Design, Synthesis, and In Vivo Evaluation of Isosteviol Derivatives as New SIRT3 Activators with Highly Potent Cardioprotective Effects

Cardiovascular diseases (CVDs) persist as the predominant cause of mortality, urging the exploration of innovative pharmaceuticals. Mitochondrial dysfunction stands as a pivotal contributor to CVDs development. Sirtuin 3 (SIRT3), a prominent mitochondrial deacetylase known for its crucial role in protecting mitochondria against damage and dysfunction, has emerged as a promising therapeutic target for CVDs treatment. Utilizing isosteviol, a natural ent-beyerene diterpenoid, 24 derivatives were synthesized and evaluated in vivo using a zebrafish model, establishing a deduced structure-activity relationship. Among these, derivative 5v exhibited significant efficacy in doxorubicin-induced cardiomyopathy in zebrafish and murine models. Subsequent investigations revealed that 5v selectively elevated SIRT3 expression, leading to the upregulation of SOD2 and OPA1 expression, effectively preventing mitochondrial dysfunction, mitigating oxidative stress, and preserving cardiomyocyte viability. As a novel structural class of SIRT3 activators with robust therapeutic effects, 5v emerges as a promising candidate for further drug development.

Tubeimoside I protects against sepsis-induced cardiac dysfunction via SIRT3

Sepsis-induced cardiac dysfunction (SICD) is one of the key complications in sepsis and it is associated with adverse outcomes and increased mortality. There is no effective drug to treat SICD. Previously, we reported that tubeimoside I (TBM) improved survival of septic mice. The aim of this study is to figure out whether TBM ameliorates SICD. Also, SIRT3 was reported to protects against SICD. Our second aim is to confirm whether SIRT3 plays essential roles in TBM's protective effects against SICD. Our results demonstrated that TBM could alleviate SICD and SICD's key pathological factor, inflammation, oxidative stress, and apoptosis were all reduced by TBM. Notably, SICD induced a significant decrease in cardiac SIRT3 expression, while TBM treatment could reverse SIRT3 expression. To clarify whether TBM provides protection via SIRT3, we injected a specific SIRT3 inhibitor 3-(1H-1,2,3-triazol-4-yl) pyridine (3-TYP) into mice before TBM treatment. Then the cardioprotective effects of TBM were largely abolished by 3-TYP. This suggests that SIRT3 plays an essential role in TBM's cardioprotective effects. In vitro, TBM also protected H9c2 cells against LPS-induced injury, and siSIRT3 diminished these protective effects. Taken together, our results demonstrate that TBM protects against SICD via SIRT3. TBM might be a potential drug candidate for SICD treatment.

Pharmacological Advantage of SIRT2-Selective versus pan-SIRT1-3 Inhibitors

Because of their involvement in various biological pathways, the sirtuin enzyme family members SIRT1, SIRT2, and SIRT3 play both tumor-promoting and tumor-suppressing roles, based on the context and experimental conditions. Thus, an interesting question is whether inhibiting one of them or inhibiting all of them would be better for treating cancers. Pharmacologically, this is difficult to address, due in part to potential off-target effects of different compounds. Compounds with almost identical properties but differing in SIRT1-3 selectivity will be useful for addressing this question. Here, we have developed a pan SIRT1-3 inhibitor (NH4-6) and a SIRT2-selective inhibitor (NH4-13) with very similar chemical structures, with the only difference being the substitution of an ester bond to an amide bond. Such a minimal difference allows us to accurately compare the anticancer effect of pan SIRT1-3 inhibition and SIRT2-selective inhibition in cellular and mouse models. NH4-6 showed stronger cytotoxicity than NH4-13 in cancer cell lines. In mice, both inhibitors showed similar anticancer efficacy. However, NH4-6 is toxic to mice, which hinders the use of higher dosages. These results highlight the advantage of SIRT2-selective inhibitors as potential anticancer therapeutics.

