Macrophage Migration Inhibitory Factor Regulates Multiple Myeloma Bone Marrow Homing

Multiple myeloma (MM) is the most frequently occurring bone cancer and is characterized by malignant antibody-producing cell accumulation in bone marrow (BM). MM is still incurable, and the disease management is complicated by crosstalk between MM cells and the BM microenvironment. Why MM cells predominantly locate in bone is not well understood. The process by which MM cells are recruited into BM and reside in bone for colonization is called MM BM homing. BM homing is also critical for MM metastasis to distal BM sites. Therefore, MM BM homing is an active process throughout the disease pathogenesis. Here for the first time, we report that macrophage migration inhibitory factor (MIF) regulates MM BM homing via a mechanism that involves MM adhesion in the BM microenvironment. More importantly, we also show that MIF might be a promising target for MM treatment.

MIF is an inflammatory cytokine, secreted by various human cell types. Our results here showed that BM biopsies from MM patients had significantly higher MIF expression, compared with that from healthy donors as determined by both immunohistochemistry of BM biopsies and ELISA of BM aspirates (P<0.05). Animal studies using a human MM SCID mouse model (ARP-1 in the SCID mouse) and a murine MM mouse model (5-TGM1 in the C57BL/KawRij mouse) showed that intravenously inoculated (IV) MIF knocked-down (MIF-KD) MM cells did NOT form tumors in bone, whereas control knocked-down cells did form tumors in bone, as determined by both in vivo bioluminescent assay and FACS analysis of BM cells. Unlike the previously identified SDF-1/CXCR4 axis, which regulates MM chemotaxis to BM, MIF knock-down did NOT affect MM cell migration to BM. However, MIF-KD MM cells showed decreased adhesion to BM stromal cells. A mechanistic study suggested that MIF regulation of MM BM homing was mediated by MIF and its receptor CXCR4, but was independent of its primary receptor CD74. The MIF/CXCR4 axis regulated a panel of adhesion molecules via NFκB and JNK intracellular signaling, and MIF-KD and CXCR4-KD MM cells had similar downstream cell signaling and adhesion molecule expression. In addition, exposing MM cells to a MIF inhibitor (4-IPP) down-regulated expression of those adhesion molecules. Further, in the murine MM model with established MM tumors in bone, administering 4-IPP resulted in extramedullary metastasis. Overall, the findings above suggest that MIF regulates MM cell homing to the BM.

We also explored MIF-targeting therapy for MM treatment. In the human MM SCID model, IV-inoculated ctrl-KD MM cells formed tumors both in BM and outside of the bone, such as in the abdomen and lung. When tumor-bearing mice were treated with the chemotherapeutic melphalan, the extramedullary tumors regressed significantly (P<0.05), whereas the BM tumors were not sensitive to treatment, probably because the BM microenvironment conferred MM chemoresistance. In the same mouse model inoculated with MIF-KD MM cells, because the MIF-KD MM cells had impaired attachment and could NOT form tumors in bone, the tumor-bearing mice had a complete response to the drug treatment as determined by tumor burden and mouse survival (P<0.05). These encouraging results suggested that MIF might be a valid target for MM treatment. To test this hypothesis, we used the murine MM mouse model and treated the tumor-bearing mice with both melphalan and MIF inhibitor. This combined therapy significantly repressed the MM tumor growth in vivo and lengthened survival (P<0.05). Thus, MIF might be a promising target for MM treatment, in combination with conventional chemotherapy.

To summarize, our findings suggest a novel function for MIF in MM BM homing regulation. MIF regulates a panel of adhesion molecules and promotes MM cell attachment within the BM. MIF inhibition disrupts the boundary between MM cells and the BM microenvironment and may raise MM cell drug sensitivity.