The leucine zipper of c-Myc is required for full inhibition of erythroleukemia differentiation

Nano-Assembly of Pamitoyl-Bioconjugated Coenzyme-A for Combinatorial Chemo-Biologics in Transcriptional Therapy

Pathogenesis, the biological mechanism that leads to the diseased state, of many cancers is driven by interruptions to the role of Myc oncoprotein, a regulator protein that codes for a transcription factor. One of the most significant biological interruptions to Myc protein is noted as its dimerization with Max protein, another important factor of family of transcription factors. Binding of this heterodimer to E-Boxes, enhancer boxes as DNA response element found in some eukaryotes that act as a protein-binding site and have been found to regulate gene expression, are interrupted to regulate cancer pathogenesis. The systemic effectiveness of potent small molecule inhibitors of Myc-Max dimerization has been limited by poor bioavailability, rapid metabolism, and inadequate target site penetration. The potential of gene therapy for targeting Myc can be fully realized by successful synthesis of a smart cargo. We developed a "nuclein" type nanoparticle "siNozyme" (45 ± 5 nm) from nanoassembly of pamitoyl-bioconjugated acetyl coenzyme-A for stable incorporation of chemotherapeutics and biologics to achieve remarkable growth inhibition of human melanoma. Results indicated that targeting transcriptional gene cMyc with siRNA with codelivery of a topoisomerase inhibitor, amonafide caused ∼90% growth inhibition and 95% protein inhibition.

Structure-based Inhibitor Design for the Intrinsically Disordered Protein c-Myc

Intrinsically disordered proteins (IDPs) are associated with various diseases and have been proposed as promising drug targets. However, conventional structure-based approaches cannot be applied directly to IDPs, due to their lack of ordered structures. Here, we describe a novel computational approach to virtually screen for compounds that can simultaneously bind to different IDP conformations. The test system used c-Myc, an oncoprotein containing a disordered basic helix-loop-helix-leucine zipper (bHLH-LZ) domain that adopts a helical conformation upon binding to Myc-associated factor X (Max). For the virtual screen, we used three binding pockets in representative conformations of c-Myc_370–409, which is part of the disordered bHLH-LZ domain. Seven compounds were found to directly bind c-Myc_370–409in vitro, and four inhibited the growth of the c-Myc-overexpressing cells by affecting cell cycle progression. Our approach of IDP conformation sampling, binding site identification, and virtual screening for compounds that can bind to multiple conformations provides a useful strategy for structure-based drug discovery targeting IDPs.

Contact-facilitated drug delivery with Sn2 lipase labile prodrugs optimize targeted lipid nanoparticle drug delivery

Sn2 lipase labile phospholipid prodrugs in conjunction with contact-facilitated drug delivery offer an important advancement in Nanomedicine. Many drugs incorporated into nanosystems, targeted or not, are substantially lost during circulation to the target. However, favorably altering the pharmacokinetics and volume of distribution of systemic drug delivery can offer greater efficacy with lower toxicity, leading to new prolonged-release nanoexcipients. However, the concept of achieving Paul Erhlich's inspired vision of a 'magic bullet' to treat disease has been largely unrealized due to unstable nanomedicines, nanosystems achieving low drug delivery to target cells, poor intracellular bioavailability of endocytosed nanoparticle payloads, and the substantial biological barriers of extravascular particle penetration into pathological sites. As shown here, Sn2 phospholipid prodrugs in conjunction with contact-facilitated drug delivery prevent premature drug diffusional loss during circulation and increase target cell bioavailability. The Sn2 phospholipid prodrug approach applies equally well for vascular constrained lipid-encapsulated particles and micelles the size of proteins that penetrate through naturally fenestrated endothelium in the bone marrow or thin-walled venules of an inflamed microcirculation. At one time Nanomedicine was considered a 'Grail Quest' by its loyal opposition and even many in the field adsorbing the pains of a long-learning curve about human biology and particles. However, Nanomedicine with innovations like Sn2 phospholipid prodrugs has finally made 'made the turn' toward meaningful translational success.

