Ghrelin receptor inverse agonists as a novel therapeutic approach against obesity-related metabolic disease

AIMS

Ghrelin is implicated in the control of energy balance and glucose homeostasis. The ghrelin receptor exhibits ligand-independent constitutive activity, which can be pharmacologically exploited to induce inverse ghrelin actions. Because ghrelin receptor inverse agonists (GHSR-IA) might be effective for the treatment of obesity-related metabolic disease, we tested 2 novel synthetic compounds GHSR-IA1 and GHSR-IA2.

MATERIALS AND METHODS

In functional cell assays, electrophysiogical and immunohistochemical experiments, we demonstrated inverse agonist activity for GHSR-IA1 and GHSR-IA2. We used healthy mice, Zucker diabetic fatty (ZDF) rats and diet-induced obese (DIO) mice to explore effects on food intake (FI), body weight (BW), conditioned taste aversion (CTA), oral glucose tolerance (OGT), pancreatic islet morphology, hepatic steatosis (HS), and blood lipids.

RESULTS

Both compounds acutely reduced FI in mice without inducing CTA. Chronic GHSR-IA1 increased metabolic rate in chow-fed mice, suppressed FI, and improved OGT in ZDF rats. Moreover, the progression of islet hyperplasia to fibrosis in ZDF rats slowed down. GHSR-IA2 reduced FI and BW in DIO mice, and reduced fasting and stimulated glucose levels compared with pair-fed and vehicle-treated mice. GHSR-IA2-treated DIO mice showed decreased blood lipids. GHSR-IA1 treatment markedly decreased HS in DIO mice.

CONCLUSIONS

Our study demonstrates therapeutic actions of novel ghrelin receptor inverse agonists, suggesting a potential to treat obesity-related metabolic disorders including diabetes mellitus.

Involvement of Amylin and Leptin in the Development of Projections from the Area Postrema to the Nucleus of the Solitary Tract

The area postrema (AP) and the nucleus of the solitary tract (NTS) are important hindbrain centers involved in the control of energy homeostasis. The AP mediates the anorectic action and the inhibitory effect on gastric emptying induced by the pancreatic hormone amylin. Amylin's target cells in the AP project to the NTS, an integrative relay center for enteroceptive signals. Perinatal hormonal and metabolic factors influence brain development. A postnatal surge of the adipocyte-derived hormone leptin represents a developmental signal for the maturation of projections between hypothalamic nuclei controlling energy balance. Amylin appears to promote neurogenesis in the AP in adult rats. Here, we examined whether amylin and leptin are required for the development of projections from the AP to the NTS in postnatal and adult mice by conducting neuronal tracing studies with DiI in amylin- (IAPP^-/-) and leptin-deficient (ob/ob) mice. Compared to wild-type littermates, postnatal (P10) and adult (P60) IAPP^-/- mice showed a significantly reduced density of AP-NTS projections. While AP projections were also reduced in postnatal (P14) ob/ob mice, AP-NTS fiber density did not differ between adult ob/ob and wild-type animals. Our findings suggest a crucial function of amylin for the maturation of neuronal brainstem pathways controlling energy balance and gastrointestinal function. The impaired postnatal development of neuronal AP-NTS projections in ob/ob mice appears to be compensated in this experimental model during later brain maturation. It remains to be elucidated whether an amylin- and leptin-dependent modulation in neuronal development translates into altered AP/NTS-mediated functions.

Alterations in energy expenditure in Roux-en-Y gastric bypass rats persist at thermoneutrality

BACKGROUND

The compensatory decrease in energy expenditure (EE) in response to body weight loss is attenuated by Roux-en-Y gastric bypass (RYGB) surgery in rats. The thermoneutral zone (TNZ) is at higher temperatures in rodents than in humans. Consequently, rodents may be under moderate cold stress if EE is measured at room temperature, leading to increased EE due to adaptive thermogenesis. We speculated that the reported alterations in EE of RYGB rats at room temperature are caused by higher adaptive thermogenesis and are therefore not present at thermoneutrality.

METHODS

Male Wistar rats were randomized for RYGB or sham surgery. Some sham rats were body weight matched (BWM) to the RYGB rats by food restriction, the others received ad libitum access to food (AL). EE, body temperature, physical activity and food intake were measured at ambient temperatures between 22 and 32 °C to determine the TNZ. Adaptive thermogenesis requires β3-adrenergic receptor-mediated uncoupling protein-1 (UCP-1) expression in brown adipose tissue (BAT). The in vivo thermogenic capacity of BAT was determined by administering the β3-adrenergic agonist CL316,243, and UCP-1 protein expression was measured at room temperature.

RESULTS

The TNZ was between 28 and 30 °C for AL and RYGB and between 30 and 32 °C for BWM rats, respectively. In contrast to AL and BWM rats, EE was not significantly higher at room temperature than at thermoneutrality in RYGB rats, reflecting a lack of adaptive thermogenesis. Consistently, both the thermogenic capacity of BAT and UCP-1 expression were decreased in RYGB compared with AL rats at room temperature.

