Mechanisms of apoptosis by c-Myc

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

c-myc induces autophagy in rat 3Y1 fibroblast cells

The proto-oncogene c-myc is a multifunctional gene that regulates cell division, cell growth, and apoptosis. Here we report a new function of c-myc: induction of autophagy. Autophagy is a bulk degradation system for intracellular proteins. Autophagy proceeds with characteristic morphologies, which begins with the formation of a double-membrane structure called the autophagosome surrounding a portion of the cytoplasm, after which its outer membrane then fuses with the lysosomal membrane to become an autolysosome. Autophagosomes and autolysosomes are generally called autophagic vacuoles. When c-Myc protein was overexpressed in rat 3Y1 fibroblasts or when the chimeric protein c-MycER was activated by estrogen, the number of autophagic vacuoles in cells increased significantly. The formation of autophagic vacuoles induced by c-Myc was completely blocked by a specific inhibitor of autophagosome formation, 3-methyladenine. A c-Myc mutant lacking Myc Box II induced neither apoptosis nor oncogenic transformation, but still stimulated autophagy. An inhibitor of caspases suppressed apoptosis but not autophagy. These results suggest that the autophagy caused by c-myc is not due to the apoptosis or tumorigenesis induced by c-myc. Taken together, our results suggest that the induction of autophagy is a novel function of c-myc.

Higher grade transformation of follicular lymphoma: phenotypic tumor progression associated with diverse genetic lesions

Higher grade histological transformation of follicular lymphoma (FL) to more aggressive diffuse large B-cell lymphomas (DLBCL) occurs in 10-60% of the cases. Review of the current knowledge of genetic and molecular alterations associated with the higher grade transformation of FCL suggests that the process that leads to clinically and phenotypically similar end-point can occur by functionally diverse genetic lesions. The most commonly identified genetic alterations associated with the FCL transformation are TP53 gene mutations, inactivation of CDKN2A and CDKN2B genes and deregulation of the C-MYC gene. These lesions affect different aspects of normal cell physiology (apoptosis, cell cycle control, and proliferation) and are potential targets for gene-specific therapies.

hMad4, c-Myc endogenous inhibitor, induces a replicative senescence-like state when overexpressed in human fibroblasts

Mad family proteins have an antagonistic action on Myc-dependent cell proliferation and transformation. We isolated a human cDNA clone, human Mad4 (hMad4), encoding a polypeptide of 209 amino acid residues, exhibiting 90% identity with mouse Mad4. Northern blot analysis shows that hMad4 probe hybridizes to a 3.8 kb message; its expression is highest in quiescent human WI38 fibroblasts. Among tissues, hMad4 mRNA is most abundant in brain, lung, and muscle. Consistent with other members of the Mad family, hMad4 can repress the transactivation activity of Myc/Max heterodimers on an E-box chloramphenicol acteyl transferase (CAT) reporter plasmid; inhibition of both proliferation and clonogenic formation of hMad4-infected cells correlates with the in vitro reporter repression. Moreover, infection of young human fibroblasts induces a replicative senescence-like state. This phenotype was accompanied by s-beta-galactosidase and PAI-1 expression. These results suggest that hMad4 might be an important regulator of replicative senescence in human cells.

Diffuse large B-cell lymphoma: insights gained from gene expression profiling

Analysis of global gene expression with DNA microarrays has great potential to improve the understanding of tumorigenesis advance tumor diagnosis and classification, and affect cancer treatment. Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin's lymphoma. However, we now realize that the disease is extremely heterogeneous. This review summarizes the progress in understanding DLBCL that has been made as a result of the application of gene expression profiling.

Skp2 regulates Myc protein stability and activity

Myc is an oncoprotein transcription factor that plays a prominent role in cancer. Like many transcription factors, Myc is an unstable protein that is destroyed by ubiquitin (Ub)-mediated proteolysis. Here, we report that the oncoprotein and Ub ligase Skp2 regulates Myc ubiquitylation and stability. Because of the growing number of Ub ligases that function as transcriptional coactivators, we speculated that Skp2 might also regulate Myc's transcriptional activity. Consistent with this model, we also show that Skp2 is a transcriptional coactivator for Myc, recognizing an essential element within the Myc activation domain and activating Myc target genes. These data suggest that Skp2 functions to connect Myc activity and destruction, and reveal an unexpected oncoprotein connection that may play an important role in controlling cell growth in normal and cancer cells.

Early modifications in the expression of mitogen-activated protein kinase (MAPK/ERK), stress-activated kinases SAPK/JNK and p38, and their phosphorylated substrates following focal cerebral ischemia

Focal ischemia induced by middle cerebral artery occlusion (MCAO) to adult rats results in necrosis at the infarct core and activation of complex signal pathways for cell death and cell survival in the penumbra. Upstream from the cell death promoters and executioners are several kinases that, once activated by phosphorylation, may activate several transcription factor substrates involved in cell death and cell survival. In the present study we examined, by immunohistochemistry, the expression of phosphorylated (active) mitogen-activated protein kinase, extracellular signal-regulated kinase (MAPK/ERK), stress-activated protein kinase (SAPK), c-Jun N-terminal kinase (JNK) and p-38 kinase at early stages (1-4 h) following 1 h of MCAO in the rat. The expression of phosphorylation-dependent, active transcription substrates of these kinases, including cyclic AMP-responsive element-binding protein (CREB) Alk-1, ATF-2, c-Myc and c-Jun was examined at early stages following reperfusion. Increased nuclear phosphorylated SAPK/JNK (SAPK/JNK-P) and c-Jun-PSer63, and reduced CREB-P, occurred in the infarct core at 1 h following reperfusion, suggesting increased phosphorylated SAPK/JNK and c-JunSer63, together with decreased phospho-CREB associated with cell death in the infarct core. However, increased cytoplasmic expression of MAPK/ERK-P, SAPK/JNK-P, p38-P, CREB-P, Elk-1-P, c-Myc-P, ATF-2-P and c-Jun-P occurred in the region bordering the infarct core (penumbra) at 4 h following reperfusion. This indicates that different signals converge in the cytoplasm of neurons located at the borders of the infarct at 4 h following reperfusion, revealing the struggle of death promoters and life facilitators at the penumbra. Whether phosphorylated kinases and specific substrates participate in promoting cell death or survival in the penumbra probably depends on additional factors and on the interaction with other proteins.

N-Myc overexpression leads to decreased beta1 integrin expression and increased apoptosis in human neuroblastoma cells

Neuroblastoma is a childhood tumor thought to arise through improper differentiation of neural crest cells. Increased N-Myc expression in neuroblastoma indicates highly malignant disease and poor patient prognosis. N-myc enhances cell growth, insulin-like growth factor type I receptor (IGF-IR) expression, and tumorigenicity in combination with Bcl-2. Despite these effects, N-Myc overexpression in SHEP neuroblastoma cells (SHEP/N-Myc cells) increases serum-withdrawal and mannitol-induced apoptosis. Although we have previously shown a protective effect of IGF-I in SHEP cells, in SHEP/N-Myc cells IGF-I rescue from mannitol-induced apoptosis is prevented. N-Myc overexpression has little effect on IGF-IR signaling pathways, but results in increased Akt phosphorylation when Bcl-2 is coexpressed. A loss of integrin-mediated adhesion promotes apoptosis in many systems. SHEP/N-Myc cells have dramatically less beta1 integrin expression than control cells, consistent with previous reports. beta1 integrin expression is decreased in more tumorigenic neuroblastoma cells lines, including IMR32 and SH-SY5Y cells. Reintroduction of beta1 integrin into the N-Myc-overexpressing cells prevents mannitol-mediated apoptosis. We speculate that N-Myc repression of beta1 integrin expression leads to a less differentiated phenotype, resulting in increased growth and tumorigenesis if properly supported or apoptosis if deprived of growth sustaining molecules.

