Nuclear location of the putative transforming protein of avian myelocytomatosis virus

MYC: there is more to it than cancer

MYC is a pleiotropic transcription factor involved in multiple cellular processes. While its mechanism of action and targets are not completely elucidated, it has a fundamental role in cellular proliferation, differentiation, metabolism, ribogenesis, and bone and vascular development. Over 4 decades of research and some 10,000 publications linking it to tumorigenesis (by searching PubMed for "MYC oncogene") have led to MYC becoming a most-wanted target for the treatment of cancer, where many of MYC's physiological functions become co-opted for tumour initiation and maintenance. In this context, an abundance of reviews describes strategies for potentially targeting MYC in the oncology field. However, its multiple roles in different aspects of cellular biology suggest that it may also play a role in many additional diseases, and other publications are indeed linking MYC to pathologies beyond cancer. Here, we review these physiological functions and the current literature linking MYC to non-oncological diseases. The intense efforts towards developing MYC inhibitors as a cancer therapy will potentially have huge implications for the treatment of other diseases. In addition, with a complementary approach, we discuss some diseases and conditions where MYC appears to play a protective role and hence its increased expression or activation could be therapeutic.

Structural and Biophysical Insights into the Function of the Intrinsically Disordered Myc Oncoprotein

Myc is a transcription factor driving growth and proliferation of cells and involved in the majority of human tumors. Despite a huge body of literature on this critical oncogene, our understanding of the exact molecular determinants and mechanisms that underlie its function is still surprisingly limited. Indubitably though, its crucial and non-redundant role in cancer biology makes it an attractive target. However, achieving successful clinical Myc inhibition has proven challenging so far, as this nuclear protein is an intrinsically disordered polypeptide devoid of any classical ligand binding pockets. Indeed, Myc only adopts a (partially) folded structure in some contexts and upon interacting with some protein partners, for instance when dimerizing with MAX to bind DNA. Here, we review the cumulative knowledge on Myc structure and biophysics and discuss the implications for its biological function and the development of improved Myc inhibitors. We focus this biophysical walkthrough mainly on the basic region helix-loop-helix leucine zipper motif (bHLHLZ), as it has been the principal target for inhibitory approaches so far.

mTOR complex 1 controls the nuclear localization and function of glycogen synthase kinase 3β

Glycogen synthase kinase 3β (GSK3β) phosphorylates and thereby regulates a wide range of protein substrates involved in diverse cellular functions. Some GSK3β substrates, such as c-Myc and Snail, are nuclear transcription factors, suggesting the possibility that GSK3β function is controlled through its nuclear localization. Here, using ARPE-19 and MDA-MB-231 human cell lines, we found that inhibition of mTOR complex 1 (mTORC1) leads to partial redistribution of GSK3β from the cytosol to the nucleus and to a GSK3β-dependent reduction of the levels of both c-Myc and Snail. mTORC1 is known to be controlled by metabolic cues, such as by AMP-activated protein kinase (AMPK) or amino acid abundance, and we observed here that AMPK activation or amino acid deprivation promotes GSK3β nuclear localization in an mTORC1-dependent manner. GSK3β was detected on several distinct endomembrane compartments, including lysosomes. Consistently, disruption of late endosomes/lysosomes through a perturbation of RAS oncogene family member 7 (Rab7) resulted in loss of GSK3β from lysosomes and in enhanced GSK3β nuclear localization as well as GSK3β-dependent reduction of c-Myc levels. These findings indicate that the nuclear localization and function of GSK3β is suppressed by mTORC1 and suggest a link between metabolic conditions sensed by mTORC1 and GSK3β-dependent regulation of transcriptional networks controlling cellular biomass production.

Two proteolytic fragments of menin coordinate the nuclear transcription and postsynaptic clustering of neurotransmitter receptors during synaptogenesis between Lymnaea neurons

Synapse formation and plasticity depend on nuclear transcription and site-specific protein targeting, but the molecular mechanisms that coordinate these steps have not been well defined. The MEN1 tumor suppressor gene, which encodes the protein menin, is known to induce synapse formation and plasticity in the CNS. This synaptogenic function has been conserved across evolution, however the underlying molecular mechanisms remain unidentified. Here, using central neurons from the invertebrate Lymnaea stagnalis, we demonstrate that menin coordinates subunit-specific transcriptional regulation and synaptic clustering of nicotinic acetylcholine receptors (nAChR) during neurotrophic factor (NTF)-dependent excitatory synaptogenesis, via two proteolytic fragments generated by calpain cleavage. Whereas menin is largely regarded as a nuclear protein, our data demonstrate a novel cytoplasmic function at central synapses. Furthermore, this study identifies a novel synaptogenic mechanism in which a single gene product coordinates the nuclear transcription and postsynaptic targeting of neurotransmitter receptors through distinct molecular functions of differentially localized proteolytic fragments.

MYC and the art of microRNA maintenance

MYC is a noncanonical transcription factor that binds to thousands of genomic loci and affects >15% of the human transcriptome, with surprisingly little overlap between MYC-bound and -regulated genes. This discordance raises the question whether MYC chooses its targets based on their individual biological effects ("a la carte") or by virtue of belonging to a certain group of genes (on a "prix fixe" basis). This review presents evidence for a prix fixe, posttranscriptional model whereby MYC initially deregulates a select number of microRNAs. These microRNAs then target a broad spectrum of genes based solely on the presence in their 3' UTRs (untranslated regions) of distinct "seed" sequences. Existing evidence suggests that there are significant microRNA components to all key MYC-driven phenotypes, including cell-cycle progression, apoptosis, metabolism, angiogenesis, metastasis, stemness, and hematopoiesis. Furthermore, each of these cell-intrinsic and -extrinsic phenotypes is likely attributable to deregulation of multiple microRNA targets acting in different, yet frequently overlapping, pathways. The habitual targeting of multiple genes within the same pathway might account for the robustness and persistence of MYC-induced phenotypes.

An overview of MYC and its interactome

This review is intended to provide a broad outline of the biological and molecular functions of MYC as well as of the larger protein network within which MYC operates. We present a view of MYC as a sensor that integrates multiple cellular signals to mediate a broad transcriptional response controlling many aspects of cell behavior. We also describe the larger transcriptional network linked to MYC with emphasis on the MXD family of MYC antagonists. Last, we discuss evidence that the network has evolved for millions of years, dating back to the emergence of animals.

