MM-1, a novel c-Myc-associating protein that represses transcriptional activity of c-Myc

Negative regulation of the Wnt signal by MM-1 through inhibiting expression of the wnt4 gene

We have reported that a novel c-Myc-binding protein, MM-1, repressed the E-box-dependent transcription activity of c-Myc through TIF1beta/KAP1, a transcriptional corepressor, and that the c-fms gene was a target gene involved in this pathway. We have also reported that a mutation of A157R in MM-1, which is often observed in patients with leukemia or lymphoma, abrogated all of the repressive activities of MM-1 toward c-Myc, indicating that MM-1 is a novel tumor suppressor. In this study, to further identify target genes of MM-1, DNA microarray analysis was carried out by comparing expression levels of genes in MM-1 knockdown and parental cells, and the wnt4 gene, a member of the Wnt-beta-catenin pathway, was identified as a target gene of MM-1. Increased expression level of the wnt4 gene, accumulation and translocation of beta-catenin to the cytoplasm and nucleus, and upregulation of TCF/Lef-1, a target protein of the Wnt-beta-catenin pathway, were found in MM-1 knockdown cells. Reporter assays using various deletion constructs of the wnt4 gene promoter showed that MM-1 recognized the region spanning -286 to -229 from a transcription start site, and MM-1 complex was found to bind to this region by chromatin immunoprecipitation and gel mobility shift assays. Furthermore, it was found that Egr-1 and MM-1 were bound to this region and that both proteins mutually down-regulate promoter activity of the wnt4 gene. Since the c-myc gene is the target gene of the Wnt-beta-catenin pathway, these findings suggest that MM-1 inhibits c-Myc by a dual mechanism.

Efficient chaperone-mediated tubulin biogenesis is essential for cell division and cell migration in C. elegans

The efficient folding of actin and tubulin in vitro and in Saccharomyces cerevisiae is known to require the molecular chaperones prefoldin and CCT, yet little is known about the functions of these chaperones in multicellular organisms. Whereas none of the six prefoldin genes are essential in yeast, where prefoldin-independent folding of actin and tubulin is sufficient for viability, we demonstrate that reducing prefoldin function by RNAi in Caenorhabditis elegans causes defects in cell division that result in embryonic lethality. Our analyses suggest that these defects result mainly from a decrease in alpha-tubulin levels and a subsequent reduction in the microtubule growth rate. Prefoldin subunit 1 (pfd-1) mutant animals with maternally contributed PFD-1 develop to the L4 larval stage with gonadogenesis defects that include aberrant distal tip cell migration. Importantly, RNAi knockdown of prefoldin, CCT or tubulin in developing animals phenocopy the pfd-1 cell migration phenotype. Furthermore, reducing CCT function causes more severe phenotypes (compared with prefoldin knockdown) in the embryo and developing gonad, consistent with a broader role for CCT in protein folding. Overall, our results suggest that efficient chaperone-mediated tubulin biogenesis is essential in C. elegans, owing to the critical role of the microtubule cytoskeleton in metazoan development.

Cyclin D1 is transcriptionally down-regulated by ZO-2 via an E box and the transcription factor c-Myc

Recent reports have indicated the participation of tight junction (TJ) proteins in the regulation of gene expression and cell proliferation. Here, we have studied the role of zona occludens (ZO)-2, a TJ peripheral protein, in the regulation of cyclin D1 transcription. We found that ZO-2 down-regulates cyclin D1 transcription in a dose-dependent manner. To understand how ZO-2 represses cyclin D1 promoter activity, we used deletion analyses and found that ZO-2 negatively regulates cyclin D1 transcription via an E box and that it diminishes cell proliferation. Because ZO-2 does not associate directly with DNA, electrophoretic mobility shift assay and chromatin immunoprecipitation (ChIP) assay were used to identify the transcription factors mediating the ZO-2-repressive effect. c-Myc was found to bind the E box present in the cyclin D1 promoter, and the overexpression of c-Myc augmented the inhibition generated by ZO-2 transfection. The presence of ZO-2 and c-Myc in the same complex was further demonstrated by immunoprecipitation. ChIP and reporter gene assays using histone deacetylases (HDACs) inhibitors demonstrated that HDACs are necessary for ZO-2 repression and that HDAC1 is recruited to the E box. We conclude that ZO-2 down-regulates cyclin D1 transcription by interacting with the c-Myc/E box element and by recruiting HDAC1.

Stimulation of c-Myc transcriptional activity by vIRF-3 of Kaposi sarcoma-associated herpesvirus

Kaposi sarcoma-associated herpesvirus is associated with two lymphoproliferative disorders, primary effusion lymphoma (PEL) and Castleman disease. In PEL, Kaposi sarcoma-associated herpesvirus is present in a latent form expressing only few viral genes. Among them is a viral homologue of cellular interferon regulatory factors, vIRF-3. To study the role of vIRF-3 in PEL lymphomagenesis, we analyzed the interaction of vIRF-3 with cellular proteins. Using yeast two-hybrid screen, we detected the association between vIRF-3 and c-Myc suppressor, MM-1alpha. The vIRF-3 and MM-1alpha interaction was also demonstrated by glutathione S-transferase pulldown assay and coimmunoprecipitation of endogenous vIRF-3 and MM-1alpha in PEL-derived cell lines. Overexpression of vIRF-3 enhanced the c-Myc-dependent transcription of the gene cdk4. Addressing the molecular mechanism of the vIRF-3-mediated stimulation, we demonstrated that the association between MM-1alpha and c-Myc was inhibited by vIRF-3. Furthermore, the recruitment of vIRF-3 to the cdk4 promoter and the elevated levels of the histone H3 acetylation suggest the direct involvement of vIRF-3 in the activation of c-Myc-mediated transcription. These findings indicate that vIRF-3 can effectively stimulate c-Myc function in PEL cells and consequently contribute to de-regulation of B-cell growth and differentiation.

Microarray analysis of the developing cortex

Abnormal development of the prefrontal cortex (PFC) is associated with a number of neuropsychiatric disorders that have an onset in childhood or adolescence. Although the basic laminar structure of the PFC is established in utero, extensive remodeling continues into adolescence. To map the overall pattern of changes in cortical gene transcripts during postnatal development, we made serial measurements of mRNA levels in mouse PFC using oligonucleotide microarrays. We observed changes in mRNA transcripts consistent with known postnatal morphological and biochemical events. Overall, most transcripts that changed significantly showed a progressive decrease in abundance after birth, with the majority of change between postnatal weeks 2 and 4. Genes with cell proliferative, cytoskeletal, extracellular matrix, plasma membrane lipid/transport, protein folding, and regulatory functions had decreases in mRNA levels. Quantitative PCR verified the microarray results for six selected genes: DNA methyltransferase 3A (Dnmt3a), procollagen, type III, alpha 1 (Col3a1), solute carrier family 16 (monocarboxylic acid transporters), member 1 (Slc16a1), MARCKS-like 1 (Marcksl1), nidogen 1 (Nid1) and 3-hydroxybutyrate dehydrogenase (heart, mitochondrial) (Bdh).

