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  • Abstract: Diabetes x
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Thierry Brun and Benoit R Gauthier

Blood glucose homeostasis is achieved by the regulation of insulin and glucagon secretion from the pancreatic islet β- and α-cells. Diabetes mellitus, which comprises a heterogeneous group of hyperglycaemic disorders, results mainly from inadequate mass and function of islet β-cells. Autoimmune destruction of β-cells causes type 1 diabetes, while type 2 is characterized by impaired insulin secretion and is often associated with diminished insulin action on its target tissues. Interestingly, similar to type 1 diabetes, a gradual loss of β-cell mass is observed in type 2 diabetes often requiring insulin therapy. Understanding the molecular mechanism that governs β-cell mass plasticity may provide a means to develop strategies to countera,ct β-cell death while increasing replication. Of particular interest is the islet-specific transcription factor paired box4 (Pax4) that was previously shown to be indispensable for the establishment of the β-cell lineage during development. However, recent accumulating evidence now suggest that Pax4 is also crucial for mature β-cell expansion and survival in response to physiological cues and that mutations or polymorphisms are associated with both type 1 and type 2 diabetes. In contrast, aberrant expression of Pax4 confers protection against apoptosis to insulinomas, whereas it promotes cell growth in lymphocytes. This review summarizes promising new published results supporting the important function of Pax4 in mature islet β-cell physiology and its contribution to pathophysiology when deregulated.

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Heather C Denroche and C Bruce Verchere

Islet amyloid polypeptide (IAPP), the main component of islet amyloid in type 2 diabetes and islet transplants, is now recognized as a contributor to beta cell dysfunction. Increasingly, evidence warrants its investigation in type 1 diabetes owing to both its immunomodulatory and metabolic actions. Autoreactive T cells to IAPP-derived epitopes have been described in humans, suggesting that IAPP is an islet autoantigen in type 1 diabetes. In addition, although aggregates of IAPP have not been implicated in type 1 diabetes, they are potent pro-inflammatory stimuli to innate immune cells, and thus, could influence autoimmunity. IAPP aggregates also occur rapidly in transplanted islets and likely contribute to islet transplant failure in type 1 diabetes through sterile inflammation. In addition, since type 1 diabetes is a disease of both insulin and IAPP deficiency, clinical trials have examined the potential benefits of IAPP replacement in type 1 diabetes with the injectable IAPP analogue, pramlintide. Pramlintide limits postprandial hyperglycemia by delaying gastric emptying and suppressing hyperglucagonemia, underlining the possible role of IAPP in postprandial glucose metabolism. Here, we review IAPP in the context of type 1 diabetes: from its potential involvement in type 1 diabetes pathogenesis, through its role in glucose metabolism and use of IAPP analogues as therapeutics, to its potential role in clinical islet transplant failure and considerations in this regard for future beta cell replacement strategies.

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Mingtong Xu, Xiaochao Chen, Li Yan, Hua Cheng and Weiqing Chen

Association between the (AC)n dinucleotide repeat polymorphism at the 5′-end of the aldose reductase gene and the occurrence of diabetic nephropathy was conducted. We examined eight studies consisting of ten Caucasian type 1 diabetes mellitus case–control comparisons and eight studies consisting of nine type 2 diabetes mellitus case–control comparisons, which were based on our inclusion criterion and available in the literature. The meta-analysis demonstrated a large heterogeneity among the studies on the type 1 diabetic subjects and a significant association was observed between the (AC)n dinucleotide repeat polymorphism at the 5′-end of the aldose reductase gene and diabetic nephropathy. The Z−2 allele appeared to be a genetic risk factor for susceptibility to diabetic nephropathy with a random effects odds ratio (OR) of 1.40 (95% confidence interval, CI (1.07, 1.84)). The Z+2 allele showed a protective effect on diabetic nephropathy with a random effects OR of 0.77 (95% CI (0.65, 0.91)). The meta-analysis, however, showed no association between the genetic polymorphism and diabetic nephropathy in type 2 diabetic subjects. Neither the risk Z−2 allele nor the protective Z+2 allele in type 1 diabetic subjects appeared to have an effect on nephropathy in type 2 diabetic subjects, while their fixed effects OR was 1.09 (95% CI (0.96, 1.22)) and 0.88 (95% CI (0.67, 1.15)) respectively. The current meta-analysis demonstrated a correlation between the (AC)n dinucleotide repeat polymorphism and the occurrence of diabetic nephropathy in Caucasian type 1 diabetic subjects in contrast to type 2 diabetic subject population in which such an association could not be demonstrated.

