Wnt and Wnt inhibitors in bone metastasis.

Bone metastasis is a clinically devastating development of progressive cancers including prostate carcinoma, breast carcinoma and multiple myeloma. Bone metastases are typically painful, lead to adverse skeletal-related events, such as fracture, and are highly resistant to therapy. A major contribution to the ability of cancers to successfully establish bone metastases is their ability to exploit mechanisms of normal bone remodeling. Wnts are a large family of morphogenic proteins that are critical for bone development and contribute to maintaining bone mass in the mature organism. Wnt function is balanced by the presence of a variety of endogenous inhibitors, such as the dickkopf family members, secreted frizzled related proteins and sclerostin. Together, these factors contribute to normal bone homeostasis, allowing for dynamic changes in bone to withstand alterations in physical forces and physiological needs. In this review, we describe the role that Wnts and their inhibitors have in normal bone biology and cancer-related bone pathology. An overview of Wnt signaling pathways is discussed and key bone microenvironment cellular players, as they pertain to Wnt biology, are examined. Finally, we describe clinical trials of several Wnt inhibitor antagonists for patients with tumor-related bone disease. As few options currently exist for the treatment of bone-metastatic disease, Wnt proteins and their inhibitors offer promise for the development of novel therapeutics.