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Division of Diabetes, Digestive and Kidney Diseases, Department of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, Kobe, Japan;
* Tulane University Health Sciences Center, New Orleans, Lousiana, USA;
Department of Medical Oncology, University of Cagliari, Cagliari, Italy;
Department of Clinical Medicine, University "La Sapienza," Rome, Italy;
Surgical Metabolism and Nutrition Lab, Neuroscience Program, Department of Surgery, University Hospital, Upstate Medical University, Syracuse, New York, USA; and
|| Department of Anatomy, Shiga University of Medical Science, Otsu, Japan
1Correspondence: Division of Diabetes, Digestive and Kidney Diseases, Department of Clinical Molecular Medicine, Kobe University Graduate School of Medicine, 7-5-1 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. E-mail: inui{at}med.kobe-u.ac.jp
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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Key Words: GHS • orexigenic signal • enteric nutrition • NYP/AgRP neurons
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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2
3
4
5
6
7
8
9
10
11
12
13)
. The hypothalamus is the center for the integration of feeding and associated autonomic, neuroendocrine, and gastrointestinal activities. Ghrelin, identified as an endogenous ligand for the growth hormone secretagogue (GHS) receptor (14)
, functions as an orexigenic (appetite-stimulating) signal from the stomach when an increase in metabolic efficiency is necessary (15
16
17)
. Ghrelin regulates, in an antagonistic manner to leptin, synthesis and secretion of several neuropeptides in the hypothalamus that regulate feeding and associated hypothalamic functions. Here, we review the evidence indicating ghrelin as a crucial missing link between the stomach and the hypothalamus, and the clinical implication of this ghrelin/GHS receptor system in the control of gastrointestinal function, energy balance and growth. |
GROWTH HORMONE SECRETAGOGUES (GHSs) |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 19)
. GHRH stimulates GH synthesis and release whereas somatostatin inhibits the release. However, several other neurotransmitters and neuropeptides are assumed to play a role in the control of GH secretion.
In the past two decades, particular attention has been given to a new family of substances, named GH secretagogues (GHSs), which show a powerful GH-releasing effect. GHSs are non-natural, synthetic compounds developed by Bowers and co-workers that directly stimulate GH secretion (20
21
22)
(Fig. 1
). The prototype GH-releasing peptide 6 (GHRP-6) has been used as a diagnostic tool since before the isolation of GHRH in 1982 (23)
. Orally active GHSs have been proposed as an alternative to GH, insulin-like growth factor I (IGF-I) and GHRH as a growth-promoting treatment in GH-deficient children, as well as an anabolic treatment in elderly patients with somatopause (19
, 24
25
26)
.
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GHSs act through specific G-protein-coupled receptor(s) (27
, 28)
distinct from that of GHRH and exert a powerful synergism when administered in addition to GHRH. The GHS receptor type Ia (GHS-RIa) transduces the GH-releasing effect of GHSs, whereas GHS-RIb is a nonspliced, nonfunctional receptor mRNA variant. GHS-RIa operates through activation of the phospholipase-IP3 pathway and inhibition of K+ channels, leading to a rise in intracellular Ca2+. GHS-RIa is predominantly expressed in the pituitary and hypothalamus and at low levels in other brain regions such as ventral tegmental area, substantia nigra, nucleus tractus solitarius, and hippocampus as well as peripheral tissues such as heart, lung, pancreas, intestine, kidney, and adipose tissue (27
28
29
30
31)
. It has been speculated that GHSs mimic an as yet unidentified endogenous hormone reflecting the presence of an additional neuroendocrine system regulating pulsatile GH secretion (32
33
34)
.
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GHRELIN IS AN ENDOGENOUS GHS AND MOTILIN-RELATED OREXIGENIC SIGNAL FROM THE STOMACH |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 35)
. Ghre is the proto-Indo-European root of the word growth. Although most people assumed that the greatest concentrations would be in the hypothalamus, the highest GHS receptor activation was found in stomach extracts (14)
. Indeed, Tomassetto and co-workers identified it separately from the stomach as the motilin-related peptide (36)
, with structural and effect-related similarities to the duodenal hormone motilin that has a key role in the regulation of gastrointestinal motility (16
, 17
, 37)
(Fig. 1)
. Motilin was also known to have a GH-releasing effect (38)
. Based on structural and effect-related similarities, motilin and ghrelin are considered to represent a novel gastrointestinal hormone family in addition to gastrin, secretin, pancreatic polypeptide (PP), insulin, epidermal growth factor (EGF), and tachykinin families (17
, 39
, 40)
.
Two molecular forms of ghrelin are found in the stomach: the 28 amino acid ghrelin having n-octanoylated serine in position 3 and the 27 amino acid des-[Gln14)] ghrelin produced by alternative splicing of the ghrelin gene (14
, 41)
. The acylation appears to be essential for GH-releasing activity in both natural forms of ghrelin although des-acyl ghrelin may have some biological activities such as those acting as a survival factor on the cardiovascular system (42)
. Other minor forms of ghrelin are present in human plasma and stomach, measured as the total immunoreactivity by the conventional radioimmunoassay based on the carboxyl-terminal fragment of ghrelin (43)
. Studies of structure activity show that amino-terminal fragments conserving the first five amino acids of the molecule display full functional activity (44)
.