Cellular MYCro economics: Balancing MYC function with MYC expression

The expression levels of the MYC oncoprotein have long been recognized to be associated with the outputs of major cellular processes including proliferation, cell growth, apoptosis, differentiation, and metabolism. Therefore, to understand how MYC operates, it is important to define quantitatively the relationship between MYC input and expression output for its targets as well as the higher-order relationships between the expression levels of subnetwork components and the flow of information and materials through those networks. Two different views of MYC are considered, first as a molecular microeconomic manager orchestrating specific positive and negative responses at individual promoters in collaboration with other transcription and chromatin components, and second, as a macroeconomic czar imposing an overarching rule onto all active genes. In either case, c-myc promoter output requires multiple inputs and exploits diverse mechanisms to tune expression to the appropriate levels relative to the thresholds of expression that separate health and disease.

MYC expression and distribution in normal mature lymphoid cells

The distribution of the product of the proto-oncogene MYC in lymphoid tissue has not been established in three decades, due to a combination of factors including low abundance, short half-life, and antibody sensitivity and specificity. We sought to validate antibodies in order to define the expression and distribution of MYC in mature normal lymphoid cells by multiparametric immunophenotyping. Having validated two antibodies for flow cytometry and for immunohistochemistry, we analysed normal tonsil tissue. MYC is expressed predominantly in B cells, some of which are interfollicular large, activated, and cycling CD30+, IRF4+, AID± blasts. Follicular mantle, isotype-switched memory B cells and FcRH4/IRTA1+ B cells express MYC in a wide range of levels and are small non-proliferating CDKN1B/p27-positive or -negative resting B lymphocytes. Germinal centre founder cells, CD30+ BCL6± AID± germinal centre blasts, and a population of GC cells in the apical light zone express MYC. MYC is expressed in all phases of the cell cycle in activated and mature B cells, but rarely in other lymphoid types and only partially fulfils the predictions derived from extractive and ex vivo experiments of the past 30 years.

Retroviral oncogenes: a historical primer

Retroviruses are the original source of oncogenes. The discovery and characterization of these genes was made possible by the introduction of quantitative cell biological and molecular techniques for the study of tumour viruses. Key features of all retroviral oncogenes were first identified in src, the oncogene of Rous sarcoma virus. These include non-involvement in viral replication, coding for a single protein and cellular origin. The MYC, RAS and ERBB oncogenes quickly followed SRC, and these together with PI3K are now recognized as crucial driving forces in human cancer.

Myc: the beauty and the beast

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

Tyrosine 23 phosphorylation-dependent cell-surface localization of annexin A2 is required for invasion and metastases of pancreatic cancer

The aggressiveness of pancreatic ductal adenocarcinoma (PDA) is characterized by its high metastatic potential and lack of effective therapies, which is the result of a lack of understanding of the mechanisms involved in promoting PDA metastases. We identified Annexin A2 (ANXA2), a member of the Annexin family of calcium-dependent phospholipid binding proteins, as a new molecule that promotes PDA invasion and metastases. We found ANXA2 to be a PDA-associated antigen recognized by post-treatment sera of patients who demonstrated prolonged survival following treatment with a PDA-specific vaccine. Cell surface ANXA2 increases with PDA development and progression. Knockdown of ANXA2 expression by RNA interference or blocking with anti-ANXA2 antibodies inhibits in vitro invasion of PDA cells. In addition, post-vaccination patient sera inhibits in vitro invasion of PDA cells, suggesting that therapeutic anti-ANXA2 antibodies are induced by the vaccine. Furthermore, cell-surface localization of ANXA2 is tyrosine 23 phosphorylation-dependent; and tyrosine 23 phosphorylation is required for PDA invasion. We demonstrated that tyrosine 23 phosphorylation resulting in surface expression of ANXA2 is required for TGFβ-induced, Rho-mediated epithelial-mesenchymal transition (EMT), linking the cellular function of ANXA2 which was previously shown to be associated with small GTPase-regulated cytoskeletal rearrangements, to the EMT process in PDA. Finally, using mouse PDA models, we showed that shRNA knock-down of ANXA2, a mutation at tyrosine 23, or anti-ANXA2 antibodies, inhibit PDA metastases and prolong mouse survival. Thus, ANXA2 is part of a novel molecular pathway underlying PDA metastases and a new target for development of PDA therapeutics.

Reflecting on 25 years with MYC

Just over 25 years ago, MYC, the human homologue of a retroviral oncogene, was identified. Since that time, MYC research has been intense and the advances impressive. On reflection, it is astonishing how each incremental insight into MYC regulation and function has also had an impact on numerous biological disciplines, including our understanding of molecular oncogenesis in general. Here we chronicle the major advances in our understanding of MYC biology, and peer into the future of MYC research.

Application of thermally responsive polypeptides directed against c-Myc transcriptional function for cancer therapy

Elastin-like polypeptides are biopolymers composed of the pentapeptide repeat Val-Pro-Gly-Xaa-Gly. Elastin-like polypeptides are soluble in aqueous solution below their transition temperature, but they hydrophobically collapse and aggregate when the temperature is raised above the transition temperature. Previous studies have suggested that the aggregation of these polypeptides in response to externally applied hyperthermia may be exploited in the use of elastin-like polypeptide for thermally targeted drug delivery. This work shows the application of elastin-like polypeptide as a delivery vehicle for a short peptide that can inhibit the transcriptional function of a specific oncogene. The coding sequence for elastin-like polypeptide was modified by the addition of the membrane translocating sequence penetratin and a peptide derived from helix 1 of the helix-loop-helix region of c-Myc (H1-S6A,F8A), known to inhibit c-Myc transcriptional function. The designed polypeptide (Pen-ELP-H1) was then expressed and purified from Escherichia coli. Cellular uptake of Pen-ELP-H1 is enhanced by both the penetratin sequence and by the hyperthermia-induced phase transition as shown by flow cytometry studies. Using immunofluorescence and reverse transcription-PCR, we show that Pen-ELP-H1 is able to disrupt the nuclear localization of c-Myc and inhibit transcriptional activation by c-Myc. Cell proliferation studies showed that Pen-ELP-H1 inhibits growth of MCF-7 cells. Furthermore, the use of hyperthermia increased the antiproliferative effect of a thermally responsive Pen-ELP-H1 approximately 2-fold compared with a nonthermally responsive control polypeptide. These studies show that genetically engineered elastin-like polypeptide carriers may provide a new way to thermally target specific oncogene inhibitors to solid tumors.