Induction of Mad expression leads to augmentation of insulin gene transcription

Insulin gene transcription is critical for the maintenance of pancreatic beta-cell differentiation and insulin production. In this study, we found that the basic helix-loop-helix transcription factor Mad, which usually acts as a repressor to c-Myc, enhances insulin gene transcription. In isolated rat islets adenoviral overexpression of Mad augmented insulin mRNA expression and insulin protein content, as well as glucokinase and GLUT2 mRNA expression. Also, Mad overexpression upregulated insulin promoter activity in beta-cell-derived cell lines, MIN6 and betaTC1, as well as in non-insulin producing liver cell line, HepG2. Mad overexpression in rat islets enhanced PDX-1 expression and its DNA binding activity. We found that Mad mediated increased PDX-1 expression by an E-box dependent transcriptional regulation of the PDX-1 gene. That the effects of Mad on insulin expression were mediated through PDX-1 was further substantiated by studies showing inhibition of insulin promoter activation by Mad in the presence of mutated PDX-1 binding site. Although Mad functions as a negative regulatory factor for multiple target genes, these studies establish the fact that Mad can also function as a positive regulatory factor for insulin gene transcription. Such regulation of insulin expression by Mad with modulation of PDX-1 expression and DNA binding activity could offer useful therapeutic and/or experimental tools to promote insulin production in appropriate cell types.

A large scale genetic analysis of c-Myc-regulated gene expression patterns

The myc proto-oncogenes encode transcriptional regulators whose inappropriate expression is correlated with a wide array of human malignancies. Up-regulation of Myc enforces growth, antagonizes cell cycle withdrawal and differentiation, and in some situations promotes apoptosis. How these phenotypes are elicited is not well understood, largely because we lack a clear picture of the biologically relevant downstream effectors. We created a new biological system for the optimal profiling of Myc target genes based on a set of isogenic c-myc knockout and conditional cell lines. The ability to modulate Myc activity from essentially null to supraphysiological resulted in a significantly increased and reproducible yield of targets and revealed a large subset of genes that respond optimally to Myc in its physiological range of expression. The total extent of transcriptional changes that can be triggered by Myc is remarkable and involves thousands of genes. Although the majority of these effects are not direct, many of the indirect targets are likely to have important roles in mediating the elicited cellular phenotypes. Myc-activated functions are indicative of a physiological state geared toward the rapid utilization of carbon sources, the biosynthesis of precursors for macromolecular synthesis, and the accumulation of cellular mass. In contrast, the majority of Myc-repressed genes are involved in the interaction and communication of cells with their external environment, and several are known to possess antiproliferative or antimetastatic properties.

Promyelocytic leukemia protein sensitizes tumor necrosis factor alpha-induced apoptosis by inhibiting the NF-kappaB survival pathway

The promyelocytic leukemia protein (PML) is a growth/tumor suppressor essential for induction of apoptosis by diverse apoptotic stimuli. The mechanism by which PML regulates cell death remains unclear. In this study we found that ectopic expression of PML potentiates cell death by apoptosis in the tumor necrosis factor alpha (TNFalpha)-resistant cell line U2OS and other cell lines. Treatment with TNFalpha significantly sensitized these cells to apoptosis in a p53-independent manner. PML/TNFalpha-induced cell death is associated with DNA fragmentation, activation of caspase-3, -7, and -8, and degradation of DNA fragmentation factor/inhibitor of CAD. PML/TNFalpha-induced cell death could be blocked by the caspase-8 inhibitors CrmA and c-FLIP but not by Bcl-2. These findings indicate that this cell death event is initiated through the death receptor-dependent apoptosis pathway. PML is a transcriptional repressor of NF-kappaB by interacting with RelA/p65 and prevents its binding to the cognate enhancer through the C terminus. Coimmunoprecipitation and double-color immunofluorescence staining demonstrated that PML physically interacts with RelA/p65 in vivo and the two proteins colocalized at the endogenous levels. Overexpression of NF-kappaB rescued cell death induced by PML/TNFalpha. Furthermore, PML(-/-) mouse embryo fibroblasts are more resistant to TNFalpha-induced apoptosis. Together this study defines a novel mechanism by which PML induces apoptosis through repression of the NF-kappaB survival pathway.

Gene profiling in spinal cord injury shows role of cell cycle in neuronal death

Spinal cord injury causes secondary biochemical changes leading to neuronal cell death. To clarify the molecular basis of this delayed injury, we subjected rats to spinal cord injury and identified gene expression patterns by high-density oligonucleotide arrays (8,800 genes studied) at 30 minutes, 4 hours, 24 hours, or 7 days after injury (total of 26 U34A profiles). Detailed analyses were limited to 4,300 genes consistently expressed above background. Temporal clustering showed rapid expression of immediate early genes (30 minutes), followed by genes associated with inflammation, oxidative stress, DNA damage, and cell cycle (4 and 24 hours). Functional clustering showed a novel pattern of cell cycle mRNAs at 4 and 24 hours after trauma. Quantitative reverse transcription polymerase chain reaction verified mRNA changes in this group, which included gadd45a, c-myc, cyclin D1 and cdk4, pcna, cyclin G, Rb, and E2F5. Changes in their protein products were quantified by Western blot, and cell-specific expression was determined by immunocytochemistry. Cell cycle proteins showed an increased expression 24 hours after injury and were, in part, colocalized in neurons showing morphological evidence of apoptosis. These findings suggest that cell cycle-related genes, induced after spinal cord injury, are involved in neuronal damage and subsequent cell death.

Apoptosis and lung cancer: a review

It is important to understand the molecular events that contribute to drug-induced apoptosis, and how tumors evade apoptotic death. Defects in apoptosis are implicated in both tumorigenesis and drug resistance, and these defects are cause of chemotherapy failures. These studies should explain the relationship between cancer genetics and treatment sensitivity, and should enable a more rational approach to anticancer drug design and therapy. Lung cancer is a major cause of cancer deaths throughout the world. Small cell lung carcinoma (SCLC) and non-small cell lung carcinoma (NSCLC) represent the two major categories of lung cancer that differ in their sensitivity to undergo apoptosis. The role of apoptosis regulation in lung cancer with major focus on the differential sensitivities of the major subtypes is reviewed.

