Prostate cancer preferentially metastasizes to bone, resulting in high mortality. Strategies to inhibit prostate cancer metastasis include targeting both tumor-induced osteoblastic lesions and underlying osteoclastic activities. We and others have previously shown that blocking receptor activator of nuclear factor-kappaB ligand (RANKL) partially blocks tumor establishment and progression in bone in murine models. However, levels of RANKL in the cell lines used in these studies were very low, suggesting that soluble factors other than RANKL may mediate the cancer-induced osteoclast activity. To identify these factors, a human cytokine antibody array was used to measure cytokine expression in conditioned medium collected from primary prostate epithelial cells (PrEC), prostate cancer LNCaP and its derivative C4-2B, and PC3 cells. All prostate cancer cells produced high amounts of monocyte chemotactic protein-1 (MCP-1) compared with PrEC cells. Furthermore, levels of interleukin (IL)-6, IL-8, GROalpha, ENA-78, and CXCL-16 were higher in PC3 than LNCaP. These results were confirmed by ELISA. Finally, human bone marrow mononuclear cells (HBMC) were cultured with PC3 conditioned medium. Although both recombinant human MCP-1 and IL-8 directly stimulated HBMC differentiation into osteoclast-like cells, IL-8, but not MCP-1, induced bone resorption on dentin slices with 21 days of culture in the absence of RANKL. However, the conditioned medium-induced bone resorption was inhibited by MCP-1 neutralizing antibody and was further synergistically inhibited with IL-8 antibody, indicating that MCP-1, in addition to IL-8, mediates tumor-induced osteoclastogenesis and bone resorption. MCP-1 may promote preosteoclast cell fusion, forming multinucleated tartrate-resistant acid phosphatase-positive osteoclast-like cells. This study may provide novel therapeutic targets for treatment of prostate cancer skeletal metastasis.