Bortezomib restores stroma-mediated APO2L/TRAIL apoptosis resistance in multiple myeloma

Objectives: Hematopoietic stroma promotes resistance to immune control by APO2L/TRAIL in multiple myeloma (MM) cells in part by increasing synthesis of the anti‐apoptotic protein c‐FLIP. Here, we tested whether bortezomib can reverse the APO2L/TRAIL environmental mediated‐immune resistance (EM‐IR). Material and methods: MM cell lines (RPMI 8226 and U266) and CD138+ patient’s MM cells were directly adhered to HS5 stroma exposed to HS5 or bone marrow stroma of patients with MM released soluble factors in a transwell system. Cells were treated with either APO2L/TRAIL (10 ng/mL), bortezomib (10 nm) or both. Results: Pretreatment with bortezomib effectively overcomes APO2L/TRAIL apoptosis resistance in myeloma cell lines and in CD138+ cells while directly adhered or in transwell assay. Bortezomib was not cytotoxic to HS5 stroma cells and only altered monocyte chemotactic protein‐2‐3 and IL‐10 levels in the stroma‐myeloma milieu. Factors released by HS5 stroma increased expression of c‐FLIP, induced STAT‐3 and ERK phosphorylation and reduced DR4 receptor expression in MM cells. HS5 stroma‐released factor(s) induced NF‐κB activation after 20 h exposure in association with an enhanced c‐FLIP transcription. Bortezomib effectively reduced c‐FLIP protein expression without affecting other proteins. Bortezomib also increased DR4 and DR5 expression in the presence of stroma. Conclusions: These findings provide the rationale to combine bortezomib and APO2L/TRAIL to disrupt the influence of the stroma microenvironment on MM cells.