Fatty acid synthase (FAS) plays a central role in fatty acid synthesis and its expression is under nutritional and hormonal control. We have investigated insulin-like growth factor-I (IGF-I) regulation of FAS by transfecting into 3T3-L1 fibroblasts chimeric genes comprising the 5′-flanking region of the FAS gene linked to a luciferase (LUC) reporter gene. First, the basal promoter activity of the 5′ serial deletions from nucleotides −318 to −19 of the FAS gene were compared. Deletions of the promoter sequences from −136 to −19 resulted in a step-wise decrease in the promoter activity, with the −67 LUC and −19 LUC plasmids retaining 40% and 16% of the luciferase activity of −136 LUC. Regulatory sequences important for the FAS basal promoter activity in 3T3-L1 fibroblasts are, therefore, located within the −136 to −19 region. Treatment with 10 mM IGF-I also increased luciferase activity 1.8 +/- 0.2-, 1.8 +/- 0.3- and 2.5 +/- 0.1-fold in 3T3-L1 fibroblasts transiently transfected with −136 LUC, −110 LUC and −67 LUC plasmids, respectively. Deletion of sequences from −67 to −19 resulted in the loss of responsiveness to IGF-I. Physiological doses of insulin (10 nM), however, did not increase luciferase activity in 3T3-L1 fibroblasts transfected with any of the above plasmids. Only upon treatment with pharmacological doses of insulin (1 microM), probably through IGF-I receptor, did luciferase activity increase 4.3 +/- 0.4-, 3.2 +/- 0.4- and 3.5 +/- 0.5-fold when transfected with −136 LUC, −110 LUC and −67 LUC plasmids, respectively; there was no increase with −19 LUC. The half-maximal effect of IGF-I on FAS promoter activity was observed at 3 nM and a maximal effect was reached at 10 nM. These results indicate that the increased promoter activities observed are probably mediated through the IGF-I receptor. Furthermore, sequences responsible for IGF-I regulation of the FAS gene are located within the proximal promoter between nucleotides −67 and −19 of the FAS gene.
Triiodothyronine stimulates transcription of the fatty acid synthase gene in chick embryo hepatocytes in culture. Insulin and insulin-like growth factor amplify that effect.
