Destabilization of MYC/MYCN by the mitochondrial inhibitors, metaiodobenzylguanidine, metformin and phenformin

In the present study, we investigated the anticancer effects of the mitochondrial inhibitors, metaiodobenzylguanidine (MIBG), metformin and phenformin. ¹³¹I-MIBG has been used for scintigraphic detection and the targeted radiotherapy of neuroblastoma (NB), a pediatric malignancy. Non-radiolabeled MIBG has been reported to be cytotoxic to NB cells in vitro and in vivo. However, the mechanisms behind its growth suppressive effects have not yet been fully elucidated. Metformin and phenformin are diabetes medications that are being considered in anticancer therapeutics. We investigated the anticancer mechanisms of action of MIBG and metformin in NB. Our data revealed that both drugs suppressed NB cell growth and that the combination drug treatment was more potent. MIBG reduced MYCN and MYC expression in MYCN-amplified and non-MYCN-amplified NB cells in a dose- and time-dependent manner. Metformin was less effective than MIBG in destabilizing MYC/MYCN. The treatment of NB cells with metformin or MIBG resulted in an increased expression of genes encoding biomarkers for favorable outcome in NB [(ephrin (EFN)B2, EFNB3, EPH receptor B6 (EPHB6), neurotrophic tyrosine kinase, receptor, type 1 (NTRK1), CD44 and Myc-interacting zinc finger protein (MIZ-1)] and tumor suppressor genes [(early growth response 1 (EGR1), EPH receptor A2 (EPHA2), growth arrest and DNA-damage-inducible, beta (GADD45B), neuregulin 1 (NRG1), TP53 apoptosis effector (PERP) and sel-1 suppressor of lin-12-like (C. elegans) (SEL1L)]. Accordingly, metformin and MIBG augmented histone H3 acetylation in these cells. Phenformin also exhibited histone modification and was more effective than metformin in destabilizing MYC/MYCN in NB cells. Our data suggest that the destabilization of MYC/MYCN by MIBG, metformin and phenformin and their effects on histone modification are important mechanisms underlying their anticancer effects.

S(+)-ibuprofen destabilizes MYC/MYCN and AKT, increases p53 expression, and induces unfolded protein response and favorable phenotype in neuroblastoma cell lines

Neuroblastoma is a common pediatric solid tumor that exhibits a striking clinical bipolarity favorable and unfavorable. The survival rate of children with unfavorable neuroblastoma remains low among all childhood cancers. MYCN and MYC play a crucial role in determining the malignancy of unfavorable neuroblastomas, whereas high-level expression of the favorable neuroblastoma genes is associated with a good disease outcome and confers growth suppression of neuroblastoma cells. A small fraction of neuroblastomas harbors TP53 mutations at diagnosis, but a higher proportion of the relapse cases acquire TP53 mutations. In this study, we investigated the effect of S(+)-ibuprofen on neuroblastoma cell lines, focusing on the expression of the MYCN, MYC, AKT, p53 proteins and the favorable neuroblastoma genes in vitro as biomarkers of malignancy. Treatment of neuroblastoma cell lines with S(+)-ibuprofen resulted in a significant growth suppression. This growth effect was accompanied by a marked decrease in the expression of MYC, MYCN, AKT and an increase in p53 expression in neuroblastoma cell lines without TP53 mutation. In addition, S(+)-ibuprofen enhanced the expression of some favorable neuroblastoma genes (EPHB6, CD44) and genes involved in growth suppression and differentiation (EGR1, EPHA2, NRG1 and SEL1L). Gene expression profile and Ingenuity pathway analyses using TP53-mutated SKNAS cells further revealed that S(+)-ibuprofen suppressed molecular pathways associated with cell growth and conversely enhanced those of cell cycle arrest and the unfolded protein response. Collectively, these results suggest that S(+)-ibuprofen or its related compounds may have the potential for therapeutic and/or palliative use for unfavorable neuroblastoma.

Neuroblastoma of undifferentiated subtype, prognostic significance of prominent nucleolar formation, and MYC/MYCN protein expression: a report from the Children's Oncology Group

BACKGROUND

This study sought to investigate biological/clinicopathological characteristics of neuroblastoma, undifferentiated subtype (NBUD).

METHODS

This study examined 157 NBUD cases filed at the Children's Oncology Group Neuroblastoma Pathology Reference Laboratory, and survival rates of the patients were analyzed with known prognostic factors. Immunostainings for MYCN and MYC protein were performed on 68 tumors.

RESULTS

NBUD cases had a poor prognosis (48.4% ± 5.0% 3-year event-free survival [EFS]; 56.5% ± 5.0% overall survival [OS]), and were often associated with high mitosis-karyorrhexis index (MKI, 65%), prominent nucleoli (PN, 83%), ≥ 18 months of age (75%), MYCN amplification (MYCN-A, 83%), diploid pattern (63%), and 1pLOH (loss of heterozygosity (72%). However, these prognostic indicators, except for MYCN status, had no significant impact on survival. Surprisingly, EFS for patients with MYCN-A tumors (53.4% ± 5.6%) was significantly better (P=.0248) than for patients with MYCN-nonamplified (MYCN-NA) tumors (31.7% ± 11.7%), with MYCN-NA and PN (+) tumors having the worst prognosis (9.3%  ± 8.8%, P=.0045). Immunohistochemically, MYCN expression was found in 42 of 48 MYCN-A tumors. In contrast, MYC expression was almost exclusively found in the MYCN-NA tumors (9 of 20) especially when they had PN (8 of 11). Those patients with only MYC-positive tumors had the worst EFS (N=8, 12.5% ± 11.7%) compared with only MYCN-positive (N=39, 49.9% ± 17.7%) and both negative tumors (N=15, 70.0% ± 17.1%) (P= .0029). High MKI was often found in only MYCN-positive (30 of 38) but rarely in only MYC-positive (2 of 8) tumors.

