Mnt, a novel Max-interacting protein is coexpressed with Myc in proliferating cells and mediates repression at Myc binding sites

Overexpression of Mad transcription factor inhibits proliferation of cultured human hepatocellular carcinoma cells along with tumor formation in immunodeficient animals

BACKGROUND

Dominant negative regulation of critical cell cycle molecules could perturb survival of cancer cells and help develop novel therapies.

METHODS

To perturb the activity of c-Myc, which regulates G0/G1 transitions, we overexpressed Mad1 protein with an adenoviral vector, AdMad. Studies were conducted with established cell lines, including HepG2, HuH-7 and PLC/PRF/5 liver cancer cells, RAT-1A embryonic fibroblasts and U373MG astrocytoma cells.

RESULTS

After AdMad-treatment, transduced cells exhibited decreased proliferation rates in culture conditions. RAT-1A embryonic fibroblasts and U373MG astrocytoma cells showed accumulations in G0/G1, whereas HepG2 and HuH-7 cells accumulated in G0/G1, and additionally in G2/M, albeit to a lesser extent. An in vitro assay using hepatocyte growth factor to stimulate proliferation in HuH-7 cells showed blunting of growth factor responsiveness, along with inhibition of cell cycle progression in AdMad-treated cells. No cytotoxicity was observed in AdMad-treated cells in culture, although cells lost clonogenic capacity in soft agar. In vivo assays using HepG2 cell tumors in immunodeficient mice showed that overexpression of AdMad prevented tumorigenesis.

CONCLUSIONS

These studies indicate roles of Mad in G2/M, as well as the potential of manipulating cell cycle controls for treating liver cancer.

Gene-target recognition among members of the myc superfamily and implications for oncogenesis

Myc and Mad family proteins regulate multiple biological processes through their capacity to influence gene expression directly. Here we show that the basic regions of Myc and Mad proteins are not functionally equivalent in oncogenesis, have separable E-box-binding activities and engage both common and distinct gene targets. Our data support the view that the opposing biological actions of Myc and Mxi1 extend beyond reciprocal regulation of common gene targets. Identification of differentially regulated gene targets provides a framework for understanding the mechanism through which the Myc superfamily governs the growth, proliferation and survival of normal and neoplastic cells.

Myc induces the nucleolin and BN51 genes: possible implications in ribosome biogenesis

The c-Myc oncoprotein and its dimerization partner Max bind the DNA core consensus sequence CACGTG (E-box) and activate gene transcription. However, the low levels of induction have hindered the identification of novel Myc target genes by differential screening techniques. Here, we describe a computer-based pre-selection of candidate Myc/Max target genes, based on two restrictive criteria: an extended E-box consensus sequence for Myc/Max binding and the occurrence of this sequence within a potential genomic CpG island. Candidate genes selected by these criteria were evaluated experimentally for their response to Myc. Two Myc target genes are characterized here in detail. These encode nucleolin, an abundant nucleolar protein, and BN51, a co-factor of RNA polymerase III. Myc activates transcription of both genes via E-boxes located in their first introns, as seen for several well-characterized Myc targets. For both genes, mutation of the E-boxes abolishes transcriptional activation by Myc as well as repression by Mad1. In addition, the BN51 promoter is selectively activated by Myc and not by USF, another E-box-binding factor. Both nucleolin and BN51 are implicated in the maturation of ribosomal RNAs, albeit in different ways. We propose that Myc, via regulation of these and probably many other transcriptional targets, may be an important regulator of ribosome biogenesis.

The aryl hydrocarbon receptor interacts with estrogen receptor alpha and orphan receptors COUP-TFI and ERRalpha1

The molecular mechanisms underlying the apparent "cross-talk" between estrogen receptor (ER)- and arylhydrocarbon receptor (AHR)-mediated activities are unknown. To determine how AHR ligand 2, 3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) may inhibit ER action and, conversely, to examine how 17-beta-estradiol (E(2)) affects AHR activity, we examined discrete activities of each receptor, i.e., protein-protein interactions, DNA binding, and transcriptional activation. We report that AHR interacts directly with ERalpha, COUP-TF, and ERRalpha1, in a ligand-specific manner in vitro. Unoccupied or beta-napthoflavone (beta-NF)-occupied AHR showed stronger interaction with ERalpha, COUP-TF, and ERRalpha1 than when AHR was occupied by the partial antagonist alpha-naphthoflavone (alpha-NF), indicating a role for ligand in AHR interaction with these proteins. We also report that AHR interacts with COUP-TF in transfected CV-1 cells. In contrast, the AHR nuclear translocator protein (ARNT) did not interact with COUP-TF, ERRalpha1, or ERalpha. We next examined the interaction of either ERalpha or COUP-TF with a consensus xenobiotic response element (XRE). Purified ERalpha did not bind the consensus XRE, but COUP-TFI bound the consensus XRE, suggesting a role for COUP-TF as a AHR/ARNT competitor for XRE binding. In transiently transfected MCF-7 human breast cancer cells, overexpression of COUP-TFI inhibited TCDD-activated reporter gene activity from the CYP1A1 promoter. TCDD inhibited estradiol (E(2))-activated reporter gene activity from a consensus ERE and from the EREs in the pS2 and Fos genes, and COUP-TFI did not block the antiestrogenic activity of TCDD. The specific interaction of COUP-TF with XREs and AHR together with the inhibition of TCDD-induced gene expression by COUP-TF suggests that COUP-TF may regulate AHR action both by direct DNA binding competition and through protein-protein interactions.

