An embryonically expressed gene is a target for c-Myc regulation via the c-Myc-binding sequence

Identification and characterization of specific DNA-binding complexes containing members of the Myc/Max/Mad network of transcriptional regulators

In the past, eukaryotic cell-derived complexes of the Myc/Max/Mad network of transcriptional regulators have largely been refractory to DNA binding studies. We have developed electrophoretic mobility shift assay conditions to measure specific DNA binding of Myc/Max/Mad network complexes using a COS7 cell-based overexpression system. With the established protocol, we have measured on- and off-rates of c-Myc/Max, Max/Max, and Mad1/Max complexes and determined relative affinities. All three complexes appeared to bind with comparable affinity to a Myc E-box sequence. Furthermore, our data derived from competition experiments suggested that the Mad3/Max and Mad4/Max complexes also possess comparable DNA binding affinities. The conditions established for COS7 cell-overexpressed proteins were then used to identify c-Myc/Max, Max/Max, and Mnt/Max complexes in HL-60 cells. However, no Mad1/Max could be detected, despite the induction of Mad1 expression during differentiation. Whereas the DNA binding activity of c-Myc/Max complexes was down-regulated, Max/Max binding increased, and Mnt/Max binding remained unchanged. In addition, we have also tested for upstream stimulatory factor (USF) binding and observed that, in agreement with published data, USF comprises a major Myc E-box-binding factor that is more abundant than any of the Myc/Max/Mad network complexes. Similar to the Mnt/Max complex, the binding activity of USF remained constant during HL-60 differentiation. Our findings establish conditions for the analysis of DNA binding of Myc/Max/Mad complexes and indicate posttranslational regulation of the Max/Max complex.

Characterization of a branched-chain amino-acid aminotransferase from Schizosaccharomyces pombe

The Saccharomyces cerevisiae genes for the cytosolic and mitochondrial branched-chain amino-acid aminotransferases (BCAT) were isolated recently. These genes show significant homology to mammalian ECA39, originally isolated as a gene regulated by the c-myc oncogene. We now report the isolation of the Schizosaccharomyces pombe eca39/BCAT gene. The S. pombe protein shows 47-52% identity to other eukaryotic BCAT proteins isolated from S. cerevisiae, nematode, mouse and man. A genetic growth assay for BCAT activity was established using an S. cerevisiae strain disrupted in both BCAT isoenzymes. Consequently, the activity of the S. pombe BCAT was demonstrated by genetic and biochemical means. Possible applications of BCAT-encoding genes as selection markers in yeast transformation are proposed.

Myc represses transcription of the growth arrest gene gas1

Cell proliferation is regulated by the induction of growth promoting genes and the suppression of growth inhibitory genes. Malignant growth can result from the altered balance of expression of these genes in favor of cell proliferation. Induction of the transcription factor, c-Myc, promotes cell proliferation and transformation by activating growth promoting genes, including the ODC and cdc25A genes. We show that c-Myc transcriptionally represses the expression of a growth arrest gene, gas1. A conserved Myc structure, Myc box 2, is required for repression of gas1, and for Myc induction of proliferation and transformation, but not for activation of ODC. Activation of a Myc-estrogen receptor fusion protein by 4-hydroxytamoxifen was sufficient to repress gas1 gene transcription. These findings suggest that transcriptional repression of growth arrest genes, including gas1, is one step in promotion of cell growth by Myc.

Role of Oncogenic Transcription Factor c-Myc in Cell Cycle Regulation, Apoptosis and Metabolism

The myc gene was initially discovered as a prototypical retrovirally transduced oncogene. Over the decades, abundant evidence has emerged to support a causal role for the activated cellular gene, c-myc, in animal and human tumors. The gene encodes an oncogenic helix-loop-helix leucine zipper transcription factor that acts as a heterodimer with its partner protein, Max, to activate genes regulating the cell cycle machinery as well as critical metabolic enzymes. The additional ability of c-Myc to repress transcription of differentiation-related genes suggest that c-Myc is a central and key molecular integrator of cell proliferation, differentiation and metabolism.

Biphasic regulation of the preproendothelin-1 gene by c-myc

Endothelin-1 (ET-1), a potent vasoconstrictive/mitogenic peptide originally isolated from vascular endothelium, stimulates the expression of immediate early response genes such as c-myc. The c-myc protooncogene participates in regulating the cascade of events that follow mitogenic stimulation of quiescent cells. Using a panel of isogenic fibroblast cell lines with differential c-myc expression levels (obtained by disrupting one c-myc gene copy with targeted homologous recombination and subsequently stably transfecting the heterozygous cells with an exogenous c-myc transgene), we demonstrate that c-Myc protein regulates ET-1 gene transcription in a biphasic fashion: as an activator at low concentrations and as a repressor at high concentrations. Using rat endothelial cells treated with antisense c-myc oligodeoxynucleotides, we also show that c-myc regulates ET-1 synthesis and secretion in a biphasic manner. The present report, therefore, demonstrates the existence of a signal transduction pathway that regulates the synthesis and secretion of ET-1 via the immediate early transcription factor, c-Myc.

c-Myc dependent initiation of genomic instability during neoplastic transformation

The dihydrofolate reductase (DHFR) gene is a target of c-Myc in genomic instability. The induced overexpression of c-Myc in cell lines is followed by the amplification and rearrangement of the DHFR gene. Furthermore, the constitutive upregulation of c-Myc protein coincides with genomic instability of the DHFR gene in lymphoid, non-lymphoid and in tumor lines. The amplification of the DHFR gene is locus-specific and independent of species origins. We have now addressed the question whether inducible deregulation of c-Myc is followed by DHFR gene amplification in vivo. We show that the DHFR gene is a target of c-Myc-dependent neoplasia in vivo and propose a role for genomic instability during the initiation of neoplastic transformation.

