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

Abstract

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 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 (TAMs). TAMs localize to solid tumors and contribute to increased tumor growth and the metastatic phenotype. In our present study, we utilized a co-culture 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 pro-metastatic variant α6pβ1 on human prostate cancer cells via the uPA/uPAR axis. Our results indicate that adherent macrophages co-cultured 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 since macrophage conditioned medium is sufficient for increased uPAR transcription and α6 cleavage dependent PC-3 cell invasion. The increased cleavage was dependent on uPAR since 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 tumor associated macrophages promote pro metastatic integrin dependent pericellular proteolysis.

The Cohesive Metastasis Phenotype in Human Prostate Cancer

“…CTCs produce proinflammatory cytokines and chemokines that draw tumor associated macrophages (TAMs) to the tumor-cell clusters from circulation [32]. Macrophages are known to aid prostate cancer tumor-cell invasion and migration through modification of the adhesion function of laminin-binding integrins that interact with laminin 511 [32], suggesting that tumor clusters themselves contain a specialized pro-metastatic microenvironment. Increased retrieval of the CTCs, including the CTC-clusters and associated cells, will likely aid in treatment stratification of prostate cancer (reviewed in [5]).…”

“…Recent work shows the functional role of one LBI (α6β4) as a mediator of endothelial cell protection in the setting of excessive mechanical stretch relative to lung injury [63]. LBI expression patterns have clinical significance for several epithelial-derived malignancies, for example the α6β1 integrin receptor is conserved in prostate cancer [39], is expressed on prostate tumor cells undergoing PNI [13], and acts as a marker in the aggressive phenotype of tumor cells during cancer progression [14,32,60,64]. The LBIs are especially associated with the invasion and metastasis of human prostate cancer, traversing through muscle [39,65–69].…”

The Role of Inflammation in Prostate Cancer

“…Cleavage of the α6β1 integrin by uPA has been shown in in vivo studies to increase tumor cell motility, invasion, and prostate cancer metastasis (Ports et al 2009). Further in vitro studies indicated that this process may be mediated by TAMs (Sroka et al 2011). Likewise, the cathepsin proteases, and specifically cathepsin K and cathepsin S, are also of interest in this regard.…”

Intracellular modifiers of integrin alpha 6p production in aggressive prostate and breast cancer cell lines

“…Recently, published work indicates that ITGA6 cleavage is closely associated with uPAR and uPA levels [12]. These observations of increased ITGA6 cleavage prompted us to determine if uPAR levels were increasing as well.…”

“…In addition to the necessary role of uPA in cleaving ITGA6, recent work by our group has shown that macrophages can stimulate uPA/uPAR production in tumor cells and increase ITGA6 cleavage. These data suggested that tumor activated macrophages promote prometastatic integrin-dependent pericellular proteolysis and the metastatic phenotype [12]. Furthermore, ITGA6 cleavage has been shown to contribute to cell invasion and migration on laminin, and inhibition of ITGA6 cleavage was shown to substantially delay the onset of bone metastasis and promote curative-type bone metastasis lesions in xenograft mouse models [13–15].…”