It is known that structural heterogeneity among species can be of major importance for motilin (45)
. Five or more positions in the 22 amino acid structure of motilin may differ among mammalian species. The situation is more complex in rats in which motilin is not identified and exogenous motilin administration does not reproduce the gastrointestinal motor effect of the peptide. However, structural heterogeneity of ghrelin appears minor (46)
, and human ghrelin is identical to rat ghrelin except for two residues (14)
(Fig. 1)
.
Ghrelin is expressed mainly in the stomach, by neuroendocrine cells (X/A-like cells in rodents and P/D1 cells in humans) in the fundus, and is secreted into the circulation (47
48
49
50)
. This is supported by the finding that gastrectomy reduces plasma ghrelin concentrations by 65% (51)
. Gastric ghrelin cells show distinctive morphology and hormone reactivity with respect to histamine enterochromaffin-like, somatostatin D, glucagon A, or serotonin enterochromaffin cells (50)
. Both open- and closed-type cells exist in the stomach (52)
, suggesting they receive both luminal and neuroendocrine information. Lower levels of ghrelin are found in the small and large intestine, pituitary gland (53)
, 
(54)
, ß (55)
, or novel (56)
cells of the pancreatic islet, and neurons of the arcuate nucleus (ARC) in the basal hypothalamus (14)
. Ghrelin has also been detected in kidney (57)
, testis (58)
, placenta (59)
, and immune cells (29)
. Like motilin, plasma ghrelin concentrations rise progressively during fasting and fall to a nadir within an hour of refeeding.
Gastric ghrelin production is regulated by nutritional and hormonal factors (17
, 49
, 60
, 61)
. Inhibitory signals seem to include somatostatin, interleukin 1ß (IL-1ß), growth hormone itself, high-fat diet, and vagal tone, whereas fasting and a low-protein diet lead to increased expression and plasma concentrations of ghrelin. Regulation of circulating ghrelin during fasting and feeding may involve distinct mechanisms and pathways. The finding that GH lowers ghrelin expression suggests a feedback loop between stomach ghrelin and pituitary GH (49)
, although systemic ghrelin concentrations appear not to be decreased in human acromegaly despite high GH concentrations (62)
. The effects of leptin on ghrelin production are variably reported, i.e., increase, no change, or decrease, which might depend on the dosage or period of leptin administration and feeding conditions of the animals (49
, 63
, 64)
. The latter may be important since increased ghrelin expression is observed after high-fat feeding for a short period that is evident only in the fasting condition (65)
. No direct involvement of insulin or glucose is demonstrated in healthy humans at physiological concentrations (66)
. The increased secretion of stomach ghrelin with fasting is unique when contrasted to a majority of gut hormones whose secretion increases with food intake and decreases with fasting.
Despite recent growth in our understanding of peripheral signals to hypothalamic pathways, ghrelin adds a new dimension to the interplay since it is the first gut peptide proved to have orexigenic properties (15
, 17)
. Although motilin can stimulate food intake after central administration, it produces the least effect when administered peripherally (67
68
69)
. Almost all other gut peptides such as cholecystokinin (CCK) (70
71
72)
, glucagon-like peptide-1 (GLP-1) (73)
, peptide YY (PYY) 3-36 (74)
, and PP (64)
, as well as neural signals derived from the gut consist of satiety systems. They influence the termination of individual meals and/or relate to the long-term regulation of body weight. Ghrelin administered into the periphery or cerebral ventricles potently stimulates food intake, leading to body weight gain in rodents (15
, 16
, 75
76
77)
. The premeal increase of ghrelin may trigger the desire to eat in animals and humans.
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GHRELIN ACTS ON NPY/AgRP NEURONS IN THE HYPOTHALAMUS |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 16
, 75
76
77)
(Fig. 2
). Hypothalamic NPY is synthesized primarily in ARC neurons that project to adjacent hypothalamic areas such as the paraventricular nucleus (PVN), lateral hypothalamic area (LHA), and brainstem, major integration areas for the regulation of feeding behavior, energy expenditure, and gastrointestinal function (78
79
80)
. NPY is a powerful orexigenic neuropeptide. Chronic central NPY administration results in sustained hyperphagia, body weight gain, and a marked increase of body fat accumulation (81)
. NPY promotes net energy gain by increasing food intake, decreasing energy expenditure, and exerting effects in peripheral tissues, including stimulation of glucocorticoid and insulin secretion, that favor further deposition of triglycerides in white adipose tissue.
