Gut hormones and the regulation of energy homeostasis

Autoantibodies against appetite-regulating peptide hormones and neuropeptides: putative modulation by gut microflora

Objective

Peptide hormones synthesized in gastrointestinal and adipose tissues in addition to neuropeptides regulate appetite and body weight. Previously, autoantibodies directed against melanocortin peptides were found in patients with eating disorders; however, it remains unknown whether autoantibodies directed against other appetite-regulating peptides are present in human sera and whether their levels are influenced by gut-related antigens.

Methods

Healthy women were studied for the presence of immunoglobulin (Ig) G and IgA autoantibodies directed against 14 key appetite-regulating peptides. The concept of molecular mimicry was applied to search in silico whether bacteria, viruses, or fungi contain proteins with amino acid sequences identical to appetite-regulating peptides. In addition, autoantibodies serum levels were studied in germ-free and specific pathogen-free rats.

Results

We found these IgG and IgA autoantibodies directed against leptin, ghrelin, peptide YY, neuropeptide Y, and other appetite-regulating peptides are present in human sera at levels of 100–900 ng/mL. Numerous cases of sequence homology with these peptides were identified among commensal and pathogenic micro-organisms including Lactobacilli, bacteroides, Helicobacter pylori, Escherichia coli, and Candida species. Decreased levels of IgA autoantibodies directed against several appetite-regulating peptides and increased levels of antighrelin IgG were found in germ-free rats compared with specific pathogen-free rats.

Conclusion

Healthy humans and rats display autoantibodies directed against appetite-regulating peptide hormones and neuropeptides, suggesting that these autoantibodies may have physiologic implications in hunger and satiety pathways. Gut-related antigens including the intestinal microflora may influence production of theses autoantibodies, suggesting a new link between the gut and appetite control.

A comparison of the effects of olanzapine and risperidone versus placebo on ghrelin plasma levels

To thoroughly investigate the phenomenon of atypical antipsychotic-associated weight gain, a feeding laboratory paradigm was developed that included obtaining plasma levels of the orexigenic peptide ghrelin that is associated with appetite and eating. This study is a randomized, double-blind, parallel group trial comparing the effects of a 2-week exposure to olanzapine, risperidone, or placebo on plasma ghrelin area under the plasma-time curve (AUC) in 28 healthy human subjects. Subjects were randomized to receive olanzapine, risperidone, or placebo and titrated over 4 days to 10 mg/d or 4 mg/d, respectively. The mean dose at end point was 8.6 + 1.8 mg/d for the olanzapine group and 2.8 + 0.8 mg/d for the risperidone group. Weight changes were significantly different between groups at end point (F2,44 = 10.193; P = 0.0001). The olanzapine group demonstrated a significant increase in weight at end point (2.25 + 1.84 kg) compared with placebo (0.13 + 1.05 kg; P = 0.007). Because of the small subject number, the comparisons between olanzapine and risperidone and risperidone and placebo did not reach statistical significance, although olanzapine's mean weight gain was numerically greater than that of risperidone (2.25 + 1.84 kg vs 1.10 + 0.99 kg) and risperidone's mean weight gain was numerically larger than placebo (1.10 + 0.99 kg vs 0.13 + 1.05 kg). The baseline adjusted Bonferroni corrected contrast of end point ghrelin AUC demonstrated a significant difference between groups (F2,24 = 4.40; P = 0.024), and the post hoc analysis revealed a significant decrease in ghrelin AUC for the olanzapine group in comparison with the risperidone group (P = 0.021) but not between risperidone and placebo or olanzapine and placebo. Ghrelin AUC values did not change significantly from baseline to end point in either of the other 2 groups. The difference between groups approached but did not reach significance (F2,23 = 3.299; P = 0.055) when body mass index change was included as a covariate, suggesting that the difference in ghrelin AUC change followed the change in body weight. Sedation associated with both active drugs (P = 0.006) and "stuffy nose" associated with risperidone (P = 0.020) were the only statistically different adverse reactions when compared with placebo. Thus, a human feeding laboratory paradigm using a brief exposure to atypical antipsychotics functions as a method to investigate pharmacologically induced weight gain and its association with changes in the orexigenic peptide ghrelin. This rejects the hypothesis that ghrelin levels are elevated by the antipsychotic and that this is a potential cause of the weight gain phenomenon.

Obesity: the hormonal milieu

PURPOSE OF REVIEW

Obesity has reached epidemic proportions throughout the world and poses significant health and economic burdens to both developed and developing societies. Most recent data from the NHANES study (2003-2004) report that 17.1% of US children are overweight and 32.2% of adults are obese, a significant increase compared with data obtained only 6 years earlier.

