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Glioblastoma (GBM; WHO grade IV) is the most common high-grade primary malignant tumor with a dismal five-year survival rate of only 5%. Patients with wild-type isocitrate dehydrogenase 1 and 2 (IDHwt) GBM have the worst survival outcomes and little progress has been made since 2005 to improve upon the current standard treatment for GBM consisting of maximal safe surgical tumor resection followed by radiation and concomitant and adjuvant temozolomide (TMZ). Therefore, there is an urgent need to develop novel treatment strategies and identify actionable targets in GBM. In our study we identified proteasome subunit alpha type-7 (PSMA7), a non-catalytic subunit of the 20S proteasome core particle involved in the regulated protein degradation via the ubiquitin-proteasome pathway and with proteasome-independent functions via interactions with HIF1α, c-Abl, and PAC4 among other oncogenic proteins, to be upregulated in IDHwt GBM. PSMA7 is frequently upregulated in other cancer types and PSMA7 silencing reduces cell proliferation in cervical, colorectal, and gastric cancer. Despite the association between PSMA7 and tumor development in multiple tumor models, the role(s) that upregulated PSMA7 expression has in GBM biology and treatment response is largely unknown. In this study we show that silencing PSMA7 sensitizes GBM cells to ionizing radiation (IR)/TMZ-induced cell death and inhibits double-strand DNA damage repair following IR treatment.

PSMA7 was identified using publicly available mRNA profiling data of two cohorts of patients with primary glioma from the CGGA database. PSMA7 was functionally validated in vitro using RNAi-mediated knockdown (KD) approaches and IDHwt GBM cell lines. Stable cell lines with shRNA-mediated PSMA7 KD were generated. Protein expression was quantified by Western blot analysis. Cell viability was assessed by MTS or ATP assays. A biological irradiator was used to treat cells with IR in vitro to evaluate response to radiation.

Primary glioma patients with IDHwt tumors had higher PSMA7 mRNA levels compared to those from patients with IDH mutant (IDHmut) tumors. Furthermore, PSMA7 mRNA levels increased as the tumor grade increased in IDHwt primary gliomas. Importantly, patients with primary IDHwt glioma and high PSMA7 mRNA levels (median split) had worse overall survival than patients with low PSMA7 mRNA levels. We found that RNAi-mediated KD of PSMA7 in IDHwt GBM cells increased IR-induced cell death in vitro. Additionally, IDHwt GBM cells with PSMA7 KD had prolonged expression of γ-H2AX after treatment with IR. Also, IDHwt GBM cells with PSMA7 KD demonstrated lower cell viability when treated with TMZ in vitro. Collectively, RNAi-mediated silencing of PSMA7 expression enhanced the sensitivity  of GBM cells to IR and TMZ.

Though PSMA7 is part of the proteasome, it has proteasome-independent protein-protein interactions that are potentially targetable for improving response of GBM to the current standard of care.