The Sirtuin 2 (SIRT2) gene was initially identified as ribonuclease mar1, which acted by preventing the expression of mating genes in yeast (1). The yeast silent information regulator 2 (SIR2) was shown to be involved in transcriptional silencing, ribosomal DNA recombination, life span and other physiological functions (2–4). It was initially hypothesized that SIR2 possessed an adenosine diphosphate (ADP)-ribosyltransferase activity that could transfer ADP-ribose from nicotinamide adenine dinucleotide (NAD) to histones (5). This activity was subsequently shown to be a low efficiency side reaction (6). It is well accepted that SIR2 is a NAD-dependent lysine deacetylase. The SIRT enzymes are highly conserved from bacteria to humans. Bacteria and archaea only express one or two SIRT, while mammals have seven SIRT homologs (SIRT1-7) (7,8). The seven mammalian SIRT have different subcellular localization patterns, including cytoplasmic (SIRT1 and 2), nuclear (SIRT1, 2, 3, 6 and 7) and mitochondrial (SIRT3, 4 and 5). Among them, SIRT1-3 belong to class I SIRTs and have higher homology to the yeast silent information regulator 2 (Sir2), histone deacetylase (Hst)1 and Hst2. All these enzymes exhibit potent deacetylase activity (7).

SIRT2 is a member of the SIRT family and is also known as SIR2, SIRT type 2 or Sir2-related protein type 2; it belongs to the classic type III deacetylases and acts in a NAD⁺ dependent manner (9). SIRT2 is unique amongst SIRTs as it is the only primary cytoplasmic enzyme with robust deacetylase activity. Recently, SIRT2 has been shown to catalyze the removal of lysine fatty acylation, including hexanoylation, decanoylation and myristoylation (10). The newly characterized de-acylation activities of SIRT2 have improved the current knowledge on the function of this enzyme and have provided novel opportunities to study the physiological and pathological role of SIRT2.

The human SIRT2 gene is located on chromosome 19 with 16 exons spanning 21 kilobases of the genomic DNA. The SIRT2 transcript undergoes alternative splicing and produces three isoforms with different cellular and tissue distributions, and different functions (11,12). The full-length isoform 1 is abundant in the skeletal muscle, while isoform 2, which lacks the N-terminal 37 residue, accumulates in the brain. Both isoforms 1 and 2 are enzymatically active and are able to shuttle between the nucleus and the cytoplasm. In contrast to these findings, isoform 5 lacks a nuclear export signal that contains the entire nuclear export signal (NES) and a short fragment of the catalytic domain, resulting in the nuclear enrichment and loss of catalytic activity (12).

X-ray crystallographic studies have revealed that the human SIRT family enzymes contain two major functional domains: A small domain that binds to zinc ions and a large domain that is responsible for NAD⁺ binding. A conserved large substrate binding groove is also presented at the interface of the two domains (13). The longest SIRT2 isoform contains 389 amino acids, of which the 65–340 amino acid sequence is the NAD-dependent catalytic domain (14). The 41–51 amino acid sequence at the N-terminal of SIRT2 is a NES, which is responsible for guiding the cytoplasmic localization of the protein (15). The detailed schematic diagram of each domain of the SIRTs family is shown in Fig. 1. Despite their conserved structural features, human SIRTs diverge significantly in their subcellular distribution, substrate targets and enzymatic activity characteristics. For example, the lipoamidase activity of SIRT4 is superior to its deacetylase activity (16). SIRT6 exhibits preferential activity for the removal of long chain fatty acyls (10). SIRT7 employs nuclear acid oligos (double-stranded DNA, ribosomal RNA and transfer RNA) as co-factors for catalysis (17). SIRT1, SIRT2 and SIRT3 possess a robust deacetylase activity.

Although SIRT2 is predominantly cytoplasmic, it shuttles into the nucleus under certain circumstances, such as mitosis or bacterial infection (18). SIRT2 predominantly regulates cellular processes through its enzymatic activity (18–23). Previous studies indicated a conserved role of SIR2 in extending the lifespan of yeast, flies and worms in a deacetylase-dependent manner (24–26). In mammals, the significant longevity gene BUB1 mitotic checkpoint serine/threonine kinase B (BubR1) was shown to be deacetylated by SIRT2. The latter deacetylates BubR1 at the K668 site, thereby stabilizing the BubR1 protein. As a consequence, the lifespan of progeroid hypomorphic BubR1 mice was largely increased by SIRT2 overexpression (27). In general, the anti-aging effect of SIRT2 seems to be conservative from yeast to mammals.