Apoptosis in Huntington's disease

Huntington's disease (HD) is an autosomal dominant, fatal disorder. Patients display increasing motor, psychiatric and cognitive impairment and at autopsy, late-stage patient brains show extensive striatal (caudate and putamen), pallidal and cortical atrophy. The initial and primary target of degeneration in HD is the striatal medium spiny GABAergic neuron, and by end stages of the disease up to 95% of these neurons are lost [J. Neuropathol. Exp. Neurol. 57 (1998) 369]. The disease is caused by an elongation of a polyglutamine tract in the N-terminal of the huntingtin gene, but it is not known how this mutation leads to such extensive, but selective, cell death [Cell 72 (1993) 971]. There is substantial evidence from in vitro studies that connects apoptotic pathways and apoptosis with the mutant protein, and theories linking apoptosis to neuronal death in HD have existed for several years. Despite this, evidence of apoptotic neuronal death in HD is scarce. It may be that the processes involved in apoptosis, rather than apoptosis per se, are more important for HD pathogenesis. Upregulation of the proapoptotic proteins could lead to cleavage of huntingtin and as recent data has shown, the consequent toxic fragment may itself elicit toxic effects on the cell by disrupting transcription. In addition, the increased levels of proapoptotic proteins could contribute to slowly developing cell death in HD, selective for the striatal medium spiny GABAergic neurons and later spreading to other areas. Here we review the evidence supporting these mechanisms of pathogenesis in HD.

c-Myc is essential but not sufficient for c-Myb-mediated block of granulocytic differentiation

The c-myb proto-oncogene plays a central role in hematopoiesis and encodes a major translational product of 75 kDa. c-Myb is highly expressed in immature hematopoietic cells, and its expression is down-regulated during terminal differentiation. Deregulated expression of c-Myb has been shown to block terminal differentiation of hematopoietic cells. Here we have studied the mechanism of action and the nature of target genes through which c-Myb mediates the block in differentiation of 32Dcl3 murine myeloid cells. We show that the ectopic overexpression of c-Myb in 32Dcl3 cells results in the overexpression of c-Myc. However, enforced expression of c-Myc in 32Dcl3 cells did not alter the normal pattern of differentiation. In addition, expression of dominant-negative mutants of c-Myc relieved c-Myb-mediated block in differentiation. These results led us to conclude that c-myc is a target gene of c-Myb and activation of the c-myc gene is a necessary event in Myb-mediated transformation. However, c-Myc expression alone is inadequate to elicit the phenotypic effects seen with Myb-mediated block in differentiation of myeloid cells, suggesting that activation of additional transcriptional targets by c-Myb plays a critical role in this process.

c-Myc-induced extrachromosomal elements carry active chromatin

Murine Pre-B lymphocytes with experimentally activated MycER show both chromosomal and extrachromosomal gene amplification. In this report, we have elucidated the size, structure, and functional components of c-Myc-induced extrachromosomal elements (EEs). Scanning electron microscopy revealed that EEs isolated from MycER-activated Pre-B+ cells are an average of 10 times larger than EEs isolated from non-MycER-activated control Pre-B- cells. We demonstrate that these large c-Myc-induced EEs are associated with histone proteins, whereas EEs of non-MycER-activated Pre B- cells are not. Immunohistochemistry and Western blot analyses using pan-histone-specific, histone H3 phosphorylation-specific, and histone H4 acetylation-specific antibodies indicate that a significant proportion of EEs analyzed from MycER-activated cells harbors transcriptionally competent and/or active chromatin. Moreover, these large, c-Myc-induced EEs carry genes. Whereas the total genetic make-up of these c-Myc-induced EEs is unknown, we found that 30.2% of them contain the dihydrofolate reductase (DHFR) gene, whereas cyclin C (CCNC) was absent. In addition, 50% of these c-Myc-activated Pre-B+ EEs incorporated bromodeoxyuridine (BrdU), identifying them as genetic structures that self-propagate. In contrast, EEs isolated from non-Myc-activated cells neither carry the DHFR gene nor incorporate BrdU, suggesting that c-Myc deregulation generates a new class of EEs.

Stability of Nucleolin protein as the basis for the differential expression of Nucleolin mRNA and protein during serum starvation

Nucleolin is a nucleolar phosphoprotein that plays a direct role in ribosome biogenesis. Our aim was to determine how its activity as a growth-promoting factor is coordinated with, if not regulated by, the cell cycle machinery. In serum starting and then rescuing these cells with serum, we found that the protein level did not drop in the same way that the mRNA level did. In addition, although the mRNA level rises during the immediate period during serum rescue, the protein level remained the same. We found that the protein level was maintained after serum starvation as a result of high stability. There was no selective enhanced translation of the remaining amount of Nucleolin mRNA. With regard to the constancy in protein level despite the rise in mRNA level during serum rescue, there is no concomitant degradation of newly synthesized or old protein and synthesis of new protein. Because Nucleolin has been documented to bind mRNA, APP mRNA being one among them, we propose a autoregulatory model in which Nucleolin regulates the translation of Nucleolin mRNA, such that during a period of excess protein, translation is inhibited through direct binding of Nucleolin protein to its mRNA.

The allelic loss of chromosome 3p25 with c-myc gain is related to the development of clear-cell renal cell carcinoma

To explore the role of allelic losses at 3p25 and genetic alterations of chromosome 8, we investigated the relationships between genetic alterations in these chromosomal regions and clinicopathologic findings (such as tumor size and grade), by employing fluorescence in situ hybridization (FISH). Fifty Japanese clear-cell renal cell carcinomas (RCCs) were examined by dual-color FISH using cosmid DNA probes for 3p25.1-25.3 combined with probes for chromosome 3 centromere, 8p12, 8p21.1, 8p21.3, 8p22 and 8q24.12-24.13 (c-myc), and chromosome 8 centromere. Deletion at 3p25.1-25.3 was detected in 38 patients (76%), while 8p12 deletion, 8p21.1 deletion, 8p21.3 deletion, 8p22 deletion and c-myc gain were detected in 23 (46%), 25 (50%), 25 (50%), 25 (50%), and 20 patients (40%), respectively. There was a significant correlation between 8p21.1 deletion, 8p21.3 deletion and 8p21.1 deletion with c-myc gain and tumor grade (p = 0.04, 0.04 and 0.02, respectively). Deletions at 8p21.1 and 8p21.3 with 3p deletion were significantly related to tumor grade; the statistical significance was identical to that of sole 8p deletion with tumor grade. The deletion at 3p25.1-25.3 with c-myc gain showed a significant correlation with tumor size, indicating an association with tumor progression. Our results suggest that the allelic loss of chromosome 3p25 with c-myc gain is related to the development of clear-cell RCC.

The transcriptional repressor gene Mad3 is a novel target for regulation by E2F1

Mad family proteins are transcriptional repressors that antagonize the activity of the c- Myc proto-oncogene product. Mad3 is expressed specifically during the S-phase of the cell cycle in both proliferating and differentiating cells, suggesting that its biological function is probably linked to processes that occur during this period. To determine the mechanisms that regulate the cell-cycle-specific transcription of Mad3, we used reporter gene assays in stably transfected fibroblasts. We show that the activation of Mad3 at the G1-S boundary is mediated by a single E2F (E2 promoter binding factor)-binding site within the 5'-flanking region of the gene. Mutation of this element eliminated transcriptional activation at S-phase, suggesting that the positively acting E2F proteins play a role in Mad3 regulation. Using electrophoretic mobility-shift assays and chromatin immunoprecipitation, we show that E2F1 binds to the Mad3 5'-flanking region both in vitro and in vivo. We thus identify Mad3 as a novel transcriptional target of E2F1.

