Influence of a cap site element on tissue-restricted expression of the glycoprotein hormone α-subunit gene

Little is known of the transcriptional regulators important for expression of the glycoprotein hormone α-subunit (GPHα) gene in nonendocrine tumors, which secrete free α-subunit at an incidence of 25-80%. Consequently, attempts were made to define cis-regulatory elements and their cognate trans-acting factors that modulate promoter activity in epithelial cell types that do not normally express the glycoprotein hormones. DNA-mediated transient expression of promoter-reporter constructs was used to identify a novel negative regulatory element located at the GPHα gene transcription start site. Mutagenesis of this element produced a 2- to 10-fold increase in promoter activity, depending on the particular mutation and the transfected tumor cell line. Electrophoretic mobility shift analysis detected a protein that binds specifically to a DNA motif encompassing the cap site. It was present at different levels in a variety of cell types. Significantly, the degree to which activity of the wild-type promoter was suppressed relative to that of the mutant promoter was proportional to the level of cap site binding protein in the collection of cell lines examined. These results indicate that a negative regulatory element centered at the GPHα gene cap site and its cognate DNA-binding protein make a significant contribution to the production of α-subunit in a variety of tumor tissues. A detailed understanding of this cis/trans pair may further suggest a mechanism to explain, at least in part, how this gene becomes activated in nonendocrine tumors.