At the cellular level, the process of bone metastasis involves many steps. Circulating cancer cells enter the marrow, proliferate, induce neovascularization, and ultimately expand into a clinically detectable, often symptomatic, metastatic deposit. Although the initial establishment and later expansion of the metastatic deposit in bone require tumor cells to possess invasive capability, the exact proteases responsible for this phenotype are not well known. The objective of our study was to take an unbiased approach to determine which proteases were expressed and functional during the initial interactions between prostate cancer cells and bone marrow stromal (BMS) cells. We found that the combination of human prostate cancer PC3 and BMS cells stimulates the invasive ability of cancer cells through type I collagen. The use of inhibitors for each of the major protease families indicated that 1 or more MMPs was/were responsible for the BMS-induced invasion. Gene profiling and semiquantitative RT-PCR analysis revealed an increased expression of several MMP genes because of PC3/BMS cell interaction. However, only MMP-12 showed an increase in protein expression. Downregulation of MMP-12 expression in PC3 cells by siRNA inhibited the enhanced invasion induced by PC3/BMS cell interaction. In vivo, MMP-12 was found to be primarily expressed by prostate cancer cells growing in bone. Our data suggest that BMS cells induce MMP-12 expression in prostate cancer cells, which results in invasive cells capable of degradation of type I collagen. ' 2008 Wiley-Liss, Inc.Key words: bone metastasis; matrix metalloproteinases (MMPs); protease inhibitors Prostate cancer is the second leading cause of cancer-related death and the most common cancer in males, resulting in~27,000 deaths of men per year in the United States.1 Almost all patients with advanced disease have bone metastasis, and most patients experience painful complications from skeletal involvement.
2Although the biology of tumor growth in bone is beginning to be unraveled, the initial steps of tumor-bone interaction that promote the establishment and expansion of the metastatic deposit are still undefined.Matrix metalloproteinases (MMPs) are endopeptidases that can cleave virtually any compound of the extracellular matrix (ECM). MMPs are produced by both tumor cells and host cells and have been shown to play a role in regulating cancer cell growth, apoptosis, invasion, metastasis and angiogenesis.3 Although ECM remodeling and invasion are thought to occur by virtue of MMPs' capacity to destroy underlying cellular foundations, it is now known that MMPs can alter signaling pathways triggered by molecules concealed in the ECM, thereby profoundly influencing the local microenvironment.4 Proliferation, survival and invasiveness of prostate cancer cells depend on their interaction with stromal cells. 5,6 This interaction affects cellular behavior through direct cell-cell contact or through diffusible factors using existent intracellular signaling pathways. 7 In any case, in orde...