Suppression of E1A-mediated transformation by the p50E4F transcription factor

Transcription factor E4F1 as a regulator of cell life and disease progression

E4F transcription factor 1 (E4F1), a member of the GLI-Kruppel family of zinc finger proteins, is now widely recognized as a transcription factor. It plays a critical role in regulating various cell processes, including cell growth, proliferation, differentiation, apoptosis and necrosis, DNA damage response, and cell metabolism. These processes involve intricate molecular regulatory networks, making E4F1 an important mediator in cell biology. Moreover, E4F1 has also been implicated in the pathogenesis of a range of human diseases. In this review, we provide an overview of the major advances in E4F1 research, from its first report to the present, including studies on its protein domains, molecular mechanisms of transcriptional regulation and biological functions, and implications for human diseases. We also address unresolved questions and potential research directions in this field. This review provides insights into the essential roles of E4F1 in human health and disease and may pave the way for facilitating E4F1 from basic research to clinical applications.

Multiple domains in the 50 kDa form of E4F1 regulate promoter-specific repression and E1A trans-activation

The 50 kDa N-terminal product of the cellular transcription factor E4F1 (p50E4F1) mediates E1A289R trans-activation of the adenovirus E4 gene, and suppresses E1A-mediated transformation by sensitizing cells to cell death. This report shows that while both E1A289R and E1A243R stimulate p50E4F1 DNA binding activity, E1A289R trans-activation, as measured using GAL-p50E4F1 fusion proteins, involves a p50E4F1 transcription regulatory (TR) region that must be promoter-bound and is dependent upon E1A CR3, CR1 and N-terminal domains. Trans-activation is promoter-specific, as GAL-p50E4F1 did not stimulate commonly used artificial promoters and was strongly repressive when competing against GAL-VP16. p50E4F1 and E1A289R stably associate in vivo using the p50E4F1 TR region and E1A CR3, although their association in vitro is indirect and paradoxically disrupted by MAP kinase phosphorylation of E1A289R, which stimulates E4 trans-activation in vivo. Multiple cellular proteins, including TBP, bind the p50E4F1 TR region in vitro. The mechanistic implications for p50E4F1 function are discussed.

E4F1 is an atypical ubiquitin ligase that modulates p53 effector functions independently of degradation

p53 is regulated by multiple posttranslational modifications, including Hdm2-mediated ubiquitylation that drives its proteasomal degradation. Here, we identify the p53-associated factor E4F1, a ubiquitously expressed zinc-finger protein first identified as a cellular target of the viral oncoprotein E1A, as an atypical ubiquitin E3 ligase for p53 that modulates its effector functions without promoting proteolysis. E4F1 stimulates oligo-ubiquitylation in the hinge region of p53 on lysine residues distinct from those targeted by Hdm2 and previously described to be acetylated by the acetyltransferase PCAF. E4F1 and PCAF mediate mutually exclusive posttranslational modifications of p53. E4F1-dependent Ub-p53 conjugates are associated with chromatin, and their stimulation coincides with the induction of a p53-dependent transcriptional program specifically involved in cell cycle arrest, and not apoptosis. Collectively, our data reveal that E4F1 is a key posttranslational regulator of p53, which modulates its effector functions involved in alternative cell fates: growth arrest or apoptosis.

The LIM-only protein FHL2 is a negative regulator of E4F1

The E1A-targeted transcription factor E4F1 is a key player in the control of mammalian embryonic and somatic cell proliferation and survival. Mouse embryos lacking E4F die at an early developmental stage, whereas enforced expression of E4F1 in various cell lines inhibits cell cycle progression. E4F1-antiproliferative effects have been shown to depend on its capacity to repress transcription and to interact with pRb and p53. Here we show that full-length E4F1 protein (p120(E4F1)) but not its E1A-activated and truncated form (p50(E4F1)), interacts directly in vitro and in vivo with the LIM-only protein FHL2, the product of the p53-responsive gene FHL2/DRAL (downregulated in rhabdomyosarcoma Lim protein). This E4F1-FHL2 association occurs in the nuclear compartment and inhibits the capacity of E4F1 to block cell proliferation. Consistent with this effect, ectopic expression of FHL2 inhibits E4F1 repressive effects on transcription and correlates with a reduction of nuclear E4F1-p53 complexes. Overall, these results suggest that FHL2/DRAL is an inhibitor of E4F1 activity. Finally, we show that endogenous E4F1-FHL2 complexes form in U2OS cells upon UV-light-induced nuclear accumulation of FHL2.

