Restoration of liver insulin signaling in Insr knockout mice fails to normalize hepatic insulin action

Partial restoration of insulin receptor Insr expression in brain, liver, and pancreatic beta cells is sufficient for rescuing Insr knockout mice from neonatal death, preventing diabetes ketoacidosis, and normalizing life span and reproductive function. However, the transgenically rescued mice (referred to as L1) have marked hyperinsulinemia, and approximately 30% develop late-onset type 2 diabetes. Analyses of protein expression indicated that L1 mice had modestly reduced Insr content but normal insulin-stimulated Akt phosphorylation in the liver. Conversely, L1 mice had a near complete ablation of Insr protein product in the arcuate and paraventricular nuclei of the hypothalamus, which was associated with a failure to undergo insulin-dependent Akt phosphorylation in the hypothalamus. To test whether reconstitution of insulin signaling in the liver is sufficient for restoring in vivo hepatic insulin action, we performed euglycemic hyperinsulinemic clamp studies in conscious L1 and WT mice. During the clamp, L1 mice required an approximately 50% lower rate of glucose infusion than did WT controls, while the rate of glucose disappearance was not significantly altered. Conversely, the rate of glucose production was increased approximately 2-fold in L1 mice. Thus, restoration of hepatic insulin signaling in Insr knockout mice fails to normalize the in vivo response to insulin.     

Upregulation of insulin receptor substrate-2 in pancreatic beta cells prevents diabetes

The insulin receptor substrate-2 (Irs2) branch of the insulin/IGF signaling system coordinates peripheral insulin action and pancreatic beta cell function, so mice lacking Irs2 display similarities to humans with type 2 diabetes. Here we show that beta cell-specific expression of Irs2 at a low or a high level delivered a graded physiologic response that promoted beta cell growth, survival, and insulin secretion that prevented diabetes in Irs2-/- mice, obese mice, and streptozotocin-treated mice; and that upon transplantation, the transgenic islets cured diabetes more effectively than WT islets. Thus, pharmacological approaches that promote Irs2 expression in beta cells, especially specific cAMP agonists, could be rational treatments for beta cell failure and diabetes.     

Effective destruction of Fas-deficient insulin-producing beta cells in type 1 diabetes

In type 1 diabetes, autoimmune T cells cause destruction of pancreatic beta cells by largely unknown mechanism. Previous analyses have shown that beta cell destruction is delayed but can occur in perforin-deficient nonobese diabetic (NOD) mice and that Fas-deficient NOD mice do not develop diabetes. However, because of possible pleiotropic functions of Fas, it was not clear whether the Fas receptor was an essential mediator of beta cell death in type 1 diabetes. To directly test this hypothesis, we have generated a beta cell-specific knockout of the Fas gene in a transgenic model of type 1 autoimmune diabetes in which CD4+ T cells with a transgenic TCR specific for influenza hemagglutinin (HA) are causing diabetes in mice that express HA under control of the rat insulin promoter. Here we show that the Fas-deficient mice develop autoimmune diabetes with slightly accelerated kinetics indicating that Fas-dependent apoptosis of beta cells is a dispensable mode of cell death in this disease.     

Relative importance of insulin actin in insulin's target tissues

Insulin is a hormone with various metabolic effects such as glucose uptake and glycogen synthesis in insulin's target tissues. Skeletal muscle has been considered to be a principal site of insulin resistance in type 2 diabetes. Studies of mice with muscle specific-knockout of the insulin receptor demonstrated that although skeletal muscle was insulin resistant, glucose tolerance was near normal at the whole body, indicating the possible importance of liver. beta cell specific disruption of the insulin receptor indicated that primary insulin resistance at the beta cells resulted in a defect of insulin secretion and impaired glucose tolerance. These animal models suggest that a combination of insulin resistance in muscle, liver, adipose cells, and beta cells appears to create the phenotype of type 2 diabetes.     

Insulin signaling and pathophysiology of type 2 diabetes mellitus

The pathophysiology of type 2 diabetes is characterized by defects in insulin action and in insulin secretion. Metabolic actions of insulin are mediated by the insulin receptor/IRS/PI 3-kinase signaling pathway in insulin's target organs including the liver, skeletal muscle and adipose tissue. Recent evidence suggests that insulin action in the brain also plays an important role in the regulation of glucose metabolism in the liver. Insulin signaling in pancreatic beta cells appears to regulate glucose-induced insulin secretion. Although the mechanism how insulin resistance develops is not fully understood, dysregulation of fatty acid metabolism, abnormalities of the function and the secretion of adipokines, as well as the increase in stress signaling might contribute to the development of insulin resistance.     

