E1A REPRESSES APOLIPOPROTEIN AI ENHANCER ACTIVITY IN LIVER-CELLS THROUGH A PRB-INDEPENDENT AND CBP-INDEPENDENT PATHWAY

The apolipoprotein Al (apoAl) promoter/enhancer contains multiple cis- acting elements on which a variety of hepatocyte-enriched and ubiquito us transcription factors function synergistically to regulate liver-sp ecific transcription. Adenovirus E1 A proteins repress tissue-specific gene expression and disrupt the differentiated state in a variety of cell types. In this study expression of E1A 12S or 13S in hepatoblasto ma HepG2 cells repressed apoAl enhancer activity 8-fold. Deletion mapp ing analysis showed that inhibition by E1A was mediated by the apoAl p romoter site B. E1 A selectively inhibited the ability of HNF3 beta an d HNF3 alpha to transactivate reporter genes controlled by the apoAl s ite B and the HNF3 binding site from the transthyretin promoter. The E 1A-mediated repression of HNF3 activity was not reversed by overexpres sion of HNF3 beta nor did E1 A alter nuclear HNF3 beta protein levels or inhibit HNF3 binding to DNA in mobility shift assays. Overexpressio n of two cofactors known to interact with E1A, pRb and CBP failed to o vercome inhibition of HNF3 activity. Similarly, mutations in E1A that disrupt its interaction with pRb or CBP did not compromise its ability to repress HNF3 beta transcriptional activity. These data suggest tha t E1A inhibits HNF3 activity by inactivating a limiting cofactor(s) di stinct from pRb or CBP.