Investigation of growth and endocrine disrupting effects of the mycotoxins zearalenone and aflatoxin B1 on breast cancer in vitro

Abstract

Despite the medical advancements, the woman breast cancer incidence rates keep rising in the past few decades. Scientists have proposed the increased exposure to endocrine disrupting chemicals as a possible factor for the rises. Zearalenone (ZEA) and aflatoxin B1 (AFB1) are common mycotoxins that are present in cereal crops worldwide. ZEA has long been recognized as a xenoestrogen, while the endocrine disrupting effects of AFB1 on steroidogenesis have been identified recently. Due to the co-occurrence and the endocrine disrupting potentials of ZEA and AFB1, the hypothesis of this project was proposed as exposure to low doses of ZEA and AFB1 might affect the growth of hormonal dependent breast cancer.

In order to address the hypothesis, the aim of the first study was to examine the ultimate effects on growth and cell cycle progression in breast cancer MCF-7 cell line, following low dose exposure to ZEA and AFB1 individually and in combination. The effects on viability, cell growth, DNA synthesis, cell cycle progression and cyclin gene expressions were determined. Significant interactions were detected for their effects on viability and DNA synthesis. While ZEA promoted growth, DNA synthesis and cell cycle progression in MCF-7 cells, AFB1 was cytotoxic and counteracted the effects of ZEA. This study confirmed the growth promoting properties of ZEA, and is the first to report the combined effects of ZEA and AFB1 on breast cancer cell growth, suggesting endocrine-disrupting mycotoxins that co-occur in human food can interact and modulate the effects of each other on human health.

The second study aimed to reveal the modulation of breast cancer genes and the underlying pathways that were directly affected by ZEA and AFB1. By using a real time reverse transcription polymerase chain reaction array, it was shown that ZEA was capable of altering the expressions of a large number of breast cancer related genes, whereas AFB1 had minimal effects on the breast cancer gene expressions. With the use of specific inhibitors, estrogen receptor α, G protein-coupled estrogen receptor 1, and mitogen-activated protein kinases (MAPKs) were found to be responsible for ZEA’s effects on cell growth and myelocytomatosis oncogene activation; while MAPK pathways might be involved in the cytotoxic effects by AFB1. Further confirmation is necessary for linking the activations of estrogen receptors and MAPKs to breast cancer cell growth by ZEA and AFB1.

The last study aimed at assessing the impacts of ZEA and AFB1 on steroidogenic and steroid metabolic enzymes in MCF-7 cells, which might result in hormonal imbalance and undesirable breast cancer cell growth. By evaluating the mRNA expressions, it was found that ZEA significantly modulated the expressions of all the steroidogenic and steroid metabolic enzymes tested while AFB1 altered the expression of cytochrome P450 (CYP) 1A1 and CYP 1B1. Coexposure of MCF-7 cells to the two mycotoxins revealed that AFB1 antagonized the effects of ZEA on expression of CYP 1A1, CYP 3A4 and CYP 1B1. Further research into the alternations of enzyme levels and activities by ZEA and AFB1 is necessary before a solid conclusion can be made.