Effects of metformin on burn-induced hepatic endoplasmic reticulum stress in male rats

Abstract

Severe burn injury causes hepatic dysfunction that results in major metabolic derangements including insulin resistance and hyperglycemia and is associated with hepatic endoplasmic reticulum (ER) stress. We have recently shown that insulin reduces ER stress and improves liver function and morphology; however, it is not clear whether these changes are directly insulin mediated or are due to glucose alterations. Metformin is an antidiabetic agent that decreases hyperglycemia by different pathways than insulin; therefore, we asked whether metformin affects postburn ER stress and hepatic metabolism. The aim of the present study is to determine the effects of metformin on postburn hepatic ER stress and metabolic markers. Male rats were randomized to sham, burn injury and burn injury plus metformin and were sacrificed at various time points. Outcomes measured were hepatic damage, function, metabolism and ER stress. Burn-induced decrease in albumin mRNA and increase in alanine transaminase (p < 0.01 versus sham) were not normalized by metformin treatment. In addition, ER stress markers were similarly increased in burn injury with or without metformin compared with sham (p < 0.05). We also found that gluconeogenesis and fatty acid metabolism gene expressions were upregulated with or without metformin compared with sham (p < 0.05). Our results indicate that, whereas thermal injury results in hepatic ER stress, metformin does not ameliorate postburn stress responses by correcting hepatic ER stress.

Figure 1

Metformin-activated hepatic AMPK after burn. Top panel: Western blot of p-AMPK and β-actin. Lower panel: Quantification of the above blots. *p < 0.05 versus burn, Student t test.

Figure 2

Metformin does not alleviate hepatic damage after burn. Albumin concentration (A) and ALT levels (B) were measured in rat livers. **p < 0.01 versus sham.

Figure 3

Metformin augments hepatic ER stress after burn. ER stress markers were assessed at the mRNA level. Chop is a downstream target of PERK, whereas Bip, Dnajb9 and Pdia3 are activated by the spliced form of Xbp-1. Both 24-h (A) and 48-h (B) postburn time points are shown. *p < 0.05, **p < 0.01 versus sham.

Figure 4

Gluconeogenesis gene expression in rat livers after burn. mRNA levels of Pgc1 (A), G6pc (B) and Pck1 (C) were evaluated. *p < 0.05 versus sham. ^†p < 0.01 versus all other groups at both time points.

Figure 5

Gene expression of markers for fatty acid metabolism (A–D) and for glucose uptake (E, F). *p < 0.05, **p < 0.01, ***p < 0.001 versus sham. †p < 0.01 versus all other groups at both time points.

Figure 6

Hepatic triglyceride content after burn. Triglyceride content was measured in the rat livers as an indicator of hepatic steatosis.