Absence of ligand-independent transcriptional activation of the estrogen receptor via the estrogen response element in pituitary lactotrophs in primary culture

The estrogen receptor (ER) is a ligand-activated transcription factor that enhances gene expression by binding to specific regulatory DNA sequences called estrogen response elements (EREs). In some cell lines, the ER is also activated in a ligand-independent manner by multiple signaling pathways. In this study, we developed a novel adenovirus-mediated assay for promoter activation, termed LASETA, which we then used to examine whether ligand-independent activation of the ER occurred in normal pituitary lactotrophs in primary culture. In the LASETA adenovirus vector, the loxP-flanked stop sequence was deleted by prolactin (PRL) promoter-regulated expression of Cre recombinase. This led to lactotroph-specific expression of a reporter gene driven by an ERE-containing promoter. Estrogen-induced expression of the reporter protein luciferase in LASETA was specific for lactotrophs and was ER-dependent. LASETA was shown to be reliable even with varying Cre recombinase expression levels, which were caused by changes in PRL promoter activity. Using LASETA, we observed no change in ERE-mediated ER activity in the absence of estrogen after treatment of normal lactotrophs with agents such as insulin-like growth factor-1, epidermal growth factor, the adenylate cyclase activator forskolin, the extracellular signal-regulated kinase kinase inhibitor U0126, and the protein kinase A inhibitor H89. The ERE-mediated ligand-independent ER activity was induced by the growth factors and forskolin in the somatolactotroph tumor cell line GH4C1 cells. These results suggest that ERE-mediated ligand-independent activation of ER does not occur in normal lactotrophs in primary culture, and is a phenomenon likely restricted to transformed cells.