The loss of Sirt1 in mouse pancreatic beta cells impairs insulin secretion by disrupting glucose sensing

Rapid Elevation in CMPF May Act As a Tipping Point in Diabetes Development

Prediabetes, a state of mild glucose intolerance, can persist for years before a sudden decline in beta cell function and rapid deterioration to overt diabetes. The mechanism underlying this tipping point of beta cell dysfunction remains unknown. Here, the furan fatty acid metabolite CMPF was evaluated in a prospective cohort. Those who developed overt diabetes had a significant increase in CMPF over time, whereas prediabetics maintained chronically elevated levels, even up to 5 years before diagnosis. To evaluate the effect of increasing CMPF on diabetes progression, we used obese, insulin-resistant models of prediabetes. CMPF accelerated diabetes development by inducing metabolic remodeling, resulting in preferential utilization of fatty acids over glucose. This was associated with diminished glucose-stimulated insulin secretion, increased ROS formation, and accumulation of proinsulin, all characteristics of human diabetes. Thus, an increase in CMPF may represent the tipping point in diabetes development by accelerating beta cell dysfunction.

SIRT1 and insulin resistance

Sirtuin 1 (SIRT1) is a prototype mammalian NAD(+)-dependent protein deacetylase that has emerged as a key metabolic sensor in various metabolic tissues. Growing evidence suggests that SIRT1 regulates glucose and lipid metabolism through its deacetylase activity. In this review, we have summarized the recent progress in SIRT1 research with a particular focus on the role of SIRT1 in insulin resistance at different metabolic tissues. Recent data indicate that activated SIRT1 improves the insulin sensitivity of liver, skeletal muscle and adipose tissues and protects the function and cell mass of pancreatic β-cells. These findings suggest that SIRT1 might be a new therapeutic target for the prevention of disease related to insulin resistance, such as metabolic syndrome and type 2 diabetes mellitus.

Sirtuin 6 regulates glucose-stimulated insulin secretion in mouse pancreatic beta cells

AIMS/HYPOTHESIS

Sirtuin 6 (SIRT6) has been implicated in ageing, DNA repair and metabolism; however, its function in pancreatic beta cells is unclear. The aim of this study is to elucidate the role of SIRT6 in pancreatic beta cells.

METHODS

To investigate the function of SIRT6 in pancreatic beta cells, we performed Sirt6 gene knockdown in MIN6 cells and generated pancreatic- and beta cell-specific Sirt6 knockout mice. Islet morphology and glucose-stimulated insulin secretion (GSIS) were analysed. Glycolysis and oxygen consumption rates in SIRT6-deficient beta cells were measured. Cytosolic calcium was monitored using the Fura-2-AM fluorescent probe (Invitrogen, Grand Island, NY, USA). Mitochondria were analysed by immunoblots and electron microscopy.

RESULTS

Sirt6 knockdown in MIN6 beta cells led to a significant decrease in GSIS. Pancreatic beta cell Sirt6 knockout mice showed a ~50% decrease in GSIS. The knockout mouse islets had lower ATP levels compared with the wild-type controls. Mitochondrial oxygen consumption rates were significantly decreased in the SIRT6-deficient beta cells. Cytosolic calcium dynamics in response to glucose or potassium chloride were attenuated in the Sirt6 knockout islets. Numbers of damaged mitochondria were increased and mitochondrial complex levels were decreased in the SIRT6-deficient islets.

CONCLUSIONS/INTERPRETATION

These data suggest that SIRT6 is important for GSIS from pancreatic beta cells and activation of SIRT6 may be useful to improve insulin secretion in diabetes.

