Domains of human c-myc protein required for autosuppression and cooperation with ras oncogenes are overlapping

MYC interaction with the tumor suppressive SWI/SNF complex member INI1 regulates transcription and cellular transformation

MYC is a key driver of cellular transformation and is deregulated in most human cancers. Studies of MYC and its interactors have provided mechanistic insight into its role as a regulator of gene transcription. MYC has been previously linked to chromatin regulation through its interaction with INI1 (SMARCB1/hSNF5/BAF47), a core member of the SWI/SNF chromatin remodeling complex. INI1 is a potent tumor suppressor that is inactivated in several types of cancers, most prominently as the hallmark alteration in pediatric malignant rhabdoid tumors. However, the molecular and functional interaction of MYC and INI1 remains unclear. Here, we characterize the MYC-INI1 interaction in mammalian cells, mapping their minimal binding domains to functionally significant regions of MYC (leucine zipper) and INI1 (repeat motifs), and demonstrating that the interaction does not interfere with MYC-MAX interaction. Protein-protein interaction network analysis expands the MYC-INI1 interaction to the SWI/SNF complex and a larger network of chromatin regulatory complexes. Genome-wide analysis reveals that the DNA-binding regions and target genes of INI1 significantly overlap with those of MYC. In an INI1-deficient rhabdoid tumor system, we observe that with re-expression of INI1, MYC and INI1 bind to common target genes and have opposing effects on gene expression. Functionally, INI1 re-expression suppresses cell proliferation and MYC-potentiated transformation. Our findings thus establish the antagonistic roles of the INI1 and MYC transcriptional regulators in mediating cellular and oncogenic functions.

Defective Hfp-dependent transcriptional repression of dMYC is fundamental to tissue overgrowth in Drosophila XPB models

Nucleotide excision DNA repair (NER) pathway mutations cause neurodegenerative and progeroid disorders (xeroderma pigmentosum (XP), Cockayne syndrome (CS) and trichothiodystrophy (TTD)), which are inexplicably associated with (XP) or without (CS/TTD) cancer. Moreover, cancer progression occurs in certain patients, but not others, with similar C-terminal mutations in the XPB helicase subunit of transcription and NER factor TFIIH. Mechanisms driving overproliferation and, therefore, cancer associated with XPB mutations are currently unknown. Here using Drosophila models, we provide evidence that C-terminally truncated Hay/XPB alleles enhance overgrowth dependent on reduced abundance of RNA recognition motif protein Hfp/FIR, which transcriptionally represses the MYC oncogene homologue, dMYC. The data demonstrate that dMYC repression and dMYC-dependent overgrowth in the Hfp hypomorph is further impaired in the C-terminal Hay/XPB mutant background. Thus, we predict defective transcriptional repression of MYC by the Hfp orthologue, FIR, might provide one mechanism for cancer progression in XP/CS.

MYC association with cancer risk and a new model of MYC-mediated repression

MYC is one of the most frequently mutated and overexpressed genes in human cancer but the regulation of MYC expression and the ability of MYC protein to repress cellular genes (including itself) have remained mysterious. Recent genome-wide association studies show that many genetic polymorphisms associated with disease risk map to distal regulatory elements that regulate the MYC promoter through large chromatin loops. Cancer risk-associated single-nucleotide polymorphisms (SNPs) contain more potent enhancer activity, promoting higher MYC levels and a greater risk of disease. The MYC promoter is also subject to complex regulatory circuits and limits its own expression by a feedback loop. A model for MYC autoregulation is discussed which involves a signaling pathway between the PTEN (phosphatase and tensin homolog) tumor suppressor and repressive histone modifications laid down by the EZH2 methyltransferase.