c-Myc localization within the nucleus: evidence for association with the PML nuclear body

Definitive localization of c-Myc within the nucleus is important to fully understand the regulation and function of this oncoprotein. Studies of c-Myc distribution, however, have produced conflicting results. To overcome technical challenges inherent in c-Myc cytology, we use here three methods to visualize c-Myc and in addition examine the impact of proteasome inhibition. EYFP or HA-tagged Myc was reintroduced by stable transfection into myc null diploid rat fibroblasts, replacing endogenous Myc with tagged Myc expressed at or near normal levels. This tagged Myc is shown to functionally replace the endogenous Myc by restoration of normal cell morphology and growth rate. We were able to confirm key findings using antibodies to the endogenous c-Myc and/or its partner, Max. Contrary to some published reports, by all three methods the c-Myc protein in rat fibroblasts distributes predominantly throughout the nucleus in a dispersed granular pattern, avoiding the nucleolus. Importantly, however, several findings provide evidence for an unanticipated relationship between c-Myc and PML nuclear bodies, which is enhanced under conditions of proteasome inhibition. Evidence of Max concentration within PML bodies is shown both with and without proteasome inhibition, strengthening the relationship between PML bodies and Myc/Max. Some accumulation of Myc and Max in nucleoli upon proteasome inhibition is also observed, although co-localization of ubiquitin was only seen with PML bodies. This work provides a comprehensive study of c-Myc distribution and also presents the first evidence of a relationship between turnover of this oncoprotein and PML nuclear bodies, known to break down in certain cancers.

dMyc is required for larval growth and endoreplication in Drosophila

Members of the Myc family of proto-oncogenes have long been implicated in regulating proliferation, apoptosis and oncogenesis. Recently, transcriptional and biological studies have suggested a direct role for Myc in regulating growth. We have used dm4, a new null allele of the Drosophila diminutive (dm) gene, which encodes dMyc on the X chromosome, to investigate a role for dMyc in larval endoreplicating tissues,where cellular growth and DNA replication occur in the absence of cell division. Hemizygous dm4/Y mutants arrest as second instar larvae, and fat body nuclei of dm4/Y mutants fail to attain normal size and normal levels of DNA, resulting from a reduced frequency of S-phase. Thus, dMyc is required for endoreplication and larval growth. In support of this, dMyc, as well as its antagonist dMnt, are expressed in larval tissues in a pattern consistent with their involvement in regulating endoreplication. Overexpression of dMyc in endoreplicating cells results in dramatic increases in nuclear DNA content and cell and nucleolar size, whereas dMnt overexpression has the opposite effect. BrdU incorporation and Cyclin E protein levels continue to oscillate in dMyc-overexpressing cells, indicating that the normal cell cycle control mechanisms are not disrupted. dMyc driven growth and endoreplication are strongly attenuated when the endocycle is blocked with Cyclin E or the cdk inhibitor p21. By contrast,the ability of dMyc to promote growth and endoreplication is only partly reduced when PI3K activity is blocked, suggesting that they influence distinct growth pathways. Our results indicate that larval growth and endoreplication are coupled processes that, although linked to cell cycle control mechanisms,are regulated by dMyc and dMnt.

Parathyroid hormone-Smad3 axis exerts anti-apoptotic action and augments anabolic action of transforming growth factor beta in osteoblasts

Although several studies indicated that parathyroid hormone (PTH) exerted anabolic action on bone, its precise mechanisms have been unknown. On the other hand, transforming growth factor beta (TGF-beta), abundantly stored in bone matrix, stimulates bone formation with a local injection in rodents. Although our previous study suggested that Smad3 is an important molecule for the stimulation of bone formation, no reports have been available about the effects of PTH on Smad3. In this present study, we examined the effects of PTH on Smad3 and the physiological significance in mouse osteoblastic cells. PTH promoted the expression of Smad3 mRNA within 10 min and the protein level in a dose-dependent manner in MC3T3-E1 and rat osteoblastic UMR-106 cells. Protein kinase A (PKA) activator as well as protein kinase C (PKC) activators increased Smad3 protein level, and both PKA and PKC inhibitors antagonized PTH-induced Smad3, indicating that PTH promotes the production of Smad3 through both PKA and PKC pathways. Next, we examined anti-apoptotic effects of PTH and Smad3 in these cells, employing trypan blue, transferase-mediated nick end labeling, and Hoechst staining. Pretreatment with PTH or overexpression of Smad3 decreased the number of apoptotic cells induced by dexamethasone and etoposide. Moreover, a dominant negative mutant, Smad3DeltaC, abrogated PTH-induced anti-apoptotic effects. On the other hand, PTH augmented TGF-beta-induced transcriptional activity. Furthermore, PTH enhanced TGF-beta-induced production of type I collagen, whereas it did not affect TGF-beta-reduced proliferation in MC3T3-E1 cells. These observations indicated that PTH amplified the anabolic effects of TGF-beta by accelerating the transcriptional activity of Smad3. In conclusion, we first demonstrated that PTH-Smad3 axis exerts anti-apoptotic effects in osteoblasts and reinforces the anabolic action by TGF-beta in osteoblasts. Hence, PTH-Smad3 axis might be involved in the bone anabolic action of PTH.

Characterization of the mouse JAB1 cDNA and protein

JAB1 was originally described as a transcriptional coactivator of c-Jun and Jun D. Recent data suggests that JAB1 is a component of a large protein complex, the JAB1 signalosome in mammals and the COP9 complex in plants. The JAB1 signalosome is implicated in the phosphorylation of selected transcription factors, while the COP9 complex is involved in repression of photomorphogenesis in Arabidopsis. In this study, we describe the partial characterization of mouse JAB1 (mJAB1). The murine JAB1 protein is encoded by a gene located on mouse chromosome 1. mJAB1 mRNA is abundantly expressed in a variety of adult tissues as well as in mouse embryos. The JAB1 protein was readily detectable in many cell types and localized to both the nucleus and cytoplasm. Endogenous JAB1 protein is relatively stable and its degradation is not perturbed by blocking 26S proteasome activity, suggesting that this protein is not degraded by the ubiquitin-mediated proteolytic pathway.

