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INVITED REVIEW Targeting type 2 diabetes: lessons from a knockout model of insulin receptor substrate 2 Joana Moitinho Oliveira, Sandra A. Rebuffat, Rosa Gasa, and Ramon Gomis

Abstract: 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. Key words: insulin receptor substrate 2, beta-cell, type 2 diabetes, survival, apoptosis. Résumé : Le substrat du récepteur de l’insuline 2 (IRS2, insuline receptor substrate 2) est une protéine largement exprimée qui régule des processus biologiques importants, y compris le métabolisme du glucose, la synthèse protéique et la survie cellulaire. IRS2 fait partie de la voie de signalisation de l’insuline–IGF et agit comme intermédiaire de l’activation des cascades de la phosphatidylinositol 3-kinase (PI3K) – Akt et de la MAPK (Ras-mitogen activated protein kinase) dans les tissus cibles de l’insuline et le pancréas. La meilleure preuve de cela est que l’élimination systémique de Irs2 chez la souris (Irs2−/−) résume la pathogenèse du diabète de type 2 (DT2), et que le diabète apparait comme conséquence de la résistance a` l’insuline combinée a` une dysfonction des cellules bêta. En effet, les travaux réalisés avec les souris knock-out ont confirmé l’importance d’IRS2 dans le contrôle de l’homéostasie du glucose et notamment, dans la survie et la fonction des cellules bêta du pancréas. Ces études ont montré que IRS2 est requis de façon importante dans la compensation des cellules bêta dans des conditions où la demande en insuline est accrue. Fait important, l’expression d’IRS2 est réduite dans les îlots isolés de patients atteints de DT2, ce qui appuie la contribution probable de la déficience de la signalisation dépendante d’IRS2 dans la dysfonction des cellules bêta chez l’humain atteint de DT2. Pour toutes ces raisons, les souris Irs2−/− ont été et seront essentielles pour élucider les interrelations entre la fonction des cellules bêta et la résistance a` l’insuline, ainsi que pour définir des stratégies thérapeutiques afin de protéger les cellules bêta durant la progression du DT2. [Traduit par la Rédaction] Mots-clés : substrat du récepteur de l’insuline 2, cellules bêta, diabète de type 2, survie, apoptose.

Introduction Diabetes mellitus is one of the most common chronic diseases in nearly all countries, which is reaching epidemic proportions at an alarming rate, with harmful effects on life expectancy and health-care costs (Shaw et al. 2010). Diabetes arises when insulin secretion from pancreatic beta-cells fails to maintain blood glucose levels in the normal range, especially when exacerbated by peripheral insulin resistance. The underlying pathophysiology of diabetes is diverse, but pancreatic beta-cell failure is the common theme. The type 2 form of the disease (T2D) is the most common one, affecting nowadays 336 million people worldwide, a number

that is expected to increase to 439 million adults by 2030 with considerably disparity between populations and regions (Shaw et al. 2010). The loss of metabolic homeostasis in T2D is often associated with obesity and resistance to the action of insulin on glucose uptake and on carbohydrate and lipid metabolism in peripheral tissues. However, T2D only occurs when insulin-secretory reserves fail to compensate for defects in insulin action (Leahy 2005). Some obese patients who are insulin-resistant do not develop diabetes because hyperinsulinemia compensates for blunted insulin action. In contrast, in a subset of insulin-resistant individuals, T2D

Received 26 March 2014. Accepted 2 June 2014. J.M. Oliveira,* S.A. Rebuffat, and R. Gasa. Diabetes and Obesity Research Laboratory, Institut d'Investigations Biomediques August Pi i Sunyer, Centre Esther Koplowitz, C/Rosselló, 149-153 08036 Barcelona, Spain; Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas, Barcelona, Spain. R. Gomis. Diabetes and Obesity Research Laboratory, Institut d'Investigations Biomediques August Pi i Sunyer, Centre Esther Koplowitz, C/Rosselló, 149-153 08036 Barcelona, Spain; Centro de Investigación Biomédica en Red de Diabetes y Enfermedades Metabólicas Asociadas, Barcelona, Spain; University of Barcelona, Hospital Clínic, Barcelona, Spain. Corresponding authors: Rosa Gasa (e-mail: [email protected]) and Ramon Gomis (e-mail: [email protected]). *Present address: Laboratory of Experimental Medicine, ULB Center for Diabetes Research, Medical Faculty, Université Libre de Bruxelles (ULB), Brussels, Belgium. Can. J. Physiol. Pharmacol. 92: 613–620 (2014) dx.doi.org/10.1139/cjpp-2014-0114

Published at www.nrcresearchpress.com/cjpp on 3 June 2014.

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develops when the compensatory beta-cell response is inadequate, owing to combined deterioration of beta-cell function and (or) decline of beta-cell mass (Leahy 2005). In fact, autopsy studies of patients with impaired fasting glucose and T2D have revealed a decrease in beta-cell mass of 50%–65% compared with body mass index-matched controls, suggesting that the deficit in beta-cell mass is an early process in the development of T2D (Matveyenko and Butler 2008). Enhanced beta-cell apoptosis is the likely process causing overall beta-cell loss, as replication and new islet formation were found to be sustained in T2D patients (Butler et al. 2003; Thomas et al. 2009). Thus, understanding how to prevent the loss and (or) how to enhance the survival of beta cells may reflect the most rational approaches for the prevention and treatment of T2D (Leahy et al. 2010). As an approach to understanding the pathogenesis of diabetes, several genetic mouse models targeting a number of insulin signaling related genes have been developed, which have provided unique opportunities not only to study the role of individual genes in tissue homeostasis but also to dissect the interplay between insulin resistance and beta-cell failure in the natural history of T2D (Kulkarni 2005). In this review we will focus on one of these genes, the gene for the insulin receptor substrate 2 (IRS2), an adaptor protein that links insulin – insulin-like growth factor (IGF) cell surface receptors with intracellular signaling cascades. Global deficiency of Irs2 in mice leads to diabetes as a consequence of combined insulin resistance and beta-cell failure in mice (Withers et al. 1998). Furthermore, conditional deletion of the Irs2 gene in a tissue-specific manner has helped delineate the differential contribution of each organ to the pathophysiology of diabetes, providing a multifaceted disease model to test new therapeutic strategies for the treatment of this disease.