Chromatin redistribution of the DEK oncoprotein represses hTERT transcription in leukemias

Although numerous factors have been found to modulate hTERT transcription, the mechanism of its repression in certain leukemias remains unknown. We show here that DEK represses hTERT transcription through its enrichment on the hTERT promoter in cells from chronic and acute myeloid leukemias, chronic lymphocytic leukemia, but not acute lymphocytic leukemias where hTERT is overexpressed. We isolated DEK from the hTERT promoter incubated with nuclear extracts derived from fresh acute myelogenous leukemia (AML) cells and from cells expressing Tax, an hTERT repressor encoded by the human T cell leukemia virus type 1. In addition to the recruitment of DEK, the displacement of two potent known hTERT transactivators from the hTERT promoter characterized both AML cells and Tax-expressing cells. Reporter and chromatin immunoprecipitation assays permitted to map the region that supports the repressive effect of DEK on hTERT transcription, which was proportionate to the level of DEK-promoter association but not with the level of DEK expression. Besides hTERT repression, this context of chromatin redistribution of DEK was found to govern about 40% of overall transcriptional modifications, including those of cancer-prone genes. In conclusion, DEK emerges as an hTERT repressor shared by various leukemia subtypes and seems involved in the deregulation of numerous genes associated with leukemogenesis.

Functions of myc:max in the control of cell proliferation and tumorigenesis

Deregulation and elevated expression of members of the Myc family of bHLHZip transcription factors are observed in a high percentage of tumors. This close association with human cancers has led to a tremendous effort to define their biological and biochemical activities. Although Myc family proteins have the capacity to elicit a wide range of cell behaviors, their principal function appears to be to drive cells into the cell cycle and to keep them there. However, forced expression of Myc profoundly sensitizes normal cells to apoptosis. Therefore, tumor formation caused by deregulated Myc expression requires cooperating events that disrupt pathways that mediate apoptosis. Myc-dependent tumor formation may also be impeded by a set of related bHLHZip proteins with the demonstrated potential to act as Myc antagonists in cell culture experiments. In this review, we examine the complex activities of Myc family proteins and how their actions might be regulated in the context of a network of bHLHZip proteins.

Low molecular weight inhibitors of Myc-Max interaction and function

c-Myc is helix-loop-helix-leucine zipper (HLH-ZIP) oncoprotein that is frequently deregulated in human cancers. In order to bind DNA, regulate target gene expression, and function in a biological context, c-Myc must dimerize with another HLH-ZIP protein, Max. A large number of c-Myc target genes have been identified, and many of the encoded proteins are transforming. Such functional redundancy, however, complicates therapeutic strategies aimed at inhibiting any single target gene product. Given this consideration, we have instead attempted to identify ways by which c-Myc itself could be effectively disabled. We have used a yeast two-hybrid approach to identify low-molecular-weight compounds that inhibit c-Myc-Max association. All of the compounds prevented transactivation by c-Myc-Max heterodimers, inhibited cell cycle progression, and prevented the in vitro growth of fibroblasts in a c-Myc-dependent manner. Several of the compounds also inhibited tumor growth in vivo. These results show that the yeast two-hybrid screen is useful for identifying compounds that can be exploited in mammalian cells. More specifically, they provide a means by which structural analogs, based upon these first-generation Myc-Max inhibitors, can be developed to enhance antitumor efficacy.

Identification of a novel liver-specific expressed gene, TCP10L, encoding a human leucine zipper protein with transcription inhibition activity

