Prostatic Acid Phosphatase Plays a Causal Role in Prostate Cancer Bone Metastases

Abstract

Abstract : Bone metastases are the major cause of morbidity and mortality in prostate cancer (PCa). While there are treatments for the osteolytic phase of PCa bone metastases, there are no therapies that inhibit the later, osteoblastic phase. Prostatic acid phosphatase (PAP) is a protein secreted by PCa cells and is highly expressed in PCa osteoblastic bone metastases. We previously demonstrated that PAP secreted by PCa cells induces the proliferation and differentiation of osteoblasts (OB). We hypothesized that prostatic acid phosphatase (PAP) secreted by prostate cancer (PCa) cells in bone metastases plays a causal role in osteoblastic bone metastases. As the RANK/RANKL/OPG system coordinately controls the balance of osteoclasts vs. osteoblasts in bone, we determined the effects of PCa-derived PAP on RANKL and OPG expression in both PCa and OB cells. We demonstrate that PAP secretion by PCa cells modulates RANKL/OPG secretion in both PCa and bone cells, favoring more OPG and less RANKL which would promote an osteoblastic phenotype. We utilized three different human PCa cell lines: 1. VCaP (PAP+, induces osteoblastic lesions) 2. PC3 (PAP negative, induces osteolytic lesions) and PC3ML (more aggressive subline of PC3, also PAP negative and induces osteolytic lesions). We successfully generated cell lines that transiently knockdown PAP expression using RNAi technology (PAP-siRNA) in high PAP-expressing human PCa cells (VCaP-luc). Conversely, for the gain-of-function experiments, PAP-negative human PCa cells (PC3-luc) and a more aggressive subline (PC3-ML-luc) were stably transfected to overexpress secretory PAP by utilizing the retroviral expression vector pLenti-GIII-CMV-hACPP (ACPP is PAP) Lentiviral Vector (PC3-pPAP) and the pLenti-III-HA Blank Control (PC3-luc-control) vector as control. Stable cell clones were selected with puromycin and characterized by qRT-PCR and western blotting. Knockout of PAP in VCaP cells induced cell cycle arrest. Knock-in of PAP in PC3 and PC3ML