The IRS signaling pathway Several studies implicate the IRS signaling system in both the response of classical insulin target tissues and in beta-cell physiology. Indeed, IRS proteins regulate many biological processes, including glucose metabolism, protein synthesis, cell survival, growth, and transformation (Myers and White 1996; White 1998; Thirone et al. 2006). There are 6 IRS proteins described: at least 3 IRS proteins occur in humans and mice, including IRS1 and IRS2, which are widely expressed, and IRS4, which is limited to the thymus, brain, kidney, and possibly beta-cells. Rodents also express IRS3, which is largely restricted to adipose tissue and displays activity similar to IRS1. IRS5 and IRS6 seem to have limited tissue expression and function in signaling. All IRS proteins lack intrinsic catalytic activity but are characterized by the presence of a highly conserved amino-terminal pleckstrin homology (PH) domain adjacent to a phosphotyrosinebinding domain (PTB), followed by a variable-length carboxylterminal tail (COOH) that contains numerous tyrosine and serine residues prone to phosphorylation. The PH and PTB domains mediate specific interactions with the insulin and IGF-I receptor kinases. The set of tyrosine phosphorylation residues on the COOH-terminal act as on–off switches to recruit and regulate various downstream signaling proteins, including the regulatory subunit of the lipid phosphatidylinositol 3-kinase (PI3K), Grb2, Nck, and SHP2 (Thirone et al. 2006). Many of these IRS signaling elements are expressed and play important roles in pancreatic beta-cell function and survival (Fig. 1). Although the IRS proteins are highly homologous and possess many similar tyrosine-phosphorylation motifs, studies in knockout mice and knockout cell lines indicate that the various IRS proteins serve complementary, rather than redundant roles (Burks and White 2001). It is clear that IRS1 and IRS2 are responsible for relaying insulin signals from the receptor to intracellular effectors, but because of their structural homology and similar tissue distribution, their distinctive contributions to glucose ho-

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meostasis are not always emphasized. However, IRS1 and IRS2 differ significantly from each other and there have been different molecular mechanisms pointed out by which each IRS protein may exert specific effects. For instance, IRS1 was shown to bind to several SH2 proteins with greater affinity than IRS2, including Grb2, the Crk adaptor protein, and phospholipase C␥39 (Sun et al. 1997; White 1998). IRS1 and IRS2 have also been shown to have a differential ability to activate various members of the atypical protein kinase C (aPKC) family (Sun et al. 1997; White 1998). The different IRS proteins also differ in their cellular compartmentalization and activation kinetics. For example, IRS1 is 2-fold more concentrated in the intracellular membrane compartment than in the cytosol, whereas IRS2 is 2-fold more concentrated in cytosol (Inoue et al. 1998). Further studies have shown that IRS2 is dephosphorylated more rapidly and activates PI3K more transiently than IRS1, thus indicating that differences in kinetics of activation may contribute to the diversity of cell signaling transduced by IRS1 and IRS2 (Ogihara et al. 1997; Inoue et al. 1998). In addition, IRS2 is structurally different from the other IRS proteins in that it possesses a unique region (between aminoacids 591–738) that interacts specifically with the kinase regulatory loop-binding (KRLB) domain of the Insulin receptor b subunit, which may partly contribute to the signaling specificity of IRS2 (Sawka-Verhelle et al. 1996). It is generally accepted that some IRS proteins play a more critical role for beta-cell survival and growth. Perhaps the best example comes from the global knockout mouse models for the Irs1 and Irs2 genes.

Global deletion of IRS1 and IRS2 in mice The analysis of metabolic and physiological parameters in tissues from global knockout mice for the Irs1 and Irs2 genes (hereinafter, Irs1−/− and Irs2−/−) has revealed the importance of the balance between beta-cell mass and insulin resistance in relation to the pathogenesis of T2D. Both Irs1−/− and Irs2−/− mice display peripheral resistance to the glucose-lowering effects of insulin. In Irs1−/− mice, decreased insulin sensitivity in muscle and adipose tissue contribute to the in vivo resistance to insulin, while the liver compensates IRS1 deficiency by increasing IRS2 levels. Both Irs1−/− and Irs2−/− mice exhibit lower peripheral glucose utilization during hyperinsulinemic–euglycemic clamps compared with wildtype (WT) mice, yet they differ in a number of other parameters. Thus, Irs2−/− mice synthesize more glycogen in the muscle, do not suppress hepatic glucose production, have reduced hepatic glycogen synthesis and storage, and show dysregulated lipid metabolism when compared with Irs1−/− mice (Previs et al. 2000). Importantly, Irs1−/− mice do not develop diabetes, and this has been ascribed to the fact that these mice present proper beta-cell mass expansion and increased insulin secretion as an adaptation to insulin resistance, as seen in nondiabetic obesity (Tamemoto et al. 1994; Kadowaki et al. 1996). In contrast, Irs2−/− mice show inadequate beta-cell mass compensation, become insulin insufficient, and develop a severe diabetic phenotype characterized by marked hyperglycemia (Withers et al. 1998; Takamoto et al. 2008). This evidence outlines the crucial role of IRS2 protein in the control of beta-cell mass in the prediabetic state. For instance, upregulation of IRS2 expression occurs in the islets of WT mice fed a high-fat (HF) diet, and HF diet-fed Irs2 (+/−) mice failed to show a sufficient increase in beta-cell mass, thus suggesting that IRS2 is critical for beta-cell hyperplasia in a situation mimicking a prediabetic state (HF diet-induced insulin resistance) (Terauchi et al. 2007). Moreover, under hyperinsulinemic conditions, there is an internal physiological mechanism that contributes to the feedback inhibition of the IRS2 signal transduction pathway, either by promoting IRS2 degradation or by reducing its expression (Rhodes et al. 2013). Thus, it is not surprising that the morphometric analysis of pancreas sections from Irs2−/− mice at different ages revealed a significant reduction in beta-cell mass as compared with Published by NRC Research Press

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Fig. 1. Activation of the insulin receptor substrate 2 (IRS2)-dependent signaling pathway is crucial for beta-cell homeostasis. The presence of a peptide ligand such as insulin or insulin-like growth factor-1 (IGF-1) activates the intrinsic tyrosine kinase activity of their receptors that then activates by tyrosine phosphorylation tyrosine phosphorylate (pY) adaptor molecules such as IRS2. Other receptors (tyrosine kinases, or receptors that activate tyrosine kinases) such as Janus kinase (not depicted in the figure) can also activate IRS2 signaling. This leads to activation of 2 major signaling cascades, the Ras–Raf–mitogen-activated protein kinase (MAPK) pathway and the phosphatidylinositol-3=kinase (PI3=K) – protein kinase-B (PKB; also known as Akt) signaling pathway. For the Ras–Raf–MAPK pathway, phospho-activated Erk-1/2 can then directly (or indirectly) phosphorylate certain transcription factors or upregulate gene transcription, some of them crucial for beta-cell survival or function, such as the Bad, Creb, MafA, and Pdx1 genes. For the PI3=K–Akt signaling pathway, Akt has a large number of substrates also involved in beta-cell survival, such as Bcl-antagonist of cell death (BAD), procaspase 9, and CREB (which controls Bcl2 expression). In addition, Akt phosphorylates the transcription factor FoxO1 causing its removal from the nucleus and promoting its degradation. In turn, this causes induction of the expression of the beta-cell transcription factor Pdx1 and inhibition of the expression of cyclin-dependent-kinase inhibitor p27 (p27KIP). Akt can also directly phosphorylate cell cycle proteins such as the cyclin-dependent-kinase inhibitor p21 (p21CIP). Phosphorylation of glycogen synthase kinase-3 (GSK3) by Akt inhibits GSK3 activity, resulting in increased cell growth. The activation of these mechanisms is highly dependent on the levels and (or) phosphorylation status of IRS2. As depicted in the figure, the control of IRS2 expression is affected by several stimuli. Thus, glucose within physiologically relevant concentrations regulates IRS2 expression, in a calciumdependent manner. Furthermore, the phosphorylation of CREB triggered by calcium-dependent signaling strongly stimulates the expression of the Irs2 gene. Glucagon-like peptide-1 (GLP-1) or the long acting GLP-1 receptor agonist exendin 4 elevate intracellular cAMP levels and activate the protein kinase A (PKA) pathway, which promotes IRS2 expression also through CREB phosphorylation. Conversely, Irs2 expression can be repressed by stress-inducible genes such as the activating transcription factor 3 (Atf3) and by feedback inhibition of normal IRS signaling. For example, serine phosphorylation of IRS2 by Erk-1/2 protein kinase or PKC␨ can promote IRS2 degradation (not depicted). Under these circumstances, there is a dramatic reduction of beta-cell survival, which in a situation of increased metabolic demand can culminate in beta-cell failure.

