Increased ghrelin signaling prolongs survival in mouse models of human aging through activation of sirtuin1.

Caloric restriction (CR) is known to retard aging and delay functional decline as well as the onset of diseases in most organisms. Ghrelin is secreted from the stomach in response to CR and regulates energy metabolism. We hypothesized that in CR ghrelin has a role in protecting aging-related diseases. We examined the physiological mechanisms underlying the ghrelin system during the aging process in three mouse strains with different genetic and biochemical backgrounds as animal models of accelerated or normal human aging. The elevated plasma ghrelin concentration was observed in both klotho-deficient and senescence-accelerated mouse prone/8 (SAMP8) mice. Ghrelin treatment failed to stimulate appetite and prolong survival in klotho-deficient mice, suggesting the existence of ghrelin resistance in the process of aging. However, ghrelin antagonist hastened death and ghrelin signaling potentiators rikkunshito and atractylodin ameliorated several age-related diseases with decreased microglial activation in the brain and prolonged survival in klotho-deficient, SAMP8 and aged ICR mice. In vitro experiments, the elevated sirtuin1 (SIRT1) activity and protein expression through the cAMP-CREB pathway was observed after ghrelin and ghrelin potentiator treatment in ghrelin receptor 1a-expressing cells and human umbilical vein endothelial cells. Furthermore, rikkunshito increased hypothalamic SIRT1 activity and SIRT1 protein expression of the heart in the all three mouse models of aging. Pericarditis, myocardial calcification and atrophy of myocardial and muscle fiber were improved by treatment with rikkunshito. Ghrelin signaling may represent one of the mechanisms activated by CR, and potentiating ghrelin signaling may be useful to extend health and lifespan.

Chronic overproduction of ghrelin in the hypothalamus leads to temporal increase in food intake and body weight.

Ghrelin is known to be a critical stimulator of feeding behavior mainly via actions in the hypothalamus. However, its functional contribution to the control of energy homeostasis under chronic elevated conditions is unknown. Here we show that overproduction of ghrelin via an AAV viral delivery system in the hypothalamus leads to an increase in food intake associated with increases in body weight. However, this increase in food intake is only temporary and is diminished and no longer significant after 3 weeks. Analysis of brain sections of mice 6 weeks after AAV-ghrelin virus injection demonstrates unaltered neuropeptide Y levels but strongly up-regulated pro-opiomelanocortin levels indicating that a compensatory mechanism has been activated to counter regulate the feeding stimulatory actions of ghrelin. This demonstrates that control mechanism exists that is activated under conditions of prolonged high ghrelin levels, which could potentially be utilized to control feeding and the development of obesity.

Potentiation of ghrelin signaling attenuates cancer anorexia-cachexia and prolongs survival.

Cancer anorexia-cachexia syndrome is characterized by decreased food intake, weight loss, muscle tissue wasting and psychological distress, and this syndrome is a major source of increased morbidity and mortality in cancer patients. This study aimed to clarify the gut-brain peptides involved in the pathogenesis of the syndrome and determine effective treatment for cancer anorexia-cachexia. We show that both ghrelin insufficiency and resistance were observed in tumor-bearing rats. Corticotropin-releasing factor (CRF) decreased the plasma level of acyl ghrelin, and its receptor antagonist, α-helical CRF, increased food intake of these rats. The serotonin 2c receptor (5-HT2cR) antagonist SB242084 decreased hypothalamic CRF level and improved anorexia, gastrointestinal (GI) dysmotility and body weight loss. The ghrelin receptor antagonist (D-Lys3)-GHRP-6 worsened anorexia and hastened death in tumor-bearing rats. Ghrelin attenuated anorexia-cachexia in the short term, but failed to prolong survival, as did SB242084 administration. In addition, the herbal medicine rikkunshito improved anorexia, GI dysmotility, muscle wasting, and anxiety-related behavior and prolonged survival in animals and patients with cancer. The appetite-stimulating effect of rikkunshito was blocked by (D-Lys3)-GHRP-6. Active components of rikkunshito, hesperidin and atractylodin, potentiated ghrelin secretion and receptor signaling, respectively, and atractylodin prolonged survival in tumor-bearing rats. Our study demonstrates that the integrated mechanism underlying cancer anorexia-cachexia involves lowered ghrelin signaling due to excessive hypothalamic interactions of 5-HT with CRF through the 5-HT2cR. Potentiation of ghrelin receptor signaling may be an attractive treatment for anorexia, muscle wasting and prolong survival in patients with cancer anorexia-cachexia.

