Myc family oncoproteins function through a common pathway to transform normal cells in culture: cross-interference by Max and trans-acting dominant mutants

MYC Family Amplification Dictates Sensitivity to BET Bromodomain Protein Inhibitor Mivebresib (ABBV075) in Small-Cell Lung Cancer

Small-cell lung cancer (SCLC) accounts for nearly 15% of all lung cancers. Although patients respond to first-line therapy readily, rapid relapse is inevitable, with few treatment options in the second-line setting. Here, we describe SCLC cell lines harboring amplification of MYC and MYCN but not MYCL1 or non-amplified MYC cell lines exhibit superior sensitivity to treatment with the pan-BET bromodomain protein inhibitor mivebresib (ABBV075). Silencing MYC and MYCN partially rescued SCLC cell lines harboring these respective amplifications from the antiproliferative effects of mivebresib. Further characterization of genome-wide binding of MYC, MYCN, and MYCL1 uncovered unique enhancer and epigenetic preferences. Implications: Our study suggests that chromatin landscapes can establish cell states with unique gene expression programs, conveying sensitivity to epigenetic inhibitors such as mivebresib.

Translation of non-canonical open reading frames as a cancer cell survival mechanism in childhood medulloblastoma

A hallmark of high-risk childhood medulloblastoma is the dysregulation of RNA translation. Currently, it is unknown whether medulloblastoma dysregulates the translation of putatively oncogenic non-canonical open reading frames (ORFs). To address this question, we performed ribosome profiling of 32 medulloblastoma tissues and cell lines and observed widespread non-canonical ORF translation. We then developed a stepwise approach using multiple CRISPR-Cas9 screens to elucidate non-canonical ORFs and putative microproteins implicated in medulloblastoma cell survival. We determined that multiple lncRNA-ORFs and upstream ORFs (uORFs) exhibited selective functionality independent of main coding sequences. A microprotein encoded by one of these ORFs, ASNSD1-uORF or ASDURF, was upregulated, associated with MYC-family oncogenes, and promoted medulloblastoma cell survival through engagement with the prefoldin-like chaperone complex. Our findings underscore the fundamental importance of non-canonical ORF translation in medulloblastoma and provide a rationale to include these ORFs in future studies seeking to define new cancer targets.

RNA N6-methyladenosine reader IGF2BP3 interacts with MYCN and facilitates neuroblastoma cell proliferation

Neuroblastoma (NB) is a kind of typical life-threatening extracranial tumor in children. N6-methyladenosine (m6A) modification is closely related to multiple cancer pathological processes. Insulin-like growth factor 2 mRNA binding protein 3 (IGF2BP3) is a top-ranked prognostic risk gene in NB; however, its function is uncertain. The expression of m6A-associated enzymes in patients with NB was analyzed using the Gene Expression Omnibus (GEO) and Therapeutically Applicable Research to Generate Effective Treatments (TARGET) database. The IGF2BP3 level in NB cell lines and primary samples was tested using quantitative real-time polymerase chain reaction (qRT-PCR), western blot method, and immunohistochemical analysis. The IGF2BP3 function in cell proliferation was clarified based on many functional in vitro and in vivo experiments. The interaction between IGF2BP3 and N-myc was researched via RNA immunoprecipitation (RIP), m6A RNA immunoprecipitation (MeRIP), and chromatin immunoprecipitation (ChIP) assays. The 16 m6A-regulated enzymes in NB were researched, and the result indicated that IGF2BP3 overexpression was related to cancer progression, COG risk, and survival based on the GEO and TARGET databases. Besides, the IGF2BP3 and MYCN levels were positively correlated. IGF2BP3 expression levels increased in MYCN-amplified NB clinical samples and cells. Knockdown of IGF2BP3 inhibited N-myc expression and NB cell proliferation in vitro and in vivo. IGF2BP3 regulates MYCN RNA stability by modifying m6A. In addition, we demonstrated that N-myc is a transcription factor that directly promotes IGF2BP3 expression in NB cells. IGF2BP3 regulates the proliferation of NB cells via m6A modification of MYCN. N-myc also acts as a transcription factor that regulates IGF2BP3 expression. A positive feedback loop between IGF2BP3 and N-myc facilitates NB cell proliferation.

Translation of non-canonical open reading frames as a cancer cell survival mechanism in childhood medulloblastoma

A hallmark of high-risk childhood medulloblastoma is the dysregulation of RNA translation. Currently, it is unknown whether medulloblastoma dysregulates the translation of putatively oncogenic non-canonical open reading frames. To address this question, we performed ribosome profiling of 32 medulloblastoma tissues and cell lines and observed widespread non-canonical ORF translation. We then developed a step-wise approach to employ multiple CRISPR-Cas9 screens to elucidate functional non-canonical ORFs implicated in medulloblastoma cell survival. We determined that multiple lncRNA-ORFs and upstream open reading frames (uORFs) exhibited selective functionality independent of the main coding sequence. One of these, ASNSD1-uORF or ASDURF, was upregulated, associated with the MYC family oncogenes, and was required for medulloblastoma cell survival through engagement with the prefoldin-like chaperone complex. Our findings underscore the fundamental importance of non-canonical ORF translation in medulloblastoma and provide a rationale to include these ORFs in future cancer genomics studies seeking to define new cancer targets.

A MYC inhibitor selectively alters the MYC and MAX cistromes and modulates the epigenomic landscape to regulate target gene expression

MYC regulates multiple gene programs, raising questions about the potential selectivity and downstream transcriptional consequences of MYC inhibitors as cancer therapeutics. Here, we examined the effect of a small-molecule MYC inhibitor, MYCi975, on the MYC/MAX cistromes, epigenome, transcriptome, and tumorigenesis. Integrating these data revealed three major classes of MYCi975-modulated gene targets: type 1 (down-regulated), type 2 (up-regulated), and type 3 (unaltered). While cell cycle and signal transduction pathways were heavily targeted by MYCi, RNA biogenesis and core transcriptional pathway genes were spared. MYCi975 altered chromatin binding of MYC and the MYC network family proteins, and chromatin accessibility and H3K27 acetylation alterations revealed MYCi975 suppression of MYC-regulated lineage factors AR/ARv7, FOXA1, and FOXM1. Consequently, MYCi975 synergistically sensitized resistant prostate cancer cells to enzalutamide and estrogen receptor-positive breast cancer cells to 4-hydroxytamoxifen. Our results demonstrate that MYCi975 selectively inhibits MYC target gene expression and provide a mechanistic rationale for potential combination therapies.

Normal and Neoplastic Growth Suppression by the Extended Myc Network

Among the first discovered and most prominent cellular oncogenes is MYC, which encodes a bHLH-ZIP transcription factor (Myc) that both activates and suppresses numerous genes involved in proliferation, energy production, metabolism and translation. Myc belongs to a small group of bHLH-ZIP transcriptional regulators (the Myc Network) that includes its obligate heterodimerization partner Max and six "Mxd proteins" (Mxd1-4, Mnt and Mga), each of which heterodimerizes with Max and largely opposes Myc's functions. More recently, a second group of bHLH-ZIP proteins (the Mlx Network) has emerged that bears many parallels with the Myc Network. It is comprised of the Myc-like factors ChREBP and MondoA, which, in association with the Max-like member Mlx, regulate smaller and more functionally restricted repertoires of target genes, some of which are shared with Myc. Opposing ChREBP and MondoA are heterodimers comprised of Mlx and Mxd1, Mxd4 and Mnt, which also structurally and operationally link the two Networks. We discuss here the functions of these "Extended Myc Network" members, with particular emphasis on their roles in suppressing normal and neoplastic growth. These roles are complex due to the temporal- and tissue-restricted expression of Extended Myc Network proteins in normal cells, their regulation of both common and unique target genes and, in some cases, their functional redundancy.

The Complex Network between MYC Oncogene and microRNAs in Gastric Cancer: An Overview

Despite the advancements in cancer treatments, gastric cancer is still one of the leading causes of death worldwide. In this context, it is of great interest to discover new and more effective ways of treating this disease. Accumulated evidences have demonstrated the amplification of 8q24.21 region in gastric tumors. Furthermore, this is the region where the widely known MYC oncogene and different microRNAs are located. MYC deregulation is key in tumorigenesis in various types of tissues, once it is associated with cell proliferation, survival, and drug resistance. microRNAs are a class of noncoding RNAs that negatively regulate the protein translation, and which deregulation is related with gastric cancer development. However, little is understood about the interactions between microRNAs and MYC. Here, we overview the MYC role and its relationship with the microRNAs network in gastric cancer aiming to identify potential targets useful to be used in clinic, not only as biomarkers, but also as molecules for development of promising therapies.