WT mice (Withers et al. 1998; Oliveira et al. 2014), which correlates with increased beta-cell apoptotic rates measured by cleaved caspase 3 staining and fluorescent DNA fragmentation assays. Therefore, the global knockout of the Irs2 gene serves as an excellent model of beta-cell failure in the context of high metabolic demand as in this model the progressive loss of beta-cells determines the progression to diabetes. Based on this evidence, it can be concluded that IRS2 but not IRS1 regulates beta-cell mass in adaptation to the metabolic homeostasis and is crucial for betacell survival. But how does IRS2 participate in beta-cell homeostasis? It was recently suggested that the tight control of IRS2 expression in beta-cells can act as a gatekeeper to control downstream beta-cell homeostasis, rather than the autocrine action of insulin or IGF1

(Rhodes et al. 2013). The expression of IRS2 in beta-cells is highly regulated and can change in accordance to variations in the metabolic situation. For instance, Irs2 expression is highly regulated by glucose, incretins such as GLP-1, and other factors that increase cytosolic (Ca2+)i and (cAMP)i in beta-cells increase IRS2 expression (Jhala et al. 2003; Amacker-Francoys et al. 2005; Lingohr et al. 2006; Demozay et al. 2011). Conversely, higher IRS2 levels in betacells distinctly enhance the rate of glucose- and IGF-I-induced beta-cell mitogenesis, implicating a significant role for IRS2 in expanding beta-cell mass (Schuppin et al. 1998). Moreover, the more pronounced effect of increasing IRS2 expression in betacells is to promote beta-cell survival, which can protect beta-cells from both streptozotocin- and free fatty acid (FFA)-induced apoptosis (Hennige et al. 2003). By contrast, IRS2 expression can be Published by NRC Research Press

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downregulated by proinflammatory cytokines and feedback inhibition of normal IRS signaling (Briaud et al. 2005; Li et al. 2008). Under these circumstances, there is marked spontaneous apoptosis and a dramatic reduction of beta-cell survival. It is known that the activation of the 2 major signaling pathways emerging downstream from IRS2, the PI3K–PDK-1–PKB and Grb2–mSOS–Ras–Raf– MEK-1–ERK pathways, well described elsewhere, are the key elements promoting beta-cell survival, with ERK1/2 having a negligible contribution (Marte and Downward 1997; Kulik and Weber 1998; White 1998; Lingohr et al. 2002; Dickson and Rhodes 2004).

Tissue-specific deletion of IRS2 Disruption of Irs2 in beta-cells The conditional knockout of Irs2 in pancreatic beta-cells was developed by using the Cre–Lox system, in which the Cre recombinase was under the control of the rat insulin II promoter (RIPCreIrs2KO) (Lin et al. 2004). As the global knockout, beta-cell specific Irs2−/− mice displayed reduced beta-cell number, confirming a cell-autonomous role of this protein in the regulation of beta-cell mass. However, the RIP promoter used to generate this mouse line is also expressed in regions of the hypothalamus, and these mice developed insulin resistance and obesity. These findings led to the suggestion that dysregulated IRS2 signaling could be a common link between obesity and beta-cell failure. Nonetheless, diabetes was prevented in this model by transgenic expression of Irs2 in beta-cells, which led to the sufficient restoration of beta-cell function to compensate for insulin resistance. These observations further underline the primary role played by the betacell compartment in the diabetic phenotype of Irs2−/− mice. Owing to the inability to clearly dissociate the beta-cell and hypothalamic phenotypes in the RIPCreIrs2KO, 2 additional mouse lines were created to ablate Irs2 in all neurons (NesCreIrs2KO using a nestin-Cre transgenic line) or in hypothalamic proopiomelanocortin (POMC) neurons (POMCCreIrs2KO using a Pomc-Cre line) (Choudhury et al. 2005). POMCCreIrs2KO did not display glucose intolerance, obesity, or altered beta cell mass as compared with age-matched controls. In contrast, NesCreIrs2KO mice, which also exhibited normal beta-cell IRS2 expression, were mildly glucose intolerant and became obese with age. Remarkably, they exhibited increased beta-cell mass in response to insulin resistance. Together, these findings demonstrate that the beta-cell mass phenotype observed in global Irs2−/− is solely dependent on the lack of Irs2 expression in beta-cells. Nonetheless, NesCreIrs2KO mice did develop elevated fasting blood glucose levels and hyperinsulinemia, which indicates that neuronal IRS2-dependent pathways are required for normal glucose homeostasis. Disruption of Irs2 in the pancreas The critical role for IRS2 signaling in beta-cell homeostasis was confirmed with another mouse model in which expression of the Irs2 gene was ablated in the whole pancreas by using a Cre recombinase driven by the promoter of the Pancreatic and Duodenal homeobox factor 1 (Pdx1; PdxCREIrs2KO) (Cantley et al. 2007). This mouse line displayed impaired glucose tolerance with reduction of beta-cell mass, impaired insulin secretion, alterations in islet gene expression, and beta-cell calcium mobilizations, but no reductions in insulin sensitivity or body mass alterations. Thus, this study demonstrated that IRS2-dependent signaling is required not only for beta-cell compensation under conditions of increased insulin but also for maintenance of a normal postnatal beta-cell mass and regulation of insulin secretion. However, PdxCREIrs2KO animals displayed normal fasting blood glucose levels and did not develop the progressive diabetes seen in global Irs2−/− mice, supporting the concept that both beta-cell failure and insulin resistance must be present for the development of T2D.

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Deletion of Irs2 in non-pancreatic tissues Although IRS2 was claimed to be the principal mediator of insulin action in the liver, liver-specific deletion of Irs2 (LivIrs2KO) does not impair hepatic glucose and lipid metabolism in mice. Indeed, LivIrs2KO had fasting and fed blood glucose levels comparable with those seen in the control animals, whilst tolerance to glucose and insulin sensitivity was normal (Simmgen et al. 2006). Moreover, even though insulin regulates hepatic metabolism partly by regulating the expression of key metabolic genes in glucose and lipid metabolism, the deletion of Irs2 in the liver had no significant effect on a number of insulin-regulated transcriptional events. Detailed analysis by performing hyperinsulinaemic–euglycaemic clamps demonstrated that whole-body glucose disposal was mildly reduced in LivIrs2KO mice, and that endogenous glucose production and glycolysis rates were not different between LivIrs2KO and control mice. These findings demonstrate that the absence of IRS2 in the liver mildly impairs whole-body insulin sensitivity, but importantly, that alone does not impact long-term glucose homeostasis. In addition to the pancreas and the liver, the Irs2 gene has been conditionally deleted in many other tissues (recently reviewed in (Guo 2014). In some cases, these conditional models have delivered unexpected insights into the contribution of specific cell types to dysregulated glucose metabolism. For instance, deletion of Irs2 in endothelial cells reduces insulin-induced glucose uptake by the skeletal muscle resulting in glucose intolerance and insulin resistance (Kubota et al. 2011). The same was only observed when an endothelial-cell-specific Irs1/Irs2 double-knockout (ETIrs1/ 2DKO) mouse was generated, suggesting that IRS2 plays a more important role in endothelial insulin signaling, at least in skeletal muscle.

