Effect of the Wnt-Signaling Inhibitor Sclerostin and Bone Marrow Adipocytes on Multiple Myeloma

Multiple Myeloma (MM) is a plasma cell tumor that originates and expands within bone marrow (BM). MM occurs primarily later in life with a median age at diagnosis in the seventh decade characterized by loss of bone tissue due to osteolytic lesions. Recent studies have suggested a positive correlation of MM with obesity. BM fat cells originate, as bone cells, from a common progenitor called Mesechymal Stem Cell (MSC). MSC commitment into osteoblastic or adipose lineage is regulated by two major signaling pathways: Wnt signaling, which promotes bone formation, and PPARγ signaling, responsible for adipocyte differentiation. Adipogenesis and osteogenesis are inversely correlated processes, thus a signaling imbalance favoring differentiation towards one lineage will tilt this critical balance. In fact, the amount of marrow adiposity increases with age such that it is 30% of the BM volume in young adults but rises up to 70% in elderly people while bone formation reduces with age. We have recently demonstrated that MM patients have increased levels of a Wnt-signaling inhibitor sclerostin (SOST) and patient MSC differentiation into osteoblastic cells is improved in the presence of a SOST neutralizing antibody (Eda et al. JBMR 2015). Sclerostin is secreted by osteolineage cells and has also been shown to increase adipogenesis of an adipogenic cell line 3T3-L1 (Ukita et al. JCB 2016). However, the role of SOST on BM adipose tissue in MM patients has not been investigated. Here we show that elevated SOST levels, induced by MM cells, increase BM adipogenesis which, in turn, supports MM progression.

To assess the role of MM cells on MSC differentiation into adipocytes, MSCs from patients and normal donors (ND) were differentiated in vitro in the presence or absence of MM.1S, a human MM cell line. Presence of MM.1S cells significantly reduced osteogenic differentiation of MSCs as assessed by quantitative and qualitative Alizarin Red S staining; by contrast, the presence of MM.1S cells significantly enhanced adipogenesis in the MSCs as assessed by Oil Red O staining quantification. At the molecular level, we observed a 2-fold increase in PPARγ gene expression in MSCs from MM patients when compared to ND samples at baseline and a 5-fold increase when MSCs from patients and NDs were cultured for 72 hours in the presence of MM.1S in trans-well. To evaluate the role of SOST on BM adiposity, we intraperitoneally injected recombinant SOST, or PBS as control, into wild-type (WT) mice daily for 3 weeks. At the end of the treatment, mice treated with SOST showed a significant increase in BM adiposity. To validate MM cell induced adipogenesis in in-vivo settings, we injected MM.1S cells into the SCID-hu immunodeficient mouse model. Four weeks after cell injection, increased BM adipocytes were observed in MM.1S treated immunodeficient mice. Interestingly, when mice injected with MM.1S cells were treated for 3 weeks with SOST neutralizing antibody the level of BM adiposity returned to the healthy control levels. Finally, MM cell engraftment and tumor development was analyzed in SOST knock-out (KO) mice. While both SOST KO and WT animals showed MM cell engraftment and extramedullary plasmacytoma formation, preliminary results suggest a lower level of MM cell engraftment in BM of SOST KO mice.

Our data suggest that sclerostin secretion increases BM adipogenesis supporting MM cell growth and survival and therefore may play a critical role in the development and progression of MM.