SIRT2 Human

What is SIRT2 and what are its primary functions in human cells?

SIRT2 is an NAD+-dependent deacetylase that belongs to the sirtuin family. It functions broadly at the interface between metabolism and cellular homeostasis primarily by deacetylating substrates, as well as by regulating various acylation modifications and other post-translational modifications (PTMs) . SIRT2 is known to act as a sensor of the cellular environment, responding to external stimuli and pathogens, facilitating rapid cellular communication processes .

SIRT2 participates in multiple cellular processes including:

• Regulation of gene expression through epigenetic mechanisms

• Metabolic pathway signal transduction

• Cell cycle progression and mitosis regulation

• DNA replication and damage response

• Immune cell function and differentiation

The multifunctional nature of SIRT2 allows it to form crosstalk between gene expression regulation and metabolism, participating in the pathological process of various diseases .

Where is SIRT2 primarily localized in human cells?

SIRT2 is predominantly found in the cytoplasm but also exhibits dynamic localization to other cellular compartments . According to proximity proteomics analysis, the majority of SIRT2-interacting proteins are annotated as nuclear or possessing dual localization to the nucleus and cytoplasm . Depending on the cellular context, approximately 15%-18% of SIRT2 interacting proteins are designated as cytoplasmic and 2.8%-3.5% as mitochondrial .

Unlike other sirtuin family members that have more restricted localizations, SIRT2 is the only sirtuin protein predominantly found in the cytoplasm but also present in the mitochondria and nucleus under specific conditions . This multi-compartment distribution enables SIRT2 to regulate diverse cellular processes through interactions with proteins in different cellular locations.

What enzymatic activities does SIRT2 possess beyond deacetylation?

While SIRT2 is primarily characterized as a deacetylase, research has revealed it possesses multiple enzymatic activities that expand its regulatory capabilities:

These diverse enzymatic capabilities allow SIRT2 to function as a multifaceted regulator of cellular processes through post-translational modifications . The spectrum of activities explains SIRT2's involvement in numerous physiological and pathological processes.

How is SIRT2 expression regulated at the transcriptional level?

SIRT2 expression is regulated by several transcription factors, with the cAMP responsive element modulator (CREM) family playing a particularly important role. Research has identified that the inducible cAMP early repressor (ICER) isoform of CREM directly binds to the SIRT2 promoter and enhances its transcription .

Genome analysis has revealed a conserved cAMP response element (CRE) motif in the promoter of the SIRT2 gene in both humans and mice . This transcriptional regulation mechanism is especially relevant in autoimmune conditions like systemic lupus erythematosus (SLE), where CREM/ICER activity is increased, leading to elevated SIRT2 expression in T cells .

Experimental evidence demonstrates that:

• SIRT2 protein expression is decreased in CREM/ICER-deficient Th17 cells

• CREM/ICER directly binds to the SIRT2 promoter and enhances its activity

• CD4+ T cells from SLE patients exhibit increased SIRT2 expression compared to healthy controls

This transcriptional regulation mechanism provides insight into potential therapeutic approaches targeting the CREM/ICER-SIRT2 axis in autoimmune diseases.

How does SIRT2 contribute to DNA replication and damage response?

SIRT2 plays critical roles in DNA replication and the cellular response to DNA damage through several mechanisms:

At the replication level, SIRT2 helps recover DNA replication from stress states following DNA damage primarily through the ataxia telangiectasia-mutated and Rad3-related (ATR) kinase checkpoint . Specifically, SIRT2:

• Deacetylates ATR-interacting protein (ATRIP) at lysine 32, driving ATR activation through binding of ATR-ATRIP to replication protein A-coated single-stranded DNA (RPA-ssDNA)

• Deacetylates CDK9 at lysine 48, partially dependent on ATR, to activate CDK9 kinase activity, promoting recovery of DNA replication

• Maintains ribonucleotide reductase (RNR) activity by deacetylating its subunit RRM2, ensuring sufficient supply of dNTP precursors for DNA synthesis

These mechanisms highlight SIRT2's importance in maintaining genomic integrity during cellular stress conditions. Dysregulation of these processes may contribute to genomic instability and potentially cancer development, explaining the observation that SIRT2 knockout mice show increased tumor development with age or when exposed to mutagenic agents .

What is the role of SIRT2 in T cell differentiation and autoimmune pathogenesis?

SIRT2 plays a critical role in T cell differentiation and contributes to autoimmune pathogenesis, particularly in systemic lupus erythematosus (SLE). Research demonstrates that SIRT2:

• Suppresses IL-2 production by CD4+ T cells while promoting their differentiation into Th17 cells

• Deacetylates p70S6K, leading to activation of the mTORC1/HIF-1α/RORγt pathway that drives Th17-cell differentiation

• Deacetylates c-Jun and histones at the Il-2 gene locus, resulting in decreased IL-2 production

These mechanisms contribute to the imbalance between Th17 cells and IL-2 production observed in SLE. CD4+ T cells from SLE patients exhibit increased expression of SIRT2 compared to cells from healthy individuals . Importantly, pharmacological inhibition of SIRT2 with the inhibitor AK-7 in CD4+ T cells from SLE patients:

• Increases the percentage of IL-2–producing CD4+ T cells

• Decreases the percentage of IL-17A–producing CD4+ T cells

In animal models, the SIRT2 inhibitor AK-7 limited disease severity in experimental autoimmune encephalomyelitis and lupus-prone MRL/lpr mice, suggesting SIRT2 as a promising molecular target for new SLE therapies .

