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. 2009 Jul 7;16(1):44-54.
doi: 10.1016/j.ccr.2009.05.009.

Proteasomal and genetic inactivation of the NF1 tumor suppressor in gliomagenesis

Lauren T McGillicuddy  1 , Jody A Fromm, Pablo E Hollstein, Sara Kubek, Rameen Beroukhim, Thomas De Raedt, Bryan W Johnson, Sybil M G Williams, Phioanh Nghiemphu, Linda M Liau, Tim F Cloughesy, Paul S Mischel, Annabel Parret, Jeanette Seiler, Gerd Moldenhauer, Klaus Scheffzek, Anat O Stemmer-Rachamimov, Charles L Sawyers, Cameron Brennan, Ludwine Messiaen, Ingo K Mellinghoff, Karen Cichowski
Affiliations

Proteasomal and genetic inactivation of the NF1 tumor suppressor in gliomagenesis

Lauren T McGillicuddy et al. Cancer Cell. .

Abstract

Loss-of-function mutations in the NF1 tumor suppressor result in deregulated Ras signaling and drive tumorigenesis in the familial cancer syndrome neurofibromatosis type I. However, the extent to which NF1 inactivation promotes sporadic tumorigenesis is unknown. Here we report that NF1 is inactivated in sporadic gliomas via two mechanisms: excessive proteasomal degradation and genetic loss. NF1 protein destabilization is triggered by the hyperactivation of protein kinase C (PKC) and confers sensitivity to PKC inhibitors. However, complete genetic loss, which only occurs when p53 is inactivated, mediates sensitivity to mTOR inhibitors. These studies reveal an expanding role for NF1 inactivation in sporadic gliomagenesis and illustrate how different mechanisms of inactivation are utilized in genetically distinct tumors, which consequently impacts therapeutic sensitivity.

