RESPONSE-SPECIFIC ANTIESTROGEN RESISTANCE IN A NEWLY CHARACTERIZED MCF-7 HUMAN BREAST-CANCER CELL-LINE RESULTING FROM LONG-TERM EXPOSURE TOTRANS-HYDROXYTAMOXIFEN

To understand better the antiestrogen-resistant phenotype that frequen tly develops in breast cancer patients receiving tamoxifen, we culture d MCF-7 breast cancer cells long-term (>1yr) in the presence of the an tiestrogen trans-hydroxytamoxifen (TOT) to generate a subline refracto ry to the growth-suppressive effects of TOT. This subline (designated MCF/TOT) showed growth stimulation, rather than inhibition, with TOT a nd diminished growth stimulation with estradiol (E(2)), yet remained a s sensitive as the parental cells to growth suppression by another ant iestrogen, ICI 164,384. Estrogen receptor (ER) levels were maintained at 40% of that in parent MCF-7 cells, but MCF/TOT cells failed to show an increase in progesterone receptor content in response to E(2) or T OT treatment. In contrast, the MCF/TOT subline behaved like parental c ells in terms of E(2) and TOT regulation of ER and pS2 expression and transactivation of a transiently transfected estrogen-responsive gene construct. DNA sequencing of the hormone binding domain of the ER from both MCF-7 and MCF/TOT cells confirmed the presence of wild-type ER a nd exon 5 and exon 7 deletion splice variants, but showed no point mut ations. Compared to the parental cells, the MCF/TOT subline showed red uced sensitivity to the growth-suppressive effects of retinoic acid an d complete resistance to exogenous TGF-beta 1. The altered growth resp onsiveness of MCF/TOT cells to TOT and TGF-beta 1 was partly to fully reversible following TOT withdrawal for 16 weeks. Our findings undersc ore the fact that antiestrogen resistance is response-specific; that l oss of growth suppression by TOT appears to be due to the acquisition of weak growth stimulation; and that resistance to TOT does not mean g lobal resistance to other more pure antiestrogens such as ICI 164,384, implying that these antiestrogens must act by somewhat different mech anisms. The association of reduced retinoic acid responsiveness and in sensitivity to exogenous TGF-beta with antiestrogen growth resistance in these cells supports the increasing evidence for interrelationships among cell regulatory pathways utilized by these three growth-suppres sive agents in breast cancer cells. In addition, our findings indicate that one mechanism of antiestrogen resistance, as seen in MCF/TOT cel ls, may involve alterations in growth factor and other hormonal pathwa ys that affect the ER response pathway. Copyright (C) 1996 Elsevier Sc ience Ltd.