Glioblastoma (GBM) is an aggressive and incurable tumor of the brain with limited treatment options. Current first-line standard of care is the DNA alkylating agent temozolomide (TMZ), but this treatment strategy adds only ~4 months to median survival due to the rapid development of resistance. While some mechanisms of TMZ resistance have been identified, they are not fully understood. There are few effective strategies to manage therapy resistant GBM, and we lack diverse preclinical models of acquired TMZ resistance in which to test therapeutic strategies on TMZ resistant GBM. In this study, we create and characterize two new GBM cell lines resistant to TMZin vitro, based on the 8MGBA and 42MGBA cell lines. Analysis of the TMZ resistant (TMZres) variants in conjunction with their parental, sensitive cell lines shows that acquisition of TMZ resistance is accompanied by broad phenotypic changes, including increased proliferation, migration, chromosomal aberrations, and secretion of cytosolic lipids. Importantly, each TMZ resistant model captures a different facet of the “go” (8MGBA-TMZres) or “grow” (42MGBA-TMZres) hypothesis of GBM behavior. Thesein vitromodel systems will be important additions to the available tools for investigators seeking to define molecular mechanisms of acquired TMZ resistance.

Glioblastoma (GBM; grade 4 glioma) is a highly aggressive and incurable tumor. GBM has recently been characterized as highly dependent on alternative splicing, a critical driver of tumor heterogeneity and plasticity. Estrogen‐related receptor β (ERR‐β) is an orphan nuclear receptor expressed in the brain, where alternative splicing of the 3' end of the pre‐mRNA leads to the production of 3 validated ERR‐β protein products: ERR‐β short form (ERR‐βsf), ERR‐β2, and ERR‐β exon 10 deleted. Our prior studies have shown the ERR‐β2 isoform to play a role in G ₂ /M cell cycle arrest and induction of apoptosis, in contrast to the function of the shorter ERR‐βsf isoform in senescence and G ₁ cell cycle arrest. In this study, we sought to better define the role of the proapoptotic ERR‐β2 isoform in GBM. We show that the ERR‐β2 isoform is located not only in the nucleus but also in the cytoplasm. ERR‐β2 suppresses GBM cell migration and interacts with the actin nucleation‐promoting factor cortactin, and an ERR‐β agonist is able to remodel the actin cytoskeleton and similarly suppress GBM cell migration. We further show that inhibition of the splicing regulatory cdc2‐like kinases in combination with an ERR‐β agonist shifts isoform expression in favor of ERR‐β2 and potentiates inhibition of growth and migration in GBM cells and intracranial tumors.—Tiek, D. M., Khatib, S. A., Trepicchio, C. J., Heckler, M. M., Divekar, S. D., Sarkaria, J. N., Glasgow, E., Riggins, R. B. Estrogen‐related receptor β activation and isoform shifting by cdc2‐like kinase inhibition restricts migration and intracranial tumor growth in glioblastoma. FASEB J. 33, 13476–13491 (2019). www.fasebj.org

Glioblastoma (GBM; grade 4 glioma) is a highly aggressive and incurable tumor. GBM has recently been characterized as highly dependent on alternative splicing, a critical driver of tumor heterogeneity and plasticity. Estrogen-related receptor beta (ERRβ, ESRRB, NR3B2) is an orphan nuclear receptor expressed in the brain, where alternative splicing of the 3’ end of the pre-mRNA leads to the production of three validated ERRβ protein products – ERRβ short form (ERRβsf), ERRβ2, and ERRβ exon 10-deleted (ERRβ-Δ10). Our prior studies have shown the ERRβ2 isoform to play a role in G2/M cell cycle arrest and induction of apoptosis, in contrast to the function of the shorter ERRβsf isoform in senescence and G1 cell cycle arrest. In this study, we sought to better define the role of the pro-apoptotic ERRβ2 isoform in GBM. We show that the ERRβ2 isoform is located in the nucleus, but also the cytoplasm. ERRβ2 suppresses GBM cell migration, interacts with the actin nucleation-promoting factor cortactin, and an ERRβ agonist is able to remodel the actin cytoskeleton and similarly suppress GBM cell migration. We further show that inhibition of the splicing regulatory cdc2-like kinases (CLKs) in combination with an ERRβ agonist shifts isoform expression in favor of ERRβ2 and potentiates inhibition of growth and migration in GBM cells and intracranial tumors. Financial Support: This work was supported by R21 CA191444 (RBR), a Georgetown University Medical Center (GUMC) Dean for Research’s Toulmin Pilot Project Award (RBR), and a Partners in Research Breakthrough Award (RBR), as well as F99 CA234799 (DMT) and a student research grant from the Medical Center Graduate Student Organization (MCGSO, to DMT). Additional fellowship funding for DMT and MMH was provided by the Tumor Biology Training Grant (T32 CA009686, PI: Dr. Anna T. Riegel). Technical services were provided by the GUMC Animal Models, Flow Cytometry & Cell Sorting, Histopathology & Tissue, Microscopy & Imaging, and Tissue Culture Shared Resources, which are supported in part by Cancer Center Support Grant P30 CA051008 (PI: Dr. Louis M. Weiner). The Mayo Clinic Brain Tumor Patient-Derived Xenograft National Resource is supported in part by P50 CA108961 (JNS, PI: Dr. Patrick O’Neill). The content of this article is the sole responsibility of the authors and does not represent the official views of the National Institutes of Health. Conflict of Interest and Author Contribution Statement: The authors declare no potential conflict of interest. DMT contributed to study design, performed experiments, analyzed data, and wrote the paper. SAK performed experiments, analyzed data, and wrote the paper. CJT performed experiments and analyzed data. MMH performed experiments. SDD performed experiments. JNS provided patient-derived xenograft samples. EG contributed to study design, and performed experiments. RBR contributed to study design, performed experiments, analyzed data, and wrote the paper. All authors reviewed, edited, and approved the manuscript.

Glioblastoma (GBM) is an aggressive and incurable tumor of the brain with limited treatment options. Current first-line standard of care is the DNA alkylating agent temozolomide (TMZ), but this treatment strategy adds only ~4 months to median survival due to the rapid development of resistance. While some mechanisms of TMZ resistance have been identified, they are not fully understood. There are few effective strategies to manage therapy resistant GBM, and we lack diverse preclinical models of acquired TMZ resistance in which to test therapeutic strategies on TMZ resistant GBM. In this study, we create and characterize two new GBM cell lines resistant to TMZ, based on the 8MGBA and 42MGBA cell lines. Analysis of the TMZ resistant (TMZres) variants in conjunction with their parental, sensitive cell lines shows that acquisition of TMZ resistance is accompanied by broad phenotypic changes, including increased proliferation, migration, chromosomal aberrations and secretion of cytosolic lipids. Importantly, each TMZ resistant model captures a different facet of the “go” (8MGBA-TMZres) or “grow” (42MGBA-TMZres) hypothesis of GBM behavior. These model systems will be important additions to the available tools for investigators seeking to define molecular mechanisms of acquired TMZ resistance.