The frequency of osteolytic bone metastasis is determined by conditions of the soil, not the number of seeds; evidence from in vivo models of breast and prostate cancer.

BACKGROUND: While both preclinical and clinical studies suggest that the frequency of growing skeletal metastases is elevated in individuals with higher bone turnover, it is unclear whether this is a result of increased numbers of tumour cells arriving in active sites or of higher numbers of tumour cells being induced to divide by the bone micro-environment. Here we have investigated how the differences in bone turnover affect seeding of tumour cells and/or development of overt osteolytic bone metastasis using in vivo models of hormone-independent breast and prostate cancer. METHODS: Cohorts of 6 (young) and 16 (mature)-week old BALB/c nude mice were culled 1, 7 and 21 days after received intracardiac injection of luciferase expressing human prostate (PC3) or breast cancer (MDA-MB-231) cell lines labelled with a fluorescent cell membrane dye (Vybrant DiD). The presence of growing bone metastases was determined by bioluminescence using an in vivo imaging system (IVIS) and followed by anatomical confirmation of tumour metastatic sites post mortem, while the presence of individual fluorescently labelled tumour cells was evaluated using two-photon microscopy ex vivo. The bone remodelling activities were compared between young and mature naïve mice (both male and female) using micro-CT analysis, ELISA and bone histomorphometry. RESULTS: Both prostate and breast cancer cells generated higher numbers of overt skeletal lesions in young mice (~80%) than in mature mice (~20%). Although mature mice presented with fewer overt bone metastases, the number of tumour cells arriving/colonizing in the tibias was comparable between young and mature animals. Young naïve mice had lower bone volume but higher bone formation and resorption activities compared to mature animals. CONCLUSIONS: Our studies suggest that higher frequencies of growing osteolytic skeletal metastases in these models are linked to increased bone turnover and not to the initial number of tumour cells entering the bone microenvironment.

Prostate cancer cells preferentially home to osteoblast-rich areas in the early stages of bone metastasis: evidence from in vivo models.

It has been suggested that metastasis-initiating cells gain a foothold in bone by homing to a metastastatic microenvironment (or "niche"). Whereas the precise nature of this niche remains to be established, it is likely to contain bone cell populations including osteoblasts and osteoclasts. In the mouse tibia, the distribution of osteoblasts on endocortical bone surfaces is non-uniform, and we hypothesize that studying co-localization of individual tumor cells with resident cell populations will reveal the identity of critical cellular components of the niche. In this study, we have mapped the distribution of three human prostate cancer cell lines (PC3-NW1, LN-CaP, and C4 2B4) colonizing the tibiae of athymic mice following intracardiac injection and evaluated their interaction with potential metastatic niches. Prostate cancer cells labeled with the fluorescent cell membrane dye (Vybrant DiD) were found by two-photon microscopy to be engrafted in the tibiae in close proximity (∼40 µm) to bone surfaces and 70% more cancer cells were detected in the lateral compared to the medial endocortical bone regions. This was associated with a 5-fold higher number of osteoblasts and 7-fold higher bone formation rate on the lateral endocortical bone surface compared to the medial side. By disrupting cellular interactions mediated by the chemokine (C-X-C motif) receptor 4 (CXCR4)/chemokine ligand 12 (CXCL12) axis with the CXCR4 inhibitor AMD3100, the preferential homing pattern of prostate cancer cells to osteoblast-rich bone surfaces was disrupted. In this study, we map the location of prostate cancer cells that home to endocortical regions in bone and our data demonstrate that homing of prostate cancer cells is associated with the presence and activity of osteoblast lineage cells, and suggest that therapies targeting osteoblast niches should be considered to prevent development of incurable prostate cancer bone metastases.