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

Neuronal overexpression of insulin receptor substrate 2 leads to increased fat mass, insulin resistance, and glucose intolerance during aging

The insulin receptor substrates (IRS) are adapter proteins mediating insulin's and IGF1's intracellular effects. Recent data suggest that IRS2 in the central nervous system (CNS) is involved in regulating fuel metabolism as well as memory formation. The present study aims to specifically define the role of chronically increased IRS2-mediated signal transduction in the CNS. We generated transgenic mice overexpressing IRS2 specifically in neurons (nIRS2 (tg)) and analyzed these in respect to energy metabolism, learning, and memory. Western blot (WB) analysis of nIRS2 (tg) brain lysates revealed increased IRS2 downstream signaling. Histopathological investigation of nIRS2 (tg) mice proved unaltered brain development and structure. Interestingly, nIRS2 (tg) mice showed decreased voluntary locomotoric activity during dark phase accompanied with decreased energy expenditure (EE) leading to increased fat mass. Accordingly, nIRS2 (tg) mice develop insulin resistance and glucose intolerance during aging. Exploratory behavior, motor function as well as food and water intake were unchanged in nIRS2 (tg) mice. Surprisingly, increased IRS2-mediated signals did not change spatial working memory in the T-maze task. Since FoxO1 is a key mediator of IRS2-transmitted signals, we additionally generated mice expressing a dominant negative mutant of FoxO1 (FoxO1DN) specifically in neurons. This mutant mimics the effect of increased IRS2 signaling on FoxO-mediated transcription. Interestingly, the phenotype observed in nIRS2 (tg) mice was not present in FoxO1DN mice. Therefore, increased neuronal IRS2 signaling causes decreased locomotoric activity in the presence of unaltered exploratory behavior and motor coordination that might lead to increased fat mass, insulin resistance, and glucose intolerance during aging independent of FoxO1-mediated transcription.

Glucokinase activation ameliorates ER stress-induced apoptosis in pancreatic β-cells

The derangement of endoplasmic reticulum (ER) homeostasis triggers β-cell apoptosis, leading to diabetes. Glucokinase upregulates insulin receptor substrate 2 (IRS-2) expression in β-cells, but the role of glucokinase and IRS-2 in ER stress has been unclear. In this study, we investigated the impact of glucokinase activation by glucokinase activator (GKA) on ER stress in β-cells. GKA administration improved β-cell apoptosis in Akita mice, a model of ER stress-mediated diabetes. GKA increased the expression of IRS-2 in β-cells, even under ER stress. Both glucokinase-deficient Akita mice and IRS-2-deficient Akita mice exhibited an increase in β-cell apoptosis, compared with Akita mice. β-cell-specific IRS-2-overexpressing (βIRS-2-Tg) Akita mice showed less β-cell apoptosis than Akita mice. IRS-2-deficient islets were vulnerable, but βIRS-2-Tg islets were resistant to ER stress-induced apoptosis. Meanwhile, GKA regulated the expressions of C/EBP homologous protein (CHOP) and other ER stress-related genes in an IRS-2-independent fashion in islets. GKA suppressed the expressions of CHOP and Bcl2-associated X protein (Bax) and protected against β-cell apoptosis under ER stress in an ERK1/2-dependent, IRS-2-independent manner. Taken together, GKA ameliorated ER stress-mediated apoptosis by harmonizing IRS-2 upregulation and the IRS-2-independent control of apoptosis in β-cells.

The role of FOXO1 in β-cell failure and type 2 diabetes mellitus

Over the past two decades, insulin resistance has been considered essential to the aetiology of type 2 diabetes mellitus (T2DM). However, insulin resistance does not lead to T2DM unless it is accompanied by pancreatic β-cell dysfunction, because healthy β cells can compensate for insulin resistance by increasing in number and functional output. Furthermore, β-cell mass is decreased in patients with diabetes mellitus, suggesting a primary role for β-cell dysfunction in the pathogenesis of T2DM. The dysfunction of β cells can develop through various mechanisms, including oxidative, endoplasmic reticulum or hypoxic stress, as well as via induction of cytokines; these processes lead to apoptosis, uncontrolled autophagy and failure to proliferate. Transdifferentiation between β cells and α cells occurs under certain pathological conditions, and emerging evidence suggests that β-cell dedifferentiation or transdifferentiation might account for the reduction in β-cell mass observed in patients with severe T2DM. FOXO1, a key transcription factor in insulin signalling, is implicated in these mechanisms. This Review discusses advances in our understanding of the contribution of FOXO1 signalling to the development of β-cell failure in T2DM.

Insulin receptor-overexpressing β-cells ameliorate hyperglycemia in diabetic rats through Wnt signaling activation

To investigate the therapeutic efficacy and mechanism of β-cells with insulin receptor (IR) overexpression on diabetes mellitus (DM), rat insulinoma (INS-1) cells were engineered to stably express human insulin receptor (INS-IR cells), and subsequently transplanted into streptozotocin- induced diabetic rats. Compared with INS-1 cells, INS-IR cells showed improved β-cell function, including the increase in glucose utilization, calcium mobilization, and insulin secretion, and exhibited a higher rate of cell proliferation, and maintained lower levels of blood glucose in diabetic rats. These results were attributed to the increase of β-catenin/PPARγ complex bindings to peroxisome proliferator response elements in rat glucokinase (GK) promoter and the prolongation of S-phase of cell cycle by cyclin D1. These events resulted from more rapid and higher phosphorylation levels of insulin-signaling intermediates, including insulin receptor substrate (IRS)-1/IRS-2/phosphotylinositol 3 kinase/v-akt murine thymoma viral oncogene homolog (AKT) 1, and the consequent enhancement of β-catenin nuclear translocation and Wnt responsive genes including GK and cyclin D1. Indeed, the higher functionality and proliferation shown in INS-IR cells were offset by β-catenin, cyclin D1, GK, AKT1, and IRS-2 gene depletion. In addition, the promotion of cell proliferation and insulin secretion by Wnt signaling activation was shown by 100 nM insulin treatment, and to a similar degree, was shown in INS-IR cells. In this regard, this study suggests that transferring INS-IR cells into diabetic animals is an effective and feasible DM treatment. Accordingly, the method might be a promising alternative strategy for treatment of DM given the adverse effects of insulin among patients, including the increased risk of modest weight gain and hypoglycemia. Additionally, this study demonstrates that the novel mechanism of cross-talk between insulin and Wnt signaling plays a primary role in the higher therapeutic efficacy of IR-overexpressing β-cells.

Direct autocrine action of insulin on β-cells: does it make physiological sense?

In recent years there has been a growing interest in the possibility of a direct autocrine effect of insulin on the pancreatic β-cell. Indeed, there have been numerous intriguing articles and several eloquent reviews written on the subject (1-3); however, the concept is still controversial. Although many in vitro experiments, a few transgenic mouse studies, and some human investigations would be supportive of the notion, there exist different insights, other studies, and circumstantial evidence that question the concept. Therefore, the idea of autocrine action of insulin remains a conundrum. Here we outline a series of thoughts, insights, and alternative interpretations of the available experimental evidence. We ask, how convincing are these, and what are the confusing issues? We agree that there is a clear contribution of certain downstream elements in the insulin signaling pathway for β-cell function and survival, but the question of whether insulin itself is actually the physiologically relevant ligand that triggers this signal transduction remains unsettled.

