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Type 2 diabetes is now a pandemic and shows no signs of abatement. In this Seminar we review the pathophysiology of this disorder, with particular attention to epidemiology, genetics, epigenetics, and molecular cell biology. Evidence is emerging that a substantial part of diabetes susceptibility is acquired early in life, probably owing to fetal or neonatal programming via epigenetic phenomena. Maternal and early childhood health might, therefore, be crucial to the development of effective prevention strategies. Diabetes develops because of inadequate islet β-cell and adipose-tissue responses to chronic fuel excess, which results in so-called nutrient spillover, insulin resistance, and metabolic stress. The latter damages multiple organs. Insulin resistance, while forcing β cells to work harder, might also have an important defensive role against nutrient-related toxic effects in tissues such as the heart. Reversal of overnutrition, healing of the β cells, and lessening of adipose tissue defects should be treatment priorities.

Metabolic stress caused by excess nutrients accelerates aging. We recently demonstrated that the newly discovered enzyme glycerol-3-phosphate phosphatase (G3PP; gene Pgp ), which operates an evolutionarily conserved glycerol shunt that hydrolyzes glucose-derived glycerol-3-phosphate to glycerol, counters metabolic stress and promotes healthy aging in C. elegans . However, the mechanism whereby G3PP activation extends healthspan and lifespan, particularly under glucotoxicity, remained unknown. Here, we show that the overexpression of the C. elegans G3PP homolog, PGPH-2, decreases fat levels and mimics, in part, the beneficial effects of calorie restriction, particularly in glucotoxicity conditions, without reducing food intake. PGPH-2 overexpression depletes glycogen stores activating AMP-activate protein kinase, which leads to the HLH-30 nuclear translocation and activation of autophagy, promoting healthy aging. Transcriptomics reveal an HLH-30-dependent longevity and catabolic gene expression signature with PGPH-2 overexpression. Thus, G3PP overexpression activates three key longevity factors, AMPK, the TFEB homolog HLH-30, and autophagy, and may be an attractive target for age-related metabolic disorders linked to excess nutrients. Activation of the glycerol shunt mimics calorie restriction, protects from glucotoxicity, and promotes healthy aging. Here, Possik et al. uncover the underlying mechanism and identify the AMPK-TFEB autophagy pathway as critical to the healthy aging phenotype.

α/β-hydrolase domain 6 (ABHD6) is a lipase linked to physiological functions affecting energy metabolism. Brain ABHD6 degrades 2-arachidonoylglycerol and thereby modifies cannabinoid receptor signalling. However, its functional role within mesoaccumbens circuitry critical for motivated behaviour and considerably modulated by endocannabinoids was unknown. Using three viral approaches, we show that control of the nucleus accumbens by neuronal ABHD6 is a key determinant of body weight and reward-directed behaviour in male mice. Contrary to expected outcomes associated with increasing endocannabinoid tone, loss of ABHD6 in nucleus accumbens, but not ventral tegmental area, neurons completely prevents diet-induced obesity, reduces food- and drug-seeking and enhances physical activity without affecting anxiodepressive behaviour. These effects are explained by attenuated inhibitory synaptic transmission onto medium spiny neurons. ABHD6 deletion in nucleus accumbens neurons and dopamine ventral tegmental area neurons produces contrasting effects on effortful responding for food. Intraventricular infusions of an ABHD6 inhibitor also restrain appetite and promote weight loss. Together, these results reveal functional specificity of pre- and post-synaptic mesoaccumbens neuronal ABHD6 to differentially control energy balance and propose ABHD6 inhibition as a potential anti-obesity tool. Endocannabinoid signalling processes are implicated in body weight regulation. Here, authors used viral-genetic tools to identify the enzyme ABHD6 in postsynaptic mesoaccumbal neurons is a key determinant of body weight and reward-relevant behaviour.

