An Acute Increase in Fructose Concentration Increases Hepatic Glucose-6-Phosphatase mRNA via Mechanisms that Are Independent of Glycogen Synthase Kinase-3 in Rats

Abstract

It appears that low amounts of fructose improve, whereas increased concentrations impair glucose tolerance and hepatic glucose metabolism. In this study, we compared directly the effects of low vs. high portal vein fructose concentrations on hepatic glucose metabolism in rats, using glucose-6-phosphatase gene expression as an endpoint. In the control group (C; n = 7), pancreatic clamps were performed using somatostatin and replacement of insulin such that basal glucose levels were maintained. In the experimental groups (n = 8/group), hyperglycemic, hyperinsulinemic pancreatic clamps were performed in which glucose (G) or glucose + fructose was infused into a jejunal vein. Fructose was infused to achieve either low (F1; <0.3 mmoVL) or high (F2; >1.0 mmol/L) portal vein concentrations. Total sugar load to the liver was equalized among the 3 experimental groups. Compared with C, liver glucose-6-phosphatase catalytic subunit mRNA was reduced by ∼55% in G and F1, whereas it was increased ∼180% in F2. F2 did not differentially affect glucose-6-phosphate translocase or phosphoenolpyruvate carboxykinase mRNA levels in liver, nor kidney glucose-6-phosphatase catalytic subunit mRNA. Livers from the F2 group were characterized by an accumulation of pentose phosphate intermediates and reduced phosphorylation of glycogen synthase kinase-3 (active form). However, in separate studies (n = 5/group), the infusion of a glycogen synthase kinase-3 inhibitor did not prevent the effects of F2 on glucose-6-phosphatase gene expression. We hypothesize that elevated fructose concentrations, similar to levels achieved after ingestion of sucrose- or fructose-enriched meals, initiate signals within the liver that elicit selective changes in hepatic gene expression.