Truncated BRCA2 is cytoplasmic: implications for cancer-linked mutations

BRCA2 mutations predispose carriers mainly to breast cancer. The vast majority of BRCA2 mutations are predicted to result in a truncated protein product. The smallest known cancer-associated deletion removes from the C terminus only 224 of the 3,418 residues constituting BRCA2, suggesting that these terminal amino acids are crucial for BRCA2 function. A series of green fluorescent protein (GFP)-tagged BRCA2 deletion mutants revealed that nuclear localization depends on two nuclear localization signals that reside within the final 156 residues of BRCA2. Consistent with this observation, an endogenous truncated BRCA2 mutant (6174delT) was found to be cytoplasmic. Together, these studies provide a simple explanation for why the vast majority of BRCA2 mutants are nonfunctional: they do not translocate into the nucleus.

Identification of a novel cytoplasmic protein that specifically binds to nuclear localization signal motifs

Active transport of proteins into the nucleus is mediated by interaction between the classical nuclear localization signals (NLSs) of the targeted proteins and the NLS receptor (importin) complex. This nuclear transport system is highly regulated and conserved in eukaryotes and is essential for cell survival. Using a fragment of BRCA1 containing the two NLS motifs as a bait for yeast two-hybrid screening, we have isolated four clones, one of which is importin alpha. Here we characterize one of the other clones identified, BRAP2, which is a novel gene and expressed as a 2-kilobase mRNA in human mammary epithelial cells and some but not all tissues of mice. The isolated full-length cDNA encodes a novel protein containing 600 amino acid residues with pI 6.04. Characteristic motifs of C2H2 zinc fingers and leucine heptad repeats are present in the middle and C-terminal regions of the protein, respectively. BRAP2 also shares significant homology with a hypothetical protein from yeast Saccharomyces cerevisiae, especially in the zinc finger region. Antibodies prepared against the C-terminal region of BRAP2 fused to glutathione S-transferase specifically recognize a cellular protein with a molecular size of 68 kDa, consistent with the size of the in vitro translated protein. Cellular BRAP2 is mainly cytoplasmic and binds to the NLS motifs of BRCA1 with similar specificity to that of importin alpha in both two-hybrid assays in yeast and glutathione S-transferase pull-down assays in vitro. Other motifs such as the SV40 large T antigen NLS motif and the bipartite NLS motif found in mitosin are also recognized by BRAP2. Similarly, the yeast homolog of BRAP2 also binds to these NLS motifs in vitro. These results imply that BRAP2 may function as a cytoplasmic retention protein and play a role in regulating transport of nuclear proteins.

Menin, the product of the MEN1 gene, is a nuclear protein

The MEN1 gene, mutations in which are responsible for multiple endocrine neoplasia type 1 (MEN1), encodes a 610-amino acid protein, denoted menin. The amino acid sequence of this putative tumor suppressor offers no clue to the function or subcellular location of the protein. We report herein, based on immunofluorescence, Western blotting of subcellular fractions, and epitope tagging with enhanced green fluorescent protein, that menin is located primarily in the nucleus. Enhanced green fluorescent protein-tagged menin deletion constructs identify at least two independent nuclear localization signals (NLS), both located in the C-terminal fourth of the protein. Among the 68 known independent disease-associated mutations, none of the 22 missense and 3 in-frame deletions affect either of the putative NLS sequences. However, if expressed, none of the truncated menin proteins resulting from the 43 known frameshift/nonsense mutations would retain both the NLSs. The precise role(s) of menin in the nucleus remain to be understood.

Cell cycle-dependent colocalization of BARD1 and BRCA1 proteins in discrete nuclear domains

Germ-line mutations of the BRCA1 gene predispose women to early-onset breast and ovarian cancer by compromising the gene's presumptive function as a tumor suppressor. Although the biochemical properties of BRCA1 polypeptides are not understood, their expression pattern and subcellular localization suggest a role in cell-cycle regulation. When resting cells are induced to proliferate, the steady-state levels of BRCA1 increase in late G1 and reach a maximum during S phase. Moreover, in S phase cells, BRCA1 polypeptides are hyperphosphorylated and accumulate into discrete subnuclear foci termed "BRCA1 nuclear dots." BRCA1 associates in vivo with a structurally related protein termed BARD1. Here we show that the steady-state levels of BARD1, unlike those of BRCA1, remain relatively constant during cell cycle progression. However, immunostaining revealed that BARD1 resides within BRCA1 nuclear dots during S phase of the cell cycle, but not during the G1 phase. Nevertheless, BARD1 polypeptides are found exclusively in the nuclear fractions of both G1- and S-phase cells. Therefore, progression to S phase is accompanied by the aggregation of nuclear BARD1 polypeptides into BRCA1 nuclear dots. This cell cycle-dependent colocalization of BARD1 and BRCA1 indicates a role for BARD1 in BRCA1-mediated tumor suppression.

HEC, a novel nuclear protein rich in leucine heptad repeats specifically involved in mitosis

The protein encoded by the human gene HEC (highly expressed in cancer) contains 642 amino acids and a long series of leucine heptad repeats at its C-terminal region. HEC protein is expressed most abundantly in the S and M phases of rapidly dividing cells but not in terminal differentiated cells. It localizes to the nuclei of interphase cells, and a portion distributes to centromeres during M phase. Inactivation of HEC by microinjection of specific monoclonal antibodies into cells during interphase severely disturbs the subsequent mitoses. Disordered sister chromatid alignment and separation, as well as the formation of nonviable cells with multiple, fragmented micronuclei, are common features observed. These results suggest that the HEC protein may play an important role in chromosome segregation during M phase.