Type 2 diabetes mellitus results from the interaction of environmental factors with a combination of genetic variants, most of which were hitherto unknown. A systematic search for these variants was recently made possible by the development of high-density arrays that permit the genotyping of hundreds of thousands of polymorphisms. We tested 392,935 single-nucleotide polymorphisms in a French case–control cohort. Markers with the most significant difference in genotype frequencies between cases of type 2 diabetes and controls were fast-tracked for testing in a second cohort. This identified four loci containing variants that confer type 2 diabetes risk, in addition to confirming the known association with the TCF7L2 gene. These loci include a non-synonymous polymorphism in the zinc transporter SLC30A8, which is expressed exclusively in insulin-producing β-cells, and two linkage disequilibrium blocks that contain genes potentially involved in β-cell development or function ( IDE–KIF11–HHEX and EXT2–ALX4 ). These associations explain a substantial portion of disease risk and constitute proof of principle for the genome-wide approach to the elucidation of complex genetic traits. Diabetes in the genes Overeating and physical inactivity are major causes of type 2 diabetes mellitus, but they affect only genetically susceptible individuals and the genetic basis of the disease is notoriously complex. Recent research has suggested that specific genes may be associated with the risk of developing the disease, however. Now a genome-wide search using high-density genotyping arrays has identified four previously unknown genes as diabetes risk factors, and confirmed a known association with the TCF7L2 gene. Together these five genes may contribute a sizeable fraction of the disease risk in type 2 diabetes, and analysis of their function should clarify the pathogenesis of diabetes and point to new drug targets. In addition, individuals shown to have these mutations could minimize their risk by adjusting diet. A survey of the entire human genome has found that five genetic loci contribute a large fraction of disease risk in type 2 diabetes.

Pyruvate kinase (PK) and the phosphoenolpyruvate (PEP) cycle play key roles in nutrient-stimulated K ATP channel closure and insulin secretion. To identify the PK isoforms involved, we generated mice lacking β-cell PKm1, PKm2, and mitochondrial PEP carboxykinase (PCK2) that generates mitochondrial PEP. Glucose metabolism was found to generate both glycolytic and mitochondrially derived PEP, which triggers K ATP closure through local PKm1 and PKm2 signaling at the plasma membrane. Amino acids, which generate mitochondrial PEP without producing glycolytic fructose 1,6-bisphosphate to allosterically activate PKm2, signal through PKm1 to raise ATP/ADP, close K ATP channels, and stimulate insulin secretion. Raising cytosolic ATP/ADP with amino acids is insufficient to close K ATP channels in the absence of PK activity or PCK2, indicating that K ATP channels are primarily regulated by PEP that provides ATP via plasma membrane-associated PK, rather than mitochondrially derived ATP. Following membrane depolarization, the PEP cycle is involved in an ‘off-switch’ that facilitates K ATP channel reopening and Ca 2+ extrusion, as shown by PK activation experiments and β-cell PCK2 deletion, which prolongs Ca 2+ oscillations and increases insulin secretion. In conclusion, the differential response of PKm1 and PKm2 to the glycolytic and mitochondrial sources of PEP influences the β-cell nutrient response, and controls the oscillatory cycle regulating insulin secretion.

Metabolic stress due to nutrient excess and lipid accumulation is at the root of many age-associated disorders and the identification of therapeutic targets that mimic the beneficial effects of calorie restriction has clinical importance. Here, using C. elegans as a model organism, we study the roles of a recently discovered enzyme at the heart of metabolism in mammalian cells, glycerol-3-phosphate phosphatase (G3PP) (gene name Pgp ) that hydrolyzes glucose-derived glycerol-3-phosphate to glycerol. We identify three Pgp homologues in C. elegans ( pgph ) and demonstrate in vivo that their protein products have G3PP activity, essential for glycerol synthesis. We demonstrate that PGPH/G3PP regulates the adaptation to various stresses, in particular hyperosmolarity and glucotoxicity. Enhanced G3PP activity reduces fat accumulation, promotes healthy aging and acts as a calorie restriction mimetic at normal food intake without altering fertility. Thus, PGP/G3PP can be considered as a target for age-related metabolic disorders. Glycerol-3-phosphate phosphatase is a recently discovered enzyme at the heart of metabolism. Here, the authors used C. elegans and showed that its activation promotes stress resistance, healthy aging and acts as a calorie restriction mimetic at normal food intake without altering fertility.

