Oestrogens induce G1 arrest in benzoapyrene-treated MCF-7 breast cells while enhancing genotoxicity and clonogenic survival.

Abstract

Carcinogens, such as benzoapyrene (BaP), allow cells to evade G1 arrest (the stealth property), thus increasing the chance that DNA damage will ultimately result in transformation. In this study we have investigated the effects of BaP in MCF-7 cells incubated in the presence or absence of oestrogens ({\ss}-oestradiol, oestrone or oestriol). The cytokinesis block micronucleus assay was used to examine cells for chromosomal damage. Micronuclei were scored in 500 binucleate cells per treatment. Increased micronucleus formation (3-fold) occurred following 24 h treatment with 10?6 M BaP alone. Following co-treatment with either 10?9 M {\ss}-oestradiol, 10?8 M oestrone or 10?8 M oestriol, 2- to 3-fold increases in micronuclei were observed with 10?8 M BaP. When MCF-7 cells were pre-incubated for 96 h with 10?9 M {\ss}-oestradiol, 10?8 M oestrone or 10?8 M oestriol prior to the addition of BaP for 24 h, up to a 5-fold enhanced sensitivity to micronucleus formation was observed with {\ss}-oestradiol and oestrone, while oestriol appeared to reduce levels of micronucleus formation. BaP-induced decreases in cell proliferation (per cent binucleate cells) and plating efficiency were reversed by all three oestrogens. Analysis of cell cycle distributions revealed that treatment with oestrogens or BaP alone did not induce marked effects on cell cycle distributions. However, in combination oestrogen and BaP induced increases in G0/G1, decreases in S phase and increases in G2/M. This work suggests that whilst oestrogens appear to enhance carcinogen-induced DNA damage, they also appear, paradoxically, to trigger mechanisms that facilitate clonogenic survival, which may be relevant to breast cancer initiation.