Unlocking the secrets of the pancreatic beta cell: man and mouse provide the key

Failure of the pancreas to secrete sufficient insulin results in type 2 diabetes, but the pathogenesis of pancreatic beta cell dysfunction is still poorly understood. New insights into beta cell failure come from defining the genes involved in rare genetic subtypes of diabetes and creating appropriate animal models. A new mouse model of transient neonatal diabetes mellitus emphasizes that both the number of beta cells and their function are critical for insulin secretion and may be regulated by imprinted genes.     

Pancreatic beta cells are important targets for the diabetogenic effects of glucocorticoids.

Abnormalities contributing to the pathogenesis of non-insulin-dependent diabetes mellitus include impaired beta cell function, peripheral insulin resistance, and increased hepatic glucose production. Glucocorticoids are diabetogenic hormones because they decrease glucose uptake and increase hepatic glucose production. In addition, they may directly inhibit insulin release. To evaluate that possible role of glucocorticoids in beta cell function independent of their other effects, transgenic mice with an increased glucocorticoid sensitivity restricted to their beta cells were generated by overexpressing the glucocorticoid receptor (GR) under the control of the insulin promoter. Intravenous glucose tolerance tests showed that the GR transgenic mice had normal fasting and postabsorptive blood glucose levels but exhibited a reduced glucose tolerance compared with their control littermates. Measurement of plasma insulin levels 5 min after intravenous glucose load demonstrated a dramatic decrease in acute insulin response in the GR transgenic mice. These results show that glucocorticoids directly inhibit insulin release in vivo and identify the pancreatic beta cell as an important target for the diabetogenic action of glucocorticoids.     

Insulin receptor substrate 2 plays a crucial role in beta cells and the hypothalamus

We previously demonstrated that insulin receptor substrate 2 (Irs2) KO mice develop diabetes associated with hepatic insulin resistance, lack of compensatory beta cell hyperplasia, and leptin resistance. To more precisely determine the roles of Irs2 in beta cells and the hypothalamus, we generated beta cell-specific Irs2 KO and hypothalamus-specific Irs2 knockdown (betaHT-IRS2) mice. Expression of Irs2 mRNA was reduced by approximately 90% in pancreatic islets and was markedly reduced in the arcuate nucleus of the hypothalamus. By contrast, Irs2 expression in liver, muscle, and adipose tissue of betaHT-IRS2 mice was indistinguishable from that of control mice. The betaHT-IRS2 mice displayed obesity and leptin resistance. At 4 weeks of age, the betaHT-IRS2 mice showed normal insulin sensitivity, but at 8 and 12 weeks, they were insulin resistant with progressive obesity. Despite their normal insulin sensitivity at 8 weeks with caloric restriction, the betaHT-IRS2 mice exhibited glucose intolerance and impaired glucose-induced insulin secretion. beta Cell mass and beta cell proliferation in the betaHT-IRS2 mice were reduced significantly at 8 and 12 weeks but not at 10 days. Insulin secretion, normalized by cell number per islet, was significantly increased at high glucose concentrations in the betaHT-IRS2 mice. We conclude that, in beta cells and the hypothalamus, Irs2 is crucially involved in the regulation of beta cell mass and leptin sensitivity.     

ATP-sensitive potassium channelopathies: focus on insulin secretion

ATP-sensitive potassium (K(ATP)) channels, so named because they are inhibited by intracellular (ATP), play key physiological roles in many tissues. In pancreatic beta cells, these channels regulate glucose-dependent insulin secretion and serve as the target for sulfonylurea drugs used to treat type 2 diabetes. This review focuses on insulin secretory disorders, such as congenital hyperinsulinemia and neonatal diabetes, that result from mutations in K(ATP) channel genes. It also considers the extent to which defective regulation of K(ATP) channel activity contributes to the etiology of type 2 diabetes.     

Concentrations of Insulin and of Insulin Receptors in the Brain are Independent of Peripheral Insulin Levels: STUDIES OF OBESE AND STREPTOZOTOCIN-TREATED RODENTS

