A specific increased expression of insulin receptor substrate 2 in pancreatic beta-cell lines is involved in mediating serum-stimulated beta-cell growth

Glucose Induces Mouse β-Cell Proliferation via IRS2, MTOR, and Cyclin D2 but Not the Insulin Receptor

An important goal in diabetes research is to understand the processes that trigger endogenous β-cell proliferation. Hyperglycemia induces β-cell replication, but the mechanism remains debated. A prime candidate is insulin, which acts locally through the insulin receptor. Having previously developed an in vivo mouse hyperglycemia model, we tested whether glucose induces β-cell proliferation through insulin signaling. By using mice lacking insulin signaling intermediate insulin receptor substrate 2 (IRS2), we confirmed that hyperglycemia-induced β-cell proliferation requires IRS2 both in vivo and ex vivo. Of note, insulin receptor activation was not required for glucose-induced proliferation, and insulin itself was not sufficient to drive replication. Glucose and insulin caused similar acute signaling in mouse islets, but chronic signaling differed markedly, with mammalian target of rapamycin (MTOR) and extracellular signal-related kinase (ERK) activation by glucose and AKT activation by insulin. MTOR but not ERK activation was required for glucose-induced proliferation. Cyclin D2 was necessary for glucose-induced β-cell proliferation. Cyclin D2 expression was reduced when either IRS2 or MTOR signaling was lost, and restoring cyclin D2 expression rescued the proliferation defect. Human islets shared many of these regulatory pathways. Taken together, these results support a model in which IRS2, MTOR, and cyclin D2, but not the insulin receptor, mediate glucose-induced proliferation.

Human β-cell proliferation and intracellular signaling: part 3

This is the third in a series of Perspectives on intracellular signaling pathways coupled to proliferation in pancreatic β-cells. We contrast the large knowledge base in rodent β-cells with the more limited human database. With the increasing incidence of type 1 diabetes and the recognition that type 2 diabetes is also due in part to a deficiency of functioning β-cells, there is great urgency to identify therapeutic approaches to expand human β-cell numbers. Therapeutic approaches might include stem cell differentiation, transdifferentiation, or expansion of cadaver islets or residual endogenous β-cells. In these Perspectives, we focus on β-cell proliferation. Past Perspectives reviewed fundamental cell cycle regulation and its upstream regulation by insulin/IGF signaling via phosphatidylinositol-3 kinase/mammalian target of rapamycin signaling, glucose, glycogen synthase kinase-3 and liver kinase B1, protein kinase Cζ, calcium-calcineurin-nuclear factor of activated T cells, epidermal growth factor/platelet-derived growth factor family members, Wnt/β-catenin, leptin, and estrogen and progesterone. Here, we emphasize Janus kinase/signal transducers and activators of transcription, Ras/Raf/extracellular signal-related kinase, cadherins and integrins, G-protein-coupled receptors, and transforming growth factor β signaling. We hope these three Perspectives will serve to introduce these pathways to new researchers and will encourage additional investigators to focus on understanding how to harness key intracellular signaling pathways for therapeutic human β-cell regeneration for diabetes.

Glycogen synthase kinase-3β is associated with the prognosis of hepatocellular carcinoma and may mediate the influence of type 2 diabetes mellitus on hepatocellular carcinoma

BACKGROUND

Although many studies have shown glycogen synthase kinase-3β (GSK-3β) was associated with type 2 diabetes mellitus (T2DM) and implicated with a wide range of cancers, the role of GSK-3β in hepatocellular carcinoma(HCC) and the correlation among GSK-3β, T2DM and HCC remains unclear. Our objectives were to identify the effect of p-Ser9-GSK-3β on the prognosis of patients with HCC and to learn more about the interaction among T2DM, GSK-3β and the prognosis of HCC.

METHODS

Firstly we used reverse transcriptase-PCR(RT-PCR) and western blotting to determine the expression levels of GSK-3β and p-Ser9-GSK-3β in human HCC samples. We then used immunohistochemical staining to evaluate the expression pattern of p-Ser9-GSK-3β in 178 patients with HCC after curative partial hepatectomy. Finally we statistically analyzed the association of p-Ser9-GSK-3β and T2DM with the prognosis of patients with HCC.

RESULTS

P-Ser9-GSK-3β was over-expressed in tumor tissues compared with their normal counterparts. Correlation and regression analysis indicated that the over-expression of p-Ser9-GSK-3β was significantly associated with T2DM, and the correlation coefficient was 0.259 (P = 0.001). Multivariate analysis showed that the over-expression of p-Ser9-GSK-3β(P<0.001) and T2DM(P = 0.008) were independently associated with poor prognosis of HCC, respectively. Further analysis demonstrated that these two variables are closely related with each other.

CONCLUSION

The over-expression of p-Ser9-GSK-3β and T2DM are strongly correlated with worse surgical outcome of HCC. P-Ser9-GSK-3β may play a significant role in mediating the influence of T2DM on the prognosis of HCC.

Targeting type 2 diabetes: lessons from a knockout model of insulin receptor substrate 2

Insulin receptor substrate 2 (IRS2) is a widely expressed protein that regulates crucial biological processes including glucose metabolism, protein synthesis, and cell survival. IRS2 is part of the insulin – insulin-like growth factor (IGF) signaling pathway and mediates the activation of the phosphotidylinositol 3-kinase (PI3K)–Akt and the Ras–mitogen-activated protein kinase (MAPK) cascades in insulin target tissues and in the pancreas. The best evidence of this is that systemic elimination of the Irs2 in mice (Irs2−/−) recapitulates the pathogenesis of type 2 diabetes (T2D), in that diabetes arises as a consequence of combined insulin resistance and beta-cell failure. Indeed, work using this knockout mouse has confirmed the importance of IRS2 in the control of glucose homeostasis and especially in the survival and function of pancreatic beta-cells. These studies have shown that IRS2 is critically required for beta-cell compensation in conditions of increased insulin demand. Importantly, islets isolated from T2D patients exhibit reduced IRS2 expression, which supports the likely contribution of altered IRS2-dependent signaling to beta-cell failure in human T2D. For all these reasons, the Irs2−/− mouse has been and will be essential for elucidating the inter-relationship between beta-cell function and insulin resistance, as well as to delineate therapeutic strategies to protect beta-cells during T2D progression.