Mlx, a novel Max-like BHLHZip protein that interacts with the Max network of transcription factors

Mad:Max heterodimers oppose the growth-promoting action of Myc:Max heterodimers by recruiting the mSin3-histone deacetylase (mSin3. HDAC) complex to DNA and functioning as potent transcriptional repressors. There are four known members of the Mad family that are indistinguishable in their abilities to interact with Max, bind DNA, repress transcription, and block Myc + Ras co-transformation. To investigate functional differences between Mad family proteins, we have identified additional proteins that interact with this family. Here we present the identification and characterization of the novel basic-helix-loop-helix zipper protein Mlx (Max-like protein x), which is structurally and functionally related to Max. The similarities between Mlx and Max include 1) broad expression in many tissues, 2) long protein half-life, and 3) formation of heterodimers with Mad family proteins that are capable of specific CACGTG binding. We show that transcriptional repression by Mad1:Mlx heterodimers is dependent on dimerization, DNA binding, and recruitment of the mSin3A.HDAC corepressor complex. In contrast with Max, Mlx interacts only with Mad1 and Mad4. Together, these findings suggest that Mlx may act to diversify Mad family function by its restricted association with a subset of the Mad family of transcriptional repressors.

A 13-amino acid amphipathic alpha-helix is required for the functional interaction between the transcriptional repressor Mad1 and mSin3A

Members of the Mad family of bHLHZip proteins heterodimerize with Max and function to repress the transcriptional and transforming activities of the Myc proto-oncogene. Mad:Max heterodimers repress transcription by recruiting a large multi-protein complex containing the histone deacetylases, HDAC1 and HDAC2, to DNA. The interaction between Mad proteins and HDAC1/2 is mediated by the corepressor mSin3A and requires sequences at the amino terminus of the Mad proteins, termed the SID, for Sin3 interaction domain, and the second of four paired amphipathic alpha-helices (PAH2) in mSin3A. To better understand the requirements for the interaction between the SID and PAH2, we have performed mutagenesis and structural studies on the SID. These studies show that amino acids 8-20 of Mad1 are sufficient for SID:PAH2 interaction. Further, this minimal 13-residue SID peptide forms an amphipathic alpha-helix in solution, and residues on the hydrophobic face of the SID helix are required for interaction with PAH2. Finally, the minimal SID can function as an autonomous and portable repression domain, demonstrating that it is sufficient to target a functional mSin3A/HDAC corepressor complex.

The MYC dualism in growth and death

Over-expression of the transcription factor c-Myc immortalizes primary cells and transforms in co-operation with activated ras. Therefore, c-myc is considered a proto-oncogene. Since its discovery c-Myc has been shown to render cells growth factor independent, accelerates passage through G1 of the cell cycle, inhibits differentiation and elicits apoptosis. Whereas the effects on immortalization, proliferation and inhibition of differentiation are in conceivable accordance with gain of function, as it is defined for a proto-oncogene, its pro-apoptotic activity disables a straight forward explanation of the physiological role of c-Myc and suggests a highly complex contribution during development. The recent accomplishments in c-Myc research shed some light on the difficile regulatory network which keeps check on c-Myc activity such as by binding to proteins some of which are transcription factors for non-c-Myc targets. Moreover, it was shown that genes are targeted by c-Myc depending on the sequence of flanking regions adjacent to the E-box or in dependence on the availability of binding partners which is most probably specific to the cellular context. Cdc25A and ornithine decarboxylase, both described to be c-Myc targets, have been brought forward as downstream effectors in the induction of proliferation under serum rich conditions, or in the induction of apoptosis when serum factors are limited. These genes seem to be regulated by c-Myc in a cell type-specific manner. H-ferritin, IRP2 and telomerase are the most recently discovered direct targets of c-Myc. The regulation of H-ferritin and IRP2 might explain the potential of c-Myc to promote proliferation and the regulation of telomerase could be responsible for the immortalizing properties of c-Myc. In the future, H-ferritin and telomerase have to be analyzed whether or not these genes are also Myc targets in other cell systems. Although the intense research efforts regarding the function of c-Myc last already two decades the role of this gene is still enigmatic.

Expression of MXI1, a Myc antagonist, is regulated by Sp1 and AP2

MXI1, a member of the MAD family of Myc antagonists, encodes a transcription factor whose expression must be tightly regulated to maintain normal cell growth and differentiation. To more closely investigate the transcriptional regulation of the human MXI1 gene, we have cloned and characterized the MXI1 promoter. After clarification of the 5'- and 3'-untranslated regions of the cDNA (indicating that the true length of the MXI1 transcript is 2643 base pairs), we identified two transcription initiation sites. We subsequently isolated the MXI1 promoter, which is GC-rich and lacks a TATA box. Although it contains at least six potential initiator sequences, functional studies indicate the proximal two initiator sequences in combination with nearby Sp1 and MED-1 sites together account for virtually all promoter activity. We also demonstrate that MXI1 promoter activity is repressed by high levels of AP2. These studies provide further insight into the complex regulatory mechanisms governing MXI1 gene expression and its role in cellular differentiation and tumor suppression.

