Over the past century, prevalent models of energy and glucose homeostasis have been developed from a better understanding of the neural circuits underlying obesity and diabetes. From the early hypothalamic lesion reports to the more recent pharmacological and molecular/genetic studies, the hypothalamic melanocortin system has been shown to play a critical role in the regulation of metabolism. This review attempts to highlight contributions to our current understanding of how numerous neuromodulators (leptin, insulin, and serotonin) integrate with the central melanocortin system to coordinate alterations in energy and glucose balance.
leptin; insulin; serotonin; obesity; diabetes; patch-clamp
Energy balance and insulin action are tightly coregulated. Leptin regulates energy intake and expenditure partly by modulation of the melanocortin pathway in the hypothalamus. Here we demonstrate potent effects of the melanocortin pathway on insulin action and body distribution of adiposity. Conscious rats received week-long infusions of either a melanocortin receptor agonist, α-melanocyte-stimulating hormone (α-MSH), or antagonist, SHU9119, in the third cerebral ventricle while food intake was maintained constant in each group. α-MSH decreased intra-abdominal fat and markedly enhanced the actions of insulin on both glucose uptake and production, while SHU9119 exerted opposite effects. Our findings elucidate a neuroendocrine network that is likely to play a central role in the coupling of energy intake and insulin action.
The central nervous melanocortin system is a neural network linking nutrient-sensing systems with hypothalamic, limbic and hindbrain neurons regulating behavior and metabolic homeostasis. Primary melanocortin neurons releasing melanocortin receptor ligands residing in the hypothalamic arcuate nucleus are regulated by nutrient-sensing and metabolic signals. A smaller group of primary neurons releasing melanocortin agonists in the nucleus tractus solitarius in the brainstem are also regulated by signals of metabolic state. Two melanocortin receptors regulate energy homeostasis. Melanocortin-4 receptors regulate satiety and autonomic outputs controlling peripheral metabolism. The functions of melanocortin-3 receptors (MC3R) expressed in hypothalamic and limbic structures are less clear. Here we discuss published data and preliminary observations from our laboratory suggesting that neural MC3R regulate inputs into systems governing the synchronization of rhythms in behavioral and metabolism with nutrient intake. Mice subjected to a restricted feeding protocol, where a limited number of calories are presented at a 24 h interval, rapidly exhibit bouts of increased wakefulness and activity which anticipate food presentation. The full expression of these responses is dependent on MC3R. Moreover, MC3R knockout mice are unique in exhibiting a dissociation of weight loss from improved glucose homeostasis when subject to a restricted feeding protocol. While mice lacking MC3R fed ad libitum exhibit normal to moderate hyperinsulinemia, when subjected to a restricted protocol they develop hyperglycemia, glucose intolerance, and dyslipidemia. Collectively, our data suggest that the central nervous melanocortin system is a point convergence in the control of energy balance and the expression of rhythms anticipating nutrient intake.
The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Pittsburg, July 2010.
Circadian rhythm; anticipation; food anticipatory activity; satiety; homeostasis; clock; melanocortin; hypothalamus
A little more than a decade ago, the molecular basis of the lipostat was largely unknown. At that time, many laboratories were at work attempting to clone the genes encoding the obesity, diabetes, fatty, tubby and agouti loci, with the hope that identification of these obesity genes would help shed light on the process of energy homeostasis, appetite and energy expenditure. Characterization of obesity and diabetes elucidated the nature of the adipostatic hormone leptin and its receptor, respectively, while cloning of the agouti gene eventually led to the identification and characterization of one of the key neural systems upon which leptin acts to regulate intake and expenditure. In this review, we describe the neural circuitry known as the central melanocortin system and discuss the current understanding of its role in feeding and other processes involved in energy homeostasis.