Guanine nucleotide exchange factor, Tiam1, directly binds to c-Myc and interferes with c-Myc-mediated apoptosis in rat-1 fibroblasts

The transcription factor c-Myc is important for the control of cell growth, cell cycle progression, neoplasia, and apoptotic cell death. Recently, c-Myc-binding proteins, which bind either to the N-terminal domain or the C-terminal domain of c-Myc, have been proposed as the key molecules to realize the mechanisms of these multiple c-Myc functions. We report in the present study on another protein, Tiam1, which is a specific guanine nucleotide exchange factor of Rac1 and which binds to c-Myc and modulates several of its biological functions. We were able to detect the direct binding and in vivo association between c-Myc and Tiam1. The necessary role in this interaction of the Myc box II of c-Myc was revealed in the cell extracts. The additional discovery of the intranuclear localization of Tiam1 in Rat1 cells and in neuronal cells of the mouse brain suggests this interaction may occur in the nucleus. Overexpression of Tiam1 repressed the luciferase activity of c-Myc and also inhibited the c-Myc apoptotic activity through this protein-protein interaction. Taken together, we concluded that Tiam1 is another c-Myc regulator, working in the nuclei to control c-Myc-related apoptosis.

Interactions of the DNA mismatch repair proteins MLH1 and MSH2 with c-MYC and MAX

MSH2 and MLH1 have a central role in correcting mismatches in DNA occurring during DNA replication and have been implicated in the engagement of apoptosis induced by a number of cytotoxic anticancer agents. The function of MLH1 is not clearly defined, although it is required for mismatch repair (MMR) and engagement of apoptosis after certain types of DNA damage. In order to identify other partners of MLH1 that may be involved in signalling MMR or apoptosis, we used human MLH1 in yeast two-hybrid screens of normal human breast and ovarian cDNA libraries. As well as known partners of MLH1 such as PMS1, MLH3 and MBD4, we identified the carboxy terminus of the human c-MYC proto-oncogene as an interacting sequence. We demonstrate, both in vitro by yeast two-hybrid and GST-fusion pull-down experiments, as well as in vivo by coimmunoprecipitation from human tumour cell extracts, that MLH1 interacts with the c-MYC protein. We further demonstrate that the heterodimeric partner of c-MYC, MAX, interacts with a different MMR protein, MSH2, both in vitro and in vivo. Using an inducible c-MYC-ER fusion gene, we show that elevated c-MYC expression leads to an increased HGPRT mutation rate of Rat1 cells and an increase in the number of frameshift mutants at the HGPRT locus. The effect on HGPRT mutation rate is small (2-3-fold), but is consistent with deregulated c-MYC expression partially inhibiting MMR activity.

Bcl-xL expression interferes with the effects of L-glutamine supplementation on hybridoma cultures

While feeding protocols and ectopic expression of anti-apoptotic genes have been used to improve the viability of hybridoma cell lines, the effect of the expression levels of survival genes on the behavior of hybridomas following nutrient supplementation is unknown. In this study, we compared the behavior of the Sp2/0-Ag14 hybridoma (Bcl-xL(low)) and the P3x63-Ag8.653 myeloma (Bcl-xL(high)) following culture supplementation with the amino acid L-glutamine (L-Gln). Our data revealed that L-Gln addition substantially increased Sp2/0-Ag14 cell viability and total cell density, concomitant with a decrease in the rate of cell death. This effect was not seen when other amino acids or D-glucose (D-Glc) replaced L-Gln. The improvement in the culture behavior of Sp2/0-Ag14 cells was attributed to a reduction in the rate of accumulation of apoptotic cells. On the other hand, L-Gln supplementation had only a limited effect on the growth of the P3x63-Ag8.653 cells. Interestingly, Sp2/0-Ag14 cells over-expressing Bcl-xL showed a culture behavior upon L-Gln complementation that was similar to the P3x63-Ag8.653 myeloma. These results suggest that the anti-apoptotic gene expression profile of hybridoma cells can markedly impact on the beneficial effects afforded by nutrient supplementation.

Molecular cloning of the mouse AMY-1 gene and identification of the synergistic activation of the AMY-1 promoter by GATA-1 and Sp1

We have reported that a novel c-Myc binding protein, AMY-1, stimulated the transcription activity of c-Myc and was translocated from the cytoplasm to the nucleus in a c-Myc-dependent manner. AMY-1 works as an inducer of human K562 cell differentiation upon induction of AraC. To characterize the expression or functional importance of AMY-1, the genomic DNA of mouse AMY-1 was cloned and characterized. Both mouse and human genomic DNAs, the latter of which was retrieved from a human DNA database, comprise five exons spanning about 11 kb. To characterize the promoter of the mouse AMY-1 gene, a series of deletion constructs of the region upstream of the first ATG was linked to the luciferase gene, and their luciferase activities were measured in human HeLa and K562 cells. The results showed that Sp1 was essential for AMY-1 expression in both cell lines and that GATA-1 is also necessary in K562 cells. Sp1 in both cell lines and GATA-1 only in K562 cells were identified as proteins binding to these sites by a mobility shift assay. Furthermore, it was found that GATA-1 stimulated AMY-1 expression synergistically with Sp1 in ectopically expressed insect cells and that both proteins were associated in K562 cells.

c-MYC apoptotic function is mediated by NRF-1 target genes

A detailed understanding of the signaling pathways by which c-Myc elicits apoptosis has proven elusive. In the current study, we have evaluated whether the activation of the mitochondrial apoptotic signaling pathway is linked to c-Myc induction of a subset of genes involved in mitochondrial biogenesis. Cytochrome c and other nuclear-encoded mitochondrial genes are regulated by the transcription factor nuclear respiratory factor-1 (NRF-1). The consensus binding sequence (T/C)GCGCA(C/T)GCGC(A/G) of NRF-1 includes a noncanonical CA(C/T)GCG Myc:MAX binding site. In this study, we establish a link between the induction of NRF-1 target genes and sensitization to apoptosis on serum depletion. We demonstrate, by using Northern analysis, transactivation assays, and in vitro and in vivo promoter binding assays that cytochrome c is a direct target of c-Myc. Like c-Myc, NRF-1 overexpression sensitizes cells to apoptosis on serum depletion. We also demonstrate that selective interference with c-Myc induction of NRF-1 target genes by using a dominant-negative NRF-1 prevented c-Myc-induced apoptosis, without affecting c-Myc-dependent proliferation. These results suggest that c-myc expression leads to mitochondrial dysfunction and apoptosis by deregulating genes involved in mitochondrial function.

Early nuclear exclusion of the transcription factor max is associated with retinal ganglion cell death independent of caspase activity

We examined the behavior of the transcription factor Max during retrograde neuronal degeneration of retinal ganglion cells. Using immunohistochemistry, we found a progressive redistribution of full-length Max from the nucleus to the cytoplasm and dendrites of the ganglion cells following axon damage. Then, the axotomized cells lose all their content of Max, while undergoing nuclear pyknosis and apoptotic cell death. After treatment of retinal explants with either anisomycin or thapsigargin, the rate of nuclear exclusion of Max accompanied the rate of cell death as modulated by either drug. Treatment with a pan-caspase inhibitor abolished both TUNEL staining and immunoreactivity for activated caspase-3, but did not affect the subcellular redistribution of Max immunoreactivity after axotomy. The data show that nuclear exclusion of the transcription factor Max is an early event, which precedes and is independent of the activation of caspases, during apoptotic cell death in the central nervous system.

