CDK12 Inhibition Sensitizes Glioblastoma Cells to Bcl-xL Targeting

Background: Resistance to mitochondrial apoptosis contributes to treatment failure in IDH-wildtype glioblastoma (GBM). Cyclin-dependent kinase 12 (CDK12) regulates transcriptional programs that sustain tumor survival, but its role in apoptotic control is largely unclear. Methods: CDK12 was inhibited in GBM patient-derived xenografts, stem-like cells, and established lines by genetic depletion or the small-molecule SR-4835. Bcl-xL was targeted using the BH3 mimetic ABT-263 (navitoclax). Viability, apoptosis, and mechanistic endpoints were assessed by combination index analysis, caspase activity, immunoblotting, and gene silencing. Results: Co-treatment with SR-4835 and ABT-263 produced a synergistic loss of viability across GBM models (combination index <0.5), whereas either agent alone had minimal effects. This synergy was associated with enhanced cleavage of initiator and effector caspases and was fully rescued by the pan-caspase inhibitor zVAD-fmk, confirming caspase-dependent apoptosis. Mechanistically, CDK12 blockade downregulated the anti-apoptotic protein Mcl-1 and induced the pro-apoptotic BH3-only protein Noxa through activation of an ATF4-driven integrated stress response (ISR). Silencing ATF4 blunted SR-4835-induced Noxa expression, and Noxa knockdown significantly reduced apoptosis triggered by the combination, demonstrating its essential role in the response. Conclusions: CDK12 inhibition activates an ISR-ATF4-Noxa signaling axis that lowers the apoptotic threshold and sensitizes GBM cells to Bcl-xL inhibition. Dual targeting of CDK12 and Bcl-xL represents a mechanistically defined strategy to overcome apoptosis resistance in GBM and may have broader therapeutic relevance across malignancies driven by Mcl-1-mediated survival. Citation Format: Jeong-Yeon Mun, Chang Shu, Emily Hsia, Mike-Andrew Westhoff, Georg Karpel-Massler, Markus David Siegelin. JM [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2026; Part 1 (Regular Abstracts); 2026 Apr 17-22; San Diego, CA. Philadelphia (PA): AACR; Cancer Res 2026;86(7 Suppl):Abstract nr 3177.