c-Myc, genome instability, and tumorigenesis: the devil is in the details

Triplex-forming oligonucleotides as an anti-gene technique for cancer therapy

Triplex-forming oligonucleotides (TFOs) can bind to the major groove of double-stranded DNA with high specificity and affinity and inhibit gene expression. Triplex-forming oligonucleotides have gained prominence because of their potential applications in antigene therapy. In particular, the target specificity of triplex-forming oligonucleotides combined with their ability to suppress oncogene expression has driven their development as anti-cancer agents. So far, triplex-forming oligonucleotides have not been used for clinical treatment and seem to be gradually snubbed in recent years. But triplex-forming oligonucleotides still represent an approach to down-regulate the expression of the target gene and a carrier of active substances. Therefore, in the present review, we will introduce the characteristics of triplex-forming oligonucleotides and their anti-cancer research progress. Then, we will discuss the challenges in their application.

Comparative analysis of compound NSC13728 as Omomyc homodimer stabilizer by molecular dynamics simulation and MM/GBSA free energy calculation

Myc is a master transcriptional regulator that controls almost all cellular processes, whose function is dependent on dimerization with its obligate partner Max. Stabilization of Max homodimer by small molecules (such as compound NSC13728) has proven an effective way to reduce the availability of Myc-Max dimer. Omomyc, a peptide inhibitor of Myc, is able to form Omomyc homodimer, which can competitively inhibit the binding of Myc-Max to the E-box of DNA. Considering the high amino acid sequence homology between Omomyc and Max, we put forward the hypothesis that Max-Max stabilizers could stabilize the Omomyc homodimer. Hence, through molecular dynamics (MD) simulation and molecular mechanics/generalized Born surface area (MM/GBSA) free energy calculation, we discovered that the stability of Omomyc-Omomyc is remarkably higher than that of Max-Max. Moreover, after adding the compound NSC13728 into the well-defined "Site 3," the binding affinity between two Omomyc monomers can be further increased. Compound NSC13728 has stronger binding interaction to Omomyc-Omomyc than to Max-Max. "Site 3" of Omomyc is more hydrophobic than that of Max, which enlightens us that the more potent Omomyc-Omomyc stabilizers may be hydrophobic in structure.

PSME4 Activates mTOR Signaling and Promotes the Malignant Progression of Hepatocellular Carcinoma

Background

As hepatocellular carcinoma (HCC) having the second-highest mortality rate globally, the early diagnosis and prognosis of HCC have always been the focus of various studies. Although PSME4 has been reported to be closely related to several malignancies, its role in HCC remains unclear.

Materials and Methods

The TCGA-LIHC database and HCC tissues were used to explore the expression of PSME4 in HCC. Gene set enrichment analysis (GSEA) was used to forecast the biological behavior of HCC cells that PSME4 might be involved in regulation. In addition, CCK-8, colony formation and flow cytometry assays were used to explore the effect of PSME4 on HCC cells. Furthermore, the underlying PSME4-related signaling pathways in HCC were further confirmed using GSEA.

Results

We found that the expression of PSME4 in HCC tissues was significantly higher than that in adjacent normal tissues, and patients with high PSME4 expression have a poor prognosis. CCK-8, colony formation and flow cytometry assays shown that knockdown of PSME4 inhibits HCC cell proliferation of HCC cells, promotes cell apoptosis and moves the cell cycle away from the S phase. Mechanistically, PSME4 may promote the development of HCC through mTOR signaling pathway.

Conclusion

The high expression of PSME4 in HCC promotes the proliferation of HCC cells via the mTOR signalling pathway. Therefore, PSME4 is an emerging tumour marker for the early diagnosis and prognosis of HCC.

Targeted Protein Degradation: An Important Tool for Drug Discovery for "Undruggable" Tumor Transcription Factors

Conventional small-molecule drugs (SMDs) are compounds characterized by low molecular weight, high cell permeability, and high selectivity. In clinical translation, SMDs are regarded as good candidates for oral drug formulation. SMD inhibitors play an important role in cancer treatment; however, resistance and low effectiveness have been major bottlenecks in clinical application. Generally, only 20% of cell proteins can potentially be targeted and have been developed as SMDs; thus, some types of tumor targets are considered “undruggable.” Among these are transcription factors (TFs), an important class of proteins that regulate the occurrence, formation, and development of tumors. It is difficult for SMDs and macromolecular drugs to identify bioactive sites in TFs and hence for use as pharmacological inhibitors in targeting TF proteins. For this reason, technologies that enable targeted protein degradation, such as proteolysis-targeting chimera or molecular glues, could serve as a potential tool to solve these conundrums.

Mitochondria and Their Relationship with Common Genetic Abnormalities in Hematologic Malignancies

Hematologic malignancies are known to be associated with numerous cytogenetic and molecular genetic changes. In addition to morphology, immunophenotype, cytochemistry and clinical characteristics, these genetic alterations are typically required to diagnose myeloid, lymphoid, and plasma cell neoplasms. According to the current World Health Organization (WHO) Classification of Tumors of Hematopoietic and Lymphoid Tissues, numerous genetic changes are highlighted, often defining a distinct subtype of a disease, or providing prognostic information. This review highlights how these molecular changes can alter mitochondrial bioenergetics, cell death pathways, mitochondrial dynamics and potentially be related to mitochondrial genetic changes. A better understanding of these processes emphasizes potential novel therapies.