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Shinobu Shimizu, Tetsuya Hosooka, Tomokazu Matsuda, Shun-ichiro Asahara, Maki Koyanagi-Kimura, Ayumi Kanno, Alberto Bartolome, Hiroaki Etoh, Megumi Fuchita, Kyoko Teruyama, Hiroaki Takahashi, Hiroyuki Inoue, Yusuke Mieda, Naoko Hashimoto, Susumu Seino and Yoshiaki Kido

The development of type 2 diabetes is accompanied by a progressive decline in β-cell mass and function. Vildagliptin, a dipeptidyl peptidase 4 inhibitor, is representative of a new class of antidiabetic agents that act through increasing the expression of glucagon-like peptide-1. The protective effect of this agent on β cells was studied in diabetic mice. Diabetic pancreatic β cell-specific C/EBPB transgenic (TG) mice exhibit decreased β-cell mass associated with increased apoptosis, decreased proliferation, and aggravated endoplasmic reticulum (ER) stress. Vildagliptin was orally administered to the TG mice for a period of 24 weeks, and the protective effects of this agent on β cells were examined, along with the potential molecular mechanism of protection. Vildagliptin ameliorated hyperglycemia in TG mice by increasing the serum concentration of insulin and decreasing the serum concentration of glucagon. This agent also markedly increased β-cell mass, improved aggravated ER stress, and restored attenuated insulin/IGF1 signaling. A decrease in pancreatic and duodenal homeobox 1 expression was also observed in β cells isolated from our mouse model, but this was also restored by vildagliptin treatment. The expression of C/EBPB protein, but not mRNA, was unexpectedly downregulated in vildagliptin-treated TG mice and in exenatide-treated MIN6 cells. Activation of the GLP1 pathway induced proteasome-dependent C/EBPB degradation in β cells as the proteasome inhibitor MG132 restored the downregulation of C/EBPB protein by exenatide. Vildagliptin elicits protective effects on pancreatic β cells, possibly through C/EBPB degradation, and has potential for preventing the progression of type 2 diabetes.

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M Tiedge and S Lenzen

ABSTRACT

RINm5F insulinoma cells show a defective physiological insulin secretory response to glucose stimulation. The short chain carbonic acid sodium butyrate induced a growth arrest during a 72-h tissue culture period. In contrast to control RINm5F cells, 2 mm glucose increased insulin secretion by more than 70% in these sodium butyrate-treated cells (1 mm) without any further increase of the secretory rate between 2 and 20 mm glucose. This effect of sodium butyrate on insulin secretion was assessed in comparison with its effect on gene expression of the GLUT1 and GLUT2 glucose transporter, hexokinase type I and type II, glucokinase and insulin. Sodium butyrate at a 1 mm concentration decreased GLUT1 gene expression by nearly 50%, but did not induce gene expression of the low-affinity GLUT2 glucose transporter above the detection limit. Furthermore, sodium butyrate increased glucokinase gene expression by more than 50% and hexokinase type II gene expression by more than 100%, while insulin gene expression was increased only by 24%. Hexokinase type II enzyme activity was increased by more than 100% without a concomitant significant change of the glucokinase enzyme activity. Sodium butyrate (2 mm) caused effects comparable with those of 1 mm sodium butyrate. Thus the improved insulin secretory responsiveness of RINm5F insulinoma cells after sodium butyrate treatment at low non-physiological millimolar glucose concentrations can be interpreted as a result of an increased hexokinase-mediated metabolic flux rate through the glycolytic chain.

