ARF is not required for apoptosis in Rb mutant mouse embryos

Deregulated E2F Activity as a Cancer-Cell Specific Therapeutic Tool

The transcription factor E2F, the principal target of the tumor suppressor pRB, plays crucial roles in cell proliferation and tumor suppression. In almost all cancers, pRB function is disabled, and E2F activity is enhanced. To specifically target cancer cells, trials have been undertaken to suppress enhanced E2F activity to restrain cell proliferation or selectively kill cancer cells, utilizing enhanced E2F activity. However, these approaches may also impact normal growing cells, since growth stimulation also inactivates pRB and enhances E2F activity. E2F activated upon the loss of pRB control (deregulated E2F) activates tumor suppressor genes, which are not activated by E2F induced by growth stimulation, inducing cellular senescence or apoptosis to protect cells from tumorigenesis. Deregulated E2F activity is tolerated in cancer cells due to inactivation of the ARF-p53 pathway, thus representing a feature unique to cancer cells. Deregulated E2F activity, which activates tumor suppressor genes, is distinct from enhanced E2F activity, which activates growth-related genes, in that deregulated E2F activity does not depend on the heterodimeric partner DP. Indeed, the ARF promoter, which is specifically activated by deregulated E2F, showed higher cancer-cell specific activity, compared to the E2F1 promoter, which is also activated by E2F induced by growth stimulation. Thus, deregulated E2F activity is an attractive potential therapeutic tool to specifically target cancer cells.

p53 in the CNS: Perspectives on Development, Stem Cells, and Cancer

The p53 tumor suppressor potently limits the growth of immature and mature neurons under conditions of cellular stress. Although loss of p53 function contributes to the pathogenesis of central nervous system (CNS) tumors, excessive p53 function is implicated in neural tube defects, embryonic lethality, and neuronal degeneration. Thus, p53 function must be tightly controlled. The anti-proliferative properties of p53 are mediated, in part, through the induction of apoptosis, cell cycle arrest, and senescence. Although there is still much to be learned about the role of p53 in these processes, recent evidence supports exciting new roles for p53 in a wide range of processes, including neural precursor cell self-renewal, differentiation, and cell fate decisions. Understanding the full repertoire of p53 function in CNS development and tumorigenesis may provide us with novel points of therapeutic intervention for human diseases of the CNS.

The E2F1/Rb and p53/MDM2 pathways in DNA repair and apoptosis: understanding the crosstalk to develop novel strategies for prostate cancer radiotherapy

Both the p53- and E2F1-signaling pathways are defective in almost all types of tumors, suggesting very important roles for their signaling networks in regulating the process of tumorigenesis and therapy response. Studies on Radiation Therapy Oncology Group tissue samples have identified aberrant expression of p53, MDM2 (an E3 ubiquitin ligase that targets p53 for proteosomal degradation), and p16 (an upstream regulator of retinoblastoma and hence E2F1 in prostate cancer); abnormal expression of these biomarkers has been associated with clinical outcome after radiotherapy ± androgen deprivation therapy. Although the proapoptotic properties of p53 are well documented, a relatively new aspect of p53 function as an active mediator of prosurvival signaling pathways is now emerging. E2F1 is a transcription factor that possesses both proapoptotic and prosurvival properties. Thus, the role of E2F1 in the process of tumorigenesis versus apoptosis is a contested issue that needs to be resolved. Furthermore, the role of E2F1 in DNA repair is being increasingly recognized. Thus, novel approaches to curb the prosurvival and DNA repair capability of E2F1 while promoting apoptotic function are of interest. In this review, we discuss the challenges involved in targeting the p53/E2F1 pathways and the crosstalk networks, and further propose potential therapeutic strategies for prostate cancer management.

p53 and E2f: partners in life and death

During tumour development cells sustain mutations that disrupt normal mechanisms controlling proliferation. Remarkably, the Rb-E2f and MDM2-p53 pathways are both defective in most, if not all, human tumours, which underscores the crucial role of these pathways in regulating cell cycle progression and viability. A simple interpretation of the observation that both pathways are deregulated is that they function independently in the control of cell fate. However, a large body of evidence indicates that, in addition to their independent effects on cell fate, there is extensive crosstalk between these two pathways, and specifically between the transcription factors E2F1 and p53, which influences vital cellular decisions. This Review discusses the molecular mechanisms that underlie the intricate interactions between E2f and p53.

E2F4 cooperates with pRB in the development of extra-embryonic tissues

The retinoblastoma gene, RB-1, was the first identified tumor suppressor. Rb(-/-) mice die in mid-gestation with defects in proliferation, differentiation and apoptosis. The activating E2F transcription factors, E2F1-3, contribute to these embryonic defects, indicating that they are key downstream targets of the retinoblastoma protein, pRB. E2F4 is the major pRB-associated E2F in vivo, yet its role in Rb(-/-) embryos is unknown. Here we establish that E2f4 deficiency reduced the lifespan of Rb(-/-) embryos by exacerbating the Rb mutant placental defect. We further show that this reflects the accumulation of trophectoderm-like cells in both Rb and Rb;E2f4 mutant placentas. Thus, Rb and E2f4 play cooperative roles in placental development. We used a conditional mouse model to allow Rb(-/-);E2f4(-/-) embryos to develop in the presence of Rb wild-type placentas. Under these conditions, Rb(-/-);E2f4(-/-) mutants survived to birth. These Rb(-/-);E2f4(-/-) embryos exhibited all of the defects characteristic of the Rb and E2f4 single mutants and had no novel defects. Taken together, our data show that pRB and E2F4 cooperate in placental development, but play largely non-overlapping roles in the development of many embryonic tissues.

