Sites of action of ghrelin receptor ligands in cardiovascular control

Obesity-Induced PVAT Dysfunction and Atherosclerosis Development: The Role of GHSR-1a in Increased Macrophage Infiltration and Adipocytokine Secretion

In obesity, recent research revealed that increased expression of the growth hormone secretagogue receptor (GHSR) in macrophages plays a pivotal role in the development of meta-inflammation, promoting macrophage infiltration and pro-inflammatory polarization. This study aimed to examine the association between GHSR-1a expression in atherosclerotic plaques and adjacent perivascular adipose tissue (PVAT) from 11 patients with obesity and peripheral artery disease (PAD) who underwent revascularization procedures. Immunohistochemistry was used to assess the expression of CD68, CD80, and CD14, while tissue homogenate levels of adiponectin, leptin, IL-6, and CRP were quantified via ELISA. Serum markers of inflammation were also measured. Among patients with GHSR-1a-positive (+) macrophages in atherosclerotic plaques, we observed significantly higher white blood cell counts and platelet-to-lymphocyte ratios in serum, a lower adiponectin-to-leptin ratio, and elevated IL-6 levels in both arterial and PVAT homogenates. Our findings suggest a link between GHSR-1a and macrophage/monocyte infiltration, macrophage polarization, and adipocytokine secretion in atherosclerotic plaques associated with obesity-induced PVAT dysfunction.

Molecular Mechanisms and Health Benefits of Ghrelin: A Narrative Review

Ghrelin, an endogenous brain-gut peptide, is secreted in large quantities, mainly from the stomach, in humans and rodents. It can perform the biological function of activating the growth hormone secretagogue receptor (GHSR). Since its discovery in 1999, ample research has focused on promoting its effects on the human appetite and pleasure-reward eating. Extensive, in-depth studies have shown that ghrelin is widely secreted and distributed in tissues. Its role in neurohumoral regulation, such as metabolic homeostasis, inflammation, cardiovascular regulation, anxiety and depression, and advanced cancer cachexia, has attracted increasing attention. However, the effects and regulatory mechanisms of ghrelin on obesity, gastrointestinal (GI) inflammation, cardiovascular disease, stress regulation, cachexia treatment, and the prognosis of advanced cancer have not been fully summarized. This review summarizes ghrelin's numerous effects in participating in a variety of biochemical pathways and the clinical significance of ghrelin in the regulation of the homeostasis of organisms. In addition, potential mechanisms are also introduced.

Effect of Ghrelin on the Cardiovascular System

Simple Summary

Ghrelin is an octanoylated peptide that was initially isolated from rat and human stomachs in the process of searching for an endogenous ligand to the orphan growth hormone secretagogue receptor (GHS-R), a G-protein-coupled receptor. Exogenous or endogenous ghrelin secreted from the stomach binds to GHS-R on gastric vagal nerve terminals, and the signals are transmitted to the central nervous system via the vagal afferent nerve to facilitate growth hormone (GH) secretion, feeding, sympathetic inhibition, parasympathetic activation, and anabolic effects. Ghrelin also binds directly to the pituitary GHS-R and stimulates GH secretion. Ghrelin has beneficial effects on the cardiovascular system, including cardioprotective effects such as anti-heart failure, anti-arrhythmic, and anti-inflammatory actions, and it enhances vascular activity via GHS-R-dependent stimulation of GH/IGF-1 (insulin-like growth factor-1) and modulation of the autonomic nervous system. The anti-heart failure effects of ghrelin could be useful as a new therapeutic strategy for chronic heart failure.

Abstract

Ghrelin, an n-octanoyl-modified 28-amino-acid-peptide, was first discovered in the human and rat stomach as an endogenous ligand for the growth hormone secretagogue receptor (GHS-R). Ghrelin-GHS-R1a signaling regulates feeding behavior and energy balance, promotes vascular activity and angiogenesis, improves arrhythmia and heart failure, and also protects against cardiovascular disease by suppressing cardiac remodeling after myocardial infarction. Ghrelin’s cardiovascular protective effects are mediated by the suppression of sympathetic activity; activation of parasympathetic activity; alleviation of vascular endothelial dysfunction; and regulation of inflammation, apoptosis, and autophagy. The physiological functions of ghrelin should be clarified to determine its pharmacological potential as a cardiovascular medication.

