Editorial Toward a Systems Biology of Secretion and Type 2 Diabetes Donald F. Steiner,1 Christian Boitard,2 Erol Cerasi,3 Suad Efendic,4 Jean-Claude Henquin,5 and Ele Ferrannini6

storage, and utilization of nutrients for either energy production or growth. lthough it is clear that ␤-cell pathology, in Appropriately, the symposium opened with an introduc- particular disordered insulin secretion, is a key tory lecture by Leopold, reviewing our current knowledge underlying pathogenetic feature of most forms of the control of metabolism and growth in the fruit fly, A of diabetes, complex interactions of the islets Drosophila melanogaster, by insulin-like and an with other organs, such as brain, , gut, and several insulin signaling pathway that is remarkably similar in peripheral tissues, are also essential for the normal inte- many of its components to that of man. No less than seven gration of metabolism. It is this area that has been Drosophila insulin-like peptides have been found. Three of explored in the Seventh Annual Servier-IGIS Meeting, these are expressed in specialized neurones in the insect which was held last Spring in St. Jean Cap Ferrat in brain, while others are expressed in larval tissues during Southern France. The focus of these meetings, since their development. Insulin-like peptides released from the brain inception in 2000, has been the ␤-cell and the mechanisms cells downregulate hemolymph sugar levels, while a glu- underlying its development and function as the source of cagon-like , the adipokinetic , from other insulin, the most essential regulator of the blood glucose neuroendocrine structures opposes it by raising sugar level. levels. Interestingly, these adipokinetic hormone-produc- The familiar pathways of glucose, lipid, and amino acid ing cells, unlike the insulin-producing cells, express an metabolism in humans and other mammals are, of course, ϩ ATP-sensitive K channel (KATP channel) that responds to fundamental to almost all organisms, except perhaps for tolbutamide to increase their secretory activity, resulting the most highly specialized bacteria, so it is not surprising in hyperglycemia. Moreover, mutations that influence that insulin-like molecules and the insulin/IGF receptor components of the pathway result in signaling system are well conserved features of all meta- reduced growth, reflected in smaller cells in smaller zoans that have been studied. With the rise of multicellular adults. Impaired insulin signaling also results in increased organisms in evolution came the need to regulate and longevity, decreased reproduction, and increased stress coordinate metabolism and growth in order to maintain resistance. Complex interactions with other endocrine both the constancy of the internal environment (ho- (ecdysone, juvenile hormone) and metabolic (TOR, etc.) meostasis) and also to respond to the external environ- pathways integrate nutrition and metabolism and eventual ment. One of the most prominent environmental stimuli organismal size. The powerful genetic tools available for had to be the availability of nutrients and fuels for survival Drosophila make it a rich experimental source for identi- and growth. The insulin-like , insulin and IGF, fication of new components of these important conserved appear to have evolved, along with a panoply of other pathways. regulatory substances, to fine-tune the efficient uptake, Much recent work has focused on the role of lipids as modulators of insulin action and as important factors in the pathogenesis of type 2 diabetes via induction of insulin From the 1Departments of Biochemistry and Medicine, University of Chicago, resistance. Shulman and coworkers have used magnetic Chicago, Illinois; 2Institut National de la Sante´ et de la Recherche Me´dicale resonance spectroscopy to explore the molecular mecha- U561, St. Vincent de Paul Hospital, Paris, France; the 3Department of nisms underlying defective glucose transport and glycogen Endocrinology and Metabolism, Hebrew University Hadassah Medical Center, Jerusalem, Israel; the 4Department of Molecular Medicine, Division of Endo- metabolism in muscle. Increased lipid metabolites such as crinology & Diabetes, Karolinska Hospital, Stockholm, Sweden; the 5Unit of fatty acyl-CoAs and diacylglycerol activate kinase cas- Endocrinology and Metabolism, University of Louvain, Brussels, Belgium; and cades that impair insulin signaling due to Ser/Thr phos- the 6Metabolism Unit, CNR Institute of Clinical Physiology, University of Pisa, Pisa, Italy. phorylation of IRS-1. Similar mechanisms may operate to Address correspondence and reprint requests to Donald F. Steiner. E-mail: impair hepatic insulin signaling due to increased hepatic [email protected]. lipids in insulin-resistant subjects. These changes then CNS, central nervous system; FFA, free fatty acid; FFAR, free fatty acid receptor; GBS, gastric bypass surgery; GIP, gastric inhibitory polypeptide; lead to relative increases in gluconeogenesis and reduced GLP, -like peptide; HGP, hepatic glucose production; IL, interleukin; hepatic glucose uptake via lowered AKT2 and increased ϩ KATP channel, ATP-sensitive K channel. FOXO transcriptional effects on several key gluconeogenic The symposium and the publication of this editorial have been made rate-controlling enzymes. In related magnetic resonance possible by an unrestricted educational grant from Servier, Paris. DOI: 10.2337/db06-S000 spectroscopy studies, significant decreases in mitochon- © 2006 by the American Diabetes Association. drial oxidative phosphorylation activity in muscles and 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 liver have been found in elderly, lean, insulin-resistant with 18 U.S.C. Section 1734 solely to indicate this fact. volunteers, in association with increased muscle and he-

