FBXW7/hCDC4 controls glioma cell proliferation in vitro and is a prognostic marker for survival in glioblastoma patients
© Hagedorn et al; licensee BioMed Central Ltd. 2007
Received: 27 October 2006
Accepted: 27 February 2007
Published: 27 February 2007
In the quest for novel molecular mediators of glioma progression, we studied the regulation of FBXW 7 (hCDC 4/hAGO/SEL 10), its association with survival of patients with glioblastoma and its potential role as a tumor suppressor gene in glioma cells. The F-box protein Fbxw7 is a component of SCFFbxw7, a Skp1-Cul1-F-box E3 ubiquitin ligase complex that tags specific proteins for proteasome degradation. FBXW 7 is mutated in several human cancers and functions as a haploinsufficient tumor suppressor in mice. Any of the identified targets, Cyclin E, c-Myc, c-Jun, Notch1/4 and Aurora-A may have oncogenic properties when accumulated in tumors with FBXW 7 loss.
We tested the expression of FBXW 7 in human glioma biopsies by quantitative PCR and compared the transcript levels of grade IV glioma (glioblastoma, G-IV) with those of grade II tumors (G-II). In more than 80% G-IV, expression of FBXW 7 was significantly reduced. In addition, levels of FBXW 7 were correlated with survival indicating a possible implication in tumor aggressiveness. Locus 4q31.3 which carries FBXW 7 was investigated by in situ hybridization on biopsy touchprints. This excluded allelic loss as the principal cause for low expression of FBXW 7 in G-IV tumors. Two targets of Fbxw7, Aurora-A and Notch4 were preferentially immunodetected in G-IV biopsies. Next, we investigated the effects of FBXW 7 misregulation in glioma cells. U87 cells overexpressing nuclear isoforms of Fbxw7 lose the expression of the proliferation markers PCNA and Ki-67, and get counterselected in vitro. This observation fits well with the hypothesis that Fbxw7 functions as a tumor suppressor in astroglial cells. Finally, FBXW 7 knockdown in U87 cells leads to defects in mitosis that may promote aneuploidy in progressing glioma.
Our results show that FBXW 7 expression is a prognostic marker for patients with glioblastoma. We suggest that loss of FBXW 7 plays an important role in glioma malignancy by allowing the accumulation of multiple oncoproteins and that interfering with Fbxw7 or its downstream targets would constitute a new therapeutic advance.
Glioblastoma (glioma grade IV, G-IV), the most common tumor arising in the central nervous system, is one of the deadliest cancers with a mean survival of less than one year . Reliable molecular predictors of survival outcome as well as novel targets for efficient therapy are urgently needed to improve life conditions of patients with glioma.
In this study, we investigated the expression of FBXW 7 (also known as hCDC 4, hAGO, SEL 10) in glioma. FBXW 7 encodes one of the 75 F-box proteins identified so far in mammals . F-box proteins represent the variable receptor component of Skp1-Cul1-F-box (SCF) complexes, that mediates binding and ubiquitination of specific proteins, which are consequently recognized and destroyed by the proteasome. In contrast to other SCF complexes such as SCFFbxl1 which target both positive and negative regulators of the cell cycle , all known targets of SCFFbxw7 – namely Cyclin E [4–6], c-Myc , c-Jun , Notch 1 and 4 [9–11] and Aurora-A  – are cell growth promoters and potential oncoproteins. Their turn-over can thus be seen as an ultimate process in tumor suppression control. Indeed, FBXW 7 itself behaves as a haploinsufficient tumor suppressor gene: the loss of one functional allele is enough to promote epithelial tumor growth in a mouse model . Given the number of its targets and the fact that FBXW 7 is translated into three different isoforms with distinct subcellular localization, possible mechanisms of tumor suppression are bound to be complex and variable depending upon the cell type in which downregulation occurs.
FBXW 7 is mutated in many cancer cell lines and human tumors such as endometrial, pancreatic and colorectal cancers [6, 14–17]. Mutations of FBXW 7 in brain tumors have not been investigated yet, however the corresponding locus – 4q31.3 – belongs to the most frequently lost portion of chromosome 4 in glioblastoma [18, 19]. As far as targets are concerned, much attention has been focused on the misregulation of Cyclin E  and c-Myc  whereas other substrates such as Aurora-A and Notch receptors have not yet been comprehensively investigated. In tumors, correlations have been demonstrated between loss of function of FBXW 7 and high levels of Cyclin E , and between Cyclin E overexpression and chromosomal instability . However, the overall effect of FBXW 7 loss in cancer cells is not straightforward and seems to vary according to cancer types [16, 22]. Fujii and coworkers recently reported the accumulation of multiple targets in several FBXW 7 mutant cell lines with a variable extent of increase in expression and even identified a different pattern of accumulation for two ovarian cancer lines, one accumulating predominantly both Cyclin E and c-Myc, the other Aurora-A solely .