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AgRP was identified as a 132 amino acid peptide with 25% homology to the agouti protein that is a competitive antagonist of the melanocortin-1 receptor in hair follicles (79
, 80)
. The melanocortin system is unique in that it consists of agonists, melanocortins such as -melanocyte-stimulating hormone (-MSH), and an endogenous antagonist, AgRP. By antagonism of the melanocortin-3 receptor and melanocortin-4 receptor, AgRP stimulates food intake and decreases energy expenditure. In the brain, AgRP is synthesized exclusively in the ARC by neurons that project to adjacent hypothalamic areas such as the PVN and LHA. The demonstration that AgRP mRNA is abundantly colocalized with NPY identifies NPY/AgRP neurons as a unique subset capable of increasing food intake via two mechanisms: by increasing orexigenic NPY signaling on the one hand and decreasing anorexigenic melanocortin signaling on the other (79
, 82)
.
Arcuate NPY/AgRP neurons appear to play an important role in energy homeostasis, transducing changes of body fat content into adaptive feeding responses that are communicated by leptin and insulin, two adipostatic hormones with their circulating levels generally proportional to energy stores in adipose tissue (79)
. The ARC is strategically positioned to be in direct communication with peripheral signals because of the absence of blood-brain barrier (BBB). Thus, the NPY/AgRP system senses and respond to a wide variety of hormones and nutrients relevant to both short- and long-term aspects of the regulation of energy balance. Defective inhibition of NPY/AgPP neurons can have major consequences: hyperphagia, reducing energy expenditure, disturbances of glucose and lipid metabolism, and obesity (7
, 9
, 11
, 78
79
80
81
82
83)
.
NPY/AgRP gene expression is up-regulated during periods of negative energy balance such as fasting and insulin-deficient diabetes, although the magnitude of the respective response may differ (79)
. Leptin and insulin subserve overlapping functions as principal inhibitors of NPY/AgRP neurons; a decrease in inhibitory input from these adiposity signals activates NPY/AgRP and induces hyperphagic responses. These responses are mediated in part by the two orexigenic peptide systems in the LHA: melanin-concentrating hormone (MCH) and orexin, and projections from the ARC NPY/AgRP synapse on the MCH-or orexin-producing neurons (6
, 11
, 84
, 85)
.
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GHRELIN IS A MISSING LINK BETWEEN ENTERIC NUTRITION AND CENTRAL REGULATION OF ENERGY BALANCE AND GROWTH |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 76
, 77)
. Consistent with this, the majority of NPY neurons in the arcuate nucleus express GHS receptor (86)
, and 
50% of cells activated (that express c-Fos mRNA) after GHS administration contain NPY mRNA (87)
. Peripherally administered ghrelin induces Fos specifically in the ARC, and 90% of the Fos-positive neurons express NPY and coexisting AgRP (88
, 89)
. NPY and AgRP mRNA expression in the ARC are increased after ghrelin administration to fed rats (16
, 76
, 77
, 90)
. The orexigenic activity of ghrelin is consistent with findings reported earlier showing that central administration of GHSs such as GHRP-6 and KP102 stimulates food intake and weight gain in rats (91
92
93
94)
. Chronic GHRP-2 treatment stimulates the accumulation of adipose tissue mass in NPY-deficient mice as in controls, paralleled by an increase of hypothalamic AgRP mRNA expression (95)
. Central GHSs also activate Orexin (but not MCH) neurons, and ghrelin-induced feeding is attenuated by pretreatment with anti-orexin (but not anti-MCH) IgGs or in orexin knockout mice, indicating that ghrelin may selectively activate certain orexigenic pathway from NPY/AgRP to orexin (6
, 11
, 96
, 97)
.
The ability to delete a gene of interest in a tissue-specific manner is a considerable advantage in studying the function of the gene (8
, 9)
. Transgenic rats were recently created using an antisense GHS receptor mRNA under the control of the promoter for tyrosine hydroxylase to selectively attenuate GHS receptor protein expression in the ARC (98)
. Transgenic rats have an expected phenotype with decreased food intake, lower body weight, and less adipose tissue; female rats show decreased GH secretion and plasma insulin-like growth factor-1 concentrations. This indicates the importance of ARC in ghrelin’s action, also supported by experiments in rats treated neonatally with monosodium glutamate (MSG) (99)
. MSG causes specific lesions in the ARC, resulting in a 70–90% destruction of neuronal cell bodies including GHRH-, NPY-, and AgRP-containing neurons (99
100
101)
. In these rats, ghrelin shows a markedly diminished GH release and a failure to stimulate food intake. These results indicate that the primary site of action of ghrelin on appetite and GH release resides in the ARC, where circulating ghrelin is able to gain access (96
, 102)
. Although peripheral hormones such as leptin and insulin have been shown to cross the BBB by saturable and nonsaturable mechanisms (103
, 104)
, the transport process for ghrelin appears to be complex with only limited ability to penetrate into the brain areas within the BBB (105)
.