The role of human papillomavirus oncoproteins E6 and E7 in apoptosis

The oncogenic potential of 'high risk' human papillomaviruses can be mainly attributed to two small proteins called E6 and E7. Even these oncoproteins have a low molecular size, they are highly promiscuous and are capable to interact with a whole variety of host cellular regulator proteins to elicit cellular immortalization and ultimately complete malignant transformation. To avoid reiterations in summarizing the biochemical and molecular biological properties of E6/E7 in terms of their influence on cell cycle control, the present review is mainly an attempt to describe some regulatory principles by which human papillomavirus (HPV) oncoproteins can interfere with apoptosis in order to escape immunological surveillance during progression to cervical cancer. The models derived from these basic cellular and molecular studies are relevant to our understanding of HPV-induced carcinogenesis. Conversely, experimental procedures aimed at relieving apoptosis resistance, can facilitate the eradication of immunologically suspicious cells and may prevent the accumulation of cervical intraepithelial cell abnormalities in future prophylactic or therapeutic approaches.

Expression of c-myc and bcl-2 in primary and advanced cutaneous melanoma

Apoptosis is an important co-factor in the pathogenesis of a plethora of malignancies. Enhanced c-myc activation can result either in proliferation or apoptosis. Coexpression with antiapoptotic bcl-2, which abrogates the apoptotic function of c-myc might lead to an enormous growth advantage of cells. In order to elucidate the role of c-myc and bcl-2 as well as the coexpression of both genes in human melanoma, their expression was studied in four samples of normal skin (SK), 15 surgical margins (SM), 20 benign melanocytic nevi (MN), 20 primary melanomas (MM), and 30 melanoma metastases (MMET) by RT-PCR. These results were compared with immunohistochemistry (IH) in 7 SK, 7 SM, 26 MN, 50 MM, and 34 MMET. Similar results were found with both methods. However, MMET expressed c-myc (PCR 28/30, IH 23/34) as well as bcl-2 (PCR 27/30, IH 24/34) more frequently. Primary melanomas showed a similar expression pattern as SM and nevi. Moreover, in contrast to SK, SM, MN, and MM coexpression of bcl-2 and c-myc was found more frequently in MMET (PCR 25/30, p < 0.01, IH 19/34, p < 0.01). These results indicate that coexpression of c-myc and bcl-2 appears to be associated with advanced melanoma and contributes to the malignant phenotype.

Loss of expression of death-inducing signaling complex (DISC) components in lung cancer cell lines and the influence of MYC amplification

We have previously reported that the key apoptosis related gene caspase 8 (CASP8) is frequently silenced in small cell lung cancer (SCLC) tumors and cell lines usually, but not always, by aberrant promoter methylation. Because CASP8 is a key component of the death-inducing signaling complex (DISC) when specific death receptors (including DR4, DR5, FAS) are activated by their specific ligands (TRAIL/FASL), we examined expression of the components of the DISC complex in lung cancer cell lines. MYC family members are frequently amplified (MYC+ve) in SCLC, and MYC is a potent inducer of apoptosis. We examined 34 SCLC lines (12 of which were MYC+ve) and 22 NSCLC lines. CASP8 gene expression was frequently lost (79%) at message and protein levels in SCLC but not in non-SCLC (NSCLC). MYC amplification was present in 45% of SCLC cell lines, which had lost CASP8 expression, but not in any of the CASP8 positive lines. The frequency of CASP8 loss was significantly higher in MYC+ve SCLC compared to MYC-ve SCLC or in NSCLC. Analyses of other DISC components showed significantly higher rates of loss of expression of CASP10, DR5, FAS and FASL in SCLC compared to NSCLC. The loss of expression of proapoptotic DISC components was significantly higher in MYC+ve SCLC cell lines and these lines were completely resistant to TRAIL. Expression of CASP10 (a caspase closely related to CASP8) was frequently absent at the protein level in both SCLC and NSCLC lines. Expression of c-FLIP (proteolytically inactive homolog of CASP8) was inversely related to expression of CASP8. Our major conclusions are: (a) The death receptor pathway is differently inactivated at multiple levels in lung cancer cell lines; and (b) MYC amplification in SCLC is associated with inactivation of most components of the DISC complex, with resistance to TRAIL and with expression of c-FLIP. These findings may have considerable clinical and therapeutic implications.

c-Myc exerts a protective function through ornithine decarboxylase against cellular insults

c-Myc is known to control cell proliferation and apoptosis, and much effort has been focused on elucidating the mechanisms by which c-Myc works. In this study, we show that c-Myc expression is induced by many cellular insults, including cisplatin, doxorubicin, paclitaxel, 5-flourouracil, H(2)O(2), and radiation, and the enhanced expression of c-Myc protects against cell death caused by these cellular insults through ornithine decarboxylase (ODC) induction. To investigate the cellular protective role of c-Myc, we constructed a stable transfectant of ODC, one of the many transcriptional targets of c-Myc in cells, and found that enhanced expression of ODC inhibited cell death induced by cellular insults such as cisplatin, H(2)O(2,) and radiation. We also found that cisplatin activated nuclear factor-kappaB, and this subsequently induced c-Myc expression, resulting in the blocking of apoptosis through ODC induction. The results herein, therefore, strongly suggest another role for c-Myc in a stress-response function; that is, it promotes cell survival under stressful conditions.

Intracellular localization and determination of a nuclear localization signal of the core protein of dengue virus

In dengue virus (DEN) particles, the core protein is a structural protein of the nucleocapsid. The core protein is known to be present in the nucleus of DEN-infected cells but there have been conflicting reports as to whether it is also present in the nucleolus. To clarify this, the intracellular location of the core protein was examined using a monoclonal antibody, 15B11, which was produced in this study. Immunofluorescence staining with this antibody demonstrated that the core protein first appeared in the cytoplasm and then in the nuclei and nucleoli of infected cells. Nuclear localization of the core protein was determined to be independent of other DEN proteins, since recombinant core proteins still entered the nuclei and nucleoli of cells transfected with only the core protein gene. Three putative nuclear localization signal motifs have been predicted to be present on the core protein. Deletion of the first one (KKAR), located at aa 6-9, and mutation of the second one (KKSK), located at aa 73-76, did not eliminate the nuclear localization property of the core protein. The third motif with a bipartite structure, RKeigrmlnilnRRRR, located at aa 85-100, was determined to be responsible for the nuclear localization of the core protein, since the core protein without this motif was located exclusively in the cytoplasm of DEN-infected cells and that this motif mediated nuclear localization of a normally cytoplasmic protein.

Pag, a putative tumor suppressor, interacts with the Myc Box II domain of c-Myc and selectively alters its biological function and target gene expression

The highly conserved Myc Box II (MBII) domain of c-Myc is critically important for transformation and transcriptional regulation. A yeast two-hybrid screen identified Pag as a MBII-interacting protein. Pag, a member of the peroxiredoxin family, has been reported previously to bind to and inhibit the cytostatic properties of the c-Abl oncoprotein. We now show that Pag promotes increased cell size and confers a proapoptotic phenotype, two hallmark features of ectopic c-Myc overexpression. Pag and c-Myc also confer resistance to oxidative stress, a previously unrecognized property of the latter protein. In contrast, Pag inhibits tumorigenesis by c-Myc-overexpressing fibroblasts and causes a broad but selective loss of c-Myc target gene regulation. Pag is therefore an MBII-interacting protein that can either mimic or enhance some of the c-Myc properties while at the same inhibiting others. These features, along with the previously identified interaction with c-Abl, provide support for the idea that Pag functions as a tumor suppressor.

