The role of the Agouti-Related Protein in energy balance regulation

Adolescent binge-like ethanol exposure reduces basal α-MSH expression in the hypothalamus and the amygdala of adult rats

Melanocortins (MC) are central peptides that have been implicated in the modulation of ethanol consumption. There is experimental evidence that chronic ethanol exposure reduces α-MSH expression in the limbic and hypothalamic brain regions and alters central pro-opiomelanocortin (POMC) mRNA activity in adult rats. Adolescence is a critical developmental period of high vulnerability in which ethanol exposure alters corticotropin releasing factor, neuropeptide Y, substance P and neurokinin neuropeptide activities, all of which have key roles in ethanol consumption. Given the involvement of MC and the endogenous inverse agonist AgRP in ethanol drinking, here we evaluate whether a binge-like pattern of ethanol treatment during adolescence has a relevant impact on basal and/or ethanol-stimulated α-MSH and AgRP activities during adulthood. To this end, adolescent Sprague-Dawley rats (beginning at PND25) were pre-treated with either saline (SP group) or binge-like ethanol exposure (BEP group; 3.0 g/kg given in intraperitoneal (i.p.) injections) of one injection per day over two consecutive days, followed by 2 days without injections, repeated for a total of 8 injections. Following 25 ethanol-free days, we evaluated α-MSH and AgRP immunoreactivity (IR) in the limbic and hypothalamic nuclei of adult rats (PND63) in response to ethanol (1.5 or 3.0 g/kgi.p.) and saline. We found that binge-like ethanol exposure during adolescence significantly reduced basal α-MSH IR in the central nucleus of the amygdala (CeA), the arcuate nucleus (Arc) and the paraventricular nucleus of the hypothalamus (PVN) during adulthood. Additionally, acute ethanol elicited AgRP IR in the Arc. Rats given the adolescent ethanol treatment required higher doses of ethanol than saline-treated rats to express AgRP. In light of previous evidence that endogenous MC and AgRP regulate ethanol intake through MC-receptor signaling, we speculate that the α-MSH and AgRP disturbances induced by binge-like ethanol exposure during adolescence may contribute to excessive ethanol consumption during adulthood.

Effects of prolonged exercise on agouti-related protein: a pilot study

Agouti-related protein (AgRP), is a signaling peptide that affects feeding behavior, energy homeostasis, and has also been shown to stimulate the hypothalamic-pituitary-adrenal axis. The purpose of this study was to determine the effects of 90 min of treadmill exercise on circulating AgRP concentrations and the relationship of AgRP responses to cortisol. Seven young males completed a preliminary trial followed by counterbalanced experimental and control trials 4-5 weeks apart. The experimental trial began 2.5 h after consumption of a standard nutrient beverage and consisted of treadmill exercise at 60 % of previously determined VO(2max) for 90 min. Blood samples were collected before (-30 and 0 min), during (18, 36, 54, 72, and 90 min), and following exercise (20, 40, and 60 min). Blood samples were collected in a resting, control trial at the same time points as the experimental trial. Plasma lactate was significantly higher in the exercise than the control trial. Although AgRP increased from 18 min of exercise to peak at 90 min, these increases were not significantly different than values in the control trial. Cortisol responses during the exercise trial were significantly higher than the control trial. AgRP concentrations during early exercise were positively correlated with cortisol levels later in recovery. The obtained data suggest that AgRP concentrations during prolonged steady-state exercise are associated with subsequent cortisol increases, but further study is required to determine whether there is a causal effect.

Melanocortin system in cancer-related cachexia

The melanocortin system plays a pivotal role in the regulation of appetite and energy balance. It was recognized to play an important role in the development of cancer-related cachexia, a debilitating condition characterized by progressive body wasting associated with anorexia, increased resting energy expediture and loss of fat as well as lean body mass that cannot be simply prevented or treated by adequate nutritional support.

The recent advances in understanding of mechanisms underlying cancer-related cachexia led to consequent recognition of the melanocortin system as an important potential therapeutic target. Several molecules have been made available for animal experiments, including those with oral bioavailability, that act at various checkpoints of the melanocortin system and that might confer singificant benefits for the patients suffering from cancer-related cachexia. The application of melanocortin 4 receptor antagonists/agouti-related peptide agonists has been however restricted to animal models and more pharmacological data will be necessary to progress to clinical trials on humans. Still, pharmacological targeting of the melanocortin system seem to represent an elegant and promising way of treatment of cancer-related cachexia.