Rox, a novel bHLHZip protein expressed in quiescent cells that heterodimerizes with Max, binds a non-canonical E box and acts as a transcriptional repressor

Proteins of the Myc and Mad family are involved in transcriptional regulation and mediate cell differentiation and proliferation. These molecules share a basic-helix-loop-helix leucine zipper domain (bHLHZip) and bind DNA at the E box (CANNTG) consensus by forming heterodimers with Max. We report the isolation, characterization and mapping of a human gene and its mouse homolog encoding a new member of this family of proteins, named Rox. Through interaction mating and immunoprecipitation techniques, we demonstrate that Rox heterodimerizes with Max and weakly homodimerizes. Interestingly, bandshift assays demonstrate that the Rox-Max heterodimer shows a novel DNA binding specificity, having a higher affinity for the CACGCG site compared with the canonical E box CACGTG site. Transcriptional studies indicate that Rox represses transcription in both human HEK293 cells and yeast. We demonstrate that repression in yeast is through interaction between the N-terminus of the protein and the Sin3 co-repressor, as previously shown for the other Mad family members. ROX is highly expressed in quiescent fibroblasts and expression markedly decreases when cells enter the cell cycle. Moreover, ROX expression appears to be induced in U937 myeloid leukemia cells stimulated to differentiate with 12-O-tetradecanoylphorbol-13-acetate. The identification of a novel Max-interacting protein adds an important piece to the puzzle of Myc/Max/Mad coordinated action and function in normal and pathological situations. Furthermore, mapping of the human gene to chromosome 17p13.3 in a region that frequently undergoes loss of heterozygosity in a number of malignancies, together with the biochemical and expression features, suggest involvement of ROX in human neoplasia.

The ataxia-telangiectasia gene product, a constitutively expressed nuclear protein that is not up-regulated following genome damage

The product of the ataxia-telangiectasia gene (ATM) was identified by using an antiserum developed to a peptide corresponding to the deduced amino acid sequence. The ATM protein is a single, high-molecular weight protein predominantly confined to the nucleus of human fibroblasts, but is present in both nuclear and microsomal fractions from human lymphoblast cells and peripheral blood lymphocytes. ATM protein levels and localization remain constant throughout all stages of the cell cycle. Truncated ATM protein was not detected in lymphoblasts from ataxia-telangiectasia patients homozygous for mutations leading to premature protein termination. Exposure of normal human cells to gamma-irradiation and the radiomimetic drug neocarzinostatin had no effect on ATM protein levels, in contrast to a noted rise in p53 levels over the same time interval. These findings are consistent with a role for the ATM protein in ensuring the fidelity of DNA repair and cell cycle regulation following genome damage.

A new member of the hsp90 family of molecular chaperones interacts with the retinoblastoma protein during mitosis and after heat shock

A gene encoding a new heat shock protein that may function as a molecular chaperone for the retinoblastoma protein (Rb) was characterized. The cDNA fragment was isolated by using the yeast two-hybrid system and Rb as bait. The open reading frame of the longest cDNA codes for a protein with substantial sequence homology to members of the hsp90 family. Antibodies prepared against fusions between glutathione S-transferase and portions of this new heat shock protein specifically recognized a 75-kDa cellular protein, hereafter designated hsp75, which is expressed ubiquitously and located in the cytoplasm. A unique LxCxE motif in hsp75, but not in other hsp90 family members, appears to be important for binding to the simian virus 40 T-antigen-binding domain of hypophosphorylated Rb, since a single mutation changing the cysteine to methionine abolishes the binding. In mammalian cells, Rb formed complexes with hsp75 under two special physiological conditions: (i) during M phase, when the envelope that separates the nuclear and cytoplasmic compartments broke down, and (ii) after heat shock, when hsp75 moved from its normal cytoplasmic location into the nucleus. In vitro, hsp75 had a biochemical activity to refold denatured Rb into its native conformation. Taken together, these results suggest that Rb may be a physiological substrate for the hsp75 chaperone molecule. The discovery of a heat shock protein that chaperones Rb identifies a mechanism, in addition to phosphorylation, by which Rb is regulated in response to progression of the cell cycle and to external stimuli.

Aberrant subcellular localization of BRCA1 in breast cancer

The BRCA1 gene product was identified as a 220-kilodalton nuclear phosphoprotein in normal cells, including breast ductal epithelial cells, and in 18 of 20 tumor cell lines derived from tissues other than breast and ovary. In 16 of 17 breast and ovarian cancer lines and 17 of 17 samples of cells obtained from malignant effusions, however, BRCA1 localized mainly in cytoplasm. Absence of BRCA1 or aberrant subcellular location was also observed to a variable extent in histological sections of many breast cancer biopsies. These findings suggest that BRCA1 abnormalities may be involved in the pathogenesis of many breast cancers, sporadic as well as familial.

Nuclear c-Myc plays an important role in the cytotoxicity of tumor necrosis factor alpha in tumor cells

The phosphoprotein c-Myc has the potential to kill cells by apoptosis. To investigate whether c-Myc is involved in tumor necrosis factor alpha (TNF-alpha)-mediated cell killing, we have examined two HeLa cell lines (D98 and H21) which show dramatic differences in their susceptibilities to TNF-alpha cytotoxicity. Northern (RNA) blot analyses showed that there were no significant differences between these cell lines in basal or TNF-alpha-induced mRNA expression for a variety of proteins, including manganous superoxide dismutase, A20 zinc finger protein, plasminogen activator inhibitor type 2, and hsp70, all of which are known to influence the susceptibility of certain cells to TNF-alpha killing. On the other hand, there was a dramatic increase in c-Myc mRNA expression in TNF-alpha-sensitive D98 cells, but not in TNF-alpha-resistant H21 cells, which was only observed when the cells were treated with cycloheximide. Western blot (immunoblot) analyses revealed that even in the absence of TNF-alpha or cycloheximide, c-Myc was detectable only in nuclear extracts of TNF-alpha-sensitive D98 cells, implying a role for preexisting c-Myc in TNF-alpha killing. In support of this interpretation, a c-myc antisense oligonucleotide specifically inhibited the TNF-alpha killing of D98 cells, provided that the oligonucleotide was added 6 h prior to TNF-alpha treatment. Either dexamethasone treatment or transient expression of c-myc antisense cDNA fragments decreased nuclear c-Myc in D98 cells and rendered the cells more resistant to TNF-alpha cytotoxicity. Nuclear c-Myc was also detectable in a TNF-alpha-sensitive human HT-1080 fibrosarcoma cell line, but it was undetectable in a derivative of HT-1080 (SS-HT-1080) known to be resistant to TNF-alpha killing because of overexpression of plasminogen activator inhibitor type 2. HT-1080 cells transfected with antisense c-myc cDNA had significantly less nuclear c-Myc and were resistant to TNF-alpha cytotoxicity. Together, these data indicate that a nuclear transcription factor, c-Myc, plays an important role in sensitizing two different tumor cell types to TNF-alpha cytotoxicity.