Menin is required for optimal processing of the microRNA let-7a

Multiple endocrine neoplasia type I (MEN1) is an inherited syndrome that includes susceptibility to pancreatic islet hyperplasia. This syndrome results from mutations in the MEN1 gene, which encodes menin protein. Menin interacts with several transcription factors, including JunD, and inhibits their activities. However, the precise mechanism by which menin suppresses gene expression is not well understood. Here, we show that menin interacts with arsenite-resistant protein 2 (ARS2), a component of the nuclear RNA CAP-binding complex that is crucial for biogenesis of certain miRNAs including let-7a. The levels of primary-let-7a (pri-let-7a) are not affected by menin; however, the levels of mature let-7a are substantially decreased upon Men1 excision. Let-7a targets, including Insr and Irs2, pro-proliferative genes that are crucial for insulin-mediated signaling, are up-regulated in Men1-excised cells. Inhibition of let-7a using anti-miRNA in wild type cells is sufficient to enhance the expression of insulin receptor substrate 2 (IRS2) to levels observed in Men1-excised cells. Depletion of menin does not affect the expression of Drosha and CBP80, but substantially impairs the processing of pri-miRNA to pre-miRNA. Ars2 knockdown decreased let-7a processing in menin-expressing cells but had little impact on let-7a levels in menin-excised cells. As IRS2 is known to mediate insulin signaling and insulin/mitogen-induced cell proliferation, these findings collectively unravel a novel mechanism whereby menin suppresses cell proliferation, at least partly by promoting the processing of certain miRNAs, including let-7a, leading to suppression of Irs2 expression and insulin signaling.

Phosphorylation of serine 1137/1138 of mouse insulin receptor substrate (IRS) 2 regulates cAMP-dependent binding to 14-3-3 proteins and IRS2 protein degradation

Insulin receptor substrate (IRS) 2 as intermediate docking platform transduces the insulin/IGF-1 (insulin like growth factor 1) signal to intracellular effector molecules that regulate glucose homeostasis, β-cell growth, and survival. Previously, IRS2 has been identified as a 14-3-3 interaction protein. 14-3-3 proteins can bind their target proteins via phosphorylated serine/threonine residues located within distinct motifs. In this study the binding of 14-3-3 to IRS2 upon stimulation with forskolin or the cAMP analog 8-(4-chlorophenylthio)-cAMP was demonstrated in HEK293 cells. Binding was reduced with PKA inhibitors H89 or Rp-8-Br-cAMPS. Phosphorylation of IRS2 on PKA consensus motifs was induced by forskolin and the PKA activator N(6)-Phe-cAMP and prevented by both PKA inhibitors. The amino acid region after position 952 on IRS2 was identified as the 14-3-3 binding region by GST-14-3-3 pulldown assays. Mass spectrometric analysis revealed serine 1137 and serine 1138 as cAMP-dependent, potential PKA phosphorylation sites. Mutation of serine 1137/1138 to alanine strongly reduced the cAMP-dependent 14-3-3 binding. Application of cycloheximide revealed that forskolin enhanced IRS2 protein stability in HEK293 cells stably expressing IRS2 as well as in primary hepatocytes. Stimulation with forskolin did not increase protein stability either in the presence of a 14-3-3 antagonist or in the double 1137/1138 alanine mutant. Thus the reduced IRS2 protein degradation was dependent on the interaction with 14-3-3 proteins and the presence of serine 1137/1138. We present serine 1137/1138 as novel cAMP-dependent phosphorylation sites on IRS2 and show their importance in 14-3-3 binding and IRS2 protein stability.

Cocaine- and amphetamine-regulated transcript (CART) protects beta cells against glucotoxicity and increases cell proliferation

Cocaine- and amphetamine-regulated transcript (CART) is an islet peptide that promotes glucose-stimulated insulin secretion in beta cells via cAMP/PKA-dependent pathways. In addition, CART is a regulator of neuronal survival. In this study, we examined the effect of exogenous CART 55-102 on beta cell viability and dissected its signaling mechanisms. Evaluation of DNA fragmentation and chromatin condensation revealed that CART 55-102 reduced glucotoxicity-induced apoptosis in both INS-1 (832/13) cells and isolated rat islets. Glucotoxicity in INS-1 (832/13) cells also caused a 50% reduction of endogenous CART protein. We show that CART increased proliferation in INS-1 (832/13) cells, an effect that was blocked by PKA, PKB, and MEK1 inhibitors. In addition, CART induced phosphorylation of CREB, IRS, PKB, FoxO1, p44/42 MAPK, and p90RSK in INS-1 (832/13) cells and isolated rat islets, all key mediators of cell survival and proliferation. Thus, we demonstrate that CART 55-102 protects beta cells against glucotoxicity and promotes proliferation. Taken together our data point to the potential use of CART in therapeutic interventions targeted at enhancing functional beta cell mass and long-term insulin secretion in T2D.

