Glioblastoma is a highly malignant tumor that easily acquires resistance to treatment. The stem-cell-like character (stemness) has been thought to be closely associated with the treatment resistance of glioblastoma cells. In this study, we determined that farnesyl diphosphate synthase (FDPS), a key enzyme in isoprenoid biosynthesis, plays an important role in maintaining glioblastoma stemness. A comparison of the mRNA expression in patient-derived glioblastoma sphere cells, which maintain stemness, and their differentiated counterparts, which lose stemness, via RNA sequencing showed that most of the altered genes were networked in the cholesterol biosynthesis pathway. We screened Federal Drug Administration (FDA)-approved drugs targeting specific enzymes in the cholesterol biosynthesis pathway for their ability to inhibit glioblastoma sphere formation. Inhibitors of FDPS, such as alendronate and zoledronate, significantly reduced the formation of glioblastoma spheres, and alendronate was effective at a lower molar concentration than zoledronate. Knockdown of FDPS using short hairpin RNA also completely inhibited the formation of secondary spheres. FDPS mRNA in patients with glioblastoma was associated with malignancy in three independent microarray data sets. RNA sequencing showed that alendronate treatment reduced the embryonic stem cell signature and activated development- and necrosis-related pathways in glioblastoma spheres. These results suggest that FDPS is important for the maintenance of glioblastoma stemness and that alendronate, a drug widely used to treat osteoporosis, can be repositioned to treat glioblastoma.
API5 (APoptosis Inhibitor 5) and nuclear FGF2 (Fibroblast Growth Factor 2) are upregulated in various human cancers and are correlated with poor prognosis. Although their physical interaction has been identified, the function related to the resulting complex is unknown. Here, we determined the crystal structure of the API5–FGF2 complex and identified critical residues driving the protein interaction. These findings provided a structural basis for the nuclear localization of the FGF2 isoform lacking a canonical nuclear localization signal and identified a cryptic nuclear localization sequence in FGF2. The interaction between API5 and FGF2 was important for mRNA nuclear export through both the TREX and eIF4E/LRPPRC mRNA export complexes, thus regulating the export of bulk mRNA and specific mRNAs containing eIF4E sensitivity elements, such as c-MYC and cyclin D1. These data show the newly identified molecular function of API5 and nuclear FGF2, and provide a clue to understanding the dynamic regulation of mRNA export.
Octamer-binding transcription factor 4 (Oct4) plays an important role in maintaining pluripotency in embryonic stem cells and is closely related to the malignancies of various cancers. Although posttranslational modifications of Oct4 have been widely studied, most of these have not yet been fully characterized, especially in cancer. In this study, we investigated the role of phosphorylation of serine 236 of OCT4 [OCT4 (S236)] in human germ cell tumors (GCTs). OCT4 was phosphorylated at S236 in a cell cycle-dependent manner in a patient sample and GCT cell lines. The substitution of endogenous OCT4 by a mimic of phosphorylated OCT4 with a serine-to-aspartate mutation at S236 (S236D) resulted in tumor cell differentiation, growth retardation, and inhibition of tumor sphere formation. GCT cells expressing OCT4 S236D instead of endogenous OCT4 were similar to cells with OCT4 depletion at the mRNA transcript level as well as in the phenotype. OCT4 S236D also induced tumor cell differentiation and growth retardation in mouse xenograft experiments. Inhibition of protein phosphatase 1 by chemicals or short hairpin RNAs increased phosphorylation at OCT4 (S236) and resulted in the differentiation of GCTs. These results reveal the role of OCT4 (S236) phosphorylation in GCTs and suggest a new strategy for suppressing OCT4 in cancer.
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