Iron Trafficking through Macrophages Regulates Signaling Pathways in Myeloma

Background

Iron is an essential element for cell growth, including cancer cells, and is present in the microenvironment. We have shown that multiple myeloma (MM) cells have abnormal iron metabolism and harbor increased intracellular iron. However, the mechanism by which MM cells retain iron has remained largely elusive.

Methods

Expression and clinical relevance of the transferrin receptor in MM samples were analyzed in publicly available microarray and RNA-sequencing databases. Macrophages were isolated from C57BL/6J mice and were induced to specific subtypes by cytokines or culturing with MM cells. The 5TGM1-KaLwRij MM mice were used to confirm whether MM cells induce macrophage polarization in vivo. Specific subtypes of macrophage and transferrin receptor expression in MM cells were assessed by flow cytometry. Expression of ferroportin (FPN1) and ferritin in MM cells and/or macrophages were analyzed by Western blots. Single-cell RNA-sequencing (scRNA-seq), RNA-seq, and gene expression profiles (GEPs) were employed to identify ferroportin-signaling pathways in both tumor cells and macrophages of primary human MM samples.

Results

MM cells induced polarization with a significant increase of CD38⁺CD206^– M1 macrophages both in vitro and in vivo. We also confirmed that the tumor associated macrophages (TAMs) were increased in the 5TGM1-KaLwRij MM mice. MM cells upregulated ferroportin expression in macrophages to provide iron to MM cells in co-culture studies and in vivo models. The transferrin receptor antibody treatment prevented MM cells from taking up iron from macrophages. scRNA-seq identified a subset of FPN1⁺ TAMs in human bone marrow aspirates, which are assumed to provide iron to MM cells. Using RNA-seq and GEPs analyses in primary human samples, multiple signaling pathways were differentially modulated in FPN1⁺ versus FPN1^– TAMs, including those related to inflammation and apoptosis

Conclusions

Increased expression of the transferrin receptor in MM cells strongly suggests that tumor cells take up iron from its environment. MM cells promote intracellular iron mobilization in macrophages, which provide iron to MM cells in a transferrin-dependent manner. Blockade of iron trafficking between MM cells and macrophages might be a promising approach to MM therapy.