Identification of putative c-Myc-responsive genes: characterization of rcl, a novel growth-related gene

The c-Myc protein is a helix-loop-helix leucine zipper oncogenic transcription factor that participates in the regulation of cell proliferation, differentiation, and apoptosis. The biochemical function of c-Myc has been well described, yet the identities of downstream effectors are just beginning to emerge. We describe the identification of a set of c-Myc-responsive genes in the Rat1a fibroblast through the application of cDNA representational difference analysis (RDA) to cDNAs isolated from nonadherent Rat1a and Rat1a-myc cells. In this system, c-Myc overexpression is sufficient to induce the transformed phenotype of anchorage-independent growth. We identified 20 differentially expressed cDNAs, several of which represent novel cDNA sequences. We further characterized one of the novel cDNAs identified in this screen, termed rcl. rcl expression is (i) directly stimulated by c-Myc; (ii) stimulated in the in vivo growth system of regenerating rat liver, as is c-myc; and (iii) elevated in human lymphoid cells that overexpress c-myc. By using an anti-Rcl antibody, immunoblot analysis, and immunofluorescence microscopy, the Rcl protein was found to be a 23-kDa nuclear protein. Ectopic expression of the protein encoded by the rcl cDNA induces anchorage-independent growth in Rat1a fibroblasts, albeit to a diminished extent compared to ectopic c-Myc expression. These data suggest a role for rcl during cellular proliferation and c-Myc-mediated transformation.

Expression of myc-family, myc-interacting, and myc-target genes during preimplantation mouse development

Previous studies indicated that members of the myc gene family may be essential for preimplantation development. Other studies revealed that preimplantation embryos lacking c-myc, N-myc, or L-myc are viable, indicating that these genes are either not essential for preimplantation development or can be substituted for functionally by other myc gene family members. To investigate the possible role of these genes during preimplantation development, we determined the temporal patterns of expression of four members of the myc gene family, genes encoding myc-associated proteins, and four putative MYC target genes. We observed a sequential pattern of myc gene expression, with the L-myc mRNA expressed as a maternal transcript, the c-myc mRNA expressed during the 4-cell through morula stages, and the B-myc mRNA expressed highly at the morula and blastocysts stages. B-myc was the predominant family member expressed during preimplantation development. The mxi mRNA was not detectable and the mad mRNA was detectable only as a maternal transcript. The max mRNA, however, was expressed both as a maternal mRNA and as an embryonic message throughout most of preimplantation development. Three putative MYC target genes (Odc, cyclin E, and prothymosin-alpha) were transcriptionally induced during the 2-cell stage, and their mRNAs increased sharply in abundance during development to the morula and blastocyst stages. Another putative MYC target gene, cyclin A, was expressed both as a maternal mRNA and as an embryonic transcript. These data support the view that the expression of myc target genes may be supported initially through the expression of maternally inherited MYC proteins and corresponding mRNAs and that subsequent stage-specific changes in expression of myc genes, myc-associated genes, and myc target genes may control early differentiative events around the time of implantation.

Myc versus USF: discrimination at the cad gene is determined by core promoter elements

Carbamoyl-phosphate synthase/aspartate carbamoyltransferase/dihydroorotase, which is encoded by the cad gene, is required for the first three rate-limiting steps of de novo pyrimidine biosynthesis. It has been previously demonstrated that cad transcription increases at the G1/S-phase boundary, as quiescent cells reenter the proliferative cell cycle. The growth-responsive element has been mapped to an E box at +65 in the hamster cad promoter. Using an in vivo UV cross-linking and immunoprecipitation assay, we show that Myc, Max, and upstream stimulatory factor (USF) bind to the chromosomal cad promoter. To determine whether binding of Myc-Max or USF is critical for cad growth regulation, we analyzed promoter constructs which contain mutations in the nucleotides flanking the E box. We demonstrate that altering nucleotides which flank the cad E box to sequences which decrease Myc-Max binding in vitro correlates with a loss of cad G1/S-phase transcriptional activation. This result supports the conclusion that binding of Myc-Max, but not USF, is essential for cad regulation. Our investigations demonstrate that the endogenous cad E box can be bound by more than one transcription factor, but growth-induced cad expression is achieved only by Myc.

E2a-Pbx1 induces aberrant expression of tissue-specific and developmentally regulated genes when expressed in NIH 3T3 fibroblasts

The E2a-Pbx1 oncoprotein contains the transactivation domain of E2a joined to the DNA-binding homeodomain (HD) of Pbx1. In mice, E2a-Pbx1 transforms T lymphoblasts and fibroblasts and blocks myeloblast differentiation. Pbx1 and E2a-Pbx1 bind DNA as heterodimers with other HD proteins whose expression is tissue specific. While the transactivation domain of E2a is required for all forms of transformation, DNA binding by the Pbx1 HD is essential for blocking myeloblast differentiation but dispensable for fibroblast or T-lymphoblast transformation. These properties suggest (i) that E2a-Pbx1 causes cellular transformation by activating gene transcription, (ii) that transcription of E2a-Pbx1 target genes is normally regulated by ubiquitous Pbx proteins and tissue-specific partners, and (iii) that DNA-binding mutants of E2a-Pbx1 activate a subset of all gene targets. To test these predictions, genes induced in NIH 3T3 fibroblasts by E2a-Pbx1 were identified and examined for tissue- and stage-specific expression and their differential abilities to be upregulated by E2a-Pbx1 in NIH 3T3 fibroblasts and myeloblasts and by a DNA-binding mutant of E2a-Pbx1 in NIH 3T3 cells. Of 12 RNAs induced by E2a-Pbx1, 4 encoded known proteins (a J-C region of the immunoglobulin kappa light chain, natriuretic peptide receptor C, mitochondrial fumarase, and the 3',5'-cyclic nucleotide phosphodiesterase, PDE1A) and 5 encoded new proteins related to angiogenin, ion channels, villin, epidermal growth factor repeat proteins, and the human 2.19 gene product. Expression of many of these genes was tissue specific or developmentally regulated, and most were not expressed in fibroblasts, indicating that E2a-Pbx1 can induce ectopic expression of genes associated with lineage-specific differentiation.

