Inhibition of muscle differentiation by the adenovirus E1a protein: repression of the transcriptional activating function of the HLH protein Myf-5

Restoring the Cell Cycle and Proliferation Competence in Terminally Differentiated Skeletal Muscle Myotubes

Terminal differentiation is an ill-defined, insufficiently characterized, nonproliferation state. Although it has been classically deemed irreversible, it is now clear that at least several terminally differentiated (TD) cell types can be brought back into the cell cycle. We are striving to uncover the molecular bases of terminal differentiation, whose fundamental understanding is a goal in itself. In addition, the field has sought to acquire the ability to make TD cells proliferate. Attaining this end would probe the very molecular mechanisms we are trying to understand. Equally important, it would be invaluable in regenerative medicine, for tissues depending on TD cells and devoid of significant self-repair capabilities. The skeletal muscle has long been used as a model system to investigate the molecular foundations of terminal differentiation. Here, we summarize more than 50 years of studies in this field.

Transcriptomic and proteomic analyses reveal new insights into the regulation of immune pathways during adenovirus type 2 infection

Background

Human adenovirus (Ad) infection leads to the changes of host cell gene expression and biosynthetic processes. Transcriptomics in adenovirus type 2 (Ad2)-infected lung fibroblasts (IMR-90) cells has previously been studied using RNA sequencing. However, this study included only two time points (12 and 24 hpi) using constrained 76 bp long sequencing reads. Therefore, a more detailed study of transcription at different phases of infection using an up-graded sequencing technique is recalled. Furthermore, the correlation between transcription and protein expression needs to be addressed.

Results

In total, 3556 unique cellular genes were identified as differentially expressed at the transcriptional level with more than 2-fold changes in Ad2-infected cells as compared to non-infected cells by using paired-end sequencing. Based on the kinetics of the gene expression changes at different times after infection, these RNAs fell into 20 clusters. Among them, cellular genes involved in immune response were highly up-regulated in the early phase before becoming down-regulated in the late phase. Comparison of differentially expressed genes at transcriptional and posttranscriptional levels revealed low correlation. Particularly genes involved in cellular immune pathways showed a negative correlation. Here, we highlight the genes which expose inconsistent expression profiles with an emphasis on key factors in cellular immune pathways including NFκB, JAK/STAT, caspases and MAVS. Different from their transcriptional profiles with up- and down-regulation in the early and late phase, respectively, these proteins were up-regulated in the early phase and were sustained in the late phase. A surprising finding was that the target genes of the sustained activators failed to show response.

Conclusion

There were features common to genes which play important roles in cellular immune pathways. Their expression was stimulated at both RNA and protein levels during the early phase. In the late phase however, their transcription was suppressed while protein levels remained stable. These results indicate that Ad2 and the host cell use different strategies to regulate cellular immune pathways. A control mechanism at the post-translational level must thus exist which is under the control of Ad2.

Electronic supplementary material

The online version of this article (10.1186/s12866-018-1375-5) contains supplementary material, which is available to authorized users.

Effects of a Novel p.A41P Mutation in the Swine Myogenic factor 5(MYF5) Gene on Protein Stabilizing, Muscle Fiber Characteristics and Meat Quality

Myogenic factor 5 (MYF5) plays an important role in regulating skeletal muscle fiber characteristics, consequently affecting meat production and quality. We identified a novel p.A41P mutation in exon1 of the porcine MYF5 gene by direct sequencing. The mutation was predicted to be destabilizing in protein structure based on the resultant amino acid substitution. We estimated the significant substitution effect of p.A41P on the energy stabilization of Myf5 protein structure. Then, we demonstrated that the mutation in Yorkshire population significantly affected muscle fiber type I composition (p<0.05), loin-eye area of lean meat content (p<0.05) and filter-fluid uptake of meat quality (p<0.01). Furthermore, dominant effects significantly influenced total muscle fiber number (p<0.05). This study suggests that the novel p.A41P mutation in porcine MYF5 may be a valuable genetic marker to affect the muscle fiber characteristics and consequently improve meat production quality and quantity.

The p300/CBP family: integrating signals with transcription factors and chromatin

Studies on the mechanisms through which the oncogene products of DNA tumour viruses subvert the physiological processes that control cell proliferation have yielded many important insights into the mammalian cell cycle. In the case of the adenovirus E1a oncoprotein, a number of distinct protein domains are required for it to exert its growth-promoting effects. These domains allow E1a to associate physically with and inactivate cellular proteins that normally restrain proliferation. Recently, a group of E1a-interacting proteins discovered in part through studies on viral oncoproteins has become a major focus of research activity. Members of this family, known as p300/CBP, function to regulate transcription and chromatin, and thereby enable diverse signals, particularly those that facilitate differentiation, to be integrated and coordinated with gene expression. Furthermore, accumulating evidence connects genes encoding p300/CBP with diseases such as cancer.

Intricate gene regulatory networks of helix-loop-helix (HLH) proteins support regulation of bone-tissue related genes during osteoblast differentiation

Helix-loop-helix (HLH) transcription factors are key regulators of neurogenesis, myogenesis and osteogenesis. Here the relative contributions of multiple classes of HLH factors to the expression of bone related genes during osteoblast maturation were compared. We examined the expression of a panel of HLH proteins (e.g., Twist1/2, USF1/2, c-Myc, Id1 approximately 4, E12/47, Stra13) and one Zn finger protein (Snail which recognizes a subset of E-boxes), during osteoblast differentiation and their functional contributions to bone phenotypic gene regulation. While expression of Twist1, Stra13, E12/47 and Snail transcripts remains relatively constant, expression of Twist2 as well as the inhibitory factors Id1, Id2, Id3, and Id4 decreases and USF1 is up-regulated during osteoblastic differentiation of MC3T3 cells. Forced expression of selected HLH transcription factors shows that Myc, Snail and USF factors increase expression of the bone markers osteocalcin (OC) and/or alkaline phosphatase (AP), while E12/47, Twist and Id factors decrease their expression. None of these factors affect Runx2 gene expression. Interestingly, Snail enhances expression of osteoblast markers, while Twist1 and Twist2 factors are cross-regulated and inhibit bone specific gene expression and other HLH proteins (e.g., Id) indirectly. Thus, our data suggest that the integrated activities of negative and positive E-box related regulatory factors control osteoblast differentiation.

