MODULATION OF LIVER-SPECIFIC TRANSCRIPTION BY INTERACTIONS BETWEEN HEPATOCYTE NUCLEAR FACTOR-III AND NUCLEAR FACTOR-I BINDING DNA IN CLOSE APPOSITION

The liver-specific enhancer of the serum albumin gene contains an essential segment, designated eH, which binds the hepatocyte nuclear factor 3alpha (HNF3alpha) and ubiquitous nuclear factor 1/CCAAT transcription factor (NF1/CTF) proteins in tight apposition. We previously showed that activation of transcription by the eH site was correlated with an increase in intracellular HNF3alpha levels during the in vitro differentiation of the hepatic cell line H2.35. We now show that transfection of an HNF3alpha cDNA expression vector into dedifferentiated H2.35 cells is sufficient to induce transcription from the eH site. Mutational analysis of the enhancer demonstrates that NF1/CTF cooperates with HNF3alpha to induce enhancer activity. However, when the eH site is removed from the context of the enhancer, NF1/CTF can inhibit transcriptional activation by HNF3alpha. We conclude that the ternary complex of HNF3alpha, NF1/CTF, and the eH site forms a novel, composite regulatory element that is sensitive to the local DNA sequence environment and suggest that the transcriptional stimulatory activity of NF1/CTF depends on its higher-order interactions with other proteins during hepatocyte differentiation.