Anke Assmann

Total insulin and IGF-I resistance in pancreatic β cells causes overt diabetes

An appropriate β cell mass is pivotal for the maintenance of glucose homeostasis. Both insulin and IGF-1 are important in regulation of β cell growth and function (reviewed in ref. 2). To define the roles of these hormones directly, we created a mouse model lacking functional receptors for both insulin and IGF-1 only in β cells (βDKO), as the hormones have overlapping mechanisms of action and activate common downstream proteins. Notably, βDKO mice were born with a normal complement of islet cells, but 3 weeks after birth, they developed diabetes, in contrast to mild phenotypes observed in single mutants. Normoglycemic 2-week-old βDKO mice manifest reduced β cell mass, reduced expression of phosphorylated Akt and the transcription factor MafA, increased apoptosis in islets and severely compromised β cell function. Analyses of compound knockouts showed a dominant role for insulin signaling in regulating β cell mass. Together, these data provide compelling genetic evidence that insulin and IGF-I–dependent pathways are not critical for development of β cells but that a loss of action of these hormones in β cells leads to diabetes. We propose that therapeutic improvement of insulin and IGF-I signaling in β cells might protect against type 2 diabetes.

Glucose Effects on Beta-Cell Growth and Survival Require Activation of Insulin Receptors and Insulin Receptor Substrate 2

ABSTRACT Insulin and insulin-like growth factor I (IGF-I) are ubiquitous hormones that regulate growth and metabolism of most mammalian cells, including pancreatic β-cells. In addition to being an insulin secretagogue, glucose regulates proliferation and survival of β-cells. However, it is unclear whether the latter effects of glucose occur secondary to autocrine activation of insulin signaling proteins by secreted insulin. To examine this possibility we studied the effects of exogenous glucose or insulin in β-cell lines completely lacking either insulin receptors (βIRKO) or insulin receptor substrate 2 (βIRS2KO). Exogenous addition of either insulin or glucose activated proteins in the insulin signaling pathway in control β-cell lines with the effects of insulin peaking earlier than glucose. Insulin stimulation of βIRKO and βIRS2KO cells led to blunted activation of phosphatidylinositol 3-kinase and Akt kinase, while surprisingly, glucose failed to activate either kinase but phosphorylated extracellular signal-regulated kinase. Control β-cells exhibited low expression of IGF-1 receptors compared to compensatory upregulation in βIRKO cells. The signaling data support the slow growth and reduced DNA and protein synthesis in βIRKO and βIRS2KO cells in response to glucose stimulation. Together, these studies provide compelling evidence that the growth and survival effects of glucose on β-cells require activation of proteins in the insulin signaling pathway.

Growth factor control of pancreatic islet regeneration and function

Type 1 and type 2 diabetes mellitus together are predicted to affect over 300 million people worldwide by the year 2020. A relative or absolute paucity of functional β-cells is a central feature of both types of disease, and identifying the pathways that mediate the embryonic origin of new β-cells and mechanisms that underlie the proliferation of existing β-cells are major efforts in the fields of developmental and islet biology. A poor secretory response of existing β-cells to nutrients and hormones and the defects in hormone processing also contribute to the hyperglycemia observed in type 2 diabetes and has prompted studies aimed at enhancing β-cell function. The factors that contribute to a greater susceptibility in aging individuals to develop diabetes is currently unclear and may be linked to a poor turnover of β-cells and/or enhanced susceptibility of β-cells to apoptosis. This review is an update on the recent work in the areas of islet/β-cell regeneration and hormone processing that are relevant to the pathophysiology of the endocrine pancreas in type 1, type 2 and obesity-associated diabetes.

Carcinoembryonic Antigen-Related Cell Adhesion Molecule 1: A Link Between Insulin and Lipid Metabolism

OBJECTIVE—Liver-specific inactivation of carcinoembryonic antigen-related cell adhesion molecule 1 (CEACAM1) by a dominant-negative transgene (l-SACC1 mice) impaired insulin clearance, caused insulin resistance, and increased hepatic lipogenesis. To discern whether this phenotype reflects a physiological function of CEACAM1 rather than the effect of the dominant-negative transgene, we characterized the metabolic phenotype of mice with null mutation of the Ceacam1 gene (Cc1−/−). RESEARCH DESIGN AND METHODS—Mice were originally generated on a mixed C57BL/6x129sv genetic background and then backcrossed 12 times onto the C57BL/6 background. More than 70 male mice of each of the Cc1−/− and wild-type Cc1+/+ groups were subjected to metabolic analyses, including insulin tolerance, hyperinsulinemic-euglycemic clamp studies, insulin secretion in response to glucose, and determination of fasting serum insulin, C-peptide, triglyceride, and free fatty acid levels. RESULTS—Like l-SACC1, Cc1−/− mice exhibited impairment of insulin clearance and hyperinsulinemia, which caused insulin resistance beginning at 2 months of age, when the mutation was maintained on a mixed C57BL/6x129sv background, but not until 5–6 months of age on a homogeneous inbred C57BL/6 genetic background. Hyperinsulinemic-euglycemic clamp studies revealed that the inbred Cc1−/− mice developed insulin resistance primarily in liver. Despite substantial expression of CEACAM1 in pancreatic β-cells, insulin secretion in response to glucose in vivo and in isolated islets was normal in Cc1−/− mice (inbred and outbred strains). CONCLUSIONS—Intact insulin secretion in response to glucose and impairment of insulin clearance in l-SACC1 and Cc1−/− mice suggest that the principal role of CEACAM1 in insulin action is to mediate insulin clearance in liver.

