c-Myc transactivation of LDH-A: implications for tumor metabolism and growth

Cancer cells are able to overproduce lactic acid aerobically, whereas normal cells undergo anaerobic glycolysis only when deprived of oxygen. Tumor aerobic glycolysis was recognized about seven decades ago; however, its molecular basis has remained elusive. The lactate dehydrogenase-A gene (LDH-A), whose product participates in normal anaerobic glycolysis and is frequently increased in human cancers, was identified as a c-Myc-responsive gene. Stably transfected Rat1a fibroblasts that overexpress LDH-A alone or those transformed by c-Myc overproduce lactic acid. LDH-A overexpression is required for c-Myc-mediated transformation because lowering its level through antisense LDH-A expression reduces soft agar clonogenicity of c-Myc-transformed Rat1a fibroblasts, c-Myc-transformed human lymphoblastoid cells, and Burkitt lymphoma cells. Although antisense expression of LDH-A did not affect the growth of c-Myc-transformed fibroblasts adherent to culture dishes under normoxic conditions, the growth of these adherent cells in hypoxia was reduced. These observations suggest that an increased LDH-A level is required for the growth of a transformed spheroid cell mass, which has a hypoxic internal microenvironment. Our studies have linked c-Myc to the induction of LDH-A, whose expression increases lactate production and is necessary for c-Myc-mediated transformation.

Max and inhibitory c-Myc mutants induce erythroid differentiation and resistance to apoptosis in human myeloid leukemia cells

We have used the human leukemia cell line K562 as a model to study the role of c-myc in differentiation and apoptosis. We have generated stable transfectants of K562 constitutively expressing two c-Myc inhibitory mutants: D106-143, that carries a deletion in the transactivation domain of the protein, and In373, that carries an insertion in the DNA-interacting region. We show here that In373 is able to compete with c-Myc for Max binding and to inhibit the transformation activity of c-Myc. K562 cells can differentiate towards erythroid or myelomonocytic lineages. K562 transfected with c-myc mutants showed a higher expression of erythroid differentiation markers, without any detectable effects in the myelomonocytic differentiation. We also transfected K562 cells with a zinc-inducible max gene. Ectopic Max overexpression resulted in an increased erythroid differentiation, thus reproducing the effects of c-myc inhibitory mutants. We also studied the role of c-myc mutants and max in apoptosis of K562 induced by okadaic acid, a protein phosphatases inhibitor. The expression of D106-143 and In373 c-myc mutants and the overexpression of max reduced the apoptosis mediated by okadaic acid. The common biochemical activity of D106-143 and In373 is to bind Max and hence to titrate out c-Myc to form non-functional Myc/Max dimers. Similarly, Max overexpression would decrease the relative levels of c-Myc/Max with respect to Max/Max. The results support a model where a threshold of functional c-Myc/Max is required to maintain K562 cells in an undifferentiated state and to undergo drug-mediated apoptosis.

Transformation by the Bmi-1 oncoprotein correlates with its subnuclear localization but not its transcriptional suppression activity

The bmi-1 oncogene cooperates with c-myc in transgenic mice, resulting in accelerated lymphoma development. Altering the expression of Bmi-1 affects normal embryogenesis. The protein product of bmi-1 is homologous to certain Drosophila Polycomb group proteins that regulate homeotic gene expression through alteration of chromatin structure. Chimeric LexA-Bmi-1 protein has previously been shown to repress transcription. How Bmi-1 functions in embryogenesis and whether this relates to the ability of Bmi-1 to mediate cellular transformation is unknown. We demonstrate here that Bmi-1 is able to transform rodent fibroblasts in vitro, providing a system that has allowed us to correlate its molecular properties with its ability to transform cells. We map functional domains of Bmi-1 involved in transcriptional suppression by using the GAL4 chimeric transcriptional regulator system. Deletion analysis shows that the centrally located helix-turn-helix-turn-helix-turn (HTHTHT) motif is necessary for transcriptional suppression whereas the N-terminal RING finger domain is not required. We demonstrate that nuclear localization requires KRMK (residues 230 to 233) and that the absence of nuclear entry ablates transformation. In addition, we find that the subnuclear localization of wild-type Bmi-1 to the rim of the nucleus requires the RING finger domain and correlates with its ability to transform. Our studies with Bmi-1 deletion mutants suggest that the ability of Bmi-1 to mediate cellular transformation correlates with its unique subnuclear localization but not its transcriptional suppression activity.

