A common E2F-1 and p73 pathway mediates cell death induced by TCR activation

Lck Is Required for Activation-Induced T Cell Death after TCR Ligation with Partial Agonists

TCR engagement can induce either T cell proliferation and differentiation or activation-induced T cell death (AICD) through apoptosis. The intracellular signaling pathways that dictate such a disparate fate after TCR engagement have only been partially elucidated. Non-FcR-binding anti-CD3 mAbs induce a partial agonist TCR signaling pattern and cause AICD on Ag-activated, cycling T cells. In this study, we examined TCR signaling during the induction of AICD by anti-CD3 fos, a non-FcR-binding anti-CD3 mAb. This mAb activates Fyn, Lck, and extracellular signal-regulated kinase, and induces phosphorylation of Src-like adapter protein, despite the inability to cause calcium mobilization or TCR polarization. Anti-CD3 fos also fails to effectively activate zeta-associated protein of 70 kDa or NF-kappaB. Using Ag-specific T cells deficient for Fyn or Lck, we provide compelling evidence that activation of Lck is required for the induction of AICD. Our data indicate that a selective and distinct TCR signaling pattern is required for AICD by TCR partial agonist ligands.

Control of apoptosis by p53

The p53 tumor suppressor acts to integrate multiple stress signals into a series of diverse antiproliferative responses. One of the most important p53 functions is its ability to activate apoptosis, and disruption of this process can promote tumor progression and chemoresistance. p53 apparently promotes apoptosis through transcription-dependent and -independent mechanisms that act in concert to ensure that the cell death program proceeds efficiently. Moreover, the apoptotic activity of p53 is tightly controlled, and is influenced by a series of quantitative and qualitative events that influence the outcome of p53 activation. Interestingly, other p53 family members can also promote apoptosis, either in parallel or in concert with p53. Although incomplete, our current understanding of p53 illustrates how apoptosis can be integrated into a larger tumor suppressor network controlled by different signals, environmental factors, and cell type. Understanding this network in more detail will provide insights into cancer and other diseases, and will identify strategies to improve their therapeutic treatment.

p73 is regulated by phosphorylation at the G2/M transition

p73 is a p53 paralog that encodes proapoptotic (transactivation-competent (TA)) and antiapoptotic (dominant negative) isoforms. TAp73 transcription factors mediate cell cycle arrest and/or apoptosis in response to DNA damage and are involved in developmental processes in the central nervous system and the immune system. p73 proteins may also play a role in the regulation of cell growth. Indeed, p73 expression is itself modulated during the cell cycle and TAp73 proteins accumulate in S phase cells. In addition, the function of p73 proteins is also regulated by post-translational modifications and protein-protein interactions in different cellular and pathophysiological contexts. Here we show that p73 is a physiological target of the p34cdc2-cyclin B mitotic kinase complex in vivo. Both p73beta and p73alpha isoforms are hyperphosphorylated in normal mitotic cells and during mitotic arrest induced by microtubule-targeting drugs. p34cdc2-cyclin B phosphorylates and associates with p73 in vivo, which results in a decreased ability of p73 to both bind DNA and activate transcription in mitotic cells. Indeed, p73 is excluded from condensed chromosomes in meta- and anaphase, redistributes throughout the mitotic cytoplasm, and unlike p53, shows no association with centrosomes. Together these results indicate that M phase-specific phosphorylation of p73 by p34cdc2-cyclin B is associated with negative regulation of its transcriptional activating function.

Adenosine deaminase, a key enzyme in DNA precursors control, is a new p73 target

The discovery of the p73 and p63 genes, homologous to p53 tumor suppressor has uncovered a family of transcription factors and widened the scenario of cell cycle control and apoptosis. We have identified a putative p53-responsive element in the human adenosine deaminase (ADA) gene, an important enzyme involved in nucleotide metabolism, the deficit of which causes the inhibition of DNA synthesis and repair. Here, we demonstrate that the ectopic expression of p73 isoforms leads to the ADA gene upregulation, showing for the first time a correlation between p73 and ADA. We found that p73 promotes ADA gene expression following a dNTP unbalance generated by ADA enzyme deficiency and 2'deoxyadenosine accumulation. The abrogation of p73 transcriptional activity by the specific dominant-negative p73DD abolishes ADA induction. By contrast, the ADA gene does not appear to be a functional p53 target in the physiological conditions we tested. On the whole, our results contribute to the emerging picture that p73 could play a different role from p53 in normal growth and development by inducing alternative target genes, which are not shared by p53.

p73 and p63 protein stability: the way to regulate function?

While the p53 homologue p73 has been found to be involved in tumorigenesis, the molecular mechanisms involved in this function are still not fully evident. The presence of two distinct promoters allows the formation of two proteins with opposite effects: while TA-p73 shows pro-apoptotic effects, DeltaN-p73 has an evident anti-apoptotic function. The relative expression of the two proteins is in fact related to the prognosis of several cancers. Since both p73 and p63, the other member of the same family, share the ability to interact with each other, it is important to understand the mechanisms that control the degradation and stability of both proteins, and their relative isoforms. p73 and p63 stability is regulated not only by protein modifications (phosphorylation, acetylation) but also by its degradation in the proteasome. To this end, the interaction with Mdm2, p300/CBP, and SUMO-1 are discussed in details.