Association Between c-Myc and Colorectal Cancer Prognosis: A Meta-Analysis

Background: There is debate as to whether c-Myc predicts prognosis in colorectal cancer (CRC). In this study, we aimed to review the association between c-Myc and CRC prognosis.

Methods: Pertinent studies were identified by searching electronic databases and carefully reviewing the reference lists of pertinent studies until March 2016. The summary hazard ratio (HR) and corresponding 95% confidence interval (CI) were calculated to study the association between c-Myc and CRC prognosis.

Results: Eight cohort studies (including seven studies about overall survival [OS] and one study about disease free survival [DFS]) were included. The pooled HR of OS was 1.13 (95% CI: 0.66–1.95). In subgroup analysis, no significant association between c-Myc and CRC prognosis was found in the studies either from Western countries (HR: 0.87, 95% CI: 0.68–1.10) or Asian countries (HR: 1.89, 95% CI: 0.62–5.77). HRs were 0.86 (95% CI: 0.38–1.94) and 1.57 (95% CI: 0.73–3.39) for the studies using univariate analysis and multivariate analysis, respectively. HR from the studies that examined DNA level was significantly different (HR: 2.05, 95% CI: 1.22–3.46); while that about RNA level or protein level was not significantly different.

Conclusion: c-Myc was not associated with CRC prognosis in this meta-analysis. However, the conclusion is preliminary and should be examined in future studies.

Regulation of c-MYC transcriptional activity by transforming growth factor-beta 1-stimulated clone 22

c‐MYC stimulates cell proliferation through the suppression of cyclin‐dependent kinase (CDK) inhibitors including P15 (CDKN2B) and P21 (CDKN1A). It also activates E‐box‐mediated transcription of various target genes including telomerase reverse transcriptase (TERT) that is involved in cellular immortality and tumorigenesis. Transforming growth factor‐beta 1 (TGF‐β1)‐stimulated clone 22 (TSC‐22/TSC22D1) encodes a highly conserved leucine zipper protein that is induced by various stimuli, including TGF‐β. TSC‐22 inhibits cell growth in mammalian cells and in Xenopus embryos. However, underlying mechanisms of growth inhibition by TSC‐22 remain unclear. Here, we show that TSC‐22 physically interacts with c‐MYC to inhibit the recruitment of c‐MYC on the P15 (CDKN2B) and P21 (CDKN1A) promoters, effectively inhibiting c‐MYC‐mediated suppression of P15 (CDKN2B) and also P21 (CDKN1A) promoter activities. In contrast, TSC‐22 enhances c‐MYC‐mediated activation of the TERT promoter. Additionally, the expression of TSC‐22 in embryonic stem cells inhibits cell growth without affecting its pluripotency‐related gene expression. These results indicate that TSC‐22 differentially regulates c‐MYC‐mediated transcriptional activity to regulate cell proliferation.

The Dual Roles of MYC in Genomic Instability and Cancer Chemoresistance

Cancer is associated with genomic instability and aging. Genomic instability stimulates tumorigenesis, whereas deregulation of oncogenes accelerates DNA replication and increases genomic instability. It is therefore reasonable to assume a positive feedback loop between genomic instability and oncogenic stress. Consistent with this premise, overexpression of the MYC transcription factor increases the phosphorylation of serine 139 in histone H2AX (member X of the core histone H2A family), which forms so-called γH2AX, the most widely recognized surrogate biomarker of double-stranded DNA breaks (DSBs). Paradoxically, oncogenic MYC can also promote the resistance of cancer cells to chemotherapeutic DNA-damaging agents such as cisplatin, clearly implying an antagonistic role of MYC in genomic instability. In this review, we summarize the underlying mechanisms of the conflicting functions of MYC in genomic instability and discuss when and how the oncoprotein exerts the contradictory roles in induction of DSBs and protection of cancer-cell genomes.

Biophysical characterization of the b-HLH-LZ of ΔMax, an alternatively spliced isoform of Max found in tumor cells: Towards the validation of a tumor suppressor role for the Max homodimers

It is classically recognized that the physiological and oncogenic functions of Myc proteins depend on specific DNA binding enabled by the dimerization of its C-terminal basic-region-Helix-Loop-Helix-Leucine Zipper (b-HLH-LZ) domain with that of Max. However, a new paradigm is emerging, where the binding of the c-Myc/Max heterodimer to non-specific sequences in enhancers and promoters drives the transcription of genes involved in diverse oncogenic programs. Importantly, Max can form a stable homodimer even in the presence of c-Myc and bind DNA (specific and non-specific) with comparable affinity to the c-Myc/Max heterodimer. Intriguingly, alterations in the Max gene by germline and somatic mutations or changes in the gene product by alternative splicing (e.g. ΔMax) were recently associated with pheochromocytoma and glioblastoma, respectively. This has led to the proposition that Max is, by itself, a tumor suppressor. However, the actual mechanism through which it exerts such an activity remains to be elucidated. Here, we show that contrary to the WT motif, the b-HLH-LZ of ΔMax does not homodimerize in the absence of DNA. In addition, although ΔMax can still bind the E-box sequence as a homodimer, it cannot bind non-specific DNA in that form, while it can heterodimerize with c-Myc and bind E-box and non-specific DNA as a heterodimer with high affinity. Taken together, our results suggest that the WT Max homodimer is important for attenuating the binding of c-Myc to specific and non-specific DNA, whereas ΔMax is unable to do so. Conversely, the splicing of Max into ΔMax could provoke an increase in overall chromatin bound c-Myc. According to the new emerging paradigm, the splicing event and the stark reduction in homodimer stability and DNA binding should promote tumorigenesis impairing the tumor suppressor activity of the WT homodimer of Max.

Therapeutic aspects of c-MYC signaling in inflammatory and cancerous colonic diseases

Colonic inflammation is required to heal infections, wounds, and maintain tissue homeostasis. As the seventh hallmark of cancer, however, it may affect all phases of tumor development, including tumor initiation, promotion, invasion and metastatic dissemination, and also evasion immune surveillance. Inflammation acts as a cellular stressor and may trigger DNA damage or genetic instability, and, further, chronic inflammation can provoke genetic mutations and epigenetic mechanisms that promote malignant cell transformation. Both sporadical and colitis-associated colorectal carcinogenesis are multi-step, complex processes arising from the uncontrolled proliferation and spreading of malignantly transformed cell clones with the obvious ability to evade the host’s protective immunity. In cells upon DNA damage several proto-oncogenes, including c-MYC are activated in parelell with the inactivation of tumor suppressor genes. The target genes of the c-MYC protein participate in different cellular functions, including cell cycle, survival, protein synthesis, cell adhesion, and micro-RNA expression. The transcriptional program regulated by c-MYC is context dependent, therefore the final cellular response to elevated c-MYC levels may range from increased proliferation to augmented apoptosis. Considering physiological intestinal homeostasis, c-MYC displays a fundamental role in the regulation of cell proliferation and crypt cell number. However, c-MYC gene is frequently deregulated in inflammation, and overexpressed in both sporadic and colitis-associated colon adenocarcinomas. Recent results demonstrated that endogenous c-MYC is essential for efficient induction of p53-dependent apoptosis following DNA damage, but c-MYC function is also involved in and regulated by autophagy-related mechanisms, while its expression is affected by DNA-methylation, or histone acetylation. Molecules directly targeting c-MYC, or agents acting on other genes involved in the c-MYC pathway could be selected for combined regiments. However, due to its context-dependent cellular function, it is clinically essential to consider which cytotoxic drugs are used in combination with c-MYC targeted agents in various tissues. Increasing our knowledge about MYC-dependent pathways might provide direction to novel anti-inflammatory and colorectal cancer therapies.

N-linked glycan profiling in neuroblastoma cell lines

Although MYCN amplification has been associated with aggressive neuroblastoma, the molecular mechanisms that differentiate low-risk, MYCN-nonamplified neuroblastoma from high-risk, MYCN-amplified disease are largely unknown. Genomic and proteomic studies have been limited in discerning differences in signaling pathways that account for this heterogeneity. N-Linked glycosylation is a common protein modification resulting from the attachment of sugars to protein residues and is important in cell signaling and immune response. Aberrant N-linked glycosylation has been routinely linked to various cancers. In particular, glycomic markers have often proven to be useful in distinguishing cancers from precancerous conditions. Here, we perform a systematic comparison of N-linked glycomic variation between MYCN-nonamplified SY5Y and MYCN-amplified NLF cell lines with the aim of identifying changes in sugar abundance linked to high-risk neuroblastoma. Through a combination of liquid chromatography-mass spectrometry and bioinformatics analysis, we identified 16 glycans that show a statistically significant change in abundance between NLF and SY5Y samples. Closer examination revealed the preference for larger (in terms of total monosaccharide count) and more sialylated glycan structures in the MYCN-amplified samples in comparison to smaller, nonsialylated glycans that are more dominant in the MYCN-nonamplified samples. These results offer clues for deriving marker candidates for accurate neuroblastoma risk diagnosis.