Human T-cell leukemia virus type I tax masks c-Myc function through a cAMP-dependent pathway

Human T-cell leukemia virus type I Tax is a pleiotropic gene regulator that functions through CREB/ATF- and NF-kappaB-mediated pathways. In most contexts, Tax is a potent gene activator. Here, we describe an unexpected finding of Myc repression by Tax. In cells that overexpress human T-cell leukemia virus type I Tax, the detection of c-Myc protein in the nucleus by a monoclonal antibody was masked. Tax prevented immunological visualization of a Myc epitope contained within amino acids 45-104, resulting in interference with Myc function in transcription and in anchorage-independent cell growth. Tax did not affect steady-state protein levels since detection of c-Myc with other antibodies was unperturbed. Four observations suggest that this Tax-Myc interaction is mediated through CREB/ATF signal transduction. 1) Tax point mutants, selectively defective for activation of CREB/ATF but not NF-kappaB, failed to mask c-Myc; 2) masking of Myc was abolished when Tax-expressing cells were treated with protein kinase inhibitor H-9; 3) Tax-specific shielding of Myc is absent in cells (B1R) that are genetically defective for cAMP signaling; and 4) forskolin treatment of cells mimicked Tax in masking the Myc epitope. Considered collectively, these findings suggest a regulation of Myc function at the level of localized protein conformation.

A link between c-Myc-mediated transcriptional repression and neoplastic transformation

Recent studies indicate that the transcription factor c-Myc contributes to oncogenesis by altering the expression of genes involved in cell proliferation, but its precise function in neoplasia remains ambiguous. The ability of c-Myc to bind the sequence CAC(G/A)TG and transactivate appears to be linked to its transforming activity; however, c-Myc also represses transcription in vitro through a pyrimidine-rich cis element termed the initiator (Inr). In transfection experiments using the adenoviral major late (adML) promoter, which contains two Myc binding sites and an Inr, we determined that c-Myc represses transcription through the initiator in vivo. This activity requires the dimerization domain and amino acids 106 to 143, which are located within the transactivation domain and are necessary for neoplastic transformation. We studied a lymphoma-derived c-Myc substitution mutation at 115-Phe, which is within the region required for transcriptional suppression, and found the mutant more effective than wild-type c-Myc in transforming rodent fibroblasts and in suppressing the adML promoter. Our studies of both loss-of-function and gain-of-function c-Myc mutations suggest a link between c-Myc-mediated neoplastic transformation and transcriptional repression through the Inr.

Genomic organization of human MXI1, a putative tumor suppressor gene

MXI1, a member of the MYC family of transcription factors, is thought to negatively regulate MYC function and may therefore be a potential tumor suppressor gene. Using detailed restriction mapping and partial DNA sequencing analysis, we have determined the genomic organization of the human MXI1 gene to facilitate a search for mutations that affect MXI1 function. The gene spans a region of approximately 60 kb on chromosome 10q24-q25 and comprises six exons. The correspondence of these exons to previously identified Mxi1 functional domains suggests that alternatively spliced transcripts may regulate Mxi1 functional activity. The presence of a cryptic ATG start codon in exon 2 suggests that a functional protein missing the SIN3-interacting domain (exon 1) may be generated by alternative splicing. Finally, we have identified two polymorphic regions within the MXI1 locus: a polymorphic CA repeat in the third intron and an AAAAC polymorphism in the noncoding region of exon 6. These findings will facilitate the analysis of tumors for the presence of inactivating mutations in MXI1 coding and regulatory sequences.

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.

The hepatitis B virus X protein increases the cellular level of TATA-binding protein, which mediates transactivation of RNA polymerase III genes