DIABETES, VOL. 55, SUPPLEMENT 2, DECEMBER 2006 S1 D.F. STEINER AND ASSOCIATES patic lipid content. Similar changes were found in young, actions, is released in large amounts during exercise. lean offspring of parents with type 2 diabetes associated Increases in IL-6 production and secretion are associated with reduced mitochondrial density, consistent with other with increases in AMPK activity in tissues such as muscle reported studies. The mechanisms underlying the reduc- and . AMPK enhances ATP generation while tion in mitochondrial biogenesis in these individuals is an inhibiting nonessential energy consuming processes via important area for further study that may lead to new phosphorylation of selected metabolic enzymes. Ruder- targets for therapeutic intervention. man and colleagues have demonstrated decreased AMPK While lipid overload can lead to insulin resistance and activity in muscle and adipose tissue in young IL-6–null impaired ␤-cell function, fatty acids and other lipids are mice and a diminished enzyme response to exercise in also important for normal ␤-cell function. It is well estab- these tissues. These animals later develop manifestations lished that fatty acids can augment glucose-stimulated of the metabolic syndrome with obesity, dyslipidemia, and insulin secretion, an effect that may be especially impor- impaired glucose tolerance. Key questions are whether tant in meeting the demands for increased insulin in these effects of IL-6 contribute to the reported benefits of compensated insulin resistance. Lipids can act both exercise in reducing the prevalence of type 2 diabetes, through their metabolism as well as via free fatty acid coronary atherosclerosis, and other concomitants of the (FFA) receptor (FFAR) activation. Prentki and associates metabolic syndrome in humans. have studied these mechanisms in detail and find that In liver, as in the ␤-cell, glucokinase plays a key role as increased cytosolic malonyl-CoA arising from glucose and a glucose sensor. However, the complex conformational lipid metabolism acts via AMPK/malonyl-CoA pathways to states and regulatory networks that control glucokinase limit fatty acid oxidation, thus increasing long-chain acyl- function differ significantly in these two tissues. As dis- CoA signaling molecules. Glucose can then enhance ester- cussed by Baltrusch and Tiedge, these range from the use ification and subsequent lipolysis of long-chain acyl-CoA of alternate promoters to regulate enzyme expression and to renew the FFA pool, which can then interact with the shuttling of a high-affinity regulatory between FFAR/GPR40, enhancing cytosolic Ca2ϩ and insulin secre- cytosol and nucleus in the liver to altered compartmental- tion. Glucose may also enhance release of arachidonic ization of glucokinase in ␤-cells and its activation by acid from phospholipids to activate yet other lipid-signal- binding of the bifunctional enzyme 6-phosphofructo-2- ing pathways in the ␤-cell. kinase/fructose-2,6-biphosphatase (PFK-2/FBPase-2) to in- Since many of the foregoing effects of lipids on insulin crease its Vmax. Some of these effects, and those of secretion depend on glucose-stimulated lipolytic activity, chemical glucokinase activators, are related to various efforts are currently underway to identify ␤-cell lipases. conformational states with altered catalytic activity, as Indeed, orlistat, a lipase inhibitor, abolishes lipolysis of tri- revealed in recent crystallographic studies. and diglycerides in islets, inhibiting insulin secretion with- This session ended with consideration of the time- out perturbing glucose metabolism. Mulder and colleagues honored conundrum as to whether the inhibitory effect of have examined hormone-sensitive lipase–null mice but insulin on hepatic glucose production (HGP) is direct or find no evidence of a ␤-cell secretory effect. Thus, further indirect. Girard carefully reviews the various known indi- studies to identify the role of other lipases involved in rect influences on HGP, which include suppression of ␤-cell stimulus-secretion coupling are needed. glucagon levels, plasma nonesterified fatty acid or glu- In addition to substrates such as FFA, various adipo- coneogenic precursors from peripheral tissues, along with kines such as tumor necrosis factor-␣ and are more recent studies on effects of various adipokines, as associated with obesity and insulin resistance, whereas well as novel central mechanisms. Infusion