Synergistic activation of the Atlantic salmon hepatocyte nuclear factor (HNF) 1 promoter by the orphan nuclear receptors HNF4 and chicken ovalbumin upstream promoter transcription factor I (COUP-TFI)

Hepatocyte nuclear factor 1 (HNF1) is a liver-enriched transcription factor that plays an important role in transcriptional networks involved in liver function. The promoters of mammalian HNF1 genes contains a single binding site for another liver-enriched transcription factor, the nuclear hormone receptor HNF4. A transcriptional hierarchy involving HNF4-mediated activation of the HNF1 promoter has been proposed to be of crucial importance in maintaining the differentiated hepatocyte phenotype. Here we present evidence that the Atlantic salmon HNF1 promoter contains three nuclear-hormone-receptor-binding sequences. Gel-shift assays showed that these motifs are recognized with different affinities by HNF4 and the orphan nuclear receptors chicken ovalbumin upstream promoter transcription factors COUP-TFI and COUP-TFII. In hepatoma cells, the site showing highest affinity for HNF4 appears to be crucial for promoter activity. Transfection experiments in non-hepatic cells indicated that the salmon HNF1 promoter was activated by both HNF4 and COUP-TFs. We also identified a promoter fragment encompassing the two more distal nuclear-hormone-binding sites that was activated by HNF4, unaffected by COUP-TF and showed a strong synergistic activation by HNF4/COUP-TF. Results are presented detailing these interactions in relation to the salmon HNF1 promoter architecture.