Myeloma Cells Exhibit Inhibitory Effect On Osteoclastogenesis

Abstract 439

In multiple myeloma (MM), an incurable bone malignancy characterized by plasma cell accumulation in the bone marrow (BM), patients have increased osteoclast (OC) activity. As a result, more than 80% of MM patients develop osteolytic bone lesions during the course of the disease. Osteolytic bone lesions cause morbidity, such as pathological fractures, bone pain, and hypercalcemia, and therefore, severely affect the patients' quality of life. A better understanding of the mechanism of MM cell-induced OC activation could lead to a novel approach to treating MM bone disease. It is commonly accepted that MM cells are responsible for OC activation. Cocultures of MM cells with monocyte-derived OC precursors (preOCs) induce OC formation and bone resorption. In addition, previous studies have demonstrated the critical roles of receptor activator of nuclear factor κB ligand (RANKL) and receptor activator of nuclear factor κB (RANK) in aberrant OC activity upregulation in MM BM. Moreover, MM-derived cytokines, such as IL-3, IL-7, and MIP-1d, have been shown to enhance OC formation in a RANKL-dependent or -independent manner. When we cocultured MM cells with monocytes, but not preOCs, and treated them with RANKL, we made the novel observation that MM cells inhibited RANKL-induced OC differentiation. Specifically, human monocytes, isolated from five different healthy donor PBMCs or murine monocytic/macrophage cell line RAW264.7 cells, were transwell-cocultured with MM cells, either human MM cell lines or primary MM cells isolated from patients, in mediums with or without RANKL (50 mg/mL) for 14 days. Mature OCs, characterized as TRAP⁺ multinuclear cells, were detected by TRAP staining and further confirmed by quantitative RT-PCR for the expressions of mature OC marker genes, such as CTSK, CALCA, and TRAP. Our results showed that coculture with MM cells inhibited the development of mature OCs from monocytes, and suppressed RANKL-induced NFκB and JNK activation. By ELISA, we analyzed the levels of soluble cytokines in the conditioning medium of MM cells and found that MM cells produced large amounts of IL-10. Adding neutralizing antibody against IL-10 significantly abrogated MM inhibition of osteoclastogenesis, whereas adding IL-10 inhibited OC differentiation and downregulated the mRNA and protein levels of RANK on monocytes/preOCs. As MM cells grow in the BM, we wondered whether bone marrow stromal cells (BMSCs) could regulate the MM cell inhibitory effect on osteoclastogenesis. Our results showed that although BMSC itself did not affect osteoclastogenesis, cocultures of monocytes with both MM cells and BMSCs significantly restored RANKL-induced osteoclastogenesis. Analysis of soluble cytokines by ELISA showed that the levels of MCP-1, but not IL-10, were significantly upregulated in medium from MM/BMSC cocultures. Adding neutralizing antibody against MCP-1 highly inhibited OC activation, while adding MCP-1 enhanced OC differentiation and upregulated the expression of RANK on monocytes/preOCs. Overall, our findings are the first to elucidate a novel mechanism by which MM cells inhibit osteoclastogenesis by producing IL-10 and the presence of BMSCs suppresses MM inhibitory effects by up-regulating MCP-1. Regulation of RANK expression on monocytes/preOCs by MCP-1 and IL-10 determines osteoclastogenesis. Our studies provide evidence that targeting BM microenvironmental cells and/or factors may be a new approach to treating MM bone lesions.