In this study, we investigated the possible misregulation of Aurora-A and Notch 4 expression in glioma. Aurora-A is a mitotic kinase required for G2-M transition, centrosome maturation and alignment of chromosomes at metaphase. Its degradation by the proteasome occurs promptly after metaphase-anaphase transition and is necessary for mitotic exit . In normal cells, the expression of Aurora-A is thus rhythmic, discrete and detected in dividing cells only. By contrast, in cancer cells, Aurora-A is frequently overexpressed indicating its possible involvement in tumorigenesis . Indeed, Aurora-A overexpression results in centrosome amplification and cytokinesis failure – both promoting tetraploidization – and transformation of cells with already acquired checkpoint defects [26, 27]. How does Aurora-A accumulate in cancer cells? In bladder cancer, AURKA/STK 15, the gene encoding Aurora-A at locus 20q13.2 is commonly amplified and the resulting overexpression is correlated with critical clinical parameters such as invasion, metastasis and poor survival . But the overexpression of Aurora-A is also frequently seen in tumors with no gain of AURKA at the DNA level suggesting that other deregulated mechanisms such as transcriptional activation or failure of a proteolysis component are responsible for Aurora-A accumulation. This may well apply to glioma: Klein and coworkers detected amplification of AURKA in 26% of malignant glioma while as much as 67% of the samples eventually overexpressed the gene .
Unlike all other targets of Fbxw7, Notch4 has not yet been investigated in human cancers though the protein is oncogenic when induced in the MMTV (mouse mammary tumor virus) mouse model . Thus far, Notch4 has been described as a vascular endothelium specific signaling receptor. Together with its ligand Delta-like 4 (Dll4), it is involved in vessel development by mediating arterial/venous specification and vascular remodeling during embryogenesis . In tumors, Notch4 signaling is activated in the endothelium and is here again responsible for vascular maturation . On the other hand, it has been recently shown that NOTCH 4 transcription can be derepressed in non-endothelial cells: in HeLa cells treated with endothelial growth factors, cell-type-specific AP-1 (activator protein 1) complexes are activated that are able to reprogram NOTCH 4 expression . This raises the possibility that tumor cells, which produce high amounts of proangiogenic factors – like malignant glioma cells – may ectopically express NOTCH 4.
Here we report that expression of FBXW 7 is strongly reduced in G-IV tumors, mostly independent of locus depletion, and that the levels of FBXW 7 expression correlate with patient survival. We find that Aurora-A and Notch4 accumulate in perivascular zones of patient glioblastoma. Proliferation is significantly impaired in glioma cells overexpressing nuclear FBXW 7 in vitro suggesting that it acts as a tumor suppressor in astroglial cells. Finally we show that knocking down FBXW 7 in cultured glioma cells destabilizes chromosome segregation at mitosis, a process controlled by several targets of SCFFbxw7 including Aurora-A.
Results and Discussion
Expression of FBXW7 in glioma patients and correlation with survival
Allelic loss is not the primary cause for low expression of FBXW7 in G-IV tumors
This analysis suggests that only for a minority of glioblastoma, monoallelic deletion of 4q31.3 may participate in reduction of FBXW 7 expression. If downregulation is required for glioblastoma progression, these events may be selected preferentially in tumor cells that retain P53 expression. Nonetheless, we conclude that allelic loss is not a recurring cause for reduced FBXW 7 expression in glioblastoma.
Aurora-A and Notch4 accumulate in G-IV tumors
All in all, two oncoproteins the mitotic kinase Aurora-A and the signaling receptor Notch4, which levels depend on FBXW 7, are strongly expressed in specific zones of G-IV tumors, particularly around blood vessels.