Stomach regulates energy balance via acylated ghrelin and desacyl ghrelin.

BACKGROUND/AIMS: The gastric peptide ghrelin, an endogenous ligand for growth-hormone secretagogue receptor, has two major molecular forms: acylated ghrelin and desacyl ghrelin. Acylated ghrelin induces a positive energy balance, while desacyl ghrelin has been reported to be devoid of any endocrine activities. The authors examined the effects of desacyl ghrelin on energy balance. METHODS: The authors measured food intake, gastric emptying, c-Fos expression in the hypothalamus, and gene expression of hypothalamic neuropeptides in mice after administration of desacyl ghrelin. To explore the effects of long term overexpression of desacyl ghrelin, transgenic mice that overexpressed desacyl ghrelin were created. RESULTS: Administration of desacyl ghrelin decreased food intake and gastric emptying rate through an action on the paraventricular nucleus and the arcuate nucleus in the hypothalamus. Gene expression of anorexigenic cocaine and amphetamine regulated transcript and urocortin in the hypothalamus was increased by desacyl ghrelin. Desacyl ghrelin overexpressing mice exhibited a decrease in body weight, food intake, and fat pad mass weight accompanied by moderately decreased linear growth. Gastric emptying was also decreased in desacyl ghrelin overexpressing mice. CONCLUSIONS: These findings indicate that in contrast to acylated ghrelin, desacyl ghrelin induces a negative energy balance by decreasing food intake and delaying gastric emptying. The effect is mediated via the hypothalamus. Although derived from the same precursor, the inverse effects of these two peptides suggest that the stomach might be involved as an endocrine organ in the regulation of the energy balance.

Emerging functions of neuropeptide Y Y(2) receptors in the brain.

The Y(2) receptor is the predominant neuropeptide Y (NPY) receptor subtype in the brain. Y(2) receptor mRNA is discretely distributed in the brain, including specific subregions of the hippocampus and the hypothalamus, and is largely consistent with the distribution of Y(2) receptor protein demonstrated by radioligand-binding methods. Y(2) receptor-mediated effects have been reported principally based on the observations using the C-terminal fragments of NPY. Recent studies indicate an involvement of the receptor in food intake, gastrointestinal motility, cardiovascular regulation, and neuronal excitability. Very recently, Y(2) receptor selective antagonist has been developed and Y(2) receptor-deficient animals have been created. These new pharmacological tools will help to clarify the roles of this receptor in brain functions.

Ghrelin is an appetite-stimulatory signal from stomach with structural resemblance to motilin.

BACKGROUND & AIMS: : Ghrelin, an endogenous ligand for growth hormone secretagogue receptor, was recently identified in the rat stomach. We examined the effects of the gastric peptide ghrelin on energy balance in association with leptin and vagal nerve activity. METHODS: : Food intake, oxygen consumption, gastric emptying, and hypothalamic neuropeptide Y (NPY) messenger RNA expression were measured after intra-third cerebroventricular or intraperitoneal injections of ghrelin in mice. The gastric vagal nerve activity was recorded after intravenous administration in rats. Gastric ghrelin gene expression was assessed by Northern blot analysis. Repeated coadministration of ghrelin and interleukin (IL)-1 beta was continued for 5 days. RESULTS: : Ghrelin exhibited gastroprokinetic activity with structural resemblance to motilin and potent orexigenic activity through action on the hypothalamic neuropeptide Y (NPY) and Y(1) receptor, which was lost after vagotomy. Ghrelin decreased gastric vagal afferent discharge in contrast to other anorexigenic peptides that increased the activity. Ghrelin gene expression in the stomach was increased by fasting and in ob/ob mice, and was decreased by administration of leptin and IL-1 beta. Peripherally administered ghrelin blocked IL-1 beta-induced anorexia and produced positive energy balance by promoting food intake and decreasing energy expenditure. CONCLUSIONS: : Ghrelin, which is negatively regulated by leptin and IL-1 beta, is secreted by the stomach and increases arcuate NPY expression, which in turn acts through Y(1) receptors to increase food intake and decrease energy expenditure. Gastric peptide ghrelin may thus function as part of the orexigenic pathway downstream from leptin and is a potential therapeutic target not only for obesity but also for anorexia and cachexia.