HELZ2 promotes K63-linked polyubiquitination of c-Myc to induce retinoblastoma tumorigenesis

Retinoblastoma is a rare ocular tumor in children that originates in the retina. Several core transcriptional regulators maintain the expansion of retinoblastoma tumors, including c-Myc. Here, we demonstrated that Helicase with zinc finger domain 2 (HELZ2) promoted retinoblastoma tumorigenesis by targeting c-Myc. HELZ2-deficient inhibited retinoblastoma cell proliferation, whereas overexpression of HELZ2 promoted retinoblastoma cell proliferation. In addition, high levels of HELZ2 promoted xenograft retinoblastoma tumorigenesis and inhibited animal survival. Mechanistically, HELZ2 interacted with c-Myc and promoted its K63-linked polyubiquitination. We indicated that HELZ2 promoted the interaction between E3 ubiquitin ligase HUWE1 and c-Myc, and HELZ2-mediated K63-linked polyubiquitination and activation of c-Myc were dependent on HUWE1. Taken together, HELZ2 plays a critical role in the regulation of retinoblastoma tumorigenesis by enhancing the activity of c-Myc.

Efficacy and mechanism of the combination of PARP and CDK4/6 inhibitors in the treatment of triple-negative breast cancer

Background

PARP inhibitors (PARPi) benefit only a fraction of breast cancer patients with BRCA mutations, and their efficacy is even more limited in triple-negative breast cancer (TNBC) due to clinical primary and acquired resistance. Here, we found that the efficacy of the PARPi olaparib in TNBC can be improved by combination with the CDK4/6 inhibitor (CDK4/6i) palbociclib.

Methods

We screened primary olaparib-sensitive and olaparib-resistant cell lines from existing BRCA^mut/TNBC cell lines and generated cells with acquired olaparib resistance by gradually increasing the concentration. The effects of the PARPi olaparib and the CDK4/6i palbociclib on BRCA^mut/TNBC cell lines were examined in both sensitive and resistant cells in vitro and in vivo. Pathway and gene alterations were assessed mechanistically and pharmacologically.

Results

We demonstrated for the first time that the combination of olaparib and palbociclib has synergistic effects against BRCA^mut/TNBC both in vitro and in vivo. In olaparib-sensitive MDA-MB-436 cells, the single agent olaparib significantly inhibited cell viability and affected cell growth due to severe DNA damage. In olaparib-resistant HCC1937 and SUM149 cells, single-agent olaparib was ineffective due to potential homologous recombination (HR) repair, and the combination of olaparib and palbociclib greatly inhibited HR during the G2 phase, increased DNA damage and inhibited tumour growth. Inadequate DNA damage caused by olaparib activated the Wnt signalling pathway and upregulated MYC. Further experiments indicated that the overexpression of β-catenin, especially its hyperphosphorylation at the Ser675 site, activated the Wnt signalling pathway and mediated olaparib resistance, which could be strongly inhibited by combined treatment with palbociclib.

Conclusions

Our data provide a rationale for clinical evaluation of the therapeutic synergy of the PARPi olaparib and CDK4/6i palbociclib in BRCA^mut/TNBCs with high Wnt signalling activation and high MYC expression that do not respond to PARPi monotherapy.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-01930-w.

Inhibition of Cyclin-dependent Kinase (CDK) Decreased Survival of NB4 Leukemic Cells: Proposing a p53-Independent Sensitivity of Leukemic Cells to Multi-CDKs Inhibitor AT7519

An unbounded number of events exist beneath the intricacy of each particular hematologic malignancy, prompting the tumor cells into an unrestrained proliferation and invasion. Aberrant expression of cyclin-dependent kinases (CDKs) is one of these events which disrupts the regulation of cell cycle and subsequently, results in cancer progression. In this study, we surveyed the repressive impact of multi-CDK inhibitor AT7519 on a panel of leukemia-derived cell lines. Our data underlined that AT7519 abated the survival of all tested cells; however, in an overview, the response rate of leukemic cells to the inhibitor was varied irrespective of p53 status. Notably, the less sensitivity of leukemia cells to AT7519 was found to be mediated partly by the compensatory activation of c-Myc oncogene which was confirmed by the induction of a superior cytotoxicity upon its suppression in less sensitive cell. The blockage of cell cycle, as announced by induction of sub-G1 arrest as well as reduced S phase, resulted in a significant decrease in survival of acute promyelocytic leukemia (APL)-derived NB4 cells, as the most sensitive cell line, either as monotherapy or in combination with arsenic trioxide. Anti-leukemic effects of the inhibitor were further verified by apoptosis analysis, where we discovered that AT7519 induced apoptosis via alteration of pro- and anti-apoptotic genes in NB4. All in all, this study proposed that AT7519 is a rewarding agent opposed to APL; however, additional examinations should be performed to determine the advantages of this inhibitor in clinical setting.

Cancer Classification at the Crossroads

Internationally accepted classifications of malignant tumors, developed by the World Health Organization (WHO) and the Union for International Cancer Control (UICC), are based on the histotype, site of origin, morphologic grade, and spread of cancer throughout the body. The WHO classifications are the foundation of cancer diagnosis and the starting point for cancer management. Starting in 2000, the WHO classifications began to include biologic and molecular–genetic features. These developments are having a strong impact on cancer diagnosis and treatment, and this impact is amplifying, given the advances in cancer genomics. Molecular–genetic profiling can be used to refine existing classifications of tumors and, for a small but increasing number of cancers, even determine the treatment irrespective of histotype. Here I discuss how cancer classifications may change in the era of cancer genomics.

