HOW DO THE TUMOUR DERIVED EXOSOMES MEDIATE BREAST CANCER METASTASIS? - A NARRATIVE REVIEW

In 90% cases of breast cancer (BrCa), the most important cause of death due to cancer is metastatic disease. This review was conducted with the aim of illuminating our understanding of the role of exosomes in the process of metastasis in breast cancer. The mechanisms of metastasis are fundamentally multifaceted, with different distant sites having a specific extracellular matrix and cellular composition favourable for metastasis and very different in characters compared to the primary site. Systemic spread of cancer cells can happen early in breast cancer even before the detection of the primary tumour. The distant organ(s) is preconditioned to receive (seed) the tumour cells in circulation. The preparation is done by exosomes which carry different types of protein, lipids and nucleic acids which on fusing with the metastatic site’s tissues’ cells alter the microenvironment and make it conducive for metastatic cancerous cells to colonize. Tumour-derived exosomes (TEX) contents and numbers vary depending on current physiological or pathological conditions. The role of these TEXs is crucial in the steps of metastatic spread of a primary tumour, extending from reprogramming of primary malignant cells, to the preparation of pre-metastatic niches and to the final implanting of adjacent and distant sites. There is detachment of the cancer cells from the primary tumour site and migration into blood vessels and lymphatics followed by movement of cancer cell from circulation into a different tissue. The seeded cancerous cells can remain in dormancy till the internal homeostasis is maintained. When that is disrupted then there is development of micro metastasis and macro metastasis. All these processes mediated by exosomes are discussed in this paper.