Background: In the field of human breast cancer, the most recent researches referred to the influence of endogenous estrogen or exposure to environmental estrogen, as risk factors. Zearalenone (ZEN) as a mycotoxin and its derivative α- zearalenol (α-ZOL) are known nonsteroidal estrogenic compounds with potential endocrine disrupting properties. Objectives: The present study was designed to investigate the effects of ZEN and α-ZOL on human breast cancer cell lines MCF-7 and MDA-MB-468. Materials and Methods: Cell lines were treated by low and high doses of ZEN and α-ZOL (0, 1, 30, 62, 125, 250 and 500 ng/ml and 1, 2, 4, 8, 15, 30, 62 and 125 μg/ml) for 24 and 48 hours. Then MTT colorimetric assay was used to evaluate the cytotoxicity effect of ZEN and α-ZOL. Furthermore, morphological changes of treated and untreated cell lines were studied under an inverted microscope. Results: The results obtained from the present study demonstrated that both ZEN and α-ZOL enhance the cell viability of MCF-7 especially at low doses (1 - 500 ng/ml) and at a high dose of 125μg/ml after 24 and 48 hours. However, this effect for α-ZOL was somewhat greater than that for ZEN. On the other hand, these estrogenic compounds did not have any effect on the cell viability of MDA-MB-468. No morphological change was observed in treated cells. Conclusions: These results show that ZEN and α-ZOL enhance the rate of cell division in ER positive cells and therefore, exposure to this mycotoxin may increase the risk of breast cancer.

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