A PCNA-derived cell permeable peptide selectively inhibits neuroblastoma cell growth

Proliferating cell nuclear antigen (PCNA), through its interaction with various proteins involved in DNA synthesis, cell cycle regulation, and DNA repair, plays a central role in maintaining genome stability. We previously reported a novel cancer associated PCNA isoform (dubbed caPCNA), which was significantly expressed in a broad range of cancer cells and tumor tissues, but not in non-malignant cells. We found that the caPCNA-specific antigenic site lies between L126 and Y133, a region within the interconnector domain of PCNA that is known to be a major binding site for many of PCNA's interacting proteins. We hypothesized that therapeutic agents targeting protein-protein interactions mediated through this region may confer differential toxicity to normal and malignant cells. To test this hypothesis, we designed a cell permeable peptide containing the PCNA L126-Y133 sequence. Here, we report that this peptide selectively kills human neuroblastoma cells, especially those with MYCN gene amplification, with much less toxicity to non-malignant human cells. Mechanistically, the peptide is able to block PCNA interactions in cancer cells. It interferes with DNA synthesis and homologous recombination-mediated double-stranded DNA break repair, resulting in S-phase arrest, accumulation of DNA damage, and enhanced sensitivity to cisplatin. These results demonstrate conceptually the utility of this peptide for treating neuroblastomas, particularly, the unfavorable MYCN-amplified tumors.

Genome recognition by MYC

MYC dimerizes with MAX to bind DNA, with a preference for the E-box consensus CACGTG and several variant motifs. In cells, MYC binds DNA preferentially within transcriptionally active promoter regions. Although several thousand promoters are bound under physiological (low MYC) conditions, these represent only a fraction of all accessible, active promoters. MYC overexpression-as commonly observed in cancer cells-leads to invasion of virtually all active promoters, as well as of distal enhancer elements. We summarize here what is currently known about the mechanisms that may guide this process. We propose that binding site recognition is determined by low-affinity protein-protein interactions between MYC/MAX dimers and components of the basal transcriptional machinery, other chromatin-associated protein complexes, and/or DNA-bound transcription factors. DNA binding occurs subsequently, without an obligate requirement for sequence recognition. Local DNA scanning then leads to preferential stabilization of the MYC/MAX dimer on high-affinity DNA elements. This model is consistent with the invasion of all active promoters that occurs at elevated MYC levels, but posits that important differences in affinity persist between physiological target sites and the newly invaded elements, which may not all be bound in a productive regulatory mode. The implications of this model for transcriptional control by MYC in normal and cancer cells are discussed in the light of the latest literature.

The action mechanism of the Myc inhibitor termed Omomyc may give clues on how to target Myc for cancer therapy

Recent evidence points to Myc--a multifaceted bHLHZip transcription factor deregulated in the majority of human cancers--as a priority target for therapy. How to target Myc is less clear, given its involvement in a variety of key functions in healthy cells. Here we report on the action mechanism of the Myc interfering molecule termed Omomyc, which demonstrated astounding therapeutic efficacy in transgenic mouse cancer models in vivo. Omomyc action is different from the one that can be obtained by gene knockout or RNA interference, approaches designed to block all functions of a gene product. This molecule--instead--appears to cause an edge-specific perturbation that destroys some protein interactions of the Myc node and keeps others intact, with the result of reshaping the Myc transcriptome. Omomyc selectively targets Myc protein interactions: it binds c- and N-Myc, Max and Miz-1, but does not bind Mad or select HLH proteins. Specifically, it prevents Myc binding to promoter E-boxes and transactivation of target genes while retaining Miz-1 dependent binding to promoters and transrepression. This is accompanied by broad epigenetic changes such as decreased acetylation and increased methylation at H3 lysine 9. In the presence of Omomyc, the Myc interactome is channeled to repression and its activity appears to switch from a pro-oncogenic to a tumor suppressive one. Given the extraordinary therapeutic impact of Omomyc in animal models, these data suggest that successfully targeting Myc for cancer therapy might require a similar twofold action, in order to prevent Myc/Max binding to E-boxes and, at the same time, keep repressing genes that would be repressed by Myc.

Co-localization of the oncogenic transcription factor MYCN and the DNA methyl binding protein MeCP2 at genomic sites in neuroblastoma

Background

MYCN is a transcription factor that is expressed during the development of the neural crest and its dysregulation plays a major role in the pathogenesis of pediatric cancers such as neuroblastoma, medulloblastoma and rhabdomyosarcoma. MeCP2 is a CpG methyl binding protein which has been associated with a number of cancers and developmental disorders, particularly Rett syndrome.

Methods and Findings

Using an integrative global genomics approach involving chromatin immunoprecipitation applied to microarrays, we have determined that MYCN and MeCP2 co-localize to gene promoter regions, as well as inter/intragenic sites, within the neuroblastoma genome (MYCN amplified Kelly cells) at high frequency (70.2% of MYCN sites were also positive for MeCP2). Intriguingly, the frequency of co-localization was significantly less at promoter regions exhibiting substantial hypermethylation (8.7%), as determined by methylated DNA immunoprecipitation (MeDIP) applied to the same microarrays. Co-immunoprecipitation of MYCN using an anti-MeCP2 antibody indicated that a MYCN/MeCP2 interaction occurs at protein level. mRNA expression profiling revealed that the median expression of genes with promoters bound by MYCN was significantly higher than for genes bound by MeCP2, and that genes bound by both proteins had intermediate expression. Pathway analysis was carried out for genes bound by MYCN, MeCP2 or MYCN/MeCP2, revealing higher order functions.

Conclusions

Our results indicate that MYCN and MeCP2 protein interact and co-localize to similar genomic sites at very high frequency, and that the patterns of binding of these proteins can be associated with significant differences in transcriptional activity. Although it is not yet known if this interaction contributes to neuroblastoma disease pathogenesis, it is intriguing that the interaction occurs at the promoter regions of several genes important for the development of neuroblastoma, including ALK, AURKA and BDNF.

Uncovering the transcriptional circuitry in skeletal muscle regeneration

Skeletal muscle has a remarkable ability to regenerate after repeated and complete destruction of the tissue. The healing phases for an injured muscle undergo an activation program controlled by a dynamically inducible transcriptional regulatory network. Mapping a complex mammalian transcriptional network is confronted by significant challenges and requires the integration of multiple experimental data types. In this work we present a system approach to describe the transcriptional circuitry during skeletal muscle regeneration using time-course expression data and motif scanning information. Time-lagged correlation analysis was utilized to evaluate the transcription factor (TF) → target associations. Our analysis identified six TFs that potentially play a central role throughout the regeneration process. Four of them have previously been described to be important for muscle regeneration and differentiation. The remaining two TFs are identified as novel regulators that may have a role in the regeneration process. We hope that our work may provide useful clues to help accelerate the recovery process in injured skeletal muscle.

A curated database of miRNA mediated feed-forward loops involving MYC as master regulator

BACKGROUND

The MYC transcription factors are known to be involved in the biology of many human cancer types. But little is known about the Myc/microRNAs cooperation in the regulation of genes at the transcriptional and post-transcriptional level.

METHODOLOGY/PRINCIPAL FINDINGS

Employing independent databases with experimentally validated data, we identified several mixed microRNA/Transcription Factor Feed-Forward Loops regulated by Myc and characterized completely by experimentally supported regulatory interactions, in human. We then studied the statistical and functional properties of these circuits and discussed in more detail a few interesting examples involving E2F1, PTEN, RB1 and VEGF.

CONCLUSIONS/SIGNIFICANCE

We have assembled and characterized a catalogue of human mixed Transcription Factor/microRNA Feed-Forward Loops, having Myc as master regulator and completely defined by experimentally verified regulatory interactions.

Silencing of the MYCN gene by siRNA delivered by folate receptor-targeted liposomes in LA-N-5 cells

INTRODUCTION

MYCN amplification is highly associated with malignancy and correlates with poor prognosis in patients with neuroblastoma.