Tailoring to RB: tumour suppressor status and therapeutic response

The retinoblastoma tumour suppressor (RB) is a crucial regulator of cell-cycle progression that is invoked in response to a myriad of anti-mitogenic signals. It has been hypothesized that perturbations of the RB pathway confer a synonymous proliferative advantage to tumour cells; however, recent findings demonstrate context-specific outcomes associated with such lesions. Particularly, loss of RB function is associated with differential response to wide-ranging therapeutic agents. Thus, the status of this tumour suppressor may be particularly informative in directing treatment regimens.

The mitogen-activated protein/extracellular signal-regulated kinase kinase inhibitor AZD6244 (ARRY-142886) induces growth arrest in melanoma cells and tumor regression when combined with docetaxel

PURPOSE

Disseminated melanoma is highly therapy resistant. The finding that 66% of melanomas harbor the activating BRAF(V600E) mutation has raised expectations for targeting the Ras/RAF/mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK pathway in melanoma. This study addresses the anti-melanoma activity of the MEK inhibitor AZD6244 (ARRY-142886).

EXPERIMENTAL DESIGN

We recently have shown that growing melanoma cells as three-dimensional collagen-implanted spheroids enhances resistance to the MEK inhibitor U0126. Here, we investigated the anti-melanoma activity of AZD6244 in two-dimensional cell culture, the three-dimensional spheroid model, and an in vivo model.

RESULTS

In two-dimensional cell culture, AZD6244 was cytostatic and reduced the growth of melanoma cells in a concentration-dependent fashion through the induction of G(1)-phase cell cycle arrest. In our three-dimensional spheroid model, the effects of AZD6244 were largely cytostatic and reversible, with drug washout leading to spheroid regrowth. Finally, 1205Lu cells were grown as tumor xenografts in severe combined immunodeficient mice. After tumor establishment, mice were dosed twice daily with 0, 10, or 30 mg/kg AZD6244 p.o. AZD6244 treatment decreased phospho-ERK in the tumors and significantly suppressed tumor growth. The original tumors remained viable, suggesting that AZD6244 monotherapy was largely cytostatic, and not proapoptotic in this model. Further studies showed that co-administration of AZD6244 (30 mg/kg) with docetaxel (15 mg/kg) led to tumor regression, indicating the potential for MEK inhibitor/chemotherapy drug combinations.

CONCLUSIONS

Inhibition of MEK is cytostatic as a monotherapy in melanoma, but cytotoxic when combined with docetaxel.

E2f4 is required for normal development of the airway epithelium

The airway epithelium is comprised of specialized cell types that play key roles in protecting the lungs from environmental insults. The cellular composition of the murine respiratory epithelium is established during development and different cell types populate specific regions along the airway. Here we show that E2f4-deficiency leads to an absence of ciliated cells from the entire airway epithelium and the epithelium of the submucosal glands in the paranasal sinuses. This defect is particularly striking in the nasal epithelium of E2f4-/- mice where ciliated cells are replaced by columnar secretory cells that produce mucin-like substances. In addition, in the proximal lung, E2f4 loss causes a reduction in Clara cell marker expression indicating that Clara cell development is also affected. These defects arise during embryogenesis and, in the nasal epithelium, appear to be independent of any changes in cell proliferation, the principal process regulated by members of the E2f family of transcription factors. We therefore conclude that E2f4 is required to determine the appropriate development of the airway epithelium. Importantly, the combination of no ciliated cells and excess mucous cells can account for the chronic rhinitis and increased susceptibility to opportunistic infections that causes the postnatal lethality of E2f4 mutant mice.

Selective requirements for E2f3 in the development and tumorigenicity of Rb-deficient chimeric tissues

The tumor suppressor function of the retinoblastoma protein pRB is largely dependent upon its capacity to inhibit the E2F transcription factors and thereby cell proliferation. Attempts to study the interplay between pRB and the E2Fs have been hampered by the prenatal death of Rb; E2f nullizygous mice. In this study, we isolated Rb; E2f3 mutant embryonic stem cells and generated Rb(-/-); E2f3(-/-) chimeric mice, thus bypassing the lethality of the Rb(-/-); E2f3(-/-) germ line mutant mice. We show that loss of E2F3 has opposing effects on two of the known developmental defects arising in Rb(-/-) chimeras; it suppresses the formation of cataracts while aggravating the retinal dysplasia. This model system also allows us to assess how E2f3 status influences tumor formation in Rb(-/-) tissues. We find that E2f3 is dispensable for the development of pRB-deficient pituitary and thyroid tumors. In contrast, E2f3 inactivation completely suppresses the pulmonary neuroendocrine hyperplasia arising in Rb(-/-) chimeric mice. This hyperproliferative state is thought to represent the preneoplastic lesion of small-cell lung carcinoma. Therefore, our observation highlights a potential role for E2F3 in the early stages of this tumor type.

