Angiogenesis, Metastasis, and the Cellular Microenvironment Macrophage-Dependent Cleavage of the Laminin Receptor a6b1 in Prostate Cancer

The laminin-binding integrin a6b1 plays a major role in determining the aggressive phenotype of tumor cells during metastasis. Our previous work has shown that cleavage of the a6b1 integrin to produce the structural variant a6pb1 on tumor cell surfaces is mediated by the serine protease urokinase plasminogen activator (uPA). Cleavage of a6b1 increases tumor cell motility, invasion, and prostate cancer metastasis, and blockage of uPA inhibits a6pb1 production. In human tumors, uPA and uPAR are expressed in tumor cells and tumor-associated macrophages (TAM). TAMs localize to solid tumors and contribute to increased tumor growth and the metastatic phenotype. In this study, we utilized a coculture system of PC-3 prostate tumor cells and macrophages [12-Otetradecanoylphorbol-13-acetate (TPA)-differentiated human leukemia HL-60 cells] to investigate the hypothesis that macrophages stimulate the production of the prometastatic variant a6pb1 on human prostate cancer cells via the uPA/uPAR axis. Our results indicate that adherent macrophages cocultured with PC-3 cells increased PC-3 uPAR mRNA, uPAR cell surface protein expression and a6 integrin cleavage. The stimulation does not require macrophage/tumor cell contact because macrophage conditioned medium is sufficient for increased uPAR transcription and a6 cleavage–dependent PC-3 cell invasion. The increased cleavage was dependent on uPAR because production was blocked by silencing RNA–targeting uPAR. These results indicate that macrophages can stimulate uPA/uPAR production in tumor cells which results in a6 integrin cleavage. These data suggest that TAMs promote prometastatic integrin-dependent pericellular proteolysis. Mol Cancer Res;