Obstructive nephropathy and renal fibrosis

Interstitial fibrosis has a major role in the progression of renal diseases. Several animal models are available for the study of renal fibrosis. The models of aminonucleoside-induced nephrotic syndrome, cyclosporin nephrotoxicity, and passive Heyman nephritis are characterized by molecular and cellular events similar to those that occur in obstructive nephropathy. Additionally, inhibition of angiotensin-converting enzyme exerts salutary effects on the progression of renal fibrosis in obstructive nephropathy. Unilateral ureteral obstruction (UUO) has emerged as an important model for the study of the mechanisms of renal fibrosis and also for the evaluation of the impact of potential therapeutic approaches to ameliorate renal disease. Many quantifiable pathophysiological events occur over the span of 1 wk of UUO, making this an attractive model for study. This paper reviews some of the ongoing studies that utilized a rodent model of UUO. Some of the findings of the animal model have been compared with observations made in patients with obstructive nephropathy. Most of the evidence suggests that the rodent model of UUO is reflective of human renal disease processes.

Synergistic and opposing regulation of the stress-responsive gene IEX-1 by p53, c-Myc, and multiple NF-kappaB/rel complexes

NF-kappaB/rel proteins, tumor suppressor p53, and oncogene c-Myc are critical transcription factors involved in coordinating cellular decision-making events in response to external stimuli. Consensus sequences for binding these three transcription factors are found in the promoter region of IEX-1 (Immediate Early response gene X-1) gene that can either suppress or induce apoptosis in a cell- and stimulus-dependent manner. Utilizing an electrophoretic mobility shift assay (EMSA) and a promoter/reporter assay, we show that the NF-kappaB/rel consensus sequence in the IEX-1 promoter is specifically bound and activated by multiple NF-kappaB/rel complexes in descending order p65-c-rel-->p65-50-->p50-50. Interestingly, NF-kappaB/rel-mediated activation of IEX-1 expression was synergized by p53, but strongly inhibited by c-Myc in a dose-dependent fashion. Moreover, the ability of c-Myc to inhibit IEX-1 expression requires the presence of functional p53, which may partially contribute to the varying effects of p53 on IEX-1 expression in different cells. In support of coordinated regulation of IEX-1 expression by these three transcription factors in vivo, binding of endogenous p53, c-Myc and NF-kappaB/rel proteins, including p50, p65 and c-rel, to the IEX-1 promoter was demonstrated in living cells by chromatin immunoprecipitation using specific antibodies. The study reveals a novel integrative regulation of specific gene expression by NF-kappaB/rel, p53 and c-Myc transcription factors.

The role of caspases in cryoinjury: caspase inhibition strongly improves the recovery of cryopreserved hematopoietic and other cells

Cryopreserved cells and tissues are increasingly used for stem cell transplantation and tissue engineering. However, their freezing, storage, and thawing is associated with severe damage, suggesting the need for better cryopreservation methods. Here, we show that activation of caspase-3 is induced during the freeze-thaw process. Moreover, we demonstrate that prevention of caspase activation by the caspase inhibitor zVAD-fmk strongly improves the recovery and survival of several cryopreserved cell types and hematopoietic progenitor cells. A short preincubation with the caspase inhibitor after thawing also enhances the colony-forming activity of hematopoietic progenitor cells up to threefold. Furthermore, overexpression of Bcl-2, but not the blockade of the death receptor signaling, confers protection, indicating that cryoinjury-associated cell death is mediated by a Bcl-2-controlled mitochondrial pathway. Thus, our data suggest the use of zVAD-fmk as an efficient cryoprotective agent. The addition of caspase inhibitors may be an important tool for the cryopreservation of living cells and advantageous in cell transplantation, tissue engineering, and other genetic technologies.

Elevated protein expression of cyclin D1 and Fra-1 but decreased expression of c-Myc in human colorectal adenocarcinomas overexpressing beta-catenin

Mutations of the adenomatous polyposis coli tumor suppressor gene, or its downstream target beta-catenin, have been implicated in the initiation of most sporadic human colorectal epithelial neoplasms. These mutations, in turn, lead to aberrant nuclear accumulation of beta-catenin and subsequent activation of the beta-catenin/Tcf transcription factor complex. In vitro studies utilizing cultured human colon cancer cell lines have identified c-myc, cyclin D1 and fra-1 as target genes of beta-catenin/Tcf signaling. In our study, 12 cases of human colorectal adenocarcinomas were examined by Western immunoblotting analysis and immunohistochemical staining to specifically investigate whether the protein expression of these target genes was indeed altered in vivo by beta-catenin dysregulation. The results show that the protein level of beta-catenin was significantly increased in all 12 tumors (3.4 +/- 1.0-fold increase compared to the control normal mucosa by Western immunoblotting, p < 0.05), and this increase was associated with positive nuclear staining by immunohistochemistry in 10 cases. Increased levels of expression of cyclin D1 and Fra-1 proteins were also demonstrated in every tumor (9.0 +/- 2.7 and 3.3 +/- 0.9-fold increases compared to normal mucosa, respectively). Surprisingly, the protein level of c-Myc was significantly decreased in all tumors examined by 49 +/- 19% (p < 0.05), but the c-myc mRNA level was increased in 8 of 12 tumors when compared to that in normal mucosa by RT-PCR. Immunohistochemical staining performed on these carcinomas and additional 27 colorectal carcinomas further demonstrated that the protein expression level of c-Myc and beta-catenin nuclear localization were not correlated. Moreover, 15 of 20 colorectal adenomas exhibited positive nuclear beta-catenin immunostaining, among which 11 also exhibited increased c-Myc protein expression. These data thus support the notion that upregulation of cyclin D1 and Fra-1 in human colorectal adenocarcinomas is driven by abnormally expressed beta-catenin. However, the regulation of c-myc expression in colorectal tumors appears to be more complex. While dysregulated beta-catenin may cause a transcriptional upregulation of the c-myc gene, the c-Myc protein expression appears to be further regulated by a posttranscriptional mechanism(s) during the process of neoplastic progression.

c-Myc sensitizes cells to tumor necrosis factor-mediated apoptosis by inhibiting nuclear factor kappa B transactivation

Nuclear factor kappaB (NF-kappaB) plays a key role in suppression of tumor necrosis factor (TNF)-mediated apoptosis by inducing a variety of anti-apoptotic genes. Expression of c-Myc has been shown to sensitize cells to TNF-mediated apoptosis by inhibiting NF-kappaB activation. However, the precise step in the NF-kappaB signaling pathway and apoptosis modified by c-Myc has not been identified. Using the inducible c-MycER system and c-Myc null fibroblasts, we found that expression of c-Myc inhibited NF-kappaB activation by interfering with RelA/p65 transactivation but not nuclear translocation of NF-kappaB. Activation of c-Myc promoted TNF-induced release of cytochrome c from mitochondria to the cytosol because of the inhibition of NF-kappaB. Furthermore, we found that NF-kappaB-inducible gene A1 was attenuated by expression of c-Myc and that the restoration of A1 expression suppressed c-Myc-induced TNF sensitization. Our results elucidate the molecular mechanisms by which c-Myc increases cell susceptibility to TNF-mediated apoptosis, indicating that c-Myc may exhibit its pro-apoptotic activities by repression of cell survival genes.

