Increased Sclerostin Secretion in Multiple Myeloma Plays a Central Role in Osteolytic Bone Disease

Abstract 3989

Bone Marrow Stromal Cells (BMSC), Osteoblasts (OB) and osteoclasts (OC) are a central part of the bone microenvironment and play a crucial role in multiple myeloma (MM) growth and survival. Their imbalance results in osteolytic lesions. Understanding the mechanisms underlying osteolytic lesions is important not only for the improvement of osteolytic bone disease but also for the treatment of MM.

The osteocyte-secreted protein sclerostin, encoded by the SOST gene, is a potent inhibitor of osteoblastogenesis.

However, the role of Sclerostin in MM remains to be elucidated. Our objective was to evaluate the role of sclerostin in MM bone disease and confirm that sclerostin directed strategies are an effective approach in MM.

We observed higher levels of sclerostin in MM patients' plasma compared to leukemia patients, gastric cancer patients and healthy volunteers. Importantly, sclerostin levels were associated with an increase in tumor burden suggesting that MM cells are associated with the increase levels of sclerostin. Sclerostin concentrations similar to those detected in MM patients' plasma inhibited OB differentiation and an anti-sclerostin neutralizing antibody (R&D Systems) reversed this effect. Furthermore, sclerostin increased TRAP positive OC numbers differentiated from MM patients' peripheral blood mononuclear cell (PBMC) and their function as detected by pit formation assay. This was associated with stimulation of Ca2+/calmodulin-dependent protein kinases II (CaMKII) and c-Jun N-terminal kinase (JNK) signaling in preosteoclasts reversed by specific inhibitors with consequent inhibition of osteoclastogenesis. Moreover, sclerostin stimulated JNK and CaMKII phosphorylation, stimulated mRNA expression of RANKL and inhibited mRNA expression of OPG in MM patient derived BMSC. RANKL plays a crucial role in promoting OC differentiation and OPG, the decoy receptor for RANKL, inhibits OC differentiation; therefore our results indicate that sclerostin accelerates OC differentiation by JNK and CaMKII signaling stimulation in BMSC in addition to its direct affect against OC.

We next examined OB derived from MM patients' BMSC cocultured with the MM cell line INA6 by using cell culture inserts to avoid cell-cell contact. INA6 inhibited OB differentiation and sclerostin neutralizing antibody reversed the INA6 effect as assessed by qPCR and alizarin red staining. Interestingly, co-culture with MM cells stimulated sclerostin mRNA expression and sclerostin protein expression in OB well as in OB cocultures with MM cells. Moreover recombinant CCL3 protein stimulated sclerostin mRNA expression in MM cells. Because CCL3 is secreted by MM cells, these data suggest in part the mechanism by which sclerostin is increased in MM-OB cocultures.

These data suggest sclerostin is secreted by MM cells and OB and inhibits osteoblastogenesis and stimulates osteoclastogenesis directly and indirectly. Neutralizing sclerostin levels reverses these effects. Taken together, our data suggest that sclerostin is a good target to inhibit myeloma bone disease and help restore normal bone homeostasis.