MYC Oncogene Contributions to Release of Cell Cycle Brakes

Promotion of the cell cycle is a major oncogenic mechanism of the oncogene c-MYC (MYC). MYC promotes the cell cycle by not only activating or inducing cyclins and CDKs but also through the downregulation or the impairment of the activity of a set of proteins that act as cell-cycle brakes. This review is focused on the role of MYC as a cell-cycle brake releaser i.e., how MYC stimulates the cell cycle mainly through the functional inactivation of cell cycle inhibitors. MYC antagonizes the activities and/or the expression levels of p15, ARF, p21, and p27. The mechanism involved differs for each protein. p15 (encoded by CDKN2B) and p21 (CDKN1A) are repressed by MYC at the transcriptional level. In contrast, MYC activates ARF, which contributes to the apoptosis induced by high MYC levels. At least in some cells types, MYC inhibits the transcription of the p27 gene (CDKN1B) but also enhances p27's degradation through the upregulation of components of ubiquitin ligases complexes. The effect of MYC on cell-cycle brakes also opens the possibility of antitumoral therapies based on synthetic lethal interactions involving MYC and CDKs, for which a series of inhibitors are being developed and tested in clinical trials.

MYC status as a determinant of synergistic response to Olaparib and Palbociclib in ovarian cancer

Background

While PARP inhibitors and CDK4/6 inhibitors, the two classes of FDA-approved agents, have shown promising clinical benefits, there is an urgent need to develop new therapeutic strategies to improve clinical response. Meanwhile, extending the utility of these inhibitors beyond their respective molecularly defined cancer types is challenging and will likely require biomarkers predictive of treatment response especially when used in a combination drug development setting.

Methods

The effects of PARP inhibitor Olaparib and CDK4/6 inhibitor Palbociclib on ovarian cancer cells lines including those of high-grade serous histology were examined in vitro and in vivo. We investigated the molecular mechanism underlying the synergistic effects of drug combination.

Findings

We show for the first time that combining PARP and CDK4/6 inhibition has synergistic effects against MYC overexpressing ovarian cancer cells both in vitro and in vivo. Mechanistically, we find that Palbociclib induces homologous recombination (HR) deficiency through downregulation of MYC-regulated HR pathway genes, causing synthetic lethality with Olaparib. We further demonstrate that MYC expression determines sensitivity to combinatorial treatment with Olaparib and Palbociclib.

Interpretation

Our data provide a rationale for clinical evaluation of therapeutic synergy of these two classes of inhibitors in ovarian cancer patients whose tumors show high MYC expression and who do not respond to PARP inhibitors or CDK4/6 inhibitors monotherapies.

Fund

This work was supported by the National Natural Science Foundation of China [81672575, 81874111, 81472447 to HC; 81572586 and 81372853 to PL], and the Liaoning Provincial Key Basic Research Program for Universities [LZ2017002 to HC].

Role of genomic instability in human carcinogenesis

IMPACT STATEMENT

This review provides various genetic and cell line data previously published in a way to explain how cellular stress can lead into genetic instability.

Transcriptomic analysis reveals key lncRNAs associated with ribosomal biogenesis and epidermis differentiation in head and neck squamous cell carcinoma

OBJECTIVE

In this study, we aimed to expand current knowledge of head and neck squamous cell carcinoma (HNSCC)-associated long noncoding RNAs (lncRNAs), and to discover potential lncRNA prognostic biomarkers for HNSCC based on next-generation RNA-seq.

METHODS

RNA-seq data of 546 samples from patients with HNSCC were downloaded from The Cancer Genome Atlas (TCGA), including 43 paired samples of tumor tissue and adjacent normal tissue. An integrated analysis incorporating differential expression, weighted gene co-expression networks, functional enrichment, clinical parameters, and survival analysis was conducted to discover HNSCC-associated lncRNAs. The function of CYTOR was verified by cell-based experiments. To further identify lncRNAs with prognostic significance, a multivariate Cox proportional hazard regression analysis was performed. The identified lncRNAs were validated with an independent cohort using clinical feature relevance analysis and multivariate Cox regression analysis.

RESULTS

We identified nine HNSCC-relevant lncRNAs likely to play pivotal roles in HNSCC onset and development. By functional enrichment analysis, we revealed that CYTOR might participate in the multistep pathological processes of cancer, such as ribosome biogenesis and maintenance of genomic stability. CYTOR was identified to be positively correlated with lymph node metastasis, and significantly negatively correlated with overall survival (OS) and disease free survival (DFS) of HNSCC patients. Moreover, CYTOR inhibited cell apoptosis following treatment with the chemotherapeutic drug diamminedichloroplatinum (DDP). HCG22, the most dramatically down-regulated lncRNA in tumor tissue, may function in epidermis differentiation. It was also significantly associated with several clinical features of patients with HNSCC, and positively correlated with patient survival. CYTOR and HCG22 maintained their prognostic values independent of several clinical features in multivariate Cox hazards analysis. Notably, validation either based on an independent HNSCC cohort or by laboratory experiments confirmed these findings.

CONCLUSIONS

Our transcriptomic analysis suggested that dysregulation of these HNSCC-associated lncRNAs might be involved in HNSCC oncogenesis and progression. Moreover, CYTOR and HCG22 were confirmed as two independent prognostic factors for HNSCC patient survival, providing new insights into the roles of these lncRNAs in HNSCC as well as clinical applications.