Sirt3 regulates adipogenesis and adipokine secretion via its enzymatic activity

The purpose of this research was to identify if Sirt3 plays a role in marrow adipogenesis and adipokines secretion, especially adiponectin using bone marrow-derived stroma (ST2) cell model. Sirt3 overexpression leads to a significant increase in adipogenesis compared to controls. The induction of adipogenesis by Sirt3 is associated with increased gene expression of adipocyte markers as well as adiponectin/adipokines. In sharp contrast, the inhibition of Sirt3 exhibited significantly decreased adipogenesis, adipocyte markers, and adiponectin/adipokines compared to the controls. Interestingly, perilipin 1 (Plin 1) expression was decreased in Sirt3 induction but increased in Sirt3 inhibition. One hundred and fifteen mitochondrial acetylated peptides from 67 mitochondrial proteins had lower levels of acetylation in adipocytes induced by Sirt3 overexpression (Sirt3OE) compared to the control. Of the 67 proteins less enriched in acetylation, 22 acetylated proteins were decreased by more than twofold. These proteins are considered potential Sirt3 substrates in adipogenesis. In conclusion, Sirt3 has a novel, important role in modulating adipogenesis and adiponectin/adipokine expression. The connection axis among Sirt3-adipogenesis-adipokines was linked to its substrates by mass spectrometry analysis. These findings contribute to the efforts of revealing Sirt3 functions and Sirt3 usage as a potential target for treatment of metabolic homeostasis and diseases including type 2 diabetes.

Design, Synthesis, and Biological Evaluation of 8-Mercapto-3,7-Dihydro-1H-Purine-2,6-Diones as Potent Inhibitors of SIRT1, SIRT2, SIRT3, and SIRT5

Sirtuins (SIRT1-7) are a family of NAD⁺-dependent deacetylases. They regulate many physiological processes and play important roles in inflammation, diabetes, cancers, and neurodegeneration diseases. Sirtuin inhibitors have potential applications in the treatment of neurodegenerative diseases and various cancers. Herein, we identified new sirtuin inhibitors based on the scaffold of 8-mercapto-3,7-dihydro-1H-purine-2,6-dione. To elucidate the inhibitory mechanism, the binding modes of the inhibitors in SIRT3 were established by molecular docking, showing that the inhibitors occupy the acetyl lysine binding site and interact with SIRT3, mainly through hydrophobic interactions. The interactions were validated by site-directed mutagenesis of SIRT3 and structure-activity relationship analysis of the inhibitors. Consistently, enzyme kinetic assays and microscale thermophoresis showed that these compounds are competitive inhibitors to the acetyl substrate, and mix-type inhibitors to NAD⁺. Furthermore, we demonstrated that the compounds are potent SIRT1/2/3/5 pan-inhibitors. This study provides novel hits for developing more potent sirtuin inhibitors.

Non-oncogene Addiction to SIRT3 Plays a Critical Role in Lymphomagenesis

Diffuse large B cell lymphomas (DLBCLs) are genetically heterogeneous and highly proliferative neoplasms derived from germinal center (GC) B cells. Here, we show that DLBCLs are dependent on mitochondrial lysine deacetylase SIRT3 for proliferation, survival, self-renewal, and tumor growth in vivo regardless of disease subtype and genetics. SIRT3 knockout attenuated B cell lymphomagenesis in VavP-Bcl2 mice without affecting normal GC formation. Mechanistically, SIRT3 depletion impaired glutamine flux to the TCA cycle via glutamate dehydrogenase and reduction in acetyl-CoA pools, which in turn induce autophagy and cell death. We developed a mitochondrial-targeted class I sirtuin inhibitor, YC8-02, which phenocopied the effects of SIRT3 depletion and killed DLBCL cells. SIRT3 is thus a metabolic non-oncogene addiction and therapeutic target for DLBCLs.