Prevention of T2D in Irs2ⴚ/ⴚ mice There is an increasing body of evidence indicating that targeting beta-cell mass failure is crucial to improve whole-body glucose tolerance in the Irs2−/− background. Thus, several studies have demonstrated that the progression to diabetes of Irs2−/− mice can be prevented by genetically modifying elements of the insulin– IGF-signaling cascade to promote compensatory beta-cell functional changes mimicking the effects of IRS2 signaling in beta-cells (summarized in Table 1A). For instance, haploinsufficiency of the transcription factor Foxo1 avoided beta-cell failure in Irs2−/− mice through partial restoration of beta-cell proliferation (Kitamura et al. 2002); deletion of p27Kip1 (regulates cell cycle progression by inhibiting the activity of cyclin-dependent kinases) increased islet mass owing to increased proliferation, which prevented the development of overt hyperglycemia in Irs2−/− mice (Uchida et al. 2005); or upregulation of Pdx1 restored beta-cell mass and function in Irs2−/− mice by inducing the expression of genes that mediate glucose-sensitive insulin secretion and survival (Kushner et al. 2002). In this same line, transgenic expression of Irs2 in beta-cells prevented diabetes in Irs2−/− mice and in streptozotocininduced diabetic mice by promoting beta-cell growth and survival and sustained compensatory insulin secretion (Hennige et al. 2003). The progression of Irs2−/− mice towards diabetes can be retarded but not prevented by decreasing the overload of glucose on betacells through genetic maneuvers or treatments with compounds that target peripheral insulin resistance (Table 1(A and B)). For instance, double-knockout mice for Irs2 and Ptp1b (Protein tyrosine phosphatase-1b) presented increased peripheral insulin sensitivity, but owing to insufficient beta-cell expansion, they remained glucose intolerant and developed diabetes (Kushner et al. 2004). Likewise, treatment with resveratrol improved systemic insulin sensitivity by restoring hepatic insulin signaling, but failed to restore glucose tolerance, consistent with a failure to improve islet morphology or beta-cell mass (Gonzalez-Rodriguez et al. 2010). In another study, Pten (phosphatase and tensin homolog, an Published by NRC Research Press

Improved Improved Improved Exendine 4 Vildagliptin Sodium tungstate

Increased insulin secretion Decreased ␤-cell apoptosis Decreased ␤-cell apoptosis; slightly increased proliferation Slightly improved Persisted Persisted

Restoration of hepatic insulin sensitivity

Delayed No Delayed/prevented

Gonzalez-Rodriguez et al. 2010 Park et al. 2006 Sato et al. 2012 Oliveira et al. 2014 Unchanged (data not shown) Unchanged Augmented 2-fold Augmented 2-fold Improved Impaired Resveratrol

Gsk-3b−/− Irs2−/− Irs2−/− Pten−/−

(B) Pharmalogical treatments

Not assessed Slightly improved

Irs2−/− Cdkn1b−/−

No

Prevented Delayed/prevented Increased ␤-cell proliferation; decreased apoptosis Increased ␤-cell proliferation Persisted Normalized

Tanabe et al. 2008 Kushner et al. 2005

Delayed ? Increased ␤-cell proliferation

Unchanged (measured as ␤-cell area) Augmented 2.5-fold Unchanged

Uchida et al. 2005

Prevented Prevented Delayed Increased ␤-cell proliferation Increased ␤-cell proliferation; decreased apoptosis Not assessed Augmented 3-fold Augmented 4-fold Augmented 1.4-fold Not assessed Not assessed Partially improved Irs2−/− Foxo1+/− Irs2−/−:rip13Irs2 Irs2−/− Ptp1b−/−

Improved but not entirely corrected Not assessed Improved Improved but not entirely corrected Not assessed Irs2−/− Pdx1tg

(A) Genetic approaches

Persisted

Prevented Increased ␤-cell proliferation Augmented 2-fold Persisted

Reference Development of T2D Mechanism ␤-cell mass Insulin resistance Glucose tolerance

Table 1. List of (A) genetic and (B) pharmaceutical strategies used to prevent type 2 diabetes (T2D) in Irs2−/− mice.

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Kitamura et al. 2002 Hennige et al. 2003 Kushner et al. 2004

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Kushner et al. 2002

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inhibitor of Akt–Pkb signaling) haploinsufficiency prolonged the life span of Irs2−/− mice by enhancing peripheral insulin sensitivity (Kushner et al. 2005). Pharmacological approaches have also been undertaken to prevent the development of diabetes in Irs2−/− mice, as summarized in Table 1B. In detail, treatment of Irs2−/− mice with exendin 4, a long acting GLP-1 agonist, was shown to retard the progress towards diabetes by enhancing GSIS (Park et al. 2006). Yet, exendin 4 failed to promote beta-cell survival in Irs2−/− mice; an effect that is thought to be key for its long-term beneficial effects upon glucose tolerance and prevention of diabetes in other mouse models. The underlying reason is that IRS2 is essential for the long-term effects of exendin 4, probably through an increase in the expression of Pdx1. Indeed, under normal circumstances, increased activation of the transcription factor CREB (cAMP response element-binding protein) by exendin 4 would strongly stimulate Irs2 gene expression in beta-cells, ultimately activating the PI3K–Akt cascade with the expected positive consequences in beta-cell growth and survival. By contrast, in an Irs2−/− background, exendin 4 would not be able to upregulate IRS2-dependent signaling. By comparison, vildagliptin, an inhibitor of the dipeptidyl peptidase-4 enzyme that degrades GLP-1 (glucagon-like peptide-1) and GIP (gastric inhibitory polypeptide), was shown to suppress beta-cell apoptosis and preserve beta-cell mass in Irs2−/− mice by IRS2-independent pathways (Sato et al. 2012). However, the study by Sato et al. used an Irs2−/− mouse strain that displayed a mild diabetic phenotype (owing to a different genetic background) and therefore, the abnormalities in glucose homeostasis were only modest. More recently, treatment with sodium tungstate has been shown to attenuate beta-cell apoptosis by decreasing the expression of proapoptotic genes in Irs2−/− islets, thus leading to an increase in beta-cell mass and the subsequent restoration of glucose homeostasis and prevention of the progression to diabetes in Irs2−/− mice (Oliveira et al. 2014). Taking into account the fundamental role of IRS2 in beta-cell survival, these latter study supports the idea that beta-cell mass can be preserved and normoglycemia maintained independently of the presence of this protein. However, it should be noted that sodium tungstate was not effective in Irs2−/− mice that were already severely diabetic at initiation of treatment. Thus, one may argue that at advanced stages of the disease, betacell apoptosis is unstoppable and normal beta-cell mass no longer recoverable, suggestive of a point of no return in beta-cell failure. Therefore, whether it is feasible to protect beta-cells from apoptosis in a clinical situation where beta-cell IRS2 signaling is expected to be impaired remains to be properly addressed.

