Investigating the role of the tumour microenvironment in breast cancer bone metastasis

Abstract

Cancer metastasis remains the largest challenge in oncology accounting for over 90% of cancer-associated deaths. The bone is the most prevalent site of metastasis in many cancers, including breast cancer where 70% of patients with advanced disease exhibit bone metastasis. With a current lack of curative treatments, patients with metastasis to the bone have a 5-year survival rate of approximately 15%. Recent focus on the tumour microenvironment (TME) has now identified an important role in cancer development. By establishing interactions with the tumour cells, the TME promotes tumour growth, metastasis and treatment resistance. However, much of the research into the TME has been performed in primary tumour models and research into the role of the TME in bone metastasis is limited.

Using in vivo experimental models of breast cancer bone metastasis complemented with in vitro assays, we investigated the role of the TME throughout breast cancer bone metastasis development, including early stage seeding, metastatic growth and during chemotherapy treatment. We first demonstrated that macrophages play a significant role during tumour cell seeding to the bone where the depletion of macrophages during tumour cell seeding resulted in reduced tumour cell number and delayed metastatic growth. Novel intravital imaging and imaging of cleared tissues were also developed and are currently being used to visualise the seeding process in real time. We next investigated the role of the TME in the growth of breast cancer bone metastasis. An increased infiltration of mesenchymal stem cells (MSCs) was observed in the bone metastasis, which when depleted using Nestin-GFP-CreERT2-iDTR mice resulted in significant decrease in bone metastatic growth in comparison to non-depleted controls. In vitro proliferation assays demonstrated that direct contact between bone metastatic breast cancer cells and MSCs promotes tumour cell growth. Furthermore, preliminary data demonstrates that the co-operation between MSCs and macrophages within the TME may also be important in tumour growth. Lastly, we investigated the effect of chemotherapy treatment on breast cancer bone metastasis. Whilst bone metastasis appears to be resistant to doxorubicin, paclitaxel treatment results in increased growth, indicating paclitaxel-induced metastatic growth. The doxorubicin resistance and paclitaxel-induced growth are both dependent on the microenvironment in the bone, therefore flow cytometry, immunostaining techniques and cell depletion models were performed to characterise the component of the TME responsible for the effects of chemotherapy treatment.

Collectively, in this project we demonstrate the role of the TME during the development of breast cancer bone metastasis including the macrophages during tumour cell seeding and MSCs during bone metastasis growth. The TME also plays a pivotal role in chemotherapy resistance and induced growth of bone metastasis. The identification of the molecular mechanisms involved in these processes may offer insight into potential novel therapeutic targets to overcome breast cancer bone metastasis.