VITO-2, a new SID domain protein, is expressed in the myogenic lineage during early mouse embryonic development

MCAT elements and its cognate binding partners, the transcription enhancer factors (TEFs) play important roles in the regulation of expression of several muscle-specific genes. The biological effects of TEFs strongly depend on different co-factors, which might act as co-activators or anti-repressors to enable transcriptional activation of target genes by TEFs. Previously, we have cloned and characterized VITO-1, which acts as a skeletal muscle-specific transcriptional co-activator of TEFs. Here we describe the cloning and expression profile of a related gene, VITO-2 (also termed Vgl-3), which shares a high homology with VITO-1 in the SID domain responsible for interaction with TEFs. During early embryonic and fetal development VITO-2 is mainly expressed in the myogenic lineage with an onset of expression in the myotomes of somites VI at E9.5 slightly later than VITO-1. At later developmental stages VITO-2 is predominantly found in the nervous system. In adult mice VITO-2 was detected in different tissues, including skeletal muscle, heart, kidney, liver and brain, where it was found in cortical and cerebellar neurons as well as in Purkinje cells. The expression of VITO-2 in the mesoderm was repressed by the notch/delta pathway and activated by Myf-5 since Dll-1 mutant showed an aberrant expression of VITO-2 but not VITO-1 in the tail bud and in the caudal neural tube at E10.5 while Myf-5 mutant mice lack expression of VITO-1 and VITO-2 in somites until E10.5.

Phylogenetic and expression analysis of the basic helix-loop-helix transcription factor gene family: genomic approach to cellular differentiation

A phylogenetic analysis of seven different species (human, mouse, rat, worm, fly, yeast, and plant) utilizing all (541) basic helix-loop-helix (bHLH) genes identified, including expressed sequence tags (EST), was performed. A super-tree involving six clades and a structural categorization involving the entire coding sequence was established. A nomenclature was developed based on clade distribution to discuss the functional and ancestral relationships of all the genes. The position/location of specific genes on the phylogenetic tree in relation to known bHLH factors allows for predictions of the potential functions of uncharacterized bHLH factors, including EST's. A genomic analysis using microarrays for four different mouse cell types (i.e. Sertoli, Schwann, thymic, and muscle) was performed and considered all known bHLH family members on the microarray for comparison. Cell-specific groups of bHLH genes helped clarify those bHLH genes potentially involved in cell specific differentiation. This phylogenetic and genomic analysis of the bHLH gene family has revealed unique aspects of the evolution and functional relationships of the different genes in the bHLH gene family.

SIRT1 is involved in glucocorticoid-mediated control of uncoupling protein-3 gene transcription

UCP3 (uncoupling protein-3) is a mitochondrial membrane transporter expressed preferentially in skeletal muscle. UCP3 lowers mitochondrial membrane potential and protects muscle cells against an overload of fatty acids, and it probably reduces excessive production of reactive oxygen species. Accordingly, ucp3 gene transcription is highly sensitive to fatty acid-dependent stimulation and also to other unrelated stress signals. In this study, glucocorticoids are identified as major inducers of ucp3 gene transcription in muscle. Glucocorticoids activate the transcription of the ucp3 gene through a glucocorticoid receptor-binding site in the promoter region. Glucocorticoids are capable of inducing ucp3 gene transcription independently from the myogenic regulatory factor MyoD, in contrast with the transcriptional activation of the ucp3 gene through other nuclear hormone receptors. An interplay of regulatory factors modulates positively (p300) or negatively (histone deacetylases) the action of glucocorticoids on ucp3 gene transcription via histone acetylation or deacetylation processes, respectively. Among them, SIRT1 acts as a major repressor of ucp3 gene expression in response to glucocorticoids. The action of SIRT1 requires its deacetylase activity and results in histone deacetylation in the ucp3 promoter. Moreover, it involves a specific impairment of association of p300 with the glucocorticoid receptor. Agents activating SIRT1, such as resveratrol, repress ucp3 gene expression. The control of SIRT1 activity via the metabolic redox status of the cell points to a novel regulatory pathway of ucp3 gene transcription in response to metabolic and stress signaling in muscle cells.

Impact of the interaction between adenovirus E1A and CtBP on host cell gene expression

In cell lines harbouring inducible adenovirus E1A genes, the cytotoxicity of wild type E1A was manifested by poor and subsiding expression of the E1A protein during prolonged induction. In contrast, cells expressing E1A deleted in the C-terminal binding protein (CtBP)-interaction domain (E1ADeltaCID) demonstrated high levels of expression for extended time. Microarray analyses of host cell gene expression demonstrated that approximately 70% of the regulated genes were increased upon E1A induction and that the majority of E1A-regulated genes were similarly regulated by wild type E1A and E1ADeltaCID. However, for 29 genes, regulation by wild type E1A and E1ADeltaCID were different. Consistent with the altered transforming capacity of E1A unable to bind CtBP, genes involved in tumour cell progression and growth suppression were found among the differently regulated genes. Moreover, promoter sequences of genes up regulated by wild type E1A and/or repressed by E1ADeltaCID demonstrated a higher prevalence of potential binding sites for the CtBP-targeted transcription factors Ets, Ikaros and/or partial differentialEF1/ZEB, suggesting that the failure to block CtBP-repression contributed to the "hyper-transforming" phenotype of E1ADeltaCID. Since E1ADeltaCID also specifically activated host cell gene expression, we find it likely that additional, possibly CtBP-independent, mechanisms contribute to the altered phenotype of E1ADeltaCID-expressing cells.

A cancer-specific transcriptional signature in human neoplasia

The molecular anatomy of cancer cells is being explored through unbiased approaches aimed at the identification of cancer-specific transcriptional signatures. An alternative biased approach is exploitation of molecular tools capable of inducing cellular transformation. Transcriptional signatures thus identified can be readily validated in real cancers and more easily reverse-engineered into signaling pathways, given preexisting molecular knowledge. We exploited the ability of the adenovirus early region 1 A protein (E1A) oncogene to force the reentry into the cell cycle of terminally differentiated cells in order to identify and characterize genes whose expression is upregulated in this process. A subset of these genes was activated through a retinoblastoma protein/E2 viral promoter required factor-independent (pRb/E2F-independent) mechanism and was overexpressed in a fraction of human cancers. Furthermore, this overexpression correlated with tumor progression in colon cancer, and 2 of these genes predicted unfavorable prognosis in breast cancer. A proof of principle biological validation was performed on one of the genes of the signature, skeletal muscle cell reentry-induced (SKIN) gene, a previously undescribed gene. SKIN was found overexpressed in some primary tumors and tumor cell lines and was amplified in a fraction of colon adenocarcinomas. Furthermore, knockdown of SKIN caused selective growth suppression in overexpressing tumor cell lines but not in tumor lines expressing physiological levels of the transcript. Thus, SKIN is a candidate oncogene in human cancer.

