SIRT1 Knockout Research Articles

SIRT1 inactivation switches reactive astrocytes to an antiinflammatory phenotype in CNS autoimmunity.

Astrocytes are highly heterogeneous in their phenotype and function, which contributes to CNS disease, repair, and aging; however, the molecular mechanism of their functional states remains largely unknown. Here, we show that activation of sirtuin 1 (SIRT1), a protein deacetylase, played an important role in the detrimental actions of reactive astrocytes, whereas its inactivation conferred these cells with antiinflammatory functions that inhibited the production of proinflammatory mediators by myeloid cells and microglia and promoted the differentiation of oligodendrocyte progenitor cells. Mice with astrocyte-specific Sirt1 knockout (Sirt1-/-) had suppressed progression of experimental autoimmune encephalomyelitis (EAE), an animal model of CNS inflammatory demyelinating disease. Ongoing EAE was also suppressed when Sirt1 expression in astrocytes was diminished by a CRISPR/Cas vector, resulting in reduced demyelination, decreased numbers of T cells, and an increased rate of IL-10-producing macrophages and microglia in the CNS, whereas the peripheral immune response remained unaffected. Mechanistically, Sirt1-/- astrocytes expressed a range of nuclear factor erythroid-derived 2-like 2 (Nfe2l2) target genes, and Nfe2l2 deficiency shifted the beneficial action of Sirt1-/- astrocytes to a detrimental one. These findings identify an approach for switching the functional state of reactive astrocytes that will facilitate the development of astrocyte-targeting therapies for inflammatory neurodegenerative diseases such as multiple sclerosis.

Sirtuin 1 aggravates hypertrophic heart failure caused by pressure overload via shifting energy metabolism

Sirtuin1 (SIRT1) is involved in regulating substrate metabolism in the cardiovascular system. Metabolic homeostasis plays a critical role in hypertrophic heart failure. We hypothesize that cardiac SIRT1 can modulate substrate metabolism during pressure overload-induced heart failure. The inducible cardiomyocyte Sirt1 knockout (icSirt1−/−) and its wild type littermates (Sirt1f/f) C57BL/6J mice were subjected to transverse aortic constriction (TAC) surgery to induce pressure overload. The pressure overload induces upregulation of cardiac SIRT1 in Sirt1f/f but not icSirt1−/− mice. The cardiac contractile dysfunctions caused by TAC-induced pressure overload occurred in Sirt1f/f but not in icSirt1−/− mice. Intriguingly, Sirt1f/f heart showed a drastic reduction in systolic contractility and electric signals during post-TAC surgery, whereas icSirt1−/− heart demonstrated significant resistance to pathological stress by TAC-induced pressure overload as evidenced by no significant changes in systolic contractile functions and electric properties. The targeted proteomics showed that the pressure overload triggered downregulation of the SIRT1-associated IDH2 (isocitrate dehydrogenase 2) that resulted in increased oxidative stress in mitochondria. Moreover, metabolic alterations were observed in Sirt1f/f but not in icSirt1−/− heart in response to TAC-induced pressure overload. Thus, SIRT1 interferes with metabolic homeostasis through mitochondrial IDH2 during pressure overload. Inhibition of SIRT1 activity benefits cardiac functions under pressure overload-related pathological conditions.

Abstract 15365: The Regulatory Role of Sirtuin 6 in Diabetic Cardiomyopathy

Introduction: DCM is a complex disease with many possible causes and multifactorial pathophysiology, but its underlying mechanisms are poorly understood, and no specific therapeutic guideline has yet been established. Increasing evidence suggests that coronary microvascular dysfunction (CMD) might be one of the primary mechanisms in DCM development. CMD is characterized by impaired endothelial-dependent vasodilation, but detailed mechanisms have yet to be elucidated. Sirtuin 6 (Sirt6) has been implicated in obesity, insulin resistance, type 2 diabetes mellitus, and cardiovascular diseases. Sirt6 protects EC from premature senescence, oxidative stress, and atherosclerosis by sustaining high eNOS levels and preserving cell replication. How Sirt6 regulates coronary microvascular function remains to be determined. Methods: Inducible Sirt6 global knockout, endothelial-specific knockout, and wild-type (WT) mice were fed a diet high in fat and sugar (HFHS), and blood lipid and glucose were measured. Coronary arteries were isolated, and endothelial-dependent vasodilation (EDD) was assessed using myography (DMT). Echocardiography and treadmill exercise exertion tests were performed to evaluate cardiac function. Myocardial blood flow (MBF) was measured by doppler. Cardiac fibrosis was detected using trichrome staining, and molecular pathways were elucidated via gene and protein analysis. Results: Our preliminary data show that Sirt6 was downregulated in diabetes. The EDD of coronary arterioles Sirt6 global knockout and endothelial-specific knockout treated with HFHS was impaired, and the mediator of coronary vasodilation switched from NO to H 2 O 2 in the Sirt6 knockout mice. Compared to the WT mice, myocardial blood flow was decreased, ejection fraction (EF) was not changed, the running distance was reduced, and E/E’ ratio was increased in Sirt6 knockout mice. The Sirt6 targeted proteins were identified. Conclusions: Our suggest Sirt6 regulated coronary microvascular function through endothelial cells, and ablation of Sirt6 in endothelial cells caused CMD and diastolic dysfunction. Further genetic profiling will elucidate the pathways and mechanisms converging with Sirt6 to regulate microvascular function.