The incidence of hepatoma is high in the Chinese population. Searching for genes involved in the functions of the liver, especially genes specifically expressed in the liver, will facilitate an insight into the molecular basis of normal and abnormal liver functions. Based on a differentially displayed cDNA fragment, which was down regulated in hepatoma tissues, we cloned a novel cDNA of 957 bp, TCP10L (T-complex protein 10 like), from the human liver cDNA library. Northern hybridization of this novel gene in 30 adult human tissues was examined. The result revealed that TCP10L expressed specifically in the human liver and testis. The TCP10L contains a 645-bp open reading frame encoding a deduced protein of 215 amino acids. As the deduced protein was analyzed further, a typical leucine zipper motif was found. We firstly examined the transcriptional function of the TCP10L protein by transfecting recombinant pM-TCP10L into mammalian cells. The subsequent analysis based on the dual luciferase assay system showed that TCP10L significantly inhibited the expression of reporter genes. Compared with that of the negative control, the luciferase activity were down regulated in HEK293 and SK-HEP-1, CHO cells by about 2.6, 9.8, and 5.5 folds respectively. A mutated type of TCP10L was also constructed. It showed that the repression of TCP10L to the expression of the reporter gene almost completely decreased, suggesting that the leucine zipper structure is critical for TCP10L to play its role in regulation function. Then we transfected the recombinant TCP10L-EGFP into cells. The results indicated that TCP10L subcellularly located in nuclei, either in HEK 293 or SK-HEP-1 cells. In addition, human TCP10L was found comprised of five exons and four introns, and mapped to chromosome 21q22.11.

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

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

Dynamic in vivo interactions among Myc network members

Members of the Myc oncoprotein network (c-Myc, Max, and Mad) play important roles in proliferation, differentiation, and apoptosis. We expressed chimeric green fluorescent protein (GFP) fusions of c-Myc, Max, and three Mad proteins in fibroblasts. Individually, c-Myc and Mad proteins localized in subnuclear speckles, whereas Max assumed a homogeneous nuclear pattern. These distributions were co-dominant and dynamic, however, as each protein assumed the pattern of its heterodimeric partner when the latter was co-expressed at a higher level. Deletion mapping of two Mad members, Mad1 and Mxi1, demonstrated that the domains responsible for nuclear localization and speckling are separable. A non-speckling Mxi1 mutant was also less effective as a transcriptional repressor than wild-type Mxi1. c-Myc nuclear speckles were distinct from SC-35 domains involved in mRNA processing. However, in the presence of co-expressed Max, c-Myc, but not Mad, co-localized to a subset of SC-35 loci. These results show that Myc network proteins comprise dynamic subnuclear structures and behave co-dominantly when co-expressed with their normal heterodimerization partners. In addition, c-Myc-Max heterodimers, but not Max-Mad heterodimers, localize to foci actively engaged in pre-mRNA transcription/processing. These findings suggest novel means by which Myc network members promote transcriptional activation or repression.

Mmip-2/Rnf-17 enhances c-Myc function and regulates some target genes in common with glucocorticoid hormones

Members of the Mad family of basic-helix-loop-helix-leucine zipper proteins inhibit the transcriptional activity of the c-Myc oncoprotein. Mmip-2/Rnf-17 is a RING-finger protein that interacts with all four known Mad proteins, redistributes them to the cytoplasm, and thus enhances c-Myc function. We generated cell lines in which Mmip-2/Rnf-17 was rendered glucocorticoid (GC)-inducible. Stable expression of Mmip-/Rnf-17 resulted in the expected transport of the most abundant endogenous mad protein, Mxi1, to the cytoplasm. Compensatory increases in Mxi1 and Mad3 transcripts, similar to those previously described in Mad1 null hematopoietic cells, were also seen. Mmip-2/Rnf-17 also sensitized cells to several different pro-apoptotic stimuli and regulated a subset of c-Myc target genes. Unexpectedly, some of these genes were also found to be modulated solely by GCs. Thus, the inhibition of Mad proteins by Mmip-2/Rnf-17 modulates c-Myc function by enhancing its ability to regulate a subset of its potential target genes. Our results also identify a previously unrecognized overlap between genes regulated by c-Myc- and GCs and provide a potential molecular basis for their regulation of common cellular functions.

The Myc/Max/Mad network and the transcriptional control of cell behavior

The Myc/Max/Mad network comprises a group of transcription factors whose distinct interactions result in gene-specific transcriptional activation or repression. A great deal of research indicates that the functions of the network play roles in cell proliferation, differentiation, and death. In this review we focus on the Myc and Mad protein families and attempt to relate their biological functions to their transcriptional activities and gene targets. Both Myc and Mad, as well as the more recently described Mnt and Mga proteins, form heterodimers with Max, permitting binding to specific DNA sequences. These DNA-bound heterodimers recruit coactivator or corepressor complexes that generate alterations in chromatin structure, which in turn modulate transcription. Initial identification of target genes suggests that the network regulates genes involved in the cell cycle, growth, life span, and morphology. Because Myc and Mad proteins are expressed in response to diverse signaling pathways, the network can be viewed as a functional module which acts to convert environmental signals into specific gene-regulatory programs.