Analysis of the Max-binding protein MNT in human medulloblastomas

Medulloblastomas (MBs) are the most frequent malignant brain tumors in children. The molecular pathogenesis of these tumors is still poorly understood. Microsatellite and restriction-fragment-length polymorphism studies have revealed allelic loss of genetic material on the short arm of chromosome 17 in the region 17p13 in approximately 50% of MBs, suggesting the presence of a tumor-suppressor gene in this region. A candidate for this putative tumor-suppressor is the MNT gene, located at 17p13.3 and encoding a Max-interacting nuclear protein with transcriptional-repressor activity. In this study, we analyzed MNT mRNA and protein expression in 44 MB samples, including 32 primary tumors, 3 recurrent tumors and 9 MB cell lines. Allelic loss at 17p13.3 was found in 49% of informative cases. RT-PCR showed MNT mRNA expression in all cases analyzed. Endogenous Mnt protein with an apparent molecular weight of 72 to 74 kDa was detected in lysates from MB cell lines. The presence and functional integrity of Mnt in MBs were tested in electrophoretic mobility-shift assays. These experiments demonstrated that Mnt interacts with Max, and that this heterodimer binds DNA specifically, suggesting a functional bHLHZip domain of MB-derived Mnt. In support, single-strand conformation-polymorphism (SSCP) analyses revealed no mutation in the bHLHZip region. Deletion of the Mnt Sin3 interaction domain was shown to convert Mnt from an inhibitor of myc/ras-co-transformation into a molecule capable of cooperating with Ras in transformation. This region therefore was screened for mutation by SSCP: again, no alterations were found. These findings indicate that the MNT gene located at 17p13.3 is not likely to be involved in the molecular pathogenesis of MBs.

The cytoplasmic Purkinje onconeural antigen cdr2 down-regulates c-Myc function: implications for neuronal and tumor cell survival

Paraneoplastic cerebellar degeneration (PCD) is a disorder in which breast or ovarian tumors express an onconeural antigen termed cdr2, which normally is expressed in cerebellar Purkinje neurons. This leads to an immune response to cdr2 that is associated with tumor immunity and autoimmune cerebellar degeneration. We have found that cdr2, a cytoplasmic protein harboring a helix-leucine zipper (HLZ) motif, interacts specifically with the HLZ motif of c-Myc. Both proteins colocalize in the cytoplasm of adult cerebellar Purkinje neurons, and coimmunoprecipitate from tumor cell lines and cerebellar extracts. cdr2 down-regulates c-Myc-dependent transcription in cotransfection assays, and redistributes Myc protein in the cytoplasm. Disease antisera from six of six PCD patients specifically blocked the interaction between cdr2 and c-Myc in vitro. These data indicate that cdr2 normally sequesters c-Myc in the neuronal cytoplasm, thereby down-regulating c-Myc activity, and suggest a mechanism whereby inhibition of cdr2 function by autoantibodies in PCD may contribute to Purkinje neuronal death.

Posttranslational Regulation of Myc Function in Response to Phorbol Ester/Interferon-γ–Induced Differentiation of v-Myc–Transformed U-937 Monoblasts

The transcription factors of the Myc/Max/Mad network are important regulators of cell growth, differentiation, and apoptosis and are frequently involved in tumor development. Constitutive expression of v-Myc blocks phorbol ester (TPA)-induced differentiation of human U-937 monoblasts. However, costimulation with interferon-γ (IFN-γ) and TPA restores terminal differentiation and G1cell-cycle arrest despite continuous expression of v-Myc. The mechanism by which TPA + IFN-γ counteract v-Myc activity has not been unravelled. Our results show that TPA + IFN-γ treatment led to an inhibition of v-Myc– and c-Myc–dependent transcription, and a specific reduction of v-Myc:Max complexes and associated DNA-binding activity, whereas the steady state level of the v-Myc protein was only marginally affected. In contrast, TPA + IFN-γ costimulation neither increased the expression of Mad1 or other mad/mnt family genes nor altered heterodimerization or DNA-binding activity of Mad1. The reduced amount of v-Myc:Max heterodimers in response to treatment was accompanied by partial dephosphorylation of v-Myc and c-Myc. Phosphatase treatment of Myc:Max complexes lead to their dissociation, thus mimicking the effect of TPA + IFN-γ. In addition to modulation of the expression of Myc/Max/Mad network proteins, posttranslational negative regulation of Myc by external signals may, therefore, be an alternative biologically important level of control with potential therapeutic relevance for hematopoietic and other tumors with deregulated Myc expression.

Posttranslational Regulation of Myc Function in Response to Phorbol Ester/Interferon-γ–Induced Differentiation of v-Myc–Transformed U-937 Monoblasts

The transcription factors of the Myc/Max/Mad network are important regulators of cell growth, differentiation, and apoptosis and are frequently involved in tumor development. Constitutive expression of v-Myc blocks phorbol ester (TPA)-induced differentiation of human U-937 monoblasts. However, costimulation with interferon-γ (IFN-γ) and TPA restores terminal differentiation and G1cell-cycle arrest despite continuous expression of v-Myc. The mechanism by which TPA + IFN-γ counteract v-Myc activity has not been unravelled. Our results show that TPA + IFN-γ treatment led to an inhibition of v-Myc– and c-Myc–dependent transcription, and a specific reduction of v-Myc:Max complexes and associated DNA-binding activity, whereas the steady state level of the v-Myc protein was only marginally affected. In contrast, TPA + IFN-γ costimulation neither increased the expression of Mad1 or other mad/mnt family genes nor altered heterodimerization or DNA-binding activity of Mad1. The reduced amount of v-Myc:Max heterodimers in response to treatment was accompanied by partial dephosphorylation of v-Myc and c-Myc. Phosphatase treatment of Myc:Max complexes lead to their dissociation, thus mimicking the effect of TPA + IFN-γ. In addition to modulation of the expression of Myc/Max/Mad network proteins, posttranslational negative regulation of Myc by external signals may, therefore, be an alternative biologically important level of control with potential therapeutic relevance for hematopoietic and other tumors with deregulated Myc expression.

