Dual role of the mitochondrial protein frataxin in astrocytic tumors.

The mitochondrial protein frataxin (FXN) is known to be involved in mitochondrial iron homeostasis and iron-sulfur cluster biogenesis. It is discussed to modulate function of the electron transport chain and production of reactive oxygen species (ROS). FXN loss in neurons and heart muscle cells causes an autosomal-dominant mitochondrial disorder, Friedreich's ataxia. Recently, tumor induction after targeted FXN deletion in liver and reversal of the tumorigenic phenotype of colonic carcinoma cells following FXN overexpression were described in the literature, suggesting a tumor suppressor function. We hypothesized that a partial reversal of the malignant phenotype of glioma cells should occur after FXN transfection, if the mitochondrial protein has tumor suppressor functions in these brain tumors. In astrocytic brain tumors and tumor cell lines, we observed reduced FXN levels compared with non-neoplastic astrocytes. Mitochondrial content (citrate synthase activity) was not significantly altered in U87MG glioblastoma cells stably overexpressing FXN (U87-FXN). Surprisingly, U87-FXN cells exhibited increased cytoplasmic ROS levels, although mitochondrial ROS release was attenuated by FXN, as expected. Higher cytoplasmic ROS levels corresponded to reduced activities of glutathione peroxidase and catalase, and lower glutathione content. The defect of antioxidative capacity resulted in increased susceptibility of U87-FXN cells against oxidative stress induced by H(2)O(2) or buthionine sulfoximine. These characteristics may explain a higher sensitivity toward staurosporine and alkylating drugs, at least in part. On the other hand, U87-FXN cells exhibited enhanced growth rates in vitro under growth factor-restricted and hypoxic conditions and in vivo using tumor xenografts in nude mice. These data contrast to a general tumor suppressor function of FXN but suggest a dual, pro-proliferative but chemosensitizing role in astrocytic tumors.

mTORC1 inhibitors suppress meningioma growth in mouse models.

PURPOSE: To evaluate the mTORC1 (mammalian target of rapamycin complex 1) pathway in meningiomas and to explore mTORC1 as a therapeutic target in meningioma cell lines and mouse models. EXPERIMENTAL DESIGN: Tissue microarrays (53 meningiomas of all WHO grades) were stained for phosphorylated polypeptides of mTOR, Akt, and the mTORC1 targets 4EBP1 and p70S6K, the latter being the consensus marker for mTORC1 activity. Expression of proteins and mRNAs was assessed by Western blotting and real-time PCR in 25 tumors. Cell lines Ben-Men-1 (benign), IOMM-Lee and KT21 (malignant), and pairs of merlin-positive or -negative meningioma cells were used to assess sensitivity toward mTORC1 inhibitors in methyl-tetrazolium and bromodeoxyuridine (BrdUrd) assays. The effect of temsirolimus (20 mg/kg daily) on tumor weight or MRI-estimated tumor volume was tested by treatment of eight nude mice (vs. 7 controls) carrying subcutaneous IOMM-Lee xenografts, or of eight (5) mice xenotransplanted intracranially with IOMM-Lee (KT21) cells in comparison to eight (5) untreated controls. RESULTS: All components of the mTORC1 pathway were expressed and activated in meningiomas, independent of their WHO grade. A significant dosage-dependent growth inhibition by temsirolimus and everolimus was observed in all cell lines. It was slightly diminished by merlin loss. In the orthotopic and subcutaneous xenograft models, temsirolimus treatment resulted in about 70% growth reduction of tumors (P < 0.01), which was paralleled by reduction of Ki67 mitotic index (P < 0.05) and reduction of mTORC1 activity (p70S6K phosphorylation) within the tumors. CONCLUSION: mTORC1 inhibitors suppress meningioma growth in mouse models, although the present study did not measure survival.

Sunitinib targets PDGF-receptor and Flt3 and reduces survival and migration of human meningioma cells.

The multitargeted tyrosine-kinase inhibitor sunitinib is a highly effective anti-angiogenic and cytostatic agent in the therapy of various tumours. While malignant gliomas have been shown to be responsive to sunitinib, detailed studies analysing human meningiomas are missing. We therefore analysed the effects of sunitinib in two benign (BenMen-1, HBL52) and two malignant (IOMM-Lee, KT21MG) human meningioma cell lines and found that DNA synthesis was significantly (p ≤ 0.001) inhibited following 1, 2 or 5 µM sunitinib, with IC(50) values between 2 and 5 µM in all cell lines. This effect was associated with a G(2)M-arrest at 10 µM for BenMen-1, HBL52 and IOMM-Lee, and 20 µM in KT21MG cells. Nuclear bisbenzimide staining revealed chromatin condensation following treatment with sunitinib concentrations of 10 µM or higher. Corresponding, cell viability assays showed a significant (p ≤ 0.001) short term decrease of viable cells (24h) only for high sunitinib concentrations with IC(50)-values between 10 and 20 µM. However, pre-irradiated meningioma cells (5 Gy) showed a sensitivity shift towards IC(50)-values around 5 µM sunitinib. We also found that 5 µM strongly reduced meningioma cell migration in vitro. Western blot analyses showed abolished platelet derived growth factor receptor (PDGFR)-autophosphorylation after sunitinib. Interestingly, the drug also inhibited the autophosphorylation of the receptor tyrosine kinase fms-like tyrosine kinase 3 (Flt3) in a dose-dependent manner. Taken together, the present data show that micromolar sunitinib has strong cytostatic and anti-migratory effects on human meningioma cells.