An appropriate beta cell mass is pivotal for the maintenance of glucose homeostasis. Both insulin and IGF-1 are important in regulation of beta 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 beta cells (betaDKO), as the hormones have overlapping mechanisms of action and activate common downstream proteins. Notably, betaDKO 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 betaDKO mice manifest reduced beta cell mass, reduced expression of phosphorylated Akt and the transcription factor MafA, increased apoptosis in islets and severely compromised beta cell function. Analyses of compound knockouts showed a dominant role for insulin signaling in regulating beta cell mass. Together, these data provide compelling genetic evidence that insulin and IGF-I-dependent pathways are not critical for development of beta cells but that a loss of action of these hormones in beta cells leads to diabetes. We propose that therapeutic improvement of insulin and IGF-I signaling in beta cells might protect against type 2 diabetes.

Insulin and insulin-like growth factor I (IGF-I) are ubiquitous hormones that regulate growth and metabolism of most mammalian cells, including pancreatic ␤-cells. In addition to being an insulin secretagogue, glucose regulates proliferation and survival of ␤-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 ␤-cell lines completely lacking either insulin receptors (␤IRKO) or insulin receptor substrate 2 (␤IRS2KO). Exogenous addition of either insulin or glucose activated proteins in the insulin signaling pathway in control ␤-cell lines with the effects of insulin peaking earlier than glucose. Insulin stimulation of ␤IRKO and ␤IRS2KO 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 ␤-cells exhibited low expression of IGF-1 receptors compared to compensatory upregulation in ␤IRKO cells. The signaling data support the slow growth and reduced DNA and protein synthesis in ␤IRKO and ␤IRS2KO cells in response to glucose stimulation. Together, these studies provide compelling evidence that the growth and survival effects of glucose on ␤-cells require activation of proteins in the insulin signaling pathway.Pancreatic islet ␤-cell regeneration and function are regulated by multiple stimuli, including nutrients, hormones, and growth factors acting via diverse intracellular signaling pathways (4, 43). Glucose is the primary regulator of insulin secretion and insulin biosynthesis, and its effects on growth and survival have been suggested to occur by activation of insulin receptor substrate 2 (IRS-2), a protein in the insulin/insulinlike growth-factor I (IGF-I) signaling pathway (40,45). Indeed, over the last decade most components in the insulin signaling pathway have been identified in murine and human pancreatic ␤-cells (1, 4, 17, 36), and their cross talk with other signaling pathways in ␤-cells is being systematically unraveled using genetic approaches in mice (reviewed in references 4 and 31). For example, insulin signaling has been reported to regulate many effects in ␤-cells that are also promoted by glucose, such as enhancing insulin gene expression, insulin secretion, proinsulin biosynthesis, and cell cycle progression (25,(28)(29)(30)38). Considering the similar effects of insulin and glucose in ␤-cells that occur by activation of largely similar proteins in the insulin/IGF-I signaling pathway, it is unclear whether the effects of glucose require activation of insulin receptors via secreted insulin.Examination of the independent effects of glucose versus insulin on ␤-cell function in vivo is limited by a lack of suitable mouse models. Further, the difficulty in separating the downstream effects of exogenous glucose from those of exogenous insulin in cultured ␤-cells ...

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 (Cc1RESEARCH 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 Ϫ/Ϫ and wild-type Cc1 ϩ/ϩ 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, Cc1Ϫ/Ϫ 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 Ϫ/Ϫ mice developed insulin resistance primarily in liver. Despite substantial expression of CEACAM1 in pancreatic ␤-cells, insulin secretion in response to glucose in vivo and in isolated islets was normal in Cc1 Ϫ/Ϫ mice (inbred and outbred strains). CONCLUSIONS-Intact insulin secretion in response to glucose and impairment of insulin clearance in L-SACC1 and Cc1Ϫ/Ϫ mice suggest that the principal role of CEACAM1 in insulin action is to mediate insulin clearance in liver.

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).

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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.

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.

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 insulinresistant states.ER stress | prohormone | bIRKO | GWAS