TheREToncogene is a critical component of transcriptional programs associated with retinoic acid–induced differentiation in neuroblastoma

Targeting the RET tyrosine kinase in neuroblastoma: A review and application of a novel selective drug design strategy

The RET (REarranged during Transfection) gene, which encodes for a transmembrane receptor tyrosine kinase, is an established oncogene associated with the etiology and progression of multiple types of cancer. Oncogenic RET mutations and rearrangements resulting in gene fusions have been identified in many adult cancers, including medullary and papillary thyroid cancers, lung adenocarcinomas, colon and breast cancers, and many others. While genetic RET aberrations are much less common in pediatric solid tumors, increased RET expression has been shown to be associated with poor prognosis in children with solid tumors such as neuroblastoma, prompting an interest in RET inhibition as a form of therapy for these children. A number of kinase inhibitors currently in use for patients with cancer have RET inhibitory activity, but these inhibitors also display activity against other kinases, resulting in unwanted side effects and limiting their safety and efficacy. Recent efforts have been focused on developing more specific RET inhibitors, but due to high levels of conservation between kinase binding pockets, specificity remains a drug design challenge. Here, we review the background of RET as a potential therapeutic target in neuroblastoma tumors and the results of recent preclinical studies and clinical trials evaluating the safety and efficacy of RET inhibition in adults and children. We also present a novel approach to drug discovery leveraging the chemical phenomenon of atropisomerism to develop specific RET inhibitors and present preliminary data demonstrating the efficacy of a novel RET inhibitor against neuroblastoma tumor cells.

Identification of a novel gene signature for neuroblastoma differentiation using a Boolean implication network

Although induction of differentiation represents an effective strategy for neuroblastoma treatment, the mechanisms underlying neuroblastoma differentiation are poorly understood. We generated a computational model of neuroblastoma differentiation consisting of interconnected gene clusters identified based on symmetric and asymmetric gene expression relationships. We identified a differentiation signature consisting of series of gene clusters comprised of 1251 independent genes that predicted neuroblastoma differentiation in independent datasets and in neuroblastoma cell lines treated with agents known to induce differentiation. This differentiation signature was associated with patient outcomes in multiple independent patient cohorts and validated the role of MYCN expression as a marker of neuroblastoma differentiation. Our results further identified novel genes associated with MYCN via asymmetric Boolean implication relationships that would not have been identified using symmetric computational approaches and that were associated with both neuroblastoma differentiation and patient outcomes. Our differentiation signature included a cluster of genes involved in intracellular signaling and growth factor receptor trafficking pathways that is strongly associated with neuroblastoma differentiation, and we validated the associations of UBE4B, a gene within this cluster, with neuroblastoma cell and tumor differentiation. Our findings demonstrate that Boolean network analyses of symmetric and asymmetric gene expression relationships can identify novel genes and pathways relevant for neuroblastoma tumor differentiation that could represent potential therapeutic targets.

Super-enhancer associated core regulatory circuits mediate susceptibility to retinoic acid in neuroblastoma cells

Neuroblastoma is a pediatric tumour that accounts for more than 15% of cancer-related deaths in children. High-risk tumours are often difficult to treat, and patients' survival chances are less than 50%. Retinoic acid treatment is part of the maintenance therapy given to neuroblastoma patients; however, not all tumours differentiate in response to retinoic acid. Within neuroblastoma tumors, two phenotypically distinct cell types have been identified based on their super-enhancer landscape and transcriptional core regulatory circuitries: adrenergic (ADRN) and mesenchymal (MES). We hypothesized that the distinct super-enhancers in these different tumour cells mediate differential response to retinoic acid. To this end, three different neuroblastoma cell lines, ADRN (MYCN amplified and non-amplified) and MES cells, were treated with retinoic acid, and changes in the super-enhancer landscape upon treatment and after subsequent removal of retinoic acid was studied. Using ChIP-seq for the active histone mark H3K27ac, paired with RNA-seq, we compared the super-enhancer landscape in cells that undergo neuronal differentiation in response to retinoic acid versus those that fail to differentiate and identified unique super-enhancers associated with neuronal differentiation. Among the ADRN cells that respond to treatment, MYCN-amplified cells remain differentiated upon removal of retinoic acid, whereas MYCN non-amplified cells revert to an undifferentiated state, allowing for the identification of super-enhancers responsible for maintaining differentiation. This study identifies key super-enhancers that are crucial for retinoic acid-mediated differentiation.

