TSP-1 in Osteoclastogenesis and Osteolytic Bone Metastasis

Bone metastasis are frequent events occurring in several tumors, especially breast and prostate cancers. Invasion of tumor cells to the bone deregulates bone homeostasis - the continuous process of bone remodeling, finely controlled by the balance between bone-building osteoblasts and bone-degrading osteoclasts - leading to the development of osteolytic or osteoblastic metastasis characterized respectively by excessive bone degradation or deposition. Osteolytic lesions are the most common type of breast cancer bone metastasis. The matricellular protein thrombospondin-1 (TSP-1) plays a pivotal role in tumor microenvironment (TME) organization and tumor-TME interactions, with context-specific pro- and anti- tumor activities. TSP-1 is composed by multiple domains, each characterized by a specific pattern of interacting ligands, and subsequent biological activities, thus conferring to TSPs a pleiotropic role in the regulation of TME and metastasis. Several evidences confirm a crucial role of TSP-1 also in the TME of bone metastasis, potentially affecting metastasis development and bone remodeling. Aim of my PhD project was to study the activity of TSP-1 in osteolytic bone metastasis, focusing on its involvement in bone remodeling. I have identified a C-terminal fragment of TSP-1 – E123CaG-1 –able to counteract osteoclast differentiation in two models of osteoclastogenesis in vitro. E123CaG-1 bound RANK, the master receptor on osteoclast precursors that induces differentiation upon RANKL binding, and impaired the signaling downstream RANK (MAPKs cascade and NFATc1 nuclear translocation). We found that E123CaG-1 also bound and potentiated the inhibitory activity of osteoprotegerin (OPG), the endogenous inhibitor of osteoclastogenesis that acts as decoy receptor of RANKL. To study the activity of E123CaG-1 on osteolytic bone metastasis in vivo, the murine mammary cancer 4T1.2 cells were engineered to express and secrete the fragment. In a model of bone metastasis, following implantation of the tumor cells in the caudal artery of mice, tumor cells expressing E123CaG-1 showed significantly reduced osteolytic lesions and delayed formation of bone metastasis, indicating the role of the TSP fragment in metastasis-associated bone remodeling. Our findings point to a role of TSP-1 as a relevant player in the microenvironment of bone metastasis, with complex mechanisms that include inhibition of osteoclast differentiation and reduction of osteolytic lesions associated with bone metastasis mediated by the C-terminal fragment. Moreover, we have identified RANK and OPG as novel ligands of TSP-1, and showed that the TSP-1/RANK and TSP-1/OPG interactions can contribute, at least in part, to the ability of TSP-1 to prevent osteoclastogenesis and osteolytic metastasis. These findings pave the way for investigating new TSP-based therapies, affecting tumor-TME interactions involved in metastasis development and to protect the bone against deregulated degradation in cancer and possibly other pathologies.