Target For Treatment Of Obesity Research Articles

Lessons from Leptins Molecular Biology: Potential Therapeutic Actions of Recombinant Leptin and Leptin-Related Compounds

Leptin, a peptide secreted by the white adipose tissue, circulates to the central nervous system and signals the status of body energy stores, regulating feeding behavior and energy balance. As human obesity is characterized by hyperleptinemia and leptin resistance, increasing leptin sensitivity is an attractive target for obesity treatment.

Saturated fatty acids inhibit induction of insulin gene transcription by JNK-mediated phosphorylation of insulin-receptor substrates

JNKs are attractive targets for treatment of obesity and type-2 diabetes. A sustained increase in JNK activity was observed in dietary and genetic models of obesity in mice, whereas JNK deficiency prevented obesity-induced insulin resistance. A similar insulin-sensitizing effect was seen upon treatment of obese mice with JNK inhibitors. We now demonstrate that treatment with the saturated fatty acid palmitic acid results in sustained JNK activation and insulin resistance in primary mouse hepatocytes and pancreatic beta-cells. In the latter, palmitic acid treatment inhibits glucose-induced insulin gene transcription, in part, by interfering with autocrine insulin signaling through phosphorylation of insulin-receptor substrates 1 and 2 at sites that interfere with binding to activated insulin receptors. This mechanism may account for the induction of central insulin resistance by free fatty acids.

Role of neuropeptides in appetite regulation and obesity – A review

Obesity represents the most prevalent nutritional problem worldwide which in the long run predisposes to development of diabetes mellitus, hypertension, endometrial carcinoma, osteoarthritis, gall stones and cardiovascular diseases. Despite significant reductions in dietary fat consumption, the prevalence of obesity is on a rise and is taking on pandemic proportions. Obesity develops when energy intake exceeds energy expenditure over time. Recently, a close evolutionary relationship between the peripheral and hypothalamic neuropeptides has become apparent. The hypothalamus being the central feeding organ mediates regulation of short-term and long-term dietary intake via synthesis of various orexigenic and anorectic neuropeptides. The structure and function of many hypothalamic peptides (neuropeptide Y (NPY), melanocortins, agouti-related peptide (AGRP), cocaine and amphetamine regulated transcript (CART), melanin concentrating hormone (MCH), orexins have been characterized in rodent models The peripheral neuropeptides such as cholecystokinin (CCK), ghrelin, peptide YY (PYY3-36), amylin, bombesin regulate important gastrointestinal functions such as motility, secretion, absorption, provide feedback to the central nervous system on availability of nutrients and may play a part in regulating food intake. The pharmacological potential of several endogenous peripheral peptides released prior to, during and/or after feeding are being explored. Long-term regulation is provided by the main circulating hormones leptin and insulin. These systems implicated in hypothalamic appetite regulation provide potential targets for treatment of obesity which could potentially pass into clinical development in the next 5 years. This review summarizes various effects and interrelationship of these central and peripheral neuropeptides in metabolism, obesity and their potential role as targets for treatment of obesity.

Neuropeptide Y in normal eating and in genetic and dietary-induced obesity

Neuropeptide Y (NPY) is one the most potent orexigenic peptides found in the brain. It stimulates food intake with a preferential effect on carbohydrate intake. It decreases latency to eat, increases motivation to eat and delays satiety by augmenting meal size. The effects on feeding are mediated through at least two receptors, the Y1 and Y5 receptors. The NPY system for feeding regulation is mostly located in the hypothalamus. It is formed of the arcuate nucleus (ARC), where the peptide is synthesized, and the paraventricular (PVN), dorsomedial (DMN) and ventromedial (VMN) nuclei and perifornical area where it is active. This activity is modulated by the hindbrain and limbic structures. It is dependent on energy availability, e.g. upregulation with food deprivation or restriction, and return to baseline with refeeding. It is also sensitive to diet composition with variable effects of carbohydrates and fats. Leptin signalling and glucose sensing which are directly linked to diet type are the most important factors involved in its regulation. Absence of leptin signalling in obesity models due to gene mutation either at the receptor level, as in the Zucker rat, the Koletsky rat or the db/db mouse, or at the peptide level, as in ob/ob mouse, is associated with increased mRNA abundance, peptide content and/or release in the ARC or PVN. Other genetic obesity models, such as the Otsuka-Long-Evans-Tokushima Fatty rat, the agouti mouse or the tubby mouse, are characterized by a diminution in NPY expression in the ARC nucleus and by a significant increase in the DMN. Further studies are necessary to determine the exact role of NPY in these latter models. Long-term exposure to high-fat or high-energy palatable diets leads to the development of adiposity and is associated with a decrease in hypothalamic NPY content or expression, consistent with the existence of a counter-regulatory mechanism to diminish energy intake and limit obesity development. On the other hand, an overactive NPY system (increased mRNA expression in the ARC associated with an upregulation of the receptors) is characteristic of rats or rodent strains sensitive to dietary-induced obesity. Finally, NPY appears to play an important role in body weight and feeding regulation, and while it does not constitute the only target for drug treatment of obesity, it may nevertheless provide a useful target in conjunction with others.

