Vildagliptin Can Alleviate Endoplasmic Reticulum Stress in the Liver Induced by a High Fat Diet

Abstract

Purpose. We investigated whether a DDP-4 inhibitor, vildagliptin, alleviated ER stress induced by a high fat diet and improved hepatic lipid deposition. Methods. C57BL/6 mice received standard chow diet (CD), high fat diet (HFD), and HFD administered with vildagliptin (50 mg/Kg) (V-HFD). After administration for 12 weeks, serum alanine aminotransferase, glucose, cholesterol, triglyceride, and insulin levels were analyzed. Samples of liver underwent histological examination and transmission electron microscopy, real-time PCR for gene expression levels, and western blots for protein expression levels. ER stress was induced in HepG2 cells with palmitic acid and the effects of vildagliptin were investigated. Results. HFD mice showed increased liver weight/body weight (20.27%) and liver triglycerides (314.75%) compared to CD mice, but these decreased by 9.27% and 21.83%, respectively, in V-HFD mice. In the liver, HFD induced the expression of ER stress indicators significantly, which were obviously decreased by vildagliptin. In vitro, the expressions of molecular indicators of ER stress were reduced in HepG2 when vildagliptin was administered. Conclusions. Vildagliptin alleviates hepatic ER stress in a mouse high fat diet model. In HepG2 cells, vildagliptin directly reduced ER stress. Therefore, vildagliptin may be a potential agent for nonalcoholic fatty liver disease.

Figure 1

Flowchart of the experiment.

Figure 2

Vildagliptin reduced hepatic lipid deposition. (a) Weights of the mice in the three groups during the experiment (n = 9 per group). (b) Liver tissue stained with H&E (upper panel, magnification ×400); liver tissue sections stained with Oil Red O (lower panel, magnification ×200). (c) Liver wet weights were measured (n = 7–9). (d) The liver index was calculated using liver wet weight/body weight (n = 7–9). (e) The triglyceride (TG) content in the liver was measured and normalized with the protein content (n = 7–9). (f) Relative mRNA levels of genes related to the synthesis of TG and dipeptidyl peptidase-4 (DDP-4). Expression values were normalized to β-actin mRNA. Data was presented as the means ± SD; ^∗P < 0.05 relative to the CD group; ^&P < 0.05 relative to the HFD group.

Figure 3

Vildagliptin ameliorated high fat diet induced endoplasmic reticulum (ER) stress. (a) Electron microscope (magnification ×15000) analyses of the ER in livers of mice from CD, HFD, and V-HFD groups. Scale bars represent 2 μm. (b) The mRNA levels of BiP measured by RT-PCR; data were normalized according to β-actin levels. (n = 5 per group). (c) The expression of BiP was assessed using immunohistochemical staining (magnification ×200 and magnification ×400). (d) The semiquantitative analysis of staining intensity was conducted using ImageJ software. (e) ER stress associated markers BiP, p-PERK, p-IRE1α, p-eIF2α, and xBP-1s expression in the livers of mice investigated by western blotting. Each expression level was quantified by densitometry, normalized with β-actin, and the relative phosphorylated protein levels were normalized with the corresponding total protein level. Data was represented as the means ± SD; ^∗P < 0.05 relative to CD group; ^&P < 0.05 relative to HFD group.

Figure 4

Vildagliptin ameliorated ER stress induced by PA or thapsigargin in HepG2 cells. (a, c) HepG2 cells were treated with 1 umol/L thapsigargin or 0.4 mmol/L PA with the indicated concentrations of vildagliptin for 24 h. Proteins were analyzed by western blotting using anti-BiP, p-PERK, p-eIF2α, p-IRE1α, and xBP-1 antibodies and quantified by densitometry (n = 3 for each condition). (b) HepG2 cells were treated with different concentrations of PA (in DMEM) for 24 h after the cells were starved for 3 h. Proteins were analyzed by western blotting using p-PERK, p-eIF2α, p-IRE1α, xBP-1, and BiP antibodies and quantified by densitometry (n = 3 for each condition). Data was represented as the means ± SD; ^∗P < 0.05 relative to PA (0 mM).