The effect of the ghrelin-receptor agonist capromorelin on glucose metabolism in healthy cats

Somatostatin secretion from islet delta cells is important in maintaining low glycemic variability (GV) by providing negative feedback to beta cells and inhibiting insulin secretion. Capromorelin is a ghrelin-receptor agonist that activates the growth hormone secretagogue receptor on delta cells. We hypothesized that in cats, capromorelin administration will result in decreased GV at the expense of reduced insulin secretion and glucose tolerance. Seven healthy cats were treated with capromorelin from days 1-30. After the first day, fasting blood glucose increased (+13 ± 3 mg/dL, P < 0.0001), insulin decreased (+128 ± 122 ng/dL, P = 0.03), and glucagon was unchanged. Blood glucose was increased throughout an intravenous glucose tolerance test on day 1 with blunting of first-phase insulin response ([FPIR] 4,931 ± 2,597 ng/L/15 min) compared with day -3 (17,437 ± 8,302 ng/L/15 min, P = 0.004). On day 30, FPIR was still blunted (9,993 ± 4,285 ng/L/15 min, P = 0.045), but glucose tolerance returned to baseline. Mean interstitial glucose was increased (+19 ± 6 mg/dL, P = 0.03) on days 2-4 but returned to baseline by days 27-29 (P = 0.3). On days 2-4, GV was increased (SD = 9.7 ± 3.2) compared with baseline (SD = 5.0 ± 1.1, P = 0.02) and returned to baseline on days 27-29 (SD = 6.1 ± 1.1, P = 0.16). In summary, capromorelin caused a decline in insulin secretion and glycemic control and an increase in glucose variability early in the course of treatment, but these effects diminished toward the end of 30 d of treatment.

Brain and kidney GHS-R1a underexpression is associated with changes in renal function and hemodynamics during neurogenic hypertension

Ghrelin is a peptide hormone whose effects are mediated by the growth hormone secretagogue receptor subtype 1a (GHS-R1a), mainly expressed in the brain but also in kidneys. The hypothesis herein raised is that GHS-R1a would be player in the renal contribution to the neurogenic hypertension pathophysiology. To investigate GHS-R1a role on renal function and hemodynamics, we used Wistar (WT) and spontaneously hypertensive rats (SHR). First, we assessed the effect of systemically injected vehicle, ghrelin, GHS-R1a antagonist PF04628935, ghrelin plus PF04628935 or GHS-R1a synthetic agonist MK-677 in WT and SHR rats housed in metabolic cages (24 h). Blood and urine samples were also analyzed. Then, we assessed the GHS-R1a contribution to the control of renal vasomotion and hemodynamics in WT and SHR. Finally, we assessed the GHS-R1a levels in brain areas, aorta, renal artery, renal cortex and medulla of WT and SHR rats using western blot. We found that ghrelin and MK-677 changed osmolarity parameters of SHR, in a GHS-R1a-dependent manner. GHS-R1a antagonism reduced the urinary Na⁺ and K⁺ and creatinine clearance in WT but not in SHR. Ghrelin reduced arterial pressure and increased renal artery conductance in SHR. GHS-R1a protein levels were decreased in the kidney and brain areas of SHR when compared to WT. Therefore, GHS-R1a role in the control of renal function and hemodynamics during neurogenic hypertension seem to be different, and this may be related to brain and kidney GHS-R1a downregulation.

Systemic acyl-ghrelin increases tail skin temperature in rats without affecting their thermoregulatory behavior in a cold environment

Fasting increases ghrelin that is a peptide hormone with two circulating isoforms, acyl and des-acyl ghrelin. We reported that fasting or des-acyl ghrelin facilitates behavioral thermoregulation in the cold in rats assessed by tail-hiding behavior that was the indicator of rats' thermoregulatory behavior in the cold; however, the effect of acyl-ghrelin on the same process remains to be elucidated. We investigated the effect of acyl-ghrelin on thermoregulatory behavior in the cold in rats. The animals received an intraperitoneal saline or 24 μg acyl-ghrelin injection and were exposed to 27 °C or 15 °C for 2 h, while their body temperature, tail skin temperature, and tail-hiding behavior were constantly monitored. cFos immunoreactive (cFos-IR) cells in the median preoptic area, medial preoptic area, paraventricular nucleus (PVN), and arcuate nucleus were counted. Body temperature and the duration of thermoregulatory behavior did not show a significant difference between the acyl-ghrelin-treated and control groups at 15 °C; however, tail skin temperature in the acyl-ghrelin-treated group was higher than that in the control group. The number of cFos-IR cells in the PVN was greater in the control group than that in the acyl-ghrelin-treated group at 27 °C. These results indicate that acyl-ghrelin did not affect behavioral thermoregulation but might affect tail skin temperature in rats in the cold.

Three-dimensional growth of breast cancer cells potentiates the anti-tumor effects of unacylated ghrelin and AZP-531

Breast cancer is the most common type of cancer in women and notwithstanding important therapeutic advances, remains the second leading cause of cancer-related death. Despite extensive research relating to the hormone ghrelin, responsible for the stimulation of growth hormone release and appetite, little is known of the effects of its unacylated form, especially in cancer. The present study aimed to characterize effects of unacylated ghrelin on breast cancer cells, define its mechanism of action, and explore the therapeutic potential of unacylated ghrelin or analog AZP-531. We report potent anti-tumor effects of unacylated ghrelin, dependent on cells being cultured in 3D in a biologically-relevant extracellular matrix. The mechanism of unacylated ghrelin-mediated growth inhibition involves activation of Gαi and suppression of MAPK signaling. AZP-531 also suppresses the growth of breast cancer cells in vitro and in xenografts, and may be a novel approach for the safe and effective treatment of breast cancer.

