A cyclin D1–CDK4/6–E2F dependency represents an acquired vulnerability in cisplatin-resistant bladder cancer

Cisplatin resistance remains a major obstacle in the treatment of bladder cancer (BC). This study aimed to define molecular drivers of cisplatin resistance and to assess potential therapeutic targets that may help restore treatment responsiveness. Integrative transcriptomic analyses were performed using The Cancer Genome Atlas (TCGA) cohort to compare cisplatin non-responders with responders, alongside a cisplatin-resistant BC cell model to identify dysregulated pathways. Functional studies, including siRNA-mediated knockdown, SRB assay, flow cytometry, western blotting, and microarray-based transcriptomics, sphere formation assay, and clonogenic assay, were used to characterize the dysregulated pathways. Both cisplatin non-responders in TCGA dataset and resistant BC cells exhibited upregulation of CCND1 and enrichment of E2F target genes. Silencing Cyclin D1 restored cisplatin sensitivity in resistant BC cells. Abemaciclib, a CDK4/6 inhibitor, selectively inhibited proliferation of cisplatin-resistant BC cells, reduced RB phosphorylation, induced sub-G1 accumulation, and suppressed expression of key regulators of cell-cycle progression and homologous recombination repair. Combined treatment with abemaciclib and cisplatin synergistically suppressed the proliferation of cisplatin-resistant BC cells in vitro and produced significantly greater tumor growth inhibition in an RT112 xenograft model in vivo. Furthermore, Abemaciclib reduced sphere-forming capability and enhanced the anti-clonogenic effect of cisplatin in cisplatin-resistant BC cells. These findings identify a Cyclin D1/CDK4/6–E2F signaling dependency as a characteristic feature of the cisplatin-resistant state in BC. Targeting this acquired vulnerability provides a mechanistic rationale for combination strategies to enhance cisplatin responsiveness for refractory BC.