Characterization of an antiestrogen-binding protein in high salt extracts of human breast cancer tissue

An antiestrogen binding protein which binds [ 3H]tamoxifen (1-[4-(2-dimethylaminoethoxy)-phenyl]1,2-diphenylbut-1(Z)-ene) with high affinity ( k d = 1.1 × 10 −9 M) is present in high salt (0.6 M KC1) extracts of washed breast cancer tissue pellets. Its concentration in high salt extract is higher than its concentration in cytosol. The characteristics of the antiestrogen binding protein from cytosol and salt extract of breast cancer tissue are indistinguishable. It specifically binds triphenylethylene and other nonsteroidal antiestrogens and displays little or no binding affinity for estrogens, progesterone, dihydrotestosterone and cortisol. The antiestrogen binding protein is of unusually large size as judged by gel filtration on agarose 0.5 m and sedimentation analysis on 5–20% sucrose density gradients. Differential centrifugation studies indicate that it is not principally microsomal in origin. This protein is more thermostable than the estrogen receptor from which it can also be distinguished by ion exchange chromatography. The antiestrogen binding protein was eluted from DEAE-Sephacel by 0.05 M KCl indicating that it is less negatively charged than the estrogen receptor which was eluted by 0.1 M KCl. Lipoprotein fractionation of breast cancer cytosol using potassium bromide density gradients did not reveal specific antiestrogen binding activity associated with any recognized class of lipoprotein. Specific [ 3H]tamoxifen binding sites were pelleted in potassium bromide gradients consistent with the apparent large size of this protein. The physical characteristics of the antiestrogen binding protein in normal human tissue (myometrium) and neoplastic tissue (breast cancer) are remarkably similar, possibly reflecting a highly conserved structure.