The efficiency and plasticity of mitochondrial energy transduction

Since it was first realized that biological energy transduction involves oxygen and ATP, opinions about the amount of ATP made per oxygen consumed have continually evolved. The coupling efficiency is crucial because it constrains mechanistic models of the electron-transport chain and ATP synthase, and underpins the physiology and ecology of how organisms prosper in a thermodynamically hostile environment. Mechanistically, we have a good model of proton pumping by complex III of the electron-transport chain and a reasonable understanding of complex IV and the ATP synthase, but remain ignorant about complex I. Energy transduction is plastic: coupling efficiency can vary. Whether this occurs physiologically by molecular slipping in the proton pumps remains controversial. However, the membrane clearly leaks protons, decreasing the energy funnelled into ATP synthesis. Up to 20% of the basal metabolic rate may be used to drive this basal leak. In addition, UCP1 (uncoupling protein 1) is used in specialized tissues to uncouple oxidative phosphorylation, causing adaptive thermogenesis. Other UCPs can also uncouple, but are tightly regulated; they may function to decrease coupling efficiency and so attenuate mitochondrial radical production. UCPs may also integrate inputs from different fuels in pancreatic beta-cells and modulate insulin secretion. They are exciting potential targets for treatment of obesity, cachexia, aging and diabetes.

Fatty acid metabolism as a target for obesity treatment

Although metabolites and energy balance have long been known to play roles in the regulation of food intake, the potential role of fatty acid metabolism in this process has been considered only recently. Fatty acid synthase (FAS) catalyzes the condensation of acetyl-CoA and malonyl-CoA to generate long-chain fatty acids in the cytoplasm, while the breakdown of fatty acids (β-oxidation) occurs in mitochondria and is regulated by carnitine palmitoyltransferase-1 (CPT-1), the rate-limiting step for the entry of fatty acids into the mitochondria. Inhibition of FAS using cerulenin or synthetic FAS inhibitors such as C75 reduces food intake and induces profound reversible weight loss. Subsequent studies reveal that C75 also stimulates CPT-1 and increases β-oxidation. Hypotheses as to the mechanisms by which C75 and cerulenin mediate their effects have been proposed. Centrally, these compounds alter the expression profiles of feeding-related neuropeptides, often inhibiting the expression of orexigenic peptides. Whether through centrally mediated or peripheral mechanisms, C75 also increases energy consumption, which contributes to weight loss. In vitro and in vivo studies demonstrate that at least part of C75's effects is mediated by modulation of AMP-activated protein kinase (AMPK), a known peripheral energy-sensing kinase. Collectively, these data suggest a role for fatty acid metabolism in the perception and regulation of energy balance.

