Bone metastasis is the major cause of morbidity and mortality in breast cancer patients. Novel therapeutic agents are urgently needed. During the past funding cycle, TGFbeta signaling was shown to increase breast cancer cell production of the osteolytic factor, PTH-rP, and breast cancer cells with attenuated TGFbeta signaling were shown to cause fewer bone metastases in vivo. TGFbeta has been shown to enhance carcinoma metastasis by acting mainly in a paracrine fashion. Bone is the body's largest source of TGFbeta, and excessive TGFbeta released from cancer cells and bone matrix after breast cancer cells metastasize to the bone has been shown to cause osteolytic lesions via PTHrP. However, the mechanism by which TGFbeta causes increased PTH-rP expression, and whether TGFbeta antagonists can be useful clinically in the prevention and treatment of breast cancer-mediated bone metastasis are still relatively unexplored. As such, our Specific Aim 1 will determine the mechanism by which TGFbeta regulates Gli2 expression to test the hypothesis that Gli2 mediates TGFbeta-induced PTH-rP expression and osteolysis. Our preliminary studies also showed that systemic administration of a TGFbeta type I receptor (Rl) kinase inhibitor significantly inhibited bone metastasis of human breast cancer cells in vivo. Our Specific Aim 2 will thus determine the comparative efficacy of different types of TGFbeta antagonists in inhibiting breast cancer-induced bone metastasis to test the hypothesis that TGFbeta antagonists may have clinical utilities for the treatment of osteolytic bone metastasis. Since abrogation of autocrine TGFbeta signaling has been shown to promote primary tumor growth, it is essential to address whether systemic administration of TGFbeta antagonists will also promote tumorigenicity of premalignant breast cells that possess the autocrine tumor-suppressive activity of TGFbeta. Therefore, our Specific Aim 3 will test the hypothesis that different modes of antagonism against TGFbeta signaling with TGFbeta binders or Rl kinase inhibitors may be exploited to inhibit tumor progression due to excessive paracrine TGFbeta activity while preserving the tumor-suppressive activity of autocrine TGFbeta. Our long-term goals are to elucidate the molecular mechanisms that drive bone metastasis and osteolysis and to ultimately develop safe, effective TGFbeta antagonists as novel agents for the prevention and treatment of breast cancer-induced bone metastasis.