Induction of pgp3 expression and reversion of the multidrug resistance phenotype in 9-OH-ellipticine-resistant Chinese hamster lung fibroblasts transfected with the MYC oncogene

Chinese hamster lung cells resistant to the DNA topoisomerase II inhibitor 9-OH-ellipticine (DC-3F/9-OH-E) are cross resistant to various drugs through the expression of the MDR phenotype. The myc oncogene was approximately 10-fold amplified and 20-fold overexpressed in parental DC-3F cells as compared with DC-3F/9-HO-E cells. Transfection of the resistant cells with a mouse c-myc gene did not alter the resistance to topoisomerase II inhibitors and, in cells with a low multidrug (MDR) expression, reversed this phenotype. Northern and Western blot analyses revealed an increased expression of pgp1 in the DC-3F/9-OH-E cells, which was not modified in the myc-transfected clones. However, myc expression in these clones resulted in an increased expression of pgp3, roughly in proportion to the level of myc expression. Transfection of the DC-3F/9-OH-E cells with the human MDR3 gene, homologous to pgp3, also resulted in the reversion of the MDR phenotype. These results show that (1) expression of the transfected myc gene positively regulates pgp3 expression but has no effect on pgp1; (2) when observed, reversion of the MDR phenotype is proportional to the levels of myc and pgp3 expression; and (3) this reversion, resulting from pgp3 expression, is associated with a decreased functional activity of the pgp1 protein and might require an appropriate balance of pgp1 and pgp3 expression.

The Myc negative autoregulation mechanism requires Myc-Max association and involves the c-myc P2 minimal promoter

Increasing evidence supports an important biological role for Myc in the downregulation of specific gene transcription. Recent studies suggest that c-Myc may suppress promoter activity through proteins of the basal transcription machinery. We have previously reported that Myc protein, in combination with additional cellular factors, suppresses transcription initiation from the c-myc promoter. To characterize the cis components of this Myc negative autoregulation pathway, fragments of the human c-myc promoter were inserted upstream of luciferase reporter genes and assayed for responsiveness to inducible MycER activation in Rat-1 fibroblasts. We found four- to fivefold suppression of a c-myc P2 minimal promoter fragment upon induction of wild-type MycER protein activity, while induction of a mutant MycER protein lacking amino acids 106 to 143 required for Myc autosuppression failed to elicit this response. This assay is physiologically significant, as it reflects Myc autosuppression of the endogenous c-myc gene with regard to kinetics, dose dependency, cell type specificity, and c-Myc functional domains. Analysis of mutations within the P2 minimal promoter indicated that the cis components of Myc autosuppression could not be ascribed to any known protein-binding motifs. In addition, to address the trans factors required for Myc negative autoregulation, we expressed MycEG and MaxEG leucine zipper dimerization mutants in Rat-1 cells and found that Myc-Max heterodimerization is obligatory for Myc autosuppression. Two models for the Myc autosuppression mechanism are discussed.

Characterization of a tumor-associated gene, a member of a novel family of genes encoding membrane glycoproteins

To isolate genes involved in tumor formation and in embryogenesis, a subtracted cDNA library was constructed from a c-myc-induced mouse brain tumor. A gene isolated in this screen, named TMP (tumor-associated membrane protein), codes for a putative glycoprotein with four transmembrane domains. The TMP gene was found to be highly expressed in brain tumor cells but not in normal brain. It is also expressed at high levels in undifferentiated embryonic stem cells, but markedly down-regulated in these cells after their differentiation into embryoid bodies. The TMP amino acid sequence bears high homology to the growth arrest specific protein PMP22/GAS-3, which is involved in several human peripheral neuropathies. The expression patterns of the TMP and PMP22 genes in NIH-3T3 fibroblasts were compared at different proliferation states. The results suggest an inverse pattern of expression for the two homologs, TMP expression being high during cell proliferation and PMP22 expression being high during growth arrest. To further characterize the TMP gene we have isolated its human homolog and examined its expression in embryonic and adult tissues. In our search for human sequences homologous to TMP and PMP22, we identified two new genes which we have named XMP and YMP. Thus, we present a novel family of membrane glycoproteins, one member of which is closely associated with proliferation and another with growth arrest.

Cell cycle: on target with Myc

Recent evidence indicates that the c-Myc proto-oncogene activates transcription of cdc25A. The Cdc25A protein phosphatase is required both for progression through mitosis and for Myc-induced apoptosis, making cdc25A the most attractive Myc target gene identified so far.

Mitochondrial and cytosolic branched-chain amino acid transaminases from yeast, homologs of the myc oncogene-regulated Eca39 protein

We have isolated a high copy suppressor of a temperature-sensitive mutation in ATM1, which codes for an ABC transporter of Saccharomyces cerevisiae mitochondria. The suppressor, termed BAT1, encodes a protein of 393 amino acid residues with an NH2-terminal extension that directs Bat1p to the mitochondrial matrix. A highly homologous protein, Bat2p, of 376 amino acid residues was found in the cytosol. Both Bat proteins show striking similarity to the mammalian protein Eca39, which is one of the few known targets of the myc oncogene. Deletion of a single BAT gene did not impair growth of yeast cells. In contrast, deletion of both genes resulted in an auxotrophy for branched-chain amino acids (Ile, Leu, and Val) and in a severe growth reduction on glucose-containing media, even after supply of these amino acids. Mitochondria and cytosol isolated from bat1 and bat2 deletion mutants, respectively, contained largely reduced activities for the conversion of branched-chain 2-ketoacids to their corresponding amino acids. Thus, the Bat proteins represent the first known isoforms of yeast branched-chain amino acid transaminases. The severe growth defect of the double deletion mutant observed even in the presence of branched-chain amino acids suggests that the Bat proteins, in addition to the supply of these amino acids, perform another important function in the cell.

