Involvement of stomach ghrelin and hypothalamic neuropeptides in tumor necrosis factor-alpha-induced hypophagia in mice

Appetite Regulation of TLR4-Induced Inflammatory Signaling

Appetite is the basis for obtaining food and maintaining normal metabolism. Toll-like receptor 4 (TLR4) is an important receptor expressed in the brain that induces inflammatory signaling after activation. Inflammation is considered to affect the homeostatic and non-homeostatic systems of appetite, which are dominated by hypothalamic and mesolimbic dopamine signaling. Although the pathological features of many types of inflammation are known, their physiological functions in appetite are largely unknown. This review mainly addresses several key issues, including the structures of the homeostatic and non-homeostatic systems. In addition, the mechanism by which TLR4-induced inflammatory signaling contributes to these two systems to regulate appetite is also discussed. This review will provide potential opportunities to develop new therapeutic interventions that control appetite under inflammatory conditions.

Increased hypothalamic serotonin turnover in inflammation-induced anorexia

BACKGROUND

Anorexia can occur as a serious complication of disease. Increasing evidence suggests that inflammation plays a major role, along with a hypothalamic dysregulation characterized by locally elevated serotonin levels. The present study was undertaken to further explore the connections between peripheral inflammation, anorexia and hypothalamic serotonin metabolism and signaling pathways. First, we investigated the response of two hypothalamic neuronal cell lines to TNFα, IL-6 and LPS. Next, we studied transcriptomic changes and serotonergic activity in the hypothalamus of mice after intraperitoneal injection with TNFα, IL-6 or a combination of TNFα and IL-6.

RESULTS

In vitro, we showed that hypothalamic neurons responded to inflammatory mediators by releasing cytokines. This inflammatory response was associated with an increased serotonin release. Mice injected with TNFα and IL-6 showed decreased food intake, associated with altered expression of inflammation-related genes in the hypothalamus. In addition, hypothalamic serotonin turnover showed to be elevated in treated mice.

CONCLUSIONS

Overall, our results underline that peripheral inflammation reaches the hypothalamus where it affects hypothalamic serotoninergic metabolism. These hypothalamic changes in serotonin pathways are associated with decreased food intake, providing evidence for a role of serotonin in inflammation-induced anorexia.

Hypothalamic inflammation and food intake regulation during chronic illness

Anorexia is a common symptom in chronic illness. It contributes to malnutrition and strongly affects survival and quality of life. A common denominator of many chronic diseases is an elevated inflammatory status, which is considered to play a pivotal role in the failure of food-intake regulating systems in the hypothalamus. In this review, we summarize findings on the role of hypothalamic inflammation on food intake regulation involving hypothalamic neuropeptide Y (NPY) and pro-opiomelanocortin (POMC). Furthermore, we outline the role of serotonin in the inability of these peptide based food-intake regulating systems to respond and adapt to changes in energy metabolism during chronic disease.

Hypothalamic inflammation: a double-edged sword to nutritional diseases

The hypothalamus is one of the master regulators of various physiological processes, including energy balance and nutrient metabolism. These regulatory functions are mediated by discrete hypothalamic regions that integrate metabolic sensing with neuroendocrine and neural controls of systemic physiology. Neurons and nonneuronal cells in these hypothalamic regions act supportively to execute metabolic regulations. Under conditions of brain and hypothalamic inflammation, which may result from overnutrition-induced intracellular stresses or disease-associated systemic inflammatory factors, extracellular and intracellular environments of hypothalamic cells are disrupted, leading to central metabolic dysregulations and various diseases. Recent research has begun to elucidate the effects of hypothalamic inflammation in causing diverse components of metabolic syndrome leading to diabetes and cardiovascular disease. These new understandings have provocatively expanded previous knowledge on the cachectic roles of brain inflammatory response in diseases, such as infections and cancers. This review describes the molecular and cellular characteristics of hypothalamic inflammation in metabolic syndrome and related diseases as opposed to cachectic diseases, and also discusses concepts and potential applications of inhibiting central/hypothalamic inflammation to treat nutritional diseases.