Critical illness myopathy and GLUT4 - significance of insulin and muscle contraction

RATIONALE Critical illness myopathy (CIM) has no known cause and no treatment. Immobilization and impaired glucose metabolism are implicated. OBJECTIVES We assessed signal transduction in skeletal muscle of patients at risk for CIM. We also investigated the effects of evoked muscle contraction. METHODS In a prospective observational and interventional pilot study, we screened 874 mechanically ventilated patients with a sepsis-related organ-failure assessment score greater than or equal to 8 for 3 consecutive days in the first 5 days of intensive care unit stay. Thirty patients at risk for CIM underwent euglycemic-hyperinsulinemic clamp, muscle microdialysis studies, and muscle biopsies. Control subjects were healthy. In five additional patients at risk for CIM, we performed corresponding analyses after 12-day, daily, unilateral electrical muscle stimulation with the contralateral leg as control. MEASUREMENTS AND MAIN RESULTS We performed successive muscle biopsies and assessed systemic insulin sensitivity and signal transduction pathways of glucose utilization at the mRNA and protein level and glucose transporter-4 (GLUT4) localization in skeletal muscle tissue. Skeletal muscle GLUT4 was trapped at perinuclear spaces, most pronounced in patients with CIM, but resided at the sarcolemma in control subjects. Glucose metabolism was not stimulated during euglycemic-hyperinsulinergic clamp. Insulin signal transduction was competent up to p-Akt activation; however, p-adenosine monophosphate-activated protein kinase (p-AMPK) was not detectable in CIM muscle. Electrical muscle stimulation increased p-AMPK, repositioned GLUT4, locally improved glucose metabolism, and prevented type-2 fiber atrophy. CONCLUSIONS Insufficient GLUT4 translocation results in decreased glucose supply in patients with CIM. Failed AMPK activation is involved. Evoked muscle contraction may prevent muscle-specific AMPK failure, restore GLUT4 disposition, and diminish protein breakdown. Clinical trial registered with http://www.controlled-trials.com (registration number ISRCTN77569430).

Insulin Signaling Regulates Mitochondrial Function in Pancreatic β-Cells

Insulin/IGF-I signaling regulates the metabolism of most mammalian tissues including pancreatic islets. To dissect the mechanisms linking insulin signaling with mitochondrial function, we first identified a mitochondria-tethering complex in β-cells that included glucokinase (GK), and the pro-apoptotic protein, BADS. Mitochondria isolated from β-cells derived from β-cell specific insulin receptor knockout (βIRKO) mice exhibited reduced BADS, GK and protein kinase A in the complex, and attenuated function. Similar alterations were evident in islets from patients with type 2 diabetes. Decreased mitochondrial GK activity in βIRKOs could be explained, in part, by reduced expression and altered phosphorylation of BADS. The elevated phosphorylation of p70S6K and JNK1 was likely due to compensatory increase in IGF-1 receptor expression. Re-expression of insulin receptors in βIRKO cells partially restored the stoichiometry of the complex and mitochondrial function. These data indicate that insulin signaling regulates mitochondrial function and have implications for β-cell dysfunction in type 2 diabetes.

Relation between fibroblast growth factor–21, adiposity, metabolism, and weight reduction☆

Fibroblast growth factor-21 (FGF-21) has been proposed as a novel metabolic regulator, and animal experiments suggested that FGF-21 may affect energy balance. In humans, FGF-21 was correlated with obesity. Until now, no data exist regarding the relationship of FGF-21 and weight reduction in humans. We therefore investigated whether FGF-21 is modified by a moderate intended weight loss in a human trial. Thirty obese individuals (24 female, 6 male) participated in a weight reduction program for 6 months. In addition to several anthropometric and metabolic parameters, FGF-21 was measured before and after weight loss. Baseline serum FGF-21 was independently associated with markers of lipid metabolism and waist circumference. The multimodal intervention induced a moderate weight loss (97.4 ± 3.1 vs 92.2 ± 3.1 kg, P < .001), which was accompanied by a significant improvement of lipid and glucose metabolism. However, FGF-21 levels were not modified by moderate weight reduction; and FGF-21 levels at baseline were not a predictive marker for subsequent weight loss. The results presented here confirmed that FGF-21 levels are associated with markers of lipid metabolism and an estimate of abdominal adiposity. The finding that moderate weight loss did not induce changes of FGF-21 levels in humans suggests that FGF-21 is not directly regulated by fat mass under those conditions.