The expressions of some metabolic hormones (leptin, ghrelin and obestatin) in the tissues of sheep tongue

In this study, we aimed to observe the localization and expression of peptide hormones-leptin, ghrelin and obestatin-in the sheep tongue by immunohistochemistry. For that purpose, tongues of ten adult sheep were used. Leptin expression of moderate intensity was observed in the basal and parabasal epithelial cells of the luminal epithelium, and leptin was strongly expressed in the taste buds of the circumvallate and fungiform papillae and in von Ebner's glands. Ghrelin was primarily expressed in some of the skeletal muscle cells and the smooth muscle cells of the middle layer of blood vessels. A strong expression was observed in the epithelial cells lining the base of the groove surrounding the circumvallate papillae. Obestatin expression was particularly strong in the epithelial cells of the salivary ducts. It was also stronger in the von Ebner's glands than in the seromucous glands. Leptin, ghrelin and obestatin were shown to be produced at varying levels in different cell types, including epithelial, stromal and skeletal muscle cells, as well as in ganglion neurons, neural plexuses and blood vessels in the sheep tongue. Cellular localization and expression of these peptide hormones have not been investigated in many species including sheep.

Ghrelin and vascular protection

Ghrelin is a small peptide with important roles in the regulation of appetite, gut motility, glucose homeostasis as well as cardiovascular protection. This review highlights the role that acyl ghrelin plays in maintaining normal endothelial function by maintaining the balance of vasodilator-vasoconstrictor factors, inhibiting inflammatory cytokine production and immune cell recruitment to sites of vascular injury and by promoting angiogenesis.

Physiological significance of ghrelin in the cardiovascular system

Ghrelin, a growth hormone-releasing peptide first discovered in rat stomach in 1999, is a ligand for the growth hormone secretagogue receptor. It participates in the regulation of diverse processes, including energy balance and body weight maintenance, and appears to be beneficial for the treatment of cardiovascular diseases. In animal models of chronic heart failure, ghrelin improves cardiac function and remodeling; these findings have been recapitulated in human patients. In other animal models, ghrelin effectively diminishes pulmonary hypertension. Moreover, ghrelin administration early after myocardial infarction decreased the frequency of fatal arrhythmia and improved survival rate. In ghrelin-deficient mice, endogenous ghrelin protects against fatal arrhythmia and promotes remodeling after myocardial infarction. Although the mechanisms underlying the effects of ghrelin on the cardiovascular system have not been fully elucidated, its beneficial effects appear to be mediated through regulation of the autonomic nervous system. Ghrelin is a promising therapeutic agent for cardiac diseases.

Ghrelin and the heart

Ghrelin, a growth hormone-releasing peptide that was first discovered in the stomach of rats in 1999, is an endogenous ligand of growth hormone secretagogue receptor. Ghrelin exerts its potent growth hormone-releasing and orexigenic activities by binding to specific receptors in the brain. Subsequent studies showed that ghrelin participates in the regulation of diverse processes, including energy balance, body weight maintenance, and glucose and fat metabolism, and demonstrated that ghrelin is beneficial for treatment of cardiac diseases. In animal models of chronic heart failure, administration of ghrelin improves cardiac function and remodeling, and these findings were recapitulated in human patients with heart failure. Also in animal models, ghrelin administration effectively diminishes pulmonary hypertension induced by monocrotaline or chronic hypoxia. In addition, repeated administration of ghrelin to cachectic chronic obstructive pulmonary disease patients has positive effects on body composition, including amelioration of muscle wasting, improvement of functional capacity, and sympathetic activity. Moreover, administration of ghrelin early after myocardial infarction decreases the frequency of fatal arrhythmia and improved the survival rate. In ghrelin-deficient mice, both exogenous and endogenous ghrelin protects against fatal arrhythmia and promotes remodeling after myocardial infarction. Although the mechanisms underlying the effects of ghrelin on the cardiovascular system have not been fully elucidated, some evidence suggests that its beneficial effects are mediated through both direct actions on cardiovascular cells and regulation of autonomic nervous system activity. Therefore, ghrelin is a promising novel therapeutic agent for cardiac disease.

Ghrelin Pre-treatment Attenuates Local Oxidative Stress and End Organ Damage During Cardiopulmonary Bypass in Anesthetized Rats