CONCLUSIONS

Our data confirm that the decrease in EE after body weight loss is attenuated by RYGB surgery and show that this effect persists at thermoneutrality. Contrary to our hypothesis, we found that adaptive thermogenesis at room temperature is reduced in RYGB rats.

Effect of bariatric surgery combined with medical therapy versus intensive medical therapy or calorie restriction and weight loss on glycemic control in Zucker diabetic fatty rats

Bariatric surgery rapidly improves Type 2 diabetes mellitus (T2DM). Our objective was to profile and compare the extent and duration of improved glycemic control following Roux-en-Y gastric (RYGB) bypass surgery and vertical sleeve gastrectomy (SG) and compare against calorie restriction/weight loss and medical combination therapy-based approaches using the Zucker diabetic fatty rat (ZDF) rodent model of advanced T2DM. Male ZDF rats underwent RYGB (n = 15) or SG surgery (n = 10) at 18 wk of age and received postsurgical insulin treatment, as required to maintain mid-light-phase glycemia within a predefined range (10-15 mmol/l). In parallel, other groups of animals underwent sham surgery with ad libitum feeding (n = 6), with body weight (n = 8), or glycemic matching (n = 8) to the RYGB group, using food restriction or a combination of insulin, metformin, and liraglutide, respectively. Both bariatric procedures decreased the daily insulin dose required to maintain mid-light-phase blood glucose levels below 15 mmol/l, compared with those required by body weight or glycemia-matched rats (P < 0.001). No difference was noted between RYGB and SG with regard to initial efficacy. SG was, however, associated with higher food intake, weight regain, and higher insulin requirements vs. RYGB at study end (P < 0.05). Severe hypoglycemia occurred in several rats after RYGB. RYGB and SG significantly improved glycemic control in a rodent model of advanced T2DM. While short-term outcomes are similar, long-term efficacy appears marginally better after RYGB, although this is tempered by the increased risk of hypoglycemia.

Bone mineral density and expression of vitamin D receptor-dependent calcium uptake mechanisms in the proximal small intestine after bariatric surgery

BACKGROUND

Roux-en-Y gastric bypass may lead to impaired calcium uptake. Therefore, operation-specific effects of gastric bypass and vertical banded gastroplasty on bone mineral density (BMD) were examined in a randomized clinical trial. Bone resorption markers and mechanisms of decreased calcium uptake after gastric bypass were investigated using blood and endoscopic samples from two additional patient cohorts.

METHODS

Total BMD and non-weight-bearing skull BMD were measured by dual-energy X-ray absorptiometry at baseline, and 1 and 6 years after gastric bypass or vertical banded gastroplasty in patients who were not receiving calcium supplements. Bone resorption markers in serum and calcium uptake mechanisms in jejunal mucosa biopsies were analysed after gastric bypass by proteomics including radioimmunoassay, gel electrophoresis and mass spectrometry.

RESULTS

One year after surgery, weight loss was similar after gastric bypass and vertical banded gastroplasty. There was a moderate decrease in skull BMD after gastric bypass, but not after vertical banded gastroplasty (P < 0·001). Between 1 and 6 years after gastric bypass, skull BMD and total BMD continued to decrease (P = 0·001). C-terminal telopeptide levels in serum had increased twofold by 18 months after gastric bypass. Proteomic analysis of the jejunal mucosa revealed decreased levels of heat-shock protein 90β, a co-activator of the vitamin D receptor, after gastric bypass. Despite increased vitamin D receptor levels, expression of the vitamin D receptor-regulated calcium transporter protein TRPV6 decreased.

CONCLUSION

BMD decreases independently of weight after gastric bypass. Bone loss might be attributed to impaired calcium absorption caused by decreased activation of vitamin D-dependent calcium absorption mechanisms mediated by heat-shock protein 90β and TRPV6.

Roux-en-Y gastric bypass surgery reduces bone mineral density and induces metabolic acidosis in rats

Roux-en-Y gastric bypass (RYGB) surgery leads to bone loss in humans, which may be caused by vitamin D and calcium malabsorption and subsequent secondary hyperparathyroidism. However, because these conditions occur frequently in obese people, it is unclear whether they are the primary causes of bone loss after RYGB. To determine the contribution of calcium and vitamin D malabsorption to bone loss in a rat RYGB model, adult male Wistar rats were randomized for RYGB surgery, sham-operation-ad libitum fed, or sham-operation-body weight-matched. Bone mineral density, calcium and phosphorus balance, acid-base status, and markers of bone turnover were assessed at different time points for 14 wk after surgery. Bone mineral density decreased for several weeks after RYGB. Intestinal calcium absorption was reduced early after surgery, but plasma calcium and parathyroid hormone levels were normal. 25-hydroxyvitamin D levels decreased, while levels of active 1,25-dihydroxyvitamin D increased after surgery. RYGB rats displayed metabolic acidosis due to increased plasma lactate levels and increased urinary calcium loss throughout the study. These results suggest that initial calcium malabsorption may play a key role in bone loss early after RYGB in rats, but other factors, including chronic metabolic acidosis, contribute to insufficient bone restoration after normalization of intestinal calcium absorption. Secondary hyperparathyroidism is not involved in postoperative bone loss. Upregulated vitamin D activation may compensate for any vitamin D malabsorption.

