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

Mxi1-0, an alternatively transcribed Mxi1 isoform, is overexpressed in glioblastomas

The c-Myc transcription factor regulates expression of genes related to cell growth, division, and apoptosis. Mxi1, a member of the Mad family, represses transcription of c-Myc-regulated genes by mediating chromatin condensation via histone deacetylase and the Sin3 corepressor. Mxi1 is a c-Myc antagonist and suppresses cell proliferation in vitro. Here, we describe the identification of Mxi1-0, a novel Mxi1 isoform that is alternatively transcribed from an upstream exon. Mxi1-0 and Mxi1 have different amino-terminal sequences, but share identical Max- and DNA-binding domains. Both isoforms are able to bind Max, to recognize E-box binding sites, and to interact with Sin3. Despite these similarities and in contrast to Mxi1, Mxi1-0 is predominantly localized to the cytoplasm and fails to repress c-Myc-dependent transcription. Although Mxi1-0 and Mxi1 are coexpressed in both human and mouse cells, the relative levels of Mxi1-0 are higher in primary glioblastoma tumors than in normal brain tissue. This variation in the levels of Mxi1-0 and Mxi1 suggests that Mxi1-0 may modulate the Myc-inhibitory activity of Mxi1. The identification of Mxi1-0 as an alternatively transcribed Mxi1 isoform has significant implications for the interpretation of previous Mxi1 studies, particularly those related to the phenotype of the mxi1 knockout mouse.

Mad 1 Inhibits Cell Growth and Proliferation but Does Not Promote Differentiation or Overall Survival in Human U-937 Monoblasts

The Mad family proteins are transcriptional repressors belonging to the basic region/helix-loop-helix/leucine zipper family. They share a common obligatory dimerization partner, Max, with the oncoprotein c-Myc and antagonize the function of Myc to activate transcription. The Myc/Max/Mad network has therefore been suggested to function as a molecular switch that regulates cell growth and differentiation by controlling a common set of genes. To study the biological consequences of Mad1 expression for hematopoietic cell growth and differentiation, we used the U-937 monocytic differentiation model to generate cells with inducible Mad1 expression using the reversed tetracycline-controlled transactivator system. The elevated expression of Mad1 in these cells resulted in increased Mad1/Max heterodimer formation correlating with reduced expression of the Myc/Mad target gene ODC. Mad1-expressing U-937 cells in suspension culture proliferated slower and exhibited an increased number of cells in the G1 phase of the cell cycle. Further, growth in semisolid medium was almost completely inhibited. Mad1-expression, however, neither enforced spontaneous differentiation nor enhanced differentiation induced by the phorbol ester 12-O-tetradecanoylphorbol-13-acetate, retinoic acid (RA), or vitamin D3 but rather led to delayed RA-stimulated differentiation. Mad1-expressing cells were further found to be reduced in cell size in all phases of the cells cycle and particularly in response to RA-induced differentiation. Unexpectedly, whereas Fas-induced apoptosis was slightly attenuated in Mad1-expressing U-937 cells, Mad1 sensitized the cells to tumor necrosis factor–α-induced apoptosis. These results suggest that Mad1 primarily regulates cell growth and proliferation in these cells, whereas its role in cellular differentiation and survival seems to be more complex.

Molecular determinants of the interaction of Mad with the PAH2 domain of mSin3

The Sin3 co-repressor acts as a protein scaffold to recruit transcription factors via its four highly homologous paired amphipathic helix (PAH) domains. PAH2 has been shown to interact strongly with the Sin3 interacting domain (SID) of the tumor suppressor Mad. This PAH2/Mad complex has been studied extensively by NMR, but the molecular determinants that dictate the specificity of interaction remain to be elucidated. To uncover residues that convey the specificity of interaction between PAH2 and Mad, PAH2 residues contacted by the Mad-SID were introduced into the PAH1 domain of mSin3b and tested for gain-of-interaction in vivo in a yeast two-hybrid setting and further confirmed in a cell-free system. This approach led to the identification of PAH2-Phe-7 as a critical residue. Stabilization of the interaction between PAH1-Phe-7 and the Mad-SID was achieved by introducing Val-14 and Gln-39 into PAH1. Substitution of PAH2 residues contacted by the Mad-SID with their respective residues in PAH1 corroborated and extended the critical role of Phe-7 and the stabilizing role of Val-14 and Gln-39. We conclude that Phe-7 is the critical determinant and provides the molecular specificity for the association between Sin3 and Mad in regulating cell growth and differentiation.

Mnt loss triggers Myc transcription targets, proliferation, apoptosis, and transformation

Myc oncoproteins are overexpressed in most cancers and are sufficient to accelerate cell proliferation and provoke transformation. However, in normal cells Myc also triggers apoptosis. All of the effects of Myc require its function as a transcription factor that dimerizes with Max. This complex induces genes containing CACGTG E-boxes, such as Ornithine decarboxylase (Odc), which harbors two of these elements. Here we report that in quiescent cells the Odc E-boxes are occupied by Max and Mnt, a putative Myc antagonist, and that this complex is displaced by Myc-Max complexes in proliferating cells. Knockdown of Mnt expression by stable retroviral RNA interference triggers many targets typical of the "Myc" response and provokes accelerated proliferation and apoptosis. Strikingly, these effects of Mnt knockdown are even manifest in cells lacking c-myc. Moreover, Mnt knockdown is sufficient to transform primary fibroblasts in conjunction with Ras. Therefore, Mnt behaves as a tumor suppressor. These findings support a model where Mnt represses Myc target genes and Myc functions as an oncogene by relieving Mnt-mediated repression.

Kinetics ofmyc-max-madgene expression during hepatocyte proliferation in vivo: Differential regulation ofmadfamily and stress-mediated induction of c-myc

Mad proteins (Mad1, Mxi1, Mad3, Mad4, Mnt/Rox) are biochemical and biological antagonists of c-Myc oncoprotein. Mad-Max dimers repress the transcription of the same target genes activated by Myc-Max dimers. Despite the critical role of Max and Mad proteins as modulators of c-Myc functions, there are no comparative data on their regulation in vivo. We carried out a systematic analysis of c-myc, max, and mad family expression in a model of synchronized cell proliferation in vivo in adult tissues, that is, rat hepatocytes after partial hepatectomy. We confirmed the previously reported early peak of c-myc expression after hepatectomy but we show that it did not correlate with hepatocyte proliferation as it also occurred in sham-operated animals as a result of surgical stresses. A second peak of c-myc expression was observed later, at the time of the wave of DNA synthesis. No such expression was detected in sham-operated rat quiescent hepatocytes. max expression increased around 4-16 h after hepatectomy, before the peaks of c-myc and DNA synthesis. mxi1 and mad4 were slightly downregulated during liver regeneration. mnt/rox expression did not change. These expression patterns suggest a role of Myc-Max for efficient mitogenic response of hepatocytes. We also analyzed the effects of Myc and Max ectopic expression on the clonogenic growth of the rat hepatoma cells. Expression of c-Myc and Max increased clonogenic growth, whereas the reduction of c-Myc levels by an antisense vector decreased growth. The results suggest nonredundant roles for mad genes in hepatocyte proliferation and point to c-Myc as a putative target for anticancer therapy of liver cancer.