Abstract 13621: Genetic and Pharmacological Induction of Endothelial Sirt3 After Onset of Hypertension Rescues Endothelial Function

Endothelial dysfunction plays critical role in the pathogenesis of hypertension and atherosclerosis. These pathological conditions are main risk factors for cardiovascular disease killing 700,000 people every year in US alone. We discovered a new control point with the potential to address this problem. We have shown that Sirt3 level is reduced in essential hypertension and endothelial Sirt3 overexpression attenuates endothelial dysfunction and hypertension. We proposed induction of endothelial Sirt3 after onset of hypertension to rescue endothelial function and reduce hypertension. To test the hypothesis, we used genetic- and pharmacological Sirt3 activation. To genetically induce the expression of endothelial Sirt3 we used Sirt3 flox/flox mice crossed with VeCad-Cre mice treated with low dose of 4-hydroxytamoxifen (4HOT, i.p. 0.3 mg/20 g daily, 5 days) which does not have off-target cardiovascular effects. Mice underwent telemetry placement and ten days later received 4-week osmotic pumps containing vehicle (saline) or angiotensin II (0.7 mg/kg/day). 14-days later half of mice received 4HOT to induce endothelial Sirt3 overexpression. Treatment of mice with 4HOT after onset of hypertension slightly reduced blood pressure but most importantly rescued endothelial-dependent relaxation, normalized mitochondrial O 2 . - and restored endothelial nitric oxide. Second, we tested if pharmacological induction of Sirt3 by hexafluoro (i.p. 8 mg/kg, 4 days) after onset of angiotensin II-induced hypertension improves endothelial function and reduces hypertension. It was found that hexafluoro substantially reduced systolic blood pressure, significantly diminished vascular mitochondrial O 2 . - and improved endothelial nitric oxide. To test the role of endothelial Sirt3 in pharmacological effect of hexafluoro we used endothelial specific Sirt3 knockout mice and angiotensin II model of hypertension. Interestingly, hexafluoro was not effective in endothelial Sirt3 deficient mice supporting specific role of endothelial Sirt3. These studies demonstrate therapeutic potential of genetic and pharmacological endothelial Sirt3 induction after onset of hypertension for treatment of endothelial dysfunction diminishing cardiovascular disease.

Abstract 9553: Sirtuin1 Aggravates Hypertrophic Heart Failure Caused by Pressure Overload via Shifting Energy Metabolism

Introduction: Sirtuin1 (SIRT1) serves as a deacetylase to regulate cardiac substrate metabolism during physiological and pathological conditions. The alteration in metabolic homeostasis plays a critical role in hypertrophic heart failure. Hypothesis: Cardiomyocyte SIRT1 modulates substrate metabolism to regulate cardiac remodeling during pressure overload-induced heart failure. Methods: The inducible cardiomyocyte specific Sirt1 knockout (icSirt1 -/- ) and its wild type littermates (Sirt1 f/f ) C57BL/6J mice were subjected to transverse aortic constriction (TAC) surgery to induce pressure overload. The cardiac functions were evaluated with echocardiography and electrocardiography measurements. Results: The pressure overload by TAC induces upregulation of cardiac SIRT1 in Sirt1 f/f but not icSirt1 -/- mice. The cardiac contractile dysfunctions caused by TAC-induced pressure overload occurred in Sirt1 f/f but not in icSirt1 -/- mice. Intriguingly, Sirt1 f/f heart showed a drastic reduction in systolic contractility and electric signals during post-TAC surgery, whereas icSirt1 -/- heart demonstrated significant resistance to the pathological stress by TAC-induced pressure overload as evidenced by no significant changes in systolic contractile functions and electric properties. The Seahorse data revealed that TAC-induced pressure overload causes a decrease in basal respiration and ATP generation, as well as reduced OXPHOS (oxidative phosphorylation) integrity of mitochondria in Sirt1 f/f but not in icSirt1 -/- cardiomyocytes. The targeted proteomics showed that the pressure overload trigged downregulation of the SIRT1-associated IDH2 (isocitrate dehydrogenase 2) that result in an increased oxidative stress in mitochondria. Moreover, a significant alteration in substrate metabolism was observed in Sirt1 f/f but not in icSirt1 -/- heart in response to TAC-induced pressure overload stress. Conclusions: SIRT1 interferes metabolic homeostasis through regulating mitochondrial IDH2 during pressure overload. Inhibition of SIRT1 activity benefits cardiac functions under pressure overload-related pathological conditions. SIRT1 antagonism could be a pharmacological approach for hypertrophic heart failure patients.

