Importancia del ratio ERα/ERβ en la función mitocondrial y el estrés oxidativo en el cáncer de mama. Influencia de la genisteína y la UCP2 en la eficacia de los tratamientos antitumorales

Abstract

Breast cancer is one of the most common diseases in developed countries. This type of cancer is usually hormone-dependent; it is influenced by the presence of hormones such as 17β-estradiol (E2), the most abundant estrogen in women, which exerts its effects in the cells through its interaction with the estrogen receptors (ER), ERα and ERβ. ERα has functions related to cell proliferation, while ERβ shows cytostatic and cell differentiation effects. As a result, the ERα/ERβ ratio may influence in the breast cancer development and progression. Moreover, the genistein, a natural phytoestrogen found in soybeans, is able to bind both ERs modulating the gene expression in breast cancer cells. The mitochondrial functionality plays a crucial role in cancer through the modulation of the oxidative stress. Thus, a lower mitochondrial functionality has been associated with a greater production of radical oxygen species (ROS), which may suppose a cell proliferation increment. However, an excessive increase in ROS levels may lead to cell death if the cell antioxidant systems are unable to counteract these ROS, damaging macromolecules. One of the proteins that is part of the antioxidant system of the cell is the uncoupling protein 2 (UCP2), whose function is to dissipate the proton gradient in the inner mitochondrial membrane, so reducing oxidative stress. The objectives of this thesis were: 1) to study how the ERα/ERβ ratio affects to the mitochondrial function and dynamics after treatment of physiological concentrations of E2 and genistein, 2) to investigate if the presence of ERβ may be a resistance factor to anticancer treatments such as cisplatin, paclitaxel and tamoxifen, whose mechanism of action is the generation of oxidative stress in cancer cells in order to provoke the cell death, 3) to review whether the UCP2 silencing could be an adjuvant treatment against breast cancer, 4) to analyze the possible interference of the genistein in the efficacy of the anticancer treatments. Results obtained in this thesis show that the presence and stimulation of the ERβ increased the mitochondrial functionality and dynamics in breast cancer cell lines treated with E2 and genistein, suggesting that a low ERα/ERβ ratio would favor the resistance of breast cancer cells to cytotoxic treatments through an improvement of the mitochondrial functionality, with the consequent reduction in the ROS production. With the exception of tamoxifen as the presence of the ERβ only determines the type of the response to this compound, apoptosis or autophagy. The UCP2 silencing produced an increase in the autophagic cell death in those cells treated with cisplatin and, especially, with tamoxifen, due to a drastic raise in the ROS production. Finally, the genistein triggered a lower response of breast cancer cells with a high ERα/ERβ ratio to anticancer treatments, displacing the E2 of the ERs, thereby reducing the oxidative stress. Furthermore, in that breast cancer cell line with a low ERα/ERβ ratio the response to tamoxifen treatment was enhanced in combination with the genistein through an increase in the apoptosis and autophagy levels. In conclusion, the UCP2 levels and the ERα/ERβ ratio, as well as the concentration of E2 and genistein which would be interacting with the estrogen receptors, play an important role in the response of breast cancer cells to anticancer treatments, modulating the mitochondrial functionality and, therefore, the ROS production.