Myc pathways provoking cell suicide and cancer

A paradox for the cancer biology field has been the revelation that oncogenes, once thought to simply provide advantages to a cancer cell, actually put it at dire risk of cell suicide. Myc is the quintessential oncogene in this respect, as in normal cells it is required for cell cycle traverse, whereas in cancers it is overexpressed and functions as the angiogenic switch. Nonetheless, Myc overexpression kills normal cells dead in their tracks. Here we review Myc-induced pathways that contribute to the apoptotic response. Molecular analysis of Myc-induced tumors has established that some of these apoptotic pathways are essential checkpoints that guard the cell from cancer, as they are selectively bypassed during tumorigenesis. The precise mechanism(s) by which Myc targets these pathways are largely unresolved, but we propose that they involve crosstalk and feedback regulatory loops between arbiters of cell death.

Expression of a dominant negative form of Daxxin vivo rescues motoneurons from Fas (CD95)-induced cell death

Fas-induced death of motoneurons in vitro has been shown to involve two signaling cascades that act together to execute the death program: a Fas-Daxx-ASK-1-p38 kinase-nNOS branch, which controls transcriptional and post-translational events, and the second classical Fas-FADD-caspase-8 branch. To analyze the role of Daxx in the developmental motoneuron cell death, we studied Fas-dependent cell death in motoneurons from transgenic mice that overexpress a dominant-negative form of Daxx. Motoneurons purified from these transgenic mice are resistant to Fas-induced death. This protective effect is specific to Fas because ultraviolet irradiation-triggered death is not affected by the transgene. The Daxx and the FADD pathways work in parallel because only Daxx, but not FADD, is involved in the transcriptional control of neuronal nitric oxide synthase and nitric oxide production. Nevertheless, we do not observe involvement of Daxx in developmental motoneuronal cell death, as the pattern of naturally occurring programmed cell death in vivo is normal in transgenic mice overexpressing the dominant negative form of Daxx, suggesting that Daxx-independent pathways are used during development.

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

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

Regionally restricted expression of the transcription factor c-myc intron 1 binding protein during brain development

Transcription factors regulate proliferation, differentiation, and regionalization of the central nervous system. In a screen of developing cerebral cortex, we identified the transcription factor c-myc intron 1 binding protein (mibp1) due to its abundant expression. In this study, we analyzed the temporal and spatial expression patterns of mibp1 mRNA in developing mouse brain to address the putative role of this transcription factor in neural differentiation. Northern hybridization studies revealed that mibp1 is expressed first in the mouse dorsal telencephalon at embryonic day (E) 14.5, during peak neuronal production. In situ hybridization experiments revealed that mibp1 expression in the cerebral wall is most abundant in postmitotic cells of the cortical plate and absent from proliferative zones. Moreover, mibp1 is restricted to dorsal telencephalon during embryogenesis with expression only in the cerebral wall, olfactory bulb, and hippocampus. N-myc, a potential target of mibp1 regulation, exhibited complementary, nonoverlapping expression patterns in the telencephalon with greatest expression in proliferating cells of the ventricular zone from E12.5 to E14.5; N-myc was absent from the telencephalon by E15.5. The specificity and timing of mibp1 expression in the cerebral cortex suggests a role in maintaining a state of neuronal differentiation in the dorsal telencephalon.

Induction of endogenous telomerase (hTERT) by c-Myc in WI-38 fibroblasts transformed with specific genetic elements

Elucidation of the mechanisms governing expression of the human telomerase reverse transcriptase (hTERT) is important for understanding cancer pathogenesis. Approximately 90% of tumors express hTERT, the major catalytic component of telomerase. Activation of telomerase is an early event, and high levels of this activity correlate with poor prognosis. Recent studies have shown that the transcription factors c-Myc and Mad1 activate and repress hTERT, respectively. It is not clear how these transcription factors compete for the same recognition sequence in the hTERT core promoter region. Studies have shown that the combined expression of SV40 large T antigen (T-Ag), hTERT, and H-Ras is able to transform human cells. In this study, we used a distinct human cell type, WI-38 fetal lung fibroblasts used extensively for senescence studies. We transduced cells with amphotropic retroviral constructs containing SV40 T antigen, hTERT, and activated H-ras. Transduced cells exhibited anchorage independence in soft agar and expressed increased levels of c-Myc and endogenous hTERT. These effects were observed by 25 population doublings (PDs) following the establishment of the neoplastic cell line. During the process of transformation, we observed a switch from Mad1/Max to c-Myc/Max binding to oligonucleotide sequences containing the hTERT promoter distal and proximal E-boxes. c-Myc can bind specifically to the hTERT promoter in vitro, indicating that c-Myc expression in tumors may account for the increased expression of hTERT observed in vivo. These findings indicate that the widely used model system of WI-38 fibroblasts can be employed for transformation studies using defined genetic elements and that the endogenous hTERT and c-Myc are induced in these cells during early tumorigenesis. Such studies should have important implications in the mechanisms of hTERT and c-Myc induction in the beginning stages of tumorigenesis and facilitate extension of these studies to novel models of tumorigenesis in cellular senescence.

Deletion of Mnt leads to disrupted cell cycle control and tumorigenesis

Mnt is a Max-interacting transcriptional repressor that has been hypothesized to function as a Myc antagonist. To investigate Mnt function we deleted the Mnt gene in mice. Since mice lacking Mnt were born severely runted and typically died within several days of birth, mouse embryo fibroblasts (MEFs) derived from these mice and conditional Mnt knockout mice were used in this study. In the absence of Mnt, MEFs prematurely entered the S phase of the cell cycle and proliferated more rapidly than Mnt(+/+) MEFs. Defective cell cycle control in the absence of Mnt is linked to upregulation of Cdk4 and cyclin E and the Cdk4 gene appears to be a direct target of Mnt-Myc antagonism. Like MEFs that overexpress Myc, Mnt(-/-) MEFs were prone to apoptosis, efficiently escaped senescence and could be transformed with oncogenic Ras alone. Consistent with Mnt functioning as a tumor suppressor, conditional inactivation of Mnt in breast epithelium led to adenocarinomas. These results demonstrate a unique negative regulatory role for Mnt in governing key Myc functions associated with cell proliferation and tumorigenesis.

