Differential bone morphology and hypoxia activity in skeletal metastases of ER+ and ER− breast cancer

Bone metastases occur in the majority of advanced breast cancer patients, and the most common complications are osteolytic bone metastases. However, due to the limitations of models and methodologies for bone metastasis studies, the intricacies of bone metastasis have not been fully acknowledged and revealed in prior work. Our earlier study indicated that certain breast cancer cells undergo a pre-osteolytic stage before the establishment of overt metastatic lesions. Here, we further identify a differential bone morphology between ER (estrogen receptor)⁺ and ER^- breast cancer. Specifically, we observe a more pronounced osteolytic phenotype in the bone metastatic lesions of ER^- cells investigated, linked to elevated hypoxia signaling that stimulates the secretion of osteolytic inducers from cancer cells. In the in vivo mouse model, the application of the hypoxia-inducible factor (HIF) inhibitor 2-methoxyestradiol demonstrates a promising efficacy in suppressing tumor growth and osteoclast differentiation in the bone lesions established by bone-tropic subpopulation of ER^- MDA-MB-231 cell. Overall, our findings explore the complexity of phenotype and morphology in bone metastatic lesions of ER⁺ and ER^- breast cancer, which offers a compelling rationale for leveraging HIF inhibitors to the treatment targeting skeletal complications of breast cancer bone metastases, especially for ER^- tumors.