MYC acts via the PTEN tumor suppressor to elicit autoregulation and genome-wide gene repression by activation of the Ezh2 methyltransferase

The control of normal cell growth is a balance between stimulatory and inhibitory signals. MYC is a pleiotropic transcription factor that both activates and represses a broad range of target genes and is indispensable for cell growth. Whereas much is known about gene activation by MYC, there is no established mechanism for the majority of MYC-repressed genes. We report that MYC transcriptionally activates the PTEN tumor suppressor in normal cells to inactivate the phosphoinositide 3-kinase (PI3K) pathway, thus suppressing AKT activation. Suppression of AKT enhances the activity of the EZH2 histone methyltransferase, a subunit of the epigenetic repressor Polycomb Repressive Complex 2 (PRC2), while simultaneously stabilizing the protein. MYC-mediated enhancement in EZH2 protein level and activity results in local and genome-wide elevation in the repressive H3K27me3 histone modification, leading to widespread gene repression including feedback autoregulation of the MYC gene itself. Depletion of either PTEN or EZH2 and inhibition of the PI3K/AKT pathway leads to gene derepression. Importantly, expression of a phospho-defective EZH2 mutant is sufficient to recapitulate nearly half of all MYC-mediated gene repression. We present a novel epigenetic model for MYC-mediated gene repression and propose that PTEN and MYC exist in homeostatic balance to control normal growth, which is disrupted in cancer cells.

Myc: the beauty and the beast

The iconic history of the Myc oncoprotein encompasses 3 decades of intense scientific discovery. There is no question that Myc has been a pioneer, advancing insight into the molecular basis of cancer as well as functioning as a critical control center for several diverse biological processes and regulatory mechanisms. This narrative chronicles the journey and milestones that have defined the understanding of Myc, and it provides an opportunity to consider future directions in this challenging yet rewarding field.

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

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

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

Emerging Concepts in the Analysis of Transcriptional Targets of the MYC Oncoprotein: Are the Targets Targetable?

Activation of the MYC oncoprotein is among the most ubiquitous events in human cancer. MYC functions in part as a sequence-specific regulator of transcription. Although early searches for direct downstream target genes that explain MYC's potent biological activity were met with enthusiasm, the postgenomic decade has brought the realization that MYC regulates the transcription of not just a manageably small handful of target genes but instead up to 15% of all active loci. As the dust has begun to settle, two important concepts have emerged that reignite hope that understanding MYC's downstream targets might still prove valuable for defining critical nodes for therapeutic intervention in cancer patients. First, it is now clear that MYC target genes are not a random sampling of the cellular transcriptome but instead fall into specific, critical biochemical pathways such as metabolism, chromatin structure, and protein translation. In retrospect, we should not have been surprised to discover that MYC rewires cell physiology in a manner designed to provide the tumor cell with greater biosynthetic properties. However, the specific details that have emerged from these studies are likely to guide the development of new clinical tools and strategies. This raises the second concept that instills renewed optimism regarding MYC target genes. It is now clear that not all MYC target genes are of equal functional relevance. Thus, it may be possible to discern, from among the thousands of potential MYC target genes, those whose inhibition will truly debilitate the tumor cell. In short, targeting the targets may ultimately be a realistic approach after all.

Myc is an essential negative regulator of platelet-derived growth factor beta receptor expression

Platelet-derived growth factor BB (PDGF BB) is a potent mitogen for fibroblasts as well as many other cell types. Interaction of PDGF BB with the PDGF beta receptor (PDGF-betaR) activates numerous signaling pathways and leads to a decrease in receptor expression on the cell surface. PDGF-betaR downregulation is effected at two levels, the immediate internalization of ligand-receptor complexes and the reduction in pdgf-betar mRNA expression. Our studies show that pdgf-betar mRNA suppression is regulated by the c-myc proto-oncogene. Both constitutive and inducible ectopic Myc protein can suppress pdgf-betar mRNA and protein. Suppression of pdgf-betar mRNA in response to Myc is specific, since expression of the related receptor pdgf-alphar is not affected. We further show that Myc suppresses pdgf-betar mRNA expression by a mechanism which is distinguishable from Myc autosuppression. Analysis of c-Myc-null fibroblasts demonstrates that Myc is required for the repression of pdgf-betar mRNA expression in quiescent fibroblasts following mitogen stimulation. In addition, it is evident that the Myc-mediated repression of pdgf-betar mRNA levels plays an important role in the regulation of basal pdgf-betar expression in proliferating cells. Thus, our studies suggest an essential role for Myc in a negative-feedback loop regulating the expression of the PDGF-betaR.