Sirtuin 3 inhibition induces mitochondrial stress in tongue cancer by targeting mitochondrial fission and the JNK-Fis1 biological axis

Sirtuin 3 (Sirt3)-modified mitochondrial fission participates in the progression of several types of cancers. However, its role in tongue cancer requires investigation. The aim of our study is to determine whether Sirt3 knockdown regulates the viability of tongue cancer cells via modulating mitochondrial fission. Two types of tongue cancer cells were used in the present study, and siRNA was transfected into the cells to suppress Sirt3 expression. Mitochondrial function and cell apoptosis were determined via immunofluorescence, Western blotting, ELISA, and qPCR assays. A pathway blocker was applied to verify the role of the JNK-Fis1 signaling pathway in regulation of mitochondrial fission. The present study showed that loss of Sirt3 promoted tongue cancer cell death in a manner dependent on mitochondrial apoptosis. Mitochondrial oxidative stress, energy metabolism disorder, mitochondrial cyt-c liberation, and mitochondrial apoptosis activation were observed after Sirt3 silencing. Furthermore, we demonstrated that Sirt3 knockdown activated mitochondrial stress via triggering Fis1-related mitochondrial fission and that inhibition of Fis1-related mitochondrial fission abrogated the pro-apoptotic effect of Sirt3 knockdown on tongue cancer cells. To this end, we found that Sirt3 modulated Fis1 expression via the c-Jun N-terminal kinases (JNK) signaling pathway and that blockade of the JNK pathway attenuated mitochondrial stress and repressed apoptosis in Sirt3 knockdown cells. Altogether, our results identified a tumor-suppressive role for Sirt3 deficiency in tongue cancer via activation of the JNK-Fis1 axis and subsequent initiation of fatal mitochondrial fission. Given these findings, strategies to repress Sirt3 activity and enhance the JNK-Fis1-mitochondrial fission cascade have clinical benefits for patients with tongue cancer.

EphB2 signaling-mediated Sirt3 expression reduces MSC senescence by maintaining mitochondrial ROS homeostasis

Disruption of mitochondrial reactive oxygen species (mtROS) homeostasis is a key factor inducing UCB-MSC senescence. Accordingly, preventing mtROS accumulation will help in suppressing the UCB-MSC senescence. In this study, we observed that the expressions of EphrinB2 and EphB2 were inversely regulated by UCB-MSC passage-dependent manner. EphB2 signaling induced mitochondrial translocation of Sirt3. The knockdown of SIRT3 inhibited the effect of EphB2 signaling in UCB-MSCs. Subsequently, EphrinB2-Fc induced the nuclear translocation of Nrf-2 via c-Src phosphorylation dependent manner, and Sirt3 expression was regulated by Nrf-2. Among Sirt3 target genes, EphB2 signaling increased MnSOD and reduced the mtROS level in UCB-MSCs. Furthermore, the deacetylase effect of Sirt3 enhanced the MnSOD activity by deacetylation at the lysine 68 residue and therapeutic effect of UCB-MSCs on skin-wound healing was increased by EphB2 activation. In conclusion, the EphB2 can serve as a novel target for the optimizing the therapeutic use of UCB-MSCs in wound repair by MnSOD-mediated mtROS scavenging through EphB2/c-Src signaling pathway and Nrf-2-dependent Sirt3 expression.

Minnelide/Triptolide Impairs Mitochondrial Function by Regulating SIRT3 in P53-Dependent Manner in Non-Small Cell Lung Cancer

Minnelide/Triptolide (TL) has recently emerged as a potent anticancer drug in non-small cell lung cancer (NSCLC). However, the precise mechanism of its action remains ambiguous. In this study, we elucidated the molecular basis for TL-induced cell death in context to p53 status. Cell death was attributed to dysfunction of mitochondrial bioenergetics in p53-deficient cells, which was characterized by decreased mitochondrial respiration, steady-state ATP level and membrane potential, but augmented reactive oxygen species (ROS). Increased ROS production resulted in oxidative stress in TL-treated cells. This was exhibited by elevated nuclear levels of a redox-sensitive transcriptional factor, NF-E2-related factor-2 (NRF2), along with diminished cellular glutathione (GSH) content. We further demonstrated that in the absence of p53, TL blunted the expression of mitochondrial SIRT3 triggering increased acetylation of NDUAF9 and succinate dehydrogenase, components of complexes I and II of the electron transport chain (ETC). TL-mediated hyperacetylation of complexes I and II proteins and these complexes displayed decreased enzymatic activities. We also provide the evidence that P53 regulate steady-state level of SIRT3 through Proteasome-Pathway. Finally, forced overexpression of Sirt3, but not deacetylase-deficient mutant of Sirt3 (H243Y), restored the deleterious effect of TL on p53-deficient cells by rescuing mitochondrial bioenergetics. On contrary, Sirt3 deficiency in the background of wild-type p53 triggered TL-induced mitochondrial impairment that echoed TL effect in p53-deficeint cells. These findings illustrate a novel mechanism by which TL exerts its potent effects on mitochondrial function and ultimately the viability of NSCLC tumor.

