The Myc:Max protein complex and cell growth regulation

Myc Supports Self-Renewal of Basal Cells in the Esophageal Epithelium

It is widely believed that cellular senescence plays a critical role in both aging and cancer, and that senescence is a fundamental, permanent growth arrest that somatic cells cannot avoid. Here we show that Myc plays an important role in self-renewal of esophageal epithelial cells, contributing to their resistance to cellular senescence. Myc is homogeneously expressed in basal cells of the esophageal epithelium and Myc positively regulates their self-renewal by maintaining their undifferentiated state. Indeed, Myc knockout induced a loss of the undifferentiated state of esophageal epithelial cells resulting in cellular senescence while forced MYC expression promoted oncogenic cell proliferation. A superoxide scavenger counteracted Myc knockout-induced senescence, therefore suggesting that a mitochondrial superoxide takes part in inducing senescence. Taken together, these analyses reveal extremely low levels of cellular senescence and senescence-associated phenotypes in the esophageal epithelium, as well as a critical role for Myc in self-renewal of basal cells in this organ. This provides new avenues for studying and understanding the links between stemness and resistance to cellular senescence.

Taking the Myc out of cancer: toward therapeutic strategies to directly inhibit c-Myc

c-Myc is a transcription factor that is constitutively and aberrantly expressed in over 70% of human cancers. Its direct inhibition has been shown to trigger rapid tumor regression in mice with only mild and fully reversible side effects, suggesting this to be a viable therapeutic strategy. Here we reassess the challenges of directly targeting c-Myc, evaluate lessons learned from current inhibitors, and explore how future strategies such as miniaturisation of Omomyc and targeting E-box binding could facilitate translation of c-Myc inhibitors into the clinic.

Mission Possible: Advances in MYC Therapeutic Targeting in Cancer

MYC is a master transcriptional regulator that controls almost all cellular processes. Over the last several decades, researchers have strived to define the context-dependent transcriptional gene programs that are controlled by MYC, as well as the mechanisms that regulate MYC function, in an effort to better understand the contribution of this oncoprotein to cancer progression. There are a wealth of data indicating that deregulation of MYC activity occurs in a large number of cancers and significantly contributes to disease progression, metastatic potential, and therapeutic resistance. Although the therapeutic targeting of MYC in cancer is highly desirable, there remain substantial structural and functional challenges that have impeded direct MYC-targeted drug development and efficacy. While efforts to drug the ‘undruggable’ may seem futile given these challenges and considering the broad reach of MYC, significant strides have been made to identify points of regulation that can be exploited for therapeutic purposes. These include targeting the deregulation of MYC transcription in cancer through small-molecule inhibitors that induce epigenetic silencing or that regulate the G-quadruplex structures within the MYC promoter. Alternatively, compounds that disrupt the DNA-binding activities of MYC have been the long-standing focus of many research groups, since this method would prevent downstream MYC oncogenic activities regardless of upstream alterations. Finally, proteins involved in the post-translational regulation of MYC have been identified as important surrogate targets to reduce MYC activity downstream of aberrant cell stimulatory signals. Given the complex regulation of the MYC signaling pathway, a combination of these approaches may provide the most durable response, but this has yet to be shown. Here, we provide a comprehensive overview of the different therapeutic strategies being employed to target oncogenic MYC function, with a focus on post-translational mechanisms.

Cell survival regulation during receptor-mediated endocytosis of chemically-modified lipoproteins associated to the formation of an Amphiphysin 2 (Bin1)/c-Myc complex

Amphiphysin 2 and members of the BAR-domain family of proteins participate in a wide array of cellular processes including cell cycle and endocytosis. Given that amphiphysin 2 is related to diverse cell responses as a result of metabolic stress, we investigated in macrophages whether oxidative stress originated by the internalization of oxidized low density lipoproteins (oxLDL) affect both, the expression of amphiphysin 2 and its binding partner c-Myc. Here we report that under oxidative stress, a complex formation between amphiphysin 2(Bin1) and c-Myc allows the cell to develop a novel survival equilibrium state established between cell proliferation and cell death. We propose that under conditions of oxidative stress given by the internalization of oxLDL, macrophages employ the formation of the amphiphysin 2(Bin1)/c-Myc complex as a control mechanism to initially avoid the process of cell death in an attempt to prolong cell survival.