Macrophage migration inhibitory factor deficiency protects pancreatic islets from cytokine-induced apoptosis in vitro

During pathogenesis of diabetes, pancreatic islets are exposed to high levels of cytokines and other inflammatory mediators that induce deterioration of insulin-producing beta cells. Macrophage migration inhibitory factor (MIF) plays a key role in the onset and development of several immunoinflammatory diseases and also controls apoptotic cell death. Because the occurrence of apoptosis plays a pathogenetic role in beta cell death during type 1 diabetes development and MIF is expressed in beta cells, we explored the influence of MIF deficiency on cytokine-induced apoptosis in pancreatic islets. The results indicated clearly that elevated MIF secretion preceded C57BL/6 pancreatic islets death induced by interferon (IFN)-γ + tumour necrosis factor (TNF)-α + interleukin (IL)-1β. Consequently, MIF-deficient [MIF-knock-out (KO)] pancreatic islets or islet cells showed significant resistance to cytokine-induced death than those isolated from C57BL/6 mice. Furthermore, upon exposure to cytokines pancreatic islets from MIF-KO mice maintained normal insulin expression and produced less cyclooxygenase-2 (COX-2) than those from wild-type C57BL6 mice. The final outcome of cytokine-induced islet apoptosis in islets from wild-type mice was the activation of mitochondrial membrane pore-forming protein Bcl-2-associated X protein and effector caspase 3. In contrast, these apoptotic mediators remained at normal levels in islets from MIF-KO mice suggesting that MIF absence prevented initiation of the mitochondrial apoptotic pathway. Additionally, the protection from apoptosis was also mediated by up-regulation of prosurvival kinase extracellular-regulated kinase 1/2 in MIF-KO islets. These data indicate that MIF is involved in the propagation of pancreatic islets apoptosis probably via nuclear factor-κB and mitochondria-related proteins.

GPR39: a Zn(2+)-activated G protein-coupled receptor that regulates pancreatic, gastrointestinal and neuronal functions

GPR39 is a vertebrate G protein-coupled receptor related to the ghrelin/neurotensin receptor subfamily. The receptor is expressed in a range of tissues including the pancreas, gut/gastrointestinal tract, liver, kidney and in some regions of the brain. GPR39 was initially thought to be the cognitive receptor for the peptide hormone, obestatin. However, subsequent in vitro studies have failed to demonstrate binding of this peptide to the receptor. Zn(2+) has been shown to be a potent stimulator of GPR39 activity via the Gα(q), Gα(12/13) and Gα(s) pathways. The potency and specificity of Zn(2+) in activating GPR39 suggest it to be a physiologically important agonist. GPR39 is now emerging as an important transducer of autocrine and paracrine Zn(2+) signals, impacting upon cellular processes such as insulin secretion, gastric emptying, neurotransmission and epithelial repair. This review focuses on the molecular, structural and biological properties of GPR39 and its various physiological functions.

Glucose, regulator of survival and phenotype of pancreatic beta cells

The key role of glucose in regulating insulin release by the pancreatic beta cell population is not only dependent on acute stimulus-secretion coupling mechanisms but also on more long-term influences on beta cell survival and phenotype. Glucose serves as a major survival factor for beta cells via at least three actions: it prevents an oxidative redox state, it suppresses a mitochondrial apoptotic program that is triggered at reduced mitochondrial metabolic activity and it induces genes needed for the cellular responsiveness to glucose and to growth factors. Glucose-regulated pathways may link protein synthetic and proliferative activities, making glucose a permissive factor for beta cell proliferation, in check with metabolic needs. Conditions of inadequate glucose metabolism in beta cells are not only leading to deregulation of acute secretory responses but should also be considered as causes for increased apoptosis and reduced formation of beta cells, and loss of their normal differentiated state.

Activation of insulin and IGF-1 signaling pathways by melatonin through MT1 receptor in isolated rat pancreatic islets

Melatonin diminishes insulin release through the activation of MT1 receptors and a reduction in cAMP production in isolated pancreatic islets of neonate and adult rats and in INS-1 cells (an insulin-secreting cell line). The pancreas of pinealectomized rats exhibits degenerative pathological changes with low islet density, indicating that melatonin plays a role to ensure the functioning of pancreatic beta cells. By using immunoprecipitation and immunoblotting analysis we demonstrated, in isolated rat pancreatic islets, that melatonin induces insulin growth factor receptor (IGF-R) and insulin receptor (IR) tyrosine phosphorylation and mediates the activities of the PI3K/AKT and MEK/ERKs pathways, which are involved in cell survival and growth, respectively. Thus, the effects of melatonin on pancreatic islets do not involve a reduction in cAMP levels only. This indoleamine may regulate growth and differentiation of pancreatic islets by activating IGF-I and insulin receptor signaling pathways.

siRNA-based gene silencing reveals specialized roles of IRS-1/Akt2 and IRS-2/Akt1 in glucose and lipid metabolism in human skeletal muscle

Type 2 diabetes is associated with defects in insulin signaling and the resulting abnormal glucose and lipid metabolism. The complexity of insulin signaling cascades is highlighted by the existence of multiple isoforms of target proteins implicated in metabolic and gene-regulatory events. We utilized siRNA to decipher the specific role of predominant insulin receptor substrates and Akt isoforms expressed in human skeletal muscle. Gene silencing revealed specialized roles of insulin signaling cascades to metabolic endpoints. IRS-1 and Akt2 were required for myoblast differentiation and glucose metabolism, whereas IRS-2 and Akt1 were dispensable. A key role of IRS-2 and Akt1 in lipid metabolism was revealed, highlighting reciprocal relationships between metabolic pathways. Unraveling the isoform-specific regulation of glucose and lipid metabolism by key elements along insulin signaling cascades through siRNA-mediated gene silencing in human tissues will facilitate the discovery of novel targets for the treatment of diabetes and related metabolic disorders.

Upregulation of rat Ccnd1 gene by exendin-4 in pancreatic beta cell line INS-1: interaction of early growth response-1 with cis-regulatory element

AIMS/HYPOTHESIS

The aim of this study was to investigate the effect of exendin-4 on the expression of cyclin D1 gene (Ccnd1), which is critical in regulating the progression of the cell cycle in INS-1 cells.

