ESTRO 2024 - Abstract Book

S5333

Radiobiology - Tumour biology

ESTRO 2024

[2] Koi L, Löck S, Linge A, Thurow C, Hering S, Baumann M, Krause M, Gurtner K. EGFR-amplification plus gene expression profiling predicts response to combined radiotherapy with EGFR-inhibition: A preclinical trial in 10 HNSCC tumour-xenograft models. Radiother Oncol. 2017 Sep ;124(3) :496-503. Doi: 10.1016/j.radonc.2017.07.009. Epub 2017 Aug 11. PMID: 28807520.

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Targeting PSMA7 in combination with RT and MDM2 inhibition enhances cell death in p53 wild-type GBM

Christian A Showalter, Priyani V Rajasekera, Heather R Manring, Saikh J Haque, Arnab Chakravarti

The Ohio State University, Department of Radiation Oncology, Columbus, USA

Purpose/Objective:

Blocking the interaction between MDM2 and p53 using MDM2 inhibitors has emerged as a promising strategy for the treatment of patients with IDH-wildtype glioblastoma (GBM) harboring wild-type p53. Importantly, p53 wild-type GBM is extremely resistant to conventional radiation and chemotherapy due to the heterogeneity of GBM and its ability to develop acquired resistance to therapy. Thus, the heterogeneous nature of GBM may also limit the potential of MDM2 inhibitors to treat this disease in patients with p53 wild-type tumors. Therefore, the most promising strategy for treating GBM and improving patient outcomes requires the combined targeting of as many therapeutic vulnerabilities in this disease as possible while limiting adverse reactions. Previously, we demonstrated that proteasome subunit alpha type-7 (PSMA7), a non-catalytic subunit of the 20S proteasome core complex involved in regulated protein degradation via the ubiquitin proteasome pathway and overexpressed in GBM, may serve as a therapeutic target in GBM. In earlier work, we showed that RNAi-mediated silencing of PSMA7 in GBM cells decreased proliferation, invasion, and resistance to ionizing radiation treatment (RT). Since MDM2 promotes ubiquitylation and proteasome dependent degradation of wild-type p53, blocking proteasome degradation through inhibition of PSMA7 may enhance the therapeutic effects of MDM2 inhibitors in p53 wild-type GBM by further elevating p53 levels and increasing p53-mediated apoptosis. Therefore, we determined whether silencing PSMA7 in combination with RT and MDM2 inhibition alters cell death in patient-derived primary GBM cells with wild-type p53.

Material/Methods:

Patient-derived primary GBM cells with shRNA-mediated PSMA7 knockdown were generated using antibiotic selection. The MDM2 inhibitor RG7388 was used to inhibit the interaction between MDM2 and p53 in cells. Protein expression was quantified by Western blot analysis. Cell viability was assessed by MTS or ATP assays. A Rad Source 2000 biological irradiator was used to treat cells with ionizing radiation in vitro. In vitro proteasome activity assays were used to measure the three major protease activities of the proteasome in GBM cells.

Results: