Macrophages are characterized by phenotypical and functional heterogeneity. In different microenvironments, macrophages can polarize into two types: classically activated macrophages (M1) or alternatively activated macrophages (M2). M1 macrophages are a well-known bacteriostatic macrophage, and conversely, M2 macrophages may play an important role in tumor growth and tissue remodeling. M1 macrophages have been reported to have high intracellular iron stores, while M2 macrophages contain lower intracellular iron. It has been well-described that disturbances of iron homeostasis are associated with altered immune function. Thus, it is important to investigate if chronic iron overload is capable of polarizing macrophages. Human monocytic leukemia THP-1 cells were maintained in culture medium that contained 100 μM ferrous sulfate heptahydrate (FeSO4) (I-THP-1) and differentiated into THP-1-derived macrophages (I-TDMs) by induction with phorbol 12-myristate 13-acetate (PMA). We characterized that I-TDMs not only enhanced the surface expression of CD163 and CD206 but also increased arginase and decreased iNOS protein expression. I-TDMs enhanced pSTAT6 expression and decreased pSTAT1 and NF-κB expressions. Furthermore, the gene expression profile of I-TDMs was comparable with M2 macrophages by performing human oligonucleotide DNA microarray analysis. Finally, functional assays demonstrated I-TDMs secreted higher levels of IL-10 but not M1 cytokines. Additionally, the conditional medium of I-TDMs had enhanced migration and increased invasion of A375 melanoma cells which was similar to the characteristics of tumor-associated macrophages. Taken together, we demonstrated that THP-1-derived macrophages polarized to a phenotype of M2-like characteristics when subjected to chronic iron overload.
