Defining the perivascular niche in the early stages of breast cancer bone metastasis

Advanced breast cancer is frequently associated with skeletal metastases. During dissemination to bone, breast cancer cells locate in a putative ‘metastatic niche’. Its components are not fully elucidated, however there is evidence of at least partial overlap between the hematopoietic stem cell (HSC), endosteal, metastatic and perivascular niches in bone. Two-photon microscopy was used to detect single tumour cells and map their location within the bone microenvironment. Irrespective of ER status, route of injection and animal age, breast cancer cells preferentially located in the trabecular region of the bone adjacent to the bone surface. Animals pre-treated with AMD3100 to mobilise HSCs prior injection of tumour cells displayed a higher number of breast cancer cells homing to bone, indicating that HSCs and breast cancer cells compete for space within the niche. The microenvironment of tumour growth-promoting (6-week old) and dormancy supporting (12-week old) animal models were investigated. In mature animals, bone volume, length and number of H-vessels and expression of CD31+ and CD34+ vasculature were reduced, while TSP-1 expression was increased, compared to the young model. To investigate the effect of Zoledronic acid (ZOL) on the bone microvasculature, young and mature animal model were treated with a single dose of ZOL and changes in the vasculature visualised with immunofluorescent protocols. In young animals ZOL did not change the microvasculature, while mature animals showed shorter and more numerous vessels. Alteration of microvascular activity, decreased with cediranib and increased with Deferoxaminemesylate, was investigated. Preliminary data did not show substantial changes in either the bone or the microvasculature structure and further studies are required. Overall, my work supports that the bone metastatic niche consists of several overlapping and interconnected niches. My data suggest that new approaches for 9 treatment of bone metastasis should evaluate the effect on several components of the bone microenvironment