melanocortin; leptin; MC4-R; agouti
The melanocortin system is crucial to regulation of energy homeostasis. The melanocortin receptor type 4 (MC4R) modulates insulin signaling via effects on c-Jun N-terminal kinase (JNK). The melanocortin agonist NDP-MSH dose-dependently inhibited JNK activity in HEK293 cells stably expressing the human MC4R; effects were reversed by melanocortin receptor antagonist. NDP-MSH time- and dose-dependently inhibited IRS-1ser307 phosphorylation, effects also reversed by a specific melanocortin receptor antagonist. NDP-MSH augmented insulin-stimulated AKT phosphorylation in vitro. The melanocortin agonist melanotan II increased insulin-stimulated AKT phosphorylation in the rat hypothalamus in vivo. NDP-MSH increased insulin-stimulated glucose uptake in hypothalamic GT1-1 cells. The current study shows that the melanocortinergic system interacts with insulin signaling via novel effects on JNK activity.
c-Jun N-terminal kinase (JNK); melanocortin receptor; AKT; insulin; insulin receptor substrate 1 (IRS-1); MT II; NDP-MSH; hypothalamus
The melanocortins, a family of peptides produced from the post-translational processing of pro-opiomelanocortin (POMC), regulate ingestive behavior and energy expenditure. Loss of function mutations of genes encoding POMC, or of either of two melanocortin receptors expressed in the central nervous system (MC3R, MC4R), are associated with obesity. The analyses of MC4R knockout mice indicate that activation of this receptor is involved in the regulation of appetite, the adaptive metabolic response to excess caloric consumption, and negative energy balance associated with cachexia induced by cytokines. In contrast, MC3R knockout mice exhibit a normal, or even exaggerated, response to signals that induce a state of negative energy balance. However, loss of the MC3R also results in an increase in adiposity. This article discusses the regulation of energy balance by the melanocortins. Published and newly presented data from studies analyzing of energy balance of MC3R and MC4R knockout mice indicate that increased adiposity observed in both models involves an imbalance in fat intake and oxidation.
Melanocortin; Energy balance; MC3R
Cannabinoids, the active components of marijuana, stimulate appetite, and cannabinoid receptor-1 (CB1-R) antagonists suppress appetite and promote weight loss. Little is known about how CB1-R antagonists affect the central neurocircuitry, specifically the melanocortin system that regulates energy balance.
Here, we show that peripherally administered CB1-R antagonist (AM251) or agonist equally suppressed or stimulated feeding respectively in Ay , which lack a functional melanocortin system, and wildtype mice, demonstrating that cannabinoid effects on feeding do not require melanocortin circuitry. CB1-R antagonist or agonist administered into the ventral tegmental area (VTA) equally suppressed or stimulated feeding respectively, in both genotypes. In addition, peripheral and central cannabinoid administration similarly induced c-Fos activation in brain sites suggesting mediation via motivational dopaminergic circuitry. Amperometry-detected increases in evoked dopamine (DA) release by the CB1-R antagonist in nucleus accumbens slices indicates that AM251 modulates DA release from VTA terminals.
Our results demonstrate that the effects of cannabinoids on energy balance are independent of hypothalamic melanocortin circuitry and is primarily driven by the reward system.
Regulation of energy balance is extremely complex, and involves multiple systems of hormones, neurotransmitters, receptors, and intracellular signals. As data have accumulated over the last two decades, the CNS melanocortin system is now identified as a prominent integrative network of energy balance controls in the mammalian brain. Here, we will review findings from rat and mouse models, which have provided an important framework in which to study melanocortin function. Perhaps most importantly, this review attempts for the first time to summarize recent advances in our understanding of the intracellular signaling pathways thought to mediate the action of melanocortin neurons and peptides in control of long term energy balance. Special attention will be paid to the roles of MC4R/MC3R, as well as downstream neurotransmitters within forebrain and hindbrain structures that illustrate the distributed control of melanocortin signaling in energy balance. In addition, distinctions and controversy between rodent species will be discussed.