The hepatitis B virus X gene product transactivates a variety of cellular and viral genes. The mechanism for X induction of RNA polymerase (pol) III genes was investigated. By using Drosophila S-2 cells stably transformed with the X gene, the transient expression of a tRNA gene is enhanced. Comparing the transcriptional activities of extracts derived from these cells, all three types of RNA pol III promoters are stimulated by X. Interestingly, both S-2 and rat 1A cells stably transformed with the X gene produce increased cellular levels of the TATA-binding protein (TBP). By using various kinase inhibitors, it was found that the X-mediated increases in both transcription and TBP are dependent upon protein kinase C activation. Since TBP is a subunit of TFIIIB, the activity of this component fractionated from extracts derived from control and X-transformed cells was analyzed. These studies reveal that TFIIIB activity is substantially more limiting in control cells and that TFIIIB isolated from X-transformed cells has increased activity in reconstitution assays compared with TFIIIB isolated from control cells. Conversely, comparison of TFIIIC from control and X-transformed cell extracts revealed that there is relatively little change in its ability either to reconstitute transcription or to bind to DNA and that there is no change in the catalytic activity of RNA pol III. Studies were performed to determine whether directly increasing cellular TBP alone could enhance RNA pol III gene transcription. Transient expression of a TBP cDNA in rat 1A cells was capable of stimulating transcription activity from the resultant extracts in vitro. Together, these results demonstrate that one mechanism by which X mediates transactivation of RNA poll III genes is by increasing limiting TBP via the activation of cellular signaling pathways. The discovery that X increases cellular TBP, the universal transcription factor, provides a novel mechanism for the function of a viral transactivator protein and may explain the ability of X to produce such large and diverse effects on cellular gene expression.

c-Myc is glycosylated at threonine 58, a known phosphorylation site and a mutational hot spot in lymphomas

c-Myc is a helix-loop leucine zipper phosphoprotein that heterodimerizes with Max and regulates gene transcription in cell proliferation, cell differentiation, and programmed cell death. Previously, we demonstrated that c-Myc is modified by O-linked N-acetylglucosamine (O-GlcNAc) within or nearby the N-terminal transcriptional activation domain (Chou, T.-Y., Dang, C.V., and Hart, G.W. (1995) Proc. Natl. Acad. Sci. U.S.A. 92, 4417-4421). In this paper, we identified the O-GlcNAc attachment site(s) on c-Myc. c-Myc purified from sf9 insect cells was trypsinized, and its GlcNAc moieties were enzymically labeled with [3H]galactose. The [3H]galactose-labeled glycopeptides were isolated by reverse phase high performance liquid chromatography and then subjected to gas-phase sequencing, manual Edman degradation, and laser desorption/ionization mass spectrometry. These analyses show that threonine 58, an in vivo phosphorylation site in the transactivation domain, is the major O-GlcNAc glycosylation site of c-Myc. Mutation of threonine 58, frequently found in retroviral v-Myc proteins and in human Burkitt and AIDS-related lymphomas, is associated with enhanced transforming activity and tumorigenicity. The reciprocal glycosylation and phosphorylation at this biologically significant amino acid residue may play an important role in the regulation of the functions of c-Myc.

A link between increased transforming activity of lymphoma-derived MYC mutant alleles, their defective regulation by p107, and altered phosphorylation of the c-Myc transactivation domain

The c-Myc protein is a transcription factor with an N-terminal transcriptional regulatory domain and C-terminal oligomerization and DNA-binding motifs. Previous studies have demonstrated that p107, a protein related to the retinoblastoma protein, binds to the c-Myc transcriptional activation domain and suppresses its activity. We sought to characterize the transforming activity and transcriptional properties of lymphoma-derived mutant MYC alleles. Alleles encoding c-Myc proteins with missense mutations in the transcriptional regulatory domain were more potent than wild-type c-Myc in transforming rodent fibroblasts. Although the mutant c-Myc proteins retained their binding to p107 in in vitro and in vivo assays, p107 failed to suppress their transcriptional activation activities. Many of the lymphoma-derived MYC alleles contain missense mutations that result in substitution for the threonine at codon 58 or affect sequences flanking this amino acid. We observed that in vivo phosphorylation of Thr-58 was absent in a lymphoma cell line with a mutant MYC allele containing a missense mutation flanking codon 58. Our in vitro studies suggest that phosphorylation of Thr-58 in wild-type c-Myc was dependent on cyclin A and required prior phosphorylation of Ser-62 by a p107-cyclin A-CDK complex. In contrast, Thr-58 remained unphosphorylated in two representative mutant c-Myc transactivation domains in vitro. Our studies suggest that missense mutations in MYC may be selected for during lymphomagenesis, because the mutant MYC proteins have altered functional interactions with p107 protein complexes and fail to be phosphorylated at Thr-58.

Cyclin A links c-Myc to adhesion-independent cell proliferation

Adhesion-independent growth is a neoplastic phenotype that is inducible in Rat 1a fibroblasts by enforced MYC expression. The c-Myc protein has been well characterized as a transcription factor, yet the molecular basis of c-Myc-induced neoplastic transformation has remained elusive. In this report, we demonstrate a link between ectopic MYC expression, deregulated cyclin A levels, and adhesion-independent growth.

