Oxidative stress induces nucleo-cytoplasmic translocation of pancreatic transcription factor PDX-1 through activation of c-Jun NH(2)-terminal kinase

Hepcidin links gluco-toxicity to pancreatic beta cell dysfunction by inhibiting Pdx-1 expression

OBJECTIVE

Gluco-toxicity is a term used to convey the detrimental effect of hyperglycemia on β-cell function through impaired insulin synthesis. Although it is known that the expression and activity of several key insulin transcription regulators is inhibited, other molecular mechanisms that mediate gluco-toxicity are poorly defined. Our objective was to explore the role of hepcidin in β-cell gluco-toxicity.

DESIGN

We first confirmed that high glucose levels inhibited hepcidin expression in the mouse insulinoma cell line, MIN6. The downregulation of hepcidin decreased Pdx-1 expression, which reduced insulin synthesis.

METHODS

MIN6 cells were exposed to high glucose concentrations (33.3 mmol/L). Glucose-stimulated insulin secretion (GSIS) and serum hepcidin levels were measured by ELISA. The mRNA levels of insulin1, insulin2, Pdx-1 and hepcidin were measured by real-time polymerase chain reaction. Western blot analysis was used to detect the changes in PDX-1 expression. Transient overexpression with hepcidin was used to reverse the downregulation of Pdx-1 and insulin synthesis induced by gluco-toxicity.

RESULTS

Exposure of MIN6 cells to high glucose significantly decreased GSIS and inhibited insulin synthesis as well as Pdx-1 transcriptional activity and expression at both the mRNA and protein levels. High glucose also decreased hepcidin expression and secretion. Hepcidin overexpression in MIN6 cells partially reversed the gluco-toxicity-induced downregulation of Pdx-1 and insulin expression and improved GSIS. The restoration of insulin synthesis by transfection of a hepcidin overexpression plasmid confirmed the role of hepcidin in mediating the gluco-toxic inhibition of insulin synthesis.

CONCLUSIONS

Our observations suggest that hepcidin is associated with gluco-toxicity-reduced pancreatic β-cell insulin synthesis in type 2 diabetes by inhibiting Pdx-1 expression.

Radioactive Iodine Therapy and Glucose Tolerance

Radioactive iodine therapy is commonly used as an adjuvant therapy in follicular and papillary thyroid carcinoma (PTC) and in the treatment of Graves’ disease (GD). The basis of this therapy is the accumulation of radioactive iodine by the sodium-iodide symporter (NIS) in the thyroid gland. Expression of NIS by extrathyroidal tissues such as islets of pancreas has been reported. Radioactive iodine uptake by pancreatic beta-cells can potentially damage these cells. In this study, we discuss the possible associations between radioactive iodine and glucose intolerance. Overall, radioactive iodine uptake by the pancreas may damage beta-cells and predispose patients to glucose intolerance or type 2 diabetes, particularly in patients exposed to radioactive iodine therapy following total thyroidectomy. Further studies are needed to clarify and confirm this association.

L-Arginine supplementation improves insulin sensitivity and beta cell function in the offspring of diabetic rats through AKT and PDX-1 activation

Maternal hyperglycemia can result in defects in glucose metabolism and pancreatic β-cell function in offspring. The purpose of this study was to evaluate the impact of maternal diabetes mellitus on pancreatic islets, muscle and adipose tissue of the offspring, with or without oral l-Arginine supplementation. The induction of diabetes was performed using streptozotocin (60mg/kg). Animals were studied at 3 months of age and treatment (sucrose or l-Arginine) was administered from weaning. We observed that l-Arg improved insulin sensitivity in the offspring of diabetic mothers (DA), reflected by higher insulin-induced phosphorylation of Akt in muscle and adipose tissue. Insulin resistance is associated with increased oxidative stress and the NADPH oxidase enzyme plays an important role. Our results showed that the augmented interaction of p47^PHOX with gp91^PHOX subunits of the enzyme in skeletal muscle tissue in the offspring of diabetic rats (DV) was abolished after l-Arg treatment in DA rats. Maternal diabetes caused alterations in the islet functionality of the offspring leading to increased insulin secretion at both low (2.8mM) and high (16.7mM) concentrations of glucose. l-Arg reverses this effect, suggesting that it may be an important modulator in the insulin secretory process. In addition it is possible that l-Arg exerts its effects directly onto essential molecules for the maintenance and survival of pancreatic islets, decreasing protein expression of p47^PHOX while increasing Akt phosphorylation and PDX-1 expression. The mechanism by which l-Arg exerts its beneficial effects may involve nitric oxide bioavailability since treatment restored NO levels in the pancreas.

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.

T-Helper Cell-Mediated Islet Inflammation Contributes to β-Cell Dysfunction in Chronic Pancreatitis

OBJECTIVES

Beta-cell dysfunction and endocrine insufficiency in chronic pancreatitis (CP) is considered as a late manifestation emanating from fibrosis. To ascertain the role of T-helper cells in β-cell dysfunction, we enumerated circulating T-cell subsets, examined their infiltration into pancreatic islets, and assessed islet functions.

METHODS

Pancreatic tissues and peripheral blood were obtained from CP patients with/without diabetes. T cells were enumerated on flow cytometry and by immunostaining. Islets were assessed for glucose-stimulated insulin release (GSIR) and apoptosis (Annexin V/caspase-3). Islet proteins were probed for insulin gene transcription factor.

RESULTS

Circulating T-helper type 1 (Th1) cells were higher (P < 0.003) in CP patients with diabetes in comparison with control and CP patients without diabetes. Intra-islet colocalization of Th1 and Th17 cells was evident. In comparison with the controls, 2% ± 0.87% β cells from CP patients without diabetes were apoptotic whereas GSIR was decreased by 60% ± 12%, and 40% ± 9% from CP patients with diabetes were apoptotic, with minimal GSIR (1.42% ± 0.9%) in the remaining 60% viable cells. Western blots of islet proteins revealed an increase in STAT1 (signal transducer and activator of transcription 1) and a decrease in phosphorylated pancreatic duodenal homeobox (Pdx-1).

CONCLUSIONS

T cell-mediated inflammation is associated with β-cell dysfunction during progression of CP.

Appropriate therapy for type 2 diabetes mellitus in view of pancreatic β-cell glucose toxicity: "the earlier, the better"

Pancreatic β-cells secrete insulin when blood glucose levels become high; however, when β-cells are chronically exposed to hyperglycemia, β-cell function gradually deteriorates, which is known as β-cell glucose toxicity. In the diabetic state, nuclear expression of the pancreatic transcription factors pancreatic and duodenal homeobox 1 (PDX-1) and v-Maf musculoaponeurotic fibrosarcoma oncogene family, protein A (MafA) is decreased. In addition, incretin receptor expression in β-cells is decreased, which is likely involved in the impairment of incretin effects in diabetes. Clinically, it is important to select appropriate therapy for type 2 diabetes mellitus (T2DM) so that β-cell function can be preserved. In addition, when appropriate pharmacological interventions against β-cell glucose toxicity are started at the early stages of diabetes, β-cell function is substantially restored, which is not observed if treatment is started at advanced stages. These observations indicate that it is likely that downregulation of pancreatic transcription factors and/or incretin receptors is involved in β-cell dysfunction observed in T2DM and it is very important to start appropriate pharmacological intervention against β-cell glucose toxicity in the early stages of diabetes.