Minireview: Cyclin D1: normal and abnormal functions

Cyclin D1 encodes the regulatory subunit of a holoenzyme that phosphorylates and inactivates the retinoblastoma protein and promotes progression through the G1-S phase of the cell cycle. Amplification or overexpression of cyclin D1 plays pivotal roles in the development of a subset of human cancers including parathyroid adenoma, breast cancer, colon cancer, lymphoma, melanoma, and prostate cancer. Of the three D-type cyclins, each of which binds cyclin-dependent kinase (CDK), it is cyclin D1 overexpression that is predominantly associated with human tumorigenesis and cellular metastases. In recent years accumulating evidence suggests that in addition to its original description as a CDK-dependent regulator of the cell cycle, cyclin D1 also conveys cell cycle or CDK-independent functions. Cyclin D1 associates with, and regulates activity of, transcription factors, coactivators and corepressors that govern histone acetylation and chromatin remodeling proteins. The recent findings that cyclin D1 regulates cellular metabolism, fat cell differentiation and cellular migration have refocused attention on novel functions of cyclin D1 and their possible role in tumorigenesis. In this review, both the classic and novel functions of cyclin D1 are discussed with emphasis on the CDK-independent functions of cyclin D1.

VITO-1 is an essential cofactor of TEF1-dependent muscle-specific gene regulation

The expression of several muscle-specific genes is partially or completely regulated by MCAT elements, which bind members of the TEF family of transcription factors. TEF1 itself is unable to activate reporter plasmids bearing TEF1-binding sites, suggesting that additional bridging or co-activating factors are necessary to allow interaction of TEF1 with the transcriptional machinery. In addition, none of the known TEF genes are exclusively expressed in the cardiac or skeletal muscle lineage to account for the muscle-specific expression of MCAT-dependent genes. Here we describe that VITO-1, a new SID (scalloped interaction domain)-containing protein, binds to TEF1 in vitro and strongly stimulates transcription of a MCAT reporter plasmid together with TEF-1. Since VITO-1 is predominantly expressed in the skeletal muscle lineage, it might serve as an essential transcriptional intermediary factor to promote muscle-specific expression via MCAT cis-regulatory elements. Although VITO-1 alone is not sufficient to initiate myogenic conversion of 10T1/2 fibroblastic cells, it enhanced MyoD-mediated myogenic conversion. In addition, interference with VITO-1 expression by siRNA attenuated differentiation of C2C12 muscle cells and MyoD-dependent myogenesis in 10T1/2 cells. We conclude that VITO-1 is a crucial new cofactor of the muscle regulatory programme.

Strategic attack on host cell gene expression during adenovirus infection

To understand the interaction between the virus and its host, we used three sources of cDNA microarrays to examine the expression of 12,309 unique genes at 6 h postinfection of HeLa cells with high multiplicities of adenovirus type 2. Seventy-six genes with significantly changed expression ratios were identified, suggesting that adenovirus only modulates expression of a limited set of cellular genes. Quantitative real-time PCR analyses on selected genes were performed to confirm the microarray results. Significantly, a pronounced transcriptional activation by the promiscuous E1A-289R transcriptional activator was not apparent. Instead, promoter sequences in 45% of the upregulated genes harbored a potential E2F binding site, suggesting that the ability of the amino-terminal domain of E1A to regulate E2F-dependent transcription may be a major pathway for regulation of cellular gene expression. CDC25A was the only upregulated gene directly involved in cell cycle control. In contrast, several genes implicated in cell growth arrest were repressed. The transforming growth factor beta superfamily was specifically affected in the expression of both the upstream ligand and an intracellular regulator. In agreement with previous reports, adenovirus also targeted the innate immune response by downregulating several cytokines, including CLL2, CXCL1, and interleukin-6. Finally, stress response genes encoding GADD45B, ATF3, and TP53AP1 were upregulated. Importantly, we also found a novel countermeasure-activation of the apoptosis inhibitor survivin.

Subcellular compartmentalization of E2F family members is required for maintenance of the postmitotic state in terminally differentiated muscle

Maintenance of cells in a quiescent state after terminal differentiation occurs through a number of mechanisms that regulate the activity of the E2F family of transcription factors. We report here that changes in the subcellular compartmentalization of the E2F family proteins are required to prevent nuclei in terminally differentiated skeletal muscle from reentering S phase. In terminally differentiated L6 myotubes, E2F-1, E2F-3, and E2F-5 were primarily cytoplasmic, E2F-2 was nuclear, whereas E2F-4 became partitioned between both compartments. In these same cells, pRB family members, pRB, p107, and p130 were also nuclear. This compartmentalization of the E2F-1 and E2F-4 in differentiated muscle cells grown in vitro reflected their observed subcellular location in situ. We determined further that exogenous E2F-1 or E2F-4 expressed in myotubes at levels fourfold greater than endogenous proteins compartmentalized identically to their endogenous counterparts. Only when overexpressed at higher levels was inappropriate subcellular location for these proteins observed. At these levels, induction of the E2F-regulated genes, cyclins A and E, and suppression of factors associated with myogenesis, myogenin, and p21(Cip1) was observed. Only at these levels of E2F expression did nuclei in these terminally differentiated cells enter S phase. These data demonstrate that regulation of the subcellular compartmentalization of E2F-family members is required to maintain nuclei in a quiescent state in terminally differentiated cells.

The molecular regulation of myogenesis

Over the past years, several studies have unraveled important mechanisms by which the four myogenic regulatory factors (MRFs: MyoD, Myf-5, myogenin, and MRF4) control the specification and the differentiation of the muscle lineage. Early experiments led to the hypothesis that these factors were redundant and could functionally replace one another. However, recent experiments using in vivo and in vitro models have demonstrated that in fact different aspects of the myogenic program are controlled by different factors in vivo, suggesting that these factors play distinct roles during myogenesis. The activity of the MRFs during proliferation and differentiation of muscle precursor cells has clearly been demonstrated to be dependent on specific cell-cycle control mechanisms as well as distinct interactions with other regulatory molecules, such as the ubiquitously expressed E proteins and several other transcription factors. Furthermore, the observation that the MRFs can recruit chromatin remodeling proteins has shed some light on the mechanisms by which the MRFs activate gene expression. Recently, a functional role for MyoD during satellite cell activation and muscle repair has been identified in vivo, which cannot be substituted for by the other MRFs. This has put forward the hypothesis that these factors also play specific biological roles following muscle injury and repair.

Transcriptional activation of the myogenin gene by MEF2-mediated recruitment of myf5 is inhibited by adenovirus E1A protein

The basic helix-loop-helix (bHLH) transcription factor myogenin plays a crucial role in terminal differentiation of committed myoblasts into mature myocytes. Transcriptional activation of the myogenin gene requires coordinate action of myocyte enhancer factor 2 (MEF2) proteins and the myogenic bHLH regulators, MyoD or Myf5. Here we show that transcription of the myogenin gene in differentiated cells correlates with MEF2 and NF1 binding to their cognate sites in the proximal myogenin promoter but not with binding of Myf5 or MyoD to the E-box. The importance of MEF2 activity was further demonstrated by expression of antisense MEF2 RNA which repressed MEF2 and Myf5-mediated MEF2 site-dependent reporter gene activation and the synergistic transactivation of a myogenin CAT reporter by Myf5 and MEF2. Adenovirus E1A which has previously been shown to specifically interfere with myogenin gene transcription also inhibited the cooperative transactivation by Myf5/MEF2 and MEF2. Consistently, coimmunoprecipitation studies revealed impaired MEF2/Myf5 protein-protein interactions. These results support a model of transcriptional activation and stabilization of myogenin expression in which DNA-bound MEF2 recruits myogenic bHLH factors into an active but E1A-sensitive transcription factor complex.