TOP2B Is Required to Maintain the Adrenergic Neural Phenotype and for ATRA-Induced Differentiation of SH-SY5Y Neuroblastoma Cells

The neuroblastoma cell line SH-SY5Y is widely used to study retinoic acid (RA)-induced gene expression and differentiation and as a tool to study neurodegenerative disorders. SH-SY5Y cells predominantly exhibit adrenergic neuronal properties, but they can also exist in an epigenetically interconvertible alternative state with more mesenchymal characteristics; as a result, these cells can be used to study gene regulation circuitry controlling neuroblastoma phenotype. Using a combination of pharmacological inhibition and targeted gene inactivation, we have probed the requirement for DNA topoisomerase IIB (TOP2B) in RA-induced gene expression and differentiation and in the balance between adrenergic neuronal versus mesenchymal transcription programmes. We found that expression of many, but not all genes that are rapidly induced by ATRA in SH-SY5Y cells was significantly reduced in the TOP2B null cells; these genes include BCL2, CYP26A1, CRABP2, and NTRK2. Comparing gene expression profiles in wild-type versus TOP2B null cells, we found that long genes and genes expressed at a high level in WT SH-SY5Y cells were disproportionately dependent on TOP2B. Notably, TOP2B null SH-SY5Y cells upregulated mesenchymal markers vimentin (VIM) and fibronectin (FN1) and components of the NOTCH signalling pathway. Enrichment analysis and comparison with the transcription profiles of other neuroblastoma-derived cell lines supported the conclusion that TOP2B is required to fully maintain the adrenergic neural-like transcriptional signature of SH-SY5Y cells and to suppress the alternative mesenchymal epithelial-like epigenetic state.

Bimodal regulation of axonal transport by the GDNF-RET signalling axis in healthy and diseased motor neurons

Deficits in axonal transport are one of the earliest pathological outcomes in several models of amyotrophic lateral sclerosis (ALS), including SOD1G93A mice. Evidence suggests that rescuing these deficits prevents disease progression, stops denervation, and extends survival. Kinase inhibitors have been previously identified as transport enhancers, and are being investigated as potential therapies for ALS. For example, inhibitors of p38 mitogen-activated protein kinase and insulin growth factor receptor 1 have been shown to rescue axonal transport deficits in vivo in symptomatic SOD1G93A mice. In this work, we investigated the impact of RET, the tyrosine kinase receptor for glial cell line-derived neurotrophic factor (GDNF), as a modifier of axonal transport. We identified the fundamental interplay between RET signalling and axonal transport in both wild-type and SOD1G93A motor neurons in vitro. We demonstrated that blockade of RET signalling using pharmacological inhibitors and genetic knockdown enhances signalling endosome transport in wild-type motor neurons and uncovered a divergence in the response of primary motor neurons to GDNF compared with cell lines. Finally, we showed that inhibition of the GDNF-RET signalling axis rescues in vivo transport deficits in early symptomatic SOD1G93A mice, promoting RET as a potential therapeutic target in the treatment of ALS.

Loss of RET Promotes Mesenchymal Identity in Neuroblastoma Cells

Aberrant activation of anaplastic lymphoma kinase (ALK) drives neuroblastoma (NB). Previous work identified the RET receptor tyrosine kinase (RTK) as a downstream target of ALK activity in NB models. We show here that ALK activation in response to ALKAL2 ligand results in the rapid phosphorylation of RET in NB cells, providing additional insight into the contribution of RET to the ALK-driven gene signature in NB. To further address the role of RET in NB, RET knockout (KO) SK-N-AS cells were generated by CRISPR/Cas9 genome engineering. Gene expression analysis of RET KO NB cells identified a reprogramming of NB cells to a mesenchymal (MES) phenotype that was characterized by increased migration and upregulation of the AXL and MNNG HOS transforming gene (MET) RTKs, as well as integrins and extracellular matrix components. Strikingly, the upregulation of AXL in the absence of RET reflects the development timeline observed in the neural crest as progenitor cells undergo differentiation during embryonic development. Together, these findings suggest that a MES phenotype is promoted in mesenchymal NB cells in the absence of RET, reflective of a less differentiated developmental status.

Differentiating Neuroblastoma: A Systematic Review of the Retinoic Acid, Its Derivatives, and Synergistic Interactions

A neuroblastoma (NB) is a solid paediatric tumour arising from undifferentiated neuronal cells. Despite the recent advances in disease management and treatment, it remains one of the leading causes of childhood cancer deaths, thereby necessitating the development of new therapeutic agents and regimens. Retinoic acid (RA), a vitamin A derivative, is a promising agent that can induce differentiation in NB cells. Its isoform, 13-cis RA or isotretinoin, is used in NB therapy; however, its effectiveness is limited to treating a minimal residual disease as maintenance therapy. As such, research focuses on RA derivatives that might increase the anti-NB action or explores the potential synergy between RA and other classes of drugs, such as cellular processes mediators, epigenetic modifiers, and immune modulators. This review summarises the in vitro, in vivo, and clinical data of RA, its derivatives, and synergising compounds, thereby establishing the most promising RA derivatives and combinations of RA for further investigation.

Regorafenib is effective against neuroblastoma in vitro and in vivo and inhibits the RAS/MAPK, PI3K/Akt/mTOR and Fos/Jun pathways

BACKGROUND

Regorafenib is an inhibitor of multiple kinases with aberrant expression and activity in neuroblastoma tumours that have potential roles in neuroblastoma pathogenesis.