Myc activity is required for maintenance of the neuromesodermal progenitor signalling network and for segmentation clock gene oscillations in mouse

The Myc transcriptional regulators are implicated in a range of cellular functions, including proliferation, cell cycle progression, metabolism and pluripotency maintenance. Here, we investigated the expression, regulation and function of the Myc family during mouse embryonic axis elongation and segmentation. Expression of both cMyc (Myc - Mouse Genome Informatics) and MycN in the domains in which neuromesodermal progenitors (NMPs) and underlying caudal pre-somitic mesoderm (cPSM) cells reside is coincident with WNT and FGF signals, factors known to maintain progenitors in an undifferentiated state. Pharmacological inhibition of Myc activity downregulates expression of WNT/FGF components. In turn, we find that cMyc expression is WNT, FGF and Notch protein regulated, placing it centrally in the signalling circuit that operates in the tail end that both sustains progenitors and drives maturation of the PSM into somites. Interfering with Myc function in the PSM, where it displays oscillatory expression, delays the timing of segmentation clock oscillations and thus of somite formation. In summary, we identify Myc as a component that links NMP maintenance and PSM maturation during the body axis elongation stages of mouse embryogenesis.

The Role for Myc in Coordinating Glycolysis, Oxidative Phosphorylation, Glutaminolysis, and Fatty Acid Metabolism in Normal and Neoplastic Tissues

That cancer cells show patterns of metabolism different from normal cells has been known for over 50 years. Yet, it is only in the past decade or so that an appreciation of the benefits of these changes has begun to emerge. Altered cancer cell metabolism was initially attributed to defective mitochondria. However, we now realize that most cancers do not have mitochondrial mutations and that normal cells can transiently adopt cancer-like metabolism during periods of rapid proliferation. Indeed, an encompassing, albeit somewhat simplified, conceptual framework to explain both normal and cancer cell metabolism rests on several simple premises. First, the metabolic pathways used by cancer cells and their normal counterparts are the same. Second, normal quiescent cells use their metabolic pathways and the energy they generate largely to maintain cellular health and organelle turnover and, in some cases, to provide secreted products necessary for the survival of the intact organism. By contrast, undifferentiated cancer cells minimize the latter functions and devote their energy to producing the anabolic substrates necessary to maintain high rates of unremitting cellular proliferation. Third, as a result of the uncontrolled proliferation of cancer cells, a larger fraction of the metabolic intermediates normally used by quiescent cells purely as a source of energy are instead channeled into competing proliferation-focused and energy-consuming anabolic pathways. Fourth, cancer cell clones with the most plastic and rapidly adaptable metabolism will eventually outcompete their less well-adapted brethren during tumor progression and evolution. This attribute becomes increasingly important as tumors grow and as their individual cells compete in a constantly changing and inimical environment marked by nutrient, oxygen, and growth factor deficits. Here, we review some of the metabolic pathways whose importance has gained center stage for tumor growth, particularly those under the control of the c-Myc (Myc) oncoprotein. We discuss how these pathways differ functionally between quiescent and proliferating normal cells, how they are kidnapped and corrupted during the course of transformation, and consider potential therapeutic strategies that take advantage of common features of neoplastic and metabolic disorders.

Early Transcriptomic Response to LDL and oxLDL in Human Vascular Smooth Muscle Cells

Background

Although nowadays it is well known that the human transcriptome can importantly vary according to external or environmental condition, the reflection of this concept when studying oxidative stress and its direct relationship with gene expression profiling during the process of atherogenesis has not been thoroughly achieved.

Objective

The ability to analyze genome-wide gene expression through transcriptomics has shown that the genome responds dynamically to diverse stimuli. Here, we describe the transcriptome of human vascular smooth muscle cells (hVSMC) stimulated by native and oxidized low-density lipoprotein (nLDL and oxLDL respectively), with the aim of assessing the early molecular changes that induce a response in this cell type resulting in a transcriptomic transformation. This expression has been demonstrated in atherosclerotic plaques in vivo and in vitro, particularly in the light of the oxidative modification hypothesis of atherosclerosis.

Approach and Results

Total RNA was isolated with TRIzol reagent (Life Technologies) and quality estimated using an Agilent 2100 bioanalyzer. The transcriptome of hVSMC under different experimental conditions (1,5 and 24 hours for nLDL and oxLDL) was obtained using the GeneChip Human Gene 1.0 ST (Affymetrix) designed to measure gene expression of 28,869 well-annotated genes. A fixed fold-change cut-off corresponding to ± 2 was used to identify genes exhibiting the most significant variation and statistical significance (P< 0.05), and 8 genes validated by qPCR using Taqman probes.

