The GOAT-Ghrelin System Is Not Essential for Hypoglycemia Prevention during Prolonged Calorie Restriction

Ghrelin acylation by ghrelin O-acyltransferase (GOAT) has recently been reported to be essential for the prevention of hypoglycemia during prolonged negative energy balance. Using a unique set of four different genetic loss-of-function models for the GOAT/ghrelin/growth hormone secretagogue receptor (GHSR) system, we thoroughly tested the hypothesis that lack-of-ghrelin activation or signaling would lead to hypoglycemia during caloric deprivation. Male and female knockout (KO) mice for GOAT, ghrelin, GHSR, or both ghrelin and GHSR (dKO) were subjected to prolonged calorie restriction (40% of ad libitum chow intake). Body weight, fat mass, and glucose levels were recorded daily and compared to wildtype (WT) controls. Forty-eight hour blood glucose profiles were generated for each individual mouse when 2% or less body fat mass was reached. Blood samples were obtained for analysis of circulating levels of acyl- and desacyl-ghrelin, IGF-1, and insulin. Chronic calorie restriction progressively decreased body weight and body fat mass in all mice regardless of genotype. When fat mass was depleted to 2% or less of body weight for 2 consecutive days, random hypoglycemic events occurred in some mice across all genotypes. There was no increase in the incidence of hypoglycemia in any of the four loss-of-function models for ghrelin signaling including GOAT KO mice. Furthermore, no differences in insulin or IGF-1 levels were observed between genotypes. The endogenous GOAT-ghrelin-GHSR system is not essential for the maintenance of euglycemia during prolonged calorie restriction

Number, age, initial vs. final body morphometry, and final acyl/desacyl-ghrelin, insulin and IGF-1 plasma levels of loss-of function ghrelin or their respective WT control mice after CR.

<p>Data are shown as Mean ± SEM. “n.d.” indicates not detectable;</p><p>*p<0.05,</p><p>**p<0.001 vs. GOAT WT;</p>†<p>p<0.05,</p>††<p>p<0.01 vs. Ghrelin-GHSR WT;</p>§<p>Number of mice removed from studies with two consecutive hypoglycemic events of blood glucose levels ≤50 mg/dL;</p>a<p>n = 4;</p>b<p>n = 3;</p>c<p>n = 4.</p

Stable blood glucose levels after partial or complete loss of body fat.

<p>Even after a partial (2% fat mass; A&B) or complete (0% fat mass; C&D) loss of body fat, calorie-restricted ghrelin-loss-of-function mice and their WT controls were able to maintain stable blood glucose levels. Values depict means ± SEMs from groups of 8 male GOAT WT, 5 female GOAT WT, 9 male GOAT KO, 8 female GOAT KO, 10 male Ghr-GHSR dWT, 20 female Ghr-GHSR dWT, 19 male Ghr KO, 18 female Ghr KO, 7 male GHSR KO, 19 female GHSR KO, 16 male Ghr-GHSR dKO and 7 female Ghr-GHSR dKO mice.</p

Change of body weight and fat mass in WT and ghrelin-loss-of-function mice after chronic CR.

<p>Male (A,B) and female (C,D) wildtype (WT) and GOAT, Ghrelin (Ghr), GHSR, or Ghr-GHSR dKO mice were subjected to chronic CR (40% of <i>ad libitum</i> calories), and changes in body weight (A,C) and fat mass (B,D) were recorded. Values are shown for each individual animal. Mice were taken out of the CR regiment when fat mass dropped to 0% for 2 consecutive days, or when mice became severely hypoglycemic. Lines depict body weight and fat mass curves for individual animals from groups of 8 male GOAT WT, 5 female GOAT WT, 9 male GOAT KO, 8 female GOAT KO, 10 male Ghr-GHSR dWT, 20 female Ghr-GHSR dWT, 19 male Ghr KO, 18 female Ghr KO, 7 male GHSR KO, 19 female GHSR KO, 16 male Ghr-GHSR dKO and 7 female Ghr-GHSR dKO mice.</p