Reduction of apoptosis in Rb-deficient embryos via Abl knockout

The retinoblastoma protein RB regulates cell proliferation, differentiation and apoptosis. Homozygous knockout of Rb in mice causes embryonic lethality owing to placental defects that result in excessive apoptosis. RB binds to a number of cellular proteins including the nuclear Abl protein and inhibits its tyrosine kinase activity. Ex vivo experiments have shown that genotoxic or inflammatory stress can activate Abl kinase to stimulate apoptosis. Employing the Rb-null embryos as an in vivo model of apoptosis, we have shown that the genetic ablation of Abl can reduce apoptosis in the developing central nervous system and the embryonic liver. These results are consistent with the inhibitory interaction between RB and Abl, and provide in vivo evidence for the proapoptotic function of Abl.

Cell cycle regulation in the developing lens

Regulation of cell proliferation is a critical aspect of the development of multicellular organisms. The ocular lens is an excellent model system in which to unravel the mechanisms controlling cell proliferation during development. In recent years, several cell cycle regulators have been shown to be essential for maintaining normal patterns of lens cell proliferation. Additionally, many growth factor signaling pathways and cell adhesion factors have been shown to have the capacity to regulate lens cell proliferation. Given this complexity, understanding the cross talk between these many signaling pathways and how they are coordinated are important directions for the future.

Apoptosis in lens development and pathology

The ocular lens is a distinct system to study cell death for the following reasons. First, during animal development, the ocular lens is crafted into its unique shape. The crafting processes include cell proliferation, cell migration, and apoptosis. Moreover, the lens epithelial cells differentiate into lens fiber cells through a process, which utilizes the same regulators as those in apoptosis at multiple signaling steps. In addition, introduction of exogenous wild-type or mutant genes or knock-out of the endogenous genes leads to apoptosis of the lens epithelial cells followed by absence of the ocular lens or formation of abnormal lens. Finally, both in vitro and in vivo studies have shown that treatment of adult lens with stress factors induces apoptosis of lens epithelial cells, which is followed by cataractogenesis. The present review summarizes the current knowledge on apoptosis in the ocular lens with emphasis on its role in lens development and pathology.

Modeling cell cycle control and cancer with pRB tumor suppressor

Cancer is a complex syndrome of diseases characterized by the increased abundance of cells that disrupts the normal tissue architecture within an organism. Defining one universal mechanism underlying cancer with the hope of designing a magic bullet against cancer is impossible, largely because there is so much variation between various types of cancer and different individuals. However, we have learned much in past decades about different journeys that a normal cell takes to become cancerous, and that the delicate balance between oncogenes and tumor suppressor is upset, favoring growth and survival of the tumor cell. One of the most important cellular barriers to cancer development is the retinoblastoma tumor suppressor (pRB) pathway, which is inactivated in a wide range of human tumors and controls cell cycle progression via repression of the E2F/DP transcription factor family. Much of the clarity with which we view tumor suppression via pRB is due to our belief in the universality of the cell cycle and our attempts to model tumor pathways in vivo, nowhere so evident as in the multitude of data emerging from mutant mouse models that have been engineered to understand how cell cycle regulators limit growth in vivo and how deregulation of these regulators facilitates cancer development. In spite of this clarity, we have witnessed with incredulity several stunning results in the last 2 years that have challenged the very foundations of the cell cycle paradigm and made us question seriously how important these cell cycle regulators actually are.

E2F1 induces MRN foci formation and a cell cycle checkpoint response in human fibroblasts

Deregulation of the Rb/E2F pathway in human fibroblasts results in an E2F1-mediated apoptosis dependent on Atm, Nbs1, Chk2 and p53. Here, we show that E2F1 expression results in MRN foci formation, which is independent of the Nbs1 interacting region and the DNA-binding domain of E2F1. E2F1-induced MRN foci are similar to irradiation-induced foci (IRIF) that result from double-strand DNA breaks because they correlate with 53BP1 and gammaH2AX foci, do not form in NBS cells, do form in AT cells and do not correlate with cell cycle entry. In fact, we find that in human fibroblasts deregulated E2F1 causes a G1 arrest, blocking serum-induced cell cycle progression, in part through an Nbs1/53BP1/p53/p21(WAF1/CIP1) checkpoint pathway. This checkpoint protects against apoptosis because depletion of 53BP1 or p21(WAF1/CIP1) increases both the rate and extent of apoptosis. Nbs1 and p53 contribute to both checkpoint and apoptosis pathways. These results suggest that E2F1-induced foci generate a cell cycle checkpoint that, with sustained E2F1 activity, eventually yields to apoptosis. Uncontrolled proliferation due to Rb/E2F deregulation as well as inactivation of both checkpoint and apoptosis programs would then be required for transformation of normal cells to tumor cells.

DNA-binding independent cell death from a minimal proapoptotic region of E2F-1

The ability to induce cell cycle progression while evading cell death is a defining characteristic of cancer. Deregulation of E2F is a common event in most human cancers. Paradoxically, this can lead to both cell cycle progression and apoptosis. Although the way in which E2F causes cell cycle progression is well characterized, the pathways by which E2F induces cell death are less well defined. Many of the known mechanisms through which E2F induces apoptosis occur through regulation of E2F target genes. However, mutants of E2F-1 that lack the transactivation domain are still able to induce cell death. To further investigate this activity, we refined a transactivation independent mutant to identify a minimal apoptotic domain. This revealed that only 75 amino acids from within the DNA-binding domain of E2F-1 is sufficient for cell death and that this activity is also present in the DNA-binding domains of E2F-2 and E2F-3. However, analysis of this domain from E2F-1 revealed it does not bind DNA and is consequently unable to transactivate, repress or de-repress E2F target genes. This provocative observation therefore defines a potential new mechanism of death from E2F and opens up new opportunities for inducing cell death in tumours for therapeutic gain.

