Insulin resistance plays a primary role in the development of type 2 diabetes and may be related to alterations in fat metabolism. Recent studies have suggested that local accumulation of fat metabolites inside skeletal muscle may activate a serine kinase cascade involving protein kinase C–θ (PKC-θ), leading to defects in insulin signaling and glucose transport in skeletal muscle. To test this hypothesis, we examined whether mice with inactivation of PKC-θ are protected from fat-induced insulin resistance in skeletal muscle. Skeletal muscle and hepatic insulin action as assessed during hyperinsulinemic-euglycemic clamps did not differ between WT and PKC-θ KO mice following saline infusion. A 5-hour lipid infusion decreased insulin-stimulated skeletal muscle glucose uptake in the WT mice that was associated with 40–50% decreases in insulin-stimulated tyrosine phosphorylation of insulin receptor substrate–1 (IRS-1) and IRS-1–associated PI3K activity. In contrast, PKC-θ inactivation prevented fat-induced defects in insulin signaling and glucose transport in skeletal muscle. In conclusion, our findings demonstrate that PKC-θ is a crucial component mediating fat-induced insulin resistance in skeletal muscle and suggest that PKC-θ is a potential therapeutic target for the treatment of type 2 diabetes.
Jason K. Kim, Jonathan J. Fillmore, Mary Jean Sunshine, Bjoern Albrecht, Takamasa Higashimori, Dong-Wook Kim, Zhen-Xiang Liu, Timothy J. Soos, Gary W. Cline, William R. O’Brien, Dan R. Littman, Gerald I. Shulman
Inadequate compensatory β cell hyperplasia in insulin-resistant states triggers the development of overt diabetes. The mechanisms that underlie this crucial adaptive response are not fully defined. Here we show that the compensatory islet-growth response to insulin resistance in 2 models — insulin receptor (IR)/IR substrate–1 (IRS-1) double heterozygous mice and liver-specific IR KO (LIRKO) mice — is severely restricted by PDX-1 heterozygosity. Six-month-old IR/IRS-1 and LIRKO mice both showed up to a 10-fold increase in β cell mass, which involved epithelial-to-mesenchymal transition. In both models, superimposition of PDX-1 haploinsufficiency upon the background of insulin resistance completely abrogated the adaptive islet hyperplastic response, and instead the β cells showed apoptosis resulting in premature death of the mice. This study shows that, in postdevelopmental states of β cell growth, PDX-1 is a critical regulator of β cell replication and is required for the compensatory response to insulin resistance.
Rohit N. Kulkarni, Ulupi S. Jhala, Jonathon N. Winnay, Stan Krajewski, Marc Montminy, C. Ronald Kahn
Diabetes in humans accelerates cardiovascular disease caused by atherosclerosis. The relative contributions of hyperglycemia and dyslipidemia to atherosclerosis in patients with diabetes are not clear, largely because there is a lack of suitable animal models. We therefore have developed a transgenic mouse model that closely mimics atherosclerosis in humans with type 1 diabetes by breeding low-density lipoprotein receptor–deficient mice with transgenic mice in which type 1 diabetes can be induced at will. These mice express a viral protein under control of the insulin promoter and, when infected by the virus, develop an autoimmune attack on the insulin-producing β cells and subsequently develop type 1 diabetes. When these mice are fed a cholesterol-free diet, diabetes, in the absence of associated lipid abnormalities, causes both accelerated lesion initiation and increased arterial macrophage accumulation. When diabetic mice are fed cholesterol-rich diets, on the other hand, they develop severe hypertriglyceridemia and advanced lesions, characterized by extensive intralesional hemorrhage. This progression to advanced lesions is largely dependent on diabetes-induced dyslipidemia, because hyperlipidemic diabetic and nondiabetic mice with similar plasma cholesterol levels show a similar extent of atherosclerosis. Thus, diabetes and diabetes-associated lipid abnormalities have distinct effects on initiation and progression of atherosclerotic lesions.