Interaction of the hepatitis B virus protein HBx with the human transcription regulatory protein p120E4F in vitro

Infection with the hepatitis B virus has been identified as one of the major causes of liver cancer. A large body of experimental work points to a central role for the virally encoded protein HBx in this form of carcinogenesis. HBx is expressed in HBV-infected liver cells and interacts with a wide range of cellular proteins, thereby interfering in cellular processes including cell signaling, cycle regulation and apoptosis. In order to identify possible new targets of the HBx protein, we performed a yeast two-hybrid screen using a truncated protein mini-HBx(18-142) as the bait. In addition to known interacting partners, such as RXR and UVDDB1, we identified several new candidates including the human transcriptional regulatory protein p120E4F, which has been implicated in the regulation of mitosis and the cell cycle. In vitro pull down experiments confirmed the interaction and transcription activation assays in the yeast demonstrated that HBx protein was able to repress GAL4AD-p120E4F-dependent activation of a reporter gene under the control of E4F binding sites found in the adenovirus E4 promoter and the HBV enhancer II region. We also showed that the cysteine residues in HBx are necessary for its interaction with UVDDB1 but not for the interaction with RXR or p120E4F. The possible functional relevance of the interaction between HBx and E4F proteins is discussed in the contexts of cellular transformation and host-virus co-evolution.

Identification of the E1A-regulated transcription factor p120 E4F as an interacting partner of the RASSF1A candidate tumor suppressor gene

Epigenetic inactivation of the candidate tumor suppressor gene RASSF1A is a frequent and critical event in the pathogenesis of many human cancers. The RASSF1A protein contains a Ras association domain, suggesting a role in Ras-like signaling pathways, and has also been implicated in cell cycle progression. However, the preliminary data suggests that the RASSF1A gene product is likely to have multiple functions. To identify novel RASSF1A functions, we have sought to identify interacting proteins by yeast two-hybrid analysis in a human brain cDNA library. We identified the E1A-regulated transcription factor p120(E4F) as a RASSF1A interacting partner in yeast and mammalian cells, and demonstrated that RASSF1A protein and p120(E4F) form a complex in vivo. The interaction between RASSF1A and p120(E4F) was confirmed by both in vitro and in vivo pull downs and coimmunoprecipitation assays. In addition, specific inactivation of RASSF1A by short interfering RNA disrupts binding of RASSF1A to p120(E4F) in coimmunoprecipitation assays. In addition, we demonstrated enhanced G(1) cell cycle arrest and S phase inhibition by propidium iodide staining of p120(E4F) in the presence of RASSF1A. As p120(E4F) has been reported previously to interact with p14ARF, retinoblastoma, and p53, these findings provide an important link between the function of RASSF1A and other major human tumor suppressor genes.

Association of p14ARF with the p120E4F transcriptional repressor enhances cell cycle inhibition

The p14(ARF) tumor suppressor is a key regulator of cellular proliferation and is frequently inactivated in human cancer. This tumor suppressor functions in the p53 and pRb cell cycle regulatory pathways and can effectively activate both pathways to induce growth arrest or cell death. We now report that p14(ARF) forms a complex with the E1A-regulated transcriptional repressor, p120(E4F). p120(E4F) contacts p14(ARF) and p53 to form a ternary complex in vivo and enhances p14(ARF)-induced G(2) cell cycle arrest in a p53-dependent manner. We suggest that the interaction of p14(ARF) and p120(E4F) forms an important link between the p14(ARF) and p53 tumor suppressor proteins, both of which exhibit enhanced cell cycle inhibitory activity in the presence of this transcriptional repressor.

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.

pRB binds to and modulates the transrepressing activity of the E1A-regulated transcription factor p120E4F

The retinoblastoma protein pRB is involved in the transcriptional control of genes essential for cell cycle progression and differentiation. pRB interacts with different transcription factors and thereby modulates their activity by sequestration, corepression, or activation. We report that pRB, but not p107 and p130, binds to and facilitates repression by p120(E4F), a ubiquitously expressed GLI-Kruppel-related protein identified as a cellular target of E1A. The interaction involves two distinct regions of p120(E4F) and the C-terminal part of pRB. In vivo pRB-p120(E4F) complexes can only be detected in growth-arrested cells, and accordingly contain the hypophosphorylated form of pRB. Repression of an E4F-responsive promoter is strongly increased by combined expression of p120(E4F) and pRB, which correlates with pRB-dependent enhancement of p120(E4F) binding activity. Elevated levels of p120(E4F) have been shown to block growth of mouse fibroblasts in G(1). We find this requires pRB, because RB(-/-) fibroblasts are significantly less sensitive to excess p120(E4F).