The
laminin-binding
integrin α6β1 plays a major role in determining the aggressive phenotype of
tumor cells during
metastasis. Our previous work has shown that cleavage of the α6β1
integrin to produce the structural variant α6pβ1 on
tumor cell surfaces is mediated by the
serine protease urokinase plasminogen activator (uPA). Cleavage of α6β1 increases
tumor cell motility, invasion, and
prostate cancer metastasis, and blockage of uPA inhibits α6pβ1 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 α6pβ1 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 α6
integrin cleavage. The stimulation does not require macrophage/
tumor cell contact because macrophage
conditioned medium is sufficient for increased uPAR transcription and α6 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 α6
integrin cleavage. These data suggest that TAMs promote prometastatic
integrin-dependent pericellular proteolysis.