Conclusions

10 molecular processes were significantly affected in hVSMC: Apoptosis and cell cycle, extracellular matrix remodeling, DNA repair, cholesterol efflux, cGMP biosynthesis, endocytic mechanisms, calcium homeostasis, redox balance, membrane trafficking and finally, the immune response to inflammation. The evidence we present supporting the hypothesis for the involvement of oxidative modification of several processes and metabolic pathways in atherosclerosis is strengthen by the fact that gene expression patterns obtained when hVSMC are incubated for a long period of time in the presence of nLDL, correspond very much the same as when cells are incubated for a short period of time in the presence of chemically modified oxLDL. Our results indicate that under physiological conditions and directly related to specific environmental conditions, LDL particles most probably suffer chemical modifications that initially serve as an alert signal to overcome a harmful stimulus that with time might get transformed to a pathological pattern and therefore consolidate a pathological condition.

Pheochromocytomas and Paragangliomas: Clinical and Genetic Approaches

Pheochromocytomas (PCCs) and paragangliomas (PGLs) are neuroendocrine tumors derived from the chromaffin tissue. Diagnosis of these tumors is extremely important as they are linked to the hypertension syndrome with great cardiovascular morbidity and mortality. A great majority of PCCs and PGLs are sporadic and benign tumors; however, the classic idea of 10% exception of these features is changing. The description of new genes linked to familial forms of PCC/PGLs, such as succinate dehydrogenase (SDH) complex subunits, KIF1Bβ, EGLN1, TMEM127, and MAX, added to the well-known PCC familial syndrome (MEN2, VHL, and neurofibromatosis type 1) presents new challenges for diagnosis. In this review, we discuss the diversity of clinical and genetic approaches to this syndrome as well the diverse criteria that should guide genetic investigation.

Mutation and association analyses of the candidate genes ESR1, ESR2, MAX, PCNA, and KAT2A in patients with unexplained MSH2-deficient tumors

Lynch syndrome (Hereditary non-polyposis colorectal cancer/HNPCC) is a cancer susceptibility syndrome which is caused by germline mutations in DNA mismatch repair (MMR) genes, in particular MLH1 and MSH2. A pathogenic germline mutation in the respective MMR gene is suggested by the finding of a loss of a mismatch repair protein in tumor tissue on immunohistochemical staining combined with an early age of onset and/or the familial occurrence of colorectal cancer. Pathogenic germline mutations are identifiable in around 60% of patients suspected of Lynch syndrome, depending on the familial occurrence. The aim of the present study was to identify novel susceptibility genes for Lynch syndrome. 64 Healthy controls and 64 Lynch syndrome patients with no pathogenic MSH2 mutation but a loss of MSH2 expression in their tumor tissue were screened for rare and disease causing germline mutations in the functional candidate genes ESR1, ESR2, MAX, PCNA, and KAT2A. Thirty variants were identified, and these were then genotyped in an independent sample of 36 mutation negative Lynch syndrome patients and 234 controls. Since a trend towards association was observed for KAT2A, an additional set of 21 tagging SNPs was analyzed at this locus in a final case-control sample of 142 mutation negative Lynch syndrome patients and 298 controls. The mutation analysis failed to reveal any rare disease-causing mutations. No association was found at the single-marker or haplotypic level for any common disease-modifying variant. The present results suggest that neither rare nor common genetic variants in ESR1, ESR2, MAX, PCNA, or KAT2A contribute to the development of Lynch syndrome.

Functional promoter polymorphisms govern differential expression of HMG-CoA reductase gene in mouse models of essential hypertension

3-Hydroxy-3-methylglutaryl-coenzyme A [HMG-CoA] reductase gene (Hmgcr) is a susceptibility gene for essential hypertension. Sequencing of the Hmgcr locus in genetically hypertensive BPH (blood pressure high), genetically hypotensive BPL (blood pressure low) and genetically normotensive BPN (blood pressure normal) mice yielded a number of single nucleotide polymorphisms (SNPs). BPH/BPL/BPN Hmgcr promoter-luciferase reporter constructs were generated and transfected into liver HepG2, ovarian CHO, kidney HEK-293 and neuronal N2A cells for functional characterization of the promoter SNPs. The BPH-Hmgcr promoter showed significantly less activity than the BPL-Hmgcr promoter under basal as well as nicotine/cholesterol-treated conditions. This finding was consistent with lower endogenous Hmgcr expression in liver and lower plasma cholesterol in BPH mice. Transfection experiments using 5′-promoter deletion constructs (strategically made to assess the functional significance of each promoter SNP) and computational analysis predicted lower binding affinities of transcription factors c-Fos, n-Myc and Max with the BPH-promoter as compared to the BPL-promoter. Corroboratively, the BPH promoter-luciferase reporter construct co-transfected with expression plasmids of these transcription factors displayed less pronounced augmentation of luciferase activity than the BPL construct, particularly at lower amounts of transcription factor plasmids. Electrophoretic mobility shift assays also showed diminished interactions of the BPH promoter with HepG2 nuclear proteins. Taken together, this study provides mechanistic basis for the differential Hmgcr expression in these mouse models of human essential hypertension and have implications for better understanding the role of this gene in regulation of blood pressure.