Glycosylation of the c-Myc transactivation domain

O-linked N-acetylglucosamine (O-GlcNAc) is an abundant and dynamic posttranslational modification composed of a single monosaccharide, GlcNAc, glycosidically composed of a single monosaccharide, GlcNAc, glycosidically linked to the side-chain hydroxyl of serine or threonine residues. Although O-GlcNAc occurs on a myriad of nuclear and cytoplasmic proteins, only a few have thus far been identified. These O-GlcNAc-bearing proteins are also modified by phosphorylation and form reversible multimeric complexes. Here we present evidence for O-GlcNAc glycosylation of the oncoprotein c-Myc, a helix-loop-helix/leucine zipper phosphoprotein that heterodimerizes with Max and participates in the regulation of gene transcription in normal and neoplastic cells. O-GlcNAc modification of c-Myc is shown by three different methods: (i) demonstration of lectin binding to in vitro translated protein using a protein-protein interaction mobility-shift assay; (ii) glycosidase or glycosyltransferase treatment of in vitro translated protein analyzed by lectin affinity chromatography; and (iii) direct characterization of the sugar moieties on purified recombinant protein overexpressed in either insect cells or Chinese hamster ovary cells. Analyses of serial deletion mutants of c-Myc further suggest that the O-GlcNAc site(s) are located within or near the N-terminal transcription activation/malignant transformation domain, a region where mutations of c-Myc that are frequently found in Burkitt and AIDS-related lymphomas cluster.

Cell density and paradoxical transcriptional properties of c-Myc and Max in cultured mouse fibroblasts

Deregulated expression of the c-Myc oncoprotein occurs in several human malignancies. The c-Myc protein behaves as a transcription factor, and undoubtedly its role in carcinogenesis involves its ability to affect the expression of genes involved in cell growth. c-Myc has been reported to both activate and repress transcription in transient transfection experiments using reporter constructs bearing multiple copies of the c-Myc binding site, CAC (G/A) TG. We investigated these apparently paradoxical effects of c-Myc by determining if they arose from differences in the cell proliferation states of transfected cells. We found that endogenous c-Myc protein levels vary inversely with the degree of cell confluency, such that at low cell confluency, where endogenous levels of c-Myc are high and presumably endogenous levels of Max are limiting, exogenous c-Myc fails to affect basal transcription. In cells at high cell confluency, in which endogenous c-Myc levels are low, exogenous c-Myc augments transactivation by titrating the relative excess endogenous Max. These observations suggest that the apparently paradoxical behavior of c-Myc in transfection experiments is partially dependent on ambient cellular levels of c-Myc.

The basic helix-loop-helix protein upstream stimulating factor regulates the cardiac ventricular myosin light-chain 2 gene via independent cis regulatory elements

Previous studies have documented that 250 bp of the rat cardiac ventricular myosin light-chain 2 (MLC-2v) promoter is sufficient to confer cardiac muscle-specific expression on a luciferase reporter gene in both transgenic mice and primary cultured neonatal rat myocardial cells. Utilizing ligation-mediated PCR to perform in vivo dimethyl sulfate footprinting, the present study has identified protein-DNA interaction within the position from -176 to -165. This region, identified as MLE1, contains a core sequence, CACGTG, which conforms to the consensus E-box site and is identical to the upstream stimulating factor (USF)-binding site of the adenovirus major late promoter. Transient assays of luciferase reporter genes containing point mutations of the site demonstrate the importance of this cis regulatory element in the transcriptional activation of this cardiac muscle gene in ventricular muscle cells. The protein complex that occupies this site is capable of binding to HF-1a and PRE B sites which are known to be required for cardiac muscle-specific expression of rat MLC-2v and alpha-myosin heavy-chain genes, respectively. This study provides direct evidence that USF, a member of the basic helix-loop-helix leucine zipper family, binds to MLE1, HF-1a, and PRE B sites and suggests that it is a component of protein complexes that may coordinately control the expression of MLC-2v and alpha-myosin heavy-chain genes. The current study also provides evidence that USF can positively and negatively regulate the MLC-2v gene via independent cis regulatory elements.