of insulin into others such as and sensitize the body to the third ventricle has been shown to inhibit HGP, an insulin. Adiponectin has been shown to be upregulated by effect reversed by inhibition of insulin receptor signaling. thiazolidinediones acting through peroxisome prolifera- Surprisingly, central infusion of activators of KATP channel tor–activated receptor-␥. As reviewed by Kadowaki and lowered blood glucose levels by inhibiting HGP, while associates, adiponectin circulates in several multimeric KATP inhibitors reduced the effects of systemic insulin. forms, of which the high–molecular weight forms are most These effects are mediated via the hepatic branch of the active in ameliorating insulin resistance through negative vagus nerve. However, recent studies with mice lacking effects on hepatic gluconeogenesis and lowering of FFA expression of insulin receptor only in liver, as well as through stimulation of skeletal muscle FFA oxidation. He clamp studies on dogs, support direct effects of insulin as and his associates have carefully dissected the effects of being of more paramount importance in suppressing HGP. pioglitazone dosage on ob/ob and adipoϪ/Ϫ ob/ob mice to Girard proposes that the relative importance of glycogen- demonstrate both adiponectin-dependent and -indepen- olysis versus gluconeogenesis in various experimental dent pathways of thiazolidinedione action on such param- protocols may account in part for various reported species eters as adipocyte size and adiponectin production, as well differences in sensitivity to direct versus indirect effects. as target organ effects on hepatic AMPK activation and Moreover, as gluconeogenesis is less sensitive to suppres- decreased gluconeogenesis, leading to improved glucose sion by insulin than glycogenolysis in type 2 diabetes, tolerance and diabetes control. therapeutic agents that suppress glucagon hypersecretion, In a session on muscle and liver, Newsholme and such as glucagon-like peptide (GLP)-1 and/or others, colleagues discussed the effects of amino acids on key should likely be clinically beneficial for lowering HGP. ␤-cell processes leading to enhanced insulin secretion, The gastrointestinal tract has recently proven to be a with special emphasis on generation of ATP and the rich source of promising endocrine substances with an- mechanisms coupling amino acid metabolic pathways tidiabetic effects. GLP-1, a product of the glucagon , is with the putative generation of messengers of mitochon- produced in the intestinal L-cells through the action on drial origin. proglucagon of prohormone convertase (PC)1/3, in con- Turning to factors arising from muscle, interleukin trast to the islet ␣-cells, where PC2 acts to process mainly (IL)-6, a cytokine having both pro- and anti-inflammatory glucagon from the same precursor. GLP-1 has emerged as

S2 DIABETES, VOL. 55, SUPPLEMENT 2, DECEMBER 2006 EDITORIAL an important hormone, augmenting the effects of both islet cell mass and function in normal and pathologic oral nutrient stimuli on insulin release. The factors that states. regulate GLP-1 secretion are clearly of considerable inter- Another aspect of extrinsic inputs into glucose sensing est, in view of its recently demonstrated efficacy in treating in the regulation of insulin secretion has been explored type 2 diabetes. Its actions include potentiation of insulin recently by Thorens and associates who have developed secretion in response to glucose, enhancement of ␤-cell ingenious methods to identify and study extrapancreatic growth and survival, and inhibition of glucagon secretion, glucose sensors in both the hepatoportal vein and the gastric emptying, and food intake. Both nutrients and central nervous system (CNS). The hepatoportal sensor is various non-nutrient peptides and neuromodulators have activated by glucose gradients between the portal vein and been implicated in GLP-1 release, as discussed in reviews peripheral veins and transmits signals via afferent by Brubaker and Reimann and coworkers in this session. branches of the hepatic vagal nerve to the CNS. It affects Clearly, elucidation of these pathways could lead to the both first-phase insulin secretion and peripheral insulin development of therapeutic GLP-1 secretogogues. sensitivity via a variety of pathways. Both extrapancreatic Gastric inhibitory polypeptide or glucose-dependent in- sensors are dependent on GLUT2 expression for their sulinotropic peptide (GIP), another member of the gluca- normalizing effects, including those on feeding behavior gon family produced in the intestinal K-cells, exerts and glucagon secretion. Identification of their precise incretin effects, both directly on the ␤-cell and via augmen- localization may provide new insights into the physiology tation of GLP-1 secretion and/or action. Seino and collab- of energy intake and metabolism. orators have studied effects of GIP receptor knockout in In addition to peripheral glucose sensors, the brain has mice and found reduced incretin effects on glucose-in- emerged as a central switchboard, integrating signals from duced insulin secretion, a defect that