Two novel combination therapies of glioblastoma multiforme from a Cx43 perspective : studies of in vitro models

Abstract: The group of patients suffering from the malignant brain tumour glioblastoma multiforme (GBM) have a median survival of only about one year. Thus, new treatment strategies are urgently needed. Gap junctions are intercellular channels, permitting intercellular transfer of water soluble substances. They are vital for cellular homesostasis, proliferation and differentiation. Gap junctions are composed of multimeric proteins termed connexins (Cxs), of which Cx43 is the most ubiquitous. Cx43 serves as a tumour suppressor independent of its ability to form gap junctions. Herpes simplex virus thymidine kinase (HSV-tk) mediated suicide gene therapy has shown promising results in preclinical research, but has as yet failed to prove clinical efficacy. Intriguingly, neural progenitor cells (NPCs) have shown tropism for intracranial tumours and could be exploited for improving the delivery of therapeutic genes or gene products. Epigenetic regulators, such as histone deacetylase inhibitors (HDACi) and receptor tyrosine kinase inhibitors (RTKIs), including gefitinib and vandetanib, have recently entered clinical trials. In the present study, primary cultures of GBM cells were established and showed positivity for neuroepithelial markers. Additionally, they were Cx43 positive and functional gap junctions were demonstrated. The bystander effect in the HSV-tk/ganciclovir (HSV-tk/GCV) suicide gene therapy system was attenuated by a specific gap junction inhibitor, AGA, showing that this phenomenon is dependent on gap junction coupling in glioma cells. The HDAC inhibitor 4-phenylbutyrate (4-PB) up regulated Cx43 and enhanced gap junction communication (GJC), a strategy which could have potential for improving efficacy of suicide gene therapy. NPCs exposed to 4-PB also increased Cx43 expression and gap junction formation. This, together with the propensity of NPCs of tracking down tumour cells, could add a new dimension to gene therapy. Cooperative antitumoural effects were evident in combination treatment of 4-PB and the RTKIs gefitinib and vandetanib. These RTKIs showed an up regulating effect on the tumour suppressor Cx43, which could show to be of importance in the downstream effects of RTKIs.