A novel 149-amino acid protein encoded by circSOD2 inhibits ferroptosis and promotes cisplatin resistance in bladder cancer

Circular RNAs (circRNAs) play pivotal roles in bladder cancer (BCa) tumorigenesis, metastasis, and chemoresistance, but their functional involvement in cisplatin-induced ferroptosis remains poorly defined. Herein, we identified circSOD2 as a significantly upregulated circRNA in cisplatin-resistant BCa cell lines and clinical specimens via RNA sequencing. Functional assays demonstrated that circSOD2 overexpression enhanced BCa cell proliferation and cisplatin resistance in vitro (CCK-8, colony formation, and ferroptosis detection assays) and in vivo (xenograft mouse models). Mechanistically, ribosome profiling and Western blot analyses confirmed the protein-coding potential of circSOD2, which translates into a novel 149-amino acid peptide (SOD2-149aa). Co-immunoprecipitation (Co-IP) and glutathione S-transferase (GST) pull-down assays verified that SOD2-149aa directly binds to the RING domain of tripartite motif 26 (TRIM26), a ubiquitin E3 ligase. This interaction activates TRIM26-mediated K63-linked polyubiquitination of glutathione peroxidase 4 (GPX4) - a key anti-ferroptotic enzyme - at lysine residue 148, thereby preventing GPX4 proteasomal degradation and enhancing its protein stability. Stabilized GPX4 efficiently scavenges lipid reactive oxygen species (ROS) and reduces iron-dependent lipid peroxidation, ultimately mitigating cisplatin-induced ferroptosis and conferring cisplatin resistance in BCa. Conversely, circSOD2 silencing or SOD2-149aa knockout abrogated TRIM26-GPX4 interaction, diminished GPX4 stability, and restored BCa cell sensitivity to cisplatin. Collectively, our findings uncover a novel circRNA-encoded peptide that regulates ferroptosis and chemoresistance via the TRIM26-GPX4 axis, highlighting circSOD2/SOD2-149aa as a promising prognostic biomarker and therapeutic target for overcoming cisplatin resistance in BCa.