MATERIALS AND METHODS

INS-1 cells were stimulated with exendin-4 (10 nmol/l). Transient transfection and luciferase reporter assays were performed to measure promoter activities of rat Ccnd1. Electrophoretic mobility shift and chromatin immunoprecipitation assays were used to examine the binding of transcription factors to sites responsive to exendin-4 in vitro and in vivo, respectively.

RESULTS

Exendin-4 increased both Ccnd1 mRNA and its protein levels in a time-dependent manner. The region from -174 to +130 of the promoter was found to contain cis-regulatory elements responsible for exendin-4-mediated gene induction. Early growth response-1 (EGR1) protein was bound to the region from -153 to -134, which includes the putative EGR1 binding site (5'-CACCCCCGC-3'). Moreover, exendin-4 recruited EGR1 protein to the promoter in vivo.

CONCLUSIONS/INTERPRETATION

These findings suggest that exendin-4 activates Ccnd1 transcription through induction of EGR1 binding to a cis-regulatory element between -153 and -134 on the rat Ccnd1 promoter. These results provide an important indication that exendin-4 is a growth factor regulating beta cell proliferation.

Overexpression of insulin receptor substrate-2 in human and murine hepatocellular carcinoma

De-regulations in insulin and insulin-like growth factor (IGF) pathways may contribute to hepatocellular carcinoma. Although intracellular insulin receptor substrate-2 (IRS-2) is the main effector of insulin signaling in the liver, its role in hepatocarcinogenesis is unknown. Here, we show that IRS-2 was overexpressed in two murine models of hepatocarcinogenesis: administration of diethylnitrosamine and hepatic overexpression of SV40 large T antigen. In both models, IRS-2 overexpression was detected in preneoplastic lesions and at higher levels in tumoral nodules. IRS-2 overexpression associated with IGF-2 and IRS-1 overexpression and with GSK-3beta inhibition. Increased expression of IRS-2 was also detected in human hepatocellular carcinoma specimens and hepatoma cell lines. In murine and human hepatoma cells, IRS-2 protein induction associated with increased IRS-2 mRNA levels. The functionality of IRS-2 was demonstrated in Hep 3 B cells, in which IRS-2 tyrosine phosphorylation and its association with phosphatidylinositol-3 kinase were induced by IGF-2. Moreover, down-regulation of IRS-2 expression increased apoptosis in these cells. In conclusion, we demonstrate that IRS-2 is overexpressed in human and murine hepatocellular carcinoma. The emergence of IRS-2 overexpression at preneoplastic stages during experimental hepatocarcinogenesis and its protective effect against apoptosis suggest that IRS-2 contributes to liver tumor progression.

Insulin-induced cell cycle progression is impaired in chinese hamster ovary cells overexpressing insulin receptor substrate-3

To analyze the roles of insulin receptor substrate (IRS) proteins in insulin-stimulated cell cycle progression, we examined the functions of rat IRS-1 and IRS-3 in Chinese hamster ovary cells overexpressing the human insulin receptor. In this type of cell overexpressing IRS-1 or IRS-3, we showed that: 1) overexpression of IRS-3, but not IRS-1, suppressed the G1/S transition induced by insulin; 2) IRS-3 was more preferentially localized to the nucleus than IRS-1; 3) phosphorylation of glycogen synthase kinase 3 and MAPK/ERK was unaffected by IRS-3 overexpression, whereas that of protein kinase B was enhanced by either IRS; 4) overexpressed IRS-3 suppressed cyclin D1 expression in response to insulin; 5) among the signaling molecules regulating cyclin D1 expression, activation of the small G protein Ral was unchanged, whereas insulin-induced gene expression of c-myc, a critical component for growth control and cell cycle progression, was suppressed by overexpressed IRS-3; and 6) insulin-induced expression of p21, a cyclin-dependent kinase inhibitor, was decreased by overexpressed IRS-3. These findings imply that: 1) IRS-3 may play a unique role in mitogenesis by inhibiting insulin-stimulated cell cycle progression via a decrease in cyclin D1 and p21 expressions as well as suppression of c-myc mRNA induction in a manner independent of the activation of MAPK, protein kinase B, glycogen synthase kinase 3 and Ral; and 2) the interaction of IRS-3 with nuclear proteins may be involved in this process.

Raf kinase inhibitory protein inhibits beta-cell proliferation

BACKGROUND

Raf-1 kinase inhibitory protein (RKIP) was recently identified as a physiologic endogenous inhibitor of the extracellular signal-regulated kinase (ERK) pathway. The expression and role of RKIP within the pancreas are unknown.

METHODS

RKIP expression in normal pancreas and human insulinomas was examined by using paraffin-embedded sections. Co-localization of RKIP within islet cell subtypes was performed by using double immunofluorescence staining with antibodies directed toward RKIP and endocrine markers. To examine the role of RKIP in beta-cell proliferation, stable expression of sense (ss) and antisense (as) RKIP was established in HIT-T15 beta cells. The effect of RKIP on the ERK-signaling pathway in beta cells was determined by Western blotting with the use of phospho-specific antibodies directed against mitogen-activated protein kinase kinase (MEK) and ERK. The role of RKIP in beta-cell proliferation was assessed by using MTS assay and FACS analysis.

RESULTS

RKIP was expressed only within pancreatic islet cells. Immunofluorescent double staining revealed that RKIP was expressed in most beta cells and a subset of pancreatic polypeptide-expressing cells. Based on the known function of RKIP, we hypothesized that RKIP expression would be downregulated in insulinomas: 8 of 9 human insulinomas demonstrated no RKIP staining, with decreased expression in 1 of 9 insulinomas. Studies using asRKIP and ssRKIP demonstrated that RKIP blocked activation of MEK and ERK by Raf-1 in beta cells. We also showed that RKIP inhibited beta-cell proliferation by altering cell cycle distribution, rather than by promoting apoptosis.

