The Role of Bone Marrow Erythroblastic Niche in Supporting metastatic Growth [scRNA-Seq]

Bone metastasis significantly compromises quality of life in advanced-stage breast cancer patients through severe pain, fractures, and decreased mobility. The role of bone-resident cells such as osteoblasts and osteoclasts in metastatic interactions is well-established, however, the contribution of hematopoietic stem cell (HSC) lineage cells in the bone marrow, which differentiate into both red and white blood cells, remains poorly understood. Employing an in vivo metastatic niche labeling system and single-cell RNA sequencing, we systematically identified and functionally validated novel stromal components critical for bone metastasis. This niche labeling system enabled metastatic cancer cells to directly label surrounding niche cells in vivo via mCherry secretion. Bone marrow cells proximal to the tumor, which absorbed the secreted mCherry, were characterized using single-cell RNA sequencing. Our analysis revealed a distinct macrophage population enriched in the metastatic niche. Further in-depth characterization of these niche-specific macrophages was performed through cell sorting and bulk RNA sequencing. This population was found to be specialized macrophages with iron metabolism capabilities, residing within the erythroid island to support erythropoiesis. We demonstrated that in the bone metastasis environment breast cancer cells hijacked the iron-rich macrophages to supply themselves with iron. To better survive in the hypoxic bone marrow microenvironment, tumor cells mimic erythroblasts by upregulating globin gene expression and heme synthesis genes. These findings underscore the critical role of iron-rich macrophages in supporting tumor growth in the bone and their significant impact on erythropoiesis, presenting novel therapeutic targets for managing bone metastasis and cancer-related anemia. The niche labeling system enabled metastatic cancer cells to directly label surrounding niche cells in vivo via mCherry secretion. E0771 breast cancer cell, engineered to express both GFP and secreted mCherry, was injected into mice via the caudal artery of 8-week-old female C57BL/6 Tyr-/- mice to develop bone metastasis. Only bone marrow cells proximal to the tumor uptook the secreted mCherry. The GFP-mCherry+ niche cells and GFP-mCherry- distal bone marrow cells were isolated from metasttic bone. Normal bone marrow cells were isolated from healthy control mice. Single cell RNAseq profiles were generated from these samples to characterize the niche composition. In addition, E0771 tumor cells from primary site mammary fat pad and bone metastasis were sampled and single cell RNAseq profile were generated to characterize tumor cell composition.