Role of Estrogen and Xenoestrogen in Chemoresistance of Breast Cancer

The resistance of cancerous cells to chemotherapeutic agents is an important issue in the treatment of breast cancer. Although estrogens and xenoestrogens have been implicated in the tumorigenesis of breast cancer, their roles in chemoresistance are not clear. These compounds can activate classical estrogen receptors as well as other types of receptors, such as aryl hydrocarbon receptor. Through the ligand–receptors activation, they control the target genes and signaling pathways. Herein, we review evidence that estrogens and xenoestrogens antagonize multiple anticancer drugs by antiapoptosis, drug efflux, and promoted proliferation. In addition, novel therapeutic targets of breast cancer have been proposed from these signaling pathways, such as ceramide/sphingosine 1-phosphate. A better understanding of the mechanisms of chemoresistance induced by estrogen or xenoestrogen could provide important insights into the strategy for breast cancer treatment.