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Differentiation of Mesenchymal Glioblastoma Multiform by Bexarotene

ABSTRACT: Glioblastoma multiforme (GBM) is the most aggressive malignant brain tumor. Differentiation can attenuate or block tumor growth; thus, phenotypic screenings were performed to search for drugs inducing differentiational morphological changes in mesenchymal recurrent GBM, a temozolomide (TMZ)-resistant brain cancer. Bexarotene, a selective retinoid X receptor agonist, showed strong inhibition of neurospheroidal colony formation and migration of cultured GBM stem cells. Bexarotene decreased nestin expression and significantly increased GFAP expression. Gene expression changes by bexarotene and all-trans retinoic acid (ATRA), a well-known differentiation inducer, were compared. Both drugs largely altered the gene expression pattern into a tumor-ameliorating direction. These drugs increased the gene expression levels of Krüppel-like factor 9 (KLF9), regulator of G-protein signaling 4 (RGS4), growth differentiation factor 15 (GDF15), angiopoietin-like protein 4 (ANGPTL4), and lowered the level of chemokine receptor type 4 (CXCR4). Both drugs also induced a potential oncogene, transglutaminase 2 (TG2), but the fold-change induced by bexarotene was considerably lesser than that by ATRA. Consistently, the TG2 activity in GBM was elevated several folds by ATRA. Because TG2 upregulation is correlated with tumor transformation and resistance, it is important to control its overexpression. Bexarotene does not upregulate TG2 as much as ATRA, and bexarotene showed in vivo tumoricidal effects in a GBM xenograft mouse model. Thus, these findings strongly suggest that bexarotene is more beneficial than ATRA and should be considered as a differentiation therapeutic agent for mesenchymal GBM.

ORGANISM(S): Homo sapiens

PROVIDER: GSE61002 | GEO | 2014/12/31

SECONDARY ACCESSION(S): PRJNA260057

REPOSITORIES: GEO

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Project description:G protein-coupled receptor 56 (GPR56/ADGRG1) is an adhesion GPCR with an essential role in brain development and cancer. Elevated expression of GPR56 was observed in the clinical specimens of Glioblastoma (GBM), a highly invasive primary brain tumor. However, we found the expression to be variable across the specimens, presumably due to the intratumor heterogeneity of GBM. Therefore, we re-examined GPR56 expression in public domain spatial gene expression data and single-cell expression data for GBM, which revealed that GPR56 expression was high in cellular tumors, infiltrating tumor cells, and proliferating cells, low in microvascular proliferation and peri-necrotic areas of the tumor, especially in hypoxic mesenchymal-like cells. To gain a better understanding of the consequences of GPR56 downregulation in tumor cells and other molecular changes associated with it, we generated a sh-RNA-mediated GPR56 knockdown in the GBM cell line U373 and performed transcriptomics, proteomics, and phospho-proteomics analysis. Our analysis revealed enrichment of gene signatures, pathways, and phosphorylation of proteins potentially associated with mesenchymal (MES) transition in the tumor and concurrent increase in cell invasion and migration behavior of the GPR56 knockdown GBM cells. Interestingly, our analysis also showed elevated expression of Transglutaminase 2 (TG2) - a known interactor of GPR56, in the knockdown cells. The inverse expression of GPR56 and TG2 was also observed in intratumoral, spatial gene expression data for GBM and in GBM cell lines cultured in vitro under hypoxic conditions. Integrating all these observations, we infer a functional link between the inverse expression of the two proteins, the hypoxic niche, and the mesenchymal status in the tumor. Hypoxia-induced downregulation of GPR56 and activation of TG2 may result in a network of molecular events that contribute to the mesenchymal transition of GBM cells, and we propose a putative model to explain this functional and regulatory relationship of the two proteins.

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Differentiation of Mesenchymal Glioblastoma Multiform by Bexarotene

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