An upstream repressor element that contributes to hepatocyte-specific expression of the rat serum amyloid A1 gene

Serum amyloid A (SAA) is a major acute-phase protein whose expression can b e dramatically induced in response to tissue damage, infection, and inflamm ation. Its expression is highly tissue-specific, restricted almost exclusiv ely to liver hepatocytes. We have shown that a 320-bp fragment of the rat S AA1 promoter could confer liver-cell-specific expression on a reporter gene when transfected into cultured cells. Here we report the identification of a 29-bp regulatory element that possesses inhibitory activities on SAA1 pr omoter in HeLa cells but has no such effects in liver cells, Moreover, this regulatory element has properties of a transcriptional repressor; in that, it can function with a heterologous promoter and is independent of orienta tion and distance from the transcription initiation site. Protein binding s tudies showed that this regulatory element can form specific protein-DNA co mplexes with nuclear proteins from several nonliver cell lines (HeLa, 10T(1 /2) and C2) and placenta. However, the same DNA-protein complex was not det ected in extracts from liver or liver-derived cell lines (HepG2 and Hep3B). Taken together, our results demonstrate the presence of a DNA-binding prot ein, termed tissue-specific repressor, found only in nonhepatocytes which m ay function to repress SAA1 gene expression by interacting with a repressor element. Thus, liver-specific expression of the SAA1 gene is apparently re gulated by both positive and negative regulatory elements and their interac ting transcription factors to ensure that it is expressed only in suitable cell types. (C) 1999 Academic Press.