METHODS

We evaluated neuroblastoma cells treated with regorafenib for cell viability and confluence, and analysed treated cells for apoptosis and cell cycle progression. We evaluated the efficacy of regorafenib in vivo using an orthotopic xenograft model. We evaluated regorafenib-mediated inhibition of kinase targets and performed reverse-phase protein array (RPPA) analysis of neuroblastoma cells treated with regorafenib. Lastly, we evaluated the efficacy and effects of the combination of regorafenib and 13-cis-retinoic acid on intracellular signalling.

RESULTS

Regorafenib treatment resulted in reduced neuroblastoma cell viability and confluence, with both induction of apoptosis and of cell cycle arrest. Regorafenib treatment inhibits known receptor tyrosine kinase targets RET and PDGFRβ and intracellular signalling through the RAS/MAPK, PI3K/Akt/mTOR and Fos/Jun pathways. Regorafenib is effective against neuroblastoma tumours in vivo, and the combination of regorafenib and 13-cis-retinoic acid demonstrates enhanced efficacy compared with regorafenib alone.

CONCLUSIONS

The effects of regorafenib on multiple intracellular signalling pathways and the potential additional efficacy when combined with 13-cis-retinoic acid represent opportunities to develop treatment regimens incorporating regorafenib for children with neuroblastoma.

Novel Therapies for Relapsed and Refractory Neuroblastoma

While recent increases in our understanding of the biology of neuroblastoma have allowed for more precise risk stratification and improved outcomes for many patients, children with high-risk neuroblastoma continue to suffer from frequent disease relapse, and despite recent advances in our understanding of neuroblastoma pathogenesis, the outcomes for children with relapsed neuroblastoma remain poor. These children with relapsed neuroblastoma, therefore, continue to need novel treatment strategies based on a better understanding of neuroblastoma biology to improve outcomes. The discovery of new tumor targets and the development of novel antibody- and cell-mediated immunotherapy agents have led to a large number of clinical trials for children with relapsed neuroblastoma, and additional clinical trials using molecular and genetic tumor profiling to target tumor-specific aberrations are ongoing. Combinations of these new therapeutic modalities with current treatment regimens will likely be needed to improve the outcomes of children with relapsed and refractory neuroblastoma.

Small molecule inhibitor regorafenib inhibits RET signaling in neuroblastoma cells and effectively suppresses tumor growth in vivo

Neuroblastoma (NB), the most common extracranial pediatric solid tumor, continues to cause significant cancer-related morbidity and mortality in children. Dysregulation of oncogenic receptor tyrosine kinases (RTKs) has been shown to contribute to tumorigenesis in various human cancers and targeting these RTKs has had therapeutic benefit. RET is an RTK which is commonly expressed in NB, and high expression of RET correlates with poor outcomes in patients with NB. Herein we report that RET is required for NB cell proliferation and that the small molecule inhibitor regorafenib (BAY 73-4506) blocks glial cell derived neurotrophic factor (GDNF)-induced RET signaling in NB cells and inhibits NB growth both in vitro and in vivo. We found that regorafenib significantly inhibited cell proliferation and colony formation ability of NB cells. Moreover, regorafenib suppressed tumor growth in both an orthotopic xenograft NB mouse model and a TH-MYCN transgenic NB mouse model. Finally, regorafenib markedly improved the overall survival of TH-MYCN transgenic tumor-bearing mice. In summary, our study suggests that RET is a potential therapeutic target in NB, and that using a novel RET inhibitor, like regorafenib, should be investigated as a therapeutic treatment option for children with NB.

Vitamin A and the epigenome

The epigenetic phenomena refer to heritable changes in gene expression other than those in the DNA sequence, such as DNA methylation and histone modifications. Major research progress in the last few years has provided further proof that environmental factors, including diet and nutrition, can influence physiologic and pathologic processes through epigenetic alterations, which in turn influence gene expression. This influence is termed nutritional epigenetics, and one prominent example is the regulation of gene transcription by vitamin A through interaction to its nuclear receptor. Vitamin A is critical throughout life. Together with its derivatives, it regulates diverse processes including reproduction, embryogenesis, vision, growth, cellular differentiation and proliferation, maintenance of epithelial cellular integrity and immune function. Here we review the epigenetic role of vitamin A in cancer, stem cells differentiation, proliferation, and immunity. The data presented here show that retinoic acid is a potent agent capable of inducing alterations in epigenetic modifications that produce various effects on the phenotype. Medical benefits of vitamin A as an epigenetic modulator, especially with respect to its chronic use as nutritional supplement, should rely on our further understanding of its epigenetic effects during health and disease, as well as through different generations.

