Control of Apolipoprotein AI Gene Expression through Synergistic Interactions between Hepatocyte Nuclear Factors 3 and 4

Abstract

Apolipoprotein AI (apoAI) gene expression in liver depends on synergistic interactions between transcription factors bound to three distinct sites (A, B, and C) within a hepatocyte-specific enhancer in the 5-flanking region of the gene. In this study, we showed that a segment spanning sites A and B retains substantial levels of enhancer activity in hepatoblastoma HepG2 cells and that sites A and B are occupied by the liver-enriched hepatocyte nuclear factors (HNFs) 4 and 3, respectively, in these cells. In non-hepatic CV-1 cells, HNF-4 and HNF-3␤ activated this minimal enhancer synergistically. This synergy was dependent upon simultaneous binding of these factors to their cognate sites, but it was not due to cooperativity in DNA binding. Separation of these sites by varying helical turns of DNA did not affect simultaneous binding of HNF-3␤ and HNF-4 nor did it influence their functional synergy. The synergy was, however, dependent upon the cell type used for functional analysis. In addition, this synergy was further potentiated by estrogen treatment of cells cotransfected with the estrogen receptor. These data indicate that a cell type-restricted intermediary factor jointly recruited by HNF-4 and HNF-3 participates in activation of the apoAI enhancer in liver cells and suggest that the activity of this factor is regulated by estrogen.An emerging hallmark of transcriptional regulation in eukaryotes is the assembly of multiprotein complexes at the enhancer and promoter regions of target genes (1). These complexes are formed and stabilized through multiple protein-DNA and protein-protein interactions. Tissue specificity of many liver-specific genes, the expression of which is restricted to the liver, is imparted by combinatorial interactions between liver-enriched and ubiquitous transcription factors (2, 3). The unique expression program of a hepatic gene is thus primarily determined by the configuration of cis-elements containing binding sites for these factors and may be fine tuned by secondary interactions among the proteins.The liver-specific enhancer of the gene encoding apolipoprotein AI (apoAI), 1 the major protein component of high density lipoprotein (reviewed in Ref. 4), contains three cis-acting elements (site A, Ϫ214 to Ϫ192; site B, Ϫ169 to Ϫ146; and site C, Ϫ134 to Ϫ119; Ref. 5). Sites A and C serve as sites of action for many nuclear receptors including retinoid X receptor ␣ (6), ARP-1/COUP-TFII (7, 8), and HNF-4 (9, 10). Site B binds the hepatocyte-enriched factor HNF-3␤ (11). Maximal activity of the apoAI enhancer depends upon the integrity of each of the sites A, B, and C, suggesting that synergistic interactions resulting from a factor binding to them dominate transcription activation from the enhancer (5).In previous studies, we showed that the apoAI enhancer can be activated in non-hepatic cell types if hepatocyte-enriched factors HNF-4 and HNF-3␤ are also provided (11). In the current report, we first define the minimum requirements for efficient enhancer function and then experimentally addre...

Partnership of PGC-1α and HNF4α in the Regulation of Lipoprotein Metabolism

“…In essence, we detect only those genes whose induction minimally requires the ectopic expression of PGC-1␣ and HNF4␣. Although apolipoproteins A-I, A-IV, C-II, and C-III have been previously identified as HNF4␣ targets (31,(35)(36)(37), they are induced robustly only with co-expressed PGC-1␣. These data suggest that HNF4␣ requires PGC-1␣ to realize its full transcriptional potential.…”

Hepatocyte Nuclear Factor 4α (Nuclear Receptor 2A1) Is Essential for Maintenance of Hepatic Gene Expression and Lipid Homeostasis

“…on May 12, 2018 by guest http://mcb.asm.org/ Downloaded from the genes are HNF4␣ responsive in transient transfection assays (9,16,42). Paradoxically, while MCAD is positively regulated by HNF4␣ in cultured cells (4), it is induced by disruption of the HNF4␣ gene (Fig.…”

Elevated Levels of Hepatocyte Nuclear Factor 3β in Mouse Hepatocytes Influence Expression of Genes Involved in Bile Acid and Glucose Homeostasis

“…The rodent HNF-3␣, -3␤, and -3␥ proteins were originally identified as factors mediating liver-specific transcription of the transthyretin (TTR) (10,27,28) gene, encoding a serum carrier protein of thyroxine and vitamin A, and were later shown to regulate the expression of other genes critical for liver function. Subsets of these hepatocyte genes include the serum carrier proteins albumin, ␣-fetoprotein, and apolipoprotein AI (21,34,39); the cholesterol 7␣-hydroxylase (Cyp7A) enzyme involved in bile acid synthesis (9,40,71); and the glucose metabolism enzymes 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase (PFK-2) and glucose 6-phosphatase (G6P) (31,33,54). HNF-3 proteins also recognize the insulin response elements of phosphoenolpyruvate carboxykinase (PEPCK) and insulin-like growth factor binding protein 1 (IGFBP-1) genes (66,72) and participate in growth hormone activation of insulin-like growth factor 1 (IGF-1) (43).…”