The isolation of ghrelin from the stomach was thus a landmark finding not only in the GH field, but also in appetite and energy metabolism regulation (15
16
17
, 60
, 75
76
77)
. During starvation, when leptin levels rapidly decline, circulating ghrelin levels increase, leading to the stimulation of NPY/AGRP production in the ARC (17)
. Therefore, ghrelin and leptin are complementary, yet antagonistic, signals of the regulatory feedback loop that has developed to inform the brain about the current status of acute and chronic energy balance (Fig. 2)
. Ghrelin is able to act on leptin-responsive arcuate neurons, the opposite effect of which may serve as a neurophysiological correlate of the orexigenic and anorexigenic effects of ghrelin and leptin (106
107
108)
. This view is supported by a recent finding that central leptin gene therapy blocks high-fat, diet-induced weight gain, hyperleptinemia, and hyperinsulinemia yet increases circulating ghrelin levels (109)
. It is likely that the ghrelin and NPY/AgRP systems respond to fasting and negative energy balance at gastric and hypothalamic levels; ghrelin secretion may be more complex, responding to dietary manipulations, since stomach ghrelin cells are exposed to the luminal contents. The effects of GH in promoting the growth of soft tissue such as cartilage, together with the potent GH-releasing and orexigenic effect of ghrelin, indicate an involvement of the stomach in the regulation of growth processes (17
, 76)
.
Ghrelin’s effect on body weight in rodents is due in part to altered metabolism and energy expenditure. Ghrelin reduces fat utilization and core body temperature in rats (15
, 96)
and reduces oxygen consumption in mice (16)
. The brain controls energy expenditure via the sympathetic nervous system, which heavily innervates thermogenic tissues such as brown adipose tissue and skeletal muscle (110)
. Ghrelin can reduce energy expenditure via NPY/AgRP pathway although the precise mechanism of the anabolic actions has yet to be fully characterized (16)
. The brain also affects energy expenditure by means of the hypothalamic-pituitary-thyroid axis. Ghrelin and other GHSs have been shown to decrease TSH levels and to stimulate the hypothalamic-pituitary-adrenal (HPA) axis, both resulting in an induction of a positive energy balance (111
, 112)
. Ghrelin may trigger increased release of GH and ACTH during fasting and be a key player connecting the nutritional state with endocrine changes. Leptin signals these endocrine axes in an opposite manner when a positive energy balance exists.
Ghrelin and leptin show antagonistic activity on gastrointestinal functions (16
, 17)
. Ghrelin increases gastric acid secretion, motility, and emptying (16
, 113)
whereas leptin decreases them. The gastro-prokinetic activity of ghrelin is independent of its GH-releasing effect and is likely to be mediated by the vagal-cholinergic muscarinic pathway (16
, 37
, 113)
. Ghrelin is a potent gastro-prokinetic agent that accelerates the normal emptying process in rats and mice with doses compatible to those required to stimulate appetite and GH release (16
, 114)
. Ghrelin also accelerates the transit of the small intestine but not that of the colon. The 28 amino acid and the des [Gln14]-27 amino acid octanoylated forms appear to be equally bioactive (114)
. It was reported that calcitonin gene-related peptide (CGRP) 8-37, an antagonist of CGRP, is among the few agents with beneficial effects on the inhibition of gastric emptying after surgery (115)
. Ghrelin is more active than CGRP 8-37 and is capable of reversing the postoperative gastric ileus in which motilin or motilin receptor agonist erythromycin is without effect (114)
. Therefore, ghrelin may be the most potent drug tested to accelerate gastric emptying in rodents.
Plasma motilin concentrations are known to be highly variable during the interdigestive fasting period (34
, 39
, 116)
. Periodic and recurrent increases in motilin are seen, which are synchronized with the migrating motor complex (MMC) of the fasting gastrointestinal tract. MMC is a recurrent pattern of cyclic occurring motor activity and can be divided into three distinct phases: phase I characterized by motor quiescence; phase II consisting of irregular motor activity; and phase III, a short burst of rhythmic contractions (37
, 117
118
119)
. In dogs and humans, the rises in circulating motilin regulate the appearance of the phase III contractions from the antroduodenal region to the distal gut (37
, 42)
. We recently examined the effect of ghrelin on the gastroduodenal motility in a freely moving conscious rat model that permits the measurement in the physiological fed and fasted states of the animals (Fig. 3
) (120
, 121)
. When ghrelin is administered into the lateral cerebral ventricle or the tail vein of fasted animals, it potentiates phase III-like contractions in the antrum and duodenum. When ghrelin is administered in fed animals, it disrupts the fed motor activity and induces phase III-like contractions despite the presence of food in the stomach. The delayed occurrence of fasted motor activity after intravenous ghrelin is due to the low intragastric pH, which is normally involved in the postprandial interruption of fasted motor activity (118
, 122)
. Vagal and nonvagal pathways may mediate these actions of ghrelin since vagotomy blocks the responses by central, but not peripheral, administration of ghrelin (114)
. Vagotomy is reported not to block the inhibition of pancreatic protein secretion by ghrelin, which may be exerted at the level of intrapancreatic neurons (123)
.