CONCLUSIONS

NBUD represents a unique subtype of neuroblastoma associated with a poor prognosis. In this subtype, MYC protein expression may be a new prognostic factor indicating more aggressive clinical behavior than MYCN amplification and subsequent MYCN protein expression.

Abstract 2654: Induced stable neuroblastoma stem cells recapitulate in vivo highly aggressive large-cell neuroblastomas

Neuroblastoma (NB) is a pediatric tumor of neural crest origin, and half of the cases are highly aggressive. Based on the degree of neuronal differentiation, NB is histopathologically divided into undifferentiated (UD), poorly differentiated (PD) and differentiating subtypes. Recently, a unique histological subset of NBs within the UD and PD subtypes has been identified. These tumors are uniformly composed of large cells having vesicular nuclei with sharply outlined nuclear membranes, and one to four prominent nucleoli, which are referred to as “Large-Cell Neuroblastomas” or LCNs. Patients with the UD neuroblastoma with the LCN appearance had a very poor prognosis regardless of age at diagnosis, clinical stage, and DNA index. Surprisingly, non-MYCN amplified UDs behaved significantly worse than MYCN amplified UDs. Our hypothesis is that the histological features and the clinical behavior of LCNs are the reminiscence of a malignancy likely driven by stem cell-like cancer cells. To gain better understanding of the nature of LCNs, we have generated phenotypically stablilized stem cell-like NB cells by treating monolayer NB cell lines with epigenetic modifiers for a short time. Without further exposure to epigenetic modifiers, stemness phenotypes of the NB cells were maintained over a year in sphere-forming culture conditions. Hence, we referred to these spheres as induced CSCs (iCSCs). We previously presented that the SKNAS iCSC had a very high tumor-initiating ability in SCID/Beige mice and SKNAS iCSC xenografts recapitulated the histological appearance of totally undifferentiated “large-cell” NBs. A cancer stem cell marker, CXCR4, was preferentially expressed in the iCSC xenografts over the monolayer counterparts. In this study, we established additional iCSCs from three NB cell lines [SKNBE(2)C, CHP134, SY5Y]. The expression of stemness factors was augmented in these NB iCSCs compared to conventional monolayer counterparts. In addition, the expression of stem cell markers (CD24, CD133) and neural crest stem cell markers (SOX9, SLUG, Musashi, p75NTR) was elevated in these NB iCSCs. Notably, SCID/Beige mice subcutaneously injected with one hundred SKNBE(2)C iCSCs formed tumors, and in some cases, SKNBE(2)C iCSCs metastasized to adrenal gland, liver, spleen, kidney, indicating an increased tumor invasiveness of these cells. CXCR4 and p75NTR expression was positive in the iCSC xenografts, although not uniformly. By histopathological and immunohistochemical examinations, we confirmed that the xenografts established from SKNAS iCSC, SKNBE(2)C iCSC, CHP134 iCSC, and SY5Y iCSC shared two common and consistent features: the LCN phenotype and high-level expression of MYC/MYCN. These characteristics may help identify the presence of CSC compartment in patients’ specimens. The established NB iCSCs may also serve as useful tools for the development of effective therapeutics against NB.

Citation Format: Naohiko Ikegaki, Hiroyuki Shimada, Xao X. Tang. Induced stable neuroblastoma stem cells recapitulate in vivo highly aggressive large-cell neuroblastomas. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2654. doi:10.1158/1538-7445.AM2013-2654

Abstract 766: Phenformin is a MYCN/MYC destabilizer that can specifically target neural crest derived tumors

We have been interested in identifying MYCN/MYC destabilizers that can specifically target neural crest derived tumors (neuroblastoma, melanoma, and malignant pheochromocytoma). MYCN amplification and its over-expression are associated with the worst neuroblastoma disease outcome. Non-MYCN amplified neuroblastomas express high-levels of MYC, and prognosis of these neuroblastomas is very poor. Melanoma and malignant pheochromocytoma are also known for high MYC expression. Interestingly, these neural crest derived tumors characteristically express the norepinephrine transporter (NET). This feature can be used to specifically target the tumor cells with agonists of NET, such as Meta-Iodo-BenzylGuanidine (MIBG). In the previous AACR meeting, we presented our finding that non-radio-labeled MIBG can suppress growth of neuroblastoma cells in vitro, and this effect is in part related to its ability to destabilize MYC and MYCN. We have also shown that FCCP, a well-known mitochondrial respiration inhibitor, destabilized MYCN and MYC in neuroblastoma cells and caused growth suppression. Recently, it has been reported that Phenformin, a Type II diabetic drug, binds NET, suggesting that NET positive cells preferentially uptake Phenformin. It is also known that Phenformin inhibits the mitochondrial electron transfer system. In this study, we have investigated a possible effect of Phenformin on growth and MYC/MYCN stability in neuroblastoma cells in vitro. SKNBE(2)C and CHP134 neuroblastoma cell lines were treated with various doses (0-2.5mM) of Phenformin for two days. MTS assay was used to assess the effect of Phenformin on growth of the cells. There was a dose-dependent growth suppression of the NB cell lines tested. Phenformin was more effective against growth of SKNBE(2)C than CHP134 cells. The effect of Phenformin on MYC/MYCN expression in four neuroblastoma cell lines [SKNBE(2)C, CHP134, SKNAS, and SY5Y] was also examined by Western blot assay. SKNBE(2)C and CHP134 express MYCN, whereas SKNAS and SY5Y express MYC at high levels. Treatment of these cells with Phenformin at the dose of 1mM for 24 hours resulted in destabilization of MYCN/MYC, suggesting that the growth suppressive effect of Phenformin on neuroblastoma cells was in part due to reduction in MYC/MYCN expression. We are currently examining whether the above observations made in neuroblastoma are also applicable to other neural crest derived tumors. In addition, we are investigating how Phenformin-related mitochondrial dysfunction causes destabilization of MYC/MYCN in neuroblastoma cells.