Regulation of histone acetyltransferases p300 and PCAF by the bHLH protein twist and adenoviral oncoprotein E1A

Histone acetyltransferases (HAT) play a critical role in transcriptional control by relieving repressive effects of chromatin, and yet how HATs themselves are regulated remains largely unknown. Here, it is shown that Twist directly binds two independent HAT domains of acetyltransferases, p300 and p300/CBP-associated factor (PCAF), and directly regulates their HAT activities. The N terminus of Twist is a primary domain interacting with both acetyltransferases, and the same domain is required for inhibition of p300-dependent transcription by Twist. Adenovirus E1A protein mimics the effects of Twist by inhibiting the HAT activities of p300 and PCAF. These findings establish a cogent argument for considering the HAT domains as a direct target for acetyltransferase regulation by both a cellular transcription factor and a viral oncoprotein.

Alteration of myogenic regulatory components in a rat myoblast GLUT 3(-) mutant

Myogenesis is a complex process characterized by both biochemical and morphological differentiation. Recent transfection studies suggested a close relationship between the GLUT 3 transporter and the myogenic ability of rat skeletal L6 myoblast. In this study, the myogenic properties of GLUT 3- mutants were examined. Studies using three different GLUT 3- mutants (D2, D9 and D23) revealed that these mutants were defective not only in the GLUT 3 transporter, but also in the expression of myogenesis-associated genes. The properties of mutant D23 were further characterized. Overexpression of an exogenous functional GLUT 3 transporter was unable to restore the myogenic defects of this mutant. This mutant was subsequently found to be altered in components acting on at least two different sites of the L6 myogenic pathway. Transfection studies revealed that mutant D23 was deficient in component(s) essential for the myogenin promoter activity. The second component was required for the transcription of muscle-specific protein genes, as overexpression of myogenin was unable to rescue the inability of this mutant to express muscle-specific genes and to form myotubes. Mutant D23 was therefore thought to be deficient in a regulatory component which controlled the expression of GLUT 3, myogenin and muscle-specific genes.

Regulation of nerve growth factor and its low-affinity receptor (p75NTR) during myogenic differentiation

In our preceding report, we have shown that nerve growth factor (NGF) and its low-affinity receptor (p75NTR) are expressed in C2C12 myoblasts and downregulated during myogenic differentiation. Furthermore, NGF affects myogenic differentiation and cell growth via p75NTR and downregulation of p75NTR is essential for myogenic differentiation (Seidl et al., 1998). Here we show that NGF and p75NTR are regulated by mechanisms preceding terminal differentiation in myogenic cells. These mechanisms include cell-density phenomena such as cell-cell contact as well as signaling of basic fibroblast growth factor (FGF-2) and its receptor (FGFR1). Downregulation of NGF and p75NTR occurred as a consequence of increasing cell density, an important trigger for the onset of myogenic differentiation. FGF-2 and FGFR1 were shown to be present in C2C12 cells and exogenous FGF-2 induced NGF and p75NTR expression, implying that FGF/FGFR signaling is an upstream regulator of the NGF/p75NTR system. The fact that FGF-2 could suspend yet not abolish density-induced downregulation indicates that cell-cell contact counteracts the FGF effect and ultimately terminates NGF/p75NTR signaling. This evidence, together with the observation that p75NTR expression is suppressed in muscle progenitors, which constitutively express adenovirus E1A proteins and thus lack the competence of myogenic differentiation, underline the important role for the NGF/p75NTR system in the interplay of multiple factors and biological systems that balance myogenic differentiation at the appropriate spatial and temporal level.

MyoD and MEF2A mediate activation and repression of the p75NGFR gene during muscle development

In an effort to clarify transient expression of the NGF low-affinity receptor p75NGFR during muscle development we have focused on the molecular mechanisms involved in the initiation and cessation of p75NGFR gene expression. Using quiescent C3H10T1/2 fibroblast as a tool, we observed that induction of differentiation competence in MyoD-transfected 10T1/2 fibroblasts was accompanied by the initiation of p75NGFR expression. Moreover, we could show that the bHLH transcription factor MyoD itself is a powerful candidate for transcriptional activation of the p75NGFR gene in muscle precursor cells. By means of MyoD-mutants we have found that both the amino terminus of the MyoD molecule as well as the bHLH-region are essential for transcriptional activity on the p75NGFR promoter. The fact that myocyte enhancer factor MEF2A inactivated MyoD-induced p75NGFR promoter activity strongly suggests that cell-specific regulation of the p75NGFR gene might be strictly dependent on the intracellular composition and balance of the appropriate bHLH-transcription factors and their modulators.

The basic domain of myogenic basic helix-loop-helix (bHLH) proteins is the novel target for direct inhibition by another bHLH protein, Twist

In vertebrates, the basic helix-loop-helix (bHLH) protein Twist may be involved in the negative regulation of cellular determination and in the differentiation of several lineages, including myogenesis, osteogenesis, and neurogenesis. Although it has been shown that mouse twist (M-Twist) (i) sequesters E proteins, thus preventing formation of myogenic E protein-MyoD complexes and (ii) inhibits the MEF2 transcription factor, a cofactor of myogenic bHLH proteins, overexpression of E proteins and MEF2 failed to rescue the inhibitory effects of M-Twist on MyoD. We report here that M-Twist physically interacts with the myogenic bHLH proteins in vitro and in vivo and that this interaction is required for the inhibition of MyoD by M-Twist. In contrast to the conventional HLH-HLH domain interaction formed in the MyoD/E12 heterodimer, this novel type of interaction uses the basic domains of the two proteins. While the MyoD HLH domain without the basic domain failed to interact with M-Twist, a MyoD peptide containing only the basic and helix 1 regions was sufficient to interact with M-Twist, suggesting that the basic domain contacts M-Twist. The replacement of three arginine residues by alanines in the M-Twist basic domain was sufficient to abolish both the binding and inhibition of MyoD by M-Twist, while the domain retained other M-Twist functions such as heterodimerization with an E protein and inhibition of MEF2 transactivation. These findings demonstrate that M-Twist interacts with MyoD through the basic domains, thereby inhibiting MyoD.