Two MAD tails: what the recent knockouts of Mad1 and Mxi1 tell us about the MYC/MAX/MAD network

Members of the MAD/MXI protein family heterodimerize with MAX and repress transcription by recruiting a chromatin-modifying co-repressor complex to specific DNA target genes. Repression mediated by MAD is thought to antagonize the transcriptional activation and proliferation-promoting functions of MYC-MAX heterodimers. Because they are induced during differentiation, it has been suggested that MAD proteins act to limit cell proliferation during terminal differentiation. There is also controversial evidence that these proteins may function as tumor suppressors. Recently, targeted gene deletions of two members of this gene family, Mad1 and Mxi1, have been carried out in mice. Although these animals display what appear to be quite different phenotypes, further analysis supports the view that both these proteins function in cell-cycle exit during terminal differentiation, and that at least MXI1 can act as a tumor suppressor.

The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation

A large body of evidence has been accumulated that demonstrates dominant effects of Myc proto-oncoproteins on different aspects of cellular growth. Myc is one of the few proteins that is sufficient to drive resting cells into the cell cycle and promote DNA synthesis. In line with this finding is that the constitutive expression of Myc in cells blocks their differentiation. These growth stimulating properties are most likely responsible for Myc's ability to initiate and promote tumor formation. Interestingly Myc can also sensitize cells to apoptosis, suggesting that this protein is part of a life-and-death switch. Molecularly Myc functions as a transcriptional regulator that needs to heterodimerize with Max to exert the biological activities described above and to regulate gene transcription. Myc and Max are just two members of a growing family of proteins referred to as the Myc/Max/Mad network. A hallmark of these proteins is that they possess a C-terminal basic region/helix-loop-helix/leucine zipper domain (bHLHZip). The bHLHZip domain specifies dimerization within the network and determines sequence specific DNA binding. Importantly this domain together with the N-terminal transactivation domain is essential for Myc biology. Here we have summarized the structural, functional, and regulatory aspects of the bHLHZip domain of Myc proteins.

New Myc-interacting proteins: a second Myc network emerges

Despite its intensive investigation for almost two decades, c-Myc remains a fascinating and enigmatic subject. A large and compelling body of evidence indicates that c-Myc is a transcription factor with central roles in the regulation of cell proliferation, differentiation, and apoptosis, but its exact function has remained elusive. In this review we survey recent advances in the identification and analysis of c-Myc-binding proteins, which suggest insights into the transcriptional roles of c-Myc but which also extend the existing functional paradigms. The C-terminal domain (CTD) of c-Myc mediates interaction with Max and physiological recognition of DNA target sequences, events needed for all biological actions. Recently described interactions between the CTD and other cellular proteins, including YY-1, AP-2, BRCA-1, TFII-I, and Miz-1, suggest levels of regulatory complexity beyond Max in controlling DNA recognition by c-Myc. The N-terminal domain (NTD), which includes the evolutionarily conserved and functionally crucial Myc Box sequences (MB1 and MB2), contains the transcription activation domain (TAD) of c-Myc as well as regions required for transcriptional repression, cell cycle regulation, transformation, and apoptosis. In addition to interaction with the retinoblastoma family protein p107, the NTD has been shown to interact with alpha-tubulin and the novel adaptor proteins Binl, MM-1, Pam, TRRAP, and AMY-1. The structure of these proteins and their effects on c-Myc actions suggest links to the transcriptional regulatory machinery as well as to cell cycle regulation, chromatin modeling, and apoptosis. Investigations of this emerging NTD-based network may reveal how c-Myc is regulated and how it affects cell fate, as well as providing tools to distinguish the physiological roles of various Myc target genes.

Cell cycle genes in chondrocyte proliferation and differentiation

Coordinated proliferation and differentiation of growth plate chondrocytes controls longitudinal growth of endochondral bones. While many extracellular factors regulating these processes have been identified, much less is known about the intracellular mechanisms transducing and integrating these extracellular signals. Recent evidence suggests that cell cycle proteins play an important role in the coordination of chondrocyte proliferation and differentiation. Our current knowledge of the function and regulation of cell cycle proteins in endochondral ossification is summarized.

Isolation and embryonic expression of the novel mouse gene Hic1, the homologue of HIC1, a candidate gene for the Miller-Dieker syndrome

The human gene HIC1 (hypermethylated in cancer) maps to chromosome 17p13.3 and is deleted in the contiguous gene disorder Miller-Dieker syndrome (MDS) [Makos-Wales et al. (1995) Nature Med., 1, 570-577; Chong et al. (1996) Genome Res., 6, 735-741]. We isolated the murine homologue Hic1, encoding a zinc-finger protein with a poxvirus and zinc-finger (POZ) domain and mapped it to mouse chromosome 11 in a region exhibiting conserved synteny to human chromosome 17. Comparison of genomic and cDNA sequences predicts two exons for the murine Hic1. The second exon exhibits 88% identity to the human HIC1 on DNA level. During embryonic development, Hic1 is expressed in mesenchymes of the sclerotomes, lateral body wall, limb and cranio-facial regions embedding the outgrowing peripheral nerves during their differentiation. During fetal development, Hic1 additionally is expressed in mesenchymes apposed to precartilaginous condensations, at many interfaces to budding epithelia of inner organs, and weakly in muscles. We observed activation of Hic1 expression in the embryonic anlagen of many tissues displaying anomalies in MDS patients. Besides lissencephaly, MDS patients exhibit facial dysmorphism and frequently additional birth defects, e.g. anomalies of the heart, kidney, gastrointestinal tract and the limbs (OMIM 247200). Thus, HIC1 activity may correlate with the defective development of the nose, jaws, extremities, gastrointestinal tract and kidney in MDS patients.