SNIP1 is a candidate modifier of the transcriptional activity of c-Myc on E box-dependent target genes

Using a yeast two-hybrid screen, we found that SNIP1 (Smad nuclear-interacting protein 1) associates with c-Myc, a key regulator of cell proliferation and transformation. We demonstrate that SNIP1 functions as an important regulator of c-Myc activity, binding the N terminus of c-Myc through its own C terminus, and that SNIP1 enhances the transcriptional activity of c-Myc both by stabilizing it against proteosomal degradation and by bridging the c-Myc/p300 complex. These effects of SNIP1 on c-Myc likely contribute to synergistic effects of SNIP1, c-Myc, and H-Ras in inducing formation of foci in an in vitro transformation assay and also in supporting anchorage-independent growth. The significant association of SNIP1 and c-Myc staining in a non-small cell lung cancer tissue array is further evidence that their activities might be linked and suggests that SNIP1 might be an important modulator of c-Myc activity in carcinogenesis.

The PVT-1 oncogene is a Myc protein target that is overexpressed in transformed cells

The human PVT-1 gene is located on chromosome 8 telomeric to the c-Myc gene and it is frequently involved in the translocations occurring in variant Burkitt's lymphomas and murine plasmacytomas. It has been proposed that PVT-1 regulates c-Myc gene transcription over a long distance. To get new insights into the functional relationships between the two genes, we have investigated PVT-1 and c-Myc expression in normal human tissues and in transformed cells. Our findings indicate that PVT-1 expression is restricted to a relative low number of normal tissues compared to the wide distribution of c-Myc mRNA, whereas the gene is highly expressed in many transformed cell types including neuroblastoma cells that do not express c-Myc. Reporter gene assays were used to dissect the PVT-1 promoter and to identify the region responsible for the elevated expression observed in transformed cells. This region contains two putative binding sites for Myc proteins. The results of transfection experiments in RAT1-MycER cells and chromatin immunoprecipitation (ChIP) assays in proliferating and differentiated neuroblastoma cells indicate that PVT-1 is a downstream target of Myc proteins.

The central domain of transcription factor FOXM1c directly interacts with itself in vivo and switches from an essential to an inhibitory domain depending on the FOXM1c binding site

We have previously shown that FOXM1c can transactivate its target genes by two different mechanisms, depending on the FOXM1c binding site. In the present study, by introducing a small 46-aa deletion, we confirm that the central domain of FOXM1c is essential for transactivation of the minimal c-myc P1 and P2 promoters via their TATA boxes, but functions as an inhibitory domain on conventional FOXM1c binding sites. Thus, distinct FOXM1c binding sites determine opposite functions of the central domain, suggesting allosteric control of its conformation by the DNA binding site. This is strongly supported by the identification of a direct in vivo interaction of the central domain with itself in the present study. In contrast, the DNA binding domain binds neither to itself nor to any other domain of FOXM1c. Transrepression by the central domain is unlikely to be achieved by recruitment of co-repressors, but instead seems to be mediated by direct interference with the basal transcription complex. Direct binding of the central domain to itself should be involved in transrepression. Finally, FOXM1c transactivates the chicken mim-1 promoter, whose TATA box represents a conventional FOXM1c binding site, so that transactivation follows neither of the above two mechanisms, but shows intermediate behavior.

Identification of candidates for tumor-specific alternative splicing in the thyroid

Alternative splicing is the differential processing of exon junctions to produce a new transcript variant from one gene. Some aberrant splicing, however, has been shown to be cancer specific. Identification of these specific splice variations will provide important insight into the molecular mechanism of normal cellular physiology as well as the disease processes. To gain knowledge about whether alternative splicing is linked to thyroid tumorigenesis, we used our prediction database to select targets for analysis. Fifteen putatively new alternative splicing isoforms were selected on the basis of their expression in thyroid libraries and/or their origin in genes previously associated with carcinogenesis. Using a set of 66 normal, benign, and malignant thyroid tissue samples, new splicing events were confirmed by RT-PCR for 13 of 15 genes (a validation rate of 87%). In addition, new alternative splicing isoforms not predicted by the system and not previously described in public databases were identified. Five genes (PTPN18, ABI3BP, PFDN5, SULF2, and ST5) presented new and/or additional unpredicted isoforms differentially expressed between malignant and benign or normal thyroid tissues, confirmed by sequencing. PTPN18, ABI3BP, and PFDN5 revealed a statistically significant differential splicing profile. In addition, real-time PCR analysis revealed that expression of an alternative PFDN5 variant was higher in malignant lesions than in benign lesions or normal tissues.

The Mad side of the Max network: antagonizing the function of Myc and more

A significant body of evidence has been accumulated that demonstrates decisive roles of members of the Myc/Max/Mad network in the control of various aspects of cell behavior, including proliferation, differentiation, and apoptosis. The components of this network serve as transcriptional regulators. Mad family members, including Mad1, Mxi1, Mad3, Mad4, Mnt, and Mga, function in part as antagonists of Myc oncoproteins. At the molecular level this antagonism is reflected by the different cofactor/chromatin remodeling complexes that are recruited by Myc and Mad family members. One important function of the latter is their ability to repress gene transcription. In this review we summarize the current view of how this repression is achieved and what the consequences of Mad action are for cell behavior. In addition, we point out some of the many aspects that have not been clarified and thus leave us with a rather incomplete picture of the functions, both molecular and at the cellular level, of Mad family members.

Altered protein profile in chronic myeloid leukemia chronic phase identified by a comparative proteomic study

Chronic myeloid leukemia is a hematological disorder in which the Ph chromosome is a marker of the disease, detected virtually in all cases. The chimeric transcripts encode a 210-kDa chimeric protein with altered tyrosine kinase activity, responsible for the disease phenotype. In this work, we tried to identify which are the molecular changes common to chronic phase patients, those that represent the chronic phase molecular phenotype. To address this problem we analyzed through a comparative proteomic approach, several CML bone marrow cells protein profile from patients in chronic phase and healthy bone marrow donors. From these results, we identified 31 differentially expressed proteins. Among these proteins, we pointed out c-Myc binding protein 1, 53BP1, Mdm4, OSBP-related protein 3 and Mortalin as putative candidates to BCR-ABL targets in chronic phase. Moreover, we describe for the first time the cytoplasmic protein map from bone marrow cells that helped in the elucidation of the changes we were looking for.

