The relative activity of lipoprotein lipase (LPL) in different tissues controls the partitioning of lipoprotein-derived fatty acids between sites of fat storage (adipose tissue) and oxidation (heart and skeletal muscle). Here we used a reverse genetic strategy to test the hypothesis that 4 angiopoietin-like proteins (ANGPTL3, -4, -5, and -6) play key roles in triglyceride (TG) metabolism in humans. We re-sequenced the coding regions of the genes encoding these proteins and identified multiple rare nonsynonymous (NS) sequence variations that were associated with low plasma TG levels but not with other metabolic phenotypes. Functional studies revealed that all mutant alleles of ANGPTL3 and ANGPTL4 that were associated with low plasma TG levels interfered either with the synthesis or secretion of the protein or with the ability of the ANGPTL protein to inhibit LPL. A total of 1% of the Dallas Heart Study population and 4% of those participants with a plasma TG in the lowest quartile had a rare loss-of-function mutation in ANGPTL3, ANGPTL4, or ANGPTL5. Thus, ANGPTL3, ANGPTL4, and ANGPTL5, but not ANGPTL6, play nonredundant roles in TG metabolism, and multiple alleles at these loci cumulatively contribute to variability in plasma TG levels in humans.
Stefano Romeo, Wu Yin, Julia Kozlitina, Len A. Pennacchio, Eric Boerwinkle, Helen H. Hobbs, Jonathan C. Cohen
Defective insulin secretion in response to glucose is an important component of the β cell dysfunction seen in type 2 diabetes. As mitochondrial oxidative phosphorylation plays a key role in glucose-stimulated insulin secretion (GSIS), oxygen-sensing pathways may modulate insulin release. The von Hippel–Lindau (VHL) protein controls the degradation of hypoxia-inducible factor (HIF) to coordinate cellular and organismal responses to altered oxygenation. To determine the role of this pathway in controlling glucose-stimulated insulin release from pancreatic β cells, we generated mice lacking Vhl in pancreatic β cells (βVhlKO mice) and mice lacking Vhl in the pancreas (PVhlKO mice). Both mouse strains developed glucose intolerance with impaired insulin secretion. Furthermore, deletion of Vhl in β cells or the pancreas altered expression of genes involved in β cell function, including those involved in glucose transport and glycolysis, and isolated βVhlKO and PVhlKO islets displayed impaired glucose uptake and defective glucose metabolism. The abnormal glucose homeostasis was dependent on upregulation of Hif-1α expression, and deletion of Hif1a in Vhl-deficient β cells restored GSIS. Consistent with this, expression of activated Hif-1α in a mouse β cell line impaired GSIS. These data suggest that VHL/HIF oxygen-sensing mechanisms play a critical role in glucose homeostasis and that activation of this pathway in response to decreased islet oxygenation may contribute to β cell dysfunction.
James Cantley, Colin Selman, Deepa Shukla, Andrey Y. Abramov, Frauke Forstreuter, Miguel A. Esteban, Marc Claret, Steven J. Lingard, Melanie Clements, Sarah K. Harten, Henry Asare-Anane, Rachel L. Batterham, Pedro L. Herrera, Shanta J. Persaud, Michael R. Duchen, Patrick H. Maxwell, Dominic J. Withers
Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K+ (KATP) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic β cells. These β-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from β-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of KATP channels in pancreatic β cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of KATP channels, closed KATP channels, elevated intracellular calcium levels, and stimulated insulin release in β-V59M β cells, indicating that events downstream of KATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic β cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. β-V59M islets also displayed a reduced percentage of β cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of β-V59M mice. Their cause requires further investigation.
Christophe A. Girard, F. Thomas Wunderlich, Kenju Shimomura, Stephan Collins, Stephan Kaizik, Peter Proks, Fernando Abdulkader, Anne Clark, Vicky Ball, Lejla Zubcevic, Liz Bentley, Rebecca Clark, Chris Church, Alison Hugill, Juris Galvanovskis, Roger Cox, Patrik Rorsman, Jens C. Brüning, Frances M. Ashcroft
Food intake is regulated by a network of signals that emanate from the gut and the brainstem. The peripheral satiety signal cholecystokinin is released from the gut following food intake and acts on fibers of the vagus nerve, which project to the brainstem and activate neurons that modulate both gastrointestinal function and appetite. In this study, we found that neurons in the nucleus tractus solitarii of the brainstem that express prolactin-releasing peptide (PrRP) are activated rapidly by food ingestion. To further examine the role of this peptide in the control of food intake and energy metabolism, we generated PrRP-deficient mice and found that they displayed late-onset obesity and adiposity, phenotypes that reflected an increase in meal size, hyperphagia, and attenuated responses to the anorexigenic signals cholecystokinin and leptin. Hypothalamic expression of 6 other appetite-regulating peptides remained unchanged in the PrRP-deficient mice. Blockade of endogenous PrRP signaling in WT rats by central injection of PrRP-specific mAb resulted in an increase in food intake, as reflected by an increase in meal size. These data suggest that PrRP relays satiety signals within the brain and that selective disturbance of this system can result in obesity and associated metabolic disorders.