Overexpression of c-myc induces epithelial mesenchymal transition in mammary epithelial cells

The c-myc gene is frequently overexpressed in human breast cancer and its target genes are involved in tumorigenesis. Epithelial mesenchymal transitions (EMT), where cells undergo a developmental switch from a polarized epithelial phenotype to a highly motile mesenchymal phenotype, are associated with invasion and motility of cancer cells. Basal E-cadherin expression was down-regulated in c-myc overexpressing MCF10A (c-myc-MCF10A) cells compared to GFP-overexpressing MCF10A (GFP-MCF10A) cells, while N-cadherin was distinctly increased in c-myc-MCF10A cells. Given that glycogen synthase kinase-3beta (GSK-3beta) and the snail axis have key roles in E-cadherin deregulation during EMT, we investigated the role of GSK-3beta/snail signaling pathways in the induction of EMT by c-myc overexpression. In contrast to GFP-MCF10A cells, both the transcriptional activity and the ubiquitination-dependent protein stability of snail were enhanced in c-myc-MCF10A cells, and this was reversed by GSK-3beta overexpression. We also found that c-myc overexpression inhibits GSK-3beta activity through activation of extracellular signal-regulated kinase (ERK). Inhibition of ERK by dominant negative mutant transfection or chemical inhibitor significantly suppressed snail gene transcription. These results suggest that c-myc overexpression during transformation of mammary epithelial cells (MEC) is involved in EMTs via ERK-dependent GSK-3beta inactivation and subsequent snail activation.

AP-2alpha Inhibits c-MYC Induced Oxidative Stress and Apoptosis in HaCaT Human Keratinocytes

AP-2α and c-MYC are important transcription factors involved in multiple cellular processes. They each display the paradoxical capacities to stimulate both cell proliferation and apoptosis under different conditions. In the present study we found that over expression of c-MYC was associated with accumulation of reactive oxygen species (ROS) and apoptosis in human keratinocytes, both of which were significantly inhibited by co-expression of AP-2. The effects of AP-2 on c-MYC were active at several levels. First, AP-2 and c-MYC were confirmed to interact at the protein level as previously described. In addition, forced expression of AP-2 significantly decreased steady state levels of c-MYC mRNA and protein. These findings suggested that AP-2 may have a direct effect on the c-myc gene. Chromatin immunoprecipitation assays demonstrated that AP-2 proteins bound to a cluster of AP-2 binding sites located within a 2 kb upstream regulatory region of c-myc These results suggest that the negative regulation of AP-2 on c-MYC activity was achieved through binding of AP-2 protein to the c-myc gene. The effects of AP-2 on c-MYC induced ROS accumulation and apoptosis in epidermal keratinocytes are likely to play an important role in cell growth, differentiation and carcinogenesis of the skin.

Global identification of Myc target genes reveals its direct role in mitochondrial biogenesis and its E-box usage in vivo

The Myc oncoprotein is a transcription factor involved in a variety of human cancers. Overexpression of Myc is associated with malignant transformation. In normal cells, Myc is induced by mitotic signals, and in turn, it regulates the expression of downstream target genes. Although diverse roles of Myc have been predicted from many previous studies, detailed functions of Myc targets are still unclear. By combining chromatin immunoprecipitation (ChIP) and promoter microarrays, we identified a total of 1469 Myc direct target genes, the majority of which are novel, in HeLa cells and human primary fibroblasts. We observed dramatic changes of Myc occupancy at its target promoters in foreskin fibroblasts in response to serum stimulation. Among the targets of Myc, 107 were nuclear encoded genes involved in mitochondrial biogenesis. Genes with important roles in mitochondrial replication and biogenesis, such as POLG, POLG2, and NRF1 were identified as direct targets of Myc, confirming a direct role for Myc in regulating mitochondrial biogenesis. Analysis of target promoter sequences revealed a strong preference for Myc occupancy at promoters containing one of several described consensus sequences, CACGTG, in vivo. This study thus sheds light on the transcriptional regulatory networks mediated by Myc in vivo.

