P38 MAPK Activity in Myeloma Cells Regulates Osteoclast and Osteoblast Activity and Induces Bone Destruction in Vivo

Abstract 566

Bone destruction is a hallmark of multiple myeloma (MM). More than 80% of MM patients have osteolysis, which is characterized by pathological fractures, severe bone pain, spinal cord compression, and hypercalcemia. These symptoms can severely compromise a patient's quality of life and performance status. It has been proposed that MM cells activate osteoclast (OC)-mediated bone resorption and inhibit osteoblast (OB)-mediated bone formation. However, the mechanism underlying the association of MM cells with development of bone lesions remains poorly elucidated. Our previous studies showed that p38 mitogen-activated protein kinase (MAPK), which is constitutively activated in MM cells, is a master regulator of MM-mediated bone destruction. Knocking down or inhibiting p38 MAPK activity in MM cells prevented MM-induced bone destruction in vivo. In the present study, we further investigated the mechanism of MM cell p38 MAPK-induced bone destruction. We hypothesized that p38 MAPK activity in MM cells can regulate OB and OC differentiation and activity by upregulating cytokine production by MM cells. In a cytokine array analysis, we examined the expression and secretion of MM-derived cytokines that regulate OB and OC differentiation. Our results showed for the first time that either knockdown or inhibition of p38 MAPK activity by p38 MAPK short hairpin RNAs or inhibitors significantly downregulated the production of dickkopf-1 (DKK-1) and monocyte chemotactic protein-1 (MCP-1) by MM cells. Real-time PCR and ELISA quantified and confirmed the array analysis results. To determine the role of p38 MAPK-upregulated DKK-1 and MCP-1 production in bone destruction, we administered treatment with neutralizing antibodies to SCID mice injected intravenously with ARP-1 or MM.1S cells. Our results showed that neutralization of DKK-1 and MCP-1 led to fewer bone lesions in these mice. Furthermore, we examined the impact of MM cell p38 MAPK activity on OB and/or OC differentiation. Our results showed that knockdown or inhibition of MM cell p38 MAPK significantly downregulated osteoclastogenesis but upregulated osteoblastogenesis in vitro and in vivo. Although DKK-1 is well known to inhibit OB differentiation, we found that DKK-1, together with MCP-1, promoted OC differentiation and bone resorption. Mechanistic studies further showed that MCP-1 upregulated RANK expression in OC precursors and that DKK-1 increased RANKL secretion from stromal cells and mature OBs, all of which led to activation of the NF-kB and MAPK signaling pathways in OCs. Thus, our study uncovered a novel mechanism by which p38 MAPK signaling in MM cells regulates osteoblastogenesis, osteoclastogenesis, and bone destruction in patients with this disease. These findings strongly suggest that disrupting and targeting MM cell p38 signaling are effective approaches to treating osteolytic bone lesions in MM patients.