Pharmacologic or genetic activation of SIRT1 attenuates the fat-induced decrease in beta-cell function in vivo

Exploring histone deacetylases in type 2 diabetes mellitus: pathophysiological insights and therapeutic avenues

Diabetes mellitus is a chronic disease that impairs metabolism, and its prevalence has reached an epidemic proportion globally. Most people affected are with type 2 diabetes mellitus (T2DM), which is caused by a decline in the numbers or functioning of pancreatic endocrine islet cells, specifically the β-cells that release insulin in sufficient quantity to overcome any insulin resistance of the metabolic tissues. Genetic and epigenetic factors have been implicated as the main contributors to the T2DM. Epigenetic modifiers, histone deacetylases (HDACs), are enzymes that remove acetyl groups from histones and play an important role in a variety of molecular processes, including pancreatic cell destiny, insulin release, insulin production, insulin signalling, and glucose metabolism. HDACs also govern other regulatory processes related to diabetes, such as oxidative stress, inflammation, apoptosis, and fibrosis, revealed by network and functional analysis. This review explains the current understanding of the function of HDACs in diabetic pathophysiology, the inhibitory role of various HDAC inhibitors (HDACi), and their functional importance as biomarkers and possible therapeutic targets for T2DM. While their role in T2DM is still emerging, a better understanding of the role of HDACi may be relevant in improving insulin sensitivity, protecting β-cells and reducing T2DM-associated complications, among others.

SIRT1: a promising therapeutic target in type 2 diabetes mellitus

A significant increase in the worldwide incidence and prevalence of type 2 diabetic mellitus (T2DM) has elevated the need for studies on novel and effective therapeutic strategies. Sirtuin 1 (SIRT1) is an NAD + dependent protein deacetylase with a critical function in the regulation of glucose/lipid metabolism, insulin resistance, inflammation, oxidative stress, and mitochondrial function. SIRT1 is also involved in the regulation of insulin secretion from pancreatic β-cells and protecting these cells from inflammation and oxidative stress-mediated tissue damages. In this regard, major SIRT1 activators have been demonstrated to exert a beneficial impact in reversing T2DM-related complications including cardiomyopathy, nephropathy, retinopathy, and neuropathy, hence treating T2DM. Therefore, an accumulating number of recent studies have investigated the efficacy of targeting SIRT1 as a therapeutic strategy in T2DM. In this review we aimed to discuss the current understanding of the physiological and biological roles of SIRT1, then its implication in the pathogenesis of T2DM, and the therapeutic potential of SIRT1 in combating T2DM.

Prebiotic Potential of Dietary Beans and Pulses and Their Resistant Starch for Aging-Associated Gut and Metabolic Health

Dietary pulses, including dry beans, lentils, chickpeas, and dry peas, have the highest proportion of fiber among different legume cultivars and are inexpensive, easily accessible, and have a long shelf-life. The inclusion of pulses in regular dietary patterns is an easy and effective solution for achieving recommended fiber intake and maintaining a healthier gut and overall health. Dietary pulses-derived resistant starch (RS) is a relatively less explored prebiotic ingredient. Several in vitro and preclinical studies have elucidated the crucial role of RS in fostering and shaping the gut microbiota composition towards homeostasis thereby improving host metabolic health. However, in humans and aged animal models, the effect of only the cereals and tubers derived RS has been studied. In this context, this review collates literature pertaining to the beneficial effects of dietary pulses and their RS on gut microbiome-metabolome signatures in preclinical and clinical studies while contemplating their potential and prospects for better aging-associated gut health. In a nutshell, the incorporation of dietary pulses and their RS in diet fosters the growth of beneficial gut bacteria and significantly enhances the production of short-chain fatty acids in the colon.