Nuclear c-Myc plays an important role in the cytotoxicity of tumor necrosis factor alpha in tumor cells

The phosphoprotein c-Myc has the potential to kill cells by apoptosis. To investigate whether c-Myc is involved in tumor necrosis factor alpha (TNF-alpha)-mediated cell killing, we have examined two HeLa cell lines (D98 and H21) which show dramatic differences in their susceptibilities to TNF-alpha cytotoxicity. Northern (RNA) blot analyses showed that there were no significant differences between these cell lines in basal or TNF-alpha-induced mRNA expression for a variety of proteins, including manganous superoxide dismutase, A20 zinc finger protein, plasminogen activator inhibitor type 2, and hsp70, all of which are known to influence the susceptibility of certain cells to TNF-alpha killing. On the other hand, there was a dramatic increase in c-Myc mRNA expression in TNF-alpha-sensitive D98 cells, but not in TNF-alpha-resistant H21 cells, which was only observed when the cells were treated with cycloheximide. Western blot (immunoblot) analyses revealed that even in the absence of TNF-alpha or cycloheximide, c-Myc was detectable only in nuclear extracts of TNF-alpha-sensitive D98 cells, implying a role for preexisting c-Myc in TNF-alpha killing. In support of this interpretation, a c-myc antisense oligonucleotide specifically inhibited the TNF-alpha killing of D98 cells, provided that the oligonucleotide was added 6 h prior to TNF-alpha treatment. Either dexamethasone treatment or transient expression of c-myc antisense cDNA fragments decreased nuclear c-Myc in D98 cells and rendered the cells more resistant to TNF-alpha cytotoxicity. Nuclear c-Myc was also detectable in a TNF-alpha-sensitive human HT-1080 fibrosarcoma cell line, but it was undetectable in a derivative of HT-1080 (SS-HT-1080) known to be resistant to TNF-alpha killing because of overexpression of plasminogen activator inhibitor type 2. HT-1080 cells transfected with antisense c-myc cDNA had significantly less nuclear c-Myc and were resistant to TNF-alpha cytotoxicity. Together, these data indicate that a nuclear transcription factor, c-Myc, plays an important role in sensitizing two different tumor cell types to TNF-alpha cytotoxicity.

An E-box element localized in the first intron mediates regulation of the prothymosin alpha gene by c-myc

In RAT1A fibroblasts, expression of the prothymosin alpha gene is under the transcriptional control of the c-myc proto-oncogene. We have now cloned the rat gene encoding prothymosin alpha and show that the cloned gene is regulated by c-myc in vivo. We find that regulation by c-myc is mediated by sequences downstream of the transcriptional start site, whereas the promoter is constitutive and not regulated by c-myc. We have identified an enhancer element within the first intron that is sufficient to mediate a response to Myc and Max in transient transfection assays and to activation of estrogen receptor-Myc chimeras in vivo. We find that this element contains a consensus Myc-binding site (CACGTG). Disruption of this site abolishes the response to Myc and Max in both transient and stable assays. Mutants of either Myc or Max that are deficient for heterodimerization fail to regulate the prothymosin alpha gene, suggesting that a heterodimer between Myc and Max activates the prothymosin alpha gene. Our data define the prothymosin alpha gene as a bona fide target gene for c-myc.

An E-box element localized in the first intron mediates regulation of the prothymosin alpha gene by c-myc

In RAT1A fibroblasts, expression of the prothymosin alpha gene is under the transcriptional control of the c-myc proto-oncogene. We have now cloned the rat gene encoding prothymosin alpha and show that the cloned gene is regulated by c-myc in vivo. We find that regulation by c-myc is mediated by sequences downstream of the transcriptional start site, whereas the promoter is constitutive and not regulated by c-myc. We have identified an enhancer element within the first intron that is sufficient to mediate a response to Myc and Max in transient transfection assays and to activation of estrogen receptor-Myc chimeras in vivo. We find that this element contains a consensus Myc-binding site (CACGTG). Disruption of this site abolishes the response to Myc and Max in both transient and stable assays. Mutants of either Myc or Max that are deficient for heterodimerization fail to regulate the prothymosin alpha gene, suggesting that a heterodimer between Myc and Max activates the prothymosin alpha gene. Our data define the prothymosin alpha gene as a bona fide target gene for c-myc.

Overproduction of v-Myc in the nucleus and its excess over Max are not required for avian fibroblast transformation

The cellular proto-oncogene c-myc can acquire transforming potential by a number of different means, including retroviral transduction. The transduced allele generally contains point mutations relative to c-myc and is overexpressed in infected cells, usually as a v-Gag-Myc fusion protein. Upon synthesis, v-Gag-Myc enters the nucleus, forms complexes with its heterodimeric partner Max, and in this complex binds to DNA in a sequence-specific manner. To delineate the role for each of these events in fibroblast transformation, we introduced several mutations into the myc gene of the avian retrovirus MC29. We observed that Gag-Myc with a mutated nuclear localization signal is confined predominantly in the cytoplasm and only about 5% of the protein could be detected in the nucleus (less than the amount of endogenous c-Myc). Consequently, only a small fraction of Max is associated with Myc. However, cells infected with this mutant exhibit a completely transformed phenotype in vitro, suggesting that production of enough v-Gag-Myc to tie up all cellular Max is not needed for transformation. While the nuclear localization signal is dispensable for transformation, minimal changes in the v-Gag-Myc DNA-binding domain completely abolish its transforming potential, consistent with a role of Myc as a transcriptional regulator. One of its potential targets might be the endogenous c-myc, which is repressed in wild-type MC29-infected cells. Our experiments with MC29 mutants demonstrate that c-myc down-regulation depends on the integrity of the v-Myc DNA-binding domain and occurs at the RNA level. Hence, it is conceivable that v-Gag-Myc, either directly or circuitously, regulates c-myc transcription.