Transcription factor Ets-1 links glucotoxicity to pancreatic beta cell dysfunction through inhibiting PDX-1 expression in rodent models

AIMS/HYPOTHESIS

'Glucotoxicity' is a term used to convey the negative effect of hyperglycaemia on beta cell function; however, the underlying molecular mechanisms that impair insulin secretion and gene expression are poorly defined. Our objective was to define the role of transcription factor v-ets avian erythroblastosis virus E26 oncogene homologue 1 (Ets-1) in beta cell glucotoxicity.

METHODS

Primary islets and Min6 cells were exposed to high glucose and Ets-1 expression was measured. Recombinant adenovirus and transgenic mice were used to upregulate Ets-1 expression in beta cells in vitro and in vivo, and insulin secretion was assessed. The binding activity of H3/H4 histone on the Ets-1 promoter, and that of forkhead box (FOX)A2, FOXO1 and Ets-1 on the Pdx-1 promoter was measured by chromatin immunoprecipitation and quantitative real-time PCR assay.

RESULTS

High glucose induced upregulation of Ets-1 expression and hyperacetylation of histone H3 and H4 at the Ets-1 gene promoter in beta cells. Ets-1 overexpression dramatically suppressed insulin secretion and biosynthesis both in vivo and in vitro. Besides, Ets-1 overexpression increased the activity of FOXO1 but decreased that of FOXA2 binding to the pancreatic and duodenal homeobox 1 (PDX-1) homology region 2 (PH2), resulting in inhibition of Pdx-1 promoter activity and downregulation of PDX-1 expression and activity. In addition, high glucose promoted the interaction of Ets-1 and FOXO1, and the activity of Ets-1 binding to the Pdx-1 promoter. Importantly, PDX-1 overexpression reversed the defect in pancreatic beta cells induced by Ets-1 excess, while knockdown of Ets-1 prevented hyperglycaemia-induced dysfunction of pancreatic beta cells.

CONCLUSIONS/INTERPRETATION

Our observations suggest that Ets-1 links glucotoxicity to pancreatic beta cell dysfunction through inhibiting PDX-1 expression in type 2 diabetes.

The IRE1α/XBP1s Pathway Is Essential for the Glucose Response and Protection of β Cells

Although glucose uniquely stimulates proinsulin biosynthesis in β cells, surprisingly little is known of the underlying mechanism(s). Here, we demonstrate that glucose activates the unfolded protein response transducer inositol-requiring enzyme 1 alpha (IRE1α) to initiate X-box-binding protein 1 (Xbp1) mRNA splicing in adult primary β cells. Using mRNA sequencing (mRNA-Seq), we show that unconventional Xbp1 mRNA splicing is required to increase and decrease the expression of several hundred mRNAs encoding functions that expand the protein secretory capacity for increased insulin production and protect from oxidative damage, respectively. At 2 wk after tamoxifen-mediated Ire1α deletion, mice develop hyperglycemia and hypoinsulinemia, due to defective β cell function that was exacerbated upon feeding and glucose stimulation. Although previous reports suggest IRE1α degrades insulin mRNAs, Ire1α deletion did not alter insulin mRNA expression either in the presence or absence of glucose stimulation. Instead, β cell failure upon Ire1α deletion was primarily due to reduced proinsulin mRNA translation primarily because of defective glucose-stimulated induction of a dozen genes required for the signal recognition particle (SRP), SRP receptors, the translocon, the signal peptidase complex, and over 100 other genes with many other intracellular functions. In contrast, Ire1α deletion in β cells increased the expression of over 300 mRNAs encoding functions that cause inflammation and oxidative stress, yet only a few of these accumulated during high glucose. Antioxidant treatment significantly reduced glucose intolerance and markers of inflammation and oxidative stress in mice with β cell-specific Ire1α deletion. The results demonstrate that glucose activates IRE1α-mediated Xbp1 splicing to expand the secretory capacity of the β cell for increased proinsulin synthesis and to limit oxidative stress that leads to β cell failure.

New Insight into the Role of Reactive Oxygen Species (ROS) in Cellular Signal-Transduction Processes

Reactive oxygen species (ROS) were once considered to be deleterious agents, contributing to a vast range of pathologies. But, now their protective effects are being appreciated. Both their damaging and beneficial effects are initiated when they target distinct molecules and consequently begin functioning as part of complex signal-transduction pathways. The recognition of ROS as signaling mediators has driven a wealth of research into their roles in both normal and pathophysiological states. The present review assesses the relevant recent literature to outline the current perspectives on redox-signaling mechanisms, physiological implications, and therapeutic strategies. This study highlights that a more fundamental knowledge about many aspects of redox signaling will allow better targeting of ROS, which would in turn improve prophylactic and pharmacotherapy for redox-associated diseases.

Role of pancreatic transcription factors in maintenance of mature β-cell function

A variety of pancreatic transcription factors including PDX-1 and MafA play crucial roles in the pancreas and function for the maintenance of mature β-cell function. However, when β-cells are chronically exposed to hyperglycemia, expression and/or activities of such transcription factors are reduced, which leads to deterioration of β-cell function. These phenomena are well known as β-cell glucose toxicity in practical medicine as well as in the islet biology research area. Here we describe the possible mechanism for β-cell glucose toxicity found in type 2 diabetes. It is likely that reduced expression levels of PDX-1 and MafA lead to suppression of insulin biosynthesis and secretion. In addition, expression levels of incretin receptors (GLP-1 and GIP receptors) in β-cells are decreased, which likely contributes to the impaired incretin effects found in diabetes. Taken together, down-regulation of insulin gene transcription factors and incretin receptors explains, at least in part, the molecular mechanism for β-cell glucose toxicity.

Regulation of postnatal pancreatic Pdx1 and downstream target genes after gestational exposure to protein restriction in rats

The study carried out in our laboratory demonstrated that protein restriction (low protein, LP) during fetal and neonatal life alters pancreatic development and impairs glucose tolerance later in life. In this study, we examined the role of the transcription factorPdx1, a master regulator of β-cell differentiation and function along with its downstream target genes insulin,Glut2and glucokinase (GK). The role(s) of these genes and protein products on the pancreata of male offspring from mothers exposed to LP diets were assessed during gestation, weaning, and adult life. Pregnant rats were allocated to two dietary treatments: control (C) 20% protein diet or LP, 8% protein diet. At birth, offspring were divided into four groups: C received control diet all life, LP1 received LP diet all life, LP2 changed the LP diet to C at weaning, and LP3 switched to C after being exposed to LP during gestation only. Body weights (bw) were significantly (P<0.001) decreased in all LP groups at birth. At weaning, only the LP3 offspring had their body weight restored to control levels.Pdx1or any of thePdx1-target genes were similar in all diets at day 21. However, at d130Pdx1mRNA expression and protein abundance were significantly decreased (P<0.05) in all LP groups. In addition, insulin mRNA and protein were decreased in LP1 and LP3 groups compared with C,Glut2mRNA and GLUT2 protein levels were decreased in LP3 and GK did not change between groups. Intraperitoneal glucose tolerance test revealed impaired glucose tolerance in LP3 males, concomitant with decreased β-cell mass, islet area, and PDX1 nuclear protein localization. Collectively, this study suggests that restoring proteins in the diet after birth in LP offspring dramatically impairs glucose homeostasis in early adulthood, by alteringPdx1expression and downstream-target genes increasing the risk to develop type 2 diabetes.

