Abstract
PCI-24781 is a novel histone deacetylase inhibitor that inhibits tumor proliferation and promotes cell apoptosis. However, it is unclear whether PCI-24781 inhibits Enhancer of Zeste 2 (EZH2) expression in malignant gliomas. In this work, three glioma cell lines were incubated with various concentrations of PCI-24781 (0, 0.25, 0.5, 1, 2.5 and 5 μM) and analyzed for cell proliferation by the MTS [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium] assay and colony formation, and cell cycle and apoptosis were assessed by flow cytometry. The expression of EZH2 and apoptosis-related proteins was assessed by western blotting. Malignant glioma cells were also transfected with EZH2 siRNA to examine how PCI-24781 suppresses tumor cells. EZH2 was highly expressed in the three glioma cell lines. Incubation with PCI-24781 reduced cell proliferation and increased cell apoptosis by down-regulating EZH2 in a concentration-dependent manner. These effects were simulated by EZH2 siRNA. In addition, PCI-24781 or EZH2 siRNA accelerated cell apoptosis by down-regulating the expression of AKT, mTOR, p70 ribosomal protein S6 kinase (p70s6k), glycogen synthase kinase 3A and B (GSK3a/b) and eukaryotic initiation factor 4E binding protein 1 (4E-BP1). These data suggest that PCI-24781 may be a promising therapeutic agent for treating gliomas by down-regulating EZH2 which promotes cell apoptosis by suppressing the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of the rapamycin (mTOR) pathway.
Highlights
Malignant gliomas are the most common and deadly brain tumor, accounting for ~80% of malignant tumors that affect the central nervous system (Dolecek et al, 2012)
All three malignant glioma cell lines were sensitive to PCI-24781 and cell growth was inhibited in a concentration-dependent manner, whereas PCI-24781 had no significant effect on Normal human astrocytes (NHA)
The expression of poly ADP-ribose polymerase (PARP) and cysteine-protease P-3, both of which are markers of cell apoptosis (Tutt et al, 2010; Steert et al, 2010), was assessed by western blotting. These two proteins were detected in the three malignant glioma cell lines, especially when they were treated with 1 mM PCI-24781 (Figure 2C)
Summary
Malignant gliomas are the most common and deadly brain tumor, accounting for ~80% of malignant tumors that affect the central nervous system (Dolecek et al, 2012). EZH2, which is frequently over-expressed in various cancers, including gliomas (Orzan et al, 2011), promotes cell proliferation and cell cycle progression by up-regulating cell division cycle 2 (Cdc2) and cyclin B1(Xia et al, 2014), blocks apoptosis by down-regulating the proapoptotic factors Puma and Bad (Hubaux et al, 2013), accelerates cell infiltration and metastasis by inhibiting Raf-1 kinase inhibitor protein (RKIP) (Ren et al, 2012) and activates tumor angiogenesis by silencing vasohibin 1 (Lu et al, 2010) These multiple actions result in poor prognosis in cancer patients (Liu et al, 2014). The development of EZH2 inhibitors is a promising therapeutic approach for treating cancer, especially gliomas
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