Overview and recent advances in the treatment of neuroblastoma

INTRODUCTION

Children with neuroblastoma have widely divergent outcomes, ranging from cure in >90% of patients with low risk disease to <50% for those with high risk disease. Recent research has shed light on the biology of neuroblastoma, allowing for more accurate risk stratification and treatment reduction in many cases, although newer treatment strategies for children with high-risk and relapsed neuroblastoma are needed to improve outcomes. Areas covered: Neuroblastoma epidemiology, diagnosis, risk stratification, and recent advances in treatment of both newly diagnosed and relapsed neuroblastoma. Expert commentary: The identification of newer tumor targets and of novel cell-mediated immunotherapy agents may lead to novel therapeutic approaches, and clinical trials for regimens designed to target individual genetic aberrations in tumors are underway. A combination of therapeutic modalities will likely be required to improve survival and cure rates for patients with high-risk neuroblastoma.

Neuroblastoma patient outcomes, tumor differentiation, and ERK activation are correlated with expression levels of the ubiquitin ligase UBE4B

BACKGROUND

UBE4B is an E3/E4 ubiquitin ligase whose gene is located in chromosome 1p36.22. We analyzed the associations of UBE4B gene and protein expression with neuroblastoma patient outcomes and with tumor prognostic features and histology.

METHODS

We evaluated the association of UBE4B gene expression with neuroblastoma patient outcomes using the R2 Platform. We screened neuroblastoma tumor samples for UBE4B protein expression using immunohistochemistry. FISH for UBE4B and 1p36 deletion was performed on tumor samples. We then evaluated UBE4B expression for associations with prognostic factors and with levels of phosphorylated ERK in neuroblastoma tumors and cell lines.

RESULTS

Low UBE4B gene expression is associated with poor outcomes in patients with neuroblastoma and with worse outcomes in all patient subgroups. UBE4B protein expression was associated with neuroblastoma tumor differentiation, and decreased UBE4B protein levels were associated with high-risk features. UBE4B protein levels were also associated with levels of phosphorylated ERK.

CONCLUSIONS

We have demonstrated associations between UBE4B gene expression and neuroblastoma patient outcomes and prognostic features. Reduced UBE4B protein expression in neuroblastoma tumors was associated with high-risk features, a lack of differentiation, and with ERK activation. These results suggest UBE4B may contribute to the poor prognosis of neuroblastoma tumors with 1p36 deletions and that UBE4B expression may mediate neuroblastoma differentiation.

Congenital neuroblastoma in a neonate with isotretinoin embryopathy

We describe a neonate with isotretinoin embryopathy and an incidental finding of congenital neuroblastoma. Diffuse liver metastases led to the decision to provide oncologic therapy followed by tumor resection. Despite the possible need for chronic care related to the comorbidities of the isotretinoin embryopathy and oncologic management, the patient remains disease-free. Because of the uncertain etiology of neuroblastoma, it remains unclear whether exposure to isotretinoin during embryogenesis and fetal development had an oncogenic effect on this patient.

A cellular model of amyloid precursor protein processing and amyloid-β peptide production

BACKGROUND

A hallmark pathologic feature of Alzheimer's disease (AD) is accumulation of neuritic senile plaques in the brain parenchyma. Neurotoxic plaque cores are composed predominantly of amyloid-β (Aβ) peptides of 40 and 42 amino acids in length, formed by sequential cleavage of amyloid precursor protein (APP) by β-, and γ-secretases. There is a great interest in approaches to modulate Aβ peptide production and develop therapeutic interventions to reduce Aβ levels to halt or slow the progression of neurodegeneration.

NEW METHOD

We characterized and present the BE(2)-M17 human neuroblastoma cell line as a novel in vitro model of the APP-cleavage cascade to support future (1) functional studies of molecular regulators in Aβ production, and (2) high-throughput screening assays of new pharmacotherapeutics.

RESULTS

In BE(2)-M17 cells, both RNA (i.e., RT-PCR, RNA sequencing) and protein analyses (i.e., Western blots, ELISA), show endogenous expression of critical components of the amyloidogenic pathway, APP-cleavage intermediates CTF83 and CTF99, and final cleavage products Aβ40 and Aβ42. We further report effects of retinoic acid-mediated differentiation on morphology and gene expression in this cell line.

COMPARISON WITH EXISTING METHOD(S)

In contrast to primary isolates or other cell lines reported in current literature, BE(2)-M17 not only sustains baseline expression of the full contingent of APP-processing components, but also remains stably adherent during culture, facilitating experimental manipulations.

CONCLUSIONS

Our evidence supports the use of BE(2)-M17 as a novel, human, cell-based model of the APP processing pathway that offers a potential streamlined approach to dissect molecular functions of endogenous regulatory pathways, and perform mechanistic studies to identify modulators of Aβ production.