IRS2 in humans The human IRS2 gene is localized on chromosome 13q34 and the human IRS2 protein contains 22 potential tyrosine phosphorylation sites, with only 13 being conserved in IRS1. The amino acid sequence identity between IRS1 and IRS2 is around 43%, which may contribute to the signaling specificity of IRS2 in humans. In mice, IRS2 expression is detectable in both beta-cells and in the ductal epithelium, whereas in the human pancreas, in addition to the ductal and islet cell compartment, IRS2 is also found in some acinar cells. Interestingly, one report in particular showed that the mRNAs coding for IRS1 and IRS2, which are proposed to be the major insulin-stimulated tyrosine phosphoproteins in humans, were expressed in only 50% and 33% of the beta-cell population, respectively (Muller et al. 2006). The expression frequency of IRS2 was higher in beta-cells than in ␣- and ␦-cells, which confirms previous reports showing the importance of this gene in beta-cells. In human pancreatic cancers, IRS2 expression is increased and especially abundant in the ductal-like cancer cells. Interestingly, 3 SNPs in the IRS2 gene have been associated with breast cancer, 2 of them being also associated with adult weight gain (Feigelson et al. 2008). This link between IRS2 and human Published by NRC Research Press

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cancers argues that this molecule may regulate mitogenic signaling, and thus overexpression of IRS2 may lead to excessive growth stimulation in these tumors. The distinct roles of IRS proteins illustrated in the mouse studies are also observed in human studies. Concerning IRS1, a link between low IRS1 protein expression and insulin resistance has been shown. In skeletal muscle from severely obese nondiabetic patients, IRS1 expression and tyrosine phosphorylation are significantly lower compared with the controls (Goodyear et al. 1995). In diabetic patients who are less obese, IRS1 expression appears to be normal but insulin-stimulated tyrosine phosphorylation of IRS1 is diminished (Bjornholm et al. 1997; Krook et al. 2000). By contrast, in adipocytes, both the expression and insulin-stimulated tyrosine phosphorylation of IRS1 are reduced in insulin-resistant and diabetic patients (Rondinone et al. 1997; Carvalho et al. 1999). Furthermore, increased frequency of IRS1 polymorphisms was linked to insulin resistance and elevated risk of diabetes in several populations (Rung et al. 2009). To date, 2 studies have examined the expression of IRS2 in T2D patients. The first one failed to find any difference in IRS2 expression in skeletal muscle from control and diabetic individuals (Krook et al. 2000). Interestingly, a second study showed reduced IRS2 mRNA levels in islets isolated from T2D patients compared with the non-diabetic controls, which led to the idea that diminished IRS2-dependent signaling in beta-cells may underlie human T2D (Gunton et al. 2005). However, to our knowledge, no additional studies have been published confirming these initial findings. Structural defects in the human IRS2 gene have not been associated with a predisposition to T2D. No genetic variability has been identified in the promoter sequence of IRS2 in T2D patients, although several amino acid variants have been described and identified in the coding region of the IRS2 gene among Caucasian and Chinese subjects (Wang et al. 2001; D'Alfonso et al. 2003). The allelic frequencies of these polymorphisms were reported not to differ between control and T2D diabetic patients. Nevertheless, middle-aged glucose-tolerant Danish subjects carrying one of these identified variants, Asp1057, in a homozygous manner were characterized by a 25% decrease in fasting insulin levels and a 17% decrease in fasting C-peptide levels compared with wild-type carriers, which remained decreased during an oral glucose tolerance test (Almind et al. 1999). Despite the reductions in serum insulin and C-peptide concentrations, plasma glucose levels were not altered in homozygous carriers of the Asp1057 IRS2 variant. Curiously, in an Italian population, the prevalence of the homozygous Asp1057 IRS2 genotype in patients with T2D compared with control individuals was observed to be dependent on the BMI: in subjects with BMI > 27 kg/m2, a higher prevalence of the homozygous Asp1057 IRS2 genotype was observed in patients with type 2 diabetes vs. control individuals (Mammarella et al. 2000). Thus, overweight seems to modify the effect of this polymorphism toward a higher risk of developing T2D. Another polymorphism in the IRS2 gene that causes a Leu to Val change at codon 647 has been identified in 3 of 413 Danish patients with T2D and in none of 280 glucose-tolerant subjects (Almind et al. 1999), and in 1 of 85 Finnish patients with T2D and in none of 82 control subjects (Wang et al. 2001). This variant is located in the KRLB domain involved in the interaction with the insulin receptor b subunit and is close to the Tyr653 residue (which belongs to the binding site for the p85 regulatory subunit of PI 3-kinase) (Sawka-Verhelle et al. 1996). However, expression of the Val647 IRS2 variant in the yeast 2-hybrid system did not affect interaction of the IRS2 KRLB domain with either the insulin receptor or p85 subunit of PI 3-kinase. Therefore, the available data indicate that polymorphisms in the IRS2 gene do not have consistent functional effects in the predisposition to T2D in humans.

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Concluding remarks Human T2D is a complex disease, where a large number of predisposing or protective genes interact with multiple environmental factors at different moments during the natural history of the disease. In-vitro studies or rodent models of insulin resistance and (or) beta-cell dysfunction provide most of the current knowledge on the mechanisms involved in the development of T2D. While these models are highly valuable for understanding the individual contributions of different tissues to the pathophysiology of diabetes, they fail to offer an integrative view of how the distinct alterations interact and contribute to the development of this disease. In this review, we have tried to highlight the importance of using physiological models that can recapitulate the pathogenesis of T2D, like the Irs2 knockout mice, where the combination of peripheral insulin resistance and dysregulated hepatic gluconeogenesis contribute to the development of diabetes, but it is pancreatic beta-cell failure that determines its onset. The IRS2 signaling pathway plays a pivotal role in coordinating beta-cell survival and in regulating the adaptation of beta-cell mass to changes in the metabolic homeostasis. Thus, experimental models as the Irs2 knockout mice model are and will be essential for revealing the inter-relationship between beta-cell and insulin resistance in the context of T2D, and to delineate therapeutic strategies to protect beta-cells during T2D progression. However, the confirmation of these findings using more advanced rodent models, human islets and beta-cells, together with genomic tools, are imperative for increasing the available information about this disease and allowing the future development of successful therapies for T2D.

Acknowledgements This work was sponsored by the Spanish Ministry of Economy and Competitiveness (SAF2010-19527 and PI13/01500) and by the Government of Catalonia (2009 SGR 1426). J.O. was recipient of a doctoral research fellowship (SFRH/BD/45845/2008) supported by the Portuguese Foundation for Science and Technology (FCT) and by the Operational Programme for Human Potential (POPH/ESF). This work was developed at the Center Esther Koplowitz in Barcelona and supported by the Sarda` Farriol Research Programme. Ciberdem is an initiative of the Instituto de Salud Carlos III.

