Exosomal MicroRNA: Diagnostic Potential and Role in Breast Cancer Dissemination

Liquid biopsy, which analyzes tumor secretomes in biological fluids, allows us to not only diagnose cancer, but also evaluate the effectiveness of antitumor therapy, predict the prognosis of the disease, and select targeted therapy. One of the promising sources for identifying tumor markers using liquid biopsy is exosomes—small extracellular vesicles (sEVs) (30–150 nm in size) that are secreted by all types of cells, including tumor cells, to exchange information. It is known that during the maturation process, mainly biologically active proteins and non-coding RNA are packaged into exosomes, and tumor cells secrete significantly more exosomes than normal cells. Taking into account the involvement of microRNAs in the mechanisms of carcinogenesis, their high stability in EVs, and ease of detection, exosomal microRNAs are the most promising tumor markers for creating panels that can serve as a guide both for clarifying diagnostics and for making therapeutic decisions on effective cancer treatment, including breast cancer (BC). The purpose of this review is to summarize information on the shortcomings of modern methods for diagnosing early BC, the involvement of exosomal microRNAs in BC dissemination (impact on the immune system, epithelial–mesenchymal transition, proliferation, invasion, migration, angiogenesis, and metastasis), and the high diagnostic potential of exosomal microRNAs for detecting early BC.