The Myc oncoprotein: a critical evaluation of transactivation and target gene regulation

Mutations which disrupt the regulation or expression level of the c-myc gene are among the most common found in human and animal cancers (reviewed in ref. Cole, 1986; Henriksson and Luscher, 1996; Marcu et al., 1992). Ectopic expression studies define numerous biological activities of the c-myc gene, including transformation, immortalization, blockage of cell differentiation and induction of apoptosis (Askew et al., 1991; Cole, 1986; Evan and Littlewood, 1993; Freytag et al., 1990; Henriksson and Luscher, 1996; Marcu et al., 1992). Furthermore, c-myc is required for efficient progression through the cell cycle (Goruppi et al., 1994; Prochownik et al., 1988; Yokoyama and Imamoto, 1987), although recent studies indicate that it is not absolutely essential (Mateyak et al., 1997). This fascinating array of biological activities makes the c-myc gene one of the most intriguing oncogenes and presents the challenging question of how a single gene can manifest so many different effects. The c-Myc protein exhibits sequence-specific DNA binding when dimerized with its partner Max, and DNA binding is mediated through the basic region, which recognizes the core sequence CACGTG (Berberich et al., 1992; Blackwell et al., 1993; Blackwood and Eisenman, 1991; Prendergast and Ziff, 1991; Prendergast et al., 1991), but exhibits somewhat higher affinity for the more extended sequence ACCACGTGGT (Berberich et al., 1992; Blackwell et al., 1993; Halazonetis and Kandil, 1991). There are three closely related Myc family proteins (c-Myc, N-Myc and L-Myc), each with documented oncogenic potential (Birrer et al., 1988; Schwab et al., 1985; Yancopoulos et al., 1985) and similar DNA binding properties (Mukherjee et al., 1992). For simplicity, we will use the term Myc to refer to all three proteins, but delineate any distinct activities where they apply. The goal of this review is to discuss Myc as a transcriptional activator and critically evaluate the evidence for the transactivation of specific target genes as direct downstream effectors. Since excellent comprehensive reviews on Myc have been published recently (Facchini and Penn, 1998; Henriksson and Luscher, 1996), we will focus on the latest observations that offer mechanistic insight into transactivation and oncogenic transformation.

c-myc null cells misregulate cad and gadd45 but not other proposed c-Myc targets

We report here that the expression of virtually all proposed c-Myc target genes is unchanged in cells containing a homozygous null deletion of c-myc. Two noteworthy exceptions are the gene cad, which has reduced log phase expression and serum induction in c-myc null cells, and the growth arrest gene gadd45, which is derepressed by c-myc knockout. Thus, cad and gadd45 are the only proposed targets of c-Myc that may contribute to the dramatic slow growth phenotype of c-myc null cells. Our results demonstrate that a loss-of-function approach is critical for the evaluation of potential c-Myc target genes.

myc boxes, which are conserved in myc family proteins, are signals for protein degradation via the proteasome

Cellular levels of the rapidly degraded c-myc protein play an important role in determining the proliferation status of cells. Increased levels of c-myc are frequently associated with rapidly proliferating tumor cells. We show here that myc boxes I and II, found in the N termini of all members of the myc protein family, function to direct the degradation of the c-myc protein. Both myc boxes I and II contain sufficient information to independently direct the degradation of otherwise stably expressed proteins to which they are fused. At least part of the myc box-directed degradation occurs via the proteasome. The mechanism of myc box-directed degradation appears to be conserved between yeast and mammalian cells. Our results suggest that the myc boxes may play an important role in regulating the level and activity of the c-myc protein.

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.

Controllable genetic manipulation of apoptosis of cells in culture

Apoptosis of mammalian cell is under the control of a wide range of intracellular and extracellular factors-amongst them proteases, protein kinases, cytokines and the protein products of oncogenes and tumour suppressor genes. The c-myc proto-oncogene encodes an essential component of the cell's proliferative machinery and its deregulated expression is implicated in many cancers. Under certain conditions, c-Myc also acts as a potent inducer of apoptosis. We have developed a 'switchable' chimaeric c-Myc protein whose activity is dependent on the synthetic ligand, 4-hydroxytamoxifen. In cells expressing this switchable c-Myc, proliferation and apoptosis in cultured fibroblasts can be regulated by addition of 4-hydroxytamoxifen. We have further demonstrated the utility of a switchable gene transcription system for the induction of proteins with pro-apoptotic effect. Myc-induced apoptosis is inhibited by the action of certain cytokines or by expresson of exogenous proteins with anti-apoptotic potential such as Bcl-2. We show that inhibition of p53 using dominant negative molecules inhibits apoptosis induced by DNA damage but has little effect on Myc-induced apoptosis. Finally, we have also been able to modulate a relatively late stage in apoptosis using inhibitors of cysteine proteases. Our data suggest a model in which the integrated activities of several proteins with diverse molecular functions may determine whether a particular cell undergoes apoptosis but that, once the actual catalytic machinery is engaged, the apoptotic process is irreversible.

