The ABCG2 transporter is a key molecular determinant of the efficacy of sonodynamic therapy with Photofrin in glioma stem-like cells

We aimed to investigate the role of the ABCG2 transporter in the efficacy of sonodynamic therapy (SDT) with Photofrin in the glioma stem-like cells (GSCs) isolated and cultured from U251 glioma cells. Immunocytochemistry and flow cytometry analyses showed that ABCG2 was overexpressed in GSCs, and the percentage of ABCG2-positive GSCs was approximately 100%. The effect of ABCG2 on Photofrin extrusion in the absence or presence of a specific inhibitor of ABCG2 (fumitremorgin C; FTC) was investigated by determining the intracellular concentration of Photofrin in GSCs incubated with 20 μg/ml Photofrin. Extrusion of Photofrin by ABCG2 was inhibited by 10 μM FTC, which significantly increased the intracellular Photofrin concentration (p < 0.05) from 0.32 ± 0.11 μg/10⁶ cells to 0.89 ± 0.13 μg/10⁶ cells. MTT and TUNEL assays showed that the antitumor effect of SDT (incubation of GSCs with 20 μg/ml Photofrin for 6 h in the dark and ultrasonic activation at 1.0 MHz and 0.5 W/cm² for 2 min) was significantly improved by FTC pretreatment (p < 0.05). Moreover, incubation of GSCs with FTC significantly increased the relative production of ROS in response to SDT. The overexpression of ABCG2 in GSCs results in efflux of Photofrin, indicating that the antitumor effect of SDT with Photofrin may be reduced in GSCs overexpressing ABCG2. However, since FTC improves the efficacy of SDT in GSCs by inhibiting ABCG2-mediated efflux of Photofrin, FTC may be useful in SDT treatment of ABCG2-expressing cancer cells.