Longevity of insulin receptor substrate1 null mice is not associated with increased basal antioxidant protection or reduced oxidative damage

Insulin receptor substrate-1 null (Irs1 (-/-)) mice are long lived and importantly they also demonstrate increased resistance to several age-related pathologies compared to wild type (WT) controls. Currently, the molecular mechanisms that underlie lifespan extension in long-lived mice are unclear although protection against oxidative damage may be important. Here, we determined both the activities of several intracellular antioxidants and levels of oxidative damage in brain, skeletal muscle, and liver of Irs1 (-/-) and WT mice at 80, 450, and 700 days of age, predicting that long-lived Irs1 (-/-) mice would be protected against oxidative damage. We measured activities of both intracellular superoxide dismutases (SOD); cytosolic (CuZnSOD) and mitochondrial (MnSOD), glutathione peroxide (GPx), glutathione reductase (GR), catalase (CAT), and reduced glutathione (GHS). Of these, only hepatic CAT was significantly altered (increased) in Irs1 (-/-) mice. In addition, the levels of protein oxidation (protein carbonyl content) and lipid peroxidation (4-hydroxynonenal) were unaltered in Irs1 (-/-) mice, although the hepatic GSH/GSSG ratio, indicating an oxidized environment, was significantly lower in long-lived Irs1 (-/-) mice. Overall, our results do not support the premise that lifespan extension in Irs1 (-/-) mice is associated with greater tissue antioxidant protection or reduced oxidative damage.

Insulin receptor substrate 2 is required for testicular development

Insulin receptor substrate (IRS) proteins are key mediators of insulin and insulin-like growth factor (IGF) signalling. In mice, deletion of Irs1 is associated with profound growth retardation and increased longevity whereas Irs2-deficiency causes diabetes and female infertility. Clinical studies suggest that diabetes and obesity diminish male fertility. However, the role of IRS proteins in male reproduction is unknown. We observed that testis weight is reduced by 45% in Irs2-deficient mice as compared with control males. The weight of these organs in Irs1(-/-) males was similar to controls; however, since Irs1-deficient mice are 50% smaller, testis weight:body weight was increased in this model. Neonatal Irs2(-/-) mice also exhibited reduced testicular size, suggesting that impairments in this model occur during development. Histological examination of testicular cross sections from Irs2(-/-) mice revealed normal cellular associations without obvious abnormalities in the seminiferous epithelium. Reduced testicular weight was associated with fewer Sertoli cells, spermatogonia, spermatocytes, elongated spermatids, and epididymal spermatozoa. However, Leydig cell number and the concentration of serum testosterone were equivalent between Irs2-deficient and control males. Testicular weight was reduced similarly in non-diabetic and diabetic Irs2(-/-) mice, indicating that hyperglycemia does not compound the effects of Irs2 deletion on impaired testis development. Expression of Irs1, Irs3, and Irs4 was comparable between experimental groups. Collectively, our results demonstrate that IRS2 plays a critical role in testicular development, potentially by mediating IGF1 signalling during embryonic and early postnatal development.

Association between insulin receptor substrate 1 Gly972Arg polymorphism and cancer risk

Epidemiological studies investigating the association between the insulin receptor substrate 1 (IRS1) gene Gly972Arg (rs1801278) polymorphism and various carcinomas risk reported conflicting results. Thus, a systemic review and meta-analysis of published studies were performed to assess the possible association. A comprehensive search was conducted to identify all eligible studies of IRS1 Gly972Arg polymorphism and cancer risk. Odds ratios (ORs) and 95 % confidence intervals (CIs) were used to assess the strength of the associations. A total of 16 independent studies, including 11,776 cases and 11,654 controls, were identified. When all studies were pooled, we found a significant association between IRS1 Gly972Arg polymorphism and increased cancer risk under dominant model (OR = 1.16, 95 %CI = 1.04-1.30, P = 0.007) and allelic model (OR = 1.16, 95 %CI = 1.02-1.30, P = 0.02). In subgroup analysis based on cancer type, increased cancer risk was found in ovarian cancer (dominant: OR = 1.55, 95 %CI = 1.17-2.05, P = 0.002; allelic: OR = 1.55, 95 %CI = 1.19-2.01, P = 0.001), breast cancer (allelic: OR = 1.12, 95 %CI = 1.00-1.26, P = 0.05), and other cancers (allelic: OR = 1.31, 95 %CI = 1.00-1.71, P = 0.05). When stratified by study types, significant associations were observed in both cohort studies (dominant: OR = 1.25, 95 %CI = 1.06-1.47, P = 0.007; allelic: OR = 1.25, 95 %CI = 1.07-1.46, P = 0.005) and case-control studies (dominant: OR = 1.15, 95 %CI = 1.01-1.31, P = 0.04). In the subgroup analyses by ethnicity, significantly increased cancer risk was suggested among both Caucasians (dominant: OR = 1.13, 95 %CI = 1.02-1.26, P = 0.02; allelic: OR = 1.13, 95 %CI = 1.03-1.25, P = 0.01) and mixed population (dominant: OR = 1.22, 95 %CI = 1.01-1.46, P = 0.04). Our investigations demonstrate that IRS1 Gly972Arg polymorphism was associated with an increased risk of cancer, and additional well-designed studies are warranted to validate these findings.

Integrating metabolism and longevity through insulin and IGF1 signaling

The insulin pathway coordinates growth, development, metabolic homoeostasis, fertility, and stress resistance, which influence life span. Compensatory hyperinsulinemia to overcome systemic insulin resistance circumvents the immediate consequences of hyperglycemia. Work on flies, nematodes, and mice indicate that excess insulin signaling damages cellular function and accelerates aging. Maintenance of the central nervous system (CNS) has particular importance for life span. Reduced insulin/IGF1 signaling in the CNS can dysregulate peripheral energy homeostasis and metabolism, promote obesity, and extend life span. Genetic manipulations of insulin/IGF1 signaling components are revealing neuronal circuits that might resolve the central regulation of systemic metabolism from organism longevity.

Selective serotonin reuptake inhibitors (SSRIs) inhibit insulin secretion and action in pancreatic β cells

Selective serotonin reuptake inhibitors (SSRIs) are antidepressants used for the treatment of mood and anxiety disorders. Here, we demonstrate that incubation (2 h) of murine islets or Min6 β cell line with the SSRIs paroxetine, fluoxetine, or sertraline inhibited insulin-induced Tyr phosphorylation of insulin receptor substrate (IRS)-2 protein and the activation of its downstream targets Akt and the ribosomal protein S6 kinase-1 (S6K1). Inhibition was dose-dependent with half-maximal effects at ∼15-20 μM. It correlated with a rapid dephosphorylation and activation of the IRS kinase GSK3β. Introduction of GSK3β siRNAs eliminated the inhibitory effects of the SSRIs. Inhibition of IRS-2 action by 30 μM SSRI was associated with a marked inhibition of glucose-stimulated insulin secretion from murine and human pancreatic islets. Secretion induced by basic secretagogues (KCl and Arg) was not affected by these drugs. Prolonged treatment (16 h) of Min6 cells with sertraline resulted in the induction of inducible nitric oxide synthase; activation of endoplasmic reticulum stress, and the initiation of the unfolded protein response, manifested by enhanced transcription of ATF4 and C/EBP homologous protein. This triggered an apoptotic process, manifested by enhanced caspase 3/7 activity, which resulted in β cell death. These findings implicate SSRIs as inhibitors of IRS protein function and insulin action through the activation of GSK3β. They further suggest that SSRIs inhibit insulin secretion; induce the unfolded protein response; activate an apoptotic process, and trigger β cell death. Given that SSRIs promote insulin resistance while inhibiting insulin secretion, these drugs might accelerate the transition from an insulin-resistant state to overt diabetes.

