Phenol red in tissue culture media is a weak estrogen: implications concerning the study of estrogen-responsive cells in culture.

TLDR

Although much attention has been paid to removing hormones from sera and developing serum‑free media for hormone‑responsive cell studies, little consideration has been given to the possibility that media components themselves may have hormonal activity. The estrogenic activity of phenol red should be considered in any studies that utilize estrogen‑responsive cells in culture. Phenol red, present at 15–45 µM in culture media, binds the estrogen receptor with ~0.001 % of estradiol’s affinity, stimulates proliferation of ER‑positive MCF‑7 cells dose‑dependently, increases cell number to 200 % and progesterone receptor content to 300 % versus phenol‑red‑free media, and causes anti‑estrogens to inhibit growth only when phenol red is present.

Abstract

Although much attention has been paid to the removal of hormones from sera and to the development of serum-free media for studies on hormone-responsive cells in culture, little consideration has been given to the possibility that the media components themselves may have hormonal activity. We have found that phenol red, which bears a structural resemblance to some nonsteroidal estrogens and which is used ubiquitously as a pH indicator in tissue culture media, has significant estrogenic activity at the concentrations (15-45 microM) at which it is found in tissue culture media. Phenol red binds to the estrogen receptor of MCF-7 human breast cancer cells with an affinity 0.001% that of estradiol (Kd = 2 X 10(-5) M). It stimulates the proliferation of estrogen receptor-positive MCF-7 breast cancer cells in a dose-dependent manner but has no effect on the growth of estrogen receptor-negative MDA-MB-231 breast cancer cells. At the concentrations present in tissue culture media, phenol red causes partial estrogenic stimulation, increasing cell number to 200% and progesterone receptor content to 300% of that found for cells grown in phenol red-free media, thereby reducing the degree to which exogenous estrogen is able to stimulate responses. The antiestrogens tamoxifen and hydroxytamoxifen inhibit cell proliferation below the control level only when cells are grown in the presence of phenol red; in the absence of phenol red, the antiestrogens do not suppress growth. The estrogenic activity of phenol red should be considered in any studies that utilize estrogen-responsive cells in culture.