Long-term pancreatic beta cell exposure to high levels of glucose but not palmitate induces DNA methylation within the insulin gene promoter and represses transcriptional activity

Recent studies have implicated epigenetics in the pathophysiology of diabetes. Furthermore, DNA methylation, which irreversibly deactivates gene transcription, of the insulin promoter, particularly the cAMP response element, is increased in diabetes patients. However, the underlying mechanism remains unclear. We aimed to investigate insulin promoter DNA methylation in an over-nutrition state. INS-1 cells, the rat pancreatic beta cell line, were cultured under normal-culture-glucose (11.2 mmol/l) or experimental-high-glucose (22.4 mmol/l) conditions for 14 days, with or without 0.4 mmol/l palmitate. DNA methylation of the rat insulin 1 gene (Ins1) promoter was investigated using bisulfite sequencing and pyrosequencing analysis. Experimental-high-glucose conditions significantly suppressed insulin mRNA and increased DNA methylation at all five CpG sites within the Ins1 promoter, including the cAMP response element, in a time-dependent and glucose concentration-dependent manner. DNA methylation under experimental-high-glucose conditions was unique to the Ins1 promoter; however, palmitate did not affect DNA methylation. Artificial methylation of Ins1 promoter significantly suppressed promoter-driven luciferase activity, and a DNA methylation inhibitor significantly improved insulin mRNA suppression by experimental-high-glucose conditions. Experimental-high-glucose conditions significantly increased DNA methyltransferase activity and decreased ten-eleven-translocation methylcytosine dioxygenase activity. Oxidative stress and endoplasmic reticulum stress did not affect DNA methylation of the Ins1 promoter. High glucose but not palmitate increased ectopic triacylglycerol accumulation parallel to DNA methylation. Metformin upregulated insulin gene expression and suppressed DNA methylation and ectopic triacylglycerol accumulation. Finally, DNA methylation of the Ins1 promoter increased in isolated islets from Zucker diabetic fatty rats. This study helps to clarify the effect of an over-nutrition state on DNA methylation of the Ins1 promoter in pancreatic beta cells. It provides new insights into the irreversible pathophysiology of diabetes.

TFP5, a peptide derived from p35, a Cdk5 neuronal activator, rescues cortical neurons from glucose toxicity

Multiple lines of evidence link the incidence of diabetes to the development of Alzheimer's disease (AD). Patients with diabetes have a 50 to 75% increased risk of developing AD. Cyclin dependent kinase 5 (Cdk5) is a serine/threonine protein kinase, which forms active complexes with p35 or p39, found principally in neurons and in pancreatic β cells. Recent studies suggest that Cdk5 hyperactivity is a possible link between neuropathology seen in AD and diabetes. Previously, we identified P5, a truncated 24-aa peptide derived from the Cdk5 activator p35, later modified as TFP5, so as to penetrate the blood-brain barrier after intraperitoneal injections in AD model mice. This treatment inhibited abnormal Cdk5 hyperactivity and significantly rescued AD pathology in these mice. The present study explores the potential of TFP5 peptide to rescue high glucose (HG)-mediated toxicity in rat embryonic cortical neurons. HG exposure leads to Cdk5-p25 hyperactivity and oxidative stress marked by increased reactive oxygen species production, and decreased glutathione levels and superoxide dismutase activity. It also induces hyperphosphorylation of tau, neuroinflammation as evident from the increased expression of inflammatory cytokines like TNF-α, IL-1β, and IL-6, and apoptosis. Pretreatment of cortical neurons with TFP5 before HG exposure inhibited Cdk5-p25 hyperactivity and significantly attenuated oxidative stress by decreasing reactive oxygen species levels, while increasing superoxide dismutase activity and glutathione. Tau hyperphosphorylation, inflammation, and apoptosis induced by HG were also considerably reduced by pretreatment with TFP5. These results suggest that TFP5 peptide may be a novel candidate for type 2 diabetes therapy.

Aurora kinase A mediates c-Myc's oncogenic effects in hepatocellular carcinoma

Dysregulation of c-Myc (Myc) has been shown to contribute to progression of hepatocellular carcinoma, however, the detailed molecular mechanism remains poorly understood. Here, we report that Myc binds to the Aurora kinase A (Aurka) promoter and induces expression of Aurka in HCC cells. Increased expression of Aurka correlates with that of Myc in HCC. Nuclear accumulation of Aurka was confirmed by subcellular protein fractionation and immunoblot experiments in HCC cells. Myc inhibition decreases the nuclear accumulation of Aurka in HCC cells. Also Aurka accumulating in the nucleus up-regulates Myc transcription by binding the Myc promoter containing the highly conserved CCCTCCCCA in the NHE region of the CpG islands. Inhibition of Myc or Aurka diminishes the malignant phenotypes of HCC cells by down-regulating some common target genes. Also Aurka and Myc mediates the effects of each other, at least partially, on proliferation, anchorage-independent soft agar growth, and ATP production. Blocking Aurka in an orthotopic model significantly impairs tumor growth in mice. These results identify a Myc-Aurka feedback loop in which Myc and Aurka regulate expression of each other at the transcriptional level and both play an important role in hepatocarcinogenesis.

Increased oxidative stress in obesity: implications for metabolic syndrome, diabetes, hypertension, dyslipidemia, atherosclerosis, and cancer

Obesity, especially of the abdominal type, is a health problem that constitutes metabolic syndrome and increases the incidence of various diseases, including diabetes, hypertension, dyslipidemia, atherosclerosis, and cancer. Various mechanisms linking obesity to these associated diseases have been postulated. One candidate is oxidative stress, which has been implicated in vascular complications of diabetes and in pancreatic -cell failure in diabetes. Notably, obese people without diabetes also display elevated levels of systemic oxidative stress. In addition, levels of oxidative stress are increased in the adipose tissue in obese mice. Treating obese mice with antioxidant agents attenuates the development of diabetes. In 3T3-L1 adipocytes, increases in reactive oxygen species (ROS) occur with lipid accumulation; the addition of free fatty acids elevates ROS generation further. Thus, adipose tissue represents an important source of ROS; ROS may contribute to the development of obesity-associated insulin resistance and type 2 diabetes. Moreover, the levels of oxidative stress present in several other types of cells or tis-sues, including those in the brain, arterial walls, and tumors, have been implicated in the pathogenesis associated with hypertension, atherosclerosis, and cancer. The increased levels of systemic oxidative stress that occur in obesity may contribute to the obesity-associated development of these diseases.

Pyrrolidine dithiocarbamate protects pancreatic β-cells from oxidative damage through regulation of FoxO1 activity in type 2 diabetes rats

Pyrrolidine dithiocarbamate (PDTC) can lower the blood glucose level and improve the insulin sensitivity in diabetic rats. However, the mechanisms underlying this effect of PDTC treatment in diabetic rats remained uncertain. In this study, we evaluated the mechanisms by which PDTC conferred protection against oxidative damage to pancreatic islet β-cells in rats with experimental type 2 diabetes mellitus (DM). DM in the rats was elicited by long-term high-fat diet accompanied with a single intraperitoneal (i.p.) injection of a low dose of streptozotocin. After a 7-day administration of PDTC (50 mg/kg/day i.p.), blood glucose levels were measured and pancreatic tissues were collected for the determination of various biochemical and enzymatic activities using immunohistochemistry, immunofluorescence, and western blot techniques. The percentage of apoptotic pancreatic islet β-cells was detected by flow cytometry. The results showed that diabetic rats had elevated blood glucose levels and insulin resistance, accompanied with an increase in malondialdehyde content, nitrotyrosine production, and inducible nitric oxide synthase expression. A decrease in superoxide dismutase and glutathione peroxidase activities was also observed in DM rats, culminating with elevated β-cell apoptosis. PDTC treatment significantly reduced the oxidative damage and the β-cell apoptosis, and also increased the insulin production through down-regulating FoxO1 acetylation and up-regulating nuclear PDX-1 level. These data suggested that PDTC can protect islet β-cells from oxidative damage and improve insulin production through regulation of PDX-1 and FoxO1 in a DM rat model.