Citation Format: Naohiko Ikegaki, Srah Lomahan, Xao X. Tang. Phenformin is a MYCN/MYC destabilizer that can specifically target neural crest derived tumors. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 766. doi:10.1158/1538-7445.AM2013-766

Abstract 2875: Meta-iodo-benzylguanidine (MIBG) is a MYCN/MYC destabilizer that specifically targets neural crest derived tumors

Melanoma and malignant pheochromocytoma are known for high MYC expression. MYCN amplification and its over-expression are associated with the worst neuroblastoma disease outcome. Meta-iodo-benzylguanidine (MIBG) is a norepinephrine analogue and a mitochondrial respiration inhibitor. 131I-MIBG has been used for scintigraphic detection of neural crest derived tumors (neuroblastoma, pheochromocytoma and melanoma) that specifically uptake MIBG by norepinephrine transporters (NET). 131I-MIBG is also used for targeted radiotherapy of neuroblastoma and pheochromocytoma. Non-radiolabeled MIBG had been reported to be cytotoxic to neuroblastoma cells in vitro and in vivo. However, the molecular mechanism of its growth suppressive effect was not elucidated. Our previous studies showed that FCCP, a well-known mitochondrial respiration inhibitor, destabilized MYCN and MYC in neuroblastoma cells and caused growth suppression. In this study we confirmed that MIBG suppressed growth of neuroblastoma cell lines (SKNBE(2)C, IMR5, Nb69, and SKNAS), and its growth suppressive effect in each cell line correlated with NET expression in these cells. In addition, MIBG suppressed growth of neuroblastoma stem cells derived from SKNBE(2)C. MIBG treatment of SKNBE(2)C and SKNAS resulted in an increase expression of growth suppressive gene (GADD45B) and genes encoding for biomarkers of favorable neuroblastoma (CD44, EPHB6, EFNB2, EFNB3, NTRK1). Treatment of MYCN amplified (SKNBE(2)C, IMR5) and non-MYCN amplified (Nb69, SKNAS) neuroblastoma cells with MIBG resulted in marked reduction of MYCN and MYC expression, respectively in dose and time dependent fashions. Collectively, our studies suggest that MIBG not only targets neural crest derived tumors, but also is a MYCN/MYC destabilizing agent.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 2875. doi:1538-7445.AM2012-2875

Abstract LB-142: Regulation of MYCN stability by reactive oxygen species in neuroblastoma

Neuroblastoma (NB) is the most common extracranial solid tumor in children. Amplification of the MYCN proto-oncogene is associated with older age, rapid tumor progression, and the worst outcome of this disease. As MYCN amplification leads to its over-expression, high-level expression of MYCN is thought to cause an aggressive behavior of MYCN amplified tumors. In fact, the forced reduction of MYCN protein expression by siRNA results in growth suppression and apoptosis of NB cells with MYCN amplification. We have identified several compounds that can rapidly destabilize the MYCN protein (within a few hours), in NB cells and cause growth suppression. The compounds include FCCP, OSU-03012, and Salinomycin. Our recent data suggest that a common effect of the above compounds appears to be the inhibition of mitochondrial functions.

Moreover, ascorbic acid, an anti-oxidant, abolishes the effect of these compounds on MYCN protein stability. It is known that inhibition of mitochondrial oxidative phosphorylation increases the production of reactive oxygen species (ROS), which include superoxide (O2-), hydroxyl radical (•OH), and hydrogen peroxide (H2O2). Anti-oxidants are also known to quench ROS. Preliminary data showed that ROS was detected when the IMR5 NB cells were treated with OSU-03012 for three hours. In addition, we have found that forced over-expression of pVHL, an E3 ubiquitin ligase, in NB cell lines results in reduction of MYCN protein levels. We are currently testing whether ROS regulates MYCN stability by hydroxylation of proline residues on MYCN, and whether pVHL recognizes the ROS-modified MYCN, which is then subjected to rapid proteasome-dependent degradation.

Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr LB-142. doi:10.1158/1538-7445.AM2011-LB-142

Biological effects of induced MYCN hyper-expression in MYCN-amplified neuroblastomas

Neuroblastoma is a childhood malignancy of the sympathetic nervous system. The tumor exhibits two different phenotypes: favorable and unfavorable. MYCN amplification is associated with rapid tumor progression and the worst neuroblastoma disease outcome. We have previously reported that inhibitors of histone deacetylase (HDAC) and proteasome enhance favorable neuroblastoma gene expression in neuroblastoma cell lines and inhibit growth of these cells. In this study, we investigated the effect of trichostatin A or TSA (an HDAC inhibitor), and epoxomycin (a proteasome inhibitor) on MYCN and p53 expression in MYCN-amplified neuroblastoma cells. It was found that TSA down-regulated MYCN expression, but Epoxomycin and the TSA/Epoxomycin combination led to MYCN hyper-expression in MYCN-amplified neuroblastoma cell lines. Despite their contrasting effects on MYCN expression, TSA and Epoxomycin caused growth suppression and cell death of the MYCN-amplified cell lines examined. Consistent with these data, forced hyper-expression of MYCN in MYCN-amplified IMR5 cells via transfection resulted in growth suppression and the increased expression of several genes known to suppress growth or induce cell death. Furthermore, Epoxomycin as a single agent and its combination with TSA enhance p53 expression in the MYCN-amplified neuroblastoma cell lines. Unexpectedly, co-transfection of TP53 and MYCN in IMR5 cells resulted in high p53 expression but a reduction of MYCN expression. Together our data suggest that either down regulation or hyper-expression of MYCN results in growth inhibition and/or apoptosis of MYCN-amplified neuroblastoma cells. In addition, elevated p53 expression has a suppressive effect on MYCN expression in these cells.

