Molecular mechanisms linking type 2 diabetes mellitus and late-onset Alzheimer's disease: A systematic review and qualitative meta-analysis

Research evidence indicating common metabolic mechanisms through which type 2 diabetes mellitus (T2DM) increases risk of late-onset Alzheimer's dementia (LOAD) has accumulated over recent decades. The aim of this systematic review is to provide a comprehensive review of common mechanisms, which have hitherto been discussed in separate perspectives, and to assemble and evaluate candidate loci and epigenetic modifications contributing to polygenic risk linkages between T2DM and LOAD. For the systematic review on pathophysiological mechanisms, both human and animal studies up to December 2023 are included. For the qualitative meta-analysis of genomic bases, human association studies were examined; for epigenetic mechanisms, data from human studies and animal models were accepted. Papers describing pathophysiological studies were identified in databases, and further literature gathered from cited work. For genomic and epigenomic studies, literature mining was conducted by formalised search codes using Boolean operators in search engines, and augmented by GeneRif citations in Entrez Gene, and other sources (WikiGenes, etc.). For the systematic review of pathophysiological mechanisms, 923 publications were evaluated, and 138 gene loci extracted for testing candidate risk linkages. 3 57 publications were evaluated for genomic association and descriptions of epigenomic modifications. Overall accumulated results highlight insulin signalling, inflammation and inflammasome pathways, proteolysis, gluconeogenesis and glycolysis, glycosylation, lipoprotein metabolism and oxidation, cell cycle regulation or survival, autophagic-lysosomal pathways, and energy. Documented findings suggest interplay between brain insulin resistance, neuroinflammation, insult compensatory mechanisms, and peripheral metabolic dysregulation in T2DM and LOAD linkage. The results allow for more streamlined longitudinal studies of T2DM-LOAD risk linkages.

Chemistry-led investigations into the mode of action of NAMPT activators, resulting in the discovery of non-pyridyl class NAMPT activators

The cofactor nicotinamide adenine dinucleotide (NAD⁺) plays a key role in a wide range of physiological processes and maintaining or enhancing NAD⁺ levels is an established approach to enhancing healthy aging. Recently, several classes of nicotinamide phosphoribosyl transferase (NAMPT) activators have been shown to increase NAD⁺ levels in vitro and in vivo and to demonstrate beneficial effects in animal models. The best validated of these compounds are structurally related to known urea-type NAMPT inhibitors, however the basis for the switch from inhibitory activity to activation is not well understood. Here we report an evaluation of the structure activity relationships of NAMPT activators by designing, synthesising and testing compounds from other NAMPT ligand chemotypes and mimetics of putative phosphoribosylated adducts of known activators. The results of these studies led us to hypothesise that these activators act via a through-water interaction in the NAMPT active site, resulting in the design of the first known urea-class NAMPT activator that does not utilise a pyridine-like warhead, which shows similar or greater activity as a NAMPT activator in biochemical and cellular assays relative to known analogues.

Modulation of sirtuins during monolayer chondrocyte culture influences cartilage regeneration upon transfer to a 3D culture environment

This study examined the role of sirtuins in the regenerative potential of articular chondrocytes. Sirtuins (SIRT1-7) play a key role in regulating cartilage homeostasis. By inhibiting pro-inflammatory pathways responsible for cartilage degradation and promoting the expression of key matrix components, sirtuins have the potential to drive a favourable balance between anabolic and catabolic processes critical to regenerative medicine. When subjected to osmolarity and glucose concentrations representative of the in vivo niche, freshly isolated bovine chondrocytes exhibited increases in SIRT1 but not SIRT3 gene expression. Replicating methods adopted for the in vitro monolayer expansion of chondrocytes for cartilage regenerative therapies, we found that SIRT1 gene expression declined during expansion. Manipulation of sirtuin activity during in vitro expansion by supplementation with the SIRT1-specific activator SRT1720, nicotinamide mononucleotide, or the pan-sirtuin inhibitor nicotinamide, significantly influenced cartilage regeneration in subsequent 3D culture. Tissue mass, cellularity and extracellular matrix content were reduced in response to sirtuin inhibition during expansion, whilst sirtuin activation enhanced these measures of cartilage tissue regeneration. Modulation of sirtuin activity during monolayer expansion influenced H3K27me3, a heterochromatin mark with an important role in development and differentiation. Unexpectedly, treatment of primary chondrocytes with sirtuin activators in 3D culture reduced their matrix synthesis. Thus, modulating sirtuin activity during the in vitro monolayer expansion phase may represent a distinct opportunity to enhance the outcome of cartilage regenerative medicine techniques.