Overexpression of FBXW7 inhibits proliferation of U87 cells in vitro
The effect of Fbxw7 on proliferation was further confirmed by showing that nuclear Fbxw7 counterselects transiently transfected cells in a time-course culture (Fig. 4L): the growth of cells overexpressing α-Fbxw7 or γ-Fbxw7 is strongly inhibited between 24 and 72 h after transfection (P < 0.0001 and P < 0.005 respectively) whereas FLAG negative internal control cells expand of more than 2-fold. By contrast, overexpression of isoform β does not significantly affect the growth of U87 cells. FLAG positive cells were screened for Aurora-A expression by double immunofluorescence with anti-FLAG and anti-Aurora-A antibodies. Amongst cells expressing FBXW 7 in the nucleus (n > 400) no Aurora-A staining was detected compared to untransfected cells out of which 10% were Aurora-A positive. This mutually exclusive expression pattern suggests that Fbxw7 targets Aurora-A in glioma cells. This may interfere with cell cycle checkpoint hence the dramatic reduction of dividing cells. Given the effect of Aurora-A knockdown, we can anticipate that cells may then be arrested in the next cycle by the post-mitotic checkpoint , hence their progressive counterselection in culture.
In summary, these results indicate that nuclear Fbxw7 exerts an inhibitory effect on proliferation of glioma cells in vitro and therefore support the hypothesis that loss of nuclear FBXW 7 expression might contribute to tumor progression in patients with astroglial tumors.
Fbxw7 knockdown causes mitotic defects in U87 cells
Possible targets that accumulation may cause checkpoint disturbances and chromosomal instability are Aurora-A and Cyclin E although they could not be quantitatively monitored in these transient transfection assays. Cyclin E accumulation has been shown to raise similar defects in colorectal cancer cells  and is accumulated in tumors of different types  including glioma with poor prognosis .
In this study, we have shown that the expression of FBXW 7 is strongly reduced in glioblastoma. Moreover, FBXW 7 emerges as a relevant survival marker for patients with glioblastoma that warrants further validation in the clinic. Our results suggest that FBXW 7 downregulation promotes gliomagenesis via the accumulation of oncogenic cell cycle regulators that control cell division such as Aurora-A and/or Cyclin E or Notch. The contribution of each of the targets remains to be investigated in details in glioma.
Gene therapy with FBXW 7 is already foreseen for cancer . Glioblastoma patients should benefit of an ameliorated survival prognosis based on FBXW 7 quantification. Future studies will tell if there is also a way to improve therapy based on Fbxw7 rescue for those with the most dismal prognosis.
Cells and tissue collection
U87 human cells glioma (ATCC/LGCpromochem, Molsheim, France) were maintained in DMEM with 10% FBS, antibiotics, and L-glutamine. Glioma biopsies were classified according to the World Health Organization , the median survival of the G-IV group was 379 days (n = 54 patients, excluding 2 patients still alive at the time of analysis). Samples were immediately snap-frozen and stored until further use for protein and RNA purification, tissue sectioning, and touch printing of interphase nuclei. All procedures complied with current French laws.
Fluorescence in situ hybridization (FISH) on interphase tumor nuclei
The FBXW 7 probe contained in BAC clone RP-11 300I24 was labeled by nick translation with SpectrumGreen dUTP (Abbott-Vysis, France) and co-hybridized with a chromosome 4 alpha-satellite probe (D4Z1) coupled to rhodamin (Qbiogene, Illkirch, France). Efficiency and specificity were assessed on metaphases and interphases from cultured human lymphocytes. Dual color FISH analysis were performed on touchprints of frozen tissues as previously described . The slides were then fixed by immersion in methanol and methanol/acetic acid baths. After drying and dehydratation, the probes were added to each slide. A coverslip was placed over each hybridization slide and sealed with rubber cement. Slides and probes were co-denatured using the Hybrite Hybridization System (Abbott-Vysis, Rungis, France) at 73°C during 5 min. Hybridization was performed 24 h at 37°C in a humidified box. Finally, slides were washed, dehydrated and nuclei were counterstained with 4,6-diamidino-2-phenyindole (DAPI) diluted in Vectashield (Abcys, Paris, France). The microscopic analysis was done by two independent observers using a fluorescent Axioplan II microscope (Zeiss, Le Pecq, France). A minimum of 50 tumour cell nuclei were evaluated for each slide. Hybridization signals of control (centromere of chromosome 4, CEN4) and test (FBXW 7) probes were counted for each nucleus. Nuclei were then classified either as 1) deleted (ratio of control and test probes 2/1, 4/2, 3/1, 4/1,...), 2) imbalanced (disproportion of the ratio of control and test probes such as 3/2, 4/3, 5/3, etc...), or 3) non-deleted (equal ratio of control and test probes signals). The cut-off value was determined as the mean + three standard deviations of the percentage of deleted nuclei on control tissues . Finally, a tumor was classified either as deleted (% of deleted nuclei ≥ cut-off), or as non-deleted (% of deleted + imbalanced nuclei < cut-off) or as imbalanced (% of imbalanced nuclei or sum of imbalanced + deleted nuclei ≥ cut-off).