Cardiopulmonary bypass (CPB) induced systemic inflammation significantly contributes to the development of postoperative complications, including respiratory failure, myocardial, renal and neurological dysfunction and ultimately can lead to failure of multiple organs. Ghrelin is a small endogenous peptide with wide ranging physiological effects on metabolism and cardiovascular regulation. Herein, we investigated the protective effects of ghrelin against CPB-induced inflammatory reactions, oxidative stress and acute organ damage. Adult male Sprague Dawley rats randomly received vehicle (n = 5) or a bolus of ghrelin (150 μg/kg, sc, n = 5) and were subjected to CPB for 4 h (protocol 1). In separate rats, ghrelin pre-treatment (protocol 2) was compared to two doses of ghrelin (protocol 3) before and after CPB for 2 h followed by recovery for 2 h. Blood samples were taken prior to CPB, and following CPB at 2 h and 4 h. Organ nitrosative stress (3-nitrotyrosine) was measured by Western blotting. CPB induced leukocytosis with increased plasma levels of tumor necrosis factor-α and interleukin-6 indicating a potent inflammatory response. Ghrelin treatment significantly reduced plasma organ damage markers (lactate dehydrogenase, aspartate aminotransferase, alanine aminotransferase) and protein levels of 3-nitrotyrosine, particularly in the brain, lung and liver, but only partly suppressed inflammatory cell invasion and did not reduce proinflammatory cytokine production. Ghrelin partially attenuated the CPB-induced elevation of epinephrine and to a lesser extent norepinephrine when compared to the CPB saline group, while dopamine levels were completely suppressed. Ghrelin treatment sustained plasma levels of reduced glutathione and decreased glutathione disulphide when compared to CPB saline rats. These results suggest that even though ghrelin only partially inhibited the large CPB induced increase in catecholamines and organ macrophage infiltration, it reduced oxidative stress and subsequent cell damage. Pre-treatment with ghrelin might provide an effective adjunct therapy for preventing widespread CPB induced organ injury.

Ghrelin receptor antagonism of hyperlocomotion in cocaine-sensitized mice requires βarrestin-2

The "brain-gut" peptide ghrelin, which mediates food-seeking behaviors, is recognized as a very strong endogenous modulator of dopamine (DA) signaling. Ghrelin binds the G protein-coupled receptor GHSR1a, and administration of ghrelin increases the rewarding properties of psychostimulants while ghrelin receptor antagonists decrease them. In addition, the GHSR1a signals through βarrestin-2 to regulate actin/stress fiber rearrangement, suggesting βarrestin-2 participation in the regulation of actin-mediated synaptic plasticity for addictive substances like cocaine. The effects of ghrelin receptor ligands on reward strongly suggest that modulation of ghrelin signaling could provide an effective strategy to ameliorate undesirable behaviors arising from addiction. To investigate this possibility, we tested the effects of ghrelin receptor antagonism in a cocaine behavioral sensitization paradigm using DA neuron-specific βarrestin-2 KO mice. Our results show that these mice sensitize to cocaine as well as wild-type littermates. The βarrestin-2 KO mice, however, no longer respond to the locomotor attenuating effects of the GHSR1a antagonist YIL781. The data presented here suggest that the separate stages of addictive behavior differ in their requirements for βarrestin-2 and show that pharmacological inhibition of βarrestin-2 function through GHSR1a antagonism is not equivalent to the loss of βarrestin-2 function achieved by genetic ablation. These data support targeting GHSR1a signaling in addiction therapy but indicate that using signaling biased compounds that modulate βarrestin-2 activity differentially from G protein activity may be required.

Cortistatin reduces atherosclerosis in hyperlipidemic ApoE-deficient mice and the formation of foam cells

Atherosclerosis is a chronic inflammatory cardiovascular disease that is responsible of high mortality worldwide. Evidence indicates that maladaptive autoimmune responses in the arterial wall play critical roles in the process of atherosclerosis. Cortistatin is a neuropeptide expressed in the vascular system and atherosclerotic plaques that regulates vascular calcification and neointimal formation, and inhibits inflammation in different experimental models of autoimmune diseases. Its role in inflammatory cardiovascular disorders is largely unexplored. The aim of this study is to investigate the potential therapeutic effects of cortistatin in two well-established preclinical models of atherosclerosis, and the molecular and cellular mechanisms involved. Systemic treatment with cortistatin reduced the number and size of atherosclerotic plaques in carotid artery, heart, aortic arch and aorta in acute and chronic atherosclerosis induced in apolipoprotein E-deficient mice fed a high-lipid diet. This effect was exerted at multiple levels. Cortistatin reduced Th1/Th17-driven inflammatory responses and increased regulatory T cells in atherosclerotic arteries and lymphoid organs. Moreover, cortistatin reduced the capacity of endothelial cells to bind and recruit immune cells to the plaque and impaired the formation of foam cells by enhancing cholesterol efflux from macrophages. Cortistatin emerges as a new candidate for the treatment of the clinical manifestations of atherosclerosis.

Effects of Food Components That Activate TRPA1 Receptors on Mucosal Ion Transport in the Mouse Intestine

