Differential response of rat strains to obesogenic diets underlines the importance of genetic makeup of an individual towards obesity

Abstract

Obesity, a multifactorial disorder, results from a chronic imbalance of energy intake vs. expenditure. Apart from excessive consumption of high calorie diet, genetic predisposition also seems to be equally important for the development of obesity. However, the role of genetic predisposition in the etiology of obesity has not been clearly delineated. The present study addresses this problem by selecting three rat strains (WNIN, F-344, SD) with different genetic backgrounds and exposing them to high calorie diets. Rat strains were fed HF, HS, and HFS diets and assessed for physical, metabolic, biochemical, inflammatory responses, and mRNA expression. Under these conditions: significant increase in body weight, visceral adiposity, oxidative stress and systemic pro-inflammatory status; the hallmarks of central obesity were noticed only in WNIN. Further, they developed altered glucose and lipid homeostasis by exhibiting insulin resistance, impaired glucose tolerance, dyslipidemia and fatty liver condition. The present study demonstrates that WNIN is more prone to develop obesity and associated co-morbidities under high calorie environment. It thus underlines the cumulative role of genetics (nature) and diet (nurture) towards the development of obesity, which is critical for understanding this epidemic and devising new strategies to control and manage this modern malady.

Figure 1

Effect of high calorie environment on feeding behavior, body composition and thermogenesis. Rats (WNIN, F-344 and SD) were fed with control, HF, HS, and HFS diets for 13 weeks and assessed (A) Mean energy intake (Kcal/day). (B) Feed efficiency ratio (FER). (C) Body weight gain. (D) Body mass index (BMI). (E) Total body fat percent. (F) Relative mRNA expression of UCP1 in brown adipose tissue. Data was presented as mean ± SEM (n = 6 per group). Diets: control; HF, high fat; HS, high sucrose; HFS, high fat sucrose. UCP1, uncoupling protein 1. *P < 0.05; **P < 0.01; ***P < 0.001 statistically significance compared to their respective controls. Groups were compared using one way ANOVA.

Figure 2

Effect of high calorie environment towards visceral adiposity and adipocyte histology in genetically different rat strains. After the completion of feeding experiment, rats (WNIN, F-344 and SD) were sacrificed and assessed visceral adiposity and adipocyte histology. (A) Dissection weights of white adipose tissue (WAT). (B) Adiposity index was calculated to measure central adiposity. (C) Hematoxylin-Eosin (H&E) staining was performed to determine the morphology of retro adipose tissue. (D & E) Mean adipocyte area and adipocyte number was calculated in retro adipose tissue. (F–H) Fraction of distribution of adipocytes in retroadipose tissue. Diets: control; HF, high fat; HS, high sucrose; HFS, high fat sucrose. *P < 0.05; **P < 0.01; ***P < 0.001 statistically significance compared to their respective controls s. Groups were compared using one way ANOVA.

Figure 3

Effect of high calorie environment towards oxidative stress and pro-inflammatory status in rat strains. After the completion of feeding experiment, blood was collected; plasma was separated and estimated the levels of pro-inflammatory cytokines. (A) Interleukin 6; IL-6. (B) Tumor necrosis factor alpha; TNFα. (C) Interleukin 1 beta; IL-1β. (D) Macrophage inflammatory protein 1 alpha; MIP-1α. (E) Vascular endothelial growth factor; VEGF. (F) IFN gamma inducible protein 10; IP-10. (G) Monocyte chemotactic protein 1; MCP1 and anti -inflammatory cytokines such as (H) Interleukin 4; IL-4 and (I) Interleukin 10; IL-10. (J) TBARS assay was performed on liver tissue lysate to study oxidative stress. Data was presented as mean ± SEM (n = 6 per group). Diets: control; HF, high fat; HS, high sucrose; HFS, high fat sucrose. TBARS, Thio Barbituricacid Reactive Substances. *P < 0.05; **P < 0.01; ***P < 0.001 statistically significance compared to their respective controls. Groups were compared using one way ANOVA.

Figure 4

Effect of high calorie environment on glucose homeostasis: glucose tolerance, insulin sensitivity, glycolysis and gluconeogenesis in genetically different rat strains. Fasting plasma was used to estimate the concentrations of (A) Glucose. (B) Insulin. (C) Adiponectin. (D) Leptin. (E–G) OGTT was performed to assess glucose tolerance. (H) AUC Glucose was calculated based on the OGTT data. (I & J) HOMA-IR and HOMA-β was calculated to assess the insulin sensitivity. Relative mRNA expression of (K) glucokinase (GCK), (L) pyruvate kinase (PK), (M) Phosphoenolpyruvate carboxykinase (PEPCK), and (N) Glucose-6-phosphotase (G6Pase) to understand the regulation of glycolysis and gluconeogensis in liver. Data was presented as mean ± SEM (n = 6 per group). Diets: control; HF, high fat; HS, high sucrose; HFS, high fat sucrose; OGTT, Oral Glucose Tolerance Test; AUC Glucose, Area under curve for glucose; HOMA-IR, Homeostasis model assessment of insulin resistance; HOMA-β, Homeostasis model assessment of β-cell function. *P < 0.05; **P < 0.01; ***P < 0.001 statistically significance compared to their respective controls. Groups were compared using one way ANOVA.

Figure 5

Effect of high calorie environment on lipid metabolism: Lipid profile, lipogenesis and beta oxidation in genetically different rat strains. Fasting plasma was used to estimate the concentrations of (A) Triglycerides. (B) HDL- Cholesterol. (C) Total - cholesterol. (D) Dissection weights of liver. (E) Liver triglycerides levels were measured to determine the fatty liver condition. (F & G) Hematoxylin-Eosin (H&E) stain and Oil Red O stain were performed on liver sections to assess hepatic lipid droplet storage. Relative mRNA expression of (H) Fatty acid synthase (FAS), (I) Stearoyl-CoA desaturase-1 (SCD-1), (J) Sterol regulatory element-binding protein 1-c (SREBP-1c), (K) Acyl-CoA Oxidase 2 (ACOX2), (L) Carnitine palmitoyltransferase 1 (CPT1), and (M) Peroxisome proliferator-activated receptor α (PPARα) to understand the regulation of hepatic lipogenesis and beta oxidation. Data was presented as mean ± SEM (n = 6 per group). Diets: control; HF, high fat; HS, high sucrose; HFS, high fat sucrose. *P < 0.05; **P < 0.01; ***P < 0.001 statistically significance compared to their respective controls. Groups were compared using one way ANOVA.