The Adrenomedullin gene is a target for negative regulation by the Myc transcription complex

The Myc family of transcription factors plays a central role in vertebrate growth and development although relatively few genetic targets of the Myc transcription complex have been identified. In this study, we used mRNA differential display to investigate gene expression changes induced by the overexpression of the MC29 v-Myc oncoprotein in C3H10T1/2 mouse fibroblasts. We identified the transcript of the adrenomedullin gene (AM) as an mRNA that is specifically down-regulated in v-Myc overexpressing C3H10T1/2 cell lines as well as in a Rat 1a cell line inducible for c-Myc. Nucleotide sequence analysis of the mouse AM promoter reveals the presence of consensus CAAT and TATA boxes as well as an initiator element (INR) with significant sequence similarity to the INR responsible for Myc-mediated repression of the adenovirus major late promoter (AdMLP). Reporter gene assays confirm that the region of the AM promoter containing the INR is the target of Myc-mediated repression. Exogenous application of AM peptide to quiescent C3H10T1/2 cultures does not stimulate growth, and constitutive expression of AM mRNA in C3H10T1/2 cells correlates with a reduced potential of the cells to be cotransformed by v-Myc and oncogenic Ras p21. Additional studies showing that AM mRNA is underrepresented in C3H10T1/2 cell lines stably transformed by Ras p21 or adenovirus E1A suggest that AM gene expression is incompatible with deregulated growth in this cell line. We propose a model in which the repression of AM gene expression by Myc is important to the role of this oncoprotein as a potentiator of cellular transformation in C3H10T1/2 and perhaps other cell lines.

Formation of alpha-Pal/Max heterodimers synergistically activates the eIF2-alpha promoter

The transcription factor alpha-Pal recognizes two tandem palindromic repeats within the promoter of eukaryotic translation initiation factor 2-alpha (eIF2-alpha). Whereas both binding sites have the same "core domain" sequence (CGCATGCG), they differ with respect to their flanking sequences. Of the two sites, the 5'-cap proximal site has a higher binding affinity for alpha-Pal than does the 5'-cap distal site (Jacob, W. F., Silverman, T. A., Cohen, R. B., and Safer, B. (1989) J. Biol. Chem. 264, 20372-20384). The well characterized transcription factor Max binds to sequences that are remarkably similar to the core domain that alpha-Pal recognizes. To date, all of the Max heterodimer partners lack DNA binding domains and are thus dependent on Max interacting with DNA. Here we report that the two alpha-Pal sites have very different binding activities with respect to the E-box-binding protein Max. The 5'-cap distal or low alpha-Pal affinity site binds both alpha-Pal and Max. Furthermore, both heterodimers and homodimers of each of these proteins bind to this site. In contrast to the low affinity site, the high affinity site does not bind Max as a homodimer. This is the first documented case where Max heterodimerizes with a transcription factor that has affinity for DNA independent of Max.

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

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

The bHLH-Zip transcription factor Tfeb is essential for placental vascularization

Tfeb is a member of the basic Helix-Loop-Helix-Zipper family of transcription factors. In vitro studies have shown that TFEB can bind DNA as a homodimer or as a heterodimer with three closely related family members: MITF, TFE3 and TFEC. While mutations of Mitf have been shown to affect the development of a number of cell types including melanocytes, osteoclasts, and masts cells, little is known about the phenotypic consequences of mutations at Tfe3, Tfeb and Tfec. Here we show that mice with a targeted disruption of Tfeb die between 9.5 and 10.5 days in embryonic development and have severe defects in placental vascularization. Tfeb is expressed at low levels in the embryo but at high levels in the labyrinthine trophoblast cells of the placenta. While labyrinthine cells are present in the mutant Tfeb placenta, they fail to express VEGF, a potent mitogen required for normal vasculogenesis of the embryo and extraembryonic tissues. In Tfeb mutant embryos the embryonic vasculature forms normally but few vessels are seen entering the placenta and those that do enter fail to thrive and branch normally. Our results indicate that Tfeb plays a critical role in the signal transduction processes required for normal vascularization of the placenta.

c-Myc target gene specificity is determined by a post-DNAbinding mechanism

Uncertainty as to which member of a family of DNA-binding transcription factors regulates a specific promoter in intact cells is a problem common to many investigators. Determining target gene specificity requires both an analysis of protein binding to the endogenous promoter as well as a characterization of the functional consequences of transcription factor binding. By using a formaldehyde crosslinking procedure and Gal4 fusion proteins, we have analyzed the timing and functional consequences of binding of Myc and upstream stimulatory factor (USF)1 to endogenous cellular genes. We demonstrate that the endogenous cad promoter can be immunoprecipitated with antibodies against Myc and USF1. We further demonstrate that although both Myc and USF1 can bind to cad, the cad promoter can respond only to the Myc transactivation domain. We also show that the amount of Myc bound to the cad promoter fluctuates in a growth-dependent manner. Thus, our data analyzing both DNA binding and promoter activity in intact cells suggest that cad is a Myc target gene. In addition, we show that Myc binding can occur at many sites in vivo but that the position of the binding site determines the functional consequences of this binding. Our data indicate that a post-DNA-binding mechanism determines Myc target gene specificity. Importantly, we have demonstrated the feasibility of analyzing the binding of site-specific transcription factors in vivo to single copy mammalian genes.

