Differences in immunoreactivity of estrogen receptor (ER) in tamoxifen-sensitive and -resistant breast carcinomas: preclinical and first clinical investigations

Inherited or acquired tamoxifen resistance is a major constraint in the end ocrinological treatment of breast carcinomas. We developed an enzyme-immuno assay that discriminates between tamoxifen-sensitive and -resistant tumors. The procedure was established and standardized using two xenografted breas t carcinomas - 3366 (highly sensitive to tamoxifen) and 3366/TAM (acquired tamoxifen resistance). The latter model was developed by treatment of 3366 tumor-bearing nude mice during serial passaging over 3 years with tamoxifen . Both lines were estrogen receptor (ER) positive (101 or 82 fmol/mg protei n), and revealed no differences in the nucleotide sequences of the hormone binding domain of the ER protein. However, while in the sensitive tumors an upregulation of ER levels was registered after estradiol treatment of tumo r bearing nude mice, the ER expression in the resistant line remained uncha nged. The tamoxifen sensitive and -resistant breast carcinoma 3366 differed , additionally, in their immunoreactivity of ER to mAB H222. While an incub ation with estradiol or tamoxifen of immobilized ER prepared from cytosols of the sensitive tumors 3366 led to a significant increase in immunoreactiv ity, samples of resistant tumors failed in the exposition of additional imm unologically reactive epitopes. These results were the basis for the develo pment of an assay for determination of the tamoxifen response in patients. Our retrospective results with 38 breast tumors from a tumor bank indicated that patients with an increase of immunoreactivity of ER more rarely had a recurrence while under going tamoxifen therapy compared with patients expe cting no increase. However, the data indicate interesting changes occurring with the ER of tam-resistant tumors that are to be explained by further mu tational or protein-chemical analysis.