The intrinsic radioresistance of certain cancer cells may be closely associated with the constitutive activation of nuclear factor–kappa B (NF-κB) activity, which may lead to protection from apoptosis. Recently, nonapoptotic cell death, or autophagy, has been revealed as a novel response of cancer cells to ionizing radiation. In the present study, the authors analyzed the effect of pitavastatin as a potential inhibitor of NF-κB activation on the radiosensitivity of A172, U87, and U251 human glioma cell lines.
The pharmacological inhibition of NF-κB activation was achieved using pitavastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. Growth and radiosensitivity assays were performed using a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. Hoechst 33258 staining, supravital acridine orange staining, and electron microscopy were performed utilizing 3 glioma cell lines with or without pitavastatin pretreatment to identify apoptosis or autophagy after irradiation.
The growth of these 3 glioma cell lines was not significantly inhibited by pitavastatin at a concentration of up to 1 μM. Treatment with 0.1 μM of pitavastatin enhanced radiation-induced cell death in all glioma cell lines, with different sensitivity. Apoptosis did not occur in any pretreated or untreated (no pitavastatin) cell line following irradiation. Instead, autophagic cell changes were observed regardless of the radiosensitivity of the cell line. An inhibitor of autophagy, 3-methyladenine suppressed the cytotoxic effect of irradiation with pitavastatin, indicating that autophagy is a result of an antitumor mechanism. Using the most radiosensitive A172 cell line, the intracellular localization of p50, a representative subunit of NF-κB, was evaluated through immunoblotting and immunofluorescence studies. The NF-κB of A172 cells was immediately activated and translocated from the cytosol to the nucleus in response to irradiation. Pitavastatin inhibited this activation and translocation of NF-κB.
Autophagic cell death rather than apoptosis is a possible mechanism of radiation-induced and pitavastatin-enhanced cell damage, and radiosensitization by the pharmacological inhibition of NF-κB activation may be a novel therapeutic strategy for malignant gliomas.