In view of the potent influences of the central nervous system on glucose metabolism and on its hormonal regulators, and our recent finding of insulin and insulin receptors throughout the central nervous systsem, we have examined extreme conditions of hyperinsulinemia (obese mice) and hypoinsulinemia (streptozotocin-treated rats) with respect to changes in brain insulin and receptor content. Sprague-Dawley rats given streptozotocin (100 mg/kg body wt) developed severe diabetes and by 48 h showed no change in brain insulin. Rats given 65 mg/kg streptozotocin also had severe diabetes, but survived longer. Both at 7 d and at 30 d after streptozotocin treatment there was no significant change in brain insulin or in brain content of insulin receptors, despite the fact that peripheral hepatic receptors were elevated and pancreatic insulin was markedly depleted.The obese mice were studied at 8-10 wk when peripheral plasma insulin concentrations were 50-fold elevated and receptors on peripheral target cells were reduced to congruent with40-50% of normal; brain insulin concentrations and receptor content were indistinguishable from those of thin littermates. Thus, brain insulin, which is typically 10 times higher than plasma insulin concentrations, and brain receptor content, which is equivalent to receptor content on peripheral tissues, appears to be regulated entirely independently of hormone and receptor in the periphery. These findings are consistent with the hypothesis that insulin in the central nervous system is synthesized by the neural elements, and plays a role in the central nervous system which is unrelated to peripheral glucose metabolism.     

Presentation of antigen by endothelial cells and chemoattraction are required for homing of insulin-specific CD8+ T cells

Activated insulin-specific CD8(+) T cells (IS-CD8(+) cells) home to the pancreas, destroy beta cells, and cause rapid diabetes upon transfer into diabetes-prone NOD mice. Surprisingly, they also cause diabetes in mouse strains that are free of preexistent inflammation. Thus, we hypothesized that islet-specific homing may be in part dependent on IS-CD8(+) cells' recognition of the cognate major histocompatibility complex (MHC)/peptide complexes presented by pancreatic endothelial cells, which acquire the antigen (insulin) from beta cells. In fact, islet-specific homing was abrogated in mice that lack MHC class I expression, or presentation of the specific peptide, or have impaired insulin secretion. Moreover, we found that IS-CD8(+) cells directly recognized pancreatic endothelial cells in islet organ cultures. Triggering of IS-CD8(+) cells' T cell receptor (TCR) led to activation of integrins expressed by these cells. In addition, chemokines, particularly SLC (CCL21), were also required for IS-CD8(+) cells' adhesion to endothelial monolayers and for successful homing in vivo. Thus, signaling through TCR and chemokine receptors work in concert to assure firm adhesion of T cells to the pancreatic endothelium. The antigen cross-presentation ability of endothelia may therefore contribute to the specificity of homing of activated T lymphocytes to the tissues where antigens are generated by other cell types.     

Mutations in the Kir6.2 subunit of the KATP channel and permanent neonatal diabetes: New insights and new treatment

Permanent neonatal diabetes (PNDM) is diagnosed in the first three months of life and is a major management problem as patients require lifelong insulin injections. Recently, activating mutations in the KCNJ11 gene which encodes the Kir6.2 subunit of the K_ATP channels in the pancreatic beta‐cells were found to be an important cause of PNDM. The mutated K_ATP channels do not close in the presence of adenosine triphosphate (ATP) so the beta‐cell membrane is hyperpolarized and insulin secretion does not occur. Some patients have DEND syndrome (developmental delay, epilepsy and neonatal diabetes) with the neurological features arising from mutated K_ATP channels in muscle, nerve and brain. Defining a genetic aetiology has not only given insights into clinical classification and disease mechanism, but has also influenced treatment. Sulphonylureas, by binding the sulphonylurea receptor, can close the K_ATP channel. This has led to patients who were insulin‐dependent being able to discontinue insulin injections and achieve excellent control with sulphonylurea tablets. In this article we discuss the work that established Kir6.2 mutations as a common cause of neonatal diabetes, the clinical features, the underlying mechanism and the impact on patient treatment.     

Defective insulin secretion and increased susceptibility to experimental diabetes are induced by reduced Akt activity in pancreatic islet beta cells

The insulin and IGF signaling pathways are critical for development and maintenance of pancreatic beta cell mass and function. The serine-threonine kinase Akt is one of several mediators regulated by these pathways. We have studied the role of Akt in pancreatic beta cell physiology by generating transgenic mice expressing a kinase-dead mutant of this enzyme in beta cells. Reduction of Akt activity in transgenic animals resulted in impaired glucose tolerance due to defective insulin secretion. The mechanisms involved in dysregulation of secretion in these mice lie at the level of insulin exocytosis and are not the result of abnormalities in glucose signaling or function of voltage-gated Ca2+ channels. Therefore, transgenic mice showed increased susceptibility to developing glucose intolerance and diabetes following fat feeding. These observations suggest that Akt plays a novel and important role in the regulation of distal components of the secretory pathway and that this enzyme represents a therapeutic target for improvement of beta cell function in diabetes.     

