The Embryonic Protein Nodal Supports Metastatic Phenotypes in Breast Cancer

Metastasis is the process by which tumour cells disseminate to distant organ sites. Aberrant expression of stem cell-associated proteins within tumours is associated with metastasis and poor patient prognosis. One example of a stem cell factor that is associated with cancer progression is Nodal, a member of the TGF-β superfamily. Nodal is normally limited to pluripotent stem cells during embryonic development, and to specialized dynamic adult tissue (such as the cycling endometrium), but is aberrantly re-expressed in multiple cancer types, including melanoma, glioma, prostate cancer, and pancreatic cancer. The central objective of this thesis is to determine the role of Nodal during various aspects of the metastatic cascade in breast cancer. First, I determined that Nodal inhibition in aggressive breast cancer cell lines impairs tumour growth in an orthotopic nude mouse model, concomitant with reduced proliferation and enhanced apoptosis. Furthermore, in an experimental metastasis assay in NOD/SCID/MPSVII mice, I determined that Nodal knockdown prevents the transition from lung micrometastases to macrometastes, by supporting a positive ratio of proliferation to apoptosis. Using numerous animal models, I then discovered that Nodal promotes angiogenesis, and that knocking down its expression in established tumours reduces vascularization and causes necrosis. Notably, Nodal protein was positively correlated with vascular density in human breast cancer lesions. Mechanistically, Nodal induced a pro-angiogenic profile in breast cancer cells by upregulating VEGF and PDGF. Finally, I investigated the role of Nodal in the regulation of EMT and invasion; phenotypes that are classically associated with this morphogen. Specifically, since Nodal is implicated in mammary gland remodeling and placentation, I examined its effects on cellular invasion in these contexts. Nodal overexpression in poorly metastatic breast cancer and choriocarcinoma cell lines enhanced invasion and EMT-associated changes in gene expression, and this effect was in part mediated by ERK signaling. Nodal inhibition in metastatic breast cancer cell lines reduced spontaneous metastasis to the liver (but not the lung) in NOD/SCID/IL2γR- mice. The results presented herein suggest that Nodal promotes several pro-metastatic processes. Given its restriction to embryonic or highly specialized adult contexts, targeting Nodal in breast cancer poses an exciting avenue for therapeutic intervention