Acute fructose intake suppresses fasting-induced hepatic gluconeogenesis through the AKT-FoxO1 pathway

Abstract

Excessive intake of fructose increases lipogenesis in the liver, leading to hepatic lipid accumulation and development of fatty liver disease. Metabolic alterations in the liver due to fructose intake have been reported in many studies, but the effect of fructose administration on hepatic gluconeogenesis is not fully understood. The aim of this study was to evaluate the acute effects of fructose administration on fasting-induced hepatic gluconeogenesis. C57BL/6J mice were administered fructose solution after 14 h of fasting and plasma insulin, glucose, free fatty acids, and ketone bodies were analysed. We also measured phosphorylated AKT and forkhead box O (FoxO) 1 protein levels and gene expression related to gluconeogenesis in the liver. Furthermore, we measured glucose production from pyruvate after fructose administration. Glucose-administered mice were used as controls. Fructose administration enhanced phosphorylation of AKT in the liver, without increase of blood insulin levels. Blood free fatty acids and ketone bodies concentrations were as high as those in the fasting group after fructose administration, suggesting that insulin-induced inhibition of lipolysis did not occur in mice administered with fructose. Fructose also enhanced phosphorylation of FoxO1 and suppressed gluconeogenic gene expression, glucose-6-phosphatase activity, and glucose production from pyruvate. The present study suggests that acute fructose administration suppresses fasting-induced hepatic gluconeogenesis in an insulin-independent manner.

Keywords: AKT; CREB, cAMP response element binding protein; ChREBP, carbohydrate response element binding protein; EDTA, ethylenediaminetetraacetic acid; FFA, free fatty acid; FoxO, forkhead box O; FoxO1; Fructose; G6Pase; G6Pase, glucose-6-phosphatase; Gluconeogenesis; Insulin; PEPCK, phosphoenolpyruvate carboxykinase; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator-1 alpha; PI3K, phosphoinositide-3-kinase; PIP 3, phosphatidylinositol-(3,4,5)-trisphosphate; SREBP, sterol-regulatory element binding protein.

Fig. 1

(A) Blood insulin levels and (B) glucose levels, before glucose (Glc) or fructose (Fru) administration, and 5 and 30 min after administration. Mice were given glucose or fructose (2.0 g/kg body weight) after 14 h of fasting. Values are presented as means ± SEM (n = 3–7). **P < 0.01; ***P < 0.001 vs. the glucose administered mice at the same time-points. ††P < 0.01; †††P < 0.001 vs. the mice under fasting condition. (C) Phosphorylation of AKT in the liver, 5 and 30 min after glucose (Glc) or fructose (Fru) administration. Phospho-AKT/total AKT in the liver was measured by western blotting. Representative blots are shown. Values in the graph are expressed as a relative intensity compared to the values at 5 and 30 min after glucose administration (n = 4–5). *P < 0.05; **P < 0.01.

Fig. 2

(A) Blood FFA, (B), ketone bodies (C) glucose levels, and (D) liver glycogen contents before and after 1–5 h of glucose (Glc) or fructose (Fru) administration. Mice were administered glucose or fructose (2.0 g/kg body weight) after 14 h of fasting. Values are presented as means ± SEM (n = 5–6). *P < 0.05; ***P < 0.001 vs. the glucose-administered mice at the same time-point. †P < 0.05; ††P < 0.01; †††P < 0.001 vs. the mice under fasting condition. FFA, free fatty acids.

Fig. 3

Phosphorylated FoxO1 protein levels in the liver before and after 1–5 h of glucose (Glc) or fructose (Fru) administration. Phosphorylated FoxO1 protein in the liver was measured by western blotting. Actin was used as an internal control. Representative blots are shown. Values in the graph are expressed as a relative intensity compared to the values before administration (n = 4). *P < 0.05 vs. the glucose administered mice at the same time-point. †P < 0.05; ††P < 0.01 vs. the mice before administration. FoxO, forkhead box O.

Fig. 4

mRNA expression levels of genes related to gluconeogenesis in the liver of the mice before and after glucose (Glc) or fructose (Fru) administration. The results are displayed as percent increase/decrease, from the level of mRNA expressed in the mice before administration considered as 100%. Values are presented as means ± SEM (n = 5–6). *P < 0.05; **P < 0.01; ***P < 0.001 vs. the glucose administered mice at the same time-point. †P < 0.05; ††P < 0.01; †††P < 0.001 vs. the mice before administration. G6Pase, glucose-6-phosphatase; PEPCK, phosphoenolpyruvate carboxykinase; FoxO, forkhead box O; PGC-1α, peroxisome proliferator-activated receptor gamma coactivator-1 alpha.

Fig. 5

G6Pase activity in the liver of the mice before and after glucose (Glc) or fructose (Fru) administration. Values are presented as means ± SEM (n = 5–6). †P < 0.05; ††P < 0.01; †††P < 0.001 vs. the mice before administration. G6Pase, glucose-6-phosphatase.

Fig. 6

Blood glucose levels after administration of pyruvate. Mice were administered glucose or fructose after 14 h fasting. After 2 h glucose or fructose administration, mice were injected with pyruvate (2.0 g/kg body weight). Blood glucose levels in the mice injected saline, glucose, or fructose administration, at 0 min were 104 ± 3, 131 ± 11 and 101 ± 5 mg/dL, respectively. Values are presented as means ± SEM (n = 4–6). *P < 0.05 vs. the saline administered mice at the same time-point.