CONCLUSIONS

RKIP is important in beta-cell proliferation, and its downregulation may play a role in islet neoplasia.

[LysB3, GluB29] insulin: a novel insulin analog with enhanced β-cell protective action

Insulin receptor substrate (IRS)-2 has been implicated in the promotion of beta-cell survival. Here we tested the hypothesis that the novel analog [LysB3, GluB29] insulin (insulin glulisine, IG) might mediate an enhanced beta-cell protective effect due to its unique property of preferential IRS-2 phosphorylation. We assessed IRS activation by IG and its anti-apoptotic activity against cytokines or palmitic acid in comparison to insulin, insulin analogs, and insulin-like growth factor (IGF)-I using INS-1 cells. IG induced a prominent IRS-2 activation without significant IRS-1 stimulation. The marked cytokine- and fatty acid-induced apoptosis was strongly (55-60%) inhibited by IG both at the level of caspase 3 activation and nucleosomal release, with only 15% inhibition of apoptosis by regular insulin. At 1nM, insulin, insulin aspart, and insulin lispro were much less effective compared to IG. In conclusion, the prominent anti-apoptotic activity of insulin glulisine might serve to counteract autoimmune- and lipotoxicity-induced beta-cell destruction.

IRS-3 inhibits IRS-2-mediated signaling in pancreatic beta-cells

IRS-2 plays an important role in the control of pancreatic beta-cell growth, however it is unclear if other IRS family members are also involved. Using recombinant adenoviruses, IRS-1, -2 and -3 expression was varied in the beta-cell line, INS-1. Increased IRS-1 expression had no appreciable effect on beta-cell growth. However, increased IRS-2 expression augmented glucose/IGF-1 induced beta-cell growth mitogenesis and decreased apoptosis due to glucose-deprivation. In contrast, increased IRS-3 expression significantly inhibited mitogenesis and increased apoptosis. IRS-3 was intransiently located to the beta-cell plasma membrane, and appeared to be inert in terms of IGF-1 induced signaling. However, increased IRS-3 expression blocked glucose/IGF-1 induced IRS-2 translocation from the cytosol to the plasma membrane, dampening IRS-2/IGF-1R interaction and subsequent activation of the PI3K/PKB/GSK3 signaling pathway. In contrast, glucose/IGF-1 induced Erk-1/-2 and p70S6K activation were unaffected by IRS-3. These data emphasize the importance of IRS-2/PI3K/PKB signal transduction for beta-cell growth and survival.

Activation of IRS-2-mediated signal transduction by IGF-1, but not TGF-alpha or EGF, augments pancreatic beta-cell proliferation

Transforming growth factor (TGF)-alpha- and epidermal growth factor (EGF)-induced signal transduction was directly compared with that of glucose and insulin-like growth factor-1 (IGF-1) in INS-1 cells. TGF-alpha/EGF transiently (<20 min) induced phosphorylation of extracellular-regulated kinase (Erk)-1/2 (>20-fold), glycogen synthase kinase (GSK)-3 (>10-fold), and protein kinase B (PKB) (Ser(473) and Thr(308)), but did not increase [(3)H]thymidine incorporation. In contrast, phosphorylation of Erk1/2, GSK-3, and PKB in response to glucose and IGF-1 was more prolonged (>24 h) and, though not as robust as TGF-alpha/EGF, did increase beta-cell proliferation. Phosphorylation of p70(S6K) was also increased by IGF-1/glucose, but not by TGF-alpha/EGF, despite upstream PKB activation. It was found that IGF-1 induced phosphatidylinositol 3-kinase (PI3K) association with insulin receptor substrate (IRS)-1 and -2 in a glucose-dependent manner, whereas TGF-alpha/EGF did not. The importance of specific IRS-2-mediated signaling events was emphasized in that adenoviral-mediated overexpression of IRS-2 further increased glucose/IGF-1-induced beta-cell proliferation (more than twofold; P < 0.05) compared with control or adenoviral-mediated IRS-1 overexpressing INS-1 cells. Neither IRS-1 nor IRS-2 overexpression induced a beta-cell proliferative response to TGF-alpha/EGF. Thus, a prolonged activation of Erk1/2 and PI3K signaling pathways is important in committing a beta-cell to a mitogenic event, and it is likely that this sustained activation is instigated by signal transduction occurring specifically through IRS-2.

Apoptosis in the beta cells: cause or consequence of insulin secretion defect in diabetes?

Pancreatic beta-cell dysfunction and insulin resistance are two interrelated defects in the pathophysiology of type 2 diabetes. Defects in peripheral insulin action precede the development of glucose intolerance, as the pancreas compensates for insulin resistance by increasing insulin production and secretion. This may be achieved by enhancing cellular secretory capacity or by increasing beta-cell mass. Over time, the pancreatic secretion of insulin becomes inadequate for the extent of insulin resistance, and the levels of fasting and postprandial glucose rise leading to the onset of frank hyperglycemia, which leads to reduction in beta-cell function and survival through a process referred to as glucose toxicity. There is increasing evidence that apoptosis is the main mode of pancreatic beta-cell death not only in type 1 but also in type 2 diabetes. Recently, studies in knockout mice, human and rat islets, and pancreatic beta-cell lines demonstrated that defective insulin signaling in beta-cells might play an important pathophysiological role by affecting both secretory function and cell survival. The purpose of this review is to present recent advances in understanding of the interrelationship between molecular mechanisms underlying defects in insulin secretion and beta-cell survival in type 2 diabetes caused by impaired activation of insulin signaling pathways.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

Effects of streptozocin diabetes and diabetes treatment by islet transplantation on in vivo insulin signaling in rat heart

