Leptin revisited its mechanism of action and potential for treating diabetes

Despite the availability of improved antidiabetic drugs, enhanced glycaemia monitoring systems and easier patient-to-physician accessibility, patients with type 2 diabetes1,2 (BOX 1) are still at a significantly higher risk of developing cardiovascular disease and cancer than non-diabetic individuals3,4. The incidence of coronary artery disease in patients suffering from type 1 diabetes5–7 (BOX 2) is also remarkably high: greater than 90% after the age of 55 years8,9. In the United States, 40% of patients diagnosed with diabetes do not achieve the accepted gly- caemic targets and 80% fail to achieve blood-pressure- and lipid-lowering goals10. Therefore, despite the fact that insulin therapy transformed the previously lethal disease of type 1 diabetes into a manageable condition, and the fact that current type 2 diabetes drugs improve glycaemic control, these interventions do not restore metabolic homeostasis and, when used over long periods of time, may cause serious comorbidities associated with diabetes. Hence, better antidiabetic approaches are urgently needed. Leptin is a hormone that is produced by adipose tissue and regulates various physiological processes and behav- iours, including appetite, body weight, neuro endocrine functions and glycaemia. These effects are mediated via actions on leptin receptors (LEPRs) expressed by neurons in the central nervous system (CNS)11,12. Among several splice variants of the Lepr gene13, the long LEPRB isoform is thought to mediate all actions of leptin via the activa- tion of multiple intracellular signalling pathways (FIG. 1). Studies in rodents have identified some of the specific neuronal targets that mediate the hormonal effects of leptin on the aforementioned parameters. For example, the actions of leptin on body weight are mediated mainly by GABA (γ-aminobutyric acid)-ergic neurons14 (the anatomical locations of which remain unclear), and its role in puberty is mediated by neurons within the ventral premammillary nucleus of the hypothalamus15. The potent effects of leptin on glucose homeostasis in the context of severe obesity and insulin resistance are predominantly mediated by pro-opiomelanocortin (POMC)-expressing neurons within the arcuate nucleus of the hypothalamus (ARH)16,17. Owing to its potent beneficial effects on glucose metabolism, as demonstrated by the administration of leptin to diabetic rodents and to humans with insulin-resistant diabetes caused by lipodystrophy18–20 (BOX 3), this 16 kDa polypeptide has the potential to become a novel and effective antidiabetic agent. In this Review, we present available results from leptin- based clinical trials. We also examine data from animal studies that led to the current understanding of the underlying mechanisms of leptin-mediated control of glucose homeostasis. In addition, because patients with type 2 diabetes are commonly obese and have 1Department of Internal Medicine, Division of Hypothalamic Research, University of Texas Southwestern Medical Center, Dallas, Texas 75390, USA. 2Department of Cellular Physiology and Metabolism, Faculty of Medicine, University of Geneva, 1211 Geneva, Switzerland. 3Center for Epigenetics and Metabolism, University of California Irvine, Irvine, California 92697, USA. 4Department of Medicine, Division of Endocrinology and Metabolism, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts 02215, USA. e-mails: roberto.coppari@ utsouthwestern.edu; cbjorbae@bidmc.harvard.edu doi:10.1038/nrd3757 Leptin revisited: its mechanism of action and potential for treating diabetes Roberto Coppari1,2,3 and Christian Bjørbæk4 Abstract | Since the discovery of leptin in 1994, we now have a better understanding of the cellular and molecular mechanisms underlying its biological effects. In addition to its established anti-obesity effects, leptin exerts antidiabetic actions that are independent of its regulation of body weight and food intake. In particular, leptin can correct diabetes in animal models of type 1 and type 2 diabetes. In addition, long-term leptin replacement therapy improves glycaemic control, insulin sensitivity and plasma triglycerides in patients with severe insulin resistance due to lipodystrophy. These results have spurred enthusiasm for the use of leptin therapy to treat diabetes. Here, we review the current understanding of the glucoregulatory functions of leptin, emphasizing its central mechanisms of action and lessons learned from clinical studies, and discuss possible therapeutic applications of leptin in the treatment of type 1 and type 2 diabetes. R E V I E W S 692 | SEPTEMBER 2012 | VOLUME 11 www.nature.com/reviews/drugdisc © 2012 Macmillan Publishers Limited. All rights reserved Type 2 diabetes An illness characterized by insulin resistance, elevated blood levels of glucose, insulin and lipids, and