Verotoxin-1 induction of apoptosis in Gb3-expressing human glioma cell lines

Abstract

The aim of this study was to examine the cytotoxicity and mechanism of apoptosis induction of verotoxin-1 (VT-1) in human glioma cell lines. VT-1 is a member of the shiga-toxin family expressed by some serotypes of Escherichia coli and Shigella dysenteriae. Shiga-toxins have been shown to induce apoptosis by binding to its membrane receptor Gb3. The human glioma cell lines SF-767, U-343 MG, and U-251 MG were studied together with BT4C, a rat glioma cell line. Cells were first screened for Gb3 expression by flow cytometry. Fluorescein diacetate was used to determine cell viability after VT-1 and irradiation exposure and apoptosis was studied by TUNEL staining, a mitochondrial membrane potential assay, and caspase activity assays. SF-767 and U-343 MG cells were found to express Gb3 and were also sensitive to VT-1-induced cytotoxicity, whereas nonGb3-expressing U-251 MG and BT4C glioma cells were not. VT-1 depolarized the mitochondrial membrane and activated caspase-9 and -3 of SF-767 and U-343 MG cells. VT-1 exposure for 72 h resulted in approx. 60 and 90% TUNEL-stained cells, respectively. D, L-Threo-1-phenyl-2-palmitoylamino-3-morpholino-1-propanol (PPMP) an inhibitor of glucosylceramide synthesis was used to block Gb3 synthesis. Two mumol/L PPMP for 72 h abolished SF-767 and U-343 MG expression of Gb3 and made the cells completely resistant to VT-1 induced apoptosis. Key components of MAP kinase signalling pathways that control BAX and mitochondrial function were investigated. VT-1 induced JNK phosphorylation in both cell lines, suggesting that survival signal pathways were overruled by VT-1-induced JNK activation leading to mitochondrial depolarization, caspase-9 activation and apoptosis. Immunohistochemistry of cryostat section from glioma biopsies demonstrated expression of Gb3 was in the vascular endothelial cells as well as tumor cells, but not in astrocytes. The high specificity and apoptosis inducing properties of verotoxin-1 indicates that the toxin may be a potential anti-neoplastic agent for Gb3-expressing gliomas.