Histone chaperone CHAF1A inhibits differentiation and promotes aggressive neuroblastoma

Neuroblastoma arises from the embryonal neural crest secondary to a block in differentiation. Long-term patient survival correlates inversely with the extent of differentiation, and treatment with retinoic acid or other prodifferentiation agents improves survival modestly. In this study, we show the histone chaperone and epigenetic regulator CHAF1A functions in maintaining the highly dedifferentiated state of this aggressive malignancy. CHAF1A is a subunit of the chromatin modifier chromatin assembly factor 1 and it regulates H3K9 trimethylation of key target genes regulating proliferation, survival, and differentiation. Elevated CHAF1A expression strongly correlated with poor prognosis. Conversely, CHAF1A loss-of-function was sufficient to drive neuronal differentiation in vitro and in vivo. Transcriptome analysis of cells lacking CHAF1A revealed repression of oncogenic signaling pathways and a normalization of glycolytic metabolism. Our findings demonstrate that CHAF1A restricts neural crest differentiation and contributes to the pathogenesis of high-risk neuroblastoma.

Changes in gene expression profiling of apoptotic genes in neuroblastoma cell lines upon retinoic acid treatment

To determine the effect of retinoic acid (RA) in neuroblastoma we treated RA sensitive neuroblastoma cell lines with 9-cis RA or ATRA for 9 days, or for 5 days followed by absence of RA for another 4 days. Both isomers induced apoptosis and reduced cell density as a result of cell differentiation and/or apoptosis. Flow cytometry revealed that 9-cis RA induced apoptosis more effectively than ATRA. The expression profile of apoptosis and survival pathways was cell line specific and depended on the isomer used.

New aspects of neuroblastoma treatment: ASPHO 2011 symposium review

Neuroblastoma is the most common extracranial solid tumor of childhood, and the outcomes for children with high-risk and relapsed disease remain poor. However, new international strategies for risk stratification and for treatment based on novel tumor targets and including immunotherapy are being employed in attempts to improve the outcomes of children with neuroblastoma. A new international neuroblastoma risk classification system has been developed which is being incorporated into cooperative group clinical trials in North America, Japan, and Europe, resulting in standardized approaches for the initial evaluation and treatment stratification of neuroblastoma patients. Furthermore, novel treatment regimens are being developed based on improved understanding of neuroblastoma biology and on the recruitment of the immune system to specifically target neuroblastoma tumors. These approaches will lead to new therapeutic strategies that likely will improve the outcomes for children with neuroblastoma worldwide.

Phosphorylation of distinct sites in MeCP2 modifies cofactor associations and the dynamics of transcriptional regulation

Mutations in the gene encoding methyl-CpG-binding protein 2 (MeCP2) lead to disrupted neuronal function and can cause the neurodevelopmental disorder Rett syndrome. MeCP2 is a transcriptional regulator that binds to methylated DNA and is most abundant in neuronal nuclei. The mechanisms by which MeCP2 regulates gene expression remain ambiguous, as it has been reported to function as a transcriptional silencer or activator and to execute these activities through both gene-specific and genome-wide mechanisms. We hypothesized that posttranslational modifications of MeCP2 may be important for reconciling these apparently contradictory functions. Our results demonstrate that MeCP2 contains multiple posttranslational modifications, including phosphorylation, acetylation, and ubiquitylation. Phosphorylation of MeCP2 at S229 or S80 influenced selective in vivo interactions with the chromatin factors HP1 and SMC3 and the cofactors Sin3A and YB-1. pS229 MeCP2 was specifically enriched at the RET promoter, and phosphorylation of MeCP2 was necessary for differentiation-induced activation and repression of the MeCP2 target genes RET and EGR2. These results demonstrate that phosphorylation is one of several factors that are important for interpreting the complexities of MeCP2 transcriptional modulation.

Gene expression analysis of mammary tissue during fetal bud formation and growth in two pig breeds--indications of prenatal initiation of postnatal phenotypic differences

BACKGROUND

The mammary gland is key to all mammal species; in particular in multiparous species like pigs the number and the shape of functional mammary gland complexes are major determinants of fitness. Accordingly, we aimed to catalog the genes relevant to mammogenesis in pigs. Moreover, we aimed to address the hypothesis that the extent and timing of proliferation, differentiation, and maturation processes during prenatal development contribute to postnatal numerical, morphological and functional properties of the mammary gland. Thus we focused on differentially expressed genes and networks relevant to mammary complex development in two breeds that are subject to different selection pressure on number, shape and function of teats and show largely different prevalence of non-functional inverted teats. The expression patterns of fetal mammary complexes obtained at 63 and 91 days post conception (dpc) from German Landrace (GL) and Pietrain (PI) were analyzed by Affymetrix GeneChip Porcine Genome Arrays.