MATERIALS AND METHODS

We developed a novel liposome-MYCN siRNA-folic acid complex, and the transfection efficacy was measured in LA-N-5 cells by cy-3 fluorescence density in each microgram of protein from the transfected cell lysate. MYCN expression and cell growth were studied with quantitative RT-PCR and MTT assays, and the expression of MYCN protein was studied with Western blot, respectively. An SCID mouse model with subcutaneous LA-N-5 xenografted tumor was established. The animals were divided into four groups (n = 5) and they were peritoneally injected with liposome-encapsulated MYCN siRNA (siRNA 125 μg/kg/day), lipid-encapsulated control siRNA, MYCN siRNA, or liposome only, respectively, for 5 consecutive days. The animals were killed 24 h after the last injection, and the expression of MYCN mRNA in tumor tissue was detected by RT-PCR.

RESULTS

Our results are as follows: the transfect efficacy reached 1808.5 ± 140.2 pg siRNA/μg protein in LA-N-5 lysates after treatment with 100 nmol/L MYCN siRNA encapsulated with lipid, and fluorescence could be visualized in 92% of LA-N-5 cells after transfection. At 72 h post-transfection, MYCN mRNA expression in LA-N-5 cells was downregulated by 79.2%, MYCN protein was downregulated by 71.3% and cell growth was inhibited by 66.2%, as measured by MTT assay. In the in vivo study, MYCN mRNA expression was knocked down 53.1% in tumor tissues with injection of liposome-encapsulated MYCN siRNA as compared to control siRNA.

CONCLUSION

These results suggest that targeted delivery of MYCN siRNA by folate receptor-targeted lipid vesicles into LA-N-5 cells is efficacious and capable of suppressing MYCN mRNA expression both in vitro and in vivo.

PeakAnalyzer: genome-wide annotation of chromatin binding and modification loci

BACKGROUND

Functional genomic studies involving high-throughput sequencing and tiling array applications, such as ChIP-seq and ChIP-chip, generate large numbers of experimentally-derived signal peaks across the genome under study. In analyzing these loci to determine their potential regulatory functions, areas of signal enrichment must be considered relative to proximal genes and regulatory elements annotated throughout the target genome Regions of chromatin association by transcriptional regulators should be distinguished as individual binding sites in order to enhance downstream analyses, such as the identification of known and novel consensus motifs.

RESULTS

PeakAnalyzer is a set of high-performance utilities for the automated processing of experimentally-derived peak regions and annotation of genomic loci. The programs can accurately subdivide multimodal regions of signal enrichment into distinct subpeaks corresponding to binding sites or chromatin modifications, retrieve genomic sequences encompassing the computed subpeak summits, and identify positional features of interest such as intersection with exon/intron gene components, proximity to up- or downstream transcriptional start sites and cis-regulatory elements. The software can be configured to run either as a pipeline component for high-throughput analyses, or as a cross-platform desktop application with an intuitive user interface.

CONCLUSIONS

PeakAnalyzer comprises a number of utilities essential for ChIP-seq and ChIP-chip data analysis. High-performance implementations are provided for Unix pipeline integration along with a GUI version for interactive use. Source code in C++ and Java is provided, as are native binaries for Linux, Mac OS X and Windows systems.

Global MYCN transcription factor binding analysis in neuroblastoma reveals association with distinct E-box motifs and regions of DNA hypermethylation

Background

Neuroblastoma, a cancer derived from precursor cells of the sympathetic nervous system, is a major cause of childhood cancer related deaths. The single most important prognostic indicator of poor clinical outcome in this disease is genomic amplification of MYCN, a member of a family of oncogenic transcription factors.

Methodology

We applied MYCN chromatin immunoprecipitation to microarrays (ChIP-chip) using MYCN amplified/non-amplified cell lines as well as a conditional knockdown cell line to determine the distribution of MYCN binding sites within all annotated promoter regions.

Conclusion

Assessment of E-box usage within consistently positive MYCN binding sites revealed a predominance for the CATGTG motif (p<0.0016), with significant enrichment of additional motifs CATTTG, CATCTG, CAACTG in the MYCN amplified state. For cell lines over-expressing MYCN, gene ontology analysis revealed enrichment for the binding of MYCN at promoter regions of numerous molecular functional groups including DNA helicases and mRNA transcriptional regulation. In order to evaluate MYCN binding with respect to other genomic features, we determined the methylation status of all annotated CpG islands and promoter sequences using methylated DNA immunoprecipitation (MeDIP). The integration of MYCN ChIP-chip and MeDIP data revealed a highly significant positive correlation between MYCN binding and DNA hypermethylation. This association was also detected in regions of hemizygous loss, indicating that the observed association occurs on the same homologue. In summary, these findings suggest that MYCN binding occurs more commonly at CATGTG as opposed to the classic CACGTG E-box motif, and that disease associated over expression of MYCN leads to aberrant binding to additional weaker affinity E-box motifs in neuroblastoma. The co-localization of MYCN binding and DNA hypermethylation further supports the dual role of MYCN, namely that of a classical transcription factor affecting the activity of individual genes, and that of a mediator of global chromatin structure.

Max-E47, a designed minimalist protein that targets the E-box DNA site in vivo and in vitro

Max-E47 is a designed hybrid protein comprising the Max DNA-binding basic region and E47 HLH dimerization subdomain. In the yeast one-hybrid system (Y1H), Max-E47 shows strong transcriptional activation from the E-box site, 5'-CACGTG, targeted by the Myc/Max/Mad network of transcription factors; two mutants, Max-E47Y and Max-E47YF, activate more weakly from the E-box in the Y1H. Quantitative fluorescence anisotropy titrations to gain free energies of protein:DNA binding gave low nanomolar K(d) values for the native MaxbHLHZ, Max-E47, and the Y and YF mutants binding to the E-box site (14, 15, 9, and 6 nM, respectively), with no detectable binding to a nonspecific control duplex. Because these minimalist, E-box-binding hybrids have no activation domain and no interactions with the c-MycbHLHZ, as shown by the yeast two-hybrid assay, they can potentially serve as dominant-negative inhibitors that suppress activation of E-box-responsive genes targeted by transcription factors including the c-Myc/Max complex. As proof-of-principle, we used our modified Y1H, which allows direct competition between two proteins vying for a DNA target, to show that Max-E47 effectively outcompetes the native MaxbHLHZ for the E-box; weaker competition is observed from the two mutants, consistent with Y1H results. These hybrids provide a minimalist scaffold for further exploration of the relationship between protein structure and DNA-binding function and may have applications as protein therapeutics or biochemical probes capable of targeting the E-box site.

Identification of functional networks of estrogen- and c-Myc-responsive genes and their relationship to response to tamoxifen therapy in breast cancer

Background

Estrogen is a pivotal regulator of cell proliferation in the normal breast and breast cancer. Endocrine therapies targeting the estrogen receptor are effective in breast cancer, but their success is limited by intrinsic and acquired resistance.

Methodology/Principal Findings

With the goal of gaining mechanistic insights into estrogen action and endocrine resistance, we classified estrogen-regulated genes by function, and determined the relationship between functionally-related genesets and the response to tamoxifen in breast cancer patients. Estrogen-responsive genes were identified by transcript profiling of MCF-7 breast cancer cells. Pathway analysis based on functional annotation of these estrogen-regulated genes identified gene signatures with known or predicted roles in cell cycle control, cell growth (i.e. ribosome biogenesis and protein synthesis), cell death/survival signaling and transcriptional regulation. Since inducible expression of c-Myc in antiestrogen-arrested cells can recapitulate many of the effects of estrogen on molecular endpoints related to cell cycle progression, the estrogen-regulated genes that were also targets of c-Myc were identified using cells inducibly expressing c-Myc. Selected genes classified as estrogen and c-Myc targets displayed similar levels of regulation by estrogen and c-Myc and were not estrogen-regulated in the presence of siMyc. Genes regulated by c-Myc accounted for 50% of all acutely estrogen-regulated genes but comprised 85% (110/129 genes) in the cell growth signature. siRNA-mediated inhibition of c-Myc induction impaired estrogen regulation of ribosome biogenesis and protein synthesis, consistent with the prediction that estrogen regulates cell growth principally via c-Myc. The ‘cell cycle’, ‘cell growth’ and ‘cell death’ gene signatures each identified patients with an attenuated response in a cohort of 246 tamoxifen-treated patients. In multivariate analysis the cell death signature was predictive independent of the cell cycle and cell growth signatures.

Conclusions/Significance

These functionally-based gene signatures can stratify patients treated with tamoxifen into groups with differing outcome, and potentially identify distinct mechanisms of tamoxifen resistance.

Natural jasmonates of different structures suppress the growth of human neuroblastoma cell line SH-SY5Y and its mechanisms

AIM

Recent evidence has indicated that members of natural jasmonates, a family of plant stress hormones, exhibit anticancer activity. The current study was undertaken to investigate the effects of jasmonates on the in vitro growth of human neuroblastomas, one of the most common solid tumors in children.