Transgenic approach to the study of body weight regulation.

Energy homeostasis is accomplished through a highly integrated and redundant neurohumoral system. Recently, novel molecular mediators and regulatory pathways for feeding and body weight regulation have been identified in the brain and the periphery. Because of the multitude and complexity of disturbances in energy intake, expenditure, and partitioning that are associated with obesity, it has been difficult to determine which abnormalities are causative versus less important phenomena that are consequences of the altered neuroendocrine and metabolic milieu. Transgenic technology has provided new opportunities to modify the complex body weight-regulating system and to assess the relative importance of the individual components. Observations of mutant mice have shed new light on the understanding of energy homeostasis equation. Once created, transgenic animal models may be useful in assessing the efficacy or determining the mode of action of potential new therapeutic agents. However, the interpretation of targeted mutation is sometimes not straightforward in unraveling the physiology because of the redundancy and compensation of the regulatory machinery, as well as the inherent problems of manipulation of the gene. Modifying the synthesis of a particular gene at all sites and developmental stages may be a relatively crude way of investigating its functions. Advanced gene-targeting strategies aimed at specific alterations (on and off) of a gene product at desired tissues and times could lead to a better understanding of the system.

Decreased food intake and body weight in pancreatic polypeptide-overexpressing mice.

BACKGROUND & AIMS: Pancreatic polypeptide (PP) is a 36-amino acid hormone produced by F cells within the pancreatic islets and the exocrine pancreas. The definitive function of PP in mammalian physiology remains to be determined. This study examined the effects of chronic overexpression of PP through the development of PP transgenic mice. METHODS: PP transgenic mice were created by using mouse PP complementary DNA under the control of the cytomegalovirus immediate early enhancer-chicken beta-actin hybrid promoter (pCAGGS expression vector). RESULTS: A unique line of transgenic mice was created that overexpresses PP in the pancreatic islets with low levels of expression in other tissues including the brain. Plasma PP concentrations were more than 20 times higher than those of control littermates. However, PP overproduction led to postnatal lethality in half of the pups because of markedly decreased milk intake. The remaining PP transgenic mice gained less weight with specifically reduced food intake and fat mass compared with controls, a result that was more evident in male than in female mice. The transgenic mice exhibited a reduced rate of gastric emptying of a solid meal but had normal oxygen consumption and fasting leptin levels. Immunoneutralization with anti-PP antiserum reversed the phenotypic changes of transgenic animals. CONCLUSIONS: PP could be involved in feeding and body weight regulation partly through regulation of gastric emptying.

Neuropeptide Y: a key molecule in anorexia and cachexia in wasting disorders?

Anorexia and body weight loss are characteristic of many diseases, including cancer and AIDS. Recent studies indicate that inflammatory cytokines, such as interleukin 1, the interleukin 6 subfamily and tumor necrosis factor, induce anorexia and cachexia by inhibiting the normal adaptive feeding response to energy deficits. Here, I discuss the evidence for and against a central role for neuropeptide Y and leptin in anorexia and cachexia.

Feeding and body-weight regulation by hypothalamic neuropeptides--mediation of the actions of leptin.