The Ticking of the Epigenetic Clock: Antipsychotic Drugs in Old Age

BACKGROUND

Exposed to antipsychotic drugs (APDs), older individuals with dementing illness are at risk of cerebrovascular adverse effects (CVAE), including sudden death. Transient microvascular dysfunctions are known to occur in younger persons exposed to APDs; however, they seldom progress to CVAE, suggesting that APDs alone are insufficient for engendering this untoward effect. It is, therefore, believed that a preexistent microvascular damage is necessary for CVAE to take place, but the exact nature of this lesion remains unclear. CNS small vessel disease (SVD) is a well-known age-related risk factor for strokes, dementia, and sudden death, which may constitute the initial CVAE-predisposing pathology. Therefore, we propose the two strikes CVAE paradigm, in which SVD represents the first strike, while exposure to APDs, the second. In this model, both strikes must be present for CVAE to take place, and the neuroimaging load of white matter hyperintensities may be directly proportional with the CVAE risk. To investigate this hypothesis at the molecular level, we focused on a seemingly unrelated phenomenon: both APDs and SVD were found protective against a similar repertoire of cancers and their spread to the brain (1-4). Since microRNA-29 has shown efficacy against the same malignancies and has been associated with small vessels pathology, we narrowed our search down to this miR, hypothesizing that the APDs mechanism of action includes miR-29 upregulation, which in turn facilitates the development of SVD.

AIM

To assess whether miR-29 can be utilized as a peripheral blood biomarker for SVD and CVAE risk.

METHOD

We conducted a search of experimentally verified miR-29 target genes utilizing the public domain tools miRanda, RNA22 and Weizemann Institute of Science miRNA Analysis. We identified in total 67 experimentally verified target genes for miR-29 family, 18 of which correlate with microvascular integrity and may be relevant for CVAE.

CONCLUSION

Upregulated microRNA-29 silences the expression of 18 genes connected with capillary stability, engendering a major vulnerability for SVD (first strike) which in turn increases the risk for CVAE after exposure to APDs (second strike).

HDAC4 mediates IFN-γ induced disruption of energy expenditure-related gene expression by repressing SIRT1 transcription in skeletal muscle cells

Metabolic homeostasis is achieved through balanced energy storage and output. Impairment of energy expenditure is a hallmark event in patients with obesity and type 2 diabetes. Previously we have shown that the pro-inflammatory cytokine interferon gamma (IFN-γ) disrupts energy expenditure in skeletal muscle cells via hypermethylated in cancer 1 (HIC1)-class II transactivator (CIITA) dependent repression of SIRT1 transcription. Here we report that repression of SIRT1 transcription by IFN-γ paralleled loss of histone acetylation on the SIRT1 promoter region with simultaneous recruitment of histone deacetylase 4 (HDAC4). IFN-γ activated HDAC4 in vitro and in vivo by up-regulating its expression and stimulating its nuclear accumulation. HIC1 and CIITA recruited HDAC4 to the SIRT1 promoter and cooperated with HDAC4 to repress SIRT1 transcription. HDAC4 depletion by small interfering RNA or pharmaceutical inhibition normalized histone acetylation on the SIRT1 promoter and restored SIRT1 expression in the presence of IFN-γ. Over-expression of HDAC4 suppressed the transcription of genes involved in energy expenditure in a SIRT1-dependent manner. In contrast, HDAC4 knockdown/inhibition neutralized the effect of IFN-γ on cellular metabolism by normalizing SIRT1 expression. Therefore, our data reveal a role for HDAC4 in regulating cellular energy output and as such provide insights into rationalized design of novel anti-diabetic therapeutics.

Pyrroloquinoline quinone increases the expression and activity of Sirt1 and -3 genes in HepG2 cells

Sirtuin (Sirt) 1 and Sirt 3 are nicotinamide adenine dinucleotide ((+))-dependent protein deacetylases that are important to a number of mitochondrial-related functions; thus, identification of sirtuin activators is important. Herein, we hypothesize that pyrroloquinoline quinone (PQQ) can act as a Sirt1/Sirt3 activator. In HepG2 cell cultures, PQQ increased the expression of Sirt1 and Sirt3 gene, protein, and activity levels (P < .05). We also observed a significant increase in nicotinamide phosphoribosyltransferase gene expression (as early as 18 hours) and increased NAD(+) activity at 24 hours. In addition, targets of Sirt1 and Sirt3 (peroxisome proliferator-activated receptor γ coactivator 1α, nuclear respiratory factor 1 and 2, and mitochondrial transcription factor A) were increased at 48 hours. This is the first report that demonstrates PQQ as an activator of Sirt1 and Sirt3 expression and activity, making it an attractive therapeutic agent for the treatment of metabolic diseases and for healthy aging. Based on our study and the available data in vivo, PQQ has the potential to serve as a therapeutic nutraceutical, when enhancing mitochondrial function.