Cell therapy in acute respiratory distress syndrome

Acute respiratory distress syndrome (ARDS) is driven by a severe pro-inflammatory response resulting in lung damage, impaired gas exchange and severe respiratory failure. There is no specific treatment that effectively improves outcome in ARDS. However, in recent years, cell therapy has shown great promise in preclinical ARDS studies. A wide range of cells have been identified as potential candidates for use, among these are mesenchymal stromal cells (MSCs), which are adult multi-lineage cells that can modulate the immune response and enhance repair of damaged tissue. The therapeutic potential of MSC therapy for sepsis and ARDS has been demonstrated in multiple in vivo models. The therapeutic effect of these cells seems to be due to two different mechanisms; direct cellular interaction, and paracrine release of different soluble products such as extracellular vesicles (EVs)/exosomes. Different approaches have also been studied to enhance the therapeutic effect of these cells, such as the over-expression of anti-inflammatory or pro-reparative molecules. Several clinical trials (phase I and II) have already shown safety of MSCs in ARDS and other diseases. However, several translational issues still need to be addressed, such as the large-scale production of cells, and their potentiality and variability, before the therapeutic potential of stem cells therapies can be realized.

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.

The T-Box factor TBX3 is important in S-phase and is regulated by c-Myc and cyclin A-CDK2

The transcription factor, TBX3, is critical for the formation of, among other structures, the heart, limbs and mammary glands and haploinsufficiency of the human TBX3 gene result in ulnar-mammary syndrome which is characterized by hypoplasia of these structures. On the other hand, the overexpression of TBX3 is a feature of a wide range of cancers and it has been implicated in several aspects of the oncogenic process. This includes its ability to function as an immortalizing gene and to promote proliferation through actively repressing negative cell cycle regulators. Together this suggests that TBX3 levels may need to be tightly regulated during the cell cycle. Here we demonstrate that this is indeed the case and that TBX3 mRNA and protein levels peak at S-phase and that the TBX3 protein is predominantly localized to the nucleus of S-phase cells. The increased levels of TBX3 in S-phase are shown to occur transcriptionally through activation by c-Myc at E-box motifs located at -1210 and -701 bps and post-translationally by cyclin A-CDK2 phosphorylation. Importantly, when TBX3 is depleted by shRNA the cells accumulate in S-phase. These results suggest that TBX3 is required for cells to transit through S-phase and that this function may be linked to its role as a pro-proliferative factor.

EBV finds a polycomb-mediated, epigenetic solution to the problem of oncogenic stress responses triggered by infection

Viruses that establish a persistent infection, involving intracellular latency, commonly stimulate cellular DNA synthesis and sometimes cell division early after infection. However, most cells of metazoans have evolved “fail-safe” responses that normally monitor unscheduled DNA synthesis and prevent cell proliferation when, for instance, cell proto-oncogenes are “activated” by mutation, amplification, or chromosomal rearrangements. These cell intrinsic defense mechanisms that reduce the risk of neoplasia and cancer are collectively called oncogenic stress responses (OSRs). Mechanisms include the activation of tumor suppressor genes and the so-called DNA damage response that together trigger pathways leading to cell cycle arrest (e.g., cell senescence) or complete elimination of cells (e.g., apoptosis). It is not surprising that viruses that can induce cellular DNA synthesis and cell division have the capacity to trigger OSR, nor is it surprising that these viruses have evolved countermeasures for inactivating or bypassing OSR. The main focus of this review is how the human tumor-associated Epstein–Barr virus manipulates the host polycomb group protein system to control – by epigenetic repression of transcription – key components of the OSR during the transformation of normal human B cells into permanent cell lines.

Acidosis decreases c-Myc oncogene expression in human lymphoma cells: a role for the proton-sensing G protein-coupled receptor TDAG8

Acidosis is a biochemical hallmark of the tumor microenvironment. Here, we report that acute acidosis decreases c-Myc oncogene expression in U937 human lymphoma cells. The level of c-Myc transcripts, but not mRNA or protein stability, contributes to c-Myc protein reduction under acidosis. The pH-sensing receptor TDAG8 (GPR65) is involved in acidosis-induced c-Myc downregulation. TDAG8 is expressed in U937 lymphoma cells, and the overexpression or knockdown of TDAG8 further decreases or partially rescues c-Myc expression, respectively. Acidic pH alone is insufficient to reduce c-Myc expression, as it does not decrease c-Myc in H1299 lung cancer cells expressing very low levels of pH-sensing G protein-coupled receptors (GPCRs). Instead, c-Myc is slightly increased by acidosis in H1299 cells, but this increase is completely inhibited by ectopic overexpression of TDAG8. Interestingly, TDAG8 expression is decreased by more than 50% in human lymphoma samples in comparison to non-tumorous lymph nodes and spleens, suggesting a potential tumor suppressor function of TDAG8 in lymphoma. Collectively, our results identify a novel mechanism of c-Myc regulation by acidosis in the tumor microenvironment and indicate that modulation of TDAG8 and related pH-sensing receptor pathways may be exploited as a new approach to inhibit Myc expression.

A high-throughput siRNA screening platform to identify MYC-synthetic lethal genes as candidate therapeutic targets

Targeted therapeutics toward specific genes and pathways represent the future of oncological treatments. However, several commonly activated oncogenes, such as MYC, have proven difficult to target by pharmacological agents. To broaden the menu of potentially druggable therapeutic targets, we describe a method to detect genes essential for the survival of MYC overexpressing cells, which we will refer to as MYC-synthetic lethal genes (MYC-SL) (Toyoshima et al., Proc Natl Acad Sci USA 109:9545-9550, 2012). These genes represent candidate targets for drug development to be utilized for MYC-driven cancers as well as probes to further our understanding of the biology of MYC-driven tumorigenesis. The discovery platform includes the following components: (1) an isogenic cell system that enables overexpression of MYC without oncogene-induced senescence (OIS) response (Benanti and Galloway, Mol Cell Biol 24:2842-2852, 2004; Benanti et al., Mol Cancer Res 5:1181-1189, 2007); (2) arrayed siRNA libraries targeting individual genes; (3) automated laboratory equipment for dispensing of cells, siRNAs, and readout assays; and (4) bioinformatics and software for data mining and visualization. This flexible platform can be readily applied to other oncogenes or tumor suppressor genes.