Physiological modulation of circulating FGF21: relevance of free fatty acids and insulin

Fibroblast growth factor 21 (FGF-21), a novel metabolic factor in obesity and fasting metabolism, has been shown to be regulated by supraphysiological levels of free fatty acids (FFAs) under hyperinsulinemic conditions. Interestingly, it is still unclear whether the observed effects of FFAs on FGF-21 are relevant under physiological conditions, and the relative functions of FFAs and insulin within this context also need to be determined. Fourteen healthy men were studied in a randomized controlled crossover trial (RCT) using lipid heparin infusion (LHI) at a dose inducing physiological elevations of FFAs vs. saline heparin infusion. In a second randomized controlled trial, FGF-21 was analyzed in 14 patients with type 1 diabetes (6 men, 8 women) during continuous insulin supply vs. discontinued insulin infusion and subsequently increased lipolysis and ketosis. Circulating FGF-21 increased during physiologically elevated FFAs induced by LHI, which was accompanied by mild hyperinsulinemia. Interestingly, a mild elevation of FFAs resulting from complete insulin deficiency also increased FGF-21 levels. These results from two independent human RCTs suggest that FFAs increase circulating FGF-21, while insulin is only of minor importance under physiological conditions. This mechanism might explain the apparent paradox of increased FGF-21 levels in obesity, insulin resistance, and starvation.

Identification of an intracellular metabolic signature impairing beta cell function in the rat beta cell line INS-1E and human islets

Aims/hypothesisChronic hyperglycaemia promotes the progressive failure of pancreatic beta cells in patients with type 2 diabetes mellitus, a clinically highly relevant phenomenon known as glucotoxicity. The intracellular metabolic consequences of a chronically high availability of glucose in beta cells are, as yet, poorly understood in its full complexity.MethodsAn unbiased metabolite profiling analysis (GC-time-of-flight-MS) was used to identify the time course of core metabolite patterns in rat beta cell line INS-1E during exposure to high glucose concentrations and its relation to insulin expression.ResultsWe report here that pentose phosphate pathway (PPP) metabolites accumulate remarkably during chronic but not acute glucose treatment, indicating altered processing of glucose through the pentose phosphate pathway. Subsequent functional studies in INS-1E cells and human islets revealed that a disturbance in this pathway contributes to decreases in insulin gene expression and a lack of glucose-stimulated insulin secretion. These effects were found to depend on the activation of extracellular-regulated-kinase (ERK1/2). Long-term inhibition of 6-phosphogluconic acid dehydrogenase resulted in accumulation of PPP metabolites, induced ERK1/2 activation independently of high glucose and impaired beta cell function. In turn, inhibition of ERK1/2 overstimulation during chronic glucose exposure partly inhibited metabolite accumulation and restored beta cell function.Conclusions/interpretationBased on unbiased metabolite analyses, the data presented here provide novel targets, namely the inhibition of PPP metabolite accumulation towards the therapeutic goal to preserve and potentially improve beta cell function in diabetes.

Insulin regulates carboxypeptidase E by modulating translation initiation scaffolding protein eIF4G1 in pancreatic β cells

Significance Elevated circulating proinsulin and a poor biological response to insulin are observed early in individuals with type 2 diabetes. Genome-wide association studies have recently identified genes associated with proinsulin processing, and clinical observations suggest that elevated proinsulin and its intermediates are markers of dysfunctional insulin-secreting β cells. Here, we propose a previously unidentified mechanism in the regulation of an enzyme that is involved in proinsulin processing called carboxypeptidase E (CPE). Disruption of insulin signaling in β cells reduces expression of a scaffolding protein, eukaryotic translation initiation factor 4 gamma 1, that is required for the initiation of translation and occurs via regulation of two transcription factors, namely, pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1. Together, these effects lead to reduced levels of CPE protein and poor proinsulin processing in β cells. Insulin resistance, hyperinsulinemia, and hyperproinsulinemia occur early in the pathogenesis of type 2 diabetes (T2D). Elevated levels of proinsulin and proinsulin intermediates are markers of β-cell dysfunction and are strongly associated with development of T2D in humans. However, the mechanism(s) underlying β-cell dysfunction leading to hyperproinsulinemia is poorly understood. Here, we show that disruption of insulin receptor (IR) expression in β cells has a direct impact on the expression of the convertase enzyme carboxypeptidase E (CPE) by inhibition of the eukaryotic translation initiation factor 4 gamma 1 translation initiation complex scaffolding protein that is mediated by the key transcription factors pancreatic and duodenal homeobox 1 and sterol regulatory element-binding protein 1, together leading to poor proinsulin processing. Reexpression of IR or restoring CPE expression each independently reverses the phenotype. Our results reveal the identity of key players that establish a previously unknown link between insulin signaling, translation initiation, and proinsulin processing, and provide previously unidentified mechanistic insight into the development of hyperproinsulinemia in insulin-resistant states.