Bidirectional Notch Signaling and Osteocyte-Derived Factors in the Bone Marrow Microenvironment Promote Tumor Cell Proliferation and Bone Destruction in Multiple Myeloma

Abstract

In multiple myeloma (MM) increased numbers of monoclonal plasma cells in the bone marrow induce localized osteolytic lesions that rarely heal, due to increased bone resorption and suppressed bone formation. Numerous studies reported the contributions that different cell types in the MM microenvironment make to MM growth and bone disease, but the role of matrix-embedded osteocytes in MM, which comprise >95% of bone cells and are major regulators of osteoclast and osteoblast activity, is unclear. We report that osteocytes in MM-bearing bones physically interact with MM cells in vivo, undergo caspase3-dependent apoptosis, and express higher RANKL and Sclerostin levels than osteocytes from control mice. Mechanistic studies revealed that osteocyte apoptosis is initiated by activation of Notch signaling in osteocytes through direct contact with MM cells, and is further amplified by MM cell-secreted TNFα. This Notch/TNFα induced osteocyte apoptosis increases osteocytic Rankl expression, the osteocytic Rankl/Opg ratio and the ability of osteocytes to attract osteoclast precursors to induce local bone resorption. Further, osteocytes in contact with MM cells express high levels of Sost/Sclerostin that decrease Wnt signaling in osteoblasts and inhibit osteoblast differentiation. Importantly, direct contact between osteocytes and MM cells reciprocally activates Notch signaling and increases Notch receptor expression in MM cells, in particular Notch3 and 4, and stimulates MM cell growth. These studies reveal a previously unknown role for bidirectional Notch signaling between MM cells and osteocytes that enhances MM growth and bone disease, and suggest the potential of targeting osteocyte-MM cell interactions as a novel MM treatment.

Inhibiting the osteocyte-specific protein sclerostin increases bone mass and fracture resistance in multiple myeloma

“…Interestingly, local osteocyte production of sclerostin has been shown to be increased when osteocytes interact directly with cancer cells. 57 This demonstrates that regulation of sclerostin expression in bone in the presence of tumor is complex, with both local and generalized responses, suggesting that sclerostin expression may be more heterogeneous in the presence of myeloma. However, the strong expression of Sost in both myelomabearing and control bones provided the rationale for investigating the ability of anti-sclerostin antibody to prevent myeloma bone disease.…”

The lateral meningocele syndrome mutation causes marked osteopenia in mice

“…Notch3 promotes tumor cell proliferation and has been implicated in the bone invasive potential of carcinoma of the breast and multiple myeloma and in the pathogenesis of mandibular torus (27,(54)(55)(56)(57). Mutations in the extracellular domain of NOTCH3 cause cerebral autosomal-dominant arteriopathy with subcortical infarcts and leukoencephalopathy syndrome (CADASIL) (58)(59)(60).…”

Role and mechanism of action of sclerostin in bone

“…Serum sclerostin levels are elevated in MM patients, which correlate with reduced osteoblast function and poor survival (124). Recent studies demonstrate that osteocytes contribute to MM generating a microenvironoment that is conducive to enhanced bone resorption and suppressed bone formation, and that osteocytes in mice bearing MM tumors express elevated Sost/sclerostin (125). Further, direct cell-to-cell contact with MM cells increases Sost/sclerostin expression in osteocytes and sclerostin accumulated in culture media decreases Wnt signaling and inhibits osteoblastic gene expression.…”