Endothelial SIRT-1 has a critical role in the maintenance of capillarization in brown adipose tissue

Brown adipose tissue (BAT) has critical roles in thermogenesis and systemic metabolism. Capillary rarefaction was reported to develop in BAT with dietary obesity, and previous studies showed that suppression of vascular endothelial growth factor A (VEGF-A) reduced capillary density in BAT, promoting the functional decline of this organ. Capillarization is regulated through the balance between angiogenesis and vasculogenesis on the one hand and apoptosis of endothelial cells (ECs) on the other; however, the role of EC apoptosis in BAT remained to be explored. In studies testing the role of boysenberry polyphenols (BoyP) in BAT, we found that BoyP decreased EC apoptosis, enhanced capillarization in BAT, and ameliorated dietary BAT dysfunction, which was associated with the upregulation of nicotinamide adenine dinucleotide-dependent protein deacetylase sirtuin 1 (SIRT-1) in ECs. Our studies suggest that EC SIRT-1 would be one of the potential targets of BoyP that contributes to BAT capillarization and function.

Abstract 046: Induction Of Endothelial Sirt3 After Onset Of Hypertension Rescues Endothelial Function

Endothelial dysfunction plays a key role in the pathogenesis of hypertension and represents a major risk factor for cardiovascular disease. Despite treatment with multiple drugs, only 1 in 4 patients has blood pressure under control. We discovered a new synergistic control point with the potential to address this problem. We have shown that Sirt3 level is reduced in essential hypertension and endothelial Sirt3 overexpression attenuates endothelial dysfunction and hypertension. We proposed that induction endothelial Sirt3 after onset of hypertension rescues endothelial function and reduces hypertension. To test this hypothesis, we used genetic- and pharmacological Sirt3 induction. To genetically induce the expression of endothelial Sirt3 we used Sirt3 flox/flox mice crossed with VeCad-Cre mice treated with low dose of 4-hydroxytamoxifen (4HOT, i.p. 0.3 mg/20 g daily, 5 days) which does not have off-target cardiovascular effects. Mice underwent telemetry placement and ten days later received 4-week osmotic pumps containing vehicle (saline) or angiotensin II (0.7 mg/kg/day). 14-days later half of mice received 4HOT to induce endothelial Sirt3 overexpression. Treatment of mice with 4HOT after onset of hypertension slightly reduced blood pressure but most importantly completely rescued endothelial-dependent relaxation, normalized mitochondrial O 2 . - and restored endothelial nitric oxide. Second, we tested if pharmacological induction of Sirt3 by hexafluoro (i.p. 8 mg/kg, 4 days) after onset of angiotensin II-induced hypertension improves endothelial function and reduces hypertension. It was found that hexafluoro substantially reduced systolic blood pressure, significantly diminished vascular mitochondrial O 2 . - and improved endothelial nitric oxide. To test the role of endothelial Sirt3 in pharmacological effect of hexafluoro we used endothelial specific Sirt3 knockout mice and angiotensin II model of hypertension. Interestingly, hexafluoro was not effective in endothelial Sirt3 deficient mice supporting critical role of endothelial Sirt3. These in vivo studies demonstrate therapeutic potential of genetic and pharmacological endothelial Sirt3 induction after onset of hypertension for treatment of endothelial dysfunction.

Sirtuin 6 protects against hepatic fibrogenesis by suppressing the YAP and TAZ function.