Distinct localizations and repression activities of MM-1 isoforms toward c-Myc

MM-1 was identified as a c-Myc-binding protein and has been reported to repress the E-box-dependent transcription activity of c-Myc by recruiting HDAC1 complex via TIF1 beta/KAP1. In this study, originally isolated MM-1 was found to be a fusion protein comprised of the N-terminal 13 amino acids from the sequence of chromosome 14 and of the rest of the amino acids from that of chromosome 12 and was found to be expressed ubiquitously in all human tissues. Four splicing isoforms of MM-1, MM-1alpha, MM-1beta, MM-1gamma, and MM-1delta, which are derived from the sequence of chromosome 12, were then identified. Of these isoforms, MM-1alpha, MM-1gamma, and MM-1delta were found to be expressed in tissue-specific manners and MM-1beta was found to be expressed ubiquitously. Although all of the isoforms potentially possessed c-Myc- and TIF1beta-binding activities, MM-1beta and MM-1delta were found to be mainly localized in the cytoplasm and MM-1alpha and MM-1gamma were found to be localized in the nucleus together with both c-Myc and TIF1beta. Furthermore, when repression activities of MM-1 isoforms toward c-Myc transcription activity were examined by reporter gene assays in HeLa cells, MM-1alpha, MM-1gamma, and MM-1gamma, but not MM-1beta, were found to repress transcription activity of c-Myc, and the degrees of repression by MM-1gamma and MM-1delta were smaller than those by MM-1 and MM-1alpha. These results suggest that each MM-1 isoform distinctly regulates c-Myc transcription activity in respective tissues.

Knockdown of Skp2 by siRNA inhibits melanoma cell growth in vitro and in vivo

BACKGROUND

Low levels of p27Kip1 expression are associated with poor prognosis in various malignancies including malignant melanoma. Recently, it has been reported that S phase kinase-interacting protein 2 (Skp2), the specific ubiquitin ligase subunit that targets p27Kip1 for degradation, was overexpressed and was inversely related to p27Kip1 levels in malignant melanoma with poor prognosis.

OBJECTIVE

We investigated whether small interfering RNA (siRNA)-mediated gene silencing of Skp2 can be employed in order to inhibit p27Kip1 down-regulation and suppress melanoma cell growth as a consequence in vitro and in vivo.

METHODS

We constructed a plasmid vector, which synthesizes siRNAs to determine the effects of decreasing the high constitutive levels of Skp2 protein in melanoma cells. Western blot and real-time RT-PCR were performed to examine the decreases of Skp2 protein and mRNA in vitro. Furthermore, melanoma cells were injected into the back of nude mice subcutaneously to examine the suppression of tumorigenicity targeting Skp2 gene silencing in vivo.

RESULTS

Skp2 protein was decreased and the p27Kip1 protein was accumulated in Skp2 siRNA transfected melanoma cells. Skp2 siRNA inhibited the cell growth of melanoma cells in vitro. Moreover, Skp2 siRNA also suppressed tumor proliferation in vivo.

CONCLUSION

Our results suggest that siRNA-mediated gene silencing of Skp2 can be a potent tool of cancer gene therapy for suppression of p27Kip1 degradation in malignant melanoma.

Molecular cloning, expression and chromosomal localization of mouse MM-1

The protooncogene product Myc associates with many proteins. The isolation of the mouse MM-1; c-Myc binding protein (Myc-Modulator 1) cDNA is described. The cDNA contains a 462 bp open reading frame that encodes a polypeptide of 154 amino acid residues. The deduced amino acid sequence indicates that mouse MM-1 has a 99% identity with the sequence of human MM-1. The expression of mouse MM-1 mRNA was detected in the fetal liver, but its level was 3-fold higher than that in the normal adult liver, and was slightly increased after a partial hepatectomy. It is expressed widely in a variety of adult mouse tissues. Thus, MM-1 may play a role in liver development and growth. A bioinformatics analysis indicates that mouse MM-1 gene consists of 6 exons. Furthermore, the chromosomal location of the mouse MM-1 gene was on the F2-F3 band of chromosome 15, as determined by fluorescence in situ hybridization.

Identification of a novel c-Myc protein interactor, JPO2, with transforming activity in medulloblastoma cells

c-myc oncogene activation is critical in the pathogenesis of a spectrum of human malignancies. The c-Myc NH2-terminal domain (MycNTD) is essential for cellular transformation, and mediates critical protein interactions that modulate c-Myc oncogenic properties. In medulloblastoma, the most common malignant pediatric brain tumor, deregulated c-myc expression is linked with poorer disease phenotypes and outcomes. The biological basis for these associations is, however, not well understood. To better understand mechanisms underlying Myc-mediated transformation of medulloblastoma, we sought to identify novel MycNTD protein interactors from a medulloblastoma cell line library using a unique two-hybrid system. We identified a novel MycNTD binding protein, JPO2, which shows nuclear colocalization with c-Myc, and interacts with c-Myc both in vitro and in mammalian cells. In Rat1a transformation assays, JPO2 potentiates c-Myc transforming activity, and can complement a transformation-defective Myc mutant. Immunohistochemical studies indicate tumor-specific JPO2 expression in human medulloblastoma, and an association of JPO2 expression with metastatic tumors. Significantly, JPO2 expression induces colony formation in UW228, a medulloblastoma cell line, whereas RNAi-mediated JPO2 knockdown impairs colony formation in UW228, and in Myc-transformed UW228 cells. These data provide evidence for biochemical and functional interaction between c-Myc and JPO2 in medulloblastoma transformation. JPO2 is closely related to JPO1, a Myc transcriptional target with transforming activity. As tumor-specific JPO1 expression in human and murine medulloblastoma has also been reported; these collective observations suggest important functional links between the novel JPO protein family and c-Myc in medulloblastoma transformation.

B7/CD28 costimulation of T cells induces a distinct proteome pattern

Effective immune strategies for the eradication of human tumors require a detailed understanding of the interaction of tumor cells with the immune system, which might lead to an optimization of T cell responses. To understand the impact of B7-mediated costimulation on T cell activation comprehensive proteome analysis of B7-primed T cell populations were performed. Using this approach we identified different classes of proteins in T cells whose expression is either elevated or reduced upon B7-1- or B7-2-mediated CD28 costimulation. The altered proteins include regulators of the cell cycle and cell proliferation, signal transducers, components of the antigen processing machinery, transporters, cytoskeletal proteins, and metabolic enzymes. A number of differentially expressed proteins are further modified by phosphorylation. Our results provide novel insights into the complexity of the CD28 costimulatory pathway of T cells and will help to identify potential targets of therapeutic interventions for modulating anti-tumor T cell activation.