TRPA1 is a ligand-activated cation channel found in the intestine and other tissues. Components of food that stimulate TRPA1 receptors (phytonutrients) include allyl isothiocyanate, cinnamaldehyde and linalool, but these may also act at other receptors. Cells lining the intestinal mucosa are immunoreactive for TRPA1 and Trpa1 mRNA occurs in mucosal extracts, suggesting that the TRPA1 receptor is the target for these agonists. However, in situ hybridisation reveals Trpa1 expression in 5-HT containing enteroendocrine cells, not enterocytes. TRPA1 agonists evoke mucosal secretion, which may be indirect (through release of 5-HT) or direct by activation of enterocytes. We investigated effects of the phytonutrients on transmucosal ion currents in mouse duodenum and colon, and the specificity of the phytonutrients in cells transfected with Trpa1, and in Trpa1-deficient mice. The phytonutrients increased currents in the duodenum with the relative potencies: allyl isothiocyanate (AITC) > cinnamaldehyde > linalool (0.1 to 300 μM). The rank order was similar in the colon, but linalool was ineffective. Responses to AITC were reduced by the TRPA1 antagonist HC-030031 (100 μM), and were greatly diminished in Trpa1−/− duodenum and colon. Responses were not reduced by tetrodotoxin, 5-HT receptor antagonists, or atropine, but inhibition of prostaglandin synthesis reduced responses. Thus, functional TRPA1 channels are expressed by enterocytes of the duodenum and colon. Activation of enterocyte TRPA1 by food components has the potential to facilitate nutrient absorption.

Serum ghrelin and prediction of metabolic parameters in over 20-year follow-up

Ghrelin is a peptide hormone from the stomach, with an ability to release growth-hormone from the pituitary. Numerous cross-sectional studies indicate that ghrelin also has a role in metabolic abnormalities, such as metabolic syndrome and type 2 diabetes, but evidence for long-term effect is scarce. We investigated, whether ghrelin concentration measured in middle age would predict the development or absence of metabolic disturbances subsequently. Study population consisted of 600 middle-aged persons, and the follow-up time was approximately 21 years. Plasma total ghrelin concentration was measured at the baseline, and divided to tertiles. Numerous anthropometric and other clinical measurements (including blood pressure), and laboratory test were made both at the baseline and at the follow-up. After the follow-up the prevalence of high systolic blood pressure according to MetS IDF-criteria was the lowest in the highest ghrelin tertile, and the highest in the first (p<0.03). When only subjects free of hypertension medication at baseline were considered, subjects belonging to the highest ghrelin tertile developed less new hypertension and high blood pressure according to IDF-criteria as well as medication for it during the follow-up (p<0.05). Although serum insulin levels were negatively correlated to ghrelin levels at both points in time (p<0.001 at baseline and p=0.003 at follow-up), plasma ghrelin concentration did not predict the development of abnormalities in glucose tolerance. The association with ghrelin and metabolic syndrome was lost during the follow-up. In conclusion, our results suggest high ghrelin to be protective against the development of hypertension in the long-term follow-up.

Ghrelin Supresses Sympathetic Hyperexcitation in Acute Heart Failure in Male Rats: Assessing Centrally and Peripherally Mediated Pathways

The hormone ghrelin prevents a dangerous increase in cardiac sympathetic nerve activity (SNA) after acute myocardial infarction (MI), although the underlying mechanisms remain unknown. This study aimed to determine whether ghrelin's sympathoinhibitory properties stem either from directly within the central nervous system, or via modulation of specific cardiac vagal inhibitory afferents. Cardiac SNA was recorded in urethane-anesthetized rats for 3 hours after the ligation of the left anterior descending coronary artery (ie, MI). Rats received ghrelin either sc (150 μg/kg) or intracerebroventricularly (5 μg/kg) immediately after the MI. In another two groups, the cervical vagi were denervated prior to the MI, followed by sc injection of either ghrelin or placebo. Acute MI induced a 188% increase in cardiac SNA, which was significantly attenuated in ghrelin-treated rats for both sc or intracerebroventricularly administration (36% and 76% increase, respectively). Consequently, mortality (47%) and the incidence of arrhythmic episodes (12 per 2 h) were improved with both routes of ghrelin administration (<13% and less than five per 2 h, respectively). Bilateral vagotomy significantly attenuated the cardiac SNA response to acute MI (99% increase). Ghrelin further attenuated the sympathetic response to MI in vagotomized rats so that the SNA response was comparable between vagotomized and vagal-intact MI rats treated with ghrelin. These results suggest that ghrelin may act primarily via a central pathway within the brain to suppress SNA after MI, although peripheral vagal afferent pathways may also contribute in part. The exact region(s) within the central nervous system whereby ghrelin inhibits SNA remains to be fully elucidated.

Integrated Approach to Early Detection of Cardiovascular Toxicity Induced by a Ghrelin Receptor Agonist