YY1 and c-Myc associate in vivo in a manner that depends on c-Myc levels

The c-Myc oncoprotein has previously been shown to associate with transcription regulator YY1 and to inhibit its activity. We show herein that endogenous c-Myc and YY1 associate in vivo and that changes in c-Myc levels, which accompany mitogenic stimulation or differentiation of cultured cells, affect the ratio of free to c-Myc-associated YY1. We have also investigated the mechanism by which association with c-Myc inhibits YY1's ability to regulate transcription. c-Myc does not block binding of YY1 to DNA. However, protein association studies suggest that c-Myc interferes with the ability of YY1 to contact basal transcription proteins TATA-binding protein and TFIIB.

Mad proteins contain a dominant transcription repression domain

Transcription repression by the basic region-helix-loop-helix-zipper (bHLHZip) protein Mad1 requires DNA binding as a ternary complex with Max and mSin3A or mSin3B, the mammalian orthologs of the Saccharomyces cerevisiae transcriptional corepressor SIN3. The interaction between Mad1 and mSin3 is mediated by three potential amphipathic alpha-helices: one in the N terminus of Mad (mSin interaction domain, or SID) and two within the second paired amphipathic helix domain (PAH2) of mSin3A. Mutations that alter the structure of the SID inhibit in vitro interaction between Mad and mSin3 and inactivate Mad's transcriptional repression activity. Here we show that a 35-residue region containing the SID represents a dominant repression domain whose activity can be transferred to a heterologous DNA binding region. A fusion protein comprising the Mad1 SID linked to a Ga14 DNA binding domain mediates repression of minimal as well as complex promoters dependent on Ga14 DNA binding sites. In addition, the SID represses the transcriptional activity of linked VP16 and c-Myc transactivation domains. When fused to a full-length c-Myc protein, the Mad1 SID specifically represses both c-Myc's transcriptional and transforming activities. Fusions between the GAL DNA binding domain and full-length mSin3 were also capable of repression. We show that the association between Mad1 and mSin3 is not only dependent on the helical SID but is also dependent on both putative helices of the mSin3 PAH2 region, suggesting that stable interaction requires all three helices. Our results indicate that the SID is necessary and sufficient for transcriptional repression mediated by the Mad protein family and that SID repression is dominant over several distinct transcriptional activators.

An essential E box in the promoter of the gene encoding the mRNA cap-binding protein (eukaryotic initiation factor 4E) is a target for activation by c-myc

The mRNA cap-binding protein (eukaryotic initiation factor 4E [eIF4E]) binds the m7 GpppN cap on mRNA, thereby initiating translation. eIF4E is essential and rate limiting for protein synthesis. Overexpression of eIF4E transforms cells, and mutations in eIF4E arrest cells in G, in cdc33 mutants. In this work, we identified the promoter region of the gene encoding eIF4E, because we previously identified eIF4E as a potential myc-regulated gene. In support of our previous data, a minimal, functional, 403-nucleotide promoter region of eIF4E was found to contain CACGTG E box repeats, and this core eIF4E promoter was myc responsive in cotransfections with c-myc. A direct role for myc in activating the eIF4E promoter was demonstrated by cotransfections with two dominant negative mutants of c-myc (MycdeltaTAD and MycdeltaBR) which equally suppressed promoter function. Furthermore, electrophoretic mobility shift assays demonstrated quantitative binding to the E box motifs that correlated with myc levels in the electrophoretic mobility shift assay extracts; supershift assays demonstrated max and USF binding to the same motif. cis mutations in the core or flank of the eIF4E E box simultaneously altered myc-max and USF binding and inactivated the promoter. Indeed, mutations of this E box inactivated the promoter in all cells tested, suggesting it is essential for expression of eIF4E. Furthermore, the GGCCACGTG(A/T)C(C/G) sequence is shared with other in vivo targets for c-myc, but unlike other targets, it is located in the immediate promoter region. Its critical function in the eIF4E promoter coupled with the known functional significance of eIF4E in growth regulation makes it a particularly interesting target for c-myc regulation.

Two yeast homologs of ECA39, a target for c-Myc regulation, code for cytosolic and mitochondrial branched-chain amino acid aminotransferases

ECA39 was isolated as a target gene for c-Myc regulation in mice. We identified two homologs for the murine ECA39 in the yeast Saccharomyces cerevisiae, ECA39 and ECA40, as well as two human homologs. These genes show a significant homology to prokaryotic branched-chain amino acid aminotransferase (BCAT) (EC). To understand the function of eukaryotic ECA39 and ECA40, we deleted either gene from the yeast genome. Activity of branched-chain amino acid aminotransferase was measured in the wild-type and mutants with either leucine, isoleucine, or valine as substrates. The results demonstrate that in S. cerevisiae ECA39 and ECA40 code for mitochondrial and cytosolic branched-chain amino acid aminotransferases, respectively. ECA39 is highly expressed during log phase and is down-regulated during the stationary phase of growth, while ECA40 shows an inverse pattern of gene expression. In agreement with these results, while we previously showed that deletion of ECA39 affected the cell cycle in proliferating cells, we do not observe a growth phenotype in eca40Delta cells. We suggest that BCAT is a target for c-Myc activity and discuss the evolutionary conservation of prokaryotic and eukaryotic BCATs and their possible involvement in regulation of cell proliferation.