Wild-type offspring of heterozygous prolactin receptor-null female mice have maladaptive β-cell responses during pregnancy

Abstract  β-Cell mass increases during pregnancy in adaptation to the insulin resistance of pregnancy. This increase is accompanied by an increase in β-cell proliferation, a process that requires intact prolactin receptor (Prlr) signalling. Previously, it was found that during pregnancy, heterozygous prolactin receptor-null (Prlr(+/-)) mice had lower number of β-cells, lower serum insulin and higher blood glucose levels than wild-type (Prlr(+/+)) mice. An unexpected observation was that the glucose homeostasis of the experimental mouse depends on the genotype of her mother, such that within the Prlr(+/+) group, the Prlr(+/+) offspring derived from Prlr(+/+) mothers (Prlr(+/+(+/+))) had higher β-cell mass and lower blood glucose than those derived from Prlr(+/-) mothers (Prlr(+/+(+/-))). Pathways that are known to regulate β-cell proliferation during pregnancy include insulin receptor substrate-2, Akt, menin, the serotonin synthetic enzyme tryptophan hydroxylase-1, Forkhead box M1 and Forkhead box D3. The aim of the present study was to determine whether dysregulation in these signalling molecules in the islets could explain the maternal effect on the phenotype of the offspring. It was found that the pregnancy-induced increases in insulin receptor substrate-2 and Akt expression in the islets were attenuated in the Prlr(+/+(+/-)) mice in comparison to the Prlr(+/+(+/+)) mice. The expression of Forkhead box D3, which plays a permissive role for β-cell proliferation during pregnancy, was also lower in the Prlr(+/+(+/-)) mice. In contrast, the pregnancy-induced increases in phospho-Jak2, tryptophan hydroxylase-1 and FoxM1, as well as the pregnancy-associated reduction in menin expression, were comparable between the two groups. There was also no difference in expression levels of genes that regulate insulin synthesis and secretion (i.e. glucose transporter 2, glucokinase and pancreatic and duodenal homeobox-1) between these two groups. Taken together, these results suggest that the in utero environment of the Prlr(+/-) mother confers long-term changes in the pancreatic islets of her offspring such that when the offspring themselves became pregnant, they cannot adapt to the increased insulin demands of their own pregnancy.

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.

Disruption of growth factor receptor-binding protein 10 in the pancreas enhances β-cell proliferation and protects mice from streptozotocin-induced β-cell apoptosis

Defects in insulin secretion and reduction in β-cell mass are associated with type 2 diabetes in humans, and understanding the basis for these dysfunctions may reveal strategies for diabetes therapy. In this study, we show that pancreas-specific knockout of growth factor receptor-binding protein 10 (Grb10), which is highly expressed in pancreas and islets, leads to elevated insulin/IGF-1 signaling in islets, enhanced β-cell mass and insulin content, and increased insulin secretion in mice. Pancreas-specific disruption of Grb10 expression also improved glucose tolerance in mice fed with a high-fat diet and protected mice from streptozotocin-induced β-cell apoptosis and body weight loss. Our study has identified Grb10 as an important regulator of β-cell proliferation and demonstrated that reducing the expression level of Grb10 could provide a novel means to increase β-cell mass and reduce β-cell apoptosis. This is critical for effective therapeutic treatment of both type 1 and 2 diabetes.

Specific silencing of the REST target genes in insulin-secreting cells uncovers their participation in beta cell survival

The absence of the transcriptional repressor RE-1 Silencing Transcription Factor (REST) in insulin-secreting beta cells is a major cue for the specific expression of a large number of genes. These REST target genes were largely ascribed to a function of neurotransmission in a neuronal context, whereas their role in pancreatic beta cells has been poorly explored. To identify their functional significance, we have generated transgenic mice expressing REST in beta cells (RIP-REST mice), and previously discovered that REST target genes are essential to insulin exocytosis. Herein we characterized a novel line of RIP-REST mice featuring diabetes. In diabetic RIP-REST mice, high levels of REST were associated with postnatal beta cell apoptosis, which resulted in gradual beta cell loss and sustained hyperglycemia in adults. Moreover, adenoviral REST transduction in INS-1E cells led to increased cell death under control conditions, and sensitized cells to death induced by cytokines. Screening for REST target genes identified several anti-apoptotic genes bearing the binding motif RE-1 that were downregulated upon REST expression in INS-1E cells, including Gjd2, Mapk8ip1, Irs2, Ptprn, and Cdk5r2. Decreased levels of Cdk5r2 in beta cells of RIP-REST mice further confirmed that it is controlled by REST, in vivo. Using siRNA-mediated knock-down in INS-1E cells, we showed that Cdk5r2 protects beta cells against cytokines and palmitate-induced apoptosis. Together, these data document that a set of REST target genes, including Cdk5r2, is important for beta cell survival.

Large-scale association analyses identify new loci influencing glycemic traits and provide insight into the underlying biological pathways

Through genome-wide association meta-analyses of up to 133,010 individuals of European ancestry without diabetes, including individuals newly genotyped using the Metabochip, we have increased the number of confirmed loci influencing glycemic traits to 53, of which 33 also increase type 2 diabetes risk (q < 0.05). Loci influencing fasting insulin concentration showed association with lipid levels and fat distribution, suggesting impact on insulin resistance. Gene-based analyses identified further biologically plausible loci, suggesting that additional loci beyond those reaching genome-wide significance are likely to represent real associations. This conclusion is supported by an excess of directionally consistent and nominally significant signals between discovery and follow-up studies. Functional analysis of these newly discovered loci will further improve our understanding of glycemic control.

Znt7-null mice are more susceptible to diet-induced glucose intolerance and insulin resistance

The Znt7 gene encodes a ubiquitously expressed zinc transporter that is involved in transporting cytoplasmic zinc into the Golgi apparatus and a ZnT7-containing vesicular compartment. Overexpression of ZnT7 in the pancreatic β-cell stimulates insulin synthesis and secretion through regulation of insulin gene transcription. In this study, we demonstrate that ZnT7 is expressed in the mouse skeletal muscle. The activity of the insulin signaling pathway was down-regulated in myocytes isolated from the femoral muscle of Znt7 knock-out (KO) mice. High fat diet consumption (45% kcal) induced weight gain in male Znt7 KO mice but not female Znt7 KO mice. Male Znt7 KO mice fed the high fat diet at 5 weeks of age for 10 weeks exhibited hyperglycemia in the non-fasting state. Oral glucose tolerance tests revealed that male Znt7 KO mice fed the high fat diet had severe glucose intolerance. Insulin tolerance tests showed that male Znt7 KO mice were insulin-resistant. Diet-induced insulin resistance in male Znt7 KO mice was paralleled by a reduction in mRNA expression of Insr, Irs2, and Akt1 in the primary skeletal myotubes isolated from the KO mice. Overexpression of ZnT7 in a rat skeletal muscle cell line (L6) increased Irs2 mRNA expression, Irs2 and Akt phosphorylation, and glucose uptake. We conclude that a combination of decreased insulin secretion and increased insulin resistance accounts for the glucose intolerance observed in Znt7 KO mice.

IRS2 signalling is required for the development of a subset of sensory spinal neurons

Insulin and insulin-like growth factor-I play important roles in the development and maintenance of neurons and glial cells of the nervous system. Both factors activate tyrosine kinase receptors, which signal through adapter proteins of the insulin receptor substrate (IRS) family. Although insulin and insulin-like growth factor-I receptors are expressed in dorsal root ganglia (DRG), the function of IRS-mediated signalling in these structures has not been studied. Here we address the role of IRS2-mediated signalling in murine DRG. Studies in cultured DRG neurons from different embryonic stages indicated that a subset of nerve growth factor-responsive neurons is also dependent on insulin for survival at very early time points. Consistent with this, increased apoptosis during gangliogenesis resulted in a partial loss of trkA-positive neurons in DRG of Irs2 mutant embryos. Analyses in adult Irs2(-/-) mice revealed that unmyelinated fibre afferents, which express calcitonin gene-related peptide/substance P and isolectin B4, as well as some myelinated afferents to the skin were affected by the mutation. The diminished innervation of glabrous skin in adult Irs2(-/-) mice correlated with longer paw withdrawal latencies in the hot-plate assay. Collectively, these findings indicate that IRS2 signalling is required for the proper development of spinal sensory neurons involved in the perception of pain.

mTORC1 signaling and regulation of pancreatic β-cell mass

The capacity of β cells to expand in response to insulin resistance is a critical factor in the development of type 2 diabetes. Proliferation of β cells is a major component for these adaptive responses in animal models. The extracellular signals responsible for β-cell expansion include growth factors, such as insulin, and nutrients, such as glucose and amino acids. AKT activation is one of the important components linking growth signals to the regulation of β-cell expansion. Downstream of AKT, tuberous sclerosis complex 1 and 2 (TSC1/2) and mechanistic target of rapamycin complex 1 (mTORC1) signaling have emerged as prime candidates in this process, because they integrate signals from growth factors and nutrients. Recent studies demonstrate the importance of mTORC1 signaling in β cells. This review will discuss recent advances in the understanding of how this pathway regulates β-cell mass and present data on the role of TSC1 in modulation of β-cell mass. Herein, we also demonstrate that deletion of Tsc1 in pancreatic β cells results in improved glucose tolerance, hyperinsulinemia and expansion of β-cell mass that persists with aging.