The targets of vitamin D depend on the differentiation and activation status of CD4 positive T cells

Vitamin D is a potent immune system regulator. The active form of vitamin D (1,25(OH)(2)D(3)) suppresses the development of animal models of human autoimmune diseases. 1,25(OH)(2)D(3) decreased the proliferation of all T helper (h) cells and decreased the production of IFN-gamma, IL-2, and IL-5. In Th2 cells 1,25(OH)(2)D(3) increased the production of IL-4. Quiescent CD4+ T cells express vitamin D receptors but only at a low level, which increased five-fold following activation. 1,25(OH)(2)D(3) treatment of Th0 cells, but not Th1 or Th2 cells, induced the expression of the transcription factor GATA-3. Microarray technology identified over 102 targets of 1,25(OH)(2)D(3) in CD4+ T cells. Of the 102 genes, 57 genes were down-regulated and 45 were up-regulated by 1,25(OH)(2)D(3) treatment of the CD4+ T cells. Two of the identified genes are regulators of NFkB. Other genes of interest included the IL-2Rbeta gene and IgE binding factor. Th2 and Th0 cells produced more IgE binding factor after treatment with 1,25(OH)(2)D(3) while Th1 cell IgE binding factor expression was unaffected by 1,25(OH)(2)D(3) addition. It is unclear why some of the genes identified are expressed in CD4+ T cells and furthermore why 1,25(OH)(2)D(3) regulates the expression of these genes. Clearly CD4+ T cells can be direct targets of vitamin D. The targets of vitamin D in CD4+ T cells depend on the state of activation and differentiation status of the cells.

The Mad and Myc basic domains are functionally equivalent

The Myc/Max/Mad family of transcription factors plays a fundamental role in the regulation of cell proliferation, oncogenic transformation, and cell differentiation. However, it remains unclear whether different heterodimers, such as Myc/Max and Mad/Max, recognize the same or different target genes in vivo. We show by chromatin immunoprecipitation that Myc target genes are also recognized by Mad1 in differentiated HL60 cells. We also substituted the complete basic region of Myc for the corresponding region of Mad. Wild-type c-Myc was then compared with c-Myc(Mad-BR) in oncogenic transformation, regulation of cell proliferation, induction of apoptosis, activation of chromosomal gene expression, and direct binding to chromosomal sites by chromatin immunoprecipitation. We find that the wild-type c-Myc and c-Myc/MadBR proteins have indistinguishable biological activity and target gene recognition in vivo. These data are consistent with a model in which Myc and Mad regulate a common set of target genes.

Detailed characterization of a homozygously deleted region corresponding to a candidate tumor suppressor locus at distal 17p13.3 in human lung cancer

17p13.3 is one of the chromosomal regions most frequently affected by allelic loss in a variety of human neoplasms including lung cancer. A number of loss of heterozygosity (LOH) analyses have suggested the existence of a tumor suppressor gene at 17p13.3, distal to the p53 locus at 17p13.1. In the present study, we characterized a homozygous deletion at 17p13.3 in a small cell lung cancer cell line by constructing a bacterial artificial chromosome (BAC) contig and a restriction map surrounding the region, as well as by utilizing publicly available draft sequences. We defined the breakpoint, assigned and analysed two known genes, 14-3-3 epsilon and CRK, and a novel gene LOST1 within or at the end of the homozygous deletion of about 170 kb in size. Marked reduction of LOST1 expression was detected in 69% (11/16) of lung cancer specimens by quantitative real-time RT-PCR, while significant DNA hypermethylation was observed at the 5' end of the LOST1 gene in four of six lung cancer cell lines with negligible LOST1 expression. We also show here that a polymorphic marker D17S1174, which resides within the homozygous deletion, was apparently located in the middle of the minimum LOH region, providing further supportive evidence for the presence of a tumor suppressor gene(s) in this region.

A novel heterodimerization domain, CRM1, and 14-3-3 control subcellular localization of the MondoA-Mlx heterocomplex

Among members of the bHLHZip family of transcriptional regulators, MondoA and Mlx have the unique property of cytoplasmic localization. We have proposed that MondoA-Mlx heterodimers accumulate in the nucleus in response to extracellular cues. Our previous work implicated heterodimerization between MondoA and Mlx and a conserved domain in the N terminus of MondoA as important determinants of MondoA-Mlx subcellular localization. MondoA and Mlx share sequence similarity in their bHLHZip domains and C termini. Here we show that for both MondoA and Mlx, this C-terminal domain has cytoplasmic localization activity that is required by the protein monomers to accumulate in the cytoplasm. This C-terminal domain is also a novel dimerization interface that functions independently of the leucine zipper to mediate heterotypic interactions between MondoA and Mlx. Dimerization between MondoA and Mlx inactivates the cytoplasmic localization activity of their C termini and is necessary for the heterocomplex to accumulate in the nucleus. MondoA-Mlx heterodimers, while poised for nuclear entry, are retained in the cytoplasm by conserved domains in the N terminus of MondoA. Mondo conserved regions (MCRs) II and III contribute to cytoplasmic localization of MondoA-Mlx by functioning as a CRM1-dependent nuclear export signal and as a novel binding site for 14-3-3 family members, respectively. We propose that the nuclear accumulation of MondoA and Mlx is a two-step process. First, heterodimerization abolishes the cytoplasmic localization activity of their C termini. Second, an extracellular signal(s) must overcome the cytoplasmic localization function imparted by CRM1 and 14-3-3 binding to the N terminus of MondoA.

N-myc is essential during neurogenesis for the rapid expansion of progenitor cell populations and the inhibition of neuronal differentiation

To address the role of N-myc in neurogenesis and in nervous system tumors, it was conditionally disrupted in neuronal progenitor cells (NPCs) with a nestin-Cre transgene. Null mice display ataxia, behavioral abnormalities, and tremors that correlate with a twofold decrease in brain mass that disproportionately affects the cerebellum (sixfold reduced in mass) and the cerebral cortex, both of which show signs of disorganization. In control mice at E12.5, we observe a domain of high N-Myc protein expression in the rapidly proliferating cerebellar primordium. Targeted deletion of N-myc results in severely compromised proliferation as shown by a striking decrease in S phase and mitotic cells as well as in cells expressing the Myc target gene cyclin D2, whereas apoptosis is unaffected. Null progenitor cells also have comparatively high levels of the cdk inhibitors p27(Kip1) and p18(Ink4c), whereas p15(Ink4b), p21(Cip1), and p19(Ink4d) levels are unaffected. Many null progenitors also exhibit altered nuclear morphology and size. In addition, loss of N-myc disrupts neuronal differentiation as evidenced by ectopic staining of the neuron specific marker betaTUBIII in the cerebrum. Furthermore, in progenitor cell cultures derived from null embryonic brain, we observe a dramatic increase in neuronal differentiation compared with controls. Thus, N-myc is essential for normal neurogenesis, regulating NPC proliferation, differentiation, and nuclear size. Its effects on proliferation and differentiation appear due, at least in part, to down-regulation of a specific subset of cyclin-dependent kinase inhibitors.

