Hypothalamic melanocortin signaling in cachexia

New ligands for melanocortin receptors

Named originally for their effects on peripheral end organs, the melanocortin system controls a diverse set of physiological processes through a series of five G-protein-coupled receptors and several sets of small peptide ligands. The central melanocortin system plays an essential role in homeostatic regulation of body weight, in which two alternative ligands, alpha-melanocyte-stimulating hormone and agouti-related protein, stimulate and inhibit receptor signaling in several key brain regions that ultimately affect food intake and energy expenditure. Much of what we know about the relationship between central melanocortin signaling and body weight regulation stems from genetic studies. Comparative genomic studies indicate that melanocortin receptors used for controlling pigmentation and body weight regulation existed more than 500 million years ago in primitive vertebrates, but that fine-grained control of melanocortin receptors through neuropeptides and endogenous antagonists developed more recently. Recent studies based on dog coat-color genetics revealed a new class of melanocortin ligands, the beta-defensins, which reveal the potential for cross talk between the melanocortin and the immune systems.

Anorexia in cancer: role of feeding-regulatory peptides

Anorexia is one of the most common symptoms in advanced cancer and is a frequent cause of discomfort for cancer patients and their families. The pathogenesis of cancer anorexia is multi-factorial and involves most of the hypothalamic neuronal signalling pathways modulating energy homeostasis. It is considered to be the result of a failure of usual appetite and satiety signals. Loss of appetite can arise from decreased taste and smell of food, as well as from dysfunctional hypothalamic signalling pathways and cytokine production. Cytokines in particular, appear to play a key role in energy balance through persistent activation of the melanocortin system and inhibition of the neuropeptide Y pathway. The imbalance between anorexigenic and orexigenic peptides leads to suppression of appetite, and increased satiety and satiation associated with marked weight loss and decline in physical performance. High levels of serotonin also appear to contribute to these effects and recent findings implicate corticotropin-releasing factor in the pathogenesis of cancer anorexia as well. Despite significant advances in our understanding of the regulation of food intake and energy expenditure, few effective therapies are available. A better appreciation of the molecular and neuronal mechanisms that control body weight homeostasis may lead to the development of new therapies for improving the survival and quality of life of these patients.

Melanocortins and agouti-related protein modulate the excitability of two arcuate nucleus neuron populations by alteration of resting potassium conductances

The hypothalamic melanocortin system is crucial for the control of appetite and body weight. Two of the five melanocortin receptors, MC3R and MC4R are involved in hypothalamic control of energy homeostasis, with the MC4R having the major influence. It is generally thought that the main impact of the melanocortin system on hypothalamic circuits is external to the arcuate nucleus, and that any effect locally in the arcuate nucleus is inhibitory on proopiomelanocortin-expressing (POMC) neurons. In contrast, using current- and voltage-clamp recordings from identified neurons, we demonstrate that MC3R and MC4R agonists depolarize arcuate POMC neurons and a separate arcuate neuronal population identified by the rat insulin 2 promoter (RIPCre) transgene expression. Furthermore, the endogenous MC3R and MC4R antagonist, agouti-related protein (AgRP), hyperpolarizes POMC and RIPCre neurons in the absence of melanocortin agonist, consistent with inverse agonism at the MC4R. A decreased transient outward (I(A)) potassium conductance, and to a lesser extent the inward rectifier (K(IR)) conductance, underlies neuronal depolarization, whereas an increase in I(A) mediates AgRP-induced hyperpolarization. Accordingly, POMC and RIPCre neurons may be targets for peptide transmitters that are possibly released locally from AgRP-expressing and POMC neurons in the arcuate nucleus, adding further previously unappreciated complexity to the arcuate system.

Cancer-Related Anorexia/Cachexia Syndrome and Oxidative Stress: An Innovative Approach beyond Current Treatment

