Macrophage-Dependent Cleavage of the Laminin Receptor α6β1 in Prostate Cancer

The laminin-binding integrin alpha 6 beta 1 plays a major role in determining the aggressive phenotype of tumor cells during metastasis. Our previous work has shown that cleavage of the alpha 6 beta 1 integrin to produce the structural variant alpha 6p beta 1 on tumor cell surfaces is mediated by the serine protease urokinase plasminogen activator (uPA). Cleavage of alpha 6 beta 1 increases tumor cell motility, invasion, and prostate cancer metastasis, and blockage of uPA inhibits alpha 6p beta 1 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-O-tetradecanoylphorbol-13-acetate (TPA)-differentiated human leukemia HL-60 cells] to investigate the hypothesis that macrophages stimulate the production of the prometastatic variant alpha 6p beta 1 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 alpha 6 integrin cleavage. The stimulation does not require macrophage/tumor cell contact because macrophage conditioned medium is sufficient for increased uPAR transcription and alpha 6 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 alpha 6 integrin cleavage. These data suggest that TAMs promote prometastatic integrin-dependent pericellular proteolysis. Mol Cancer Res; 9(10); 1319-28 (C) 2011 AACR.