|
Previous studies demonstrated that food intake and gastrointestinal fed and fasted motor activities are closely related (124
, 125)
. NPY not only induces food intake in fed animals but also changes gastroduodenal motor activity from fed to fasted pattern (Fig. 3)
(120)
. This motor effect of NPY is mediated by the Y2 receptor rather than the Y1 and Y5 receptors, which mediate appetite stimulation by the peptide (78)
. Administration of anti-NPY antiserum into the cerebral ventricle abolishes the fasted motor activity and induces fed-like contractions, indicating an involvement of the endogenous NPY and Y2 receptor system in the generation of fasted motor activity (120)
. AgRP is also able to induce fasted motor activity in fed animals (unpublished data). Therefore, the interdigestive phasic contraction can be correlated well with the onset of hunger sensation and initiation of feeding. On the other hand, urocortin, a potent anorexigenic peptide belonging to corticotropin-releasing factor (CRF) family, induces fed-like motor activity when administered centrally or peripherally in fasted animals (126)
. CCK is another example that suppresses feeding and MMC following either route of administration (127
128
129)
. The finding that the gastroduodenal motor effect of ghrelin is blocked not only by the GHS receptor antagonist [D-Lys3] GHRP-6, but also by immunoneutralization of NPY in the brain, indicates that NPY neurons mediates this effect under the presence of the vagus nerve (Fig. 3)
(121)
. Since ghrelin antagonist/antiserum treatment reduces body adiposity and gastric emptying, ghrelin may be critically involved in these alterations (17
, 65)
. Ghrelin thus illustrates the importance of functional interdependency of the stomach and hypothalamus for the integration of feeding and associated autonomic, neuroendocrine, and gastrointestinal functions (Fig. 2)
.
It is reported that NPY and Y2 agonist inhibit gastric emptying of solid or liquid meals (130)
and a Y2 agonist applied to the dorsal vagal complex suppresses thyrotropin-releasing hormone (TRH) -stimulated gastric motility (131)
. Since the ghrelin-NPY pathway is down-regulated after feeding, it is not known whether it plays an important role in postprandial gastrointestinal physiology. It is generally agreed that motilin’s physiological effect is limited to the interdigestive state (132)
. Although PYY3-36 is a physiological satiety factor acting on the NPY Y2 receptor in the ARC, it does not affect gastric emptying (74)
.
Although peripherally administered ghrelin has no effect on gastric acid secretion in pylorus-ligated conscious rats, central ghrelin produces a long-lasting gastric inhibitory action (133)
(though stimulatory under anesthesia; 134
, 135
). Distinct pathways may be involved in gastric and feeding activities, since EP40737, a hexapeptide that stimulates GH release but is devoid of effects on feeding (136)
, is able to reduce gastric acid secretion (133)
. It remains to be determined whether a small amount of ghrelin produced in the hypothalamus (which hardly contributes to circulating ghrelin) (62)
is physiologically relevant and involved in the regulatory feedback loop (Fig. 2)
. Ghrelin-like immunoreactive neurons are observed in the ventral part of the ARC (137)
or the internuclear space between the PVN, ARC, VMH, and DMH (108)
. The axons of hypothalamic ghrelin neurons abut NPY axons presynaptically in the ARC and PVN and may increase secretion of GABA, NPY, and AgRP (108)
.
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THE STOMACH IS AN IMPORTANT COMPONENT OF ENERGY HOMEOSTASIS EQUATION |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 124
, 126
, 138
139
140
141
142
143
144
145
146)
. Gastric distension acts as a satiety signal to inhibit food intake, and rapid gastric emptying is closely related to overeating and obesity (139
, 140)
. Lesions of the ventromedial nucleus (VMH) of the hypothalamus are known to disrupt autonomic output controlling the stomach and increase the normally slow daylight rate of gastric emptying of regular diet (140)
. The abbreviated satiating effect of food resulting in such gastric acceleration is thought to be a major cause in the obese VMH syndrome, along with other autonomic defects such as insulin hypersecretion and reductions in fat mobilization and thermogenic capacity (5
, 7
, 9)
.
Most anorexigenic molecules such as CCK, CRF, and IL-1 not only suppress feeding but also decrease gastric transit1 (24
, 126
, 139
, 141
142
143
144
145
146
147)
. The inhibitory effect of peripherally administered CCK-8 on the rate of gastric emptying has been shown to contribute to its ability to inhibit food intake in various species (16
, 138
, 148
150
151)
. The inhibitory effects on gastric emptying by CRF, CRF type2 receptor agonist urocortin, and their analogs paralleled the inhibitory effects on food intake (139)
. The ob/ob mice are another example that shows rapid gastric emptying and overeating at the development of obesity (1
, 6
, 16
, 139)
. Urocortin potently decreased food intake and reversed rapid gastric emptying in the animals (139)
. Repeated administration of urocortin decreased body weight and improved glycemic control, indicating that the decreased absorption rate of nutrients is beneficial not only to decrease food intake but also to decrease the requirements of insulin in obese diabetic animals. GLP-1 or its potent analogs may be effective in treating such diabetic animal models and humans (73
, 152)
although they may affect sympathetic outflow (153)
.