RESULTS

The expression of 11,731 probe sets was analysed between the two stages within and among breeds. The analysis showed the largest distinction of samples of the breed GL at 63 dpc from all other samples. According to Ingenuity Pathways Analysis transcripts with abundance at the four comparisons made (GL63-GL91, PI63-PI93, GL63-PI63 and GL91-PI91) were predominantly assigned to biofunctions relevant to 'cell maintenance, proliferation, differentiation and replacement', 'organismal, organ and tissue development' and 'genetic information and nucleic acid processing'. Moreover, these transcripts almost exclusively belong to canonical pathways related to signaling rather than metabolic pathways. The accumulation of transcripts that are up-regulated in GL compared to PI indicate a higher proliferating activity in GL, whereas processes related to differentiation, maturation and maintenance of cells are more prominent in PI. Differential expression was validated by quantitative RT-PCR of five genes (GAB1, MAPK9, PIK3C2B, PIK3C3 and PRKCH) that are involved in several relevant signaling pathways.

CONCLUSIONS

The results indicate that mammary complex development in PI precedes GL. The differential expression between the two breeds at fetal stages likely reflects the prenatal initiation of postnatal phenotypes concerning the number and shape as well as functionality of teats.

Steroid hormone modulation of RET through two estrogen responsive enhancers in breast cancer

RET, a gene causatively mutated in Hirschsprung disease and cancer, has recently been implicated in breast cancer estrogen (E2) independence and tamoxifen resistance. RET displays both E2 and retinoic acid (RA)-dependent transcriptional modulation in E2-responsive breast cancers. However, the regulatory elements through which the steroid hormone transcriptional regulation of RET is mediated are poorly defined. Recent genome-wide chromatin immunoprecipitation-based studies have identified 10 putative E2 receptor-alpha (ESR1) and RA receptor alpha-binding sites at the RET locus, of which we demonstrate only two (RET -49.8 and RET +32.8) display significant E2 regulatory response when assayed independently in MCF-7 breast cancer cells. We demonstrate that endogenous RET expression and RET -49.8 regulatory activity are cooperatively regulated by E2 and RA in breast cancer cells. We identify key sequences that are required for RET -49.8 and RET +32.8 E2 responsiveness, including motifs known to be bound by ESR1, FOXA1 and TFAP2C. We also report that both RET -49.8 regulatory activity and endogenous RET expression are completely dependent on ESR1 for their (E2)-induction and that ESR1 is sufficient to mediate the E2-induced enhancer activity of RET -49.8 and RET +32.8. Finally, using zebrafish transgenesis, we also demonstrate that RET -49.8 directs reporter expression in the central nervous system and peripheral nervous system consistent with the endogenous ret expression. Taken collectively, these data suggest that RET transcription in breast cancer cells is modulated by E2 via ESR1 acting on multiple elements collectively.

MicroRNAs-10a and -10b contribute to retinoic acid-induced differentiation of neuroblastoma cells and target the alternative splicing regulatory factor SFRS1 (SF2/ASF)

MicroRNAs (miRNAs) are an emerging class of non-coding endogenous RNAs involved in multiple cellular processes, including cell differentiation. Treatment with retinoic acid (RA) results in neural differentiation of neuroblastoma cells. We wanted to elucidate whether miRNAs contribute to the gene expression changes induced by RA in neuroblastoma cells and whether miRNA regulation is involved in the transduction of the RA signal. We show here that RA treatment of SH-SY5Y neuroblastoma cells results in profound changes in the expression pattern of miRNAs. Up to 42 different miRNA species significantly changed their expression (26 up-regulated and 16 down-regulated). Among them, the closely related miR-10a and -10b showed the most prominent expression changes. Induction of miR-10a and -10b by RA also could be detected in LA-N-1 neuroblastoma cells. Loss of function experiments demonstrated that miR-10a and -10b are essential mediators of RA-induced neuroblastoma differentiation and of the associated changes in migration, invasion, and in vivo metastasis. In addition, we found that the SR-family splicing factor SFRS1 (SF2/ASF) is a target for miR-10a -and -10b in HeLa and SH-SY5Y neuroblastoma cells. We show here that changes in miR-10a and -10b expression levels may regulate SFRS1-dependent alternative splicing and translational functions. Taken together, our results give support to the idea that miRNA regulation plays a key role in RA-induced neuroblastoma cell differentiation. The discovery of SFRS1 as direct target of miR-10a and -10b supports the emerging functional interaction between two post-transcriptional mechanisms, microRNAs and splicing, in the neuronal differentiation context.

Chromatin and DNA methylation dynamics during retinoic acid-induced RET gene transcriptional activation in neuroblastoma cells

Although it is well known that RET gene is strongly activated by retinoic acid (RA) in neuroblastoma cells, the mechanisms underlying such activation are still poorly understood. Here we show that a complex series of molecular events, that include modifications of both chromatin and DNA methylation state, accompany RA-mediated RET activation. Our results indicate that the primary epigenetic determinants of RA-induced RET activation differ between enhancer and promoter regions. At promoter region, the main mark of RET activation was the increase of H3K4me3 levels while no significant changes of the methylation state of H3K27 and H3K9 were observed. At RET enhancer region a bipartite chromatin domain was detected in unstimulated cells and a prompt demethylation of H3K27me3 marked RET gene activation upon RA exposure. Moreover, ChIP experiments demonstrated that EZH2 and MeCP2 repressor complexes were associated to the heavily methylated enhancer region in the absence of RA while both complexes were displaced during RA stimulation. Finally, our data show that a demethylation of a specific CpG site at the enhancer region could favor the displacement of MeCP2 from the heavily methylated RET enhancer region providing a novel potential mechanism for transcriptional regulation of methylated RA-regulated loci.