Gene expression profile of adipocyte differentiation and its regulation by peroxisome proliferator-activated receptor-gamma agonists

PPAR gamma is an adipocyte-specific nuclear hormone receptor. Agonists of PPAR gamma, such as thiazolidinediones (TZDs), promote adipocyte differentiation and have insulin-sensitizing effects in animals and diabetic patients. Affymetrix oligonucleotide arrays representing 6347 genes were employed to profile the gene expression responses of mature 3T3-L1 adipocytes and differentiating preadipocytes to a TZD PPAR gamma agonist in vitro. The expression of 579 genes was significantly up- or down-regulated by more than 1.5-fold during differentiation and/or by treatment with TZD, and these genes were organized into 32 clusters that demonstrated concerted changes in expression of genes controlling cell growth or lipid metabolism. Quantitative PCR was employed to further characterize gene expression and led to the identification of beta-catenin as a new PPAR gamma target gene. Both mRNA and protein levels for beta-catenin were down-regulated in 3T3-L1 adipocytes compared with fibroblasts and were further decreased by treatment of adipocytes with PPAR gamma agonists. Treatment of db/db mice with a PPAR gamma agonist also resulted in reduction of beta-catenin mRNA levels in adipose tissue. These results suggest that beta-catenin plays an important role in the regulation of adipogenesis. Thus, the transcriptional patterns revealed in this study further the understanding of adipogenesis process and the function of PPAR gamma activation.

The proto-oncogene c-myc in hematopoietic development and leukemogenesis

The proto-oncogene c-myc has been shown to play a pivotal role in cell cycle regulation, metabolism, apoptosis, differentiation, cell adhesion, and tumorigenesis, and participates in regulating hematopoietic homeostasis. It is a transcription regulator that is part of an extensive network of interacting factors. Most probably, different biological responses are elicited by different overlapping subsets of c-Myc target genes, both induced and suppressed. Results obtained from studies employing mouse models are consistent with the need for at least one, and possibly two, mutations in addition to deregulated c-myc for malignant tumor formation. Repression of c-myc is required for terminal differentiation of many cell types, including hematopoietic cells. It has been shown that deregulated expression of c-myc in both M1 myeloid leukemic cells and normal myeloid cells derived from murine bone marrow, not only blocked terminal differentiation and its associated growth arrest, but also induced apoptosis, which is dependent on the Fas/CD95 pathway. There is evidence to suggest that the CD95/Fas death receptor pathway is an integral part of the apoptotic response associated with the end of the normal terminal myeloid differentiation program, and that deregulated c-myc expression can activate this signaling pathway prematurely. The ability of egr-1 to promote terminal myeloid differentiation when co-expressed with c-myc, and of c-fos to partially abrogate the block imparted by deregulated c-myc on myeloid differentiation, make these two genes candidate tumor suppressors. Several different transcription factors have been implicated in the down-regulation of c-myc expression during differentiation, including C/EBPalpha, CTCF, BLIMP-1, and RFX1. Alterations in the expression and/or function of these transcription factors, or of the c-Myc and Max interacting proteins, such as MM-1 and Mxi1, can influence the neoplastic process. Understanding how c-Myc controls cellular phenotypes, including the leukemic phenotype, should provide novel tools for designing drugs to promote differentiation and/or apoptosis of leukemic cells.

Characterization of the c-MYC-regulated transcriptome by SAGE: identification and analysis of c-MYC target genes

To identify target genes of the oncogenic transcription factor c-MYC, serial analysis of gene expression (SAGE) was performed after adenoviral expression of c-MYC in primary human umbilical vein endothelial cells: 216 different SAGE tags, corresponding to unique mRNAs, were induced, whereas 260 tags were repressed after c-MYC expression (P < 0.05). The induction of 53 genes was confirmed by using microarray analysis and quantitative real-time PCR: among these genes was MetAP2/p67, which encodes an activator of translational initiation and represents a validated target for inhibition of neovascularization. Furthermore, c-MYC induced the cell cycle regulatory genes CDC2-L1, Cyclin E binding protein 1, and Cyclin B1. The DNA repair genes BRCA1, MSH2, and APEX were induced by c-MYC, suggesting that c-MYC couples DNA replication to processes preserving the integrity of the genome. MNT, a MAX-binding antagonist of c-MYC function, was up-regulated, implying a negative feedback loop. In vivo promoter occupancy by c-MYC was detected by chromatin immunoprecipitation for CDK4, Prohibitin, MNT, Cyclin B1, and Cyclin E binding protein 1, showing that these genes are direct c-MYC targets. The c-MYC-regulated genes/tags identified here will help to define the set of bona fide c-MYC targets and may have potential therapeutic value for inhibition of cancer cell proliferation, tumor-vascularization, and restenosis.

Repression of in vivo growth of Myc/Ras transformed tumor cells by Mad1

The Myc/Max/Mad network of transcriptional regulatory proteins plays an essential role in cell proliferation, growth, apoptosis, and differentiation. Whereas Myc proteins affect cell cycle progression positively, Mad proteins are negative regulators of cell proliferation. It has been shown in several in vitro systems that Mad proteins antagonize c-Myc functions. In this report we describe the inhibition of tumor cell outgrowth in vivo by Mad1 expression. Transformed cell lines were generated by co-transfection of c-myc, c-H-ras, and a chimeric mad1ER construct into primary rat embryo cells (MRMad1ER cells). Activation of Mad1 by 4-Hydroxy-Tamoxifen (OHT) resulted in abrogation of telomerase activity, reduced cloning efficiency, and decreased proportion of cells in S phase. Injection of MRMad1ER cells into syngenic rats induced aggressively growing tumors after a short latency period. This tumor growth was inhibited by OHT-treatment of animals, with the extent of inhibition correlating with the amount of OHT injected. No effect of OHT on tumor growth was observed with similarly transformed Myc/Ras cell lines which did not express Mad1ER. These data demonstrate that Mad1 is able to suppress Myc/Ras-mediated transformation under in vivo conditions.