The N-terminal domain of c-Myc associates with alpha-tubulin and microtubules in vivo and in vitro

The polymerization of alpha- and beta-tubulin into microtubules results in a complex network of microfibrils that have important structural and functional roles in all eukaryotic cells. In addition, microtubules can interact with a diverse family of polypeptides which are believed to directly promote the assembly of microtubules and to modulate their functional activity. We have demonstrated that the c-Myc oncoprotein interacts in vivo and in vitro with alpha-tubulin and with polymerized microtubules and have defined the binding site to the N-terminal region within the transactivation domain of c-Myc. In addition, we have shown that c-Myc colocalizes with microtubules and remains tightly bound to the microtubule network after detergent extraction of intact cells. These findings suggest a potential role for Myc-tubulin interaction in vivo.

Mammary tumor suppression by transforming growth factor beta 1 transgene expression

In cell culture, type alpha transforming growth factor (TGF-alpha) stimulates epithelial cell growth, whereas TGF-beta 1 overrides this stimulatory effect and is growth inhibitory. Transgenic mice that overexpress TGF-alpha under control of the mouse mammary tumor virus (MMTV) promoter/enhancer exhibit mammary ductal hyperplasia and stochastic development of mammary carcinomas, a process that can be accelerated by administration of the chemical carcinogen 7,12-dimethylbenz[a]anthracene. MMTV-TGF-beta 1 transgenic mice display mammary ductal hypoplasia and do not develop mammary tumors. We report that in crossbreeding experiments involving the production of mice carrying both the MMTV-TGF-beta 1 and MMTV-TGF-alpha transgenes, there is marked suppression of mammary tumor formation and that MMTV-TGF-beta 1 transgenic mice are resistant to 7,12-dimethylbenz[a]anthracene-induced mammary tumor formation. These data demonstrate that overexpression of TGF-beta 1 in vivo can markedly suppress mammary tumor development.

Overexpression of the c-Myc oncoprotein blocks the growth-inhibitory response but is required for the mitogenic effects of transforming growth factor beta 1

One of the more intriguing aspects of transforming growth factor beta 1 (TGF beta 1) is its ability to function as both a mitogenic factor for certain mesenchymal cells and a potent growth inhibitor of lymphoid, endothelial, and epithelial cells. Data are presented indicating that c-myc may play a pivotal role in both the mitogenic and antiproliferative actions of TGF beta 1. In agreement with previous studies using C3H/10T1/2 fibroblasts constitutively expressing an exogenous c-myc cDNA, we show that AKR-2B fibroblasts expressing a chimeric estrogen-inducible form of c-myc (mycER) are able to form colonies in soft agar in the presence of TGF beta 1 only when c-myc is activated by hormone. Whereas these findings support a synergistic role for c-myc in mitogenic responses to TGF beta 1, we also find that c-myc can antagonize the growth-inhibitory response to TGF beta 1. Mouse keratinocytes (BALB/MK), which are normally growth-arrested by TGF beta 1, are rendered insensitive to the growth-inhibitory effects of TGF beta 1 upon mycER activation. This ability of mycER activation to block TGF beta 1-induced growth arrest was found to occur only when the fusion protein was induced with hormone in the early part of G1. Addition of estradiol late in G1 had no suppressive effect on TGF beta 1-induced growth inhibition.

