title: Characterisation of a tamoxifen-resistant variant of the ZR-75-1 human breast cancer cell line (ZR-75-9a1) and ability of the resistant phenotype.
abstract: A 6-month exposure of ZR-75-1 human breast cancer cells to tamoxifen (1 microM rising to 2 microM). resulted in a fall in oestrogen receptor (ER) levels from 225 fmol mg protein-1 to 56 fmol mg protein-1 while progesterone receptor (PGR) concentration fell from 63 fmol mg protein-1 to undetectable levels. Sensitivity to the anti-proliferative effects of tamoxifen was unchanged. A further 6 months' exposure to 4 microM tamoxifen resulted in loss of detectable ER and PGR and development of resistance to tamoxifen. Resistant cells, designated ZR-75-9a1, displayed morphological changes consistent with the acquisition of a less well differentiated phenotype. Flow cytometric studies demonstrated that the cell cycle distribution pattern of the resistant variant growing in the presence of 8 microM tamoxifen was identical to that of the untreated parent line, which showed marked accumulation of cells in G0/G1 when exposed to 8 microM tamoxifen. The resistant phenotype was not stable if cells were transferred to complete drug-free medium, but remained stable for at least 3 months in the presence of medium lacking oestrogenic activity. ZR-75-9a1 cells differ from previously reported tamoxifen-resistant variants of the MCF-7 line which retain ER and may prove a valuable model for the study of the development and stability of tamoxifen resistance in human breast cancer.
authors.affiliation: Department of Therapeutics and Pharmacology, Queen's University of Belfast, Northern Ireland.
authors.forename: H W
authors.initials: HW
authors.lastname: van den Berg
authors.forename: M
authors.initials: M
authors.lastname: Lynch
authors.forename: J
authors.initials: J
authors.lastname: Martin
authors.forename: J
authors.initials: J
authors.lastname: Nelson
authors.forename: G R
authors.initials: GR
authors.lastname: Dickson
authors.forename: A D
authors.initials: AD
authors.lastname: Crockard
mesh_terms: D001943:Breast Neoplasms / Q000032:analysis / Q000188:drug therapy / Q000473:pathology*; D002453:Cell Cycle; D002460:Cell Line; D004351:Drug Resistance; D006801:Humans; D011960:Receptors, Estrogen / Q000032:analysis; D011980:Receptors, Progesterone / Q000032:analysis; D013629:Tamoxifen / Q000494:pharmacology*; D014407:Tumor Cells, Cultured / Q000032:analysis / Q000187:drug effects / Q000473:pathology*
chemical_list: D011960:Receptors, Estrogen; D011980:Receptors, Progesterone; D013629:Tamoxifen