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Abstract
Since its discovery, mitophagy has been viewed as a protective mechanism used by cancer cells to prevent the induction of mitochondrial apoptosis. Most cancer treatments directly or indirectly cause mitochondrial dysfunction in order to trigger signals for cell death. Elimination of these dysfunctional mitochondria by mitophagy could thus prevent the initiation of the apoptotic cascade. In breast cancer patients, resistance to doxorubicin (DOX), one of the most widely used cancer drugs, is an important cause of poor clinical outcomes. However, the role played by mitophagy in the context of DOX resistance in breast cancer cells is not well understood. We therefore tried to determine whether an increase in mitophagic flux was associated with the resistance of breast cancer cells to DOX. Our first objective was to explore whether DOX-resistant breast cancer cells were characterized by conditions that favor mitophagy induction. We next tried to determine whether mitophagic flux was increased in DOX-resistant cells in response to DOX treatment. For this purpose, the parental (MCF-7) and DOX-resistant (MCF-7dox) breast cancer cell lines were used. Our results show that mitochondrial reactive oxygen species (ROS) production and hypoxia-inducible factor-1 alpha (HIF-1 alpha) expression are higher in MCF-7dox in a basal condition compared to MCF-7, suggesting DOX-resistant breast cancer cells are prone to stimuli to induce a mitophagy-related event. Our results also showed that, in response to DOX, autophagolysosome formation is induced in DOX-resistant breast cancer cells. This mitophagic step following DOX treatment seems to be partly due to mitochondrial ROS production as autophagolysosome formation is moderately decreased by the mitochondrial antioxidant mitoTEMPO.
Highlights
We evidenced a structural impairment of respiratory chain complexes in MCF7-dox cells since blue native PAGE analysis revealed a significant reduction of the quantity of complex I and complex IV holoenzyme in MCF-7dox compared to MCF-7dox than in their DOX-sensitive counterparts (MCF-7) (−71% and −60%, respectively) (Figure 1c)
We show that in DOX-resistant breast cancer cells, autophagolysosome formation is increased in response to DOX treatment
It seems that DOX-resistant breast cancer cells are prone to stimuli to initiate mitophagy since mitochondrial reactive oxygen species (ROS) production and HIF-1 alpha expression are higher in these cells in a basal condition
Summary
Mitophagy appears to be tumor-promoting or tumorsuppressive depending on tumor type or stage of progression. Preserving a basal level of mitophagy facilitates cell survival, but excessive mitophagy may activate apoptosis pathways [2]. In breast cancer, the most commonly diagnosed cancer in women worldwide, it has been shown that reduced/defective mitophagy promoted tumor growth and metastasis and predicted poor metastasis-free survival [3,4]. Data showed activation of mitophagy reduced drug-induced apoptosis in breast cancer cells, suggesting that once a tumor has progressed to an advanced stage, mitophagy may mitigate the treatment-induced stress and participate in the chemoresistance of cancer cells [5,6]
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