Stimulation of c-Myc transcriptional activity by vIRF-3 of Kaposi sarcoma-associated herpesvirus

Kaposi sarcoma-associated herpesvirus is associated with two lymphoproliferative disorders, primary effusion lymphoma (PEL) and Castleman disease. In PEL, Kaposi sarcoma-associated herpesvirus is present in a latent form expressing only few viral genes. Among them is a viral homologue of cellular interferon regulatory factors, vIRF-3. To study the role of vIRF-3 in PEL lymphomagenesis, we analyzed the interaction of vIRF-3 with cellular proteins. Using yeast two-hybrid screen, we detected the association between vIRF-3 and c-Myc suppressor, MM-1alpha. The vIRF-3 and MM-1alpha interaction was also demonstrated by glutathione S-transferase pulldown assay and coimmunoprecipitation of endogenous vIRF-3 and MM-1alpha in PEL-derived cell lines. Overexpression of vIRF-3 enhanced the c-Myc-dependent transcription of the gene cdk4. Addressing the molecular mechanism of the vIRF-3-mediated stimulation, we demonstrated that the association between MM-1alpha and c-Myc was inhibited by vIRF-3. Furthermore, the recruitment of vIRF-3 to the cdk4 promoter and the elevated levels of the histone H3 acetylation suggest the direct involvement of vIRF-3 in the activation of c-Myc-mediated transcription. These findings indicate that vIRF-3 can effectively stimulate c-Myc function in PEL cells and consequently contribute to de-regulation of B-cell growth and differentiation.

Proteomic discovery of Max as a novel interacting partner of C/EBPalpha: a Myc/Max/Mad link

The transcription factor CCAAT/enhancer binding protein a (C/EBPalpha) is important in the regulation of granulopoiesis and is disrupted in human acute myeloid leukemia. In the present study, we sought to identify novel C/EBPalpha interacting proteins in vivo through immunoprecipitation using mass spectrometry-based proteomic techniques. We identified Max, a heterodimeric partner of Myc, as one of the interacting proteins of C/EBPalpha in our screen. We confirmed the in vivo interaction of C/EBPalpha with Max and showed that this interaction involves the basic region of C/EBPalpha. Endogenous C/EBPalpha and Max, but not Myc and Max, colocalize in intranuclear structures during granulocytic differentiation of myeloid U937 cells. Max enhanced the transactivation capacity of C/EBPalpha on a minimal promoter. A chromatin immunoprecipitation assay revealed occupancy of the human C/EBPalpha promoter in vivo by Max and Myc under cellular settings and by C/EBPalpha and Max under retinoic acid induced granulocytic differentiation. Interestingly, enforced expression of Max and C/EBPalpha results in granulocytic differentiation of the human hematopoietic CD34(+) cells, as evidenced by CD11b, CD15 and granulocyte colony-stimulating factor receptor expression. Silencing of Max by short hairpin RNA in CD34(+) and U937 cells strongly reduced the differentiation-inducing potential of C/EBPalpha, indicating the importance of C/EBPalpha-Max in myeloid progenitor differentiation. Taken together, our data reveal Max as a novel co-activator of C/EBPalpha functions, thereby suggesting a possible link between C/EBPalpha and Myc-Max-Mad network.

A credit-card library approach for disrupting protein-protein interactions

Protein-protein interfaces are prominent in many therapeutically important targets. Using small organic molecules to disrupt protein-protein interactions is a current challenge in chemical biology. An important example of protein-protein interactions is provided by the Myc protein, which is frequently deregulated in human cancers. Myc belongs to the family of basic helix-loop-helix leucine zipper (bHLH-ZIP) transcription factors. It is biologically active only as heterodimer with the bHLH-ZIP protein Max. Herein, we report a new strategy for the disruption of protein-protein interactions that has been corroborated through the design and synthesis of a small parallel library composed of 'credit-card' compounds. These compounds are derived from a planar, aromatic scaffold and functionalized with four points of diversity. From a 285 membered library, several hits were obtained that disrupted the c-Myc-Max interaction and cellular functions of c-Myc. The IC50 values determined for this small focused library for the disruption of Myc-Max dimerization are quite potent, especially since small molecule antagonists of protein-protein interactions are notoriously difficult to find. Furthermore, several of the compounds were active at the cellular level as shown by their biological effects on Myc action in chicken embryo fibroblast assays. In light of our findings, this approach is considered a valuable addition to the armamentarium of new molecules being developed to interact with protein-protein interfaces. Finally, this strategy for disrupting protein-protein interactions should prove applicable to other families of proteins.