ECA39, a conserved gene regulated by c-Myc in mice, is involved in G1/S cell cycle regulation in yeast

The c-myc oncogene has been shown to play a role in cell proliferation and apoptosis. The realization that myc oncogenes may control the level of expression of other genes has opened the field to search for genetic targets for Myc regulation. Recently, using a subtraction/coexpression strategy, a murine genetic target for Myc regulation, called EC439, was isolated. To further characterize the ECA39 gene, we set out to determine the evolutionary conservation of its regulatory and coding sequences. We describe the human, nematode, and budding yeast homologs of the mouse ECA39 gene. Identities between the mouse ECA39 protein and the human, nematode, or yeast proteins are 79%, 52%, and 49%, respectively. Interestingly, the recognition site for Myc binding, located 3' to the start site of transcription in the mouse gene, is also conserved in the human homolog. This regulatory element is missing in the ECA39 homologs from nematode or yeast, which also lack the regulator c-myc. To understand the function of ECA39, we deleted the gene from the yeast genome. Disruption of ECA39 which is a recessive mutation that leads to a marked alteration in the cell cycle. Mutant haploids and homozygous diploids have a faster growth rate than isogenic wild-type strains. Fluorescence-activated cell sorter analyses indicate that the mutation shortens the G1 stage in the cell cycle. Moreover, mutant strains show higher rates of UV-induced mutations. The results suggest that the product of ECA39 is involved in the regulation of G1 to S transition.

c-Myc expression is controlled by the mitogenic cAMP-cascade in thyrocytes

In dog thyroid epithelial cells in primary culture, thyrotropin (TSH), acting through cAMP, induces proliferation and differentiation expression, whereas epidermal growth factor (EGF) and phorbol esters induce proliferation and dedifferentiation. In these cells, we have detailed the regulation by cAMP of the c-myc protooncogene mRNA and protein. The cAMP signaling pathway induces a biphasic increase of c-myc mRNA and protein. c-Myc protein accumulation follows the abundance and kinetics of its mRNA expression. Using in vitro elongation of nascent transcripts to measure transcription and actinomycin D (AcD) chase experiments to study mRNA stability, we have shown that in the first phase cAMP releases a transcriptional elongation block. No modification of transcriptional initiation was observed. After 30 min of treatment with TSH, c-myc mRNA was also stabilized. During the second phase, cAMP stabilization of the mRNA disappears and transcription is again shut off. Thus, in a tissue in which it stimulates proliferation and specific gene expression, cAMP regulates biphasically c-myc expression by mechanisms operating at the transcriptional and posttranscriptional levels.

A novel function for Myc: inhibition of C/EBP-dependent gene activation

We have investigated the effect of the v-Myc oncoprotein on gene expression in myelomonocytic cells. We find that v-Myc dramatically down-regulates the expression of myelomonocytic-specific genes, such as the chicken mim-1 and lysozyme genes, both of which are known targets for C/EBP transcription factors. We present evidence that Myc downregulates these genes by inhibiting the function of C/EBP transcription factors. Detailed examination of the inhibitory mechanism shows that amino-terminal sequences of v-Myc, but not its DNA-binding domain, are required for the suppression of C/EBP-dependent transactivation. Our findings identify a new function for Myc and reveal a novel mechanism by which Myc affects the expression of other genes.

Dominant negative mutants of Myc inhibit cooperation of both Myc and adenovirus serotype-5 E1a with Ras

We have used dominant negative Myc mutants to analyze the Myc and E1a mechanisms of cooperation with Ras. We show that mutants of Myc with an altered basic region (BR; RR366, 367EE) or deletion of the leucine zipper (LZ; delta aa 414-439), changes which modify the DNA binding domain, or with deletions in the Myc amino terminal conserved regions box 1 (dlMB1; delta aa 46-55) and box 2 (dlMB2; delta aa 132-140) inhibit cooperation of wt Myc and activated Ras to transform rat embryo fibroblasts (REF). Expression of the amino terminal 104 aa had no effect whereas wt Myc stimulated focus formation. Mutant dlMB1 cooperated with Ras with one half wt efficiency while dlMB2 was inactive. No mutant tested was toxic during neomycin cotransformation of REF to G418 resistance. Interestingly, these Myc mutants exerted a parallel inhibition of E1a-Ras cooperation to transform REF. This suggests that the Myc-Ras and E1a-Ras cooperation pathways intersect and require common protein factors. A Myc box 2 deletion mutant which is a wt transactivator of the Myc responsive ornithine decarboxylase promoter, but unlike the wt does not repress the adenovirus-2 core promoter (Li et al., 1994, EMBO J., 13:4070-4079), inhibits Myc-Ras and E1a-Ras cooperation. This suggests that a box 2-dependent step, potentially gene repression, is required for both the E1a- and Myc-Ras cooperation mechanisms.

Expression and activity of L-Myc in normal mouse development

To determine the role of L-Myc in normal mammalian development and its functional relationship to other members of the Myc family, we determined the normal patterns of L-myc gene expression in the developing mouse by RNA in situ hybridization and assessed the phenotypic impact of L-Myc deficiency produced through standard gene targeting methodology. L-myc transcripts were detected in the developing kidney and lung as well as in both the proliferative and the differentiative zones of the brain and neural tube. Despite significant expression of L-myc in developing mouse tissue, homozygous null L-myc mice were found to be viable, reproductively competent, and represented in expected frequencies from heterozygous matings. A detailed histological survey of embryonic and adult tissues, characterization of an embryonic neuronal marker, and measurement of cellular proliferation in situ did not reveal any congenital abnormalities. The lack of an apparent phenotype associated with L-Myc deficiency indicates that L-Myc is dispensable for gross morphological development and argues against a unique role for L-Myc in early central nervous system development as had been previously suggested. Although overlapping expression patterns among myc family members raise the possibility of complementation of L-Myc deficiency by other Myc oncoproteins, compensatory changes in the levels of c- and/or N-myc transcripts were not detected in homozygous null L-myc mice.

Do products of the myc proto-oncogene play a role in transcriptional regulation of the prothymosin alpha gene?