A novel therapeutic combination for neuroblastoma: the vascular endothelial growth factor receptor/epidermal growth factor receptor/rearranged during transfection inhibitor vandetanib with 13-cis-retinoic acid

BACKGROUND

High-risk cases of neuroblastoma have poor survival rates, and novel therapies are needed. Vandetanib (ZD6474, Zactima) is an inhibitor of the vascular endothelial growth factor receptor, epidermal growth factor receptor, and rearranged during transfection (RET) tyrosine kinases, which have each been implicated in neuroblastoma pathogenesis. The authors hypothesized that vandetanib combined with 13-cis-retinoic acid (CRA), a differentiating agent used in most current neuroblastoma treatment regimens, would be effective against neuroblastoma tumor models.

METHODS

The authors evaluated the effects of vandetanib with and without CRA on RET phosphorylation and on the proliferation and survival of human neuroblastoma cell lines in vitro. Using a subcutaneous mouse xenograft model of human neuroblastoma, they analyzed tumors treated with CRA, vandetanib, and the combination of vandetanib plus CRA for growth, gross and histologic appearance, vascularity, and apoptosis.

RESULTS

Vandetanib treatment inhibited RET phosphorylation and resulted in induction of apoptosis in the majority of neuroblastoma cell lines in vitro, whereas CRA treatment induced morphologic differentiation and cell-cycle arrest. Treatment with vandetanib plus CRA resulted in more significant reduction in neuroblastoma cell viability than either alone. In a mouse xenograft model, the combination of vandetanib with CRA demonstrated significantly more growth inhibition than either alone, via both reduction in tumor vascularity and induction of apoptosis.

CONCLUSIONS

Vandetanib induces neuroblastoma tumor cell death in vitro and reduces tumor growth and vascularity in vivo. The combination of vandetanib with CRA was more effective in reducing tumor growth than either treatment alone. The antitumor effects of vandetanib plus CRA suggest a novel combination for use in neuroblastoma patients.

Enhancement of ATRA-induced differentiation of neuroblastoma cells with LOX/COX inhibitors: an expression profiling study

BACKGROUND

We performed expression profiling of two neuroblastoma cell lines, SK-N-BE(2) and SH-SY5Y, after combined treatment with all-trans retinoic acid (ATRA) and inhibitors of lipoxygenases (LOX) and cyclooxygenases (COX). This study is a continuation of our previous work confirming the possibility of enhancing ATRA-induced cell differentiation in these cell lines by the application of LOX/COX inhibitors and brings more detailed information concerning the mechanisms of the enhancement of ATRA-induced differentiation of neuroblastoma cells.

METHODS

Caffeic acid, as an inhibitor of 5-lipoxygenase, and celecoxib, as an inhibitor on cyclooxygenase-2, were used in this study. Expression profiling was performed using Human Cancer Oligo GEArray membranes that cover 440 cancer-related genes.

RESULTS

Cluster analyses of the changes in gene expression showed the concentration-dependent increase in genes known to be involved in the process of retinoid-induced neuronal differentiation, especially in cytoskeleton remodeling. These changes were detected in both cell lines, and they were independent of the type of specific inhibitors, suggesting a common mechanism of ATRA-induced differentiation enhancement. Furthermore, we also found overexpression of some genes in the same cell line (SK-N-BE(2) or SH-SY5Y) after combined treatment with both ATRA and CA, or ATRA and CX. Finally, we also detected that gene expression was changed after treatment with the same inhibitor (CA or CX) in combination with ATRA in both cell lines.

CONCLUSIONS

Obtained results confirmed our initial hypothesis of the common mechanism of enhancement in ATRA-induced cell differentiation via inhibition of arachidonic acid metabolic pathway.

