AN INDIRECT NEGATIVE AUTOREGULATORY MECHANISM INVOLVED IN HEPATOCYTE NUCLEAR FACTOR-I GENE-EXPRESSION

Recent studies have revealed that hepatocyte nuclear factor 4 (HNF-4) is an essential positive regulator of another liver enriched transcrip tion factor HNF-1, defining a transcriptional hierarchy between the tw o factors operating in hepatocytes. To assess the possible autoregulat ion of the HNF-1 gene we have examined the effect of HNF-1 on its own transcription. In transient transfection assays, HNF-1 strongly downre gulated transcription driven by its own promoter in HepG2 cells. In ad dition HNF-1 also repressed the activity of HNF-4 dependent ApoCIII an d ApoAI promoters. The same effect was observed using vHNF-1, a distin ct but highly related protein to HNF-1. Both HNF-1 and vHNF-1 downregu lated HNF-4 activated transcription from intact and chimeric promoter constructs carrying various HNF-4 binding sites implying that they act by impeding HNF-4 binding or activity. DNA binding and cell free tran scription experiments however failed to demonstrate any direct or indi rect interaction of HNF-1 and vHNF-1 with the above regulatory regions . Both factors repressed HNF-4 induced transcription of the ApoCIII an d HNF-1 genes in HeLa cells, arguing against the requirement of a hepa tocyte specific function. These findings define an indirect negative a utoregulatory mechanism involved in HNF-1 gene expression, which in tu rn may affect HNF-4 dependent transcription of other liver specific ge nes.