Expression of the aldehyde dehydrogenase 2-like gene is controlled by BOB.1/OBF.1 in B lymphocytes

BOB.1/OBF.1 is a lymphocyte-restricted transcriptional coactivator. It binds to the Oct1 and Oct2 transcription factors and increases their transactivation potential. Targeted gene disruption experiments revealed that BOB.1/OBF.1 is critical at different stages of B cell development. A large number of genes expressed in B cells contain octamer motifs in their regulatory regions. However, only few genes have been described so far whose expression is dependent on BOB.1/OBF.1. To understand the molecular basis of BOB.1/OBF.1 function in B cell development, we searched for BOB.1/OBF.1 target genes by expression profiling. We have identified genes both induced and repressed by BOB.1/OBF.1. Using different genetic systems, we demonstrate regulation of a selection of these genes. Identified targets included genes encoding Ahd2-like, AKR1C13, Rbp1, Sdh, Idh2, protocadherin gamma, alpha-catenin, Ptprs, Id3, and Creg. Classification of BOB.1/OBF.1 target genes by function suggests that they affect various aspects of B cell physiology such as cellular metabolism, cell adhesion, and differentiation. To better understand the mechanism of BOB.1/OBF.1 action, we cloned the promoter of the gene encoding Ahd2-like, the gene showing the strongest regulation by BOB.1/OBF.1. This promoter indeed contains a perfect octamer motif. Furthermore, the motif was recognized by the Oct transcription factors as well as BOB.1/OBF.1 in vitro and in vivo, as shown by electromobility shift and chromatin immunoprecipitation assays. Transient transfections confirm that this promoter is activated by BOB.1/OBF.1. Our observations suggest that by regulating genes in different functional pathways, BOB.1/OBF.1 has a widespread effect on B cell development and function.

Apoptotic and mitogenic stimuli inactivate Rb by differential utilization of p38 and cyclin-dependent kinases

Inactivation of the retinoblastoma (Rb) tumor suppressor protein is essential for the G1/S transition during mammalian cell cycle progression. Although Rb is inactivated by phosphorylation by cyclins D and E and their associated kinases during cell cycle progression, we find that Rb is inactivated upon apoptotic stimulation by Fas through the mediation of p38 kinase, independent of cyclins and cyclin-dependent kinases (cdks). Inactivation by p38 kinase coincided with increased phosphorylation of Rb leading to dissociation of E2F and increased transcriptional activity; such p38-mediated changes in Rb function occurred only during Fas stimulation but not mitogenic progression. p38 kinase targets Rb preferentially and had minimal effects on p107 and had no effect on p130 function. We also find that phosphorylation site mutants of Rb (PSM7LP and PSM9-Rb) that cannot be inactivated by cdks can be targeted by Fas and p38 kinase, suggesting that Rb inactivation by these kinases is biochemically and functionally distinct. It appears that Rb inactivation is achieved by different kinase cascades in response to mitogenic and apoptotic signals.

A naturally occurring p73 mutation in a p73-p53 double-mutant lung cancer cell line encodes p73 alpha protein with a dominant-negative function

p73, a close homolog of p53 tumor suppressor, induces growth arrest and apoptosis. However, its role in cancers is controversial because of the rarity of p73 mutations, lack of tumors in p73-knockout mice, and the presence of multiple isotypes, among which Delta N isotypes inhibit the function of TA isotypes. We analyzed three naturally occurring p73 mutants found in lung cancer cell lines, NCI-H1155, DMS 92 and A427. NCI-H1155 is a cell line that has a p73 mutation [p73(G264W)] in the DNA-binding domain, as well as a p53 mutation [p53(R273H)], which is frequently found in human cancers and has a "gain-of-function" characteristic. p73 alpha(G264W) not only lacks transactivation activity itself, but also suppressed the transactivation activity of the wild-type p73 alpha in a dose-dependent manner, indicating that p73 alpha(G264W) is a dominant-negative mutant. p73 alpha(G264W) failed to suppress colony formation. We tested two other mutations, p73(Del418) in DMS 92 and p73(Del603) in A427. Both mutants retained similar levels of transactivation activity and suppression of colony formation to those of wild-type p73. The biological significance of these two mutations is unclear. In NCI-H1155 cells the coexistence of mutations that abrogate the normal functions of p73 and p53 may indicate that each mutation confers an additive growth advantage upon the cells.

deltaNp73 facilitates cell immortalization and cooperates with oncogenic Ras in cellular transformation in vivo

TP73, despite significant homology to TP53, is not a classic tumor suppressor gene, since it exhibits upregulation of nonmutated products in human tumors and lacks a tumor phenotype in p73-deficient mice. We recently reported that an N-terminally truncated isoform, DeltaNp73, is upregulated in breast and gynecological cancers. We further showed that DeltaNp73 is a potent transdominant inhibitor of wild-type p53 and TAp73 in cultured human tumor cells by efficiently counteracting their target gene transactivations, apoptosis, and growth suppression functions (A. I. Zaika et al., J. Exp. Med. 6:765-780, 2002). Although these data strongly suggest oncogenic properties of DeltaNp73, this can only be directly shown in primary cells. We report here that DeltaNp73 confers resistance to spontaneous replicative senescence of primary mouse embryo fibroblasts (MEFs) and immortalizes MEFs at a 1,000-fold-higher frequency than occurs spontaneously. DeltaNp73 cooperates with cMyc and E1A in promoting primary cell proliferation and colony formation and compromises p53-dependent MEF apoptosis. Importantly, DeltaNp73 rescues Ras-induced senescence. Moreover, DeltaNp73 cooperates with oncogenic Ras in transforming primary fibroblasts in vitro and in inducing MEF-derived fibrosarcomas in vivo in nude mice. Wild-type p53 is likely a major target of DeltaNp73 inhibition in primary fibroblasts since deletion of p53 or its requisite upstream activator ARF abrogates the growth-promoting effect of DeltaNp73. Taken together, DeltaNp73 behaves as an oncogene that targets p53 that might explain why DeltaNp73 upregulation may be selected for during tumorigenesis of human cancers.