Participation of cyclin A in Myc-induced apoptosis

The involvement of c-Myc in cellular proliferation or apoptosis has been linked to differential cyclin gene expression. We observed that in both proliferating cells and cells undergoing apoptosis, cyclin A (but not B, C, D1, and E) mRNA level was elevated in unsynchronized Myc-overexpressing cells when compared with parental Rat1a fibroblasts. We further demonstrated that Zn(2+)-inducible cyclin A expression was sufficient to cause apoptosis. When Myc-induced apoptosis was blocked by coexpression of Bcl-2, the levels of cyclin C, D1, and E mRNAs were also elevated. Thus, while apoptosis induced by c-Myc is associated with an elevated cyclin A mRNA level, protection from apoptosis by coexpressed Bcl-2 is associated with a complementary increase in cyclin C, D1, and E mRNAs.

Differential binding of c-Myc and Max to nucleosomal DNA

The ability of a transcription factor to function in vivo must be determined in part by its ability to bind to its recognition site in chromatin. We have used Max and derivatives of c-Myc to characterize the effect of changes of dimerization partner on binding to nucleosomal DNA templates. We find that homo- and heterodimeric complexes of these proteins bind to the CACGTG sequence in free DNA with similar affinities. Although Max homodimers bind to nucleosomes, truncated c-Myc homodimers do not. Surprisingly, modifying the c-Myc dimerization interface or changing its dimerization partner to Max enables nucleosomal DNA binding. Thus, changes in dimer structure or dimerization efficiency can have significant effects on nucleosome binding that are not predicted from their affinity for free DNA. We conclude that domains other than the basic region per se influence the ability of a transcription factor to bind to nucleosomal DNA and that changes of dimerization partner can directly affect the ability of a factor to occupy nucleosomal binding sites.

Binding and suppression of the Myc transcriptional activation domain by p107

An amino-terminal transactivation domain is required for Myc to function as a transcription factor controlling cell proliferation, differentiation, and apoptosis. A complementary DNA expression library was screened with a Myc fusion protein to identify proteins interacting with this domain, and a clone encoding the Rb-related p107 protein was isolated. The p107 protein was shown to associate with Myc in vivo and to suppress the activity of the Myc transactivation domain. However, mutant forms of Myc from Burkitt lymphoma cells, which contain sequence alterations in the transactivation domain, were resistant to p107-mediated suppression. Thus, disruption of a regulatory interaction between Myc and p107 may be important in tumorigenesis.

B-myc inhibits neoplastic transformation and transcriptional activation by c-myc

B-myc is a recently described myc gene whose product has not been functionally characterized. The predicted product of B-myc is a 168-amino-acid protein with extensive homology to the c-Myc amino-terminal region, previously shown to contain a transcriptional activation domain. We hypothesized that B-Myc might also function in transcriptional regulation, although its role in regulating gene expression is predicted to be unique, because B-Myc lacks the specific DNA-binding motif found in other Myc proteins. To determine whether B-Myc could interact with the transcriptional machinery, we studied the transcriptional activation properties of a chimeric protein containing B-Myc sequences fused to the DNA-binding domain of the yeast transcriptional activator GAL4 (GAL4-B-Myc). We found that GAL4-B-Myc strongly activated expression of a GAL4-regulated reporter gene in mammalian cells. In addition, full-length B-Myc was able to inhibit or squelch reporter gene activation by a GAL4 chimeric protein containing the c-Myc transcriptional activation domain. We also observed that B-Myc dramatically inhibited the neoplastic cotransforming activity of c-Myc and activated Ras in rat embryo cells. Because B-Myc inhibits both neoplastic transformation and transcriptional activation by c-Myc, we suggest that the transforming activity of c-Myc is related to its ability to regulate transcription. Whether B-Myc functions biologically to squelch transcription and/or to regulate transcription through a specific DNA-binding protein remains unestablished.

Detection and modulation in vivo of helix-loop-helix protein-protein interactions

Studies are described that allow for the in vivo detection of helix-loop-helix (HLH) protein-protein interaction. The assay used requires HLH protein-protein interaction to reconstitute a functional GAL4 transcriptional activator, which in turn activates a reporter gene placed downstream of GAL4 DNA binding sequences. Using this assay, we are able to detect intracellular heterodimerization but not homodimerization of the MyoD, E12, and Id gene products. In addition, using this system we are unable to detect stable heterodimerization between MyoD and c-Jun. We also show that expression of activated rasH gene product does not inhibit and may stabilize HLH protein-protein interaction. This system may be of general utility in studying the modulation of transcription factor interactions.

