Establishment and validation of an in vitro co-culture model to study the interactions between bone and prostate cancer cells.

Bone is the preferred site for prostate cancer (PCa) metastases. Once the tumor has established itself within the bone there is virtually no cure. To better understand the interactions between the PCa cells and bone environment in the metastatic process new model systems are needed. We have established a two-compartment in vitro co-culturing model that can be used to follow the trans-activation of bone and/or tumor cells. The model was validated using two PCa tumor cell lines (PC-3; lytic and LNCaP; mixed/osteoblastic) and one osteolytic inducing factor, 1,25-dihydroxyvitamin D(3) (D3). Results were in accordance with the expected bone phenotypes; PC-3 cells and D3 gave osteolytic gene expression profiles in calvariae, with up-regulation of genes needed for osteoclast differentiation, activation and function; Rankl, CathK, Trap and MMP-9, and down-regulation of genes associated with osteoblast differentiation and bone mineralization; Alp, Ocl and Dkk-1. LNCaP cells activated genes in the calvarial bones associated with osteoblast differentiation and mineralization, with marginal effects on osteolytic genes. The results were strengthened by similar changes in protein expression for a selection of the analyzed genes. Furthermore, the osteolytic gene expression profiles in calvarial bones co-cultured with PC-3 cells or with D3 were correlated with the actual ongoing resorptive process, as assessed by the release of collagen fragments from the calvariae. Our results show that the model can be used to follow tumor-induced bone remodeling, and by measuring changes in gene expression in the tumor cells we can also study how they respond to the bone microenvironment.