Mitochondrial Oxidative Stress Alters the Phosphoproteome and Reverses Epithelial–Mesenchymal Transition in Human Breast Cancer Cells via the β-Catenin/c-Myc Axis

Mitochondrial redox status plays a critical role in cancer progression, yet the effects of mitochondrial oxidative stress on the epithelial-mesenchymal transition (EMT), a key step in metastasis, remain elusive. We have investigated the phosphoproteomic landscape of breast cancer cells exposed to mitochondrial oxidative stress induced by mitochondria-targeted curcumin (mitocurcumin (MC)) and explored its potential as a druggable target. Mitocurcumin led to altered cell morphology, reduced migration, and shift to a cobblestone-like epithelial morphology, indicating EMT reversal. Label-free mass spectrometry-based global phosphoproteomic analysis revealed upregulation of phosphoproteins involved in DNA damage-related processes, transcription termination, and induction of oxidative stress, while downregulated phosphoproteins were linked to translation and amino acid metabolism. Processes related to EMT reversal, such as cytoskeleton organization, cell-cell adhesion, focal adhesion assembly and cell-cell junction organization/assembly, and establishment/maintenance of epithelial cell apical-basal polarity, were enriched. The RAC2/RAC3 GTPase cycle, important for cell migration, was downregulated, along with the Wnt/β-catenin signaling pathway. A decrease in the levels of mesenchymal markers, β-catenin, and its target gene, c-Myc, confirmed the suppression of the mesenchymal phenotype. In conclusion, mitochondrial oxidative stress inhibits the migratory capacity of breast cancer cells by targeting the β-catenin/c-Myc axis, indicating its potential as a novel druggable target to prevent cancer metastasis.