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Free access

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.

Free access

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.

Free access

Jan Christiansen, Astrid M Kolte, Thomas v O Hansen and Finn C Nielsen

Recent genome-wide association (GWA) studies of type 2 diabetes (T2D) have implicated IGF2 mRNA-binding protein 2 (IMP2/IGF2BP2) as one of the several factors in the etiology of late onset diabetes. IMP2 belongs to a family of oncofetal mRNA-binding proteins implicated in RNA localization, stability, and translation that are essential for normal embryonic growth and development. This review provides a background to the IMP protein family with an emphasis on human IMP2, followed by a closer look at the GWA studies to evaluate the significance, if any, of the proposed correlation between IMP2 and T2D.

Restricted access

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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K J Parker, P M Jones, C H Hunton, S J Persaud, C G Taylor and S L Howell

ABSTRACT

The liberation of arachidonic acid (AA), by phospholipase A2 (PLA2), is the rate-limiting step in a number of cell signalling pathways. In the pancreatic β-cell, AA itself is thought to participate in the regulation of insulin secretion. Recently a Ca2+-sensitive, AA-selective cytosolic PLA2 (type IV cPLA2) has been isolated from the human monocyte U937 cell line. Although the DNA sequence of this enzyme implies a molecular weight of 85 kDa, the protein migrates with a molecular weight of 100-110 kDa on SDS-PAGE. In many cell types, cPLA2s which are reactive towards antibodies raised against the type IV cPLA2 have been shown to hydrolyse AA from membrane glycerophospholipids. Using a polyclonal antibody raised against a recombinant form of type IV cPLA2, we have detected an immunoreactive protein with a molecular weight of 93·5 kDa in rat islets of Langerhans. Furthermore, we have detected similar immunoreactive proteins in insulin-secreting β-cell lines and have shown co-expression of type IV cPLA2 immunoreactivity and insulin immunoreactivity in rat pancreatic β-cells. Under non-stimulatory conditions the 93·5 kDa immunoreactive protein detected in rat islets of Langerhans was located predominantly in the cytosolic fraction. We have shown that immunoprecipitation of the rat immunoreactive protein from rat islet homogenates significantly decreases the total dithiothreitol/β-mercaptoethanol-insensitive PLA2 activity by 56·4±7% This provides further evidence that the immunoreactive rat protein is a type IV cPLA2 and is responsible for a large component of the PLA2 activity in rat islets of Langerhans. It is possible that, in the rat β-cell, type IV cPLA2 couples the increase in intracellular Ca2+, brought about by insulin secretagogues, to the liberation of AA and the subsequent release of insulin.

Free access

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.

Free access

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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Giulia Cantini, Martina Trabucco, Alessandra Di Franco, Edoardo Mannucci and Michaela Luconi

Glucagon-like peptide 1 receptor agonists (GLP-1RAs), which are currently used for the treatment of type 2 diabetes, have recently been proposed as anti-obesity drugs, due to their relevant effects on weight loss. Furthermore, dual agonists for both GLP-1R and glucagon receptor (GCGR) are under investigation for their promising action on adiposity, although underlying mechanisms still need to be clarified. We have recently demonstrated that GLP-1 and liraglutide interfere with the proliferation and differentiation of human adipose precursors, supporting the hypothesis of a peripheral action of GLP-1RA on weight. Here, we investigated glucagon activity in an in vitro model of primary human adipose-derived stem cells (ASCs). Glucagon significantly inhibited ASC proliferation in a dose- and time-dependent manner, as evaluated by cell count and thymidine incorporation. When added during in vitro-induced adipogenesis, glucagon significantly reduced adipocyte differentiation, as demonstrated by the evaluation of intracellular fat content and quantitative expression of early and mature adipocyte markers (PPARγ and FABP4, HSL). Notably, the inhibitory effect of glucagon on cell proliferation and adipogenesis was reversed by specific GLP-1R (exendin-9) and GCGR (des-His1-Glu9-glucagon(1–29)) antagonists. The presence of both receptors was demonstrated by Western blot, immunofluorescence and cytofluorimetric analysis of ASCs. In conclusion, we demonstrated a direct inhibitory action of glucagon on the proliferation and differentiation of human adipose precursors, which seems to involve both GLP-1R and GCGR. These findings suggest that the adipose stem compartment is a novel target of glucagon, possibly contributing to the weight loss obtained in vivo with dual GLP-1R/glucagon agonists.

Free access

LC Bollheimer, S Troll, H Landauer, CE Wrede, J Scholmerich and R Buettner

Thiazolidinediones (TZDs) have been suggested to act beneficially on pancreatic islet function and on beta-cell viability but data concerning direct effects on isolated islets are controversial. Therefore, we have examined parameters of pancreatic insulin and glucagon secretion and biosynthesis in TZD-exposed rat pancreatic islets under physiological glucose level conditions and under conditions of glucolipotoxicity. Primary rat islets were incubated for 2.5 h with or without troglitazone (10 microM) in 5.6 mM glucose (standard glucose levels) and 16.7 mM glucose (high glucose levels); a subgroup was additionally treated with oleate (200 microM) to simulate acute glucolipotoxicity. Insulin and glucagon secretion, intracellular content and their respective mRNAs were quantified. Newly synthesized insulin was determined by pulse-labeling experiments. Troglitazone reduced insulin secretion at standard and high glucose levels by about one-third (P<or=0.05). Insulin content was decreased at 5.6 mM glucose but increased at 16.7 mM glucose by the presence of troglitazone (P<or=0.05). Newly synthesized insulin mRNA and preproinsulin mRNA decreased by about 20% at standard glucose levels (P<or=0.05). Glucagon secretion was augmented by troglitazone in islets under high glucose conditions by an additional 50% (P<or=0.05). No clear beneficial troglitazone effects were observed under glucolipotoxic conditions. The reduced insulin secretion and biosynthesis at standard glucose levels can be interpreted as an insulin-sparing effect. Troglitazone effects were less pronounced at high glucose alone or in combination with oleate. From a clinical point of view, these results indicate a greater benefit of troglitazone for beta-cell function in hyperinsulinemic, but normoglycemic patients with insulin resistance or early type 2 diabetes without major insulin secretion deficits and/or pronounced hyperglycemia.

Free access

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.