References Almind, K., Frederiksen, S.K., Bernal, D., Hansen, T., Ambye, L., Urhammer, S., et al. 1999. Search for variants of the gene-promoter and the potential phosphotyrosine encoding sequence of the insulin receptor substrate-2 gene: evaluation of their relation with alterations in insulin secretion and insulin sensitivity. Diabetologia, 42(10): 1244–1249. doi:10.1007/s001250051299. PMID: 10525667. Amacker-Francoys, I., Mohanty, S., Niessen, M., Spinas, G.A., and Trub, T. 2005. The metabolisable hexoses D-glucose and D-mannose enhance the expression of IRS-2 but not of IRS-1 in pancreatic beta-cells. Exp. Clin. Endocrinol. Diabetes, 113(8): 423–429. doi:10.1055/s-2005-865803. PMID:16151975. Bjornholm, M., Kawano, Y., Lehtihet, M., and Zierath, J.R. 1997. Insulin receptor substrate-1 phosphorylation and phosphatidylinositol 3-kinase activity in skeletal muscle from NIDDM subjects after in vivo insulin stimulation. Diabetes, 46(3): 524–527. doi:10.2337/diab.46.3.524. PMID:9032113. Briaud, I., Dickson, L.M., Lingohr, M.K., McCuaig, J.F., Lawrence, J.C., and Rhodes, C.J. 2005. Insulin receptor substrate-2 proteasomal degradation mediated by a mammalian target of rapamycin (mTOR)-induced negative feedback down-regulates protein kinase B-mediated signaling pathway in beta-cells. J. Biol. Chem. 280(3): 2282–2293. doi:10.1074/jbc.M412179200. PMID:15537654. Burks, D.J., and White, M.F. 2001. IRS proteins and beta-cell function. Diabetes, 50(Suppl. 1): S140–S145. doi:10.2337/diabetes.50.2007.S140. PMID:11272176. Butler, A.E., Janson, J., Bonner-Weir, S., Ritzel, R., Rizza, R.A., and Butler, P.C. 2003. Beta-cell deficit and increased beta-cell apoptosis in humans with type 2 diabetes. Diabetes, 52(1): 102–110. doi:10.2337/diabetes.52.1.102. PMID:12502499. Cantley, J., Choudhury, A.I., Asare-Anane, H., Selman, C., Lingard, S., Heffron, H., et al. 2007. Pancreatic deletion of insulin receptor substrate 2 reduces beta and alpha cell mass and impairs glucose homeostasis in mice. Diabetologia, 50(6): 1248–1256. doi:10.1007/s00125-007-0637-9. PMID:17393136. Carvalho, E., Jansson, P.A., Axelsen, M., Eriksson, J.W., Huang, X., Groop, L., et al. Published by NRC Research Press

Can. J. Physiol. Pharmacol. Downloaded from www.nrcresearchpress.com by MICHIGAN STATE UNIVERSITY on 02/28/15 For personal use only.

Oliveira et al.

1999. Low cellular IRS 1 gene and protein expression predict insulin resistance and NIDDM. FASEB J. 13(15): 2173–2178. PMID:10593864. Choudhury, A.I., Heffron, H., Smith, M.A., Al-Qassab, H., Xu, A.W., Selman, C., et al. 2005. The role of insulin receptor substrate 2 in hypothalamic and ␤ cell function. J. Clin. Invest. 115(4): 940–950. doi:10.1172/JCI200524445. PMID: 15841180. D'Alfonso, R., Marini, M.A., Frittitta, L., Sorge, R., Frontoni, S., Porzio, O., et al. 2003. Polymorphisms of the insulin receptor substrate-2 in patients with type 2 diabetes. J. Clin. Endocrinol. Metab. 88(1): 317–322. doi:10.1210/jc.2002020807. PMID:12519871. Demozay, D., Tsunekawa, S., Briaud, I., Shah, R., and Rhodes, C.J. 2011. Specific glucose-induced control of insulin receptor substrate-2 expression is mediated via Ca2+-dependent calcineurin/NFAT signaling in primary pancreatic islet beta-cells. Diabetes, 60(11): 2892–2902. doi:10.2337/db11-0341. PMID: 21940781. Dickson, L.M., and Rhodes, C.J. 2004. Pancreatic beta-cell growth and survival in the onset of type 2 diabetes: a role for protein kinase B in the Akt? Am. J. Physiol. Endocrinol. Metab. 287(2): E192–E198. doi:10.1152/ajpendo.00031. 2004. PMID:15271644. Feigelson, H.S., Teras, L.R., Diver, W.R., Tang, W., Patel, A.V., Stevens, V.L., et al. 2008. Genetic variation in candidate obesity genes ADRB2, ADRB3, GHRL, HSD11B1, IRS1, IRS2, and SHC1 and risk for breast cancer in the Cancer Prevention Study II. Breast Cancer Res. 10(4): R57. doi:10.1186/bcr2114. PMID: 18611262. Gonzalez-Rodriguez, A., Mas Gutierrez, J.A., Sanz-Gonzalez, S., Ros, M., Burks, D.J., and Valverde, A.M. 2010. Inhibition of PTP1B restores IRS1mediated hepatic insulin signaling in IRS2-deficient mice. Diabetes, 59(3): 588–599. doi:10.2337/db09-0796. PMID:20028942. Goodyear, L.J., Giorgino, F., Sherman, L.A., Carey, J., Smith, R.J., and Dohm, G.L. 1995. Insulin receptor phosphorylation, insulin receptor substrate-1 phosphorylation, and phosphatidylinositol 3-kinase activity are decreased in intact skeletal muscle strips from obese subjects. J. Clin. Invest. 95(5): 2195– 2204. doi:10.1172/JCI117909. PMID:7537758. Gunton, J.E., Kulkarni, R.N., Yim, S., Okada, T., Hawthorne, W.J., Tseng, Y.H., et al. 2005. Loss of ARNT/HIF1beta mediates altered gene expression and pancreatic-islet dysfunction in human type 2 diabetes. Cell, 122(3): 337–349. doi:10.1016/j.cell.2005.05.027. PMID:16096055. Guo, S. 2014. Insulin signaling, resistance, and the metabolic syndrome: insights from mouse models into disease mechanisms. J. Endocrinol. 220(2): T1–T23. doi:10.1530/JOE-13-0327. PMID:24281010. Hennige, A.M., Burks, D.J., Ozcan, U., Kulkarni, R.N., Ye, J., Park, S., et al. 2003. Upregulation of insulin receptor substrate-2 in pancreatic ␤ cells prevents diabetes. J. Clin. Invest. 112(10): 1521–1532. doi:10.1172/JCI200318581. PMID: 14617753. Inoue, G., Cheatham, B., Emkey, R., and Kahn, C.R. 1998. Dynamics of insulin signaling in 3T3-L1 adipocytes. Differential compartmentalization and trafficking of insulin receptor substrate (IRS)-1 and IRS-2. J. Biol. Chem. 273(19): 11548–11555. doi:10.1074/jbc.273.19.11548. PMID:9565570. Jhala, U.S., Canettieri, G., Screaton, R.A., Kulkarni, R.N., Krajewski, S., Reed, J., et al. 2003. cAMP promotes pancreatic beta-cell survival via CREB-mediated induction of IRS2. Genes Dev. 17(13): 1575–1580. doi:10.1101/gad.1097103. PMID:12842910. Kadowaki, T., Tamemoto, H., Tobe, K., Terauchi, Y., Ueki, K., Kaburagi, Y., et al. 1996. Insulin resistance and growth retardation in mice lacking insulin receptor substrate-1 and identification of insulin receptor substrate-2. Diabet. Med. 13(9 Suppl. 6): S103–S108. PMID:8894493. Kitamura, T., Nakae, J., Kitamura, Y., Kido, Y., Biggs, W.H., Wright, C.V.E., et al. 2002. The forkhead transcription factor Foxo1 links insulin signaling to Pdx1 regulation of pancreatic ␤ cell growth. J. Clin. Invest. 110(12): 1839–1847. doi:10.1172/JCI200216857. PMID:12488434. Krook, A., Bjornholm, M., Galuska, D., Jiang, X.J., Fahlman, R., Myers, M.G., Jr., et al. 2000. Characterization of signal transduction and glucose transport in skeletal muscle from type 2 diabetic patients. Diabetes, 49(2): 284–292. doi: 10.2337/diabetes.49.2.284. PMID:10868945. Kubota, T., Kubota, N., Kumagai, H., Yamaguchi, S., Kozono, H., Takahashi, T., et al. 2011. Impaired insulin signaling in endothelial cells reduces insulininduced glucose uptake by skeletal muscle. Cell Metab. 13(3): 294–307. doi: 10.1016/j.cmet.2011.01.018. PMID:21356519. Kulik, G., and Weber, M.J. 1998. Akt-dependent and -independent survival signaling pathways utilized by insulin-like growth factor I. Mol. Cell. Biol. 18(11): 6711–6718. PMID:9774684. Kulkarni, R. 2005. New insights into the roles of insulin/IGF-I in the development and maintenance of ␤-cell mass. Rev. Endocr. Metab. Disord. 6(3): 199–210. doi:10.1007/s11154-005-3051-y. PMID:16151624. Kushner, J.A., Ye, J., Schubert, M., Burks, D.J., Dow, M.A., Flint, C.L., et al. 2002. Pdx1 restores beta cell function in Irs2 knockout mice. J. Clin. Invest. 109(9): 1193–1201. doi:10.1172/JCI14439. PMID:11994408. Kushner, J.A., Haj, F.G., Klaman, L.D., Dow, M.A., Kahn, B.B., Neel, B.G., and White, M.F. 2004. Islet-sparing effects of protein tyrosine phosphatase-1b deficiency delays onset of diabetes in IRS2 knockout mice. Diabetes, 53(1): 61–66. doi:10.2337/diabetes.53.1.61. PMID:14693698. Kushner, J.A., Simpson, L., Wartschow, L.M., Guo, S., Rankin, M.M., Parsons, R., and White, M.F. 2005. Phosphatase and tensin homolog regulation of islet