The Myc protein has been reported to activate transcription of the rat prothymosin alpha gene by binding to an enhancer element or E box (CACGTG) located in the first intron (S. Gaubatz et al., Mol. Cell. Biol. 14:3853-3862, 1994). The human prothymosin alpha gene contains two such motifs: in the promoter region at kb -1.2 and in intron 1, approximately 2 kb downstream of the transcriptional start site in a region which otherwise bears little homology to the rat gene. Using chloramphenicol acetyltransferase (CAT) reporter constructs driven either by the 5-kb human prothymosin alpha promoter or by a series of truncated promoters, we showed that removal of the E-box sequence had no effect on transient expression of CAT activity in mouse L cells. When intron 1 of the prothymosin alpha gene was inserted into the most extensive promoter construct downstream of the CAT coding region, a diminution in transcription, which remained virtually unchanged upon disruption of the E boxes, was observed. CAT constructs driven by the native prothymosin alpha promoter or the native promoter and intron were indifferent to Myc; equivalent CAT activity was observed in the presence of ectopic normal or mutant Myc genes. Similarly, expression of a transiently transfected wild-type prothymosin alpha gene as the reporter was not affected by a repertoire of myc-derived genes, including myc itself and dominant or recessive negative myc mutants. In COS-1 cells, equivalent amounts of the protein were produced from transfected prothymosin alpha genes regardless of whether genomic E boxes were disrupted, intron 1 was removed, or a repertoire of myc-derived genes was included in the transfection cocktail. More importantly, cotransfection of a dominant negative Max gene failed to reduce transcription of the endogenous prothymosin alpha gene in COS cells or the wild-type transfected gene in COS or L cells. Taken together, the data do not support the idea that Myc activates transcription of the intact human prothymosin alpha gene or reporter constructs that mimic its structure. Rather, they suggest that the human prothymosin alpha promoter and downstream elements are buffered so as to respond poorly, if at all, to transient fluctuations in transcription factors which regulate other genes.

Induction of apoptosis by the c-Myc helix-loop-helix/leucine zipper domain in mouse 3T3-L1 fibroblasts

The cellular proto-oncogene c-myc is involved in cell proliferation and transformation but is also implicated in the induction of programmed cell death (apoptosis). The c-Myc protein is a transcriptional activator with a carboxyl-terminal basic region/helix-loop-helix (HLH)/leucine zipper (LZ) domain. It forms heterodimers with the HLH/LZ protein Max and transactivates gene expression after binding DNA E-box elements. We have studied the phenotype of dominant-negative mutants of c-Myc and Max in microinjection experiments. Max mutants with a deleted or mutated basic region inhibited DNA synthesis in serum-stimulated 3T3-L1 mouse fibroblasts. In contrast, mutants of c-Myc expressing only the basic region/HLH/LZ or HLH/LZ domains rapidly induced apoptosis at low and high serum levels. Co-expression of the HLH/LZ domains of c-Myc and Max failed to do so. We suggest that the c-Myc HLH/LZ domain induces apoptosis by specific interaction with cellular factors different to Max.

C-myc represses transiently transfected HLA class I promoter sequences not locus-specifically

Overexpression of the c-myc oncogene is frequently accompanied by downregulation of Major Histocompatibility Complex (MHC, HLA in humans) class I antigens. In human melanoma c-myc overexpression downmodulates HLA-B expression, whereas HLA-A is hardly affected. Repression of HLA-B is mediated through the core promoter, containing a CAAT-box and a non-conventional TATA-box. We show evidence that in transient transfection assays the HLA-A2 and HLA-B7 promoters are repressed by c-myc to the same extent. Therefore, other sequences of the HLA-A and HLA-B genes, possibly intron/exon sequences, should contribute to the locus B-specificity of the downregulation. Furthermore, c-myc does not seem to alter binding of protein complexes to the CAAT- or TATA-box of HLA-B7 or HLA-A2 in gel retardation assays. Comparison of promoters repressed by c-myc reveals a weak consensus sequence of the initiator (Inr) element: TCA(+1)YYYNY. The presence of a TCA sequence in the initiator region of the MHC class I promoter makes downregulation by c-myc through the Inr likely. We speculate that the Inr contributes to MHC class I promoter activity by stimulating recruitment of TFIID to the weak, non-conventional TATA-box, thereby making it susceptible to repression by c-myc through the Inr.

An E-box-mediated increase in cad transcription at the G1/S-phase boundary is suppressed by inhibitory c-Myc mutants

To better understand the signaling pathways which lead to DNA synthesis in mammalian cells, we have studied the transcriptional activation of genes needed during the S phase of the cell cycle. Transcription of the gene encoding a pyrimidine biosynthetic enzyme, carbamoyl-phosphate synthase (glutamine-hydrolyzing)/aspartate carbamoyltransferase/dihydroorotase (cad), increases at the G1/S-phase boundary. We have mapped the growth-dependent response element in the hamster cad gene to the extended palindromic E-box sequence, CCACGTGG, which is centered at +65 in the 5' untranslated sequence. Mutation of the E box abolished growth-dependent transcription, and an oligonucleotide corresponding to the cad sequence at +55 to +75 (+55/+75) restored growth-dependent regulation to nonresponsive cad promoter mutants when placed down-stream of the transcription start site. The same oligonucleotide conferred less G1/S-phase induction when placed upstream of basal promoter elements. An analogous oligonucleotide containing the mutant E box had no effect in either location. Nuclear proteins bound the cad +55/+75 element in a cell cycle-dependent manner in electromobility shift assays; antibodies specific to USF and Max blocked the DNA-binding activity of different growth-regulated protein-DNA complexes. Expression of c-Myc mutants which have been shown to dominantly interfere with the function of c-Myc and Max significantly inhibited cad transcription during S phase but had no effect on transcription from another G1/S-phase-activated promoter, dhfr. These data support a model whereby E-box-binding proteins activate serum-induced transcription from the cad promoter at the G1/S-phase boundary and suggest that a Max-associated protein complex contributes to the serum response.