Biological significance of EPHA2 expression in neuroblastoma

Neuroblastoma is a pediatric solid tumor that exhibits striking clinical bipolarity. Despite extensive efforts to treat unfavorable neuroblastoma, survival rate of children with the disease is among the lowest. Previous studies suggest that EPHA2, a member of the EPH family receptor kinases, can either promote or suppress cancer cell growth depending on cellular contexts. In this study, we investigated the biological significance of EPHA2 in neuroblastoma. It was found that tumorigenic N-type neuroblastoma cell lines expressed low levels of EPHA2, whereas hypo-tumorigenic S-type neuroblastoma cell lines expressed high levels of EPHA2 (p<0.005). Notably, inhibitors of DNA methylation and histone deacetylase enhanced EPHA2 expression in N-type cells, suggesting that EPHA2 is epigenetically silenced in unfavorable neuroblastoma cells. Furthermore, ectopic high-level expression of EPHA2 in N-type neuroblastoma cell lines resulted in significant growth suppression. However, Kaplan-Meier survival analysis showed that high EPHA2 expression was not associated with a good disease outcome of neuroblastoma, indicating that EPHA2 is not a favorable neuroblastoma gene, but a growth suppressive gene for neuroblastoma. Accordingly, EPHA2 expression was markedly augmented in vitro in neuroblastoma cells treated with doxorubicin, which is commonly used for treating unfavorable neuroblastoma. Taken together, EPHA2 is one of the effectors of chemotherapeutic agents (e.g., gene silencing inhibitors and DNA damaging agents). EPHA2 expression may thus serve as a biomarker of drug responsiveness for neuroblastoma during the course of chemotherapy. In addition, pharmaceutical enhancement of EPHA2 by non-cytotoxic agents may offer an effective therapeutic approach in the treatment of children with unfavorable neuroblastoma.

De novo identification of MIZ-1 (ZBTB17) encoding a MYC-interacting zinc-finger protein as a new favorable neuroblastoma gene

PURPOSE

Neuroblastoma is a childhood cancer that exhibits either a favorable or an unfavorable phenotype. Favorable neuroblastoma genes (EPHB6, EFNB2, EFNB3, NTRK1, and CD44) are genes whose high-level expression predicts favorable neuroblastoma disease outcome. Accordingly, the forced expression of these genes or their reactivation by gene silencing inhibitors in unfavorable neuroblastoma cells results in suppression of tumor growth and metastases. This study was undertaken to design an experimental strategy to identify additional favorable neuroblastoma genes.

EXPERIMENTAL DESIGN

Favorable neuroblastoma gene candidates were first identified by gene expression profiling analysis on IMR5 neuroblastoma cells treated with inhibitors of DNA methylation and histone deacetylase against the untreated control cells. Among the candidates, we focused on MIZ-1, which encodes a MYC-interacting zinc-finger protein, because it is known to enhance the expression of growth suppressive genes, such as CDKN1A.

RESULTS

High-level MIZ-1 expression was associated with favorable disease outcome of neuroblastoma (P = 0.0048). Forced MIZ-1 expression suppressed in vitro growth of neuroblastoma cell lines. High MIZ-1 expression was correlated with the small-size neuroblastoma xenografts treated with gene silencing inhibitors or a glucocorticoid. In addition, forced MIZ-1 expression enhanced the expression of CD44 and EFNB2 in neuroblastoma cell lines in vitro. Furthermore, MIZ-1 expression was positively correlated with the expression of favorable neuroblastoma genes (EFNB2, EFNB3, EPHB6, and NTRK1) in the human neuroblastoma xenograft therapeutic models.

CONCLUSION

MIZ-1 is a new favorable neuroblastoma gene, which may directly or indirectly regulate the expression of other favorable neuroblastoma genes.

The MYCN Enigma: Significance of MYCN Expression in Neuroblastoma

MYCN amplification strongly predicts adverse outcome of neuroblastoma. However, the significance of MYCN expression in the clinical and biological behavior of neuroblastoma has been unclear. To address this question, we first examined the expression of MYCN in combination with TrkA (a favorable prognostic indicator of neuroblastoma) in 91 primary neuroblastoma by quantitative reverse transcription-PCR and investigated the relationship among patient survival, MYCN, and TrkA expressions. Three subsets of neuroblastoma were defined based on MYCN and TrkA expression. Neuroblastoma expressing the highest level of MYCN but little TrkA were MYCN-amplified cases, which had a 5-year survival of 9.3%. Interestingly, MYCN and TrkA expression showed a linear correlation (r = 0.5664, P < 0.00005) in neuroblastoma lacking MYCN amplification, and the 5-year survival of neuroblastoma patients with low MYCN and low TrkA expressions was 63.7%, whereas those with high expression of both had a 5-year survival of 88.1% (P < 0.00005). This nonlinear distribution of disease outcome relative to MYCN expression in neuroblastoma explains why MYCN expression is not predictive of neuroblastoma disease outcome by dichotomous division of the neuroblastoma cohort. However, high-level MYCN expression is associated with favorable outcome in neuroblastoma lacking MYCN amplification. Furthermore, forced expression of MYCN significantly suppresses growth of neuroblastoma cells lacking MYCN amplification by inducing apoptosis and enhancing favorable neuroblastoma gene expression. Collectively, these data suggest that high-level MYCN expression in neuroblastoma lacking MYCN amplification results in a benign phenotype. Thus, the high MYCN expression confers the opposite biological consequence in neuroblastoma, depending on whether or not MYCN is amplified.