Catherine B. Renard, Farah Kramer, Fredrik Johansson, Najib Lamharzi, Lisa R. Tannock, Matthias G. von Herrath, Alan Chait, Karin E. Bornfeldt
The regulation of insulin secretion by pancreatic β cells is perturbed in several diseases, including adult-onset (type 2) diabetes and persistent hyperinsulinemic hypoglycemia of infancy (PHHI). The first mouse model for PHHI has a conditional deletion of the gene encoding the winged-helix transcription factor Foxa2 (Forkhead box a2, formerly Hepatocyte nuclear factor 3β) in pancreatic β cells. Using isolated islets, we found that Foxa2 deficiency resulted in excessive insulin release in response to amino acids and complete loss of glucose-stimulated insulin secretion. Most PHHI cases are associated with mutations in SUR1 (Sulfonylurea receptor 1) or KIR6.2 (Inward rectifier K+ channel member 6.2), which encode the subunits of the ATP-sensitive K+ channel, and RNA in situ hybridization of mutant mouse islets revealed that expression of both genes is Foxa2 dependent. We utilized expression profiling to identify additional targets of Foxa2. Strikingly, one of these genes, Hadhsc, encodes short-chain L-3-hydroxyacyl-coenzyme A dehydrogenase, deficiency of which has been shown to cause PHHI in humans. Hadhsc is a direct target of Foxa2, as demonstrated by cotransfection as well as in vivo chromatin immunoprecipitation experiments using isolated islets. Thus, we have established Foxa2 as an essential activator of genes that function in multiple pathways governing insulin secretion.
Kristen A. Lantz, Marko Z. Vatamaniuk, John E. Brestelli, Joshua R. Friedman, Franz M. Matschinsky, Klaus H. Kaestner
Children at risk for type 1 diabetes can develop early insulin autoantibodies (IAAs). Many, but not all, of these children subsequently develop multiple islet autoantibodies and diabetes. To determine whether disease progression is reflected by autoantibody maturity, IAA affinity was measured by competitive radiobinding assay in first and subsequent IAA-positive samples from children followed from birth in the BABYDIAB cohort. IAA affinity in first positive samples ranged from less than 106 l/mol to more than 1011 l/mol. High affinity was associated with HLA DRB1*04, young age of IAA appearance, and subsequent progression to multiple islet autoantibodies or type 1 diabetes. IAA affinity in multiple antibody–positive children was on average 100-fold higher than in children who remained single IAA positive or became autoantibody negative. All high-affinity IAAs required conservation of human insulin A chain residues 8–13 and were reactive with proinsulin. In contrast, most lower-affinity IAAs were dependent on COOH-terminal B chain residues and did not bind proinsulin. These data are consistent with the concept that type 1 diabetes is associated with sustained early exposure to (pro)insulin in the context of HLA DR4 and show that high-affinity proinsulin-reactive IAAs identify children with the highest diabetes risk.
Peter Achenbach, Kerstin Koczwara, Annette Knopff, Heike Naserke, Anette-G. Ziegler, Ezio Bonifacio
Transient neonatal diabetes mellitus (TNDM) is a rare inherited diabetic syndrome apparent in the first weeks of life and again during early adulthood. The relative contributions of reduced islet β cell number and impaired β cell function to the observed hypoinsulinemia are unclear. The inheritance pattern of this imprinted disorder implicates overexpression of one or both genes within the TNDM locus: ZAC, which encodes a proapoptotic zinc finger protein, and HYMAI, which encodes an untranslated mRNA. To investigate the consequences for pancreatic function, we have developed a high-copy transgenic mouse line, TNDM29, carrying the human TNDM locus. TNDM29 neonates display hyperglycemia, and older adults, impaired glucose tolerance. Neonatal hyperglycemia occurs only on paternal transmission, analogous to paternal dependence of TNDM in humans. Embryonic pancreata of TNDM29 mice showed reductions in expression of endocrine differentiation factors and numbers of insulin-staining structures. By contrast, β cell mass was normal or elevated at all postnatal stages, whereas pancreatic insulin content in neonates and peak serum insulin levels after glucose infusion in adults were reduced. Expression of human ZAC and HYMAI in these transgenic mice thus recapitulates key features of TNDM and implicates impaired development of the endocrine pancreas and β cell function in disease pathogenesis.
Dan Ma, Julian P.H. Shield, Wendy Dean, Isabelle Leclerc, Claude Knauf, Rémy Burcelin, Guy A. Rutter, Gavin Kelsey
Regulation of energy balance by leptin involves regulation of several neuropeptides, including thyrotropin-releasing hormone (TRH). Synthesized from a larger inactive precursor, its maturation requires proteolytic cleavage by prohormone convertases 1 and 2 (PC1 and PC2). Since this maturation in response to leptin requires prohormone processing, we hypothesized that leptin might regulate hypothalamic PC1 and PC2 expression, ultimately leading to coordinated processing of prohormones into mature peptides. Using hypothalamic neurons, we found that leptin stimulated PC1 and PC2 mRNA and protein expression and also increased PC1 and PC2 promoter activities in transfected 293T cells. Starvation of rats, leading to low serum leptin levels, decreased PC1 and PC2 gene and protein expression in the paraventricular nucleus (PVN) of the hypothalamus. Exogenous administration of leptin to fasted animals restored PC1 levels in the median eminence (ME) and the PVN to approximately the level found in fed control animals. Consistent with this regulation of PCs in the PVN, concentrations of TRH in the PVN and ME were substantially reduced in the fasted animals relative to the fed animals, and leptin reversed this decrease. Further analysis showed that proteolytic cleavage of pro–thyrotropin-releasing hormone (proTRH) at known PC cleavage sites was reduced by fasting and increased in animals given leptin. Combined, these findings suggest that leptin-dependent stimulation of hypothalamic TRH expression involves both activation of trh transcription and stimulation of PC1 and PC2 expression, which lead to enhanced processing of proTRH into mature TRH.