Asn(78) and His(81) form a destabilizing locus within the Max HLH-LZ homodimer

The human Max protein lies at the center of the Myc/Max/Mad family of transcription factors. Its role at the center of this regulatory network is dependent on the helix-loop-helix leucine zipper (HLH-LZ) dimerization domain. The Max LZ contains three residues that deviate from the pattern of hydrophobic amino acids normally present at the interface of LZ dimers: Asn(78), His(81) and Asn(92). In contrast to interfacial Asn residues in other LZ proteins, we have shown that Asn(92) does not act to destabilize the homodimer. Here we describe thermal denaturation experiments performed on Asn(78) and His(81) mutants demonstrating that these residues are involved in actively destabilizing the Max homodimer.

The Mad1 transcription factor is a novel target of activin and TGF-beta action in keratinocytes: possible role of Mad1 in wound repair and psoriasis

Activin A, a member of the transforming growth factor beta (TGF-beta) superfamily, affects keratinocyte proliferation and differentiation in vitro and in vivo. However, little is known about the mechanisms of activin action in keratinocytes, and its target genes have not been identified. In this study, we demonstrate that activin A and TGF-beta1 directly induce the expression and activity of Mad1, an antagonist of the c-Myc transcription factor, in the human HaCaT keratinocyte cell line. Expression and activity of Mad1 was strongly induced by both factors in keratinocytes, although the intensity of induction was different for activin A and TGF-beta1. To determine a possible role of activin and TGF-beta in the regulation of mad1 expression in vivo, we analysed its expression during cutaneous wound repair when high levels of these factors are present. Expression of mad1 mRNA and protein, but not of other mad genes, increased significantly after skin injury, particularly in polymorphonuclear leukocytes and in suprabasal keratinocytes of the hyperproliferative epithelium. Elevated levels of mad1 mRNA were also detected in the hyperthickened epidermis of psoriatic patients. Since Mad1 regulates proliferation and/or differentiation of various cell types, our results suggest that this transcription factor mediates at least in the part the anti-mitotic and/or differentiation-inducing activities of TGF-beta and activin in keratinocytes.

New members of the Drosophila Myc transcription factor subfamily revealed by a genome-wide examination for basic helix-loop-helix genes

The basic helix-loop-helix (bHLH) proteins represent an evolutionary conserved class of transcription factors that are known to play important roles in cell determination and differentiation during animal embryonic development. Following an exhaustive search of the complete Drosophila genome sequence using a PSI-BLAST strategy, we identified 19 new genes, bringing the total number of bHLH-encoding genes in the Drosophila genome to 56. These new genes belong to various subfamilies of bHLH transcription factors, such as the Daughterless, Hairy-Enhancer of split, bHLH-PAS or bHLHZip subfamilies. The embryonic expression pattern of each of these new genes has been analyzed by in situ hybridization. By looking for closely structurally related motifs, we found two genes that represent likely orthologues of vertebrate Mnt and Mlx. Together with previous reports, our data suggest that, similar to networks involved in neurogenesis and myogenesis, the network of Myc-related genes has been globally conserved throughout evolution.

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

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

Structure, function, and dynamics of the dimerization and DNA-binding domain of oncogenic transcription factor v-Myc

The protein product (c-Myc) of the protooncogene c-myc is a transcriptional regulator playing a key role in cellular growth, differentiation, and apoptosis. Deregulated myc genes, like the transduced retroviral v-myc allele, are oncogenic and cause cell transformation. The C-terminal bHLHZip domain of v-Myc, encompassing protein dimerization (helix-loop-helix, leucine zipper) and DNA contact (basic region) surfaces, was expressed in bacteria as a highly soluble p15(v-myc )recombinant protein. Dissociation constants (K(d)) for the heterodimer formed with the recombinant bHLHZip domain of the Myc binding partner Max (p14(max)) and for the Myc-Max-DNA complex were estimated using circular dichroism (CD) spectroscopy and quantitative electrophoretic mobility shift assay (EMSA). Multi-dimensional NMR spectroscopy was used to characterize the solution structural and dynamic properties of the v-Myc bHLHZip domain. Significant secondary chemical shifts indicate the presence of two separated alpha-helical regions. The C-terminal leucine zipper region forms a compact alpha-helix, while the N-terminal basic region exhibits conformational averaging with substantial alpha-helical content. Both helices lack stabilizing tertiary side-chain interactions and represent exceptional examples for loosely coupled secondary structural segments in a native protein. These results and CD thermal denaturation data indicate a monomeric state of the v-Myc bHLHZip domain. The (15)N relaxation data revealed backbone mobilities which corroborate the existence of a partially folded state, and suggest a "beads-on-a-string" motional behaviour of the v-Myc bHLHZip domain in solution. The preformation of alpha-helical regions was confirmed by CD thermal denaturation studies, and quantification of the entropy changes caused by the hydrophobic effect and the reduction of conformational entropy upon protein dimerization. The restricted conformational space of the v-Myc bHLHZip domain reduces the entropy penalty associated with heterodimerization and allows rapid and accurate recognition by the authentic Myc binding partner Max.

Switch from Myc/Max to Mad1/Max binding and decrease in histone acetylation at the telomerase reverse transcriptase promoter during differentiation of HL60 cells

Recent evidence suggests that the Myc and Mad1 proteins are implicated in the regulation of the gene encoding the human telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase. We have analyzed the in vivo interaction between endogenous c-Myc and Mad1 proteins and the hTERT promoter in HL60 cells with the use of the chromatin immunoprecipitation assay. The E-boxes at the hTERT proximal promoter were occupied in vivo by c-Myc in exponentially proliferating HL60 cells but not in cells induced to differentiate by DMSO. In contrast, Mad1 protein was induced and bound to the hTERT promoter in differentiated HL60 cells. Concomitantly, the acetylation of the histones at the promoter was significantly reduced. These data suggest that the reciprocal E-box occupancy by c-Myc and Mad1 is responsible for activation and repression of the hTERT gene in proliferating and differentiated HL60 cells, respectively. Furthermore, the histone deacetylase inhibitor trichostatin A inhibited deacetylation of histones at the hTERT promoter and attenuated the repression of hTERT transcription during HL60 cell differentiation. In addition, trichostatin A treatment activated hTERT transcription in resting human lymphocytes and fibroblasts. Taken together, these results indicate that acetylation/deacetylation of histones is operative in the regulation of hTERT expression.