Insulin-like growth factor and fibroblast growth factor expression profiles in growth-restricted fetal sheep pancreas

Placental insufficiency results in intrauterine growth restriction (IUGR), impaired fetal insulin secretion and less fetal pancreatic β-cell mass, partly due to lower β-cell proliferation rates. Insulin-like growth factors (IGFs) and fibroblast growth factors (FGFs) regulate fetal β-cell proliferation and pancreas development, along with transcription factors, such as pancreatic and duodenal homeobox 1 (PDX-1). We determined expression levels for these growth factors, their receptors and IGF binding proteins in ovine fetal pancreas and isolated islets. In the IUGR pancreas, relative mRNA expression levels of IGF-I, PDX-1, FGF7 and FGFR2IIIb were 64% (P < 0.01), 76% (P < 0.05), 76% (P < 0.05) and 52% (P < 0.01) lower, respectively, compared with control fetuses. Conversely, insulin-like growth factor binding protein 2 (IGFBP-2) mRNA and protein concentrations were 2.25- and 1.2-fold greater (P < 0.05) in the IUGR pancreas compared with controls. In isolated islets from IUGR fetuses, IGF-II and IGFBP-2 mRNA concentrations were 1.5- and 3.7-fold greater (P < 0.05), and insulin mRNA was 56% less (P < 0.05) than control islets. The growth factor expression profiles for IGF and FGF signaling pathways indicate that declines in β-cell mass are due to decreased growth factor signals for both pancreatic progenitor epithelial cell and mature β-cell replication.

IRS2 signaling in LepR-b neurons suppresses FoxO1 to control energy balance independently of leptin action

Irs2-mediated insulin/IGF1 signaling in the CNS modulates energy balance and glucose homeostasis; however, the site for Irs2 function is unknown. The hormone leptin mediates energy balance by acting on leptin receptor (LepR-b)-expressing neurons. To determine whether LepR-b neurons mediate the metabolic actions of Irs2 in the brain, we utilized Lepr(cre) together with Irs2(L/L) to ablate Irs2 expression in LepR-b neurons (Lepr(ΔIrs2)). Lepr(ΔIrs2) mice developed obesity, glucose intolerance, and insulin resistance. Leptin action was not altered in young Lepr(ΔIrs2) mice, although insulin-stimulated FoxO1 nuclear exclusion was reduced in Lepr(ΔIrs2) mice. Indeed, deletion of Foxo1 from LepR-b neurons in Lepr(ΔIrs2) mice normalized energy balance, glucose homeostasis, and arcuate nucleus gene expression. Thus, Irs2 signaling in LepR-b neurons plays a crucial role in metabolic sensing and regulation. While not required for leptin action, Irs2 suppresses FoxO1 signaling in LepR-b neurons to promote energy balance and metabolism.

Enhanced beta cell proliferation in mice overexpressing a constitutively active form of Akt and one allele of p21Cip

AIMS/HYPOTHESIS

The ability of pancreatic beta cells to proliferate is critical both for normal tissue maintenance and in conditions where there is an increased demand for insulin. Protein kinase B(Akt) plays a major role in promoting proliferation in many cell types, including the insulin-producing beta cells. We have previously reported that mice overexpressing a constitutively active form of Akt(caAkt (Tg)) show enhanced beta cell proliferation that is associated with increased protein levels of cyclin D1, cyclin D2 and cyclin-dependent kinase inhibitor 1A (p21(Cip)). In the present study, we sought to assess the mechanisms responsible for augmented p21(Cip) levels in caAkt(Tg) mice and test the role of p21(Cip) in the proliferative responses induced by activation of Akt signalling.

METHODS

To gain a greater understanding of the relationship between Akt and p21(Cip), we evaluated the mechanisms involved in the modulation of p2(Cip) by Akt and the in vivo role of reduced p21(Cip) in proliferative responses induced by Akt.

RESULTS

Our experiments showed that Akt signalling regulates p21(Cip) transcription and protein stability. caAkt(Tg) /p21(Cip+/-) mice exhibited fasting and fed hypoglycaemia as well as hyperinsulinaemia when compared with caAkt(Tg) mice. Glucose tolerance tests revealed improved glucose tolerance in caAkt(Tg)/p21(Cip+/-) mice compared with caAkt (Tg). These changes resulted from increased proliferation, survival and beta cell mass in caAkt(Tg)/p21(Cip+/-) compared with caAkt(Tg) mice.

CONCLUSIONS/INTERPRETATION

Our data indicate that increased p21(Cip) levels in caAkt(Tg) mice act as a compensatory brake, protecting beta cells from unrestrained proliferation. These studies imply that p21(Cip) could play important roles in the adaptive responses of beta cells to proliferate in conditions such as in insulin resistance.

Genetic correlates of early accelerated infant growth associated with juvenile-onset type 1 diabetes

OBJECTIVE

We previously showed that accelerated growth predisposing to development of childhood-onset type 1 diabetes (T1D) is restricted to the first year after birth. We assessed whether this phenomenon of increased early growth is associated with variants of two genes, insulin-like growth factor-1 (IGF1) and insulin variable number of tandem repeats (INS-VNTR), whose products are components of the growth axis.

PATIENTS AND METHODS

Patients and their siblings were genotyped for the INS-VNTR and for an IGF1 microsatellite. We tested for difference in first year growth, i.e., increased weight standard deviation score (SDS), a reliable measure of especially first year growth, between carriers and non-carriers of these gene variants, using a repeated measurement and regression analysis.

RESULTS

In patients, growth did not differ between carriers and non-carriers of the INS-VNTR*III allele, while carriership of this allele in siblings was positively associated with increased first year growth. In both patients and siblings, non-carriership of the IGF1*194 allele was positively associated with growth. Birth size was not associated with either variant.

CONCLUSIONS/DISCUSSION

Non-carriership of the IGF1*194 allele was positively associated with accelerated first year growth in both patients and siblings, independent of disease. This IGF1 variant may therefore contribute to increased first year growth, but cannot explain the association of first year growth with diabetes. An effect on growth of the INS-VNTR was detected in healthy siblings, but not in patients, suggesting that disease supersedes a growth effect of INS-VNTR.

