Estrogen receptor transformation in MCF-7 breast cancer cells: Characterization by immunochemical and sedimentation analyses

We have examined the sedimentation properties of MCF-7 cell cytosol estrogen receptors (ERc) and salt-extracted nuclear estrogen receptors (ERn), and the interaction of these receptors with a monoclonal antibody, as assessed on 0.4 M KC1 sucrose gradients prepared in different buffers. ERc labeled with [³H]estradiol (E₂) sediments as a 4 S species in all gradient buffers tested, whereas ERn extracted from cells incubated with [³H]E₂ sediments as a 4 S species in sucrose gradients prepared in phosphate-thioglycerol-glycerol buffer, but as a 5.4 S species in Tris-EDTA buffered gradients. Interaction of ERc with antibody D547SpY results in formation of a 7.2 S complex, whereas the antibody reacts with ERn to form complexes that are 8.5 S in sucrose gradients prepared in either buffer. These findings are consistent with the hypothesis that estrogen receptors from MCF-7 cells undergo a 4 S to 5.4 S transformation during nuclear interaction, but that the 5.4 S ERn is converted to a 4 S form during centrifugation in phosphate-buffered gradients. Antibody interaction blocks conversion of receptor to the 4 S form, since only 8.5 S complexes are observed, regardless of the gradient buffer composition. These results highlight that the pattern of receptor transformation by E₂ in these cancer cells need not differ from that seen in normal estrogen target tissues such as uterus.