BIGH3-induced Osteocyte Apoptosis and Gap Junction Failure Foster Osteolysis in Clear Cell Renal Cell Carcinoma (ccRCC) Bone Metastasis

Metastatic progression in bone involves crosstalk between tumor cells and osteoblasts, osteoclasts, and osteocytes. Renal cell carcinoma bone metastases (RCCBM) are predominantly osteolytic and often treatment resistant. We previously reported that inhibition of osteoblast differentiation by BIGH3 (transforming growth factor-beta-induced protein ig-h3) secreted by colonizing 786-O RCC cells promotes tumor progression in bone. Here we report that BIGH3 induces osteocyte apoptosis in both human specimens and mice and inhibits gap junction (GJ) protein Cx43 expression and function, disrupting bone homeostasis. As proof-of-concept, lysosomal inhibitors, such as hydroxychloroquine (HCQ) and NH4Cl, reversed BIGH3-mediated GJ dysfunction in vitro; and inhibited tumor growth and reversed Bo-786 RCC-induced osteolysis in vivo. HCQ in combination with CBZ, but not alone, reversed the RCCBM-mediated decreases in bone mineral density and osteoblast numbers, and decreased osteocyte apoptosis and osteoclast recruitment. These BIGH3 associated impacts on the bone microenvironment comprise a noncanonical mechanism of metastasis progression.