4-hydroxytamoxifen differentially exerts estrogenic and antiestrogenic effects on discrete subpopulations of human breast cancer cells

Functional heterogeneity within populations of breast cancer cells contribu te to the seemingly paradoxical effects of antiestrogens and the developmen t of antiestrogen "resistance." Our objectives were to determine the degree to which T-47D cells may respond inappropriately (positively) to the antie strogen 4-hydroxytamoxifen (HOT) alone, and whether all cells that respond to the stimulatory effects of estradiol-17 beta (E-2) are inhibited by the addition of HOT. Single, living T-47D cells were transfected by microinject ion with an estrogen response element (ERE)-driven luciferase reporter plas mid, Transfected cells were then treated with medium alone, HOT, E-2 or a c ombination thereof on consecutive days, exposed to the substrate luciferin and subjected to quantification of photonic emissions reflective of ERE-sti mulated activity. This analysis revealed a subpopulation of cells that exhi bited increased ERE-driven photonic activity in response to HOT. In compani on studies, E-2-stimulated ERE activity was reversed (on average) with HOT addition. However, analysis of individual cells revealed that although HOT reduced photonic activity in the majority (89.2%) of E-2-responsive cells, there was a small subset (10.8% of the population) that was stimulated by E -2 + HOT co-treatment. Our data support the hypothesis that these cells pos sess an intrinsic "resistance" to antiestrogenic agents, and that this coul d contribute to the remodeling of tumor cell populations toward a "resistan t" phenotype.