Increased β-cell replication and β-cell mass regeneration in syngeneically transplanted rat islets overexpressing insulin-like growth factor II

Insulin-like growth factor II (IGF2) is a growth-promoting peptide that increases β-cell proliferation and survival. The aim of the study was to determine the effect of IGF2 overexpression on β-cell mass in transplanted islets. Islets infected with adenovirus encoding for IGF2 (Ad-IGF2 group), for luciferase (Ad-Luc control group), or with uninfected islets (control group) were syngeneically transplanted to streptozotocin-diabetic Lewis rats. Eight hundred islets, a minimal mass model to restore normoglycemia, or 500 islets, a clearly insufficient mass, were transplanted. Rats transplanted with 800 Ad-IGF2 islets showed a better metabolic evolution than control groups. As expected, rats transplanted with 500 Ad-IGF2 or control islets maintained similar hyperglycemia throughout the study, ensuring comparable metabolic conditions among both groups. β-Cell replication was higher in Ad-IGF2 group than in control group on days 3 [1.45% (IQR: 0.26) vs. 0.58% (IQR: 0.18), p = 0.006], 10 [1.58% (IQR: 1.40) vs. 0.90% (IQR: 0.61), p = 0.035], and 28 [1.35% (IQR: 0.35) vs. 0.64% (IQR: 0.28), p = 0.004] after transplantation. β-Cell mass was similarly reduced on day 3 after transplantation in Ad-IGF2 and control group [0.36 mg (IQR: 0.26) vs. 0.38 mg (IQR: 0.19)], it increased on day 10, and on day 28 it was higher in Ad-IGF2 than in control group [0.63 mg (IQR: 0.38) vs. 0.42 mg (IQR: 0.31), p = 0.008]. Apoptosis was similarly increased in Ad-IGF2 and control islets after transplantation. No differences in insulin secretion were found between Ad-IGF2 and uninfected control islets. In summary, IGF2 overexpression in transplanted islets increased β-cell replication, induced the regeneration of the transplanted β-cell mass, and had a beneficial effect on the metabolic outcome reducing the β-cell mass needed to achieve normoglycemia.

Insulin-like growth factor axis gene polymorphisms modify risk of pancreatic cancer

OBJECTIVE

Insulin-like growth factor (IGF)-axis genes plays a critical role in cancer development and progression via their impact on the RAS/MAPK/ERK and PI3K/AKT/mTOR signaling pathways. We hypothesized that IGF-axis genetic variants modify individual susceptibility to pancreatic cancer.

METHODS

We retrospectively genotyped 41 single-nucleotide polymorphisms of 10 IGF-axis genes (IGF1, IGF2, IGF1R, IGF2R, IGFBP1, IGFBP3, IGFBP5, IRS1, IRS2, and IRS4) in 706 pancreatic cancer patients and 706 cancer-free controls using Sequenom and TaqMan technology. The association between genotype and pancreatic cancer risk was evaluated using multivariate logistic regression. A P value ≤.007 at a false discovery rate of 10% was set as the significance level.

RESULTS

We observed that the IGF1 *10212C>A and Ex4+2776G>A and IGF1R IVS2-70184A>G and IVS2+46329T>C variant genotypes were significantly associated with decreased pancreatic cancer risk (odds ratio [OR] range, 0.60-0.75) and that IGFBP1 Ex4+111A>G (I253M) was significantly associated with increased pancreatic cancer risk (OR=1.46) after adjusted for other risk factors and multiple comparisons (P≤.007). IGF2R and IGFBP3 variant haplotypes were associated with increased and decreased pancreatic cancer risk, respectively (P<.001). We also observed a weak interaction of the IGF1R IVS2+46329T>C and IGF2R Ex45+11C>T (L2222L) genotypes with diabetes (P(interaction)=.05) and interaction of IGF2R and IRS1 genotypes with alcohol consumption (P(interaction)=.03 and .019, respectively) on increased pancreatic cancer risk.

CONCLUSION

These findings support our hypothesis that polymorphic variants of IGF-axis genes act alone or jointly with other risk factors to affect susceptibility to pancreatic cancer.

Insulin receptor substrate 2 (IRS2)-deficient mice show sensorineural hearing loss that is delayed by concomitant protein tyrosine phosphatase 1B (PTP1B) loss of function

The insulin receptor substrate (IRS) proteins are key mediators of insulin and insulinlike growth factor 1 (IGF-1) signaling. Protein tyrosine phosphatase (PTP)-1B dephosphorylates and inactivates both insulin and IGF-1 receptors. IRS2-deficient mice present altered hepatic insulin signaling and β-cell failure and develop type 2-like diabetes. In addition, IRS2 deficiency leads to developmental defects in the nervous system. IGF1 gene mutations cause syndromic sensorineural hearing loss in humans and mice. However, the involvement of IRS2 and PTP1B, two IGF-1 downstream signaling mediators, in hearing onset and loss has not been studied. Our objective was to study the hearing function and cochlear morphology of Irs2-null mice and the impact of PTP1B deficiency. We have studied the auditory brainstem responses and the cochlear morphology of systemic Irs2⁻/⁻Ptpn1⁺/⁺, Irs2⁺/⁺Ptpn1⁻/⁻ and Irs2⁻/⁻Ptpn1⁻/⁻ mice at different postnatal ages. The results indicated that Irs2⁻/⁻Ptpn1⁺/⁺ mice present a profound congenital sensorineural deafness before the onset of diabetes and altered cochlear morphology with hypoinnervation of the cochlear ganglion and aberrant stria vascularis, compared with wild-type mice. Simultaneous PTP1B deficiency in Irs2⁻/⁻Ptpn1⁻/⁻ mice delays the onset of deafness. We show for the first time that IRS2 is essential for hearing and that PTP1B inhibition may be useful for treating deafness associated with hyperglycemia and type 2 diabetes.

The many faces of insulin-like peptide signalling in the brain

Central and peripheral insulin-like peptides (ILPs), which include insulin, insulin-like growth factor 1 (IGF1) and IGF2, exert many effects in the brain. Through their actions on brain growth and differentiation, ILPs contribute to building circuitries that subserve metabolic and behavioural adaptation to internal and external cues of energy availability. In the adult brain each ILP has distinct effects, but together their actions ultimately regulate energy homeostasis - they affect nutrient sensing and regulate neuronal plasticity to modulate adaptive behaviours involved in food seeking, including high-level cognitive operations such as spatial memory. In essence, the multifaceted activity of ILPs in the brain may be viewed as a system organization involved in the control of energy allocation.

Impact of the dipeptidyl peptidase-4 inhibitor vildagliptin on glucose tolerance and β-cell function and mass in insulin receptor substrate-2-knockout mice fed a high-fat diet

Type 2 diabetes is characterized by diminished pancreatic β-cell mass and function. Glucagon-like peptide-1 has been reported to increase islet cell proliferation and reduce apoptosis of β-cells in rodents. In this study, we explored the effect of chronic administration of the dipeptidyl peptidase-4 inhibitor vildagliptin on glucose tolerance, β-cell function, and β-cell mass in Irs2-knockout (Irs2(-/-)) mice. Wild-type and Irs2(-/-) mice were fed a high-fat diet for 20 wk, with or without vildagliptin. In both genotypes of mice, vildagliptin significantly decreased the area under the curve (0-120 min) of blood glucose and increased the insulin response to glucose during the oral glucose tolerance test. In the oral glucose tolerance test performed 1 d after discontinuation of vildagliptin administration, the area under the curve (0-120 min) of blood glucose was still significantly decreased and the insulin response to glucose was significantly increased in the Irs2(-/-) mice treated with vildagliptin as compared with the values in the mice not treated with vildagliptin. Histochemical analysis of the pancreatic islets revealed significant increase of the β-cell mass and decrease in the proportion of terminal deoxynucleotidyl transferase dUTP nick end labeling-positive β-cells but no significant increase of the bromodeoxyuridine incorporation in Irs2(-/-) mice treated with vildagliptin. Our results suggest that vildagliptin improved glucose tolerance and increased the β-cell mass by reducing β-cell apoptosis in the Irs2(-/-) mice, and that the reduction of β-cell apoptosis by vildagliptin was independent of the Irs2 expression in the cells.

Importance of oestrogen receptors to preserve functional β-cell mass in diabetes

Protecting the functional mass of insulin-producing β cells of the pancreas is a major therapeutic challenge in patients with type 1 (T1DM) or type 2 diabetes mellitus (T2DM). The gonadal hormone 17β-oestradiol (E2) is involved in reproductive, bone, cardiovascular and neuronal physiology. In rodent models of T1DM and T2DM, treatment with E2 protects pancreatic β cells against oxidative stress, amyloid polypeptide toxicity, lipotoxicity and apoptosis. Three oestrogen receptors (ERs)--ERα, ERβ and the G protein-coupled ER (GPER)--have been identified in rodent and human β cells. Whereas activation of ERα enhances glucose-stimulated insulin biosynthesis, reduces islet toxic lipid accumulation and promotes β-cell survival from proapoptotic stimuli, activation of ERβ increases glucose-stimulated insulin secretion. However, activation of GPER protects β cells from apoptosis, raises glucose-stimulated insulin secretion and lipid homeostasis without affecting insulin biosynthesis. Oestrogens are also improving islet engraftment in rodent models of pancreatic islet transplantation. This Review describes developments in the role of ERs in islet insulin biosynthesis and secretion, lipid homeostasis and survival. Moreover, we discuss why and how enhancing ER action in β cells without the undesirable effect of general oestrogen therapy is a therapeutic avenue to preserve functional β-cell mass in patients with diabetes mellitus.

Specific glucose-induced control of insulin receptor substrate-2 expression is mediated via Ca2+-dependent calcineurin/NFAT signaling in primary pancreatic islet β-cells

OBJECTIVE

Insulin receptor substrate-2 (IRS-2) plays an essential role in pancreatic islet β-cells by promoting growth and survival. IRS-2 turnover is rapid in primary β-cells, but its expression is highly regulated at the transcriptional level, especially by glucose. The aim was to investigate the molecular mechanism on how glucose regulates IRS-2 gene expression in β-cells.

RESEARCH DESIGN AND METHODS

Rat islets were exposed to inhibitors or subjected to adenoviral vector-mediated gene manipulations and then to glucose-induced IRS-2 expression analyzed by real-time PCR and immunoblotting. Transcription factor nuclear factor of activated T cells (NFAT) interaction with IRS-2 promoter was analyzed by chromatin immunoprecipitation assay and glucose-induced NFAT translocation by immunohistochemistry.

RESULTS

Glucose-induced IRS-2 expression occurred in pancreatic islet β-cells in vivo but not in liver. Modulating rat islet β-cell Ca(2+) influx with nifedipine or depolarization demonstrated that glucose-induced IRS-2 gene expression was dependent on a rise in intracellular calcium concentration derived from extracellular sources. Calcineurin inhibitors (FK506, cyclosporin A, and a peptide calcineurin inhibitor [CAIN]) abolished glucose-induced IRS-2 mRNA and protein levels, whereas expression of a constitutively active calcineurin increased them. Specific inhibition of NFAT with the peptide inhibitor VIVIT prevented a glucose-induced IRS-2 transcription. NFATc1 translocation to the nucleus in response to glucose and association of NFATc1 to conserved NFAT binding sites in the IRS-2 promoter were demonstrated.

CONCLUSIONS

The mechanism behind glucose-induced transcriptional control of IRS-2 gene expression specific to the islet β-cell is mediated by the Ca(2+)/calcineurin/NFAT pathway. This insight into the IRS-2 regulation could provide novel therapeutic means in type 2 diabetes to maintain an adequate functional mass.

FoxO feedback control of basal IRS-2 expression in pancreatic β-cells is distinct from that in hepatocytes

OBJECTIVE

Appropriate regulation of insulin receptor substrate 2 (IRS-2) expression in pancreatic β-cells is essential to adequately compensate for insulin resistance. In liver, basal IRS-2 expression is controlled via a temporal negative feedback of sterol regulatory element-binding protein 1 (SREBP-1) to antagonize transcription factors forkhead box class O (FoxO)1/FoxO3a at an insulin response element (IRE) on the IRS-2 promoter. The purpose of the study was to examine if a similar mechanism controlled IRS-2 expression in β-cells.

RESEARCH DESIGN AND METHODS

IRS-2 mRNA and protein expression, as well as IRS-2 gene promoter activity, were examined in isolated rat islets. Specific transcription factor association with the IRE on the IRS-2 promoter was examined by chromatin immunoprecipitation (ChIP) assay, and their nuclear translocation was examined by immunofluorescence. A direct in vivo effect of insulin on control of IRS-2 expression in liver and pancreatic islets was also investigated.

RESULTS

In IRS-2 promoter-reporter assays conducted in isolated islets, removal of the IRE decreased basal IRS-2 promoter activity in β-cells up to 80%. Activation of IRS signaling in isolated rat islets by insulin/IGF-I (used as an experimental in vitro tool) or downstream constitutive activation of protein kinase B (PKB) significantly decreased IRS-2 expression. In contrast, inhibition of phosphatidylinositol 3-kinase (PI3K) or PKB significantly increased IRS-2 levels in β-cells. ChIP assays indicated that transcription factors FoxO1 and FoxO3a associated with the IRE on the IRS-2 promoter in β-cells in a PI3K/PKB-dependent manner, whereas others, such as SREBP-1, the transcription factor binding to immunoglobulin heavy chain enhancer 3', and the aryl hydrocarbon receptor nuclear translocator (ARNT), did not. However, only FoxO3a, not FoxO1, was capable of driving IRS-2 promoter activity via the IRE in β-cells. In vivo studies showed insulin was able to suppress IRS-2 expression via activation of SREBP-1 in the liver, but this mechanism was not apparent in pancreatic islets from the same animal.

CONCLUSIONS

The molecular mechanism for feedback control of IRS signaling to decrease IRS-2 expression in liver and β-cells is quite distinct, with a predominant role played by FoxO3a in β-cells.

IRS-2 Deficiency impairs NMDA receptor-dependent long-term potentiation

The beneficial effects of insulin and insulin-like growth factor I on cognition have been documented in humans and animal models. Conversely, obesity, hyperinsulinemia, and diabetes increase the risk for neurodegenerative disorders including Alzheimer's disease (AD). However, the mechanisms by which insulin regulates synaptic plasticity are not well understood. Here, we report that complete disruption of insulin receptor substrate 2 (Irs2) in mice impairs long-term potentiation (LTP) of synaptic transmission in the hippocampus. Basal synaptic transmission and paired-pulse facilitation were similar between the 2 groups of mice. Induction of LTP by high-frequency conditioning tetanus did not activate postsynaptic N-methyl-D-aspartate (NMDA) receptors in hippocampus slices from Irs2(-/-) mice, although the expression of NR2A, NR2B, and PSD95 was equivalent to wild-type controls. Activation of Fyn, AKT, and MAPK in response to tetanus stimulation was defective in Irs2(-/-) mice. Interestingly, IRS2 was phosphorylated during induction of LTP in control mice, revealing a potential new component of the signaling machinery which modulates synaptic plasticity. Given that IRS2 expression is diminished in Type 2 diabetics as well as in AD patients, these data may reveal an explanation for the prevalence of cognitive decline in humans with metabolic disorders by providing a mechanistic link between insulin resistance and impaired synaptic transmission.