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Figures

Figure 1
Figure 1. PKC regulates the proteasomal degradation of neurofibromin
(A) Serum starved NIH3T3 cells were pre-treated with 1μM bortezomib or vehicle and stimulated with serum for increasing amounts of time. Immunoblots for neurofibromin and p120 (as a control) are shown. The neurofibromin antibody used was UP69, and recognizes the last 8 amino acids of neurofibromin. (B) Serum starved NIH3T3 cells were stimulated with serum for increasing amounts of time. A second antibody (NF1-5.16) that recognizes an epitope within the N-terminus of neurofibromin was used. A lentiviral NF1 shRNA construct was used to demonstrate specificity (left). (C) An in vitro ubiquitination assay was performed as described using immunopreciptated neurofibromin as a substrate (Cichowski et al., 2003). (D) Serum starved NIH3T3 cells were pre-treated with Bis I or vehicle (DMSO) and stimulated with PDGF or LPA for increasing amounts of time. Immunoblots for neurofibromin (UP69) and pMARCKS, a PKC substrate, are shown. (E) Serum starved NIH3T3 cells were pre-treated with Ro-31-8220 or vehicle (DMSO) and stimulated with LPA for increasing amounts of time. (F) Serum starved NIH3T3 cells were acutely treated with PMA for increasing amounts of time. (G) NIH3T3 cells were infected with a control retrovirus or a retrovirus expressing a constitutively activated PKCα allele tagged with an HA epitope (PKCα). Relative neurofibromin levels were assessed by immunoblot. Ectopic PKC expression was confirmed by an HA immunoblot.
Figure 2
Figure 2. PKC regulates Ras activation via neurofibromin degradation
(A) Fibroblasts were infected with DRNF1-FLAG. Cells were cultured in .25% serum for 24 hours and treated with PMA for increasing amounts of time. Levels of endogenous neurofibromin and p120RasGAP were assessed by immunoblotting (IB) protein from total cell lysates (TCL). DRNF1-FLAG was detected by immunoprecipitating (IP) this fragment from the same lysates, followed by a FLAG immunoblot. (B) Relative levels of Ras-GTP were assessed in cells expressing DRNF1-FLAG or a control retrovirus (pbabe) in response to PMA as described in A by performing a Ras pull-down assay. Ras-GTP levels were quantified as described in Experimental Procedures (C) Relative levels of endogenous neurofibromin were assessed in cells expressing DRNF1-FLAG or the pbabe control retrovirus (cont). (D) Relative levels of Ras activation were assessed in cells infected with a control lentivirus (cont) or a lentivirus expressing an shRNA directed against NF1, in low serum. Ras immunoblots from Ras pull-down assays are shown (Ras-GTP), along with a Ras immunoblots from total cell lysates (Ras IB (TCL)) as loading controls. An NF1 immunoblot from total cell lysates (NF1 IB (TCL)) was performed to confirm knock-down.
Figure 3
Figure 3. PKC-mediated neurofibromin instability in glioblastoma
(A) U87 cells were treated with proteasome inhibitors (1μM bortezomib and 10μM MG132) for increasing amounts of time and neurofibromin levels were assessed by immunoblot. (Left) Lysates were prepared in the presence of deubiquitinase inhibitor (2mM NEM) to preserve ubiquitinated species in vitro. (Right) Lysates were prepared in the absence of NEM to allow quantification using Image j software. (B) U87 cells were exposed to the PKC inhibitor Bis I for increasing amounts of time. (C) A panel of GBM cell lines were exposed to two additional PKC inhibitors (Ro-31-8220 or Bis II) and neurofibromin levels were assessed by immunoblot. (D) Table summarizing data acquired using multiple PKC inhibitors (Bis I, Ro-31-8220, Bis II). The cell lines listed in this table were subjected to the same analysis described in B and C. “NF1 stabilization” observed means that multiple PKC inhibitors promoted an accumulation of neurofibromin. Relative PKC activity was assessed by quantifying the phosphorylation of proteins recognized by PKC-substrate antibodies as previously performed (Ikenoue et al., 2008). (E) U87 cells were infected with a control lentivirus or lentiviruses expressing one of two shRNA sequences directed against PKCα. PKCα knock-down was confirmed by immunoblot (IB). Neurofibromin levels and p120 levels are shown. (F) (Bottom Left) U87 cells were infected with a pbabe retrovirus or a retrovirus that expresses the DRNF1 protein. Expression was confirmed by immunoblot (M2-IP, M2-IB). (Top Left) Proliferation curves and colony number was quantified as described. (top right, bottom right) Box plot representing the fold tumor volume (day 21 versus day 0 of tumor measurement) of tumors formed after subcutaneous injection of U87 cells either expressing the DRNF1-FLAG protein or control retrovirus. (G) Primary GBM neurosphere cultures (TS543) were cultured as described in Experimental Procedures and were treated with either Ro-31-8220 (PKC inhib) or the proteasome inhibitors MG132 and bortezomib (prot inhib) as described in A for the indicated amounts of time. Neurofibromin levels were assessed by immunoblot (Bethyl). p85 levels are shown as a loading control. (H) Neurofibromin expression was assessed by immunoblot in tissue from untreated grade IV human tumors. Lysates were prepared in the presence of the deubiquitinase inhibitor NEM. An immunoreactive smear consistent with ubiquitinated protein is shown. GAPDH serves as loading control. While lane 5 is slightly under-loaded, little to no neurofibromin was detected even under the darkest exposures.
Figure 4
Figure 4. Complete NF1 inactivation promotes senescence in vitro and in vivo in cells with an intact p53 pathway
(A) U87 cells were infected with a control or a lentivirus expressing an shRNA directed against NF1. Neurofibromin knock-down was confirmed by immunoblot in C. Proliferation curves were generated as described. Senescence associated β-galactosidase (SA-βgal) activity was assessed as described previously (Dimri et al., 1995). (B) Dbtrg-05mg or Gli36 cells were infected with a control or a lentivirus expressing an shRNA directed against NF1. Neurofibromin knock-down was confirmed in by immunoblot in C. SA-βgal assays were performed and phase photographs are shown. (C) GBM cell lines were infected with a control a lentivirus expressing an shRNA directed against NF1. Neurofibromin knock-down was confirmed by immunoblot. p53 activity was assessed by examining the levels of p21 in response to doxorubicin (DR) treatment. Cells that became senescent in response to NF1 inactivation are noted (bottom). (D) Dbtrg-05mg cells were infected with a control (vector) or a lentivirus expressing an shRNA directed against p53. p53 knock-down was confirmed by immunoblot (data not shown). Cells were then infected with a control lentivirus (control) or a lentivirus expressing an shRNA directed against NF1. SA-βgal assays were performed and the percentage of SA-βgal expressing cells was quantified as described. (E) Paraffin sections of pilocytic astrocytomas from NF1 patients were stained with a control antibody (control, counterstained with H&E), a p53 antibody (no counterstain), a p16 antibody (counterstained with H&E) or a p15 antibody (no counterstain).
Figure 5
Figure 5. NF1 genomic alterations and drug sensitivity in GBMs
(A) Table detailing detected mutations in NF1 in GBM cell lines. (B) Table detailing detected mutations in NF1 in primary GBM tumor samples. (C) IC50 curves were generated using three NF1+/+ (red) GBM cell lines treated with Ro-31-8220 or vehicle for 18hrs before assessing viability by a Cell Titer-Glo Luminescent Cell viability assay. (D) IC50 curves were generated using three NF1-/- (blue) GBM cell lines treated with Ro-31-8220 or vehicle for 18hrs before assessing viability by a Cell Titer-Glo Luminescent Cell viability assay. (E) Gli36 cells were infected with a control (c, red) or a lentivirus expressing an shRNA directed against NF1 (N, blue) and were treated with Ro-31-8220 or vehicle for 18 hrs before assessing the percentage of viable cells. An immunoblot confirming knock-down is shown. The top panel illustrates the dramatic difference in viability observed at 1.25 μM Ro-31-8220. (F) Primary GBM cultures that are NF1 +/+ (red, TS-543) or NF1 -/- (blue, TS-565) were treated with Ro-31- 8220 or vehicle for 24 hours before assessing the percentage of viable cells as described. (G) NF1 -/- GBM cell lines were infected with a control retrovirus or a retrovirus expressing the DRNF1 protein (rescued). Cells were selected with puromycin prior to seeding for drug sensitivity assay. IC50 values were calculated as described.
Figure 6
Figure 6. Model illustrating how complete genetic loss and proteasomal degradation of NF1 functions in gliomagenesis
We hypothesize that NF1 inactivation participates in gliomagenesis via three distinct mechanisms. (Left) Because NF1 patients are born with an NF1 mutation, the majority of tumors that arise are likely to be driven by a second hit mutation in the remaining NF1 allele. This event would be predicted to result in an initial burst of proliferation and ultimately the development of pilocytic astrocytomas. If the p53 pathway is intact (expected in the majority of cases) tumors undergo senescence and are unable to progress. (Middle) However, neurofibromin can also be excessively destabilized by PKC and the proteasome. Heterozygous mutations in NF1 may further decrease protein levels in some tumors. Importantly, neurofibromin instability promotes tumorigenesis but is not sufficient to trigger a senescence response. Data generated from GBM cell lines and primary cultures further suggest that tumors with an intact NF1 gene, but destabilized protein, may be sensitive to PKC inhibitors. (Right) Heterozygous deletions or mutations in NF1 also occur. We hypothesize that mutations in the p53 pathway would be permissive for a second hit mutation in NF1. Such tumors would effectively evade the senescence response. Our pre-clinical studies further suggest that NF1-deficient GBMs may be sensitive to mTOR inhibitors.
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References