Hepatic fibrosis occurs in response to prolonged tissue injury in the liver, which results in abnormal accumulation of extracellular matrix. Hepatic stellate cells (HSCs) have been suggested to play a major role in liver fibrosis. However, the molecular mechanisms remain incompletely understood. Sirtuin 6 (SIRT6), an NAD+‐dependent deacetylase, has been previously implicated in the regulation of the transforming growth factor β (TGFβ)‐SMAD3 pathway that plays a significant role in liver fibrosis. In this work, we aimed to identify other important players during hepatic fibrogenesis, which are modulated by SIRT6. Yes‐associated protein (YAP) and transcriptional coactivator with PDZ‐binding motif (TAZ or WWTR1), key players in the Hippo pathway, have been implicated in the promotion of hepatic fibrosis. Our data show that HSC‐specific Sirt6 knockout mice are more susceptible to high‐fat‐cholesterol‐cholate diet‐induced hepatic fibrosis than their wildtype counterparts. Our signaling analyses suggest that in addition to the TGFβ‐SMAD3 pathway, YAP and TAZ are also highly activated in the SIRT6‐deficient HSCs. As it is not clear how SIRT6 might regulate YAP and TAZ, we have decided to elucidate the mechanism underlying the regulation of YAP and TAZ by SIRT6 in HSCs. Overexpression or knockdown of SIRT6 corroborates the role of SIRT6 in the negative regulation of YAP and TAZ. Further biochemical analyses reveal that SIRT6 deacetylates YAP and TAZ and reprograms the composition of the TEA domain transcription factor complex to suppress their downstream target genes, particularly those involved in hepatic fibrosis. In conclusion, our data suggest that SIRT6 plays a critical role in the regulation of the Hippo pathway to protect against hepatic fibrosis.

Spinal Sirtuin 3 Contributes to Electroacupuncture Analgesia in Mice With Chronic Constriction Injury–Induced Neuropathic Pain

BackgroundElectroacupuncture (EA) is a traditional Chinese therapeutic technique that has a beneficial effect on neuropathic pain; however, the specific mechanism remains unclear. In this study, we investigated whether EA inhibits spinal Ca/calmodulin-dependent protein kinase II (CaMKIIα) phosphorylation through Sirtuin 3 (SIRT3) protein, thus relieving neuropathic pain. Materials and MethodsWe used wild-type and SIRT3 knockout (SIRT3−/−) mice and used chronic constriction injury (CCI) as a pain model. We performed Western blotting, immunostaining, von Frey, and Hargreaves tests to gather biochemical and behavioral data. Downregulation and overexpression and spinal SIRT3 protein were achieved by intraspinal injection of SIRT3 small interfering RNA and intraspinal injection of lentivirus-SIRT3. To test the hypothesis that CaMKIIα signaling was involved in the analgesic effects of EA, we expressed CaMKIIα-specific designer receptors exclusively activated by designer drugs (DREADDs) in the spinal dorsal horn (SDH) of mice. ResultsThese results showed that the mechanical and thermal hyperalgesia induced by CCI was related to the decreased spinal SIRT3 expression in the SDH of mice. A significant reduction of mechanical and thermal thresholds was found in the SIRT3−/− mice. SIRT3 overexpression or EA treatment alleviated CCI-induced neuropathic pain and prevented the spinal CaMKIIα phosphorylation. Most importantly, EA increased the expression of spinal SIRT3 protein in the SDH. Downregulation of spinal SIRT3 or CaMKIIα Gq-DREADD activation inhibited the regulatory effect of EA on neuropathic pain. ConclusionOur results showed that CaMKIIα phosphorylation was inhibited by spinal SIRT3 in neuropathic pain and that EA attenuated CCI-induced neuropathic pain mainly by upregulating spinal SIRT3 expression in the SDH of mice.

Abstract P3138: The Regulatory Role Of Sirtuin 6 In Coronary Microvascular Dysfunction

Introduction: Coronary microvascular dysfunction (CMD) is characterized by impaired endothelial-dependent vasodilation. These impairments are seen in diabetic cardiomyopathy, ischemia with no obstructive coronary artery disease, and heart failure with preserved ejection fraction (HFpEF), but detailed mechanisms have yet to be elucidated. Sirtuin 6 (Sirt6), an important member of the Sirtuin family, has been implicated in obesity, insulin resistance, type 2 diabetes mellitus (T2DM), and cardiovascular diseases. Sirt6 protects EC from premature senescence, oxidative stress, and atherosclerosis by sustaining high eNOS levels and preserving cell replication. Sirt6 was reported to regulate endothelial dilation in atherosclerosis in Sirt6 heterozygous mice, but how Sirt6 regulates coronary microvascular function remains to be determined. Methods: Sirt6 knockout and wild-type (WT) mice were fed a diet high in fat and sugar (HFHS) for six months, and blood lipid and glucose were measured. Coronary arteries were isolated, and endothelial-dependent vasodilation (EDD) was assessed using myography (DMT). Echocardiography and treadmill exercise exertion tests were performed to evaluate cardiac function. Myocardial blood flow (MBF) was measured by doppler. Cardiac fibrosis was detected using trichrome staining, and molecular pathways were elucidated via gene and protein analysis. Results: Our preliminary data show that Sirt6 was downregulated in diabetes. The EDD of coronary arterioles was impaired, and the mediator of coronary vasodilation switched from NO to H 2 O 2 in the Sirt6 knockout mice. Compared to the WT mice, myocardial blood flow is decreased, ejection fraction (EF) was not changed, the running distance was reduced, and E/E’ ratio was increased in Sirt6 knockout mice. The Sirt6 targeted proteins were identified. Conclusions: Sirt6 regulated coronary microvascular function, and ablation of Sirt6 caused CMD and diastolic dysfunction. Further genetic profiling will elucidate the pathways and mechanisms converging with Sirt6 to regulate microvascular function.