Cardiovascular (CV) safety concerns are among the leading causes of compound attrition in drug development. This work describes a strategy of applying novel end points to a 7-day rodent study to increase the opportunity to detect and characterize CV injury observed in a longer term (ie, 28 days) study. Using a ghrelin receptor agonist (GSK894281), a compound that produces myocardial degeneration/necrosis in rats after 28 days at doses of 0.3, 1, 10, or 60 mg/kg/d, we dosed rats across a range of similar doses (0, 0.3, 60, or 150 mg/kg/d) for 7 days to determine whether CV toxicity could be detected in a shorter study. End points included light and electron microscopies of the heart; heart weight; serum concentrations of fatty acid-binding protein 3 (FABP3), cardiac troponin I (cTnI), cardiac troponin T (cTnT), and N-terminal proatrial natriuretic peptide (NT-proANP); and a targeted transcriptional assessment of heart tissue. Histologic evaluation revealed a minimal increase in the incidence and/or severity of cardiac necrosis in animals administered 150 mg/kg/d. Ultrastructurally, mitochondrial membrane whorls and mitochondrial degeneration were observed in rats given 60 or 150 mg/kg/d. The FABP3 was elevated in rats given 150 mg/kg/d. Cardiac transcriptomics revealed evidence of mitochondrial dysfunction coincident with histologic lesions in the heart, and along with the ultrastructural results support a mechanism of mitochondrial injury. There were no changes in cTnI, cTnT, NT-proANP, or heart weight. In summary, enhancing a study design with novel end points provides a more integrated evaluation in short-term repeat dose studies, potentially leading to earlier nonclinical detection of structural CV toxicity.

Ghrelin-related peptides exert protective effects in the cerebral circulation of male mice through a nonclassical ghrelin receptor(s)

The ghrelin-related peptides, acylated ghrelin, des-acylated ghrelin, and obestatin, are novel gastrointestinal hormones. We firstly investigated whether the ghrelin gene, ghrelin O-acyltransferase, and the ghrelin receptor (GH secretagogue receptor 1a [GHSR1a]) are expressed in mouse cerebral arteries. Secondly, we assessed the cerebrovascular actions of ghrelin-related peptides by examining their effects on vasodilator nitric oxide (NO) and superoxide production. Using RT-PCR, we found the ghrelin gene and ghrelin O-acyltransferase to be expressed at negligible levels in cerebral arteries from male wild-type mice. mRNA expression of GHSR1a was also found to be low in cerebral arteries, and GHSR protein was undetectable in GHSR-enhanced green fluorescent protein mice. We next found that exogenous acylated ghrelin had no effect on the tone of perfused cerebral arteries or superoxide production. By contrast, exogenous des-acylated ghrelin or obestatin elicited powerful vasodilator responses (EC50 < 10 pmol/L) that were abolished by the NO synthase inhibitor N(ω)-nitro-L-arginine methyl ester. Furthermore, exogenous des-acylated ghrelin suppressed superoxide production in cerebral arteries. Consistent with our GHSR expression data, vasodilator effects of des-acylated ghrelin or obestatin were sustained in the presence of YIL-781 (GHSR1a antagonist) and in arteries from Ghsr-deficient mice. Using ghrelin-deficient (Ghrl(-/-)) mice, we also found that endogenous production of ghrelin-related peptides regulates NO bioactivity and superoxide levels in the cerebral circulation. Specifically, we show that NO bioactivity was markedly reduced in Ghrl(-/-) vs wild-type mice, and superoxide levels were elevated. These findings reveal protective actions of exogenous and endogenous ghrelin-related peptides in the cerebral circulation and show the existence of a novel ghrelin receptor(s) in the cerebral endothelium.

Obesity-related hypertension: possible pathophysiological mechanisms

Hypertension is one of the major risk factors of cardiovascular diseases, but despite a century of clinical and basic research, the discrete etiology of this disease is still not fully understood. The same is true for obesity, which is recognized as a major global epidemic health problem nowadays. Obesity is associated with an increasing prevalence of the metabolic syndrome, a cluster of risk factors including hypertension, abdominal obesity, dyslipidemia, and hyperglycemia. Epidemiological studies have shown that excess weight gain predicts future development of hypertension, and the relationship between BMI and blood pressure (BP) appears to be almost linear in different populations. There is no doubt that obesity-related hypertension is a multifactorial and polygenic trait, and multiple potential pathogenetic mechanisms probably contribute to the development of higher BP in obese humans. These include hyperinsulinemia, activation of the renin-angiotensin-aldosterone system, sympathetic nervous system stimulation, abnormal levels of certain adipokines such as leptin, or cytokines acting at the vascular endothelial level. Moreover, some genetic and epigenetic mechanisms are also in play. Although the full manifestation of both hypertension and obesity occurs predominantly in adulthood, their roots can be traced back to early ontogeny. The detailed knowledge of alterations occurring in the organism of experimental animals during particular critical periods (developmental windows) could help to solve this phenomenon in humans and might facilitate the age-specific prevention of human obesity-related hypertension. In addition, better understanding of particular pathophysiological mechanisms might be useful in so-called personalized medicine.

