Current Pharmaceutical Design - Volume 18, Issue 31, 2012

The growth hormone secretagogue receptor (GHS-R) is a component of the ghrelin signaling pathway and is involved in mediating the pleiotropic effects of ghrelin. Two isoforms have been identified, but only GHS-R1a binds with acyl ghrelin and transduces its message. However, the inactive variant of GHS-R, GHS-R1b, appears to play a critical role in modulating the activity of GHS-R1a by forming heterodimeric complexes which attenuates trafficking of the active variant to the cell surface. The molecular mechanisms of signal transduction are complex and are specific of the tissues where GHS-R1a is expressed. The potent induction of GH secretion and the stimulation of appetite are the most intensively studied functions of GHS-R1a. However, the tissue distribution of GHS-R1a extends beyond the pituitary and the hypothalamus, and reflects the different biological functions of the ghrelin/GHS-R system. GHS-R1a is also expressed in other brain areas, in the pancreas, adipose tissue, immune cells and cardiovascular system, and modulates learning and memory, glucose and lipid metabolism, inflammatory response and cardiac performance. The pleiotropic effects of the ghrelin/GHS-R system suggest their exploitation to prevent and treat a number of clinical conditions. Among many other syndromes and diseases, cancer cachexia, aging related cognitive decline, obesity and diabetes may significantly benefit from the use of GHS-R1a agonists or antagonists.

Human ghrelin and human motilin, belonging to the ghrelin/motilin-related peptide family, share 36% amino acid sequence identity, while the human ghrelin receptor exhibits a remarkable 50% overall identity with the human motilin receptor. In addition to their structural resemblance, ghrelin and motilin are the only two mammalian hormones known to decrease in the postprandial period. Ghrelin and motilin participate in initiating the migrating motor complex in the stomach, and stimulate gastrointestinal motility, accelerate gastric emptying, and induce “gastric hunger”. In addition to modulating the release of growth hormone and gut motility, ghrelin plays a crucial role in the secretion and protection of the stomach and colon. Ghrelin mimetics and motilin agonists are currently being developed to reverse gastrointestinal hypomotility disorders. With additional appetite-enhancing, adiposity-promoting, and anti-inflammatory effects, ghrelin and rikkunshito (a traditional Japanese herb enhancing acyl ghrelin signaling) are superior to motilin in the treatment of cancerrelated anorexia and cachexia, post-chemotherapy symptoms, rheumatological diseases, age-related frailty, as well as post-operative, septic, and post-burn gut ileus.

Complex mechanisms have evolved that control feeding and energy homeostasis in mammals. Centrally, particularly in the hypothalamus, numerous neurotransmitters have been identified that regulate appetite and energy homeostasis. On the other hand, hormones released from the gut signal states of hunger and satiety to the brain. From the large number of players involved in this interplay, peptides from the neuropeptide Y (NPY) family are unique, with the predominantly neuronally expressed NPY being one of the most strongly stimulating agents for food intake while its two other closely related family members peptide YY (PYY) and pancreatic polypeptide (PP) released from the gut induce satiety. Another major player in this circuitry is ghrelin, which is released from the stomach and is the only known hormone that signals hunger to the brain. It is doing this by stimulating hypothalamic NPY production and release, subsequently leading to increased appetite and feeding behaviour. Deregulation of these processes can lead to either the development of obesity or the other extreme, anorexia. The aim of this review is to summarize the recent literature on NPY and ghrelin and its involvement in anorexia nervosa.

Ghrelin is a stomach-derived hormone that acts at the ghrelin receptor (formerly called the Growth Hormone Secretagogue (GHS)-1a receptor) in multiple tissues throughout the body, exhibiting pleotropic effects potentially beneficial as a treatment in human disease states. Given its properties including increasing appetite, decreasing systemic inflammation, decreasing vascular resistance, increasing cardiac output, and increasing growth hormone and IGF-1 levels, ghrelin has been tested as a treatment in animal models of multiple disease states that produce the deficits in these processes. Thus, the efficacy of ghrelin has been testing in diseases involving anorexia, negative energy balance, cardiovascular compromise, systemic inflammation and gastroparesis. These diseases include cancer cachexia, chronic heart failure, chronic renal failure, chemotherapy, arthritis, gastroparesis and inflammatory bowel disease. Across this wide variety of diseases treatment with ghrelin and ghrelin agonists have produced benefits, though given ghrelin's widespread effects, the exact mechanisms behind ghrelin's action in these settings is frequently difficult to determine. Further investigation using animal models may help to determine mechanisms that are most operative in these disease states and narrow treatment parameters helpful for human application.

