Notch3 Promotes Prostate Cancer-Induced Bone Osteoblastic Metastasis in an MMP-3-Dependent Manner

Purpose: Over ninety percent of prostate cancer metastasis is localized in the bone where it induces a unique osteoblastic response. Elevated Notch signaling is associated with advanced high-grade disease and metastasis, but how it contributes to osteoblastic bone metastasis is unknown. Experimental Design: To determine how Notch affects prostate cancer bone metastasis, Notch expression was manipulated in mouse tibia xenografts and tumor growth, lesion phenotype, and tumor microenvironment assessed. Patient bone metastases were assessed for Notch3 and MMP-3 expression. Results: Osteoblastic tumor cells expressed 5-6 times more Notch3, than osteolytic tumor cells. Expression of active Notch3 (NICD3) in osteolytic tumors reduced osteolytic lesion area and enhanced osteoblastogenesis, while loss of Notch3 in osteoblastic tumors enhanced osteolytic lesion area and decreased osteoblastogenesis. This was accompanied by a respective decrease and increase in number of active osteoclasts at the tumor-bone interface, without any effect on tumor proliferation. Conditioned medium from NICD3-expressing cells enhanced osteoblast differentiation and proliferation in vitro, while simultaneously inhibiting osteoclastogenesis. MMP-3 was specifically elevated in NICD3-expressing tumors, and inhibition of MMP-3 rescued the NICD3-induced block in osteolytic lesions, suppressed osteoblast proliferation, and stimulated osteoclastogenesis. Human bone metastasis samples had higher levels of Notch3 and MMP-3 compared to patient matched visceral metastases. Conclusions: We identified a Notch3-MMP-3 axis in human prostate cancer bone metastases that promotes osteoblastic lesion formation by blocking osteoclast differentiation, while stimulating osteoblastogenesis. This newly identified Notch3-MMP-3 axis could be used as a therapeutic target for inhibiting osteoblastic bone metastasis.