Golgi stress induces SIRT2 to counteract Shigella infection via defatty-acylation

Enzymes from pathogens often modulate host protein post-translational modifications (PTMs), facilitating survival and proliferation of pathogens. Shigella virulence factors IpaJ and IcsB induce proteolytic cleavage and lysine fatty acylation on host proteins, which cause Golgi stress and suppress innate immunity, respectively. However, it is unknown whether host enzymes could reverse such modifications introduced by pathogens’ virulence factors to suppress pathogenesis. Herein, we report that SIRT2, a potent lysine defatty-acylase, is upregulated by the transcription factor CREB3 under Golgi stress induced by Shigella infection. SIRT2 in turn removes the lysine fatty acylation introduced by Shigella virulence factor IcsB to enhance host innate immunity. SIRT2 knockout mice are more susceptible to Shigella infection than wildtype mice, demonstrating the importance of SIRT2 to counteract Shigella infection.

Sirtuin 1 deletion increases inflammation and mortality in sepsis.

Sepsis is a hyperinflammatory response to infection that can lead to multiorgan failure and eventually death. Often, the onset of multiorgan failure is heralded by renal dysfunction. Sirtuin 1 (SIRT1) promotes cellular stress resilience by inhibiting inflammation and promoting mitochondrial function. We hypothesize that SIRT1 plays an important role in limiting the inflammatory responses that drive organ failure in sepsis, predominantly via expression in myeloid cells. We performed cecal ligation and puncture (CLP) on whole body SIRT1 knockout (S1KO) and myeloid cell-specific S1KO (S1KO-LysMCre) mice on a C57BL/6J background. Serum interleukin (IL)-6 was quantified by enzyme-linked immunosorbent assay. Renal mitochondrial complex activity was measured using Oxygraph-2k (Oroboros Instruments, Innsbruck, Austria). Blood urea nitrogen (BUN) was measured from serum. Survival was monitored for up to 5 days. Following CLP, S1KO mice had decreased renal mitochondrial complex I-dependent respiratory capacity (241.7 vs. 418.3 mmolO2/mg/min, p = 0.018) and renal mitochondrial complex II-dependent respiratory capacity (932.3 vs. 1,178.4, p = 0.027), as well as reduced rates of fatty acid oxidation (187.3 vs. 250.3, p = 0.022). Sirtuin 1 knockout mice also had increased BUN (48.0 mg/dL vs. 16.0 mg/dL, p = 0.049). Interleukin-6 levels were elevated in S1KO mice (96.5 ng/mL vs. 45.6 ng/mL, p = 0.028) and S1KO-LysMCre mice (35.8 ng/mL vs. 24.5 ng/mL, p = 0.033) compared with controls 12 hours after surgery. Five-day survival in S1KO (33.3% vs. 83.3%, p = 0.025) and S1KO-LysMCre (60% vs. 100%, p = 0.049) mice was decreased compared with controls. Sirtuin 1 deletion increases systemic inflammation in sepsis. Renal mitochondrial dysfunction, kidney injury, and mortality following CLP were all exacerbated by SIRT1 deletion. Similar effects on inflammation and survival were seen following myeloid cell-specific SIRT1 deletion, indicating that SIRT1 activity in myeloid cells may be a significant contributor for the protective effects of SIRT1 in sepsis.

Sirt6 deficiency contributes to mitochondrial fission and oxidative damage in podocytes via ROCK1-Drp1 signalling pathway.