Ghrelin as a human natural hormone is involved in fundamental regulatory processes of eating and energy balance. Ghrelin signals the nutrient availability from the gastrointestinal tract to the central nervous system, up-regulates food intake and lowers energy expenditure mainly through hypothalamic mediators acting both centrally and peripherally including the gastrointestinal tract (motility, epithelium), promotes both neuro-endocrine and inflammatory signals to increase skeletal muscle growth and decrease protein breakdown, and increases lipolysis while body fat utilization is reduced. Ghrelin does more to exert its probably sentinel role around “human energy”: it influences through mainly extra-hypothalamic actions the hedonic and incentive value of food, mood and anxiety, sleep-wake regulation, learning and memory, and neurogenesis. Recently numerous ghrelin gene-derived peptides were discovered, demonstrating the complexity within the ghrelin/ghrelin receptor axis. For clinical applications, not only the natural ghrelin and its slice variants, but also several modified or artificial molecules acting at ghrelin-associated receptors were and are developed. Current clinical applications are limited to clinical studies, focusing mainly on cachexia in chronic heart failure, COPD, cancer, endstage- renal-disease or cystic fibrosis, but also on frailty in elderly, gastrointestinal motility (e.g., gastroparesis, functional dyspepsia, postoperative ileus), after curative gastrectomy, anorexia nervosa, growth hormone deficient patients, alcohol craving, sleep-wake regulation (e.g. major depression), or sympathetic nervous activity in obesity. The results of completed, preliminary studies support the clinical potential of ghrelin, ghrelin gene-derived peptides, and artificial analogues, suggesting that larger clinical trials are demanded to move ghrelin towards an available and reimbursed pharmaceutical intervention.

Cachexia influences morbidity, mortality and quality of life of cancer patients at advanced stage of disease. Therefore, the knowledge of its pathophysiology is critical to develop effective therapies to be integrated in the comprehensive approach of cancer patients. Oxidative stress, unless counteracted by effective antioxidant therapies, contributes to the development of anorexia and cachexia in cancer patients. In the present review the potential role of targeting oxidative stress in the treatment of cachexia is reported. Efficacy data on the use of antioxidants in advanced cancer patients are promising. However, the optimal dosage and route of administration as well as the timing and the most effective combination are not well established. Moreover, since cachexia is a multifactorial syndrome, targeting only oxidative stress as a contributing factor would be inadequate and likely to achieve a limited clinical therapeutic benefit. According to this rationale, antioxidants should be included as essential components of a multitargeted combined treatment of cancer cachexia, which has been shown to be the most successful approach for this syndrome.

Cancer-related cachexia, a condition in which the body is consumed by deranged carbohydrate, lipid and protein metabolism that is induced by inflammatory cytokines. Cachexia is associated with poor treatment outcome, fatigue and poor quality of life. Pharmacological intervention in the treatment and/or prevention of cachexia has been mainly aimed at the use of appetite enhancers to increase oral nutritional intake so far. Herbal remedies are part of traditional and folk healing methods with long histories of use. In this report, we have assessed which herbal approaches have had associated cancer cachexia case reports. Commonly used herbal medicines in western countries include essiac, iscador, pau d’arco tea, cannabinoids and so on. Some Kampo herbs and formulations are commonly used by cancer patients reduce the side effects and complications during the antitumor therapy. The relevant herbal medicines include ginseng, C. rhizome and radix astragali, and the related herbal remedies, such as TJ-48, TJ-41, PHY906 and Rikkunshito. However, there still have some adverse effects caused or amplified by herb and drug interactions that are difficult to separate. However, randomized effectiveness of herbal medicines shall be further identified in controlled clinical trials involving cancer patients with cachexia.