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Jianling Xie, Norhan M El Sayed, Cheng Qi, Xuechan Zhao, Claire E Moore and Terence P Herbert

Glucagon-like peptide 1 receptor (GLP1R) agonists, such as exendin-4, potentiate glucose-stimulated insulin secretion and are currently used in the management of type 2 diabetes. Interestingly, GLP1R agonists also have the ability to augment β-cell mass. In this report, we provide evidence that in the presence of glucose, exendin-4 stimulates rodent islet cell DNA replication via the activation of ribosomal protein S6 kinase 1 (S6K1) and that this is mediated by the protein kinase B (PKB)-dependent activation of mTOR complex 1 (mTORC1). We show that activation of this pathway is caused by the autocrine or paracrine activation of the IGF1 receptor (IGF1R), as siRNA-mediated knockdown of the IGF1R effectively blocked exendin-4-stimulated PKB and mTORC1 activation. In contrast, pharmacological inactivation of the epidermal growth factor receptor has no discernible effect on exendin-4-stimulated PKB or mTORC1 activation. Therefore, we conclude that GLP1R agonists stimulate β-cell proliferation via the PKB-dependent stimulation of mTORC1/S6K1 whose activation is mediated through the autocrine/paracrine activation of the IGF1R. This work provides a better understanding of the molecular basis of GLP1 agonist-induced β-cell proliferation which could potentially be exploited in the identification of novel drug targets that increase β-cell mass.

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D T Furuya, A C Poletto, H S Freitas and U F Machado

Evidences have suggested that the endocannabinoid system is overactive in obesity, resulting in enhanced endocannabinoid levels in both circulation and visceral adipose tissue. The blockade of cannabinoid receptor type 1 (CB1) has been proposed for the treatment of obesity. Besides loss of body weight, CB1 antagonism improves insulin sensitivity, in which the glucose transporter type 4 (GLUT4) plays a key role. The aim of this study was to investigate the modulation of GLUT4-encoded gene (Slc2a4 gene) expression by CB1 receptor. For this, 3T3-L1 adipocytes were incubated in the presence of a highly selective CB1 receptor agonist (1 μM arachidonyl-2′-chloroethylamide) and/or a CB1 receptor antagonist/inverse agonist (0.1, 0.5, or 1 μM AM251, 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-1-piperidinyl-1H-pyrazole-3-carboxamide). After acute (2 and 4 h) and chronic (24 h) treatments, cells were harvested to evaluate: i) Slc2a4, Cnr1 (CB1 receptor-encoded gene), and Srebf1 type a (SREBP-1a type-encoded gene) mRNAs (real-time PCR); ii) GLUT4 protein (western blotting); and iii) binding activity of nuclear factor (NF)-κB and sterol regulatory element-binding protein (SREBP)-1 specifically in the promoter of Slc2a4 gene (electrophoretic mobility shift assay). Results revealed that both acute and chronic CB1 receptor antagonism greatly increased (∼2.5-fold) Slc2a4 mRNA and protein content. Additionally, CB1-induced upregulation of Slc2a4 was accompanied by decreased binding activity of NF-κB at 2 and 24 h, and by increased binding activity of the SREBP-1 at 24 h. In conclusion, these findings reveal that the blockade of CB1 receptor markedly increases Slc2a4/GLUT4 expression in adipocytes, a feature that involves NF-κB and SREBP-1 transcriptional regulation.

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B Lee, PG Bradford and SG Laychock

The inositol 1,4,5-trisphosphate receptor (InsP3R) is an intracellular Ca2+ channel that plays a role in the regulation of insulin secretion. In rat isolated pancreatic islets the expression of types I, II and III InsP3R mRNA was identified by reverse transcriptase-polymerase chain reaction and confirmed by cDNA cloning and sequencing. The islet ratios of types I, II and III InsP3R mRNA to beta-actin mRNA were 0.08 +/- 0.02, 0.08 +/- 0.03 and 0.25 +/- 0.04 respectively. Types I, II and III InsP3R mRNA were also expressed in rat (RINm5F) and mouse (betaHC9) pancreatic beta-cell lines, and rat cerebellum. Type III InsP3R mRNA was quantitatively the most abundant form in rat islets and RINm5F cells. In betaHC9 cells, types II and III InsP3R mRNA were expressed at similar levels, and in much greater abundance than type I mRNA. Type III was the least abundant InsP3R mRNA in cerebellum. Culture of betaHC9 cells for 5 days at 2.8 and 25 mM glucose, or RINm5F cells for 7 days at 5.5 and 20 mM glucose, resulted in significantly enhanced expression of type III, but not types I and II, InsP3R mRNA in the cells at the higher glucose concentrations. During short-term (0.5-2 h) incubations, betaHC9 cell type III InsP3R mRNA levels increased in response to glucose in a time- and concentration-dependent manner. Actinomycin D inhibited the glucose response. Alpha-ketoisocaproic acid also stimulated betaHC9 cell type III InsP3R mRNA expression in a concentration-dependent manner, whereas 2-deoxyglucose and 3-O-methylglucose were without effect. The different levels of expression of mRNA for three InsP3R isoforms in islets and insulinoma cells, and the influence of glucose and alpha-ketoisocaproic acid on the expression of type III mRNA, suggests that nutrient metabolism plays a role in the regulation of this gene and that the function of InsP3R subtypes may be unique with each playing a distinct role in beta-cell signal transduction and insulin secretion.