619

growth and glucose homeostasis. J. Biol. Chem. 280(47): 39388–39393. doi: 10.1074/jbc.M504155200. PMID:16170201. Leahy, J.L. 2005. Pathogenesis of type 2 diabetes mellitus. Arch. Med. Res. 36(3): 197–209. doi:10.1016/j.arcmed.2005.01.003. PMID:15925010. Leahy, J.L., Hirsch, I.B., Peterson, K.A., and Schneider, D. 2010. Targeting beta-cell function early in the course of therapy for type 2 diabetes mellitus. J. Clin. Endocrinol. Metab. 95(9): 4206–4216. doi:10.1210/jc.2010-0668. PMID:20739389. Li, D., Yin, X., Zmuda, E.J., Wolford, C.C., Dong, X., White, M.F., and Hai, T. 2008. The repression of IRS2 gene by ATF3, a stress-inducible gene, contributes to pancreatic ␤-cell apoptosis. Diabetes, 57(3): 635–644. doi:10.2337/db07-0717. PMID:18057093. Lin, X., Taguchi, A., Park, S., Kushner, J.A., Li, F., Li, Y., and White, M.F. 2004. Dysregulation of insulin receptor substrate 2 in beta cells and brain causes obesity and diabetes. J. Clin. Invest. 114(7): 908–916. doi:10.1172/JCI200422217. PMID:15467829. Lingohr, M.K., Buettner, R., and Rhodes, C.J. 2002. Pancreatic beta-cell growth and survival — a role in obesity-linked type 2 diabetes? Trends Mol. Med. 8(8): 375–384. doi:10.1016/S1471-4914(02)02377-8. PMID:12127723. Lingohr, M.K., Briaud, I., Dickson, L.M., McCuaig, J.F., Alarcon, C., Wicksteed, B.L., and Rhodes, C.J. 2006. Specific regulation of IRS-2 expression by glucose in rat primary pancreatic islet beta-cells. J. Biol. Chem. 281(23): 15884–15892. doi:10.1074/jbc.M600356200. PMID:16574657. Mammarella, S., Romano, F., Di Valerio, A., Creati, B., Esposito, D.L., Palmirotta, R., et al. 2000. Interaction between the G1057D variant of IRS-2 and overweight in the pathogenesis of type 2 diabetes. Hum. Mol. Genet. 9(17): 2517–2521. doi:10.1093/hmg/9.17.2517. PMID:11030756. Marte, B.M., and Downward, J. 1997. PKB/Akt: connecting phosphoinositide 3-kinase to cell survival and beyond. Trends Biochem. Sci. 22(9): 355–358. doi:10.1016/S0968-0004(97)01097-9. PMID:9301337. Matveyenko, A.V., and Butler, P.C. 2008. Relationship between beta-cell mass and diabetes onset. Diabetes Obes. Metab. 10(Suppl. 4): 23–31. doi:10.1111/j. 1463-1326.2008.00939.x. PMID:18834430. Muller, D., Huang, G.C., Amiel, S., Jones, P.M., and Persaud, S.J. 2006. Identification of insulin signaling elements in human beta-cells: autocrine regulation of insulin gene expression. Diabetes, 55(10): 2835–2842. doi:10.2337/db060532. PMID:17003350. Myers, M.G., and White, M.F. 1996. Insulin signal transduction and the IRS proteins. Annu. Rev. Pharmacol. Toxicol. 36(1): 615–658. doi:10.1146/annurev. pa.36.040196.003151. PMID:8725404. Ogihara, T., Shin, B.C., Anai, M., Katagiri, H., Inukai, K., Funaki, M., et al. 1997. Insulin receptor substrate (IRS)-2 is dephosphorylated more rapidly than IRS-1 via its association with phosphatidylinositol 3-kinase in skeletal muscle cells. J. Biol. Chem. 272(19): 12868–12873. doi:10.1074/jbc.272.19.12868. PMID: 9139749. Oliveira, J.M., Rebuffat, S.A., Gasa, R., Burks, D.J., Garcia, A., Kalko, S.G., et al. 2014. Tungstate promotes beta-cell survival in Irs2−/− mice. Am. J. Physiol. Endocrinol. Metab. 306(1): E36–E47. doi:10.1152/ajpendo.00409.2013. PMID: 24253047. Park, S., Dong, X., Fisher, T.L., Dunn, S., Omer, A.K., Weir, G., and White, M.F. 2006. Exendin-4 uses Irs2 signaling to mediate pancreatic beta cell growth and function. J. Biol. Chem. 281(2): 1159–1168. doi:10.1074/jbc.M508307200. PMID:16272563. Previs, S.F., Withers, D.J., Ren, J.M., White, M.F., and Shulman, G.I. 2000. Contrasting effects of IRS-1 versus IRS-2 gene disruption on carbohydrate and lipid metabolism in vivo. J. Biol. Chem. 275(50): 38990–38994. doi:10.1074/jbc. M006490200. PMID:10995761. Rhodes, C.J., White, M.F., Leahy, J.L., and Kahn, S.E. 2013. Direct autocrine action of insulin on beta-cells: does it make physiological sense? Diabetes, 62(7): 2157–2163. doi:10.2337/db13-0246. PMID:23801714. Rondinone, C.M., Wang, L.M., Lonnroth, P., Wesslau, C., Pierce, J.H., and Smith, U. 1997. Insulin receptor substrate (IRS) 1 is reduced and IRS-2 is the main docking protein for phosphatidylinositol 3-kinase in adipocytes from subjects with non-insulin-dependent diabetes mellitus. Proc. Natl. Acad. Sci. U.S.A. 94(8): 4171–4175. doi:10.1073/pnas.94.8.4171. PMID:9108124. Rung, J., Cauchi, S., Albrechtsen, A., Shen, L., Rocheleau, G., Cavalcanti-Proenca, C., et al. 2009. Genetic variant near IRS1 is associated with type 2 diabetes, insulin resistance and hyperinsulinemia. Nat. Genet. 41(10): 1110–1115. doi:10.1038/ng.443. PMID:19734900. Sato, K., Nakamura, A., Shirakawa, J., Muraoka, T., Togashi, Y., Shinoda, K., et al. 2012. Impact of the dipeptidyl peptidase-4 inhibitor vildagliptin on glucose tolerance and beta-cell function and mass in insulin receptor substrate-2knockout mice fed a high-fat diet. Endocrinology, 153(3): 1093–1102. doi:10. 1210/en.2011-1712. PMID:22315446. Sawka-Verhelle, D., Tartare-Deckert, S., White, M.F., and Van Obberghen, E. 1996. Insulin receptor substrate-2 binds to the insulin receptor through its phosphotyrosine-binding domain and through a newly identified domain comprising amino acids 591-786. J. Biol. Chem. 271(11): 5980–5983. doi:10. 1074/jbc.271.11.5980. PMID:8626379. Schuppin, G.T., Pons, S., Hugl, S., Aiello, L.P., King, G.L., White, M., and Rhodes, C.J. 1998. A specific increased expression of insulin receptor substrate 2 in pancreatic beta-cell lines is involved in mediating serumstimulated beta-cell growth. Diabetes, 47(7): 1074–1085. doi:10.2337/diabetes. 47.7.1074. PMID:9648831. Published by NRC Research Press

