Bortezomib resistance in a myeloma cell line is associated to PSMβ5 overexpression and polyploidy

Bortezomib is a proteasome inhibitor important to the therapy of multiple myeloma (MM), though a number of patients show resistance to this drug. To study the cellular basis of this resistance we have generated a MM cell line displaying enhanced (5-6-fold) resistance to bortezomib by serial cultivation of RPMI 8226 cells with increasing concentrations of this drug. Bortezomib-resistant cells (8226/7B) became bigger in size than parental cells and nearly doubled the amount of DNA per cell, evolving from hypotriploidy to near-tetraploidy. 8226/7B displayed lowered Noxa accumulation and reduced caspase-3 activation in response to bortezomib. Resistant 8226/7B cells overexpressed the PSMβ5 proteasome subunit, the molecular target of bortezomib, both at the mRNA and protein level. No mutations were detected in the PSMβ5 gene. Bortezomib-resistant cells were roughly as sensitive as parental cells to other chemotherapeutic drugs, including doxorubicin, melphalan, vincristine, BMS-214662 and BMS-345541. 8226/7B cells showed partial and high cross-resistance to the proteasome inhibitors epoxomicin and MG-132, respectively. Co-treatment with the histone deacetylase inhibitor trichostatin A (TSA) potentiated bortezomib-induced apoptosis in parental RPMI 8226 cells but did not revert bortezomib resistance in 8226/7B cells. Therefore, treatment of bortezomib-refractory myeloma with drugs targeting molecular structures other than proteasome seems to be the more suitable therapeutic strategy to overcome bortezomib resistance.