Will Hlavinka

Background: Glioblastoma multiforme (GBM) is the most common primary intracranial malignancy in adults.¹ GBM is a grade IV astrocytoma with a dismal mean survival time and two-year survival rate of 9-15 months and less than 10%, respectively.^2,3,4,5 The current standard of care for GBM is surgical resection followed by concomitant treatment with the chemotherapeutic temozolomide (TMZ).^1,6,7,8 TMZ is an alkylating agent responsible for adding alkyl groups to the nitrogen base guanine in DNA, inducing base pair mismatches and subsequent cell death.^1,2 It has been well-documented that O6-methylguanine-DNA methyltransferase, MGMT, is an endogenous protein responsible for repairing DNA damage induced by TMZ, thus promoting GBM cell survival and growth.^1,2,4,6-9 Increased 2-microglobulin signaling and overexpression of TGF-β1 have been observed in TMZ resistant GBM.^5,8,9 These findings could highlight possible therapeutic targets to increase TMZ sensitivity and improve patient outcomes.

Objective: In this review, we investigated the biomolecular mechanisms behind MGMT-induced TMZ resistance in GBM cells.

Search Methods: Online research was performed through the PubMed MeSH Database for recent articles published between 2017-2022 using the terms: “Glioblastoma”, “drug therapy”, “immunology”, “pathology”, and “therapy”. The keywords “drug resistance” were also used.

Results: Studies demonstrate that MGMT is the most prominent factor in TMZ resistance and that overexpressed signaling pathways may be the root cause.² Preferential expression of 2-microglobulin by glioblastoma stem cells (GSCs) was observed in surgically resected GBM cells.⁵ 2-microglobulin was subsequently found to promote maintenance and growth of GSCs through activation of the PI3K/AKT signaling cascade with downstream upregulation of the growth-promoting MYC gene.⁵ MYC directly influenced the upregulation of TGF-β1, and knockdown 2-microglobulin correlated with decreased levels of MYC and TGF-β1.⁵ Overexpression of TGF-β1 demonstrated increased TMZ resistance in both in vitro and in vivo models.⁹ TGF-β1 alters miRNA-198 processing by disrupting K-homology splicing protein (KSRP) function.⁹ KSRP is responsible for the maturation of miRNA-198.⁹ TGF-β1 directly upregulated competitive inhibitors of the KSRP/miRNA-198 interaction, decreased KSRP affinity for miRNA-198, and decreased nucleocytoplasmic of KSRP into the nucleus.⁹ MiRNA-198 was found to have a predicted binding site in the 3’-UTR of MGMT mRNA, demonstrating its direct functions in post-transcriptional silencing of the MGMT gene.⁴ Both cell and mouse models exhibiting upregulated miRNA-198 showed significant increases in TMZ sensitivity and TMZ-induced growth inhibition in vivo.⁴ One study found that TGF-β1 inhibitors, such as OKN-007, could serve as an adjunct therapy to TMZ to modulate its effectiveness.⁸ This study saw significant increases in survival and decreases in tumor volume for in vivo models that underwent a combination therapy including OKN-007 and TMZ.⁸

Conclusions: Studies have demonstrated a detailed mechanism that controls MGMT upregulation in TMZ-resistant GBM cells leading to dismal prognoses. Inhibition of TGF-β1, one of the major contributors in this pathway, shows considerable promise in reversing MGMT-mediated TMZ resistance. Further investigation of drug therapies targeting TGF-β1 or other mediators of this mechanism could lead to significant improvements in the treatment of GBM and patient outcomes.

Works Cited:

1. Melhem JM, Detsky J, Lim-Fat MJ, Perry JR. Updates in IDH-Wildtype Glioblastoma. Neurotherapeutics. 2022;19(6):1705-1723. doi:10.1007/s13311-022-01251-6
2. Tomar MS, Kumar A, Srivastava C, Shrivastava A. Elucidating the mechanisms of Temozolomide resistance in gliomas and the strategies to overcome the resistance. Biochim Biophys Acta Rev Cancer. 2021;1876(2):188616. doi:10.1016/j.bbcan.2021.188616
3. Ghosh M, Shubham S, Mandal K, Trivedi V, Chauhan R, Naseera S. Survival and prognostic factors for glioblastoma multiforme: Retrospective single-institutional study. Indian J Cancer. 2017;54(1):362-367. doi:10.4103/ijc.IJC_157_17
4. Nie E, Jin X, Wu W, et al. MiR-198 enhances temozolomide sensitivity in glioblastoma by targeting MGMT. J Neurooncol. 2017;133(1):59-68. doi:10.1007/s11060-017-2425-9
5. Li D, Zhang Q, Li L, et al. β2-Microglobulin Maintains Glioblastoma Stem Cells and Induces M2-like Polarization of Tumor-Associated Macrophages. Cancer Res. 2022;82(18):3321-3334. doi:10.1158/0008-5472.CAN-22-0507
6. Szylberg M, Sokal P, Śledzińska P, et al. MGMT Promoter Methylation as a Prognostic Factor in Primary Glioblastoma: A Single-Institution Observational Study. Biomedicines. 2022;10(8):2030. Published 2022 Aug 20. doi:10.3390/biomedicines10082030
7. McKinnon C, Nandhabalan M, Murray SA, Plaha P. Glioblastoma: clinical presentation, diagnosis, and management. BMJ. 2021;374:n1560. Published 2021 Jul 14. doi:10.1136/bmj.n1560
8. Towner RA, Smith N, Saunders D, et al. OKN-007 Increases temozolomide (TMZ) Sensitivity and Suppresses TMZ-Resistant Glioblastoma (GBM) Tumor Growth. Transl Oncol. 2019;12(2):320-335. doi:10.1016/j.tranon.2018.10.002
9. Nie E, Jin X, Miao F, et al. TGF-β1 modulates temozolomide resistance in glioblastoma via altered microRNA processing and elevated MGMT. Neuro Oncol. 2021;23(3):435-446. doi:10.1093/neuonc/noaa198