The mammalian Cut homeodomain protein functions as a cell-cycle-dependent transcriptional repressor which downmodulates p21WAF1/CIP1/SDI1 in S phase

Cut is a homeodomain transcription factor which has the unusual property of containing several DNA-binding domains: three regions called Cut repeats and the Cut homeodomain. Genetic studies in Drosophila melanogaster indicate that cut plays important roles in the determination and maintenance of cell-type specificity. In the present study, we show that mammalian Cut proteins may yet play another biological role, specifically in proliferating cells. We found that the binding of Cut to a consensus binding site varies during the cell cycle. Binding was virtually undetectable in G0 and early G1, but became very strong as cells reached S phase. This was shown to result both from an increase in Cut expression and dephosphorylation of the Cut homeodomain by the Cdc25A phosphatase. We also show that the increase in Cut activity coincides with a decrease in p21WAF1/CIP1/SDI1 mRNAs. In co-transfection experiments, Cut proteins repressed p21WAF1/CIP1/SDI1 gene expression through binding to a sequence that overlaps the TATA box. Moreover, p21WAF1/CIP1/SDI1 expression was repressed equally well by either Cdc25A or Cut. Altogether, these results suggest a model by which Cdc25A activates the Cut repressor which then downregulates transcription of p21WAF1/CIP1/SDI1 in S phase. Thus, in addition to their role during cellular differentiation, Cut proteins also serve as cell-cycle-dependent transcriptional factors in proliferating cells.

Insights into the mechanism of heterodimerization from the 1H-NMR solution structure of the c-Myc-Max heterodimeric leucine zipper

The oncoprotein c-Myc (a member of the helix-loop-helix-leucine zipper (b-HLH-LZ) family of transcription factors) must heterodimerize with the b-HLH-LZ Max protein to bind DNA and activate transcription. It has been shown that the LZ domains of the c-Myc and Max proteins specifically form a heterodimeric LZ at 20 degreesC and neutral pH. This suggests that the LZ domains of the c-Myc and Max proteins are playing an important role in the heterodimerization of the corresponding gene products in vivo. Initially, to gain an insight into the energetics of heterodimerization, we studied the stability of N-terminal disulfide-linked versions of the c-Myc and Max homodimeric LZs and c-Myc-Max heterodimeric LZ by fitting the temperature-induced denaturation curves monitored by circular dichroism spectroscopy. The c-Myc LZ does not homodimerize (as previously reported) and the c-Myc-Max heterodimeric LZ is more stable than the Max homodimeric LZ at 20 degreesC and pH 7.0. In order to determine the critical interhelical interactions responsible for the molecular recognition between the c-Myc and Max LZs, the solution structure of the disulfide-linked c-Myc-Max heterodimeric LZ was solved by two-dimensional 1H-NMR techniques at 25 degreesC and pH 4.7. Both LZs are alpha-helical and the tertiary structure depicts the typical left-handed super-helical twist of a two-stranded parallel alpha-helical coiled-coil. A buried salt bridge involving a histidine on the Max LZ and two glutamate residues on the c-Myc LZ is observed at the interface of the heterodimeric LZ. A buried H-bond between an asparagine side-chain and a backbone carbonyl is also observed. Moreover, evidence for e-g interhelical salt bridges is reported. These specific interactions give insights into the preferential heterodimerization process of the two LZs. The low stabilities of the Max homodimeric LZ and the c-Myc-Max heterodimeric LZ as well as the specific interactions observed are discussed with regard to regulation of transcription in this family of transcription factors.

Control of cell proliferation by Myc proteins

Taken together, the available data appear to be consistent with a model in which Myc proteins function downstream of D-type cyclins and synergize with E2F proteins in the activation of the cyclin E/cdk2 kinase. This view of Myc proteins appears strikingly similar to established models for the E2F/DP family of proteins. However, it should be noted that there are clear differences and several predictions of such a model that have been critically tested for E2F proteins are still untested for Myc in this model. First, it appears that at least some target genes of Myc implicated in this process are still unknown; second, clear data from knockout cells that link p107 to Myc function are missing; and third, we are not aware of studies of tumour samples that clarify whether mutations in myc genes relieve the requirement for mutations in the cyclin D/p16 pathway.

Control of cell proliferation by Myc

Myc proteins are key regulators of mammalian cell proliferation. They are transcription factors that activate genes as part of a heterodimeric complex with the protein Max. This review summarizes recent progress in understanding how Myc stimulates cell proliferation and how this might contribute to cellular transformation and tumorigenesis.

Sequences flanking the E-box contribute to cooperative binding by c-Myc/Max heterodimers to adjacent binding sites

Previously, we have shown that c-Myc/Max heterodimers, bind cooperatively to the two adjacent, canonical E-boxes (CACGTG) located in the rat ornithine decarboxylase (ODC) gene. In order to study this in more detail, we changed the length of the linker that separates the two E-boxes, as well as their flanking sequences. We found that high affinity, cooperative binding requires a minimal linker length of 1-4 bp and that the binding affinity is influenced by E-box flanking sequences. Binding to the c-Myc responsive element of prothymosin alpha, containing both a canonical and a noncanonical E-box (CAAGTG) was also studied. As shown by DNAseI footprinting analysis, only the canonical E-box is bound by c-Myc/Max and c-Max/Max dimers. Replacing the noncanonical site with a canonical E-box only partially restored high affinity, cooperative binding. By making hybrid fragments between ODC and prothymosin alpha, we found that nucleotides in the linker between the E-boxes influence the affinity of c-Myc/Max heterodimers. Taken together, our results show that E-box sequences and sequences in the linker separating both E-boxes influence cooperative, high affinity binding by c-Myc/Max dimers.

