Rx-induced autophagy enhances the treshold for apoptosis in melanoma

Advanced malignant melanoma is an aggressive neoplasm resistant to conventional anti-cancer treatment. We, previously, found that an immunophilin, namely FKBP51, was a factor of resistance to cell death induced by the genotoxic drug doxorubicin. FKBP51 has isomerase activity and is important for NF-κΒ activation. Moreover, it is expressed at high levels in melanoma. In order to elucidate the mechanism responsible for radioresistance of melanoma, we investigated the cellular response to ionizing radiation (Rx) in melanoma cells, depleted or not of FKBP51 by gene silencing. We found that Rx activated autophagy, as indicated by an increase of Beclin1 levels and the electrophoretic mobility change of Light Chain 3 (LC3) from non autophagic (LC3-I) to autophagosome membrane recruited (LC3-II). Autophagy was confirmed by transmission electron microscopy. By contrast, apoptosis was observed in irradiated FKBP51-silenced cells, as suggested by annexin V binding and activation of caspases 3, 7 and 9. Consistently, FKBP51-silenced cells exhibited a reduced clonogenic potential after irradiation compared to non-silenced cells. We show that FKBP51 was required for activation of Rx-induced NF-κB, which inhibited apoptosis by stimulating xIAP transcription. We also show that NF-κB up-regulated Beclin-1, at both mRNA and protein level, thereby stimulating autophagy, which in turn inhibited apoptosis through Bax degradation. Using a tumor-xenograft mouse model, the in vivo pre-treatment of tumors with FKBP51-siRNA provoked massive apoptosis after irradiation. In conclusion, our study proposes a crosstalk between autophagy and apoptosis in irradiated melanoma cells and identifies FKBP51 as a possible molecular target for radio-sensitizing strategies against malignant melanoma.