METHODS

Cellular proliferation was measured by 3-(4,5-dimethylthiazol-2-yl)-2,5- diphenyltetrazolium bromide colorimetry and colony formation assay. Apoptosis was detected by Hoechst 33258 staining and flow cytometry. Western blotting was applied to assay gene expression.

RESULTS

The administration of natural jasmonates, methyl jasmonate, cis-jasmone, and jasmonic acid to cultured neuroblastoma cell line SH-SY5Y, resulted in a decrease of cell proliferation in a doseand time-dependent manner. However, the in vitro growth of cultured human embryonic kidney (HEK) cell line HEK 293 was not affected by jasmonates. The cell cycles of jasmonate-treated SH-SY5Y cells were arrested at the G2/M phase. The incubation of SH-SY5Y cells with jasmonates resulted in characteristic changes of apoptosis. The anticancer activities of natural jasmonates on SH-SY5Y cells are as follows: methyl jasmonate>cis-jasmone>jasmonic acid. In addition, the expressions of proliferating cell nuclear antigen and N-myc were downregulated by methyl jasmonate. Moreover, methyl jasmonate decreased the expression of the Xlinked inhibitor of apoptosis protein and survivin, critical members of inhibitors of the apoptosis protein family, in SH-SY5Y cells.

CONCLUSION

Jasmonates suppress the growth of human neuroblastoma cell line SH-SY5Y via inhibiting cell proliferation and inducing apoptosis, which lays the groundwork for further investigation into the anticancer activities and its mechanisms of natural jasmonates on human neuroblastomas.

Translocations targeting CCND2, CCND3, and MYCN do occur in t(11;14)-negative mantle cell lymphomas

The genetics of t(11;14)(q13;q32)/cyclin D1-negative mantle cell lymphoma (MCL) is poorly understood. We report here 8 MCL cases lacking t(11;14) or variant CCND1 rearrangement that showed expression of cyclin D1 (2 cases), D2 (2 cases), and D3 (3 cases). One case was cyclin D negative. Cytogenetics and fluorescence in situ hybridization detected t(2;12)(p11;p13)/IGK-CCND2 in one of the cyclin D2-positive cases and t(6;14)(p21;q32)/IGH-CCND3 in one of the cyclin D3-positive cases. Moreover, we identified a novel cryptic t(2;14)(p24;q32) targeting MYCN in 2 blastoid MCLs: one negative for cyclin D and one expressing cyclin D3. Interestingly, both cases showed expression of cyclin E. Notably, all 3 blastoid MCLs showed a monoallelic deletion of RB1 associated with a lack of expression of RB1 protein and monoallelic loss of p16. In sum-mary, this study confirms frequent aberrant expression of cyclin D2 and D3 in t(11;14)-negative MCLs and shows a t(11;14)-independent expression of cy-clin D1 in 25% of present cases. Novel findings include cyclin E expression in 2 t(11;14)-negative MCLs characterized by a cryptic t(2;14)(p24;q32) and identification of MYCN as a new lymphoma oncogene associated with a blastoid MCL. Clinically important is a predisposition of t(11;14)-negative MCLs to the central nervous system involvement.

Uncovering a macrophage transcriptional program by integrating evidence from motif scanning and expression dynamics

Macrophages are versatile immune cells that can detect a variety of pathogen-associated molecular patterns through their Toll-like receptors (TLRs). In response to microbial challenge, the TLR-stimulated macrophage undergoes an activation program controlled by a dynamically inducible transcriptional regulatory network. Mapping a complex mammalian transcriptional network poses significant challenges and requires the integration of multiple experimental data types. In this work, we inferred a transcriptional network underlying TLR-stimulated murine macrophage activation. Microarray-based expression profiling and transcription factor binding site motif scanning were used to infer a network of associations between transcription factor genes and clusters of co-expressed target genes. The time-lagged correlation was used to analyze temporal expression data in order to identify potential causal influences in the network. A novel statistical test was developed to assess the significance of the time-lagged correlation. Several associations in the resulting inferred network were validated using targeted ChIP-on-chip experiments. The network incorporates known regulators and gives insight into the transcriptional control of macrophage activation. Our analysis identified a novel regulator (TGIF1) that may have a role in macrophage activation.

Macrophages play a vital role in host defense against infection by recognizing pathogens through pattern recognition receptors, such as the Toll-like receptors (TLRs), and mounting an immune response. Stimulation of TLRs initiates a complex transcriptional program in which induced transcription factor genes dynamically regulate downstream genes. Microarray-based transcriptional profiling has proved useful for mapping such transcriptional programs in simpler model organisms; however, mammalian systems present difficulties such as post-translational regulation of transcription factors, combinatorial gene regulation, and a paucity of available gene-knockout expression data. Additional evidence sources, such as DNA sequence-based identification of transcription factor binding sites, are needed. In this work, we computationally inferred a transcriptional network for TLR-stimulated murine macrophages. Our approach combined sequence scanning with time-course expression data in a probabilistic framework. Expression data were analyzed using the time-lagged correlation. A novel, unbiased method was developed to assess the significance of the time-lagged correlation. The inferred network of associations between transcription factor genes and co-expressed gene clusters was validated with targeted ChIP-on-chip experiments, and yielded insights into the macrophage activation program, including a potential novel regulator. Our general approach could be used to analyze other complex mammalian systems for which time-course expression data are available.

Differential effects of Mxi1-SRalpha and Mxi1-SRbeta in Myc antagonism

Mxi1 belongs to the Myc-Max-Mad transcription factor network. Two Mxi1 protein isoforms, Mxi1-SRalpha and Mxi1-SRbeta, have been described as sharing many biological properties. Here, we assign differential functions to these isoforms with respect to two distinct levels of Myc antagonism. Unlike Mxi1-SRbeta, Mxi1-SRalpha is not a potent suppressor of the cellular transformation activity of Myc. Furthermore, although Mxi1-SRbeta exhibits a repressive effect on the MYC promoter in transient expression assays, Mxi1-SRalpha activates this promoter. A specific domain of Mxi1-SRalpha contributes to these differences. Moreover, glyceraldehyde-3-phosphate dehydrogenase interacts with Mxi1-SRalpha and enhances its ability to activate the Myc promoter. Our findings suggest that Mxi1 gains functional complexity by encoding isoforms with shared and distinct activities.

Max is acetylated by p300 at several nuclear localization residues

Max is a ubiquitous transcription factor with a bHLHZip [basic HLH (helix-loop-helix) leucine zipper] DNA-binding/dimerization domain and the central component of the Myc/Max/Mad transcription factor network that controls cell growth, proliferation, differentiation and apoptotic cell death in metazoans. Max is the obligatory DNA-binding and dimerization partner for all the bHLHZip regulators of the Myc/Max/Mad network, including the Myc family of oncoproteins and the Mad family of Myc antagonists, which recognize E-box DNA elements in the regulatory regions of target genes. Max lacks a transcription regulatory domain and is the only member of the network that efficiently homodimerizes. Binding of Max homodimers to E-box elements suppresses the transcription regulatory functions of its network partners and of other non-network E-box-binding regulators. In contrast with its highly regulated partners, Max is a constitutively expressed and phosphorylated protein. Phosphorylation is, however, the only Max post-translational modification identified so far. In the present study, we have analysed Max posttranslational modifications by MS. We have found that Max is acetylated at several lysine residues (Lys-57, Lys-144 and Lys-145) in mammalian cells. Max acetylation is stimulated by inhibitors of histone deacetylases and by overexpression of the p300 co-activator/HAT (histone acetyltransferase). The p300 HAT also directly acetylates Max in vitro at these three residues. Interestingly, the three Max residues acetylated in vivo and in vitro by p300 are important for Max nuclear localization and Max-mediated suppression of Myc transactivation. These results uncover novel post-translational modifications of Max and suggest the potential regulation of specific Max complexes by p300 and reversible acetylation.

c-Myc overexpression and endocrine resistance in breast cancer

The oncoprotein c-Myc is frequently overexpressed in breast cancer and ectopic expression in breast cancer cell lines attenuates responses to antiestrogen treatment. Here, we review preliminary data aimed at further elucidating a potential role for c-Myc in clinical endocrine resistance in breast cancer. Immunohistochemical and semi-quantitative PCR revealed that c-Myc protein and c-myc mRNA were frequently overexpressed in both ER-positive and ER-negative breast carcinoma. Furthermore, both constitutive and inducible c-Myc overexpression in MCF-7 breast cancer cell lines markedly reduced their sensitivity to the growth inhibitory effects of the pure antiestrogen ICI 182,780. In order to identify potential downstream targets of c-Myc that mediate this effect, Affymetrix microarrays were employed to examine the patterns of gene expression shared by MCF-7 cells stimulated by estrogen, or by induction of c-Myc. Approximately 50% of estrogen target genes identified 6h after treatment were also regulated by c-Myc. One novel target, EMU4, was transcriptionally regulated by c-Myc. In addition, there was a strong correlation between c-myc and EMU4 mRNA expression in a battery of breast cancer cell lines. These data confirm that c-Myc overexpression is a common event in breast cancer, and that this is associated with resistance to antiestrogens in vitro. Furthermore, the development of an experimental paradigm for the discovery of c-Myc and estrogen target genes associated with endocrine resistance provides a framework for the discovery and validation of genes involved in estrogen signalling, and c-Myc-mediated-antiestrogen resistance.

