SummaryTo elucidate the mechanisms involved in anti-oestrogen resistance, two human breast cancer cell lines MCF-7 and the ICI 182780-resistant cell line, MCF-7/182 R -6, have been compared with regard to oestrogen receptor (ER) expression, ER function, ER regulation, growth requirements and differentially expressed gene products. MCF-7/182 R -6 cells express a reduced level of ER protein. The ER protein is functional with respect to binding of oestradiol and the anti-oestrogens tamoxifen, 4-hydroxy-tamoxifen and ICI 182780, whereas expression and oestrogen induction of the progesterone receptor is lost in MCF-7/182 R -6 cells. The ER protein and the ER mRNA are regulated similarly in the two cell lines when subjected to treatment with oestradiol or ICI 182780. Oestradiol down-regulates ER mRNA and ER protein expression. ICI 182780 has no initial effect on ER mRNA expression whereas the ER protein level decreases rapidly in cells treated with ICI 182780, indicating a severely decreased stability of the ER protein when bound to ICI 182780. In vitro growth experiments revealed that the ICI 182780-resistant cell line had evolved to an oestradiol-independent phenotype, able to grow with close to maximal growth rate both in the absence of oestradiol and in the presence of ICI 182780. Comparison of gene expression between the two cell lines revealed relatively few differences, indicating that a limited number of changes is involved in the development of anti-oestrogen resistance. Identification of the differentially expressed gene products are currently in progress.

To elucidate the mechanisms responsible for the development of anti‐estrogen resistance, we have cloned and established 3 stable ICI‐182,780‐resistant sub‐lines, MCF‐7/182R‐1, MCF‐7/182R‐6 and MCF‐7/182R‐7 from the estrogen‐receptor(ER)‐positive and estrogen‐responsive human breast‐cancer MCF‐7 cell line by long‐term treatment with 10−7 M ICI 182,780. The ICI‐182,780‐resistant MCF‐7 sub‐lines express ER, but compared with MCF‐7 cells the level is significantly lower in all 3 sub‐lines. In the MCF‐7 cell line we find that ER expression is regulated by estrogen and anti‐estrogens at the transcriptional and post‐transcriptional level. This is in contrast to the ICI‐182,780‐resistant sub‐lines, in which we find very little hormonal effects on the ER mRNA expression level. The resistant sub‐lines also deviate from parent characteristics by the complete lack of expression of progesterone receptor even when grown in the presence of estradiol. All 3 resistant sub‐lines have a lower basal expression of cathepsin‐D mRNA comparable with the lower ER expression, but, in contrast, they have higher basal expression of the pS2 mRNA than the parent MCF‐7 cell line. Although there are different basal expression levels of the pS2 and cathepsin‐D genes, the resistant sub‐lines behave like the parent MCF‐7 cell line with respect to the hormonal regulation of both genes. The estrogen receptors in the resistant sub‐lines have also maintained wild‐type characteristics with respect to estrogen and anti‐estrogen regulation of the estrogen‐regulated proteins procathepsin D, α1‐anti‐trypsin and a 42‐kDa protein. The resistant cells require estrogen for growth in athymic nude mice. Our results clearly demonstrate that the ER in the resistant sub‐lines have a normal function for most parameters investigated, supporting our earlier observation that only wild‐type ER protein is expressed in these cells. The few observed differences in ER function between the parent MCF‐7 cell line and the resistant sub‐lines are not likely to be responsible for the ICI‐182,780‐resistant phenotype. Int. J. Cancer 72:1129–1136, 1997. © 1997 Wiley‐Liss, Inc.