Endothelial remodeling in osteosarcomas: insights from patient samples and in vitro studies

Osteosarcomas (OS) are the most frequent malignant primary bone sarcomas with an overall poor prognostic for high-risk patients. The current therapeutic management combining chemotherapy and surgery remains partially inefficient. OS are very heterogeneous tumors, evolving in a complex and specific highly vascularized microenvironment. Upon microenvironmental signals, remodeling of tumor vessels may occur through angiogenic processes but also through endothelial differentiation process namely the endothelial-to-mesenchymal transition (EndoMT). In a patient cohort of ten high-grade OS samples (at diagnosis, after surgery, and/or metastasis), we detected by a multiplexing immunohistochemistry approach the presence of endothelial cells co-expressing endothelial CD31/EMCN and mesenchymal ASMA/FSP1 markers. In order to partially mimic an OS microenvironment in vitro, we exposed human umbilical vein endothelial cells (HUVECs) to secreted factors of OS tumor or stromal cells. In this cellular model, we established that the secretome from stromal cells did not induce EndoMT in primary ECs. Nevertheless, soluble factors from the OS cell line KHOS were able to induce in ECs some of the EndoMT hallmarks such as induction of mesenchymal markers associated to increased migration, but without inhibition of tubulogenesis. In conclusion, this study identified the presence of endothelial-mesenchymal cells in the tumor microenvironment of OS patients and give cues for further investigation of the regulation and consequences of this remodeling in the biology of OS.