Transcripts from a human primordial follicle cDNA library

BACKGROUND

Human primordial follicles (PFs) or the oocyte-pre-granulosa complex, constitute the earliest and most immature stage of human oogenesis. The factors, signalling networks and the precise role of the oocyte and the pre-granulosa cells in initiating growth and recruitment from this finite resting pool remain largely unknown at present.

METHODS

To obtain a gene resource of this oogenesis stage and thereby determine a molecular blueprint of the human PF, a cDNA library was constructed from 50 isolated human PFs using the phagemid vector pTriplEx2.

RESULTS

Sequence analysis showed that 46.67% of these clones corresponded to known genes while 29.48% were uncharacterized genes that included hypothetical proteins, human cDNA clones and novel genes. Bioinformatics analysis revealed a preponderance of mitochondrial genes and repeat elements followed by ribosomal proteins, transcription and translation genes. Transcripts for heat shock proteins, cell cycle, embryogenesis genes and apoptosis genes were identified. Members of the ubiquitin-proteasome pathway, MAPK, p38/JNK, GPCR, Wnt, NF-kappaB and notch signalling pathways were identified. A mitochondrial pathway and a transcription factor pathway in the human PF were generated. The gene networks in the transcription factor pathway provided a first glimpse of the balance between proliferation and cell death/apoptosis in this earliest stage of oogenesis.

CONCLUSIONS

The abundance and diversity of retroviral elements and transcriptional repressor genes in the human PF suggest these could contribute to the maintainance of this oogenesis stage. The role of these genes in initial recruitment and in subsequent oogenesis stages will be greatly facilitated and elucidated by printing a human PF cDNA array of the sequenced clones and using it for gene profiling.

Large-scale Transcriptome Analyses Reveal New Genetic Marker Candidates of Head, Neck, and Thyroid Cancer

A detailed genome mapping analysis of 213,636 expressed sequence tags (EST) derived from nontumor and tumor tissues of the oral cavity, larynx, pharynx, and thyroid was done. Transcripts matching known human genes were identified; potential new splice variants were flagged and subjected to manual curation, pointing to 788 putatively new alternative splicing isoforms, the majority (75%) being insertion events. A subset of 34 new splicing isoforms (5% of 788 events) was selected and 23 (68%) were confirmed by reverse transcription-PCR and DNA sequencing. Putative new genes were revealed, including six transcripts mapped to well-studied chromosomes such as 22, as well as transcripts that mapped to 253 intergenic regions. In addition, 2,251 noncoding intronic RNAs, eventually involved in transcriptional regulation, were found. A set of 250 candidate markers for loss of heterozygosis or gene amplification was selected by identifying transcripts that mapped to genomic regions previously known to be frequently amplified or deleted in head, neck, and thyroid tumors. Three of these markers were evaluated by quantitative reverse transcription-PCR in an independent set of individual samples. Along with detailed clinical data about tumor origin, the information reported here is now publicly available on a dedicated Web site as a resource for further biological investigation. This first in silico reconstruction of the head, neck, and thyroid transcriptomes points to a wealth of new candidate markers that can be used for future studies on the molecular basis of these tumors. Similar analysis is warranted for a number of other tumors for which large EST data sets are available.

N and C-terminal Sub-regions in the c-Myc Transactivation Region and their Joint Role in Creating Versatility in Folding and Binding

The proto-oncogene c-myc governs the expression of a number of genes targeting cell growth and apoptosis, and its expression levels are distorted in many cancer forms. The current investigation presents an analysis by proteolysis, circular dichroism, fluorescence and Biacore of the folding and ligand-binding properties of the N-terminal transactivation domain (TAD) in the c-Myc protein. A c-Myc sub-region comprising residues 1-167 (Myc1-167) has been investigated that includes the unstructured c-Myc transactivation domain (TAD, residues 1-143) together with a C-terminal segment, which appears to promote increased folding. Myc1-167 is partly helical, binds both to the target proteins Myc modulator-1 (MM-1) and TATA box-binding protein (TBP), and displays the characteristics of a molten globule. Limited proteolysis divides Myc1-167 in two halves, by cleaving in a predicted linker region between two hotspot mutation regions: Myc box I (MBI) and Myc box II (MBII). The N-terminal half (Myc1-88) is unfolded and does not alone bind to target proteins, whereas the C-terminal half (Myc92-167) has a partly helical fold and specifically binds both MM-1 and TBP. Although this might suggest a bipartite organization in the c-Myc TAD, none of the N and C-terminal fragments bind target protein with as high affinity as the entire Myc1-167, or display molten globule properties. Furthermore, merely linking the MBI with the C-terminal region, in Myc38-167, is not sufficient to achieve binding and folding properties as in Myc1-167. Thus, the entire N and C-terminal regions of c-Myc TAD act in concert to achieve high specificity and affinity to two structurally and functionally orthogonal target proteins, TBP and MM-1, possibly through a mechanism involving molten globule formation. This hints towards understanding how binding of a range of targets can be accomplished to a single transactivation domain.

PAP-1, the mutated gene underlying the RP9 form of dominant retinitis pigmentosa, is a splicing factor

PAP-1 is an in vitro phosphorylation target of the Pim-1 oncogene. Although PAP-1 binds to Pim-1, it is not a substrate for phosphorylation by Pim-1 in vivo. PAP-1 has recently been implicated as the defective gene in RP9, one type of autosomal dominant retinitis pigmentosa (adRP). However, RP9 is a rare disease and only two missense mutations have been described, so the report of a link between PAP-1 and RP9 was tentative. The precise cellular role of PAP-1 was also unknown at that time. We now report that PAP-1 localizes in nuclear speckles containing the splicing factor SC35 and interacts directly with another splicing factor, U2AF35. Furthermore, we used in vitro and in vivo splicing assays to show that PAP-1 has an activity, which alters the pattern of pre-mRNA splicing and that this activity is dependent on the phosphorylation state of PAP-1. We used the same splicing assay to examine the activities of two mutant forms of PAP-1 found in RP9 patients. The results showed that while one of the mutations, H137L, had no effect on splicing activity compared with that of wild-type PAP-1, the other, D170G, resulted in both a defect in splicing activity and a decreased proportion of phosphorylated PAP-1. The D170G mutation may therefore cause RP by altering splicing of retinal genes through a decrease in PAP-1 phosphorylation. These results demonstrate that PAP-1 has a role in pre-mRNA splicing and, given that three other splicing factors have been implicated in adRP, this finding provides compelling further evidence that PAP-1 is indeed the RP9 gene.