Role of c-myc in simian virus 40 large tumor antigen-induced DNA synthesis in quiescent 3T3-L1 mouse fibroblasts

Stably transfected NIH 3T3-L1 mouse fibroblasts (L1 cells) expressing the simian virus 40 large tumor antigen (LTAg) maintain c-myc expression and proliferation in low serum, whereas cells expressing the mutant form LTAg-K1, defective in binding of the retinoblastoma suppressor gene product pRb, showed reduced levels of c-myc RNA and only background levels of DNA synthesis in low serum. The role of the c-Myc protein in LTAg-induced DNA synthesis was studied in microinjection experiments. Expression of LTAg induced cellular DNA synthesis in > 95% of microinjected serum-starved L1 cells, whereas the mutant LTAg-K1 could not induce DNA synthesis. Coexpression of dominant negative c-Myc or Max mutants with LTAg inhibited DNA synthesis, indicating that functional c-Myc is necessary for induction of DNA synthesis by LTAg. Expression of c-Myc induced programmed cell death (apoptosis) in serum-starved L1 cells. Coexpression of c-Myc with LTAg-K1 restored induction of DNA synthesis without apoptosis. Expression of a truncated LTAg, LTAg-(1-259), defective in binding of the tumor suppressor gene product p53, failed to prevent c-Myc-induced apoptosis. The data indicate that c-Myc can restore the ability of LTAg-K1 to induce DNA synthesis and that LTAg-K1 prevents c-Myc-induced apoptosis in serum-starved L1 cells by its interaction with p53.

c-Myc-induced apoptosis in fibroblasts is inhibited by specific cytokines

We have investigated the mechanism by which deregulated expression of c-Myc induces death by apoptosis in serum-deprived fibroblasts. We demonstrate that Myc-induced apoptosis in low serum is inhibited by a restricted group of cytokines, principally the insulin-like growth factors and PDGF. Cytokine-mediated protection from apoptosis is not linked to the cytokines' abilities to promote growth. Protection from apoptosis is evident in the post-commitment (mitogen-independent) S/G2/M phases of the cell cycle and also in cells that are profoundly blocked in cell cycle progression by drugs. Moreover, IGF-I inhibition of apoptosis occurs in the absence of protein synthesis, and so does not require immediate early gene expression. We conclude that c-Myc-induced apoptosis does not result from a conflict of growth signals but appears to be a normal physiological aspect of c-Myc function whose execution is regulated by the availability of survival factors. We discuss the possible implications of these findings for models of mammalian cell growth in vivo.

Functional analysis of the AUG- and CUG-initiated forms of the c-Myc protein

Activation of the c-myc proto-oncogene by chromosomal translocation or proviral insertion frequently results in the separation of the c-myc coding region from its normal regulatory elements. Such rearrangements are often accompanied by loss or mutation of c-myc exon 1 sequences. These genetic alterations do not affect synthesis of the major c-myc protein, p64, which is initiated from the first AUG codon in exon 2. However they can result in mutation or loss of the CUG codon located in exon 1 that normally serves as an alternative translational initiation codon for synthesis of an N-terminally extended form of c-Myc (p67). It has been hypothesized that p67 is a functionally distinct form of c-Myc whose specific loss during c-myc rearrangements confers a selective growth advantage. Here we describe experiments designed to test the functional properties of the two c-Myc protein forms. We introduced mutations within the translational initiation codons of a normal human c-myc cDNA that alter the pattern of Myc protein synthesis (p64 vs. p67). The functions of each of these proteins were experimentally addressed using co-transformation and transcriptional activation assays. Both the p64 and p67 c-Myc proteins were independently able to collaborate with bcr-abl in the transformation of Rat-1 fibroblasts. In addition, both the exon 1- and exon 2-initiated forms of the c-Myc protein stimulated transcription of a Myc/Max-responsive reporter construct to a similar level. Given the apparent absence of functional differences between p64 and p67, we conclude that the basis for c-Myc oncogenic activation lies primarily in the overall deregulation of its expression and not in alterations in the protein. The existence of the CUG translational initiator may reflect a mechanism for the continued synthesis of c-Myc protein under conditions where AUG initiation is inhibited.