Yuki Takayanagi, Hirokazu Matsumoto, Masanori Nakata, Takashi Mera, Shoji Fukusumi, Shuji Hinuma, Yoichi Ueta, Toshihiko Yada, Gareth Leng, Tatsushi Onaka
The progression from insulin resistance to type 2 diabetes is caused by the failure of pancreatic β cells to produce sufficient levels of insulin to meet the metabolic demand. Recent studies indicate that nutrient fluctuations and insulin resistance increase proinsulin synthesis in β cells beyond the capacity for folding of nascent polypeptides within the endoplasmic reticulum (ER) lumen, thereby disrupting ER homeostasis and triggering the unfolded protein response (UPR). Chronic ER stress promotes apoptosis, at least in part through the UPR-induced transcription factor C/EBP homologous protein (CHOP). We assessed the effect of Chop deletion in multiple mouse models of type 2 diabetes and found that Chop–/– mice had improved glycemic control and expanded β cell mass in all conditions analyzed. In both genetic and diet-induced models of insulin resistance, CHOP deficiency improved β cell ultrastructure and promoted cell survival. In addition, we found that isolated islets from Chop–/– mice displayed increased expression of UPR and oxidative stress response genes and reduced levels of oxidative damage. These findings suggest that CHOP is a fundamental factor that links protein misfolding in the ER to oxidative stress and apoptosis in β cells under conditions of increased insulin demand.
Benbo Song, Donalyn Scheuner, David Ron, Subramaniam Pennathur, Randal J. Kaufman
Diet-induced obesity is associated with fatty liver, insulin resistance, leptin resistance, and changes in plasma lipid profile. Endocannabinoids have been implicated in the development of these associated phenotypes, because mice deficient for the cannabinoid receptor CB1 (CB1–/–) do not display these changes in association with diet-induced obesity. The target tissues that mediate these effects, however, remain unknown. We therefore investigated the relative role of hepatic versus extrahepatic CB1 receptors in the metabolic consequences of a high-fat diet, using liver-specific CB1 knockout (LCB1–/–) mice. LCB1–/– mice fed a high-fat diet developed a similar degree of obesity as that of wild-type mice, but, similar to CB1–/– mice, had less steatosis, hyperglycemia, dyslipidemia, and insulin and leptin resistance than did wild-type mice fed a high-fat diet. CB1 agonist–induced increase in de novo hepatic lipogenesis and decrease in the activity of carnitine palmitoyltransferase–1 and total energy expenditure were absent in both CB1–/– and LCB1–/– mice. We conclude that endocannabinoid activation of hepatic CB1 receptors contributes to the diet-induced steatosis and associated hormonal and metabolic changes, but not to the increase in adiposity, observed with high-fat diet feeding. Theses studies suggest that peripheral CB1 receptors could be selectively targeted for the treatment of fatty liver, impaired glucose homeostasis, and dyslipidemia in order to minimize the neuropsychiatric side effects of nonselective CB1 blockade during treatment of obesity-associated conditions.
Douglas Osei-Hyiaman, Jie Liu, Liang Zhou, Grzegorz Godlewski, Judith Harvey-White, Won-il Jeong, Sándor Bátkai, Giovanni Marsicano, Beat Lutz, Christoph Buettner, George Kunos
The pregnane X receptor (PXR) and the constitutive androstane receptor (CAR) are closely related orphan nuclear hormone receptors that play a critical role as xenobiotic sensors in mammals. Both receptors regulate the expression of genes involved in the biotransformation of chemicals in a ligand-dependent manner. As the ligand specificity of PXR and CAR have diverged between species, the prediction of in vivo PXR and CAR interactions with a drug are difficult to extrapolate from animals to humans. We report the development of what we believe are novel PXR- and CAR-humanized mice, generated using a knockin strategy, and Pxr- and Car-KO mice as well as a panel of mice including all possible combinations of these genetic alterations. The expression of human CAR and PXR was in the predicted tissues at physiological levels, and splice variants of both human receptors were expressed. The panel of mice will allow the dissection of the crosstalk between PXR and CAR in the response to different drugs. To demonstrate the utility of this panel of mice, we used the mice to show that the in vivo induction of Cyp3a11 and Cyp2b10 by phenobarbital was only mediated by CAR, although this compound is described as a PXR and CAR activator in vitro. This panel of mouse models is a useful tool to evaluate the roles of CAR and PXR in drug bioavailability, toxicity, and efficacy in humans.
