Role of Sclerostin in Multiple Myeloma Progression

Multiple Myeloma (MM) is a tumor of plasma cells that originates and expands within bone marrow (BM). The BM microenvironment, including its constituent stromal cells and cytokines contribute to disease progression. The BM stromal cells include osteolineage cells, osteoclasts, adipocytes, endothelial cells etc. Among these, cells of the osteoblast lineage have been identified as the most versatile regulators of hematopoietic cells. The osteolineage cells include preosteoblasts, committed osteoblasts, mature osteoblasts, and the terminally differentiated osteocytes, each with its own characteristic biological activity. The osteolineage cells have been shown to regulate commitment and differentiation of hematopoietic lineage cells including hematopoietic stem cells, B-cells, and myeloid lineage cells. Previously published data have shown that loss of parathyroid hormone (PTH) signaling in osteolineage cells adversely affects B-cell development (Panaroni et al., 2016). PTH is also a strong inhibitor of sclerostin, a Wnt-signaling inhibitor secreted by osteocytes. Moreover, mice lacking osteocytes show decreased lymphopoiesis, which may in part be regulated by sclerostin. A sclerostin-neutralizing antibody has shown promise for treatment for age-induced osteoporosis. We have recently demonstrated that MM patients have increased levels of sclerostin and that treatment with a SOST neutralizing antibody resulted in increased osteoblastic differentiation (Eda et al., 2015) and decreased tumor burden. Based upon these findings, we hypothesized that the osteolineage cells contribute to MM progression, and that this may in part occur via sclerostin. To test this hypothesis, we have used genetic mouse models of osteolineage or sclerostin deficiency. Mouse MM cell line 5TGM1 was used to examine MM progression in these mouse models. Global sclerostin deficient (SOST KO) mice were generated as described previously. Given that sclerostin acts as a negative regulator of bone formation, SOST KO mice displayed robust increase in trabecular and cortical bone density as analyzed by micro-computed tomography. Co-culture with 5TGM1 MM cells showed that the BM stromal cells from SOST KO mice significantly decreased proliferation of MM cells raising the possibility that sclerostin deficiency may decrease MM progression in vivo. The SOST KO mice showed significantly reduced BM cellularity compared to the controls. However, relevant to MM progression, overall B-lymphopoiesis, proliferation or apoptosis was unchanged during any stages of B-cell development in BM of 3-month old SOST KO mice. To determine if sclerostin deficiency alters MM engraftment in vivo, 5TGM1 cells were injected into the tibia of 3-month old control or SOST KO mice and engraftment was followed for 2-months. Complete blood counts (CBC) showed no significant changes between the control and SOST KO mice injected with the MM cells. The 5TGM1 cells were fluorescently tagged before in vivo injections allowing us to track the cells by whole body bioluminescence imaging and by contralateral tibia BM flow cytometry analysis. Both the analyses showed no changes in the distant MM engraftment between the control and the SOST KO mice. Ongoing work is examining local MM engraftment in the tibia with injected MM cells. These data suggest that the sclerostin deficiency does not alter systemic MM engraftment after 2 months of follow-up observations. It is possible, however, that the local progression of MM cells may have altered. We are investigating this possibility. We are also examining MM engraftment in mice lacking mature osteoblasts and osteocytes to identify sclerostin-dependent and independent factors controlling MM progression. Disclosures Raje: Millenium: Consultancy; Amgen: Consultancy; Celgene: Consultancy; Onyx: Consultancy.