Betalain-rich dragon fruit (pitaya) consumption improves vascular function in men and women: a double-blind, randomized controlled crossover trial

BACKGROUND

Betalains are natural red color pigments abundant in red-fleshed dragon fruit (Hylocereus polyrhizus). Recent research has shown that dragon fruit consumption may help improve blood glucose and lipid profile. However, investigations of its cardioprotective properties in human trials, especially in nutritionally achievable amounts, remain nonexistent.

OBJECTIVES

The aim of this study was to investigate the effects of acute and short-term consumption of dragon fruit on vascular function in a healthy population.

METHODS

A randomized, double-blind, placebo-controlled, crossover trial was conducted in 19 young, healthy, nonsmoking men and women assigned to consume 24 g whole dragon fruit powder (33 mg betalains) or a nutrient-matched placebo, daily for 14 d. Flow-mediated dilation (FMD), arterial stiffness, and blood pressure (BP) were measured at 0 h, 1 h, 2 h, 3 h, and 4 h and finally at 14 d after daily consumption.

RESULTS

A total of 18 participants completed the trial. Dragon fruit consumption significantly improved acute FMD at 2 h (+0.8 ± 0.3%, P = 0.01), 3 h (+1.0 ± 0.3%, P = 0.001), and 4 h (+1.3 ± 0.4%, P < 0.001) postconsumption compared with placebo. This effect was sustained up until 14 d (+1.3 ± 0.2%, P < 0.001). Pulse-wave velocity was acutely significantly reduced at 3 h (-0.5 ± 0.2 m/s, P = 0.003), whereas augmentation index (AIx) also improved after 14 d (-7.0 ± 3.3%, P = 0.02) when compared with placebo. No differences were found in either peripheral or central BP across all time points.

CONCLUSIONS

Acute and short-term consumption of dragon fruit in dietary achievable amounts improved endothelial function and arterial stiffness in healthy individuals. This implies that regular dragon fruit consumption may have a meaningful impact on cardiovascular disease risk likely due to the high betalain content. This trial was registered at ClinicalTrials.gov as NCT03995602.

Carnosine protects stimulus-secretion coupling through prevention of protein carbonyl adduction events in cells under metabolic stress

Type 2 diabetes is characterised by failure to control glucose homeostasis, with numerous diabetic complications attributable to the resulting exposure of cells and tissues to chronic elevated concentrations of glucose and fatty acids. This, in part, results from formation of advanced glycation and advanced lipidation end-products that are able to modify protein, lipid, or DNA structure, and disrupt normal cellular function. Herein we used mass spectrometry to identify proteins modified by two such adduction events in serum of individuals with obesity, type 2 diabetes, and gestational diabetes, along with similar analyses of human and mouse skeletal muscle cells and mouse pancreatic islets exposed to glucolipotoxic stress. We also report that carnosine, a histidine containing dipeptide, prevented 65-90% of 4-hydroxynonenal and 3-nitrotyrosine adduction events, and that this in turn preserved mitochondrial function and protected stimulus-secretion coupling in cells exposed to metabolic stress. Carnosine therefore offers significant therapeutic potential against metabolic diseases.

Psoriatic skin inflammation induces a pre-diabetic phenotype via the endocrine actions of skin secretome

OBJECTIVE

Psoriasis is a chronic inflammatory skin disease that is thought to affect ∼2% of the global population. Psoriasis has been associated with ∼30% increased risk of developing type 2 diabetes (T2D), with numerous studies reporting that psoriasis is an independent risk-factor for T2D, separate from underlying obesity. Separately, studies of skin-specific transgenic mice have reported altered whole-body glucose homeostasis in these models. These studies imply a direct role for skin inflammation and dysfunction in mediating the onset of T2D in psoriasis patients, potentially via the endocrine effects of the skin secretome on key metabolic tissues. We used a combination of in vivo and ex vivo mouse models and ex vivo human imiquimod (IMQ) models to investigate the effects of psoriasis-mediated changes in the skin secretome on whole-body metabolic function.

METHODS

To induce psoriatic skin inflammation, mice were topically administered 75 mg of 5% IMQ cream (or Vaseline control) to a shaved dorsal region for 4 consecutive days. On day 5, mice were fasted for glucose and insulin tolerance testing, or sacrificed in the fed state with blood and tissues collected for analysis. To determine effects of the skin secretome, mouse skin was collected at day 5 from IMQ mice and cultured for 24 h. Conditioned media (CM) was collected and used 1:1 with fresh media to treat mouse explant subcutaneous adipose tissue (sAT) and isolated pancreatic islets. For human CM experiments, human skin was exposed to 5% IMQ cream for 20 min, ex vivo, to induce a psoriatic phenotype, then cultured for 24 h. CM was collected, combined 1:1 with fresh media and used to treat human sAT ex vivo. Markers of tissue inflammation and metabolic function were determined by qPCR. Beta cell function in isolated islets was measured by dynamic insulin secretion. Beta-cell proliferation was determined by measurement of Ki67 immunofluorescence histochemistry and BrDU uptake, whilst islet apoptosis was assessed by caspase 3/7 activity. All data is expressed as mean ± SEM.

