Growth hormone-releasing hormone deficiency confers extended lifespan and metabolic resilience during high-fat feeding in mid and late life

Abstract

Growth hormone-releasing hormone-deficient (GHRH-KO) mice have previously been characterized by lower body weight, disproportionately high body fat accumulation, preferential metabolism of lipids compared to carbohydrates, improved insulin sensitivity, and an extended lifespan. That these mice are long-lived and insulin-sensitive conflicts with the notion that adipose tissue accumulation drives the health detriments associated with obesity (i.e., diabetes), and indicates that GH signaling may be necessary for the development of adverse effects linked to obesity. This prompts investigation into the ultimate effect of diet-induced obesity on the lifespan of these long-lived mice. To this end, we initiated high-fat feeding in mid and late-life in GHRH-KO and wild-type (WT) mice. We carried out extensive lifespan analysis coupled with glucose/insulin tolerance testing and indirect calorimetry to gauge the metabolic effect of high-fat dietary stress through adulthood on these mice. We show that under high-fat diet (HFD) conditions, GHRH-KO mice display extended lifespans relative to WT controls. We also show that GHRH-KO mice are more insulin-sensitive and display less dramatic changes in their metabolism relative to WT mice, with GHRH-KO mice fed HFD displaying respiratory exchange ratios and glucose oxidation rates comparable to control-diet fed GHRH-KO mice, while WT mice fed HFD showed significant reductions in these parameters. Our results indicate that GH deficiency protects against the adverse effects of diet-induced obesity in later life.

Keywords: aging; growth hormone‐releasing hormone; high fat diet; insulin; longevity.

FIGURE 1

Effects of high‐fat diet on longevity. Pooled male and female mouse survival for wild‐type (WT) and growth hormone‐releasing hormone knockout (GHRH‐KO) mice fed high‐fat diet (HFD) or control diet (CD) (a). Survival analysis of female mice (b), and male mice (c). Shaded regions represent 95% CI. Sample sizes, statistical analyses employed, and p‐values are provided in Table 1.

FIGURE 2

Glucose and insulin tolerance tests. Glycemic response to an intraperitoneal (IP) glucose challenge in female GHRH‐KO (a) and wild‐type (WT) (b) mice after 2.5 months of dietary intervention. Area under the curve (AUC) analysis of the glucose tolerance tests (c). Glycemic response to an IP insulin challenge in female GHRH‐KO (d) and WT (e) mice after 2.5 months of dietary intervention. Area under the curve analyses of the insulin tolerance tests (f). Data presented as mean ± SEM, with points representing individual mice. #p < 0.1; *p < 0.05 by Student's t test. N = 5 for all groups.

FIGURE 3

Growth hormone‐releasing hormone (GHRH) deletion or high‐fat diet reduces respiratory exchange ratio (RER). RER (calculated as VCO₂/VO₂) for male wild‐type (WT) (a) and male GHRH‐knockout (KO) (b) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean RER at each hour in a 24‐h day in WT males (c) and GHRH‐KO males (d). RER for female WT (e) and female GHRH‐KO (f) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean RER at each hour in a 24‐h day in WT females (g) and GHRH‐KO females (h). Shaded regions indicate dark cycles. Data presented as mean ± SEM. Statistical significance assessed by repeated measure two‐way ANOVA; *p < 0.05 as determined by pairwise comparisons with the Benjamini–Hochberg false discovery rate correction applied. N = 4–9 (males), N = 5–6 (females).

FIGURE 4

High‐fat diet induced changes in glucose oxidation rate. Body weight normalized glucose oxidation rate, calculated as 4.57 × VCO₂–3.23 × VO₂, for male wild‐type (WT) (a) and male growth hormone‐releasing hormone‐knockout (GHRH‐KO) (b) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized glucose oxidation rate at each hour in a 24‐h day in WT males (c) and GHRH‐KO males (d). Bodyweight normalized glucose oxidation rate for female WT (e) and female GHRH‐KO (f) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized glucose oxidation rate at each hour in a 24‐h day in WT females (g) and GHRH‐KO females (h). Shaded regions indicate dark cycles. Data presented as mean ± SEM. Statistical significance assessed by repeated measure two‐way ANOVA; *p < 0.05 as determined by pairwise comparisons with the Benjamini–Hochberg false discovery rate correction applied. N = 4–9 (males), N = 5–6 (females).

FIGURE 5

High‐fat diet induced changes in fatty acid oxidation. Body weight normalized fatty acid oxidation rate, calculated as 1.69 × VO₂–1.69 × VCO₂, for male wild‐type (WT) (a) and male GHRH‐KO (b) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized fatty acid oxidation rate at each hour in a 24‐h day in WT males (c) and growth hormone‐releasing hormone‐knockout (GHRH‐KO) males (d). Bodyweight normalized fatty acid oxidation rate for female WT (e) and female GHRH‐KO (f) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized fatty acid oxidation rate at each hour in a 24‐h day in WT females (g) and GHRH‐KO females (h). Shaded regions indicate dark cycles. Data presented as mean ± SEM. Statistical significance assessed by repeated measure two‐way ANOVA; *p < 0.05 as determined by pairwise comparisons with the Benjamini–Hochberg false discovery rate correction applied. N = 4–9 (males), N = 5–6 (females).

FIGURE 6

High‐fat diet induced changes in rate of energy expenditure. Body weight normalized energy expenditure, calculated as VO₂ × (3.815 + 1.232 × RER), for male wild‐type (WT) (a) and male growth hormone‐releasing hormone‐knockout (GHRH‐KO) (b) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized energy expenditure at each hour in a 24‐h day in WT males (c) and GHRH‐KO males (d). Bodyweight normalized energy expenditure for female WT (e) and female GHRH‐KO (f) mice over the 6‐day indirect calorimetry data collection period. Pairwise comparisons for mean bodyweight normalized energy expenditure at each hour in a 24‐h day in WT females (g) and GHRH‐KO females (h). Total activity, measured as the number of beam breaks across the indirect calorimetry data collection period, for WT males (i), GHRH‐KO males (j), WT females (k), and GHRH‐KO females (l). Shaded regions indicate dark cycles. Data presented as mean ± SEM. Statistical significance assessed by repeated measure two‐way ANOVA; *p < 0.05 as determined by pairwise comparisons with the Benjamini–Hochberg false discovery rate correction applied. N = 4–9 (males), N = 5–6 (females).