Ghrelin - a pleiotropic hormone secreted from endocrine x/a-like cells of the stomach

The gastric X/A-like endocrine cell receives growing attention due to its peptide products with ghrelin being the best characterized. This peptide hormone was identified a decade ago as a stimulator of food intake and to date remains the only known peripherally produced and centrally acting orexigenic hormone. In addition, subsequent studies identified numerous other functions of this peptide including the stimulation of gastrointestinal motility, the maintenance of energy homeostasis and an impact on reproduction. Moreover, ghrelin is also involved in the response to stress and assumed to play a role in coping functions and exert a modulatory action on immune pathways. Our knowledge on the regulation of ghrelin has markedly advanced during the past years by the identification of the ghrelin acylating enzyme, ghrelin-O-acyltransferase, and by the description of changes in expression, activation, and release under different metabolic as well as physically and psychically challenging conditions. However, our insight on regulatory processes of ghrelin at the cellular and subcellular levels is still very limited and warrants further investigation.

Repletion of TNFα or leptin in calorically restricted mice suppresses post-restriction hyperphagia

The causes of post-restriction hyperphagia (PRH) represent a target for drug-based therapies to prevent obesity. However, the factors causing PRH are poorly understood. We show that, in mice, the extent of PRH was independent of the time under restriction, but depended on its severity, suggesting that PRH was driven by signals from altered body composition. Signals related to fat mass were important drivers. Circulating levels of leptin and TNFα were significantly depleted following caloric restriction (CR). We experimentally repleted their levels to match those of controls, and found that in both treatment groups the level of PRH was significantly blunted. These data establish a role for TNFα and leptin in the non-pathological regulation of energy homeostasis. Signals from adipose tissue, including but not limited to leptin and TNFα, regulate PRH and might be targets for therapies that support people engaged in CR to reduce obesity.

Ghrelin levels in patients with juvenile idiopathic arthritis: relation to anti-tumor necrosis factor treatment and disease activity

Studies in adults with rheumatoid arthritis reported low serum ghrelin that increased following anti-tumor necrosis factor (TNF) infusion. Data on juvenile idiopathic arthritis (JIA) are lacking. The aim of this pilot study was to explore serum ghrelin levels in patients with JIA and the possible association with anti-TNF treatment, disease activity, and nutritional status. Fifty-two patients with JIA (14/52 on anti-TNF treatment) were studied. Juvenile idiopathic arthritis was inactive in 3 of 14 anti-TNF-treated patients and in 11 of 38 non-anti-TNF-treated patients. The nutritional status, energy intake/requirements, appetite, and fasting serum ghrelin levels were assessed. Ghrelin control values were obtained from 50 individuals with minor illness matched for age, sex, and body mass index. Ghrelin levels in patients with JIA were significantly lower than in controls (P < .001, confidence interval [CI] = -101 to -331). Analysis according to anti-TNF treatment and disease activity showed that ghrelin levels were comparable to control values only in 3 patients with anti-TNF-induced remission. Ghrelin in non-anti-TNF-treated patients in remission was low. Multiple regression analysis showed that disease activity (P = .002, CI = -84.16 to -20.01) and anti-TNF treatment (P = .003, CI = -82.51 to -18.33) were significant independent predictors of ghrelin after adjusting for other potential confounders. Ghrelin did not correlate with nutritional status, energy balance, and appetite. Serum ghrelin is low in patients with JIA and is restored to values similar to those in controls following anti-TNF-induced remission. Our study provides evidence that TNF blockade is independently associated with serum ghrelin, which possibly contributes to anti-TNF-induced remission. These preliminary results could form the basis for future research.