ObjectivesIncreasing evidence suggests that mitochondrial dysfunction is the key driver of angiotensin II (Ang II)‐induced kidney injury. This study was designed to investigate whether Sirtuin 6 (Sirt6) could affect Ang II‐induced mitochondrial damage and the potential mechanisms.Materials and MethodsPodocyte‐specific Sirt6 knockout mice were infused with Ang II and cultured podocytes were stimulated with Ang II to evaluate the effects of Sirt6 on mitochondrial structure and function in podocytes. Immunofluorescence staining was used to detect protein expression and mitochondrial morphology in vitro. Electron microscopy was used to assess mitochondrial morphology in mice. Western blotting was used to quantify protein expression.ResultsMitochondrial fission and decreased Sirt6 expression were observed in podocytes from Ang II‐infused mice. In Sirt6‐deficient mice, Ang II infusion induced increased apoptosis and mitochondrial fragmentation in podocytes than that in Ang II‐infused wild‐type mice. In cultured human podocytes, Sirt6 knockdown exacerbated Ang II‐induced mitochondrial fission, whereas Sirt6 overexpression ameliorated the Ang II‐induced changes in the balance between mitochondrial fusion and fission. Functional studies revealed that Sirt6 deficiency exacerbated mitochondrial fission by promoting dynamin‐related protein 1 (Drp1) phosphorylation. Furthermore, Sirt6 mediated Drp1 phosphorylation by promoting Rho‐associated coiled coil‐containing protein kinase 1 (ROCK1) expression.ConclusionOur study has identified Sirt6 as a vital factor that protects against Ang II‐induced mitochondrial fission and apoptosis in podocytes via the ROCK1‐Drp1 signalling pathway.

SIRT2 Deficiency Exacerbates Hepatic Steatosis via a Putative Role of the ER Stress Pathway.

Nonalcoholic fatty liver disease (NAFLD), a condition strongly associated with obesity and insulin resistance, is characterized by hepatic lipid accumulation and activation of the endoplasmic reticulum (ER) stress response. The sirtuin 2 (SIRT2) protein deacetylase is emerging as a new player in metabolic homeostasis, but its role in the development of hepatic steatosis and its link with ER stress activation remains unknown. SIRT2-knockout (SIRT2-KO) and wild-type mice were fed either a control or a high-fat diet (HFD) for 4 weeks. Genetic manipulation of SIRT2 levels was performed in human hepatic cells. Although apparently normal under a control diet, SIRT2-KO mice showed accelerated body weight gain and adiposity on a HFD, accompanied by severe insulin resistance. Importantly, SIRT2-KO mice exhibited worsened hepatic steatosis independently from diet, consistent with upregulated gene expression of lipogenic enzymes and increased expression of ER stress markers. Exposure of hepatic cells to palmitate induced lipid accumulation, increased ER stress, and decreased SIRT2 expression. Moreover, SIRT2-silenced cells showed enhanced lipid accumulation and ER stress activation under basal conditions, whereas SIRT2 overexpression abrogated palmitate-induced lipid deposition and ER stress activation. Our findings reveal a role for SIRT2 in the regulation of hepatic lipid homeostasis, potentially through the ER stress response, suggesting that SIRT2 activation might constitute a therapeutic strategy against obesity and its metabolic complications.

Knockout of Sirt2 alleviates traumatic brain injury in mice.

Sirtuin 2 (SIRT2) inhibition or Sirt2 knockout in animal models protects against the development of neurodegenerative diseases and cerebral ischemia. However, the role of SIRT2 in traumatic brain injury (TBI) remains unclear. In this study, we found that knockout of Sirt2 in a mouse model of TBI reduced brain edema, attenuated disruption of the blood-brain barrier, decreased expression of the nucleotide-binding oligomerization domain-like receptor protein 3 (NLRP3) inflammasome, reduced the activity of the effector caspase-1, reduced neuroinflammation and neuronal pyroptosis, and improved neurological function. Knockout of Sirt2 in a mechanical stretch injury cell model in vitro also decreased expression of the NLRP3 inflammasome and pyroptosis. Our findings suggest that knockout of Sirt2 is neuroprotective against TBI; therefore, Sirt2 could be a novel target for TBI treatment.

Sirt2 positively regulates muscle regeneration after Notexin-induced muscle injury

Sirt2 regulates various biological processes by deacetylating target genes. Despite roles in regulating proliferation, cell cycle, and glucose metabolism, which are closely associated with skeletal muscle physiology, Sirt2 functions in this tissue remain unclear. In this study, genetic deletion of Sirt2 delayed muscle regeneration after Notexin-induced muscle injury. Gene expressions of myogenic regulatory factors, including Myf5, MyoD, and Myogenin, and cell cycle regulators, such as cyclin D1 and CDK2, were repressed in Sirt2 knockout mice after injury. Also, Sirt2 knockout mice presented muscle atrophy after muscle injury which is associated with the down-regulation of anabolic signaling and the up-regulation of catabolic signaling, in particular, increased atrogin1 transcriptional expression. Thus, Sirt2 positively regulated skeletal muscle regeneration after muscle injury by regulating transcriptional expression involved in myogenesis, cell cycle, and anabolic and catabolic signaling. Based on the in vivo analyses, Sirt2 could function as an interventional therapeutic for chronic myopathy, which is characterized by impaired muscle regeneration and muscle atrophy.

