J Endocrinol. 2016 228(2):115-125

Ghrelin O-acyltransferase knockout mice show resistance to obesity when fed high-sucrose diet

Tetsuya Kouno1, 3, Nobuteru Akiyama1, Takahito Ito1, Tomohiko Okuda2, Isamu Nanchi2, Mitsuru Notoya1, Shogo Oka3, Hideo Yukioka1

1Discovery Research Laboratory for Innovative Frontier Medicines, 2Discovery Research Laboratory for Core Therapeutic Areas, Shionogi & Co., Ltd., 3-1-1 Futaba-cho, Toyonaka, Osaka 561-0825, Japan.

3Department of Biological Chemistry, Human Health Sciences, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606-8507, Japan.



Ghrelin is an appetite-stimulating hormone secreted from stomach. Since the discovery that acylation of the serine-3 residue by ghrelin O-acyltransferase (GOAT) is essential for exerting its functions, GOAT has been regarded as an therapeutic target for attenuating appetite, and thus for the treatment of obesity and diabetes. However, contrary to the expectations, GOAT-knockout (KO) mice have not shown meaningful body weight reduction, under high-fat diet. Here, in this study, we sought to determine whether GOAT has a role in body weight regulation and glucose metabolism with a focus on dietary sucrose, because macronutrient composition of diet is important for appetite regulation. We found that peripherally administered acylated-ghrelin, but not unacylated one, stimulated sucrose consumption in a two-bottle-drinking test. The role of acylated-ghrelin in sucrose preference was further supported by the finding that GOAT KO mice consumed less sucrose solution compared with wild-type (WT) littermates. Then, we investigated the effect of dietary composition of sucrose on food intake and body weight in GOAT KO and WT mice. As a result, when fed on high-fat diet, food intake and body weight were similar between GOAT KO and WT mice. However, when fed on high-fat, high-sucrose diet, GOAT KO mice showed significantly reduced food intake and marked resistance to obesity, leading to amelioration of glucose metabolism. These results suggest that blockade of acylated-ghrelin production offers therapeutic potential for obesity and metabolic disorders caused by overeating of palatable food.

KEYWORDS: body weight; food intake; ghrelin; insulin sensitivity; sucrose

PMID: 26645250



Acylated-ghrelin administered exogenously has definitely been demonstrated to stimulate food intake in humans and rodents (1-3), but its endogenous role in regulating food intake and body weight has remained unclear. For instance, several studies using mice deficient in ghrelin or ghrelin O-acyltransferase (GOAT), the enzyme responsible for ghrelin acylation, have shown no significant changes in food intake and body weight when given a high-fat (HF) diet or standard chow as compared to wild type (WT) littermates (4-7). We then asked the question whether acylated-ghrelin affects the consumption of different macronutrient composition of diet, e.g. HF diet supplemented with sweet sugars including sucrose, or not? That was because ghrelin receptor, GHSR-1a, has been reported to be expressed in brain areas involved in “hedonic feeding” as well as “homeostatic feeding” (8, 9). Therefore, we first tested a role of ghrelin signaling in consumption of sucrose solution with a two-bottle drinking test and revealed that mice deficient in GOAT consumed less sucrose solution than WT mice, in addition to higher consumption of sucrose solution in mice received an intraperitoneal injection of acylated-ghrelin rather than saline control (10). These findings suggest that an endogenous ghrelin/GOAT system could play a significant role in the pathological process of obesity elicited by overconsumption of sugar-sweetened high-calorie foods. We then investigated the effect of chronic feeding of several kinds of diets (Figure 1) containing high-fat, high-sucrose, or medium-chain triglyceride, which is used for acyl-modification of ghrelin, on the development of obesity in GOAT KO mice. Interestingly, we have found that GOAT knockout (KO) mice protected from obesity with accompanying decrease in food intake on a high-sucrose diet, while the mice showed no significant changes in food intake and body weight on a high-fat diet in accordance with a previous report (4, 5). This is a novel finding that the endogenous ghrelin/GOAT system may play a major role in the regulation of appetite for sucrose containing diets.


What is the mechanism behind the reduction of sucrose intake in GOAT KO mice? In order to examine the influence on preference for sweetness, we investigated the effect of ghrelin on the intake of saccharin, a non-nutritive sweetener, in the two-bottle drinking test. Interestingly, we found that GOAT KO mice exhibited similar preference for saccharin solution compared to WT mice, indicating that ghrelin signal may stimulate rewarding system in response to nutritional value, but not sweetness, of sucrose. Some studies have indicated that ghrelin stimulates sucrose intake via dopamine release from nucleus accumbens (NAc) in the midbrain (11, 12), and caloric value of sucrose is necessary to induce dopamine release (13). Further investigation is needed to elucidate the underlying mechanism in regulating sucrose consumption by ghrelin.


The importance of this study: our findings revealed that ghrelin/GOAT system may be associated with overconsumption of sucrose-containing foods and drinks. Recently, it has been reported that increasing consumption of sugar-sweetened beverages are associated with overweight and obesity (14), and a newly updated guideline from the World Health Organization (WHO) strongly recommends reducing the intake of free sugars such as sucrose and fructose (15). In the light of social demands to cut down on sugar intake, our present finding is noteworthy for shedding light on a physiological function of ghrelin in the regulation of sugar intake and body weight control, and also showing that inhibition of enzymatic activity of GOAT is proved to be a potential strategy to treat obesity.




Figure 1 Dietary composition. A high-fat diet, most commonly used in mice, contains 60% kcal% fat, predominantly lard. A medium-chain triglyceride (MCT) diet contains 58% kcal% fat from coconut, which is the source of medium-chain triglyceride. A high-fat + high-sucrose diet consists of 40% lard and 40% sucrose. A MCT + high-sucrose diet consists of 40% coconut oil and 40% sucrose.



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Correspondence to:

Tetsuya Kouno, E-mail: tetsuya.kouno@shionogi.co.jp


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