RESULTS

Topical treatment with IMQ induced a psoriatic-like phenotype in mouse skin, evidenced by thickening, erythema and inflammation of the skin. Topical IMQ treatment induced inflammation and signs of metabolic dysfunction in sub-cutaneous and epidydimal adipose tissue, liver, skeletal muscle and gut tissue. However, consistent with islet compensation and a pre-diabetic phenotype, IMQ mice displayed improved glucose tolerance, increased insulin and c-peptide response to glucose, and increased beta cell proliferation. Treatment of sAT with psoriatic mouse or human skin-CM replicated the in vivo phenotype, leading to increased inflammation and metabolic dysfunction in mouse and human sAT. Treatment of pancreatic islets with psoriatic mouse skin-CM induced increases in beta-proliferation and apoptosis, thus partially replicating the in vivo phenotype.

CONCLUSIONS

Psoriasis-like skin inflammation induces a pre-diabetic phenotype, characterised by tissue inflammation and markers of metabolic dysfunction, together with islet compensation in mice. The in vivo phenotype is partially replicated by exposure of sAT and pancreatic islets to psoriatic-skin conditioned media. These results support the hypothesis that psoriatic skin inflammation, potentially via the endocrine actions of the skin secretome, may constitute a novel pathophysiological pathway mediating the development of T2D.

Immunohistochemistry of Kidney a-SMA, Collagen 1, and Collagen 3, in A Novel Mouse Model of Reno-cardiac Syndrome

Cardiorenal syndrome defines a synergistic pathology of the heart and kidneys where failure of one organ causes failure in the other. The incidence of cardiovascular mortality caused by this syndrome, is 20 fold higher in the end stage renal disease (ESRD) population compared to the population as a whole thus necessitating the need for improved therapeutic strategies to combat reno-cardiac pathologies. Murine in vivo models play a major role in such research permitting precise genetic modification thus reducing miscellany, however presently there is no steadfast model of reno-cardiac syndrome in the most common genetically modified mouse strain, the C57BL/6 mouse. In this study we have modified an established model of chronic renal disease using adenine diet and extended the associated pathology achieving chronic renal failure and consequent reno-cardiac syndrome in the C57BL/6 mouse. Eight week-old male C57BL/6 mice were acclimatized for 7 days before administration of a 0.15% adenine diet or control diet for 20 weeks after which the experiment was terminated and blood, urine and organs were collected and analyzed biochemically and by immunohistochemistry. Administration of 0.15% adenine diet caused progressive renal failure resulting in a reno-cardiac syndrome confirmed by a significantly increased heart to body weight ratio (P < 0.0001). Blood biochemistry showed that adenine fed mice had significantly increased serum creatinine, urea (P < 0.0001), and a significantly reduced glomerular filtration rate (P < 0.05), while immunohistochemistry of the kidneys for α-SMA, collagen 1 and collagen 3 showed severe fibrosis. We present a novel regimen of adenine diet which induces both chronic kidney disease and reno-cardiac syndrome in the C57BL/6 mouse strain. The non-surgical nature of this model makes it highly reproducible compared to other models currently available.

The effects of betalain-rich cacti (dragon fruit and cactus pear) on endothelial and vascular function: a systematic review of animal and human studies

Dragon fruit (Hylocereus) and cactus pear (Opuntia) are cacti species that have been widely used globally as a reliable source of food as well as traditional folk remedies. They have become of scientific interest recently due to their high levels of bioactive phytochemical compounds, in particular betalains. Earlier systematic reviews have explored the impact of supplementation of these cactus species on obesity, type-2 diabetes mellitus and cardiovascular risk factors: body weight and composition, serum triglycerides, cholesterol, blood glucose and blood pressure. However, effects on vascular health and endothelial function have yet to be reviewed. In order to address this gap in the literature, a systematic review has been conducted to evaluate the physiological effects of Hylocereus and Opuntia cacti on endothelial and vascular function in in vivo animal models and human studies. An electronic search was performed in the following databases: PubMed (MEDLINE), EMBASE (via Ovid), CINAHL, Scopus, Web of Science®, and The Cochrane Library (CENTRAL). All journals were searched since inceptions up to January 2020 without language restriction. Outcomes of interest were blood pressure, arterial stiffness, vascular reactivity and biochemical markers of endothelial dysfunction. Two investigators independently performed the study selection and data extraction. From 394 references, only 16 studies (9 animal and 7 human) fulfilled the eligibility criteria. Animal studies suggested a potential increase in vasodilation and serum nitric oxide and a reduction in vascular stiffness and blood pressure. The small number of human studies showed a reduction in heart rate as well as an increase in heart rate variability. Although these findings appear to indicate improvement in vascular health, there is a severe lack of robust, randomised human intervention studies to identify underlying mechanisms, optimal dose and long-term effects of cacti consumption.