An E box element is required for the expression of the ad4bp gene, a mammalian homologue of ftz-f1 gene, which is essential for adrenal and gonadal development

Ad4BP, also known as SF-1, is a cell type-specific transcription factor regulating all the steroidogenic P-450 genes. Recently, the targeted disruption of the mouse ftz-f1 gene encoding Ad4BP/SF-1 has established its essential function in both adrenal and gonadal development and sexual differentiation. As an initial step toward understanding its role in the cascade of gene activations necessary for the differentiation of the steroidogenic tissues and the sex differentiation of the gonads, we isolated and characterized the rat ad4bp gene. A sequence analysis of the ad4bp gene revealed that another nuclear factor ELP was also transcribed from the same gene by alternative promoter usage and splicing. The promoter of the ad4bp gene showed activities in the steroidogenic cells such as Y-1 adrenocortical cells and I-10 testicular Leydig cells when examined by transient transfection assays. Using deletion analysis and site-directed mutagenesis, we identified a cis-element at the position from -82 bp to -77 bp in the 5'-upstream region. The cis-element was identical to the consensus E box element, which is the binding site for the basic-helix-loop-helix proteins. Gel mobility shift analyses revealed the amount of a binding factor to this E box in the nuclear extract prepared from the rat testes attained a maximal level 1 week after birth and then decreased dramatically thereafter, and only trace amounts were detected in adult rats. In contrast, the binding factor in the ovaries attained a maximal level just after birth and kept its level thereafter. These dimorphic expressions of the binding factor to the E box correlated well with those of Ad4BP, and thus suggested that the expression of Ad4BP, and thus suggested that the expression of Ad4BP is transcriptionally regulated through this E box element.

Identification of a cDNA for SSRP1, an HMG-box protein, by interaction with the c-Myc oncoprotein in a novel bacterial expression screen

We describe a system for screening cDNA expression libraries in Escherichia coli based on protein-protein interactions. The system utilizes fusion proteins containing the DNA binding domain of the lambda phage cl repressor and a heterologous dimerization domain, which is the target of the screen. Such chimeric proteins were functional as transcriptional repressors in E.coli; function was dependent on the presence of the heterologous dimerization domain, and function of the chimeras was disrupted by expression of excess dimerization domain. A screen was designed to identify factors that could interact with the heterologous dimerization domain and thereby inactivate the chimeric repressor. We used this screen to identify factors that could interact with the basic helix-loop-helix/leucine zipper domains of c-Myc, and isolated the cDNA for a previously characterized HMG domain protein that interacts specifically with c-Myc in this system. This screening method could be used with proteins that have the ability to homo- or heterodimerize.

The c-myc protein represses the lambda 5 and TdT initiators

The lambda 5 promoter initiates transcription at multiple sites and confers expression in all cell types. Two lambda 5 promoter-derived oligonucleotides (Inr lambda 5:1 and Inr lambda 5:2), each with a transcription start site, could promote transcription in transient transfection assays. In contrast, a third oligonucleotide (+90 lambda 5), without a transcription initiation site, was inactive. The Inr lambda 5:1 and Inr lambda 5:2 oligonucleotides formed a major DNA-protein complex B' in gel retardation analyses; no protein-DNA complexes were observed with the inactive +90 lambda 5 oligonucleotide. The B' complexes of Inr lambda 5:1 and Inr lambda 5:2 each contained c-myc and myn (murine homologue of Max) proteins. The c-myc and myn proteins were also found to bind the TdT initiator (InrTdT). Using mutated oligonucleotides, we found that the c-myc/myn proteins bound to the transcription initiation site of both Inr lambda 5:1 and InrTdT, however, these mutated oligonucleotides were inactive in transfection assays. This suggested that, in this system, transcription depended both on a transcription initiation site and appropriate flanking sequences. The significance of c-myc binding to the respective initiator was analysed by overexpressing c-myc in co-transfection assays. Under these conditions the transcriptional activity of both the lambda 5 and the TdT initiator was repressed.

BK1: an FGF-responsive central nervous system-derived cell line

Fibroblast growth factors (FGF) are expressed at high levels in the central nervous system (CNS), however their function in the CNS is not well understood. The immortalized neuronal cell line (BK1), derived from a transgenic mouse central nervous system tumor, expresses high levels of FGF receptor 1 (FGFR1) and demonstrates both morphologic and biochemical changes when treated with basic FGF (FGF-2). We have derived subclones of BK1 cells with varying degrees of FGF responsiveness by transfecting either a wild type (FRW) or a truncated (FRX) form of FGFR1. Cells expressing high levels of FGFR1 rapidly and uniformly respond to FGF, while cells expressing FRX fail to respond to FGF, either morphologically or by the expression of molecular markers. These BK1 subclones will prove useful to study FGFR mediated signal transduction and FGFR responsive genes in a CNS derived cell. These studies also demonstrate that a dominant negative FGF receptor can be used as a tool to elucidate the function of FGF in the central nervous system.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

Transactivation of the human p53 tumor suppressor gene by c-Myc/Max contributes to elevated mutant p53 expression in some tumors

Elevated levels of mutant forms of the p53 tumor suppressor are a hallmark of many transformed cells. Multiple mechanisms such as increased stability of the protein and increased transcription of the gene can account for elevated p53 expression. Recent findings indicate that c-Myc/Max heterodimers can bind to an essential CA(C/T)GTG-containing site in the p53 promoter and elevate its expression. We have addressed the possibility that elevated mutant p53 expression is due to deregulated c-Myc expression. Here we demonstrate that the human p53 promoter is transactivated by high c-Myc expression and repressed by high Max expression. In examining the relative levels of c-Myc and p53 in human Burkitt's lymphomas and other B-lymphoid lines, we found that there is a correlation between the levels of c-Myc protein and p53 mRNA expression. In particular, cells that express very low levels of c-Myc protein also express low levels of p53 mRNA, while cells that express high levels of c-Myc tend to express high levels of p53 mRNA. To determine whether the p53 gene can be a target for c-Myc in vivo, we assayed the effects of antisense c-myc RNA on the levels of endogenous p53 mRNA. The results indicate that the presence of antisense c-myc RNA leads to a reduction in the levels of c-Myc protein, p53 mRNA, and expression from the p53 promoter. Taken together, our findings support a direct role for c-Myc in elevating expression of the mutant p53 gene in some tumors.