Deregulated expression of the c-myc oncogene abolishes inhibition of proliferation of rat vascular smooth muscle cells by serum reduction, interferon-gamma, heparin, and cyclic nucleotide analogues and induces apoptosis

We have investigated the requirement for c-myc downregulation in the growth arrest of vascular smooth muscle cells (VSMCs). Rat VSMCs were infected with a retrovirus vector directing constitutive expression of either the complete human c-Myc protein (VSM-myc cells) or the c-Myc deletion mutant D106-143, which is inactive in cotransformation and autosuppression assays (VSM-D106-143 myc cells). Clones of transfected VSM-myc cells were isolated that constitutively expressed a range of levels of c-Myc protein from that observed in normal proliferating VSMCs to approximately seven times normal. The growth rates of these clones and their responses to growth inhibitors were then assessed. VSM-myc clones possessed a shorter mean intermitotic time than normal cells, which was inversely correlated (P < .05) with the level of c-Myc protein expressed. VSM-myc cells also expressed lower levels of alpha-smooth muscle actin mRNA and protein and exhibited an altered morphology. The proliferation of normal VSMCs and VSM-D106-143 myc cells was inhibited by serum reduction (0.5% fetal calf serum) and also by treatment with interferon-gamma (100 IU/mL), heparin (50 micrograms/mL), 8-bromo-cAMP (0.1 mmol/L), or 8-bromo-cGMP (0.1 mmol/L). In contrast, proliferation of VSM-myc cells was not inhibited by any of these agents, even if present at 10-fold higher concentrations. However, approximately 75% of VSM-myc cells expressing levels of c-Myc protein seen in normal proliferating VSMCs underwent apoptosis after 4 days of serum reduction or treatment with interferon-gamma. The results show that constitutive c-myc expression induces continuous cell proliferation, reduction in alpha-smooth muscle actin expression and apoptosis in VSMCs. We conclude that downregulation of c-myc is a prerequisite for growth arrest and subsequent survival of VSMCs. Conversely, deregulated c-myc expression may be important in the proliferation and death of VSMCs--characteristics of the pathogenesis of atherosclerosis.

Inhibition of vascular smooth muscle cell proliferation in vitro and in vivo by c-myc antisense oligodeoxynucleotides

Restenosis after angioplasty is due predominantly to accumulation of vascular smooth muscle cells (VSMCs). The resistance of restenosis to pharmacological treatment has prompted investigation of genes involved in VSMC proliferation. We have examined the effect on VSMC proliferation of blocking expression of the c-myc proto-oncogene with antisense oligodeoxynucleotides, both in vitro and in a rat carotid artery injury model of angioplasty restenosis. Antisense c-myc oligodeoxynucleotides reduced average cell levels of c-myc mRNA and protein by 50-55% and inhibited proliferation of VSMCs when mitogenically stimulated from quiescence or when proliferating logarithmically (IC50 = 10 micrograms/ml). Corresponding sense c-myc, two-base-pair mismatch antisense c-myc, antisense alpha-actin or glyceraldehyde phosphate dehydrogenase oligodeoxynucleotides did not suppress c-myc expression or inhibit VSMC proliferation. Antisense c-myc inhibition was relieved by overexpression of an exogenous c-myc gene. After balloon catheter injury, peak c-myc mRNA expression occurred at 2 h. Antisense c-myc applied in a pluronic gel to the arterial adventitia reduced peak c-myc expression by 75% and significantly reduced neointimal formation at 14 d, compared with sense c-myc and gel application alone. We conclude that c-myc expression is required for VSMC proliferation in vitro and in the vessel wall. C-myc is a therefore a potential target for adjunctive therapy to reduce angioplasty restenosis.

Overexpression of Myc suppresses CCAAT transcription factor/nuclear factor 1-dependent promoters in vivo

Overexpression of Myc in cells can suppress the transcription of specific genes. Because several of these genes have common transcriptional regulatory elements, we investigated the possibility that this effect of Myc is mediated through a specific transcription factor. In vitro DNA-binding assays detect only one form of CCAAT transcription factor/nuclear factor 1 (CTF/NF-1) in quiescent 3T3-L1 cells. By contrast, quiescent 3T3-L1 cells that stably overexpress either c-Myc or N-Myc contain at least three forms of CTF/NF-1. Biochemical characterization of the various CTF/NF-1 forms showed that they have the same native molecular weight but differ in charge density. The more negatively charged CTF/NF-1 forms present in Myc-overexpressing cells are converted into that found in normal cells by treatment with acid phosphatase, suggesting that they represent a more phosphorylated form of the CTF/NF-1 protein. The various CTF/NF-1 forms have a similar DNA-binding affinity. Transfection experiments demonstrated that transcription from CTF/NF-1-dependent promoters is specifically suppressed in cells that stably overexpress c-Myc. This effect requires CTF/NF-1 binding. CTF/NF-1-dependent promoter activity is also suppressed in 3T3-L1 cells during active growth (relative to the quiescent state). Interestingly, actively growing 3T3-L1 cells contain forms of CTF/NF-1 similar to those in quiescent cells that stably overexpress c-Myc. Thus, the CTF/NF-1 forms present in cells that express high amounts of c-Myc correlate with a lower transcription rate of CTF/NF-1-dependent promoters in vivo. Our results provide a basis for the suppression of specific gene transcription by c-Myc.