melanocortin; POMC; ARC; NTS; leptin; MC4R; MC3R; obesity
Central circuits known to regulate food intake and energy expenditure also affect central cardiovascular regulation. For example, both the melanocortin and neuropeptide Y (NPY) peptide families, known to regulate food intake, also produce central hypertensive effects. Members of both families share a similar C-terminal amino acid residue sequence, RF(Y) amide, a sequence distinct from that required for melanocortin receptor binding. A recently delineated family of RFamide receptors recognizes both of these C-terminal motifs. We now present evidence that an antagonist with Y1 and RFamide receptor activity, BIBO3304, will attenuate the central cardiovascular effects of both gamma-melanocyte stimulating hormone (γ-MSH) and NPY. The use of synthetic melanocortin and NPY peptide analogs excluded an interaction with melanocortin or Y family receptors. We suggest that the anatomical convergence of NPY and melanocortin neurons on cardiovascular control centers may have pathophysiological implications through a common or similar RFamide receptor(s), much as they converge on other nuclei to coordinately control energy homeostasis.
γ-MSH; Neuropeptide Y; Hypertensive effects; Central nervous system; RFamide peptides; RFamide receptors; Y1 antagonists; Central vasopressin system
The burden of type 2 diabetes and its associated premature morbidity and mortality is rapidly growing, and the need for novel efficacious treatments is pressing. We report here that serotonin 2C receptor (5-HT2CR) agonists, typically investigated for their anorectic properties, significantly improve glucose tolerance and reduce plasma insulin in murine models of obesity and type 2 diabetes. Importantly, 5-HT2CR agonist-induced improvements in glucose homeostasis occurred at concentrations of agonist that had no effect on ingestive behavior, energy expenditure, locomotor activity, body weight, or fat mass. We determined that this primary effect on glucose homeostasis requires downstream activation of melanocortin-4 receptors (MC4Rs), but not MC3Rs. These findings suggest that pharmacological targeting of 5-HT2CRs may enhance glucose tolerance independently of alterations in body weight and that this may prove an effective and mechanistically novel strategy in the treatment of type 2 diabetes.
Hypothalamic AMP-activated protein kinase (AMPK) has been suggested to act as a key sensing mechanism, responding to hormones and nutrients in the regulation of energy homeostasis. However, the precise neuronal populations and cellular mechanisms involved are unclear. The effects of long-term manipulation of hypothalamic AMPK on energy balance are also unknown. To directly address such issues, we generated POMCα2KO and AgRPα2KO mice lacking AMPKα2 in proopiomelanocortin– (POMC-) and agouti-related protein–expressing (AgRP-expressing) neurons, key regulators of energy homeostasis. POMCα2KO mice developed obesity due to reduced energy expenditure and dysregulated food intake but remained sensitive to leptin. In contrast, AgRPα2KO mice developed an age-dependent lean phenotype with increased sensitivity to a melanocortin agonist. Electrophysiological studies in AMPKα2-deficient POMC or AgRP neurons revealed normal leptin or insulin action but absent responses to alterations in extracellular glucose levels, showing that glucose-sensing signaling mechanisms in these neurons are distinct from those pathways utilized by leptin or insulin. Taken together with the divergent phenotypes of POMCα2KO and AgRPα2KO mice, our findings suggest that while AMPK plays a key role in hypothalamic function, it does not act as a general sensor and integrator of energy homeostasis in the mediobasal hypothalamus.
The hypothalamic melanocortin system plays a fundamental role in the regulation of energy homeostasis. Orexins (hypocretins) are also involved in a diverse range of physiological processes, including food intake. Previous evidence has suggested that hypothalamic orexin expression may be influenced by the central melanocortin system. Here, we studied orexin mRNA levels in Pomc−/− mice, a mouse model lacking all endogenously produced melanocortin peptides. Orexin expression in the lateral hypothalamus was significantly increased in corticosterone deficient Pomc−/− mice. Further, when circulating glucocorticoids were restored to levels within the physiological range, orexin expression remained elevated. However, ICV administration of the melanocortin α-MSH to Pomc−/− mice reduced orexin expression back down to WT levels. This was independent of the effects of α-MSH on food intake as in Pomc−/− mice pair-fed to α-MSH-treated animals elevated orexin expression persisted. These data indicate that α-MSH may play a role in the regulation of orexin expression in Pomc−/−, with an elevation in orexin levels contributing to the hyperphagia seen in these animals.
