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

Heterogeneity of the NIH3T3 Fibroblast Cell Line

The embryonic mouse fibroblast cell line NIH3T3 is widely used in life science research, including the study of cell cycle control and primary cilia. Fibroblasts are the most important cell type in connective tissue, as they produce components of the extracellular matrix and determine tissue architecture. However, they are very heterogeneous and consist of subtypes specific to their organ of residence, among others. The NIH3T3 cell line was derived from whole mouse embryos that developed to pre-birth and is therefore most likely composed of different fibroblast subtypes. Furthermore, prolonged proliferation may have influenced their cellular composition. A heterogeneous cell population is unsuitable for any sophisticated research project. We found that the proportion of ciliated cells in the total NIH3T3 cell population was highly variable and asked whether this was a consequence of cellular heterogeneity and what molecular signatures were associated with it. We have established sub-cell lines by clonal expansion of single cells and characterized them morphologically and molecularly. Eventually, a myofibroblast-like and a fibroblast-like cell line were generated that differ in ciliation and proliferation. These homogeneous cell lines are valuable for a more detailed study of their molecular signatures, not least to uncover further the molecular pathways that contribute to the formation of the primary cilium.

Paradoxical Behavior of Oncogenes Undermines the Somatic Mutation Theory

The currently accepted theory on the influence of DNA mutations on carcinogenesis (the Somatic Mutation Theory, SMT) is facing an increasing number of controversial results that undermine the explanatory power of mutated genes considered as "causative" factors. Intriguing results have demonstrated that several critical genes may act differently, as oncogenes or tumor suppressors, while phenotypic reversion of cancerous cells/tissues can be achieved by modifying the microenvironment, the mutations they are carrying notwithstanding. Furthermore, a high burden of mutations has been identified in many non-cancerous tissues without any apparent pathological consequence. All things considered, a relevant body of unexplained inconsistencies calls for an in depth rewiring of our theoretical models. Ignoring these paradoxes is no longer sustainable. By avoiding these conundrums, the scientific community will deprive itself of the opportunity to achieve real progress in this important biomedical field. To remedy this situation, we need to embrace new theoretical perspectives, taking the cell-microenvironment interplay as the privileged pathogenetic level of observation, and by assuming new explanatory models based on truly different premises. New theoretical frameworks dawned in the last two decades principally focus on the complex interaction between cells and their microenvironment, which is thought to be the critical level from which carcinogenesis arises. Indeed, both molecular and biophysical components of the stroma can dramatically drive cell fate commitment and cell outcome in opposite directions, even in the presence of the same stimulus. Therefore, such a novel approach can help in solving apparently inextricable paradoxes that are increasingly observed in cancer biology.

β-catenin links cell seeding density to global gene expression during mouse embryonic stem cell differentiation

Although cell density is known to affect numerous biological processes including gene expression and cell fate specification, mechanistic understanding of what factors link cell density to global gene regulation is lacking. Here, we reveal that the expression of thousands of genes in mouse embryonic stem cells (mESCs) is affected by cell seeding density and that low cell density enhances the efficiency of differentiation. Mechanistically, β-catenin is localized primarily to adherens junctions during both self-renewal and differentiation at high density. However, when mESCs differentiate at low density, β-catenin translocates to the nucleus and associates with Tcf7l1, inducing co-occupied lineage markers. Meanwhile, Esrrb sustains the expression of pluripotency-associated genes while repressing lineage markers at high density, and its association with DNA decreases at low density. Our results provide new insights into the previously neglected but pervasive phenomenon of density-dependent gene regulation.

Co-emergence and Collapse: The Mesoscopic Approach for Conceptualizing and Investigating the Functional Integration of Organisms

