Oleanolic acid enhance the radiosensitivity of C6 rat glioma cells by down-regulating HIF-1α expression

Objectives The aim of this study was to identify whether oleanolic acid (OA) enhances the radiosensitivity of C6 rat glioma cells, and to elucidate the molecular mechanism.

Methods In vitro, C6 rat glioma cells were treated with OA, radiation, or both. Cell viability was detected by MTT assay. Radiosensitization was detected by clonogenic survival assay. The expression of hypoxia-inducible factor-1α was evaluated by western blot and real time PCR. In vivo, C6 rat glioma cells were implanted into the right flank of rats, and treated with OA, radiation (12 Gy), both, or no treatment. Tumor-bearing rats were monitored with 18F-FDG PET/CT and 18F-FMISO PET/CT imaging. The activities of glucose transporter member 1 (GLUT-1) and HIF-1α were detected by immunohistochemistry.

Results In vitro, co-treatment with OA led to more cell death after 7 days . Co-treatment with radiation (2 Gy) and 8.4 or 12.6 μg/mL OA led to sensitization enhancement ratios of 1.61 and 1.81 in normoxic C6 rat glioma cells, and 1.57 and 1.72 in hypoxic C6 rat glioma cells, respectively. The protein expression of HIF-1α was decreased when treated with OA and radiation (P < 0.01). The mRNA level of HIF-1α was also decreased in the co-treatment group. In vivo, 24 h post-treatment, the maximal standardized uptake values detected by 18F-FDG PET/CT decreased in the co-treatment group compared to the control and OA groups , but had no significant difference with the radiation group . The tumor to muscle ratios, which were detected by 18F-FMISO PET/CT, decreased in the co-treatment group at 48 h post-treatment. The SUVmax values of the combined group decreased more significantly than that of the other groups at 7 d post-treatment . The expression of HIF-1α and GLUT-1 were lowest in the co-treatment group. Treatment with OA and radiation led to significant tumor growth inhibition.

Conclusions Co-treatment led to significant improvements in tumor growth inhibition and survival. These results suggest that OA may be a promising radiosensitizing adjuvant for radiotherapy.