Insulin receptor substrate 2 and FoxO3a signalling are involved in E-cadherin expression and transforming growth factor-β1-induced repression in kidney epithelial cells

Insulin receptor substrate (IRS) proteins comprise a family of adaptor molecules that integrate extracellular signals from insulin and other ligands to intracellular effectors such as phosphoinositide 3-kinase and mitogen-activated protein kinase. The predominant forms of IRS protein in humans, IRS1 and IRS2, are widely expressed. Despite structural similarities, IRS1 and IRS2 display distinct signalling modalities, and mice lacking these proteins present with distinct phenotypes. Transforming growth factor (TGF)-β1 is the primary cytokine shown to induce epithelial-mesenchymal transition. Recent data have demonstrated a role for IRS1 in TGF-β1-induced epithelial-mesenchymal transition in lung epithelial cells. In the present study, we report data showing that TGF-β1 signals via IRS2 in kidney epithelial cells. Small interfering RNA (siRNA)-mediated targeting of IRS2 increased E-cadherin expression, although it did not alter TGF-β1-mediated E-cadherin repression. Phosphorylation of the downstream target of IRS2/Akt signalling, FoxO3a, was induced on Ser253 and, to a lesser extent, on Thr32. Transfection of FoxO3aThr32Ala mutant for 24 h greatly reduced FoxO3a phosphorylation on Ser253 but over-expression of FoxO3a Ser253Ala did not effect Thr32 phosphorylation, suggesting that a distinct order of phosphorylation of FoxO3a is required for physiological function in cells. Transfection of FoxO3a Ser253Ala mutant partially inhibited TGF-β1-mediated E-cadherin repression at 24 h. Taken together, these data highlight novel roles for IRS2 and FoxO3a in the regulation of kidney epithelial cells by E-cadherin.

IRS2 and PTP1B: Two opposite modulators of hepatic insulin signalling

Type 2 Diabetes mellitus (T2D) is the most common endocrine disorder associated to metabolic syndrome (MS) and occurs when insulin secretion can no compensate peripheral insulin resistance. Among peripheral tissues, the liver controls glucose homeostasis due to its ability to consume and produce glucose. The molecular mechanism underlying hepatic insulin resistance is not completely understood; however, it involves the impairment of the insulin signalling network. Among the critical nodes of hepatic insulin signalling, insulin receptor substrate 2 (IRS2) and protein tyrosine phosphatase 1B (PTP1B) modulate the phosphatidylinositol (PI) 3-kinase/Akt/Foxo1 pathway that controls the suppression of gluconeogenic genes. In this review, we will focus on recent findings regarding the molecular mechanism by which IRS2 and PTP1B elicit opposite effects on carbohydrate metabolism in the liver in response to insulin. Finally, we will discuss the involvement of the critical nodes of insulin signalling in non-alcoholic fatty liver disease (NAFLD) in humans.

Tissue-specificity of insulin action and resistance

Insulin resistance is the most important pathophysiological feature in many pre-diabetic states. Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion by pancreatic beta cells. The creation of monogenic or polygenic genetically manipulated mice models in a tissue-specific manner was of great help to elucidate the tissue-specificity of insulin action and its contribution to the overall insulin resistance. However, complete understanding of the molecular bases of the insulin action and resistance requires the identification of the intracellular pathways that regulate insulin-stimulated proliferation, differentiation and metabolism. Accordingly, cell lines derived from insulin target tissues such as brown adipose tissue, liver and beta islets lacking insulin receptors or sensitive candidate genes such as IRS-1, IRS-2, IRS-3, IR and PTP1B were developed. Indeed, these cell lines have been also very useful to understand the tissue-specificity of insulin action and inaction.

Inducible nitric-oxide synthase and nitric oxide donor decrease insulin receptor substrate-2 protein expression by promoting proteasome-dependent degradation in pancreatic beta-cells: involvement of glycogen synthase kinase-3beta

Insulin receptor substrate-2 (IRS-2) plays a critical role in the survival and function of pancreatic β-cells. Gene disruption of IRS-2 results in failure of the β-cell compensatory mechanism and diabetes. Nonetheless, the regulation of IRS-2 protein expression in β-cells remains largely unknown. Inducible nitric-oxide synthase (iNOS), a major mediator of inflammation, has been implicated in β-cell damage in type 1 and type 2 diabetes. The effects of iNOS on IRS-2 expression have not yet been investigated in β-cells. Here, we show that iNOS and NO donor decreased IRS-2 protein expression in INS-1/832 insulinoma cells and mouse islets, whereas IRS-2 mRNA levels were not altered. Interleukin-1β (IL-1β), alone or in combination with interferon-γ (IFN-γ), reduced IRS-2 protein expression in an iNOS-dependent manner without altering IRS-2 mRNA levels. Proteasome inhibitors, MG132 and lactacystin, blocked the NO donor-induced reduction in IRS-2 protein expression. Treatment with NO donor led to activation of glycogen synthase kinase-3β (GSK-3β) and c-Jun N-terminal kinase (JNK/SAPK) in β-cells. Inhibition of GSK-3β by pharmacological inhibitors or siRNA-mediated knockdown significantly prevented NO donor-induced reduction in IRS-2 expression in β-cells. In contrast, a JNK inhibitor, SP600125, did not effectively block reduced IRS-2 expression in NO donor-treated β-cells. These data indicate that iNOS-derived NO reduces IRS-2 expression by promoting protein degradation, at least in part, through a GSK-3β-dependent mechanism. Our findings suggest that iNOS-mediated decreased IRS-2 expression may contribute to the progression and/or exacerbation of β-cell failure in diabetes.

The insulin receptor substrate (IRS) proteins: at the intersection of metabolism and cancer

Increasing evidence supports a connection between cancer and metabolism and emphasizes the need to understand how tumors respond to the metabolic microenvironment and how tumor cell metabolism is regulated. The insulin receptor (IR) and its close family member the insulin-like growth factor-1 receptor (IGF-1R) mediate the cellular response to insulin in normal cells and their function is tightly regulated to maintain metabolic homeostasis. These receptors are also expressed on tumor cells and their expression correlates with tumor progression and poor prognosis. Understanding how the IR/IGF-1R pathway functions in tumors is increasing in importance as the efficacy of drugs that target metabolic pathways, such as metformin, are investigated in prospective clinical trials. This review will focus on key signaling intermediates of the IR and IGF-1R, the Insulin Receptor Substrate (IRS) proteins, with an emphasis on IRS-2, and discuss how these adaptor proteins play a pivotal role at the intersection of metabolism and cancer.

Human IGF binding protein-3 overexpression impairs glucose regulation in mice via an inhibition of insulin secretion

Human IGF binding protein-3 (hIGFBP-3) overexpression in mice causes hyperglycemia, but its effect on β-cell function is unknown. We compared wild-type mice with mice overexpressing hIGFBP-3 [phoshoglycerate kinase (PGK)BP3] and mutant (Gly⁵⁶/Gly⁸⁰/Gly⁸¹)hIGFBP-3 devoid of IGF binding affinity (PGKmBP3). Intraperitoneal glucose and insulin tolerance tests were performed, and glucose, IGFBP-3, IGF-I, and insulin were determined. Pancreatic sections were used for islet histomorphometry and stained with antibodies against insulin, glucagon, and hIGFBP-3. Pancreatic islets were isolated to determine the expression of IGFBP-3, and glucose-stimulated insulin secretion was measured using both islet batch incubation and perifusion. IGFBP-3 was expressed in β-cells but not in other islet cell types. Fasting glucose concentration was elevated in PGKBP3 mice (6.27 ± 0.31 mm) compared with PGKmBP3 mice (3.98 ± 0.36 mm) and wild-type mice (4.84 ± 0.07 mm). During glucose tolerance test, glucose declined more slowly in PGKBP3 and PGKmBP3 mice than in wild-type mice, and insulin secretion was impaired in PGKBP3 mice. During insulin tolerance test, insulin declined more slowly in both transgenic mice compared with wild-type mice. Insulin secretion in islets incubated with 3.3 mm glucose was similar among groups, but islet insulin response to 16.7 mm glucose alone, or with carbachol and cAMP enhancers, was reduced in PGKBP3 and PGKmBP3 mice compared with wild-type controls. ATP content, Akt phosphorylation, and phosphoglucose isomerase activity were reduced in islets from both transgenic mice. Thus, overexpression of hIGFBP-3 in mice delays in vivo insulin clearance and reduces glucose-stimulated insulin secretion in pancreatic islets by both IGF-dependent and IGF-independent mechanisms.

