Increases in estrogen receptor-alpha concentration in breast cancer cells promote serine 118/104/106-independent AF-1 transactivation and growth in the absence of estrogen.

A common phenotype in breast cancer is the expansion of the estrogen receptor-alpha (ER+) cell population and an inappropriate elevation of ERalpha protein, the latter predisposing patients for a poorer prognosis than those with lower levels of the receptor. A tetracycline-inducible ERalpha overexpression model was developed in the MCF-7 cell line to assess induction of endogenous gene activation and growth in response to elevations in ERalpha protein. Heightened levels of ERalpha resulted in aberrant promoter occupancy and gene activation in the absence of hormone, which was independent of ligand and AF-2 function. This increased receptor activity required the amino-terminal A/B domain and was not inhibited by tamoxifen, which supports an enhancement of AF-1 function, yet was independent of serine-104, 106, and 118 phosphorylation. Ligand-independent transcription was accompanied by an increase in growth in the absence of hormonal stimulation. The results suggest that elevated levels of ERalpha in breast cancer cells can result in activation of receptor transcriptional function in a manner distinct from classical mechanisms that involve ligand binding or growth factor-induced phosphorylation. Further, they describe a potential mechanism whereby increases in ERalpha concentration may provide a proliferative advantage by augmenting ERalpha function regardless of ligand status.