Potential role of the OPG/RANK/RANKL axis in prostate cancer invasion and bone metastasis

Abstract

Receptor activator of NF-κB (RANK), RANK ligand (RANKL) and osteoprotegerin (OPG) are key regulators of bone metabolism under both normal and pathological conditions, including prostate cancer (PCa) bone metastases. However, little is known concerning the expression and function of these regulators in prostate tumor samples and PCa cells and their correlation with invasion and bone metastasis. In the present study, we determined the expression of RANK, RANKL and OPG in 3 human PCa cell lines and 40 PCa patient samples by immunohistochemistry and enzyme-linked immunosorbent assay (ELISA). As controls, samples from 20 patients with benign prostate hyperplasia (BPH) and normal prostate epithelial RWPE2 cells were also included in the analyses. The effects of soluble RANKL (sRANKL) and OPG as well as RANK knockdown on PCa invasion were examined in Transwell assays. Immunohistochemical staining detected little RANK, OPG and RANKL expression in hyperplasia prostate while the percentages of positivity were increased to 50, 45 and 52.5%, respectively, in prostate tumor tissues. OPG and sRANKL levels in the prostate tumor samples as measured by ELISA were ∼10-fold that in the BPHs (P<0.01) and the levels were higher in aggressive tumors than nonaggressive ones (P<0.05). The sRANKL level in the serum of PCa patients was the same as that in the patients with BPH, yet the serum OPG levels correlated with the tissue levels (R2=0.620, P<0.01, which both showed a 10-fold increase in PCa over BPH (P<0.01) with higher levels in aggressive PCa than non-aggressive ones (P<0.05). Consistent with the tissue analyses, expression levels of RANK mRNA and protein were detected in multiple human PCa cell lines by RT-PCR and western blotting, respectively. The treatment of PCa cells with RANKL significantly increased the number of invaded cells (P<0.01), which was suppressed by the decoy receptor OPG. RANK siRNA transfection dramatically dampened the stimulatory effect of RANKL on PCa cell invasion. Our findings indicate that the expression of RANK, RANKL and OPG may be used as diagnostic markers to identify patients at high risk for aggressive PCa and that the effective suppression of PCa cell migration by OPG via the blockage of RANKL activity represents a potential therapeutic strategy for interfering with prostate tumor metastasis and progression to bone.