Cyclin-dependent kinases phosphorylate p73 at threonine 86 in a cell cycle-dependent manner and negatively regulate p73

p73 transcription factors are members of the p53 family and participate in developmental processes and DNA damage response. p73 expression was shown to be regulated during the cell cycle, suggesting that p73 might play a role in cell growth and might be a target for cyclin-dependent kinases. Consistent with this hypothesis, we discovered that p73 interacts physically with various cyclins (A, B, D, and E). Furthermore, cyclin A/CDK1/2, cyclin B/CDK1/2, and cyclin E/CDK2 complexes can phosphorylate multiple p73 isoforms in vitro at threonine 86. A specific antibody directed against phosphorylated Thr86 showed that this site is phosphorylated in vivo and that such phosphorylation is regulated in a cell cycle-dependent manner. Thr86 phosphorylation is induced during S phase and is maximal in the G2/M phase. Accordingly inhibitors of cell growth, such as p16 and serum starvation, reduce Thr86 phosphorylation. Finally, we found that cyclin-dependent kinase (CDK)-dependent Thr86 phosphorylation represses the ability of p73 to induce endogenous p21 expression. Our results demonstrate that p73 proteins are targets of CDK complexes and that phosphorylation on Thr86 by CDKs regulates p73 functions.

Exclusion of c-Abl from the nucleus restrains the p73 tumor suppression function

The p73alpha protein is a functional homolog of the p53 tumor suppressor. Although the TP53 gene is frequently mutated in human cancers, the TP73 gene is rarely inactivated. We have found that p73alpha is highly expressed in a significant fraction of anaplastic thyroid cancer, whereas it is not detectable in normal thyroid epithelial cells or in papillary and follicular thyroid cancer cells. Interestingly, the tumor suppression function of p73alpha is actively restrained in anaplastic thyroid cancer cells. We have also found that c-Abl tyrosine kinase, an activator of p73, is excluded from the nucleus of p73alpha-positive thyroid cancer cells; whereas c-Abl undergoes nuclear-cytoplasmic shuttling in normal thyroid and p73-negative thyroid cancer cells. We constructed an AblNuk-FK506-binding protein (FKBP) fusion protein to enforce the nuclear accumulation of an inducible Abl kinase. Activation of this nuclear AblNuk-FKBP by dimerization with AP20187 in anaplastic thyroid cancer cells increased the levels of p73alpha and p21Cip1 and caused p73-dependent apoptosis. These results suggest subcellular segregation of c-Abl from p73 to be a strategy for disrupting the tumor suppression function of p73alpha.

The requirements for Fas-associated death domain signaling in mature T cell activation and survival

Fas-associated death domain (FADD) is a death domain containing cytoplasmic adapter molecule required for the induction of apoptosis by death receptors. Paradoxically, FADD also plays a crucial role in the development and proliferation of T cells. Using T cells from mice expressing a dominant negative form of FADD (FADDdd), activation with anti-TCR Ab and costimulation or exogenous cytokines is profoundly diminished. This is also seen in wild-type primary T cells transduced with the same transgene, demonstrating that FADD signaling is required in normally differentiated T cells. The defective proliferation does not appear to be related to the early events associated with TCR stimulation. Rather, with a block in FADD signaling, stimulated T cells exhibit a high rate of cell death corresponding to the initiation of cell division. Although CD4 T cells exhibit a moderate deficiency, this effect is most profound in CD8 T cells. In vivo, the extent of this defective accumulation is most apparent; lymphocytic choriomenigitis virus-infected FADDdd-expressing mice completely fail to mount an Ag-specific response. These results show that, in a highly regulated fashion, FADD, and most likely caspases, can transduce either a signal for survival or one that leads directly to apoptosis and that the balance between these opposing outcomes is crucial to adaptive immunity.

Activation-induced cell death in T cells

A properly functioning immune system is dependent on programmed cell death at virtually every stage of lymphocyte development and activity. This review addresses the phenomenon of activation-induced cell death (AICD) in T lymphocytes, in which activation through the T-cell receptor results in apoptosis. AICD can occur in a cell-autonomous manner and is influenced by the nature of the initial T-cell activation events. It plays essential roles in both central and peripheral deletion events involved in tolerance and homeostasis, although it is likely that different forms of AICD proceed via different mechanisms. For example, while AICD in peripheral T cells is often caused by the induction of expression of the death ligand, Fas ligand (CD95 ligand, FasL), it does not appear to be involved in AICD in thymocytes. This and other mechanisms of AICD are discussed. One emerging model that may complement other forms of AICD involves the inducible expression of FasL by nonlymphoid tissues in response to activated T lymphocytes. Induction of nonlymphoid FasL in this manner may serve as a sensing mechanism for immune cell infiltration, which contributes to peripheral deletion.

Differential regulation of E2F1 apoptotic target genes in response to DNA damage

E2F1, a member of the E2F family of transcription factors, in addition to its established proliferative effect, has also been implicated in the induction of apoptosis through p53-dependent and p53-independent pathways. Several genes involved in the activation or execution of the apoptotic programme have recently been shown to be upregulated at the transcriptional level by E2F1 overexpression, including the genes encoding INK4a/ARF, Apaf-1, caspase 7 and p73 (refs 3-5). E2F1 is stabilized in response to DNA damage but it has not been established how this translates into the activation of specific subsets of E2F target genes. Here, we applied a chromatin immunoprecipitation approach to show that, in response to DNA damage, E2F1 is directed from cell cycle progression to apoptotic E2F target genes. We identify p73 as an important E2F1 apoptotic target gene in DNA damage response and we show that acetylation is required for E2F1 recruitment on the P1p73 promoter and for its transcriptional activation.

p53 deficiency and defective mitotic checkpoint in proliferating T lymphocytes increase chromosomal instability through aberrant exit from mitotic arrest

During the proliferation of T cells for successful immune responses against pathogens, the fine regulation of cell cycle is important to the maintenance of T cell homeostasis and the prevention of lymphoproliferative disorders. However, it remains to be elucidated how the cell cycle is controlled at the mitotic phase in proliferating T cells. Here, we show that during the proliferation of primary T cells, the disruption of the mitotic spindle leads to cell-cycle arrest at mitosis and that prolonged mitotic arrest results in not only apoptosis but also the form of chromosomal instability observed in human cancers. It is interesting that in response to spindle damage, the phosphorylation of BubR1, a mitotic checkpoint kinase, was significantly induced in proliferating T cells, and the expression of the dominant-negative mutant of BubR1 compromised mitotic arrest and subsequent apoptosis and thus led to the augmentation of polyploidy formation. We also show that in response to prolonged spindle damage, the expression of p53 but not of p73 was significantly induced. In addition, following sustained mitotic arrest, p53-deficient T cells were found to be more susceptible to polyploidy formation than the wild type. These results suggest that during flourishing immune response, mitotic checkpoint and p53 play important roles in the prevention of chromosomal instability and in the maintenance of the genomic integrity of proliferating T cells.