A role for virally induced reactive oxygen species in Kaposi's sarcoma herpesvirus tumorigenesis

AIMS

Kaposi's sarcoma (KS), caused by the Kaposi's sarcoma herpesvirus (KSHV), is an AIDS-associated cancer characterized by angiogenesis and proliferation of spindle cells. Rac1-activated reactive oxygen species (ROS) production has been implicated in KS tumorigenesis. We used an animal model of KSHV-induced Kaposi's sarcomagenesis (mECK36) to study the role of ROS in KS and the efficacy of N-acetyl l-cysteine (NAC) in inhibiting or preventing KS.

RESULTS

Signaling by the KSHV early lytic gene viral G protein-coupled receptor (vGPCR) activated ROS production in mECK36 cells via a Rac1-NADPH oxidase pathway. Induction of the lytic cycle in KSHV-infected KS spindle cells upregulated ROS along with upregulation of vGPCR expression. We also found that expression of the major latent transcript in 293 cells increased ROS levels. ROS scavenging with NAC halted mECK36 tumor growth in a KSHV-specific manner. NAC inhibited KSHV latent gene expression as well as tumor angiogenesis and lymphangiogenesis. These effects correlated with the reduction of vascular endothelial growth factor (VEGF), c-myc, and cyclin D1, and could be explained on the basis of inhibition of STAT3 tyrosine phosphorylation. NAC prevented mECK36 de novo tumor formation. Molecular analysis of NAC-resistant tumors revealed a strong upregulation of Rac1 and p40(PHOX).

INNOVATION AND CONCLUSION

Our results demonstrate that ROS-induction by KSHV plays a causal role in KS oncogenesis by promoting proliferation and angiogenesis. Our results show that both ROS and their molecular sources can be targeted therapeutically using NAC or other Food and Drug Administration (FDA)-approved inhibitors for prevention and treatment of AIDS-KS.

Identification of cis-Regulatory Elements in the dmyc Gene of Drosophila Melanogaster

Myc is a crucial regulator of growth and proliferation during animal development. Many signals and transcription factors lead to changes in the expression levels of Drosophila myc, yet no clear model exists to explain the complexity of its regulation at the level of transcription. In this study we used Drosophila genetic tools to track the dmyc cis-regulatory elements. Bioinformatics analyses identified conserved sequence blocks in the noncoding regions of the dmyc gene. Investigation of lacZ reporter activity driven by upstream, downstream, and intronic sequences of the dmyc gene in embryonic, larval imaginal discs, larval brain, and adult ovaries, revealed that it is likely to be transcribed from multiple transcription initiation units including a far upstream regulatory region, a TATA box containing proximal complex and a TATA-less downstream promoter element in conjunction with an initiator within the intron 2 region. Our data provide evidence for a modular organization of dmyc regulatory sequences; these modules will most likely be required to generate the tissue-specific patterns of dmyc transcripts. The far upstream region is active in late embryogenesis, while activity of other cis elements is evident during embryogenesis, in specific larval imaginal tissues and during oogenesis. These data provide a framework for further investigation of the transcriptional regulatory mechanisms of dmyc.

Amino acid-anticodon binding specificity: rationale for a new class of therapeutic agent

In this article a new class of anticancer and antiviral drugs is discussed. These new drugs consist of small di- and tri-peptides, designed to bind to single-stranded (ss) regions that are crucial for the expression of genes such as the c-myc oncogene in cancers and start sites (and other ss regions) of viral pathogenic genes. The components (i.e. the amino acids and the sequences they form) of these peptides could be dictated by the specific binding of amino acids to their ss anticodons in tRNA. Cancer cell viability depends on the continued overexpression of the c-myc oncogene, and thus this gene is a target of opportunity for anticancer agents. Sharply reducing the overexpression of c-myc leads to the death of cancer cells. To achieve this end the following rationale is suggested: crucial regions of the c-myc promoters (to which activating proteins must bind for expression to occur) are single stranded and thus strongly resemble the anticodon loop of tRNA. It was found that amino acids chemically bind to their cognate tRNA anticodons. Regarding the ss regions of c-myc as a series of adjacent 'anticodons', di- and tri-peptides are proposed to be aligned to their cognate 'anticodons' in the proper order. For example, if the ss region of a promoter is hypothetically TTT-GGG-CCC, the tripeptide Lys-Pro-Gly could be expected to bind to it and deny access of the promoter to all activating proteins, thereby blocking c-myc expression and all the cancers dependent on such overexpression. Similarly, it is reported that in the initial phase of gene expression the start sites of the genes are single stranded (before and after and spanning the start site). Thus, invoking the amino acid cognate anticodon binding specificity (ACABS) principle as described above, a series of small peptides are suggested that could span the start sites of pathogenic viral genes (e.g. the oris region of herpes simplex virus (HSV)) to deny access of the gene to the transcription elements. This would inactivate the toxic effect of the virus and thereby constitute a promising approach to antiviral therapy, where the start sites (or other ss regions of pathogenic genes) have been sequenced. The ACABS principle (for peptide-nucleic-acid interaction) enables us to focus on probable effective small peptides rather than having to screen a large number of randomly chosen small peptides to find probable anticancer and antiviral therapeutic agents.