The fall of reductionist approaches to explanation leaves biology with an unescapable challenge: how to decipher complex systems. This entails a number of very critical questions, the most basic ones being: "What do we mean by 'complex'?" and "What is the system we should look for?" In complex systems, constraints belong to a higher level that the molecular one and their effect reduces and constrains the manifold of the accessible internal states of the system itself. Function is related but not deterministically imposed by the underlying structure. It is quite unlikely that such kind of complexity could be grasped by current approaches focusing on a single organization scale. The natural co-emergence of systems, parts and properties can be adopted as a hypothesis-free conceptual framework to understand functional integration of organisms, including their hierarchical or multilevel patterns, and including the way scientific practice proceeds in approaching such complexity. External, "driving" factors - order parameters and control parameters provided by the surrounding microenvironment - are always required to "push" the components' fate into well-defined developmental directions. In the negative, we see that in pathological processes such as cancer, organizational fluidity, collapse of levels and dynamic heterogeneity make it hard to even find a level of observation for a stable explanandum to persist in scientific practice. Parts and the system both lose their properties once the system is destabilized. The mesoscopic approach is our proposal to conceptualizing, investigating and explaining in biology. "Mesoscopic way of thinking" is increasingly popular in the epistemology of biology and corresponds to looking for an explanation (and possibly a prediction) where "non-trivial determinism is maximal": the "most microscopic" level of organization is not necessarily the place where "the most relevant facts do happen." A fundamental re-thinking of the concept of causality is also due for order parameters to be carefully and correctly identified. In the biological realm, entities have relational properties only, as they depend ontologically on the context they happen to be in. The basic idea of a relational ontology is that, in our inventory of the world, relations are somehow prior to the relata (i.e., entities).

High expression of density-regulated re-initiation and release factor drives tumourigenesis and affects clinical outcome

Previously, certain experiments have suggested that density-regulated re-initiation and release factor (DENR) could serve important roles in cancer, however, to the best of our knowledge, a comprehensive analysis of DENR and its association with cancer patient survival is lacking. The aim of the current study was to investigate the expression of DENR in multiple tumour types and to evaluate the effects of DENR on survival in malignancies. Sample expression profiles were downloaded from the Gene Expression Omnibus database. Association between DENR expression and clinicopathological features was analysed by Chi-square tests. The effects of DENR on survival were evaluated by Kaplan-Meier analysis. The results of the current study demonstrate that DENR expression was upregulated in nine cancer types. High DENR expression indicated poor prognosis of patients. The results of the present study demonstrated that DENR is highly expressed in multiple tumour types and may be used as a potential prognostic marker and therapeutic target.

Cellular and viral oncogenes: the key to unlocking unknowns of Kaposi's sarcoma-associated herpesvirus pathogenesis

Oncogenic viruses carry an extensive arsenal of oncogenes for hijacking cellular pathways. Notably, variations in oncogenes among tumor-producing viruses give rise to different mechanisms for cellular transformation. Specifically, Kaposi's sarcoma-associated herpesvirus (KSHV) is an oncogenic virus able to infect and transform a variety of cell types. The oncogenicity of KSHV disseminates from the virus' ability to induce and encode a wide variety of both cellular and viral oncogenes. Such an array of cellular and viral oncogenes enables KSHV to induce the malignant phenotype of a KSHV-associated cancer. Evolutionarily, KSHV has acquired many oncogenic homologues capable of inducing cell proliferation, cell differentiation, cell survival, and immune evasion. Integration between inducing and encoding oncogenes plays a vital role in KSHV pathogenicity. KSHV is alleged to harbor the highest number of potential oncogenes by which a virus promotes cellular transformation and malignancy. Many KSHV inducing/encoding oncogenes are mainly expressed during the latent phase of KSHV infection, a period required for virus establishment of malignant cellular transformation. Elucidation of the exact mechanism(s) by which oncogenes promote KSHV pathogenicity would not only give rise to potential novel therapeutic targets/drugs but would also add to our understanding of cancer biology. The scope of this review is to examine the roles of the most important cellular and viral oncogenes involved in KSHV pathogenicity.

The HMGA1-COX-2 axis: a key molecular pathway and potential target in pancreatic adenocarcinoma

CONTEXT

Although pancreatic cancer is a common, highly lethal malignancy, the molecular events that enable precursor lesions to become invasive carcinoma remain unclear. We previously reported that the high-mobility group A1 (HMGA1) protein is overexpressed in >90% of primary pancreatic cancers, with absent or low levels in early precursor lesions.

METHODS

Here, we investigate the role of HMGA1 in reprogramming pancreatic epithelium into invasive cancer cells. We assessed oncogenic properties induced by HMGA1 in non-transformed pancreatic epithelial cells expressing activated K-RAS. We also explored the HMGA1-cyclooxygenase (COX-2) pathway in human pancreatic cancer cells and the therapeutic effects of COX-2 inhibitors in xenograft tumorigenesis.