Structure-functional changes in eNAMPT at high concentrations mediate mouse and human beta cell dysfunction in type 2 diabetes

AIMS/HYPOTHESIS

Progressive decline in functional beta cell mass is central to the development of type 2 diabetes. Elevated serum levels of extracellular nicotinamide phosphoribosyltransferase (eNAMPT) are associated with beta cell failure in type 2 diabetes and eNAMPT immuno-neutralisation improves glucose tolerance in mouse models of diabetes. Despite this, the effects of eNAMPT on functional beta cell mass are poorly elucidated, with some studies having separately reported beta cell-protective effects of eNAMPT. eNAMPT exists in structurally and functionally distinct monomeric and dimeric forms. Dimerisation is essential for the NAD-biosynthetic capacity of NAMPT. Monomeric eNAMPT does not possess NAD-biosynthetic capacity and may exert distinct NAD-independent effects. This study aimed to fully characterise the structure-functional effects of eNAMPT on pancreatic beta cell functional mass and to relate these to beta cell failure in type 2 diabetes.

METHODS

CD-1 mice and serum from obese humans who were without diabetes, with impaired fasting glucose (IFG) or with type 2 diabetes (from the Body Fat, Surgery and Hormone [BodyFatS&H] study) or with or at risk of developing type 2 diabetes (from the VaSera trial) were used in this study. We generated recombinant wild-type and monomeric eNAMPT to explore the effects of eNAMPT on functional beta cell mass in isolated mouse and human islets. Beta cell function was determined by static and dynamic insulin secretion and intracellular calcium microfluorimetry. NAD-biosynthetic capacity of eNAMPT was assessed by colorimetric and fluorescent assays and by native mass spectrometry. Islet cell number was determined by immunohistochemical staining for insulin, glucagon and somatostatin, with islet apoptosis determined by caspase 3/7 activity. Markers of inflammation and beta cell identity were determined by quantitative reverse transcription PCR. Total, monomeric and dimeric eNAMPT and nicotinamide mononucleotide (NMN) were evaluated by ELISA, western blot and fluorometric assay using serum from non-diabetic, glucose intolerant and type 2 diabetic individuals.

RESULTS

eNAMPT exerts bimodal and concentration- and structure-functional-dependent effects on beta cell functional mass. At low physiological concentrations (~1 ng/ml), as seen in serum from humans without diabetes, eNAMPT enhances beta cell function through NAD-dependent mechanisms, consistent with eNAMPT being present as a dimer. However, as eNAMPT concentrations rise to ~5 ng/ml, as in type 2 diabetes, eNAMPT begins to adopt a monomeric form and mediates beta cell dysfunction, reduced beta cell identity and number, increased alpha cell number and increased apoptosis, through NAD-independent proinflammatory mechanisms.

CONCLUSIONS/INTERPRETATION

We have characterised a novel mechanism of beta cell dysfunction in type 2 diabetes. At low physiological levels, eNAMPT exists in dimer form and maintains beta cell function and identity through NAD-dependent mechanisms. However, as eNAMPT levels rise, as in type 2 diabetes, structure-functional changes occur resulting in marked elevation of monomeric eNAMPT, which induces a diabetic phenotype in pancreatic islets. Strategies to selectively target monomeric eNAMPT could represent promising therapeutic strategies for the treatment of type 2 diabetes.

A novel model of reno-cardiac syndrome in the C57BL/ 6 mouse strain

Background

The end stage renal disease population has a 20 fold higher incidence of cardiovascular mortality compared to the overall population. The development of reno-cardiac syndrome in these patients will result in cardiovascular events to be the cause of 50% of fatalities. There is therefore a need to research improved therapeutic strategies to combat renal cardiac pathologies. Murine in vivo models contribute greatly to such research allowing for specific genetic modification and reduced miscellany, however there is currently no reliable model of reno-cardiac syndrome in the most common genetically modified mouse strain, the C57BL/6. In this study we have manipulated an established model of chronic renal disease using adenine infused diet and prolonged the course of its pathology achieving chronic renal failure and subsequent reno-cardiac syndrome in the C57BL/6 mouse.

Methods

Eight week-old male C57BL/ 6 mice were acclimatised for 7 days before administration of a 0.15% adenine diet or control diet for 20 weeks.

Cardiac function was assessed in mice at week 20 by echocardiography. At experiment termination blood and urine samples were analysed biochemically and organ dysfunction/injury was determined using immunoblotting and immunohistochemistry.

