Bone is a highly specialized organ that supports haematopoiesis but can also serve as a metastatic niche for tumour cells. Metastatic cells must overcome the hypoxic, nutrient-limited conditions in the bone marrow to establish secondary growth. Phenotypic mimicry of stromal cells has been proposed as an adaptive mechanism of bone metastatic tumour cells, but the metabolic strategies used in this context remain poorly defined.

In a recent preprint (not peer-reviewed), Han et al. show that tumour cells mimic erythroblasts to hijack iron from bone marrow macrophages and facilitate metastatic progression. Using a model of breast cancer bone metastasis that enables in vivo metastatic niche labelling and single-cell RNA sequencing, the authors identified an iron-exporting macrophage population (iMACs) enriched in the metastatic niche. Notably, these iMACs were present under steady-state conditions and maintained their phenotype in the presence of metastases, which suggests that they are pre-existing components of the bone marrow niche that are co-opted by cancer cells.