The furan fatty acid metabolite CMPF is elevated in diabetes and induces β cell dysfunction

Gestational diabetes (GDM) results from failure of the β cells to adapt to increased metabolic demands; however, the cause of GDM and the extremely high rate of progression to type 2 diabetes (T2D) remains unknown. Using metabolomics, we show that the furan fatty acid metabolite 3-carboxy-4-methyl-5-propyl-2-furanpropanoic acid (CMPF) is elevated in the plasma of humans with GDM, as well as impaired glucose-tolerant and T2D patients. In mice, diabetic levels of plasma CMPF induced glucose intolerance, impaired glucose-stimulated insulin secretion, and decreased glucose utilization. Mechanistically, we show that CMPF acts directly on the β cell, causing impaired mitochondrial function, decreasing glucose-induced ATP accumulation, and inducing oxidative stress, resulting in dysregulation of key transcription factors and ultimately reduced insulin biosynthesis. Importantly, specifically blocking its transport through OAT3 or antioxidant treatment could prevent CMPF-induced β cell dysfunction. Thus, CMPF provides a link between β cell dysfunction and GDM/T2D that could be targeted therapeutically.

Age-associated changes of islet endocrine cells and the effects of body mass index in Japanese

AIMS/INTRODUCTION

Impaired growth and premature death of β-cells are implicated in the progression of islet pathology in type 2 diabetes. It remains unclear, however, how aging affects islet cells, or whether the islet change in diabetes is an augmented process of aging. We studied age-related changes of the islet structure in Japanese non-diabetic subjects and explored the underlying mechanism of the changes.

MATERIALS AND METHODS

A total of 115 non-diabetic autopsy cases were subjected to morphometric analysis for volume densities of islets, β- and non-β-cells, as well as their masses. Proliferation activity identified by Ki67, and expressions of pancreatic and duodenal homeobox (PDX)-1, cell cycle inhibitor P16, and oxidative stress marker γH2AX were also examined.

RESULTS

There was a gradual and marginal decline of volume densities of islets, β- and non-β-cells with aging, while masses of these components were increased during maturation and slowly decreased after the 40s. Islet density was high in the young, but reduced after maturation. There was only a minimal influence of increased body mass index (BMI) on the increase in β-cell mass, but not on the other variables. Ki67 positivity and PDX-1 expressions were high in the young, but low after maturation, whereas expressions of P16 and γH2AX were elevated in the aged.

CONCLUSIONS

Age-associated decline of β-cell mass is marginal after maturation, and the reduction of β-cell mass could be a specific process in diabetes. The impact of BMI on the islet structure is limited in Japanese with normal glucose tolerance.

Effects of a novel curcumin derivative on insulin synthesis and secretion in streptozotocin-treated rat pancreatic islets in vitro

Background

Hyperglycemia induces activation of the c-Jun N-terminal kinase (JNK) pathway, which suppresses insulin gene expression and reduces DNA binding of pancreatic and duodenal homeobox factor (PDX)-1. This study aims to investigate the effects of a novel curcumin derivative (NCD) on JNK signaling pathway on insulin synthesis and secretion in streptozotocin (STZ)-treated rat pancreatic islets in vitro.

Methods

Isolated rat pancreatic islets were divided into five groups: untreated control group; group treated with NCD (10 μM); group exposed to STZ (5 mM); group treated with NCD (10 μM) and then exposed to STZ (5 mM); and group exposed to STZ (5 mM) and then treated with NCD (10 μM). The pancreatic islets from all groups were used for DNA fragmentation assays and quantitative assessments of the JNK, Pdx1, glucose transporter-2 (GLUT2), heme oxygenase (HO)-1, transcription factor 7-like 2 (TCF7L2), and glucagon-like peptide (GLP)-1 gene expression levels. The intracellular calcium, zinc, and the phosphorylated and total JNK protein levels were assessed. The insulin (secreted/total) and C-peptide levels were examined in islet culture medium.

Results

NCD protected pancreatic islets against STZ-induced DNA damage, improved total insulin (P = 0.001), secreted insulin (P = 0.001), and C-peptide levels (P = 0.001), normalized mRNA expressions of insulin, Pdx1, and GLUT2 (P = 0.0001), and significantly elevated calcium and zinc levels (P = 0.0001). All effects were significant when islets were treated with NCD before STZ (P = 0.05). JNK gene overexpression and JNK protein levels induced by STZ were significantly inhibited after NCD treatment of islets ( P = 0.0001). NCD-treated islets showed significantly elevated gene expressions of HO-1, TCF7L2, and GLP-1 (P = 0.0001), and these upregulated gene expressions were more significantly elevated with NCD treatment before STZ than after STZ (P = 0.05).

Conclusions

NCD improved insulin synthesis and secretion in vitro in isolated pancreatic islets treated with STZ through inhibition of the JNK pathway, up-regulation of the gene expressions of HO-1, TCF7L2, and GLP-1 and enhancing effects on calcium and zinc levels.

Pancreatic and duodenal homeobox-1 nuclear localization is regulated by glucose in dispersed rat islets but not in insulin-secreting cell lines

The transcription factor Pancreatic and Duodenal Homeobox-1 (PDX-1) plays a major role in the development and function of pancreatic β-cells and its mutation results in diabetes. In adult β-cells, glucose stimulates transcription of the insulin gene in part by regulating PDX-1 expression, stability and activity. Glucose is also thought to modulate PDX-1 nuclear translocation but in vitro studies examining nucleo-cytoplasmic shuttling of endogenous or ectopically expressed PDX-1 in insulin-secreting cell lines have led to conflicting results. Here we show that endogenous PDX-1 undergoes translocation from the cytoplasm to the nucleus in response to glucose in dispersed rat islets but not in insulin-secreting MIN6, HIT-T15, or INS832/13 cells. Interestingly, however, we found that a PDX-1-GFP fusion protein can shuttle from the cytoplasm to the nucleus in response to glucose stimulation in HIT-T15 cells. Our results suggest that the regulation of endogenous PDX-1 sub-cellular localization by glucose is observed in primary islets and that care should be taken when interpreting data from insulin-secreting cell lines.

Obesity, insulin resistance, and metabolic syndrome: a study in WNIN/Ob rats from a pancreatic perspective

Alterations in pancreatic milieu to adapt to physiological shifts occurring in conditions of obesity and metabolic syndrome (MS) have been documented, though mechanisms leading to such a state have remained elusive so far. The data presented here tries to look at the gravity of metabolic insult during the early and prolonged phases of obesity/insulin resistance (IR) depicted in WNIN/Ob strain of rats—an obese euglycemic mutant rat model developed indigenously at our institute which is highly vulnerable for a variety of degenerative diseases. The present results in situ show the participation of several confounding factors in the pancreatic milieu that collectively coprecipitates for a state of profound inflammation in the pancreas (among Mutant compared to Lean/Control) which gets worsened with age. These include hypertrophy, macrophage infiltration (CD11b/TNFα/IL6), apoptosis, β-cell vacuolation, hyperinsulinemia (HI), and stress markers (RL-77/HSP104/TBARS) all of which correlated well with indices for obesity (2-3 fold), IR (1.5-3 fold), and HI (2-3 fold). Further, supportive data was also obtained from in vitro studies using islet cell cultures amongst phenotypes. Taken together, these results advocate that inflammation was the major precipitating factor to cause islet cell dysfunctions (in situ and in vitro) in these Mutant rats compared to their Lean littermates and parental Control.