CIR, a corepressor of CBF1, binds to PAP-1 and effects alternative splicing

We have reported that PAP-1, a product of a causative gene for autosomal retinitis pigmentosa, plays a role in splicing. In this study, CIR, a protein originally identified as a CBF1-interacting protein and reported to act as a transcriptional corepressor, was identified as a PAP-1 binding protein and its function as a splicing factor was investigated. In addition to a basic lysine and acidic serine-rich (BA) domain and a zinc knuckle-like motif, CIR has an arginine/serine dipeptide repeat (RS) domain in its C terminal region. The RS domain has been reported to be present in the superfamily of SR proteins, which are involved in splicing reactions. We generated CIR mutants with deletions of each BA and RS domain and studied their subcellular localizations and interactions with PAP-1 and other SR proteins, including SC35, SF2/ASF, and U2AF35. CIR was found to interact with U2AF35 through the BA domain, with SC35 and SF2/ASF through the RS domain, and with PAP-1 outside the BA domain in vivo and in vitro. CIR was found to be colocalized with SC35 and PAP-1 in nuclear speckles. Then the effect of CIR on splicing was investigated using the E1a minigene as a reporter in HeLa cells. Ectopic expression of CIR with the E1a minigene changed the ratio of spliced isoforms of E1a that were produced by alternative selection of 5'-splice sites. These results indicate that CIR is a member of the family of SR-related proteins and that CIR plays a role in splicing regulation.

Repression of the c-fms gene in fibroblast cells by c-Myc-MM-1-TIF1beta complex

MM-1 has been reported to repress the E-box-dependent transcription activity of c-Myc by recruiting histone deacetylase 1 complex via TIF1beta/KAP1. In this study, to identify target genes for c-Myc-MM-1-TIF1beta, we established rat-1 cells harboring the dominant-negative form of TIF1beta to abrogate the pathway from TIF1beta to MM-1-c-Myc. This cell line, in which transcription activity of c-Myc was activated, was found to be tumorigenic. By DNA-microarray analysis of this cell line, expression and promoter activity of the c-fms oncogene were found to be upregulated. Of the two promoters, pE1 and pE2, in the c-fms gene, pE1 promoter activity was found to be activated in an E-box-dependent manner.

Phosphorylation-dependent degradation of c-Myc is mediated by the F-box protein Fbw7

The F-box protein Skp2 mediates c-Myc ubiquitylation by binding to the MB2 domain. However, the turnover of c-Myc is largely dependent on phosphorylation of threonine-58 and serine-62 in MB1, residues that are often mutated in cancer. We now show that the F-box protein Fbw7 interacts with and thereby destabilizes c-Myc in a manner dependent on phosphorylation of MB1. Whereas wild-type Fbw7 promoted c-Myc turnover in cells, an Fbw7 mutant lacking the F-box domain delayed it. Furthermore, depletion of Fbw7 by RNA interference increased both the abundance and transactivation activity of c-Myc. Accumulation of c-Myc was also apparent in mouse Fbw7-/- embryonic stem cells. These observations suggest that two F-box proteins, Fbw7 and Skp2, differentially regulate c-Myc stability by targeting MB1 and MB2, respectively.

Molecular clamp mechanism of substrate binding by hydrophobic coiled-coil residues of the archaeal chaperone prefoldin

Prefoldin (PFD) is a jellyfish-shaped molecular chaperone that has been proposed to play a general role in de novo protein folding in archaea and is known to assist the biogenesis of actins, tubulins, and potentially other proteins in eukaryotes. Using point mutants, chimeras, and intradomain swap variants, we show that the six coiled-coil tentacles of archaeal PFD act in concert to bind and stabilize nonnative proteins near the opening of the cavity they form. Importantly, the interaction between chaperone and substrate depends on the mostly buried interhelical hydrophobic residues of the coiled coils. We also show by electron microscopy that the tentacles can undergo an en bloc movement to accommodate an unfolded substrate. Our data reveal how archael PFD uses its unique architecture and intrinsic coiled-coil properties to interact with nonnative polypeptides.

Guanine nucleotide exchange factor, Tiam1, directly binds to c-Myc and interferes with c-Myc-mediated apoptosis in rat-1 fibroblasts

The transcription factor c-Myc is important for the control of cell growth, cell cycle progression, neoplasia, and apoptotic cell death. Recently, c-Myc-binding proteins, which bind either to the N-terminal domain or the C-terminal domain of c-Myc, have been proposed as the key molecules to realize the mechanisms of these multiple c-Myc functions. We report in the present study on another protein, Tiam1, which is a specific guanine nucleotide exchange factor of Rac1 and which binds to c-Myc and modulates several of its biological functions. We were able to detect the direct binding and in vivo association between c-Myc and Tiam1. The necessary role in this interaction of the Myc box II of c-Myc was revealed in the cell extracts. The additional discovery of the intranuclear localization of Tiam1 in Rat1 cells and in neuronal cells of the mouse brain suggests this interaction may occur in the nucleus. Overexpression of Tiam1 repressed the luciferase activity of c-Myc and also inhibited the c-Myc apoptotic activity through this protein-protein interaction. Taken together, we concluded that Tiam1 is another c-Myc regulator, working in the nuclei to control c-Myc-related apoptosis.

Cloning and characterization of Disc1, the mouse ortholog of DISC1 (Disrupted-in-Schizophrenia 1)

We cloned the mouse ortholog of DISC1 (Disrupted-in-Schizophrenia 1), a candidate gene for schizophrenia. Disc1 is 3163 nucleotides long and has 60% identity with the human DISC1. Disc1 encodes 851 amino acids and has 56% identity with the human protein. Disc1 maps to the DISC1 syntenic region in the mouse, and genomic structure is conserved. A Disc1 splice variant deletes a portion of Disc1 beginning at amino acids orthologous to the human truncation. Bioinformatic analysis and cross-species comparisons revealed sequence conservation distributed across the genes and conservation of leucine zipper and coiled-coil domains in both orthologs. In situ hybridization in adult mouse brain revealed a restricted expression pattern, with highest levels in the dentate gyrus of the hippocampus and lower expression in CA1-CA3 of the hippocampus, cerebellum, cerebral cortex, and olfactory bulbs. Identification of Disc1 will facilitate the study of DISC1's function and creation of mouse models of DISC1 disruption.

