An acute bout of prolonged exercise has been shown to decrease hepatic fatty acid synthase (FAS) mRNA and activity induced by high carbohydrate diets. The purpose of the current study was to examine the role of insulin in this exercise down-regulation of FAS. Sixty-four male Wistar rats were randomly divided into normal and streptozotocin (STZ)-treated diabetic groups. After being starved for 48 h and refed a high cornstarch (C) or fructose (F) diet for 10 h, one half of each group of rats was killed after an acute bout of prolonged exercise (E), while the other half of the group was killed in the rested state. STZ treatment suppressed plasma insulin and elevated plasma glucagon levels along with a severe hyperglycemia. FAS mRNA levels decreased by 60% (P < 0.05) with STZ treatment but were 250% higher in F-fed versus C-fed rats. E abolished F-induced FAS mRNA levels in both normal and STZ rats and decreased plasma glucose concentration in STZ rats (P < 0.05). F-fed normal rats showed twofold higher hepatic FAS activity than did C-fed normal rats and this dietary induction was abolished by STZ (P < 0.05). FAS activity in normal rats was not affect by E and was increased with E in STZ rats. Nuclear protein binding to the insulin response sequence was not affected by STZ or diet and increased with E (P < 0.05). Carbohydrate response element binding was greater with F- versus C-feeding (P < 0.05) but unaffected by E. E enhanced inverted CCAAT-box element binding regardless of diet and STZ. We conclude that although insulin status had a great influence on FAS gene expression, E-induced down-regulation of FAS mRNA was not mediated by altered insulin response sequence binding but primarily by increased inverted CCAAT-box element binding to the FAS promoter and/or decreased concentration of carbohydrate metabolites.
- Fatty acid synthase
- Transcription factor