CONTROL OF APOLIPOPROTEIN AI GENE-EXPRESSION THROUGH SYNERGISTIC INTERACTIONS BETWEEN HEPATOCYTE NUCLEAR FACTOR-3 AND FACTOR-4

Apolipoprotein AI (apoAI) gene expression in liver depends on synergis tic interactions between transcription factors bound to three distinct sites (A, B, and C) within a hepatocyte-specific enhancer in the 5'-f lanking region of the gene, In this study, we showed that a segment sp anning sites A and B retains substantial levels of enhancer activity i n hepatoblastoma HepG2 cells and that sites A and B are occupied by th e liver-enriched hepatocyte nuclear factors (HNFs) 4 and 3, respective ly, in these cells, In non-hepatic CV-1 cells, HNF-4 and HNF-3 beta ac tivated this minimal enhancer synergistically, This synergy was depend ent upon simultaneous binding of these factors to their cognate sites, but it was not due to cooperativity in DNA binding, Separation of the se sites by varying helical turns of DNA did not affect simultaneous b inding of HNF-3 beta and HNF-4 nor did it influence their functional s ynergy, The synergy was, however, dependent upon the cell type used fo r functional analysis, In addition, this synergy was further potentiat ed by estrogen treatment of cells cotransfected with the estrogen rece ptor, These data indicate that a cell type-restricted intermediary fac tor jointly recruited by HNF-4 and HNF-3 participates in activation of the apoAI enhancer in liver cells and suggest that the activity of th is factor is regulated by estrogen.