Bone morphogenetic proteins in breast cancer: dual role in tumourigenesis?

The human bone morphogenetic protein (BMP) family consists of over 20 growth factor proteins that are involved in bone formation and developmental processes. BMPs are extracellular signalling molecules that are able to regulate various cellular functions, proliferation, differentiation, apoptosis and migration. For the last 10 years, these powerful cytokines have increasingly been studied in several cancers, and aberrant expression patterns of BMPs have been reported. Functional studies have suggested that BMPs are involved in both cancer promotion and inhibition. The role these signalling molecules play in breast cancer is only starting to emerge: thus far, studies have been even contradictory. Different BMP ligands have been shown to decrease as well as increase cancer cell growth and migration. Furthermore, they are involved in bone metastases, which are a common feature in breast cancer. In this sense, BMPs resemble a closely related protein transforming growth factor beta, which possesses a bidirectional role in cancer cell regulation. In this review, we focus on the current knowledge of BMP expression, functional roles and involvement in bone metastasis in breast cancer.