Neuropeptides are essential for the regulation of appetite and body weight within the hypothalamus. The understanding of the neuropeptide regulation of energy homeostasis has been greatly advanced by the recent discovery of leptin, the protein product of the obese gene (ob). Significant new insights into the relationship between peripheral adiposity signals and their impact on the hypothalamic neuropeptide signaling circuitry have provided some crucial missing links in the negative-feedback regulation of appetite and body weight. The neuropeptide Y orexigenic network is a final common pathway for this signaling cascade and, along with feeding-inhibitory neuropeptides such as melanocortin, corticotropin-releasing factor and glucagon-like peptide 1, it is a major target through which leptin exerts a regulatory tonic restraint on body adiposity.

[A study on PP family peptide receptor: I. The distribution of peptide YY (PYY) receptor in the brain].

Peptide YY (PYY) was discovered in the porcine gut by Tatemoto in 1982. PYY has recently been found in the central nervous system. This has provoked studies of PYY effects when centrally administrated. We investigated the specific binding of radioactive PYY (125I-PYY) to brain membranes in pigs and dogs. PYY chiefly bound to the hippocampus, as well as to the pituitary gland, hypothalamus, and amygdala, suggesting that PYY acts on the limbic-hypothalamic-pituitary axis. PYY binding in the brain had a high-affinity and a low-affinity component (dissociation constant, 1.39 X 10(-10) M and 3.72 X 10(-8) M, respectively). The binding sites were highly specific for PYY and neuropeptide Y (NPY), but not for pancreatic polypeptide (PP) or other peptide hormones such as cholecystokinin octapeptide, methionine enkephalin, adrenocorticotropic hormone, and thyrotropic releasing hormone. The similar affinities for PYY and NPY imply that these peptides regulate brain functions through interaction with common receptor site(s).

Peptide YY receptors in the brain.

Radiolabelled ligand binding studies demonstrated that specific receptors for peptide YY are present in the porcine as well as the canine brains. Peptide YY was bound to brain tissue membranes via high-affinity (dissociation constant, 1.39 X 10(-10)M) and low-affinity (dissociation constant, 3.72 X 10(-8)M) components. The binding sites showed a high specificity for peptide YY and neuropeptide Y, but not for pancreatic polypeptide or structurally unrelated peptides. The specific activity of peptide YY binding was highest in the hippocampus, followed by the pituitary gland, the hypothalamus, and the amygdala of the porcine brain, this pattern being similarly observed in the canine brain. The results suggest that peptide YY and neuropeptide Y may regulate the function of these regions of the brain through interaction with a common receptor site.

[Regional distribution of pancreatic polypeptide-like immunoreactivity in the canine brain].

Recently many gut hormones have been found in the brain, and there is some evidence to suggest that pancreatic polypeptide-like immunoreactivity (PP-LI) is also present in the brain. Although in mammals, confirmative evidence is not yet shown. In the present paper we report the distribution and tissue localization of PP-LI in the canine brain by radio-immunoassay (RIA) and immunohistochemistry. Normal, fasted mongrel dogs were used. Brain tissue was extracted by boiling water. High concentrations of PP-LI were found in the pituitary gland (3.67 +/- 1.10 ng/g wet wt), substantia nigra (1.58 +/- 0.36 ng/g wet wt), hypothalamus (0.74 +/- 0.28 ng/g wet wt) and olfactory lobe (0.58 +/- 0.21 ng/g wet wt). PP-LI was not detectable in the frontal lobe, parietal lobe, striatum, thalamus, hippocampus, pons, cerebellum and medulla oblongata. The amounts of PP-LI in the brain were more less than the amounts of PP present in the pancreas (duodenal lobe, 29.3 +/- 1.1 microgram/g wet wt). The dilution curve of the brain tissue extracts showed parallelism with the standard curve of human and porcine PP on the RIA system. On Bio-Gel P-30 column chromatography, PP-LI from the pituitary gland and olfactory lobe eluted as a single peak coincided with highly purified bovine PP. In immunohistochemical study, PP-LI was found in the intermediate lobe and the stalk of the pituitary gland by means of anti-bovine PP antiserum. These findings of the specific regional localization suggest that PP or PP-LI may have a physiological role in the central nervous system.