Physiological and behavioral responses to intermittent starvation in C57BL/6J mice

The dual intervention point model states that body mass is controlled by upper and lower intervention points, above and below which animals (and humans) intervene physiologically to bring their body mass back into the acceptable range. It has been further suggested that the lower intervention point may be defined by the risk of starvation, while the upper intervention point may be defined by the risk of predation. The objective of the present study was to test whether the risk of starvation determines the lower intervention point and to examine the physiological and behavioral mechanisms that underpin the regulation of body mass, when the risk of starvation is increased. Sixty-four mice were exposed to random days of complete fasting or 50% food restriction and their body mass and fat mass responses were measured. Food intake, physical activity and body temperature were measured throughout the experiment. In addition, plasma leptin and insulin, triglyceride and non-esterified fatty acids, along with hypothalamic neuropeptides gene expression in the arcuate nucleus were assessed after 13 and 42 days of treatment. We found that C57BL/6J mice increased body mass and fatness in response to a short-term (13 days) intermittent fasting, which was restored to baseline as the treatment was prolonged. In contrast, intermittently 50% food restricted mice showed no significant changes in body mass or fatness. Over the first 13 days of treatment the data were consistent with the dual intervention point model as the mice showed both increased body mass and adiposity over this period. Over the more protracted period of 42 days the effect waned and was therefore inconsistent with the model. The body mass and fat mass gains in intermittently fasted mice were mainly accounted for by increased food intake. Elevated NPY gene expression after 13 days (three 24 h fasting events) may have driven the increase in food intake. However, no changes were observed in such neuropeptides as POMC, CART, AgRP, Ob-Rb and SOCS 3 or circulating levels of leptin, insulin, NEFA and TG. Hypothermia during fasting days may have also contributed to the increase in body mass. Over 42 days of treatment (nine 24 h fasting events) cumulative food intake was not affected by intermittent starvation. However physical activity, mainly activity during the light phase was lowered suggesting an adaptation to unpredictable starvation. Overall, mice exhibited different behavioral and physiological responses to intermittent starvation depending on the duration of treatment.

Molecular mechanisms for activation of the agouti-related protein and stimulation of appetite

OBJECTIVE

The agouti-related protein (Agrp) is a powerful orexigenic peptide, but little is known about its transcriptional regulation. The objective of this study was to determine molecular mechanisms for the activation of hypothalamic Agrp and identify compounds that stimulate appetite.

RESEARCH DESIGN AND METHODS

We used promoter analyses methods, hypothalamic cell culture and transfection, immunohistochemistry, luciferase-expressing transgenic mice, in vivo bioluminescence, anitisense RNA, mouse feeding studies, indirect calorimetry, real-time PCR, and Western blots.

RESULTS

We found that the Krüppel-like factor 4 (Klf4) is a potent activator of Agrp by binding to a specific CACCC-box in its minimal promoter. We also found that an extract of tarragon, termed PMI-5011, activated hypothalamic Klf4 and Agrp. In vivo, PMI-5011 increased Agrp promoter activity in luciferase-expressing transgenic mice, increased hypothalamic Klf4 and Agrp expression, increased hypothalamic Orexin and melanin-concentrating hormone, increased food intake, reduced circulating insulin and leptin levels, attenuated energy expenditure, and enhanced body weight but only when using a high-fat diet.

CONCLUSIONS

These data show that Klf4 augmented hypothalamic Agrp by binding to a specific CACCC-box onto its minimal promoter. In addition, the tarragon extract PMI-5011 activated Klf4 and orexigenic neuropeptides and reduced peripheral insulin and leptin levels leading to positive energy balance.

Solid-phase peptide head-to-side chain cyclodimerization: discovery of C(2)-symmetric cyclic lactam hybrid α-melanocyte-stimulating hormone (MSH)/agouti-signaling protein (ASIP) analogues with potent activities at the human melanocortin receptors

A novel hybrid melanocortin pharmacophore was designed based on the pharmacophores of the agouti-signaling protein (ASIP), an endogenous melanocortin antagonist, and α-melanocyte-stimulating hormone (α-MSH), an endogenous melanocortin agonist. The designed hybrid ASIP/MSH pharmacophore was explored in monomeric cyclic, and cyclodimeric templates. The monomeric cyclic disulfide series yielded peptides with hMC3R-selective non-competitive binding affinities. The direct on-resin peptide lactam cyclodimerization yielded nanomolar range (25-120 nM) hMC1R-selective full and partial agonists in the cyclodimeric lactam series which demonstrates an improvement over the previous attempts at hybridization of MSH and agouti protein sequences. The secondary structure-oriented pharmacophore hybridization strategy will prove useful in development of unique allosteric and orthosteric melanocortin receptor modulators. This report also illustrates the utility of peptide cyclodimerization for the development of novel GPCR peptide ligands.

High fat diet altered the mechanism of energy homeostasis induced by nicotine and withdrawal in C57BL/6 mice

Nicotine treatment has known to produce an inverse relationship between body weight and food intake in rodents. Present study determined the effect of repeated treatment with nicotine and withdrawal in control and obese mice, on: (1) body weight, caloric intake and energy expenditure; (2) hypothalamic neuropeptides mRNA expression; and (3) serum leptin. 21-week-old C57BL/6 mice (n = 65) received nicotine (3.0 mg/kg/day; 2 weeks) and saline (1 ml/kg/day; 2 weeks) subcutaneously. Animals were given either a normal-fat (10% kcal from fat, NF) or a high-fat diet (45% kcal from fat, HF) from the 12th week to 25th week. While, nicotine treatment for 14 days induced an increase in hypothalamic agouti-related protein, cocaine- and amphetamine- regulated transcript, pro-opiomelanocortin mRNA expressions, nicotine also produced a reducing effect in body weight gain and leptin concentration in NF mice. High-fat diet induced obese mice showed a blunted hypothalamic and leptin response to nicotine. Remarkable weight loss in obese mice was mediated not just by decreasing caloric intake, but also by increasing total energy expenditure (EE). During nicotine withdrawal period, weight gain occurred in NF and HF groups, which was ascribed to a decrease in EE rather than changes in caloric intake. Hypothalamic AgRP might play a role for maintaining energy balance under the nicotine-induced negative energy status.