Overproduction of v-Myc in the nucleus and its excess over Max are not required for avian fibroblast transformation

The cellular proto-oncogene c-myc can acquire transforming potential by a number of different means, including retroviral transduction. The transduced allele generally contains point mutations relative to c-myc and is overexpressed in infected cells, usually as a v-Gag-Myc fusion protein. Upon synthesis, v-Gag-Myc enters the nucleus, forms complexes with its heterodimeric partner Max, and in this complex binds to DNA in a sequence-specific manner. To delineate the role for each of these events in fibroblast transformation, we introduced several mutations into the myc gene of the avian retrovirus MC29. We observed that Gag-Myc with a mutated nuclear localization signal is confined predominantly in the cytoplasm and only about 5% of the protein could be detected in the nucleus (less than the amount of endogenous c-Myc). Consequently, only a small fraction of Max is associated with Myc. However, cells infected with this mutant exhibit a completely transformed phenotype in vitro, suggesting that production of enough v-Gag-Myc to tie up all cellular Max is not needed for transformation. While the nuclear localization signal is dispensable for transformation, minimal changes in the v-Gag-Myc DNA-binding domain completely abolish its transforming potential, consistent with a role of Myc as a transcriptional regulator. One of its potential targets might be the endogenous c-myc, which is repressed in wild-type MC29-infected cells. Our experiments with MC29 mutants demonstrate that c-myc down-regulation depends on the integrity of the v-Myc DNA-binding domain and occurs at the RNA level. Hence, it is conceivable that v-Gag-Myc, either directly or circuitously, regulates c-myc transcription.

Mxi1, a protein that specifically interacts with Max to bind Myc-Max recognition sites

We used the interaction trap to isolate a novel human protein that specifically interacts with Max. This protein, Mxi1 (for Max interactor 1), contains a bHLH-Zip motif that is similar to that found in Myc family proteins. Mxi1 interacts specifically with Max to form heterodimers that efficiently bind to the Myc-Max consensus recognition site. When bound to DNA by a LexA moiety in yeast, Mxi1 does not stimulate transcription. mxi1 mRNA is expressed in many tissues, and its expression is elevated in U-937 myeloid leukemia cells that have been stimulated to differentiate. These facts are consistent with a model in which Mxi1-Max heterodimers indirectly inhibit Myc function in two ways: first, by sequestering Max, thus preventing the formation of Myc-Max heterodimers, and second, by competing with Myc-Max heterodimers for binding to target sites.

Mutations within the 5' half of the avian retrovirus MC29 v-myc gene alter or abolish transformation of chicken embryo fibroblasts and macrophages

Avian myelocytomatosis virus MC29 induces a wide variety of neoplastic diseases in infected birds and transforms cells of the macrophage lineage as well as fibroblasts and epithelial cells. A biological and biochemical analysis, carried out on a series of in-frame insertion and deletion mutations within the gag-myc gene of MC29, revealed several mutations within the 5' portion of the v-myc gene that encode proteins either completely defective for transformation or compromised in their ability to transform chicken embryo fibroblasts but not macrophages. Mutations within the 3' end of the v-myc gene which disrupt sequences encoding the basic/helix-loop-helix region were defective for transformation of both fibroblasts and macrophages. Eight variants were cloned into the replication-competent avian expression vector RCAS. Analysis of cells infected with transformation-defective, replication-competent viruses confirmed the expression of functionally defective Myc proteins. Further, expression of the transformation defective variant dl91-137 in chicken fibroblasts inhibited subsequent transformation by wild-type MC29. The results reported herein support the hypothesis that Myc proteins function as regulators of transcription in a variety of cell types and clearly point out the necessity of putative regulatory domains within the amino-terminal half of the Myc protein.

Myc and Max associate in vivo

Max is a helix-loop-helix zipper protein that associates in vitro with Myc family proteins to form a sequence-specific DNA-binding complex. We show here, by means of a coimmunoprecipitation assay with anti-Myc and anti-Max antibodies, that Myc and Max are associated in vivo and essentially all of the newly synthesized Myc can be detected in a complex with Max. This complex possesses specific DNA-binding activity for CACGTG-containing oligonucleotides. Although Max itself is a highly stable protein, Myc is rapidly degraded during or after its association with Max. In vivo Max is shown to be a nuclear protein phosphorylated by casein kinase II, and alternatively spliced forms of Max are expressed in cells. Furthermore, the levels of Max expression are equivalent in quiescent, mitogen-stimulated, and cycling cells. We conclude that the highly regulated rate of Myc biosynthesis is likely to be a limiting step in the formation of Myc:Max complexes.

Interferon gamma abrogates the differentiation block in v-myc-expressing U-937 monoblasts

Extensive studies suggest a role for the myc protooncogene family in the control of cell proliferation and differentiation in vertebrates. Previously, deregulated expression of exogenous myc genes has been shown to inhibit induced differentiation in Friend erythroleukemia (MEL) cells and in human monoblastic U-937 cells. To examine the nature of the block of phorbol 12-myristate 13-acetate-induced differentiation in v-myc-expressing U-937 cells, we have studied the effect of other inducers utilizing signal pathways distinct from phorbol 12-myristate 13-acetate (i.e., 1 alpha, 25-dihydroxycholecalciferol and retinoic acid). We show that v-myc also inhibits differentiation associated with these inducers. However, the v-myc-associated block of phorbol 12-myristate 13-acetate-, 1 alpha,25-dihydroxycholecalciferol-, and retinoic acid-induced differentiation retinoic acid-induced differentiation can be overcome by adding interferon gamma as a costimulatory factor. Costimulation with interferon gamma restores terminal differentiation, as shown by acquisition of a macrophage phenotype and an irreversible growth arrest in the G0/G1 phase of the cell cycle, but induces only limited differentiation on its own. The differentiation is accomplished without altering the expression or nuclear localization of the v-myc protein. These results argue against the widely held view that down-regulation of myc expression is a general prerequisite for terminal differentiation of hematopoietic cells and suggests that interferon gamma induces a signal(s) that circumvents the v-myc activity.