SIRT1, 2, 3 protect mouse oocytes from postovulatory aging

The quality of metaphase II oocytes will undergo a time-dependent deterioration following ovulation as the result of the oocyte aging process. In this study, we determined that the expression of sirtuin family members (SIRT1, 2, 3) was dramatically reduced in mouse oocytes aged in vivo or in vitro. Increased intracellular ROS was observed when SIRT1, 2, 3 activity was inhibited. Increased frequency of spindle defects and disturbed distribution of mitochondria were also observed in MII oocytes aged in vitro after treatment with Nicotinamide (NAM), indicating that inhibition of SIRT1, 2, 3 may accelerate postovulatory oocyte aging. Interestingly, when MII oocytes were exposed to caffeine, the decline of SIRT1, 2, 3 mRNA levels was delayed and the aging-associated defective phenotypes could be improved. The results suggest that the SIRT1, 2, 3 pathway may play a potential protective role against postovulatory oocyte aging by controlling ROS generation.

A Novel Sirtuin-3 Inhibitor, LC-0296, Inhibits Cell Survival and Proliferation, and Promotes Apoptosis of Head and Neck Cancer Cells

BACKGROUND

The survival rate of patients with head and neck squamous cell carcinoma (HNSCC) stands at approximately 50% and this has not improved in decades. This study developed a novel sirtuin-3 (SIRT3) inhibitor (LC-0296) and examined its role in altering HNSCC tumorigenesis.

MATERIALS AND METHODS

The effect of the SIRT3 inhibitor, LC-0296, on cell survival, proliferation, and apoptosis, and reactive oxygen species levels in HNSCC cells were studied.

RESULTS

LC-0296 reduces cell proliferation and promotes apoptosis of HNSCC cells but not of normal human oral keratinocytes. This inhibitory effect is mediated, in part, via modulation of reactive oxygen species levels. Additionally, LC-0296 works synergistically to increase the sensitivity of HNSCC cells to radiation and cisplatin treatment.

CONCLUSION

Development of novel SIRT3 inhibitors, such as LC-0296, might enable the development of new targeted therapies to treat and improve the survival rate of patients with head and neck cancer.

Mechanisms of cell death pathway activation following drug-induced inhibition of mitochondrial complex I

Respiratory complex I inhibition by drugs and other chemicals has been implicated as a frequent mode of mitochondria-mediated cell injury. However, the exact mechanisms leading to the activation of cell death pathways are incompletely understood. This study was designed to explore the relative contributions to cell injury of three distinct consequences of complex I inhibition, i.e., impairment of ATP biosynthesis, increased formation of superoxide and, hence, peroxynitrite, and inhibition of the mitochondrial protein deacetylase, Sirt3, due to imbalance of the NADH/NAD(+) ratio. We used the antiviral drug efavirenz (EFV) to model drug-induced complex I inhibition. Exposure of cultured mouse hepatocytes to EFV resulted in a rapid onset of cell injury, featuring a no-effect level at 30µM EFV and submaximal effects at 50µM EFV. EFV caused a concentration-dependent decrease in cellular ATP levels. Furthermore, EFV resulted in increased formation of peroxynitrite and oxidation of mitochondrial protein thiols, including cyclophilin D (CypD). This was prevented by the superoxide scavenger, Fe-TCP, or the peroxynitrite decomposition catalyst, Fe-TMPyP. Both ferroporphyrins completely protected from EFV-induced cell injury, suggesting that peroxynitrite contributed to the cell injury. Finally, EFV increased the NADH/NAD(+) ratio, inhibited Sirt3 activity, and led to hyperacetylated lysine residues, including those in CypD. However, hepatocytes isolated from Sirt3-null mice were protected against 40µM EFV as compared to their wild-type controls. In conclusion, these data are compatible with the concept that chemical inhibition of complex I activates multiple pathways leading to cell injury; among these, peroxynitrite formation may be the most critical.