Novel Regulatory Role of SIRT3 on Cardiomyocyte Mitochondrial Frataxin and Ferroptosis

AIMSSirtuin 3 (SIRT3) has been shown to contribute to the mitochondrial cardiomyopathy of Friedreich’s Ataxia (FRDA), which is characterized by a deficiency in mitochondrial frataxin along with mitochondrial hyperacetylation. Using a cardiomyocyte‐specific SIRT3 knockout (SIRT3cKO) mouse model, we addressed two key questions: (1) whether SIRT3‐induced acetylation of proteins is specific to the mitochondria; and (2) if mitochondrial SIRT3 is a key regulator of mitochondrial frataxin which impacts mitochondrial iron homeostasis and ferroptosis.METHODS AND RESULTSThe mitochondrial and cytosolic fractions were isolated from the ventricles of the hearts of SIRT3cKO mice. The mitochondrial and cytosolic proteins and mitochondrial iron levels were analyzed by comparison to SIRT3‐Loxp wild‐type (WT) control mice. Cardiac function study showed that SIRT3cKO mice developed heart failure as evidenced by reduction of ejection fraction (EF) and fraction shortening (FS) and increased isovolumic relaxation time (IVRT) and myocardial performance index (MPI) when compared to WT controls. Comparison of the mitochondrial and cytosolic fractions of the SIRT3cKO model to those of the WT control shows that, upon loss of SIRT3, mitochondrial, but not cytosolic, total lysine acetylation was significantly increased in the heart. Similarly, acetylated p53 (p53ace) was significantly upregulated only in the mitochondria, while levels of p53 were not altered in either compartment. These data demonstrate that SIRT3 is the primary mitochondrial deacetylase, while acetylation in the cytosol is independent of SIRT3 in cardiomyocytes.Most importantly, loss of SIRT3 in the mitochondria resulted in significant reduction of the protein frataxin and the Iron (II) export protein ferroportin (FPN). Furthermore, levels of glutathione peroxidase 4 (GPX4) were also downregulated in the mitochondria. This was accompanied by a significant increase in levels of 4‐hydroxynonenal (4‐HNE), an indicator of lipid peroxidation, and suggestive of upregulated mitochondrial ferroptosis in SIRT3cKO mouse hearts. Additionally, mitophagy marker beclin‐1 expression was significantly reduced in the mitochondria of SIRT3cKO mice. Levels of glucose transporter‐1 (GLUT1), which functions to deliver the antioxidant Vitamin C to the mitochondria and reduce ROS production, were also diminished in the mitochondria. Mechanistically, mitochondrial levels of hypoxia inducible factor‐2α (HIF‐2α) were downregulated, while those of HIF‐1α remained unchanged in SIRT3cKO mouse hearts. Treatment with ferroptosis inhibitor ferrostatin‐1 for 14 days significantly reduced 4‐HNE levels and rescued preexisting impaired cardiac function in SIRT3cKO mice.CONCLUSIONSFor the first time, we have demonstrated that cardiomyocyte SIRT3 deficiency causes mitochondrion‐specific acetylation and impairment of frataxin and ferroportin potentially via downregulation of HIF‐2α. Our results suggest that the SIRT3‐ferroptosis pathway may be a novel target for the mitochondrial cardiomyopathy of FRDA.

SIRT1‐mediated Lysine Crotonylation is Involved in the Regulation of CaMKII Activity in Myocardium