Food intake; glucocorticoids; hypothalamus; melanocortins; orexins; proopiomelanocortin
Sirt1 is an evolutionarily conserved NAD+ dependent deacetylase involved in a wide range of processes including cellular differentiation, apoptosis, as well as metabolism, and aging. In this study, we investigated the role of hypothalamic Sirt1 in energy balance. Pharmacological inhibition or siRNA mediated knock down of hypothalamic Sirt1 showed to decrease food intake and body weight gain. Central administration of a specific melanocortin antagonist, SHU9119, reversed the anorectic effect of hypothalamic Sirt1 inhibition, suggesting that Sirt1 regulates food intake through the central melanocortin signaling. We also showed that fasting increases hypothalamic Sirt1 expression and decreases FoxO1 (Forkhead transcription factor) acetylation suggesting that Sirt1 regulates the central melanocortin system in a FoxO1 dependent manner. In addition, hypothalamic Sirt1 showed to regulate S6K signaling such that inhibition of the fasting induced Sirt1 activity results in up-regulation of the S6K pathway. Thus, this is the first study providing a novel role for the hypothalamic Sirt1 in the regulation of food intake and body weight. Given the role of Sirt1 in several peripheral tissues and hypothalamus, potential therapies centered on Sirt1 regulation might provide promising therapies in the treatment of metabolic diseases including obesity.
Albright hereditary osteodystrophy is a monogenic obesity disorder due to heterozygous mutations of Gsα, the G protein which mediates receptor-stimulated cAMP generation, in which obesity only develops when the mutation is on the maternal allele. Likewise, mice with maternal (but not paternal) germline Gsα mutation develop obesity, insulin resistance, and diabetes. These parent-of-origin effects are due to Gsα imprinting with preferential expression from the maternal allele in some tissues. As Gsα is ubiquitously expressed, the tissue involved in this metabolic imprinting effect is unknown. Using brain-specific Gsα knockout mice we show that Gsα imprinting within the central nervous system underlies these effects and that Gsα is imprinted in the paraventricular nucleus of the hypothalamus. Maternal Gsα mutation impaired melanocortin stimulation of energy expenditure but did not affect melanocortin's effect on food intake, suggesting that melanocortins may regulate energy balance in the central nervous system through both Gsα-dependent and -independent pathways.
The hypothalamic melanocortin system is known for its role in regulating energy homeostasis through it actions within hypothalamic brain centers. However, emerging evidence suggests that this system regulates addictive behaviors through signaling within mesolimbic neurons. Here, we hypothesized the melanocortin system modulates feeding behavior through its actions on mesolimbic neurons. In particular, we predicted that central administration of the melanocortin antagonist agouti-related peptide (AgRP) would activate midbrain dopamine neurons, increase mesolimbic dopamine turnover, and alter food seeking behaviors. We found that intraventricular administration of agouti-related peptide increased neuronal activation within midbrain dopamine neurons in addition to increasing dopamine turnover in the medial prefrontal cortex. Additionally, using the conditioned place preference paradigm to assay food seeking behavior, we report that central injection of agouti-related peptide attenuates the acquisition of a conditioned place preference for sucrose, but not high fat diet. These results suggest that the melanocortin system is capable of regulating mesocorticolimbic activity and food seeking behavior.
melanocortins; MC4R; reward; conditioned place preference; Food Seeking Behavior; Mesocorticolimbic; Dopamine
Data from mice lacking all endogenous melanocortin peptides suggest Agouti-related-peptide acts in vivo as a melanocortin antagonist rather than an inverse agonist.