Function of p73, not of p53, is inhibited by the physical interaction with RACK1 and its inhibitory effect is counteracted by pRB

The newly identified p53-related gene, p73, encodes a nuclear transcription factor. Unlike p53, p73 has various isoforms with different NH(2)- and COOH-terminal tails. p73alpha with the longest COOH-terminal extension is most abundantly expressed in many tissues and cells among those splicing isoforms of p73 and the COOH-terminal region appears to have an autoregulatory function. To isolate and characterize the cellular protein(s) that interacts with the unique COOH-terminal region of p73alpha, we employed a yeast two-hybrid screen with a human fetal brain and 293 cell cDNA libraries. We identified the receptor for activated C kinase (RACK1) as a new member of p73alpha-binding proteins. The interaction was confirmed by coimmunoprecipitation experiments, whereas RACK1 did not interact with p53 or p73beta. Ectopic overexpression of RACK1 in SAOS-2 cells reduced the p73alpha-mediated transcription from the p53/p73-responsive promoters, and inhibited the p73alpha-dependent apoptosis. On the other hand, the p53-dependent transcriptional activation as well as apoptosis was unaffected in the presence of RACK1. Furthermore, we found that pRB physically bound to RACK1, and repressed the RACK1-dependent inhibition of p73alpha. Taken together, our observations suggest that pRB diminishes the RACK1-mediated inhibition of p73alpha activity through the interaction with RACK1.

Regulation of the p53 homolog p73 by adenoviral oncogene E1A

p73 is a p53 homolog, as they are similar structurally and functionally. Unlike p53, p73 is not inactivated by the products of viral oncogenes such as SV40 T antigen and human papilloma virus E6. Here we show that the product of adenoviral oncogene E1A inhibits the transcriptional activation by both p73alpha and p73beta. Electrophoretic mobility shift assays revealed that E1A does not inhibit the sequence-specific DNA binding by p73. Transcriptional activation by a fusion protein containing the Gal4 DNA-binding domain and either of the activation domains of p73 was inhibited by wild-type (WT) E1A, but not by the N-terminal deletion mutant E1A(Delta2-36). E1A(Delta2-36), which does not bind to the p300/CBP family of coactivators, failed to inhibit p73-mediated transcription, whereas E1A(DeltaCR2), a deletion mutant that does not bind to the pRb family of proteins, inhibited p73-mediated transcription as efficiently as WT E1A. Consistent with these observations, growth arrest induced by p73 expressed from a recombinant adenovirus was abrogated by WT E1A, which correlated with inhibition of p73-mediated induction of p21(WAF1/CIP1) by E1A. However, p73 was able to induce p21(WAF1/CIP1) and to mediate growth arrest in the presence of E1A(Delta2-36). Furthermore, the expression of either wild-type E1A or E1A(Delta2-36) resulted in the stabilization of endogenous p73. However, p73 stabilized in response to the expression of E1A(Delta2-36), but not WT E1A, was able to activate the expression of p21(WAF1/CIP1). These results suggest that the transcriptional activation function of p73 is specifically targeted by E1A through a mechanism involving p300/CBP proteins during the process of transformation and that p73 may have a role to play as a tumor suppressor.

Defective gene expression, S phase progression, and maturation during hematopoiesis in E2F1/E2F2 mutant mice

E2F plays critical roles in cell cycle progression by regulating the expression of genes involved in nucleotide synthesis, DNA replication, and cell cycle control. We show that the combined loss of E2F1 and E2F2 in mice leads to profound cell-autonomous defects in the hematopoietic development of multiple cell lineages. E2F2 mutant mice show erythroid maturation defects that are comparable with those observed in patients with megaloblastic anemia. Importantly, hematopoietic defects observed in E2F1/E2F2 double-knockout (DKO) mice appear to result from impeded S phase progression in hematopoietic progenitor cells. During DKO B-cell maturation, differentiation beyond the large pre-BII-cell stage is defective, presumably due to failed cell cycle exit, and the cells undergo apoptosis. However, apoptosis appears to be the consequence of failed maturation, not the cause. Despite the accumulation of hematopoietic progenitor cells in S phase, the combined loss of E2F1 and E2F2 results in significantly decreased expression and activities of several E2F target genes including cyclin A2. Our results indicate specific roles for E2F1 and E2F2 in the induction of E2F target genes, which contribute to efficient expansion and maturation of hematopoietic progenitor cells. Thus, E2F1 and E2F2 play essential and redundant roles in the proper coordination of cell cycle progression with differentiation which is necessary for efficient hematopoiesis.

Chk2 activates E2F-1 in response to DNA damage

The E2F-1 transcription factor is regulated during cell cycle progression and induced by cellular stress, such as DNA damage. We report that checkpoint kinase 2 (Chk2) regulates E2F-1 activity in response to the DNA-damaging agent etoposide. A Chk2 consensus phosphorylation site in E2F-1 is phosphorylated in response to DNA damage, resulting in protein stabilization, increased half-life, transcriptional activation and localization of phosphorylated E2F-1 to discrete nuclear structures. Expression of a dominant-negative Chk2 mutant blocks induction of E2F-1 and prevents E2F-1-dependent apoptosis. Moreover, E2F-1 is resistant to induction by etoposide in tumour cells expressing mutant chk2. Therefore, Chk2 phosphorylates and activates E2F-1 in response to DNA damage, resulting in apoptosis. These results suggest a role for E2F-1 in checkpoint control and provide a plausible explanation for the tumour suppressor activity of E2F-1.