ObjectivesPost‐translational modifications (PTMs) can modify proteins and regulate their functions. Lysine crotonylation is a novel protein PTM that can be regulated by silent mating type information regulation 2 homolog 1 (SIRT1). Ca2+/calmodulin‐dependent protein kinase II (CaMKII) plays a significant role in mediating cellular responses to external signaling molecules. Little is known regarding the effect of SIRT1 on CaMKII crotonylation and the impact of crotonylation on the activity of this enzyme.MethodsCre‐Loxp technology was used to generate conditional cardiac‐specific SIRT1 knockout mice (SIRT1 KO). Protein crotonylation modification was studied by PTMomics. CaMKII activity was determined by ELISA. Western blot and co‐immunoprecipitation (CO‐IP) were performed to examine the expression and crotonylation of CaMKIIResultsPTMomics showed changes in protein crotonylation profiles in the myocardium of cardiac‐specific SIRT1 knockout mice. Crotonylation of CaMKII was significantly increased at lysine 301 and the enhanced CaMKII crotonylation was further confirmed by CO‐IP (2.37±0.25 in SIRT1 KO vs. 0.96±0.10 in wild‐type and 1.04±0.10 in SIRT1fl/fl mice, p<0.01). Cardiac‐specific knockout of SIRT1 did not affect the protein expression of CaMKII whereas significantly increased CaMKII activity (2.75±0.17 m units/μL in SIRT1 KO vs. 2.10±0.13 m units/μL in wild‐type and 1.98±0.17 m units/μL in SIRT1fl/fl mice, p<0.05). CO‐IP showed no physical interaction between SIRT1 and CaMKII.ConclusionsSIRT1 deficiency increases CaMKII activity in myocardium at least partially through enhancing lysine crotonylation of CaMKII, which advances our understanding of the role played by PTM in the regulation of CaMKII.

Loss of SIRT1 inhibits hematopoietic stem cell aging and age-dependent mixed phenotype acute leukemia

Aging of hematopoietic stem cells (HSCs) is linked to various blood disorders and malignancies. SIRT1 has been implicated in healthy aging, but its role in HSC aging is poorly understood. Surprisingly, we found that Sirt1 knockout improved the maintenance of quiescence of aging HSCs and their functionality as well as mouse survival in serial bone marrow transplantation (BMT) recipients. The majority of secondary and tertiary BMT recipients of aging wild type donor cells developed B/myeloid mixed phenotype acute leukemia (MPAL), which was markedly inhibited by Sirt1 knockout. SIRT1 inhibition also reduced the growth and survival of human B/myeloid MPAL cells. Sirt1 knockout suppressed global gene activation in old HSCs, prominently the genes regulating protein synthesis and oxidative metabolism, which may involve multiple downstream transcriptional factors. Our results demonstrate an unexpected role of SIRT1 in promoting HSC aging and age-dependent MPAL and suggest SIRT1 may be a new therapeutic target for modulating functions of aging HSCs and treatment of MPAL.

Autotaxin signaling facilitates β cell dedifferentiation and dysfunction induced by Sirtuin 3 deficiency

Objectiveβ cell dedifferentiation may underlie the reversible reduction in pancreatic β cell mass and function in type 2 diabetes (T2D). We previously reported that β cell-specific Sirt3 knockout (Sirt3f/f;Cre/+) mice developed impaired glucose tolerance and glucose-stimulated insulin secretion after feeding with high fat diet (HFD). RNA sequencing showed that Sirt3-deficient islets had enhanced expression of Enpp2 (Autotaxin, or ATX), a secreted lysophospholipase which produces lysophosphatidic acid (LPA). Here, we hypothesized that activation of the ATX/LPA pathway contributed to pancreatic β cell dedifferentiation in Sirt3-deficient β cells. MethodsWe applied LPA, or lysophosphatidylcoline (LPC), the substrate of ATX for producing LPA, to MIN6 cell line and mouse islets with altered Sirt3 expression to investigate the effect of LPA on β cell dedifferentiation and its underlying mechanisms. To examine the pathological effects of ATX/LPA pathway, we injected the β cell selective adeno-associated virus (AAV-Atx-shRNA) or negative control AAV-scramble in Sirt3f/f and Sirt3f/f;Cre/+ mice followed by 6-week of HFD feeding. ResultsIn Sirt3f/f;Cre/+ mouse islets and Sirt3 knockdown MIN6 cells, ATX upregulation led to increased LPC with increased production of LPA. The latter not only induced reversible dedifferentiation in MIN6 cells and mouse islets, but also reduced glucose-stimulated insulin secretion from islets. In MIN6 cells, LPA induced phosphorylation of JNK/p38 MAPK which was accompanied by β cell dedifferentiation. The latter was suppressed by inhibitors of LPA receptor, JNK, and p38 MAPK. Importantly, inhibiting ATX in vivo improved insulin secretion and reduced β cell dedifferentiation in HFD-fed Sirt3f/f;Cre/+ mice. ConclusionsSirt3 prevents β cell dedifferentiation by inhibiting ATX expression and upregulation of LPA. These findings support a long-range signaling effect of Sirt3 which modulates the ATX-LPA pathway to reverse β cell dysfunction associated with glucolipotoxicity.