| Summary: | Understanding the mechanisms of cancer initiation will help to prevent and manage the disease. At present, the role of the breast microenvironment in transformation remains unknown. As BMP2 and BMP4 are important regulators of stem cells and their niches in many tissues, we investigated their function in early phases of breast cancer. BMP2 production by tumor microenvironment appeared to be specifically upregulated in luminal tumors. Chronic exposure of immature human mammary epithelial cells to high BMP2 levels initiated transformation toward a luminal tumor-like phenotype, mediated by the receptor BMPR1B. Under physiological conditions, BMP2 controlled the maintenance and differentiation of early luminal progenitors, while BMP4 acted on stem cells/myoepithelial progenitors. Our data also suggest that microenvironment-induced overexpression of BMP2 may result from carcinogenic exposure. We reveal a role for BMP2 and the breast microenvironment in the initiation of stem cell transformation, thus providing insight into the etiology of luminal breast cancer.
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