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

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.

Inhibition of MITF transcriptional activity independent of targeting p300/CBP coactivators

Microphthalmia-associated transcription factor (MITF) activates the expression of melanocyte-specific markers and promotes the survival of embryonic, adult and malignant melanocytes. Although numerous MITF-dependent downstream genes have been identified, the mechanisms by which the MITF activity is coregulated remain elusive. Here we used a non-melanocytic cell line U2-OS as a model in which MITF evokes transcription of a paradigmatic MITF target tyrosinase and show that the adenoviral E1A protein represses the MITF-driven transcription in these cells. The E1A CR1 domain (which alone is insufficient to bind p300) was sufficient for repression, while the N-terminus, through which E1A binds the p300/CBP proteins and other coactivators, was unable to repress. Correspondingly, CR1 inhibited colony formation of MITF-positive, but not MITF-negative, melanoma cells. The repression by CR1 was largely independent of the PCAF-binding motif, previously recognized to be necessary for suppression of muscle-specific enhancer. Interestingly, CR1 conferred transcriptional competence to the MITF-CR1 chimera in which the MITF portion was rendered transcription-deficient. Moreover, MITF mutants defective in binding to p300/CBP in vivo still activated transcription, further supporting a p300/CBP-independent coactivation of MITF targets. MITF is amplified in a subset of melanomas and is thought to be required for sustained proliferation of malignant melanocytes. Our results suggest that understanding how CR1 represses Mitf activity may reveal a route to melanoma therapy.

RB, the conductor that orchestrates life, death and differentiation

The retinoblastoma susceptibility gene was the first tumor suppressor gene identified in humans and the first tumor suppressor gene knocked out by targeted deletion in mice. RB serves as a transducer between the cell cycle machinery and promoter-specific transcription factors, its most documented activity being the repression of the E2F family of transcription factors, which regulate the expression of genes involved in cell proliferation and survival. Recent investigations of RB function suggest that it works as a fundamental regulator to coordinate pathways of cellular growth and differentiation. In this review, we unravel the novel role of an equally important aspect of RB in downregulating the differentiation inhibitor EID-1 during cellular differentiation by teasing apart the signal, which elicit differentiation and limit cell cycle progression, since the molecular mechanisms relating to RB activation of differentiation is much less understood. We review the various roles for RB in differentiation of neurons, muscle, adipose tissue, and the retina. In addition, we provide an update for the current models of the role of RB in cell cycle to entry and exit, extending the view toward chromatin remodeling and expose the dichotomies in the regulation of RB family members. We conclude with a discussion of a novel RB regulatory network, incorporating the dynamic contribution of EID family proteins.

p400 Is Required for E1A to Promote Apoptosis

The adenovirus E1A oncoprotein promotes proliferation and transformation by binding cellular proteins, including members of the retinoblastoma protein family, the p300/CREB-binding protein transcriptional coactivators, and the p400-TRRAP chromatin-remodeling complex. E1A also promotes apoptosis, in part, by engaging the ARF-p53 tumor suppressor pathway. We show that E1A induces ARF and p53 and promotes apoptosis in normal fibroblasts by physically associating with the retinoblastoma protein and a p400-TRRAP complex and that its interaction with p300 is largely dispensable for these effects. We further show that E1A increases p400 expression and, conversely, that suppression of p400 using stable RNA interference reduces the levels of ARF, p53, and apoptosis in E1A-expressing cells. Therefore, whereas E1A inactivates the retinoblastoma protein, it requires p400 to efficiently promote cell death. These results identify p400 as a regulator of the ARF-p53 pathway and a component of the cellular machinery that couples proliferation to cell death.

Tumor suppression activity of adenovirus E1a protein: anoikis and the epithelial phenotype

Adenovirus E1a proteins reverse-transform diverse human tumor cells in culture. This has stimulated interest in the arenas of clinical and basic cancer research. Clinically, cancer gene therapy trials on E1a are in progress, and drug discovery strategies based on E1a are being considered. Biologically, the effect of E1a is unique in that it overrides most or all oncogenic signaling pathways to yield nontumorigenic cells. Apparently, this is a consequence of the ability of E1a to reprogram transcription in tumor cells so as to produce an epithelial phenotype that is refractory to oncogenic growth stimulation. The molecular basis for this effect is emerging.

