SIRTing out the link between autophagy and ageing

Effect of exercise and curcumin on cardiomyocyte molecular mediators associated with oxidative stress and autophagy in aged male rats

AIM

Aging can origin changes in the heart that may increase risk of developing cardiovascular disease. This study aimed to characterize autophagy alterations and related molecular mediators in the heart tissue in the aging alone or in combination with exercise and curcumin treatment.

METHODS

Seven young and twenty-eight elderly male Wistar rats were assigned into five groups, namely: young control, age, exercise, curcumin, and curcumin+exercise. Aged rats in exercise group run on treadmill (17 m/min) and in the curcumin group received curcumin (50 mg/kg) by gavage daily for 8 weeks for 2 months. At the end, heart samples were collected and used for determination of autophagy by immunostaining for LC3-phosphatidylethanolamine conjugate (LC3-II), apoptosis by TUNEL assay, Malondialdehyde (MDA) level by enzymatic assay and determination of mediators' molecules by ELISA for NADPH Oxidase 4 (NOX4), sirtuin 1 (SIRT-1), phosphorylated nuclear factor kappa-light-chain-enhancer of activated B cells (p-NF-Ƙb) protein levels and Sequestosome-1 (P62). Also, histological changes such as fibrosis evaluated by Masson trichrome staining.

RESULTS

Our results showed that autophagy, SIRT-1 level were significantly decreased and MDA, NOX4, p-NF-Ƙb and P62 levels were significantly increased in heart of aged group compared to young group. Also, significant increased apoptosis and fibrosis levels in the heart of aged rats were observed compared with young rats, whereas, these undesirable changes were improved by exercise and curcumin. Also, combination therapy of aged rats with curcumin and exercise showed more significant prominent effect on molecular mediators and histological changes in the heart compared with monotherapy.

CONCLUSION

These findings indicate that stress oxidative increase and autophagy decrease in the heart tissue of aged rats. The age induced the mentioned changes in the heart may in part be associated with down-expression of SIRT-1 and overexpression of NOX4 proteins. It was also showed that these age induced effects can be alleviated by treatment with exercise and curcumin. Since NF-Ƙb increased in both the age and treatment groups, it seems the age heart increased NF-Ƙb to be due to a compensatory mechanism.

Cellular Stresses and Stress Responses in the Pathogenesis of Insulin Resistance

Insulin resistance (IR), a key component of the metabolic syndrome, precedes the development of diabetes, cardiovascular disease, and Alzheimer's disease. Its etiological pathways are not well defined, although many contributory mechanisms have been established. This article summarizes such mechanisms into the hypothesis that factors like nutrient overload, physical inactivity, hypoxia, psychological stress, and environmental pollutants induce a network of cellular stresses, stress responses, and stress response dysregulations that jointly inhibit insulin signaling in insulin target cells including endothelial cells, hepatocytes, myocytes, hypothalamic neurons, and adipocytes. The insulin resistance-inducing cellular stresses include oxidative, nitrosative, carbonyl/electrophilic, genotoxic, and endoplasmic reticulum stresses; the stress responses include the ubiquitin-proteasome pathway, the DNA damage response, the unfolded protein response, apoptosis, inflammasome activation, and pyroptosis, while the dysregulated responses include the heat shock response, autophagy, and nuclear factor erythroid-2-related factor 2 signaling. Insulin target cells also produce metabolites that exacerbate cellular stress generation both locally and systemically, partly through recruitment and activation of myeloid cells which sustain a state of chronic inflammation. Thus, insulin resistance may be prevented or attenuated by multiple approaches targeting the different cellular stresses and stress responses.

Autophagy and Tubular Cell Death in the Kidney

Many common renal insults such as ischemia and toxic injury primarily target the tubular epithelial cells, especially the highly metabolically active proximal tubular segment. Tubular epithelial cells are particularly dependent on autophagy to maintain homeostasis and respond to stressors. The pattern of autophagy in the kidney has a unique spatial and chronologic signature. Recent evidence has shown that there is complex cross-talk between autophagy and various cell death pathways. This review specifically discusses the interplay between autophagy and cell death in the renal tubular epithelia. It is imperative to review this topic because recent discoveries have improved our mechanistic understanding of the autophagic process and have highlighted its broad clinical applications, making autophagy a major target for drug development.

Sirtuin 1 ablation in endothelial cells is associated with impaired angiogenesis and diastolic dysfunction

Discordant myocardial growth and angiogenesis can explain left ventricular (LV) hypertrophy progressing toward heart failure with aging. Sirtuin 1 expression declines with age; therefore we explored the role played by angiogenesis and Sirtuin 1 in the development of cardiomyopathy. We compared the cardiac function of 10- to 15-wk-old (wo), 30-40 wo, and 61-70 wo endothelial Sirtuin 1-deleted (Sirt1(endo-/-)) mice and their corresponding knockout controls (Sirt1(Flox/Flox)). After 30-40 wk, Sirt1(endo-/-) animals exhibited diastolic dysfunction (DD), decreased mRNA expression of Serca2a in the LV, and decreased capillary density compared with control animals despite a similar VEGFa mRNA expression. However, LV fibrosis and hypoxia-inducible factor (HIF)1α expression were not different. The creation of a transverse aortic constriction (TAC) provoked more severe DD and LV fibrosis in Sirt1(endo-/-) compared with control TAC animals. Although the VEGFa mRNA expression was not different and the protein expression of HIF1α was higher in the Sirt1(endo-/-) TAC animals, capillary density remained reduced. In cultured endothelial cells administration of Sirtuin 1 inhibitor decreased mRNA expression of VEGF receptors FLT 1 and FLK 1. Ex vivo capillary sprouting from aortic explants showed impaired angiogenic response to VEGF in the Sirt1(endo-/-) mice. In conclusion, the data demonstrate 1) a defect in angiogenesis preceding development of DD; 2) dispensability of endothelial Sirtuin 1 under unstressed conditions and during normal aging; and 3) impaired angiogenic adaptation and aggravated DD in Sirt1(endo-/-) mice challenged with LV overload.