Results

Administration of 0.15% adenine diet caused progressive renal failure resulting in reno-cardiac syndrome. At endpoint uraemia was confirmed by blood biochemistry which in the adenine fed mice showed significant increases in serum creatinine, urea, calcium (P < 0.0001) potassium (P < 0.05), and a significantly reduced glomerular filtration rate (P < 0.05). Reno-cardiac syndrome was confirmed by a significantly increased heart to body weight ratio (P < 0.0001) and echocardiography which showed significant reductions in percentage of ejection fraction, fractional shortening, fractional area change, (P < 0.0001) and an increase in left ventricular end diastolic volume (P < 0.05). Immunoblotting of kidney and heart tissue showed increased apoptosis (caspase 3) and fibrosis (fibronectin) and increases in the cardiac levels of phosphorylated Akt, and renal total Akt. Immunohistochemistry for α-SMA, collagen 1 and collagen 3 further confirmed fibrosis.

Conclusions

We present a novel regimen of adenine diet which induces both chronic kidney disease and reno-cardiac syndrome in the C57/BL6 mouse strain. The non-surgical nature of this model makes it highly reproducible compared to other models currently available.

Electronic supplementary material

The online version of this article (10.1186/s12882-018-1155-3) contains supplementary material, which is available to authorized users.

Can the skin make you fat? A role for the skin in regulating adipose tissue function and whole-body glucose and lipid homeostasis

Prevalence of obesity and related complications such as type 2 diabetes (T2D) has increased dramatically in recent decades. Metabolic complications of obesity arise in part due to subcutaneous adipose tissue (SAT) dysfunction. However, it is currently unclear why some obese individuals develop insulin resistance and T2D and others do not. In this review, we discuss the role of the skin in regulating SAT function, and whether presence of inflammatory skin diseases such as psoriasis represent a novel risk mechanism mediating development of obesity-related complications.

Monomeric eNAMPT in the development of experimental diabetes in mice: a potential target for type 2 diabetes treatment

AIMS/HYPOTHESIS

Serum extracellular nicotinamide phosphoribosyltransferase (eNAMPT) concentrations are elevated in type 2 diabetes. However, the relationship between abnormally elevated serum eNAMPT and type 2 diabetes pathophysiology is unclear. eNAMPT circulates in functionally and structurally distinct monomeric and dimeric forms. Dimeric eNAMPT promotes NAD biosynthesis. The role of eNAMPT-monomer is unclear but it may have NAD-independent proinflammatory effects. However, studies of eNAMPT in type 2 diabetes have not distinguished between monomeric and dimeric forms. Since type 2 diabetes is characterised by chronic inflammation, we hypothesised a selective NAD-independent role for eNAMPT-monomer in type 2 diabetes.

METHODS

Two mouse models were used to examine the role of eNAMPT-monomer in type 2 diabetes; (1) a mouse model of diabetes fed a high-fat diet (HFD) for 10 weeks received i.p. injections with an anti-monomeric-eNAMPT antibody; and (2) lean non-diabetic mice received i.p. injections with recombinant monomeric eNAMPT daily for 14 days.

RESULTS

Serum monomeric eNAMPT levels were elevated in HFD-fed mouse models of diabetes, whilst eNAMPT-dimer levels were unchanged. eNAMPT-monomer neutralisation in HFD-fed mice resulted in lower blood glucose levels, amelioration of impaired glucose tolerance (IGT) and whole-body insulin resistance, improved pancreatic islet function, and reduced inflammation. These effects were maintained for at least 3 weeks post-treatment. eNAMPT-monomer administration induced a diabetic phenotype in mice, characterised by elevated blood glucose, IGT, impaired pancreatic insulin secretion and the presence of systemic and tissue inflammation, without changes in NAD levels.

CONCLUSIONS/INTERPRETATION

We demonstrate that elevation of monomeric-eNAMPT plays an important role in the pathogenesis of diet-induced diabetes via proinflammatory mechanisms. These data provide proof-of-concept evidence that the eNAMPT-monomer represents a potential therapeutic target for type 2 diabetes.

Early-life nutrition modulates the epigenetic state of specific rDNA genetic variants in mice

A suboptimal early-life environment, due to poor nutrition or stress during pregnancy, can influence lifelong phenotypes in the progeny. Epigenetic factors are thought to be key mediators of these effects. We show that protein restriction in mice from conception until weaning induces a linear correlation between growth restriction and DNA methylation at ribosomal DNA (rDNA). This epigenetic response remains into adulthood and is restricted to rDNA copies associated with a specific genetic variant within the promoter. Related effects are also found in models of maternal high-fat or obesogenic diets. Our work identifies environmentally induced epigenetic dynamics that are dependent on underlying genetic variation and establishes rDNA as a genomic target of nutritional insults.

