Mitochondrial Proteins as Exosomal Cargo: New Breast Cancer Biomarkers & Crucial Players in Carcinogenesis?

Mitochondrial proteins (mtPs) packaged into extracellular vesicles (EVs), particularly exosomes, have emerged as promising biomarkers and functional mediators in cancer biology. In this study, we investigated whether breast cancer (BC)-derived exosomes selectively incorporate mtPs, thereby providing insights into their diagnostic potential and role in tumor progression. Exosomes were isolated from conditioned media of multiple BC cell lines and non-tumorigenic breast epithelial cells using ultracentrifugation and were characterized by transmission electron microscopy and flow cytometry for canonical markers CD9 and CD81. Proteomic profiling by MALDI-TOF mass spectrometry revealed a distinct enrichment of mtPs in exosomes derived exclusively from tumor cells, while non-tumorigenic cells lacked such cargo. Identified proteins were predominantly associated with the oxidative phosphorylation system, with Complex I subunits most frequently detected, whereas Complex II components were entirely absent. These findings highlight a cancer-specific mechanism of exosomal mtP incorporation, potentially reflecting altered energy metabolism and stress responses in malignant cells. We conclude that mtPs in exosomes represent both functional contributors to tumor progression and promising candidates for liquid biopsy-based BC diagnostics.