Transcription and cancer

The normal growth, development and function of an organism requires precise and co-ordinated control of gene expression. A major part of this control is exerted by regulating messenger RNA (mRNA) production and involves complex interactions between an array of transcriptionally active proteins and specific regulatory DNA sequences. The combination of such proteins and DNA sequences is specific for given gene or group of genes in a particular cell type and the proteins regulating the same gene may vary between cell types. In addition the expression or activity of these regulatory proteins may be modified depending on the state of differentiation of a cell or in response to an external stimulus. Thus, the differentiation of embryonic cells into diverse tissues is achieved and the mature structure and function of the organism is maintained. This review focusses on the role of perturbations of these transcriptional controls in neoplasia. Deregulation of transcription may result in the failure to express genes responsible for cellular differentiation, or alternatively, in the transcription of genes involved in cell division, through the inappropriate expression or activation of positively acting transcription factors and nuclear oncogenes. Whether the biochemical abnormalities that lead to the disordered growth and differentiation of a malignant tumour affect cell surface receptors, membrane or cytoplasmic signalling proteins or nuclear transcription factors, the end result is the inappropriate expression of some genes and failure to express others. Current research is starting to elucidate which of the elements of this complicated system are important in neoplasia.

Max: a helix-loop-helix zipper protein that forms a sequence-specific DNA-binding complex with Myc

The myc protooncogene family has been implicated in cell proliferation, differentiation, and neoplasia, but its mechanism of function at the molecular level is unknown. The carboxyl terminus of Myc family proteins contains a basic region helix-loop-helix leucine zipper motif (bHLH-Zip), which has DNA-binding activity and has been predicted to mediate protein-protein interactions. The bHLH-Zip region of c-Myc was used to screen a complementary DNA (cDNA) expression library, and a bHLH-Zip protein, termed Max, was identified. Max specifically associated with c-Myc, N-Myc, and L-Myc proteins, but not with a number of other bHLH, bZip, or bHLH-Zip proteins. The interaction between Max and c-Myc was dependent on the integrity of the c-Myc HLH-Zip domain, but not on the basic region or other sequences outside the domain. Furthermore, the Myc-Max complex bound to DNA in a sequence-specific manner under conditions where neither Max nor Myc exhibited appreciable binding. The DNA-binding activity of the complex was dependent on both the dimerization domain and the basic region of c-Myc. These results suggest that Myc family proteins undergo a restricted set of interactions in the cell and may belong to the more general class of eukaryotic DNA-binding transcription factors.

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

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

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

The leucine zipper motif has been observed in a number of proteins thought to function as eucaryotic transcription factors. Mutation of the leucine zipper interferes with protein dimerization and DNA binding. We examined the effect of point mutations in the leucine zipper of c-Myc on its ability to dimerize in vitro and to inhibit Friend murine erythroleukemia (F-MEL) differentiation. Glutaraldehyde cross-linking studies failed to provide evidence for homodimerization of in vitro-synthesized c-Myc protein, although it was readily demonstrated for c-Jun. Nevertheless, whereas transfected wild-type c-myc sequences strongly inhibited F-MEL differentiation, those with single or multiple mutations in the leucine zipper were only partially effective in this regard. Since the leucine zipper domain of c-Myc is essential for its cooperative effect in ras oncogene-mediated transformation, this study emphasizes the close relationship that exists between transformation and hematopoietic commitment and differentiation. c-Myc may produce its effects on F-MEL differentiation through leucine zipper-mediated heterodimeric associations rather than homodimeric ones.

Neoplastic transformation of mouse mammary epithelial cells by deregulated myc expression

A spontaneously immortalized, nontumorigenic mouse mammary epithelial cell line (MMEC) was transfected with an activated myc construct by electroporation. Constitutive expression of myc in MMEC resulted in anchorage independence in soft agar and tumorigenicity in nude mice. The myc-expressing MMEC showed higher saturation density, faster growth rate, and partial abrogation of serum-derived growth factor(s) requirement compared with parent MMEC. Epidermal growth factor or transforming growth factor alpha stimulated the anchorage-independent growth, but not the anchorage-dependent growth, of MMEC-myc cells. Type 1 transforming growth factor beta, on the other hand, inhibited both the anchorage-independent and anchorage-dependent growth of MMEC-myc cells. These results demonstrate that deregulated expression of myc results in neoplastic transformation iin mammary epithelial cells. Accompanying the transformation is altered sensitivity to polypeptide growth factors.

Distinct and different effects of the oncogenes v-myc and v-src on avian sympathetic neurons: retroviral transfer of v-myc stimulates neuronal proliferation whereas v-src transfer enhances neuronal differentiation

Immature avian sympathetic neurons are able to proliferate in culture for a limited number of divisions albeit expressing several neuron-specific properties. The effect of avian retroviral transfer of oncogenes on proliferation and differentiation of sympathetic neurons was investigated. Primary cultures of 6-d-old quail sympathetic ganglia, consisting of 90% neuronal cells, were infected by Myelocytomatosis virus (MC29), which contains the oncogene v-myc, and by the v-src-containing Rous sarcoma virus (RSV). RSV infection, in contrast to findings in other cellular systems, resulted in a reduction of neuronal proliferation as determined by 3H-thymidine incorporation (50% of control 4 d after infection) and in increased morphological differentiation. This is reflected by increased neurite production, cell size, and expression of neurofilament protein. In addition, RSV-infected neurons, unlike uninfected cells, are able to survive in culture for time periods up to 14 d in the absence of added neurotrophic factors. In contrast, retroviral transfer of v-myc stimulated the proliferation of immature sympathetic neurons preserving many properties of uninfected cells. The neuron-specific cell surface antigen Q211 and the adrenergic marker enzyme tyrosine hydroxylase were maintained in MC29-infected cells and in the presence of chick embryo extract the cells could be propagated over several weeks and five passages. Within 7 d after infection, the number of Q211-positive neurons increased approximately 100-fold. These data demonstrate distinct and different effects of v-src and v-myc-containing retroviruses on proliferation and differentiation of sympathetic neurons: v-src transfer results in increased differentiation, whereas v-myc transfer maintains an immature status reflected by proliferation, immature morphology, and complex growth requirements. The possibility of expanding immature neuronal populations by transfer of v-myc will be of considerable importance for the molecular analysis of neuronal proliferation and differentiation.