A key role for interferon regulatory factors in mediating early-life metabolic defects in male offspring of maternal protein restricted rats

An adverse intra-uterine environment, induced by maternal consumption of diets high in saturated fat or low in protein have been implicated as a potential trigger for development of metabolic disease in later life. However, the underlying mechanisms responsible for this programming of obesity have yet to be described. Recent studies have demonstrated that interferon regulatory factors 3 (IRF3) and 4 (IRF4) function to repress adipogenesis. We investigated whether impaired IRF3 and IRF4 function may predispose to development of metabolic disease in a model of programmed obesity. Changes in IRF3 and IRF4 levels, adipogenic gene expression, and adiponectin signalling were measured in white adipose tissue from programmed male offspring of rat dams fed a low-protein diet (MLP), which are predisposed to obesity. 3T3L1 adipocytes were used to determine novel regulatory mechanisms governing IRF expression. IRF3 and IRF4 levels were suppressed in MLP rats, together with raised lipogenic and adipogenic gene expression. Adiponectin and adiponectin receptor 1 and 2 mRNA levels were reduced in MLP rats, along with levels of PPARα and activity of AMP-activated protein kinase (AMPK), 2 downstream targets of adiponectin. Further studies determined that both IRF3 and IRF4 are induced by adiponectin, with adiponectin-AMPK and adiponectin-PPARα signalling regulating IRF3 and IRF4, respectively. We have demonstrated that impaired ability to repress adipogenesis and lipogenesis, through dysregulated adiponectin-PPARα-AMPK-IRF signalling, may play a causal role in predisposing MLP offspring to development of obesity and metabolic disease in later life.

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.

Metformin opposes impaired AMPK and SIRT1 function and deleterious changes in core clock protein expression in white adipose tissue of genetically-obese db/db mice

AIM

AMPK activates SIRT1 in liver and skeletal muscle. Impaired circadian function is associated with development of obesity. SIRT1 regulates circadian function and is suppressed in white adipose tissue (WAT) of obese patients. We examined the potential role of AMPK and SIRT1 in regulation of circadian components in WAT of obese db/db mice and in mice fed a high-fat diet (HFD), and investigated whether metformin-mediated activation of AMPK opposed any deleterious changes in the WAT clock mechanism.

METHODS

db/+ and db/db mice were administered metformin (250 mg/kg/day; 7 days). Separately, mice were fed HFD for 16-weeks. 3T3-L1 adipocytes were incubated with metformin, EX527 or FK866, inhibitors of SIRT1 and NAMPT, respectively. Gene and protein expression were measured by qRT-PCR and immunoblotting.

RESULTS

AMPK activity, NAMPT expression and SIRT1 expression were decreased in WAT of db/db and HFD mice, in association with suppressed expression of the core circadian components CLOCK and BMAL1. Expression of Pparγ and the adipogenic repressors Irf3 and Irf4 were also suppressed. Metformin increased AMPK activity in WAT of db/db mice and in metformin-treated adipocytes, with increased NAMPT, SIRT1 and circadian component expression. Metformin-mediated induction of Clock mRNA in adipocytes was blocked by inhibition of NAMPT and SIRT1.

CONCLUSIONS

Decreased AMPK-SIRT1 signalling in db/db and HFD mice impacts WAT circadian function causing dysregulated lipid regulation, favouring an obese phenotype. Metformin mediates a phenotypic shift away from lipid accretion through AMPK-NAMPT-SIRT1 mediated changes in clock components, supporting chronotherapeutic treatment approaches for obesity.

Nicotinamide mononucleotide protects against pro-inflammatory cytokine-mediated impairment of mouse islet function

AIMS/HYPOTHESIS

Nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme for NAD(+) biosynthesis, exists as intracellular NAMPT (iNAMPT) and extracellular NAMPT (eNAMPT). eNAMPT, secreted from adipose tissue, promotes insulin secretion. Administration of nicotinamide mononucleotide (NMN), a product of the eNAMPT reaction, corrects impaired islet function in Nampt ( +/- ) mice. One of its potential targets is the NAD(+)-dependent deacetylase sirtuin 1. We hypothesised that altered NAMPT activity might contribute to the suppression of islet function associated with inflammation, and aimed to determine whether NMN could improve cytokine-mediated islet dysfunction.

METHODS

Acute effects of NMN on cytokine-mediated islet dysfunction were examined in islets incubated with TNFα and IL1β, and in mice fed a fructose-rich diet (FRD) for 16 weeks. Changes in iNAMPT, eNAMPT and inflammation levels were determined in FRD-fed mice.