Antioxidants prevent oxidative DNA damage and cellular transformation elicited by the over-expression of c-MYC

Reactive oxygen species (ROS)-induced genomic damage may have important consequences in the initiation and progression of cancer. Deregulated expression of the proto-oncogene c-MYC is associated with intracellular oxidative stress and increased DNA damage. However, the protective role of antioxidants such as Vitamin C against MYC-induced genomic damage has not been fully investigated. In a variety of cell lines, we show that ectopic MYC over-expression results in the elevation of intracellular ROS levels and a concomitant increase in oxidative DNA damage, as assessed by levels of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxo-dG) in the genomic DNA. Loading cells with ascorbic acid (AA) relieved MYC-elicited intracellular oxidative stress and conferred genomic protection. A mitochondrially targeted Vitamin E analog, TPPB, also protected cells from MYC-elicited oxidative DNA damage, suggesting the involvement of mitochondria in increased ROS production. We found that deregulated MYC expression resulted in the attenuation of intracellular glutathione levels, which was reversed by loading cells with Vitamin C. Additionally, cells over-expressing MYC had elevated levels of intracellular superoxide, which was significantly quenched by Vitamin C or the selective superoxide quencher, Tiron. Consequently, Vitamin C and other antioxidants protected cells from MYC-induced cellular transformation. Our studies implicate a role for ROS, and superoxide in particular, in MYC-elicited oxidative DNA damage and cellular transformation, and point to a pharmacological role of antioxidants in cancer chemoprevention.

Targeted expression of human MYCN selectively causes pancreatic neuroendocrine tumors in transgenic zebrafish

The zebrafish model organism has been used extensively for studies of genetic pathways in development, indicating its potential applicability to cancer. Here we show that targeted expression of MYCN in cells of the pancreatic islet induces neuroendocrine carcinoma. Four transgenic fish developed abdominal tumors between 4 and 6 months of age, and histologic analysis revealed lobulated arrangements of neoplastic cells with expression of the MYCN transgene. The tumors also expressed insulin mRNA, and pancreatic exocrine cells and ducts were identified within the neoplasms, indicating a pancreatic origin for the tumor. Transmission electron microscopy revealed cytoplasmic, endocrine-dense core granules, analogous to those found in human neuroendocrine tumors. Our studies establish a zebrafish transgenic model of pancreatic neuroendocrine carcinoma, setting the stage to evaluate molecular pathways downstream of MYCN in this vertebrate forward genetic model system.

Autoregulation of the N-myc gene is operative in neuroblastoma and involves histone deacetylase 2

BACKGROUND

Autoregulation of the myc gene family is a negative feedback mechanism known to occur at high levels of Myc expression. Loss of this mechanism and associated Myc overexpression has been observed in human tumors, thereby contributing to tumorigenesis. The childhood tumor neuroblastoma is characterized by N-myc amplification in aggressive and highly proliferative tumors that occur in a subset of patients. The precise molecular mechanism of autoregulation is unknown, and previous observations indicated that N-myc autoregulation was intact only in single-copy neuroblastoma cell lines.

METHODS

Transient reporter assays and trichostatin A (TSA) experiments were performed to evaluate several candidate genes, including Mxi1, c-myc promoter binding protein 1 (MBP-1), Miz, and histone deacetylase 2 (HDAC2), for their involvement in N-myc autoregulation. Mxi1 and HDAC2 were examined further for their expression levels and effects on endogenous N-myc levels. Finally, their recruitments to the N-myc promoter were investigated by chromatin immunoprecipitation (ChIP).

RESULTS

The autoregulatory circuit was operative, even in amplified cell lines. Mxi1 consistently showed a modest effect in down-regulating N-myc in transient reporter assays. Overexpression of the c-myc, Mxi1, and mHDAC2 genes resulted in a threefold to fourfold decrease in endogenous N-myc levels. Mxi1 and HDAC2 were up-regulated by N-Myc in an myc-inducible cell line and in N-myc-expressing cell lines. In addition, down-regulation of the N-myc promoter was relieved in the presence of TSA. Increased association of HDAC2 with the autoregulatory region within the N-myc promoter by ChIP was observed upon down-regulation of endogenous N-myc.

CONCLUSIONS

The autoregulatory circuit was intact in both amplified and single-copy neuroblastoma cell lines. Furthermore, myc gene autoregulation occurred through histone deacetylation.

Identification of genes that are regulated transcriptionally by Myc in childhood tumors

BACKGROUND

Amplification of the N-myc oncogene is associated with adverse outcomes in the common childhood tumor, neuroblastoma. Because the transforming properties of Myc are related to its ability to modulate gene expression, the authors used cDNA microarrays to identify potential Myc target genes.

METHODS

Expression levels of 4608 genes were analyzed in a series of neuroblastoma cell lines. Identical analyses were performed in a panel of medulloblastoma cell lines to identify c-Myc targets and to determine the extent to which N-Myc targets and c-Myc targets were shared. Comparisons were made between cell lines with high levels versus low levels of Myc protein expression.

RESULTS

Array analyses yielded 121 genes with increased expression levels (>or= 1.65-fold) and 9 genes with decreased expression levels in N-Myc-expressing versus nonexpressing cell lines. Many of these were newly identified targets of biologic interest. Fifty percent of the N-Myc targets (60 of 121) were mutual c-Myc targets. A significant correlation between the level of N-myc and selected target gene expression was demonstrated independently in 27 neuroblastoma tumor samples and in an N-myc-inducible cell line system.

CONCLUSIONS

A number of diverse pathways are modulated by N-Myc in neuroblastoma. Although, overall, there was significant correlation between myc and target transcript expression among cohorts of tumors, great variability in levels of target expression was seen among individual tumor samples, and this biologic heterogeneity in the levels of target gene expression may offer insight into differences in the clinical behavior of neuroblastoma and may prove to be of prognostic significance in the future.

The MYCN oncoprotein as a drug development target

The transcription factor and proto-oncogene MYCN is reviewed as a potential specific target for cancer therapy. Amplification of MYCN is frequently found in a number of advanced-stage tumours, including neuroblastoma (25%), small cell lung cancers (7%), alveolar rhabdomyosarcoma and retinoblastoma. It is associated with rapid tumour progression and poor outcome in human neuroblastoma. MYCN is a member of the myc family of proto-oncogenes which encode nuclear proteins that form heterodimers with MAX protein through their conserved HLHZip domains. The MYC/MAX complexes transactivate a number of MYC-target genes in a sequence-specific manner. MYC-MAX interaction is essential for MYC-induced cell cycle progression, cellular transformation, and transcriptional activation. A causal link between the transformed phenotype and MYCN has been established by a range of in vitro and in vivo studies, including a transgenic model of neuroblastoma in which MYCN overexpression is targeted to neuronal tissue by the use of a tyrosine hydroxylase promoter. Downregulation of MYCN expression either by antisense treatment targeted against MYCN mRNA or by retinoids has been shown to decrease proliferation and/or induce neuronal differentiation of neuroblastoma cells. Inhibition of MYC-MAX dimerisation by small-molecule antagonists has recently been shown to interfere with MYC-induced transformation of chick embryo fibroblasts, indicating that functional inhibitors of the MYC family of oncoproteins have potential as therapeutic agents. Finally, we describe the development and validation of a functional MYCN reporter gene assay using neuroblastoma cells (NGP) which have been stably transfected with a luciferase gene construct under control of the ornithine decarboxylase gene promoter. This assay has been used for a pilot screen of 2800 compounds from the Cancer Research-UK collection, identifying five compounds showing a consistent significant reduction of MYCN-dependent luciferase activity (>50%) in repeated screens. This cell-based, MYCN reporter gene assay will be scaled up for high throughput screens of compound libraries and will aid in the future development of specific therapeutic strategies in neuroblastoma and other tumours in which MYCN amplification has been implicated.

p38 Mitogen-activated protein kinase regulation of JB6 Cl41 cell transformation promoted by epidermal growth factor