Role of INK4a locus in normal eye development and cataract genesis

The murine INK4a locus encodes the critical tumor suppressor proteins, p16(INK4a) and p19(ARF). Mice lacking both p16(INK4a) and p19(ARF) (INK4a-/-) in their FVB/NJ genetic backgrounds developed cataracts and microophthalmia. Histopathologically, INK4a-/- mice showed defects in the developmental regression of the hyaloid vascular system (HVS), retinal dysplasia, and cataracts with numerous vacuolations, closely resembling human persistent hyperplastic primary vitreous (PHPV). Ocular defects, such as retinal fold and abnormal migration of lens fiber cells, were observed as early as embryonic day (E) 15.5, thereby resulting in the abnormal differentiation of the lens. We also found that ectopic expression of p16(INK4a) resulted in the induction of gammaF-crystallin, suggesting an important role of INK4a locus during mouse eye development, and also providing insights into the potential genetic basis of human cataract genesis.

E2F1 suppresses skin carcinogenesis via the ARF-p53 pathway

The E2F1 transcription factor, which is deregulated in most human cancers by mutations in the p16-cyclin D-Rb pathway, has both oncogenic and tumor-suppressive properties. This is dramatically illustrated by the phenotype of an E2F1 transgenic mouse model that spontaneously develops tumors in the skin and other epithelial tissues but is resistant to papilloma formation when subjected to a two-stage carcinogenesis protocol. Here, this E2F1 transgenic model was used to further explore the tumor-suppressive property of E2F1. Transgenic expression of E2F1 was found to inhibit ras-driven skin carcinogenesis at the promotion stage independent of the type of promoting agent used. E2F1 transgenic epidermis displayed increased expression of p19(ARF), p53, and p21(Cip1). Inactivation of either p53 or Arf in E2F1 transgenic mice restored sensitivity to two-stage skin carcinogenesis. While Arf inactivation impaired tumor suppression and p21 induction by E2F1, it did not reduce the level of apoptosis observed in E2F1 transgenic mice. Based on these findings, we propose that E2F1 suppresses ras-driven skin carcinogenesis through a nonapoptotic mechanism involving ARF and p53.

The retinoblastoma gene family in cell cycle regulation and suppression of tumorigenesis

Since its discovery in 1986, as the first tumor suppressor gene, the retinoblastoma gene (Rb) has been extensively studied. Numerous biochemical and genetic studies have elucidated in great detail the function of the Rb gene and placed it at the heart of the molecular machinery controlling the cell cycle. As more insight was gained into the genetic events required for oncogenic transformation, it became clear that the retinoblastoma gene is connected to biochemical pathways that are dysfunctional in virtually all tumor types. Besides regulating the E2F transcription factors, pRb is involved in numerous biological processes such as apoptosis, DNA repair, chromatin modification, and differentiation. Further complexity was added to the system with the discovery of p107 and p130, two close homologs of Rb. Although the three family members share similar functions, it is becoming clear that these proteins also have unique functions in differentiation and regulation of transcription. In contrast to Rb, p107 and p130 are rarely found inactivated in human tumors. Yet, evidence is accumulating that these proteins are part of a "tumor-surveillance" mechanism and can suppress tumorigenesis. Here we provide an overview of the knowledge obtained from studies involving the retinoblastoma gene family with particular focus on its role in suppressing tumorigenesis.

Involvement of E2F transcription factor family in cancer

The E2F family of transcription factors is a central modulator of important cellular events, including cell cycle progression, apoptosis and DNA damage response. The role of E2F family members in various human malignancies is yet unclear and may provide vital clues to the diagnosis, prognosis and therapy of cancer patients. In this review we provide a brief but concise overview of E2F function and its putative role in the most common human tumour types.

ASPP1 and ASPP2 are new transcriptional targets of E2F

The E2F family of transcription factors regulates the expression of a number of genes whose products are involved in cell cycle control, DNA replication and apoptosis. We show here that E2F-1 binds in vivo the promoters of ASPP1 and ASPP2 genes, two activators of p53-mediated apoptosis, E2F-1, E2F-2 and E2F-3 all activate the isolated ASPP1 and ASPP2 promoters. Overexpression or deregulation of E2F-1 increased the expression levels of ASPP1 and ASPP2 mRNA and proteins. The identification of ASPP1 and ASPP2 genes as transcriptional targets of E2F provides another mechanism by which E2F cooperates with p53 to induce apoptosis.

