Differentiation therapy targeting indoleamine 2,3-dioxygenase 1 in glioblastoma cells in vitro

Glioblastoma cells strongly express the tryptophan-metabolizing enzyme indoleamine 2,3-dioxygenase 1 (IDO1), which is important in enabling immune evasion. Notably, glioma stem-like cells (GSCs) express higher levels of IDO1 than differentiated glioblastoma cells (DGCs); therefore, it was hypothesized that IDO1 inhibition in GSCs may induce differentiation. The present study used the glioblastoma cell line U-87MG, and newly established GSC lines 0125-GSC and 0222-GSC. A model GSC line, Rev-U-87MG, was created by culturing U-87MG in serum-free medium. Rev-U-87MG-IDO(-) was generated by continuous treatment with an IDO1 inhibitor. The DGC lines 0125-DGC and 0222-DGC were derived by culturing 0125-GSC and 0222-GSC in serum-containing medium. IDO1-inhibited GSC lines 0125-GSC-IDO(-) and 0222-GSC-IDO(-) were generated by continuous IDO1 inhibitor exposure. The expression levels of IDO1 and stem cell markers (Nestin, Nanog and Sox2) were measured by western blotting, and temozolomide sensitivity was evaluated by cell counting. Notably, U-87MG, 0125-DGC and 0222-DGC adhered without sphere formation. By contrast, Rev-U-87MG, 0125-GSC and 0222-GSC formed spheres, whereas IDO1-inhibited cells showed partial adhesion with reduced sphere formation. IDO1 and stem cell marker expression were reduced in all IDO1-inhibited cell lines. Rev-U-87MG exhibited decreased temozolomide sensitivity compared with U-87MG, whereas Rev-U-87MG-IDO(-) showed increased sensitivity. Similarly, 0125-DGC and 0222-DGC were more sensitive than their parent GSC lines, and 0125-GSC-IDO(-) and 0222-GSC-IDO(-) also exhibited enhanced drug sensitivity compared with their parent GSC lines at multiple concentrations. These findings suggest that IDO1 inhibition may induce the differentiation of GSCs and reduce their resistance to temozolomide.