Beta-Catenin Depended and Independent Effects Induced by Myeloma Cells in Human and Murine Osteoblasts and Osteoblast Progenitors.

Osteoblast impairment occurs within myeloma (MM) cell infiltration into the bone marrow (BM). Wnt signaling is involved in the regulation of osteoblast formation. Canonical Wnt signaling pathway is activated by Wnt 1/3a that induce the activation of GSK3/Axin complex leading to the stabilization and nuclear translocation of beta-catenin that in turn activates the transcription system Lef1/TCF. Recently it has been reported that MM cells produce the Wnt inhibitors DKK-1 demonstrating a correlation between its expression and the presence of bone lesions in MM patients. However the effect of MM cells on Wnt signaling cascade in osteoblasts and osteoblast progenitors has not been investigated.

To clarify this issue, first we checked DKK-1 production by human myeloma cell lines (HMCLs), purified CD138+ MM cells and BM plasma of MM patients by PCR and ELISA. Following we performed a co-culture system with HMCLs or CD138+ MM cells and either human osteoblast line (HOBIT) and with BM osteoprogenitor cells (PreOB) obtained after differentiation from mesenchymal cells or murine osteoprogenitor cell lines C2C12 and MC3T3. Both DKK-1 positive HMCLs (XG-1 and JJN3) and negative ones (RPMI-8226, OPM-2) have been used in co-culture as well as DKK-1 positive and negative purified CD138+ MM cells. Similarly we tested the effect of BM plasma of MM patients positive and negative for DKK-1 production on both human and murine cells. Wnt signaling in osteoblasts and osteoblast progenitors was evaluated either at mRNA level by specific human and murine Wnt Array kits and by quantitative PCR or at protein one by Western blot analysis for GSK3b/Axin and LEF-1/TCF expression. We evaluated active de-phosphorylated beta-catenin and inactive phosphorilated one by westernblot and by ELISA in cytosolic and nuclear extracts.

DKK-1 median levels detected in the conditioned media of XG-1 and JJN3, MM cells and in BM plasma of DKK-1 positve MM patients were 0.60 ng/mL and 0.38 and 8.84 (range: 1.55–91) ng/mL respectively. Any significant inhibitory effect on WNT signaling and active beta-catenin expression and levels was not observed in HOBIT and human PreOB after co-culture with both HMCLs and MM cells or BM plasma independently to DKK-1 expression. On the contrary DKK-1 positive MM cells or BM plasma suppressed active beta-catenin expression in murine osteoprogenitor cell lines in presence of BMP-2. Consistently Wnt3a stimulation as well as anti-DKK-1 abs. did not restore the inhibitory effects on osteoblast formation and differentiation induced by MM cells in human PreOB. Consistently any significant difference was not detected on beta-catenin expression by stromal/osteoblastic cells on bone biopsies by immunohistochemistry between osteolytic (n°=10) and non-osteolytic (N°=10) MM patients.

The different behavior between human and murine osteoblastic cells was further investigated. We found that both cells expressed significant levels of active beta-catenin however DKK-1 suppressed active nuclear and cytosol beta-catenin at concentration of 20–30 ng/mL in C2C12 and MC3T3 whereas only DKK-1 concentrations higher to 500 ng/mL are able to inhibited beta-catenin in HOBIT and human PreOB as well as osteoblast formation and differentiation in human BM cultures.

In conclusion our data indicate that MM cells block canonical Wnt signaling in murine osteoblastic cells but not in human osteoblasts and osteoblast progenitors. Beta-catenin independent mechanisms could be involved in DKK-1 mediated bone destruction in MM patients.