We propose that the ETC is a suitable therapeutic target in Her2 disease. Antioxid. Redox Signal. 26, 84-103.

Background/Aims: MiRNA-301a-3p is an oncogenic miRNA whose expression is associated with tumor development, metastases and overall poor prognosis. Estrogen receptor α (ERα) is one of the estrogen hormone-activated transcription factors, which regulates a large number of genes and is involved in the mammary gland development. Expression of ERα is considered to be a good indicator for endocrine therapy and breast cancer survival. Loss of ERα in breast cancer patients indicates invasiveness and poor prognosis. In this study, we focus on the regulation of ERα by miR-301a and its role in transition from estrogen-dependent to estrogen-independent breast cancer. Methods: Expression of miR-301a-3p was measured by qRT-PCR in tumor tissue samples from 111 patients with primary breast carcinoma and in mammospheres representing in vitro model of cancer stem-like cells. Dual reporter luciferase assay and complementary experiments were performed to validate ESR1 as a direct target of miR-301a-3p. The effect of miR-301a-3p on estrogen signaling was evaluated on the level of gene and protein expression and growth response to estrogens. Finally, the effect of miR-301a-3p expression on tumor growth was studied in nude mice. Results: We identified ESR1 as a direct target of miR-301a-3p. Ectopic miR-301a-3p causes a decrease in ESR1 mRNA and protein level and modulates the expression of ERα target genes in ERα positive breast cancer cells. Consistently, miR-301a-3p causes a decrease in sensitivity of MCF7 cells to 17β-estradiol and inhibits the growth of estrogen dependent tumor in nude mice. Yet, the mice tumors have significantly increased expression of genes related to cancer stem-like cells and epithelial to mesenchymal transition suggesting enrichment of the population of cells with more invasive properties, in line with our observation that miR-301a-3p expression is highly increased in mammospheres which show a decrease in estrogenic signaling. Importantly, miR-301a-3P level is also increased in primary breast cancer samples exhibiting an ER/PR negative phenotype. Conclusion: Our results confirm ESR1 as a direct target of miR-301a-3p and suggest that miR-301a-3p likely contributes to development of estrogen independence, which leads to a more invasive phenotype of breast cancer.

Mitochondrial electron transport chain (ETC) targeting shows a great promise in cancer therapy. It is particularly effective in tumors with high ETC activity where ETC-derived reactive oxygen species (ROS) are efficiently induced. Why modern ETC-targeted compounds are tolerated on the organismal level remains unclear. As most somatic cells are in non-proliferative state, the features associated with the ETC in quiescence could account for some of the specificity observed. Here we report that quiescent cells, despite increased utilization of the ETC and enhanced supercomplex assembly, are less susceptible to cell death induced by ETC disruption when glucose is not limiting. Mechanistically, this is mediated by the increased detoxification of ETC-derived ROS by mitochondrial antioxidant defense, principally by the superoxide dismutase 2 - thioredoxin axis. In contrast, under conditions of glucose limitation, cell death is induced preferentially in quiescent cells and is correlated with intracellular ATP depletion but not with ROS. This is related to the inability of quiescent cells to compensate for the lost mitochondrial ATP production by the upregulation of glucose uptake. Hence, elevated ROS, not the loss of mitochondrially-generated ATP, are responsible for cell death induction by ETC disruption in ample nutrients condition, e.g. in well perfused healthy tissues, where antioxidant defense imparts specificity. However, in conditions of limited glucose, e.g. in poorly perfused tumors, ETC disruption causes rapid depletion of cellular ATP, optimizing impact towards tumor-associated dormant cells. In summary, we propose that antioxidant defense in quiescent cells is aided by local glucose limitations to ensure selectivity of ETC inhibition-induced cell death.