is additive with that many regions of the body (liver, gut, adipose tissue, and induced by GLP-1 receptor knockout. These investigators islets) conveyed by afferent nerves and neurotransmitters, as well as by circulating hormones and the major nutrients have also documented extra-pancreatic effects of GIP on (glucose, amino acids, and free fatty acids) to regulate the accumulation of fat in adipose tissue and of calcium energy homeostasis, food intake, reproduction, and even into bone, indicating a broader role for this gut-derived learning and memory. The reports in this section are peptide in regulating nutrient uptake. These effects sug- focused on various aspects of this expanding area of great gest that GIP is the product of a “thrifty gene” and thus current interest. may contribute to the incidence of obesity and diabetes. Needless to say, insulin and leptin are key players in this Crossing GIP receptor–null mice with leptin-deficient informational game, and they act primarily via receptors ob/ob mice resulted in significant amelioration of obesity localized in hypothalamic centers such as the arcuate and dyslipidemia, accompanied by increased insulin sen- nucleus that control feeding behavior, satiety, and energy sitivity and glucose tolerance. The authors conclude that expenditure. Elevated insulin and leptin signal the avail- effects of both GIP receptor agonists and/or antagonists ability of excess nutrients and thus tend to reduce food may provide beneficial effects in certain forms of diabetes. intake and enhance energy expenditure, as discussed by Another approach to therapy of obesity and/or type 2 Wood and his coworkers. They point out that insulin with diabetes is gastric bypass surgery (GBS) and related its rapid fluctuations in secretion and short half-life is a surgical procedures. Naslund and Kral review the effects monitor of blood glucose and ongoing metabolism, as well of GBS with special emphasis on its effects on gastroin- as of body adiposity, while leptin’s longer half-life and testinal peptide levels, including and three incretin secretion from adipocytes conveys information on both fat hormones—GLP-1, GLP-2, and peptide YY. This approach stores and adipocyte metabolic activity. Centers in the to therapy in a majority of cases is curative and results arcuate nucleus regulate satiety and respond to these generally in reduced ghrelin levels and enhanced incretin anorexigenic signals by downregulating secretion of orexi- effects, which likely contribute to the favorably altered genic factors such as Y and agouti-related physiologic state induced by GBS. Alterations in adipo- peptide and upregulating the release of other factors kines and neuroregulatory circuits may also contribute to promoting anorexia, such as ␣-MSH (melanocortin) and its positive effects. CART (cocaine- and amphetamine-related transcript). The islets are the focus of a larger number of peripheral Other peripheral orexigenic hormones, especially ghrelin, and central inputs in addition to those discussed above. are released from the and upper gut before The islets are richly innervated by both parasympathetic mealtimes and tend to oppose waning insulin/leptin ac- and sympathetic nerve fibers, which act through the tions in the , stimulating new cycles of classical neurotransmitters acetylcholine and norepineph- feeding. Woods et al. also discuss possible consequences rine, respectively, which exert either stimulatory or inhib- of insulin signaling in the hippocampus in relation to itory effect on the ␤-cells. A variety of neuroregulatory cognitive function, pointing out the possibility that en- peptides also modulate both insulin and glucagon secre- hanced learning may facilitate foraging for food. Olfactory tion. The latter are the major focus of a comprehensive insulin signaling may likewise enhance associations be- review by Ahren and coworkers. The possibility of altered tween certain foods and specific odors. Clearly, the devel- islet innervation in various models of insulin resistance opment of obesity and insulin resistance may also occur and type 2 diabetes has been investigated by these authors centrally, resulting in impairment of normal regulatory in several animal models of diabetes (GK rats and db/db processes, as well as cognitive functions. mice) with findings indicative of increased islet innerva- Levin and coworkers go on to consider in greater detail tion. They propose that “augmented expression of neuro- the properties of glucose-sensing neurons, both inhibitory transmitters in the islets is a sign of islet adaptation for and excitatory. In early classical studies of the hypothala- normalization of glucose tolerance” and conclude that mus using lesions or electrical stimulation, certain areas further exploration of this area may yield new insights into were associated with food intake and energy expenditure. neural mechanisms that contribute to the regulation of Lesions of the ventromedial hypothalamus led to hy-

