RANKL-induced c-Src activation contributes to conventional anti-cancer drug resistance and dasatinib overcomes this resistance in RANK-expressing multiple myeloma cells.

The survival and growth of multiple myeloma (MM) cells are facilitated by cell-cell interactions with bone marrow stromal cells and the bone marrow microenvironment. These interactions induce de novo drug resistance known as cell adhesion-mediated drug resistance. Our previous results recently revealed that the receptor activator of NF-κB (RANK) ligand (RANKL), which is expressed by bone marrow stromal cells, contributes to anti-cancer drug resistance through the activation of various signaling molecules and suppression of Bim expression in RANK-expressing MM cells. However, the detailed mechanisms underlying RANKL-induced drug resistance remain uncharacterized. In the present study, we investigated the mechanism of RANKL-induced drug resistance in RANK-expressing MM cell lines. We found treatment of MM cells with RANKL-induced c-Src phosphorylation and activation of the downstream signaling molecules Akt, mTOR, STAT3, JNK, and NF-κB. In addition, treatment with dasatinib, a c-Src inhibitor, overcame RANKL- and bone marrow stromal cell-induced drug resistance to adriamycin, vincristine, dexamethasone, and melphalan by suppressing c-Src, Akt, mTOR, STAT3, JNK, and NF-κB activation and enhancing expression of Bim. Overall, RANKL- and bone marrow stromal cell-induced drug resistance correlated with the activation of c-Src signaling pathways, which caused a decrease in Bim expression. Dasatinib treatment of RANK-expressing MM cells re-sensitized them to anti-cancer drugs. Therefore, inhibition of c-Src may be a new therapeutic approach for overcoming RANKL-induced drug resistance in patients with MM.