The insulin signaling cascade was investigated in rat myocardium in vivo in the presence of streptozocin (STZ)-induced diabetes and after diabetes treatment by islet transplantation under the kidney capsule. The levels of insulin-stimulated tyrosine phosphorylation of the insulin receptor beta-subunit, insulin receptor substrate (IRS)-2, and p52(Shc) were increased in diabetic compared with control heart, whereas tyrosine phosphorylation of IRS-1 was unchanged. The amount of the p85 subunit of phosphatidylinositol 3-kinase (PI 3-kinase) and the level of PI 3-kinase activity associated with IRS-2 were also elevated in diabetes, whereas no changes in IRS-1-associated PI 3-kinase were observed. Insulin-induced phosphorylation of Akt on Thr-308 was increased fivefold in diabetic heart, whereas Akt phosphorylation on Ser-473 was normal. In contrast with Akt phosphorylation, insulin-induced phosphorylation of glycogen synthase kinase (GSK)-3, a major cellular substrate of Akt, was markedly reduced in diabetes. In islet-transplanted rats, the majority of the alterations in insulin-signaling proteins found in diabetic rats were normalized, but insulin stimulation of IRS-2 tyrosine phosphorylation and association with PI 3-kinase was blunted. In conclusion, in the diabetic heart, 1) IRS-1, IRS-2, and p52(Shc) are differently altered, 2) the levels of Akt phosphorylation on Ser-473 and Thr-308, respectively, are not coordinately regulated, and 3) the increased activity of proximal-signaling proteins (i.e., IRS-2 and PI 3-kinase) is not propagated distally to GSK-3. Islet transplantation under the kidney capsule is a potentially effective therapy to correct several diabetes-induced abnormalities of insulin signaling in cardiac muscle but does not restore the responsiveness of all signaling reactions to insulin.

Protein kinase Czeta activation mediates glucagon-like peptide-1-induced pancreatic beta-cell proliferation

Glucagon-like peptide-1 (GLP-1), an insulinotropic and glucoincretin hormone, is a potentially important therapeutic agent in the treatment of diabetes. We previously provided evidence that GLP-1 induces pancreatic beta-cell growth nonadditively with glucose in a phosphatidylinositol-3 kinase (PI-3K)-dependent manner. In the present study, we investigated the downstream effectors of PI-3K to determine the precise signal transduction pathways that mediate the action of GLP-1 on beta-cell proliferation. GLP-1 increased extracellular signal-related kinase 1/2, p38 mitogen-activated protein kinase (MAPK), and protein kinase B activities nonadditively with glucose in pancreatic beta(INS 832/13) cells. GLP-1 also caused nuclear translocation of the atypical protein kinase C (aPKC) zeta isoform in INS as well as in dissociated normal rat beta-cells as shown by immunolocalization and Western immunoblotting analysis. Tritiated thymidine incorporation measurements showed that the p38 MAPK inhibitor SB203580 suppressed GLP-1-induced beta-cell proliferation. Further investigation was performed using isoform-specific pseudosubstrates of classical (alpha, beta, and gamma) or zeta aPKC isoforms. The PKCzeta pseudosubstrate suppressed the proliferative action of GLP-1, whereas the inhibitor of classical PKC isoforms had no effect. Overexpression of a kinase-dead PKCzeta acting as a dominant negative protein suppressed GLP-1-induced proliferation. In addition, ectopic expression of a constitutively active PKCzeta mutant stimulated tritiated thymidine incorporation to the same extent as GLP-1, and the glucoincretin had no growth-promoting action under this condition. The data indicate that GLP-1-induced activation of PKCzeta is implicated in the beta-cell proliferative signal of the insulinotropic hormone. The results are consistent with a model in which GLP-1-induced PI-3K activation results in PKCzeta translocation to the nucleus, which may play a role in the pleiotropic effects (DNA synthesis, metabolic enzymes, and insulin gene expression) of the glucoincretin.

Defects of the insulin receptor substrate (IRS) system in human metabolic disorders

Insulin receptor substrate (IRS) molecules are key mediators in insulin signaling and play a central role in maintaining basic cellular functions such as growth, survival, and metabolism. They act as docking proteins between the insulin receptor and a complex network of intracellular signaling molecules containing Src homology 2 (SH2) domains. Four members (IRS-1, IRS-2, IRS-3, IRS-4) of this family have been identified that differ as to tissue distribution, subcellular localization, developmental expression, binding to the insulin receptor, and interaction with SH2 domain-containing proteins. Results from targeted disruption of the IRS genes in mice have provided important clues to the functional differences among these related molecules, suggesting they play different and specific roles in vivo. The available data are consistent with the notion that IRS-1 and IRS-2 are not functionally interchangeable in tissues that are responsible for glucose production (liver), glucose uptake (skeletal muscle and adipose tissue), and insulin production (pancreatic beta cells). In fact, IRS-1 appears to have its major role in skeletal muscle whereas IRS-2 appears to regulate hepatic insulin action as well as pancreatic beta cell development and survival. By contrast, IRS-3 and IRS-4 genes appear to play a redundant role in the IRS signaling system. Defects in muscle IRS-1 expression and function have been reported in insulin-resistant states such as obesity and type 2 diabetes. Several polymorphisms in the IRS genes have been identified, but only the Gly-->Arg972 substitution of IRS-1, interacting with environmental factors, seems to have a pathogenic role in the development of type 2 diabetes. In contrast, polymorphisms of the other IRS genes do not appear to contribute to type 2 diabetes.

Preserved pancreatic beta-cell development and function in mice lacking the insulin receptor-related receptor

Receptors of the insulin/insulinlike growth factor (IGF) family have been implicated in the regulation of pancreatic beta-cell growth and insulin secretion. The insulin receptor-related receptor (IRR) is an orphan receptor of the insulin receptor gene (Ir) subfamily. It is expressed at considerably higher levels in beta cells than either insulin or IGF-1 receptors, and it has been shown to engage in heterodimer formation with insulin or IGF-1 receptors. To address whether IRR plays a physiologic role in beta-cell development and regulation of insulin secretion, we have characterized mice lacking IRR and generated a combined knockout of Ir and Irr. We report that islet morphology, beta-cell mass, and secretory function are not affected in IRR-deficient mice. Moreover, lack of IRR does not impair compensatory beta-cell hyperplasia in insulin-resistant Ir(+/-) mice, nor does it affect beta-cell development and function in Ir(-/-) mice. We conclude that glucose-stimulated insulin secretion and embryonic beta-cell development occur normally in mice lacking Irr.