RECENT FINDINGS

The neurohormonal control of appetite, body composition, and glucose homeostasis is mediated by hormones secreted from adipose tissue, endocrine glands, and enteroendocrine cells, which converge at the vagus nerve, brainstem and hypothalamus to modulate complex interactions of neurotransmitters and central appetite-regulating peptides. These hormonal signals are tightly regulated to maintain body weight/adiposity within a narrow, individually defined range that may be further impacted by variables such as ingested calories, meal composition, and lifestyle.

SUMMARY

Clinical manifestations of obesity, the metabolic syndrome and impaired glucose tolerance reflect biochemical alterations in a complex hormonal milieu. Elucidation of these hormonal perturbations in obese patients has already provided novel pharmacologic treatments to improve weight management and address the metabolic sequelae of obesity. The remarkable redundancy of these hormones, however, and their interactions make a monopharmaceutical approach unlikely to be successful.

Gastrointestinal peptides controlling body weight homeostasis

Obesity has become an international public health problem. Unfortunately, effective treatment options are limited. In the last 20 years, research in obesity and associated pathologies has derived in a significant increase in the knowledge of the physiological and molecular mechanism regulating body mass, such as gastrointestinal-neuroendocrine communications. Gut-brain peptides may provide attractive therapeutic targets against this disease. This review summarizes research into energy balance through gastrointestinal tract peptides. Understanding these molecular mechanisms will provide new pharmacological targets for the treatment of obesity and appetite disorders.

Inhibitory effects of lipopolysaccharide on hypothalamic nuclei implicated in the control of food intake

The arcuate nucleus (Arc) and the lateral hypothalamic area (LHA), two key hypothalamic nuclei regulating feeding behavior, express c-Fos, a marker of neuronal activation in fasted animals. This is reversed by refeeding. In the present study we tested whether an anorectic dose of lipopolysaccharide (LPS), the cell wall component of Gram-negative bacteria, also inhibits fasting-induced c-Fos expression in these hypothalamic nuclei. This would suggest that they are involved in anorexia during bacterial infections as well. We also studied whether LPS modulates the activity of orexin-A positive (OX+) LHA neurons. Food deprived BALB/c mice were injected with LPS or saline and were sacrificed 4 or 6h later. Four hours after injection, LPS reduced the number of c-Fos positive cells in the Arc and in the LHA, but had no effect on c-Fos in OX+ neurons. Six hours after injection, LPS reduced c-Fos expression in the LHA, both in the OX- and OX+ neurons, but not in the Arc. These results show that LPS modulates neuronal activity in the Arc and LHA similar to feeding-related stimuli, suggesting that the observed effects might contribute to the anorectic effect of LPS. Thus, physiological satiety signals released during refeeding and anorexia during bacterial infection seem to engage similar neuronal substrates.

Evidence that lipopolysaccharide-induced anorexia depends upon central, rather than peripheral, inflammatory signals

Systemic inflammatory stimuli cause anorexia and weight loss by disrupting the physiological regulation of energy balance. Mice lacking MyD88, an intracellular mediator of signal transduction activated by Toll-like receptor 4 or IL-1beta receptors, are resistant to anorexia induced by the bacterial endotoxin lipopolysaccharide (LPS), despite a significant circulating cytokine response. Thus, we hypothesized that induction of a peripheral inflammatory response is insufficient to cause LPS-induced anorexia when MyD88 signaling in the central nervous system and other tissues is absent. To test this hypothesis, we used bone marrow transplantation (BMT) to determine if LPS-induced anorexia can be restored to MyD88-deficient mice by reconstituting their bone marrow with wild-type (WT) immune cells. We found that restoring WT circulating immune cells to mice lacking MyD88 conferred only a mild, short-lived anorexia in response to LPS, such that food intake was fully normalized by 20 h post injection (LPS 4.1 +/- 0.5 g vs. vehicle 4.3 +/- 0.3 g), whereas LPS-induced anorexia was profound and sustained in WT controls after either autologous BMT or sham BMT. Similarly, LPS-mediated induction of hypothalamic mRNA encoding IL-1beta and TNFalpha was robust in both WT control groups but was absent in chimeric MyD88 mice, despite comparable peripheral inflammatory responses across the three groups. We conclude that LPS reduces food intake via a mechanism dependent on MyD88 signaling within brain and/or other tissues and that in the absence of this effect, robust stimulation of circulating immune cells cannot induce sustained anorexia.

