EPOR deficiency on glioma cells induces sensitivity to ionizing radiation and temozolomide

The standard of care for glioblastoma (GBM) patients consists primarily of surgical resection followed by radiotherapy and concomitant temozolomide (TMZ) which provides a modest improvement in survival compared to radiation alone. One prognostic factor identified as a reliable biomarker for GBM sensitivity to TMZ is the methylation status of O6-methylguanine-methyl-transferase (MGMT). On the other hand, recombinant EPO has been, until recently, widely used in clinic treatment of anemia associated with cancer, with a potential effect on the increase in tumor oxygenation and thereby an improvement of radiosensitivity. Recently, we and other demonstrated that glioma cells express EPOR and that targeting EPOR on glioma cells reduces tumor growth. Accordingly, the purpose of this study was to evaluate whether silencing EPOR on glioma cells might contribute to modulate sensitivity of these tumor cells to either radiation or chemotherapy. We showed that silencing EPOR on U251 or U87 cells, two human glioma cell lines, led to a reduction in tumor growth both in vitro and in vivo which might be in part due to a G2/M arrest of these cells, an increase in apoptosis and/or senescence. Our results also demonstrated that the inhibition of EPOR of these cells (shEPOR-U87 or shEPOR-U251) increased their radiosensitivity to X-Rays and, more importantly, counteracted the hypoxia-induced radioresistance. On the other hand, when U87 or U251 cells, which exhibit a methylation of MGMT promoter, were exposed to TMZ, EPOR inhibition reinforced the efficacy of TMZ treatment in vitro. In vivo, we showed that animals which received an ortho-topic implantation of shEPOR-U251 cells in the striatum, and treated with a TMZ regimen closed to that of glioma patients, displayed a reduction in tumor volume as compared to scrambled-U251 tumor-bearing animals (scrambled-U251 + TMZ group: 57%; shEPOR-U251 + TMZ group: 77%, p<0.01) accompanied with an increase in survival median of 34% versus scrambled-U251 animals treated with TMZ (p<0.005). In conclusion, our results suggest that EPOR on glioma cells might be one of the candidate targets for radiotherapy combined with TMZ for patients with TMZ-resistant GBM and/or radiotherapy-resistant GBM. (authors)