Sirtuin function and metabolism: Role in pancreas, liver, and adipose tissue and their crosstalk impacting bone homeostasis

Mammalian sirtuins (SIRT1-7) are members of the nicotine adenine dinucleotide (NAD+)-dependent family of enzymes critical for histone deacetylation and posttranslational modification of proteins. Sirtuin family members regulate a wide spectrum of biological processes and are best known for maintaining longevity. Sirtuins are well characterized in metabolic tissues such as the pancreas, liver and adipose tissue (AT). They are regulated by a diverse range of stimuli, including nutrients and metabolic changes within the organism. Indeed, nutrient-associated conditions, such as obesity and anorexia nervosa (AN), were found to be associated with bone fragility development in osteoporosis. Interestingly, it has also been demonstrated that sirtuins, more specifically SIRT1, can regulate bone activity. Various studies have demonstrated the importance of sirtuins in bone in the regulation of bone homeostasis and maintenance of the balance between bone resorption and bone formation. However, to understand the molecular mechanisms involved in the negative regulation of bone homeostasis during overnutrition (obesity) or undernutrition, it is crucial to examine a wider picture and to determine the pancreatic, liver and adipose tissue pathway crosstalk responsible for bone loss. Particularly, under AN conditions, sirtuin family members are highly expressed in metabolic tissue, but this phenomenon is reversed in bone, and severe bone loss has been observed in human subjects. AN-associated bone loss may be connected to SIRT1 deficiency; however, additional factors may interfere with bone homeostasis. Thus, in this review, we focus on sirtuin activity in the pancreas, liver and AT in cases of over- and undernutrition, especially the regulation of their secretome by sirtuins. Furthermore, we examine how the secretome of the pancreas, liver and AT affects bone homeostasis, focusing on undernutrition. This review aims to lead to a better understanding of the crosstalk between sirtuins, metabolic organs and bone. In long term prospective it should contribute to promote improvement of therapeutic strategies for the prevention of metabolic diseases and the development of osteoporosis.

Nicotinamide Mononucleotide Prevents Free Fatty Acid-Induced Reduction in Glucose Tolerance by Decreasing Insulin Clearance

The NAD-dependent deacetylase SIRT1 improves β cell function. Accordingly, nicotinamide mononucleotide (NMN), the product of the rate-limiting step in NAD synthesis, prevents β cell dysfunction and glucose intolerance in mice fed a high-fat diet. The current study was performed to assess the effects of NMN on β cell dysfunction and glucose intolerance that are caused specifically by increased circulating free fatty acids (FFAs). NMN was intravenously infused, with or without oleate, in C57BL/6J mice over a 48-h-period to elevate intracellular NAD levels and consequently increase SIRT1 activity. Administration of NMN in the context of elevated plasma FFA levels considerably improved glucose tolerance. This was due not only to partial protection from FFA-induced β cell dysfunction but also, unexpectedly, to a significant decrease in insulin clearance. However, in conditions of normal FFA levels, NMN impaired glucose tolerance due to decreased β cell function. The presence of this dual action of NMN suggests caution in its proposed therapeutic use in humans.

Effects of Obesogenic Feeding and Free Fatty Acids on Circadian Secretion of Metabolic Hormones: Implications for the Development of Type 2 Diabetes

Circadian rhythms are 24-h internal biological rhythms within organisms that govern virtually all aspects of physiology. Interestingly, metabolic tissues have been found to express cell-autonomous clocks that govern their rhythmic activity throughout the day. Disruption of normal circadian rhythmicity, as induced by environmental factors such as shift work, significantly increases the risk for the development of metabolic diseases, including type 2 diabetes and obesity. More recently, obesogenic feeding and its fatty acid components have also been shown to be potent disruptors of normal circadian biology. Two key hormones that are released in response to nutrient intake are the anti-diabetic incretin hormone glucagon-like peptide-1, from intestinal L cells, and insulin secreted by pancreatic β cells, both of which are required for the maintenance of metabolic homeostasis. This review will focus on the circadian function of the L and β cells and how both obesogenic feeding and the saturated fatty acid, palmitate, affect their circadian clock and function. Following introduction of the core biological clock and the hierarchical organization of the mammalian circadian system, the circadian regulation of normal L and β cell function and the importance of GLP-1 and insulin in establishing metabolic control are discussed. The central focus of the review then considers the circadian-disrupting effects of obesogenic feeding and palmitate exposure in L and β cells, while providing insight into the potential causative role in the development of metabolic disease.