Nico Scheer, Jillian Ross, Anja Rode, Branko Zevnik, Sandra Niehaves, Nicole Faust, C. Roland Wolf
Prolonged activation of p70 S6 kinase (S6K) by insulin and nutrients leads to inhibition of insulin signaling via negative feedback input to the signaling factor IRS-1. Systemic deletion of S6K protects against diet-induced obesity and enhances insulin sensitivity in mice. Herein, we present evidence suggesting that hypothalamic S6K activation is involved in the pathogenesis of diet-induced hepatic insulin resistance. Extending previous findings that insulin suppresses hepatic glucose production (HGP) partly via its effect in the hypothalamus, we report that this effect was blunted by short-term high-fat diet (HFD) feeding, with concomitant suppression of insulin signaling and activation of S6K in the mediobasal hypothalamus (MBH). Constitutive activation of S6K in the MBH mimicked the effect of the HFD in normal chow–fed animals, while suppression of S6K by overexpression of dominant-negative S6K or dominant-negative raptor in the MBH restored the ability of MBH insulin to suppress HGP after HFD feeding. These results suggest that activation of hypothalamic S6K contributes to hepatic insulin resistance in response to short-term nutrient excess.
Hiraku Ono, Alessandro Pocai, Yuhua Wang, Hideyuki Sakoda, Tomoichiro Asano, Jonathan M. Backer, Gary J. Schwartz, Luciano Rossetti
Adipose tissue inflammation is a characteristic of obesity. However, the mechanisms that regulate this inflammatory response and link adipose inflammation to systemic metabolic consequences are not fully understood. In this study, we have taken advantage of the highly restricted coexpression of adipocyte/macrophage fatty acid–binding proteins (FABPs) aP2 (FABP4) and mal1 (FABP5) to examine the contribution of these lipid chaperones in macrophages and adipocytes to local and systemic inflammation and metabolic homeostasis in mice. Deletion of FABPs in adipocytes resulted in reduced expression of inflammatory cytokines in macrophages, whereas the same deletion in macrophages led to enhanced insulin signaling and glucose uptake in adipocytes. Using radiation chimerism through bone marrow transplantation, we generated mice with FABP deficiency in bone marrow and stroma-derived elements in vivo and studied the impact of each cellular target on local and systemic insulin action and glucose metabolism in dietary obesity. The results of these experiments indicated that neither macrophages nor adipocytes individually could account for the total impact of FABPs on systemic metabolism and suggest that interactions between these 2 cell types, particularly in adipose tissue, are critical for the inflammatory basis of metabolic deterioration.
Masato Furuhashi, Raquel Fucho, Cem Z. Görgün, Gürol Tuncman, Haiming Cao, Gökhan S. Hotamisligil
Identifying the genetic variants that regulate fasting glucose concentrations may further our understanding of the pathogenesis of diabetes. We therefore investigated the association of fasting glucose levels with SNPs in 2 genome-wide scans including a total of 5,088 nondiabetic individuals from Finland and Sardinia. We found a significant association between the SNP rs563694 and fasting glucose concentrations (P = 3.5 × 10–7). This association was further investigated in an additional 18,436 nondiabetic individuals of mixed European descent from 7 different studies. The combined P value for association in these follow-up samples was 6.9 × 10–26, and combining results from all studies resulted in an overall P value for association of 6.4 × 10–33. Across these studies, fasting glucose concentrations increased 0.01–0.16 mM with each copy of the major allele, accounting for approximately 1% of the total variation in fasting glucose. The rs563694 SNP is located between the genes glucose-6-phosphatase catalytic subunit 2 (G6PC2) and ATP-binding cassette, subfamily B (MDR/TAP), member 11 (ABCB11). Our results in combination with data reported in the literature suggest that G6PC2, a glucose-6-phosphatase almost exclusively expressed in pancreatic islet cells, may underlie variation in fasting glucose, though it is possible that ABCB11, which is expressed primarily in liver, may also contribute to such variation.
Wei-Min Chen, Michael R. Erdos, Anne U. Jackson, Richa Saxena, Serena Sanna, Kristi D. Silver, Nicholas J. Timpson, Torben Hansen, Marco Orrù, Maria Grazia Piras, Lori L. Bonnycastle, Cristen J. Willer, Valeriya Lyssenko, Haiqing Shen, Johanna Kuusisto, Shah Ebrahim, Natascia Sestu, William L. Duren, Maria Cristina Spada, Heather M. Stringham, Laura J. Scott, Nazario Olla, Amy J. Swift, Samer Najjar, Braxton D. Mitchell, Debbie A. Lawlor, George Davey Smith, Yoav Ben-Shlomo, Gitte Andersen, Knut Borch-Johnsen, Torben Jørgensen, Jouko Saramies, Timo T. Valle, Thomas A. Buchanan, Alan R. Shuldiner, Edward Lakatta, Richard N. Bergman, Manuela Uda, Jaakko Tuomilehto, Oluf Pedersen, Antonio Cao, Leif Groop, Karen L. Mohlke, Markku Laakso, David Schlessinger, Francis S. Collins, David Altshuler, Gonçalo R. Abecasis, Michael Boehnke, Angelo Scuteri, Richard M. Watanabe