Fatty Acid Signaling in the β-Cell and Insulin Secretion Christopher J. Nolan 1 , Murthy S.R. Madiraju 2 , Viviane Delghingaro-Augusto 2 , Marie-Line Peyot 2 and Marc Prentki 2 1 Department of Endocrinology, The Canberra Hospital, and the Medical School, The Australian National University, Garran, Australia 2 Molecular Nutrition Unit and the Montreal Diabetes Research Center, University of Montreal, and the Centre Hospitalier de l’Université de Montréal, Montreal, Quebec, Canada Address correspondence and reprint requests to Marc Prentki, CR-CHUM, Pavillon de Sève Y-4603, 1560 Sherbrooke Est, Montreal, Quebec H2L 4M1, Canada. E-mail: marc.prentki{at}umontreal.ca Abstract Fatty acids (FAs) and other lipid molecules are important for many cellular functions, including vesicle exocytosis. For the pancreatic β-cell, while the presence of some FAs is essential for glucose-stimulated insulin secretion, FAs have enormous capacity to amplify glucose-stimulated insulin secretion, which is particularly operative in situations of β-cell compensation for insulin resistance. In this review, we propose that FAs do this via three interdependent processes, which we have assigned to a “trident model” of β-cell lipid signaling. The first two arms of the model implicate intracellular metabolism of FAs, whereas the third is related to membrane free fatty acid receptor (FFAR) activation. The first arm involves the AMP-activated protein kinase/malonyl-CoA/long-chain acyl-CoA (LC-CoA) signaling network in which glucose, together with other anaplerotic fuels, increases cytosolic malonyl-CoA, which inhibits FA partitioning into oxidation, thus increasing the availability of LC-CoA for signaling purposes. The second involves glucose-responsive triglyceride (TG)/free fatty acid (FFA) cycling. In this pathway, glucose promotes LC-CoA esterification to complex lipids such as TG and diacylglycerol, concomitant with glucose stimulation of lipolysis of the esterification products, with renewal of the intracellular FFA pool for reactivation to LC-CoA. The third arm involves FFA stimulation of the G-protein–coupled receptor GPR40/FFAR1, which results in enhancement of glucose-stimulated accumulation of cytosolic Ca 2+ and consequently insulin secretion. It is possible that FFA released by the lipolysis arm of TG/FFA cycling is partly “secreted” and, via an autocrine/paracrine mechanism, is additive to exogenous FFAs in activating the FFAR1 pathway. Glucose-stimulated release of arachidonic acid from phospholipids by calcium-independent phospholipase A 2 and/or from TG/FFA cycling may also be involved. Improved knowledge of lipid signaling in the β-cell will allow a better understanding of the mechanisms of β-cell compensation and failure in diabetes. AA, arachidonic acid AMPK, AMP-activated protein kinase CPT, carnitine palmitoyl-transferase DAG, diacylglycerol FA, fatty acid FFA, free fatty acid FFAR, free fatty acid receptor GSIS, glucose-stimulated insulin secretion HSL, hormone-sensitive lipase iPLA2, calcium-independent ATP-stimulated phospholipase A2 KATP channel, ATP-sensitive K+ channel LC-CoA, long-chain acyl-CoA MCD, malonyl-CoA decarboxylase RNAi, RNA interference TG, triglyceride Footnotes This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact. Accepted May 26, 2006. Received March 31, 2006. DIABETES

Glucagon-Like Peptide 1 Induces Pancreatic β-Cell Proliferation Via Transactivation of the Epidermal Growth Factor Receptor Jean Buteau , Sylvain Foisy , Erik Joly and Marc Prentki From the Molecular Nutrition Unit, Departments of Nutrition and Biochemistry, University of Montreal, the Centre de Recherche du CHUM and Institut du Cancer, Montreal, Quebec, Canada Abstract We previously provided evidence that glucagon-like peptide 1 (GLP-1) induces pancreatic β-cell growth nonadditively with glucose in a phosphatidylinositol (PI) 3-kinase- and protein kinase C ζ-dependent manner. However, the exact mechanism by which the GLP-1 receptor (GLP-1R), a member of the G protein-coupled receptor (GPCR) superfamily, activates the PI 3-kinase signaling pathway to promote β-cell growth remains unknown. We hypothesized that the GLP-1R could activate PI 3-kinase and promote β-cell proliferation through transactivation of the epidermal growth factor (EGF) receptor (EGFR), an event possibly linked to GPCRs via activation of c-Src and the production of putative endogenous EGF-like ligands. Both the c-Src inhibitor PP1 and the EGFR-specific inhibitor AG1478 blocked GLP-1-induced [ 3 H]thymidine incorporation in INS(832/13) cells as well as in isolated rat islets, while only AG1478 inhibited the proliferative action of betacellulin (BTC), an EGFR agonist. Both compounds also suppressed GLP-1-induced PI 3-kinase activation. A time-dependent increase in tyrosine phosphorylation of the EGFR in response to GLP-1 was observed in INS(832/13) cells. This transactivation of the EGFR was sensitive to both the pharmacological agents PP1 and AG1478. The action of GLP-1 and BTC on INS cell proliferation was found to be not additive. Overexpression of a dominant-negative EGFR in INS cells with a retroviral expression vector curtailed GLP-1-induced β-cell proliferation. GLP-1 treatment of INS cells caused a decrease in cell surface-associated BTC, as shown by FACS analysis. Also, the metalloproteinase inhibitor GM6001 and an anti-BTC neutralizing antibody suppressed the GLP-1 proliferative effect. Finally, coculturing the prostatic cancer cell line LNCaP that lacks GLP-1 responsiveness with INS cells increased LNCaP cell proliferation in the presence of GLP-1, thus revealing that INS cells secrete a growth factor in response to GLP-1. GM6001 and an anti-BTC neutralizing antibody suppressed increased LNCaP cell proliferation in the presence of GLP-1 in the coculture experiments. The results are consistent with a model in which GLP-1 increases PI 3-kinase activity and enhances β-cell proliferation via transactivation of the EGFR that would require the proteolytic processing of membrane-anchored BTC or other EGF-like ligands. Footnotes Address correspondence and reprint requests to Dr. Marc Prentki, CR-CHUM, Pavillon de Sève, Y-4603, 1560 Sherbrooke East, Montreal, PQ H2L 4M1, Canada. E-mail: marc.prentki{at}umontreal.ca . Received for publication 20 December 2001 and accepted in revised form 14 October 2002. BTC, betacellulin; EGF, epidermal growth factor; EGFR, EGF receptor; GLP-1, glucagon-like peptide 1; GLP-1R, GLP-1 receptor; GPCR, G protein-coupled receptor; KRBH, Krebs-Ringer bicarbonate HEPES; PI, phosphatidylinositol. DIABETES