The role of the multidrug resistance-associated protein 1 gene in neuroblastoma biology and clinical outcome

Multidrug resistance is a major obstacle to cancer treatment and leads to poor prognosis for the patient. Multidrug resistance-associated protein 1 (MRP1) can confer drug resistance in vitro and MRP1 may play a role in the development of drug resistance in several cancers including acute myeloid leukaemia, small cell lung cancer, T-cell leukaemia and neuroblastoma. The majority of patients with neuroblastoma present with widely disseminated disease at diagnosis and despite intensive treatment, the prognosis for such patients is dismal. There is increasing evidence for the involvement of the MYCN oncogene, and its down-stream target, MRP1, in the development of multidrug resistance in neuroblastoma. Given the importance of MRP1 overexpression in neuroblastoma, MRP1 inhibition may be a clinically relevant approach to improving patient outcome in this disease.

Improving the thermodynamic stability of the leucine zipper of max increases the stability of its b-HLH-LZ:E-box complex

Max is a member of the b-HLH-LZ (basic region-helix1-loop-helix2-leucine zipper) family of eukaryotic transcription factors. It is the obligate partner of the related b-HLH-LZ proteins, c-Myc and Mad1, with which it forms heterodimers on target DNA. While c-Myc and Mad1 require Max for DNA-binding, Max itself can form a homodimer that recognizes E-box DNA sequences (CACGTG) in gene promoters that are targeted by c-Myc. Evidence suggests that this mode of binding by Max may repress c-Myc transcriptional activity, and this may have applications in the control of the aberrant activity of c-Myc during certain oncogenic transformations. To enhance this repressive potential of Max, we sought to stabilize Max homodimers. We have designed a double mutant (N78V/H81L) located in the coiled-coil interface of the leucine zipper domain and we demonstrate that these mutations do indeed increase the stability of the protein. The mutations also improve the stability of the complex with cognate DNA. Thermal denaturations monitored by circular dichroism reveal two transitions that are due to intermediate folding states for both the wild-type and mutant proteins; this is supported by detailed thermodynamic analyses. A formalism to characterize the temperature-dependence of the unfolding, including the effect of intermediates, is presented.

Overexpression of c-Myc alters G(1)/S arrest following ionizing radiation

Study of the mechanism(s) of genomic instability induced by the c-myc proto-oncogene has the potential to shed new light on its well-known oncogenic activity. However, an underlying mechanism(s) for this phenotype is largely unknown. In the present study, we investigated the effects of c-Myc overexpression on the DNA damage-induced G(1)/S checkpoint, in order to obtain mechanistic insights into how deregulated c-Myc destabilizes the cellular genome. The DNA damage-induced checkpoints are among the primary safeguard mechanisms for genomic stability, and alterations of cell cycle checkpoints are known to be crucial for certain types of genomic instability, such as gene amplification. The effects of c-Myc overexpression were studied in human mammary epithelial cells (HMEC) as one approach to understanding the c-Myc-induced genomic instability in the context of mammary tumorigenesis. Initially, flow-cytometric analyses were used with two c-Myc-overexpressing, nontransformed immortal lines (184A1N4 and MCF10A) to determine whether c-Myc overexpression leads to alteration of cell cycle arrest following ionizing radiation (IR). Inappropriate entry into S phase was then confirmed with a bromodeoxyuridine incorporation assay measuring de novo DNA synthesis following IR. Direct involvement of c-Myc overexpression in alteration of the G(1)/S checkpoint was then confirmed by utilizing the MycER construct, a regulatable c-Myc. A transient excess of c-Myc activity, provided by the activated MycER, was similarly able to induce the inappropriate de novo DNA synthesis following IR. Significantly, the transient expression of full-length c-Myc in normal mortal HMECs also facilitated entry into S phase and the inappropriate de novo DNA synthesis following IR. Furthermore, irradiated, c-Myc-infected, normal HMECs developed a sub-G(1) population and a >4N population of cells. The c-Myc-induced alteration of the G(1)/S checkpoint was also compared to the effects of expression of MycS (N-terminally truncated c-Myc) and p53DD (a dominant negative p53) in the HMECs. We observed inappropriate hyperphosphorylation of retinoblastoma protein and then the reappearance of cyclin A, following IR, selectively in full-length c-Myc- and p53DD-overexpressing MCF10A cells. Based on these results, we propose that c-Myc attenuates a safeguard mechanism for genomic stability; this property may contribute to c-Myc-induced genomic instability and to the potent oncogenic activity of c-Myc.

