Cabozantinib Reverses Renal Cell Carcinoma-Mediated Osteoblast Inhibition in Three-Dimensional Co-culture In Vitro and Reduces Bone Osteolysis In Vivo

Renal cell carcinoma (RCC) bone metastases (RCCBM) are typically osteolytic. We previously showed that BIGH3 (beta Ig-h3/TGFbI), secreted by 786-O RCC, plays a role in osteolytic bone lesion in RCCBM through inhibition of osteoblast (OSB) differentiation. To study this interaction, we employed three-dimensional (3D) hydrogels to co-culture bone-derived 786-O RCC cells (Bo-786) with MC3T3-E1 pre-osteoblasts (OSB). Culturing pre-osteoblasts in the 3D hydrogels preserved their ability to differentiate into mature OSB; however, this process was decreased when pre-osteoblasts were co-cultured with Bo-786 cells. Knockdown of BIGH3 in Bo-786 cells recovered OSB differentiation. Further, treatment with bone morphogenetic protein 4 (BMP4), which stimulates osteoblast differentiation, or cabozantinib (CBZ), which inhibits VEGFR1 and MET tyrosine kinase activities, also increased OSB differentiation in the co-culture. CBZ also inhibited pre-osteoclast RAW264.7 cell differentiation. Using RCCBM mouse models, we showed that CBZ inhibited Bo-786 tumor growth in bone. CBZ treatment also increased bone volume and OSB number, and decreased osteoclast number and blood vessel density. When tested in SN12PM6 RCC cells that have been transduced to overexpress BIGH3, CBZ also inhibited SN12PM6 tumor growth in bone. These observations suggest that enhancing OSB differentiation could be one of the therapeutic strategies for treating RCCBM that exhibit OSB inhibition characteristics, and that this 3D co-culture system is an effective tool for screening osteoanabolic agents for further in vivo studies.