Dysregulation of insulin receptor substrate 2 in beta cells and brain causes obesity and diabetes

The molecular link between obesity and beta cell failure that causes diabetes is difficult to establish. Here we show that a conditional knockout of insulin receptor substrate 2 (Irs2) in mouse pancreas beta cells and parts of the brain--including the hypothalamus--increased appetite, lean and fat body mass, linear growth, and insulin resistance that progressed to diabetes. Diabetes resolved when the mice were between 6 and 10 months of age: functional beta cells expressing Irs2 repopulated the pancreas, restoring sufficient beta cell function to compensate for insulin resistance in the obese mice. Thus, Irs2 signaling promotes regeneration of adult beta cells and central control of nutrient homeostasis, which can prevent obesity and diabetes in mice.     

Disruption of leptin receptor expression in the pancreas directly affects beta cell growth and function in mice

Obesity is characterized by hyperinsulinemia, hyperleptinemia, and an increase in islet volume. While the mechanisms that hasten the onset of diabetes in obese individuals are not known, it is possible that the adipose-derived hormone leptin plays a role. In addition to its central actions, leptin exerts biological effects by acting in peripheral tissues including the endocrine pancreas. To explore the impact of disrupting leptin signaling in the pancreas on beta cell growth and/or function, we created pancreas-specific leptin receptor (ObR) KOs using mice expressing Cre recombinase under the control of the pancreatic and duodenal homeobox 1 (Pdx1) promoter. The KOs exhibited improved glucose tolerance due to enhanced early-phase insulin secretion, and a greater beta cell mass secondary to increased beta cell size and enhanced expression and phosphorylation of p70S6K. Similar effects on p70S6K were observed in MIN6 beta cells with knockdown of the ObR gene, suggesting crosstalk between leptin and insulin signaling pathways. Surprisingly, challenging the KOs with a high-fat diet led to attenuated acute insulin secretory response to glucose, poor compensatory islet growth, and glucose intolerance. Together, these data provide direct genetic evidence, from a unique mouse model lacking ObRs only in the pancreas, for a critical role for leptin signaling in islet biology and suggest that altered leptin action in islets is one factor that contributes to obesity-associated diabetes.     

Hypoplasia of pancreatic islets in transgenic mice expressing activin receptor mutants.

Activin, a member of the TGF-beta superfamily, regulates the growth and differentiation of a variety of cell types. Based on the expression of activin in pancreatic rudiments of rat embryos and stimulation of insulin secretion from adult rat pancreatic islets by activin, activin is implicated in the development and function of islets. To examine the significance of activin signaling in the fetal and postnatal development of islets, transgenic mice expressing a dominant negative form of activin receptor (dn-ActR) or a constitutively active form of activin receptor (ActR-T206D) in islets were generated together with the transgenic mice expressing intact activin receptor (intact ActR) as a negative control. Transgenic mice with both dn-ActR and ActR-T206D showed lower survival rates, smaller islet area, and lower insulin content in the whole pancreas with impaired glucose tolerance when compared with transgenic mice with intact ActR or littermates, but they showed the same alpha cell/beta cell ratios as their littermates. In addition to islet hypoplasia, the insulin response to glucose was severely impaired in dn-ActR transgenic mice. It is suggested that a precisely regulated intensity of activin signaling is necessary for the normal development of islets at the stage before differentiation into alpha and beta cells, and that activin plays a role in the postnatal functional maturation of islet beta cells.     

PKClambda regulates glucose-induced insulin secretion through modulation of gene expression in pancreatic beta cells

Altered regulation of insulin secretion by glucose is characteristic of individuals with type 2 diabetes mellitus, although the mechanisms that underlie this change remain unclear. We have now generated mice that lack the lambda isoform of PKC in pancreatic beta cells (betaPKClambda(-/-) mice) and show that these animals manifest impaired glucose tolerance and hypoinsulinemia. Furthermore, insulin secretion in response to high concentrations of glucose was impaired, whereas the basal rate of insulin release was increased, in islets isolated from betaPKClambda(-/-) mice. Neither the beta cell mass nor the islet insulin content of betaPKClambda(-/-) mice differed from that of control mice, however. The abundance of mRNAs for Glut2 and HNF3beta was reduced in islets of betaPKClambda(-/-) mice, and the expression of genes regulated by HNF3beta was also affected (that of Sur1 and Kir6.2 genes was reduced, whereas that of hexokinase 1 and hexokinase 2 genes was increased). Normalization of HNF3beta expression by infection of islets from betaPKClambda(-/-) mice with an adenoviral vector significantly reversed the defect in glucose-stimulated insulin secretion. These results indicate that PKClambda plays a prominent role in regulation of glucose-induced insulin secretion by modulating the expression of genes important for beta cell function.