The Extracellular Vesicles (EVs) and their role in primary and metastatic bone tumors

Extracellular vesicles (EVs) are defined as spherical structures surrounded by a lipid bilayer released by cells into various biological fluids. EVs represent an important means of communication and transport of signal molecules between cells, playing a crucial role as mediators of many physiological and pathological processes, including cancer. In recent years, several studies have been emerging on their involvement in the onset and progression of primary bone tumors and metastases, both addressed in Chapter 1 of this PhD thesis. In a previous work conducted in our lab, it was shown that there is extensive EV-mediated communication among the cells of the bone tissue. Since the latter is one of the most common sites for breast cancer metastases, we set out to evaluate the EVs-mediated interaction between MDA-MB-231 cells, a human osteotropic breast cancer cell line, and bone resident cells. In particular, we showed that EVs from MDA-MB-231 have a direct role on bone cell physiology, stimulating osteoclastogenesis, angiogenesis, and inhibiting osteoblast differentiation. We also observed that soluble factors, released by MDA-MB-231 cells, educate osteoblasts, increasing the osteoclastogenic potential of the EVs isolated from the latter and thus creating a microenvironment conducive to bone destruction and to tumor growth. These results are reported in more detail in Chapter 2. Subsequently, we focused on the mechanisms underlying the progression of primary bone tumors, studying the EVs-mediated interaction between the human osteosarcoma cell line MNNG/HOS and the bone cells. We treated osteoblasts with EVs from MNNG/HOS, observing a significant reduction in their number, as well as their metabolic and alkaline phosphatase (Alp) activity. We found a reduction in the expression of genes associated with the cell cycle and osteoblast differentiation. Furthermore, we observed that osteoblasts were induced by tumor EVs to produce and release pro-inflammatory and pro-tumoral factors in the bone microenvironment. In addition to these results, described in Chapter 3, we found that EVs from MNNG/HOS increase the expression of Serpin b2 in osteoblasts, a protein often overexpressed in tumor tissues and still under our investigation. Furthermore, we found that EVs from MNNG/HOS promote angiogenesis, both in vitro and in vivo. In conclusion, the results obtained during my PhD underline the importance of tumor-derived EVs in the crosstalk between cancer cells and resident cells in the bone microenvironment, and show their role in influencing bone cells behavior, in particular of osteoblasts, and their release of determining factors, hopefully useful in advancing the current research on therapeutic and prognostic potential of EVs in cancer treatment.