Human evolution: thrifty genes and the dairy queen

Two new studies of genes that have experienced positive selection since the origin of pastoral agriculture help explain the incidence of lactose tolerance and diabetes, but cast considerable doubt on the popular thrifty genes hypothesis.

Obestatin and ghrelin in obese and in pregnant women

We identified, through qPCR, receptor mRNA for a number of gut peptides in female human omental fat: the incretins, GIP and GLP-1, the orexigenic peptides PYY-Y1 and -Y2 and ghrelin, and the anorexigenic peptide obestatin. Four cohorts of women were examined: lean controls (BMI<23), obese (BMI>41), obese diabetic and term pregnant women. Human fat expressed receptor mRNAs for all six peptides. Pregnant women expressed roughly three times as much orphan GPR-39 receptor, a proposed obestatin receptor, than other women and less than half as much of the ghrelin receptor (GHSR-1a). An immunoblot probed with a GPR-39 selective antibody yielded a single band corresponding to the correct molecular weight (52 kDa) for the proposed obestatin receptor. Fluorescent immunohistochemistry of human fat employing the same antibody indicated the receptor protein was localized to the adipocyte cell membrane. The concentration of obestatin circulating in blood was measured in the same cohort of women and was significantly lower in obese and obese diabetic women compared to control.

Orexins control intestinal glucose transport by distinct neuronal, endocrine, and direct epithelial pathways

OBJECTIVE

Orexins are neuropeptides involved in energy homeostasis. We investigated the effect of orexin A (OxA) and orexin B (OxB) on intestinal glucose transport in the rat.

RESEARCH DESIGN AND METHODS AND RESULTS

Injection of orexins led to a decrease in the blood glucose level in oral glucose tolerance tests (OGTTs). Effects of orexins on glucose entry were analyzed in Ussing chambers using the Na(+)-dependent increase in short-circuit current (Isc) to quantify jejunal glucose transport. The rapid and marked increase in Isc induced by luminal glucose was inhibited by 10 nmol/l OxA or OxB (53 and 59%, respectively). Response curves to OxA and OxB were not significantly different with half-maximal inhibitory concentrations at 0.9 and 0.4 nmol/l, respectively. On the one hand, OxA-induced inhibition of Isc was reduced by the neuronal blocker tetrodotoxin (TTX) and by a cholecystokinin (CCK) 2R antagonist, indicating involvement of neuronal and endocrine CCK-releasing cells. The OX(1)R antagonist SB334867 had no effect on OxA-induced inhibition, which is likely to occur via a neuronal and/or endocrine OX(2)R. On the other hand, SB334867 induced a significant right shift of the concentration-effect curve for OxB. This OxB-preferring OX(1)R pathway was not sensitive to TTX or to CCKR antagonists, suggesting that OxB may act directly on enterocytic OX(1)R. These distinct effects of OxA and OxB are consistent with the expression of OX(1)R and OX(2)R mRNA in the epithelial and nonepithelial tissues, respectively.

CONCLUSIONS

Our data delineate a new function for orexins as inhibitors of intestinal glucose absorption and provide a new basis for orexin-induced short-term control of energy homeostasis.

The regulation of body mass and its relation to the development of obesity

A testtömeg, ezen belül a zsírtömeg regulálása egy visszacsatolásos rendszerben valósul meg, melyben a zsírtömeg nagyságáról adipositasszignálok (leptin, inzulin, amylin), a pillanatnyi tápláltsági állapotról intestinalis peptidhormonok (ghrelin, PYY, PP, GLP-1, OXM, CCK) és a n. vagus informálják a központi idegrendszert mint központi szabályozót. A hypothalamus nucleus arcuatusának két jól elkülöníthető neuroncsoportja fogadja a zsírtömeg nagyságával arányos mennyiségben termelődő leptin közvetítette afferens információt. A leptinkötődés a sejtek felszínén elhelyezkedő leptinreceptorokhoz intracelluláris szignálmechanizmusokon keresztül a targetgének aktiválódását és anorexigén (POMC, CART) neuropeptidek termelését, majd másodlagos központokon keresztül az energiafelvétel és -leadás folyamatainak efferens regulációs mechanizmusait generálja. A testtömeg-szabályozás összetett és redundáns rendszer, számos más neuroendokrin folyamattal (növekedés, mellékvese- és pajzsmirigyműködés, reproduktív funkciók stb.), memória-, valamint jutalmazási és függőségi mechanizmussal áll kapcsolatban, így a szabályozórendszer egyes elemeinek befolyásolása, pl. gyógyszerekkel, más rendszerek működésére is hatással lehet, mellékhatások felléptére lehet számítani. Az obesitas világméretű epidémiája – mely elsősorban a magas energiasűrűségű élelmiszerek bőségével és a mozgásszegény életmóddal áll összefüggésben, melyhez a testtömeg-szabályozó rendszerünk nem tud megfelelően alkalmazkodni – intenzív kutatásokra ösztönzi az akadémiai és gyógyszeripari kutatóközpontokat annak érdekében, hogy új gyógyszerek, hatékony testsúlycsökkentő kezelési eljárások álljanak rendelkezésre az életmód-változtatási stratégiákon (diéta, fizikai aktivitás, magatartásterápia) túlmenően.