The effects of ghrelin on inflammation and the immune system

A number of hormones and metabolic mediators signal the brain of changes in the body's energy status and when an imbalance occurs; the brain coordinates the appropriate changes in energy intake and utilization via the control of appetite and food consumption. Under conditions of chronic inflammation and immune activation, there is often a significant loss of body mass and appetite suggesting the presence of shared ligands and signaling pathways mediating "crosstalk" between the immune and neuroendocrine systems. Ghrelin, the endogenous ligand for growth hormone secretagogue receptor (GHS-R), is produced primarily by cells in the stomach and serves as a potent circulating orexigenic hormone controlling food intake, energy expenditure, adiposity and GH secretion. The functional roles of ghrelin and other growth hormone secretagogues (GHS) within the immune system and under states of inflammatory stress and injury are only now coming to light. A number of reports over the past decade have described ghrelin to be a potent anti-inflammatory mediator both in vitro and in vivo and a promising therapeutic agent in the treatment of inflammatory diseases and injury. Moreover, ghrelin has also been shown to promote lymphocyte development in the primary lymphoid organs (bone marrow and thymus) and to ablate age-associated thymic involution. In the current report, we review the literature supporting a role for ghrelin as an anti-inflammatory agent and immunoregulatory hormone/cytokine and its potential use in the treatment of inflammatory diseases and injury.

Disassociated relation between plasma tumor necrosis factor-alpha, interleukin-6 and increased body weight in Amerindian women: A long-term prospective study of natural body weight variation and impaired glucose tolerance

BACKGROUND

Inflammatory cytokines are linked to obesity-related insulin resistance and may predict type 2 diabetes independently of obesity. We previously reported that a majority of a cohort of 73 non-diabetic women with normal plasma (p-)glucose with Amerindian heritage in Lima, Peru, during a 5-year period increased both body weight and p-glucose levels, yet p-insulin was unaltered. A high proportion of palmitoleic acid (16:1n-7) in serum (s) and systolic blood pressure (SBP) were independent predictors of high p-glucose. Whether cytokines also contributed is, however, not known.

METHODS

During 5 years we prospectively investigated the relation between changed concentrations of p-tumor necrosis factor (TNF)-alpha, p-interleukin (IL)-6 and circulating insulin and glucose in relation to the natural variation of body weight. Study variables included anthropometric measurements, p-insulin, TNF-alpha, IL-6, SBP and the proportion of 16:1n-7 in s-fatty acid composition.

RESULTS

Weight and waist differences correlated negatively to the difference in p-TNF-alpha but positively to differences in p-IL-6 and p-insulin, whereas the increase of p-glucose from baseline to follow-up did not correlate with changes in levels of the two cytokines. In multiple regression analysis changes of TNF-alpha and insulin contributed independently to the variance in weight. P-insulin at baseline and weight change were determinants of fasting p-insulin at follow-up. Multiple regression analysis revealed that weight change (t-value = - 2.42; P = 0.018) and waist change (t-value = 2.41; P = 0.019) together with S-16:1n-7 (p < 0.0001) and SBP (p = 0.0005) at baseline were significant predictors of p-glucose at follow-up.

CONCLUSION

Our prospective study of Amerindian women revealed disassociations between changes in p-TNF-alpha and p-IL-6 in relation to variation in body weight. A high proportion of s-16:1n-7, SBP at baseline together with weight and waist changes were independent predictors of p-glucose at follow-up. The exact role of the opposite effects and clinical impact of p-TNF-alpha and p-IL-6 on loss and gain of body weight and indirectly on the development of glucose intolerance is not known.

Rejuvenation of the aging thymus: growth hormone-mediated and ghrelin-mediated signaling pathways

One of the major fundamental causes for the aging of the immune system is the structural and functional involution of the thymus, and the associated decline in de novo naïve T-lymphocyte output. This loss of naïve T-cell production weakens the ability of the adaptive immune system to respond to new antigenic stimuli and eventually leads to a peripheral T-cell bias to the memory phenotype. While the precise mechanisms responsible for age-associated thymic involution remain unknown, a variety of theories have been forwarded including the loss of expression of various growth factors and hormones that influence the lymphoid compartment and promote thymic function. Extensive studies examining two hormones, namely growth hormone (GH) and ghrelin (GRL), have demonstrated their contributions to thymus biology. In the current review, we discuss the literature supporting a role for these hormones in thymic physiology and age-associated thymic involution and their potential use in the restoration of thymic function in aged and immunocompromised individuals.