Favorable neuroblastoma genes and molecular therapeutics of neuroblastoma

PURPOSE AND EXPERIMENTAL DESIGN

Neuroblastoma (NB) is a common pediatric solid tumor that exhibits a striking clinical bipolarity: favorable and unfavorable. Favorable NB genes (EPHB6, EFNB2, EFNB3, NTRK1, and CD44) are genes whose high-level expression predicts favorable NB outcome, and forced expression of these genes inhibits growth of unfavorable NB cells. In this study, we investigated whether favorable NB gene expression could be augmented in unfavorable NB cells by chemical compounds and whether an increased expression of these genes was associated with suppression of NB growth and metastasis.

RESULTS

We found that inhibitors of DNA methylation [5-aza-2'-deoxycytidine (5AdC)], histone deacetylase (HDAC) [4-phenylbutyrate (4PB)], and proteasome (MG262) enhanced the expression of favorable NB genes in NB cell lines and inhibited the growth of these cells in vitro (P < 0.0005). The growth-inhibitory effects of 5AdC and 4PB in vitro were in part due to caspase-dependent cell death and inhibition of DNA synthesis. Administration of 5AdC and/or 4PB also suppressed growth of subcutaneous NB xenografts in nude mice (P < 0.001), which was accompanied by enhanced favorable NB gene expression and an increase in apoptosis. Moreover, 4PB suppressed bone marrow and liver metastases of NB cells in severe combined immunodeficient/Beige mice (P = 0.007 and P = 0.008, respectively). The growth-suppressive activity of HDAC inhibitors on NB was further confirmed by the efficacy of trichostatin A, a potent and specific HDAC inhibitor.

CONCLUSIONS

Collectively, these observations further emphasize the link between the elevated favorable NB gene expression and a benign phenotype of NB.

Effect of 20-epi-1alpha,25-dihydroxyvitamin D3 on the proliferation of human neuroblastoma: role of cell cycle regulators and the Myc-Id2 pathway

The antiproliferative effects of 1alpha,25-dihydroxyvitamin D(3) [1,25(OH)(2)D(3)] and its epimer, 20-epi-1alpha,25-dihydroxyvitamin D(3) [20-epi-1,25(OH)(2)D(3)], in six human neuroblastoma (NB) cell lines (SH-SY5Y, NB69, SK-N-AS, IMR5, CHP134, and NGP) were investigated. We determined the ability of 1,25(OH)(2)D(3) and 20-epi-1,25(OH)(2)D(3) to influence cell viability by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay, cell proliferation by bromodeoxyuridine (BrdU) incorporation, and their antineoplastic effect on colony formation in a soft agar assay. A concentration-dependent decrease in cell viability, inhibition of DNA synthesis, and suppression of clonal proliferation was observed with both compounds. 20-epi-1,25(OH)(2)D(3) was more potent in suppressing the proliferation of all six NB cell lines. To understand the mechanisms of action, we examined the effect of 20-epi-1,25(OH)(2)D(3) on the Myc-Id2 cell proliferative network and also on key regulators of the cell cycle. For the first time, we show that 20-epi-1,25(OH)(2)D(3) down-regulated Myc and Id2 expression by western blot analysis. Semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis revealed that 20-epi-1,25(OH)(2)D(3) induced the expression of retinoic acid receptor-beta and p21(Cip1), and down-regulated the expression of cyclin D1 resulting in decreased phosphorylation of retinoblastoma protein (pRB). In sum, we show that 20-epi-1,25(OH)(2)D(3) exerts strong antiproliferative effects by regulating key growth control networks (Myc-Id2-pRB) in NB cells.

Anti-proliferative effects of 20-epi-vitamin-D3 analogue, KH1060 in human neuroblastoma: induction of RAR-beta and p21(Cip1)

We determined the in vitro biological activities of 1 alpha, 25-dihdroxyvitamin D(3) (1,25-D(3)) and its analogue, 20-epi-22-oxa-24a, 26a, 27a-trihomo-1 alpha, 25 (OH)(2) vitamin D(3) (KH1060) in six human neuroblastoma (NB) cell lines (SH-SY5Y, NB69, SK-N-AS, IMR5, CHP-134, NGP). The ability of these compounds to inhibit cell growth and DNA synthesis was evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide assay and BrdU incorporation, respectively. The induction of cell death was monitored by caspase-3 activity. Their antineoplastic effect was assessed by clonal proliferation in soft agar. KH1060 was more effective than 1,25 D(3) in inhibiting cell growth and DNA synthesis. The IC-(50) (inhibition of 50% cell viability) indicated that KH1060 was about 10-20-fold more potent than 1,25 D(3). This growth inhibition was also accompanied by induction of caspase-3 activity, indicating that these compounds induce cell death in a caspase-dependent fashion. Moreover, KH1060 exerted potent antineoplastic activity by suppressing the clonal proliferation of the six NB cells. For the first time we demonstrate that KH1060 induces the expression of retinoic acid receptor-beta and p21(Cip1) suggesting that these proteins in part mediate the growth inhibitory effects. Taken together, all the six NB cells were more susceptible to growth inhibition by KH1060 than 1,25-D(3), suggesting its possible use in NB to potentiate the action of retinoids, which are in clinical use for this disease.

