Camptothecin Triggers Apoptosis in Human and Mouse Drug-resistant Glioblastoma Cells via ROS-mediated Activation of the p53-p21-CD1/CDK2-E2F1-Bcl-xL Signaling Axis

Background/Aim: Glioblastoma multiforme (GBM) is the most aggressive brain tumor. Temozolomide (TMZ) is the first-line treatment for GBM. However, most patients with GBM develop drug resistance. Our previous study showed the effects of camptothecin (CPT) and CRLX101, a nanoparticle of CPT, in suppressing GBM growth by targeting drug-sensitive glioblastoma cells. This study evaluated the effects of CPT on drug-resistant glioblastoma cells and explored the underlying molecular mechanisms. Materials and Methods: Expression of type I topoisomerase (Topo-1) gene in GBM was analyzed using the UALCAN database. Human U87MG-R and mouse GL261-R TMZ-resistant glioblastoma cells were developed. After CPT treatment, apoptotic events were successively determined. The role of the p53-p21-cyclin D1 (CD1)/cyclin-dependent kinase 6 (CDK6)-E2F1-Bcl-xL signaling axis was subsequently investigated. Results: The expression of Topo-1 gene was up-regulated in human GBM compared to normal human brains. Treatment of human U87MG-R cells with CPT decreased cell viability. Sequentially, exposure to CPT led to activation of caspase-3, fragmentation of chromosomal DNA, and cell apoptosis. Furthermore, intracellular reactive oxygen species (iROS) were augmented following CPT treatment. Suppression of iROS production concurrently alleviated CPT-triggered apoptotic insults. CPT enhanced the levels of p53, phosphorylated p53, and p21. In contrast, levels of CDK6, CD1, E2F1, and Bcl-xL were decreased by CPT. Attenuating p53 transactivation activity using pifithrin-α also mitigated the CPT-induced apoptosis. The effects of CPT on killing drug-resistant glioblastoma cells were further confirmed in mouse GL261-R cells. Conclusion: CPT could effectively induce apoptosis in drug-resistant glioblastoma cells via iROS-mediated activation of the p53-p21-CD1/CDK6-E2F1-Bcl-xL axis.