Promotion of growth and apoptosis in c-myc nullizygous fibroblasts by other members of the myc oncoprotein family

c-myc nullizygous fibroblasts (KO cells) were used to compare the abilities of c-myc, N-myc and L-myc oncoproteins to accelerate growth, promote apoptosis, revert morphology, and regulate the expression of previously described c-myc target genes. All three myc oncoproteins were expressed following retroviral transduction of KO cells. The proteins all enhanced the growth rate of KO cells and significantly shortened the cell cycle transition time. They also accelerated apoptosis following serum deprivation, reverted the abnormal KO cell morphology, and modulated the expression of previously described c-myc target genes. In most cases, L-myc was equivalent to c-myc and N-myc in restoring all of the c-myc-dependent activities. These findings contrast with the previously reported weak transforming and transactivating properties of L-myc. Myc oncoproteins may thus impart both highly similar as well as dissimilar signals to the cells in which they are expressed.

Distinct Apoptotic Responses Imparted by c-myc and max

The c-myc oncoprotein accelerates programmed cell death (apoptosis) after growth factor deprivation or pharmacological insult in many cell lines. We have shown that max, the obligate c-myc heterodimeric partner protein, also promotes apoptosis after serum withdrawal in NIH3T3 fibroblasts or cytokine deprivation in interleukin-3 (IL-3)-dependent 32D murine myeloid cells. We now show that c-myc– and max-overexpressing 32D cells differ in the nature of their apoptotic responses after IL-3 removal or treatment with chemotherapeutic compounds. In the presence of IL-3, c-myc overexpression enhances the sensitivity of 32D cells to Etoposide (Sigma, St Louis, MO), Adriamycin (Pharmacia, Columbus, OH), and Camptothecin (Sigma), whereas max overexpression increases sensitivity only to Camptothecin. Drug treatment of c-myc–overexpressing cells in the absence of IL-3 did not alter the spectrum of drug sensitivity other than to additively accelerate cell death. In contrast, enhanced sensitivity to Adriamycin, Etoposide, and Taxol (Bristol-Meyers Squibb, Princeton, NJ) was revealed in max-overexpressing cells concurrently deprived of IL-3. Differential rates of apoptosis were not strictly correlated with the ability of the drugs to promote G1 or G2/M arrest. Ectopic expression of Bcl-2 or Bcl-XL blocked drug-induced apoptosis in both cell lines. In contrast, whereas Bcl-2 blocked apoptosis in both cell lines in response to IL-3 withdrawal, Bcl-XL blocked apoptosis in max-overexpressing cells but not in c-myc–overexpressing cells. These results provide mechanistic underpinnings for the idea that c-myc and max modulate distinct apoptotic pathways.

© 1998 by The American Society of Hematology.

Distinct Apoptotic Responses Imparted by c-myc and max

The c-myc oncoprotein accelerates programmed cell death (apoptosis) after growth factor deprivation or pharmacological insult in many cell lines. We have shown that max, the obligate c-myc heterodimeric partner protein, also promotes apoptosis after serum withdrawal in NIH3T3 fibroblasts or cytokine deprivation in interleukin-3 (IL-3)-dependent 32D murine myeloid cells. We now show that c-myc– and max-overexpressing 32D cells differ in the nature of their apoptotic responses after IL-3 removal or treatment with chemotherapeutic compounds. In the presence of IL-3, c-myc overexpression enhances the sensitivity of 32D cells to Etoposide (Sigma, St Louis, MO), Adriamycin (Pharmacia, Columbus, OH), and Camptothecin (Sigma), whereas max overexpression increases sensitivity only to Camptothecin. Drug treatment of c-myc–overexpressing cells in the absence of IL-3 did not alter the spectrum of drug sensitivity other than to additively accelerate cell death. In contrast, enhanced sensitivity to Adriamycin, Etoposide, and Taxol (Bristol-Meyers Squibb, Princeton, NJ) was revealed in max-overexpressing cells concurrently deprived of IL-3. Differential rates of apoptosis were not strictly correlated with the ability of the drugs to promote G1 or G2/M arrest. Ectopic expression of Bcl-2 or Bcl-XL blocked drug-induced apoptosis in both cell lines. In contrast, whereas Bcl-2 blocked apoptosis in both cell lines in response to IL-3 withdrawal, Bcl-XL blocked apoptosis in max-overexpressing cells but not in c-myc–overexpressing cells. These results provide mechanistic underpinnings for the idea that c-myc and max modulate distinct apoptotic pathways.