Transcriptional coactivator p300 stimulates cell type-specific gene expression in cardiac myocytes

Terminal differentiation is characterized by cell cycle arrest and the expression of cell type-specific genes. Previous work has suggested that the p300 family of transcriptional coactivators plays an important role in preventing the re-initiation of DNA synthesis in terminally differentiated cardiac myocytes. In this study, we investigated whether p300 proteins are also involved in the transcriptional activation of cell type-specific genes in these cells. Since p300 function can be abrogated through direct binding by the adenovirus E1A protein, we overexpressed E1A in cardiac myocytes using recombinant adenoviral vectors. The expression of transfected reporter genes driven by alpha- or beta-myosin heavy chain promoters was markedly diminished by expression of the 12 S E1A protein. In contrast, the activity of a promoter derived from the ubiquitously expressed beta-actin gene was affected only modestly. While an E1A mutant unable to bind members of the retinoblastoma family of pocket proteins decreased the activity of alpha- and beta-myosin heavy chain promoters to nearly the same extent as wild type 12 S E1A, transcriptional repression by a mutant defective for p300 binding was severely impaired. Furthermore, overexpression of p300 and, to an even greater extent, p300del33, a mutant lacking residues required for binding by E1A, relieved E1A's repression of beta-myosin heavy chain promoter activity while having no effect on the activity of the beta-actin promoter. Thus, E1A's transcriptional repression of cell type-specific genes in cardiac myocytes is mediated through its binding of p300 proteins, and these proteins appear to be involved in maintaining both cell type-specific gene expression and cell cycle arrest in cardiac myocytes.

Restoration of interferon responses of adenovirus E1A-expressing HT1080 cell lines by overexpression of p48 protein

We have previously shown that both alpha interferon (IFN-alpha) and IFN-gamma signaling pathways are blocked in HeLa cells expressing the adenovirus E1A proteins (G. T. Leonard and G. C. Sen, Virology 224:25-33, 1996). Here, we report that in two other E1A-expressing cell lines derived from the HT1080 cells, neither IFN-alpha nor IFN-gamma could induce the transcription of genes containing the IFN-stimulated response element (ISRE). In contrast, IFN-gamma-mediated signaling to the gamma-activated sequence was unimpaired in these cells. This dichotomy was due to a lowered level of functional p48 protein but not of STAT1 protein in the E1A-expressing HT1080 cells. When p48 was overexpressed in those cells by stably transfecting a p48 expression vector, both types of IFN could effectively induce the transcription of ISRE-driven genes. Consequently, IFN-alpha was highly effective in inhibiting the replication of encephelomyocarditis virus in the E1A-expressing cells, which also overexpressed p48. These results reinforce the general conclusion that adenovirus E1A proteins block IFN signaling pathways by lowering the functional levels of one or more components of the trans-acting complexes that activate the transcription of IFN-stimulated genes.

Carcinoembryonic antigen, a human tumor marker, cooperates with Myc and Bcl-2 in cellular transformation

Carcinoembryonic antigen (CEA) is a tumor marker that is overexpressed in many human cancers and functions in vitro as a homotypic intercellular adhesion molecule. We have investigated the possibility of synergy between CEA, v-Myc, and Bcl-2 in the transformation of cells with differentiation capacity. We find that v-Myc increases the cell division rate and maximum density of rat L6 myoblasts but also markedly stimulates both apoptosis and surprisingly, differentiation, thus preventing transformation. The superposition of Bcl-2 blocks the apoptotic stimulation of v-Myc and independently promotes further cell division at confluence, but still allows differentiation. The further expression of CEA has a dominant effect in blocking differentiation, regardless of the presence of the other activated oncogenes, generating cells that enter a reversible quiescent G0-like state in medium promoting differentiation. Transfectants expressing CEA with or without v-myc and bcl-2 allow the emergence of cells with the property of heritable, efficient, anchorage-independent growth in soft agar and the ability to markedly reduce the latency for tumor formation in nude mice. We propose that by prolonging cell survival in the presence of differentiation signals, CEA represents a novel class of dominant differentiation-blocking oncogene.

Molecular mechanisms of myogenic coactivation by p300: direct interaction with the activation domain of MyoD and with the MADS box of MEF2C

By searching for molecules that assist MyoD in converting fibroblasts to muscle cells, we have found that p300 and CBP, two related molecules that act as transcriptional adapters, coactivate the myogenic basic-helix-loop-helix (bHLH) proteins. Coactivation by p300 involves novel physical interactions between p300 and the amino-terminal activation domain of MyoD. In particular, disruption of the FYD domain, a group of three amino acids conserved in the activation domains of other myogenic bHLH proteins, drastically diminishes the transactivation potential of MyoD and abolishes both p300-mediated coactivation and the physical interaction between MyoD and p300. Two domains of p300, at its amino and carboxy terminals, independently function to both mediate coactivation and physically interact with MyoD. A truncated segment of p300, unable to bind MyoD, acts as a dominant negative mutation and abrogates both myogenic conversion and transactivation by MyoD, suggesting that endogenous p300 is a required coactivator for MyoD function. The p300 dominant negative peptide forms multimers with intact p300. p300 and CBP serve as coactivators of another class of transcriptional activators critical for myogenesis, myocyte enhancer factor 2 (MEF2). In fact, transactivation mediated by the MEF2C protein is potentiated by the two coactivators, and this phenomenon is associated with the ability of p300 to interact with the MADS domain of MEF2C. Our results suggest that p300 and CBP may positively influence myogenesis by reinforcing the transcriptional autoregulatory loop established between the myogenic bHLH and the MEF2 factors.

p300 is required for MyoD-dependent cell cycle arrest and muscle-specific gene transcription

The nuclear phosphoprotein p300 is a new member of a family of 'co-activators' (which also includes the CREB binding protein CBP), that directly modulate transcription by interacting with components of the basal transcriptional machinery. Both p300 and CBP are targeted by the adenovirus E1A protein, and binding to p300 is required for E1A to inhibit terminal differentiation in both keratinocytes and myoblasts. Here we demonstrate that, in differentiating skeletal muscle cells, p300 physically interacts with the myogenic basic helix-loop-helix (bHLH) regulatory protein MyoD at its DNA binding sites. During muscle differentiation, MyoD plays a dual role: besides activating muscle-specific transcription, it induces permanent cell cycle arrest by up-regulating the cyclin-dependent kinase inhibitor p21. We show that p300 is involved in both these activities. Indeed, E1A mutants lacking the ability to bind p300 are greatly impaired in the repression of E-box-driven transcription, and p300 overexpression rescues the wild-type E1A-mediated repression. Moreover, p300 potentiates MyoD- and myogenin-dependent activation of transcription from E-box-containing reporter genes. We also provide evidence, obtained by microinjection of anti-p300 antibodies, that p300 is required for MyoD-dependent cell cycle arrest in either myogenic cells induced to differentiate or in MyoD-converted C3H10T1/2 fibroblasts, but is dispensable for maintenance of the postmitotic state of myotubes.