E2F activity is essential for survival of Myc-overexpressing human cancer cells

Effective cell cycle completion requires both Myc and E2F activities. However, whether these two activities interact to regulate cell survival remains to be tested. Here we have analysed survival of inducible c-Myc-overexpressing cell lines derived from U2OS human osteosarcoma cells, which carry wild-type pRb and p53 and are deficient for p16 and ARF expression. Induced U2OS-Myc cells neither underwent apoptosis spontaneously nor upon reconstitution of the ARF-p53 axis and/or serum-starvation. However, they died massively when concomitantly exposed to inhibitors of E2F activity, including a constitutively active pRb (RbDeltacdk) mutant, p16, a stable p27 (p27T187A) mutant, a dominant-negative (dn) CDK2, or dnDP-1. Similar apoptotic effect was observed upon down-modulation of endogenous E2Fs through overexpression of E2F binding site oligonucleotides in U2OS-Myc cells, upon expression of RbDeltacdk or dnDP-1 in the Myc-amplified HL-60 (ARF-; p53-) human leukemia cells, and upon co-transfection of Myc and RbDeltacdk in SAOS-2 (ARF+; p53-) human osteosarcoma cells but not in human primary fibroblasts. Consistent with these results, a dnp53 mutant did not abrogate the Myc-induced apoptotic phenotype, which instead strictly depended on caspase-3-like proteases and on Myc transcriptional activity. Our data indicate that in contrast to normal cells, Myc-overexpressing human cancer cells need E2F activity for their survival, regardless of their ARF and p53 status, a notion that may have important implications for antineoplastic treatment strategies.

The nucleophosmin-anaplastic lymphoma kinase fusion protein induces c-Myc expression in pediatric anaplastic large cell lymphomas

The majority of pediatric anaplastic large cell lymphomas (ALCLs) carry the t(2;5)(p23;q35) chromosomal translocation that juxtaposes the dimerization domain of nucleophosmin with anaplastic lymphoma kinase (ALK). The nucleophosmin-ALK fusion induces constitutive, ligand-independent activation of the ALK tyrosine kinase leading to aberrant activation of cellular signaling pathways. To study the early consequences of ectopic ALK activation, a GyrB-ALK fusion was constructed that allowed regulated dimerization with the addition of coumermycin. Expression of the fusion protein caused a coumermycin-dependent increase in cellular tyrosine phosphorylation and c-Myc immunoreactivity, which was paralleled by a rise in c-myc RNA. To assess the clinical relevance of this observation, c-Myc expression was determined in pediatric ALK-positive and -negative lymphomas. Co-expression of c-Myc and ALK was seen in tumor cells in 15 of 15 (100%) ALK-positive ALCL samples, whereas no expression of either ALK or c-Myc was seen in six of six cases of ALK-negative T-cell lymphoma. C-Myc may be a downstream target of ALK signaling and its expression a defining characteristic of ALK-positive ALCLs.

A novel form of the RelA nuclear factor kappaB subunit is induced by and forms a complex with the proto-oncogene c-Myc

Members of both Myc and nuclear factor kappaB (NF-kappaB) families of transcription factors are found overexpressed or inappropriately activated in many forms of human cancer. Furthermore, NF-kappaB can induce c-Myc gene expression, suggesting that the activities of these factors are functionally linked. We have discovered that both c-Myc and v-Myc can induce a previously undescribed, truncated form of the RelA(p65) NF-kappaB subunit, RelA(p37). RelA(p37) encodes the N-terminal DNA binding and dimerization domain of RelA(p65) and would be expected to function as a trans-dominant negative inhibitor of NF-kappaB. Surprisingly, we found that RelA(p37) no longer binds to kappaB elements. This result is explained, however, by the observation that RelA(p37), but not RelA(p65), forms a high-molecular-mass complex with c-Myc. These results demonstrate a previously unknown functional and physical interaction between RelA and c-Myc with many significant implications for our understanding of the role that both proteins play in the molecular events underlying tumourigenesis.

TIP49, but not TRRAP, modulates c-Myc and E2F1 dependent apoptosis

We previously described two nuclear cofactors, TRRAP and TIP49, that have functional roles in Myc-mediated oncogenesis. We have now expanded the analysis of these Myc-associated cofactors to investigate their roles in apoptosis and cell proliferation. Although TRRAP and TIP49 are both essential for transformation, TIP49 modulates c-Myc-mediated apoptosis whereas disruption of TRRAP activity has no apparent effect on apoptosis. We extended our analysis of TIP49 to show that it also binds to the E2F1 transactivation domain and modulates both transforming and apoptotic activities. These results indicate that individual cofactors differentially potentiate c-Myc and E2F1 functions.

The myc oncogene: MarvelouslY Complex

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

Transformation of follicular lymphoma to diffuse large-cell lymphoma: alternative patterns with increased or decreased expression of c-myc and its regulated genes

The natural history of follicular lymphoma (FL) is frequently characterized by transformation to a more aggressive diffuse large B cell lymphoma (DLBCL). We compared the gene-expression profiles between transformed DLBCL and their antecedent FL. No genes were observed to increase or decrease their expression in all of the cases of histological transformation. However, two different gene-expression profiles associated with the transformation process were defined, one in which c-myc and genes regulated by c-myc showed increased expression and one in which these same genes showed decreased expression. Further, there was a striking difference in gene-expression profiles between transformed DLBCL and de novo DLBCL, because the gene-expression profile of transformed DLBCL was more similar to their antecedent FL than to de novo DLBCL. This study demonstrates that transformation from FL to DLBCL can occur by alternative pathways and that transformed DLBCL and de novo DLBCL have very different gene-expression profiles that may underlie the different clinical behaviors of these two types of morphologically similar lymphomas.

A computerized database-scan to identify c-MYC targets

The c-MYC oncogene plays a pivotal role in the malignant transformation of various types of human cancer. It is also a key regulator of cellular proliferation, embryonic differentiation and apoptosis. c-MYC encodes a transcription factor that activates target genes in a sequence specific manner through heterodimerization with the ubiquitously expressed factor MAX. Identifying c-MYC target genes is therefore crucial for elucidating the molecular pathways that are downstream of MYC. Most of the c-MYC targets isolated to date as well as targets of other transcription factors have been identified by differential expression or the candidate gene approach. In this paper, we outline a computer-based scan that allows us to create a pool of putative target genes for a transcription factor. The scan is based on a set of criteria including sequence specificity of the c-MYC transcription factor, sequence location and evolutionary conservation of these regulatory elements. Using this procedure, we have identified 12 putative targets for c-MYC. Expression analyses, DNA binding assays and chimeric promoter-reporter experiments suggest that two genes, NM23-H2 and N-RAS, may indeed be direct targets for c-MYC activation. This type of computer-based scan may have a general use to identify targets for other transcription factors.

