Mice with hyperghrelinemia are hyperphagic and glucose intolerant and have reduced leptin sensitivity

Abstract

Objective: Ghrelin is the only known peripheral hormone to increase ingestive behavior. However, its role in the physiological regulation of energy homeostasis is unclear because deletion of ghrelin or its receptor does not alter food intake or body weight in mice fed a normal chow diet. We hypothesized that overexpression of ghrelin in its physiological tissues would increase food intake and body weight.

Research design and methods: We used bacterial artificial chromosome transgenesis to generate a mouse model with increased ghrelin expression and production in the stomach and brain. We investigated the effect of ghrelin overexpression on food intake and body weight. We also measured energy expenditure and determined glucose tolerance, glucose stimulated insulin release, and peripheral insulin sensitivity.

Results: Ghrelin transgenic (Tg) mice exhibited increased circulating bioactive ghrelin, which was associated with hyperphagia, increased energy expenditure, glucose intolerance, decreased glucose stimulated insulin secretion, and reduced leptin sensitivity.

Conclusions: This is the first report of a Tg approach suggesting that ghrelin regulates appetite under normal feeding conditions and provides evidence that ghrelin plays a fundamental role in regulating beta-cell function.

FIG. 1.

Total and bioactive ghrelin levels are increased in Tg mice. Ghrelin stomach mRNA expression was significantly increased in Tg mice compared with Wt littermates (A). In addition, both total ghrelin (B) and octanoylated ghrelin (C) were significantly increased in stomach extracts. Plasma total ghrelin concentrations were increased in male mice (D). Fasting plasma octanoylated ghrelin concentrations were increased in both female (E) and male (F) mice. Ghrelin expression was found exclusively in the hypothalamus and stomach of Tg and Wt mice (G). The results are means ± SEM; n = 6. *P < 0.05, **P < 0.01 Tg and WT controls at 16 weeks of age. int, intestine; RQ, relative quantification compared with control stomach.

FIG. 2.

Overexpression of bioactive ghrelin increases food intake. Cumulative food intake was measured from 5 to 16 weeks of age in both male (A) and female (C) mice fed on regular chow. Body growth curves from 5 to 16 weeks of age for male (B) and female (D) mice. Body composition (E; □, lean; ▨, protein; ■, fat) and plasma leptin concentrations (F) were measured in 16-week-old male mice. ● (solid lines), Wt; ◇ (broken lines), Tg. The results are means ± SEM; n = 6–8. *P < 0.05, **P < 0.01 Tg and Wt controls.

FIG. 3.

Overexpression of bioactive ghrelin increases energy expenditure. Brown adipose tissue UCP-1 mRNA expression in 16-week-old mice (A) and average oxygen consumption measured during one 24-h period (B) and average hourly oxygen consumption (C) using the comprehensive lab animal monitoring system in 16-week-old mice. Average ambulatory activity estimated as X beam breaks during either the light or dark period (D). Respiratory exchange ratio was calculated as Vco₂ / Vo₂ (E). The results are means ± SEM; n = 6–8. *P < 0.05, **P < 0.01, Tg and Wt controls, respectively.

FIG. 4.

Overexpression of bioactive ghrelin attenuates glucose-stimulated insulin release. After an 18-h fast, 16-week-old male mice were injected with glucose (2 g/kg). Plasma glucose (A) was measured and AUC calculated (B). Plasma glucose was measured during an insulin tolerance test (insulin 1.5 units/kg) (C) and AUC calculated (D). After intraperitoneal glucose tolerance test, plasma glucose (E) and insulin (F) were measured and AUC for insulin release was calculated (G). The results are presented as means ± SEM; n = 6–10. *P < 0.05, **P < 0.01, Tg and Wt controls, respectively. Dashed lines, Tg; solid lines, Wt.

FIG. 5.

Ghrelin overexpressing Tg mice are sensitive to ghrelin but have reduced leptin sensitivity. After intraperitoneal injection of ghrelin (0.3 nmol/g) or saline to fed 16-week-old male Tg and Wt mice, 0- to 1-h food intake was measured (A). After intraperitoneal administration of leptin (3 μg/g) or saline to fasted 16-week-old male Tg and Wt mice, 0- to 1-h food intake was measured (B) as well as 0- to 4-h food intake (C). The results are means ± SEM; n = 9–10. *P < 0.05 saline versus ghrelin or leptin treatment.