Cyclin-mediated inhibition of muscle gene expression via a mechanism that is independent of pRB hyperphosphorylation

It was recently demonstrated that ectopic expression of cyclin D1 inhibits skeletal muscle differentiation and, conversely, that expression of cyclin-dependent kinase (cdk) inhibitors facilitates activation of this differentiation program (S. S. Rao, C. Chu, and D. S. Kohtz, Mol. Cell. Biol. 14:5259-5267, 1994; S. S. Rao and D. S. Kohtz, J. Biol. Chem. 270:4093-4100, 1995; S. X. Skapek, J. Rhee, D. B. Spicer, and A. B. Lassar, Science 267:1022-1024, 1995). Here we demonstrate that cyclin D1 inhibits muscle gene expression without affecting MyoD DNA binding activity. Ectopic expression of cyclin D1 inhibits muscle gene activation by both MyoD and myogenin, including a mutated form of myogenin in which two potential inhibitory cdk phosphorylation sites are absent. Because the retinoblastoma gene product, pRB, is a known target for cyclin D1-cdk phosphorylation, we determined whether cyclin D1-mediated inhibition of myogenesis was due to hyperphosphorylation of pRB. In pRB-deficient fibroblasts, the ability of MyoD to activate the expression of muscle-specific genes requires coexpression of ectopic pRB (B. G. Novitch, G. J. Mulligan, T. Jacks, and A. B. Lassar, J. Cell Biol., 135:441-456, 1996). In these cells, the expression of cyclins A and E can lead to pRB hyperphosphorylation and can inhibit muscle gene expression. The negative effects of cyclins A or E on muscle gene expression are, however, reversed by the presence of a mutated form of pRB which cannot be hyperphosphorylated. In contrast, cyclin D1 can inhibit muscle gene expression in the presence of the nonhyperphosphorylatable form of pRB. On the basis of these results we propose that G1 cyclin-cdk activity blocks the initiation of skeletal muscle differentiation by two distinct mechanisms: one that is dependent on pRB hyperphosphorylation and one that is independent of pRB hyperphosphorylation.

Skeletal muscle cells lacking the retinoblastoma protein display defects in muscle gene expression and accumulate in S and G2 phases of the cell cycle

Viral oncoproteins that inactivate the retinoblastoma tumor suppressor protein (pRb) family both block skeletal muscle differentiation and promote cell cycle progression. To clarify the dependence of terminal differentiation on the presence of the different pRb-related proteins, we have studied myogenesis using isogenic primary fibroblasts derived from mouse embryos individually deficient for pRb, p107, or p130. When ectopically expressed in fibroblasts lacking pRb, MyoD induces an aberrant skeletal muscle differentiation program characterized by normal expression of early differentiation markers such as myogenin and p21, but attenuated expression of late differentiation markers such as myosin heavy chain (MHC). Similar defects in MHC expression were not observed in cells lacking either p107 or p130, indicating that the defect is specific to the loss of pRb. In contrast to wild-type, p107-deficient, or p130-deficient differentiated myocytes that are permanently withdrawn from the cell cycle, differentiated myocytes lacking pRb accumulate in S and G2 phases and express extremely high levels of cyclins A and B, cyclin-dependent kinase (Cdk2), and Cdc2, but fail to readily proceed to mitosis. Administration of caffeine, an agent that removes inhibitory phosphorylations on inactive Cdc2/cyclin B complexes, specifically induced mitotic catastrophe in pRb-deficient myocytes, consistent with the observation that the majority of pRb-deficient myocytes arrest in S and G2. Together, these findings indicate that pRb is required for the expression of late skeletal muscle differentiation markers and for the inhibition of DNA synthesis, but that a pRb-independent mechanism restricts entry of differentiated myocytes into mitosis.

Interaction and functional collaboration of p300/CBP and bHLH proteins in muscle and B-cell differentiation

Differentiation of skeletal muscle cells and B lymphocytes is regulated by basic helix-loop-helix (bHLH) proteins. Both differentiation programs are inhibited by the adenovirus E1A oncoprotein. Analysis of E1A mutants has implicated two of its cellular-binding proteins, p300 and CBP, in controlling certain aspects of differentiation. We find that p300 can cooperate with tissue-specific bHLH proteins in activating target genes and requires only the bHLH domain of such proteins to stimulate E box-directed transcription. Importantly, the ability of bHLH proteins to activate transcription correlates with the presence of p300/CBP in E box-dependent DNA-binding complexes, because both phenomena require at least two adjacent E-box motifs. Microinjection of p300/CBP antibodies into myoblasts blocks terminal differentiation, cell fusion, and transcriptional activity of myogenic bHLH proteins. These results suggest that the function of p300/CBP is essential for the execution of key aspects of cellular differentiation.

A novel E1A domain mediates skeletal-muscle-specific enhancer repression independently of pRB and p300 binding

The adenovirus E1A oncoprotein completely blocks muscle differentiation and specifically inhibits the transactivating function of myogenic basic helix-loop-helix (bHLH) transcription factors. This inhibition is dependent on the conserved region CR1 of E1A, which also constitutes part of the binding sites for the pocket proteins pRB, p107, and p130 and the transcriptional coactivators p300 and CBP. Here we report a detailed mutational analysis of E1A and the identification of a muscle inhibition motif within CR1. This motif encompasses amino acids 38 to 62 and inhibits Myf-5- or MyoD-mediated activation of myogenin and the muscle creatine kinase gene. Overexpression of this E1A region also inhibits the conversion of 10T1/2 fibroblasts to the myogenic lineage. The sequence motif EPDNEE (amino acids 55 to 60) within CR1 appears to be particularly important, because point mutations of this sequence diminish the E1A inhibitory activity. Interactions of E1A with pRB and with p300 do not seem to be necessary for the muscle-specific enhancer repression, because E1A mutants which lack these interactions still inhibit Myf-5- and MyoD-mediated transactivation. Moreover, overexpression of p300 fails to overcome muscle-specific inhibition by wild-type E1A and mutant E1A protein which lacks pRB binding. Since we have no evidence for direct E1A interaction with bHLH proteins, we propose that E1A may target a necessary cofactor of the muscle-specific bHLH transcription complex.

Induction of p18INK4c and its predominant association with CDK4 and CDK6 during myogenic differentiation

Terminal cell differentiation involves permanent withdrawal from the cell division cycle. The inhibitors of cyclin-dependent kinases (CDKs) are potential molecules functioning to couple cell cycle arrest and cell differentiation. In murine C2C12 myoblast cells, G1 CDK enzymes (CDK2, CDK4, and CDK6) associate with four CDK inhibitors: p18INK4c, p19INK4d, p21, and p27Kip1. During induced myogenesis, p21 and its associated CDK proteins underwent an initial increase followed by a decrease as cells became terminally differentiated. The level of p27 protein gradually increased, but the amount of total associated CDK proteins remained unchanged. p19 protein decreased gradually during differentiation, as did its associated CDK4 protein. In contrast, p18 protein increased 50-fold, from negligible levels in proliferating myoblasts to clearly detectable levels within 8-12 h of myogenic induction. This initial rise was followed by a precipitous increase between 12 and 24 h postinduction, with p18 protein finally accumulating to its highest level in terminally differentiated cells. Induction of p18 correlated with increased and sequential complex formation--first increasing association with CDK6 and then with CDK4 over the course of myogenic differentiation. All of the CDK6 and half of the CDK4 were complexed with p18 in terminally differentiated C2C12 cells as well as in adult mouse muscle tissue. Finally, kinase activity of CDK2 and CDK4 decreases as C2C12 cells differentiate, whereas the CDK6 kinase activity is low in both proliferating myoblasts and differentiated myotubes. Our results indicate that p18 may play a critical role in causing and/or maintaining permanent cell cycle arrest associated with mature muscle formation.

