Multiple myeloma (MM) is characterized by the impaired osteogenic differentiation of human mesenchymal stromal cells (hMSCs). Canonical Wnt signaling is critical for the regulation of bone formation, however, recent evidence suggests that the non-canonical Wnt agonist Wnt5a stimulates human osteoblastogenesis through its co-receptor Ror2. The effects of MM cells on non-canonical Wnt signaling and the effect of the activation of this pathway on MM-induced osteoblast exhaustion are not known and were investigated in this study. We found that the osteogenic differentiation of bone marrow hMSCs toward osteoprogenitor cells (PreOB) significantly increased Ror2 expression, and that MM cells inhibit Ror2 expression by PreOB in co-culture by inhibiting the non-canonical Wnt5a signaling. The activation of the non-canonical Wnt pathway in hMSCs by means of Wnt5a treatment and the overexpression of Wnt5 or Ror2 by lentiviral vectors increased the osteogenic differentiation of hMSCs and blunted the inhibitory effect of MM in co-culture. Consistently, Wnt5a inhibition by specific small interfering RNA reduced the hMSC expression of osteogenic markers. Our findings demonstrate that the Wnt5a/Ror2 pathway is involved in the pathophysiology of MM-induced bone disease and that the activation of the non-canonical Wnt5a/Ror2 pathway in hMSCs increases osteogenic differentiation and may counterbalance the inhibitory effect of MM cells.
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