Myc-mediated apoptosis requires wild-type p53 in a manner independent of cell cycle arrest and the ability of p53 to induce p21waf1/cip1

Deregulated expression of the c-myc proto-oncogene can lead to apoptosis under certain physiological conditions. By introducing a conditionally active Myc allele into primary embryo fibroblasts null for p53, and into fibroblasts without endogenous p53 expression but ectopically expressing a temperature-sensitive p53 allele, we show that expression of wild-type p53 is required for susceptibility to Myc-mediated apoptosis. Although ectopic expression of wild-type p53 blocked cells in the G1 phase of the cell cycle, G1 arrest by isoleucine starvation, in a manner independent of p53, did not confer susceptibility to apoptosis. Thus, growth arrest per se is not sufficient to induce Myc-mediated apoptosis; instead, a property intrinsic to p53 is specifically required. Moreover, apoptosis did not require induction of p53 target proteins, including the cyclin-dependent kinase inhibitor p21waf1/cip1. Therefore, the role of p53 in apoptosis may be distinct from its role in cell cycle arrest.

Overexpression of c-myc precedes amplification of the gene encoding dihydrofolate reductase

An experimentally inducible model system was generated in which Chinese hamster ovary cells (CHO-9) were stably transfected with an inducible c-myc cDNA. The induction of c-myc in these transfectants is followed by the enhanced binding of c-Myc/Max-containing protein complexes to 5'flanking E-box sequences of the gene encoding dihydrofolate reductase (DHFR). Moreover, DHFR is transiently amplified. The inappropriate overproduction of the oncoprotein, therefore, seems to plays a role in induced DHFR amplification.

The alternatively initiated c-Myc proteins differentially regulate transcription through a noncanonical DNA-binding site

The myc proto-oncogene family has been implicated in multiple cellular processes, including proliferation, differentiation, and apoptosis. The Myc proteins, as heterodimers with Max protein, have been shown to function as activators of transcription through an E-box DNA-binding element, CACGTG. We have now found that the c-Myc proteins regulate transcription through another, noncanonical, DNA sequence. The non-AUG-initiated form of the c-Myc protein, c-Myc 1, strongly and specifically activates transcription of the C/EBP sequences within the EFII enhancer element of the Rous sarcoma virus long terminal repeat. In contrast, comparable amounts of the AUG-initiated form, c-Myc 2, fail to significantly affect enhancer activity. However, both c-Myc proteins trans-activate the CACGTG sequence comparably. In addition, Myc/Max heterodimers, but not Max homodimers, bind to the EFII enhancer sequence in vitro. Finally, c-Myc 1 overexpression, but not c-Myc 2 overexpression, significantly inhibits cell growth. These results reveal new transcriptional activities for the Myc proteins and demonstrate that the different forms of the Myc protein are functionally distinct. These results also suggest an interplay between two different growth regulatory transcription factor families.

Mediation of c-Myc-induced apoptosis by p53

The cellular proto-oncogene c-myc is involved in cell proliferation and transformation but is also implicated in the induction of programmed cell death (apoptosis). The same characteristics have been described for the tumor suppressor gene p53, the most commonly mutated gene in human cancer. In quiescent mouse fibroblasts expressing wild-type p53 protein, activation of c-Myc was found to induce apoptosis and cell cycle reentry, preceded by stabilization of p53. In contrast, in quiescent p53-null fibroblasts, activation of c-Myc induced cell cycle reentry but not apoptosis. These results suggest that p53 mediates apoptosis as a safeguard mechanism to prevent cell proliferation induced by oncogene activation.

Effects of c-myc expression on cell cycle progression

We used targeted homologous recombination to disrupt one c-myc gene copy in a diploid fibroblast cell line and found that a twofold reduction in Myc expression resulted in lower exponential growth rates and a lengthening of the G0-to-S-phase transition (M. Shichiri, K. D. Hanson and J. M. Sedivy, Cell Growth Differ. 4:93-104, 1993). Myc is a transcription factor, and the number of target genes whose regulation could result in differential growth rates may be very large. We have approached this problem by examining effects of reduced c-myc expression in three broad areas: (i) secretion of growth factors, (ii) expression of growth factor receptors, and (iii) intracellular signal transduction between Myc and components of the intrinsic cell cycle clock. We have found no evidence that differential medium conditioning can account for the growth phenotypes. Likewise, the expression of receptors for platelet-derived growth factor, epidermal growth factor, basic fibroblast growth factor, and insulin-like growth factor I was the same in diploid and heterozygous cells (platelet-derived growth factor, epidermal growth factor, fibroblast growth factor, and insulin-like growth factor are the sole growth factors required by these cells for growth in serum-free medium). In contrast, expression of cyclin E, cyclin A, and Rb phosphorylation were delayed when quiescent c-myc heterozygous cells were stimulated to enter the cell cycle. Expression of cyclin D1, cyclin D3, and Cdk2 was not affected. The timing of cyclin E induction was the earliest observable effect of reduced Myc expression. Our data indicate that Myc contributes to regulation of proliferation by a cell-autonomous mechanism that involves the modulation of cyclin E expression and, consequently, progression through the restriction point of the cell cycle.