The role of the transcription factor Miz-1 in lymphocyte development and lymphomagenesis-Binding Myc makes the difference

The Myc interacting zinc finger protein 1 (Miz-1) is a BTB/POZ domain containing transcription factor that can function as an activator or repressor depending on its binding partners. In a complex with co-factors such as nuclophosmin or p300, Miz-1 stimulates transcription of genes that encode regulators of cell cycle progression such as p21(Cip1) or p15(Ink4b) or inhibitors of apoptosis such as Bcl-2. In contrast, Miz-1 becomes a transcriptional repressor when it binds to c-Myc or Bcl-6, which replace nucleophosmin or p300. During lymphocyte development, Miz-1 functions as a regulator of the IL-7 signaling pathway at very early steps in the bone marrow and thymus. When the IL-7 receptor (IL-7R) recognizes its cognate cytokine, a cascade of events is initiated that involves the recruitment of janus kinases (JAK) to the cytoplasmic part of the IL-7R, the phosphorylation of Stat5, its dimerization and relocation to the nucleus, enabling a transcriptional programming that governs commitment, survival and proliferation of lymphoid lineage cells. Miz-1 is critical in this signal transduction pathway, since it controls the expression of Socs1, an inhibitor of JAKs and thus of Stat5 activation and Bcl-2 expression. A lack of Miz-1 blocks IL-7 mediated signaling, which is detrimental for early B- and T-lymphoid development. These functions of Miz-1 during early lymphocyte development are c-Myc-independent. In contrast, when c-Myc is constitutively over-expressed, for instance during c-Myc induced lymphomagenesis, the interaction between Miz-1 and c-Myc becomes important and critical for the initiation and maintenance of c-Myc-dependent lymphoid malignancies.

Gain of MYC underlies recurrent trisomy of the MYC chromosome in acute promyelocytic leukemia

The leukemogenic effects of Myc drive recurrent trisomy in a mouse model of acute myeloid leukemia.

Gain of chromosome 8 is the most common chromosomal gain in human acute myeloid leukemia (AML). It has been hypothesized that gain of the MYC protooncogene is of central importance in trisomy 8, but the experimental data to support this are limited and controversial. In a mouse model of promyelocytic leukemia in which the MRP8 promoter drives expression of the PML-RARA fusion gene in myeloid cells, a Myc allele is gained in approximately two-thirds of cases as a result of trisomy for mouse chromosome 15. We used this model to test the idea that MYC underlies acquisition of trisomy in AML. We used a retroviral vector to drive expression of wild-type, hypermorphic, or hypomorphic MYC in bone marrow that expressed the PML-RARA transgene. MYC retroviruses cooperated in myeloid leukemogenesis and suppressed gain of chromosome 15. When the PML-RARA transgene was expressed in a Myc haploinsufficient background, we observed selection for increased copies of the wild-type Myc allele concomitant with leukemic transformation. In addition, we found that human myeloid leukemias with trisomy 8 have increased MYC. These data show that gain of MYC can contribute to the pathogenic effect of the most common trisomy of human AML.

Increased radioresistance and accelerated B cell lymphomas in mice with Mdmx mutations that prevent modifications by DNA-damage-activated kinases

Mdmx is a critical negative regulator of the p53 pathway that is stoichiometrically limiting in some tissues. Posttranslational modification and degradation of Mdmx after DNA damage have been proposed to be essential for p53 activation. We tested this model in vivo, where critical stoichiometric relationships are preserved. We generated an Mdmx mutant mouse in which three conserved serines (S341, S367, S402) targeted by DNA-damage-activated kinases were replaced by alanines to investigate whether modifications of these residues are important for Mdmx degradation and p53 activation. The mutant mice were remarkably resistant to radiation, and very susceptible to Myc-induced lymphomagenesis. These data demonstrate that Mdmx downregulation is crucial for effective p53-mediated radiation responses and tumor suppression in vivo.

MYC prevents apoptosis and enhances endoreduplication induced by paclitaxel

Background

The role of the MYC oncogene in the apoptotic pathways is not fully understood. MYC has been reported to protect cells from apoptosis activation but also to sensitize cells to apoptotic stimuli. We have previously demonstrated that the down-regulation of Myc protein activates apoptosis in melanoma cells and increases the susceptibility of cells to various antitumoral treatments. Beyond the well-known role in the G1→S transition, MYC is also involved in the G2-M cell cycle phases regulation.

Methodology/Principal Findings

In this study we have investigated how MYC could influence cell survival signalling during G2 and M phases. We used the microtubules damaging agent paclitaxel (PTX), to arrest the cells in the M phase, in a p53 mutated melanoma cell line with modulated Myc level and activity. An overexpression of Myc protein is able to increase endoreduplication favoring the survival of cells exposed to antimitotic poisoning. The PTX-induced endoreduplication is associated in Myc overexpressing cells with a reduced expression of MAD2, essential component of the molecular core of the spindle assembly checkpoint (SAC), indicating an impairment of this checkpoint. In addition, for the first time we have localized Myc protein at the spindle poles (centrosomes) during pro-metaphase in different cell lines.

Conclusions

The presence of Myc at the poles during the prometaphase could be necessary for the Myc-mediated attenuation of the SAC and the subsequent induction of endoreduplication. In addition, our data strongly suggest that the use of taxane in antitumor therapeutic strategies should be rationally based on the molecular profile of the individual tumor by specifically analyzing Myc expression levels.