Objective: Cancer-related anorexia/cachexia syndrome and oxidative stress play a key role in the progression and outcome of neoplastic disease. Patients and Methods: On the basis of our previously published studies and clinical experience, we have developed an innovative approach consisting of diet with high polyphenol content (400 mg), p.o. pharmaconutritional support enriched with n − 3 fatty acids (eicosapentaenoic acid and docosahexaenoic acid) 2 cans (237 mL each) per day, medroxiprogesterone acetate 500 mg/d, antioxidant treatment with α-lipoic acid 300 mg/d plus carbocysteine lysine salt 2.7 g/d plus vitamin E 400 mg/d plus vitamin A 30,000 IU/d plus vitamin C 500 mg/d, and selective cyclooxygenase-2 inhibitor Celecoxib 200 mg/d. The treatment is administered for 16 weeks. The following variables are evaluated: (a) clinical variables (stage and Eastern Cooperative Oncology Group performance status); (b) nutritional variables (lean body mass, appetite, and resting energy expenditure); (c) laboratory variables (serum levels of proinflammatory cytokines, C-reactive protein, and leptin and blood levels of reactive oxygen species and antioxidant enzymes); and (d) quality of life variables (European Organization for Research and Treatment of Cancer QLQ-C30, EQ-5Dindex, and EQ-5DVAS). A phase II nonrandomized study has been designed to enroll 40 patients with advanced cancer at different sites with symptoms of cancer-related anorexia/cachexia syndrome and oxidative stress. Results: As of January 2004, 28 patients have been enrolled: 25 patients were evaluable and 14 of them have completed the treatment (20 patients have completed 2 months of treatment). As for clinical response, five patients improved, three patients remained unchanged, and six patients worsened. The Eastern Cooperative Oncology Group performance status (grade) 1 remained unchanged. As for nutritional/functional variables, the lean body mass increased significantly at 2 and 4 months. As for laboratory variables, reactive oxygen species decreased significantly and proinflammatory cytokines interleukin-6 and tumor necrosis factor-α decreased significantly. As for quality of life, it comprehensively improved after treatment. Conclusions: The treatment has been shown to be effective for clinical response, increase of lean body mass, decrease of reactive oxygen species and proinflammatory cytokines, and improvement of quality of life. The treatment has been shown to be safe with good compliance of patients. The study is in progress (14 further patients will be included).

Synthesis and biological evaluation of imidazole-based small molecule antagonists of the melanocortin 4 receptor (MC4-R)

A novel series of imidazole-based small molecule antagonists of the melanocortin 4 receptor (MC4-R) is reported. Members of this series have been identified, which exhibit sub-micromolar binding affinity for the MC4-R, functional potency <100nM, and good oral exposure in rat. Antagonists of the MC4-R are potentially useful in the therapeutic treatment of involuntary weight loss due to advanced age or disease (e.g. cancer or AIDS), an area of large, unmet medical need.

Ciliary neurotrophic factor and leptin induce distinct patterns of immediate early gene expression in the brain

Ciliary neurotrophic factor (CNTF) and leptin decrease food intake and body weight. Lipopolysaccharide (LPS) is a potent exogenous pyrogen and produces anorexia via cytokine production. CNTF-, leptin-, and LPS-induced cytokines all act on type I cytokine receptors. However, it is not known if these cytokines engage similar central nervous system (CNS) pathways to exert their effects. To assess mechanisms by which these cytokines act, we examined the patterns of immediate early gene expression (SOCS-3, c-fos, and tis-11) in the brain following intravenous administration. CNTF and LPS induced gene expression in circumventricular organs; ependymal cells of the ventricles, meninges, and choroid plexus; and the arcuate nucleus of the hypothalamus. CNTF administration also induced fever and cyclooxygenase-2 mRNA expression. In contrast, we found no evidence of leptin-induced inflammation. CNTF and leptin are being assessed as potential therapeutic anti-obesity agents, and both potently reduce food intake. Our findings support the hypothesis that CNTF and leptin engage distinct CNS sites and CNTF possesses inflammatory properties distinct from leptin.

Roles of the melanocortin-4 receptor in antipyretic and hyperthermic actions of centrally administered α-MSH

Activation of central melanocortin receptors (MCR) inhibits fever but can also stimulate thermogenesis, and the mechanisms involved are unknown. To determine whether the long-recognized antipyretic effect of exogenous alpha-MSH is mediated by the melanocortin-4 receptor (MC4R), and what thermoeffector systems are involved, we tested the effects of intracerebroventricular (i.c.v.) injection of alpha-MSH on lipopolysaccharide (LPS, 30 microg/kg i.p.)-induced fever in rats, in the presence and absence of the selective MC4R antagonist HS014. Treatment with alpha-MSH (1 microg, i.c.v.) suppressed LPS-induced increases in core body temperature (Tc), whereas a lower dose (300 ng) was ineffective. Nevertheless, both alpha-MSH doses effectively inhibited LPS-induced peripheral vasoconstriction, the principal heat-conserving thermoeffector, as determined by changes in tail skin temperature (Tsk). This implies that the net antipyretic effect of alpha-MSH cannot be accounted for solely by modulation of heat loss effectors, but also involves other mechanisms. Surprisingly, central MC4-R blockade by coinjected HS014 (1 microg) not only prevented, but reversed the effect of alpha-MSH (1 microg) on Tc, thus resulting in augmented LPS-induced fever. In afebrile rats, alpha-MSH infusion caused a modest transient increase in Tc that was blocked by coinjected HS014, but was not accompanied by altered Tsk. Overall, the results support the hypothesis that the MC4R mediates the antipyretic effects of alpha-MSH. Paradoxically, in the presence of pharmacological MC4-R blockade during fever, exogenous alpha-MSH can exacerbate fever, probably by acting via other central MCR subtype(s). In normal animals, centrally injected alpha-MSH exerts a hyperthermic effect that is mediated by the MC4R, consistent with recent evidence that MC4R activation promotes energy expenditure in normal states through stimulation of thermogenesis.

