Runx2 Deficiency in Osteoblasts Promotes Myeloma Resistance to Bortezomib by Increasing TSP-1-Dependent TGFβ1 Activation and Suppressing Immunity in Bone Marrow.

Multiple myeloma is a plasma cell malignancy that thrives in the bone marrow (BM). The proteasome inhibitor bortezomib is one of the most effective first-line chemotherapeutic drugs for multiple myeloma; however, 15% to 20% of high-risk patients do not respond to or become resistant to this drug and the mechanisms of chemoresistance remain unclear. We previously demonstrated that multiple myeloma cells inhibit Runt-related transcription factor 2 (Runx2) in pre- and immature osteoblasts (OB), and that this OB-Runx2 deficiency induces a cytokine-rich and immunosuppressive microenvironment in the BM. In the current study, we assessed the impact of OB-Runx2 deficiency on the outcome of bortezomib treatment using OB-Runx2+/+ and OB-Runx2-/- mouse models of multiple myeloma. In vitro and in vivo experiments revealed that OB-Runx2 deficiency induces multiple myeloma cell resistance to bortezomib via the upregulation of immunosuppressive myeloid-derived suppressor cells (MDSCs), downregulation of cytotoxic T cells, and activation of TGFβ1 in the BM. In multiple myeloma tumor-bearing OB-Runx2-/- mice, treatment with SRI31277, an antagonist of thrombospondin-1 (TSP-1)-mediated TGFβ1 activation, reversed the BM immunosuppression and significantly reduced tumor burden. Furthermore, treatment with SRI31277 combined with bortezomib alleviated multiple myeloma cell resistance to bortezomib-induced apoptosis caused by OB-Runx2 deficiency in cocultured cells and produced a synergistic effect on tumor burden in OB-Runx2-/- mice. Depletion of MDSCs by 5-fluorouracil or gemcitabine similarly reversed the immunosuppressive effects and bortezomib resistance induced by OB-Runx2 deficiency in tumor-bearing mice, indicating the importance of the immune environment for drug resistance and suggesting new strategies to overcome bortezomib resistance in the treatment of multiple myeloma.