Endoplasmic reticulum stress may be involved in insulin resistance and lipid metabolism disorders of the white adipose tissues induced by high-fat diet containing industrial trans-fatty acids

Abstract

Background: Consumption of industrially produced trans-fatty acids (iTFAs) can result in alteration to lipid profile and glucose metabolism. Moreover, a diet high in iTFAs could increase the risk of obesity, cardiovascular diseases (CVDs) and type 2 diabetes mellitus. Glucose and lipid metabolism are closely linked in white adipose tissue (WAT), yet the underlying mechanisms of the effect of iTFAs in WAT are poorly understood.

Materials and methods: Parameters of glucose homeostasis, lipid profiles and markers of endoplasmic reticulum (ER) stress of WAT were measured in rats maintained on a high-fat diet containing margarine (HFD-M) (n=10) compared to controls maintained on standard chow (n=10) over 16 weeks.

Results: Fat mass and body weight was significantly increased in rats maintained on the HFD-M compared to controls (P<0.01). HFD-M rats had increased levels of insulin (INS), homeostasis model assessment of insulin resistance and serum lipid profile was significantly altered. The expression of glucose-regulated protein 78 (GRP78) and the phosphorylation of inositol-requiring enzyme 1-alpha and c-Jun N-terminal kinase (JNK) were significantly increased in subcutaneous and retroperitoneal adipose depots of HFD-M-fed rats. In vitro, wider ER lumens were observed in 100μmol/L elaidic acid (EA)-treated human mature adipocytes. We observed activation of ER stress markers, impaired INS receptor signaling and increased lipogenesis in adipocytes after EA exposure. These effects could be alleviated by inhibiting ER stress in adipocytes in vitro.

Conclusion: Collectively these data suggest that ER stress may be involved in INS resistance and lipid metabolism disorders induced by high-fat diet containing iTFAs. These findings suggest that WAT could be regarded as a key target organ for inhibiting ER stress to reverse the impaired INS receptor signaling, alleviate lipid metabolism disorders, and provide a novel approach to prevent and treat INS resistance and dyslipidemia-related chronic diseases such as T2MD and CVDs.

Keywords: ER stress; adipocytes; dyslipidemia; elaidic acid; insulin resistance; obesity.

Figure 1

Body weight and retroperitoneal fat mass in control (n=10) and HFD-M (n=10) rats. The comparison between CON and HFD-M groups in (A) Body weight; (B) body weight gain; (C)retroperitoneal fat weight; (D) retroperitoneal fat mass (% of total body weight). *P<0.05, **P<0.01. Abbreviations: CON, control; HFD-M, high-fat diets containing margarine.

Figure 2

Effects of HFD-M on the expression of ER stress-related proteins in subcutaneous fat tissue of rats. (A) Representative GRP78, p-IRE1-α, p-JNK Western blots; Quantification of the Western blot membranes for (B) GRP78; (C) p-IRE1-α; (D) p-JNK. Data were presented as mean ± SD, n=6 each group; *P<0.05, **P<0.01. Abbreviations: ER, endoplasmic reticulum; GRP78, glucose-regulated protein 78; p-IRE1-α, phosphorylated Inositol-requiring enzyme 1-alpha; p-JNK, phosphorylated c-Jun n-terminal kinase.

Figure 3

Effects of HFD-M on the expression of ER stress-related proteins in rat retroperitoneal fat tissue. (A) Representative GRP78, p-IRE1-α, p-JNK Western blots; Quantification of the Western blot membranes for (B) GRP78; (C) p-IRE1-α; (D) p-JNK. Data were presented as mean ± SD, n=6 each group; **P<0.01. Abbreviations: ER, endoplasmic reticulum; GRP78, glucose-regulated protein 78; p-IRE1-α, phosphorylated Inositol-requiring enzyme 1-alpha; p-JNK, phosphorylated c-Jun n-terminal kinase.

Figure 4

ER stress induced by EA in human mature adipocytes. (A) Control group; (B) treated with 100 μmol/L of EA for 24 hrs; (C) quantitative analysis of ER luminal diameter, as a surrogate marker for ER stress per cell using transmission electron microscope images. Results are representative averages of at least six images performed from three independent experiments and are displayed as mean ± SD. **P<0.01. White arrow shows the ER lumen. Abbreviations: ER, endoplasmic reticulum; EA, elaidic acid; LD, lipid droplets.

Figure 5

EA exposure activated markers of ER stress pathways in human adipocytes. (A) Representative GRP78, p-IRE1-α, p-JNK Western blots; quantification of the Western blot membranes for (B) GRP78; (C) p-IRE1-α; (D) p-JNK. Results are representative averages of at least three independent experiments and displayed as means ± SD. Compared to control group *P<0.05, **P<0.01; compared to EA-treated group, ^#P<0.05, ^##P<0.01. Abbreviations: ER, endoplasmic reticulum; EA, elaidic acid; 4-PBA, 4-phenylbutyric acid; GRP78, glucose-regulated protein 78; p-IRE1-α, phosphorylated Inositol-requiring enzyme 1-alpha; p-JNK, phosphorylated c-Jun n-terminal kinase.

Figure 6

ER stress involves impaired insulin receptor signaling in human adipocytes. (A) Glucose uptake capacity; (n=6); (B) representative Western blots of GLUT4 and IRS-1 in human mature adipocytes; (C) quantification of the Western blot for GLUT4; (D) quantification of the Western blot for IRS-1. (n=3); *P<0.05 compared to control group. ^#P<0.05 compared to EA group. Abbreviations: EA, elaidic acid; 4-PBA, 4-phenylbutyric acid; GLUT4, glucose transporter 4; IRS-1, insulin receptor substrate 1; p-IRS-1, phosphorylated insulin receptor substrate 1.

Figure 7

Effects of EA exposure on fat accumulation of human mature adipocytes. (A) TG content. (n=6); (B) relative expression of SREBP-1c mRNA. (n=6); (C) representative FAS Western blots. (D) Quantification of the Western blot membranes for FAS. (n=3); Data are presented as means ± SD; Compared to control group **P<0.01; compared to EA group ^##P<0.01. Abbreviations: TG, triglyceride; EA, elaidic acid; 4-PBA, 4-phenylbutyric acid; SREBP-1c, sterol regulatory element binding protein 1c; FAS, fatty acid synthase.