Inhibition of postconfluent focus production in cultures of MCF-7 human breast cancer cells by 2,3,7,8-tetrachlorodibenzo-p-dioxin

Summary 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD), a potent inducer of differentiation and an antiestrogen, is shown to suppressin vitro postconfluent cell accumulation in the estrogen-dependent MCF-7 human breast tumor cell line. This dose-responsive suppression is apparent by 14 days of exposure with an EC₅₀ between 10^–10 and 10^–11 M TCDD, and is characterized by reduced cell density (approximately 60% of controls after 14 days). This was attributed to a reduced formation in TCDD-treated cultures of multicellular foci which are chracteristic of cancer cell growthin vitro (less than 1/mm² compared to control levels of 40/mm²). Preconfluent cell growth and viability of MCF-7 cells is not affected by 10^–9 M TCDD. These results suggest that the principle of TCDD's activity may be useful in the study and possibly the management of estrogen-dependent breast tumors.