A small-molecule inducer of PDX1 expression identified by high-throughput screening

Pancreatic and duodenal homeobox 1 (PDX1), a member of the homeodomain-containing transcription factor family, is a key transcription factor important for both pancreas development and mature β cell function. The ectopic overexpression of Pdx1, Neurog3, and MafA in mice reprograms acinar cells to insulin-producing cells. We developed a quantitative PCR-based gene expression assay to screen more than 60,000 compounds for expression of each of these genes in the human PANC-1 ductal carcinoma cell line. We identified BRD7552, which upregulated PDX1 expression in both primary human islets and ductal cells, and induced epigenetic changes in the PDX1 promoter consistent with transcriptional activation. Prolonged compound treatment induced both insulin mRNA and protein and also enhanced insulin expression induced by the three-gene combination. These results provide a proof of principle for identifying small molecules that induce expression of transcription factors to control cellular reprogramming.

Ebselen treatment prevents islet apoptosis, maintains intranuclear Pdx-1 and MafA levels, and preserves β-cell mass and function in ZDF rats

We reported earlier that β-cell-specific overexpression of glutathione peroxidase (GPx)-1 significantly ameliorated hyperglycemia in diabetic db/db mice and prevented glucotoxicity-induced deterioration of β-cell mass and function. We have now ascertained whether early treatment of Zucker diabetic fatty (ZDF) rats with ebselen, an oral GPx mimetic, will prevent β-cell deterioration. No other antihyperglycemic treatment was given. Ebselen ameliorated fasting hyperglycemia, sustained nonfasting insulin levels, lowered nonfasting glucose levels, and lowered HbA1c levels with no effects on body weight. Ebselen doubled β-cell mass, prevented apoptosis, prevented expression of oxidative stress markers, and enhanced intranuclear localization of pancreatic and duodenal homeobox (Pdx)-1 and v-maf musculoaponeurotic fibrosarcoma oncogene family, protein A (MafA), two critical insulin transcription factors. Minimal β-cell replication was observed in both groups. These findings indicate that prevention of oxidative stress is the mechanism whereby ebselen prevents apoptosis and preserves intranuclear Pdx-1 and MafA, which, in turn, is a likely explanation for the beneficial effects of ebselen on β-cell mass and function. Since ebselen is an oral antioxidant currently used in clinical trials, it is a novel therapeutic candidate to ameliorate fasting hyperglycemia and further deterioration of β-cell mass and function in humans undergoing the onset of type 2 diabetes.

Redox regulation of SIRT1 in inflammation and cellular senescence

Sirtuin 1 (SIRT1) regulates inflammation, aging (life span and health span), calorie restriction/energetics, mitochondrial biogenesis, stress resistance, cellular senescence, endothelial functions, apoptosis/autophagy, and circadian rhythms through deacetylation of transcription factors and histones. SIRT1 level and activity are decreased in chronic inflammatory conditions and aging, in which oxidative stress occurs. SIRT1 is regulated by a NAD(+)-dependent DNA repair enzyme, poly(ADP-ribose) polymerase-1 (PARP1), and subsequent NAD(+) depletion by oxidative stress may have consequent effects on inflammatory and stress responses as well as cellular senescence. SIRT1 has been shown to undergo covalent oxidative modifications by cigarette smoke-derived oxidants/aldehydes, leading to posttranslational modifications, inactivation, and protein degradation. Furthermore, oxidant/carbonyl stress-mediated reduction of SIRT1 leads to the loss of its control on acetylation of target proteins including p53, RelA/p65, and FOXO3, thereby enhancing the inflammatory, prosenescent, and apoptotic responses, as well as endothelial dysfunction. In this review, the mechanisms of cigarette smoke/oxidant-mediated redox posttranslational modifications of SIRT1 and its roles in PARP1 and NF-κB activation, and FOXO3 and eNOS regulation, as well as chromatin remodeling/histone modifications during inflammaging, are discussed. Furthermore, we have also discussed various novel ways to activate SIRT1 either directly or indirectly, which may have therapeutic potential in attenuating inflammation and premature senescence involved in chronic lung diseases.

In vivo JNK activation in pancreatic β-cells leads to glucose intolerance caused by insulin resistance in pancreas

Insulin resistance is a key condition in the development of type 2 diabetes. It is well established that exacerbated Jun NH2-terminal kinase (JNK) activity is involved in promoting insulin resistance in peripheral insulin-target tissues; however, this involvement is less documented in pancreatic β-cells. Using a transgenic mouse model, here we show that JNK activation in β-cells led to glucose intolerance as a result of impaired capacity to increase insulinemia in response to hyperglycemia. Pancreatic islets from these mice showed no obvious morphostructural abnormalities or decreased insulin content. In contrast, these islets failed to secrete insulin in response to glucose or insulin but were competent in succinate-, ketoisocaproate-, 3-isobutyl-1-methylxanthine (IBMX-), KCl-, and tolbutamide-induced insulin secretion. At the molecular level, JNK activation in β-cells inhibited insulin-induced Akt phosphorylation, pancreatic and duodenal homeobox 1 nucleocytoplasmic shuttling, and transcription of insulin-target genes. Remarkably, rosiglitazone restored insulin secretion in response to hyperglycemia in mice and insulin-induced insulin secretion and signaling in isolated islets. In conclusion, the mere activation of JNK suffices to induce insulin resistance in pancreatic β-cells by inhibition of insulin signaling in these cells, but it is not sufficient to elicit β-cell death. In addition, we provide the first evidence that thiazolidinediones exert insulin-sensitizing action directly on pancreatic β-cells.

Update on the protective molecular pathways improving pancreatic beta-cell dysfunction

The primary function of pancreatic beta-cells is to produce and release insulin in response to increment in extracellular glucose concentrations, thus maintaining glucose homeostasis. Deficient beta-cell function can have profound metabolic consequences, leading to the development of hyperglycemia and, ultimately, diabetes mellitus. Therefore, strategies targeting the maintenance of the normal function and protecting pancreatic beta-cells from injury or death might be crucial in the treatment of diabetes. This narrative review will update evidence from the recently identified molecular regulators preserving beta-cell mass and function recovery in order to suggest potential therapeutic targets against diabetes. This review will also highlight the relevance for novel molecular pathways potentially improving beta-cell dysfunction.

PPAR alpha: a novel radiation target in locally exposed Mus musculus heart revealed by quantitative proteomics

Radiation exposure of the thorax is associated with a markedly increased risk of cardiac morbidity and mortality with a latency period of decades. Although many studies have confirmed the damaging effect of ionizing radiation on the myocardium and cardiac endothelial structure and function, the molecular mechanism behind this damage is not yet elucidated. Peroxisome proliferator-activated receptor alpha (PPAR alpha), a transcriptional regulator of lipid metabolism in heart tissue, has recently received great attention in the development of cardiovascular disease. The goal of this study was to investigate radiation-induced cardiac damage in general and the role of PPAR alpha in this process in particular. C57BL/6 mice received local heart irradiation with X-ray doses of 8 and 16 gray (Gy) at the age of 8 weeks. The mice were sacrificed 16 weeks later. Radiation-induced changes in the cardiac proteome were quantified using the Isotope Coded Protein Label (ICPL) method followed by mass spectrometry and software analysis. Significant alterations were observed in proteins involved in lipid metabolism and oxidative phosphorylation. Ionizing radiation markedly changed the phosphorylation and ubiquitination status of PPAR alpha. This was reflected as decreased expression of its target genes involved in energy metabolism and mitochondrial respiratory chain confirming the proteomics data. This study suggests that persistent alteration of cardiac metabolism due to impaired PPAR alpha activity contributes to the heart pathology after radiation.

