Mapping glucocorticoid response sequences (GRE) in the 5′-flanking region of rat Fatty Acid Synthase (FAS) gene

The enhancement of lipogenesis associated with fasting and refeeding of carbohydrate requires both insulin and glucocorticoids. In this regard we have found that insulin and glucocorticoids function synergistically to induce liver FAS gene transcription. However, the DNA sequences required for this synergistic response have not been located. Thus our objective was to map the GREs of FAS gene and examine their synergy with the insulin response element (IRE). The IRE and GREs for hepatic transcription of FAS were localized by using primary cultures of rat hepatocytes transfected (n=3-5 cultures per construct) with various regions of the 5′-flanking sequence of the FAS gene (-9700 to +65) fused to chloramphenicol acetyl transferase (CAT) reporter. The region of -265 to +65 (basal promoter) of FAS was found to induce CAT activity 2-3 fold in response to insulin but was unresponsive to dexamethasone. Further mapping showed that the fragments -7300 to -6300 and -6300 to -4606 linked to the basal promoter each increased CAT expression by 2-fold. However, full induction (10-fold) was achieved with the -7300 to -4606 fragment. Sequence analysis of the region -7300 to -4606, revealed the presence of 3 consensus GREs at -6699,-6640 and -6183 bp. Thus the synergistic induction of hepatic FAS transcription by insulin plus glucocorticoids appears to involve IRE sequences in the proximal (-265 to +65) and GRE sequences in distal (-7300 to -4606) promoter of FAS.