Role of integrin VLA-6 in suppression of bone formation in myeloma

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 DESCRIPTION (provided by applicant): The goal of this project is to investigate the mechanism by which myeloma cells alter the balance of adipogenesis and osteoblastogenesis, thereby suppressing bone formation. Multiple myeloma is characterized by a clonal expansion of malignant plasma cells. Bone destruction is a hallmark of this disease, and has a severe impact on patients' quality of life and survival. Unfortunately, current treatment only offers moderate palliative effects, and bone disease associated with myeloma remains incurable. The bone changes in myeloma patients results from increased osteoclast-mediated bone resorption and decreased osteoblast-mediated bone formation. In particular, new bone formation that usually occurs at sites of previously resorbed bones is deeply suppressed; as a result areas of bone destruction rarely heal. Previous studies have shown that myeloma cells inhibit osteoblast differentiation from mesenchymal stem cells (MSCs) by myeloma cells produce dickkopf-1, which can suppress Wnt/beta-catenin signaling pathway. However, the role of dickkopf-1 in suppressed bone formation remains controversial, because myeloma cells alone do not produce sufficient dickkopf-1 to suppress osteoblast differentiation, and administration of anti-dickkopf-1 antibody in patients failed to completely restore new bone formation, indicating there must be an additional mechanism for inhibition of osteoblast differentiation seen in myeloma. We observed that myeloma patients and myeloma-bearing mice had not only decreased osteoblastic deposits but also increased adipocytic deposits in bone marrow. In vitro co-culture of MSCs with myeloma cells not only inhibited osteoblast but also enhanced adipocyte differentiation. We further identified that integrin alpha-6 in myeloma cells is a novel factor to regulate adipocyte and osteoblast differentiation. Alpha-6 can bind with alpha-1 to form the integrin very late antigen-6, a cell surface heterodimer highly expressed in myeloma cells. We observed that p38 MAPK, a signaling pathway that is shown to inhibit osteoblastogenesis, upregulated alpha-6 expression. Blocking alpha-6 by an antibody significantly reduced adipocyte but enhanced osteoblast differentiation. We therefore hypothesize that myeloma cell alpha-6 enhances adipocyte differentiation, as a result less MSC differentiation into osteoblasts and less new bone formation. We propose to determine the role and mechanism of alpha-6 in adipogenesis and osteoblastogenesis, and develop the novel strategies to enhance bone formation by targeting alpha-6 and the related signaling molecules. The knowledge gained with the successful completion of the proposed work will provide a rationale for a therapeutic to enhance bone formation, filling in the defects caused by myeloma-induced osteoclast-mediated resorption. Of equal importance, as the increased adipocytic deposit is also found in bone marrow of patients with bone-metastatic breast cancer and non-malignant diabetes mellitus and obesity, the proposed work may have direct relevance to reduced bone formation observed in these diseases, and targeting alpha-6 may prove to be a novel approach in treating these patients also.