Novel link between E2F and p53: proapoptotic cofactors of p53 are transcriptionally upregulated by E2F

The E2F1 transcription factor is a critical downstream target of the tumor suppressor RB. When activated, E2F1 induces cell proliferation. In addition, E2F1 can induce apoptosis via both p53-dependent and p53-independent pathways. A number of E2F-regulated genes, including ARF, ATM and Chk2, contribute to E2F-induced p53 stabilization. However, it is not known how E2F directs p53 activity towards apoptosis rather than growth arrest. We show that E2F1 upregulates the expression of four proapoptotic cofactors of p53--ASPP1, ASPP2, JMY and TP53INP1--through a direct transcriptional mechanism. Adenovirus E1A protein also induces upregulation of these genes, implicating endogenous E2F in this effect. TP53INP1 was shown to mediate phosphorylation of p53 on serine 46. We demonstrate that activation of E2F1 leads to phosphorylation of p53 on serine 46 and this modification is important for E2F1-p53 cooperation in apoptosis. Overall, these data provide novel functional links between RB/E2F pathway and p53-induced apoptosis.

The E2F transcriptional network: old acquaintances with new faces

The E2 factor (E2F) family of transcription factors are downstream targets of the retinoblastoma protein. E2F factors have been known for several years to be important regulators of S-phase entry. Recent studies have improved our understanding of the molecular mechanisms of action used by this transcriptional network. In addition, they have given us an appreciation of the fact that E2F has functions that reach beyond G1/S control and impact cell proliferation in several different ways. The discovery of new family members with unusual properties, the unexpected phenotypes of mutant animals, a diverse collection of biological activities, a large number of new putative target genes and the new modes of transcriptional regulation have all contributed to an increasingly complex view of E2F function. In this review, we will discuss these recent developments and describe how they are beginning to shape a new and revised picture of the E2F transcriptional program.

Selection of novel mediators of E2F1-induced apoptosis through retroviral expression of an antisense cDNA library

The E2F1 transcription factor is an essential mediator of p53-dependent and p53-independent apoptosis as part of an anti-tumour safeguard mechanism. In this study, a functional so-called technical knockout (TKO) approach was applied to Saos-2ERE2F1 cells that conditionally activate E2F1 by the addition of 4-hydroxytamoxifen to search for p53-independent pro-apoptotic E2F1 targets. The approach was based on random inactivation of genes after retroviral transfer of an antisense cDNA library enriched of E2F1-induced genes, followed by the selection of Saos-2ERE2F1 cells that survive in the presence of the apoptotic stimulus. We identified 13 novel E2F1 target genes encoding proteins of known cellular function, including apoptosis and RNA binding. FACS analysis revealed that E2F1-induced apoptosis was significantly attenuated in cell clones containing the antisense cDNA fragments of these genes, demonstrating their participation in E2F1 death pathways. Moreover, inactivation of the target genes resulted in a clear increase of cell viability (>80%) in response to E2F1 activation compared with controls (∼30%). Four genes showed an increase in expression intensity in the presence of cycloheximide, suggesting a direct effect of E2F1 on gene transcription, whereas one gene was identified as an indirect target. Our data provide new insight in the regulation of E2F1-induced apoptosis.

The emerging role of E2F-1 in the DNA damage response and checkpoint control

Genotoxic stress triggers a myriad of cellular responses including cell cycle arrest, stimulation of DNA repair and apoptosis. A central role for the E2F-1 transcription factor in the DNA damage response pathway is gaining support. E2F-1 is phosphorylated by DNA damage responsive protein kinases, which leads to E2F-1 accumulation and the induction of apoptosis. In addition, emerging information suggests that E2F-1 may play a role in the detection and subsequent repair of damaged DNA.

Hepatitis delta virus antigen is methylated at arginine residues, and methylation regulates subcellular localization and RNA replication

Hepatitis delta virus (HDV) contains a circular RNA which encodes a single protein, hepatitis delta antigen (HDAg). HDAg exists in two forms, a small form (S-HDAg) and a large form (L-HDAg). S-HDAg can transactivate HDV RNA replication. Recent studies have shown that posttranslational modifications, such as phosphorylation and acetylation, of S-HDAg can modulate HDV RNA replication. Here we show that S-HDAg can be methylated by protein arginine methyltransferase (PRMT1) in vitro and in vivo. The major methylation site is at arginine-13 (R13), which is in the RGGR motif of an RNA-binding domain. The methylation of S-HDAg is essential for HDV RNA replication, especially for replication of the antigenomic RNA strand to form the genomic RNA strand. An R13A mutation in S-HDAg inhibited HDV RNA replication. The presence of a methylation inhibitor, S-adenosyl-homocysteine, also inhibited HDV RNA replication. We further found that the methylation of S-HDAg affected its subcellular localization. Methylation-defective HDAg lost the ability to form a speckled structure in the nucleus and also permeated into the cytoplasm. These results thus revealed a novel posttranslational modification of HDAg and indicated its importance for HDV RNA replication. This and other results further showed that, unlike replication of the HDV genomic RNA strand, replication of the antigenomic RNA strand requires multiple types of posttranslational modification, including the phosphorylation and methylation of HDAg.

