During dissemination to the skeleton, breast cancer cells are proposed to localise in a putative metastatic niche, situated in close proximity to the endosteal bone surface. To assess whether tumour cells compete for the space in this niche, we mapped the number and location of tumour cells in the tibia 5 out of 12 days following injection of human breast cancer cells in immunocompromised mice. Female 12-week old BALB/c nude mice were injected i.v. with 1×105 MDA-MB-231-IV breast cancer cells labelled with a lipophilic dye (Vybrant-DiD/Vybrant-CM-DiI). The number and location of tumour cells was mapped in three different regions of the tibia by multiphoton microscopy. Using Volocity 3D Image Analysis Software we measured the distance between tumour cells and the nearest bone surface, and to other tumour cells. Competition studies were performed by partially occupying the niche by injecting 1×105 DiD labelled MDA-MB-231-IV cells and repeating the injection seven days later with cells labelled with CM-DiI, allowing separate identification of both cell batches in the niche. The tumour cells preferentially homed to the trabecular area of the bones rather than to the growth plate (P≤0.005). In animals receiving two batches of tumour cells, the number of cells homing to bone from the second batch was significantly lower compared with the cells from the first injection (P≤0.005). Moreover, the preferential homing pattern changed, with tumour cells evenly located in different regions of the bone. Tumour cells were located significantly closer to the bone surface than to other tumour cells (P<0.05 and P<0.01), regardless of whether the niche was empty or partially occupied. Our results show that the preferential pattern of tumour cell homing is modified when the niche is partially occupied, suggesting a degree of competition for space in the bone metastatic niche. (In vivo work covered by UK Home Office license PPL 40/3462).
Disclosure: The authors declared no competing interests. MarieCurie ITN Training Network Agreement number: 264817 (Bone-Net) and the CRUK program entitled Defining the Bone Metastasis Niche.