P10.33.a targeted inhibition of three major proliferation pathways in glioblastoma induces senescence through a mitochondria-dependent mechanism

Abstract BACKGROUND Glioblastoma (GBM) is a fatal brain tumor with a great unmet need for better therapies. Research on the genomic landscape of GBM has revealed that three major proliferation pathways — the PI3K-mTOR-AKT, the MAPK, and the CDK4/6-RB pathways — are overactive in the majority of GBM patients. Based on that knowledge we have developed a multi-target combination therapy (MTCT) by employing inhibitors of these three pathways. When these inhibitors are used in combination, lower, clinically more relevant concentrations of each drug are sufficient for target inhibition and proliferation arrest than with single agents.Excitingly, we find that MTCT not only shuts down proliferation but also causes permanent growth inhibition by the induction of senescence and the secretion of a senescence-associated secretory phenotype (SASP). MATERIAL AND METHODS Senescence is identified in a multimodal way, including morphological characterization, SASP production, beta-galactosidase activity, and colony outgrowth potential. RNAseq together with (phospho)proteomics are also applied to investigate the molecular mechanism in combination with techniques such as DNA damage assays, flow cytometry, western blot, metabolic assays, and fluorescence microscopy. RESULTS MTCT-senescent cells have increased mitochondrial reactive oxygen species (mtROS). However, they do not acquire nuclear DNA damage but only local mitochondrial DNA damage. Addition of antioxidants that deplete mtROS resulted in senescence rescue both in terms of morphology and proliferation arrest. MTCT-senescent cells display an enhancement in total mitochondrial mass, mitochondrial DNA copy numbers, and the transcription of mitochondrial proteins. Despite this enhancement, the basal respiration rate of these cells is comparable to untreated cells, indicating an impaired maximal respiration capacity in MTCT-senescent cells. Concordantly, these cells display striking disturbances of their mitochondrial network.Intriguingly, depletion of functional mitochondria prevents the induction of mtROS and senescence by MTCT but not by classical senescence inducers such as ionizing radiation or high concentrations of CDK4/6 inhibitors. CONCLUSION MTCT induces senescence in GBM cells through a unique nuclear DNA damage-independent mechanism for which the mitochondria are essential.