Histone acetylation and the control of the cell cycle

The critical steps of the cell cycle are generally controlled through the transcriptional regulation of specific subsets of genes. Transcriptional regulation has been recently linked to acetylation or deacetylation of core histone tails: acetylated histone tails are generally associated with active chromatin, whereas deacetylated histone tails are associated with silent parts of the genome. A number of transcriptional co-regulators are histone acetyl-transferases or histone deacetylases. Here, we discuss some of the critical cell cycle steps in which these enzymes are involved.

Mxi1 is a repressor of the c-Myc promoter and reverses activation by USF

The basic region/helix-loop-helix/leucine zipper (B-HLH-LZ) oncoprotein c-Myc is abundant in proliferating cells and forms heterodimers with Max protein that bind to E-box sites in DNA and stimulate genes required for proliferation. A second B-HLH-LZ protein, Mxi1, is induced during terminal differentiation, and forms heterodimers with Max that also bind E-boxes but tether the mSin3 transcriptional repressor protein along with histone deacetylase thereby antagonizing Myc-dependent activation. We show that Mxi1 also antagonizes Myc by a second pathway, repression of transcription from the major c-myc promoter, P2. Repression was independent of Mxi1 binding to mSin3 but dependent on the Mxi1 LZ and COOH-terminal sequences, including putative casein kinase II phosphorylation sites. Repression targeted elements of the myc P2 promoter core (-35/+10), where it reversed transactivation by the constitutive transcription factor, USF. We show that Zn2+ induction of a stably transfected, metallothionein promoter-regulated mxi1 gene blocked the ability of serum to induce transcription of the endogenous c-myc gene and cell entry into S phase. Thus, induction of Mxi1 in terminally differentiating cells may block Myc function by repressing the c-myc gene P2 promoter, as well as by antagonizing Myc-dependent transactivation through E-boxes.

Control of cell proliferation by Myc proteins

Taken together, the available data appear to be consistent with a model in which Myc proteins function downstream of D-type cyclins and synergize with E2F proteins in the activation of the cyclin E/cdk2 kinase. This view of Myc proteins appears strikingly similar to established models for the E2F/DP family of proteins. However, it should be noted that there are clear differences and several predictions of such a model that have been critically tested for E2F proteins are still untested for Myc in this model. First, it appears that at least some target genes of Myc implicated in this process are still unknown; second, clear data from knockout cells that link p107 to Myc function are missing; and third, we are not aware of studies of tumour samples that clarify whether mutations in myc genes relieve the requirement for mutations in the cyclin D/p16 pathway.

Transcription of the transforming growth factor-beta2 gene is dependent on an E-box located between an essential cAMP response element/activating transcription factor motif and the TATA box of the gene

Transforming growth factor-beta2 (TGF-beta2) is an important regulator of cell proliferation and differentiation; however, its transcriptional regulation is not well understood. Here we report characterization of an essential E-box motif, positioned at -50/-45 between a previously described functional cAMP response element/activating transcription factor site and the TATA box of the human TGF-beta2 promoter. By site-directed mutagenesis, we demonstrate that this E-box motif is necessary for the promoter activity, not only in differentiated cells derived from embryonal carcinoma cells, but also in choriocarcinoma cells and in MCF-7 breast carcinoma cells. We also demonstrate that the transcription factors USF1 and USF2 bind to this E-box motif in vitro when nuclear extracts from each of these cell lines are examined by gel retardation assays. Moreover, using a dominant-negative USF2 protein, we show that USF proteins are critical for TGF-beta2 promoter activity in vivo. The importance of the E-box motif described in this study is supported by the presence of an E-box motif in the same position in the chicken TGF-beta2 gene promoter.

Myc-mediated apoptosis

Mammalian cells contain an intron-less myc gene, such as the rat s-myc gene and human myc L2 gene, which are expressed in rat embryo chondrocytes and human testis, respectively. Our recent findings demonstrated that s-Myc expression suppresses the growth activity and tumorigenicity of glioma cells, indicating that s-Myc acts as a negative regulator in tumor growth. In addition, we found that s-Myc overexpression can effectively induce apoptotic cell death in human and rat glioma cells without serum deprivation, which is distinct from c-Myc-mediated apoptosis.