Minireview: From anorexia to obesity--the yin and yang of body weight control

Over the past decade, there has been a tremendous increase in the understanding of the molecular and neural mechanisms that control food intake and body weight. Yet eating disorders and cachexia are still common, and obesity cases are rising at alarming rates. Thus, despite recent progress, an increased understanding of the molecular and neural substrates that control body weight homeostasis is a major public health goal. In this review, we discuss the mechanisms by which metabolic signals interact with key behavioral, neuroendocrine, and autonomic regulatory regions of the central nervous system. Additionally, we offer a model in which hormones such as leptin and ghrelin interact with similar central nervous system circuits and engage them in such a way as to maintain an appropriate and tight regulation of body weight and food intake. Our model predicts that overstimulation or understimulation of these central pathways can result in obesity, anorexia, or cachexia.

The central melanocortin system and fever

Fever is a phylogenetically ancient response that is mounted upon exposure of the host to pathogens or inflammatory agents. Melanocortin agonists act centrally to inhibit fever by acting at receptors, including the melanocortin-4 receptor, which is prominently expressed in key hypothalamic thermoregulatory centers. Furthermore, endogenous melanocortins act centrally as physiological modulators of fever, recruited during the febrile response to restrain its intensity. Functionally, these actions lie at the interface between the anti-inflammatory effects of melanocortins, which involve suppression of the synthesis and actions of proinflammatory cytokines, and the central control of thermoregulation. Considering the extensive neuroanatomic and functional overlaps between central pathways and peripheral effectors involved in thermoregulation and energy balance, it is not surprising that melanocortins have been found to influence the metabolic economy profoundly in pathological as well as normal states. For example, despite suppressing endotoxin-induced fever, endogenous melanocortins appear to mediate the associated anorexia, a classic component of the "illness syndrome" accompanying acute infections, and promote a negative energy balance. The thermoregulatory actions of melanocortins are in several respects functionally opposed, and are remarkably dependent on physiological state, indicating that responsiveness to melanocortins is a physiologically modulated variable. Elucidating the anti-inflammatory and thermoregulatory roles of central melanocortin receptors during inflammatory states may lead to novel pharmacotherapeutic targets based on selective targeting of melanocortin receptor subtypes, for clinical benefit in human disease states involving neuroinflammatory components and metabolic wasting.

Cancer-related cachexia and oxidative stress: beyond current therapeutic options

Cancer-related anorexia/cachexia syndrome is a complex phenomenon in which metabolic abnormalities, proinflammatory cytokines produced by the host immune system, circulating tumor-derived catabolic factors, decreased food intake and probably additional unknown factors all play different roles. This review examines the mechanisms of cancer-related anorexia/cachexia syndrome and the mainstays of its management. The two major options for pharmacological therapy have been progestational agents and corticosteroids. Agents currently under investigation are nonsteroidal anti-inflammatory drugs, N-3 fatty acids, branched-chain amino acids and thalidomide. On the basis of several previously published studies and clinical experience, an innovative treatment approach, which consists of an integrated nutritional and pharmacological treatment, has been developed. This approach, based upon multiple components each targeted at different factors involved, may be effective in both improving objective clinical symptoms, such as lean body mass, and subjective symptoms such as quality of life. A Phase II study has been initiated and Phase III study designed.

The melanocortin system

The melanocortin system consists of melanocortin peptides derived from the proopiomelanocortin gene, five melanocortin receptors, two endogenous antagonists, and two ancillary proteins. This review provides an abbreviated account of the basic biochemistry, pharmacology, and physiology of the melanocortin system and highlights progress made in four areas. In particular, recent pharmacological and genetic studies have affirmed the role of melanocortins in pigmentation, inflammation, energy homeostasis, and sexual function. Development of selective agonists and antagonists is expected to further facilitate the investigation of these complex physiological functions and provide an experimental basis for new pharmacotherapies.

The need to feed: homeostatic and hedonic control of eating

Feeding provides substrate for energy metabolism, which is vital to the survival of every living animal and therefore is subject to intense regulation by brain homeostatic and hedonic systems. Over the last decade, our understanding of the circuits and molecules involved in this process has changed dramatically, in large part due to the availability of animal models with genetic lesions. In this review, we examine the role played in homeostatic regulation of feeding by systemic mediators such as leptin and ghrelin, which act on brain systems utilizing neuropeptide Y, agouti-related peptide, melanocortins, orexins, and melanin concentrating hormone, among other mediators. We also examine the mechanisms for taste and reward systems that provide food with its intrinsically reinforcing properties and explore the links between the homeostatic and hedonic systems that ensure intake of adequate nutrition.