Cold acclimation reduces hepatic protein Kinase B and AMP-activated protein kinase phosphorylation and increases gluconeogenesis in Rats

Abstract

This study investigated the molecular and metabolic responses of the liver to cold-induced thermogenesis. To accomplish that, male Wistar rats were exposed to cold (4°C) for 7 days. Livers were then extracted and used for the determination of glucose and fatty acid oxidation, glycogen content, the expression and content of proteins involved in insulin signaling, as well as in the regulation of gluconeogenesis and de novo lipid synthesis. Despite being hyperphagic, cold-acclimated rats displayed normoglycemia with reduced insulinemia, which suggests improved whole-body insulin sensitivity. However, liver protein kinase B (AKT) and glycogen synthase kinase 3 (GSK3) phosphorylations were markedly reduced along with the expressions of the insulin receptor (IR) and its substrates IRS1 and IRS2, whereas glycogen synthase (GS) phosphorylation increased. Thus, major signaling steps of the glycogen synthesis pathway in the liver were inhibited. Furthermore, glucagonemia and hepatic glucose and fatty acid oxidation were increased, whereas liver glycogen content was reduced by cold acclimation. This was accompanied by significantly elevated expressions of the gluconeogenic transcription regulators CRTC2, PGC-1α, and FoxO1, as well as of major gluconeogenic enzymes (G6Pase, FBP1, and PEPCK). Conversely, phosphorylation and contents of AMP-activated protein kinase (AMPK) and acetyl-CoA carboxylase (ACC) and fatty acid synthase (FAS) content were markedly downregulated in livers of cold-acclimated rats. In conclusion, cold acclimation suppressed hepatic glycogen synthesis and promoted profound metabolic changes in the liver so the organ could sustain its ability to regulate whole-body glucose and lipid metabolism under conditions of high-energy demand in thermogenic tissues.

Keywords: AMPK; HNF4α; PGC-1α; fatty acid oxidation; glycogen; liver.

Figure 1

Cold acclimation increases palmitate (A) and glucose (B) oxidation and reduces glycogen content (C) in rat livers. Con = control, n = 18 rats per condition. *P < 0.05, t‐test.

Figure 2

Cold acclimation increases PEPCK (A) and PGC‐1α (B) contents and reduces the phosphorylation and contents of AMPK (C) and ACC (D) in rat livers. Con = control, n = 6 rats per condition. *P < 0.05, t‐test.

Figure 3

Cold acclimation reduces the phosphorylation of AKT (A) and GSK3α (B) and increases the phosphorylation of GS (C) in rat livers. Con = control, n = 6 rats per condition. *P < 0.05, t‐test.

Figure 4

Cold acclimation reduces the mRNA expression of proteins involved in insulin signaling (IR (A), IRS1 (B), and IRS2 (C)), increases the mRNA expression of proteins in the gluconeogenic pathway (G6Pase (D), and FBP1 (E)), and also markedly reduces GK (F) and PDK4 (G) in rat livers. Con = control, n = 6–8 rats per condition. *P < 0.05, t‐test.

Figure 5

Cold acclimation increases the mRNA expression of CRTC2 (A), PGC‐1α (B), and FoxO1 (C), but it does not alter the mRNA expression of PPARα, PPARγ, HNF3β, and HNF4α in rat livers. Con = control, n = 6–8 rats per condition. *P < 0.05, t‐test.

Figure 6

Cold exposure reduces the content and expression of FAS (A and B), as well as the expression of ACC (C), SREBP‐1c (D), and ELOVL3 (E) in rat livers. Con = control, n = 6–8 rats per condition. *P < 0.05, t‐test.

Figure 7

Cold exposure increases mRNA expression of CPT1 (A), ACOT2 (B), and COX6c (C) in rat livers. Con = control. n = 6 rats per condition. *P < 0.05, t‐test.

Figure 8

Time course analysis of circulating glucose, insulin, and glucagon in control (Con) and cold‐exposed (Cold) rats. Cold exposure did not increase glycemia (A and B), but it reduced insulinemia (C and D) and increased plasma glucagon (E and F) in rats. Area under the curve = AUC. n = 6 rats per condition.*P < 0.05, t‐test.