Hypothalamic mechanisms in cachexia

The role of nutrition and balanced metabolism in normal growth, development, and health maintenance is well known. Patients affected with either acute or chronic diseases often show disorders of nutrient balance. In some cases, a devastating state of malnutrition known as cachexia arises, brought about by a synergistic combination of a dramatic decrease in appetite and an increase in metabolism of fat and lean body mass. Other common features that are not required for the diagnosis include decreases in voluntary movement, insulin resistance, and anhedonia. This combination is found in a number of disorders including cancer, cystic fibrosis, AIDS, rheumatoid arthritis, renal failure, and Alzheimer's disease. The severity of cachexia in these illnesses is often the primary determining factor in both quality of life, and in eventual mortality. Indeed, body mass retention in AIDS patients has a stronger association with survival than any other current measure of the disease. This has led to intense investigation of cachexia and the proposal of numerous hypotheses regarding its etiology. Most authors suggest that cytokines released during inflammation and malignancy act on the central nervous system to alter the release and function of a number of neurotransmitters, thereby altering both appetite and metabolic rate. This review will discuss the salient features of cachexia in human diseases, and review the mechanisms whereby inflammation alters the function of key brain regions to produce stereotypical illness behavior. The paper represents an invited review by a symposium, award winner or keynote speaker at the Society for the Study of Ingestive Behavior [SSIB] Annual Meeting in Portland, July 2009.

Are ghrelin and leptin involved in food intake and body mass index in maintenance hemodialysis?

OBJECTIVES

Both leptin and ghrelin (in the forms of acyl ghrelin and des-acyl ghrelin) are involved in food intake, and appear to be dysregulated in chronic kidney disease. This study describes plasma leptin, acyl, and des-acyl ghrelin concentrations in relation to protein intake and body mass index (BMI) in hemodialysis (HD) patients.

DESIGN

This was a cross-sectional study.

SETTING

This study was conducted during the baseline phase of the French multicenter Influence of a High-Flux Dialyzer on Long-Term Leptin Levels Study.

PATIENTS

We studied 125 HD patients (aged 72.5+/-11.7 years; 59% males).

MAIN OUTCOME MEASURE

Blood samples were collected during fasting, and before a regular HD session. Plasma ghrelin and leptin were evaluated. The protein equivalents of total nitrogen appearance and BMI were calculated.

RESULTS

Patients demonstrated elevated serum leptin (48.0+/-49.0 ng/mL) and des-acyl ghrelin (646.6+/-489.5 pg/mL) levels, and low acyl ghrelin levels (29.8+/-58.5 pg/mL), according to normal values. Acyl ghrelin was negatively correlated with C-reactive protein (r=-0.34, P < .001). The des-acyl to acyl ghrelin ratio was negatively correlated with protein intake, as estimated by normalized Protein Nitrogen Appearance (r=-0.22, P=.01). Serum leptin exhibited its well-described positive correlation with BMI and waist circumference, but the other hormones did not.

CONCLUSIONS

This study reports high des-acyl ghrelin and leptin levels and low acyl ghrelin levels in HD patients, a finding potentially associated with inflammation and food intake.

