INVESTIGATION OF ANGIOGENESIS INDUCTION FOLLOWING MITOCHONDRIAL INHIBITION IN GLIOBLASTOMA

Abstract: Glioblastoma (GBM) is the most common and lethal primary malignant adult brain tumour. Current standard of care for GBM consists of tumour resection followed by concomitant radiotherapy and chemotherapy with temozolomide which have led to a modest improvement in overall survival. To improve the outcome of GBM patients, new treatments are urgently required. Mitochondrial inhibition by ME-344 (MEI Pharma), a second generation isoflavone analogue, has shown cytotoxic activity in pre-clinical in vitro and in vivo models of ovarian cancer and leukemia. Preliminary studies have shown that GBM cells are highly sensitive to ME-344 in vitro ME-344 exhibited therapeutic efficacy as a single agent in U87MGLuc-2 orthotopic GBM xenografts by prolonging mouse survival compared to vehicle, although to a lesser extent than expected from in vitro models, suggesting a mechanism of resistance activated in vivo ME-344 treatment induces the hypoxia-inducible factor 1 alpha (HIF-1a) both in vitro in patient-derived cell lines and in vivo in a U87MGLuc mouse model. HIF-1a is the inducible subunit of the HIF-1 transcription factor that regulates genes involved in the response to hypoxia, including those that promote angiogenesis, a process of new blood vessel formation, such as the vascular endothelial growth factor (VEGF). Understanding the tumour physiology following mitochondrial inhibition might provide an insight into the mechanisms of resistance observed in vivo in GBM models. The objective of this study was to investigate the pro-angiogenic growth factor (VEGF-A) and VEGF receptor-2 (VEGFR-2) signalling induced by ME-344 treatment of the established glioblastoma cell line U87MGLuc-2 in a novel in vitro three-dimensional (3D) model that more closely reflects clinical phenotypes and in vivo in an orthotopic GBM mouse model. In vitro, ME-344 induced HIF-1a expression and up-regulated pro-angiogenic growth factor VEGF-A protein expression. However, no increase in VEGF-A secretion was detected following ME-344 treatment as demonstrated by ELISA assay. Immunohistochemical analysis of VEGF-A expression in intracranial U87MGLuc tumours showed no significant changes. Although ME-344 induced HIF-1 a and VEGF-A protein expression both in vitro and in vivo in the U87MGLuc cell line, no increase in secretion could be detected in vitro suggesting that angiogenesis is not activated following ME-344 treatment.