Enhancement in efferocytosis of oxidized low-density lipoprotein-induced apoptotic RAW264.7 cells through Sirt1-mediated autophagy

Macrophages play a key role in atherosclerotic plaque formation and rupture. These phagocytic cells are important in the scavenging of modified lipoproteins, unwanted or dead cells and cellular debris through efferocytosis. Sirtuin1 (Sirt1), a member of the conserved sirtuin family and a key regulator in the progression of atherosclerosis exerts protective effects by regulating autophagy, a well-known survival mechanism. Inhibition of autophagy may also result in defective efferocytosis. This study aimed to investigate the effect of Sirt1 on the efferocytosis of oxidized low-density lipoprotein (ox-LDL)-induced apoptotic RAW264.7 cells through upregulation of autophagy. The apoptotic cells were incubated with high and low concentrations of Sirt1 activator resveratrol (RSV) and Sirt1 inhibitor nicotinamide (NAM) as well as autophagy inhibitor 3-methyladenine (3-MA) + low concentration RSV. Apoptosis was determined by flow cytometry (FCM) of annexin-V/propidium iodide (AV/PI) dual staining. Total proteins were extracted and protein levels were detected through western blot analysis. The ox-LDL uptake and efferocytosis of apoptotic RAW264.7 cells were detected by oil red O staining and calculation of the phagocytic index of apoptotic RAW264.7 cells. The expression of Sirt1 and autophagy marker proteins was simultaneously increased with the stimulation of low concentration RSV (all P<0.05) and decreased in low and high NAM groups (all P<0.05), compared with the control group. Efferocytosis was highest in the low concentration RSV group (P<0.001) and relatively lower in the low and high concentration NAM groups (both P<0.05) compared with the control group, which was similar to the change in the expression of Sirt1 and autophagy marker proteins. The results showed that the efferocytosis of apoptotic RAW264.7 cells was significantly improved with the upregulation of Sirt1-mediated autophagy. Therefore, Sirt1 may serve as a novel therapeutic target for the treatment of atherosclerosis.

Forms, crosstalks, and the role of phospholipid biosynthesis in autophagy

Autophagy is a highly conserved cellular process occurring during periods of stress to ensure a cell's survival by recycling cytosolic constituents and making products that can be used in energy generation and other essential processes. Three major forms of autophagy exist according to the specific mechanism through which cytoplasmic material is transported to a lysosome. Chaperone-mediated autophagy is a highly selective form of autophagy that delivers specific proteins for lysosomal degradation. Microautophagy is a less selective form of autophagy that occurs through lysosomal membrane invaginations, forming tubes and directly engulfing cytoplasm. Finally, macroautophagy involves formation of new membrane bilayers (autophagosomes) that engulf cytosolic material and deliver it to lysosomes. This review provides new insights on the crosstalks between different forms of autophagy and the significance of bilayer-forming phospholipid synthesis in autophagosomal membrane formation.

Sirtuins: molecular traffic lights in the crossroad of oxidative stress, chromatin remodeling, and transcription

Transcription is regulated by acetylation/deacetylation reactions of histone and nonhistone proteins mediated by enzymes called KATs and HDACs, respectively. As a major mechanism of transcriptional regulation, protein acetylation is a key controller of physiological processes such as cell cycle, DNA damage response, metabolism, apoptosis, and autophagy. The deacetylase activity of class III histone deacetylases or sirtuins depends on the presence of NAD⁺ (nicotinamide adenine dinucleotide), and therefore, their function is closely linked to cellular energy consumption. This activity of sirtuins connects the modulation of chromatin dynamics and transcriptional regulation under oxidative stress to cellular lifespan, glucose homeostasis, inflammation, and multiple aging-related diseases including cancer. Here we provide an overview of the recent developments in relation to the diverse biological activities associated with sirtuin enzymes and stress responsive transcription factors, DNA damage, and oxidative stress and relate the involvement of sirtuins in the regulation of these processes to oncogenesis. Since the majority of the molecular mechanisms implicated in these pathways have been described for Sirt1, this sirtuin family member is more extensively presented in this paper.

Sirtuin 1 in lipid metabolism and obesity

Sirtuin 1 (SIRT1), the mammalian ortholog of yeast Sir2, is a highly conserved NAD(+)-dependent protein deacetylase that has emerged as a key metabolic sensor that directly links environmental nutrient signals to animal metabolic homeostasis. SIRT1 is known to be involved in gluconeogenesis in the liver, fat mobilization in white adipose tissue, and insulin secretion in the pancreas. Recent studies have shown SIRT1 to regulate fatty acid oxidation in the liver, sense nutrient availability in the hypothalamus, influence obesity-induced inflammation in macrophages, and modulate the activity of the circadian clock in metabolic tissues. The activity of SIRT1 also appears to be under the control of AMPK and adiponectin. This review focuses on the involvement of SIRT1 in regulating metabolic diseases associated with obesity. It includes brief overviews of sirtuin signaling, with emphasis on SIRT1's role in the liver, macrophage, brain, and adipose tissue as it relates to obesity.