Radiotherapy-Resistant Breast Cancer Cells Enhance Tumor Progression by Enhancing Premetastatic Niche Formation through the HIF-1α-LOX Axis

Cancer stem cells (CSCs) exist in solid tumors and contribute to therapeutic resistance and disease recurrence. Previously, we reported that radiotherapy-resistant (RT-R)-MDA-MB-231 cells from highly metastatic MDA-MB-231 cells produced more CSCs than any other RT-R-breast cancer cells and showed therapeutic resistance and enhanced invasiveness. Hypoxia inducible factor-1α (HIF-1α) induced in the tumor microenvironment leads to the release of lysyl oxidase (LOX), which mediates collagen crosslinking at distant sites to facilitate environmental changes that allow cancer cells to easily metastasize. Therefore, in this study, we investigated whether RT-R-MDA-MB-231 cells induce greater HIF-1α expression, LOX secretion, and premetastatic niche formation than MDA-MB-231 cells do. RT-R-MDA-MB-231 cells increased HIF-1α expression and LOX secretion compared with MDA-MB-231 cells. Mice harboring RT-R-MDA-MB-231 cell xenografts showed enhanced tumor growth and higher expression of the CSC markers, CD44, Notch-4, and Oct3/4. In addition, mice injected with RT-R-MDA-MB-231 cells exhibited a higher level of HIF-1α in tumor tissue, increased secretion of LOX in plasma, higher induced levels of crosslinked collagen, and a higher population of CD11b(+) BMDC recruitment around lung tissue, compared with those injected with MDA-MB-231 cells. These results suggest that RT-R-MDA-MB-231 cells contribute to tumor progression by enhancing premetastatic niche formation through the HIF-1α-LOX axis.