Flow-Stimulated Osteocytes Promote Trans-Endothelial Migration of Breast Cancer Cells into Bone

Mei, Junyi 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

Breast cancer cells disseminate early in the progression of cancer and have been detected in the bone marrow, where the constitutive presence of cell adhesion molecules and slow blood flow in sinusoidal spaces trap disseminated tumor cells. Only a subset of these cells will migrate into bone, and a subset of this subset will develop into incurable, debilitating bone metastases. Bone is one of the most common sites of metastasis in breast, lung, and prostate cancers, which often leads to malignancies such as painful fractures and spinal nerve compression. It is imperative to understand what makes these disseminated tumor cells enter bone tissue to be able to develop therapies to prevent this crucial step of bone metastasis. Bi-directional interactions of tumor cells with endothelial cells cause changes in gene expression that can promote trans-endothelial migration. In bone, the master regulators of bone function are matrix-embedded osteocytes, cells that form extensive networks which associate intimately with blood vessels. Osteocytes communicate with endothelial cells to orchestrate long-range signaling, providing basis for the endocrine function of bone. However, it is unclear how osteocytic signaling affects disseminated tumor cells. This novel research establishes how osteocyte signaling via endothelial mediation alters gene expression in disseminated tumor cells to promote trans-endothelial migration into bone.

Due to their dendritic morphology, osteocytes are mechanical force-sensing cells. A physiological mechanical stimulus is applied to osteocytes in the form of oscillatory fluid flow to mimic routine physical activity. Their soluble signals are captured as conditioned media at various time points to be then applied to endothelial cell culture. The conditioned media collected from the endothelial cell culture at various time points is subsequently applied to breast cancer cells. RNA-seq is a technique that provides a comprehensive picture of global gene expression between different conditions. It will be supplemented by RT-PCR of select gene transcripts. RNA-seq analysis will be used to uncover differences in gene expression between breast cancer cells treated with conditioned media arising from mechanically-stimulated osteocytes, non-mechanically-stimulated osteocytes, and a no osteocyte negative control. In particular, transcripts involved in migration and invasion, such as IL-11 and CTGF, and in bone-metastasis-predictive genes signatures, such as TTF1 and FGFR3, are expected to demonstrate differential expression between these conditions.

Once we determine the role of osteocytic signaling in promoting bone metastasis, we open the floor to developing novel therapies or exercise intervention programs for breast cancer patients. These therapies will focus on interfering with this crucial arm of signaling in preventing the development of bone metastases.