Stearoyl‐CoA desaturase as a new drug target for obesity treatment

Stearoyl-CoA desaturase (SCD), the rate-limiting enzyme in monounsaturated fatty acid synthesis, has recently been shown to be the critical control point regulating hepatic lipogenesis and lipid oxidation. As several manifestations of the metabolic syndrome and type 2 diabetes mellitus are associated with alterations in intracellular lipid partitioning, we propose that SCD1 may be a potential therapeutic target in the treatment of obesity and the metabolic syndrome. In support of this notion, we have shown that SCD1-deficient mice have increased energy expenditure, reduced body adiposity, increased insulin sensitivity and are resistant to diet-induced obesity and liver steatosis. Furthermore, SCD1 was found to be specifically repressed during leptin-mediated weight loss, and leptin-deficient ob/ob mice lacking SCD1 showed marked correction of the hypometabolic phenotype and hepatic steatosis. Much evidence indicates that the direct anti-steatotic effect of SCD1 deficiency stems from increased fatty acid oxidation and decreased lipid synthesis. All of these findings reveal that pharmacological manipulation of SCD activity might be of benefit in the treatment of obesity, diabetes, liver steatosis and other diseases of the metabolic syndrome.

Apolipoprotein C3 Deficiency Results in Diet-Induced Obesity and Aggravated Insulin Resistance in Mice

Our aim was to study whether the absence of apolipoprotein (apo) C3, a strong inhibitor of lipoprotein lipase (LPL), accelerates the development of obesity and consequently insulin resistance. Apoc3(-/-) mice and wild-type littermates were fed a high-fat (46 energy %) diet for 20 weeks. After 20 weeks of high-fat feeding, apoc3(-/-) mice showed decreased plasma triglyceride levels (0.11 +/- 0.02 vs. 0.29 +/- 0.04 mmol, P < 0.05) and were more obese (42.8 +/- 3.2 vs. 35.2 +/- 3.3 g; P < 0.05) compared with wild-type littermates. This increase in body weight was entirely explained by increased body lipid mass (16.2 +/- 5.9 vs. 10.0 +/- 1.8 g; P < 0.05). LPL-dependent uptake of triglyceride-derived fatty acids by adipose tissue was significantly higher in apoc3(-/-) mice. LPL-independent uptake of albumin-bound fatty acids did not differ. It is interesting that whole-body insulin sensitivity using hyperinsulinemic-euglycemic clamps was decreased by 43% and that suppression of endogenous glucose production was decreased by 25% in apoc3(-/-) mice compared with control mice. Absence of apoC3, the natural LPL inhibitor, enhances fatty acid uptake from plasma triglycerides in adipose tissue, which leads to higher susceptibility to diet-induced obesity followed by more severe development of insulin resistance. Therefore, apoC3 is a potential target for treatment of obesity and insulin resistance.

Discovery of Bicycloalkyl Urea Melanin Concentrating Hormone Receptor Antagonists: Orally Efficacious Antiobesity Therapeutics

Melanin concentrating hormone (MCH) is involved in regulation of food intake and energy homeostasis. Antagonists of the MCH receptor are expected to affect food intake and weight gain, making MCH-R1 an attractive target for obesity treatment. Herein, we report the discovery of a novel, orally active series of MCH-R1 antagonists that exhibit in vivo efficacy in rodent obesity models.

The corticotropin-releasing factor system as a potential target for antiobesity drugs

Corticotropin-releasing factor (CRF)-related peptides are involved in numerous physiological and behavioral actions, including activation of the pituitary-adrenal axis, stimulation of anxiety-related behaviors and modulation of cardiovascular and gastrointestinal functions. They are also capable of strong anorectic and thermogenic effects. In fact, the CRF system, which promotes a negative energy profile upon activation, could represent a potential target for the pharmacological treatment of obesity. The recent identification of two endogenous ligands for the CRF(2) receptor, urocortins 2 and 3, and the development of selective CRF receptor antagonists, has paved the way for improving our understanding of the specific physiological roles played by each CRF receptor. Based on recent progress, we conclude that the CRF(2) receptor could be a potential target for the development of an antiobesity drug.

Central pre-proglucagon derived peptides: opportunities for treatment of obesity.