RESULTS

HMGA1 cooperates with activated K-RAS to induce migration, invasion, and anchorage-independent cell growth in a cell line derived from normal human pancreatic epithelium. Moreover, HMGA1 and COX-2 expression are positively correlated in pancreatic cancer cell lines (r(2) = 0.93; p < 0.001). HMGA1 binds directly to the COX-2 promoter at an AT-rich region in vivo in three pancreatic cancer cell lines. In addition, HMGA1 induces COX-2 expression in pancreatic epithelial cells, while knock-down of HMGA1 results in repression of COX-2 in pancreatic cancer cells. Strikingly, we also discovered that Sulindac (a COX-1/COX-2 inhibitor) or Celecoxib (a more specific COX-2 inhibitor) block xenograft tumorigenesis from pancreatic cancer cells expressing high levels of HMGA1.

CONCLUSIONS

Our studies identify for the first time an important role for the HMGA1-COX-2 pathway in pancreatic cancer and suggest that targeting this pathway could be effective to treat, or even prevent, pancreatic cancer.

Fatty acids induce apoptosis in human smooth muscle cells depending on chain length, saturation, and duration of exposure

OBJECTIVE

Plasma free fatty acid (FFA) concentrations are increased in states of insulin resistance. Therefore, this study evaluated apoptosis and underlying mechanisms induced by selected nutritional FFAs, a defined FFA-mix, and human plasma containing high FFA concentrations in human smooth muscle cells (HSMCs).

RESEARCH DESIGN AND METHODS

HSMCs were incubated (24-72 h) with selected FFAs (100-300 micromol/l), an FFA-mix (palmitic-/stearic-/oleic-/linoleic-/alpha-linolenic acid=2.6/1/3.6/9/1; 300-900 micromol/l), or with high FFA-plasma (600 micromol/l) versus respective control cultures. Apoptosis, caspase activation, and protein expression were determined by DNA-fragmentation assays, flow cytometry, and Western blots, respectively.

RESULTS

Exposure (24h) of HSMCs to 300 micromol/l stearic-, oleic-, linoleic-, alpha-linolenic-, and arachidonic acid induced apoptosis, correlating (p<0.01) with the FFAs' chain length (r=0.602) and number of FFA double bonds (r=0.956). After 48 h, 100 micromol/l of all tested FFAs - including palmitic acid - were already sufficient to trigger HSMCs' cell death. FFA-exposure resulted in activation of caspases and apoptosis was completely abolished by co-incubation with caspase inhibitors and negatively correlated (p<0.01) with the base-excision repair protein XRCC1 (r=-0.765) and with c-myc's antagonist mad (r=-0.916), whereas positive correlations (p<0.01) were found for protein expression of the proto-oncogene c-myc (r=0.972) and the transcription factor E2F-1 (r=0.971). Exposure of HSMCs to the defined FFA-mix and to plasma samples from individuals with elevated plasma FFAs supported the results obtained by defined FFA stimulation.

CONCLUSIONS

Since smooth muscle cells surround the macrophage/foam cell/lipid-laden artheromatous core of atherosclerotic lesions with a protective fibrous cap, their FFA-induced HSMC apoptosis could contribute to progression of atherosclerosis by thinning of the fibrous cap and subsequent plaque destabilization.

Function of the c-Myc oncogenic transcription factor

The c-myc gene and the expression of the c-Myc protein are frequently altered in human cancers. The c-myc gene encodes the transcription factor c-Myc, which heterodimerizes with a partner protein, termed Max, to regulate gene expression. Max also heterodimerizes with the Mad family of proteins to repress transcription, antagonize c-Myc, and promote cellular differentiation. The constitutive activation of c-myc expression is key to the genesis of many cancers, and hence the understanding of c-Myc function depends on our understanding of its target genes. In this review, we attempt to place the putative target genes of c-Myc in the context of c-Myc-mediated phenotypes. From this perspective, c-Myc emerges as an oncogenic transcription factor that integrates the cell cycle machinery with cell adhesion, cellular metabolism, and the apoptotic pathways.

drp, a novel protein expressed at high cell density but not during growth arrest

Contact is a vital mechanism used by cells to interact with their environment. Contact with living and nonliving elements adjacent to a cell is the basis for many common biological events ranging from growth regulation to metastasis to embryonic pattern formation. We describe the cloning and characterization of a novel density-regulated protein (drp) whose expression is increased in cultured cells at high density compared with cells at low density. A drp cDNA was isolated from the human teratocarcinoma cell line PA-1. Northern analysis with a drp probe revealed transcripts of 2.8 and 3.2 kb. The drp RNA was expressed in a variety of tissues, with the highest amounts in skeletal and cardiac muscle. Using antipeptide antisera, increasing amounts of a 70-kDa protein were detected using several experimental approaches in several cells lines as cell density is increased. Conditioned medium from high-density cells was unable to induce expression of drp in cells growing at low density. Similarly, growth arrest by serum starvation or transforming growth factor-beta (TGF-beta) treatment failed to elicit drp expression. We conclude that drp is a novel protein whose expression is increased at high cell density but not growth arrest.