Repression of the Arf tumor suppressor by E2F3 is required for normal cell cycle kinetics

Tumor development is dependent upon the inactivation of two key tumor-suppressor networks, p16(Ink4a)-cycD/cdk4-pRB-E2F and p19(Arf)-mdm2-p53, that regulate cellular proliferation and the tumor surveillance response. These networks are known to intersect with one another, but the mechanisms are poorly understood. Here, we show that E2F directly participates in the transcriptional control of Arf in both normal and transformed cells. This occurs in a manner that is significantly different from the regulation of classic E2F-responsive targets. In wild-type mouse embryonic fibroblasts (MEFs), the Arf promoter is occupied by E2F3 and not other E2F family members. In quiescent cells, this role is largely fulfilled by E2F3b, an E2F3 isoform whose function was previously undetermined. E2f3 loss is sufficient to derepress Arf, triggering activation of p53 and expression of p21(Cip1). Thus, E2F3 is a key repressor of the p19(Arf)-p53 pathway in normal cells. Consistent with this notion, Arf mutation suppresses the activation of p53 and p21(Cip1) in E2f3-deficient MEFs. Arf loss also rescues the known cell cycle re-entry defect of E2f3(-/-) cells, and this correlates with restoration of appropriate activation of classic E2F-responsive genes. Our data also demonstrate a direct role for E2F in the oncogenic activation of Arf. Specifically, we observe recruitment of the endogenous activating E2Fs, E2F1, and E2F3a, to the Arf promoter. Thus, distinct E2F complexes directly contribute to the normal repression and oncogenic activation of Arf. We propose that monitoring of E2F levels and/or activity is a key component of Arf's ability to respond to inappropriate, but not normal cellular proliferation.

Apoptosis associated with deregulated E2F activity is dependent on E2F1 and Atm/Nbs1/Chk2

The retinoblastoma protein (Rb)/E2F pathway links cellular proliferation control to apoptosis and is critical for normal development and cancer prevention. Here we define a transcription-mediated pathway in which deregulation of E2F1 by ectopic E2F expression or Rb inactivation by E7 of human papillomavirus type 16 signals apoptosis by inducing the expression of Chk2, a component of the DNA damage response. E2F1- and E7-mediated apoptosis are compromised in cells from patients with the related disorders ataxia telangiectasia and Nijmegen breakage syndrome lacking functional Atm and Nbs1 gene products, respectively. Both Atm and Nbs1 contribute to Chk2 activation and p53 phosphorylation following deregulation of normal Rb growth control. E2F2, a related E2F family member that does not induce apoptosis, also activates Atm, resulting in phosphorylation of p53. However, we found that the key commitment step in apoptosis induction is the ability of E2F1, and not E2F2, to upregulate Chk2 expression. Our results suggest that E2F1 plays a central role in signaling disturbances in the Rb growth control pathway and, by upregulation of Chk2, may sensitize cells to undergo apoptosis.

E2F1 Uses the ATM Signaling Pathway to Induce p53 and Chk2 Phosphorylation and Apoptosis

The p53 tumor suppressor protein is phosphorylated and activated by several DNA damage-inducible kinases, such as ATM, and is a key effector of the DNA damage response by promoting cell cycle arrest or apoptosis. Deregulation of the Rb-E2F1 pathway also results in the activation of p53 and the promotion of apoptosis, and this contributes to the suppression of tumor development. Here, we describe a novel connection between E2F1 and the ATM DNA damage response pathway. In primary human fibroblasts lacking functional ATM, the ability of E2F1 to induce the phosphorylation of p53 and apoptosis is impaired. In contrast, ATM status has no effect on transcriptional activation of target genes or the stimulation of DNA synthesis by E2F1. Cells containing mutant Nijmegen breakage syndrome protein (NBS1), a component of the Mre11-Rad50 DNA repair complex, also have attenuated p53 phosphorylation and apoptosis in response to E2F1 expression. Moreover, E2F1 induces ATM- and NBS1-dependent phosphorylation of the checkpoint kinase Chk2 at Thr68, a phosphorylation site that stimulates Chk2 activity. Delayed γH2AX phosphorylation and absence of ATM autophosphorylation at Ser1981 suggest that E2F1 stimulates ATM through a unique mechanism that is distinct from agents that cause DNA double-strand breaks. These findings identify new roles for several DNA damage response factors by demonstrating that they also participate in the oncogenic stress signaling pathway between E2F1 and p53.

Additive effect of p53, p21 and Rb deletion in triple knockout primary hepatocytes

Using Cre-Lox technology to inducibly delete Rb from wild-type, p21- and/or p53-deficient primary hepatocytes, we investigated the role of p53, p21 and pRb in the regulation of liver cell proliferation, polyploidization and death. These cellular decisions are critical to maintaining liver cell replacement in disease, and in determining the likelihood of carcinogenesis in chronic liver injury. Clearly, the present study shows a complex interplay between p53, p21 and pRb, which regulates the likelihood of hepatocytes stimulated from quiescence, to proliferate, undergo polyploidy or die. It reveals that these proteins act both in concert and independently, demonstrating that a small set of key cellular players is common to diverse cell decisions of fundamental importance to disease.

E2F3 loss has opposing effects on different pRB-deficient tumors, resulting in suppression of pituitary tumors but metastasis of medullary thyroid carcinomas

The E2F transcription factors are key downstream targets of the retinoblastoma protein (pRB) tumor suppressor. We have previously shown that E2F3 plays a critical role in mediating the mitogen-induced activation of E2F-responsive genes and contributes to both the inappropriate proliferation and the p53-dependent apoptosis that arise in pRB-deficient embryos. Here we show that E2F3 also has a significant effect on the phenotype of tumor-prone Rb(+/-) mice. The absence of E2F3 results in a significant expansion in the life spans of these animals that correlates with a dramatic alteration in the tumor spectrum. E2F3 loss suppresses the development of the pituitary tumors that normally account for the death of Rb(+/-) mice. However, it also promotes the development of medullary thyroid carcinomas yielding metastases at a high frequency. This increased aggressiveness does not seem to result from any change in p53 levels or activity in these tumors. We show that, instead, E2F3 loss leads to an increase in the rate of tumor initiation. Finally, analysis of Rb(+/-); E2f3(+/-) mice shows that this tumor-suppressive function of E2F3 is dose dependent.

