Functional interactions of an upstream enhancer of the mouse glycoprotein hormone α-subunit gene with proximal promoter sequences

Abstract

Transcription of the glycoprotein hormone α-subunit gene in the pituitary is governed by different promoter elements in thyrotropes and gonadotropes. We recently identified an upstream enhancer that directs a high level of cell type specific expression in transgenic mice and stimulates proximal promoter activity in cultured αTSH and αT3 cells. To assess the contribution of promoter sequences that functionally interact with the enhancer, we mutated two proximal elements shown to be important in both thyrotrope and gonadotrope cells. Disruption of the pituitary glycoprotein hormone basal element (PGBE), which binds a LIM homeodomain protein, resulted in a decrease in basal promoter activity in both αTSH and αT3 cells. Enhancer function was completely abolished by the PGBE site mutation in αT3 gonadotropes, whereas some stimulatory activity remained in αTSH thyrotropes. Mutation of the gonadotrope specific element (GSE), which binds SF1 and is important for basal activity in gonadotropes and TRH response in thyrotropes, resulted in declines in basal and enhanced promoter activity only in αT3 cells and not in αTSH cells. Despite this decrease in enhanced activity, the GSE mutated promoter still retained some enhancer stimulated activity, suggesting that the PGBE site still functionally interacts in the absence of an intact GSE. This mutation had no effect in αTSH cells. These data suggest that although the enhancer works in both cell types it exhibits cell type specific functional characteristics.