Inhibition of cell growth in CaCO2 cells by the polyamine analogue N1,N12-bis(ethyl)spermine is preceded by a reduction in MYC oncoprotein levels

The polyamine analogue N1,N12bis(ethyl)spermine (BESpm) is a potent inhibitor of cell proliferation and is representative of a class of agents currently in clinical trials. Previous studies have demonstrated that BESpm treatment can produce a decrease in the mRNA levels of the protooncogene c-myc resulting from decreased transcription. Investigation into the mechanism of the antiproliferative effect of BESpm in the colon cancer cell line CaCO2 indicated that significant reduction in MYC protein, but not c-myc mRNA levels, preceded cytostasis. Specificity of the downregulation of MYC expression by BESpm treatment was demonstrated by comparison to effects on the polyamine catabolic enzyme spermidine/spermine N1-acetyltransferase (SSAT) and the polyamine biosynthetic enzyme ornithine decarboxylase (ODC). SSAT activity rapidly increased while levels of ODC activity decreased after BESpm treatment. Measurement of intracellular polyamines demonstrated significant uptake of the analogue after 24 hours, which was concurrent with a reduction of spermine and spermidine levels. Thus, cellular uptake of BESpm mediated a reduction of polyamine levels that was associated with a decrease of MYC protein at the post-transcriptional level.

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.

Distinct roles for MAX protein isoforms in proliferation and apoptosis

MAX is a basic helix-loop-helix-leucine zipper protein that plays a central role in the transcriptional control of Myc oncoproteins. MYC-MAX heterodimers stimulate transcription, whereas MAX homodimers, or heterodimers between MAX and members of the MAD family of basic helix-loop-helix-leucine zipper proteins, repress transcription. Max exists in two major isomeric forms, MAX(L) and MAX(S), which differ from one another only by a 9-amino acid insertion/deletion. We show here that MAX(L) is much more effective at homodimeric DNA binding than MAX(S). In NIH3T3 cells, MAX(L) was able to repress a c-Myc-responsive reporter gene whereas MAX(S) either stimulated the reporter gene or had little effect on its expression. In comparison to control cell lines or those stably over-expressing MAX(S), MAX(L)-over-expressing cell lines showed reduced expression of transiently expressed or endogenous c-Myc responsive genes, grew more slowly, possessed a higher growth factor requirement, and showed accelerated apoptosis following growth factor deprivation. Differential effects on growth and apoptosis represent two previously unrecognized properties of MAX proteins. These can at least partly be explained by the differences in their DNA binding abilities and their effects on target gene expression.

An improved quantitative RT-PCR fluorescent method for analysis of gene transcripts in the STS-65 space shuttle experiment

We describe a reverse transcription polymerase chain reaction (RT-PCR) technique using fluorescent dUTP to examine changes in mRNA level in samples. In this procedure, the amplified product is identified by the fluorescent detection system in an automated DNA sequencer, and if the MW of the DNA/RNA or the fluorescent dye is different, several samples can be analyzed in a single lane. The basis for this technique is similar to that of radiolabeled methods, and we applied this technique for the comparison of the expression level of the rat c-myc gene in osteoblasts exposed to microgravity and unit gravity conditions. Using the fluorescent- and radiolabeled methods, the level of rat c-myc mRNA were compared quantitatively and the results demonstrated that the c-myc expression level was not altered by microgravity. Therefore, this fluorescent RT-PCR technique is useful for gene expression analysis particularly when the samples are limited, such as in space flight experiments. The method also allows for rapid assessment of mRNA changes in many samples simultaneously.

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.