    1. Andersen LB, Fountain JW, Gutmann DH, Tarle SA, Glover TW, Dracopoli NC, Housman DE, Collins FS. Mutations in the neurofibromatosis 1 gene in sporadic malignant melanoma cell lines. Nat Genet. 1993;3:118–121. - PubMed
    1. Basu TN, Gutmann DH, Fletcher JA, Glover TW, Collins FS, Downward J. Aberrant regulation of ras proteins in malignant tumour cells from type 1 neurofibromatosis patients. Nature. 1992;356:713–715. - PubMed
    1. Berger R, Febbo PG, Majumder PK, Zhao JJ, Mukherjee S, Signoretti S, Campbell KT, Sellers WR, Roberts TM, Loda M, et al. Androgen-induced differentiation and tumorigenicity of human prostate epithelial cells. Cancer Res. 2004;64:8867–8875. - PubMed
    1. Beroukhim R, Getz G, Nghiemphu L, Barretina J, Hsueh T, Linhart D, Vivanco I, Lee JC, Huang JH, Alexander S, et al. Assessing the significance of chromosomal aberrations in cancer: methodology and application to glioma. Proc Natl Acad Sci U S A. 2007;104:20007–20012. - PMC - PubMed
    1. Cawthon RM, Weiss R, Xu GF, Viskochil D, Culver M, Stevens J, Robertson M, Dunn D, Gesteland R, O’Connell P, et al. A major segment of the neurofibromatosis type 1 gene: cDNA sequence, genomic structure, and point mutations. Cell. 1990;62:193–201. - PubMed

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