Pancreas-Specific Sirt1-Deficiency in Mice Compromises Beta-Cell Function without Development of Hyperglycemia

AIMS/HYPOTHESIS

Sirtuin 1 (Sirt1) has been reported to be a critical positive regulator of glucose-stimulated insulin secretion in pancreatic beta-cells. The effects on islet cells and blood glucose levels when Sirt1 is deleted specifically in the pancreas are still unclear.

METHODS

This study examined islet glucose responsiveness, blood glucose levels, pancreatic islet histology and gene expression in Pdx1Cre; Sirt1ex4F/F mice that have loss of function and loss of expression of Sirt1 specifically in the pancreas.

RESULTS

We found that in the Pdx1Cre; Sirt1ex4F/F mice, the relative insulin positive area and the islet size distribution were unchanged. However, beta-cells were functionally impaired, presenting with lower glucose-stimulated insulin secretion. This defect was not due to a reduced expression of insulin but was associated with a decreased expression of the glucose transporter Slc2a2/Glut2 and of the Glucagon like peptide-1 receptor (Glp1r) as well as a marked down regulation of endoplasmic reticulum (ER) chaperones that participate in the Unfolded Protein Response (UPR) pathway. Counter intuitively, the Sirt1-deficient mice did not develop hyperglycemia. Pancreatic polypeptide (PP) cells were the only other islet cells affected, with reduced numbers in the Sirt1-deficient pancreas.

CONCLUSIONS/INTERPRETATION

This study provides new mechanistic insights showing that beta-cell function in Sirt1-deficient pancreas is affected due to altered glucose sensing and deregulation of the UPR pathway. Interestingly, we uncovered a context in which impaired beta-cell function is not accompanied by increased glycemia. This points to a unique compensatory mechanism. Given the reduction in PP, investigation of its role in the control of blood glucose is warranted.

Characterization of Zinc Influx Transporters (ZIPs) in Pancreatic β Cells: ROLES IN REGULATING CYTOSOLIC ZINC HOMEOSTASIS AND INSULIN SECRETION

Zinc plays an essential role in the regulation of pancreatic β cell function, affecting important processes including insulin biosynthesis, glucose-stimulated insulin secretion, and cell viability. Mutations in the zinc efflux transport protein ZnT8 have been linked with both type 1 and type 2 diabetes, further supporting an important role for zinc in glucose homeostasis. However, very little is known about how cytosolic zinc is controlled by zinc influx transporters (ZIPs). In this study, we examined the β cell and islet ZIP transcriptome and show consistent high expression of ZIP6 (Slc39a6) and ZIP7 (Slc39a7) genes across human and mouse islets and MIN6 β cells. Modulation of ZIP6 and ZIP7 expression significantly altered cytosolic zinc influx in pancreatic β cells, indicating an important role for ZIP6 and ZIP7 in regulating cellular zinc homeostasis. Functionally, this dysregulated cytosolic zinc homeostasis led to impaired insulin secretion. In parallel studies, we identified both ZIP6 and ZIP7 as potential interacting proteins with GLP-1R by a membrane yeast two-hybrid assay. Knock-down of ZIP6 but not ZIP7 in MIN6 β cells impaired the protective effects of GLP-1 on fatty acid-induced cell apoptosis, possibly via reduced activation of the p-ERK pathway. Therefore, our data suggest that ZIP6 and ZIP7 function as two important zinc influx transporters to regulate cytosolic zinc concentrations and insulin secretion in β cells. In particular, ZIP6 is also capable of directly interacting with GLP-1R to facilitate the protective effect of GLP-1 on β cell survival.