Pag, a putative tumor suppressor, interacts with the Myc Box II domain of c-Myc and selectively alters its biological function and target gene expression

The highly conserved Myc Box II (MBII) domain of c-Myc is critically important for transformation and transcriptional regulation. A yeast two-hybrid screen identified Pag as a MBII-interacting protein. Pag, a member of the peroxiredoxin family, has been reported previously to bind to and inhibit the cytostatic properties of the c-Abl oncoprotein. We now show that Pag promotes increased cell size and confers a proapoptotic phenotype, two hallmark features of ectopic c-Myc overexpression. Pag and c-Myc also confer resistance to oxidative stress, a previously unrecognized property of the latter protein. In contrast, Pag inhibits tumorigenesis by c-Myc-overexpressing fibroblasts and causes a broad but selective loss of c-Myc target gene regulation. Pag is therefore an MBII-interacting protein that can either mimic or enhance some of the c-Myc properties while at the same inhibiting others. These features, along with the previously identified interaction with c-Abl, provide support for the idea that Pag functions as a tumor suppressor.

The myc oncogene: MarvelouslY Complex

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

Recruitment of Gcn5-containing complexes during c-Myc-dependent gene activation. Structure and function aspects

The N-terminal domain of c-Myc plays a key role in cellular transformation and is involved in both activation and repression of target genes as well as in modulated proteolysis of c-Myc via the proteasome. Given this functional complexity, it has been difficult to clarify the structures within the N terminus that contribute to these different processes as well as the mechanisms by which they function. We have used a simplified yeast model system to identify the primary determinants within the N terminus for (i) chromatin remodeling of a promoter, (ii) gene activation from a chromatin template in vivo, and (iii) interaction with highly purified Gcn5 complexes as well as other chromatin-remodeling complexes in vitro. The results identify two regions that contain autonomous chromatin opening and gene activation activity, but both regions are required for efficient interaction with chromatin-remodeling complexes in vitro. The conserved Myc boxes do not play a direct role in gene activation, and Myc box II is not generally required for in vitro interactions with remodeling complexes. The yeast SAGA complex, which is orthologous to the human GCN5-TRRAP complex that interacts with Myc in human cells, plays a role in Myc-mediated chromatin opening at the promoter but may also be involved in later steps of gene 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.

Physical interaction of p73 with c-Myc and MM1, a c-Myc-binding protein, and modulation of the p73 function

p73 shares high sequence homology with the tumor suppressor p53. Like p53, ectopic overexpression of p73 induces cell cycle arrest and/or apoptosis, and these biological activities are linked to its sequence-specific transactivation function. The COOH-terminal region of p73 is unique and has a function to modulate DNA-binding ability and transactivation activity. To identify and characterize cellular proteins that interact with the COOH-terminal region of p73 alpha and regulate its activity, we employed a yeast-based two-hybrid screen with a human fetal brain cDNA library. We found MM1, a nuclear c-Myc-binding protein, was associated with p73 alpha in both yeast two-hybrid and in vitro pull-down assays. In mammalian cells, MM1 co-immunoprecipitated with p73 alpha, whereas p73 beta and tumor suppressor p53 did not interact with MM1. Overexpression of MM1 in p53-deficient osteosarcoma SAOS-2 cells enhanced the p73 alpha-dependent transcription from the p53/p73-responsive Bax and PG13 promoters, whereas p73 beta- and p53-mediated transcriptional activation was unaffected in the presence of MM1. MM1 also stimulated the p73 alpha-mediated growth suppression in SAOS-2 cells. More importantly, we found that c-Myc was physically associated with p73 alpha and significantly impaired the transcriptional activity of p73 alpha on Bax and p21(waf1) promoters. Expression of MM1 strongly reduced the c-Myc-mediated inhibitory activity on p73 alpha. These results suggest that MM1 may act as a molecular partner for p73 to prevent the c-Myc-mediated inhibitory effect on its activity.

The Raf/MEK/ERK signal transduction cascade as a target for chemotherapeutic intervention in leukemia

The Raf/MEK/ERK (MAPK) signal transduction cascade is a vital mediator of a number of cellular fates including growth, proliferation and survival, among others. The focus of this review centers on the MAPK signal transduction pathway, its mechanisms of activation, downstream mediators of signaling, and the transcription factors that ultimately alter gene expression. Furthermore, negative regulators of this cascade, including phosphatases, are discussed with an emphasis placed upon chemotherapeutic intervention at various points along the pathway. In addition, mounting evidence suggests that the PI3K/Akt pathway may play a role in the effects elicited via MAPK signaling; as such, potential interactions and their possible cellular ramifications are discussed.

A novel transrepression pathway of c-Myc. Recruitment of a transcriptional corepressor complex to c-Myc by MM-1, a c-Myc-binding protein

The protooncogene product c-Myc plays a role in transcription regulation both for activation and repression. While transactivation pathways of c-Myc either from the N-proximal or the C-proximal region that is linked to the chromatin remodeling complex have been identified, a transrepression pathway had been identified only from the C-proximal region via Max and Mad that recruit the histone deacetylase (HDAC) complex. We have reported that a novel c-Myc-binding protein, MM-1, repressed the E-box-dependent transcription activity of c-Myc (Mori, K., Maeda, Y., Kitaura, H., Taira, T., Iguchi-Ariga, S. M. M., and Ariga, H. (1998) J. Biol. Chem. 273, 29794-29800). To clarify the molecular mechanisms of MM-1 toward c-Myc, cDNAs encoding MM-1-binding proteins were screened by the two-hybrid method with MM-1 as a bait using a human HeLa cDNA library, and a cDNA encoding TIF1 beta/KAP1, a transcriptional corepressor, was obtained. MM-1 was found to bind to the central portion of TIF1 beta in vitro and in vivo, and these proteins were found to be colocalized in the nucleus. MM-1 and TIF1 beta complex in human HeLa cells was found to also contain c-Myc, mSin3, and HDAC1. Introduction of the C-terminal half of TIF1 beta as a dominant negative form abrogated the inhibitory activity of MM-1 toward c-Myc and greatly stimulated the transcription activity of c-Myc. Moreover, the inhibitory activity of MM-1 toward c-Myc was canceled by trichostatin A, an inhibitor of HDAC1. These results indicate that MM-1 is a connecting factor that forms a novel transcription repression pathway of c-Myc.