Adenovirus-mediated transfer of CCAAT/enhancer-binding protein-alpha identifies a dominant antiproliferative role for this isoform in hepatocytes

CCAAT/enhancer-binding protein (C/EBP) isoforms are thought to be important regulators of the hepatocyte phenotype. However, the specific physiological roles of different isoforms are poorly understood because hepatocytes express multiple C/EBPs, and various isoforms have overlapping functions. To identify the functions of C/EBPalpha in mature hepatocytes, replication-defective adenovirus vectors were used to efficiently and homogeneously overexpress the mouse C/EBPalpha gene in a SV40 virus-conditionally transformed rat hepatocyte line that can be induced to express C/EBPbeta and C/EBPdelta but that has little endogenous C/EBPalpha expression. Hepatocytes were infected with a recombinant adenovirus vector carrying the cDNA for C/EBPalpha driven by Rous sarcoma virus promoter elements (AdCEBPalpha) or a similar vector carrying the Escherichia coli lacZ gene (Adbetagal). Staining for beta-galactosidase demonstrated an infection efficiency of 100% at a multiplicity of infection of 25 plaque-forming units/cell and persistence of foreign gene expression for at least 9 days. Cultures infected with AdCEBPalpha had 50-fold higher levels of C/EBPalpha mRNA and protein than those infected with Ad-beta-gal, but similar expression of C/EBP-beta. Infection with AdCEBPalpha inhibited proliferation in cells expressing little C/EBPbeta, even when proliferation was driven by the SV40 transforming antigen, and also blunted mitogenic induction of the c-myc proto-oncogene in nontransformed cells with high levels of C/EBPbeta. Although overexpression of C/EBPalpha consistently increased C/EBPalpha DNA binding activity, it was not sufficient for albumin expression. Infection with AdCEBPalpha only increased albumin mRNA levels in nontransformed cells that also expressed relatively high levels of C/EBPbeta. Thus, in hepatocytes, C/EBPalpha has a dominant antiproliferative function, but must interact with other factors to regulate hepatocyte-specific gene expression.

Discrimination between different E-box-binding proteins at an endogenous target gene of c-myc

c-myc plans a key role in regulating mammalian cell proliferation and apoptosis. The gene codes for a transcription factor, Myc, that belongs to the helix-loop-helix/leucine zipper (HLH/LZ) family of proteins. Myc heterodimerizes with a partner protein termed Max; the heterodimeric complex binds to CAC(G/A)TG (E-box) sequences and activates transcription from these sites. However, several other HLH/LZ proteins, including USF and TFE-3, bind to and trans-activate from the same element, yet have no documented effect on cell proliferation or apoptosis. Therefore, it is likely that mechanisms exist that discriminate between these proteins for activation of natural target genes of Myc. We now show that trans-activation from the E-box in the rat prothymosin-alpha intron enhancer is indeed specific for Myc, and identify both the distance from the start site of transcription and a second E-box element adjacent to that recognized by Myc as critical determinants of specificity. Surprisingly, transcription activation domains required for Myc to activate from this distal enhancer position differ from previously mapped domains and closely correlate with those domains essential for transformation. As observed in transformation assays, Myc and Max strongly synergize in activation from a distal enhancer position. Our data suggest that trans-activation from the prothymosin intron enhancer is a faithful reflection of the transforming properties of the Myc protein.

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.

Variant Max protein, derived by alternative splicing, associates with c-Myc in vivo and inhibits transactivation

Max (Myc-associated factor X) is a basic helix-loop-helix/leucine zipper protein that has been shown to play a central role in the functional activity of c-Myc as a transcriptional activator. Max potentiates the binding of Myc-Max heterodimers through its basic region to its specific E-box Myc site (EMS), enabling c-Myc to transactivate effectively. In addition to the alternatively spliced exon a, several naturally occurring forms of alternatively spliced max mRNAs have been reported, but variant protein products from these transcripts have not been detected. Using Western blot (immunoblot) and immunoprecipitation analysis, we have identified a variant form of Max protein (16 to 17 kDa), termed dMax, in detergent nuclear extracts of murine B-lymphoma cells, normal B lymphocytes, and NIH 3T3 fibroblasts. Cloning and sequencing revealed that dMax contains a deletion spanning the basic region and helix 1 and the loop of the helix-loop-helix region, presumably as a result of alternative splicing of max RNA. S1 nuclease analysis confirmed the presence of the mRNA for dMax in cells. The dMax protein, prepared via in vitro transcription and translation, associated with bacterially synthesized Myc-glutathione S-transferase. Coimmunoprecipitation of dMax and c-Myc indicated their intracellular association. In vitro-synthesized dMax failed to bind EMS DNA, presumably because of the absence of the basic region. Coexpression of dMax inhibited EMS-mediated transactivation by c-Myc. Thus dMax, which can interact with c-Myc, appears to function as a dominant negative regulator, providing an additional level of regulation to the transactivation potential of c-Myc.