Mammalian SIRT2 has been shown to restrain cell cycle progression in a deacetylase activity-dependent manner (28). SIRT2 is associated with chromatin during mitosis and facilitates chromatin condensation by deacetylating Histone H4 lysine (H4K) 16 (29). The deacetylation of H4K16 is in turn essential for the mitotic deposition of H4K20 methylation (30). The upregulation of the level of mitotic regulators in SIRT2-knockout mice confirmed the cell cycle regulatory function of SIRT2 (31). It was proposed that SIRT2 regulates mitosis by deacetylating cadherin-1 and cell division cycle protein 20 homolog, which are two adenomatous polyposis coli/C coactivators (31). In addition to its action as a mitotic regulator, SIRT2 also deacetylates ribonucleotide reductase regulatory subunit M2 at the K95 site during the S phase of the cell cycle. This deacetylation increases the dNTP pool size and accelerates DNA replication fork progression by enhancing ribonucleotide reductase activity, which ultimately results in an increased cancer cell proliferative rate (32).

A number of previous studies have shown that SIRT2 also plays a role in regulating cell metabolism (33,34). SIRT2 can promote the metastasis of gastric cancer through the RAS/ERK/JNK/matrix metalloproteinase-9 pathway by increasing phosphoenolpyruvate carboxykinase 1-associated metabolism (35). SIRT2 promotes glycolysis and tumor growth by deacetylating the K305 site of pyruvate kinase M2, a hallmark enzyme that bridges metabolism and immunity (36). A comprehensive understanding of the function and mechanism of SIRT2 in the progression of various cancer types is imminent. Clinical data and mechanisms of action for SIRT2 in cancer are shown in Table I. Glucose-6-phosphate dehydrogenase (G6PD) is a key enzyme that is involved in the pentose phosphate pathway and is responsible for producing NADPH. Deacetylation of the residue K403 of G6PD by SIRT2 increases its activity and enhances NADPH production (37). It is important to note that SIRT2 can inhibit glycolysis and metabolic reprogramming of induced pluripotent stem cells (iPSCs) by deacetylating the following four key glycolytic enzymes: Aldolase, phosphoglycerate kinase 1, enolase and GAPDH (19). A recent study demonstrated that SIRT2 is a master organizer of T-cell metabolism, since it inhibits T-cell glycolysis and impairs T-cell effector functions by deacetylating a number of metabolic enzymes, such as phosphofructokinase, α-ketoglutarate dehydrogenase, succinate dehydrogenase complex, subunit A, flavoprotein variant and succinyl-CoA ligase (GDP-forming) subunit α mitochondrial (38).

In addition to its classical deacetylase activity, SIRT2 has also been shown to catalyze the removal of long chain fatty acyls from Kras4a and Ras like proto-oncogene B (RalB) (39,40). Furthermore, the de-myristoylation activity of SIRT2 towards ADP-ribosylation factor 6 lysine 3 was recently identified (41). The diverse functions of SIRT2 in the nervous system, mitosis, genome integrity, cell differentiation, cell homeostasis, aging, infection, inflammation, oxidative stress and autophagy have been previously reviewed (42–44).

Dysregulation of SIRT2 in cases of gene amplification and mutation, protein overexpression and mislocalization has been associated with the progression of various cancer types. Cross-cancer analysis of The Cancer Genome Atlas database indicates that the SIRT2 gene is amplified in ~9% (52 out of 584 cases) of ovarian epithelial tumors and 4% (41 out of 1,053 cases) of NSCLCs (https://www.cancer.gov/about-nci/organization/ccg/research/structural-genomics/tcga). The gene locus of SIRT2 is known to be frequently deleted in human oligodendrogliomas (45). In addition, a number of somatic mutations within SIRT2 are found in endometrial carcinoma, melanoma, leukemia and NSCLC (cbioportal website) (http://www.cbioportal.org/). Particularly, multiple cancer-associated SIRT2 mutations at evolutionarily conserved sites have been reported as functionally significant (46). For example, the R42L mutation found in lung cancer decreased the protein levels of SIRT2, while the P128L mutation found in both colon and uterine cancer types abrogated the enzymatic activity of SIRT2 (46).

