Mechanically stimulated osteocytes reduce the bone‐metastatic potential of breast cancer cells in vitro by signaling through endothelial cells

Abstract

Bone metastases occur in 65% to 75% of patients with advanced breast cancer and significantly worsen their survival and quality of life. We previously showed that conditioned medium (CM) from osteocytes stimulated with oscillatory fluid flow, mimicking bone mechanical loading during routine physical activities, reduced the transendothelial migration of breast cancer cells. Endothelial cells are situated at an ideal location to mediate signals between osteocytes in the bone matrix and metastasizing cancer cells in the blood vessels. In this study, we investigated the specific effects of flow‐stimulated osteocytes on the interaction between endothelial cells and breast cancer cells in vitro. We observed that CM from flow‐stimulated osteocytes reduced endothelial permeability by 15% and breast cancer cell adhesion onto endothelial monolayers by 18%. The difference in adhesion was abolished with anti‐intercellular adhesion molecule 1 (ICAM‐1) neutralizing antibodies. Furthermore, CM from endothelial cells conditioned in CM from flow‐stimulated osteocytes significantly altered the gene expression in bone‐metastatic breast cancer cells, as shown by RNA sequencing. Specifically, breast cancer cell expression of matrix metallopeptidase 9 (MMP‐9) was downregulated by 62%, and frizzled‐4 (FZD4) by 61%, when the osteocytes were stimulated with flow. The invasion of these breast cancer cells across Matrigel was also reduced by 47%, and this difference was abolished by MMP‐9 inhibitors. In conclusion, we demonstrated that flow‐stimulated osteocytes downregulate the bone‐metastatic potential of breast cancer cells by signaling through endothelial cells. This provides insights into the capability of bone mechanical regulation in preventing bone metastases; and may assist in prescribing exercise or bone‐loading regimens to patients with breast cancers.

Pathological Crosstalk between Metastatic Breast Cancer Cells and the Bone Microenvironment

“…In addition, osteocyte conditioned medium reduced endothelial cell permeability, as well as breast cancer cell invasion in vitro. Furthermore, the crosstalk between mechanically stimulated osteocytes and endothelial cells decreased the expression of MMP-9, an enzyme involved in metastatic cancer cell migration, in breast cancer cells [58]. These findings not only strengthen the importance of mechanical loading (i.e., exercise) in preventing bone metastases, but also highlight a key role of osteocytes in the establishment of metastatic breast cancer.…”

“…Osteocytes could also be involved in regulating the early stages of breast cancer bone metastasis including the homing and colonization to bone. Interestingly, conditioned medium from mechanically stimulated MLO-Y4 osteocyte-like cells was able to reduce the adhesion of metastatic MDA-MB-231 breast cancer cells to human umbilical vein endothelial cells (HUVECs) [58]. In addition, osteocyte conditioned medium reduced endothelial cell permeability, as well as breast cancer cell invasion in vitro.…”

“…Although osteocytes were originally not included in the concept of the vicious cycle of bone metastasis, several aspects support their role in breast cancer bone metastasis. Whether this is due to a direct interaction with disseminated tumor cells or through indirect actions on bone cells including endothelial cells, osteoblasts, and osteoclasts remains to be elucidated [58].…”

The Emerging Role of Osteocytes in Cancer in Bone

“…However, alternatively it is also possible that osteocytes prevent the migration and arrival to bone of metastatic breast cancer cells. In vitro studies show that mechanically stimulated osteocytes reduced the transendothelial migration of breast cancer cells . Further, interactions between osteocytes and endothelial cells decrease the expression of matrix metalloproteinase 9 (MMP‐9), an enzyme known to facilitate the movement of metastatic cancer cells through the extracellular matrix .…”

“…In vitro studies show that mechanically stimulated osteocytes reduced the transendothelial migration of breast cancer cells . Further, interactions between osteocytes and endothelial cells decrease the expression of matrix metalloproteinase 9 (MMP‐9), an enzyme known to facilitate the movement of metastatic cancer cells through the extracellular matrix . The disparity between these findings suggest that interactions between osteocytes and tumor cells are context and cancer specific.…”