Vanesa C. Sanchez, Jorge Goldstein, Ronald C. Stuart, Virginia Hovanesian, Lihong Huo, Heike Munzberg, Theodore C. Friedman, Christian Bjorbaek, Eduardo A. Nillni
The role of different tissues in insulin action and their contribution to the pathogenesis of diabetes remain unclear. To examine this question, we have used genetic reconstitution experiments in mice. Genetic ablation of insulin receptors causes early postnatal death from diabetic ketoacidosis. We show that combined restoration of insulin receptor function in brain, liver, and pancreatic β cells rescues insulin receptor knockout mice from neonatal death, prevents diabetes in a majority of animals, and normalizes adipose tissue content, lifespan, and reproductive function. In contrast, mice with insulin receptor expression limited to brain or liver and pancreatic β cells are rescued from neonatal death, but develop lipoatrophic diabetes and die prematurely. These data indicate, surprisingly, that insulin receptor signaling in noncanonical insulin target tissues is sufficient to maintain fuel homeostasis and prevent diabetes.
Haruka Okamoto, Jun Nakae, Tadahiro Kitamura, Byung-Chul Park, Ioannis Dragatsis, Domenico Accili
We investigated the chronic in vivo effect of resistin on insulin sensitivity and glucose metabolism by overexpressing resistin protein in male Wistar rats using intravenous administration of an adenovirus encoding mouse resistin. After 7 days of elevated resistin levels at a supraphysiological concentration, the animals displayed glucose intolerance and hyperinsulinemia during glucose tolerance tests, and insulin tolerance tests demonstrated an impaired glucose-lowering effect of insulin. The glucose clamp studies were performed at submaximal (4 mU/kg/min) and maximal (25 mU/kg/min) insulin infusion rates and demonstrated the presence of insulin resistance induced by elevated resistin levels. Indeed, the insulin-stimulated glucose infusion rate was decreased by 12–31%; suppression of hepatic glucose output was attenuated by 28–55%; and insulin suppression of circulating FFA levels was inhibited by 7%. Insulin receptor substrate–1 and –2 phosphorylation and Akt activation were impaired in muscle and adipose tissue. Interestingly, activation of AMP-activated protein kinase in skeletal muscle, liver, and adipose tissue was also significantly downregulated. Together, these results indicate that chronic “hyper-resistinemia” leads to whole-body insulin resistance involving impaired insulin signaling in skeletal muscle, liver, and adipose tissue, resulting in glucose intolerance, hyperinsulinemia, and hypertriglyceridemia. Thus elevated resistin levels in normal rats fed a regular chow diet produce many of the features of human syndrome X.
Hiroaki Satoh, M.T. Audrey Nguyen, Philip D.G. Miles, Takeshi Imamura, Isao Usui, Jerrold M. Olefsky
Resistin is an adipose-derived hormone postulated to link adiposity to insulin resistance. To determine whether resistin plays a causative role in the development of diet-induced insulin resistance, we lowered circulating resistin levels in mice by use of a specific antisense oligodeoxynucleotide (ASO) directed against resistin mRNA and assessed in vivo insulin action by the insulin-clamp technique. After 3 weeks on a high-fat (HF) diet, mice displayed severe insulin resistance associated with an approximately 80% increase in plasma resistin levels. In particular, the rate of endogenous glucose production (GP) increased more than twofold compared with that in mice fed a standard chow. Treatment with the resistin ASO for 1 week normalized the plasma resistin levels and completely reversed the hepatic insulin resistance. Importantly, in this group of mice, the acute infusion of purified recombinant mouse resistin, designed to acutely elevate the levels of circulating resistin up to those observed in the HF-fed mice, was sufficient to reconstitute hepatic insulin resistance. These results provide strong support for a physiological role of resistin in the development of hepatic insulin resistance in this model.
Evan D. Muse, Silvana Obici, Sanjay Bhanot, Brett P. Monia, Robert A. McKay, Michael W. Rajala, Philipp E. Scherer, Luciano Rossetti