Myocardial infarction as a problem of growth control: cell cycle therapy for cardiac myocytes?

Pump failure after myocardial infarction ultimately can be ascribed, in large part, to the inability of ventricular muscle to regenerate functional mass through cell proliferation. Recent studies using adenoviral gene transfer have provided direct evidence for the operation of two growth-suppressing pathways in cardiac muscle, via "pocket proteins," including the retinoblastoma gene product, and via a less well understood protein, p300. An understanding of molecular mechanisms that confer a virtually irreversible lock to the proliferative cell cycle in "postmitotic" cardiac muscle, together with improved means for delivery of exogenous genes to the heart, suggests the long-term potential for manipulating cardiac growth to achieve a therapeutic benefit.

Defective myogenesis in NFB-s mutant associated with a saturable suppression of MYF5 activity

Myogenic cell lines have proved to be useful tools for investigating the molecular mechanisms that control cellular differentiation. NFB-s is a mutant myogenic cell line which fails to differentiate in vitro, and can repress differentiation in normal myogenic cells when fused to form heterokaryons. The NFB-s cell line was used here to study the molecular mechanisms underlying such myogenic repression. Using muscle-specific reporter genes, we show that NFB-s cells fail to activate fully the muscle differentiation program at a transcriptional level, although muscle-specific transcription can be enhanced by regulators of differentiation such as pertussis toxin. Paradoxically we find that the myogenic regulator myf5 is expressed at constitutively high levels in NFB-s cells, and retains DNA binding activity. Expression plasmids encoding NFB-derived myf5 cDNA can rescue the myogenic phenotype in NFB-s cells, demonstrating that a threshold level of positive regulators must be reached before the myogenic program is activated. Thus, the dominant negative phenotype does not appear to result from defective myf5, but is due to a dosage-dependent saturable mechanism that interferes with myf5 function. These studies demonstrate that the stoichiometric ratio of positive and negative regulators is critical for determining the myogenic differentiation state.

The CtBP binding domain in the adenovirus E1A protein controls CR1-dependent transactivation

The adenovirus E1A-243R protein has the ability to force a resting cell into uncontrolled proliferation by modulating the activity of key targets in cell cycle control. Most of these regulatory mechanisms are dependent on activities mapping to conserved region 1 (CR1) and the non-conserved N-terminal region of E1A. We have previously shown that CR1 functions as a very patent transactivator when it is tethered to a promoter through a heterologous DNA binding domain. However, artificial DNA binding was not sufficient to convert full-length E1A-243R to a transactivator. Thus, an additional function(s) of the E1A-243R protein modulates the effect of CR1 in transcription regulation. Here we demonstrate that a 44 amino acid region at the extreme C-terminus of ElA inhibited transactivation by a Gal4-CR1 fusion protein. Inhibition correlated with binding of the nuclear 48 kDa C-terminal binding protein (CtBP), which has been implicated in E1A-mediated suppression of the metastazing potential of tumour cells. This might suggest that CtBP binding can regulate E1A-mediated transformation by modulating CR1-dependent control of transcription.

Induction of DNA synthesis and apoptosis in cardiac myocytes by E1A oncoprotein

Beginning during the second half of gestation, increasing numbers of cardiac myocytes withdraw from the cell cycle such that DNA synthesis is no longer detectable in these cells by neonatal day 17 in vivo. The mechanisms that exclude these and other terminally differentiated cells from the cell division cycle are poorly understood. To begin to explore the molecular basis of the barrier to G1/S progression in cardiac myocytes, we used adenoviruses to express wild-type and mutant E1A proteins in primary cultures from embryonic day 20 rats. While most of these cardiac myocytes are ordinarily refractory to DNA synthesis, even in the presence of serum growth factors, expression of wild-type E1A stimulates DNA synthesis in up to 94% or almost all successfully transduced cells. Rather than complete the cell cycle, however, these cells undergo apoptosis. Apoptosis is limited to those cells that engage in DNA synthesis, and the kinetics of the two processes suggest that DNA synthesis precedes apoptosis. Mutations in E1A that disable it from binding Rb and related pocket proteins have little effect on its ability to stimulate DNA synthesis in cardiac myocytes. In contrast, mutants that are defective in binding the cellular protein p300 stimulate DNA synthesis 2.4-4.1-fold less efficiently, even in the context of retained E1A pocket protein binding. In the absence of ElA pocket protein binding, the usual situation in the cell, loss of p300 binding severely decreases the ability of ElA to stimulate DNA synthesis. These results suggest that the barrier to G1/S progression in cardiac myocytes is mediated. at least in part, by the same molecules that gate the G1/S transition in actively cycling cells, and that p300 or related family members play an important role in this process.

The inhibition of cultured myoblast differentiation by the simian virus 40 large T antigen occurs after myogenin expression and Rb up-regulation and is not exerted by transformation-competent cytoplasmic mutants

We have investigated the mechanism by which the simian virus 40 large T antigen (SVLT) interferes with the differentiation of C2 myoblasts. SVLT mutants, defective either in the Rb binding site, near the N-terminal end, in a region that affects binding to p53, or in the nuclear transport signal, were also employed to determine whether the interference was especially dependent on these functional domains. It was found that wild-type (wt) SVLT strongly inhibited the terminal differentiation of mouse C2 myoblasts, but this arrest occurred only after the synthesis of myogenin, an initial step in biochemical differentiation. Neither the synthesis nor some basic activities of MyoD appeared to be affected by wt SVLT. In these transformants, mitogen depletion elicited an increase in the Rb level comparable to that in normal C2 cells; wt SVLT, however, promoted the phosphorylation of a large part of the induced Rb. Mutations affecting nuclear transport were far more critical for the ability to interfere with myogenic differentiation than were those affecting the transforming potential; cytoplasmic SVLT expression was fully compatible with the terminal differentiation of C2 cells, despite enabling them to grow in semisolid medium, thus showing that the myogenesis-inhibiting property can be dissociated from transforming competence. The remaining SVLT mutants presented different degrees of ability to inhibit differentiation (as shown by the expression of tissue-specific markers in transformants). The inhibiting mutants, including the Rb binding site mutant, were able to promote a higher state of Rb phosphorylation than that observed in either normal cells or cytoplasmic-SVLT transformants.

