CDK9 inhibitor A09-003 overcomes TRAIL resistance via dual Mcl-1 suppression in breast cancer cells

BACKGROUND: Resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a main limitation in breast cancer therapy. Identifying effective sensitizers is crucial to overcome this hurdle. Cyclin-dependent kinase-9 (CDK-9) inhibition has recently emerged as a potential strategy to enhance apoptotic responses in cancer cells. MATERIALS AND METHODS: We investigated the effect of A09-003, a novel CDK9 inhibitor, on TRAIL-induced apoptosis and cell signaling in TNBC breast cancer cells. Cell viability was assessed by CellTiter-Glo assays, while apoptotic induction was evaluated using Flow cytometry analysis with Annexin V/PI staining, DNA fragmentation, and caspase activity assay. Protein expression levels of Mcl-1 and death receptors were analyzed via Western blotting and real time PCR. Mechanistic studies explored Mcl-1 protein degradation and transcriptional regulation through CDK-9 inhibition. RESULTS: A09-003 significantly enhanced TRAIL-induced apoptosis in tested breast cancer cells by increasing death receptor 5 expression, promoting caspase activation, and DNA fragmentation. Mechanistically, A09-003 downregulated Mcl-1 expression through dual pathways: promoting its proteasomal degradation and suppressing transcription via inhibition of CDK9-dependent RNA polymerase II phosphorylation. CONCLUSION: A09-003 restores TRAIL sensitivity through Mcl-1 downregulation, identifying CDK-9 as a therapeutic target in resistant breast cancers.