The Proteasome Is Revealed as a Therapeutic Target in Recurrent Glioblastoma Xenografts

Radiation remains a primary treatment for glioblastoma (GBM), yet tumors frequently recur within 2 years. In this study, orthotopic xenografts initiated from glioma stem-like cells (GSC) implanted into the right striatum of nude mice were used to investigate the biology of recurrent GBM. In this model, untreated tumors showed diffuse growth pattern across the right hemisphere and olfactory bulb (OB), whereas postirradiation tumors (10 Gy) regrew predominantly within the OB, exhibiting increased cell density and a well-demarcated border indicative of an altered growth pattern. Transcriptomes of untreated and recurrent tumors were assessed using spatial profiling. Comparison of gene expression across regions of interest revealed that recurrent tumors are less heterogeneous and exhibit a distinct transcriptional profile compared with untreated tumors. A total of 463 genes were differentially expressed, and gene set enrichment analysis revealed significant enrichment of pathways related to cell-cycle regulation in the recurrent as compared with untreated tumors. Further analysis of those pathways revealed a significant upregulation of 22 proteasome-related genes in recurrent tumors. Moreover, functional assays revealed significantly higher proteasome activity in recurrent compared with untreated tumors, suggesting the proteasome as a potential therapeutic target unique to recurrent GBM. To evaluate the therapeutic relevance, mice were treated with the combination of radiation followed by the proteasome inhibitor ixazomib. Whereas ixazomib had no effect on untreated tumors, its administration after irradiation significantly prolonged survival in two GSC xenograft models. These results illustrate how defining molecular alterations that develop in recurrent GBM xenografts can lead to the identification of a novel therapeutic target.