Transcriptional mechanisms responsible for the overexpression of the keratin 18 gene in cells of a human colon carcinoma cell line

The keratin 18 (K18) gene is overexpressed in cells of tumorigenic clones i solated from the SW613-S human colon carcinoma cell line, compared to cells of nontumorigenic clones, The isolated minimal promoter (TATA box and init iation site) of the K18 gene has by itself at differential activity in tumo rigenic and nontumorigenic cells. An Sp1 binding site located upstream of t he TATA box contributes to the high level of expression of the gene in tumo rigenic cells, We report here that the Sp1 gene is not differentially expre ssed between the two cell types and that this is also the case for genes co ding for factors of the preinitiation complex known to directly interact wi th the Sp1 protein. Further, DNase I footprinting experiments and mutagenes is analysis indicated that the mechanism responsible for the differential a ctivity of the minimal K18 promoter apparently does not involve the binding of a factor to a specific sequence. During the course of these experiments , it was found that the initiation site of the K18 promoter is actually loc ated 11 bp upstream of the +1 position previously reported and that the TAT A box is the only essential element of the minimal promoter. Treatment of t he cells with histone deacetylase inhibitors was more efficient at stimulat ing the activity of the K18 promoter in nontumorigenic cells than in tumori genic cells. We propose that overexpression of the K18 gene in tumorigenic cells could result from of a high level of acetylation of histones and/or o f factors controlling the activity of the transcription complex. (C) 1999 A cademic Press.