The human T cell leukemia/lymphotropic virus type 1 Tax protein represses MyoD-dependent transcription by inhibiting MyoD-binding to the KIX domain of p300. A potential mechanism for Tax-mediated repression of the transcriptional activity of basic helix-loop-helix factors

The human T cell leukemia/lymphotropic virus type 1 (HTLV-1) Tax protein strongly activates viral and cellular gene transcription. It mainly functions by interacting with cellular transcription factors and the KIX domain of the p300/CBP coactivators. Tax can also repress the transcription of cellular genes through the basic helix-loop-helix (bHLH) protein family. To investigate the molecular mechanisms of this Tax-mediated inhibition, we analyzed its effect on the transcriptional activity of the myogenic MyoD protein, which was used as a paradigm of bHLH factors. In this study, we show that overexpression of the p300 coactivator in transient transfection assays was sufficient to rescue MyoD repression by Tax. Furthermore, an N-terminal domain of p300 (amino acids 379-654) containing the region of KIX serving as the Tax binding site was found, when overexpressed, to potentiate Tax-mediated transactivation of HTLV-1 proviral as well as MyoD-dependent transcription, and to antagonize the inhibition by Tax of the transcriptional activity of MyoD. These results revealing the presence of an N-terminal MyoD binding site were confirmed by in vitro protein-protein interaction assays that demonstrate that MyoD binds to the KIX domain of p300 and that Tax competes with MyoD binding in a nonreciprocal manner. These observations provide evidence that Tax binding to the KIX domain of p300 prevents bHLH proteins from contacting this N-terminal domain of the coactivator, thus resulting in their transcriptional repression. As bHLH proteins are implicated in many developmental fate decisions, especially during thymopoiesis, Tax-mediated inhibition of their transcriptional activity may contribute to the induction of HTLV-1-linked leukemogenesis.

Long-term fate of terminally differentiated skeletal muscle cells following E1A-initiated cell cycle reactivation

We have previously shown that E1A reactivates the cell cycle in 'irreversibly' growth arrested, terminally differentiated (TD) cells. The molecular events following E1A-mediated reactivation of TD skeletal muscle cells have been extensively investigated. However, the long-term fate of the reactivated cells has not been directly determined. In this paper, E1A is used to reactivate TD myotubes derived from established cell lines or primary myoblasts. We show that the reactivated muscle cells continue proliferating beyond the end of the first cell cycle and progress through at least a second one. Experiments performed with an inducible E1A/estrogen receptor chimera indicate that the reactivated cell cycle is self-sustained, since E1A is exclusively necessary to reactivate TD cells, but is dispensable for both the continuation of the first cycle and the progression into the following one. Finally, we report that E1A-mediated reactivation of muscle cells results in apoptotic cell death that can be delayed by the antiapoptotic, adenoviral E1B 55 kDa oncogene.

p300 collaborates with Sp1 and Sp3 in p21(waf1/cip1) promoter activation induced by histone deacetylase inhibitor

We have reported that histone acetylation induced by trichostatin A (TSA) promotes p21(waf1/cip1) (p21) expression, the GC-box located just upstream of TATA box was responsible for TSA-induced promoter activation, and both Sp1 and Sp3 were the working activator of this GC-box. To understand the molecular pathway from histone acetylation to this Sp1 family factors-mediated promoter activation, we investigated the function of p300, one of the histone acetyltransferase, in the present work. The evidence supporting the linkage between p300 and TSA-induced p21 promoter activation were realized from the following findings: 1) cotransfection of p300 elevated p21 promoter activity, and this elevation was dependent on TSA-responsive GC-box; 2) TSA-induced promoter activation was blocked by the introduction of p300 dominant-negative mutant into cells; and 3) Sp1- or Sp3-mediated activation was also suppressed by this p300 dominant-negative mutant. Our data also suggested that p300 collaborates with Sp1 in a way which is different from that when p300 collaborates with p53 in p21 transcription.

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.

Activation of Xenopus genes required for lateral inhibition and neuronal differentiation during primary neurogenesis

XNGN-1, a member of the neurogenin family of basic helix-loop-helix proteins, plays a critical role in promoting neuronal differentiation in Xenopus embryos. When ectopically expressed, XNGN-1 induces the expression of a set of genes required for neuronal differentiation such as XMyT1 and NeuroD. At the same time, however, XNGN-1 induces the expression of genes that antagonize neuronal differentiation by a process called lateral inhibition. Here, we present evidence that XNGN-1 activates the expression of genes required for differentiation and lateral inhibition by recruiting transcriptional coactivators p300/CBP (CREB-binding protein) or PCAF (p3OO/CBP-associated protein), both of which contain histone acetyltransferase (HAT) activity. Significantly, transcriptional activation of the genes in the lateral inhibitory pathway is less dependent on the HAT activity than is the activation of the genes that mediate differentiation. We propose that this difference enables the genes in the lateral inhibition pathway to be induced prior to the genes that promote differentiation, thus enabling lateral inhibition to establish a negative feedback loop and restrict the number of cells undergoing neuronal differentiation.