Overexpression of Myc suppresses CCAAT transcription factor/nuclear factor 1-dependent promoters in vivo

Overexpression of Myc in cells can suppress the transcription of specific genes. Because several of these genes have common transcriptional regulatory elements, we investigated the possibility that this effect of Myc is mediated through a specific transcription factor. In vitro DNA-binding assays detect only one form of CCAAT transcription factor/nuclear factor 1 (CTF/NF-1) in quiescent 3T3-L1 cells. By contrast, quiescent 3T3-L1 cells that stably overexpress either c-Myc or N-Myc contain at least three forms of CTF/NF-1. Biochemical characterization of the various CTF/NF-1 forms showed that they have the same native molecular weight but differ in charge density. The more negatively charged CTF/NF-1 forms present in Myc-overexpressing cells are converted into that found in normal cells by treatment with acid phosphatase, suggesting that they represent a more phosphorylated form of the CTF/NF-1 protein. The various CTF/NF-1 forms have a similar DNA-binding affinity. Transfection experiments demonstrated that transcription from CTF/NF-1-dependent promoters is specifically suppressed in cells that stably overexpress c-Myc. This effect requires CTF/NF-1 binding. CTF/NF-1-dependent promoter activity is also suppressed in 3T3-L1 cells during active growth (relative to the quiescent state). Interestingly, actively growing 3T3-L1 cells contain forms of CTF/NF-1 similar to those in quiescent cells that stably overexpress c-Myc. Thus, the CTF/NF-1 forms present in cells that express high amounts of c-Myc correlate with a lower transcription rate of CTF/NF-1-dependent promoters in vivo. Our results provide a basis for the suppression of specific gene transcription by c-Myc.

Sequence-specific transcriptional activation by Myc and repression by Max

The c-Myc oncoprotein, which is required for cellular proliferation, resembles in its structure a growing number of transcription factors. However, the mechanism of its action in vivo is not yet clear. The discovery of the specific cognate DNA-binding site for Myc and its specific heterodimerization partner, Max, enabled the use of direct experiments to elucidate how Myc functions in vivo and how this function is modulated by Max. Here we demonstrate that exogenously expressed Myc is capable of activating transcription in vivo through its specific DNA-binding site. Moreover, transcriptional activation by Myc is dependent on the basic region, the integrity of the helix-loop-helix and leucine zipper dimerization motifs located in the carboxy-terminal portion of the protein, and the regions in the amino terminus conserved among Myc family proteins. In contrast to Myc, exogenously expressed Max elicited transcriptional repression and blocked transcriptional activation by Myc through the same DNA-binding site. Our results suggest a functional antagonism between Myc and Max which is mediated by their relative levels in the cells. A model for the activity of Myc and Max in vivo is presented.

Sequence-specific transcriptional activation by Myc and repression by Max

The c-Myc oncoprotein, which is required for cellular proliferation, resembles in its structure a growing number of transcription factors. However, the mechanism of its action in vivo is not yet clear. The discovery of the specific cognate DNA-binding site for Myc and its specific heterodimerization partner, Max, enabled the use of direct experiments to elucidate how Myc functions in vivo and how this function is modulated by Max. Here we demonstrate that exogenously expressed Myc is capable of activating transcription in vivo through its specific DNA-binding site. Moreover, transcriptional activation by Myc is dependent on the basic region, the integrity of the helix-loop-helix and leucine zipper dimerization motifs located in the carboxy-terminal portion of the protein, and the regions in the amino terminus conserved among Myc family proteins. In contrast to Myc, exogenously expressed Max elicited transcriptional repression and blocked transcriptional activation by Myc through the same DNA-binding site. Our results suggest a functional antagonism between Myc and Max which is mediated by their relative levels in the cells. A model for the activity of Myc and Max in vivo is presented.