E2F and cell cycle control: a double-edged sword

The E2F family of transcription factors plays a central role in regulating cellular proliferation by controlling the expression of both the genes required for cell cycle progression, particularly DNA synthesis, and the genes involved with apoptosis. E2F is regulated in a cell cycle-dependent manner, principally through its temporal association with pocket protein family members, the prototype member being the retinoblastoma tumor suppressor protein. Pocket proteins are, in turn, regulated through phosphorylation by cyclin-dependent kinase (cdk). The kinase activity of cyclin/cdk complexes is negatively regulated by cdk inhibitors, and thus both positive and negative growth regulatory signals impinge on E2F activity. Different E2F family members exhibit distinct cell cycle and apoptotic activities. Thus, E2F appears to play a pivotal role in coordinating events connected with proliferation, cell cycle arrest, and apoptosis.

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).

The mitochondrial apoptosis-inducing factor plays a role in E2F-1-induced apoptosis in human colon cancer cells

BACKGROUND

Overexpression of the transcription factor E2F-1 provokes apoptosis in cancer cells; the mechanism, however, is not completely understood. We sought to evaluate E2F-1 gene therapy in human colon cancer and to investigate the apoptotic pathway involved.

METHODS

Adenoviral vectors were used to transfect the E2F-1 gene (Ad5E2F-1) or the control gene luciferase (Ad5Luc) into four human colon carcinoma cell lines. Apoptosis was confirmed by flow cytometry and poly (ADP-ribose) polymerase cleavage. Expression of apoptotic factors was determined with Western blot analysis. Inhibitory assays were used to determine the involvement of caspases in the apoptotic pathway.

RESULTS

Overexpression of E2F-1 was evident in all cells treated with Ad5E2F-1; upregulation of Bcl-2, and activation of caspases were noted. The apoptosis-inducing factor in the cytosolic fraction was markedly upregulated after Ad5E2F-1 treatment. E2F-1 overexpression inhibited proliferation and induced significant apoptosis in all cell lines (P <.005). This apoptotic response could be only partially blocked by caspase inhibitors.

CONCLUSIONS

These findings demonstrate that E2F-1 induces apoptosis and inhibits proliferation in human colon cancer cell lines. The marked upregulation of apoptosis-inducing factor and the fact that E2F-1-induced apoptosis is incompletely blocked by caspase inhibitors suggest a caspase-independent pathway of E2F-1-mediated apoptosis, reported here for the first time.

Genetic alterations in ovarian cancer cells that might account for sensitivity to chemotherapy in patients

The cellular response to anticancer agent treatments is determined by many factors that could be altered in tumor cells. The induction of apoptosis of cancer cells is thought to be important for the overall response of these cells. Despite the introduction of new and potent anticancer agents, the survival rate for patients with ovarian cancer remains poor. In general ovarian cancer cells present a poor propensity to undergo apoptosis, which could be one of the reasons for this relatively poor response observed in the clinic. Induction of apoptosis is the result of activation and repression of pro- and antiapoptotic genes, which are regulated by complex mechanisms. Many cancer cells activate a "survival" program to escape disruption and allow propagation. In this review we have analyzed the role of genetic alterations observed in ovarian cancer cells in determining cellular response to drug treatment.

Death and NF-kappaB in T cell activation: life at the edge

Nuclear Factor kappa-B appears to be required to prevent p73-induced apoptosis triggered by primary T cell activation. Here we explore what this state of affairs might do for the regulation of the immune system.

The survival of antigen-stimulated T cells requires NFkappaB-mediated inhibition of p73 expression

We have explored the interactions between the NFkappaB and Cdk-Rb-E2F pathways in controlling T cell fate following antigen stimulation. The inhibition of NFkappaB in antigen-stimulated T cells results in apoptosis but does not inhibit E2F activation and S phase entry. IkappaB-induced apoptosis coincides with the superinduction of p73 expression and activity. G1 Cdk activity is required for IkappaB-induced apoptosis and the induction of p73. Importantly, p73 deficiency rescues activated T cells from the apoptosis resulting from the inhibition of NFkappaB. Thus, Cdk2 activation sends signals for both cell cycle progression and apoptosis, the latter of which must be blocked by NFkappaB to allow for proliferation.

Novel cell-specific and dominant negative anti-apoptotic roles of p73 in transformed leukemia cells

Although extensive homology exists between related genes p53 and p73, recent data suggest that the family members have divergent roles. We demonstrate that the differential regulatory roles of p53 family member p73 are highly cell-context and promoter-specific. Full-length p73 expressed in the transformed leukemia cell line Jurkat behaves as a specific dominant negative transcriptional repressor of the cell cycle inhibitor gene p21 and blocks p53-mediated apoptosis. These findings provide evidence for a new mechanism in oncogenesis through which the functional properties of p73 can be altered in an inheritable and cell-specific fashion independent of transcriptional coding.

Overexpression of p73 causes apoptosis in vascular smooth muscle cells

Abnormal vascular smooth muscle (VSM) cell proliferation contributes to the development of atherosclerosis and its associated disorders, including angioplasty restenosis. The tumor-suppressor protein p53 has been linked to the development of atherosclerotic lesions, and its homolog, p73, is proving to have contrasting functions in a variety of tissues. As an outgrowth of our previous finding that p73 is increased in serum-stimulated VSM cells and human atherosclerotic tissue, we examined p73 overexpression in VSM cells to elucidate causality of p73 expression with growth response. Overexpression of p73 results in decreased cell cycle transit and is accompanied by apoptosis. The apoptotic changes in p73 overexpressing VSM cells are independent of p53 and are associated with a decrease in levels of p21(waf1/cip1). In conjunction with our previous data finding that p73 is increased in serum-stimulated VSM cells, this work suggests a role for p73 in vascular proliferative diseases.