Small-molecule inhibitors at the PSD-95/nNOS interface attenuate MPP+-induced neuronal injury through Sirt3 mediated inhibition of mitochondrial dysfunction

Post-synaptic density protein 95 (PSD-95) links neuronal nitric oxide synthase (nNOS) with the N-methyl-D-aspartic acid (NMDA) receptor in the central nervous system, and this molecular complex has been implicated in regulating neuronal excitability in several neurological disorders. Here, small-molecule inhibitors of the PSD-95/nNOS interaction, IC87201 and ZL006 were tested for neuroprotective effects in an in vitro Parkinson's disease (PD) model. We now report that IC87201 and ZL006 reduced MPP(+)-induced neuronal injury and apoptotic cell death in a dose-dependent manner in cultured cortical neurons. These protective effects were associated with suppressed mitochondrial dysfunction, as evidenced by decreased reactive oxygen species (ROS) generation, cytochrome c release, mitochondrial membrane potential (MMP) collapse, and the preserved mitochondrial complex I activity and ATP synthesis. IC87201 and ZL006 also preserved intracellular homeostasis through mitigating mitochondrial Ca(2+) uptake and promoting mitochondrial Ca(2+) buffering capacity. Moreover, treatment with IC87201 and ZL006 significantly increased the expression of Sirt3 after MPP(+) exposure, and knockdown of Sirt3 using specific targeted small interfere RNA (siRNA) partially nullified the protective effects induced by these two inhibitors. These data strongly support the hypothesis that targeting the PSD-95/nNOS interaction produces neuroprotective effects and may represent a novel class of therapeutics for PD as well as other neurological diseases where detrimental NMDA receptor signaling plays a major role.

Mechanism of inhibition of the human sirtuin enzyme SIRT3 by nicotinamide: computational and experimental studies

Sirtuins are key regulators of many cellular functions including cell growth, apoptosis, metabolism, and genetic control of age-related diseases. Sirtuins are themselves regulated by their cofactor nicotinamide adenine dinucleotide (NAD+) as well as their reaction product nicotinamide (NAM), the physiological concentrations of which vary during the process of aging. Nicotinamide inhibits sirtuins through the so-called base exchange pathway, wherein rebinding of the reaction product to the enzyme accelerates the reverse reaction. We investigated the mechanism of nicotinamide inhibition of human SIRT3, the major mitochondrial sirtuin deacetylase, in vitro and in silico using experimental kinetic analysis and Molecular Mechanics-Poisson Boltzmann/Generalized Born Surface Area (MM-PB(GB)SA) binding affinity calculations with molecular dynamics sampling. Through experimental kinetic studies, we demonstrate that NAM inhibition of SIRT3 involves apparent competition between the inhibitor and the enzyme cofactor NAD+, contrary to the traditional characterization of base exchange as noncompetitive inhibition. We report a model for base exchange inhibition that relates such kinetic properties to physicochemical properties, including the free energies of enzyme-ligand binding, and estimate the latter through the first reported computational binding affinity calculations for SIRT3:NAD+, SIRT3:NAM, and analogous complexes for Sir2. The computational results support our kinetic model, establishing foundations for quantitative modeling of NAD+/NAM regulation of mammalian sirtuins during aging and the computational design of sirtuin activators that operate through alleviation of base exchange inhibition.