estimated to affect more than 300 million people worldwide. Type 1 diabetes An illness characterized by the loss of pancreatic β-cells, lack of insulin, hyperglycaemia, cachexia and ketoacidosis, and estimated to affect millions of people worldwide. Lipodystrophy A rare condition that can be inherited or acquired, and is typically characterized by varying adipose tissue loss and distribution. Hyperleptinaemia A condition in which leptin levels in the blood are elevated, typically in obesity. Leptin resistance A condition in which endogenous and exogenous leptin is less effective at mediating its actions: for example, at reducing food intake or lowering glucose and lipids levels in the blood. Hypoleptinaemia A condition in which the level of leptin in the blood is below normal owing to reduced fat mass: for example, in patients with anorexia, lipodystrophy or hypothalamic amenorrhea. hyperleptinaemia, and are thus resistant to the meta- bolic actions of leptin, we deliberate on the possible mechanisms responsible for leptin resistance. A detailed understanding of this issue is probably crucial for the successful development and therapeutic use of leptin, or compounds targeting downstream pathways engaged by the hormone, for treating diabetes. Finally, we touch upon the therapeutic potential and limitations of lep- tin in becoming an addition to the pharmacopoeia of antidiabetic agents. Leptin in clinical settings Below, we discuss results from leptin-based clinical trials, data from which are available from the ClinicalTrials.gov website and from the PubMed database. Although these results indicate that leptin therapy can effectively improve energy and/or glucose imbalances in individu- als who have severe hypoleptinaemia, they also suggest that hyperleptinaemic individuals (who represent the vast majority of the obese and diabetic population) are refractory to therapy (TABLE 1). These observations high- light the need to overcome the ‘leptin-resistant’ obstacle if leptin therapy is to become an effective and widely used tool against obesity and diabetes. Effects on obesity. Because of reproducible preclinical results showing that leptin exerted potent anti-obesity effects, the hormone was initially heralded as the pana- cea for the obesity pandemic. In humans, leptin therapy indeed rescued obesity and several other endocrine defects (for example, pubertal delay and infertility) present in the very few obese people suffering from congenital leptin deficiency21–23. However, most of the individuals suffering from obesity have elevated levels of circulating leptin and are therefore likely to be leptin- resistant24. Indeed, obese patients with hyperleptinaemia show poor responses (in terms of anorectic effects and body weight suppression) to exogenously administered leptin25,26; these results diminished expectations from leptin-based anti-obesity approaches. Box 1 | Type 2 diabetes: current treatments, complications and limitations Type 2 diabetes is caused by a combination of insufficient insulin production by the pancreas and diminished responsiveness to insulin in target tissues and organs. The illness is characterized by insulin resistance, hyperglycaemia and elevated circulating lipid levels; in later stages of the disease the loss of pancreatic β-cells can also occur1. There are several treatments available to patients with type 2 diabetes151. The most widely prescribed drug for the management of this metabolic defect is metformin, a biguanide that enhances hepatic insulin sensitivity and hence curtails the elevated hepatic glucose production typically observed in patients with type 2 diabetes152,153. Metformin inhibits complex I of the mitochondrial oxidative phosphorylation machinery and causes an increased AMP/ATP ratio154, an effect that induces the activation of AMP-activated protein kinase (AMPK). The antidiabetic action of metformin stems from the activation of hepatic liver kinase B1 (LKB1)–AMPK pathways; this leads to inhibition of histone deacetylases (HDACs), which leads to the suppression of forkhead box protein O1 (FOXO1) activity, which causes reduced gluconeogenic gene expression, ultimately resulting in diminished hepatic glucose output153,155–157. Metformin causes minor side effects (such as gastrointestinal disturbances) if it is administered to patients who are not prone to developing lactic acidosis158,159. Thiazolidinediones (TZDs; for example, rosiglitazone (Avandia; GlaxoSmithKline), pioglitazone (Actos; Takeda) and troglitazone) are synthetic ligands of the nuclear receptor transcription factor peroxisome proliferator-activated receptor-γ (PPARγ)160 that increase insulin sensitivity161–163. Recent studies have unveiled some of the mechanisms underlying the unwanted (for example, augmenting body adiposity) and wanted (alleviation of diabetes) actions of TZDs. For example, increased appetite and body