Myc and E2F1 induce p53 through p14ARF-independent mechanisms in human fibroblasts

p19ARF is induced in response to oncogene activation or during cellular senescence in mouse embryo fibroblasts, triggering p53-dependent and p53-independent cell cycle arrest and apoptosis. We have studied the involvement of human p14ARF as a regulator of p53 activity in normal human skin fibroblasts (NHFs) or WI38 lung embryonic fibroblasts expressing conditional Myc or E2F1 estrogen receptor fusion proteins. Both Myc and E2F1 activation rapidly induced p53 phosphorylation at Ser-15, p53 protein accumulation, and upregulation of the p53 target genes MDM2 and p21. Activation of E2F1 induced p14ARF mRNA and protein levels. In contrast, Myc activation did not induce any significant increase in p14ARF mRNA or protein levels in neither NHFs nor WI38 fibroblasts within 48 h. Myc and E2F1 induced p53 and cell cycle arrest even after silencing of p14ARF using short-interfering RNA. Treatment with the ATM/ATR kinase inhibitor caffeine prevented p53 accumulation upon activation of Myc or E2F1. Our results indicate that p53 phosphorylation, but not p14ARF, plays a major role for the induction of p53 in response to Myc and E2F1 activation in normal human fibroblasts.

Myc-Mediated Proliferation and Lymphomagenesis, but Not Apoptosis, Are Compromised by E2f1 Loss

Myc and E2f1 promote cell cycle progression, but overexpression of either can trigger p53-dependent apoptosis. Mice expressing an Emu-Myc transgene in B lymphocytes develop lymphomas, the majority of which sustain mutations of either the Arf or p53 tumor suppressors. Emu-Myc transgenic mice lacking one or both E2f1 alleles exhibited a slower onset of lymphoma development associated with increased expression of the cyclin-dependent kinase inhibitor p27(Kip1) and a reduced S phase fraction in precancerous B cells. In contrast, Myc-induced apoptosis and the frequency of Arf and p53 mutations in lymphomas were unaffected by E2f1 loss. Therefore, Myc does not require E2f1 to induce Arf, p53, or apoptosis in B cells, but depends upon E2f1 to accelerate cell cycle progression and downregulate p27(Kip1).

Macrophage migration inhibitory factor deficiency is associated with altered cell growth and reduced susceptibility to Ras-mediated transformation

Macrophage migration inhibitory factor (MIF) has been shown to functionally inactivate the p53 tumor suppressor and to inhibit p53-responsive gene expression and apoptosis. To better understand the role of MIF in cell growth and tumor biology, we evaluated MIF-null embryonic fibroblasts with respect to their immortalization and transformation properties. Although minor deviations in the growth characteristics of MIF(-/-) fibroblasts were observed under normal culture conditions, MIF-deficient cells were growth-impaired following the introduction of immortalizing oncogenes. The growth retardation by the immortalized MIF(-/-) cultures correlated with their reduced susceptibility to Ras-mediated transformation. Our results identify E2F as part of the restraining mechanism that is activated in response to oncogenic signaling and show that the biological consequences of E2F induction in MIF(-/-) fibroblasts vary depending on the p53 status, inducing predominantly G(1) arrest or apoptosis in p53-positive cells. This E2F activity is independent of Rb binding, but contingent on binding DNA. Resistance to oncogenic transformation by MIF(-/-) cells could be overcome by concomitant interference with p53- and E2F-responsive transcriptional control. Our results demonstrate that MIF plays a role in an E2F/p53 pathway that operates downstream of Rb regulation and implicate MIF as a mediator of normal and malignant cell growth.

Conditional mutation of Rb causes cell cycle defects without apoptosis in the central nervous system

Targeted disruption of the retinoblastoma gene in mice leads to embryonic lethality in midgestation accompanied by defective erythropoiesis. Rb(-/-) embryos also exhibit inappropriate cell cycle activity and apoptosis in the central nervous system (CNS), peripheral nervous system (PNS), and ocular lens. Loss of p53 can prevent the apoptosis in the CNS and lens; however, the specific signals leading to p53 activation have not been determined. Here we test the hypothesis that hypoxia caused by defective erythropoiesis in Rb-null embryos contributes to p53-dependent apoptosis. We show evidence of hypoxia in CNS tissue from Rb(-/-) embryos. The Cre-loxP system was then used to generate embryos in which Rb was deleted in the CNS, PNS and lens, in the presence of normal erythropoiesis. In contrast to the massive CNS apoptosis in Rb-null embryos at embryonic day 13.5 (E13.5), conditional mutants did not have elevated apoptosis in this tissue. There was still significant apoptosis in the PNS and lens, however. Rb(-/-) cells in the CNS, PNS, and lens underwent inappropriate S-phase entry in the conditional mutants at E13.5. By E18.5, conditional mutants had increased brain size and weight as well as defects in skeletal muscle development. These data support a model in which hypoxia is a necessary cofactor in the death of CNS neurons in the developing Rb mutant embryo.

