Platelet TSP-1 Controls Prostate Cancer-Induced Osteoclast Differentiation and Bone Marrow-Derived Cell Mobilization through TGFβ-1

The development of distant metastasis is the main cause of prostate cancer (CaP)-related death with the skeleton being the primary site of metastasis. While the progression of primary tumors and the growth of bone metastatic tumors are well described, the mechanisms controlling pre-metastatic niche formation and homing of CaP to bone remain unclear. Through prior studies, we demonstrated that platelet secretion was required for ongoing tumor growth and pre-metastatic tumor-induce bone formation and bone marrow-derived cell mobilization to cancers supporting angiogenesis. We hypothesized that proteins released by the platelet granules were responsible for inducing changes in the pre-metastatic bone niche. We found that the classically anti-angiogenic protein thrombospondin (TSP)-1 was significantly increased in the platelets of mice bearing tumors. To determine the role of increased TSP-1, we implanted tumors in TSP-1 null animals and assessed changes in tumor growth and pre-metastatic niche formation. TSP-1 loss resulted in increased tumor size and enhanced angiogenesis but reduced bone marrow-derived cell mobilization and tumor-induced bone formation with enhanced osteoclast formation. We hypothesized that these changes in the pre-metastatic niche were due to the retention of TGF-{beta}1 in the platelets of mice with TSP-1 deleted. To assess the importance of platelet-derived TGF-{beta}1, we implanted CaP tumors in mice with platelet-specific deletion of TGF-{beta}1. Similar to TSP-1 deletion, loss of platelet TGF-{beta}1 resulted in increased angiogenesis with a milder effect on tumor size and BMDC release. Within the bone microenvironment, platelet TGF-{beta}1 deletion prevented tumor-induced bone formation due to increased osteoclastogenesis. Thus, we demonstrate that the TSP-1/TGF-{beta}1 axis regulates pre-metastatic niche formation and tumor-induced bone turnover. Targeting the platelet release of TSP-1 or TGF-{beta}1 represents a potential method to interfere with the process of CaP metastasis to bone.