Resistance to chemotherapy mediated by TrkB in neuroblastomas

Neuroblastoma is a common childhood tumor derived from the peripheral nervous system. Favorable neuroblastomas usually express TrkA, the receptor for nerve growth factor (NGF), whereas unfavorable, MYCN-amplified neuroblastomas usually express TrkB and its ligand, brain-derived neurotrophic factor (BDNF). Here, we provide evidence that the TrkB-BDNF pathway is associated with enhanced survival and resistance to chemotherapy in neuroblastoma. We transfected the neuroblastoma line SH-SY5Y, which has endogenous expression of BDNF, with a full-length TrkB expression vector, and obtained clones with moderate or high levels of expression. Cells were exposed in vitro to chemotherapy agents used to treat neuroblastomas: doxorubicin, etoposide (VP16), and cisplatin. Chemoresistance was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for cell survival and by ELISA for cell death. In all cases, the TrkB-expressing subclones were more resistant to treatment than the parent line. Furthermore, when the TrkB tyrosine kinase was blocked with the Trk-specific inhibitor CEP-2563, or by neutralizing antibody to BDNF, sensitivity to chemotherapy was significantly increased. We also found constitutive phosphorylation of AKT at the Ser-473 site in TrkB transfectants, whereas there was only a minimal level of constitutive phosphorylation of AKT in SY5Y cells. These results show that the TrkB-BDNF pathway provides a survival advantage when exposed to DNA-damaging reagents, and, therefore, this autocrine pathway may play an important role in mediating the drug-resistant phenotype associated with TrkB-expressing neuroblastomas. Activation of PI3K/AKT survival pathway may contribute to the increased drug resistance in TrkB-expressing neuroblastomas.

Expression of the neurotrophin receptor TrkA down-regulates expression and function of angiogenic stimulators in SH-SY5Y neuroblastoma cells

Angiogenesis is essential for tumor growth and metastasis and depends on the production of angiogenic factors. Mechanisms regulating the expression of angiogenic factors in tumor cells are largely unknown. High expression of the neurotrophin receptor TrkA in neuroblastomas (NBs) is associated with a favorable prognosis, whereas TrkB is mainly expressed on aggressive, MYCN-amplified NBs. To investigate the biological effects of TrkA and TrkB expression on angiogenesis in NB, we examined the expression of angiogenic factors in the human NB cell line SY5Y and its TrkA and TrkB transfectants. In comparison with parental SY5Y cells, mRNA and protein levels of the examined angiogenic factors were significantly reduced in SY5Y-TrkA cells, whereas SY5Y-TrkB cells did not demonstrate a significant change. Conditioned medium of TrkB transfectants and parental SY5Y cells induced endothelial cell proliferation and migration, but this effect was completely absent in SY5Y-TrkA cells. TrkA expression also resulted in severely impaired tumorigenicity in a mouse xenograft model and was associated with reduced angiogenic factor expression and vascularization of tumors, as determined by immunohistochemistry and an in vivo Matrigel assay. TrkA expression inhibits angiogenesis and tumor growth in SY5Y NB cells by down-regulation of angiogenic factors, whereas expression of TrkB does not down-regulate the production of these angiogenic factors. The biologically different behavior of TrkA- and TrkB-expressing NBs may be explained in part by their effects on angiogenesis.

Expression of Apo-3 and Apo-3L in primitive neuroectodermal tumours of the central and peripheral nervous system

Deregulation of apoptosis has been implicated in the pathogenesis, spontaneous regression and treatment resistance of neuroblastoma. A newly recognised member of the tumour necrosis factor (TNF)-family of death receptors known as Apo-3 has been mapped to human chromosome 1p36.3, a region commonly deleted in aggressive neuroblastoma. Based on its localisation and function, Apo-3 is a candidate for the putative neuroblastoma tumour suppressor gene. Therefore we analysed mRNA expression of the Apo-3 receptor/ligand (Apo-3/Apo-3L) system in a representative panel of 18 neuroblastoma cell lines, 41 primary neuroblastoma and 13 ganglioneuromas/ganglioneuroblastomas by semi-quantitative RT-PCR. We compared the level of expression with the well-established prognostic factors age, stage, histology, MYCN-amplification and TrkA expression, as well as outcome. For comparison, we studied Apo-3/Apo-3L expression in 27 central nervous system (CNS) primitive neuroectodermal tumours/medulloblastomas (PNET/medulloblastoma) and in six normal brain samples. Neuroblastoma cell lines with 1p deletion and MYCN-amplification expressed significantly lower levels of Apo-3 (P=0.009 and P=0.03, respectively) compared with neuroblastoma cell lines without 1p deletion or MYCN-amplification. The mean expression level of Apo-3L was significantly higher in ganglioneuromas/ganglioneuroblastomas compared with neuroblastomas (P=0.001) and in normal brain compared with PNET/medulloblastoma (P<0.0001). Expression of Apo-3L was significantly associated with survival in neuroblastomas (P<0.049) and in PNET/medulloblastomas (P=0.01). Expression of Apo-3 was significantly associated with survival in PNET/medulloblastomas (P=0.03). Thus, the Apo-3 receptor/ligand system might be involved in the regulation of apoptosis in neuroblastomas and PNET.

Resistance to tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-induced apoptosis in neuroblastoma cells correlates with a loss of caspase-8 expression

Disruption of apoptotic pathways may be involved in tumor formation, regression, and treatment resistance of neuroblastoma (NB). Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in cancer cell lines, whereas normal cells are not sensitive to TRAIL-mediated apoptosis. In this study we analyzed the expression and function of TRAIL and its agonistic and antagonistic receptors as well as expression of cellular FLICE-like inhibitory protein and caspase-2, -3, -8, -9, and -10 in 18 NB cell lines. Semiquantitative RT-PCR revealed that TRAIL-R2 and TRAIL-R3 are the main TRAIL-receptors used by NB cells. Sensitivity to TRAIL-induced apoptosis did not correlate with mRNA expression of TRAIL receptors or cellular FLICE-like inhibitory protein. Surprisingly, caspase-8 and caspase-10 mRNA expression was detected in only 5 of 18 NB cell lines. Interestingly, only these five NB cell lines were susceptible to TRAIL-induced apoptosis in a time- and dose-dependent manner. Treatment with 5-aza-2'-deoxycytidine restored mRNA and protein expression of caspase-8 and TRAIL sensitivity of resistant cell lines, suggesting that gene methylation is involved in caspase inactivation. The TRAIL system seems to be functional in NB cells expressing caspase-8 and/or caspase-10. Because many cytotoxic drugs induce caspase-dependent apoptosis, failure to express caspase-8 and/or caspase-10 might be an important mechanism of resistance to chemotherapy in NB.

