IL-32γ Induces M2 Macrophage Polarization with Enhanced Ability of Macrophage-Mediated Drug Resistance in Myeloma Cells through STAT3 and NF-κB Activation

Background: Macrophages(MΦs) are an abundant and important component of multiple myeloma (MM) microenvironment, and support the survival of MM cells and promote their resistance to chemotherapy. It has been proposed that MΦs have great plasticity and can differentiate into 2 major polarized states, M1 ("classically activated") and M2 ("alternatively activated"). M1 macrophages are considered to be anti-tumoural and pro-inflammatory, whereas M2 macrophages are characterized by efficient activity of tumour promotion and suppression of adaptive immunity.IL-32, a novel proinflammatory cytokine, has been found to play a crucial role in a variety of inflammatory diseases and cancers. In this study, we investigated the immunomodulatory effects and underlying mechanisms of IL-32γ on macrophage polarization and MΦ-mediated drug resistance in myeloma cells.

Results: First, we found that both bone marrow supernatant and peripheral blood supernatant of MM patients showed significantly higher levels of IL-32 as compared with those of healthy controls. We also detected IL-32 in MM cell lines and M-CSF-induced MØs by qRT-PCR and Western blot, which showed that IL-32 expression was higher in MM cell lines than in MØs. We next investigated the polarization profiles and functional activities of IL-32γ-treated MØs. After stimulated with recombinant IL-32γ,MØs exhibited upregulated expression of cell-surface molecules and cytokines associated with M2 polarization(CD206,Dectin-1, and IL-10), while M1-polarizing marker(CD86) were down-regulated.

Further, Western blot was applied to detect the inflammation molecular pathway in MØs, which showed that IL-32γ treatment activated the JAK-STAT3 pathway and NF-κB pathway. This pro-M2MØ polarization effect of IL-32γ was inhibited by the addition of specific inhibitors of STAT3 and NF-kB. Moreover, co-culture experiments revealed that IL-32γ-educated MØs provided a survival benefit to myeloma cells, as tested with several anti-myeloma agents including bortezomib and doxorubicin. The pro-survival effect was associated with less cleavage of caspase-3 and PARP in myeloma cells, and thus less apoptosis. The STAT3 and NF-κB inhibitors partially inhibited the observed IL-32γ-induced and MØs -mediated resistance to bortezomib in MM cells.

Conclusions:IL-32γ could effectively induce the polarization of M2 MØs, leading to enhanced MØ-mediated drug resistance in myeloma cells as the result of activating the JAK-STAT3 and NF-κB pathways in MØs.