RESULTS

FRD-fed mice displayed markedly lower levels of circulating eNAMPT, with impaired insulin secretion and raised islet expression of Il1b. NMN administration lowered Il1b expression and restored suppressed insulin secretion in FRD-fed mice. NMN also restored insulin secretion in islets cultured with pro-inflammatory cytokines. The changes in islet function corresponded with changes in key markers of islet function and differentiation. The anti-inflammatory effects of NMN were partially blocked by inhibition of sirtuin 1.

CONCLUSIONS/INTERPRETATION

Chronic fructose feeding causes severe islet dysfunction in mice. Onset of beta cell failure in FRD-fed mice may occur via lowered secretion of eNAMPT, leading to increased islet inflammation and impaired beta cell function. Administration of exogenous NMN to FRD-fed mice corrects inflammation-induced islet dysfunction. Modulation of this pathway may be an attractive target for amelioration of islet dysfunction associated with inflammation.

Fructose induces gluconeogenesis and lipogenesis through a SIRT1-dependent mechanism

Consumption of a fructose-rich diet leads to insulin resistance and dyslipidemia in part due to elevated gluconeogenesis and lipogenesis. SIRT1, an NAD(+)-dependent protein deacetylase, can induce gluconeogenesis and lipogenesis. The aim of this study was to determine whether fructose increased hepatic SIRT1, leading to induction of gluconeogenesis and lipogenesis. Rat hepatocytes were incubated with fructose (1-5 mM). SIRT1 protein, SIRT1 activity, and NAD(+)/NADH ratio were measured. The effects of SIRT1 inhibitors (EX-527 and nicotinamide) and activators (SIRT1 activator 3 and SRT1720) and the mitochondrial complex I inhibitor rotenone were examined on fructose-induced increases in gluconeogenesis and lipogenesis. Fructose increased SIRT1 protein, SIRT1 activity, and NAD(+)/NADH ratio. Fructose also induced gluconeogenesis, with increases in peroxisome proliferator-activated receptor coactivator 1-alpha (PGC1α) and phosphoenolpyruvate carboxykinase (PEPCK; gene code Pck1) gene expression, PEPCK activity, and hepatocyte glucose production. In addition, levels of 3-hydroxy-3-methylglutaryl coenzyme A reductase (Hmgcr) and acetyl-coA carboxylase (Acc) mRNA, and intracellular cholesterol were increased. Increases in gluconeogenesis, Hmgcr, Acc, and cholesterol were abolished by SIRT1 inhibitors and rotenone, while SIRT1 activators increased gluconeogenesis, Hmgcr, Acc, Pgc1β, and sterol regulatory element-binding protein 1c (Srebp1c) gene expression. In conclusion, fructose induces gluconeogenesis and lipogenesis through a SIRT1-dependent mechanism, suggesting that induction of hepatic SIRT1 could play a pivotal role in the metabolic changes observed in humans and animals consuming a fructose-rich diet. These results highlight the need for a greater understanding of the role of SIRT1 in metabolic regulation and indicate the potential for adverse effects of SIRT1 activators if used therapeutically.

Regulation of vascular endothelial function by procyanidin-rich foods and beverages

Flavonoid-rich diets are associated with a lower mortality from cardiovascular disease. This has been linked to improvements in endothelial function. However, the specific flavonoids, or biologically active metabolites, conferring these beneficial effects have yet to be fully defined. In this experimental study of the effect of flavonoids on endothelial function cultured endothelial cells have been used as a bioassay with endothelin-1 (ET-1) synthesis being measured an index of the response. Evaluation of the relative effects of extracts of cranberry juice compared to apple, cocoa, red wine, and green tea showed inhibition of ET-1 synthesis was dependent primarily on their oligomeric procyanidin content. Procyanidin-rich extracts of cranberry juice triggered morphological changes in endothelial cells with reorganization of the actin cytoskeleton and increased immunostaining for phosphotyrosine residues. These actions were independent of antioxidant activity. Comparison of the effects of apple procyanidin monomers through heptamer showed a clear structure-activity relationship. Although monomer, dimer, and trimer had little effect on ET-1 synthesis, procyanidin tetramer, pentamer, hexamer, and heptamer produced concentration-dependent decreases with IC(50) values of 5.4, 1.6, 0.9, and 0.7 microM, respectively. Levels of ET-1 mRNA showed a similar pattern of decreases, which were inversely correlated with increased expression of Kruppel-like factor 2 (KLF2), a key endothelial transcription factor with a broad range of antiatherosclerotic actions including suppression of ET-1 synthesis. Future investigations of procyanidin-rich products should assess the role KLF2 induction plays in the beneficial vascular effects of high flavonoid consumption.

