The human estrogen receptor is a steroid nuclear receptor found in breast cancer and a variety of other tissues. Located in the nucleus, it can exist either loosely or tightly associated with the nuclear matrix depending on whether or not it is bound to ligand. When bound to ligand, it is responsible for the transcriptional regulation of estrogen-responsive genes through recruitment of coactivators and corepressors of transcription. The estrogen receptor is also capable of ligand-independent transcriptional activation via the mitogen-activated protein kinase pathway. Ligands have been implicated in the regulation of estrogen receptor levels via changing the levels and stability of estrogen receptor mRNA and protein. The resulting levels of estrogen receptor and the type of ligand bound to it have a direct impact on the transcription of estrogen-responsive genes.

Tagging hormone receptors with the green fluorescent protein (GFP) has increased our knowledge of ligand dependent sub-cellular trafficking of hormone receptors. However, the effect of the tagged hormone receptor expression on the corresponding wild type hormone receptor and endogenous gene expression has not been investigated. In this study, we constructed a MCF-7 cell line stably expressing GFP-tagged human estrogen receptor-alpha (ER) under control of the tetracycline-on system to determine the effect of GFP-ER expression on cell proliferation and expression of endogenous ER and hormone-responsive genes. Further, the inducible system was applied to determine the ligand dependent turnover rates of GFP-ER protein and mRNA. Our results demonstrate that GFP-ER expression did not affect cell cycling. Independent of ligand, GFP-ER markedly reduced the level of endogenous ER mRNA and protein, suggesting that ER negatively autoregulates its expression. Cisplatin cross-linking studies showed that GFP-ER is associated with nuclear DNA in situ, suggesting that GFP-ER is partially replacing ER at estrogen response elements. Furthermore, GFP-ER expression did not affect the estradiol induced temporal expression of hormone responsive genes c-myc and pS2.