Off-target effect of the BMI1 inhibitor PTC596 drives epithelial-mesenchymal transition in glioblastoma multiforme

Anthony Flamier,Rimi Hamam,Gilbert Bernier,Mohamed Abdouh,Roy Hanna,Niraj Patel,Andy Gao,Andrea Barabino

doi:10.1038/s41698-019-0106-1

Abstract

Glioblastoma multiforme (GBM) is an incurable primary brain tumor containing a sub-population of cancer stem cells (CSCs). Polycomb Repressive Complex (PRC) proteins BMI1 and EZH2 are enriched in CSCs, promoting clonogenic growth and resistance to genotoxic therapies. We report here that when used at appropriate concentrations, pharmaceutical inhibitors of BMI1 could efficiently prevent GBM colony growth and CSC self-renewal in vitro and significantly extend lifespan in terminally ill tumor-bearing mice. Notably, molecular analyses revealed that the commonly used PTC596 molecule targeted both BMI1 and EZH2, possibly providing beneficial therapeutic effects in some contexts. On the other hand, treatment with PTC596 resulted in instant reactivation of EZH2 target genes and induction of a molecular program of epithelial–mesenchymal transition (EMT), possibly explaining the modified phenotype of some PTC596-treated tumors. Treatment with a related but more specific BMI1 inhibitor resulted in tumor regression and maintenance of cell identity. We conclude that inhibition of BMI1 alone is efficient at inducing GBM regression, and that dual inhibition of BMI1 and EZH2 using PTC596 may be also beneficial but only in specific contexts.

Highlights

Glioblastoma multiforme (GBM) represents the most common brain malignancy in adults
To evaluate the effect on cancer stem cells (CSCs) self-renewal, cancerous neurospheres of the GBM1205 cell line were treated for 7 days with 5 nM of drugs, washed, and maintained for an additional 25 days in drugfree media
Remaining spheres were dissociated at 4600 viable cells/well in fresh media and maintained for an extra 18 days. We found that both drugs dramatically affected the number and size of newly formed colonies, suggesting depletion of the CSC population (Fig. 1c, d)

Summary

Introduction

Glioblastoma multiforme (GBM) represents the most common brain malignancy in adults. Current treatments are mostly effective at reducing intracranial brain pressure and the alkylating agent Temozolide (Temodal) can increase lifespan by ~4 months. The median lifespan of patients at the time of diagnosis is still 9–12 months. An effective treatment is critically needed.[1,2,3] GBMs are highly heterogeneous tumors containing a relatively rare sub-population of cancer-initiating cells expressing the CD133 (PROM1) cell surface antigen.[4,5,6] Based on cell culture and xenotransplantation experiments, it was shown that CD133+ cells behave like neural stem cells, express stem cell markers, and are able to generate new brain tumors in serial transplantations.[5,6,7] The CD133+ cancer stem cell (CSC) fraction represents the radio-resistant cell population in GBM and is believed to be responsible for brain tumor reoccurrence after radiotherapy treatments.[8,9,10] Importantly, cell lines grown under serum conditions are not representative of the phenotype of primary GBM tumors.[7]

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Results

Conclusion