Neuroanatomical characterization of a growth hormone secretagogue receptor-green fluorescent protein reporter mouse

Growth hormone secretagogue receptor (GHSR) 1a is the only molecularly identified receptor for ghrelin, mediating ghrelin-related effects on eating, body weight, and blood glucose control, among others. The expression pattern of GHSR within the brain has been assessed previously by several neuroanatomical techniques. However, inherent limitations to these techniques and the lack of reliable anti-GHSR antibodies and reporter rodent models that identify GHSR-containing neurons have prevented a more comprehensive functional characterization of ghrelin-responsive neurons. Here we have systematically characterized the brain expression of an enhanced green fluorescence protein (eGFP) transgene controlled by the Ghsr promoter in a recently reported GHSR reporter mouse. Expression of eGFP in coronal brain sections was compared with GHSR mRNA expression detected in the same sections by in situ hybridization histochemistry. eGFP immunoreactivity was detected in several areas, including the prefrontal cortex, insular cortex, olfactory bulb, amygdala, and hippocampus, which showed no or low GHSR mRNA expression. In contrast, eGFP expression was low in several midbrain regions and in several hypothalamic nuclei, particularly the arcuate nucleus, where robust GHSR mRNA expression has been well-characterized. eGFP expression in several brainstem nuclei showed high to moderate degrees of colocalization with GHSR mRNA labeling. Further quantitative PCR and electrophysiological analyses of eGFP-labeled hippocampal cells confirmed faithful expression of eGFP within GHSR-containing, ghrelin-responsive neurons. In summary, the GHSR-eGFP reporter mouse model may be a useful tool for studying GHSR function, particularly within the brainstem and hippocampus; however, it underrepresents GHSR expression in nuclei within the hypothalamus and midbrain.

Hypotensive effects of ghrelin receptor agonists mediated through a novel receptor

BACKGROUND AND PURPOSE

Some agonists of ghrelin receptors cause rapid decreases in BP. The mechanisms by which they cause hypotension and the pharmacology of the receptors are unknown.

EXPERIMENTAL APPROACH

The effects of ligands of ghrelin receptors were investigated in rats in vivo, on isolated blood vessels and on cells transfected with the only molecularly defined ghrelin receptor, growth hormone secretagogue receptor 1a (GHSR1a).

KEY RESULTS

Three agonists of GHSR1a receptors, ulimorelin, capromorelin and CP464709, caused a rapid decrease in BP in the anaesthetized rat. The effect was not reduced by either of two GHSR1a antagonists, JMV2959 or YIL781, at doses that blocked effects on colorectal motility, in vivo. The rapid hypotension was not mimicked by ghrelin, unacylated ghrelin or the unacylated ghrelin receptor agonist, AZP531. The early hypotension preceded a decrease in sympathetic nerve activity. Early hypotension was not reduced by hexamethonium or by baroreceptor (sino-aortic) denervation. Ulimorelin also relaxed isolated segments of rat mesenteric artery, and, less potently, relaxed aorta segments. The vascular relaxation was not reduced by JMV2959 or YIL781. Ulimorelin, capromorelin and CP464709 activated GHSR1a in transfected HEK293 cells at nanomolar concentrations. JMV2959 and YIL781 both antagonized effects in these cells, with their pA2 values at the GHSR1a receptor being 6.55 and 7.84.

CONCLUSIONS AND IMPLICATIONS

Our results indicate a novel vascular receptor or receptors whose activation by ulimorelin, capromorelin and CP464709 lowered BP. This receptor is activated by low MW GHSR1a agonists, but is not activated by ghrelin.

Novel and conventional receptors for ghrelin, desacyl-ghrelin, and pharmacologically related compounds

The only molecularly identified ghrelin receptor is the growth hormone secretagogue receptor GHSR1a. Its natural ligand, ghrelin, is an acylated peptide whose unacylated counterpart (UAG) is almost inactive at GHSR1a. A truncated, nonfunctional receptor, GHSR1b, derives from the same gene. We have critically evaluated evidence for effects of ghrelin receptor ligands that are not consistent with actions at GHSR1a. Effects of ghrelin are observed in cells or tissues where the expression of GHSR1a is not detectable or after the Ghsr gene has been inactivated. In several, effects of ghrelin are mimicked by UAG, and ghrelin binding is competitively reduced by UAG. Effects in the absence of GHSR1a and sites at which ghrelin and UAG have similar potency suggest the presence of novel nonspecific ghrelin receptors (ghrelin receptor-like receptors [GRLRs]). A third class of receptor, the UAG receptors, at which UAG, but not ghrelin, is an agonist has been proposed. None of the novel receptors, with the exception of the glycoprotein CD36, which accounts for ghrelin action at a limited number of sites, have been identified. GHSR1a and GHSR1b combine with other G protein-coupled receptors to form heterodimers, whose pharmacologies differ from their components. Thus, it is feasible some GRLRs and some UAG receptors are heterodimers. Effects mediated through GRLRs or UAG receptors include adipocyte lipid accumulation, myoblast differentiation, osteoblast proliferation, insulin release, cardioprotection, coronary artery constriction, vascular endothelial cell proliferation, and tumor cell proliferation. The molecular identification and pharmacologic characterization of novel ghrelin receptors are thus important objectives.

Unexpected effect of the appetite-stimulating hormone ghrelin on ENaC: hunger for sodium?

The orexigenic hormone ghrelin acts like a hunger signal, released by the stomach in response to nutritional status. However, ghrelin and its receptor in the kidney may play other biological roles. Kemp and colleagues identify that ghrelin stimulates renal Na+ absorption through cAMP-dependent trafficking of ENaC in the cortical collecting duct. While ghrelin seems to be a physiological regulator of ENaC, future studies are necessary to clarify its physiological and pathological roles in sodium homeostasis.