DNA binding by the Myc oncoproteins

The c-Myc protein is a potential activator of transcription, with the ability to bind in a heterodimer form with Max to DNA sequences containing the core hexanucleotide sequence CAC(G/A)TG. These properties are shared with L-Myc, a homologous oncoprotein expressed in small cell lung carcinoma cells; with N-Myc, expressed in neuroblastoma cells; and with avian v-Myc, the c-Myc homolog expressed by a chicken retrovirus. The c-Myc, and probably v-Myc, proteins also have nonspecific DNA binding function, which may improve the kinetics of specific DNA binding. Curiously, this domain appears not to be conserved in L-Myc or N-Myc [22]. The data that have accumulated to date are consistent with a model in which a c-Myc/Max heterodimer positively regulates the transcription of growth-related genes, with Max homodimer functioning as a negative regulator of the same genes (Fig. 4) [55]. Max is expressed constitutively at low levels, whereas c-Myc is expressed at low levels in quiescent cells, but high levels of c-Myc are induced by mitogenic stimulation [56]. Thus, in proliferating cells c-Myc/Max heterodimers might bind to the regulatory elements of growth-related genes, where the c-Myc TAD might stimulate transcription. Conversely, in quiescent cells with little c-Myc present, Max homodimers might predominate. They might bind to exactly the same regulatory elements, but due to the apparent absence of a TAD in Max [36], transcription might be repressed. Validation of this model will require the demonstration of clear regulation of a physiological promoter of a growth-related gene by c-Myc/Max. Although it is widely believed that Myc proteins function as transcriptional activators, this hypothesis has only been conclusively supported recently [57, 58]. A theory that c-Myc plays a role in DNA replication is not as well substantiated at this point. It is even possible that Myc might be involved in both transcription and replication. Although the function of these fascinating proteins has been enigmatic for a decade, the rate of progress in our understanding of Myc function is accelerating. Such progress will undoubtedly lead to a deeper appreciation of this protein, which lies at the crossroads of cellular proliferation and oncogenesis.

Function of the c-Myc oncoprotein

The c-Myc protein, the product of the c-myc proto-oncogene, is a nuclear phosphoprotein with DNA binding properties. Deregulated c-myc expression participates in the development of experimentally induced tumors, and its expression appears to be abnormal in many naturally occurring malignancies. Although the precise molecular mechanism of c-Myc activity in oncogenesis and in normal cell proliferation is unknown, recent advances have uncovered a series of molecular and cellular properties of c-Myc. These properties include nuclear localization, transcriptional activation, oligomerization nonspecific and specific DNA binding. Recently, the c-Myc protein was found to heterodimerize with Max, a protein that cooperates with c-Myc to bind specifically to a core DNA sequence, CAC(G/A)TG. These characteristics suggest that c-Myc participates in the regulation of gene transcription in normal and neoplastic cells.

Opposite orientations of DNA bending by c-Myc and Max

The control of gene transcription requires specific protein-protein and protein-DNA interactions. c-Myc, the protein product of the c-myc protooncogene, is a member of the basic helix-loop-helix leucine-zipper class of transcription factors. Although c-Myc is able to bind to a specific core hexanucleotide DNA sequence (CACGTG), its precise function in modulating transcription remains unclear. The recent discovery of Max, a basic helix-loop-helix leucine-zipper partner protein for c-Myc, suggests that the ability of c-Myc to regulate transcription is modulated by the presence of Max. By taking advantage of the altered mobility of protein-bound DNA in the mobility-shift assay, we demonstrate the homo- and heterodimeric complexes of c-Myc and Max are able to cause increased DNA flexure as measured by the circular permutation assay. Based on phasing analysis, c-Myc and Max homodimers bend DNA in opposite orientations, whereas c-Myc-Max heterodimers cause a smaller bend, in an orientation similar to that induced by Max homodimers. To address the possibility that the apparent opposite orientation of bending was the result of DNA unwinding by one of the proteins, we measured the ability of c-Myc and Max homodimers to affect DNA unwinding; we were unable to show any specific unwinding caused by c-Myc or Max. In addition to demonstrating that members of the basic helix-loop-helix leucine-zipper class of transcription factors are able to induce DNA bending, these results suggest that different transcription factor dimers are able to bind to identical DNA sequences and yet have distinct structural effects.