NOX, NOX Who is There? The Contribution of NADPH Oxidase One to Beta Cell Dysfunction

Predictions of diabetes prevalence over the next decades warrant the aggressive discovery of new approaches to stop or reverse loss of functional beta cell mass. Beta cells are recognized to have a relatively high sensitivity to reactive oxygen species (ROS) and become dysfunctional under oxidative stress conditions. New discoveries have identified NADPH oxidases in beta cells as contributors to elevated cellular ROS. Reviewed are recent reports that evidence a role for NADPH oxidase-1 (NOX-1) in beta cell dysfunction. NOX-1 is stimulated by inflammatory cytokines that are elevated in diabetes. First, regulation of cytokine-stimulated NOX-1 expression has been linked to inflammatory lipid mediators derived from 12-lipoxygenase activity. For the first time in beta cells these data integrate distinct pathways associated with beta cell dysfunction. Second, regulation of NOX-1 in beta cells involves feed-forward control linked to elevated ROS and Src-kinase activation. This potentially results in unbridled ROS generation and identifies candidate targets for pharmacologic intervention. Third, consideration is provided of new, first-in-class, selective inhibitors of NOX-1. These compounds could have an important role in assessing a disruption of NOX-1/ROS signaling as a new approach to preserve and protect beta cell mass in diabetes.

Decreased expression of insulin and increased expression of pancreatic transcription factor PDX-1 in islets in patients with liver cirrhosis: a comparative investigation using human autopsy specimens

BACKGROUND

Glucose intolerance in patients with liver cirrhosis (LC), known as hepatogenous diabetes, is thought to be distinct from type 2 diabetes (T2DM) in some aspects. Hyperinsulinemia and/or insulin resistance in liver disease is associated with hepatocarcinogenesis, growth of hepatocellular carcinoma, and poor prognosis. However, the pathophysiological processes in islets that are responsible for hyperinsulinemia in LC are still not precisely known. Therefore, we investigated the histopathological differences in islets of Langerhans cells between LC and T2DM.

METHODS

A total of 35 human autopsy pancreatic tissue samples were used in this study (control, n = 18; T2DM, n = 6; LC, n = 11). The expression of insulin, glucagon, somatostatin, pancreatic duodenal homeobox-1 (PDX-1), proliferating cell nuclear antigen (PCNA), and Ki-67 was examined using immunohistochemistry and quantitated by image analysis.

RESULTS

Islet hypertrophy and a significant increase in PCNA-positive cells in islets were observed in the tissues from LC cases. The insulin-positive areas in islets were significantly decreased in LC cases compared with control and T2DM cases (P = 0.001, P = 0.035, respectively), whereas the PDX-1-positive area was significantly increased in LC cases (P = 0.001) compared with the control. Furthermore, disorganization of pancreatic endocrine cells and nucleocytoplasmic translocation of PDX-1 were both seen in the LC subjects.

CONCLUSIONS

In LC, islets undergo hypertrophy and exhibit paradoxical expression of insulin and PDX-1. In the subjects autopsied, insulin expression was decreased, whereas expression of the pancreatic transcription factor PDX-1 was increased in LC. These results point to important distinctions between LC and T2DM.

The molecular mechanisms of pancreatic β-cell glucotoxicity: recent findings and future research directions

It is well established that regular physiological stimulation by glucose plays a crucial role in the maintenance of the β-cell differentiated phenotype. In contrast, prolonged or repeated exposure to elevated glucose concentrations both in vitro and in vivo exerts deleterious or toxic effects on the β-cell phenotype, a concept termed as glucotoxicity. Evidence indicates that the latter may greatly contribute to the pathogenesis of type 2 diabetes. Through the activation of several mechanisms and signaling pathways, high glucose levels exert deleterious effects on β-cell function and survival and thereby, lead to the worsening of the disease over time. While the role of high glucose-induced β-cell overstimulation, oxidative stress, excessive Unfolded Protein Response (UPR) activation, and loss of differentiation in the alteration of the β-cell phenotype is well ascertained, at least in vitro and in animal models of type 2 diabetes, the role of other mechanisms such as inflammation, O-GlcNacylation, PKC activation, and amyloidogenesis requires further confirmation. On the other hand, protein glycation is an emerging mechanism that may play an important role in the glucotoxic deterioration of the β-cell phenotype. Finally, our recent evidence suggests that hypoxia may also be a new mechanism of β-cell glucotoxicity. Deciphering these molecular mechanisms of β-cell glucotoxicity is a mandatory first step toward the development of therapeutic strategies to protect β-cells and improve the functional β-cell mass in type 2 diabetes.

[Effects of Chinese herbal medicine Danzhi Jiangtang Capsule and exercise on JNK signaling pathway in pancreatic tissues of diabetic rats]

OBJECTIVE

To explore the effects of exercise and Danzhi Jiangtang Capsule (DJC), a compound traditional herbal medicine, on the JNK signaling pathway in pancreatic tissues of diabetic rats and to investigate the possible mechanisms of exercise and DJC in treating diabetes.

METHODS

Seventy-eight male Wistar rats were injected with low dose of streptozotocin and fed a high-fat diet to establish a diabetic model in rats. Then 60 diabetic rats were divided into diabetes group, exercise group, DJC group and exercise combined with DJC group. Another twelve rats were used as normal control. After eight months of treatment, the expression levels of phosphor-c-Jun N-terminal kinase (p-JNK), pancreatic and duodenal homeobox-1 (PDX-1), and insulin protein in pancreatic tissues from rats were detected by immunohistochemical method and Western blotting.

RESULTS

In pancreatic tissues of diabetes group, the expression level of p-JNK protein was significantly higher than that in the normal group (P<0.01), and the expression levels of PDX-1 and insulin protein were significantly decreased (P<0.01). After administration of exercise and DJC, the expression level of p-JNK protein in pancreatic tissues of the diabetes group was decreased significantly, while the expression levels of PDX-1 and insulin protein were increased significantly (P<0.05 or P<0.01).

CONCLUSION

Exercise and DJC effectively protect isletβ-cell function in diabetic rats, which might be due to a decreased JNK signaling pathway.

Involvement of oxidative stress in suppression of insulin biosynthesis under diabetic conditions

Type 2 diabetes is characterized by pancreatic β-cell dysfunction and insulin resistance, and the number of patients has markedly increased worldwide. In the diabetic state, hyperglycemia per se and subsequent induction of oxidative stress decrease insulin biosynthesis and secretion, leading to the aggravation of Type 2 diabetes. In addition, there is substantial reduction in expression and/or activities of several insulin gene transcription factors. This process is known as β-cell glucose toxicity, which is often observed under diabetic conditions. Taken together, it is likely that oxidative stress explains, at least in part, the molecular mechanism for β-cell glucose toxicity, which is often observed in Type 2 diabetes.

Transcriptional and translational regulation of cytokine signaling in inflammatory β-cell dysfunction and apoptosis

Disease is conventionally viewed as the chaotic inappropriate outcome of deranged tissue function resulting from aberrancies in cellular processes. Yet the patho-biology of cellular dysfunction and death encompasses a coordinated network no less sophisticated and regulated than maintenance of homeostatic balance. Cellular demise is far from passive subordination to stress but requires controlled coordination of energy-requiring activities including gene transcription and protein translation that determine the graded transition between defensive mechanisms, cell cycle regulation, dedifferentiation and ultimately to the activation of death programmes. In fact, most stressors stimulate both homeostasis and regeneration on one hand and impairment and destruction on the other, depending on the ambient circumstances. Here we illustrate this bimodal ambiguity in cell response by reviewing recent progress in our understanding of how the pancreatic β cell copes with inflammatory stress by changing gene transcription and protein translation by the differential and interconnected action of reactive oxygen and nitric oxide species, microRNAs and posttranslational protein modifications.