© 1998 by The American Society of Hematology.

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.

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

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

Mutation of the MXI1 gene in prostate cancer

The Mxi1 protein negatively regulates Myc oncoprotein activity and thus potentially serves a tumour suppressor function. MXI1 maps to chromosome 10q24-q25, a region that is deleted in some cases of prostate cancer. We have detected mutations in the retained MXI1 alleles in four primary prostate tumours with 10q24-q25 deletions. Two tumours contained inactivating mutations, whereas two others contained the identical missense mutation. Fluorescence in situ hybridization also demonstrated loss of one MXI1 allele in an additional tumour lacking chromosome 10 abnormalities. MXI1 thus displays allelic loss and mutation in some cases of prostate cancer that may contribute to the pathogenesis or neoplastic evolution of this common malignancy.

Analysis of c-Myc domains involved in stimulating SV40 replication

We have demonstrated previously that overproduction of c-Myc, N-Myc and, to a lesser extent, L-Myc facilitates the replication of simian virus 40 (SV40)-based vectors in human lymphoid cells. Using a series of c-myc deletion mutants, we investigated which c-Myc regions are important in stimulating SV40 replication. The ability of c-Myc to promote SV40 replication was significantly reduced by deletions in the second exon domain, formerly shown to be crucial for c-Myc's transforming capacity. The c-myc mutants with a disrupted basic region (b) or leucine zipper (Zip) motif were also unable to stimulate SV40 replication. These regions are implicated in protein-DNA and protein-protein interactions, respectively, suggesting that the c-Myc protein might be associated with the DNA-protein replication complex. We present data obtained from gel mobility shift assays and from an immunocomplex-binding assay substantiating this hypothesis.

Constitutive expression of c-myc does not relieve cAMP-mediated growth arrest in human lymphoid Reh cells

The Reh cell system is suitable for evaluating events important for control of proliferation independently of mechanisms involved in differentiation, as Reh cells are unable to differentiate. In the human pre-B cell line Reh, activation of adenylate cyclase by forskolin induces a five to tenfold rapid, transient down-regulation of steady-state c-myc RNA within 4 hours. Concurrently, the cells are strongly growth arrested in the G1 phase of the cell cycle. To clarify if the observed growth arrest could be relieved by constitutive expression of c-myc, an exogenous c-myc gene under constitutive promoter control was introduced into Reh cells by electroporation. The c-myc-expressing construct pDMmycHyg contained human c-myc exons 2 and 3 driven by the Mo-MLV LTR and conferred hygromycin resistance. Exogenous c-myc RNA transcripts and protein were constitutively expressed in the transfected clones at levels roughly twice as high as the level in nontransfected cells. Total c-myc protein levels were unchanged upon treatment of transfected clones with forskolin. Yet, the transfected cells were not released from growth arrest. Furthermore, the transfected Reh cells did not differentiate upon forskolin treatment. Constitutive overexpression of c-myc is therefore not sufficient for relieving forskolin-mediated effects on growth arrest in Reh cells.

