ-based quantitative proteomics analysis revealed that 24-mCAF inhibits cell proliferation and activates cell death and apoptosis. 24-mCAF induces up-regulation of Myb binding protein 1A (MYBBP1A), a tumor suppressor that halts cancer progression. 24-mCAF inhibits the activity of AKT and Aurora B kinase, two Ser/Thr kinases involved in MYBBP1A regulation and that represent important targets in NSCLC. This study provides the first insight of the effect of 24-mCAF, the main component of rice bran oil, on A459 cells at the cellular and molecular levels.