Tamoxifen alters lysosomes of breast cancer cells by a mechanism independent of its anti-estrogenic activity

Abstract

Breast cancer is one of the most important causes of morbidity and mortality worldwide. It has been shown that the cells of some tumors have an increased lysosomal biogenesis in response to metabolic alterations, which also has an impact on the integrity and/or lysosomal functionality, showing increased levels of lysosomal proteases, such as cathepsin D (CatD). It has been demonstrated that this enzyme induces apoptosis when is released into the cytoplasm. Since the lysosomes could play a role either as initiators or executors of apoptotic processes when the membrane integrity is altered, this organelle could be taken as a potential therapeutic target against tumors. In breast cancer cell lines positive to estrogen receptor RE (REα), CatD is positively regulated by this hormone, while in cell lines negative for REα the enzyme is constitutively overexpressed. Tamoxifen (TAM) is one of the most common anti-estrogenic drugs used in breast cancer therapy. It interacts with ER and inhibits transcriptional activity in the mammary gland. The aim of this study was to evaluate the effect of estrogens and tamoxifen on lysosomal acidification and CatD processing in breast cancer cells. Mammary cell lines MCF-7 (tumorigenic expressing REα), MDA-MB231 (tumorigenic non-expressing REα) and MCF-10A (non-tumorigenic) were treated in the presence or absence of 17-β-estradiol and/or TAM. For quantification of acidic lysosomes, cells were treated with Lysotracker. Cultures were subjected to immunoblot analysis and fluorescence microscopy. As expected, TAM blocked the effect of estrogen on CatD processing in MCF-7 cells. However, TAM used alone, also altered CatD processing and decreased the number of acidic lysosomes in both cell lines. Neither effect of TAM was observed on MCF-10A cell line. In addition, a decreased level of another lysosomal protein, GM2AP (related to the development of tumors), was observed in the cells due to TAM. All these results suggest that TAM has additional effects independent of its anti-estrogenic activity, possibly due to lysosomotropic action.