Function of the c-Myc oncoprotein in chromatin remodeling and transcription

Deregulated expression of the c-myc proto-oncogene contributes to malignant progression of a variety of tumors. The c-Myc protein (or Myc) is a transcription factor that positively or negatively regulates expression of distinct sets of target genes. Transcriptional activation by Myc is mediated through dimerization with Max and binding to the DNA consensus sequence CA(C/T)GTG (the E-box). Transcriptional inhibition is mediated through distinct DNA elements, and may be due to functional interference with factors that transactivate via these sequences. We review here our current knowledge on these transcriptional activities of Myc and their relationship to its biological function. The findings that Myc interacts with subunits of histone acetyl-transferase (HAT) complexes and of the ATP-dependent chromatin remodeling complex, SWI/SNF, suggest that localized changes in chromatin structure may mediate Myc function. We present a working hypothesis for the concerted action of HAT and SWI/SNF complexes in Myc-activated transcription and argue that this model should prompt re-thinking of the experimental strategies and criteria used to identify Myc target genes.

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

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

Identification of the Sin3-binding site in Ume6 defines a two-step process for conversion of Ume6 from a transcriptional repressor to an activator in yeast

The DNA-binding protein Ume6 is required for both repression and activation of meiosis-specific genes, through interaction with the Sin3 corepressor and Rpd3 histone deacetylase and the meiotic activator Ime1. Here we show that fusion of a heterologous activation domain to Ume6 is unable to convert it into a constitutive activator of early meiotic gene transcription, indicating that an additional function is needed to overcome repression at these promoters. Mutations in UME6 allowing the fusion to activate lie in a predicted amphipathic alpha helix and specifically disrupt interaction with Sin3 but not with Teal, an activator of Ty transcription also found to interact with Ume6 in a two-hybrid screen. The mutations cause a loss of repression by Ume6 and precisely identify the Ume6 Sin3-binding domain, which we show interacts with the paired amphipathic helix 2 region of Sin3. Analysis of these mutants indicates that conversion of Ume6 to an activator involves two genetically distinct steps that act to relieve Sin3-mediated repression and provide an activation domain to Ume6. The mutants further demonstrate that premature expression and lack of subsequent rerepression of Ume6-Sin3-regulated genes are not deleterious to meiotic progression and suggest that the essential role of Sin3 in meiosis is independent of Ume6. The model for Ume6 function arising from these studies indicates that Ume6 is similar in many respects to metazoan regulators that utilize Sin3, such as the Myc-Mad-Max system and nuclear hormone receptors, and provides new insights into the control of transcriptional repression and activation by the Ume6-URS1 regulatory complex in yeast.

An internal ribosome entry segment in the 5' untranslated region of the mnt gene

Mnt is a transcriptional repressor related to the Myc/Mad family of transcription factors. It is expressed in proliferating, resting and differentiating cells and is believed to antagonize the function of Myc. Here we have characterized the major transcription initiation site of the mnt gene. In doing so we noted a remarkable level of sequence conservation between the murine and human 5' untranslated regions. Our experiments revealed that this sequence contains an internal ribosome entry segment (IRES). In addition, we show that sequences at both the 5' and 3' end of the IRES are essential for its function. These findings indicate that mnt can be translated by internal initiation. Such a mechanism may allow efficient Mnt synthesis when cap-dependent translation initiation is reduced.

Targeted deletion of the S-phase-specific Myc antagonist Mad3 sensitizes neuronal and lymphoid cells to radiation-induced apoptosis

The Mad family comprises four basic-helix-loop-helix/leucine zipper proteins, Mad1, Mxi1, Mad3, and Mad4, which heterodimerize with Max and function as transcriptional repressors. The balance between Myc-Max and Mad-Max complexes has been postulated to influence cell proliferation and differentiation. The expression patterns of Mad family genes are complex, but in general, the induction of most family members is linked to cell cycle exit and differentiation. The expression pattern of mad3 is unusual in that mad3 mRNA and protein were found to be restricted to proliferating cells prior to differentiation. We show here that during murine development mad3 is specifically expressed in the S phase of the cell cycle in neuronal progenitor cells that are committed to differentiation. To investigate mad3 function, we disrupted the mad3 gene by homologous recombination in mice. No defect in cell cycle exit and differentiation could be detected in mad3 homozygous mutant mice. However, upon gamma irradiation, increased cell death of thymocytes and neural progenitor cells was observed, implicating mad3 in the regulation of the cellular response to DNA damage.

A novel 16-kilodalton cellular protein physically interacts with and antagonizes the functional activity of c-myc promoter-binding protein 1

We initially identified c-myc promoter-binding protein 1 (MBP-1) from a human cervical carcinoma cell expression library which negatively regulates c-myc promoter activity. A recent study demonstrated that MBP-1 acts as a general transcriptional repressor (A. K. Ghosh, R. Steele, and R. B. Ray, Mol. Cell. Biol. 19:2880-2886, 1999). In order to identify the cellular protein(s) interacting with MBP-1 for transcriptional regulation, a HeLa cell cDNA expression library was screened using a yeast two-hybrid system. An MBP-1-interacting cDNA encoding a polypeptide of 140 amino acid residues with an approximate molecular mass of 16 kDa was identified and named MBP-1 interacting protein-2A (MIP-2A). MIP-2A has a sequence similarity with an unknown mRNA and SEDL. Mutations in the SEDL gene, located at human chromosome Xp22, has recently been implicated with an X-linked genetic disease, although the function of SEDL gene product was not determined (A. K. Gedeon et al., Nat. Genet. 22:400-404, 1999). However, our results suggested the localization of MIP-2A at human chromosome 19. The specificity of interaction between MBP-1 and MIP-2A was verified by an in vitro glutathione S-transferase pulldown experiment, a mammalian two-hybrid analysis, and in vivo coimmunoprecipitation assays. Further analysis revealed that the amino-terminal domain of MBP-1 (amino acids 1 to 95) interacts with MIP-2A. Immunofluorescent staining suggested colocalization of MIP-2A and MBP-1 primarily in the perinuclear membrane of cells. Functional analysis demonstrated that MIP-2A relieves MBP-1 mediated transcriptional repression on c-myc promoter. Additionally, MIP-2A antagonizes cell growth regulatory role of MBP-1. Taken together, these results suggest the functional interaction of MIP-2A and MBP-1 in cell growth regulation.