The hypothalamic melanocortin system is unique among neuropeptide systems controlling energy homeostasis, in that both anorexigenic proopiomelanocortin (POMC)-derived and orexigenic Agouti related-peptide (AgRP)-derived ligands act at the same receptors, namely melanocortin 3 and 4 receptors (MC3/4R). AgRP clearly acts as a competitive antagonist at MC3R and MC4R but may also have an inverse agonist action at these receptors. The physiological relevance of this remains uncertain. We generated a mouse lacking both POMC and AgRP [double knockout (DKO) mouse]. Phenotyping was performed in the absence and presence of glucocorticoids, and the response to central peptide administration was studied. The phenotype of DKO mice is indistinguishable from that of mice lacking Pomc alone, with both exhibiting highly similar degrees of hyperphagia and increased body length, fat, and lean mass compared with wild-type controls. After a 24-h fast, there was no difference in the refeeding response between Pomc−/− and DKO mice. Similarly, corticosterone supplementation caused an equivalent increase in food intake and body weight in both genotypes. Although the central administration of [Nle4, d-Phe7]-α-MSH to DKO mice caused a decrease in food intake and an increase in brown adipose tissue Ucp1 expression, both of which could be antagonized with the coadministration of AgRP, there was no effect of AgRP alone. These data suggest AgRP acts predominantly as a melanocortin antagonist. If AgRP has significant melanocortin-independent actions, these are of insufficient magnitude in vivo to impact any of the detailed phenotypes we have measured under a wide variety of conditions.
Energy homeostasis is a complex physiological function coordinated at multiple levels. The issue of genetic regulation of nutrition and metabolism is attracting increasing interest and new energy homeostasis-regulatory genes are continuously identified. Among these genes, vgf is gaining increasing interest following two observations: (1) VGF-/- mice have a lean and hypermetabolic phenotype; (2) the first VGF-derived peptide involved in energy homeostasis, named TLQP-21, has been identified. The aim of this review will be to discuss the role of the vgf gene and VGF derived peptides in metabolic and nutritional functions. In particular we will: (1) provide a brief overview on the central systems regulating energy homeostasis and nutrition particularly focusing on the melanocortin system; (2) introduce the structure and molecular characteristic of vgf; (3) describe the phenotype of VGF deficient mice; (4) present recent data on the metabolic role of VGF-derived peptides, particularly focusing on one peptide named TLQP-21.
Protein tyrosine phosphatases (PTPs) are important regulators of intracellular signaling pathways via the dephosphorylation of phosphotyrosyl residues on various receptor and non-receptor substrates. The phosphorylation state of central nervous system (CNS) signaling components underlies the molecular mechanisms of a variety of physiological functions including the control of energy balance and glucose homeostasis. In this review, we summarize the current evidence implicating PTPs as central regulators of metabolism, specifically highlighting their interactions with the neuronal leptin and insulin signaling pathways. We discuss the role of a number of PTPs (PTP1B, SHP2, TCPTP, RPTPe, and PTEN), reviewing the findings from genetic mouse models and in vitro studies which highlight these phosphatases as key central regulators of energy homeostasis.
protein tyrosine phosphatase; leptin signaling; insulin signaling; PTP1B; SHP2; PTEN; TCPTP; RPTPe
Growing evidence suggests that a complex relationship exists between the central nervous system and peripheral organs involved in energy homeostasis. It consists in the balance between food intake and energy expenditure and includes the regulation of nutrient levels in storage organs, as well as in blood, in particular blood glucose. Therefore, food intake, energy expenditure, and glucose homeostasis are strictly connected to each other. Several hormones, such as leptin, adiponectin, resistin, and ghrelin, are involved in this complex regulation. These hormones play a role in the regulation of glucose metabolism and are involved in the development of obesity, diabetes, and metabolic syndrome. Recently, their presence in breast milk has been detected, suggesting that they may be involved in the regulation of growth in early infancy and could influence the programming of energy balance later in life. This paper focuses on hormones present in breast milk and their role in glucose homeostasis.
