Hepatocyte nuclear factor-4α contributes to carbohydrate-induced transcriptional activation of hepatic fatty acid synthase

Abstract

Refeeding a carbohydrate-rich meal after a fast produces a co-ordinated induction of key glycolytic and lipogenic genes in the liver. The transcriptional response is mediated by insulin and increased glucose oxidation, and both signals are necessary for optimal induction of FAS (fatty acid synthase). The glucose-regulated component of FAS promoter activation is mediated in part by ChREBP [ChoRE (carbohydrate response element)-binding protein], which binds to a ChoRE between −7300 and −7000 base-pairs in a carbohydrate-dependent manner. Using in vivo footprinting with nuclei from fasted and refed rats, we identify an imperfect DR-1 (direct repeat-1) element between −7110 and −7090 bp that is protected upon carbohydrate refeeding. Electrophoretic mobility-shift assays establish that this DR-1 element binds HNF-4α (hepatocyte nuclear factor 4α), and chromatin immunoprecipitation establishes that HNF-4α binding to this site is increased approx. 3-fold by glucose refeeding. HNF-4α transactivates reporter constructs containing the distal FAS promoter in a DR-1-dependent manner, and this DR-1 is required for full glucose induction of the FAS promoter in primary hepatocytes. In addition, a 3-fold knockdown of hepatocyte HNF-4α by small interfering RNA produces a corresponding decrease in FAS gene induction by glucose. Co-immunoprecipitation experiments demonstrate a physical interaction between HNF-4α and ChREBP in primary hepatocytes, further supporting an important complementary role for HNF-4α in glucose-induced activation of FAS transcription. Taken together, these observations establish for the first time that HNF-4α functions in vivo through a DR-1 element in the distal FAS promoter to enhance gene transcription following refeeding of glucose to fasted rats. The findings support the broader view that HNF-4α is an integral component of the hepatic nutrient sensing system that co-ordinates transcriptional responses to transitions between nutritional states.