Osteoblast-Derived Paracrine and Juxtacrine Signals Protect Disseminated Breast Cancer Cells from Stress

SIMPLE SUMMARY: Bone metastasis is a debilitating, incurable complication that occurs in breast cancer patients with a variable period of latency after treatment of the primary tumor. The seeding of individual cancer cells in the bone marrow from the primary tumor occurs early in disease progression. How disseminated cancer cells are able to remain dormant in the bone marrow and then reactivate decades later to form destructive bone lesions is not fully understood. We have used model systems to identify bone resident cell types, and molecular mediators, that influence the survival and fate of disseminated breast cancer cells. The identification of such cellular and molecular support mechanisms can inform the design of new therapeutic strategies aimed at eliminating indolent cancer cells from the bone marrow, preventing progression to overt metastases. ABSTRACT: Metastatic breast cancer in bone is incurable and there is an urgent need to develop new therapeutic approaches to improve survival. Key to this is understanding the mechanisms governing cancer cell survival and growth in bone, which involves interplay between malignant and accessory cell types. Here, we performed a cellular and molecular comparison of the bone microenvironment in mouse models representing either metastatic indolence or growth, to identify mechanisms regulating cancer cell survival and fate. In vivo, we show that regardless of their fate, breast cancer cells in bone occupy niches rich in osteoblastic cells. As the number of osteoblasts in bone declines, so does the ability to sustain large numbers of breast cancer cells and support metastatic outgrowth. In vitro, osteoblasts protected breast cancer cells from death induced by cell stress and signaling via gap junctions was found to provide important juxtacrine protective mechanisms between osteoblasts and both MDA-MB-231 (TNBC) and MCF7 (ER(+)) breast cancer cells. Combined with mathematical modelling, these findings indicate that the fate of DTCs is not controlled through the association with specific vessel subtypes. Instead, numbers of osteoblasts dictate availability of protective niches which breast cancer cells can colonize prior to stimulation of metastatic outgrowth.