Due to tumor heterogeneity, the SIRT2 expression pattern is not always consistent. According to two recent review articles, SIRT2 expression was elevated in neuroblastoma, uveal melanoma, renal cell carcinoma and acute myeloid leukemia, while it was decreased in glioma, neck squamous cell carcinoma, breast cancer, prostate cancer and liver cancer (47,48). In addition, dual SIRT2 expression patterns have been noted in NSCLC. Li et al (49) demonstrated that SIRT2 mRNA and protein expression levels were downregulated in NSCLC. Grbesa et al (50) demonstrated that the expression levels of the SIRT2 proteins were significantly higher in lung primary tumors than those noted in normal tissues. It is notable that Gao et al (51) demonstrated patients with NSCLC with low SIRT2 expression had longer overall survival (OS) compared with those with high SIRT2 expression, according to a survival analysis of 1,926 patients and SIRT2 expression levels were significantly related to the survival time of patients with lung adenocarcinoma (ADC) but not squamous cell carcinoma (SCC). Despite nuclear and cytoplasmic shuttling, SIRT2 is in fact a primary cytoplasmic protein. High levels of mislocalized nuclear SIRT2 protein were associated with shorter disease-free survival time in ER-negative breast cancer (52). Gong et al (53) revealed that the combination of SIRT1 and SIRT2 was an improved recurrence-free survival prediction model for NSCLC. It is also notable that SIRT2 has been identified as a candidate plasma biomarker in invasive cervical cancer (54).

The role of SIRT2 in cancer is controversial. SIRT2 has been reported to exert either tumor suppressor or oncogenic functions. It was initially proposed as a tumor suppressor due to its regulatory role on the mitotic checkpoint and due to its deacetylase activity on histone H3K56, which is a frequent modification noted in cancer cells (55,56). The tumor suppressive role of SIRT2 was supported by genetic experiments demonstrating that Sirt2-deficient mice exhibited increased tumor incidence (30). In contrast to these observations, the tumor promoting activity of SIRT2 was supported by its ability to deacetylate p53 and downregulate its transcriptional activity (57). Moreover, SIRT2 deacetylates K5 of lactate dehydrogenase A and increases its activity and protein levels, thereby accelerating glycolysis and lactate production, which in turn leads to increased cancer cell proliferation and migration (58). Moreover, SIRT2 inhibitors have been shown to have broad anticancer activity (38,59), suggesting their therapeutic potential in cancer cells.

These seemingly opposite observations may reflect a context-specific role of SIRT2 in cancer progression. Researchers are therefore encouraged to appropriately assess the clinical and molecular features in order to determine SIRT2 protein abnormalities in NSCLC.

As aforementioned, NSCLC is a malignant cancer with a complicated etiology and poor prognosis, accounting for ~80% of the total incidence of lung cancer. SIRT2 functions either as a tumor suppressor or as an oncogene in NSCLC, depending on the experimental conditions. It is notable that SIRT2 inhibitors exhibit a protective effect in patients with lung cancer and high SIRT2 expression, providing a theoretical basis for successful cancer therapy.

SIRT2 is mainly localized in the cytoplasm. However, it is also found in the nucleus and mitochondria. A secretome study performed in mouse macrophages revealed 775 proteins including SIRT2, which were reproducibly detected in the culture medium following lipopolysaccharide stimulation (85), suggesting a potential role of SIRT2 in the extracellular compartment. The applications of high-throughput liquid chromatography tandem mass spectrometry analysis have resulted in the identification of a large number of extracellular matrix and peripheral proteins that are acetylated in both healthy and diseased tissues (86–88). However, additional studies are required to assess whether the SIRT2 deacetylase activity in the extracellular tumor microenvironment can regulate tumorigenesis.

In conclusion, SIRT2 is implicated in a wide range of physiological and pathological processes via deacetylation or fatty deacylation of specific substrates. Dysregulation of SIRT2 is closely associated with NSCLC progression. SIRT2 exhibits both oncogenic and tumor suppressive functions depending on the cancer stage, cell molecular characteristics and experimental conditions. Therefore, despite the broad anticancer activity of SIRT2 inhibitors, extensive research is still required to validate the potential of SIRT2 as a target for NSCLC treatment.