ARF mutation accelerates pituitary tumor development in Rb+/- mice

Mice heterozygous for the retinoblastoma (Rb) tumor suppressor gene develop pituitary and thyroid tumors with high penetrance. We demonstrate here that loss of the ARF tumor suppressor strongly accelerates intermediate lobe pituitary tumorigenesis in Rb heterozygous mice. These effects in the pituitary are greater than those conferred by p53 loss in that Rb+-;ARF-- mice display significantly more early atypical lesions than Rb+-; p53-- mice. Also, Rb+-;ARF-- compound mutants do not develop many of the novel tumors or precancerous lesions seen in Rb+-;p53-- compound mutants. Although complete loss of ARF expression is not obligatory for pituitary tumorigenesis in Rb+- mice, alterations of the ARF locus are observed in tumors from Rb+-;ARF+- mice, consistent with a selective advantage of ARF inactivation in this context. We conclude that inactivation of ARF acts more broadly than that of p53 in connecting abrogation of the Rb pathway to tumorigenesis.

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?

Apaf-1 is a mediator of E2F-1-induced apoptosis

E2F-1 is capable of promoting both cell cycle progression and apoptosis. The latter is important for suppressing untoward expansion of proliferating cells. In this study, we investigated its underlying mechanisms. E2F-1-induced apoptosis was accompanied by caspase-9 activation and inhibited by a specific inhibitor of caspase-9 in K562 sublines overexpressing E2F-1. E2F-1 enhanced the expression of Apaf-1 without the cytosolic accumulation of cytochrome c. Apaf-1-deficient melanoma cell lines were resistant to E2F-1, indicating that Apaf-1 is an essential element of E2F-1-mediated apoptosis. Finally, we isolated the promoter region of the Apaf-1 gene and found a putative binding site for E2F. A chromatin immunoprecipitation assay revealed that E2F-1 bound to Apaf-1 promoter upon E2F-1 overexpression, suggesting that Apaf-1 is under transcriptional regulation of E2F-1. These data demonstrate a novel mechanism of apoptosis in which an increase in Apaf-1 levels results in direct activation of caspase-9 without mitochondrial damage, leading to the initiation of a caspase cascade.

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.

Role of cell cycle regulator p19ARF in regulating T cell responses

Although it is well established that the processes of cellular proliferation and apoptosis are linked, the role of cell cycle regulators in T cell responses in vivo is not well understood. In recent years, tumor suppressor molecule p19(ARF) has emerged as a key cell cycle regulator important in cellular apoptosis against strong mitogenic stimuli. In this study, we compared the antigen-specific T cell responses between wild type (+/+) and p19(ARF)-deficient (p19-/-) mice following an acute infection with lymphocytic choriomeningitis virus (LCMV). p19-/- mice mounted a potent CD8 T cell response and the magnitude of expansion of LCMV-specific CD8 T cells was comparable to that of +/+ mice. Further, the clonal downsizing of the expanded virus-specific CD8 T cells and establishment of long-term T cell memory were minimally affected by p19(ARF) deficiency. Therefore, p19(ARF) function is not essential to regulate T cell responses following an acute viral infection.

DeltaNp73, a dominant-negative inhibitor of wild-type p53 and TAp73, is up-regulated in human tumors

p73 has significant homology to p53. However, tumor-associated up-regulation of p73 and genetic data from human tumors and p73-deficient mice exclude a classical Knudson-type tumor suppressor role. We report that the human TP73 gene generates an NH(2) terminally truncated isoform. DeltaNp73 derives from an alternative promoter in intron 3 and lacks the transactivation domain of full-length TAp73. DeltaNp73 is frequently overexpressed in a variety of human cancers, but not in normal tissues. DeltaNp73 acts as a potent transdominant inhibitor of wild-type p53 and transactivation-competent TAp73. DeltaNp73 efficiently counteracts transactivation function, apoptosis, and growth suppression mediated by wild-type p53 and TAp73, and confers drug resistance to wild-type p53 harboring tumor cells. Conversely, down-regulation of endogenous DeltaNp73 levels by antisense methods alleviates its suppressive action and enhances p53- and TAp73-mediated apoptosis. DeltaNp73 is complexed with wild-type p53, as demonstrated by coimmunoprecipitation from cultured cells and primary tumors. Thus, DeltaNp73 mediates a novel inactivation mechanism of p53 and TAp73 via a dominant-negative family network. Deregulated expression of DeltaNp73 can bestow oncogenic activity upon the TP73 gene by functionally inactivating the suppressor action of p53 and TAp73. This trait might be selected for in human cancers.

Evaluation of strategies for the intracellular delivery of proteins

PURPOSE

The intracellular delivery of functionally active protein represents an important emerging strategy for laboratory investigation and therapeutic applications. Although a number of promising approaches for protein delivery have been developed, thus far there has been no attempt to compare the merits of the various deliver technologies. This issue is addressed in the current study.

METHODS

In this study we utilize a sensitive luciferase reporter gene assay to provide unambiguous and quantitative evaluation of several strategies for the intracellular delivery of a biologically active protein comprised of the Gal4 DNA binding domain and the VP16 transactivating domain.

RESULTS

Both a cationic lipid supramolecular complex and a poly meric complex were able to effectively deliver the chimeric transcription factor to cultured cells. In addition, protein chimeras containing the Tat cell penetrating peptide, but not those containing the VP22 peptide, were somewhat effective in delivery.

