Involvement of a Unique Carbohydrate-responsive Factor in the Glucose Regulation of Rat Liver Fatty-acid Synthase Gene Transcription

Refeeding carbohydrate to fasted rats induces the transcription of genes encoding enzymes of fatty acid biosynthesis, e.g. fatty-acid synthase (FAS). Part of this transcriptional induction is mediated by insulin. An insulin response element has been described for the fatty-acid synthase gene region of -600 to +65, but the 2-3-fold increase in fatty-acid synthase promoter activity attributable to this region is small compared with the 20-30-fold induction in fatty-acid synthase gene transcription observed in fasted rats refed carbohydrate. We have previously reported that the fatty-acid synthase gene region between -7382 and -6970 was essential for achieving high in vivo rates of gene transcription. The studies of the current report demonstrate that the region of -7382 to -6970 of the fatty-acid synthase gene contains a carbohydrate response element (CHO-RE_FAS) with a palindrome sequence (CATGTGn₅GGCGTG) that is nearly identical to the CHO-RE of the L-type pyruvate kinase and S₁₄ genes. The glucose responsiveness imparted by CHO-RE_FAS was independent of insulin. Moreover, CHO-RE_FAS conferred glucose responsiveness to a heterologous promoter (i.e. L-type pyruvate kinase). Electrophoretic mobility shift assays demonstrated that CHO-RE_FAS readily bound a unique hepatic ChoRF and that CHO-RE_FAS competed with the CHO-RE of the L-type pyruvate kinase and S₁₄ genes for ChoRF binding. In vivo footprinting revealed that fasting reduced and refeeding increased ChoRF binding to CHO-RE_FAS. Thus, carbohydrate responsiveness of rat liver fatty-acid synthase appears to require both insulin and glucose signaling pathways. More importantly, a unique hepatic ChoRF has now been shown to recognize glucose responsive sequences that are common to three different genes: fatty-acid synthase, L-type pyruvate kinase, and S₁₄.