ATM is a target for positive regulation by E2F-1

The E2F-1 transcription factor is a critical downstream target of the tumor suppressor, RB. When activated, E2F-1 induces cell proliferation. In addition, deregulation of E2F-1 constitutes an oncogenic stress that can induce apoptosis. The protein kinase ATM is a pivotal mediator of the response to another type of stress, genotoxic stress. In response to ionizing radiation, ATM activates the tumor suppressor p53, a key player in the control of cell growth and viability. We show here that E2F-1 elevates ATM promoter activity and induces an increase in ATM mRNA and protein levels. This is accompanied by an E2F-induced increase in p53 phosphorylation. Expression of the E7 protein of HPV16, which dissociates RB/E2F complexes, also induces the elevation of ATM levels and p53 phosphorylation, implicating endogenous E2F in these phenomena. These data demonstrate that ATM is transcriptionally regulated by E2F-1 and suggest that ATM serves as a novel, ARF-independent functional link between the RB/E2F pathway and p53.

Role of the RB tumor suppressor in cancer

Apart from their coordinated inactivation by DNA tumor viral oncoproteins, the pRB and p53 tumor suppressor pathways were not known to be connected ten years ago. Within the last decade, our appreciation of how these pathways are interconnected has grown substantially. The checks and balances that exist between pRB and p53 involve the regulation of the G1/S transition and its checkpoints, and much of this is under the control of the E2F transcription factor family. Following DNA damage, the p53-dependent induction of p21CIP1 regulates cyclin E/Cdk2 and cyclin A/Cdk2 complexes both of which phosphorylate pRB, leading to E2F-mediated activation. Similarly, E2F1-dependent induction of p19ARF antagonizes the ability of mdm2 to degrade p53, leading to p53 stabilization and potentially p53-mediated apoptosis or cell cycle arrest. From the existing mouse models discussed above, we also know that proliferation, cell death and differentiation of distinct tissues are also intimately linked through entrance and exit from the cell cycle, and thus through pRB and p53 pathways. Virtually all human tumors deregulate either the pRB or p53 pathway, and often times both pathways simultaneously, which is critical for crippling cellular defense against neoplasia. The next decade of cancer research will likely see these two tumor suppressor pathways only merge even more.

A revised picture of the E2F transcriptional network and RB function

New techniques have enhanced our picture of E2F regulation. These studies have shed light on the roles played by individual E2F and retinoblastoma family members and implicate these proteins in processes extending well beyond the G1/S transition. One thorny issue remains: do our current molecular models of E2F and retinoblastoma action explain all of the functions of these proteins in vivo?

E2F1 pathways to apoptosis

The E2F family of transcription factors plays a pivotal role in the regulation of cell proliferation and their activity is often deregulated in human tumors. Recent studies demonstrate that E2F1 can induce both proliferation and apoptosis. E2F1-induced apoptosis occurs via multiple pathways, some of which induce stabilization and activation of the tumor suppressor p53. The pro-apoptotic activity of E2F1 suggests that its deregulation constitutes an oncogenic stress that may target pre-malignant cells to undergo apoptosis, thus preventing tumor development.

E2F1 induces phosphorylation of p53 that is coincident with p53 accumulation and apoptosis

It has been proposed that the E2F1 transcription factor serves as a link between the Rb/E2F proliferation pathway and the p53 apoptosis pathway by inducing the expression of p19ARF, a protein that regulates p53 stability. We find that although p19ARF contributes to p53 accumulation in response to E2F expression, p19ARF is not required for E2F1-mediated apoptosis. E2F1 can signal p53 phosphorylation in the absence of p19ARF, similar to the observed modifications to p53 in response to DNA damage. These modifications are not observed in the absence of p19ARF following expression of E2F2, an E2F family member that does not induce apoptosis in mouse embryo fibroblasts but can induce p19ARF and p53 protein expression. p53 modification is found to be crucial for E2F1-mediated apoptosis, and this apoptosis is compromised when E2F1 is coexpressed with a p53 mutant lacking many N- and C-terminal phosphorylation sites. Additionally, E2F1-mediated apoptosis is abolished in the presence of caffeine, an inhibitor of phosphatidylinositol 3-kinase-related kinases that phosphorylate p53. These findings suggest that p53 phosphorylation is a key step in E2F1-mediated apoptosis and that this modification can occur in the absence of p19ARF.

The RB and p53 pathways in cancer

The life history of cancer cells encompasses a series of genetic missteps in which normal cells are progressively transformed into tumor cells that invade surrounding tissues and become malignant. Most prominent among the regulators disrupted in cancer cells are two tumor suppressors, the retinoblastoma protein (RB) and the p53 transcription factor. Here, we discuss interconnecting signaling pathways controlled by RB and p53, attempting to explain their potentially universal involvement in the etiology of cancer. Pinpointing the various ways by which the functions of RB and p53 are subverted in individual tumors should provide a rational basis for developing more refined tumor-specific therapies.

Sibling rivalry in the E2F family

The E2F transcription factor family determines whether or not a cell will divide by controlling the expression of key cell-cycle regulators. The individual E2Fs can be divided into distinct subgroups that act in direct opposition to one another to promote either cellular proliferation or cell-cycle exit and terminal differentiation. What is the underlying molecular basis of this 'push-me-pull-you' regulation, and what are its biological consequences?