Characterizing The Role of Cancer-Associated Osteoblasts (CAOs) in Facilitating Bone Colonization of Metastasizing Breast Cancer Cells:

Mortality from breast cancer is almost exclusively due to distant metastasis. Bone metastasis occur in up to 85% of patients with advanced breast cancer, resulting in severe skeletal complications, and are mostly untreatable. Breast cancer-induced bone metastases are essentially characterized by osteolytic lesions induced by reciprocal interactions between cancer and bone cells, leading to changes in the bone metastatic microenvironment. Osteoblasts were found to over-activate osteoclasts to resorb bone, thus releasing growth factors that can in turn help cancer cells to survive and invade. However, the early interactions of metastasizing cancer cells with stromal cells in the bone-marrow, and their reprogramming to cancer-associated osteoblasts (CAOs) is still largely unresolved.

In this study we investigate the role of paracrine signaling by bone-metastasizing breast cancer cell-secreted factors on activation of osteoblasts. We found that activation of osteoblasts included a switch in their morphology and gene expression towards a pro-inflammatory phenotype, reminiscent of CAFs. CAOs demonstrated an upregulation of multiple pro-inflammatory genes and a downregulation of genes associated with osteoblast development, suggesting a cancer-induced inhibition of normal osteoblast differentiation and maturation. To study the interactions of breast cancer cells with the bone microenvironment in vivo, we generated bone-seeking variants of 4T1 cells (4T1.2 and 4T1.3). Analysis of the changes in the populations of cells in BM of mice with bone metastases revealed an increase in the population of granulocytic BMDCs and a decrease in T cells in 4T1.3 injected mice, suggesting that bone metastases induce immune suppression. Interestingly, we found that activated osteoblasts express significantly higher levels of multiple immune-suppressive genes, implying that activated osteoblasts play a role in the formation of an immune suppressed niche. Uncovering the mechanisms that underlie the early events of breast cancer bone metastasis may provide the mechanistic basis for therapeutic targeting to reduce bone metastatic relapse.