Protease‐activated receptor‐2 dependent and independent responses of bone cells to prostate cancer cell secretory products

Background Metastatic prostate cancer lesions in the skeleton are frequently characterized by excessive formation of bone. Prostate cancer cells secrete factors, including serine proteases, that are capable of influencing the behavior of surrounding cells. Some of these proteases activate protease‐activated receptor‐2 (PAR2), which is expressed by osteoblasts (bone‐forming cells) and precursors of osteoclasts (bone‐resorbing cells). The aim of the current study was to investigate a possible role for PAR2 in regulating the behavior of bone cells exposed to metastatic prostate cancer cells. Methods The effect of medium conditioned by the PC3, DU145, and MDA‐PCa‐2b prostate cancer cell lines was investigated in assays of bone cell function using cells isolated from wildtype and PAR2‐null mice. Osteoclast differentiation was assessed by counting tartrate‐resistant acid phosphatase‐positive multinucleate cells in bone marrow cultured in osteoclastogenic medium. Osteoblasts were isolated from calvariae of neonatal mice, and BrdU incorporation was used to assess their proliferation. Assays of alkaline phosphatase activity and quantitative PCR analysis of osteoblastic gene expression were used to assess osteoblast differentiation. Responses of osteoblasts to medium conditioned by MDA‐PCa‐2b cells were analyzed by RNAseq. Results Conditioned medium (CM) from all three cell lines inhibited osteoclast differentiation independently of PAR2. Media from PC3 and DU145 cells had no effect on assays of osteoblast function. Medium conditioned by MDA‐PCa‐2b cells stimulated BrdU incorporation in both wildtype and PAR2‐null osteoblasts but increased alkaline phosphatase activity and Runx2 and Col1a1 expression in wildtype but not PAR2‐null cells. Functional enrichment analysis of RNAseq data identified enrichment of multiple gene ontology terms associated with lysosomal function in both wildtype and PAR2‐null cells in response to MDA‐PCa‐2b‐CM. Analysis of individual genes identified osteogenesis‐associated genes that were either upregulated by MDA‐PCa‐2b‐CM selectively in wildtype cells or downregulated selectively in PAR2‐null cells. Conclusions Factors secreted by prostate cancer cells influence bone cell behavior through both PAR2‐dependent and ‐independent mechanisms. Both PAR2‐independent suppression of osteoclast differentiation and PAR2‐dependent stimulation of osteogenesis are likely to determine the nature of prostate cancer metastases in bone.