Long-term high animal protein diet reduces body weight gain and insulin secretion in diet-induced obese rats

BACKGROUND

The effects of a high protein diet on insulin secretion and glucose metabolism have been quite controversial. The aim of this study was to evaluate the effects of long-term isocaloric high animal protein intake on insulin secretion in diet-induced obese rats.

RESULTS

After the experimental period (24 weeks), the high-fat diet-induced obese rats that were fed isocaloric high-protein diets (HP) had lower body weight gain (P < 0.01) and lower visceral fat (P < 0.05) than normal protein (NP) rats. Fasting plasma glucagon-like peptide-1 (GLP-1) was also reduced significantly (P < 0.05), as well as serum insulin levels at 5 min and 10 min by intravenous insulin releasing test. In addition, insulin mRNA and pancreatic duodenal homeodomain-1 (PDX-1), GLP-1 protein expression were both markedly lower in HP rats (P < 0.05), while PDX-1 mRNA in HP rats had no difference from NP rats.

CONCLUSION

These results suggest that long-term isocaloric high animal protein intake reduces the acute insulin response in obese rats and the decrease of insulin is associated with both reduced weight gain and inhibition of PDX-1 expression. GLP-1 might be a negative feedback for the balance of energy metabolism secondary to changes of body weight and visceral fat.

NADPH oxidase 2 plays a critical role in dysfunction and apoptosis of pancreatic β-cells induced by very low-density lipoprotein

In type 2 diabetes, pancreatic β-cells cannot secret enough insulin compensate for insulin resistance, which are often accompanied by abnormality in lipid metabolism such as hypertriglyceridemia. It is reported that oxidative stress is involved in pancreatic β-cell dysfunction. However, molecular mechanisms linking between excessive generations of reactive oxygen species (ROS) and β-cell dysfunction and apoptosis induced by high levels of very low-density lipoprotein (VLDL) are poorly understood. In this study, we test the hypothesis that NADPH oxidase 2 (NOX2)-derived ROS may play a key role in dysfunction and apoptosis of pancreatic β-cell induced by VLDL. Our results show that the ApoCIII transgenic mice displayed increased serum TG levels, enhanced generation of ROS and impaired insulin content in pancreatic β-cells. In vitro, the treatment of pancreatic NIT-1 cells with 1 mg/ml VLDL for 12 h stimulated NOX2-derived ROS generation, decreased expression and secretion of insulin. Furthermore, we found that VLDL induced dysfunction and apoptosis of pancreatic β-cells through JNK and p53 pathways, which were rescued by siRNA-mediated NOX2 reduction. In conclusion, our data demonstrate a critical role of NOX2-derived ROS in dysfunction and apoptosis through JNK and p53 pathways in pancreatic β-cells induced by VLDL.

JNK3 maintains expression of the insulin receptor substrate 2 (IRS2) in insulin-secreting cells: functional consequences for insulin signaling

We have recently shown that silencing of the brain/islet specific c-Jun N-terminal Kinase3 (JNK3) isoform enhances both basal and cytokine-induced beta-cell apoptosis, whereas silencing of JNK1 or JNK2 has opposite effects. While it is known that JNK1 or JNK2 may promote apoptosis by inhibiting the activity of the pro-survival Akt pathway, the effect of JNK3 on Akt has not been documented. This study aims to determine the involvement of individual JNKs and specifically JNK3 in the regulation of the Akt signaling pathway in insulin-secreting cells. JNK3 silencing strongly decreases Insulin Receptor Substrate 2 (IRS2) protein expression, and blocks Akt2 but not Akt1 activation by insulin, while the silencing of JNK1 or JNK2 activates both Akt1 and Akt2. Concomitantly, the silencing of JNK1 or JNK2, but not of JNK3, potently phosphorylates the glycogen synthase kinase3 (GSK3β). JNK3 silencing also decreases the activity of the transcription factor Forkhead BoxO3A (FoxO3A) that is known to control IRS2 expression, in addition to increasing c-Jun levels that are known to inhibit insulin gene expression. In conclusion, we propose that JNK1/2 on one hand and JNK3 on the other hand, have opposite effects on insulin-signaling in insulin-secreting cells; JNK3 protects beta-cells from apoptosis and dysfunction mainly through maintenance of a normal IRS2 to Akt2 signaling pathway. It seems that JNK3 mediates its effects mainly at the transcriptional level, while JNK1 or JNK2 appear to mediate their pro-apoptotic effect in the cytoplasm.

Hyperthermia with mild electrical stimulation protects pancreatic β-cells from cell stresses and apoptosis

Induction of heat shock protein (HSP) 72 improves metabolic profiles in diabetic model mice. However, its effect on pancreatic β-cells is not known. The current study investigated whether HSP72 induction can reduce β-cell stress signaling and apoptosis and preserve β-cell mass. MIN6 cells and db/db mice were sham-treated or treated with heat shock (HS) and mild electrical stimulation (MES) (HS+MES) to induce HSP72. Several cellular markers, metabolic parameters, and β-cell mass were evaluated. HS+MES treatment or HSP72 overexpression increased HSP72 protein levels and decreased tumor necrosis factor (TNF)-α-induced Jun NH(2)-terminal kinase (JNK) phosphorylation, endoplasmic reticulum (ER) stress, and proapoptotic signal in MIN6 cells. In db/db mice, HS+MES treatment for 12 weeks significantly improved insulin sensitivity and glucose homeostasis. Upon glucose challenge, a significant increase in insulin secretion was observed in vivo. Compared with sham treatment, levels of HSP72, insulin, pancreatic duodenal homeobox-1, GLUT2, and insulin receptor substrate-2 were upregulated in the pancreatic islets of HS+MES-treated mice, whereas JNK phosphorylation, nuclear translocation of forkhead box class O-1, and nuclear factor-κB p65 were reduced. Apoptotic signals, ER stress, and oxidative stress markers were attenuated. Thus, HSP72 induction by HS+MES treatment protects β-cells from apoptosis by attenuating JNK activation and cell stresses. HS+MES combination therapy may preserve pancreatic β-cell volume to ameliorate glucose homeostasis in diabetes.

Redox control of the survival of healthy and diseased cells

Abstract Cellular redox homeostasis is the first line of defense against diverse stimuli and is crucial for various biological processes. Reactive oxygen species (ROS), byproducts of numerous cellular events, may serve in turn as signaling molecules to regulate cellular processes such as proliferation, differentiation, and apoptosis. However, when overproduced ROS fail to be scavenged by the antioxidant system, they may damage cellular components, giving rise to senescent, degenerative, or fatal lesions in cells. Accordingly, this review not only covers general mechanisms of ROS production under different conditions, but also focuses on various types of ROS-involved diseases, including atherosclerosis, ischemia/reperfusion injury, diabetes mellitus, neurodegenerative diseases, and cancer. In addition, potentially therapeutic agents and approaches are reviewed in a relatively comprehensive manner. However, due to the complexity of ROS and their cellular impacts, we believe that the goal to design more effective approaches or agents may require a better understanding of mechanisms of ROS production, particularly their multifaceted impacts in disease at biochemical, molecular, genetic, and epigenetic levels. Thus, it requires additional tools of omics in systems biology to achieve such a goal. Antioxid. Redox Signal. 15, 2867-2908.