Inhibition of neuronal cell death after retinoic acid-induced down-regulation of P2X7 nucleotide receptor expression

Apoptosis is a major mechanism for cell death in the nervous system during development. P2X(7) nucleotide receptors are ionotropic ATP receptors that mediate cell death under pathological conditions. We developed an in vitro protocol to investigate the expression and functional responses of P2X(7) nucleotide receptors during retinoic acid (RA)-induced neuronal differentiation of human SH-SY5Y neuroblastoma cells. Neuronal differentiation was examined measuring cellular growth arrest and neuritic processes elongation. We found that SH-SY5Y cells treated for 5 days with RA under low serum content exhibited a neuron-like phenotype with neurites extending more than twice the length of the cell body and cell growth arrest. Concurrently, we detected the abolishment of intracellular-free calcium mobilization and the down-regulation of P2X(7) nucleotide receptor protein expression that protected differentiated cells from neuronal cell death and reduced caspase-3 cleavage-induced by P2X(7) nucleotide receptor agonist. The role of P2X(7) nucleotide receptors in neuronal death was established by selectively antagonizing the receptor with KN-62 prior to its activation. We assessed the involvement of protein kinases and found that p38 signaling was activated in undifferentiated after nucleotide stimulation, but abolished by the differentiating RA pretreatment. Importantly, P2X(7) receptor-induced caspase-3 cleavage was blocked by the p38 protein kinase specific inhibitor PD169316. Taken together, our results suggest that RA treatment of human SH-SY5Y cells leads to decreased P2X(7) nucleotide receptor protein expression thus protecting differentiated cells from extracellular nucleotide-induced neuronal death, and p38 signaling pathway is critically involved in this protection of RA-differentiated cells.

Enteric neural crest differentiation in ganglioneuromas implicates Hedgehog signaling in peripheral neuroblastic tumor pathogenesis

Peripheral neuroblastic tumors (PNTs) share a common origin in the sympathetic nervous system, but manifest variable differentiation and growth potential. Malignant neuroblastoma (NB) and benign ganglioneuroma (GN) stand at opposite ends of the clinical spectrum. We hypothesize that a common PNT progenitor is driven to variable differentiation by specific developmental signaling pathways. To elucidate developmental pathways that direct PNTs along the differentiation spectrum, we compared the expression of genes related to neural crest development in GN and NB. In GNs, we found relatively low expression of sympathetic markers including adrenergic biosynthesis enzymes, indicating divergence from sympathetic fate. In contrast, GNs expressed relatively high levels of enteric neuropeptides and key constituents of the Hedgehog (HH) signaling pathway, including Dhh, Gli1 and Gli3. Predicted HH targets were also differentially expressed in GN, consistent with transcriptional response to HH signaling. These findings indicate that HH signaling is specifically active in GN. Together with the known role of HH activity in enteric neural development, these findings further suggested a role for HH activity in directing PNTs away from the sympathetic lineage toward a benign GN phenotype resembling enteric ganglia. We tested the potential for HH signaling to advance differentiation in PNTs by transducing NB cell lines with Gli1 and determining phenotypic and transcriptional response. Gli1 inhibited proliferation of NB cells, and induced a pattern of gene expression that resembled the differential pattern of gene expression of GN, compared to NB (p<0.00001). Moreover, the transcriptional response of SY5Y cells to Gli1 transduction closely resembled the transcriptional response to the differentiation agent retinoic acid (p<0.00001). Notably, Gli1 did not induce N-MYC expression in neuroblastoma cells, but strongly induced RET, a known mediator of RA effect. The decrease in NB cell proliferation induced by Gli1, and the similarity in the patterns of gene expression induced by Gli1 and by RA, corroborated by closely matched gene sets in GN tumors, all support a model in which HH signaling suppresses PNT growth by promoting differentiation along alternative neural crest pathways.

Potent antitumor effects of ZD6474 on neuroblastoma via dual targeting of tumor cells and tumor endothelium

Among children with relapsed or refractory neuroblastoma, the prognosis is poor and novel therapeutic strategies are needed to improve long-term survival. As with other solid tumors, high vascular density within neuroblastoma is associated with advanced disease, and therapeutic regimens directed against the tumor vasculature may provide clinical benefit. The receptor tyrosine kinase RET is widely expressed in neuroblastoma and is known to activate key signal transduction pathways involved in tumor cell survival and progression including Ras/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/Akt. We investigated the effect of dual targeting of tumor cells and tumor endothelium with ZD6474, a small-molecule tyrosine kinase inhibitor of vascular endothelial growth factor (VEGF) receptor 2, epidermal growth factor receptor, and RET. ZD6474 inhibited the phosphorylation of RET in neuroblastoma cells and had a direct effect on tumor cell viability in seven neuroblastoma cell lines. In a human neuroblastoma xenograft model, ZD6474 inhibited tumor growth by 85% compared with treatment with vehicle alone. In contrast, no significant inhibition of tumor growth was observed after treatment with bevacizumab, an antihuman VEGF monoclonal antibody, or the epidermal growth factor receptor inhibitor erlotinib, either alone or in combination. Immunohistochemical analysis showed that ZD6474 treatment led to an increase in endothelial cell apoptosis along with inhibition of VEGF receptor-2 activation on tumor endothelium. In conclusion, dual targeting of tumor cells, potentially through RET inhibition, and tumor vasculature with ZD6474 leads to potent antitumor effects. This approach merits further investigation for patients with neuroblastoma.