DIABETES, VOL. 55, SUPPLEMENT 2, DECEMBER 2006 S3 D.F. STEINER AND ASSOCIATES perphagia and obesity, while lesions in the lateral hypo- central fuel sensing. Thus, central dysregulation of fatty thalamic area reduced food intake and increased acid signaling could be a factor leading to impaired autonomic activity, leading to lower body weight. Glucose- glucoregulation of insulin secretion. sensing neurons are widely distributed in forebrain and Ahima and coworkers review the central effects of brainstem nuclei, where they integrate both “hard-wired” adipocytokines on metabolism and energy homeostasis, inputs from the periphery with hormonal, neuropeptide, focusing mainly on leptin and adiponectin. A great deal is and substrate signals. These populations appear to ex- now known about their mechanisms of production, their press glucose-sensing systems, such as glucokinase and various circulating plasma forms, and their putative recep- ␤ ␣ KATP channels, similar to those of normal - and -cells. tors and signaling pathways. Leptin normally acts to They respond to other stimuli such as lactate from glial prevent the negative changes associated with starvation cells, fatty acids, and ketone bodies, as well as to both and weight reduction, including reduced energy expendi- insulin and leptin. Their output, in combination with other ture, insulin resistance, hyperlipidemia, and fertility. Cen- glucose-sensing neurones, is via a variety of efferent neural tral leptin administration in ob/ob mice suppresses hepatic pathways that impact all aspects of energy homeostasis— glucose production followed by food intake and weight intake, storage, and expenditure. loss, while restoring insulin sensitivity. Both leptin and An interesting related issue, reviewed by Pe´nicaud and adiponectin act peripherally to increase AMP-activated colleagues, is the discovery of the expression of the protein kinase and other enzymes involved in lipid metab- translocatable glucose transporter GLUT4 and more re- olism in liver and muscle, as well as centrally. The mode of cently the related transporter, GLUT8, in the brain. These entry of adiponectin into the CNS remains unsettled, but appear to function by translocation from intracellular adiponectin receptors are present in cerebral microves- pools to the plasma membrane, and their expression has sels. These issues and a variety of other adipocytokines been reported to be influenced by glucose and insulin, but and putative hormones produced by fat cells remain to be whether they respond to insulin in a classical manner is still controversial. GLUT8 has also been noted to translo- studied in order to clarify their roles in human metabolic cate to the endoplasmic reticulum, suggesting a possible pathophysiology. role in glycoprotein biosynthesis and/or degradation. Gene The conference concluded with an excellent personal disruption strategies are needed now to shed new light on overview by Porte of the history and development of our the potential physiological roles of these transporters in current concepts regarding the central regulation of en- regulating the brain’s metabolism and/or signaling functions. ergy homeostasis and the key role of insulin in this process In addition to glucose, FFAs are well known to influence as they have developed during the past 30 years of his carbohydrate metabolism and energy homeostasis via research. He and his colleagues, as well as many others central mechanisms. This area is nicely reviewed by drawn into this field, have made great strides in elucidat- Magnan and coworkers. These authors have previously ing the mechanisms by which insulin and other more shown that infusion of lipids such as oleic acid centrally in recently discovered hormones/cytokines enter and act in rats leads to inhibition of food intake and glucose produc- the brain to regulate and integrate the various organ tion, while infusion of triglycerides for 24 h leads to systems of the body that must all work harmoniously hepatic insulin resistance associated with increased glu- together to maintain an optimal normal state. Porte con- cose-stimulated insulin secretion and accompanied by cludes that “the overwhelming evidence that insulin plays decreased splanchnic sympathetic nerve activity, the latter a key dual role in the regulation of carbohydrate metabo- effects being dependent on ␤-oxidation of the substrate. lism and body weight suggests that further analysis of its Transient increases in plasma insulin could be induced by CNS effects will continue to be a fruitful area for study and a single intracarotid injection of oleic acid without potentially therapeutic intervention.” changes in plasma glucose, indicating a direct effect of We are indebted to the Secretary of the IGIS group, Dr. FFAs on neural control of insulin secretion. These authors Alain Ktorza, and to Laurence Alliot’s team at Servier for have also provided evidence for the existence of hypotha- their outstanding assistance with the organization of the lamic subpopulations of neurones that are either excited Symposium, as well as to Catriona Donagh for the editorial or inhibited by FFAs in vitro, consistent with their role in management of this supplement.

S4 DIABETES, VOL. 55, SUPPLEMENT 2, DECEMBER 2006