FER-1, an enhancer of the ferritin H gene and a target of E1A-mediated transcriptional repression

Ferritin, the major intracellular iron storage protein of eucaryotic cells, is regulated during inflammation and malignancy. We previously reported that transcription of the H subunit of ferritin (ferritin H) is negatively regulated by the adenovirus E1A oncogene in mouse NIH 3T3 fibroblasts (Y. Tsuji, E. Kwak, T. Saika, S. V. Torti, and F. M. Torti, J. Biol. Chem. 268:7270-7275, 1993). To elucidate the mechanism of transcriptional repression of the ferritin H gene by E1A, a series of deletions in the 5' flanking region of the mouse ferritin H gene were constructed, fused to the chloramphenicol acetyltransferase (CAT) gene, and transiently cotransfected into NIH 3T3 cells with an E1A expression plasmid. The results indicate that the E1A-responsive region is located approximately 4.1 kb 5' to the transcription initiation site of the ferritin H gene. Further analyses revealed that a 37-bp region, termed FER-1, is the target of E1A-mediated repression. This region also serves as an enhancer, augmenting ferritin H transcription independently of position and orientation. FER-1 was dissected into two component elements, i.e., a 22-bp dyad symmetry element and a 7-bp AP1-like sequence. Insertion of these DNA sequences into a ferritin H-CAT chimeric gene lacking an E1A-responsive region indicated that (i) the 22-bp dyad symmetry sequence by itself has no enhancer activity, (ii) the AP1-like sequence has moderate enhancer activity which is repressed by E1A, and (iii) the combination of the dyad symmetry element and the AP1-like sequence is required for maximal enhancer activity and repression by E1A. Gel retardation assays and cotransfection experiments with c-fos and c-jun expression vectors suggested that members of the Fos and Jun families bind to the AP1-like element of FER-1 and contribute to its regulation. In addition, gel retardation assays showed that E1A reduces the ability of nuclear proteins to bind to the AP1-like sequence without affecting the levels of nuclear factors that recognize the 22-bp dyad symmetry element. Taken together, these results demonstrate that FER-1 serves as both an enhancer of ferritin H transcription and a target for E1A-mediated repression.

Adenovirus E1A represses cardiac gene transcription and reactivates DNA synthesis in ventricular myocytes, via alternative pocket protein- and p300-binding domains

To examine the potential impact of disrupting "pocket" protein function on cardiac differentiation and growth, we introduced 12 S E1A genes into neonatal ventricular myocytes, by adenoviral gene transfer. In the absence of E1B, E1A was cytotoxic, with features typical of apoptosis. In the presence of E1B, E1A preferentially inhibited transcription of cardiac-restricted alpha-actin promoters, and reactivated DNA synthesis in cardiac myocytes, without cell death. Mutations that abrogate known activities of the amino terminus of E1A, versus conserved region 2, demonstrate that the "pocket" protein- and p300-binding domains each suffice, in the absence of the other, for transcriptional repression and re-entry into S phase.

Melanocyte-specific expression of the human tyrosinase promoter: activation by the microphthalmia gene product and role of the initiator

The tyrosinase gene is expressed specifically in melanocytes and the cells of the retinal pigment epithelium, which together are responsible for skin, hair, and eye color. By using a combination of DNase I footprinting and band shift assays coupled with mutagenesis of specific DNA elements, we examined the requirements for melanocyte-specific expression of the human tyrosinase promoter. We found that as little as 115 bp of the upstream sequence was sufficient to direct tissue-specific expression. This 115-bp stretch contains three positive elements: the M box, a conserved element found in other melanocyte-specific promoters; an Sp1 site; and a highly evolutionarily conserved element located between -14 and +1 comprising an E-box motif and an overlapping octamer element. In addition, two further elements, one positive and one negative, are located between positions -185 and -150 and positions -150 and -115, respectively. We also found that the basic helix-loop-helix factor encoded by the microphthalmia gene, which is essential for melanocyte differentiation, can transactivate the tyrosinase promoter via the M box and the conserved E box located close to the initiator. Since in vitro assays failed to identify any melanocyte-specific DNA-binding activity, the possibility that the specific arrangement of elements within the basal tyrosinase promoter determines melanocyte-specific expression is discussed.

Regulatory mechanisms that coordinate skeletal muscle differentiation and cell cycle withdrawal

Skeletal muscle differentiation entails the coupling of muscle-specific gene expression to terminal withdrawal from the cell cycle. Several models have recently been proposed which attempt to explain how regulated expression and function of myogenic transcription factors ensures that proliferation and differentiation of skeletal muscle cells are mutually exclusive processes.

Melanocyte-specific expression of the human tyrosinase promoter: activation by the microphthalmia gene product and role of the initiator

The tyrosinase gene is expressed specifically in melanocytes and the cells of the retinal pigment epithelium, which together are responsible for skin, hair, and eye color. By using a combination of DNase I footprinting and band shift assays coupled with mutagenesis of specific DNA elements, we examined the requirements for melanocyte-specific expression of the human tyrosinase promoter. We found that as little as 115 bp of the upstream sequence was sufficient to direct tissue-specific expression. This 115-bp stretch contains three positive elements: the M box, a conserved element found in other melanocyte-specific promoters; an Sp1 site; and a highly evolutionarily conserved element located between -14 and +1 comprising an E-box motif and an overlapping octamer element. In addition, two further elements, one positive and one negative, are located between positions -185 and -150 and positions -150 and -115, respectively. We also found that the basic helix-loop-helix factor encoded by the microphthalmia gene, which is essential for melanocyte differentiation, can transactivate the tyrosinase promoter via the M box and the conserved E box located close to the initiator. Since in vitro assays failed to identify any melanocyte-specific DNA-binding activity, the possibility that the specific arrangement of elements within the basal tyrosinase promoter determines melanocyte-specific expression is discussed.

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.

The cellular transcription factor USF cooperates with varicella-zoster virus immediate-early protein 62 to symmetrically activate a bidirectional viral promoter

The mechanisms governing the function of cellular USF and herpesvirus immediate-early transcription factors are subjects of considerable interest. In this regard, we identified a novel form of coordinate gene regulation involving a cooperative interplay between cellular USF and the varicella-zoster virus immediate-early protein 62 (IE 62). A single USF-binding site defines the potential level of IE 62-dependent activation of a bidirectional viral early promoter of the DNA polymerase and major DNA-binding protein genes. We also report a dominant negative USF-2 mutant lacking the DNA-binding domain that permits the delineation of the biological role of both USF-1 and USF-2 in this activation process. The symmetrical stimulation of the bidirectional viral promoter by IE 62 is achieved at concentrations of USF-1 (43 kDa) or USF-2 (44 kDa) already existing in cells. Our observations support the notion that cellular USF can intervene in and possibly target promoters for activation by a herpesvirus immediate-early protein.