Constitutive Activation of p38 MAPK in Myeloma Cells Contributes to Myeloma-Induced Osteolytic Bone Lesions.

Abstract 740

Bone destruction is common in osteolytic tumors such as multiple myeloma. Myeloma cells that reside in the bone cause osteolysis, which is characterized by severe bone pain, multiple fractures, and hypercalcemia in patients and thereby, compromises patient quality of life. While myeloma cells have been proposed to activate osteoclasts and/or inhibit osteoblasts leading to bone resorption, the mechanism underlying myeloma-induced osteolytic bone lesions is still poorly elucidated. As p38 mitogen-activated protein kinase (MAPK) is constitutively activated in myeloma, we hypothesized that p38 MAPK activation in myeloma cells might be responsible for myeloma-induced osteolytic bone lesions. Human myeloma cell lines ARP-1 and MM.1S caused bone lesions in SCID mice after intravenous injection. However, once p38 MAPK was knocked down by small hairpin RNAs (shRNAs) in these cell lines, myeloma was established but failed to cause bone lesions in SCID mice, whereas the wild-type or vector control myeloma cells established myeloma (with similar tumor burdens as myeloma cells with knocked-down p38 MAPK) and caused bone lesions in tibial bones, as measured by radiograph and peripheral quantitative m-computed tomography and further confirmed by histological examination. In vivo examination revealed that the number of bone surface-covering osteoclasts, osteoclast size, cellular nuclear numbers in per osteoclast, and levels of circulating collagen type I (bone resorption marker) and TRAP5b (active osteoclast marker) were all reduced in mice injected with myeloma cells with knocked-down p38 MAPK as compared with controls. Consistently, we found that tumor p38 MAPK had active effects on in vitro osteoclast differentiation and bone resorption, and inhibited osteoblast differentiation and function, suggesting that constitutive activation of p38 MAPK in myeloma cells induces osteoclastogenesis. By functional studies and protein array analysis, we showed that monocyte chemotactic protein-1 (MCP-1) and dickkopf-1 (DKK-1) are upregulated downstream of p38 MAPK activation in myeloma cells and are responsible for enhanced osteoclastogenesis. Our results showed that p38 MAPK activation in myeloma cells upregulate MCP-1 and DKK-1 expression and production, which are released into the microenvironment. MCP-1 enhanced RANK expression on osteoclast precursors and DKK-1 increased RANKL secretion from stromal cells, all of which led to activation of NF-kB and MAPK signaling pathways in osteoclasts. In vivo blockade of MCP-1 and DKK-1 by specific antibodies significantly abrogated tumor p38 MAPK-induced osteoclast activation and bone lesions in established myeloma-SCID mice. Thus, our results have elucidated a novel mechanism that p38 MAPK activity in myeloma cells contributes to myeloma-induced osteolytic bone lesions. This study indicates that disruption of tumor p38 MAPK may be a new therapeutic approach to treat osteolytic bone lesions in myeloma.