Alanyl-glutamine improves pancreatic β-cell function following ex vivo inflammatory challenge

Obesity-associated diabetes and concomitant inflammation may compromise pancreatic β-cell integrity and function. l-glutamine and l-alanine are potent insulin secretagogues, with antioxidant and cytoprotective properties. Herein, we studied whether the dipeptide l-alanyl-l-glutamine (Ala-Gln) could exert protective effects via sirtuin 1/HUR (SIRT1/HUR) signalling in β-cells, against detrimental responses following ex vivo stimulation with inflammatory mediators derived from macrophages (IMMs). The macrophages were derived from blood obtained from obese subjects. Macrophages were exposed (or not) to lipopolysaccharide (LPS) to generate a pro-inflammatory cytokine cocktail. The cytokine profile was determined following analysis by flow cytometry. Insulin-secreting BRIN-BD11 β-cells were exposed to IMMs and then cultured with or without Ala-Gln for 24 h. Chronic insulin secretion, the l-glutamine-glutathione (GSH) axis, and the level of insulin receptor β (IR-β), heat shock protein 70 (HSP70), SIRT1/HUR, CCAAT-enhancer-binding protein homologous protein (CHOP) and cytochrome c oxidase IV (COX IV) were evaluated. Concentrations of cytokines, including interleukin 1β (IL1β), IL6, IL10 and tumour necrosis factor alpha (TNFα) in the IMMs, were higher following exposure to LPS. Subsequently, when β-cells were exposed to IMMs, chronic insulin secretion, and IR-β and COX IV levels were decreased, but these effects were partially or fully attenuated by the addition of Ala-Gln. The glutamine-GSH axis and HSP70 levels, which were compromised by IMMs, were also restored by Ala-Gln, possibly due to protection of SIRT1/HUR levels, and a reduction of CHOP expression. Using an ex vivo inflammatory approach, we have demonstrated Ala-Gln-dependent β-cell protection mediated by coordinated effects on the glutamine-GSH axis, and the HSP pathway, maintenance of mitochondrial metabolism and stimulus-secretion coupling essential for insulin release.

Upregulation of Unc-51-like kinase 1 by nitric oxide stabilizes SIRT1, independent of autophagy

SIRT1 is central to the lifespan and vascular health, but undergoes degradation that contributes to several medical conditions, including diabetes. How SIRT1 turnover is regulated remains unclear. However, emerging evidence suggests that endothelial nitric oxide synthase (eNOS) positively regulates SIRT1 protein expression. We recently identified NO as an endogenous inhibitor of 26S proteasome functionality with a cellular reporter system. Here we extended this finding to a novel pathway that regulates SIRT1 protein breakdown. In cycloheximide (CHX)-treated endothelial cells, NONOate, an NO donor, and A23187, an eNOS activator, significantly stabilized SIRT1 protein. Similarly, NO enhanced SIRT1 protein, but not mRNA expression, in CHX-free cells. NO also stabilized an autophagy-related protein unc-51 like kinase (ULK1), but did not restore SIRT1 protein levels in ULK1-siRNA-treated cells or in mouse embryonic fibroblasts (MEF) from Ulk1-/- mice. This suggests that ULK1 mediated the NO regulation of SIRT1. Furthermore, adenoviral overexpression of ULK1 increased SIRT1 protein expression, while ULK1 siRNA treatment decreased it. Rapamycin-induced autophagy did not mimic these effects, suggesting that the effects of ULK1 were autophagy-independent. Treatment with MG132, a proteasome inhibitor, or siRNA of β-TrCP1, an E3 ligase, prevented SIRT1 reduction induced by ULK1-siRNA. Mechanistically, ULK1 negatively regulated 26S proteasome functionality, which was at least partly mediated by O-linked-GlcNAc transferase (OGT), probably by increased O-GlcNAc modification of proteasomal subunit Rpt2. The NO-ULK1-SIRT1 axis was likely operative in the whole animal: both ULK1 and SIRT1 protein levels were significantly reduced in tissue homogenates in eNOS-knockout mice (lung) and in db/db mice where eNOS is downregulated (lung and heart). Taken together, the results show that NO stabilizes SIRT1 by regulating 26S proteasome functionality through ULK1 and OGT, but not autophagy, in endothelial cells.