Nuclear colocalization of c-myc protein and hsp70 in cells transfected with human wild-type and mutant c-myc genes

Using immunofluorescence and electron microscopy we have studied the localization of wild-type and mutant c-myc proteins transiently expressed in CV-1 cells. In agreement with our previous observations, wild-type c-myc protein accumulated in large amorphous globules in the nucleus. All mutant proteins tested accumulated in the nucleus as well, but gave rise to morphologically different inclusion bodies. Many small globules appeared in cells transfected with D145-262 (deletion of amino acids 145-262), while cells transfected with D371-412 or D414-433 generated structures looking like a fine network or like beads on a string. In addition, a particulate cytoplasmic staining appeared in some cells transfected with the wild-type gene and in cells transfected with mutants D145-262 or D414-433. Since the c-myc protein has been reported to stimulate expression of exogenous hsp70 protein, we also examined the intracellular distribution of hsp70 in the transfected cells. Double immunofluorescence microscopy revealed that hsp70 codistributed with the c-myc protein in distinct globules in the nucleus of many but not all myc-positive cells. However, the levels of hsp70 transcripts were not significantly raised compared to nontransfected and vector-transfected cells. Likewise, the levels of hsp70 protein did not vary significantly. These findings indicate that overexpression of c-myc stimulates translocation of preexisting hsp70 from the cytoplasm into the nucleus, rather than influencing hsp70 expression. Conceivably, this may represent one of several mechanisms whereby the cell deals with excessive amounts of c-myc protein.

Inhibition of murine erythroleukemia cell differentiation by normal and partially deleted c-myc genes

In our study of normal and partially deleted myc genes we found that N-myc, similarly to L-myc, can substitute for c-myc and inhibit MEL cell differentiation. All of the known putative functional domains of c-myc seem to be required for this inhibition. It is conceivable that c-myc inhibits differentiation by a mechanism that is related to its normal role in the cell, possibly by regulating transcription of genes involved in growth promotion. As was previously found for all of the other known activities of c-Myc, the HLH and LZ dimerization motifs are absolutely necessary for inhibition of MEL cell differentiation. Heterodimerization of Myc with Max or Max-like proteins could be a prerequisite for such inhibition. It is, therefore, of interest to study the regulation of max in MEL cells expressing normal and deregulated myc genes.

c-myc oncoprotein function

Genetic alterations of the c-myc locus in various malignancies and the ability of c-myc to transform cultured cells and induce tumors in transgenic animals attest to its central role in many neoplasms. By dissecting the c-Myc protein, a number of critical functional domains of c-Myc have been identified and characterized; these findings suggest a model for c-Myc function and intracellular activity (Fig. 4). c-Myc is synthesized in the cytoplasm and undergoes oligomerization another protein such as Max. Its nuclear localization signal allows c-Myc to be targeted to and retained in the nucleus, where the protein seeks out and binds to specific DNA sites, perhaps facilitated by c-Myc's ability to bind non-specifically to DNA. Once bound to specific DNA sequences, c-Myc then activates or inhibits transcription of a number of target genes, with consequent alterations in cell growth and differentiation. Continued studies of c-Myc and its partner Max should further elucidate the mechanisms by which c-Myc can contribute both to the regulation of normal cell growth and the alteration in that regulation in neoplasia.

Protein—protein interactions: structural motifs and molecular recognition

The past year has brought new insights into common structural motifs used for protein-protein interactions by DNA-binding proteins. In addition, there have been significant advances in our understanding of antibody-protein complexes.

Association of Myn, the murine homolog of max, with c-Myc stimulates methylation-sensitive DNA binding and ras cotransformation

Myn, a novel murine approximately 18 kd basic/helix-loop-helix/"leucine zipper" (B/HLH/LZ) protein, forms a specific DNA-binding complex with the c-Myc oncoprotein through the HLH/LZ motif in both proteins. c-Myc/Myn recognizes a c-Myc-binding site (GACCACGTGGTC) with higher affinity than either protein by itself. CpG methylation of the recognition site greatly inhibits DNA binding, suggesting that DNA methylation may regulate the c-Myc/Myn complex in vivo. In 3T3 fibroblasts, Myn mRNA levels are induced several-fold by serum with delayed early kinetics, suggesting regulation by immediate-early gene products. Coexpression of Myn in a myc/ras rat embryo fibroblast focus formation assay specifically augmented c-myc transforming activity. We suggest that interaction of Myn with c-Myc stabilizes sequence-specific DNA binding in vivo.