Modern societies have moved from famine to feast and obesity and its co-morbidities now sweep the world as a global epidemic. Numerous scientific laboratories and pharmaceutical companies have taken the challenge and are now exploiting novel molecular targets for treatment of obesity. The pre-proglucagon system constitutes interesting candidates as potential targets for new anti-obesity drugs. In the periphery, pre-proglucagon derived peptides, Glucagon-Like Peptide-1 (GLP-1), Glucagon-Like Peptide-2 (GLP-2) and oxyntomodulin (OXM) are involved in a wide variety of physiological functions, including glucose homeostasis, gastric emptying, intestinal growth, insulin secretion as well as the regulation of food intake. Peripheral administration of GLP-1 derivatives and analogues to both rodents and man have shown promising effects on food intake and body weight suggesting that such therapies constitute potential anti-obesity treatment. In the central nervous system, pre-proglucagon and hence GLP-1, GLP-2 and OXM are exclusively found in a small population of nerve cells in the nucleus of the solitary tract. These constitute a neural pathway linking the "viscero-sensory" brainstem to hypothalamic nuclei involved in energy homeostasis. Intracerebroventricular administration of all of the three derived peptides robustly decrease food intake. It is evident that central GLP-1 agonism probably in combination with GLP-2 and/or OXM agonism constitute a potential pharmacological tool to reduce food intake and maybe also enhance energy expenditure. This and other aspects of the current state of the role of central pre-proglucagon in energy homeostasis are reviewed.

CCK1R agonists: a promising target for the pharmacological treatment of obesity.

Almost 30 years have passed since Gibbs, Young, and Smith demonstrated the ability of exogenously administered cholecystokinin (CCK) to inhibit food intake in rats. This observation was the beginning of very extensive studies into the role CCK plays in the regulation of food intake in mammals. CCK is a brain-gut peptide, which exists in multiple forms. CCK peptides exert their action on two distinct receptor subtypes: CCK-A (Alimentary) now called the CCK1R, mostly expressed peripherally; and CCK-B (Brain), renamed the CCK2R, which is primarily present in the brain. Through the use of subtype-selective agonists and antagonists for the CCK receptor, it was determined that the effect of CCK on feeding was dependent on agonist induced activation of peripheral CCK1 receptors. This discovery was followed by intense research with the goal of identifying small molecule agonists on the CCK1 receptor as potentially useful agents for the treatment of obesity. This review will attempt to summarize the results of this research.

Peroxisome-Proliferator-Activated Receptor δ Activates Fat Metabolism to Prevent Obesity

In contrast to the well-established roles of PPARγ and PPARα in lipid metabolism, little is known for PPARδ in this process. We show here that targeted activation of PPARδ in adipose tissue specifically induces expression of genes required for fatty acid oxidation and energy dissipation, which in turn leads to improved lipid profiles and reduced adiposity. Importantly, these animals are completely resistant to both high-fat diet-induced and genetically predisposed ( Lepr db/db ) obesity. As predicted, acute treatment of Lepr db/db mice with a PPARδ agonist depletes lipid accumulation. In parallel, PPARδ-deficient mice challenged with high-fat diet show reduced energy uncoupling and are prone to obesity. In vitro, activation of PPARδ in adipocytes and skeletal muscle cells promotes fatty acid oxidation and utilization. Our findings suggest that PPARδ serves as a widespread regulator of fat burning and identify PPARδ as a potential target in treatment of obesity and its associated disorders.

The role of uncoupling protein 3 in human physiology.

Obesity is simply understood as an imbalance between energy intake and expenditure in favor of weight accretion. However, the human biological interface between food consumption and energy dissipation results in broad individual differences in eating behavior, physical activity, and efficiency of fuel storage and metabolism. In particular, the basal metabolic rate, which accounts for the greatest portion of overall energy expenditure, can vary almost twofold among individuals. Classically, three major biochemical systems are believed to contribute to basal thermogenesis: futile cycles, Na+/K+ATPase activity, and mitochondrial proton leak. The latter is the most important quantitative contributor and can explain up to 50% of the basal metabolic rate (1). The molecular basis of mitochondrial proton leak is unclear, despite its importance in the understanding of energy balance and its potential as a therapeutic target for obesity treatment. The article by Hesselink and colleagues in this issue of the JCI (2) addresses whether uncoupling protein 3 contributes to mitochondrial proton leak in human skeletal muscle.

Protection against diet-induced obesity and obesity- related insulin resistance in Group 1B PLA2-deficient mice.

