HNF1alpha and HNF4 Regulate HNF1alpha Expression

The HNF-1 gene contains a relatively short promoter segment located between positions -82 and -40 which directs cell type-specific HNF-1alpha transcription. This region contains a single site for HNF-4alpha (Tian and Schibler, 1991). Transfection experiments revealed that a short region between -118 and -8 is crucial for cell type-specific expression of the HNF-1alpha gene in HepG2 cells. This region contains two positive cis elements called site A, an HNF-4alpha binding site, and site B, an HNF-1alpha binding site. Mutational analyses of these sites and cotransfection assays showed that HNF-4 and HNF-1alpha can trans-activate the HNF-1alpha gene (Miura and Tanaka, 1993). It could be demonstrated that HNF-1alpha negatively regulates its own expression in transient transfection experiments as well as the expression of HNF-4-dependent genes (ApoCllland ApoAI) that lack HNF-1alpha binding sites in their promoter region. DNA binding and cell-free transcription experiments failed to demonstrate any direct or indirect interaction of HNF-1alpha with the regulatory regions of ApoCIII or ApoAI. From these observations it was assumed that HNF-1alpha is able to impede HNF-4 binding or activity. An indirect negative auto-regulatory mechanism for HNF-1alpha expression was described, which in turn may affect HNF-4-dependent transcription of other liver-specific genes (Kritis et al., 1993). Later, it could be found that this repression is exerted by a direct interaction of HNF-1alpha with AF-2, the main activation domain of HNF-4. The dual functions of gene activation and repression suggest that HNF-1alpha is a global regulator of the transcriptional network involved in the maintenance of the hepatocyte-specific phe-notype (Ktistaki and Talianidis, 1997a). Figure 14.1 depicts an assumed molecular

interaction involved in the regulation of the HNF-lalpha gene. Numerous coactiva-tors, as well as the positive HNF-4 ligands, appear to be necessary for optimal HNF-lalpha expression. In this context it is interesting to note that the HNF-4 coactivators p300/CBP, as well as SRC-1 and SRC-3, bind to the activation domain AF-2 of HNF-4. It may well be that HNF-lalpha competes with coactivator binding at the activation domain AF-2 of HNF-4 and thus exerts its indirect negative autoregulation. Additionally, it might be that this hypothetical competition is further modulated by tissue-specific coactivator availability.

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