Obesity is characterized by an excess storage of body fat and promotes the risk for complex disease traits such as diabetes mellitus and cardiovascular diseases. The obesity prevalence in Europe is rising and meanwhile ranges from 10 to 20% in men and 15–25% in women. Body fat accumulation occurs in states of positive energy balance and is favored by interactions among environmental, psychosocial and genetic factors. Energy balance is regulated by a complex neuronal network of anorexigenic and orexigenic neurons which integrates peripheral and central hormonal and neuronal signals relaying information on the metabolic status of organs and tissues in the body. A key component of this network is the central melanocortin pathway in the hypothalamus that elicits metabolic and behavioral adaptations for the maintenance of energy homeostasis. Genetic defects in this system cause obesity in mice and humans. In this review we emphasize mouse models with spontaneous natural mutations as well as targeted mutations that contributed to our understanding of the central melanocortin system function in the control of energy balance.
Energy balance; Melanocortin; Mouse models; Obesity
Prolonged fasting is associated with a downregulation of the hypothalamo-pituitary thyroid (H-P-T) axis, which is reversed by administration of leptin. The hypothalamic melanocortin system regulates energy balance and mediates a number of central effects of leptin. In this study, we show that hypothalamic melanocortins can stimulate the thyroid axis and that their antagonist, agouti-related peptide (Agrp), can inhibit it. Intracerebroventricular (ICV) administration of Agrp (83-132) decreased plasma thyroid stimulating hormone (TSH) in fed male rats. Intraparaventricular nuclear administration of Agrp (83-132) produced a long-lasting suppression of plasma TSH, and plasma T4. ICV administration of a stable α-MSH analogue increased plasma TSH in 24-hour–fasted rats. In vitro, α-MSH increased thyrotropin releasing hormone (TRH) release from hypothalamic explants. Agrp (83-132) alone caused no change in TRH release but antagonized the effect of α-MSH on TRH release. Leptin increased TRH release from hypothalami harvested from 48-hour–fasted rats. Agrp (83-132) blocked this effect. These data suggest a role for the hypothalamic melanocortin system in the fasting-induced suppression of the H-P-T axis.
OBJECTIVE—Xenin, a 25–amino acid peptide, was initially isolated from human gastric mucosa. Plasma levels of xenin rise after a meal in humans, and administration of xenin inhibits feeding in rats and chicks. However, little is known about the mechanism by which xenin regulates food intake. Signaling pathways including leptin and melanocortins play a pivotal role in the regulation of energy balance. Therefore, we addressed the hypothesis that xenin functions as a satiety factor by acting through the melanocortin system or by interacting with leptin.
RESEARCH DESIGN AND METHODS—The effect of intracerebroventricular and intraperitoneal administration of xenin on food intake was examined in wild-type, agouti, and ob/ob mice. The effect of intracerebroventricular injection of SHU9119, a melanocortin receptor antagonist, on xenin-induced anorexia was also examined in wild-type mice. To determine whether the hypothalamus mediates the anorectic effect of xenin, we examined the effect of intraperitoneal xenin on hypothalamic Fos expression.
RESULTS—Both intracerebroventricular and intraperitoneal administration of xenin inhibited fasting-induced hyperphagia in wild-type mice in a dose-dependent manner. The intraperitoneal injection of xenin also reduced nocturnal intake in ad libitum–fed wild-type mice. The intraperitoneal injection of xenin increased Fos immunoreactivity in hypothalamic nuclei, including the paraventricular nucleus and the arcuate nucleus. Xenin reduced food intake in agouti and ob/ob mice. SHU9119 did not block xenin-induced anorexia.
CONCLUSIONS—Our data suggest that xenin reduces food intake partly by acting through the hypothalamus but via signaling pathways that are independent of those used by leptin or melanocortins.
