Mechanical Regulation of Breast Cancer Bone Metastasis via Osteocytes’ Signaling to Endothelial Cells

Ma, Yu-Heng 1; Xu, Liangchen 1; Mei, Xueting 1; You, Lidan 1, 2

1. Institute of Biomaterials and Biomedical Engineering, University of Toronto; 
2. Department of Mechanical and Industrial Engineering, University of Toronto

INTRODUCTION: Bone metastases, the migration of cancers to the bone, occur in 65-75% of patients with advanced breast cancer and significantly increase the morbidity and mortality of patients. Bone mechanical loading during exercise, commonly suggested for cancer patients, stimulate osteocytes, the major population of cells in the bone. We previously showed that mechanically stimulated osteocytes affect breast cancer cell both directly through chemokines and indirectly through osteoclasts and endothelial cells [1]. Since endothelial cells comprise the first barrier encountered by metastasizing cancer cells in blood vessels, we will be investigating the mechanism underlying mechanically stimulated osteocytes’ impact on the interaction between endothelial cells and breast cancer cells during metastasis. 

METHODS: To simulate the mechanical stimulation experienced by osteocytes during mild exercise, MLO-Y4 osteocyte-like cells (gift of Dr. Lynda Bonewald, Indiana University) were placed in flow chambers and subjected to oscillatory fluid flow (1Pa; 1Hz; 2 hours). Osteocytes were then cultured for 24 hours and medium was extracted (conditioned medium; CM). Controls were MLO-Y4 cells not placed in flow chambers. Human umbilical vein endothelial cells (HUVECs; gift of Dr. Young, University of Toronto) were conditioned in osteocyte CM. Endothelial permeability was measured with fluorescein-dextran, 40 kDa. The strength of MDA-MB-231 breast cancer cells’ adhesion to endothelial cells was tested by applying 30 minutes of oscillatory fluid flow (1Pa; 1Hz) after 30 minutes of adhesion. Gene expression of MDA-MB-231/1833 bone-metastatic breast cancer cells (gift of Dr. Seth, University of Toronto) conditioned in CM from endothelial cells conditioned in osteocyte CM was determined with PCR. 

PRELIMINARY RESULTS: CM from mechanically stimulated osteocytes reduces the adhesion of breast cancer cells (by 18%) to endothelial cells by lowering endothelial cell’ expression of ICAM-1 (intercellular adhesion molecule 1). This aligns well with our previous result demonstrating the reduction of breast cancer cell extravasation towards CM from mechanically stimulated osteocytes [1]. Furthermore, CM from endothelial cells conditioned in CM form mechanically stimulated osteocytes lowered the expression of MMP-9 (matrix metallopeptidase 9; important for cancer cells to degrade the matrix during invasion into the bone) by bone-metastatic breast cancer cells (by 54%). 

CONCLUSION: We have demonstrated that signaling from mechanically stimulated osteocytes reduces the ability of breast cancer cells to adhere to endothelial cells and degrade the bone. This provides insights into the impact of exercises on bone metastases through osteocyte and endothelial cell signaling. Examining the mechanism will assist in designing cancer intervention programs that lower the risk for bone metastases. 

REFERENCES: [1] Ma. J Cell Biochem. 2018.