Roux-en-Y gastric bypass surgery in rats alters gut microbiota profile along the intestine

Roux-en-Y gastric bypass (RYGB) surgery might modify the gut microbiota composition differently in the three distinct anatomical sections of the small intestine compared to sham surgery. We showed that RYGB induced changes in the microbiota of the alimentary limb and the common channel resembling those seen after prebiotic treatment or weight loss by dieting. These changes may be associated with altered production of intestinal hormones known to control energy balance. Postsurgical modulation of gut microbiota may significantly contribute to the beneficial metabolic effects of RYGB surgery.

Acute peripheral GLP-1 receptor agonism or antagonism does not alter energy expenditure in rats after Roux-en-Y gastric bypass

Compared to traditional weight loss strategies, the compensatory decrease in energy expenditure in response to body weight loss is markedly attenuated after Roux-en-Y gastric bypass surgery (RYGB). Because basal and postprandial levels of glucagon-like peptide-1 (GLP-1) are increased after RYGB surgery, and because GLP-1 has been shown to increase energy expenditure, we investigated if increased GLP-1 levels are involved in the alterations in energy expenditure after RYGB. Adult male Wistar rats were randomized for RYGB (n=8) or sham surgery (n=17). Part of the sham-operated rats were food restricted and body weight-matched (n=8) to the RYGB animals. The effects of acute subcutaneous administration of the GLP-1 antagonist Exendin (9-39) (Ex-9, 30μg/kg) or the GLP-1 agonist Exendin-4 (Ex-4, 5μg/kg), respectively, on energy expenditure were tested using indirect calorimetry. We found that Ex-9 increased food intake in RYGB, but not in sham-operated rats. Energy expenditure was lower in RYGB and sham-operated body weight-matched rats compared to sham-operated ad libitum fed rats, but significantly higher in RYGB rats compared to sham-operated body weight-matched rats. There was no effect of Ex-9 treatment on energy expenditure in either group of animals. Similarly, Ex-4 decreased food intake more in RYGB than in sham-operated rats, but Ex-4 did not modulate energy expenditure in any surgical group. We conclude that acute modulation of GLP-1 signaling is not directly involved in altered energy expenditure after RYGB surgery in rats.

Estradiol increases body weight loss and gut-peptide satiation after Roux-en-Y gastric bypass in ovariectomized rats

Despite the fact that ∼85% of bariatric operations are performed in women, the effects of the reproductive axis function on outcome of bariatric surgery remain to be determined. Here we developed the first published model of Roux-en-Y gastric bypass (RYGB) in female rats. We show in ovariectomized rats receiving estradiol or control treatment that (1) RYGB-induced body weight loss and (2) the satiating efficacy of endogenous glucagon-like peptide-1 and cholecystokinin satiation were significantly increased in estradiol-treated rats. These data are relevant to the care of obese women, in particular perimenopausal women, undergoing bariatric surgery.

Roux-en-Y gastric bypass operation in rats

Currently, the most effective therapy for the treatment of morbid obesity to induce significant and maintained body weight loss with a proven mortality benefit is bariatric surgery. Consequently, there has been a steady rise in the number of bariatric operations done worldwide in recent years with the Roux-en-Y gastric bypass (gastric bypass) being the most commonly performed operation. Against this background, it is important to understand the physiological mechanisms by which gastric bypass induces and maintains body weight loss. These mechanisms are yet not fully understood, but may include reduced hunger and increased satiation, increased energy expenditure, altered preference for food high in fat and sugar, altered salt and water handling of the kidney as well as alterations in gut microbiota. Such changes seen after gastric bypass may at least partly stem from how the surgery alters the hormonal milieu because gastric bypass increases the postprandial release of peptide-YY (PYY) and glucagon-like-peptide-1 (GLP-1), hormones that are released by the gut in the presence of nutrients and that reduce eating. During the last two decades numerous studies using rats have been carried out to further investigate physiological changes after gastric bypass. The gastric bypass rat model has proven to be a valuable experimental tool not least as it closely mimics the time profile and magnitude of human weight loss, but also allows researchers to control and manipulate critical anatomic and physiologic factors including the use of appropriate controls. Consequently, there is a wide array of rat gastric bypass models available in the literature reviewed elsewhere in more detail. The description of the exact surgical technique of these models varies widely and differs e.g. in terms of pouch size, limb lengths, and the preservation of the vagal nerve. If reported, mortality rates seem to range from 0 to 35%. Furthermore, surgery has been carried out almost exclusively in male rats of different strains and ages. Pre- and postoperative diets also varied significantly. Technical and experimental variations in published gastric bypass rat models complicate the comparison and identification of potential physiological mechanisms involved in gastric bypass. There is no clear evidence that any of these models is superior, but there is an emerging need for standardization of the procedure to achieve consistent and comparable data. This article therefore aims to summarize and discuss technical and experimental details of our previously validated and published gastric bypass rat model.