Sirtuins-Mediated System-Level Regulation of Mammalian Tissues at the Interface between Metabolism and Cell Cycle: A Systematic Review

Sirtuins are a family of highly conserved NAD+-dependent proteins and this dependency links Sirtuins directly to metabolism. Sirtuins' activity has been shown to extend the lifespan of several organisms and mainly through the post-translational modification of their many target proteins, with deacetylation being the most common modification. The seven mammalian Sirtuins, SIRT1 through SIRT7, have been implicated in regulating physiological responses to metabolism and stress by acting as nutrient sensors, linking environmental and nutrient signals to mammalian metabolic homeostasis. Furthermore, mammalian Sirtuins have been implicated in playing major roles in mammalian pathophysiological conditions such as inflammation, obesity and cancer. Mammalian Sirtuins are expressed heterogeneously among different organs and tissues, and the same holds true for their substrates. Thus, the function of mammalian Sirtuins together with their substrates is expected to vary among tissues. Any therapy depending on Sirtuins could therefore have different local as well as systemic effects. Here, an introduction to processes relevant for the actions of Sirtuins, such as metabolism and cell cycle, will be followed by reasoning on the system-level function of Sirtuins and their substrates in different mammalian tissues. Their involvement in the healthy metabolism and metabolic disorders will be reviewed and critically discussed.

Sirt1-PPARS Cross-Talk in Complex Metabolic Diseases and Inherited Disorders of the One Carbon Metabolism

Sirtuin1 (Sirt1) has a NAD (+) binding domain and modulates the acetylation status of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC1α) and Fork Head Box O1 transcription factor (Foxo1) according to the nutritional status. Sirt1 is decreased in obese patients and increased in weight loss. Its decreased expression explains part of the pathomechanisms of the metabolic syndrome, diabetes mellitus type 2 (DT2), cardiovascular diseases and nonalcoholic liver disease. Sirt1 plays an important role in the differentiation of adipocytes and in insulin signaling regulated by Foxo1 and phosphatidylinositol 3′-kinase (PI3K) signaling. Its overexpression attenuates inflammation and macrophage infiltration induced by a high fat diet. Its decreased expression plays a prominent role in the heart, liver and brain of rat as manifestations of fetal programming produced by deficit in vitamin B12 and folate during pregnancy and lactation through imbalanced methylation/acetylation of PGC1α and altered expression and methylation of nuclear receptors. The decreased expression of Sirt1 produced by impaired cellular availability of vitamin B12 results from endoplasmic reticulum stress through subcellular mislocalization of ELAVL1/HuR protein that shuttles Sirt1 mRNA between the nucleus and cytoplasm. Preclinical and clinical studies of Sirt1 agonists have produced contrasted results in the treatment of the metabolic syndrome. A preclinical study has produced promising results in the treatment of inherited disorders of vitamin B12 metabolism.

Proposed Tandem Effect of Physical Activity and Sirtuin 1 and 3 Activation in Regulating Glucose Homeostasis

Sirtuins (SIRTs) are seven nicotinamide adenine dinucleotide (NAD⁺)-dependent protein deacetylases enzymes (SIRT1–7) that play an important role in maintaining cellular homeostasis. Among those, the most studied are SIRT1 and SIRT3, a nuclear SIRT and a mitochondrial SIRT, respectively, which significantly impact with an increase in mammals’ lifespan by modulating metabolic cellular processes. Particularly, when activated, both SIRT1 and 3 enhance pancreatic β-cells’ insulin release and reduce inflammation and oxidative stress pancreatic damage, maintaining then glucose homeostasis. Therefore, SIRT1 and 3 activators have been proposed to prevent and counteract metabolic age-related diseases, such as type 2 diabetes mellitus (T2DM). Physical activity (PA) has a well-established beneficial effect on phenotypes of aging like β-cell dysfunction and diabetes mellitus. Recent experimental and clinical evidence reports that PA increases the expression levels of both SIRT1 and 3, suggesting that PA may exert its healthy contribute even by activating SIRTs. Therefore, in the present article, we discuss the role of SIRT1, SIRT3, and PA on β-cell function and on diabetes. We also discuss the possible interaction between PA and activation of SIRTs as a possible therapeutic strategy to maintain glucose hemostasis and to prevent T2DM and its complications, especially in the elderly population.