Extracellular synaptic factors induce clustering of acetylcholine receptors stably expressed in fibroblasts

The clustering of nicotinic acetylcholine receptors (AChRs) is one of the first events observed during formation of the neuromuscular junction. To determine the mechanism involved in AChR clustering, we established a nonmuscle cell line (mouse fibroblast L cells) that stably expresses just one muscle-specific gene product, the AChR. We have shown that when Torpedo californica AChRs are expressed in fibroblasts, their immunological, biochemical, and electrophysiological properties all indicate that fully functional cell surface AChRs are produced. In the present study, the cell surface distribution and stability of Torpedo AChRs expressed in fibroblasts (AChR-fibroblasts) were analyzed and shown to be similar to nonclustered AChRs expressed in muscle cells. AChR-fibroblasts incubated with antibodies directed against the AChR induced the formation of small AChR microclusters (less than 0.5 micron 2) and caused an increase in the internalization rate and degradation of surface AChRs (antigenic modulation) in a manner similar to that observed in muscle cells. Two disparate sources of AChR clustering factors, extracellular matrix isolated from Torpedo electric organ and conditioned media from a rodent neuroblastoma-glioma hybrid cell line, each induced large (1-3 microns 2), stable AChR clusters with no change in the level of surface AChR expression. By exploiting the temperature-sensitive nature of Torpedo AChR assembly, we were able to demonstrate that factor-induced clusters were produced by mobilization of preexisting surface AChRs, not by directed insertion of newly synthesized AChRs. AChR clusters were never observed in the absence of extracellular synaptic factors. Our results suggest that these factors can interact directly with the AChR.

Analysis of early events in acetylcholine receptor assembly

Mammalian cell lines expressing nicotinic acetylcholine receptor (AChR) subunit cDNAs from Torpedo californica were used to study early events in AChR assembly. To test the hypothesis that individual subunits form homooligomeric intermediates before assembling into alpha 2 beta gamma delta pentamers, we analyzed the sedimentation on sucrose density gradients of each subunit expressed separately in cell lines. We have shown previously that the acute temperature sensitivity of Torpedo AChR subunit assembly is due, in part, to misfolding of the polypeptide chains (Paulson, H.L., and T. Claudio. 1990. J. Cell Biol. 110:1705-1717). We use this phenomenon to further analyze putative assembly-competent intermediates. In nonionic detergent at an assembly-permissive temperature, the majority of alpha, beta, gamma, and delta subunits sediment neither as 3-4S monomers nor as 9S complexes, but rather as 6S species whether synthesized in fibroblasts, myoblasts, or differentiated myosyncytia. Several results indicate that the 6S species are complexes comprised predominantly of incorrectly folded subunit polypeptides. The complexes represent homoaggregates which form rapidly within the cell, are stable to mild SDS treatment and, in the case of alpha, contain some disulfide-linked subunits. The coprecipitation of alpha subunit with BiP or GRP78, a resident protein of the ER, further indicates that at least some of these internally sequestered subunits also associated with an endogenous protein implicated in protein folding. The majority of subunits expressed in these cell lines appear to be aggregates of subunits which are not assembly intermediates and are not assembly-competent. The portion which migrates as monomer, in contrast, appears to be the fraction which is assembly competent. This fraction increases at temperatures more permissive for assembly, further indicating the importance of the monomer as the precursor to assembly of alpha 2 beta gamma delta pentamers.

Transcriptional suppression of cellular gene expression by c-Myc

High levels of c-Myc in mouse 3T3-L1 cells specifically suppress the expression of three collagen genes. This effect is exerted through collagen promoter sequences and requires the leucine zipper motif of c-Myc. Our data suggest that an important aspect of c-Myc transforming activity is the ability to suppress specific cellular gene transcription.

Transcriptional suppression of cellular gene expression by c-Myc

High levels of c-Myc in mouse 3T3-L1 cells specifically suppress the expression of three collagen genes. This effect is exerted through collagen promoter sequences and requires the leucine zipper motif of c-Myc. Our data suggest that an important aspect of c-Myc transforming activity is the ability to suppress specific cellular gene transcription.