Aims/hypothesis To directly assess the role of beta cell lipolysis in insulin secretion and whole-body energy homeostasis, inducible beta cell-specific adipose triglyceride lipase (ATGL)-deficient (B- Atgl- KO) mice were studied under normal diet (ND) and high-fat diet (HFD) conditions. Methods Atgl flox/flox mice were cross-bred with Mip-Cre -ERT mice to generate Mip-Cre -ERT /+ ; Atgl flox/flox mice. At 8 weeks of age, these mice were injected with tamoxifen to induce deletion of beta cell-specific Atgl (also known as Pnpla 2), and the mice were fed an ND or HFD. Results ND-fed male B- Atgl -KO mice showed decreased insulinaemia and glucose-induced insulin secretion (GSIS) in vivo. Changes in GSIS correlated with the islet content of long-chain saturated monoacylglycerol (MAG) species that have been proposed to be metabolic coupling factors for insulin secretion. Exogenous MAGs restored GSIS in B- Atgl -KO islets. B- Atgl -KO male mice fed an HFD showed reduced insulinaemia, glycaemia in the fasted and fed states and after glucose challenge, as well as enhanced insulin sensitivity. Moreover, decreased insulinaemia in B- Atgl- KO mice was associated with increased energy expenditure, and lipid metabolism in brown (BAT) and white (WAT) adipose tissues, leading to reduced fat mass and body weight. Conclusions/interpretation ATGL in beta cells regulates insulin secretion via the production of signalling MAGs. Decreased insulinaemia due to lowered GSIS protects B- Atgl- KO mice from diet-induced obesity, improves insulin sensitivity, increases lipid mobilisation from WAT and causes BAT activation. The results support the concept that fuel excess can drive obesity and diabetes via hyperinsulinaemia, and that an islet beta cell ATGL-lipolysis/adipose tissue axis controls energy homeostasis and body weight via insulin secretion.

Many metabolic studies employ tissue-specific gene knockout mice, which requires breeding of floxed gene mice, available mostly on C57BL/6N (NN) genetic background, with cre or Flp recombinase-expressing mice, available on C57BL/6J (JJ) background, resulting in the generation of mixed C57BL/6NJ (NJ) genetic background mice. Recent awareness of many genetic differences between NN and JJ strains including the deletion of nicotinamide nucleotide transhydrogenase (nnt), necessitates examination of the consequence of mixed NJ background on glucose tolerance, beta cell function and other metabolic parameters. Male mice with NN and NJ genetic background were fed with normal or high fat diets (HFD) for 12 weeks and glucose and insulin homeostasis were studied. Genotype had no effect on body weight and food intake in mice fed normal or high fat diets. Insulinemia in the fed and fasted states and after a glucose challenge was lower in HFD-fed NJ mice, even though their glycemia and insulin sensitivity were similar to NN mice. NJ mice showed mild glucose intolerance. Moreover, glucose- but not KCl-stimulated insulin secretion in isolated islets was decreased in HFD-fed NJ vs NN mice without changes in insulin content and beta cell mass. Under normal diet, besides reduced fed insulinemia, NN and NJ mice presented similar metabolic parameters. However, HFD-fed NJ mice displayed lower fed and fasted insulinemia and glucose-induced insulin secretion in vivo and ex vivo, as compared to NN mice. These results strongly caution against using unmatched mixed genetic background C57BL/6 mice for comparisons, particularly under HFD conditions.