Interaction of phenol red with estrogenic and antiestrogenic action on growth of human breast cancer cells ZR-75-1 and T-47-D.

Studies reported here confirm that the pH indicator, phenol red, acts as a weak estrogen and reexamine the significance of estrogenic and antiestrogenic effects on growth of the human breast cancer cell lines ZR-75-1 and T-47-D in the absence of phenol red. Removal of phenol red reduces but does not immediately eliminate cell growth in the absence of estradiol. Basal cell growth can be reduced for T-47-D cells and eliminated for ZR-75-1 cells by prior growth in the absence of steroid and phenol red for 3 weeks, demonstrating that estrogens can have long lasting effects on cells in culture (termed "steroid memory") and that there exist both cells which are responsive (T-47-D) and dependent (ZR-75-1) on estradiol for growth. Antiestrogen action in these cell lines is affected by at least four parameters: (a) presence of phenol red; (b) time in culture; (c) cell density; (d) steroid memory effects. At high cell density, antiestrogens suppress phenol red-stimulated activity but have little effect in the absence of phenol red. However, at low cell density in the absence of phenol red, tamoxifen has a biphasic action: initial weak stimulation, later inhibition. trans-Hydroxytamoxifen does not stimulate but inhibition increases with time in culture. Following deprivation for 3 weeks of phenol red and steroid, antiestrogen action on ZR-75-1 cells at low density became much more inhibitory. Such responses to antiestrogens are discussed in relation to possible autocrine/paracrine growth regulation of the cells. Clinical relevance is suggested.