Role of transcription factor modifications in the pathogenesis of insulin resistance

Non-alcoholic fatty liver disease (NAFLD) is characterized by fat accumulation in the liver not due to alcohol abuse. NAFLD is accompanied by variety of symptoms related to metabolic syndrome. Although the metabolic link between NAFLD and insulin resistance is not fully understood, it is clear that NAFLD is one of the main cause of insulin resistance. NAFLD is shown to affect the functions of other organs, including pancreas, adipose tissue, muscle and inflammatory systems. Currently efforts are being made to understand molecular mechanism of interrelationship between NAFLD and insulin resistance at the transcriptional level with specific focus on post-translational modification (PTM) of transcription factors. PTM of transcription factors plays a key role in controlling numerous biological events, including cellular energy metabolism, cell-cycle progression, and organ development. Cell type- and tissue-specific reversible modifications include lysine acetylation, methylation, ubiquitination, and SUMOylation. Moreover, phosphorylation and O-GlcNAcylation on serine and threonine residues have been shown to affect protein stability, subcellular distribution, DNA-binding affinity, and transcriptional activity. PTMs of transcription factors involved in insulin-sensitive tissues confer specific adaptive mechanisms in response to internal or external stimuli. Our understanding of the interplay between these modifications and their effects on transcriptional regulation is growing. Here, we summarize the diverse roles of PTMs in insulin-sensitive tissues and their involvement in the pathogenesis of insulin resistance.

ATF3 inhibits PDX-1-stimulated transactivation

Chronic endoplasmic reticulum (ER) stress leads to β-cell failure via reduction of pancreatic and duodenal homeobox-1 (PDX-1) activity, which contributes to the pathogenesis of type 2 diabetes. However, the exact mechanisms by which ER stress reduces PDX-1 activity in pancreatic β-cells are unclear. Previously, we showed that ATF3 downregulates PDX-1 gene expression in MIN6N8 pancreatic β-cells. Here, we investigated another role of ATF3 on the regulation of PDX-1 activity. ATF3 significantly inhibited PDX-1-stimulated transactivation of reporter plasmid containing promoters for PDX-1 binding element and the PDX-1 target gene glucokinase, which is dependent on C-terminal domain of ATF3. ATF3 interacted with PDX-1, and effectively inhibited p300-mediated transcriptional coactivation of the PBE-containing promoter, whereas C-terminal domain-deleted ATF3 did not inhibit the transcoactivation of p300. ATF3 decreased the interaction of p300 with PDX-1 in MIN6N8 cells coexpressing PDX-1 and ATF3. In addition, chromatin immunoprecipitation analysis demonstrated that both tunicamycin treatment and ATF3 overexpression inhibited the recruitment of p300 to PDX-1 on the insulin promoter in MIN6N8 cells. Taken together, these results suggest that ATF3 inhibits PDX-1-mediated transactivation through the inhibition of p300-stimulated coactivation, which may lead to β-cell dysfunction by ER stress.

Overexpression of serine racemase in retina and overproduction of D-serine in eyes of streptozotocin-induced diabetic retinopathy

Background

Recent data indicate that inflammatory mechanisms contribute to diabetic retinopathy (DR). We have determined that serine racemase (SR) expression is increased by inflammatory stimuli including liposaccharide (LPS), amyloid β-peptide (A-beta), and secreted amyloid precursor protein (sAPP); expression is decreased by the anti-inflammatory drug, dexamethasone. We tested possibility that SR and its product, D-serine, were altered in a rat model of DR.

Methods

Intraperitoneal injection of streptozotocin (STZ; 70 mg/kg body weight) to Sprague-Dawley rats produced type-I diabetic mellitus (fasting blood sugar higher than 300 mg/dL). At 3 and 5 months after STZ or saline injection, retinas from some rats were subjected to cryosectioning for immunofluorescent analysis of SR and TUNEL assay of apoptosis. Retinal homogenates were used to detect SR levels and Jun N-terminal kinase (JNK) activation by immunoblotting. Aqueous humor and retina were also collected to assay for neurotransmitters, including glutamate and D-serine, by reverse-phase HPLC.

Results

Compared to saline-injected rats, STZ-injected (diabetic) rats showed elevation of SR protein levels in retinal homogenates, attributed to the inner nuclear layer (INL) by immunofluorescence. Aqueous humor fluid from STZ-injected rats contained significantly higher levels of glutamate and D-serine compared to controls; by contrast, D-serine levels in retinas did not differ. Levels of activated JNK were elevated in diabetic retinas compared to controls.

Conclusions

Increased expression of SR in retina and higher levels of glutamate and D-serine in aqueous humor of STZ-treated rats may result from activation of the JNK pathway in diabetic sequelae. Our data suggest that the inflammatory conditions that prevail during DR result in elevation of D-serine, a neurotransmitter contributing to glutamate toxicity, potentially exacerbating the death of retinal ganglion cells in this condition.

ER stress and its functional link to mitochondria: role in cell survival and death

The endoplasmic reticulum (ER) is the primary site for synthesis and folding of secreted and membrane-bound proteins. Proteins are translocated into ER lumen in an unfolded state and require protein chaperones and catalysts of protein folding to assist in proper folding. Properly folded proteins traffic from the ER to the Golgi apparatus; misfolded proteins are targeted to degradation. Unfolded protein response (UPR) is a highly regulated intracellular signaling pathway that prevents accumulation of misfolded proteins in the ER lumen. UPR provides an adaptive mechanism by which cells can augment protein folding and processing capacities of the ER. If protein misfolding is not resolved, the UPR triggers apoptotic cascades. Although the molecular mechanisms underlying ER stress-induced apoptosis are not completely understood, increasing evidence suggests that ER and mitochondria cooperate to signal cell death. Mitochondria and ER form structural and functional networks (mitochondria-associated ER membranes [MAMs]) essential to maintain cellular homeostasis and determine cell fate under various pathophysiological conditions. Regulated Ca(2+) transfer from the ER to the mitochondria is important in maintaining control of prosurvival/prodeath pathways. We discuss the signaling/communication between the ER and mitochondria and focus on the role of the mitochondrial permeability transition pore in these complex processes.

Myristoylated methionine sulfoxide reductase A protects the heart from ischemia-reperfusion injury

Methionine sulfoxide reductase A (MsrA) catalytically scavenges reactive oxygen species and also repairs oxidized methionines in proteins. Increasing MsrA protects cells and organs from a variety of oxidative stresses while decreasing MsrA enhances damage, but the mechanisms of action have not been elucidated. A single gene encodes MsrA of which ∼25% is targeted to the mitochondria, a major site of reactive oxygen species production. The other ∼75% is targeted to the cytosol and is posttranslationally modified by myristoylation. To determine the relative importance of MsrA in each compartment in protecting against ischemia-reperfusion damage, we created a series of transgenic mice overexpressing MsrA targeted to the mitochondria or the cytosol. We used a Langendorff model of ischemia-reperfusion and assayed both the rate pressure product and infarct size following ischemia and reperfusion as measures of injury. While the mitochondrially targeted MsrA was expected to be protective, it was not. Notably, the cytosolic form was protective but only if myristoylated. The nonmyristoylated, cytosolic form offered no protection against injury. We conclude that cytosolic MsrA protects the heart from ischemia-reperfusion damage. The requirement for myristoylation suggests that MsrA must interact with a hydrophobic domain to provide protection.