Tumor-associated macrophages induce multiple myeloma cell survival and STAT3 activation

Macrophages are an abundant component of the myeloma bone marrow microenvironment and a prognostic factor for multiple myeloma (MM) patients. They express more M2 markers like CD206 (mannose scavenger receptor) and CD163 (hemoglobin scavenger receptor) which correlates with an “alternatively activated state” and immunosuppressive and pro-angiogenic function. They are also involved in drug resistance to several anti-myeloma agents like melphalan (alkylating agent) and bortezomib (proteasome inhibitor). The goal of our project is to further unravel survival mechanisms in MM cells induced by tumor-associated macrophages (TAMs) and to target TAMs. An increase in survival of murine MM cells (5T33MMvv and 5T33MMvt) is observed after 24h of coculture with macrophages independently of their polarization state. The biggest effects were observed with TAMs obtained from bone marrow derived macrophages of C57BL/KaLwRij mice and polarized with 5T33MMvt conditioned medium. This survival benefit for myeloma cells is maintained when bortezomib and melphalan are added for 24h and is confirmed by a reduced cleavage of caspase-3. This drug resistance is cell contact mediated and independent of interleukin-6 and -10. Western blot analysis revealed an increased phosphorylation of STAT3 in 5T33MMvv and 5T33MMvt cells and also an increased phosphorylation of Akt in 5T33MMvv cells in coculture with TAMs while no changes in (p)ERk or MCL-1 were observed. We further investigated the involvement of pSTAT3 in the induced survival. AZD1480 (an ATP-competitive JAK2 inhibitor) targets both MM cells and TAMs. Interestingly, 5T33MMvv/vt cells were more sensitive to AZD1480 induced apoptosis when cocultured with TAMs. The inhibition of phosphorylation STAT3 in 5T33MMvv/vt cells could be a possible explanation for this increased sensitivity. In conclusion, TAMs induce the survival of MM cells with the activation of STAT3 as a possible underlying mechanism. STAT3 activation is an interesting target not only for targeting MM cells but also TAMs.