Evidence for a signaling axis by which intestinal phosphate rapidly modulates renal phosphate reabsorption

The mechanisms by which phosphorus homeostasis is preserved in mammals are not completely understood. We demonstrate the presence of a mechanism by which the intestine detects the presence of increased dietary phosphate and rapidly increases renal phosphate excretion. The mechanism is of physiological relevance because it maintains plasma phosphate concentrations in the normal range after ingestion of a phosphate-containing meal. When inorganic phosphate is infused into the duodenum, there is a rapid increase in the renal fractional excretion of phosphate (FE Pi). The phosphaturic effect of intestinal phosphate is specific for phosphate because administration of sodium chloride does not elicit a similar response. Phosphaturia after intestinal phosphate administration occurs in thyro-parathyroidectomized rats, demonstrating that parathyroid hormone is not essential for this effect. The increase in renal FE Pi in response to the intestinal administration of phosphate occurs without changes in plasma concentrations of phosphate (filtered load), parathyroid hormone, FGF-23, or secreted frizzled related protein-4. Denervation of the kidney does not attenuate phosphaturia elicited after intestinal phosphate administration. Phosphaturia is not elicited when phosphate is instilled in other parts of the gastrointestinal tract such as the stomach. Infusion of homogenates of the duodenal mucosa increases FE Pi, which demonstrates the presence of one or more substances within the intestinal mucosa that directly modulate renal phosphate reabsorption. Our experiments demonstrate the presence of a previously unrecognized phosphate gut-renal axis that rapidly modulates renal phosphate excretion after the intestinal administration of phosphate.

Drug discovery in the metabolic syndrome: context and some recent developments

The metabolic syndrome, encompassing the clinically distinct but related areas of dyslipidaemia, insulin resistance, obesity and vascular disease, offers a wide arena for drug discovery. There is substantial and growing unmet medical need, particularly as the worldwide epidemic of obesity continues to develop. There are also many targets and biological mechanisms that can be exploited. However, the context for clinical development is challenging because of the many ways in which the syndrome can be approached. As with most therapeutic areas, preclinical data provide only limited confidence in the potential of a novel target in humans. In this review, the author outlines the context for drug discovery in the metabolic syndrome, the clinical and biological scope and recent developments in preclinical models. Finally, existing examples of drug targets for a range of biological mechanisms are considered, outlining their biology and points relevant to lead identification and optimisation and clinical development.

Exercise-induced suppression of acylated ghrelin in humans

Ghrelin is an orexigenic hormone secreted from endocrine cells in the stomach and other tissues. Acylation of ghrelin is essential for appetite regulation. Vigorous exercise induces appetite suppression, but this does not appear to be related to suppressed concentrations of total ghrelin. This study examined the effect of exercise and feeding on plasma acylated ghrelin and appetite. Nine male subjects aged 19-25 yr participated in two, 9-h trials (exercise and control) in a random crossover design. Trials began at 0800 in the morning after an overnight fast. In the exercise trial, subjects ran for 60 min at 72% of maximum oxygen uptake between 0800 and 0900. After this, they rested for 8 h and consumed a test meal at 1100. In the control trial, subjects rested for 9 h and consumed a test meal at 1100. Area under the curve values for plasma acylated ghrelin concentration (assessed from venous blood samples) were lower over the first 3 h and the full 9 h of the exercise trial compared with the control trial: 317+/-135 vs. 510+/-186 pg.ml(-1).3 h and 917+/-342 vs. 1,401+/-521 pg.ml(-1).9 h (means+/-SE) respectively (P<0.05). Area under the curve values for hunger (assessed using a visual scale) were lower over the first 3 h of the exercise trial compared with the control trial (P=0.013). These findings demonstrate that plasma acylated ghrelin concentration and hunger are suppressed during running.