Pharmacologically impeding glioblastoma tumour motility through simultaneous inhibition of aquaporin-1 and ion channels

Abstract

Glioblastoma multiforme (GBM) is an invasive tumour derived from neuroglial progenitor stem cells. Despite increasingly intricate treatment strategies, rapid infiltration of glioma cells into healthy brain tissue remains a major clinical challenge. I hypothesise that therapies which target cellular motility pathways could effectively slow tumour dispersal and widen the time window for administration of frontline treatments aimed at direct eradication of primary tumours. The array of signal transduction pathways that control cellular motility include aquaporins and ion channels. These protein classes could therefore be prime candidates as pharmacological targets to restrain cell motility in glioblastoma. Identifying optimal combinations of inhibitory agents to use against selected channel targets, and developing drug delivery systems that have minimal side effects in the complex environment of the brain, could control glioblastoma motility without disrupting finely tuned activities of neuroglial networks. This thesis explores a selection of ion channel and aquaporin channel blockers as putative inhibitors of in vitro glioblastoma invasion. Results here define novel agents that potently impair GBM cell motility. Natural compounds (xanthurenic acid and caelestine C) and semi-synthetic amides (SN00756563, SN00756564 and SN00756565) decreased invasion in U87-MG and U251-MG glioblastoma cell lines, revealing previously unknown anti-invasive activities of these agents. When combined with the aquaporin-1 inhibitor AqB013, xanthurenic acid and caelestine C produced a synergistic block of invasion in both U87-MG and U251-MG. Ion channel blockers nifedipine, amiloride, apamin, 4-aminopyridine, and AMPA/kainate receptor inhibitor cyanquixaline significantly decreased invasion in U87-MG and U251-MG, effects that were additively enhanced upon co-treatment with AqB013. Interestingly, when solid tumour spheroids of U87-MG and U251-MG were treated with the same agents, no significant additive or synergistic effects were observed. Some combinations of inhibitors blocked invasion in U87-MG more effectively than in U251- MG and vice versa. This differential effect could reflect the array of protein factors that affect the motility of glioblastoma tumour cells, including unique expression patterns of aquaporins and ion channels. This work presents the novel finding that these pharmacological inhibitors, natural compounds, or semi-synthetics, administered at low doses either individually or in combinations, produced no significant cytotoxicity in cultured glioblastoma cells or astrocytes. This could indicate the capacity to minimise off-target effects associated with these agents. Implementation of site-specific drug delivery systems could further reduce off-target effects. pHsensitive gating mechanisms in many channel proteins is attributable to pore-lining histidine residues. Site-directed mutagenesis experiments described in this thesis identified sites within AQP1 where introduction or removal of histidine residues potentiated or abolished cGMPinduced ionic currents in the presence of Ni2+. Histidine could serve as a promising pH-sensing agent to be incorporated into the design of drug delivery systems that are activated by the hallmark acidity of the tumour microenvironment. Impeding glioblastoma tumour dispersal by targeting enriched signalling proteins with functions in key cellular motility pathways could constitute a powerful adjunct therapy when applied in parallel with existing procedures. Additionally, coupling these novel inhibitors of glioblastoma invasion to carrier molecules containing pH-sensitive histidine residues could maximise controlled release of treatment agents to glioblastoma tumour cells and limit off-target effects.