Group 1B phospholipase A2 (PLA2) is an abundant lipolytic enzyme that is well characterized biochemically and structurally. Because of its high level of expression in the pancreas, it has been presumed that PLA2 plays a role in the digestion of dietary lipids, but in vivo data have been lacking to support this theory. Our initial study on mice lacking PLA2 demonstrated no abnormalities in dietary lipid absorption in mice consuming a chow diet. However, the effects of PLA2 deficiency on animals consuming a high-fat diet have not been studied. To investigate this, PLA2(+/+) and PLA2(-/-) mice were fed a western diet for 16 wk. The results showed that PLA2(-/-) mice were resistant to high-fat diet-induced obesity. This observed weight difference was due to decreased adiposity present in the PLA2(-/-) mice. Compared with PLA2(+/+) mice, the PLA2(-/-) mice had 60% lower plasma insulin and 72% lower plasma leptin levels after high-fat diet feeding. The PLA2(-/-) mice also did not exhibit impaired glucose tolerance associated with the development of obesity-related insulin resistance as observed in the PLA2(+/+) mice. To investigate the mechanism by which PLA(2)(-/-) mice exhibit decreased weight gain while on a high-fat diet, fat absorption studies were performed. The PLA(2)(-/-) mice displayed 50 and 35% decreased plasma [(3)H]triglyceride concentrations 4 and 6 h, respectively, after feeding on a lipid-rich meal containing [(3)H]triolein. The PLA(2)(-/-) mice also displayed increased lipid content in the stool, thus indicating decreased fat absorption in these animals. These results suggest a novel role for PLA(2) in the protection against diet-induced obesity and obesity-related insulin resistance, thereby offering a new target for treatment of obesity and diabetes.

Expression profiling during adipocyte differentiation of 3T3-L1 fibroblasts

The 3T3-L1 cell line is a well-established and commonly used in vitro model to assess adipocyte differentiation. Over the course of several days confluent 3T3-L1 cells can be converted to adipocytes in the presence of an adipogenic cocktail. Changes in gene expression were measured by DNA microarrays at three time points (24 h, 4 days, and 1 week) during the course of differentiation from preadipocytes to mature adipocytes. Several functional categories of genes were affected by adipocyte conversion. In addition, seven genes were found to be commonly altered by 5-fold or more by adipocyte conversion at all three time points. Lipocalin 2, haptoglobin, serum amyloid A3, stearoyl-CoA desaturase, and 11β-hydroxysteroid dehydrogenase 1 were induced while actin α2 and procollagen VIII α1 were suppressed by adipocyte differentiation. Further study of the regulation of these genes and pathways will lead to an increased understanding of the biochemical pathways involved in adipocyte differentiation and possibly to the identification of new therapeutic targets for treatment of obesity and other metabolic diseases.

A solid-phase approach to mouse melanocortin receptor agonists derived from a novel thioether cyclized peptidomimetic scaffold.

The solid-phase synthesis of a novel thioether cyclized peptidomimetic scaffold, displaying functionality at the i to i + 3 positions, is reported. The thioether bridge is formed on-bead by an intramolecular reaction between a chloroacetylated reduced peptide bond and the free thiol from a cysteine. The crude products were obtained in moderate to very high purity. A series of 19 compounds were prepared and tested for agonist activity at the mouse melanocortin receptors 1, 3, 4, and 5 (mMC1-5R). From these results, several compounds were identified as having low micromolar agonist activity at the mMC1R and mMC4R. The former is involved in skin pigmentation and animal coat coloration. The latter is involved in the regulation of appetite and food intake and is currently a drug target for potential treatment of obesity. The most potent compound 1n with the pharmacophore motif "His-DPhe-Arg-Trp" was identified as having an EC(50) value of 165 nM at mMC1R, 7600 nM at mMC3R, 650 nM at mMC4R, and 335 nM at mMC5R. In addition, some of the compounds showed moderate selectivity for the mMC1R.

Strategies and potential molecular targets for obesity treatment.