Can. J. Physiol. Pharmacol. Downloaded from www.nrcresearchpress.com by MICHIGAN STATE UNIVERSITY on 02/28/15 For personal use only.

620

Shaw, J.E., Sicree, R.A., and Zimmet, P.Z. 2010. Global estimates of the prevalence of diabetes for 2010 and 2030. Diabetes Res. Clin. Pract. 87(1): 4–14. doi:10. 1016/j.diabres.2009.10.007. PMID:19896746. Simmgen, M., Knauf, C., Lopez, M., Choudhury, A.I., Charalambous, M., Cantley, J., et al. 2006. Liver-specific deletion of insulin receptor substrate 2 does not impair hepatic glucose and lipid metabolism in mice. Diabetologia, 49(3): 552–561. doi:10.1007/s00125-005-0084-4. PMID:16404553. Sun, X.J., Pons, S., Wang, L.M., Zhang, Y., Yenush, L., Burks, D., et al. 1997. The IRS-2 gene on murine chromosome 8 encodes a unique signaling adapter for insulin and cytokine action. Mol. Endocrinol. 11(2): 251–262. doi:10. 1210/mend.11.2.9885. PMID:9013772. Tanabe, K., Liu, Z., Patel, S., Doble, B.W., Li, L., Cras-Meneur, C., Martinez, S.C., Welling, C.M., White, M.F., Bernal-Mizrachi, E., Woodgett, J.R., and Permutt, M.A. 2008. Genetic deficiency of glycogen synthase kinase-3beta corrects diabetes in mouse models of insulin resistance. PLoS Biol. 6(2): e37. doi:10.1371/journal.pbio.0060037. Takamoto, I., Terauchi, Y., Kubota, N., Ohsugi, M., Ueki, K., and Kadowaki, T. 2008. Crucial role of insulin receptor substrate-2 in compensatory beta-cell hyperplasia in response to high fat diet-induced insulin resistance. Diabetes Obes. Metab. 10(Suppl. 4): 147–156. doi:10.1111/j.1463-1326.2008.00951.x. PMID: 18834442. Tamemoto, H., Kadowaki, T., Tobe, K., Yagi, T., Sakura, H., Hayakawa, T., et al. 1994. Insulin resistance and growth retardation in mice lacking insulin receptor substrate-1. Nature, 372(6502): 182–186. doi:10.1038/372182a0. PMID: 7969452.

Can. J. Physiol. Pharmacol. Vol. 92, 2014

Terauchi, Y., Takamoto, I., Kubota, N., Matsui, J., Suzuki, R., Komeda, K., et al. 2007. Glucokinase and IRS-2 are required for compensatory beta cell hyperplasia in response to high-fat diet-induced insulin resistance. J. Clin. Invest. 117(1): 246–257. doi:10.1172/JCI17645. PMID:17200721. Thirone, A.C., Huang, C., and Klip, A. 2006. Tissue-specific roles of IRS proteins in insulin signaling and glucose transport. Trends Endocrinol. Metab. 17(2): 72–78. doi:10.1016/j.tem.2006.01.005. PMID:16458527. Thomas, H.E., McKenzie, M.D., Angstetra, E., Campbell, P.D., and Kay, T.W. 2009. Beta cell apoptosis in diabetes. Apoptosis, 14(12): 1389–1404. doi:10.1007/ s10495-009-0339-5. PMID:19322660. Uchida, T., Nakamura, T., Hashimoto, N., Matsuda, T., Kotani, K., Sakaue, H., et al. 2005. Deletion of Cdkn1b ameliorates hyperglycemia by maintaining compensatory hyperinsulinemia in diabetic mice. Nat. Med. 11(2): 175–182. doi:10.1038/nm1187. PMID:15685168. Wang, H., Rissanen, J., Miettinen, R., Karkkainen, P., Kekalainen, P., Kuusisto, J., et al. 2001. New amino acid substitutions in the IRS-2 gene in Finnish and Chinese subjects with late-onset type 2 diabetes. Diabetes, 50(8): 1949–1951. doi:10.2337/diabetes.50.8.1949. PMID:11473060. White, M.F. 1998. The IRS-signalling system: a network of docking proteins that mediate insulin action. Mol. Cell. Biochem. 182(1–2): 3–11. doi:10.1023/ A:1006806722619. PMID:9609109. Withers, D.J., Gutierrez, J.S., Towery, H., Burks, D.J., Ren, J.M., Previs, S., et al. 1998. Disruption of IRS-2 causes type 2 diabetes in mice. Nature, 391(6670): 900–904. doi:10.1038/36116. PMID:9495343.

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