The relationship between cell transformation and p38 MAP kinase, a major mitogen-activated protein (MAP) kinase pathway converting signals of various extracellular stimuli into expression of specific target genes through activation of transcription factors, still remains unclear. The aim of the present study was to investigate the role of the p38 MAP kinase pathway in epidermal growth factor (EGF)-induced cell transformation in JB6 cells. Our data show that a dominant negative mutant of p38 MAP (DN-p38) kinase inhibits EGF-promoted JB6 Cl41 cell transformation and that SB202190, an inhibitor of p38 MAP kinase, also inhibits JB6 Cl41 cell transformation in a dose-dependent manner. Moreover, our results show that DN-p38 MAP kinase inhibits the phosphorylation of EGF-stimulated activating transcription factor-2 (ATF-2) and signal transducer and activator of transcription 1 (STAT1). Additionally, DN-p38 MAP kinase inhibits EGF-induced phosphorylation of c-Myc (Thr58/Ser62). Gel shift assays indicate that DN-p38 MAP kinase inhibits EGF-induced activator protein-1 (AP-1) DNA binding in a dose-dependent manner. These results show that p38 MAP kinase plays a key role in the regulation of EGF-induced cell transformation in JB6 cells through regulation of phosphorylation of p38 MAP kinase and activation of its target genes in phosphorylation, c-Myc cell transformation-related genes, and AP-1 binding ability.

Transcriptional regulation of the ornithine decarboxylase gene by c-Myc/Max/Mad network and retinoblastoma protein interacting with c-Myc

c-Myc is an oncogenic transcription factor involved in the regulation of cell proliferation, differentiation and apoptosis. The direct targets of c-Myc mediating these various processes are slowly being unravelled. This study indicates that the ornithine decarboxylase (ODC) gene is a physiological transcriptional target of c-Myc in association with induction of cell proliferation and transformation, but not with induction of apoptosis. In addition to the two conserved CACGTG c-Myc-binding sites in the first intron, the CATGTG motif in the 5'-flanking region of the murine odc is also shown to be a functional c-Myc response element. odc is thus a c-Myc target with three binding sites a distance apart. Transient transfection studies with different c-Myc, Max and Mad constructs in COS-7 cells showed that the balance between c-Myc/Max, Max/Max and Max/Mad complexes is crucial for the regulation, resulting in either transactivation or transrepression of an ODC-CAT reporter gene. Transcription of both ODC-CAT and endogenous odc was strongly induced in HeLa cells expressing tetracycline-regulated c-Myc, concomitant with c-Myc promoting the S-phase entry of the cells. Transformation of NIH3T3 cells by c-Ha-ras-(Val12) oncogene was reversed by expression of transcriptionally inactive c-Myc, which was associated with repression of ODC-CAT expression. Further, the c-Myc-induced transactivation of ODC-CAT in COS-7 cells was suppressed by co-expression of the retinoblastoma tumor suppresser pRb, evidently as a result of pRb directly or indirectly interacting with c-Myc. Importantly, the endogenous c-Myc and pRb proteins were also found to associate in Colo 320HSR cells under physiological conditions. These results suggest that c-Myc and pRb can interact in vivo, and may in part control some aspects of cell proliferation and transformation through modulation of odc expression.

Oral cancer: reviewing the present understanding of its molecular mechanism and exploring the future directions for its effective management

The present review aims to analyze the information available regarding the molecular mechanisms of Oral Carcinogenesis and explore the future directions where the field of Cancer Biology is venturing. Oncologists have excellently followed the proverb "Necessity is the mother of Invention". The desire to be more precise and comprehensive in their studies has led to the invention of some of the most innovative techniques like laser capture microdissection, comparative genomic hybridization, microarrays, and protein chips etc. Various Biotech companies and Cancer Institutes are on a hunt for anti-cancer drugs and molecular markers for cancers. These revolutionary approaches and the new breed of Oncologists have made the field very exciting and have generated the hope that finally the war against cancer would be won. In the end it is urged that the lead taken in other cancers like colon, breast, leukemia will be emulated in oral cancer. This is expected to provide a molecular blueprint for HNSCC, thus helping to identify suitable markers for the early detection of pre-neoplastic lesions, as well as novel targets for its pharmacological intervention.

Phosphorylated c-MYC expression in Alzheimer disease, Pick's disease, progressive supranuclear palsy and corticobasal degeneration

Phosphorylated c-Myc (c-Myc-P) expression has been examined by immunohistochemistry, using an antibody that recognizes phosphorylated c-Myc at Thr58 and Ser62, in the brains of Alzheimer disease (AD), Pick's disease (PiD), progressive supranuclear palsy (PSP), corticobasal degeneration (CBD) and age-matched control cases, as well as in human medulloblastomas and central neuroblastomas. Strong c-Myc-P immunoreactivity was seen in dystrophic neurites and neurones with neurofibrillary tangles in AD, and in neurones and glial cells bearing abnormal tau deposits in PiD, PSP and CBD. Previous studies have shown active Ras and increased mitogen-activated protein kinase (MAPK/ERK) expression in neurones and glial cells with abnormal tau deposition in AD and other tauopathies. Since MAPKs phosphorylate c-Myc at Thr58 and Ser62, these observations implicate the Ras/MAP kinase pathway in c-Myc phosphorylation and accumulation in AD and other tauopathies. Previous studies have also shown activation of cell cycle associated proteins in neuronal death. The present results have shown colocalization of nuclear c-Myc-P and active, cleaved caspase-3, a major executioner of apoptosis, in medulloblastomas and central neuroblastomas, thus suggesting phosphorylated c-Myc expression in caspase-3-dependent apoptosis of tumour cells. However, no evidence of caspase-3 activation has been observed in neurones and glial cells with strong phosphorylated c-Myc immunoreactivity in AD, PiD, PSP and CBD. Therefore, it is not clear that the activation of the Ras/MAPK/c-Myc subprogramme leads to neuronal death in AD and other tauopathies.

Pathways of apoptosis and the modulation of cell death in cancer

Although cell death once was viewed exclusively as the disordered, chaotic outcome of metabolic catastrophe, apoptosis now is recognized as a highly ordered, evolutionarily conserved, and genetically selected program that is essential for normal development. The death receptor pathway of apoptosis, cytotoxic T cells, prolife survival signals, Bcl-2 family of regulators, p53 and regulated cell death in cancer, and oncogenes are reviewed. Future prospects in this arena also are discussed.

Analysis of C-MYC function in normal cells via conditional gene-targeted mutation

Germline inactivation of c-myc in mice causes embryonic lethality. Therefore, we developed a LoxP/Cre-based conditional mutation approach to test the role of c-myc in mouse embryonic fibroblasts (MEFs) and mature B lymphocytes. Cre expression resulted in reduced proliferation of wild-type MEFs, but c-Myc-deficient MEFs showed a further reduction. In contrast to fibroblasts, Cre expression had no apparent affect on wild-type B cell proliferation. Deletion of both c-Myc genes in B cells led to severely impaired proliferation in response to anti-CD40 plus IL-4. However, treated cells did upregulate several early activation markers but not CD95 or CD95 ligand. We discuss these findings with respect to potential c-Myc functions in proliferation and apoptosis and also discuss potential limitations in the Cre-mediated gene inactivation approach.

MondoA, a novel basic helix-loop-helix-leucine zipper transcriptional activator that constitutes a positive branch of a max-like network

Max is a common dimerization partner for a family of transcription factors (Myc, Mad [or Mxi]), and Mnt [or Rox] proteins) that regulate cell growth, proliferation, and apoptosis. We recently characterized a novel Max-like protein, Mlx, which interacts with Mad1 and Mad4. Here we describe the cloning and functional characterization of a new family of basic helix-loop-helix-leucine zipper heterodimeric partners for Mlx termed the Mondo family. MondoA forms homodimers weakly and does not interact with Max or members of the Myc or Mad families. MondoA and Mlx associate in vivo, and surprisingly, they are localized primarily to the cytoplasm of cultured mammalian cells. Treatment of cells with the nuclear export inhibitor leptomycin B results in the nuclear accumulation of MondoA and Mlx, demonstrating that they shuttle between the cytoplasmic and nuclear compartments rather than having exclusively cytoplasmic localization. MondoA preferentially forms heterodimers with Mlx, and this heterocomplex can bind to, and activate transcription from, CACGTG E-boxes when targeted to the nucleus via a heterologous nuclear localization signal. The amino termini of the Mondo proteins are highly conserved among family members and contain separable and autonomous cytoplasmic localization and transcription activation domains. Therefore, Mlx can mediate transcriptional repression in conjunction with the Mad family and can mediate transcriptional activation via the Mondo family. We propose that Mlx, like Max, functions as the center of a transcription factor network.

