Contribution of proximal promoter elements to the regulation of basal and differential glutathione S-transferase P1 gene expression in human breast cancer cells

Abstract

Glutathione S-transferase P1 (GST P1-1) is normally expressed exclusively in estrogen receptor negative (ER−) but not receptor positive (ER+) cultured breast cancer cells. We examined the role of proximal promoter elements in GST P1 gene expression in MCF7 (ER+, GST P1−) and HS578T (ER−, GST P1+) breast cancer cells. Transient transfection of GST P1 promoter–CAT reporter genes confirmed that the GST P1 TRE (−69 to −60) and the adjacent distal GC box (−56 to −51) are required for basal promoter activity in both cell lines. Other studies identified differences in the GST P1 promoter activity and DNA–protein interactions between the two cell lines. Electrophoretic mobility shift assay revealed a protein–TRE interaction that is unique to nuclear proteins derived from GST P1 expressing HS578T cells. Furthermore, a putative silencer region contained within sequences −130 to −70 selectively reduced GST P1 promoter–CAT reporter gene expression in MCF7 but not HS578T cells. While this cell-line specific silencer contributed to the level of GST P1 promoter activity observed in the two cell lines, analysis of cells stably transfected with a novel genomic GST P1 minigene vector established that the silencer is insufficient to completely repress GST P1 transcription in ER+, MCF7 cells that do not normally express endogenous GST P1.