p300 functions as a transcriptional coactivator for the TAL1/SCL oncoprotein

Activation of the TAL1 (or SCL) gene, originally identified through its involvement by a recurrent chromosomal translocation, is the most frequent gain-of-function mutation recognized in T-cell acute lymphoblastic leukemia (T-ALL). The TAL1 proteins contain a basic helix - loop - helix (bHLH) motif characteristic of a large family of transcription factors that control transcription from an E box target element as heterodimers with the E2A- and HEB-encoded gene products. Gene knockout studies in mice indicate that this transcription factor is required for embryonic and adult hematopoiesis, and considerable evidence suggests it has specific functions in terminal erythroid differentiation. We investigated whether the broadly expressed nuclear protein p300, known to function as a coactivator for other bHLH proteins involved in cellular differentiation, also interacts with TAL1. p300 was found to coimmunoprecipitate with Tal1 in extracts from murine erythroleukemia (MEL) cells induced to differentiate with dimethylsulfoxide (DMSO), and p300 and Tal1 were observed in a common E box DNA-binding complex in extracts from differentiating MEL cells. p300 also interacted with Tal1 in protein pulldown assays, suggesting this was a direct interaction. Finally, p300 augmented transcription by Tal1 from an E box-containing promoter and by a GAL4-Tal1 fusion from a promoter containing the GAL4 DNA-binding element. Deletion analysis identified the bHLH domain of Tal1 and amino-terminal sequences of p300 as necessary for p300-stimulated transactivation and Tal1-p300 interaction in vitro. These results indicate that recruitment of the transcriptional coactivator p300 can positively regulate TAL1-directed gene expression. The dependence of their interaction in MEL cells on addition of a differentiation inducer suggests, further, that this TAL1-p300 complex may have an important role in terminal erythroid differentiation.

E1A directly binds and regulates the P/CAF acetyltransferase

The P/CAF protein has intrinsic histone acetyltransferase (HAT) activity and is capable of binding the transcriptional co-activator CBP. Here we show that P/CAF can regulate transcription and that this function is independent of its binding to CBP. The HAT domain of P/CAF has transcriptional activation potential in yeast. In mammalian cells P/CAF can stimulate transcription of the RSV promoter, using the activity of its HAT domain. We show that the adenovirus protein E1A targets P/CAF and sequesters its transcriptional activity. Binding of E1A to P/CAF is direct, independent of CBP and requires residues within E1A conserved region 1. We find that the P/CAF binding residues in E1A are within a motif shown to be essential for efficient disruption of myogenesis by E1A. The fact that E1A can directly bind and regulate the activity of P/CAF, independently of its regulation of CBP, highlights an important role for P/CAF in the process of cell differentiation.

The transcriptional co-activator proteins p300 and CBP stimulate adenovirus E1A conserved region 1 transactivation independent of a direct interaction

p300 and CBP are two related transcriptional co-activator proteins required by many cellular transcription factors for activity. The adenovirus E1A protein binds p300 and CBP through its amino-terminus and conserved region (CR) 1. Fusing CR1 to a heterologous DNA-binding domain creates a potent transcriptional activator, suggesting that CR1 might activate transcription by recruiting p300/CBP to the promoter. We show that both p300 and CBP enhances CR1-dependent transactivation. However, this enhancement occurs independently of a direct interaction with E1A and does not correlate with the CR1 activator function.

The epithelial cell default-phenotype hypothesis and its implications for cancer

The expression of epithelial cell adhesion and cytoskeletal genes is orchestrated by an apparently unique set of rules. No tissue-specific transactivator proteins have been found to drive them; only ubiquitous factors are utilized. In non-epithelial cells, they are actively repressed. Moreover, it was recently found that a single protein (adenovirus E1a) coordinately represses non-epithelial genes while inducing epithelial genes. A simple model is offered to explain how epithelial gene expression is coordinated. Under this model, the epithelial cell gene expression program is a transcriptional 'default'; that is, it occurs in the absence of tissue-specific transactivation. Conversion to this default requires only that mesenchymal transactivators are not expressed, or that central 'integrator' proteins are inactive. In their absence, mesenchymal gene expression cannot occur. Moreover, because the repressors cease to be expressed, the epithelial genes are induced. Oncogenes generally cause the breakdown of the epithelial phenotype--generating carcinomas--so genes such as E1a that cause epithelial conversion may prove useful for both understanding and controlling cancer.