Physiological and cell biological aspects of perfusion culture technique employed to generate differentiated tissues for long term biomaterial testing and tissue engineering

Optimal results in biomaterial testing and tissue engineering under in vitro conditions can only be expected when the tissue generated resembles the original tissue as closely as possible. However, most of the presently used stagnant cell culture models do not produce the necessary degree of cellular differentiation, since important morphological, physiological, and biochemical characteristics disappear, while atypical features arise. To reach a high degree of cellular differentiation and to optimize the cellular environment, an advanced culture technology allowing the regulation of differentiation on different cellular levels was developed. By the use of tissue carriers, a variety of biomaterials or individually selected scaffolds could be tested for optimal tissue development. The tissue carriers are to be placed in perfusion culture containers, which are constantly supplied with fresh medium to avoid an accumulation of harmful metabolic products. The perfusion of medium creates a constant microenvironment with serum-containing or serum-free media. By this technique, tissues could be used for biomaterial or scaffold testing either in a proliferative or in a postmitotic phase, as is observed during natural development. The present paper summarizes technical developments, physiological parameters, cell biological reactions, and theoretical considerations for an optimal tissue development in the field of perfusion culture.

Insulin receptor substrate (IRS) transduction system: distinct and overlapping signaling potential

Insulin receptor substrate (IRS) proteins play a central role in maintaining basic cellular functions such as growth and metabolism. They act as an interface between multiple growth factor receptors possessing tyrosine kinase activity, such as the insulin receptor, and a complex network of intracellular signalling molecules containing Src homology 2 (SH2) domains. Four members (IRS-1, IRS-2, IRS-3, IRS-4) of this family have been identified which differ in their subcellular distribution and interaction with SH2 domain proteins. In addition, differential IRS tissue- and developmental-specific expression patterns may contribute to specificity in their signaling potential.

A model of beta-cell mass, insulin, and glucose kinetics: pathways to diabetes

Diabetes is a disease of the glucose regulatory system that is associated with increased morbidity and early mortality. The primary variables of this system are beta-cell mass, plasma insulin concentrations, and plasma glucose concentrations. Existing mathematical models of glucose regulation incorporate only glucose and/or insulin dynamics. Here we develop a novel model of beta -cell mass, insulin, and glucose dynamics, which consists of a system of three nonlinear ordinary differential equations, where glucose and insulin dynamics are fast relative to beta-cell mass dynamics. For normal parameter values, the model has two stable fixed points (representing physiological and pathological steady states), separated on a slow manifold by a saddle point. Mild hyperglycemia leads to the growth of the beta -cell mass (negative feedback) while extreme hyperglycemia leads to the reduction of the beta-cell mass (positive feedback). The model predicts that there are three pathways in prolonged hyperglycemia: (1) the physiological fixed point can be shifted to a hyperglycemic level (regulated hyperglycemia), (2) the physiological and saddle points can be eliminated (bifurcation), and (3) progressive defects in glucose and/or insulin dynamics can drive glucose levels up at a rate faster than the adaptation of the beta -cell mass which can drive glucose levels down (dynamical hyperglycemia).

Insulin receptor substrate polymorphisms and type 2 diabetes mellitus

Insulin receptor substrate (IRS) molecules are key mediators in insulin signalling and play a central role in maintaining basic cellular functions, such as growth, survival and metabolism. They act as docking proteins for the insulin receptor and a complex network of intracellular signalling molecules containing Src homology 2 (SH2) domains. Four members (IRS-1, IRS-2, IRS-3 and IRS-4) of this family have been identified that differ in tissue distribution, subcellular localisation, developmental expression, binding to the insulin receptor and interaction with SH2 domain-containing proteins. Results from targeted disruption of the IRS genes in mice have provided important clues as to the functional differences among these related molecules and suggest that they play very different roles in vivo. The available data are consistent with the notion that both IRS-1 and IRS-2 are important for insulin action and glucose homeostasis in vivo, whereas IRS-and IRS-4 appear to play a redundant role in the IRS signalling system. Considering their key role in both insulin action and insulin secretion, IRS-1 and IRS-2 molecules have been considered plausible candidate genes involved in the pathogenesis of Type 2 diabetes. Several polymorphisms in the IRS genes have been identified, but only the Gly --> Arg72 substitution of IRS-1, acting with environmental factors, seems to have a pathogenic role in the development of Type 2 diabetes. In contrast, polymorphisms of the other IRS genes do not appear to contribute to Type 2 diabetes.

Implantation of rat insulinoma cell line into cyclosporine treated rats. Effect of the in vivo environment on beta-cell specific gene expression

BACKGROUND

Transplantation of engineered beta cell-derived lines is a promising modality for cell-based therapy of diabetes mellitus. The in vivo environment and antirejection and other medications may have significant effects on the differentiation and proliferation of the transplanted beta cells, thus affecting their function. The effect of the in vivo environment on expression of genes encoding proteins involved in insulin production, secretion, and glucose sensing were analyzed in the RIN 104638 cell line.

METHODS

RIN 104638 cells, were used for s.c. implantation in cyclosporine treated rats and for parallel in vitro culture. The differential expression of the insulin, PDX-1, GLUT-2, and glucokinase genes were assessed by quantitative reverse transcription polymerase chain reaction.

