Glucose regulation of transforming growth factor-alpha expression is mediated by products of the hexosamine biosynthesis pathway.

We have recently shown that glucose and glucosamine regulate the transcription of transforming growth factor-alpha (TGF alpha) in rat aortic smooth muscle (RASM) cells. Based on the increased potency of glucosamine compared to glucose, we hypothesized that stimulation of TGF alpha transcription by glucose is mediated through the hexosamine biosynthesis pathway. The yeast cDNA for the rate-limiting enzyme of this pathway, glutamine:fructose-6-phosphate amidotransferase (GFA), was therefore expressed in RASM cells. GFA-transfected cells showed an increase in GFA activity, exhibiting a 2.2-fold increase in the synthesis of glucosamine-6-phosphate, the first product of the hexosamine biosynthetic pathway. To test the effect of GFA overexpression on TGF alpha transcriptional activity, cells were transiently cotransfected with GFA along with a reporter plasmid containing the firefly luciferase gene under control of the TGF alpha promoter. GFA-transfected cells exhibited a glucose-dependent 2-fold increase in TGF alpha activity compared to control cells. Maximal stimulation of TGF alpha-luciferase activity by glucosamine, however, was equivalent in GFA-and control-transfected cells, confirming that the stimulation observed by both agents operated through the same pathway. This increase in TGF alpha activity was inhibited (85% at 0.5 mM glucose and 69% at 30 mM glucose) by the glutamine analog and inhibitor of GFA, 6-diazo-5-oxonorleucine (10 microM). Control studies confirmed that the increased TGF alpha-luciferase activity in the GFA-expressing cells was not an artifact of altered growth, survival, or transfection efficiency.(ABSTRACT TRUNCATED AT 250 WORDS)