Insulin-induced serine phosphorylation of IRS-2 via ERK1/2 and mTOR: studies on the function of Ser675 and Ser907

The identity of specific serine phosphorylation residues of insulin receptor substrate (IRS)-2 and their impact on insulin signal transduction are largely unknown. Ser(675) and Ser(907) of mouse IRS-2 are adjacent to PI 3-kinase or Grb2 binding domains, respectively. Using monoclonal phosphosite-specific antibodies, we demonstrated the phosphorylation of both serines after stimulation of Fao hepatoma cells with insulin, anisomycin, or phorbol esters. Phosphorylation of both sites was a late and prolonged event during insulin treatment and was also detected in liver tissue of insulin-treated as well as refed mice. Inhibition and siRNA-mediated knockdown of ERK1/2 indicated that the insulin-induced phosphorylation of Ser(907) was ERK dependent. Phosphorylation of Ser(907) did not prevent the insulin-induced association of IRS-2 with Grb2, but phosphorylation of the adjacent Tyr(911) was proved to be crucial in HEK 293 cells expressing IRS-2 Ala mutants. The insulin-induced phosphorylation of Ser(675) was prevented by inhibition and siRNA-mediated knockdown of mTOR but not of p70(S6K1). Mutation of Ser(675) to Ala did not affect downstream insulin signaling but increased the half-life of the protein, suggesting an involvement of phospho-Ser(675) in an accelerated degradation of IRS-2. Moreover, the insulin-induced degradation of IRS-2 was blocked by inhibition of mTOR. We conclude that the two novel insulin-dependent serine phosphorylation sites of IRS-2 were not involved in the regulation of the adjacent PI 3-kinase and Grb2 binding domains but might be implicated in the ERK- and mTOR-mediated negative feedback control.

α3(V) collagen is critical for glucose homeostasis in mice due to effects in pancreatic islets and peripheral tissues

Collagen V, broadly expressed as α1(V)2 α2(V) heterotrimers that regulate collagen fibril geometry and strength, also occurs in some tissues, such as white adipose tissue (WAT), pancreatic islets, and skeletal muscle, as the poorly characterized α1(V) α2(V) α3(V) heterotrimer. Here, we investigate the role of α3(V) collagen chains by generating mice with a null allele of the α3(V) gene Col5a3 (Col5a3–/– mice). Female Col5a3–/– mice had reduced dermal fat and were resistant to high-fat diet–induced weight gain. Male and female mutant mice were glucose intolerant, insulin-resistant, and hyperglycemic, and these metabolic defects worsened with age. Col5a3–/– mice demonstrated decreased numbers of pancreatic islets, which were more susceptible to streptozotocin-induced apoptosis, and islets isolated from mutant mice displayed blunted glucose-stimulated insulin secretion. Moreover, Col5a3–/– WAT and skeletal muscle were defective in glucose uptake and mobilization of intracellular GLUT4 glucose transporter to the plasma membrane in response to insulin. Our results underscore the emerging view of the importance of ECM to the microenvironments that inform proper development/functioning of specialized cells, such as adipocytes, β cells, and skeletal muscle.

Tissue specificity on insulin action and resistance: past to recent mechanisms

Insulin resistance is the most important pathophysiological feature in many pre-diabetic states. Type 2 diabetes mellitus is a complex metabolic disease and its pathogenesis involves abnormalities in both peripheral insulin action and insulin secretion by pancreatic β-cells. The creation of monogenic or polygenic genetically manipulated mice models in a tissue-specific manner was of great help to elucidate the tissue specificity of insulin action and its contribution to the overall insulin resistance. However, a complete understanding of the molecular bases of insulin action and resistance requires the identification of intracellular pathways that regulate insulin-stimulated proliferation, differentiation and metabolism. Accordingly, cell lines derived from insulin target tissues such as brown adipose tissue, liver and beta islets lacking insulin resistance or sensitive candidate genes such as IRS-1, IRS-2, IRS-3, IR and PTP1B have been developed. Indeed, these cell lines have also been very useful to understand the tissue specificity of insulin action and inaction. Obesity is a risk factor for several components of the metabolic syndromes such as type 2 diabetes, dyslipidaemia and systolic hypertension, because white and brown adipose tissues as endocrine organs express and secrete a variety of adipocytokines that can act at both local and systemic levels, modulating the insulin sensitivity. Recent studies revealed that the subjects with the highest transcription rates of genes encoding TNF-α and IL-6 were prone to develop obesity, insulin resistance and type 2 diabetes. Accordingly, we specifically focus in this review on the impact of those adipocytokines on the modulation of insulin action in skeletal muscle.

Participation of Akt, menin, and p21 in pregnancy-induced beta-cell proliferation

β-Cell mass increases during pregnancy to accommodate for insulin resistance. This increase is mainly due to β-cell proliferation, a process that requires intact prolactin receptor (Prlr) signaling. Signaling molecules that are known to regulate β-cell proliferation include Jak2, Akt, the tumor suppressor menin, and cell cycle proteins. Whether these pathways are involved in prolactin-mediated β-cell proliferation is unknown. Using the heterozygous prolactin receptor-null (Prlr(+/-)) mice, we isolated pancreatic islets from both Prlr(+/+) and Prlr(+/-) mice on d 0 and 15 of pregnancy and examined the expression levels of these signaling molecules. In the wild-type mice (Prlr(+/+)), both phospho-Jak2 and phospho-Akt expression in pancreatic islets increased during pregnancy, which were attenuated in the pregnant Prlr(+/-) mice. During pregnancy, menin expression was reduced by 50 and 20% in the Prlr(+/+) and the Prlr(+/-) mice, respectively, and the pregnant Prlr(+/-) mice had higher islet p18 levels than the Prlr(+/+) mice. Interestingly, between d 0 and 15 of pregnancy, expression of cyclin inhibitory protein p21(cip) was increased in the Prlr(+/+) mice, but this increase was blunted in the Prlr(+/-) mice. Lastly, we did not find any difference in the expression levels of cyclins D1, D2, and inhibitory kinases between the pregnant Prlr(+/+) and Prlr(+/-) mice. Therefore, we conclude that during pregnancy, placental hormones act through the prolactin receptor to increase β-cell mass by up regulating β-cell proliferation by engaging Jak2, Akt, menin/p18, and p21. Future studies will determine the relative contribution of these molecules in maintaining normal glucose homeostasis during pregnancy.

Pancreatic beta-cell failure in obese mice with human-like CMP-Neu5Ac hydroxylase deficiency

Type 2 diabetes is highly prevalent in human populations, particularly in obese individuals, and is characterized by progressive pancreatic β-cell dysfunction and insulin resistance. Most mammals, including Old World primates, express two major kinds of sialic acids, N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc), typically found at the distal ends of glycoconjugate chains at the cell surface. Humans are uniquely unable to produce endogenous Neu5Gc due to an inactivating mutation in the CMP-Neu5Ac hydroxylase (CMAH) gene. The CMAH enzyme catalyzes the generation of CMP-Neu5Gc by the transfer of a single oxygen atom to the acyl group of CMP-Neu5Ac. Here, we show that mice bearing a human-like deletion of the Cmah gene exhibit fasting hyperglycemia and glucose intolerance following a high-fat diet. This phenotype is caused not by worsened insulin resistance but by compromised pancreatic β-cell function associated with a 65% decrease in islet size and area and 50% decrease in islet number. Obese Cmah-null mice also show an ∼40% reduction in response to insulin secretagogues in vivo. These findings show that human evolution-like changes in sialic acid composition impair pancreatic β-cell function and exacerbate glucose intolerance in mice. This may lend insight into the pathogenesis of type 2 diabetes in obese humans.

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.