The human ROX gene: genomic structure and mutation analysis in human breast tumors

We have recently isolated a human gene, ROX, encoding a new member of the basic helix-loop-helix leucine zipper protein family. ROX is capable of heterodimerizing with Max and acts as a transcriptional repressor in an E-box-driven reporter gene system, while it was found to activate transcription in HeLa cells. ROX expression levels vary during the cell cycle, being down-regulated in proliferating cells. These biological properties of ROX suggest a possible involvement of this gene in cell proliferation and differentiation. The ROX gene maps to chromosome 17p13.3, a region frequently deleted in human malignancies. Here we report the genomic structure of the human ROX gene, which is composed of six exons and spans a genomic region of less than 40 kb. In an attempt to identify possible inactivating mutations in the ROX gene in human breast cancer, we performed a single-strand conformation polymorphism analysis of its coding region in 16 sporadic breast carcinomas showing loss of heterozygosity in the 17p13.3 region. No mutations were found in this analysis. Five nucleotide polymorphisms were identified in the ROX gene, three of which caused an amino acid substitution. These nucleotide changes were present in the peripheral blood DNAs of both the patients and the control individuals. In vitro translated assays did not show a significant decrease in the ability of the ROX mutant proteins to bind DNA or to repress transcription of a driven reporter gene in HEK293 cells. Despite experimental evidence that ROX might act as a tumor suppressor gene, our data suggest that mutations in the coding region of ROX are uncommon in human breast tumorigenesis.

Molecular analysis of a Myc antagonist, ROX/Mnt, at 17p13.3 in human lung cancers

The chromosome region 17p13 is known to be frequently deleted in lung cancers. We recently showed the presence of an independent, commonly deleted region at 17p13.3, suggesting that in addition to p53 at 17p13.1 an as-yet-unidentified tumor suppressor gene may reside in this telomeric region. Interestingly, the chromosomal location of a recently isolated novel myc antagonist gene, termed ROX/Mnt, coincides exactly with the centromeric border of the commonly deleted region at 17p13.3 in lung cancers. In conjunction with the generally acknowledged roles of myc genes in the pathogenesis of lung cancers, these findings led us to investigate whether ROX/Mnt is altered in lung cancers. Despite an extensive search for alterations in 52 lung cancer specimens. somatic mutations of ROX/Mnt could not be identified. We conclude that ROX/Mnt itself is not a frequent target for 17p13.3 deletions in lung cancers and that further explorations are required to identify the putative tumor suppressor gene at 17p13.3.

Identification and characterization of specific DNA-binding complexes containing members of the Myc/Max/Mad network of transcriptional regulators

In the past, eukaryotic cell-derived complexes of the Myc/Max/Mad network of transcriptional regulators have largely been refractory to DNA binding studies. We have developed electrophoretic mobility shift assay conditions to measure specific DNA binding of Myc/Max/Mad network complexes using a COS7 cell-based overexpression system. With the established protocol, we have measured on- and off-rates of c-Myc/Max, Max/Max, and Mad1/Max complexes and determined relative affinities. All three complexes appeared to bind with comparable affinity to a Myc E-box sequence. Furthermore, our data derived from competition experiments suggested that the Mad3/Max and Mad4/Max complexes also possess comparable DNA binding affinities. The conditions established for COS7 cell-overexpressed proteins were then used to identify c-Myc/Max, Max/Max, and Mnt/Max complexes in HL-60 cells. However, no Mad1/Max could be detected, despite the induction of Mad1 expression during differentiation. Whereas the DNA binding activity of c-Myc/Max complexes was down-regulated, Max/Max binding increased, and Mnt/Max binding remained unchanged. In addition, we have also tested for upstream stimulatory factor (USF) binding and observed that, in agreement with published data, USF comprises a major Myc E-box-binding factor that is more abundant than any of the Myc/Max/Mad network complexes. Similar to the Mnt/Max complex, the binding activity of USF remained constant during HL-60 differentiation. Our findings establish conditions for the analysis of DNA binding of Myc/Max/Mad complexes and indicate posttranslational regulation of the Max/Max complex.

Accelerated proliferative senescence of rat embryo fibroblasts after stable transfection of multiple copies of the c-Myc DNA-binding sequence

The protooncogene c-myc positively regulates cellular proliferation whereas it exhibits negative effects on both cellular senescence and differentiation. Ectopic overexpression of c-myc in transfection experiments or titration of the c-myc mRNA by antisense oligonucleotides has demonstrated that small changes of the concentration of cellular c-myc mRNA or protein levels can be crucial for these processes. In view of the role of c-Myc as a transcription factor, most of these effects may be mediated via its binding to specific DNA sequences. Here we studied the cellular reactions after manipulating the cellular concentration of c-Myc DNA-binding sites. Multiple copies of the c-Myc-binding sequence GACCACGTGGTC or, alternatively, the control sequence GACCAGCTGGTC that displays only a poor affinity for c-Myc were stably introduced into the genome of rat embryo fibroblasts. Transfection with the c-Myc-binding sequence yielded much lower clone numbers and sizes than transfection with the control sequence. After polyclonal selection and further subcultivation cells transfected with c-Myc-binding sequence exhibited a reduced growth rate and achieved less than two-thirds of the cumulative population doublings before becoming senescent and irreversibly growth arrested compared to the controls. Southern blot analysis demonstrated that 30 binding sequences on average were integrated into the cellular genome. Our results can be interpreted as competition of the ectopically introduced c-Myc-binding sequences with the functional genomic ones and assume that a fairly low number of the latter exist in the normal cellular genome. Hence, only a low copy number of introduced c-Myc-binding sequences is sufficient to cause signs of accelerated proliferative senescence.