MondoA, a novel basic helix-loop-helix-leucine zipper transcriptional activator that constitutes a positive branch of a max-like network

Max is a common dimerization partner for a family of transcription factors (Myc, Mad [or Mxi]), and Mnt [or Rox] proteins) that regulate cell growth, proliferation, and apoptosis. We recently characterized a novel Max-like protein, Mlx, which interacts with Mad1 and Mad4. Here we describe the cloning and functional characterization of a new family of basic helix-loop-helix-leucine zipper heterodimeric partners for Mlx termed the Mondo family. MondoA forms homodimers weakly and does not interact with Max or members of the Myc or Mad families. MondoA and Mlx associate in vivo, and surprisingly, they are localized primarily to the cytoplasm of cultured mammalian cells. Treatment of cells with the nuclear export inhibitor leptomycin B results in the nuclear accumulation of MondoA and Mlx, demonstrating that they shuttle between the cytoplasmic and nuclear compartments rather than having exclusively cytoplasmic localization. MondoA preferentially forms heterodimers with Mlx, and this heterocomplex can bind to, and activate transcription from, CACGTG E-boxes when targeted to the nucleus via a heterologous nuclear localization signal. The amino termini of the Mondo proteins are highly conserved among family members and contain separable and autonomous cytoplasmic localization and transcription activation domains. Therefore, Mlx can mediate transcriptional repression in conjunction with the Mad family and can mediate transcriptional activation via the Mondo family. We propose that Mlx, like Max, functions as the center of a transcription factor network.

Solution structure of the interacting domains of the Mad-Sin3 complex: implications for recruitment of a chromatin-modifying complex

Gene-specific targeting of the Sin3 corepressor complex by DNA-bound repressors is an important mechanism of gene silencing in eukaryotes. The Sin3 corepressor specifically associates with a diverse group of transcriptional repressors, including members of the Mad family, that play crucial roles in development. The NMR structure of the complex formed by the PAH2 domain of mammalian Sin3A with the transrepression domain (SID) of human Mad1 reveals that both domains undergo mutual folding transitions upon complex formation generating an unusual left-handed four-helix bundle structure and an amphipathic alpha helix, respectively. The SID helix is wedged within a deep hydrophobic pocket defined by two PAH2 helices. Structure-function analyses of the Mad-Sin3 complex provide a basis for understanding the underlying mechanism(s) that lead to gene silencing.

Differential activity of conditional MYC and its variant MYC-S in human mortal fibroblasts

We have explored the effects of the conditional MYC-estrogen receptor fusion protein, MYC-ERTM, in human mortal fibroblasts, WI38, on cell-cycle entry, apoptosis and gene expression. The results indicate that activation of MYC-ERTM in WI38 cells is sufficient to cause S phase entry of quiescent cells, which is preceded by phosphorylation of Rb and activation of the Cdk2-associated kinase. We also analysed the MYC protein variant, MYC-S, which lacks part of the transcriptional activation domain but includes the conserved MYC box II and 26 amino acids N-terminal to it. MYC-S was previously shown to promote proliferation and apoptosis of immortalized rodent cell lines. The results indicate that MYC-S has undetectable activity as an inducer of S phase or apoptosis of quiescent WI38 cells. However, Myc-S stimulates proliferation of WI38 cells in the presence of 10% fetal calf serum. Surprisingly, we found that MYC-S, previously considered solely a repressor of specific reporter genes, is instead a weak transactivator of endogenous target genes both in mortal and immortalized cells. In addition, MYC-S exhibit a weak repressor activity upon an endogenous target gene only in immortalized cells. MYC-S transcriptional properties suggest that MYC box II and the adjacent N-terminal amino acids, while not sufficient for full repression function, participate in transactivation of endogenous target genes.

Inhibition of cell growth and apoptosis by inducible expression of the transcriptional repressor Mad1

Mad1 is a Myc antagonist that heterodimerizes with Max and functions as a transcriptional repressor. We have studied the effects of Mad1 on cell growth, cell cycle distribution, and apoptosis using Mad1-inducible cell lines. Expression of Mad1 inhibited cell proliferation, S-phase entry, and colony formation, changes that were accompanied by a reduction in CDK2 activity. The inhibition of Mad1 on cell proliferation was potentiated by serum starvation and was paralleled by accumulation of cells in the G0/G1 and the G2 phases of the cell cycle. Mad1 also reduced apoptosis induced by serum withdrawal and by the cytostatic drug cisplatinum. The effects on both cell growth and apoptosis were dependent on the mSin3 interaction domain of Mad1, which is necessary for recruitment of histone deacetylases and corepressors, suggesting that transcriptional repression is mediating these functions. Taken together with the expression pattern of Mad1, these results suggest that Mad1 plays an important role during initiation of differentiation by inhibiting cell proliferation and blocking apoptosis.

Co-repressors 2000

In the last 5 years, many co-repressors have been identified in eukaryotes that function in a wide range of species, from yeast to Drosophila and humans. Co-repressors are coregulators that are recruited by DNA-bound transcriptional silencers and play essential roles in many pathways including differentiation, proliferation, programmed cell death, and cell cycle. Accordingly, it has been shown that aberrant interactions of co-repressors with transcriptional silencers provide the molecular basis of a variety of human diseases. Co-repressors mediate transcriptional silencing by mechanisms that include direct inhibition of the basal transcription machinery and recruitment of chromatin-modifying enzymes. Chromatin modification includes histone deacetylation, which is thought to lead to a compact chromatin structure to which the accessibility of transcriptional activators is impaired. In a general mechanistic view, the overall picture suggests that transcriptional silencers and co-repressors act in analogy to transcriptional activators and coactivators, but with the opposite effect leading to gene silencing. We provide a comprehensive overview of the currently known higher eukaryotic co-repressors, their mechanism of action, and their involvement in biological and pathophysiological pathways. We also show the different pathways that lead to the regulation of co-repressor-silencer complex formation.

Mlx, a new Max-like bHLHZip family member: the center stage of a novel transcription factors regulatory pathway?

The Myc proto-oncogene family members have been identified as the cellular homologs of the transforming oncogene of avian retroviruses. They encode central regulators of mammalian cell proliferation and apoptosis, and they associate with the bHLHZip protein Max to bind specific DNA sequences and regulate the expression of genes important for cell cycle progression. The other family members, Mad1, Mxi1, Mad3, Mad4 and Rox (Mnt) antagonize their activities. The Mads and Rox compete with Myc in heterodimerizing with Max and in binding to the same specific target sequences. These Mads:Max and Rox:Max dimers repress transcription through binding to the mSIN3 corepressor protein and by tethering histone deacetylase-containing complexes to the DNA. In a screen for Rox interactors we isolated Mlx, a bHLHZip protein previously identified in a screen for Mad1 interactors. In the present work we extend the known dimerization partners of Mlx by demonstrating its ability to interact with Rox. Moreover, we show that contrary to previous reports Mlx is able to homodimerize and to bind E-box sequences at low concentration levels. The possible role of Mlx in an emerging regulatory pathway and acting parallel to the Max driven network is discussed.

