VEGF receptor-stimulated signal transduction in human oesophageal squamous cell carcinoma cells
Tinderholm, Claire Ann
Signal transduction pathways are essential for regulation of many biological processes, the disruption of which underlies the development of cancer. The autocrine signalling mechanism induced by vascular endothelial growth factor (VEGF) has been implicated in carcinogenesis and cancer progression by contributing to increased cell proliferation, apoptosis prevention, metastasis and increased resistance to treatment. VEGF receptor-stimulated signalling may be an important factor contributing to human oesophageal squamous cell carcinoma (HOSCC) progression and metastasis. HOSCC is prevalent worldwide and a primary cause of cancer death among South African Black males. This form of cancer is renowned for its aggressiveness and high rate of metastasis. Current therapies including surgery, radiotherapy and chemotherapy are largely ineffective and the prognosis is generally poor therefore effective HOSCC prevention and treatment is essential. The ability of VEGFR stimulation to aid in HOSCC progression was evaluated by analysing abundance of active signalling intermediates though western blotting in combination with assessment of cellular proliferation via an MTT assay and observation of cellular migration through scratch-wound assays. An autocrine VEGF/VEGFR signalling axis was found to exist in HOSCCs, where autocrine VEGFR-stimulated signal transduction enhances HOSCC proliferation and initiates HOSCC metastasis. Abrogation of VEGFR induced PI3K/PKB and MAPK/ERK signalling pathways at various specific intermediates revealed an intricate balancing, wherein if one pathway is shutdown VEGFR stimulation may initiate the other to compensate for its loss. In this manner VEGFR signalling contributes to proliferation of HOSCC cells via both the PI3K/PKB and MAPK/ERK/RSK signalling pathways. The VEGFR triggered HOSCC metastatic response however, occurs primarily through the PI3K signalling pathway. Combined inhibition of VEGFR and PI3K/PKB signalling was found to synergistically reduce HOSCC proliferation, reduce HOSCC initiation of metastasis and prevent compensation via salvage pathways. Combination therapy targeting both the VEGFR signalling pathway and the PI3K/PKB signalling pathway is thus a promising option in the treatment of human oesophageal squamous cell carcinoma where the therapies would act together to reduce metastasis, curb drug resistance and synergistically decrease HOSCC proliferation.
A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfillment of the requirements for the degree of Doctor of Philosophy. Johannesburg, March 2019