PURPL Represses Radiation-induced Apoptosis to Promote Radioresistance in Cutaneous Melanoma by Direct Interfering with BID Cleavage

During the treatment of cutaneous melanoma, the presence of radioresistance compromised the therapeutic effects of radiotherapy. Yet, the mechanism of radioresistance in melanoma still remains unclear. Bioinformatic analysis of the transcriptomic sequencing showed PURPL was top up-regulated genes in response to ionizing radiation (IR) treatment in melanoma cells. Loss of PURPL notably repressed melanoma cell viability, colony formation, migration and invasiveness which potentiated PURPL’s role in radioresistance of melanoma. Further, loss-of- and gain-of-function analysis indicated that PURPL repressed IR-induced DNA damage and apoptosis. Mechanistically, RNA pulldown was performed and BID was identified as the interacting protein partner of PURPL. As the core gene of the mitochondrial apoptosis pathway, BID could be cleaved by Caspase-8 to generate tBID, which promotes the permeability of the mitochondrial membrane and the release of cytochrome c. IR treatment promotes the interaction between BID and Caspase-8 which was further strengthened by the loss of PURPL. The in vivo assays further verified the in vitro findings. Collectively, our study supports the pro-radioresistance role of PURPL in melanoma by inhibiting the interaction between BID and Caspase-8, which may provide a novel target for clinical radiotherapy. In order to investigate the mechanism of melanoma developing resistance to ionizing radiation, we conducted transcriptome sequencing on melanoma cells A375 after irradiation, and analyzed the differentially expressed genes.