HMG-1 stimulates estrogen response element binding by estrogen receptor from stably transfected HeLa cells

Estrogen receptor (ER) toxicity has hampered the development of vertebrate cell lines stably expressing substantial levels of recombinant wild-type ER. To isolate clonal lines of HeLa cells stably expressing epitope-tagged ER, we used a construction encoding a single bicistronic mRNA, in which FLAG-epitope-tagged human ERα (fER) was translated from a 5'-translation initiation site and fused to the neomycin resistance gene, which was translated from an internal ribosome entry site. One stable HeLa-ER-positive cell line (HeLa-ER1) produces 1,300,000 molecules of fER/cell (~20-fold more ER than MCF-7 cells). The HeLa fER is biologically active in vivo, as judged by rapid death of the cells in the presence of either 17β-estradiol or trans-hydroxytamoxifen and the ability of the cell line to activate a transfected estrogen response element (ERE)-containing reporter gene. The FLAG-tagged ER was purified to near homogeneity in a single step by immunoaffinity chromatography with anti-FLAG monoclonal antibody. Purified fER exhibited a distribution constant (K(D)) for 17β-estradiol of 0.45 nM. Purified HeLa fER and HeLa fER in crude nuclear extracts exhibit similar K(D) values for the ERE (0.8 nM and I nM, respectively), which are approximately 10 times lower than the K(D) of 10 nM we determined for purified ER expressed using the baculovirus system. HMG-1 strongly stimulated binding of both crude and purified HeLa fER to the ERE (K(D) of 0.25 nM). In transfected HeLa cells, HMG-1 exhibited a dose-dependent stimulation of 17β-estradiol-dependent transactivation. At high levels of transfected HMG-1 expression plasmid, transactivation by ER became partially ligand-independent, and transactivation by trans-hydroxytamoxifen was increased by more than 25-fold. These data describe a system in which ER, stably expressed in HeLa cells and easily purified, exhibits extremely high affinity for the ERE, and suggest that intracellular levels of HMG-1 may be limiting for ER action.