PO-45 - The role of microvesicles in multiple myeloma progression

Introduction Multiple myeloma (MM) is an incurable, genetically heterogeneous malignancy of plasma cells that secrete non-functioning immunoglobulins and present high proteasome activity. MM is characterized by bone marrow infiltration leading to multiple lytic bone lesions, cytopenia and increased rate of thrombotic events. Microvesicles (MVs) include exosomes (30-100 nm) and microparticles (0.1-1 micron) shed from various cells and expressing antigens reflecting their cellular origin. MVs are involved in thrombosis, inflammation and cancer.However, the effect of MM-MVs on disease progression and their mechanism of action are unclear. We assume that MVs play a role in the interaction between malignant plasma cells and mesenchymal and endothelial cells (EC). Aim To characterize MM-MVs and investigate their effects on microenvironment cells. Materials and Methods MVs were isolated from MM cell line RPMI 8226 untreated or treated with bortezomib and from peripheral blood (PB) and bone marrow (BM) of MM patients (n=13) and healthy controls (n=14). MM-MV size, concentration and cell origin were measured by Nanosite and FACS. Protein content was evaluated by protein arrays and ELISA. Coagulation and proteasome activity were assessed using chromogenic assays. Migratory capacity (migration assay), proliferative rate (XTT assay) and cell-signaling effects (Western blot analysis) of MVs on BM-mesenchymal and ECs were analyzed. Results MM cells exhibited high MV shedding rate, which further increased with the exposure to bortezomib. Significant elevation in MV production was found in MM patients compared to controls. MM-MVs expressed membrane MM markers (syndecan-1/ CD138, CD38), coagulation factor (TF, TFPI, EPCR, TM) and angiogenic factors (VEGFR1, VEGFR2, and CD31). MM-MVs contained high levels of growth factors (Angiogenin, PDGF-BB and VEGF) and displayed procoagulant and proteasome activity. MM-MVs penetrated cells and affected their function. MVs of untreated cells and patient MVs increased EC and mesenchymal cell migration and EC proliferation, while MVs obtained from bortezomib-treated cells decreased these effects. MVs of untreated cells increased ERK1/2 and c-Jun phosphorylation in ECs (by 6.15 and 1.84 fold) but did not affect MAPKAPK-2. MVs of bortezomib-treated cells reduced c-Jun phosphorylation in ECs. Conclusions MM cells are characterized by high shedding rate of MVs. They are pro-coagulants and increase EC thrombogenicity, suggesting their involvement in MM-related thrombosis. MVs contain high levels of angiogenic factors that affect mesenchymal and EC, induce cell migration and proliferation via specific signal transductions. MVs exposed to bortezomib display lower levels of angiogenic factors, which limits proliferation and migration of MVs, reflecting the efficacy of therapy and MM dynamics.