MM-1, a c-Myc-binding protein, is a candidate for a tumor suppressor in leukemia/lymphoma and tongue cancer

The c-myc oncogene product (c-Myc) is a transcription factor that dimerizes with Max and recognizes the E-box sequence, and it plays key functions in cell proliferation, differentiation, and apoptosis. We previously showed that MM-1 bound to myc box II within the transactivation domain of c-Myc and repressed the E-box-dependent transcriptional activity of c-Myc. Here we report that MM-1 showed features of a tumor suppressor. In an EST data base search for cDNAs homologous to MM-1, we found a frequent substitution of amino acid 157 of MM-1, from alanine to arginine (A157R), and the substitution was observed more in tumor cells than in normal cells. A survey of the A157R mutation of MM-1 in 57 cultured cancer cells and 90 tissues from cancer patients showed that the A157R was present in about 50-60% of leukemia/lymphoma cells and in more than 75% of squamous cell carcinoma of tongue cancer. Although both the A157R and the wild-type MM-1 bound to c-Myc, only A157R lost the activities to repress both the E-box-dependent transcriptional activity of c-Myc and the myc/ras cooperative transforming activity in rat 3Y1 cells. Furthermore, the wild-type MM-1, but not A157R, arrested the growth of 3Y1 cells. The human MM-1 gene was mapped at chromosome 12q12-12q13, where many chromosome abnormalities in cancer cells have been reported. The results suggest that MM-1 is a novel candidate for a tumor suppressor that controls the transcriptional activity of c-Myc.

Pathways of apoptosis and the modulation of cell death in cancer

Although cell death once was viewed exclusively as the disordered, chaotic outcome of metabolic catastrophe, apoptosis now is recognized as a highly ordered, evolutionarily conserved, and genetically selected program that is essential for normal development. The death receptor pathway of apoptosis, cytotoxic T cells, prolife survival signals, Bcl-2 family of regulators, p53 and regulated cell death in cancer, and oncogenes are reviewed. Future prospects in this arena also are discussed.

AMY-1, a c-Myc-binding protein, is localized in the mitochondria of sperm by association with S-AKAP84, an anchor protein of cAMP-dependent protein kinase

We have reported that a novel c-Myc-binding protein, AMY-1 (associate of Myc-1), stimulated the transcription activity of c-Myc. To access the molecular function of AMY-1, a two-hybrid screening of cDNAs encoding AMY-1-binding proteins was carried out with AMY-1 as a bait using a human HeLa cDNA library, and a clone encoding cAMP-dependent protein kinase anchor protein 149 (AKAP149), was obtained. AMY-1 was found to bind in vitro and in vivo to the regulatory subunit II binding region of AKAP149 and S-AKAP84, a splicing variant of AKAP149 expressed in the testis. AMY-1 was expressed postmeiotically in the testis, as S-AKAP84 was expressed. Furthermore, S-AKAP84 and regulatory subunit II, a regulatory subunit of cAMP-dependent protein kinase, made a ternary complex in cells, and AMY-1 was localized in the mitochondria of HeLa and sperm in association with AKAP149 and S-AKAP84, respectively. These results suggest that AMY-1 plays a role in spermatogenesis.

Bob1, a Gim5/MM-1/Pfd5 homolog, interacts with the MAP kinase kinase Byr1 to regulate sexual differentiation in the fission yeast, Schizosaccharomyces pombe

The MAPKK Byr1 is an essential component of a Ras-dependent MAPK module required for sexual differentiation in the fission yeast, Schizosaccharomyces pombe. Here we describe the genetic and molecular characterization of a highly conserved protein, Bob1, which was identified from a two-hybrid screen for Byr1-interacting proteins. Byrl and Bobl proteins coprecipitate from S. pombe cell lysates, and both proteins localize to the tips and septa of S. pombe cells. S. pombe bob1 null (bob1delta) mutants lack obvious growth defects but exhibit a significant mating deficiency, which can be suppressed by overexpression of Byrl. Overexpression of Bob1 also leads to inhibition of mating in S. pombe, and this defect is likewise suppressed by Byrl overexpression. Bob1 is highly homologous in structure to the mammalian MM-1/Pfd5 and budding yeast Gim5/Pfd5-Sc proteins, which have been implicated as regulators of actin and tubulins. Similar to budding yeast gim5/pfd5-Sc mutants, S. pombe bob1delta cells have cytoskeletal defects, as judged by hypersensitivity to cytoskeletal disrupting drugs. byr1delta mutants do not share this characteristic with bob1delta mutants, and byr1delta bob1delta mutants are not significantly more sensitive to cytoskeletal disrupting drugs than cells carrying only the bob1delta mutation. Taken together, our results suggest that Bob1 has Byr1-related function(s) required for proper mating response of S. pombe cells and Byrl-independent function(s) required for normal cytoskeletal control. We show that the human MM-1/Pfd5 protein can substitute for its counterpart in fission yeast, providing evidence that the functions of Bob1-related proteins have been highly conserved through evolution. Our results lead us to propose that Bob1-related proteins may play diverse roles in eukaryotic organisms.

CDC6 interacts with c-Myc to inhibit E-box-dependent transcription by abrogating c-Myc/Max complex

The c-myc oncogene product (c-Myc) is a transcription factor that dimerizes with Max and recognizes the E-box sequence. It plays key functions in cell proliferation, differentiation and apoptosis. It is generally thought that c-Myc transactivates genes encoding proteins essential to cell-cycle progression by binding to the E-boxes that control them. The functions of c-Myc are also thought to be modulated by its associated proteins, several of which have recently been identified. In this study, we found that c-Myc directly bound in vivo and in vitro to the N-terminal region of human CDC6, a component of the pre-replication complex, and that both co-localized in cell nuclei. CDC6 bound to the C-proximal region of c-Myc, thereby competing with Max on the E-box sequence and changing c-Myc/Max heterodimer to a Max/Max homodimer. In consequence, the E-box-dependent transcription activity of c-Myc was abrogated. These results suggest that, in addition to its DNA replication activity, CDC6 also has a role as a transcriptional suppressor of c-Myc.

MSSP, a protein binding to an origin of replication in the c-myc gene, interacts with a catalytic subunit of DNA polymerase alpha and stimulates its polymerase activity

MSSP has been identified as a protein that binds to both single- and double-stranded sequences of a putative DNA replication origin sequence in the human c-myc gene. MSSP possesses versatile functions, including stimulation of DNA replication, transcriptional regulation, apoptosis induction, and cell transformation coordinated by c-Myc. MSSP contains two RNP domains, RNP1-A and RNP1-B, both of which are necessary for all of the functions of MSSP. In this study, we found that MSSP binds to the N-terminal region of a catalytic subunit of a human DNA polymerase alpha via its RNP domains both in vitro and in human cells. Furthermore, MSSP was released from the putative DNA replication origin of the c-myc gene after it complexed with DNA polymerase alpha, and MSSP stimulated DNA polymerase activity in vitro.