Tumorigenesis and anti-tumor immune responses in Xenopus

Despite intense study, the role of the immune system in detecting (immunosurveillance), controlling and remodeling (immunoediting) neoplasia remains elusive. We present here a comparative view of the complex interactions between neoplasia and the host immune system. We provide evidence, in the amphibian Xenopus laevis, consistent with an evolutionarily conserved and crucial role of the immune system in controlling neoplasia, which involves a striking variety of anti-tumoral immune effectors including conventional CTLs, classical MHC class Ia unrestricted CTLs (CCU-CTLs) that interact with nonclassical MHC class Ib molecules, CD8 NKT-like cells and NK cells. We also review the tumors found in X. laevis with an emphasis on thymic lymphoid tumors and a rare ovarian dysgerminoma. Finally, we consider the use of X. laevis for in vivo study of tumorigenesis. Given our current knowledge, the experimental systems already established in X. laevis, and the rapid accumulation of genetic resources for the sister species Silurana (Xenopus) tropicalis, it is our conviction that these species provide an ideal alternative to the murine system for studying tumorigenesis and tumor immunity.

Suppression of Myc oncogenic activity by ribosomal protein haploinsufficiency

The Myc oncogene regulates the expression of multiple components of the protein synthetic machinery, including ribosomal proteins, initiation factors of translation, Pol III, and rDNA1,2. An outstanding question is whether and how increasing the cellular protein synthesis capacity can affect the multi-step process leading to cancer. We utilized ribosomal protein heterozygote mice as a genetic tool to restore increased protein synthesis in Eμ–Myc/+ transgenic mice to normal levels and show that in this context Myc's oncogenic potential is suppressed. Our findings demonstrate that the ability of Myc to increase protein synthesis directly augments cell size and is sufficient to accelerate cell cycle progression independently of known cell cycle targets transcriptionally regulated by Myc. In addition, when protein synthesis is restored to normal levels, Myc overexpressing precancerous cells are more efficiently eliminated by programmed cell death. Our findings reveal a novel paradigm that links increases in general protein synthesis rates downstream of an oncogenic signal to a specific molecular impairment in the modality of translation initiation employed to regulate the expression of selective mRNAs. We show that an aberrant increase in cap-dependent translation downstream Myc hyperactivation specifically impairs the translational switch to internal ribosomal entry site (IRES)-dependent translation required for accurate mitotic progression. Failure of this translational switch results in reduced mitotic-specific expression of the endogenous IRES-dependent form of Cdk11 (p58-PITSLRE)3-5, which leads to cytokinesis defects and is associated with increased centrosome numbers and genome instability in Eμ–Myc/+ mice. When accurate translational control is re-established in Eμ–Myc/+ mice, genome instability is suppressed. Our findings reveal how perturbations in translational control provide a highly specific outcome on gene expression, genome stability, and cancer initiation that have important implications for understanding the molecular mechanism of cancer formation at the post-genomic level.

Myc increases self-renewal in neural progenitor cells through Miz-1

The mechanisms underlying the decision of a stem or progenitor cell to either self-renew or differentiate are incompletely understood. To address the role of Myc in this process, we expressed different forms of the proto-oncogene Myc in multipotent neural progenitor cells (NPCs) using retroviral transduction. Expression of Myc in neurospheres increased the proportion of self-renewing cells fivefold, and 1% of the Myc-overexpressing cells, but none of the control cells, retained self-renewal capacity even under differentiation-inducing conditions. A Myc mutant (MycV394D) deficient in binding to Miz-1, did not increase the percentage of self-renewing cells but was able to stimulate proliferation of NPCs as efficiently as wild-type Myc, indicating that these two cellular phenomena are regulated by at least partially different pathways. Our results suggest that Myc, through Miz-1, enhances self-renewal of NPCs and influences the way progenitor cells react to the environmental cues that normally dictate the cellular identity of tissues containing self-renewing cells.

Myc oncogene-induced genomic instability: DNA palindromes in bursal lymphomagenesis

Genetic instability plays a key role in the formation of naturally occurring cancer. The formation of long DNA palindromes is a rate-limiting step in gene amplification, a common form of tumor-associated genetic instability. Genome-wide analysis of palindrome formation (GAPF) has detected both extensive palindrome formation and gene amplification, beginning early in tumorigenesis, in an experimental Myc-induced model tumor system in the chicken bursa of Fabricius. We determined that GAPF-detected palindromes are abundant and distributed nonrandomly throughout the genome of bursal lymphoma cells, frequently at preexisting short inverted repeats. By combining GAPF with chromatin immunoprecipitation (ChIP), we found a significant association between occupancy of gene-proximal Myc binding sites and the formation of palindromes. Numbers of palindromic loci correlate with increases in both levels of Myc over-expression and ChIP-detected occupancy of Myc binding sites in bursal cells. However, clonal analysis of chick DF-1 fibroblasts suggests that palindrome formation is a stochastic process occurring in individual cells at a small number of loci relative to much larger numbers of susceptible loci in the cell population and that the induction of palindromes is not involved in Myc-induced acute fibroblast transformation. GAPF-detected palindromes at the highly oncogenic bic/miR-155 locus in all of our preneoplastic and neoplastic bursal samples, but not in DNA from normal and other transformed cell types. This finding indicates very strong selection during bursal lymphomagenesis. Therefore, in addition to providing a platform for gene copy number change, palindromes may alter microRNA genes in a fashion that can contribute to cancer development.

A C. elegans Myc-like network cooperates with semaphorin and Wnt signaling pathways to control cell migration

Myc and Mondo proteins are key regulators of cell growth, proliferation, and energy metabolism, yet often overlooked is their vital role in cell migration. Complex networks of protein-protein and protein-DNA interactions control the transcriptional activity of Myc and MondoA confounding their functional analysis in higher eukaryotes. Here we report the identification of the transcriptional activation arm of a simplified Myc-like network in Caenorhabditis elegans. This network comprises an Mlx ortholog, named MXL-2 for Max-like 2, and a protein that has sequence features of both Myc and Mondo proteins, named MML-1 for Myc and Mondo-like 1. MML-1/MXL-2 complexes have a primary function in regulating migration of the ray 1 precursor cells in the male tail. MML-1/MXL-2 complexes control expression of ECM components in the non-migratory epidermis, which we propose contributes to the substratum required for migration of the neighboring ray 1 precursor cells. Furthermore, we show that pro-migratory Wnt/beta-catenin and semaphorin signaling pathways interact genetically with MML-1/MXL-2 to determine ray 1 position. This first functional analysis of the Myc superfamily in C. elegans suggests that MondoA and Myc may have more predominant roles in cell migration than previously appreciated, and their cooperation with other pro-migratory pathways provides a more integrated view of their role in cell migration.

