Circ0008399 Interaction with WTAP Promotes Assembly and Activity of the m6A Methyltransferase Complex and Promotes Cisplatin Resistance in Bladder Cancer

Cisplatin (CDDP)-based chemotherapy is the first-line treatment for muscle-invasive and metastatic bladder cancer, yet most patients rapidly develop resistance. N6-methyladenosine (m6A) methylation is a pervasive RNA modification, and its specific role and potential mechanism in the regulation of CDDP chemosensitivity in bladder cancer remain unclear. Furthermore, studies have not yet fully elucidated whether circular RNA (circRNA) can directly regulate m6A modification of mRNA. Here we report upregulation of a novel circRNA, hsa_circ_0008399 (circ0008399), by eukaryotic translation initiation factor 4A3 (EIF4A3) in bladder cancer tissues and cell lines. Functionally, circ0008399 inhibited apoptosis of bladder cancer cells. Mechanistically, circ0008399 bound Wilms' tumor 1-associating protein (WTAP) to promote formation of the WTAP/METTL3/METTL14 m6A methyltransferase complex. Circ0008399 increased expression of TNF alpha-induced protein 3 (TNFAIP3) by increasing its mRNA stability in an m6A-dependent manner. In patients with bladder cancer, high expression of circ0008399 and WTAP was associated with poor outcomes. Importantly, activation of the circ0008399/WTAP/TNFAIP3 pathway decreased bladder cancer chemosensitivity to CDDP, and targeting the circ0008399/WTAP/TNFAIP3 axis enhanced the CDDP efficacy. Collectively, these findings give novel insights into circRNA-mediated regulation of m6A modifications and provide potential therapeutic targets for bladder cancer. SIGNIFICANCE: A newly characterized circRNA circ0008399 binds WTAP to modulate expression of target RNA through m6A modification and reduce cisplatin sensitivity in bladder cancer, implicating the potential therapeutic value of targeting this axis.