The melanocortin-4 receptor: physiology, pharmacology, and pathophysiology

The melanocortin-4 receptor (MC4R) was cloned in 1993 by degenerate PCR; however, its function was unknown. Subsequent studies suggest that the MC4R might be involved in regulating energy homeostasis. This hypothesis was confirmed in 1997 by a series of seminal studies in mice. In 1998, human genetic studies demonstrated that mutations in the MC4R gene can cause monogenic obesity. We now know that mutations in the MC4R are the most common monogenic form of obesity, with more than 150 distinct mutations reported thus far. This review will summarize the studies on the MC4R, from its cloning and tissue distribution to its physiological roles in regulating energy homeostasis, cachexia, cardiovascular function, glucose and lipid homeostasis, reproduction and sexual function, drug abuse, pain perception, brain inflammation, and anxiety. I will then review the studies on the pharmacology of the receptor, including ligand binding and receptor activation, signaling pathways, as well as its regulation. Finally, the pathophysiology of the MC4R in obesity pathogenesis will be reviewed. Functional studies of the mutant MC4Rs and the therapeutic implications, including small molecules in correcting binding and signaling defect, and their potential as pharmacological chaperones in rescuing intracellularly retained mutants, will be highlighted.

Treadmill training enhances rat agouti-related protein in plasma and reduces ghrelin levels in plasma and soleus muscle

Ghrelin and agouti-related protein (AgRP) are orexigenic peptides secreted from stomach mucosa and the arcuate nucleus of the hypothalamus, respectively. Both peptides affect feeding behavior and play a role in energy balance, glucose homeostasis, and adiposity. The purpose of the current study was to determine the effects of moderate-term (6 weeks) running regimen on resting levels of ghrelin, AgRP, adenosine triphosphate, and glycogen in soleus muscle as well as plasma concentrations of the orexigenic hormones. Eighteen adult Wistar male rats (12 weeks old, 235-255 g) were randomly assigned to training (n = 10) and control (n = 8) groups. The training group ran for 60 min/d, 5d/wk at 25 m/min and 0% grade for 6 weeks. Forty-eight hours after the last exercise session, rats were killed; and soleus muscle and plasma were collected and frozen in liquid nitrogen for later analysis. Results demonstrated that 6 weeks of treadmill exercise reduced ghrelin and increased AgRP levels in plasma. Trained rat soleus muscle had higher levels of glycogen but not adenosine triphosphate or AgRP compared with untrained controls. Data indicate that training lowers ghrelin levels in rat soleus and plasma, which is accompanied by higher plasma AgRP and soleus glycogen content.

Role of central melanocortin pathways in energy homeostasis

The rise in the global prevalence of human obesity has emphasized the need for a greater understanding of the physiological mechanisms that underlie energy homeostasis. Numerous circulating nutritional cues and central neuromodulatory signals are integrated within the brain to regulate both short- and long-term nutritional state. The central melanocortin system represents a crucial point of convergence for these signals and, thus, has a fundamental role in regulating body weight. The melanocortin ligands, synthesized in discrete neuronal populations within the hypothalamus and brainstem, modulate downstream homeostatic signalling via their action at central melanocortin-3 and -4 receptors. Intimately involved in both ingestive behaviour and energy expenditure, the melanocortin system has garnered much interest as a potential therapeutic target for human obesity.

Control elements in the neighboring ATPase gene influence spatiotemporal expression of the human agouti-related protein

The agouti-related protein (AgRP) is an orexigenic peptide that plays a significant role in the regulation of energy balance. It is expressed in the hypothalamus, the adrenal glands, and the testis, but sequences determining its spatial and temporal expression have not been identified. Using an elaborate in vitro screening approach, we show here that two adjacent enhancers inside the first intron of the neighboring (1.4 kb downstream) ATPase gene (ATP6V0D1) modulate the human AgRP promoter with profound spatiotemporal variation despite their diminutive sizes (221 and 231 nt). In transgenic mice, the proximal enhancer displayed specificity for the testis, tail, and ears, and the distal one for the testis, front feet, bone, heart, muscle, brain, spinal cord, and tongue, while dietary fat and overnight fasting had differential effects on enhancer activities. AgRP in the testis was localized to pachytene spermatocytes and in the tongue to epithelial cells. Comparative sequence analysis showed that the AgRP-ATP6V0D1 intergenic region is two times longer in humans than in mice and that the two enhancers are conserved in the rhesus monkey genome but not in the mouse genome. These data show that spatiotemporal expression of the human AgRP gene is influenced by diversified primate-specific intronic sequences in its neighboring ATP6V0D1 gene.