The central melanocortin system plays a key role in the regulation of food intake and energy homeostasis. We investigated whether genetic or pharmacologic blockade of central melanocortin signaling attenuates cardiac cachexia in mice and rats with heart failure. Permanent ligation of the left coronary artery (myocardial infarction (MI)) or sham operation was performed in wild-type (WT) or melanocortin-4 receptor (MC4R) knockout mice. Eight weeks after surgery, WT-Sham mice had significant increases in lean body mass (LBM; P<0·05) and fat mass (P<0·05), whereas WT-MI did not gain significant amounts of LBM or fat mass. Resting basal metabolic rate (BMR) was significantly lower in WT-Sham mice compared to WT-MI mice (P<0·001). In contrast, both MC4-Sham and MC4-MI mice gained significant amounts of LBM (P<0·05) and fat mass (P<0·05) over the study period. There was no significant difference in the BMR between MC4-Sham and MC4-MI mice. In the second experiment, rats received aortic bands or sham operations, and after recovery received i.c.v. injections of either artificial cerebrospinal fluid (aCSF) or the melanocortin antagonist agouti-related protein (AGRP) for 2 weeks. Banded rats receiving AGRP gained significant amount of LBM (P<0·05) and fat mass (P<0·05) over the treatment period, whereas banded rats receiving aCSF did not gain significant amounts of LBM or fat mass. These results demonstrated that genetic and pharmacologic blockade of melanocortin signaling attenuated the metabolic manifestations of cardiac cachexia in murine and rat models of heart failure.
Cachexia is a devastating syndrome of body wasting that is associated with multiple common chronic diseases including cancer, chronic kidney disease, and chronic heart failure. These underlying diseases are associated with increased levels of inflammatory cytokines and result in anorexia, increased resting energy expenditure and loss of fat and lean body mass. Prior experiments have implicated the central melanocortin system in the hypothalamus with the propagation of these symptoms of cachexia. Pharmacologic blockade of this system using melanocortin antagonists causes attenuation of the signs of cachexia in laboratory models. Recent advances in our knowledge of this disease process have involved further elucidation of the pathophysiology of melanocortin activation and demonstration of the efficacy of melanocortin antagonists in new models of cachexia, including cardiac cachexia. Additionally, small molecule antagonists of the melanocortin-4 receptor (MC4R) continue to be introduced, including ones with oral bioavailability. These developments generate optimism that melanocortin antagonism will be used to treat humans with disease-associated cachexia. However, to date human application has remained elusive and it is unclear when we will know whether humans with cachexia would benefit from treatment with these compounds.
Cachexia is among the most debilitating and life-threatening aspects of cancer. It represents a metabolic syndrome affecting essential functional circuits involved in the regulation of homeostasis, and includes anorexia, fat and muscle tissue wasting. The anorexigenic peptide α-MSH is believed to be crucially involved in the normal and pathologic regulation of food intake. It was speculated that blockade of its central physiological target, the melanocortin (MC)-4 receptor, might provide a promising anti-cachexia treatment strategy. This idea is supported by the fact that in animal studies, agouti-related protein (AgRP), the endogenous inverse agonist at the MC-4 receptor, was found to affect two hallmark features of cachexia, i.e. to increase food intake and to reduce energy expenditure.
SNT207707 and SNT209858 are two recently discovered, non peptidic, chemically unrelated, orally active MC-4 receptor antagonists penetrating the blood brain barrier. Both compounds were found to distinctly increase food intake in healthy mice. Moreover, in mice subcutaneously implanted with C26 adenocarcinoma cells, repeated oral administration (starting the day after tumor implantation) of each of the two compounds almost completely prevented tumor induced weight loss, and diminished loss of lean body mass and fat mass.
In contrast to the previously reported peptidic and small molecule MC-4 antagonists, the compounds described here work by the oral administration route. Orally active compounds might offer a considerable advantage for the treatment of cachexia patients.