Tumour-Endothelial Interactions in Metastasis

Metastatic progression of cancer occurs in a vast number of patients and accounts for 90% of cancer related deaths. Understanding metastasis therefore remains a priority for cancer research and treatment. Tumour cells need to bind to and cross endothelial cell barriers during metastasis. The aim of this work was to investigate tumour-endothelial interactions in vitro and in vivo and understand their implications on metastatic progression. The type I transmembrane receptor CD99 is expressed by leucocytes and endothelial cells (EC) and participates in inflammatory transendothelial migration (TEM). CD99 is also expressed by tumour cells and we have analysed the role of CD99 in tumour progression and cancer cell TEM. In a xenograft model, CD99 negatively regulated metastatic progression of human breast cancer. In vitro, tumour cell CD99 was required for adhesion to EC. However, CD99 negatively regulated post-adhesion events of tumour TEM, namely invasiveness of the endothelial barrier by metastatic cancer cells and TEM itself. Furthermore, tumour cell CD99 depletion was associated with cytoskeletal remodelling. For EC, loss of CD99 enhanced endothelial barrier function and reduced tumour cell TEM. The loss of CD99 enhanced expression and activity of the Rho GTPase Cdc42, a known cytoskeletal organiser. As a signal transduction hub, Cdc42 activity impacts upon many of the hallmarks of cancer. The functional link between CD99 and Cdc42 now implicates CD99 in these diverse functions. Further work in this thesis focussed on brain metastatic breast cancer cells. A major site for breast cancer metastasis is the brain where a specialised endothelial barrier, the blood brain barrier, operates. A surface screen of adhesion molecules revealed a role for CD146 in the adhesion and transmigration of breast cancer cells across the endothelium. Analysis of clinical data indicated that CD146 may of prognostic value in evaluating breast cancer metastasis. This work uncovers a potentially novel role for CD146 in breast cancer brain metastasis through mediating the interaction between cancer cells and the brain endothelium. The immortalised cell line hCMEC/D3 is a commonly used model of brain barrier endothelium. However, this cell line lacks key endothelial functions, for example being poorly responsive to VEGF signalling. It is shown here that p53 plays a key role in regulating the endothelial phenotype of this cell line and that stable p53 knockdown enhances the endothelial responses of this cell line thereby increasing the utility of this model system.