Real-time quantitative PCR (qPCR)
RNA was purified from biopsies or cultured cells using RNeasy columns (Qiagen, Courtaboeuf Cedex, France). RNA quality was checked by electrophoresis and any sample displaying degraded RNA was excluded from the study. RNA was reversed-transcribed with SuperScript II RNase H-Reverse Transcriptase (Invitrogen, Cergy Pontoise Cedex, France) by using oligo (dT)15 priming. Human-specific primers for qPCR were designed and evaluated for amplification efficiency. Primer sequences were: α-tubulin (NM_006082), 5'-GAGTGCATCTCCATCCACGTT-3', 5'-TAGAGCTCCCAGCAGGCATT-3', FBXW 7 (target sequence common to all isoforms), 5'-CCACTGGGCTTGTACCATGTT-3', 5'-CAGATGTAATTCGGCGTCGTT-3'. Real-time PCR was carried out in a MX3000P thermocycler (Stratagene) by using SYBR Green dye (ABgene, Courtaboeuf Cedex; France). FBXW7 expression was steady in all G-II samples tested (mean ΔCt = 6.27, SD = 1). Hence, five controls were individually run in parallel with every G-IV PCR assay, and the mean ΔCt served as normalization value. Normalization and quantification were calculated as described previously . All G-IV samples were tested in a minimum of two independent experiments.
U87 cells were seeded in LabTek chambers (100000 cells per well) and transfected the following day with 100 ng (α) or 500 ng (β and γ) of plasmid containing FLAG-FBXW 7 cDNA , Lipofectamine and Reagent Plus (Invitrogen). For knockdown experiments, human FBXW 7 specific  and negative control siRNAs were purchased from Eurogentec (Angers, France) and used at a concentration of 100 nM as previously described . Knockdown was verified 48 h after transfection by real-time PCR as described above.
Western blotting and Immunohistochemistry
Primary antibodies were mouse monoclonal anti-FLAG (Sigma, clone M2), anti-Aurora-A , anti-α-SMA (DakoCytomation, clone M0851), anti-Ki-67 (DakoCytomation, clone M7240), anti-PCNA (Santa Cruz sc-56), rabbit polyclonal anti-FLAG (Sigma), anti-Notch4 (Santa Cruz, clone H-225), goat polyclonal anti-actin (Santa Cruz, clone I-19),
For western blotting, 20 μg of proteins from each tumor were separated on a 15% SDS-polyacrylamide gel, then electrotransferred onto a Hybond-S membrane (Amersham, Les Ulis, France). Membranes were blocked in PBST containing 5% skim milk for 2 h at 4°C, and incubated with primary antibodies at 1:200 dilution. After washing, immunocomplexes were identified with secondary antibodies coupled to peroxidase. The blots were visualized using chemiluminescence (ECL, Amersham – GE Healthcare Europe, Orsay, France). Immunofluorescence was carried out on 10 μm cryosections of tumors or on U87 cells grown in Labtek chambers. Tissue samples and cells were fixed in paraformaldehyde, formaline-ethanol (PCNA detection) or methanol-acetone (Aurora-A) and incubated after saturation with the primary antibodies at the concentration recommended by the supplier. After washing, slides were incubated with secondary antibodies (Alexa Fluor 488, and 546; 1:2000, Molecular Probes – Invitrogen) and mounted with Vectashield containing DAPI for nuclei staining.
All statistical analysis and graphs were performed using GraphPad Prism software.
We thank Markus Welcker for FBXW 7 expressing plasmids and kind advises, Frédéric Chibon for helping with BAC extraction and labeling. Part of this work was done on anonymous frozen tumors provided through the Tumor Bank of the Centre Hospitalier Universitaire de Bordeaux in compliance with institutional ethical guidelines.
This work was supported by La Ligue Régionale contre le Cancer, comité de la Dordogne.
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