Protooncogenes as mediators of apoptosis

Apoptosis has been well established as a vital biological phenomenon that is important in the maintenance of cellular homeostasis. Three major protooncogene families and their encoded proteins function as mediators of apoptosis in various cell types and are the subject of this chapter. Protooncogenic proteins such as c-Myc/Max, c-Fos/c-Jun, and Bcl-2/Bax utilize a synergetic effect to enhance their roles in the pro- or antiapoptotic action. These family members activate and repress the expression of their target genes, control cell cycle progression, and execute programmed cell death. Repression or overproduction of these protooncogenic proteins induces apoptosis, which may vary as a result of either cell type specificity or the nature of the apoptotic stimuli. The proapoptotic and antiapoptotic proteins exert their effects in the membrane of cellular organelles. Here they generate cell-type-specific signals that activate the caspase family of proteases and their regulators for the execution of apoptosis.

Mad1 function is regulated through elements within the carboxy terminus

Myc and Mad are basic helix-loop-helix leucine zipper (bHLH-LZ) proteins that heterodimerize with Max to bind DNA and thereby influence the transcription of Myc-responsive genes. Myc-Max dimers transactivate whereas Mad-Max-mSin3 complexes repress Myc-mediated transcriptional activation. We have previously shown that the N-terminal mSin3 binding domain and the centrally located bHLH-LZ are required for Mad1 to function during a molecular switch from proliferation to differentiation. Here we demonstrate that the carboxy terminus (CT) of Mad1 contains previously unidentified motifs necessary for the regulation of Mad1 function. We show that removal of the last 18 amino acids of Mad1 (region V) abolishes the growth-inhibitory function of the protein and the ability to reverse a Myc-imposed differentiation block. Moreover, deletion of region V results in a protein that binds DNA weakly and no longer represses Myc-dependent transcriptional activation. In contrast, deletion of the preceding 24 amino acids (region IV) together with region V restores DNA binding and transcriptional repression, suggesting a functional interplay between these two regions. Furthermore, phosphorylation within region IV appears to mediate this interplay. These findings indicate that novel regulatory elements are present in the Mad1 CT.

Analysis of Myc/Max/Mad network members in adipogenesis: inhibition of the proliferative burst and differentiation by ectopically expressed Mad1

Transcription factors of the Myc/Max/Mad network affect multiple aspects of cellular behavior, including proliferation, differentiation, and apoptosis. Recent studies have shown that Mad proteins can inhibit cellular growth and transformation and thus antagonize the function of Myc proteins. To define further the contribution of these proteins to cellular growth control, we have studied the expression of the respective genes and proteins in 3T3-L1 cells, both upon serum stimulation of quiescent cells and during adipocytic differentiation in response to insulin, dexamethasone, and isobutylmethylxanthine. We found distinct expression patterns for the mad genes. Mad4 was induced when cells exit the cell cycle and, together with mad1, during the late phase of differentiation. In contrast, mad3 expression was associated with progression through S phase and the proliferative burst of differentiating preadipocytes, overlapping in part c-myc expression. DNA binding analyses revealed that the most prominent network complex both in cycling and in differentiating cells was Mnt/Max, whereas c-Myc/Max complexes were detectable only during peak c-Myc expression periods. Ectopic expression of Mad1 in preadipocytes resulted in the inhibition of S phase and the proliferation associated with the proliferative burst; as a consequence, adipocytic differentiation was significantly inhibited. Our findings suggest that the precise temporal regulation of Myc/Max/Mad network proteins is critical for determining cellular behavior.

Inhibition of proliferation and apoptosis by the transcriptional repressor Mad1. Repression of Fas-induced caspase-8 activation

Mad1 is a member of the Myc/Max/Mad network of transcriptional regulators that play a central role in the control of cellular behavior. Mad proteins are thought to antagonize Myc functions at least in part by repressing gene transcription. To systematically examine the function of Mad1 in growth control and during apoptosis, we have generated U2OS cell clones that express Mad1 under a tetracyline-regulatable promoter (UTA-Mad1). Mad1 was induced rapidly and efficiently, localized to the nucleus, and bound to DNA as a heterodimer with Max. The induction of Mad1 reduced cellular growth and, more profoundly, inhibited colony formation of UTA-Mad1 cells. Conditioned medium neutralized this inhibitory effect implying that Mad1 function is regulated by extracellular signals. In addition Mad1 interfered with Fas-, TRAIL-, and UV-induced apoptosis, which coincided with a reduced activation of caspase-8 during Fas-mediated apoptosis in response to Mad1 expression. Furthermore, microinjection of Mad1-expressing plasmids into fibroblasts inhibited apoptosis induced by the oncoproteins c-Myc and E1A. Thus, Mad1 not only interferes with cellular proliferation but also with apoptosis, which defines a novel aspect of Mad1 function.

Hepatocyte transplantation improves survival in mice with liver toxicity induced by hepatic overexpression of Mad1 transcription factor

Hepatic overexpression of Mad1 with an adenoviral vector, AdMad, induced liver toxicity in immunodeficient mice. Transduction of cultured hepatocytes with AdMad inhibited cellular DNA synthesis and cell cycling, along with increased lactate dehydrogenase release, indicating cytotoxicity. When dipeptidyl peptidase IV-deficient F344 rat hepatocytes were transplanted into the liver of immunodeficient mice after treatment with AdMad, significant portions of the liver were repopulated. This was in agreement with corresponding losses of host hepatocytes, which showed increased apoptosis rates. Mortality in mice following AdMad treatment decreased significantly when animals were subjected to hepatocyte transplantation. The findings indicated that Mad1 overexpression perturbed hepatocyte survival. Investigation of pathophysiological mechanisms concerning specific cell cycle regulators in acute liver toxicity will thus be appropriate. Cell therapy has potential for treating acute liver injury under suitable circumstances.

Expression of Fas ligand in activated T cells is regulated by c-Myc

The transcription factor c-Myc is important for the control of cell cycle progression, neoplasia, and apoptotic cell death. c-Myc dimerizes with its partner Max to form an active transcription factor complex. Little is known, however, about the transcriptional targets of c-Myc and their roles in c-Myc-induced cell death. Here we demonstrate that T cell activation-induced expression of Fas ligand (FasL, CD95-L, APO-1-L), which can induce apoptotic cell death in many different cell types, is regulated by c-Myc. Down-modulation of c-Myc protein via antisense oligonucleotides blocked activation-induced FasL mRNA and protein expression and functional FasL expression in activated T cells and T cell lines. Further, FasL promoter activity in T cells is driven by overexpression of c-Myc and inhibited by expression of dominant-negative mutants of c-Myc and Max. Our findings indicate that c-Myc controls apoptotic cell death in T cells through regulation of FasL expression.