Fructose-induced stress signaling in the liver involves methylglyoxal

Fructose produces hepatic insulin resistance in humans and animals. We have proposed that the selective metabolism of fructose by the liver can, under conditions of elevated fructose delivery, inflict a metabolic insult that is localized to the hepatocyte. The present study was designed to identify potential cellular effectors of this insult. Primary hepatocytes were incubated with 8 mM glucose and 0.12% inulin (G, n = 6) or 8 mM glucose, 0.12% inulin and 28 mU of inulinase (GF, n = 6) in the presence or absence of insulin for 0, 2, or 4 h. GF produced fructose concentrations of ~0.7 mM over the 4 h experiment. GF induced phosphorylation of MKK7 and JNK, phosphorylation of serine307 on IRS-1, and reduced tyrosine phosphorylation of IRS-1 and -2. GF increased ceramide levels and reactive oxygen species (ROS); however inhibitors of ceramide synthesis or ROS accumulation did not prevent GF-mediated changes in MKK7, JNK or IRS proteins. GF increased cellular methylglyoxal concentrations and a selective increase in methylglyoxal recapitulated the GF-induced changes in MKK7, JNK and IRS proteins. We hypothesize that GF-mediated changes in stress signaling involve methylglyoxal in primary hepatocytes.