Inflammatory stimuli induce inhibitory S-nitrosylation of the deacetylase SIRT1 to increase acetylation and activation of p53 and p65

Inflammation increases the abundance of inducible nitric oxide synthase (iNOS), leading to enhanced production of nitric oxide (NO), which can modify proteins by S-nitrosylation. Enhanced NO production increases the activities of the transcription factors p53 and nuclear factor κB (NF-κB) in several models of disease-associated inflammation. S-nitrosylation inhibits the activity of the protein deacetylase SIRT1. SIRT1 limits apoptosis and inflammation by deacetylating p53 and p65 (also known as RelA), a subunit of NF-κB. We showed in multiple cultured mammalian cell lines that NO donors or inflammatory stimuli induced S-nitrosylation of SIRT1 within CXXC motifs, which inhibited SIRT1 by disrupting its ability to bind zinc. Inhibition of SIRT1 reduced deacetylation and promoted activation of p53 and p65, leading to apoptosis and increased expression of proinflammatory genes. In rodent models of systemic inflammation, Parkinson's disease, or aging-related muscular atrophy, S-nitrosylation of SIRT1 correlated with increased acetylation of p53 and p65 and activation of p53 and NF-κB target genes, suggesting that S-nitrosylation of SIRT1 may represent a proinflammatory switch common to many diseases and aging.

Cross-talk between SIRT1 and endocrine factors: effects on energy homeostasis

The mammalian sirtuins (SIRT1-7) are a family of highly conserved nicotine adenine dinucleotide (NAD(+))-dependent deacetylases that act as cellular sensors to detect energy availability. SIRT1 is a multifaceted protein that is involved in a wide variety of cellular processes. SIRT1 is activated in response to caloric restriction, acting on multiple targets in a wide range of tissues. SIRT1 regulates the role of multiple hormones implicated in energy balance, including glucose and lipid metabolism. Here, we review the relevant role of SIRT1 as a mediator of endocrine function of several hormones to modulate energy balance. In addition, we analyze the potential of targeting SIRT1 for the treatment of obesity and type 2 diabetes mellitus.

GABA protects pancreatic beta cells against apoptosis by increasing SIRT1 expression and activity

We have previously shown that GABA protects pancreatic islet cells against apoptosis and exerts anti-inflammatory effects. Notably, GABA inhibited the activation of NF-κB in both islet cells and lymphocytes. NF-κB activation is detrimental to beta cells by promoting apoptosis. However, the mechanisms by which GABA mediates these effects are unknown. Because the above-mentioned effects mimic the activity of sirtuin 1 (SIRT1) in beta cells, we investigated whether it is involved. SIRT1 is an NAD(+)-dependent deacetylase that enhances insulin secretion, and counteracts inflammatory signals in beta cells. We found that the incubation of a clonal beta-cell line (rat INS-1) with GABA increased the expression of SIRT1, as did GABA receptor agonists acting on either type A or B receptors. NAD(+) (an essential cofactor of SIRT1) was also increased. GABA augmented SIRT1 enzymatic activity, which resulted in deacetylation of the p65 component of NF-κB, and this is known to interfere with the activation this pathway. GABA increased insulin production and reduced drug-induced apoptosis, and these actions were reversed by SIRT1 inhibitors. We examined whether SIRT1 is similarly induced in newly isolated human islet cells. Indeed, GABA increased both NAD(+) and SIRT1 (but not sirtuins 2, 3 and 6). It protected human islet cells against spontaneous apoptosis in culture, and this was negated by a SIRT1 inhibitor. Thus, our findings suggest that major beneficial effects of GABA on beta cells are due to increased SIRT1 and NAD(+), and point to a new pathway for diabetes therapy.

SIRT1 metabolic actions: Integrating recent advances from mouse models

SIRT1 has attracted a lot of interest since it was discovered as a mammalian homolog of Sir2, a protein that influences longevity in yeast. Intensive early research suggested a key role of SIRT1 in mammalian development, metabolic flexibility and oxidative metabolism. However, it is the growing body of transgenic models that are allowing us to clearly define the true range of SIRT1 actions. In this review we aim to summarize the most recent lessons that transgenic animal models have taught us about the role of SIRT1 in mammalian metabolic homeostasis and lifespan.