Inhibition of N-myc expression and induction of apoptosis by iron chelation in human neuroblastoma cells

Neuroblastoma is the second most common solid malignancy of childhood. Enhanced expression of the amplified N-myc gene in the tumor cells may be associated with poor patient prognosis and may contribute to tumor development and progression. The use of deferoxamine mesylate (DFO), an iron chelator, to treat neuroblastoma is being investigated in national clinical studies. We show here by TUNEL assay and DNA laddering that DFO induces apoptosis in cultured human neuroblastoma cells, which is preceded by a decrease in the expression of N-myc and the altered expression of some other oncogenes (up-regulating c-fos and down-regulating c-myb) but not housekeeping genes. The decrease in N-myc expression is iron-specific but does not result from inhibition of ribonucleotide reductase, because specific inhibition of this iron-containing enzyme by hydroxyurea does not affect N-myc protein levels. Nuclear run-on and transient reporter gene expression experiments show that the decrease in N-myc expression occurs at the level of initiation of transcription and by inhibiting N-myc promoter activity. Comparison across neuroblastoma cell lines of the amount of residual cellular N-myc protein with the extent of apoptosis measured as pan-caspase activity after 48 h of iron chelation reveals no correlation, suggesting that the decrease in N-myc expression is unlikely to mediate apoptosis. In conclusion, chelation of cellular iron by DFO may alter the expression of multiple genes affecting the malignant phenotype by multiple pathways. Given the clinical importance of N-myc overexpression in neuroblastoma malignancy, decreasing N-myc expression by DFO might be useful as an adjunct to current

Expression of the neurotrophin receptor TrkB is associated with unfavorable outcome in Wilms' tumor

PURPOSE

Neurotrophins and their receptors regulate the proliferation, differentiation, and death of neuronal cells, and they have been implicated in the pathogenesis and prognosis of neuroblastomas and medulloblastomas. Tyrosine kinase (Trk) receptors also are expressed in extraneural tissues.

PATIENTS AND METHODS

To study the role of neurotrophin receptors and ligands in Wilms' tumor (WT), we determined their expression by semiquantitative duplex reverse transcriptase polymerase chain reaction in 39 patients with primary WT. Comparison of mRNA expression levels with clinical variables was performed by use of Cox regression analysis.

RESULTS

Children with WT that expressed high levels of full-length TrkB mRNA (TrkBfull) had a significantly greater risk of death than children whose tumors had little or no TrkBfull expression (hazard ratio, 9.7; P =.02). The 5-year relapse-free survival was 100% versus 65% for patients with low versus high tumor expression of TrkBfull (P <.003). Conversely, children with tumors that expressed high mRNA levels of a functionally inactive truncated TrkB receptor (TrkBtrunc) had a greater chance of survival than children with low levels of TrkBtrunc (hazard ratio, 0.08; P =.005). The 5-year relapse-free survival was 95% versus 68% for patients with high versus low levels of TrkBtrunc (P =.01). The hazard ratios for TrkBfull and TrkBtrunc remained significant after they were adjusted for tumor stage (P =.01 and P =.017, respectively). All WTs with high levels of TrkB expression also expressed the brain-derived nerve growth factor ligand.

CONCLUSION

Expression of TrkBfull in WT is associated with worse outcome, perhaps because it provides an autocrine survival pathway. Conversely, TrkBtrunc expression is associated with excellent outcome, perhaps as a result of a dominant negative effect.

Association among EPHB2, TrkA, and MYCN expression in low-stage neuroblastomas

BACKGROUND

The EPH family is the largest subfamily of receptor protein-tyrosine kinases, consisting of EPHA and EPHB subgroups. Ligands of EPH family receptors are called ephrins, which include ephrin-A and ephrin-B subgroups. We recently found that transcripts encoding the EPHB subgroup (EPHB) and the ephrin-B subgroup (EFNB) were expressed together in neuroblastoma (NB) cell lines.

PROCEDURE

In this study, we examined the expression of EPHB and EFNB transcripts in 24 NB specimens representing all clinical stages. We found that several EPHB and EFNB transcripts were expressed together in all NBs examined.

RESULTS

Among the transcripts examined, EPHB6 expression was most significantly associated with low stage tumors (stages 1, 2, and 4S; P = 0.0048). TrkA expression was significantly correlated with EPHB6, EFNB2, and EFNB3 expression (P < 0.01 in each case). Taken together, these data indicate that the expression of EPHB6, EFNB2, and EFNB3 may serve as prognostic indicators of favorable NBs. In the low-stage NBs without MYCN amplification, EPHB2 expression was correlated both with MYCN expression and with TrkA expression (P < 0.01 in each case). Moreover, MYCN expression was correlated with TrkA expression (P < 0.01) in the low-stage NBs.

CONCLUSIONS

This observation points to the possibility that MYCN expression might contribute to favorable outcome of low-stage NBs.

Effect of CEP-751 (KT-6587) on neuroblastoma xenografts expressing TrkB

BACKGROUND

The compound CEP-751 (KT-6587), a potent and selective inhibitor of the Trk family of tyrosine kinases, has been shown to inhibit the growth of human neuroblastoma (NB) xenografts in nude mice [1].