Recently leptin and its receptor were identified in gastric mucosa in animals (154
, 155)
and humans (156
, 157)
(Fig. 2)
. Gastric leptin can be released into the blood and gastric juice after feeding, insulin-induced hypoglycemia, or infusion of CCK-8, pentagastrin, and secretin, providing evidence that leptin is a stomach-derived protein (154
, 156
, 157)
. Gastric leptin may have a role in appetite regulation by acting directly in the hypothalamus or in synergy with CCK via the vagal pathway (72
, 157
158
159)
or by modifying absorption of dietary protein and fats (160)
. How leptin interacts with ghrelin in the stomach needs to be determined. However, the existence of a regulatory feedback loop involving the stomach suggests a fascinating role of ghrelin and leptin as an interface between regulatory centers in the brain of nutritional intake, gastrointestinal function, metabolic control, and growth regulation.
Enterostatin may be another component of this system, formed through the processing of pancreatic procolipase, a protein necessary for intestinal fat digestion (161)
. It is also produced in chief cells of the stomach as part of procolipase as well as in enterochromaffine cells independent of procolipase, most frequently in the antral part of the stomach extending all along the intestine down to the ileum (162)
. Enterostatin increases during fat feeding, where it is found in circulating blood as well as in intestinal lumen and the lymph. When reducing food intake by either peripheral or central administration, enterostatin behaves as a physiological satiety substance, inducing early satiety (163)
. During long-term treatment, enterostatin is able to reduce body weight in rodents when fed a high-fat diet. A reduced body weight may be explained by a decreased food intake involving delayed gastric emptying (164)
and an increased energy expenditure through activation of the sympathetic nervous system and increased expression of uncoupling protein 1 in brown adipose tissue. The mechanism of action for restriction of fat intake may involve serotonin, opioids and dopamine, which are components of the reward system (161)
. A more complete understanding of how the stomach regulates energy balance and other brain functions thus depends upon the elucidation of the gastric machinery and network of which molecules like ghrelin, leptin, and enterostatin are a part.
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VAGAL AFFERENT NEURONS ARE AN IMPORTANT TARGET FOR PEPTIDE ACTIONS |
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 158
, 165)
. Abdominal vagal afferents have their cell bodies in the nodose ganglion. Neurotransmitters and receptors synthesized within the ganglion and subsequently transported to the nerve terminals are being identified (166)
. The presence of leptin within the gut, the discovery of the leptin receptor on the vagus nerve, the modulatory effects of leptin on intestinal vagal mechanoreceptors, and the known involvement of the vagus and other afferents in transmitting satiety all suggest a need to evaluate the potential of other feeding regulatory hormones located within the hypothalamus and/or gut on vagal nerve function (166
169
170)
.
Orexins may be another example, produced in neurons and endocrine cells in the gut (171)
in addition to neurons in the LHA. A recent study localized the orexin-1 receptors to vagal afferent neurons that also expressed CCK-1 (A) and leptin receptors and showed that orexin-A inhibited the response of afferent nerve fibers to CCK (172)
. The mechanism by which CCK signals to the brain on satiety is known to include activation of CCK-1(A) receptors produced in vagal afferent cell soma and transported peripherally (173
174
175)
. The dual CCK-leptin signals are transmitted to the nucleus tractus solitarius, then hypothalamic sites, of which the PVN appears to be the primary target, orchestrating appropriate food intake alterations (159)
.
Ghrelin receptors are also synthesized in vagal afferent cell soma and intravenous ghrelin suppresses firing of the vagal afferents (16
, 165)
. Selective chemical blockade by capsaicin or surgical deafferentation of the gastric vagal nerve blocks the Fos expression in the ARC and the feeding response to peripherally administered ghrelin. The finding that vagotomy eliminates the GH response to ghrelin indicates a novel vagus-mediated GH secretion pathway (165)
in addition to the known vagus-mediated regulation of gastrointestinal motility and secretion, behavioral responses such as satiety, and pathophysiological sensations and reflexes such as nausea and vomiting (166)
. These data stress the importance of the vagus nerve in conveying ghrelin signals to the brain although the extent of the dependence needs to be further examined.
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TOPABSTRACTINTRODUCTIONGROWTH HORMONE SECRETAGOGUES...GHRELIN IS AN ENDOGENOUS...GHRELIN ACTS ON NPY/AgRP...GHRELIN IS A MISSING...THE STOMACH IS AN...VAGAL AFFERENT NEURONS ARE...CLINICAL IMPLICATIONSPERSPECTIVEREFERENCES |
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, 176
, 177)
. It may be a convenient, safe, and reliable test for the diagnosis of GH deficiency. Administration of GHS is preferable to direct administration of GH because it induces a more physiological profile of GH secretion, not bypassing the feedback loop controlling the GH-IGF-1 axis (178)
. Potential targets for GHS compounds include children (179)
and adults with GH deficiency associated with alterations in body composition, increased prevalence of cardiovascular morbidity, and shortened life expectancy (180
, 181)
. The majority of GH-deficient subjects respond to GHS, which can be administered intravenously, intramuscularly, subcutaneously, orally, intranasally, or transdermally (17
, 19
, 182)
. GHS analogs could be used in all pathophysiological states where the administration of moderate GH doses has been shown to be effective such as aging, catabolic states, and osteoporosis. Although ghrelin levels in GH-deficient subjects are not significantly different from controls (183)
, it remains to be seen whether some subjects can lack ghrelin and have growth-retarded phenotype and whether the ectopic production of ghrelin can lead to acromegaly (23)
. Ghrelin has been shown to regulate expression of a pituitary-specific transcription factor, Pit-1, suggesting a role in somatotroph differentiation in addition to GH secretion (184)
. Transgenic animals that overexpress or delete the ghrelin gene may help address these issues.
