Photodynamic therapy improves sensitivity to temozolomide in glioblastoma cells via inhibiting autophagy

Chemotherapy resistance from temozolomide (TMZ) continues to be a major cause of glioblastoma (GBM) recurrence. Emerging evidence indicates that cytoprotective autophagy constitutes a key resistance mechanism​ to TMZ. Photodynamic therapy (PDT) has been shown to modulate the sensitivity of glioblastoma cells (GCs) to TMZ through multiple pathways. This study aims to explore how PDT regulates the sensitivity of GCs to TMZ chemotherapy by inhibiting autophagy formation.

The vitality and proliferation ability of U251 cells were evaluated under different treatments using cell counting kit-8 (CCK-8), colony formation, and fluorescence assays, respectively. The reactive oxygen species (ROS) levels were measured using the 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) fluorescence probes. The features of autophagy were assessed via lysosomal fluorescent probes, monodansylcadaverine (MDC) staining, and transmission electron microscopy (TEM).

Compared to TMZ group, samples treated with the combination of TMZ and PDT showed a marked reduction in both cell viability and proliferative ability. Despite this anti-tumor effect, cell death, as assessed by propidium iodide (PI) staining, was not significantly increased during the first 4 h of the combination treatment. But the fluorescence intensity of ROS was significantly higher in Combination group, compared with the other group. The fluorescent analyses and TEM observations confirmed an increase in cytoprotective autophagy in the late phase of TMZ therapy. However, the PI staining assay revealed that the TMZ with synergistic PDT enhanced the percentage of GCs death.

The combination of TMZ and PDT suppresses autophagy by increasing ROS levels, thereby improving the sensitivity of GCs to TMZ chemotherapy. This finding represents a novel biomedical engineering strategy to overcome TMZ resistance.