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MT Travers and MC Barber

Transcription of the acetyl-CoA carboxylase (ACC)-alpha gene is initiated from two promoters, promoter I (PI) and promoter II (PII) such that transcripts demonstrate heterogeneity in their 5' untranslated regions (UTR). Exons 1 and 2 (E1 and E2) are the primary exons in transcripts initiated from PI and PII respectively; E5 is the first coding exon present in all transcripts. In addition alternative exon splicing results in transcripts that either include or exclude a 47 nucleotide sequence corresponding to E4, such that E[1/4/5] and E[1/5] type transcripts result from PI activity, whereas transcripts containing E[2/4/5] or E[2/5] in the 5'UTR result from PII. In subcutaneous adipose tissue from non-pregnant non-lactating sheep approximately 60% of ACC-alpha transcripts are derived from PI, of which 85% are the E[1/5] type. Lactation resulted in an 88% reduction in total PI transcripts, of which the E[1/5] type was reduced 90% and the E[1/4/5] type 80%. By contrast lactation reduced the total levels of PII transcripts by only 50%. Culture of explants from the subcutaneous depot of lactating sheep with insulin plus dexamethasone for 72 h resulted in an 8-fold increase in both E[1/4/5] and E[1/5] types when compared with explants prior to culture. PII transcripts, by contrast, were increased 2-fold by culture in insulin plus dexamethasone and this was entirely attributed to an increase in the expression of the E[2/4/5] type. Dexamethasone acts to potentiate the action of insulin on PI and PII transcript abundance and this effect is greatest for PI transcripts. This study has demonstrated that repression of the ACC-alpha gene in adipose tissue during lactation is largely achieved through attenuation of PI transcript abundance and may be related, in part, to a change in the sensitivity of the apparatus that regulates PI transcript steady-state levels to insulin.

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Hongjie Zhang, Jing Li, Xiangying Liang, Yun Luo, Ke Zen and Chen-Yu Zhang

It is known that endogenous levels of the incretin hormone glucagon-like peptide 1 (GLP1) can be enhanced by various secretagogues, but the mechanism underlying GLP1 secretion is still not fully understood. We assessed the possible effect of uncoupling protein 2 (UCP2) on GLP1 secretion in mouse intestinal tract and NCI-H716 cells, a well-characterized human enteroendocrine L cell model. Localization of UCP2 and GLP1 in the gastrointestinal tract was assessed by immunofluorescence staining. Ucp2 mRNA levels in gut were analyzed by quantitative RT-PCR. Human NCI-H716 cells were transiently transfected with siRNAs targeting UCP2. The plasma and ileum tissue levels of GLP1 (7–36) amide were measured using an ELISA kit. UCP2 was primarily expressed in the mucosal layer and colocalized with GLP1 in gastrointestinal mucosa. L cells secreting GLP1 also expressed UCP2. After glucose administration, UCP2-deficient mice showed increased glucose-induced GLP1 secretion compared with wild-type littermates. GLP1 secretion increased after NCI-H716 cells were transfected with siRNAs targeting UCP2. UCP2 was markedly upregulated in ileum tissue from ob/ob mice, and GLP1 secretion decreased compared with normal mice. Furthermore, GLP1 secretion increased after administration of genipin by oral gavage. Taken together, these results reveal an inhibitory role of UCP2 in glucose-induced GLP1 secretion.