Rikkunshito is a kampo herbal medicine which is widely used in Japan for the treatment of the upper gastrointestinal symptoms of patients with functional dyspepsia (FD), gastroesophageal reflux disease (GERD), dyspeptic symptoms of postgastrointestinal surgery patients, and chemotherapy-induced dyspepsia in cancer patients. Recently, very unique characteristics of rikkunshito have been unveiled; oral administration of rikkunshito potentiates orexigenic action of ghrelin through several different mechanisms. In addition, several lines of evidence obtained from both animal and human studies indicate that rikkunshito can be an attractive and promising therapeutic option for the anorectic conditions including cisplatin-induced dyspepsia, anorexia of aging, stress-induced hypophagia, cancer cachexia-anorexia syndrome. In this review, we will highlight what is known about the orexigenic effect of rikkunshito with a special focus on an interaction with ghrelin signaling system.

Traditional Japanese herbal, or Kampo medicine was developed and modified from Chinese herbal medicine. After the Japanese government approved Kampo for clinical use, much attention has been paid to establishing scientific evidence for the effectiveness of these medicines. Recent progress has been made in elucidating the mechanisms of action of some types of Kampo medicine, including rikkunshito (RKT), daikenchuto, and yokukansan. In this review, we focused on identifying the target molecules and the active ingredients of RKT. Thus far, many target molecules have been implicated in the mechanism of action of Kampo medicines, such as ion channels, enzymes, and receptors. In particular, G protein-coupled receptors are attractive candidates for explaining herbal medicine activity. This is particularly true of RKT, which is composed of 8 independent, crude drug extracts. Recent reports have shown that RKT elicits its effects through dual action to the G protein-coupled receptors: inhibition of serotonergic 5-HT2C and 5-HT2B receptors and activation of ghrelin receptors via specific ingredients of RKT. In addition, we suggest that the identification of the effective ingredients from Kampo medicines could contribute to the discovery and development of new drugs by means of modern high-throughput drug screening technology.

Ghrelin is the orexigenic peptide produced in the periphery, and its plasma level shows remarkable pre/postprandial changes. Ghrelin is considered a pivotal signal to the brain to stimulate feeding. Hence, characterizing the target neurons for ghrelin in the hypothalamic feeding center and the signaling cascade in the target neurons are essential for understanding the mechanisms regulating appetite. Anorexia and cachexia associated with gastric surgery, stress-related diseases, and use of anti-cancer drugs cause the health problems, markedly deteriorating the quality of life. The anorexia involves several neurotransmitters and neuropeptides in the hypothalamic feeding center, in which corticotropin-releasing hormone (CRH), urocortine, serotonin (5HT) and brain-derived neurotrophic factor (BDNF) play a pivotal role. A Japanese herbal medicine, rikkunshito, has been reported to ameliorate the anorexia by promoting the appetite. This review describes 1) the interaction of ghrelin with the orexigenic neuropeptide Y (NPY) neurons in the hypothalamic arcuate nucleus (ARC) and underlying signaling cascade in NPY neurons, 2) the anorectic pathway driven by BDNF-CRH/urocortine and 5HTCRH/ urocortine pathways, 3) the effect of rikkunshito on the interaction of ghrelin and NPY neurons in ARC, and 4) the effect of rikkunshito on the interaction of 5HT on CRH neurons in paraventricular nucleus (PVN).

Kampo, a form of traditional herbal medical practice, has become a substance of interest for scientific research. Although earlier clinical reports concerning Kampo are abundant, the scientific investigation of Kampo has a very short history. However, the process of acquiring quantifiable clinical trial evidence on herbal medicine is now clearly underway. The development of multi-component herbal medicines capable of targeting multiple sites could be useful both for future drug discovery and for the potential management of complex diseases. Additionally, mechanistic studies and the identification of active compounds could lead to new discoveries in the biological and biomedical sciences. Modern translational research on herbal medicines beyond basic science and clinical perspectives will contribute to the development of new medicines. This review covers the translational aspects of herbal medicine with a focus on cancer anorexiacachexia. The review gives perspective on a new horizon for herbal medicine from a scientific point of view and a basis for the further development of complementary and alternative medicine (CAM) for patients.