Ghrelin and des-acyl ghrelin inhibit aromatase expression and activity in human adipose stromal cells: suppression of cAMP as a possible mechanism

Aromatase converts androgens into estrogens and its expression within adipose stromal cells (ASCs) is believed to be the major driver of estrogen-dependent cancers in older women. Ghrelin is a gut-hormone that is involved in the regulation of appetite and known to bind to and activate the cognate ghrelin receptor, GHSR1a. The unacylated form of ghrelin, des-acyl ghrelin, binds weakly to GHSR1a but has been shown to play an important role in regulating a number of physiological processes. The aim of this study was to determine the effect of ghrelin and des-acyl ghrelin on aromatase in primary human ASCs. Primary human ASCs were isolated from adipose tissue of women undergoing cosmetic surgery. Real-time PCR and tritiated water-release assays were performed to examine the effect of treatment on aromatase transcript expression and aromatase activity, respectively. Treatments included ghrelin, des-acyl ghrelin, obestatin, and capromorelin (GHSR1a agonist). GHSR1a protein expression was assessed by Western blot and effects of treatment on Ca(2+) and cAMP second messenger systems were examined using the Flexstation assay and the Lance Ultra cAMP kit, respectively. Results demonstrate that pM concentrations of ghrelin and des-acyl ghrelin inhibit aromatase transcript expression and activity in ASCs under basal conditions and in PGE2-stimulated cells. Moreover, the effects of ghrelin and des-acyl ghrelin are mediated via effects on aromatase promoter PII-specific transcripts. Neither the GHSR1a-specific agonist capromorelin nor obestatin had any effect on aromatase transcript expression or activity. Moreover, GHSR1a protein was undetectable by Western blot and neither ghrelin nor capromorelin elicited a calcium response in ASCs. Finally, ghrelin caused a significant decrease in basal and forskolin-stimulated cAMP in ASC. These findings suggest that ghrelin acts at alternate receptors in ASCs by decreasing intracellular cAMP levels. Ghrelin mimetics may be useful in the treatment of estrogen-dependent breast cancer.

Growth hormone secretagogue receptor deficiency in mice protects against obesity-induced hypertension

Abstract Growth hormone secretagogue receptor (GHS-R) signaling has been associated with growth hormone release, increases in food intake and pleiotropic cardiovascular effects. Recent data demonstrated that acute GHS-R antagonism leads to increases in mean arterial pressure mediated by the sympathetic nervous system in rats; a highly undesirable effect if GHS-R antagonism was to be used as a therapeutic approach to reducing food intake in an already obese, hypertensive patient population. However, our data in conscious, freely moving GHS-R deficient mice demonstrate that chronic absence of GHS-R signaling is protective against obesity-induced hypertension. GHS-R deficiency leads to reduced systolic blood pressure variability (SBPV); in response to acute high-fat diet (HFD)-feeding, increases in the sympathetic control of SBPV are suppressed in GHS-R KO mice. Our data further suggest that GHS-R signaling dampens the immediate HFD-mediated increase in spontaneous baroreflex sensitivity. In diet-induced obesity, absence of GHS-R signaling leads to reductions in obesity-mediated hypertension and tachycardia. Collectively, our findings thus suggest that chronic blockade of GHS-R signaling may not result in adverse cardiovascular effects in obesity.

Ghrelin: central and peripheral implications in anorexia nervosa

Increasing clinical and therapeutic interest in the neurobiology of eating disorders reflects their dramatic impact on health. Chronic food restriction resulting in severe weight loss is a major symptom described in restrictive anorexia nervosa (AN) patients, and they also suffer from metabolic disturbances, infertility, osteopenia, and osteoporosis. Restrictive AN, mostly observed in young women, is the third largest cause of chronic illness in teenagers of industrialized countries. From a neurobiological perspective, AN-linked behaviors can be considered an adaptation that permits the endurance of reduced energy supply, involving central and/or peripheral reprograming. The severe weight loss observed in AN patients is accompanied by significant changes in hormones involved in energy balance, feeding behavior, and bone formation, all of which can be replicated in animals models. Increasing evidence suggests that AN could be an addictive behavior disorder, potentially linking defects in the reward mechanism with suppressed food intake, heightened physical activity, and mood disorder. Surprisingly, the plasma levels of ghrelin, an orexigenic hormone that drives food-motivated behavior, are increased. This increase in plasma ghrelin levels seems paradoxical in light of the restrained eating adopted by AN patients, and may rather result from an adaptation to the disease. The aim of this review is to describe the role played by ghrelin in AN focusing on its central vs. peripheral actions. In AN patients and in rodent AN models, chronic food restriction induces profound alterations in the « ghrelin » signaling that leads to the development of inappropriate behaviors like hyperactivity or addiction to food starvation and therefore a greater depletion in energy reserves. The question of a transient insensitivity to ghrelin and/or a potential metabolic reprograming is discussed in regard of new clinical treatments currently investigated.