Expression, regulation and biological actions of growth hormone (GH) and ghrelin in the immune system

Immune and neuroendocrine systems have bidirectional communications. Growth hormone (GH) and an orexigenic hormone ghrelin are expressed in various immune cells such as T lymphocytes, B lymphocytes, monocytes and neutrophils. These immune cells also bear receptors for hormones: growth hormone receptor (GHR) for GH and growth hormone secretagogue receptor (GHS-R) for ghrelin. The expression of GH in immune cells is stimulated by ghrelin as in anterior pituitary cells, whereas the regulation of GH secretion in the immune system by other peptides seems to be different from that in the anterior pituitary gland. Cytokines and mitogens enhance GH secretion from immune cells. GH has several biological actions in the immune system: enhancing thymopoiesis and T cell development, modulating cytokine production, enhancing B cell development and antibody production, priming neutrophils and monocytes for superoxide anion secretion, enhancing neutrophil adhesion and monocyte migration and anti-apoptotic action. Biological actions of ghrelin include attenuation of septic shock and anti-inflammatory actions, modulating phagocytosis, and enhancing thymopoiesis. The effect of ghrelin may be direct or through GH production, and that of GH may be direct or through insulin like growth factor-I (IGF-I) production. Elucidation of the roles of GH and ghrelin in the immune system may shed light on the treatment and prevention of immunological disorders such as AIDS and organ damages due to obesity/ageing-related chronic inflammation.

Increased plasma ghrelin following infliximab in Crohn's disease

BACKGROUND

Ghrelin, a potent orexigenic peptide produced by the stomach, may be affected by circulating inflammatory mediators.

AIM

To assess the effect of an anti-TNFα antibody on ghrelin in patients with Crohn's disease (CD).

METHODS

Fifteen patients with Crohn's receiving infliximab were studied before and 1 week after infusion. Following an overnight fast, blood was sampled before a meal and then every 20 min for 2 h. Total ghrelin and CRP were measured using ELISA. Acylated ghrelin and TNFα, IFNγ, IL-1β and IL-6 were measured with bioplex. Harvey Bradshaw Activity Index was assessed.

RESULTS

Median (95% CI) 2-h integrated plasma total ghrelin increased from 162 (99-311) before infliximab to 200 (128-387) pg/mL h, (P = 0.02) after. Following infliximab, 20 min postmeal, median acylated ghrelin decreased from 50.3 (24-64) to 38.6 (26-82) pg/mL, (P = 0.04) thus reverting to a traditional meal related ghrelin curve. Median (range) disease activity decreased from 5 (2-28) before to 3 (0-22), (P = 0.0001) and Median (95% CI) TNFα decreased from 2.8 (1.89-4.48) to 1.31 (0.73-2.06) pg/mL (P = 0.002).

CONCLUSIONS

Infliximab increases circulating total ghrelin by 25% in CD and restores the postprandial response of acylated ghrelin to food intake. Acylated and de-sacyl ghrelin remain unchanged, suggesting that an alternate isoform could be affected by infliximab.

The role of ghrelin and ghrelin analogues in wasting disease

PURPOSE OF REVIEW

The purpose of this review is to summarize recent studies that investigated the role of ghrelin and ghrelin analogs in wasting conditions.

RECENT FINDINGS

Numerous studies have demonstrated potential beneficial effects exerted by ghrelin in a number of diseases associated with wasting. Besides ghrelin's orexigenic effect, anabolic as well as anti-inflammatory activity mediated by ghrelin have been investigated in wasting conditions such as cancer, diabetes mellitus, malabsorptive diseases, chronic obstructive pulmonary disease, anorexia nervosa, renal failure, liver failure, and chronic heart failure. Encouraging results have been obtained from experimental studies and a few clinical trials using subcutaneous administration of ghrelin and ghrelin agonists in cachexia. In-vitro studies have shown ghrelin and des-acyl ghrelin biological activities on proliferation of tumor cells and abnormal tissues.

SUMMARY

The recent studies support the possible positive effects of ghrelin in therapeutic approaches and adjunct treatment of a number of diseases associated with wasting. Utilization of agonists of the ghrelin receptor growth hormone secretagogue-1a is a promising approach for clinical use. Randomized and placebo-controlled studies, including large number of patients are further required.