Obesity
Obesity is characterized by a blunted GH secretion that might help to maintain this state and is reversed by weight loss (24
, 185)
. Fasting plasma ghrelin concentrations in obesity are significantly lower and are negatively correlated with body mass index, percent body fat, and/or fasting insulin and leptin concentrations (180
, 187)
. The low levels of ghrelin might contribute to the decreased GH secretion in obese patients. Plasma ghrelin levels are also negatively correlated with plasminogen activator 1 (PAI-1) levels that are elevated in insulin-resistant subjects and associated with increased cardiovascular risk of atherothrombosis (188)
. The decreased ghrelin secretion in established obesity may be a physiological adaptation to long-term positive energy balance although a particular subset of obesity may be associated with high levels of ghrelin (189)
. Despite its low levels, ghrelin may act to maintain the increased body weight as with NPY (190
, 191)
, since weight loss increases ghrelin levels that are correlated with the extent of weight loss (189)
. The lack of suppression of plasma ghrelin after a meal in obese subjects may contribute to increased food consumption (192)
.
A recent study suggests that ghrelin is a long-term regulator of body weight rather than a short-term, meal-related orexigenic signal in humans (193)
. Gastric bypass surgery is an important treatment for morbid obesity that can produce prolonged weight reduction (194)
. Gastric bypass subjects exhibit markedly different ghrelin secretary profiles, with markedly reduced cumulative secretion as well as loss of premeal increase or diurnal variability that normally peaks between 12 and 2 AM and nadirs at 9–10 AM. Conversely, bypass subjects exhibit normal postprandial insulin secretion and diurnal leptin cycling. Therefore, suppression of plasma ghrelin by gastric bypass surgery can contribute to the efficacy of this procedure as weight reduction therapy in addition to the physical restriction of the stomach (gastric pouch) and the reduction of nutrient digestion and absorption (long Roux-en-Y) (195)
.
Prader-Willi syndrome is the most common form of human syndromic obesity, occurring in 1 of 10,000–16,000 live births (196
197
198)
. It is characterized by severe hyperphagia, GH deficiency, hypogonadism, neonatal hypotonia, dysmorphic features and cognitive impairment. Although Prader-Willi syndrome arises from functional loss of several paternally expressed genes in an imprinted domain on chromosome 15, mediators of the phenotype are not well known (9
, 196)
. Recent studies demonstrated that while obesity per se is associated with low ghrelin levels, that accompanied by Prader-Willi syndrome is associated with elevated ghrelin with no decrease after a meal (197
, 198)
. The increased ghrelin levels are comparable to or higher than those reported to stimulate appetite and food intake during exogenous ghrelin administration in humans (199)
, suggesting a role of ghrelin in the pathogenesis of hyperphagia in Prader-Willi syndrome. It remains to be determined whether elevated ghrelin participates in the GH deficiency by such mechanisms as paradoxical override inhibition described in continuous GHRH stimulation of GH (197
, 200)
. Interventions that lower plasma ghrelin levels or antagonize its orexigenic effects warrant consideration in treatment of this type of obesity. Gastric bypass may have less efficacy in Prader-Willi patients than in other obese children and adolescents (201
, 202)
. Ghrelin levels are low or normal in patients with other genetic obesity such as those with leptin receptor mutation or melanocortin 4 receptor mutation (197)
.
Rapid gastric emptying may have a role in apparent leptin resistance since the gastromotor abnormality is quantitatively the most important precondition of excess fat deposition and the hyperphagic pattern of frequent meals in some animal models of obesity (140)
. Rapid gastric emptying has been reported in clinically obese populations, and the appetite suppressant, antiobesity agent fenfluramine (although withdrawn from the global market due to the unfortunate development of valvuloplasty) (10
, 203)
may reduce food intake by slowing gastric emptying and so extending the satiating effect of a meal (140
, 204
, 205)
. Gastric emptying half-time as assessed by 13C-octanoic acid breath test is correlated with fasting plasma ghrelin levels in healthy humans, though this could be interpreted as a physiological reaction to balance of gastric motor function (206)
. A failure to demonstrate the prokinetic effect of a physiological dose of exogenous ghrelin is reported when gastric emptying is measured using the paracetamol absorption test (199)
. More studies, especially those using radionuclide techniques (207)
, are needed to evaluate the actions of ghrelin/GHS on gastric emptying in humans. It remains to be examined whether ghrelin/GHS antagonist is effective in slowing abnormal gastric emptying and ameliorating excessive food intake and fat deposition in humans as it does in animals (61)
.