N-myc can functionally replace c-myc in murine development, cellular growth, and differentiation

Members of the myc family of cellular oncogenes have been implicated as transcriptional regulators in pathways that govern cellular proliferation and death. In addition, N-myc andc-myc are essential for completion of murine embryonic development. However, the basis for the evolutionary conservation ofmyc gene family has remained unclear. To elucidate this issue, we have generated mice in which the endogenous c-myccoding sequences have been replaced with N-myc coding sequences. Strikingly, mice homozygous for this replacement mutation can survive into adulthood and reproduce. Moreover, when expressed from the c-myc locus, N-myc is similarly regulated and functionally complementary to c-myc in the context of various cellular growth and differentiation processes. Therefore, themyc gene family must have evolved, to a large extent, to facilitate differential patterns of expression.

Mad1 function is regulated through elements within the carboxy terminus

Myc and Mad are basic helix-loop-helix leucine zipper (bHLH-LZ) proteins that heterodimerize with Max to bind DNA and thereby influence the transcription of Myc-responsive genes. Myc-Max dimers transactivate whereas Mad-Max-mSin3 complexes repress Myc-mediated transcriptional activation. We have previously shown that the N-terminal mSin3 binding domain and the centrally located bHLH-LZ are required for Mad1 to function during a molecular switch from proliferation to differentiation. Here we demonstrate that the carboxy terminus (CT) of Mad1 contains previously unidentified motifs necessary for the regulation of Mad1 function. We show that removal of the last 18 amino acids of Mad1 (region V) abolishes the growth-inhibitory function of the protein and the ability to reverse a Myc-imposed differentiation block. Moreover, deletion of region V results in a protein that binds DNA weakly and no longer represses Myc-dependent transcriptional activation. In contrast, deletion of the preceding 24 amino acids (region IV) together with region V restores DNA binding and transcriptional repression, suggesting a functional interplay between these two regions. Furthermore, phosphorylation within region IV appears to mediate this interplay. These findings indicate that novel regulatory elements are present in the Mad1 CT.

N-Myc shares cellular functions wiht c-Myc

N-Myc is a member of the myc family of proto-oncogenes involved in initiation and progression of tumors. While c-MYC, the most characterized member of the family, is well known for its role in cellular proliferation and apoptosis, the function of N-MYC in differentiation and proliferation remains unclear. N-Myc mutant mice present a phenotype more consistent with a role of N-MYC protein in proliferation of precursor populations than in differentiation per se. Recent studies have also shown that N-MYC can enhance apoptosis and shorten the G1 phase of the cell cycle. However, the role of N-MYC in instigating cell-cycle progression has not been clearly demonstrated. Here, we demonstrate that overexpression of N-myc or activation of inducible N-MYC proteins is sufficient to induce apoptosis in serum-starved fibroblast cells, an effect that can be counteracted by overexpression of Bcl-2. Moreover, N-MYC can induce the reentry of quiescent cells into the cell cycle even in the absence of external stimuli. These results indicate that N-MYC and c-MYC share many properties, supporting the model that MYC-specific roles during embryonic development are mediated, at least in part, via their specific profile of expression rather than by their different protein functions.

Gene-target recognition among members of the myc superfamily and implications for oncogenesis

Myc and Mad family proteins regulate multiple biological processes through their capacity to influence gene expression directly. Here we show that the basic regions of Myc and Mad proteins are not functionally equivalent in oncogenesis, have separable E-box-binding activities and engage both common and distinct gene targets. Our data support the view that the opposing biological actions of Myc and Mxi1 extend beyond reciprocal regulation of common gene targets. Identification of differentially regulated gene targets provides a framework for understanding the mechanism through which the Myc superfamily governs the growth, proliferation and survival of normal and neoplastic cells.

Mlx, a novel Max-like BHLHZip protein that interacts with the Max network of transcription factors

Mad:Max heterodimers oppose the growth-promoting action of Myc:Max heterodimers by recruiting the mSin3-histone deacetylase (mSin3. HDAC) complex to DNA and functioning as potent transcriptional repressors. There are four known members of the Mad family that are indistinguishable in their abilities to interact with Max, bind DNA, repress transcription, and block Myc + Ras co-transformation. To investigate functional differences between Mad family proteins, we have identified additional proteins that interact with this family. Here we present the identification and characterization of the novel basic-helix-loop-helix zipper protein Mlx (Max-like protein x), which is structurally and functionally related to Max. The similarities between Mlx and Max include 1) broad expression in many tissues, 2) long protein half-life, and 3) formation of heterodimers with Mad family proteins that are capable of specific CACGTG binding. We show that transcriptional repression by Mad1:Mlx heterodimers is dependent on dimerization, DNA binding, and recruitment of the mSin3A.HDAC corepressor complex. In contrast with Max, Mlx interacts only with Mad1 and Mad4. Together, these findings suggest that Mlx may act to diversify Mad family function by its restricted association with a subset of the Mad family of transcriptional repressors.

The MYC dualism in growth and death

Over-expression of the transcription factor c-Myc immortalizes primary cells and transforms in co-operation with activated ras. Therefore, c-myc is considered a proto-oncogene. Since its discovery c-Myc has been shown to render cells growth factor independent, accelerates passage through G1 of the cell cycle, inhibits differentiation and elicits apoptosis. Whereas the effects on immortalization, proliferation and inhibition of differentiation are in conceivable accordance with gain of function, as it is defined for a proto-oncogene, its pro-apoptotic activity disables a straight forward explanation of the physiological role of c-Myc and suggests a highly complex contribution during development. The recent accomplishments in c-Myc research shed some light on the difficile regulatory network which keeps check on c-Myc activity such as by binding to proteins some of which are transcription factors for non-c-Myc targets. Moreover, it was shown that genes are targeted by c-Myc depending on the sequence of flanking regions adjacent to the E-box or in dependence on the availability of binding partners which is most probably specific to the cellular context. Cdc25A and ornithine decarboxylase, both described to be c-Myc targets, have been brought forward as downstream effectors in the induction of proliferation under serum rich conditions, or in the induction of apoptosis when serum factors are limited. These genes seem to be regulated by c-Myc in a cell type-specific manner. H-ferritin, IRP2 and telomerase are the most recently discovered direct targets of c-Myc. The regulation of H-ferritin and IRP2 might explain the potential of c-Myc to promote proliferation and the regulation of telomerase could be responsible for the immortalizing properties of c-Myc. In the future, H-ferritin and telomerase have to be analyzed whether or not these genes are also Myc targets in other cell systems. Although the intense research efforts regarding the function of c-Myc last already two decades the role of this gene is still enigmatic.

Tissue-selective expression of dominant-negative proteins for the regulation of vascular smooth muscle cell proliferation

The transcription factors c-myb and c-myc are essential for vascular smooth muscle cell (VSMC) replication and are rapidly induced following mitogenic stimulation of quiescent VSMCs in vitro and in vivo following balloon catheter injury. Consequently, interference with c-myb and c-myc function provides a possible avenue for the prevention of VSMC proliferation associated with intimal hyperplasia. We have carried out studies focused on the inhibition of VSMC proliferation using dominant-negative gene constructs incorporating the DNA-binding domains of the c-myb or c-myc genes fused to the repressor domain of the Drosophila engrailed gene. Transient transfection of rat, rabbit and human vascular SMCs results in a dramatic inhibition of proliferation for at least 72 h after transfection. Furthermore, this inhibition of cellular proliferation was found to be due, at least in part, to the induction of apoptosis. Coupling expression of the chimeric dominant-negative proteins to transcriptional regulatory elements of the human vascular smooth muscle alpha-actin gene allows specific targeting of vascular smooth muscle cells.

p53 deficiency rescues the adverse effects of telomere loss and cooperates with telomere dysfunction to accelerate carcinogenesis

Maintenance of telomere length and function is critical for the efficient proliferation of eukaryotic cells. Here, we examine the interactions between telomere dysfunction and p53 in cells and organs of telomerase-deficient mice. Coincident with severe telomere shortening and associated genomic instability, p53 is activated, leading to growth arrest and/or apoptosis. Deletion of p53 significantly attenuated the adverse cellular and organismal effects of telomere dysfunction, but only during the earliest stages of genetic crisis. Correspondingly, the loss of telomere function and p53 deficiency cooperated to initiate the transformation process. Together, these studies establish a key role for p53 in the cellular response to telomere dysfunction in both normal and neoplastic cells, question the significance of crisis as a tumor suppressor mechanism, and identify a biologically relevant stage of advanced crisis, termed genetic catastrophe.