Sirtuin 3 regulates mouse pancreatic beta cell function and is suppressed in pancreatic islets isolated from human type 2 diabetic patients

AIMS/HYPOTHESIS

Sirtuin (SIRT)3 is a mitochondrial protein deacetylase that regulates reactive oxygen species (ROS) production and exerts anti-inflammatory effects. As chronic inflammation and mitochondrial dysfunction are key factors mediating pancreatic beta cell impairment in type 2 diabetes, we investigated the role of SIRT3 in the maintenance of beta cell function and mass in type 2 diabetes.

METHODS

We analysed changes in SIRT3 expression in experimental models of type 2 diabetes and in human islets isolated from type 2 diabetic patients. We also determined the effects of SIRT3 knockdown on beta cell function and mass in INS1 cells.

RESULTS

SIRT3 expression was markedly decreased in islets isolated from type 2 diabetes patients, as well as in mouse islets or INS1 cells incubated with IL1β and TNFα. SIRT3 knockdown in INS1 cells resulted in lowered insulin secretion, increased beta cell apoptosis and reduced expression of key beta cell genes. SIRT3 knockdown also blocked the protective effects of nicotinamide mononucleotide on pro-inflammatory cytokines in beta cells. The deleterious effects of SIRT3 knockdown were mediated by increased levels of cellular ROS and IL1β.

CONCLUSIONS/INTERPRETATION

Decreased beta cell SIRT3 levels could be a key step in the onset of beta cell dysfunction, occurring via abnormal elevation of ROS levels and amplification of beta cell IL1β synthesis. Strategies to increase the activity or levels of SIRT3 could generate attractive therapies for type 2 diabetes.

Oroxylin A induces dissociation of hexokinase II from the mitochondria and inhibits glycolysis by SIRT3-mediated deacetylation of cyclophilin D in breast carcinoma

Oroxylin A is a major active component of the Chinese traditional medicinal plant Scutellaria baicalensis Georgi, which has been reported as a potential anticancer drug. We demonstrated that, Oroxylin A inhibited the glycolysis and the binding of hexokinase II (HK II) with mitochondria in human breast carcinoma cell lines, which was dependent on sirtuin-3 (SIRT3). The level of SIRT3 in mitochondria was increased by Oroxylin A. Then SIRT3 deacetylated cyclophilin D, diminished its peptidyl-prolyl cis-trans isomerase activity and induced its dissociation from the adenine nucleotide translocator. Finally, SIRT3-induced inactivation of cyclophilin D resulted in the detachment of mitochondrial HK II and the inhibition of glycolysis. These results have important implications for the metabolism reprogramming effect and the susceptibility to Oroxylin A-induced mitochondrial cytotoxicity through the regulation of SIRT3 in breast carcinoma.

SIRT3 protects cardiomyocytes from oxidative stress-mediated cell death by activating NF-κB

Oxidative stress-mediated cell death in cardiomyocytes reportedly plays an important role in many cardiac pathologies. Our previous report demonstrated that mitochondrial SIRT3 plays an essential role in mediating cell survival in cardiac myocytes, and that resveratrol protects cardiomyocytes from oxidative stress-induced apoptosis by activating SIRT3. However, the exact mechanism by which SIRT3 prevents oxidative stress remains unknown. Here, we show that exposure of H9c2 cells to 50 μM H(2)O(2) for 6h caused a significant increase in cell death and the down-regulation of SIRT3. Reactive oxygen species (ROS)-mediated NF-κB activation was involved in this SIRT3 down-regulation. The SIRT3 activator, resveratrol, which is considered an important antioxidant, protected against H(2)O(2)-induced cell death, whereas the SIRT inhibitor, nicotinamide, enhanced cell death. Moreover, resveratrol negatively regulated H(2)O(2)-induced NF-κB activation, whereas nicotinamide enhanced H(2)O(2)-induced NF-κB activation. We also found that SOD2, Bcl-2 and Bax, the downstream genes of NF-κB, were involved in this pathological process. These results suggest that SIRT3 protects cardiomyocytes exposed to oxidative stress from apoptosis via a mechanism that may involve the NF-κB pathway.