fat mass are frequent undesired effects of TZDs164–169. These untoward outcomes are likely to be due to the action of TZDs on PPARγ in the brain170,171. TZDs influence PPARγ activity and consequentially glucose metabolism by blocking the phosphorylation of PPARγ by cyclin-dependent kinase 5 (CDK5); CDK5 activity increases in the adipose tissue of obese and diabetic individuals163. New compounds tailored to target only the CDK5–PPARγ interaction site have been successfully tested in preclinical studies172. These findings may pave the way for the development of better antidiabetic compounds that exert their effects via PPARγ. Indeed, because of its known detrimental effects on the heart, the use of rosiglitazone-containing medicines was recently restricted in the United States and suspended in the European Union173. Other classes of approved antidiabetic (type 2) drugs • Agonists of the glucagon-like peptide 1 (GLP1) receptor (for example, exenatide (Byetta; Amylin) and liraglutide (Victoza; Novo Nordisk) have moderate efficacy; exenatide is now approved in both Europe and the United States as a once-weekly injection, but can cause nausea and vomiting151. • Inhibitors of the endogenous GLP1-inactivating protein dipetidyl peptidase 4 (DDP4) also have moderate efficacy but can cause serious side effects such as an increased risk of developing pancreatitis151. • Insulin secretagogues (such as sulphonylureas), by lowering the open probability of ATP-sensitive K+ channels, diminish the threshold for glucose-induced insulin secretion; these compounds have good efficacy but substantially increase the risks of hypoglycaemia (an event that can be life-threatening). • Insulin also has moderate efficacy but needs to be injected multiple times a day; it can cause increased ectopic lipid deposition and hypoglycaemia. Other classes of drugs are currently in clinical trials (reviewed in REFS 151,174,175). In addition to the drawbacks mentioned above, the patients’ requirement of currently available antidiabetic (type 2) drugs (either alone or in combination) usually augments with the progression of the disease, hence increasing the risks of unwanted effects175. R E V I E W S NATURE REVIEWS | DRUG DISCOVERY VOLUME 11 | SEPTEMBER 2012 | 693 © 2012 Macmillan Publishers Limited. All rights reserved Amylin A hormone that is co-secreted with insulin from pancreatic β-cells; amylin slows gastric emptying and promotes satiety. Euglycaemia Normal levels of glucose in the blood. Nevertheless, more recent clinical data have indicated that combination treatment with leptin and pramlintide (Symlin; Amylin Pharmaceuticals) (an amylin analogue) significantly reduces body weight (up to 12% of pre- treatment body weight) in leptin-resistant obese indi- viduals27,28. Because the duration of therapy was limited to 20 weeks, it is unclear whether a longer treatment period would result in maintenance or further reduc- tion of body weight or cause negative side effects. Although longer-term clinical trials have been initi- ated (ClinicalTrials.gov identifiers: NCT00819234 and NCT00673387) to address these issues, results are not yet available. In addition, the efficacy of leptin monotherapy or leptin and pramlintide bi-therapy in obese individuals with low or normal levels of circulating leptin (repre- senting ~10% of the obese population)11,24 is unknown. Therefore, although there are still chances for possible improvements, it seems as though treatment with leptin alone is not an effective approach for the treatment of obesity in the vast majority of obese individuals who also have hyperleptinaemia (fasting serum levels of leptin >15 ng per ml). Effects on lipodystrophy. Soon after the publication of the remarkable preclinical results obtained by Shimomura and colleagues18 — who showed that leptin administra- tion corrects the severe insulin resistance and hypergly- caemia displayed by rodents with congenital generalized lipodystrophy (BOX 3) — the clinical effects of the hor- mone were analysed. The clinical outcomes satisfied, to some extent, the expectations engendered by the preclini- cal results. For example, the insulin resistance, hyper- insulinaemia, hyperglycaemia and hypertriglycer idaemia present in patients with severe hypoleptinaemia (fasting serum levels of leptin <4 ng per ml) and lipodystrophy were all improved (without adverse effects) following daily subcutaneous administration of recombinant methionyl human leptin19,20. Of note, the improvement in diabetes observed in these leptin-treated lipodys- trophic patients was achieved even after discontinuation of previously administered antidiabetic therapy, hence ruling out the possibility of a synergistic or additive posi- tive effect of leptin and antidiabetic drugs on glucose and lipid imbalances20. Results from additional studies have established the beneficial effects of leptin therapy on