Myc and Max: molecular evolution of a family of proto-oncogene products and their dimerization partner

The myc gene family encodes a group of transcription factors that regulate cell proliferation and differentiation. These genes are widely studied because of their importance as proto-oncogenes. Phylogenetic analyses are described here for 45 Myc protein sequences representing c-, N-, L-, S-, and B-myc genes. A gene duplication early in vertebrate evolution produced the c-myc lineage and another lineage that later gave rise to the N- and L-myc lineages by another gene duplication. Evolutionary divergence in the myc gene family corresponds closely to the known branching order of the major vertebrate groups. The patterns of sequence evolution are described for five separate highly conserved regions, and these analyses show that differential rates of sequence divergence (= mosaic evolution) have occurred among conserved motifs. Further, the closely related dimerization partner protein Max exhibits significantly less sequence variability than Myc. It is suggested that the reduced variability in max stems from natural selection acting to preserve dimerization capability with products of myc and related genes.

The myc intron-binding polypeptide associates with RFX1 in vivo and binds to the major histocompatibility complex class II promoter region, to the hepatitis B virus enhancer, and to regulatory regions of several distinct viral genes

We demonstrated that MIF-1, identified initially as a binding activity that associated with the intron I element of the c-myc gene, consists of two polypeptides, the myc intron-binding peptide (MIBP1) and the major histocompatibility class II promoter-binding protein, RFX1. Using a polyclonal antiserum directed against either oligonucleotide affinity-purified MIBP1 or a peptide derived from RFX1, we showed that MIBP1 and RFX1 are distinct molecules that associate in vivo and are both present in DNA-protein complexes at the c-myc (MIF-1) and major histocompatibility complex class II (RFX1) binding sites. We have also found that MIBP1 and RFX1 bind to a regulatory site (termed EP) required for enhancer activity of hepatitis B virus. In addition, we have identified MIF-1-like sequences within regulatory regions of several other viral genes and have shown that MIBP1 binds to these sites in cytomegalovirus, Epstein-Barr virus, and polyomavirus. We have also demonstrated that the MIF-1 and EP elements can function as silencers in the hepatocarcinoma HepG2 and the cervical carcinoma HeLa cell lines. These findings indicate that MIBP1 and EP/RFX1 can associate in vivo and may regulate the expression of several distinct cellular and viral genes.

Identification of a rel-related protein in the nucleus during the S phase of the cell cycle

The c-rel proto-oncogene encodes a 75-kDa protein (p75c-rel) which is present in the cytosol of chick embryo fibroblasts (CEF) associated with a distinct set of cellular proteins with molecular masses of 40, 115, and 124 kDa. CEF cultures arrested in S phase of the cell cycle, or enriched for G2 or mitotic cells, were examined to determine whether the expression of c-rel was altered during the cell cycle. Levels of p75c-rel remained constant in all portions of the cell cycle examined; however, a Rel-related protein with an apparent molecular mass of 64 kDa was detected in nuclei of S-phase cells. As cells enter G2, the level of this protein in the nucleus decreases. This protein reacts with antiserum generated against the carboxy terminus of p75c-rel in radioimmunoprecipitations and Western immunoblot experiments and was also detected in a Western immunoblot with antiserum generated against the first 161 amino acids of pp59v-rel. Thus, unlike other Rel/NF-kappa B family members, p64 has carboxy-terminal homology with c-Rel. The majority of peptides generated by partial proteolytic cleavage of p64 are shared with peptides generated by digestion of p75c-rel and/or pp59v-rel. We suggest that this protein represents a new member of the Rel family of transcription factors and is located in the nucleus of avian fibroblasts during S phase of the cell cycle.

Identification of a rel-related protein in the nucleus during the S phase of the cell cycle

The c-rel proto-oncogene encodes a 75-kDa protein (p75c-rel) which is present in the cytosol of chick embryo fibroblasts (CEF) associated with a distinct set of cellular proteins with molecular masses of 40, 115, and 124 kDa. CEF cultures arrested in S phase of the cell cycle, or enriched for G2 or mitotic cells, were examined to determine whether the expression of c-rel was altered during the cell cycle. Levels of p75c-rel remained constant in all portions of the cell cycle examined; however, a Rel-related protein with an apparent molecular mass of 64 kDa was detected in nuclei of S-phase cells. As cells enter G2, the level of this protein in the nucleus decreases. This protein reacts with antiserum generated against the carboxy terminus of p75c-rel in radioimmunoprecipitations and Western immunoblot experiments and was also detected in a Western immunoblot with antiserum generated against the first 161 amino acids of pp59v-rel. Thus, unlike other Rel/NF-kappa B family members, p64 has carboxy-terminal homology with c-Rel. The majority of peptides generated by partial proteolytic cleavage of p64 are shared with peptides generated by digestion of p75c-rel and/or pp59v-rel. We suggest that this protein represents a new member of the Rel family of transcription factors and is located in the nucleus of avian fibroblasts during S phase of the cell cycle.