Insulin stimulation of glutamine: Fructose-6-phosphate amidotransferase occurs via an insulin-like growth factor-1 pathway in rat fibroblasts

The biosynthetic pathway for hexosamine mediates some of the adverse effect s of high glucose. The rate limiting enzyme in this pathway is glutamine : fructose-6-phosphate amidotransferase (GFA). Using HPLC, the regulation of GFA activity by glucose and insulin was studied in wild type and rat-1 fibr oblasts overexpressing human insulin receptors (HIRcB cells). In wild type cells only maximal doses of insulin (580 ng/ml) resulted in an increase in GFA activity (51.0 +/- 40.6%). In HIRcB cells insulin led to a dose depende nt increase in GFA activity that was enhanced when compared to wild type (8 9 +/- 5% (p<0.001) increase at 580 ng/ml). Insulin's action was glucose dep endent and required prolonged serum deprivation. HIRcB's cultured in 0 mM g lucose had a 58.2% (p<0.001) decrease in insulin stimulation. However, when present the concentration of glucose (2-20 mM) did not affect insulin stim ulation of GFA activity. Most of insulin's effects occur by way of the IGF- I receptor as a two-fold stimulation of GFA activity was seen with signific antly lower doses (10 ng/ml) of IGF-1. We conclude that GFA enzyme activity is upregulated by insulin and this may occur via a IGF-1 receptor mediated pathway.