Transcription factor AP-2 functions as a repressor that contributes to theliver-specific expression of serum amyloid A1 gene

We previously identified transcription factor AP-2 as the nuclear factor th at interacts with the tissue-specific repressor element in the rat serum am yloid Al (SAA1) promoter. In this report, we provide evidence for a second AP-2-binding site and show that both AP-2 sites participate in mediating th e transcription repression of SAA1 promoter. This proximal AP-2 site overla ps with the NF kappaB-binding site known to be essential for SAA1 promoter activity. Protein binding competition experiments demonstrated that AP-2 an d NF kappaB binding to these overlapping sites were mutually exclusive. Fur thermore, the addition of AP-2 easily displaced prebound NF kappaB, whereas NF kappaB could not displace AP-2, These results thus suggest that one mec hanism by which AP-2 negatively regulates SAA1 promoter activity may be by antagonizing the function of NF kappaB, Consistent with a repression functi on, transient expression of AP-2 in HepG2 cells inhibited conditioned mediu m-induced SAA1 promoter activation. This inhibition was dependent on functi onal AP-2-binding sites, since mutation of AP-a-binding sites abolished inh ibitory effects of AP-2 in HepG2 cells as well as resulted in derepression of the SAA1 promoter in HeLa cells. In addition to SAA1, we found that seve ral other liver gene promoters also contain putative AP-2-binding sites. So me of these sequences could specifically inhibit AP-2-DNA complex formation , and for the human complement C3 promoter, overexpression of AP-2 also cou ld repress its cytokine-mediated activation. Finally, stable expression of AP-2 in hepatoma cells significantly reduced the expression of endogenous S AA, albumin, and alpha -fetoprotein genes. Taken together, our results sugg est that AP-2 may function as a transcription repressor to inhibit the expr ession of not only SAA1 gene but also other liver genes in nonhepatic cells .