How does SIRT2 interact with viral replication mechanisms?

SIRT2 exhibits context-dependent roles in viral infections, showing both pro-viral and anti-viral activities depending on the specific virus. The evidence shows:

• SIRT2 promotes human cytomegalovirus (HCMV) replication by regulating HCMV-mediated cell cycle dysregulation through its deacetylase activity

• SIRT2 enhances the replication of hepatitis B virus and human immunodeficiency virus

• Conversely, SIRT2 inhibits influenza A virus replication, demonstrating its virus-specific effects

In the context of HCMV infection, SIRT2 interacts with known substrates and regulators of gene expression, metabolism, and cell cycle progression during early stages of viral replication . Interestingly, previous research suggested that knockdown of SIRT2 resulted in elevated HCMV virus titers, which appears contradictory to more recent findings indicating a pro-viral role . This highlights the complex and possibly stage-specific roles of SIRT2 in viral infections.

The mechanisms by which SIRT2 modulates viral replication likely involve its ability to deacetylate specific cellular proteins that regulate antiviral responses or viral proteins directly, though further research is needed to fully elucidate these pathways.

What is known about SIRT2 dimerization and its functional consequences?

Recent research has revealed that SIRT2 can form dimers in human cells, with significant implications for its enzymatic activities . Key findings include:

• SIRT2 dimerization affects its deacylase activities, potentially altering substrate specificity

• The SIRT2 dimer typically escapes detection during standard chromatographic purification of recombinant protein

• Dimerization may represent an additional regulatory mechanism controlling SIRT2 function in different cellular contexts

This dimerization capability helps explain the diverse and sometimes contradictory roles of SIRT2 observed in different biological processes and disease states. The structural basis for SIRT2 dimerization and how it affects the enzyme's catalytic pocket or substrate binding domains remains an active area of investigation.

Methodologically, researchers studying SIRT2 should consider the potential impact of dimerization on experimental outcomes, particularly when using recombinant SIRT2 for in vitro enzymatic assays or when interpreting results from SIRT2 overexpression studies.

What methodological approaches are most effective for studying SIRT2 interactions?

Several methodological approaches have proven effective for investigating SIRT2 interactions with other proteins and its diverse functions:

• Co-immunoprecipitation coupled with mass spectrometry: Particularly useful for identifying novel SIRT2-interacting proteins during specific cellular processes or in response to stimuli. This approach has successfully identified 380 proteins that passed specificity thresholds in HCMV infection studies .

• Yeast two-hybrid screening: Has identified interactions between SIRT2 and proteins like Homeobox Transcription Factor HOXA10, contributing to understanding SIRT2's role in cell cycle progression .

• Chromatin immunoprecipitation (ChIP): Essential for investigating SIRT2's interactions with specific gene loci and understanding its role in transcriptional regulation.

• Primary cell cultures: While many SIRT2 studies have been conducted in immortalized cell lines, investigations in primary human cells provide more physiologically relevant insights into SIRT2 function. This is particularly important given SIRT2's context-dependent functions .

• Small molecule inhibitor studies: Using specific SIRT2 inhibitors like AK-7 has proven valuable for dissecting SIRT2 functions in various disease models, including autoimmune conditions .

When designing experiments to study SIRT2, researchers should consider the cell type, cellular compartment, and physiological context relevant to their hypothesis, as SIRT2 functions differ significantly depending on these factors.

Why are there contradictory findings regarding SIRT2's role in cancer progression?

The contradictory findings regarding SIRT2's role in cancer progression appear to stem from its context-dependent functions and diverse cellular activities. Evidence supports both tumor suppressor and tumor promoter roles:

As a tumor suppressor:

• SIRT2 knockout mice show increased tumor development with age

• SIRT2 knockout mice are more susceptible to tumorigenesis when challenged with mutagenic agents

• Decreased SIRT2 expression has been observed in certain cancers like gliomas

As a tumor promoter:

• Disruption of SIRT2 activity has shown anti-cancer effects in some experimental models

• SIRT2 may support cancer cell survival through metabolic adaptations in certain contexts

These contradictions likely result from SIRT2's multifaceted roles in:

• Cell cycle regulation

• Metabolic reprogramming

• Genomic integrity maintenance

• Inflammatory processes

• Cell differentiation

The specific role of SIRT2 in cancer depends on cancer type, genetic background, tumor microenvironment, and stage of disease progression. Methodologically, researchers investigating SIRT2 in cancer should carefully define the context of their study and consider multiple experimental approaches to fully characterize SIRT2's impact on specific cancer types or stages.