The pathophysiology of obesity is known to be sustained excess of nutrient intake over expenditure. It has been shown that a “Western diet” with a high fat content is associated with an increased risk of obesity. We previously reported that repeated administration of ghrelin significantly increased body weight compared with saline treated controls under a standard diet.4
Fat pad mass, serum insulin, and cholesterol levels showed a tendency to increase by 31.8%, 76.2%, and 12.9%, respectively, although they failed to reach statistical significance. In this study, we found that repeated administration of ghrelin significantly increased adiposity with a concomitant increase in cholesterol and insulin levels under a high fat diet.
Leptin, adiponectin, and resistin are known to be associated with insulin resistance as adipocytokines, in WAT. Leptin is thought to be a critical molecule in the body weight regulatory system as an adiposity signal from the periphery to the hypothalamus.2,14–16
On the other hand, previous studies have shown that hypoadiponectinaemia is closely related to hyperinsulinaemia and insulin resistance, which is ameliorated by adiponectin treatment.17
Resistin, a new adipose secreted polypeptide, has been reported to have a causative role in insulin resistance through an as yet unknown mechanism.18
However, this was recently challenged by conflicting data on resistin gene expression. While tumour necrosis factor α and FFA, which contribute to insulin resistance, have a suppressive effect on resistin mRNA levels in adipocytes, several types of peroxisome proliferator activated receptor gamma agonists, antidiabetic drugs with an insulin sensitising action, increase adipose resistin mRNA expression.19,20
In addition, although serum levels of resistin are increased in genetic and diet induced forms of obesity, resistin gene expression is reported to be suppressed by insulin and obesity.18,21,22
In the present study, ghrelin elevated leptin mRNA expression, as well as reduced resistin mRNA expression in WAT with insulin resistance, which might be compatible with the latter postulate. Recently, Lee et al
have reported that under ad libitum fed condition, gastric ghrelin gene expression was decreased by a high fat diet.23
In contrast, we have shown here that under fasted condition, ghrelin gene expression in the stomach is increased by a high fat diet. These observations indicate that ghrelin appears to be closely related to excess weight gain, adiposity, and insulin resistance, particularly under a high fat diet and in the dynamic stage. If so, GHS-R may represent a target for pharmacological intervention in the treatment of obesity and related disorders.
Assuming that GHS-R antagonists would induce a state of negative energy balance, we examined the effects of GHS-R antagonists on feeding. As expected, GHS-R antagonists decreased feeding in lean mice and in mice rendered obese by a high fat diet. Previous reports have shown that GHS-R is present in the hypothalamus, heart, lung, pancreas, intestine, and adipose tissue.1,2
In the hypothalamus, GHS-R is located in the arcuate nucleus (ARC), where two orexigenic peptides, neuropeptide Y (NPY) and agouti related protein (AGRP), are synthesised in the neurone.1,2,16
In addition, non-peptide GH secretagogues act in the hypothalamus to alter the electrical activity of ARC neurones and activate expression of c- fos.2,24
To date, ghrelin has been reported to stimulate feeding behaviour with its mechanism of action involving direct activation of hypothalamic NPY and AGRP neurones in the ARC where the blood-brain barrier is less effective.2,4,5
However, an alternative pathway for ghrelin signalling from the stomach is via an ascending neural network through the vagus nerve and brainstem nuclei that ultimately reaches the hypothalamus.2,4
In our study, centrally administered GHS-R antagonist abolished the stimulatory effects on feeding induced by peripherally administered ghrelin. These results suggest that ghrelin may act through GHS-R in the brain. We also demonstrated that peripherally administered GHS-R antagonist decreased gastric emptying rate which may contribute to its anorexigenic effect. Considerable evidence has accumulated to indicate that gastric distention acts as a satiety signal to inhibit food intake, and rapid gastric emptying is closely related to overeating and obesity, as is delayed gastric emptying to anorexia and cachexia.25
Previous studies have shown that ghrelin increases gastric emptying rate and motility through vagal pathways.2,4
In addition, ICV administered NPY affects gastroduodenal contractile activity, changing fed patterns of irregular contractions into phasic contractions characterised as fasted patterns.26
Thus our findings suggest that GHS-R has a role in the control of feeding behaviour and that antagonism of GHS-R may be a promising approach for treating obesity.
Finally, we demonstrated that peripherally administered GHS-R antagonists produced anorexigenic effects and lowered body weight gain and blood glucose concentrations in ob/ob
obese mice, which is a known genetic model of obesity and diabetes with insulin resistance and rapid gastric emptying.16
This remarkable reduction in glucose levels, accompanied by a moderate decrease in serum insulin levels, implicates GHS-R antagonists in the amelioration of insulin resistance. In contrast, it has been shown that elevations of plasma FFA induce insulin resistance through inhibition of glucose transport activity with its mechanism of action involving reduction of phosphatidylinositol 3-kinase activity.27
Recently, elevated circulating FFA concentration has been reported to be an independent risk factor for sudden death in middle aged men in a long term cohort study.28
In the present study, GHS-R antagonists produced a remarkable decrease in FFA levels of ob/ob
obese mice. In addition, GHS-R antagonists demonstrated no significant effects on water intake, general behaviour, or anxiety, compared with saline treated controls, suggesting the specific nature of the effect.
In conclusion, we found that peripherally administered GHS-R antagonists [D-Lys-3]- GHRP-6 and [D-Arg-1, D-Phe-5, D-Trp-7, 9, Leu-11] substance P decreased food intake in lean mice, in mice with diet induced obesity, and in ob/ob
obese mice. We also showed that repeated administration of [D-Lys-3]-GHRP-6 decreased body weight gain and improved glycaemic control in ob/ob
obese mice. In contrast, repeated administration of ghrelin, an endogenous ligand for GHS-R, induced remarkable adiposity and affected glycaemic control under a high fat diet. Moreover, gastric ghrelin gene expression during fasting was elevated by a high fat diet. Together with the findings that the stomach is not only a source of ghrelin but also a source of leptin,29
the stomach as well as adipose tissue plays a crucial role in the regulation of energy balance as an endocrine organ. Hansen et al
have recently reported that weight loss in obese human subjects increases fasting plasma ghrelin levels.30
Moreover, transgenic rats expressing an antisense GHS-R mRNA have been reported to exhibit lower body weight and less adipose tissue than control rats.31
Although further studies are needed to determine the possibility of GHS-R antagonists acting as toxins, these observations suggest that gastric peptide ghrelin and GHS-R may be promising targets for pharmacological intervention, not only in the treatment of anorexia-cachexia but also in the treatment of obesity and type 2 diabetes, which are increasingly prevalent in the world.