ORC1 interacts with c-Myc to inhibit E-box-dependent transcription by abrogating c-Myc-SNF5/INI1 interaction

BACKGROUND

The c-myc oncogene product (c-Myc) is a transcription factor that forms a complex with Max and recognizes the E-box sequence. c-Myc plays key functions in cell proliferation, differentiation and apoptosis. As for its activity towards cell proliferation, it is generally thought that c-Myc transactivates the E-box-containing genes that encode proteins essential to cell-cycle progression. Despite the characterization of candidate genes regulated by c-Myc in culture cells, these have still not been firmly recognized as real target genes for c-Myc.

RESULTS

We found that c-Myc directly bound to the N-terminal region of origin recognition complex-1 (ORC1), a region that is responsible for gene silencing, in a state of complex containing other ORC subunits and Max in vivo and in vitro. Furthermore, ORC1 inhibited E-box-dependent transcription activity of c-Myc by competitive binding to the C-terminal region of c-Myc with SNF5, a component of chromatin remodelling complex SNF/Swi1.

CONCLUSIONS

These results suggest that ORC1 suppresses the transcription activity of c-Myc by its recruitment into an inactive form of chromatin during some stage of the cell cycle.

Reciprocal regulation via protein-protein interaction between c-Myc and p21(cip1/waf1/sdi1) in DNA replication and transcription

The c-myc protooncogene product (c-Myc) is a transcription factor and is rapidly induced in resting cells following various mitogenic stimuli. c-Myc is thus suggested to play an important role in the transition from quiescence to proliferation. Despite numerous studies, including those on the connection between cyclin E/cyclin-dependent kinase 2 and c-Myc, little has been clarified about c-Myc in terms of the cell cycle regulation. Here we show that c-Myc can directly bind to the carboxyl-terminal region of the cyclin-dependent kinase inhibitor p21(cip1/waf1/sdi1) and thus partially relieves the p21 of the inhibitory effect on DNA synthesis directed by the proliferating cell nuclear antigen-dependent DNA polymerase delta. As for transcription, on the other hand, the p21 binding to the Myc box II region of c-Myc blocks c-Myc-Max complex formation on the E-box and thereby suppresses the transcriptional activation from the E-box by c-Myc. These results suggest that c-Myc activates DNA replication via inactivation of p21 and that p21, vice versa, represses the transcriptional activity of c-Myc. The balance of the reciprocal inactivation between c-Myc and p21 may determine the course of cellular processes such as cell proliferation, differentiation, and apoptosis.

Identification of heterochromatin protein 1 (HP1) as a phosphorylation target by Pim-1 kinase and the effect of phosphorylation on the transcriptional repression function of HP1(1)

Pim-1, a protooncogene product, is a serine/threonine kinase and is thought to play a role in signal transduction in blood cells. Few phosphorylated target proteins for Pim-1, however, have been identified. In the present study, two-hybrid screening to clone cDNAs encoding proteins binding to Pim-1 was carried out, and a cDNA for heterochromatin protein 1gamma (HP1gamma) was obtained. Binding assays both in yeast and in vitro pull-down using the purified HP1gamma and Pim-1 expressed in Escherichia coli showed that Pim-1 directly bound to the chromo shadow domain of HP1gamma. HP1gamma was also associated with Pim-1 in human HeLa cells and the serine clusters located at the center of HP1gamma were phosphorylated by Pim-1 in vitro. Furthermore, a transcription repression activity of HP1gamma was further stimulated by the deletion of the serine clusters targeted by Pim-1. These results suggest that Pim-1 affects the structure or silencing of chromatin by phosphorylating HP1.

MSSP promotes ras/myc cooperative cell transforming activity by binding to c-Myc

BACKGROUND

MSSPs, myc single strand binding proteins, were originally identified as proteins recognizing a putative replication origin/transcriptional enhancer in the human c-Myc gene. The cDNAs encoding four of the family proteins, MSSP-1, MSSP-2, Scr2 and Scr3, were cloned. These proteins carry two copies of the putative RNA binding domains, RNP-A and RNP-B, and have been suggested to participate in DNA replication and cell cycle progression from the G1 to the S phase.

RESULTS

We report that MSSP-1 and MSSP-2 bound directly to the C-terminal portion of c-Myc, along with Max, side by side. MSSP, c-Myc and Max formed a ternary complex in vivo, although MSSP did not directly associate with Max. The MSSP/Myc/Max ternary complex lost the binding activity to the E-box sequence-the recognition sequence of c-Myc/Max complex-thereby abrogating the E-box-dependent transcription activity of c-Myc. MSSP specifically stimulated the cooperative transforming activity of c-myc with ras, in a manner dependent upon the RNP sequences, while mssp itself showed no transforming activity in mouse NIH3T3 cells. The NIH3T3 transformants, together with ras, myc and mssp, grew to form very large colonies in soft agar, as compared to those with ras plus myc or ras alone.

CONCLUSIONS

MSSP is a modulator of c-Myc and the c-Myc/MSSP complex may deregulate cell cycle controls and lead cells towards transforming pathways.

Cell cycle-dependent switch of up-and down-regulation of human hsp70 gene expression by interaction between c-Myc and CBF/NF-Y

A CCAAT box-binding protein subunit, CBF-C/NF-YC, was cloned as a protein involved in the c-Myc complex formed on the G(1)-specific enhancer in the human hsp70 gene. CBF-C/NF-YC directly bound to c-Myc in vitro and in vivo in cultured cells. The CBF/NF-Y.c-Myc complex required the HSP-MYC-B element as well as CCAAT in the hsp70 G(1)-enhancer, while the purified CBF subunits recognized only CCAAT even in the presence of c-Myc. Both the HSP-MYC-B and CCAAT elements were also required for the enhancer activity. In transient transfection experiments, the CBF/NF-Y.c-Myc complex, as well as transcription due to the G(1)-enhancer, was increased by the introduction of c-Myc at low doses but decreased at high doses. The repression of both complex formation and transcription by c-Myc at high doses was abrogated by the introduction of CBF/NF-Y in a dose-dependent manner. Furthermore, the CBF/NF-Y.c-Myc complex bound to the G(1)-enhancer appeared in the early G(1) phase of the cell cycle when c-Myc was not higly expressed and gradually disappeared after the c-Myc expression reached its maximum. The results indicate that the cell cycle-dependent expression of the hsp70 gene is regulated by the intracellular amount of c-Myc through the complex formation states between CBF/NF-Y and c-Myc.

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.