Gastrointestinal disorders
The beneficial effect of ghrelin in an animal ileus model (107)
is interesting. Abdominal surgery inhibits gastric emptying and digestive motor activity in experimental animals and humans (114
, 208
, 209)
. Abdominal surgery with manipulation of viscera induces postoperatively a state of digestive motor inactivity that may occasionally be overly prolonged while associated with significant morbidity; these patients could certainly benefit from a specific and effective treatment. Attempts by various prokinetics such as acetylcholine, cisapride, and motilide to stimulate directly or via the afferent neural pathways frequently gave disappointing results (208
209
210)
. However, ghrelin is the most potent drug to reverse postoperative gastric ileus in the animal model (114)
. It should be examined whether ghrelin or GHS compounds could help prevent postoperative gastric ileus in humans.
Gastroparesis is associated with many diseases, the most frequent cause of which is diabetes mellitus. About one-half of patients with insulin-dependent or non-insulin-dependent diabetes have delayed gastric emptying. Diabetic gastroparesis is generally ascribed as a consequence of autonomic neuropathy although hyperglycemia per se may contribute to the pathogenesis of disordered gastric motility (211)
. Motilin and erythromycin are able to accelerate gastric emptying in such patients probably by direct effects on smooth muscles (132
, 207
, 212
, 213)
. Due to the potent prokinetic activity in animals, ghrelin and GHS compounds may also offer therapeutic potentials to treat the disorder.
Short bowel syndrome is a condition that occurs after resection of a substantial portion of small intestine and is characterized by malnutrition. The malnourished patients have lower plasma ghrelin concentrations in contrast to the expected increase (214)
. This indicates that significant amounts of ghrelin are produced by neuroendocrine cells in the intestine and the decrease in plasma ghrelin levels may contribute to decreased appetite in these patients. It is possible that the short bowel feeds back on the release of ghrelin.
Helicobacter pylori is known to modify leptin expression in the gastric mucosa (215)
. After eradication of H. pylori, median and integrated plasma ghrelin is increased significantly, making it likely that the association of H. pylori and appetite is mediated in part through ghrelin. Although there are no studies in adults illustrating whether H. pylori-positive subjects have a lower body weight than negative controls, H. pylori children may have a high incidence of growth retardation (216
217
218
219
220
221)
, as demonstrated in experimental animals (222)
.
A recent study indicates a potent protective effect against ethanol-induced gastric lesions by central ghrelin and a partial peripheral protective effect (223)
. This effect of ghrelin is mediated by endogenous nitric oxide (NO) release and requires the integrity of sensory nerve fibers. Since acute hemorrhagic and erosive gastropathy may represent a potentially life-threatening condition, ghrelin may deserve attention as a new pharmacological tool for the treatment.
Cachexia
Recent studies suggest that ghrelin may be a clinical marker of catabolism (17)
. Elevated levels of circulating ghrelin are observed in cachexia associated with chronic heart failure (224)
or liver cirrhosis. Indeed, GHSs such as MK-0677 and GHRP-2 are reported to reverse diet-induced catabolism and to improve alterations in the somatotrophic axis and protein catabolism in patients with prolonged, severe illnesses (24
, 225
, 226)
.
GH has been used as a potential anabolic agent to counteract muscle wasting associated with surgical stress, sepsis, glucocorticoid administration, AIDS, and cancer (227
, 228)
. GH stimulates whole body and muscle protein synthesis under some conditions. Ghrelin or ghrelin/GHS compounds may improve the treatment of such patients, especially aging patients in whom reduced GH secretion, reduced muscle mass, and anorexia are often observed (227)
. The age-related decline of plasma ghrelin may explain in par, the somatotrophic dysregulation and anorexia of elderly subjects (229)
. Chronic treatment of elderly with MK-0677 is reported to reverse age-related changes of the GH/IGF-1 axis and to improve the quality of sleep in healthy elderly subjects (25
, 230)
.
The impressive orexigenic effect of ghrelin administered peripherally in rodents indicates it warrants evaluation in treating conditions associated with excessive weight loss such as cancer. Weight loss is a potent stimulus to food intake in normal humans and animals. Therefore, the persistence of anorexia in cancer implies a failure of this adaptive feeding response (147
, 231)
. Cytokines, proposed mediators of the cachectic process, may play a pivotal role in long-term inhibition of feeding by mimicking the hypothalamic effect of excess negative feedback signaling fr
) is replaced by the fed motor activity consisting of irregular contractions after food ingestion (A). Administration of ghrelin into the tail vein (B) or the cerebral ventricle of fed rats powerfully affects the motility, changing the fed pattern into the fasted pattern despite the presence of food in the stomach. This effect is mediated by the ghrelin receptor since the GHS antagonist [D-Ly3] GHRP-6 administered by the same route blocks it (C). Administration of NPY into the cerebral ventricle also produces the MMC in fed rats (D). Immunoneutralization by anti-NPY antiserum administered intracerebroventricularly blocks not only the MMC induced by exogenous NPY, but also the MMC induced by exogenous ghrelin (E) or that occurring physiologically during fasting (F) in the animals.