Targeted disruption of the MYC antagonist MAD1 inhibits cell cycle exit during granulocyte differentiation

The switch from transcriptionally activating MYC-MAX to transcriptionally repressing MAD1-MAX protein heterodimers has been correlated with the initiation of terminal differentiation in many cell types. To investigate the function of MAD1-MAX dimers during differentiation, we disrupted the Mad1 gene by homologous recombination in mice. Analysis of hematopoietic differentiation in homozygous mutant animals revealed that cell cycle exit of granulocytic precursors was inhibited following the colony-forming cell stage, resulting in increased proliferation and delayed terminal differentiation of low proliferative potential cluster-forming cells. Surprisingly, the numbers of terminally differentiated bone marrow and peripheral blood granulocytes were essentially unchanged in Mad1 null mice. This imbalance between the frequencies of precursor and mature granulocytes was correlated with a compensatory decrease in granulocytic cluster-forming cell survival under apoptosis-inducing conditions. In addition, recovery of the peripheral granulocyte compartment following bone marrow ablation was significantly enhanced in Mad1 knockout mice. Two Mad1-related genes, Mxi1 and Mad3, were found to be expressed ectopically in adult spleen, indicating that functional redundancy and cross-regulation between MAD family members may allow for apparently normal differentiation in the absence of MAD1. These findings demonstrate that MAD1 regulates cell cycle withdrawal during a late stage of granulocyte differentiation, and suggest that the relative levels of MYC versus MAD1 mediate a balance between cell proliferation and terminal differentiation.

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.

Cell growth inhibition by the Mad/Max complex through recruitment of histone deacetylase activity

BACKGROUND

The organization of chromatin is crucial for the regulation of gene expression. In particular, both the positioning and properties of nucleosomes influence promoter-specific transcription. The acetylation of core histones has been suggested to alter the properties of nucleosomes and affect the access of DNA-binding transcriptional regulators to promoters. A recently identified mammalian histone deacetylase (HD1) shows homology to the yeast Rpd3 protein, which together with Sin3 affects the transcription of several genes. Mammalian Sin3 proteins interact with the Mad components of the Myc/Max/Mad network of cell growth regulators. Mad/Max complexes may recruit mammalian Rpd3-like enzymes, therefore, directing histone deacetylase activity to promoters and negatively regulating cell growth.

RESULTS

We report the identification of a tetrameric complex composed of Max, Mad1, Sin3B and HD1. This complex has histone deacetylase activity which can be blocked by the histone deacetylase inhibitors trichostatin A and sodium butyrate. The inhibition of cell growth by Mad1 is enhanced by Sin3B and HD1, as measured by colony formation assays. Furthermore, a Mad1-induced block of S-phase progression can be overcome by trichostatin A, as shown in microinjection experiments.

CONCLUSIONS

The recruitment of a histone deacetylase by sequence-specific DNA-binding proteins provides a mechanism by which the state of acetylation of histones in nucleosomes and hence the activity of specific promoters can be influenced. The finding that Mad/Max complexes interact with Sin3 and HD1 in vivo suggests a model for the role of Mad proteins in antagonizing the function of Myc proteins.

Genomic organization of the murine Miller-Dieker/lissencephaly region: conservation of linkage with the human region

Several human syndromes are associated with haploinsufficiency of chromosomal regions secondary to microdeletions. Isolated lissencephaly sequence (ILS), a human developmental disease characterized by a smooth cerebral surface (classical lissencephaly) and microscopic evidence of incomplete neuronal migration, is often associated with small deletions or translocations at chromosome 17p13.3. Miller-Dieker syndrome (MDS) is associated with larger deletions of 17p13.3 and consists of classical lissencephaly with additional phenotypes including facial abnormalities. We have isolated the murine homologs of three genes located inside and outside the MDS region: Lis1, Mnt/Rox, and 14-3-3 epsilon. These genes are all located on mouse chromosome 11B2, as determined by metaphase FISH, and the relative order and approximate gene distance was determined by interphase FISH analysis. The transcriptional orientation and intergenic distance of Lis1 and Mnt/Rox were ascertained by fragmentation analysis of a mouse yeast artificial chromosome containing both genes. To determine the distance and orientation of 14-3-3 epsilon with respect to Lis1 and Mnt/Rox, we introduced a super-rare cutter site (VDE) that is unique in the mouse genome into 14-3-3 epsilon by gene targeting. Using the introduced VDE site, the orientation of this gene was determined by pulsed field gel electrophoresis and Southern blot analysis. Our results demonstrate that the MDS region is conserved between human and mouse. This conservation of linkage suggests that the mouse can be used to model microdeletions that occur in ILS and MDS.

Histone deacetylases and SAP18, a novel polypeptide, are components of a human Sin3 complex

An important event in gene expression is the covalent modification of histone proteins. We have found that the mammalian transcriptional repressor Sin3 (mSin3) exists in a complex with histone deacetylases HDAC1 and HDAC2. Consistent with the observation that mSin3-mediated repression of transcription involves the modification of histone polypeptides, we found that the mSin3-containing complex includes polypeptides that tether the mSin3 complex to core histone proteins. In addition, two novel mSin3-associated polypeptides, SAP18 and SAP30, were identified. We isolated a cDNA encoding human SAP18 and found that SAP18 is a component of an mSin3-containing complex in vivo. Moreover, we demonstrate a direct interaction between SAP18 and mSin3. SAP18 represses transcription in vivo when tethered to the promoter, consistent with the ability of SAP18 to interact with mSin3.

Histone deacetylases associated with the mSin3 corepressor mediate mad transcriptional repression

Transcriptional repression by Mad-Max heterodimers requires interaction of Mad with the corepressors mSin3A/B. Sin3p, the S. cerevisiae homolog of mSin3, functions in the same pathway as Rpd3p, a protein related to two recently identified mammalian histone deacetylases, HDAC1 and HDAC2. Here, we demonstrate that mSin3A and HDAC1/2 are associated in vivo. HDAC2 binding requires a conserved region of mSin3A capable of mediating transcriptional repression. In addition, Mad1 forms a complex with mSin3 and HDAC2 that contains histone deacetylase activity. Trichostatin A, an inhibitor of histone deacetylases, abolishes Mad repression. We propose that Mad-Max functions by recruiting the mSin3-HDAC corepressor complex that deacetylates nucleosomal histones, producing alterations in chromatin structure that block transcription.