Obesity is an increasingly prevalent and important health problem. Although treatment is available, the long-term maintenance of medically significant weight loss (5 to 10 percent of initial body weight) is rare. Since 1995 there has been an explosion of research focused on the regulation of energy balance and fat mass. Characterization of obesity-associated gene products has revealed new biochemical pathways and molecular targets for pharmacological intervention that will likely lead to new treatments. Ideally, these treatments will be viewed as adjuncts to behavioral and lifestyle changes aimed at maintenance of weight loss and improved health.

Strosberg et al. reply

The letter by Galitzky et al. summarizes the article published last year by the same authors[1xGalitzky, J. Br. J. Pharmacol. 1997; 122: 1244–1250Crossref | PubMed | Scopus (74)See all References[1], which listed a number of pharmacological arguments for the existence of a fourth β-adrenoceptor. The re-opening of this debate will certainly boost the research in the field and may lead to the discovery of a new β-adrenoceptor or another type of receptor. Before this is achieved, however, one needs to pay attention to the following points:•The authors claim that CGP12177A binds to a novel, putative β-adrenoceptor. However, no binding data with the commonly available tritiated CGP12177A or any other radioligand have been reported to support this hypothesis.•The putative β-adrenoceptor would exist only by virtue of the fact that SR59230A does not block all the CGP12177A-induced lipolysis. Two assumptions are thus made: CGP12177A would be a β3-adrenoceptor-selective agonist at any concentration and SR59230A would be a β3-adrenoceptor-selective antagonist.In their discussion the authors disregard three key facts: (1) CGP12177A has, at high concentrations, also been shown to act as a β1-adrenoceptor agonist[2xPak, M.D and Fishman, P.H. J. Recept. Signal Transduction Res. 1996; 16: 1–23Crossref | PubMedSee all References[2], and this by itself could explain most of the observations made by Galitzky et al.; (2) the antagonist activity of SR59230A towards the β3-adrenoceptor could not be confirmed in model systems[3xStrosberg, A.D and Pietri-Rouxel, F. Trends Pharmacol. Sci. 1997; 18: 52Abstract | Full Text PDF | PubMedSee all References[3]; and (3) the selectivity of SR59230A for the β3-adrenoceptor has not yet been confirmed on cloned receptor subtypes. Even in human immortalized brown adipocytes (PAZ-6 adipocytes), this compound was found to block 30–50% of catecholamine-induced adenylate cyclase activity. A sizeable portion of the resident β1- and β2-adrenoceptors are also blocked by SR59230A ([3xStrosberg, A.D and Pietri-Rouxel, F. Trends Pharmacol. Sci. 1997; 18: 52Abstract | Full Text PDF | PubMedSee all References, 4xZilberfarb, V et al. J. Cell Sci. 1997; 110: 801–807PubMedSee all References]and unpublished observations).•The putative additional β-adrenoceptor described by Galitzky et al. is not sensitive to adrenaline or noradrenaline. A protein that binds several β-adrenoceptor ligands, including the often-used radiolabelled cyanopindolol, but not catecholamines, has recently been described in rat colon[5xSugasawa, T et al. J. Biol. Chem. 1997; 272: 21244–21252Crossref | PubMed | Scopus (9)See all References[5]. Such a protein does not qualify as a β-adrenoceptor.To conclude, a fourth β-adrenoceptor might well exist, but its functional role cannot be demonstrated in whole tissues using ligands such as CGP12177A or SR59230A. The discovery of a β4-adrenoceptor will have to await cloning of the gene and characterization of the recombinant proteins in a model cell. This is precisely what had to be done for the β-adrenoceptor[6xEmorine, L.J. Science. 1989; 245: 1118–1121Crossref | PubMedSee all References[6], for which the functional role is now generally well recognized in adipocytes[7xArch, J.R.S and Kaumann, A.J. Med. Res. Rev. 1993; 13: 663–729Crossref | PubMed | Scopus (306)See all References, 8xLonnqvist, F, Thorne, A, Nilsell, K, Hoffstedt, J, and Arner, P. J. Clin. Invest. 1995; 95: 1109–1116Crossref | PubMedSee all References, 9xStrosberg, A.D. Annu. Rev. Pharmacol. Toxicol. 1997; 37: 421–450Crossref | PubMedSee all References]. At the current state of knowledge the β-adrenoceptor is thus still a very promising target for the pharmacological treatment of obesity.