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.

c-Myc-mediated genomic instability proceeds via a megakaryocytic endomitosis pathway involving Gp1balpha

Genomic instability (GI) is essential for the initiation and evolution of many cancers and often precedes frank neoplastic conversion. Although GI can occur at several levels, the most conspicuous examples involve gains or losses of entire chromosomes (aneuploidy), the antecedent of which may be whole genome duplication (tetraploidy). Through largely undefined mechanisms, the c-Myc oncoprotein and its downstream target, MTMC1, promote tetraploidy and other forms of GI. In myeloid cells, c-Myc and MTMC1 also regulate a common, small subset of c-Myc target genes including GP1Balpha, which encodes a subunit of the von Willebrand's factor receptor complex that mediates platelet adhesion and aggregation. Gp1balpha also participates in megakaryocyte endomitosis, a form of controlled and precise whole-genome amplification. In this article, we show that both c-Myc and MTMC1 strongly up-regulate Gp1balpha concurrent with their promotion of tetraploidy. shRNA-mediated inhibition of Gp1balpha prevents tetraploidy by both c-Myc and MTMC1, whereas Gp1balpha overexpression alone is sufficient to induce tetraploidy in established and primary cells. Once acquired, tetraploidy persists in most cases examined. Our results indicate that chromosome-level GI, induced by c-Myc overexpression, proceeds by means of the sequential up-regulation of MTMC1 and Gp1balpha and further suggest that the pathways leading to megakaryocytic endomitosis and c-Myc-induced tetraploidy are mechanistically linked by their reliance on Gp1balpha.

The expression and purification of the N-terminal activation domain of the transcription factor c-Myc: a model substrate for exploring ERK2 docking interactions

ERK2 is a mitogen-activated protein kinase (MAPK) that plays pivotal roles in cell signal transduction, where it mediates effects on proliferation and differentiation by growth factors and hormones. An important substrate of ERK2 is the transcription factor c-Myc, which mediates cell cycle progression. The phosphorylation of Ser-62 on c-Myc by ERK2 is thought to contribute to the increased stability of c-Myc during the cell cycle and is thus a critical cellular event. However, the mode of c-Myc recognition by ERK2 is not understood. Early studies by Gupta and Davis concluded that ERK2 specificity determinants are located in residues 1-100 of c-Myc, its activation domain. To pursue both structural and kinetic studies a rapid, but efficient purification method, for the production of the activation domain of c-Myc from an Escherichia coli source, was developed. We chose the minimal number of high-resolution steps to maximize both yield and efficiency without sacrificing purity. Thus, GST-(c-MycDelta2-99)-His(6) was expressed in E. coli, and purified using glutathione-agarose affinity chromatography. Cleavage of the GST fusion protein by thrombin and subsequent purification by nickel-agarose affinity chromatography yielded 8 mg of purified (c-MycDelta2-99)-His(6) from one liter of LB culture. Rigorous characterization demonstrated that under standard assay conditions (c-MycDelta2-99)-His(6) is phosphorylated by ERK2 with the following Michaelis parameters: k(cat)=10.4s(-1), K(M)(c-Myc)=57.4 microM. In summary, a rapid procedure is outlined for the preparation of (c-MycDelta2-99)-His(6) that will be useful for mechanistic and biophysical studies of ERK2.

The mechanism of discrimination between cognate and non-specific DNA by dimeric b/HLH/LZ transcription factors

The Myc/Max/Mad proteins are basic region-helix-loop-helix-leucine zipper (b/HLH/LZ) transcription factors that regulate the transcription of numerous genes involved in cell growth and proliferation. The Max protein is the obligate heterodimeric partner of the Myc and Mad proteins. Heterodimerization and DNA binding to target gene promoters are mediated by the b/HLH/LZ domains. Max can also form a homodimeric b/HLH/LZ. The enhanced expression of Myc and binding to promoters of target genes contribute to almost every aspect of tumor biology. However, the detailed mechanism by which dimeric and heterodimeric b/HLH/LZs discriminate cognate DNA (E-Box: CACGTG) from non-specific sequences in the target gene promoters is still unknown. Here, we use the Max b/HLH/LZ homodimer as a model for this class of transcription factors in the characterization and understanding of the mechanism of discrimination between the E-Box and non-specific DNA sequences. We report the characterization of a cognate and a non-specific Max b/HLH/LZ/DNA complex by EMSA, CD and NMR. Our results support a detailed mechanism by which dimeric b/HLH/LZ transcription factors can discriminate E-Box sequences from non-specific DNA. The mechanism proceeds via the conformational selection of fitting b/HLH/LZ homodimers with the basic region only partially helical. Next, the basic region undergoes a DNA-assisted folding or induced-fit. It is this step that provides the discrimination by stabilizing and destabilizing the alpha-helical conformation of the basic region in the cognate and non-specific complexes, respectively. This leads to a low affinity complex with a higher probability of being dissociated and hence to discrimination. A description of the side-chains and nucleotides proposed to be involved in the discrimination process is provided.