glucose and lipid metabolism in lipodystrophic patients with hypoleptinaemia29,30. Nevertheless, lipodystrophy refers to a very hetero- geneous group of disorders that may be accompanied by diverse changes in the amount of circulating leptin31 (BOX 3). Indeed, not all patients with lipodystrophy have reduced leptin levels32. Thus, concerns pertinent to the efficacy of leptin therapy in all types of lipodystrophies have been raised; more specifically, the antidiabetic potential of leptin administration in lipodystrophic patients who do not have severe hypoleptinaemia has been questioned. Supporting these concerns, recent findings indicate that even though leptin therapy is very effective in ameliorating lipid profiles, it does not improve hyperglycaemia in lipodystrophic individuals with moderately low levels of circulating leptin (fasting serum levels of leptin ~5 ng per ml)32. Thus, people who develop lipodystrophy without having severe hypoleptin aemia are expected to respond poorly to the hyperglycaemia-lowering effect of the hormone yet still benefit from its hypertriglyceridaemia-lowering effect. This potential limitation of leptin therapy in the context of lipodystrophy underscores the necessity of develop- ing better approaches to treat metabolic imbalances in individuals affected by this disorder. To this end, combination therapy may be a better option. For example, the most prevalent type of lipo- dystrophy is the acquired form, which affects patients with HIV or AIDS who have been taking highly active antiretroviral treatments33. These patients usually have subcutaneous fat loss and increased abdominal fat. Results from a small clinical trial indicate that leptin administration improves glucose metabolism but not lipid parameters in lipodystrophic patients with HIV or AIDS34. Conversely, the administration of tesamo- relin (Egrifta; Theratechnologies) (a growth hormone- releasing factor analogue approved by the US Food and Box 2 | Type 1 diabetes: current treatments, complications and limitations Type 1 diabetes is caused by the loss of pancreatic β-cells, which results in a lack of insulin and a lethal catabolic outcome if untreated. A classical endocrinological approach for treating an illness resulting from the lack of a given hormone is to replace levels of the hormone therapeutically; type 1 diabetes clinical practice strictly abides by this paradigm. Thus, insulin administration is part of the daily activities of virtually all patients with type 1 diabetes6,176. Despite its undisputable life-saving action, insulin therapy does not restore metabolic homeostasis in patients with type 1 diabetes, as these individuals are at a much higher risk of developing challenging morbidities (such as heart disease, blindness, kidney failure, neuropathy and hypertension) than normal unaffected individuals 177–179. It has yet to be resolved whether these complications are due to the volatility of euglycaemia (wide fluctuations in blood glucose levels are commonly seen in patients with type 1 diabetes), direct consequences of therapy or a combination of both. In addition to the euglycaemic volatility, drawbacks of insulin therapy stem from its lipogenic actions, as insulin stimulates the transcription of genes that encode enzymes involved in the biosynthesis of lipids180. Thus, long-term insulin treatment may underlie the excessive ectopic lipid deposition (that is, in non-adipose tissues)181 and the high incidence of coronary artery disease observed in patients with type 1 diabetes8,9. Of note, these lipogenic actions of insulin are likely to promote a vicious cycle of fatty-acid-induced insulin resistance in insulin target cells (for example, hepatocytes and myocytes) and hence lead to increased insulin requirements in the enduring management of patients with type 1 diabetes182–184. In part owing to the potent, fast-acting, glycaemia-lowering effect of the hormone, intensive insulin therapy also substantially increases the risk of hypoglycaemia — an event that can be life-threatening6,185–188. R E V I E W S 694 | SEPTEMBER 2012 | VOLUME 11 www.nature.com/reviews/drugdisc © 2012 Macmillan Publishers Limited. All rights reserved P PTP1B P P P P SOCS3 P P SFK PTPase LEPRB PTPase P P P P P P P P P SH2B JAK2 Tyr985 Tyr1077 Tyr1138 1166 863 SFK STAT3 SHP2 STAT5 Nature Reviews | Drug Discovery LeptinLEPRB a Resting b Activation c Inactivation P P P P P P Proteins: • PI3K–AKT • GRB2–ERK • FOXO1 • p90S6K • p70S6K and S6 proteins Genes: • Pomc • Socs3 • Agrp • Npy • Fos • Cpe Cellular functions: • Transcription • Translation • Neuronal activity Downstream pathways and cellular processes Amenorrhea A condition characterized by the absence of menstrual periods in a woman of reproductive age. Drug Administration for the treatment of lipodystrophy in patients with HIV or AIDS) has been shown to improve visceral fat, triglyceride and choleste