PROCEDURE

To address its mechanism of action, we studied SY5Y, a human NB cell line with no detectable Trk expression, and two subclones transfected with TrkB. The transfected clones, SY5Y (G8) and SY5Y (G12), expressed moderate and high levels, respectively, of TrkB mRNA and protein. These TrkB-expressing subclones and the parental line were then grown as xenografts in nude mice, and CEP-751 was used to inhibit TrkB tyrosine kinase activity in these xenografts. Animals were treated twice a day with CEP-751 (21 mg/kg), or with the carrier vehicle as a control. TrkB expression in the resultant tumors was examined by quantitative RT-PCR. The effect of CEP-751 on TrkB activation by BDNF was examined in G12 cells in culture by immunoprecipitation with antipan Trk antiserum, followed by Western blot analysis using antiphosphotyrosine antibodies. To determine if CEP-751 was causing apoptosis, the TUNEL assay was used.

RESULTS

CEP-751 had little effect on the growth of SY5Y tumors, but did slow the growth rate of the C8 and G12 tumors. The daily growth rate of the treated tumors was 0.16, 0.13, and 0.10 cm3, respectively, for the SY5Y, G8, and G12 tumors. RT PCR analysis confirmed the expression of TrkB in G8 and G12, but not in SY5Y tumors. Activation of TrkB by BDNF in G12 cells was inhibited by CEP-751 in a dose dependent fashion. The treated tumors showed marked evidence of apoptosis.

CONCLUSIONS

These data suggest that the effect of CEP-751 is due, at least in part, to its inhibition of TrkB kinase, and that CEP-751 may become a useful therapeutic tool for the treatment of aggressive neuroblastomas, which often express TrkB.

TrkA signal transduction pathways in neuroblastoma

BACKGROUND

Favorable neuroblastomas frequently express high levels of the TrkA receptor, and these tumors have a propensity to either differentiate or regress, but the mechanisms responsible for these two fates are unclear.

PROCEDURE

To study TrkA signal transduction in neuroblastoma (nb), we stably expressed wild-type TrkA and five TrkA mutants in the human nb cell line SH-SY5Y. Resulting single cell clones were characterized by TrkA mRNA and protein expression and by autophosphorylation of the receptor.

RESULTS

Introduction of TrkA restored nerve growth factor (NGF) responsiveness of SH-SY5Y cells, demonstrated by morphological differentiation and induction of immediate-early genes. TrkA overexpression leads to growth inhibition in the absence of NGF, whereas NGF treatment results in increased proliferation.

CONCLUSIONS

Analysis of downstream signaling elements in mutated TrkA receptors indicates that NGF-induced differentiation is dependent on TrkA kinase activity, but several redundant pathways seem to be used farther downstream. This suggests differences from TrkA pathways identified in PC12 cells.

Resistance to TRAIL-induced apoptosis in neuroblastoma cells correlates with a loss of caspase-8 expression

BACKGROUND

Disruption of apoptotic pathways may be involved in tumor formation, regression, and treatment resistance of neuroblastoma (NB). TNF-related apoptosis-inducing ligand (TRAIL) is a potent inducer of apoptosis in cancer cell lines.

PROCEDURE

In this study we analyzed the expression and function of TRAIL, its agonistic and antagonistic receptors, and important intracellular signaling elements in 18 NB cell lines.

RESULTS

Semiquantitative RT-PCR revealed that TRAIL-R2 and TRAIL-R3 are the main TRAIL-receptors used by NB cells. Sensitivity to TRAIL-induced apoptosis did not correlate with mRNA expression of TRAIL receptors or cFLIP. Surprisingly, caspase-8 and caspase-10 mRNA was detected in only 5 of 18 NB cell lines. Interestingly, only these five NB cell lines were susceptible to TRAIL-induced apoptosis in a time- and dose-dependent manner.

CONCLUSIONS

Treatment with 5-aza-2'-deoxycytidine restored mRNA expression of caspase-8 and -10 and TRAIL sensitivity of resistant cell lines, suggesting that gene methylation is involved in caspase inactivation. Since many cytotoxic drugs induce caspase-dependent apoptosis, failure to express caspase-8 and/or caspase-10 might be an important mechanism of resistance to chemotherapy in NB.

Different effects of TrkA expression in neuroblastoma cell lines with or without MYCN amplification

BACKGROUND

Expression of the neurotrophin receptor TrkA is associated with a favorable prognosis in neuroblastoma (NB) and promotes growth inhibition and neuronal differentiation. Aggressive, MYCN-amplified NB tumors express little or no TrkA mRNA, suggesting that MYCN overexpression may inhibit TrkA expression.

PROCEDURE

To study the interactions of TrkA expression and MYCN amplification in NB, we stably expressed the TrkA receptor in the MYCN single copy cell lines SH-SY5Y and NB69 as well as in the MYCN amplified cell lines CHP134 and IMR5.

RESULTS

All four transfected cell lines demonstrated high TrkA expression and similar activation of the TrkA receptor and of mitogen-activated protein kinases as well as induction of immediate-early genes in response to nerve growth factor (NGF). Introduction of TrkA restored NGF responsiveness of SH-SY5Y and NB69 cells, as demonstrated by morphologic differentiation, growth inhibition, and enhanced survival in serum-free medium. However, no morphologic, growth, or survival responses to NGF were detected in MYCN-amplified CHP134 and IMR5 TrkA transfectants.

CONCLUSIONS

Thus, transfection of TrkA into MYCN amplified NB cell lines only partly restored the TrkA/NGF signaling pathway, suggesting additional inhibitory effects of MYCN overexpression on TrkA signaling.