Metformin suppresses hepatic gluconeogenesis through induction of SIRT1 and GCN5

Abnormal elevation of hepatic gluconeogenesis is central to the onset of hyperglycaemia in patients with type 2 diabetes mellitus (T2DM). Metformin corrects hyperglycaemia through inhibition of gluconeogenesis, but its mechanism of action is yet to be fully described. SIRT1 and GCN5 (listed as KAT2A in the MGI Database) have recently been identified as regulators of gluconeogenic gene expression through modulation of levels and activity of the coactivators cAMP-response element binding protein-regulated transcription coactivator 2 (TORC2 or CRTC2 as listed in the MGI Database) and peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC1alpha or PPARGC1A as listed in the MGI Database). We report that in db/db mice, metformin (250 mg/kg per day; 7 days) increases hepatic levels of GCN5 protein and mRNA compared with the untreated db/db mice, as well as increases levels of SIRT1 protein and activity relative to controls and untreated db/db mice. These changes were associated with reduced TORC2 protein level and decreased gene expression and activation of the PGC1alpha gene target phosphoenolpyruvate carboxykinase, and lower plasma glucose and insulin. Inhibition of SIRT1 partially blocked the effects of metformin on gluconeogenesis. SIRT1 was increased through an AMP-activated protein kinase-mediated increase in gene expression of nicotinamide phosphoribosyltransferase, the rate-limiting enzyme of the salvage pathway for NAD(+). Moreover, levels of GCN5 were dramatically reduced in db/db mice compared with the controls. This indicates that loss of GCN5-mediated inhibition of gluconeogenesis appears to constitute a major mechanism for the onset of abnormally elevated hepatic glucose production in db/db mice. In conclusion, induction of GCN5 and SIRT1 potentially represents a critical mechanism of action of metformin. In addition, these data identify induction of hepatic GCN5 as a potential therapeutic strategy for treatment of T2DM.

PPAR control: it's SIRTainly as easy as PGC

This review describes recent advances in our knowledge of the regulatory interactions influencing the expression of peroxisome proliferator-activated receptor (PPAR)-regulated genes. We address recent advances highlighting the role of PPARgamma (PPARG) coactivator-1 (PGC-1) and lipin-1 in co-ordinating the expression of genes controlling nutrient handling. We evaluate the possibility that SIRT1 lies at the heart of a regulatory loop involving PPARalpha, PGC-1alpha (PPARA, PPARGC1A as given in the HUGO Database), and lipin-1 (LPIN1 as listed in the HUGO Database) that ultimately controls the metabolic response to varying nutrient and physiological signals via a common mechanism mediated by post-translation modifications (deacetylation) of both PPARalpha and PGC-1s. Finally, we comment on the potential of pharmaceutical manipulation of these targets as well as the possible problems associated with this strategy.

Phenobarbital pre-treatment prevents kainic acid-induced impairments in acquisition learning

This study examined the protective effect of phenobarbital on kainic acid-induced deficits in acquisition learning. A single kainic acid injection (9 mg/kg i.p.) was administered five days prior to testing using the Morris water maze test. Kainic acid produced deficits in the acquisition of spatial information observed as an increase in latency to a hidden escape platform. Daily phenobarbital treatment (20 mg/kg i.p.) initiated 45 minutes prior to the kainic acid injection blocked the kainic acid-induced deficits in acquisition learning. When daily phenobarbital treatment was initiated 2-3 hours after kainic acid seizure development it did not block the kainic acid induced-deficits in water maze performance. Daily administration of phenobarbital alone at the moderate concentration used in this study did not cause alterations in behavioral performance in the Morris water maze. These studies indicate that phenobarbital pre-treatment results in a behavioral neuroprotection against kainic acid-induced neurotoxicity.

Involvement of nitric oxide in nitrous oxide anxiolysis in the elevated plus-maze

We recently reported that inhibition of nitric oxide (NO) production by the NO synthase (NOS) inhibitor L-NG-nitro arginine (L-NOARG) antagonized the behavioral effects of a benzodiazepine (BZ) in a mouse paradigm for screening anxiolytic drug activity. Because other research has found that the anesthetic gas nitrous oxide (N2O) also produces BZ-like behavioral effects, the present research was conducted to ascertain whether NO might also be involved in N2O anxiolysis. Male Swiss-Webster mice were tested in an elevated plus-maze inside an inflatable glovebag. Exposure to N2O significantly increased exploratory activity on the open arms of the plus-maze, as measured by the number of entries into the open arms and the time spent on the open arms. Pretreatment with L-NOARG significantly reduced the N2O-induced elevation in open arm activity. This antagonism of the N2O effect was reversed by ICV treatment of L-NOARG-pretreated mice with L-arginine but not D-arginine. These findings indicate that NO possibly mediates behavioral effects of N2O in an animal model for anxiety.