CONCLUSIONS

Both supramolecular protein-carrier complexes and protein chimeras with certain cell penetrating peptides can support intracellular delivery of proteins. In the cell culture setting the supramolecular complexes are more effective, but their large size may present problems for in vivo applications.

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.

Differential regulation of Rb family proteins and prohibitin during camptothecin-induced apoptosis

Prohibitin, a potential tumor suppressor, is known to induce growth suppression and repress E2F-mediated transcription. These growth regulatory functions of prohibitin require a physical interaction with the Rb protein. We now find that prohibitin protects cells from apoptosis mediated by camptothecin, a topoisomerase I inhibitor. Camptothecin treatment of Ramos B cells leads to the degradation of Rb protein and phosphorylation of its family members, p107 and p130. This correlates with an increase in the levels of cyclin E as well as the kinase activity associated with it. Inactivation of Rb leads to the dissociation and release of free E2F. We find also that E2F activity is induced upon camptothecin treatment, but this increase is absent in prohibitin overexpressing cells. It thus appears that prohibitin may be inhibiting apoptosis by downregulating E2F activity when Rb family members are inactive.

Induction of p73beta by a naphthoquinone analog is mediated by E2F-1 and triggers apoptosis in HeLa cells

Recently, p73 was identified as a structural and functional homolog of p53. The p73 protein activates the transcription of genes downstream of p53 and induces apoptosis when overexpressed in several cell lines, similar to the tumor suppressor p53. However, the extracellular stimuli and molecular mechanisms regulating p73 activity remain to be elucidated. In this paper, we present evidence that the naphthoquinone analog, 2,3-dichloro-5,8-dihydroxy-1,4-naphthoquinone (NA), is a novel apoptotic stimulus that induces p73beta expression. Treatment with NA induced the expression of p73beta mRNA and protein and its downstream genes, p21 and bax, in HeLa cells. Similar results were obtained in MCF7 cells (p53(+/+), p73(+/+)). In the MCF7 cells, p53 protein level was rather decreased by NA treatment. Overexpression of p73beta led to the apoptosis of HeLa cells and enhancement of NA-induced cell death. Expression of p73beta was mediated by E2F-1, which was activated via release from pRB after exposure of cells to NA. We additionally observed that overexpression of pRB inhibited NA-induced apoptosis. These results imply that p53-independent p73beta-dependent p21 expression is involved in NA-induced apoptosis of HeLa cells.

p14(ARF) regulates E2F activity

The ARF protein product of the ink4a/arf locus is induced by a variety of oncogenic signals. ARF facilitates growth arrest through the p53 pathway by hindering the down-regulation of p53 activity mediated by MDM2, through the formation of a protein complex with MDM2. Here we have explored the possibility that human p14(ARF) activity is integrated with growth regulating pathways other than p53, and report our results that p14(ARF) can control the activity of the E2F transcription factor. p14(ARF) regulates E2F activity in different cell-types, including p53(-/-)/mdm(-/-) MEFs, thus excluding that the effects of p14(ARF) are indirectly caused through MDM2 modulation. p14(ARF) down-regulates E2F-dependent transcription, and in cells undergoing E2F-dependent apoptosis prompts cell cycle arrest. p14(ARF) possesses multiple binding domains for E2F-1, one of which resides within the N-terminal region and coincides with the regulation of E2F activity. A mutational analysis of p14(ARF) indicates that the E2F-1 and MDM2 binding domains can be distinguished. These results highlight the potential interplay between p14(ARF) and E2F, and establish p14(ARF) as a pleiotrophic regulator of cell growth that acts by targetting at least two key pathways in the control of proliferation, namely E2F and p53.

Induction of apoptosis by telomere 3' overhang-specific DNA

Telomeres are tandem repeats of a specific TTAGGG nucleotide sequence at the ends of chromosomes. Telomere shortening is proposed to act as a biological clock and cancer prevention mechanism by inducing a nonproliferative, senescent phenotype after a limited number of cellular divisions. Recent evidence also suggests that telomere disruption can trigger apoptosis in certain cell types, mimicking a major cellular response to DNA damage. Here, we show that addition of DNA oligonucleotides homologous to the telomere 3' overhang sequence causes lymphocytic (Jurkat) cells to undergo apoptosis, as described for lymphocytes following telomere loop disruption. We further implicate the p53 tumor suppressor and transcription factor, as well as the p53 homolog p73 and the E2F1 transcription factor, in mediating the apoptotic response. We propose that exposure of the telomere 3' overhang due to opening of the normal telomere loop structure is a physiologic signal for these DNA damage-like responses in vivo and that oligonucleotides partially or completely homologous to the telomere overhang mimic this signal in the absence of DNA damage or telomere disruption.

TP53 polymorphism of exon 4 at codon 72 in cutaneous squamous cell carcinoma and benign epithelial lesions of renal transplant recipients and immunocompetent individuals: lack of correlation with human papillomavirus status

A common polymorphism at codon 72 of exon 4 encoding either arginine or proline has been shown to confer a susceptibility to the development of skin tumor in renal transplant recipients. Moreover, this polymorphism may affect proteolytic degradation of p53 promoted by E6 protein from mucosal human papillomaviruses and represent a risk factor for human-papillomavirus-induced carcinogenesis. In this study, we analyzed the human papillomavirus presence and the TP53 allele distribution in cutaneous squamous cell carcinoma of renal transplant recipients and immunocompetent patients. Fifty-three squamous cell carcinomas from 40 renal transplant recipients, 50 benign epithelial skin lesions from 50 renal transplant recipients with no history of skin cancer, 51 squamous cell carcinomas from immunocompetent patients, and 29 blood samples from immunocompetent individuals without skin cancer were investigated. Human papillomavirus DNA was detected using polymerase chain reaction performed with two pairs of primers (MY09-MY11 and FAP59-FAP64). TP53 allele distribution was studied by denaturing gradient gel electrophoresis assay, followed by sequencing analysis. Human papillomavirus DNA was detected in 64% of squamous cell carcinoma and 79% of benign epithelial lesions from renal transplant recipients (NS) and only in 37% of squamous cell carcinoma from immunocompetent patients (p < 0.05). Mucosal oncogenic human papillomavirus types were predominant in squamous cell carcinoma from both renal transplant recipients and immunocompetent patients. Rate of arginine homozygosity in squamous cell carcinoma from renal transplant recipients was significantly higher (83%) than in immunocompetent patients with or without squamous cell carcinoma (60% and 59%, respectively). Our results suggest that TP53 arginine/arginine genotype could represent a potential risk factor for the development of squamous cell carcinoma in renal transplant recipients compared to immunocompetent patients. No association between TP53 arginine/arginine genotype and human papillomavirus status could be determined, however.