What retroviruses teach us about the involvement of c-Myc in leukemias and lymphomas

Overexpression of the cellular oncogene c-Myc frequently occurs during induction of leukemias and lymphomas in many species. Retroviruses have enhanced our understanding of the role of c-Myc in such tumors. Leukemias and lymphomas induced by retroviruses activate c-Myc by: (1) use of virally specified proteins that increase c-Myc transcription, (2) transduction and modification of c-Myc to generate a virally encoded form of the gene, v-Myc, and (3) proviral integration in or near c-Myc. Proviral integrations elevate transcription by insertion of retroviral enhancers found in the long terminal repeat (LTR). Studies of the LTR enhancer elements from these retroviruses have revealed the importance of these elements for c-Mycactivation in several cell types. Retroviruses also have been used to identify genes that collaborate with c-Myc during development and progression of leukemias and lymphomas. In these experiments, animals that are transgenic for c-Mycoverexpression (often in combination with the overexpression or deletion of known proto-oncogenes) have been infected with retroviruses that then insertionally activate novel co-operating cellular genes. The retrovirus then acts as a molecular 'tag' for cloning of these genes. This review covers several aspects of c-Myc involvement in retrovirally induced leukemias and lymphomas.

Myc target in myeloid cells-1, a novel c-Myc target, recapitulates multiple c-Myc phenotypes

Using cDNA microarrays, we recently identified a large number of transcripts that are regulated differentially by the c-Myc oncoprotein in myeloid cells. Here, we characterize one of these, termed MT-MC1 (Myc Target in Myeloid Cells-1). MT-MC1 is a widely expressed nuclear protein whose overexpression, unlike that of c-Myc targets reported previously, recapitulates multiple c-Myc phenotypes. These include promotion of apoptosis, alteration of morphology, enhancement of anchorage-independent growth, tumorigenic conversion, promotion of genomic instability, and inhibition of hematopoietic differentiation. The MT-MC1 promoter is a direct c-Myc target; it contains two consensus E-box elements, both of which bind c-Myc.Max heterodimers. Mutation of either site abrogates DNA binding by c-Myc.Max and renders the promoter c-Myc unresponsive. Finally, MT-MC1 regulates the expression of several other c-Myc target genes. MT-MC1 represents a proximal and direct c-Myc target that recapitulates many of the properties typically associated with Myc oncoprotein overexpression.

The Ewing's sarcoma oncoprotein EWS/FLI induces a p53-dependent growth arrest in primary human fibroblasts

Ewing's sarcoma is associated with a fusion between the EWS and FLI1 genes, forming an EWS/FLI fusion protein. We developed a system for the identification of cooperative mutations in this tumor through expression of EWS/FLI in primary human fibroblasts. Gene expression profiling demonstrated that this system recapitulates many features of Ewing's sarcoma. EWS/FLI-expressing cells underwent growth arrest, suggesting that growth arrest-abrogating collaborative mutations may be required for tumorigenesis. Expression profiling identified transcriptional upregulation of p53, and the growth arrest was rescued by inhibition of p53. These data support a role for p53 as a tumor suppressor in Ewing's sarcoma and demonstrate the use of transcriptional profiling of model systems in the identification of cooperating mutations in human cancer.

C-myc gene expression alone is sufficient to confer resistance to antiestrogen in human breast cancer cells

C-myc is implicated in the initiation, progression and estrogen response of breast cancer. To further investigate the role of c-myc in breast cancer, we have developed clonal MCF-7 human breast cancer cell lines harboring a stably-transfected human c-myc gene, whose expression was stringently controlled by the bacterial reverse tetracycline transcription activator protein. The expression of the endogenous genomic c-myc gene in MCF-7 cells was abolished by the potent pure estrogen antagonist, ICI 182,780. Functional c-Myc protein was identified by both Western immunoblotting and by its ability to transactivate a chimeric plasmid consisting of E-box sequences upstream of the luciferase reporter gene. One MCF-7 clone, 35im, was chosen for further characterization. C-myc induction by doxycycline was rapid and dose dependent; c-myc mRNA appeared as early as 30 min after doxycycline addition and stimulation of c-myc expression required as little as 50 ng/ml doxycycline, with c-myc mRNA levels reaching a plateau at 2.5 microg/ml doxycycline. ICI 182,780 or doxycycline (a tetracycline analog) treatment did not alter the mRNA levels of Max, the c-myc binding partner. As in wildtype MCF-7 cells, the growth of clone 35im was inhibited by 1 microM or less of ICI 182,780 and stimulated by 10 nM to 1 microM 17beta-estradiol. When maintained in a complete medium containing 5% normal fetal bovine serum (FBS) and ICI 182,780, doxycycline induced cell growth by 400% in an 8-day assay. A similar level of growth was achieved with doxycycline treatment in cells that were arrested by the use of charcoal-stripped FBS. Doxycycline had no effect on the growth of a control MCF-7 clone (18c). Apoptosis, assessed by caspase-dependent cleavage of poly(ADP-ribose) polymerase, was unchanged in clone 35im cells after treatments with doxycycline or ICI 182,780. The present study demonstrates that c-myc alone is sufficient to confer antiestrogen resistance in human breast cancer. Our novel c-myc-inducible MCF-7 cell model offers a unique opportunity to study the diverse actions of the c-myc proto-oncogene in human breast cancer.

Wnt signaling promotes oncogenic transformation by inhibiting c-Myc-induced apoptosis

Aberrant activation of the Wnt/beta-catenin signaling pathway is associated with numerous human cancers and often correlates with the overexpression or amplification of the c-myc oncogene. Paradoxical to the cellular transformation potential of c-Myc is its ability to also induce apoptosis. Using an inducible c-MycER expression system, we found that Wnt/beta-catenin signaling suppressed apoptosis by inhibiting c-Myc-induced release of cytochrome c and caspase activation. Both cyclooxygenase 2 and WISP-1 were identified as effectors of the Wnt-mediated antiapoptotic signal. Soft agar assays showed that neither c-Myc nor Wnt-1 alone was sufficient to induce cellular transformation, but that Wnt and c-Myc coordinated in inducing transformation. Furthermore, coexpression of Wnt-1 and c-Myc induced high-frequency and rapid tumor growth in nude mice. Extensive apoptotic bodies were characteristic of c-Myc-induced tumors, but not tumors induced by coactivation of c-Myc and Wnt-1, indicating that the antiapoptotic function of Wnt-1 plays a critical role in the synergetic action between c-Myc and Wnt-1. These results elucidate the molecular mechanisms by which Wnt/beta-catenin inhibits apoptosis and provide new insight into Wnt signaling-mediated oncogenesis.

Bovine papillomavirus type 1 E6-induced sensitization to apoptosis is distinct from its transforming activity

The bovine papillomavirus type 1 (BPV-1) E6 oncoprotein induces tumorigenic transformation of murine C127 cells and stimulates transcription when targeted to a promoter. We have previously shown that C127 cells expressing BPV-1 E6 exhibited increased tumor necrosis factor alpha (TNF)-mediated apoptosis. To understand the mechanisms by which BPV-1 E6 sensitizes cells to apoptosis and to investigate the relevance of E6-enhanced apoptosis to its other biological activities, we analyzed a BPV-1 E6 mutant (491, with four amino acids deleted at the C-terminus) for its ability to sensitize C127 cells to apoptosis. The result was then compared with the E6 mutant's ability to transform cells, to activate transcription, and to associate with known cellular binding proteins. Our data indicated that the transcriptional activation function of BPV-1 E6 correlated with sensitization of cells to TNF-mediated apoptosis. Moreover, functions required for BPV-1 E6-mediated sensitization of cells to apoptosis are distinct from those required for transformation. A potential role of paxillin in E6 sensitization of cells to apoptosis is implicated. These results thus indicate that sensitization of cells to TNF-induced apoptosis represents a novel function of BPV-1 E6.