RESULTS

The in vivo environment of cyclosporine-treated rats, preserved most of the differentiated characteristics of the implanted cells. Insulin and glucokinase gene expression were maintained at high levels, although GLUT-2 expression decreased. This was in contrast to the substantial decrease of all the three genes expression when cultured in vitro. Cyclosporine treatment reduced insulin and GLUT-2 gene expression in in vitro culture.

CONCLUSIONS

Beta cell implantation in cyclosporine-treated rats induces alteration in expression of genes pivotal to insulin production and secretion and the glucose sensing abilities. The normal in vivo environment improves the implanted b cell function by increasing the insulin gene expression and content. Furthermore, it reverses some of the dedifferentiating changes caused by the in vitro culture. This may have a positive effect on the therapeutic efficiency of this cell line.

Signaling through the p38 and p42/44 mitogen-activated families of protein kinases in pancreatic beta-cell proliferation

The present study has focused on the role of the 42- and 44-kDa mitogen-activated protein kinases (p42/44 MAPKs) and the 38-kDa mitogen-activated protein kinase (p38 MAPK) in the proliferation of the pancreatic beta-cell line MIN6. MIN6 beta-cell proliferation was assessed by measuring 5-bromo-2'-deoxyuridine (BrdU) incorporation into cellular DNA. Inhibition of both the p42/44 MAPK pathway using the MEK inhibitor PD098059 (PD) and the p38 MAPK pathway using the p38 inhibitor SB203580 (SB) caused a marked, concentration-dependent reduction in the BrdU immunostaining observed in the presence of 15% FCS when assessed using fluorescence immunocytochemistry. These data provide direct evidence of a role for p42/44 MAPKs in the mitogenic response of MIN6 beta-cells to FCS. Furthermore, these data also suggest a novel role for the p38 MAPK pathway in MIN6 beta-cell proliferation.

Exercise-induced changes in expression and activity of proteins involved in insulin signal transduction in skeletal muscle: differential effects on insulin-receptor substrates 1 and 2

Level of physical activity is linked to improved glucose homeostasis. We determined whether exercise alters the expression and/or activity of proteins involved in insulin-signal transduction in skeletal muscle. Wistar rats swam 6 h per day for 1 or 5 days. Epitrochlearis muscles were excised 16 h after the last exercise bout, and were incubated with or without insulin (120 nM). Insulin-stimulated glucose transport increased 30% and 50% after 1 and 5 days of exercise, respectively. Glycogen content increased 2- and 4-fold after 1 and 5 days of exercise, with no change in glycogen synthase expression. Protein expression of the glucose transporter GLUT4 and the insulin receptor increased 2-fold after 1 day, with no further change after 5 days of exercise. Insulin-stimulated receptor tyrosine phosphorylation increased 2-fold after 5 days of exercise. Insulin-stimulated tyrosine phosphorylation of insulin-receptor substrate (IRS) 1 and associated phosphatidylinositol (PI) 3-kinase activity increased 2.5- and 3. 5-fold after 1 and 5 days of exercise, despite reduced (50%) IRS-1 protein content after 5 days of exercise. After 1 day of exercise, IRS-2 protein expression increased 2.6-fold and basal and insulin-stimulated IRS-2 associated PI 3-kinase activity increased 2. 8-fold and 9-fold, respectively. In contrast to IRS-1, IRS-2 expression and associated PI 3-kinase activity normalized to sedentary levels after 5 days of exercise. Insulin-stimulated Akt phosphorylation increased 5-fold after 5 days of exercise. In conclusion, increased insulin-stimulated glucose transport after exercise is not limited to increased GLUT4 expression. Exercise leads to increased expression and function of several proteins involved in insulin-signal transduction. Furthermore, the differential response of IRS-1 and IRS-2 to exercise suggests that these molecules have specialized, rather than redundant, roles in insulin signaling in skeletal muscle.

Irs-2 coordinates Igf-1 receptor-mediated beta-cell development and peripheral insulin signalling

Insulin receptor substrates (Irs proteins) mediate the pleiotropic effects of insulin and Igf-1 (insulin-like growth factor-1), including regulation of glucose homeostasis and cell growth and survival. We intercrossed mice heterozygous for two null alleles (Irs1+/- and Irs2+/-) and investigated growth and glucose metabolism in mice with viable genotypes. Our experiments revealed that Irs-1 and Irs-2 are critical for embryonic and post-natal growth, with Irs-1 having the predominant role. By contrast, both Irs-1 and Irs-2 function in peripheral carbohydrate metabolism, but Irs-2 has the major role in beta-cell development and compensation for peripheral insulin resistance. To establish a role for the Igf-1 receptor in beta-cells, we intercrossed mice heterozygous for null alleles of Igf1r and Irs2. Our results reveal that Igf-1 receptors promote beta-cell development and survival through the Irs-2 signalling pathway. Thus, Irs-2 integrates the effects of insulin in peripheral target tissues with Igf-1 in pancreatic beta-cells to maintain glucose homeostasis.

Pancreas development is promoted by cyclopamine, a hedgehog signaling inhibitor

Exposure to cyclopamine, a steroid alkaloid that blocks Sonic hedgehog (Shh) signaling, promotes pancreatic expansion in embryonic chicks. Heterotopic development of pancreatic endocrine and exocrine structures occurs in regions adjacent to the pancreas including stomach and duodenum, and insulin-producing islets in the pancreas are enlarged. The homeodomain transcription factor PDX1, required for pancreas development, is expressed broadly in the posterior foregut but pancreas development normally initiates only in a restricted region of PDX1-expressing posterior foregut where endodermal Shh expression is repressed. The results suggests that cyclopamine expands the endodermal region where Shh signaling does not occur, resulting in pancreatic differentiation in a larger region of PDX1-expressing foregut endoderm. Cyclopamine reveals the capacity of a broad region of the posterior embryonic foregut to form pancreatic cells and provides a means for expanding embryonic pancreas development.