Stimulation of Osteoblastic Bone Metastasis by Canine Prostate Cancer

Abstract

Prostate cancer metastases primarily localize in the bone where they induce a unique osteoblastic response. The mechanisms are not well understood; however, they depend on a complex interplay between prostate cancer cells and the cellular microenvironment in the bone marrow. We hypothesize that prostate cancer cells secreted biological factors that induce new bone formation and either induce or stimulate osteoclastic bone resorption to permit growth of bone metastasis. Dogs are the only other species that develop spontaneous prostate cancer with bone metastases in addition to men. Previously, our laboratory has developed three canine prostate cancer cell lines that mimic human osteoblastic bone metastases when injected into the left cardiac ventricle of nude mice. This study then investigated the effect of three canine prostate cancer cell lines (Ace‐1, Probasco, and LuMa) on bone formation and resorption. Mouse calvaria were treated with conditioned medium (CM) collected from cell lines. A novel calcein uptake assay was developed to measure bone formation and mineralization in vitro. Ace‐1 CM induced predominantly bone resorption in calvaria, while Probasco CM induced marked bone formation, mineralization, and healing of calvaria defects. The expression of osteoblast‐related genes in calvaria showed that Probasco CM stimulated the maturation and differentiation of osteoblasts and inhibited osteoclastogenesis. Both bone modeling and remodeling were involved in Probasco‐induced bone formation and mineralization by inhibiting remodeling with zoledronic acid. RNAseq results from CM treated osteoblasts indicated that the WNT signaling pathway was activated by Probasco compared to either control or Ace‐1. In addition, Calvaria treated with Probasco+hDKK‐1 CM, Probasco transduced with human DKK‐1 cDNA (a WNT antagonist), showed decreased bone formation. In vivo, Our preliminary data showed that DKK‐1 significantly increased the growth of Probasco tumors after intratibial injection of Probasco+hDKK‐1 cells in nude mice, which suggested the autocrine effect of DKK‐1 on tumor cells. In conclusion, Probasco cells induced bone formation and mineralization in vitro that depended on the WNT signaling pathway. Probasco cells serve as a valuable model for studying the mechanisms of osteoblastic bone metastasis in prostate cancer.