Abstract
Glioblastoma (GBM) is the most aggressive brain tumor, with a strong invasiveness and high tolerance to chemotherapy. With the current standard treatment combining temozolomide (TMZ) and radiotherapy, glioblastoma can be incurable due to drug resistance. The existence of glioma stem‐like cells (GSCs) is considered the major reason for drug resistance. However, the mechanism of GSC enrichment remains unclear. Herein, we found that the expression and secretion of angiopoietin‐like 4 protein (ANGPTL4) were obviously increased in GSCs. The overexpression of ANGPTL4 induced GSC enrichment that was characterized by BMI‐1 and SOX2 expression, resulting in TMZ resistance in GBM. Furthermore, epidermal growth factor (EGFR) phosphorylation induced 4E‐BP1 phosphorylation that was required for ANGPTL4‐induced GSC enrichment. In particular, ANGPTL4 induced 4E‐BP1 phosphorylation by activating PI3K/AKT and ERK cascades for inducing stemness. To elucidate the mechanism contributing to ANGPTL4 upregulation in GSCs, chromatin immunoprecipitation coupled with sequencing (ChIP‐Seq) revealed that specificity protein 4 (Sp4) associates with the promoter region, −979 to −606, and the luciferase reporter assay revealed that Sp4 positively regulates activity of the ANGPTL4 promoter. Moreover, both ANGPTL4 and Sp4 were highly expressed in GBM and resulted in a poor prognosis. Taken together, Sp4‐mediated ANGPTL4 upregulation induces GSC enrichment through the EGFR/AKT/4E‐BP1 cascade.Support or Funding InformationThis research was supported by the Ministry of Science and Technology of Taiwan (MOST 106‐2320‐B‐038 ‐003 ‐MY2, 107‐2320‐B‐038‐001, and 108‐2628‐B‐038‐005‐)
Highlights
Gliomas are the most common type of tumors occurring in the brain, and are classified by the Word Health Organization as grade I–IV, depending on the level of malignancy
We found that rANGPTL4 (5 μg/mL) enhanced spheroid formation and increased the protein expression of CD133, BMI-1, and SOX2 (Figure 2D,E)
We found that the phosphorylation of epidermal growth factor receptor (EGFR) was increased by angiopoietin-like 4 protein (ANGPTL4) overexpression (Figure 3A) in parallel with an increase in 4E (eIF4E)-binding protein 1 (4E-BP1) phosphorylation (Figure 3B)
Summary
Gliomas are the most common type of tumors occurring in the brain, and are classified by the Word Health Organization as grade I–IV, depending on the level of malignancy. Glioblastoma (GBM) is considered a grade IV glioma, with the highest proliferative and invasive capacity among all types of brain tumors [1]. The current standard therapy for GBM includes surgical resection combined with radiation and chemotherapy. With the current standard treatment, GBMs remain incurable due to drug resistance. Glioma cells are capable of transforming into GSCs under the treatment of TMZ, resulting in drug resistance [4]. SOX2 and BMI-1 inhibit the differentiation signal and keep GSCs in an undifferentiated state, which increases the tolerance of tumors to TMZ-mediated chemotherapy [5,6]
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