DN-p73 is activated after DNA damage in a p53-dependent manner to regulate p53-induced cell cycle arrest

p53 and p73 genes are both activated in response to DNA damage to induce either cell cycle arrest or apoptosis, depending on the strength and the quality of the damaging stimulus. p53/p73 transcriptional activity must be tightly regulated to ensure that the appropriate biological response is achieved and to allow the cell to re-enter into the cell cycle after the damage has been repaired. In addition to multiple transcriptionally active (TA) isoforms, dominant negative (DN) variants, that lack the amino-terminal transactivation domain and function as trans-repressors of p53, p63 and p73, are expressed from a second internal promoter (P2-p73Pr). Here we show that, in response to a non apoptotic DNA damage induced by low doses of doxorubicin, p53 binds in vivo, as detected by a p53-specific chromatin immunoprecipitation assay, and activates the P2-p73 promoter. DN-p73alpha protein accumulates under the same conditions and exogenously expressed DN-p73alpha is able to counteract the p53-induced activation of the P2-p73Pr. These results suggest that DN-p73 may contribute to the autoregulatory loops responsible for the termination of p53/p73 responses in cells that do not undergo apoptosis. Accordingly, the activation of the P2-p73Pr is markedly enhanced in both p73-/- murine fibroblasts and in human cells in which p73 transcripts are selectively knocked-out by p73-specific small interfering RNAs.

Activation of ARF by oncogenic stress in mouse fibroblasts is independent of E2F1 and E2F2

The ARF tumour suppressor protein (p14(ARF) in human and p19(ARF) in mouse) is a major mediator of the activation of p53 in response to oncogenic stress. Little is known about the signalling pathways connecting oncogenic stimuli to the activation of ARF. Regulation of ARF occurs primarily at the transcriptional level and several modulators of ARF transcription have been identified. Notably, ectopic expression of E2F1 upregulates ARF transcriptionally, and both E2F1 and ARF have been implicated in apoptosis and cell-cycle arrest. We have used primary mouse fibroblasts deficient for E2F1, E2F2, or both to determine the possible role of these E2F proteins as upstream regulators of ARF in response to oncogenic stimuli and other stresses. In particular, we have studied the effects of oncogenic Ras and the viral oncoprotein E1A on ARF levels, neoplastic transformation, and sensitization to apoptosis. We have also examined the behaviour of the E2F-deficient MEFs with respect to immortalization and sensitivity to DNA damage. None of the ARF-mediated responses that we have analysed is significantly affected in E2F1(-/-), E2F2(-/-) or E2F1/2(-/-) MEFs, and ARF is upregulated normally in all cases. Taken together, our results indicate that the activation of ARF in response to oncogenic stress can occur by E2F1- and E2F2-independent mechanisms. This challenges previous suggestions implicating E2F factors as key mediators in the activation of ARF by oncogenic stress.

E2F1 and c-Myc potentiate apoptosis through inhibition of NF-kappaB activity that facilitates MnSOD-mediated ROS elimination

Overexpression of c-Myc or E2F1 sensitizes host cells to various types of apoptosis. Here, we found that overexpressed c-Myc or E2F1 induces accumulation of reactive oxygen species (ROS) and thereby enhances serum-deprived apoptosis in NIH3T3 and Saos-2. During serum deprivation, MnSOD mRNA was induced by NF-kappaB in mock-transfected NIH3T3, while this induction was inhibited in NIH3T3 overexpressing c-Myc or E2F1. In these clones, E2F1 inhibited NF-kappaB activity by binding to its subunit p65 in competition with a heterodimeric partner p50. In addition to overexpressed E2F1, endogenous E2F1 released from Rb was also found to inhibit NF-kappaB activity in a cell cycle-dependent manner by using E2F1(+/+) and E2F1(-/-) murine embryonic fibroblasts. These results indicate that E2F1 promotes apoptosis by inhibiting NF-kappaB activity.

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

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

Gene expression changes in response to E2F1 activation

The p16/RB/E2F regulatory pathway, which controls transit through the G1 restriction point of the cell cycle, is one of the most frequent targets of genetic alterations in human cancer. Any of these alterations results in the deregulated expression of the transcription factor E2F, one of the key mediators of cell cycle progression. Under these conditions, E2F1 also participates in the induction of apoptosis by a p53-dependent pathway, and independently of p53. Recently, we identified the p53-homolog p73 as a first direct target of p53-independent apoptosis. Here, we used a cDNA microarray to screen an inducible E2F1-expressing Saos-2 cell line for E2F1 target genes. Expression analysis by cDNA microarray and RT-PCR revealed novel E2F1 target genes involved in E2F1-regulated cellular functions such as cell cycle control, DNA replication and apoptosis. In addition, the identification of novel E2F1 target genes participating in the processes of angiogenesis, invasion and metastasis supports the view that E2F1 plays a central role in many aspects of cancer development. These results provide new insight into the role of E2F1 in tumorigenesis as a basis for the development of novel anti-cancer therapeutics.