Epithelial ovarian cancer (EOC) frequently metastasises to the omentum. Metastatic growth requires angiogenesis (new blood vessel formation), to provide nutrients and oxygen to the growing secondary tumour. This process requires the activation of local microvascular endothelial cells (ECs), by tumour cell secreted pro-angiogenic factors, such as vascular endothelial growth factor (VEGF). The prognosis for patients with advanced metastatic EOC remains poor and the search for effective treatments is on-going. However, therapeutic targeting of angiogenesis and, in particular, the VEGF pathway has been relatively unsuccessful, suggesting the involvement of other pro-angiogenic factors that may offer alternative therapeutic targets.
Cathepsin-L (CL), a lysosomal protease, has been shown to induce galectin-1 (gal-1) secretion, a small glycoprotein, from disease-relevant human omental microvascular ECs (HOMECs). Gal-1 is upregulated in advanced EOC, and has been implicated in metastatic processes, including angiogenesis. The mechanisms by which gal-1 may contribute to HOMEC pro-metastatic and angiogenic activity are unknown, and therefore the aims of this thesis were to a) improve a method to isolate HOMECs from omental samples, b) to examine pro-metastatic and angiogenic effects of gal-1 in EOC cells and HOMECs, and c) to identify pro-proliferative gal-1 activated receptors and downstream signalling pathways in HOMECs.
HOMEC cultures were obtained by enzymatic digestions and immunoselection, and characterised with immunocytochemistry (ICC). A2780 and SKOV3 EOC cell lines secreted gal-1, and HOMECs secreted significantly more gal-1 in response to CL. Gal-1 pre-treatment of either A2780/SKOV3 cells or HOMEC monolayers, significantly increased A2780/SKOV3 cell adhesiveness to HOMEC monolayers. CL significantly increased extracellular surface gal-1 on HOMECs, and HOMECs were shown to be able to bind exogenous gal-1 to their cell surface. Exogenous gal-1 significantly induced HOMEC proliferation in WST-1 and BrdU assays, as well as HOMEC migration in chamber but not scratch assays. Receptor tyrosine kinase and intracellular phosphokinase arrays identified gal-1 induced phosphorylation of VEGF receptor 2 (VEGFR2) and phospholipase Cγ1 (PLCγ1) independently of VEGF, findings which were confirmed by ELISA and flow cytometry. It was hypothesised that gal-1 was inducing pro-proliferative effects by binding to complex N-glycans on the cell surface (possibly on VEGFR2). Inhibition of complex N-glycan synthesis with swainsonine (SW) significantly inhibited gal-1 induced HOMEC proliferation, phosphorylation of VEGFR2, and preliminarily suggested inhibition of PLCγ1 phosphorylation. Gal-1 also significantly increased retention of VEGF activated VEGFR2 complexes at the cell surface.
In conclusion, these data suggest that gal-1 is a potential pro-metastatic and pro-angiogenic molecule that may contribute to metastasis of EOC to, and within the omentum. In disease-relevant microvascular ECs (HOMECs) gal-1 activates pro-angiogenic responses via the VEGF receptor independently of VEGF, potentially by binding to complex N-glycans on VEGFR2 and initiating pro-proliferative intracellular pathways. Thus gal-1 may represent a new therapeutic target for the treatment of advanced EOC.